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NBS SPECIAL PUBLICATION 435

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

Laser Induced Damage

in Optical Materials:

1975
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Laser Induced Damage aatoo

In Optical Materials: 1975 ao hsb

Proceedings of a Symposium Sponsored by:

National Bureau of Standards

American Society for Testing and Materials

Office of Naval Research

Energy Research and Development Administration

July 29-31, 1975

NBS Boulder, Colorado 80302

Edited by

Alexander J. Glass

Lawrence Livermore Laboratory

Livermore, California 94550

and

Arthur H. Guenther

Air Force Weapons Laboratory

Kirtland AFB, New Mexico 87117

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

James A. Baker, III, Under Secreiary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

U5 NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued April 1976
Library of Congress Cataloging in Publication Data

Symposium on Laser Damage in Optical Materials, 7th, Boulder,

Colo., 1975. Laser induced damage in optical materials, 1975.

(NBS special publication; 435)

1. Optical materials—Defects—Congresses. 2. Optical materials.

Effect of radiation on—Congresses. 3. Laser materials—Defects

Congresses. 4. Laser materials. Effect of radiation on—Congresses.

I. Glass, Alexander, J. 1933- . IL Guenther, Arthur Henry,

1931- . IIL United States. National Bureau of Standards. IV.

Title. V. Series: United States. National Bureau of Standards.

Special publication : 435. QC100.U57 No. 435 [QC374] 602'.ls [621.36'6'028]

76-4985

National Bureau of Standards Special Publication 435

Nat. Bur. Stand. (U.S.), Spec. Publ. 435, 437 pages (Apr. 1976)

CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1976

For sale by the Superintendent of Documents, U.S. Government Printing Ofilce Washington, D.C. 20402 - Price $5.60
Foreword

These proceedings report in detail the formal papers presented at the 7th Annual

Symposium on Laser Damage in Optical Materials held at the National Bureau of Standards,

Boulder, Colorado, July 29-31, 1975. This meeting was jointly sponsored by the

Office of Naval Research, National Bureau of Standards, the Energy Research and

Development Administration, and the American Society of Testing and Materials. The

major topics covered were optical fabrication, use of metal mirrors for high power

lasers, window materials for infrared lasers, measurement of laser induced damage in

the infrared, damage in dielectric film and at exposed surfaces, avalanche ionization,

and multiphoton absorption as a damage mechanism.

The Co-chairperson, 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, July 13-15,

1976, 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 and surface damage, machined optics,

the problem of cleaning optical materials and components, as well as address ourselves

to the subject of avalanche and multiphoton processes particularly as they relate to

short wavelength application. Hopefully, additional 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

DISCLAIMER

Certain commercial equipment, instruments, or materials are identified in this

publication in order to adequately specify the experimental procedure. In no case does such

identification imply recommendation or endorsement by the National Bureau of Standards, nor

does it imply that the material or equipment identified is necessarily the best available

for the purpose.

Ill

CONTENTS

Section p^gg

Foreword j^^^^ A. H. Guenther

Summary and Conclusions _

. • • • A. J. Glass and A. H. Guenther

0.1 Opening Remarks

^ A. H. Guenther

FABRICATION

1.1 Fabrication of Laser Optics at Lawrence Livermore Laboratory 3 Norman J. Brown

1.2 Standard Industrial Polishing of High Energy Laser Optics 10

Walter J. Spawr

1.3 Ion Planing and Coating of Sodium Chloride 14

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

1.4 Polishing Studies and Backscatter Measurements on Alkali-Halide Windows 20

M. J. Soileau, H. E. Bennett, J. M. Bethke, & J. Shaffer

METAL MIRRORS

2.1 1.06 um psec Laser Damage Study of Diamond Turned, Diamond Turned/Polished and

Polished Metal Mirrors 29

T. T. Saito, D. Milam, P. Baker, & G. Murphy

2.2 Pulsed Laser Damage to Uncoated Metallic Reflectors 41

I. Goldstein, Don Bua, & F. A. Horrigan

2.3 Diamond-Turned Mirrors 49

H. E. Bennett, M. J. Soileau, & P. C. Archibald

2.4 Results of Optical Measurements of Surface Quality and Figure of Diamond-Turned Mirrors. 57

R. E. Sladky & R. H. Dean

2.5 Dielectric Coated Diamond Turned Mirrors 66

J. R. Buckmelter, T. T. Saito, R. Esposito, L.P, Mott, & R. Strandlund

2.6 Slide-Position Errors Degrade Machined Optical Component Quality 75

J. B. Arnold, P. J. Steger, & R. R. Burleson

2.7 Ultrasonic Cleaning of Optical Surfaces 90

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

IR WINDOW MATERIALS

3.1 Thermal Diffusivity of Germanium, Gallium Arsenide and Cadmium Telluride Over

the Temperature Range 80 K - 900 K 98

R. Doussain & H. P. LeBodo

3.2 Damage to 10.6 Um Window Materials Due to CO2 Tea Laser Pulses 107

K. M. Leung, M. Bass, & A. G. J. Balbin-Villaverde

3.3 Improvements in the Breakdown Threshold in Alkali Halides at 10.6 pm 118

V. Wang, G. R. Giuliano, S. D. Allen, & R. C. Pastor

3.4 Optical Distortion by Laser Heated Windows 126

J. S. Loomis & E. Bernal G.

3.5 Thermal Distortion Studies of ZnSe Windows by Far Field Irradiance Measurements .... 142

J. A. Detrio & R. D. Petty

3.6 Spectral Emittance Measurements with a Cryogenically Cooled Instrument 148

D. L. Stierwalt

3.7 Surface Studies with Acoustic Probe Techniques 157

J. H. Parks & D. A. Rockwell

3.8 Photoelastic Constants of Infrared Materials 164

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

3.9 Refractive Index and Temperature Coefficient of Index of CVD Zinc Selenide 170

M. J. Dodge & I. H. Malitson

3.10 Laser Window Test Facility 175

J. L. Zar

V
IR DAMAGE TESTS

4.1 Optical Material Damage from 10.6 pm CW Radiation 189

C. A. Huguley & J. S. Loomis

4.2 10.6 um Component Damage from a 20 psec Rapidly Pulsed Laser 202

A. B. Callander

4.3 Laser Damage Measurements at CO2 and DF Wavelengths 207

J. 0. Porteus, M. J. Soileau, H. E. Bennett, & M. Bass

DAMAGE IN DIELECTRIC FILMS AND AT EXPOSED SURFACES

5.1 Single and Multilongitudinal Mode Damage in Multilayer Reflectors at 10.6 ym as a Function of Spot Size and Pulse Duration 216

V. Wang, C. R. Giuliano, & B. Garcia

5.2 Temperature and Wavelength Dependence of the Reflectance of Multilayer Dielectric

Mirrors for Infrared Laser Applications 230

D. L. Decker

5.3 Preparation and Evaluation of ZnS/CeFs AR Coatings for 10.6 micron KCl Laser Windows. . . 236

A. Golubovic, W. Ewing, R. Bradbury, I. Berman, J. Bruce, & J. J. Comer

5.4 AS2S3 Coatings on KCl 244

A, D. Baer, T. M. Donovan, & M. J. Soileau

5.5 Design for High Power Resistance 248

A. L. Bloom & V. R. Costich

5.6 Influence of Standing-Wave Fields on the Laser Damage Resistance of Dielectric Films. . . 254

B. E. Newnam, D. H. Gill, & G. Faulkner

5.7 Threshold Ambiguities in Absorptive Laser Damage to Dielectric Films . 272

R. H, Picard, D. Milam, R. A. Bradbury, & J. C. C. Fan

5.8 Time Resolved Study of Laser-Induced Structural Damage in Thin Films 284

N. Alyassini S. J. H. Parks

5.9 The Importance of Pefractive Index, Number Density, and Surface Roughness in the

Laser-Induced Damage of Thin Films and Bare Surfaces 289

J. R. Bettis, R. A. House, A. H. Guenther, and R. Austin

5.10 Investigation of the Damage Properties of Multilayer Dielectric Coatings for Use in

High Power Nd: Glass Lasers 296

C. E. Thomas, B. Guscott, K. Moncur, S. Hildum, & R. Sigler

5.11 Correlation of Laser-Induced Damage with Surface Structure and Preparation Techniques

of Several Optical Glasses at 1.06 ym 305

R. A. House, J. R. Bettis, A. H. Guenther, & R. Austin

AVALANCHE IONIZATION and MULTIPHOTON PROCESSES

6.1 Picosecond Breakdown Studies: Threshold and Nonlinear Refractive Index Measurements

and Damage Morphology 321

W. L. Smith, J. H. Bechtel, & N. Bloembergen

6.2 Current Status of Electron-Avalanche-Breakdown Theories ...... 331

M. Sparks

6.3 A Statistical Analysis of Laser Induced Gas Breakdown - A Test of the Lucky Electron

Theory of Avalanche Formation 347

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

6.4 Measurement of Free Electron Density at the Onset of Laser-Induced Surface

Damage in BSC-2 356

N. Alyassini & J. H. Parks

6.5 Multi-pulse Optical Damage of NaCl 362

P. Braiinlich & P. Kelly

6.6 Do Multi-photon Induced Collision Chains Lead to Pre-breakdown Material

Modifications in Alkali Halides? • 366

A. Schmid, P. Branlich, & P. K. Rol

6.7 Multiphoton Absorption Coefficients in Compound Semiconductors from Ruby to CO2

in Laser Wavelengths 369

R. A. Shatas, S. S. Mitra, & L. M. Narducci

6.8 Electroabsorption: A Possible Damage Consideration 389

R. P. Benedict 5, A. H. Guenther

6.9 Irradiance Limits for Vacuum Ultraviolet Material Failure 395

C. J. Duthler & M. Sparks

VI
APPENDIX I

Participants ^06

VII
Laser Induced Damage in Optical Materials

7th ASTM Symposiuin

July 29-31, 1975

The Seventh ERDA-ASTM-ONR-NBS Symposiuin on Laser Induced Damage in Optical

Materials was held at the National Bureau of Standards in Boulder, Colorado,

on July 29-31 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 ChairDerson of

Subcommittee II is Haynes Lee, of Owens-Illinois, Inc. Co-chairpersons for the

Damage Symposia 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 Symposium heard 42 papers on topics relating

fabrication procedures to laser induced damage in optical materials; on

metal mirrors; in infrared window materials; the multipulse, wavelength

and pulse length dependence of damage thresholds; damage in dielectric films

and at exposed surfaces; as well as theoretical discussions on avalanche

ionization and multiphoton processes of importance at shorter wavelengths.

Of particular importance was the sealing relations developed from several

parametric studies relating fundamental properties (refractive index,

surface roughness, etc.) to the damage threshold. This year many of the

extrinsic influences tending to reduce a material damage resistance were

isolated such that a measure of its egregious nature could be quantified.

Unfortunately, it was evident that much still needs to be accomplished to

improve processing and fabrication procedures to allow a measurable approach

to a materials intrinsic strength to be demonstrated.

Key words: Avalanche ionization; IR windows and mirrors; laser damage; laser

materials; multiphoton processes; self -focusing ; thin films.

1. Principal Conclusions ^

The seventh consecutive Symposium on Damage in Laser Materials was held at the

National Bureau of Standards in Boulder, Colorado, on July 29-31, 1975. This symposium,

held each year since 1969, was jointly sponsored by the National Bureau of Standards,

the Office of Naval Research and the Energy Research and Development Administration, and

is an activity of Subcommittee II of the Araerican Society of Testing and Materials, on Lasers and Laser Materials. The syuposium co-persons are Dr. Arthur H. Guenther,

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

of the Theory and Design Analysis Group in Y-Program at the Lawrence Livermore Laboratory. The symposium was attended by about 150 scientists v/ho heard papers on a variety

of subjects relating to the suitability of various materials and components for highpower laser applications. Primary interest was concentrated on problems attendant to

large glass lasers for fusion research, and large gas lasers operating in the infrared,

for military and industrial applications . Forty-five papers were presented during the

two and one-half day symposium, representing research efforts at twenty-five institutions in the US, France, and Australia. Next year's symposium will be held at the

National Bureau of Standards in Boulder, Colorado, on July 13 to 15, 1976.

The topics discussed at this year's symposium included details on techniques of

optical fabrication, use of metal mirrors for high power lasers, window materials for

IR lasers, measurement of laser induced damage in the IR, damage in dielectric films and

at exposed surfaces, avalanche ionization, and multiphoton absorption as a damage

mechanism. The sessions on metal mirrors and fabrication were moderated by Capt.

T. A. Saito of the Air Force Weapons Laboratory, who is currently at the Lawrence

Livermore Laboratory. The increasing use of metal mirrors for both IR gas and Ndglass lasers is due primarily to improved fabrication methods, especially diamondturning or micomachining . In fact, diamond- turned metal mirrors are now being

employed for high level laser fusion illumination systems. Experimental results

indicate that damage thresholds on quality diamond-turned metal surfaces are comparable

to dielectric surface values, and as expected in excess of those seen with dielectric

coated metal surfaces

.

VIII
Where the reflectivity of the bare metal is sufficiently high, diamond turning

offers an attractive means of mirror generation, especially for spherical, ellipsoidal,

or other non-planar geometries. Enhancing the performance of metal mirrors, however, by dielectric coatings, is hard to achieve without lowering the damage threshold.

This is due to poorer adhesions of coatings, and to the multitude of problems associated

with the use of dielectric coatings at high power levels, as detailed below in this

article. In the fiiicromachining process itself, machine control is still a problem,

especially for large, non-planar figures. Inspection techniques need considerable

improvement, especially for fast optics

.

Similar considerations apply to the fabrication of large transmitting elements,

such as laser disks, and gas laser windows. Detailed understanding is emerging of

the influence of the fabrication process on the damage threshold of the element, and

in many cases, the preferred processes have been identified. To be useful, however,

these processes must be suitable for volume production of large pieces. Similarly,

inspection techniques must advance correspondingly, and must be capable of identifying

those features to which the damage process is sensitive, such as micro-irregularities

and particulate impurities.

In infrared window materials, surface absorption is seen to play a limiting role

in establishing the damage threshold. Damage in most cases results not from thermal

fracture of the element, but from thermal distortion. New methods of surface preparation have been developed, especially for alkali halides, to decrease surface absorption.

Surface cleaning, with acid etching, ion-beam polishing, or even flame polishing, can increase the damage resistance of surfaces, by removing impurities, but can also alter

the optical character of the surface. Prolonged ultrasonic cleaning can expose imperfect

sub-surface features, yielding a lower damage threshold. Fundamental measurements of

material properties relevant to thermal damage are beginning to be tabulated. These

include values of thermal diffusivity, bulk absorption, surface absorption, the refractive index and its temperature variation, and photoelastic constants. These measurements provide the necessary data base for the evaluation of material performance.

Care must be taken in both obtaining and applying the measured values of parameters

like absorption coefficients, due to the strong effect of impurities.

Several laboratories have established dedicated facilities for damage testing.

These include the Michelson Laboratory at the Naval Weapons Center in China Lake, the

Air Force Weapons Laboratory, Avco-Eyerett Research Laboratory, and Lawrence Livermore

Laboratory. All laser fusion laboratories are perforce damage testing facilities, as was evidenced by the paper presented by C. E. Thomas of KMS Fusion.

Present systems are primarily limited by failure at optical surfaces. It was pointed out at this year's symposium that surface damage, especially where coated

surfaces and multi-layer dielectrics are involved, is a complicated phenomenon dependent

on many parameters of the system. Thus, in high repetition rate CO2 lasers, surface

damage via plasma formation shows hysteresis effects, with plasma formation and damage

on successive pulses being dependent on conditions created by previous pulses. Plasma

shielding of surfaces is seen, and plasma propagation toward the laser is observed.

In thermal failure, coated elements exhibit lower damage thresholds, due to the poor

thermal conductivity across the interface between coating and- substrate

.

Nowhere is the complexity of damage phenomena more evident than in multilayer

dielectric coatings. These film materials are variable in structure, depending on

the process by which they are deposited. Their morphology is seen to change, after

deposition, due to residual stress, thermal cycling during a laser pulse, or even on

the shelf. Thin film damage is seen to depend on substrate roughness, even down to

o rms roughness values of less than 100 A, on interlayer roughness, standing wave intensity, and residual stress. Empirical relations are emerging, however, which allow

one to isolate the damage dependence on each separate variable, in order to assess

the relative importance of each of the contributing factors. The dependence of surface

breakdown electric field E on surface roughness, for example, is found to take the

"HI

i form E 'V a , where a is the rms roughness, and m 0.5. This expression appears to

o o be valid for 10 A < o < 300 A, although a complete theoretical justification is yet

unavailable.

The dependence on surface roughness is an example of the cycle of improvement of

optical materials. The relation between the process of fabrication and the measured

parameters, in this case, the roughness, is established via careful measurement. The

relation between the measured parameters and the damage threshold is elucidated by

careful experiment, and then summarized in an empirical relationship of convenient

IX
simplicity. Thus the fabrication process is linked directly to the performance characteristics via an intermediate scientific hypothesis. The laser user can specify to the

vendor the required fabrication method, and can measure the resultant character of the

element involved. Unless this cycle is complete, specification of high power laser

components is essentially impossible.

An observation emerging from the investigations of several laboratories concern

the use of Zr02 vs. Ti02 for the high index layer of multilayer film. Although Zr02

has a slightly lower index than Ti02 , its damage level, in thin film form, is consistently lower. This is attributed to poorer homogeneity and purity in this material.

Some evidence is emerging of multiphoton absorption in Zr02 films, even though the

band structure of the material would seem to preclude this effect. A preference for

Ti02 over Zr02 has been indicated by several manufacturers and purchasers of thin

films for high power use. Whenever thin films are involved, it is of great importance to distinguish between bulk properties of the material and thin film properties. Fundamental characteristics of optical materials, such as refractive index, are seen to vary significantly from bulk values depending on the nature of the film deposition process, the

substrate, the thermal environments, and other variables of the coating process.

All properties of thin films are severely affected by impurities, particulate inclusions, and defects, both in the films and at the film-substrate interface.

The ultimate failure mode of dielectric surfaces in short pulses is by avalanche

ionization. Evidence is accumulating that this process is more complicated than previously thought. The process whereby the avalanche is initiated, by impurity ionization, multiphoton ionization, or excitation of electrons from shallow traps and color

centers, is seen to play a large role in determining the threshold for avalanche

ionization, the statistical character of the avalanche, and the wavelength dependence

of the breakdown threshold. Avalanche ionization in solids, and at dielectric surfaces, will be a subject of continuing, vigorous investigation.

Interest in uv lasers has spurred renewed investigation of multiphoton processes.

Here again, the theoretical picture is uncertain, and accurate experimental data are

just beginning to appear. As more powerful short wavelength lasers are developed,

multiphoton processes will be more clearly revealed.

A vital lesson can be learned from the series of symposia on laser damage, one

that may not be fully appreciated. At present, optical material development retains

an empirical flavor, and lacks the underlying base of continuing scientific support

the field requires. The tendency is to attack problems as they arise in the engineering

development of large systems, rather than attempting to develop the base of scientific

understanding. This is a consequence of the way funds are allocated, rather than the

proclivities of the research community. In the long run, the absence of an underlying scientific program in optical materials development will increase the cost and

decrease the performance and reliability of large laser systems. It is fervently hoped

that this situation can be remedied in the not-too-distant future.

2 . 0 Summary of Papers

The papers presented at the 19 75 Symposium were organized under six topics:

Fabrication; Metal Mirrors; IR Window Materials, IR Damage Tests; Damage in Dielectric

Films and at Exposed Surfaces, and Avalanche Ionization and Multiphoton Processes.

Because of the large number of papers submitted and an intent to avoid concurrent sessions, several papers were presented by title only. These papers have been inserted

and summarized under the most appropriate topical heading. As in the 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 these papers of greatest interest

to him.

2 . 1 Fabrication

The influence of fabrication procedures on resultant optical quality and damage

resistance was described in four papers in the initial session of the Symposium.

These early presentations stressed the processes and test employed at four organizations involved in the generation of high power optical components for use from

1.06 pm to 10.6 um. It is obvious to most knowledgeable laser specialists that

fabrication variation is the largest contributing factor in the non-reproducibility

and ultimate damage resistance of high power optical elements.

X
To lead off the session on Fabrication, N. J. Brown of the Lawrence Livermore

Laboratory discussed the procedures and techniques presently employed at LLL in the

fabrication of laser optics. Techniques developed at LLL for laser optics fabrication

are strongly tied to the stringent requirements of figure control, geometric (parallelism)

control, and cosmetic quality for both dielectric and metallic elements. Details relating to the importance of temperature stability and component blocking were presented in light of A/10 figure and 2-3 seconds of parallelism tolerances.

For example, polishing elliptically shaped laser disks to good figure is difficult

because of fluctuating thermal gradients across the disk. A typical radius to thickness

o ratio of 4 at A 6328 A in BK7 leads to about 4 1/2 fringes/degree for a 20 cm diameter

workpiece. Thus it is easily seen that very small thermal gradients can indeed cause

considerable difficulty in fabricating good figure elements, particularly if the workpiece is removed from the lap for intermittent testing and placed in a slightly different

temperature environment. Many other fabrication concerns and limits were described.

W. J. Spawr of Spawr Optical Research, Inc., discussed standard industrial polishing

procedures employed at his company. His message stressed the major importance of the

skilled and careful optician who through experience or knowledge of the fabrication

process can produce high quality optics in a timely manner. In the case of metal

mirrors, they have essentially eliminated the grinding process by the use of standard

3 2 machine tools. Machining to a /~ finish allows the grinding step to be eliminated.

The component proceeds directly to the polishing operation. This procedure has consistenly yielded damage resistant elements.

Ion polishing has been employed in several cases to improve the surface finish

of optical materials, most notably glasses, metals and hard crystalline substances such

as sapphire. R. A. Hoffman, W. J. Lange and W. J. Choyke of Westinghouse have explored

this technique on single crystal NaCl, using low energy grazing incidence Xe ions. The

main purpose of this study was not only improving surface finish but to evaluate the

adhesion of films to ion planed vs. conventionally finished surfaces. Results indicated that ion planing and coating (AS2S2) in situ of NaCl surfaces minimizes contamination at the film-substrate interface, greatly improves film adherence and protects NaCl

substrates against moisture attack. While previously introduced surface scratches were

removed, they were replaced by other raised surface features, which may be the result of

different sputtering rates at dislocations introduced in the grinding phase or from

contaminates. This area requires further study.

Surface aborption, optical figure and optical scatter from alkali-halide windows

are particularly important in high average power 10.6 pm lasers. These factors were

i considered from a polishing standpoint in a paper by M. J. Soileau, H. E. Bennett,

J. M. Bethke, and J. Shaffer of the Michelson Lab at the Naval Weapons Center, Through

a chemical-mechanical surface finishing technique they have developed alkali-halide

-4

surfaces which have less than 1 x 10 surface absorption, are parallel to < 3 seconds

of arc and flat to better than A/4 in the visible and achieve scatter levels below

-4

10 at 10.6 pm. While the recommended procedure is too lengthy to detail here, it

involves first removal of all surface contamination and mechanical damage by etching

in concentrated HCl. The requisite optical figure is then produced with a minimum of

surface working. In this procedure, the surface is initially ground to approximate

flatness by various controlled grinding with different grit sizes (20 5 ym) of

^^2^3 kerosene on a Pyrex plate . The sample is then etched for 3 minutes and

rinsed in ethanol, then blown dry with nitrogen. Thence, several steps proceed to

the final polishing and cleaning state. If the sample does not reach the desired

figure in 4 hours of polishing it is best to re-etch and then repolish again. This

procedure allows the fabrication of alkali halide surfaces of acceptably high quality.

Several important ancillary conclusions were drawn from this work. They are:

(1) scratches significantly increase IR scattering in KCl windows as well as resulting

-4

in lower damage thresholds; (2) in the absence of scratches scattering above 5 x 10

is determined by microirregularities , while below this by particulates; (3) components

used in reflection (mirrors) are much more sensitive to microirregularities than lowindex materials used in transmission (windows) . For example, surface a values of

o o 80 to 100 A rms can be tolerated on windows while o values of 20 A rms or less are

desirable for mirrors; and finally (4) the magnitude of backscattering from windows is

approximately equal to the magnitude of forward scattering.

XI
2.2 Metal Mirrors

The growing interest in metal mirrors for both high peak power and high average

power applications was evident in the second topical session. Results of damage testing

under these two conditions was reported along with detailed optical measurements of

diamond turned surfaces. Problems relating to the actual turning and cleaning of

micromachined surfaces were revealed. These two areas are in need of further emphasis

if damage resistant coatings for enhanced reflectivity of high optical quality metal

mirrors are to be achieved.

Although metal substrate mirror development is generally driven by high average

power infrared applications (primarily because of better thermal conductivity and

evidenced high reflectivity of these elements) , they may as well offer considerable

advantages as laser fusion system components as well. In this vain, the damage

resistance of diamond turned and polished metal mirrors to 150 psec, 1.06 pm laser

pulses was the subject of a report by T. T. Saito, D. Milam, P. Baker and G. Murphy of

the Lawrence Livermore Laboratory. Several diamond turned, diamond turned/polished and

polished copper and silver mirrors were evaluated both at near normal and 45° angles

of incidence. As a corollary to the experimental damage assessments a one-dimensional

neat flow treatment was developed. In this case to attain satisfactory agreement between theory and experiment, it was deemed necessary to include the heat of fusion into

the computed damage threshold. From the experimental observations, diamond turned

surfaces had the highest damage threshold of the types of surfaces tested, with silver

(4 J/cm^ ) being the best. Similar to other reports at this symposium an increased

damage resistance at a 45° angle of incidence for s polarization was found. Thus

these elements compare favorably with dielectric mirrors. This work demonstrated that

various defects introduced during fabrication degrade the damage resistance of silver.

A multiplicative degradation factor D was estimated for each effect. A reduction of the

tneoretical damage threshold was observed as follows: (a) slight electroplating

defects (D=0.8); (b) diamond turning marks (D=0.7); (c) scratches introduced during

cleaning (D=0.5); and (d) final polishing (D=0.1^0.2).

Threshold damage measurements on a variety of polished bulk copper and thin gold

film reflectors were reported by I. Goldstein and D. Bua of Raytheon and F. A. Horrigan

of Science Applications. The laser employed was a Kd:YAG system operating in the TEM^q

mode with a pulse width of 11 nsec FWHM. Experimental results agreed quite well with

a simple thermal model assuming a critical surface temperature at damage threshold.

From their results a figure of merit (FOM) for the selection of metal mirrors materials

AT /kC

useful in pulsed laser applications was tendered. This FOM was equal to = —, where

J. — K

AT^ = critical temperature rise at the surface, K the thermal conductivity in watt/

cm °K, C is the specific heat per area, in joules/cm^, and R the reflectivity of

the surface. Based upon this analysis, the ranking of normally used materials shows

copper to be the optimum, followed by silver and gold. Sample tests were performed

to validate the suggested analytical approach, together with damage measurements as

a function of angle of incidence and polarization. They found damage thresholds as high as 44 j/cm^at a 70° angle of incidence. Furthermore, increasing the reflecting

film thickness also causes an increase damage resistance until a thickness of about

1 ym is reached. A basic conclusion of this study was that use of reflecting metal

mirrors at large angles of incidence, such as in toric optics, can lead to greater

throughput power/operative area than more normal incidence focusing configuration.

The group at the Michelson Laboratories, Naval Weapon Center, have applied their

extensive array of optical evaluation facilities to assess diamond turned mirrors.

H. E. Bennett, M. J. Soileau, and P. C. Archibald investigated the surface figure,

scattering characteristics, reflectivity, laser damage resistance and environmental

resistance of several high quality micromachined elements. Generally, optical figure

better than A/20 at 10.6 pro for ^ 4 cm diameter samples and one wave for 10 cm samples was noted on a large number of samples from the Y-12 plant at Oak Ridge.

Scattered light levels in the infrared as low as 2 x 10 ^ were noted, comparable to

o values of good conventionally polished mirrors. Typical roughness was 20-30 A rms

o with one sample showing only a 12 A rms level from scattering measurements. In addition, infrared reflection values as high or higher than those of conventional evaporated

or sputtered metallic coatings have been achieved and one mirror exhibited an absorption of only 0.006 at 10.6 ym. It is conjectured that the damage threshold of micromachined mirrors may well be higher than those produced by other techniques. This

specific area as well as environmental durability and coatability of these surfaces

requires further assessment.

XII
R. E. Sladky and R. H. Dean of the Oak Ridge Y-12 plant discussed optical measurements on high-quality, large diamond-turned mirrors. They pointed out that in order

to produce acceptable super finished metal mirrors by single point turning, five

requirements must be met. They are: (1) precise machine movements; (2) complete

static and dynamic vibration analysis; (3) dynamic balancing; (4) temperature control;

and finally (5) sharp diamond tools. However, after the component is fabricated,

testing is necessary to insure that it meets specification as well as allowing immediate

diagnostic information to be made available to the machine tool engineer. A most

important test has been surface finish and surface figure. A recently fabricated flat

15.5 X 25.75 inch, elliptical mirror, made of copper electroplated on an aluminum

substrate was inspected by both a Leitz interference miroscope and a Davidson piano

interferometer. Average deviation from flatness was 77 microinches (saddle-shaped)

.

This result is thought to be due to the shape of the mirror, and its blocking resulting

in mirror flexing during fabrication. Spherical mirrors are normally tested with the

classic Foucault knife-edge test. For a 15" diameter spherical mirror with a radius

of curvature of 133 inches, a maximum error of 31 yinch (A/13 at 10.6 ym) was obtained.

Other measurements frequently made are blur circle testing, scattering and diffraction.

Diamond turned surfaces are quite similar to a circular diffraction grating. A typical

cusp spacing might be 0.001 inch resulting in a '^^ 1.2 pinch finish. Further studies

are under way to assess the utility of diffraction testing of these surfaces as an indication of mirror quality.

Because of the outstanding potential of diamond turned metal mirrors as high average

power laser elements, an obvious possibility for improved performance and durability

would be to enhance their reflectivity and protect them against environmental degradation by dielectric coatings. Thus, the damage resistance of coated diamond turned

mirrors to a CW CO^ laser beam was evaluated by J. R. Buckmelter, T. T. Saito and

R. Esposito of the Air Force Weapons Laboratory and L. R. Mott and R. Strundlund of

Optical Coating Laboratory. While the samples tested are too numerous to describe

in this review, certain major conclusions can be stated. The initial results of multilayer 10.6 pm dielectric coatings on diamond turned mirrors demonstrated serious problems

of coating adhesion. Auger analysis has shown the presence of hydrocarbon contamination probably as residuals from cutting fluids. Furthermore, stress relief of

electroplated/diamond turned surfaces contributes to coating adhesion problems. CW

2.aser damage tests at 10.6 pm demonstrated threshold only up to 46 KW/cm^ , which is

considerably less than the damage threshold of similar coatings on polished bare

substrates. Adhesion problems are identified by peeling and erratic (non-uniform)

optical properties. This problem bears further study if present metal mirror reflectivities of 99.3% are to be raised to levels of 99.9%, typical of good multilayer

dielectric enhanced surfaces. Generally this initial endeavor into dielectric coating

of metal mirrors was disappointing. This is usually the case, when dielectric coatings

are employed in new situations. However, if coatings could be deposited on diamondmachined elements without defects such as pinholes, inclusions, absorption and with

good adherence, laser damage thresholds of these elements can be expected to increase

substantially.

An obvious concern in machined optics is the introduction of systematic or random

out-of-tolerance variations by the machine tool itself. The degradation of resulting

component quality from slide position errors was treated by J. B. Arnold, P. J. Sieger

and R. R. Burleson of the Y-12 plant. After reviewing the important criteria for

diamond turning, they described various inspection techniques which are suitable for

identifying error sources in the drive train. As a consequence of this procedure,

the stepping motors employed in setting the slide position was identified as a prime

culprit in causing poor quality mirror surfaces. This source of error has been

greatly reduced by employing encoded DC torque motors. It is intended that a further

reduction in this source of error will be achieved by the use of laser interferometry

to set the slide position accurately. The correction of these errors must be

accomplished before contoured surface finishes comparable to present-day flat and

cylindrical surface quality can be fabricated by diamond machining techniques.

Several individuals, including Ted Saito of LLL, R. House, J. Bettis, and

A. Guenther of AFWL, and W. E. K. Gibbs and A. D. McLachlan of the Material Research

Laboratory, Department of Defense, Commonwealth of Australia, have recently noted the

degradation of polished optical surfaces by ultrasonic cleaning. In a communication

from the last two authors, surfaces of OFHC copper, beryllium/copper, zirconium/

copper, stainless steel, kanigen, and fused quartz were investigated using Nomarski

microscopy. In all cases somewhat mild ultrasonic cleaning caused a discernible

degradation of surface finish after periods of 45 or 90 minutes in the cleaning bath.

Subsurface structures, obviously introduced during the grinding process, were subsequently revealed for fused quartz after ultrasonic cleaning, and several of the

metallic reflectors exhibited enhanced grain boundaries and the appearance of imperfections similar to etch pits.

XIII
2.3 IR Window Materials

As might be expected, a large number of presentations addressed the multi-faceted

subject of laser windows. Topics ranged from fundamental thermophysical and optical

properties of materials, to pulsed and CW damage and distortion studies. Several

improved instruments for making fundamental measurements were described together with

a novel technique for ascertaining surface absorption, a subject of growing importance.

Physical parameters such as the thermal diffusivity, thermal conductivity, optical

absorption coefficient, thermal expansion coefficient, critical rupture stress and

Young's modulus are all important characteristics in the thermally induced failure of

optical components. R. Doussain and H. P. LeBodo of the Laboratoire National D'Essais,

Paris, France, reported on careful measurements of the thermal diffusivity of germanium,

gallium arsenide and cadmium telluride over the temperature range 80 °K - 900 °K. Their

experimental procedure employed a laser flash technique in which a thin, disk-shaped

specimen was uniformly irradiated by a pulsed (0.5 - 1.3 msec) Nd-glass laser system.

The resultant temperature at the rear surface of the sample was recorded and analyzed

to produce the thermal diffusivity values. As a check of this procedure, Armco iron and

OFHC copper were also investigated. Typical values for the thermal diffusivity @ 300 °K

were 0.195 for Armco iron; 1.14 for OFHC copper; 0.340 for germanium; 0.225 for gallium

arsenide and 0.046 for cadmium telluride all in units of cm /sec. Estimated accuracy was better than ± 8%.

In another attempt at finding a suitable damage indicator, K. M. Leung, M. Bass,

and A. G. J. Balbin-Villeverde of the University of Southern California employed a comparison of the incident and transmitted laser pulse waveforms to exhibit the on set and type of laser induced damage to several 10.6 pm window materials. Employing

a TEMqq, CO2 TEA laser emitting up to 100 mj in a FWHM gain switched pulse of 92 nsec,

they reported results on improved ZnSe, commercial and RAP Grown KCl, and commercial

NaCl. The self mode-locked output typically consisted of twenty- three pulses of duration < 2.5 nsec, separated by 8 nsec. From previous work, indications were that variations in the transmitted pulse shape through a window under test can be used to identify

either intrinsic, electron avalanche or extrinsic, inclusion dominated damage. A summary

of their results based on survival times (before damage) or bulk breakdown on the

tested materials led to the conclusions that the NaCl damage was intrinsic in nature

but that the failure of the KCl and ZnSe samples were inclusion dominated. The NaCl

samples, polished conventionally, damaged between an axial peak power density of 840

and 1600 MW/cm^ on the surface and between 3900 and 6100 MW/cm^ in the bulk. Conventionally polished RAP grown KCl samples damaged at 460 MW/cm^ on the surface and

bulk between 1500 and 4600 MW/cm^ , while conventionally polished commercial KCL damaged

at 560 MW/cm^ on the surface and from < 190 to 650 MW/cm in the bulk. For conventionally polished ZnSe, surface damage was 270 MW/cra^ , diamond polished 350 MW/cm^ and

super polished also 350 MW/cm ^ . Bulk values ranged from 230 MW/cm^ for an old sample

to 460 MW/cm^ for more recently produced ZnSe. Obviously, for several of the samples

the surface values were controlled by contamination.

Continuing laser induced damage studies on alkali halides at 10.6 pm by V. Wang,

C. R. Giuliano, S. D. Allen, and R. C. Pastor have again stressed the dominant role

played by material purity and processing variables in limiting the damage resistance

of KBr, KCl, and NaCl exposed to pulsed CO2 TEA lasers. Large variations in threshold

were observed for samples grown by different methods. Unfortunately it was not possible to ascertain whether the large fluctuations observed were due to spatial inhomogeneities or from purely statistical processes. In essentially all cases the

performance of RAP grown samples was superior to the commercially available materials.

The highest observed breakdown fields (in Mv/cm) for KBr, KCl, and NaCl at 0.6 usee and 4 ysec were 0.8/0.94, 0.42/0.44; 1.8/2.7, 1.03; and 2.5/2.9, > 1.08.

Inasmuch as most present-day candidate window materials for high power 10.6 ym

CW laser application exhibit sufficient strength to preclude their fracture before

unacceptable laser induced optical distortion, J. S. Loomis of the Air Force Weapons

Laboratory and E. Bernal of Honeywell have performed a detailed analysis and experimental verification of optical distortion as a function of incident power and

material properties of ZnSe, KCl, NaCl, CaF2 / SrF2 and BaF2 windows. Changes in

interferometric patterns were attributed to: (a) isotropic phase shifts resulting

from local temperature rises; (b) anisotropic effects due to induced birefrigence , and

(c) localized phase shifts at the entrance and exit points due to surface absorption.

The effects of thermally induced birefringence are shown to be dominant in the case

of alkaline earth fluorides. For KCl, NaCl and ZnSe relative shifts in fringes/watt

were in the ratio of 1:1.11:6.30 with KCl and NaCl displaying negative lensing and

ZnSe positive. Again in this research bare windows performed better than coated

elements because of decreased surface absorption.

XIV
To arrive at a quantitative understanding of the ultimate concern in windows used

in high average power laser systems, J. A. Detrio and R. D. Petty of the University of

Dayton evaluated the thermal distortion in ZnSe windows by measuring variations in

the far field irradiance of a 10.6 ym laser beam as a function of input power and

absorption coefficient. Results were compared to theoretical predictions based on a figure of merit analysis previously proposed by Sparks. This analysis treats two

modes of failure, the first being a mechanical fracture due to thermally induced

stresses and the second failure due to unacceptable optical distortions resulting in

a reduction in far field intensity. The latter mode was the concern of this paper.

It was not possible to make an exact comparison between theory and experiments, since

experimental conditions did not conform to the assumptions of the theory. It was concluded that ZnSe fails primarily in the optical distortion mode, however.

D. L. Stierwalt of the Naval Electronics Laboratory Center has continued his program

on the accurate measurement of the spectral emittances of laser window materials. Until

the present time, the sensitivity of his measurements was limited by background noise

caused by thermal fluctuations in the temperature of the total instrument. To reduce

this limitation a new instrument was constructed allowing operation at 77°K. Through

this reduction in background level, new measurements were reported for NaF, NaCl, and

KBr over the ranges 200°K to 3730K in temperature, and 2.5 to 14 ym in wavelength with

a spectral resolution of 1.5%. The noise equivalent absorption coefficient was esti-

— 6 —1 —5 ~1

mated to 10 cm at 10.6 \im and about 10 cm at 5 . 0 ym. A number of new previously unreported bands were observed in the above materials

.

In a report describing a rather elegant technique for measuring surface absorption

as a function of depth, J. H. Parks and D. A. Rockwell of the University of Southern

California discussed the use of acoustic probe techniques to evaluate absorption levels

-4

down to 1 X 10 in thin surface layers of bulk KCl polished by different procedures.

It was pointed out that while infrared absorption measurement techniques are available

which avoid surface contributions, no sensitive method has been devised which allowed

the direct assessment of surface absorption (1 - 10 ym depths) as might be modified

from the bulk value by environmental or finishing processes. Consideration was given

to any non uniform temperature gradient over the surface wave penetration depth,

which could produce dispersive effects in the surface wave propagation. These con- siderations are generally arrived at through recorded phase changes. A principal

conclusion of initial studies on KCl indicated that chemical etching can reduce surface

absorption by a factor of 3-4 over that of conventionally polished surfaces.

In their continuing DARPA sponsored program on the precise measurement of the

optical and thermal properties of optical materials, A. Feldman, D. Horowitz, and

R. m. Waxier of the National Bureau of Standards reported on the determination of

photoelastic constants (importance in birefrigence considerations) of Ge , RAP grown

KCl and KCl doped with KI . Present techniques employ null methods using TwymannGreen and/or Fizeau interferometers. These procedures have several advantages over

previous techniques which were not independent of the intensity fluctuations frequently present in the radiant thermal source. From the evaluation of the various

combinations of photo-elastic coefficients, q^^'s, several elasto-optic coefficients,

p^j's, were computed. For germanium, ^j^j's at 10.6 ym were determined and for KCl

at 10.6, 0.644, and 0.633 ym.

Another paper dealing with fundamental material properties was presented by

M. J. Dodge and I. H. Malitson also of the National Bureau of Standards. In their

work, the refractive index of CVD zinc selenide and its temperature coefficient was

investigated. Measurements of the refractive index of this important window material

were accomplished over the range 0.5086 to 18.2 ym by the normal minimum deviation

method employing prismatic samples. For the evaluation of the temperature coefficient

of index change, data were obtained at 20°C and 34°C and subsequently fit to a three

term Sellmeir-type dispersion equation. For two samples investigated the index of

refraction ranged from 2.67544 at 0.54 ym to 2.32781 at 18.2 ym and was 2.40778 at

10.6 ym. For the temperature coefficient of refractive index at 10.6 ym values ranged

-5 -5

from a low of 6.2 x 10 /°C to a high of 10.0 x 10 /°C. The average of all reported

values being 7.8 x 10 /°C. Agreement between their measurement and those of several

other published values was good. Any differences between reported values are un- doubtedly due in large part to differences in the test materials, as well as some

possible instrumental factors.

XV
In a paper, "by title only," J. L. Zar of the Arco-Everett Research Laboratory

communicated information concerning the establishment of a test facility specifically

constructed for testing laser window materials at high power levels. Among several

available lasers the facility primarily utilizes a smooth intensity profile focusable

15 KW CW laser operating at 10.6 pm, with provisions for cooling of windows and

evaluation of the mechanical and optical response of test objects. Another laser is

available for testing with 80 j at 10.6 \im. in 100 ysec pulses. In an initial experimental sequence of tests, forty-seven diamonds were evaluated to attempt a correlation

between the UV absorption spectrum, the IR absorption spectrum, and the absorption coefficient 3 at 10.6 pm. Preliminary results indicate a rough correlation between 3 at

10.6 ym and the sample transparency at 0.253 ym. This correlation is only a rough

guide, since impurities, in the form of interstitial nitrogen in diamond, have a prefound effect in the IR transmission. In materials with greater tolerance for impurities, it is expected that infrared absorption will be only weakly correlated with

UV absorption.

2.4 IR Damage Tests

Three very interesting papers discussed various aspects of optical material damage.

For the first time, information on the repetivitiyely pulsed damage response of com- ponents was heard, as well as IR wavelength dependent studies at HF, DF, and

wavelengths. Both these presentations portend greater emphasis in these areas in

the future

.

A very comprehensive set of CW 10 pm damage tests on a large variety of optical

components was reported by C. A. Hugeley and J. S. Loomis of the Air Force Weapons

Laboratory. In their experiment they subjected several samples to the output of a 1000-

watt gas transport laser for periods 2 or 30 seconds at discrete power levels of 120,

240, 360, 480 and 600 watts on 1 mm diameter spots. These included seventeen coated

and uncoated windows of ZnSe and KCl, eleven abraded and varying absorptance KCl windows

twelve coated mirrors and eleven gratings. Briefly, in almost all cases, samples

exhibiting higher absorption failed at low incident laser powers as expected. Materials

with opaque coatings of Ge or CdTe did not exhibit a high damage resistance. These

materials are low-band gap semiconductors and damage is most probably caused by thermal

runaway. Results from the abraded KCl samples suggest that some damage to coated KCl

windows may be due to higher absorption at the substrate surface and consequent

failure at the surface, rather than the failure of the coating producing subsequent

failure of the substrate. From the mirror tests it was obvious that damage resistance

was apparently more a factor of who prepared the sample than the sample's composition

and structure. This again points out the dominance of processing variables in ultimate

damage resistance of fabricated optical components. For the samples tested, one can conclude that under CW irradiation dielectric coated mirrors can survive intensity

levels comparable to bare metal mirrors; however, extrinsic factors are more likely

to be present in coated elements. Finally gratings damaged in a manner similar to metal

mirrors, but with damage levels generally at or above 150 KW/cm^

.

For the first time in this symposium series, results on the laser induced damage

of components subjected toa rapidly pulsed high energy 10.6 pm laser were reported.

The results reported by A. B. Callender of the Air Force Weapons Laboratory clearly

indicated that high rate multi-exposure damage thresholds differ considerably from

single shot values primarily due to the formation of breakdown plasmas in the vicinity

of the component surface after several repetitive irradiations. Repetitive pulsing is an interesting region of damage testing intermediate between single high peak power

exposures and the high average power situation. As was pointed out, under the conditions of this experiment, a new laser parameter must be introduced, namely the duty

cycle, to evaluate the damage mechanism properly. Depending on the specific conditions

of the test, other factors may also play a role. They are primarily the environmental

flow rate over the component under test, and its thermal time constant. The laser

employed in this study was an electric discharge laser (EDL) emitting 60 joule pulses

of 20 ysec duration at rates up to 150 pps over sufficiently large areas. The pulse

shape consisted of a leading 100 nsec pulse followed by a 20 ysec duration flat topped

pulse, the initial peak being between two and four times the average power. In this

study consideration was given to surface temperature rises during an experimental run

to determine if substantial vapor was released to aid plasma formation. Calculations

indicated that this situation was unlikely unless influences such as evaporation at

surface irregularities come into play, which they can if the pulses are short enough,

i.e., transition from thermal to electrical breakdown behavior. The variation of

surface temperature rise was investigated experimentally by controlling the firing

rate. Generally, conclusions from the test results were encouraging for mirrors and

somewhat discouraging for windows. Bare copper samples exhibited surface plasmas for

low repetition rate loading above 200 J/cm^ ('\' 10 mW/cm^) but no permanent visible

XVI
damage to 290 J/cm^. Dielectric coated mirrors showed much lower thresholds as can

be expected in that the dielectric film was evaporated producing a visible substrate

degradation. Window tests were carried out in a strongly focused beam. Bubble tracks

were observed for impurity-doped alkali halide samples, but not for undoped KCl, NaCl

or ZnSe . The author attributes these tracks to self -focusing in the samples, but in

light of the low value of n2 for these materials, and its susceptibility to impurities,

this explanation seems unlikely.

J. O. Porteus, M. J. Soileau and H. E. Bennett of the Michelson Laboratories,

Naval Weapons Center, reported on the establishment of a new well instrumented test

facility at China Lake. Optical samples are mounted in a bakeable ultra-high vacuum

chamber. In situ measurements of the response of the test object is accomplished by

scanning electron imaging, optical microscopy, a profiling Auger system and charged

particle collectors. The output and focused characteristics of the test lasers are well controlled. Typical laser outputs are 1.5 J at 10.6 ym (CO2) in 100 nsec and

0.5 J at 2.8 ym or 3.8 ym (HF, DF) in a range of pulse widths from 200 to 800 nsec. Preliminary results using machined Al or Al coated mirrors have been performed. At

10.6 ym polished or machined 2024A1 damaged at 14 J/cm^ while DC sputtered Al

o o (1500 A thick) on an aluminum substrate damaged at 8 J/cm , DC sputtered Al (1500 A),

o 0 RF sputtered Al (2000 A) and UHV evaporated Al (1400 A) deposited on high quality

o quartz substrates (a < 20 A rms) failed at 7, 6 and 4 J/cm respectively. At 3.8 ym

polished 2024 Al failed at 63 J/cm^ . These threshold damage levels correspond to the

observance of visible pitting. Even though the coated elements have a greater surface

purity, uniformity and reflectance than the bare Al substrates, it is seen that coated

elements have considerably lower damage thresholds. This was attributed to the

coating — substrate interface which acts as a barrier to heat flow. We look forward

to more results from this facility in the future.

2.5 Damage in Dielectric Films and at Exposed Surfaces

As can be expected the most populated session of the 1975 Damage Symposium was concerned with damage at bare and coated surfaces. It is well known that this is the area where the major extrinsic damage initiating features are introduced. Looking for

guidelines and scaling relationships, several parametric studies addressed the

influence of the following variables: materials, aging, mode structure, spot size,

pulse length, wavelength, temperature, environment, reflectance, coating design,

refractive index, and surface roughness. Both pulsed and CW high power tests from

1.06 ym to 10.6 ym were reported. Several very important results were obtained as a consequence of these efforts.

Investigation of damage in ZnSe/ThF^ multilayer dielectric enhanced reflectors

{'^ 99.8%) as a function of mode structure, spot size, and pulse duration were reported

by V. Wang, C. R. Giuliano, and G. Garcia of the Hughes Research Laboratory. This

parametric study, using a 10.6 ym CO2 TEA laser operated in either a single or multilongitudinal mode over the temporal domain from 0.6 ysec to 6 ysec and over spot sizes

from 50 ym to 255 ym, was performed to elucidate the appropriate damage mechanism of

dielectric coated reflectors under the conditions of the experiment. In tests on samples

placed in an evacuated ('^ 20 m torr) chamber to avoid extraneous dust or air breakdown processes, it was observed that ZnSe/ThF^ dielectric coatings deposited on silver

overcoated molybdenum substrates displayed an essentially constant damaging energy

threshold for pulses of 0.6, 1.0, 4.0, and 6.0 ysec duration. These results are in

agreement with a thermal damage mechanism as opposed to an electric field dominated

breakdown processes. By performing additional pulsed (6.0 ysec) irradiations over

50, 72, 140, and 255 ym diameter spot sizes, and using an analysis similar to that

previously reported by DeShazer, et al., the average inclusion spacing was 25 ym with

90 J/cm^ being the defect limited threshold, with an intrinsic level of 4.4 kj/cm^,

it being the highest threshold recorded. Thus, for this class of optical elements

tested under the above conditions, it was concluded that performance is limited by

j the presence of localized absorbing defects

.

i It is surprising to note that despite the considerable concern for thermal disj tortion and damage to high average power windows and mirrors, no one had performed a thermal sensitivity study of the changes in optical properties of enchanced reflectivity coatings as a function of temperature. However, this year D. L. Decker of

the Michelson Laboratories accomplished a careful evaluation of the temperature and

wavelength dependence of the reflectance of multilayer dielectric mirrors for use

in the infrared. It is well known that the absorptance of an optical element in

high average power application controls both the thermal heat load and the

XVII
associated distortion. Thus one must evaluate this property both at room temperature

and under operating conditions to assess adequately the performance of the total

system as well as of the individual component. Thus data are needed on the change

in absorptance and/or reflectivity with temperature, together with any wavelength

dependence for broad band reflectors. Employing an instrument of his own design

capable of detecting changes in reflectivity and temperature with a precision of

better than 7 x 10 ^ over the temperature range 25 °C to 125°C, Decker analyzed

several commercially available dielectric multilayer designs as well as a thin

film Ag coating. The multilayer designs (R's) included ZnS/ThF^/Ag (R^) ZnSe/ThF^/Ag (R^)

and Si/SlOx/Ag (R^ ) coatings on ULE glass with the bulk of the measurements being made

around 3.8 ym. Results are given in terms of absolute reflectance as a function of

temperature for the original film, after storage for one year under nitrogen and

after cycling to 120°C. Design R^^^ was much less stable than designs R^ or R^ . The

observed change in reflectivity was strongly wavelength dependent. The variation in

reflectivity could not be associated to any degree with changes in scattering and

this was thus eliminated as a reason for the observations. From a critical analysis

of possible causes it was concluded that since design R^ produced an irreversible

variation in mirror performance with temperature, the dominant mechanism was a change

in structure of composition as a result of possible intra or inter-layer diffusion

producing a concurrent index change. The total change was also highly wavelength

dependent. In the case of design R. the variation with temperature was nearly wave-

^ -5

length independent with a value of dR/dT '^^ +3 x 10 /°C. Decker pointed out, in an additional experiment, that an analysis of the behavior of a metal coated mirror

would be instructive in corroborating the results. Theory predicts a dR/dT of

-1.5 X 10 /°C in the near infrared for silver at temperatures near room temperature and indeed at 10 ym a value of -1 x 10 ^/°C was observed. This is in agreement

with electron-phonon scattering considerations. At 4.0 ym, however, results obtained

did not agree either in magnitude or sign. No unambiguous explanation of this un- expected result is presently available and thus it warrants further study. Summarizing

Decker's results, design R^ was clearly superior since it exhibited the smallest

reversible temperature dependency and lowest apparent absorption, leading to highest

damage thresholds. It is obvious that temperature dependent studies such as those

reported by Decker are not only welcome but strongly suggested for other future

investigations.

In an attempt to find isotropic, homogeneous, stable, and suitably damage resistant

coatings for 10.6 pm windows, A. Golubovic, W. Ewing, R. Bradbury, I. Herman, J. Bruce,

and J. J. Comer of the Air Force Cambridge Research Laboratories investigated the

ZnS/CeF^ bi-layer anti-reflection coating. The study investigated both preparation

effects and a damage evaluation of the finished coating on well characterized RAP

grown KCl substrates. Both sublimed and sputtered ZnS layers were first deposited

with CeF^ layers thermally evaporated on both bare and ZnS coated KCl. From a large

variety of instrvjnental investigations they were able to analyze the resulting films

as a function of deposition conditions. Sputtered films of ZnS on KCl were identified

as being in the cubic form with a preferred {111} orientation regardless of substrate

o orientation providing the rate of deposition was > 1 A/sec. Surface roughness of the

coating is strongly dependent on deposition rate, e.g., there is a decrease in grain

size with increased deposition rate from 2.5 to 6.5 A°/sec. Evaporated ZnS films

displayed preferred orientation of [110] planes parallel to the substrate, and tended

to be smoother than sputtered films, but had a greater tendency to vary in crystalliniti

and orientation. CeF., coatings on KCl were also crystalline, being hexagonal with

o a preferred orientation of [100] , fairly smooth and fine grained (< 300 A) . For CeF^ on ZnS coated KCl, the films were polycrystalline with no preferred orientation. All

the anti-reflection coatings were found to be environmentally stable, adhering well

to the substrate. Damage tests were accomplished on the samples employing a 65 mj,

100 nsec FWHM, TEMqq, 1 PPS CO2 TEA laser focused to a 180 ps spot- size. Single CeF^

and single evaporated ZnS coatings both damaged at 35 j/cm^ . Sputtered single

films of ZnS damaged from 40 to 65 J/cm^ depending on deposition rate, and the

CeF^-sputtered ZnS AR coatings failed over a range from 5 to 13 J/cm^ depending on

ZnS deposition rate while the CeF^-evaporated ZnS failed at < 6 J/cm^ . The results of

this paper point out that the mechanical properties of both substrate and individual

coatings must be considered (particularly stress) and that interface topography may

contribute significantly to the total absorption of the coating stack.

XVIII
I AS2^3 coatings are frequently considered for infrared applications because of

their ability to withstand high levels of laser power, while offering the added

advantage of environmental durability. A, D. Bear, T. M. Donovan, and M. J. Soileau

of the Michelson Laboratories reported on their studies of the effects of various

deposition procedures on the quality and optical characteristics of AS2S2 coatings

applied to KCl substrates. Taking a lead from previously reported results that the

stoichiometric behavior of KCl surfaces was probably a distinct function of temperature, they concentrated their efforts on an evaluation of this point for both KCl

and fused quartz by applying thermally evaporated AS2S2 coatings on substrates at different temperatures. The conclusions of these studies were as follows. The

stoichiometry of KCl surfaces under electron bombardment is a function of temperature.

As Golubovic previously had reported for room temperature KCl surfaces coated by

either sputtering or e-beam evaporation, they have a K-rich layer. Coatings deposited

on windows held at 80°C are more durable and less absorbing than those on 22°C substrates. Bulk absorption of AS2S2 is 1.1 cm while for AS2S2 coatings on 22°C KCl

substrates it is 2.9 - 11 cm ^, while for As_S, coatings on 80° KCl substrates it is

-1

1.7 cm , and the film index is more nearly that of the bulk. Little difference was

noted on fused quartz at two temperatures. The index of refraction of the AS2S2

coating is a function of the substrate temperature for KCl, but not for quartz, with

the coating on quartz having a higher index than either the 22°C or 80°C coatings on

KCl.

A. L. Bloom and V. R. Costich, of Coherent Radiation, described the sensitivity

of high power coating performance to structural design and materials parameters. They

analyzed the influence of thermal conductivity, thermal expansivity, intrinsic stress,

and Young's modulus on the damage resistance of metal, metal-dielectric, and dielec-

<-ric multilayer coatings. The main points put forward in regard to the short pulse

performance of multilayer dielectric films were as follows. The stress in a coating

before illumination is the sum of the intrinsic stresses of the components, which can

be either tensile or compressive, and the thermal stress. The latter arises due to the

differences in thermal expansivity between the coating components and the substrate,

which produces additional stress upon cooling from the deposition temperature to the

ambient temperature. Upon illumination, the coating is further compressed by

heating from the laser pulse.

Calculations were presented for Ti02/Si02 coatings on quartz and glass substrates.

The Si02 film is expected to be less resistant to damage, since it is under compressive

stress before illumination, and is further compressed during illumination, while the

;ri02 film is tensile. This analysis is consistent with experimental results presented

at an earlier damage symposium by Austin and Guenther, on damage threshold as a function of residual stress.

Due to its high thermal conductivity, silver would seem a good coating material

[for mirror use at 10.6 \im. However, its performance degrades at 100 °C, so that gold

;and copper are competitive, if not clearly superior. At 1.06 ym, Ti02/Si02 on fused

silica seems the best coating. ZnS/ThF^ is too compressive to resist damage, while

Zr02 """^ ^° non-reproducible that the authors state that it is no longer used for good

quality, high power optics.

Another aspect of coating design was emphasis by B. E. Newnam, D. H. Gill and

\?. Faulkner of the Los Alamos Scientific Laboratory. They showed that the standing

vave pattern established in a dielectric film influences the observed damage threshold.

Experiments were carried out on films of Ti02, Zr02 , Si02 , and MgF2 , deposited by

2lectron-gun evaporation on substrates of BK-7 glass. The thresholds observed for

fi02 films of odd quarter-wave thickness were generally greater than those observed

for even quarter-wave thickness, as expected from the internal field distributions.

Zr02 showed the same trend, but with much greater scatter in the data. Si02

land MgF_ films showed little effect, as expected from their low indices.

i

XIX
The variability of Zr02 has already been discussed in connection with the

previous paper. An overall observation concerning the data presented on these film

damage thresholds is in order. For each coating, i.e., of a given thickness, of a given material, on a given substrate, tbe reported damage thresholds varied by a factor of 1.5 to 2. In previous damage symposia, both Newnam and A. Leppelmeier,

of the Lawrence Livermore Laboratory, have reported on the large variation of damage

resistance from vendor to vendor for the same film design. It is therefore not

prudent to attach absolute significance to the actual damage levels reported in any

given study; and intercomparison is especially unreliable.

Newnam, et al., did study the variation of threshold on individual samples as a function of various parameters. In most cases, no difference was seen in damage

threshold for films illuminated from the coating side or the substrate side. This is

not consistent with the standing wave picture. The authors attribute this discrepancy to the presence of absorbing inclusions in the films masking out the standing

wave effects. In another study, they looked for differences in threshold between

linear and circularly polarized light, as evidence of multi-photon absorption. No

significant difference was seen except for Zr02 films. (Other evidence for twophoton absorption in Zr02 films has been reported in the following paper at this

symposium. ) No clear cut dependence on the plane of polarization of light obliquely

incident on the film was seen in any sample, indicating that photoemission plays a minimal role in film damage.

It should be noted that in this work, no consideration was given to film stress

effects, which were discussed in the previous paper. Three aspects of thin film

damage which have been emphasized in these proceedings over the years are a) the

large number of significant variables, b) the wide variation in sample quality, and

c) the "Achilles heel" aspect of the damage process, which always finds the weakest

point in the structure. Due to these factors, damage studies in thin films are much

harder to interpret than studies in bulk materials or even at surfaces. Careful

studies such as those carried out in this paper, where an attempt is made to isolate

individual variables, are of great value in unraveling a very complicated situation.

A careful study of thin film damage was reported by R. H. Picard and R. A.

Bradbury of the Air Force Cambridge Research Laboratories, J. C. C. Fan of the

Lincoln Laboratory, and D. Milam of the Lawrence Livermore Laboratory. They measured

the time to the appearance of damage in single-layer Si02, AI2O2 and Zr02 films;

RF sputtered on glass substrates. Damage was generated using a 20-30 nsec ruby laser

producing a rectangular temporal waveform described in last year's proceedings.

Thus, the damage is created at a constant power level, and the time to survival is

a linear measure of the deposited energy density.

In this paper the authors tested two hypotheses. They restricted their investigations to samples which exhibited homogeneous absorption, rather than defect or inclusion damage. They first tested the effect of heat conduction out of the film, and

found that the observations were consistent with uniform heating only when heat con- duction to the substrate was taken into account. Secondly, they tested the hypotheses

that two-photon absorption plays a role in thin-film damage. Here the results were

somewhat more ambiguous. The data for Zr02 films seem to show a clear two-photon

dependence, with a nonlinear absorption coefficient of 2.4 to 2.8 cm/GW. This number

is remarkably high, especially for a material in which the energy gap exceeds twice

the photon energy, as is discussed in the papers at the end of this proceedings. Furth

more, they found evidence of two-photon absorption in some (but not all) Si02 samples.

Thus, we have two reports of possible two-photon absorption in thin film structure, in materials in which two-photon absorption is not expected in the bulk material.

Clearly, this effect will require further study and experimental observation before the

physics is made clear.

It has come to be accepted that laser induced scattering (LIS) provides a sensitive probe for detecting laser damage. N. Alyassini and J. H. Parks of the University

of Southern California reported on a technique of damage detection more sensitive than

conventional LIS. In this method, a He-Ne probe laser is focused in the region to be

illuminated by a pulsed ruby laser. The probe laser is internally incident on the

interface between a thin film and substrate. The films were quarter-wave ZnS and MgF2j

a half-wave MgF2 on a quarter-wave ZnS, and a 17 layer ZnS/ThF^ multilayer stack. All

films were thermally deposited on BSC-2 Dove prisms.

XX
Observations of changes of the reflected intensity of the probe beam were compared

to spark observation and transmission changes in the ruby laser beam. The probe reflection was detected with a temporal resolution of less than 1 nsec. Changes in probe

reflection were observed in cases where no visible spark or ruby laser attenuation was

observed. It was also noted that the probe signal often followed the damaging laser

pulse by as much as 25 nsec. The probe scattering signal is associated with morphological changes in the film

structure, either through melting or cracking due to thermal stresses. In the case of

pronounced delay, it would appear that time was required for the stress to reach a critical value at some weak point in the film, which served as an initiation point

for crack propagation. In a later paper, discussed below, the probe technique was applied to the study of avalanche initiation, providing a means of detecting the buildup of the conduction electron concentration in the damage area.

J. R. Bettis, R. A. House, and A. H. Guenther, of the Air Force Weapons Laboratory, along with R. R. Austin of Perkin-Elmer , presented data on the dependence of

surface damage in both vacuum-deposited thin films and bare surfaces on material

parameters and method of preparation. The research presented represented a continuation of earlier work by the authors, attempting to develop a description of the

dependence of the breakdown field strengths on such variables as surface roughness,

number density and refractive index of the material. A Nd-doped glass laser,

generating approximately 400 mj in 40 nsec was used in these experiments. The laser

was carefully mode-selected to give single-mode output. Half-wave films of "^^O^i

MgF2, liiF and ThF^ were deposited on various substrates, including fused silica,

sapphire and BK-7. AI2O2 on sapphire, and ZnSe on ZnSe substrate were also tested.

The data obtained were fit to the form empirically derived earlier by Bettis,

which predicts that the threshold field for damage should vary like N/(n*-l) where N

is the number density, and n the refractive index of the material. Half -wave films

were used, to produce a controlled standing-wave effect. The effect of surface

roughness as discussed in a later paper by these same authors was found to modify the

threshold field and the threshold field was found to vary as a ^, where 0.5 < m < 0.75.

These empirical relations apply over a broad range of variables, except in cases where

extrinsic factors, such as imperfections or inclusions in the film lower the damage

threshold. The presence of OH radicals, indicated by spectrophotometric measurements,

was found to have an overriding influencing effect on thin film performance, completely

dominating standing wave effects produced by alterations in stack geometry. It was generally found that "good" films exhibit higher damage thresholds than "bad" films,

where good and bad refer to scattering by inhomogeneities . Application of these

expressions, using experimental intrinsic bulk damage levels for several materials of

varying index from LiF to ZnSe gave excellent agreement. However, when applied to these

materials in thin film form, agreement was also good except in the case of inhomogeneous

films or films of low packing factor (generally the fluorides of Li, Mg, Ba, Sr) . There

variations from the bulk performance are process related. Through these relationships

it is possible to estimate the intrinsic damage level of materials and determine,

for example, how much improvement can be made in a marginally performing fabricated

component (thin film device, etc.). Again the isolation of individual variables,

carefully done, is of great service in uncovering salient factors affecting the

;laser induced damage of thin film devices

.

The primary motivation of laser damage studies at 1.06 ym is the development of

large Nd-glass lasers for fusion research. The laboratories currently active in laser

'fusion are the major consumers of coated elements for high power operation at this

rfavelength. They also are active in supporting developmental efforts in the coating

Industry to improve the quality of hard coatings for fusion lasers. An excellent

example of the cooperative effort between consumer and producer in the coating area

*as provided in the paper given by C. E. Thomas, B. Guscott, K. Moncur, S. Hildum, and * ^. Sigler of KMS Fusion.

The authors first reviewed the main attributes of the KMSF laser system. To

[Stress the importance of coated elements , they pointed out that in its present configuration their system includes 91 dielectric coatings (mirrors, AR coatings, beamsplitters, and polarizers) in the main beam. The laser produces a series of pulses of

30 psec FWHM, 40 psec apart. Generally, 6 pulses comprise the total laser output,

damage tests were carred out with a beam of 90 mm diameter, the spatially filtered

putput of the final CGE rod amplifier. The beam is focused with a 25 cm focal length

Lens for damage testing, and samples are placed in the converging beam at diameters of

LO, 20, or 40 mm. Due to temporal and spatial variations in the beam, the peak power

iensity is typically 10 to 12 dB above the average.

XXI
Damage tests were carried out on high power mirrors, polarizers, and AR coating.

Samples tested included full-sized pieces (production coatings) up to 30 cm in diameter,

and test pieces (developmental coatings) of 5 cm diameter. Samples were obtained from

four of the leading vendors of high power coatings. As would be expected, there was significant variation from vendor to vendor on the relative merit of given coating

designs and on the absolute performance of individual compositions and designs. Nevertheless, some fairly general conclusions were drawn.

In mirror coatings, Ti02/Si02 coatings were generally better than 2x02/510^

coatings. One of the four vendors, however, consistently produced higher damage

thresholds for Zi02/Si02 than Ti02/Si02 . Higher damage thresholds were obtained for

Ti02 coatings on Pyrex than on BK-7, which would seem to follow the stress analysis

given previously by Bloom and Costich. Note that in polarizers, this effect was not

observed. As expected for short-pulse applications, mirror damage levels were not

affected by predeposition substrate cleaning.

Polarizers exhibited lower thresholds than mirrors. A serious problem encountered

with polarizers is a dimensional shift with age. Thus a polarizer designed for optimum

"p" transmission at 60° fell from 90% transmission at that angle to 78% in five months,

and the optimum angle shifted from 60° to 64°. The same effect was observed whether

the coating was exposed to laser radiation or remained on the shelf. With AR coatings,

the damage levels were higher than most polarizers, and predeposition cleaning seemed to

improve performance.

In general, coatings with higher thresholds exhibited less scattering. Some exceptions were noted. One vendor consistently produced mirrors with relatively high

scattering, but high damage thresholds. Analysis of damage threshold vs. pulse

length for pulse durations from 80 to 48 psec showed neither constant-energy nor constant-power behavior. As is pointed out above, in the paper of Picard, et al .

,

this effect requires detailed analysis, and is currently not satisfactorily understood.

The authors conclude that polarizers are the most vulnerable elements in the system,

with AR coatings the next most vulnerable. Other laboratories with fusion lasers

have reported similar observations.

It is important to distinguish between parametric studies of specially prepared

coatings, such as those reported by the other authors in this session, and acceptance

testing of production coatings or testing of developmental coatings, such as the KMSF

authors reported. The former studies advance the understanding of the detailed physics

of damage phenomena, but the latter aid the industry in process improvement, and

provide an operational test of coating quality where it counts, in the actual system

in which it is intended to operate.

A further study of damage dependence on structural properties and preparation

techniques in optical glasses was presented by R.A. House, J. R. Bettis and A. H.

Guenther of the Air Force Weapons Laboratory, and R. R. Austin of Perkin-Elmer

.

Materials investigated included fused silica, BK-7, ED-2, ED-4, and Cer-Vit. Samples

were prepared using controlled grinding, chemical etching, overcoating, conventional

polishing, ion polishing, bowl-feed polishing, and flame polishing. The laser used

for the investigation was the same one previously described by the authors in a paper

discussed above.

Four methods of damage detection were used, listed here in order of increasing

sensitivity. These were laser-induced scattering, phase contrast detection of scattered

light, visible observation of the luminous plasma, and the breath test.

The damage threshold electric field was found to vary as , where a is the rms surface roughness, and 0.4 < m < 0.6 for conventionally polished fused silica, Si02-

overcoated fused silica, and MgF„-overcoated fused silica. The relation was observed

o ^ o to hold for o-values from 335 A to 13.75 A. For reputed bulk damage thresholds, a

o value of a equal to the lattice spacing (3.57 A) was seen to fit the general curve. Etched samples were not found to obey the relation E '^' a'^, although the trend for

lower thresholds on rougher surfaces was observed.

Among the general observations reported in this paper were the following. Nitric

acid etching resulted in a 21% improvement of breakdown field strength, for all

samples. Flame polishing greatly increased the damage threshold, raising the breakdown field strength to eight times its former value, but generally destroyed the optical

figure of the surface. Overcoating by vacuum deposition caused a marked reduction in

damage level, for both Si02 and MgF2 . Ion polishing yielded a higher damage threshold,

when surface roughness was taken into account. As other reports at this Symposium have

indicated, prolonged ultrasonic cleaning resulted in a decrease in surface damage

threshold. vytt
The conclusions one can draw from a systematic study such as reported here are fairly general. Increased surface smoothness yields a higher damage level. Absence

of impurities on the surface yields higher damage levels. Thin films damage more

easily than does bulk material. The damage value is sensitive to properties on the

exposed surface of the material, not in the subsurface layers. The success of

empirical relation E , aTM for a broad range of surface roughness values prepared

tn Q

by different processes is surprising, especially for a < 100 A. It was pointed out,

o o however, that even for a < 100 A surfaces there are many features > 100 A in dimension.

In the present work, the relation was extended to bulk damage values with a equal to

the lattice spacing. This implies that the damage threshold is responding to the

short range order or periodicity of the surface material, probably through variation

of electron mobility near the surface. This last point warrants continuing investigations. The above relationship and m value should be of great utility in selecting the

most appropriate finishing procedure. It also allows an estimate of the increase in

damage resistance as a function of surface roughness level, which frequently is

economic in nature

.

2.6 Avalanche Ionization and Multi-photon Processes

In short-pulse laser operation, performance is limited by the creation of high

18 —3 concentrations (> 10 cm ) of conduction electrons in nominally transparent dielectrics. Since window and laser host materials are generally chosen to exhibit a band

gap far in excess of the photon energy, this internal plasma formation must arise

either through a relatively improbable multiphoton ionization process, or through the

repeated absorption of energy from the optical field by the few conduction electrons

already present, until these electrons are sufficiently energetic to cause secondary

ionization to occur. This latter process is called avalanche ionization or optical

breakdown. Both multi-photon ionization and avalanche ionization have been extensively

studied for a long time, there are still serious questions concerning the accuracy of

the existing theories, and the significance of reported measurements. For this reason,

the final session of the Symposium was devoted to these two topics.

M. Sparks, of Xonics, Inc., provided a critical review of existing theories of

avalanche ionization. He first outlined the basic tenets of avalanche theory, that

the threshold field is determined by equating the electron energy gain to its loss,

and that the effective ionization rate is determined by the amount by which the intensity exceeds the threshold value. He also reviewed the assumptions concerning frequency dependence and necessary electron concentration. He further reviewed the

"lucky electron" theory of Schockley, and its extension to optical frequencies by Bass.

He showed at every step that inconsistencies appear in the theory, which cannot be

resolved simply by adjustment of parameters. The main points at issue were the origin

of the initiating electrons, and the role of collisions in controlling the rate at

which conduction electrons can gain energy from the field.

Sparks then proposed a new theory, in which electrons gain energy from the light

field continuously, through electron-phonon-photon collisions. This process, just

proposed by Wolstein, is essentially a three-body collision, in which the presence of

the phonon allows the electron to gain a large amount of energy from the photon, while

still conserving momentum. In this sense, the concept of a quasi-free electron is

abandoned, in favor of an electron continuously interacting with the lattice. This

theory is most attractive for polar crystals, where longitudinal optical phonons do

interact strongly with conduction electrons, forming polarons . It is also in these

materials that the intermittent collision model is least satisfactory, since the

collision mean free paths one arrives at are comparable to the lattice spacing.

Sparks further proposed that the starting electrons are generated by a variety of

multi-photon process, either across the gap directly or from impurities, depending

on the incident optical frequency. In this way, the lack of a strong frequency

dependence observed in many experiments can be understood. A few cases were presented

in which good agreement between theory and experiment was obtained without adjusting

parameters. Dr. Sparks indicated, however, that this success of the revised theory

must be viewed as tentative, if encouraging, until further tests can be made.

Much of the difficulty encountered in understanding avalanche ionization at optical

frequencies arises from the paucity of reliable data, particularly, comparative data

for families of materials. This situation has been remedied somewhat by the continuing efforts of the Harvard group. W. L. Smith, J. H. Bechtel, and N. Bloembergen

presented an account of breakdown studies carried out in alkali halides using 30 psec

pulses from a well-characterized Nd:YAG laser.

XXIII
An interesting feature of this study was the simultaneous measurement of breakdown field and index nonlinearity . The laser output was focused into the sample using

a lens of given power. In the focal region, however, self -focusing alters the intensity distribution, and it is this altered distribution which must be analyzed to determine the breakdown threshold. From the constant shape approximation for weak selffocusing, the effective focal area is given by A (1-P/P„_,) , where A is the low-power O UK O

focal area, is the Talanov critical power for the medium.. The peak intensity at

the final focus, I , is then simply given in terms of total power P and critical power

^ -1 -1 -1 -1

P by the relation, P = I A + P„„ . By varying the lens power and recording the CR O O CK

laser power at which breakdown occurs, assuming that no other factors, such as focal

spot size are important, both the breakdown field and critical power can be determined.

The latter yields a value of the index nonlinearity, n^.

The authors reported values of breakdown fields for 14 materials, including 9 alkali halides, fused silica, ED-4 glass, YAG, CaF2 and KDP. Breakdown field strengths

varied from 3.4 MV/cm for Rbl to 22.3 MV/cm for KDP. Index nonlinearity values, which

-12 -13

were only determined to ± 50%, ranged from 10 to 10 esu for all materials tested.

Agreement with other measurements of n2 was generally consistent with the accuracy

quoted for this measurement, but some exceptions were observed.

A significant difference exists between breakdown investigations using picosecond

pulses and those using nanosecond pulses. In the latter case, damage occurs early in

the pulse, and the rest of the laser energy goes into excavating the damage site. In

the former case, the damage site morphology is more nearly representative of the

geometry in which the breakdown initially occurred. Thus, post-mortem investigations

can yield information about the breakdown process in the picosecond case . The authors

reported the observation of isolated, discrete damage loci, which they identify as

11 -3 arising from individual electron avalanches, at a density of about 2 x 10 cm in

NaCl. This number is consistent with current estimates of "starting electron" densities . (The reader should note the comments concerning the previous paper in this

regard, however.) Further evidence from the size of the damage sites attested to the

identification of the damage process as avalanche ionization, and not heating of

absorbing impurities

.

Damage via avalanche ionization is expected to be a statistical process. If a sample is irradiated n times at a given power level, damage will occur in some fraction

of cases. This fraction, the probability of damage, goes from zero at low power to

unity at very high power. The probability of survival, given by one minus the damage

probability, decreases monotonically from unity at low power to zero at high power.

The lucky electron theory of avalanche ionization predicts that the logarithm of the

survival probability, after irradiation for a given time at a given field strength, E,

should decay exponentially, like exp(-K/E), where K depends on the material and laser

frequency

.

D. Milam, of the Lawrence Livermore Laboratory, and R. A. Bradbury and R. H. Picard,

of the Air Force Cambridge Research Laboratories, had noted in the past that damage

statistics in solid dielectrics did not obey the predictions of the lucky electron

theory. They decided to examine the survival time statistics in gas breakdown to see

if similar discrepancies appeared.

Gas breakdown survival statistics were reported for argon at pressures from 20 ps;

to 500 psi, and N2 at 150 psi, with and without an added DC field of one-half the

breakdown value. As the argon pressure was increased, a marked variation in survival

statistics occurred. At low pressure, the breakdown was almost causal, i.e., at a given po^ex level, breakdown occurred over a very small range of times for many trials.

As the pressure was increased, and correspondingly the breakdown field decreased, the

time of breakdown became less and less well defined, and the survival statistics took

on the character of those similar for solid dielectric breakdown. The authors

attribute this behavior to the fact that as the pressure increases and the field at

which the breakdown occurs decreases, the rate of photoionization , which starts the

avalanche is reduced. Thus the statistics which are observed reflect the statistical

character of the initiation process rather than the avalanche process, and do not

reflect the lucky electron hypothesis. In the case of an added DC field, electrons

are swept out of the volume- as ionization occurs, and correspondingly, the spread in

breakdown times is increased. The data reported in gas agree with this model.

1

.XXIV
Among the crucial parameters of any avalanche ionization theory is the density of

electrons formed in the process. N. Alyassini and J. Parks of the University of

Southern California reported a measurement of the build up of the electron concentration at the surface of a glass sample under ruby laser illumination. The electron

density was probed by observing variation in the reflectivity of a He-Ne probe beam

reflected at the critical angle in internal reflection. This method is sensitive to

-5 -4

changes in index of the order of 10 to 10 , corresponding to electron concentra17 18 tions of 10 to 10 . The refractive index change was observed to precede the attenuation of the ruby laser beam by as much as 2.4 nsec. This delay time is associated

with the build up of an absorbing plasma in the solid. Build up times varied from

one to two nsec in duration.

In a short contribution by P. Braunlich of the Bendix Research Laboratories and

the Research Institute for Engineering Sciences at Wayne State University, and P. Kelly

of the National Research Council, further evidence was put forward that breakdown

thresholds are sensitive to the process of initiation as well as the process of build

up. The authors show that recent results of Penzkover, in which multi-pulse damage

thresholds were systematically lower than single pulse thresholds, are entirely

explicable from avalanche ionization theory, if appropriate source terms for conduction electron generation are included. The model neglected any diffusion of conduction

electrons out of the irradiated volume, which was consistent with the spot size and

pulse duration employed in the experiments. It was shown that as much as a ten-fold

reduction in the peak power density required for damage resulted from using a train

of ten pulses instead of a single pulse. The authors cautioned against the interpretation of multi-pulse damage threshold values as being single-pulse values. They con- clude that no extrinsic mechanism, such as inclusion damage, is required to explain

Penzkover 's results.

A series of papers relating to multi-photon processes in dielectric solids was presented at the Symposium. A Schmidt, P. Braunlich, and P. Rol of the Research

Institute for Engineering Sciences, at Wayne State, and of the Bendix Research

Laboratories, reported a hitherto unobserved phenomenon in laser-irradiated alkali

halides. The effect proceeds as follows. By multi-photon absorption of laser light,

a negative halide ion is generated. This ion subsequently forms a negatively charged

F-center, or self -trapped exciton. The exciton decays nonradiatively , launching a Pooley-Hersh collision chain along those directions in the crystal, [211] and [110],

in which a chain of halide atoms occurs. If this chain lies near the surface, a halide atom is ejected from the surface. In KCl, in which the absorption is via four

photons, as many as 10 chloride atoms have been detected at incident laser fluxes well

below the damage threshold. The authors report cases in which surface damage occurred,

not due to plasma formation, but instead due to concentrated energy of these collision

chains at the crystal surface.

R. A. Shatas, S. S. Mitra, and L. M. Narducci of the Physical Sciences Directorate

of the U.S. Army Missile Command offered a critical review of the theory of multiphoton absorption in direct-band semiconductors. A comparison was made of theoretical

perturbation calculations of Braunstein and Ockman, and Basov, and the empirical theory

of Keldysh, which employed "dressed" states to represent the effect of the applied field

on the states of the system. The Braunstein and Ockman theory assumes a model of transitions between the valence band and the lowest-lying conduction band, with an intermediate state in a higher-lying conduction band, while the Basov model assumes that

only intraband transitions enter in the intermediate state. Comparison of the three

calculations with experimental results for GaAs and InSb show that the Basov model

leads to a substantial overestimate of the two-photon absorption, while the Braunstein

and Ockman model leads to an underestimate. The Keldysh model, however, provides

surprisingly good agreement with experiment. It also gives good agreement for bandedge absorption, without parameter adjustment. Furthermore, the Keldysh model allows

the calculation of higher-order multi-photon effects more directly than is possible via

: perturbation theory. For a large class of II-VI and III-V semiconductors, two-photon

— 8

j( absorption coefficients at wavelengths near 1 pm are of the order of 10 cm/watt.

Another paper, read by title only, also dealt with multi-photon absorption, and

the validity of the Keldysh theory. In this paper, by R. P. Benedict and A. H.

Guenther of the Air Force Weapons Laboratory, the effect of electroabsorption (the

Franz-Keldysh effect) as possible damage mechanism was considered. The authors coni elude that the important manifestation of electroabsorption at optical frequencies

is just multi-photon absorption. They further conclude that, for IR materials, multiphoton absorption is not important as a damage mechanism, being less probable than

avalanche ionization except for extremely short pulses. For lasers in the visible and

uv, two-photon absorption can be the limiting phenomenon.

XXV
Benedict and Guenther agree with the previous paper in their evaluation of the

applicability of the Keldysh formulation to two- and three-photon absorption. Like j Shatas et al., they found the Keldysh theory to be within an order of magnitude of I experimental results for several materials. They gave several sample calculations

for materials used both in the IR and visible region of the spectrum. , Two-photon absorption as a limiting damage mechanism for /acuum uv laser component:!

was considered by C. J. Duthler and M, Sparks, of Xonics, Inc. They estimated the

two-photon absorption coefficient from the theoretical perturbation expression of

Braunstein, referenced above. They examined in detail the implications of twophoton absorption for the performance of an uv laser with a photon energy of 7.2 eV,

with windows of LiF, which has a band gap of 13.0 eV, less than twice the photon

energy. The authors list a number of other possible uv window materials, all of which i exhibit smaller band gap energies than LiF.

The authors first discuss the use of metallic reflectors in the vacuum uv. Alumini

which has the best reflecting properties at photon energies above 6 eV, is found to have a melting threshold of 20 MW/cm^ , too low for practical use at high power density.

Multilayer dielectric mirrors will be severely limited for vacuum wavelengths. The

authors conclude that the most damaging consequence of two-photon absorption is optical i distortion due to the creation of free carriers . The threshold for avalanche ioniza- I tion is estimated to be an order of magnitude higher than that for free carrier production. Other effects, such as bulk heating from two-photon absorption, valence electron

contributions to the nonlinear refractive index, and stimulated scattering processes,

are also considered.

In light of considerations put forth in the preceding papers, the estimate of the

two-photon transition probability in this paper is only to be interpreted as a qualita-l;

tive estimate. Shatas et al. have shown that discrepancies greater than two orders ' of magnitude can arise from different choices of models for two-photon processes.

Nevertheless, Duthler and Sparks have reemphasized the fact that two-photon absorption is the most severe limitation for short wavelength laser operation.

3 . Recommendations

The recommendations forthcoming from this year's discussion are not substantially

different from those of previous years. The problems of optical surface preparation,

coating and evaluation continue to be of great interest and importance. Advances have

been made in scientific understanding and fabrication processes, but much improvement

can still be gained. We shall attempt to set down a subjective view of the subjects of

great interest and the areas of greatest potential advance.

Fabrication processes continue to play a dominant role in determining the damage

sensitivity and mechanisms of exposed optical surfaces. In order for a process to be

useful commercially it must be reproducible, suitable for volume production, and , capable of uniformaly preparing large size elements economically. Quality assurance ^

requires suitable test procedures for monitoring the process. Process development, I both for the preparation of bulk materials and for preparation and finishing of surfaces

must continue in close association with the advance of characterization techniques and

damage studies. Economic factors will become important in many instances and thus

damage resistance versus process cost will have to be determined.

As lasers evolve with different operating characteristics such as new wavelengths

or different pulse durations, new materials become attractive for specific laser

applications. These in turn require new approaches for fabrication. A prime example

is the recent interest in diamond- turned metal mirrors for 1.06 ym optics. We can look

for a similar upsurge in interest in low index optical materials for use with uv lasers

and with 1.06 pm lasers, the latter to reduce the index nonlinearity in passive optics.

f

Techniques for the fabrication, coating, and testing of large aperture optics made from

crystalline flourides, for example, to meet high power laser specifications deserve

greater attention and emphasis than they have been receiving, particularly since in

many cases flouride coatings tend to exhibit considerably different optical properties

than bulk flouride characteristics. What is obviously needed besides an accurate tabulation of bulk optical and physical properties is a tabulation of optical and physical

properties of materials in thin film form as a function of process variables.

Diamond turning itself is still far from a closed subject. The turning of nonplanar surfacess such as ellopsoids and paraboloids require further development. The

application of micromachining techniques to non-metallic materials requires further

development. The question of coatings for metal mirrors must be clarified with

particular attention to substrate preparation techniques and the problem of the

differential expansion of the metal substrate and dielectric coating. For high average

power application, cleaning procedures must be developed to completely remove all

traces of contaminates introduced in the machining process, such as cutting oils,

before sufficiently adhering coatings can be applied.

XXVI
similar problems remain in the area of window materials and their preparation.

Presently the residual absorption in these materials is dominated by surface impurities.

Further development of surface treatment is required, including chemical etching, ultrasonic cleaning, and optical finishing. The chemical basis of surface absorption is

still not clear and requires elucidation. A quantitative evaluation of the efficacy

of ultrasonic cleaning is necessary. This study should relate material type, duration

of cleaning, pH, and type of bath, power and frequency with the resultant surface

characterization for surfaces prepared by different methods.

Great strides have been made in understanding damage processes. The empirical

scaling laws recently evolved relating damage levels to observable characteristics

of the material, such as index, dispersion, or surface topography, have provided a very useful guide to the user in choosing materials and processes. These results are

a powerful incentive to the research scientist to understand surface damage phenomena

better. Further work along this line is needed. The temperature dependence of damage

thresholds is of great interest, as revealing the role of phor^on processes, especially

in electron avalanches. Data collection on samples of high purity and known and well

characterized impurity levels should be undertaken to evaluate the impurity role in

damage, again with emphasis on avalanche ionization, then existing scaling relations

can be placed on a firmer scientific basis.

No where is the potential for improvement as great as in the area of thin films.

Studies reported in this year's symposium show the successive isolation of several

factors which determine the performance of a coating. These include the substrate

material and preparation, method of deposition, temperature, coating design, field

configuration, residual stress, structure, density, and others. Scattering continues

to be a valuable measure of quality in films, but low scattering and high quality are not always correlated. Roughness and impurity effects need further elucidation. The

problem of dimensional changes in the coating apparently associated with the coalescing

of minute crystalline regions emerged from this year's discussion as an important consideration. This is part of the general problem of film stability which is of great

impact. The relative importance of surface absorption vis a vis bulk absorption for

high average power bare and coated elements needs further study as relates to its

response at high levels of irradiation.

Finally, it is appropriate to remind ourselves of the purpose of this series of

symposia. These meetings and their consequent publications are designed to provide a forum for the transfer of information regarding optical materials for lasers from the

research scientist to the practical user, to stimulate the collection of data regarding

these materials, and to tabulate, organize, and explain those data where possible. By

providing a forum and a mechanism for this exchange of information we can hopefully

avoid the expense and pain of relearning old truths in new guise each time a new

material or laser emerges.

4 . Acknowledgment

The editors would like to thank Capt. T. A. Saito of the U.S. Air Force for his

kind service as moderator of the sessions on fabrication and metal mirrors, areas with

which he is intimately familiar. We would like to acknowledge the invaluable assistance

of Dr. Harold S. Boyne, Mr. Wilbur Anson, 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. We would like to take special note of Sue A. DiLullo and Paula J.

Chukoski, who added measurably 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 the support of those named above.

XXVII
5. Bibliography

"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. H. Guenther, Editors,

NBS Special Publication 341, 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).

"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).

"High Power Infrared Laser Windows,"

N. Bloembergen, National Materials

Advisory Board Publication NMAB-292,

National Academy of Sciences,

Washington, D.C. (1972).

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: 1972," A. J. Glass and

A. H. Guenther, Editors, NBS

Special Publication 372, U. S.

Government Printing Office,

Washington, D.C. (1972).

[8] "Laser Induced Damage of Optical

Elements, A Status Report," A. J.

Glass and A. H. Guenther, Applied

Optics 12, pp. 637-649 (1973).

[9] "Laser Induced Damage in Optical

Materials: 1973," A. J. Glass and

A. H. Guenther, Editors, NBS Specia

Publication 387, U. S. Government

Printing Office, Washington, D.C.

(1973) .

[10] "Laser Induced Damage to Optical

Materials, 1973: a Conference

Report," A. J. Glass and A. H.

Guenther, Applied Optics, 14 , pp. 74-88 (1974)

.

[11] "Laser Induced Damage in Optical

Materials: 1974," A. J. Glass and

A. H. Guenther, Editors, NBS

Special Publication 414, U. S.

Government Printing Office,

Washington, D.C. (1974).

[12] "Laser Induced Damage in Optical

Materials: 6th ASTM Symposium,"

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

Applied Optics, 14^, pp. 698-715

(1975) .

XXVIII
0.1 Opening Remarks

Arthur H. Guenther

Kirtland Air Force Base, New Mexico 87117

Unfortunately this year Haynes Lee, Chairperson of Subcominittee 2 on Lasers of the ASTM, could not be

with us to personally open the conference. He has asked me to express his regrets, but he is sure that

the conference will carry on in its characteristic manner. I am sure he would want me to mention that

the ASTM looks to this conference for the technical details concerning proposed and possible standards

for evaluating the damage sensitivity of optical components and materials used in conjunction with high

power lasers. In particular, the ASTM finds the damage symposium most useful as a sounding board for

evaluating diagnostic techniques and instruments which can be transferred to viable standards, procedures and recommended practices. Of course, the prime goal is non-destructive testing methods if

possible.

Speaking for Alex Glass and myself, we would like to welcome you to the Seventh Annual Symposium

on Damage in Laser Materials. As you can see from the program, there has been an increase in the

number of sponsoring agencies. We have already covered the interest of the ASTM and its role in supplying the initiative for these meetings. Certainly these meetings would not have achieved their prominence without the excellent support and facilities made available to us by the National Bureau of

Standards here at Boulder. We particularly wish to express our appreciation to Dr. Hal Boyne, Chief of

the Electromagnetic Division, for sponsoring our annual conclave. Furthermore, this year we have the

benefit of Mrs. Florence Indorf who is now filling the role formerly held by Pauline Smith. Mrs. Indorf

works for Mr. Wilbur Anson who has marshalled the resources for the operation of the meeting, including

Paula Chukoski, and Sue Dilullo who will carry our roving microphones. Of course, we are most grateful

that the Office of Naval Research has again supported us financially to help defray the cost of publishing the Symposium proceedings. I imagine because of our popularity and success, this year the Energy

Research and Development Administration has also seen fit to aid us financially. We are glad to add

them to our list of sponsors.

We would also like to issue a warm welcome to our international attendees from Canada, Great

Britain and France.

In the event that we remain to receive support from these, as well as possibly from other agencies,

we are finding it somewhat of a tongue twister to refer to this as the Seventh Annual ASTM-ERDA-NBS-ONR

Symposium on Damage in Laser Materials. So to preclude this from becoming known as the alphabet conference, I suggest that in the future we simply refer to this conference as the Boulder Meeting on Laser

Damage

.

Each year we wonder if there will be enough interest for another meeting the following year, but

our experience has been that each year more papers and more attendees have been the general rule. In fact, this year we received in excess of fifty submitted papers. Because of our past emphasis on damage in optical components used in high power laser systems, we found it necessary to reject several

papers dealing more with engineering and ancillary component damage, such as photo-detectors, since

there are already several meetings for which these papers would be likely candidates. In addition, we found several papers of high interest, but because of the press of the limited duration of our meeting,

as well as our desire to avoid concurrent sessions, we have accepted several papers by title only. The

abstracts of these papers will be found in your handouts. It is our intention that these papers be

published in full in the Symposium proceedings. Even after these two courses of action, we found that

to present the remaining papers while allotting sufficient time for discussion we had to extend the meeting an additional half day.

The success of this meeting I believe primarily stems from our flexibility in changing the emphasis each year to concentrate on the major accomplishments and near-term large-scale efforts. These

major programs today are obviously the high peak power laser systems of the fusion program primarily

at 1.06 \M and 10.6 ym and the numerous government and industrial applications of high average power

lasers, to date mainly in the infrared region. Of course, we would expect both of these areas to move

to shorter wavelengths for a variety of technical and engineering reasons. This will come about as soon as the several large efforts underway in the search for new efficient short wavelength lasers

bear fruit.

From the program you will note that the meeting has been organized into sessions on Fabrication,

Metal Mirrors, IR Window Materials, IR Damage Tests, Damage in Dielectric Films and at Exposed

Surfaces and finally Avalanche Ionization. Thus the subjects of interest to the high average power

-1-
community come early in the conference, while the high peak power subjects conclude our meeting with

the dielectric film and surface papers forming a bridge between the far extremes. We do hope, however,

that you will see fit to attend the full two and one-half days.

This year there is little or no in-depth discussion of bulk and self-focusing laser-induced damage.

These concerns have either been successfully eliminated by improved processing procedures or sufficiently understood theoretically to allow us to design around them. This situation fortunately is

complemented by a considerable tabulation of salient material properties which allow the necessary

assessment of their import in high power laser system design and performance.

This year we have elevated the subject of metal mirrors from the workshop sessions held last year

to a formal session here at the Symposium and have enlisted the services of Captain Ted Saito who,

besides being concerned with the Air Force high energy laser program, is presently on assignment to Lawrence Livermore Laboratory to aid in coordinating the metal mirror activities which bear on both

major program areas.

Several years ago the deleterious effects of rouge were identified as a damage initiator. This

situation led to the most widely used rule of thumb in specifying optics to be used in high power

lasers and that is the total avoidance of rouge polishing. This year several discussions may center

on yet another process which may possibly degrade the damage resistance of optical elements and that

is ultrasonic cleaning, particularly as it applies to metal mirrors. I am sure several discussions 4

will concern themselves with this topic during the next two and one-half days. In addition, this meeting will identify many other important processing variables which control factors that can be identified

as damage indicators. Therefore, this correlation is useful in both an analytic and an engineering j

sense

.

I

But we should remember that the prime reason for this forum is the transfer of information relative

to the development of standards by which proper specifications can be written and optimum processes

identified together with the development of diagnostic techniques suitable as a non-destructive test.

This year sees several papers dealing with the collation of these several factors.

-2-
1.1 Fabrication of Laser Optics at Lawrence Livermore Laboratory*

Norman J. Brown

Lawrence Livermore Laboratory

Box 808

Livermore, California 94550

This paper summarizes fabrication techniques in use at Lawrence

Livermore Laboratory for laser discs, windows, and mirrors, and

polishing techniques used with metal surfaces.

Key words: Chamfering; continuous polishing; grinding; metal

polishing; optical fabrication.

1. Introduction

To achieve a near diffraction-limited beam in a laser chain composed of dozens of elements requires

optical components having surfaces that are ultra flat, parallel and of high cosmetic quality. This

paper summarizes the results of studies conducted by LLL to determine methods for fabrication production

quantities of these components.

There are seven basic steps in piano-parallel glass fabrication:

1. The flat surfaces are rough ground with bonded diamond.

2. An edge chamfer is ground and polished.

3. One surface is ground flat to a few fringes in several steps of free abrasive.

4. The second surface is similarly ground, with some blocking or fixturing technique to obtain

parallelism in the second range.

5. Both sides are prepolished or greyed out.

6. The first side is polished flat to specification.

7. The second side is polished to a figure consistent with specified transmission figure with

special care to maintain parallelism.

These processes will be described in inverse order.

2. Figure Control

The 20 cm diameter flat shown in the interferogram (figure 1) is one of eight that were brought to X/20 (at 633 nm) peak-to-valley figure. Two flats were processed at a time, four times in succession,

with a total polishing time of six hours per pair. Both the figure and the low edge roll are typical.

The material is fused silica; this is the only material for which we can regard the above performance as routine, but it does establish the fact that the polishing process can be controlled.

Our processing is done on a 48" single planetary lap consisting of a pitch covered annulus whose

width is one-third the lap diameter. A two-foot low expansion glass tool continuously trues the pitch

which passes at 2 to 3 rpm under 3 workrings. Slurry is composed of rare earth oxide in a water vehicle.

Tool, rings and lap are independently driven, usually at the same rpm. Lap figure is controlled by

moving the tool in the radial direction, the outermost position producing a convex lap and the innermost producing a concave lap. Material removal rates are near one-tenth micron per minute, and workpieces can range in size up to 1/2 to 2/3 of the lap annular width.

When we attempted to polish elliptically shaped laser glass we found that half wave surfaces were easily-obtained, but 1/10 to 1/20 wave surfaces were approached erratically. It soon became obvious that

the erratic results were due to a fluctuating thermal gradient across the disc. The sag for a circular

disc of radius r and thickness t in terms of coefficient of thermal expansion a and temperature gradient

AT is given by

Assuming BK7 glass, a typical radius-to-thickness ratio of 4 and a wavelength of 633 nm, the ex- pression becomes:

6/(xaT) = .224r, (2)

or about 4-1/2 fringes per degree C for a 20 cm diameter workpiece.

* Work performed under the auspices of the U. S. Energy Research and Development Administration.

-3-
It is easily seen that fluctuating gradients of 1/50 to 1/100 degree C can be troublesome.

Identically shaped pieces of different materials that are simultaneously polished tend to come off the

lap with residual curvatures a linear function of their coefficients of thermal expansion. The thermal

gradients due to polishing action appear to be quite small, most of the heat being removed by the

slurry. The real culprit is a fluctuating temperature difference between air and slurry.

We attempt to control this to 0.1 °C. It is difficult to assess the effectiveness of this control

but two independent estimates in our shop after six months experience agree that it decreases the

polishing time by at least one-half. There are a number of ways to control this temperature difference.

We enclose the lap in a plastic cover. The air above the lap is slightly warmer than that outside the cover due to heat from the polishing. We raise the temperature of the slurry to match this within

0. 1°C. The slurry control consists of a thermistor in one leg of a Wheatstone bridge, a commerical

unit costing under $100. This operates either a 750 watt immersion heater or a solenoid valve permitting chilled water to flow through a 50 ft. length of 1/2 inch copper tubing. Heater and chiller buck

each other in a typical cycle of 5 seconds heat and 15 seconds cool.

There are several less obvious sources of thermal error. One is the transient effect occurring

when a part is moved from one environment to another. An examination of various slab transient temperature solutions shows that for most common initial distributions where one side is suddenly fixed and the

other insulated, the distribution will decay to a near linear distribution with the insulated side 10%

of the initial difference between the fixed and insulated sides at a time e when the Fourier number

(Ke/(pCpt2)) approaches unity [l]\Assuming BK7 glass and a typical radius-to-thickness ratio of 4, the

time e to accomplish this change in minutes is approximately

e % r^/5 (r in cm). (3)

Thus a 20 cm workpiece has a relaxation time of 20 minutes or so. Assume such a workpiece is returned

to the lap after a brief test at a temperature only a half a degree different from the lap. Almost

immediately upon contact with the slurry, it will bow several fringes. The direction of bowing will

depend on whether the temperature difference is positive or negative. During the tens of minutes for

this to relax to equilibrium, several microns of material will be removed from the disc in its deformed

shape. Thus, the figure of a part nearing completion can be completely destroyed in the last minutes.

We monitor the lap figure constantly with Cervit discs, removing the workpieces as seldom as possible. On return to the lap in final stages of polishing, the part should be brought to equilibrium

by a brief soak in lap air at least, and preferably in the lap slurry.

Another source of thermal error is the power supply in the base of most commercial piano interferometers. Placing the work to be examined on an insulating pad can reduce this effect considerably.

Attempts to compensate for significant thermal gradients by adjusting the curvature of the lap are never completely satisfactory. There are always variations in thermal gradients at the edges of the workpieces. A flat figure obtained with varying edge gradients will relax into a different shape, and

smoothly upturned edges are sometimes seen. This problem is more pronounced in elliptical and sharpcornered workpieces. Note that the interferogram for the fused silica rounds show a remarkable lack of

turned edges, and this is quite characteristic of materials such as fused silica which have a low co- efficient of expansion..

Another source of figure variation is a ground glass surface on the opposite side. A ground sur- face is a mass of subsurface fractures which tend to expand the surface. Moisture complicates this

behavior as capillary pressures further expand the surface in different areas in a manner which can vary with time. In addition, there is a strong tensile stress developed in a polished surface. It is an old optician's trick to bring in the figure of small or medium mirrors by judicious polishing of the

back surface. Final figuring should not be attempted until both surfaces are thoroughly greyed out.

In the case of thin parts, the edge should also be etched prior to final figuring since an expanding

edge scallops and leads to potato chipping. Etching should also be considered for the back of thin

mirrors

.

Another source of figure errors is lack of rotational synchronism. Consider a circular disc ro- tated on a stationary flat plate. The velocity and resulting wear at any point is a function of the

radius; the edges will wear at a faster rate than the center. Only pure translation will result in even wear. Rotation during translation will also result in a wear pattern that increases from the

center of rotation. Consider the wear along two axes, one parallel to the direction of translation

and one normal to it. Along the normal there will be wear variations that are linear across

the diameter, canceling out on a full revolution. Along the axis parallel to the direction of translation there will be a vectorial addition of velocities that does not cancel out. In the case of a part resting on a rotating lap, it can be shown that the radial variations in wear are proportional to the absolute value of the angular velocity difference between the part and the lap. This can easily be

verified by observing the synchronous rotation that occurs when a part, pinned at the center of

gravity of area, is free to rotate on a rotating lap. This point is crucial to the understanding of

planetary lapping.

1. Figures in brackets indicate the literature references at the end o£ the paper.

-4-
The large circular tool will tend to rotate in synchronization if it is free, but if it is not

separately driven, extreme care must be taken to see that its restraints are truly frictionless . In the case of workrings, these should either be driven, or kept loaded such that they rotate as much as possible about the center of gravity of their loads; the latter method is not too feasible as different pieces may have different weights and may suck down on the lap with different pressures

during their processing. Synchronization is especially important in the case of odd shaped parts

restrained from independent rotation within the workring, but even circular parts free to rotate tend

to be dragged around at higher angular velocity than the lap by contact with the edges of the workring

or inserts. 3_ Parallelism Control

This results both in a figure effect and in a parallelism effect. There is a second parallelism

effect due to lack of synchronization that is very pronounced. Polishing results in a shearing

pressure at the interface between part and lap. This is reacted by a load above the surface resulting

in a moment supported by a linearly varying pressure distribution across the interface. This pressure

variation is in opposite directions relative to the disc as it rotates in the workring. At synchronous

velocity these wear distribution variations cancel out resulting in even wear over one revolution. At nonsynchronous rotation they can result in up to 10 seconds of wedge per hour for 1% - 11 angular

velocity difference.

So far we have discussed maintaining parallelism in the polishing operation. This parallelism is

initially set up in the free abrasive grinding operation. The traditional methods of achieving this

are by contact or color blocking parts to a larger plate. This does not work too well with laser

glasses due to their combination of high expansion, low strength, and softness. Contact blocking is

particularly unsuitable since it usually results in material being pulled out of the glass. We have

devised a very simple mechanical fixture to supplant this method. This is a vacuum chuck free to rotate

within a lapmaster workring. The chuck is held in a precision sleeve precisely oriented orthogonal to the ring face. A workring was first lapped parallel. The bearing assembly was rough machined and a final cut was taken on the sleeve and bearing assembly face in a precision lathe. The chuck was then

inserted and its face lapped parallel to the ring face. We grind the glass on one face and apply this

to the chuck face. A five-minute grind in the finest grit generally reduces wedge to 2 - 3 seconds of

arc.

4. Cosmetic Quality

Most of our surfaces are of 10/5 scratch/dig quality and the majority are probably nearer 5/2.

This is achieved by keeping the lap under a filter hood supposedly good to class 100 conditions.

There is a second plastic thermal barrier over the lap. Even with this we found scratches. We traced

most of these occurring on the lap to breakout from the edge chamfer. With soft laser 'glass in particular we find that a finely ground and polished edge chamfer is a necessity. These materials grind

rapidly. If the chamfer is lost in this operation, the edge will break away on striking the edges of

lap facets, whether they are metal or pitch. Polishing the chamfers results in very few breakouts and

reduces polishing time 2 to 4 hours per disc.

There was another class of scratches we found resulting from grinding laps which were cast iron. We began picking inclusions out of the cast iron. The scratches were not immediately visible in the

ground surface, but became visible as the surface was polished out. These are readily distinguishable

from scratches started in polishing since the latter tend to be clean and continuous while the

former tend to be a series of irregularly shaped pits.

We had one other problem with grinding laps that was peculiar to laser glasses, glazing or premature fusing during the grinding operation. These glasses are gummy and soft and grind very fast.

When the abrasive size is decreased below a certain point the material is not flushed out rapidly

enough and burnishing of the glass against its own frit and glazing takes place. Observations of this

glazing material filling pits of known size lead us to conclude that this layer is microns thick. It

is very hard to polish through and there is a danger that figure may be achieved before polishing into

the base material. This fused material contains particles of grinding compound and scrapings of the

lap material, iron, which can lead to laser damage.

In addition to iron, we have tried copper, tin, aluminum, brass, and ceramic. These were not

exhaustive tests, but we believe brass is giving us our best results. It is more uniform than iron

and we find we can go to. 5 micron silicon carbide abrasive without encountering glazing. The brass re- quires much more care than iron. It has a tendency to pick up and imbed grit, so it should only be

used with a single size abrasive unless resurfacing tools (e.g., coarse diamond faced rings) are available.

Due to the fast grinding action on soft laser glasses we grind in only two free abrasive stages,

30 micron and 5 micron. Five minutes on 30 micron is usually sufficient. We then go to 5 micron

abrasive for 15 to 20 minutes removing initially about two microns of stock per minute. The first 10

minutes remove the tiulk of the coarse grind and the second 10 minutes show a stock removal rate of

about one- half of a micron per minute.

-5-
Ceramic tile laps work almost as well, seldom causing glazing and showing removal rates about

half those of brass. They do, however, show a greater tendency to scratch.

In addition to the problem of eliminating scratches and digs, we must detect those that are

present. The inspection illumination scheme used in most shops is usually that of MIL-0-13830A. This

was designed for clear glass and is insufficient to show up the scratches and digs of the same spec in

colored laser glass. We illuminate the glass with a high intensity projection lamp while looking

through the glass at a dark background with a 5 - 8 power eye loop or binocular magnifier. Dust is a problem and anti-static or laminar flow-filtered air equipment is a great help. It should be

emphasized that this is not the overly stringent test that it appears. It is an attempt to overcome

the absorption of the glass.

The attention to cleanliness is important. The combination of the filtered hood and recirculating

slurry leads to surfaces that are almost superpol ished. Under conditions of extreme cleanliness we

are able to use the slurry for a period of weeks gradually breaking it down and approaching the con- ditions of a bowl feed process. We recently examined a cylindrical lens that we fabricated and coated

on both sides. The lens was later damaged by beam hot spots. The front cylindrical surface had been

polished on a conventional cylindrical lap with normal attention to cleanliness. It was a good sur- face with a 30 to 40 A finish viewed on the Nomarski microscope. The back surface was polished flat

on the continuous lap under the hood. The front surface shows damage clearly originating in sleeks

and shows these literally burned out. The back surface showed a completely different kind of damage.

There is no indication of propagation along sleek lines indicating a surface of uniform damage resistance. This was obviously not a controlled test, but presumably the back surface saw higher fields due

to the exit effect and some self-focusing.

5. Metal Finishing

We have recently been working gold, copper, silver, aluminum and nickel material using diamond

and low viscosity silicon oils. We have used pitch and beeswax, both separately and covered with fine

unsized silk. We have had excellent results, copper showing consistent reflectivities in the 99.2% to 99.3% range uncoated at 10.6 micron. We have had our best results with 1/4 micron natural diamond and

1 centistoke silicon oil using pitch and silk. The opticians prefer the silicon oil to other vehicles

with the diamond finding that although cutting may be slower, there is very little tendency for the lap to work through the fluid and grab the metal surface. The oil protects the surfaces without the need

for chemical buffers. We find sleeking is always a problem, but that it can be significantly reduced by

working under a plastic tent fitted over the quill. The use of small powered polishing pads keeps a thicker film of oil between lap and workpiece and also reduces sleeking. To date these surfaces have

a bulk reflectivity at 10 ym that is equivalent to surface turned with a single-point diamond tool but

their damage threshold is lower than that of the turned surfaces by a factor of about four.

6. Acknowledgments

The author gratefully acknowledges the many helpful suggestions of Mr. Frank Cooke of Frank Cooke,

Inc., by Mr. Saul Laufer and the staff of Zygo Inc., manufacturers of the LLL continuous polisher, and

the skill of Messrs. Procknow, Baker, Stolcis, Blackman, Whistler, and Gillespie of the LLL Optical

Fabrication Facility.

7. References

[1] Carslaw, H. S. and V. C. Jaeger,

"Conduction of Heat in Solids," 2nd ed.

Chap. 3 (Oxford Univ. Press, London and

New York, 1959).

NOTE: Micron(s) as used in this paper expresses micrometer (s ).

-6-
8. Figures

Figure 3. Curvature Induced by thermal

gradients through flatware. Figure 4. Continuous ring polisher in clean booth.

' TEMPERATURE CONTROL SYSTEM

Figure 5. Temperature control scheme in use

at Lawrence Livermore Laboratory.

1!9 TRANSIENT BEHAVIOR

Note that for a Fourier Number

approaching unity, each solution

From Carslaw & Jaeger, Conduction of Heat in Solids

Figure 6. Thermal transient decay for

flatware.

-7-

Figure 10. Typical chamfers (600x) generated by

mutal bonded diamond, and by 15 micron

diamond belt followed by buff

polishing.

Figure 11. Coating damage on front and back

surfaces (500x & 600 x) of cylindrical

lens. Front surface (Nomarski photo)

shows sleek damage initiation.

NO COMMENTS ON PAPER BV BROWN

-9-
STANDARD INDUSTRIAL POLISHING OF

HIGH ENERGY LASER OPTICS

Walter J. Spawr

SPAWR Optical Research, Inc. Corona, California 91720

The polishing and figuring of optical materials to produce state-ofthe-art surface quality is truly an art, a scientific art, but an art nonetheless.

To attain success, the optician must have a clear mental model of the

processes and associated techniques, and use this model as his most important tool. Second only to that is the combination of discipline and determination that no harmful or deleterious influences are allowed to effect

the processes. New sophisticated polishing machines used in "Metrology

Laboratory" environments are of little or no assistance. Polishing machines

built 50 years ago work as well as the latest designs.

o Today, moderately skilled technicians routinely produce 30A RMS sur- faces on copper, molybdenum, and zinc selenide. These surfaces are free

of orange peel, scratches, and digs; and are produced on conventional

polishing machinery. Such surfaces are shown [3]^ to exhibit exceedinf^ly hifh

laser damage thresholds from 1. O^m to 10. ^m. This is accomplished on a daily

basis in a time frame comparable to the commercial polishing of ordinary

optical glasses.

Key words: Copper; molybdenum; optician; polishing; surfaces; zinc selenide.

Introduction

The limitations in polishing state-of-the-art surfaces in any type of material are primarily imposed only by the optician. The present standard materials, environments , and polishing machines are

of sufficient quality to produce high energy laser (HEL) components, provided the optician understands

the techniques involved.

Polishing

An optician is not necessarily limited to any specific type of material. Both very hard and very

soft polycrystalline materials can be polished to super-smooth (scratch free) surfaces. The highest

damage resistant surfaces can be produced using the standard polishing techniques established many year

ago [Ij. State-of-the-art surfaces of various geometries are produced on a daily schedule in our optica

polishing lab. This demonstrates that new technological breakthroughs are not necessarily required for

many present requirements that have been considered unattainable in the industry.

Photo ;Jo. 1 shows a 10" diameter molybdenum (bulk material) mirror and a 2.2" diameter zinc selenid

(CVD) window. The molybdenum mirror was polished flat to X/2 (A = 5461A) with a surface quality (or

smoothness) of 30A RMS. The zinc selenide window in the foreground was polished (both sides) flat to X/4 (X = 546lA) with 20A RMS surface smoothness. Both the molybdenum mirror and the zinc selenide

window were fabricated in 1972.

Photo No. 2 is a Nomarski photograph [5J of the surface of the zinc selenide (shown in photo No. 1)

using 98X.

Multiple element blocks, which is a standard practice in the industry, can also be used to meet high

volume production requirements for copper and molybdenum without sacrificing optical sxirface figure or damage threshold [1],

We have also found that experienced opticians are not limited to polishing simple geometries.

1. Figures in brackets indicate the literature references at the end of the paper.

-10-
Photo No. 3 is of a water-cooled copper elliptical mirror with a large output coupling hole.

Mirrors of this geometrical configuration are normally considered to be very costly because of the

extra detailed work involved in mounting a plug for the central hole, and "fill material" for the

surrounding area. When this coupler was fabricated we chose to use no plugs or polishing aids (fill

material) of any kind. The mirror was polished flat to 3 fringes overall with a 3 fringe rolloff

extending <0.050" out from the hole. This was well within the design tolerances specified for this

mirror.

Aspheric configurations including parabolic cylinders and toric surfaces can also be produced.

Photo No. 4 illustrates interference fringes for a cylindrical figure in a water-cooled molybdenum

mirror. This mirror was accurate to X/8 (NaD) . The surface quality was <.35A RMS and <50/40 scratch

and dig per MIL-O-13830A. Though this was the first of its kind, research and development was not

required to produce it.

Based on the results of diamond turning experiments [2], we decided to obtain better surfaces from

machine tools (lathes and mills) to cut down on the grinding and polishing time for copper mirrors. We

found that with surfaces no better than a machine finish as illustrated in photo No. 5, the entire

grinding process could be eliminated. We can now take machined surfaces (in copper) from the machine

shop and go right into the polishing operation.

The polishing processes used by opticians in our lab do not seem to be limited to small sizes.

They have fabricated several 12" and 16" diameter copper mirrors polished to <3 5X RMS, 40/20 scratch

and dig, and flat to 5500A. The largest mirrors fabricated to date measured 25.5" x 15.5" and offered

no additional problems to polish [1].

The reported [3, A ] reflectance and damage threshold values for polished copper surfaces are ^0.99

and >250 J/cm^ (? 600 nsec, respectively. Molybdenum reflectance and damage threshold is reported [3, A] to be 2.0.98 and >200 KW/cm^ CW for 10 sec. uncooled.

Summary

When copper or molybdenum is properly polished it offers high damage thresholds [3], The opticians

in our lab are taught that the basic limitations in polishing and figuring HEL optical components

is with their understanding. An optician's technical capability is a direct function of the optical

figure and damage threshold achieved.

References

[1] Spawr, W. J., "The Art of Metal Polishing,"

presented at SPIE, August 1975.

[2] Saito, T. T., APPL. OPT. lA, 1973.

[3] Spawr, W. J. and Pierce, R. L., "Metal

Mirror Selection Guide," SOR Report No. 7A-00A, Spawr Optical Research, Inc.,

Corona, CA.

[A] Saito, T. T., "Microinch Machining of Optical

Components for Infrared Optics," Air Force

Weapons Laboratory Tech Report AFWL-TR-73-290,

Air Force Weapons Laboratory, Kirtland Air

Force Base, NM (February 197A).

[5] Compliments Capt. R. Selleck, Kirtland Air

Force Base, NM.

-11-
Figures

Photo No. 3 - Copper Laser Coupling Mirror Photo No. 4 - Interference Fringes

of a Cylindrical Mirror

Photo No. 5 - Machined Surfaces of

Copper Mirrors

-12-
COmENTS OW PAPER BY SPAWR

The. i>pzakeA mcUntcu.md that in kit, zxpe/iizncz rruAAou pfizpoAzd bg thAj> technique, have not damaged

In leQuZoA. (Me at WiumZvuvtion teveZii up to {^ouA on. ^ive jouZeA pen. iquane aentAjneten. VanJ^ation^ in

n.e{,lecXi.v-U:y have been noted in coppen. nuMAonj> pn.epaA.ed by the method deicnibed. The^e vaniation^ axe attributed to details ojj the pnopnletoAy poliihing pn.oceM. SuA(,aee n.oughneii ii moniton.ed u^ing a viiual obieAvatyion Oj$ light icatteAing. An accuAacy ± 2 A wai claimed. The technique 0(5 inipectton uioi de&cAibed,

-13-
1.3 Ion Planing and Coating of Sodium Chloride

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

Westinghouse Electric Corp.

Research Laboratories

Pittsburgh, Pennsylvania 15235

Ion planing of single crystal sodium chloride, using low energy Xe ions at grazing

incidence, is effective in removing scratches from surfaces when the sample is rotated.

Surface features Wtiich develop, probably due to defects such as polishing grit and

dislocations, can b'e minimized and the scratches eliminated by optimizing the amount

of material removed. Ion planing and depositing overcoating films "in situ" greatly

improves film adherence and protection of sodium chloride substrates against moisture

attack.

Key words: Arsenic trisulphide; ion planing; protective films; moisture resistance;

sodium chloride,

1. Introduction

Several studies have indicated that ion planing of optical components can produce beneficial

results. In a previous paper [l]^we have shown that the combination of ion planing and vacuum annealing

of single crystal copper samples results in a smoother surface with significantly lower optical

absorption at 10.6 ym. Giuliano [2] has demonstrated that ion planing of sapphire yielded samples with

improved laser damage threshold. Finally, Bruce and co-workers [3] have reported that ion planing

of alkali-halide windows results in a better surface finish with lower optical absorption. The object

of this study is to demonstrate another use for ion planing, namely , the improvement in film adherence

and protection afforded by the combination of ion planing and depositing overcoating films "in situ."

Ion planing and overcoating "in situ" offers great potential for producing more durable laser

components. In addition to a better surface finish, the technique should yield a clean film-substrate

interface free from contamination, thus giving rise to improved film adhesion, lower absorptance> and

increased laser damage threshold. This is especially true for alkali-halide laser windows where

surface preparation and film adhesion are major problems because of substrate hardness and moisture

sensitivity. Hence, it was decided first to investigate the effects of ion planing on the surface

morphology of single crystal NaCl and then to study the influence of ion planing and overcoating

"in situ" on film adhesion and moisture protection.

2. Experimental Procedures

The ion planing apparatus was constructed to provide an experimental laboratory facility for

removing material from surfaces in a controlled manner. The apparatus, which is fully described

elsewhere [1], is shown schematically in figure 1 along with typical operating conditions. Briefly,

the planer employs sputtering by a focused beam of relatively low energy Xe ions incident on the

sample surface at glancing angle, 12° from the plane of surface. The beam is rastered across the

surface and the sample is rotated to produce uniform removal of material. The combination of low ion

energy, glancing angle of incidence, rastering, and sample rotation are expected to produce a smoothing

of the surface features while minimizing damage to the surface. Figure 2 depicts the sample chamber

of the ion planer in the configuration for depositing overcoating films. Shown in the figure are a basket evaporation source with shutter, a quartz crystal film thickness monitor, a neutralization

filament used when planing insulating samples, and a phosphor screen used for focusing and positioning

the ion beam. In practice, ion planing at a low removal rate is continued during film deposition to minimize contamination at the interface and in the film, while sample rotation provides better film

thickness uniformity over the sample.

One series of experiments was performed using a commercial "Microetch" unit purchased from the

Veeco Corporation. A sample holder has been added to this equipment to provide sample rotation and

variable angle of incidence of the ions which form a broad area beam. Typical operating conditions

used 950 eV Xe ions incident on the rotating sample at 40° from the normal. The removal rate, using

either apparatus, was determined by masking a portion of the sample during planing, and then using a profilometer to measure the amount of material removed.

Two instruments were used to examine the surfaces of the NaCl samples: an optical microscope

with an interference contrast attachment and magnification up to 500X, and a scanning electron

microscope having magnification up to 20,000X. Of course, insulating samples such as NaCl must be

coated with a conducting film, gold for example, before the surface can be examined in the SEM which

limits the usefulness of this instrument.

The humidity test chamber consisted of a bell jar containing dishes of water, a humidity meter>

and a thermometer. A small opening was provided under the bell jar rim to permit some circulation

and prevent the relative humidity from reaching 100%. During testing, the relative humidity was

typically in the range 90-95%.

1. Figures in brackets indicate the literature references at the end of the paper.

-14-
3. Ion Planing Results

The samples used in this study were single crystal NaCl disks, 2.54 cm in diameter, which were

first mechanically polished. Most of the sample faces had a crystallographic orientation close to

(100) except for those especially prepared with (110) and (111) faces for the orientation dependence

study. These samples were selected because NaCl represented a typical laser window material with

polishing and moisture sensitivity problems, they were readily available, and it was

felt that the results on single crystal material would be less complicated and easier to interpret.

A typical surface, which was ion planed to remove about 3 ym and then coated with about 40oX of Au, is

shown in the scanning electron micrograph in figure 3. The surface shown in the left hand side of the

figure was masked off during ion planing while that on the right hand side was ion planed. It is seen

that the surface is drastically different after ion planing with all the scratches removed and raised

features (hummocks) appearing predominantly along previous polishing scratches. Figure 4 shows

scanning electron micrographs of the same area of the ion planed surface, but with higher magnifications.

Several different types of hummocks can be seen in these photographs, ranging from those with pointed

tops to those with a broad rounded profile. The hummocks are caused by regions with a lower sputtering

yield than the surrounding area and emerge out of the background as it is sputtered away. One can speculate that polishing grit having a lower sputtering yield and trapped in the surface would evolve

into a pointed feature while dislocations, induced by mechanical polishing of the surface, might lead

to the rounded features that predominate along polishing scratches.

The change in surface morphology with the amount of material removed by ion planing is shown in

figure 5 which consists of interference contrast micrographs of NaCl surfaces with 0.4, 1.5, 4.1, and

7 ym removed. It can be seen that as more material is removed from the surface the scratches disappear

and the number and size of the hummocks grow until for large amounts of material removed the entire

surface is covered with them. This suggests that there is an optimum amount of material to be removed

which would eliminate the scratches and damage layer and minimize the growth of hummocks. The

crystallographic orientation of the sample with 1.5 ym removed was (110) while the other samples had

(100) orientation. It is seen that the growth of hummocks is similar for both orientations, but the

background texture appears to be different. These same observations can be made from examination of

figure 6 which shows interference contrast micrographs of (100), (110) and (111) samples which were ion

planed to remove about 4 ym. It must be noted that the quality of the surface before ion planing was worse for the (110) and (111) orientations since mechanical polishing of these orientations was found

to be more difficult.

It was observed that the density of the hummocks varied dramatically over some of the ion planed

surfaces. This is seen in figure 7 which shows interference contrast micrographs of two' areas of a (110) NaCl crystal which was ion planed to remove about 4 ym. The hummock density was much greater

near the edge of the sample (left-hajid photograph) than in the center of the sample (right-hand

photograph) . It is possible that hummock growth is related to damage in the sample induced by mechanical

polishing which can be greater at the sample edges. Davisson [4] reported that damage which resulted

from mechanical polishing of NaCl windows had to be annealed out before chemical polishing would give

satisfactory surfaces. Therefore, a NaCl sample was annealed at 650°C for 1/2 hour in air prior to ion

planing, but the surface both before and after ion planing seemed worse after annealing. It is not

known what happened to this sample, but the effects of annealing prior to ion planing should be

investigated further.

Several samples were ion planed, with the Xe ion beam incident at 40° from the surface normal,

using the commercial apparatus described previously. Typical surfaces obtained with this equipment

are shown in interference contrast micrographs in figure 8. By comparing figures 5 and 8, it is seen

that for similar amounts of material removed, ion planing at 40° is not as effective as that at 78°

(from the surface normal) in removing scratches from the surface. Also the raised features on the

surface, although they have similar density and disposition in both cases, seem sharper in the

former case. Hence, we conclude that ion planing at glancing angles results in a smoother surface

than obtained by planing at more normal incidence.

Finally, the interference contrast micrographs of figure 9 show the effects of not rotating the

sample. On the left hand side, the NaCl sample was not rotated and the scanned beam was incident

in the horizontal direction in the photograph. The surface in the top half of the photograph was

masked off during ion planing which removed about 3 ym from the lower portion of the sample. It is seen that the scratches that run perpendicular to the ion beam direction are "washed out" while those

running in the direc,tion of the ion beam are still sharp. The right hand photograph of figure 9 shows

a sample that was planed with the ion beam scan narrower than the sample width in one direction; thus,

the edges of the sample were struck by ions traveling predominantly in one direction. In the

photograph the surface features are elongated in the direction of the ion beam. It can be concluded

that sample rotation during ion planing is necessary for obtaining smooth, scratch-free surfaces.

-15-
A. Ion Planing and Overcoating "In Situ"

The principal purpose of this work was to determine what effect ion planing of NaCl and overcoating

"in situ" had on film adherence and protection. First, two NaCl disks with a rough mechanical polish were coated on one face and edges with a 3.7 pm thick film of As252' uncoated face of one sample

was ion planed and after removing about 7 pm, a 2.2 ym thick film of AS2S2 was deposited on both

samples in the ion planer at the same time. Both samples were subjected to the humidity test previously

described with the relative humidity at about 95%. Interference contrast micrographs of the surfaces

of the two samples after 7 hours in the humidity test are shown in figure 10. The As-S_ film is

cracked and lifting off the sample that was not ion polished (left hand photograph), while the ion

polished surface appeared to have a deposit on it (light areas in the photograph) , but the film remained

intact. The samples were returned to the humidity test chamber and after several hours it was observed

that small water droplets were condensing on the surface of the ion polished sample » while it looked

as if moisture had penetrated the film on the sample that was not ion polished. This is shown in

figure 11 which is a photograph of the samples in the humidity chamber after 39 hours of test. The

AS2S2 film on the sample on the left, not ion planed, appears to be floating on water, while the

surface of the ion planed sample on the right has condensed water droplets on the film surface

predominantly along the remnants of two large polishing scratches. Figure 12 shows interference

contrast micrographs of the two samples after the 39 hours of humidity testing. The film on the sample

that was not ion polished was badly blistered and lifting making it difficult to photograph. The ion

polished sample had what appeared to be NaCl crystals on the film surface as shown in the right-hand

side of figure 12. This probably indicates that moisture had penetrated the AS2S3 coating through

film defects in discrete spots, but the film was still intact and flat even in the vicinity of the

defect. Therefore, we conclude that ion planing and coating "in situ" does markedly improve film

adherence and protection of NaCl substrates against moisture, and that even better protection of alkalihalide windows can be expected if the film can be made free from defects, perhaps by the proper

combination of mechanical polishing and ion planing.

5. Conclusions

We conclude that ion planing, using low energy inert gas ions incident at a glancing angle on a rotating sample, is effective in removing scratches in single crystal NaCl. The influence of the

raised surface features that develop, probably due to defects such as polishing grit and dislocations,

can be minimized by optimizing the amount of material removed and could possibly be eliminated by

proper annealing prior to ion planing. Ion planing and depositing overcoating films "in situ", which

minimizes contamination at the film-substrate interface, greatly improves film adherence and protection

of NaCl substrates against moisture attack and offers promise of increased laser damage thresholds.

6. References

[1] Hoffman, R. A. , Lange, W. J. and

Choyke, W. J., To be published Appl. Opt.

Aug. (1975).

[2] Giuliano, C. R. , Appl. Phys . Lett. 21, 39

(1972)

.

[3] Bruce, J. A., Comer, J. J., Collins, C. V.

,

and Lipson, H. G. , Proc. Third Laser Window

Conf. AFCRL-TR-74-0085(lll) , 757 (1974).

[A] Davisson, J. W. , Proc. Second Laser Window

Conf. AFCRL-TR-73-0372(ll) , 525 (1973).

-16-
7. Figures

LN2 Trap

Diffusion Pump —]lOcm[—

Figure 1. Ion polishing apparatus. The vacuum envelope, except for the large glass cross on

the right, is stainless steel at ground potential. The LN2-trapped, oil diffusion

pump provides a base pressure measured by the ion gauge (BAG) of <_5 x 10~^ Torr

(6 X 10~6 Pa). The system is inclined to the horizontal at 12°, so that the

sample is held on the rotatable holder by gravity alone. Electrons supplied by

the filament, F, maintain a discharge in the anode. A, into which gas, G, is

admitted to maintain a pressure of 'vl x 10"-^ Torr, some 500 times that in the

remainder of the system. Cylinders C]^, C2, and comprise the ion lens. Two

pairs of plates, and Dy, provide deflection of the beam in the x and y directions; for rastering, frequencies of 1000 and 50 Hz, respectively, are used on D and D . Typical operating conditions using Xe as the sputtering

gas are: filament at +3975 V, anode at +4000 V, at -2000 V, at +2600 V,

C3 at ground, target at ground, ion current 10 yA, sample rotated at 6 rpm.

Figure 2. Apparatus for depositing overcoating

films in ion planer.

-17-
20 urn Mot Ion Polished Ion Pollehed

Figure 3. SEM photomicrographs of sodium chloride.

Right-hand side was ion planed to remove 3 ym.

Figure 4. SEM photomicrographs of sodium chloride

which was ion planed to remove 3 ym.

0,4 uD reooved

I I

I i

Figure 6. Interference contrast photomicrographs

of ion planed sodium chloride showing

dependence on crystallographic

orientation.

It, I un removed

Figure 5. Interference contrast photomicrographs

of sodium chloride showing dependence

on amount of material removed by ion

planing.

Figure 7. Interference contrast photomicrographs

of two areas of ion planed sodium

chloride sample. Left side is near

edge and right side is in center.

-18-
1.7 um removed 20 um,

4 . 3 Mm removed

Figure 8. Interference contrast photomicrographs

of sodium chloride which was ion planed

at A0° from the surface normal.

Figure 9. Interference contrast photomicrographs

of sodium chloride showing effects of

not rotating the sample during ion

planing. Sample on left was not rotated,

on right was partially rotated.

Figure 10. Interference contrast photomicrograDhs

of sodium chloride coated with AS2S3

after 7 hours of humidity testing.

Figure 11. Photograph of sodium chloride coated

with AS2S3 after 39 hours of humidity

testing. Sample on right was ion

planed, that on left was not.

Hot lOD PoUshed

Ion Polished

Figure 12. Interference contrast photomicrographs

of sodium chloride coated with AS2S3

after 39 hours of humidity testing.

COmEHTS ON PAPER BY HOFFMAW, U al

Iho. 6pe.akeA wm oifeecf a,{, ^uA^ace iiqaxe. wcu, mcUvLtcUmd du/Ung thz potUking pnocuM,. Hz K.e.ptizd

that iuA^ace iiguJit Itieli iMoa not mowctoKtd, but that unlionm-Ltij Kmoval wcu> moyuXonnd and a variation up to 10% iMoi obieAvzd ovdH. thd iuAiace. w-uth a total KmovaZ o{, ^ouA tnicKomeXi^MA o{^

matzJvLal, Tkn pattzm o^ nmoval moa dutgnzd to be z6izntA,ally fiandom. Hzatlng 0^ thz i,amplz wcu

not thought to bz a izAlouA pKobtzjn, Thz quzition 0|5 bubblz fionmatxon daz to iubiuAf^acz pznztJiatlon

0(1 thz ion bzam moa dAJ,zuMzd and iX. woi zoncladzd that thz Ion nangz at thz znzAgy uizd woA not

liU{^{^tcA.znt to zauiiz iignidlcant iubiuA{,aaz pznztJiatLon and conizquznt bubbtz (^omaZion.

-19-
l.A Polishing Studies and Backscatter Measurements on Alkall-Hallde Windows

M. J. Solleau, H. E. Bennett, J. M. Bethke, and J. Shaffer

Michelson Laboratories, Naval Weapons Center

China Lake, California 93555

Surface absorption, optical figure, and optical scatter from windows are particularly important in high energy lasers. Surface absorption can result in window

failure due to thermal distortion and In some cases catastrophic failure. Excellent

optical figure is required for each component in the laser system optical train in

order to maintain maximum on axis intensity of the focused laser beam. Backscatter

from windows and other optical components can cause depumping of the active medium,

and in the case of amplified chains, catastrophic damage to the laser amplifiers. A

technique for polishing alkali-hallde and quantitative measurements of backscatter

for alkali-hallde windows from the visible to 10.6 ym will be presented. This

polishing procedure produces alkali-hallde surfaces which have less than 1 x 10"^

surface absorption, are parallel to less than 3 seconds of arc and are flat to better

than a quarter wave In the visible. Scattering levels below 10~^ at 10.6 ym have

been achieved.

Key words: Alkali-halide; KCl; laser windows; optical figure; polishing; scattered

light; surface absorption.

1. Introduction

A technique has been developed for producing HEL window surfaces which have low surface absorption,

low scatter, excellent optical figure, and are parallel. This procedure, which employs a combination

chemical-mechanical polish, has been successfully applied to pure KCl (single and polycrystalllne) , KCl

alloys (single and polycrystalllne), and NaCl windows. Surfaces which have less than 1 x 10"^ surface

absorption, total backscatter below 1 x 10~^ at 10.6 ym, are parallel to less than 3 seconds of arc and

aye flat to better than X/4 have been achieved. The details of the polishing procedure, experimental

results on a variety of alkali-hallde laser windows and backscatter and absorption measurements will be

discussed

.

2. Polishing Technique for Alkall-Hallde Windows

We have developed a technique for polishing alkali-halide windows with less than 1 x 10~^ surface

absorption, parallel to less than 3 seconds of arc and flat to better than a quarter wave in the visible.

This technique is a hybrid of the mechanical procedure previously reported by Michelson Laboratory [1]^,

and the chemical etch procedure reported by the Naval Research Laboratory and Hughes Research Laboratory

[2,3].

Previously we described a technique which yielded well-figured KCl which has surface roughness a factor of four less than conventional polished KCl [1]. This technique employed a pitch lap, AI2O3

abrasive, and triacetin as a polishing vehicle. Initial measurements of the 10.6-ym absorptance of KCl

polished using this technique indicated significant surface absorption. Previous results by Davlsson

and Allen have shown that the mechanical polishing damage and surface contamination, which give rise to surface absorption, can be removed by careful etching [2,3]. The primary difficulty with etching is

that material is removed at a hi^h rate, thus making figure control extremely difficult.

The basic idea of the technique reported here is to first remove all surface contamination and

mechanical damage by etching in concentrated HCl, and then produce the optical figure with a minimum

working of the surface (usually no more than h hours polishing on each side). In practice the specimen

is ground to approximate flatness and wedge using 20-, 11-, and 5-ym AI2O3 grit in kerosene on a Pyrex

plate. After grinding, the specimen is etched for 3 minutes in concentrated HCl and rinsed in reagentgrade ethanol in the manner described by Davisson. When the specimen is removed from the ethanol it is

blown dry with clean, dry nitrogen. Boil-off from liquid nitrogen is used in conjunction with a delonizlng nozzle to minimize buildup of static charges, thus minimizing dust accumulation on the surface.

The etching procedure sometimes results in enhanced grain boundaries (particularly in fine grain

polycrystalllne materials) and sharp edges. If visual inspection reveals any such edges the specimen

is buffed on a Polytex cloth using 0.3-ym AI2O3 in triacetin until the surface is smooth enough to reveal optical figure. This usually takes 1 to 2 minutes and Is done only when indicated by visual

inspection. If sharp edges are present and are not removed damage to the polishing lap will result.

The specimen is then blocked by taping it to a felt-covered metal disc of the same diameter. The

part is then polished on a paraffin lap with 0.3-ym AI2O3 abrasive in a triacetin vehicle. The speci- men is polished until desired figure is obtained and optimum surface quality (determined by inspection

1. Figures in brackets indicate the literature references at the end of the paper.

-20-
with a Nomarski microscope) is achieved. Experience has shown that if the desired figure and surface

quality are not achieved after approximately 4 hours of polishing, it is best to re-etch the specimen

and repolish it. When the desired surface finish and figure are obtained the sample is cleaned with

benzene. A soft tissue is saturated with benzene and gently rubbed over the specimen until the residual

polishing material has been removed.

After cleaning, the polished side is coated with Carbofilm and the specimen Is blocked as previously

described. The polishing process Is repeated for the second side. The procedures are summarized In tables 1 and 2, and are illustrated in figures 1 and 2.

Table 1. General Procedure for Polishing Alkall-Hallde Windows.

Rough grind Flat glass lap, AI2O3 abrasive (20, 11, and

5 ym) , kerosene vehicle.

Chemical etch 3 minutes in concentrated HCl, rinse in ethanol,

dry with clean, dry N2 gas.

Cloth buff Polytex cloth, O.S-pm AI2O3 abrasive, trlacetin

vehicle.

Block Coat one side with Carbofilm and tape to felt

covered metal plate.

Polish Paraffin lap, 0.3-um AI2O3 abrasive, trlacetin

vehicle, bowl feed or fresh feed.

Table 2. Details of Polishing Procedure.

Lap Paraffin, pressed flat, cut to form 1/2-ln. squares

with approximately 1/8-ln. spacing.

Polish Spindle speed 28-30 rpm, arm speed 1/A spindle speed,

1 to 1.5 lb/in2 load, bowl feed with paddle in con- tinuously or fresh feed by brushing on slurry, avoid

polishing more than 4 hours per side.

Clean Wipe clean with soft tissue saturated with reagentgrade benzene.

3. Surface Microroughness Requirements for KCl and NaCl

Although we have demonstrated that it is possible to polish KCl and NaCl surfaces using the pitchlap, triacetin-polish technique to rms roughness values under 30 A rms, we have not succeeded in producing surfaces with low surface absorption which are this smooth. The goal to achieve roughnesses

under 20 X rms was set by analogy with roughness requirements for mirrors. Infrared absorption of

silver coatings increases rapidly with roughness for rms values above 20 A. This anomalous absorption

may result from a resonant dipole phenomenon [4] , from limitation of the electronic mean free path by

surface irregularities with lateral dimensions in the 100 A range [5] , or possibly by some other phenomenon, such as two-carrier absorption. None of these mechanisms should cause surface absorption in

dielectrics.

Another reason for minimizing surface roughness is to increase the threshold for damage resistance.

Scratches and other gross surface imperfections increase the local field in dielectrics, resulting in a lowered threshold for dielectric breakdown. The minimum size of irregularities for which this mechanism

is estimated to be important in the infrared is about 100 A [6].

The final reason for minimizing surface roughness on polished dielectric surfaces is to minimize

scattered light. Both forward and backscattering are Important in a high energy beam. In addition,

light scattered at the surface into the dielectric at angles larger than the critical angle will be

trapped and eventually absorbed in the window and its mounting. For low absorption surfaces a significant fraction of the apparent "surface absorption" may result from this mechanism. However, as discussed in a later section of this paper, the dominant scattering mechanism at 10.6 ym is not necessarily

surface microroughness. If the contribution to scattered light from microirregularities is below about

10~5, other scattering mechanisms dominate. The fraction of the light incident externally on the surface which is scattered by microirregularities is

R ,. .v2
where AR is the fraction of the reflected light Rg which is scattered, Iq

is the rms height of surface irregularities, and X is the wavelength.

Ro = 0.04 so that

is the incident intensity, 6 For KCl and NaCl at 10.6 pm.

AR

In 10 for 6 < 130 X.

We thus conclude that although the criterion that surface microirregularities be under 20 k rms is

a valid objective for mirrors used in HEL systems, it is unnecessarily severe for NaCl and KCl windows

and may be relaxed. A requirement that the microroughness be under 100 A seems adequate for HEL

windows made from these materials.

4. Experimental Results

4.1 Single Crystal KCl

Initial efforts were directed at polishing pure KCl and emphasis was placed on obtaining very smooth

flat surfaces. This approach proved successful and yielded surfaces flat to one-eighth wave in the

visible and with surface microroughness of less than 30 K rms. The next step was to determine the

effects of the polishing procedure on surface absorption. A laser calorimeter was assembled for measuring absorptance of the polished material. A schematic diagram of this calorimeter is shown in figure 3. The calorimeter and the method of data reduction are based on earlier work by John Loomis [7,8].

The first absorptance measurement on a triacetin-polished single crystal KCl yielded total absorptance of 0.0062 to 0.014 (measurements at different spots) on a 5-mm-thick piece. Hope that the measure- ments were wrong was vanquished by measurements on a similar specimen polished by Harshaw which

indicated total absorptance of 0.0011. Since the specimen with the high absorptance had been polished

for weeks in order to try to optimize surface finish, the logical thing to assume was that extensive

surface damage had resulted and the surface had been contaminated.

This theory was tested by etching the high absorptance specimen. After 3 minutes of etching the

total absorptance was reduced to 0.00077, thus indicating surface absorptance in the polished piece on

the order of 0.005 per surface. This high surface absorptance cast serious doubts as to the usefulness

of the polishing technique. However, as we were to discover, the technique could be used successfully

if polishing time could be restricted to less than 8 hours and preferably 4 hours per surface.

After etch, the specimen discussed above was polished until flat to one-half wave in the visible.

The absorptance was measured after polish and found to be 0.00053. Since this value was less than that

of the freshly etched piece, it seemed that all the surface contamination was not removed in the 3-

mlnute etch. The specimen was etched again for 3 minutes and the absorptance went down to 0.00041. The

specimen was polished again on both sides to a flatness of less than \/2 in the visible on each surface.

The specimen was polished a total of 8 hours using fresh feed and the total absorptance was 0.00066,

indicating surface absorptance on the order of 1.2 x 10~^ per surface. These results are sunmiarized in

table 3.

Table 3. Polishing Results on Single Crystal KCl. This specimen was

pure KCl from Harshaw. The specimen was 1.5 inches

in diameter and 5 mm thick.

Surface treatment

Optical Total absorptance

figure at 10.6 wm

Several weeks of triacetin

polish using bowl feed

X/8^ 0.0062-0.014

3 min HCl etch

6 hours triacetin polish

using fresh feed

A/2

0.00077

0.00053

3 min HCl etch

8 hours triacetin polish

using fresh feed

X/2

0.00041

0.00066

X is in the visible.

-22-
4.2 KCl Alloys

The results obtained on single crystal, pure KCl were applied to Eu"'"'' and RbCl doped KCl in single

and polycrystalline form. This material was provided by the Air Force Weapons Laboratory. The single

crystal material was grown by Harshaw and was hot pressed by the Honeywell Ceramics Center [9].

At this point the NWC technique consisted of etching in HCl and polishing with the trlacetin slurry

using fresh feed. Bowl feed polishing sometimes produced severe scratching. We later found that this was caused by "tearing" or "chipping" of the lap by sharp edges produced in the etching process. These

pieces of lap material were then thrown back in the lap by the stirring action of the paddle in the

bowl. This problem was eliminated by buffing the part on a Polytex cloth for approximately 1 minute to remove the sharp edges. This buffing procedure should only be performed if visual inspection indicates

the presence of sharp edges, since any handling of the KCl is likely to cause some surface contamination

and increased absorptance.

Excellent results have been achieved in polishing the KCl alloy materials using the technique outlined in section 2 with bowl feed and fresh feed. Single crystal and polycrystalline specimens have

been polished to a good optical figure, while keeping the total surface absorption in the 1 x 10~^ range.

Figure 4 summarizes the results obtained on polycrystalline Eu''"'' doped KCl. The total absorptance of

this specimen after being polished to X/4 in the visible was only 7 x 10~5 more than the absorptance

after chemical etching. Thus, the increase in surface absorptance by the mechanical polishing was only

3.5 X 10~5 per surface. Similar results have been obtained on single crystal Eu doped single and polycrystalline RbCl/KCl alloy material. The results of our efforts to polish polycrystalline alloy

materials are summarized in table 4.

Table 4. KCl Alloy Polishing Results.

Optical figure X/8

-4

Surface absorption < 1 X 10

per surface

Surface roughness 27 X rms

Scale-up 6 in. diameter

Wedge < 3 sec

Although the results obtained to date have been encouraging, several problems remain to be solved;

First, even in best case results some increased absorptance is caused by the optical figuring process.

The origin of this residual absorption is not well understood and will be subject to further research.

Second, there appears to be a serious problem with the "coatability" of polished surfaces. The absorptance of coatings deposited on the polished surfaces is as much as a factor of 10 higher than that of

chemically etched surfaces. We are presently pursuing this problem. Two possible mechanisms for the

higher coating absorptance are (1) inadequate cleaning of the polished surface, and (2) enhanced trapping of scattered light by the addition of a quarter-wave optical thickness, high index coating (in

this case AS2S3).

4.3 Polycrystalline NaCl

The procedures developed for polishing KCl were applied to Harshaw "Polytran" provided to us by

Dr. Walter Reichelt of Los Alamos Scientific Laboratory (LASL) . In general, the NaCl was easier to polish than the KCl. Both figure and surface quality were easier to control; however, the problem of

"coatability" discussed in the previous paragraph is as serious for NaCl as for KCl.

The results of the work on NaCl surfaces are shown in table 5 and figure 5. The value of the absorptance of the NWC-polished specimen presented in table 5 indicates no increase over that for the chemically etched surface. However, since the total absorptance is so high (0.0016) it would be difficult to detect increases in absorptance on the order of 1 x 10~^. Thus, one cannot conclude from this data that

no surface absorptance is introduced by the optical figuring process. The absolute accuracy of the calorimeter used to measure the absorptance is on the order of il0%, thus one can conclude that the total

induced absorptance is less than 3 x 10~^, i.e., less than 1.5 x 10~^ per surface.

-23-
Table 5. Results of NaCl Polishing.

Absorptance of etched NaCl 0.0016

(1 cm thick)

Absorptance after NWC polish U . UU-LD

Optical figure A/ 8^

Surface roughness 26 A rms

'^A is in the visible.

5. Backscattering From Alkali Halide Windows

Backscattering from windows and other optical components is particularly important in high energy

lasers, since it can cause depumping of the active medium, and in the case of amplified chains, catastrophic damage to the laser amplifiers. The surface microirregularities for both NaCl and KCl have

been shown by FECO interf erometry to have Gaussian height distribution functions [10], so scattering

levels from this source should fall exponentially with increasing wavelength and exhibit a 1/a2 dependence. Scattering from scratches would be expected to show little wavelength dependence. Particulate

scattering was investigated previously for mirrors [11] and also showed little wavelength dependence.

To our knowledge, quantitative measurements for alkali halide windows have not been previously reported.

To obtain an approximate value of the level of backscattering to be expected from single crystal and

polycrystalline KCl windows, measurements were made on the Optical Evaluation Facility (OEF) [12] using

a conical absorber mounted approximately 10 cm behind the window to prevent light transmitted through

the window from reaching the scattered light detector. The contribution from light scattered from the

back surface of the transparent sample was determined by measuring the apparent scattering level from a mirror surface as the mirror was moved back from the Coblentz sphere focus. For a window of 1 cm thick- ness with index of refraction of 1.5, the effective air path would be 1/1.5 = 6.67 mm and the signal

from the back side at that effective distance was determined to be 0.79th of that from the front surface

The scattered light results reported are total values observed on the OEF, and hence are 1.79 times the

values from a single surface. However, the roughness values quoted are obtained by dividing the observe

scattering levels by 1.790 to correct for back surface reflection.

The first sample measured was polycrystalline KCl doped with Eu and hot forged by Honejrwell. It had

been polished commercially, was heavily scratched, and had a figure of about 5 fringes across its 38-mm

diameter. The observed backscattering is given by the circles in figure 6 and follows an exponential

curve, Indicated by the dashed line. The slope of the dashed line is less than would be predicted for

microirregularities, probably because of the scratches, which result in scattering levels with little

wavelength dependence [11]. Forward scattering from this sample was also measured at a wavelength of

6328 A on the Optical Functional Tester [13] and was in excellent agreement with the backscattering

values. This result is important since it suggests that forward and backscattering values for windows

may often be nearly identical.

After the initial measurements the KCl sample was given an NWC polish, described in section 2 of

this paper, and had a figure of about 1/2 fringe before etching. It was etched for 3 minutes as described in section 2 of this paper and then remeasured. The results are shown by the square points

in figure 6. The scatter in the visible region is reduced by over a factor of 2, and falls more rapidly

in the infrared region than did the commercially polished sample. It fits the theoretical line for

scattering from microirregularities of height 183 A rms very well both in the visible and infrared

regions. Under a Nomarski microscope the etched surface appeared virtually free from scratches, suggesting that microirregularities should be the dominant scattering mechanism.

The difference in scattering from polycrystalline and single-crystal surfaces is shown in figure 7. Both Eu-doped samples were etched for 3 minutes in HCl. The polycrystalline surface scatters in a manner similar to the polycrystalline surface in figure 6. However, scattering from the single-crystal

surface is lower by a factor of 3 in the visible and near infrared. At longer wavelengths, scattering

from the single-crystal surface departs from the theoretical predictions for scattering from microirregularities and is a factor of A higher than that from the polycrystalline surface at 10 ym. Since Eu is

a much larger atom than K and will cause significant distortion of the lattice, the same experiment was carried out with another dopant, Rb. The results are shown in figure 8. Again the single-crystal surface scattered much less than did the polycrystalline one in the visible region. Again scattering from

the single-crystal surface departed from the theoretical predictions at about 5 x 10~^ scattering level.

This time, however, the polycrystalline surface scattering also departed from theory and virtually

coincided with that from the single-crystal surface in the 3-10 ym wavelength region.

The average deviation in scattering from various points on the surface is also plotted in figure 8. The circles correspond to the polycrystalline sample and the squares to the single-crystal sample. In

the case of the single-crystal sample, the average deviation in the points is almost Independent of

wavelength. Scatter in the polycrystalline points is about an order of magnitude lower than the

-24-
scattering values in the visible region, and decreases with wavelength until it reaches the singlecrystal values, at which point it also becomes independent of wavelength. A similar behavior has been

observed for mirrors; when the scattering values reach the 10~^ region they tend to stabilize and do not

decrease further. We suspect that the variations from point-to-point are caused largely by scattering

from particulates, such as dust, or possibly microscopic salt protuberances, rather than from microirregularities. If the scattering is from dust, then surface finishing beyond a certain point is not productive

and the scattering level of the component will be determined by the environment in which it is used,

rather than by the way in which it is polished. Work is continuing on this question.

6. Conclusions

A technique has been developed at Michelson Laboratory for producing HEL window surfaces which have

low surface absorption (1 x 10~^ per surface or less), excellent optical figure (X/8 in the visible),

and are parallel (less than 3 seconds of wedge). The major problems remaining are the coatability of

these surfaces, improvements in surface quality, and scale-up to larger sizes.

Our tentative conclusions regarding scattering from halide windows are: (1) Scratches can significantly increase the infrared scattering level in KCl windows as well as presumably lower the damage

threshold and should be minimized; (2) In the absence of scratches scattering above about 5 x 10"^ is

determined by microirregularities and below this value may be governed by particulates. In the 10 ym

wavelength region particulate scattering may well be the dominant scattering mechanism; (3) Since the

index of refraction and hence the reflection coefficient from the surface is low, requirements on microirregularities for window surfaces are much lower than for mirror surfaces. For example, a window

with a reflection coefficient of A% can have surface irregularities with rms values four to five times

those permissible for a good mirror surface scattering the same amount of light. Thus as far as scattered light goes, surfaces of 80-100 K rms can be tolerated for windows, whereas roughnesses of 20 X rms

or less are desirable for mirrors; (4) The magnitude of backscattering from windows is approximately

equal to the magnitude of forward scattering.

7. Acknowledgments

The authors wish to acknowledge the support of the following agencies and individuals: ARPA,

Dr. C. Martin Stickley, program monitor; AFWL, Major James Stapp, program monitor; ERDA, Dr. Walter

Reichelt of LASL, program monitor.

8. References

[1] Shaffer, J., et al. , "High Energy Laser

Mirrors and Windows," (Semi-Annual Report

No. A, ARPA Order 2175, Naval Weapons

Center, March 197A), pp. 103-109.

[2] Davisson, J. W. , Conference on High Power IR

Laser Window Materials, Oct. 30, 31 and

Nov. 1, 1972, Vol. 2, AFCRL, June 1973, pp. 525-534.

[3] Allen, S. D. , M. Braunstein, C. Giuliano,

and V. Wang, "Laser Induced Damage in

Optical Materials: 1974," NBS Spec. Publ. 414

(1974), pp. 66-75.

[4] 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.

[5] Tinkham, M. , "Report of the Materials

Research Council (1974)," (ARPA Order 2341,

University of Michigan, December 1974), p. 37.

[6] Bloembergen, N. , Appl. Opt. 12, 661 (1973).

[7] Loomis, J., Appl. Opt. 12, 877 (1973).

[8] Loomis, J., "Laser Division Digest, Spring

1972," (Air Force Special Weapons Center,

Klrtland AFB, N. M. , TR LRD-72-1, 1972),

pp. 29-41.

[9] Harrison, W. B. , et al.. Fourth Annual

Conference on Infrared Laser Window Materials, ARPA, Jan. 1975, pp. 599-610.

[10] Bennett, J. M. , "Laser Induced Damage in

Optical Materials: 1974," NBS Spec. Publ.

AlA (197A), pp. 157-162.

[11] Bennett, H. E. and M. J. Soileau, "High

Energy Laser Mirrors and Windows," (SemiAnnual Report No. 6, ARPA Order 2175,

Naval Weapons Center, May 1975), pp. 156-

172.

[12] Bennett, H. E. and P. C. Archibald, ibid,,

pp. 1A5-155.

[13] Soileau, M. J. and H. E. Bennett, "Laser

Induced Damage in Optical Materials: 197A,"

NBS Spec. Publ. AlA (1974), pp. 149-156.

-25-
Figure 1. Bowl feed polishing of alkali-halide

windows, (a) A 1.5" diameter piece of KCl

is being polished using a submerged paraffin lap and the blocking procedure described in the text, (b) The "paddle" can

be seen; in this case a paintbrush is

used to stir the slurry. Solid paddles

are also used and give similar results. Kb)

-26-
SPRING CONTACT

BEFORE

ABSORPTION : 11 X 10"

FIGURE : 5X (X/4 « lO.Bimil SCATTERING : 16 X KT^ O 6328X

ABSORPTION : 3.9 X 1(r* FIGURE : YUK 1 SCATTERING : 1.7 X 1(r' • 632aX

Figure 3. Window calorimeter. This is a schematic

of the calorimeter used to measure

10.6 ym absorptance in alkali-halide

materials. Copper-constantan thermocouples are used. Pressure contact by a plastic-tipped screw is used to attach

the thermocouple to the specimen. The

thermocouple voltage is read by a digital

voltmeter (DVM) and displayed, along with

the transmitted laser power, on a strip

chart recorder. The laser source is a Coherent Radiation Model 41 CO2 laser

with continuous wave output up to 250

watts

.

NWC POLISH

ABSORPTION

FIGURE SCATTERING

4.8 X 10-* V4 (VM • 10.« Kin)

3 X 10-^ e 8328X

J

•POLYCRYSTALLINE Eu ++ DOPED KCI 1 CM THICK

Figure 4. Polished Eu"*^ doped KCI. The polished sur- faces exhibit only a slight increase in

absorptance as compared to chemically

etched surface. Similar results have been

obtained on single crystal Eu doped

material and single and polycrystalline

RbCl alloy material. The magnification of

the micrographs is 360X.

100 MICRONS

Figure 5. NWC polished NaCl. The picture on the

right is a 360X Nomarski micrograph of a NaCl specimen polished for 4 hours

using the NWC triacetin slurry. The

picture on the left is an interf erogram

of the optical figure of this specimen.

The interf erogram was taken with a YZGO

interferometer with a He-Ne laser o source (X = 6328 A).

NOTE: Microns as expressed in

figure 5 expresses micrometers.

10- o

z

<

10-

-r -r T—I—

r

COMMERCIAL POLISH O BACKSCATTER A FORWARD SCATTER

3 MIN HCI ETCH BACKSCATTER

.THEORY 183A rim

Figure 6. Backscatter from Eu-doped KCI after

commercial polish and after 3-minute

HCI etch.

-27-
-1—I—I—I—

r

POLYCRYSTALLINE

2.0 WAVELENGTH l(im)

Figure 8. Backscatter from etched, Rb-doped for

polycrystalline and single crystal

samples. The open circles and squares

represent the average deviation in

scattering from different points on the

surface of the polycrystalline and

single-crystal samples, respectively.

COmEMTS OM PAPER BV SOJLEAU, eX at

In KUpomn to a queJ>tlon aona^Knlng thz potuking dope.d atkatl-haJLid^i, V4 puAe aZkaZikoLLdu thz ipzakkA indZcatzd that thz iomz tzchnlquz uicu> cU>zd {^on both ctoAiU o{, mautzxiaJUi, He

alio -indicatzd that bzcauAZ in tkU, mzthod thz potiihzd iuA^azz U, submzAgzd thz znv-OLOnmzntaZ

vaJujutlon obizAvzd in othzA potiikLng tzchniqazi, {^on. zxamplz, thz iiiiczptibitity to aXmoiphzAA^c

humidity wa6 glzoutly fizdaczd by tkU pwczii. It woi {^uAtkzA commzntzd that bzcaiuz thz {^inaZ

iigoAing oj^ thz iuA{,acz ii> pzfi^oKmzd a{,tzA thz ztcking dzgmx.daXA,on o^ iuA{,aaz {^iguAz ii, minimizzd.

-28-
2.1 1.06ym 150 psec Laser Damage Study of Diamond Turned, Diamond Turned/Polished

and Polished Metal Mirrors

T. T. Saito*, D. Milam, P. Baker, and G. Murphy, Lawrence Livermore Laboratory

Livermore, California 94550

Using a well characterized 1.06 ym 150 psec glass laser pulse we have

studied the damage characteristics for diamond turned, diamond turned/polished,

and polished copper and silver mirrors less than 5 cm diameter. Although most

samples were tested with a normal angle of incidence, some were tested at 45°

with different linear polarization showing an increase in damage threshold for

S polarization. Different damage mechanisms observed will be discussed. Laser

damage is related to residual surface influences of the fabrication process.

Our first attempts to polish diamond turned surfaces resulted in a significant

decrease in laser damage threshold. The importance of including the heat of

fusion in the one dimensional heat analysis of the theoretical damage threshold

and how close our samples came to the theoretical damage threshold will be

discussed.

Key words: Copper mirror; damage threshold; dark field photography; diamond

turned optics; 1.06 \im. polishing.

1. Introduction

Metal mirrors are of interest to the high power pulsed laser programs because of the high reflectivity and good thermal conductivity. Diamond [1]' turning of metals offers expanded design flexibility to incorporate fast parabolas or unusual shapes such as compound axicons or ellipses. In addition, diamond

turning may offer considerable cost benefits over dielectrics for large diameter optics which will re- quire refinishing due to laser damage.

This paper presents our study of laser damage on metal mirrors. We shall discuss the experimental

arrangement, preparation of the samples, and demonstrate the degradation effects of polishing or even rubbing/cleaning diamond turned silver. Comparisons of our results to other studies of polished copper

at different wavelength and pulse length indicate that a similar damage mechanism is present for less

than 500 nsec pulses.

2. Experimental Apparatus

A schematic of the damage facility in the laboratory is shown in figure 1. The source consists of a mode-locked oscillator and pulse selector, two YAG preamps, and two glass rod amplifiers. Fast diodes

A and B monitor waveforms of the pulse train and of the switched-out pulse. Integrating diodes C and D measure the energy of the switched-out pulse before and after amplification. The output energy is con- trolled by setting the amplification. Typical beam parameters in a plane normal to the beam at the test

sample's surface are:

Energy - .0 - 0.5 joules

Beam profile - nearly Gaussian

Beam diameter - mm at 1/e level

Pulse duration - 150 ± 25 psec

The damage experiment is also shown in figure 1. A Galilean telescope is adjusted to serve as a focusing element with the beam waist being 4-8 meters distant as required. The reflection from the

front surface of a bare BK-7 wedge is centered on an aperture placed in front of the calorimeter. The

aperture is chosen to correspond to the approximate diameter of the beam at the e"2 intensity level,

and the centering of the aperture is photographically certified at the beginning of each day of operation. This calorimeter serves as the fundamental monitor of pulse energy.

The reflection from the rear surface of the BK-7 wedge is incident on a 40% - 100% mirror pair used

to form multiple replicas of the beam profile existent in the sample plane. The images are recorded on polaroid film so that the results of each firing can be viewed immediately thereby avoiding the possibility of operating with significant undetected problems with the beam profile. The poloroid photos

also furnish an instant estimate of the incident energy density. We have found these estimates to be

reproducible to 20% as substantiated with much more ellaborate methods.

The replicas are also recorded on 1-Z plates. From five to nine firings can be recorded on each

plate. The most intensely exposed of these is reduced using the Photocal routine [2]. That serves to calibrate absolutely the exposure vs. film density curve for that particular plate. The film densities

I resulting from the remaining shots recorded on the same plate can then be interpreted without recourse

to Photocal

.

*Detached duty from the Air Force Weapons Laboratory, Laser Division, Kirtland AFB, NM 87117

1. Figures in brackets indicate the literature references at the end of the paper.

-29-
Photocal is a sophisticated generalized computer routine developed by our colleague, Dr. Joe

Weaver, for providing the energy density on the sample. After the exposure versus film density (i.e.,

the H & D curve) for the plate is determined, a hundred by hundred matrix of optical density versus position is established by repeated scans of a spot. The optical density and H & D curves are used to yield the relative intensity as a function of position in the sample plane and a three-dimensional com- puter picture is produced. Using the total energy measurements from the calorimeter the program can then calculate the peak energy density taking into consideration intensity spikes and other departures

from true Gaussian profiles. ^

Values obtained from the reduction of 1-Z plates are qualitatively compared with the poloroid beam

photographs to insure that no significant data reduction errors have been made. The incident energy

densities of a set of firings can be ranked from least to greatest by simply counting the number of

replicas recorded on poloroid film for each firing. The reduction of 1-Z data should yield the same

ranking.

Since the Gunn calorimeters appear to be accurate to within about 3-5%,energy densities determined

in these experiments should mirror that accuracy. Estimates of the energy made from the same 1-Z plate,

but without the 100 x 100 matrix are accurate to 10%. The estimates of energy density from just the

poloroid pictures are accurate to 20%. |

The pulse width is determined by using a streak camera. We did not measure the pulse width for I each shot. We have many thousands of laser firings, and have periodically measured the pulse width to

be 150 ± 25 psec.

3. Experimental Technique

Sample Preparation. The diamond turned samoles were all electroplated on copper or brass substrates. Lea RonaP silver and copper pyrophosphate electroplating were used. Most of the samples were

heat treated by placing them in a beaker with oil and slowly raising the temperature to 150° C, maintaining 150° C for one hour, and then removing the beaker from the heat source and allowing the beaker/

oil /mirror blank to cool to room temperature. The heat treating process was used to relieve the residual stresses from the electroplating [3]. The samples were then diamond turned. Some of the

samples were diamond turned simultaneously in batches of 18. After they are diamond turned and while

the spindle is spinning, isopropyl alcohol is used to rinse the cutting oil off the samples.

Some of the diamond turned samples were polished using techniques which will be described in de- 4

tail elsewhere [4]. Based on the success of others [5, 6] in using india ink, we used suspended

carbon particles (Acqua Dag) to polish some of our samples. Other samples were polished with diamond

and silicon oil. Our lap was made of pitch, bees wax, and silk.

Immediately before the laser damage test, the samples were rinsed with various solvents in an attempt to remove the residual film. Eastman lens cleaner left such a bad residue on some parts that

they were not tested. We found the best results came from rinsing the samples first with acetone, and

then removing the residual acetone film with alcohol. Further studies of cleaning of diamond turned

optics are being planned.

Laser Damage Testing . Unless specified, all tests were performed at a nominally normal angle of

incidence. Experiments were also performed with a 45° (±2°) angle of incidence with p or s polarization. The sample was placed on a stage which had translation stages in directions perpendicular to the

direction of laser propagation. Each shot number and the corresponding coordinates of illumination were recorded along with any observations made before firing, during firing (viewed through laser

safety glasses), or after firing. We found small emissions of light could be seen during the firing

without seeing any damage using a dark field illumination technique. Before and after the firing we

inspected the surface by dimming the room lights and illuminating the sample with a bright (white)

microscope light. Residue films from the cleaning process, scratches as well as laser damage could be

easily seen. Newnam has reported that the most sensitive technique in his experiments was the visual

determination of scattering of an auxilary low power cw laser [7]. Similar to Newnam, we found light

scattering to be the best technique for determining damage.

Dark Field Illumination Photographs . We have adapted Hizney's photographic technique to present

the surface defects which we saw during the laser tests [8,9]. The simple experimental set-up is

shown in figure 2. A white light source illuminates the sample such that the specular beam misses the camera which photographs the scattered light. Figure 3 is a series of photographs taken in sequence.

A diamond turned copper mirror was photographed as received from the diamond turning lab as shown in

figure 3a. Figures 3c and 3d were taken immediately after 3b, but by increasing the exposure time.

This technique has several advantages:

a. Produced by Sel-Rex Corporation, Nutley, N.J.
1. It is easy and economical to set up.

2. It may lend itself to an easily describable>and therefore. reproducible technique.

3. It can give 100 % part inspection.

4. It presents artifacts (especially residual films) which are not easily observed with even

Nomarski microscopy.

We plan further investigations of this technique with the possibility of establishing a quantitative technique for measuring cleanliness.

It was very difficult to find the laser damage sites when using a microscope, even with a Nomarski

differential interference contrast accessory. The best success in doing optical microscopy was to place

the sample on the translation stage while illuminating it from the side with the high intensity white

light source. We could then see the laser damage spot and translate the sample until the light from the

microscope coincided with the site of interest. Simply scanning the sample with normal observation

usually resulted in overlooking the damage sites because of the very small differences in the background surface and laser damage in some sites.

4. Theoretical Calculations

Since our 150 psec pulse length is so short, a one-dimensional heat transfer model is adequate for

calculating the damage threshold. The laser light is absorbed and heats a cylinder whose base is the same as the cross section of the laser on the mirror and whose height is (Trat/4)V2 where t is the

pulse length and a is the thermal diffusivity in cm2/sec. The incident laser energy density, E] , with

pulse width t needed for a temperature rise, AT, to the melting temperature is a function of the

density p, the specific heat s, and the absorption A of the laser light as given by eq. (1).

E^ = ^

(.at)l/2 (1)

In order to change phase from a solid to a liquid, the heat of fusion H must be included requiring additional energy Eg.

c - Hp , . J/2

^2 2A

^'^"^^ (2)

We have not included the heat of vaporization because we are calculating the threshold energy density,

E^;h' which is the sum of Ei and E2. (It is not necessary to vaporize the mirror in order to see a change in the surface.)

-th (ATS + H) (3)

This calculation assumes a uniform intensity beam, but is valid for our experiment since the 1/e radius

is so much greater than the thermal diffusion length. Note the t^/^ time dependance of the damage

threshold.

Theoretical damage thresholds are calculated for various metals and are presented in table 1 using

typical absorption for 1 ym. These absorption values are based on Goldstein's work, 1 ym reflectivity

and light scattering measurements [10],

The requirement for including the heat of fusion has been demonstrated by our experiments as silver

samples have been illuminated at 4 j/cm^ without exhibiting laser damage.

If copper, gold, and silver mirrors each had the same absorption, copper would have the highest

theoretical damage threshold. Copper is theoretically better than silver or gold because of the higher

specific heat and heat of fusion. Gold has the lowest theoretical damage threshold of the three.

Table 1. Theoretical Prediction of Damage Threshold for 150 psec 1 ym Pulse

Material A

a

2 cm /sec

P gm/cm^

AT

°C

S j/gm °c

^1 2 j/cm

H j/gm

h

j/cm2

^th

j/cm2

Al uminum 0.06 0.86 2.70 635 0.899 0.26 396 0.18 0.44

Copper 0.011 1.18 8.97 1058 0.384 3.9 212 2.0 5.9

Gold 0.016 1 .14 19.3 1038 0,13 1.9 67.5 0.94 2.8

Silver o.on 1.71 10.5 936 0.23 2.9 104 1.4 4.3

-31-
5. Results and Discussion

Figure 4 is a dark field photograph of the Spawr copper. The damage sites are clearly visible

along the edge of the mirror. Figure 5a is a regular micrograph of a damage site illuminated at

2 j/cm2 on the Spawr mirror. Figure 5b and 5c are dark-field illumination photomicrographs at the same magnification as 5a at the center and edge respectively, demonstrating the laser enhanced scratches

in the damaged area. Figure 5d is a Nomarski photomicrograph taken at about twice the magnification of

5a demonstrating that many small laser enhanced scratches are evident as well as some pitting. These

scratches are very similar in appearance to what Brown reported observing laser damage of an anti-re- fl flection coated cylindrical lens [11], I

The effect of scratches on damage threshold is further demonstrated in figure 6 for sample LLL-Ag-5.

LLL-Ag-5,6, and 7' were all diamond turned simultaneously in a fixture which can hold eighteen mirrors.

Sample 5 was cleaned with alcohol and optical tissue by lightly wiping the surface. Fig. 6a is the

dark field illumination photograph of the mirror which shows the (mainly) uni-directional scratches

from the cleaning. Figures 6b and 6c are Nomarski photomicrographs taken at the same magnification of the same pi ace, but by rotating the part so as to emphasize the diamond turning marks in 6b and show the ^

background in 6c. The damage of site 793 has clearly been initiated along the scratches of the clean- I

ing process and at one-half the threshold for just diamond turned silver as discussed below. I

Electroplating defects can effect the laser damage threshold as shown in figure 7. The pox marks are evident across all the surface, but are more pronounced in the area where the laser damage occurs. This

Nomarski photomicrograph was taken with the background contrast such that only the laser effected pox

marks showed.

Diamond turning marks and electroplating defects in silver are shown in Fig. 8. Fig. 8a is a dark

field illumination of the mirror taken at about the same time as 6a and with identical illumination and camera settings. Attempts were made to rinse the mirror clean with acetone and alcohol, blowing off the

residual solvent with canned methane or freon. The residual film on LLL-Ag-7' has been removed in some

spots by the laser without damaging the surface. We inspected the surface with the light scattering

technique and with the Nomarski microscope without finding damage. We plan to investigate these sites

with a double objective two beam (Linnick) micro-interferometer to see if we can detect any changes in

the surface. Site 837 shown in Fig. 9 on LLL-Ag-7', illuminated at 3.9 j/cm^, demonstrates damage along

the diamond turning lines. I translated the sample and followed one of the heavier damage lines into an undamaged region where I could see that the diamond turning mark was one of the darker lines in the

field of view. Light scattering may offer a quantitative measure of the important parameters of the

particular diamond turning marks which seem to create damage at a lower threshold. Fig. 9 shows damage

similar to that of the copper mirror in Fig. 7. These small marks are felt to be due to electroplating

defects.

Table 2 summarizes our findings. We have listed the lowest level energy density, E^, which caused

damage and the highest level at which laser damage was not observed, Epj. Due to inhomogeneities in the

laser damage threshold across the surface, E^j can be less than Epj as for LLL-Cu-78. The 1 ym absorption

is estimated from the 1 ym reflectivity which we 'neasured [12], and estimating the scattering as 0.001-

0.002 for the diamond turned mirrors and for as much as 0.004 for the polished mirrors. Qualitatively

we know the diamond turned mirrors scatter much less than the polished mirrors, because during the

reflectivity measurements an S-1 diode "sniperscope" was used to view the 1 ym 0.5 cm diameter, 100 mw, cw laser beam. The scattering from the polished sample was readily observable. but we could see no 1 ym

scattering from the diamond turned samples. Diamond turned samples have given some of the lowest light

scattering of metal mirrors. with the exception of one super-polished piece of kanigen [3,13],

One of the most encouraging results is the 4.0 j/cm damage threshold for diamond turned silver,

which is 80% of the theoretical damage threshold. Higher damage thresholds may possibly be achieved by

better cleaning procedures, as well as improvements in electroplating surface defects. Goldstein and co- workers have demonstrated that with lym 11 ns .pulses the damage thresholds appear to follow

absorption [10],

2 The results for copper are especially disappointing. The damage threshold of about 2.5 j/cm is

only about 40% of the theoretical value. We do not know why the threshold is proportionally so much

lower than for the case of silver. Studies of optical properties at 10.6 ym have indicated ^-hat Cubath (a) has given the highest reflectivity by as much as 0.004. We plan to test Selrex Cubath in

the future.

(a) Produced by Sel-Rex Corporation, Nutley, N.J.
Table 2. Summary of Laser Damage Threshold

Sample A ^dam

j/cm'^

^no dam

j/cm2

^theory

j/cm^ Comments

Si 1 ver

LLL-Ag-7' 0.010 4.0^^ 4.0^ 4.8 EP/HT/DT/R

3.9^ 1.6^ 45° P polarization

5.4^ 4.8'^ 45° S polarization

LLL-Ag-5 0.009 2.2'' 2.2'' 5.3 EP/HT/DT/R/cleaned

LLL-Ag-6 0.020 0.43^ 0.24*^ 2.4 EP/HT/DT/R/P/cleaned

Copper

LLL-Cu-78 0.01 2.0' 2.6*^ 6.5 EP/HT/DT/R

2.0*^ 1.6^ 45° R polarization

5.9' 4.9*^ 45° S polarization

LLL-Cu-44 0.042 0.]6^ None 1.4 EP/DT/R/C/P/cleaned

Baker 10 0.026 0.7' 0.4'' 2.5 P

Spawr Cu 0.018 0.39'' 0.41^^ 3.6 P

a = ± 5% EP = Electroplate P = Polished

b = ± 10% DT = Diamond Turned c = Coated

c = ± 20% R = Rinsed

We have not optimized our polishing parameters and hope to improve laser damage threshold for

polished smaples. LLL-Ag-6 and LLL-Cu-44 were both polished with suspended carbon particles. The

especially high absorption for LLL-Cu-44 coupled with its damage threshold so low that we could not

determine it with this experimental arrangement has led us to conclude that our technique with carbon

particles will not yield good high power metal laser components. The results for Baker 10, polished

with diamond and silicon oil, compare favorably with damage data on other polished copper.

Based on the work of Goldstein, at al., [10] we tested some samples at 45° angle of incidence using

first p and then s polarization. Similar to Goldstein we found an improvement in the damage threshold

for s polarization. This improvement is due to the decreased absorption as well as the increased area

of illumination. The values given in table 2 are the energy density in the beam measured in a plane

perpendicular to the laser propagation direction. We concur with Goldstein's findings that use of

metal turning mirrors may be very competitive with dielectrics in terms of laser damage. In the case

of diamond turning, the initial fabrication cost and especially the refinishing cost would be better

than for dielectrics.

It may be possible to take advantage of the periodic nature of the surface finish of a dimaond

turned mirror. Although the total amount of scattering is low (less than 0.002), the mirror may serve

as a low efficiency diffraction grating and the diffracted beam be used for beam diagnostics.

Comparison of laser damage thresholds at different wavelengths and pulse lengths can be compared

to our results using eq. (3). Table 3 normalizes results for other polished copper reported

mainly by Spawr and Pierce [141 and one from Stewart [IS] of 10.6 \im, 470 ns pulsed study. The values

have been normalized to our values of 0.018 absorption and 150 psec. Our results seem to be consistent with others except for the pulses longer than 500 ns. There is a possibility that at the longer

pulse length, a different mechanism is associated with the damage than with the shorter pulse lengths.

It is now tempting to speculate on the cause of the lower damage threshold around micro-scratches.

Bloembergen has discussed the effects of pits and scratches enhancing the electric field and lowering

laser damage [16], It may be possible that scratches have a higher absorption to pulses than is

measured by low power cw methods, and therefore the scratches have lower damage thresholds than the

theoretical value.

-33-
Table 3. Polished Copper Laser Damage Data Normalized to A = 0.018 5 - 150 psec

Testor

X pm

t nsecA

% ^exp

j/cm2

^normal i zed

u/cm2 Ref

.

Our Data 1 .06 0.15 1.8 0.4 - 0.6 0.4 - 0.6 -

Cincinnati

Electronics 1 .06 17 2 2 - 4 0.3 13

Raytheon 1 .06 11 1 .4 2.9 0.3 8

Los Alamos 1.06 0.05 % 2 0.4 0.7 13

Los Alamos 3 50 1.8 10 0.6 13

Northrop 5 10^ 1 .5 1000 1 13

Los Alamos 10.6 1 0.9 > 4 >0.3 13

Battelle NW 10.6 470 1.7 35 0.6 14

Hughes

Research 10.6 600 'V 0.9 260 2.1 13

6. Conclusions

Diamond turned metals have the highest laser damage threshold for metal mirrors which we have

tested. Diamond turned silver competes favorably with dielectric mirrors especially for turning S polarized beams.

Our work has demonstrated that various fabrication effects degrade the laser damage of silver.

The degradation factor, D, is estimated and presented in table 4. One can estimate the damage

threshold due to the effect by multiplying the theoretical damage threshold by D.

Table 4. Fabrication Degradation Factor for Silver

Fabrication Effect D

Slight Electroplating Defects 0 8

Diamond Turning Marks 0 7 Scratches During Cleaning 0 5 Polishing 0 1 - 0.2

In contrast to Stewart [14] we were unsuccessful in polishing with suspended carbon particles in

that although we achieved smooth surfaces low damage threshold resulted.

7. Acknowledgment

We are grateful for the donation of a sample and helpful discussions with Mr. Walter Spawr, the

advanced use of the data of Dr. Irv Goldstein, and the assistance of our colleague Mr. Wayne Whistler

in making the reflectivity measurements.

8. References

[1] T. T. Saito, "Machining of Optics: An Introduction" to be published in Appl Opt

14:1773 (1975)

-tit- -i^

[2] J. Weaver, G. Sommargren, and E. Bliss,

"Self Calibration and Analysis of Image

Formation in the Sub-nanosecond Domain"

Proceedings of the Society of Photo-Optical

Engineers, 18th Annual Technical Meeting 48 : p-63.

[3] T. T. Saito "Characterizations of Diamond

Turned Optics" to be published AF Weapons

Laboratory Technical Report.

[4] P. Baker, J. Sonderman, and T. T. Saito,

"Finishing of Precision Generated Optical

Components" to be presented to the Soc. PhotoOptical Instrumentation Eng. Symposium on the

"Design, Manufacture and Application of Metal

Optics" San Diego, Aug. 1975.

[5] F. Twyman, "Prism and Lens Making", 2nd Edition, Hilger & Watts Ltd., London p-46.(1952_

[6] R. W. Stewart "Investigate Materials Systems

for Mirrors Used in High Power CO & CO2

Lasers," Technical Report ARPA Order No. 2175

Battelle Pacific Northwest Laboratories,

Richland, Washington (Dec. 1972).

-34-
[7] B. E. Newnam "Damage Resistance of Dielectric

Reflectors for Picosecond Pulses" in Laser

Induced Damage in Optical Materials 1974, A. Glass and A. H. Guenther Editors, National

Bureau of Standards Special Publication 414,

1974.

[8] J. Hizny" (Characterization of Low Scatter

Optics" given at ASTM Mini -Symposium "The

Possibility of Using Light Scattering to Establish a Surface Characterization Standard" Dallas, Tex, Jan. 1975.

[9] J. Z. Hizny "Substrate Throughprint in Low

Scatter Mirrors" Society of Photo-Optical

Instrumentation Engineers Symposium "The

Design, Manufacture and Application of Metal

Optics" at San Diego, August, 1975.

[10] I. Goldstein, D. Sua, and F. A. Horrigan

"Pulsed Laser Damage to Uncoated Metallic Reflectors" 7th Annual Laser Induced Damage in

Optical Materials, 1975.

[11] N. J. Brown "Fabrication of Laser Optics at Lawrence Livermore Laboratory" 7th Laser In- duced Damage in Optical Materials Symposium,

1975.

[12] T. T. Saito, "1.06 ym Reflectivity Measurements of Metal Optics" UCID 16815, Lawrence

Livermore Laboratory, Livermore, CA 1975.

113] H. E. Bennett "High Energy Laser Mirrors and

Windows" Semi -Annual Report No. 6, ARPS Order

2175, Michelson Laboratory, Naval Weapons

Center, China Lake, CA March 1975.

[14] W. J. Spawr and R. L. Pierce, Metal Mirror

Selection Guide, SOR Report #74-004, 30 Dec

74, published in part as "Guide to Metal

Mirrors" Laser Focus, March, 1975, p-37

(1 975)

[15] R. W. Stewart, "Investigate Material Systems

for Mirrors Used in High Power CO and CO2

Lasers" Semiannual Technical Report, August

1973, ARPA Order No. 2175, Battelle Pacific'

Northwest Laboratories, Richland, Washington

[16] N. Boemberger, "Role of Cracks, Pores, and

Absorbing Inclusions on Laser Damage Threshold at Surfaces of Transparent Dielectrics"

Appi Opt 12:661 (1973)

8. Figures

15 mm rod

ampl i f ier

32 mm rod

ampl i f ier

Laser source

ye cell

apodi zer

• R = 407»

Calorimeter

I-Z beam

photographs

wedge

Sampl

e

plane

roid beam

photographs

Damage experiment

Figure 1. Schematic of experimental apparatus

showing the mode locked oscillator

which generates the 150 psec pulse.

Diagnostics include calorimeters,

1-Z photographic plates, and

polaroid pictures.

Microscope

Light

Figure 2. Schematic for the dark field photographic

scheme for investigating laser damage

specimen.

-35-

Camera

Specular

Reflection

Sample
Vapor degreaser cleaning of

LLL-CU

c. 90 sec cleaning d . 90 sec cleaning

(t = 1/4 sec) (t = 1/2 sec)

Figure 3. The dark field photographs were taken of a diamond turned copper mirror (a) as received from our diamond turning laboratory, and (b,c,d) after 90 seconds of

cleaning in a vapor degreaser using TF freon at 50°C. Figure a and b were taken

at the same photographic conditions. Figure c and d were taken at longer exposure

times to demonstrate the residue on the mirror which still has not been removed.

Figure 4. Dark field photograph of the Spawr copper demonstrating

the rows of laser damage spots along the bottom.

-36-
SPAWR COPPER

Figure 5. Site 793 on the Spawr copper was Illuminated at 2 j/cm^. The laser dama<;e is manifested by

the enhanced scratches shown in (a) which is a regular photomicrograph and (b) which is taken

using dark field illumination. (c) shows the edge of the damage area and the background

smoothness of the undamaged area, (d) is a Nomarski photomicrograph of site 793.

-37-
i CO

ai

to

01

cn COeo -oc

(U

n) ^

42I U 10 60 -H ^ ^ c O T3 C I a

COu

00ouo

x:a

43 73 ^ ao C 0

•H to •H

cn 4-1 •H

tn Ti

o

CO CO

to

n

o

4J

4J

CO

V4 <u O 4-1

c cn

c

•H <u

cu •H

(U x: V4 rH u O

•H oo 01

tn

c i-l •H o aB

to CO

cn

to 00 (U

to 4= B 44

c to •H -o 4= c 44

to >-l •H

(U (V

s

tH cn O to 01

7-1 CJ <U to

Q)

H • 43H o

>^ (U •—y

43 • cn T3 tn cn

<U iH (U

CD

a <uB 3 B 4= 4=

to to 4J 44

tJ cn 0

CM

O

CO

o

s

(U <u cn j: > cn

ej 4= T3

4-1 •H

to tn& §o

u T3 00 I-l cn (U o OD C u 4^

(U

M

oa

42 3 •H CO H 4-1 E • VO

0)U3

ao

•H

-38-
Figure 7. The damage on the diamond turned copper

is due to electroplating artifacts which

appear as pox marks in this Nomarski

photocicrograr>h.

(b)

Figure 8. The removal without any apparent damage of the residual film from the diamond

turned silver LLL-Ag-7 sample is shown in the dark field photograph (a) , The

damage in the slight electroplating defects is shown in (b).

-39-
COmENTS OM PAPER BV SAITO, zt at

(See aommznti on papeA by GotdAteJ-n, zt al Mkich i{o££ow4)

-40-
2.2 Pulsed Laser Damage to Uncoated Metallic Reflectors

Irving Goldstein

Raytheon Co. Missile Systems Division, Bedford, Mass.

Don Bua

Raytheon Research Division, Waltham, Mass.

Frank A. Horrigan

Science Applications, Inc., Bedford, Mass.

Threshold damage measurements were made on a variety of polished bulk

copper and thin gold film reflectors using a TEMqq mode Q-switched Nd:YAG laser.

Results correlate well with a simple thermal model based on the assumed existence of a critical surface temperature. Direct confirmation of this thermal

model was obtained through systematic variations of the polarization and angle

of incidence of the incident laser beam and calorimetric measurements of the

surface optical absorption coefficients. The "Damage Threshold" was defined

through a careful procedure of attenuating the incident beam and varying all

parameters (e.g., focus, position on the surface) until the sample no longer

could be "damaged" as assessed visually with a microscope. In one striking

case the damage threshold of two "identical" samples was shown to correlate

with the measured surface absorption and not with the visual surface quality.

The implications of these results to the design of high power optical

systems which use large angles of incidence (e.g., axicon or toric optical

elements) are discussed. In particular, for the polarization associated with

Brewster's phenomenon the damage threshold was found to remain constant over wide variations of the angle of incidence, e.g., from 0 to 70° or 80°,

Key words: Angle of incidence; polarization; pulsed laser damage; thermal

model.

1 . Introduction and Summary

The purpose of this work was to determine the feasibility of using metal

mirrors with a high energy, pulse laser at X = 1. 06um for axicon or toric optical

systems. As a consequence, a program was set up at Raytheon to measure both

the absorption and pulse (t = 11 nanosec) damage threshold of a series of

sample mirrors at varying incidence angles. Damage thresholds on Cu mirrors

have been measured in the range of 44 to 70 joules/cm2 at an incidence angle

of 80° for and 11 nanosec pulse. It was found that the damage threshold did

follow the pattern of absorption.

2 . Technical Approach

Initially there was a selection of the different metals to be tested in

the program and figure of merit (FOM) was assigned to each material. This

POM derived below from the Surface Heating Relationship which is eq. (1)

.

2 (1 - R) I /r

AT =

°

(1)

/ttKC

2 where I^ = watts/cm in the laser beam

t = Pulse width in sec. R = Reflectivity of the surface

K = Thermal conductivity watt/cm°K

2 2 C = joules/cm °K = watt, sec/cm °K

AT = Temperature rise at the surface.

-41-
Regrouping eq. (1) into laser variables and material variables

resulted in the following definition of a Figure of Merit (FOM) as the

material parameter of equation (1)

AT

FOM = (2)

With appropriate values put into eq . (2) the following bar chart

(figure 0) indicates the ranking of the materials with copper being the

optimum followed by silver and gold. Experiments were performed on a number

of these metals.

Next the absorption change with incidence angle was investigated and one

of the key results obtained was that two polarizations have to be considered

for the metal mirrors that we were testing for the following reasons:

In a toric optical system, for example, let the input polarization vector

be vertical as shown in figure 1.

There will be regions in plane (A-A) and plane (B-B) where the absorption

will be different for polarization vector E. The following diagram (figure 2)

should help clarify the meaning of the terms to be used namely, E|| and Ej_.

Ej_ is defined as the electric field vector which is perpendicular to the plane

orthogonal to the mirror surface, and E|| is defined as the electric field

vector parallel to plane orthogonal to the mirror surface. Figure 3 shows

the absorption versus polarization and angle of incidence for a mirror surface

incidentally where gj_ and g| | are defined as the absorption coefficients for

the different polarizations as previously defined. The circles and squares

indicate the experimental data points and the solid and dashed lines the

computed values . As can be seen, there is a reasonably good agreement between

the expected and measured points and the theoretical curves of absorption

are based on a solution of the Fresnel reflection formulas with complex index

of refraction and absorption.

Next, the damage thresholds for the two different polarizations have been

calculated as shown in figure 4. The interesting curve is E| | in which the

increase in mirror area is directly compensated by the increase in absorption

(S||) with incidence angle.

Experimental results are shown in figure 5. This data was taken on a gold plated mirror with a poor surface finish. Next threshold damage data

was taken on two Cu mirrors purchased from Spawr Optical Company in California.

The specifications on the mirrors were very good optical finish with little

attenation paid to optical figure, since the damage would be confined to

70p diameter spot.

Schematic of the experimental laser damage set up is shown in figures

6 and 7. The sample was placed on a stage which has a number of degrees of

freedom of movement and the reason for this was that the Damage Measurement

Procedure shown in figure 8 required such freedom in order to make meaningful

measurements at the grazing angle. For each threshold damage point there are

at least 5 or 6 measurements made. The laser parameters are shown in figure 9' and figure 10 shows the Intensity distribution of the YAG laser used. The

absorption values for the different mirrors were obtained from measurements

that were made on a set up shown in figure 11.

The results of the tests on the two Spawr samples are shown in figures

12 and 13. For mirror #1 a damage threshold of 44 joules/cm^ was observed for an incidence angle of 80°, for mirror #2 damage thresholds of 43 joules/cm^,

and 69 joules/cm^ for incidence angles of 75° and 80°, respectively. Photomicrographs were taken on both these mirrors as shown in figure 14. Note

that the finish of Spawr mirror #1 appears to be better than Spawr mirror #2,

and yet mirror #2 has a higher threshold of damage level. We attribute this

to the absorption of the mirror surface rather than the optical finish,

-42-
a la the Bloembergen criteria. At 0° angle of incidence the absorption of

Spawr #1 and Spawr #2 was 2.1% and 1.42%, respectively, and the damage for

Spawr #1 and #2 was 2.12 joules/cm and 2.36 joules/cm2, respectively.

Figure 15 is interesting since the simple thermal model correlates well

with the experimental data and surface finish does not play a significant role

in determining damage threshold. This finding was also corroborated by the

damage data taken on the two copper Spawr mirrors

.

Since the competing technique to the use of metal mirrors are dielectric

coatings either on refractive or reflective optics, the damage threshold

obtained at Raytheon was compared to the results obtained by other workers.

Figure 16 indicates the typical data obtained on other materials [1,2,3]

for different pulse widths. The slope of damage threshold versus pulse

width was obtained from the reference sighted and agrees quite well with a proportionality to /pulse width. The Raytheon data for 80° incidence angle - when

extrapolated to the picosecond range from the nanosec, indicate definite promise

of being able to achieve damage threshold at least equal, if not better, than

the dielectric materials.

Now let us reexamine the meaning of the input polarization vector for a toric symmetrical mirror (see figure 1) . It can be seen that there will be

portions of the input mirror when the B| | absorption coefficient will increase

and g_L absorption coefficient will decrease with incidence angle. Hence» for

a high power CW case there will be potential distortion problem with parts

of the mirror and as to the pulse case, there are the problems of different

damage thresholds because the different values of 3i and g|| previously

described. The asymmetrical toric optical design provides a solution to the

varying absorption of the symmetric case, provided an operating point is

assigned to the mirrors in the asymmetrical system. Figure 17 indicates

an operating point for an asymmetrical derived from the threshold damage data

taken to date, and it is a ratio E|/E| , - 20 for an incidence angle in the range

of 75° to 80°.

3. Conclusions

There are a number of conclusions that can be drawn from this program

and these are as follows:

1) Damage threshold appears to follow absorption.

2) Metal mirrors at grazing angles can be used with pulsewidths in the

nanosec range at X = 1.06y. Toric Optics can provide this grazing

angle very conveniently.

3) Metal mirrors at grazing angles can be expected to show acceptable

damage thresholds in the picosec pulsewidth range.

4) The incidence angle has not been optimized.

5) Reflecting surface thicknesses oflpmor greater are sufficient

for good mirror performance.

6) The operating point for an asymmetrical toric optical system has

been prescribed.

4. References

[1] "Dielectric mirror damage by laser

radiation over a range of pulse

durations and beam radii," Applied

Optics 12, no. 4 (April 1973)

.

[2] Milam, D., Lawrence Livermore Lab,

Private communication.

[3] Saito, T. , Lawrence Livermore Lab,

Private communication.

-43-
Cu Ac Au Ia Rh \a Iv ito Pd Al In Cd

Figure 0. Figure of merit for different

metals at A = 1.06y.

"7

MIRROR SURFACE

PLANE ORTHOGONAL

TO THE MIRROR

Figure 2. Definition of polarization

vectors

.

POLARIZATION POLARIZATION VECTOR VECTOR

B

E 7E

A (~~ f ^^J— — A

INPUT MIRROR B SIDE VIEW FRONT VIEW

Figure 1. Toric or axicon mirror.

2 f

r

I

- fl(0)/cc s 6 \4-

^^^^^

1

Sj_ le) - 3(0) cos e

20 40 60 ANGLE OF INCIDENCE 9

Figure 3. Absorption at 1. 06^m polarization

and angle of incidence copper

mirror (LLL)

.

Figure A. Anticipated variation of damage

threshold energy for a simple

thermal failure model.

Figure 5. Experimental damage threshold

energies vs polarization and

angle of incidence.
,. BOlAtABLE SAMPLE HOIOEB

IMERMOCOUPLE

MICROVOLT

METER

POLARIZATION ROTATOR

SHUTTER

POLAHIZtD YAG

LASER

Figure 6. Laser damage experiment.

C»OSCOfE OBJEClrvE

Figure 7. Experimental setup.

ENERGY I

(J/cm ) Ir

1

90°

THRESHOLD IS DEFINED TO BE THE LOWEST ENERGY WHICH CAN STILL DAMAGE THE SAMPLE UNDER ALL VARIATIONS OF PARAMETERS - FOCUS, SAMPLE SITE, ETC.

WAVELENGTH ND:YAG

1,06 ^(M

ENERGY TEHqq MODE ^: 1.5 MILLIJOULES

BEAM DIAMETER AT OUTPUT MIRROR TEMqq MODE 0,8 MM

POLARIZATION LINEAR

PULSE REPETITION RATE 1 PPS

PULSE DURATION IN TEMog MODE ill NSEC (FHHP)

PULSE-TO-PULSE ENERGY REPRODUCIBILITY i 7 PERCENT

Figure 9. Laser parameters.

Figure 8, Damage measurement procedure.

s

DISTANCE FROM PULSE CENTER (mlU) SQUARE OF RADIAL DISTANCE FROM BEAM CENTER (mill)

(b)

Figure 10. Intensity distribution of the

YAG laser.

-45-
MODIFIED _

MICROSCOPE

1 .5" FL LENS

BE^M SPLITTER COATED

FOR 100% REFLEC.

I ,06 /I 45°

VARIABLE ATTENUATOR

(FINE ADJUST)

CALIBRATED ATTENUATORS

SAMPLE ON

MOVABLE STAGE

Q-5WITCHED YAG LASER

Figure 11. Absorption measurements test setup.

Cre20 M4030«070«>W 0° 10 20 30 40 50 60 70 80 90° ANGU Of iNCiOtNCt

»

ANGLE OF INCIDENCE d

Figure 12. Experimental damage threshold ^^8"^^ 13. Experimental damage threshold

energies vs. polarization and angle energies vs polarization and angle

of incidence (#1). °^ incidence («2).

-46-
SPAWR #1 SPAWR #2

Figure 14. Photomicrographs of two solid copper mirrors polished by SPAWR,

-7-10 0 2 \0 10 10 10 10'^ 10

PULSE LENGTH T (nsec) ^ d^sec)

Figure 15. Theroetlcal curve and experimental points.

-47-
-

Q

I 1

A

1 -

- -

Domoge Ttireshold vs Thickness -"insuloted" subslrote / ^ Film- A.J + 300 A Ti OUUblrUlc ^2 UpilCUl llul lAJD^m Nd TAG loser

• 7 6 r^sec pulse width 8,7 nsec pulse width */

A 1 2 3 n sec pulse width

Least-square best fit Thermol Model

.. 1 .... 1 11 1 1/im 2fj,m 3^m 4fj.m S/xm

Film ttiickness (microns) -

Figure 16. Comparison of damage threshold for metal and dielectrics vs pulse width.

Figure 17. Operating point data for an asymmetrical system.

COmENT ON PAVERS BV SAJTO, U al and GOLVSTETN, ei al

It Mcu, mpkcUfiztd tkat the. damaQO. thAuhoZd -c6 dominated by 6uA{^ace ab6on.ptA.on ftathzn. than the

total Ke{tlectiv<Xy , Jhz lattz/i aj> atio 4en4-ctcue to icatteAlng, HoMeveK, icAotahu on the. ivJif^ace. may in^laznce both icatteAing and the damage thAeAhold iince both the eZecXxlc iieJLd and eZect/ion

6catteAA.ng in the iabiuA^jace mateAlaJL oAe incAeased in the vlcyinvty o^ the icAotch.

-48-
2 . 3 Diamond-Turned Mirrors

H. E. Bennett, M. J. Soileau, and P. C. Archibald

Michelson Laboratories, Naval Weapons Center

China Lake, California 93555

Diamond-turned, micromachined metal mirrors offer a new approach to optical

fabrication and may have significant advantages for use as laser components. A

laser mirror must have (1) excellent optical figure, (2) low scattered light,

(3) high reflectance, (4) high damage threshold, and (5) good environmental

resistance. We have measured optical figures as good as l/20th wave over a 3.86-cm

diameter sample and better than a wave over a 10.2-cm diameter sample on micromachined mirrors produced by Oak Ridge National Laboratories. Scattered light

levels in the infrared can be as low or lower than those obtained on conventional

mirrors and values as low as 2 x 10"^ have been observed at a wavelength of 10.6

pm. Infrared reflectance values as high or higher than those of conventional

evaporated or sputtered coatings have been achieved and one mirror had an absorption at 10.6 iJm of 0.006. The laser damage threshold of micromachined mirrors may

well be higher than that obtainable by other techniques, but this question and the

environmental resistance and "coatability" of a micromachined surface require

further investigation.

Key words: Diamond turning; laser damage; metal mirrors; micromachining ; optical

figure; scattered light.

Introduction

Diamond-turned optics represent a relatively new development in precision optics manufacture. The

potential of this method is not yet clear, partly because improvements are still being made at a rapid

rate. However, the outlook for this production technique is very favorable and it could conceivably

displace the more established methods for high quality and economical metal mirror fabrication. Mirrors

for use in laser systems must satisfy the following requirements: (1) good optical figure, (2) low

scattered light, and hence both minimum surface microroughness and minimum scratches and other macroirregularities, (3) high reflectance, (4) high damage threshold, and (5) good environmental resistance.

Let us consider how diamond-turned metal optics meet these objectives.

We have now measured 23 different 3.86-cm diameter, diamond-turned test samples and four larger

aluminum mirrors. All but one of the mirrors were made at Y-12 Division of Oak Ridge National Laboratories. Most of the small mirrors were machined on platings deposited on brass substrates. The larger

mirrors were all machined from solid aluminum. Of the 23 test samples, 10 were copper coated, 5 were

silvered, 3 were gold coated, 2 coatings were of unknown composition, and 3 were machined out of solid

copper. We have measured optical figure, surface microroughness, scattered light, and reflectance.

The general conclusions to be drawn from this study so far are: (1) Extremely smooth, low scatter surfaces can be prepared by diamond turning. Typical surfaces have equivalent optical rms roughness values

in the 20-30 A range. These values are in good agreement with those determined interferometrically [1]

The best sample we have seen had an average roughness of 12 K rms. This sample scattered only

1.1 X 10~5 of the reflected light at 10.6 ym and is nearly as low at this wavelength as the best con- ventional mirror we have measured. (2) The infrared absorption of diamond-turned mirrors is much lower

than that of typical optically polished metal surfaces, and in the best cases it approaches the minimum

theoretical absorption. At 10.6 um absorption values as low as 6 x 10"^ have been observed. (3) The

major problem with diamond-machined mirrors until recently was figure control. Typical values on the

earlier samples were 6-8 fringes across a 3.86-cm diameter. Recently, considerable effort has been

spent in improving figure control, and we have measured figures as good as 1/lOth fringe across a 3.86-cm diameter flat and less than 2 fringes across a 10.2-cm diameter flat. Since diamond turning is

a relatively new technique, even further improvement may be anticipated.

Measurement Techniques

Optical figure, surface topography, equivalent roughness of the optical surface, and scattering

level as a function of wavelength were measured in this study. The figure measurements were made on a Zygo Model GH Interferometer System and the initial surface topographic measurements on a Zeiss Differential Interference Contrast (Nomarski) microscope. To determine the equivalent surface roughness and

scattering level we used the Optical Evaluation Facility described below.

A schematic diagram of the Optical Evaluation Facility (OFF) used to measure scattered light is

shown in figure 1 and an overall view in figure 2. Light from one of five lasers is focused on sample

Sj at nearly normal incidence. An angle of incidence of 20 min is usually used to prevent instabilities in the CO2 and HeCd lasers caused by reflected light reentering the laser cavity. All turning

mirrors used with the CO2 and DF lasers are 50 mm or more in projected diameter to minimize edge effects

1. Figures in brackets indicate the literature references at the end of the paper.

-49-
An aperture d of 9.5 mm diameter is mounted directly in front of the beveled hole in the Coblentz

sphere, also 9.5 mm in diameter, to act as a beam trap for oblique rays. A blackened conical mask of

the same inner diameter is mounted inside the 114 mm radius of curvature sphere to ensure that the

wings of the reflected beam are either absorbed or reflected back outside the hole in the sphere. The

mask subtends 11° at the sample surface.

Light scattered by the sample is imaged by the Coblentz sphere on pyroelectric detector Di, which is

blackened and mounted inside a spherical BaF2 dome to minimize the angular sensitivity of detector

response. A second pyroelectric detector is mounted at D2 together with a silicon detector on a movable

arm to measure scatter as a function of angle both in the incident and reflected beams. D2 registered

less than 10~^ of the incident flux at angles greater than 11°, the angle subtended by the outer edge of

the mask inside the Coblentz sphere, for all wavelengths except the CO2 laser line, which was about

10~^ at the mask angle. Most of the diffracted light which did not pass back through the hole in the

sphere was thus trapped by the cone and did not reach the detector.

The OEF was calibrated at 4416 X by measuring the scattering level of an aluminized spray-painted

glass sample. The angular distribution function of this sample was measured and found to be very nearly

Lambertian with virtually no specular component. Knowing the reflectance of the aluminum thus gave the

desired calibration. As a check, an optically polished DF2 glass was measured using this calibration and

found to have a scatter of 10.6 x 10~3. The measured scattering level of this sample on the NWC visible

light scatterometer [1] at 4358 K. was 9.3 X 10-3. The measurements made on slightly different points of

the sample at slightly different wavelengths using two entirely different instruments thus agreed to within 13%.

To compare measurements made at different wavelengths, it is necessary to know the wavelength dependence of the ratio of the signal from detector to D3, the reference detector. Since both are pyroelectric detectors of similar design, they should be wavelength independent. However, to reduce the

signal of D3 to manageable proportions, a screen transmitting approximately 2.9% of the incident energy

and a CaF2 filter for reducing the 10.6 vm signal were used. The wavelength dependence of this combination was determined by comparing the signal from D3 to that from a Scientech Model 3610 power meter

which employs an optical cavity and thus has a response which is nearly independent of wavelength.

Origins of Scattered Light

Light scattering may be caused by various mechanisms. In high quality optics scattered light in

the visible region results primarily from surface microirregularities only a few tens of angstroms in

height, but extending over the entire surface area. If the height distribution function for these microirregularities is Gaussian, as is nearly always the case to a good approximation, the total integrated

scatter in all directions (TIS) should fall off inversely as the square of the wavelength of incident

light. In the visible region these predictions are usually in good agreement with theory. Infrared

scattered light measurements have been few in number and plagued by experimental uncertainties, but

have indicated that in the infrared the experimental TIS values usually do not obey the theory. Often

the TIS stops decreasing in the near infrared and approaches a nearly constant value. An example is

given in figure 3. In the visible region the experimental scattering values for this relatively rough

aluminized dense flint glass mirror fit the theoretical predictions (straight dashed line) quite well.

However, in the infrared they reach a nearly constant value and at 10.6 ym were about 20 times higher

than the theoretical predictions. The squares indicate the difference between the average and minimum

values of scattered light at a given wavelength as determined from measurements at ten points across

the mirror surface each separated by 1 mm. This difference value is nearly independent of wavelength,

and when the total scatter value reaches the difference value it stops decreasing and becomes nearly

constant. Under microscopic examination numerous particulates were observed on the mirror surface and

by examining the surface under a microscope in dark field using grazing incidence illumination many more

scattering centers could be detected. The total particulate count was about 10^ particles/mm^ and the

particles ranged in size from several micrometers in diameter to submicroscopic. They could not be

removed by a jet of ionized air and many of them may have been deposited during the coating process.

The small bump, in the neighborhood of 1 um, is often seen. Increased scattering at this wavelength

would be expected if many of the particulates were about 1 ym in diameter. Dust is about this size.

Dust particles are used as optical spacers between our FECO interferometer plates and the minimum order

of interference, which is normally obtained in our laboratory, four, corresponds to a one micron diameter particle.

Scattering From Micromachined Mirrors

The scattering behavior of micromachined mirrors in the infrared is typically somewhat different

from that of the evaporated coating shown in figure 3. The scattered light does deviate from the

theoretical curve for microirregularities having a Gaussian height distribution as shown in figure 4,

but the average deviation in the scattered light from point to point on the surface is not independent

of wavelength. Instead it has the same shape as the scattering curve. This behavior suggests that

particulate scattering is not as important as for the evaporated film shown in figure 3. Microscopic

examination reveals there are almost no large particulates on the micromachined surface, although

many were visible on the evaporated film surface of figure 3. How then to explain the deviation of the

-50-
infrared scattering from the theoretical curve in the case of the micromachined surface? Scanning FECO

interferometry may furnish an answer. The height distribution functions of microirregularities can be

determined by this technique. Usually the distribution function is quite close to a Gaussian. However,

for micromachined surfaces the measured height distribution in the wings is higher than for a Gaussian

function. There are thus more large irregularities than would be predicted on a Gaussian model and

hence the infrared scattering, which should be most sensitive to these large microirregularity heights,

would be expected to be somewhat larger than for a purely Gaussian surface. Qualitatively this explanation is in agreement with the experimental data. Quantitative analysis has not yet been performed.

If this explanation is correct, however, it suggests that by controlling the tool shape and machining

parameters the infrared scattering from micromachined mirrors might be reduced even further.

Solid Copper Mirrors

The most recent 3.86-cm diameter Y-12 mirrors we have measured were machined from massive copper on

a new Moore diamond point turning machine. The three we measured were part of a set of 18 which were

potted on a large plate with PPR rubber and machined simultaneously [2]. Figure control was excellent,

the best samples were flat to about 1/lOth fringe or l/20th wave, which compares favorably with the best

optical figures normally obtainable on glass or quartz using conventional optical polishing techniques.

In addition, unlike conventionally polished optics there is no rounded edge on any of the diamond turned

samples we have examined. An interferogram and a Nomarski micrograph of one of the optical surfaces is

shown in figure 5. The reflectance of this sample is shown as a function of wavelength in figure 6. It is significantly higher than typical polished, sputtered, or evaporated copper films and is as high at 10.6 ym as the best copper surfaces we have measured. The equivalent rms roughness of this surface was

21.0 A and its scattering behavior as a function of wavelength is shown in figure 4.

Copper-Coated Mirrors

Most of the mirrors tested thus far have been machined from brass blanks coated with electrolytically deposited copper. The coated samples were received in two batches. In most cases the effective

rms roughness, defined as the value obtained from scattering measurements made in the visible region,

ranged from 20 to 25 A rms. However, one copper-plated sample had a value of 117 A, and thus a visible

scattering level 20 times that of the other samples. The smoothest copper-plated sample averaged 16 A

rms. However, its absorption at 10.6 ym was 2.6 times that of the best sample. When the same plating

technique was repeated on another sample the same low infrared reflectance was achieved. The mechanical

condition of the sample during plating may also be important. For example, one sample was peened on the

plated surface as it was being plated. This sample has reflectance and short wavelength scattering

values which were comparable to the others, but the scattering level at 10.6 ym was an order of magnitude

higher, possibly because of the increased sensitivity at longer infrared wavelengths to long range

irregularities

.

Based on the evidence we have, it would appear that low infrared absorption, low scatter surfaces

can be obtained by using either a solid copper blank or copper plating from either a high copper con- centration, low acid concentration bath, or a low copper, high acid bath. Absorption values at 10.6 ym

as low as those for the best evaporated copper films and very close to the theoretical value of 0.0065

calculated from the Drude-Zener theory [3] can be obtained on plated diamond-point-machined copper surfaces. The surfaces obtained are quite uniform, with average deviations in scattering from point-topoint an order of magnitude lower than the scattered light level even at 10.6 ym.

Silver-Coated Mirrors

Five silver, diamond-point machined mirrors were tested. Four of these were machined by Y-12 and

one by Lawrence Livermore Laboratory (l3) . The lowest scatter diamond-turned mirror yet tested, sample

9, was in this group. It had an equivalent roughness of 12 A rms and scattered 2 x 10~5 of the

reflected light at 10.6 ym. Its absorption at 10.6 ym was 0.008, about twice the theoretical value of

0.0040 calculated from the Drude-Zener theory [4]. The machine marks were still evident in this sample

(figure 7), but it was quite uniform in scattering level at successive points on the surface even at 10.6 ym where the average deviation in scattering was about 1 x 10~5. This sample was measured before

and after being coated with "Carbofilm", a strippable protective coating manufactured by the American

Drug and Chemical Company and useful for removing dust from optical surfaces without damaging them.

Only a very slight improvement in scattering levels at 10.6 ym was observed.

None of the other silver samples had surface roughnesses comparable to sample 9, although all of the

silvered Y-12 samples were somewhat better than the plated copper samples. The silver mirror micromachined at Lawrence Livermore Laboratory (l3) had an equivalent roughness value of 85 A, four times as rough as most of the silver Y-12 samples and seven times as rough as sample 9. The scattering level

goes as the square of the roughness so that the scattering level of the l3 sample was 16 to 50 times as high as the Y-12 samples in the visible region and possibly in the infrared also. Unfortunately, the

scattering level at 10.6 ym of this sample could not be measured before it was returned to Y-12. The

reflectance at 10.6 ym was measured, however, and was as high or higher than any of the coated Y-12

-51-
samples and the figure was more uniform. Since this sample was machined we have heard that additional

improvements have been made in the diamond turning technique at L^, although we have not measured any

of their newer samples.

Gold-Coated Mirrors

Three gold-coated mirrors were measured. Gold [4] has a theoretical absorption at 10.6 ym of 0.0059,

but only one sample was within a factor of two of this value. It, however, had nearly as low absorption

as any sample tested. Scattering levels for this sample were equal to or better than those of most of

the other samples tested, but were six times as high at 10.6 pm as those measured for silver sample 9. Gold has the definite advantage that unlike both silver and copper it does not form a heavy tarnish

film upon exposure to air and thus may not require a protective coating. Gold coatings frequently have

much lower reflectance than would be expected theoretically. However, good diamond-point-machined

mirror surfaces have been demonstrated on gold-coated mirrors and the fabrication of such mirrors appear

to hold considerable promise.

Operational Size Mirrors I

The micromachined surfaces discussed above have been all 3.86-cm diameter laboratory samples. In many applications mirrors significantly larger in diameter than this will be required. Surface uniformity and figure control are more difficult to achieve in any optical fabrication process as mirror size

increases, but significant achievements are being made in micromachining medium size mirrors as well as laboratory samples. Figure 8 shows the figure and surface character of two such mirrors. Both are 10.2-cm in diameter and were turned from solid aluminum. The best one has a figure of better than one wave across its entire surface. The other one is a two wave surface but, the figure is quite uniform,

the surface is smoother and the machining marks are less pronounced. The depth of the final cut on the

smoother surface was only one ten thousandth of an inch, which illustrates the type of precision

required for micromachining operations.

Damage Threshold and Tarnish Resistance

The properties of diamond turned surfaces have as yet not been extensively explored. Preliminary . measurements indicate [5] that the laser damage threshold for micromachined surfaces is as high or ^

higher than that of any type of coated mirror. Tarnish resistance and environmental stability are also

unclear. It has been suggested that micromachined silver mirrors tarnish much less readily than do

other silver coated mirrors. However, preliminary studies of chemically deposited silver layers which

had been micromachined indicate [6] that the initial rate of tarnish formation is the same as that on evaporated silver coatings. Protective overcoatings can presumably halt, or at least greatly inhibit,

tarnish formation on micromachined surfaces. However, since cutting oil is normally used in the machining process and some of it may penetrate the microcracks in the metal surface and be difficult to remove, the "coatability" of micromachined surfaces is at present unclear and should be explored.

Conclusions

Diamond turned micromachined surfaces offer a new approach to optical fabrication. Mirrors used in

lasers must have (1) excellent optical figure, (2) low scattered light, (3) high reflectance, (4) a high , damage threshold, and (5) good environmental resistance. At present, micromachined mirrors can be produced which are better than a wave over a 10.2-cm diameter and l/20th wave over a 3.86-cm diameter.

Infrared scattered light levels as low as those obtained on conventional mirrors have been demonstrated*

and because of the uniformity of the machining process there is hope that even lower infrared scattering levels may be achieved. Infrared reflectance values as high or higher than those obtained by con- ventional evaporation or sputtered coatings have been demonstrated and the laser damage threshold of

micromachined mirrors may well be higher than that obtainable by any coating technique. The damage

threshold, environmental resistance, and "coatability" of micromachined surfaces, however, require further

investigation.

Acknowledgment

The authors are grateful to Mr. R. E. Sladky of Oak Ridge National Laboratory, Y-12 Division, for

furnishing the micromachined samples for evaluation and to Dr. W. Reichelt of the Los Alamos Scientific

Laboratory for encouragement and project support. This program was sponsored by LASL under ERDA Order

LS-75-4.

I

-52-
References

[1] Bennett, H. E., J. L. Stanford, and J. M. Bennett, in Space Optics (National Academy

of Sciences, Washington, D. C. , 1974), pp. 717-738.

[2] Sladky, R. , Y-12 Division, Oak Ridge National

Laboratory (private communication).

[3] Bennett, H. E. , in Symposium on Thermal

Radiation in Solids, S. Katzoff, ed.

(National Aeronautics and Space Administration, Washington, D. C, 1965), pp. 145-152.

[4] Bennett, H. E. and J. M. Bennett, in Optical

Properties and Electronic Structure of

Metals and Alloys, F. Abeles, ed. (NorthHolland Publishing Co., Amsterdam, 1966),

pp. 175-188.

[5] Soileau, M. J. and V. Wang, Appl. Opt. 13,

1286 (1974).

[6] Burge, D. K. , in High Energy Laser Mirrors

and Windows (Semi -Annual Report No. 6, ARPA

Order 2175, Naval Weapons Center, May 1975),

pp. 63-67.

Tables and Figures

0.3507 0.5208 0.7S25

0.3564 0.5309 0.7931 0.4762 0.5682 0.7993

0.4825 0.6471 0.48S0(Ar| 0.6764

Figure 1. Schematic of optical evaluation facility.

-53-
0.4 1.0 2.0 4.0 10.0 WAVELENGTH Oiml Figure 3. Scattering from an aluminized dense flint

glass (DF2) mirror.

1 1—I—I—I—r-i 1 1 1 1—I—I—I—I—

r

WAVELENGTH (|im)

Figure 4. Scattering from diamond-turned, solid

copper mirror.

Figure 5. Interf erogram and Nomarskl micrograph of 3.86-cm diameter

diamond-turned, solid copper mirror.

-54-
1.00 1—1

; 1 1 1 1 1 1 1 1 r

WAVELENGTH (micromoterj)

Figure 6. Infrared reflectance of diamond-turned,

solid copper mirror compared with

reflectance of copper mirrors prepared

by other means. The other curves shown

are for a polished bulk sample, an ev- aporated film, and two sputtered films.

Figure 7. Nomarski micrograph of diamond-turned

silver mirror.

-55-
Figure 8. Interferograms and llomarski micrographs of two 10.2-cin diameter,

diamond-turned, solid aluminum mirrors.

COMMENTS OW PAPER 8^ 5EWNE7T, al

(See commeitti on papeA by Sladky, eJ: al which (^oltom)

-56-
2 , 4 RESULTS OF OPTICAL MEASUREMENTS OF SURFACE QUALITY AND FIGURE

OF DIAMOND-TURNED MIRRORS

R. E.SIadky

Fabrication Systems Development Department

Y-12 Development Division

R. H. Dean

Materials Testing Department

Production Certification Division

Oak Ridge Y-12 Plant Oak Ridge, Tennessee 37830

Abstract

During the past year, the Oak Ridge Y-12 Plant'^' has diamond turned mirrors for the LASL

High-Energy Laser and for other projects. The largest of these mirrors are flat, elliptical in shape, and

654 mm (25.5 in) on the major axis and 394 mm (1 5.5 in) on the minor axis. The concave mirrors are 394 mm OD. These mirrors were inspected for surface finish and figure. Results of this work will be

summarized. In addition, blur circle was measured. Measurements of scattered light have also been

made. In some cases, the diffraction pattern was related to the waves formed in the surface by the machining process. This information has been useful to the engineers doing research on the diamond

turning of laser materials.

Key words: Blur circle; circular diffraction grating; diamond- turned mirrors;

diffraction pattern; machine cusp; scattered light; surface figure.

1. Introduction

Development efforts in diamond turning at the Oak Ridge Y-12 Plant, which date back to 1962, were initiated to study the

engineering principles related to precision machine tools. Diamond turning can be described as "superfinishing a metal surface by

single-point tooling". However, a diamond tool alone, used in a high-quality machine, cannot produce the surface characteristics that

are required for some applications such as the optical surfaces of mirrors needed for high-energy lasers. There are five requirements

for successful diamond turning; (1) precise machine movements, (2) a vibration analysis, both static and dynamic, (3) dynamic

balancing, (4) temperature control, and (5) sharp diamond tools.

The final test of how well these requirements have been met, or whether they are adequate for the desired results, depends on

the performance of the part in its particular application. These results are usually not available for a long time after the part has been

completed. Optical inspection in the laboratory provides immediate diagnostic information to the machine-tool engineer; and, the test data which are accumulated over a period of time will show long-range performance characteristics of the machines, individual deviations, and areas where improvements are needed. For these reasons, we have been working to make our optical testing program more comprehensive.

This paper will describe, first, the data that were accumulated in testing a series of mirrors that were made for a laser, and then

describe additional tests that reveal the effects of machine parameters on mirror performance by their effects on the diffraction

pattern.

2. Discussion

Surface Finish and Surface Figure

These data were obtained from a set of mirrors that were diamond turned for the LASL High-Energy Laser Project. The mirrors

consist of an aluminum substrate electroplated with copper. A flat mirror of this set is shown in figure 1 . It was made elliptical to fit into a tube. The dimensions are 394 by 654 mm (1 5.5 by 25.75 in). The surface finish of all the large mirrors was measured using a

Leitz interference microscope, as shown in figure 2, at five positions across the surface. The flatness of the surface was measured with

a 1 2-inch-diameter Davidson piano interferometer. The results of these measurements are shown in table 1 . The average surface finish of these mirrors is 2.5 microinches peak to valley, and the data show that in some cases the surface finish is better in the center where the cut was continuous than at the ends where the cut was interrupted. The average deviation from flatness of these mirrors was 77 microinches.

The figure of the mirrors as revealed by the piano interferometer was saddle shaped. The ends of the major axis were lower

than the center of the mirror and the ends of the minor axis were higher. The axis of the saddle was rotated somewhat clockwise

(usually about 30 degrees with respect to the axis of the mirror). The reason for this saddle-shaped figure is believed to be due to the

(a) Operated by the Union Carbide Corporation's Nuclear Division for the US Energy Research and Development Administration.

-57-
Table 1. Surface Finish and Flatness of Large Diamond-Turned Mirrors

[Elliptical Shape, 394 x 654 mm (15.5 x 25.75 in)]

Left Left Right Right

Mirror End Midpoint Center Midpoint End Average Flatness Number Date (Min) (Min) (/jin) (//in) (piin) (A/in) (//in)

538* 02-18-75 3.7 3.4 3.6 3.3 3.7 3.5 80

539 10-18-74 4.1 2.7 1.3 2.7 3.2 2.8 72

540 12-02-74 2.1 2.5 2.5 2.4 2.2 2.3 90

541 12-16-74 2.3 2.1 1.8 2.0 2.6 2.2 75

542 12-02-74 2.4 2.8 2.2 2.2 2.9 2.5 60

543 1 2-02-74 2.9 2.5 2.6 2.3 3.2 2.7 50

544 10-23-74 2.4 2.7 1.7 3.1 2.5 2.5 87

545 11-18-74 2.0 1.9 1.0 1.5 2.7 1.8 ICQ

AVERAGE 2.6 2.5 1.9 2.3 2.8 2.5 77

*This mirror was turned after a new and larger spindle was installed.

elliptical shape of the mirror and the recess cut in the back of the mirror for the holding fixture. These characteristics provided a situation in which the body of the mirror would flex when it was rotated on the machine.

The spherical mirrors were fabricated with radii of curvature of 133 and 418 inches. The surface finish was measured in the same manner as it was for the flat mirrors. The figure was obtained from the classic Foucault knife-edge test. The knife edge was mounted

on a compound slide at the center of curvature. The radius of curvature was not uniform from the edge to the center of the mirror,

generally being longer at the center. Masks were placed in front of the mirror so that it could be studied one zone at a time, and the

knife edge was moved back and forth on the compound slide to position it at the center of curvature of the zone. These data for a mirror with a radius of curvature of 133 inches are shown in figure 3, which illustrates that from the edge to the center of the mirror

the radius of curvature varies about one-and one-half inch. A computer program was developed to relate the variations in radius of curvature to the contour accuracy of the mirror and the results of this analysis are shown in table 2. The computer program placed a perfectly spherical surface at such a position that the error in the actual surface is a minimum and then calculated the error for each

zone. Thus, the maximum contour error for this mirror was 31 microinches, or 1/13 wavelength for CO2 lasers.

Table 2. Contour Error of Mirror 0548

Distance from Center Contour Error

(in) (//in)

0.50 0

1.50 - 5 2.25 - 1

1

2.75 - 12

i3.50 - 13

4.25 - 19

4.75 - 27

5.25 - 31 5.75 - 27

6.25 - 18

7.00 - 11 7.63 - 1

A summary of the data for the six mirrors is given in table 3. The surface finish of the spherical mirrors is about the same as it was for the flat mirrors, but the surface finish is not as good toward the edge of the mirrors with the short radius of curvature. The

contour error is given both in microinches and in fractions of a wavelength for the CO2 laser.

Blur Circle

The blur circle was the last item that was measured, and the time frame was such that it was measured on only one pair of

mirrors. When a lens or mirror is illuminated with collimated light, this light will be focused to a spot which is called the blur circle. In the practical sense, the diameter of the blur circle depends upon the optical perfection of the indivual components in the system and

the design of the system to eliminate or minimize those conditions that cause aberrations such as astigmatism, coma, and spherical aberration. The experimental arrangement is shown in figure 4. It consisted of a 6 mW He/Ne laser and a spatial filter/beam expander

which expanded the beam to a 2.5-cm diameter. This beam advanced to the convex mirror which further expanded the beam,

reflecting it to the 10.64-m radius-of-curvature mirror with a diameter of 394 mm, filling its aperture. The light coming from the

latter mirror was collimated. This collimation was accomplished by adjusting the distance between these two mirrors to be equal to -58-
Table 3. Surface Finish and Figure of Large Diamond-Turned Mirrors

[Concave, 394 mm (IS. 5 in) in Diameter)]

Mirror 1" 3" 5" Center Average Contour Error Number Date in'in) (Min) (/iin (Min) (piin) (//in) K, CU2

Radius of Curvature: 0.6 m (418.86 in)

535 10-28-74 2.9 3.0 2.4 2.1 2.8 - 76 1/5

536 11-18-74 2.7 2.8 2.3 2.4 2.6 + 50 1/8

537 11-25-74 2.3 2.5 1.9 2.4 2.3 -1- 10 1/42

AVERAGE 2.6 2.8 2.2 2.3 - -

Radius of Curvature: 3.4 m (133.8 in)

546 10-26-74 3.5 3.2 2.9 2.1 2.9 + 13 1/32

547 11-18-74 2.6 3.4 2.9 2.4 2.8 - 24 1/17

548 12-03-74 3.3 3.1 2.4 2.5 2.8 - 31 1/13

AVERAGE 3.1 3.2 2.7 2.3

the difference in their focal lengths. The collimated light proceeded to the 3.4-m radius-of-curvature mirror which focused the light to

a small spot. The diameter of this spot was measured. All mirrors in the system were placed as close together as the width of the beams would allow to reduce astigmatism. The diameter of the blur spot was measured with the use of small apertures. The device

holding the apertures was mounted on a compound slide so that the minimum diameter of the spot could be located. The aperture was then moved across the spot in steps of 0.001 inch and the power read from a Spectra-Physics power meter. The data obtained

with a 0.6-mm aperture is shown in figure 5. The power transmitted through this pinhole is 76% of the power measured at this point

if the pinhole is absent. When the 0.8-mm aperture is used (figure 6) the top of the peak is flat and 82% of the power is transmitted

through the aperture.

Scattering and Diffraction

Let us begin the subject of scattering by saying that a diamond-turned surface is similar to a circular diffraction grating. As the

disk turns on the spindle, the slide moves the diamond tool and a small spiral cusp remains on the surface. The spacings that remain on the surface may cause it to act as a grating. For example (figure 7), in one case the spindle speed was 1 12 rpm and the slide speed was 0.122 ipm. Dividing 0.122 by 1 12 and the cusp is 0.001 inch. This cusp spacing can be seen very easily with a surface-finish microscope at 50X. The theoretical surface finish (if the bit has a 1/8-inch nose radius) is equal to the square of the cusp spacing or

tool advance, and for this example is 1.2 microinches. The surface of this specimen is shown in figure 8. When the beam of a He/Ne

laser falls normally upon this surface a diffraction pattern is produced. The equipment for recording this diffraction pattern shown

in figure 9, consists simply of an uncollimated laser with a one-millimeter aperture in the beam, the specimen, and the detector. The

detector is at the end of a 45-cm arm attached to a rotary table, and the specimen is mounted over the center of the rotary table and

aligned so that the angle of incidence is known, usually zero degree. The rotary table is motor driven and a scan can be made through

±70 degrees. The first scan (figure 10) to demonstrate the results obtained from this system is from a 14,500-groove/inch

diffraction grating from the spectrographic laboratory. Observe that there are two orders of diffraction on both sides of the

zero-order peak and that the lines are tall and slim with almost no hash around the base. The diffraction pattern can be calculated from the grating equation:

n X = d sin 7,

since in this work the angle of incidence was kept at zero degree. Where there is a diffraction pattern and monochromatic light is used, it is interesting to calculate the order numbers to see if they can be obtained from the turning parameters. For the example

mentioned (figure 11) the order number was:

d 2767 X 10-6 cm n=—sin7 = ———— sin 7, or X 63.28 x 10-6 cm

n = 43.72 sin 7.

If the value for the tool advance that was obtained by counting fringes in the photograph shown before (figure 8) is used:

n = 45.15 sin 7. The diffraction pattern that was obtained from this copper mirror is shown in figure 12 with the order of diffraction marked

above each peak. Although the peaks are very small, the orders were calculated up to the 16th order for this specimen. The value of n from the surface finish photograph was used to obtain whole numbers for the orders, rather than the value from the machine

parameters. The latter are not accurate enough because the spindle speed is not calibrated accurately for each setting of the speed controL -59-
i

Admittedly, these are not very good surfaces. A much better surface was investigated (figure 13) and no diffracted light was

found that could be correlated with the machine parameters of a 0.210-ipm slide speed and an 800-rpm spindle speed where the cusp

spacing is 263 microinches. It is clear, however, that there are some peaks near the zero-order peak, shown in figure 14. How can they be accounted for? Among the surface-finish pictures that were made, some were taken with the Reichert metallurgical

microscope using the Nomarski attachment. This picture showed a series of waves (figure 15) at a magnification of 236X. The

number of waves over a length of the picture was counted, and from this the wave spacing was determined to be 912

microinches— nearly four times the spacing calculated from the machine parameters. The large peaks on both sides of the zero-order peak were calculated to be exactly the first order, based on this wave spacing.

What about the actual cusp spacing? Can it be seen? At 240X, the cusps should be 1 .6 mm apart on the Nomarski photographs,

but the faint lines that are visible are spaced almost exactly 1.0 mm apart. The theoretical surface finish is 0.06 micromch or 15 A,

which would be visible on the Nomarski, but which could not be observed with the Leitz microscope which has a height resolution of one microinch. What then is the "grass" that is observed on the photograph (figure 16) from the Leitz surface-finish microscope

which we have been reading as surface finish? Again, the spikes were counted in a length of picture and the spacing was the same as for the waves in the Nomarski picture at 240X. In other words, what we were calling "surface finish" and what we had thought were

"tool cusps" are actually waves in the surface, and the surface finish is the height of the waves.

Unfortunately, most turned surfaces do not show the even waves displayed here; in fact, these are not even waves. If the width

of each wave is measured, there is a fairly large distribution about the average which probably accounts for the additional small peaks

shown around the large diffraction peaks. As a final comment, the machine engineers say that the spacing does not correspond-to

anything in the machine such as gear ratios, ratio of spindle rpm to motor rpm, and drive-belt speeds. So the cause of this pattern has not yet been determined.

A final important question is: Do these diffraction patterns made with a He/Ne laser relate to the performance of these mirrors

in the infrared at 10.6 micrometres? We are starting to study that problem, and perhaps others can provide more information than we can at the present time. Dr. Bennett has point out that dust on the surface is an extremely important factor, and in his work he is evaluating all the factors that affect scattering and diffraction. We have a CO2 laser with an output of 45 watts. When this was

directed normally to the surface of several specimens we could not detect any light at the angles calculated for the first and

second-order diffraction peaks using the grating equation discussed before. We are working to improve the sensitivity of the detector,

and we think we know where to put it.

3. Conclusions

We have examined many more diamond-turned surfaces than we could summarize in this report. Some of them displayed

superior surfaces in terms of flatness and finish. Possibly you will hear about them in other talks at this meeting. All the work we do

will hopefully show what is necessary for turning better surfaces. It has also shown what we have to do to evaluate them better. We

look forward to discussing this with you next year.

-60-
4. Figures
Figure 2. Measuring the surface finish of a large

elliptical mirror with a Leitz surface

finish interference microscope.

1 .'

1

1 i

i

-

at tr BUS nm

- 1) 7 K.Bje :

1

1

J

i

Lib d: KUU —i

1 Ua*M 4^*

^ of ItU-rar

dt KUror -

- 1J.5 iMk> U90-81-O5W ujn

i

1

j Hi

«

8 i lis

. VIM «M r mm MUM "r r-

.

1

: 1 . p..

Figure 3. Knife-edge test variation of radius of

curvature with distance from center of

mirror. (Radius of curvature-133. 858",

diameter of irirror-15.5", number of

mirror-7190-81-0548, date-November 13,

1974.)

Spollol Flllar

Colllmotlng MIrfor (5320 mm focol l.ngth) -llurClrcIa

Slide 7 . EXPERIMENTAL ARMANQEMENT FOR THE ILUR^IRCIE MEASUREMINTt.

TmI Mirror (1700 mm fecel

l.rtgth)

Figure A. Experimental arrangement for the blurcircle measurements.
Slide 10 . Calculation of Cusp Spacing

Machine Parameters

Slide Speed: 0.122 inch/mlrfute

Spindle Speed: 112 rpm

Cusp Spacing 0.122 1pm

= 0.001 089 in = 0.002 767 cm

Tool Advance (TA) 200 rpm

When the surface is examined with a 50X microscope:

0.001 089 X 50 = 0.051* inch

The Theoretical Surface Finish is (for an I/8" R tool):

Surface Finish = (TA)^ = 0.001 089)^ = 0.000 001 2 A in, p to v

Figure 7. Calculation of cusp spacing.

Figure 8. Surface finish of a copper diamondturned surface with grating-like

characteristics. (The cusp spacing

on the surface is 0.001". On this

picture, taken at SOX, the cusps

are plainly visible.)

Figure 10. Scan from a 14, 500-groove/inch

diffraction grating.

Figure 9. Plan and elevation views of

apparatus for measuring scattered

light.

-63-
Slide Ik. Calculations with Orating Equation

n A . d sin r (Normal Incidence)

n " d sin 7 A

When d - TA . 2767 x IQ-^ en:

n - 2767 X IQ-^ cm sin 7 bi.ZO X 10"" cm

n - It}. 72 sin 7

From Surface Finish Riotograph:

There were 21 grooves In 5 cm at 50X

d - 3 cm = 2857 X 10"^ cm

21 X 50

n = 1*5.15 sin 7

Figure 11. Calculations with grating equation.

Figure 12. Diffraction pattern from the

copper mirror.

Figure 13. Diffraction pattern from a highquality diamond- turned mirror.

(100-mm-diameter , flat, unplated

copper, chart speed-1 nnn/sec.)

Figure 14. Diffraction pattern near the zero- order peak of a high-quality 100-

mm-diameter copper diamond- turned

mirror. (Chart speed, 50 nan/sec.)

-64-
Figure 15, Nomarski polarization contrast,

(This is a lOO-nun-diameter

diamond-turned copper disc, 236X.)

Figure 16, Leitz surface finish microscope,

(This is the same lOO-mm-diameter

copper-turned mirror as in the

previous slide. The structure in

these fringes are waves in the

surface, and not tool cusos, SOX.)

COMMENT ON PAPERS BV BENNETT, zt al and SIKVKV , zt aZ

Se.vM-aZ po-inti weAe fiouiiizd i.n the. dAAciLiiion concoAninQ the. {^acXou wlvich aontAot tk& iWi^acz

abi>on.ption ^^{jte.cM.v-vty and damage tlvieAhold o{, diamond tuAnzd miAfiom,. Thz pnJ,ncA.plQ, cont/iyibtution

to the. damage -u, thought to be ^-^om iubMiA^ace itAuctuAai de^ecJ^ induced by the diamond tuAning

pfLOCzii, The KeiidiMiZ damage level -U, thought to extend to a depth about {,ive tA.meA the. depth o^

the tool-cuZ, It Mas pointed out that annealing may help reduce the damage thAe^hold by Keduclng

the extent od thai damage latet. HoMzveA, annealing may alio geneAally degrade the opticaZ {^igune

0(5 the iuA^^ace. The chanxicteAlzation oj$ the ian-^ace includes both the deteAxninatlon o^ the widely

diMKibwted micAoiAAeguloJLvtiei on the 6uK{jace oa chaAacteJvized by an Ami naughnei,i> and the

occasional digs and i,cnatchej> . Both contAibute to icattening, but the net ed(,ect ij> dui^^ieAent in

the in^AOAed {lAom the visible, SuA^ace chatacteAization nequiAeA both a ipeci^icatyion o\ the

micKoiAAeguloAitiei and the macAoiAAeguloAitie^ . ScAatch and dig chaAacteAization alone is too

simple a de^cAiptton to be geneAolly useful in deteA/nining the damage tl1Ae^hold. It had been

obieAved in polished coppeA miAKOKS that impAoved the imoothncis on. reduced Ams Aoughnesi yields

a lowei Aefilectivi-ty and pAobably a Iomza damage thAeshold, evidence Oj^ an increase suA^^ace

absoAption due to continued polLihing o{, the suA{,ace. This I'Joi put (^omoAd as an example o(, the

complexity 0^ suAf^ace choAactsAization Aequt/tcd to deteAmine the peA{^0Amance o{, metal miAAoAi ^oA

high poMCA laseA use.

-65-
2.5 Dielectric Coated Diamond Turned Mirrors

James R. Buckmelter, Theodore T. Saito, Richard Esposito

Air Force Weapons Laboratory

Kirtland APB, MM 87117

and

Leonard P. Mott, R. Strand! und

Optical Coating Laboratory Inc. Santa Rosa, CA 95403 ,

Adherence of multilayer dielectric enhanced mirror coatings

to diamond turned metallic surfaces was discovered to be a problem. Initial coating results demonstrated peeling and

erratic optical properties for a variety of coating designs

deposited on ERDA diamond turned copper, silver, and gold. The

same coating designs deposited on polished molybdenum had excellent adherence and consistent optical properties. The adherence

problem was determined to derive from two factors intimately

connected with the diamond turning process, namely, oil adsorption and stress in the electroplated metal surface. These factors can be eliminated by stress relief in a heated oil bath

followed by thorough cleaning in freon or trichloroethylene

prior to coating. Procedures used to investigate dielectric

coated diamond turned mirrors included absorption calorimetry,

damage measurements with a 10 kW laser and Auger Spectroscopy.

Key words: Adherence; diamond machining; dielectric mirror;

electroplating; laser damage resistance; stress relief.

1 . Introduction

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 produce a finished

optical component [1,2]\ Diamond turning has two distinct advantages when compared to conventional optical fabrication. Because of machine accuracies many figures of revolution

can be easily fabricated. For example, a very fast parabola [3], ellipses and compound

axicons have been diamond turned. The second advantage is in the area of economy of

fabricating large (greater than six inch) diameter flats. Diamond turning is also more

economical when fabricating parts with unusual cross sections or parts which have holes

in them. The optical properties of diamond turned surfaces are outstanding. Reflectivity

at 1 and 10 microns compares favorably with ultra-high vacuum evaporated coatings of gold . and silver and is better than most super-polished samples of copper and silver. Laser

damage properties have also proven to be very competitive with conventionally fabricated

optics [4].

To date, diamond turning has been useful only on soft metals such as gold, silver,or copper,

and such metal mirrors are limited to experimental reflectivities at 10.6 microns of about 99.3

percent; whereas, multilayer dielectric enhanced mirrors have achieved 99.9 percent reflectivity

[5]. It would be advantageous to combine the economies of diamond turning with the improved

performance of multilayer dielectric enhanced mirrors. Accordingly a total of 32 diamond turned

metal mirrors were coated at the Optical Coating Laboratory, Inc., in an initial attempt

to produce superior multilayer dielectric enhanced mirrors for use at 10.6 microns. The results were unexpected and disappointing.

1. Figures in brackets indicate the literature references at the end of the paper.

NOTE: Micron(s) as used in this paper expresses micrometer (s ), -66-
2. Experimental

2.1. Sample Description

A total of 32 diamond turned mirrors, including 18 copper, 12 silver, and two gold, were

coated with seven slightly different multilayer dielectric enhanced mirror designs. Fifteen of

these mirrors exhibited complete peeling of the multilayer coating and were not tested at all.

The remaining 17 mirrors were subjected to a variety of evaluation procedures in spite of very

uneven quality. The 17 "good" mirrors included several that had only partially adherent

coatings and many that had cosmetic defects that apparently were caused by inadequate cleaning

of the metal surface prior to coating, but these were evaluated anyway due to the limited

number of samples. For comparison, 12 polished molybdenum and six glass witness samples were

also coated at the same time with the multilayer designs and all of these samples demonstrated

good adherence and cosmetic quality. These samples were evaluated in parallel with the coated

diamond turned mirrors. The substrates and coating designs are described in table 1 where the

glass and molybdenum substrates are polished, and the copper, silver, and gold are diamond

turned by either the Lawrence Livermore Laboratories (LLL) or by Union Carbide Y-12 at Oak

Ridge, Tennessee.

2.2. Optical Evaluation

The multilayer dielectric coated samples were evaluated with a variety of techniques and

the results are given in table 1. The 10.6 microns reflectivity was measured with a multiple

bounce reflectometer which basically compares a reference signal to a sample signal which is

varied by adjusting the number of times a 10.6 micron laser beam reflects between a known

mirror and the sample mirror [6]. The reflectance of the pair of mirrors is then derived from

the slope of the log of the signal ratio versus the number of reflections. This reflectometer

is accurate to + .2 percent, but typically gives lower reflectances on dielectric mirrors

than absorption calorimetry (1-A) because the reflectometer sees only specular reflectance

after several bounces; whereas, an absorption calorimeter can also include most of the scattered

radiation.

R-6328 is the reflectivity at .6328 microns (HeNe) as compared to a good silver coating.

The silver standard had a measured reflectance at 10.6 micron of 99.2 percent, so based on standard plots of reflectance versus wavelength, this standard would have an absolute

reflectance at .6328 microns of 98.5 percent. This standard silver sample was arbitrarily

set at 100 percent in a Gary 14 Spectrophotometer with a reflectance attachment and then all

the dielectric mirrors were simply compared to the baseline set by the standard. This is not

a particularly accurate measurement, perhaps ± 1.0 percent, but the values obtained are useful

as an indication of how easily such optics might be aligned in a real system. Although no effort was made to design coatings which would have high reflectance at .6328 microns, as well

as at 10.6 microns, several coating designs did have appreciable reflectance at the HeNe wavelength.

TIS is the total integrated scatter at .6328 microns as collected in an integrating

sphere and compared to the total reflectance from a dielectric mirror [7]. A 100 percent

reflecting dielectric mirror for .6328 microns is used to reflect a HeNe beam at the inside

of an integrating sphere combined with a photomultipl ier tube. The signal produced by

the dielectric mirror is arbitrarily set at 100 percent and the sample mirrors are then

introduced into the system, except that the reflected beam is aimed through a hole in the

integrating sphere so that only the scattered light from the sample is collected. A typical

polished metal surface might have a TIS near .5 percent and some diamond turned metal

surfaces have had TIS values nearly indistinguishable from the background level (blue

velvet) of about .04 percent. The TIS values are useful mostly as a qualitative tool since it

is extremely sensitive to the ambient environment or to sample position. Different spots

on the same sample can easily vary by factors of 2-5. Furthermore, there is no convenient

method to relate a sample's performance at .6328 microns to that at 10.6 microns, except

that 10.6 micron scatter is much lower [8]. The TIS values are useful for distinguishing

coatings or surfaces that contain numerous imperfections that are not always apparent to the eye. It should be noted that dielectric coatings will never improve the scatter from

the base substrate, due to the tendency of the coating to reproduce or even magnify original

substrate defects as well as introduce new flaws.

-67-
The design wavelength for all samples was 10.6 microns, but due to the experimental

difficulties involved in depositing an exact quarterwave optical thickness many of the

coating designs had a design bandwidth that was not centered at 10.6 microns. However, since

the maximum reflectance band is usually about 2 microns wide for the number of coating pairs

involved in this study, the maximum reflectance of any sample was not seriously affected by

the detuning of the multilayer stack. The effects of coating thickness variations or detuning

were negligible compared to absorption effects or gross film imperfections such as pinholes or inclusions; but, the extent of detuning is an excellent indication of the control that the

manufacturer has over his process. The tuning control of the manufacturing process is indicated

in table 1 by x-Scan. This was determined with a Digilab FTS-20 infrared spectrophotometer

used in a reflectance mode. The A-Scan value is the center of the reflectance band taken at

half-maximum value.

An absorption calorimeter was used to measure the 1 -Absorptance (1-A) values presented in

table 1 [9]. This equipment employs a Coherent Radiation Model 42L C0„ laser as a light source,

and temperature monitors and heat capacities of the mirrors are calibrated in situ. The

energy of the reflected beam is measured calorimetrically within 4 percent accuracy. The

absolute accuracy of the measurement is ± 10 percent of the absorption measured and the precision

is 0.0005.

The laser damage measurements were done with the AFWL 15kW, CW, COr,, coaxial, electrical

discharge laser (EDCL) [10]. This laser was operated at approximately lo kW for all damage

shots in order to get reasonably stable operation for the time periods involved. Most shots were run for 5 seconds, except that the 12 kW/cm shots were run for 10 seconds and others,

as noted in table 1. Laser intensity was varied by focusing the output beam with a one meter focal length copper mirror, but for the highest intensity, 46 kw/cm , a 25 cm focal

length salt lens was used in conjunction with the one meter mirror to produce a beam area

of 24 cm . Beam area was measured simply and directly by the use of plexiglass burn patterns

taken at the same position as the mirror samples. The worst case accuracy for the intensity

measurements was + 30 percent, due mostly to variations in the laser output from run to run and

to errors in beam area measurement. The variations in laser output were known, but were averaged for the sake of convenience in reporting the data. Nominal intensities used were

5, 12, 19, and 46 kW/cm ; however, a few samples were run in a different test series with the same laser and the intensities were slightly different. These are noted in table 1. Also, the

intensities given are all corrected for reflection losses at the various optical interfaces,

such as the output beam splitter, focusing mirror, or lens. The laser power was measured with

a Coherent Radiation model 213 power meter to better than + 5 percent; and, since the EDCL

lases with a uniform flat top intensity output, no corrections were necessary to account for

a Gaussian distribution. This is to be contrasted to most other laser damage measurements where

extensive calibration is required to define a highly focused Gaussian beam [11,12].

Laser damage in this study is simply defined as catastrophic or microscopic. Catastrophic

damage involves a damaged area at least as large as the incident laser beam area; whereas,

microscopic damage consisted of burn spots that were much smaller than the irradiated area. Samples that survived catastrophic damage are noted in table 1 with a greater than (>) sign,

otherwise the intensity listed implies that the sample survived catastrophic damage at the preceeding intensity. Only four samples survived catastrophic damage and only one of these was a dielectric coated diamond turned mirror. Microscopic damage was extensive on these four samples

and also on other samples in areas outside the catastrophic damage spot.

2.3. Microscopic and Metalurgical Evaluation

Several of the dielectric coated diamond turned mirrors with especially poor coating

adherence were examined with a Nomarski phase contrast microscope. This revealed that an extensive orange-peel effect had developed in the metal surface during the coating process.

This observation led to an experiment in which diamond turned gold and copper samples were

heat treated in vacuum to 200°C for one hour. The gold sample developed an extensive orangepeel and the copper sample developed a pitted appearance. Other diamond turned substrates

that were annealed in an oil bath at 150°C for one hour did not develop any surface changes.

Since the dielectric coatings were deposited on unannealed substrates in vacuum at ISCC over

-68-
a period of several hours, the coating adherence was almost surely affected by the changes in

the substrates brought about by the coating chamber environment. The vacuum heat treatment also

made some metal surfaces noticeably dirtier. Evidently a prior cleaning in trichloroethylene

and alcohol was not sufficient to remove all of the cutting fluid associated with the diamond

turning process. Again the conditions necessary for depositing dielectric coatings probably

caused the cutting fluid or other contaminant to migrate to the surface being coated and this

resulted in poor cosmetic quality and also poor coating adherence.

Additional work was done on single crystal copper, one piece of which was diamond turned,

and an adjacent piece from the same single crystal which was polished in a conventional manner

[13]. Nomarski microscopy revealed numerous flaws, scratches, pits, etc., on t?ie mechanically

polished sample, while the diamond turned sample was of excellent cosmetic and optical quality.

Laue back reflection x-ray patterns, however, demonstrated considerably more smearing for the

diamond turned sample as compared to the conventionally polished sample indicating that the

diamond turning process introduces more work-hardening than conventional polishing. X-ray

reflection topography was also attempted with the result that the diamond turned surface

produced a very weak broad peak extending over a degree or more. Only small random areas of

the surface with very little detail were shown in the resulting topograph. In contrast, x-ray

topographs of the mechanically polished surface showed an image for the entire surface which

again indicates that diamond turning introduces considerably more damage into the metal surface.

Finally, knoop micro-hardness measurements were made on both surfaces using a 15 gram test

load. An average of five measurements on each surface resulted in a knoop hardness of 82

for the mechanically polished surface and 126 for the diamond turned sample; thus Laue

patterns, hardness measurements, and x-ray topography all indicate that there is significantly

more "cold work" in the diamond turned mirror than in the mechanically polished mirror regardless of the cosmetic or optical quality. The 10.6 micron reflectivity of the diamond turned

copper was greater than 99.1 percent.

3. Discussion

The application of multilayer dielectric enhanced coatings to diamond turned surfaces has

been unexpectedly difficult for reasons that are inherent in the diamond turning process.

Most important, the diamond turned surfaces are severely cold worked and this results in

extensive atomic mobility during the coating operation, which in turn adversely affects the

adherence of the coating. Secondly, the diamond turning process only works so far on the soft

metals such as gold, copper, silver, or aluminum. These materials cannot be mechanically

scrubbed and so must be cleaned with a non-contact process which has not been sufficient. In addition, the best surfaces for the diamond turning process are electroplated since this gives

a very fine grained structure amenable to diamond turning. Such electroplated layers can be

highly stressed and must be annealed prior to coating.

The samples in this study which were successfully coated had erratic optical properties.

Several dielectric coated diamond turned mirrors had very large 10.6 micron absorption and

these mirrors also had lower laser damage thresholds as would be expected. Other mirrors with

absorption comparable to the coated polished molybdenum samples also had roughly comparable

laser damage thresholds. This indicates that the defect structure of the coating dominated the

defects introduced by the diamond turning process for laser damage resistance of samples

with comparable absorption.

Based on the information accumulated with the first group of samples, a second set of six

polished molybdenum, four diamond turned gold, and four diamond turned copper substrates were

coated. The diamond turned substrates in this set were all fabricated by LLL using electroplated layers of gold or copper; however, these samples were annealed at 150°C in an oil bath

for one hour prior to cleaning and coating. The cleaning procedure consisted of an ultrasonic bath in trichloroethylene and a soaking in alcohol. Two multilayer coating designs were

used and all substrates were coated with adherent dielectric coatings, but the cosmetic quality

of the coated diamond turned mirrors was still unsatisfactory. These samples are described

-69-
in table 2. The cosmetic quality of these samples was not particularly good, but the most re^ cent cleaning procedures were not used. There are two techniques that will produce suitably

clean oil free surfaces. One consists of ordinary rinsing in trichloroethylene and alcohol

followed by heating in vacuum to about 100°C followed by another rinse in clean trichloroethylene and alcohol. Observation under blue light indicates that this produces a clean

surface. The second procedure involves cleaning for about 30 seconds with clean TF freon

at 50°C in a vapor degreaser.

4. Conclusion

Dielectric coatings can be applied to diamond turned metal surfaces to form adherent multilayer dielectric enhanced mirrors that will be cosmetically and optically good. Such mirrors

should be limited in laser damage thresholds by the same defects that limit conventionally

polished surfaces. These limiting coating defects include the usual pinholes, inclusions,

scratches, coating absorption, etc., that cause presently available coatings to damage at GV

10.6 micron irradiation levels of about 30 kW/cm^.

The recommended substrates for diamond turning are oxygen free high conductivity (OFHC)

copper, or electroplated gold, or silver, all of which should be annealed and thoroughly cleaned.

A recommended multilayer dielectric coating would be (ThF./ZnS) or (ThF./ZnSe) with a metallic binder undercoating of perhaps gold, chromium,or titanium, but not silver. If such

coatings can be deposited without defects, laser damage thresholds can be expected to increase

substantial ly.

5 . Acknowledgments

Several people contributed to this effort, but special mention should be made of Auger

Analysis and hardness measurements by Dr. W. Ewing and Ms. J. Bruce of the Air Force

Cambridge Research Lab; the optical measurements by Mr. R. Lester, Mr. M. Guadagnol i, and

Mr. W. Kunzler of AFWL; the damage testing with Mr. R. Carlson of AFWL,and conversations

with Mr. R. Austin of Perkin-Elmer.

6. References

[1] Saito, T.T., "An Introduction to the

Machining of Optics," to be published

in August issue of Applied Optics

14, 1975.

[2] Saito, T.T. and Simmons, L.B.,

"Performance Characteristics of Single

Point Diamond Machined Metal Mirrors

for Infrared Laser Applications,"

Ap. Opt. 13, 2647, 1974.

[3] Bryan, J.B., Donaldson, R.R., McClure

E.R., Wayland, H.A., Clouser, R.W.,

"Diamond Turning of Parabolic Mirrors,"

Presented at Society of Parabolic

Instrumentation Engineers, August 1973

Available as UCRL-51213 from Lawrence

Livermore Laboratory.

[4] Saito, T.T., Milan, D., Baker, P., Murphy,

G., "10. 6u 15 psec Laser Damage Study

of Diamond Turned, Diamond Turned/ Polished

and Polished Metal Mirrors," to be presented

to the 1975 Laser Damage Symposium at

Boulder, Colorado, July 1975.

[5] Braunstein, M., "Multilayer Enhanced Dielectric Mirrors," AFWL-TR-75-196, Air

Force Weapons Laboratory, 1975.

[6] Wetzel, M.G., Saito, T.T. and Patterson,

S.R., "Improvement of Absolute Accuracy

for a Multiple Bounce Reflectometer Through

a Detailed Effort to Reduce Systematic

Errors," App. Opt. 12: 1558, 1973.

-70-
[7] Kurdock, J..R., "Low Absorptance

Metallic Coatings for Metallic Substrates," AFWL-TR-74-28, Air Force

Weapons Laboratory, 1974.

[8] Scheeley, S., "Scattering Characteristics of Mirrors and the Associated

Inverse Scattering Problem," Ph.D.

Thesis, USC, 1973.

[9] Saito, T.T., Callender, A.B. and

Simmons L.B., "Calorimeter to Measure

the 10.6 Micron Absorption of Metal

Substrate Mirrors," Appl . Opt. 14:

721, 1975.

[10] Carlson, R. "A 15 Kilowatt CW, CO^

Coaxial, Electrical Discharge Laser,"

to be published as Air Force Weapons

Laboratory Technical Report.

[11] Huguley, C-. and Loomis, J., "Optical

Material Damage From 10.6 Micron CW

Radiation," to be presented at NTSS

Symposium on Damage in Laser Materials,

Boulder Colorado 1975.

[12] Saito, T.T., Charlton, G and Loomis, J. "10.6 Micron Cw Laser Damage Studies

of Metal Substrate Mirrors," Laser Induced

Damage in Optical Materials, ed. Geunther

A. NBS Special Publication 414, 1974.

[13] Bruce, J., Private communication

-71-
7, Figures and Tables

C. D.

Figure 1. Nomarski photomicrographs of various diamond turned surfaces all at lOOX.

A. Dielectric coated diamond turned LLL-Cu-54.

B. Diamond turned electroplated LLL-Au-3, vacuum baked 200°C one hour.

C. Diamond turned electroplated LLL-Au-28 oil bath annealed 50°C, vacuum baked

200°C one hour.

D. Diamond turned electroplated LLL-Cu-85 oil bath annealed 50°C, vacuum baked

200°C one hour.

-72-
Table 1. San^le Description and Optical Properties

Source

Dielectric

oUDStra.re ucsign K- 1 U , D TTQ AoC3n 1 A i-A

Damagi

kw/ cm

OCLI Glass (ZnS/Ge)

99.9 10.2 5.1

AFWL Mo-156 99.2 25. .4 10.3 99.81 46

AFWL Mo-134 99.5 25. .4 10.3 99.62 46

LLL Cu-62 99.2 25. .3 10.3 99.73 46

LLL Cu-66 99.4 26. .3 10.3 99.70 46

ULLX ri'^cc All +r7-nQ /rci'\ ^ uiass /AU^i.t.no/bej QO Q in X lU . o

C 1 J . 1

ArrVL I*I0-i3U yy . 0 jO , 7 in 7 QQ ZA yy . i)4 A40

AFWL Mo-147 99.7 37. .9 10.3 99.87 46

LLL Ag-13 93.3 26. 6.3 10.3 93.72 5

LLL Cu-58 99.6 37. .5 10.3 99.84 46

LLL Cu-60 99.7 37. .3 10.3 99.82 19

Y-12 AU-C6 93.6 .3 9.8 93.90 19

OCLI Glass (ThF./ZnSe)^ 99.6 10.6 10.8

AFWL Mo- 137 99.4 78. 1.3 10.6 99.78 >46, 5 sees

AFWL Mo-157 99.6 79. 4.5 10.6 99.68 46

LLL Ag-14 99.5 63. 5.6 10.6 99.70 46

LLL Ag-15 99.6 62. 4.0 10.6 99.71 46

OCLI Glass Au+fThF,/ZnS)^

4 99.4 10.5 10.8

AFWL Mo-135 99.3 75. 1.7 10.5 98.90 >46, 5 sees

AFWL

Y-12

Mo-155

An-^Q22/12

99.4

99.3

75.

72.

4.6

3.0

10.5

10.5

99.59

99.60

>40,

>40

,

46

10 sees

5 sees

LLL Ag-3 99.2 56. 2.5 10.5 99.60 >46, 5 sees

npT T biaSS yLno/ be

J

yy . o

1 n ft

AFWL Mo-132 99.7 27. .7 10.4 99.82 46

AFWL Mo-136 99.8 27. .6 10.4 99.78 46

LLL Ag-10 94.0 24, 10.7 10.4 96.86 19

Y-12 Ag-M123/7 92.5 26. 4.9 10.4 96.28 12

LLL Cu-50 98.6 28. .6 10.4 99.85 46
Table 1. Sample Description

Dielectric

Source Substrate Design R-10.6

OCLI Glass Ag+ (ThF^/ZnS) ^ 99.3

AFWL Mo-149 99.2

AFWL Mo-133 99.5

LLL Ag-11 99.5

LLL Ag-10 99.3

LLL Cu-49 Ti+ (ZnS/Ge) '^+ZnS 99.7

LLL Cu-51 99.5

and Optical Properties (Continued]

Damag(

R-.6328 TIS X Scan 1-A kw/cm'

10,,4 10.

1

88. 6.2 10..4 99.,81 46

90. 1.7 10.,4 99.,62 >40

46

52. 5.5 10,,4 99.,59 46

52. 5.2 10,.4 99..56 46

.5 10,.6 99,.89 46

.5 10,.5 99,.87 46

Table 2. Sample Description and Optical Properties Group 2

AFWL Mo-189 (TliF^/ZnS) 99,.3 92. 2.7 10.2 99.,68

AFWL Mo-192 99 .4 80. 1.8 10.3 99.,74

AFWL Mo-193 99 .4 77. 1.7 10.3 99.,66

LLL Au-20 99,.5 96. 1.5 10.3 99.,74

LLL Au-24 99 .4 85. 1.6 10.3 99. 64

LLL Cu-71 99 .2 84. 2.0 10.3 99. 57

LLL Cu-72 99 .2 48. 1.8 10.3 99.,71

AFWL Mo-178 (ZnS/Ge) 99 .3 32. .9 9.7 99,,64

AFWL Mo-182 99 .5 34. 2.8 9.7 99.,67

AFWL Mo-185 99 .6 29. .5 9.6 99.,63

LLL Au-21 99 .6 38. .4 9.7 99,,62

LLL Au-25 99 .4 23. .4 9.7 99.,62

LLL Cu-69 99 .4 37. .5 9.9 99.,58

LLL Au-73 99 .2 23. 3.6 9.8 99.,45

COMMEWT ON PAPER BV BUCKMELTER, eX al

A CLOvmmt wcu, mad& HdQaxding the cleaning iuA{,acu a{,tM. pfie.paA.cution and aX mch mpha&iztd

that tho. cJiQ.avu.nQ miL&t be pufif^oimzd bd^^ofio. the. iani^aco. dAA-U oiit. Any contaminate, iihldn. li) on the

6uA{,ace in the iom ol a iolmUon Mkich iabieqaently dfUeA out i^iZJi be extAemely dtdd-LcuZt to nejnove.

It wa6 alio pointed oat that hydAocaxbon contamlnaJxon can be meoiuAed veA.y fieadily in the wavelength

region ^n.om 3.4 to 3.5 micAometeAi due to the it/iong abdoKption OjJ hydAocoAbom at that wavelength

and that probing on 3.S micAometeAi may ititl pnoviAe a iemitive teit o(, hydAocanbon conX.aminaXion, A {^uAtheK comment wkich woi made o^j^eAi the intAigulng poaibiZiXy that the reduced coKAOi,lon o^

dUxmond-tuAned miAAoHM may actuaJUiy be due to the ei$)$ect oi the n.eiidual oil layeA which nemalnk on the miAAofi af,teA the tuAning pAoce^i.

-74-
2 . 6 SLIDE-POSITION ERRORS DEGRADE MACHINED OPTICAL COMPONENT QUALITY

J. B. Arnold

P. J. Steger

Fabrication Systems Development Department

Y-12 Development Division

R. R. Burleson

Laboratory Development Department

Y-12 Development Division

Oak Ridge Y-12 Plant Oak Ridge, Tennessee 37830

An ultraprecision lathe is being developed at the Oak Ridge Y-12 Plant'^' to fabricate optical

components for use in high-energy laser systems. The lathe has the capability to produce virtually any

shape mirror which is symmetrical about an axis of revolution. Two basic types of mirrors are fabricated on the lathe; namely, (1) the mirrors which are machined using a single slide motion (such as

flats and cylinders), and (2) the mirrors which are produced by two-coordinated slide motions (such as hyperbolic reflectors; large, true-radius reflectors, and other contoured-surface reflectors). The

surface-finish quality of typical mirrors machined by a single axis of motion is better than 13 nm, peak

to valley, which is an order of magnitude better than the surface finishes of mirrors produced by two

axes of motion. Surface finish refers to short-wavelength-figure errors that are visibly detectable. The

primary cause of the inability to produce significantly better surface finishes on contoured mirrors has been determined as positional errors which exist in the slide positioning systems. The correction of these errors must be accomplished before contoured surface finishes comparable to the flat and

cylinder can be machined on the lathe.

Key words: Controlled environment; diamond machining; machined optics; nondistortion part support; position errors; quality and compatible part material; quality

diamond tools.

1. Introduction

Exceptional-quality mirrors are required for high-energy laser-optics systems. This need has motivated research and development

to improve existing production methods and to study and evaluate new techniques that may be applicable. Until recently, because of

the intimate relationship between quality of the surface and the performance of the optics, experience has dictated that

state-of-the-art polishing techniques will be needed to produce the desired surfaces. New developments in precision machining now

indicate that surfaces of competitive quality can be generated using single-crystal diamond tools and specially constructed turning

machines.

The Oak Ridge Y-12 Plant, since it is a sophisticated production facility, has the responsibility to develop and utilize precision

machining technology. As a result of this obligation, the Plant has maintained a continuing program in the study of machine tools, their theory and operation, with particular attention to analyzing the behavior of machine-tool elements in relation to their overall performance. Part of this program was to study engineering principles as they apply to ultraprecise machining processes where

maximum deviation from an idealized shape and size is not to exceed a few microinches. These studies, which involved basic diamond-machining experience using high-quality-edge, single-crystal diamond tools, helped to determine basic requirements for precision machining and led to the development of improved machine components. Significant advances have been made in precision-part machining technology. The data acquired served to establish engineering guidelines for building computer and

numerically controlled machine tools that are capable of diamond turning shapes of revolution to a high quality.

Refinements of these machines and development of a total process for machining metal mirrors to the final quality required for

laser optics is the motivation for the present development efforts.

2. Basic Criteria for Machining Optics

The basic criteria required for machining optics have been established for several years. Researchers have had reports published

that discuss these criteria and define the relative importance of each criterion to the machining process. [l»2] Most researchers agree

that at least five basic requirements must be met to achieve the capability to produce high-quality machined optics, namely: (1) use

of a quality tool, (2) use of a quality and compatible part material, (3) use of a nondistortion part support technique, (4) prevent

detrimental influence of the environment on the machining process, and (5) use of a quality machine.

(a) Operated by the Union Carbide Corporation's Nuclear Division for the US Energy Research and Development Administration.

1. Figures in brackets indicate the literature references at the end of the paper.

-75-
Quality Diamond Tools

I Development of fabrication techniques to produce angstrom-quality edges on diamond tools was a significant milestone in the chain of events leading to optic fabrication by machining. Only with the single-crystal natural diamond has the necessary edge

sharpness been produced and maintained to generate high-quality machined surfaces. Even when all the other machining requirements

are met, it is not possible to produce quality optics using other known tool materials. The forerunner to the diamond tools used at Y-12 was the biological diamond knife. This knife was employed on a microtome machine which was used to slice very thin layers of suscepted cancer tissue in preparation for evaluation.

The first diamond tools were very similar to the biological diamond knives, with straight cutting edges and small, included edge

angles. The use of these tools led to improved revisions; and, in 1967, the first curved tool was used at Y-12. The success obtained

with the curved tools encouraged the design and development of various shapes and styles.

A view of two basic styles of diamond tools used at the Y-12 Plant is given in figure 1 . The 45° shank tool has been commonly

referred to as a "diamond knife" because of its close similarity to the biological diamond knife. The shank is orientated 45° with

respect to the part when performing a machining operation. It is generally fabricated with an included edge angle of 78°, a 6°

clearance angle, and a 6° positive back rake. Both straight and curved-edge tools are used. The curved-edge tools have radii of 1/16,

1/8, and 3/16 inch and arc lengths of 0.060 and 0.100 inch.

The horizontal-shank diamond tools have edge angles ranging from 77 to 85°, clearance angles ranging from 6 to 16°, and a 3°

negative back rake. These tools are obtained with nose radii ranging from 0.001 to 0.032 inch, over 1 10° of arc, and to infinity for arc lengths of 0.060 to 0.100 inch. If particular needs dictate, special tool geometries and sizes other than the ones described are ordered.

Figure 2 provides a comparison of the edge quality of several items which are known for edge sharpness. It can be seen from the

figure that the natural diamond edge is significantly better than the edges on the other items. The answer to the question: "How good

are the natural diamond tools?", has been sought for many years. Various microscopes using different magnifications have been

employed with limited success until the scanning electron microscope was used. Coating the opaque edge of the diamond with 150 A

of carbon and then 1 50 A of gold made it possible to evaluate the edge of the tool at magnifications as high as 30,000X. Figure 3 is a compilation of several electron microscope photographs of a typical natural diamond tool. It can be seen from the figure that the diamond edge has flaws approximately 1/3 microinch (8 nm) in amplitude and 1 microinch (25 nm) in length. These silhouette electron microscope photographs are among the first made of a diamond edge and are believed to be true representations of the edge.

Quality and Compatible Material

Several materials have been machined at Y-12 to determine if they are compatible to the diamond turning process. High-purity,

soft, face-center-cubic metals such as gold, silver, copper, lead, and aluminum have been found to be very compatible. Other metals such as high-purity iron, platinum, and nickel were diamond turned but failed to produce the desired optical properties.

Nondistortion Part Support

As the requirements for mirror-figure quality increased, so does the need for the use of nondistortion-part-support techniques. If

a mirror is held in a strain during the machining operation, a near-perfect geometry could be machined on the mirror surface which

would only distort once the mirror is removed from its machining fixture. To prevent such an incident from happening, several nondistortion support techniques have been investigated and utilized. Of these techniques, the use of a refined conventional vacuum

system and the potting of the mirrors into a fixture using two-part castable rubber to hold the mirror in place have been found to have significant advantages over other evaluated techniques.

Vacuum fixtures have been used very successfully to hold mirrors which have a flat back. The face of the fixture must be

machined with a network of narrow vacuum grooves which will allow adequate vacuum support with an acceptable amount of mirror

distortion. The contact surface of the vacuum fixture is machined flat in order not to distort the back of the mirrors. The back of the

mirrors are hand lapped to assure the highest degree of flatness. The vacuum-fixturing technique provides a significant advantage in the fact that it is relatively easy to remove the mirrors from the fixture after machining is completed.

Two-part castable rubber, such as Rubberjel'b) and DPR 336'^', has been used to hold mirrors in place during machining.

Rubber in a liquid form is placed in a cavity in either the back of the mirror or in the fixture face prior to forcing these surfaces

together. The rubber is allowed to cure at room temperature before the mirror and fixture are mounted on the machine. Metal safety catches are installed "finger tight" prior to machining. Once the machining process is complete, the mirror and fixture must be

removed by forcing water into special part-removal grooves which have been provided in the face of the 'fixture. Figure 4 is an

illustration of a nondistortion-part-support fixture.

(b) A product of the L. D. Caulk Company, Milford, Delaware 19963

(c) A product of Hardman Incorporated, Belleville, New Jersey 07109

-76-
Controlled Environment

Any influence of the environment must be controlled to prevent a detrimental effect on the diamond machining process. Effects of external vibration, temperature variation, humidity, and cleanliness are controlled on the diamond turning machines. External

vibration is reduced by supporting the machines on pneumatic isolators (air bags), and the machines are housed in laboratories which

are equipped with control systems that maintain a constant temperature, low humidity, and high cleanliness. Cutting fluids used in the machining process are closely controlled both for thermal stability and cleanliness. In areas where the environment cannot be

satisfactorily controlled, partial to total flooding of critical components with thermally controlled fluids will successfully maintain

machine stability [3]

.

Quality Machine

The machine on which diamond turning is performed must have the capability to rotate a part and position a tool accurately and

vibration free.

The air-bearing spindle with an isolated motor and noninfluencing drive, has been used successfully to meet the part-rotating accuracy requirements. To eliminate vibration, which could be caused by an unbalanced rotary part, the parts are dynamically

balanced at the speed at which they will be machined. A noncontact capacitance probe and storage oscilloscope are used in this dynamic balancing process. The radial motion of the lathe faceplate is measured at a low rpm and stored in the oscilloscope. The

radial motion is then measured at the speed at which the part will be machined and again stored in the oscilloscope. If the recorded

trace of the radial motions do not coincide, balance weights are added to the spindle. This process is repeated until the traces coincide

within a couple of microinches ] •

Several machines have been used for machining optics which employ various tool positioning techniques. The Moore lathe, which is one of Y-12's most recently developed optical machining lathes, is seen in figures 5 and 6. The lathe consists primarily of a standard Model 3 Moore measuring machine base, modified for turning by the addition of an air-bearing spindle mounted horizontally on a cut-down vertical column. An ultraprecision Moore rotary table (C motion) has been installed on the bed of the lathe to allow

the diamond tool to be rotated as it is moved in an X-Y plane, where the X motion is normal to and the Y motion is parallel to the

spindle axis of rotation. A minicomputer is used in conjunction with positioning motors to control the X, Y, and C positional

displacement of the tool when performing complex contouring operations; but, to perform simple facing and cylindrical operations,

variable-speed (0 to 8.5 in/min) DC slewing motors are employed. The lathe has the capability to machine a 0.5-microinch

(13-nm)-peak-to-valley (p-v) finish on a flat mirror, and a 2 to 4-microinch (50 to 100-nm)-p-v finish on a contoured mirror. Although

this machine incorporates one of the most advanced and accurate positioning systems used at Y-1 2, errors in the system are believed

to be the primary limiting factor in producing better-quality contoured surfaces.

An investigation was conducted to correctly define the X and Y positioning errors and to determine the cause of these errors. The first steps of the investigation consisted of the fabrication and evaluation of two large spherical radius mirrors. The mirrors were

machined on the Moore lathe using stepping motors to drive the X and Y lead screws, and were first evaluated using the knife-edge

inspection technique. A photograph of the inspection results of one mirror is presented in figure 7. The evaluation results indicated

that periodic surface irregularities existed across the face of the mirrors. The knife-edge inspection technique is basically qualitative in nature; therefore, to define the amplitude of the wave, a precision-air-bearing LVDT with a sensitive amplifier and recorder was used.

Once the errors were defined, an additional large-radius mirror was machined on the lathe. The outer portion of the mirror

surface was machined using the same parameters as for the first mirrors, while the inner portion was machined with the environmental

control system turned off. It was first thought that the cycling of the temperature controller may have caused the geometry of the machine to fluctuate during the machining operation. Inspection results of the mirror revealed that the environmental control system

had no effect on the surface irregularities in question. Efforts to correlate the irregularities with features of the Moore lathe were

conducted by using a precision optical flat and the air-bearing LVDT. The air-bearing LVDT was fastened to the bed of the lathe in such a position that it would indicate Y movement of the bed as it moved along the face of the optical flat, which was fastened to the

lathe column and parallel to the X travel. The LVDT indicated that the lathe bed had very little Y movement as it advanced in the X

direction. A total indicator reading of less than 6 microinches (150 nm) was recorded over 14 inches (355.6 mm) of travel. The

irregularities detected on the surfaces of the contoured mirrors could not be attributed to this straightness error.

Due to the fact that these contoured mirrors were produced by movements of both the X and Y axes, it was decided that the bed movement should be evaluated with both axes in motion. To aid in acquiring these data, a special control tape was fabricated that would drive the X axis 33 1/3 microinches (830 nm) while the Y axis advanced only 1/3 microinch (8.3 nm). This slope is representative of the range of slopes involved on the test mirrors. By positioning the optical flat at a 100-to-1 slope (with respect to the X axis) it was possible to measure and record incorrect positional movement of the bed in the Y direction as the LVDT moved

along the optical flat. Figure 8 shows the results obtained from this measuring setup. It can be seen from the figure that the predominant errors, which repeat every 0.25 inch (6.35 mm) of the table travel in the X direction, have an amplitude of 4

microinches (100 nm), and errors which repeat every 1/12 inch (0.083 inch, 2.1 mm) have an amplitude of 2 microinches (50 nm).

Much smaller errors of approximately 1 microinch (25 nm) in amplitude are very irregular and appear to repeat every 0.1 inch (0.25

mm). A careful evaluation of the figure will reveal that there is not one small wave which is repeating in every 0.1 inch (0.25 mm),

but about 25 small waves which are repeating every 0.25 inch (6.35 mm).

To aid in determining the cause of the errors, a schematic diagram of the Moore lathe's X and Y positioning systems was drawn

for evaluation (see figure 9). By labeling each component of the positioning systems with the proper number of revolutions made as -77-
the bed advanced 1 inch (25.4 mm) in the X direction, it can be seen that the repeatable errors coincide identically with the

revolutions and teeth engagement of the Y-axis drive train components. The Y primary worm makes four revolutions as the bed

advances 1 inch (25.4 mm) in the X direction, and 12 teeth of the Y primary gear engage with the triple-thread worm over the same

distance. The smaller waves correspond to each revolution of the Y secondary worm which makes 100 revolutions as the table travels

1 inch in the X direction.

Several revisions have been made to minimize the positioning errors. DC-encoded torque motors were installed on both X and Y

axes, and an ultraprecision (200:1 reduction) primary worm and gear was installed on the Y axis. A series of slope tests were

conducted to determine the value of each revision. These tests were first conducted before any revisions were made. Then, tests were

rerun when the torque motors were installed and also when the new worm-and-gear system was installed. Results of these tests are reported in figures 10 through 13.

It can be seen from the figures that small-wavelength errors were eliminated when the DC torque motors were used. The reason

that these errors were eliminated was because the secondary worm-and-gear system, which caused the errors, was bypassed, as shown

in figure 14. It can also be seen from figures 10 through 13 that the new primary worm and gear did not reduce cyclic errors as had

been anticipated. The amplitude of th§ errors produced by the new worm and gear is about the same as the amplitude of the errors produced by the original worm and gear, but have a shorter wavelength. The increased error frequency did cause the new worm and

gear to be less desirable. It can be concluded from the test results that the most desirable surface finish could be produced when the DC torque motors and the original (40: 1 reduction) worm and gear are used.

Two contoured mirrors, one with a 133-inch (3.38-m) radius and the other with a 418-inch (10.6-m) radius were machined and

evaluated to determine if the surface finish of contoured mirrors would be improved, as had been indicated by later slope test results. Leitz-surface-finish photographs of the two mirrors were compared to surface-finish photographs of respective radius mirrors which

had been machined using the original stepping motors to drive the lathe's positioning systems. A significant improvement in the

longer-wavelength surface (figure 15) was observed. The data are reported in figures 15 and 16. The surface variations of the mirrors

machined using the DC torque motor's are estimated at 2 to 2.5 microinches (50 to 75 nm), peak to valley (p-v), whereas the surface

finish of the mirrors machined using the stepping motors is estimated at 2.4 to 3.2 microinches (60 to 80 nm), p-v, with the

difference due primarily to an absence of the longer-wavelength ripple. A higher degree of improvement was observed for the

smaller-radius mirror than for the larger-radius or flatter-surface mirror due to the greater slope of the surface.

3. Future Work

The success obtained by using the DC-encoded torque motors to improve the surface finish of parts machined on the Moore

lathe has been very encouraging. It appears likely that if continued short-wavelength contour improvement takes place, a better power

transfer medium other than the existing worms and gears must be used. The possibility and practicality of using reduction rollers to

drive the lead screws have been discussed with engineers from the Moore Special Tool Company. It appears practical at this time, but

needs to be studied further. The possibility of incorporating a control system which will have laser interferometers as its positional

sensing components will also be studied. Hardware required for the installation of laser interferometers on the Moore lathe has been

designed and fabricated, and installation is under way. The basic layout of the installation is shown in figures 17 and 18. It can be

seen in the figures that both X and Y table displacement will be monitored in horizontal planes which are relatively close to the

respective slide levels. It was first felt that the laser interferometers should monitor slide displacement at tool height; but, after considerable study, such a setup was deemed to be impractical. Straightness tests of the slides indicate that pitch is very low and also repeatable; therefore. Abbe offset should not be a problem.

After installation, the laser interferometers will determine the X and Y tool displacement. The tool-displacement signals will be

transmitted to a dynamic error-monitor system. The system will resolve an error signal by comparing the measured tool displacement

to the commanded displacement. The error signal will be used to modify the command signal, thereby reducing the error in displacement. The electrical hardware for the system has been fabricated and set up temporarily on the Moore lathe to measure the

slide-positioning errors. The temporary setup consisted of a laser interferometer, a fringes-to-inches converter, an error monitor, and a chart recorder. A block diagram of the setup, used to determine the errors in the Y-axis positioning system, is given in figure 19. This

setup compares where the machine tool control system commanded the slide to go with where the slide actually went, as seen by the

laser interferometer. This comparison is done on a continuous basis. The basic resolution of the system is one microinch when using a resolution extender on the laser interferometer. Inspection results obtained from the evaluation of both the X and Y axes are reported in figures 20 and 21. It can be seen from the figures that both axes have cyclic errors which correspond to each revolution of the lead screws. Short-wavelength errors are more apparent in the "b" figures where the horizontal scale is amplified. It can be seen

in these figures that these short-wavelength errors correspond to each revolution of the triple-thread worms which drive the primary

gears and the engagement of each thread on the worm. These errors, which have an amplitude of less than one microinch, will not be

eliminated by this error-monitoring system. The small errors must be eliminated by the use of better power-transfer mediums such as reduction rollers.

4. Conclusions

Mirror manufacturing by the single-point diamond machining process is in an early stage of development, but success which has already been achieved proves that this process has a significant advantage in producing large, flat mirrors and contoured mirrors, such

as hyperboles and parabolas, over conventional fabrication techniques.

-78-
5. References

[1] Bryan, J. B., Clouser, R. R., and Holland, E., "Spindle

Accuracy", American Machinist, pp 149 - 164; McGraw-Hill

Inc, New York, Special Report Number 612; December 4, 1967.

[2] Warmbrod, W. M. and Williams, R. L.; Laser Fusion IVIirror Machining, Y-DA-5130; Union Carbide Corporation-Nuclear

Division, Oak Ridge Y-12 Plant, Oak Ridge, Tennessee; January TO, 1973.

6.

(a) Typical diamond tools used for microinch

machining.

Figure 1. View of two typical diamond tools

[3] Bryan, J. B., Donaldson, R. R., McClure, E. R. and Clouser,

R. R.; /4 Practical Solution to the Thermal Stability

Problem in Machine Tools, MR72-138; Society of Manufacturing Engineers, 20501 Ford Road, Dearborn,

Michigan 48128 (1972).

[4] Lewis, T. G.; Ultra-Precision Machine Tool Elements and

Machining Processes, Y-1611; Union Carbide CorporationNuclear Division, Oak Ridge Y-12 Plant, Oak Ridge, Tennessee (1968).

Figures

(b) Photomicrograph of edge of diamond tool; 475X,

a photomicrograph of the edge of a diamond tool.

(c) Commercial diamond tool, (d) Natural diamond tool.

Figure 2. Comparison of the edge quality of four items (425X)

.

-79-
(a) 30,000X. (b) lO.OOOX

(c) 3,000X. (d) l.OOOX.

Figure 3, Several electron microscope photographs of a typical natural diamond tool.

Figure 4. Part support for mirror machining.

-80-
01

01u

o

<u

s

o

!•o

u

eg

u

01

-81-
-82-
Figure 7. The knife-edge photograph indicates waves across the machined surface.

[Radius of curvature 133.8 inch (3.398 m) ; diameter-lS. 5 inch (0.39 m) ]

.

1" (25.4 mm) of X-Trovel

Figure 8. Error plot of the Y-axis positioning system.

-83-
Precision Optical Plot (sloped 100 to 1 with respect to the X oxis)

Precision Air Bearing LVDT

(mounted to the lathe bed)

Moore Lothe Bed Y-AxIs Secondary Gear

(50 teeth, 4 rev/In of X travel)

Y-Axis Secondary Worm (double

threod, 100 rev/in of X travel)

X-Axis Leodscrew

(10 pitch, 10 rev/in of X trovel)

X-Axis Primary Geor

(120 teeth, 10 rev/in of X trovel)

X-Axis Primary Worm

(triple-threod worm,

400 rev/in of X trovel)

-Y-Axis Stepping Motor

(100 rev/in of X travel, 30,000 pulses/in of X trovel)

^

X-Axls Stepping Motor (10,000 Vev/in of X trovel, 3,000,000

pulses/in of X travel)

X-Axis Secondary Geor

(50 teeth, 400 rev/In of X trovel)

X-Axls Secondary Worm

(double thread, 10,000 rev/in of X travel)

Figure 9. Moore lathe's X and Y drive systems and the measuring setup used to determine the positioning errors illustrated in figure 8,

Stepping Motors

100 to 1 Slope Test 7-17-74

40: 1 Reduction Gear 60mm 0.001" of Y Travel (horlzontol scale) 20p" Full Scole (vertical) Air Bearing LVDT on Glass Bor

LSL I \ \ I

Figure 10. Slope test results. [The stepping motors and the original (40:1 reduction) worm and gear were used during this test,]

I r D.C. Motors

100 to 1 Slope Test 1-9-75

40; 1 Reduction Gear 314mm ' 1 In. on X-AxIs or 0.01 in. of Y Trovel (horlzontol) 2Qy" Full Scale (vertical) MIcrosense on Gloss Bor

Figure 11. Slope test results. [The new dc encoded torque motors and the original

(40:1 reduction) worm and gear were used during this test.]

-84-
T

D.C. Motors

100 to I Slope Test 2-6-75 11:00 o.m.

200:1 Redjclion Geor Instolled

One Rev. of Primary Worm (Y) 0.0005" of Y Travel

or 0.05" of X Travel

T

14.86mni ^ 0.0005" of Y Travel (horizontal) 20m" full Scale (vertical)

1 mm/sec - Chart Speed

Microsense on Glass Bar

T

Figure 12. Slope test results. [The new encoded dc torque motors and the new

(200:1 reduction) worm and gear were used during this test.]

Stepping Motors

lOO to 1 Slope Test 2-<6-75 1:00 p.m.

200:1 Reduction Gear Installed 14mm - 0.0005" on Y-Axis or

1 Rev. of Y Worm (horizontol)

20|j" fuW Scale (vertical) Microsense on Gloss Bar 2m (— One Rev. Y Primory Worm

0.0005" of Y Trovel

or 0.05" of X Trovel

-0.00)" of Y Travel

_L

Figure 13, Slope test results, [The stepping motors and the new (200:1 reduction) worm and gear were used during this test.]

Spindle

Prec Ision OpHcol Flat (sloped 100 to 1 with respect to the X-axis

Precision Air Beorlng LVDT

(mounted to the lathe bed)

Moore Lathe Bed

Y-Axis Electric Clutch Disengaged When Torque Motors ore Used

Y-Axis Secondary Gear

(50 teeth, 4 rev/in of X travel)

Y-Axis Secondary Worm (do thread, 100 rev/in of X travel)

Y-Axis Stepping Motor

(100 rev/in of X travel, 30,000 pulses/in of X travel)

' Y-Axis Primary Gear (120 teeth,

0. 1 rev/in of X travel) -Y-Axis Primary Worm (triple-thread worm, 4 rev/in of X travel)

X-Axis Leadscrew

(10 pitch, 10 rev/in of X travel) ^X-Axis D.C. Encoded Torque Motors / (3,200,000 pulses/in of X travel, 8,000

pulses/rev encoder) — X-Axis Primary Gear

i l20 teeth, 10 rev/in of X travel) X-Axls Primary Worm ^triple-thread worm , 400 rev/in of X travel)

X-Axis Electric Clutch Disengaged When

Torque Motors are Used

X-Axis Stepping Motor (10,000

rev/in of X travel, 3,000,000

pulses/in of X trovel)

X-Axis Secondary Gear

(50 teeth, 400 rev/in of X travel)

X-Axis Secondary Worm (double

thread, 10,000 rev/in of X travel)

Figure 14, Schematic of equipment arrangement for slope tests.

-85-
Near the center

3.0 microinches, P to V Stepping motors (SOX)

Four inches from the center

3.2 microinches, P to V

Stepping motors (SOX)

Near the center

2.S microinches, P to V

dc torque motors (SOX)

Four inches from the center

2.0 microinches, P to V

dc torque motors (SOX) 1

Figure 15. The surface finish of a 133-lnch-radlus mirror machined using dc torque motors to drive the

lathe slides is better than the surface finish of the mirror machined using stepping motors.

Near the center Four inches from the center Near the center

2. A microinches, P to V 2.8 microinches, P to V 2 microinches, P to V

Stepping motors (SOX) Stepping motors (SOX) dc troque motors (SOX)

Four inches from the center

2.2 microinches, P to V

dc torque motors (SOX)

Figure 16. The surface finish of the 418- inch-radius mirror

machined using dc torque motors to drive the

lathe slides is better than the surface finish of

the mirror machined using stepping motors.

-86-
tacoN

•HUO

.ac

•HO

0) •H

a

-a >

•H (U

cn

C -H

n) 4Jo X 0)p.

0)

o (1) 4J •HCOe

a)

CO

m

p

o

<U i-l

4-1

o

(De >^ O -! U 0)

(U >

^1 4-1 0)c <u •H M M (U m CDo

a)M

-87-
Figure 18. The components of the laser interferometer system are arranged to determine

displacement of both the "X" and "Y" axes.

Spindle

Inches Converter

Machine Tool

Control System Error Monitor

Chart Recorder

Figure 19. Dynamic error monitor block diagram.

-88-
the X-Axis Leadscrew

a. Positioning error of X-axis lead screw.

Triple-Thread Primary Worm

One Revolution of the X-Axis Leadscrew

b. Positioning error of X axis, primarily worm and gear.

Figure 20. Cyclic positional errors as detected by the dynamic error system correspond to revolutions of X-axis drive train components.

One Revolution of I

the Y-Axis Leadscrewf^

a. Positioning error of the Y-axis lead screw.

_L

4 One Revolution of the Triple-Thread Primary Worm ~r~

One Revolution of the Y-Axis Leadscrew *

b. Positioning error of the Y axis, primarily worm and gear.

Figure 21. Cyclic positional errors correspond to revolution of drive train components.

COMMEWTS ON PAPER BV AmOLV, at

A numbeA OjJ tha. pcimtLcala/u, thz diamond tuAvving pKocuM, and thd mackinz poJiamtteAA uizd -in thJj>

-89-
2.7 ULTRASONIC CLEANING OF OPTICAL SURFACES

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

Materials Research Laboratories

Department of Defense

Ascot Vale, Victoria, Australia

During the formal proceedings of the 1975 Symposium on Laser Induced

Damage in Optical Materials, Dr. A. H. Guenther presented some recent results

on the degradation of polished optical surfaces by ultrasonic cleaning procedures. This information was supplied to him by Dr's. W. E. K. Gibbs and

A. D. McLachlan of the Materials Research Laboratories of the Department of

Defense, Commonwealth of Australia. Materials studied included OFHC copper,

beryllium copper, zirconium copper, stainless steel, kanigen and fused

quartz. Results indicate that ultrasonic cleaning degrades polished surfaces, and if the cleaning process is vigorous enough for a sufficient duration of time, subsurface fracture introduced in the grinding process is

made quite evident. At the conclusion of the transmission of this correspondence, a discussion on the subject of ultrasonic cleaning ensued.

Key words: Laser damage; metal mirrors; subsurface structure; ultrasonic

cleaning.

1. Introduction

This ad hoc preliminary investigation was to determine the extent of ultrasonic

cleaning damage on various materials . Damage had previously been observed on OFHC

copper

.

2 . Samples

Samples tested were: OFHC copper, beryllium copper (Berylco 25-hardened) , zirconium copper, stainless steel, "Kanigen" electroless nickel on copper, fused quartz

(Vitreosil) . The metal samples were 1" diameter x 1/2" thick, while the quartz was

1/2" X 1/2" X 1/8".

3 . Polishing

Metal samples were polished on a "2 mm"-pitch lap using glycerol as the medium for

"Linde A" alumina abrasive. Final polishing was with a "5 mm"-pitch lap using "Linde B"

abrasive with glycerol as the medium. The quartz was polished on a "5 mm"-pitch lap

with cerium oxide and water,

4 . Microscopy and Cleaning

The metal samples were indexed to locate a given spot on the polished surface and

photographs were taken on a Reichert MeF microscope by using the interference contrast

technique (Nomarski) . Two samples of fused quartz were polished, only one of which was ultrasonically cleaned. The quartz samples were coated with a thin coating of aluminiom

in order to enhance the reflectivity of the surface. Ultrasonic cleaning consisted of

3 intervals of cleaning of 15 minutes each in detergent, distilled water, and AR methanol. Samples were individually cleaned in a Branson Model D-lOO ultrasonic cleaner.

5 . Results

Micrographs clearly show the presence of pitting after ultrasonic cleaning, particularly in the softer materials, e.g., zirconium copper and OFHC copper. The beryllium

copper showed little damage after 45 minutes of cleaning, and was thus cleaned for a further 4 5 minutes to give the results shown. The damage could be seen with the naked

eye under glancing illumination on most of the samples (including fused quartz)

.

In interpreting the micrographs, it should be noted that the Nomarski technique

renders some deep pits as bright spots for the contrast setting chosen.

-90-
6 . Figures

Figure 2. Another sample of OFHC copper. (a) As polished. Magnification 500 X. of ultrasonic cleaning. Magnification AOOX.

(b) After 45 minut
V ^ '

(a) (b)

H i-

,

-til

Figure 3. OFHC copper, (a) After 45 minutes of ultrasonic deaniiag eschlltitliig an area typlcaiL of a

more severely damaged region, Ifagniflcation 150X, (b) Same area. Magnlficattioim

(a) (b)

Figure 4. Beryllium copper, (a) As polished. (b) After 90 minutes of ultrasonic cleaning of an area typical of the average damage to the sample. Magnification 150X,

-92-
Figure 5. Beryllium copper (Berylco 25-hardened) , (a) As polished. (b) After 90 minutes of

ultrasonic cleaning. Magnification 500X,

Figure 6. Zirconium copper, (a) As polished. (b) After 45 minutes of ultrasonic cleaning.

Magnification 150X.

-93-
(a) (b)

Figure 8. Stainless steel, (a) As polished. (b) After A5 minutes of ultrasonic cleaning,

(Dark smudges are due to dust In the microscope; however, note the appearance

of a nev7 plt„) Magnification 150X,

-94-
(a) (b)

Figure 9. Kanlgen. (a) As polished. (b) After 45 minutes of ultrasonic cleaning. Note the

appearance of a new pit and the increased apparent relief of the grinding marks.

Magnification 500X.

(a) (b)

Figure 10. Kanigen. (a) As polished. (b) After 45 minutes of ultrasonic cleaning. (Again dark

smudges are due to dust in the microscope.) Note the appearance of two major pits

uncovered during the ultrasonic cleaning. Magnification ISOX.

-95-
(a)

mi 'iiiiiiiifttiiiii

Figure 11, Fused quartz (Vltreosll) . (a) A typical general area of the orevious sample after

90 minutes of ultrasonic cleaning. (b) An area typical of the more severely

damaged areas of the previous sample after the same minute ultrasonic cleaning.

Magnification 500X.

(a)

Figure 12. Fused auartz (Vltreosll). (a) A typical general area of the previous sample after

90 minutes of ultrasonic cleaning. (b) An area typical of the more severely

damaged areas of the previous sample after the same minute ultrasonic cleaning.

Magnification 500X.
(a) (b)

Figure 13. Another sample of fused quartz. (a) As polished. (b) An area typical of the more

severly damaged after 90 minutes of ultrasonic cleaning. Magnification 500X,

COMMENT ON PAPER BV GIBBS, U al

A numbzfL OjJ people obieAvzd that pfLolongzd uttAcu,on-Lc clzayiyLnq can 6e.v2Azly d^gfiadz optyical iaA(^acu.

ExpoiuAeA 0^ t&yn minutu in uttAaj,onic clzaning can le.ad to a total dutAuctton 0(5 the. optical

Aatjjace and diAKuption o^ bondi beM^ccn layzu o{) mateAixxZ. UZtfuuonicaZZy cZcancd iuJi^acu iubjcctcd to

pKotongzd cleaning do not take, dielectric coatingi ivell. It woi commented that a bfiied altfiaionic cleaning Mith a period o^ a minute or two can lead to an impnovement in the optical. 6uA(,ace by the removal o^

iuA{)ace dOvt., but that once a critical expoiure hoi occurred the degradation o{i the iuAf^ace occun> dramatically and sharply. Prolonged ultrasonic cleaning hai even been obieAved to datroy the diamond

tooti used in the diamond-turning process.

-97-
3 . 1 THERMAL DIFFUSIVITY OF

GERMMIUM, GALLIUM ARSEMIDE AND CADMIUM TELLURIDE

OVER THE TEMPERATURE RAMGE 80 K - 900 K

Robert DOUSSAIN, Hubert P. LE BODO

Service des Essais Therraiques

LABORATOIRE NATIONAL D' ESSAIS

750 15 PARIS - FRANCE

The laser flash technique was used to determine the thermal diffusivity of

germanium, gallium arsenide and cadmium telluride over the temperature range 80 K- 1000 K. The main features of the experimental equipment are described. Values of thermal

diffusivity (8O-9OO K) are reported.

Key words: Cadmium telluride; gallium arsenide; germanium; laser flash method;

laser windows; thermal diffusivity.

1 . INTRODUCTION

The extensive studies relative to laser induced damage in optical materials

carried out by a number of laboratories have shown the importance of several physical

parameters such as thermal diffusivity, thermal conductivity, optical absorption

coefficient, thermal expansion coefficient, critical rupture stress and Young's

modulus

.

In application of a wide research program achieved under contract and

involving 10 French laboratories, an experimental equipment was developed at the

Laboratoire National d'Essais in order to measure the thermal diffusivity of semiconducting compounds.

This property is defined as a = , where

" c p

. . 2-1

a IS the thermal diffusivity, cm s

k the thermal conductivity, Wcm ^ K

^

c the specific heat, J g

'' K

^

-3

and p the density, g cm •

Germanium, gallium arsenide* and cadmium telluride samples were studied

and the results obtained were compared with available data from the Thermophysical

Properties Research Center (TPRC) as well as with previously reported data.

-98-
2. THEORETICAL

In the flash method first reported ty Parker, et al., [2]' who used a xenon flash lamp as the source of radiant energy, the front face of a thin diskshaped specimen is uniformly irradiated by a very short pulse and the resultant

temperature history of the rear face is recorded by means of an adequate detector.

The front face is previously blackened in order to absorb the energy

deposited by the beam. The thermal diffusivity of the sample is determined from

the study of the temperature- versus- time curve, according to the boundary conditions.

The general mathematical formula used to determine thermal diffusivity is:

l2

a = u where

^1/2

1 is the thickness of the specimen>

and ^1/2 time required for the back face of the sample to

reach one-half the maximum temperature rise.

In case of negligible heat losses at the front and rear faces and if we consider the laser pulse time as a Dirac function, the solution of the fundamental

heat transfer equation may be written as follows [1]:

T (l,t) = -S- [1+2 l(-^fe -"^"^'^t/l^

] where

n=1

T (l,t) is the instantaneous back surface temperature rise at time t

,

Q the surface density of deposited energy,

c the specific heat,

p the density,

a the thermal diffusivity

and n successive integers.

With the previously mentioned assumptions , the value for to derived from

this relation is 0.1388.

Since the shape of the temperature response curve being altered by heat losses,

several mathematical models have been developed to take them into account.

The models from 'Cape and Lehman [3], Cowan [4,5], and Koyama and Larson [6] were applied to the studied materials, and a model including linear heat losses

and the pulse duration corresponding to the neodynlum glass laser used was developed.

The resultant solutions were handled into a Hewlett-Packard HP 30 computer.

1. Figures in brackets indicate the literature references at the end of this paper.

-99-
3, EXPERIMENTAL

Thermal diffusivity measurements were achieved by means of the laser

heat-pulse apparatus shown in figures 1 and 2.

The experimental assembly is constituted by the 3 following main parts

:

the laser source, the temperature regulated furnace, and the detection device.

- A neodynium glass laser operating in a conventional mode without Q-switching at

1 . 06 ym was used to heat the sample.

The size of the laser rod was: 23mm in diameter aod-'SISmm in lengthThe duration of the laser pulses could be varied from O.5 to 1.3 ms and the energy

delivered by the beam from 5 to 18O Joules.

For some useful energetical levels, the beam uniformity was tested by

means of a special paper.

The time distribution of the laser beam could be represented by the

equation used by Koyama and Larson and the peak-time was estimated, in most cases,

to be 190 us.

The radiajit energy delivered by the laser rod was monitored by meajis

of a dual cone coated with a graphite layer and equipped with a thermopile.

- The main part of the regulated enclosure is a cylindrical inconel furnace ( lUsOmm

in length and 77/88 in inner/outer diameter). This furnace is thermally insulated

with a refractory material.

For operation above 300 K , the tube furnace was electrically heated so as to obtain a uniform temperature in the central zone over 250 mm approximately.

The furnace temperature profile was determined by means of 13 chromel-alumel

thermocouples connected to an automatic potentiometer.

The ends of the enclosure, as well as the outer jacket, were cooled by a water flow.

The measurements were made in vacuo up to 1000 K.

For operation below 300 K the apparatus was equipped with copper constantan

( 0.2 mm in diameter) thermocouples.

The cooling of the enclosure was obtained by a slow longitudinal liquid nitrogen

flow, the rate of which could be adjusted by means of a valve lying between the

diffusimeter and the liquid nitrogen container.

The temperature of the central zone was made uniform by an additional electrical

heating. Nitrogen was drained out of the enclosure through an aperture located

close to the window. In order to avoid freezing effect, the window was fitted with

heating coils.

-100-
- The sample is located at the core of a stainless steel ring by use of 3 alumina

tubular pins.

This ring lies on a cradle rigidly locked with a movable tube constituting

a quite independant assembly.

Radiation shields made of nickel-chromium reflecting disks are located

at the ends of the inner cylinder.

Before each laser shot , the front face of the sample was coated with a very thin colloidal graphite layer absorbing the larger part of the laser radiant

energy

.

This coating was achieved by care of reproducibility on metallic

samples considered as reference materials ( Armco iron and OFHC Copper). For the studies'

performed on semiconductors , this thin layer was spread in order to avoid any transmission of the optical flux through the sample.

- Over the temperature range 300 K - 1000 K, where commercially available optical

pyrometers lack sensitivity and time response requirements, chromel alximel thermocouples

(0.2 mm in diameter) were used to measure the temperature rise of the back face of the

sample as also the initial steady temperature.

The chamfered thermocouple wires were separated 1 to 2 millimeters in order

to avoid temperature response errors and pressed against the rear face of the sample.

The output signal of the detector was connected to a high gain differential

amplifier and then recorded on a 5103 N Tektronix storage oscilloscope equipped with

a C-5 Polaroid camera.

h. RESULTS

Calibration of the system was carried out with Armco iron and OFHC copper

saniples

.

The average values of the thermal diffusivity measurements results were

coii5)ared with published and recommended values.

These materials are generally considered by most experimentalists as reference materials for thermophyslcal properties comparisons.

U.I. Armco iron

Measurements on Armco iron were achieved from 80 K to 8U0 K and the data obtained

(table n'l) were compared with the values from Degiovanni, Morrison, et al., [7],

Rawuka and Gaz [8], Shanks [9], as well as with TPRC data [10],

The overall estimated errors on the experimental results are less

than + 8% for all the tested materials.

A plot of the thermal diffusivity of Armco iron versus temperature is shown in figure n°3.

Micrographs of the studied samples were performed.

-101-
k,2. OFHC Copper

The thermal diffusivity of OFHC copper samples was determined over the

temperature range 80 K to 750 K. Data obtained are reported in table n°2 and a plot

of the thermal di'ffusivity versus temperature is presented In figure n°A.

Comparisons were principally performed with the data reported by Butler and Inn [11],

Adam» and Wyman [12], as well as with the TPRC values.

.3 • Germanium

The tested samples were cut from an n-type germanium ingot exhibiting an electrical resistivity gradually decreasing from 33 ^cm at one end to 15 ficm at the

other end. The electrical resistivity of the monocrystalline samples was measured to be

20 ficm at 300 K. The crystalline plane was (1.0.0). The disks were machined with a precision of + 100 pm and buffed on both siarfaces.

A qualitative analysis of the impurities included in the various samples was performed by UV emission spectroscopy.

Arranging them in the decreasing Intensity order, the detected elements were:

- Magnesium, silicon

- Aluminum, titanium

- Calcium, copper, iron (this latter element being hardly detectable)

It is to be mentioned that the detection intensity magnitudes may not

coincide, in all cases, with concentration magnitudes.

The results of the thermal diffusivity measurements are shown in table n° 3 and a plot of these values versus temperature is given in figure n°5.

Comparison was achieved with the data from Abeles, et al., [13], Meddins and

Parrott [14], and with TPRC recommended values.

h'k. Gallium arsenide

7 The electrical resistivity of the tested GaAs samples was above 10 ficm at 300 K and their crystalline plane was (1.1.1.). As for germanium, GaAs specimens were spectroscopically analyzed. The main impurities showed off were:

- Magnesium, palladium

- Gallium, silicon

- Calciiim, copper, titanium

- Aluminum, silver, iron, and germanium ( these k elements being hardly detectable)

The results obtained from 80 K to 85O K are shown on table n° h and

plotted vers\is temperature in figure n'S.

Comparison was achieved with data from Timberlake and Davis [15] from 300 K to 850 K and our measured values closely agree with these later ones.

U. 5 • Cadmium telluride

Measurements were achieved on several polycrystalline undoped samples.

The studied samples contained the following impurities:

- Aluminum, silver, magnesium, silicon

- Germanium

~ Calcium J copper, titanium

- Iron ( in very low traces) . . „ , , , The data from our measurements are listed m table n 5 and plotted versus temperature in figure n°7.

-102-
5. CONCLUSIONS

The laser flash method was used to determine the thermal diffusivity of

germanium, gallium arsenide, and cadmium telluride over the temperature range 80 K-9OO K. A device was designed and developed at the Laboratoire National d'Essais (France)

in order to perform these measurements. Calibration of this system was carried out

with Armco iron and OFHC copper specimens over the temperature range investigated

and the meas\ired values were compared with reported data. The estimated errors on the

experimental results are less than + &% for the studied materials.

Improvements of the data acquisition system are expected which will induce

a significant gain in sensitivity.

The investigation of the laser beam uniformity is planned in order to estimate

errors in thermal diffusivity arising from a deviation from ideal spatial uniformity

of the heating pulse.

6 . ACKNOWLEDGMENTS

The authors wish to thank the Direction des Recherches et Moyens d'Essais

du Ministere de la Defense which supported our work and gave its authorization for

the presentation of this paper.

7. REF

[1] Carslaw, H. S. and J. C, Jaeger, "Conduction

of heat in solids," 2nd Ed. (Clarendon Press,

Oxford, 1959).

[2] Parker, W. J., R. J. Jenkins, C. P. Butler,

and G. L. Abbott, J. Appl. Phys. 32, 1679-84

(1961).

[3] Cape, J. A. and G. W, Lehman, J. Appl. Phys.

34, 1909-13 (1963).

[4] Cowan, R. D., J. Appl. Phys. 32, 1363-70 (1961).

[5] Cowan, R. D., J. Appl. Phys. 3±, 926-7 (1963).

[6] Koyama, K. and K. B. Larson, J. Appl. Phys. 38 , 465-74 (1967).

[7] Morrison, B. H. , D. J. Klein, and L. R. Cowder,

(Proc. 6th Conf . on Thermal Conductivity,

Oct. 19-21) 673-99 (1966).

[8] Rawuka, A. C. and R. A. Gaz, (Proc. 6th Conf.

on Temp, Measurements Society) 55-67 (1969).

[9] Shanks, H. R., A. H. Klein, and G.C. Danielson,

J. Appl. Phys. 38, no. 7, 2885-92 (1967).

[10] Touloukian, Y. S. ed.. Handbook of the Thermophyslcal Properties of Matter, _10 (Plenum,

New York 1973).

[11] Butler, C. P. and E. C. Y. Inn, USNRDL TR 177,

1-27 (1957).

[12] Adams, C. H. and M. E. Wyman, J. Appl. Phys.

40, no. 1, 344-50 (1969).

[13] Abeles, B. et al.. Bi-monthly Prog. Rep.,

RCA Lab., Princeton, N. J., no's. 1 and 2j

1-36 (1959).

[14] Meddlns, H. R. and J. E. Parrott, Brit. J. Appl. Phys. 2, no. 5, 691-97 (1969).

[15] Timberlake, A. B., P. W. Davis, and

T. S. Shllllday, Advanced Energy Conversion 2,

45-51 (1962).

-103-
8. TABLES and FIGURES

Table 1. Thermal diffuslvity

data for Armco iron.

1 2 -^s

0. \ cm s }

100 0.550

150 0.339

200 0.255

250 0.219

300 0. 195

350 O.iTT

hOO 0. 162

U50 0. 11+9

500 0. 136

550 0. 122

600 0.110

650 0. 100

700 0.091

750 0.083

800 0.076

Table 2. Thermal diffuslvity

data for OFHC copper.

T (K)

, 2 a \ cm

100 2.02

150 1.1+8

200 1 .29

250 1 .20

300 l.lU

350 1 . 10

Uoo 1 .07

U50 1.05

500 1 .02

550 1 .00

600 0.99

650 0.98

700 0.96

750 0.95

800 0.9k

Table 3. Thermal diffuslvity

data for germanium.

T (K) 1 2 -K a (, cm s )

100 1 .22

150 0.720

200 0.500

250 0.1+00

300 O.3I+O

350 0.300

i^OO 0.270

U50 0.230

500 0.210

550 0. 180

600 0. 160

650 0. 1U0

700 0. 120

750 0. 1 10

800 0. 106

850 0. 103

900 0. 100

950 0.098

1000 0.093

Table 4, Thermal diffuslvity

data for gallium

arsenide.

T (K) 1 2 -Is

a i cm s J

100 1 .350

150 0.1+85

200 O.3I+O

250 0.270

300 0.225

350 0.200

1+00 0. 170

1+50 0. 11+5

500 0. 122

550 0. 103

600 0.085

650 0.075

700 0.066

750 0.057

800 0.050

Table 5, Thermal diffuslvity

data for cadmium

telluride.

T (K) 1 2 -Ixa (cm s )

100 O.2I+O

150 0. 160

200 0.1 10

250 0.071+

300 O.OI+6

350 0.033

1+00 0.025

1+50 0.022

500 0.020

550 0.018

600 0.016

650 0.015

700 O.Ollt
Figure 1, Schematic of the apparatus designed for thermal diffusivity measurements

above room temperature,

1, Laser head

2, Laser power supply

3, Cooling unit

A, Laser trigger

5, Furnace

6, Specimen

7. Electrical power supply

8. Vacuum system

9. Temperature readout

10. High gain differential amplifier

11. Storage oscilloscope

12. Camera record

Figure 2. Schematic of the apparatus designed for thermal diffusivity measurements

below room temperature.

From 1 to 12 (see captions of figure 1)

13. Liquid nitrogen container

lA. Aperture for the exit of nitrogen

-105-
2 2 -1

0.5

0.4

0.3

0.2

0.1

* —

•

•

-T

I 100 200 300 400 500 600 700 800 T(K)

Figure 3. Thermal diffusivity of Armco iron.

Crosses represent the TPRC data.

2.-1)

1.5 -

1.0

0.5

•

in.

2«.

"»—+<. 4*

0 300 600 900 T(K)

Figure 5. Thermal diffusivity of n-type germanium.

Crosses represent the TPRC data.

/ 2 -1,

a(cm s )

2.5

2.0

1.5

1 .0

0.5

-I

\

•\—

0 100 200 300 400 500 600 700 T(K)

Figure 4. Thermal diffusivity of OFHC copper.

Crosses represent the TPRC data.

I 2 -1> a(cm s )

I

1 .0

0.5

100 200 300 400 500 600 700 T(K)

Figure 6. Thermal diffusivity of gallium

arsenide. Comparisons were achieved

with the results of Timberlake, et al.

0 100 200 300 400 500 600 T(K)

Figure 7. Thermal diffusivity of cadmium

telluride.

NO COMMEMT ON PAPER VOUSSKIU and UBOdO

-106-
3.2 Damage to 10. 6|jm Window Materials Due to CO TEA Laser Pulses''

K. M. Leung, M. Bass, and A. G. J. Balbtn- Villaverde f

Center for Laser Studies

University of Southern California

Los Angeles, California 90007

Laser irradiation Induced damage to several materials of Interest for use as 10.6 (jm laser system windows was studied. A pulsed CO2 TEA laser, operating In the TEMqq mode was the irradiation source in these experiments. The light was

focused onto the surfaces or into the bulk of the samples and the waveform of the

transmitted pulse was monitored. Comparison of the Incident and transmitted laser

pulse waveforms shows the onset of laser Induced damage as a distortion of the latter.

Damage threshold data and a discussion of possible damage mechanisms for improved

ZnSe, commercial and RAP grown KCl,and commercial NaCl are presented.

Key words: KCl; NaCl; pulsed CO2 TEA laser damage; transmitted pulse; waveform

distortion; ZnSe.

1. Introduction

One of the commonly used diagnostic techniques for monitoring laser-Induced damage Is to ex- amine the temporal shapes of laser pulses that are transmitted through transparent dielectrics.

When damage occurs on sample surfaces or In bulk materials, the transmitted pulse Is attenuated In amplitude as well as distorted In waveform. In recent years Bass and Fradln [1]^ have demonstrated that when damage occurs, the transmitted ruby laser pulse Is attenuated in a manner that is characteristic of the source of damage. They further showed that pulses which are attenuated very

rapidly form damage regions In the bulk which are characteristic of the Intrinsic mechanism such as an electron-avalanche breakdown. Other damaging pulses are attenuated in a very different manner

and produce damage regions that appear to result from Inclusion absorption. Last year at this

symposium, Milam, et al., [2] described a new experimental technique for determining the cause of

la ser - Induced damage. The new procedure was based upon this statistical nature of the times re- quired to produce damage at many sites which were Irradiated by equally-intense, square-waveform

pulses. This permits identification of the damage mechanisms without requiring a threshold measurement. In the Ir region Yablonovltch [3] observed similar statistical properties for damage due

to CO2 TEA laser pulses passing through alkall-hallde crystals. However, the experimental observation was complicated by several longitudinal modes which contributed a time structure to the laser

pulse.

At use, a setup based on Milam's technique [2] to extract square -waveform pulses from the

most Intense portion of a CO2 TEA laser pulse is under construction. This will make possible measurement of 10. 6|jm survival curves In different Ir materials. To date, however, we have employed

the recent development of fast detection electronics to examine simultaneously both the transmitted

and Incident pulses of a CO2 TEA laser. In this paper we describe the technique and discuss possible

damage mechanisms for 10. 6 |jm window materials. The experimental results will be divided Into

' This work was supported by Defense Advanced Research Projects Agency and was monitored by Air

Force Cambridge Research Laboratories.

i" Present address: Instltuto de Flslca, Universidade Estadual de Campinas, Campinas, S. P. , Brazil

^ Figures in brackets indicate the literature references at the end of this paper.

-107-
two sections, one based on the transmitted pulse waveform analysis and the other on threshold measurements for surface and bulk damage on ten samples of three different ir window materials. Some

damage morphology on typical samples will be discussed.

2. Experimental

The experimental setup for conducting pulsed damage experiments is shown in figure 1. The basic apparatus consists of a pulsed CO^ TEA laser, a beam attenuator, a test vacuum chamber,

and an energy/waveform monitoring system. Three measurements are provided by using this setup:

the incident energy at the sample, the spatial profile of the laser beam, and the waveforms of the

incident and transmitted pulses. Details of individual components as well as test procedure are discussed below.

2.1. TEA Laser Pulses

The performance and characteristics of the TEA laser used in this work are described in table 1.

Table 1. TEA laser parameters and performance data

Discharge length (double Rogowski electrodes)

Energy storage capacitance

Flow rate ratio (He:N2:C02)

Mirrors:

Brewster windows

Intracavity aperture diameter for TEMqq mode

Cavity length

Typical input energy

Typical output energy (9 mm aperture) for TEMqo mode

Typical width for gain- switched pulse (FWHM)

41 cm

0. 08 ^f

8. 7:2. 4:2. 6 jj/min

100%R - Flat/Si

80%R - 10 meter /Ge

KCl

6 to 9 mm

120 cm

21 J

100 mJ

92 ns

The spatial profile of the laser beam was measured by a pinhole scan detector. Figure 2 shows that

the beam profile was a Gaussian distribution. When a 9 mm intracavity aperture was placed 15 cm

from the flat 100% reflecting mirror, the spot-size (l/e2 rad ius of the intensity profile) at the focusing lens was measured to be 2. 9 mm and 2.4 mm for a 6 mm intracavity aperture. The output pulse

of the laser consisted of a 92 nsec gain- switched pulse followed by a long tail of ~ 1. 6 |j.sec duration.

Several longitudinal modes were able to oscillate and so the pulse waveform showed structure similar

to that observed by Yablonovitch [3]. The self-locked output waveform was caused by the fact that the

absorption of CO2 molecules In the laslng medium can saturate [4]. The mode-locked waveform was

composed of pulses having < 2. 5 nsec duration and separated by 8 nsec, the cavity round-trip time.

The fraction of the total energy in the ga in- switched pulse was determined by measuring the areas

under the gain- switched pulse and the long tail. These areas were equal, showing that half the total

energy was in the gain-switched pulse. The total incident energy was measured with a thermopile

(Hadron Model 100) calorimeter having an accuracy of ±5%.

2. 2. Experimental Procedures

The energy of the laser beam incident on the sample was selected by a beam attenuator consisting of Brewster angle ZnSe polarizers which provide extinction ratio of 1:16, A discussion of this

device is given in Appendix A. Each laser shot was either focused onto the surface or into the bulk

of a test specimen using a 3. 8 cm focal length AR coated Ge lens. The focal position was determined by varying the lens to sample separation and observing burn spots on a piece of black photographic film (unexposed Poloroid Type 410) attached to the surface of the sample. The focal spotsize of the Gaussian mode beam Wq was calculated by the following equation:

- 4X f. 1. 2w = — X —

0 n 2w

-108-
where w is the measured laser spot size on the focusing lens. The calculated wq was in excellent

agreement with the measured radius of many surface damage sites on ZnSe. All samples were

studied in a plexiglas chamber evacuated by a mechanical pump to eliminate spurious breakdown due

to dust or gas molecules near the surface of a sample. Onset of damage was monitored by (1) observing the visible plasma and changing appearance of the sample from the focal region, (2) examining the damaged region under the microscope, and (3) comparison of the incident and transmitted

laser pulse waveforms. In this work, the waveform diagnostic technique was emphasized and found

to be useful in understanding the damage process. A ZnSe beamsplitter located in front of the beam

attenuator was used to direct a portion of the incident beam onto a photon drag detector fPD 1). The

transmitted beam was re- coUimated by a ZnSe lens and monitored by another photon drag detector

(PD 2). Both detectors have similar response and rise time of 500 ps. Signals corresponding to Incident and transmitted pulses were simultaneously displayed on upper and lower traces of a highspeed (400 MHz) dual-beam oscilloscope (Tektronix Model 7844), respectively. Typical traces are shown in upper corner of figure 3. For convenience, the transmitted pulse waveform was inverted

and when no damage was produced the waveforms were registered with respect to each other in time.

For this work, only single shot on single site testing is reported. ^

3. Waveforms Analysis and Possible Damage Mechanisms

3. 1. Comparison of Incident and Transmitted Pulses

Since the high-speed detector dual-beam scope combination can provide a time resolution of

2. 5 nsec, it Is possible to examine waveform distortion on this time scale and to measure the survival time of each damage site. For this reason, the analysis is based upon the comparison of incident and transmitted mode-locked pulses in the gain-switched pulse when no damage occurs during

the experiment. There are typically 23 mode-locked pulses in a recorded signal. The transmission

of Individual mode-locked pulses Is normalized to that at t = 0. The normalized transmission of the

l*-*^ pulse Is defined as

JAt = t ) T. s —!— for I = 0, 1, 2, . . . -^0

and

I , (t.)

i I. (t.) m I

where 1-^^^ and 1^^^. are the amplitudes of individual mode-locked pulses at time t = t-. For Illustration, the normalized transmission of a pulse which did not cause damage is plotted against time in figure 3. The solid line gives a good Indication that no distortion of the transmitted waveform oc- curred (all Tj^ are very close to unity). The dashed curve shown on the same plot gives the normalized waveform of the gain- switched pulse. In general, waveform distortion was studied over a range

of incident laser power from below threshold to 4 times threshold.

Since laser Induced gas breakdown Is thought to be caused by electron avalanche breakdown, we

have examined the waveform distortion due to air breakdown pulses at 22 cm Hg pressure. The

transmitted waveform distortion is depicted in figure 4. It is easy to see that air is completely

broken down within one pulse at = 72 nsec. We shall refer to the time when damage occurs, t^j, as

the survival time.

3. 2. Surface Damage on IR Window Materials

2 For pulsed laser damage experiments, three types of threshold measurement have been practiced

by different workers. They are single shot on single site (one to one), multiple shots with variable

incident Intensity on single site (n to l)>and multiple shots with constant Incident intensity on single

site (n on 1). -109-
Seven surfaces of three ir window materials fZnSe, KCl, and NaCl) were studied in these laser

damage experiments. The waveform distortions for surface damage on NaCl, KCl, and ZnSe.as well

as air breakdown, are shown in figure 5. To show the distortion more clearly three transmission

curves are plotted in figure 6 for surface damage on three ir window materials at threshold. All

samples survived until ~ 50 nsec and the complete attenuation took place within one to two pulses,

except for KCl. If the incident power was raised above threshold, the survival time was reduced

and transmission cut-off occurred in the second or third mode-locked pulse. A blue-white plasma

was observed at or above threshold in all cases. The waveform distortion analysis for surface damage suggests that the damage mechanism is very similar to air breakdown.

3. 3 Bulk Damage in IR Window Materials

Typical traces for bulk damage in NaCl, KCl, and ZnSe both at threshold and above threshold are shown in figure 7. For commercially fabricated NaCl, sharp cutoff at threshold, as well as above threshold,wer e characteristics of the transmitted pulse waveform distortion. In general,

both ZnSe and KCl show very slow attenuation. For ZnSe, this slow cutoff was expected since inclusion absorption is the dominant cause of material failure. However, in the case of RAP grown KCl

and commercial KCl, the observed slow cutoff suggests that very small inclusions in the bulk still play a dominant role in the damage process. In all cases, if no spark was observed for a particular

site, waveform distortion could not be detected either. The spark in NaCl was yellow as expected

for a sodium spark. Bulk damage in ZnSe was always accompanied by a yellow spark inside the

medium. The observed color of this spark is due to the fact that it is surrounded by a medium which

absorbs the blue and green parts of the spectrum.

For a more detailed view of damaging waveform distortion figures 8, 9, and 10 display transmission curves for NaCl, KCl, and ZnSe, respectively. The results of waveform distortion analysis

for bulk damage in these materials is summarized in table 2. In addition, figure 11 gives the transmission versus gain-switched pulse energy for the case of bulk damage in commercial NaCl,

RAP grown KCl, and old ZnSe as well as air breakdown. It implies that damage in these ir materials occurs when nearly half the energy contained in the gain-switched pulse has passed or just after the

peak of the pulse.

Table 2. Bulk damage characteristics in ir window materials due to CO^ TEA laser pulses

^ , ,• ^ ,. . Time for

Color of Condition „ . , ^. a t-far,,,o(-;r>„ i Survival Time Attenuation Possible Damage

„

Spark ^ . „

of Site

t , at threshold to Reach. its Mechanism u TM

Formation Fracture d Maximum

IR Materials

NaCl (commercial) yellow severe and

large

56 50 nsec intrinsic breakdown

KCl (commercial)

KCl (RAP)

blue-pink small 24 nsec 136 nsec inclusion absorption

80 nsec > 100 nsec

ZnSe (Raytheon 1974) yellow small and 56 nsec

in the bulk

ZnSe (Raytheon 1975) 40 nsec

30 nsec inclusion absorption

> 100 nsec

4. Measured Damage Threshold in IR Window Materials

Table 3 summarizes our measurements of surface and bulk damage thresholds in three ir window materials, ZnSe, KCl, and NaCl. In this paper we report the axial peak power density

Po(MW/cm^) which is defined

P =2XP, =2 (-^^"2 J ^otal

0 Ave rr At

-110-
Here At is the FWHM for the gain- switched pulse of the TEA laser, ^^^otal

'^^ '^^^ energy density in J/cm^ deposited at the focal region of the sample during the gain-switched pulse. We emphasize again that the test results are reported for a single shot on a single site in contrast to other

reports [5]. The damage threshold is defined as the lowest power density which damages 50% of the

sites studied. The numbers in parenthesis in table 3 are the power densities which cause damage to

1 0% of the sites.

Table 3, Measured damage threshold on ir window materials due to CO TEA laser pulses

Sample Material Absorption^

^total'"TM '

Surface Finishing

Method

Axial Peak Power Density

Pq (MW/cm2)

On Surface In Bulk

ZnSe (Raytheon 1974)

ZnSe (Raytheon 1975)

ZnSe (Raytheon 1975)

- - — ,

..

- .,

0. 005

0. 0005

0. 0005

conventional

diamond -poll shed

super -polished

270

350 (270)

350 (270)

230

460 (350)

460 (350)

KCl (HRL-RAP)

KCl (HRL-RAP)

KCl (Harshaw Ecpnoflat)

KCl (Harshaw)

KCl (Harshaw Comm.)

0. 00014

0. 00026

0. 001

0. 0008

0. 0023

conventional

conventional

460 (350)

560 (165)

4600

1500 (760)

650 (150)

< 190

< 190

NaCl (Harshaw Econoflat)

NaCl (Harshaw Spectro. )

0. 004

0. 004

conventional

conventional

1600 (1200)

840

6100 (5600)

3900

^Values are provided by respective suppliers to be considered for reference only.

4. I4 Damage in ZnSe

We have performed 10.6 (jm damage experiments on three different surfaces of ZnSe crystals and

in two different bulk materials. Raytheon Company furnished samples of its 1974 ZnSe material and

its improved 1975 ZnSe. It should be noted that the improved ZnSe has much lower absorption than

the 1974 material.

As seen in table 3, no significant difference was found in damage threshold for different surface

finishing methods. Figure 12 shows the super-polished and diamond-polished surfaces obtained on improved ZnSe crystals for these experiments. The conventional surface on an old ZnSe crystal

appeared to be very similar to the diamond-polished surface.

We have also investigated both visible and 10.6 [am scattering using the Optical Functional Tester

at NWC described last year [6]. Bulk scattering was dominant in both old and improved ZnSe suggesting that inclusions in ZnSe may still be the major determinant of the 10.6 (jm damage threshold. However, the improved ZnSe was the first sample of this material which could be damaged on the surface

and not in the bulk. The morphology of the damage sites in ZnSe (see figure 13) further supports the

fact that inclusions near the surface or inside the bulk play an important role in the damage mechanism.

4. 2 Damage in KCl and NaCl

Since conventionally polished KCl surfaces are easily contaminated even under normal laboratory

environments, only two surfaces were used for the experiment. Our interests in alkali halides are in

understanding the damage mechanism,but not in the threshold comparison. Therefore, one should not

be too serious about the comparison between KCl and NaCl listed in table 3. In general, conventionally grown NaCl crystals still have higher threshold values. The RAP grown KCl crystals are superior

to conventionally grown KCl by an order of magnitude in damage threshold. Damage morphology on

these crystal surfaces is quite similar and in general the damage site is larger than the laser beam

cross section. Figures 14 and 15 show such morphology for KCl and NaCl surfaces. In bulk damage,

-111-
NaCl shows larger fracture along cleavage planes than KCl,

5. Summary and Conclusions

With a high-speed detector /dual-beam oscilloscope detection system we have begun to observe

the dynamics of 10.6 |jm laser induced damage In Ir window materials. Comparison of the Incident

and transmitted laser pulse waveforms shows the onset of laser Induced damage as a distortion of the

latter. Survival time of surface and bulk damage for different materials can be accurately determined using this technique.

With the samples confined In an evacuated chamber, surface damage of NaCl, KCl, and ZnSe was

similar to air breakdown, showing a sharp cutoff In the transmitted pulse waveform. Surface con- taminants may cause early breakdown and generate a plasma similar to that In a gas breakdown. For

bulk damage In old and Improved ZnSe, the distortion of the transmitted pulse waveform suggests that

two different kinds of Inclusions are Involved In the damage process. Measured threshold values of

these ZnSe samples suggested that the Improved material Is Indeed out-performing the old one. Damaging pulse waveform distortion for HR L RAP grown KCl Is very slow and suggests that Inclusions

still play an Important role In limiting the damage resistance of this material. The waveform distortion observed for Harshaw NaCl Is fast and resembles that due to Intrinsic breakdown. Measured

threshold values of these alkali halldes suggest that the breakdown field of NaCl may be greater than

that of KCl In contrast to the result of "n on 1" experiments reported last year [7]. This conclusion,

however, depends upon which samples are compared and so reports of relative breakdown fields must

be cautiously evaluated.

In conclusion, we feel that the current results still do not provide a quantitative understanding of

damage to 10.6 |jm window materials due to CO2 TEA laser pulses. A better statistical technique [2]

will be applied at this wavelength In the coming year.

We further conclude that since different damage threshold values for a given material can be

easily found In the literature, It Is essential to exchange test specimens or work jointly on a particular

experiment using different diagnostic techniques and equipment. To this end, USC and HR L have Initiated a joint experiment to perform damage measurements on RAP grown KCl crystals and other

samples.

6. Acknowledgments

The authors are grateful to Raytheon Company for providing the ZnSe samples, Hughes Research

Lab. at Mallbu, CA, for the RAP grown KCl samples, and Mr, Walter Spawr for his effort in polishing

ZnSe for this work. The authors also thank many Individuals at NWC, China Lake, CA , for their

technical assistance In scattering experiments and Nomarskl microscopy.

At the University of Southern California, we thank C. C. Tang and M. Chang for their assistance

In data acquisition, and Professor J. Ward who has provided many fruitful discussions is especially

acknowledged.

7. Appendix A. 10. 6 \j,m Attenuator

For laser-Induced damage experiments, It Is desirable to have a variable attenuator which will

control the Incident laser energy continuously without affecting the duration, the polar Ization, and the

spatial distribution of the laser pulse. In the visible and near ir region, the combination of one rotatable and one fixed Clan polarizer was used as a variable attenuator [8]. For 10.6|am radiation, a pair of wire-grid polarizers on ZnSe substrates can provide two orders of magnitude attenuation [7].

However, these polarizers are very costly and are limited by low resistance to laser damage. In this

work, we have prepared two relatively inexpensive ZnSe reflection polarizers as a beam attenuator.

A detailed discussion of how this attenuator works is presented below.

It Is well known that when a plane wave Is Incident upon a dielectric surface, the reflection coefficients can be described by Fresnel's equations. Let cp be the angle of Incidence, r the angle of

-112-
refraction, and n the refractive index. Then the reflection coefficient for linearly polarized incident

light with electric vibration perpendicular and parallel to the plane of incidence can be expressed as,

^ sin^ (CP - r)

__ tan^ (cp - r)^ sin (cp + r) tan (cp + r)

respectively. If the angle of incidence is Brewster's angle, cp = arc tan(n)and Rii is zero. For ZnSe,

cp^ = arctan(2. 40)at ]0.6um andR

B ± A typical arrangement of reflection polarizers employs two oppositely sloping sets of plates

mounted at Brewster's angle. Let the fixed polarizer be oriented to transmit the laser polarization

and = 0° be the angle of the rotating polarizer giving the maximum transmission through the combination attenuator. Then the transmitted laser intensity, lout' ^'^V rotation angle Q about the beam

axis can be determined by means of Jones calculas as follows:

cos 9 -singi/l 0

sin 9 cos 9/ \0 p

,

cos 9 s in 9 \ / 1

-sin 9 cos 9/ \ 0

2. ^ . 2 a

cos e + sm 9p cos 9 s in 9 ( 1 - p^)p^ ^

and

out

(cos^e + sin^9p^)^ + [cos 9 sin 9(1- p°^)p^]^

Here, p is related to the principal transmittance: = 1 - - p a is the number of ZnSe surfaces in the rotatable polarizer and g the number in the fixed polarizer. For our case, p^ = f, a = 4,

3=2 and the normalized transmitted laser intensity is

I J9) out 4„ ^ 1 . 4„ ^ 41 2. . 2, cos 9 + TT sin 9 + tt cos 9 sm

16 64

This expression agrees with the measured calibration intensity as a function of rotation angle about

the beam axis. The extinction ratio of our attenuator is 1:16.

One final remark on the design of this attenuator is to avoid the use of plates with parallel faces.

A scheme proposed by Bird and Schurcliff [9] can be considered by readers for obtaining better performance polarizers. They employed two oppositely sloping groups of plates that were each slightly

wedged by angle about 1°. In addition, each group of plates was also "fanned" slightly, As a result, no reflected rays would follow the optical path of the main transmitted beam and produce unwanted

interferences.

8. References

[1] Bass, M. and Fradin, D. W. , IEEE J. Quantum Electron. QE-9, 890 (1973).

[2] Milam, D. , Bradbury, R.A., Picard, R.H.,

and Bass, M. , in Proc. 6thONR/ASTM/

NBS Symposium on Laser Induced Damage

in Optical Materials: 1974 , NBS Special

Pub. 414, 169 (1974).

[3] Yablonovitch, E. , Appl. Phys. Lett. 1_9

,

495 (1971).

[4] Gilbert, J. and Lachambre, J. L. , Appl.

Phys. Lett. ]J_. 187 (1971).

[5] For example, see Wang, V. and Giuliano, C.

R. , "Measurements of Damage at 10. 6 ^m

in Three Alkali Halides over a Range of

Pulse Durations, " this Proceedings.

[6] Soileau, M. J. and Bennett, H. E. , in Proc.

6th ONR/ASTM/NBS Symposium on Laser

Induced Damage in Optical Materials: 1974 , NBS Special Pub. 414, 149 (1974).

[7] Allen, S. D. , Braunstein, M. , Giuliano, C,

and Wang, V. , in Proc. 6th ONR/ASTM/

NBS Symposium on Laser Induced Damage in

Optical Materials: 1974, NBS Special Pub.

414, 66 (1974).

-113-
[8] Bass, M. and Barrett, H. H. , IEEE J. Quantum Electron. QE-8, 338 (1971).

[9] Bird, G.R. and Shur cliff, W.A., J. Opt.

Soc. Am. 49, 235 (1959).

9. Figures

CO, TEA LASER

7ft44

PDl

Figure 1. Schematic of the damage experiment.

Both PD 1 and PD 2 are Ge photon

drag detectors for monitoring the

incident and transmitted pulses.

5.0

z

Si

4 BEAM

S 6 Id KAPIUS SQllAttD (mm)'

Figure 2. Pinhole scan of the CO2 laser

beam.

laser pulse. Dashed curve Tn^^t (ntic) represents the laser pulse. At

Figure 4. Transmission as a function of

time for air breakdown.

-114-
Mr BR£AKPOk/»l

5URF4CE VMAGL

Had

KCl

ZnSe

SUBfACE D«<A&t

Z»5e

Figure 6. Transmission of mode-locked pulses

as a function of time for surface

damage on NaCl, KCl, and 2nSe.

NoCl BULK DAMAGE

Figure 5. Typical oscilloscope traces for air

breakdown and surface damage on NaCl,

KCl, and ZnSe. The upper beam is

the incident pulse and the lower

beam is the transmitted Dulse.

BULK

At i:UreMJi

DAMA&E.

NaCl

KCl

ZnSe

Figure 7. Typical waveforms for bulk damage

in NaCl, KCl, and ZnSe at damage

threshold md above threshold.

120 \V> l32 M

TIML (n$ec)

Figure 8, Normalized transmission of CO2 TEA

laser pulse as a function of

survival time for NaCl bulk damage.

KCl BULK. DAMAGt

0 "'^ II n n II

665 (m Rip.'u.)

At ft

'

\ 665

\ « ft

TIME. ("MC;

Figure 9. Normalized transmission of CO2 TEA

laser pulses as a function of

_]^]^5_ survival time for KCl bulk damage.
T?.^fh«o« 1975 M Ik

Figure 10. Noraalized transmission of CO2

TEA laser pulses as a function

of survival time for ZnSe

bulk damage.

TK;^rtSNIISSio>l tloEMALIIlP 12) That Whi^J f<0 [Mdicr MAS nistp

VEB5J5 TKACTjol Of e'^IIfl''' Ti<t CA'*I 5»JrTtftttP ?OLSt

V^I4ICI4 HAS PAS^LP TrtSOUGH Bulk Pa.a^> La CMRLKAPj-.-)

4ie 3aAia>ortM

FRACTION Of EHfRCY

Figure 11, Normalized transmission versus fraction of energy contained in

the gain-switched pulse for air

breakdown, and bulk damage in

NaCl, KCl, and ZnSe.

100,

S\>M^ 5uPER-P0UStJl>«l 1?AYWeOl4 : PlAMOMP-POtlSHlfWr

Figure 12. Nomarski micrograph of surfaces

on improved ZnSe prepared by

both super-polishing and diamondpolishing. The spots on the

diamond-polished surface are probably due to inadequate cleaning of the protective Opticoat

coating.

100 Mm

h

^

Figure 13. Damage site at threshold on a super-polishing surface of 1975

ZnSe observed with an optical

microscope.

-116-
KCl HRL RAP grown

Figure 14, Nomarski micrograph showing

a damage site on surface of

RAP grown KCL (B70) at threshold.

Figure 15. Damage site at threshold on a con- ventional surface of Harshaw NaCl

observed under an optical

microscope.

COMMEWT OW PAPER BV LEUNG, U al

Thexz Mcu a bfu.e.{, dUcuMlon thz d^tcuJii 0(5 sample. pK^pa/uxtion aizd in thl!> mtk.

-117-
3.3 Improvements in the Breakdown Threshold in Alkali Halides at 10. 6 |j.m'

V. Wang, C.R. Giuliano, S.D. Allen, andR.C. Pastor

Hughes Research Laboratories

3011 Malibu Canyon Road

Malibu, California 90265

Evidence has been found that the threshold for laser- induced breakdown in the

bulk of single crystals is strongly dependent upon processing. Potassium bromide,

KCl, and NaCl have been examined over a range of pulse widths (from 0. 2 (j.s to 6 fis in

selected cases and for a variety of pulse shapes using both single longitudinal mode and

partially mode-locked multilongitudinal mode pulses. A breakdown threshold of over

12 GW/cm^ (2.5 MV/cm) for KCl far exceeds the best values obtained in previously

available commercial material at these pulse lengths, A comparison of breakdown

threshold for conventional and reactive atmosphere processed (RAP) halides is

made with comment upon the present ambiguities in breakdown mechanism. In addition to the measurement of damage thresholds of materials from different sources,

the temporal profile of transmitted pulses at the time of damage was monitored.

Key words: Alkali halides; bulk damage; longitudinal mode control; pulsed 10,6 ym laser

damage; RAP materials; transmitted pulse cutoff; variable pulse duration.

Introduction

Increasing evidence has linked laser-induced breakdown in pure materials such as the alkali

halides to an avalanche process, which is thought to be similar to the process responsible for dc electrical breakdown. The breakdown threshold has been interpreted as an "intrinsic" property of the

material, dependent primarily upon its band gap. The following evidence may suggest that laserinduced breakdown depends strongly upon material purity and that commercial materials available in

the past are limited by localized effects. Previous evidence of unusually high thresholds has been

found in ultrapure materials [l, 2]\

The purified materials used in this study were prepared by a reactive atmospheric process (RAP)

developed to lower 10. 6 fjim absorption in laser windows by removing water from the growth process

[3,4], The commercial materials are standard low cost spectrophotometer materials that are commonly available. There is evidence from others [5], as well as our own experience, that low absorption laser quality crystals can andusually do tend to perform worse than the common spectrophotometer

grades in laser breakdown measurements, apparently because of inclusions.

A significant improvement in the quality of the following measurements is gained by improved

control of the CO2 TEA. laser output. By control of the pump duration and insertion of a longitudinal

mode selector described elsewhere [6], pulses of different temporal shape and duration can be provided. Figure 1 illustrates the five pulse shapes chosen here. A typical, partially mode-locked multilongitudinal single transverse mode pulse consists of a gain switched spike of about 200 ns duration

(FWHM) followed by a tail of several microseconds duration. The typical TEA laser pvilse used in the

experiments described contains a somewhat greater fraction of energy in its tail than usually seen in

commercial TEIA lasers because of details of the pump system, gas mix, and pressure. Figure 1(b)

shows a single longitudinal mode pulse of the same overall envelope profile as figure 1(a), achieved by

operating the low pressure intracavity longitudinal mode selector slightly below threshold. Operation of the discharge above the cw threshold results in the suppression of the gain- switched spike as seen in fig- ure 1(c), and a more nearly square pulse of 1 \j.s followed by a tail of several fis[7]. Changes in the las- er operating conditions allow selection of a longer 4 (is multilongitudinal mode pulse, as well as a 6 |jis smooth single longitudinal mode pulse (figures 1(d), 1(e), respectively). All pulses used were single

transverse mode, measured and monitored as described in [6]. The lens used was a 1 in. f . 1, neardiffraction-limited f/l germanium doublet from Laser Optics, Inc., with a spot size of 34 (im. Care was

j taken to properly calibrate the on-axis flux density of the system.

Preliminary evidence indicates that the laser may operate on more than one rotational line in the

gain- switched mode. All measurements were made by irradiating a single site with progressively | increasing flux levels until breakdown was reached. A new site is then chosen and irradiation is

repeated starting with a fl\ix well below breakdown.

Work supported by the Defense Advanced Research Projects Agency through Air Force Cambridge

Research Laboratories under Contract No. F19628-75-C-01 35.

1. Figures in brackets indicate the literature references at the end of the paper.

-118-
Peak power quoted in the following section was determined by using the measurement of total

pulse energy together with a monitor of pulse shape and duration using a photon drag detector and

oscilloscope, system rise time is ~1 ns. By using a planimeter to determine the area under a typical

recorded pulse, the peak power can be determined. An "equivalent" pulse length is also cited for a rectangular pulse of height and area equal to the original pulse. For the multilongitudinal pulse with

rapid oscillations a slower detector which averages these rapid oscillations is used. Thus, for these

pulses the instantaneous peak power is about eight times higher than that for the averaged

pulse (figure 1(f)).

Test Results

An effort was made to test three alkali halides grown by two processes (RAP and Kyropoulos,

unless otherwise stated) at several pulse lengths and using both single and multiple longitudinal mode

pulses. A damage threshold in power flux dependent upon tV^ suggestive of a localized thermal

damage mechanism. Comparison of single and multiple longitudinal mode thresholds, or gain- switched

I TEIA pulses versus smooth pulses gives large changes in the ratio of peak to average power. A comI parison of these results gives a comparison of the effect on thresholds for 2 ns pulse trains, 150 ns,

; 1 p.s,and 6 |as pulses, an appreciable range of pulse lengths.

; A sampling of the type of results that are typical is shown in table 1 for KBr. Considerable

: scatter is typical of the data, but nevertheless the damage resistance of RAP KBr is clearly higher

i than that of the other materials. Although the data are incomplete, they suggest that the single longitudinal mode thresholds are far higher than the partially mode-locked multiple longitudinal mode

results. This would suggest that the damage threshold is actually determined by the peak power in

the 2 ns pulses that make up the mode-locked pulse trains, rather than by the average power. RememI ber that the power in the mode-locked pulse is measured by averaging over the high speed modulation.

' The commercial material doe s not show thi s strong time dependence , which might indicate that a slower

I mechanism is operative here, such as thermal inclusions precipitated failure. However, RAP KCl

appeared to have the identical threshold for the first two types of pulses, which is compatible with both

thermal and avalanche time constants. Breakdown was not reached for longer single mode pulses.

2 Table 1, Potassium Bromide Thresholds for Four Types of Pulses (GW/cm )

Pulse Shape Mode Locked Single Mode 1 fJ-S 6 fJLS

Commercial

High Purity-RAP

0. 4 to 1.5

1. 35 to 2.

5

0. 7 to 1. 2

>6. 2

2. 6

5 to 1 1+

0. 2 to 0. 4

>4

Table 2 compares the performance of these two sources of material for three materials at one

pulse length, listed in order of increasing band gap. Again an improvement is indicated with the

most noticeable case being KCl. (RAP NaCl has received relatively little development to date. ) Of

particular interest is the large and consistent difference seen for the two types of NaCl commercially

available. One type is stated by the manufacturer to be grown by the Stockbarger method in vacuum

for low absorption laser windows. The other type is air grown by the Kyropoulos method for spectrophotometers. The latter is three times more resistant in these multiple shot tests. Table 3 displays

!a similar set of data for a single-longitudinal mode gain- switched pulse. Again, measured values for

i the commercial materials are comparable with previous literature [8]. However, the breakdown field

for RAP KCl exceeds 3. 5 MV/cm compared with reported values of about 1.4 MV/cm. As a further

' elaboration of this trend, table 4 presents the damage thresholds of KCl for a variety of sources using

; one type of pulse. Most commercially available material we have tested suffers from inclusion1 initiated damage in the bulk which is clearly visible under a microscope as a string of small damage

• sites. The maximum thresholds observed have been 0.5 GW/cm^. In passing, we have noted that

1 some of these same crystals can give the appearance of a reproducible bulk damage (by the criterion

of [9]) when tested at 0. 69 \xm.. RAP prepared KCl crystals possess breakdown thresholds com-

: parable to the best NaCl samples available, with some evidence that the most reproducible samples

result from recent improvements in processing.

i

-119-
Table 2. Mode- Locked Pulse Train (0.6 \±s)

Commercial Purity (GW/cm^) High Purity-RAP (GW/cm^)

KBr 0.4 to 1. 5 1. 35 to 2. 5

U, b to 1,5 3 to 14

NaCl 5(a)

5 to 16

12 to 16<^'

'^^Stockbarger. ^^^Kyropoulos.

Table 3. Single Longitudinal Mode Gain Switched Pulse

2 Commercial Purity (GW/cm ) High Purity-RAP (GW/cm^)

KBr 0. 7 to 1. 2 >6. 2

KCl 1.6 >12

NaCl >17

Table 4. KCl Damage Thresholds for 0. 6 |j.s Mode- Locked Pulses

2 Damage Threshold (GW/cm )

Commercial KCl

(Inclusion type damage)

0, 5

Commercial KCl 0. 7 to 1.

5

Cornmercial Europium

Doped Polycrystalline

1.8

RAP B65-3 3

RAP B63-2

RAP B63-9

4 to 7

8 to 14

RAP B53-1 10 to 14

RAP B78-2

(CO^ Carrier Gas)

10 to 14

As a further example of the type of pulse length dependence we have observed, table 5 sum- marizes the measurements made using two multilongitudinal mode pulses. Here, the large decrease

in breakdown threshold for the longer pulse might suggest that some mechanism other than electron

avalanche is responsible. Electron avalanche is expected to be a rapid process which would s.ijggest

little decrease in threshold for pulses of this length. Yet the increases do not follow the t ^/^dependence expected for localized thermal mechanisms such as inclusions, nor do they follow the energy

dependence of homogeneous absorption proportional to t. However, the relatively complex temporal

pulse shape of the pulses in this set of data may mask the true pulse length dependence of this

mechamism.

-120-
Table 5. 10. 6 (xm Bulk Damage Thresholds for Three Alkali Halides

at Two Pulse Lengths

Material

Damage Threshold

(Power Density and Electric Field Strength)

Pulse Length = 0. 6 |j.s Pulse Length = 4 p.s Comments

KBr

Har shaw

RAP-HRL

1. 03

0. 73

1. 25/1.

7

0. 8/0. 94

GW/cm

MV/cm

GW/cm'^

MV/cm

0. 36

0. 43

0. 34/0. 37

0. 42/0. 44

GW/cm

MV/cm

GW/cm'^

MV/cm

KCl

Har shaw

RAP-HRL

<0.52'^>

<0. 52(a)

6. 08 to 14

1. 8 to 2.

7

GW/cm

MV/cm

GW/cm"^

MV/cm

<0. 046

<0. 15(a)

0.095<^>

0. 22(b)

2. 05

1. 03

(a) GW/cm

MV/cm

GW/cm'

MV/cm

GW/cm'

MV/cm

Damage byinclusions

Damage by

inclusions

NaCl

Har shaw

RAP-HRL

4. 6^'^'

1. 5(a)

12 to 16. o'^'

2. 5 to 2. 9*^'

4. 9 to 16

1. 6 to 2.

9

GW/cm'

MV/cm

GW/cm'

MV/cm

GW/cm'

MV/cm

>2.3(^'

>1. 08(a)

GW/cm

MV/cm

(a)

(b)

Stockbarger.

Kyropoulos,

Temporal Characteristics of Transmitted Pulses

To provide insight into the avalanche process as well as to indicate whether the breakdown process

was an inclusion mediated processor a homogeneous process, the transmitted light through the sample

was monitored with a fast detector and preamplifier. It was found, as expected [10], that where inclusion damage was obvious from the morphology of the damaged sample (multiple damage sites from one

shot), the transmitted light was not sharply cut off. Some samples with only one apparent damage site

also exhibited the gradual cutoff characteristic of inclusion damage. Perhaps these samples contained

more closely spaced defects of higher damage resistance resulting in the merger of these defect sites.

Figure 2 is a set of pulses transmitted through conventional KCl for three types of pulses. The

upper photographs represent the last pulse before breakdown occurs and the lower row shows the sharp

cutoff at breakdown. The multilongitudinal mode pulses on the left have two values for power density

quoted. The numbers in parentheses represent direct measurements made on this photograph at the

moment before damage. The other power density below is based upon the measured energy and the

equivalent pulse length of 0. 6 |j.s obtained from a typical pulse, with the rapid modulation averaged.

Thus, the peak power from the train of partially mode-locked 2 ns pulses is much higher than the

averaged power, as expected. A pulse with the same gain- switched envelope profile is shown in the

center pair, but without modulation. Here the two power densities are more nearly equal. These

smaller discrepancies are probably due to fluctuations in pulse shape and thus in equivalent pulse

length. The third pulse is a 1 |ji.s pulse followed by a small tail of several microseconds. The upper

picture of the last pair also includes a blocked beam trace indicating that the residual modulation visible is caused by electrical noise. The important point illustrated here is in the comparison of breakdown values for the different pulses. For instance, the first two pulses, multimode and single mode,

have a large difference in peak power, but essentially the same average power. This is consistent with

a breakdown mechanism which responds to the averaged waveform and is consistent with both electrical avalanche mechanisms and thermal mechanisms. Since this material exhibited a similar peak

power breakdown for the 1 (j.s pulse on the right, it is tempting to conclude that longer pulses in this

range do not result in a decrease of breakdown threshold. This would be consistent only with a rapid

process such as electrical avalanche with a nearly constant power flux threshold at these long |jis pulse

lengths, but inconsistent with a thermal model. This set of data points, taken by itself, would seem

to provide strong support for an avalanche hypothesis. However, table 5 provides contradictory

evidence.

-121-
A comparison of transmitted pulses for conventional KBr is shown in figure 3. Using 1 ys single

longitudinal mode pulses, a pulse below threshold is shown in addition to four examples of pulses that cause breakdown. In the last case the pulse energy was purposely raised a factor of Z above the

threshold. For this sample breakdown occurred consistently between 1. 6 and 1.8 kj/cm^, corresponding to a power of about 0. 8 to 0. 9 GW/ cm^ when calculated by dividing energy by an equivalent

pulse length of 1.9 p.s. The value for power obtained from the photon drag detector oscilloscope traces

in these cases agrees only to within a factor of two with that obtained from dividing energy by pulse

duration. The reason for this discrepancy has not been firmly established. Nevertheless, the point

of special interest in this series of oscillographs is the large variation in breakdown times seen in the

first three examples of figure 3. In the fourth instance, doubling the pulse energy did not result in

simply halving the time to breakdown, at least in this instance, suggesting that the mechanism for

breakdown is not likely to be energy dependent (such as simple absorption heating). Statistical fluctuations on this long time scale suggest further study of this aspect is warranted.

A direct comparison of modulated and smooth pulses of 0. 6 ^xs duration was made on several

samples. While the thresholds are often very close, the morphology of the damage site can be very

different. A sample believed to fail by inclusion-mediated damage, RAP KCl B65-3 is shown in fig- ure 4. Figure 4(a) shows the small microscopic site often obtained with a single longitudinal mode

gain- switched pulse, as compared with the millimeter sized fracture typical of multilongitudinal pulses

having the same envelope and energy (figure 4(b)).Longer single and multilongitudinal mode pulses were

damage in a manner similar to figure A(b), Larger fractures are expected with pulses of greater

energy, as would arise from longer pulses or more damage resistant materials, but the strong morphological effect seen for the two pulses which differ only in their detailed temporal structure remains

unexplained.

Svimmary

The scope of phenomena we have seen suggests that considerable work remains to be done before

a damage mechanism and breakdown "threshold" can be meaningfully assigned to an alkali halide at

10. 6 [j.m. Preliminary conclusions that may be drawn from this work include the following points.

• There is a large variation in threshold from samples grown by different methods.

Some samples grown with special care can have consistent breakdown thresholds

many times higher than conventional materials.

• There is a fairly large fluctuation in threshold for most materials. This might

occur from spatial inhomogeneities or from purely statistical processes.

• The observation of a single damage site from a laser pulse has not always been

accompanied by a sharp cutoff in the transmitted beam. While the appearance of a string of scattered damage sites from a laser pulse is always accompanied with an incomplete or undetectable cutoff, samples have been found to exhibit damage

thresholds sufficiently high to be attributable to intrinsic mechanisms and to have

a single damage site, and yet to occasionally have the incomplete cutoff characteristic of inclusions. It woxjid seem that the morphology of the site and the

behavior of the transmitted light cannot be used to unequivocally identify intrinsic

processes.

The availability of relatively smooth, well-behaved pulse shapes at 10.6 [im suggests that

detailed survival time studies of breakdown processes in these materials can now be carried out.

Acknowledgment

We gratefully acknowledge the contribution of A. Braunstein, M. Braunstein, and A. Pastor in

the areas of technical management, materials growth, fabrication and preparation, technical discussion, and support measurements.

-122-
References

[1] Olness, D., "Laser-induced breakdown in

transparent dielectrics," J. Appl. Phys. 39 , 1 (1967).

[2] Allen, S. D., Braunstein, M. , Giuliano, C,

Wang, v., "Pulsed CO2 laser damage studies of

RAP grown KCl," NBS Spec. Pub. 414, Laser

Induced Damage in Optical Materials: Dec. 1974.

[3] Pastor, R. C. and Pastor, A. C, "Crystal

growth in a reactive atmosphere," Mat. Res. Bull. 10 (Pergamon Press, Inc., 1975)

pp. 117-124.

[4] Pastor, R. C. and Pastor, A. C, "Crystal

growth of KCl in a reactive atmosphere,"

Mat. Res. Bull. 1£, (Pergamon Press Inc.,

1975) pp. 251-256.

[5] Private correspondence, E. Yablonovitch (1974).

[6] Wang, v., Giuliano, C. R. and Gracia, B.,

"Single and multilongitudinal mode damage in

multilayer reflector at 10.6 ym as a function

of spot size and pulse duration," Proc. 1975

Laser Damage Symposium, Boulder, Colorado,

July 1975.

[7] Gondhalekar, A., Heckenberg, N. R. and

Holzhauer, E. , "The mechanism of singlefrequency operation of the hybrid-C02 laser,"

J, Quantum Electron. QE-11, 3 (1975).

[8] Yablonovitch, E., "Optical dielectric strength

of alkali-halide crystals obtained by laserinduced breakdown," Appl. Phys. Lett. 19 , No. 11, Dec. 1971.

[9] Fradin, D. W. and Bass, M. , "Electron

avalanche breakdown induced by ruby laser light,"

Appl. Phys. Lett. 22^, No. 5, 1 March 1973.

[10] Bloembergen, N. , "Laser- induced electric breakdown in solids," IEEE J. Quantum Electron.

QE-10, No. 3, March 1974.

Figures

aw

ilii^ki

Figure 1. OscilloscoDe traces of the five types of pulses used. (a) 0.6 ps multilongitudinal gain switched pulse. (b) 0.6 ys single longitudinal mode gain

switched pulse. (c) 1.9 ps single longitudinal mode pulse. (d) 4 Us

multilongitudinal mode gain-switched pulse. (e) 6 ps single longitudinal

mode pulse. (f) expanded view of typical multilongitudinal partially modelocked pulse train of (a) and (d)

.

-123-
-124-
Figure 4. Breakdown sites in reactive atmosphere

processed KCl B65-3.

COUUEMTS ON PAPER BV WAMG, dt al

Ai hcU) been mzvitioniid In puzvioai, yeoAA the. chafLacXeAA.zation optical mattnx.aJif> cu, polgcAgitaJiinz

OK iinqlz cAyitatine. miut be oied M^th iomz coAd wheAz IcUtM. damagz i>tadiu oAe Involved. UnleJ>i the. d^miiom, 0() the mtcKocHijitaLi, -in the polLjcALjitcULtine mateAial axe imalZ compaxed M-iXh the eZzcXnon

mean (^nee path in the maX.eJu.aZ theie -ci no dLitlncJU-on between itngle and polycAyitaZLine mateAtat.

ThiU, in pxevioiu yexvu, i.t hai, been Keponted that the. damage thxeAhold ^o>i {ixded illtca woi iigni(,icantlij

htgheA than that obieAved in cAyitattine qaoAtz. HoMeveA, in iodium chZontde and otheA alkatL-haLid&i

no diittnctton Mai ieen between io-aaZled polycuyitaJLtine mateAial and iingle cAyitattine matexiat. It MOA {^uAtheA brought oat in the di.iCLU>iion that the morphology o^ the damage iite ii pftima/iiZy iemitive

to the total eneAgy {^alLing on the damage itte duAing the puJU,e. latheA than the detcULb o^ the damage

pfLoceAi,

-125-
3.4 Optical Distortion by Laser Heated Windows

j J. S. Loomis 1 Air Force Weapons Laboratory |

Kirtland AFB, New Mexico 87117

'

E. Bernal gJ

Honeywell Corporate Research Center

10701 Lyndale Avenue South

Bloomington, Minnesota 55420

The optical distortion introduced in a collimated He-Ne beam by windows irradiated

with a C0_ laser has been measured interf erometrically . Materials measured included

ZnSe, KCl, NaCl, CaFj, SrF-, and BaF2. Flux densities from the CO2 laser were in the

range 10-60 KW/cm2 with total powers from 250 to 1000 watts for the ZnSe and the halides.

The changes in the interf erometric patterns observed can be attributed to a) isotropic

phase shifts proportional to the local temperature, b) anistropic effects due to induced

birefringence that affect both the symmetry of the interferometric pattern and the

visibility of the fringes, c) localized phase shifts at the entrance and exit points due

to surface absorption.

An analysis of the experimental observations is given using a scalar optics approach

and temperature rise functions derived previously by the authors. Experimental com- parisons of thermal sensitivity of various materials are given. Also, the relative

value of the anisotropic changes in optical path as the window aperture is filled at constant flux density is calculated. This result allows the prediction of window behavior

in large lasers from the results of interf erometric tests with focused beams of the same

flux density.

Key words: Infrared lasers; interferometry; stress birefringence; thermal distortion;

windows

.

I . INTRODUCTION

The optical aberration introduced by laser heated windows has been the subject of numerous recent

theoretical and experimental studies [1-8]^. The theoretical studies have fallen into two categories.

The earlier treatments emphasized the development of figures of merit expressions for window materials

based on simplified temperature prof lies, and often, on estimated values of materials parameters. This

approach yielded valuable broad guidelines on the relative potential of various materials [1,2]. More

detailed treatments of thermally induced aberrations yielded more insight on the effects of the

aberration on the far field power distribution, albeit at the loss of analytical expressions for figures

of merit [3,4].

The bulk of the experimental results reported to date have emphasized the experimental verification

of the predicted laser-induced window aberrations via either interf erometric measurements [5,7,8] or measurements of the far-field intensity [5,6]. However, in order to produce measurable effects to verify theory with laboratoty-type lasers having single transverse mode beams and power outputs in the

hundreds of watts, it has often been necessary to use samples with much higher absorption than are available from state-of-the-art materials or, to artificially raise the surface absorption by applying

an absorbing layer on the surface of the sample. Thus, while the experimental results have largely

confirmed the occurrence of various types of aberrations at high irradiances, the throughput capability

of the materials tested has not been established for lack of good documentation on the power absorbed

by the sample, the effect of coatings, and the absence of a scale-up model to predict the magnitude of

the aberrations to be expected when the window aperture is filled by a beam having the same peak flux

density as the tightly focused beam used to make the distortion measurements in the laboratory.

It is now clear that the CW power that can be trasnmitted through a solid window will be limited by

laser- induced optical distortion. This is because the strength of candidate window materials has been

increased to the point that laser-induced fracture would occur at powers above those that produce

unacceptable beam aberrations. Therefore, in this paper we endeavor to present qualitative experimental

results on the optical distortion produced by various window materials when irradiated by a laser beam

focused to a spot size much smaller than the window diameter. The effects of thermally induced birefringence are shown to be dominant in the case of alkaline earth fluorides. We also give the results

of a model that attempts to predict the increase in optical distortion to be expected when the diameter

of the laser beam is increased from the focused case to an aribtrary value while keeping the flux

density constant. If this model is correct, it should provide a method of predicting component perfor- mance in a high power laser from laboratory measurements. Finally, the interf erometric observation of

surface absorption is also reported.

^Work supported by Defense Advanced Research Projects Agency under Contract #DAHC-15-73-C-0464

.

1. Figures in brackets indicate the literature references at the end of the paper.

-126-
II. ANALYSIS

The purpose of the analysis of the dynamic behavior of the interferogram during laser irradiation

is to provide a common model for comparison of the response of various materials in terms of their

physical properties.

Once the model is refined sufficiently to reproduce the results of interferometric measurements

obtained during irradiation of windows with focused beams from laboratory type lasers (200-1000 watts)

,

it can be used to predict the optical distortion and stress levels to be expected when the full aperture of the window is filled with the beam of a high power laser. This is a most important aspect of

the analytical approach, because it reduces the amount of testing required in a high power laser facility.

The main task of the model is to produce expressions for the change in optical path produced by the

absorption of laser radiation. This is because all the window aberrations produced by laser irradiation are caused by the non-uniform temperature rise resulting from absorption of energy from the beam.

Thus, the total model for the window aberration requires the following components: a realistic calculation of the temperature rise of the window resulting from laser irradiation, calculation of the

stresses resulting from the non-uniform temperature rise and, expressions for the change in refractive

index resulting from the above. We deal with each of those questions below.

A. Temperature Rise of the Window

The interf erometric measurements of laser-induced aberrations in solid windows are typically conducted with undamped windows that have very small heat transfer to the mount. This is accomplished

by having the window in a holder such as the ones used for calorlmetric measurement of the absorption

coefficient [9]. Assuming that the window is in the form of a disc of radius b and length 2£, the heat

flow problem associated with the calculation of the temperature rise resulting from absorption of

energy from the beam is the same found in calorimetry. The boundary value problem to be solved is

therefore [10]

V^T + (Q/k) = (1/k) (9T/3t) 0 £ r £ b (la)

0 <_ z <_ Si

k(3T/3r) + hT = 0 r = b, (lb)

-k(3T/3z) - 0 z = 0, (Ic)

k(3T/3z) + hT = 0 z = «- (Id)

where h = heat transfer coefficient

k = thermal conductivity

K = k/pC = thermal diffusivity

p = density

C = specific heat capacity

Solutions for the temperature rise resulting from a gaussian heat input function with radius a used in

equations (la-Id) have been obtained before by the method of finite Fourier transforms [10] for beam

radii smaller than, or equal, to half the window radius (2a <_ b) and by a finite element method for

2a >^ b [11]. The reason for using the two methods is due to optimal rate of convergence of the

solutions in their respective regimes. The results obtained have been verified to agree in the overlapping region near 2a = b to within better than 5%.

B. Thermal Stresses

The thermal stresses are calculated here based on the isotropic thermoelastic model for plane stress

given by Boley and Wiener [12] and used extensively by others. This approach is known to be inaccurate

for several of the window materials of interest, which although possessing a cubic structure, exhibit

strong elastic anisotropy. Thus, although isotropy is assumed in order to obtain analytic expressions

for the thermal stresses, any experimental observations related to crystalline orientation cannot be

expected to be describable by this model.

For the range of laser beam diameters 0<2a<b, the stresses were obtained by term-by-term integration

of the temperature distribution, eq. (10), [10], substituted into the thermal stress equations of [12].

-127-
C. Refractive Index

The changes In refractive index resulting from the inhomogeneous temperature rise and associated

thermal stresses are calculated using expressions given by Jasperse and Gianino [2]. Because of the

assumed elastic isotropy and the cylindrical symmetry of the temperature rise, the eigenvectors of the

index ellipsoid correspond to radial and azimuthal polarizations and are given by [2]

'6n^(r,z,t)^

^(5ng(r,z,t)^

no

2E

3n l ano'

9T ~2 (P 11^^12^]

3 /Pir2^Pl2 '12

T(r,z,t) I

V(p^^+p^2)

Pl2-^ ^Pll-'Pl2> (Pir2^Pl2)'

/o_(r,z,t)^

^QCr.z.t]

(2)

where p..'s are stress-optic constants [I3] and Jt, is the 2x2 identity matrix; v is Poisson's ratio, and

E is the-'modulus of elasticity.

The equation above has a polarization-independent term proportional to the temperature rise and a polarization dependent term proportional to the thermal stresses. The relative magnitudes of the two

terms will determine whether or not birefringence is important.

D. Transmitted Wave

The expressions above are based on a paraxial ray approximation. This is quite appropriate to the

analysis of the interf erometric measurements because the windows are illuminated with a well collimated

beam in such experiments. Because of these experimental considerations, the spatial dependence of each

electric polarization component of the electric field of the monochromatic wave from the HeNe laser source used in the interferometer can be written as

E(z) E e o

ikz

where k =

2Trn (r , z)

X

(3)

(4)

Thus, a change in refractive index 6n in a window of thickness 2. simply introduces a phase factor on the

propagating wave such that

E(z) E(z) e

2TT ^ = T-^ P6n(r ,z)d2

^ -I

(5)

(6)

Following Flannery and Marburger [4], we use the Jones matrix formalism in a cartesian coordinate system

Then the field

resulting from propagation of the input field

through the laser-heated window is given by

-128-
where

and

A = e

^cos 6 + i sin 5 cos 29

V

sin S sin 29

<t> = h (<1)^-H>q)

6 = Is (V*9)

/E

X = .|

E'

j

i sin

cos 6 - i sin 5 ;os 29y

(7)

(8)

(9)

(10)

27T r

(() (r,t) = T— I 6n (r,z,t)dz

r ^ r

2Tr (i)g(r,t) ^ ~ J

'5ng(r,z,t)dz

(11)

(12)

(j) is therefore the average phase retardation and 6 is the phase difference, which is proportional to the

induced birefringence. We show below how these two quantities lead to polarization independent and

polarization dependent effects, respectively.

E. Intensity Distribution in Interf erogram

To calculate the intensity distribution in the observation plane of the interferometer, we assume

that the input wave to the window and the reference wave are linearly polarized in the y-direction,

i.e.,

E. = E , m ref E(z)

where E(z) is given by eq. (3). Then the intensity is obtained by the interference of the wave propagating through the sample, eq. (7) above, and a reference wave of the form

Ej^(r,z) = E^ exp ^i(kz+f2(r,9)j

\1

(13)

where Q (r,9) is a function used to introduce bias fringes for reference purposes in the interf erogram.

These fringes are typically either equally spaced straight lines obtained by introducing a wedge in an arm of the interferometer, or circular fringes introduced by changing the curvature of the reference

beam . For these two cases the form of n is

2TTn r cos9 2Trn r n(r,e) = i- + (14)

where n^ and n- are the number of straight-line and circular fringe bias, respectively. In general the

intensity resulting from the interference procedure is given by

K E + E,

where K is a constant.

After some algebraic manipulation, it is found that the intensity for the fields of eqs. (7 and 13)

is given by -129-
I(r,z,t) = 2KE |l+cos((t)+fi)cos6+sin((J)+n)sin6cos 9] ' (15)

o I J

with (j) and 6 as defined in eqs. (9 and 10),

An equivalent expression in terms of (j)^ and ^q, as defined in eqs. (11 and 12) is

I(r,z,t) = 2KE^ I l+cos((t) +fi)sin^9+cos((f)„+fi)cos^9l. (16)

, o I. r t) I

Examination of eqs. (15 and 16) is very useful in terms of predicting qualitatively the effect

of the laser irradiation on the intensity patterns observed interf erometrically . Specifically, for the case of a circular fringe bias, when the first term of eg. (lA) is zero, we see from ea . (15) that the

fringe pattern remains circularly S3rmmetric in the absence of birefringence. Thus, the effect of laser

irradiation on the observed interferometric pattern will be that the circular bias fringes will move radially in a direction determined by whether the laser-induced lenslng is positive or negative. The

spacing between the reference fringes will measure the strength of the lens. Fringe motion is con- trolled strictly by the local temperature rise. From eq . (15) it is also clear that local fringe

contrast becomes dependent on the angle 6 as the birefringence increases, due to the rotation of the

plane of polarization of the beam passing through the sample relative to the fixed polarization of the

reference beam.

Similarly, examination of eq. (16) reveals that:

(i) As the phase retardation for r and 9 polarizations become unequal

(birefringence non-zero), the circular bias fringes will distort

first into ellipses and then into more complicated shapes.

(ii) As the difference in phase retardation for the two polarizations

becomes >^1A, the fringe that started as a circle can divide into

two distinct fringes as in biological cell division.

(iii) The interf erometric pattern always retains a fourfold symmetry.

Hence, only one quadrant needs to be analyzed.

(iv) The principal axes of the fringe patterns are parallel to the x and y axes, respectively. This results from the assumption of

elastic isotropy and is not consistent with experimental results,

as discussed below.

III. EXPERIMENTAL RESULTS

A. Procedures

The interferometric measurements were made by placing samples in one arm of either a holographic or

a Twyman-Green interferometer. The arrangement in either case has been described before [14, 11].

The results obtained with either type of interferometer are equivalent. The holographic interferometer

allows interf erometric measurements on samples that are not perfectly polished and it permits more flexibility in the introduction of bias fringes, although at the expense of increased complexity.

Table I lists the properties of the samples used in the measurements and some parameters appropriate

to the experiments. The potassium and sodium chloride samples were single crystals with (100) surfaces;

the fluorides were also single crystals, but their crystallographic axes did not have any simple

relationship to the surface; the zinc selenide was polycrystalline material prepared by the CVD

process [15].

The lasers used to irradiate all samples to induce thermal distortion were CW CO. devices with

power outputs up to 250 watts. Therefore, the laser power had to be considerably reduced when testing

flourides since they are highly absorbing at the 10 ym wavelength and are not intended for use as windows

in this region. The experimental results were all obtained by focusing the nearly TEMoo beam from the

laser to a spot .02cm2 area at the sample position using a 50cm focal length lens made of potassium

chloride.

B. Results

The discussion in this section will be qualitative, dealing in comparisons of materials and

symmetry effects related to birefringence. This is because the quantitative reduction of the interf ero- -130-
grams is not complete.

Figures 1-6 show a sequence of interf erograms photographed at various times after the CO. laser was

turned on. The laser power and spot size at the sample are given in table 1. The first photo in each

figure shows the circular fringe bias introduced for reference purposes prior to turning on the

laser. The fringes in that first photo are numbered sequentially. In subsequent photos in each figure

the fringes are numbered by keeping track of fringe motion on a video tape recorder playback from which

the photos are obtained.

1. Alkali Halides and ZnSe

The first three materials in table 1, potassium chloride with a protective/AR coating of TlI-ThF^,

sodium chloride, and ZnSe are all window candidates for the laser. Hence, we discuss the nature

and magnitude of the optical distortion. The interf erograms of optical distortion produced by the three

materials under identical conditions of laser power and spot size, given in table 1, are shown in

figures 1-3. Examination of the inter f erograms shows that the fringes retain their circular symmetry

during irradiation which, as discussed in the previous section, indicates that birefringence effects are negligible for all three materials at the energy deposition rates of these experiments. Thus, one may

expect to be able to correct for the window distortion using adaptive mirrors.

Comparison of figures 1-3 shows a very small change in optical path for sodium chloride; potassium

chloride and zinc selenide show changes of the same magnitude, but the difference in sign of the fringes

is indicative of the fact that the lensing is negative for the former and positive for the latter.

However, a simple comparison of the photos is not sufficient to assign figures of merit to these

materials

.

Examination of table 1 reveals that the power absorbed in the three samples varies widely with a very substantial input into the coated KCl. The high absorption in the KCl case was totally dominated

by the 0.5% loss in the ThF, layer which is process-dependent and in our experience can be lowered to 0.1%. ^

A better basis for material comparison Is the number of fringes shifted for a given laser exposure

per watt absorbed. This quantity, given in table 1, shows that sodium and potassium chloride are comparable (with a larger uncertainty in the value for sodium chloride due to the small change produced),

while zinc selenide is about five times more sensitive to the dissipated power. This is consistent

with previous experimental results on the optical distortion of uncoated halides and zinc selenide [8,15], as well as the theoretical predictions of relative sensitivity of semiconductors and ionic crystals to absorbed power [1]. The uncoated sodium chloride sample, due to its low absorption and excellent can- cellation of the thermo-optic and thermal expansion contributions to the optical path change has the

lowest distortion. It corresponds to less than 1/10 wave at 10.6pm.

The results for potassium chloride, figure 1, are particularly significant because |hey demonstrate

the ability of the TlI-ThF^ protective/AR coating to withstand exposure to the 12 KW/cm flux density

without damage in spite of the high absorption of the ThF^ layer used. Comparison of figures 1 and 2 clearly shows that at the present time coating absorption is the limiting factor in halide performance.

Furthermore, the low index ThF^ coating contributes essentially all of the absorption, and is therefore

the material that must be replaced to improve performance

.

2. Alkaline Earth Fluorides

The interf erometric results for fluorides are shown in figures 4-6. For these materials we will

only discuss the birefringence effects, since as discussed above, their absorption at 10.6lJm is very

high and the absorbed power is high and not uniform along the thickness

.

The most important observation in figures 4-6 is the verification of strong birefringence effects

apparent for all fluorides as demonstrated by the evolution of birefringence which proceeds through the

stage of fringe division as predicted for strongly birefringent materials in the last section. Hence,

while the magnitude of the optical distortion in fluorides is small per watt of absorbed power, the

polarization dependence will increase the difficulty involved in compensation.

A very significant feature that was observed in the optical distortion measurements on fluorides is

that the principal axes of the elliptical fringes were not parallel to the polarization vector of the

Interferometer source (vertically polarized He-Ne laser) . This observation is in disagreement with the

prediction of the elastic isotropy model and is believed due to the elastic anisotropy of the fluorides,

since the principal axes of the ellipses rotate as the samples are rotated in the holder. It clearly

indicates the need to develop a model that handles the elastic anisotropy. We have not attempted any

work along these lines because of the complexity involved, and because as pointed out before, the

crystalline axes of the fluoride samples used did not have a simple relationship to the sample faces.

The lack of a simple relation between crystal axes and samplfe geometry is also why all the photos in

figures 4-6 were rotated to make their major axes vertical.

-131-
3. Surface Absorption

The interf erometrlc observation of surface absorption is reported here because of the increasing

need for measuring absorption of surfaces, coatings, and the potential value of interferometry for

that purpose. The results reported here are not in the mainstream of this report and have not been

analyzed in detail.

Figure 7 shows the relationship of the sample, viewing direction (interferometer axis) and laser

beam used to irradiate the sample inside the interferometer. The important point to notice is that the

angle of incidence of the CO^ laser beam is made large enough that the entrance and exit points at the

two sample surfaces are separated in the field of view. For this arrangement, bulk absorption should

produce optical distortion continuously over the area traversed by the beam. On the other hand, surface

absorption would produce two non-overlapping centers of optical distortion, as observed in the photos

of figure 8. The interf erograms in the left side of the figure corresponds to the initial fringe bias

before the laser is turned on. The right side is the Interf erogram after irradiation for 25

seconds with a 300 watt beam focused to a diameter of 1mm. The sample used was a 1cm thick NaCl window

with high surface absorption.

The interpretation of the optical distortion caused by surface absorption requires the calculation

of the temperature distribution in the bulk. A more favorable arrangement for this type of measurement

should be one where the interferometer probes the sample at right angles to the propagation direction

of the laser beam. Such a geometry would show how the temperature varies with depth.

IV. COMPUTATIONAL RESULTS

In this section we present the results of computations, based on the model of section I, concerning

the spatial and polarization dependence of the optical distortion of window materials. We also present

results on the magnitude of the optical distortion as the window aperture is filled at constant flux

density.

For simplicity, we have chosen KCl and CaF^ as characteristic of halides and fluorides. The

numerical results presented represent the changes occurring 10 sec. after a lOKW/cm laser beam is

^ turned on to a 2cm radius, 1cm thick sample. The absorption coefficient 3 is assumed to be 3=0. 001cm

The heat transfer coefficient to the air was taken as h=7.5xlO watts/cm'^, which is the measured value

for KCl'^. The other material parameters used are from [1, 2, 17, 18, and 19].

Figure 9 shows the spatial and polarization dependence of the optical distortion in KCl and CaF^

when irradiated with a beam having a 1cm radius. The distortion is normalized to the peak distortion

for each material. The difference in behavior of the two materials is clearly apparent. Potassium

chloride behaves as a negative lens with a small amount of polarization dependence. Calcium fluoride

behaves as a positive lens for 6-polarization and as a hybrid (positive in the center, negative for

most of the aperture) for the r-polarization. Furthermore, the difference in phase retardation for the

two polarizations is large, in agreement with the experimental results of the previous section.

Figure 10 is a plot of the difference in phase retardation between center and edge of the window

versus the ratio of Gaussian beam diameter to window diameter at constant flux density. The phase re- tardation is given in units of the 63288 wavelength of the interferometer source. This type of calculation, if proven valid, is what should allow the prediction of window performance in a large laser system

from test results obtained using focused beams from laboratory sized lasers. The curves in the figure

show that for equal thermal inputs the magnitude of the optical distortion is greater in KCl than in

CaF^ ,but that the birefringence is much more severe in the latter.

The difference in thermal stresses, normalized to the stress at the center, is shown in figure 11

as a function of the ratio of beam to window radius for potassium chloride. This difference determines

the magnitude of the birefringence. The calculation in this case has been carried out to beam radii

larger than the window radius in order to simulate the effect of more uniform illumination. The results

show that the stress difference increases monotonically with beam radius. However, the slope clearly

decreases for beam radii larger than the window radius as a result of the more uniform temperature rise

produced by such a beam.

V. SUMMARY AND CONCLUSIONS

In this report we have presented experimental results of laser-induced thermal distortion by various

candidate window materials. The results were used to compare the relative merits of the materials and

to demonstrate the strong birefringence effects occurring in fluorides. We also presented a model that

should be useful in predicting component behavior in large laser systems based on laboratory test

results. Finally, we demonstrated the use of interferometry to observe surface absorption. ;

The results .show conclusively the superiority of uncoated alkali halides for use at 10.6Mm. However,

absorption by the ThF^ layer in coated halides seriously degrades their performance. Therefore, it is

imperative that a low index coating material with lower loss than that of ThF^ at 10,6lJm be found since

-132-
ThF^ Is currently the low index material for zinc selenide as well as halide coatings.

The preliminary results presented here show clearly that polarization-dependent optical distortion

due to induced birefringence is indeed likely to be significant in fluoride laser windows. Further work

needs to be done to learn more about this problem and its relationship to the elastic anisotropy of those

materials

.

Finally, this report has emphasized the optical distortion effects produced by laser irradiation of

various materials and how they are likely to scale-up. However, the relative merits of various

materials in actual applications will be determined by many other factors as well, such as strength,

fracture energy, environmental stability> and others. The need exists to perform nondestructive tests

on fully coated components in large laser systems in order to reassess the areas in which further

Improvements in materials and processes is required.

VI . REFERENCES

[1] M. Sparks, J. Appl. Phys. 42, 5029 (1971).

[2]

[4]

[5]

[6]

[7]

[8]

[9]

[11] J. S. Loomis, C. A. Huguley, "Laser Induced

Damage in Optical Materials: 1974", p. 94,

J. R. Jasperse and P. D. Gianino, J, Appl. Phys. NBS Special Publication 414, U.S. Govt. Print43, 1686 (1972). ing Office, Washington (1974).

[3] B. Bendow and P. D. Gianino, J. El. Matls. 2,

87 (1973).

M. Flannery, J. H. Marburger, "Laser Induced

Damage in Optical Materials: 1974", p. 31, NBS

Special Publication 414, U.S. Government

Printing Office, Washington, D.C. (1974).

There are some phase factors missing in the

aberration matrix given in this reference. The

correct form is given above.

A. Hordvik, Proc. First Conf. on High Power IR Laser Window Materials, p. 389, Hyannis, Mass.

(1971), Air Force Cambridge Research Laboratories Report #AFCRL-7 1-0592.

L. Skolnik, B. Bendow, P. D. Gianino,

A. Hordvik, E. F. Cross, Proc. 2nd. Conf. on High Power IR Laser Window Materials, p. 15,

Hyannis, Mass. (1972), Air Force Cambridge

Research Laboratories Report #AFCRL-TR-73-

0372(1).

J. S. Loomis, C. A. Huguley, "Laser Induced

Damage in Optical Materials: 1974", p. 94, NBS

Special Publication 414, U.S. Government Printing Office, Washington, D.C. (1974).

E. Bernal G., Proc. Fourth Conf.

Infrared Laser Window Materials,

Arizona (1974).

on High Power

p. 193, Tucson,

E. Bernal G., B. G. Koepke, R. H. Anderson,

R. J. Stokes, "Preparation and Characterization

of Polycrystalline Halides", Quarterly Tech.

Report #2, ARPA Contrach DAHC15-72-C-0227

,

Honeywell Corporate Research Center, Oct. 1972.

[12] B. A. Boley and J. H. Wiener, "Theory of

Thermal Stresses", Wiley, New York, (1960).

[13] R. S. Krishnan, "Progress in Crystal Physics",

Vol. 1, Viswanathan, Madras, India (1958).

[14] E. Bernal G., B. G. Koepke, R. J. Stokes,

R. H. Anderson, Proc. Second Conf. on High

Power IR Laser Window Materials, p. 413,

Hyannis, Mass. (1972). Air Force Cambridge

Research Laboratories Report //AFCRL-TR-73-

0372 (II).

[15] The authors are indebted to Mr. John Fenter,

Air Force Materials Laboratory, Dayton, Ohio,

for kindly supplying this material.

[16] E. Bernal G., J. H. Chaffin, B. G. Koepke,

R. B. Maciolek, R. J. Stokes, -Honeywell Semiannual Technical Report #2 on ARPA Contract

//DAHC-73-C-0464, Jan. 1975.

[17] M. Sparks, H. C. Chow, J. Appl. Phys. 45, 1510

(1974) .

[18] 0. V. Shakin, M. F. Bryzhina, V. V. Lemanov,

Sov. Phys. - Solid State l^, 3141 (1972).

[19] C. S. Sahagian, C. A. Pitha, "Compendium on High Power Infrared Laser Window Materials,

Air Force Cambridge Research Laboratories

Report #AFCRL-72-0170 (1972).

[10] E. Bernal G., Appl. Optics 14, 314, (1975),

-133-
VII. TABLE AND FIGURES

CO

tu

•IC

4J (U >H ^

o

-5 o <;

so

M S M O

S

ow

T3

<V 4-1 ^ cd u :s O

"

CO <

I

I

in o

CO CM

1-H ON -a- u-l

I

u u

dl 01 4-1 Ul 3 4-1 O n) iJ S

nj

4J

S-i <=< O O OQ. H in

(U

4-1 <U B

o

CO

CO

CJ O

•H

W

C

•H

4J

(fl O

O -H

•O V4 C OJ •HS to

o o o o o in

to in in CM

CN CM

I I o o o

rH 1-4

!«: in in o

O <r CM

vO tH ON

00 O H- CO O < 00 .-1 00 00 00

CM m m m cn

<UU

n)

flcr C

(0 o

•H

14-1 4J 0 CO •H O CM in in a- cu T3 in 0^ C3^ o^ CTi T3 CU CO •H r-l M O O o o o O CO M

•H

CO •H CO M-l0 C 4-1 o CO CO •H CU

CO 4-1 4-1 C o 3

<u a. C

e to •H

OJ 0) (U 01 <U •H e

C c C C C a CM o o o 0 O g

cn M IZ iH CM H <u O (U

1-4 4-> M-l 1

O CO

4-1 (0 T3

-o <U

cu cu M m v£) oo <r >-l CO D

1 1 1 n) 3 CO CM CM CM V CO

1-1 o CO Pu lil cr o CU O CD a (S CD CO lb S

US U CO fO H— •K *

-134-
60 sec 3 min

TLI6-4

Figure 1. Interferograms of laser-induced optical distortion introduced by single crystal potassium

chloride coated with protective/AR coating of Tll-ThFi,. Upper left photo shows the fringe

bias introduced for reference prior to irradiation.

-135-
2

3

T = 0 5 sec 10 Sec

— 0

60 sec NaC£ 36 3 min

Figure 2. Interferograms of laser-induced optical distortion introduced by uncoated sinele crystal

sodium chloride. Upper left photo shows the fringe bias introduced for reference prior

to irradiation.

T = 0 10 Sec mm

-2

0

-3

-2

2 min ZnSe 3 m in Figure 3. Interf erograms of laser-induced optical distortion introduced by uncoated polycrystalline

ZnSe prepared by CVD. Upper left photo shows the fringe bias introduced for reference

Drier to irradiation.

-136-
5 sec CaF2 7 sec

Figure A. Interferograms of laser-induced optical distortion introduced by uncoated single crystal

calcium flouride. Upper left photo shows the fringe bias introduced for reference prior

to irradiation.

5 sec SrF2 -8 10 sec

Figure 5. Interferograms of laser-induced optical distortion introduced by uncoated single crystal

strontium flouride. Upper left photo shows the fringe bias introduced for reference prior

to irradiation.

-137-
T = 0 I sec 2 sec

5 sec BAF 2 10 sec

Figure 6. Interferograms of laser-Induced optical distortion introduced by uncoated single crystal

barium flouride. Upper left photo shows the fringe bias introduced for reference prior

to irradiation.

Figure 7. Diagram showing relative orientations of sample, CO2 laser beam and

interferometer axis (viewing direction) for observation of surface

absorption.

-138-
BEFORE

Figure 8. Interf erograms showing surface absorption of CO

DURING

2 laser radiation by sodium chloride window.

Left photo shows the bias fringe pattern introduced prior to irradiation. Right photo

shows two localized centers of fringe distortion corresponding to entrance and exit points

of beam.

o

Q

<

Lul q:

UJ

tn<X

Q.O

UJN

cro

0.8

0.4 -

Figure 9. Spatial and polarization dependence

of normalized optical path change

in potassium chloride and calcium

f loride.

-139-

0 . .5 1.0 1.5 2.0 2.5 o/b

Figure 11. Normalized difference in principal stresses versus ratio of beam to window radius. (Constant flux density.)

NO COmENT ON PAPER BV LOOMIS, at

-141-
3.5 Thermal Distortion Studies of ZnSe Windows by Far Field Ir radiance Measurements*

John A. Detrio and Roger D. Petty

University of Dayton Research Institute

Dayton, Ohio 45469

We have experimentally measured the effect of thermal lensing in ZnSe windows

on the far field irradiance (at 10. 6 pxn) as a function of input power and absorption

coefficient. These preliminary measurements are compared to the theoretical predictions of the figures of merit analysis. The undesirable effect produced by interference between reflections from the sample faces is successfully minimized by the use of wedged samples and ratiometric detection techniques. Beam slewing, which

may also mask the desired data, is minimized by the use of a scanning slit at a primary mirror focus and a detector preceded by a diffuser and integrating sphere.

Keywords: Far field intensity; laser windcfws ; thermal distortion; thermal

lensing; ZnSe.

1. Introduction

The objective of the work reported here is to experimentally test the predictions of the figure of

merit analysis [l, 2]^ and thus determine its validity or limitations in its use as a tool for selecting one laser window material over another. In particular, we wished to test the comparison between

samples of the same material which have different absorption coefficients. Additional experiments

are planned to test the predicted variations between different materials, the scaling of the theory

with respect to sample diameter, thickness, and the effect of surface absorption.

Two modes of failure are treated by the FOM analysis. These are the mechanical failures due to

the thermally induced stresses fracturing the window or a failure due to optical distortions reducing

the target intensity to the extent that the system fails to perform its desired function. The mechanical failures are being examined in a companion experimental study and will not be discussed here.

In [l]. Sparks suggests that the theory should apply quite accurately (about 20%) to the covalently

bonded materials such as ZnSe,but that the ionic materials' performance would be predicted with less

accuracy (100%). The present work conforms so loosely to the theory that no estimate of the error

can be made at this time. The experimental approach employed in this work was also suggested by

Sparks [l] and it represents an extension of thermal lensing measurements previously made in our

laboratories.

2. Summary of the FOM Analysis

The details of the figure of merit (FOM) analysis are contained in the references, but the salient

features and assumptions of [1] will be summarized here to facilitate comparisons between

the theory and the experimental attempts to quantitatively verify the FOM analysis.

The parameters employed in calculating the figure of merit include optical properties such as the

refractive index, n, thermooptic coefficient, dn/dT, absorption coefficient, p , and mechanical and

thermal properties such as the yield strength, cr. Young' s modulas, E, linear expansion coefficient,

a, specific heat, C , and thermal conductivity. In [2], six separate cases are considered:

Pulsed and CW ope?ation as determined by the laser pulse length compared to the thermal time constant of the window, two cases of sample cooling, either the edge or face being exposed to the cooling

medium, and the two modes of failures discussed above- - optical or mechanical fracture. The result

of performing the FOM calculation for a given material is the maximum power (watts) that the sample can transmit under specific conditions of beam size and profile and with specific cooling conditions and pulse lengths for an "optimum" thickness of the material. The theoretical results are summarized in table 1 which is taken from [2].

This work is supported by the Air Force Materials Laboratory.

Figures in brackets indicate the literature references at the end

-142-

of this paper.
Table 1. Summary of the F(M analysis [2],

Laser Failure

Operation N. Mode

Mode \(AT, ) \^ les s

Fracture

AT

f

Optical

AT

o

pulse

TT Co- 2

TT D C X 2S t , a E(SF)

pulse 32P t , la ^ ^ pulse 1 nT gr

P

cw

Cooling

Mode

8tt Ko- TT K \

Edge

t, = less EE(SF)

1 nTl gr

Face

less ^

2

TT D h 0- r.2 , TT D h.\

2B a E(SF) I gr

32P la „l

1 nT| gr

gr

is the greater of and I where:

i ^ 0. 842 D

o C(D/2)^

1) (P/E) (D/\) 1/5

4K

Cf

(1 +—^)

nT

AT,

3K

(dn/dT) + a (1 +v) (n - 1) + S

2o- a E (SF)

AT

8f

gr nTl

The assumptions used in formulating the FOM analysis are summarized in table 2. The problem

is graphically depicted in figure 1. The radially symmetric laser beam profile incident on the at)Sorbing window will give rise to a radially symmetric thermal profile. The sketch shows a calculated

thermal profile using the FOM assumptions and the material parameters for ZnSe listed in tables 2 and 3, respectively.

Table 2. Assumptions of FOM analysis.

1. A diffraction limited, untruncated Gaussian beam incident on the window.

Intensity and temperature distributions across the window are radially

symmetric and parabolic.

3. The intensity of the beam at the window edge (D/2) is l/3rd of the central intensity.

4. The window is elastically and optically isotropic.

5. The window is thin f < < D.

6. The window is clamped at the edge with a 1/2 atmosphere pressure applied.

7. A safety factor (SF) of four is assumed in the mechanical design.

8. The system optics are focused on a distant target through the window.

9. The power at which the window distortion reduces the far field intensity to one-half is defined as the FOM. , , -143-
Table 3. Parameters of Interest for ZnSe. These values are taken from measurements

in our laboratories.

n = 2. 406 a ^ = 9. 32 X 10" ^/K C 0. 347 J/g K

nT p

6 7 o = 7. 2x10" /K K = 0. 17 w/cm K E = 1 x 10 psi

p = 5. 267 g cm

- 3

cr = 6.,2,1 x 10 psi v = 0. 3

The thermal profile is approximately parabolic and gives rise to a change in optical path differ- ence which is parabolic in the radial coordinate. If one neglects the role of stress optic effects, this

parabolic variation will produce a focusing effect. The window focal length is F . The system then

possesses an overall focal length given by:

F"^ = F"\ + F"'^ (1)

t tel w

where F is the telescope focal length,

tel

For thermally induced lensing with a long focal length, the intensity ratio at the target will be

given by:

2 -, - 1

where

A = dn/dT + » ( 1 + v) (n - 1)

t = thickness

X = wavelength (10.6 |j,m)

AT - temperature difference between the center and edge of the sample.

The temperature difference is proportional to the incident power, P. . The predicted variation

of eq. (2) is plotted in figure 2. The nearly linear variation of I/Io vs. power from 0. 85 to 0. 5 has

been useful in extrapolating the data to determine the experimental value of the FOM.

3. Experimental Approach

The experimental details are outlined in figures 3 and 4 which show, in the form of block diagrams, the optical layout and electronic circuitry, respectively. It is very helpful to have samples

that are slightly wedged to minimize interference effects and to use ratiometric techniques to further

reduce these effects. A variable delay gate and scanning slit at the primary mirror focus allow the

peak far field intensity to be monitored. Variations in beam position on the detector would tend to produce intensity variations if it were not for the use of integrating sphere collecting optics and a ZnSe diffuser.

Our initial experiments were done with F*yrex substrate mirrors and thermally induced distortion

in the mirrors was readily observed. The use of good metal substrate mirrors greatly reduced the

influence of this type of lensing on our results. The mirrors tend to produce a negative focal length

distortion while the ZnSe (and CdTe) undergo a positive lensing effect. The net result is a partial

cancellation of the window lensing by the mirrors. Figure 5 shows the effect of mirror and laser

output coupler lensing which is the system baseline when no window is present.

The distance to the target is about one meter. Longer focal length systems were les s- convenient

and not as sensitive, while shorter focal length systems tended to be "noisy" with a great deal of

signal jitter. The more intense focus also tended to cause difficulties at the scanning slit.

4. Results and Discussions

The results of the far field irradiance measurements (FFIM) are shown in figure 6. The theoretical curve is that predicted for samples 38 mm in diameter under CW irradiation in accordance

with the other conditions specified by the FOM analysis. The data shown in figure 6 were obtained

from FFIM made on samples of various values of absorption and which were plotted as shown in

-144-
figure 7. These curves were extrapolated (by a least squares technique) assuming a linear dependence of I/Io on the incident power. The value of P. for the extrapolated curve for I/Io equal to unity was substracted from the value of P. corresponding to I/Io equal to one half. This difference

was then plotted as a function of 1/(3 in figure 6.

The most dramatic example of optical distortion yet observed occurred in a sample of low quality

CdTe which also underwent thermal runaway. Due to the hazard posed by CdO vapor, which may

result from destroying a CdTe sample, no further studies of this material were undertaken.

For reasons of economy and experimental simplicity, we have not precisely matched any of the

theoretical assumptions in these initial efforts. The effect of not conforming to the theoretically

assumed conditions may be examined by scaling the theory and by computational analysis. By using

a system baseline scan with no window present, the effect of non- diffraction limited optics are partially mitigated. Using a beam size that is less than that specified in the theory produces a pair of

offsetting effects. The smaller beam will tend to produce a shorter focal length distortion for a given

total power,but the effect of the thermal conductivity tends to make the radial thermal profile more

gradual with respect to the beam power profile and thus tends to increase the focal length for a given

total power.

For windows which fail optically, such as ZnSe, the mechanical assumptions are not critical.

Placing the focusing mirror after the window did not seem to have significant effects since both configurations (beam focused through window and focusing optics following the mirror) were employed

with little difference. The latter configuration greatly simplified our experimental set-up.

5. Acknowledgments

The authors wish to thank John Loomis, formerly of the Air Force Weapons Laboratory, for a very stimulating discussion of the problem and Mary Wendeln for performing the computations.

6. References

[1] Sparks, M. , J. Appl. Phys. 42_, 5029 [2] Sparks, M. and H. C. Chow, Proc. 3rd Conf.

[1971]. on High Power Infrared Laser Window Materials,

III , 1083 (1973).

7. Figures

THERMAL LENSING IN LASER WINDOW MATERIALS

_BEAM

POWER

PROFILE

THERMAL

PROFILE

Figure 1. A schematic presentation of thermal

lensing in a laser window. The far

field intensities are only artistic

interpretations of the intensity distributions. The beam power and

thermal profile shown are taken from

calculations made for ZnSe.

-145-
COMMERCIAL POWER

METER

SIGNAL CONDITIONING

TRANSMITTED POWER lo

FAR FIELD INTENSITY I

RATIOMETER

SCANNING

SLIT TRIG. PULSE ~

PYROELECTRIC

DETECTOR REF CHOPPER

TRIG PULSE

AMPLIFIER

SAMPLING

CIRCUITRY RECORDER

SIGNAL

CONDITIONING

Figure 4, Block diagram of the electronics used

to obtain the far field intensity

ratio. The ratio-meter, power meter,

and recorder are the only items not

custom fabricated for this experiment.

I METER

FOCUSING MIRROR

MIRROR CHOPPER (50%)

Figure 5. The effect of mirror substrates on the

system's baseline leasing when no winSCANNING

SLIT.

PYROELECTRIC

DETECTOR

SAMPLE

BEAM

DUMP

METAL MIRROR

REF CHOPPER

TRIG OUTPUT APERTURE

SHUTTER

COa LASER

Figure 3. Schematic view of the optical layout

employed in the far field intensity

measurements

.

.20

. 18

, 16

14

12

. 10 .08

,06

04

02

0

o METAL MIRRORS

o PYREX MIRRORS

20 40 60 80 100 120 140 INCIDENT POWER (WATTS)

160 180 200

-146-
10' -1—r^r I I I I r ~r Mill

5 10'

10'

n—I—I I I I I.

I X 10 IxlO^ (cm)

o DISC

o SQUARE

o RECTANGLE

1x10^

- Figure 6, The measured and predicted variation

of the power at which the far field intensity is reduced to 0.5 of the no- distortion value as a function of the

absorption coefficient 6.

Figure 7. The far field intensity ratio measured

as a function of input power. The data

are normalized to samples of equal

length in order to clarify the display.

This plot should be compared to figure 2.

1.4 L2

o 10

-^.9 8 0.9 6

I-

>:.92 1 .90

.88 5.86 Q

^ 84 ^ .82

if. 80

.78

.76

134

100 200 300 NORMALIZED POWER

4 00 500

COMMEWTS OH PAPER By VETKIO , al

It woj, polntzd out by E. BeAnaZ o{, HonzijwoM. that the. ipzakoA had oMuimd itAUi ei$((ecXi oAe

mgUglble. in zinc, ielenide.. The. thenmal dJj,tontlon Induced In that maXenlal li loAge, but li

geneAcMy liotAop-ic.

-147-
3.6 Spectral Emittance Measurements With a Cryogenical ly Cooled Instrument

D. L. Stierwalt

Naval Electronics Laboratory Center

San Diego, California 92129

We have been measuring spectral emittance of laser window materials for several

years. The sensitivity of our apparatus was limited not by detector noise, but by

background fluctuations caused by minute fluctuations in the temperature of the

instrument. Because of this limitation a new instrument has been built which operates

at 77°K. This greatly reduces the background radiation and allows us to take advantage of the higher sensitivity of a cooled detector. This instrument covers the

spectral region from 2.5 to 14.0 microns with a spectral resolution of about 1-1/2%.

The noise equivalent absorption coefficient appears to be about 10"^ cm"' at 10.6 microns and about 10"^ cm"^ at 5.0 microns.

Several samples of NaF, NaC£, and KBr have been measured at temperatures from

200°K to 373°K. A number of bands were observed in these samples, some of which have

been previously reported and some have not. These spectra will be presented as well

as a description of the instrument.

Key words: Cryogenically cooled; spectral emittance.

Previously reported spectral emittance measurements were made using an infrared spectrophotometer

operated at room temperature. The lowest value of emittance which could be measured with this apparatus was limited not by detector noise, but by noise caused by temperature fluctuations in the instrument. To eliminate this source of noise, an instrument was designed and constructed to operate at

liquid nitrogen temperature. At this temperature the background radiation from the instrument is so low that temperature fluctuations do not contribute significantly to the noise. Cryogenic operation

also allows us to use a photoconductive detector operating at 4.2°K and take advantage of the increased

sensitivity under low background conditions.

The monochromator for the new instrument is a narrow bandpass circular variable filter (CVF)

covering the 2.5 to 14.5 micron spectral region. The spectral bandwidth is about one percent of the

wavelength over the entire range. The filter is made in three sections, each covering about one octave.

The three sections can be seen mounted on the large gear in figure 1„ The sample mirror is mounted on

the optical bench. The tuning fork chopper is shown unmounted. In the lower left of figure 1 is the

slit plate.

In figure 2 the slit plate and chopper are in place. The detector mirror can be seen in this

figure.

Figure 3 shows the optical bench in place in the liquid nitrogen vessel. The sample holder and

detector are not shown in these figures since they are mounted on the end plates of the cold chamber.

All parts of the cold chamber are aluminum except for the mirrors which are copper. The cold chamber

is suspended inside a stainless steel vacuum vessel.

Some preliminary data taken with the instrument are presented here. Figure 4 shows the absorption

coefficient of a sodium fluoride crystal grown at NRL, identified as number 87. When this sample was run previously in the room temperature apparatus a shoulder was noted at about 6.5 microns, suggesting

the presence of an unresolved band. The higher resolution of the new instrument shows a sharp band at

6.4 microns and an additional small band at 7 microns.

The results of several measurements indicate that the signal from 4.5 to 5.5 microns is too high

because of long wavelength energy leaking through the CVF. Sodium fluoride has an emittance of nearly

unity from about 12 to 20 microns. This is near the peak of the Plankian distribution so that the outof-band energy from the sample is several orders of magnitude higher than the in-band energy. The outof-band transmittance of the CVF has been measured and found to be less than 10"= at the long wavelengths, but still more rejection is needed. Steps are being taken to install additional filtering.

The same is true of the 2.5 to 4.5 micron segment of the CVF. For this reason only the sharp

bands at 3.4 and 3.5 microns are shown in figure 4. In figure 5 these two bands are shown on an expanded

wavelength scale along with data for two other sodium fluoride crystals also grown at NRL. These

three curves were all taken at 294°K. The large variation in absorption coefficient indicates that the

bands are not intrinsic properties of the sodium fluoride.

Figure 6 shows the absorption coefficient at three temperatures for a freshly cleaved sample of

sodium chloride of unknown origin. Again the absolute level is somewhat uncertain because of

-148-
insufficient filter rejection. Sharp bands can be observed at 4.7, 6.3, and 6.5 microns. The bands at 3.4 and 3.5 microns were also observed in this sample, although much weaker than in the sodium fluoride

samples.

In figure 7 we see data on commercially grown potassium bromide. These curves cover a wider temperature range than those in the previous figures. This data demonstrates dramatically the effect of long

wavelength leakage through the CVF. The apparent absorption coefficient at 4.5 microns is nearly

proportional to the ratio of in-band to out-of-band energy for the three temperatures. This suggests

that the signal is due almost entirely to long wavelength energy and the actual absorption coefficient

at this wavelength is undetermined, but probably less than 10"^. The only reliable data, then, is the

band at 7.2 microns. No trace of bands at 3.4 and 3.5 microns was seen in this sample.

In summary, the preliminary data show that the new instrument has much higher resolution than the

earlier instrument, and greater sensitivity at the longer wavelengths. Sensitivity at the shorter

wavelengths is limited at present by long wavelength energy leaking through the CVF. Steps are being

taken to add additional filtering to the instrument. This is expected to improve the short wavelength

sensitivity by at least two orders of magnitude.

NOTE: Micron(s) as used in this paper expresses micrometer(s).

-149-
Figures

-150-
-151-
-152-
Figure 4. Absorptance coefficient of sodium fluoride at 294 Kelvin.

NOTE: Microns as used on figures 4-7

expresses micrometers.

-153-
Figure 5, Absorptance coefficient of three samples of sodium

fluoride in the 3.5 micron region.

-154-
Figure 6. Absorptance coefficient of sodium chloride at 323, 373, and

294 Kelvin.

-155-
coMUEtrrs on paper 8/ stjerwalt

In fL2^poni& to a queition {,fiom the. OLadie^nca. thu 6pzak2A indicated that no 6pe,clal tAzatment o^

i,ampl(L i,uA{,aceA cUiidz {^fiom nonmal cleaning pKoczduAsj) wca lu^d. The pKumcz oi abioKptlon bandi cut 3.4 and 3.5 mtaKotmt&Ai might be du.e to contamination oi tkd iuA^acd bij hydAocjaAboni.

-156-
3 . 7 Surface Studies with Acoustic Probe Techniques

J. H. Parks and D. A. Rockwell

Departments of Physics and Electrical Engineering

University of Southern California

Los Angeles, California 90007

A new measurement technique will be discussed which utilizes

acoustic surface waves to detect the surface depth dependence of

radiative absorption. This will include a theoretical analysis of

the detection process which relates these measurements to a quantitative model of surface absorption phenomena. This technique is

generally applicable to study weak surface absorption at wavelengths

for which a material is essentially transparent. Surface wave phase

variations induced by the absorption of 10. 6 yum radiation have been

measured on KCl surfaces with a laser pulse energy of only '^'50 mJ. The surface absorption a of these samples were measured by calorimetry to be 0-2 x 10"^ and acoustic probe measurements on the same

samples yielded a~l x 10" . Initial results of recent studies on both polished and etched alkali halide surfaces are discussed.

Kev words: Acoustic probe techniques; infrared window laser absorption;

KCl absorption; laser induced damage; surface absorption.

1. Introduction

Infrared absorption measurement techniques have been developed [I]''" which avoid the effects of

the surface contribution. However, the surface absorption represents a practical limitation of high

power laser optical materials and an understanding of the details is needed. The principle effort of

our research program has been to develop a new measurement process [2,3] which utilizes acoustic surface waves to detect the surface depth dependence of radiative absorption. In addition, a detailed

theoretical analysis of this detection process was derived to relate these measurements to a quantitative model of surface absorption phenomena.

These new experimental techniques and the related theoretical analysis involved advances beyond

the current state of acoustic surface wave technology. In particular, it was necessary to develop

techniques to propagate surface waves on non-piezoelectric solids which did not alter the surface characteristics under study nor require any special surface treatment.

Initial results of recent studies are presented which measure radiative absorption on KCl surfaces. Surface absorption of 10.6pm radiation was measured for both mechanically polished and chemically etched surfaces.

2. Surface Wave Detection of Radiative Absorption

A fundamental understanding of the physics involved in the surface wave detection of radiation

was obtained in quantitative studies [2, A] on well known acoustic materials (a-quartz, lithium niobate)

.

It was shown that surface wave propagation in a region heated by radiative absorption could be accurately described by a laser induced transient phase change given by

L

A(l)(t) = f [e^(t)-a^AT(t)]dr. (1) so o

Equation (1) is valid when the temperature distribution is essentially uniform over the surface wave

penetration depth, or 9T/3z«T/X . Here X is the acoustic wavelength and z is a distance normal to the surface. The phase change arises from a change in propagation path length via thermoelastic

strains, e^(.t) , and also a change in wave velocity through the temperature variation of the elastic

coefficients represented by a AT(t). In eq. (1) V is the frequency and v is the velocity of the sur- face wave, 2L is the acoustic path length, and r is the distance along the propagation path. The laser

induced temperature change AT(t) is found by solving the heat equation, and has the spatial dependence

of the Gaussian laser profile. The factor fg accounts for the incomplete overlap of the acoustic beamwidth and the temperature profile.

* Research supported by Advanced Research Projects Agency.

1. Figures in brackets indicate the literature references at the end of this paper.

-157-
Experimental measurements of A(}) on a-quartz and lithium niobate were compared with theoretical

calculations of eq. (1) using no adjustable parameters. The quantitative agreement between theory and

experiment was within 15%. In these experiments the radiative pulse widths were long enough for heat

diffusion to produce a uniform temperature within the surface wave penetration depth. In this way the

need to account for propagation in the presence of an axial temperature gradient was avoided, and the

resulting analysis provided a clear picture of the fundamental surface wave thermal interaction leading

to the induced phase changes.

2.1. The Effect of a Temperature Gradient

Depending on the surface wave frequency and the extent of thermal diffusion, it may not be valid

to assume that the temperature is uniform over the surface wave penetration depth. In the presence of

such a spatial inhomogeneity, surface wave propagation becomes dispersive. In this case, measurement

of the frequency dependence of the induced phase change yields information about the temperature distribution, and consequently, the surface absorption properties.

The contribution of the velocity variation to the phase change in eq. (1) cannot be represented

by c^AT for propagation in the presence of an arbitrary temperature gradient. In this case the phase

change is given by

L Av

A* = - ^

f, V B

s V

. SOJ

dr, (2)

X

s when the strain contribution is negligible. The wavelength dependent velocity variation [AVg/Vg^Jj^

follows from a perturbative treatment [5] of the wave equation in which the elastic coefficients exhibit spatial variation through their known temperature dependence. The perturbation approach allows a precise model of radiative absorption to be easily incorporated into the analysis.

Experimental measurements of the dispersive phase change have been made on Y-cut a-quartz [4]. In this case the exceptionally high bulk absorption coefficient of 241 cm"! at 10.6 \sm in quartz is

responsible for a significant temperature gradient over the acoustic penetration depth. In the following section it will be shown that a surface absorption layer extending only several microns into a relatively transparent solid will also induce a temperature gradient and dispersive wave propagation.

3. Infrared Surface Absorption

In weakly absorbing materials the radiative absorption coefficient within 1-lOym of the surface

can be significantly different than values for bulk absorption. The extent of this surface absorption

depends on surface preparation including mechanical polishing and chemical etching procedures, immediate environmental impurity adsorption, and perhaps on the effects of surface dislocations induced

during sample preparation. The resulting effect of these surface perturbations can be described by

taking the radiative absorption coefficient to be a function of depth below the surface 3(z). A simple

absorption model for weakly absorbing materials is shown in figure 1. The highly absorbing surface

region, 3,cm~-'-, extends a characteristic distance d into the material at which a transition to the

bulk value PrjCm" occurs. This model adequately represents the material absorption properties in the

region probed by the surface wave. Although the sharp transition region from 6^ to gg may be somewhat

unphysical, thermal diffusion during the laser puslewidth causes the acoustic probe measurement to be

relatively insensitive to the finer details of the model.

Applying this model to express the intensity absorbed, we solve a one-dimensional heat equation

to find the resulting temperature increase in terms of the parameters AT(6g,d,Bg). The peak dispersive

velocity variation is then derived using this temperature evaluated at the time t=Tp which yields

:^AT^(Bg,d,3g). A^CXg/Zrf) . (3) n

Here the sum is extended over the fourier temperature coefficients ATj^ which characterize the absorption depth distribution. The factor A^ contains the dispersive behavior as a function of the variable

Xg/Z'p, where Z.p(Tp) is the characteristic depth of the temperature distribution at t=Tp. This depth

is determined primarily by thermal diffusion and thus dependent on the pulsewidth Tp. The variable

becomes \q/Zj«1 for long times during which the temperature becomes uniform over the surface wave penetration depth, or for high frequency surface waves. In this case Aj^ approaches a constant independent of wavelength indicating the ordinary regime of dispersionless surface wave propagation.

When the pulsewidth Tp is short enough so that d:£Zrp<<Xs, the measured phase changes will be

sensitive to each of the parameters ggj d, and 3g independently, and the complete distribution of the

absorption coefficient may be determined. However, if the pulsewidth is long enough so that d«Z'j^Xs,

-158-

Av

_ so_
dispersive propagation may be observed but the phase change will essentially be sensitive only to the

product a=6gd. This parameter a, usually referred to as the surface absorption, is included in calorimetric measurements of the percentage power absorbed by a sample of length L in the form

^ = 2a + BgL. (4)

When the surface absorption, 2a, from both surfaces is comparable to the bulk absorption 3gL, the

surface contribution is not separable by calorimetric techniques. However, the acoustic probe technique

will yield a independently even in this case, since Bgd<<Bgd is usually satisfied in weakly absorbing

materials.

3.1. Application of the Absorption Model to KCl

In KCl the bulk absorption of good samples is known [6] to be as low as 7xlO~^cm However,

radiative absorption occurring in a thin surface layer is significantly greater, and leads to appreciable temperature gradients within the acoustic penetration depth. As an example the absorption model

shown in figure 1 is applied to radiative absorption at 10.6ym in KCl using the parameters Bg=10cm~-'-,

d=2ym, and 3g=2xlO~3cm~l. The resulting temperature depth profiles are shown in figure 2 for radiative

pulsewidths of 10 msec and 100 msec. In the absence of a highly absorbing surface layer, the presence

of such temperature gradients at 10 msec would require a bulk absorption coefficient of ~200 cm"-'-. The

measurement of dispersive surface wave propagation resulting from these temperature gradients is then

a direct indication of the existence of highly absorbing surface layers on weakly absorbing materials.

The depth profiles of the acoustic energy are also shown in figure 2 for surface waves of frequency

28 MHz and 124 MHz. For Tp=10 msec we note that d<<Z^~Xg resulting in the conditions for dispersive

propagation. However, for Tp=100 msec, dispersion would not be observed for these frequencies since

Zip»Xg. In both these cases Zx>>d, which implies the acoustic probe phase changes would measure the

magnitude of a and not the complete set of distribution parameters.

The phase changes A4) induced by the temperature gradient are shown in figure 3. The phase change

per input laser power is plotted versus radiative pulsewidth. The curviture in these graphs is a measure of the effect of thermal diffusion. The acoustic probe at 28 MHz, with a penetration depth of

'V'lSOum, is much less sensitive to the redistribution of thermal energy than the 124 MHz probe having a penetration depth of '^'35iJm.

In the limit of long pulsewidths, we note that (A(l)28MHz/Act)124MHz)= 0.23 which is simply the

frequency ratio as predicted by eq. (2) for the phase change in this non-dispersive limit. However,

for Tp=10 msec, we find (A(})28MHz/A(j)124MHz)=: 0.19 which indicates the dispersion effects the order of

20%. To emphasize the effect of dispersion, figure 4 shows a log-log plot of the reduced phase change

(A(j) per unit input power and per unit frequency), which eliminates the explicit frequency dependence

given in eq.(2). These curves should coincide as dispersion vanishes for long T^. The increasing

effect of dispersion for short Xp reaches '^'43% at Tp=0.1 msec.

4. KCl Surface Studies

4.1. Coupling Surface Waves to KCl

The application of the surface wave measurement technique to KCl requires the ability to propagate waves on the surface of a non-piezoelectric material. This wave propagation must be generated by

a technique which does not affect the surface properties under analysis. The technique should also be

easily applicable to samples without additional preparation. A fluid coupling technique has been

developed which satisfies both these requirements. As shown in figure 5, the surface wave is initially

excited on a piezoelectric material (a-quartz), and subsequently coupled through a fluid layer onto the

KCl surface. The inverse coupling returns the wave to the detection transducer. Although this method

has previously received relatively little development, it is of a particular advantage for our purpose.

Our research has included an experimental study [7] of a fluid coupling technique which has

resulted in successfully coupling surface waves to KCl. Phase changes on piezoelectric samples using

the fluid coupling technique were compared with the results using direct surface wave excitation by

interdigital transducers. This comparison verified that the laser induced phase change was reliably

and reproducibly coupled through the fluid regions. Ortho-dichlorobenzene was used as the coupling

fluid in these experiments. An experimental transducer configuration has been designed which accepts

arbitrary samples sized and provides ample access to the sample surface for the Infrared laser sources.

-159-
4.2. KCl Experimental Results

Experimental measurements of induced phase changes on KCl were obtained as shown in figure 6. The fluid layer used to couple waves to the KCl surface has been deleted for clarity. The RF reference

signal and the transmitted RF signal are combined in a double balanced mixer which provides an output

proportional to the sine of the phase change. The amplitude Vq was typically ~200 mV. The peak phase

change signal was detected by a digital peak voltmeter and averaged by a PAR Boxcar. Surface wave

frequencies of 124 MHz, 62MHz, and 28MHz were used in these studies.

A CO2 laser pulse of ~65 watts was incident on the KCl samples with pulsewidths ranging from 2 msec to 100 msec. The pulse repetition rate was varied to maintain a constant duty cycle, D=.05, and

the average power for each pulsewidth value was <P>=;3 watts. The signal/noise improvement ratio provided by the Boxcar (during 80 sec exposures and a time constant of 1 sec) was 15-30 for the shortest

pulsewidths. Typical KCl phase change signals detected after amplification by the peak detector are shown in figure 7. Note that the signal detected for a pulsewidth of Tp=0.5 msec required a CO2 laser

pulse energy of only ~35 mJ. The RMS phase noise at 124 MHz observed in the absence of incident

radiation was 6 (A(J) ) 0 . 2 mrad.

Induced phase change data on KCl are shown in figure 8. Polished and etched samples of Harshaw

KCl were measured and are referred to as Chemical Polish. These samples were then repolished mechanically at use using standard techniques and measurements taken are referred to as Mechanical Polish.

Although 3- frequency data were obtained for the Chemical Polish sample, mechanical repolishing introduced enough surface roughness to cause excessive scattering at 124 MHz. Previous measurements on expertly polished Hughes samples have been obtained routinely at 124 MHz. The absolute experimental

uncertainty in the phase change measurements was typically 15%.

The data indicate that chemically etched surfaces provide a reduction of the surface absorption

a by at least a factor of 3-4. Similar results have been observed [8] in calorimetric measurements

of KCl absorption. Figure 9 shows evidence of dispersive phase changes measured on the chemically

etched sample. This is interesting since it indicates the presence of a surface absorption layer even

after several microns of material have been removed during a 60 sec etch in concentrated HCl. An

initial analysis of this data yields a surface absorption value,a= 1x10"^, for the chemically polished

Harshaw sample. Calorimetric measurements on the same sample yield consistent results,a= 2x10" , assuming a value of 6b=10~-^ cm"-*- used in eq. (4).

5. Acoustic Probe Sensitivity

The present results indicate the sensitivity of these surface wave techniques. If It is only required to obtain a relative measurement of surface absorption for different samples, the measurement

can be taken at a single frequency and a single pulsewidth. We have found a phase change A(J)=60 mrad at

124 MHz for a 100 msec, 70 watt pulse on a surface with o~lxl0~^. A surface absorption of a~2xlO"6

could be detected, Acjj-l mrad, with the same laser pulse. This limit could be reduced further by operating at higher laser power. It is also important to point out that a laser pulse energy of only <25 mJ at Tp=l msec would be required to measure a~lxlO"^ with a phase change A<j)~l mrad.

However, dispersive phase change measurements require data at lower frequencies and shorter

pulsewidths. Since the phase change is directly proportional to the frequency, the sensitivity will be

reduced if depth dependence of the absorptive properties is of interest. For example, the smallest

phase change at 28 MHz was measured for a 2 msec, 70 watt pulse on a surface with a~lxlO~ . Since this was a factor of 2-3 above the noise, a surface absorption of a~5xl0~5 represents a lower limit at this

power for measuring an accurate, absolute value for a from dispersive phase changes. This limit is

comparable to the lowest surface absorption obtained on etched samples [6]. If it is desirable to determine the surface absorption parameters Bs and d independently, it is necessary to use radiative

pulsewidths on the order of Tp<10 ysec. Present TEA lasers (CO2, HF, and DF) can provide sufficiently

short pulses of high enough energy for such measurements.

6. Concluding Remarks and Planned Applications

The principle accomplishment of this research program has been the successful development of the

surface wave measurement process and its associated theoretical description as a reliable research tool

to detect and measure the details of weak surface absorption. This technique has been applied to stud- ies of KCl after a method was developed to couple surface waves to non-piezoelectric samples. Results

on KCl indicate a high sensitivity to the details of low level surface absorption. Current research

plans include:

- measurement of 6(z) distribution on KCl surfaces

- measurement of a on ZnSe

- absorption in thin films

- impurity diffusion into alkali-halide surfaces.

-160-
7. Acknowledgments

The authors wish to thank S. Allen, and A. Timber, Hughes Research Laboratories, for the KCl sam- ples and their preparation. They also thank D. TVhite and R. King, China Lake, NWC, for transducer

fabrication.

8. References

1. M. Haas, J. W, Davisson, H. B. Rosenstock, and

J. Babiskin, Proc. Int. Conf. Opt. Prop. Highly Transparent Sol . , paper VI-B (Waterville,

N.H. 1975).

2. J. H. Parks, D. A. Rockwell, T. S. Colbert, K, M. Lakin, and D. Mih, Appl. Phys. Lett. ^5,

537 (1974).

3. D. A. Rockwell, T. S. Colbert, and J. H. Parks,

in Proceedings of the International Conference

on Optical Properties of Highly Transparent

Solids , edited by S. S. Mitra and B. Bendow

(Plenum, New York, 1975).

A. D. A. Rockwell and J. H. Parks, "Theory of

Acoustic Surface Wave Detection of Radiative

Absorption" (submitted for publication in J. Appl. Phys.).

5. B. A. Auld, Acoustic Fields and Waves in

Solids , (Wiley, New York, 1973) Vol. II, p. 294.

6. T. F. Deutsch, Appl. Phys. Lett. 25,109(1974).

7. D. A. Rockwell, and J. H. Parks, "A Technique

for Acoustic Surface Studies of Non-Piezoelectric Materials" (to be published in J. Appl.

Phys . )

.

8. H. G. Lipson, private communication.

8. Figures

ABSORPTtON MODEL- IR W/NC>OI^ MATERIALS

Surface Absorption Bulk Absorption , Coefficient (crn'M » Coefficient (cm"M

Pa

Figure 1. Absorption model for infrared window

materials. The distribution 6(z) is

used to calculate the temperature distribution and the induced phase

changes measured on KCl surfaces.

Figure 2. Temperature depth profiles for 10 and

100 msec laser pulsewidths calculated

using the absorption model parameters

6s - 10 cm"-'-, d = 2 ym and

3b = 2 X 10" -^cm"-"-* The surface wave

energy profiles at frequencies of 28 MHz

are shown for comparison.

TEMPEI^ATURE AA/D SURFACE IVAVE DEPTH PROFILES OA/ KCt

30 IZO DEPTH (yum)

-161-
5

Figure 3.

I r LASER INDUCED ON KC-I

d = 2 yum

' 2 '/O

1 r PHASE MANGE

eo -fo 60 eo LASER PULSEH'/DTH T Cmsrc)

Laser Induced peak phase change Aej) (mrad per incident laser power) versus

laser pulsewidth is shown for surface wave frequencies of 28 MHz and 12A MHz.

LASER PULSEIVIDTH Tf, (msec)

Figure 4. Log-log plot of neak reduced phase

change (A(|) per frequency and per incident laser power) versus laser pulsewidth. Dispersive propagation is indicated by divergence of graphs for

different surface wave frequencies.

TRANSDUCER CRYSTALLASER ACCESS HOLE

SAMPLE

FLUID COUPLING REGION

i-iNTERDiGiTAL TRANSDUCER

Figure 5. Schematic of the fluid coupling technique. Fluid interfaces couple the

surface wave from (to) the piezoelectric transducer crystal to (from) the

KCl sample.

TRANSDUCER

CRYSTAL

PEAK

DET. STABILIZED VHP GENERATOR

Figure 6. Experimental apparatus for measuring

laser induced phase changes on KCl

surfaces

.

KCl - HARSHAW , CHEMICAL POLISH

I0.6f^^ , 10\M , QAUSS/AN (HWHH-I U-Myr.)

"Cp- /oo >MS

Figure 7. Typical oscilloscorie data traces of the

transient phase change induced on KCl

by laser pulses of 0.5 and 100 msec pulsewidths. Scales are indicated for

each trace as we'll as other pertinent

data. 6(Ac|))fj is the RMS noise value of

the phase change measured in the

absence of radiation.

-162-
KCl- CUCMICML POLISH

KCI- MeCHANICAL POLISH

io *o to oo too LASER PULSEi¥IOTH TT f'rwj-.cj

» 10

1

1 1 r

PHASE CHAUGE DISPERSION ON KC£

T

chem/cal polish

\

\

2S MHz

LASER PULSEW/DTH Tf. (msrc)

Figure 9. Phase change dispersion versus laser

pulsewidth is indicated by the increasing divergence of A(J) as Tp decreases.

This data was taken on a chemically

polished surface and experimental un- certainty is ± 15%.

Figure 8. Phase change data versus pulsewidth at various frequencies for chemically

polished and mechanically polished KCl

surfaces. Bars indicate ± 15%

experimental uncertainty.

COMMEWT OW PAPER 8/ PARKS, oJL

In mnk ca/iAi^d out cut thz kUi Tones. Cojnb'Udg^ RuzaJich LaboKatonJ^U aX wa6 obiz/iv^d that

abiofiptlon dfLoppe.d by iignidicant ^actou Mhm iuA(,ace. cZzanlng wcu> apptie.d to tht i,ampt^ giving

{^afithoA zvldmcz tht dominant eo'j^eci o;$ iuA^acz abioKption In doXeAminlng thz n^i,ldwat ton

in theAe. matznJjxlif>

.

-163-
3.8 photoelastic Constants of Infrared Materials*

Albert Feldman, Deane Horowitz, and Roy M. Waxier

National Bureau of Standards

Washington, D.C. 20234

Optical distortion occurs in infrared laser windows due to thermal gradients

established by the absorption of high power radiation. Principal sources of distortion are the stresses produced by the thermal gradients. Because of the photoelastic

effect, these stresses, which vary from point-to-point in the material, produce

changes in the refractive index. We are measuring the photoelastic constants of

infrared window materials. The measurements employ null techniques which have several

advantages over other techniques that have been employed in the past. We present data

obtained on Ge, reactive atmosphere processed (RAP) KCl, and KCl doped with KI.

Key words: Birefringence; germanium; KCl; refractive index; single crystal; stress

optic constants

.

1 . Introduction

Stress can produce inhomogeneous refractive index changes that may be highly deleterious in optical

components intended for diffraction limited operation. In high power laser windows, stresses may

result from thermal gradients caused by the absorption of intense laser radiation [1]^. The stressoptical constants are important parameters needed by laser system designers for calculating the effects

of stress on the optical properties of these windows. In this paper we present the methods we employ

for measuring stress-optical constants of important infrared window materials. These methods employ

null techniques, which have the advantage of being relatively independent of intensity fluctuations in

the radiation sources used [2] . Most of the techniques have been discussed previously [3] , but are included for completeness. We present data obtained at 10.6 pm on specimens of single crystal Ge, single

crystal KCl grown by the reactive atmosphere process (RAP), and single crystal KCl doped with KI . Data are also given for both types of KCl at 0.633 ym and 0.644 ym. Measurements will be continued on these

materials so as to improve the precision of the data.

2. The Stress-Optical Constants

The stress-optical effect arises from the dependence on stress, Oj^^.' °^ optical dielectric tenso) K. .. By convention t^is relationship is expressed in terms of the change of the reciprocal of the

di^lectric tensor, K. . (or dielectric impermeability) , thus

6k"-!- = q. ... a (1) 13 ^i]kl kl

where the stress optical constants q. .^^ are the components of a fourth rank tensor and the indices

ijkl each take on values 1, 2, 3. tIiZs relationship is frequently expressed in a system of contracted

indices (also called the Voigt notation) , thus

«k"^= qo^ (2) m Tnn n

w'nere m and n each take on values 1-6. Nye [4] discusses in detail the relationships between the

tensors in the full notation and in the contracted notation. For the purposes of this discussion, we

use the contracted notation.

of the

Incubic

general,

class

there

m3m,

exist

there

36areindependent

only three

stress-optical

independent constants,

constants,

<3-].i'

but

'^12'^"*^

for our

^^44'

crystals,

obtain

which

all

areI

these coefficients requires three independent measurements of the change of refractive index with stress,

but at least one experiment must measure an absolute change of refractive index. j

* This work is supported in part by the Advanced Research Projects Agency of the Department of Defense.

1. Figures in brackets indicate the literature references at the end of this paper.

-164-
3. Stress-Induced Birefringence

In the present context, stress-induced birefringence is the measure of the relative refractive index change caused by a uniaxial stress. The change is relative because it measures the difference between the changes in refractive index of the two normal polarization modes of radiation propagating

through a stressed crystal.

In this experiment, stress is applied along the axis of a specimen in the form of a rectangular

prism that has a square cross-section perpendicular to the long axis. The ratio of height to width of

a prism is approximately 3:1. For a cubic material two specimens are used with the axis of one speci- men along the [100] direction and the axis of the other specimen along the [111] direction. These axes

are chosen because the refractive indices are independent of the direction of radiation propagation in

the plane perpendicular to the stress axis. The radiation that propagates into the specimen is polarized at 45° with respect to the stress axis. The state of polarization of the emerging radiation, which

is determined by the stress-induced birefringence, is analyzed by any of several methods, depending upon

the size of the birefringence, and wavelength of the radiation. In the case of Ge, where the stressoptical effect is large and the specimen can support a large stress, one need only to place an analyzer

at 90° with respect to the incident polarization angle and photometrically detect nulls or fringes as a function of applied stress. The fringe count, N , per unit applied stress is

B

_ m n'^ { 'i - 'J.-^-^ ~ [100] stress

da 2 A (3)

•q = q^^ for [111] stress

where n is the zero stress refractive index, X is the wavelength, t is the specimen thickness, and m, which is the number of passes the radiation makes through the specimen, equals one. Increased sensitivity is obtained by allowing multiple passes.

In the case of KCl, it is impossible to obtain a minimum of one fringe, even for multiple passes of

the radiation, because the material cannot support large stresses. We .therefore, employ a birefringent

compensator technique for measuring the induced birefringence. For measurements at 10.6 ym we place a specimen of Ge in a stress frame between the KCl specimen and the analyzer. With zero load on the KCl, we stress the Ge to the first null position. After applying a stress to the specimen which removes the

null, we adjust the stress of the Ge until the null is restored. Because the Ge has been previously

calibrated, we can compute the stress-optical constants of the KCl. For measurements using the above

technique in the visible, we employ either a stressed plate of fused silica or a Soleil-Babinet compensator .

Even greater accuracy and precision is possible in the visible if we employ another technique which

makes use of a de Senarmont compensator. However, the apparatus for this technique is unavailable for

the infrared.

In all cases, multiple passes of the specimen increases the sensitivity of measurement. In figure

1 we show an 8 pass arrangement of the birefringent compensator technique. Multiple passes, however,

do not always help if internal strains occur in the specimens. These internal strains can be a major

source of error [5]

.

4. Absolute Change of Refractive Index

In order to calculate the individual values of q-j^-^ and qi2' "® require a measurement of the absolute change of the refractive of index. For this purpose we use Twyman-Green and/or Fizeau interferometers .

The Twyman-Green interferometer is assembled from components with a laser as the radiation source. The specimen, which must have sufficient optical quality so that fringes are observed at the interferometer output, is mounted in one arm of the interferometer. The shift of fringes as a function of stress

is either detected photometrically or viewed on an imaging device. Pyroelectric detectors are used in

the infrared at 10.6 ym and photomultipliers are used in the visible and near infrared. Fringes at

10.6 urn are viewed with a thermal image plate or a liquid crystal sheet, while in the visible and near

infrared, fringes are viewed with a Si matrix tube and TV-monitor.

-165-
The fringe shifts per unit applied [100] stress for stresses parallel and perpendicular to the

polarization of the radiation are, respectively,

AN

Aa

1 2t fn^ , 1 - = — [— %1 - "12J

(4)

"^2 2t Aa - X

1^2 ^12 - ^12j '

where s-j^2 ^ component of the elastic compliance tensor. If s^2 "^^^ known, then it may be determined with Fizeau interferometry. The sample itself functions as a Fizeau interferometer if the faces are polished sufficiently parallel. Fringes are then obtained from the interference between reflections

from the front and back surfaces, and the shift of these fringes as a function of stress is measured in

the same manner as the shift of the Twyman-Green fringes. The fringe shifts per unit applied [100]

stress for stresses parallel and perpendicular to the polarization of the radiation are, respectively,

'\ 2t [J

A^ = ~ ~ %1 " ^12

r

3

[2 %2 - AN

2 2t

= r- %2 - ^2

(6)

(7)

With eqs. (4-7) we can obtain 'i-^-^' ^^2' ^^'^ ^12'

In certain instances, such as in the case of KCl, we are unable to produce a minimum shift of one fringe using standard Twyman-Green or Fizeau interferometers. For this case we have constructed a modified Twyman-Green interferometer that is capable of detecting a 0.01 fringe shift at 10.6 pm. A

schematic diagram of the interferometer for use with a CO2 laser is shown in figure 2. The two arms of the interferometer are in close proximity in order to minimize instabilities due to air currents and

vibrations. The effects of vibrations are also minimized by mounting the diagonal mirror onto the same base as the beam splitter and by mounting the two end mirrors on a common base. The specimen arm end

mirror, which is mounted on a piezoelectric translator, undergoes a sinusoidal translation

thus modulating the output intensity of the interferometer. The reference specimen at 10.6 ym

is a crystal of a Ge in a compression apparatus. In the visible the reference specimen is fused Si02-

We calibrate the reference specimen by measuring the force necessary to produce an integral number

fringe shift. Fractional fringes are then obtained by linear interpolation. In operation, the reference

specimen is stressed until the interferometer is at a null, which occurs when the fundamental harmonic

of the output intensity is zero. A given stress applied to the unknown specimen will shift the interferometer away from null, whereupon we compensate for this shift by applying an incremental stress to the

Ge which brings the interferometer back to null. From these data we then calculate the stress-optical

constant using eqs. (4) or (5) provided s^^ known.

5. Results

In table I we show the results we have obtained for Ge compared with earlier reported results [6,7].

We have studied Ge primarily because it is the reference material used in our measurements of the

photoelastic constants of other materials in the infrared. Ge is important also as an infrared window

material and an infrared acousto-optical material..

Our measurements of the relative stress-optical constants at 10.6 ym are compared with earlier

measurements [5] at 2.5 ym. From that earlier work, it can be shown that q2^2.~'5l2 ^ large dispersion, whereas q/^^ has a small dispersion. This is verified in our results, where the values of q^^/^ at 10.6 ym and 2.5 ym agree to within the experimental error, while the value of qii-qi2 10.5 ym contin- ues the trend of the dispersion of shorter wavelength values.

From our measurements of stress-optical constants, qn, qi2' and q^^, we have calculated the

elasto-optic constants [4], P-^i' P]_2 ^44' ^'^'^ find a significant difference with the corresponding

values obtained by Abrams and Pinnow [7] . Our results indicate that Ge has a smaller figure of merit

as an acousto-optic modulator than they calculate from their data.

In table II are presented the values we have obtained for the stress-optical constants of RAP KCl

and KCl doped with KI together with earlier reported values [5,8-15] . From our results we cannot infer

any difference in the coefficients of these two types of materials. The values for q, , ~

q, , are exper- imentally determined. ^

-166-
-12 3 Table I. Photoelastic Constants of Ge in Units of 10 m /N.

Coefficients 10.6 Mm 2.5 ym

^11

^111-^12

%4

-.84 ± .04

-.48 ± .03

-.36 + .02

-1.09 ± .05

-.20

-1.12^

'11

^12

44

.126

.154

.073

.27

.235^

.125

[6].

'^Absolute Values - [7],

Table II. Stress-Optical Constants of KCl in

Units of 10~"''^m^/N.

^11 ^12 ^11 ~ ^^12 i44 X (um) Ref

.

4.63

5.23

2.92

3.58

4.6 +.2 2.7 +.8

4.6 +.2* 2.8 +.2*

1.70

1.71

1.66

1.47

1.42

1.65/1.57

1.81

1.7 +.4

1.9 +.1

1.9 +.2*

1.9 +.1*

2.0

2.5 +1.0

1.5 +0.6*

-4.31

-3.62

-4.42

-4.94

-4.49/-4.74

-3.9 +.8

-4.4 +.2

-4.6 +.2*

-4.7 +.2*

-2.62

-4.3 +.9

0.589

0.589

0.589

0. 589

0.480

0.589

0.633

0.633

0.644

0.633*

0.644*

10.6

10.6

10.6

10.6*

8

9

10

11

12

13

5

This work

This work

This work

This work

14

15

This work

This work

Doped with KI.

-167-
There have been only two reported measurements of the absolute stress-optical constants q and q

but the values of these respective coefficients in the two reports differ [9,13] . Our results agree

most closely with values calculated from the results of Iyengar [9] who actually measured the elastooptical constants.

Our values for ^^-^'^-^2 ^'^ visible tend to be slightly higher than earlier reported values.

At 10.6 vjm we at present cannot attribute any significance to the different values we obtain and the

difference of our values from an earlier measurement [14]

.

Our values for q^^ in the visible appear to fall within the range of earlier reported values. At

10.6 pm, however, our values suggest a small dispersion when compared to the visible. This latter

result is in disagreement with an earlier measurement [15]

.

6. Acknowledgments

We thank Phillip Klein of the Naval Research Laboratory for supplying the RAP grown KCl and E. Berna of Honeywell for supplying the KCl doped with KI

.

7. References

[1] Bendow, B. , P. D. Gianino, A. Hordvik and

L. H. Sholnik, Optics Comm. 219 (1973) ; Bendow, B., and P. D. Gianino, Applied

Physics 2, 1 (1973)

.

[9] lyenger, K. S., Nature 176 , 1119 (1955).

[10] Bhagavantam, S. and Y. Krishna Murty, Proc. Indian Acad. Sci. A 46, 399 (1957)

.

[2] Birnbaum, G. , E. Cory and K. Gow, Appl.

Optics 13_, 1660 (1974).

[11] Bansigir, K. G. and K. S. Iyengar, Proc.

Phys. Soc. London 71B, 225 (1958).

[3] Feldman, A., I. Malitson, D. Horowitz,

R. M. Waxier, and M. Dodge, Laser Induced

Damage in Optical Materials : 1974 , NBS

Spec. Publ. 414 (1974)

.

[4] Nye, J. F. , Physical Properties of Crystals

(Oxford University Press, London 1957)

pp. 243-254.

[12] Srinivasan, R. , Zeit, f. Physik 155 , 281-289

(1959) .

[13] Rao, K. V. Krishna and V. G. Krishna Murty,

Proc. Indian Acad. Sci., Sci. 6£, 24 (1966).

[14] Chen, C. S., J. P. Czcznesniak, and J. C. Corelli, J. Appl. Phys. 46, 303 (1975).

[5] Wilkening, W. W. , J. Friedman, and C. A. Pitha, "Measurement of Stress-Induced

Birefringence in Alkali Halides," Third Conf. on High Power Infrared Laser Window Materials,

1973, AFCEL-TR-74-0085-I.

[6] Feldman, A., Phys. Rev. 150 , 748 (1966).

[7] Abrams, R. L. and D. A. Pinnow, J. Appl.

Phys. 41, 2765 (1970)

.

[15] Pitha, C. A. and J. D. Friedman, "StressOptical Coefficients of KCl, BaF , CaF , CdTe, TI-1120 and TI-1173," Proc. of the

Fourth Annual Conf. on Infrared Laser Window

Materials, Nov. 18-20, 1974. Compiled by

C. R. Andrews and C. L. Strecker (1975)

.

[8] Pockels, F. , Lehrbuch der Kristalloptik , Leipzig: Teubner (1906)

.

-168-

I
LENS BEAMSPLITTER

LASER I /

(

SAMPLE

\ .MIRRORS

LENS

COMPENSATOR'^ /

ANALYSER

Figure 1. Eight pass measurement of stress

birefringence.

OSCILLOSCOPE

LOCK-IN AMPLIFIER AC AMPLIFIER

MIRROR ON PZT DRIVE

Figure 2. Modified Twyman-Green interferometer for

measuring fractional wavelength changes

of optic path at 10.6 micrometers.

WO comitfr on paper BV FELPMAN, eX at

-169-
3.9 Refractive Index and Temperature Coefficient of Index of CVD Zinc Selenlde

Marilyn J. Dodge and Irving H. Malltson

National Bureau of Standards

Optical Physics Division

Washington, D.C. 20234

The refractive index of each of two prismatic samples of CVD ZnSe was measured from

0.5086 to 18.2 pm by means of the minimum-deviation method on a precision spectrometer.

Data were obtained at temperatures near 20''C and 34''C and each set of data was fitted to

a three-term Sellmeier-type dispersion equation, which permits refractive index interpolation within several parts In 10"'. Using the data obtained at the two temperatures, i dn/dT was calculated for both samples. A comparison of refractive index and dn/dT is | made with other types of ZnSe,

Key words: Refractive index; temperature coefficient of refractive index; zinc solenlde,

1. Introduction

The performance of a high-power laser sysl^em could be limited because of optical distortion of

the window material. It has been reported [1] that the effect of a heated window can distort the

beam and thus degrade the system at powers less than those required to fracture or melt the window.

To predict the amount of distortion which can occur, it is necessary to know the refractive index, n, temperature coefficient of index, dn/dT, and the thermal-expansion and stress-optical coefficients of

the material under consideration. An optical materials characterization program [2] is currently in

progress at NBS to determine these pertinent optical properties. Although much work has been done to predict the oretically the dn/dT of candidate laser window materials, there is a lack of experimental

data on refractive index and dn/dT to back up the theoretical calculations [3]. The ref ractometry 1 laboratory at NBS is determining the index of refraction and dn/dT of selected window materials over d

limited temperature range. 1

Chemical vapor deposited (CVD) zinc selenlde is an important candidate window material. This

polycrystalline material has a practical transmission range from 0.5 to 20.0 ym. A literature search

has revealed that a comprehensive study of the refractive index and dn/dT of this material over its

entire useful transmission range has not been reported. In addition to predicting distortion effects,

this data is needed for designing lenses, prisms, and other transmitting components for use in laser

systems operating in the 2 - 14 um range. This paper will present a study on ^he refractive index and

dn/dT for two samples of CVD ZnSe. Both samples were made by Raytheon Company and are arbitrarily

designated as sample A and sample B. Sample A was manufactured about 1971 and sample B in 1974.

Sample B appeared to be optically better material than sample A and is considered by the manufacturer

to be representative of what is now commercially available.

2. Experimental Technique

Both specimens were in prismatic form and were measured by means of the minimum-deviation method

using a precision spectrometer shown schematically in figure 1 [4] . In the visible and near Infrared

regions of the spectrum, the index was measured at known emission wavelengths of mercury, cadmium, and

helium. Beyond 2 pm, a glo-bar was used for the radiant-energy source, and measurements were made at known absorption bands of water, carbon dioxide, polystyrene, and 1,2-4 trichlorobenzene . A series of

narrow-band interference filters was also used between 3.5 and 10.6 ym. A thermocouple with a cesium

iodide window was used for the detector. This instrument has a scale that is accurate to within one second of arg. Therefore, the refractive index of good optical material can be measured within

1 to 2 X 10 over a wide wavelength range.

The index was determined for both samples at selected wavelengths from 0.5086 ym to 18.2 ym and

at controlled room temperatures near 20°C and 34°C. Each set of experimental data wa.s fitted by a least square solution to a three-term Sellmeier-type dispersion equation [5] of the form

This project is sponsored in part by the Defense Advanced Research Projects Agency.

1. Figures in brackets indicate the literature references at the end of this paper.

2. The use of company and brand names in this paper are for identification purposes only and in no case does it imply recommendation or endorsement by the National Bureau of Standards, and it does not

imply that the materials used in this study are necessarily the best available.

-170-
A.

A

N

I 2 2-

3=1 X -A.

3

The index of refraction is represented by n, A is the wavelength of interest in micrometers, the A.'s

are the calculated wavelengths of maximum absorption and the A.'s are the calculated oscillator

strengths corresponding to the absorption bands. The A.'s and~'the A.'s are not intended to have any

physical significance and are influenced by the wavelength range covered by the experimental data.

Primary emphasis is given to procuring a mathematical fit of the measured data useful for interpolation.

This equation will fail when an attempt is made to fit data too close to an absorption edge. Bec;^use

ZnSe is almost totally absorbing below 0.5 ym, it was not possible to fit the data below 0.5461 pm

for either sample.

3. Index Data

The constants calculated for each dispersion equation, the specified wavelength range, the number

of wavelengths, and the overall average absolute residual (the average difference between the experimental values and the calculated values) are given in table I.

Table 1: Constants for Dispersion Equations

Constant Sample A Sample B

20.3°C 30.8°C 20.8°C 33.5°C

4.2980149

0.62776557

2.8955633

0.19206300

0.37878260

46.994595

4.2466487

0.68520488

3.3114671

0.18807700

0.37665162

49.694957

4.4639521

0.46132463

2.8828867

0.20107634

0.39210520

47.047590

4.2366336

0.69295365

2.7641692

0.19283319

0.36954401

46.148359

No. of Wavelengths 33 19 38 34

Wavelength

Range (ym)

0.54-18.2 0.54-18.2 0.54-18.2 1.0-18.2

Average Absolute

Residual of 6.2

index X 10

15.6 4.1 7.4

It should be emphasized that these constants refer specifically to these two samples of CVD ZnSe under

the stated experimental conditions. The average residual is indicative of the accuracy of the experimental data.

The refractive index was calculated at regular wavelength intervals using each set of fitted

parameters. Figure 2 shows the refractive index plotted as a function of wavelength for sample B near

21°C. The index values range from 2.67544 at 0.54 ym to 2.32781 at 18.2 um and is 2.40278 at 10.6 ym.

In figure 3, the refractive index of sample B is compared with that of sample A and results obtained by other experimenters. The values for sample B are represented by the zero line. All of the data were reduced to a common temperature of 20°C, using the dn/dT values determined for sample B which will

be discussed below. Both single crystals studied by Marple [6] were in the form of prisms and the

index w^s measured at selected wavelengths from the visible to the near IR with a stated accuracy of

4 X 10 . From about 1.2 to 2.5 ym the indice^ of the two samples are about the same, differing from

the NBS sample by approximately -8 to -4 x 10 . A prism of Irtran 4, which is hot-pressed polycrystalline Zn^e, was measured by Hilton [7] at selected wavelengths with an estimated uncertainty

of 3 X 10 . Hilton's data were published in graphical form and the data shown were extracted from

the smooth curve. The indices are generally higher than the NBS values by about 4 x 10 and agree

with the data published on Irtran 4 by Kodak [8] which isn't shown here. The older CVD specimen of

-171-
ZnSe, sample A, is generally higher than_^he newer material, sample B, by 30 x 10 ^£^"1 the visible to

1 ym, then levels off to within + 8 x 10 between 1.6 and 10.8 ym, rising to 65 x 10 at 18.2 ym. At

10.6 ym, sample A is higher than B by 9 x 10

4. Temperature Coefficient of Index

Sample B was measured at 38 wavelengths at an average room temperature of 33. 5° C. An attempt was made to fit the data over the same wavelength range as at 20.8°C, but an acceptable fit could not be

obtained. Dropping all wavelengths below 1.0 ym and fitting only_^he longer IR data a fit was attained

with 34 wavelengths with an average absolute residual of 7.4 x 10 . The calculated index values at regular wavelength intervals for both temperatures were used to determine dn/dT as a function of wavelength from 1.0 to 18.2 ym. This data is shown graphically in figure 4. The solid portion of the curve represents the data calculated from the fitted values and the dashed portion represents values

calculated from the raw data. For a mean temperature of 27°C, dn/dT steadily decreases from about 15 x

10 /°C at 0.5461 ym tg a value of 7.4 x 10 /°C at 1.6 ym, remains fairly constant to 12.2 y^ then

decreases to 4.3 x 10 /°C at 18.2 ym. These values are considered accurate within 1.5 x 10 . However

the change in wavelength of the absorption bands used for wavelength calibration as a result of the i increase in temperature is not known. The positive dn/dT is typical of covalent crystals and is

indicative of a shift of the strong absorption edges to longer wavelengths. This could explain the

inability to fit the data in the visible region of the spectrum.

The dn/dT values obtained for the two CVD specimens in this study are compared with those of other

researchers [9,10] at 10.6 ym in table II,

Table II: Temperature Coefficient of Refractive Index of CVD Zinc Selenide at 10.6 ym

Reference

Dodge and' Maiitson

(Sample B)

(Sample A)

Method

Minimum-Dev ia t ion

Minimiim-Dev iat ion

Temperature Range (°C)

20.8-33.5

20.8-30.8

dn/dT X

10-^

7.4

6.8

Feldman, Waxier & Horowitz

Skolnik & Clark

Fizeau Inter ferometry

Laser-Doppler Interferometry

20-200

23-62

6.2

10.0

Kurdock Off-Normal Interference 24-69 8.0-9.1

The values range from a Igw of 6.2 x 10~^/°C to a high of 10^^ x 10~^/°C. The average of all the

values given is 7.8 x 10 /°C which is close to the 7.4 x 10 /°C obtained for sample B and the 8.0 x

10 /°C determined by Kurdock. The same method of measurement was not used in any two laboratories,

which could account for the discrepancies.

_j Skolnik and Clark [10] have also determined dn/dT for Irtran IV and found the value to^be 5.4 x

10 /°C at 10.6 ym. This is in fairly good agreement with a value of approximately 5 x 10 /"C determined by Hilton and Jones.

5. Conclusions

_Jhe results of this study indicate that the two samples of CVD ZnSe generally agree within 1 to 3 x 10 from 0.9 to 15 ym, the spectral region of current interest for use in high-power IR laser

systems. The discrepancies in the values of dn/dT determined by different observers could be due to ^

differences in the material; however, the different methods of measurement should not be ruled out as the cause. These data for the refractive index and dn/dT are only valid within the stated accuracies

for the specimens reported in this study.

-172-

I
6. References

[1] Sparks, M. , J. Appl, Phys. 42, 5029 (1971).

[2] Feldman, A., I. Malitson, D. Horowitz,

R. M. Waxier, and M. Dodge, Laser Induced

Damage in Optical Materials: 1974, NBS

Special Pub. 414, 141, (1974).

[3] Tsay, Y. , B. Bendow, and S. S. Mitra, Phys.

Rev. B, 8, 2688 (1972).

[4] Rodney, William S. and Robert J. Spindler,

J. Res. Nat'l. Bur. Stds. (U.S.), 51, 123,

(1953).

[5] Sutton, Loyd E. and Orestes N. Stavroudis,

J. Opt. See. Am. 51, 901 (1961).

[6] Marple, D.T.F., J. Appl. Phys., 35, (3), Pt.l,

(1964)

.

[7] Hilton, A. Ray, and Charlie E. Jones, Appl.

Opt., 6, 1513 (1967).

[8] Kodak Irtran Infrared Optical Materials,

Kodak Publication U-72 (1971)

.

[9] Feldman, A., I. H. Malitson, D. Horowitz,

R. M. Waxier, and M. J. Dodge, Proc. 4th Ann.

Conf . on IR Laser Window Materials, ARPA,

117 (1974).

[10] Skolnik, L.H. and 0. M. Clark, Appl. Opt., 13,

1999L (1974).

7. Figures

SCANNING

DEVICE

Figure 1. Schematic diagram of the modified

Gaertner precision spectometer showing

optical path. The prism is rotated at one-half the rotation rate of the telescope assembly by gear system, thus

maintaining the condition of minimum

deviation for any wavelength. The

scanning device drives the assembly

which scans the spectrum to Identify

lines or bands and determine their

approximate scale positions.

Figure 2, Refractive Index of CVD ZnSe (sample B) as a function of wavelength (logarithmic

scale). Data at 21°C were calculated

from the dispersion equation.

-173-
.006

.004

.002

0

.002

.004

.006

.008

.010

.012

.014

.REFRACTIVE INDEX COMPARISON

ZINC SELENIDE

All

V

I J L

0.1 0.5 1.0 5.0 10.0 WAVELENGTH, M m

16

12 -

CVD ZINC SELENIDE

TEMPERATURE COEFFICIENT OF INDEX

J L

0.1 0.5 1.0 5.0 10.0

WAVELENGTH, urn

50.0

50.0

Figure 3. Comparison of NBS index values for CVD

ZnSe (sample B) with NBS sample A and

values reported by other observers.

All data were reduced to 20 °C. Refractive index of sample B is represented

by the zero line. +NBS sample A,

CVD ZnSe; OMarple, single crystal;

Marple, single crystal; AHilton,

Irtran 4.

Figure 4. Temperature coefficient of refractive

index of CVD ZnSe (sample B) at a mean

temperature of 27°C. The solid portion

of the curve represents data calculated

from index values at two temperatures

determined from dispersion eauations,

and the dashed portion indicates values

calculated from raw data.

COmENTS ON PAPER BV VOVGE and MALUSOM

TheAz w)eAe iwo queJ>t^n& itlating to thz {^iXtlng 0($ thz Kz(,>iaaav2. indzx data untng the. SoJUbnoA-OA

e.qLLation. In fLZ4>poniz to a question conceAnJjiQ the. nwmbeJi o(^ te-Umi LU>zd tn the. i-it tkz ipmkzA Kupondzd

that -it u)a^ fiOund by t/Ual and eAAoi that a tlfiez-teAm SoJUbne/ieA equation pnoMtdad the. but i-it CLOni>Aj>tznt Mtth the accuAacy o{, the data. The. second que-itton axoie conceAning the a&e. o{, the

SeZbneA.eA equation mXhout conJitcJxng jjo/i the local f^tzld eHectA,. The ipeakeA indicated that tkii, point

had not been exploded iully, but that a iatlkdactofiy {^It woi obtained u-slng the SellmeleA e.quatlon without local (^leld conjiecXlom, , AnotheA question anoie conceAnlng the poalble te/rtpe/iatuAe nange oven, which minimum devlallon meoAuAemenX^ could be. made In the ipeakeA'i labonatoAy. The Aupome woi thaX. although to date only Koom tempeAatu/ie. mexu>uA.ement-!> had been made, It MOi poiilble to canxy out thzie.

meoiuAementi oveA a tesnpeAotuxe. nange iAom IB'C to 34°C.

-174-
3.10 Laser Window Test Facility^

Jacob L. Zar

Avco Everett Research Laboratory, Inc. 2385 Revere Beach Parkway

Everett, Massachusetts 02149

A test facility has been constructed for testing laser window material.

This utilizes the focused beam from a 15 kW CW laser at 10. 6 fxm or other

available lasers. Provision for cooling the window is included and also

instrumentation for measuring optical and mechanical effects.

The irradiated area may be varied by positioning the specimen. The

beam profile is very close to Gaussian. The intensity and beam profile is

given as a function of diameter and distance from the focus.

Forty-seven natural diamonds have been tested to find the correlation

between the UV absorption spectriim, the IR absorption spectrum and the

absorption coefficient (3 at 10. 6 fim. Transparency at 0 . 253 ym is roughly

correlated with low values of j3. A water cooled diamond window holder is

also described.

Key words: Diamond absorption coefficient; laser window heating;

laser window testing; Type II diamond windows.

1. Introduction

The development of high power lasers has simultaneously generated a requirement for the de- . velopment of windows capable of transmitting the laser beam from one environment to another without

significant distortion of the wave front. At one time, the best available windows were aerodynamic

windows. However, they are not suited to all interfaces. Therefore, an extensive program of

material development was funded by the U. S. Government to develop solid-state windows capable of

transmitting high energy and high intensity laser beams for all useful wavelengths. 'These programs

have born fruit in that there are now available improved window materials and coatings. There

has arisen the need for a facility where windows may be tested for suitability for both pulsed and

CW lasers.

This test facility should have the capability to measure changes in optical and mechanical

properties of the window while transmitting the laser beam. For this, one needs access to high power

pulsed and CW lasers, such as those available at the Avco Everett Research Laboratory, Inc. Therefore, an integrated research program has been authorized which calls for modification of an existing

10. 6 |j.m high power laser test facility to test windows of various types. It will also be available under

the authorization of our contract monitor to other laboratories who wish to test and evaluate their own

materials.

One of the available lasers at Avco is the HPL industrial laser. This can deliver a continuous

beam of radiation at 10. 6 \xm with a power of about 15 kilowatts. A separate test station has been constructed for window testing. This is described in the next section. The experiment is mounted on a special moveable optical stand so that it can be transferred to other lasers for testing at other wavelengths or with pulsed beams. One of the available pulsed lasers is the 10. 6 |j.m "breakdown" laser

which can produce 80 J pulses of about 100 microseconds duration. When focused, it yields intensities

approaching 10° W/cm^. Other lasers with even shorter pulse lengths, higher power, or other wavelengths are available when scheduled.

This program is supported by the Advanced Research Projects Agency and monitored by AFCRL

under contract No. Fl 9628-75-C-0066.

*Trademark of AERL, Inc. -175-
2. Description of HPL Industrial Laser Test Facility

The laser is a closed cycle CO^ nnachine producing a 10. 6 |am beam of approximately 15 kW [1] . The output beam is coUimated with a mameter of 3. 8 cm and is transferred by mirrors and actuators

to several work stations. Each work station may command the laser and has its own safety interlock

system. When one is in use, the other stations may safely be entered for test preparation. The laser

power control has closed loop feedback. The power, its rate of rise and fall and the exposure time may

all be varied by controls at each test station.

The window test station is the third to be added to this laser. It is housed in a room about 6 ft. sq. with lucite panel walls. The experimentor may view the experiment and operate the laser from outside the room with complete safety.

The laser beam is brought to a sharp focus with an f/21 telescope. Diamonds and other small

specimens may be put at the focus. For larger beams, the test specimen can be located off-focus. The

laser mirrors and the holder for the test specimen are cooled by circulating water through a closed

cycle heat exchanger. After passing through the test specimen, the beam is received in a calorimeter

and the power measured. Alternatively, it may be absorbed in a beam dump. An exhaust is provided

for removal of fumes generated either from the test specimen or from the beam dump. Compressed

gases are available for flushing the surface of test samples.

For instrumentation of the test, a second laser beam from a visible laser such as helium neon,

is passed through the window at an angle to the heating beam. The window is included in one leg of a Mach-Zehnder interferometer. Changes in the window are observed by interference and may be photographed. There is also available one or more thermocouples and a strip chart recorder for recording

temperatures. Other types of instrumentation may be added as required by the experiment.

Figure 1 is a photograph of the facility used for testing small windows at the focus of the laser

beam. A small diamond plate is mounted in the center of a holder and is edge cooled. The laser beam

is focused through the window by two copper mirrors seen at the right above the sample holder. The

interferometer and probe laser are positioned around the specimen holder.

A test was made of the power and distribution of the beam at several distances from the focus.

This was done by allowing the laser beam to pass through apertures in polished copper plates and

received on a flat plate calorimeter. The calorimeter has an anoidized aluminum surface and absorbs

. 995 of the instant laser energy at 10. 6 u-m [2] . The temperature rise of the plate is compared with

that of a reference plate in the laser shadow by twelve differential thermocouples in series. Correction

is made for the cooling of the calorimeter during the exposure time by extrapolating the cooling curve measured after the laser beam has been extinguished back to the midpoint of the beam duration.

The power transmitted through the aperture, P^^ was measured at a power setting P, of 2000

watts for exposure of 2. 0 seconds. The ratio of P/P is shown as a function of the hole radius r^^ in

figure 2. Observations were taken at 12.2 cm, 2^8 cm and 53 cm from the fccus.

Assume that the intensity profile is a Gaussian intensity distribution

P 2/2

I - = ^ e o

TT r

(1)

where r determines the spreading of the beam and P is the peak central power. The power

O

1 O Q through a hole is then

I dr = P

r o l-exp (-r^/rj') (2)

The data was then fitted by a least squares calculation and the constants P^ and r^ were determined

from the data. The curves so obtained are shown plotted as the solid lines in figure 2. In particular,

the calculated value of P could be independently compared to the value measured with a full beam. The

difference between thes° quantities is a sensitive measure of whether the beam profile is correctly

described by the laser function. In two cases the difference was unmeasurable and in the third case was about 6% and is believed to be due to experimental misalignments rather than to any inherent

property of the laser beam. Figure 3 shows the beam profiles obtained from the fitted Gaussian profiles.

Figures in brackets indicate the literature references at the end of the paper.

-176-
Figure 4 is a plot of r vs. the distance from the focus. The data points are shown as well as

a straight line fitted by least squares. The linearity of this data shows that diffraction effect are

j negligible at these distances. It is difficult to measure the beam profile close to the focus because of

I the high intensity and small size. However, burn patterns were made on blackened steel with the beam

at the focus of the f/?. 1 telescope. These showed a diameter of 1 mm which is consistent with the size

of a diffraction limited beam of 10. 6 [xm and the type of laser cavity employed in the HPL industrial

i laser. At 15 kW , the intensity at the focus for a 1 mm beam diameter is 2 x 10" W/cm^. The peak

intensity is somewhat higher and it may be further increased, if desired, by using optics of shorter

focal length.

3. Diamond Spectra

We have examined 47 natural diamonds to determine the infrared, the ultraviolet spectra and

have measured the absorption coefficient for 10. 6 |j.m radiation. The diamonds (clear natural

stones)* were in the shape of discs from 2 to 2.5 mm diameter by approximately 0.5 mm thick. The UV

spectra were obtained with a Gary Model 1756 spectrometer and the infrared spectra with a Perkin

Elmer Model 457 spectrometer. Because the diamonds were small, it was difficult to concentrate

sufficient light intensity through them. Therefore, a Perkin Elmer 4X, All Reflecting Beam Condenser

was used with the spectrometers and the instruments adjusted to give at least half scale deflection of

the recorder by reducing the intensity of the reference light beam.

Figure 5 shows the ultraviolet spectra obtained with representative diamonds. These are shifted

by an arbitrary amount along the vertical (intensity) scale so that they do not overlap. The number on

the right is the identifying serial number of the diamond. It will become apparent from the measurement

of p, the absorption coefficient at 10. 6 \xm described in the next section, that diamonds #13 and 28 have

the lowest values of p. Diamonds #43 and 38 have much higher p's and the others fall in between.

Figure 5 shows that diamonds with a high p (#43 and 38) also have a high absorption coefficient at . 25 |im,

whereas diamonds with low p (#13 and 28) also have a low absorption at this wavelength. This figure

further shows that the precise shape of the ultraviolet absorption edge is variable. For instance,

diamonds 46 and 23 have very similar shaped absorption edges, whereas we shall find a factor 3 difference

in the values of p.

It would be very useful if the transparency at . 25 |J.m could be used as a basis for selection of the

uncut stones. In order to characterize the spectrum by a single number, the following definition was

used. The transmission at .25 [xm, I was compared to the transmission at .40 |J.m, I ^q, measured

with the spectrometer. The relative JiBsorption at . 25 |xm was defined as

A.25 = ^ - <^ 25/^40)

Table I lists all of the data obtained for the 47 diamonds. The values of A 25 ^i^® given in column 3.

In a similar way, the infrared absorption spectrum was obtained from 2. 5 |JLm to 15 fj.m. These

spectra are shown in figure 6 for the same diamonds. Again, the curves are displaced along the intensity scale to prevent overlapping. (The discontinuity in the spectra at 5. 0 tJ-m is due to automatic

switching of the spectrometer optics. ) Some diamonds, notably #'s 13, 28, 42, and 39 show no significant absorption in the region

from 7. 5 to 8. 5 iJ.m. Others, notably #'s 38 and 43 show a very well developed absorption band

and still others show intermediate amounts of absorption. This band has been attributed to interstitial

nitrogen atoms lying in the (100) planes in concentrations up to several times 10^0 atoms/cm3 [ 3] . The nitrogen impurity in natural diamonds reduces the thermal conductivity at low temperatures by

about an order of magnitude, presumably by scattering of phonons. It also increases the indent hardness

and reduces the plastic flow by interfering with the migration of dislocations. A pronounced absorption

band at 8 [o-m may also have a significant tail extending to 10. 6 [xm and thus increase p for the window

at this wavelength.

All of the diamonds were transparent at 12. 5|jim. The spectra have been characterized by a relative absorption A^ „ which compares the transmission at 7. 9 fim to the transmission at 12. 5 [s.m as read from the spectrogram.

^7.9 = ^ - <^7.9/^12.5)

The values of A.^ ^ are given in table I, column 4.

These were kindly lent to us by Lazare Kaplan & Sons, Inc. -177-
The correlation between the absorption at . 25 fjim and at 7. 9 l^-m is presented in figure 7. Here,

the circles represent the individual diamond numbers and the relative absorption values are cross plotted.

One sees that those diamonds with low values of A namely diamond #'s 13, 28, 39, and 42, also

have low values of A_ „. Presumably these are Type II diamonds and should also have low values of p. There is a group of aiamonds with values of „ only slightly higher which have higher values of A ^g.

Finally, there is a very large group of diamonds" with high values of absorption at both wavelengths. ' These latter are very likely to be Type I diamonds.

4. Direct Absorption Measurements

The diamonds that were chosen for a direct determination of (3 at 10. 6 fim were all of those with

low and intermediate values of A ^c, =^rid some of those with a high absorption. Thus, the distribution

of p's could reveal whether the transparency to the . 25 fim mercury line is correlated with p.

The experiment consisted in passing a focused 10. 6 |am laser beam through the diamond, measuring the incident and transmitted intensity and the heating resulting from absorption. ITie diamonds

were sufficiently parallel across the small irradiated spot diameter so that internal reflection would

interfere to an unknown degree with the transmitted beam. A correction was therefore applied to the

measurements to give corrected values of p. The correction was calculated by the procedure for multipl

internal reflection except that allowance was made for absorption within the plate [4] .

The laser beam was approximately 20 watts, obtained from a Perkin Elmer Model 6200 CO^

laser. A collimated beam, 1. 3 cm diameter was sharply focused through the diamond with a copper

mirror of 10 cm focal length. Approximately 12% of the laser power was transmitted through a hole

in the mirror into a differential thermocouple calorimeter so that the laser power could be monitored.

After passing through the diamond, the energy was received on a calibrated absorber and integrated.

The temperature rise of the absorber was recorded and converted to the transmitted beam intensity.

The specimens were clamped between two thin copper discs having a hole for transmission of the laser

beam. A copper-constantan thermocouple was attached to the holder. Absorption of energy by the

diamond was transmitted to the holder and the temperature recorded. The weight of the copper diamond

holder was . 59g. Its heat capacity was . 23j/°C. The heat capacity of an average size diamond was

. 0036j/°C. For the incident beam intensity, the diamond was removed from the sample holder and

the transmitted beam measured directly.

A correction is needed for the amount of energy that was intercepted by the diamond holder from

fringes of the laser beam. Therefore, a calibration run was made in which the heating of the holder

alone was measured as a function of the exposure time. The power absorbed by the holder alone was

about . 08 watts. This was substracted from the power absorbed by the diamond plus holder to give the

absorption of the diamond alone.

The results of the direct absorption experiment are also shown in table I, columns 5 thru 11. Column 5 is the exposure time, 6 gives the incident laser power; column 7 is the transmitted power;

column 8 is the temperature rise of the diamond and holder; column 9 is the power absorbed in the

diamond alone. Column 10 is the value of p obtained without correction for interference between the

transmitted and internally reflected beam. Column 11 is the value of p obtained after correction for

this effect. The values of p are plotted against the relative absorption in the infrared, A „ in figure

8 and against the relative absorption in the ultraviolet A ^5 in figure 5. Note that the scattering in the

data is amplified by the use of a logarithmic plot. The lowest value of p that has been observed is .021 for diamond #28.

The data shows that A is a. rough guide to the value of S,but it is not quantitatively reliable.

For example, diamonds # 28' and 13 have similar values of A ^5 but a factor 3 difference in p.

Approximately 20% of the diamonds have values of p less than .5; however, if a selection had

been made of those diamonds with values of A ^3 ^ . 75, only half of the low p diamonds would have

been found. The conclusion of this spectroscopic study is that if diamonds show a high absorption in

the ultraviolet, they also have a high absorption at 10. 6 (J.m. If they show a high transparency in the

ultraviolet, they also have a low absorption at 10. 6 |J.m. However, there are a number of diamonds

for which the ultraviolet spectrum is an unreliable guide to the 10. 6 |jLm absorption.

5. Design of Diamond Holder

Figures 8 and 9 show that we can expect to find diamonds for windows with p < . 3 cm ^

. The

diamond cutters say that they may have difficulty in cutting plates thinner than 2 mm because of the

danger of cracking. We may therefore expect an absorption by the window of 6% or less of the transmitted power. The diamond temperature may not exceed 800 °C, the temperature at which transition

to diamond becomes accelerated. The transmitted power therefore depends on the efficiency of cooling

of the window that can be provided.

^ -178-
Table 1, Data for Diamond Spectra and Absorption

Specimen Thickness A_ ^5 Exposure Incident Transmitted AT Absorbed Uncorrected p ' " Time Power Power Power p — sec. watts watts 0^C watts

_ 1 cm_ J cm

1 . 974 . 321

2 . 914 . 262

3 . 989 . 384

4 . . 048 . 876 . 408 10. 3 33. 1 17.4 62 6 1 37 . 81

5 . 053 . 885 . 209 10. 2 25,6 18.6 47 8 1 m 7 A . ' D . 72

6 . 930 . 421

78

. 995

. 996

. 458

. 340 «

9 1 . ODD . 583

10 1 . 000 . 576

11 . 920 . 277

12 . 060 . 747 . 102 15. 1 32. 2 1 9. 9 37 2 u . D 0 . 30 . 34 13 . 042 . 480 . 006 43. 9 34. 9 16 6 0. 068 . 045 . 067 14 . 925 . 277

15 . 986 . 310

16 1., 000 . 900 Diamond turned black in a few sec onds

.

17 , 985 . 407

18 , 998 . 440

19 977 . 326 20 . 036 914 . 259 15. 1 29. 8 18.0 7 c

. 1 J . 71 . 80 21 951 . 253

22 1. 000 . 586 23 . 079 830 . 1 96 15.4 25. 9 1 ft A

. 32 . 15 . 16 24 . 051 , 938 . 207 15. 3 27. 9

110.7 f) Q J 1 . 4 . 46 . 33 . 35 25 1., 000 . 973 26 , 975 . 336

27 , 936 . 327 28 . 052 , 446 . 048 39. 5 36. 2 16.8 23 5 n 7 Q .015 . 021 1.23 29 . 070 1. 000 . 624 15. 2 24. 1 1 6. 5 123.0 2 07 I . CO

30 943 . 273

31 . 053 . 847 .113 -^4. 9 32. 3 1 7. 9 47. 3 . 46 . 27 34

32 . 988 . 406

33 . 048 , 863 . 1 30 15. 3 33. 3 20. 2 25. 4 . 35 . 22 .27

34 1,. 000 . 330

35 . 051 , 857 . 123 15. 2 27. 1 19. 2 40. 9 . 63 .46 .45

36 , 989 . 406

37 1., 000 . 442

38 . 039 1., 000 . 481 13. 8 29. 1 18. I 65. 8 . 1. 08 .97 1. 07

39 . 056 636 . 025 9. 5 34. 9 18. 8 25. 3 . 46 .24 . 30 40 841 . 1 54 41 990 . 371 42 . 047 526 . 033 13. 7 37. 2 18. 5 21.9 . 26 . 15 .21 43 . 056 1. 000 . 533 8. 0 25. 0 15. 9 99. 5 2. 56 1. 92 1. 95 44 982 . 391 45 979 . 297

46 . 060 813 .210 15. 2 25. 5 18. 3 66.4 1. 08 . 72 . 81 47 846 . 277

Not tested for 0

A water-cooled window holder has been designed in which the diamond is clamped at the edges of

its parallel faces between two water-cooled copper discs. This is shown in figure 10. Thin annular

gold washers distribute the clamping force uniformly to the window surface. Water flowing at high

velocity is brought into contact with the edge of the diamond by constructing the holder in the shape of a nozzle with the window located at the smallest cross section. It is estimated that a Reynolds Number

of about 40, 000 will be reached by the water flowing over the diamond surface. The expected heat

transfer coefficient will be about 6. 7 watts/cm deg. If we allow the diamond temperature rise to be

600 °C, the heat transfer to water will be 1500 watts. For an absorption of . 06, the transmitted laser

power may be 25 kW. In addition to the heat transfer to the water, there is also conduction heat transfer to the cooled metal surfaces that clamp the edge of the diamond. However, in this case, the heat

must be transferred through metal interfaces. According to McAdams [ 5] the contact coefficient for

heat transfer between machined surfaces under moderate pressure is h =1. 7. In our diamond holder,

there are two such surfaces in series and two in parallel giving 1. 7 as an estimate of the resultant

conduction heat transfer coefficient. The clamped surface will have about the same area as the watercooled surface; therefore, it is estimated that the conduction heat removal will only be about 25% of the

heat transferred directly to water at the edge of the window. This 25% additional margin is more than

sufficient to compensate for another source of temperature to be discussed next.

-179-
In addition to the A T at the surface, there is also a radial temperature distribution A T(r) due

to the heat conduction within the diamond. The maximum value of A T(r) depends on the distribution of

laser flux. It is clear that the smaller the spot diameter, the larger will be the conduction AT. For

a focal spot of . Z cm radius, the central temperature rise is about 35 °C or about 7% of that at the

surface. The conclusions of these estimates is that it should be easy to transmit the laser beam from

our 15 kW HPL industrial laser through a diamond window.

Figure 11 shows a photograph of two of the diamond plates that have been obtained for testing as laser windows. Information on the test results of these and of some zinc selenide windows will be

made available as soon as the tests can be done.

6. References

[1] Hoag, E., H. Pease, S. Staal, and J. Zar,

Performance characteristics of a 10 kW

industrial CO2 laser system. Applied Optics 13_ (1959), (197A).

[2] Jacob, J., E, Pugh, J. Daugherty, and

D. Northam, R.S.I. 4A, 471 (1973).

[3] Strong, H. M. and R. M. Chrenko, Further

studies on diamond growth rates and physical

properties of laboratory-made diamonds,

J. Phys. Chem. 75, 1838 (1971).

[4] Zar, Jacob L,, Investigations into the feasibility of high power laser window materials,

AFCRL-23-75-0264 (1975),

[5] McAdams, W. H., Heat transmission, (McGraw-Hill,

New York 1954).

4

-180-

1.6

1.4

1.2

h .8

.6

.2

+

/

[/

DISTANCE FROM

FOCUS

O I2.2CM

A28.0

/

+ D3

0 .4 E8340.8 1.2 I.e 2.0 2.4 (cm)

Figure 2. Intensity of laser beam transmitted through aperatures of radius r.

OlSTANC E FROM

FOG US

12.2 CM

28.0

53

N\

\\

\\

\

N

0

il3

P

0

E8342

.8 1.2 1.0 2.0

r(cm)

2.4 2JS, 3.0

Figure 3, Gaussian beam profile at various distances from focus.

-182-
.20

E7I3I

.24 .28 .32 WAVELENGTH fj.M

.36 Figure 5. UV absorption spectrum of^'^ representative sample diamond

discs

.

-183-
INFRARED ABSORPTION SPECTRA- DIAMOND

2.5 3.0 4.0 5 0 6.0 7 0 8,0 10,0 150 •"> WAVELENGTH /j,M

Figure 6, IR absorption spectrum of representative diamond discs.

-184-
-185-
Figure 8. Distribution of absorption coefficient, B versus absorption at

.25 pm for Type I and Type II diamonds.

-186-
Figure 9. Distribution of absorption coefficient, 3 versus absorption at 7.9 lim for Type I and Type II diamonds.

-187-
WATER GAS NOZZLE

Figure 10, Water cooled diamond

window and window

holder.

THERMOCOUR.E

c«[mi]imjnnnn|nn|ni

SPECIMEN. JDfi

Figure 11. Type II diamond discs to be tested as windows.

-188-
4.1 Optical Material Damage From 10.6pm CW Radiation

Carl A. Huguley and John S. Loomis

Air Force Weapons Laboratory

Kirtland Air Force Base, New Mexico 87117

Damage studies of a variety of optical materials including KCL, ZnSe,

NaCl , Mo, and Cu substrates were conducted. Most of the samples were

substrates with optical coatings, and several were gratings. Typically,

a rectangular grid of thirty irradiation sites was used. An analysis

of damage thresholds was accomplished.

Key words: CO2 laser radiation; Cu; CW laser damage; grating; infrared

windows; KCl; mirror; Mo; NaCl; window; ZnSe.

1. Introduction

During the past few years, an effort was made to explore the problem of damage to optical

components, such as windows and mirrors, that results from continuous irradiation at 10.6pm

[1, 2, 3]^. During the current series of measurements, the goal was to examine the effects of

irradiating each sample at a number of locations with various incident powers, rather than

to irradiate the center of the sample several times with increasing power levels. In this

manner, it was hoped to learn something about the statistical nature of CW damage--and to look for microscopic damage as a precursor of catastrophic damage.

In subsequent portions of this paper, the experimental apparatus is described first

followed by a discussion of measurements of power and spot size--the maximum power achieved

after the focusing optics was 600 watts, with a spot size of 1-mm diameter. Next, the

criteria for defining damage and the sequence of measurements is explained, and calculations

of the temperature rise at the center of an irradiated spot is presented. Finally, the results

of measurements on a variety of windows, mirrors, and gratings are given.

2. Apparatus

The test apparatus described herein, shown in figure 1, is very similar to that used

earlier [1, 2, 3]. A Sylvania, Model 971, Gas Transport Laser (GTL) was used as the radiation

source to produce a continuous power of 1000 watts at 10.6 microns. The laser's beampower

was continuously monitored throughout the test by using a mirror chopper (in lieu of a beam

splitter)to deflect a portion of the beam into a Coherent Radiation, Model 201, Power Meter.

Control of the beam's access to the sample was effected by a solenoid-actuated, mirroredsurface shutter, which could be switched within 10 msec. After the shutter, the beam passed

through a 1 .27-cm diameter graphite aperture and onto a 25-cm focal length, NaCl, planoconvex, uncoated lens, which was used to focus the CO2 beam 1.7 cm in front of the sample.

The sample was so mounted that the beam impinged upon it at an angle of incidence of

approximately 8°, which allowed the reflected portion of the beam to be directed into a graphite

dump. The sample was supported in an x-y positioning device that was used to move the sample

into different irradiating locations.

1. Figures in brackets indicate the literature references at the end of the paper.

NOTE: Micron(s) as used in this paper expresses micrometer (s).

-189-
3. Power Measurements

The power meters used during these tests required periodic calibration because the

calibration constant had earlier been observed to change with age and usage. To combat this

drift, these meters were intercompared every few months or at the beginning of a test series.

A mirrored chopper with a one-percent duty cycle reflected a uniform portion of the beam into

these power meters for continuous measurements during the test series. The power after the

lens (see figure 1) was measured to be a factor of 60 (+ 3) times the power reflected from

the mirror chopper.

4. Measurement of Beam Radius

A scanning edge technique [1, 4] was used to measure the diameter of the CO^ laser beam

at the location of the damage tests. A massive aluminum block with a 45° milled edge was mounted on a translation stage, and a power meter behind this edge was used to measure the

laser power transmitted past the edge. Total power was monitored by the previously mentioned

chopper/power meter technique, while the ratio of transmitted power to total power and the

position of the edge were recorded as the edge was moved in discrete steps into the beam.

The focused intensity distribution was assumed to be Gaussian

I = I exp

0 (x2 + /) /b2 (1)

where x and y are the coordinates from the center of the beam, and b is the radius (1/e ), The power transmitted past the aperture is given by

P = /:/. I (x,y) dxdy (2)

erfc

where

P = -L TT b^ I 0 2 t

The solid line in figure 2 is a graph of eq. (2), near the midpoint of the curve (optic

axis). For power ratios of 0.3 < P/P < 0.7, the curve can be approximated by a straight line

given by

_P. = 0.5 + 0.8 x P„ b (3)

and beam radius is then calculated from the slope of this line. The result of this measurement

was a beam radius of 0.50, + 0.05 mm, which correlates with the previously measured value [1].

-190-
5. Test Procedures

Each test specimen was first examined for defects in the coating or substrate and then

for optical quality.

Next, an irradiation conditioning run was made with the lens moved upstream (from the

sample) to a point where the diverging beam was the same size as the specimen. The specimen

was then irradiated--with power levels beginning at 120 watts, then raised to 600 watts over

a 30-second period, and then maintained at 600 watts for an additional 30 seconds. Throughout

the conditioning phase, the peak intensity was below 130 watts/cm^, and no damage or scintillations were observed. Specimen conditioning had earlier been observed [1] to increase the

damage threshold.

The damage test itself was a series of 30 shots (each shot is a 2 second or 30 second

pulse of laser power on the sample), with the test piece being moved after each shot to produce

a five-by-six matrix shot array as shown in figure 3. Each row (beginning at top left)

received a successively higher power unless substantial damage was observed, at which time

subsequent shots were made at lower power levels. After the damage test, each specimen was examined with a Nomarski microscope.

6. Damage Criteria

Specimen damage was first correlated to the 5X6 shot array--defects not lying at matrix

locations were not identified as damage sites. Damage to the specimen was then categorized

by the size and the nature of the damaged area. Damage that was large enough to be seen with

the naked eye (i.e., of the same size as the beam) was classified as "macro" damage, while

damage that required magnification to be visible was called "micro" damage. At the majority

of damage locations, characteristic ash marks were in evidence; but with the halide samples,

many damage spots indicated a local fracturing or cratering of the substrate.

The determination of a damage threshold was often a value judgment, since the sample

might be damaged twice at one power and not at all at the next higher power. Such inconsistency

was ascribed to inhomogeneity of the sample. Thus, whenever possible, the following criteria

was applied in determining "damage threshold."

(a) If only one damage spot (resulting from the three, equally-timed pulses) at a given power was found, it was categorized as below the average threshold.

(b) If two damage spots at a given power were found, such power/time combination was categorized as being at^ the average damage threshold.

(c) If three damage spots at a given power/time combination were found, such level was considered to be above the damage threshold.

7. Heat Conduction Effects

The temperature rise at the center of an irradiated spot on a window or mirror depends

upon the difference between the heat flow into the material (from absorbed laser radiation)

and the conduction of heat away from the irradiated spot (into the cooler surrounding material)

a steady-state thermal profile develops within a few seconds [2]. When the heat flow reaches

the outer edges of the specimen (see figure 4), the temperature at all points rises uniformly

with time until the heat loss to the external world becomes significant. The rate of dT/dt

rise is given by

dL'lA. (4)

dt C

-191-
where P. is the power absorbed (watts) and C is the heat capacity of the specimen (Joules/°C).

The initial temperature rise (or temperature offset between different points on the

specimen) depends upon the laser beam geometry, on the thermal conductivity of the specimen,

and on the beam radius. Figure 5 is an example of this dependence on beam radius, where the

calculation assumed a KCl -specimen irradiated for 10 seconds. The ratio of temperature to absorbed power is plotted because the temperature is proportional to the total absorbed power.

For spot radius (B) larger than the specimen radius (A), the temperature rise at the center

(or at any spot) is given approximately by eq. (4); but, for laser spot sizes much smaller

than the specimen radius, the temperature at constant power becomes very 1arge--increasing

rapidly as the spot size decreases. At constant intensity, however, the temperature rise

decreases with a decrease of spot size because of heat conduction. For the beam size used

in these tests (and assuming a sample in which the absorbed power is distributed evenly

throughout the thickness of the specimen) the temperature at the center during steady-state

rise is given by

T = 0.556 + ^A^ (5)

kd C

where k is the thermal conductivity, and d is the specimen thickness. Equation (5) accurately

represents ttie thermal situation of test windows after 30 seconds and prior to 300 seconds

(the external time constant). For metal mirrors, heat conduction is much more effective in

reducing temperature gradients because the metallic thermal conductivity is so much higher

than are the materials in the window. On the other hand, absorption occurs on the surface,

which has the effect of increasing the temperature near the surface.

Table 1 presents the calculated temperature rise (per watt absorbed power) in three

different materials after a 2-second and 30-second irradiation. An external heat transfer

coefficient of 0.001 watt/cm2 - °C and a specimen radius of 1.9 cm ms assumed for all three

materials. An eigenfunction series solution to the heat conduction equation (expressed

in cylindrical geometry with axial symmetry) was used [5, 6]. The front surface temperature

rise was calculated for molybdenum, but this temperature rise was not very different from the

three materials (in table 1). In each case, the temperature rise after 30 seconds was only

half again as much as the temperature rise during the first 2 seconds.

8. General Results

The results of the damage tests on different kinds of specimens are presented in a series of tables. Each table gives the total absorption (when available) of the specimen

and the peak intensity at which damage was observed As di'scussed in section 6, isolated

spots at lower intensity were not assumed to be the actual damage threshold, but an asterisk

in the table denotes those samples where such damage was observed. For several specimens,

although damage from lower intensities at multiple locations was clearly evident, isolated

undamaged higher-intensity sites could be observed. This phenomenon is denoted in the

tables by a "plus" sign.

High-power optical components that have sustained isolated damage sites (much smaller

than the actual beam size) have been observed in use, yet they have shown no deterioration

in optical quality or absorption. Thus, the first appearance of damage--especial ly microscopic damage--need not indicate the end of a component's usefulness, and such damage should

not be used as the definition of damage threshold.

As with past studies, the type of damage observed has almost always involved burning

or melting of the substrate and coating. At lower intensities (and sometimes independent

of any microscopic damage), occasional sparkles (from the specimen) were observed [2] with

durations much less than the irradiation time. With most specimens, however, microscopic

damage was accompanied by sparkling of almost continuous duration. Of course, macroscopic damage was generally marked by a highly visible plume and the production of smoke.

-192-
9. Window Results

Table 2 presents the results of damage tests to four ZnSe windows, two of which were

uncoated and two were ThF4/ZnS anti -reflection coated. Only one of tfiese samples sustained

damage--a2Coated window that reached melt temperatures (1100°C) in less than 2 seconds at 120 kW/cm . Such temperatures are far in excess of the nominal 20°C temperature rise that

was calculated for this sample's measured absorption rate (a local absorption value in excess

of 10 percent would have been required to generate enough heat to reach the melting temperature

of ZnSe). No blemishes or microscopic defects were observed at that site prior to irradiation.

Furthermore, blemishes on other ZnSe samples have been irradiated without catastrophic

damage, although local melting has been observed on a few other coated ZnSe windows [2, 3].

Interferometric techniques that measure local absorption as a function of position on the

sample are currently being developed to determine those inhomogeneities in absorption that

could be used to predict potential damage sites.

Also presented in table 2 are results of tests on thirteen coated KCl windows. Multilayer anti-reflection coatings, each of a different design, were applied to seven of these

samples; while the other six had single-layer coatings of various materials applied to them.

Two of the specimens (KCl 298 and KCl 300) had AFWL-designed AseSo/KCl/As S3 coatings [7].

These coatings had a very hazy appearance, but their total absorption was unchanged (or even slightly lower) from the measured value for the uncoated substrates. Another specimen,

which had a ZnSe/Ge coating design specified, failed at a very low intensity as had other

specimens with coatings that used Ge. The remaining four anti -reflection coated specimens were coated with proprietary commercial materials.

Figure 6A, a photograph of the full KCl §292 window (3.8-cm daimeter), shows one of

the more ideal damage patterns. Note that this picture shows an isolated damage spot at

90 kW/cm'^ (top row), no damage at 120 kW/cm2 for 2 seconds, but macroscopic damage for all

of the 30-second shots, and damage over the entire row at 150 kW/cm^. These damage sites were areas of burned and melted material, approximately 0.08 cm in diameter (about the

size of the incident beam), as shown in figure 6B.

The results of this survey of anti-reflection coated materials indicate that the damage

threshold is lower for these specimens that have higher absorption levels. Materials with

opaque coatings of Ge or CdTe do not have a high resistance to laser-induced damage.

Opaque semiconductors all have a relatively low band-gap, which suggests that the damage

mechanism may be that„of thermal runaway. The single-layer KCl samples all showed low, on the order of 10 kW/cm'-, damage thresholds.

10. Abraded KCl Window Results

To assess the possible effects of surface absorption on the damage process, a series of

samples were prepared by lapping one surface of each sample on a polishing wheel charged with

various grades of alumina grit. The resulting surfaces had the appearance of ground glass,

with greatly increased scatter and absorption over that of the untreated material (nominally

0.15-percent absorption). The damage test was conducted as described in paragraph 5, and

table 3 lists the resulting absorption and damage intensity. As expected, samples with

larger grit sizes demonstrated higher absorption (except for those charged with the smallest

grit size).

The circles in figure 7 show the damage intensity for each specimen as a function of

its absorption. Although these data would appear to be fitted by a smooth curve, as illustrated,

they do not correlate with the hyperbolic dependence expected if peak temperature rise was

the sole factor in producing damage. The damage mechanism in all of the abraded samples was

thermal -induced fracture, with some pitting of the abraded surface. The x-marked data points

in figure 7 are taken from some of the data for coated KCl windows in table 2. These points

-193-
can be construed to lie along the same general curve as do these abraded specimens, albeit

with much higher scatter about the curve. This behavior suggests that the damage to coated

KCl windows may be due to the higher absorption of the substrate because of the presence

of the coating--rather than by the coating failing first and producing subsequent failure

in the substrate. This hypothesis is substantiated by earlier observations [2] of AS2S3-

coated KCl windows that failed with little or no damage to the coating itself.

11 . Mirror Results

The mirrors tested in this study were composed of (ThF4/ZnSe)" and (Th^ /ZnS) multilayer dielectric stacks deposited on molybdenum or TZM substrates. Some of these substrates

had been Ag and ThF^ coated, prior to the deposition of the multi-layer dielectric stack.

The mirror testing was conducted as outlined in paragraph 5, and these results are summarized in table 4, Some of the mirrors had previously been tested at 46 kW/cm^ (uniform

illumination) without damage [9], but these results showed failures from about 60 kW/cm to greater than 150 kW/cm^ . There seems to be (from table 4) more of a correlation between

the grouping by identification number (i.e., manufacturer or batch group) and damage

level than with any other characteristic of the mirror. One can conclude that, under

continuous radiation [1], the dielectric-coated mirrors can survive intensity levels as high

as can bare metal mirrors, but that extrinsic effects are more likely to be present (with

the coated mirrors).

12. Grating Results

For these tests, gratings of the RMG series were furnished by PTR Optics, and the

others were made by Perkin-Elmer [8]. Table 5 summarizes the results of damage tests

(conducted as previously described) on these gratings.

In general, the damage characteristics of gratings were very similar to those of metal

mirrors, but the damage levels were generally at or above 150 kW/cm^. The RMG gratings were

not intended for high-pov/er applications so it was not surprising that some of these samples

did not endure well. It should be noted that with metal substrates, the resistance to CW

radiation can persist even for high-absorption surfaces (like GBC-26), which have an estimated

surface temperature rise of 650°C after a 30-second focused irradiation of 600 watts. For

example, this particular (GBC-26) grating began to glow after 10 seconds at a 600-watt power

level. More nearly typical damage morphology is shown in figures 8 and 9.

Figure 8 shows a conventionally-ruled grating that has a spacing of 118.2 lines/nm

and a groove depth of O.llvim (GT-6). It also shows that the ThF4 coating has started tojnelt

near the damage site. Figure 9 shows the damage to an ion-polished grating having rectangularshaped grooves with a spacing of 127 lines/mm and a depth of 0.12um. The damage spots in

both cases are burn pits about 0.008 cm in diameter, which is about one-tenth the size of the

incident beam.

13. Conclusions

It has been shown that, for many types of optical components--including windows, mirrors,

and gratings--damage resistance to focused continuous radiation beyond 150 kW/cm^ can be

achieved. For windows, damage resistance deteriorated at absorptions higher than about 0.5

percent. However, bare-metal surfaces on mirrors and gratings would survive temperature

rises resulting from much-higher (6 to 10 percent) absorption. The coatings on all types of

optical materials were shown to survive to the limits of the substrate, but overall damage

was much more likely to result earlier (at lower intensities). Damage to KCl windows was

shown to be highly correlated with the total absorption of the window.

-194-
14 . Acknowledgments

The authors gratefully acknowledge the assistance of L.A. Marrero, Z.M. Green, J. P. Buehler,

. Lester, and G.B. Charlton in collecting and reducing the data for this paper.

15. References

Saito, T.T., Charlton, 6.B., and

Loomis, J.S., Proc. 6th ASTM Sytnp.

Damage in Laser Mat's., NBS Spec.

Pub. 414 (1974).

Loomis, J.S. and C.A. Huguley, Proc.

6th ASTM Symp. Damage in Laser Mat's.,

NBS Spec. Pub. 414 (1974)

Loomis, J.S. and T.T., Saito, "Third

Conference on High Power Infrared

Window Materials," vol. 3, Air Force

Cambridge Research Labs, AFCRL-TR0084 (III) (1973).

Skinner, D.R. and R.E. Wictcher,

"Measurement of the Radius of a High-Power Laser Beam Near the Focus

of a Lens", J. Phys. E: Sci . Instrum.

5, 237 (1972).

Bernal , 6., Enrique, "Heat Flow

Analysis oif Laser Absorption

Calorimetry" , Appl . Optics lj[, 314

(1975).

[6] Laughlin, W.T. and R.C. Vrem, "Unsteadystate Heating of a Finite Disc by Continuous and Repeatively Pulsed Laser Radiation,

Air Force Weapons Laboratory, AFWL-TR-72-

236 (1973)

[7] Loomis, J.S., "Anti-reflection Coatings

for Potassium Chloride Infrared Windows,"

Laser Digest, Air Force Weapons Laboratory,

to be published.

[8] Harris, J.S., Slomba, A.F., et al .

,

"Diffraction Grating Development," AFV/LTR-74-218 (1975).

[9] Buckmelter, J.R., et al., "Dielectric

Coated Diamond Turned Mirrors", to be

published elsewhere in this volume.

-195-
16. Tables and Figures

Table 1. Temperature Rise per Watt Absorbed Power for Some Typical Materials.

Temperature Rise/

Thermal Heat Watt Absorbed

Conductivity Capacity Thickness 2 sec 30 sec Material (watt/cm-"^ ( jou1e/cm-^-°C) (cm) (°C/watt) (°C/watt)

ZnSe 0.12 1.90 0.8 4.97 7.53

KCl 0.07 1.35 1.0 6.59 9.90

Mo 1.44 2.57 0.6 5.45 7.01

Table 2. Damage Results for Windows

Sample

Identi fi cation

Absorption

(percent)

Damage

Intensity

(KW/cm^) Remarks

7»^C/i cor 111 Q/\ >150 Uncoated

ZnSe 58C-WL-7A 0.13 >150 Uncoated

ZnSe 82A-1G 1.30 >150 AR Coated

ZnSe 76A-3B 0.86 120* AR Coated

KCl 300 0.21 150* AR (1) (ASgSj/KCl/ASgSj)

KCl 298 0.29 >150* - AR (2) (AS2S3/KCI/AS2S3)

KCl 297 0.46 90+* AR (3) (AS2S3/KCI/AS2S3)

KCl 292 0.66 120* AR (4) (AS2S3/KCI/AS2S3)

KCl - SS 0.41 >150* AR Coated (5)

KCl 41 2.1 23 AR Coated (6) Opaque

KCl 33 1 .2 10* AnS/Ge AR Coated (7) Opaque

KCl 299 0.16 90+* AS2S3 /2

KCl 34 3.4 10+* 12 Coated Opaque

KCl 50 1.8 14* 11 Coated Opaque

KCl 35 3.0 10+* /2 Coated Opaque

KCl 51 2.3 10* CdTe Coated

KCl 49 3.0 7.5 Ge 11

* Isolated Damage spots at lower intensity

+ Isolated undamaged spots at higher intensity

-196-
Table 3. Damage Results for Abraded KCl Windows

Sample

jntification

Absorption

(percent)

Damage

Intensity

(KW/cm2) Remarks

KCl 202 1.53 11 * 25 m grit

KCl 203 1.34 8 +

* 25 m grit

KCl 204 1.14 20 + 25 m grit

KCl 205 0.58 60 + 10 m grit

KCl 206 0.53 70 * 10 m grit

KCl 207 0.60 70 +

* 10 m grit

KCl 208 0.54 60 * 1 m grit

KCl 209 0.47 90 * 1 m grit

KCl 210 0.47 80 * 1 m grit

KCl 211 0.88 30 * 0.1 m grit

KCl 212 0.61 >15 * 0.1 m grit

* Isolated damage spots at lower intensity

+ Isolated undamaged spots at higher intensity

Table 4. Damage

Sample

Identification

Absorption

(percent)

M 130 0.85

OCLI 4A 1.10

OCLI 3A 0.22

AM 51 0.14

AM 18 0.13

AM 23 0.13

AM 40 0.13

AM 53 0.25

T 86 0.14

T 85 0.15

. T 80 0.29

T 78 0.07

Isolated damage spots at lower

intensity

-197-

for Mirrors

Damage

Intensity

(KW/cm2) Remarks

150 Au(ThF4/ZnS)^

90 + Au(ThF4/SnS)2

150 * Mo(ThF4/ZnSe)^

>150

3 Mo(ThF /ZnSe)

4 >150 * Ag+ThF^(ThF^/ZnSe)3

>150 * Ag+ThF4(ThF^/ZnSe)^

>150 Mo(ThF^/ZnSe)^

140 * Mo(ThF^/ZnSe)^

90 * + Ag+ThF^(ThF^/ZnSe)^

60 + TZM (ThF^/ZnSe)^

120 * + TZM (ThF ./ZnSe)^

4 150 * Ag+ThF^(ThF^/ZnSe)^

Isolated undamaged spots at higher

intensity
Table 5. Damage Results for Gratings

Damage

Sample Absorption Intensity

Identification (percent) (KW/cm^)

RMG 1 >150

RMG 2 >150

RMG 3 60

RMG 4 <15

RMG 5 <1.5

GT-6 0.97 >150 *

GM-7 0,88 >150 *

GT-7 1.15 >150

GM-8 1.07 150

GBC-26 13.4 150

GM-18 1.0 >150

* Isolated damage spots at lower intensity

Remarks

Solid Al ruled grating

OFHC-Kanigen-Al ruled

grating

Al -Kanigen-Al ruled

grating

Si-Al ruled grating

Glass-Epoxy-Al Replica

grating

TZM/Au ruled grating

ThF. over coat

4

moly/Al ruled then Au over coat

TZM/Au ruled grating

ion -polished moly/Au

over coat

BeCu ruled/Au over coat

ion polished moly/UHV

Au over coat

-198-
SHUTTER

LASER CO,

1% CHOPPER

:\

I

1

LENS

SAMPLE

GRAPHITE APERTURE

POWER METER Figure 1. Diagram of damage- testing apparatus,

0 r

0.5

2 SEC S 3 0 SE CS

WATTS /

WATTS /

WATTS

WATTS

WATTS \

Figure 3. Irradiation matrix for damage study.

Sequence began at top left corner.

Figure 2. Power transmitted past an edge when

the edge is near the center of the

laser beam (x = 0).

2 3

TIME I SECONOSI

Figure 4. Average temperature at the center

and edge of a potassium chloride

window as a function of time.

<rQ.

Figure 5. The ratio of peak temperature rise to absorbed power as a function of beam

radius ratio to specimen radius after ten

seconds of Irradiation, The specimen was assumed to be potassium chloride.

-199-
Figure 6. Damage pattern produced on KCl #292. Picture A shows the entire specimen

(3.8-cm diameter). Picture B shows close-up of a damage site.

150,

6. 0.: i.b \:j Absorption (percent)

Figure 7. Damage intensity versus absorption for

abraded KCl specimens (circles) and

coated KCl windows (crosses)

.

Figure 8. Damage spot on a conven- tionally ruled grating (line

spacing 118.11/mm). The

damage spot is ab out 0.08 nnr in diameter.

4

1

1

Figure 9. Damage spot on an ion-polished grating

(line spacing 127/mm). The damage — .Zm-m —^ spot is about 0.08 mm in diameter.

-200-
COmENTS ON PAPER BV HUGULEV, ei: al

The. phyiiaal boi-ci thu pie.- conditioning eiiecX noted at the obieAved level o^ InAadtaution li> not

cJieajL. The leveJL 0($ InAadlation at vokich thii e^iecJ: -is obieAvzd ieemi anomalous Ig loM in tenm

cAeating micAoploAma-fs at the {^uA^ace. knothen. obieAvation conc&ined damage leveLi, on gfiatingi.

TKaditA-onally it ha-d been held that due to thz naLing{> gmtingi, ihould ex.hA.bit. a loMeA damage thfie^hold

than the conAe.i ponding iubitAote mateAial. In thJji mfik evidence Moi ieen exactly to the confiaAy.

No explanation Oj^ thiA diicAepancy woi ionthcoming.

-201-
4.2 10.6Min COMPONENT DAMAGE FROM

A 20 VI sec RAPIDLY PULSED LASER

Dr. Alan B. Callender

Air Force Weapons Laboratory

Kirtland AFB, New Mexico 87117

Extrinsic lower bound thresholds have been found for some window materials

and coated and uncoated metal mirrors. The results differ from single shot data

in that formation of breakdown plasmas in the vicinity of the components appear to

be more likely after several high repetition rate shots have been made. No major

conditioning effects have been observed.

Key words: Pulsed lasers; mirror damage; window damage.

1. Introduction _

Rapidly pulsed electric discharge lasers (RPEDLs) provide an intermediate regime for laser damage

study mechanisms between very high peak power single pulse lasers and high average power CW lasers.

Although the peak power densities are much less than those encountered with single pulse systems, pulse

damage still does occur. Also, the average power densities are very much less than those of interest

for past CW damage studies; but it appears that the average power density can, in fact, couple possibly

through either of two mechanisms to reduce the damage threshold drastically after the first few pulses

in a high-rate burst.

A new parameter, the laser's duty cycle, must be considered in explanation of damage mechanisms and

thresholds. Air flow flush time is also a factor for one candidate mechanism. A third factor, which

will strongly influence the importance of the duty cycle, is the thermal time constant of the component.

The experimental work reported here was performed on the AFWL EDL which has been described previously [1]' . At the time of this work this device delivered 60J pulses of 20 psec duration at rates up

to 150 pps.

Most AFWL interest has been associated with mirror surface damage rather than (surface or bulk)

window damage. Some tests have been made on candidate window material, but this work has been limited

by the lack of acceptable cleaning equipment on site.

2. Models

The power density regime at which single pulse damage to metal mirrors has been observed is approxi

mately l-15MW/cm for 20ysec pulses. Some CO damage tests on mirrors [2 ] and on unfinished metal targets [3 1 with the Northrop lOOysec pulse length EDL show that a damaging plasma is struck at less than

4 MW/cm . Other CO2 work has shown much higher (power density) thresholds at 0.6visec [4T . Here the ^ bare metal failure point was estimated at about 60 MW/cm with later results showing somewhat higher "

levels [5]. All of these power densities are low for air breakdown, but surface temperature rises may

be high enough to yield substantial metal vapor near the surface which could aid in plasma formation [6].

To check this possibility surface temperature rises were calculated for the pulse lengths involved

for polished copper. The values are all surprisingly low ('\jlOO't:),but they show only 20% deviations

from the mean. By contrast, the power densities vary by more than an order of magnitude. The inadequacy of temporal data make closer comparison difficult. New instrumentation is needed to discriminate

between power-dependent damage mechanisms and surface temperature rise mechanisms. Pre-heated and prechilled mirror damage experiments will be conducted at AFWL later this year to test the temperature rise

mechanism further. Care will, , of course, be taken to ensure that the surfaces are free of condensed

water.

If the mechanism is surface heating the temperature rises ('\'100°C for copper) are rather low for

metal vapor formation unless evaporation is occurring at surface proturberances . As at least some of

the tests have been made on surfaces which had RMS roughness less than 5nm with no significant difference in results, one should not expect large defects to be essential, though they are known to help.

Unfortunately, thermal transport parameters become somewhat nebulous, like electron transport parameters, near the metal surface; but one should expect that features on the surface which show poor electron

transport, and thus higher optical absorption, will also show poor heat transfer with consequent runaway.

The dependence of surface temperature on duty cycle and cooling place the RPEDL in a mixed category.

A calculation procedure is presented in section 4. If the duty cycle exceeds 1 x 10"^ for an uncooled

mirror, for example, the temperature at the end of the 26th pulse will be about 14% greater than for an equivalent single shot. Since the AFWL EDL can exceed a duty cycle of 2.5 x 10~3 this effect was studied

by varying the repetition rate. Cooling works to reduce the effect of pulse pile up in surface

1. Figures in brackets indicate the literature references at the end of the paper.

-202-
temperature by allowing a rapid approach to a steady state in which the average surface temperature is . set by the average flux and cooling characteristics and the peak temperature is limited to the single

shot rise plus the average temperature rise associated with the average power level.

Unfortunately, even though the uncooled mirror will reach a higher final temperature after a short

burst than the well cooled mirror, the presence of vapor build-up in front of the mirror may make

either mirror fail early. This would be shown by the presence of a smaller plasma followed by thorough

breakdown on each pulse for the remainder of the burst if no flushing is used. This was observed

in the experiments reported here.

Future experiments should include fast power detectors and either data collection equipment which

can sample at rates of ten samples per cavity round-trip time (to account for the presence of modelocked trains) or real-time high bandwidth analog analyzers to derive the surface temperature from optical power with only a minor time lag. Comparisons of data on different lasers or on the same laser with

slight random variations in its gas mixture are otherwise very difficult to make.

3. 10.6ym EDL Damage to Components

The preliminary experiments which have been made at AFWL were primarily made to determine safe

limits for system design. These experiments have been conducted on window and mirror material candidates

which were thus selected on the basis of contractor experience and with special emphasis on component

survivability under occasional (possibly dust induced) surface plasmas. For this reason only a few

coated samples were tried.

In the experiments an RdF large cone calorimeter was used to measure the net energy in a pulse or series of pulses. For mirror tests the calorimeter was located about 60 cm along the beam path after

reflection from the mirror. In window tests a mask of brightly finished tungsten foil (0.25 mm thick

X 2 X 5 cm) was constructed to aperture the beam to a square cross-section with edge of 2.0 cm, about 1 cm before the window, and the calorimeter measured the energy transmitted 30 cm behind the window.

The RdF calorimeter was calibrated by comparison with absorbed energy measurements on a 5 cm

diameter plane molybdenum mirror whose absorption had previously been measured.

Window tests were made about 2.0 m before the focal plane of a 10 m focal length spherical mirror

which received the (relayed) output of the AFWL EDL. At that time the output was a near-field annulus,

about 7 cm outside diameter and 6 cm inside diameter, and definitely somewhat more intense near the inner

edge. Single-shot bum patterns in front of and behind the tungsten mask showed substantial uniformity

over a 2 cm diameter circle with a single hot spot about 1 mm diameter located 5 mm off-pattern center.

Unfortunately, the relative intensity cannot be estimated.

Mirror tests were made in the focal plane of the 10 m spherical mirror. Some diagnostics had been

made on the machine output using transmission grating and wedged window beamsplitters. The results were

that 20% or more of the integrated output over a single pulse or over up to ten at high rate (> 50 pps) was within the central peak of the pattern which would be expected from an annular plane wave [7].

This should be compared with the expected value of O.AO for the central peak energy to total energy

ratio. In this sense the laser output was "50% of diffraction limited" performance. Thus, although

the shape was not measured within this spot size, it is still somewhat reasonable to assume that the peak

on axis intensity is about half of the value to be expected from an annular plane wave with the given

total energy. This spot is calculated and measured, in the split-off beam, to be about 1.3 mm radius

which agrees reasonably with the observed burn pattern radius of 1.4 mm. The peak energy density is then,

per joule incident on the focusing mirror or seen in the calorimeter, about 9.6 x 0.5 = 4.8 J/cm^. The

first factor, 9.6, is the calculated on-axis intensity gain due to the mirror and the second factor, 0.5,

is to account for the observation that only 50% of the calculated fraction of the total energy was within the first dark ring.

The temporal output of the AFWL EDL consisted of a leading power peak of about 100 nsec duration

followed by a 20 ysec pulse with a fairly flat top and fast falling edge. The leading peak was at least twice and sometimes as much as four times the average power. Some pulses had a rapid oscillation

(100 nsec to 1 ysec period) during the main pulse which had a peak-to-peak amplitude which was about

equal to the average.

The results were encouraging for mirrors and somewhat discouraging for windows. A summary is presented in tables I and II.

Mirror tests began with the bare polished molybdenum sample, which had been used just before the

damage test to calibrate the RdF calorimeter. For the calibration series this mirror was thermally

isolated, instrumented, and used about 3 m before focus at energies up to 60 J/pulse. The energy

density may be guessed to be of the order of 3 J/cm^ maximum. Absorbed energy was observed during the

damage tests onthis mirror and compared with the previous EDL sustainer voltage-energy curve. The

first surface flash created a melt pit about 2 mm diameter x 0.5 mm deep and the temperature rise was

eight times the expected value.

-203-
The bare copper samples showed surface plasmas for low repetition rate loading above 200 J/cm

(average beam axis power density 10 MW/cm^ during pulse), but no visible damage was created up to the

available limit of 290 J/cm^.

These samples might well have had rather less input from the last pulses in a sequence due to ^

breakdown of the disturbed air in front of the sample. They showed also that the calorimeter was too

close to the sample in that the calorimeter would break down at about 180 J/cm^, and during a burst of

pulses, the breakdowns would occur outside of the calorimeter but, after the test mirror. This might

well have been caused by neutral copper vapor from the calorimeter's previous breakdowns. Energy

estimates were based on the single pulse values (without breakdowns) for the same laser operating condl

tions.

The silver-plated mirror test was accompanied by many breakdowns between the calorimeter and mirror

but none on the mirror. At the end of the test ('^ 60 breakdown pulses in all) a 2 cm diameter circular area of the mirror appeared to have been plated with copper and copper oxides.

Dielectric coated mirrors showed much lower thresholds. In both cases the first surface plasma not only evaporated the coating over a 3 mm diameter circle, but also evaporated some of the substrate materl

as well. It appears that thermal coupling from the plasma to the metal was much enhanced by the presence of the coating. Unfortunately, lack of sufficient energy output for large area loading at realistic fluence did not allow further testing of these damaged samples to determine whether the damage

would spread. ^

A mistake in planning of geometry for the window tests was discovered [8] after the tests in that

the strongly converging beam used will greatly reduce self-focusing damage levels in comparison to the

collimated or expanding beam cases. This was nonetheless fortunate in that self-focusing damage was observed for nearly all impurity doped (for strength enhancement) samples, but not for the NaCl or KCl

undoped samples, or for ZnSe (undoped) . A power density dependent effect, like self-focusing, may have

been the cause of the anti-reflection coating failure on the ZnSe sample, as the actual failure area was about 1 mm diameter and in the same location as the bubble tracks. The hot spot in the bum pattern

mentioned earlier did not correspond to the bubble tracks. It might be that a very short output spike

has a quite different divergence from the rest of the pulse. Further tests will be made with spatially

filtered diverging or collimated beams.

The preliminary tests reported here will be followed by further tests with better beam quality.

Tests and development on mirrors will concentrate on means of combining massive polished or machined

copper or gold coatings on molybdenum substrates. Window efforts will continue with ZnSe and various

halides

.

Table 1. Mirror damage performance.

Mirror

1. Bare polished Mo. (Source in parentheses)

2. Bare polished Cu

(Spawr)

3. Bare diamond-turned

Cu (Y12)

Failed Survived

(S) 240 J/cm^ (S) 190 J/cm^

Could not damage rep. 10 @ 10 pps

290 J/cm2

Remarks

Observed 8X enhanced absorption on damaging pulse, which was the first

pulse to strike a surface plasma.

Struck plasmas but could not find

damage with 400X Nomarski microscope.

Diamond turned Ag

electroplated onto

Cu blank (LLL)

Polished Cu coated

with ThF^ (Spawr)

OCLI design #2 on diamond-turned Au

electro dep. on Cu

rep 10 @ 10 pps

290 J/cm2

rep. 10 (3 10 pps (S) 42 J/cm^

38 J/cm2

(S) 22 J/cm^

Many plasmas struck in calorimeter

bombarded sample and plated Cu.

Surface flashed and coating blistered.

Surface flash, coating completely

destroyed on first shot.

Test Modes: S

rep.

single pulse

repetitively pulsed N pulses (? M pulses /second.

-204-
Table II. Window damage performance.

KCl uncoated

single crystal,

old sample with

some haze.

Uncoated single

Crystal NaCl

Uncoated single

crystal KCl

4. Uncoated single

crystal rubidium

doped KCl

5.' Uncoated poly^

crystalline

rubidium doped

KCl

6. Uncoated Sr doped

KCl (very clean

surface)

7. Uncoated ZnSe

8. Anti-reflection

coated ZnSe

Test Modes: S rep.

3 J/cm single shot

rep. 7 @ 150 pps

5 J/cm2

rep. 10 @ 150 pps

5 J/cm2

rep. 4 @ 150 pps

8 J/cm2

rep 10 (? 150 pps

6 J/cm2

(S) 3 J/cm

(S) 8 J/cm

rep. 100 @ 150 pps

passed > 3 J/cm2

died 'V 5 J/cm^

Hazed surface ablated uniformly

over 2 cm diameter irradiated area.

Surface impurities visibly burned

away at this level. Could not cause

gross cracking or ablation.

Surface bum pits. Could not damage

bulk at maximum available fluence = 10 J/cm2.

Small bum pit.

1 mm diameter interior

bubble track.

1 mm diameter Interior

bubble track.

Small surface bum pits where

flashes occurred.

Catastrophic coating failure

on rear surface with window

breaking.

single pulse

repetitively pulsed N pulses @ M pulses/second.

4. Surface Temperature Rise

For times such that the characteristic length /4Kt is much less than either the laser spot size or the thickness of the plate one can use the solution given by Ready [9] when the incident beam is uniform in the plane of the surface and a rectangular pulse in time. This is, for surface temperature

Z (O.t)

1/2

z(o,t) = ^

(f-) ' (1)

Substitution of the heat capacity per unit volume pCp into the diffusivlty k then leads to

1/2

Z(0,t) 2aF ( ) (2)

TTpCpk

a is the optical absorption coefficient.

After the pulse is over the appropriate solution includes a cut-off:

Z(0,t) = [2aF/(irpC k)^''^] {t^^^ - (t-b)^/^ 0(t-b)}

P

(3)

Extending this result in the usual way for periodic problems, the periodic input surface temperature is

V(0,t) = Z Z (0,t-MT) e(t-MT)

M^6

(4)

where T is the period of the repetition.

-205-
One estimates surface temperature pulse pile-up using eq. (A) provided that t is not too large.

5. Conclusions

In all repetitive irradiation tests there was a breakdown on each pulse following the (relatively

weak) initial breakdown pulse. This held even for repetition rates as low as 1 pps. At high repetition

rates the later breakdowns would occur progressively further up the beam from the sample. This is

probably indicative of the presence of target vapor moving up the beam in this unflushed system.

Future tests will incorporate dry nitrogen flushing.

Thus, these experiments do not discriminate between the possibly gradual build-up of target vapor,

followed by a series of breakdowns, and the cumulative temperature rise of the surface alone. Discrimination will be achieved in the future by flushing of vapor build-up and by the use of. cooled targets.

6. Acknowledgments

The author gratefully acknowledges helpful discussions with Raymond V. Dennis Riley.

Wick, Ralph R. Rudder, and

References

[1] Rudder, R. R. , "The Interaction of Multiple

Laser Pulses with Solid Matter," Laser Digest , Fall 1974, (Jan 75) AFWL-TR-74-344, Air Force

Weapons Laboratory.

[5] Wang, v., J. E. Rudisill, C. R. Giuliano, M. .. Braunstein, and A. Braunstein, Proc. 6th ASTMONR-NBS Symposium on Laser Induced Damage in

Optical Materials, NBS Spec. Pub. 414 (1974).

[2] Holmes, S. and P. Kraatz, Proc. 5th ASTM-ONRNBS Symposium on Laser Induced Damage in

Optical Materials, NBS Spec. Pub. 387 (1973).

[6] Riley, Dennis, private communication, AVCOEverett Research Laboratory, Everett, MA,

February 1974.

[3] Rudder, R. R. , "Initial experimental observations of the interaction of pulsed five

micron radiation with solid targets," Laser

Digest, Summer 1974, AFWL-TR-74-241, Air Force

Weapons Laboratory, Kirtland Air Force Base,

MM, September 1974.

[4] Wang, v., A. Braunstein, M, Braunstein, J. E. Rudisill, and J. Y. Wada, Proc. 5th ASTMONR-NBS Symposium on Laser Induced Damage in

Optical Materials, NBS Spec. Pub. 387 (1973).

[7] Hanlon, J. A. , private communication, AFWL/

ALO, Kirtland Air Force Base, NM, March 1975.

[8] Wick, R. v., private communication, AFWL/LRE,

Kirtland Air Force Base, NM, November 1974.

F9] Ready, J. F. , "Effects of high power laser

radiation," (Academic Press, New York, 1971).

COmENTS ON PAPER BY CALLENVAR

The. qaution Mcu, oik^d concdAwinQ thz compa/iU>on o{j thz neMjJlMi obtiUnzd In thii mnk w-cth tiiz lULbttf, izpontzd by ScuXo tanZlM. in the, (ion(,ii>i(incz ^OK ihonjt palie. ilngle ihot damago.. Tho. ipaahtfi

fie^pondtd that due. to thz di{,{)eAznt iamplz choAacUeAA^tici It wad not poi^ilblz to makz a compcuvUon,

HoweueA, he mpha^izzd that a ilgnZ(,lcanA: fizdacvtion In the. damage thfie^hold oveA the single, ihot vaJLae woi obieAved in thli mn.k and thli W(X4 atXAibuted both to an incAeaiz In t&mpeAatuAe Mh^ich accumulated

dwung thz couM>e o{^ ifiAadtation and thz buildup ab-ioKblng vapon. ne.aA the. iuA^^ace. dxie. to fLe.petAJxvt

puLilng.

-206-
4 . 3 Laser Damage Measurements at CO. and DF Wavelengths

J. 0. Porteus, M. J. Soileau, and H. E. Bennett

Michelson Laboratories, Naval Weapons Center

China Lake, California 93555

and

M. Bass

Center for Laser Studies, University of Southern California

Los Angeles, California 90007

Preliminary results from laser damage tests conducted with CO2 and DF lasers on samples mounted in a newly constructed ultrahigh vacuum sample chamber are reported.

The lasers operate single mode and the pulse lengths are 100 nsec for CO2 and about

500 nsec for DF. A precise method of focusing which permits accurate determination of

power densities is described. Incorporated in the test chamber are a scanning electron

imager, an optical microscope, a profiling Auger system, and a charged particle collector

for in situ analysis. Analytical results are presented and implications for physical

damage processes are discussed. Measurements in nitrogen on aluminum samples indicate

that the threshold for N2 breakdown is reduced some 20 times in the vicinity of the

aluminum surface. Laser supported gas plasma waves are formed which shield the surface,

and no physical damage occurs for single 100 nsec pulses even at power levels of 10

GW/cm^. However, in vacuum damage does occur at about the expected power levels. No

significant difference between the damage threshold for polished and machined aluminum

surfaces was found.

Key words: Al mirrors; Auger spectroscopy; damage morphology; electron emission;

electron imaging; ion emission; laser-induced damage; surface characterization;

surface temperature.

1. Introduction

Improvement in the performance of laser-damage-resistant materials requires a better understanding

of the damage process and how it relates to material characteristics. This requires the ability to critically examine various aspects of damage on well-characterized samples under well controlled conditions. In this paper we describe a newly completed facility based on this approach, and discuss early

results that we have obtained on aluminum mirrors. Experiments are performed both in vacuum and in

1 atm of N2 with pulsed laser sources operating at wavelengths of 10.6 and 3.8 ym. The arrangement of

the various optical components is described in section 2. In section 3 we describe the temporal and

spatial characteristics of the lasers and outline an accurate method of focusing the beam on the sample.

In section 4 we describe the test chamber and its facilities for sample characterization and for monitoring damage phenomena. Conditions under which the various samples were prepared are described in

section 5. In section 6 we present data on damage thresholds. Damage morphology and the related

phenomena of electron and ion emission are discussed in section 7, together with implications regarding

the damage process. Finally, section 8 is a summary of the work.

2. Optical Arrangement

The optical arrangement of our laser damage analysis facility is shown in figure 1. Either of two

pulsed laser sources are used, one operating with CO2 for 10.6 ym radiation, and the other operating

either with HF or DF for radiation at 2.8 or 3.8 ym, respectively. A HeNe laser is also provided for

convenience in aligning the optics. Temporal beam characteristics are measured by a photon drag detector. Beam intensity is controlled by a rotating polarizer and analyzer, following which the pulse

energy is measured by a pyroelectric energy meter. The beam then enters the test chamber where it is

focused on the sample by a ZnSe lens having a focal length of 12 cm.

3. Laser Characteristics and Focusing

The lasers- operate single-mode with reproducible temporal and spatial characteristics. Figure 2 shows the measured dependence of intensity on time, together with functions which describe this dependence analytically. The latter provide a convenient basis for accurate heat flow calculations. Available power levels are more than adequate for damage testing with 1.5 J in 100 nsec from the CO2 laser

and 0.5 J in 200 to 800 nsec at the DF wavelength. Figure 3 shows the measured spatial profile of the

CO2 laser beam, which is represented by a Gaussian to within 4%. When focused by the 12 cm lens the

profile at the sample surface is represented by exp[-(r/93 ym)2]. The spatial profile of the DF laser

beam is similar, but has not yet been characterized with comparable accuracy.

Proper focusing is accomplished by a method illustrated in figure 4. The beam from a CW CO2 laser

passes through the lens along its optic axis, is reflected from the tilted sample and returns off-axis

through the lens to cross the incoming beam at a distance q in front of the lens. If q can be determined, the focal length f is calculable from the simple lens formula and the corresponding distance

-207-
p(>f) separating the lens and sample. The crossover distance is established by a thermocouple detector

whose two junctions straddle the incoming beam. With p fixed, an oscillating signal is produced by the

changing position of the return ray with respect to the detector as the sample is rocked about an axis

perpendicular to the line connecting the detector junctions. As illustrated by the signals a and b,

which were obtained with the crossover point on opposite sides of the detector, a phase reversal (and

signal null) occurs at the position of coincidence. A focusing accuracy of better than 10 Mm, which is

well within the focal range of the lens, is easily achieved.

4. Test Chamber

The test chamber and the analytical instrumentation are indicated schematically in figure 5. The

chamber is designed for ultrahigh vacuum, including bakeout. However, present measurements were made

without baking, either at pressures below 1 x 10"^ Torr or in 1 atm of The sample is mounted on a precision manipulator and may be placed alternatively in positions S]^ or S2. Position Si permits

irradiation and simultaneous monitoring of laser- induced effects. Visible changes in the sample surface

are monitored by scattered light observed with a 20X optical microscope. A biased Faraday cup with

grounded entrance grid provides intensities and time-of-f light energies of charged particles emitted by

the sample. Other features planned, but not available in time for the present measurements, are a mass

analyzer, electronic scattered light detector, and an evaporator. Auger analysis and sputter profiling,

if desired, are possible with the sample in position S2.

Figure 6 is a photograph of the test chamber in its present form. The laser entrance port faces

right. Surrounding this are four smaller ports containing a viewing window, microscope, particle

collector, and scattered light source. The Auger analyzer and coaxial electron gun are in the large

port to the left of the photograph. In smaller ports above this are the sputter gun for Auger profiling

and a glancing electron gun used for Auger spectra of insulating samples. The sample manipulator is

mounted in the top of the chamber.

An electron imager operating in conjunction with the Auger probe beam permits precise positioning

for analysis of selected surface features. Ultimate spatial resolution is 5 pm. Figure 7 shows an electron image of the central portion of a large damage crater with an optical (Nomarski) micrograph of

the entire crater for comparison. The electron imager also provides valuable information on the electron emission characteristics and its variation across the surface, which, as will be shown, is related

to damage phenomena.

Surface contaminants, as revealed by Auger analysis, may enhance optical absorption and lower damage

thresholds. Figures 8a and b show examples of Auger spectra from undamaged portions of polished and

sputter-deposited Al surfaces, respectively. The absence of C, N, and Mg on the sputter-deposited surface is noteworthy. Since samples are exposed to air when they are transferred from the preparation

facility to the test chamber, surface oxidation is unavoidable. Future work is planned on samples

deposited, or sputter-cleaned in the test chamber to determine the effect of the oxide layer on damage

phenomena.

5. Sample Preparation

The five different types of samples used are listed in table 1. Machined surfaces were cut on a lathe. Polished Al surfaces for testing and for substrates were prepared from the machined surfaces by

loose grinding and polishing using bowl feed, pitch lap, and 0.3 ym AI2O3 compound. Care was taken to grind long enough with each grit to remove the subsurface damage left by the previous grinding operation.

Quartz substrates were polished using bowl feed also, and had equivalent surface roughnesses under 20 X

rms. DC sputtered aluminum coatings were nominally 1500 X thick, deposited in a previously baked

system at a rate of 60 A/min in a 3 ym Ar atmosphere. The RF sputtered aluminum coating was nominally

2000 A thick and was deposited in an Ultek 2400 system at a rate of 1600 A/min in a 20 ym Ar atmosphere. The UHV evaporated aluminum coating was 1400 K thick and was deposited at a rate of 720 A/min

in a vacuum of 2 x 10~^ Torr.

Table 1 . Damage Thresholds on Aluminum.

Threshold Energy Peak Power

X Surface Substrate (1-R) Density (J/cm2) (MW)

10.6 ym Polished 2024 0.026 14 140

Machined 2024 14 140

DC Sputtered Al 0.017 8 80

DC Sputtered Qtz. 0.015 7 70 RF Sputtered Qtz. 0.024 6 60

UHV Evaporated Qtz. 0.012 4 40

3.8 ym Polished 2024 0.038 63 204

-208-
6. Damage Thresholds

Damage threshold results obtained in vacuum are summarized in table 1. The threshold energy density

or peak power is defined as that required to produce visible pitting. Thresholds of machined and polished surfaces are about the same. In the case of coated surfaces the threshold is significantly lower

in spite of the lower absorptivity (1-R) of the coating. Coating thicknesses are estimated to be less

than 10% of the thermal diffusion depth where a is the thermal diffusivity and tp is the duration

of the laser pulse Consequently, a thermal barrier at the substrate interface would cause a larger

amount of energy to be deposited within the coatings than within an equivalent layer of the uncoated

surfaces. In the case of the Al-coated Al substrates the oxide layer at the interface may introduce

such a barrier. Comparison of results for the sputtered vs. UHV deposited coatings suggests that the

thermal barrier is mitigated by sputtering leading to an increased threshold, again in spite of a higher absorptivity. The higher threshold at 3.8 ym is mainly a result of the longer pulse duration,

which provides more time for the energy to dissipate.

Attempts to damage the polished sample at 10.6 ym in 1 atm of N2 failed, even at power densities as high as 470 MW/cm^. The reason for this is a drastic lowering of the N2 breakdown threshold by the

presence of the Al sample. Numerical results are given in table 2. The low level breakdown is triggered by electrons emitted by the sample. Results with the sample tilted 60° are consistent with

thermionic emission, the N2 breakdown threshold rising roughly as the secant of the angle, i.e., in

accordance with the power increase required to maintain a given power density at the sample surface.

Thermionic emission has been directly observed in experiments performed in vacuum, as discussed in

section 7. We were able to damage the sample in N2 at 3.8 ym. In this situation successive pulses at the same position on the surface produced damage at power levels lower than that of the Initial pulse.

This may be attributed to removal of the oxide layer, which has a lower work function and hence produces

a higher electron emission than the bare Al. This interpretation is supported by electron imaging, in

which damaged areas appear darker than the surrounding undamaged areas.

Table 2. Nitrogen Breakdown Threshold at 10.6 ym

Without Polished Aluminum Sample 272 J/cm^ 272 MW

With Sample 8 J/cm^ 8 MW

Sample Tilted 60° 13 J/cm2 13 MW

Damage Threshold >470 J/cm^ >470 MW

7 . Damage Morphology and Related Phenomena

Damage craters produced at 10.6 ym on the sputtered and polished Al surfaces are compared in

figure 9. Generally, craters consist of a central depression surrounded by a raised rim. Craters on

the polished Al surface, either with or without the sputtered coating, are less regular than on the

quartz substrates, obviously a result of the poorer finish and uniformity.

An important bit of information for damage mechanisms is the relationship between pulse energy and

crater size. This is presented in figure 10 for the polished Al surface at 10.6 ym. The large scatter

in the data points is characteristic of polished Al and corresponds to the nonuniformity noted in

figure 9. The solid line is the result of a one-dimensional heat flow calculation of the energy

required to produce melting based on the beam spatial profile, the Drude temperature variation of

absorptance, and the temporal characteristics of the laser pulse. Although the threshold region is

fairly well represented, the larger craters extend well outside the predicted melt area. Possible

explanations for this aside from possible shortcomings of the model, are expulsion of molten material

beyond the melt zone and distortion of unmelted material. Attempts to resolve this discrepancy are in

progress

.

Perhaps the most important key to understanding damage mechanisms is the sample temperature. In principle, temperature can be inferred from the laser-induced electron emission if the surface is well

characterized and the emission process is known. Figure 11 reproduces an oscilloscope trace of the

laser output signal and the corresponding collected electron signal. Note that the electron signal

follows the individual mode-locked laser pulses. If the emission is purely thermionic, the relationship between temperature and electron pulse height is as given in figure 12. Here we have plotted the

Richardson-Dushman equation [2] using the known geometrical and electrical characteristics of the

detector, and assuming that the emitting area is delimited by the 1/e radius of the beam profile.

Unfortunately, the value of the work function, $, which parameterizes this relationship, is not known

for the oxidized surface, but is indicated by electron imaging to be below the value for clean Al. The

latter, fortunately, has been measured at room temperature, although no data is available on temperature dependence [2]. Multiphoton photoelectric and field-assisted emission (Schottky effect), which

have been neglected, tend to lower the effective work function. Space charge effects, which have also

been neglected, tend to limit the electron emission and thus appear to raise the work function. However, due to the pulsed emission and high fields present, the influence of space charge is probably

small. In consideration of these uncertainties, the Al curve in figure 12 at best provides an upper

limit on the temperature for a given pulse height. A maximum temperature of approximately 2350 K is

obtained for a typical peak electron signal of 500 mV,

1. Figures in brackets indicate the literature references at the end of the paper.

-209-
Another revealing symptom is the crater profile. Figure 13 compares the profile of a crater

produced by a 20 mJ CO2 pulse with the profile of the laser beam and a model calculation. Integration

of the observed profile over the crater volume indicates no net removal of material, i.e., the volume

of material in the cusps essentially cancels that in the central hole. Calculation from simple kinetic

theory of the amount of material evaporated within the times and temperatures available confirms this

result. The conclusion is that the crater is formed by pressure applied to the molten metal, rather

than by vaporization or ablation.

Models of laser damage due to the reaction pressure of vaporizing material have been proposed in the

literature [3,4]. The calculated surface pressure from vaporization of Al amounts to 0.07 atm at 2350 K. Another source of pressure is the momentum of the radiation itself. This amounts to 0.25 atm at the

center of the focused 20 mJ pulse if the radiation is fully absorbed, and up to 0.5 atm if partially m

reflected. These surprisingly large values are due to the combination of small focal area and short "

pulse duration. In comparing the relative importance of these two sources of pressure, one must consider

the impulse, i.e., the relative times available for the respective pressures to act. A simple calculation indicates that the vapor pressure will fall by an order of magnitude within approximately 20 nsec

of the termination of the laser pulse. Assuming that vaporization is suppressed by radiation pressure

during the 100 nsec pulse, the time available for the radiation pressure to act dominates. We conclude

that if our estimate of maximum temperature is correct, radiation pressure is the major cause of

cratering at CO2 wavelengths.

To demonstrate that the radiation pressure can produce a crater of the type observed, we have

modeled the surface profile of a molten pool of Al responding statically to the combined effects of

radiation pressure, surface tension, and confinement (figure 13). The boundary of the melt zone is

taken as the outside edge of the large cusps in the observed profile. The case depicted corresponds to total absorption of the radiation, which produces a crater of minimum depth. Although it does not

accurately represent the dynamical situation, the model strongly suggests that our basic interpretation

of the damage morphology is correct.

Although no significant material removal can be detected from crater profiles, the particle collector detects the emission of a small number of ions. Figure 14 shows the collector signal due to ^

positive ions from a sputtered coating at a pulse energy of 15 mJ. Such ion signals are commonly "

observed at power levels insufficient to produce a visible plasma, and occasionally, even when visible

damage is not observed. The most remarkable feature is the high energies of the ions, as indicated by

the time-of-flight corresponding to the peaks. Such energies cannot be accounted for by thermal vaporization and definitely indicate the involvement of an electric field. The laser field strengths are commonly in the range of 10^ V/cm in these experiments, which may be adequate for thermally-assisted

field-ion desorption. This is consistent with the magnitude of the ion signals observed near threshold,

which commonly correspond to desorption of less than 1% of an atomic monolayer of Al. The possibility

that fields developed across the oxide layer may play a significant role must not be overlooked, however,

and will be investigated in future work.

8 . Summary

A laser damage facility which can provide definitive information on failure modes of damageresistant materials is now operational. Well-characterized pulsed laser sources combined with wellcontrolled and comprehensively monitored testing are key features. Damage studies of polished and

coated Al surfaces have been made at 10.6 ym with some preliminary testing at 3.8 ym. Damage thresholds of coated surfaces are generally lower than those of polished or machined surfaces, in spite of

greater

substrate

surface

Interface,

purity

which

and uniformity, and higher reflectance. This is attributed to the coating- ^

acts as a barrier to heat flow. Damage in 1 atm of N2 is inhibited by gas- jH

breakdown induced by thermionic emission from the sample surface. Dimensions of damage craters Hj

obtained in vacuum are in fair agreement with calculated dimensions of the melt zone near threshold,

but exceed the calculated dimensions at higher pulse energies. A rough estimate of the maximum surface

temperature has been obtained from the electron emission. Based on this estimate and the shape of the

crater profiles, the dominant damage mechanism at 10.6 ym is found to be melting, accompanied by displacement of material due to radiation pressure. However, a small amount of material removal has been

detected in the form of positive ions.

9. Acknowledgment

The authors are indebted to W. R. Compton for the heat flow calculations. The theoretical aspects

of this work and the DF measurements were supported by the Office of Naval Research under Task Area

No. R0220202 and the experimental measurements at C0„ wavelengths by the Naval Air Systems Command

under Task Area No. R02201001.

-210-
10. References

[1] Ames, W. F., Nonlinear Partial Differential [3]

Equations in Engineering (Academic Press,

New York, 1965), pp. 158-62.

[2] Riviere, J. C, "Work Function: Measurements [4]

and Results," in Solid State Surface Soienee,

Vol. 1 (Dekker, New York, 1969), pp. 179-289.

Ready, John F. Effects of High-Power Laser

Radiation (Academic Press, New York, 1971),

pp. 109-14.

Batanov, V. A., F. V. Bunkin, A. M. Prokhorov,

and V. B. Fedorov, Soviet Phys. JEPT 36, 311

(1973).

11. Figures

<X>2 TEA I.ASER

760 J/cm2, 10 GW/ctn^, WITH 12 an LENS

HF/DF TEA LASER

1500 J/cmf 2.7 GW/cm^

WITH 12 cm LENS

PHOTON DRAG

DETECTORDIAPHRAGM

PYROELECTRIC ENERGY

METER

ROTATING ANALYZER

POLARIZER

ALIGNMENT

LASER

FOCUSING'

LENS, f - 12 cm

Figure 1. Optical arrangement of laser damage facility, showing

TEA lasers, beam line, beam control, and monitoring

instrumentation and the test chamber.

EQUAL ENERGY PULSE SHAPES FOR DF AND COj LASERS

0.5 I

CO2 PULSE

f Ij t/58 0 < t S 58 nsac

ll^e -0.0215 lt-581 58 < t < 00 WHERE 1 = 0.0132 x PULSE ENERGY

DF PULSE

t/160

0-0629(1-1601"

0 < t < 160 nwc

160 < t < 00 WHERE ln=0.00321 x PULSE ENERGY

600 800 TIME (nsac)

Figure 2. Temporal profiles of output from CO2 and DF lasers,

showing empirically derived functions used for

analytical representation of the measured data points.

-8-4 0 4 8 POSITION FROM BEAM CENTER (mm)

Figure 3. Spatial profile of output from CO2 laser, showing

Gaussian fit to measured data points. Circles and

squares indicate profiles measured in two orthogonal planes.

-211-
-SAMPLE ROTATION AXIS THERMOCOUPLE

DETECTOR - CHART

RECORDER

CROSSOVER IN IMAGE PLANE

FRONT NODAL POINT

- BACK NODAL POINT

DETECTOR LINE

Figure 4. Procedure for focusing the beam on the

sample. The detector remains fixed and p is varied until the crossover distance q coincides with the detector line as shown

in the figure, using the method outlined

in the text. The proper sample-to-lens

distance f is then calculated using the

lens formula.

UHV TEST CHAMBER

MICROSCOPE,

TIME-OF-FLIGHT- ANALYZER

AUGER ANALYZER

AND

ELECTRON IMAGER

ION GUN

EVAPORATOR

MASS ANALYZER

Figure 5. Schematic of the test chamber in its

completed form.

Figure 6. Laser-damage test chamber.

-212-
Figure 7. Comparison of Nomarski micrograph of large damage crater (left)

with electron image of central portion of the same crater (right),

AUGER ANALYSIS

1 1

J

1 1

i 1 1 1 1 1 1 1 1 T

1 1 1

POLISHED ALUMINUM

AC

1 1 c t 1 1 0

1 1 1 1 1 1

1

1

1 1 1 800 i;200 AUGER ELECTRON ENERGY, eV

1.600

Figure 8(a). Auger spectrum of polished Al surface.

AUGER ANALYSIS

1 1 1 1

1 1 1 1 1 1 1 1 1 1 r

1

SPUTTERED ALUMINUM

Al

1 1 1 1 1

0

1 I I 1 1 1 1 1 1 1 1 800 1,200 AUGER ELECTRON ENERGY, eV

1,600

Figure 8(b). Auger spectrum of sputter-deposited Al

surface. Note the absence of C, N2, and

Mg, as compared to figure 8(a).

-213-
POLISHED 202') At DC SPUTTERED M ON 2024 AV

DC SPUTTERED AV OM QUARTZ RF SPUTTERED Al ON QUARTZ

Figure 9. Comparison of damage craters produced on different Al sample

types at 10.6 ym.

CRATER AREA »s. INCIDENT ENERGY FOR POLISHED Al

BEAM AREA FOR l/e RADIUS

40 60 AREA K lO-^cm^

100 LASER INDUCED ELECTRON EMISSION OF 2024 Al

Figure 10. Pulse energy vs. crater area on polished

Al at 10.6 um.

Figure 11. Simultaneous oscilloscope traces of laser-induced

electron emission (upper trace) and laser intensity

(lower trace). The synchronization of the two traces

is not necessarily significant due to possible

differences in cabling length from the two signal sources

.

-214-

200 300 TIME (ntac)
SURFACE TEMPERATURE vs. ELECTRON EMISSION NEGLECTING SCHOTTKY

EFFECT AND SPACE CHARGE

5000 1

1 1 1 1 1 1

3000

2000

1000

OBSERVED SIGNAL 500mV ,

10" PULSE HEIGHT (VOLTS)

Iff"

BEAM AND CRATER PROFILES IN Al 20 mj, 100 nsec LASER PULSE

BEAM

. OBSERVED A ^ MODEL r

CRATER /r/y/

/ MELTED ZONE ' / / / / / /////////////

_

1 1 1 1 1 1 1

Figure 12. Surface temperature vs. height of laserinduced electron pulses based on thermionic emission and known experimental parameters. The upper curve represents the emission of an unoxidized

Al surface (((> = 4.2 eV) , while the lower curve represents an arbitrary surface

with (|) = 3.2 eV.

LASER INDUCED ION EMISSION

ENERGY OF At ATOM, 8V

1.000 100 10 5

-1 ' 1

1

-100 RADIAL DISTANCE Wm)

. 1.2 X 10* °K

SPUTTERED ALUMINUM

Figure 13. Comparison of beam profile with observted

and model profiles of a crater produced

in polished Al with a 20 mJ, 100 nsec

laser pulse at 10.6 ym. The observed

profile was obtained from a profilometer trace. The model profile indicates

the combined effects of radiation

pressure, surface tension and confinement of the molten metal (uniform

pressure) in a static situation.

0 4 8 12 16 TIME OF FLIGHT. pSEC Figure 14. Laser-induced ion emission from a sputter-deposited surface. The

energy scale at the top of the

figure applies to Al ions having

tlme-of-f light indicated by the

scale at the bottom. The lower

trace is the intensity of the laser

pulse, which is essentially zero after 0.1 ysec.

comims OH paver BV PORTEUS, at

It woi polntzd out by PetM. Bfiaunlich oi{ th^ Btndix ReAnoAch LaboiatofUti that the. obieAvatlon

fie.pofitzd -in tkli> papzn that the. Mon.k {function o{ baxe aluminum li htgheA than that o{, the oxtde. con- tmdlcti fiecent fieAutU o^ Baxtex GeneAal Hotou Re^eaxah LabonjotoKle^, In BaxteA'i mik the

oppo-ilte obieAvatton u)ai made. Mo explanation of, thti, dlicAepancy ivoi available. It wa4 alio bn.ought

out In the dlscuiilon that metati, can be expected to Kemaln good Kedlectofn u)etl above the melting

point. The theoretical bai,L(> {^oh. tkli is the Vfiude the.ofiij which -ci a-iiume.d to apply In thz liquid

Atate OA weLt aA beloM the melting point.

-215-
5.1 Single and Multilongitudinal Mode Damage in Multilayer Reflectors at 10.6 (Jim as a Function of Spot Size and Pulse Duration'"

V. Wang, C.R. Giuliano, and B. Garcia

Hughes Research Laboratories

3011 Malibu Canyon Road

Malibu, California 90265

An extensive series of measurements was carried out upon several high reflectivity

multilayer dielectric (ZnSe/XhF^) enhanced reflectors (typically 99.8%) using a CO2

TEA laser. Measurements were made using smooth single longitudinal mode pulses,

partially modelocked multilongitudinal mode pulses, and a range of pulse lengths

extending from 0. 6 (is to 6 h^s. Studies were conducted at spot size diameters ranging from 50 (i.m to 255 [xm. It is observed that damage thresholds are independent of

pulse duration or pulse temporal shape (energy dependent) and evidence is presented

that the damage is caused by inclusions, and that the average separation of inclusions

,

their absorption, and the film absorption can be inferred. A defect- limited damage

threshold (90 j/cm^) and a lower limit to the defect-free threshold (4.4 kj/cm2) for

the film is established.

Key words: Absorbing inclusions; damage statistics; laser damage; reflectors; 10.6 ym;

thin film; ThF^; ZnSe

Introduction

Previous papers [1-3]' have reported on the development of enhanced multilayer dielectric mirrors.

The goal was to develop multilayer dielectric coating materials and deposition techniques in order to reproducibly fabricate enhanced metallic reflectors with minimum absorptance at a wavelength of

10. 6 H-m and maximum resistance to damage from CO2 laser irradiation. The measurement of damage

threshold on these reflectors as a function of beam focal spot size, pulse length, and pulse shape are reported here.

Interpretation of the experimental data allows extrapolation of the damage properties of these

reflectors to large-scale beams and longer pulses than those employed in the damage experiments

described here.

Experimental Conditions

The experimental setup for the laser used in this experiment is shown in figure I. The pulsed laser

used to perform these damage threshold measurements has been refined for more than 2 years for

this purpose. The uv preionized CO2 TEA laser typically yields 200 mj at a pressure of 550 Torr

from a 50 cm x 5 cm x 2. 5 cm transverse discharge. A 2. 5 m approximately hemiconfocal cavity, an uncoated Ge resonant output reflector, and an adjustable aperture set at about 1. 2 cm selects the

TEMqq mode while providing an essentially untruncated output. An intracavity longitudinal mode

selector consisting of a I m low pressure longitudinal discharge tube installed between the high pressure transverse discharge and the output coupler provides a single longitudinal mode smooth output of

175 mJ with no mode beating evident, and also serves to suppress the initial gain- switched spike

characteristic of TEA lasers [4]. A beam attenuator consisting of two wire grid polarizers on ZnSe

substrates allows attenuation without rotation of the output polarization [5], Wedged NaCl beamsplitters allow monitoring of the energy in each pulse by a polyvinylfluoride (PVF) pyroelectric detector. Absolute calibration of the pyroelectric monitor is provided by a Hadron Model 100 cone thermopile with a 1 cm aperture.

Tests are conducted in an evacuated sample chamber (~20 mTorr) to eliminate extraneous effects

from dust and air breakdown. The sample is mounted on a translator at the focal point of the Ge

focusing lens and observed through an off-axis 50x microscope. The appearance of any microscopic

visible lesion in the film is regarded as an indication of damage threshold.

By control of the transverse discharge, variation of the gas mixture and total pressure, and operation of the low pressure discharge for longitudinal mode control, four types of pulses could be

selected. These are shown in figure 2. The basic pulse consists of a typical TEA laser gain- switched

partially mode-locked pulse as used previously [1-3] with an equivalent pulse length of 0.6 fxs. (The

equivalent pulse length is defined by integrating the area under a pulse and defining a rectangular

pulse of equal area and of height equal to the peak of the pulse. For rapidly modulated mode-locked

pulses the modulation is averaged. Power density is obtained by dividing the energy density by the

equivalent pulse length. ) A pulse consisting of a 1 |j.s single longitudinal mode pulse followed by a

Work supported by Air Force Weapons Laboratory, Kirtland Air Force Base, New Mexico under

Contract No. F29601 -74-C-0060.

1. Figures in brackets indicate the literature references at the end of the paper,

-216-
tail (equivalent pulse length of 1.9 f-Ls), a 4 [as equivalent pulse length gain - switched pulse , and a 6 [jls single longitudinal mode pulse are available by modifications to the laser parameters and operation of

the low pressure discharge section.

No provisions for rotational line control are included in the laser and preliminary measurements

indicate the possibility of multiple rotational line operation during the gain- switched modes of

operation

.

A near diffraction-limited beam is available and the focal spot sizes are measured at the beam

waist by the method discussed in [6], These results are tabulated in table 1.

Table 1. Ratio of Measured-to-Calculated Spot Size

Lens f . 1 .

, Maximum rms

Wavefront

Aberration, X

Spot Size a

, M-m Measured/Calculated

cm Calculated Diffraction

Limit Measured Spot Size

Ratio

3. 81 0. 13 37 50 1. 35

6. 35 0. 025 61 72 1. 18

12. 7 0. 036 123 140 1. 14

25. 4 (nil) 246 255 1. 04

a -

1

e diameter for intensity, 26.

Summary of Laser Performance

Control of a large number of operating parameters of the laser has been necessary to achieve the

present output, pulse length, and pulse shape. Some relevant changes which improved those desirable

aspects of the laser output include electrical pump pulse shape and duration, electrode profile and

spacing, gas mixture and pressure (especially CO2 fraction and the addition of H2), and a modest

increase in mirror radius.

In its final form this system has allowed us for the first time

• To study the influence pulse length over a 10:1 range

• To compare the effect of pulse modulation and the high instantaneous power

in a gain- switched mode-locked pulse with a smooth single longitudinal mode

pulse of easily characterized instantaneous intensity

• To compare the effect of beam spot size upon damage threshold over a range

of five to one.

To our knowledge, this is the first time a single transverse mode CO2 TEA laser has been used to study laser damage with a variable pulse length and single longitudinal mode control. It also represents the first study of spot size on damage threshold at 10. 6 fimThreshold Measurement Procedure

From time to time evidence arises suggesting that the damage resistance of a particular region of a material may be affected by its past history of laser irradiation. This type of effect which has been

reported previously [2], but has not been studied in great detail can result in either positive or negative conditioning. Generally, the kinds of conditioning effects observed are positive; i.e. , the level at which irreversible damage occurs is higher if the irradiated region is subjected to a number of pulses

of relatively low intensity prior to the damaging pulse than it would be if the site had never been previously irradiated at some relatively low level that is believed to be insufficient to cause damage.

In an attempt to determine whether significant conditioning occurs a specific threshold measurement sequence was employed as follows. A sample site is irradiated in steps increasing by ~ 1 5% until damage is observed. The threshold value obtained in this manner is referred to a s the multipleshot threshold. Following the observation of damage at a given site the beam is relocated to a fresh

region and the sample is irradiated again, this time with a pulse that is 1 5 to 30% less intense than

the one that was previously seen to cause damage in the adjacent site. If damage then occurs at the

new site, the sample is moved again to a new site and the power reduced still further. This procedure

-217-
continues until no damage is found at which time the power is then increased at the same site until

damage occurs. This is referred to as the single-shot damage threshold.

Hence some sites are subjected to a number of nondamaging pulses prior to damage and others are damaged on a single shot. Figure 3 illustrates schematically an idealized sequence of measurements that might be taken for two kinds of samples, one which shows a well defined threshold and

another which shows a marked positive conditioning effect.

Fabrication of Multilayer Reflector Samples

Because the goals of the program included the need to generate reflectors having the highest possible damage threshold, good environmental stability, and good adherence so that the resulting mirrors

could be easily cleaned, materials meeting these rigid demands had to be found.

The materials presently available that are suitable for use in low absorptance coatings at 10.6 |J.m are very limited. Thorium tetrafluoride and ZnSe have proved to be some of the best candidates for

this work. Therefore, the ThF^/ZnSe system was selected for concentrated effort because of its

potential for meeting all the goals. These dielectric materials were deposited directly on polished

molybdenum mirror blanks. Earlier emphasis was placed on the ability to prepare reproducible ThF^

films with minimal absorption. Film layers having lower absorption than previously realized had to be synthesized and applied in stable configurations involving a minimum of six to eight layers to achieve the goals of the program. Minimization of the number of layers was important since the

physical thicknesses required at 10. 6 fJ-m ( 1 to 2 [im) are an order of magnitude larger than those used

at visible wavelengths, and many films of this thickness tend to develop texture, surface roughness,

and cracks because of strain and expansion coefficient differentials.

This minimization of layers is realized through maximization of the difference in refractive index

between the dielectric materials employed in the alternate layers and also through maximization of

the reflectivity of the surface of the metal substrate. During the course of this experimental effort,

the reflectance of many Mo blanks were measured. The range observed was 97 . 8 to 98. 8%.

Considerable effort was expended to control the starting materials used for coating. Deposition

techniques were also found to be essential for the preparation of the high reflectance, low absorptance, damage resistant mirrors. All depositions were done in a Balzers 510 vacuum system.

Determination of Absorption Index of Dielectric Films

We feel the first consideration in the development of multilayer designs is to determine the absorption indices in single layer fibms of quarter-wave or half-wave optical thickness. This is done prior

to the fabrication of enhanced metallic reflectors.

ThF^

With the rapid development of damage-resistant high-index materials of low absorption, the

absorption coefficient of ThF^ became the limiting factor in achieving lower loss coatings and also

the limiting factor in the damage resistance of low absorptance designs. Absorption index values of

this material can be improved substantially by elimination of the effects of water vapor and other contaminants. The purity of the starting material is also essential for improved damage resistance. In past work experimental results with reflector systems were poor as a result of difficulty with the

ThF, starting material. To meet the goals of the program the best starting material had to be

found. Three different sources of ThF^ were evaluated, and the results are summarized in table 2.

Table 2. ThF. Evaluation

Run Number ThF^ Source ThF^ Absorption Coefficient, cm"-^

4-4-74 Poly-Research 16.6 to 20. 7 Pressed Powder

4-18-74 Cerac Inc. 11.5

TS-108

4-23-74 HRL Crystal Chunks 16.6

-218-
The ThF, absorption coefficient (cm~^) values in the table above are a factor of 3 to 4 higher than

needed to fabricate 99. 9% reflectors routinely. Since the HRL crystal chunk material is known to be

free of major impurities and has produced consistently good results in the past, the deposition conditions were suspect rather than the starting materials. The Poly-Research pressed powder had a dirty

exterior appearance and black residue was left in the source after each deposition. The material was

considered inferior and was eliminated from further consideration.

To lower the ThF^ absorption index values, work was begun to find methods of eliminating residual

water during deposition. Lower ultimate base pressures in the vacuum system became of prime importance for the elimination of suspected residual water. Long-term pumpdowns became a necessity.

With these long-term pumpdowns, pressure ranges of 6 to 7 x 10"^ Torr were achieved. Typical system pressures after a run were 2 to 3 x 10"^ Torr. An extensive series of experiments showed that

another important variable in controlling the water content, and thus the 10. 6 |J.m absorption of

deposited ThF^ films, is substrate temperature. The optimum temperature is IBQOC. With the lower

base pressures and the optimum substrate temperatures the minim\xm absorption coefficients in the

films were found to be near 1 cm"^. With absorption coefficients in this range multilayer work could

continue.

ZnSe

To determine the absorption in the ZnSe film material, a number of film deposition runs were made

in which quarter wavelength films were deposited on KCl substrates. The ZnSe source material

(Raytheon CVD) produced films that had too small an absorption to distinguish from the KCl substrate.

The upper limit on the ZnSe film absorption index which this implies is 0.6 cm~^. An absorption

coefficient of this order is considered excellent, and no problem was foreseen with the use of this

material.

Coating Techniques

During the course of this experimental effort, the objective was to prepare films which might be

incorporated in low absorptance coatings, compatible with each other, and finally compatible with

molybdenum mirror blanks. All of these requirements were met by ThF^ and ZnSe.

ThF^ Films

The deposition technique used for this material was evaporation by a resistance-heated tantalum box

source. Typical deposition rates were 6300 S./min.

ZnSe Films

The deposition technique used for this material was evaporation by a resistant-heated molybdenum

source with an aluminum oxide crucible insert. There was no advantage in depositing this material

onto an elevated temperature substrate. The absorption index would remain the same regardless of

temperature. However, all films were deposited at elevated temperatures (150 C) because this parameter was needed for the ThF^ films. Typical deposition rates were 7700 ^/min.

This experimental effort came to a successful completion with the fabrication of reflectors on the

Mo mirrors with 99. 92 and 99. 93% reflectance. Reflectors with the same design have been shown previously to have pulsed CO2 laser damage thresholds in localized areas more than twice as high as the

best metal mirrors; with reduced absorption, particularly of the ThF^ material, even higher damage

thresholds might be expected. At the reflectance levels now attainable it is possible to consider the use of uncooled reflectors in many applications where water-cooled reflectors had previously been

used.

Experimental Results

Damage Measurements for Different Pulse Durations

One of the means by which different types of damage mechanisms can be distinguished is by the performance of damage threshold measurements for pulses of different duration. A typical set of data is

presented in table 3 for a single thorium fluoride- zinc selenide dielectric enhanced reflector on a silver overcoated molybdenum substrate surface. Although there is a wide range of values over which

damage occurs,the ranges for this sample overlap substantially for the different pulse lengths employed, indicating essentially a constant energy density damage threshold. A constant energy density

threshold was also observed for the other samples having the same coating design as that for which

data are presented in table 3.

NOTE: 1 torr =

101 325

7^^ Pa.

-219-
Table 3. Reflector Mo AM-56 Mo/Ag /(ThF^/ZnSe)^

Absorption = 0. 30%

Spot Size = 140 |JLm diameter

4209-15 DAMAGE THRESHOLD

0.6 Ais MODE-LOCKED

1 MS SINGLE

LONGITUDINAL

MODE

4 fjs MODE-LOCKED

6 MS SINGLE

LONGITUDINAL

MODE

MULTIPLE SHOT

THRESHOLD

50-290 J/cm^

80-480 MW/cm^

110-460 J/cm^

110-460 MW/cm^

110-200 J/cm^

17-70 MW/cm^

70-290 J/cm^

28-40 MW/cm^

SINGLE SHOT

THRESHOLD

15-75 J/cm^

25-125 MW/cm^

-70 J/cm^ ~ 70 MW/cm^

(70-110 J/cm^)

(17-275 MW/cm^)

(50-90 J/cm^l

(8-15 MW/ cm^)

0.6 MS 1ms 4 )is 6 MS

This observation of a constant energy density threshold over a range of different pulse durations is consistent with two different interpretations that involve a thermal damage mechanism.

1. The light is absorbed in a uniformly lossy film that is essentially being

heated uniformly throughout during the time of the laser pulse. Damage

then occurs when the film reaches some critical temperature (e.g. , melting point). This case would also apply to a film that has local defects or inclusions that are small and so close together than the thermal

diffusion length is comparable to the mean defect spacing.

2. The damage is caused by the heating of a local defect or inclusion that

is essentially uniformly heated throughout its volume during the time of

the irradiating pulse and during which time its rate of heat loss to the

surrounding host material is negligibly small.

The constant energy density threshold, combined with the data for different spot sizes presented,

next suggest that mechanism 2 is the more likely choice.

From the work of Duthler and Sparks [7] it is possible using a few simple assumptions to obtain

limits on the sizes of defects or absorbing inclusions responsible for the damage in the mirrors

studied.

The following expressions are taken from [7], The approximate time required for heat to diffuse from the inclusion surface to the center is

T - 4 aV9TTKj , (1)

The approximate time required for heat to diffuse from the inclusion into the host is

= Cj^a2/3Kj^r| , (2)

where a is the inclusion radius, C and K are heat capacity and thermal conductivity, respectively,

and T] represents a thermal coupling efficiency between the inclusion and the host.

-220-
The constant energy density threshold is equivalent to the expression

^a V

<^ ' (3)

where t is the laser pulse duration. Statement (3) must be valid over the range of pulses employed in

the darrRge experiments, 0.6 to 6 ^s.

Using the above considerations one can derive an expression for the range of inclusion radii

where t, and t^ are the lower and upper extremes of pulse durations over which the constant energy

density threshold is observed, 0.6 and 6 \i.s

, respectively.

2 2 Taking values of Kj^/Cj^ ~ 0.04 cm /s and Kj/Cj ~ 1 cm /s, typical of dielectrics (ThF^) and

metals (Cu), respectively, and a value of r| ~ 0.1, we obtain 2.7 (J.m « a « 21 [J-m.

It should be pointed out that the limits obtained for the inclusion radii from (4) are only to be taken

as approximations and are given as an indication of the kind of data available from pulse duration

damage studies in these types of materials. In fact, the value taken for r| which determines a lower

limit for a is at best an order of magnitude estimate. A value for r\ of 0. 01 would extend the low end

and widen the range to 0. 85 t-i-m « a « 21 fxm. It should also be pointed out that the wide range of

possible values for inclusion radii indicates only that particles within this range will behave with a constant energy density threshold over the pulse durations studied,and should not be interpreted to mean that the films actually contain inclusions having this wide a range of radii. In fact, because of

the lack of obvious inclusions on microscopic inspection of these films, the low scattering, and thick- ness of the individual layers, it is likely that the defects are very small, of the order of 1 [J.m or less.

The value of the defect dominated damage threshold obtained next and the modeling of inclusion

heating dynamics developed in [2] both support the thesis that the inclusion defects are limited to a

relatively small range of sizes of the order of 1 |J.m. The presence of a large range of inclusion sizes

should imply an observable pulse length dependence, rather than a constant energy density threshold.

The damage morphology for these reflectors is shown in figures 4 and 5.

The Effect of Spot Size Upon Damage

It has been long recognized that the apparent damage threshold of many materials changes with the

diameter of the illuminating beam [8] . The difficulty in obtaining a diffraction limited focus increases

with more tightly focused beams, as well as the difficulty in finding the beam waist, measuring the

spot size, and detecting damage threshold. However, beyond these effects there still can exist a marked increase in damage threshold for small beams when there is a random distribution of defects or localized absorption sites. For example, if a very tightly focused laser beam is used, and defects or inclusions are small and spaced far apart in comparison, then the preponderance of damage

thresholds would register the intrinsic threshold. Conversely, a large beam of uniform illumination

would be expected to cause damage at the weakest defect or inclusion. The damage threshold with

large beams, whether due to some intrinsic property of the material, or due to localized weaknesses,

is sharp and deterministic. Small tightly focused beams might tend to have large fluctuations in

damage threshold, whether from inherent statistical mechanisms or from randomly scattered localized weaknesses. Since the present multilayer dielectric reflectors show statistical variation in

damage threshold with the spot sizes we have used indicating'an inclusion/defect spacing comparable

to the beam size, a study of the spot size dependence has been made to better characterize the defect

limited damage threshold and the damage threshold obtained in regions that are free of defects,

referred to here as the "intrinsic" film threshold.

Seven similar 6- and 8-layer ThF^/ZnSe reflectors have been tested at four focal lengths, of

3.81 cm, 6. 35 cm, 12. 7 cm, and 25. 4 cm, which gave measured spot sizes of 50 jim, 72 nm,

140 nm, and 255 (jim, respectively. These tests were performed using a smooth 6 fJ.s single longitudinal mode pulse.

The results of several thousand test shots on these reflectors is presented in figure 6. The energy

density for threshold is plotted versus the size of the illuminating spot. The spot size 26 is the

diameter at the e"^ intensity points of the illuminating beam and the energy density is the on-axis

value obtained by dividing the total energy by Tr52. The data from these seven reflectors are combined in figure 7 where the standard deviation cr , the mean, and the extremes are shown.

Using the expressions developed in [8] (shown In figure 7) by DeShazer and coworkers, we can

derive an average inclusion spacing d^ from the curve drawn through these data. Using 90 j/cm^ as

-221-
the defect limited threshold and 4.4 kj/cm^, the highest threshold recorded for the "intrinsic" film

threshold, we arrive at an average inclusion spacing of d^ = 25 |j.m.

Coincidentally, this value is similar to values found in thin films of ZnS, MgF2, ZrO^, and Ti02/

SiO by DeShazer. However, the ratio of "intrinsic" to defect-limited damage threshold*^ for these

systems is about 50, as compared with the value of 3 assumed by DeShazer.

"Unconditioned" single shot damage data are summarized for the same seven reflectors in figure 8 These data give similar but lower results with

I. = 1.3 5 kj/cm^

= 40 j/cm^

d = 28 MJn . o

Several assumptions, which are only approximately true, are implicit in the expression in figure 7 First, a Poisson distribution of point defects is assumed by the expression, whereas other evidence

seems to indicate particles with a size not very far removed from the mean spacing. Second, it is

assumed that each defect possesses the same damage threshold. Third, a two-dimensional sheet of

particles is assumed, whereas the actual sample is a three-dimensional stack of films. A situation

could be encountered where the beam encounters no defects at the first maximum in field (at the

outermost interface of high and low index films), but fails to damage a defect several layers below

which happens to be below the inclusion damage threshold. A subsequent shot at a higher flux level

damages this imbedded inclusion. This would give the effect of defects with a range of damage

thresholds. Thus, the interpretation of the spot size statistics used here is only semiquantitatively

correct; the inclusion spacing should be viewed as an approximation,but the two limiting thresholds

should remain correct.

In the limiting case where the mean inclusion spacing is very large compared with the beam diameter (a situation that hopefully will be achieved as mirror technology advances for 10,6 |jum reflectors), the distribution of measured thresholds would be expected to cluster about the two extreme

values given by either the intrinsic or the inclusion-limited damage thresholds.

Summary

j Variations of pulse length, pulse shape, and spot size have led to a model where these multilayer

dielectric reflectors are limited in both absorption and damage threshold by the presence of localized

absorbing defects. The damage threshold for pulses typical of e-beam TEA lasers is expected to be

about 90 j/cm^ for large beams. This threshold is apparently independent of pulse length and the

usual variations in pulse temporal shape over the range 0.6 to 6 [jls tested here.

The pulse length study, together with absorption measurements, suggests that the local defects are possibly metallic, highly absorbing particles of ~1 |i.m in radius. This is consistent with an observed threshold of ~90 j/cm'^. The measurements of damage threshold as a function of spot size

indicate that the mean defect/inclusion spacing is 25 to 30 |am.

The intrinsic limitation of these films is expected to be as high as 4400 j/cm , and perhaps

limited by only linear absorption in the ThF^ or ZnSe.

Acknowledgments U

The authors wish to acknowledge helpful technical discussions with A. Braunstein and \

M. Braunstein. The skilled technical assistance of T.E. Horne in construction of the apparatus

and in taking damage threshold measurements is acknowledged as is the assistance of Donald Williams

in fabrication of the multilayer samples.
Reference s

[1]

[2]

[3]

[4]

Wang, v., Braunstein, A.I.,

Braunstein, M. and Wada, J. Y. , "Investigations of Pulsed Laser Damage of

Metal and Dielectric - Coated Mirrors,"

Proceedings Laser Induced Damage in [5]

Optical Materials: NBS Spec. Pub. 372

(1972) .

Braunstein, A.E., Wang, V,, Braunstein,

M. , Rudisill, J.E., and Wada, J. Y. , [6]

"Pulsed CO^ Laser Damage Studies of

Windows and Window Coatings," and

Wang, V. , Braunstein, A. , Braunstein,

M. , Rudisill, J.E. , and Wada, J. Y. , "Pulsed CO, Laser Damage Studies of

Metal and melectric Coated Mirrors," [7]

Proceedings Laser Induced Damage in

Optical Materials: NBS Spec. Pub. 387

(1973) .

Wang, v., Rudisill, J.E., Giuliano, C.R. , Braunstein, M. , and Braunstein, A, , "Pulsed CO2 Laser Damage in Windows,

Reflectors, and Coatings, " proceedings

Laser Induced Damage in Optical [8]

Materials: NBS Spec. Pub. 414 (1974).

Gondhalekar, A.

Holzhauer, E. ,

, Heckenberg, N.R.

'The Mechanism of

and

Single-Frequency Operation of the HybridCO-, Laser," J. Quantum Electron. QE- 1

1

, 3 (r975).

Garvin, H. L. and Kiefer, J.E., "WireGrid Polarizers for 10.6 fxm Radiation, " presented at IEEE/OSA CLEA Conference

May 30 - June 1, 1973, Washington, D. C.

C.R. Giuliano, and D. Y. Tseng, "Damage

in Lithium lodate With and Without Second

Harmonic Generation," p. 84, proceedings

from Symposium on Laser Damage, NBS

Spec. Pub. 387 (1973).

Duthler, C.J. and Sparks, J., "Theory of

Material Failure in Crystals Containing

Infrared Absorbing Inclusions," Proceedings of a Symposium Sponsored by Office

of Naval Research, May 15-16, 1973,

Boulder, Colorado, NBS Spec. Pub, 387.

DeShazer, L. G. , Newnam, B.E., and

Leung, K. M. , "The Role of Coating

Defects in Laser-Induced Damage to Thin

Films," Laser Induced Damage in Optical

Materials: NBS Spec. Pub. 387 (1973).

Figures

2.5 m LASER CAVITY

0.5 m HIGH PRESSURE DISCHARGE

1 m, LOW PRESSURE DISCHARGE

WIRE

GRID POLARIZER

(ROTATABLEl

_ 1,2 cm APERTURE

99% OR 20% REFLECTOR

RADIUS =LECTOR

VACUUM ENCLOSURE

VACUUM

FEEDTHROUGHS

OBSERVER

Figure 1. Schematic of laser, optical cavity, optical train, attenuators, detectors,

alignment system, and sample chamber with microscope and focusing lens.

-223-
Multilongitudinal mode pulses

1 . 9 M-S ^

Single longitudinal mode pulses

Figure 2. Oscilloscope traces of the four types of pulses used.

-224-
4209-2

>-O

cc

HIz

111 LU

t/5

I I I I I I I I I I

CASE A, STRONG CONDITIONING

4 = DAMAGE • = NO DAMAGE

I \

\

START SEQUENCE

I I I I I I I I I I I

0 1 234 567 89 10 11 12

SITE NUMBER

I I I I I I I I I

CASE B, NO CONDITIONING

i^* ;V

START SEQUENCE

I I I I I I 01 23456789 10

SITE NUMBER

Figure 3. Sequence for irradiating sample sites. Case A, strong

"conditioning." Case B, no "conditioning."

Figure 4. Damage in No. AM-57 Mo/Ag/ (ThF^/ZnSe) "*

,

1300 J/cm^, pulse length is 6 Ms, single

longitudinal mode, 50 ym beam diameter.

Figure 5. Damage in sample Mo AM-57, Mo/Ag/ (ThFi,/ZnSe) "*

,

70 J/cm^, pulse length is 6 ys, single

longitudinal mode, 255 ym beam diameter.

-225-
200 240 280 0 40

SPOT DIAMETER, e"' INTENSITY.

Figure 6 (continued on next page).

-226-
Figure 6 (continued)

1700

1600 -

1500 -

1400 -

1300 -

1200 -

1100 -

1000 -

. 900 -

' 800 -

700 -

600 -

500 -

400 -

300 -

200 -

100 -

0

I J L

I

-

80 120 160 200 240

SPOT DIAMETER, e'' INTENSITY, fim

80 120 160 200

SPOT DIAMETER, e

' INTENSITY, iim

Figure 6. Spot size dependence of damage threshold for multiple shot damage thresholds.
T

0 20 40 60 80100 200 300

e

"" SPOT DIAMETER, fJm

(INTENSITY)

Figure 7. Summary of damage thresholds for seven reflectors obtained for multiple thresholds.

Curve represents best fit of above expression for chosen values of I, and I,,

-228-
40 80 120 160 200 240 280 SPOT DIAMETER, /um

(intensity)

Figure 8. Single shot damage thresholds as a function of spot size.

COMMENTS ON PAPER BY UJANG, oA al

AZXhough thd -idzntUiij thz damaging dc^ec;t4 in tkLi wonk uncZaafi th^ ip^akeA obicAued that

cjxxe. 'in the chotce. itoAting mateAlaZi and coAtiul control o{, the. pfLoaeiitng o{, thz matQAlat did tj-LeZd

i>-igni{iica.Yit ^pnovmznt Zn damage th/in^hold. Su/iiaceA iubifiate matunJjxJU weK& pfiepoAzd MJjth

aoAe-iut attzwtion to tniyUmczZng the occuAAenceA icnatchoj, and digi and typtcaZ i>iiAiac.<i AoagkneMU

mojid. kept to the oKden. o{^ 20 A nmi. The queitZon Mai nailed a6 to why the damage Moi attAibiLted to

the pAeAence 0i5 meXat Atch deieet^ that Aegtoni, o{, high metaZ concentAotion weAe not ieen tn the

v-LcA-nAty o^ damage iiXei,, In the emulng diicuM-Lon it: woa pAopoied that thii might be ottAibutable to

the {lact that it iA not metal indLiLdioni which oAe damaging, bat instead pin holej> and deiecti in the

dieZectAic coating. A {,uAtheA qaeition wa6 futided expAejsiing conceAn that the obieAved valuei>

4uA($ace damage when exXAapolated to zeAo ipot iize gave valuej> higheA than the obieAved balk damage

in zinc ^etevLide. The amweA wai that the balk damage in tkii matetial ii limiXed by de{,ect6, and

theAeioAe would be expected to be loweA than the extAapotated value oj) the iuA^^ace damage.

-229-
5 . 2 Temperature and Wavelength Dependence of the Reflectance of Multilayer

Dielectric Mirrors for Infrared Laser Applications

D. L. Decker

Michelson Laboratories, Naval Weapons Center

China Lake, California 93555

The absorptance of mirrors used in high energy laser applications is significant

in determining the heat load which must be removed by cooling and the magnitude of

the associated thermal figure distortion. For design purposes it is necessary to know the absorptance not only at room temperature, but also at temperatures up to and

above the actual operating temperature of the mirror. The damage threshold is also

related to the absorptance and to its variation with temperature, dA/dT. Since

temperature and wavelength dependence is needed, it is most convenient to infer the

absorptance from absolute reflectance and scattering measurements, rather than from

direct measurement. The present paper describes instrumentation capable of making

relative reflectance vs. temperature measurements with a precision and accuracy of

7 X 10"^. Temperature dependence data over the range from 25°C to 125°C is presented

and analyzed for several commercially obtained dielectric multilayer mirror designs

intended for use at 3.8 um. These mirrors have absolute reflectances above 0.997

and show a reflectance variation with temperature, |dR/dT| of 5 x 10~^ or less.

Key words: Multilayer dielectric mirror; optical absorptance; reflectance;

scattering; temperature dependence; wavelength dependence.

1. Introduction

Metal mirrors have in the past held near exclusive province in high energy laser applications as a consequence of high thermal conductivity and relatively low absorption. However, in the mid infrared

increasing use of multilayer dielectric mirrors is being made as technology improves and old prejudices

die. The usual multilayer dielectric mirror consists of many quarter-wave thick layers of alternate

low and high index [1,2]? The response of the "stack" is, of course, defined by the superposition of

constructively interfering waves transmitted through and reflected from the layers of the mirror. The

design reflectance can, in principle, be increased to values as close to one as desired by increasing

the number of layers in the stack, limited only by scattering from imperfections, residual absorption

in the films, and practical limitations on the uniformity of the optical thicknesses of the films of

the stack. Reflectances greater than 0.999 are possible with current technology. The viability of a multilayer dielectric mirror in high energy applications depends upon maintenance of its Initial very

low absorptance. Even very small changes in absorptance (representing large fractional changes) as occur, for example, as the temperature increases during operation are important. Since temperature and

wavelength dependences are needed it is most convenient to infer the absorptance from absolute reflectance and scattering measurements rather than by direct measurement. Experimentally, the challenges

presented here are nontrivial: for an initial absorptance of 10~^ , a 10% change is only 10"^ absorptance

units. This paper provides results and analysis of three commercially produced multilayer dielectric

mirrors intended for use at the DF laser wavelength (3.8 ym), together with some experimental detail.

2. Experimental Program

The designs are ZnS/ThF4/Ag (Rl) , ZnSe/ThF4/Ag (R3),and Si/SiOx/Ag (R4) deposited on ultralow

expansion (ULE) glass. They were designed and produced by Optical Coating Laboratories, Inc. (OCLI)

,

Santa Rosa, California. Figure 1 presents measured values of absolute reflectance for design R3 from

3.6 to A. 2 pm. Three sets of data are plotted. Original absolute reflectance measurements "A" were

made in March 1974, measurements made one year later "o" after storage at room temperature in dry

nitrogen, and the points "o" after cycling to 120+''C. Figure 2 presents similar data for design Rl and

figure 3 for design R4. It is obvious that designs R3 and R4 are much more stable than Rl in both storage and upon temperature cycling.

Before proceeding further it is important to briefly describe the measurements performed to generate

this data. The points denoted by "A" and "O" are the results of near normal incidence absolute reflectance measurements performed in our laboratory at room temperature and have an uncertainty of

±0.0005 [3]. In addition, relative reflectance measurements have been made using a near normal incidence ref lectometer recently designed and constructed specifically for temperature dependence measurements [4] (see figure 4). This instrument measures the ratio of the absolute reflectance Rg of the

sample at temperature T, to the absolute reflectance Rr of the reference at temperature T^, RgCl) /R.^(.Tq)

The uncertainty of the reflectance ratio measurement is ±3 x 10"^. If Rr(To) is known, the absolute

reflectance of the sample as a function of temperature can be computed:

1. Figures in brackets indicate the literature references at the end of the paper.

-230-
R (T) = [r (T)/R (T )]. s Is r o J (1)

The values "o" in figures 1, 2, and 3 were computed from the changes in measured relative reflectance,

and absolute reflectance measured before temperature cycling, using eq. (1). The uncertainty in the

computed absolute reflectance (eq. 1) is completely dominated by the uncertainty in the measurement of

Rr('^o) ^rid is thus i0.0005. The temperature dependence of mirror performance is provided by evaluation

of not only thermal cycling stability, but also the absorptance and scattering at elevated temperature.

Figure 5 presents raw relative reflectance data as a function of temperature for design Rl at 4.2 pm. Sample and reference in each set of measurements reported here are nominally identical, having been

sliced to dimension by a diamond wafering saw from an original 1.52-inch diameter sample. The initial

departure from unity is thus a measure of the spatial uniformity of the original sample. Design Rl in

fact displays a much larger initial departure from unity than either R3 or R4. This departure is

strongly wavelength dependent, varying from a part in 10^ at 3.6 pm to a part in 10^ at A. 2 um. The

temperature coefficient of reflectance of Rl at 4.2 ym, dR/dT ~ -5.3 x 10~^/°C is larger than for any

other design and wavelength.

The reflectance and its temperature dependence are obviously both more or less strongly wavelength

dependent. The temperature dependence of such a device can come from a number of sources: most simply,

from the temperature dependence of the optical thickness of the various layers of the stack, with the

resulting change in "tuning." It is clear that such an effect will be wavelength dependent, and in fact

may result in the high reflectance band shifting in wavelength. From the linear coefficient of expansion (a) for a single layer, and the variation of index of refraction dn/dT, the variation of optical

thickness (nd) of that layer can be computed

(nd) dT n dT

If the film design is known and a multilayer design program is available, then changes in response can be readily evaluated. Of course, if the reflectance is limited by absorption in the stack or by scattering, the behavior of the reflectance of the device with temperature is not described so simply. If

significant absorption is present, regardless of the specific mechanisms, the temperature dependence of

the stack reflectance will be relatively large. This is simply a consequence of all dispersive mechanisms being more or less strongly temperature dependent. On the other hand, if the reflectance is

limited by scattering, the temperature dependence of the device reflectance would be expected to be much

smaller. This can be seen from the following argument. Visible and near infrared scattering in high

quality optics is nearly always the consequence of distributed microirregularities or dust; i.e.,

scratches and other macroscopic cosmetic blemishes are usually not important. In the case of distributed microirregularities, the ratio of the fractional intensity AR of light scattered to Rq, that

reflected, is given by the expression [5]

AR/Rq = (4Tr6/X)2, . (3)

where 6 is the rms roughness of the surface microirregularities and A is the wavelength. From this

expression one can simply compute the temperature dependence:

dAR/dT = (dRo/dT) (AR/Rq) • (4)

If we assume that all of the incoming beam which is not reflected is scattered, AR/Rq ~ 10~3 for the

multilayer dielectric mirror designs considered here. A precise value for dR^/dT for the present

purposes is not necessary, and we can set dRo/dT ~ 10~^/°C as a very conservative maximum estimate.

The temperature dependence of the fractional intensity of the light scattered from distributed microirregularities, dAR/dT is then 10"^/°C x 10-3 ~ IQ-I /"c. a similar calculation can be made of the

temperature dependence of the scattered light associated with localized microirregularities, yielding

a temperature coefficient ~ 10-^/°C, again assuming that all light which is not specularly reflected is

scattered [6]. These values, especially the lattei^ are very much smaller than any value of dR/dT which

could be reliably measured by the present experimental apparatus over even a 1000°C temperature increment. Any strong reversible temperature or wavelength dependence in the reflectance of a multilayer

dielectric mirror must be due to absorption or changes in stack "tuning." There is one exception to this observation: If we are near a dipole resonance of the localized scattering particles the scattered light intensity will be strongly temperature and wavelength dependent [7],

All of the mechanisms considered up to this point apply to reversible temperature effects. Any

changes in structure or composition as a consequence, for example, of intra- or inter-layer diffusion

will likely result in index changes in the layers involved. Such effects will be temperature activated,

and result in irreversihle variation of mirror performance with temperature, as is very evident in the

temperature cycling data for design Rl (figure 2). Figure 6 presents computed absolute reflectance vs. temperature for designs Rl and R3 obtained from relative reflectance measurements at two selected wavelengths. The difference in performance of Rl at wavelengths separated by only 0.4 ym is striking.

Since many DF lasers have an output which extends from 3.6 to over 4.0 ym, the requirement for detailed

examination of performance with wavelength is obvious. Figure 7 is a plot of similar information for

-231-
design RA . The variation of the reflectance with temperature is nearly wavelength independent, and has

a value, dR/dT ~ + 3 x 10-5/°C, ignoring hysteresis.

It is instructive to compare the temperature dependence of the multilayer dielectric mirror designs

considered here with the performance of a metal coated mirror. The absorption in a good metal (e.g.,

silver) in the infrared is a consequence of electron-phonon scattering, the same mechanism which limits

the electrical conductivity to finite value. The variation of the absorptance (or reflectance) with

temperature can be simply related to the variation of the dc conductivity with temperature. For sil- ver in the near infrared, and at temperatures near room temperature the calculated variation of reflectance with temperature, dR/dT, is -1.5 x lO'^/'C [7]. Figure 8 is a plot of the change in reflectance

from an initial value as a function of temperature at 10.0 ym wavelength. The calculated slope falls

between the slopes of the data taken with ascending temperature "•" and desending temperature "o". However, at 4.0 wm wavelength, results are obtained which do not agree even to sign with theoretical

predictions (see figure 9). This data is particularly interesting since silver is the initial film in

all three multilayer designs, and since there is evidence that the fields at the silver film are not

negligible, i.e., the silver film is participating in the response of the mirror. In fact, the measured

temperature dependence of the reflectance of silver films at 4.0 pm is identical with the temperature

dependence of design R4 near 4.0 pm. This may be coincidental, but could also possibly be the origin of

the effect. With the exception of the temperature dependence data on silver films, there are no measured values of the corresponding dependence available for the other materials in the dielectric

mirror designs considered here, as thin films, and in some cases the information even for bulk is not

available. This, combined with incomplete information concerning the stack design, has limited theoretical calculations to a few simple estimates.

Total hemispherical (integrated) scattering measurements (TIS) at a wavelength of 3.39 pm have also

been made. Preliminary differential scattering measurements at the same wavelength indicate that most

of the scattered light is contained in a narrow cone about the specular direction. Most of the scattered light is thus collected by the optics of the absolute ref lectometer , and the quantity, 1-R, is to the first approximation equal to the absorptance, at least for designs Rl and R4. Table 1 contains a summary of several important characteristics of the three designs, dR/dT, where it provides a meaningful

description of the reversible temperature dependence, AR, the irreversible change in reflectance upon

temperature cycling to 120+°C, values of 1-R, and TIS measurements at 3.39 pm. Notice that the TIS

level for R3 is more or less a half order of magnitude larger than the value of 1-R, and since a precise

correction for the amount of scattered light falling outside the collection angle of the absolute

ref lectometer is not available, the actual absorptance of R3 may be significantly less than 1-R. In other words, the measured value of reflectance of R3 may be determined by scattering. This possibility

is further reinforced by the observation of a very small temperature dependence of the reflectance

dR/dT, and very high stability on temperature cycling. In the case of design Rl, the computed "absolute absorptance" 1-R is a factor of two or three larger at all wavelengths than the TIS at 3.39 urn. It is interesting that the largest irreversible and reversible changes with temperature in this design

occurred at wavelengths for which the initial reflectance was highest. This is in fact what would be

expected from "detuning" effects—the stack would be most sensitive to "detuning" where its "Q" was

greatest. Like R3, design R4 does not appear scatter limited, and the nearly constant and large values

of dR/dT and relatively high stability on thermal cycling suggest that its performance is determined

not by stack detuning but perhaps by absorption, possibly in the initial silver film.

Table 1. Summary of experimental results for OCLI dielectric

mirror designs Rl, R3 and R4 at various

wavelengths near 3.8 \lm.

MIRROR

DESIGN

3.6 3.8

WAVELENGTH, jim

4.0 4.2 4.4 4.6

SCATTERED

LIGHT AT

3.39 fitn

R-1 dR/dT 7 ? -0.2 -0.53 ZnS/ThF^/Ag AR -1.0 -5.9 -17.0 -15.3 1-R 25.9 20.7 29.8 35.1 8.5

R-3 dR/dT 0.00 0.06 0.03 0.00 ZnS«/ThF4/Ag An 1.9 2.9 0.3 -4.7 1-R 0.9 -1.3 3.1 9.5 19.0

R-4 dR/dT 0.25 0.27 0.29 0.40 0.24 0.25 Si/SiO^/Ag AR -1.4 0.7 -07 -3.1 -1.7 0.0 1-R 18.6 9.0 5.0 6.8 14.2 52.2 11.0

ALL ENTRIES < 10-*

-232-
3. Conclusions

In the evaluation performed here, design R3 emerges clearly superior in most respects, possessing

the smallest reversible temperature dependence, lowest apparent absorption, and very small irreversible

changes upon temperature cycling. The first two factors presumably also relate to a higher damage

threshold. However, the total light scattered by R3 is a factor of two larger than from the other

designs. The conjecture that R4 might be plagued with a thermal runaway problem is laid to rest with

the discovery that the design possesses a nearly wavelength independent, large and positive temperature

coefficient of reflectance. In a given application, scattered light, or perhaps stability upon temperature cycling may be the most important consideration, and thus determine which design would be optimum.

Since a quantitative predictive capability for the dielectric mirror parameters evaluated in this

paper is not yet available, only by direct measurement can mirror performance be determined. This is

especially true of the dependence of mirror characteristics with temperature. Of course, this latter

comment is not restricted to dielectric mirrors, but applies to high energy laser optics in general.

The small changes in absorption which occur, for example, when the temperature is changed, represent

large fractional changes, and are very important from a design standpoint. Determination of performance

at elevated or depressed temperature is consequently an essential complement to room temperature evaluation. It is hoped that this paper will encourage a greater effort to characterize materials and com- ponents at actual operating temperatures.

4. References

[1] Vaslcek, A., Optics of Thin Films (North- [5]

Holland Publishing Co., Amsterdam, 1960),

pp. 234-239.

[2] Costich, V. R. , in Handbook of Lasers, R. J. Pressley, ed. (The Chemical Rubber Co., [6]

Cleveland, Ohio, 1971), pp. 155-170.

[3] Bennett, H. E. and W. F. Koehler, "Precision

Measurement of Absolute Specular Reflectance

with Minimized Systematic Errors," J. Opt. [7]

Soc. Am. 50, 1 (1960).

[4] Decker, D. L. , in High Energy Laser Mirrors

and Windows (Semi-Annual Report No. 2, ARPA

Order 2175, Naval Weapons Center, April

1973), pp. 34-49.

5. Figures

Bennett, H. E. and J. 0. Porteus, "Relation

Between Surface Roughness and Specular

Reflectance at Normal Incidence," J. Opt.

Soc. Am. 51, 123 (1961).

Decker, D. L. , in High Energy Laser Mirrors

and Windows (Semi-Annual Report No. 6, ARPA

Order 2175, Naval Weapons Center, May 1975),

pp. 25-44.

Decker, D. L., in High Energy Laser Mirrors

and Windows (Semi-Annual Report No. 4,

ARPA Order 2175, Naval Weapons Center,

March 1974), pp. 66-83.

1.0000 -

5 0.9990 - o

<

0.9980

R3(ZnSe/ThF4/A9)

O...

CC ~-0-^

3.6

ABSOLUTE REFLECTANCE

UNCERTAINTY

-J 1 I L. 3.8 4.0 WAVELENGTH (pm)

Figure 1. Absolute reflectance of OCLI

mirror design R3 vs wavelength.

Symbols are described in text.

<

I-o

c 0.9980

0.9970

0.9960

-1 1 1 1 1 1

R1(ZiiS/ThF4/A9)

°x

ABSOLUTE REFLECTANCE

UNCERTAINTY

3.8 4.0 4.2 WAVELENGTH (jim)

Figure 2. Absolute reflectance of OCLI

mirror design Rl vs wavelength.

Symbols are described in text.

-233-
0.9990

U 0.99S0

0.9970

0.9960

T

\—I—

r

T—I—I—

r

A

\\

R4(Si/SiO /Ag)

ABSOLUTE REFLECTANCE

UNCERTAINTY

\

W j

I I I I I I

\

\ 0.99360 J A3.6 3.8 4.0 4.2 4.4 WAVELENGTH. ;im

Figure 3. Absolute reflectance of OCLI

mirror design R4 vs wavelength.

Symbols are described in text.

Figure 4. Schematic of the relative ref lactometer used

in temperature dependence studies. Light from

the monochromator exit slit SL is focused by

Ml at S, where the sample and reference can be

alternately positioned. The reflected light is

then passed to either a thermocouple detector

Di or a photomultiplier Da. AC is an autocollimator used for the critical alignment of

sample and reference.

n—I—I—I—I—I—I—I—I—I—I—I—I

I RELATIVE REFLECTANCE UNCERTAINTY

\\\

_l I I I I I I L. J I I L. 80 110 TEMPERATURE (°CI

Figure 5. Experimental relative reflectance

of OCLI dielectric mirror dssign R-1

vs. temperature at a wavelength of

4.2 vm. Sample and reference are nominally identical. Thermal/

temporal history as follows: "O"

ascending temperature, "o"

decending temperature, "A" ascending

temperature, "o" decending temperature, '" 60 hours after return to room temperature.

1.001

1.000

0.999

E 0.998

S 0.999 -o- o

-T [—

a,

R-3 4.2(im

1 I I L.

O

•A<

0.998

R-1 3.8(im

60 100

IT''C) 140 60 100

(T°C) 140

Figure 6. Computed absolute reflectance of OCLI

mirror designs R3 and Rl, at the wavelength 3.8 and 4.2 ym. Thermal /temporal

history as indicated in figure 5. Uncertainty in computed absolute reflectance is ± 0.0005.

-234-
0.997

I

0.996

4.6Mni 7^

/

V—

_l I I I L.

3

O

< 1.001

R-4

3.8Mm

20 60 100 140 TEMPERATURE, "C

1.003 -

100 TEMPERATURE, 'C

Figure 7. Computed absolute reflectances of

OCLI mirror design R4 at the wave- lengths 3.8, 4.2, and 4.6 ym.

Thermal/temporal history is as indicated in figure 5. Uncertainty

in computed absolute reflectance

is ± 0.0005.

Figure 8. Change in normal incidence

absolute reflectance of standard vacuum evaporated thin

film silver deposited on supersmooth fused quartz as a function of temperature at 10.0 ym

wavelength. The points marked

(•) were taken with increasing

temperature and the points

marked (o) with temperature

decreasing.

-«

T -8 o

-12 c

< -16

-20

-24

"T 1 1 r

dR/dT - -2.29 X Ifl-^C

dR/dT - -1.0 X 10-Vc

6

60 70 80 TEMPERATURE. "C

32

1 1 1 1 1 1 1 1 1 1 1

28 •o

24

20

16 o

12

8

4 o

0 o

-4 • \ o o Figure 9. Change in the normal absolute

-8 ^ dR/dT - +3.33 X 10~^/"C reflectance of standard^ vacuum ev-

-12 aporated thin film silver deposited

-16 on supersmooth fused quartz as a

-20

-24

function of temperature at 4.0 ym

wavelength. The points marked (•)

1

1 1 1 1 1 1 1 1 1 1 were taken with increasing temper20 30 40 60 60 70 SO TEMPERATURE, <>C

90 100 110 120 ature and the points marked (o)

with temperature decreasing.

COMMEWTS OW PAPER BV CECfCER

It 1006 pointed out that inAzvzAd-iblz changu In the. iUbn ob6tfLve,d oveA timu tika hou/u mag be rfue to theMmat itAoln accumulation. A^^tzA heating the {^ilm oveA tongzA pe/viodf, o<5 ttme, iuch ai dayi,

itMuin >ieLie(i occuM and the ithn xeveAti to iXi original, itate and oniqtnat n.e(,Zectxvitij . It Mai

(^o/itheA pointed out tlvxt inteA-tayeA. dA.lluA)lon led to permanent long tern changeA in ^ilm ne^^ltcttvitg

upon theAmaJL cycling. ThuA (^OK example, in iome ^Ubni, a change in abiolption up to a {^acton o/5 2 wa4

obieAved i{, the iiZm wo* heated to 120° ^on. one-hat{f houA. fanthefi theAmal cycling inc^eoAed the

abioA.ption up to a {^actofi Oj) 3 oue/i iXA inAJxaZ value.

-235-
5 . 3 PREPARATION AND EVALUATION OF ZnS/CeF^ AR

COATINGS FOR 10. 6 MICRON KCI LASER WINDOWS

A. Golubovic, W. Ewing, R. Bradbury, I. Berman, J. Bruce aid J. J. Comer

Air Force Cambridge Research Laboratories

Hanscom AFB MA 01731

The possibility of using ZnS/CeFs as a 2-layer antireflection coating for 10.6

micron laser window materials was investigated. Both sputtered and sublimed layers

of ZnS were deposited on well characterized Bridgman RAP grown KCI substrates.

CeF3 layers were thermally evaporated onto both bare and ZnS coded KCI substrates.

The chemistry and structural quality of the coatings were examined by several characterization techniques including x-ray diffraction, electron microscopy and diffraction,

scanning electron microscopy. Auger spectroscopy, Nomarski microscopy. and emittance spectroscopy. Pulsed laser damage studies at 10.6 microns were performed to evaluate the damage threshold of the coatings.

Key words: CeF3; CO2 pulsed laser damage; coating topography; KCI;

laser window coatings; ZnS.

1. Introduction

Potassium chloride is a prime candidate for 10.6 fim laser window material due to its low optical

absorption and high damage threshold at 10.6 /im. However, its application in high energy laser

optics requires antireflection coatings which at the appropriate wavelength - 10. 6 /im, must be nonabsorbing as well as isotropic, homogeneous>and stable to environmental stress -intensive conditions.

The simplest form of antireflection coating is a single layer quarter wave coating whose refractive

index is equal to the square root of that of the substrate. Since KCI has a refractive index, n, of

1.454, single layer coating designs cannot be implemented because md:erials with the appropriate

index are not available. With a 2-layer AR coating, a high index/low index combination of materials

of suitable thicknesses can provide zero reflectance at a single wavelength. One such coating which

has been investigated on KCI is ZnSe/ThF4 [l] i Results have shown that there is significant absorption due to this coating. One of the contributing factors may be the dissociation of ThF4 upon deposition.

The rare earth fluorides as a class, are low refractive index materials. As a possible alternative to ZnSe/ThF4, an AR coating stack based on CeFs was investigated. Of the rare earth fluoride

which we have studied, CeF3 was chosen because it adhered well and did not dissociate on deposition.

Although it is a hexagonal material, the difference between nQ^-^jj^j^j^j^y and ngj^^pQ^^^^^y is approximately 4% and should not effect the performance of this material as a coating. For these studies,

ZnS was selected as a first layer material. This paper evaluates the damage threshold of rare earth

fluoride-based AR coating stacks, particularly ZnS/CeF3 on KCI.

2. Deposition of Coatings

KCI substrates were prepared from boules of RAP Bridgman material grown at AFCRL. The

substrates were polished and etched as described previously [2]. Typical sample substrates were

single crystal disks in diameter and ^" thick, with various crystallographic orientations.

ZnS coatings were deposited by both evaporation and RF sputtering techniques. All sputtered

depositions of ZnS were performed with a turbo-pumped system under an argon atmosphere. Depositions made at RF power levels from 50 to 200 watts yielded different deposition rates and substrate

temperatures as given in table 1.

1. Figures in brackets indicate the literature references at the end of the paper.

-236-
Table 1. Deposition conditions for sputtering of ZnS.

RF Power 50 W 100 w 125 W 150 W 200 W

Deposition Rate

(A/sec)

1.0 2. 75 3. 53 4. 50 6. 17

Substrate temperature

(°C) < 100° -120° 120° -140° -150° 150° - 170°

At power levels above 200 watts, the coatings did not adhere to the substrate. Prior to deposition,

the substrates were sputter etched for 60 seconds by argon bombardment at 50 watts RF power.

Thermally evaporated ZnS coatings were produced in a 10"^ Torr coating chamber using an indirect pancake heater of tantalum over a vitreous carbon crucible. Deposition rate of 6 A/ sec was

controlled to ± 1 i/sec. Samples were cleaned in- situ by AC Ar"*" bombardment prior to deposition.

The substrate temperature was at room temperature to start>but rose to 100°C during the coating

process due to radiant heating from the ZnS source.

Deposition of CeFs was performed at 1-2x10"'^ Torr by evaporation of CeF^ (CERAC/PURE 99,9%)

from a tantulum tube source. Attempts at sputtering CeFg coatings produced dissociation of the CeF3

and were abandoned. The thickness of the deposition layer was controlled by a quartz crystal thick- ness monitor and measured after each run by stylus (Dektak) and Tolansky microscope techniques.

The sublimation temperature was 1200°C. The deposition rate was 2.25 A/ sec on KCl and 2,0 1/ sec

on ZnS coated KCl. The KCl substrate was heated to 150° during deposition, but the ZnS coated KCl

substrate was kept at 100°C.

3. Characterization of Coatings

The chemistry and structure of the CeFg and ZnS laser window coatings were investigated by

several different techniques. X-ray and electron diffraction were used to determine composition,

crystallinity and the presence of preferred orientation. X-ray techniques involved either x-ray diffractometry of the coated surface, or x-ray powder diffraction of just the coating after removal from

the substrate. Reflection or glancing angle electron diffraction patterns were obtained from the

coating before removal from the substrate. Identification was made by comparison of the diffraction

data with the ASTM Powder Diffraction file. Also the preferred orientation of the film with respect to the substrate was determined from the diffraction data. Both the surface chemistry and a profile of

the chemistry throughout the film thickness and interface were investigated with Auger spectroscopy.

Transmission and scanning electron microscopy, as well as Nomarski interference contrast microscopy were used to reveal coating and substrate surface topography. Spectral scans were taken of

each coated specimen, and some preliminary measurements of optical absorption at 10. 6 microns

were made. Finally pulsed laser damage at 10. 6 microns was done to compare and evaluate the

various coatings and coating parameters. The results of these various analyses are summarized below.

Sputtered ZnS coatings on KCl were identified as the cubic form of ZnS with a strong preferred

orientation of [ill] planes parallel to the substrate surface. This (111) preferred orientation was

observed regardless of the substrate crystallographic orientation, and was also observed on glass

substrates. Only those coatings sputtered at a low rate (1 i/sec) deviated from this preferred orientation. The surface texture was rough, regardless of deposition rate. Transmission electron micrographs of figure 1, showing replicas of ZnS surfaces deposited at various rates, dramatically reveal

the details of the coating topography. It can be seen that the surface roughness varies with deposition

rate. There is a decrease in grain size with an increased rate of sputtering in the range of 2.5 1/sec

(100 W) to 6. 5 k/ sec (200 W), resulting in a decrease in roughness. An image of the electron

diffraction pattern, attached to the corner of each micrograph, shows the (111) orientation of each

layer. However, a change in both orientation and texture is observed upon decreasing the deposition

rate to 0. 9 1/sec (50 W), Scanning electron microscopy of a cross- section of a typical sputtered ZnS

-237-
film (150 W) revealed the growth structure of these films. Figure 2 shows the columnar growth of

the ZnS crystals normal to the substrate along the [ill] direction, and the surface roughness related

to these individual crystals.

The presence of contaminants in sputtered ZnS films was examined both on the film surface and

throughout its thickness including the interface by Auger spectroscopy. The spectrum of figure 3 shows the principle Auger structure of ZnS. The major contaminants are oxygen (probably present

in the argon sputter gas during deposition), aid argon from the sputter cleaning gun in the Auger system. The oxygen level shown was consistent throughout the ZnS layer thickness, reaching a peak at the ZnS/KCl interface. This can be seen in the Auger profile (figure 4) of the ZnS/KCl interface

The interface appears to be potassium-rich, probably due to CI desorption during the first stages

of sputtering.

Some preliminary measurements of the 10. 6 /im optical absorption coefficient of sputtered ZnS

coatings on KCl were made. The average value for the three coatings measured was 28±4 cm"^. No

differences were observed on coatings sputtered at different rates.

Evaporated ZnS films were quite different in both texture and crystallographic orientation from

those films prepared by sputtering. Many of the evaporated films exhibited a preferred orientation

of {no} planes parallel to the plane of the substrate in contrast to the [ill} structure of the sputtered films. The surface topography was less pronounced as seen in figure 5, in which electron micrographs of replicas of ZnS coatings prepared by both methods are shown. The evaporated films,

although smoother, were often found to vary in crystallinity and orientation.

CeF3 coatings were prepared on both bare and ZnS coated KCl substrates. On KCl, they were

found to be crystalline, hexagonal structure CeF3, with usually a preferred orientation of [lO.O}

planes approximately parallel to the substrate surface. The coating surface was fairly smooth and

fine grained; average grain size was less than 300 A. In the 2-layer coatings, CeF3 deposited on ZnS

is poly crystalline with no preferred orientation. Electron micrographs of surface replicas (figure 7)

reveal that the smaller CeFs crystallites tended to fill in valleys between the sputtered ZnS peaks,

smoothing out somewhat the roughness of the ZnS surface.

Spectral scans were made of each coated specimen as a first evaluation test. Figure 8 shows the

spectral response of a ZnS/CeFg AR coated KCl which gives a peak transmission at approximately

10. 6 Mm.

Spectral emissivity measurements were made on three coated samples. To eliminate the effects

of substrate and instrumental variations between measurements, one sample was half coated with

CeF3 allowing measurements to be made on both coated and uncoated KCl in one run. Spectral emittance scans of both coated and uncoated areas of this sample are shown in figure 6. The relatively

high baseline of the spectrum for the uncoated area indicates that this substrate is of rather poor

quality. By comparison, the emittance of the coated area in the 10. 6 (im region shows an increase of

about 50%. This implies a higher overall absorption for the coated sample in this region.

All of these coatings were stable and adhered well to the substrate during numerous experiments

in our New England humidity. When failure occurred, it often did so along specific linear and parallel

directions. The optical micrographs of f igure 9 illustrate this failure in a ZnS coating; light gray

areas are coating, dark gray areas are substrate. The first micrograph contains a series of linear

and parallel faults visible on the surface of the coating - these are not scratches. The next two

micrographs illustrate that failure of the coating has occurred along these faults which are visible in

the substrate as well as the coating. By x-ray diffraction, these faults were found to be parallel to the trace of substrate {llO} type planes, and may indicate slip along these [llO} planes caused by

stress associated with the coating.

4. Laser Damage of Coatings

Pulsed laser damage testing was used to evaluate the relative merits of the various coatings

described above. The experimental arrangement used is shown in figure 10. A TEA laser, operated

in the TEMqq mode was used, combined with a 10 cm focal length lens to focus the beam gn the sample surface. CaF2 attenuator plates were used to select the amount of laser energy incident on the

sample surface. Characteristics of the TEA laser are listed in table 2.

-238-
Table 2. Characteristics of TEA CO„ Laser Pulses

Pulse Energy ~65 millijoules, reproducible to 5%

Repetition Rate 1 pps

Spatial Mode TEM , Gaussian to I/I ^0.01

oo o Mode Diameter 5. 5 mm

Longitudinal Modes Many

Pulse Width 100 nsec, FWHM

The spot size in the focal plane of the lens was 180 nui, which was calculated both from cavity

parameters and from a beam scan with good agreement. The sample was adjusted so as to be in the

waist of the beam, which is 0. 5 mm long for a 10% variation in the beam diameter. The sample and

indexing head were adjusted so that the sample was normal and moved perpendicular to the beam.

Energy was measured using a Scientech Model 362 calorimeter located on the sample side of the 10 cm

lens.

Damage experiments were performed by irradiating selected sites with known combinations of

attenuators in the beam. At least 10 sites were irradiated for each selected energy density. The

presence of damage was determined by examination of the surface at a magnification of lOOx. The

resulting threshold values are summarized in table 3.

Table 3. 10.6 Micron Pulsed Laser Damage

Thresholds for Coated KCl

COATING THRESHOLD (J/cm^)

CeFg 35

ZnS, sputtered (50 W) 65

(100 W) 40

" (150 W) 40

(200 W) 40

ZnS, evaporated 35

CeFo & ZnS (50 W) 5-6

(100 W) 6-7

(125 W) 7-10

" (150 W) 13-15

" (200 W) 10-13

CeFg & ZnS, evaporated < 6

The following observations can be made from the data of table 3:

(1) Both CeF„ and ZnS single layer coatings on KCl have approximately the same damage threshold,

35-40 J/cm^.

2

(2) The sputtered ZnS coatings all had similar threshold levels, 40 J/cm .independent of their

deposition rate, except for the 50 W sample which had a higher threshold of 60 J/cm^. The evaporated ZnS layers damaged at a slightly lower energy than the sputtered layers.

-239-
(3) When the CeFg and ZnS were combined to form a 2-layer AR coating, the damage threshold of

5-15 J/cm^ was significantly lower than all values observed for single layers regardless of material

or deposition method. However, threshold values increased with increasing ZnS sputter deposition

rate. For these 2-layer coatings, damage usually occurred only in the top or CeF3 layer and these are the energy values reported here. Figure H shows typical single and double layer damage sites,

clearly differentiated by varying shades of gray.

The sensitivity of the damage threshold to the deposition rate of the ZnS first layer, and the damage occurring primarQy in the CeF3 second layer may both be explained by examination of the first

layer surface topography. In figure 2 electron micrographs of replicas of ZnS surfaces sputtered at various deposition rates, showed that ZnS crystallite size decreased with increased deposition rates.

These crystallites were crystallographically oriented with their [ill} planes approximately parallel

to the substrate surface, and had a surface morphology of [lOO] facets. Thus the surface of the

coating is composed of these "pyramids", on the order of \ micron across with surface roughness

decreasing with decreasing crystallite size or increasing deposition rate. The CeF3 was deposited

on this irregular surface. During laser damage testing the laser beam was scattered at this rough,

CeF3/ZnS interface, causing the low level damage observed in the CeF3 layer. However. the reason

for the low damage threshold in the 2-layer coatings composed of a much smoother evaporated ZnS

is not understood. Even though they are smoother, several anomalies have been observed in these

films such as amorphous material, unidentified phases»and foreign particles on the surface. Future

experiments with stricter control of the vacuum and higher purity raw materials may result in improved damage threshold levels in these evaporated coatings.

5. Conclusions

This experiment points out two very important items to be concerned with in the development of

coated laser optics.

(1) The mechanical properties of both the substrate and the individual coatings must be considered.

Stress developed in the coatings can cause severe changes in the substrate, such as slip.

(2) Interface topography may contribute significantly to the total absorption of the coating stack.

This points to the requirement that each layer in a coating be investigated individually as a coating

stack is developed.

In summary, an antireflective coating stack based on CeF3 may be a viable alternative to ThF4.

Most of the difficulties encountered in these experiments were with the ZnS first layer, where further

studies are needed.

6. Acknowledgments

The authors thank H. Posen for msny valuable suggestions and discussions, J. Larkin, who provided KCl substrate material, N. Pickering for the SEM picture, H. Lipson for the optical absorption

data, R. Brown for the emittance spectroscopy data, and J. Fitzgerald and C. Bergeron for technical assistance.

7. References

[l] M. Braunstein and J. E. Rudisill, Proc. of [2] A. Golubovic et al, Proc. of Conf. on

4th Annual Conf. on IR Window Materials, Laser Induced Damage in Optical Mate- 1974. rials, 1974.

NOTE: Micron(s) as used in this paper expresses micrometer(s).

101 325

1 torr

760

Pa.

-240-
1 um 6.2 A/SEC. (200 W) 1.5 X/SEC.(150 W) Figure 1. Electron micrographs Illustrating

orientation and texture of sputtered

ZnS as a function of deposition rate.

« 2.5

211

Snirmfs ZnS on KCl

ISO WTT fmtH UVCL

700 800 900 1000 UOO

)0D WO an 600

CLEOra EIEKV. iV. Figure 3. Auger spectrum of sputtered ZnS on KCl.

1 um

Figure 2, Scanning electron micrograph of sputtered ZnS illustrating columnar

growth perpendicular to the substrate,

surface.

25

Sputteked ZnS m KCl 150 MtTT Depositiw POKtll a

+ • X

. oOOOo?§«»S" "

2 « 6 g 10 12 14 16 U 20 22

a; snina tic hiiwtb

Figure 4. Auger profile of ZnS/KCl interface.

-241-
25 X

10"

S 10

7 8 9 10

UAVELFNOm (niCRONG)

Figure 6(a). Spectral emittance scan of

uncoated KCl.

NOTE: Microns as used in figures 6a and

6b expresses micrometers.

25 X 10

20

7 8 9

WAVELENGTH (niCRONS)

Figure 6(b). Spectral emittance scan of CeFs

coated KCl; coating thickness

Is 1,3 microns.

ZnS ON KCl

100

SnCTRM. RESPMSC GF Z.S/CiF,

MTIREFlfaiOl COATED KCl

• THEORETICAL

- EXPERIHENTAL

10.6 w

t.F. 1.12W ».H • • 1.57 {Li i.mu.U-i.7a

I all SLKimis i" •

10 11

WAVELENGTH, WCMK

12 13

Figure 7. Electron micrographs of surface

texture after first and second

layer deposition on KCl.

Figure 8. Spectral response of ZnS/CeFs antireflection coated KCl.

ZnS COATING ON KCl FAILED COATING FAILED COATING

Figure 9. Nomarski micrographs of ZnS coating

on KCl illustrating failure due to slip.

-242-
TEA CO2

LASER

CaF- ATTEN.

10 an

LENS

OAHAGE SAMPLE EXPERIMENTAL SET-UP FOR PULSED CO2

LASER DAMAi",E EXPERIMENTS Figure 10. Experimental set-up for pulsed CO2

laser danage experiment.

Figure 11. Micrographs illustrating damage

sites on a 2-layer ZnS/CeFj AR

coating on KCl: left one shows

only top layer damage; right one show 2-layer damage, dark gray area is KCl substrate. 100

NO COMMEWT OM PAPER BV GOLUBOVIC, zX. at

-243-
5.4 AS2S3 Coatings on KCl

A. D. Baer, T. M. Donovan, and M. J. Soileau

Michelson Laboratories, Naval Weapons Center

China Lake, California 93555

Potassium chloride (KCl) and quartz windows were coated with AS2S3 at deposition

temperatures of 80°C and 22°C. The index of refraction of AS2S2 coatings is a function of deposition temperature for KCl windows, but not for quartz windows. Coatings

deposited on KCl at 80°C are more durable and less absorbing than those deposited at 22°C. It is suggested that deposition temperature affects the properties of coatings

on KCl because the composition of the surface of polished KCl windows changes with

temperature. AS2S3 coatings were produced with an optical absorption of 1.7 cm"-'- at a wavelength of 10.6 pm, as compared with 1.1 cm~l reported for bulk AS2S3.

Key words: AS2S3; coating deposition; index of refraction; laser windows; 10.6 ym;

surface composition.

1. Introduction

AS2S3 coatings show promise for use with high-power lasers because they can tolerate high-intensity

radiation, and protect alkali halide surfaces from water vapor [1]'. Since the absorption of thin-film

AS2S3 is higher than that of bulk AS2S3, the optical performance of AS2S3 coatings does not appear to be limited by intrinsic properties of AS2S3. Some feature of the thin films seems to be degrading

their optical performance. For thin films on KCl windows, the composition of the window surface during

deposition may be a critical feature. A KCl surface or interface is not necessarily stoichiometric.

Palmberg and Rhodin found that when KCl is bombarded with electrons, the surface quickly reaches a com- position characteristic of the temperature: potassium rich at room temperature, and stoichiometric at high temperatures [2]. The composition changes quickly because bombardment with electrons, even electrons with energy less than 50 eV, caused chlorine and potassium to be liberated [2]. Very mild bombardment produces a potas slum-rich surface at room temperature. Consequently, it is not surprising

that Golubovic and co-workers found a potassium-rich layer between KCl windows and germanium coatings

deposited by sputtering and by electron beam evaporation [3]. Here we confirm that the surface of

polished KCl is nearly stoichiometric at 80°C, and show that AS2S3 coatings deposited on polished KCl

at 80°C are more durable and less absorbing than coatings deposited at 22°C. Deposition temperature is

shown to significantly affect the properties of coatings on KCl, but not the properties of similar

coatings on quartz.

2. Measurements

The stability of AS2S3 coatings on KCl was determined by raising the temperature of coated disks of

KCl single crystals until the coatings re-evanorated. The disks were 7 mm in diameter and 2 mm thick.

Coatings were deposited and maintained in a vacuum of less than 10"'^ Pa (10~^ torr) . The temperature

of a coated disk was monitored with a thermocouple mounted at the edge of the disk. Voltage from the

thermocouple electronically controlled a heater which warmed the disk. This feedback system held temperature constant within 1°C, until manually changed; response time of the system to manual temperature

changes was 3 minutes. Using this system, the temperature of coated disks was adjusted upwards from

100° C in 10°C increments, waiting at least 2 hours at each temperature. The coatings re-evaporated

during the wait at 150°C. Re-vaporization at 150°C compares with dissociation of bulk AS2S3 at 340°C [4]

and softening of bulk AS2S3 at 200°C [5]. The instability of AS2S3 coatings sets an upper limit of

150°C on deposition temperature.

Stoichiometry of uncoated KCl surfaces was studied by comparing the heights of potassium and

chlorine peaks on Auger spectra. Single crystal disks of KCl were mounted in the vacuum system

described above. Auger spectra were measured on a disk at various temperatures, both in increasing and

decreasing sequence. After a temperature change, the temperature of the disk was allowed to equilibrate

for at least an hour before an Auger spectrum was recorded. The Auger electron beam was 1 ma at an energy of 2 keV. Figure 1 shows Auger spectra recorded with the KCl cold and hot. Two changes occur

in the surface composition as the temperature is increased: water, indicated by the 0 peak, vanishes;

and the relative number of chlorine atoms on the surface increases. The relative number of chlorine

atoms is proportional to the ratio of the chlorine to potassium Auger peak heights [6]. This ratio,

obtained from measured Auger spectra, is plotted as a function of temperature on figure 2. As shown,

the relative number of chlorine atoms increases until 90°C, after which it remains approximately constant. Previous workers found the stoichiometry of the surface to behave as shown by the dashed line

on figure 2, and that the surface was stoichiometric at temperatures above 65°C [2]. Present work

finds the stoichiometric edge to be at 90°C rather than 65°C. In either case, these data show that a KCl surface under light bombardment is stoichiometric above 90°C and potassium rich at room temperature.

1. Figures in brackets indicate the literature references at the end of the paper.

-244-
Interpretation of the Auger data Is complicated because the electron beam was large enough to produce changes in the surface during measurement. Chlorine and potassium are removed from the surface

by both thermal and electron-induced desorption, and with present instrumentation it is difficult to determine which is more important. At low temperatures the electron-induced desorption seems to be

dominant, but at 90°C some other mechanism, perhaps thermal desorption, becomes dominant and causes the

surface to be stoichiometric. In this case, it would be possible to make the KCl surface stoichiometric

by heating it above 90°C.

In order to test the effect of deposition temperature on window coatings, KCl windows were coated

at both low and high temperatures. AS2S3 coatings 2 ym thick were deposited on window blanks, 39 mm in

diameter and 5-8 mm thick, in the bell jar diagrammed in figure 3. The vacuum system includes a diffusion pump with liquid nitrogen trap and has a base pressure of 10~6 Pa (10~^ torr) . Arsenic trisulfide,

99.999% pure, was evaporated at a rate of 1.4 nm/sec from a quartz bottle with a tantalum insert heater

[7]. The windows were supported by a stainless steel plate and heated with an infrared lamp, while the

temperature of the steel plate was monitored. Room temperature coatings were deposited with the lamp

off. High-temperature coatings were deposited after thesteel plate had been warmed to 80°C for at least 2 hours; after deposition, the window was allowed to cool for at least 2 hours before removal

from the vacuum system. The same apparatus was used for coating all the windows discussed.

The transmission of the coated windows was measured using a Perkin-Elmer 180 spectrometer. The

index of refraction, n, was calculated from the ratio of optical transmission measured at adjacent

maxima and minima in a spectral region with low absorption and dispersion, and from known values of n

for the windows; this method for calculating n was chosen because it does not require a knowledge of

the thickness of the coating. All points plotted for KCl and the points plotted at 3.5 vim for quartz

in figure 4 were determined from the position of a maximum or minimum, and the value of n at 3.5 um. By using the ratio method, uncertainties due to error in the estimate of film thickness are eliminated.

The film thickness and film uniformity were also measured in separate experiments. Over a distance of

0.5 inch, the coatings deposited at 22°C and 80°C were uniform to better than 1%.

The value of the indices of refraction shown on figure 4 could be in error for several reasons.

First, the index of refraction may be significantly different at the two wavelengths of the adjacent

maximum and minimum; however, in the present case at 3.5 and 8.5 um, the index of refraction is essentially constant for AS2S3 in both bulk and coating form [8]. Second, the formula relating a ratio to the index of refraction was derived assuming lossless media. In the present case, the fraction of light

absorbed in the coatings was measured calorimetrically and found to be between 4 x 10~^ and 1.5 x 10~-^. Whether the losses are assumed to occur at the interface or to be spread throughout the film, the cor- rection for absorption increases our estimate of n, but by no more than 0.2%. This source of error is

small relative to the 2.5% difference in n for different deposition temperatures shown in figure 4. Third, the wavelength calibration of the Perkin-Elmer 180 could be in error, introducing an uncertainty

in the index of refraction of the windows. This possible source of error is insignificant. However,

uncertainties in transmission of +0.2% may add as much as 0.3% uncertainty to n in our particular case. Finally, there may be systematic errors. For example, the value of the index of refraction of AS2S3 on quartz at 3.5 ym found with the ratio method is 1.5% lower than that found using thickness measurements

and the position of the transmission maxima, and 3.5% lower than the index of refraction reported in

the literature [8]. However, the values plotted in figure 4 were obtained using the same method of

analysis from data measured under identical conditions, so that systematic errors would be the same. For the purpose of comparison done here, the values of n in figure 4 are accurate to ±0.5%, sufficiently

accurate to document the changes of 2.5% actually found.

As shown in figure 4, the deposition temperature significantly affects n for AS2S3 on KCl, but not on quartz. Evidently the elevated temperature affects the coating because it alters the KCl, but not the

quartz. The index of the coatings on KCl appears to be approaching that of coatings on quartz as the

deposition temperature is increased; that is, when excess potassium is likely to be present on the KCl

during deposition, the index of refraction of the coating deviates further from that of the quartz

coatings

.

Durability of high- and low-temperature coatings was estimated by comparing how long each would

last when subjected to a standard buffing. Two 2.2-pm coatings were buffed off KCl windows using

Linde A polishing compound with a triacetin slurry and a polishing wheel. Under identical conditions,

it took four times as long to remove the high-temperature coating as it did the low-temperature

coating.

Optical absorption at 10.6 um was determined from calorimetric measurements [9,10]. The absorption

of the 80°C window increased 0.04% when it was coated with a 2.2-um AS2S3 film. Assuming that all the

Increased absorption is in the film, the 6 of this film is 1.7 cm"-'- [9]. Values of 6 for various

coatings are compared with that for bulk AS2S3 in table 1. The range of values shown for KCl with a 22°C deposition temperature represents a large number of coatings. The best coating produced at 22°C,

using the most careful handling procedures, had an absorption of 2.9 cm"-'-. The absorption of coatings

produced at 80°C, 1.7 cm~l, is significantly lower than the 2.9 cm"l and close to the absorption

reported for bulk AsoSt.

-245-
Table 1. Absorption of AS2S3 prepared In various ways.

Coatings are approximately 2.2 ym thick.

Form Window Deposition

temp (°C) B (cm '")

Coating NaCl 22 8 Coating KCl 22 2.9—11

Coating KCl 80 1.7

Bulk 1.1*

*

P. A. Young [1].

3. Discussion

The measurements presented above indicate three empirical relationships: (1) The stoichiometry of a KCl surface under electron bombardment is a function of temperature; room temperature surfaces have

excess potassium. (2) Coatings deposited on windows held at 80°C are more durable and less absorbing

than coatings deposited on windows held at 22°C. (3) The index of refraction of AS2S3 coatings is a function of deposition temperature on KCl, but not on fused quartz. Deposition temperature affects the

coating because it alters KCl; otherwise one would expect the index of refraction of coatings on quartz

to depend as strongly on deposition temperature as does the index of refraction of coatings on KCl.

Some critical property of the KCl surfaces must depend strongly on temperature between 22°C and 80°C.

This property could be long-range order, or lattice parameter, or composition. Composition is a prime

candidate because it can change so much between 22°C and 93°C (more than a factor of four in figure 1)

,

We tentatively conclude that it is excess potassium on KCl surfaces during deposition that degrades the

optical and mechanical properties of optical coatings.

This conclusion can be verified by probing the surface more delicately with electron and photon

beams. The large electron beam, 1 \ik, required for standard Auger spectroscopy interacts too strongly

with AS2S3 coatings and KCl surfaces; the surface changes significantly during the measurement. Consequently, Auger measurements will have to be done with more sensitive instrumentation so that measurements can be made with a beam current smaller than 1 pA. A photon beam is less damaging than an electron beam, but photoemission measurements require special high-energy light sources. Both kinds of

measurement can be done, but they require the application of improved instrumentation.

4 . Acknowledgments

The authors gratefully acknowledge the assistance of E. J. Ashley and J. M. Bethke.

5. References

[1] Young, P. A., Thin Solid Films 6, 423 (1970).

[2] Palmberg, P. W. and T. N. Rhodin, J. Phys.

Chem. Solids 29, 1917 (1968).

[3] Golubovic, A., W. Ewing, J. Bruce, J. Comer

and D. Milan, Proc. Symp. Laser Induced

Damage in Optical Materials, NBS Spec. Pub.

414 (1974), p. 76.

[4] The stability of AS2S3 used here was determined by differential thermal analysis.

[5] Savage, J. A. and S. Nielsen, Infrared Phys.

5, 195 (1965).

[7] The design of the AS2S3 evaporator was based on information obtained from the

Servo Corporation of America.

[8] Young, P. A. , J. 4, 92 (1971).

Phys. C: Solid State Phys.

[9] Loomis, J. S., Appl. Opt. 12, 877 (1973).

[10] Loomis, J. S., "Calorimetric Absorption

Measurement of Infrared Optical Materials,"

Laser Division Digest, Air Force Special

Weapons Center, Kirtland, AFB, N. M.

,

TR LRD-72-1 (1972), p. 29.

[6] Palmberg, P. W. , G. E. Riach, R. E. Weber

and N. C. MacNeal, "Handbook of Auger

Electron Spectroscopy," (Physical Electronics

Industries, Inc., Edina, Minn., 1972).

-246-
Z

I

<

ENERGY, (V

600

PALMBERG AND RHODIN

, o

UNSTABLE COATING yyyyyyyyyyyy>}?

STOICHIOMETRIC SURFACE /////////y/////

100 200 TEMPERATURE, "C

300

Figure 1. Auger spectra measured with 1-yA,

2-keV electron beam on singlecrystal, KCl disks. The mix of

atomic species on the surface

changes with temperature.

Figure 2. Ratio of CI and K peak heights for Auger

spectra recorded on KCl at various temperatures. Circular data points were taken

from Auger spectra reported by Palmberg

and Rhodin who found the surface to be

stoichiometric at temperatures above

65°C [2].

Figure 3. Dibgram showing bell jar in which

trial depositions of AS2S3 on KCl

and quartz windows were performed.

o 2.3 X

uiO

2.2

80<>C 22°C QUARTZ

Kce, «f>c

\— KCB, 22°C

WAVELENGTH, (im

Figure 4. Index of refraction of AS2S3 coatings

deposited at different temperatures on KCl and quartz windows. Points with error bars were determined from the ratio of

transmission maxima to minima as measured

with a Perkin-Elmer 180 spectrometer.

Other points were determined from the wave- length of a maximum or minimum.

NO COMME^^ OW PAPER W 8AER, oX at

-247-
5.5 DESIGN FOR HIGH POWER RESISTANCE

By

Arnol d L. Bloom and

Verne R. Costich

Coherent Radiation, Palo Alto, Calif.

The influence of materials parameters such as thermal conductivity, intrinsic stress,

thermal expansivity, and Young's modulus on coating performance is discussed. Metal,

metal-dielectric, and dielectric multilayer films at both 1.06 \im and 10.6 jim are con- sidered. Calculations are presented for various combinations of coatings and substrates.

Key words: Damage threshold; optical coatings; thin films.

Coatings develop intrinsic stresses during deposition in a vacuum chamber and are further stressed

when they are cooled from their deposition temperature to ambient temperature due to mismatch between]

their expansion coefficient and that of the substrate. It is thfs prestressed mul ti layer whi ch is exposed to a high power pulse for a short period of time. We will indicate the prestresses existing

in common 1.06nm and 10o£iVTxcoati ng designs and the additional stresses introduced during laser pulses.

Silver Reflector for 10.6 ym

The most commonly used 10.6 m mirror is silver with a very thin dielectric overcoating of thorium

fluoride. The absorption occurs in a very thin region near the surface of the silver called the

skin depth (about 0.07 ym for silver exposed to 10.6 \m pulses). The temperature rise can be computed

for a very short pulse of one Joules /cm2 by use of the equation:

where ^E^ is the square of the electric field in the film, with <^E^ = 1 in air, n - ik is the

complex index of the film, Cp is the specific heat,^ is the density in grams/cm3, and X'ts the

wavelength of the incident radiation.

Silver has such great thermal conductivity, that in a one microsecond pulse, the heat absorbed in the

skin depth of 0.07 ym is conducted to a depth of 2.6 ym. The computed temnerature profiles versus depth are illustrated in figure 1. Note that the surface temperature rises in proportion to the square root of the pulse duration as does the depth of penetration of the temperature distribution. A one

microsecond pulse of energy density 10.0 joules/cm2 will introduce 0.1 joules/cm^ in a 2.6 ym deep

region and thus cause the surface temperature to be raised by 220°C, at which temperature the film

will start to degrade. Longer pulses will allow more energy to be conducted into the film. Therefore,

the energy density damage threshold is proportional to the square root of the pulse length if the

film is thick enough, i.e., of greater thickness than the penetration depth. Conclusion: use thicker

films of silver for long pulses or check out the thermal conductivity of the substrate.

Coating Materials

For 1.06 ym coatings, Ti02;and Si02 are commonly used, while at 10.6 ym, ZnS and ThF.* are used, sometimes

in conjunction with silver. The relevant parameters of these coatings are given in table 1. Note

that some of the data has been estimated from known data on similar materials. Note that ZnS has

negative intrinsic stress (compressive) which makes it damage easily at 1.06 ym as will be seen below.

-248-
Dielectric films of quarter wavelength optical thickness have physical thicknesses of 0.12 ym and

0.20 ym, respectively, for TiOa and Si02 tuned at 1.06 pm, 1.2 pm, and 2.0 \m, respectively, for ZnS and

TliFi, tuned at 10.6 ym. Figure 2 shows the temperature rise in a maximum reflectance stack of such

layers at 1.06ym, Since the absorption in Si02 is much smaller than that in Ti02, there is a discontinuity at the odd-numbered interfaces. The even-nurrtbe red interfaces, being at nodes of the electric

field, have no immediate temperature rise. Note that the shape of the temperature rise in the first

two layers is repeated many times down through the stack, but the amplitude is decreased by a factor

of about (n|-|/n|_)'' in each succeeding repetition. In a short|time conduction will smooth the temperature excursions versus depth. The time taken to smooth to ^ of the original excursions was computed

for this coating and all the coatings considered below. Table II gives these thermal diffusion

time constants for the materials and designs considered here. Note that for ZnS/ThFz,, which could

be used at both wavelengths, the time constant for 10,6 ymis two orders of magnitude larger than the

time constant for 1.06ym. This is logical, since, if both distributions have 1°C excursions, the

distribution caused by the lOx more gross wavelength can be smoothed only by the conduction of heat

lOx as fao but it has temperature gradients only 1/10 as great. Note from the table that for

picosecond pulses the heat always "stays in place", while for microsecond pulses one can assume a uniform temperature throughout the multilayer, but that time has not permitted any of the heat to

be conducted away into the substrate.

Stresses in Coatings for 1.06 ym

There are three sources of stress in coatings exposed to 1.06 ym pulses:

1. Intrinsic stress due to the deposition process. Values for this component of the

stress are given in table 1.

2. Thermally induced stress due to the mismatch of expansion (contraction) coefficient

between the coating and the substrate. This component is given by the equation ^ - YC(*4'(~«ts^^T^ where «l.f is the coating's expansion coefficient from table 1, A.S is the substrate's expansion coefficient, AT is the temperature of deposition

minus that of ambient (before the pulse) in degrees centigrade, and Y is Young's

modulus of the coating as given in Table I.

3. Stress induced by the pulse. This is given by where is the

the coating's expansion coefficient from table 1, V is Young's modulus from table 1,

and AT is the temperature rise, which in the short pulse domain is a non-uniform

function within each layer. Note that since AT , , and Y > ^re always positive, the

induced stress must always be compressive.

The computed stresses in a Ti02/Si02 maximum reflector deposited at 300°C on Bk-7 and used at 20°C are shown in figure 3, The dotted line indicates the intrinsic stress components. The solid

line indicates the combination of intrinsic stresses and the stresses introduced by the cooling

from the temperature of deposition to that of ambient, i.e. the stresses in the coating before the

pulse. The dashed line indicates the stresses in the coating immediately after a five joule/cm^

short pulse. Note that even though the Si02 layers were heated, their expansion coefficient is so low that the pulse-induced stress is negligible." The stress at each level in the multilayer

is the integral of the plotted curves"from the air surface down to that depth. The integral is tensile at all interfaces. If the pulse energy density were increased to 50 Joules/cm , the top

layer becomes highly compressive and the stress on the first interface surpasses its strength

(estimated at lo5 dynes/cm). In fact, real coatings probably have areas with greater absorption

and less strength, which will damage at lower energy densities. The damage will show the characteristics of compressive buckling. That is, it will look like a burst paint blister, and will be more

' likely to occur near the edge of a large beam than in the central region of the beam where there

should be no asymmetrical shear forces.

Figure 4 shows the same stresses in the same coating deposited onto fused silica. Note that

this coating is so tensile that it is about to craze (form hairline cracks of tensile rupture). The

laser pulse again causes the high index layers to become less tensile (eventually more compressive).

Note that the highly tensile prestress in this coating on fused silica causes it to be about twice as damage resistant as the same coating on Bk-7.

The temperature rise in a 1.06 anti reflection coating of Ti02 and SiO^ deposited on fused silica is shown in figure 5. A non-quarter wave design is used because it has Tower reflectance.

-249-
The intrinsic, ambient and pulse- i nduced stresses in this coating are shown in figure 6, Being highly

tensible, this AR should be highly damage resistant. It damages, when the substrate/Ti O2 interface

can no longer stand the stress. This interface is believed to be less strong than the more chemically pure interfaces between the layers (k x lo'* dynes/cm is our guess). Figure 7 shows the

stresses in this AR coating on Bk-7. The stress integral is compressive, becomes more so during a pulse and is therefore not as damage resistant as it would be on fused silica.

Coatings for Use at 10.6 Microns

Since 10. Gym pulses are usually longer than 20 nsec, table II indicates that the long pulse domain

is of interest. The intrinsic and ambient stresses computed per a ZnS/ThF^ reflector, deposited at 150°C on ZnSe, are shown in figure 8. Note that the stress scale has been changed. ZnS is used at

10, 6vim because it has a compressive intrinsic stress which will compensate the low index material's

tensile intrinsic stress. This allows thicl< coatings to be deposited without buckling or crazing.

If after a long laser pulse, the coating is heated to 100°C above ambient, the compressive pulseinduced stress will be -0.035 x 10^0 dynes/cm2 for the ZnS layers and -0.012 x 10'° dynes/cm^ for the

ThF/j layers. Failure will take place at the substrate interface. Increasing the ThF^^ layer thicknesses will not improve matters because that would increase the absorption, and thus increase the

temperature and the pul se- i nduced compressive stress.

The stresses in an enhanced metal reflector of ZnS/ThFjj on silver are shown in figure 9. This is a poor design since it is so compressive even before the pulse, because of silver's large expansion

coefficient. When deposited on molybdenum this coating is less compressive as shown in figure 10.

In fact, use of a lower expansion coefficient would have produced a tensile design which would be more resistant to laser pulses of high energy density. In these last two figures, the effect of a 100°C temperature rise in the coating but not the substrate is the same as above, -0.035 x lO'O dynes/

cm2 for ZnS and -0.012 x lo'O dynes/cm^ for ThFz,.

Conclusions and Recommendations

Of the metal films used to make reflectors for 10.6ym, silver has the best thermal conductivity and

will therefore rise less in temperature. But, since silver starts to degrade at 200°C, copper or gold

may be better and should be considered.

Of the dielectric materials used at l.OGym, ZnS/ThF/^ shows too much compressive strain to be very

resistant. Ti02/Si02 used at I.06 has a short (good) thermal diffusion time constant and is highly

tensile on fused silica, yielding better damage resistance. To maintain surface figure with a highly

tensile coating the substrate must be at least of 6 : 1 = diameter: thickness. Zr02 was not considered

due to lack of basic data and because it is so non-reproducable that it is no longer commonly used in

the production of high power resistant optics.

At 10. Gym, mirrors made of ZnS/ThF/^ maintain their integrity, but are slightly compressive. It would be better if they were produced with such high tensile stress as to be almost crazing. Twice

as high a damage threshold would result from such a design. For very long pulses (1 millisecond

or longer) or cw exposure, the thermal conductances of the substrate should be optimized, enhanced

metal reflectors with no more than four dielectric layers on top of a metal that is stronger, less

expansive and less temperature sensitive than silver should be used.

We have learned much in the course of this work and would like to thank C.E. Thomas and KMS Fusion

for their timely testing of various coating experiments. Two references we have found useful are "The Mechanical Properties of Thin Condensed Films" by R.W. Hoffman in Physics of Thin Films, Volume 3,

1966, and "Stresses Developed in Optical Film Coatings" by A.E. Ennos , Applied Optics, I966.

-250-
Figures and Tables

HEAT CONDUCTION INTO SILVER

SURF ° V

''TM

PENETRATION ° V

^"^^

"penetration -2.61131 l.Onsec

2 3 DEPTH INTO COATING (microns)

Figure 1. Computed temperature rise in silver versus depth and time.

Ti02/Si02 - MR at ^.06|l AFTER 5 joule/cm^ SHORT PULSE

SUBSTRATE

0 1 2 3 4 5 6 7 DEPTH INTO COATING (QUARTER WAVES! Figure 2, Computed temperature rise after a

5 joule/cm^ pulse for Ti02/Si02

maximum reflector at 1.06 ym.

0.3

0.0

Ti02/Si02 - MR at 1.06p on Bk-7 GLASS

INTRINSIC AMBIENT AFTER 5 joule SHORT PULSE

L H SUBSTRATE

0 1 2 3 4 5 6 DEPTH INTO COATING (OUARTER WAVES!

Figure 3. Computed stresses in the coating of

figure 2, when deposited on BK-7,

cooled to ambient, then exposed to

a 5 joule/cm^ pulse.

i.Or0.6

0.2

0.0

Ti02/Si02 - MR at T06ji on FUSED SILICA

INTRINSIC AMBIENT AFTER 5 joule SHORT PULSE

L H L SUBSTRATE

I I 0 1 2 3 4 5 6 DEPTH INTO COATING (QUARTER WAVES)

Figure 4. Computed stresses in the coating of

figure 2, when deposited on fused

silica, cooled to ambient, then

exposed to a 5 joule/cm^ pulse.

Si02/Ti02 - AR at 1.06(1 AFTER 5 joule/cm2 SHORT PULSE

AIR

1.30 L SUBSTRATE

1 DEPTH INTO COATING (QUARTER WAVES)

Figure 5.

251-

Computed temperature rise after a

5 joule/cm^ pulse for a Ti02/Si02

anti-reflection coating at 1.06 ym.
1.0

0.6

o 0.4

0.2

0.0

INTRINSIC AMBIENT

AFTER 5 joule SHORT PULSE

AIR H SUBSTRATE

0 1 : DEPTH INTO COATING IQUARTER WAVES)

Figure 6. Computed stresses in the coating of

figure 5, vjhen deposited on fused

silica, cooled to ambient, then ex- posed to a 5 joule/cm^ pulse.

0.2

0.1

0.0

-0.1

-0.2

ZnS/ThF. OUTPUT REFLECTOR at 10.6m on ZnSe

L ZnSe

• INTRINSIC -AMBIENT

_L

0 1 2 3 4 5 DEPTH INTO COATING (QUARTER WAVES)

Figure 8, Computed stresses in a ZnS/ThFH

output mirror coating in 10.6 pm

deposited on ZnSe and cooled to ambient.

_ 0.3

p 0.2

o 0.1

0.0

-0.1

Si02/Ti02 - AR at 1.06(1 on Bk-7 GLASS

INTRINSIC

- AMBIENT

- AFTER 5 joule SHORT PULSE

0 1 2 DEPTH INTO COATING (QUARTER WAVES)

Figure 7. Computed stresses in the coating of

figure 5, vrhen deposited on BK-7,

cooled to ambient, then exposed to a 5 joule/cm^ pulse.

0.2

0.1

0.0

-0.1

-0.2

-0.3

ZnS/ThF^ ENHAIMCED REFLECTOR at 10.6^ on SILVER

AIR SILVER

. INTRINSIC

AlVlBIENT

0 12 3 4 DEPTH INTO COATING (QUARTER WAVES)

Figure 9. Computed stresses in a ZnS/ThFi,

enhanced metal reflector at 10.6 vim deposited in silver and cooled to ambient.

-252-
0.2

0.1

E

0.0 O

'o

in

in

LU

cr

1-in -0.1

-0.2

AIR MOLYBDENUM

INTRINSIC

. AMBIENT

0 1 2 3 4 5 DEPTH INTO COATING (QUARTER WAVES!

Figure 10. Computed stresses in the coating of

figure 9, when deposited on molybdenum and cooled to ambient.

Table 1, Characteristics of coating materials.

CHARACTERISTICS OF COATING MATERIALS

TiOj SiOj ZnS ThF^ Mo Ag ZnSe

Density 4.3 2.7 3.98 6.3 10.2 10.5 5.42

Cp 0.169 0,22 0.12 0.048- 0.059 0.056 0.08

Heal Conductivitv 0.10 0.015 0.05* 0.05- 1.4 4.18 0.05

Expansion Coefficient (xlO-^/°Cl

8.8 0.5 7.1 4- 5 20 8.5

Intrinsic Stress (10'0dvne/cm2)

0.30 «0 -0.2 0.145 0.55 0.085 -0.2"

Young's Modulus

(x 10'° dvne/cm^) 230 108 50 30- 340 75 50-

•

* estimated ** taken same as ZnS

Table II, Thermal diffusion time constants for some common coating designs.

THERMAL DIFFUSION TIME CONSTANTS

Coating Design

Tuned Wavelength

1.06m 10.6a/

TiOj/SiOj -MR 36 psec

ZnS/ThF^ -MR 200 psec 20 nsec

Si02/Ti02 - AR 6.5 psec

ThF4/ZnS - AR 200 psec 20 nsec

Silver 500 psec 150 psec

Molybdenum 0.06 psec

COmEUTS OH PAPER BV BLOOM and COSTKH

Thz po-Lnt wai Kcua,zd that itKUi and cAyitatlogiapky o(, a thin (^-ilm oAt iViongly de,pzndz.nt on the. mzthod 0^ dzpoiiition. Undan thz cuAcumitanceA iX usoi oikud how can one meoiuAe tho. iVieAi accuAateZy.

The. author ^.e^MAed the. qazitLoneA to an ofvticte by R. W. Ho^^man in Vol. 3 o^ the Vhyitdi, o{) Thin

TUbm,. Tku> atticle -c6 entitled "Mechanical PKope/utieA of, Thin Film." The question ^^)a^ then fiaiied

04 to how imponXant the iPieAi ef^f^ect ij> in the damage pn.oceii comidefUng that othen efdecti iuch oi

impuAAjtieM and deiectM tend to maA>k the comideAatiom pfie^iented in thij> papeA. The authou emphasized

the ^act that iyneAgiitic e{,(,ecti oAe pneAent in the damage pKoceAi and that the va/Uoui cau^e^ Oj$ damage inteAact, bat that in geneAal highly itAtaed {^ilm tend to {^aiZ mote neadily than iilmi without ne^idaaZ itteAi whateveA the principal caiu,e Oj$ the damage. A fu/vtheA point wai naif>ed conceAning

the application o{, b'xlk pn.ope>itieyi> to the thin film. HeAe the authou indicated that they only could

make genenat in{,eAenceA with negoAd to the appticability 0)5 tia£fe pKopeAtie^ in design o^ thin dilmi,

but they {^elt that in geneAal they pn.ovided a uieful guide to the design of, multilayeA coatingi. The

method put {jOmoAd in thii> papeA hai> been used to achieve improved coatingi in pfuactical caies.

-253-
5.6 INFLUENCE OF STANDING-WAVE FIELDS ON THE LASER

DAMAGE RESISTANCE OF DIELECTRIC FILMS*

Brian E. Newnam, Dennis H. Gill and George Faulkner

University of California

Los Alamos Scientific Laboratory

Los Alamos, NM 87545

The influence of standing-wave electric fields on the damage

resistance of dielectric thin films was evaluated for the case of

30-ps laser pulses at 1.06 ym. Single-layer films of Ti02 / Zr02

,

SiOa, and MgFj were deposited by state-of-the-art electron-gun

evaporation on BK-7 glass substrates with uniform surface preparation. The film thicknesses ranged from one to five quarter-wave

increments

.

The thresholds for TiOz films of odd quarter-wave thickness

were greater than for even multiples which correlated well with

the calculated internal maximum electric fields. Threshold variations for ZrOa films were apparent, but not as distinctly periodic

with film thickness. Negligible variations were obtained for Si02

films, again correlating with electric-field calculations.

Additional tests allowed comparisons of thresholds for 1) backand front-surface films for normal incidence; 2) S- and P-polarized

radiation at an incidence angle of 60°; and 3) circular and linear

polarizations for normal incidence. The thresholds were compared

with calculated standing-wave field patterns at various locations

in the films. A definite correlation was generally found between

the internal field maxima and the thresholds, but in a few coatings

defects apparently decreased or prevented any correlation.

Key words: Damage thesholds; dielectric thin films; electric fields;

electron-gun evaporation; picosecond pulses; rf sputtering; standingwave patterns.

1. Introduction

The purpose of this experimental study was to determine whether or not there is

a direct correlation of the standing-wave (SW) electric field distributions in optical

coatings with their laser-induced damage thresholds. Standing-wave field distributions are quasi steady-state interference patterns produced by superposition of

traveling waves interref lected at various parallel impedance discontinuities, e.g.,

air-film and film-substrate interfaces.

As early as 1971, Turner [1]^ reported that half-wave (HW) optical films had

lower damage thresholds than quarter-wave (QW) films, from which he suggested a possible correlation with the internal SW field patterns. In 1972 Newnam and

DeShazer [2] observed that both QW and 3QW films of TiOa had higher thresholds than

* This work was performed under the auspices of the Energy Research and Development

Administration

.

^Figures in brackets indicate the literature references at the end of this paper.

-254-
the HW films for 13-ns duration ruby laser pulses. Further damage experiments were

conducted at 0.6943 ym by Newnam [3] with various front- and back-surface films,

and the thresholds were found to be in qualitative agreement with calculated SW field

patterns.

Notwithstanding, these correlations of damage thresholds and SW field distributions, Milam, et al., [4], suggested that state-of-the-art coatings (1973) of Ti02

,

ZrOa / and Si02 contained absorbing inclusions of such number and size as to preclude

the build-up of standing-wave fields. The absorbing inclusions were deduced to be

the sites of early damage to the films. Furthermore, they demonstrated a scaling

law in thin films between the laser pulsewidth t and the size of a film defect most

likely to be damaged first. This relationship, originally formulated by Hopper and

Uhlmann [5] for bulk materials, predicts that large defects are damaged most easily

by long pulses, and vice versa, in proportion to \h . Further research by DeShazer,

et al.,[6], produced a correlation of the defect density with the measured damage

threshold dependence on laser spot-size radius.

At present, the general agreement on laser-induced damage in thin film coatings

is that the thresholds are limited by absorbing inclusions. However, it is difficult

to ignore the aforementioned threshold correlations with the standing-wave electric

field patterns. Now, the state-of-the-art of thin film deposition techniques yield

optical coatings with fewer defects and with increased damage resistance compared to

that of a few years ago. It is important to determine whether SW field distributions

affect the thresholds of these coatings or if their effect is inoperative in the

presence of film defects.

Since small film defects are more numerous than large ones and are more susceptible to damage by short laser pulses, damage experiments were conducted with 30-ps

pulses at 1.06 ym. It was considered that damage measurements at this pulsewidth

would be a severe test of the presence of SW field effects. In addition, thermal

diffusion of absorbed heat from the locations of maximum fields is minimal during this

pulse duration.

The test samples were single-layer films of TiOa, ZrOa , Si02, and MgFa deposited

by state-of-the-art electron-gun evaporation by CVI Laser, Inc., on BK-7 glass substrates with uniform surface preparation by bowl-feed polishing. In addition, a few

Ti02 coatings prepared by rf sputtering on BK-7 glass and Optocil I fused silica

substrates were included. All coating substrates had a 2° wedge to prevent SW interference effects with the second uncoated substrate surface.

Damage thresholds were measured as a function of film thickness (from one to

five quarter-waves) , direction of laser incidence (back- and- front-surface films for

normal incidence), and for S- and P-polarized radiation incident at 60°. Additionally,

tests with circular versus linear polarizations for normal incidence were performed

to determine if multiphoton absorption was an important physical process leading to

damage. Such a process would further emphasize the electric-field distributions in

the films.

The measured thresholds were compared with the corresponding calculated SW field

distributions. A definite correlation was generally found between the internal field

maxima and the thresholds, but in a few coatings defects apparently decreased or prevented any correlation.

In section 2, the experimental parameters of the laser and damage detection

'scheme are described. The experimental results are given in section 3 and compared

with the SW field calculations. Section 4 includes an analysis of the results in

terms of single-pass flux density versus SW fields, and a summary of our findings

appears in section 5. The equations derived for the standing-wave fields are included

in an appendix, section 7.

2. Experimental Parameters

2.1. Laser Apparatus

The laser consisted of a mode-locked, Nd:YAG oscillator from which a single pulse

was extracted by a Pockels cell/electronic trigger arrangement and augmented by three

YAG amplifiers. The net output was approximately 75 mJ in a 30-ps pulse with a TEMqo

spatial distribution. The energy was measured with an accuracy of ± 5% with a calorimeter of LASL design [7] , and a two-photon fluorescence camera measured the pulsewidth. A fast Si photodiode and a Tektronix 519 oscilloscope monitored the modelocked pulse train.
The output of the laser was focused with a 2-in lens to a spot-size radius of

0.5 mm at the sample plane, located in front of the lens focal plane (figure 1).

The beam profile was measured at the sample plane by directing a small portion of the

beam onto a Reticon RL 256EC linear diode array (resolution of 0.05 mm) centered on

the beam with an X-Y translation device. The signal was recorded on a Tektronix 7633

storage oscilloscope and the resultant photograph allowed a measure of the beam spotsize and shape as shown in figure 2. We found it necessary to measure the beam

profile on every shot since the beamwidth was not constant. Beamwidth variations were

directly related to shot-to-shot variations in the switched-out oscillator pulse

energy and the resultant degree of gain saturation in the third YAG amplifier.

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.

2.2.. Detection of Damage

Damage was detected by two methods simultaneously (figure 1) . Optical breakdown,

accompanied by a high-temperature spark, was detected with a photomultipLier (RCA6655A)

with a purple-transmitting filter (peaked at 4180 A, 160 A bandwidth) to discriminate

against scattered flashlamp radiation and the laser pulse. The photoelectric signal,

proportional to the energy in the laser-induced spark emission, was measured with a Tektronix 556 oscilloscope (figure 3) . The sensitivity of the spark detection system

was such as to detect a single spark 1 ym in diameter with black-body temperature

equal to the melting point of ZrOa (2983°K)

.

Evidence of laser-induced disruption of the surface of a coating was manifested

by increased scattering of a probe beam from a low-power (2 mW) He-Ne laser aligned

collinearly with the pulsed laser beam. By comparing the level of scattering before,

during (with N.D 9 safety goggles), and after pulse irradiation, changes in the

coating structure could be detected. This method has been called laser-induced

scatter (LIS) detection [2]

.

We found that for films with high damage resistance and low initial scatter, the

sensitivity of these two methods of damage detection were comparable. For films with

low damage resistance, the LIS method was more sensitive, which indicated that film

damage occurred before very high spark temperatures were reached.

2 . 3. Test Procedure

Each sample was first wiped once with lens tissue moistened with reagent grade

acetone and gently blown with a jet of Freon gas to remove any adhering dust

particles. For laser irradiation experiments at normal incidence, the samples were

oriented at an actual angle of 1° to prevent retroref lection into the laser system.

Great care was taken to absorb the reflected and transmitted laser beams with Schott

glass filters located far from the view of the spark-detecting photomultiplier system.

In determining the damage thresholds, a given area of a sample was irradiated by one

shot only. Sites of irradiation were spaced several millimeters apart. The energy

of the incident 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 an average of 4 0 shots.

3. Experimental Results

The damage resistance of each thin film coating is reported here as a range of

threshold energy densities rather than a single value. This provides a more realistic

description of this film property which was found to vary over the surface area. The

top end of the threshold range was defined as the maximum energy density for which

neither visible damage nor spark radiation of a minimum magnitude were detected. The

bottom end of the threshold range was defined as the minimum energy density for which

damage was detected either by the LIS method or by the occurrence of a spark with at

least the prescribed radiant energy. Since the laser spot-size was large (0.5 mm), no statistical nature of damage should be inferred [8]. The range of threshold values is

believed to be a measure of the variable damage resistance of the coatings from one

area to another. -oqc
For purposes of immediate comparison, each series of test results is accompanied

by illustrations of the corresponding calculated SW electric-field distributions in

the films and in the surrounding media. The field distributions are all normalized

to the incident electric field-squared in air, IEo"*"!^. The mathematical expressions

for these fields are given in the appendix.

3.1. Damage Threshold Versus Film Thickness

The results for Ti02 films are shown in figure 4, and it is apparent that films

with odd quarter-wave thicknesses have greater damage resistance than those with

even multiples. The corresponding SW field distributions calculated for 1-, 2-, and

3-QW thicknesses are illustrated in figure 5. (The distributions for 4- and 5-QW

films are just sinusoidal extensions of the fields in the 2- and 3-QW films, respectively.) It is evident that the SW field pattern in the half-wave film is relatively

larger than those in the odd quarter-wave films. For a refractive index of 2.32

measured at 1.06 ym for these films, the ratio of the maximum values of |e/Eo'''|^ for

half- and quarter-wave films is 1.4. (The selection of the field maxima for comparison

rather than values at particular locations, such as the air-film interface, will be

discussed in section 4) . As justified in the next section, for linear absorption it

is appropriate to compare the inverse of this ratio with the damage thresholds using

the quarter-wave thresholds (upper and lower) as the references. The dashed lines

in figure 4 are the damage thresholds predicted by a linear dependence on the SW

electric fields. Good qualitative agreement is seen, but a higher order dependence

could fit better.

It is not reasonable to expect absolute quantitative agreement in these tests

because we are comparing the damage thresholds of coatings deposited on consecutive

coating runs. Slight variations in the coating deposition parameters can produce films

with some variability.

The results for ZTO^ films are shown in figure 6, and though an oscillation of

the damage thresholds is apparent with increasing thickness, it is not as definite

as seen for Ti02 films. This is consistent with the lower refractive index for Zr02

of 1.90. One of the 3QW films is seen to have a notably low threshold. This might

indicate higher film absorption or absorbing inclusions. The 5QW films had much lower

initial scatter than the other samples which could explain their higher damage threshold ranges. The corresponding SW field distributions for QW and HW films of ZrOa are shown in figures 7 and 8 along with other film materials for comparison. The dashed

lines in figure 6 are the predicted thresholds due to linear absorption at the locations of the maximum electric fields in the films. Qualitative agreement is seen.

The results for Si02 films are shown in figure 9. The high end of the thresholds

is nearly constant with film thickness. This is in good quantitative agreement with

the dashed line calculated from the SW field maxima in figures 7 and 8. The film

index, 1.45, is very close to that of the substrate, 1.506. The low end of the thresholds shows no recognizable thickness dependence. The randomness is likely due to

variable film quality. This is not unexpected since the bottom end of the threshold

range is a measure of film imperfections, e.g., absorbing inclusions and sites of

enhanced electric fields. The high ends of the threshold ranges would be more likely

to show a consistent dependence such as that predicted by the SW field distributions.

Figures 10 and 11 are plots of previous results reported in 1972 [2] for irradiation of TiOa and ZrOa coatings at the ruby laser wavelength of 0.694 ym with 13-ns

pulses. Though the number of thresholds plotted is small, the same general dependence

with film thickness reported herein for 30-ps pulses of 1.06 ym radiation is obvious.

For 13-ns pulses, however, any temperature patterns replicating the absorbed SW fields

in the films would be largely smeared out by thermal diffusion since (Dt)^ = 2. IX and

1.2X the physical thicknesses of QW films of Ti02 and Zr02, respectively, at the ruby

wavelength. The ratio of the average fields in HW and QW thick films would remain

nearly constant during this longer pulse, however. This ratio, 1.4, is identical

to that for the SW field maxima (observe the sinusoidal dependence of the SW field

expressions in the appendix) . Therefore, the measured cyclic variation of the damage

thresholds with film thickness observed in figure 10 is not unexpected.

The observed thickness dependence of the damage thresholds allows us to state

that the total laser energy absorbed in thin dielectric films is not the important

quantity for predicting damage resistance. If total absorption was important in the

damage process, then a 5QW-thick film, which absorbs 5X more energy than a IQW-thick

film (both have the same SW-field pattern) , would have a much lower damage threshold.

This is contrary to the experimental results presented in figures 4, 6, and 9 in which

-257-
the thresholds for QW, 3QW, and 5QW films of each respective material are nearly the

same

.

3.2. Damage Resistance of Back- Versus Front-Surface Coatings

When radiation is incident upon a coating from the substrate side, the SW field

distributions set up in the coating differ greatly in magnitude from those for frontsurface films. The fields calculated for QW back-surface coatings shown in figure 12

may be compared with those in figure 7 for front-surface QW films. The SW maxima

in figure 12 are from 20 to 40% greater, so that reduced damage thresholds are predicted. The damage thresholds for both front- and back-surface irradiation at normal

incidence are listed in table 1 for QW films. In each case, the upper threshold

value for front-surface films exceeded that for back-surface films. The values were

corrected for the 4% reflection loss at the bare glass surface in the case of the

back-surface films. The correction for enhanced axial intensity by large-scale selffocusing in the substrate was determined to be only 2 to 3%. Except for the sputtered

coating, B-32, the lower threshold values were nearly the same for both orientations,

however, suggesting limitations due to film defects. The meaning of the values in

the last column will be discussed in section 4.

Table 1. Damage thresholds for front- and back-surface coatings.

Sample t Coating

Peak Energy

Density (J/cm^)

Front Back

Ratio of

Thresholds

Front/Back

Ratio of Max

.

E-Fields

Squared

Back/Front

Ratio of t^

Back/Front

B-32 Ti02, A/

4

5.5-9.2 2.5-3.4 2.2-2.7 1.4 1.1

C-la Ti02, A/

4

3.3-6.8 3.9-5.2 .85-1.3 1.4 1.1

C-6 Zr02, A/

4

3.5-7.9 3.4-5.6 1.0-1.4 1.4 1.2

C-2 MgF2, A/

4

2.6-7.3 2.5-6.9 1.05 1.2 1.0

t Sample B-32 was rf sputtered onto Optocil I; others were E-gun deposited onto BK-7

.

3.3. Damage Threshold Versus Polarization State for Non-Normal Incidence

A second way to generate different SW field distributions in a given coating

sample is to irradiate with S- and P-polarized light at a large angle of incidence.

The SW field patterns for both polarizations and an angle of incidence of 60° are

shown in figures 13 to 16. Except for the case of the QW Ti02 film, the magnitudes

of the fields are not very different. If SW fields dominate over other considerations

in determining the damage threshold, then the films illustrated in figures 14 to 16

would be expected to have equal thresholds for S- and P-light.

However, an additional phenomena, the vectorial field effect, must be considered

when using P-polarized light, which has an electric field component normal to the

film surface. According to the vectorial field effect [9] , the component of the

electric field normal to the surface is much more efficient in producing photoemission

of electrons than the tangential component. If photoemission were important in initiating laser-induced damage in the films being considered, then the damage thresholds

for P-polarized light would be less than for S-polarization . For 60° incidence, the

incident normal electric field component Eq^"*" equals 0.867 of the magnitude of the

total incident field, or | Eq^'^'/Eq"'' | ^ = 0.75, a large fraction.

The experimental results for this test series are listed in table 2. For the

last three samples, the P-thresholds were equal to, or slightly greater than the

corresponding S-thresholds . This result suggests that photoemission is not important

in initiating damage in Ti02 , Zr02 , and MgF2 thin films.- Rather the ratios of the

P- and S-thresholds are in agreement with the ratios of the SW electric fields within
Table 2. Damage thresholds vs linear polarization state for an incidence angle of 60°.

CL ^ -1- 1 1

Peak Energy

Density (J/cm2)

S-Pol P-Pol

Ratio of

*-s ^ p

Ratio of Max,

E-Fields

|E /E |2

' p^ s

1

Ratio of

t 2/t 2 P s

c-la TiO-, A/4 2.1-6.9 1.5- 5 1.4 1.7 1.6

C-lb Ti02, A/2 1.1-4.0 1.6-5.0 0.7-0.8 1.0 1.6

C-2b ZrO^, A/2 2.2-4.4 2.3-5.5 1.0-0.8 1.1 1.4

C-3a MgF^, A/2 3.0-7.7 2.9-7.7 1.0 1. 05 1.15

If

3.4.

multiphoton

Damage by Circularly

absorption processes

and Linearly Polarized Light

lead to damage in dielectric films, then

maxima in the internal SW electric field distributions would be even more important

than for linear absorption. For example, the absorption coefficient for two-photon

absorption is proportional to |e|^. For one-electron atoms, Parzynski [10] has predicted that the ionization probability (from the ground state) for N photons is

different for circularly and linearly polarized light. His theoretical ratios of the

probabilities for these two irradiation conditions are given in table 3 along with

experimental data for cesium [11,12]. The data agree well with Parzynski's ratios.

Note that no difference was predicted or observed for linear (N=l) absorption.

Talrile 3. Multiphoton ionization by circularly and linearly polarized light.

Data Reported

for Cesium [11,12]

1

1.28

2.15

For one-electron atom in ground state.

The theory and results for one-electron atoms suggested a comparison of the

damage thresholds for linearly and circularly polarized laser radiation for the optical thin film materials being considered. These tests were performed with the

recognition that the theory was not developed for these materials, but that if any

differences were measured for the two polarization conditions, an applicable theory

should be sought.

The damage thresholds for three QW film materials are listed in table 4. If

Parzynski's theory were applicable, the threshold ratios should be 0.88, 0.46, and

>> 1 for 2-, 3-, and s 4-photon ionization, respectively. The ratio of the thresholds, ^q/€t, for MgFa and TiOz are equal to or slightly greater than 1.0. For Zr02

the ratio of the upper thresholds, 0.76, is very close to that predicted for twophoton ionization.

We conclude that either linear absorption leads to damage first for MgF2 and TiOa/ or the theory does not apply. However, two-photon absorption may possibly be present

prior to damage in Zr02 films. (Evidence for two-photon absorption at 0.694 ym in Zr02

is presented elsewhere in these proceedings by Picard, et.al.) A firm conclusion

based on the present results will require modification of Parzynski's theory for the

materials tested. -259-

N

Theoretical Ionization

Probability Ratio''" [10]

P /P,

•

circ Im

1 1

2 1.14

3 2 . 18

s 4 << 1
Table 4. Damage thresholds for circularly and linearly polarized light.

Sample Coating

Description

Peak Energy

(J/cm

Density

2) Ratio of Thresholds

Linear Circular /(?

C-6

C-2 •

B-32

B-33

ZnO^, A/4

Ti02, A/4

TiO^, A/4

3.5- 7.9

2.6- 5.7

5,5-9.2

6.3-10.1

4.8-6.0

2.6-5.7

6.1-9.1

7.6-12.2

1.4-0.76

1.0

1.1-1.0

1.2

4. Analysis

In this section, theoretical expressions for the damage threshold are alternately

derived in terms of the SW fields and the field of the first-pass traveling wave (TW)

in the film medium.' Soth predictions are then compared with the measured damage

thresholds in each of the preceding experiments. We assume absorption to be the

mechanism leading to damage, but a similar analysis could be attempted in terms of

elastic properties of the film materials.

The power absorbed per unit volume from an electromagnetic wave of intensity I traveling in a medium having a complex refractive index n is = -dl/dz = al. The

quantity a is the absorption coefficient (cm'^j which can be a function of the intensity as lN-1 for N-photon absorption. For linear absorption, a is related to the

imaginary part of n (= n-ik) by the relation a = 4Trk/A. For a plane wave, I = E^/n

where E^ is the mean square of the electric field averaged over several cycles

(^|e|2), and n = (y/e)^ is the wave impedance. In terms of the incident intensity

Iq (= I Eq''' 1 2/2rio) , the power absorbed (watts/cm^) at a distance z in a weakly absorbing medium is

P (z) = an

A similar expression for the energy absorbed per unit volume (joules/cm' ) from a light

pulse with incident energy density is

( z ) = an

a

* By rearranging, we obtain the incident energy density at the threshold of damage, ^

E(z)

Eo

(1)

E(z)

(2)

{E^ (z)/an) E(Z)

-2

(3)

In analyzing the measured damage thresholds for a possible correlation with the

SW field distributions, it was first necessary to detennine what location in the films

was most susceptible to early damage. Therefore, we made a preliminary comparison of

the thresholds with the SW field values at several film planes. The comparisons were

made with the SW fields at the: 1) air-film interface; 2) film-substrate interface;

3) planes of field maxima; and 4) average field-squared. In almost all cases, the

best correlation was obtained with the field maxima. Thus, only the maximum value of

|e/Eo+|2 for each film material and sample orientation is used in the ensuing analysis

Now, damage may occur either during the first pass of the laser beam through the

film as a traveling wave or as the field evolves to a standing-wave distribution. The

value of Ie/Eq+P of the first-pass TW immediately inside the film is equal to t^

,

where t is the Fresnel transmission coefficient. (Expressions for t are listed in the

appendix.) Then by eq. (3), the damage threshold for a traveling wave is reached when

-260-
e = ^ /ant'^ o a

(4)

If the film is weakly absorbing and inclusion free, the traveling wave has nearly

constant intensity across the film thickness. For such films, however, it is possible

to show that the maximum value of the corresponding SW field-squared is always larger

than t2 . Examples of this fact are given in table 5 in which the computed ratios of

I E/Eq"*" I ^max/t^ for each film material and sample orientation used in this experimental

program are tabulated. The ratios for the low-index films, SiOa and MgF2, are generally only a little larger than 1.0, so that SW-field effects on their damage thresholds might be hard to distinguish. The ratios for the high-index films, ZrOa and

Ti02, are considerably greater than 1.0, so that SW-field effects should be obvious

in the damage thresholds of such ideal films.

Table 5. Comparison of standing-wave and traveling-wave fields in thin films.

Square of

Coating Description t Angle of

Incidence

Direction

of

Incidence

Maximum

E-Field

|e/E +|2

' o

'

Fresnel

Factor

t2

Ratio

|e/e +|2/t^ 1 ' o

'

Ti02(A/4, 3A/4, 5A/4) 0° Front .46 .36 1.28

TiO (k/2 X) von "t" 11 . 7fi / o TiO^ {A/4) 0° Rear .65 .40 1.63

Ti02 (A/2) 0° Rear .92 .40 2.30

Tin ± J.v-'2 A(\/d) A/ ^ } T i. Uil u . J-1 7/ 1 . Z X TiO^ (A/4) 60°, P-pol. Front .30 .23 1.30

Ti02 (A/2) 60°, S-pol. Front .32 .14 2.29

Ti02(A/2) 60°, P-pol. Front .32 .23 1.39

Zr02(A/4, 3A/4, 5A/4) 0° Front .55 .48 1.15

Zr02(A/2, A) 0° Front .64 .48 1.33

ZrO_ (A/4) 0° Rear .80 .56 1.43

Zr02 (A/2) 0° Rear .92 .56 1.64

Zr02 (A/2) 60°, S-pol. Front .32 .21 1.52

Zr02 (A/2) 60°, P-pol. Front .34 .30 1.13

Si02(A/4, 3A/4) 0° Front .69 .67 1.03

Si02(A/2, A) 0° Front . 68 .67 1.01

MgF2(A/4) 0° Front .78 .71 1.10

MgF2(A/2) 0° Front .76 .71 1.07

MgF2(A/4) 0° Rear . 94 .69 1.36

MgF2(A/2) 60°, S-pol. Front .46 .40 1.15

MgF2(A/2) 60°, P-pol. Front .48 .46 1.04

^Substrate is BK-7 glass , refractive index n = 1

.

506.

In state-of-the-art optical thin films in which absorbing inclusions are known

to be present, it is possible that the first-pass TW may be sufficiently absorbed by

an inclusion to prevent a SW field from evolving at that location. If the absorptance

of the inclusion is sufficiently great and its diameter appropriately scaled to the

laser pulsewidth, the temperature of the inclusion can increase sufficiently to damage

the surrounding film material [4,5]. However, it is also conceivable that damage could

first occur to defect-free regions of film material adjacent to an absorbing inclusion

-261-
if 1) the pulsewidth-to-inclusion-diameter scaling is not optimum and 2) the SW field

in the adjacent material is sufficiently greater than the traveling wave striking the

inclusion

.

Whether or not SW field effects are important in the films tested in this study

can be evaluated by comparing the threshold ratios shown in figures 4, 6> and 9, and in

tables 1 and 2 with the corresponding ratios of maximum SW electric fields and t2

ratios

.

4.1. Evaluation of TiOa Films

In figure 4, the evidence of SW field effects in TiOz films is pronounced. The

calculated SW field maxima in even-QW films are 40% greater than those in odd-QW films,

whereas for the first-pass TW fields t2 is a constant value (0.36) with thickness.

The observed ratios (odd to even QW) for the upper thresholds ranged from 1.4 to 1.8; the

ratios for the lower thresholds ranged from 1.5 to 2.0. These values compare very well

with the 1.4 value predicted by SW field theory.

In table 1, the ratio of front- and back-surface coating thresholds for the sputtered TiOa film greatly exceed both the values predicted for the ratios of the SW field

maxima and the traveling waves, 1.4 and 1.1, respectively. This is not fully understood. The coating had very low scatter and presumably fewer defects than E-gun produced coatings. Possibly nonlinear absorption could be emphasized at the SW field

maxima, or the air-film interface absorbed abnormally to result in the 2.2-2.7 threshold

ratio obtained. |l

The results for the E-gun deposited TiOa film (C-la) were reasonable. The upper I

threshold ratio, 1.3, compares well with the 1.4 value predicted by SW fields, but the

lower ratio of 0.8 5 is closer to the 1.1 TW prediction. It is understandable that the

lower end of the threshold range could be a measure of the limitations posed by absorbing defects.

In table 2 the threshold ratio, 1.4, for the QW Ti02 coating is somewhat lower

than that predicted for both SW and TW fields, 1.7 and 1.6, respectively. A definite

correlation is not possible here. However, threshold ratios for the HW film, 0.7-0.8,

match the SW ratio of 1.0 much better than the TW ratio of 1.6.

In summary, the measured thresholds for Ti02 coatings consistently correlated well

with SW field maxima.

4.2. Evaluation of Zr02 Films

In figure 6 the threshold variations with thickness are generally in accord with

those predicted by SW theory, but are not as uniformly distinct as for TiOa coatings.

This is expected since the values of | E/Eq"^ | ^n,ax/t^ are smaller, which means the

possible influences of SW fields compared to the first-pass TW field cannot be as great.

The calculated SW field maxima in even-QW films are 16% greater than those in odd-QW

films, whereas for the first-pass TW fields t^ is a constant value (0.48) with thickness. The observed ratios for the upper thresholds ranged from 1.15 to 1.5; the ratios

for the lower thresholds ranged from 1.0 to 1.8. These ratios have a wide range, but

they generally were greater than 1.0 with one notable exception, which indicates the ^ influence of SW fields. M

In table 1 the upper threshold ratio for the QW ZrOa coating is 1.4 in agreement

with the SW ratio. The lower threshold ratio of 1.0 is closer to 1.2, the ratio of TW

field t^ values. This implies that the lower threshold is controlled by absorbing

defects

.

In table 2 the HW Zr02 coating has a threshold ratio of 1.0-0.8 which is in near

accord with the ratio of SW fields, 1.1.

In summary, the measured upper thresholds for Zr02 coatings generally correlated

well with SW field maxima, but there was some evidence that the lower thresholds were

restricted by defects.

-262-
4.3. Evaluation of MgFj Films

The difference between the influence of SW and TW fields in MgFa films cannot be

very great since the values of | E/Eo"*" | ^max/^^ not very much greater than 1.0 (table

5). In table 1 the threshold ratio for the QW MgF2 coating, 1,05, is very close to the

1.0 ratio of t2 values for the first-pass TW fields. The ratio for the SW fields, 1.2,

is also not too different. In table 3, the HW MgF2 coating has the same damage threshold for both P- and S-polarizations . This ratio of unity compares well with the ratio

of SW fields, 1.05, but the ratio of t2 values is also not too distant at 1.15.

In summary, the measured thresholds for MgF2 coatings showed minor SW field influence. This was predictable, with or without film defects, because of the small refractive index, 1.38, of this material relative to air and glass.

4.4. Evaluation of SiOz Films

The only threshold measurements performed on SiOz coatings are shown in figure 9 as a function of thickness. Due to this material's index of refraction being close to

that of the glass substrate, not much variation was expected or observed at the upper

threshold end. The random variation of the lower thresholds was presumably due to

film defects.

5 . Summary

The specific findings of this experimental study using 30-ps, 1.06-ym laser pulses

were

:

a. A definite correlation was generally found between the internal standing-wave electric field maxima and the damage thresholds, but in a few coatings defects apparently decreased or prevented any correlation. Designers of coatings for intense

laser beams cannot afford to neglect standing-wave field considerations.

b. Damage thresholds of Ti02 films showed a strong dependence on the internal standingwave electric field maxima.

c. The higher end of the damage threshold range for a given coating of Zr02 correlated

well with the internal SW field maxima, but there was some evidence that the lower

end of the threshold range was restricted by film defects.

Damage thresholds of MgF2 coatings were influenced by SW field maxima to a minor

degree. This was due to either or both the low refractive index and film defects.

e. Damage thresholds of Si02 coatings on glass were not influenced by SW fields due

to the refractive index being nearly equal to that of the substrate. Random variations of the lower thresholds were presumably due to film defects.

f. Damage thresholds for circularly polarized light directed at normal incidence on

Ti02 and MgF2 coatings did not differ from their respective thresholds for linearly

polarized light. By Parzynski ' s theory, linear absorption only is prominent in

these films. A lower threshold for circularly polarized light incident on a Zr02

coating may indicate the presence of two-photon absorption.

g. Photoemission via the vectorial photoeffect was not important in initiating damage

in TiOa , Zr02 , and MgF2 thin films.

6. Acknowledgments

The authors are indebted to Y. H. Hahn of CVI Laser, Inc., for supplying optical

coatings deposited by the electron-gun method and to W. J. Coleman and J. S. Hartman

of Battelle Pacific Northwest Laboratories for Ti02 coatings deposited by rf sputtering. The skillful preparation of the typed manuscript by Jo Ann Painter is also

gratefully acknowledged.

-263-
7 . Appendix

Calculation of Electric-Field Distributions

I

The standing-wave electric-field intensity distributions were calculated using the

equations below as derived in reference 3 . Subscript o refers to the medium from

which the beam is incident (air for a front surface coating) , subscript 1 refers to

the dielectric film, and subscript 2 refers to the following medium (substrate for a front surface coating)

.

A. Normal Incidence

The field expressions are

o

2 2 A + B

o

t^^ [1 + + 2r2 cos 2(6^ - k^z)]

and

t

2 t

2 1 ^2

|D|2

where

(1 + [cos (6^ k^z) + r^^ cos (6^ + k^z)] ,

B = (1 - r^) [sin (6^ k^z) + r^ sin (6^ + k^z)] ,

D|^ = 1 + r^^r^^ + 2r^r2 cos 26.

r - n , - n m-1 m

"m n , + n m-1 m

2nm-l

t = -

m n , + n„ m-1 m

6 = k d

m mm 6=0 o

k = m

2_ITo

\

- "m

and d = layer thickness

m

Parameters r„ and t are the Fresnel reflection and transmission coefficients, respec-

. • , m m tively

.

B. Non-Normal Incidence, S-Polarization

The field expressions are the same as for normal incidence, but the Fresnel

coefficients are redefined as i

r =

n , cos 4 , - n cos ([> m-1 m-1 m m m n , cos 4 , + n cos 4 m-1 m-1 m m

-264-
and

^ m-i m-i m n_ 1 cos d) n + n cos <b m-1 m

6 m

= k

z ,m

dm ,5=0, o

cos <{).m

2-n 'z,m \^ m

n cos

'm

(n„ - n sm ) m o o m

1/2

C. Non-Normal Incidence, P-Polarization

The field expressions are

o

2 2 A + B

1^o

2 2 [1 + r2 + 2r2 cos(2k^z - 26^ +Tr)]

2 1^1 2 n, D

and

o

2 2 t t

1 ^2

2 I ^ I 2

where

A = (1 + r_)21[cos(6, - k z - tt) + r, cos(6, + k z + 77)] , o 1 1 o

B = (1 - r_) [sinCS^ - k z - tt) + r, sin(6, + k z + tt) ] ,

d|^ = 1 + r,^r-^ + 2r,r„ cos 26, , 1 "2 1^2

and

r = m

t = m

n cos A 1 - n , cos * m ^m-1 m-1 Ijn n cos d) T + n , cos *

' m ^m-1 'mi m-1 m

2n cos m m-1

n cos d) . + n , cos cb m m-1 m-1 m

D. Fresnel Transmission Coefficients

When calculating the Fresnel transmission coefficient, t^, for comparison with the

maximxim relative electric field squared, | E/Eq''" i 2 , as explained in the text, the above

formulas can be used except in case C, non-normal incidence, P-polarization . For case

C, the equation above is actually for the H field, t^ . The correct value to be used

(for this direct comparison) is ^

-265-
E. Exit-Surface Calculations

For exit-surface coating calculations, the equations as presented give electric

fields normalized to the incident field in the substrate, Ej g . For normalization

to the incident field in air, eJ^^ , the correct relation is

E+

o , s = t E+a , s o , a

where for normal incidence

a,s 1 + n

All calculations reported in the text are referenced to air. This same normalization

factor must be used for the Fresnel transmission coefficients when calculated for

direct comparison with E-field ratios, thus

t ^ = t t ^ a, f a , s s ,

f

8. References

[1] Turner, A. F. , Damage in Laser Materials; 1971 , NBS Spec. Pub. No. 356,

119 (1971).

[2] Newnam, B. E. and DeShazer, L. G.

,

Laser Induced Damage in Optical

Materials: 1972 , NBS Spec. Pub. No.

372, 123 (1972).

[3] Newnam, B. E. , "Laser Induced Damage

Phenomena in Dielectric Films, Solids

and Inorganic Liquids," dissertation.

University Microfilm, Ann Arbor, MI.

[4] Milam, D., Bradbury, R. A., and Bass,

M., Appl. Phys. Lett. 23^, 654 (1973).

[5] Hopper, .R. W. , and Uhlmann, D. R.

,

J. Appl. Phys. 41, 4023 (1970).

[6] DeShazer, L. G. , Newnam, B. E. , and

Leung, K. M. , Appl. Phys. Lett. 23,

607 (1973).

[7] Watt, B. E., Appl. Opt. 12^, 2372

(1973) .

[8] Bass, M. , and Barrett, H. H. , Laser

Induced Damage in Optical Materials

1972 , NBS Spec. Pub. 372, 58 (1972)

[9] Fischer, T. E. , J. Vacuum Sci. and

Tech. 9, 860 (1972)

.

[10] Parzynski, R. , Opt. Commun. 8_, 79

(1973)

.

[11] Fox, R. A., Kogan, R. M. and Robinson, E. J., Phys. Rev. Lett. 26 , 1416 (1971) .

[12] Kogan, R. M. , Fox, R. A., Burnham.,

G. T. , and Robinson, E. J., Bull.

Am. Phys. Soc. 16, 1411 (1971).

9. Figures

Experimentol room

Domage detection Figure 1, Schematic diagram of laserinduced damage system showing

laser beam diagnostics and

damage detection methods.

-266-
Figure ?.. Laser beam profile as measured by linear diode array located at equivalent sample

position. Diodes are spaced .05 mm center-to-center, thus this pulse had a diameter of 0.70 mm at the 1/e height.

-267-
I -

I r

Ti on BK-7 i 30 ps, I 06 fim / /1\ W = 0 5mm ^/

\ / V

3X 5^

4 Film Optical Thickness

Figure 4. Damage threshold versus film optical

thickness for Ti02; 1.06-Min, 30-ps

pulses.

Figure 5. Standing-wave electric field intensity

distributions for A/4, A/2, and 3A/4

films of TiOa. The distributions are normalized to the incident electric

field-squared in air, |e+|^.

Air

ZrOj on BK-7 30 ps, I 06 /im Spot size = 0 5fnm

Film Optical Thickness

Film Substrate % thick BSC-2

Figure 6. Damage threshold versus film optical

thickness for ZrOa; 1.06-)Jm, 30-ps

pulses.

Figure 7. Standing-wave electric field intensity

distributions for A/4 films of Ti02,

Zr02, Si02, and MgF2. For the distributions in air, the intensity scale is

broken at 1.6 and the peaks have been

displaced downward and plotted on the

scale in parenthesis.

-268-
I , hilm Substrate

Figure 9. Damage threshold versus film optical

Figure 8. Standing-wave electric field intensity thickness for SiOz; I.p6-Mm, 30-ps

distributions for X/2 films of TiOa, pulses.

Zr02, Si02, and MgF2.

50

40

30

1 20 h

10

Figure 10.

TIO2 on BSC -2

13 ns, 0 6943 fj.m W = 0 10 mm

3\

4 Film Optical Tliickness

Damage threshold versus film optical

thickness for Ti02; 0.6943-ym, 13-ns

pulses

.

50

40

E 30 -

20 -

o

Eoo 10 -

13 ns, 0 6943

W = 0072 mm

4 2

Film Optical Thickness

Figure 11. Damage threshold versus film optical

thickness for ZrOj; 0.6943-ym, 13-ns

pulses

.

-269-
Substrate BSC -2

Glass

0

Incidence

Incidence: 60/'

Figure 12. Standing-wave electric field intensity Figure 13. Standing-wave electric field intensity

distributions for X/4 back-surface

coatings of Ti02, ZrOa, SiOa, and MgFa.

Substrate

distributions for X/4 front-surface

coating of Ti02 , for an angle of

incidence of 60°. Both S- and Ppolarizations are shown. For the distribution in air, the scale is broken

at 1.8 and the peaks have been displaced

downward and plotted on the scale in

parenthesis.

Sub$trole BK-7 Glass

Figure lA, Standing-wave electric field intensity

distributions for X/2 front-surface

coating of Ti02, for an angle of incidence of 60°. Both S- and P- polarizations are shown. The scale is broken

at 1.8 in figure 13. Figure 15. Standing-wave electric field intensity

distributions for X/2 front-surface

coating of ZtOz , for an angle of incidence of 60°. Both S- and P- polarizations are shown. The scale is broken

at 1.8 as in figure 13.

-270-
Figure 16. Standing-wave electric field intensity

distributions for X/2 front-surface

coating of HgFz , for an angle of incidence of 60°. Both S- and P- polarizations are shown.

COMMENTS ON PAPER BV WEWNAM, al

TfieAe m& dxtHYiAlvz dL&caiilon o{, the. qaution 0($ wheXheA 01 not one. mixld expect a di^eAtnt

moiphotoQij to be ob&eAvzd the. domaqe occanjied by avalanche ionization icmiXive only to the etect/iic

{^ieJLd int^nAity oh. i^ the damage occaAAed at the iite 0^ a d&{,ect oA an incia&ion. The point woi

hai.&e.d that in the ihont puJU>eJ> aied in thii expeAJjment the. electAic (^ieJid induced e^f^ect maid alwayt,

dominate and that theAe Moutd not be mi^iicient zneAQy in the put&e to came defect mediated damage.

TuAtheAmofie, even in ZongeA put&eyi iince the damage wa6 occuAAing at thAe^hold it might be poaible

to teJUL (tOA mall de{^ect^ which. moAs the dominant eiisCct. The Keai>on ii 06 iottom'- the itanding wave

patteAn ij> A&iponiive to the aveAage pAopeAtieA o^ the thin {^ilm itAuctuAe, and the ^ield ij, moit

intense at the pexik o{, the itanding wave. The damage occuaj, at a weak point in the thin ^ilm whetheA.

it i& an incZuiion, a defect, on a Kegion o{, impuAlty. ThuA, whateveA the cauae of, the damage on a local icale it ii AeApomive to the. aveAage pAopeAtleA, o^ the electAic {^ield diitAibution, foA the^e

Kza^oni it mold ieem to be. extAemely di^f^icuJU. to obtain moAphological evidence o{^ the initial caixie

0){ tkt damage. pAoceM.

-271-
5.7 Threshold Ambiguities in Absorptive Laser Damage to Dielectric Films"*^

R,H. Picard, D. Milam , and R.A. BradburyAir Force Cambridge Research Laboratories

Hanscom AFB, Massachusetts 01731

and

John C.C. Fan

Lincoln Laboratory, Massachusetts Institute of Technology

Lexington, Massachusetts 02173

The threshold energy density for laser- induced damage

arising from homogeneous absorption in thin dielectric films

is dependent upon the laser irradiance. This is illustrated

by measurements of the time to breakdown of rf sputtered

films irradiated by tightly focussed square-waveform ruby

laser pulses. We are able to show that the increase of

threshold with decreasing irradiance is due to two

mechanisms: (1) cooling of the film by conduction of heat

to the substrate and (2) two-photon absorption in the film

itself.

Key words: Damage threshold; dielectric films; heat conduction;

laser damage; multiphoton absorption; rf sputtering.

1. Introduction

It has been shown previously by a number of investigators that the breakdown of

dielectrics due to irradiation in the small focal spot of a tightly focussed laser

beam is a statistical process. This was shown first for the case of intrinsic

electron-avalanche breakdown Cll^and then for the cases of damage due to absorbing

inclusions and dielectric structural defects £2, 3, 4]]. In all these cases the

damage threshold can only be defined statistically, and hence the concept of a threshold is of limited utility. When irradiating apparently equivalent sites with

equivalent laser waveforms, damage may occur at any time during the pulse or pulse

train.

On the other hand, breakdown due to homogeneous absorption in the irradiated

material is non-statistical, and materials which damage due to homogeneous

absorption should exhibit a sharp damage threshold, even in small focal spots LW],

Nevertheless, one must still exercise caution in quoting threshold energy densities

for damage in this case since the threshold depends on the laser irradiance, or equivalently on the laser pulse duration. We wish to illustrate this point by

describing the results of damage experiments on a series of rf- sputtered singlelayer dielectric films which we subjected to irradiation by a constant laser field.

In [4] it was shown that in such an experiment one could determine

whether the mechanism responsible for breakdown was homogeneous absorption, or one

of the other mechanisms mentioned above by analyzing the statistical variation of

the time until damage, or survival time, for different sites on the sample. Hence, we were able to select those sputtered films in our study which damaged due to

homogeneous absorption. Concentrating on the homogeneous absorbers, we varied the

survival time by varying the irradiance and determined the dependence of the energy

densiuy threshold on the survival time. This is equivalent to a measurement of the

dependence of the threshold energy density on pulse duration.

* Present address: Lawrence Livermore Laboratory, Livermore, California 94550.

t The Lincoln Laboratory portion of the work was sponsored by the Department of

the Air Force.

1. Figures in brackets indicate the literature references at the end of the paper.

-272-
Although the pulse-duration dependence of the damage threshold has been discussed £ 53 and measured (for example, £6, 7, 32) many times before, these authors

were invariably concerned either with damage to bulk dielectrics or with situations

where the cause of damage was unknown or known not to be homogeneous absorption.

Moreover, in our measurements we have been able to identify the reasons for the

irradiance or pulse-duration dependence of the damage threshold by comparison with

model calculations.

2. Experimental Procedure

The apparatus and the experimental technique are the same as described in

[4]. The samples are irradiated by 20 - 30 nsec, square-waveform ruby

laser pulses. A typical pulse waveform is shown in figure 1. This pulse is obtained by switching out the central portion of the output from a single-longitudinalmode, single-transverse-mode Q-switched laser by means of a Pockels cell shutter.

As shown in figure 2 the pulse is focussed onto the sample and recollimated by a pair of lenses, then detected and displayed. A portion of the light is picked off

by a beam splitter prior to focussing and displayed on the same oscilloscope trace

to serve as a reference.

An idealized scope trace is shown in figure 3. The time delay before damage

occurs, or survival time, tj is measured from the width of the transmitted pulse

shown on the right, and the beam power is determined from the height of the

reference pulse shown on the left by using a radiometer to calibrate the detector.

Since the beam is known to have a Gaussian spatial profile, the irradiance I at the

center of the focal spot may be determined from the beam power and the spot size.

A 6 ym focal spot (FWHM) was used in this series of experiments.

After irradiating a large number of sites on the same sample, each with one pulse, we are able to select those samples in which damage is due to homogeneous

absorption. In figure 4 we show data from a sample in which breakdown is not due

to homogeneous absorption. Even though all five sites shown are subject to the same irradiance, damage occurs at random times during the pulse. fin figures 4 and

6 the positions of the transmitted and reference pulses are interchanged, relative

to their positions in figure 3.) We show a typical plot of t;^ versus I in figure 5 for a sample which is damaging due to some process other than uniform absorption.

Figures 4 and 5 are to be contrasted with figures 6 and 7 which show data

characteristic of damage due to homogeneous absorption. In the latter case, breakdown is quite deterministic, a given irradiance leading to a well-defined time delay before damage. We shall refer to a line drawn through the data points of

figure 7 as the heating curve for the sample; assuming that damage occurs when the

material reaches a given temperature T^j, the heating curve determines how long it

takes to heat the sample to T^j as a function of the level of irradiation. An

alternate way to represent the data in figures 5 and 7 is in the form of a survival

curve, or plot of the fractional number of sites surviving as a function of time

In figure 8 we show survival curves for the sample of figure 7 at three

different irradiances. The absorptive nature of the breakdown process is also

apparent from the survival curve; however, we will show only heating curves for

samples in the rest of this paper.

3. Sample Preparation

The samples used in our experiments were single-layer dielectric films prepared by rf sputtering in a Class 100 clean room at Lincoln Laboratory. Some con- ditions of the deposition process and properties of the samples produced are listed

in table 1.

-273-
Table 1. Conditions of Preparation of Sputtered Films

^ l«X LJ O I X. CX I i i iOL L- V— A, -1— CG 0211 or CG 7059 fire-polished glass.

Substrate preparation Ultrasonic cleaning in hot detergent solution. for

less than 5 min. Ultrasonic rinse in distilled

deionized water. Dry in N2 gas boiled off from

liquid. Sputter etch in Ar/Oa mixture.

Target diameter 5 in.

Target (film) material Si02j ZrOa , or AI2O3

Flntittprinp' M ^ U> ^ 1- LL "dowpt ^ W ^ J- 100-200 W

Sputter bias 0-307o of sputtering power

Sputtering pressure 10"^ Torr

Ar or At/ 10 vol7 n Deposition rate 15-50 8/min

Film thickness 440-1500 A

The bare substrates were tested and found to be very resistant to damage. A

prime motivation of the study was to determine the influence of preparation con- ditions, in particular the presence of a bias and the sputtering gas, on damage

characteristics; this subject will be treated in a future publication. In this

paper we confine ourselves to analyzing the heating curves of those samples which

damaged due to uniform absorption.

4. Results and Analysis

We have already shown the heating curve for one of the AI2O3 samples in our study

in figure 7. Since the laser pulses have a square temporal waveform, we need only

multiply the ordinates in figure 7 by the corresponding abscissae to obtain the

threshold energy density as a function of irradiation time (or, alternately, as a function of irradiance) . The result is shown in curve (f) of figure 9. We also

show the time dependence of threshold for five other samples in this figure

(curves (a) - (e)). In all cases the threshold energy density increases by a factor of two or three as the survival time increases from the shortest to the

largest values observed in the experiment. Of the two ZrOa samples shown, sample

(a) has a threshold twice as high as sample (b) for 20 nsec pulses, but the

thresholds are comparable for pulses shorter than 4-5 nsec. Moreover, Si02 sample

(c) appears to have a higher threshold than Si02 sample (d) for the longer pulses,

while the reverse appears to be true for the shorter pulses.

In an attempt to understand the time dependence of threshold, we assume that

damage occurs when some part of the sample reaches a damaging temperature Tj. Then

the heat Q deposited per unit volume in the material by the laser pulse is given by

Q = c^ pAT^ = alt^, (1)

where c is the specific heat, p the mass density, a the linear absorption

coefficient of the sample, and AT^ is the temperature rise needed to reach temperature T^. This simple model predicts a hyperbolic heating curve. In figure 10 we show a least-squares fit to the data points for Si02 sample (d) of figure 9. The

absorption coefficient obtained if one assumes that T, is the melting point of the

material (a = 38.9 cm"-') is shown in the figure. It is clear that the theoretical

curve does not fit the data. This should also have been clear from figure 9, since

eq. (1) predicts that the threshold energy density is independent of time.

-274-
One of the most obvious possible mechanisms to explain the observed behavior

is cooling of the sample by heat cgnduction. The characteristic length for heat

diffusion irt a time t is £p = (Dt)2, where D E k/(c^p) is the thermal diffusivity,

K being the thermal conductivj.ty . Assuming D = 5 x 10"^ cm^/sec and t = 30 nsec, we obtain £ = 0.1 ym = 1000 A. Since 1-q '^'^ ^ the radius of our focal spot, we can ignore radial heat loss; on the other hand, since H.-q = d, the thickness of the

film, we must consider longitudinal losses by thermal conduction into the nonabsorbing substrate.

The one-dimensional diffusion equation for this slab geometry can be solved

exactly by Laplace transformation 18], if we assume a uniform rate of heat deposition

in the film by the laser beam. In general, the solution is in the form of an infinite series 191, but it takes on a particularly simple form it we make the

following reasonable assumptions: (1) No heat loss at the front surface of the film

(air) ; (2) infinite surface conductance at the rear surface (substrate) ; (3)

thermal impedance matching at the film-substrate interface (that is, values of D

and K for film and substrate approximately equal) . The equation for the heating

curve is then

Q = c^ p AT^ = alt^f (Z). (2)

where l W}

f (Z) = (1+2Z2) erf Z + ^

Z e"^' -2Z^ , (3)

and

Z = p . (4)

2 (Dt^) ^

Comparing eqs. (2) and (1) and noting that f (Z) £ 1, we see that cooling steepens

the heating curve, as required to fit the data points in figure 10. In figure 11

we show the results of performing a least-squares fit of the data points in

figure 10 to eq. (2) rather than eq. (1), still with the single adjustable parameter a. The improvement in the fit is dramatic, and we can conclude that the

temporal dependence of threshold for this sample is due to thermal conduction to

the substrate.

The next obvious question to ask is whether conductive cooling can account for

the increase of energy threshold for damage with time for all of the samples shown

in figure 9. To answer this we look at the data for sample (a), a ZrOa film with

d = 600 A. The data points are shown in figure 12 along with the best fit assuming

cooling to the substrate, indicated by the solid line with triangles (A). Even

though the film is thin and the effect of cooling is appreciable, we find that the

predicted heating curve is not steep enough and that the fit is poor.

We are led to ask what other mechanisms might steepen the heating curves. One

possible mechanism is nonlinear absorption which makes the curves steeper by

lowering the threshold energy density for short times (high irradiances) . If we

stop at the lowest-order nonlinearity , that is two-photon absorption, the heating

curve, eq. (2), is modified to the form

Q 5 c^ p AT^ = (al+31^) t^ f (Z) , (5)

where B is the two-photon absorption coefficient. The best least-square fit with

adjustable a and g is indicated by the solid line without symbols in figure 12.

This curve fits the data well. The one- and two-photon absorption coefficients

-275-
required for the best fit (a = 9.1 cm"-', 6 = 2.4 cm/GW) and indicated on the figure.

To determine what fraction of the threshold variation is due to two-photon

absorption and what fraction due to cooling, we also show in figure 12 a fit with

a = 0, indicated by a solid line with x's (X). We see that this fit with B = 2.8

cm/GW is nearly indistinguishable from that in which we had allowed a % 0. We are

led to conclude that this sample is a two-photon absorber with little or no linear

absorption. By contrast, with sample Cd) we had found no need to invoke any twophoton absorption (6 = 0)

.

In general, the situation is not so simple, and we find that a given sample

exhibits both linear and two-photon absorption. In figure 13 we show the data

points for sample (e) , figure 9 (SiOz , d = 550 A), with the same three fits. The

best fit for this sample is midway between the fit with B = 0 and the one with a = 0.

5. Summary and Conclusions

Letting W = It, be the threshold energy density, we may rewrite eq. (5) to show

explicitly how the threshold energy density changes,

^ =

f (Z)^(a+ei) ' (6)

Hence, there are two factors which change the value of W, cooling by heat losses to

the substrate, represented by f (Z) , and nonlinear absorption, represented by a+31.

We may write the ratio R of the maximum and minimum thresholds observed as a product

of two factors, the first representing the effect of cooling and the second the effect of nonlinear absorption, that is

R = W /W . = R . R T , (7) max mxn c nl' ^ '

where

R = f /f • , R n = Ca+BI )/Ca+6I . ). (8)

c max' min' nl ^ max" ^ mm ^

'

In table 2 we summarize the fractional increase of threshold energy density

for the six samples of figure 9 and identify that part of the increase due to

cooling and that part due to two-photon absorption. We take Wj^ax ^ ^ ^A. ~ nsec and Wjj,^^ = W for t^j = 4 nsec, even though in some cases our data extends over

a greater range.

The variationR in threshold energy density ranges from a factor of 1.77 for

sample (d) to a factor of 2.78 for sample (a). There is no simple correlation

between film thickness and threshold change, although cooling is significant for

all of the samples, giving rise to a 26% increase in threshold for the thickest

sample and an 81% increase for the thinnest. To account for the complete time

dependence of the threshold energy density, one must assume that nonlinear

absorption occurs in the film. For three of our samples the increase of threshold

due to nonlinear absorption exceeds 507o. On the other hand, only a 5.5% threshold

variation could be attributed to two-photon absorption in the case of sample (d)

,

a sample we have already identified as a linear absorber in section 4.

In a future publication we will attempt to identify correlations between the

behavior of our samples under irradiation and the conditions under which they were

prepared.

-276-
Table 2. Sources of Variation of Threshold Energy Density W

Percent increase in threshold between 4 and 20 nsec

a b Sample Material Thickness Total Due to cooling Due to nonlinear ab-

(A) sorption'^

(R-1) (R^-1) (\l-l)

(a) ZxOz 600 178 64 66

(b) Zr02 1150 92 30 52

(c) Si02 440 131 81 29

(d) SiOz 600 77 66 5.5

(e) SiOz 550 135 72 35

(f) AI2O3 1500 119 26 71

^ Read from figure 9.

^ Calculated from eqs. (7) and the values of a and B determined from least-squares

fits

.

6. Refe

[1} Bass, M. , and H. H. Barrett, IEEE

J. Quantum Electron., QE-8 , 338

(1972) .

y_2j DeShazer, L.G., B.E. Newman, and

K.M. Leung, Laser Induced Damage

in Optical Materials: 1973~ edited

by A.J. Glass and A.H. Guenther

(U.S. Govt. Printing Office,

Washington, 1973), p. 114; Appl.

Phys. Lett. 23, 607 (1973).

l32 Bliss, E.S., D. Milam, and R.A.

Bradbury, Appl. Opt. 12, 677 (1973).

t4] Milam, D. , R.A. Bradbury, R.H.

Picard, and M. Bass, Third Conference on High Power Infrared

Laser Window Materials, November

12-14, _ 1973 , Vol. Ill, edited by

C. A. Pitha , H. Posen, and A.

Armington (Air Force Cambridge Research Laboratories, Bedford, Mass.,

1974), p. 1011; Laser Induced

Damage in Optical Materials: 1974 , edited by A.J. Glass and A.H.

Guenther (U.S. Govt. Printing

Office, Washington, 1974), p. 169.

Us] Bliss, E.S., Damage in Laser Glass , edited by A.J~ Glass , A.H. Guenther

,

CM. Stickley, and J.D. Myers

(American Society for Testing and

Materials, Philadelphia, 1969), p.

9; Opto-Electronics 3, 99 (1971).

[6] Avi zonis,P.V., and T. Farrington,

Appl. Phys. Lett. 7, 205 (1965).

[ 7j Zverev, G.M. , T.N. Mikhailova,

V.A. Pashkov, and N.M. Solov'eva,

Zh. Eksp. Teor. Fiz. 53, 1849

(1967) iSov. Phys.-JETP 26,

1053 (1968)j. ~

\ Carslaw, H.S., and J.C. Jaeger,

Conduction of Heat in Solids , 2nd edition (Clarendon Press,

Oxford, 1959), pp. 297-326.

[93 An analysis similar in some

respects for the heating of

metals has been carried out by

M. Sparks and C.J. Duthler,

Fourth Technical Report, Contract No. DAHC15-73-C-0127

(Xonics, Inc., Van Nuys,

Calif., 1974).

[lOl If Z <<1 (film very thin or time

long or dif fusivity large) , then

we have f (Z) ~ Z, and eqs. (2)

through (4) give rise to T^ ~ Q

~ t^^, as is used in simple

diffusion models of metal

heating. (See, for example

paper by T.T. Saito, D. Milam,

P. Baker, and G. Murphy in these

Proceedings.) However, this

simplification is not available

to us here.

-277-
7. Figures

20 nsec/div

Figure 1. Square-waveform pulse selected by shutter from 0-swltched ruby laser pulse.

Atten plates

scope Figure 2. Set-up for damage experiment. The

sample is placed at the common focus

of the lens pair.

Figure 3. Idealized oscilloscope display recorded

during damage experiment. Irradiance I is determined from reference pulse (on

the left) and time delay until damage

tjj from pulse which transversed sample

(on the right).

-278-

Pu/se Bjpass/ng Pulse Through

Sample Sample
J I I I L

10 nsec/cm

J

Figure 4. Typical data for a sample which is not

damaging due to uniform absorption.

The reference pulse is on the right, and

the pulse passing through the sample is on the left.

Figure 5. Typical plot of data points for time

delay and on-axis irradiance for a sample damaging due to some process

other than uniform absorption. It is

not possible to draw a curve through

the points, demonstrating the usual

statistical character of small-spot

damage

.

2 4

GW/em* ON-AXIS

-279-
I L

10 nsec/cm

Figure 6. Typical data for a sample damaging due

to uniform absorption. Positions of the

two pulses are the same as shown in

figure 4.

Figure 7, Plot of time delay versus Irradlance

for a sample which has damaged due to uniform absorption. Breakdown is nonstatistical, and one can draw a smooth

curve (the heating curve) through the

data points. The laser power Is indicated on the upper horizontal axis.

0.00

POWER( KW

0.50 1.00 1 .50

1 r1 1

\ " o

o \ ALUMINUM OXIDE

"V== 1500 R

0 B LJ \ D \ a LlJ

\ °

\, 0 o tS\ a

CC

CC a \ a a ok ^0

Q S\

o

1

LU " ^l = z:

' ' o M« a 1—

o

'J- 0

o

o

1 1 1 1 1 0.00 1.00 2.00 3.00 4.00

IRRflDIflNCEt GW/CM ^)

-280-
1.0

.5

o

tij <

s

<

a

z3

trUJ

ffi S3

<

O<

.2

.05

.02

.01

1 V TT- 1

o\

-

\ 0

-

" 3.2 GW/cm^

2.8 GW/cm*

2 2 GW/ cm* —

0 -

Al,0, Single Layer

J

1 •

600 A

5 10

TIME, nanoseconds

15

TIME TO DAMAGE (Msec)

Figure 9. Threshold energy density versus time

delay for six sputtered single-layer

films, showing the dependence of

threshold energy for damage on survival

time. The sample shown in figures 7 and 8 is represented by curve (f )

,

Figure 8, Survival curves, or plots of survival

probability versus time, for the

absorbing sample of figure 7.

POHERt KW J 3.00 4.00 5.00

O

O

C_) UJ2 o

o

CC

CC o

LlJ

SILICON DIOXIDE

600 fl

-flLPHflr38 .9

o- o

o

o

6.00 9.00 12.00 IS. 00 16.00 IRRflOIRNCEICW/CM'')

Figure 10. Data points for SIO2 film (sample (d) ' of figure 9) with heating curve obtained

by least-squares fit to data points,

assuming heating rate proportional to irradiance and no heat losses. Best

estimate for absorption coefficient a. is listed on plot in cm~^.

281-
POWER! KH

)

0 ,00 1 '00 ? '00 3 '00 4.00 5.00 6 .00 7 .00

LJ

hJCO

cr

cc o

o- o

o

o

\ °

\ °

\ SILICON DIOXIDE

"V"" 600 R

\

°

o 0 a

Q Q a

"

(B \\O \

f)LPHfl = 58.8 /^^a i

a

= ° \H°

a OB

a

0.00

Zo

C3

CE

cr

PONER

8 .00 H

KW

12.00

H

16.00 —I

ZIRCONIUM OXIDE

500 n

-flLPHfl=73 .6 BETR= 0.0

-flLPHfl=0 .0 BETB= 2.8

-PLPMfl=9.1

SETfi= 2.4

O.OC 10.00 20.00 30.00

IRRflDIflNCEl GW/Cn'

)

-+- 40 .00

t^o.oo 6.00 9.00 12.00

I RRPID IHNCEl GW/Cn' )

Figure 11. Data for sample of figure 10 fit to a heating curve which assumes cooling of

the film by thermal conduction to the

substrate.

POWERC KW ) 0.00 1.00 2.00 3.00 4.00

1 1—t—B-M 1 1

(M

O . . (M

C3

cr

cr .

SILICON DIOXIDE

550 R

-fiLPHRz94 .2 BETfl= 0.0

-flLPHR=0.0

BETfl= 14.1

-RLPMfl=57.3

BETfl= S.6

Figure 12. Data points for ZrOa film (sample (a)

of figure 9) fit to three different

heating curves assuming (1) linear

absorption only (—A—), (2) two-photon

absorption only (—K—)

, and (3) both

one- and two-photon absorption (solid

line without symbols) . The linear

and two-photon absorption coefficients

a and 6 are listed for each of the

plots, ct in cm~^ and 6 in cm/GW. All

three fits allow for conductive cooling to the substrate.

-o.oo 2.00 4.00 6.00 8.00 IRRflDIflNCEt OW/CM"

)

10.00

Figure 13. Data points for Si02 film (sample (e)

of figure 9) fit to three different

heating curves. The fits used are the same as in figure 12.

-282-
COmEffTS ON PAPER 8/ PI CARP, U at

In tiiU, pap&fL ew-trfence wcu> p^eAzntzd 0(5 tMo-photon abion.ptA.on in matz>viaJi(> in Mliich two-pkoton

abioKption iM znzKgeXiaMy ^ofLbiddzn in thz balk matzniaZ. The. quution u)au> theAzdone. ncuj,Q.d cu, to wheXhzi one. mold expect to 4ee a di(i(,eAznt band itAucXuAe in the. thin (^itm mateAial than i(> obieAved

in thz bulk matenJjal. The point wa6 nxuAed that \ja/u.ationi, 0(5 the enexgy level itAuctuAe diom the

batk pKopa/itiu might indeed be expected to occuA in thin {,ilm. One poaible came o^ new leveti

woi the phenomenon of, band bending due to thz abiupt tenmina-tian ojj the lattice in the inteAsnittent

itAuctuAe 01$ the thin fiilm. The authou o^ the papeK ^elt that thLf> wa4 one poaible caiue o{^ the

miMxphoton abioKption, but that otheA mediating impuAitieA on. de(,ecti might alio play a lole.

WhateveA the cauie 01 location 0($ the intenmediate itateyi wa6 theAe u)en.e obieAved to be homogeneously

dUtAibutzd thAoughout the mateAialA on the icale o(, the expenJjnent, The nesolution 0^ the

expeAunent MOi d&tenmined by the (^uZZ width hal^ maximum ipot iize 0|$ the loieA piobe aied in thli

iMfik, which Moi 6 micAomeXeAi . The question wcw fuxlied ai to whelheA on not the electnlc {field diiPUbution in the thin {jilm had been expliclty taken into account in thli mnk. The authonA

KeAponded that it had not, but due to the thlckneM oi the iilmf, they did not {^eel that it would be a iignlfiicant eHzct,

-283-
5.8 Time Resolved Study of Laser-Induced Structural Damage in Thin Films

Nabll Alyassini and Joel H. Parks

Departments of Physics and Electrical Engineering

University of Southern California

Los Angeles, California 90007

A time resolved optical probe technique reported previously was used to study laser induced structural damage in thin film dielectric

coatings. Experimental data indicates that thin film damage frequently occurs without any detactable spark or distortion to the transmitted

damaging ruby pulse and that structural damage to thin film can occur

up to 40 nsec after passage of the damaging Q-switched ruby pulse through

the site. In addition damage threshold of X/4 ZnS single layer film indicates the presence of a film/substrate adhesion effect.

Key words: Adhesion effect; laser induced damage; optical probe technique;

thin film; time resolved damage.

1. Introduction

This paper presents the results of time resolved damage studies of thin films using optical probe

techniques [1-3] . These techniques allow the detailed temporal development of laser induced damage

to be monitored. The time resolution can include effects occurring during the laser pulse and can also be extended to observe phenomena well after the damaging laser pulse has past, covering a range

0.5-50 nsec. In addition, these optical probe methods proved to be an extremely sensitive detector of

small film damage occurring with site diameters ~lym.

In section 2 the experimental technique is described and the film configurations studied are presented. The primary results discussed in section 3 include an indication of the sensitivity and

utility of the probe technique. In the concluding section 4 we summarize the primary results of

these studies.

2. Experimental Technique

Figure 1 shows the experimental arrangement used in this study [1-3]. In order to monitor structural changes in the film during and after irradiation with the single mode TEMqo Gaussian ruby laser,

a 3 mW unpolarized TEMqq He-Ne laser was employed to probe the film internally at the critical angle

for the film/air interface. The reflected probe beam was detected with a silicon PIN photodiode

(HP 5082-4207) which was dc coupled to a 500 MHz Tek. 7904 oscilloscope. The optical probe detection

system had a measured risetime of less than 1 nsec and was synchronized [3] in time to the damaging

ruby pulse with a measured accuracy of better than ±0.25 nsec.

Internal probing of the film at its critical angle was chosen in order to: -Eliminate any possible spark perturbation of the probe beam,

-Maximize sensitivity of probe to structural variations at film/air interface since the reflectivity angular gradient is maximum at the critical angle,

-Maximize resolution of probe detection system since detected signal is maximum at this angle.

The calculated spot size radius of the probe at the film/air interface was less than 4ym and the

measured spot size radius of the ruby at the film was 35ym. Each site was irradiated only once. Tests were conducted on X/4 ZnS, A/4 MgF2, bi-layer of XI 2 MgF2 on top of X/4 Zns, and a 17-layer film

of ZnS/T F^ G(HL)^HA. All the coatings were tested as entrance and exit surface film, except the bilayer coating which was tested only as an entrance surface film. All the films were thermally deposited on BSC-2 dove prisms as shown in figure 1. More detailed description of the experimental arrangement can be found in [3].

3. Experimental Results and Discussion

Figure 2 shows a typical data set for an entrance surface X/4 MgF2 film damage of =30lJm diameter.

The traces shown are exact replicas copied from original photographs of the oscilloscope traces and

Partially supported by Advanced Research Projects Agency and Joint Services Electronics Program at

U.S.C.

1. Figures in brackets indicate the literature references at the end of this paper.

-284-
arranged to emphasize the important times in the evolution of damage. Such a data set is typical of

the thin films coatings studies when the damage site is relatively large. In such a case distortion

of the transmitted damaging ruby pulse was always present as shown in trace (a) (output pulse).

Figure 3 shows typical data set for a smaller damage site, which in this case was 10 ym diameter

entrance surface X/4 ZnS damage. Note that the transmitted damaging ruby pulse shows no detectable

distortion even though a spark was detected with this damage. The fall time of the probe trace for

this site is much slower than the site shown in figure 2.

It should also be pointed out that no major detectable difference between the probe traces for

entrance and exit sites was found in the collected data for all the films studied.

3.1. Sensitivity of Optical Probe Technique

The dip in the transmitted damaging ruby pulse could not be due to absorption by the visible

spark that sometimes was detected with thin film damage since a spark was also detected at many sites

without any detectable dip in transmitted damaging ruby pulse. It is also not due to absorption by

the film or absorbing inclusions since the energy needed to thermally damage a film site is negligible

relative to energy of the damaging ruby pulse, and in addition, the extraction of such energy would be

occurring continuously during the pulse. ^gEven when ^ hin film damage is due to an avalanche type

process, the energy needed to create 10 - 10 / cm free electrons by ionization, and the subsequent

energy absorbed by such a density of free electrons in the irradiated film site is too small to be

detectable in our set-up.

Hence we conclude that the dip in transmitted damaging ruby pulse for the thin film case is due

to scattering by the deforming site. It appears from our results though, that the damaging ruby pulse

is not very sensitive to relatively slow deformation in the film on which it was normally incident.

Only when the deformation was relatively violent, reaching a peak in less than 10 nsec, did it cause detectable scattering of the damaging ruby pulse.

The probe beam is insensitive to changes in the film index since it was incident at the critical

angle for the film/air interface. Linear absorption by the film or an absorbing inclusion should be

going on continuously and would not explain the sudden dip in the reflected probe intensity. However,

laser induced deformation in the film could cause excessive scattering of the reflected probe intensity and a sudden dip in probe trace. This deformation could result from melting, vaporization, or cracking in the film due to thermal stresses. It can be assumed that the probe beam was scattered

by these deformations as they were taking place. When the resulting damaged site was relatively free

of debris or bubbles, and larger than the probe cross section at the film, the probe trace recovered

almost completely. When the site appeared with considerable debris or bubbles or was smaller than

the probe cross section at the film, the probe trace showed only slight or negligible recovery (see

figure 2 and 3). The start of the dip in the probe trace indicates the instant at which the laser

induced deformation in the film effected the probe. The fall time of the probe gives a measure of

how long a time this deformation of the irradiated site takes to reach a peak or plateau as far as the effect in scattering the probe is concerned. The recovery time of the probe indicates the time

needed for such deformation effects to subside.

Distortion to the transmitted damaging laser pulse has been used as an indicator of laser induced

damage to dielectric thin films [4]. Our dielectric thin film data indicates that this is not a positive indication of small damage near threshold. In general, the spark that sometimes accompanies

thin film damage is a better indicator, even though it is well known that the spark itself is a less

sensitive detector of damage than the LIS method [5]. We have also employed the LIS method as a damage indicator throughout this study [6]. In the few cases when the LIS method failed to detect

damage, the site induced scattering was clearly revealed by a drop in the probe trace. Such sites

which occurred in the X/4 ZnS and the 17-layer film had a diameter of less than 2-3 ym and relatively

smooth edges. Figure 4 shows the morphology and data set of such a site which was revealed only by

the probe method. The calculated minimum detectable damage size was 1 . 5y m in diameter [3].

We have also found that the lack of distortion of the transmitted damaging ruby pulse, or the

lack of an accompanying spark, does not necessarily occur only at lower power intensities or energy

densities. We have detected damage in X/4 ZnS film without a spark or distortion to the transmitted

damaging ruby pulse at slightly higher energy densities than other sites in the same film at which

a spark and/or distortion was detected.

3.2. Time Evolution of Structural Damage

Another interesting result is that the structural damage of the irradiated site as revealed by

the dip in the probe trace, always occurs during the time of interaction of the damaging ruby

pulse and the film. The data set for such a site in X/4 MgF2 film is shown in figure 5. Note that

the structural damage which is indicated by the drop in the probe trace starts -25 nsec after the

passage of the damaging ruby pulse. The site temperature probably induced thermal stresses which

reached a critical value about 25 nsec after passage of ruby pulse causing the crack shown in the

-285-
SEM photo of the site. On the average a lower incident field intensity implied a longer time delay

between the start of the dip in the probe trace and the peak of the ruby pulse.

3.3. Possible Film/Substrate Adhesion Effect

Figure 6 shows the standing wave field in an entrance and exit A/4 ZnS film. The measured peak

threshold intensity at points of maximum field inside the film are also shown. The exit surface

film exhibit a higher threshold density than the same film used on the entrance surface. This

difference in the measured threshold could be due to an adhesion effect at the film/substrate interface. This could decrease the entrance surface film threshold since the standing wave field is

maximum at the film/substrate interface in that case. The other films tested did not reveal any

difference in their threshold data.

The importance of the standing wave field on the morphology of the film damage is shown in figure

7. The standing wave field in this case is maximum in the layer nearer the laser source and drops

off progressively in successive layers. In the entrance film case the top few layers seem to have

fused together, while in the exit film case the inner few layers changed phase and the resulting

stresses ruptured the film.

4. Conclusions

Damage to thin dielectric films can occur without any detectable distortion in transmitted

damaging laser pulse and/or an accompanying spark. Thus both are questionable indicators of weak

damage.

For damage that is the order of 2-3 um in diameter or greater, the optical probe technique P50^

to be as sensitive as the LIS method. In cases when the damaged site was of smaller size, the

damage was revealed by the probe trace behavior and undetected by the LIS method.

Damage does not necessarily occur during the time of interaction between the laser pulse and the

film. The development of laser induced thermal stresses can cause damage well after passage of the

laser pulse through the site.

A possible adhesion effect in the film/substrate interface of X/4 ZnS film was detected causing

a lower threshold intensity when film was used as an entrance surface relative to its value when

used as exit surface film. The effect of the field standing wave on morphology of film damage was

also pointed out.

5. Acknowledgments

We would like to acknowledge discussions with Prof. J. Marburger and Prof. R. W. Hellwarth.

The film depositions were prepared by Mr. H. R. Owen and SEM photographs were taken by Mr. Jack

Worrall, both of U.S.C.

6. References

[1] J. H. Parks and N. Alyasslni, 4th. ASTM

Symp., Damage in Laser Mat 'Is., NBS Spec.

Pub. 372, 104 (1972).

[2] N. Alyasslni, J. H. Parks and L. G. DeShazer,

5th. ASTM Symp. Damage in Laser Mat 'Is.,

NBS Spec. Pub. 387, 133 (1973).

[3] N. Alyasslni, Ph.D. Thesis, University of

Southern California, 1975 (University

Microfilm, Ann Arbor, Mich).

[4] D. Milam, R. A. Bradbury, R. H. Picard and

M. Bass, 6th. ASTM Symp. Damage in Laser

Mat 'Is., NBS Spec. Pub. 414, 169 (1974).

[ 5 ] B . E . Newnam and L . G . DeShazer , Op . Cit .

,

123 (1972).

[6] We found experimentally that using a fixed

40X telescope to examine the LIS level

before and after Irradiation is easier and more sensitive to changes in the scattering

level than using the unaided eye.

-286-
7. Figures

TEK. 7904

TRIGGER'

TEM.o RUBY

VWm = 11.4 nsec

HE-NE

PROBE

Figure 1. Schematic of experimental arrangement

and optical probe technique for

study of thin film damage.

(a)

(b)

(c)

A yy

(d)

Fli'HM ==9.1 ns(H;

(d)

Figure 2. Typical entrance site damage in X/y

MgF2 film; site C3,9. (a) output (left)

and input pulses of damaging ruby pulse,

(b) optical probe trace, (c) damaging

ruby pulse, (d) SEM photograph. A spark

was detected with this site. The

incident peak field measured 0.624 mW/cm,

10 nsec

Figure 3, Typical entrance site damage data in

X/4 ZnS film; site F5, 5. (a) output

(left) and input pulses of the damaging

ruby pulse, (b) optical probe trace,

(c) damaging ruby pulse, (d) SEM photograph. Note that no distortion is detectable in the output pulse although

a spark was detected with this site.

The incident peak field measured

0.615 mV/cm.

(c)

Figure 4.

-287-

Data set of an entrance site damage in

a 17-layer film, site H6,5. (a) optical

probe trace, (b) damaging ruby pulse,

(c) SEM photograph. This damage site was not detected by the LIS method although the probe trace indicates the occurrence of damage. The incident

peak , field measured 0.59 mV/cm,
STANDING WAVE FIELD

(a)

(b)

10 mV

10 nsec

(c)

BSC-2 BSC-2

LASER

AIR

ENTRANCE EXIT

PEAK THRESHOLD INTENSITY

, 2 0.35-0.55 GW/CM' 0.5- 1.0 GW/CM'

Figure 6. Measured peak threshold intensity at point of maximum standing wave field

for A/4 ZnS film.

Figure 5. Exit site data set for X/A MgFj film

damage of the "crack" type, site Rll,5

(a) optical probe trace, (b) damaging

ruby pulse, (c) SEM photograph. The

incident peak field measured 0156 mV/cm.

(b)

Figure 7. Effect of standing wave field on damage

morphology in the 17-layer film for

(a) entrance site (HI, 7), (b) exit site

(M12,5).

COMMENTS ON PAVER BV ALVASSWJ, e-t al

In tho. dlicuMlon that iottowzd tka> papoji ajL imcu> emphcuiz^d tkcut thz tKamrnUttd putiz ihapt Id a veAy pool mdoMiAe, oi damage mcvi thtuhold, white, the back-i caUeAed tight li veAy iemZjtivz to thz

omeX. ofi the, damage pnozeAi,.

-288-
5.9 The Importance of Refractive Index, Number Density, and Surface

Roughness in the Laser- Induced Damage of Thin Films and Bare Surfaces

J. R. Bettis*, R. A. House, and A. H. Guenther

Air Force Weapons Laboratory

Kirtland AFB, NM 87117

and

R. Austin

Perkin-Elmer Corp.

Norwalk, Conn. 16851

Results of a laser-induced damage study to optical surfaces and thin-film coatings are reported. Ten different half wave films, placed on fused silica, sapphire, BK-7, and ZnSe, were subjected to 1.06 ym radiation in a 147 pm spot size. Nine different optical materials

in bulk form including four glasses were also tested. The results were compared to a phenomenological equation relating threshold optical field to number density, refractive

index, and root mean square surface roughness. The films were measured for inhomogeneity

and were deposited by r.f. sputtering, electron beam heating, and thermal evaporation. Very

good correlation was obtained between predicted and observed behavior.

Key words: Damage thresholds; laser damage; refractive index; surface damage; surface

roughness; thin films.

1. Introduction

The research reported herein is a continuation of work reported at the 1974 Damage Symposium [1]\

In that brief note a postulation of the expected behavior of the laser- induced damage threshold electric field as a function of the refractive index and number density of polarizable species in an optical material was given. At that time the thesis was tested against data on damage threshold

values reported by a number of investigators on a variety of materials in various forms. The agreement was generally good for data from thin-films at surfaces and within bulk dielectric materials.

This year a comprehensive examination of the working hypothesis was performed by damaging a set of

coated and uncoated samples prepared especially for such tests. As noted last year, fluorides, as a class of materials, did not appear to obey the suggested relationship, thus a set of fluoride materials

in bulk and thin film form ranging in refractive index from 1.38 to 1.62 was examined to determine

whether fluorides behaved contrary to the expected behavior. In addition, the scatter inherent in

thin-film coatings was investigated as a possible indicator of relative sensitivity to laser-induced

damage, as well as a study to correlate substrate roughness with thin film threshold, compare surface

threshold for bare surfaces to surface threshold for the material in thin-film form, and an evaluation

of thin-film thresholds with deposition technique.

2. Experimental Method

For the experimental portion of _^his study the arrangement shown schematically in figure 1 was

employed. The laser consists of a Nd ^ doped glass oscillator with a 16 mm in diameter by 300 mm

long rod and one amplifier stage with 16 mm in diameter by 500 mm long rod.** The lowest order TEM

mode was selected by an intracavity aperture of 1.9 mm diameter. The far field intensity distribution

was measured with a 135 ym pinhole [2] and found to be Gaussian to within 13%. The laser was operated

at a constant pump level with the target energy being controlled by a set of non-saturating Schott

attenuating filters used in pairs for minimal beam steering. The temporal pulse shape was monitored

by a pair of ITT biplanar photodiodes, one of which was supplied with an integrating circuit to monitor

total energy and detect pre- or post-lasing caused by regenerative feedback. Over a series of 6000

shots the pulse width was stable at 40+1 nsec. The target energy was monitored with a pyroelectric

detector.* Its digital output was correlated against a carbon-cone thermopile calorimeter as well as against an additional pyroelectric detector. The variation in measured energy for the thermopile

calorimeter and integrating photodiode was +8.5 and 9% when compared to the two pyroelectric detectors

* Based upon portions of a PhD thesis submitted to the Graduate Engineering School of the Air Force

Institute of Technology, Wright-Patterson AFB, Ohio 45433.

** Normal oscillator and first amplifier of a Compagnie General d'Electric VD 640 laser system.

*** Laser Precision Engineering model RPK 3230.

1. Figures in brackets indicate the literature references at the end of the paper.

-289-
while the pyroelectric detectors tracked each other to + 4%. The specific filters employed served as an additional check on the credibility of the readout from the pyroelectric detector. The helium-neon

lasers were used as aids in detecting damage through laser induced scatter (LIS) [3] and a phasecontrast method discussed in a separate paper presented at this symposium [4]

.

-2

The spot size from the 503.5 mm focusing lens was 147 ym to the e power points. This spot size was large enough to obviate any dependence of threshold on the spot size [5] • Total energy wa.s O.A J nominal

.

Two principal observations have become apparent as a result of this investigation. One is that %

the characterization of a substrate material is important in predicting both its damage threshold and

the threshold of any film applied to it. The second is that without special control over more parameters than has been proposed, thin films may exhibit greatly different optical characteristics

compared to the same material in bulk form, and as such, the careful determination of a variety of

resultant film properties is needed. Partially because of these considerations each bare surface was measured for roughness by the total integrated scatter (TIS) [6] technique and a representative number were measured by the fringes of equal chromatic order (FECO) [7] technique. The films were measured

for internal scatter by TIS, for physical thickness with a Tallystep weighted stylus depth gauge, and

for spectral response to determine optical thickness and homogeneity with a Carey 14 spectrophotometer.

Results of the reflection spectra for four films are shown in figures 2 and 3. Figure 2 is the

spectral response of a pair of good films, Zr02 and ThF^ while figure 3 depicts the generally inhomogeneous films of LiF and MgF2. The behavior observed in figure 3 can be adequately modeled by a refractive index variation of 7% increasing from the substrate outward. Although this variation in the

refractive index is not sufficient in itself to lower the expected threshold to the values observed

(figure 5) , it is an indication that the film will behave in a less than ideal or predictable fashion

from first principles (i.e., assuming that the material in thin-film form is identical to the bulk

optical and physical properties)

.

Studies are continuing on the samples utilizing Auger spectroscopy, ellipsometry , SEM, and a possibly revealing study on the OH contamination on surfaces as it affects coating deposition and

composition, and ultimately damage threshold.

I

The preparation of the bare substrates is described in a separate paper presented at this

symposium [ 4 ] . For this portion of the report the films were deposited on samples prepared to bestfinish specifications (typically <X/5wave flatness and 15 A rms roughness). The dielectric films were deposited by one of three techniques: radio frequency sputtering, electron beam heating, and

thermal evaporation. A set of MgF2 deposition technique films were prepared by each of the three

methods. The remainder of the films were applied by electron beam heating. For the majority of

samples, each dielectric film was specified to be a half-wave in optical thickness at the 1.06 ym

wavelength used in the study. This parameter was monitored by observing the reflectance of the filmsubstrate system as the film was being deposited. For films whose index is greater than the substrate

index a minimum in reflectance at 1.06 ym is obtained when a half- wave optical thickness is reached.

For homogeneous films this minimum corresponds to the reflectance of the substrate. The half wave

thickness was chosen because this assured that the optical electric field at the substrate-film

interface was the same as the optical electric field at the film-air interface. Because to an electric

field a half-wave window is apparently not there, one would expect the character of the substrate to have a very great effect on the field at the surface of the film. Thus one would expect the roughness

of the substrate to materially affect the threshold of the film. This may not be as critical in

multilayer reflector films since the electric field tends to "pile up" in the first few (outer) layers

of the stack. It is also the thickness which gives the least variation in the electric field of a standing wave pattern for slight variations in the thickness. For films whose index is less than the

substrate index a reflectance maximum is obtained when a half- wave optical thickness is reached. For

films whose index is the same as, or very near the index of the substrate, it is necessary to use a separate witness plate (substrate) with a different index varying as much as possible from that of the

film to realize maximum sensitivity in the measurement of the optical thickness. In using a separate

witness plate one should match the mechanical properties of the surface (e.g., coefficient of thermal

expansion, roughness, crystallinity, if any, and orientation) so that such effects as different residual

stress of the film on the sample and the film on the witness plate do not induce changes in film

density and refractive index between the two films.

A total of 25 irradiations were taken on each sample-repositioning on a virgin site for each

exposure. The sites were shielded from each other with an apertured face plate [4] so that plasma

blowoff and ultraviolet radiation from any breakdown plasmas would not affect adjacent sites. The

25 shots were sufficient to determine electric field threshold to with + 12%. An increase in the

number of shots by a factor of four failed to significantly reduce the statistical limits of the value.

-290-
3 . Theory

Dielectric systems respond to an electric field through their polarizability . A semi-classical

approach to the problem of displacement of a bound electron in an alternating field of amplitude, E^,

and frequency* w, yields the following equation for the charge displacement x,

iwt _ n^ - 1 3£o T, ii^t = ^i^^TT Nqf h ^ (3.1)

where the polarizability, x, has been related to refractive index, n, and number density, N, by the

Clausius-Mosotti equation, and the subscript, £, denotes the local electric field. In a more standard

form one can relate the macroscopic electric field to the local electric field by the Lorentz local

field correction. In terms of measurable quantities eq. (3.1) may be rewritten as

ioit n^ - 1 f-o 2 ,„ , Z , 2

i<»>t zo ox xe = — -H ——V ig.A/Sw/m-^ e (3.2) N q^ I n + 1 J

where S is the power density incident on the dielectric and the expression in brackets is the macroscopic field, E, within the dielectric. Upon inverting this expression one obtains an expression

directly relating the macroscopic field to certain material parameters.

(3.3)

Because the damage symposium functions as a sounding board dedicated to the interchange of ideas,

the authors do not think it inappropriate to suggest two ways in which eq. (3.3) can be interpreted to predict laser damage thresholds.

Equation (3.3) predicts a direct relationship between the displacement of the optical electron in

a dielectric and the applied field. It is obvious that there exists a displacement beyond which this

equation is not applicable. Assuming that such a critical displacement x^j. does exist the electron would be

sufficiently liberated from the atom by action of this threshold field. In order to quantify this

proposition, recourse is made to order of magnitude arguments advanced by Seitz [8] and Bloembergen

[9]. Seitz states that it is safe to assume that breakdown by electron avalanche will occur if a primary electron produces one electron by collisional ionization for each 10^ atoms which it encounters.

For most solid dielectrics with atomic densities of 5 - 12 x 10^^ cm~^ this requirement implies that

electron densities of 5 x 10^^ to 1.2 x 10^^ cm~^ are required for breakdown. Bloembergen states that

breakdown may be defined as occurring when the electron density exceeds lO'^^^ cm~^. It is interesting

to note that Seitz was dealing with dc breakdown of dielectrics while Bloembergen was involved with

laser-induced damage to dielectrics. It is also of interest to note that Alyassini and Parks [10] used

the measured specular reflectivity decrease at the onset of surface damage in BSC-2 to calculate the

free electron density as 10^^ - 10^^ cm~^. Thus, not only is it assured that it is unnecessary to ionize every atom in the focal volume of a laser to achieve damage? but one is able to ascribe an order

of magnitude estimate of that percentage which is required. To account for this in eg. (3.3) consider

that the total energy delivered to the target volume is reduced by that fraction of atoms which when

Ionized will result in damage. Taking Seitz criteria eq. (3.3) becomes

E,^ = ^ r 5£ X /lO^ . (3.4)

th n^ - 1 cr

For these orders of magnitude calculations one need not probe deeply into quantum theory to ascribe

a critical displacement. From the work of Wunsch [11] the dipole moments of polarizable entities do not

saturate until the displacement of an electron distribution of an atom in most optical dielectrics

reaches about 0.75 - 1.0 A. Thus it would seem that the distance x^^. must be greater than this amount

in order for the electron to be "freed" from the atom. It is possible and perhaps reasonable to suppose that the critical displacement is of the order of half the molecular spacing in the solid.

Since these^distances tend to be 3.5 - 4.0 X this gives a reasonable value for x^j. of the order of

1.75 - 2.0 A which exceeds the limit for saturation of the polarizability. Using this for purposes of

comparison with experimental work results in one final form of eq. (3.3) as

E^^ = -4

^

/lF5" . (3.5) th n'' - 1 2

As an alternative statement of eq. (3. 3) consider the energy expended in displacing an electron a distance x. For purposes of identification of the functional relation it is sufficient to claim that

2 mx2 , (3.6)

-291-
and since x and x are related through

xe^'^'} (3.7)

one obtains

^^^=4 . ^^J^-Jw^ (3.8) th - 1 oj ' m ^

where the Seitz criteria has again been applied. Since laser-induced damage disrupts the lattice and

has been shown [12] to cause plasmas of electrons and ions to be ejected from surface damage sites, the

authors suggest that the heat of formation (or cohesive energy) related to the ions of the species is a likely candidate for the energy necessary to impart to 1 in 10^ of the atoms in a solid in order to cause damage.

N

It should be noted that the ^2_-^ term is the dominant term in both eqs. (3.5 and 3.8). While this

term varies by factors up to 15 in the materials tested for this report the molecular separation and

the square root of the heats of formation vary by factors of only three.

4. Results

In testing the relationship given by eqs. (3.5 and 3.8), 10 different materials in half-wave films

applied to four different substrates were subjected to damaging laser pulses in the experimental

arrangement shown in figure 1. As a check on the quality of the thin films the surfaces of five of

these materials in uncoated bulk form were also damaged. To observe the major factor in eqs, (3,5 and

3.8) four optical glasses were also tested. From the spectral response of the MgF2 and LiF films

shown in figure 3 one might expect that the film quality evidenced there would result in anomalous or lower than expected threshold fields. In table 1 the ratio of film to bare substrate thresholds is

displayed and one should note that the LiF and MgF2 exhibit the lowest ratio. The LiF is only a partial

result^since the effect of surface roughness is currently being assessed and its quite rough nature

("v lOOA) will lower the LiF ratio even more. It is of interest to note that ZnSe alone has a higher

threshold in film than in bulk form. This result is not completely unexpected due to the difficulty in producing clear, unhanded, and homogeneous bulk ZnSe [13].

Table 1. Ratio of Film to Bare Surface Thresholds

Material Bare Surface Threshold Film Threshold Oj.^'

,75 Ratio

FS on FS 1.55 MV/cm 0.892 MV/cm 0.575

MgF2 on FS 2.365 0.899 0.38

LiF on FS 1.27* 0.516 0.41

A£203 on Sapphire 0.608 0.439 0.72

ZnSe on ZnSe 0.116 0.122 1.06

*Film substrate much smoother than bare surface which is uncorrected for roughness

In figure 4 the threshold electric field for films versus'yL ^/ ^ is plotted. The fit is not

unreasonable with three exceptions: namely, the low values of ^^2^3 require further explanation.

For the course of this research it has been found that a critical parameter in determining threshold

fields for surface damage depends on the surface roughness value. In the paper by House et. al.,[4]

a relationship was presented between threshold electric field and a power of the root mean square

surface roughness. Over a range of surface roughness from 13 A rms to 335 A rms as measured by FECO

interferometry, the threshold field for a Si02 overcoated surface varied from 0.9 MV/cm to 0.24 MV/cm,

or a ratio in energy density of 14. To account for this trend one simply modifies the threshold field

by (0/13.75)°* where 0 is the root mean square roughness and m was taken as 0 . 75, although postsymposium work indicates 0.5 is more^appropriate. This factor has the effect of modifying each surface

to an equivalent roughness of 13.75 A rms. Since most of the films were placed on^l3.75A rms fused

silica blanks the only values changed significantly are the on Sapphire (43 A) , the ZnSe on ZnSe (30 X), and the Si02 on BK-7 (21 t) .

A final form of eq, (3.5) including the roughness correction is

(4.1)

where is a standard roughness against which the measured roughness is ratioed.
To demonstrate the major trend for bare uncoated surfaces one must plot the threshold field

modified for roughness against N . These results shown in figure 6 indicate that the bulk of

n2-l

eqs. (3.5 and 3.8) holds for materials ranging from ZnSe (n = 2.485) to MgF2 (n= 1.38) when each

surface is referenced to a standard surface roughness. Although not present here, LiF at ^iL_=13.356

n2_i has been damaged and is currently being measured for roughness. Preliminary indications are that

LiF will fit the expected behavior.

Data is currently being analyzed to compare the full functional forms of eqs. (3.5 and 3.8) to the

experimental results. Preliminary calculations indicate that LiF and kJL2^2i hold the key to which

one, if either, of eqs. (3.5 and 3.8) represents an acceptable view.

To demonstrate the order of magnitude fit of these equations the predicted value for fused silica

was calculated by each one. From eq, (3.5)the predicted value is 6.14 MV/cm while eq_. (3. 8) predicts

40.1 MV/cm. These values compare with 5.2 MV/cm measured for bulk Si02 by Fradin [14] and indicate

that eq. (3.5) may more nearly approximate the true nature of the damage process.

The final data to be presented involves MgF2 films deposited by three techniques. In retrospect,

MgF2 may not have been a good choice for this evaluation since the MgF2 films generally appear to be

inhomogeneous in form. The three techniques were: rf sputter, thermal evaporation, and electron beam

heating. The results in table 2 are at best inconclusive. There simply are too many extrinsic

variables present in film deposition to state with assurance which technique gives best films of MgF2

or any other material. (It was a good idea to use MgF2> however, because it does give anomalous results

and this was a search, however unsuccessful.)

Work is continuing to analyze the data and the many variables as yet not fully considered. From

the work of Guenther et. al., [15] it appeared that film stress was an important factor in determining

film thresholds. The present work tends to substantiate that claim to some degree. From the data

analyzed so far, and from the reanalysis of previously published data in terms of the hypothesis

advanced in this paper, it appears that the major factor in determining a film's threshold is expressed

by some form of eq. (3. 3) , whether the displacement is used explicitly or implicitly, while the stress

in the film is a frequently secondary effect.

Table 2. Thresholds for MgF2 Films Versus Deposition Technique

Technique Threshold Electric Field

Electron Gun 0.737 MV/cm

Thermal Evaporation 0.797 MV/cm

R-F Sputter 0.899 MV/cm

Essentially the same comments apply to the use of scatter in the films as damage predictors. However, just as a poorly deposited film can have such bad stress characteristics so as to overcome the

intrinsic effects so can a film possess enough scatter to overcome the intrinsic effects. It is

important to observe that in film thicknesses other than A/2, scatter, roughness, and stresa can play

a much greater role.

5. Conclusions

There are three major conclusions to draw from this study. It is obvious that the relationship

expressed by eq. (3. 3) is a major determining factor in the threshold of film surfaces, bare surfaces,

and, from the work presented last year, bulk materials. Some form of the cohesive energy - whether

expressed in relation to the free atoms, free ions, neutral gases, or standard form of the constituents

is to be determined - may be important in the total functional form. Secondly, fluorides as a group

do not exhibit anomalous behavior. Rather some fluorides make better films than other fluorides. An

obvious sub-conclusion is that films with near theoretical spectral response (i.e., response similar

to that exhibited in the bulk form) exhibit higher damage thresholds. A third major conclusion is

that values for surface and thin film thresholds are not meaningful unto themselves unless the surface

roughness is specified. Obviously other extrinsic factors such as stress, etc., will be important in

certain cases. Note that although MgF2 and LiF do not follow the predicted dependence in thin-film

form they both, at least preliminarily, do fit when the surface of the bulk materials is damaged.

It cannot be over stressed that the compelling reason for this behavior lies in the inhomogeneities of

the thin films as revealed by figure 3,

-293-
6. Acknowledgments

The authors are indebted to the following people for the time and energy they spared: Norm Boling

of Owens-Illinois shared some particularly fruitful discussions as did Alex Glass of Lawrence Livermore;

Bryan Newnam of Los Alamos Scientific Laboratory helped design the experimental arrangement as did

Del Owyoung of Sandia Laboratories; Hal and Jean Bennett and Jane Bruce performed various measurements

for us; and Dr. Karl Mader of Schott Optical supplied impossible to find data on their optical glasses.

7. References

[1] Bettis, J. R., Guenther, A. H., and Glass,

A. J., NBS Special Publication 414 , U.S. GPO,

Washington, D.C. (1974), p. 214

[2] Owyoung, A., NBS Special Publication 387 . U.S. GPO, Washington, D.C. (1973), p. 11

[3] Newnam, Brian E. and DeShazer, L. G.,

NBS Special Publication 372 , U.S. GPO,

Washington, D.C. (1972), p. 123

[4] House, R. A., Bettis, J. R., Guenther, A. H.,

and Austin, R., This Symposium

[5] DeShazer, L. G., Newnam, B. E., and Leung,

K. M., NBS Special Publication 387 , U. S. GPO,

Washington, D. C. (1973), p. 114

[6] Bennett, H. E. and Porteus, J. 0., JOSA 51, No. 2 (1961), p. 123

[7] Holland, L., The Properties of Glass Surfaces , Chapman and Hall, London (1966), p. 113

[8] Seitz, F., The Modern Theory of Solids , McGraw Hill, N.Y. (1940)

[9] Bloembergen, N., IEEE J. of Quantum Electronics

QE-10 , No. 3, (Mar 1974), p. 375

[10] Alyassini, Nabil and Parks, Joel H., This

Symposium

[11] Wunsch, Donald C., Doctoral Dissertation submitted to New Mexico State University (1969)

,

[12] Avizonis, P. V., and Farrington, T., Applied

Physics Letters 7 , (1965), p. 204

[13] Braunstein, M. , Laser Window Surface Finishing

and Coating Technology , ARPA Contract

F19628-73-C-0234

[14] Fradin, D. W. and Bass, M. , NBS Special Publi- cation 387 , GPO, Washington, D.C. (1973),

p. 225

[15] Austin, R. Russel, Michaud, Raymond C, I

Guenther, Arthur H., Putnam, Joseph M. , and

Harniman, Richard, NBS Special Publication 372 , GPO, Washington, D.C. (1972), p. 135

[Note added in proof: Further analysis of the surface roughness correction factor (aTM) indicates that

an exponent value m in eq. (4.1) of 0.5 is probably more appropriate than 0.75 as employed in figures

4, 5, and 6. Little of the discussion is changed except to make eq. (3.8) less likely than eq. (3,5) in

modeling the behavior observed.] .

8. Figures

'

40 nsec F>HM ^0 4j PULSE

osc AMP

Figure 1. Experimental arrangement showing

diagnostics and damage detection

apparatus. WflVELENGIH l^m) Figure 2, Percent reflectance versus wavelength

for homogeneous half-wave Zr02 and

ThFij compared to fused silica substrate.

A systematic offset has the value of

substrate reflectance at 4% rather than

294- the true value of 3.36%.
Figure 3. Percent reflectance versus wavelength

for Inhomogeneous half-v;ave LIF and MgF2

compared to fused silica substrate. A

systematic offset has the values of substrate reflectance at 4% rather than the

true value of 3.36%.

-T"

lR£l BOUGHrtESSlO '5 IHRESH01.0 FIE1.C

(COHESIVE ErJERGY)'^ ATOMIC DENSITY

(RELATIVE UNITS)

THRESHOLD ELECTRIC f I E LC VS ATOMIC DENSITY (COHESIVE ENEHGYf'.-

100 120 140 160 180 200 VT _N_

n'-l Figure A. Threshold electric field versus

(cohesive energy) "'^ ^-j

— for half- wave films of 10 materials on four substrates. The materials in ascending

order of the dependent variable are: ZnSe, ZrOa, BaF2 , CeF^, ThFi, , SiOa (2) , PrFs, AI2O3, MgF2, and LiF.

(REL ROUGHNESS)" THRESHOLD flELD

VS ATOMIC DENSITY

Figure 5. (Roughness relative to 13.75 A rms)°*^^ x threshold field

. u • a/2 N versus (cohesive energy)

n"^-l for the 10 materials in figure 4.

Figure 6. (Roughness relative to 13.75 A rms)"'^' x threshold field

atomic density (N) , versus 2 7 —^— for (in

n —1

ascending order of dependent variable)

the bare surfaces of: ZnSe, ZnS,

LASFe, SF-14, SF-12, AI2O3, BK-7,

Si02, and MgF2

.

COMMENTS ON PAPER BY 8ETTIS, eJ: al

It voous, polntzd out that ipattnizd {,11m oAz not aliAJayi the. but and that {oK a glvm muIJulayeA

dizlnctxic iUjn compoiltlon and duign one. muAt chooie the appiopulate method o{, depoiition whetheA It

li, iputtzAlng, evapoiated depoiUxon, on election beam depoiltion. It waj> duAtheA pointed oat that

the meXkod oi depo6ltlon l6 not i>aiil(u.ZYit to chaAacteAize the thin ilbn pAoceM, but In addUlon the

conditions ol depoiiXlon including the piepaAatlon o(5 the iubitAote must also be coniideAed, A point

Moi Aolied conceAnlng eZectnlc dleld InZenslilcatZon In the pAesence oi iuAjface AoughneAi and the

auZhou Indicated that that eU^-ct had not been explicitly comldeAed. The lliteneAi weAe AeieAJied to

the subsequent papeA by House, &t al., ioK iuAtheA deXcuIs conceAnlng this seAA.es 0(5 Investigations.

-295-
5 . 10 Investigation of the Damage Properties of Multilayer ^ Dielectric Coatings for Use in High Power Wd:Glass Lasers

C. E. Thomas, B. Guscott, K. Moncur, S. Hildum, and R. Sigler

KMS Fusion, Inc.

P. 0. Box 1567

Ann Arbor, Michigan U8IO6

The development of high-power Nd:glass laser systems has reached a point

where the damage in multilayer dielectric coatings limits the available energy

from the laser. KMSF has initiated a testing program to identify the parameters

that affect the damage levels of dielectric coatings . We will report the measured

damage levels as a function of predeposition substrate cleaning, the dielectric

material used (TiO^ vs ZrO^), the substrate material (Pyrex vs BK-7 or quartz),

and the laser pulse length (80 to U80 picoseconds). Damage levels will be com- pared with the light -scattering properties of the coatings.

Key words: Coating damage; laser damage; mirrors; multilayer dielectric coatings;

polarizers.

1. Introduction

This paper presents experimental data on the laser damage levels of multilayer dielectric

coatings. These coatings are considered critical to the operation of our high-power Nd :glass

laser system since the coatings damage before any other component, and therefore, limit the energy

which can be delivered to our laser-fusion targets. We will report on the current damage levels

of commercial coatings on large substrates obtained from several vendors, as well as on sample

coatings which have been manufactured under contract to KMS Fusion with controlled variation of

parameters to test the influence of substrate cleaning, coating materials, substrate material, and

laser pulse length.

2. The KMSF Laser

A block diagram of the current KMS Fusion Nd:glass laser system is shown in figure 1. Damage

tests are run with this laser using the 90-mm-output beam from the spatial filter.

The pulse shape generated from this system is formed by stacking 30 picosecond Gaussian pulses

with variable time separations. Unless otherwise noted, all damage testing reported here used

six equal -amplitude pulses spaced by I4O picoseconds forming a square pulse envelope about 250

picoseconds long. There is temporal ripple on this pulse, however, so the peak power densities

are somewhat higher than the quoted average power densities.

The importance of multilayer dielectric coatings to the KMSF system is illustrated by table 1,

which summarizes the 91 coated surfaces which are in the main laser beam.

Table 1. Multilayer Dielectric Coatings in the KMS Fusion Laser System

Mirrors 31

Anti -Reflection (AR) Coated Surfaces hk

Beamsplitters 7 Polarizers _9

Total No. of Coatings 91

3. Damage Test Procedure

Dielectric coatings are damage tested by focussing the main laser onto the part as illustrated in

figure 2. The parts are placed in one of three different positions corresponding to beam diameters

of 20, lU.or 10 mm. Each part is oriented at the angle of incidence and polarization for which

it was designed. If the part does not damage at the 20 mm position, it is moved to a higher-intensity

position and a new portion of the surface until visible damage occurs on one shot . Based on the

degree of damage at the lowest power density for which damage was observed, we estimate the maximum

level at which no visual damage would be expected in a single shot . Using different beam diameters

to vary the incident power density avoids the need for large changes in the laser output power which

* The work was supported in part by the United States Energy Research and Development Administration

under Contract E(ll-l) 2709,
could change the laser-beam spatial distribution.

A microdensitometer trace of a typical IR photographic plate which illustrates the laser-beam

spatial distribution at the 90-mm-beam position is shown in figure 3. The "hot spots" seen in this

photograph are characteristic of self -focussing, which occurs in high-power glass-laser beams. The

spatial filter has removed the high-frequency fluctuations. Due to the relatively high intensity

of the lower spatial frequencies in the Fourier transform plane, they cannot be blocked by conventional

spatial filters without substantial erosion of the pinhole. The plasma generated by small pinholes

affects the laser-beam temporal and spatial shape. Thus we currently operate with a relatively large

pinhole and accept the intensity fluctuations shown in figure 3. Folding the density variation

through the H & D curve for the IR plates (measured with 250 picosecond exposures), the peak-to-peak

intensity variation is estimated at 7 dB.

The remaining self -focussing peaks have focal points tens of meters from the spatial filter.

The 10-meter lens used in damage testing effectively shortens this focal distance and figure 4 shows that the intensity fluctuations at the 10 mm test position have increased slightly. From this

trace we estimate that the intensity fluctuates about ^. dB over the beam at the test positions.

Quoted power densities are those calculated perpendicular to the beam axis. The power density

on oblique surfaces will be lower than the quoted values by the cosine of the angle of incidence.

In summary, the laser beam which we use for damage testing has 8 to 10 dB spatial-intensity

fluctuations and 20 to temporal fluctuations. All power densities and energy densities quoted

here are values averaged over the time and space of the laser beam. The peak power density in the

hot spots where the damage invariably occurs- in the coatings is higher than the quoted average

values by 10 to 12 dB.

Damage Test Results

Damage measurements of mirrors, polarizers, and anti -reflection coatings are reported in the

next three sections followed by damage measurements as a function of laser pulse length and the

predamage scattering characterisitcs of the coatings.

k.l Damage Measurements of High Power Mirrors

Our mirror-damage testing activity has been divided into two areas: first, the routine testing

of full-sized purchased mirrors and mirror samples supplied periodically by various coating vendors,

and secondly, the testing of a controlled set of mirror samples coated under contract from KMS Fusion.

In the controlled experiments 6 to 9 two-inch samples were included in each coating run. The

substrate material, predeposition cleaning, and postdeposition processing were varied for each sample.

In the case of the mirror-coating runs, some samples were BK-7 and some were Pyrex, some were cleaned

with hydrofluoric acid, some with nitric acid, and some with boiling water before being coated.

Finally, some of the substrates were baked after coating to increase oxidization.

The damage-testing measurements for two mirror -coating runs are presented in figure 5, The

ordinate is our subjective estimation of the degree of damage after one shot. Medium (M) damage

corresponds roughly to 50^ of the damaged area being destroyed, very light (Vl) damage corresponds

to two or three hot-spot damage sites, and very, very light (WL) damage means that only one faint

blemish is visible to the eye. All damage measurements were made with "p" polarized light at 1+5 angle of incidence. The abscissa of figure 5 is the average power level calculated perpendicular

to the beam axis, as previously discussed.

The data of figure 5 include samples from two coating runs. The first run (solid lines)

used 35 layers of ZrO^/SiO . The second run (dashed lines) used 11 layers of TiO^/siO^. The

general conclusions from these data are

;

1) The eleven layers of TiO /SiO (dashed lines) are consistently better than the 35 layers

of ZrOg/SiOg.

2) The TiO /SiO samples (replotted in figure 6) on pyrex substrates (dashed lines of figure

6) consistently show higher damage levels than similar samples on BK-7 substrates.

3) Baking the sample after deposition (a technique which previously was thought to improve

damage level) had no effect of the BK-7 sample, and actually degraded the pyrex-sample

damage level to the BK-7 levels

.

k) Predeposition cleaning had no significant effect on mirror-damage level.

Prior to this experiment, our standard high powered mirrors incorporated ZrO^ on either BK-7

or pyrex substrates with postdeposition baking. According to the data of figure 5j this combination

of parameters would yield mirror-damage levels between 5 and 10 gigawatts/cm^. We have now changed

all our mirrors to Ti02/Si02 on pyrex without any postdeposition baking with a corresponding

increase in damage level up to 20 to 25 gigawatts/cmS in 250 picoseconds

.

-297-
Damage levels for full-sized mirrors or vendor -supplied mirror samples are plotted in figure

7 as a function of delivery time over the last six months . Four different coating vendors have been

used as indicated by the letters A through D . Each letter generally represents a batch of up

to 12 mirrors. The circled letters denote normal -incidence mirrors; all the others denote k'f mirrors

The subscript "s" indicates a sample; all other mirrors are full-sized mirrors. The letters Z and T indicate whether Zr02 or SiOg was used in the mirror, although we do not always know the

exact composition. There are several interesting trends in figure 7. First, if we exclude the sample

and normal- incidence mirrors, there is a general trend toward higher damage level for the 1+5° mirrors actually used in our system. Thus in January none of our tested mirrors exceeded 20 GW/cm , but by June some of our mirrors were approaching UO GW/cm2.

Another interesting sequence involves vendor "c". In mid-February we tested several small,

normal -incidence , samples, which typically have higher damage levels than 1+5° mirrors. The best

mirror survived over 60 GW/cm2. Next, the vendor supplied a 1+5° sample and the damage level fell

to 35 GW/cm2. Finally, vendor "c" supplied a full-sized mirror on a 20 cm substrate and the damage

level fell again to l8 GW/cm^. Other vendors have exhibited similar sequences.

Notice also that the three Zr02 normal incidence mirror samples supplied by vendor "c" suffered

damage between 1+5 and 65 gigawatts/cm^, while the three TiOo/SiOg mirrors shown in figure 7 suffered

damage between 20 and 1+0 gigawatts/cm^ . This result (Zr02 better than Ti02) is just the opposite

of what our controlled experiments with vendor "a" show. Vendor "a's" production mirrors tested

in May and June show that for their process Ti02 is better than a composite Zr02 design. Thus

a vendor's coating procedure may be more important at this point in time than the actual materials

used.

1+.2 Damage Measurements of Polarizers

In addition to achieving high damage levels, polarizer coating layers must be made more uniform

than mirror layers to produce acceptable optical properties since polarizers operate at the

edge of the absorption band. Our current optical specifications for polarizers include "p" transmission greater than 95^' and "s" transmission less than 1%. The "p" transmission versus angle of

incidence at the center of a typical 12 -inch polarizer is shown in figure 8. At 58° angle of

incidence, the above specification is met. However, the "p" reflection exceeds the maximum allowable

5^ value over most of the polarizer outside a central l-to-2-cm area as shown in figure 9, In

the past, 50 to 60% of our polarizers have been rejected after not meeting the optical specifications.

Substantial improvements in coating uniformity have been made by three of our coating vendors.

The transmission versus angle of one particularly good I8 x 36 cm polarizer is shown in figure 10,

and its positional scan In figure 11. The. entire part meets the specification of less than 3% "p"

reflection, having less than 0.5'^^ over almost all of the useful area.

Initial acceptance is not always sufficient, however. We have observed some aging on all

polarizers which have been retested. The transmission curves consistently shift to longer wavelengths or larger angles of incidence as shown in figures 12 through I5. For the polarizer of figure

12, the "p" transmission at the initial optimum angle of 60° fell from 99'^ to only 78^ after five

months in our laser system. We subsequently found that the optimum angle shifted from 60° to 61+°. Note that the "s" rejection actually improved by a factor of more than 10. The polarizers of

figures 13, lU, and I5 were unused spares; their performance shifted without any exposure to

laser radiation.

Returning to damage considerations, the measured damage levels of sample polarizers from our test program obtained in two different coating runs (all Ti02/Si02and composite Ti02/Si02 plus

Zr02/Si02) are summarized in figure 16, The damage levels are consistently low compared to mirror

damage levels. None of the parameter variations, including a low coefficient of expansion substrate

(quartz), predeposition cleaning, or postdeposition baking had any significant effect on damage level.

The damage levels of vendor -supplied samples and full-size polarizers over the last six months

are shown in figure 17. Only one vendor has been able to supply polarizers with over 20 gigawatts/cm^

damage levels at 2l+0 psec; however, this vendor cannot currently achieve satisfactory uniformity for

parts larger than about 25 cm due to the limited size of his coating chambers. Note that with only

one exception over six months, even the sample polarizers on 2-inch-diameter substrates do not reach

the 20 gigawatts/cm2 level whichwe have found necessary for sustained operation in our laser system.

U.3 Damage Measurements on Anti -Reflection (AR) Coatings

The damage measurements for two AR coating runs are summarized in figure 18. The damage

levels are consistently higher than for polarizers, presumably because fewer layers are used. The

Ti02/Si02 layers (dashed lines) tend to have higher damage levels than the Zr02/Si02j except for

the Zr02 coating on the HF -cleaned substrate. In general, predeposition cleaning seems to raise

the damage level, although the HF cleaning which significantly increased the Zr02 damage level

-298-
showed only a small improvement in the Ti02 run. More data on the effects of acid cleaning are needed

.

h-h Damage Level versus Scattering

It would be desirable to be able to estimate the damage level of dielectric coatings by measuring

some other property such as light scattering. This is particularly true for the coating vendors,

since no vendor currently has the capability of damage testing his own product. Therefore, we measured the back-scattering characteristics of selected coatings which were subsequently damage

tested

.

The scattering -measurement apparatus is diagrammed in figure 19. Each part was illuminated with

3-nim-diameter helium-neon laser beam at 632.8 nm. A lens then imaged the dielectric layers via

the ^+5° back-scattered light onto an RCA 931A photomultiplier tube . For each part we calculated

the ratio of the measured back-scattered power to the ratio of the detected solid angle to I4 tt. For multilayer dielectric coatings, typically this back-scatter ratio is 10"^ at compared to

about 10"^ for a polished glass surface.

The damage level versus scattering for 1+5° mirrors is shown in figure 20. The sample pieces

(X's) showed a strong correlation between low scatter and high damage levels. The scattering must

be below I.3 x 10"'+ for damage levels above 10 gigawatts/cra^. However, at least one full-sized mirror,

.(O's), made by "c" showed a high damage level with relatively high scattering.

There was no correlation between scattering and damage for polarizers (figure 21), although

none of the samples tested had scattering less than I.3 x 10"^. On the other hand, all the AE

coatings scattered less than 3 x 10"5 (figure 22) and all their damage levels were above 10

gigawatts/cm2.

In summary, for three of our vendors a back-scattering level at 1+5° of less than I.3 x lO"'^ relative to an isotropic scatterer appears to be necessary, but not sufficient for damage levels

above 10 gigawatts/cm^. The fourth vendor is able to manufacture high power mirrors with scatter

above I.3 x 10"^ and yet with a damage level above 10 gigawatts/cm^ at 250 psec.

1+.5 Damage Levels versus Laser Pulse Length

Measurement of laser damage level versus pulse length is important both to improve laser

operation whenever possible, and hopefully to better understand the damage mechanisms in the

dielectric coatings. The results of damage measurements at three pulse lengths (80 psec, 2l|0 psec,

1+80 psec) are summarized in figure 23. The increased power damage- level at shorter pulse lengths

has important operational implications for our laser system. Assuming that polarizer damage limits

the available power, we can increase the power delivered to our fusion targets by reducing laser

pulse length.

The data of figure 23 are replotted In figure 24 as energy damage -level (instead of power damage

level) versus pulse length, showing that the damage levels are neither constant -power nor constantenergy phenomena in the 80 to I+80 psec regime.

5 . Summary

Figures 23 and 2l+ also serve as a summary of our current status with respect to damage of

multilayer dielectric coatings. Polarizers are currently our limiting component. To date our experimental program with one vendor has shown no significant variation in polarizer damage level with

the substrate material ( quartz vs BK-7), the substrate cleaning process, the materials used, and

the postdeposition baking process

AE coatings are the next most susceptible component. As laser output energies increase, the

pulse length should increase according to current laser fusion-scaling theories. At longer pulse

lengths the AR coating power damage level will not be adequate, so improvements will be needed.

Our program has shown that substrate cleaning may improve damage levels, although the data are inconclusive

.

Finally, for the three different coating designs tested here, mirror coatings currently have

the highest damage level. This work has shown ways in which these coatings may improved. Our

materials study showed TiO better than ZrO^ with respect to damage for one vendor, while another

had much better damage levels with Zr02. Furthermore, a pyrex substrate can yield higher damage

levels than a BK-7 substrate, while cleaning has no significant effect.

6.0 Acknowledgments

We gratefully acknowledge the support of the following coating vendors who have supplied

-299-
samples and have iniproved the damage characteristics of multilayer dielectric coatings : Coherent

Radiation, CVI Laser Corporation, Spectra Physics, and Optical Coating Laboratory, Inc. We also

wish to thank Verne R. Costich, Gary W. DeBell, and Yu H. Hahn for irany valuable technical discussions

and guidance

.

7 . Figures

|lstg I 0' I 5inm HARD

FORMING APERTURE.

I6mm~j ] 16mm |~ H '6 mm |~V

. ()|^-| 80mm|^64mm"}-«-(TRy«-

[ ISmm^

lll*^ 32mm [-j|^23mm [»-(pcyj—[gmm \

-.

COLLIMATING LENS

SPATIAL nLTER

SOFT

APERTURE

+10m -e

CHIKKXKD-®- DISC AMPS #oi>n-miHii— DISC AMPS MOVEABLE

MIRROR LEGEND

LTSQ- LASER TRiOOER SPARK QAP FR - FARADAY ROTATOR PC POCKELS CELL OSC OSCILLATOR

CILAS VK 640 LASER

Figure 1, Block diagram of KMSF laser system,

June 1975.

High Power Spatial Filter

-90 MM Beam from Laser

Part Under

Test

"Photodiode

Storage Oscilloscope

Figure 2. Block diagram of damage testing

apparatus

.

90 HM Beam Profile ;

Figure 3. Microdensitometer trace across photographic plate of 90 mm beam.

10 w Beam Profile

Figure 4. Microdensitometer trace across plate

of the beam condensed to 10 mm.

-300-
15° nimoit Samples

35 Layers of Z11O2/S1O2

" 10 Layers'of T1O2/S1O2

H - Heavy Damage

M - Medium Damage

L = Light Dakage

VL - Very Light Damage

Pulse Length - 210 ps

X - Control on BK-7

• = Baked BK-7 ®" Control on Pyrex A- Pyrex, HF Cleaned O- Pyrex, HNO3 Cleaned 0= Pyrex, H2O Cleaned 0» Pyrex, Baked

ML

L

VL WL

i

'/•i' -. Hi i

'—7

POWER DENSITY ( nw/em* ) —«-

Figure 5. Damage levels of 45° mirror samples.

= BK-7 Substrates

Pyrex Substrates

19 Layers T1O2 ^ - l.OM^M

Pulse Length - 210 ps

T1O2 15" Mirrors

X Control on BK-7

• - Baked 3k-7 ®= Control on Pyrex A " Pyrex, HF Cleaned

Pyrex, HNO, Cleaned O- Pyrex, Cleaned O" Pyrex, Paked

—7*

—

-^^

/f'

7'

M

ML

L

VL WL

15 20 29 30 35 40 45 POWER DENSITY (GW/cm* ) —

Figure 6. Damage levels of TiOj 45°

samples

.

Mirrors

X - Vendor "X", 15° "P" NrRROR ® = Normal Mirror

X's = Sample 15° Mirror

T = TiOj/SiOj

fM 40

E

2 30

20

2-©©

• Z

D

T-©,®. »-T

AA

A

c

8 ®.

JAN FEB MAR APRIL MAY JUNE JULY AUG

TIME —

Figure 7. Mirror damage level versus time of

manufacture

.

100

80

t= 60

10

20

30 CM Polarizer, ,P129 c

.

Transmi SI ON

I A

50 52 511 56 58 60

Angle of Incidence —»-

62 61

Note: < O.O.U for all angles

Figure 8. Transmission vs angle for 30 cm polarizer P129.

60

50

10

30

20

10 hX \« tLL

1

Sample Polarizer

Beam Position (cm) —».

Figure 9. Positional variation across polarizer

P129o

100

e 60

S10

e - 51"

T. 99.7:

'P' Trahsmi sion

18 »

"S' RANSMIS ION X 1 )—i^J

, —:

,

0 50 52 51 56 58 60 62

Angle of Incidence —»•

Figure 10. Transmission versus angle for

18 X 36 cm polarizer sample.

-301-
2.0

1.0

1

- Refelc ION ACR ISS 18 C 1 Sectio N

Refl

36 c

CTION A

< Sectio

:ross —«

1 0 2 1 6 8 10 12 It 16 IS

Beam Position (cm)

Figure 11. Positional variation across

18 X 36 cm polarizer sample.

100

60

10

20

lAL "P RAhSHISSldN

Transh

RAISHISSIC* AFTER r

SSION X

23 cw Polarizer, P1C7

100 15 io'it:is liter

15 MONTI'S

Trai

5F '^8 60 62 61 66 6g

Angle of Incidence

Figure 12. Transmission versus angle for

polarizer P107.

100

30

60

'!0

20

— 10 CM Polarizer, P113 ~r= > _

.

/ / -Initia /—

/

/

// T ter W MONTHS A

>

f

I

//

/

///

/ T, X 10 ) after

r MONTH'; Hn,t AL X

1

t

i r

0 50 52 5'l 56 58 60 62 Gi{

Angle of Incidence —*-

Figure 13. Transmission versus angle for

polarizer P113.

!60

10

20

10 CM Polarizer.

0^ 1

NITIAL-4

,x 10

.0 g Moi ns

52 " 5i| 56 58 60 62

Angle of Incidence —»-

Figure 14. Transmission versus angle for

polarizer P123.

100

60

10

20

10 CM POLARIZFR, P12')

"

1 r

i-

)- —-<

IwiTIAiL

). 1 ^Tp AFTI R 8 h

rlNITlA . T, X 1 }

i

^ L..-. ^—

(

r

' ^

^

AF TER 8 MC

0

NTH!

0 50 52 51 56 58 60 62

Angle of Incidence

Figure 15. Transmission versus angle for

polarizer P124.

POURIZERS

All T1O2/S1O2

. Composite T1O2/S1O2 ano

ZRO2/S1O2 X - 1.051>(H

Pulse Length - 210 RS

X - Control om BK-7 A - HP Cleaned

- HNO3 Cleaned O- H2O Cleaned • - Baked ®- Control on Suartz

VH

t "

it| MH I M o

u. ML O

u t. u

S VL

'& WL

-p

/

IS 20 29 30 35 POWER DENSITY ( OW/cm* ) —

40 45

Figure 16. Damage levels of sample polarizers.

-302-

J
POURIZERS ^ - 1.061«M X = Full-Size ''olaiiizer from Vehdor "X*

Pulse Length = 2V> ps X's - Sample from Vendor "X"

AR Coatings

90

ui 30

-I

20

S 10

—^

B

B 0

I c

•

B —et A A,

A.

>

C

t

JAN FEB MAR APRL MAY JUNE JULY AUO

TIME

ZRO2/S1O2 — - T1O2/S1O2 A - i.0Wh«

Normal Incidence

X - BK-7 Control a - HF Cleanins

- HHO3 (iEAWNG O- H2O Cleaning • - Baked

15 20 29 30 35 POWER DENSITY t8W/cm2 ) 1-

Figure 17, Polarizer damage level versus time

of manufacture.

Figure 18, Damage levels of sample antireflection coatings.

Coated Sanple

3 MM Aperture

632.8 NH Bandpass Filter

RCA 931' Photomultiplier

Figure 19, Apparatus for measuring coating backscatter.

s<

o

MIRRORS

^ » ACTUAL MIRROR FROM VENDOR A

X SAMPLE MIRRORS

SCATTERING WAVELENGTH 632.8 NM

rD 1 0

t

1

X

—A—

X o

X

K X

I

X

X

SCATTERING RELATIVE TO ISOTROPIC SCATTERERIrIO"")—^

Figure 20, Damage level versus scattering for

mirrors.

POLARIZERS 40

39 M 30

5eW20

u IS

3 10

SCATTERING RELATIVE TO ISOTROPIC SCATTERER(xI0'*) —»-

Figure 21. Damage level versus scattering for

polarizers

.

AR COATINGS 40

35

30

u^25

o

^ 20 u

_i

ixj 15

i

10

2 3 4 5 6 7 SCATTERING RELATIVE TO ISOTROPIC SCAT TERER ( « 10' 5) —»-

Figure 22, Damage level versus scattering for

AR coatings.

-303-
COmEMTS ON PAPER BV THOMAS, oX al

The d^^zc^ ofi tho. iabiqiatz. (fiyuAking weA.e covnidzizd to be mgtigiblz in all caiu ilncz -in aZJL ccaeA iubitwXz ic-cUteA-ing woi ob^zn.vzd to be down by at IzoMt an ondzK o{^ magnitadz beJLoM tho. £eue£

0|5 icatt&AA.ng obieAvzd u)-itk thz coating -in pZacz. TheAe. wa6 4ome dUcLuiiion od thz po44-tb£e caMU

ol the, long tenm aging ef^f^ect which uioi obieAved in tkli KUzoAch. Hypothz!>iJ> which wzKz put ioK.- waAd includzd oxidation OjJ thz iiZm and thz inf^luzncz of, abio^bzd watzA vapon. on thz {,iZm. Thz

obizAvzd ipzctAal ihi{it izzmzd to be comi^tznt with thz hypothziii o{, watzK vapon. in^^luzncing thz

{film pKopzntie^ lathzA than oxidation, and thzAzioiz it woj, piopoizd that thz {^iMm could be izMtoizd

to thziK oniginal design pfiopzAJxei by baking out.

-304-
5 . 11 Correlation of Laser-Induced Damage with Surface Structure

and Preparation Techniques of Several Optical Glasses at 1.06 ym

R. A. House*, J. R. Bettis, and A. H. Guenther

Air Force Weapons Laboratory

Kirtland AFB, NM 87117

and

R. Austin

Perkin-Elmer Corp.

Norwalk, Conn. 06851

An extensive experimental investigation has been conducted to correlate laser-induced

surface damage with surface structural properties in a range of transparent dielectric

materials. Materials investigated were Fused Silica (Corning and Optosil) ; BK-7 (Schott

"P" quality); ED-2, ED-4, and Cervit (all Owens-Illinois). Samples were prepared using

controlled grinding, conventional polishing, ion polishing, and bowl-feed polishing. Major

parameters of interest were the polishing compound used, the rms surface roughness, the

subsurface fracture zone, etching, flame polishing, overcoating with X/2 of the substrate

material, and the possibly deleterious effect of ultrasonic cleaning. The damaging radiation

was a focused 1.06 ym laser beam of about 40 ns width in the TEMqq transverse mode. Target

sites on a given sample were shielded from each other against blowoff and UV radiation.

Key words: Damage thresholds; fused silica; ion polishing; surface damage; surface

roughness; surface structure; thin films; ultrasonic cleaning.

1 . Introduction

The subject of short-pulse laser damage at the surfaces of dielectric materials has received con- siderable attention in the last few years. Many experiments have been performed on a wide variety of

coated and uncoated materials, and because of those experiments three ideas have become increasingly

apparent. (1) There are many extrinsic variables affecting laser surface damage [l-7]\ (2) The

electric field is the primary initiator of the pulsed-damage process [8-12]. (3) There has been a growing tendency to discount surface topography as unimportant - if the substrates have the "best"

optical finish, such as from superpolishing. This is perhaps partly due to a general misinterpretation of Bloembergen' s hypothesis on surface flaws and their limiting sizes.

The purpose of the research reported in this paper is to correlate laser-induced surface damage

with surface and near-surface (subsurface) structural properties in a variety of transparent dielectric

materials. The material of emphasis, however, is fused silica. In order to. perform a meaningful set

of experiments, it was necessary to prepare a series of samples according to specific fabrication procedures. In addition, a large number of physical measurements had to be made, both before and after

laser irradiation. This paper will concern itself primarily with correlations between damage threshold

and roughness, subsurface disorder, and various finishing processes such as etching, overcoating, ion

polishing, bowl-feed polishing, and ultrasonic cleaning. Complete details on sample preparations will

be given, not because of their intrinsic value in fabricating optics, but because of the necessity to completely describe the samples and their resultant surface character.

2 . Experiment

The experimental arrangement used in this research is shown in figure 1 and is described in detail

elsewhere [13]

.

An important feature of this research and that of [13] was the use of a special target mask.

Figure 2 shows the mask mounted on the sample holding ring. The purpose of the mask was to shield target

sites from one another - both optically and physically. In preliminary experiments, it was found that even the smallest breakdowns produced effects to a range as much as 100 times the actual crater diameter. Each aperture in the mask was isolated by means of an annular-shaped piece of printed circuit

design tape. These tape pads were forcibly pressed onto the target face. Use of the breath test

(described below) on several preliminary samples damaged during use of the mask showed that indeed the

various target sites were isolated.

*Based upon portions of a PhD thesis submitted to the Graduate Engineering School of the Air Force

Institute of Technology, Wright-Patterson AFB, Ohio 45433

1. Figures in brackets indicate the literature references at the end of the paper.

-305-
Figure 3 shows the typical form of the raw data for a given sample. The standard experiment was

25 shots, with a new site for each shot. A ratio, R, was calculated for each sample, where

_ lowest energy which produced damage

highest energy at which there was no damage

The reported energy threshold was obtained by joining the lowest damage energy with the highest nondamage energy and reading where the line crossed ordinate zero. Ratio R indicated how threshold-like

the sample was, and it was used to derive error bars for the data.

Two comments are required concerning this method of data taking. Firstly, the described simple

method of obtaining the threshold value was chosen because many other methods of analysis were tried

and all of them were no more accurate or precise. In addition, they were all more time-consuming.

Secondly, 25 shots seemed sufficient for accuracy to about ±20%. In a study such as this, one is

not looking for small differences but rather major differences - factors of 2, for instance. For

differences of this magnitude, a greater number of exposures is not necessary.

3. Damage Indicators

A recurrent problem in recent years has been what constitutes the onset of damage. With or without the appearance of a plasma, it is generally accepted that damage is an irreversible change in the

physical properties of the surface. In this experiment we monitored the appearance of surface

response in four independent ways: (a) direct visual observation of plasma by two observers; (b) a LIS/diffraction method; (c) a phase contrast method; and (3) the breath test.

a. Plasma observation . One observer watched through the sample from the back, just below the

beam line. The other observer watched from the front, through the phase contrast focusing lens.

Consensus on the appearance of a plasma was sufficient for a determination that damage occurred. Disagreement was decided by one of the other methods.

b. LIS/diffraction . This method made use of a He-Ne beam directed through the second turning

mirror and focused on the target by the target lens. Damage was indicated by the appearance of laserinduced scatter - (LIS) - both forward and backward - and by the appearance of diffraction rings in

the transmitted beam.

c. Phase Contrast . This method was developed by us specifically for the purpose of detecting

surface damage. Used in conjunction with a photo-detector, the method can also be used to monitor the

formation of damage in a time-resolved manner as well as the fact that it occurred. The phase contrast

arrangement is shown in figure A. The reflected He-Ne beam is focused onto the phase plate by the lens

The quarter-wave dot on the phase plate retards the zero-order diffracted light (specular component)

relative to the higher-order diffracted, or scattered light, resulting in a diffraction/interference

pattern.

Figures 5 and 6 show typical patterns for bare and overcoated samples. In each figure the

pattern on the left is the virgin site and the pattern on the right is the damaged site. The overall

elliptical pattern shape and background diffraction are due to the fact that the indicent He-Ne beam is

vignetted at non-normal incidence by the target mask.

d. Breath test . This test consisted of steaming the damaged surface with one's breath. The

breath test was used only after the various sample analyses were made and was not a real-time test.

We employed this test because preliminary experiments showed it to be the most sensitive indicator of

the presence of surface change. There was never an ambiguity about whether or not damage had occurredeven if one or more of the other indicators failed. We believe that a damage event alters surface bond

and charge distributions, and hence, locally alters the contact angle for water.

After approximately 6000 shots, our judgment is that the four damage indicators rank in increasing

sensitivity as follows: LIS/diffraction, phase contrast, visual observation of plasma, breath test.

4. Sample Preparations

Samples for this research were fabricated with eight types of surface preparation: conventional

grinding and finishing, overcoating with Si02 and MgF2, flame polishing, etching, subsurface disorder,

ion polishing, bowl-feed polishing, and ultrasonic cleaning. Sample dimensions were 1.5 inches in

diameter and either 0.375 or 0.300 inches in thickness. All samples, except those which were flame

polished, were prepared to a flatness figure of X/5 or better. Substrate materials used were (1) fused

silica: Optosil I and Corning 7940; (2) BK-7: Schott "P" quality; (3) ED-2, ED-4, and Cervit: OwensIllinois .

-306-
a. Conventional grinding/ flnlshlnR . These were the samples used for the basic study of threshold

vs. roughness. They were subjected to a controlled grinding procedure (see below) to minimize the

possible effects of subsurface disorder. Then they were conventionally polished to the desired roughness values and overcoated in the center with a 0.5-lnch diameter, opaque thin-film dot of molybdenum.

A representative number of samples from each batch were measured for surface roughness by the fringes

of equal chromatic order (FECO) technique, and all samples were measured for surface scatter by the

total Integrated scatter (TIS) technique.

The controlled grinding procedure was followed to minimize the final expected thickness of the

subsurface disorder region. At each stage, a thickness of material at least three times the previous

grit size was removed. The final grinding stage was a 3 )jm grit stage. The rule of three derives

from the empirical rule of thumb that a given grit size produces a fracture zone of thickness about

three times the grit size. Hence, each stage in the controlled-grlnding process attempts to remove

the fracture zone produced by the previous stage. The last five grinding stages used alumina grits.

Following controlled grinding, conventional polishing was done on Swiss pitch using milled barnesite and distilled water. Normal cleaning procedure after polishing was a rinse in distilled water,

followed by vapor degreaslng in freon. All samples on which molybdenum was deposited were cleaned by

a mild sputter-etch with argon before the molybdenum was sputtered on. The sputter etch conditions were 2 mln., 20 ym argon, and 150 W.The tnolybdenum deposition conditions were 7 min. , 20 y m argon,

1200 VDC target voltage, and 180 VDC substrate bias.

b. Overcoating . A set of samples with a range of surface roughness values was overcoated with

half-wave thicknesses of Si02 and MgF2. These samples were control ground, conventionally polished,

overcoated with a molybdenum dot, and measured by FECO and TIS. Then they were overcoated at 250° C by E-gun deposition. For SIO2 the vacuum was 5 x 10~^ torr and the deposition time was 5 mln.; for

MgF2, the vacuum was 5 x 10"^ torr and the deposition time was 10 mln. The starting materials were

MgF2 - new, Patlnal, 99.9% pure; and Si02 - CERAC. Following the dielectric deposition, another TIS

measurement was made. The difference in the two TIS measurements yielded information about film

scatter. The half-wave thickness was chosen to facilitate comparison of breakdown intensities with

those obtained for bare substrates.

c. Flame polishing . Following the controlled grinding and conventional polishing to desired

roughness values (determined by FECO), the samples were cleaned and then flame-polished. This was done

by spinning each sample in a glass-blower's lathe and playing an acetylene torch over the surface until

the surface softened. Then the samples were overcoated with a molybdenum dot and measured by TIS.

Obviously, this was not a very quantitative process and the resultant surface figure was poor. In addition, although the surface scatter and roughness were extremely low near the center, the scatter

Increased radially outward. Data were taken only on the inner ring of target sites for these samples.

These samples were expected to exhibit higher thresholds than any of the other samples.

d. Etching . A series of samples identical to the surface roughness samples was subjected to an

etching procedure after conventional polishing and FECO: (1) a wash in Solveno and acetone, (2) a wash in Orvus and a rinse with distilled water, (3) an ultrasonic cleaning in benzene, (A) a 10-minute

etch in a 10% nitric acid solution and distilled water, (5) a rinse in distilled water, with the

sample being blown dry. Then the samples were overcoated with a Molybdenum dot. These samples were

expected to be higher in threshold than the surface roughness samples because of an expected reduction

in surface contamination and disorder.

e. Subsurface disorder . These samples were fabricated to test the effect of the disorder region

(caused by grinding) which lies below the Beilby polish layer. The samples were prepared by^ interrupting the controlled grinding procedure at given stages and then polishing to better than 20 A surface

roughness. Then the usual FECO, molybdenum dot, and TIS were done.

f. Ion-polishing . While the above samples were all fused silica, the ion-polished and bowl-feed

polished samples were sets of five different materials: fused silica, ED-2, ED-4, BK-7, and Cervit.

The ion-polished samples were control-ground and conventionally polished with rouge, with the conventional polishing removing about 10 of materia^ j T^e ion polishing conditions were as follows:

70° angle of incidence; doses of 10 /cm^ and 10 /cm ; energies of 20KV and 30KV; and ions of H, Ar,

and Xe. These samples were overcoated in the center with aluminum and measured by TIS.

g. ' Bowl-feed polishing . These samples were control ground and then bowl-feed polished using rouge.

A total of four polishing times for each material type were used. Then the samples were coated with

aluminum and measured by TIS.

h. Ultrasonic cleaning . Impelled by recent findings of Saito [14] and Gibbs and McLachlan [15] on the deleterious effects on surface topography produced by ultrasonic cleaning, a rudimentary

101 325

NOTE: 1 torr= Pa.

-307-

760
experiment on some samples of fused silica was performed. Two of the previous roughness samples, two

samples provided by R. Austin (of Perkin-Elmer Corp.)* and three samples provided by H. Bennett (of

the Michelson Labs, NWC) were ultrasonically cleaned in two commercial solutions (Micro and Radiac)

for varying periods of time. Then the thresholds of the controls and ultrasonically cleaned samples were determined. The ultrasonic cleaner was a 250W Bransonic 52.

5. Experimental Results

Figure 7 shows the results for the conventional grinding/finishing samples. ^ is the incident

energy threshold, in mJ, and a is the surface roughness. Because V^is proportional to the electric

field (the irradiation geometry and material index of refraction were constant in each experiment), the

plot shows the existence of a close log-log relationship between the electric field and the surface

roughness. ^

This is not surprising since one may consider that surface scattering is a measure of the tendency

of surfaces to "absorb" laser light - by field enhancement at surface flaws, plasmon excitation, etc.

One assumes that

scat . (5.1)

where m>0 and I ^ is the scattered light intensity. From scattering/roughness theory [16] it is scat

known that for sufficiently small roughness, a, one can write

R ^ „ I ^ AtoV

scat scat ~/ I

, /c o\

where R is the reflectance of the surface for scattered light

scat " R^ is the reflectance of a completely smooth surface of the same material

I is the incident intensity,

o

o For X = 1.06 ym, the condition O < 250 A appears to be sufficient. This leads to

(5.3)

where E is the electric field in MV/cm. Hence, one expects

k/^ % const. (5.4)

In considering eq. (5. A), one can assume that the smoothest possible surface will have a limiting roughness of several X (e.g., the average interatomic distance). Then the threshold should

approach the bulk threshold. Fradin [17] has recently reported the bulk threshold^ of fused silica to be 5.2 MV/cm. From the density of fused silica^ an average atomic spacing of 3.57A is obtained. The

lattice parameter for crystalline quartz is 4.9A. Using these values, two data points are plotted as squares in figure 7. The straight-line fit is for the five roughness samples only. It is encouraging to note that both points for the bulk breakdown fit the surface data reasonably well.

The precise value of m for a given set of samples depends, of course, on the actual physical state

of the surface. But the relationship should hold for a wide variety of surface types. In figure 8 are plotted the data for the Si02-overcoated samples, in figure 9 are the data for the MgF2-overcoated samples

and in figure 10 are the data for the flame-polished samples. The line-fit in figure 9 includes only the

E-gun films. The^ significant observation concerning figure 10 is that all four samples were well under

Bloembergen' s lOOA limit, and yet the thresholds were strikingly ordered by the roughness values. The

result seems surprising since Bloembergen [10] pointed out that surface flaws and inclusions with

typical dimension lOOA or less should not affect the damage threshold.

Unfortunately, the etched samples did not closely fit relationship eq. (5.4), although the bverall

trend of the data was similar. The results are presented in table 1. We do not yet know the explanation for the lack of a similar fit the simple relationship of eq. (5.4).

-308-
Table 1. Etched Samples: Breakdown field vs. Surface Roughness

a(A) E(MV/cm)

42

82

243

325

1.11

0.34

0.41

0.29

In table 2 is presented a process vs. threshold correlation for various roughness ranges, using

data from the conventionally-finished, overcoated, flame-polished, and etched samples. For the flamepolished samples, the indicated roughness range denotes the roughness before the flame-polishing stage was carried out. The final roughness values (inferred from TIS measurements) of the flame-polished

samples are given in parentheses, after the breakdown field values.

Table 2. Process vs. Threshold (MV/cm) for Various Roughness Ranges

o 14A Range

Conv . : 1 . 60

Si02 : 0.89

MgF2 : 0.64

40A Range

Flame: 2.12 (17. 7A)

Etch : 1.11

Conv.: 0.98

Si02 : 0.66

MgF : 0.34

80A Range

Flame: 1.73 (60. 6A)

Si02 : 0.42

Etch : 0.35

MgF2 : 0.26

140A Range 240A Range 330A Range

Conv.

MgF2

0.36

0.23

Flame: 1.78 (56A)

Etch : 0.41

Conv.: 0.32

Si02 : 0.29

Flame: 2.24 (13. 3A)

Etch : 0.29

Conv.: 0.24

SiO„ 0.24

Two sets of subsurface disorder samples were irradiated and the results are given in table 3. There is no obvious relation between threshold and roughness of the form eq. (5.4), nor is there

an obvious telationship between threshold and grit size, g. There does seem to be a reasonable fit

of the data to a relationship of the form

m n ^ff const. (5.5)

Where m - 0.5, n = 0.023, however, the analysis has not proceeded sufficiently to warrant the assertion

that eq. (5.5) is meaningful. Comparison of the breakdown values for the 20 um grit samples with

those for the other samples (and keeping in mind the general effect of roughness on threshold) , it is seen that the 20 ym values are abnormally low.

Table 3. Results for the Subsurface Disorder Samples

g (ym) o(A) E (MV/cm)

3 15.9 1.60

5 10.1 1.87

5 10.1 1.76

12 9.15 1.85

12 9.15 2.19

20 10.9 1.23

20 10.9 1.42

30 8.87 2.10

30 8.87 1.76

-309-
The results for the ion-polished S2imples are given in table 4. The first five entries show the

material effects, since all these samples were identically polished. The fifth through eighth samples

show the effects of varying the dose and energy on the material fused silica. Samples five and eight

through twelve show the effects on fused silica of varying the ion and of proceeding from low doselow energy to high dose-high energy. Comparing only the electric field data f ©refused silica with the

reference sample (conventional finish, barnesite polished, 1.60 MV/cm, and 13.75A roughness), we see

that samples 5, 6, 8, 11, and 12 all had slightly lower thresholds. Both argon-polished samples had

slightly higher thresholds. The more interesting result is the comparison of the quantities

(aO'5)(E) for the reference and ion-polished samples. These quantities constitute a "roughness

correction," based on the results of figures 7,8, and 9. Only sample 5 is lower than the reference in this

comparison. In fact, for each ion used, the progression from low dose-low energy to high dose-high

energy leads to a decrease in the corrected field strength. The argon-polished samples showed an

increase in corrected field strength of 88%, with the Xenon-polished samples having an increase of

36%, and the hydrogen-polished samples having an increase of 31%. These comments, however, are not

conclusive yet, because we have not yet examined the samples for probable refractive index change

[18, 19].

Table 4. Results for Ion Polishing. All Samples Were Previously Rouge Polished.

Sample Material Ion

_2

Dose (cm ) Energy (KV) a (A) E (MV/cm)

1 BK-7 10^^ 20 18 1.43

2 Cervit 10^^ 20 0.55

3 ED-4 10^^ 20 47 1.18

4 ED2

10^^ 20 0.84

5 FS 10^^ 20 14 1.32 4.94

6 FS 10^^ 30 25 1.53 7.65

7 FS ioi« 20 11 2.34 7.76

8 FS ioi« 30 25 1.39 6.95

9 FS Ar-^ 10^^ 20 31 2.00 11.14

10 FS Ar-^ ioi« 30 36 1.68 10.08

11 FS Xe+ 10^^ 20 36 1.34 8.04

12 FS Xe+ 10^« 30 31 1.46 8.13

Ref. (FS) FS 13.75 1.60 5.93

Table 5 shows the results for the bowl-feed polished samples. Except in the case of Cervit, the

rouge-polished samples are significantly lower in threshold than the ion polished samples. Since

roughness values for Cervit are not available, a much larger roughness for the ion-polished case could

easily explain the apparent discrepancy. Note the absence of significant variation in threshold as a function of polishing time for the fused silica. Note also that the corrected values are significantly lower than the reference corrected value.

Table 5. Results for Bowl-Feed Polished Samples.

Samples were Polished with Rouge.

Sample Time (hrs) a (A) E (MV/cm) aO-^E

BK-7 40 30 0.93

Cervit 46.5 0.55

ED-4 45 40 0.77

ED-2 48 0.62

FS 48 13 0.86 3.10

FS 72 13 0.90 3.24

FS 96 11 0.87 2.88

FS 120 18 0.86 3.65

Ref. (FS) 13.75 1.60 5.93

310-
Table 6 gives the results for the ultrasonic cleaning experiment. Sample PE#3 was originally

cleaned for 3/4 hours, damaged, cleaned for an additional 16 3/4 hours, and damaged again. The rougepolished PE samples showed no significant variation even for long cleaning times, even though the

roughness variation was a factor of 5. The rouge contamination probably dominated. However, the two

barnesite-polished samples showed a possible threshold reduction after only a short time in the

cleaner. The dramatic threshold reduction occurred for the HB samples - 21% for Micro solution and

32% for Radiac solution. These reductions in electric field correspond to energy reductions of 38%

and 54%.

Table 6. Ultrasonic Experiment on Fused Silica

Sample Polishing Time (hrs) Solution a (A) E(MV/cm)

PE//1 C,R 0 25 0.73

PE//3 C,R 3/4 MI 'V25* 0.72

PE#3 C,R 17 1/2 MI 119 0.75

077 C,B 0 16 0.87

082 C,B 1/2 RA 0.80

HB#5 C,B 0 1.74

HB//6 C,B 16 3/4 MI 1.37

HB#7 C,B 17 RA ^-20* 1.18

C = Conventional

B = Barnesite

R = Rouge

MI = Micro

RA = Radiac

* Approximate, not measured yet

Finally, figures 11-16 show the types of surfaces obtained for the conventionally finished,

etched, flame-polished, subsurface, ion-polished, and bowl-feed polished samples. ^Note that on even

the smoothest surfaces can often be found defects with dimensions greater than lOOA.

6. Conclusions

a. Roughness . The existence of a relationship of the form of eq. (5.4) is strong evidence

of the importance of the electric field in surface breakdown and of the enhancement of the electric

field at surface scattering sites. The relationship quite obviously holds over as much as two orders

of magnitude in roughness, and in particular it holds as the roughness approaches zero. However, the

fact that roughness may be small does not imply that Bloembergen's surface flaw model and its con- clusions are necessarily wrong. This is because a->-o does not necessarily imply the absence of defects

with typical dimensions more than lOOA (see figures 11-16)

.

b. Etching . The nitric acid etching procedure yielded an improvement in electric field threshold

of about 21%, but the magnitude of the improvement could possibly have been adversely affected by the

ultrasonic cleaning stage of the procedure.

c. Flame polishing . Although bad for general surface figure, flame polishing did drastically

reduce surface roughness and yielded a dramatic increase of as much as 830% in electric field

threshold. This occurred for the largest initial roughness values. At the low roughness end the in- crease was 115%. Possible explanations would include superficial smoothing by glass flow, defect

annealing, and contaminant removal due to vaporization.

d. Overcoating . Both types of films, Si02 and MgF2, generated significant reductions in

threshold. Si02 showed an average reduction of 21% in electric field threshold and MgF2 showed an average reduction of 54%. Correlations of the thin-film data with index inhomogeneity , film scatter,

etc., have not yet been done. So the explanation for threshold reduction in these films has not yet

i been determined

.

e. Subsurface disorder . There was only a weak correlation of threshold with roughness and grit

size. The thresholds were generally higher than for the minimum disorder samples. Since measurements

of the actual extent of the subsurface disorder region have not been completed, we conclude the following: If large subsurface zones of disorder do exist, then the effect of this disorder on threshold is

secondary to the effect of the topography of the exposed finish layer.

f. Ion polishing . The results for ion-polishing at high energies were somewhat erratic, at least

in terms of electric field breakdown strength only. Three of the hydrogen-polished samples and both of

-311-
the Xenon-polished samples showed a decrease in threshold compared to the reference samples. Making

a roughness correction to the field strength values yielded a more ordered picture - as pointed out

in the discussion of table A. The corrected breakdown values showed that all three ions used resulted

in improved damage thresholds. This is in keeping with Giuliano's results on sapphire [7] and with

the expectation that ion-polished surfaces should have a reduced level of mechanical damage (due to previous grinding and abrasive polishing) and a reduced level of surface contamination. This last

comment is especially telling when it is recalled that all of the ion-polished samples had previously

been rouge polished. Rouge contamination is known to reduce the threshold for breakdown at 1.06 ym

[1, and below]

.

g. Bowl-feed polishing . In general, a comparison of tables 4 and 5 shows that the presence

of rouge contamination in the bowl-feed polished samples caused a reduction of threshold. For fused

silica the reduction averaged about 45%, and there was no dependence on polishing time, within

experimental accuracy, in the range from 48 to 120 hours.

h. Ultrasonic cleaning . The results were somewhat erratic, but generally did show that for some conditions of duration and type of cleaning solution considerable threshold reduction occurred. An

interesting conclusion is suggested by comparison of corrected field values for the PE samples. The

corrected fields are 3.65 MV/cm, for PE #1 (0 hours) and 8.18 MV/cm, for PE //3 (17.5 hours). The first

value correlates with the results for rouge polished fused silica (table 5), and the second value

correlates with the ion-polished fused silica (table 4). Not only did the ultrasonic cleaning in Micro

solution lead to an increase in surface topography (i.e., an increase in roughness), but it apparently

brought about a drastic reduction in contamination by rouge.

i. In general . The overall general conclusion of this research is that the electric field at breakdown is intimately related to the surface roughness - from large roughness values down to and

including the smoothest surfaces. The corollary is that a complete statement of conditions in a surface damage experiment must include the value of the surface roughness.

Additional measurements, including Auger spectrometry, SEM, Nomarski microscopy, surface

topography, ellipsometry , and d£image morphology are currently being conducted on the samples. In addition, ultrasonic cleaning experiments of a more quantitative nature are being planned.

7. Acknowledgments

The authors would like to express their deep appreciation for fruitful discussions and various

sample analyses to the Dr's. Bennett of the Michelson Labs, to Dr. Harold Posen and co-workers at AFCRL, to Dr. John Reichert of Texas Tech, and to Dr. Robert Hengehold of the Air Force Institute of

Technology. In addition, we would like to acknowledge the technical support of Perkin-Elmer Corp.

and the Materials Branch of the Air Force Weapons Laboratory.

8. References

[1] Boling, N. L., Dube, G., and Crisp, M. D., | NBS Special Publication No. 387 (U.S., GPO,

Wash., D.C., 1973) , p. 69.

[2] Ringlien, J. A., Boling, N. L., and Dube, G. , 1 Appl, Phys. Lett. 25, #10, 598 (1974).

[3] Fersman, I. A. and Khazov, L. D., Sov. J. Opt. | Tech. 37, 627 (1970)

.

[4] Khan, J. M., NBS Special Publication No. 372

(U.S. GPO, Wash., D.C., 1972), p. 75.

I [5] Davit, J., J. Appl. Phys. 39, #13, 6052 (1968).

[6] Swain, J. E., ASTM Special Technical Publication No. 469 (ASTM, Phila., Pa., 1969), p. 69.

[7] Giuliano, C. R., NBS Special Publication No.

372 (U.S. GPO, Wash., D.C., 1972), p. 55.

[8] Crisp, M. D., IEEE J. Quant. Elect. QE-10 , #1, 57 (1974).

[9] Newnam, B. E., NBS Special Publication No. 414

(U.S. GPO, Wash., D.C., 1974), p. 39.

[10] Bloembergen, N., Appl. Opt. 12, #4, 661 (1973).

-312-

[11] Bettis, J. R., Guenther, A. H., and Glass,

A. J., NBS Special Publication No. 414 (U.S.

GPO, Wash., D.C., 1974), p. 214.

[12] Fradin, D. W. and Bass, M., Appl. Phys. Lett.

23, #11, 604 (1973)

.

[13] Bettis, J. R., House, R. A., Guenther, A. H.,

and Austin, R. , This Symposium.

[14] Saito, T. T., Private communication.

[15] Gibbs, W. E. K. and McLachlan, A. D., This

Symposium.

[16] Bennett, H. E. and Porteus, J. 0., JOSA 51,

123 (1961).

[17] Fradin, D. W. and Bass, M. , Appl. Phys. Lett.

22^, #4, 157 (1973).

[18] Bayly, A. R., Radiation Effects 18, 111 (1973)

[19] Presby, H. M. and Brown, W. C, Appl. Phys.

Lett. 24, #10, 511 (1974).
[Note added In proof: Subsequent to the presentation of this paper at the symposium, results of

further measurements of roughness on these samples became available. TIS measurements were made at Perkin-Elmer and at China Lake, FECO measurements were made at China Lake, and Tallystep measurements were made at AFWL. In general, the various methods of inferring roughness showed the trend exhibited

in figure 7. However, there tended to be a wide variation in inferred roughness values for any given

sample. The data for this paper made use of the original FECO (PE) numbers, except in the case of the

flame-polished samples. For these samples, only the TIS values represented the final surface topography. The FECO measurements are the only ones considered to be consistent over our whole roughness

range. TIS measurements were done in the visible and are not considered reliable above about 100 X

roughness. Tallystep measurements become suspect below about 100 A. In any event, the important con- clusion still holds. Surface breakdown does definitely correlate with at least some measure of surface

scattering - be it TIS values or inferred roughness. However, because of the difficulty in determining

and specifying the true roughness over the total range of values, the numerical slope (ln\/? vs Ina)

should not be considered absolute. Recall that TIS and FECO estimates are in good agreement at low

surface roughness. What obviously is called for are better scatter-roughness determinations together

with auto-covariance measurements of each of the differently processed surfaces as well as a sensitive measure of their impurity content and a detailed determination of surface structure (crystallinity

,

etc. ) o ] 9. Figures

OSC

Figure 1. Experimental arrangement for damage studies. BS denotes beam splitters;

Ml and M2 are dielectric turning mirrors, optimized at 45° incidence angle

for 1.06 pm; Li is the target lens, of focal length 513.5 mm; L2 is the

phase-contrast focusing lens, of focal length 250 mm; PC is the phase

contrast plate (see figure 4 for details).

-313-
(a) (b)

Figure 2. Target mask, mounted on sample holder ring. In actual practice, the positioning screws are nylon - metal screws are shown for clarity. Small apertures (for target sites) are

2 mm in diameter. Large central aperture permits access to metal dot on sample. See

figure A for view of mask on sample. Black rings around each aperture in (b) are sealing

pads, described in text.

a. Front face - toward laser beam. b. Back face - pressed against sample surface.

1 \ I I I r

^^^^^ O 00 OO ® (S> O 9

\

RATIO 0.692

46.74

(NO) \ O O OOOOOD oooo o

J I L

0 10 20 30 40 50 60 70 ENERGY f INCIDENT ON SAMPLE, mJ

Figure 3. Typical raw data plot

-314-

I
-315-
(a) (b)

Figure 6. Phase contrast for overcoated surface.

a. Before damage. b. After damage.

Figure 7. Electric field threshold vs roughness for the conventionallyfinished samples. Circles with error bars denote our data.

Squares denote Fradin's value for bulk breakdown, using assumed

roughness values (see text) . Data points are labeled by roughness values and breakdown electric fields (in parentheses). The

equation is the fitted straight line - fitted only to our data.

Electric fields are macroscopic (include the index of refraction)

.

-316-
Figure 8. Electric field threshold vs roughness of the substrate, for the

half-wave SiOa-overcoated samples. Data points are labeled by

roughness values and breakdown electric fields (parentheses)

.

Electric fields are macroscopic (include the index of refraction),

The equation is the fitted straight line.

-0,4

2.0

"o

' (G.B7MV/CM)

(0.77MV/CM)

P(0.64MV/CM)

O SPUTTERED

EVAPORATED

O E-GUN

^0.4549^ = 7.55

( + 9.9%)

9\(0.34i,W/CM)

,{0.26MV/CM)

<i(G.22MV/CM) _

6.0

Figure 9. Electric field threshold vs roughness

of the substrate, for the half-wave

MgF2~overcoated samples. Data points

are labeled by roughness values and

breakdown electric fields (parentheses).

Electric fields are macroscopic (include

the index of refraction) . The equation

is the straight-line fit to only the

E-gun data points.

-317-
Figure 10, Electric field threshold vs roughness

for the flame—polished samples. Data

points are labeled by roughness values

and breakdown electric fields

(parentheses) , Electric fields are macroscopic (include the index of re- fraction). The equation is the fitted

straight line.

Figure 12. Fused silica, etched, a = 82 A.

Figure 13. Fused silica, flame-polished.

Before flame-polish, a = 310 1.

After flame-polish, a = 13.3 A.

-318-
-319-
COMMENTS ON PAPER BY HOUSE, eX al

Noim E^oMn iKom th& Lawn.mcz LivzAmoKe. LaboAjoutofiy madz iome. 6-Lgvii{j-icant commznti conceAnlng thz

potiiking p^ocdii and iti -cni^^uewce on obieM.ve.d damage thAuholdi . He polntad out that \)zny o^tzn

thz potiihing pn.oceAi tzndi to mooth oveA ^ub^uAj^ace damagz compacting the. iuAj^ace layzK and thii

e.{j(,zct AJi moit pfLonounccd uiith coauzt gKlti. He atio cauutioncd the Li(>tzneAA> to be eoviei^u^ in the. oie o(i imaZZ boMl-^ezd machA.neA. In thoie machines w-ith imatZ iQyieAvO'iu the. potUihing pKoceAi

pHoceedi too lapidly and thzKz Ld a iign^^icant chance that the potij>hing compound bz (,ui>zd i

thz i>uA(,acz. A May oi getting oAound thli li to -incAza^z thz izieAvo-Oi o{^ the KzcAAcuZjJuting ilwuiy

and to CAJiculatz thz ituHAy ui-ing a tubing pump io that u)hzn thz iluAAy AJi changzd thz tubing aJUo

can bz changed. Hz {^uAtheA zmphnM-izzd thz impoKtancz of, thz choicz of potiihing compound in

dztzAmining thz iuAfacz pKopeJitizi , It iA of coufue MztZ known thaX thz uiz of jewzlz'u' Kougz

Izadi to an zaiily damaged iuAface. BA.om pointed out that he hoi obieAved a 6uAface of bzttzA

quality obtained with a ziAconium oxide iluAAy than uiith a ceAtim oxidz ilufOiy, at Zzait with the

potiihing compoundi iuppZied f^om one manufactuAzn., Twithzi dijicuiiion zniued conczAning thz

degradation of optical figuAe in the flame poliihing pioceJ>.!>. It wa6 brought out in thz dticuiiion

that fZamz poitihed gZa^i iWifaczi aAz among thz moit damagz iz^iitant onz can obtain, but that thz

typical flatnzM availabZz, for zxamplz, in fZamz poZiihzd micAoicopz iZideA which oAe commeAcialZy

availablz i^ of thz ordzA of 5 to 10 micAOM, fZatneM variation, ThzAz -ci furthzA zvidencz that

a iiuAfacz roughnzM ii the determinant in damage properties rathzr than thz prziencz or abizncz

of iubiuAface structure or damagz in the material, Thii i& comiitent with the obieAvation zoALizr

reported that in ultroAonic cleaning thz iurfacz Zjxyzr may be disrupted revealing iubiuAface

itructurz, and thzrzby rzducing the damagz thrzihoZd, However, it is also important to remembzr

that iubiurface damage ihould bz kzpt at a minimum in any optical iurfacz in ordzA to znsure good

optical prcpzrties quite apart from questions of ZaszA damagz.

-320-
6.1 Picosecond Breakdown Studies: Threshold and Nonlinear Refractive

Index Measurements and Damage Morphology'''

W. L. Smith, J. H. Bechtel, and N. Bloembergen

Gordon McKay Laboratory

Harvard University

Cambridge, Mass. 02138

Single picosecond pulses from a well-calibrated Nd:YAG laser have been used to study

the bulk dielectric properties of 14 transparent solids. Measured values of the breakdown

thresholds and nonlinear refractive indices are presented. Photomicrographic data of the

damage morphology yield information concerning spatial and temporal microplasma growth

from initiating electrons, and concerning the spatial density of initiating sites. The data

indicate avalanche ionization is the damage mechanism. Preliminary results from a breakdown study at 0, 53 /xm are discussed.

Keywords: Avalanche ionization; dielectric breakdown; inclusions; laser damage threshold;

morphology; nonlinear refractive index; picosecond laser pulses.

1. Introduction

We wish to present results which have been obtained in a study of laser-induced damage in transparent solids with picosecond pulses. Aspects of the 1. 06 jim investigation which have been completed

to date and which are discussed include breakdown threshold measurements, nonlinear refractive index

measurements, and microscopic observations of the damage morphology In addition, comments

concerning a similar study at the second-harmonic wavelength, 0. 532 /im, are included.

2. Experiment

The experimental technique that we have used is one first mentioned by Zverev and Paskkov in

1969 [2], and later by Fradin in 1973 [3]. The rationale of the technique will now be described.

Consider a low power, diffraction-limited, Gaussian laser pulse of wavelength X with an initial

intensity e" ^ radius p. A corrected lens of focal length f will focus this pulse, as shown in figure 1, to a spot having a radius w equal to Xf/2TTp. If the pulse power is P and we define a focal area A = TTw^, then the focal plane intensity distribution is given by

I(r) = I e-<^/^)\ (1) ^ ' max

^max power limit will equal P/A.

This simple picture is modified, however, for all bulk breakdown experiments at wavelengths

shorter than roughly one micron by the effects of self-focusing. The characteristic quantity for this

self-action, the critical power for self -focusing, is given by

P = cX^/32n^n, . (2) cr 2

where is the nonlinear refractive index. For an input pulse power less than Pcr» focal _^

situation is modified as shown in figure 2. The low-power focal area A is reduced to A' = A(1-PP^^),

thereby increasing Imax • Thus we arrive at a relation for Imax which may be written

p-^ = A"^ + P"^ , (3) max cr

of the form y = ax + b. Therefore, a plot of the reciprocal measured power necessary to cause breakdown, as a function of reciprocal focal area, should yield the breakdown electric field

= t^max/Vo^]'^'

from the reciprocal slope I^ax- In addition, the vertical-axis intercept yields the nonlinear refractive index by eq, (2).

A diagram of the experimental apparatus is illustrated in figure 3 and will be described briefly.

The Brewster-ended Nd:YAG mode-locked (Kodak 9860) oscillator is constrained to oscillate In the

TEMqo mode by an intracavity aperture, A single pulse is excised from the train by a Pockels cell,

and is amplified as needed up to approximately 300 MW by two single-pass Nd:YAG amplifiers. A

Tektronix 519 scope (CRO-1) is used to insure that only data from clean, single pulses are used. The

photodiode PD-3 measured the energy incident on the lens L, and was calibrated against two Eppley

1: Figures in brackets indicate the literature references at the end of the paper.

-321-
Laboratory thermopiles. The pulse spatial profile was measured exactingly and determined to be

Gaussian to the e"^ points. The average temporal duration of the laser was carefully measured by

two-photon absorption and second-harmonic autocorrelation techniques to be 30 ± 6 psec. The temporal

p rofile fits well to a Gaussian shape. Because of inherent small fluctuations in the pulse duration of

the laser, a nonlinear-optic monitor of pulse duration was devised and added to the laser system [4]. The fundamental pulse energy content signal, from PD-3 in figure 3 is used in conjunction with

the second-harmonic pulse energy content signal, <^gj^> from PD-2 to record the ratio

R = ^li ^ u

" (5)

f sh

for each laser pulse. This relation is easily derived by using the known Gaussian temporal profile of our laser output. The quantity q is a collection of constants for a given experiment, involving phasematching angle, etc. After each breakdown experiment, involving about 150 shots, the average of the

individual pulse R values was normalized to a 30 psec. FWHM duration, thereby providing individual

pulse duration values. This technique was important in obtaining accurate breakdown threshold

measurements.

Three lenses were used in the experiment for each material. The focal lengths were 0. 5, 1. 0,

1. 5 inches, with focal areas of 34, 48 and 108 square microns, respectively. All aberrations were

carefully considered and the focal area for the smallest focal length lens was measured to verify the

calculated area. Each material was tested at least twice. The procedure involved firing pulses into

each sample while recording various signals and visually monitoring damage spark occurrence as the

pulse power was brought up through the threshold level. The sample was moved between shots so that

each pulse sampled a new voliime element. Coordinates of each probed site in the crystal were recorde

and later a high-power microscopic examination was performed to verify uncertain sightings. The

reproducibility of the data when probing different specimens and different volume elements in a single

specimen indicates that a property characteristic of the bulk material is observed. Typically 50 shots

were used with each lens. The data were then plotted as shown in figure A, an example of KCl. The

sharpness of the threshold is typical of the behavior encountered in these experiments. The observed

uncertainty in the breakdown threshold field was less than or equal to the experimental uncertainty of ± 15%. Data from such graphs were then plotted as shown in figure 5, for this same material. The

desired quantities are extracted as discussed above.

3. Breakdown Threshold Results

First let us consider the pulse duration dependence of the breakdown field in NaCl at 1. 06 /im.

Figure 6 displays measurements at four pulse durations made by other investigators and the measure- ment made in the present study at 30 psec. The solid curve in figure 6 is the result of an approximate

theoretical calculation discussed in [5], The present work verifies the behavior of the threshold in the picosecond region,and in addition, makes available similar data for five other alkali-halides, as shown. The reciprocal pulse duration can be related to the avalanche ionization rate as will be discussed later.

Next, let us examine the variation of the 1. 06 jxm breakdown threshold for picosecond pulses across the periodic table. Our results are presented in table 1, We have tested 14 materials: 9 alkal halides and 5 other materials of optical importance. The threshold field varies by a factor of over 6, from 3. 4 MV/cm for RBI to 22. 3 MV/cm for KDP. The latter value is so high as to approach the

value thought necessary "to induce multiphoton ionization [6]. Figure 7 displays the trend of the normalized thresholds across the periodic table. Previous data from DC [7], and Q-switched CO2 [8], ruby

[9], and Nd:YAG [lO] laser damage studies are presented for comparison. Note that all the data from

a single experiment are normalized to the NaCl threshold of that same data set. The systematic variation throughout the alkali-halides is the resxilt of the systematic variation of the parameters of the

avalanche: the band gap, phonon densities of states, and the deformation potential. The similar chemi

cal dependence of the DC and optical thresholds is a compelling statement in favor of the intrinsic ava- lanche ionization nature of the data.

Table 1. Rms breakdown electric field strengths (Ej^ uncertainty, ± 15%; field ratio

uncertainty, ± 10%).

E^(MV/cm)

E^/ENaCl

b b

E (MV/cm)

E

/ENaCl

b b

NaF NaCl NaBr KF KCl KBr KI

10. 77 7. 34 5. 67 8. 34 5. 86 5. 33 5. 87

1. 47 1. 00 0. 77 1. 14 0. 08 0. 73 0. 80

LiF Rbl ED-4

glass

YAG:Nd Fused

Si02 CaF2 KDP

12. 24 3. 40 9. 90 9. 82 11. 68 14. 44 22. 28

1. 67 0. 46 1. 35 1. 34 I. 59 1. 97 3. 04

-322-
4. Nonlinear Refractive Index Results

For the breakdown measurements that we performed at 1. 06 ytm, the input power was typically a few tenths (but never greater than 0. 9) of the critical power, Per- Due to the \^ dependence of P^v

it is clear that accurate measurements of P^j. are of increasing importance to breakdown experiments

for X< 1.06 fim, which we are undertaking presently. We have measured for 12 materials and the

data are presented in table 2. Due to our experimental conditions that the laser pulse duration is much

shorter than the electrostrictive response time (~ 10-9 sec), and that the materials tested are ionic

solids, our n2 values are of electronic origin only. Values of n2 obtained by others with different

techniques and from theory are listed for comparison. The smallest value of n2 we measured was

that of NaF, a factor of 15 less than that of KBr. Agreement with the measurements of others is reasonable in almost all cases (LiF excepted), considering the rather large experimental uncertainties listed.

The expected chemical-dependence in the alkali-halides is observed.

1

3

Table 2. Measured values of nonlinear refractive index (in units of 10' esu).

Mat'l Present Work Other Work

Experiment^ Theory^

I 11 III IV V VI I II ± 50% ± 300% ± 20% ± ~ 20% ± 7% ± 20% ± 10%

NaBr 9. 6

NaCl 6. 5 3. 0 4. 2 2. 0 4. 9

NaF 0. 95 0. 71 0. 37 0. 86

KI 11. 2 7. 1 20. 7 KBr 14. 2 6. 8 7. 3 3. 2 9. 0 KCl 3. 3 3. 1 4. 8 2, 3 4. 8 LiF 2. 4 0. 82 0. 54 0. 92 0. 76 0. 82

CaF2 2. 8 0. 92 1. 13 1. 1 1. 1 Si02 1. 4 1. 8 1. 82 1. 00 3. 6 2. 9

KDP 3. 6

ED-4 2. 1 2. 60 1. 73

YAG:Nd<^ 3. 5 4. 51 4. 10 3. 16 3. 47

I. Values from third-harmonic generation, Wang and Baardsen [ll];

Values from three-wave mixing, Maker and Terhune [12];

Values from three-wave mixing, Levenson and Bloembergen [13];

Values from intensity-dependent ellipse rotation, Owyoung [l4];

Value from time-resolved interferometry. Bliss et al . , [15];

Value from time-resolved interferometry, Moren et al. , [16].

II, III. IV.

V.

VI.

I.

II. Calculated from generalized Miller's rule, Wang [17].Calculated from Wang's rule, Wang [17].

^ All experimental values in this row, with one exception, are for pulse propagation along a [ill]

direction, as in the usual laser rod configuration; in these cases the measured nonlinear susceptibility is

[3) 2 ^''^iiii ^^1221

,(3)

2X

(3)

) 1122'

,(3) The value in the column designated III is for the single element Xii iii however. The present work

investigated a YAG:Nd crystal; all other values in the row pertain to undoped YAG crystal.

5. Damage Morphology

To date, not a great deal of use has been made of bulk damage morphology for obtaining data on damaging processes themselves, except in the area of inclusion damage. Early workers were quick

to recognize inclusion damage by its vestige of specks randomly placed in the focal volume.

Regarding the needle-like tracks of more intrinsic origin, however, the morphology has been

employed only as a simple yes-or-no indicator of the occurrence of damage. One reason for this

situation is that a great degree of control and calibration of the laser source is necessary to produce

damage very close to threshold. Precise control in prior years was not always achievable for reasons such as (even slight) mode beating. Damage tracks from TEA CO2 laser pulses are of large (~ . 5 mm)

dimension even at threshold, due to plasma absorption of the energy in the long trailing edge of the

TEA pulse. Information about the plasma growth mechanism is therefore obscured or lost by the severe disruption of the focal region.

An intriguing question is whether it is possible to capture information about plasma growth inside a solid material by allowing it to develop only through early stages, and not to the point of

-323-
excessive disruptive heating. Current estimates of the density of free or slightly bound electrons

which may act as nucleation centers for the avalanche process are in the 10°^- 10^^ per cm3 range [6], Larger values due to bleachable F -center concentrations have been discussed [18]. If we assume a starting center density of lO^^/cm-^, say, then we obtain a linear density of one nucleation center

per micron. Shoxold an avalanche begin from such a center, it would need to produce a light scattering

region of roughly 5000 A = d' diameter in order to be observed microscopically. If we use

d(t) = 2r(t) = zViDt (6)

as a simple random-walk spatial growth estimate, then we require

5x lO'^cm ~ d' ^ d i i ~ lO"^ cm (7)

i

in order for the individual microplasma remnants to be observable.

Using D ~ 1 to 10 cm^-sec"^ under high electric fields [19], we conclude that for a pulse duration of a few tens of picoseconds, we perhaps wo\ild be able to see microscopically the vestiges of

individual, resolved microplasmas . From such data we should be able to extract information regarding the actual density of starting sites, spatial and temporal plasma growth, etc. Figure 8 is a high-resolution optical photomicrograph of a damage track produced in NaCl. The

damaging pulse had a maximum electric field at the focal point, K^, of 8. 7 MV/cm, 18% greater

than the measured threshold field of 7. 3 MV/cm for NaCl. The total power in this pulse was 86. 3 kW,

It was focussed by a 1/2" focal length lens. The pulse propagation direction is from

left to right. The photograph was taken with blue light using a Leitz Ortholux microscope with a 0. 4

N.A. objective "and 340 power total magnification. The calculated resolution (.61X/N, A. ) is ~ 0. 7|i0

The scale lines on the figure are 10 (im apart, and were transferred to this photograph from one of an objective micrometer which was made under identical magnification conditions.

One observes in the figure discrete specks, which are the vestiges of microplasmas resulting

from avalanche ionization. We believe that each of the microspots originated from a single electron.

From the linear density of microspots, one obtains a volume density of nucleation centers of about

2 X lOll cm"^, which is consistent with the estimates mentioned earlier. To our knowledge, this

measurement technique represents the only way to determine the actual density of avalanche nucleation

centers in a material. This density will, of course, be a function of incident pvilse intensity and frequency [18].

The next feature of the damage track to be discussed is the diameter of the microspots. The

largest diameter in figure 8 is about 1. 5 jim; the smallest measurable diameter, limited by microscopic resolving power, is about 0. 5 fim. We have not attempted a detailed calculation for an expected

diameter. Several difficulties would beset such a program. In general, the same set of difficulties

that at present prevent detailed calculations of other aspects of avalanche phenomena in solids also

plagues this problem: incomplete knowledge of high-field collision rates, diffusion parameters, etc. In this preliminary report we simply note that the observed average diameter of about 1. 0 fxm is in

agreement with the very crude diffusion model estimate of 2V4Dt 0. 8 (Xm, taking 10 cm^/sec for

the diffusion coefficient and 38. 7 psec, the 1/e duration of the intensity, for the diffusion duration.

The last feature of figure 8 to be discussed is the perimeter of the track. The results of several

calculations, using the parameters of the pulse that produced the damage track of figure 8, are shown

in figure 9. In this figure the point z = r = 0 corresponds to the focal point in the NaCl material.

Distance away from the focal point in the laser pulse propagation direction is plotted on the horizontal

axis, and distance away from the focal point, in the plane normal to the propagation axis, is plotted

along the vertical axis. Therefore, the plane of figure 9 is also that of figure 8, but with modified

z and r scales. Isothermal curves for a temperature of 1000°C are plotted for three heating mechanisms: joule heating by avalanche produced electrons, and metallic inclusion heating from spherical

inclusions of two different radii a. The dashed curve is the actual damage track perimeter from

figure 8. The isotherms for inclusion heating cases are derived from work of Sparks and Duthler [20].

The curve designated by a = 100 A represe^nts a cross section of a three-dimensional surface, within

which any sodium inclusions of radius 100 A would heat the surrounding NaCl to a temperature of

1000°C. The calculation was performed using thermal and optical parameters of Na as the inclusion

material, although the results would not differ greatly for other metals (Al, Ca, Fe, Mg, Cu) which

are possible impurities in NaCl [21]. All temperature dependence of the thermal parameters was

neglected. In the case of a 100 A inclusion radius, the radius is of the order of magnitude as the

1,06^ absorption 'depth of ~ 150 A in Na, and smaller than the radiation wavelength in the host.

Therefore, a model assuming uniform heat generation within the inclusion is an appropriate approximation, and results in an inclusion temperature rise above ambient temperature [20] of

T = Mt_, (8)

^ 4rra^Cj.

Here I is the incident laser intensity; t represents elapsed time from the beginning of the laser

pulse at t = 0 when the temperature is taken as 0°C. Cj is the heat capacity per unit volume of Na

-324-
(1, 23 J/cm'^°C). The absorption cross section a is derived from Mie scattering theory and is given

by

3 2 3

1 2tt n-T u Fa

CT = ^ , (9)

p

for metals with 2TTnjTa/X « 1. In the above expressions c is the vacuum speed of light, is the

linear index of refraction of the host, X is the vacuum wavelength, w is laser angular frequency,

(J is the Na plasma frequency (~ 9x10'-^ sec"'- [22]). F is^ the electron relaxation frequency which is approximately 1. 3 x lO-'-'^ sec" for an Na particle of 100 A radius [20]. For the purpose of the inclusion heating calc\ilations, the laser temporal profile was taken to be a square wave having duration

equal to the 1/e duration of the actual intensity profile (2t = 39. 7 psec for the track under discussion)

and having intensity equal to the actual peak intensity. Using the above expressions and that for the

electric field in the focal region, one arrives at an equation for the temperature attained by an inclusion

located at some point (r, z) in the focal voliome:

4^ 2^ . -{r2/[p^l-PP;^^)(l-Hz^z-^^^)]}

18 n^ €_TU FE e ^1 = i——2 (10)

In this equation eg the free space permittivity, E£ is the maximum rms electric field at the focal

point, p is the intensity 1/e spatial radius in the focal plane (3. 3 ;jm), and Zr£; is the focal Rayleigh

distance of 2TTp^np./X for the electric field (98. 5 ^m). The curve in figure 9 for a = 100 A was

found by setting t\ in eq. (10) eq\ml to lOOO'-'C and solving for^ r as a function of z. The melting

temperature of NaCl and Na is SOO'-'C. This curve, for a = 100 A, is for the most easily damaging

inclusion size for our experimental conditions [20].

o The curve designated by a = 1000 A in figure 9 was computed in a similar manner. However,

because the radius a exceeds the skin depth, a model assuming a delta function source heating a slab

of metal was employed. The quantity F is 5x 10^^ sec"l in this case [20]. The equations for the case of a = 1000 A which correspond to eqs. (9) and (10) are

'^2 ^ 3/2 2^^* 172 T7T ' ^^^^

^ 2n^'^ a'^((Cj^K^'^ + (C^Kj)^'^

3.1ld. 7 7 ^ 7 7 4 2,^2 -tr'/[p^i"Pp;M(i+-'^-i^)]}

6v2t Orre -u F E. e

2 „l/2 2..„ „ ^1/2,,^ K .1/2,,,, 2 -2 , ^'"^ ^p(^ H

^ I I^ )^ ^RE^

In the above equations Cj^ is the host heat capacity per unit volume (1. 85 J/cm^°C) and K^.

(1. 34W/cm°C) and Kj^ (0. 065W/cm°C) are the thermal conductivity of the inclusion and NaCl host,

respectively. Equation (12) was^ likewise set equal to 1000°C and solved for r as a function of z to produce the curve for a = 1000 A in figure 9. The curve identified by avalanche ionization in figure 9 is from a calculation given by

2 2 -Cr'/[p'(l-PP;r)(^^-'^RE)" oo e tE e r

-(t'/r)^

Cj^md + t. T^)(l + z zj^g)

where the number of electrons per unit volume is

t I

N(t) = Nq I e^tE(t')}

_ (^4)

The ionization rate r|{E(t)} above was obtained from the data in figure 6. The ionization rate is related to the pulse duration by t] = l^^p^j^j^, as discussed in [5], B'or the

numerical integration of eq. (O), ritElt)} was represented by the function

LogTi{E(t)3 = 3. 97 (Log E) - 15. 62 , (15)

-325-
which fits the experimental data well for E between 1. 0 and 9. 0 MV/cm. In eq. (13), m and e are the electron mass and charge, and t is the effective momentum-change relaxation time (taken

here to be w" ^ [6]). The quantity Nq in eq, (lA) has before been taken to be the initial number of g

free or almost free electrons per unit volume, in a continuum approximation, and was found earlier ' from figure 8 to be about 2x 10^^ cm"-^. However, in light of the new data of figure 8 showing discrete microplasma structure, it is useful to interpret Nq differently. The average volume of the

single microspots in figure 8 is about 10"^^ cm . Therefore, if we consider all the energy absorbed

by the microplasma to be deposited into that volume of NaCl, neglecting all heat loss processes into

the surrounding NaCl, then the temperature reached by that NaCl sphere is given by eq. (13). In this

spirit, Nq was given the value lO'^^ cm" in the computation for the avalanche ionization isotherm.

That isotherm was produced by a numerical integration over time of eq. (13), at various values of the

coordinates (r, z). The agreement between the observed damage perimeter and the calculated avalanche ionization

perimeter for a temperature rise of 1000°C is quite good, both in shape and extent. The two inclusion

isotherms predict that any metallic inclusion of the appropriate size within the designated curve should

have been melted along with the surrounding NaCl, and, therefore, should have produced a microscopically observable signature. However, in only one track in fifty in this sample was a speck sighted far

outside the avalanche ionization curve. This observed fact, and the additional observed fact that the

illustrated damage track appearance was observed in all the tested materials, lead us to believe that

inclusion effects in these experiments are negligible. Certainly crystal imperfections in the form of

inclusions with dimension of a few angstroms up to perhaps 1000 A exist. Nevertheless, these experiments imply that their densities are small enough that their role is of no significance greater than

perhaps as shallow traps for initial electrons.

6. Picosecond Breakdown at 0. 53fJ.m

Preliminary breakdown experiments have been performed at 0. 532|im with picosecond pulses

obtained by harmonic generation. The tested materials were LiF, NaF, NaCl, KCl, KBr, KDP

CaF2 and fused quartz. A great difficulty inherent in this experiment is the effect of self-focusing on

the focal area. Analysis of the data has only begun. However, it appears certain that the breakdown

thresholds at 0. 532i-im exceed those at 1. 06|am, by a factor larger than that due to pulse duration

dependence of the threshold alone. In terms of the familiar qxialitative expression for the frequency

depende nee of the threshold [6],

E(w) - Vl + w^T^ E^^ , (16)

it appears that at 0. 532Mm the breakdown threshold departs measurably from the DC limit. These

remarks will be made quantitative in the near future. The threshold increase for the doubled

frequency implies that multiphoton ionization does not take a dominant role in the 0. 532[j.m damage, as that effect would produce decrease in the threshold. It is possible that multiphoton ionization plays an

initial role in the avalanche process which goes undetected due to the avalanche process becoming

dominant after the first electrons are produced [l8]. The damage morphology at 0. 532M.m exhibits the

same features as the 1. 064g.m morphology. A

7. Summary H

Dielectric breakdown threshold measurements have been presented for 14 transparent solids.

The measurements were made with single, 30 psec average FWHM duration, 1. 064^im laser pulses,

in a manner which corrects for the effects of self -focusing. The threshold values are those of an I

avalanche ionization process. Nonlinear refractive index measurements, also obtained in the breakdown experiments, were presented. Photographic data from a microscopic study of the damage

tracks in NaCl was presented. Information from this data regarding the number of initiation sites

for plasma growth, approximately 10^2 per cm^ in the investigated sample, was discussed. The

damage perimeter was also shown to indicate that avalanche ionization is the damage mechanism.

8. Acknowledgments

The authors are grateful to Patrick Heck of Owens-Illinois, Inc. , for supplying a sample of

ED-4 glass, and to S. Maurici for his skillful sample preparation. Helpful discussions with Prof.

Eli Yablonovitch of Harvard are most appreciated. The authors are indebted to Prof. Lance Taylor

of Harvard for his generous loan of photomicrographic equipment and collaboration.

Research supported in part by the National Aeronautics and Space Administration (under

contract NGR-22-007- 1 17) and by the Joint Services Electronics Program (under contract

N00014-67-A-0298-0006),

-326-
9. References

A detailed account of the experiment reported here, except for material concerning

damage morphology, is to be published in

the Physical Review B- 15 , July (1975).

G. M. Z verev and V. A. Pashkov, Zh. Eksp.

Teor. Fiz. 5J_, 1128 (1969) [Sov. Phys. - JETP 30, 616 (1970)].

D. W. Fradin, IEEE J. Quantum Electron.

QE-9 , 954 (1973). Note that Eq. (2) in this

I

.

reference should read = Io(l-P/Per^ 1

W. L. Smith and J. H. Bechtel, submitted

for publication in J. App. Phys. The proportionality expressed by Eq. (5) was noted by

M.J. Brienza [Appl. Phys. Lett. 10_, 221

(1967)] and used by C. V. Shank and E. P.

Ippen [Appl. Phys. Lett. 24, 373 (1974)] in

conjunction with a cw mode-locked dye laser.

D. W. Fradin, N. Bloembergen, and J. P.

Letellier, Appl. Phys. Lett. 22^, 635 (1973).

N. Bloembergen, IEEE, J. Quant. Elect.

QE-10 , 375 (1974).

A. vonHippel, J. Appl. Phys. 8^, 815 (1937).

E. Yablonovitch, Appl. Phys. Lett. 495

(1971).

D.W. Fradin and M. Bass, Appl. Phys. Lett.

22^, 206 (1973).

D.W. Fradin, E. Yablonovitch, and M. Bass,

Appl. Opt. rz, 700 (19 73).

11] C.C. Wang and E.L. Baardsen, Phys. Rev.

185 , 1079 (1969), and Phys. Rev. Bl_, 2827

(1970).

12] P. D. Maker and R. W. Terhune, Phys. Rev.

137 , A801 (1965),

13] M. D. Levenson, IEEE, J. Quant. Elect.

QE-10 , 110(1074), andM.D. Levenson and

N. Bloembergen, Phys. Rev. B 10 , 4447

(1974).

14] A. Owyoung, IEEE, J. Quant. Elect. QE -9 , 1064 (1973).

15] E.S. Bliss, D.R. Speck and W.W. Simmons,

Appl. Phys. Lett. 2_5. 728 (1974).

16] M.J. Moran, C. She, and R. L. Carman,

IEEE, J. Quant. Electron QE - 1

1

, 259 (1975).

17] C.C. Wang, Phys. Rev. B2, 2045 (1970).

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

Appl. Phys. Lett. 26, 150 (1975).

19] F. Seitz, Phys. Rev. 2i. 1376 (1949).

20] M. Sparks and C.J. Duthler, J. Appl. Phys.

44, 3038 (1973).

21] Personal communication with D. Cope,

Harshaw Chemical Co. , Solon, Ohio.

22] C. Kittel, "Introduction to Solid State Physics, " 4th ed. (John Wiley and Sons, New

York, 1971).

10. Figures

in limit P- 0

Figure 1. Illustration of the external focusing

of a Gaussian laser pulse into a medium.

-327-
A' = A(l-f

)

'cr

Figure 2. Illustration of the external focusing

into a material of a Gaussian laser

pulse having power P that is nonnegligible compared to Pqj-. Self-focusing reduces the focal area from A to A'

,

thereby increasing I^ax*

z

So h— 3 J = BREAKDOWN THRESHOLD a 1 ENERGY (98 8kW)

^ © ©

m ©

PULSE ENERGY (f^J)

1 28

1 1 1 3.0 1 3.2

1 1 1 3.4 3G 3 8

BREAKDOWN

• I

1

KCf

f = l.5.n o

z

Figure 4, Results of breakdown experiment in KCL

for f = 1.5" lens. The three circled

points resulted from atypically short

pulses.

46

AMP 1

PC

LTSG

S Nd; YAG MLO

AMP2

CRO-1 PD-2

PD-1 1KDP

BS-1 BS-2

a

PD-3

L S

BS-3

Figure 3. Schematic diagram of the experimental

arrangement, MLO, mode- locked Nd:YAG

oscillator; PC, Pockels cell; LTSG,

laser-triggered spark gap; AMP, Nd:YAG

amplifier; BS, beam splitter; F, filter

PD, biplanar photodiode; CRO, oscilloscope; KDP, second-harmonic generation

crystal; A, double Glan-prism variable

attenuator, L, focusing lens; S,

dielectric sample.

S

Figure 5. Results of breakdown experiments in KCl

for three lenses. The solid line is a least-squares fit to the data. The slope

equals the reciprocal threshold breakdown intensity and the vertical-axis

intercept equals the reciprocal critical

power for self-focusing.

30.0

25.0

20.0

15.0

100

50

SLOPE (=>

—I 1 [—

DAMAGE INTENSITY = 1.35 x 10 W/cm^

DAMAGE FIELD = 5 86 + .88 MV/cm

KCf

C:> P- =3221161 kW

01 .02 .03

A"' (m"!'^)

-328-
10

12

10

U 10

10 o

iro

CO

X

5

10=

10'

1 1 1 1 1 1 MI 1 1 1 1 1 1 1

1 1 1 1 Milt

: O DATA FROM REF 17

-

I a KI -

] A KBr

• KCI

- X NaBr

: + NaCI

-

: * NaF • /

-

r

ff)

II

1

1

1

1

1

1

1

1

1

@@©®

1

1

1 1 1 1 1 1 1 1

1

1 1 1 1 1 M 1 1 1

1 1 1 1 1

1

Figure 6. Functional relationship between the breakdown threshold electric field strength and

the pulse duration. The solid curve Is a semiemplrlcal prediction for NaCI,

discussed In [5].

1.0 10 100

RMS OPTICAL ELECTRIC FIELD (MV/cm)

2.5r

2.0 -

Z 1-5 o2 J3

<

t0.5 -

+ lOSMfi, psec (Present Work)

X 1.06^m,nsec ^ .69 ^m, nsec

llO.e.nsec

*

I D.C

X

Nal NaBr

(0)

NaCf NaF

Figure 7. Experimental trend of the breakdown field

through a) the Na-halldes and b) the Khalldes. Shown for comparison Is data

from previous studies: (x) , [10]; (A), [9];

(•), dc data from [7], 10.6 pm data from

[8], In the figure, nsec denotes a pulse

duration on the order of nanoseconds, and

psec, picoseconds. Each set of data Is normalized to the field for NaCI of that same data set. Error bars for the present

work are shown displaced to the right.

i.5r

1.0 -

0.5 -

tl

_1_ KI KBr

(b)

KCI KF

Alkoli-halide Thresholds Normalized to NoCf Threshold -329-
Figure 8. Photomicrograph of a damage track in NaCl produced by a 32.2 psec (FWHM) , 1,06 Vim, 86,3 kW laser pulse. The pulse was focused to a 1/e intensity diameter in the focal

plane of 6.6 ym. The maximum rms electric field reached at the focal point was 8.7

MV/cm, 18% above the threshold field. The pulse propagation direction is from left

to right. The scale marks are spaced by a distance of 10.0 pm.

INCLUSION

a= lOOi

300 400

Figure 9 . Calculated isothermal curves for three

possible heating mechanisms for the

damage track shown in figure 8. The

z axis is laser propagation axis; the

point z = r = 0.0 is the focal point.

The actual damage perimeter is indicated

by the dashed curve.

COMMEMTS OH PAPER W SMITH, at

(See comrmnti on papeA by Spa/iki, wkick {^ottom]

-330-
6 • 2 Current Status of Electron-Avalanche-Breakdown Theories

M. Sparks

Xonics, Incorporated, Van Nuys, California 91406

Evidence is presented that current theories of electron-avalanche breakdown in dielectrics are inadequate to explain existing data or to be predictive. Specific diffi- culties include the following: (1) The theoretical value of the ionization frequencies are

too small, by tens of orders of magnitude in some cases, to explain the experimental

damage results even when the large initial conduction electron densities in (4) below

are assumed. (2) The theoretical result for the frequency dependence of the breakdown electric field Eg ~ ( 1 + w2t2 )1/2 disagrees with experimental results. The

explanation in terms of an anomalously small electron relaxation frequency is inconsistent with the value of T required to explain the magnitude of Eg, with calculated

values of T (by two orders of magnitude), and with the difference between the dc and

10. 6 ^m experimental values of Eg. (3) The temperature dependence of Eg is incorrect. (4) The assumed value of the electron density n^, = lO^^lOlOcm'S required to initiate the avalanche is in conflict with results of photoconductivity measurements.

Bounds on n^, set by estimates of the value of the electronic conductivity indicate that the probability of finding an electron in the focal volume during the pulse is less than

10"", possibly much less, in some cases. (5) The theories are not predictive. Tentative new theoretical results hold promise of explaining existing experiments and being

predictive.

Key words: Electron-avalanche breakdown; dielectrics; frequency dependence;

magnitude.

1. Introduction

The motivation for the study of electron- avalanche breakdown was an attempt to predict the magnitude of

the breakdown electric field, Eg, and its temperature and frequency dependence and to predict whether the effect

would be intrinsic or extrinsic, all in the vacuum-ultraviolet region. It was found that current theories could not

afford this information. The rather serious problems with current theories are discussed, and a tentative theory

of avalanche breakdown that appears to alleviate the difficulties is presented briefly.

The practical importance of breakdown in limiting the performance of material in numerous applications

and fundamental interest in the subject have stimulated enormous interest in electrical breakdown in dielectrics.

The laser studies of avalanche breakdown in the last five years have supplied invaluable new experimental data.

No attempt is made to review the literature on electron-avalanche breakdown, which extends from the early studies

of A. von Hippel [1 ] in 1931 to the present. In fact, in the first volume of Annalen der Physik in 1799, A. Van

Marum [2 ] reported the laboratory observation of electrical breakdown in glass. There are a numh)er of textbooks

and reviews. The newer of J. J. O'Dwyer's [3] books, the reviews by N. Bloembergen [4] and N, Klein [5], and

the report by D. Fradin [6] should lead the reader into the literature.

At least part of an apparent disagreement in the literature and among investigators in the field as to how

good the current theories of electrical breakdown in solids are, stems from what is expected of the theory. If the

goal of the theory is to explain the magnitude of the breakdown field, Eg, within an order of magnitude or so, then

any one of many theories that have been proposed will suffice. This is, in fact, to be expected since small changes

*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.

-331-
in the value of the electric field give rise to large changes in the values of the physical quantities of interest, such

as the ionization frequency. The electric field appears in an exponent in most theories, and even a factor-of-two

change in E can change the probability for damage by tens of orders of magnitude in some cases. For example,

see figure 2 in section 3. This situation is analogous to the fact that the value of the temperature n*st be known more accurately than within a factor of ten in most cases, as in thermal emission over a barrier or in the human

environment, where a one percent change is large , a ten percent change is intolerable, and a factor of two is

exocrematory.

If, on the other hand, the goal of the theory is to explain the magnitude of avalanche ionization frequency,

co^,in the expression for the conduction -electron density, n^ = n^^ exp(co^t), to within an order of magnitude at

the observed value of E or to explain the dependence of Eg on the laser frequency co, temperature T , laserpulse duration t , or material parameters, or to be predictive, then current theories are inadequate. The problem is more serious than simply an inability to calculate the values of or Eg to within some required accuracy,

as it was found in the early attempts to understand the existing experimental data that even in being overly generous with the values of all parameters, current theories still failed. If a theory cannot explain the experimental ^

results to within the estimated errors of the experimental and theoretical results, the theory will be considered

as unsatisfactory. A factor of three to ten difference in Eg is not sufficient.

2. The Avalanche Process

The simplest current explanation of electron -avalanche breakdown is as follows. In the presence of an electric field E and electron-phonon collisions, an electron drifts in the direction of E gaining energy £ as it goes.

The electron absorbs energy from the electric field, as illustrated schematically in figure la. The rate of energy

gain from the field is given by the well known relation

(d£\ = ^^^k^^ , ' ^2 1)

where co is the frequency of the electric field and T^^ is the electron relaxation frequency for large-angle scattering. When the electron attains sufficient energy £j , it cair excite another conduction across the electronic energy

gap from the valence band. Repetition of this multiplication process continues to increase the number of electrons

until breakdown occurs. Breakdown has been assumed to occur when this process increases the energy density, n , from an initial value of 10 - 10 cm to 10 cm , as discussed at the end of section 3.

For the dc case illustrated in figure 1, electron-phonon collisions prevent the electron from accelerating

very rapidly to high energies by changing its direction of travel every seconds on the average. If the electron were to go without suffering a collision for a long time, its energy would be increased more efficiently. Such a

"lucky" event, shown at L Ln figure la, is discussed in section 6. Small-angle collisions are less effective in

inhibiting the rapid build up of energy. However, both small-angle and large- angle collisions reduce the electron

energy since phonons (having energies fico^ ) are excited in the scattering process. In figure 1, the build-up of en- ergy impeded by the large-angle scattering and the energy loss, are visualized individually in the (a) and (b) parts

of the figure. In passing, notice that the electron drift velocity v^ = ^E , where n

= eT^/m is the mobility, is determined by the increase in the component k^ of wave vector k in the direction of E , which is related to the

curvature of the trajectory in figure la.

The rate of loss of energy to the lattice phonons is

(de/dt)^ = -RWp/T^ , . (2.2)

-332-
where iia) is an average phonon energy and T, is the time constant that includes small- and large-angle scatter- P ^

ing. The time constants Tj^ and are calculated in [7] . Equating (2. 1) and (2.2) and solving for E = E^^^

gives the value

of E at Miiiich the net rate of change of £ is zero. For E > E^.^^, the electron gains energy on the average, and

vice versa. If the electron gains energy on the average at each collision, it will accelerate until £ = £j , Thus,

the simplest criterion for breakdown is that E > Eg with

Eg = EgL . (2.4)

The energy dependence of the relaxation frequencies will be considered in section 8, Values of Eg calculated from

(2.4) and (2.3) typically are a factor of three to ten times greater than observed values. For example, at 1,06 pm

with [7] Tj^ = 1.36x10 sec and T^^ = 8,77 x 10 sec for sodium chloride at room temperature, (2.3) and

(2.4) give

Eg = 9. 0 MV/cm ,

which is a factor of 4.3 times greater than the experimental value [6] of 2,1 MV/cm,

3. Ionization Frequency

Current theories do not explain the experimentally determined values of the ionization frequency co^ , as

will now be shown. The ionization frequency can be obtained from the model in section 2 by integrating

f = (ff) -(f) • E L

for £ = 0 to £j , which corresponds to t = 0 to tj , where tj is the time required for the electron to gain energy

£j in the presence of the field and of phonon collisions. The values of (d£/dt)g and (d£/dt)j^ in (3, 1) are

given by (2. 1) and (2.2). Assuming that Tj^ and Tj^ are constants, the integration gives

e^T

m ( 1 + CO Tj, )

Since the number of conduction electrons doubles every tj seconds, n^ is equal to

t /t,

n,= n^o(2)P

which can be written as

n^ = n^ge , (3.3)

-333-
with

2 2

'^c = 0-6^3

^ —2 . for E > EgL . (3.4)

I L E„

,

EL

and oj =0 for E < E^, . Forr7]-Rco = 1/40 eV, £, = 8eV, T, = 8. 77 x lO"-^^ sec, T. = 1. 36 x lO"^^ sec

C tj L. p 1 K (see figure 5 helov/), and X = l,06fxm, (2,3) gives E^j^ = 9.04 MV/cm. With these values and E = 1. 1 E^,^.

(3.4) gives oj^ = 5. 19 x 10^^ sec ^. The agreement with experiment is poor, as illustrated in figure 2 and discussed below.

In comparing the result (3.4) for o)^ with experimental results of the dependence of Eg on the laser-pulse

duration t , the relation h)etween co and t is needed. This is commonly obtained [6] by assuming that n is P "^n 1A^^ o 18-3

increased from an initial value of 10°- 10 cm to a final value of 10 cm , as already mentioned. Taking the

9 -3 18 -1 logarithm of (3. 3) with n „ = 10 cm and n (t ) = 10 cm gives the required expression

o) = t'-^ £n (n /n „) - 20.7 t"-^ . (3.5)

c p c cO p

The experimental values [6] of tp^ for sodium chloride at room temperature and 1.06 Jim are shown in

figure 2. The dc results, which will be discussed in section 6, are included here for comparison with the 1.06pn

results. The theoretical result for t^ from (3.4) and (3.5) is plotted as a solid curve. The theoretical results of

Holway and Fradin [8] obtained from a numerical solution of a transport equation, with one parameter adjusted,

2 also are shown on figure 2. The agreement between experiment and either theory is poor. At E„ s= 2 MV/cm , - 1 8-1

(3.4) gives tp =0, compared with the experimental value of ~10 sec . Extrapolating the Holway- Fradin

curve to E = 2 MV/cm (which surely gives an inaccurate value for such a great extrapolation) gives a value of

tp^ that is 18 orders of magnitude smaller than the experimental value. The relatively smaller factor of ~3. 7 between the lowest experimental point and the theoretical curve illustrates the assertion made in section 1 , that order-of-magnitude accuracy in the value of Ep is very weak testimony at best for the validity of a theory of ava18 lanche breakdown. It is perhaps worth mentioning that appears in an exponent, and a factor of 10 error in

o) makes a factor of

c

g 10

( 10^

)

error in n . c

Finally, consider the criterion for damage. The one used above [ 6 ]

°c/°cO = (3.6)

is one of the commonly used criteria. Seitz [9] introduced the "40 generation" criterion in which an electron

starting at the cathode must undergo 40 multiplications before reaching the anode. This corresponds to

n /n _ = (2)^°

c cO

on the average. With

(2)30 =10^^

-334-
the criterion (3. 6) corresponds to 30 generations during the laser-pulse duration. Another commonly used

criterion [5, 10, 11] is that the sample temperature must be raised to the melting temperature. There are many

other possible criteria, some of which are discussed in [7]

.

4. Frequency Dependence of the Breakdown Field

It will be shown in this section that current theories give a much stronger frequency dependence of Eg than

observed experimentally. It is currently believed that the frequency dependence of Eg should be given by

2 2 Eg ~ (1 + co^ Tp , (4. 1)

which follows from (2,4) and (2.3). The increase in the value of Eg from the first 10. 6 Jim laser experiments

over the dc values was attributed to this frequency factor in (4. 1). It was then surprising that there was little additional increase in Eg, first at 1. 06 ^im , then at 0. 694 ^Jm .

At X = 0.694 fim, the value of 1 /oo is 3.69 x 10 '^^ sec. According to (4. 1) and the experimental result

that Eg changes little between 10.6, 1.06, and 0.694 ym, the value of T would have to satisfy

< 2. lOx 10'^^ sec (4.2)

(even for a 15-percent increase in Eg from 10.6 to 0.694 pim) for values of £ between ~l/40 eV and ~8 eV, It is extremely unlikely that T, is this small. The calculations in [7] indicate that T, is an order-of-magnitude

-16 K greater than 2 x 10 sec for all values of £ on this range, and that is two orders of magnitude greater than

the value in (4.2) at £ s 8 eV. See figure 5 below.

Furthermore, the values of T^^ required to explain the observed values of Eg are inconsistent with (4.2).

At 10. 6fim, the experimental value of Eg is [12] 1.95 MV/cm for NaCl. From (2.4) and (2,3) with = and

ficOp = 1/40 eV, the value of Tj^ required to give this experimental value of Eg is

Tj^ = 2.00 X

10'-^^ sec'-^ , (4,3)

The frequency dependence of Eg calculated from (2.3) with this value of Tj^ is in poor agreement with experiment,

as seen in figure 3. For example, even though the theoretical curve was fit to the experimental result at 10, 6fim,

the theoretical value of Eg = 11, 1 MW/cm at 0, 694pTi is much greater than the experimental value of 2, 3 MV/cm,

If the difference between Tj^ and is included, the agreement is even poorer. Including the energy dependence of

and Tj^ makes little difference in this result.

5. Initial Electron Density

In this section it will be shown that the initial density of conduction electrons is so small that the probability of finding an electron in a typical focal volume is much less than unity. This is in disagreement with current

theories, in which the initial density of electrons n^Q in (3. 3) required to start the avalanche is usually assumed

to have the value

n^Q = 10^-10^*^ cm"^ . (5,1)

18 -3

It is also assumed that the avalanche process must increase the value of n to 10 cm , From (3. 3), the ionization frequency w = £n(n /n „) is rather weakly dependent on the initial electron density n „. With

c p c cu 335
18 -1 9-3 4 -3 = 10 cm , changing the value of n^^ from 10 cm to 10 cm changes co^ by 56 pjercent. However, there

is the more serious problem with small electron densities that the probability of an electron being in the focal

volume during the pulse becomes negligibly small for the small values of n^Q , as discussed below.

The assumed value of n^^ in (5, 1) would at first appear to be a reasonable density, which could result

from thermal ionization of impurity levels, for example. Under closer examination, this density is seen to be un8 10

reasonably large. First, with 10 -10 conduction electrons per cubic centimeter, previous observations of photocurrents would have been impossible.

It is not difficult to see why lower densities are expected. For F centers in sodium chloride for the best

case of the chemical potential having the value j

(below the conduction band), vjheve £p — 2eV is the energy

of the F center below the conduction band, the theoretical value of n^ at room temperature is [ 13 ]

13 19 1/2 n = (5 X 10 " 2.41 X 10^) exp

-3

10 cm

4(2) (40)

13 -3

which is negligibly small. The density 5 x 10 cm of F centers is the greatest value the crystal can have without being colored, as shown in [7] . Shallower imperfection levels could contribute more electrons, but it is 8 10 "3

currently believed that there is not a sufficient density of shallow levels, and 10 -10 cm from shallow levels

is inconsistent with photoconductivity experiments.

Measurements of the dc conductivity a would afford sufficiently accurate estimates of the value of n^

.

However, room-temperature values of ct were not found in the literature. In the absence of experimental values,

8 10 -3 -1 6 a simple estimate further suggests that n =10-10 cm is unlikely. Values of ct in (ohm cm) are [13] 10

2 9

^ 14 22 26

for a good conductor, 10 to 10 for semiconductors, 10 to 10 for insulators, and 10 for extrapolation

8 10 -3

from high-temperature ionic conductivity values [3]. For n^ = 10 -10 cm , a lies in the semiconductor

range rather than the insulator range, which Is not reasonable for alkali halides.

-14 -1 -3 -1 For a in the insulator range, that is a < 10 (ohm cm) = 9 X 10 sec , the conductivity relation [ 13 ]

gives

/ 2 n^ = ma/e Tj^

n^ < 2X 10^ cm"^ , (5.2)

For this limit of n^ , the probability of finding an electron in the focal volume can be estimated as follows.

Since the electron-avalanche process is highly nonlinear, only the region of the focal volume with E near the max2 imum is effective, and the diameter d^ and length &^ of the high-field region are less than the 1/e or 1/e values.

Reasonable values for 1. 06 (im experiments [14], which also correspond to the damage volume, are d- = 2fim

- in ^

^ and £j = SOpn. With s 2 x 10 cm and (5.2), the value of n^V^ is

n V. < 4 X 10'^ . c f

This negligibly small probability of finding an electron in the focal volume Indicates that there is not a sufficient

electron density to initiate the avalanche. Furthermore, the value of n from (5.2) is at least eight orders of magnitude too small to explain the number of nucleation centers reported by Smith, Bechtel, and Bloembergen in the

following paper in these proceedings. -3 36-
This problem of an insufficient prepulse electron density is one of the less serious difficulties with current

theories, since there are mechanisms for generating the starting electrons as discussed in section 9. However,

8 10 -3

the source of the starting electrons can determine the value of E^; then assuming that n^Q = 10 -10 cm

would give an incorrect value of Eg.

6. Lucky-Electron Avalanche at dc

Several of the attempts to alleviate the difficulties in the simple theory discussed in the previous sections

will be considered here and in the following two sections. In the present section it will be shown that current luckyelectron theories do not explain the experimental values of Eg. The value of Tj^ is the average time between collisions, with each individual time between collisions being different from ij^ in general. Seitz [9] and Shockley [ 15]

considered the effects of the deviations from the average in dc experiments. In the discussion of figure 1 in section

2, it was fjointed out that collisions limit the rate of energy absorption from the field Ln the dc case. Thus, a few

"lucky" electrons that have collision times much greater than the average value can gain energy Gj even when

(d&/dt)g < (d£/dt)j^ is satisfied on the average. Thus Eg can be less than the value of E^.^^ in (2.3).

A simple demonstration of the large energy gained by the lucky electrons is obtained from the equation of

motion between collisions for the component k^ of the electron wave vector k along the direction of the dc field

E = zE

dk^/dt = eE/fi . (6. 1)

The solution betAveen collisions is

The corresponding electron energy is

£ = Tm

which shows that the greater the value of t between collisions, the greater the value of £. In passing, notice that

the energy gained

m

(6.2)

is large for small-angle collisions, for which k^Q is large on the average. Also, for isotropic scattering, for

which k^Q =0 on the average, (6.2) with t = T.^^ and (d£/dt)g = Ae,/T^^^ give

2 2 e^T. E^

ISO 2m (6.3)

in agreement with (2. 1) for j^jg^ = and w = 0.

It is not difficult to show that the probability that the electron goes for time t without undergoing a colli

sion is [ 9, 7 ] •t<l/Tj^>

P^(t) = e (6.4)

•337-
where (1/t, ) is the time average of 1/t, . The time t required for an electron to gain energy £ 2: 8eV in the K K nc absence of collisions is from (6.2) with k^^ negligible

V= (ii!^) ,.54X.O-»sec . ,6.5,

The time average (1/t, ) can be approximated by the average over the electron wave vector k [7] . From

1 A 1 figure 5 below, the average from £ = kgT = 1/40 eV to £ = £j = 8. leV is <1/Tj^> = 3x 10 sec"\ Thus, (6.4)

and (6. 5) give

P^(t ) = exp

T nc' ^ .28.6 (LM^) (6.6)

-13

This small probability of ionization per "try" of 3. 8 x 10 for E = 1 MV/cm must be multiplied by the number

40

of "tries," which can be estimated as follows. Seitz [ 9 ] has argued that 40 multiplications, which results in ( 2 ) 12 = 10 electrons in a small volume is a typical value for breakdown. In other words, the electron must undergo

-2 40 multiplications before being swept into the anode by the field. Thus, for a sample 3. 2 X 10 cm thick [ 16 ] and

2 6

electrons with average drift velocity [3]v=(iE = 20 (cm / Vsec) 10 V/cm, the electron energy must reach

£j ( in the presence of collisions ) in time

^IMV^j ^

I

1 MV/cm

j 4.00x10-11 1 3. 2x10'^ cm , ^^..^^ J ^ , J 4.00 X 10 " sec

' 2 X 10 cm/sec

4 During this time tj, the electron maJces tj/lj^ = 2.93x10 collisions on the average. After each collision, it has

another try to accelerate without a collision. Thus, the number of tries is tj/^j^- The probability of ionization Pj

is the product of the probability of ionization per try times the number of tries:

?! =

Pi-^'nc^ = 1.1 X 10 " « 1 . (6.7)

i

This small value of Pj at the experimental value of E = 1 MV/cm shows that the lucky -electron mechanism of

avalanche is clearly inadequate in this example.

The ionization frequency ,0)^, is approximately equal to the product of the number of tries per unit time

1/

Tj^ and the probability of ionization per try

nc K w - — e

^ ^k

(6.8)

where the coefficient 1/Tj^ is the thermal equilibrium value. Thus, from (6.6) and (6. 8)

to^ = 7. 33 X lO^"* exp 28.6 |- MV/cmsec

-1

(6.9)

The comparison in figure 4 of this ionization frequency,co^, in (6. 9) with experimental values [16] illustrates the

poor agreement. The experimental dc results are obtained by converting the sample length X to the time Tj^ required for the electron to drift across the sample by using the expression

t^ = X/fiE , -338-
where p is the electron mobility. The ionization frequency, co^> is then obtained from (3.5) with t^ = t^. Values

of t. for the curve marked " ^ const" were calculated using the constant, low field experimental value [3] of

2

IX = 20 cm /V sec for the mobility. For the = fi(E ) curve, the theoretical expression for the field dependence

mobility is given on p. 122 of [3] . In the variable- ^ result used in [17] it was assumed that the mobility

decreases with increasing field, as it should for nonpolar solids. In the theoretical result used here, the mobility

increases with increasing field. The physical reason is that = er^/m and increasing the field increases the

average value of Tj^ in polar solids by forcing the electrons into the higher k region in figure 5 of section 8 where

the relaxation frequency 1/Tj^ is smaller. Thus, the lower curve in (d) of figure 1 in ref. [ 17] appears to be a reasonable lower limit in general, but it apparently is not approached in the alkali halides. In figure 2, only the

constant \x curve was shown for clarity in comparing the dc and l.OSitm results.

7. Lucky Reversing-Electron Avalanche at Laser Frequencies

It will h»e shown that the lucky reversing-electron theory [18] cannot explain the 0. 694^m breakdown results.

The mechanism probably is important only at frequencies below the -laser frequency, and then only with the

modifications discussed in section 9.

If an electron in an ac electric field does not undergo collisions, its wave vector oscillates with some peak

value kpj^ , rather than increasing continuously as in the dc case. Thus, the wave vector can be Increased between

two collisions by the peak-to-peak value 2kpj^, at most. The value of

2k , = ^^^^ = ^ r TT^^r— 3.50X10^ cm"l (7.1) pk fioJ 2.2MV/cm-l 0.694^m

is easily obtained by integrating (6. 1) with E(t) = E^j^ exp(itot) and using E^j^ = i/T Ej^j^g with E = Ej^j^g.

According to the lucky-reversing-electron theory, the electron must suffer a backscattering collision

during the time the value of k is near kp . Then the electron is traveling in the same direction as E during two

successive half-cycles. It is, therefore, accelerated rather than deaccelerated as it would be if its direction had

not been reversed. This process must he repeated until k = kj , where kj is the value of k required to generate

a second electron, as before. The sequence is so unlikely that a very rough estimate is sufficient to demonstrate

that it is negligible.

For an electron that has a lucky reversing collision when k^ = - k^j^ , the next lucky reversing collision

must occur at k = + k , . For the case of cot. » 1 , the electron makes several oscillations before suffering a pK K

2 2 collision, on the average. Thus the probability of a collision during the time tpj^ - j ^ t to t^j^ + A t , where

t = tpj^ at k = +kpj^, is approximately equal to v^t = a)At/2iT. Here 1/u is the period of oscillation. For

CO Tj^ « 1 it can be shown that the probability is even lower.

-

1

Selecting At to make k = 0. 8 kpj^ at the two ends of the time interval At and using 2 cos 0. 8 = 73. 7°

gives vAt = 0.205. A generous estimate of the effective fractional solid angle Afi/4ir for backscatter near 180°

is 1/5 , which gives

Pg = i vAt = 4 X 10"^

for the probability of the backscattering collision.

The number of such collisions required to make k = kj is greater than kj/2kpj^. The inequality results from the fact that the change in k between the reversing collision is less than 2 kpj^, since the collisions were

assumed to occur at [k | > 0. 8k . and collisions with scattering angle 6 ^ 180° are less effective. Thus an pK

-339-
overestimate is obtained by using = k.^/2k^^. The value of kj is ~1.46x 10 cm" for £ s: 8eV, and

2k 2: 3. SOX 10^ cm" according to (7. 1). Thus N„ = 41.9.

P The probability for k reachin'g kj is therefore

N

, 'C -2 41.9 .59

(Pg) =(4X10'') =2.67X10^^ . (7.2)

Multiplying by the number of tries t / T. = 5 x 10^ during the pulse duration t , which is again an overestimate

since cot^^ » 1 gives Pg = 1. 3 x 10 for the probability of success of attaining k = kj, which is negligible

as stated.

A previous estimate [ 18 ] used much smaller values of the number of reversing collisions ( M in the

reference), which appears in the exponent in (7.2). Values of = 1 to 7 for various materials, apparently ob

tained from a fit to experimental results, are much smaller than our lower limit of N^., = 42 ( 0.694/1.06) = 27.

Also, the tacit assumption that to Tj^ « 1 is satisfied appears to have been made in one part of the previous

calculation. These two differences account for tens of orders of magnitude difference in the two results.

8. Energy Dependence of Electron Relaxation Frequencies

At the early stages of the electron-avalanche-breakdown investigation reported in [7] , a number of

modifications of the existing theories were tried in hopes of solving the problems of these theories. Only slight

improvements could be made. For example, consider the effects of including the energy dependence of the relaxation frequencies. A calculation in [7] of the values of i^ and gives the results in figure 5.

Including this energy dependence of and Tj^ in (2. 1) and (2.2) gives the results sketched in figure 6.

Increasing the value of E raises the curve (de/dt)g ~ E^ and leaves (de/dt)j^ unchanged. For sufficiently

great values of E, the curve (d£/dt)g lies above (de/dt)^^ at all values of k as marked "greater E" on figure 6, and breakdown surely would occur in most cases of interest.

For the value of E in figure 6, electrons with ^ l^g lenergy on the average, while those

with k < k^ and l^gj^ ^ ^ energy on the average. Thus, the lucky-electron process must

take the electron across the barrier between k^ and kgj^, shown shaded in figure 6. This is considerably more

probable in general than making k = kj as was required in sections 6 and 7. However, even this easier process

has a probability that is too low to explain experiments. In [7] .it is shown that if the electrons generated in

the avalanche process have k > kg^. then the lucky-electron mechanism, with the starting electrons supplied

by cathode or impurity emission, can explain the dc data.

2 2 "1

For the case of laser frequencies, the factor (1 + (jO ) in (2. 1) reduces the value of (de/dt)^, , the

reduction being especially great at large k where Tj^ is large. There is typically another barrier, or region of

(d£/dt)p < (d£/dt), , at high values of k, as illustrated in figure 6. At 10.6pn with E = 2 MV/cm, this 8-1 2 2-1

second barrier starts at k s; kj = 1. 6 X 10 cm . At higher frequencies, the value of ( 1 + co Tj^ ) becomes

so small that the two barriers merge, and all electrons with k > k^ lose energy on the average. The luckyelectron process then must carry k all the way to kj , which is very unlikely as already shown.

9. Preliminary New Theory

The difficulties of the current theories discussed in the previous sections are removed by a new theory

that includes mechanisms for generating starting electrons and for initiating and sustaining an avalanche. The

major new mechanisms at laser frequencies are the photon-electron-phonon process illustrated schematically in

-340-
figure 7 and inter-conduction-band transitions. The photon-electron-phonon process was considered for

acoustical phonons in 1954 by Holstein Ln studies of metals.

The significance of these processes is that the electron can absorb the large energy fiu) of the photon,

which is not possible for a single-vertex intra-band electron-photon process (first vertex in figure 7) because

energy and momentum cannot be conserved. In the Holstein process, an electron interacts with both a phonon and

a photon. The large wave vector of the phonon allows wave vector to be conserved so that the large energy of the

photon can he absorbed by the electron. A schematic illustration of the increase in the energy of an electron by

the combined effects of the Holstein process H, the electron-phonon scattering P, and the vertical interband process V is shown in figure 8.

In the dc expjeriments on sodium chloride at room temperature, the starting electrons are supplied principally by thermionic emission from the cathode. The lucky -electron process accelerates the electron to the top

of the barrier at £ = £pT , after which the average -electron acceleration raises the energy to £ = £y + £

An important factor is that the excess energy Sgj^^, above the minimum £j required for ionization must be sufficiently great that the ionized electrons will have £ > £

g ^ ^° '•^^^ ^ ''^'^^ ^ accelerated from initial

energy £ > £gj^ to £j by the average-electron process. Thus, it is necessary to obtain only one or a few electrons with £ > £g,-j^. By contrast, in the laser experiments every ionizing electron must be accelerated to

At 10. 6|im the starting electrons are excited from F centers by multiphoton absorption or tunnel emission. The Holstein process is sufficient to accelerate the electron to ~ 5''^BZ ^EL ~ l.SeV). The

threshold appears to be determined by the condition that (^EL^upper ~ ^1* '^^^ *" ^B' barrier at

high electron energies in figure 6 extends to such low energies that the electrons are not accelerated to £ = £j.

However, the value of E required to obtain £ > (^EL^ower approximately equal to the value of E at vfhich

(GpT ) = St. Thus, either condition could control since both give the same value of to within the accu-

^ ti^ 'upper I ° a racy of the calculations.

At 1.06pii the starting electrons are generated by two-photon absorption by the F centers, and these electrons gain energy £ > £j by a series of three Holstein processes plus approximately ten vertical inter- conductionband transitions as in figure 8.

At 0.694 pm the starting electrons are generated by five -photon absorption across the gap, which is a

faster process than two-photon absorption by F centers. The starting electrons are accelerated to £ s 3eV by

the Holstein process acting twice. Then the vertical inter-conduction-band process accelerates the electrons to

£ = £j, as at 1.06fim.

Finally, at 0. 172 jim (xenon laser, with -hoo = 7.21eV), the starting electrons are generated by twophoton absorption across the gap. The electron energy is increased to £ > £j by one Holstein event plus one vertical Inter-conduction-band transition. These processes for generating the starting electrons and for accelerating

them to £ = £ ^ are summarized in figure 9.

In all cases the electron density at time zero when the electric field is turned on was assumed to be zero,

and the criterion for damage was that the temperature was raised to the melting point. A square pulse of duration

equal to one^third the full-width-half-power pulse width was used since only the high-intensity region of the pulse

is effective in the highly nonlinear processes. The electric field values quoted are the RMS values at the maximum

intensity of the pulse. The intensity I is related to the RMS electric field E by the relation 1= cn^E /47r.

The results of the calculations that have been completed to date are shown in figures 2-4. The dependence

of the breakdown field on the laser-pulse duration at 1.06fjm is shown in figure 2. The agreement is excellent,

especially in view of the facts that no parameters were adjusted and that previous theories with parameters adjusted disagreed with e:q)eriment by many orders of magnitude over most of the curve. Such good

-341-
agreement in likely to be fortuitous, as discussed below. It is important that no parameters have been adjusted.

Adjusting the value of parameters to improve agreement between theory and experiment reduces the credibility of

any theory. This is especially relevant in the present case in which the results are so sensitive to small changes

in the value of E

.

The comparison between the theoretical and experimental frequency dependence of the breakdown field in

figure 3 for lOnsec pulses shows good agreement. The experimental point at the ruby-laser frequency is the 10

nsec-pulse value of 2. 1 MW/cm rather than the 4.7 nsec-pulse value of 2. 3 MV/cm as in Fradin's [6] original

comparison of the experimental values of Eg at different frequencies. In figure 4 the theoretical result for the

single point calculated to date lies on the experimental curve.

The agreement with experiment of all the theoretical results obtained to date is better than had been expected in view of the possible inaccuracies in the calculated values of Tj^, t^, and the frequency oo^j for the

Holstein process, and in view of the rather gross approximations used at this early stage in the theory.

The theory presented here removes all the difficulties of previous theories and Che tentative results obtained to date are encouraging. However, additional results from a number of other studies in progress are needed in order to further evaluate the theory.

10 . Acknowledgments

1 am indebted to many people who kindly discussed electron-avalanche breakdown. It was realized that the electron-phonon-optical-phonon process was one of the missing keys to the theory while discussing the Joule heating aspect of the problem with Dr. Norton Moise. Dr. Douglas Mills pointed out that the electron -photon -acoustical

phonon process had been studied by Dr. Theodore Holstein in studies of metals. Dr. Holstein subsequently became

involved in the present program. He suggested that the inter-conduction-band processes should be investigated,

developed a transport theory to include the effect of large-energy transfer processes, and contributed in numerous

discussions. Several helpful discussions with Dr. David Fradin are gratefully acknowledged. Discussions with Drs.

M. Bass, N. Boling, P. Braunlich, C. J. Duthler, F. Luty, and J. Marburger are appreciated.

11. References

[1] von Hippel, A., Z. Physik 67, 707 (1931); J. Appl. Phys. [10] Braunlich, P. F. , Bendix Research Labs Final Report, Contract F19628-73-C-0032, August (1974);

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

8, 815 (1937).

[2] VanMarum, M., Ann. Physik 1, 68 (1799). Phys. Lett. 26, 150 (1975).

[3] O'Dwyer, J. J. , The Theory of Electrical Conduction [11] Sparks, M. and Duthler, C.J., J. Appl. Phys. 44,

and Breakdown in Solid Dielectrics ( Clarendon, 3038 (1973).

Oxford, l^rS).

[12] Yablonovitch, E. , Appl. Phys. Lett. 19, 495 (1971).

[4] Bloembergen, N. , IEEE J. Quantum Elect. QE-10 , 375 (1974).

[5] Klein, N. , Adv. in Elect, and Elect. Phys. 26, 309

(1969).

[13] Kittel, C. , Introduction to Solid State Physics, 4th

ed. (Wiley, New York, 1971).

[14] Smith, W, L. , Bechtel, J. H. , and Bloembergen, N,

,

Riys, Rev, , in press.

[6] Fradin, D. W., Harvard Univ. Technical Report No.

643, Contract N00014-67-A-0298-0006, May (1973); [15] Shockley, W. , Czech. J. Phys. Bll., 81 (1961) and

Fradin, D. W. , Yablonovitch, E. , and Bass, M.

,

Appl. Opt. 12, 700 (1973).

[7] Sparks, M. and Duthler, C.J. , Xonics Fifth Technical Report, Contract DAHC15-73-C-0127, June

(1975).

[8] Holway, L. H. and Fradin, D. W. , J. Appl. Phys. 46,

279 (1975). ~

SoL St. Elect. 2, 35 (1961).

[16] Watson, D, B, and Heyes, W. , J. Phys. Chem. Solids

31, 2531 (1970),

[17] Yablonovitch, E, and Bloembergen, N. , Phys. Rev,

Lett. 29, 907 (1972).

[9] Seitz, F. , Riys. Rev. 76 , 1376 (1949).

[18] Bass, M. and Barrett, H. H, , IEEE J, Quant. Elect.

QE-8, 338 (1971),

-342-
12. Figures

(a) (dC/df)

(bl (dC/dt)|_

Figure 1. Schematic representation of an electron's path in the presence of an electric field E and collisions

with phonons: Part (a) illustrates

the increase in the electron's energy

by a dc electric field and (b) shows

the energy loss to phonons. At L in

(a) the electron is "lucky," that is

it does not suffer a collision for a long time, thereby gaining great

energy from the field.

Figure 2. Comparison of 1.06 ym [6] (points

®) and dc [16] (dashed curve)]*]

experimental values of the inverse

pulse duration tp"-'- with two solid

theoretical curves showing poor

agreement for both. The preliminary

new theoretical results (points

and dotted curve) are discussed in

section 9.

DC EXPERIMENTS

SJ^ 1

PRESENT THEORY 0

...X/. -r^

HOLWAYFRADIN

, 1.06 ;im THEORY

PREVIOUS THEORY,

EQ.(3.4), 1.06 pm V

1.06pm EXPERIMENTS ®

_L

NaCI 300K

4 6 8 BREAKDOWN ELECTRIC FIELD Eg IMV/cm) -343-
12

figure 3, Comparison of the experimental results

[6] with the theoretical results (2.4)

and (2.3) fit at 10.6 ym showing poor

agreement for the frequency dependence

of the breakdown electric field.

10'

10'

10"

10»

EXPERIMENT, (1 =;j(EI

EXPERIMENT

ti = CONST.

2 3 4 6 BREAKDOWN ELECTRIC FIELD (MV/cml

Figure 4. Comparison of the experimental results

[16] with the result eq. (6.9) of the

lucky-electron theory. The agreement

is improved from that of eq. (3.4) for

the average electron, but is still poor.

PREVIOUS THEORY,

FIT AT CO2

PRESENT THEORY B

EXPERIMENTS®^ NEODYMIUM RUBY

XENON

0.1 1 LASER PHOTON ENERGY (eV)

1

-1.14 X

10'=" UK

1/31 «V 1 tV 2>V 3aV 4«V 5 e(«V)

kj 1.4

6 7 8.1 aV

Figure 5. Dependence of electron relaxation

frequencies on electron energy.

-344-
electron

\

Figure 7, Schematic illustration of the Holstein

process in which an electron gains

energy -ho) in an interaction vjith a photon and a phonon.

ELECTRON WAVE VECTOR k

Figure 6. Wave-vector dependence of the electron

energy gain and loss showing the

shaded energy barriers of net average

loss.

Figure 8. Schematic illustration of the processes

in which an electron gains energy

5 > Ci by Holstein collisions H, vertical

inter-conduction-band collisions V, and ,,0 phonon collision P.

WAVE VECTOR

STARTING ELECTRONS

(el 0.172 /jm (7 21 eVI

(cl 1.06 fjm

Figure 9. Summary of the processes of generating the starting

electrons and increasing their energies to the ionization

value.

-345-
COMMENTS ON PAPERS 8/ SPARJCS and SMITH, &t al

It tM(U pO'intzd oat that the. obizxvzd valLLZ thz th^e^hold doK damage aj> not a veAy ieni'Utive

dttecXol o{i the pfioaeAi 0|$ tnUiiation the damage, outs ion. example, mulitphoton ionization, but that

the damage moKphology ti, a much moKZ ien^ttive indxcajton o^ the nataxe o^ the damage pn.oceJ>i>, ThiA

point wilL be dUcuMed ^uxthefi in the next pap-eA in thij> pfioceedingi. The viAujiZ inApection

technique uAcd by Smith to deteAmine damage thKe^hold wa6 ^ound to be exXAemtly ien^itive. Id the

amount o^ itAay Light pneMent in the labonatony a)a6 kept to a minimum and the obieAvation woi made

Mcth a dajik adapted eye, Smith ^ound that he could .iee visible evidence o^ damage pKoceAieA which

iMfie veJiy di(,iicult and in iome coJ>eA ijmpoiiibZe to detect by 6ubiequent micAoicopic investigation.

In the genexal dLicuMion a point wai nailed conceAning tiie apparent diiciepancy beM*)een ixueA

damage data and the -iomewhaX. oldeA data on dc b^eakdoMn. The .tienie Oj$ tixe dJj>cuAi,ion wcu, that the

dc bfieakdoMn data, was put in question by unceAtaintiei in elecJAode eHecJ^, impuAity ed^ect^, and

choAacteAization o^ ti\e 6ampleJ>. In all probability modeAn loieA breakdown data is much more reliable than the old dc data.

-346-
6.3a Statistical Analysis of Laser Induced Gas Breakdown - A Test of the Lucky Electron Theory of Avalanche Formation*

D. Milam''"

Lawrence Livermore Laboratory, University of California

Livermore, California 9^550

and

R. A. Bradbury and R. H, Picard

Air Force Cambridge Research Laboratories

Hanscom Field, Bedford, Massachusetts 91730

It is shown that the statistical distribution of breakdown times of gas

volumes irradiated with square waveform ruby laser pulses can be explained

by accounting for the probability that initial electrons are reliably produced by photoionization . Furthermore, the nature of the statistics of gas

breakdown are closely related to those predicted by the lucky electron

theory, and to the experimentally observed statistics of breakdown in solid dielectrics.

It is s\;ggested, therefore, that the statistics of intrinisic breakdown in both

gases and solids may be related to the probability of obtaining initial charge

rather than purely a manifestation of the lucky electron statistical process.

Key words: Electron avalanche; laser-induced gas breakdown; photoionization.

1. Introduction

The lucky-electron theory of avalanche formation assumes that free electrons of unspecified

origin are forced to undergo repetitive accelerations and lattice collisions under the driving influence of the incident laser field. The average free electron gains little energy from the applied

field since collisions are, in general, frequent and lossy. Occasionally, a free electron obtains

ionizing energy through a sequence of "lucky" momentum-reversing collisions and accelerations.

Avalanche breakdown is ass-umed to be virtually assured once a single electron in the focal volume is

capable of ionizing its surround. In this paper we describe an experimental test of the lucky-electron

theory.

2. Statement of the Lucky-Electron Theory

Since most collisions are lossy, the forced motion of a free electron in an applied field can be

visualized as a sequence of trials, each with some small but finite probability of resulting in an

ionizing electron energy. A typical trial will last only until the first collision. The probability

per trial that an individual electron will reach ionization energy e has been stated [l]'' as

= A^f-e-K/^ • (1)

where f is the fraction of all collisions that are "lucky", m is the number of half-cycles required to produce ionizing energy, K is a material constant, and E is the applied field. The number m is itself

inversely dependent on E, so that we can restate eq. (l) in the form,

<til = A^e ' . (2)

On a particular focal site there will be ng free electrons each independently undergoing trials

at a rate approximately equal to the collision rate t^""'" . The probability Pjjj) (t) of avoiding damage

by avalanche breakdown until time t is simply the cumulative probability that all trials preceeding

time t are unsuccessful;

*This work was performed at Air Force Cambridge Research Laboratories with support from both the Air

Force and the Advanced Projects Research Agency.

tBnployed by AFCRL while this research was performed.

1. Figures in brackets indicate the literature references at the end of the paper.

-347-
n t/T

= (1 - ' ' . (3)

Operationally one envisions irradiating sites, and determining the number N(t) still undamaged at

time t, so that

!!-»-<» o

o

For convenience we shall chose to use logarithmic plots, and rewrite Pj^{t) in the form

Pj^(t) = e-^*, r = ^ |jln(l - c^^)| . (5)

c

We observe that when ng, t^,, and <()]_ are constant, the logarithmic plots of Pjj]-) will be linear curves. Since (fi^ is very small, the dependence of the slope T upon the applied field strength E is given to

high accuracy by the expanded form,

^ -~ ^ ^'

c

when only the first term is retained. Relative slopes, found at different values of E, should there

fore, be calculable from the equation,

r.^^e-K'/E

T

C

Logarithmic plots of Pj^(t) will be nonlinear if n^ or E varies in time or space due to any of

the following situations:

a. Spatial variation of E due to field enhancement on some focal sites.

b. Spatial variation in n^ due to electron traps or easily ionizable impurities,

c. Variation of ng in time at all sites due to ionization of the basic dielectric itself.

Cases a and b produce functions In P-^it) which are more steeply sloped at early times than at later

times, due to the larger damage rates present on some sites. Note that the factor T is in fact a damage rate since

Nit)=e-^^ (8) o

where we have used eqs. (3) and (5). Condition c above will result in functions Jin Pp^p which slope more steeply at later times due to the constantly increasing damage rate.

Experimental Test of the Lucky-Electron Theory

It is possible to test the above hypotheses by experimentally generating the functional forms

PUD^'t) =

^

, using a technique described [2] at the preceeding Boulder Conference. That technique

is illustrated in figure 1. A large number of small sites are irradiated using square-waveform

ruby laser pulses of strength Eq. The survival time of each site is observed by recording the

damage-induced termination of the laser pulse transmitted through the site. A table of survival

times is then organized to allow plotting of N{t)/NQ. All data is sorted after the experiment to eliminate firings for which varied significantly during the pulse; all remaining data are included

in the analysis. -348-
Data for Transparent Dielectrics

We have applied this technique to record functions P]jq(E, t) for a wide range of dielectric films

and polished surfaces, and for the bulk of such materials as fused quartz, sapphire, ED-2, ED-U, BK-7,

CaF^, KCL, KBr, and other randomly chosen glasses. In only tvo instances do we observe functions Pup

which are closely related to the predictions of the lucky-electron theory: namely, bulk fused quartz

and sapphire. For even these cases, the agreement is incomplete unless one allows selected variations

of ng in both time and space. Consider data shown for fused quartz in figure 2. The functions PupCt)

found at 31*. 5 and 39 kW are reasonably linear in time, indicating potential agreement with luckyelectron theory. Using an effective avalanche constant k' computed from the slopes of data at those

two power levels, one can calculate the slope expected for data at 1+2.5 kW. The third data set exhibits

two problems: the slope is too large to agree with the curve predicted by the lucky-electron theory,

and many of the sites irradiated at h2.5 kW damaged during the rise of the pulse. These early damages

are most likely due to inclusions or to a large spatial variation in ionizability . Field enhancement

by defects or a time-constant spatial variation of ng cannot explain the early damages since we must

assume that similiar sites were sampled at the two lower power levels, and that their effects were

incorporated into the effective k' . As a result, the slope can change with changes in E only predicted by e-K /E when damage is due to such time constant spatial variations. The second problem,

too large a slope for those sites not damaged during the risetime can be explained by the luckyelectron theory, only by assuming that n^, and therefore the damage rate, are increasing in time.

Three data sets generated in fused quartz all yield the same result.

The reader should be aware that we have in the past erroneously identified [2] a combination of

lucky-electron avalanche breakdown and electric field enhancement at defects as the cause of data such as that in figure 2. For reasons stated above, it is clear that that combination of mechanisms will not

totally explain the data.

In sapphire the agreement between data and the lucky-electron theory is excellent within

restricted time intervals, as shown in, figure 3. Initial damages at each power level lie on linear

curves whose slopes are related by e"^ Z-^. However, all sites surviving an initial irradiation

interval damaged at a well defined time, which again demands that ng be increasing in time if the

lucky-electron effect is invoked to describe damages

,

h. Introduction to Gas Breakdown

Based on the failure to find close general agreement with the lucky-electron theory when solid

dielectrics were studied, we decided to investigate the statistical nature of laser-induced gas breakdown. The arguments used to formulate the lucky-electron model are equally applicable to breakdown

in gases. Indeed, the concept of an electron traveling under the influence of the applied field, and

suffering collisions in which the duration of the interaction is small relative to the mean time

between collisions, is perhaps more applicable to a gas. Furthermore, the gas pressure, and therefore

the collision rate can be varied at will, so that the relationship between breakdown characteristics

and collision time can be determined.

Laser-induced gas breakdown has been extensively studied and review papers are available [3,1+].

Since it is not our piarpose to survey the field of laser breakdown in gases, we shall rely on these

reviews for the few facts needed here. The accomplishments of individual workers in the field are acknowledged in the extensive bibliographies attached to the reviews.

Theoretical treatments of gas breakdown assume that the plasma development is well described by

the solution of rate equations. The increase in the free electron population, ne» is assumed to result

from multiphoton ionization and from collision-assisted ionization, with the latter rate taken to have the functional form found in microwave breakdown. Losses in the electron population are due to diffusion, recombination, and attachment. Solutions have been found to describe the growth in ng

under a variety of assumptions concerning the relative contributions of the various gains and losses.

These solutions adequately accoxmt for the functional dependencies of the threshold breakdown fields,

and generally account for the magnitude of such fields. Ready [1+] notes, for example, that even the

simplest assumptions result in an expression for the threshold flux F^j^,

10 mc ^ 2 ^th = -T- I

-^c ' ^9)

ire TP

which agrees with the experimental dependences found for pulse duration Tp, laser frequency o), ionization potential I, and the pressure of the gas which is inversly porportional to . These dependencies

hold when collision assisted ionization is dominant.

-349-
5. Statistics in Gas Breakdown

Stastical arguments have not been used to describe laser-induced gas breakdown. It is assumed

that time independent average rates are observed for each process, resiilting in a precisely defined

time of damage for each precisely defined incident laser intensity. For moderate gas pressures, in

the range 10 torr < p < 10^ - 105 torr, the collision-assisted cascade growth process dominates

plasma formation for Q-switched pulsed having durations of tens of nanoseconds. As the pressure or pulse duration is reduced, direct photo ionization becomes important. In the transition region,

the cascade process is richly seeded with starting electrons, so that any statistical effects resulting from a lack of initial electrons for the cascade process, or in the photolonization process

itself, are unlikely.

One can, therefore, from the most general principals predict several features that should be

observable in the statistics of laser-induced breakdown. If the rate equations with time constant

rates are an adequate representation of plasma growth, the time of breakdown will be precisely related

to the pulse intensity. Since some rates are very nonlinear in intensity, significant spread can be

expected in the breakdown times even if stringent precautions are taken to include only data taken at

a preselected field strength E. The spread in breakdown times should have a predictable dependence

on the average time of breakdown. If all rates involved are all sufficiently large to be adequately

represented by time averages, the least experimental scatter will be obtained by insuring that damage, on the average, occurs at early times during the square-pulse irradiation. Larger scatter in breakdown time will be observed at lower intensities where lesser rates are allowed to compete over long

times

.

The spread in breakdown times will also have a predictable dependence on pressure, since the

threshold for collision-assisted breakdown is inversely dependent on pressure [U] while the photolonization rate is only very weakly pressure dependent C*] . As a result, the threshold irradiation

field increases as the pressure is decreased. The number of electrons created by direct photolonization will increase very rapidly as the irradiation level increases, since photolonization is proportional to the intensity raised to some large power. Photolonization in argon, for example, sho\ild be

proportional to the ninth power of intensity. One can, therefore, predict a pressure dependence in

the scatter of breakdown times. At low pressures, approximately one torr, photolonization predominatf over collision-assisted ionization, with the result that the accumulation of electrons will depend

critically on intensity, so that even with careful selection of irradiation levels, a large scatter

in breakdown times will be observed. As the pressure is increased, both the field necessary to produce breakdown and the photolonization rate will be reduced until one begins to see the statistical

featiires associated with single photolonization events. At that point, one again expects to see a large scatter in breakdown times associated with a small spread in irradiation intensity.

Based on the arguments above, one should be able to find breakdown occurring at a well defined

time at intermediate pressure levels ('v 10^ torr) . As the pressure is increased, the time of breakdown will become less well determined at each intensity level. One should eventually reach the

situation corresponding to observation of actual atomic statistics in the production of starting

electrons. Since the photolonization rate for individual atoms is constant in time, the breakdown

can occur at any time during irradiation. This latter state is then closely related to the statistics predictable for the lucky-electron' process

.

6. Measurement of the Distribution of Breakdown

Tim.es in Gas Using a Ruby Laser

Using the technique described above, we have measured the distribution of breakdown times in

argon at several pressures and in nitrogen at one pressure. The gas was contained in a high

pressure housing designed for use as a laser-triggered spark gap. Filling was accomplished in a series of evacuation-flush steps to furnish a reasonable assurance that the gas was at the maximm

purity level possible. A controlled leak was used to move the gas so that each gas volxime was irradiated only once. The lens used to focus the beam produced a beam waist of approximately

Gaussian profile which was = 6.0 um in diameter at half-maximum. A detailed reduction [5] of

magnified images of the beam profile indicate that the peak on-axis intensity was 2.3 Gw/cm^ for

each kW into the system. The time of breakdown in each focal volume was monitored in the same fashion as described for dielectrics in section 3.

T . Data and Conclusions

Plots of N(t)/NQ, the fractional number of gas volumes which have not suffered breakdown, are

shown in figures U,5,6, and 7 for argon at pressures of 1820, 28IO, 851O, and 26,600 torr, respectively. The data, taken as a whole, shows exactly those trends associated with the assumption that

gas breakdown is adequately described by rate equations. Breakdown at 1820 torr was deterministic,

occurring at a well-defined time at each power level. The spread in breakdown times became greater

NOTE: 1 torr = Pa

760

-350-
when breakdown occurred later during the laser pulse. We recall that the collision-assisted impact

ionization process dominates breakdown [U] at this pressure. Photoionization, with its strong intensity dependence, must have served only to furnish a copious supply of initial free electrons, since

breakdown occurs at a sharply defined time. It is evident that we did not, at this pressure, observe

the large fluctuations in breakdown time predicted if photoionization were furnishing a major portion of the breakdown plasma, or if the photoionization rate were so low that free charge was not reliably produced.

The statistics of breakdown in argon at 28l0 torr as shown in figure 5 were essentially the same

as those at 18OO torr. Breakdown was largely deterministic; both the average breakdown time and the

time jitter were slightly increased by decreasing the beam intensity.

By making a significant pressure change from 2810 torr to 85IO torr, the statistical characterists of breakdown in argon were markedly altered as shown in figure 6. The time of breakdown

was poorly defined with N(t)/Ng having the approximate form predicted for lucky-electron avalanche

breakdown. Data at 103 Gw/cm2 does not, however, have the slope predicted by the factor e-K /E^

although the data may be too few to serve as an adequate test.

Data taken at 26,600 torr as shown in figure 7 confirms the trend toward less deterministic

behavior at higher pressures and lower intensities. Ample data was obtained at each of four intensity

levels to show that the probability of breakdown per unit time, or the breakdown rate, was constant

over at least the ih nsec interval investigated. Two possible explanations are available, but the

relative slopes still do not agree with the exponential factor related to the lucky-electron theory.

Agreement with that tljieory could be found by assuming that the lucky-electron damage rate had increased more rapidly than e"'^ /-^ (j^e to a power dependent change in the number of electrons undergoing trials , Such charge, if produced by photoionization, must acciimulate in time except under a very caref\illy

chosen balance of gains and losses which could not exist at all power levels. It woul-d seem, therefore,

that an argument which assumed that the lucky-electron rate increased faster than e-K /E -^o intensity dependent photoionization, also would demand that the rate increase in time at some intensity

levels. Since this combination of events did not occur, we must assume that the statistics of breakdown probably did not result directly from the lucky-electron effect.

An alternate explanation for data in figure 7 is to simply assume that the photoionization was so improbable that an initial free electron population was not reliably established in each focal

volume. Since the photoionization of individual atoms is described by a constant probability per unit

time, the linear ctirves in figure 7 would result if we had been producing only a few charges in each

focal volume. Alternatively, if the photoionization was improbable, a large experimental jitter

in breakdown time would be predicted due to the intensity nonlinearity of photoionization. These

two effects, while conceptually different, are probably very difficult to separate experimentally.

If one assumes that the collision-assisted ionization rate has the form of eq. 9, so that the

breakdown threshold is inversely proportior.al to pressure, one can make a rough estimate of the

fractional ionization occurring by photoionization at 26,600 torr. To accomplish the estimate, we assume that we are able to directly ionize * .1 the neutral atoms in the focal volume when the pressure

is 1 torr. The number of ionizations, .1 N^V, is related to the breakdown intensity at 1 torr by an equation of the form [U],

.1 N^V = C (NqV) F^^^ , (10)

where N is the density of neutrals, V is the effective volume for a ninth order process, and F^^ is

the breakdown intensity which is assumed to expressible as an inverse function of pressure,

F^, = 5/P, 6 = constant, (11

)

th

if only pressure is allowed to vary. The constant C in eq. (lO)is a product of the pulse duration and

the ionization rate. Using this form, the fraction X of the atoms which woiild be ionized at a higher

pressure Pg is calculable from the fraction ionized at Pi = 1 torr;

C F

9 p

9

^= th,l = (-2_) , (12) ^ C F

^

^ ^th,2

-351-
so that X " .iCPj/Pg) in order of magnitude. Using eq. (12), tt^e fractional ionization at 26,600 torr

is X = 1.8 X 10~^^. More conservative estimates that .1 the focal volume is directly ionized at P^^ = 10 torr or at T^. ~ iO*^ 'torr lead to estimates X = 1.5 x 10~31 ^j^^ X = 1.5 x 10~22 respectively.

One may, therefore, reasonably expect to have trouble producing starting electrons in a focal volume

of approximately 10~9 cm3 which contains only 10l2 atoms at a pressure of 26,600 torr.

We also measured the distribution of breakdown times in nitrogen at 8,210 torr. That data is

shown in figure 8. Again we observed a constant probability per unit time for breakdown, and in this

instance the slopes were in rather close agreement with the predictions of the lucky-electron theory.

We note, however, that there seems to be no apriori reason to assume that we cannot use the argument

concerning a lack of consistent photoionization to describe the breakdown data in figure 8.

Finally, we measured the distribution of breakdown times with dc electric fields of "various

magnitude applied to the gas volume. The dc field should have two effects [6]. It should sweep

naturally occurring electrons from the focal volume, and it should furnish a forced one-way drift loss

during plasma build-up. The field could also possibly remove charged dirt, but we have no evidence

that dirt contributed to our measiirements is such small volumes. The distribution of breakdown times

in nitrogen in the presence of a strong dc field is shown in figure 9. The data again is explicable by assuming that the starting charge density is unreliably produced by photoionization and

that some of the electrons produced are lost by one-way drift.

8. Final Conclusions

-k'/e The simple damage rate associated with the lucky-electron theory, T = e~ , is closely

related to the observed aspects of damage in a limited class of high-quality, solid, transparent

dielectrics. The expression does not, however, account in detail for the statistics of the total

breakdown process unless special assumptions are made concerning spatial and temporal variations of

the free electron population.

In laser-induced gas breakdown, statistical results similiar to those found in solid dielectrics

are found at pressures above a few thousand torr. The statistical fluctuation in the time of breakdown'

in high pressure gases can be understood as a result of a low probability for photoionization at the

breakdown field strengths associated with those pressures. The form of the breakdown statistics in

gas at high pressure is, however. Identical with that predicted for the lucky-electron effect in some cases and closely related in other cases.

The data suggests that statistics observed in laser damage are a real phenomena, and that they

may be more closely associated with the production of starting electrons than with the statistical

model predicted by the lucky-electron effect.

9. Acknowledgments

The authors acknowledge extensive discussions with Dr. Norman Boling of Owens Illinois, Dr.

Michael Bass of the Center for Laser Studies at the University of Southern California, and Dr. Alex

Glass of Livermore Laboratories, during which the ramifications and formulations of the lucky-electron theory were discussed at length. Dr. David C. Smith of United Aircraft Research Laboratories

furnished significant discussion concerning the gas breakdown data. We are further grateful to the

secretarial and photographic personnel at Livermore who aided in preparation of the manuscript.

10. References

[l] Bass, M., and H. H. Barrett, IEEE J. Quantum Electron., QE-8 , 338 (1972).

[2] Milam, D., R. A. Bradbury, R. H. Picard, and

M. Bass., Laser Induced Damage in Optical

Materials: 197^ , edited by A. J. Glass and

A. H. Guenther (U. S. Govt. Printing Office,

Washington, 197^+), p. I69.

[3] deMichelis, C, IEEE J. Quantim Electron.,

QE-5 , 188 (1969).

[U] Ready, J. F., Effects of High-Power

Laser Radiation , Chapter 5, Academic

Press, New York, 1971.

[5] Weaver, H. J., G. E. Sommargren, and

E. S. Bliss, Proc. of SPIE, lt8 63,

(197'+).

[6] Tulip, J., and H. Seguin, Appl . Phys

.

Lett., 23, 135 (1973).

-352-
11. Figures

rCLD STfiENGTH £<, APPLIED TO SITES 1.0

SITE MA«ER

SURVIVAL TWC

D**UCEO

Ml)

[Figure 1. Schematic representation of the

experiment used to measure Pi,j])(t)/Np,

the probability of avoiding damage

until time t. Survival time is

measured for each of Nq sites.

.05 _

.02

.01

Predicted for 42.5 kW

^

\

Suprasil quartz

input power

+ 34.5 kW

o 39.0 kW

a 42.5 k\N

8 12

Survival time, nsec

14

Figure 2. Survival curves for suprasil fused

quartz. Data for t > 2 nsec is

reasonably linear, but the slope

increases more rapidly than predicted

by e-kVe The slopes predicted by

nput power

+ 49.0 kW

o 52.0 kW

69.0 kW

the exponential factor for data at 42.5 kW is shown by the dashed curve.

1.0

.5

S .1

.05

5 10

Survival time, nsec

15

Figure 3. Survival curves for single crystal

sapphire. Initial data agrees with

the lucky-electron slope factor as shown by agreement between data at

69 kW and the dashed curve.

.02

.01

Argon 20 psi

203 Gw/cm^

231 Gw/cm*

297 Gw/cm'

J ^ L

4 8

Survival time, nsec

12

Figure 4. Survival curves for argon at a pressure of 1820 torr (20 psi overpressure) at three irradiation levels.

-353-
1.0

.5

2

$ .1

Figure 5.

.05 -

.02

.01

1.0

Argon 40 psi + 184 Gw/cm^

o 210 Gw/cm^

o 248 Gw/cm^

4 8

Survival time, nsec

12

Survival curves for argon at a pressure

of 2810 torr (40 psi overpressure) at three irradiation levels.

z^ 0.1

.05

Figure 6.

.02

.01

•+-+.

\\\

\

Slope predicted

for 103 Gw/cm^ \

Argon 150 psi

+ 81.4 Gw/cm^

o 89.2 Gw/cm^

o 103 Gw/cm^

4 8

Survival time, nsec

12

Survival curves for argon at a pressure

of 8510 torr (150 psi overpressure) at three irradiation levels. Slope pre- M

dieted by the lucky-electron factor ^ e-K'/E for ^^^g^ ]^03 Gw/cm2 is shown

by a dashed line.

1.0

.5

.2

1 .1

.05

.02

.01

Slope predicted

for 98 Gw/cm^

1.0

Argon 500 psi

59.0 Gw/cm^

70.8 Gw/cm^

84.2 Gw/cm^

98.0 Gw/cm^

4 8 12

Survival time, nsec

oZ^ .1 _

.05 -

Figure 7. Survival curves for argon at a pressure

of 26600 torr (500 psi overpressure) at three irradiation levels. Slope predicted by the lucky-electron factor

e-K'/E is shown by a dashed line.

Figure 8.

.02 -

.01

^^MIT"—I—

^ r

^§ Calculated slope

vfor 224 Gw/cm^

150 psi

+ 224 Gw/cm^

o 248 Gw/cm^

^ 276 Gw/cm^

4 8 12

Survival time, nsec

16

Survival curves for N2 at a pressure

of 8210 torr (150 psi overpressure) for

three irradiation levels. The data is

in close agreement with the luckyelectron slope factor e~K'/E shown

by agreement between the dashed curve

and data at 224 Gw/cm2.

-354-
1.0

$ .1

.05

.02

.01

- Nj 150 psi

o 248 Gw/cm^, without d.c. field • 248 Gw/cm^, d.c. field equal

1/2 self-breakdown

8 12

Survival time, nsec

16

Figure 9. Survival curves for N2 at a pressure of

8210 torr (150 psi overpressure) , both

with and without the presence of a transverse dc electric field applied

to the gas volume.

MO comEhrrs on paper by mi lam, al

-355-
6 . 4 Measurement of Free Electron Density at the Onset of Laser-Induced Surface Damage in BSC-2*

Nabil Alyassini and Joel H. Parks

Departments of Physics and Electrical Engineering

University of Southern California

Los Angeles, California 90007

A study of transient processes which occur during laser induced

surface damage to BSC-2 glass was conducted. An optical probe technique was employed in this study which used a He-Ne laser to internally

probe the surface at the critical angle. The variations in the detected He-Ne beam revealed a decrease in the site specular reflectivity

which precedes structural damage by as much as 2. A nsec. This is

shown to be the result of a laser induced increase in the free electron

density at the irradiated surface site with a measured average build

up time of 1.6 nsec, resulting in a detectable decrease in the site

refraction index (An=10-5-10 -5). From the measured specular reflectivity decrease, a calculation of the free electron density at the onset

of damage indicates a density of 10 -10 /cm . It is shovm that the

resulting free electron absorption can transfer a damaging amount of

energy from the laser pulse to the sample causing at least melting of

the irradiated surface site.

key words: Damage electron density; electron density transient; free ' electron absorption; laser induced surface damage; optical probe

technique; refractive index variation,

1. Introduction

Presently, it is widely accepted that free electrons are the cause of laser induced damage to intrinsically transparent dielectrics in a self focusing free situation and in absence of absorbing

inclusions. Although there is not broad agreement [1-8]' on the process by which the free electron

density increases in the presence of an intense laser field, it has been suggested [6] that the free

electron density must reach lO-'-^-lO-'-^/cc in order to cause damage to the dielectric by heating due

to free electron absorption.

Such a density of free electrons will Induce a detectable decrease in the refractive Index of

the dielectric in addition to making it lossy. This paper reports on the successful measurement of

the decrease in specular reflectivity of a BSC-2 surface site which was subjected to a damaging Qswitched ruby laser pulse. This reflectivity change was observed to occur a few nanoseconds prior

to the onset of catastrophic damage. It is shown that the specular reflectivity decrease is due to

a decrease in refractive index of the irradiated surface site which was caused by a laser induced

increase in the free electron density. A measure of the free electron density at onset of damage

was calculated, and the amount of energy transferred by such a density of free electrons from the

damaging laser pulse to the lattice was found to be sufficient to at least cause melting of the site.

This study also provides a measure of the free electron build up time.

2. Experimental Technique

Figure 1 shows the optical probe technique previously discussed [9-11] which was used to monitor

the specular reflectivity decrease of a BSC-2 surface site during the presence of an intense Qswitched TEMqo single mode Gaussian ruby pulse. The surfaced site to be damaged was probed Internally

at its critical angle by a 3 mW unpolarized He-Ne laser. The reflected probe intensity was measured

with a photodiode detector which had a measured rise time of less than 1 nsec, and which was synchronized to the damaging ruby pulse with a measured accuracy of better than ±0.25 nsec [11]. Entrance

and exit surface damage were studied and each site was irradiated only once. The measured rubv spot

size and the calculated probe spot size at the irradiated sample surface were 34pm and less than 4pm,

respectively.

* Partially supported by Advanced Research Projects Agency and the Joint Services Electronics

Program at USC.

1. Figures in brackets indicate the literature references at the end of this paper.

-356-
One of the reasons for probing the BSC-2 surface internally at its critical angle is shown in

figure 2. As can be seen, the absolute decrease in the BSC-2 sample reflectivity due to a decrease

in its index is maximum at the critical angle for both polarizations. That is, the sensitivity of the

reflected probe intensity to a decrease in the index of the site is maximum at the critical angle.

Also, since reflected probe intensity is maximum at critical angle, the sensitivity of the optical

probe detection is also maximum at this angle. In addition, perturbation of the probe by the spark

that was found to always accompany surface damage in BSC-2 is eliminated when probing internally.

3. Experimental Results

Figures 3 and 4 show typical data sets for BSC-2 entrance and exit surface sites damage,

respectively. These traces have been copied from the original photographs for clarity of presentation.

The values shown in the figures were measured from the original photographs. An important finding of

this study was the observation in over half the damaged sites of a time delay At between the start of

the dip in the optical probe trace and the start of the dip in the transmitted damaging ruby pulse,

with the ruby trace delayed relative to the probe trace. During this interval of time the probe trace

dropped AV(mV).

The point in time at which the transmitted damaging ruby gulsegStarted its dip is the point at which the free electron density reached a critical value ( 10 -10 /crri^) causing sharp attenuation

of the damaging ruby pulse [6,11,12]. The onset of damage apparently occurs at this point, since

the resulting Joule heating of the irradiated site becomes appreciable at this density, eventually

leading to irreversible damage.

At the start of the dip in the reflected probe intensity, the free electron density at the

irradiated site was too low to cause any appreciable heating of the site (~10 /cm^). However, this

density was enough' for the resulting decrease in the refractive index to cause a detectable decrease

in the site reflectivity when probed at the critical angle, as will be shown. The damaging ruby pulse

is relatively insensitive to such a decrease in the index since it was normally incident at the

sample surface (see figure 2). The delay At ranged in value between 0.5 - 2.4 nsec (lower value is

limit of our measurement accuracy) with an average value of 1.6 nsec. The detected probe signal

AV ranged in value between 0-6 mV with an average salue of 3.3 mV.

4. Analysis

An originally lossless dielectric of index n plus a free electron density Nq can be

described by an "extrinsic" index nj^ and extinction coefficient kj^. It can be shown [11,13] using

Maxwell's equations for a wave of radian frequency (i) and the classical free electron model [14] that

and kj^ are given by (MKS units):

k," = ^ (1)

+ k? (2)

where

A =

0) ^ P c

1 + W^T ^ c

B = n-^ - A

0)

N e

2 _ O

e m

o

and T Is the free electron lattice collision time. The plasma frequency OJp depends on the electron

charge and masse andm, respectively, and the free space dielectric constant (eo=8. 854x10"! F/m).

For the case of interest here < lO^^/cm^, thus at the ruby or He-Ne radian frequency a)p«(0 and

-357-
A << 1. Therefore, even for a collision time as small as 10 sec, eqs. (1) and (2) are accurately

approximated by:

1 2(i)T n

c (3)

and

1 - 2n

(A)

Thus, the refractive index variation is simply

An = 2n (5)

It can be shown that the reflectivity of a substrate having an index (n - An) at the angle of

total internal reflection, 6 = sin (1/n) is given by:

-

- (£ - 0 (IS - ^)

-, 2

1

-I 2

for a beam polarized parallel or perpendicular to" the plane of incidence, respectively . These can be

approximated in the case of BSC-2 glass (n = 1.513) for An < 10" by:

^11 = 1 Anv^ An"^ (6)

^1

A/lK An^

(7)

Since the time constant of the polarization fluctuations of the unpolarized probe beam is much smaller than

the probe detector rise time, the measured reflectivity of the sample R is given by the average value

R = h(R|

I + Rp.

After algebraic manipulation of eqs. (6) and (7) we obtain,

^ n(n^ - 1)(1 - R) '

_ 8(n2 + 1)2

Where the measured value of R at onset of damage is given by:

_ V - AV

(8)

(9)

V being the initial reflected probe signal before irradiation (V = 33-34 mV)

.

-358-
Hence from the measured value of AV, R is calculated using eq, (9), which is then used to obtain

an estimate of An from eq. (8). Values of the parameter A are then calculated using eq. (5). The

values of R, An, and A are all based on measured and known quantities. Assuming a value for the electron lattice collision time, an estimate for the free electron density N^^ and extinction coefficient

at the onset of damage can finally be obtained from the analytical expression for A.

Table 1 lists the measured and calculated values of R, An, Nq, and ki for site D64,9 and P7,8,

whose data sets are shown in figures 3 and 4, respectively. The value of w used in calculating ki was

that of the ruby laser (w = 2.71 x lO-'-^ rps), and the frequency used in calculating was that of the

He-Ne laser (co = 2.98 x lO-'-^ rps). Values of the electron-lattice collision time have been taken

from the literature [6].

Table 1. T = lO"

c

15 sec T =^^3

c w

3 x 10-16 sec

Site R An (cm"^) •^1 N^Ccm""^) \

D64,9 0. 888 2.8x10"^ 2.7x10^^ l.Oxlo"'^ 5.0x10^^ 3.2x10"

4

P7,8 0 918 1.6x10"'^ 1.5x10^^ 5,9x10"^ 2.7x10^® 1.8xl0"

4

^ For the sites that showed a dglay At > 0.5 sec. An varied in value between 1.2 x 10 and 6.8 x

10 with a mean value of 2.7 x 10 . Using a collision time of T = 10" '-^ sec, Nq varied between

1.1 X 10^^ and 6.7 x 10l^(cm-3) with a mean value of 2.6 x 10^^ (cm" 3), while k-^ range was 4.3 x 10"^ - 2.6 X 10"^ with a mean value of 1.0 x 10"^. For T^, = w"-*- 3.3 x 10"^^ sec, was in the range 2.0 x

lOl^ - 1.2 X 10l^(cm-3) with a mean value of 4.8 x lO-*- , and the k-^ range was 1.3 x 10"^ - 7.8 x 10"'* with a mean value of 3.1 x 10~*. Note that using a shorter collision time in these calculations re- sults in higher values for the free electron density Nq and the extinction coefficient k-j^.

The measured value of An, the calculated values of the free electron density, and extinction

coefficient are all lower estimates. First, due to the morphology of the BSC-2 entrance and exit

surface damage, it was difficult to exactly observe the location of the probe beam relative to the

damaged region using the 40X telescope. If the probe beam was slightly displaced from the central

portion of the ruby laser pulse, the probe sampled a lower field site area with a lower free electron

density. Second, it is also probable that the probe beam was incident at an angle slightly off the

critical angle, and hence was less sensitive to changes in the index of the irradiated site (see

figure 2). Finally, the free electron could have continued increasing for one or two nanoseconds

beyond the start of the dip in the transmitted damaging ruby pulse.

We have solved the one dimensional heat equation with a heat source that is Gaussian in time

[11], and found that the values of k^ calculated for each damaged site at onset of damage was more

than enough to subsequently cause at least melting of most of the sites that showed a time delay At.

5. Conclusions

The ability of the optical probe technique to measure time dependent index variations in the

range An = 10 - 10"^ on the nanosecond timescale provides a sensitive, non-destructive method of

examining the transient properties of materials.

The utility of the probe technique to obtain detailed temporal data of laser induced damage has

resulted in the observation of the buildup of a free electron density several nanosejgnds prior to the onset of surface damage in BSC-2 glass. The magnitude of the density, in the 10 - lO-'-^/cm^

range, is consistent with measured index variation and the Joule heating necessary to cause melting at

the surface site. The buildup time, in the 1-2 nanosecond range, is interesting in view of recent

publications [7,8] which are critical of current theories of electron-avalanche breakdown as the

mechanism for this buildup. Sparks [7] suggests a collision process requiring several nanoseconds to establish an electron density sufficient to initiate damage.

6 . Acknowledgments

We would like to acknowledge valuable discussions with Prof. J. Marburger and Prof. R. W. Hellwarth.

-359-
References

[1] Hellwarth, R. W. , "Role of photo- [9]

electrons in optical damage," Damage

in Laser Materials , NBS Spec. Pub

.

341, 67 (1970)

.

[2] Wasserman, A., "A mechanism for

damage in solids by intense light," [10]

Appl. Phys. Lett. 10^, 132 (1967).

[3] Keldysh, L. V., "Ionization in the

field of a strong electromagnetic

wave," Sov. Phys. JETP 20i, 1307

(1965)

.

[4] Bass, M. and H. H. Barrett, "Avalanche

breakdown and the probabilistic nature

of laser-induced damage,"

IEEE J. QE 8, 338 (1972) .

[5] Holway, L. H. , Jr., "High frequency

breakdown in ionic crystals," J. Appl.

Phys. 45, 677 (1967)

.

[6] Bloembergen, N. , "Laser-induced

electric break down in solids," IEEE

J. QE 10, 375 (1974)

.

[7] Sparks, M. , "Current status of [13]

electron-avalanche-breakdown theories ,

this publication.

[8] Holing, N. L. , et al. , "Statistics in

laser-induced dielectric breakdown,"

Appl. Phys. Lett. 27, 191 (1975). [14]

[11]

[12]

Parks, J. H. and N. Alyassini, "Time

resolved damage studies of thin film

and substrate surfaces," Laser- Induced ! Damage in Optical Materials , NBS Spec.

Pub. 372 , 104 (197rn

Alyassini, N. , J. H. Parks and

L. G. DeShazer, "Time resolution of

laser-induced damage to thin films, '|

Laser-Induced Damage in Optical

Materials, NBS Spec. Pub. 387, 133

(1973) .

Alyassini, N., "A study of laserinduced damage to dielectric surfaces

and thin films," Ph.D. Dissertation,

U. of So. California, 1975 (University

Microfilm, Ann Arbor, Mich.).

Giuliano, C. R. , "The dynamics of

transmitted, reflected and scattered

laser pulses above and below damage

threshold : The search for precatastrophic damage," NBS Spec. Pub.

414, 179 (1974)

.

V.

Calkin,

S. Vavilov

G. N. , andL. M.

A.

Blinov,

G. Golovashkin

^

"Plasma resonance on nonequilibrium

carriers in semiconductors , " JETP

Lett. 7, 69 (1968)

.

Kittel, C. , Introduction to Solid

State Physici (John Wiley & Son,

3rd ed.

I

9687

8. Figures

TEK. 7904

TEK. 556 P. SPARK

r — sDET. o

trigger;

TEM.. RUBY

TEK. 519 HE-^E

PROBE

>ITT F4000

Figure 1. Schematic of experimental arrangement

and optical probe technique for study

of surface damage In scllicate glass

BSC-2.

'Incident field parallel to plane of incidence

"Incident field perpendicular to plane of incidence

-j10 20 30 40 Incidence Angle e (degrees)

Figure 2, Absolute change In reflectivity AR of

a BSC-2 sample of Index n = 1.513 due

to a decrease of .003 in Its Index, versus the Incidence angle 6, At each

angle of Incidence 6, a decrease In

the Index will cause a change In the

reflectivity; AR Is the absolute

magnitude of this change.

-360-
(a)

(b)

(c) 10 mV

FWHM = 10.6 nsec

10 nsec

(a)

(b)

(c)

10 mV

—FWHM = 12.4 nsec

10 nsec

Figure 3. Entrance surface damage data, site

D64,9. (a) output (left) and input

pulse traces of a non-damaging pulse,

(b) output and input pulse traces of

the damaging pulse, (c) optical probe

trace, (d) damaging ruby pulse. The

time scale for traces (a) and (b) is

10 nsec/8.5 mm. Measured values for

this data site are At = 1,5 nsec,

AV = 3,5-4.0 mV,

Figure 4. Exit surface damage data, site P7,8,

(a) output (left) and input pulse

trace of a non-damaging pulse,

(b) output and input pulse traces of

the damaging pulse, (c) optical probe

trace, (d) damaging ruby pulse. The

time scale for traces (a) and (b) is

10 nsec/8.5 mm. The measured values

for this site are At = 2.4 nsec and

AV = 2,5-3,0 mV,

NO COmENTS ON PAPER 8/ AL/AS5IWI and PARKS

-361-
6.5 Multi-pulse Optical Damage of NaCl*

P. BrSunlich

Bendix Research Laboratories

Southfield, Michigan A8076

and

Research Institute for Eng. Sciences

College of Engineering

Wayne State University

Detroit, Michigan 48202

and

P, Kelly

Physics Division

National Research Council

Ottawa, Canada KIA OSl

Single-shot intrinsic laser damage of NaCl at 1,06 ym is caused by avalanche

ionization. The reduction of the damage threshold of an order of magnitude in case the

sample is exposed to a mode-locked train of ten pulses, observed by Penzkofer, was assumed

to be caused by absorbing inclusions [1]'. In this paper we show that the dependence of

the damage threshold on the number of pulses in a mode-locked train can be satisfactorily

explained by the avalanche breakdown mechanism.

Key words: Avalanche breakdown; intrinsic damage; mode-locked pulse trains.

Recently Penzkofer [1] reported intrinsic damage thresholds for NaCl that is exposed to a train of

up to 10 mode-locked pulses of 6 psec FWHM duration from a Nd : Yag laser. He measures the following

damage thresholds.

2 1) 1.5 X 10 W/ ^m for a single pulse

ii) 4 X lO-*- cm ^ for damage occurring in the second pulse

iii) 1.5 X 10 W/ cm for damage occurring in the 10th pulse

In agreement with recent work by the Raytheon-Harvard group [2], the single pulse damage is interpreted as avalanche breakdown; however, the multi-pulse thresholds as due to inclusion heating [1],

This inconsistency is unacceptable in view of the fact that in both cases the probability for hitting an absorbing inclusion is identical. A closer examination of the problem appears therefore warranted. We

will show in this paper that the dependence of the damage threshold on the number of pulses in the pulse

train can be explained quite adequately by the avalanche breakdown mechanism [2,3,4],

For the computation of the damaging photon fluxes, we employed the same electron kinetic approach

as used in [3,4,8], A detailed discussion of this computational procedure was given in

[4], While this type of analysis does not permit to calculate absolute value of the damage threshold with high accuracy, it is well suited to study the change of a threshold with parameters such as the pulse width, etc., or, as in the present case, with the number of pulses in the mode-locked train.

The calculations reported here were performed with 15 psec FWHM indj^vidual pulses having a pulse separation of 300 nsec. All pulses in the train are of equal height A (peak photon flux) and are given by

F = A* sin^ (iTt/t ) (1) P where t^ = 30 psec is the full pulse duration.

The electron kinetic equations used for this purpose were described in [4]:

dT/dt = n^e^TE^/mpk (1 + u^t^) (2)

dn /dt = n 0) + n to, + n^o) - n (N^-X-n^)u + Xii)_ (3) c V vc c i t tc c t t ct Fc

dn^/dt = -n^o) + n (N- -X-n^)a) ^ (4)

t t tc c t t ct dX/dt = -Xa)_ (5)

where m is the effective electron mass, t the electron-phonon collision time, u the laser frequency, e the electron charge, E the rms optical field strength, p the sample density^ and k its heat capacity.

The temperature increases during the laser pulse^and when the melting point Tj^ = 1074 K is reached

irreversible damage occurs [4,7],

* supported by the Division of Materials Research of the National Science Foundation

1. Figures in brackets indicate the literature references at the end of the paper.

-362-
The w.j^'s are electron transition rates in appropriate units. The subscripts designate the type

of transition. X and n^. are the density of F-centers in the ground level and in, the, excited level,

respectively. is the density ofj^gl~-Yacancies which we assume to be 5 x 10 cm . The best crystals available have at least 1 x 10 cm" Cl~ vacancies.

The avalanche ionization rate is given by [4,5,6]

In a)^(E) = 8.17 + A. 22 In E - 0.823 (In E)^; 1 ^ E £ 50 (6)

from which lo^ (sec *") is obtained by using the rms optical field strength E in the units MV/cm.

At the ruby wavelength conduction electrons (n^) are generated from valence electrons (n ) by fivephoton absorption (transition rate uj^), by two-photon absorption (co ) from F-centers in^the ground

level (X), and by single-photon absorption (to ) from excited F-centers (n ). lC t The numerical values of the involved transition rates are listed in table 1.

We have considered three different cases

• X = 1.06^m; here all multi-photon terms in eq, (3)-(4) are neglected

• X = 6943 A; we have calculated two mechanisms in this case, (a) avalanche ionization

only, no five-photon photo carrier generation^ and (b) five-photon assisted avalanche

ionization.

On first sight the avalanche breakdown mechanism apparently cannot account for the one order of

magnitude reduction in the damaging photon flux when the damage occurs in the 10th pulse of the train as compared to single pulse damage. That this is not so will be shown in the following section.

Single-pulse damage is commonly observed at around t = t /2, and we assume this was the case in

Penzkofer's experiments also. For two-pulse damage, the probEbility for it to occur around t /2 of the

second pulse is also very high [7]. In both cases, damage may also occur at considerably smaller peak

fluxes than those producing breakdown at tp/2,' however, the probability for damage to occur late in the

pulse decreases accordingly [7]. As the peak flux of the pulses in a train is reduced, damage occurs

later in the train. The temperature of the exposed spot in the crystal increases from pulse to pulse.

It may either rise up to the melting point, T , and result in irreversible material modifications

(damage), or it may reach a steady-state equilibrium at which temperature losses (e.g., by diffusion)

and carrier recombination prevent further increase of T during the duration of the train.

* In order to establish the relation between the peak flux A and the time of damage occurrence In a multi-pulse train, we have calculated Tjjj(t) as a function of A , The chosen pulse trains (up to 10

pulses of 15 psec FWHM, 300 nsec pulse separation) have a duration of up to .3 psec. Carrier relaxation

(the life time of free electrons and of excited F-centers is 10"^ sec. [8]) effects the results somewhat. However, smaller separation times (<100 nsec) between pulses result in negligible decay of n and

during the duration of the pulse train. Temperature diffusion as well as electron diffusion is

negligible for pulse trains of <1 usee duration [3] provided the laser spot radius is larger than 10 ym.

Some results are compiled in table 2. Clearly, Penzkofer's results can be explained by avalanche

breakdown alone. Our "avalanche only" data clearly indicate that at A*/10, ten pulses produce breakdown

where A is one shot damage threshold at t /2, No alternate damage mechanism appears necessary to account for the large difference in the brlakdown threshold for simple pulse damage as compared to multipulse damage. For completeness, some results for "avalanche only" and for multi-photon assisted ava- lanche breakdown at the ruby frequency are listed as well. The decrease in A*^^^ with the number of

pulses in the train is again significant.

Figure 1 shows the relation between the damaging peak flux A and the time of damage occurrence in

the pulse train. Since no damage occurs between pulses, the separation times between pulses have been

neglected in this graph.

In view of these results it appears appropriate to re-examine the classic experiment by

Yablonovitch [9] who found a systematic correlation between dc breakdown and laser breakdown at

10. 6um. He used a train of short pulses from a C0„ laser having a total length of 100 nsec, which is

short enough to neglect relaxation effects. The width of the individual pulses was a few nsec. The

thresholds he measures are multi-pulse values. Typically, damage appeared to have occurred in the

fourth pulse. In view of the above calculations we expect the single-pulse damage thresholds for all

alkali halides to be higher than the values reported in [11] and thus appreciably higher than the

dc thresholds. The correlation with the dc results, however, is expected to be unaltered.

Refer ences

[1] A.Penzkofer; Optics Com. 11, 265 (1975). [3] P. Braunlich; Final Report for Air Force Cam-

[2] N. Bloembergen; IEEE Quant. Electron. ^E^^IO, Y^'K.TrT''

AFCRF-TR-0266. 1974

375 (1974) _3g3_ (""Published)
[4] P. BrSunlich, A. Schmid, P. Kelly; Appl.Phys.

Lett 26, 150 (1975).

[5] E. Yablonovitch and N. Bloembergen; Phys. Rev,

Lett 29, 907 (1972)

.

[6] D. Fradin, Harvard University Technical Report

No. 643, 1973 (ONR-372-0012) (unpublished)

[7] P. Kelly, P. BrSunlich and A. Schmid; Appl.

Phys. Lett 2_6, 233 (1975).

Tables and

[8] I. M. Catalano, A. Cingolani, and A.N.

Minafra, Phys. Rev. B5, 1620 (1972).

[9] E, Yablonovitch, Appl. Phys. Lett 19, 495

(1971).

Figures

Table 1. Values of transition rates. The superscript 0 denotes the optical and th the

thermal part of the transition rate.

Photon- i nduced transitions

X = 0.694y

05 = 0.5 X lO-^^^^cm^Osec^

"v"vc

" ^-^^ 10"^^^F^(sec"^cm"^)

^ 10"''^F(sec~'')

a)°(. z 1.6 X lO'^^F^Csec"'')

Thermal ly- i nduced transitions

th

'^tc = 7 X lO^T^/^ exp(-0. 1 leV/kT)

(sec-"')

tc , "PC ^ 7 X lO^T^''^ exp(-l .94eV/kT)

(sec-b

'^ct

1 45 X 10"^(sec'^cm^)

-364-
Table 2. Laser peak photon flux A*j,„ required for dam ^ pulse train damage of NaCl

No. of pulses

in train

Avalanche Nd(l . 17 eV)

A* t dam dam

10^°(photon/ (psec)

sec cm )

Avalanche

A*

dam

10'^°{photon/

„ 2 \ sec cm )

Ruby(1.78 eV)

t Ju d rn (psec)

five-photon assisted

Ava 1 anche

A* t dam dam

10-^°(pl)oton/ (psec)

2 sec cm )

1 11.9 1 5 7.77 15 1 . 44 13.7

2 5.95

4.75

3.60

7.4

1424

3.9

3.1 2.4

7.8

1424

.72 .66517.1

23

10 1.19 17.5 .777 24 .359

. 360

22.7

23.2

COmENTS ON PAPER BV BRAUWLICH and KELLV

Th^ 4peafeeA mpha^lz^d th^ iact that th^ cJUUcaZ paJuimtt2A^ In dMln^ng miMlpuL&^ dmagz OAZ

dliiulion U.^, both a.s^on Ui^lon In tk^ c^e avalanchz

^J, f^^^^f^l^rc^cA

Vol IncluJ^lon damage th^ abio^ibing Incimion int^giatti ov^A tkt paUt tAo^n io that damage, can occuJi

ZtTJSZ^e. SuUmMon az\ncZu..on. that wUZ not lead to drn'^B^-fj-^ I'^Luon ta.e WiimlmUon A ^mlloJi eUee;C can occuA In thz caiz oi avalancnc ^on^zatxon. }i the i>epaMuUon t^me

C^eT^cce^U.e pc^e. 15 .hoU co.poKed ^Uh the eZccXAon d^H^^on t^c ofjij^c .^larr^o^

PleclAon tARmina kme then the plasma mill continue to ^ncJieoAe ^n deni^ &iom paUe to puUe.

HoJ^eT ^enTftS conLlon not .aXA^iied thexe mM ,tUZ be an integ^on Hi^f- J'^f

Zec^on avalanche pKoceM heat^ the tatUce and U the pixUe iepaAat^on ^ ihonZ compaAcd to the

Te^cl\^U^To^^ ^I«xee tempeMe u^ill continue to ni^e i^om puUe to puUe even though the

^e^on avMe p^oce.. the cauoe oi the damage, ,o

fj^^ /^^'^"^-J^J^f^^"2/^^^,^

acluaUy p^e^ent. Whatever mean6 lead, to the heaUng oi

^ff-J^^' f^^^^ ^ te Z£cTTd^-

avalanche heating ok IncluMon hexUing. once a cAUtcat tejnpeM^uxe aAA^ved at the Utt^ce am Oam

Kupted and InJievznMlbte damage occuu.

-365-
4

6.6 Do Multi-photon Induced Collision Chains Lead to Pre-breakdown Material Modifications in Alkali Halides?

A. Schmid and P. Braunlich

Research Institute for Engineering Sciences

College of Engineering

Wayne State University

Detroit, Michigan 48202

and

Bendix Research Laboratories

Southfield, Michigan 48076

and

P. K. Rol

Research Institute for Engineering Sciences

College of Engineering

Wayne State University M

Detroit, Michigan 48202

Experiments are described that indicate the existence of a new phenomenon

in certain alkali halides: the multi-photon induced collision chain along [211]

and [110] directions of the halide sublattice at power densities below the oneshot damage threshold. A preliminary model description of the involved physical

processes is given. The sequence of events starts with the multi-photon generation

of self-trapped excitons and leads, via nonradiative exci ton-decay, to the formation

of new color centers and/or to the ejection of atomic species from the surface.

Possible implications which these pre-breakdown material modifications may have on intrinsic optical breakdown are discussed.

Key words: Alkali halides; collision chains; halogen emission; intrinsic damage;

multi-photon absorption.

Laser damage reveals itself through one or more of the following phenomena, all of which indicate

an Irreversible modification of the damaged material [1]^:

i) a sudden decrease of the transmitted laser power

ii) the formation of a hot plasma

iii) the formation of bubbles and cracks (or craters on the surface).

If none of these phenomena is observed, the material is believed to be uneffected aside from some temporary increase in temperature, possible electric charging (due to emission of photoelectron)* or evaporation of contaminants from the surface, etc.

Holing et al [2] have recently investigated the formation of color centers which may be formed by

electron or hole trapping in existing point defects; e.g., the formation of F-centers in NaCl by

trapping of conduction electrons in Cl~-vacancies . These color centers represent an electronic

material modification which can reduce the damage threshold in repeated shot experiments. This is, of

course, one of the reasons why studies of intrinsic laser damage should be performed on sample sites

which were not previously exposed to intense laser photon fluxes. Also, color center formation may be one of the origins of the statistical character of laser damage[2]. Nevertheless, repeated-shot

damage thresholds are of considerable practical interest for they provide information on the susceptibility of optical components to laser damage under repetitive use.

In the present paper we discuss experimental evidence for another type of pre-breakdown material

modification that we believe may have a profound effect on the damage characteristics of certain

optical materials. This laser induced change of the sample is not electronic in character (such as electron trapping in Cl~-vacancies), but rather consists of laser induced formation of new or additional

absorbing defects by removing lattice constituents from their normal sites. The removal of lattice

constituents can be particularly efficient on the surface as we have found in the case of NaCl, KCl, and

KBr that are exposed to intense fluxes from a Q-switched ruby laser. The range of fluxes for which we

were able to detect this phenomenon extends from the one-shot damage threshold down to about one-tenth

of this value.

The experimental arrangement which we used is shown in figure 1. Inside a vacuum chamber (i2 x 10

^

Torr) the crystal is mounted on a sample holder. The beam from a Q-switched ruby laser is focused onto

the sample and the emitted species are analyzed with a minature Mattauch-Herzog mass spectrometer [3]

which is positioned such that it receives atoms and ions being^emitted from the fee alkali-halide in

the [110] direction. The halogen atom signals increase with A , where A is the peak laser flux and N is an integer, which depends on the particular alkali halide and is larger than two. For KCl we find N=4.

*Supported by the Division of Materials Research of the National Science Foundation

1. Figures in brackets indicate the literature references at the end of the paper.

-366-
A preliminary calibration of the detection system showed that at the highest flux, just below the damage

threshold, up to IOI6 halogen atoms may be emitted per laser shot.

The directionality of the halogen emission was established with the aid of microchannel plates in

conjunction with a phosphor screen or with specially modified detectors having high spatial resolution.

A typical result from an undistorted surface region of KCl is shown in figure 2. Emission occurs in

four [211] directions and the four [110] directions that penetrate the sample surface. Only four of the

twelve possible [211] spots are observed because of geometrical detector limitation. The center hole in

the detector is required for passage of the laser beam.

We believe the following simple model describes the physical processes that occur in the sample.

During exposure to the laser pulse, self-trapped excitons are formed by multi-photon absorption. In alkali halides this self-trapped exciton may also be regarded as a V color center plus one electron [4].

Since this in itself is an excited state, it will decay radiatively or non-radiatively to the ground

state. The multi-photon excitation is expected to lead to the same final state of the exciton as observed by electron bombardment [5] , x-ray[6] or UV-excitation[7 ] , At room temperature the probability

for non-radiative decay is high. It was shown that the non-radiative decay of the V~ center leads to the formation of separated F and H centers via a halogen atom replacement collision sequence[8]. If we

apply this model to our experiment, and if we assume that the crystal surface intercepts the replacement

collision sequence, then not only the emission of halogen ions and neutrals can be explained, but also its

structural directionality.

In summary, the following sequence of events appears to occur:

1) Multi-photon excitation of an electron from the valence band during the duration of the

laser pulse.

2) The resulting Cl-ion which has the excited electron only weakly bound finds its energetic

saddle point in the formation of a CI2 molecule-ion or a Vj^ center with an orbital electron attached. This center can also be understood as a self-trapped exciton.

3) At room temperature the lifetime of the self-trapped exciton is very short and two options of exciton decay are open:

a) radiative decay: results in luminescence

b) non-radiative decay: results in kinetic energy for the CI partners of the

original CI" molecule- ion.

4) Non-radiative decay may launch a Pooley-Hersh collision chain[9] along [211] and [110]

direction in fee alkali halides. o 5) If the laser excitation occurs close enough to the surface ("^ 40 A) the crystal surface

may intercept the collision chain and CI is released into the mentioned specific crystallographic directions in ionized and neutral form. In the bulk, the formation of new CI -vacancies and Cl-interstials as a result of the collision chain is

expected to reduce the damage threshold from shot to shot. However, we have not yet incorporated a color

center creation rate in our kinetic damage model, and a dynamic description [2,10] of this effect is

not available at this time.

On the surface a pit is eventually formed after one or more shots, and according to Bloembergen[l]

,

the threshold flux for avalanche ionization is reduced as soon as the dimensions of this groove reach a few hundred A. At sufficiently high fluxes this may occur even during one shot of 30 nsec duration so that ablation in conjunction with avalanche ionization, in fact, appears to be the damage mechanism at least in KCl, KBr, and NaCl at the ruby frequency. With a laser spot radius of .1 mm, we calculate the area affected by collision chains to be .015 mm . A fourth-order excitation process is assumed in this

case. In KCl we have 2.5 x 10-^ lattice cells per mm or .5 x 10 Cl-ions per mm^ of surface area. Therefore, the measured emission corresponds to more than 1000 ablated layers per laser shot at the

upper pre-breakdown flux limit. The effected areas may not be detected with the usual optical methods

used to detect the occurrence of breakdown until the diiuensions of the pit approach 1 or more.

Finally, we want to point out that a new mechanism appeared to be effective in some of the observed

damage events . Figure 3 shows a damage site in the [110] plane of KCl, The emission pattern observed

during that particular event was very similar to the one in figure 2. We must conclude from the symmetric

appearance of the damage site that no plasma was formed which would have caused deposition of material

in a circular pattern[ll]. Rather, we believe that damage occurred due to a dense flux of simultaneously propagating collision chains with no indication of avalanche breakdown. The latter process

would have resulted in melting and plasma formation.

References

[1] N. Bloembergen, IEEE Quant. Electron. QE-I9, (1974).

375, (1974).

[2] N. Boling, P. Braunlich, A. Schmid and P. [4] R. G. Fuller, R. T. Williams, and M. N.Kabler;"

Kelly, Appl. Phys. Lett. (Aug. 15, 1975) to Phys. Rev. Letter 25, 446 (1970).

be published,

[5] See, for example: R. T. Williams, and M. N,

[3] J. P. Carrico, S. Booker, J. Rice and E. Kabler; Phys. Rev. B9, 1897 (1974) and Solid

Schaefer; J. Phys. E. Sci. Instrum., 7, 471 State Comm. 10, 49, (1972).

-367-
I

I

[6] A. Wasiela, G. Ascarelli, and Y. Merle [9] R. Smoluchowskl, 0. W. Lazareth, R. D. Hatcher

d'Aubigne; Phys . Rev. Lett. 31, 993 (1973). and G. J. Dienes; Phys. Rev. Lett 27, 1288,

(1971). ~

[7] V. V. Bichevin, KL. F. Kaambre, Ch. B. Lushchik,

and E. S. Tlisler, Soviet Physics - Solid State [10] P. Braunlich, A. Schmid and P. Kelly, Appl.

12, 2332, (1971). Phys. Lett 26, 150 (1975) and Appl. Phys.

Lett. 26, 253 (1975).

[8] Y. Al Jammal, and P. D. Townsend; J. Phys. C. Solid State b, 955 (1973) [11] N, Boling, G. Dube, and M. D. Crisp; Appl.

Phys. Lett. 21_, 487 (1972 and references

therein.

Figures

Figure 1. Schematic of experimental

arrangement.

Figure 2. Cl-emission pattern during exposure of

KCl to an intense pulse from a Q-switched ruby laser.

n

Figure 3. Damage site on exit surface of KCl

after exposure to one pulse from a Q-switched ruby laser.

COmEHTS ON PAPER BV SCHMWT, oX cut

TkeAZ Ui a clzoA Imptication in tkU mik that a sample, mold damage at a iigyii^ticantly loMeA

thfiuhold on thz 6e.cond ihot a^toA -it had been Ajvuxdlatiid once and tho. colon, czntox domation pfiopoi^d

In tka> nuQ,aACLh had taknn place.. The. ipcaken. indicated that that zxpeAmcnt had not be.e.n coMltd out. We iantheA indicated that T-ccnteAi oAe thought to be iJo-Ymed In thif> pfioccii no coloration oi thz

cMjital could be 4een bzcauic oi the iact that all thz T=-ccntMA would be (,omcd in a vM.g mall volume,

oi the cKyital and theAeione the net abionptlon induced by the F-centeu li veAy mall. The ipeakeA.

iantheA indicated that a 6eMA.e6 of, expeAlmenti mole extenAlve than thoie indicated In the papeA have

been coAAled out Including the iAAadlatlon ojj potoMlum cItloAlde, potoMlum bKomide, iodlum chlontde.,

and lithium {^louAlde. The pantlcle emliiion hoi bee.n obieAved In potoMium chloride and potaiilum

bfLomide, but not In lithium f^luoilde* The iodlum chlonlde n.e^ulti obtained at the time pi the

presentation o{j the paper weAe i>tllt amblguoui. The lack o/J obieAvation of, particle emiMion In the

case Oj$ lithium ^louAlde If, consistent with the veAy large eneAgy gap o^ that material.

-368-
6.7 Multiphoton Absorption Coefficients in Compound Semiconductors

from Ruby to C0_ Laser Wavelengths

R. A. Shatas. S. S. Mitra,* and L. M. Narducci

Quantum Physics, Physical Sciences Directorate

U. S. Army Missile Command

Redstone Arsenal, Alabama 35809

A critical review of multiphoton absorption coefficients for direct band semiconductors of interest in laser optical applications is presented. The two-photon

nonlinear absorption coefficients have been calculated in the second order perturbation employing interconduction band transition model of Braunstein and Ockman, two- valence band intraconduction or intravalence band transition model of Basov et al.,

and the Keldysh electromagnetic field "dressed" valence and conduction band wave- function model. In all models, corrections to the original calculations were needed

to account for effective masses, dielectric constants and the energy band dispersion

relations. Comparison of theoretical predictions at 0.694, 1.06, 1.318, and 10.6 |im

laser wavelengths shows that, in general, the Basov model slightly over-estimates, the

Braunstein model substantially under-estimates the nonlinear absorption, and the Keldysh model yields second order nonlinear absorption about three times larger than

Braunstein. It is shown that the second order nonlinear absorption coefficient in

absorption coefficients up to the fifth order have been estimated from the Keldysh

model. The utility of the Keldysh model is demonstrated by the surprisingly good

prediction of the band-edge absorption in GaAs and InSb without any adjustable

parameters.

Key words: Multiphoton processes; nonlinear absorption; nonlinear optical

properties; semiconductor optical properties; two-photon absorption

coefficients.

Under illumination by intense coherent infrared radiation sources presently available, many optoelectronic materials are driven into a nonlinear response region. In this paper, we analyze processes

contributing to the nonlinear absorption of light by electronic transitions taking place as a result of

a multiquantal process in a dielectric or a semiconductor material usually transparent to low intensity

radiation. We also show that the absorbance at high radiation fluxes is describable by a nonlinear differential equation. For the case of a weak radiation field, this equation can be linearized and its

solution yields the Lambert-Beer law. At high intensities, however, nonlinear terms dominate, and con- sequently, a general analytic solution is not available. Physical arguments must be invoked to separate

terms in powers of intensity. For each particular ratio of materials properties to radiation flux

parameters, one particular term dominates the absorption. Electronic transition rates induced by photon

processes of appropriate multiplicity enter as coefficients in the nonlinear differential equation

describing the absorption. Their calculation can be undertaken in the framework of the time-dependent

perturbation theory of wave mechanics; also, a semiphenomenological approach employing the rudiments of

the "dressed state" concept of the quantum field theory is parametrized in terms of the one-electron

effective mass approximation. In the perturbation-theoretic approach, a semiclassical treatment of the

interaction operator is undertaken in v^ich the vector potential of the Maxwell field is used to describe

the optical radiation, and the oscillating electron in the momentum representation accounts for the

electronic motion. The description is complicated by the need to employ the effective mass parametrization of the energy band theory of the solid state. Within the limits imposed by these fundamental difficulties, numerical methods were developed to calculate multiphoton absorption for a number of technologically important semiconductors. Because of a rather unsatisfactory state of the fundamental knowledge in the nonlinear quantum optics from both the physical and the mathematical points of view, a deeper

insight through the employ of methods leaning towards the axiomatic quantum field theory was sought. One

significant aspect of this approach is that the dynamic eigenlevel shifts of the originally quasi-

*Permanent address: Department of Electrical Engineering, University of Rhode Island, Kingston, RI.

''"Permanent address: Department of Physics, Worcester Polytechnic Institute, Worcester, MA.

1. Introduction

-369-
stationary states caused by the turn-on of the radiation field can be incorporated at the beginning of

calculations. This fast-varying modulation of the eigenfunction of a discrete state of the matter field

is not usually accessible by the perturbation expansion of the first-quantized wave mechanics simply

because the order of perturbation expansion usually is not carried out beyond the first-nonzero contribution. Computerized numerical solutions for transition probabilities in various approaches are given

and compared with experimental data; disagreements between theory and experiment are shown to arise

from certain inadequacies of the theoretical approach. Suggestions with respect to the further development of the nonlinear quantum optics of solids are offered. In reviewing the research literature on the

multiphoton absorption processes in gases, we note extensive investigations in recent years [1-3].'''

Unfortunately, despite numerous theoretical and experimental contributions, there is a wide scatter between both the measured and the predicted values of multiphoton absorption. Because of the many applications of pulsed lasers, there is a need to identify theoretical approaches v^ich are suitable to predict at least the order of magnitude of multiphoton absorption in optically transparent materials. In

the case of gases, the ratio of the ionization potential to the photon energy of a typical pulsed laser

is rather high; therefore, atomic photoionization experiments are not suitable to test theories construct'

ed for optical electromagnetic fields of moderate intensity. Specifically in what follows, we compare

two low-order perturbation treatments with a semiclassical procedure in which the band distortion due to the electromagnetic field is taken into account. Within the context of models proposed by Keldysh [4]

,

Braunstein and Ockman [5], and Basov et al. [6,7], we calculate second order absorption coefficients for

a number of direct-bandgap semiconductors and compare theoretical predictions with available experimental results. In section 2, we consider the rate equations describing the nonlinear absorption. In section 3, we outline a general formulation of the multiphoton absorption within the semiclassical radiation theory in the electric dipole approximation. Section 4 deals with the Keldysh "exact" model which

employs conduction and valence band electronic wavefunction "dressed" by the radiation field. Section

5 contains the perturbation-theoretic approaches of Braunstein and Ockman. Comparison between the theoretically predicted and the experimentally determined values of nonlinear absorption coefficients is

given in section 6.

2. Attentuation of Light by Nonlinear Absorption

In a transparent material, residual absorption of light may be related to a number of independent

processes such as multiphoton and -phonon excitations, presence of residual free carriers or impurities,

creation of excitonic states, and perhaps phonon parametric amplification. At sufficiently high intensities of the optical field, free carriers created through the elementary excitation processes enumerated above, may contribute to the time dependence of the absorption through the free carrier interaction

with the electromagnetic field (so-called inverse Bremsstrahlung) . The time dependence of this part of

absorption follows from the rate equations describing the free carrier density and shows typically

transients of lO"-'-^ to 10"^ sec duration reflecting the trapping and recombination rates in solid materials. In these considerations, we will neglect these transient contributions.

For the steady-state description of the nonlinear absorption, we argue that for a given material

with a fixed bandgap between the valence and conduction bands Eg and a given incident photon energy hu,

the photon absorption on v-th order dominates. Here v denotes the ratio (Eg/hu + 1), and the brackets

( ) stand for the integer part of this ratio. Although linear energy losses caused by nonresonant processes will always be present even in the purest materials, low-intensity measurements can yield reliable values of the linear absorption coefficient. Hence observed intensity loss due to multiphoton

absorption should be satisfactorily described by the phenomenological rate equation

g = - (a^l^ + O'/ .f") = , (2.1)

v=l

where i represents the spatial coordinate along the direction of travel of the light in the material,

and tty is the v-photon absorption coefficient. The total experimentally observed intensity attenuation

should be corrected for the contribution due to nonresonant losses. The corrected value provides the

rate of intensity loss dl/d£ due to nonlinear absorption processes.

_2

In the above rate equation, I denotes the incident intensity (light flux in units of W cm ).

The dimension of the v-th order absorption coefficient o'^ is (length)2^~^/(power)^"-'-. If the multiphoton process of order v dominates, we can express the attenuation rate of the light flux as

dl _£_

Figures in brackets indicate the literature references at the end of this paper.

-370-
where c is the velocity of light in vacuum, and e„ is the high-frequency dielectric constant of the

material. The relation between the flux in the material I [W cm~2] and the peak electric field amplitude Eq [V cm-1] in practical electromagnetic units is given by I = %(e| yi^/Rp) , where is the vacu- um impedance (377 Q) . The relation between the flux I [W cm"^] and the photon number density Npjj [cm"3]

is given by I = (c/ ^s^) hi) N-j^. Accordingly, the rate of photon absorption per unit volume is expressed

by

dN ^ a dN

ph V V _ e dt " to dt

where Ng designates the number density of free carriers created by across the gap ionization through

the v-photon absorption. The right-hand side of the above equation is also a consequence of the energy

balance condition since the rate of the photon absorption must be v-times larger than the rate of the

free carrier creation; hence dNp^^/dt + v(dNg/dt) = 0. We also designate

dN ,

where w is the multiphoton transition rate; consequently, the multiphoton absorption coefficient for

the v-th order process is given by

2v ho(2R )^ w(E^^) \' v/2%v ° • (2.2)

00 O

In eq. (2,2) e„ is the high frequency dielectric constant, and the factor of 2 is introduced to account

for the free carrier spin degeneracy. Note that despite the apparent dependence of the absorption

coefficient upon E^, the actual calculated numerical values of given in section 6 are field

independent.

3. Multiphoton Absorption by the Semiclassical Time-Dependent Perturbation

The theoretical description of multiphoton absorption is based on the employ of second- and higherorder time-dependent quantum mechanical perturbation theory. In the case of the two-photon process, it

has been first shown by M. Goeppert-Mayer [8] that the interaction term coupling tjje momentum of the

electron p with the vector potential of the Maxwell field A , given by (e/mc) p . A, can be canonically transformed to ? ,2. This transformation is valid when the linear extent of the interacting

electron-ion system is small with respect to the wavelength of the radiation field (this condition is

readily satisfied for the electric dipole mediated transitions into far ultraviolet wavelengths) ; subsequently, either of these interaction terms can be used in perturbation expansion. Here P denotes

the polarization of the medium.

At first we shall review the salient features of the perturbation theory. Let there be two eigenstates of energy and Ej; we are concerned with an electric dipole transition between them induced by

photons of energy hto < Ef - . Such transitions can then only take place by the participation of more

than one photon. The order or the photon process is obviously given by (1 + (Ef - E£)/huj) where ( ) indicates the integer part. The probability of such transitions will depend on the number density of

photons or the electric field intensity of the radiation. Large field intensities will also introduce

level shifts.

Let us consider the time-dependent Schrodinger equation

3Ct = ih II . (3.1)

perturbation term

The Hamiltonian 3C consists of the unperturbed time-independent Hamiltonian !Kf and a time-dependent

3C'(r,t). If ^°ir) defined by

^0,0,-»v _, ,o K"t;;(r) = E^*;(r) (3.2)

are the eigenstates ofid^, i|f°(r,t) is given by

-371-
^^(^.t) =^°(^) exp [-i^t] (3.3)

The general state function, which is the solution of eq. (3.1), can be expressed as a superposition of

states

I

Kr,t) =^ a^(t) *°(r,t) . (3.4)

The probability that at any time t the system is in a state with energy E^^ is given by Iajj(t)| , Substituting eq. (3.4) in eq. (3,1), and making use of eqs. (3.2) and (3.3), one obtains

(3.5)

Equation (3.5) may be used to solve a^ to any order. The zeroth order coefficients a^°^ (0) at the time

t = 0 are zero except for the initial state, for which

a(°) (0) = 1 .

The first order coefficient a^'''^ (t) is therefore n

(3.6)

To obtain the second order correction to the coefficient ajj(t), one substitutes the above value of a^^\t)

in the right-hand side of eq, (3.1) and solves for a^^) (t) . Continuing this iterative process, one can write for a general n-th-order correction (the summation over all repeated indices is implied)

I

t ) dt",..

'^m o

n-1

(3.7)

The repeated indices of the wave functions are the so-called intermediate states. With the use of eq,

(3.3), eq. (3.7) may be rewritten in terms of the unperturbed wave functions as

.<"'(t, = (i)

E - E ± hu , . . .£tw n r 1 n

E - E. ± nu), S 1 1_

(E^ - ± hJDj^...±hD^)t'

df , (3.8)

For the sake of generality, every one of the n-photons has been assigned a different frequency cu^ ^ u)^.

The transition probability rate may be readily obtained from eq, (3,8) and is given by

(3,9)

So w^f will contain square of the expression in the square bracket of eq, (3.8), and the integral in eq,

(3,8) will introduce a 6- function
6 (E^ "

'^i

• •^^)

to satisfy the energy conservation requirement.

For optical transitions in a crystalline solid, the initial and final states are obviously the

valence and the conduction bands, respectively. The choice of the intermediate states accounts for

different results in different calculational schemes. The order of the transition equals the number of

quanta of the perturbing field absorbed or emitted. The intermediate states do not appear in the 6-

function,but in the denominator of the square of the pre-integral terms of eq. (3.8). Therefore the

contribution of the intermediate states to the transition probability becomes significant when they are located close to the initial or final states.

In the presence of a radiation field described by the vector potential A , the kinetic energy of

the electron is modified to

which as a result introduces a perturbation Hamiltonian in eq. (3.1) of the type

K' = — p . A (3.10) mc

where 'k = A^ej exp [ik . r - out]. Here, ^ and u) are photon wave vector ^nd frequency, respectively,

and e its unit polarization vector. In eq. (3.10) we have neglected the | A| term, which is usually

very small at the wavelengths and intensities in question. The amplitude eA^ of the vector potential is

related to the photon density N^j^ by

- 2rT N hc^

K'-n^— ^^'^-'-^ (3.11)

where Soo is the dielectric constant. The calculation ^f the transition probability rate then boils

down to the calculation of the matrix elements of p . A between the valence band (or bands) . the chosen

intermediate levels and the conduction band. This is essentially what Braunstein et al., [5] and Basov

et al,, [6,7] models pertain to do,

Keldysh treatment [4] , however, differs from the above scheme as it also includes the level shifts

caused by the perturbing electric fiel^ E which waj neglected by Braunstein and Basov. The energy

shift of a level induced by the field E = /? Eq exp [i(k • r - (ut)] is given by

|p -ll' Ip .e| ' '^mn o' ,

' ^mn o' E - E - hi) n m

E + tw m

(3.12)

where the summation extends to all other states of the system. In the case of a semiconductor, these

level shifts account for the change in bandgap in the presence of a field, which in the case of static

fields is fairly well known, and is termed the Franz-Keldysh [9] effect. Keldysh' s treatment for the

multiphoton transition probability between shifted levels incorporates the dynamic Franz-Keldysh effect,

and will be described in section 4.

4, The Keldysh "Dressed State" Model

Keldysh [4] treated the multiphoton absorption as the high frequency limit of the time-dependent

tunneling induced by the oscillatory electric field of the laser radiation. In other words, a unified

description is available for the autoionization process under the influence of a strong low-frequency

field and for the multiphoton ionization induced by a strong high-frequency field. Keldysh's treatment

applies to both isolated atoms and crystalline solids (e.g., semiconductors). In both cases Keldysh

calculates the probability of direct electron excitation from the ground state (or valence band) to the

continuum (or conduction band) and the excitation probability through intermediate states of the discrete

spectrum. The intermediate states could be higher excited states of the Coulomb field of isolated

atoms, or impurity levels and excitonic states in crystalline solids.

-373-
Keldysh considers a system described by a Hamiltonian of the form

K = + JC^ + JCj, (4.1)

where Kq is the kinetic energy term, Kq is the Coulomb contribution to the potential energy (or the

crystal field in solids), and JCp is the interaction energy of the bound charge in a periodic electric

field E(t) = Eq cos 'Jut. The electron is assumed to be initially in the unperturbed Coulomb ground state

given by the wave function

<|(^(r,t) = to(r) exp (-LS^t/h)

= ^

exp (-r/a ) exp (-W tAi) (4.2)

(na^)

2 2 where ap = h /(me z) is the z-charged atom Bohr radius and its ionization potential.

In the case of a crystalline solid, the Bloch wave function Is modified by the presence of the

electromagnetic field as follows. (In a sense, the bare electron state is "dressed" by the electric

field component of the e.m. field.)

t^'^^r.t) = u^^^^(^) expji(p(t) . r - dTE^[p(T)])| (4.3)

where

p(t) = p + — sin (ut . (4.4)

(v) ~*

"* The amplitude functions u , (r) correspond to a valence electron with momentum p(t) and have the translational symmetry of the lattice. The principal difference between the traditional treatment of

this problem and the Keldysh approach is that he recognizes the modification of the unperturbed bands

under^the influence of the field. This modification amounts to a replacement of the unperturbed momentum p with the time -dependent momentum given by eq. (4.4). The excited electronic states associated

with the continuum of an atom or in the conduction band of a solid are assumed to be unaffected by the

electrostatic Coulomb energy, but are modified by the perturbing effect of the electromagnetic field.

Thus, in the case of isolated atoms, the ionized electron is described by the solution of the Schrodlnger

equation

|i=.(^iv^..l(t,.?) -vt:^ +eE(t) . r;t (4.5)

which is given by

'l'p(r,t) = exp

. / e? \

X exp

For the case of a solid, the conduction electron is described by the Bloch function

t^*=^r,t) = u^^^j(r) exp [p(t) '^-J^ dTE^[p(T)]| . (4.7)

One again notices that the wave function of the final state is also dressed by the perturbing action of

the field. As a consequence, the electronic transition does not occur between unperturbed states, but

rather between nonstationary states in which the electron acceleration due to the field is taken into

-374-
account. Finally, the transition probability rate is calculated according to first order perturbation

theory. The calculated transition probability is then summed over all possible final momenta of the

quasi-free electron.

In order to arrive at an explicit expression for the transition probability, Keldysh used the following parabolic energy-band relation for a solid

E(k) = E (4.8)

where m-- designates the reduced effective mass of the electron-hole pair l/nc'< = 1/m* + Imf , E denotes

_5he wjdth of the bandgap, and the momentum and k-space representations are connected by the rllation

p = hk.

In the limiting case when the parameter

Y = ^

(2m*E )^

o

° (4.9)

is much larger than unity (a condition which is readily satisfied for most of crystalline solids), Kel

dush's transition rate (electronic transition probability per unit volume and per unit time) is given by

w ±„ (H^)%[(2(x+ 1) - 2x)^]

2^2

e E

o

, 15m''<Uj^E

.(x+1)

exp 2(x + 1) 1 (4.10)

The meaning of symbols in eqs. (4.9) and (4.10) is given below: y = 6.45 X 10^ (m*E )%/ (XE^)

,

where m* is given in units of electron mass, E„ in eV, X-wavelength of incident light in vacuum (in \im)

,

2 r.z y2

_ e dy is the DaW'

2 2 2 \ o son integral, x = Eg/ha)(l + e E^/(4m''^ E„)l , and (...) is the integer of the argument.

-1 -z E^ is the electric field amplitude in the ^material in units of MV m , §(z) =e

Up to five photon transition probability rates have been calculated by means of relation (4.10) for

a nvunber of semiconductors as function of wavelength of light at a given field intensity of 10 MV m"-*- in

the material. The relevant band masses and bandgaps used in these calculations are listed in table 1.

Table 1. Material parameters used in calculation of multiphoton transition rates as function of energy

of incident laser light shorn in figures 1 to 6.

Effective mass ratio

Bandgap Reduced

Semiconductor (eV) Electron Hole pair mass

GaAs 1.53 0.058 0.5 0.05

GaSb 0.8 0.047 0.5 0.043

InAs 0.45 0.02 0.41 0.019

PbS 0.34 0.55 0.5 0.364

InSb 0.26 0.013 0.5 0.0127

PbTe 0.24 0.22 0.29 0.125

The lo^ w vs photon energy plots are given in figures 1 to 6. Numbers written above each plateau of the

transition rate indicate the integer value of (Eg/h(u + 1). Apart from quantitative differences, the

qualitative functional dependence in figures 1 to 6 is strinkingly similar in that a quasi-resonant behavior at integer Eg/huo values is displayed. This reminds us of one of the most obvious features of the

multiphoton transition probability calculated by Bebb and Gold [10] for hydrogen-like atoms which shows

the distinctly resonant dependence of the excitation probability as a function of the incident photon

energy. While the energy levels of isolated atoms are clearly quite different from the bands of

-375-
semiconductors considered in Keldysh's model, a quasi-resonant dependence of the transition rate on the

incident photon energy seen in our calculations is qualitatively correct.

It should be also remarked that Weiler at al.,[ll] have extended Keldysh's calculation to include

the effects of longitudinal or transverse magnetic fields on the interband electronic transition.

Weiler' s conclusions are in agreement with Keldysh's results for zero applied magnetic field. One ex- pects a series of edge absorptions of the form ~ (\ftiu - Eg) 2 where v is the photon multiplicity. Such

a behavior is indeed found in the negative slope of the transition rate after each quasi-resonant "peak"

as seen in figures 1 to 6, It has been argued that, at best, Keldysh's theory should provide acceptable

results only for processes of fairly high photon multiplicity and that its application to two-photon

absorption processes should not yield more than qualitative agreement with the experimental data. Recent experiments [12] involving four-photon transition in ZnS are at variance with Keldysh's prediction,

while they are in fair agreement with perturbative semiclassical calculations [13], However, a good

agreement with three-photon absorption in CdS has been found. C. H. Lee measured [14J the three-photon

absorption coefficient = 1.3 . 10"^ cm^/GW^ for single-crystal CdS, Our calculations based on the

evaluation of eqs. (2.2) and (4.10) yielded a value of Q'3 = 0,2 • 10"^ cm We have used Eg = 2,42

eV, = 5,32 and m* = 0,192 as CdS material parameters in this calculation, A more systematic comparison for the two-photon absorption coefficient is presented in section 6, An additional feature of the

Keldysh model was found (which was rather unexpected) that it predicts very well the one-photon absorption coefficient near the band edge for two thus far analyzed semiconductors, GaAs and InSb, It is

known that near the fundamental absorption edge, the one-photon absorption coefficient can be expressed

as

= AQw - Eg)^ (4,11)

where hu is the photon energy, and y is a constant which equals 1/2 and 3/2 for allowed direct transitions and forbidden direct transitions, respectively. We specialize eq. (4.11) for the case hu) > Eg.

In addition, the exponent Y equals 2 for indirect phonon assisted transitions and 1/2 for allowed indirect transitions to exciton states.

Near the absorption edge, where the values of (Juu > E ) become comparable to the binding energy of

the exciton, the Coulomb interaction between the free hole and the electron must be taken into account.

For hu) < Eg the absorption merges continuously into that due to the higher excited states of the exciton;

where hu » Eg, higher energy bands will participate in the transition process and complicated band

structures will be reflected in the absorption coefficient. The least square fit of absorbance vs photon

energy curves for photon energies near the band edge of GaAs yields the following values for A and y : A = 44 , 103, Y = 0.499, and A = 44.7 . 10^, Y = 0.505 from Moss and Hawkins' [15] and from Sturge's [I6]

measurements, respectively. Numerical evaluation of eqs. (2.2) and (4,10) for the one-photon transition

case V = 1 can be expressed by the functional dependence shown in eq, (4,11) with A = 44 • 103 and Y

=

1/2. Hence, the Keldysh model besides giving the correct value of the absorption coefficient at the band

edge of GaAs describes very well its wavelength dependence also. This agreement is seen in figure 7 where

unadjusted calculated absorption coefficients are compared with Moss and Hawkins' experimental values for

photon energies between 1.42 to 1.48 eV. GaAs parameters used in calculating the theoretical absorption

coefficient of figure 7 are listed in table 2.

Table 2. List of parameters for GaAs used in the theoretical fit of figure 7. The values for

reduced effective mass m* and dielectric constant e have been obtained from [18] and

[19], respectively.

00

Data source E [eVj m*

L151 1,403 0.059 10.9

[I6J 1.521 0.059 10.9

[16 1 1.435 0.059 10.9

For the comparison of theoretical and experimental one-photon absorption coefficient of InSb, the experimental data points have been taken from figure 3 of [20]. The empirical equation which fits the data

around the band edge is listed in table 3 together with the reduced effective mass and the high frequency dielectric constant which are needed for the theoretical comparison.

-376-
Table 3. Empirical relation for the absorption coefficients of InSb reported in [20], The bandgap

energy Eg has been calculated by a least square fitting procedure. The reduced effective

mass m* and the high-frequency dielectric constant e^a have been obtained from [19] and

[21], respectively.

« = 2.026 . lo"^ (h« - Eg)^ Lcm-lj

hu) , E in eV

g E = 0.2248 LeVj

S

m* = 0.0113 m ; m = 9.108 • lo"^^ [gj

Geo = 15.7

In fig. 8 we have plotted the experimental data together with the results of the numerical evaluation of

eqs. (2.2) and (4.10). We note that the quantitative agreement between the Keldysh theory and the meas- ured data is rather good, particularly if we consider the large range of incident photon energies included in the comparison. A further discussion on the agreement of Keldysh model with the second-order

perturbation theory is given in section 6.

5. Second-Order Perturbation Models.

In this section, we consider two second-order perturbation models first proposed by Braunstein and

Ockman fSl and by Basov et al.,L6,7]. As explained in section 3, in the second-order perturbation an intermediate state is required to complete the transition from the initial to the final state of the system perturbed by the radiation field. A proper accounting of intermediate states becomes Important when

their energy eigenstates are close to initial or final states of the system, or if they coincide with

other real states encountered in practical materials such as exciton or impurity states. Therefore, the

agreement between the calculated and the observed nonlinear absorption coefficients will depend largely

upon the inclusion of the appropriate details of the energy band structure.

Braunstein and Ockman [5] consider vertical transitions between unperturbed parabolic bands. They assume that the only significant intermediate state is a higher conduction band designated with n in

figure 9.

E

Fig'

k

9. The Interconduction band transition

model of Braunstein; v , c , and n refer to the valence, lowest con- duction, and intermediate conduction

bands, respectively. All energy

bands are taken to be parabolic in

the k-space.

Second-order perturbation theory yields the transition rate per unit volume for vertical transitions

at a given value of k ; subsequently, the k-dependent probability is integrated over all values of k . For all owed -all owed transitions, the result is

' nc ' vn

Srrh^

I (2bu - E )^ / a + a \"

-Hi AE - to + ~r-^ (^'^ - EJ

J2 U + a )^ V ^c + \ S /

(5.1)

-377-
In eq. (5.1), ^ and 3C are the matrix elements of the interaction Hamlltonian between the conduction

nc vti c and the valence v bands, and the intermediate conduction band n , respectively. The free electron mass is designated with m , and (i = c,v,n) is the reciprocal effective mass ratio m/m|. The

energy gap Ae is the energy difference between the top of the heavy hole band (the only valence band included in this calculation) and the bottom of the intermediate conduction band.

The interaction Hamiltonian is taken to be of the form

K= e_p . A

mc

We will evaluate the matrix elements of the interaction term by the so-called k . P method [22] , this method, the matrix elements of the effective Hamiltonian are given by

In

(5.2)

Consider a solid consisting of two energy bands denoted by 0 and 1 each having an extremum in the re ciprocal lattice space at k^ = 0 and at corresponding momenta Pq-^ = p^^ = p taken to be isotropic. We

choose the zero of energy axis to be at the top of the lower band 0 such that Eq(^^) = 0 and = Eg for the upper band. Eg being equal to the forbidden gap width. These assumptions restrict our treatment to solids with allowed direct transitions between the two bands. The Hamiltonian operator is then

given by a 2 X 2 matrix shown below

2 2 h k'' 2m

h 7» — k . p m

m

g 2m

(5.3)

Its diagonalization yields the following eigenvalues of the matrix (5.3)

E- .(k) = E /2 + hV/2m ± + hVp^ /m^ . (5.4)

0,1 g 'S

For small values of k , the square root in eq. (5.4) can be expanded in a power series about E /2.

Retention of the first-term only yields the following expression for the lower energy band

E^(k)

mEg 2m J (5.5)

and

^i(^> = ^g- ^Li.^] (5.6)

for the upper energy band. The effective reciprocal mass tensor of the carrier in n-th band is defined

by

(m/m* )

3 E (k) n

>i2 <^\dv^

-378-

(5.7)
Because of the assumption of isotropic matrix elements in the Hamiltonian (5.3) of the two-band model,

eqs. (5.5) to (5.7) yield scalar reciprocal effective masses for free carriers, [m/m* ] = 1 - 2p^/(mE ), ° o

^

2 and [m/m*] = 1 + 2p / (mE ) for the lower and the upper energy bands, respectively. We take the lower

1 ^ band 0 to be the highest valence band and the upper band 1 the lowest conduction band of a solid

with a direct energy bandgap. The effective reciprocal masses of holes h in the valence v and electrons e in the conduction c bands are thus given by the scalar relations

[m/m* 1 = 1 - 2p^/(mE ) (5.8) h'

V

and

[m/m*] = 1 + 2p^/(mE ) , (5.9)

8

respectively.

The band curvatures of the valence and the conduction bands are of opposite signs which accounts

for the opposite signs of the p2 term in the eqs. (5.8) and (5.9). Keeping in mind the opposite curvatures, we can rewrite eqs. (5.8) and (5.9) in absolute values to yield

[m/m*! = 2p^/(mE ) - 1 (5.8a)

and

|m/m*I = 2p^/(mE ) + 1 . (5.9a)

e

2 From (5.8a) and (5.9a), one readily obtains m/m* = 4p / (mE ). The energy-momentum relation needed to evaluate the matrix elements of the interaction Hamiltonian in terms of the one-electron effective mass approximation of the solid state theory is therefore given by

2 E E- = _£ (_S.) , (5.10)

2m 8 m*

which is quite satisfactory for small values of k . We proceed in evaluating the matrix elements of

eq. (5.1) with the help of eq. (5.10) and substitute the transition rate into eq. (2.2) for the case

V = 2. The second-order nonlinear absoprtion coefficient is given by

E ) AE

-I f f 2 nc vn

(2ha) - E )

'

-/m*

AE

a + a

hu) + -2—

- (2to - E ) a + a g

c V °

.

(5.11)

The f and f denote the oscillator strengths for the transitions mediated by JC and K , respectnc vn vn nc

ively. In numerical calculations given in section 6, it is taken that f = f =1. All parameters ' " ' nc vn

in eq. (5.11) are expressed in c.g.s. electrostatic units.

Equation (5.11) has been specialized for the case of single photon beam of frequency u) in the

material (rather than two beams at two different frequencies and m^). Braunstein and Ockman's expression for the nonlinear absorbance given by eq. (7) of [5] must be multiplied by a factor of

2 f \ 2

I m/(2m*)] to obtain our eq. (5.11). The first term in the corrective factor is due to the

different formulation of absorption coefficient in our eq. (2.2) as compared with eq. (2) of [5],

and the second term arises because we evaluated the matrix elements of allowed transitions in the

-379-
-» -» k • p approximation rather than by employing eq. (4) of ref. 5. Our eq, (5.11) includes appropriate

factors for spin orientation and photon multiplicity.

A generalization of Braunstein's calculation for anisotropic energy bands has been given by

Hassan [23]. His results agree with our eq. (5.11) in the limit of zero anisotropy.

In Basov et al.,[6,7] model, the two highest valence bands Vj^ and and the lowest conduction

band c are coupled by the radiation field. Furthermore, the intermediate states are conduction (or

valence) intraband states as shown in figure 10.

Figure 10, The energy band model employed by

Basov et al. Parabolic energy bands

^1' ^2 ^^^^^ ^° bound electrons

in valence band, and c is the con- duction band. Intermediate levels

are provided by intraconduction and

intravalence states. Only intraconduction band transitions are ex- plicitly indicated.

Second order perturbation calculation of this model leads to the following expression for the sec- ond order absorption coefficient

2^ ./2 TT e^(m*)^(2hii)

(^e

e c (ru)

)

(5.12)

where e is the unit polarization vector of the incident radiation, p is the momentum operator of the

electron and the index 1=1,2 refers to the initial state of the electron in the valence bands. Contrary to Basov et. al . , we use eq. (5.10) to evaluate the momentum matrix elements, and furthermore we average over all directions in the k-space. The total absorption coefficient must contain summation over both valence band contributions, a 2 = ^^o/^^ . Since each valence band contributes about equally,

we need to calculate only o'^'''^ . Therefore, values listed in table 7 of section 6 must be multiplied by

a factor close to 2 to obtain the total second-order absorption coefficient in this model.

In our eq. (5.12), factor of 2 has been introduced to account for the spin degeneracy. This fac' tor has been omitted In the original work by Basov et al. Further differences between our eq. (5.12)

and Basov et al., expressions arise because of an Incorrect numerical factor used in relating the flux

intensity with the magnitude of vector potential in [6]. This gives a multiplicative correction of

1/16. Averaging the scalar product |e • pj ^ over all directions in the k-space yields another multlpli

cative correction of 1/3. Hence, Basov et al., expression for the second-order absorbance in the context of eq. (6) of [6] must be multiplied by a factor of 1/48.

This has been noted previously by Lee and Fan [24] and Fossum and Chang [21j. The need for this

correction does not arise in connection with our eq. (5.12) because our absorption coefficient a2 is

intensity-independent and it was calculated from our eq. (2.2) rather than eqs. (6) and (8) of [6],

and the averaging over the k-space has been also performed in the calculations listed in sectl<]|^ 6. further difference between Basov et al.,work and our eq. (5.12) arises from our use of the k . p metha

in evaluating the matrix elements of the Interaction operator. Because of eq. (5.10), we evaluate

-380-

1
I (e . p) c-v.

1

trtiereas Basov et al.,use the relation p /2m = m*/m in connection with eq. (5) and the relation p /2in =

3Eg/4m* in the experimental results section of [6]. In section 6, we report the numerical evaluation

of eq. (5.12).

6. Comparison Between Calculated and Measured Absorption Coefficients

In this section, we compare absorption coefficients calculated from eq. (2.2) with available ex- perimental data for the second-order nonlinear absorption. First, we evaluate the transition rates for

photon multiplicities v = 1 to 3 in the perturbed valence and conduction band wavefunction model of

Keldysh, eq. (4.10). For the second-order calculation, we also use the perturbation-theoretic approaches

for the two different band models, eqs. (5.11) and (5.12).

In the Keldysh model calculation, it is necessary to evaluate the Dawson integral numerically by

using the trapezoidal rule of integration. In general, the accuracy of the numerical integration is of

the order of a few parts in 10^. if it is desired to increase the accuracy of the Integration, increase

the number of intervals N in the program line 260. The computation time will increase accordingly.

The computer program listed in table 4 has been written in a modified BASIC programming language

for the HP 9830A calculator equipped with ROM's containing the mathematics package and the plotter control. This program (less lines 1000 and above, which contain the calculation of the absorption coefficient), with an appropriate modification of the plotting routine, has also been employed in plotting

figures 1 to 6 of section 2.

Table 4. HP 9830A computer program used to calculate and to plot the wavelength dependence of absorption

coefficients of the order 1 to 3 from eqs. (4.10) and (2.2). The computer system consists of

extended memory core HP 9830A, built-in mathematics and plotter control ROM's, the HP 9866A

printer and the HP 9862A plotter. Instructions for use are contained in lines 30 to 255.

30 REM THIS PROGRAM CALCULATES THE TRANSITION RATE PREDICTED BY KELDYSH

40 REM AND PLOTS VS PHOTON ENERGY IN ELECTRON VOLTS; KEY IN RUN AND FOLLOW INSTRUCTIONS

42 ElEM IT ALSO PLOTS ABSORPTION COEFF. VS PHOTON ENERGY IN ELECTRON VOLTS

50 REM NOTE- AT LEAST ONE INCREMENT MUST BE ASSIGNED

70 PRINT "INPUT PLOTTER SCALE XMIN, XMAX, YMIN, YMAX"

71 PRINT

72 PRINT

80 INPUT Si, S2, S3, S4

90 SCALE SI, S2, S3, S4

95 XAXIS S3, S2/10, SI, S2

100 YAXIS SI, S4/2, S3, S4

115 FLOAT 3 120 PRINT "ENTER GAP IN EV, LAMBDA IN MICRONS"

130 PRINT "ENTER EL. FIELD IN UNITS OF MEGAVOLTS/M, MASS IN EL. MASS"

140 PRINT

150 INPUT G, L, E, M

160 PRINT

170 PRINT "IF YOU WANT TO INCREMENT GAP, KEY IN DELTAGAP; IF NOT KEY IN ZERO"

180 PRINT

190 INPUT Gl

200 PRINT "IF YOU WANT TO INCREMENT LAMBDA, KEP IN DELTALAMBDA; IF NOT, KEY IN ZERO"

210 PRINT

220 INPUT LI

230 PRINT "IF YOU WANT TO INCREMENT EL. FIELD, KEY IN DELTAFIELD; IF NOT, ZERO"

240 PRINT

250 INPUT El

252 PRINT "TO PLOT ABSORPTION COEFF. INPUT DIELECTRIC CONST (E2) ; IF NOT, ZERO"

253 PRINT

254 PRINT

255 INPUT E2

260 N=20

270 PRINT "GAP="; G, "LAMBDA=";L

280 PRINT "EL. FIELD=";E,"MASS=";M -381-
290 PRINT "G1=G1;"L1="L1;"E1="E1

295 PRINT "SI="S1;"S2="S2;"S3="S3;"S4="S4

297 PRINT "E2="E2

300 PRINT

210 PRINT

320 A=8.76057E+36*(M/L)Tl.5/L

330 B=1.23752E-08*(E*L)t2/ (M*G)

340 X=0.806015*G*L*(1+B)

350 Rl=INT(X+l)

360 Z=(2*Rl-2'^X)t0.5

370 W1=A*(B/4)TR1*EXP(2*R1*(1-B)-Z*Z

)

380 Y=Z/N

390 P=(1+EXP(Z*Z))*Z/(2*N)

400 F=0

410 FOR 1=1 TO N-1

420 F=F+EXP((I*Z/N)t2)

430 NEXT I 440 W2=P+F*Z/N

450 W=W1*W2

460 IF G1=0 THEN 520

470 PLOT G.LGTW

490 IF 05 THEN 630

500 G=G+G1

510 GOTO 320

520 IF L1=0 THEN 570

524 IF E2#0 THEN 530

525 PLOT 1.2395/L,LGTW

527 IF E2=0 THEN 540

530 GOSUB 1000

540 IF L>21 THEN 630

550 L=L+L1

560 GOTO 320

570 IF El=0 THEN 630

580 PRINT E.LGTW

600 IF E>50 THEN 630

610 E=E+E1

620 GOTO 320

630 END

1000 X=INT(G/(1.2395/L))+1

1010 A=2*X*1 . 2395/L*l . 602E-19*754t X*W/ (SQRE2tX*(E*lE+04) t (2*X)

)

1020 PLOT 1.2395/L,LGTA

1030 RETURN

GaAs absorption coefficient for photon processes from 1 to 4 order calculated with the program listed

in table 4 is shown in figure 11. The material parameters used in this calculation are listed in table 5 Experimental data for the absorption coefficient of GaAs are available for the first-order and secondorder transitions only; they are entered as a dot, a vertical bar, and a circle in figure 11. It is seen

that the agreement at 1.5 eV (one-photon linear absorption) and 0.94 eV (two-photon nonlinear absorption) between the Keldysh model prediction and the experiment is quite good. The sharp decline in absorption at the increase of the order of the photon multiplicity will be moderated by the contribution

from the excitonic states. Since the calculation does not include exciton effects, one can expect that

the experimental values will not agree with the predictions in the range of photon energies up to 0.1

eV less than that of the absorption edge for a particular order of photon multiplicity. Furthermore,

impurities giving rise to energy levels within the forbidden gap would also Increase the absorption co- efficient. Undoubtedly, they also contribute to the large scatter of experimental values seen at 1.17

eV in figure 11. Because none of these contributions are accounted for in the Keldysh model, the predict

ed values of the absorption coefficients should be regarded as lower limits expected in the case of

very pure materials. By inspection of fig. 11, one can estimate that the second-order absorption will be

significant at light fluxes above 10^ w cm"^ and the third order - above 10- W cm"^ for the intrinsic

nonlinear electronic transition processes in direct-bandgap materials. Absorption coefficients for sev- eral semiconductors of this type are listed in tables 5 and 6 for fixed laser wavelengths of doubled

Nd-^ -glass (0.53 nm), ruby (0.694 M.m) , Nd-^-glass (1.06 nm), Nd^-YAG (1.318 , HF (2.8 nm) , DF

(3.9 tJ,m), CO (5.3 M.m) , and CO2 (10.6 |j,m)

,

Since the order of the transition is the dominant factor in determining the magnitude of the absorption, a large variation in the bandgap with the temperature implies a very large temperature dependence of the absorption coefficient. This is demonstrated in the last entry of table 6 by listing absorption coefficients of PbS calculated at 300 K and 0 K. Because the perturbatlve methods have been

-382-
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-383-
carried out to the second order only, comparison of the second-order nonlinear absorption coefficient

calculated for the three models of sections 4 and 5 is given in table 7. For material parameters used

in these calculations and comparison with the available experimental data, the reader is referred to

[32]. We note that the absorption coefficient calculated from the Keldysh model generally falls inbetween that of the two perturbation models of Braunstein and Basov. However, the Basov model usually ov« estimates the absorption coefficient; therefore, the Keldysh model values yield currently the best estimate of the lower limit of the nonlinear absorption coefficients.

7. Conclusions

Comparison of theoretical models of Keldysh, Braunstein, and Basov in calculating the nonlinear absorption coefficients for direct band semiconductors with forbidden energy bandgaps between 2.6 to 0.15

eV show that the second order perturbation models of Braunstein and Basov differ in their prediction for

Q'2 by almost three orders of magnitude. The Keldysh model prediction for lies between the two perturbation models and is usually slightly lower than the experimental value. A large scatter in reported ex- perimental values of (perhaps attributable to the presence of impurities in real materials) hinders

in selecting the best theoretical approach. However, generally it can be stated that if the interference terms in calculating transition rates are neglected, the absorption constants are underestimated

(Braunstein and Ockman) . Calculations allowing transitions from several bands (Basov) generally over- estimate the absorption. The Keldysh method in which the effect of the optical electromagnetic field

on the eigenfunctions of the unperturbed system is incorporated at the beginning of the calculation

gives a slight underestimate of the two-photon absorption. It describes very well the band edge absorption of the one-photon process. Intuitively, one would concede that the perturbation of the eigenstates

of the noninteracting system by the light intensities of interest is significant and that the perturbed

eigenfunctions should be introduced at the onset of calculations as it is done in the Keldysh model. In addition, the Keldysh model is the only one currently available to'calculate absorption coefficients high er than second order since there has been no published work carrying perturbation theory to a higher

than second order in semiconductors.

Table 7. Comparison of calculated two-photon absorption coefficient »2 [cm w" ]for direct bandgap semiconductors. Absorption coefficients were calculated from eqs (2.2), (4.10)7 (5.11), and (5,12

for Keldysh, Braunstein, and Basov models, respectively. Contribution from only one valence band is listed under Basov; the total absorption coefficient should include transitions

from both valence bands and should be nearly twice as large.

»2 [cm W ] calculated from model of

Wavelength

Material in |im Keldysh Braunstein Basov

ZnSe 0,694 6.4 10-9 2.2 • 10-^ 4.5 • 10"^

CdS 0.694 6.1 10-9 2.2 • 10-9 4.4 • 10-'^

CdSe 1.06 1.7 • 10"^ 1.2 • 10-^

1.318 1.65 10-^ 10-^

CdTe 1.318 2.7 • 10-^ 9.6 • 10-9 1.8 • lo"^

GaAs 1.06 1.9 • 10-8 6.7 • 10-9 1.4 • 10"^

1.318 2.3 • 10"^ 8.25 • 10-9 1.5 • 10-^

InP 1.06 2.6 • 10-3 8.4 • 10-9 1.8 • 10"^

InSb 10.6 2.1 • 10-^ 0.72 • 10-^ 1.8 • 10-^

9 —2

However, because intensities above 10 W cm ^ are required to attain a signficant absorption in the

third order in pure materials, such calculations may be superfluous for practical applications because

of the onset of Drude absorption by free carriers created through the avalanche multiplication induced

by the optical electric field. In fact, the notion that a single physical process can explain the

effects of an intense laser pulse on a given optical material is too simplistic to hold because, for

example, the multiphoton absorption and the avalanche ionization are two competing processes having a different time dependence.

-384-
On the other hand, this contribution demonstrates that it is important to include properly the band

parameters of the solid state into the calculation. In particular, if contributions from excitonic

states and impurity levels should be included, a summation over all corresponding elements of the interaction Hamiltonian must be incorporated into the calculation of absorption coefficients. The outline of

such a procedure is given in [33]. Furthermore, up to the third order, it is sufficient to use the

P . A interaction term in the electric dipole approximation.

8. Acknowledgments

The authors are thankful to Dr. C. C. C. Lee for pointing out inconsistencies in the original

Basov et al.,work and for his comments on the contributions of excitonic states to the nonlinear absorption. Also, thanks are due to Mr, J. E. Williams who has programmed several calculations reported m

this work.

9. References

[I] V. I. Bredikhin, M. D. Galanin, and V. N. Genkin, Usp. Fiz. Nauk 110 , 3 (1973) [Sov.

Phys.-Usp. 16, 299 (1973)].

[2] J. M. Worlock, Two-photon Spectroscopy, in

Laser Handbook , Vol. 2, Arecchi and SchulzDuBois, eds. (North-Holland, 1972).

[3] S. Bakos, Multiphoton Ionization of Atoms,

Adv. in Electronics and Electron Physics , Vol. 36, L. Marton, ed. , (Academic Press,

1971+)

.

[h] L. V. Keldysh, Zh. Eksp. Teor. Fiz. hj_ {l96h)

[Sov. Phys.-JETP 20, 1307 (1965)].

[5] R. Braunstein and N. Ockman, Phys . Rev. 13^ , Ak99 (196I+).

[6] N. G. Basov, A. Z. Grasyuk, I. G. Zubarev,

V. A. Katulin, and 0. N. Krokhin, Zh. Eksp.

Teor, Fiz. 50, 551 (1966) [Sov. Phys.-JETP

23, 366 (1966)].

[7] N. G. Basov, A. Z. Grasyiik, V. F. Efimkov,

I. G. Zubarev, V. A. Katulin, and J. M. Popov, Proceedings of the International

Conference on the Physics of Semiconductors,

Kyoto 1966, J. Phys. Soc. Japan 21 Suppl .

,

277 (1966).

[8] M. Goeppert-Mayer , Naturwissenschaften 17 , 932 (1929); Annalen der Physik 9, 273 (l93l).

[9] W. Franz, Zeitschr.f. Naturforschg. 13a , kSh (1958).

L. V. Keldysh, Zh. Eksp. Teor. Fiz. 3^i, 1138

(1958) [Sov. Phys.-JETP 7, 768 (1958)].

[10] H. B. Bebb and A. Gold, Phys. Rev. lh3 , 1 (1966).

[II] M. H. Weiler, M. Peine, B. Lax, Phys. Rev.

171 , 9^9 (1968).

[12] I. M. Catalano, A. Cingolani, and A. Minafra,

Solid State Comm. I6 (1975).

[13] J. H. Yee, Phys. Rev. B3, 355 (1971).

[ik] C. H. Lee, "Interaction of Intense Picosecond

Light Pulse with Materials," Progress Reports,

Defense Documentation Center #AD7^1391 and

AD750Ulit (1972). -

[[15] T. S. Moss and T. D. F. Hawkins, Infrared

Phys. 1, 111 (1961).

[16] M. D. Sturge, Phys. Rev. 127, 768 (1962).

[17] I. Kudman and T. Seidel, J. Appl. Phys. 33., 771 (1962).

[18] D. Long, Energy Bands in Semiconductors

(J. Wiley, N.Y. , 1968)

.

[19] E. Kartheuser, in Polarons in Ionic Crystals

and Polar Semiconductors , ed., J. T. Devreese

Tlorth-Holland Publ . , 1972 )

•

[20] G. W. Gobeli and H. Y. Fan, Phys. Rev. 119 . 613 (i960).

[21] H. J. Fossum and D. B. Chang, Phys. Rev. B8,

28U2 (1963).

[22] J. Callaway, Quantum Theory of the Solid

State , Part A, pp. 2U8-252 (Academic Press,

New York, 1971*).

[23] A. R. Hassan, II Nuovo Cimento 70B, 21 (1970).

[2h] C. C. Lee and H. Y. Fan, Appl. Phys. Letters

20, 18 (1972).

[25] Y. A. Oksman, A. A. Semenov, V. N. Smirnov,

and '0. M. Smirnov, Fiz. Tekh. Poluprov. 6_, 731 (1972) [Sov. Phys. Semicond. 6, 629

(1972) .

[26] C. C. Lee and H. Y. Fan, Phys. Rev. B9, 3502

(197U).

[27] A. Z. Grasyuk, I. G. Zubarev, V. V. Lobko

,

Yu. A. Matveets, A. B. Mironov, and 0. B. Shatberashvili, ZhETF Pis. Red. 17_, 58U

(1973) [Sov. Phys. JEPT Lett. 17, U16 (1973)].

[ 28 ] S . Jayaraman and C . H . Lee , Appl . Phys

.

Letters 20, 392 (1972).

[29] D. A. Kleinman, R. C. Miller, and W. A. Nordland, Appl. Phys. Letters 23, 21*3 (1973).

[30] J. M. Ralston and R. K. Chang, Appl. Phys.

Letters 15., 16I* (1969)-
[ 31 ] J . I . Pankove , Optical Processes in Semiconductors (Prentice Hall, Englewood Cliffs,

N.Y., 1971).

[32] S. S. Mitra, L. M. Narducci, R. A. Shatas,

Y. F. Tsay, and A. Vaidyanathan, Applied

Optics, 1975, in press.

.33] C. C. C. Lee, "Two Photon Absorption and

Second Harmonic Generation in Semiconductors,'

Doctoral Dissertation, Purdue University (May

197^*) (unpublished).

10. Figures

<

z

o

o

28

26

24

22

20

18

n

I

<

CCzo

32

30

28

0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

H

a 24 o

22

PHOTON ENERGY (eV) Figure 1. GaAs multiphoton transition rate (2-nd

20

to 4-th order) in cm'_ 3 induced by

a constant optical electric field of

lo"* V cm"' amplitude and varied

frequency.

0.2

Figure 2,

0.3 0.4 0.5 0.6 0.7 0.8 0.9

I

<

oczo

z<

oc

t-a

o

31

30

29

28

27

26

25

24

PHOTON ENERGY (eV) GaSb multiphoton transition rate (1-st

to 4-th order) in cm~^ s~' induced by

a constant optical electrical field of

lO"* V cm~^ amplitude and varied

frequency.

0.1 0.2 0.3 0.4 0.5

PHOTON ENERGY (eV)

0.6 Figure 3. InAs multiphoton transition rate (1-st

to 4-th order) in cm~^ s~' induced by £ constant optical electrical field of

lO"* V cm~^ amplitude and varied

frequency.

386-

I
32

7 30

E

<

oc 28 z

o

CO

oc

I-o

o_l 24

22

Figure 4.

7

Eu

0.1 0.2 0.3 0.4 PHOTON ENERGY (eV)

0.5 0.6

PbS multiphoton transition rate (1-st

to 4-th order) in cm~^ s~' induced by a constant optical electric field of

10** V cm~^ amplitude and varied

frequency.

o

31

30

- 29h UJ

I-<

— 28 —

z

<

oc

27 -

0.1 0.2 0.3 0.4 PHOTON ENERGY (eV)

0.5 0.6

Figure 5. InSb multiphoton transition rate (1-st

to 4-th order) in cm"^ s-' induced by a constant optical electric field of

lO"* V cui"^ amplitude and varied

frequency.

0.1 0.2 0.3 0.4 PHOTON ENERGY (eV)

0.5

Figure 6. PbTe multiphoton transition rate (1-st

to 4-th order) in cm~^ s~' induced by

a constant optical electric field of

lO"* V cm~^ amplitude and varied

frequency

.

J 5-10^ oH

Z

lUu

o

u

a.

oc

CO<

Figure 7.

1.41 1.43

PHOTON ENERGY (eV)

1.45

Comparison between the experimental

values of the absorption coefficient

of GaAs at room temperature [15]

(solid curve) and the Keldysh model

calculation (dashed curve)

.

-387-
0.25 0.35 0.45 0.55 PHOTON ENERGY (eV)

Figure 8. Comparison between the exoerimental

values of the absorption coefficient

of pure InSb taken from [20] (solid

curve) and the Keldysh model calculation (dashed curve).

O -18 O

K -24

8

m

<o -30

-36

v = 3

0.4 0.8 1.2 PHOTON ENERGY (eV)

1.6 2.0

Figure 11. Multiphoton absorption coefficient of

GaAs at room temperature calculated

from eqs. (4.10 and 2.2). See

table 5 for material parameters used

in the calculation. The dimension

of the absorption coefficient is

given in units of cm^^~' W*~^, with

V designating the order of the / absorption process. Experimental

data are shown for 1.5 eV photon

energy [16] (dot), 1.17 eV photon

energy [6, 25, 28] (vertical bar),

and 0.94 eV [29, 30] (circle).

comims ON paper BV SHATAS, qX al

Thz i,pzak2A aomrmntzd that the. utaion no compcuviion wai gtvm b^twzm th^oKij and ^xpMA.me.nt (^on. la/Lge. gap mateAiat6 iuch ai, olkaLi-hoLidu WOA that the. abimcz Ojj hlgh-poMSA. ihoKt-Mavzlmgth ta6e.U

iMntteA the. avaJMiblLctLj txp^ximzntaZ data. Hz potntQ.d oat tkat (^on itgnl'i^-icant two-photon

abiofiptton to occuA euew Mhm iX. Moi ^mAgeJXcalty alZowzd itaxej> ojj the. oKdeA 0|$ megotootti peA iquoKz

cdntAjnztQA. weAe tzqiuJizd. TeXzA BH.aantick 0() thz Bzndix RuzaAch labonatonloA polntzd oat that twophoton data on alkatL-hatidu oAe. avalZable and In iacJ: abandant In thu titumtaAd, bat tkat thuz weAe

obtained a(>tng ond tntdniz iouAcz iach ai an tn{^>iaAe.d on. v-utblt loiM. and an InaoheAznt 6oaAcz

uttn.a-vtol^ tight. It Mai {^anJ±eA potnte.d oat In the. dlicuMton that with the. advances o{i -ihoitMavetength ta^ex technology data on two-photon abion.ptA.on In a vaJvietg o^ matentaLi mold be {,onthcomtng.

-388-
6.8 Electroabsorption: A Possible Damage Consideration

Rettig P. Benedict* and Arthur H. Guenther

Air Force Weapons Laboratory

Klrtland AFB, NM 87117

Electroabsorption, also known as photon-assisted tunneling or the Franz-Keldysh

effect, is the phenomenon in which the application of a large electric field produces

a change in the absorption coefficient of a solid material. This effect has been

proposed as a possible laser damage mechanism since the effective absorption coefficient

will be dependent upon the laser intensity (optically related electric field) rather

than the constant it is usually taken to be. The constant value is normally determined from low laser intensity calorimetric or emittance measurements.

A theoretical investigation of the interaction of an EM field with a material

shows that the dc field limit is a tunneling effect and the high-frequency limit is

multiphoton absorption so that electroabsorption is closely related to the latter.

The result being that one should primarily consider multiphoton absorption rather

than electroabsorption for potential damage implications in most situations. Calculations are presented that indicate the relative importance of multiphoton absorption

for window materials for both visible and IR lasers. The results indicate that multiphoton absorption will usually be unimportant for IR materials except in cases of ex- tremely high fields and no other competing damage mechanisms. For materials used with

visible lasers, the phenomenon must be considered as a potential cause of damage.

Key words: Electroabsorption; IR optics; laser damage; multiphoton urocesses; uv.

1. Introduction

The question has been raised recently whether the Franz-Keldysh effect (also known as electroabsorption or photon-assisted tunneling) could affect the damage thresholds of laser window materials.

[1]\ As shown theoretically by Franz [2] and Keldysh in 1958 [3], a uniform dc electric

field applied to a crystal will change the optical absorption coefficient significantly in the

neighborhood of the energy gap. Subsequently, the existence of the "Franz-Keldys^Q effect has been

confirmed experimentally not only at dc but also at fiel-ds with frequencies to 10 Hz. At optical

frequencies ('^lO Hz) the existence of this phenomenon could result in an intensity-dependent absorption coefficient thereby possibly invalidating the extrapolation of absorption measurements made at low power. It will be shown that this is unlikely for lasers operating at frequencies far below the

gap. At these frequencies, the interaction of the electric field with the medium is characterized by

multiphoton absorption, which will be seen to be the high frequency limit of the Franz-Keldysh effect

[4]. The theory and previously published experimental results are discussed as well as some calculations pertaining to multiphoton absorption in IR, visible, and uv window materials

»

2 . Theory

Using an elementary model, one can assess the differences between electric field induced tunneling at low and high frequencies. The width (£) of the barrier through which the valence electron

must tunnel can be characterized by £ = Eg/Fe from a consideration of the distance it takes an electron to acquire E of energy under a field F, ignoring coll isions . During this acceleration by

the field, the averagl electron velocity can be written as v = v'Eg2m. Using these expressions, we can associate a tunneling frequency oi^ = Fe//2mEg. At low frequencies (w << oo^) , the electron has

sufficient time to tunnel in less than one cycle; however, at very high frequencies (lo >> w^) the

electron does not acquire Eg of energy in a single cycle. The effect now is more one of the simultaneous absorption of several photons. Keldysh developed the initial theory in this area and con- cluded that dc photon-assisted tunneling and multiphoton absorption were the low and high frequency

limits of the same interaction.

In his 1965 paper, Keldysh examined the transition probability of a valence electron to the

conduction band continuum as the result of the interaction with a sinusoidally time varying uniform

*Presently completing the thesis requirements for the PhD degree at the Air Force Institute of

Technology, Wright-Patterson AFB, Ohio 45433.

'Figures in brackets indicate the literature references at the end of the paper.

-389-
electric field. The initial state was described by a Bloch wave function for a valence electron.

The final state was a non-stationary conduction band state that directly accounts for the electric

field effects. This allows intraband and interband transitions to be considered. The Houston wave

functions for an electron in a solid under the influence of an electric field were employed with the

inclusion of a time varying field. First-order perturbation theory was then used to determine the

interband transition probability from the valence band to all possible final states in the conduction

band. In the process, Keldysh made the approximation that ha)<<Eg so that for the multiphoton case, several photons are required. Keldysh 's results usually are assumed to apply for five or more

photons only. The result is given in his paper as follows [4]:

2oj|

9i\ ^ fioo + 1>

where y = /mEg/eF (note 1)

.

(1)

0^ _ 2 /I +

TT g Y Vl + Y^'"

<x> denotes the integer part of x

m* is the effective electronic mass

K, E are complete elliptic integrals of the first and second kind

Q (y, X) = 2K (l //rT72)y^^x 2exp | - tt [k

( y /

AT^)- E

(^y

^A~^^'^n^

E (l / A + Y^)} X $1 (2<x + 1>- 2x + n) / 2K (l / /l + y^ ) x E ( 1 / /l + y^)]

1/2

(x)

Keldysh examined two limiting cases: y«l and y>>l where y /mE /eF = oj/to^. The former case corresponds to low frequencies (and strong fields) and his res§lts reduce to the standard equation

for dc photon-assisted tunneling (electroabsorption) . For y>>l, high frequencies, the transition

probability is given by Keldysh as

1/2. A

(2)

This approach of calculating multiphoton transition probabilities was also followed by Weiler, et al.

[5] who extended the analysis to include magnetic fields. Her results for zero magnetic field are essentially the same as those of Keldysh.

Kovarskii and Perlin [6] follow Keldysh' s path, but did not constrain themselves to a twoband approximation; that is, the matrix elements for the transitions using the actual crystal energy

bands were calculated and used. Again the results are similar to those of Keldysh, with some small

corrections. They are also valid for fewer than five photons, a region where the Keldysh result

could be significantly in error. (Bychov and Dykhne also looked at this problem with no substantial

differences —[7] .)

The second approach to calculating N-photon transition probabilities is to use n'^ order perturbation theory between stationary initial and final states. Several people have done this such as Basov [8], Braunstein [9] and Yee [10] for two, two and four-photon absorption, respectively. To date, no one has published calculations of this type for more than four photons that we

discovered. Direct comparisons between these results and those of Keldysh et al are questionable

since Keldysh's results are only valid for large N, at least five, but it does give some measure of

Consistency. In the case of four-photon absorption, Yee calculated the transition probabilities

for ZnS(Eg = 3.8eV) and Nd:YAG laser light (1.06pm) and got W = 10^^ cm'^ sec'^ by his perturbation

-390-
theory and W = 10^^ cm ^ sec"^ from eq. (2). He also states that If the average lifetime is on the

Order of 10~ sec then his results of lO'^ e/cm^ agree favorably with photoconductivity experiments.

Others have made numerical calculations for two photon absorption and in most cases also found

Keldysh's equation gave a result that was about a factor of one hundred low, not too surprising for

this small an N.

3. Experimental Results

Experimental measurements of multlphoton absorption have been made almost since the discovery of

the laser. Usually, only two or three photon processes are observed. Some of the earliest work was

by Hopfield and Worlock [11] looking at two-photon absorption in KI and Csl with a ruby laser.

The ruby was used to apply the high frequency field (1.8 eV) and the change in absorpt ion from 4eV

up to 6.5eV (the gap energy) was examined with a Xenon arc lamp. The theory for this two-beam

experiment was later developed by Yacoby— [12]. Two-photon absorption has also been observed in

several IR window materials such as Si, GaAs, CdTe, and, CdSe [13]. Specifically for ZnSe, an intensity dependent absorption coefficient was measured to be 0.04 cm/MW in the neighborhood of 10®

watts/cm [lA]. Thus at 100 MW/cm^, we expect an absorption coefficient due to two-photon absorption of 4cm '

. Since this is for an electric field of l.lxlO^v/cm in the material, we can compare

it with the prediction, based on Keldysh's work of 0.5 cm-1 . The absorption coefficient for N-Dhoton

absorption is given by ttj, = (8TTNha)/nc<F2>)W where <Y^> is the average value of the square of the local

field, taken to be the square of the external field (not exactly correct) [15].

One paper presented breakdown experiments, which involved N-photon absorption for N as high as eleven [16]. The experiments were performed in air and KDP at 1.06 ym and 0.533 \im using picosecond pulses given in table 1. In most cases, the Keldysh predictions are in good agreement. This

stems both from the fact that this theory fits rather better for high N, and that multlphoton absorption is important as a breakdown mechanism for short pulse widths where electron avalanche rate effects

become Important

.

12 2 Table 1. Breakdown Threshold (10 watts/cm ) (After Orlov~[16])

AIR

KDP

^ N Keldysh Exp

1.06ijm

0.533ym

11

6

23

40

70

30

1.06ym

0.533vim

7.6

6.8

7-10

4-6

The primary conclusion from the experimental work to date is that Keldysh's theory seems to give

a fair approximation for N-photon absorption for N>2, and no worse than a factor of one hundred low

for two-photon absorption.

4. Implications for High Power Laser Windows

Generally, materials considered for high power IR laser windows have large band gaps so that the

laser frequency is far below the fundamental absorption edge. This will also reduce any possible

problems from multlphoton absorption because of the large number of photons required. For example, we look at ZnSe at 10.6um and 4um at 10^ volts/cm

at 10.6ym:

N = 22 Eg = 2.6ev m* = 0,133m

W = 5.4 X lO'/cm^ sec [y = 1.76 so one uses the exact expression rather than eq, (1)]

a ~ 7 X 10~^Vcm-^

-391-
at more 4um:

N = 8 all else the same

W = 3 X lO'^/cm' sec

a = 3 X 10"'^ cm"*

These numbers are extremely small and uninteresting from a practical point of view, even at lO^v/ cm. The magnitude is not surprising if we examine eq, (2) again:

5/2 W = const w

2N 2^ or for Y>>1 W = cons*" • = ' , ^ 5/2 / L.\ t u. yi^^J

and since (1/4y)« 1, W depends on a small number to a large power. We do notice, however, that upon

substituting for Y*^niEg/eF

2N 2(N-1)

W ~ F or a - F

therefore, everything is strongly field dependent, if y»1.

The field strength of lO^v/cm corresponds to about 6GW/cm^in ZnSe. At this level other damage

mechanisms such as avalanche ionization would be very important in intrinsic material failure [1].

For example, short laser pulse intrinsic breakdown in alkali halides occurs about 2 x lO^v/cm at 10.6pm and is caused primarily by an electron avalanche [17]. In ZnSe to date, typical failure

levels are near 200-300MW/cm^ or at fields of about 2.3 x lO^v/cm. The damage observed has frequently

been associated with inclusions [18]. With the strong field dependence of W, the transition

probabilities at this field strength are significantly lower than the already calculated low values

at lO^v/cm.

Clearly, it does not appear that multiphoton absorption will contribute noticeably to damage levels

in IR materials due to direct absorption. Another consideration might be whether this phenomenon could

produce enough carriers to initiate an electron avalanche, i.e., would multiphoton absorption produce more carriers than are already there by other means. In the case of intrinsic material, photoionization is probably the major source of carriers in ZnSe at 300 K. For example, direct sunlight could

produce 10^^-10^' e/cm^ sec, about the same as that produced by A pm radiation at 10^ volts/cm. Again

multiphoton absorption appears insignificant. However, for low temperature high purity windows, shielded from photoionization production, multiphoton absorption could contribute the first few carriers.

The calculations given above have been specifically for ZnSe; however, the effect is even smaller

for other IR window materials such as the alkali halides as the energy gap is much wider in these

materials. Bloembergen completely discounts multiphoton absorption in these materials since N'V'lOO [19]. Other materials such as Ge and Si could possibly experience problems, but they are not being

actively considered for high average power applications. Overall multiphoton absorption should be

considered negligible for high average power IR windows, with large band gaps.

In the visible and especially the uv range of the spectrum, multiphoton absorption can become

extremely important at high laser powers. This follows, of course, since the number of photons which

must be simultaneously absorbed is reduced considerably. Two materials that are heavily used in the

visible and near uv are quartz and MgF2. Taking the energy gaps to be 7.3 eV and 11 eV, for SOOOA

radiation we are concerned with three- and five-photon absorption, respectively . Using Keldysh's

frequency approximation (y>>1) we get for the transition probability and absorption coefficient at

5000A

Quartz MgF^

.<c.-., A. 10-" E-(y"' 3 ,

10-79 ,e

r i.j loi" E-i 2.1 X lo"10 ^m*il/2 ^-1

(E in volts /m) .

Again using 10^ v/cm a (quartz) 10 ^ cw~^ and a (MgF2) 10"^^ cm~^ assuming an m* = m. These are still not large contributions, but if the effective mass is small and considering Keldysh's estimates

are usually two orders of magnitude small, at high power levels, this effect may become important. If

-392-
we look more toward the uv, e.g., 2500 A, the contribution from multiphoton absorption becomes much

more pronounced. For the same 10^ v/cm, we get a (MgF2) 10"^ cm"-'^ (m* = m) and a (quartz) ~ 0,5 cm"-'- (m* = m) . Thus, as higher power lasers become available in the visible and near uv, where

the energy gap of the materials to be used as optical components is at best only a few times that of

the photon energy, multiphoton absorption will become an extremely important damage consideration.

5 . Summary

We have seen how the photon-assisted tunneling in a dc electric field, otherwise denoted electroabsorption or the Franz-Keldysh effect, is the low frequency limit of the same phenomenon that gives

rise to multiphoton absorption in high frequency electric fields, such as those associated with a high

power laser. The magnitude of this multiphoton absorption has been computed by several theories and

to some extent checked by experiment. Agreement is good for many photon absorption and within a factor of one-hundred for two photon absorption. Calculations of the change in absorption in IR window materials were presented and the results shown to be negligible even if one allowed a factor

of one hundred increase. The primary reason is the large number of photons required. For lasers

operating at frequencies within factors of 2 or 3 of the energy gap of the window material, multiple

photon absorption will become significant, especially for very short pulse lasers (picoseconds) where

normal avalanche breakdown is highly rate dependent. In the future, as very high power pulsed lasers

are developed in the visible and uv, multiphoton absorption must be considered as a contributor to laser induced damage.

6. Acknowledgments

The first author would like to acknowledge his appreciation for the hours of consultation given

him by his professors, Dr R. H. Hengehold and Maj.K. Jungling at AFIT.

7. References

(1) Bettis, J. R., Guenther, A. H., and Glass,

A. J., "The Influence of the Optical Electric

Field on Pulsed Laser Induced Damage Processes," IEEE '74 Region Six Conference.

(2) Franz, W., "The Effect of an Electric Field

on an Optical Absorption Edge," Z. Naturforschung 13a:484 (1958).

(3) Keldysh, L. V., "The Effect of a Strong Electric Field in the Optical Properties of

Insulating Crystals," Soviet Physics JETP

2:788 (1958).

(4) Keldysh, L. V., "Ionization in the Field of a Strong Electromagnetic Wave," Soviet

Physics - JETP 20 :1307 (1965).

(5) Weiler, M., Reine, M. , and Lax, B., "Theory

of Multiphoton Magnetoabsorption in Semiconductors," Physical Review 171:949 (1968).

(6) Kovarskii, V. A. . and Perlin, E. Yu, "Multiphoton Interband Optical Transitions in

Crystals," Physica Status Solidi 45:47

(1971).

(7) Bychov, Yu A. and Dykhne, A. M. , "Breakdown

in Semiconductors in an Alternating Electric

Field," Soviet Physics JETP 31 :928 (1970).

(8) Basov, N. G. et al, "Semiconductor Quantum

Generator with Two-Photon Optical Excitation, " Sovi^t_Phxsixs_JETP_23: 366 (1966).

(9) Braunstein, R. , "Optical Double-Photon Absorption in CdS," Physical Review 134 :A499

(1964).

(11) Hopfield, J. J. and Worlock, J. M., "TwoQuantum Absorption Spectrum of KI and Csl,"

Physical Review 137 :A1455 (1965).

(12) Yacoby, Y., "High-Frequency Franz-Keldysh

Effect," Physical Review 169 :610 (1968).

(13) Ralston, J. M. and Chang, R. K., "Optical

Limiting in Semiconductors," Applied Physics

Letters 15 :164 (1969).

(14) Arsen'ev, V. V. et al, "Nonlinear Absorption

and Limitation of Light Intensity in Semiconductors," Soviet Physics JETP 29 :413

(1969) .

(15) Wooten, F. , Optical Properties of Solids , Academic Press, New York, Chapter 3, 1972.

(16) Orlov, R. Yu, et al, "Investigation of Breakdown Produced in Dielectrics by Ultra Short

Laser Pulses," Soviet Physics JETP 34 :418

(1972) .

(17) Yablonovitch, E., "Optical Dielectric

Strength of Alkali Halide Crystals Obtained

by Laser-Induced Breakdown," Applied Physics

Letters 19 :495 (1971).

(18) Fradin, D. W. et al, "Optical Damage in Laser

Material," Conference on High Power IR Laser

Window Materials , ed. C. A. Pitha, A. Armington and H. Posen, Vol III, p 1047, 1974.

(19) Bloembergen, N., "Laser-Induced Electric

Breakdown in Solids," IEEE Journal of

Quantum Electronics, QE-10 :375 (1974).

(10) Yee, J. H., "Four-Photon Transition in Semiconductors," Ph£sixal_Jeview_J_3: 355 (1971).

-393-
Keldysh originally defines y for gases and Includes the )/2 as given on the first page

However, he later redefines y without the /l for solids.

In this paper F refers to the peak field strength and E the rms field strength. All

numerical values are rms values.

-394-
I

6 . 9 Irradiance Limits for Vacuum Ultraviolet Material Failure*

C. J. Duthler and M. Sparks

Xonics, Incorporated, Van Nuys, California 91406

Calculated values of irradiance I at which materials fail in 7,2 eV pulses of

10 nsec duration by various mechanisms indicate that metallic mirrors melt at very

low values, typically I ^ 20 MW/cm. Improving the aluminum, which is the only good

metallic vuv reflector, would increase I by a factor of only three. The lowest of the

thresholds for transparent materials are ~140 MW/cm^ for the intrinsic reversible

process of optical distortion caused by the conduction electrons generated by twophoton absorption and ~200 MW/cm^ for the intrinsic irreversible process of thermal

fracture by two-photon absorption. Other thresholds are: ~lGW/cm2 for fracture „ from enhanced stimulated Raman scattering in Raman-active materials; ~1,6 GW/cm

for melting from two -photon absorption; ~2 GW/cm^ for optical distortion from the

nonlinear refractive index ( bound electrons ); and ~ 100 GW/cm^ for thermal fracture

from extrinsic one-photon absorption with j3 = 0. 1 cm"l. Included in the calculations are the Joule heating by the generated electrons, which is greater than the direct heating by the absorption process, and electron-avalanche multiplication. Thermally in- duced optical distortion has a higher threshold than that for the optical distortion by

generated conduction electrons.

Key words: Nonlinear index of refraction; optical distortion; thermal

fracture; two-photon absorption.

1. Introduction

Our theoretical studies of materials damage by high-power, vacuum-ultraviolet (vuv) radiation were motivated by the successful operation of lasers in this wavelength region [ 1]^. High output powers have been achieved

from the rare-gas excimer lasers with wavelengths of 174 nm (-fito = 7.2 eV) for the xenon laser [2 ], 146 nm

(8.5eV) for the krypton laser [3], and 126 nm (9. 8eV) for the argon laser [4]. At the present time there have

been no experiments on materials damage in the vuv, other than that observed with the first lasers, v/hich were

materials limited [2,5].

The laser cavity-mode pattern was burned into the thin-film aluminum mirrors in these first lasers. The

2 threshold irradiance (intensity) for this damage was I s 20 MW/cm . Since orders-of-magnitude greater power

would be feasible if suitable mirrors were available, there is great interest in obtaining improved reflectors.

Transparent materials for high-power use as windows, lenses, reflecting devices, and other optical components

are of even greater interest.

Consider the intrinsic limit for metallic reflectors in the vuv. For a metal to be a good reflector it must

have a plasma frequency large compared to the frequency of interest, and it must not have interband transitions in

the region of interest. These conditions are best satisfied in the vuv by aluminum. Silver and gold, which are commonly used in the visible and infrared, are transparent in the vuv since their plasma wavelengths are greater

than 200 nm.

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.

-395-
The reflectance of aluminum as a function of frequency is shorn in figure 1. In the infrared, values of the

reflectance as large as 99 percent are obtained. Aluminum is somewhat anomalous in that it has interband transitions in the near infrared. With extreme care in sample preparation under exacting laboratory conditions, values

of reflectance approaching 92 percent have been obtained from the visible to the vuv [6]. Below the plasma wavelength of 81.4 nm (15.2eV), aluminum is transparent except for interband transitions.

Chow and Sparks [7] have shown that the 92 percent reflectance, observed with the best samples at 174 nm,

is within two percent of the intrinsic limit. In commercially available mirrors the absorptance in the vuv typically is 20 percent or greater, as denoted by the dashed curves in figure 1. Mills and Maradudin [8] have studied

these greater values of absorptance, which result from enhanced coupling to aluminum surface plasmons caused by

a rough surface or rough ^^2^3 ^S^2 overcoating.

In view of the extreme difficulty of achieving the intrinsic reflectance limit of aluminum and considering

the large absorptance at the intrinsic limit, significant improvements in the failure thresholds of metallic reflectors do not seem possible. Improved reflectance has been obtained using a stack of very thin aluminum layers

separated by approximately one-quarter wave layers of MgF2 [9], The damage threshold of these multilayer

structures remains to be investigated. However, even greater damage thresholds should be attainable from the

use of transparent materials as multilayer dielectric reflectors, or as total internal reflection devices.

Problems of transparent materials for high-power vuv use also are severe. There are few materials with

sufficiently great electronic band gap E to avoid the great linear absorption above the absorption edge. Those

having the largest band gaps are ionic materials composed of light ions from the edges of the periodic table. Materials having Eg greater than 7.2 eV include: LiF (~13), MgF2 (~11), KF(10.9), NaF(>10,5), RbF (10.4),

CsF (10), LiCl (~10), CaF2 (~10), SrF2 (>9), BaF2 (>9), NaCl (8,6), KCl (8.5), LiBr (~8.5), AI2O3 (8.3),

RbCl (8.2), KBr (7.8), NaBr (7.7), Si02 (^7.7), and MgO (7.3). All transparent materials will suffer the effects

of two-photon absorption at the xenon laser frequency. The 13 eV band gap of LiF, which is the largest band gap of

all transparent solids, is less than twice the photon energy of the xenon laser with 2-fi60 = 14. 4 eV.

Many of these materials are not expected to be useful as practical high-power vuv windows. The alkalineearth halides are superior to the alkali halides with respect to hygroscopic properties and fracture strengths.

Sapphire and quartz have highly desirable chemical and mechanical properties,but their band gaps of 8. 3 eV and

7.7 eV, respectively, are very close to the xenon laser photon energy of 7.2 eV. The steepness of the electronic

absorption edge appears to allow their use as xenon laser windows unless further study reveals such effects as anomalous frequency dependence of the absorption coefficient or a large nonlinear refractive index.

For construction of dielectric reflectors, there is a paucity of large index materials so that it may be nec- essary to use moderately absorbing materials. The absorption coefficient of such materials as NdF^ and LaFg

needs further investigation.

Aged LiF can suffer increased absorption from vuv induced color centers. It is not yet known if twophoton absorption at 7. 2 eV will result in similar creation of color centers in LiF or other materials.

The frequency dependence of the linear absorption coefficient j3 of such ionic materials as alkali halides

is shown schematically in figure 2. In the infrared, the intrinsic absorption mechanism is the creation of phonons

which results in a large peak in j3 at the reststrahl frequency followed by a broad, exponentially decreasing multiphonon tail. At the laser frequency many materials are impurity limited with absorption coefficients as low

as 10~'^cm~'^ t)eing obtained. In the ultraviolet the intrinsic absorption mechanism is the excitation of electrons

from the valence band to the conduction band. In contrast to the infrared, the electronic "Urbach tail" is relatively

8 -

1

sharp (typically ~10 cm /eV for the rate of reduction of the absorption coefficient ^) so that materials can be

used close to the edge without suffering excessive linear intrinsic absorption.

Presently available materials have extrinsic absorption coefficients ranging from 0. 1 to 1 cm ^ for vuv

frequencies below the intrinsic absorption edge [ 10 ] . Even though these values are quite large, improving the

-396-
materials to reduce the linear absorption will not result in a greater threshold for the case of a single 10 nsec

pulse, unless the one-photon induced optical-distortion mechanism of section 3D is operative. Even then, the

2 2 threshold would be increased from ~60 MW/cm to only ~140 GW/cm , which is set by the two-photon induced

electron optical distortion. However, in cw or repeated pulse systems it is expected that lowering the value of jS

should increase the threshold substantially. If such systems become of interest, the present analyses could be

extended easily to include these cases.

2. Metallic Reflectors

Materials damage in the first xenon lasers occurred with the laser cavity-mode pattern being burned into

the thin-film aluminum mirrors. The mechanism for damage in this case can be understood with the aid of figure 3, where a cross section of one of the laser-cavity mirrors is sketched. The mirror consists of a coating of

aluminum approximately 100 A thick on a magnesium fluoride substrate. At the xenon-laser wavelength of 174 nm,

commercially available aluminum reflectors typically have 20 percent absorptance, and the intrinsic limit is approximately 6 percent.

In order to determine the threshold for metallic-reflector damage, the surface temperature T of the mirror

is calculated by dividing the energy absorbed lACtt by the volume of heated material C(d. Here I is the incident

1/2

irradiance, A the absorptance, (J. the surface area that is irradiated, t is the pulse duration, and d = (itK t/4 C) '

= 2 X 10 ^ Jim ( for a 10 nsec pulse in MgF2 ) is the thermal diffusion depth. Also, K is the thermal conductivity and

C the heat capacity per unit volume. Equating this energy density lAt/d to CT gives

T = (4t/7rCK)^/^ lA . (1)

Notice that the energy is absorbed in the thin aluminum film, while the volume of heated material is determined by

the thermal diffusion into the magnesium fluoride.

For short pulse durations, failure occurs by melting of the aluminum film since aluminum has a lower

melting point than MgF„ , and thermal stresses in the small volume of heated material are insufficient to cause

3

fracture. Using A = 0.2, K = 0. 11 W/cm K, and C = 4. 1 J/cm K in eq. (1) yields a failure intensity of 20

2 MW/cm . The failure intensity decreases as the square root of the pulse duration, while the power density in1/2 creases as t

'

.

3. Transparent Materials

In addition to use in thin-film dielectric reflectors, transparent materials are of course needed for windows, lenses, and other optical components. Single-shot thresholds are calculated in the following for failure of

bulk transparent materials by various mechanisms. These bulk thresholds can be used as a guide for thin films

or for multiple shots.

A. Two-Photon Absorption

The two-photon absorption coefficient ^2 ^ calculated using golden-rule perturbation theory which

yields the formal expression [ 11, 12 ]

1/2

4ff c fic n. i f 6(E) S

h

IT IT fh hi

"hco (2)

where € is the dielectric constant, n^^ is the laser-photon occupation number, and 6(E) is an energy-conserving

delta function. In eq. (2), K^^ and are matrix elements of the interaction Hamiltonian (which can be written

-397-
in terms of dipole matrix elements) between the initial state i (valence band) or the final state f (conduction band)

and an intermediate state h . The matrix elements are summed over all possible initial, final, and intermediate

states that conserve over-all energy. Possible intermediate states are higher conduction bands, exciton levels,

lower valence bands, and the conduction band itself with intra-band transitions.

Present knowledge of the band structure of large -band-gap insulators, even LiF, is insufficient to predict ^

which intermediate states are dominant or to calculate the two-photon absorption coefficient to better than an order

of-magnitude accuracy. Tentatively, it appears that a lower valence band may be the dominant intermediate state

for LiF at the xenon-laser frequency. Simple arguments indicate that two-photon absorption is parity forbidden at

the absorption edge in LiF. For slightly larger energies, two-photon absorption is allowed. Neglecting the wave1/2 vector dependence of the transition matrix elements gives the frequency dependence ^2 ~ ^20 ^'^ ~ '^g^^ ^ ' where is constant and is the band gap. This approximate frequency dependence, which is obtained from

the joint density of states of parabolic conduction and valence bands, agrees qualitatively with experimental results

for two-photon absorption coefficient in heavier alkali halides [ 13 ] .

The two-photon absorption coefficient is proportional to the intensity and can be written formally as

^2 = (I/Iq) 1 cm"^ . (3)

At the xenon-laser frequency in LiF, an order-of- magnitude estimate of the characteristic intensity at which

j3 = 1 cm"^ is Ig = 1,2 GW/cm^ [12].

B. Optical Distortion 1

There are two nonlinear mechanisms for optical distortion in the vacuum ultraviolet. First, the process of

two-photon absorption creates conduction electrons,which in turn, can scatter incident radiation. The second mechanism is the usual nonlinear index, n2 , of the bound electrons in the solid, which is the dominant mechanism for ^

optical distortion and catastrophic self focusing in infrared Nd-glass lasers. In the vicinity of two-photon absorption bands, the nonlinear index can be resonantly enhanced. The two mechanisms are not independent since n2 is

obtained from the real part of the third-order susceptibility, and two-photon absorption from the imaginary part.

The change in the refractive index from conduction electrons created by two-photon absorption is estimated

using the Drude-theory result

6 = -co^/2n„co^ , (4) n 0 p

where.

2 2

CO = 4iTNe/m

P

2

In eq. (4), co^ is the plasma frequency, e is the electronic charge, m is the reduced effective mass of the electrons, and nQ is the linear refractive index. At the threshold for optical distortion, the number of free carriers

is approximately equal to the integrated number of photons absorbed, neglecting relaxation, although the number is

slightly larger as a result of electron- avalanche creation of additional pairs.

Our criterion for optical distortion is that the change in the optical path length at the center of the beam is

1/8 wave:

X/8 = ( 6n) z , (5)

where z is the thickness of the material, which is taken to be 1 cm. Using the absorption coefficient in eq. (3) for

2 LiF yields the free carrier threshold intensity I = 160 MW/cm for a 10 nsec duration xenon-laser pulse.

-398-
The change in the refractive index from the nonlinear index of bound electrons is 6n = n„ E , where E is

- 13 the rms electric field. The typical value of n„ = 10 esu, observed in the visible and infrared, yields a thresh2 old of I = 7 GW/cm for optical distortion. However, resonant enhancement of by two-photon absorption in the vuv reduces this threshold.

A simple model of the nonlinear index that preserves much of basic physics has been considered by

Fournier and Snitzer. [ 14] This model considers a solid composed of noninteracting molecules having three

electronic levels. The nonlinear index in this case is

47TN

n,0

nQ+1

) ^3 (2m)

f f 01 12

2 22

/ 2 _ 2,

(cOq + 5co )

(coj - 4 J)

+ 2 (6)

where n^ is the linear refractive index, ficOp is the energy of the electronic excited states, 'fiWj2 is the energy

difference of the excited states, and fp^ and f ^2 the oscillator strengths for the transitions between subscripted levels. Since our primary interest is in the frequency dependence of n2 , eq, (6) is written in terms of

the low frequency nonlinear index 1^20'

13 "20

0

/ 2 ^2, (cOq + 5w )

A 2 , 4co )

+ 2 (7)

Methods of estimating n2Q from linear optical constants have been proposed [ 14, 15 ]

-

The first term within the square brackets of eq. (7) has a resonance at one-half the one-photon absorption

frequency oJq. The nonlinear index is plotted as a function of frequency from eq. (7) in figure 4. In the absence

of damping very large values of nu^ would be obtained near (^/2 . To keep n2 finite, a small amount of damping

has been added, which leads to a broadened two-photon absorption line at ^^0^/2 shown by the dashed curve in

figure 4.

The molecular model is not realistic for solids such as alkali halides, which have broad two-photon absorption bands. For LiF, two-photon transitions from the valence band to the lowest conduction band extend from

CO /2 to approximately co . To estimate the frequency dependence of n„ in this case, eq. (7) is integrated over

S S 1/2 a distribution of oscillator frequencies having density (w - o) ) , which yields

"20

2(aj,m

CO ) g

372

.co_ A, , a/2

CO (O) - CO )

(co^

(co'^ + 5co^ ) „

(co - 4co )

dco' (8)

This integral is approximated by replacing co' in the slowly varying terms of the integrand by co^, which gives

2

3 "20

CO

(co^

1_ 2 "20

4/ 2 , ^ 2,

, ,3/2, 2 2~2

(cOj^-COg) (CO -COg)

m (CO - co„) dco'

(CO

,2 4co^)

g

(9)

Notice that eq. (9) has the form of a Kramers -Kronig integral over the two-photon absorption band. Even though

the individual oscillators diverge at one-half the resonant frequency, the integral over the oscillators remains

finite, except at co , and has the value s

-399-
and

cJ(to /2 <

g

CO <

1 1/2 - ^g^ = 2^ (a5g + 2co) tan

1

\ CO + 2co /

1 2co (2co - (joi^^JLn A/2 , COg) -(co^

CO ) 1/2

1/2 (10)

The nonlinear index obtained from eqs. (9) and (10) is plotted as a function of frequency In figure 5. The

maximum positive value of n2 - 3n2g occurs at • As the frequency increases, n2 passes through zero (no

optical distortion) and attains large negative values near co . However, close to co , one -photon effects will domig ^ -13

nate. The xenon-laser frequency equals 0. 55oOg in LiF. Using = 3^20 ^^^^ '^20 ~ yields a threshold of I = 2 GW/cm^ for optical distortion in LiF. This is a factor of 12 greater than the estimated threshold for

optical distortion from conduction electrons.

Although optical distortion from the nonlinear index of bound electrons is negligible at the xenon-laser frequency in LiF, there are other cases for which it may be important. The degree of resonance enhancement at

CO /2 increases with decreasing two-photon absorption band width. If a material has a strong sharp two-photon

absorption band with the laser frequency adjacent tojbut not within,the band, two-photon creation of free carriers

will not occur, and the nonlinear index will he considerably enhanced. Large negative values of n2 are predicted

for CO just below co^ for broad-band materials. This negative value of n2 could yield strong nonlinear defocusing,

but the three-band model used may not be valid in this frequency range. Then a more realistic model including

effects of higher bands is needed for accurate predictions.

The frequency dependence of the nonlinear index has not been measured experimentally. A factor of 100

resonant enhancement of three-wave mixing (also obtained from the third-order susceptibility) has been observed

in diamond by Levenson et al.,[ 16] when the difference frequency was resonant with the Raman frequency.

Optical distortion from heating of the material by absorbed radiation, which is the dominant mechanism for

distortion in cw 10. 6)im lasers, is negligible on the 10 nsec time scale of the xenon laser (except for rapidly repeated pulses). The lack of thermal distortion results from the fact,that, first, recombination of the electron-hole

pairs is negligible on this time scale. Even if the pairs degraded rapidly into heat, there would be no optical distortion since the thermal expansion of the lattice (which dominates dn/dT) is slow.

C. Irreversible Damage Thresholds

Heat generated by two-photon absorption will result in thermal fracture of transparent materials. This will

occur on a time scale of microseconds during which the thermal stresses in the material approach their equilibrium values. The previous analysis of Sparks and Chow [ 17 ] indicates that a LiF window fractures w^ien the temperature at the center of the window is 7 K greater than at the edge. Using the two-photon absorption coefficient in

eq. (3), and allowing for electron-avalanche creation of additional conduction electrons, and Joule heating of these

2 electrons yields a fracture threshold of I = 1.6GW/cm for LiF.

-400-
The electron-avalanche creation of conduction electrons and Joule heating of these carriers is highly nonlinear [ 18 ]. A factor of only three greater irradiance will result in two orders of magnitude greater heating

in LiF.

Local fracture from absorbing inclusions, if present, will have the lowest permanent damage threshold.

Since absorbing inclusions are expected to absorb at least as much energy in the vuv as in the infrared, the failure 2 2 power density of 2J/cm (200 MW/cm with a 10 nsec pulse) observed for many materials in the infrared, is ex- pected to apply in the vuv [19]. This failure mechanism, being extrinsic, can be removed by materials improvement, in principle.

D. Other Failure Mechanisms

In Raman-active crystals, enhanced stimulated Raman scattering can lead to materials damage from heat2 ing by phonons created in the scattering process. Sparks has estimated the threshold irradiance of 2 GW/cm for

thermal fracture by this mechanism [20], Of the candidate 7.2 eV window materials, Raman scattering is symmetry allowed in the alkaline -earth halides, but not in the alkali halides.

Another form of optical distortion is nonlinear generation of harmonic frequencies in transparent materials.

Frequency doubling occurs in materials lacking a center of inversion wdiich, of the candidate materials, includes

only crystalline Si02. Frequency tripling can occur in materials having a center of inversion, but the effects of

frequency tripling are expected to be less severe than those from two-photon absorption.

Linear, extrinsic absorption can lead to optical distortion and thermal fracture, as with intrinsic twophoton absorption. The efficiency of generation of conduction electrons by linear, extrinsic absorption is not known.

If each photon absorbed generates a conduction electron, it is estimated that an impurity absorption coefficient of

15 cm ^ will yield an optical distortion threshold of 60MW/cm^. At higher intensities nonlinear mechanisms

2 dominate. An irradiance of I = 30 GW/cm is required for thermal fracture from extrinsic one-photon heating

-1 2 with ^ = 0. 15 cm , compared to I = 2 GW/cm for two-photon heating.

Such crystalline defects as F centers have been observed to be created by light absorption [21]. It is possible that unavoidable two-photon absorption will create defects.which in turn yield an increasing extrinsic absorption coefficient at 7.2eV, This remains to be investigated.

4. Summary and Conclusions

The various failure thresholds are presented graphically in figure 6. Unless otherwise noted, these thresh

olds are most appropriate for a single, 10 nsec duration, xenon-laser pulse in LiF. The mechanism having the

2 lowest threshold of I = 20 MW/cm is melting of thin-film aluminum mirrors. With transparent materials, the

2 lowest reversible threshold is 1 = 140 MW/cm for optical distortion by free carriers created by intrinsic, twophoton absorption. Depending on the efficiency for the creation of free carriers, extrinsic absorption coefficients

greater than approximately j3 = 10 ^ cm ^ are required for lower optical distortion thresholds. Light absorption

is known to create crystalline defects. Two-phonon creation of defects may lead to unacceptably large linear absorption coefficients in aged materials.

2 The lowest irreversible damage threshold of 200 MW/cm occurs for localized fracture from absorbing

inclusions. This mechanism being extrinsic, can be eliminated in principle. The lowest intrinsic threshold re2 suits from thermal fracture from two-photon heating and has the threshold of 1,6 GW/cm for LiF. Assuming a comparable two-photon absorption coefficient for other materials, the threshold for thermal fracture is expected

to be greater for the materials having greater fracture strengths. The threshold in MgF„ is estimated as 4

2 -1 GW/cm . Thermal fracture from an extrinsic absorption coefficient of ^ = 0. 1 cm is negligible compared to

that from two-photon absorption.

-401-
The threshold of 2 GW/cm for optical distortion from the nonlinear index of bound electrons in LiF is

negligible compared to the distortion from free carriers. This mechanism may be dominant in other materials if

the laser frequency is adjacent to, but not within, a sharp, strong two-photon absorption line. Another nonlinear

mechanism that can lead to failure is enhanced stimulated Raman scattering. In materials with allowed Raman

scattering ( includes alkaline-earth halides ), heating from Raman-created phonons results in fracture at

1 = 2 GW/cm^.

An accuracy of only one order of magnitude is expected for the above thresholds because of an order-ofmagnitude uncertainty in the two-photon absorption coefficient. Our present efforts are directed toward a better

understanding of the linear optical properties of wide-band-gap materials so that better estimates of two-photon

absorption and other nonlinear effects can be made.

5. References

[1] Rhodes, C, K. , IEEE J. Quant. Elect. QE-10, 153

(1974).

[2] Hughes, W. M., Shannon, J.,Kolbi, A., Ault, E., and

Bhaumik, M., Appl. Phys. Lett. 23, 385 (1973).

[3] Hoff, P. W., Swingle, J. C, and Rhodes, C. K. , Appl.

Phys. Lett. 23, 245 (1973).

[4] Hughes, W. M. , Shannon, J. , and Hunter, R. , Appl.

Phys. Lett. 24, 488 (1974).

[5] Laser Focus, p. 10, May (1973).

[6] Feuerbacher, B. P. and Steinmann, W. , Opt.

Ctommun. 1, 81 (1969).

[7] Chow, H. C. and Sparks, M. , J. Appl. Phys. 46

,

1307 (1975).

[8] Maradudin, A. A. and Mills, D. L. , Phys. Rev. B 11^, 1392 (1975); Mills, D. L. and Maradudin, A. A.

,

Phys. Rev. , in press.

[9] Spiller, E. , Optik 39, 118 (1973).

[10] Tomiki, T. and Miyata, T. , J. Phys. Soc. Japan

658 (1969).

[11] Braunstein, R. , Phys. Rev. 125, 475 (1962).

[12] Sparks, M. and Sham, L.J, , to be published.

[13] Hopfield, J. J. and Worlock, J. M, , Phys. Rev. 137,

A1455 (1965); Frblich, D, and Staginnus, B. , Phys,

Rev. Lett. 19, 496 (1967).

[14] Fournier, J, T, and Snitzer, E. , IEEE J, Quant,

Elect. QE-10, 473 (1974).

[15] Wang, C. C. , Phys. Rev. B 2 , 2045 (1970).

[16] Levenson, M, D. , Flytzanis, C., and Bloembergen,

N., Phys, Rev. B 6, 3962 (1972).

[17] Sparks, M. and Chow, H. C. , J. Appl. Phys. 45,

1510 (1974).

[18] Sparks, M, , these proceedings, and to be published,

[19] Sparks, M, and Duthler, C, J. , J. Appl. Phys. 44,

3038 (1973); Duthler, C. J, , Appl. Phys. Lett. 24,

5 (1974).

[20] Sparks, M, , J, Appl, Phys, 46, 2134 (1975),

[21] Warneck, P., J. Opt. Soc, Am, 55, 921 (1965).

-402-
6. Figures

0)o

c

TO

0)

IR 8% Absorption

Surface-Plasmon

Absorption

7.2 eV

Frequency OJ

Figure 1. Reflectance of aluminum as a function

of frequency. At 7.2 eV, the

intrinsic absorptance is 8%, More

typically, the absorptance is limited

by surface plasmons as shown by

dashed curves.

Figure 2, Absorption coefficient of an ionic solid as a function of

frequency.

MgF2

E

10"

CO. ^ 10'' c

0)

<uoo

c2a

o<

10"

10"

10.6 Mtn (0.1 eV)

Extrinsic

7.2 eV (Xenon)

Extrinsic

10 102

Frequency, co/cjf

J Figure 3. Sketch of aluminum-film laser-cavity mirror.

The heat diffusion dept is 6.

-403-

103

t
I\

I \

I I

I

\

H h0.2 4-

\

I N I

0.6 0.8 1.0 <j/u_

Figure 4, Frequency dependence of the nonlinear

refractive Index for a solid consisting of molecular oscillators. Twophoton absorption is denoted by

dashed curve

„

X

aiQ

IT<

Zo

z

ure 5. Frequency dependence of the nonlinear refractive index for a broad-band solid, such as LIF.

Two-photon absorption is denoted

by dashed curve.

3 -

-404-
ALUMINUM FILM MELTING

EXTRINSIC ONE-PHOTON, OPTICAL DISTORTION*

TWO-PHOTON. OPTICAL DISTORTION

] MASCR0SC0P1C INCLUSIONS

I TWO-PHOTON, THERMAL FRACTURE

I ES RAMAN SCATTERING THERMAL FRACTURE

I SELF-FOCUSING, OPTICAL DISTORTION

I ELECTRON AVALANCHE, MELTING

-1 ONE-PHOTON (0.1 cin"'l -I THERMAL FRACTURE

10 NSEC PULSE

10^ 10^ 10^ 10^ 10^" 10^'

FAILURE INTENSITY (W/cm^)

*ONLY IF ELECTRONS GENERATED; SEE TEXT.

Figure 6. Failure irradiances for various mechanisms.

Except for mirror melting and Raman scattering fracture, these thresholds are most

appropriate for a single 7.2 eV xenon-pulse

incident on LiF,

COMMENTS OM PAPER 8^' VUTHLER and SVARKS

In fLUpoYibz. to a qaution iiom thz audcence thz ip^aktu Indicatzd that ^tlmxZatzd bfuIZoain

ica^tXeAing Moi, coni-Ld^Aexi a6 a potzntxal damage me.chanl-im and not ^oand to bz ilgni{,laant.

-405-
APPENDIX I

PARTICIPANTS

C. M. Allred Electromagnetics Division, National Bureau of Standards, Boulder, CO 80302

N. Alyassini Phys. Dept., SSC, USC, Los Angeles, CA 90024

L. J. Aplet Hughes Aircraft Co., M.S. E119, Centinela & Teale Sts ., Culver City, CA 90230

J. B. Arnold Union Carbide Corporation, P.O. Box Y, Oak Ridge, TN 37830

D. M. Aspinwall Lockheed Missiles & Space Co., D52-01 - B201, 3251 Hanover St., Palo Alto, CA 94304

R. R. Austin Perkin-Elmer Corp. Main Ave., Norwalk, CT 06897

D. G. Aviv Aerospace Corporation, P.O. Box 92957, Los Angeles, CA 90009

B

A. D. Baer

W. P. Barnes, Jr.

M. Bass

H. E. Bennett

E. Bernal G. J. R. Bettis

A. L. Bloom

L. C. Bobb

N. Boling

W. T. Boord

R. J. Bouchard

H. S. Boyne

R. A. Bradbury

P. Braunlich

M. Braunstein

N. J. Brown

J. Bruce

J. R. Buckmelter

Naval Weapons Center, Code 6018, China Lake, CA 93555

ITEK Corporation, 10 Maguire Road, Lexington, MA 02173

Univ. of Southern Calif. Center for Laser Studies, Los Angeles, CA 90007

Michelson Lab. Naval Weapons Center, Code 6018, China Lake, CA 93555

Honeywell Corp. Res. Ctr., 70701 Lyndale Ave. So., Bloomington, MN 55420

USAF, Kirtland AFB, NM 87117

Coherent Radiation, Inc., 3210 Porter Drive, Palo Alto, CA 94304

Frankford Arsenal, L5000, Bridge & Tacony Sts., Philadelphia, PA 19137

Owens-Illinois, 1700 N. Westwood Ave., Toledo, OH 43666

Honeywell Systems and Res. Center, R2340, 2600 Ridgway Parkway, Minneapolis, MS 55413

Rockwell International, Box 464, Golden, CO 80401

Electromagnetics Division, National Bureau of Standards, Boulder, CO 80302

Air Force Cambridge Res. Labs., Hanscom AFB, MA 01731

Engineering Physics Dept., Bendix Research Labs, Southfield, MI 48075

Chem. - Physics, Hughes Research Labs, 3011 S. Malibu Canyon Rd. , Malibu, CA 90291

Lawrence Livermore Lab., Box 808, Livermore, CA 94550

AFCRL/LQO, Hanscom AFB, MA 01730

Air Force Weapons Lab., Optical Systems Branch (LRE) , Kirtland AFB, NM 87117

A. B. Callender

A. Carlson

G. E. Chamberlain

C. F. Cline

A. R. Cook

V. Costich

AFWL/LRE, Kirtland AFB, NM 87117

Cleveland Crystals, Inc., Box 17157, Euclid, OH 44117

Electromagnetics Division, National Bureau of Standards, Boulder, CO 80302

Lawrence Livermore Lab., L-401, P.O. Box 808, Livermore, CA 94500

Electromagnetics Division, National Bureau of Standards, Boulder, CO 80302

Coherent Radiation, Inc., 3210 Porter Dr., Palo Alto, CA 94304

B. L. Danielson

G. W. Day

D. L. Decker

R. K. DeLong

J. A. Detrio

M. J. Dodge

R. Doussain

C. E. Durbin

C. J. Duthler

Electromagnetics Division, National Bureau of Standards, Boulder, CO 80302

Electromagnetics Division, National Bureau of Standards, Boulder, CO 80302

Naval Weapons Center, Code 6018, China Lake, CA 93555

TRW Systems, One Space Park, Mail Station 01/2070, Redondo Beach, CA 90278

University of Dayton, Research Institute, 300 College Park, Dayton, OH 45469

National Bureau of Standards, U.V. Physics Sec, A251 Physics Bldg. , Washington,

D.C. 20234

Laboratoire National D'Essais, 1, Rue Gaston Boisseir, Paris, France

Rockwell International, Machining R&D, P.O. Box 464, Golden, CO 80401

Xonics Inc., 6837 Hayvenhurst Ave., Van Nuys , CA 91406

D . F . Edwards

R. R. Esposito

W. S. Ewing

Los Alamos Scientific Lab, MS 564, Los Alamos, NM 87545

International Laser Systems, 729 Sagebrush Trail, Albuquerque, NM 87123

AFCRL, LQO/30, Bedford, MA 01739

J. Feldman

J. R. Fenter

A. M. Frank

D. L. Franzen

Electro-Optics Dept., EM Labs, 500 Executive Blvd., Elmsford, NY 10523

Air Force Materials Lab., Wright-Patterson AFB, OH 45433

Lawrence Livermore Lab., Optics Group, Box 808, Livermore, CA 94550

Electromagnetics Division, National Bureau of Standards, Boulder, CO 80302

-406-
G

D. H. Gill

C. R. Giuliano

A. J. Glass

A. T. Glassman

I. Goldstein

A. Golubovic

W. V. Goodell

P. M. Gruzensky

A. H. Guenther

A. Gundjian

H

Y. Hahn

T. Harris

G. L. Hennessey

R. Hilton

R. A. Hoffman

S. J. Holmes

D. Horowitz

R. A. House II

C. A. Huguley

I

J

R. H. Johnson

K

T. Kardos

J. C. Kershenstein

J. M. Khan

G. E. Kuhl

J . Kurdock

L

J. Lancelot

J. K. Larin

R. A. Lawton

M. Le Bodo

K. M. Leung

R. L. Lewis

D. H. Liebenberg

R. M. Linford

H. G. Lipson

J. D. Lytte

M

R.W. MacPherson

I. H. Malitson

G. E. Marx

F. J. McClung

D. Milam

P. Miles

K. Moncur

M. Monsler

D. Morelli

N

N. S. Nahman

B . E . Newnam

T. E. Norwood

Los Alamos Scientific Laboratory, Los Alamos, NM 87544

Hughes Res. Labs, 3011 Malibu Canyon Road, Malibu, CA 90265

Univ. of Calif., P.O. Box 808, Livermore, CA 94550

ME Dept. Optics-Group, Lawrence Livermore Lab., Box 808, Livermore, CA 94550

Raytheon Missile Systems Div. , Huntwell Rd. , Bedford, MA 01730

AFCRL, LQO, Hanscom AFB, MA 01730

Optical Sciences Div., NRL, Washington, D.C.

Electromagnetics Division, National Bureau of Standards, Boulder, CO 80302

Air Force Weapons Lab./CA, Kirtland AFB, NM 87117

McGill University, Sherbrooke Street, Montreal P. Q. Canada

CVI Laser Corp., 200 Dorado PI. SE. , Albuquerque, NM 87123

Raytheon Company, 130 Second Avenue, Waltham, MA 02154

Valpey Corp. 1244 Highland St., Holliston, MA 01746

Texas Instruments Inc., P.O. Box 6015, MS 406, Dallas, TX 75222

Westinghouse Electric Corp., R&D Center, Beulah Road, Pittsburgh, PA 15235

Northrop Research & Technology, 3401 W. Broadway, Hawthorne, CA 90250

National Bureau of Standards, Bldg. 223, Rm. A-259, Washington, D.C. 20234

AFWL, DYX, Kirtland, AFB, NM 87117

Laser Div./LRE, Air Force Weapons Lab., Kirtland AFB, NM 87117

Northrop Research & Technology, 3401 W. Broadway, Hawthorne, CA 90250

Broomer Research, 23 Sheer Plaza, Plainview, NY 11803

U.S. Naval Research Lab., Code 5503, Washington, D.C. 20234

Lawrence Livermore Lab., L-503, P.O. Box 808, Livermore, CA 94550

USAF, AFML/LPO, Wright Patterson AFB, OH 4543 3 Perkin-Elmer Corp., 77 Danbury Road, So. Wilton, CT 06897

CEA/CEL, B.P. 27, Villeneuvi St. Georges 94, France

Laser Optics Inc., P.O. Box #3, Danbury, CT 06476

Electromagnetics Division, National Bureau of Standards, Boulder, CO 80302

Laboratoire National D'Essais, 1, Rue Gaston Boisseir, Paris, France

U. of So. Calif., Center for Laser Studies, University Park, Los Angeles, CA 90007

Electromagnetics Division, National Bureau of Standards, Boulder, CO 80302

Los Alamos Scientific Lab., P.O. Box 1663, Los Alamos, NM 87544

Dept. E457, McDonnell Douglas, P.O. Box 516, St. Louis, MO 63166

Air Force Cambridge Res. Lab. LQS , Hanscom AFB, MA 017 31

Dept. 62-30, Lockheed Missiles & Space Co., Bldg. 151, P.O. Box 504, Sxmnyvale,

CA 95014

Electro-Optics Div., DREV/CRDV, P.O. Box 880, Courcellette , Quebec, Canada GOA IRO

Optical Physics Div., National Bureau of Standards, A 251 Physics Bldg.,

Washington, D.C. 20234

Westinghouse Elect. Corp., Adv. Tech. Labs, P.O. Box 1521, M.S. 3714, Baltimore,

MD 21203

Adaptive & High Power Optics Dept., Laser Div., Hughes Aircraft, Culver City,

CA 90230

Y-Div. , Lawrence Livermore Lab., Livermore, CA 94550

Raytheon Co., Hartwell Road, Bedford, MA 01730

KMS Fusion, 3941 Research Park Drive, T^n Arbor, MI 48106

Science Applications Inc., 3 Preston Ct., Bedford, MA 01730

Optical Coating Lab., Inc., 2789 Giffen Ave., Santa Rosa, CA 95403

Electromagnetics Division, National Bureau of Standards, Boulder, CO 80302

Los Alamos Scientific Lab., L-2 , P.O. Box 1663, MS 552, Los Alamos, NM 87544

U.S. Army High Energy Laser Pro j . , AMCPM-HEL, U.S. Army Missile Command, Redstone

Arsenal, AL 35809

-407-
o

D. F. O'Brien

R. W. O'Neil

P

J. H. Parks

R. H. Picard

H. S. Pilloff

J. O. Porteus

H. Posen

R. F. Prater

Q

R

W. L. Ramer

D. W. Readey

W. Reichect

D. K. Rice

S

T. Saito

A. A. Sanders

A. Schmid

R. A. Shatas

G. H. Sherman

R. E. Sladky

D. H. Sliney

R. L. Smith

W. L. Smith

M. J. Soileau

M. Sparks

W. J. Spawr

J. L. Stapp

J. Starling

D. L. Stierwalt

J. C. Stover

R. Strandlund

T

A. Takenouche

C. E. Thomas

U

V

K. Vedam

W

F. B. Waldrop

E. P. Wallerstein

V. Wang

S. W. Warren

R. Webb

C. B. Willingham

T. D. Wise

R. M. Wolff

S. M. Wong

D. L. Wright

X

Y

Yu Chung

USAF, Materials Lab/LPO, Wright-Patterson AFB, OH 45433

Optics Div. , MIT Lincoln Laboratory, P.O. Box 73, Lexington, MA 07179

Physics Dept., Univ. Southern Calif., University Park, Los Angeles, CA 90007

AF Cambridge Research Labs., OPL, Hanscom AFB, MA 01731

Office of Naval Res., Code 421, 800 N. Quincy St., Arlington, VA 22217

Naval Weapons Center, Code 6017, China Lake, CA 93555

USAF Cambridge Research Labs., Solid State Sci/LQO, Hanscom AFB, MA 01731

AFWL/LRE, Kirtland AFB, NM 87117

Rockwell Int., Rocky Flats Plant, P.O. Box 464, Golden, CO 80401

U.S. ERDA/DPR, Materials Sciences Program, Washington, D.C. 20545

Univ. of Calif., LASL, L-1, Los Alamos, NM 87545

Northrop Research & Technology Center, 3401 W. Broadway, Hawthorne, CA 90250

U.S. Air Force, Lawrence Livermore Labs, L-140, P.O. Box 808, Livermore, CA 94550

Electromagnetics Division, National Bureau of Standards, Boulder, CO 80302

Bendix Corp. — Research Labs, Southfield, MI 48076

Physical Sciences Div., U.S. Army Missile Comm., Bldg. 77770, AMSML-RRD,

Redstone Arsenal, AL 35802

II-VI, Inc., 207 Garden Lane, Glenshaw, PA 15116

Union Carbide Corp, Y-12 Plant, P.O. Box Y, Bldg. 9998, Oak Ridge, TN 37830

U.S. Army Environmental Hygiene Agency, Aberdeen Proving Ground, MD 21010

Electromagnetics Division, National Bureau of Standards, Boulder, CO 80302

Div. Engineer & Applied Physics, Harvard Univ., 321 McKay Lab, Cambridge, MA 02138

Naval Weapons Center, Code 6018, China Lake, CA 93555

Xonics, Inc., 6837 Hayvenhurst Ave., Van Nuys, CA 91406

Spawr Optical Research, Inc., 1521 Pomona Rd., Corona, CA 91720

USAF Weapons Lab., LRE, Kirtland AFB, NM 87108

Ceramics Dept. — V1864, Honeywell Inc., 1885 Douglas Dr. N., Golden Valley, NM 55422

Naval Electronics Laboratory Center, Code 4600, 271 Catalina Blvd., San Diego,

CA 92152

Rockwell International, RSD, Bldg. 881, P.O. Box 464, Golden, CO 80401

Optical Coating Lab., Inc., 2789 Giffen Ave., Santa Rosa, CA 95403

Electrical Eng., Colo. State Univ., 719 S. Washington St., Apt. 119, Fort

Collins, CO 80521

KMS Fusion, P.O. Box 1567, Ann Arbor, MI 48106

Penn. State Univ., 276 Materials Res. Lab, University Park, PA 16802

Develop. Div., Union Carbide Nuclear, P.O. Box Y, Bldg. 9202, Oak Ridge, TN 37830

Laser Div., L-543, Lawrence Livermore Labs., P.O. Box 808, Livermore, CA 94550

Hughes Research Lab., 30-25-00, 3011 Malibu Canyon Rd. , Malibu, CA 90265

Electrical Products Div., ITT Component, Edinburgh Way, Harlow, Essex, England

Holobeam Laser, Inc., 560 Winters Ave., Paramus, NJ 07652

Research Div., Raytheon Co., 28 Seyon St., Waltham, MA 02154

Ball Bros. Res. Corp, P.O. Box 1062, Boulder, CO 80302

Valpey Corp., Box 2311, Newport Beach, CA 92663

Rockwell International, Bldg. 779, P.O. Box 464, Golden, CO 80401

Dept. of EE, Colorado State University, Fort Collins, CO 80523

EE Dept., North Carolina AST State Univ., Greensboro, NC 27411

-408-
z

J. L. Zar AVCO Everett Res. Lab., Everett, MA 02149

R. W. Zimmerer Scientech, Inc., 5649 Arapahoe, Boulder, CO 80303

-409-

HBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

I. PUBLICATION OR REPORT NO. Special Publication 435

2. Gov't Accession

No.

4. TITLE AND SIJBTITLE.

Laser Induced Damage in Optical l^aterials: 1975

3. Recipient's Accession No.

5. Publication Date April 1976

6. Performing Organization Code

7. AUTHOR(S)

Edited by Alexander J. Glass and Arthur H. Guenther

8. Performing Organ. Report No

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2769569

n. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Electromagnetics Division, National Bureau of Standards,

Boulder, CO 80302

(American Society for Testing & Materials, Phila., PA 19103

Dept. of the Navy, Office of Naval Research, Arlington, VA 22217

D. C. 20234

13. Type of Report & Period Covered Final

14. Sponsoring Agency Code ^^WpMrnJ^^MT^"^'^"'''' Admin. > Waohingtc

Library of Congress Catalog Card Number: 76-4985

16. ABSTRACT (A 200-word or less factual summary ot most significant information. If document includes a significant bibliography or literature survey, mention it here.; The Seventh ERDA-ASTM-ONE-NBS Symposiutii on Laser

Induced Damage in Optical Materials was held at the National Bureau of Standards in

Boulder, Colorado, on July 29-31 of this year. These Symposia are held as part of the

ictivities in Subcommittee II on Lasers and Laser Materials, of the ASTM. Subcommittee

CI is charged with the responsibilities of formiilating 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 Symposia are Dr. Arthur

juenther. Chief Scientist of the Air Force Weapons Laboratory, and Dr. Alexander J. jlass. Head, Theoretical Studies, Y Division, Lawrence Livermore Laboratory.

Over 150 attendees at the Symposium heard h2 papers on topics relating fabrication

procedures to laser induced damage in optical materials; on metal mirrors; in infrared

window materials; the multipiilse, wavelength and pulse length dependence of damage

thresholds; damage in dielectric films and at exposed sixrfaces; as well as theoretical

iiscussions on avalanche ionization and multiphoton processes of importance at shorter

cavelengths . Of particular importance was the sealing relations developed from several

parametric studies relating fundamental properties (refractive index, surface roughness,

tc.) to the damage threshold. This year many of the extrinsic influences tending to

reduce a material damage resistance were isolated such that a measure of its egregious

lature could be quantified. Unfortunately, it was evident that much still needs to be

iccomplished to improve processing and fabrication procedures to allow a measurable

approach to a materials Intrinsic strength to be demonstrated.

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)

Ivalanche ionization; IR windows and mirrors; laser damage; laser materials;

aultiphoton processes; self-focusing; thin films.

18. AVAILABILITY |^ Unlimited

I

' For Official Distribution. Do Not Release to NTIS

[2 Order From Sup. of Doc, U.S. Government PtijyjJg.OOffice Washington, D.C. 20402, SD Cat. No. CH • 1U*435

I I Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

20. SECURITY CLASS

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UNCLASSIFIED

21. NO. OF PAGES

437

22. Price $5. 60

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