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Laser Induced Damage

in Optical Materials:

1984

BOULDER DAMAGE SYMPOSIUM STP 954
m he National Bureau of Standards' was established by an act of Congress on March 3, 1901. The

f Bureau's overall goal is to strengthen and advance the nation's science and technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research and provides: (1) a

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The Bureau's technical work is performed by the National Measurement Laboratory, the National

Engineering Laboratory, the Institute for Computer Sciences and Technology, and the Institute for Materials

Science and Engineering

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Provides the national system of physical and chemical measurement;

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• Basic Standards2

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Provides technology and technical services to the public and private sectors to address national needs and t<? solve national problems; conducts research in engineering and applied science in support of these efforts; builds and maintains competence in the necessary disciplines required to carry out this research and technical service; develops engineering data and measurement

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Conducts research and provides measurements, data, standards, reference

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its programs. The Institute consists of the following Divisions:

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Polymers

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'Headquarters and I anoratories ai ( laithersburg, Nil), unless otherwise noted: mailing address

C.ailhcrsburg, Ml) 20899.

: Somc divisions within the eenier are loeated at Boulder, CO 80303.

*| OCated at Boulder, CO, with some elements at Ciaithersburg, MD,
Laser Induced Damage

In Optical Materials: 1984

Proceedings of a Symposium Sponsored by:

National Bureau of Standards

American Society for Testing and Materials

Office of Naval Research

Department of Energy

Defense Advanced Research Project Agency

Air Force Office of Scientific Research

October 15-17, 1984

NBS, Boulder, Colorado 80303

Edited by:

Harold E. Bennett

Naval Weapons Center

China Lake, California 93555

Arthur H. Guenther

Air Force Weapons Laboratory

Kirtland Air Force Base, New Mexico 87117

David Milam

Lawrence Livermore National Laboratory

Livermore, California 94550

Brian E. Newnam

Los Alamos National Laboratory

Los Alamos, New Mexico 87545

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

October 1986
Library of Congress Catalog Card Number: 86-600587

National Bureau of Standards Special Publication 727

Natl. Bur. Stand. (U.S.), Spec. Publ. 727, 444 pages (Oct. 1986)

CODEN: XNBSAV

U.S. Government Printing Office

Washington: 1986

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

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

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

October 15-17, 1984. The Symposium was jointly sponsored by the National Bureau of Standards, the

American Society for Testing and Materials, the Office of Naval Research, the Defense Advanced Research

Projects Agency, the Department of Energy, and the Air Force Office of Scientific Research. The

Symposium was attended by approximately 200 scientists from the United States, the United Kingdom,

France, West Germany, and the Netherlands. It was divided into sessions devoted to the following

topics: Materials and Measurements, Mirrors and Surfaces, Thin Films, and finally Fundamental Mechanisms.

The Symposium Co-chairmen were Dr. Harold E. Bennett of the Naval Weapons Center, Dr. Arthur H. Guenther

of the Air Force Weapons Laboratory, Dr. David Milam of the Lawrence Livermore National Laboratory,

and Dr. Brian E. Newnam of the Los Alamos National Laboratory. They also served as editors of this

report. Dr. Alexander J. Glass of KMS Fusion acts as Conference Treasurer with Aaron A. Sanders of

the National Bureau of Standards as the Conference Coordinator.

The editors assume full responsibility for the summary, conclusions, and recommendations contained

in the report and for the summaries of discussion found at the end of each paper. The manuscripts of

the papers presented at the Symposium have been prepared by the designated authors, and questions

pertaining to their content should be addressed to those authors. The interested reader is referred

to the bibliography at the end of the summary article for general references to the literature of laser

damage studies. The Seventeenth Annual Symposium on this topic will be held in Boulder, Colorado,

from October 28-30, 1985. A concerted effort will be made to ensure closer liaison between the

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

The principal topics to be considered as contributed papers in 1985 do not differ drastically from

those enumerated above. We expect to hear more about improved scaling relations as a function of pulse

duration, area, and wavelength, and to see a continuing transfer of information from research activities

to industrial practice. New sources at shorter wavelengths continue to be developed, and a corresponding

shift in emphasis to short wavelength and repetitively pulsed damage problems is anticipated. Fabrication

and test procedures will continue to be developed, particularly in the diamond- turned optics and thinfilm areas. Comprehensive modeling studies are, as well, anticipated.

The purpose of these symposia is to exchange information about optical materials for high power

lasers. The editors will welcome comment and criticism from all interested readers relevant to this

purpose, and particularly relative to our plans for the Seventeenth Annual Symposium.

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

D. Milam, and B. E. Newnam

Co-Chairmen

DISCLAIMER

Certain papers contributed to this publication have been prepared by non-NBS authors. These papers

have not been reviewed or edited by NBS; therefore, the National Bureau of Standards accepts no res- ponsibility for comments or recommendations contained therein.

Certain commercial equipment, instruments, and materials are identified in this publication in

order to explain the experimental procedure adequately. Such identification in no way implies approval,

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

instruments, or materials identified are necessarily the best available for the purpose.

iii
CONTENTS

Page

Foreword iii

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

Disclaimer iii

Symposium Welcome and Perceptions of Future Research vii

Brian E. Newnam

Summary of Meeting 1 H. E. Bennett, A. H. Guenther, D. Milan, and B. E. Newnam

I. Introduction 1

2. Principal Conclusions 1

3. Summary of Papers 3

3.1 Materials and Measurements 3

3.2 Mirrors and Surfaces. 6

3.3 Thin Films 7

3.4 Fundamental Mechanisms 11

4. Recommendations 12

5. Acknowledgments 12

6. References 13

Materials and Measurements

Retrofit of a High Power Nd:Glass Laser System with Liquid Crystal Polarizers 15

S. D. Jacobs, K. A. Cerqua, T. J. Kessler, W. Seka, and R. Bahr

Nonlinear Rotatory Power of 90° Twisted Nematic Liquid Crystals 23

S.-T. Wu and L. D. Hess

Picosecond Damage in Y 0 Stabilized Cubic Zirconia 31

N. Mansour, M. J. Sofleau, and E. W. Van Stryland

Pulsed Laser Induced Optical Damage Threshold in Nd:CR:GSGG Crystal 39

M. A. Acharekar, D. P. McCarthy, and R. Blachman

Laser Damage Studies of Several Methacrylate Polymeric Materials 49

R. M. O'Connell, R. V. Ellis, A. B. Romberger, T. F. Deaton, K. E. Siegenthaler,

A. A. Shaffer, B. W. Mullins, and T. T. Saito

Leed Analysis of Pulsed Laser Damage to Mo(100) Surfaces 59

A. L. Helms, Jr., C.-C. Cho, and S. L. Bernasek

The Effect of Defects on the Laser Damage Performance of Metal Mirror Surfaces 66

H. H. Hurt

Automated Pulsed Testing Using a Scatter-Probe Damage Monitor 71

J. B. Franck, S. C. Seitel, V. A. Hodgkin, W. N. Faith, and J. 0. Porteus

Application of a Computer-Automated IR Sensitive Camera to the Determination of

Pulsed Laser Surface Damage Thresholds 77

P. C. Filbert, J. L. Guttman , C. L. Navoda, and B. A. Watson

i v
High-Precision Damage-Resistant Multiple-Pass Ultraviolet Ref lectometer 85

L. J. Jolin and S. R. Foltyn

A Low Loss, High Power Variable Laser Attenuator 93

S. R. Foltyn, J. E. Griggs, L. J. Jolin, J. H. Roberts, and D. Keaton

Studies of CO2 Laser Induced Damage to Infrared Optical Materials and Coatings 100

D. R. Gibson and A. D. Wilson

Mirrors and Surfaces

Surface Potential as a Laser Damage Diagnostic H6

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

Charge Emission and Accumulation in Multiple-Pulse Damage of Silicon

Y. K. Jhee, M. F. Becker, and R. M. Walser 127

Laser Generated Ripple Patterns on Dielectrics and Intermediate Band Gap Semiconductors 137

N. Mansour, G. Reali, P. Aiello, and M. J. Soileau

Surface Damage Mechanisms in Nontransparent Media 147

P. M. Fauchet and A. E. Siegman

Picosecond-Pulse Damage Studies of Diffraction Gratings 154

Dennis H. Gill and Brian E. Newnam

Thin Films

Assessment of Optical Coatings Prepared by Molecular Beam Techniques 162

K. L. Lewis, J. A. Savage, A. G. Cullis, N. G. Chew, L. Charlwood, and D. W. Craig

Some Recent Observations on the Properties of UHV-Deposited Zr02 171

T. Raj, L. D. Weaver, S. R. Tuenge, J. S. Price, and K. C. Jungling

Oxygen Threshold for Ion-Beam Sputter Deposited Oxide Coatings 180

H. Demiryont and J. R. Sites

Tomorrow's Coatings Today— They Need Help 187

A. F. Stewart, A. H. Guenther

High Damage Threshold Porous Silica Anti reflective Coating 205

I. M. Thomas, J. G. Wilder, W. H. Lowdermilk, and M. C. Staggs

Pulse Duration Dependence of 1064-nm Laser Damage Thresholds of Porous Silica

Anti reflection Coatings on Fused Silica Substrates

D. Milam, I. M. Thomas, C. Weinzapfel, and J. G. Wilder

Variation of Transmittance Spectra of Porous Anti reflection Coatings with Index Profile 217

D. Milam, G. G. Peterson

Characterization of Aluminum Nitride/Aluminum Oxide Reactively Sputtered

Anti reflection Coatings 233

L. G. Koshigoe, L. F. Johnson, T. M. Donovan, and C. D. Marrs

Crystallization of Titania Films by Thermal Heating 253

R. Rujkorakarn, L. S. Hsu, and C. Y. She

Time Resolved Raman Studies of Laser Induced Damage in Ti02 Optical Coatings 262

G. J. Exarhos and P. L. Morse

Ultraviolet Thin Film Coating Characterization 272

A. F. Stewart and D. J. Gallant

Effect of Overcoats on 355-nm Reflectors 285

C. K. Carniglia, T. T. Hart, and M. C. Staggs

v
Thermal Properties of Optical Thin Film Materials.

D. L. Decker, L. G. Koshigoe, and E. J. Ashley

291

Interface and Bulk Absorption of Oxide Layers and Correlation to Damage Threshold

at 1.06 u m 298

D. Ristau, X. C. Dang, and J. Ebert

Effect of Anistropic Stress on Thin Film Damage Thresholds 313

A. J. Heiney, J. Eastman, and C. W. Gabel

Design of High Power Laser Coatings for Grazing Incidence Mirrors 322

W. H. Southwell

The Design of Optimum High Reflectivity Coatings for

Grazing Angles of Incidence 330

J. 8. Shellan

Angular Dependence of Multilayer-Reflector Damage Threshold 342

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

Survey of Laser Damage Thresholds for High Reflector Films at 1.315 Microns 352

T. F. Deaton

Scanning Electron Microscopy Studies of Laser Damage Initiating Defects in ZnSe/ThF^

and Si H/Si O2 Multilayer Coatings 356

L. F. Johnson, E. J. Ashley, T. M. Donovan, J. B. Franck, R. W. Woolever,

and R. Z. Dal bey

Single Pulse Laser Induced Damage in IR Coatings at 10.6 v m 371

H. Deng, M. Bass, and N. Koumvakalis

Light Stripping of UV Dielectric Coatings 377

J. F. Asmus and J. R. Oldenettel

Fundamental Mechanisms

An Initial Study of: The Inclusion Model for Repetitively Pulsed Laser Damage 382

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

Self-Focusing in Damage Experiments Revisited 394

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

Two-Photon Absorption, Nonlinear Refraction and Optical Limiting in Semiconductors 404

E. W. Van Stryland, H. Vanherzeele, M. A. Woodall, M. J. Soileau, A. L. Smirl,

S. Guha, and T. F. Boggess

APPENDIX I. List of Attendees 424

vi
Symposium Welcome and Perceptions of Future Research

Brian E. Newnam

Los Alamos National Laboratory

Los Alamos, New Mexico 87545

On behalf of the Symposium Steering Committee of Drs. Harold Bennett, Arthur Guenther,

David Milam, and myself, I welcome all of you to the 16th Annual Symposium on Optical Materials for

High Power Lasers. Having participated in these Symposia since 1971, I recognize among you a number of persistent old timers as well as many new faces. Obviously, these Symposia are still of

great interest to a significant number of scientists in the laser community. For the last several

years, we have comprised between 180 and 200 participants and between 40 and 60 papers, as revealed

by figure 1, a chart of our experience.

In his opening remarks at last year's Boulder Damage Symposium, Art Guenther reviewed the main

themes of laser damage research of the previous 15 years. This year I want to describe my perceptions of the progress needed in the near future. Do you recall the fervor of activity in the late

1960's associated with America's Apollo spacecraft mission? A major concern in those days was the

impact of the space environment on the thermal control surfaces such as white reflective paints and

second-surface metal mirrors. The potentially dangerous elements of the space environment included

solar ultraviolet radiation, electrons and protons in the Van Allen radiation belts, micrometeorites, and the vacuum of space. A number of aerospace companies assembled space effects laboratories, including high-pressure arc lamps, to simulate the ultraviolet portion of the solar radiation

and charged-particle simulation chambers. These environmental simulations produced a large amount

of very useful data on radiation resistance. Thus, adequately damage-resistant materials were identified or subsequently developed, and the mission to the moon was a resounding success.

Today, similar attention is beginning to be focused on the effects of machine-produced highenergy radiation environments on the window materials and reflective coatings in lasers driven by

electron accelerators. Currently, there is much concern about optical elements in the large

e- beam-driven excimer lasers for 248 and 351 nm, in particular. Besides having adequate damage

resistance for the primary uv laser wavelength, optical coatings must survive the impact of electrons of 100 to >300 kV energies and the resultant x rays. To add further insult, some of the

coatings must function in a fluorine gas environment! Laboratory research on coating degradation

for e- beam-pumped excimer lasers has been proceeding now for only one or two years.

Recently, another very important accelerator-driven device, the free-electron laser (FEL) , has

become prominent. Having been associated with the FEL research program at Los Alamos for the last

five years, I am very optimistic about its future role in such applications as materials research,

industrial chemistry, medical surgery, as well as a potential military tool. Since the first operation of an FEL oscillator at 3-4 ym at Stanford University in 1977 [1], there has been a number

of FEL oscillator demonstrations extending from the visible through the sub-millimeter range.

Table 1 lists the experimental progress. For more information, the interested reader should con- sult the review article of Charles Brau [9].

FELs require electron accelerators with energies ranging from a few MeVs for submill imeter waves to 100 to 200 MeV for visible wavelengths. Upon collision with various materials in the ac- celerator structure, electrons with such energies naturally produce other high-energy radiations,

such as x rays, gamma rays, and neutrons. Additionally, the periodic wiggling motion of the electrons traveling through the magnetic undulator (gain region) naturally produces harmonics of the

fundamental lasing wavelength. For example, for an FEL lasing at 500 nm, there can be significant

power in the harmonics in the extreme-ultraviolet range (<100 nm). The optical elements composing

the oscillator cavity and the external directing mirrors cannot be allowed to degrade significantly

by any of these radiations.

In the first tests with an infrared (10 vim) FEL oscillator at Los Alamos, electrons caused

some permanent damage and x rays caused temporary color centers in the NaCl Brewster windows. The

presence of transient absorption was also suspected. With the resulting round-trip losses, the FEL

could not reach the lasing threshold. To avoid these losses and attain vigorous lasing, it was necessary to remove the Brewster windows, leaving the cavity mirrors in the vacuum [6,8].

vii
70

Hi

n

200

g 20- r M

1969 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84

SYMPOSIUM YEAR

Figure 1. Boulder damage symposium history.

v i i i
Although radiation-induced degradation of output windows between the laser and atmosphere will

still be a concern for powerful, visible-wavelength FELs, the primary future requirement will be for

cavity reflectors with reflectances of the order of 99.9% which are not significantly degraded by

the multiple environments. We should take note that in the last year FEL researchers at the University of Paris at Orsay have already had to contend with actual degradation of resonator mirror coatings (TiC^/SiC^) induced by ~120~nm photons associated with lasing at 640 nm.

The frontier for FELs, as well as for laser oscillators based on harmonic generation and fourwave mixing, is in the VUV and XUV. Attainment of sufficiently high reflectance below 100 nm to

permit lasing is a present limitation. However, the recent attainment by Bar bee, et al. [10] of over 50% reflectance at near-normal incidence at 1 7 nm by a Mo/Si multilayer reflector is very en- couraging. Such a reflectance over a large enough surface could make an XUV FEL oscillator possible. Who knows? Perhaps one of the topics about which we will hear in future Laser Damage Symposia

will be the damage resistance of 100-nm reflectors!

In concluding, I wish to extend our gratitude to the staff of the National Bureau of Standards

at Boulder for making these fine facilities available, and for their great assistance in the planning, arrangements, and conduct of our meeting plus their part in preparing the proceedings for

publication. These include Dr. Robert Kamper , Chief of NBS-Boulder, Aaron Sanders, Head of the Optical Electronic Metrology Group and the NBS Coordinator of these meetings, and Susie Rivera,

Aaron's able administrative assistant who has dealt with almost all facets of these Symposia over

the years. Other involved NBS staff are Ann Mannos, Kathy Sherlock, and Shirley Deeg. Pat Whited

of the Air Force Weapons Laboratory has also participated in the meeting planning and arrangements.

References

[1] D. A. G. Deacon, L. R. Elias, J. M. J. Madey, G. J. Ramian, H. A. Schwettman, and T. I. Smith,

Phys. Rev. Lett. 3.8, 892 (1977).

[2] M. Billardon, P. Elleaume, J. M. Ortega, C. Bazin, M. Bergher, Y. Petroff, M . Velghe, D. A. G.

Deacon, K. E. Robinson, and J. M. J. Madey, Phys. Rev. Lett. 5J_, 1652 (1983).

[31 J. A. Edighoffer, G. R. Neil, C. E. Hess, T. I. Smith, S. W. Fornaca, and H. A. Schwettman,

Phys. Rev. Lett. 52., 344 (1984).

[4] R. W. Warren, B. E. Newnam, W. E. Stein, J. G. Winston, R. L. Sheffield, M. T. Lynch, J. C.

Goldstein, M. C. Whitehead, 0. R. Norris, G. Luedemann, T. 0. Gibson, and C. M. Humphry, in

Proc. of the Int'l. Conf. on Lasers '83, R. C. Powell, Ed. (STS Press, McLean, VA) , pp. 31 6-

320, 1983.

[5] A. Ts. Amatuni, M. L. Petrosian, B. V. Petrosian, N. T. Shahbazian, and K. B. Oganesian,

Yerevan Phys. Inst. Rept. 727 (42)-84, Yerevan, USSR 1984.

[6] B. E. Newnam, R. W. Warren, J. C. Goldstein, R. L. Sheffield, and C. A. Brau, Nucl . Inst, and

Methods in Phys. Res. A237 , 187 (1985).

[7] L. R. Elias, J. Hu, and G. Ramian, Nucl. Inst, and Methods in Phys. Res. A237 , 203 (1985).

[8] B. E. Newnam, R. W. Warren, R. L. Sheffield, W. E. Stein, M. T. Lynch, J. S. Fraser, J. C.

Goldstein, J. E. Sollid, T. A. Swann, J. M. Watson, and C. A. Brau, IEEE J. Quantum Electron.

QE-21 , 867 (1985).

[9] C A. Brau, IEEE J. Quantum Electron. QE-21 , 824 (1985).

[10] T. W. Barbee, Jr., S. Mrowka, and M. C. Hettrick, Appl. Opt. 24, 883 (1985).

i x
Table 1

Summary of Visible and Infrared Free-Electron Laser Oscillators

Wavelength Peak Power Average Power*

1977

1983

(June

1983

(July)

1983

(November

1984

1984

(March

1984

(August

1 984

(October

Stanford University [1]

University of Paris

and Stanford

University [2]

TRW Inc. and Stanford

University [3]

Los Alamos [4]

Yerevan Phys. Inst.

(Armenia) [5]

Los Alamos [6]

University of

California at

Santa Barbara [7]

Los Alamos [8]

3.4 pm

640 to 655 nm

1.6 urn

9 to 1 1 pm

-40 pm

9 to 11 pm

400 urn

9 to 35 pm

400 kW

16 W

1.2 MW

700 kW

-6 kW

5 MW

-10 kW (est.)

10 MW

5 W

5 x 10 4 W

80 W

1000 W

10 W

3000 W

3000 W

6000 W

*Average over electron macropulse
Laser- Induced Damage in Optical Materials

Sixteenth ASTM Symposium

October 15-17, 1984

The Sixteenth Annual Symposium on Optical Materials for High Power Lasers (Boulder

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

October 15-17, 1984. The Symposium was held under the auspices of ASTM Committee F-l,

Subcommittee on Laser Standards, with the joint sponsorship of NBS, the Defense Advanced

Research Project Agency, the Department of Energy, the Office of Naval Research, and the

Air Force Office of Scientific Research. Approximately 180 scientists attended the

Symposium, including representatives from England, France, The Netherlands, Scotland, and

West Germany. The Symposium was divided into sessions concerning Materials and Measurements, Mirrors and Surfaces, Thin Films, and Fundamental Mechanisms. As in previous

years, the emphasis of the papers presented at the Symposium was directed toward new frontiers and new developments. Particular emphasis was given to materials for high

power apparatus. The wavelength range of the prime interest was from 10.6 urn to the uv region. Highlights included surface characterization, thin film-substrate boundaries,

and advances in fundamental laser-matter threshold interactions and mechanisms. Harold

E. Bennett of the Naval Weapons Center, Arthur H. Guenther of the Air Force Weapons Laboratory, David Milam of the Lawrence Livermore National Laboratory, and Brian E. Newnam

of the Los Alamos National Laboratory were co-chairmen of the Symposium. The Seventeenth

Annual Symposium is scheduled for October 28-30, 1985, at the National Bureau of Standards, Boulder, Colorado.

Key words: laser damage; laser interaction; optical components; optical fabrication;

optical materials and properties; thin film coatings.

1. Introduction

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

Symposium) was held, as in previous years, at the National Bureau of Standards in Boulder,

Colorado, October 15-17, 1984. The Symposium was held under the auspices of the ASTM Committee

F-l, Subcommittee on Laser Standards, with the joint sponsorship of NBS, the Defense Advanced

Research Projects Agency, the Department of Energy, the Office of Naval Research, and the Air Force

Office of Scientific Research. Approximately 180 scientists attended the Symposium, including

representatives from England, France, The Netherlands, Scotland, and West Germany. The Symposium

was divided into sessions concerning Materials and Measurements, Mirrors and Surfaces, Thin Films,

and finally, Fundamental Mechanisms. In all, approximately 45 technical presentations were made.

Harold E. Bennett of the Naval Weapons Center, Arthur H. Guenther of the Air Force Weapons Laboratory, David Milam of the Lawrence Livermore National Laboratory, and Brian E. Newnam of the Los

Alamos National Laboratory were co-chairmen of the Symposium. Alexander J. Glass of KMS Fusion is

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

Coordinator.

The purpose of these symposia is to exchange information about optical materials for high

power lasers. The authors welcome comments and criticism from all interested readers relevant to

this purpose and particularly relative to our plans for the Seventeenth Annual Symposium, scheduled

for October 28-30, 1985, at the National Bureau of Standards, Boulder, Colorado.

2. Principal Conclusions

The cataloging of properties and damage thresholds of bulk materials continued to be of interest. Authors at this conference discussed studies of liquid crystals, plastics, cubic zirconia,

and a laser host crystal, gallium scandium gadolinium garnet. Of particular interest was the

successful fabrication of polarizers from liquid crystals.

Laboratories for studying laser-induced damage have continued to become more sophisticated.

Four authors described automated experiments in which computers control the operating parameters of

the source laser, the devices used to diagnose the laser pulses, the manipulation of the damage

sample, and the determination as to whether damage occurred at a particular site. A multiplebounce ref lectometer usable with intense 248-nm laser pulses, and a variable attenuator for the

248-nm wavelength were also described.

Detection of the defects responsible for damage initiation has been an elusive goal. Lowenergy electron diffraction (LEED) was used to study recrystall ization of sites on metal surfaces

that had been irradiated at fluences below those required to cause apparent damage. A closely

1
related paper described a study by transmission electron microscopy of metal samples that were thinned after having been damaged. Localized damage was found to have occurred at grain boundary

sites that frequently contained impurities such as Fe.

It is clear that emphasis has shifted from single-pulse, one-on-one experiments to the much

more difficult multipulse laser damage. The exploration of the mechanism of multipulse damage

utilizing electron or ion emission from the damaged surface has continued. Apparently, simple

probability as evidenced by the 1972 lucky-electron theory of Bass and Barrett will not explain

multi-pulse damage, and the cumulative effect of excitations must be considered. Damage levels were related to pulse energy but not to pulse repetition rate and may be related to evaporation at

localized hot spots, at least in silicon. A generalized theoretical framework for analyzing multipulse thermal events was provided, but the mechanism of initiation remains unknown. In addition,

surface-potential measurements were explored as an alternative to charged particle emission. It was demonstrated that the influence of multipulse damage extends far beyond the diameter of the

initiating laser beam, at least for small spot sizes. Further exploration of multipulse damage

using these tools can be anticipated.

Other, more traditional questions in the field of laser damage were also clarified. Careful

work on the theory of two-photon absorption resulted in theoretical values calculated from material

parameters which are in good agreement in all cases with experimental values. Eleven semiconductor

materials of various types were studied. Several other longstanding problems have been attacked in

the same spirit.

Self- focusing was examined utilizing a theory reported at this meeting last year. In cases where sufficient data were available, the experimental results of various authors were recalculated,

yielding improved values. It was shown that previous analyses gave results which may be in error

by an order of magnitude. Ripples associated with single- and multiple-pulse damage were also

investigated, and it was shown that, whereas single-pulse-derived ripples are correctly predicted

by present theory, multipul se-generated ripples are not. Finally, multipulse damage measurements

using 30-ps pulse lengths at 1.06 urn on ruled and holographic gratings were reported. This data is

of immediate importance in optical system design. A theoretical analysis of these results has yet

to be reported, but is anticipated shortly.

It is gratifying to note that, along with improved damage ratings, increased resistance to

adverse environments is being addressed as well. This is particularly true for corrosive chemicals

and electron beams as in excimer lasers as well as to abrasion and humidity as in outdoor applications. However, the true advances have come in improved understanding of the effect of deposition

parameters (particularly in the novel techniques of molecular-beam epitaxy, laser evaporation,

sputtering, and ion-beam-assisted processes) on film structure such as substrate temperature,

photon or particle energy, background-gas composition, and pressure, as well as post-deposition

techniques like annealing. This work is leading to denser, more damage-resistant, robust films

with lower absorption.

We see as well a correlation of damage susceptibility with residual stress in films and can identify initiating defect types and location by the morphology of damage. Most importantly besides the new processes, materials, and correlations, we are realizing the benefit of diagnostic

techniques well-suited to films, e.g., Raman scattering to map structure and time resolve thermal

processes. This latter point is most important because we now have the approaches in hand to study

the thermophysical properties of thin films, like thermal conductivity, and to determine if they

are near bulk values.

An outstanding development in optical coatings has been the process for producing poroussilica antireflection coatings. When properly made, they have exhibited damage thresholds equal

to that of bare fused silica surfaces, and their usefulness has been proven from the ultraviolet

to the near ultraviolet (0.2 to 1.2 urn).

Not yet understood, however, is the physics behind the wavelength-dependent influence of overcoats. As reported at the last conference and again this year, use of either overcoats of MgF 2 or Si0 2 resulted in a moderate reduction (2 to 20%) in damage threshold at 355 nm. This contrasts

with the 50% to 100% average improvements previously demonstrated at 248 and 1064 nm.

One controversial topic of discussion was the theoretical versus actual performance of intracavity, grazing-incidence reflectors, now considered essential for high-average power operation of

free-electron lasers (FEL). The optimized designs for multilayer dielectric reflectors for use at

grazing-incidence angles, advocated at this meeting, predict large (>100X) increases in damage

thresold over that possible at normal incidence. However, careful damage tests at 351 nm, also

reported at this conference, revealed a very modest (only 2.5X) increase in threshold at 85°

2
compared to normal incidence. (About double this enhancement was measured in other unpublished tests

at 248 nm. ) Possible explanations have been suggested, but these results are still not entirely

understood. Until future tests with other dielectric mirrors show considerable improvement, metal

mirrors must be considered the conservative choice for grazing-incidence applications, particularly

in the presence of the other high-energy radiation environments of FEL resonators.

3. Summary of Papers

The subject matter of the Sixteenth Annual Symposium covered four broad areas of interest to

the high-power laser community: (1) Materials and Measurements, (2) Mirrors and Surfaces, (3) Thin

Films, and (4) Fundamental Mechanisms. These conference proceedings are organized accordingly. In

this section, a concise summary of each paper is provided, and closely related papers are discussed

together, whenever possible. The interested reader is referred to the complete manuscript of any

paper for further details. Our intention here is to provide the reader with an overview of the

Symposium and to identify the topics of current interest, the authors, and their organizations. To

highlight the presented papers, each topical area is discussed with a brief statement of the underlying problems and the status of understanding within the area of interest.

3.1 Materials and Measurements

In the category of optical materials and measurement techniques, four general topics received

attention this year. These included: (1) liquid crystals whose properties are being exploited for

use as polarizers, electro-optical shutters, and modulators, (2) bulk damage resistance of new laser

crystals and windows, (3) application of two types of electron microscopy for analysis of metal

surfaces to deduce the cause of laser damage, and (4) automation of laser damage facilities including development of advanced diagnostic measuring devices.

Development of novel materials for laser applications continues to be a major goal of those

participating in these annual symposia. Installation of polarizers made from liquid crystals in

the high-power glass development laser (GDL) at the University of Rochester was discussed by

S. Jacobs , K. Cerqua , T. Kessler , W. Seka , and R. Bahr of the Laboratory for Laser Energetics. The

polarizers consisted of an 11-um-thick layer of a base nematic doped with either left-handed (LH)

or right-handed (RH) chiral additive to produce a helical twist structure in the crystal. A circularly polarized beam with wavelength A = nP, where n is the average refractive index and P is

the pitch dimension of the helix, is di frt.se ly scattered by the crystal if wave rotation is in the same sense as the helix, but is transmitted when the wave rotation and helix are opposed. This

property of liquid-crystal polarizers allows them to be used in polarizer/analyzer roles with circularly polarized light. Extinction of ~104 for the crossed polarization case was reported. They

have been installed in GDL as the crossed polarizers associated with Pockeis-ceil shutters and as single elements to serve as isolators for waves propagating backward through the chain.

The reported advantages of liquid crystal polarizers over Brewster-angle thin-film polarizers

were low cost and easy replacement due to in-house fabrication, insensitivity to tip angles as large as 20° off normal, insensitivity to temperature change (0.35 to 1.4 nm/°C in the position of

the central maxima), high transmittance (-98%), and the capability to propagate circularly

polarized light over most of the path length in a laser arm. Laser damage thresholds (1054-nm,

1-ns) were 3 J/cm2 for the RH units, but <1 J/cm2 for LH units, a problem that the authors

circumvented by fabricating LH polarizers as RH polarizers on quartz halfwave plates.

Additional data on liquid crystals were presented by S. Wu and L. Hess of Hughes Research

Laboratories. They described a study at 10.6 urn of the nonlinearity of the optical rotatory power

of a 90° twisted nematic liquid crystal. The crystal (BDH-E7) exhibited the expected dependence of

rotatory power on crystal thickness when subjected to low power (<0.3 kW/cm2

) irradiation at

10.6 um. When the intensity was increased, the rotatory power decreased due to alignment of the

liquid crystal molecules with the polarization of the laser beam. The incident power required to

obtain a given loss of rotatory power depended on the inverse square of the crystal length. The

loss of rotatory power was reversible for intensities up to 7.5 kW/cm2

, at which level a crystal

with 52-um thickness underwent a phase change into an isotropic material. Recovery of the

anisotropic state required about 2 min. At a higher intensity, ~8.5 kW/cm2 for the 52-pm crystal,

permanent damage occurred, but crystals with thicknesses of either 24 um or 13.5 um were not

altered at that power level. The mechanism for the permanent damage was not known.

Other authors discussed their efforts to produce and characterize improved crystals for use as laser hosts or windows. N. Mansour , M. Soileau , and E. Van Stryland of North Texas State

University reported measurements with 45-ps, 1064-nm pulses of the bulk damage thresholds for cubic

zirconia crystals stabilized by the addition of 9.4 to 2136 of Y2 0 3

. The beam diameter at the waist

formed by their focusing lens was 7.2 um (half width at I Q

/e). Breakdown thresholds were the same

3
for either linearly or circularly polarized beams, indicating that sel f-focusi ng was not a factor,

and thresholds were also independent of the Y2 03 concentration. When the beam waist was increased

in size to 10 pm, sel f-focusi ng dominated the experiment. Measurement of the critical power for

self-focusing yielded an estimate of the value of n 2 in the stabilized zirconia of 8 x 10 13 esu. This value is in reasonable agreement with a theoretical estimate made by the authors of 10 x 10 13

esu, which value is the same as predicted using an empirical equation, derived previously, that

relates n2 to the Abbe number and linear refractive index.

M. Acharekar and D. McCarthy of Litton Systems, and R. Blachman of Material Progress Corporation described damage tests of crystals of gadolinium scandium gallium garnet doped with both

chromium and neodymium (Nd: Cr: GSGG) , and the performance of lasers using this crystal. Their

Q-switched laser equipped with a 5 mm x 65 mm rod provided 135 mJ/pulse at 1061 nm with 2.2

milliradian divergence when pumped with 8-J pulses at 10 Hz. The typical pulsewidth (FWHM) was

17 ns. These 17-ns pulses were used to measure both the bulk and surface damage thresholds for

crystals of Nd:Cr:GSGG that had been polished with 0.3-um alumina powder. Beam sizes used in the

test were not specified. The bulk damage threshold for this laser crystal was reported to be

greater than that of BK-7 glass but less than that of fused silica. It was also reported that a 10% reduction in Nd concentration led to a 13% increase in bulk damage threshold. The surface

damage threshold was 14 J/cm2

.

Methacrylate polymers were studied in a collaborative effort by R. O'Connell and R. Ellis of

the University of Missouri at Columbia, A. Romberger of Pennsylvania State University (Berks

Campus), T. Deaton , K. Siegenthaler , A. Shaffer and B. Mull ins of the USAF Academy, and T. Saito of

Lawrence Livermore National Laboratory. These authors prepared samples of polymethylmethacrylate

(PMMA), polyethyl methacrylate (PEMA) and polycyclohexylmethacrylate (PCMA), some containing up to

10 wt% of a plasticizer, dibutylpthalate (DBP). The monomers were initially distilled and, in some cases, also passed through filters with pore sizes of 0.40 or 0.22 pm. Both heating (40°C) and

ultraviolet light were used to obtain polymerization. The samples were tested with 1064- nm, 8-ns

pulses focused to a beam size of 33 pm (radius at I /e2 ), and tests were conducted to insure that

the experiments were not influenced by sel f- focusing. Thresholds for either single-shot or multiple-shot tests were defined to be the fluence necessary to cause damage at 50% of the sites, a convention that leads to an overestimate of the large-area damage thresholds. Using this convention, the authors found single-shot thresholds of 29, 67, and 86 J/cm2 for samples of PMMA made from

distilled monomer and subjected to, respectively, 7.1, 0.5, and 0.1 particles/mm3

, which correlated

well with the thresholds. However, the lowest fluences for which damage occurred in the samples,

12, 28, and 21 J/cm2

, respectively, only partially correlated with dust content, and these minimum

fluences are the best indication of how the materials would perform in actual usage.

These authors also reported that single-shot thresholds of UV-polymerized samples were systematically greater than those of thermally polymerized samples, and that multiple-shot (250) thresholds were independent of the technique used for polymerization and much less than single-shot

thresholds . Multiple-shot thresholds were also found to be independent of the amount of plasticizer used, although the authors felt this observation was possibly invalid due to unequal dust

concentrations in the plasticized samples.

A portion of each preceding conference has been given to discussions of experimental techniques for characterizing materials and measuring damage thresholds. At this conference, A. Helms , C. Cho , and S. Bernasek of Frick Chemical Laboratory at Princeton University described analysis by

low energy electron diffraction (LEED) of sites on molybdenum surfaces that had been laser irradiated. They studied two samples (A and B) that had carefully prepared 100-<j>riented surfaces and a third (C) with similar orientation whose surface had been disordered by Ar ion bombardment. The

surfaces were initially analyzed by both LEED and by Auger Electron Spectroscopy (AES), and then

moved under vacuum into a sample chamber which was used to transport the sample under vacuum to a separate laboratory for laser irradiation. One half of each surface was irradiated by 140-ns,

second-harmonic pulses from a Nd:YAG laser operating at 5 kHz. The laser spot size was approximately 25 pm. Area coverage was obtained by a raster motion of the samples. The two crystalline

surfaces A and B were irradiated at, respectively, 15 MW/cm2 and 60 MW/cm2

, intensities that are below and above that necessary to produce cratering. The ion-bombarded surface was irradiated at

75 MW/cm2

. Subsequent LEED evaluation of surfaces A and B indicated that both contained randomly

distributed islands of raised 100-oriented material whose edges were oriented with crystalline

planes, indicating both the occurrence of damage during irradiation at subthreshold intensities and

the regrowth of 100-oriented material from melts produced at higher intensities. Laser annealing

of the disoriented surface on Sample C produced a partial recovery of crystal 1 i ni ty.

H. Hurt of the Michel son Laboratory at the Naval Weapons Center described the use of transmission electron microscopy (TEM) to investigate the defects responsible for site-selective damage

on copper mirrors. The surfaces studied were on small (3-mm diameter x 0.25-mm thick) samples of

4
either OFHC copper or OFHC copper plated with a UBAC (Ultra-Bright Acid Copper) copper film. These

surfaces were diamond turned at NWC using tool rake angles of either -5° or -15°, and then damaged

with 100-ns, 10.6-um C02 laser pulses. The damaged areas were thinned from the back and examined

by TEM. Sites on a UBAC surface which appeared by Nomarski microscopy to have wavy slip were found

to have been melted. One pit at a selective damage site was centered on a grain boundary and con- tained small particles of iron. In tests of additional 1.5-inch diameter samples "apparent" slip was observed as the limiting damage mechanism and occurred at 8 and 10 J/cm2 in OFHC copper turned

with rake angles of, respectively, -5° and -15°, and at 19 J/cm2 in UBAC copper.

Several papers discussed automated laser damage experiments or improved apparatus for use in

damage studies. J. Franck , S. Seitel , V. Hodgkin , W. Faith , and J. Porteus of the Naval Weapons

Center have automated an infrared damage facility equipped with C0 2 , HF, and DF lasers. A computer

system controls the fluence of the laser pulses and raster scanning of the sample to allow

multiple-site irradiation. Their system automatically detects occurrence of damage by detecting a laser-induced alteration of the scattering of light from the beam of a HeNe laser that irradiates

the test site. For samples in which damage consisted of micropits, this technique for detecting

damage was found to give results equivalent to those obtained by post- irradiation inspection of

test sites by Nomarski microscopy. Scattering was a less suitable detector of spatially uniform

damage, but a cursory examination of the sample was able to detect whether failure by this mode had

influenced the automated damage detection. The stated intent of the group was to speed testing so that 400-800 sites could be tested on each sample, thereby providing an accurate and statistically

meaningful large-area threshold.

P. Filbert , J. Guttman , C. Navoda , and B. Watson of the Lockheed Palo Alto Research Center

reported use of a computer-automated IR-sensitive camera to record fluence distributions in the

beam of a large Nd-glass laser being used in damage tests. A fraction of the beam was directed to

a diffusely reflecting screen, and a down-magnified image of this screen was recorded by 100 x 200

pixel si 1 icon-photodiode array. The computer processed image of the beam profile, appropriately

scaled in size, was superimposed on an enlarged photograph of the damage site, allowing the damage

to be correlated with fluence contours in the beam. The authors indicated that the ability to measure thresholds with large beams of varied profile would allow determination of the importance

of material stress gradients in damage.

A high-precision multiple-pass ultraviolet reflectometer was described by L. Jolin and

S. Foltyn of Los Alamos National Laboratory. The instrument was based on the White cell and

allowed computation of the net loss in a dielectric reflector through measurement of the intensity

of a 351-nm laser beam as a function of the number of times (>100) it had interacted with the re- flector. To obtain damage resistance, dielectric thin-film reflectors were used as the secondary

mirrors needed in a White cell arrangement. Repeated measurements by several operators indicated

that the precision in reflectance measurements was ±0.0007. The authors reported 351-nm reflectances ranging from 0.8498 to 0.9976 that were measured on 17 multiple-layer HR coatings.

In another instrument paper from Los Alamos National Laboratory, S. Foltyn , J. Griggs , L. Jolin , J. Roberts , and D. Keaton described a new attenuator for controlling the fluence in the

divergent and usually unpolarized beams emitted by excimer lasers. Their device employs two

reflectors operated at incidence angles near 40°. The incident beam is deflected by the first

reflector onto the second reflector, which is oriented to restore the initial direction of propagation. Because the reflectance of a multilayer dielectric reflector varies with incidence angle,

beam attenuation could be accomplished by tuning the incidence angles at the reflectors. The re- flectors were mounted in an assembly that allowed angle tuning by stepper motor control. Transmi ttance at 248 nm was measured as a function of incidence angle for an attenuator equipped with

49-layer Al 2 0 3 /Si02 reflectors designed to be reflective at 248 nm for normally incident beams.

Transmi ttance varied from 99% to 3% as incidence angle was tuned from 34° to 44°, and beam steering

was less than 0.5 mr. A transmi ssive version of the device was also described.

Variable pulsewidth was an important feature of the recently completed C0 2 laser damage test

facility at Barr and Stroud, Ltd., as described by D. Gibson and A. Wilson . Through variations in

the gas fill, their laser can be made to provide pulses with durations of ~100-ns, either with or without a 2-us tail. Introducing a locking signal from a small C02 laser into the principal C02 source laser provided 6-us pulses. Pulses with tunable duration <90 ns were obtained with a plasma

shutter in which the attenuating plasma was induced by irradiation of a dielectric. The laser

provided output energies ranging from 10 J at 10 Hz to 15 J at 1 Hz, which allowed deposition of

damaging fluences (1 - 100 J/cm2

) over areas about 1 mm in diameter. Their stated intent was to

compile a data base suitable for use in laser design applications, and initial test results were

reported for a wide range of window materials and optical films.

5
3.2 Mirrors and Surfaces

Three themes were evident in the contributions concerning the effects of intense laser irradiation of surfaces. The first was a continuation of the experiments to extend our understanding of

the physical processes involved at high laser intensities. This included evaluation of surfacepotential as a predamage indicator and multiple-pulse accumulation effects in silicon. Two papers

treated the topic of laser-induced ripple structure, primarily for semiconductors. Adequate understanding of this phenomenon will undoubtedly lead to practical applications. The third and related

topic was picosecond-pulse damage to manufactured diffraction gratings, both ruled and holographic.

Surface-potential modification as a precatastrophic damage indicator was studied by M. Becker

of the University of Texas, J. Kardach , of the Air Force Institute of Technology, and A. Stewart

and A. Guenther of the Air Force Weapons Laboratory. Two-dimensional surface-potential measurements were made using a vibrating Kelvin probe on OFHC diamond- turned copper mirrors, singlecrystal silicon, and MgF 2 , Hf02 , and ThF 4 half-wave films on fused Si02 , oriented MgF 2 , and

crystalline and amorphous quartz substrates. In damaged areas a positive surface potential was observed for the diamond-turned Cu, while all others gave negative values. Except in the case of one copper sample, no predamage changes in surface potential on irradiated surfaces were detected,

and not all damaged areas exhibited changes in surface potential. Although the laser beam had a diameter of only 0.39 mm FWHM, changes in surface potential extended over a 4-6 mm range and small

changes could be observed several millimeters beyond that, suggesting care in site spacing for

multiple-site damage tests, at least on dielectrics. Decay relaxation times for the modified sur- face potentials were over an hour for dielectrics and "permanent", i.e. long duration, changes at

damage sites were observed.

Y. Jhee , M. Becker and R. Walser of the University of Texas continued to explore the mechanism

of multiple-pulse laser damage of silicon using pulse trains of 60-ps, 1.06-pm pulses produced by a cw-model ocked Nd:YAG laser. Initial damage as seen using an SEM was correlated to the onset of

electron and ion emission. This emission was subsequently monitored as a damage indicator. They

encountered multiple-pulse damage thresholds below the one-on-one damage threshold for crystalline

silicon. In addition, the probability of damage for single events at various sites was not the

same as that for multiple-shot damage (i.e., temporal and spatial probabilities were unequal).

Thus, the 1972 lucky-electron theory of Bass and Barrett could not explain these results. No evidence of slip accumulation as evidenced by an increase in surface roughness as a precursor of damage

was found either. They conjectured that a cumulative effect of excitations, not simple probability,

is required to explain the results, even though damage does depend on pulse energy but not on pulse

repetition frequency. They concluded that a multiphonon process is unlikely and the damage mechanism may have been as simple as thermal evaporation at localized hot spots in Si. The charged

particle emission followed an Arrhenius equation, supporting the concept of laser-induced defect

sites that then become evaporation sites. The mechanism for formation of these sites and their

character was not identified.

N. Mansour , G. Reali , P. Aiello , and M. Soileau all of North Texas State University, examined

in detail the ripple structure formed near the onset of laser damage in wide gap dielectrics and

the intermediate-gap semiconductors ZnS and ZnSe. For the dielectrics the ripple spacing is con- sistently (within 5%) equal to the wavelength of incident light in the medium. The ripple direction is normal to the direction of polarization of the incident light for smooth surfaces but can

be perturbed by scratches or surface defects. The results can be explained by a simple electrostatic-dipole model but not by the competing surface excitation model. By contrast, results for

the intermediate-gap semiconductors are not easily explained by any model. They differ at front

and back surfaces, spacings vary by over a factor of 2 at the same surface for ZnSe and the back

surface ripples are not oriented perpendicular to the polarization and may be parallel to it. No

explanation is offered for these anomalies.

The analysis of laser-induced surface ripples for Si, Ge, GaAs, and Cu in terms of the interference between the incident light and surface waves in the sample was presented by P. Fauchet of

Princeton University and A. Siegman of Stanford University. Unfortunately, the authors did not

adequately reference previous and alternate explanations of the phenomena. Single-pulse ripples

fit the presented theory quite well for materials studied. However, ripples formed by multipulse

illumination were not explained by the simple theory. Irradiation levels below which ripples did

not form under multipulse illumination were found to be one-third of the single-pulse threshold for

crystalline materials and as low as one-tenth the single pulse threshold for amorphous materials.

The authors pointed out that two possible hypotheses reported by others are not inconsistent with

their data, but they offered no experimental data, such as an increase in surface roughness or small crystallite formation, which would support these hypotheses.

6
The results of extensive measurements of the laser damage resistance of diffraction gratings

were presented for the first time at these symposia. At the Los Alamos National Laboratory, D. Gill

and B. Newnam tested holographic and ruled gratings using 30-psec, 1.06-um wavelength radiation.

Holographic gratings had thresholds 1.5 to 5 times higher than ruled gratings and the replicas had

higher damage thresholds still, sometimes by over a factor of 2, than did the master holographic

grating. S-polarized light had a threshold from 1.5 to 6 times higher than P-polarized light, and

first-order gratings had as much as 5 times the damage threshold of more finely ruled gratings used

at the same angle in third order. At 1.06-um gold-coated gratings had 1.4 to 9.7 times higher

damage threshold than aluminum-coated gratings. No quantitative theoretical analysis of these

results was given.

3.3 Thin Films

Thin films continued to lead the interest at these annual symposia with over 50% of the presented papers addressing various aspects of this complex topic. A full third of these dealt with

evaluation of the properties of films produced by new processes and comparing them with those of

standard deposition methods. Others examined the possible correlation of laser damage with coating

absorption and variations of film stress over small areas. Multiple papers also addressed the

requirements for intracavity reflectors for high-average power operation of free-electron lasers.

Of special interest was a measurement study of the thermal properties of films, including thermal

conductivity and specific heat, and the comparison with bulk properties.

We begin by considering the numerous papers involving development, demonstration, and evaluation of advanced coating deposition methods. K. Lewis , J. Savage , A. Cullis , N. Chew , L. Charlwood , and D. Craig of the Royal Signals and Radar Establishment in Malvern, England, have pioneered the

growth of optical thin films by molecular-beam techniques. The films, primarily ZnSe, had negligible values of absorption at 10.6 urn and exhibited very high damage thresholds. The authors attributed the latter, in part, to the high-density character of the films of polycrystalline,

columnar morphology whose crystallite size is dependent primarily on the deposition temperature.

Also, the films showed no evidence of pinholes and had a very low impurity level.

As an indication of how good these films are, the laser-induced damage threshold of 1-pm-thick

ZnSe films on polycrystalline ZnSe substrates was in the range of 60 - 73 J/cm2 (1/e2 diameter

^102 urn) which compared well with 50 - 60 J/cm2 for the uncoated region of the sample. This observed difference was real and repeatable. For these tests, the temporal pulse from the gainswitched C02 laser included an initial spike, 33-ns wide FWHM, followed by a long tail extending to

1.7 us. For simple pulses of ~340-ns duration, damage of the films occurred at ~31-38 J/cm2 (180 urn diameter). Finally, although no voids could be detected in the films, high densities of

micro-twins and stacking faults were evident. Thus, further improvements in damage threshold may

be feasible.

Variations of novel deposition techniques and well-regarded procedures continue to be investigated for potential improvements on present-day methods. T. Raj , L. Weaver , S. Tuenge , J. Price , and K. Jungling of the Martin-Marietta Aerospace Company reported on coatings of Zr02 produced in

ultra-high vacuum (UHV) of 5 x 10 10 torr using either electron-beam or laser evaporation by a 100-W

cw C0 2 laser. Substrates were maintained at either 150°C or 250°C while background 02 pressures

were varied between 0 and 5 x 10- 5 torr. Coatings were evaluated for stoichiometry and impurities

by AES and for refractive index by el 1 ipsometry. Some coatings were damage tested at 351 nm. Results obtained led to the conclusion that the stoichiometry of Zr02 films deposited by either

technique is not affected by variations in 02 backfill pressure to 5 x 10- 5 torr. However, stoichiometry was superior at 150°C than for higher substrate temperatures, and the refractive index

was closer to bulk values at this lower temperature. Another observation was that coatings

produced under UHV conditions had at least an order-of-magnitude lower contamination level than

coatings produced in normal HV chambers. In all cases, free metal and sub-oxides of Zr02 were

evident. Unfortunately, damage trends were inconclusive. These investigations should be continued

with a larger sized sample set.

As we continue progressing in our separation of the controlling variables in the coating

process, we can focus on optimization of the variables if they are sufficiently independent. In

ion-beam deposition of refractory oxides, one such well-established variable is beam composition.

H. Demiryont and J. Sites of Colorado State University have identified a critical level of oxygen

necessary in the beam to produce stoichiometric films of Ti02 , Ta2 0 5 , and Si0 2 when using both

elemental or oxide targets. The threshold was easily seen in the film composition, refractive

index, and optical band-gap character. There was strong evidence that an oxide forms on elemental

targets and this is subsequently sputtered onto the substrate.

7
The demand for higher quality thin films has stimulated experimentation with a number of novel

coating deposition techniques that offer the possibility of fine control of the film properties.

A. Stewart and A. Guenther of the Air Force Weapons Laboratory conducted a limited survey of characteristics of single-layer films of Zr02 and A1 2 03 produced by a number of these processes. Besides the conventional electron beam technique, the other processes evaluated included electron

beam in ultrahigh vacuum, electron beam with ion-beam assist, cw laser in ultrahigh vacuum, and an advanced epitaxial technique. Characterization of the film properties included total -integrated

scatter, film transmittance near the uv absorption edge, x-ray diffractometry, and multiple-pulse

laser damage thresholds varied by only a factor of two for each material. The results of this

study will serve as a benchmark for future progress. As the parameters of the newer processes are optimized for maximum damage resistance over the next several years, any relative advantages should

become manifest.

Substantial progress in the development of porous silica antireflection coatings for largescale optics used in high-power laser systems was reported by I. Thomas , J. Wilder , W. Lowdermilk , and M. Staggs of Lawrence Livermore National Laboratory. To maximize laser damage resistance by

minimizing the chances of any carbonaceous residue, a highly purified organic silicate, tetraethyl

orthosi 1 icate, Si(0C2 H 5 ) 4 was hydrolyzed by a basic catalyst to yield a colloidal suspension of

silica particles in anhydrous ethanol. When applied in quarterwave thicknesses to optical com- ponents made of fused silica and KDP by a dip or spin process at room temperature, no further

treatment was necessary. Damage thresholds measured at 248, 346, and 1064 nm were approximately

the same as for the uncoated substrates.

In a second experimental study of porous-silica antireflection coatings, D. Milam, L. Thomas , C. Weinzapfel , and J. Wilder of Lawrence Livermore National Laboratory determined the pulseduration scaling of laser damage for a series of coatings using pulse durations of 1, 5, and 9 ns. Ten porous-silica antireflection coatings were deposited from both methanol and ethanol solutions

containing silica particles with diameters of 10 to 20 nm. The median thresholds measured at the

three pulse durations were 11, 26.5, and 38 J/cm2

, respectively. The observed threshold scaling to

the 0.56 power of the pulsewidth is, within experimental error, close to the 0.5 power previously

measured for both bare, polished glass surfaces and those with graded-index made by etching.

Porous silica coatings can be deposited from alcoholic solutions containing either silica

microspheres with diameters of 10-20 nm or smaller, polymer silica molecules. To further develop

the versatility of these as antireflection coatings, D. Milam and G. Peterson of Lawrence Livermore

National Laboratory studied how a mixture of these two forms could be optimized to maximize

the spectral bandwidth of high transmission over the 0.2- to 1.2-um range. Calculations were made

for three single-layer density gradients: (1) homogeneous refractive index, (2) index profiles that

increase monotonically with distance into the coating, and (3) index profiles that rise in discrete

steps. They determined that the maximum bandwidth occurs for the linear-ramp coating whose index

rises linearly from that of air to that of the substrate. In current practice, however, this desig

can not be realized because a sufficiently porous film at the air-film interface cannot be attained

For currently attainable fractional silica contents, estimated to range from 0.4 to 0.5, the

optimum bandwidth results from a coating with a single discrete step in its index profile.

Besides the study of new coating procedures, new coating materials and combinations are being

investigated for specific applications as more novel techniques become commonplace. To this end,

multilayer AR coatings of A1N3 /A1 2 03, prepared by reactive sputtering, were characterized for

scratch-resistance by L. Koshigoe , L. Johnson , T. Donovan , and C. Marrs of the Michel son Laboratory

of the Naval Weapons Center. These materials appeared to have adequate stability. A variety of

sputtering techniques were employed from the use of dc magnetrons to ion-beam and rf diodes. An

important aspect of this study was a determination of the resistances of these materials and coatings to laser radiation, fluorine, electron-beam irradiation, and humidity, while coating composition and stoichiometry were evaluated by SAM, SEM, EDX, XPS, and Nomarski microscopy. Substrates

included fused silica, silicon, and calcium fluoride. The first tests on these films were encourag

ing. However, considerable work needs to be done to reduce the number of damage- initiating defect

sites. It was pointed out that in most cases, the oxide layer should be the outer layer because it

is more impervious to environmental degradation.

Raman scattering continues to gain in utility as a routine diagnostic technique for characterizing thin films. Its use to study annealing and phase transformation, such as crystallization,

was reported by R. Rujkorakarn , L. Hsu , and C. She of Colorado State University. Two types of

low-loss, low-scatter, amorphous sub-micron-thick titania films were thermally annealed in air.

The transition to anatase/ruti 1 e crystalline phases coincided with a decrease in optical transmission, small changes in refractive indices and energy gap in the film, along with a 100x increase

in inelastic scattering.

8
Raman scattering has also found considerable use as an analytical measurement technique to

probe the structure, thickness, and homogeneity of optical thin films. G. Exarhos of Battelle

Pacific Northwest Laboratory and P. Morse of the Air Force Weapons Laboratory added a new twist by

employing time resolution to investigate laser-induced damage in Ti02 coatings. The technique

involved two lasers which allowed one to follow surface transformation/relaxation phenomena, etc.

One laser was used as a Raman probe while another pulsed laser stimulated the disturbance. The

authors observed that anatase-phase coatings have lower damage thresholds than those with the

rutile phase, and the former failed via an irreversible phase transition to the rutile structure at

temperatures below 910°C. On the other hand, rutile-phase coatings apparently accommodate energy

in an excited electronic state resulting in "non-equilibrium" phase transformation to an "anataselike" state.

In support of high power laser development for operation in the near ultraviolet, A. Stewart

of the Air Force Weapons Laboratory and D. Gallant of Rocketdyne Division of Rockwell International

have assembled a laboratory of instrumentation to characterize low-loss optical surfaces and thin

film coatings. These were used to measure scatter, absorption and reflectance at 351 nm of maximum

reflectors deposited by commercial vendors on super-polished silicon and fused silica substrates.

Ion-beam sputter-deposited reflectors realized the lowest losses and exhibited the highest reflectance of 0.999 on the average. However, laser damage tests on these reflectors, conducted by

S. Foltyn's group at Los Alamos National Laboratory, determined that electron-beam-produced coatings are presently more damage resistant by a factor of 2. Since the ion-beam deposition process

is in the early phases of optimization for damage resistance, large improvements may well be

attained by persistent effort.

As reported at several previous Boulder Damage Symposia, the damage resistance of multilayerdielectric reflectors is substantially increased by addition of half-wave thick overcoats of lowindex material. Experiments at 1064 nm and 248 nm have demonstrated that overcoats result in

average improvements of approximately 50% and 100%, respectively. However, at the 1983 Symposium,

C. Carniglia and T. Hart of the Optical Coating Laboratory, Inc., and M. Staggs of the Lawrence

Livermore National Laboratory reported that halfwave overcoats of Si02 on high reflectors con- taining Zr02 amd Ta2 0 5 caused about a 20% reduction in damage thresholds at the intermediate wavelength of 355 nm. This very surprising result prompted these authors to conduct another and even

more careful series of coating and damage experiments using a scandia/silica/magnesium fluoride

reflector design which had experienced the 100% threshold enhancement with overcoats of both MgF 2 and Si02 . Again, on the average the overcoated reflectors had slightly lower (2 to 7%) thresholds.

The physics behind the wavelength-dependent influence of overcoats is still not understood and

offers the coating researcher a challenge.

In a paper of considerable interest to the thin-film damage community, D. Decker , L. Koshigoe , and E. Ashley of the Michel son Laboratory, Naval Weapons Center, reported on measurements of the

thermal properties of optical material in thin film form and compared them to tabulated bulk material properties. Materials initially analyzed were the important refractory oxides A1 2 0 3 and Si02

. Measured values of Si02 thin-film thermal conductivity were found to be lower by a factor of 5 to 8

when compared to bulk fused silica while the observed decrease for A1 2 03 was on the order of 80.

The large variations were ascribed to the greater disorder present in thin films than in bulk materials. In point of fact, the A1 2 0 3 films were quite amorphous suggesting support for the workers'

contention. Specific-heat values for the films were quite comparable with bulk values since this

property should be largely independent of structural characteristics and density. The conclusions

drawn in this work have far-reaching implications when modeling the laser interaction and thermal

response in thin film damage experiments. Clearly, more work is needed to corroborate the present

results, determine the extent of variation in other materials, such as fluorides, and to address

discontinuities between layers and the substrate/film system boundary. We were most pleased to see this work under way and reported.

For many years, scientists have been trying to relate absorption to damage sensitivity. While

this has been relatively easy in windows and for some surfaces, no strong correlation has been

found for thin films perhaps because all films have been of uniformly poor quality. However, a new attack on this problem was reported this year by D. Ristau , X. Dang , and J. Ebert of the University

of Hannover, Federal Republic of Germany. Employing an infrared, scanning- line technique, they

measured the temporal and spatial development of surface-temperature increases of coated material

irradiated with a cw Nd:YAG laser. In this way, they were able to separate the interface and bulk

absorption of single and multiple layers and relate them to laser damage thresholds for 15-ns,

1.064-um pulses. All oxide samples of Ti , Ta, Hf, Al , and Si were prepared by electron-beam and

ion-beam assisted deposition. Thermal conductivities much lower than bulk values were recorded for

the oxides of Si and Al , with the e-beam-deposited films exhibiting higher absorption and lower

thermal conductivity. Interestingly, absorption was much higher in the bulk of the films than at

interfaces between layers.

9
With the exception of Hf02 and the co-evaporated Ti0 2 layers, there was a "strong correlation"

between absorption and damage threshold. Damage morphology affords evidence of inclusions in the

bulk while for stacks, interface damage is frequently manifest, probably due to it being a region

of high stress. Co-evaporation may help alleviate this problem.

A. Heiney , J. Eastman , and C. Gabel of the Laboratory for Laser Energetics at the University

of Rochester attempted a study of the effect of anisotropic stress in thin films on laser- induced

damage sensitivity. Use was made of a microscope-based, modulated ellipsometer to determine the

directional dependence of the stress in the plane of the film via induced birefringence. Interestingly, areas with negative anisotropic stress were found to have higher damage sensitivities more frequently than regions of positive anisotropic stress. The authors reported that damage thresholds changed from 1.3 to 6.9 J/cm2 within 100 pm. The width of measured bands in this work ranged

form 0.5 to 2.5 J/cm2

. Pulse lengths were 30 ps at 1.06 pm, and the test samples were generally

Ti02 /Si0 2 reflectors.

Intracavity grazing-incidence mirrors appear necessary to handle the high-average powers

expected in future free-electron laser (FEL) oscillators. With slight curvature of the figure,

these mirrors diverge the reflected beam to tolerable intensity levels at the cavity end mirrors.

However, these intracavity reflectors now become potentially very susceptible to laser-induced

damage and/or thermal distortion. W. Southwell of Rockwell International Science Center evaluated

the advantages of multilayer-dielectric reflectors for this purpose and presented an optimized

design for the layer thicknesses. With the standard design using quarter-wave layer thicknesses,

very high reflectance was predicted for s-polarized radiation with drastically reduced internal

standing-wave (SW) electric fields. For a reflector designed for 86° incidence and film-material

pairs with refractive indices of 2.0 and 1.38, the calculated peak SW fields-squared were reduced

by a factor of 160 relative to normal-incidence reflectors, but only for s-polarization. A design

with improved performance for both s- and p-polarizations was also devised, but a smaller fieldsquared reduction factor of 40 was predicted. Based upon previous correlations of laser damage and

the internal SW electric fields, the author predicted that these designs should be much less sus- ceptible to laser damage and thermal distortion.

With FELs still in mind, the theoretical design of intracavity grazing-incidence reflectors

using multilayer dielectric coatings was also treated by J. Shellan of W. J. Schafer Associates.

His criterion was to maximize the reflectance of the multilayer stack to minimize the power

transmitted to an absorbing substrate. With coating absorption assumed negligible compared to that

of the substrate, he derived analytical expressions for the thickness of the outermost layer of

otherwise all quarter-wave- layer reflectors and benchmarked the predicted reflectance against that

generated using numerical methods of nonlinear optimization. For 100% s-polarized light, maximum

reflectance requires the outer layer to have quarter-wave optical thickness, whereas it should have

half-wave thickness for 100% p-polarization. Shellan's expressions are most helpful for analyzing

the most likely configuration where the fraction of p-polarized light is small, but nonzero. Even

if only 0.1% of the incident flux is p-polarized, optimization of the top layer thickness will

reduce the reflector transmittance, and thereby the substrate absorption, by at least an order of

magnitude.

One would expect that the angular dependence of damage to thin film coatings would be some- thing one could calculate in a straight forward, if somewhat involved manner. But as B. Newnam , S. Foltyn , D. Gill , and L. Jolin of Los Alamos National Laboratory showed, first principle calculations and experimental verifications don't always agree! The results of their study was not only

surprising but has wide-reaching implications since it was expected that near-grazing angles of

incidence could be employed to diverge the tightly focused beams such as are present in free electron lasers and thereby realize a higher intensity throughput. Their work centered on measuring

the damage resistance of Hf02 /Si02 multilayer reflectors subjected to a 10-ns pulsed 351-nm XeF

laser operating at 35 pps. Several angles of incidence between 0° and 85° were employed using an S-polarized beam. Calculations of the expected damage threshold increase with angle of incidence

included both areal intensity decrease (l/cos8) and appropriate electric field variations in the

thin film/substrate system. In one case, a multilayer designed and tested at 85° afforded only a 2.5x increase in damage threshold compared to normal -incidence reflectors while one could con- ceivably expect a factor of 100x. Potential explanations for this discrepancy included: effects

due to increased irradiation area at large angles and physically thicker layers, both resulting in

the interception of more defects, plus scattered- and trapped-1 ight effects. Obviously, more work

needs to be accomplished to shed light on this problem.

It is readily accepted that defects are bad in optical elements used in high-power/high-energy

laser systems, so that anything we can learn about them is useful whether it enables us to determine their origin or to model their influence. The powerful use of scanning-electron microscopy

was employed by L. Johnson , E. Ashley , T. Donovan , J. Franck , R. Woolever , and R. Dalbey of the

10
Michel son Laboratory of the Naval Weapons Laboratory to study damage- initiating defects in ZnSe/

ThF4 and SiH/Si02 multilayers.

For ZnSe/ThF4 multilayer mirrors, oblong-shaped damage sites, oriented perpendicular to the

laser electric field, were associated with particulates at or near the surface while circularshaped damage sites were related to those defects below the top layer. A third type of damage was identified as pinholes. Stress-related cracking was frequently observed as well. Frequently,

damage related to nodule defects was seen in Si H/Si02 multilayers. Good correlation was seen between nodule density and damage sensitivity. Excellent micrographs of the different types of

damage morphology related to different types of defects were presented.

Laser damage data for pulsewidths longer than a few-hundred nanoseconds are scarce, especially

for visible-wavelength or near-infrared lasers. Thus, the results of a multilayer reflector damage

survey using 8-us (FWHM) pulses from an atomic iodine laser at 1.315 urn, presented by T. Deaton of

the U.S. Air Force Academy, were especially interesting. Large 2.8-mm spot diameters were used in

the single-shot-per-site mode. A number of coating materials combinations and designs from several

vendors, each deposited on silicon and molybdenum substrates, were tested. Using the onset of

pitting visible by Nomarski microscopy as the damage criterion, threshold fluences ranged widely

from 5 J/cm2 to over 140 J/cm2

, with uncertainties typically about 15%. Titania/si 1 ica coatings

had some of the highest thresholds, and reflectors incorporating lead fluoride layers had the

lowest damage resistance. As might be expected, very little dependence on the substrate material was evident. Hopefully, this survey will be followed by optimization of the best reflector

designs.

A contribution from H. Deng , M. Bass , and N. Koumvakalis of the Center for Laser Studies at

the University of Southern California concerned the measurement of the 10.6-um damage resistance of

multilayer-dielectric reflectors comprised of ZnSe and ThF4 as a function of substrate material,

reflectance, and defect density. Testing was conducted with a C02 laser restricted to operate in

the TEMqo node with a pulse duration of 230 ns. The single-shot-per-site mode of testing with a small -diameter beam (120 urn at l 0 /e level) was used. For these conditions, the average threshold

was 90 J/cm2 with no apparent dependence on substrate material (Si, Ge, and ZnSe), reflectance

(ranging from 65% to 100%), or defect density. Anti reflection coatings of these two materials on ZnSe had a much lower threshold (40 J/cm2 ), which was probably due to laser absorption by embedded

impurities at the substrate/coating interface. Additional tests to relate visible-light scatter to

damage susceptibility revealed no correlation.

The problem of economical recovery of damaged coated optics for recoating has been awaiting a satisfactory solution. This is especially important when large-diameter substrates, figured to the

tolerances necessary to ultraviolet coatings, are involved. J. Asmus and J. Oldenettel of Maxwell

Laboratories evaluated the effectiveness of a pulsed xenon flashlamp for removal of a number of

dielectric coatings including Si02 , LaF 3 , MgF 2 , and Hf0 2 . They found that a train of exposures

with 30 J/cm2 fluence and 600-us pulsewidth is sufficient to dislodge these coatings without

damaging fused silica or Zerodur substrates. The authors suggested that coating removal results

from absorption near the ultraviolet bandedge with consequent differential heating and expansion.

In some instances, a thin coating residue remained after irradiation indicating the need for further refinement of this technique.

3.4 Fundamental Mechanisms

With the practical problems of thin film coatings drawing so much attention this year, only

three papers addressed the fundamental physics questions of importance. These included calculations

of the thermal response of thin films under repetitive-pulse irradiation, theoretical analysis

and critical review of past work concerning self-focusing in transparent solids, and calculation of

two-photon absorption coefficients for semiconductors.

Extending their thermal modeling work for dielectric thin films M. Lange , and J. Mclver of the

University of New Mexico and A. Guenther of the Air Force Weapons Laboratory solved the thermal

diffusion equation for repetitive pulses. The possibility of irreversible changes occurring with

each pulse was included in the theory. In this case, the spatial dependence of the absorption from

pulse to pulse is a key parameter. The special case of spherical or cylindrical absorption

centers was shown to lead to an elegant closed-form solution. Various models of absorption proportional to number of pulses were explored and the basis was laid for further work on multipulse

damage analysis.

M. Soileau , W. Williams and E. Van Stryland of North Texas State University continued to

address nonlinear electro-optical processes. Theoretical analysis, indicating that the critical

parameter of self-focusing is the second critical power P 2 (presented in 1983), was applied to

11
various experimental studies reported in the literature and to measurements made on Si02 , NaCl

,

BK-7 glass and CS2

. In cases where sufficient data were available the recalculations were made,

yielding improved values. It was shown that some previous analyses yield results which are both

logically inconsistent and in error by as much as an order of magnitude.

An analytical relationship for the two-photon absorption coefficient based upon material

parameters such as energy gap, linear index of refraction, wavelength, and a parameter f was derived by E. Van Stryland , H. Vanherzeele , M. Woodall , M. Soileau , A. Smirl , S. Guha and T. Boggess

of North Texas State University. (The parameter f is a function of the assumed band structure and

hence those optical states which are coupled.) Using this relation, the two-photon absorption co- efficients of ZnTe, CdTe, CdS/Se mixtures, GaAs, ZnS, ZnSe, CdS, and ZnO were calculated using

published parameters and compared against measured pulses. Good agreement was found in all cases,

although a dependence on crystal type and single or polycrystall i ne structure was observed. Calculated values for ZnS, ZnSe, CdS and ZnO were reported for a wavelength of 0.53 urn, and values for

the others for a wavelength of 1.06 urn. Data on InSb, reported previously, was shown to fit the

theory, also. The nonlinear real part of the index was calculated assuming that it arises from

two-photon generated free carriers. An optical li miter made of GaAs, which employs both two-photon

absorption and self-defocusing transmitting 33% of the incident light at low intensities, was demonstrated. For energies above 10 uJ the transmitted energy was constant at 10 uJ over an incident energy range of at least 20. In comparison, GaAs melted above an energy of 100 uJ (0.9 J/cm2 ),

but the device continued to function. Response time was shown to be in the picosecond range.

4. Recommendations

We should retain a commitment to the extensive and lengthy development required to adapt

promising new materials to practical applications. The successful and novel use of liquid crystals

as polarizers is a significant example of the advantage obtained through concentrated study of a new material. In addition, increased use of surface diagnostic devices, particularly those capable

of studying damage precursors, is needed to find the causes of damage. We hasten to add that

automation of damage test laboratories will provide a needed increase in the national capability

for significant damage testing. However, such labs should use procedures that contain adequate

redundancy to insure detection of error due to malfunctions of the apparatus.

The increasing use of diffraction gratings in high-power laser resonators motivates more laser

damage testing for a wide range of laser pulsewidths. Based on the results reported at this con- ference, an especially valuable achievement would be development of a process for fabricating

large-size, blazed holographic gratings directly on high-conductance substrates such as silicon,

silicon carbide, or molybdenum. Elimination of the photoresist or epoxy layer used in current

fabrication would greatly increase the damage threshold for long-pulse and cw laser applications.

While it was gratifying to note the many correlations between deposition process variables and

film structure on one hand and optical properties on the other, insufficient attention was paid to

correlating damage resistance with these process-dependent characteristics. We all agree that

lower absorption, denser, less-stressed films are good goals. However, there are some conflicting,

or at least not readily consistent, results regarding the magnitude of the stress dependence of

damage. This should be pursued with attention given to stress compensation in multilayers. Continued development of new processes, materials, and new diagnostics is warranted. UHV and

multiple-beam epitaxy look promising as does ion-beam-assisted deposition; too little is known

about laser evaporation as yet, but laser-induced desorption or substrate modification appear promising. The most advantageous of the diagnostics is Raman scattering, particularly when timeresolved, and surface emission as an indicator of surface cleanliness or pre-catastrophic damage.

But most importantly, we must resolve the issue of thin-film thermophysical properties. The early

results seem to say that thermal conductivities are on one hand of similar magnitude as those of

bulk materials while on the other hand they are orders-of-magnitude lower. These statements have

not been applied to the same materials. Quality measurements, those in which the community can

have confidence, are most important in modeling thermal failure, material selection, and film

design. They must certainly be structure dependent.

Finally, two unexplained results concerning the damage resistance of multilayer dielectric

reflectors, described at the beginning of this review, should be pursued vigorously with careful

experimentation and analysis. These are the wavelength-dependent influence of overcoat layers and

the angular dependence, especially at angles beyond 60°.

5. Acknowledgments

The editors would like to acknowledge the invaluable assistance of Mr. Aaron A. Sanders and

the other involved staff members of the National Bureau of Standards in Boulder, Colorado, for

12
their interest, support, and untiring efforts in the professional operation of the symposium.

Particular thanks to to Ms. Susie Rivera for her part in the preparation and publication of the

proceedings and to Ms. Pat Whited of the Air Force Weapons Laboratory for conference coordination.

6. References

[1] Glass, A. J.; Guenther, A. H. , eds. Damage in Laser Glass, ASTM Spec. Tech. Publ. 469, ASTM,

Philadelphia. PA; 1969.

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

Publ. 341; 1970.

[3] Bloembergen, N. Fundamentals of Damage in Laser Glass, National Materials Advisory Board

Publ. NMAB-271, National Academy of Sciences; 1970.

[4] Glass, A. J.; Guenther, A. H. , eds. Damage in Laser Materials: 1971, Nat. Bur. Stand.

(U.S.) Spec. Publ. 356; 1971.

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

NMAB-356; 1971.

[6] Glass, A. J.; Guenther, A. H. , eds. Laser Induced Damage in Optical Materials: 1972, Nat.

Bur. Stand. (U.S.) Spec. Publ. 372; 1972.

[7] Glass, A. J.; Guenther, A. H. , eds. Laser Induced Damage in Optical Materials: 1973, Nat.

Bur. Stand. (U.S.) Spec. Publ. 387; 1973.

[8] Glass, A. J.; Guenther, A. H. Laser Induced Damage in Optical Materials, 1973: A Conference

Report. Appl. Opt. 13 (1): 74-88; 1974.

[9] Glass, A. J.; Guenther, A. H. , eds. Laser Induced Damage in Optical Materials; 1974, Nat.

Bur. Stand. (U.S.) Spec. Publ. 414; 1974.

[10] Glass, A. J.; Guenther, A. H. Laser Induced Damage in Optical Materials: 6th ASTM

Symposium. Appl. Opt. 14 (3): 698-715; 1975.

[11] Glass, A. J.; Guenther, A. H. , eds. Laser Induced Damage in Optical Materials: 1975, Nat.

Bur. Stand. (U.S.) Spec. Publ. 435; 1975.

[12] Glass, A. J.; Guenther, A. H. Laser Induced Damage in Optical Materials: 7th ASTM

Symposium. Appl. Opt. 15(6): 1510-1529; 1976.

[13] Glass, A. J.; Guenther, A. H. , eds. Laser Induced Damage in Optical Materials: 1976. Nat.

Bur. Stand. (U.S.) Spec. Publ. 462; 1976.

[14] Glass, A. J.; Guenther, A. H. Laser Induced Damage in Optical Materials: 8th ASTM

Symposium, Appl. Opt. 16(5): 1214-1231; 1977.

[15] Glass, A. J.; Guenther, A. H. , eds. Laser Induced Damage in Optical Materials: 1977, Nat.

Bur. Stand. (U.S.) Spec. Publ. 509; 1977.

[16] Glass, A. J.; Guenther, A. H. Laser Induced Damage in Optical Materials: 9th ASTM

Symposium, Appl. Opt. 17(15): 2386-2411; 1978.

[17] Glass, A. J.; Guenther, A. H. Laser Induced Damage in Optical Materials: 1978, Nat. Bur.

Stand. (U.S.) Spec. Publ. 541; 1978.

[18] Glass, A. J.; Guenther, A. H. , eds. Laser Induced Damage in Optical Materials; 10th ASTM

Symposium, Appl. Opt. 18(13): 2212-2229; 1979.

[19] Bennett, H. E. ; Glass, A. J.; Guenther, A. H. ; Newnam, B. E. Laser Induced Damage in Optical

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

[20] Bennett, H. E. ; Glass, A. J.; Guenther, A. H. ; Newnam, B. E. Laser Induced Damage in Optical

Materials: 11th ASTM Symposium, Appl. Opt. 19(14): 2375-2397; 1980.

13
[21]

[22]

[23]

[24]

[25]

[26]

[27]

[28]

Bennett

Materia

Bennett

Materia

Bennett

Materia

Bennett

Materia

Bennett

Materia

Bennett

Materia

Bennett

Materia

Bennett

Materia

H. E. ; Glass, A. J.; Guenther, A. H. ; Newnam, B. E. Laser Induced Damage in Optical

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

H. E. ; Glass, A. J.; Guenther, A. H. ; Newnam, B. E. Laser Induced Damage in Optical

s: 12th ATSM Symposium, Appl . Opt. 20(17): 3003-3019; 1981.

H. E. ; Guenther, A. H.

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

H. E. ; Guenther, A. H.

s: 13th ASTM Symposium,

H. E. : Guenther, A. H.

s: 1982, Nat. Bur. Stand. (U.S.) Spec. Publ. 669; 1984.

H. E. ; Guenther, A. H.

H. E. ; Guenther, A. H.

s: 1983, Nat. Bur. STan

H. E. ; Guenther, A. H.

Milam, D. ; Newnam, B. E. Laser Induced Damage in Optical

Milam, D. ; Newnam, B. E. Laser Induced Damage in Optical

Appl. Opt. 22(20): 3276-3296; 1983.

Milam, D. ; Newnam, B. E. Laser Induced Damage in Optical

Milam, D. ; Newnam, B. E. Laser Induced Damage in Optical

s: 14th ASTM Symposium, Appl. Opt. 23(21): 3782-3795; 1984.

Milam, D. ; Newnam, B. E. Laser Induced Damage in Optical

. (U.S.) Spec. Publ. 688; 1985.

Milam, D. ; Newnam, B. E. Laser Induced Damage in Optical

s: 15th ASTM Symposium, Appl. Opt. 25(2): 258-275; 1986.

14
Manuscript Received

2-11-85 SAR

Retrofit of a High Power Nd:Glass Laser System with Liquid Crystal Polarizers

S. D. Jacobs, K. A. Cerqua, T. J. Kessler, W. Seka, R. Bahr

Laboratory for Laser Energetics

University of Rochester

250 East River Road

Rochester, New York 14623

The glass development laser (GDL) , has been operating at the Laboratory for Laser

Energetics since 1978. This Nd:phosphate glass system produces high peak power optical

radiation at X = 1054 nm or X = 351 nm for use in studying the interaction physics of

intense laser beams with matter. The amplifier staging incorporates the propagation of

linearly and circularly polarized light in rod amplifiers which vary in diameter from

16 mm to 90 mm. Numerous quartz or mica quarter waveplates and Brewster angle dielectric

thin film polarizers are required to limit accumulated phase retardation between amplification stages, and to accommodate interstage Pockels' cell isolation switches.

We have recently replaced most of the waveplate-dielectric polarizer combinations in

GDL with liquid crystal polarizers. Comprised of 11 ym thick cholesteric fluids sandwiched between optical quality glass plates, liquid crystal polarizers provide excellent

polarization properties, low insertion loss, angular insensitivity, and laser damage re- sistance at x = 1054 nm.

The design, fabrication and performance of left-handed and right-handed circular polarizers will be discussed.

Key words: cholesterics ; circular polarizers; liquid crystals; Nd:glass laser; optical

isolators; polarizers; selective reflection.

1. Introduction

Brewster angle linear polarizers composed of alternating high and low index, thin film layers

which have been deposited on glass substrates have been utilized in the glass development laser

(GDL) system at LLE since it was first constructed [1]. Produced by standard e-gun deposition technology in apertures ranging from 2 cm to 20 cm, these optical elements routinely achieve polarization levels of 200 to 500:1 at 1054 nm with good laser damage resistance. They are essential to the

operation of electro-optic switching devices such as Pockels' cells. Three disadvantages to

Brewster angle polarizers include environmental instability, tendency to displace the laser system

beamline, and physical space requirements.

We have recently replaced six Brewster angle linear polarizers in GDL with liquid crystal (LC)

polarizers, varying in clear aperture from 38 mm to 100 mm. Figure 1 shows the locations of the

liquid crystal cells in the system, which has been reconfigured to drive active mirror amplifiers

15
SF5 90 mm V//,

45 J 1054 nm

1 ns

Active Mirrors 350 J 175 J 1054 nm 351 nm

1 ns

64|p mm SF4 \

SF,

LH

40 mm

30 mm

)ZZ&ZZl 64, mm ^^>\>

LH RH

zzzj SF3 Y7777^,

• Circular polarization

• Isolation

• Ease of use

SFn H«-|-^p|-| ( 16 mm )-^« Osc. LH RH

Figure 1. GDL Nckglass laser system with liquid crystal polarizers.

The 3 left-handed (LH) and 3 right-handed (RH) circular

polarizers vary in clear aperture from 38 mm to 100 mm and

permit the propagation of circularly polarized laser

radiation through most of the beamline.

Liquid crystal polarizers have none of the disadvantages associated with Brewster angle polar

izers, and offer additional features not available with any other polarizer technology. Figure 2 shows how, in addition to serving as polarization elements for electro-optic switches, liquid crys

tal cells can provide optical isolation or back reflection protection.

Polarization Isolation

LH incident

LH

back

reflection

G1466

Figure 2. Dual function of liquid crystal polarizers. Left-handed (LH) and right-handed

(RH) circular polarizer elements may be fabricated and installed to perform

traditional polarizer/analyzer functions in electro-optic switch-out devices.

In addition, they act as optical isolators to prevent beamline component damage

due to specular back-reflections from fusion targets or ghosts.

16
This versatility has permitted us to incorporate the following changes in reconfiguring the GDL

system:

1. propagation of circularly polarized laser radiation through the entire front end, minimizing stress-birefringence induced by the laser rods,

2. relocation of the Faraday rotator toward the output end of the system, providing for better

protection against system back reflection,

3. reduced concern with back reflected ghosts, due to the isolation properties of the LC polarizers,

4. improved throttle adjustment at the oscillator input to the beamline with the use of circular polarization, and

5. ease of installation and alignment.

In this article we describe the in-house design, fabrication, installation, and testing of liquid

crystal polarizers on GDL.

2. Selective Reflection in Cholesterics

The use of selective reflection in liquid crystals for high power laser applications has been

discussed in the literature [3,4,5]. Briefly reviewed, selective reflection occurs in organic

liquid crystal compounds when LH or RH chiral additives are mixed into base nematics so that a helical twist structure results.

Figure 3 shows how, if the pitch, P, characteristic of this twisted structure, satisfies the

equation

\

Q

= nP (1)

where n is the average refractive index of the liquid, then incident laser radiation at wavelength,

X0 > experiences Bragg-like interference and diffuse reflection for that circularly polarized component which rotates in the same sense as the helical structure.

Molecular planes order

in RH helix

A, laser wavelength n, average refractive index lor liquid crystal

a 0 , wavelength of selective reflection P, pitch of structure

G1469

Figure 3. Selective reflection and Bragg-like interference. The chiral structure of liquid crystals will selectively reflect incident laser

radiation if the laser's sense of circular polarization and wavelength equal the product of helical pitch times average liquid re- fractive index. Otherwise, no interaction occurs.

17
By using two cells in tandem, each filled with a fluid layer whose pitch and refractive index

are tuned by compound blending to satisfy eq (1) but with opposi te chirality, laser radiation of any

arbitrary polarization state can be polarized and rejected by the cells. They act as a pair of

crossed, circular polarizers at A 0

. Figure 4 shows spectral absorptance in unpolarized light for LH

and RH cells tuned to 1064 nm. Scanned separately, each fluid exhibits a selective reflection peak

with few other features between 300 nm and 2000 nm. When scanned together the composite exhibits

strong extinction near 1064 nm.

0.0 1 1 1 I I I I I I 300 500 700 900 1100 1300 1500 1700 1900

WAVELENGTH (nm)

Figure 4. Spectral transmission through liquid crystal polarizers.

Single LH and RH element scans indicate that, apart from

regions of increased optical density due to selective re- flection, the spectral transmission of liquid crystal polarizers is featureless. Crossed polarizers exhibit

blocking extinction at any desired wavelength, which is

determined by tuning liquid crystal pitch with composition mixing.

3. Cell Fabrication

Cell fabrication begins with selection of the compound. Table 1 lists the pure, base nematics

which we have selected from commercial sources. They represent eutectic mixtures of long, rod-like

molecules whose terminal and bridging groups are chosen to optimize properties such as melting and

clearing points, viscosity, birefringence, optical transmittance, and environmental stability. The

base nematics are all liquids at room temperature. The LH and RH chiral additives listed in table 1 may be solids (wax or powder) or liquids. The quantity of additive required for pitch tuning depends upon the intrinsic pitch of the substance (150 nm for CB15, 800 nm for C15), but it is usually

on the order of 10-20 wt.%. Compound mixing is performed at elevated temperatures near 70°C to promote dissolution of the chiral additive.

18
Table 1. Base Nematics and Chiral Additives

Nematics

X^O)-A-B^O^ Y

X,Y

Cyano-CN

Alkyl-CH3(CH2) n Alkoxy-CH3(CH2) nO

A-B

Schiff-CH = NAzoxy-N = N(O)-

Ester-COOLH Additives

COC

ZLI-811

C15

CN

wan

powder

CH3(CH2 ) 7COOC27 H45

Merck: Licristal V, ZLI1646

BDH: E7, E8 E44

Roche: TN-403, TN701

RH Additives

CAA ) S1082 > solid

CE1, 2, ... 7 )

CB15 fluid

CH3 CH2CHCH2 -(0/^0/- CN

CH3

The recipe for circular polarizer fabrication is as follows:

1. choose supporting substrate type (usually BK-7) and diameter (20 mm to 100 mm for present

work)

2. unidirectionally buff inner substrate surfaces with 1/10 micron diamond paste to create a preferred alignment direction and to provide wall anchoring for the liquid crystal

molecules

3. heat substrates and tuned liquid crystal compound to ^ 70°C, apply 12 ym thick "Mylar"

spacer tabs, and form an air-gap sandwich

4. fill air-gap with heated fluid using capillary action

5. quench cell to room temperature and shear one substrate with respect to the other by

1/2 mm to orient the pitch structure along the beam propagation direction (see fig. 5)

adjust cell for zero wedge in an interferometer and seal the fluid edge with epoxy,

19
Figure 5. Liquid crystal polarizer fabrication. Substrate

inner surface rubbing helps to orient the rodlike liquid crystal molecules which contact the

cell boundaries. A mechanical shear after cell

assembly tips molecular helices normal to the

laser beam propagation direction, thereby minimizing scatter losses for the preserved polarization

and maximizing rejection for the rejected polarization .

4. Performance

We have determined that fluid paths of 11 to 18 microns provide optimum polarization for the

1000 nm wavelength regime. The uniformity of fluid thickness across the polarizer clear aperture

does not affect performance, as long as the layer exceeds a minimum of 10 pitch lengths. Transmission for the preserved polarization through a single element approaches 98%. Blocking extinction

through crossed polarizers approaches 10\

The angular sensitivity of liquid crystal polarizers can be minimized by using base nematics

whose birefringence, An, is large. Composition detuning to the long wavelength side of the lasc

wavelength may also be employed to create polarizer elements whose angular performance resembles

that shown in fig. 6. Extinction from 0 to ± 20° off normal in excess of 10 3 has been demonstrated.

20
10*

(0 BEc

o

O

c

U

O

m

it-*-

i ' t i—

10^

10<

LICRISTAL V (EM)

Xp = 1076 nm

• Measurement wavelength: 1064 nm

• Laser spot size: 5/7 mm

• LH/RH fluid layer thicknesses: 11 /zm

10 J L J I I L J I L 10 20

Angle of Incidence, 6 (degrees)

Figure 6. Blocking extinction and angular sensitivity.

Liquid fluid paths of between 11 and 18 ym are adequate to provide crossed polarizer blocking

extinction levels of TO4

. The use of highly

birefringent base nematics promotes selective

reflection bandwidth in excess of 150 nm

(FWHM). This in turn enables liquid crystal

polarizers to be relatively insensitive to

angle of incidence effects.

We have found that the temperature sensitivity of liquid crystal polarizers depends upon the

type of base nematic and the type and quantity of chiral additive. Center wavelength temperature

shifts from 0.35 to 1.4 nm per °C have been measured. This level of sensitivity presents no problem

to the use of these devices in our laboratory, where temperature is controlled to ± 3°C.

A series of on-line laser damage tests was performed on 50-100 mm diameter cells using the

1054 nm output of the GDL system. The RH combination of CB-15 in nematic E7 was found to be damage

resistant to average fluences of 3 J/cm 2

(1 nsec). LH polarizer cells failed at substantially lower

fluences of less than 0.2 J/cm2. Failure is determined by polariscopic observation of bubble growth

in irradiated cells. We suspect that the primary reason for low damage thresholds in LH fluids is

the lack of a room temperature, liquid chiral additive, or inadequate dissolution of the LH solid

during fluid composition tuning. Our interim solution to this problem has been to employ RH element/

half waveplate combinations to simulate the three LH elements indicated in fig. 1.

5. High Power Polarizer Technologies

A comparison of liquid crystal polarizers with other polarizer technologies is given in table 2.

Liquid crystals offer the advantages of high optical quality at large apertures, high contrast with

angular insensitivity, high transmission for the preserved polarization, and environmental stability.

They represent the only polarizer technology which, by itself, can provide back-reflection protection

during standard use. Work is presently being conducted to understand and solve the LH element laser

damage problem.

21
Table 2. Comparison of Polarizer Technologies

Crystal

Attribute Prism

• Optical quality at no

large apertures

• High contrast with no

angular insensitivity

• High transmission for yes

passed polarization

• Environmental stability yes

• Laser-damage resistance some

• Back-reflection protection no

Brewster Dielectric Dyed Liquid

Thin Film Cube Plastic Crystal

yes no no yes

no no yes yes

? yes no yes

no yes yes yes

yes some no adequate

no no no yes

This work was supported by the U.S. Department of Energy Office of Inertia! Fusion under agreement No. DE-FC08-85DP40200 and by the Laser Fusion Feasibility Project at the Laboratory for Laser

Energetics which has the following sponsors: Empire State Electric Energy Research Corporation,

General Electric Company, New York State Energy Research and Development Authority, Northeast Utilities Service Company, Ontario Hydro, Southern California Edison Company, The Standard Oil Company,

and the University of Rochester. Such support does not imply endorsement of the content by any of

the above parties.

6. References

[1] Seka, W. , et al., "High-power phosphate-glass laser system: design and performance characteristics", Appl . Opt. 1_9; 409-419; 1 February 1980.

[2] Abate, J. A., et al . , "Active mirror: a large-aperture medium-repetition rate Nd:glass amplifier", Appl. Opt. 20; 351-361; 15 January 1981.

[3] Jacobs, S. D., "Liquid Crystals for Laser Applications", in CRC Handbook Series of Laser Science

and Technology; III; M. J. Weber, Ed.; in press.

[4] Jacobs, S. D., "Liquid Crystals as Large Aperture Waveplates and Circular Polarizers", SPIE 307;

98-105; 28 August 1981

[5] Jacobs, S. D., Abate, J. A., Bauer, K. A., Bossert, R. P., and Rinefierd, J. M. , "Liquid Crystal

Isolator for Fusion Lasers", Technical Digest - CLE0S/ICF '80; 128-129; 26 February 1980.

Is the liquid crystal polarizer temperature dependent? The author stated that there is no significant dependence between -20C and +S0C. The uniformity of the polarizer film is also not

important provided that the film thickness is 10 of more pitch lengths. The rejected light is

reflected diffusely, not specularly, an important advantage in device applications. Finally it was suggested that stacked plate polarizers should be added to the list of possible polarizing methods.

Extinction ratios as high as 40,000 to 1 have been obtained with these in the infrared.

22
NONLINEAR ROTATORY POWER

OF 90" TWISTED NEMATIC LIQUID CRYSTALS

Shin-Tson Wu and LaVerne D. Hess

Hughes Research Laboratories

301 1 Malibu Canyon Road

Malibu, California 90265

The optical rotatory power of 90° twisted nematic liquid crystal cells was measured with a CW C02 laser and found to be intensity dependent for intensities > 1 kW/cm2

. We found that the rotatory power

can either decrease monotonically or increase first and then decrease as the laser intensity increases;

this effect depends on the liquid crystal layer thickness. A threshold-like transition was observed

and the threshold power was found to be linearly proportional to the inverse square of the liquid

crystal layer thickness. These results provide evidence that optical field induced molecular

reorientation and thermal effect are responsible for the observed nonlinear optical effect. Selfdefocusing, self-phase modulation and laser-induced damage were also observed with the use of a HeNe

laser probe beam.

Key words: C0 2 laser; diffractions; dynamic recovery; liquid crystals; nonlinear rotatory power; phase

transition.

1 . Introduction

The optical rotatory power of liquid crystals (LCs) is a measure of the capability of a liquid crystal

layer with a twisted molecular orientation to rotate the angle of incident polarized light. It is an important

physical quantity for LC devices such as electro-optic shutters or spatial light modulators using the twisted

nematic liquid crystal (TNLC) configuration. In particular the contrast ratio of these devices is critically

influenced by the rotatory power of the LC cell used. Under the condition of low light intensities, nonlinear

effects are absent and the optical rotatory power is a function of LC birefringence, film thickness, twist

angle , and wavelength . [ 1 ]

In this paper, we report experimental measurements of intensity dependent rotatory power of LC cells having

the 90° TNLC configuration. The studies were conducted with a C02 laser and with LC cells of different

thicknesses. Results indicate that the rotatory power is constant when the incident light intensity is below a threshold value (this threshold-like transition is analogous to the Freedericksz transition [2] which occurs with a magnetic or electric field). Above this threshold, the rotatory power either decreases monotonically or increases first and then decreases depending on the thickness of the LC layer. The threshold intensity was

found to be linearly dependent on the inverse square of the LC layer thickness in agreement with theoretical

predictions. [3, 4] These observations provide direct evidence that a laser-induced Freedericksz transition

occurs, and is the physical mechanism responsible for the observed intensity dependent rotatory power of TNLC

cells. A HeNe laser beam was used to probe the C02 laser/liquid crystal interaction region. Diffraction

fringes associated with self-defocusing [5,6,7] of the HeNe probe laser beam were studied at different

intensities until C0 2 laser-induced phase transition or damage to the liquid crystal occurred.

2. Experiment

The experimental setup is sketched in Fig. 1. A vertically polarized tunable C02 laser operating at

10.59 ym is attenuated by a pair of crossed polarizers and focused onto a LC cell by a 254 mm focal length,

NaCl lens. The spot size is 155 ym in diameter at the 1/e2 intensity points. The LC director at the front

surface of the 90° TNLC cell is aligned parallel to that of the laser polarization axis. Two transmission

measurements were made at each laser intensity when the analyzer was (1) perpendicular, and (2) parallel to the

polarization axis of the incident beam. Details of the experimental technique and sample preparation process

have been reported earlier. [8] In the present study the highly absorbing conductive electrode (indium-tinoxide) layer was not deposited on the KC1 windows in order to minimize thermal effects on the rotatory power

measurements. A positive dielectric anisotropic liquid crystal mixture, designated as BDH-E7, was used in this

study. The optical absorption coefficient of this material was measured to be 5 x 10"3 ym~ l at 10.59 ym for

the ordinary ray, i.e., when the polarization of the laser beam is perpendicular to the liquid crystal

director.

The rotatory power represents the basic modulation capability of a polarizer-LC cell-analyzer system, and

represents the fraction of incident light intensity whose polarization has been rotated by 90° in passing

through the liquid crystal cell. Thus, with a polarizer-TNLC cell-crossed analyzer arrangement, the rotatory

power will be proportional to the transmission, Tj_, through the analyzer. The intensity dependent

transmission, Tj_ (normalized transmission [8]) of BDH-E7 liquid crystal is shown in Fig. 2 for three LC film

thicknesses, 13-5, 24.0 and 52.0 ym. The transmission is constant at low intensities for all three cells.

However, when the intensity exceeds particular threshold values, the transmission of the 13.5 ym and 24.0 ym

thickness cells decreases monotonically. For the 52.0 ym cell, the transmission first increases to about unity

at a power of 300 mW and then decreases to <5% at a power of 2.5 W, corresponding to an intensity of

Manuscript Received

7~l-85 SAB

23
7.5 kW/cm* . Note that this process is reversible as indicated by the two sets of experimental points plotted

in the figure.

3. Theory

To understand these intensity dependent phenomena, we first compare transmission data obtained at low

intensities with the relevant theoretical expression [1] for optical transmission for our experimental

configuration.

sin2

[<t)v/l +u

2

] 1+u 2 :d

where u = irdAn/A<t>; d is the thickness of the LC layer, An is the LC birefringence which has been measured to be

0.205 for BDH-E7 at 10.59 ym,[9] X is the wavelength, and <|> is the twist angle of the liquid crystal in the LC

cell; for the present case, <j> = ir/2.

As shown by the plot in Fig. 3 there is good agreement between experimental measurements and theoretical

calculations. It is readily seen that the first maximum of the transmission appears at a thickness of 44.7 ym.

Hence, the transmission of the 52 pm thickness cell is less than unity at low laser intensities (Fig. 2). Once

the intensity exceeds a certain threshold ( 0.3 kW/cm2 ), however, the liquid crystal molecules tend to line up

along the polarization axis of the laser due to an optical field induced torque, [10]

r = 7p (£-n) (E x n) (2)

-> where Ae = e

n - e

i is the dielectric anisotropy of the liquid crystal, E is the electric field of the incident

light, and n is the unit vector of the liquid crystal director. Due to this optical field induced molecular

reorientation, the effective LC thickness (dAn) of the 90° TNLC cell is reduced, which results in an increased

transmission as expected from a reduction in effective cell length (Fig. 3). and indeed observed (Fig. 2).

When the intensity is increased further, more molecules are aligned along the laser polarization direction, and

the effective twisted cell thickness is further decreased leading to decreased transmission at high

intensities.

4. Threshold Power

It is also shown by the data plotted in Fig. 2 that the measured threshold power depends on the liquid

crystal thickness. The thinner the cell, the higher the threshold. The specific functional dependence of this

relationship is illustrated in Fig. 4 where the threshold power is plotted as a function of the inverse square

of the LC thickness. The threshold power is defined as the power where the transmission, T begins to deviate

from the constant value at low intensities. In our experiments, a 10? accuracy was estimated and is indicated

by the error bars shown in the figure. A linear dependence is obtained, and the slope is determined by the

dielectric constants and Frank constants of the liquid crystal under study. This linear relationship between

the threshold power and inverse square of the LC thickness provides evidence that the observed intensity

dependent rotatory power is a consequence of optical field induced molecular reorientation.

5. Diffraction

In addition to the direct measurements of the intensity dependent rotatory power described above, a HeNe

laser beam was used to probe the interaction region as shown in Fig. 1. The 4-mm-thick KC1 windows are used as beam splitters to reduce the reflection losses at 10.59 ym. The reflected HeNe laser beam from the front

surface of the KC1 beam splitter was aligned coincident with the C0 2 laser beam. Another reflected HeNe laser

beam from the back surface of the beam splitter served as the reference beam as shown in the right hand side of

each photograph in Fig. 5. These photographs were taken from a screen which is located 60 cm away from the

52-ym liquid crystal cell. When the incident C02 laser power is below threshold (--100mW) no diffraction ring

appears (Fig. 5a). As the power exceeds this threshold, diffraction rings associated with the divergence of

the HeNe laser beam change drastically as indicated in Fig. 5b, c, and d for the powers of 200 mW, 700 mW, and

2.0 W, respectively. The threshold powers determined by the appearance of the first diffraction ring are also

plotted in Fig. 3 (the triangle data points) for three LC film thicknesses. Good agreement is found between

these two methods of measurement. Further increase in the incident power may cause a liquid crystal phase

transition to occur (due to thermal effects) resulting in formation of the isotropic state. Fig. 5e reveals

such a transition at an intensity of 7.5 kW/cm 2

. If the LC cell is irradiated at this intensity for a short

time (few seconds), recovery to the original anisotropic state occurs in about 2 minutes after the incident

power is turned off; this indicates that the process is reversible.

24
6. Dynamic Recovery

The dynamic recovery processes are illustrated by the data shown in Fig. 6a, b, and c. These photographs

were taken within an interval of 30 seconds after the phase transition (Fig. 5e) occurs (the C0 2 laser beam was turned off immediately after the phase transition was observed). Once the liquid crystal recovers (from

Fig. 6c to Fig. 5a), it can withstand about 20% higher optical power without a further phase transition

occurring. This observation may be explained as resulting from laser- induced purification of the liquid

crystal mixture. Small particulates in the interaction region may be heated by the C02 laser beam. This

induced temperature change results in a lower viscosity causing the small particulates to preferentially leave

the interaction volume. However, if the incident power is further increased, an irreversible process occurs.

Fig. 5f shows the diffraction pattern of the HeNe laser beam after the LC cell was irradiated at 8.5 kW/cm2

for about one second. This pattern remains unchanged and corresponds to the pinhole diffraction pattern with a pinhole diameter of 100 vim, which is in good agreement with the result measured directly from a microscope. No

damage of 24-ym and 13.5-um cells was detected at this intensity level. This indicates that the damage to the

liquid crystal is not due to the breakdown of surface alignment layers deposited on the KC1 windows. The

detailed mechanism corresponding to the damage to the LC layer is so far not clear, and is still under

investigation. Therefore, the thinner cells have not only higher Freedericksz transition thresholds but higher

damage thresholds. This fact is particularly important for high power laser applications of liquid crystal

devices. In order to know whether the self-defocusing effect occurs in the C02 laser beam, knife edge beam

scans were performed at a constant location (150 mm from the focal plane) to determine the transmitted beam

size. The following conditions were used: (1) the LC cell was removed from the beam path; (2) a 24-ym LC cell was irradiated at 200 mW (which is below the threshold intensity); and (3) same as (2) except for higher power,

1 W (above the threshold intensity). The measured beam sizes were equal within 5% for all three conditions.

This indicates that the critical power for self-action effect to occur is higher than the power used in these

experiments.

7. Conclusion

In summary, we have studied the intensity dependent rotatory power of 90° twisted nematic liquid crystal cells

at a wavelength of 10.59 \m. The rotatory power was found to be constant at intensities below a threshold

intensity. Above this threshold, the rotatory power can either increase or decrease as the intensity

increases. The threshold intensity is proportional to the inverse square of the LC layer thickness. These

observations provide direct evidence that both laser field induced molecular reorientation, analogous to a.c. electric field induced molecular reorientation, and thermal effect are the basic mechanisms of the observed

intensity dependent rotatory power. The intensity dependent transmission measurements discussed above

represent spatially averaged effects. Diffraction patterns with a HeNe probe laser provide further details

about the nonlinear process within the spatial profile of the Gaussian shaped C02 laser beam. Damage phenomena

resulting from laser interactions with liquid crystal materials were also investigated. It was found that

thinner cells have both higher Freedericksz transition thresholds and higher damage thresholds; this is

particularly important for high power laser applications of liquid crystal devices.

References

1. Gooch, C. H. , and Tarry, H. A., "The optical properties of twisted nematic liquid crystal structures with

twist angles <90°," J. Phys. D. 8_, 1575 (1975).

2. Freedericksz, V., and Zolina, V., Trans. Faraday Soc. 29_, 919 (1933).

3- Ya Zel'dovich, B. , and Tabiryan, N. V., "Theory of optically induced Freedericksz transition," Sov. Phys.

JETP 55, 656 (1982).

4. Barbero, G. , Simoni, F. , and Aiello, P., "Nonlinear optical reorientation in hybrid aligned nematies," J. Appl. Phys. 55, 301 (1984).

5. Vol terra, V., and Wiener-Avnear , E. , "CW thermal lens effect in thin layer of nematic liquid crystal,"

Optics Comm. J_2, 194 (1974).

6. Durbin, S. D. , Arakelian, S. M. , and Shen, Y. R. , "Laser-induced diffraction rings from a nematic-liquidcrystal film," Opt. Lett. 6_, 411 (1981).

7. Khoo, I. C, "Theory of optically induced molecular reorientations and quantitative experiments on wave mixing and the self-focusing of light," Phys. Rev. A25, 1 636 (1982).

8. Wu, S. T., Efron, U., and Hess, L. D. , "Optical rotatory power of 90° twisted nematic liquid crystals,"

Appl. Phys. Lett. 44, 842 (1984).

9. Wu, S. T., Efron, U., and Hess, L. D., "Infrared birefringence of liquid crystals," Appl. Phys. Lett. 44,

1033 (1984).

10. deGennes, P. G. , The Physics of Liquid Crystals (Clarendon Press, Oxford, 1974), p. 96.

25
DETECTOR

POLARIZERS

B/S . LC CE LL LENS

B/g /\_

7

/

ANALYZE R

TO SCREEN

He-Ne —

co2

LASER

Figure 1. Schematic diagram of the experimental apparatus.

100

90

80

70

60

50

40

30

20

10

0

14044-2

d = 52

•O

_ •

o

•o

o °.

, d = 24jim

d = 13.5)Ltm a

A A A A A A A A A OA a- A A

* - A

• o

J

A

1 1 1

1 1 1

° »o.• o o

I I I I I I I

I 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0

LASER POWER, W

Figure 2. Transmission of the nematic liquid crystal mixture,

BDH-E7, as a function of C02 laser power for three

cell thicknesses having a twist angle of it/2; the

dots represent the reversibility of intensity

dependent transmissions of the liquid crystal under

study; the laser spot size is 155 pm (1/e 2 intensity

points); T = 2H°C.

26
14044-4

CO

CO

CO<

cc

hTHICKNESS,yu.m

Figure 3. Normalized transmission of BDH-E7 as a function of cell thickness for low C02 laser (10.59 w)

intensities; solid curve is theoretical, points are experimental; T = 24 °C.

DC

LUO

Q_Q

_l

O

I

CO

LUcc

2 3 4 5

(THICKNESS)-2x 103

, /im"2

Figure 4. Observed threshold C02 laser (10.59 ym) powers for nonlinear transmission of BDH-E7 plotted as a function of 1 /d 2 where d is the cell thickness; A represents the threshold power determined by the

HeNe laser probe technique; the laser spot size is 155 pm (1/e2 intensity points); T = 24°C.

27
03

m

#* 1

ED

El

1

m

*

Figure 5. Diffraction patterns observed with a HeNe laser

probe beam coincident with the C02 laser beam

(normal incidence) for five power levels: (a) 50 mW,

(b) 200 mW, (c) 700 mW, (d) 2W, and (e) 2.5 W; laser

spot sizes are 155 urn and ~10 ym for the C02 and

HeNe lasers, respectively. (f) is the diffraction

pattern of the HeNe laser beam when a small hole in

the liquid crystal cell has been created by the C02 laser beam. LC cell thickness = 52 pm, screen is

located 60 cm from the cell.

28
D3

14044-7

Figure 6. Dynamic recovery of liquid crystal after a thermal induced phase transition occurs. Photographs were taken at the same conditions as those shown in Fig. 5 with

an interval of 30 seconds from (a) to (c).

The complete recovery is the same as that

shown in Fig. 5a.

29
The observed absorption coefficient was clarified SxlO~^'/micrometer is SO cm~^. The rational

for assigning the observed long relaxation time toreorientational rather than thermal effects

was also discussed. The author pointed out that if the effect was thermaly one should see a temperature rise. No phase transition was observed except at much higher temperatures than

used here.

30
Manuscript Beaeived

2-25-85 SAR

Picosecond Damage in Y2O3 Stabilized Cubic Zirconia

Nastaran Mansour, M. J. Soileau, and Eric W. Van Stryland

Center for Applied Quantum Electronics

Department of Physics

North Texas State University

Denton, Texas 76203

The hardness, strength, and transparency of cubic zirconia ZrCU makes it a good

candidate as a laser window material; however, to obtain these qualities a relatively

large percentage of Y2O3 is added to stabilize the crystals and increase their mechanical strength. As impurities are often regarded as the initiators of laser-induced

damage we have performed a study of the damage thresholds of optical quality ZrC^

stabilized with Y2O3. These thresholds have been measured with picosecond 1.06 ym

laser pulses in crystals having Y2O0 concentrations of 9.4%, 12%, 15%, 18%, and 21%.

We found that the addition of Y2O3 for increasing the mechanical strength of zirconia

does not necessarily lead to a decrease in the damaging irradiance. In addition, the

thresholds for Zr02 are comparable to those of NaCl.

Key Words: laser damage; picosecond pulses; Zr0

2 stabilized with Y2O3; estimation of

n2; 1«06 ym.

1. Introduction

The ideal optical materials for use with high power lasers should be transparent over a broad spectral range, mechanically strong, environmentally stable, and resistant to laser induced

damage. Few materials meet all these criteria. For example, NaCl has low absorption from the

infrared to the ultraviolet, is resistant to pulsed laser damage but has poor mechanical strength

and is hydroscopic. ZnSe has good mechanical properties and broad band transmission but damages

easily by pulsed laser irradiation. Fused silica is resistant to pulsed laser damage and has

good mechanical properties but has high transparency only in the visible and near infrared

spectral region.

Cubic zirconia stabilized with Y

2 03 is mechanically strong, transparent from the UV to the

mid-infrared and is now available in the industrial market. These factors make this material a good candidate for use as a high power laser window. The laser damage thresholds of this material had not previously been measured. In this paper we present measurements of the bulk laserinduced damage threshold of cubic zirconia stabilized with Y2O3 having concentrations of 9.4%,

12%, 15%, 18%, and 21%. Measurements were conducted using picosecond pulses at 1.06 ym.

2. Experiment

The laser source used in this study was a passively mode-locked, microprocessor-controlled,

neodymi urn :yttri urn aluminum garnet (Nd:YAG) oscillator-amplifier laser system operating at

1.06 pm. A single pulse of measured Gaussian spatial and temporal irradiance distribution was switched from the mode-locked train and amplified. The temporal pulsewidth was 45 psec [full

width at half maximum (FWHM)]. The width of each pulse was monitored by measuring the ratio of

the square of the energy in the fundamental (1.06 ym) to the energy in the second harmonic,

produced in a Li IO3 crystal. The ratio was calibrated by measuring the pulsewidth using an autocorrelation scan. Essential details are described in reference 1.

The laser half-angle beam divergence was 0.18 mrad. The beam divergence and the spatial

beam spot size were determined by pinhole scans at several different positions along the propagation direction. A 37 mm focal length lens designed for minimum spherical aberration was used to

focus the light into the bulk of the materials. The calculated focal spot radius using this lens

is 7.2 ym (HW 1/e M in irradiance). A rotating half-wave plate, polarizer combination was used

to vary the irradiance on the sample. The energy incident on the sample was continuously monitored by a sensitive photodiode peak-and-hold detector, and was calibrated with respect to a pyroelectric energy monitor. The incident beam polarization on the sample was changed from

31
linear to circular by adding a quarter wave plate in front of the focusing lens. Each site was irradiated only once, and damage was defined by the observation of scattered light from a coaxial

HeNe laser as viewed through a lOx microscope.

3. Experimental Results and Discussion

Tables 1 and 2 summarize the results of measurements of the laser induced breakdown

threshold for ZrOo stabilized with Y0O3. The Zr0 2 samples having Yo^ concentrations of 9.4%,

12%, 15%, 18%, ana 21% were studied for linearly and circularly polarized light at 1.06 ym. The

uncertainties listed in the tables of data are the relative errors obtained by the method used in

reference 9. For all the data presented in tables 1 and 2 we used 45 picosecond (FWHM) pulses

and a calculated 7.2 ym focused spot size (HW 1/e M in irradiance). The values given for the

electric field are rms fields corresponding to the peak-on-axis irradiance. As indicated in the

tables of data, increasing the percentage of Y 2 0o does not significantly change the breakdown

threshold. The breakdown fields for linearly and circularly polarized light are displayed in

figure 2. As clearly represented by bar graphs, the breakdown field remains unchanged (to within

±10%) for Y2O3 concentrations ranging from 9.4 to 21%.

In this work, we carried out the same measurements for circularly as well as linearly

polarized light in order to determine the effect of sel f -focusi ng on laser-induced damaged

threshold measurements. For the data listed here, we have done the polarization dependence test

(2,6) which insures the absence of self-focusing in bulk damage measurements. Figure 3 is the

experimentally determined ratio of breakdown electric field for circular polarized light to that

for linearly polarized light. The ratios are constant (within uncertainties of experiment), and

approximately equal to 1 for all different concentrations.* This implies that for our experimental conditions self-focusing did not dominate the damage process in these measurements. Note

that we carried out the same experiment for a different focused spot size (10 ym). The ratios

increased by a factor of 1.2 to 1.5 for all the samples tested. The polarization dependence of

self-focusing has already been seen experimentally in a variety of materials. For example, for

NaCl the ratio of the critical powers for circular to linear polarization is 1.37 to 1.46 [6].

For various laser glasses this ratio is equal to 1.50 [10]. Therefore, we concluded that for the

larger focal spot size self-focusing is controlling the damage process.

The critical power for self-focusing of a focused Gaussian beam within a Rayleigh range, is

obtained from numerical solutions of the nonlinear wave equation [4], and is given by

P

2

= 3.77cA2 /32Tr

2

n 2

where no is the nonlinear refractive index in esu, X is the laser wavelength, and c is the speed

of light in vacuum. If for the 10 ym focused spot size, self-focusing is the dominant process

(as seems to be indicated by the polarization dependence), then the measured breakdown power (Pg)

should be the same as the critical power P 2

. We used the values of the measured breakdown power

and the equation above to calculate an estimated value of no for the Zr0 2 samples at 1.06 ym.

Using the assumption that P B

~ P

2 we find that n 2 = 8 x 10"" esu. This value is in agreement

with the rough theoretical estimation of n

2 in the next paragraph.

Sel f -focusi ng is an induced lensing effect in materials resulting from the changes of the

optical dielectric constant when an electric field is applied. In this case, the total dielectric constant can be written as

e = e + e0 <E • E> 0 2

where <E • E> is the time average of the square of the optical field. The induced polarization

in the medium is

*The 15% concentration sample was tested near the samples' edge for circularly polarized light

where the optical figure of the front surface was poor. This probably accounts for the slightly

greater than unity ratio for this sample shown in figure 3.

32
p = x

( 1 )e + x

(3) <E • E> E

where

£ = 1+47^ ^ and e = 4ttX^ 3 o l

and the refractive index n is

n = SZ~ n + n 0 <E • E>

o 2

In solid materials, the n2 can be roughly estimated by using the argument given in reference 8. There it is assumed that the nonlinear polarization, x' ' <E * E> E, becomes strong when the

electrostatic energy of the applied optical field e 0

/8ir <E • E> V (V is the volume of the atom)

is comparable to the electronic transition energy, ftu)0

. The nonlinear polarization will then be

comparable to the linear polarization so that

X^ <E • E> E e /8tt <E • E> V o

X

(1 ) E ft(D0

( 3) With n^ = 2irx v / n 0

> tn is gives

n

2

- "o^o"1 )

16irflu N 0

where n 0 is the linear index of refraction and N=l/V is the number density of atoms. For the

Zr02 sample with n 0= 2.12 [11] and m 0

- 55 x lO 1 ^ Hz, we estimate n

2 to be 10 x 10" 13 esu which

agrees with our experimental estimate of 8 x 10" 13

esu. The validity of this estimation proceThe estimate of n

2 for NaCl is 1.3 x 10"^ 3 esu and for Si0 2 It is 1.0 x 10"^ esu which compares

to experimentally determined n 2

's of (1.37 ± .15) x 10" 13 esu and (.62 ± .03) x 10" 13 esu,

respectively [7], Another empirical expression which has accurately given the nonlinear refractive index for a wide variety of transparent insulating materials is

-13 (n -lKnW

n- (10 esu) = K ^= 2

v[1.517 + (nj+2)(n

d+l) v/6n y

f

where vis the Abbe number, nj is the helium d line refractive index and K=68 [12]. This

expression predicts an n 2 for Zr0 2 of 13 x 10" 13 esu which again agrees with our estimate.

4. Summary

Laser induced breakdown was studied at 1.06 ym for cubic zirconia stabilized with Y 203.

Samples having concentrations of 9.4%, 12%, 15%, 18%, and 21% were investigated. We find that

increasing the percentage of Y 203 to stabilize the zirconia does not decrease the breakdown

threshold. The breakdown field is unchanged (to within ±10%) for Y 203 concentration ranging from

9.4% to 21%. The breakdown fields for circular and linear polarized light were found to be

approximately the same for the small spot size used. The implication of this is that for these

conditions self-focusing did not play a major role in the measurements. Also, we estimate the

nonlinear index of refraction n

2 for the Zr02 samples. For this estimate we used the breakdown

data for which self-focusing was a dominant process. The value for n

2 agrees with theoretical

estimates.

33
The authors acknowledge the support of the Office of Naval Research and the North Texas

State University Faculty Research Fund. These samples were polished at the AFWL Developmental

Optics Facility. We are grateful to Dr. Alan Stewart of AFWL for this help in the sample

preparation

5. References

[I] Van Stryland, E. W.; Soileau, M. J.; Smirl, A. L; Williams, W. E. Pulsewidth and focal - volume dependences of laser-induced breakdown. Phys. Rev. B 23(5): 2144-2151; 1981.

[2] Soileau, M. J.; Williams, W. E.; Van Stryland, E. W.; Boggess, T. F.; Smirl, A. L. Picosecond damage studies at .5 and 1 m. Opt. Eng. 22(4): 424-430; 1983.

[3] Soileau, M. J.; Williams, W. E.; Van Stryland, E. W. Optical power limiter with picosecond

response time. IEEE J. Quant. Elect. QE-19(4): 731-735; 1983.

[4] Marburger, J. H. Self-focusing: Theory. Progress in Quantum Electronics, edited by

Sanders, J. H.; Stenhold, S. Pergamon. Oxford. 4(1): 35-110; 1975.

[5] Williams, W. E.; Soileau, M. J.; Van Stryland, E. W. Optical switching and n2 measurements

in CS 2

. Optics Communications. 50(4): 256-260; 1984.

[6] Williams, W. E.; Soileau, M. J.; Van Stryland, E. W. The effect of self-focusing on laserinduced breakdown. Natl. Bur. Stand. (U.S.) Spec. Pub.; 1983.

[7] Williams, W. E.; Soileau, M. J.; Van Stryland, E. W. Simple direct measurements of n

2

. Natl. Bur. Stand. (U.S.) Spec. Pub.; 1983.

[8] Yariv, A. Quantum Electronics, 2nd ed. New York; Wiley; 498-507; 1975.

[9] Porteus, J. 0.; Jernigan, J. L; Faith, W. N. Multithreshold measurement and analysis of

pulsed laser damage on optical surface. Natl. Bur. Stand. (U.S.) Spec. Pub. 568; 507-515;

1979.

[10] Moran, M. J.; She, C. Y.; Carman, R. L. Interferometric measurements of the nonlinear

refractive-index coefficient relative to CS 2 in laser-system-related materials. IEEE J. Quant. Elect. QE-11: 259-263; 1975.

[II] Wood, D. L.; Nassau, K. Refractive index of cubic zirconia stabilized with yttria. Applied

Optics. 21(16):2978-2981; 1982.

[12] Milam, D. ; Weber, M. J.; Glass, A. J. Nonlinear refractive index of fluoride crystals.

Appl. Phys. Lett. 31(12): 822-824; 1977.
Nd=YAG

LASER SYSTEM

-*>—^ j*-

t

\

k

ENERGY

mmm

HeNe

tp

] A/2 PLATE

LENS SAMPLE

VIEWER

Figure 1. Experiment.

35
Linear-Polarization

X =1.06 fi m

t p =45±15psec

Circular-Polarization

\=1.06/xm

tp=45±15psec

21%

18%

15%

12%

9.4%

21%

18%

15%

12%

9.4%

0 1 2 3 4 5 6 7

EBL(MV/cm)

8

i i 0 1 3 4 5 6 7

EBC(MV/cm)

8

Figure 2. Experimentally determined breakdown electric field for linearly and circularly

polarized light, Eg j_ and Egr, for cubic zirconia ZrOp stabilized with Y0O3 having

concentrations of 9.4%, 12%, 15%, 18%, and 21%.

36
\=1.06/*m

tp=45±15psec

0 0.5 1.0 1.5

EBC/E BL

Figure 3. Experimentally determined ratio of breakdown electric field for linearly and circularly

polarized light, Egr/Eg|_, for cubic zirconia stabilized with Y2O3 having concentrations

of 9.4%, 12%, 15%, 18%, and 21%. (E BC is the breakdown field for circularly polarized

light and E3L is the breakdown field for linear polarized light.) Note that Er£ - Egi.

The lack of polarization dependence indicates that its breakdown power is significantly

less than its critical power for self-focusing.

Table 1. Laser induced damage data for Zr0 2 stabilized with Y2O3 at 1.06 ym using linearly

polarized light.

Y203 'bl E BL PBL FBL

Concentration GW/cm2 MV/cm KW J/cm2

9.4% 265+/I8 6.910.2 438130 12.710.9

12.0% 257130 6.7jj0.4 423149 12.311.4

15.0% 222tl 1 6.310.2 366119 10.610.6

18.0% 237124 6.510.3 389140 11.3+1.2

21.0% 230117 6.410.2 380128 11.010.8

'BL

= breakdown irradiance (peak on-axis irradiance)

Eg|_ = breakdown field

Pbl = breakdown power

Fg[_ = breakdown fluence

37
e 2. Laser induced damage data for Zr0

2 stabilized with Y

2 0o at 1.06 y m using circularl

polarized light.

Y203 •bc E BC Pbc F BC

Concentration GW/cm2 MV/cm KW J/cm 2

9.4% 282117 7.110.2 465127 13.510.7

12.0% 260117 6.810.2 430128 12.410.8

15.0% 278113 7.010.1 458122 13.310.6

18.0% 257125 6.810.3 424142 12.311.2

21.0% 238116 6.510.2 393126 11.410.7

'BC = breakdown irradiance (peak on axis irradiance)

Egc = breakdown field

Pbc = breakdown power

Fgc = breakdown fluence

38
Manuscript Received

12-14-84 SAR

Pulsed Laser Induced Optical Damage Threshold

In Nd:Cr:GSGG Crystal

M.A. Acharekar and D.P. McCarthy

Litton Systems, Inc.

International Laser Systems Division

3404 N. Orange Blossom Trail

Orlando, FL 32804

and

R. Blachman

Material Progress Corporation

1395 N. Dutton Avenue

Santa Rosa, CA 95401

Gadolinium scandium gallium garnet (GSGG) doubled doped with neodymium

(Nd) and chromium (Cr) shows higher efficiency than the commonly used neodymium doped yttrium aluminum garnet (Nd:YAG) laser. The performance characteristics of 6.3 mm diameter by 100 mm long and 5 mm diameter by 65 mm long

Nd:Cr:GSGG rods were obtained. A flashlamp pumped Nd:Cr:GSGG laser was operated in both normal and Q-switch modes. The performance data for the material

will be presented and compared with Nd:YAG laser rod.

Q-switch output pulses of Nd:Cr:GSGG laser with 17 ns pulse width and

1.061 ym wavelength were used to study surface and bulk damage in coated and

uncoated, polished samples of Nd:Cr:GSGG crystal. These optical damage threshold measurements were compared by measuring damage threshold of fused silica

and BK-7 glass samples using the same experimental set up. Nd:Cr:GSGG damage

threshold was higher than BK-7 glass and slightly lower than fused silica.

Key Words: Nd:Cr:GSGG, Co-doped GSGG, 1.061 ym laser, Laser Damage.

1. Introduction

Neodymium doped yttrium aluminum garnet (Nd:YAG) lasers have found numerous uses in scientific, military, and industrial applications. The

typical overall efficiency for Nd:YAG is presently limited to 2-3 percent.

The increase in this overall efficiency would be a very desirable improvement

for reducing size and output energy of the system. The concept of improving

3+ 3+ efficiency by co-doping a neodymium (Nd ) laser crystal with chromium (Cr ) 3+ ions and transferring excitation absorbed by the broad Cr absorption bands over to the Nd

3+

ions is nearly as old as the Nd

3+

laser itself.'-

1

-' No

dramatic efficiency improvement was achieved with the host crystal YAG,

3+ 2 because all Cr excitation was deposited in E level and the spin-forbidden

2 4 nature of the E

f A^ transition resulted in an inefficient transfer

process. However, nearly 100 percent transfer efficiency could be achieved in

the co-doped gadolinium scandium gallium garnet (GSGG).

39
Figure 1. shows a schematic energy level diagram of octahedral ly

coordinated Cr in a conf igurational coordinate model.

A series of measurements performed indicated that co-doped GSGG

laser rods provide several times higher efficiency under flashlamp pumped

conditions as compared to the more common host, Nd: YAG . In this paper, some

of the performance data collected at International Laser Systems (ILS) for

Nd: Cr: GSGG laser rods will be summarized. The data collected indicates that

Nd:Cr:GSGG crystal can be used in high power applications if the optical

damage threshold for the material is suitable for the applications.

Therefore, pulsed laser induced optical damage threshold for the material was measured. These measurements are discussed in the paper.

P9455

Figure 1. Energy Level Diagram of the Donor Cr3+

and Acceptor Nd3+jn GSGG System.

40
2. Spectrophotometry Measurements

The transmission spectra for Nd:Cr:GSGG at room temperature was obtained

using Varian model 2390 spectrophotmeter. The sample 1- J was 1.58 mm thick

with concentrations for Nd

3+

of 1.5 ± 0.5 X 10

20

cm"

3 and Cr

3+

of 1 X 10

19

o 3+

cm" . The transmission spectra clearly shows narrow Nd transitions at 810,

800, 750 and 690 nm while the broad absorption observed at 460 and 645 nm is

of Cr+3 for ^Tj, and ^1 electronic levels.

Fluorescence spectra for Nd:Cr:GSGG was studied also. The spectra

comprises a wide band having maximum at 750 nm of Cr

3+

fluorescence spectrum

2

due to a transition from T

9 excited state of the Cr ion to the A? ground

4 4 3+

state. The F-, /9 —^ I n/1 , transition in Nd ions is seen at 1.06ym

wavelength range in the fluorescence spectra. Also, the F.^ —^ ^9/2

transition in Nd

3+

ions is observed at 946 nm wavelength range. As expected,

.3+. ,3+ the efficient Cr"" —> Nd"" steady state energy transfer reduces the intensity

of the chromium fluorescence at 750 nm in the spectra observed for Nd:Cr:GSGG

crystal. Fluorescence spectrum at 300°K for 0.63 cm diameter by 10.16 cm long

Nd:YAG and Nd:Cr:GSGG laser rods were obtained by using Xenon gas filled

flashlamp to pump the laser rods and a Farrand Optical Company Model 117182

grating monochromator to measure the fluorescence. The data obtained is shown

in figure 2. Although, the monochromator resolution was 1.0 nm it appears

that only the major fluorescence lines at 1.06 ym and 0.93 ym with high

effective cross-section are observed.

>

<

70

60

5 50 —

<

2 40 —

30 —

20 —

10 —

0 —j.... i n |

|

i 1 1 1 1 1 1 1

1

1 1 1 1 \ *

|

| | "•'|" pi

400 500 600 700 800 900 1000 1100 1200 1300 1400 WAVELENGTH (nm)

* - Nd:YAG

+ - Nd:Cr P9456

Figure 2. Fluorescence Spectrum Nd:YAG Vs. Nd:Cr.

41
3. Lasing Properties of Nd:Cr:GSGG

The lasing properties of Nd:Cr:GSGG were investigated in the free lasing

regime in the 1.06 ym wavelength range in the following two modes:

i) Pulsed normal mode operation

ii) Q-switohed mode of operation

The experimental data was collected for the pulse normal mode operation

using 0.63 cm by 10.16 cm Nd:Cr:GSGG rod. The experimental set up as shown in

figure 3. The laser rod was installed in a single elliptical pump cavity.

The same pump cavity is used for the Nd:YAG laser systems manufactured by ILS.

Thus, the performance characteristics for the pump cavity using 0.63 cm by

10.16 cm Nd:YAG laser rod are known. The pump cavity, laser rod, and flashlamp were cooled using cooling system that consisted of a heat exchanger, pump,

and coolant reservoir. The 50:50 ethylene glycol and water coolant was used

with a 1 liter capacity reservoir. The pulse forming network (PFN) of 35.5 yF

capacitor and 39 yH inductor was used with a Xenon gas filled at 400 torr

pressure in cerium doped fused silica envelope flashlamp. The input-output

characteristics for the resonator consisted of 5 meter radius mirror coated

for maximum reflectivity spaced 40 cm apart with an output coupling mirror of

40 percent is shown in figure 4. The comparison of Nd:YAG and Nd:Cr:GSGG

laser rods of the indentical size in exactly same set-up is shown in figure 5.

The experimental data was collected for Q-switch mode operation using 0.5

cm diameter by 6.50 cm long Nd:Cr:GSGG laser rod. In the resonator a lithium

niobate pockel cell with a polarizer was added. Also, the PFN was changed,

capacitor 49.6 yF and inductor 29.3 yH was used. Q-switched output energy of

135 mJ was obtained in beam divergence of 2.18 mR with 8 J/P input energy at

10 pps. The pulse width of the Q-switched pulses was measured using EG&G 580

radiometer and Tektronix Model 454A oscilloscope. The typical full width at

half aplitude (FWHA) pulse width for the laser was 17.0 ns.

4. Laser Damage Threshold for Nd:Cr:GSGG

4. 1 Test Specimens

For measuring damage threshold of Nd:Cr:GSGG two types of samples were

prepared. For the bulk damage threshold measurements rods of 0.63 cm diameter

by 0.5 cm long were optically polished and single layer magnesium floride

anti reflection (AR) coated. The samples with two different concentrations of

3+

Nd dopant were used. For the surface damage threshold measurements

optically polished discs of 0.38 cm diameter by 0.1 cm thick were used. It may be noted that all the samples were inspected under cross polarizer to make

42
5M

MAX. R c

COOLANT

RESERVOIR

LAMP

ND: YAG

OPTICAL BED

POWER

SUPPLY

PULSE

GENERATOR

1PUMP

lHEAT EXCHANGER

FLAT

40%R

PULSE

COUNTER

i7o n p.

D [ POWER

METER

P6855A

1000

Figure 3. Experimental Setup.

4 8 12 16 20 24 28

INPUT ENERGY (J/P)

P9453

Figure 4. Nd:Cr:GSGG Laser Input-Output Characteristics.

43
300

250

5 200 —

1

1

>-a

1 150

° 100

50

0

Nd: YAG

d: Cr: GSGG

i

1 2 3 4 5 6 7 8 9

INPUT ENERGY (J/P)

P9450 Figure 5. Comparison of Nd:Cr:GSGG to Nd:YAG Laser.

TEST FLUENCE (J /cm 2 )*

Figure 6. Bulk Damage Measurements for Nd:Cr:GSGG.

* DoD has classified the data.

44
sure that they were free of stress and crystal defects. The surfaces polished

were perpendicular to crystal axis of growth, thus the samples were oriented

in the same direction as the laser rod during its operation. The samples were

polished using 0.3 vim size alumina (A1,,0

3

) on a pitch polisher and flatness of

A/2 was obtained.

4.2 Experimental Techniques

The Q-switched, Nd:Cr:GSGG laser system described above was used. As

mentioned before the laser provided 135 mJ output energy in 17 ns pulse width

at 1.061 urn. The desired power density on the test sample was obtained using

focusing lenses of different focal lengths. The spot size was calculated and

actually measured on a photosensitive film, however, calculated spot size was

used in obtaining the damage threshold.

The surface damage threshold measurements were conducted using single

shot while in the bulk damage threshold measurements multiple shots were used.

In this work "damage" is defined by the appearance of a flash of light and the

attenuation of transmitted pulse through the sample. This definition and

method is widely used in the study of single and multiple shot damage and the

details of the methods can be found in reference ^-L

4.3 Experimental Results

Figure 6. represents the results of bulk damage threshold measurements

using multiple shots (average of 4-6 shots) on the samples of Nd:Cr:GSGG. As

seen in the figure, high and low doped GSGG crystal samples were investigated.

Actually, the samples came from two ends of a five inch long Nd:Cr:GSGG

crystal. The chemical analysis of the samples indicated that Cr-dopant was uniformly distributed within the crystal, however, concentration of Nd showed

10 percent difference in the samples. The damage threshold was 12.5% lower

for the higher Nd-doped crystal.

The surface damage threshold data obtained for Nd:Cr:GSGG samples is

shown in figure 7. All the samples tested for the surface damage were tested

using a single shot. The comparison of the single-shot surface damage

threshold obtain for BK-7 and fused silica is also provided in the figure.

The front surface damage threshold of Nd:Cr:GSGG was observed to 14.0 J/cm .

It may be noted that the samples tested for the single-shot surface damage

threshold were low-doped Nd:Cr:GSGG.

45
FUSED SILICA Nd:Cr:GSGG BK 7 GLASS

Figure 7. Damage Threshold Comparison Chart.

P9452

* DoD has classified the data.

5. Conclusions

The following conclusions can be drawn based on the data collected for

Nd:Cr:GSGG crystal

1. The spectrophotometry and the f luoremetric data collected show that for

1.5 ± 0.5 X 10

20

cm"

3

Nd

3+

concentration and 1.0 X 10

19

cm"

3

Cr

3+

concentration in GSGG host, almost 100 percent transfer efficiency is

observed.

2. The lasing data collected for the Nd:Cr:GSGG laser rod shows 4 percent

laser efficiency for the normal mode operation. The experimental data

also shows that over 7.5 J/p input energy Nd:Cr:GSGG laser performance

was superior to the same size Nd:YAG laser rod.

46
3. The pockel cell Q-switch used with the 5 mm X 65 mm Nd:Cr:GSGG laser rod

provided output energy and beam divergence equal to a typical 6.3 mm X

1016 mm Nd: YAG laser rod.

4. Multiple shot bulk damage threshold for the Co-doped GSGG crystal

measured indicates its dependance on the Nd concentration. For 10

percent increase in Nd concentration bulk damage threshold of was

decreased by 12 percent.

5. Single-shot surface damage threshold for Nd:Cr:GSGG crystal was measured

The damage threshold was compared with fused silica and BK-7 glass.

Nd:Cr:GSGG surface damage threshold is slightly higher than BK-7 glass

but lower than fused silica.

6. The damage morphology (figure 8) shows the uniform surface damage. This

indicates that the polishing technique used for Nd:Cr:GSGG was adequate.

Also the excellent energy profile obtained in the Nd:Cr:GSGG laser was

used for the damage measurements.

Figure 8. Damage Morphology Near Threshold.

47
References

[1] Z.J. Kiss and R.C. Duncan, Appl . Phys. Lett. 5, 200 (1964).

[2] D. Pruss, V.V. Laptev, et. al . , Appl. Phys. B28, 355 (1982).

[3] P.F. Moulton, Laser Focus 19, 83 (1983).

[4] E.V. Zharikov, V.V. Laptev et. al., SOV. J. Quant. Elec. 13, 82

(1983).

[5] The material was grown by Material Progress Corp, CA.

[6] L.D. Merkle, M. Bass and R.T. Swimm, NBS Spec. Publ . 669, 50 (1982).

This work was supported by and conducted at International Laser Systems

Division of Litton Systems, Inc., Orlando, FL.

The author also acknowledges help from S. Latham, P. Milligan and J. Trudell in preparation of the manuscript.

Since chromium absorbs strongly In the ultraviolet, one might expect a degraded

beam quality. However, the author replied that they were pleasantly sur- prised with how good the beam quality was. They were not cleared to discuss

it further. It was suggested that chromium fluorescence in the ultraviolet

_ would be expected and its absence required an explanation. The author replied

that perhaps the concentration of chromium was just optimum to prevent fluorescence. The surface damage was found to be lower than the bulk threshold.

The surface was polished by Virgo Optics using 0,3 alumina on a pitch lap.

The samples were flat to about a half wave.

48
Manuscript Received

6-2-85 SAE

Laser Damage Studies of Several Methacrylate Polymeric Materials

R;M. O'Connell* and R.V. Ell is

Electrical Engineering Department

University of Missouri -Columbia

Columbia, Missouri 65201

A.B. Romberger

The Pennsylvania State University, Berks Campus

Reading, Pennsylvania 19608

T.F. Deaton and K.E. Siegenthaler

Frank J. Seiler Research Laboratory, USAF Academy

Colorado Springs, Colorado 80840-6528

A. A. Shaffer

Department of Chemistry, USAF Academy

Colorado Springs, Colorado 80840-5791

B. W. Mullins

Department of Physics, USAF Academy

Colorado Springs, Colorado 80840-5821

T.T. Saito

Lawrence Livermore National Laboratory

Livermore, California 94550

The bulk damage resistance of several acrylic polymers was investigated with a 1.06 y m wavelength pulsed Hd:YAG laser having a pulse length of 8-9 ns and a spot size

of 33 ym. These materials v/ere synthesized in bulk form using either thermal or ultraviolet initiation after extensive distillation and/or filtration at the monomers

with sub-micron porosity membrane filters. Samples of pure poly (methyl )methacryl ate,

poly (ethyl )methacryl ate and polyfcyclohexyl )methacry 1 ate as well as plasticized polymers

were studied. Details of synthesis methods and results of the damage measurements are presented, and the influence of synthesis methods on damage properties is discussed. In

addition, a simple technique that uses scattered, depolarized helium-neon laser light to

quickly estimate the impurity content of these samples is described and its ability to

predict simple-shot damage resistance is discussed. Finally, a somewhat different method

of analyzing multiple-shot damage data is applied and the results are discussed.

Key Words: bulk laser damage; mu1 ti pie-shot; single-shot; plastics; polymethylmethacrylate

1. Optical System For Laser Damage Measurements

The bulk laser damage properties of the polymerized samples were measured using the Frank J.

Seiler Research Laboratory (FJSRL) laser damage facility'*^ which is shown schematically in

figure 1. Briefly, the facility consists of a 1.06 ym wavelength 0-switched Hd:YAG laser that

provides 8+2 nsec FWHM pulses with up to 300 mj/pulse at a pulse repetition frequency of up to

10 pulses per second (pps). A 25 cm focal length lens L-j focuses the beam into the test sample

S. Careful knife-edge measurements of the focused 1.06 ym beam showed that its spatial profile

is reasonable Gaussian in and near the focal plane with a 1/e^ radius (spot size) of 33 + 1.7 ym

and an associated focal volume (in PMMA) of approximately 0.01 mnr. The spot sTze was determined by least-squares-fitting a Gaussian profile to the numerically differentiated

knife-edge data. The peak fluence level in joules/cm^ for each pulse was obtained by

49
normalizing the measured spatial profile to the total pulse energy, which v/as measured

calorimetrically. Based upon uncertainties in the spot size and the power meter P.M. used to

monitor pulse energy, a standard deviation of 15% was obtained for these values.

A collinear helium-neon laser probe-* is used to monitor damage and to help align the 1 .06 um

beam optics. Lenses Lj> and L3 precondition the probe beam's focus to approximately coincide

with that of the MdrYAG beam. The components from the dielectric turning mirror M3 to the

photodiode (PD) monitor damage by measuring the onset of damage-induced probe-beam scatter out of

the focal volume.

Nd:

YAG He-Ne

Figure 1. The optical system for laser damage experiments.

M-j ,fl2 and M3 - flat turning mirrors; P - polarizer

attenuator; W - silica wedge beam spMtter;

Lt,L2,L3,l_4 - lenses, S - sample, A - aperture,

F - attenuating filter;

P.D. - photo diode; P.M. - power meter.

2. Materials and Synthesis Methods

Several polymer samples were prepared from monomer which in some cases included a plasticizer. The polymers prepared for this study in^uded poly (methyl )methacryl ate (PMMA),

poly (ethyl )methacryl ate (PEMA), poly(cyclohexyl )methacryl ate (PCHMA) and samples of each with

either 5 or 10% by weight dibuty 1 pthal ate (DBP) added as a plasticizer. Groups of samples were prepared under identical conditions, and all samples were prepared using very similar procedures.

In order to reduce impurity levels and remove chemical inhibitors (hydroquinone, for example),

the materials were all fractionally distilled either under 1 atmosphere of nitrogen (MMA, EMA) or under partial vacuum (MMA, CHMA, DBP). Prior to distillation, some samples were dried with CaMj>

to remove water (table l). Many samples were filtered with membrane filters of mean porosity of

0.40 ym or 0.22 um. The prepared monomers, with or without p

1 asticizers, and without any chemical

initiators, were then sealed in cylindrical ampoules (typically 6 cm long x 17 mm in diameter)

under nitrogen atmosphere or in a vacuum and bu^k polymerized either thermally in a temperature

controlled bath or by exposure to ultraviolet. (UV) light. The thermal bath was held at 40°c

and, because no chemical initiators were used, took approximately two weeks to produce fulTy

50
solidified samples. UV light (300-400 nm) was provided by either a fluorescent "black light" or a laboratory mercury lamp. These sources produced fully solidified samples in approximately three

days.

All of the synthesized PMMA and PEMA samples were hard, odorless, bubble-free, colorless and

transparent. The PCHMA samples v/ere hard, odorless, and bubble-free, too, but they turned

slighfy cloudy upon fully solidifying. The sample with 10 weight percent DBP took the longest

time to turn cloudy, which leads us to speculate that the cloudiness was due to partial

crystallization, which the plasticizer could delay but not prevent.

Table 1. Summary of synthesis methods

Distillation

Method

Monomer-plasticizer Dried Filtered Polymerization method

MMA*

MMA + 5% DBP

MMA + 10% DBP

EMA

EMA + 5% DBP

EMA + 10% DBP

CHMA

CHMA + 5% DBP

CHMA + 10% DBP

Some samples N? atmosphere

or vacuum

Ho M 2 (MMA)

Vacuum (DBP)

Yes (EMA only) Vacuum

No Vacuum

Some Not Thermal or Some with 0.44 Ultraviolet (UV)

ym filter

Some with 0.22

ym filter

No

No

No

UV

UV

Thermal or UV

UV

*one sample of PMMA (R-3) was handled in a sealed vacuum system using a procedure similar to that

of reference 4.

3. Dust Content Estimation from Scattered Depolarized Helium-Meon Laser Light

To evaluate the effectiveness of the various distillations and filtrations in removing

impurities, as well as to avoid performing unnecessary damage tests on contaminated materials, a method was sought for quickly and easily estimating the dust content of the monomer and polymer

samples. The optical arrangement shown in figure 2 was found to be well -suited to the task. The

helium-neon laser beam was polarized with a sheet of polaroid, focused into the sample with a 30 cm focal length lens to somewhat concentrate the beam, and observed with a 25X microscope mounted

with its observation axis perpendicular to the scatter track and parallel tn the electric field

vector of the polarized light. This arrangement allowed observation of scattered depolarized

light from the micron-sized particles of interest against a minimized background of

Rayleigh-scattered light from molecular matter and particles much smaller than those of interest.

Thus the dust content of a sample could be estimated simply by counting the relatively bright

scattering centers seen with the 25X microscope in the approximately 0.35 mrn^ volume occupied by

a 5 mm length of scatter track.

This scheme proved to be most useful when it was used to estimate the relative dust content of

two or more samples. For example, in a recent study of the effects of simp 1 e monomer purification

(i.e., drying and single distillation) on the laser damage properties of PMMA^, it was found

that the purified sample had significantly greater resistance to laser damage than the unpurified

sample. To confirm that the purification effort had actually been successful at removing dust,

the depolarized scattered light scheme was used to estimate the relative dust content of the two

samples. The unpurified material was found to contain approximately six times as many dust

particles as the purified material, thereby establishing a direct correlation between dust content

and susceptibility to laser damage. Similar results were obtained with the material used in the

work of this report, as will be shown later. (See tables 2 and 3.)

51
SAMPLE LENS

MASK

He-Ne LASER

SCATTERED

LIGHT

POLARIZER

VOBSERVOR

Figure 2. Experimental arrangement for observation of scattered depolarized

helium-neon laser light.

4. Investigation for Self-Focusing

The data reported here have not been corrected to account for self-focusing. A check for the

possible significance of self-focusing in PUMA at threshold fluence levels was performed by

comparing single-shot damage thresholds determined for linearly polarized and circularly polarized

laser irradiation, respectively. Circular polarization was obtained by the insertion of a quarter-wave plate just before the focusing lens. The resulting data for a particular PMMA sample

are shown in figure 3. There appears to be no significant difference between the damage

thresholds for the two polarization conditions, verifying that self-focusing is not a significant

factor in this work. This result is as expected because the sign of the thermo-optic coefficient

dn/dT for PMMA is negative, ° and therefore, self-focusing should not occur.'

52
1.0

uj 0.8

<

0.6 -

_j 0.4 -

GO<

COO

ccQ- 0.2 -

O CIRCULAR POLARIZATION

LINEAR POLARIZATION

O O

CD

_L

10 20 30 40 50

PEAK FLUENCE (j/cm 2

)

60 70

Figure 3. Comparison of damage data for PMMA under irradiation by linearly

polarized and circularly polarized single pulses.

5. The Effects of Monomer Filtration on the Single-Shot Damage Thresholds of PMMA

In the above mentioned study of the effects of simple monomer drying and distillation on the

laser damage properties of PMMA^, it was found that both the single- and multiple-shot behavior

could be significantly improved. The next step was to determine whether micropore filtration of

the monomer prior to polymerization could produce further improvement over distillation. To study

this, three PMMA samples were synthesized at the University of Missouri and laser-damage-tested at

FJSRL. All three samples v/ere made from carefully distilled monomers which were then passed

through a filter apparatus containing either no filter, a 0.40 ym filter, or a 0.22 ym filter,

respectively. Polymerization was by UV initiation without any free-radical initiator.

The occurence of laser damage was defined as a large enough permanent change in the material

to be detected with the collinear helium-neon laser damage probe described in section 1, which was

found to be reliable in detecting damage sites as small as 5-10 ym in diameter. Besides using

the laser probe, which suffers from high sensitivity to slight misa 1 ignment, the occurrence of

damage was also indicated by a spark or flash, and/or by the appearance of one or more bright

scattering centers in the beam path after irradiation.

To measure single-shot damage thresholds, defined as laser fluence levels (J/cm?) with a 50%

probability of causing damage in one pulse, approximately 10 sites in each test sample were

irradiated with one pulse each at a given fluence level and evaluated as discussed above for the occurrence of damage. This process was repeated at several fluence levels. A plot of the

resultant damage statistics is shown in figure 4 along with the results of "least-squares-f itting

(LSF) straight lines to the data. The statistical nature of the data is caused by the 15%

standard deviation in the peak fluence values discussed in section 1; by the subjectivity inherent

in deciding whether damage occurred in marginal cases, which introduces no more than a 5% error in

the damage probabilities; and by site-to-site variations in the material, as discussed in

reference 5. The damage thresholds, obtained from the midpoints of the LSF lines, are given in

table 2 along with the dust content of each sample, as determined with the depolarized scattered

light scheme. Each dust content number in the table is the average of six readings taken with the

scattered light probe.

53
PEAK FLUENCE (J/cm 2

)

Figure 4. Single-shot damage statistics for PMMA in the mononer-filtration study.

Table 2. Dust Content and Single-Shot Damage Thresholds of PMMA in the Monomer

Filtration Study

Sample

Monomer

Description

Dust Content

(Particles/mm^)

Single-Shot Damage

Threshold (J/cm-*)

E20 distill only 7.1 ?9

E22 distill and 0.40 ym filter 0.5 67

F24 distill and 0.22 ym filter OJ 86

The data in table 2 show a clear correlation betv/een decreasing filter pore-size,

significantly decreased dust content, and significantly increased single-shot damage thresholds.

When combined with the results of reference 5 that distil"!ation alone can significantly raise the

damage threshold of PMMA, these results provide strong evidence that micropore filtration can significantly reduce dust and raise the single-shot damage threshold beyond, i.e., in addition to,

the level of improvement possible with distillation alone.

Multiple-shot damage measurements on the three samp^s of table 2 were begun, too, but an analysis of the data showed that it was insufficient for any firm conclusions to be drawn for

inclusion in this report.

6. The Effects of Polymerization Method on Damage Thresholds in PMMA

Table 3 presents a summary of the single- and multiple-shot damage thresholds and dust

contents for 9 samples of PMMA. They were chosen for their low and very similar dust contents,

and are listed in increasing order of their single-shot damage thresholds. Clearly, the

IJV-polymerized samp 1 es were superior in that regard. To explain this, the samples were carefully

54
examined with a 100x microscope. The thermally polymerized samples all contained flaws, not seen

in any of the UV-polymerized samples, that range in size up to approximately 100 un. We suspect

that these flaws act as the nuclei of damage sites, thereby reducing thresholds. The flaws may be

vapor bubbles that become generated and trapped in the viscous mixture of monomer and polymer

while the reaction proceeds at the 40°C bath temperature. During UV-polymerization, the medium

is much cooler (approximately 30°C) and therefore less likely to bubble.

The multiple-shot damage threshold was defined as the fluence leve1 with a 50% probability of

causing damage to a given site within 250 shots. F/Fjh is the ratio of the peak fluence for the

multiple-shot damage to peak fluence for the single-shot damage threshold. Table 3 contains data

for two pulse repetition frequencies, 1 pps and ""O pps. Unlike the sing^-shot data, the

multiple-shot data favor neither of the polymerization methods.

Sample R-3 is unusual in that the multiple-shot damage resistance is very high. R-3 v/as the

only sample prepared in a vacuum using the multiple distillation technique of reference 4. The

reasons for its high multiple-shot damage thresholds are unclear and require further study.

Table 3. Comparison of the Damage Resistance of Several PMMA Samples

Sample Polymerization Fj\] F/Fjh F/Fjh Oust Content

Method at 1 pps at 10 pps (Particles (mm^))

S-47 thermal 44 0.22 0.20 0.6

S-46 thermal 48 0.1 0.9

S-43 thermal 49 0.5 1.7

E-9 uv 54 <0.01 1.1

R-3 uv 57 0.75 0.61 <0.1

E-6 uv 62 0.1 0.5

E-22 uv 67 0.5

E-5 uv 79 <0.01 1.4

E-24 uv 86 0.2 0.21 0.1

7. The Effects of Plasticization on the Single-Shot Damage Thresholds of PMMA and PEMA

It has been asserted** that plasticization should improve a material's laser damage

resistance by allowing the relief of thermoelastic stresses. Supposedly, plasticization should be

especially useful in the case of low fluence level, multiple-shot damage, which is very important

from an applications point of view. Since no domestic research in this area had been reported to

date, the initiation of such was recently established as part of the FJSRL plastics damage

program. In this report we present the results of measurements on PMMA and PEMA. Three samples

of each were plasticized with 0, 5, and 10 weight percent dibutyl phthalate, respectively, and UV

polymerized as described in section 2. The maximum plasticizer content was limited to 10 weight

percent because plasticizers are known to reduce glass transition temperatures rather sharply^,

and a material whose glass transition temperature is near room temperature would not have the

desirable thermal and mechanical properties mentioned above.

Damage tests on the PMMA and PEMA samples were performed exactly as described in the

filtration study of section 5. LSF straight lines were fit to single-shot damage statistics, and

damage thresholds were obtained from the midpoints of the LSF lines. Table 3 lists the six

samples tested, their single-shot damage thresholds, and their respective dust contents as determined with the depolarized scattered light scheme.

55
The relatively high dust contents of the samples in table 4 as compared to those in table 2 is

probably because the samples in table 4 were not micropore-filtered before polymerization.

Despite the high dust contents, the data in table 3 allow an interesting observation to be made.

Whereas the plasticized materials all had significantly higher dust contents than their

unplasticized counterparts, their thresholds were not significantly lower.

Table 4. Dust Content and Single-Shot Damage Thresholds of PMMA and PEMA in the

Plasticization Study

Dust Content Single-Shot Damage

Sample Description (Particles/mm^) Threshold (J/cm?)

RO-1 PMMA 20.0 32

RO-2 PMMA + 5% DBP 37.1 29

RO-3 PMMA + 10% DBP 31 .4 32

RO-8 PEMA 72.9 19

RO-9 PEMA + 5% DBP 96.6 17

RO-10 PEMA + 10% DBP 97.7 17

For example, sample RO-3 contained more than 50% more dust than RO-1, but their single-shot damage

thresholds were identical. Using the results from reference 5 and section 5 above that damage

thresholds are always inversely correlated with dust content, the results here suggest that

plasticization can significantly increase single-shot damage thresholds. Because the dust

contents and damage thresholds of unplasticized PMMA and PEMA were so different, no valid

comparison of the single-shot behavior of the two materials can be made with the data in table 3.

Preliminary multiple-shot damage measurements were also made on the six samples in table 3.

Whereas the PMMA samples showed virtually no plasticizer-dependent multiple-shot behavior, the

plasticized PEMA was somev/hat more damage resistant than its unplasticized counterpart. No strong

conclusion can be drawn, hov/ever, because of the differences in dust content. To study the

effects of plasticizer alone, both the unplasticized and plasticized samples would have to have

identical, or at least similar, dust contents. Table 3 shows that such was not the case here.

8. Multiple-Shot Data Analysis

The multiple-shot threshold measurements were conducted, as described earlier (section 6), in

such a way that the shot number at which damage was first detected was recorded. Survival curves were then compiled from that data for the arbitrarily selected values of 10, 20, 30, 50, 100, and

200 shots. A least-squares fit of a straight line through the data for each case yielded the 50%

probability point, analogous to the reported single-shot threshold. Sample data for 1 pps

irradiation are shown in figures 5 and 6.

56
1.0

UJ 0.8 O

<

-

<

D

O

>-

0.6

d 0.4

<

CQO

DC

°- 0.2

10

O 10 SHOTS

50 SHOTS

20 30 40 50

PEAK FLUENCE (j/cm2

)

60 70

Figure 5. MuUiple-shot damage data for a typical sample.

N CR

10 20 30

PEAK FLUENCE ( J/cm2

)

Figure 6. Plot of Nqr versus peak fluence for the same sample as in figure 5. Fluence plotted for each Mcr is that value of peak fluence at

which the probability of damage is 0.5.

57
9. Summary and Conclusions

1. The scattered depolarized helium-neon laser scheme is a simple yet reliable indicator of a sample's micron-sized impurity content and prediction of its single-shot damage resistance.

2. Micropore filtration of distilled monomer before UV-initiated polymerization can improve

the single-shot damage threshold beyond the level possible with distillation alone.

3. UV-initiated PMMA is generally more single-shot damage-resistant than thermally

polymerized PMMA, presumably due to the presence of fewer thermally generated voids in the

UV-initiated material.

4. PEMA samples had good mechanical and optical properties, but relatively large dust content

and, consequently, relatively low damage resistance. Further study is needed.

5. PI asticization of PMHA and PEMA with DBP can improve their single-shot damage resistance.

This result and the effects on multiple-shot behavior both need further study.

6. Although UV-initiated PCHMA po1 ymerized slightly cloudy, it was hard, odorless and

bubble-free, thereby warranting further work.

* "Research sponsored in part by the Air Force Office of Scientific Research/AFSC, United

States Air Force, under Contract F49620-82-C-0035. The United States Government is

authorized to reproduce and distribute reprints for governmental purposes notwithstanding

any copyright notation hereon."

10. References

[1 ] O'Connell, R.M., Oeaton, T.F. and Saito, T.T., "Single- and Multiple-Shot Laser-Damage

Properties of Commercial Grade PMMA," Appl . Opt. 23(5), 682 (1984).

T2] O'Connell, R.M., Saito, T.T., Oeaton, T.F., Siegentha^er, K.E., McMally, J.J., and Shaffer,

A. A., "Laser Damage in Plastics at the F.J. Seiler Research Laboratory (FJSRL)," in

Proceedings of the 15th Annual Symposium on Optical Materials for High Power Lasers, Boulder,

CO, 14-16 Nov 83.

T3] Mucins, B.W., and Richert, B.A., "Strehl Ratio Measurements of Laser Damaged Plastics," in

Proceedings of the 15th Annual Symposium on Optical Materials for High Power Lasers, Boulder,

CO, 14-16 Nov 83.

[4] Aardsma, G. and Stevens, J.R., "Preparation of Clean Polymers for Rayleigh-Brillouin Light

Scattering Studies," Appl. Polym. Sci. 23, 2533 (1979).

T5] O'Connell, R.M., Romberqer, A.B., Shaffer, A. A., Saito, T.T., Deaton, T.F., and Siegenthaler,

K.E., "Improved Laser-Damage-Resistant Polymethyimthacrylate," J. Opt. Soc. Am. B. 1(6), 853

(1984).

rfi] Waxier, R.M., Horowitz, D., and Feldman, A., "Optical and Physical Parameters of Piexiglas 55

and Lexan," Appl. Opt. 18(1), 101 (1979).

T7] Akhmanov, S.A., Krindach, D.P., MiquMn, A.V., Sukhorokov, A. P., and Khokhiov, R.V., "Thermal

Self-Action of Laser Beams," IEEE J. Quantum Electron., 0E-4(10), 568 (1968).

T8] Adoshin, M.I., Gerasimov, B.G., Manenkov, A. A., and Nechitailo, V.S., "Decisive Importance of

the Viscoeiastic Properties of Polymers in their Laser Oamaqe Mechanism," Sov. J. Ouantun

Electron. 9(9), i

1 02 (1979).

[9] Bruins, P.F., Plasticizer Technology, Reinhoid, Hew York, 1965.

by
Manuscript Received

12-28-84 SAR

LEED ANALYSIS OF PULSED LASER DAMAGE TO Mo(100) SURFACES

Aubrey L. Helms, Jr., Chih-Chen Cho, and Steven L. Bernasek

Department of Chemistry, Frick Chemical Laboratory, Princeton University, Princeton, NJ 08544

Low Energy Electron Diffraction (LEED) and Auger Electron Spectroscopy

(AES) have been used to investigate the effect of Q-switched, frequency doubled

Nd:YAG laser pulses on the surface structure and composition of Mo(100). The

experiments were conducted in a special ultra-high vacuum (UHV) chamber which

allowed the characterized surfaces to be irradiated in an UHV environment. Good

epitaxial regrowth of the Mo(100) surface was observed after laser surface

melting as evidenced by LEED. Changes in the spot profiles and their dependence

on incident electron energy after irradiation suggest the incorporation of

defects and the formation of random islands on the surface. The study included

laser fluences both above and below the melt threshold of the surface.

Additionally, the annealing action of laser pulses on surfaces that had been

heavily damaged by Ar+ ion bombardment was investigated.

Key Words: dislocations, LEED, molybdenum, pulsed laser damage, Q-switched laser

INTRODUCTION

The demand for better materials for use in high power laser systems has grown dramatically in

the past few years. With the development of high power laser systems, one has to consider the

damage introduced in the materials used in these systems. This has been the topic of many studies

presented in the proceedings of this conference. The peak power attained in such a system depends

heavily on the thresholds for different types of damage in the components. (1) Typical responses of

metals to pulsed laser irradiation are slip (2), crater formation (3), pitting, dislocation motion

and multiplication (4), and increased vacancy concentration (5). The optical properties and

response of the metal to laser irradiation are sensitive to the condition of the surface. This

study investigated the response of Mo(100) single crystals to pulsed laser irradiation.

The development of spot profile analysis (SPA) applied to LEED has made the use of this technique to study defect structures on surfaces possible (6-10). Careful analysis of the angular profile of the LEED spot in reciprocal space as a function of incident electron energy allows a qualitative determination of the structure of the surface. The technique is very surface sensitive,

probing only the top few atomic layers of the surface. It allows the distinction of random point

defects, ordered terraces, ordered islands, random islands, and facets (9).

LEED-SPA, AES, Rutherford Backscattering Spectrometry (RBS), and Nomarski Interference Contrast

Microscopy have been used to characterize the response of Mo(100) single crystals to pulsed laser

irradiation at laser fluences both above and below the visible melt threshold. The use of LEED has

been shown to be very sensitive to the introduction of disorder. The results indicate that random

islands form on the surface after pulse laser irradiation, even at laser fluences below the melt

threshold. These islands coalesce back toward the flat surface after long periods of heating at

1000°C. A sample which had been disordered by Ar+ ion implantation was annealed by pulsed laser

irradiation in a manner similar to the laser annealing of silicon (11). The LEED analysis indicates

random island formation in the irradiated region of this sample.

EXPERIMENTAL

The samples were cut from a single crystal boule oriented to within 1° of the (100) face by means of the back reflection Laue method. Standard metallographic techniques were employed to

obtain a mirror finish. The crystals were spot welded to thin tantalum foils which were mounted on

a special transfer block. The sample was heated by thermal conduction from the tantalum foil which

59
could be resistively heated. The temperature was monitored using an optical pyrometer ^cussed on

the sample though an 8" viewport at the front of the system. Sample temperatures in excess of

1000°C were routinely used for cleaning and annealing purposes.

The experiments were conducted in a conventional ion pumped UHV ^^ii1 ^^^?

supplied by

P a titanium sublimation^and liquid Mtrogen cryotrap (J^.J^

SlXT^SJSLfS^ LSLrlSb S ^rr^drup^irmasslspectrometer for residual gas

analysis, a 3 KeV ion gun for Ar+ sputtering and a variable leak valve.

The chamber was equipped with standard 4-grid LEED

CMA-AES ION PUMP

LEAK VALVE

ION GUN

MASS SPECTROMETER

Figure 1. Schematic of the experimental apparatus showing the main chamber, transfer

system, and laser chamber.

Attached to the main system was a small UHV laser chamber and associated transfer systems.

This chamber could be isolated from the main chamber by means of high vacuum valves. The laser

chamber was independently pumped by a 20 1/s ion pump. A detailed description of the design and

capabilities of the laser chamber and its associated transfer system will appear elsewhere (12).

Briefly, the samples were cleaned and characterized in the main chamber and then transferred to

the laser chamber under UHV conditions. The laser chamber was then detached from the transfer

system and transported to AT & T Technologies Engineering Research Center where the samples were

pulse laser irradiated. The system was then transported back to Princeton where the sample was transferred back to the main chamber for analysis. UHV conditions were maintained throughout the

experiment.

The laser system used for these studies was a Q-switched, frequency doubled Nd:YAG that is part

of an ESI laser trimming system. The laser was typically operated at a repetition rate of 5 KHz

with a pulse width of 140 nsec. The spot diameter was approximately 25 microns. To ensure complete

coverage of the surface, the beam was rastered across the surface by means of computer controlled

linear induction motors. Parameters such as raster speed, displacement, and the coordinates of the

spot pattern were chosen to guarantee good spot-to-spot overlap. The top half of the crystal was irradiated, leaving the bottom for in situ comparison. To further ensure uniform modification, the

process was repeated five times.

Samples were pulsed laser irradiated under three different conditions. One sample was irradiated at 15 MW/cm , which was below the threshold for surface melting as defined by the appearance

of a crater. This sample will be designated as 'Sample A

1 in the following discussion. A second

sample was irradiated at 60 MW/cm which was well above the threshold for crater formation. This

sample will be designated as 'Sample B' . The third sample was a sample that had been Ar+ bombarded

at 3 KeV in 1.0 X 10"^ torr Ar for one hour. This bombardment produced a disordered surface as evidence^ by the absence of a LEED pattern. This sample was laser irradiated at a laser fluence of 75

MW/cm and will be designated as 'Sample C

The LEED patterns were photographed using a 35 mm camera mounted in front of the 8" viewport.

The negatives were then evaluated using a computer interfaced Vidicon camera system described previously (13). The data obtained with this system included the spot profile, spot to spot distance,

and relative intensity between the spot and the background. The photographic technique provides a permanent record of the experiment while allowing the collection of a large amount of data In a very

60
short time. This ensures that the system will change very little in the course of the data collection.

The composition of the surface was monitored using AES. Great care was taken to make certain

that the conditions for taking the spectra remained constant. The surface concentrations of carbon

sulfur, and oxygen were calculated by evaluating the ratio of the peak-to-peak heights of these ele

ments to the peak-to-peak height of the Mo-220 eV transition. The ratios could then be compared

with literature values to give concentrations in units of monolayers (14). Additional calibrations

could be obtained in the case where a clear overlayer structure was observed in the LEED pattern.

The typical procedure involved characterizing the surface using both AES and LEED followed by

transfer and laser irradiation. After the sample was returned to the main chamber, AES was used to

determine compositional differences in the laser irradiated versus the virgin areas of the crystal.

The sample was then heated to 1000°C for two minutes to desorb contamination accumulated during

transfer and laser irradiation. Photographic data was taken as a function of incident electron

energy for both the laser irradiated and virgin areas. Sequences of heating, AES, and LEED were repeated to a total accumulated heating time of three hours.

The analysis was accomplished by comparing the spot profile and trends with the work of

Henzler, Lagally, and others (6-10). By restricting the study to the spot profiles and neglecting

the use of integral intensities, they have calculated the expected diffraction profiles for a variety of surface defect structures within the kinematical approximation. Figure 2 summarizes the

profiles expected from a variety of defect structures (9). The results to be described in the

following section were compared with their calculations, thus indicating the surface structure

resulting from pulsed laser irradiation under a variety of experimental conditions.

information extracted fram LEED spots

surface structure

• A

ideal surface

• • A'A

regulor steps

• random steps

0 AM i_n_j i__r

regular size

?

r island!

regular distance •

random size

and distance

islands

Figure 2. Table fro- Gro„„al<i and „e nzler „, summarlzl „g the infor„atlo„ ^

RESULTS

evidenced by AES. The level nf hL« lnc

^ease on both the virgin and laser irradiated surfaces as

contamination

the proces/o, U^LS ZZS^ZttSZ ZlXrl^T *f" on both surfaces dronned m rt. It V I !

irradiated region. The levels of

1000°C for two minutes!

PP the P re "irrad ^ted values after heating the sample to

substrate!

8

^

lndlcates that the melt ***** "grew epitaxially on the (100)

lar proriLT^r'trirraH'^r I* *?* indlcated that ln each ***** the half-width of the the angutlllllH ? cating the

irradiated regions were greater than that for the virgin areas (Fig. 3) indi- incorporation of surface disorder in the irradiated regions.

61
15 MW/CM2

NORMAL AREA

15 MW/CM2

LASER AREA

"tiToo 40 00 60.00 120.00 160.00 200.00 POSITION - ARB. UNITS

"boo "40.00 80.00 120.00 160.00 200.00 POSITION - PRB. UNITS

Figure 3. A plot of intensity versus position for typical spot profiles from both the

virgin and irradiated regions of sample A.

A plot of relative half-width versus incident electron energy for first-order spots in the irradiated regions showed an oscillatory behavior indicating the formation of stepped structures on the

surfaces (9) (Fig 4).

'c

-d

to

JZ

•*-> "D

(0 x:

* * * * o

o

o

o

o

o

o

*

* * *

100 150

eV

Figure A. A plot of relative half-width (half-width divided by the intensity) versus

incident electron energy for several heating times. ( * ) 5 minutes, (<>) 20 minutes, ( * ) 35

minutes, (#) 80 minutes.

b2
The amplitude and frequency of the oscillations decreased as a function of heating time as the

defect structures coalesced into a flat surface.

Careful analysis of the shape of the spots indicated that the spots in the irradiated regions

contained 'shoulders' which were aligned along the <010> and <001> directions indicating the orientation of terrace edges along the major crystallographic directions (Fig. 5).

Figure 5. A plot of intensity versus position for a spot in the irradiated region of sample B.

Both the half-width and the prominence of the shoulders were observed to decrease as a function of

heating time, again, an indication of the annealing of the surface defect structures.

The LEED patterns in the laser irradiated regions were slightly expanded relative to the patterns for the virgin areas as evidenced by an increase in the spot-to-spot distance. The expansion

was 3.4% for sample A and 7.7% for sample B. The expansion was isotropic and was determined not to

be an experimental artifact due to sample positioning or errors in incident electron energy. The

difference in the patterns indicates that the irradiated surfaces are slightly contracted parallel

to the surface in real space relative to the normal regions.

The carbon and oxygen contamination levels for sample C followed the same trends observed for

the other two samples. A LEED pattern was observed in the irradiated region and not observed for

the virgin area. This indicates that the melted region extended beyond the damage range and then

underwent liquid phase epitaxial regrowth. A very poor LEED pattern was observed for the virgin

area after a heating time of seven minutes and remained more diffuse than the pattern for the irradiated area throughout the experiment. The half-widths of the LEED spots from the irradiated region

were smaller than the half-widths of the LEED for spots in the virgin region indicating that the

laser had annealed some of the disorder introduced by the ion bombardment. The irradiated region

was still damaged relative to the well annealed single crystal before Ar+ implantation. The halfwidth of the spots in both regions decreased as a function of heating time.

The increase in the levels of carbon and oxygen on the sample after the transfer and laser

irradiation is due to the adsorption of residual gases on the surface. The conductance of the valve

that separates the 20 1/s ion pump from the laser chamber is very small. The base pressure in this

chamber is on the order of 1.0 X 10~8 torr. At these pressures many monolayers of gas will be incident on the surface in the time required for the experiment. The lower levels observed on the irraDISCUSSION

63
diated region indicate that the surface was cleaned by laser induced desorption. The features

observed in the LEED pattern cannot be attributed to this difference in absorbate concentrations

because the carbon and oxygen levels in both regions decreased to values equal to the pre-irradiated

surface after a short heating to 1000°C.

The craters observed on samples B and C with Nomarski Interference Contrast Microscopy are clear indications that the surface has melted. The appearance of a LEED pattern for the irradiated

regions of these two surfaces implies that epitaxial regrowth took place during the solidification

phase. The geometry of the pattern is consistent with that of the (100) substrate. The epitaxial

regrowth is further evidenced by the low backscattered yield observed in the channeling spectra.

Nomarski Interference Contrast Microscopy showed that the irradiated surface of sample A was visually identical to the virgin region indicating that the surface had not melted.

By carefully applying the principles of LEED-SPA, it is possible to derive information about

the morphology and defect structure of the surface from the shape and energy dependence of the LEED

spots. The technique is very sensitive and allows the investigation of the defect structure introduced at laser fluences well below the melt threshold of the surface. The increased half-width and

decrease in intensity of the spots from the irradiated region of samples A and B is an indication of an increase in disorder (8). The half-width oscillated as a function of incident electron energy

which indicates the formation of steps on the surface. The spot profiles show sharp central spikes

with broad shoulders. This profile is typical of that expected for a surface with a random distribution of randomly sized islands. These shoulders are aligned along the major crystallographic

directions, implying that the edges of the islands are aligned along the major crystallographic

directions.

The half-width of a first-order spot was followed as a function of heating time. The halfwidth decreased as a function of heating time which is consistent with the coalescence of these

multi-tiered islands into a flat surface. The trends observed in samples A and B were similar

implying that the features observed In the LEED patterns were not simply due to the macroscopic

roughness of the melted surface.

The appearance of a LEED pattern for the irradiated region of sample C indicates that the surface melted to a depth below the disordered region to the good single crystal below. As the melt

solidified, liquid phase epitaxial regrowth was nucleated on the single crystal substrate and

resulted in an annealing of the surface. Although the crystalline quality was better than the

disordered, implanted region, the irradiated region still contained many defects and damage relative

to the well annealed surface prior to implantation. The spot profiles were similar to those

observed for samples A and B indicating the formation of random islands. The profiles differed in

the fact that the shoulders were not aligned in the crystallographic directions, but were circularly

symmetric. The symmetry of the shoulders implies that the preferential orientation of the edges of

the islands formed on this sample is not as strong as in the other two samples.

It has been shown previously that the damage introduced in metals after pulsed laser melting is

in the form of dislocation networks and vacancies (15). The damage is introduced by thermal

stresses introduced by the temperature gradients created by the absorbtion of the laser pulse. When

these stresses exceed the yield point for plastic deformation, the sample is irreversibly damaged.

It has been shown for the case of copper that this yield point can be reached for very small temperature excursions at the surface, on the order of 20°C (16). The deformation occurs through a mechanism involving dislocation motion and multiplication which sometimes results in the formation

of slip lines on the surface.

The disorder and island formation in sample A may be due to the motion of disloca

tions caused by the stress introduced when the hot surface under the laser beam expands against the

cold bulk material. The movement of a dislocation to the surface results in the formation of a ledge. The slip systems in body centered cubic metals are not as well defined as in face centered

cubic systems which may explain the lack of slip lines on the surface and the formation of randomly

sized islands.

The disorder and island formation in samples B and C must be due to stresses introduced during

the solidification of the melt puddle because the liquid cannot support shear or tensile stresses.

These two samples probably also contain a large number of vacancies. The mechanism for island formation on these samples is the same as for sample A. The magnitude of the damage is greater because

the temperatures are higher and the introduced stresses greater.

CONCLUSIONS

The method of LEED-SPA has been successfully applied in the investigation of the response of

Mo(100) to pulsed laser irradiation. It has been shown that this method is sensitive to surface

disorder introduced by laser fluences ranging from below the melt threshold to fluences where deep

64
cratering of the surface is observed. The behavior of the half-width as a function of incident

electron energy and the profile of the spots after irradiation indicate the formation of multitiered, randomly sized, randomly distributed islands on the surface with their edges aligned along

the major crystallographic axes. The appearance of a sharp LEED pattern in the irradiated region of

a sample which had been previously disordered by 3 KeV Ar+ ion bombardment indicates that the melt

extended beyond the disordered region and underwent liquid phase epitaxial regrowth resulting in an ordered surface. The expansion of the LEED pattern is an indication of a contraction parallel to

the surface of the laser irradiated surface in real space.

ACKNOWLEDGEMENTS

The expertise and patience of Ms. Lisa Kennedy '86 of Princeton University is gratefully

acknowledged in the preparation of high quality single crystal Mo surfaces. We also gratefully

acknowledge the help and guidance of Mr. Randy Crisici and Dr. Clif Draper of AT & T Technologies in

the operation of the laser system and helpful discussions on the topic of laser-metal interactions.

REFERENCES

1. J.O. Porteus, D.L. Decker, J.L. Jernigan, W.N. Faith, and M. Bass, IEEE J. of Quantum

Electronics, QE-14, 776 (1978).

2. J.O. Porteus, M.J. Soileau, and C.W. Fountain, Appl. Phys. Lett., 29, 156 (1976).

3. M.K. Chun, and K. Rose, J. of Appl. Phys., 41, 614 (1970).

4. F. Haessner, and W. Seitz, J. of Mat. Sci. , 6, 16 (1971).

5. S.A. Metz, and F.A. Smidt, Jr., Appl. Phys. Lett., 19, 207 (1971).

6. M. Henzler, Appl. Surf. Sci., 11/12, 450 (1982).

7. M. Lagally, Appl. Surf. Sci., 13, 260 (1982).

8. M. Henzler in ELECTRON SPECTROSCOPY FOR SURFACE ANALYSIS, Ibach ed. , (Springer, Berlin, 1977),

ch. 4.

9. K.D. Gronwald, and M. Henzler, Surf. Sci., 117, 180 (1982).

10. J.F. Houston, and R.L. Park, Surf. Sci., 21, 209 (1970).

11. G. Foti, and E. Rimini in LASER ANNEALING OF SEMICONDUCTORS, Poate, and Mayer eds. , (Academic,

New York, 1982), ch. 7.

12. A.L. Helms, Jr., W.A. Schiedt, B.M. Biwer, and S.L. Bernasek, to be published.

13. T.N. Tommet, G.B. Olszewski, P. A. Chadwick, and S.L. Bernasek, Rev. Sci. Instrum. , 50, 147

(1979).

14. M. Salmeron, G.A. Somorjai, and R.R. Chianelli, Surf. Sci., 127, 526 (1983).

15. L. Buene, E.N. Kaufmann, CM. Preece, and C.W. Draper in LASER AND ELECTRON-BEAM SOLID

INTERACTIONS AND MATERIALS PROCESSING, Gibbons, Hess, and Sigmon eds., (Elsevier North Holland,

New York, 1981), pp. 591.

16. H.M. Musal, Jr., Symp. on Optical Materials for High Power Lasers, Boulder, (1979).

65
Manuscript Received

3-17-86

The Effect of Defects on the Laser Damage Performance of Metal Mirror Surfaces*

H. H. Hurt

Michelson Laboratory, Physics Division

Naval Weapons Center, China Lake, California 93555-6001

From a laser damage standpoint, the optical surface is not isotropic as evidenced by

the selected area damage phenomenon. This is because the modern processes that generate

specular surfaces and coatings inherently introduce defects and damage into the bulk

material. These defects then act as preferential sites for the onset of laser damage, and

the distribution of these defects appears to affect almost all laser damage parameters.

This paper presents the development of high resolution electron microscopy methods to

identify deleterious imperfections in typical metal-mirror surfaces. Pulsed-laser damage

testing of identical surfaces shows systematic changes in the damage resistance of the

material that can be correlated directly to defects in the near-surface region. By this

method, it is possible to identify trace impurities that are deleterious to high fluence

optical components. In addition, one can now specify process parameters that will control

not only surface roughness and figure, but will also control laser damage resistance.

Key words: defects; diamond turning; laser damage; optical materials.

Introduction

The study of the optical properties of metal-mirror surfaces has been extensive [1]. From these

studies it has been apparent that the surface finishing method changes both the reflectance of the

surface and its laser damage resistance. Copper mirrors that are diamond turned, for example, have

higher reflectance and damage thresholds than conventionally polished copper surfaces [2] . It is also

known that changing the machining conditions of the diamond-turning process changes the laser damage

resistance. The selected area damage phenomenon is also related to the process parameters. These

empirical data suggest that the performance of these optical surfaces is directly tied to the state of

the mirror material in the near-surface region that "sees" the incident radiation.

Through the process of generating the mirror surface, damage and disorder have been introduced

into the starting material. The amount and type of damage are dependent on the mechanical/chemical

finishing conditions and are caused by the forces exerted on the surface, heat generated, and chemical

reactions with the fresh surface. Trace impurities in the mirror base material can also be a problem

as they tend to collect at sinks such as grain boundaries so that concentrations are sufficient to act

as nucleation sites for laser damage.

Consider that an optical component is designed to meet a wide range of environmental, structural,

and optical requirements in addition to laser damage resistance. Except for electronic materials,

defects are not necessarily bad. Defects can give strength, fatigue resistance, corrosion resistance,

and other desirable properties. The control and manipulation of defects in engineering materials has

been important since ancient times and is the basis for many modern manufacturing processes. Science

has unraveled the secret behind the medieval swordmaker who forged a blade that had both great

strength and good impact resistance. It was not a piece of single-crystal iron, but had a very high

point defect concentration. In this investigation, the role of defects in optical materials is

explored to reach the same type of goal: higher laser damage threshold.

Small-Spot Laser Damage Testing of Diamond-Turned Surfaces

Copper mirror surfaces were fabricated on the Naval Weapons Center's (NWC) advanced diamondturning machine under highly controlled conditions to ensure correlation of data as the machining

conditions were altered to change the surface structure. The mirror blanks were 1 .5-in. -diameter OFHC

copper.

Work supported by Naval Weapons Center Independent Research Funds.

Numbers In brackets indicate the literature references at the end of the paper.

66
Several of the blanks were overplated with fine grained UBAC copper that was subsequently diamond

machined. The UBAC copper machines better than the softer OFHC copper, and it is popular as a diamond-machinable cladding for heat exchanger structures. The machining conditions are those that are known to produce the best optical surfaces. Previous studies have shown a large difference in the

IR reflectance and the microstructure of the diamond-turned surface as the tool rake angle is made

more negative [1,3]. Therefore, the OFHC surfaces were machined at two different rake settings.

The laser damage behavior of the metal-mirror surfaces was then determined using the NWC automated pulsed-laser damage facility. Details on the design and function of this facility are included

in another paper in these proceedings [4], A 10.6 ym C0 2 TEA laser with a pulse length of 100 ns was employed. The 1/e^ spot size on the sample was 250 ym. A nominal vacuum of 250 microns was achieved

using a roughing pump on the sample chamber. Several hundred irradiation sites were needed per sample

to generate valid statistical data for the the plotting of damage-probability curves. The use of a HeNe scatter probe to detect which irradiation sites had failed was successful in detecting melt and

selective damage. Unfortunately, the surface roughening caused by microslip was below the detection

level of the scatter probe device. This necessitated the laborious inspection of each irradiation

site with a Nomarski phase interference microscope.

The other part of this investigation involved looking at the laser damage with electron microscopy methods. To do this, specialized 3-mm-diameter by 0.010-in. -thick copper disks were prepared

from the mirror base materials. These small disks were then diamond machined in a manner identical to

the larger laser damage test mirrors. The small mirror disks were loaded into a holder in the laser

damage test chamber and then irradiated at fluence levels to cause damage. By carefully electropolishing the disks from the backside, electron transparent regions of 500 to 1000 A thickness could be

produced in the damage areas. Since the surface was untouched by the thinning operation, microtopography changes that accompany the microstructural changes caused by the laser damage were easily correlated in the electron microscope.

The electron microscopy was performed on a Phillips EM 420T located at the University of Southern California, Center for Electron Microscopy and Microanalysis. The capabilities of the instrument

utilized in this investigation included transmission electron microscopy (TEM), scanning electron

microscopy (SEM), and energy dispersive X-ray analysis (EDX).

Laser Damage Test Results

A plot for the melt thresholds for the diamond-turned copper surfaces is shown in figure 1. The

two characteristics in this plot are the steep rise in damage frequency that characterizes the bulk

damage threshold and the low-damage frequency "tail" that extends to very low fluences. These two

parts of the curve correspond to two different damage types. The bulk damage is characterized by a large melt pool, while the salient characteristics of melt damage at the lower fluences are small melt

pits that do not correspond to the intensity maximum of the laser beam. What this means is that the

defects that fail at the lower fluences are widely distributed in comparison to the 250-m spot size.

What is important is the realization that if this test was scaled up to large spot size, the optic

would always damage at a very low fluence.

As demonstrated in an earlier study, the -15-deg rake surface has a much higher bulk damage

threshold than the -5-deg surface [1]. The electroplated UBAC coating falls approximately in the

middle. Strangely, the -5-deg rake surface showed no selective damage behavior. As will be shown,

this selective damage phenomenon is linked with impurities in the base material and the -5-deg

machined mirror may have come from different parent material.

Microslip is the other damage characteristic of metal-mirror surfaces and is displayed in figure 2. Slip is visually defined as a roughening of the surface that is not characterized by surface

tension delineations that accompany melting. At higher fluences, slip is very evident. At the lower

fluences, the detection of slip in the Nomarski microscope is very subjective. It is best accomplished by rotating the sample so that the periodic structure of the diamond-machining grooves disappears. It can be seen from the graph that the onset of slip is very steep but shows some slope. This

is caused by the fact that the yield strength of copper is very anistropic, and the orientation of the

grains with relation to the surface determines the resistance of slip.

The data in figure 2 shows that the fine grain size of the electroplated UBAC copper results in a more slip-resistant surface. Surprisingly, the OFHC surfaces do not follow the inverse relationship

between slip and melt. This relationship was determined in earlier testing of a variety of copper

surfaces and showed that a surface that had a higher melt threshold necessarily had a lower slip

threshold [1]. The highest melt threshold surface — an electropolished single crystal — was so soft

that its yield stress was low; therefore, the slip threshold was at very low fluence. A diamondturned surface, by comparison, was much more slip resistant because of its harder, deformed surface.

The results of this study do not agree with the earlier observations. The -15-deg rake surface is

superior in both melt and slip performance to the -5-deg rake surface. This is not an artifact of the

67
testing procedure but is the result of surface microstructure modification when machining at high

negative rake angles.

Electron Microscopy of the Laser Damage Sites

In the electron microscopy study, the main concern was identifying the causes of the selective

damage. The bulk structure of the diamond-turned surfaces has been investigated and reported on in

last year's conference [3]. The study showed that the -5-deg rake surface was highly deformed in the

near-surface region. The -15-deg rake surface structure was very interesting, as the top surface

layer had recrystallized during machining. The resulting structure then had a low defect, high

reflectance outermost layer backed by a strong, highly deformed base. The surface microstructure can explain the bulk melting behavior of these surfaces as will be described.

As for the selected damage initiators, the defects were widely dispersed and would be almost

impossible to find with the electron microscopy methods. And if one did find a defect, is it a type

that leads to catastrophic laser damage? The answer was to let the laser "find" the defects at a fluence high enough to initiate damage but not to let the damage sites grow appreciably. Hopefully,

there would be enough information in the postmortem analysis to reconstruct the nature of the initiating defect. As indicated previously, the microscopy test samples were laser damaged and prepared for

electron microscopy. Figure 3 shows the selective damage on a macroscopic scale with a Nomarski

microscope. The figure on the left is what has been described as "wavy slip" and is from the fine

grain UBAC sample. The right figure is an OFHC surface and shows selective damage pits aligned on the

grain boundaries. Figure 4 is a TEM micrograph of the "slip" damage in the UBAC. It is evident that

what looks like slip in the Nomarski is actually laser melting and solidification as the grain size

has increased a hundredfold, and there are few defects. The rest of this discussion will concern

itself with the selective damage pits in the OFHC surface.

Figure 5 is a SEM micrograph of one of the selective damage pits. The grain boundary can clearly

be seen as a line bisecting the pit in the horizontal, whatever initiated the laser damage must have

been located at the grain boundary. Rings of blast debri are evident around the pit. At higher magnification (fig. 6), the bottom of the crater contains melted material and particles. TEM analysis

showed the melted material to be amorphous and almost all copper. An analysis of the particles, however, showed the interesting result that they are predominantly iron. The EDX analysis of this

particle is shown in figure 7. In addition to iron, significant amounts of zinc and phosphorous are present.

Discussion

The direct correlation of the laser damage test data and the microscopy data has allowed the

following statements to be made about the laser damage behavior of metal mirror surfaces:

Diamond turning or, for that matter, any surface finishing operation inherently imparts energy in

the form of deformation and heat into the mirror surface. When machining at a certain condition

(-5-deg rake angle), the surface is highly deformed and the dislocation density is high. The resulting material is stronger and therefore more laser-slip resistant. However, the dislocations also act

as scattering sites for the "free" electrons in the metal and cause increased Drude absorption even at room temperature [1]. This means that the melting threshold of a deformed material is lower than if

the material were defect-free. Compounding this effect is the released stored energy of the dislocations that will tend to drive the melting or recrystallization event once initiated.

From this it would appear that for a given material one is faced with equal trade-offs between

slip and melt performance. That is, improvements in one are paid for by degradation in the other.

There are, however, ways around the problem. The simplest is to use the known slip resistance of a fine grained material such as electroplated UBAC copper. The laser damage test data confirm this as the UBAC has very high slip resistance and good melt thresholds. Another more subtle way is to impart

enough deformation and heat into the surface during machining to recrystallize the surface layer. The

low defect recrystallized layer has low absorption and high melt behavior and is backed by the strong,

deformed underlayer that resists slip.

It looks like what appears to be slip in the Nomarski can actually be melting, as shown in figure 4. This is illustrative of the need for more than one method for characterizing the type of laser

damage. For thin films on metal substrates, the number of types of failure are much greater than for

the simple bare metal case. Hopefully, the microscopy methods described can be extended to provide a better understanding of thin-film damage.

The selective damage in the large-grained OFHC copper is caused by the impurities that have

collected at the grain boundaries. It is obvious now that even a 99.99% pure substance is inadequate

as an optical material if the impurities can collect in sufficient concentrations and are highly

absorbing. For mirrors and transmissive optics that are crystalline, the problem of grain boundary

68
segregation will continue. However, knowledge of which impurities are deleterious and those which are benign will be of great use.

The main lesson to be learned is the fact that controlled processes that produce laser optics can produce components with known and repeatable optical and laser damage properties. From the design

standpoint, the optic can be irradiated at higher fluences because of more confidence in the actual

damage threshold. Also, control of the materials properties allows one the possibility to specify the

type of laser damage resistance desired in an optic: melt, slip, pulsed or CW, etc.

Conclusions

The electron microscopy methods illustrated here can completely characterize the state of the

surface of the copper mirrors. The pulsed laser damage testing on identical surfaces has allowed a direct correlation of damage test data with known microscopic defects in the material. This is the

first attempt to define and improve the laser damage performance of optical materials by a nonempirical method. This methodology can and is being extended to look at optical thin films and window

materials.

Acknowledgments

The author would like to acknowledge the input of J. B. Franck in performing the pulsed-laser

damage testing at the Naval Weapons Center.

References

[1] Decker, D. L.; Hurt, H. H.; Porteus, J. 0.; Grandjean, D. L. "Optical properties of diamond

machined metal surfaces and their relationship to physical and chemical surface perfection," in

Proceeding of the 15th annual symposium on optical materials for high power lasers , Bennett,

H. E.; Guenther, A. H. ; Milam, D. ; Newnam, B. E. , ed. 1983 November 14-16; Boulder, CO. Nat.

Bur. Stand. (U.S.) Spec. Publ. 688; 1985 November. Pp. 147-56.

[2] Porteus, J. 0.; Fountain, C. W.; Jernigan, J. L.; Faith, W. N.; Bennett, H. E. "Pulsed-laser

stress phenomena on highly reflecting metal and alloy surfaces," in Proceedings of the 9th annual

symposium on optical materials for high power lasers , Glass, A. J.; Guenther, A. H., ed. 1977

October 4-6; Boulder, CO. Nat. Bur. Stand. (U.S.) Spec. Publ. 509; 1977 December. Pp. 204-14.

[3] Hurt, H. H. ; Decker, D. L. "Inherent mechanical damage in diamond machine optical surfaces," in

Proceedings of the 15th annual symposium on optical materials for high power lasers , Bennett,

H. E.; Guenther, A. H. ; Milam, D. ; Newnam, B. E. , ed. 1983 November 14-16; Boulder, CO. Nat.

Bur. Stand. (U.S.) Spec. Publ. 688; 1985 November. Pp. 140-46.

[4] Franck, J. B.; Seitel, S. C; Hodgkin, V. A.; Faith, W. N.; Porteus, J. 0. "Automated pulse

testing using a scatter-probe damage monitor," these proceedings.

DAMAGE

FREQUENCY

A -5° RAKE OFHC

- 1 5* RAKE OFHC

CU ELECTROPLATE /

AXIAL FLUENCE (J/CWT)

Figure 1. Melt threshold laser damage performance for diamond-turned

copper.

AXIAL FLUENCE (J 'CM

Figure 2. Slip threshold laser damage performance for diamond-turned

copper.

69
Figure 3. Nomarski micrograph showing low frequency damage in electroplated UBAC (right) and

OFHC copper (left). Note the damage pits aligned

along the grain boundary.

Figure 4. TEM micrograph showing the recrystallized and low defect raicrostructure of the

laser-damaged UBAC for figure 3.

Figure 5. SEM micrograph of a selective damage

pit aligned along a grain boundary. Figure 6. SEM micrograph of the bottom of the

pit showing melted material and small particles.

Figure 7. EDX spectra of one of the particles in

figure 6 showing a high concentration of iron and

trace amounts of phosphorus and zinc.

70
Manusaript Reaeived

7-16-84 SAR

Automated Pulsed Testing Using a Scatter-Probe Damage Monitor*

J. B. Franck, S. C. Seitel, V. A. Hodgkin, W. N. Faith, and J. 0. Porteus

Michelson Laboratory, Physics Division

Naval Weapons Center, China Lake, California 93555

This paper describes automated pulsed testing of infrared mirrors based on scatterprobe detection of laser damage. Advantages of our approach are (1) ability to monitor

minute changes in surface condition without an observer, (2) rapid sampling of a large

matrix of test fluences and test positions on the mirror, (3) automatic selection of test

fluences, (4) automatic control of laser intensity drift, and (5) compatibility with a vacuum test environment.

Automated determination of damage frequencies and relative damage onsets will be

described for single-layer-coated and uncoated metal and semiconductor mirrors. Tests were performed at 2.7 and 10.6 ym with 1/e 2 focal spot diameters of 150 and 240 ym, respectively, using a nominal 100-ns pulse length in all cases. Results obtained with the scatter probe will be compared with those obtained from Nomarski postdamage analysis.

Key words: automation; C0 2 ; DF; HeNe scatter; HF; laser; laser-induced damage; multilayer

dielectric coating; onset; scatter probe.

Introduction

Scattered radiation has been used in the past [1-3], 1 to detect laser-induced, low-level damage

to test-sample surfaces. Specifically, an auxiliary helium-neon (HeNe) laser was used to produce

visible scatter from damaged areas. In this paper, we discuss an improved HeNe laser scatter probe

recently developed at the Naval Weapons Center (NWC) for automated damage detection. The device has

been used in one-on-one tests performed in vacuum at wavelengths of 2.7 and 10.6 ym, with Gaussian

spatial profiles having 150 and 240 ym 1/e 2 diameters, respectively. Data taken in the automated

arrangement were analyzed using an algorithm developed at NWC [4,5] to determine the onset of laser

damage. Moreover, a direct correlation was found between the scatter probe data and observation of

selective damage using Nomarski microscopy after testing. Damage onsets derived from the scatter

probe are thus in excellent agreement with those obtained via the much more laborious method of

microscopic postdamage analysis. Further, low-level damage assessment can be performed in real time,

which aids in the selection of optimum laser test fluences during a test run. Such automation is

almost mandatory when the sample area to be tested greatly exceeds the area of the spatial profile.

Background

The development of an automated laser-damage test facility was driven by the need to increase the

sample throughput for tests at NWC where large numbers of samples were to be tested. For manual

systems, speed and accuracy are limited by the individuals performing the testing, including the time

required for visual damage monitoring. Fatigue becomes an increasingly important factor when testing

for several hours unabated. "Off-the-shelf" hardware for computer-controlled sample positioning has

been available for several years, and computer monitoring and control of the fluence levels delivered

to a sample are also relatively straightforward. The fact that our system was to be built around an existing test facility was a major constraint. This required that the sample-positioning hardware fit

into an existing lucite vacuum chamber, since avoidance of air breakdown was desired. Again,

off-the-shelf and easily obtained hardware (1) meets the dimensional requirements, (2) has adequate

positioning accuracy, and (3) is vacuum compatible. The more severe requirement of damage-assessment

automation was fulfilled by the development of the scatter probe.

Shown schematically in figure 1 is the scatter probe ray diagram. The pulsed laser source is

directed into the evacuated chamber through a NaCl window at Brewsters angle and is focused onto the

surface of sample S by lens L3. For positioning adjustment, a HeNe laser is directed into the chamber

by a periscope and is focused onto a small beam block (negative pinhole) by a combination of lens LI,

mirror Ml, and the surface of sample S. By a simple translation of lens LI and mirror Ml, the 1/e

diameter of the HeNe probe beam can be made to match approximately that of the test beam. Tilt

adjustment of mirror Ml allows both beams to be coincident on the sample surface. Thus, by adjusting

*Work supported by Defense Advanced Research Projects Agency Order 3815-H.

lumbers in brackets indicate the literature references at the end of the paper.

71
tilt and translation, the probe beam can reflect off the same region as the testing beam. The specularly reflected HeNe beam is blocked by the negative pinhole. The scattered component, however, is

distributed over 2tt steradians with the intensity maximum in the specular direction. A portion of the

scattered radiation which is not blocked by the negative pinhole is collected and focused by lens L2

onto a photodetector via mirror M2

. The operating principle is shown in figure 2, where a smalldamage

pit is represented on the sample surface, the result of laser irradiation by the test beam. The probe

beam is reflected off the sample surface, and the focused beam is blocked by the negative pinhole.

The shaded area represents the scattered light from the pit, while the unshaded area represents the

scattered radiation collected by the lens and focused onto a detector. Note the shadow of the negative pinhole.

The Automated System

Shown in figure 3 is a schematic diagram of the laser beam line. The test laser sources are a C0 2 laser [6] or an HF/DF laser [7] operating at 10.6 and 2.7/3.8 ym, respectively. The energy output

from the laser source is controlled by a computer-driven attenuator consisting of a set of Ge plates

at Brewsters angle in a polarizer/analyzer arrangement. The polarizer plate is mounted in a rotatable periscope which directs the output beam along a fixed axis. The analyzer plate A2 is fixed and

gives a plane-polarized output. An alignment HeNe laser beam is made coaxial with the test laser beam

after the attenuator. A portion of the test beam, which is reflected off of a wedged NaCl beamsplitter B, is sent to detector D 1 which is calibrated against a standard calorimeter. The remainder of

the beam is sent to the experiment test station. Figure 4 is a block diagram of the automated test

facility. The computer is shown (1) monitoring and controlling the laser energy delivered to the sample, (2) keeping the distance from the lens to the front surface of the sample constant while testing

nonplanar samples, (3) translating the sample in the x- and/or y-direction to expose fresh test sites,

and (4) monitoring the scattered light signal from the scatter-probe detector prior to and after test

irradiation. Damage is recorded when the change in scatter exceeds a predetermined value entered into

the computer.

The scatter level change corresponding to damage is determined for a family of samples by irradiating a matrix of sites at varying known fluences on a sample typical of the family. The sites are then inspected under Nomarski microscopy to compare scatter-probe signal to damage morphology. In

general, certain types of morphology (i.e., pitting, erosion, substrate slip) produce specific ranges

of scatter-probe signals. While there may be some overlapping from one range to another, a lower

bound for the limiting type of damage can be chosen. Some subsequent microscopy is generally required

to validate the selected lower bound. The software controlling the measurement sequencing is designed

to generate damage frequency data used to obtain the onset of laser damage [4]. If desired by the

operator, this software has the ability to predict appropriate fluence values for the damage frequency

curve from evolving data trends. Typically, to develop an adequate damage frequency curve, 400 to

1200 sites must be sampled depending on the mean defect separation relative to the focal spot size and on the uniformity of the sample tested. This provides from 8 to 14 points on the curve.

Experiment

Reflective coatings were tested at 2.7 and 10.6 ym in vacuum at normal incidence. For all tests

reported here, each site was irradiated once only at one of several fixed (±20%) pulse energies.

Shown in figure 5 is a plot of damage frequency versus fixed energy density at the peak of the Gaussian spatial profile, i.e., axial fluence, in arbitrary units. The sample is a single-layer A1 2 0 3 coating on diamond-turned, plated Cu irradiated at 2.7 ym. The shaded circles represent an increase

in scatter as seen by the scatter probe, while the darkened squares represent visible damage as seen

by Nomarski microscopy. Both techniques show agreement for lower values of axial fluence, but the curves diverge for upper levels of axial fluence. The reason for this divergence can be seen in figure 6 where damage frequencies for different types of visible damage are plotted. The crosses here

represent an increase in scatter as seen by the scatter probe. The visible damage of figure 5, as seen by Nomarski microscopy, has been separated into uniform erosion (dark squares) and selective pitting related to defects (shaded circles). The increase in scatter correlates closely with the pitting

but begins to diverge once uniform erosion occurs. This occurs because the scatter probe is relatively insensitive to uniform erosion until fully developed. An analysis of the data of figures 5 and

6 for the onset of damage is shown in figure 7. The solid and shaded curves indicate the results of

least-squares modeling of the pitting and increased scatter data, respectively. To minimize distortion of the results by the uniform erosion damage mechanism, only the four lowest fluence data points

in each case have been used in the analysis. As indicated in the figure, the damage onset based on the scatter probe agrees closely with that based on Nomarski microscopy.

Damage frequencies from a sample exhibiting predominantly selective damage (pitting) are shown in

figure 8, where the results of modeling the Nomarski and scatter-probe damage data are again

compared. The sample in this case consists of a single layer each of Cu, Au, and A1 2 0

3 deposited on a polished Mo substrate. Damage testing was performed at 2.7 ym. Increased scatter as seen by the

scatter probe is represented by the shaded circles and damage visible in Nomarski microscopy by the

dark squares. Reasonably good agreement is obtained, as indicated.

72
For the two examples just discussed, damage generally produced an increase in local scatter. If

the sample contains strongly scattering defects, the local scatter can decrease when damage occurs.

The damage frequencies shown in figure 9 exhibit both effects. The sample represented here is an enhanced reflective coating on Si tested at 10.6 ym. The damage frequencies based on scatter change

(shaded circles) (i.e. representing either an increase or decrease) provide a better representation of

visible damage than those based on increased scatter alone (shaded squares). All three frequency

distributions provide virtually the same damage onset when analyzed by the method of reference 4.

The automated laser damage facility was developed primarily for determining the damage onset of

multilayer dielectric mirror coatings. Figure 10 shows damage frequencies from such a sample using

both Nomarski and the scatter probe. The sample is a ThF^/ZnSe paired multilayer on a polished and

Au-coated Mo substrate. For this sample, visible damage as seen by Nomarski microscopy exactly

matches the increase in scatter signal as seen by the scatter probe. Selective pitting was the only

type of damage exhibited by this sample. The onset of damage, of course, is the same for both methods

of damage determination.

Shown in figure 11 for the same multilayer sample is a plot of damage frequency versus initial,

i.e., predamage local scatter. Squares represent damage frequencies averaged over half-unit scatter

increments. The data shown were taken at a constant fluence of 3.5 times the damage onset level

indicated in figure 10. This correlation between initial scatter and damage frequency may be expected

to hold for coatings where defects that scatter strongly fail at fluences below the damage levels of

other defects or of the intrinsic material.

Conclusion

This study has validated the utility of an automated scatter probe for determining damage

frequencies used to determine the onset of laser-induced damage in mirror coatings. The following

conderations apply.

(1) The scatter probe is very useful for monitoring defect-initiated selective damage, i.e.,

pitting, which is ordinarily the limiting failure mode of dielectric coatings.

(2) While available improvements in signal processing indicate that sensitivity of the scatter

probe for uniform damage can be greatly improved, this sensitivity is inherently less than that for

selective damage. Apparently, the uniform damage observed here represents subtle surface

modification, possibly including recrystallization [8] that only, slightly affects the surface

microscatter as seen by the scatter probe.

(3) Care must be taken to correlate the amplitude and sign of the scatter signal to the damage

morphology which limits the operational viability of the optical surface under test.

(4) While the scatter probe may be useful in predicting damage via initial scatter, this may

apply only to a limited class of samples.

When using the scatter probe to obtain damage frequency data in the automated system, four times

as many samples can be tested with improved statistics as compared to manual testing.

References

[1] Newnam, B. E. "Damage resistance of dielectric reflectors for picosecond pulses." Glass, A. J.;

Guenther, A. H. , ed. Proceedings of the 6th annual symposium on optical materials for high power

lasers; 1974 May 22-23; Boulder, CO. Nat. Bur. Stand. (U.S.) Spec. Publ. 414; 1974 December.

39-47.

[2] Saito, T. T.; Milam, D; Baker, P.; Murphy, G. "1.06 ym 150 psec laser damage study of diamond

turned, diamond turned/polished and polished metal mirrors." Glass, A. J.; Guenther, A. H., ed.

Proceedings of the 7th annual symposium on optical materials for high power lasers; 1975 July

29-31; Boulder, CO. Nat. Bur. Stand. (U.S.) Spec. Publ. 435; 1976 April. 29-40.

[3] Newnam, B. E.; Gill, D. H.; Faulkner, G. "Influence of standing-wave fields on the laser damage

resistance of dielectric films." Glass, A. J.; Guenther, A. H. , ed. Proceedings of the 7th

annual symposium on optical materials for high power lasers; 1975 July 29-31; Boulder, CO. Nat.

Bur. Stand. (U.S.) Spec. Publ. 435; 1976 April. 254-271.

[4] Porteus, J. 0.; Seitel, S. C. "Absolute onset of optical surface damage using distributed defect

ensembles." Appl. Opt. 23; 3796-3805; 1984 November.

[5] Porteus, J. 0.; Franck, J. B.; Seitel, S. C; Allen, S. D. "Defect characteristics of optical

surfaces using pulsed laser damage methods." Proceedings of SPIE's 1985 L. A. symposium on optical and electro-optical engineering; 1985 January 20-25; Los Angeles, CA (in process).

73
[6] Porteus, J. 0.; Decker, D. L. ; Jernigan, J. L. ; Faith, W. N. ; Bass, M. "Evaluation of metal

mirrors for high power applications by multithreshold damage analysis," IEEE J. Quantum

Electron. QE-14; 776-782; 1978.

[7] Porteus, J. 0.; Donovan, T. M. ; Jernigan, J. L. ; Faith, W. N. "Multithreshold evaluation of

100-nsec pulsed laser damage to coating materials at 2.7- and 3.8-ym wavelengths." Glass, A. J.;

Guenther, A. H. , ed. Proceedings of the 10th annual symposium on optical materials for high

power lasers; 1978 September 12-14; Boulder, CO. Nat. Bur. Stand. (U.S.) Spec. Publ. 541; 1978

December. 202-211.

[8] Hurt, H. H. ; Franck, J. B. "High resolution electron microscopy of defects in optical

materials." Proceedings of the 16th annual symposium on optical materials for high power lasers;

1984 October 15-17; Boulder, CO (in process).

PULSED LASER SOURCE

Figure 1. Block diagram of test chamber showing

laser-damaging source focused by lens L3 and HeNe

laser scatter-probe arrangement consisting of the

HeNe laser, periscope, optical train (L1-M2), and

apertured detector.

M 1 LASER SOURCE

COMPUTER

ATTENUATOR

TO EXPERIMENT STATION

(

M

1 M 7

) - Cu TURNING MIRRORS

Figure 3. Schematic diagram of the laser system. The laser source can be either the HF/DF or

the CO2 laser. Also shown is the continuously

variable attenuator with a constant polarization

output off the germanium plate (A2) at Brewster's

angle. The energy during the experiment is monitored by detector Dj and calibrated versus an isoparabolic calorimeter C.

Figure 2. Close-up schematic diagram of the

scatter-probe arrangement. The focused specular

beam is blocked by the negative pinhole, and the

light scattered by the laser-induced pit is collected by a lens system. The collected beam is

spatially filtered and sent to a photodiode detector.

SCATTER PROBE

DETECTOR

COMPUTER

CONTROLLED

DIGITAL VOLTMETER

Figure 4. Block diagram of the automated test

station. The computer monitors and maintains a required energy level based on an ongoing ten-shot

average. The computer translates the test sample

and maintains focal distance of the lens (z-axis)

to the sample if nonplanar samples are to be

tested. On-line damage assessment is determined

from changes seen by the scatter probe from the

irradiated surface.

74
DAMAGE

FREQUENCY

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

I VISIBLE DAMAGE

INCREASED SCATTER

_l_ 0.0 0.2 0.4 0.6 0.8 AXIAL FLUENCE (ARBITRARY UNITS)

1.0

DAMAGE

FREQUENCY

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

EROSION (UNIFORM)

PITTING (SELECTIVE)

+ INCREASED SCATTER

0.0 0.2 0.4 0.6 0.8

AXIAL FLUENCE (ARBITRARY UNITS)

1.0

Figure 5. Damage frequencies determined by the

scatter probe compared with those determined from

Nomarski microscopy. The sample is a singlelayer A1 2 0 3 coating on plated and diamond-turned

Cu tested at 2.7 ym.

DAMAGE

FREQUENCY

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

I PITTING (SELECTIVE)

ONSET=0.18 ±0.07 INCREASED SCATTER

ONSET=0.22 +-0.04

0.0 0.2 0.4 0.6 0.8

AXIAL FLUENCE (ARBITRARY UNITS)

1.0

Figure 7. Data of figure 6 analyzed for the

onset of laser damage using the model described

in ref. [4]. Results of applying the analysis to

pitting damage determined by Nomarski and damage

detected by the scatter probe are compared. The

uncertainties in the onsets are model dependent.

DAMAGE

FREQUENCY

1.0

0.9 h

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

+ VISIBLE DAMAGE

ONSET=0.11 ±0.01

El INCREASED SCATTER

ONSET=0.09±0.03

CHANGE IN SCATTER

ONSET=0.09 ±0.01

0.0 0.2 0.4 0.6 0.8

AXIAL FLUENCE (ARBITRARY UNITS)

1.0

Figure 6. Data of figure 5 where the visible damage determined by Nomarski has been resolved into

the two different morphological types observed on this sample. Since the scatter probe is more sensitive to pitting, it provides a better representation of the frequency of this type of damage.

DAMAGE

FREQUENCY

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

1 VISIBLE DAMAGE

/ ONSET=0.26

11

INCREASED SCATTER

ONSET=0.30

1 1 0.0 0.2 0.4 0.6 0.8

AXIAL FLUENCE (ARBITRARY UNITS)

1.0

Figure 8. Comparison of damage frequencies

obtained from Nomarski with those obtained from

the scatter probe on a sample where pitting predominates. The measured damage frequencies were analyzed by the method of ref. [4].

Figure 9. Damage frequencies with onset analyses

based on Nomarski and on scatter-probe measurements from a sample which exhibited either positive or negative scatter changes with damage.

Inclusion of scatter decrease as an indication of

damage (change in scatter) provides a better

representation of visible damage than increased

scatter alone.

75
Figure 10. Identical damage frequencies with

onset analysis obtained via Nomarski and the

scatter probe from a multilayer dielectric mirror

at 2.7 ym. The uncertainty in the onset is model

dependent [4],

Figure 11. Damage frequency versus local scatter

before damage as measured by the scatter probe.

The curve represents the damage onset model of

ref. [4], which has been applied arbitrarily to

indicate the data trend.

Was the range of total integrated scatter from the sample measured before the damage measurement? No.

The onset model was used to determine the threshold using least squares fits. In all but one case the

model was degenerate and all the defects failed at once. The onset model uses a three parameter fit.

If the wavelength were changed the damage threshold might well change. In response to another

question the speaker replied that the system had been running for about 4-6 months and 30 or more

samples had been tested. The main point of the paper was that agreement was found in these tests

between the scatter probe results and previous microscopy . A questioner pointed out that Newnam in the

period 1972-74 presented work with 6943 A 10 ns pulses and 1.06 urn 30 ps pulses in which the

correlation of damage threshold with scatter was only slight. Initial scatter of the sample was

thus not a reliable monitor. Was sample dependence or wavelength dependence the cause of the

discrepancy? The author did not know but pointed out3 that in their setup they were looking among the

specular direction at the scatter3 which might give better correlation than at an angle.

76
Manuscript Received

1-10-85 SM

Application of a Computer-Automated IR Sensitive Camera

To the Determination of Pulsed Laser Surface Damage Thresholds

P. C. Filbert, J. L. Guttman, C. L. Navoda, and B. A. Watson

Lockheed Palo Alto Research Laboratory

3251 Hanover Street

Palo Alto, California 94304

An inexpensive, easily constructed infrared-sensitive camera system which as an

active electronic readout is described in detail and examples of its utility in the

determination of surface damage thresholds are presented. A small fraction of the pulsed

1.06 ym laser beam incident on the sample is split off and directed to a screen. The beam

spatial profile of each shot is then imaged onto a 100 x 100 photodiode array whose output

is digitized and stored by a PDP 11/23 computer-controlled data acquisition system.

Further processing of the image data by a VAX 11/780 computer yields a high resolution

fluence contour map of the laser beam incident on the sample. This map is especially useful when irradiating areas of several cm

2 during material testing experiments. The map,

showing the variation of beam fluence over its cross-section, can be superposed over the

damaged sample providing an accurate value for the surface damage threshold for the

material or coating. Examples will be presented and additional advantages of large area

irradiation discussed.

Key words: beam profiling; data acquisition; large area irradiation; pulsed laser damage;

surface damage thresholds.

1. Introduction

Recent materials testing at the Lockheed Palo Alto Research Laboratory requires the irradiation

of numerous samples with pulsed 1.06 ym laser light over an area of several cm

2 to approximately

10 cm

2

. In order to deduce surface damage thresholds over such a large spot size, an accurate

spatial intensity map of the incident laser beam is needed. To this end, an infrared-sensitive

camera with a 100 x 100 photodiode array at the focal plane is used to image the beam on a shot by

shot basis. The image data is acquired and processed under computer control and a fluence contour

map generated.

This paper will first describe the experimental configuration used in the materials testing

experiments. Second, a detailed description of the camera and data acquisition system is given. The

third section describes the procedure to reduce the image intensity data to absolute fluence values

which are input to a contour plotting routine. Fourth, the application to the determination of

surface damage thresholds is demonstrated. Last, the additional advantages of large area irradiation

testing will be discussed.

2. Experimental Configuration

Figure 1 is a schematic of the target chamber and the laser beam diagnostics used to perform

pulsed laser testing. The Nd glass laser system used in consists of an oscillator and 5 amplifiers.

The system can be operated in one of three modes. First is the relaxation-oscillation mode which

produces a 200 ys pulse train composed of approximately 100 ys pulses spaced 2 ys apart. The

maximum energy available in this mode is 400 Joules. Second is a Q-switched mode which produces a

77
single 20 ns pulse with a maximum energy of roughly 175 Joules. Third is a pulse-sliced mode which

provides a 3 ns pulse with ^ 100 Joules maximum energy.

The temporal profile of the beam is monitored by an SI response photodiode, the beam energy is

measured by a total energy calorimeter and the beam spatial profile is obtained by the camera. The

data from these diagnostics are fed into computer controlled data acquistion system.

For testing, samples are placed into the center of the vacuum chamber which is then evacuated to

2 x 10 -5 Torr. The ports at 45° can be used to field varous target diagnostics as the testing

requires

.

3. The Camera System

The camera system described here is similar to that described in reference [1]. A small fraction of the laser beam is deflected by a glass pickoff onto a screen which is then focussed by a

50 mm camera lens onto a 100 x 100 silicon photodiode array manufactured by E.G.&G. Reticon. The

array is square with an inter-diode spacing of 60 ym in the x and y directions. The array is selfscanning with the integration time of each diode beginning upon diode readout. The required wave- form and clocking to drive the array are supplied by a readout circuit board which is also available

from Reticon (model RC502A)

.

Figure 2 shows a block diagram of the array and data aquisition system used. The output of the

RC502A readout board is fed into a LeCroy 8212/4 Fast Data Logger which digitizes the video signals

to 12-bit accuracy at a maximum rate of 10 5 Hz. The clock pulse from the RC502A is used to initiate

the A to D conversion. After conversion, the data are stored in a LeCroy model 8800A/12 memory

module which can store up to 32768 12-bit words of data. The Data Logger uses a stop-trigger signal

which is generated by the firing of the laser to select the desired frame of video data to containing

the beam profile.

The data is read from the memory module into a Kinetic System LSI-11/23 microprocessor based

computer system run under the RT-11 operating system. Communication between the computer and the

data modules is accomplished by a Kinetic Systems model 1510 power crate and model 2923 crate

controller. The crate provides all power and communication busses for the modules placed in it

while the controller provides the interface between the modules and the computer system. The entire

data acquisition system conforms to the international CAMAC (Computer Automated Measurement And

Control) Standards which specify the basic bus structure and the form of the commands at the byte

level used to control the instrumentation.

The LSI-11/23 computer provides master control of the data acquisition, storage of the data onto

floppy disk, and preliminary image processing. A Tektronix 4025 graphics terminal is used as the

keyboard and image display device while a tektronix 4632 Video Hardcopy unit is used to obtain a

permanant record of images displayed on the terminal screen.

An Ethernet data link allows communication between the LSI 11/23 and a VAX 11/780 computer which

is used to generate equal fluence contours from the image data. A licensed commercial graphics

package (DISSPLA ) is used which derives the contours from the image data and performs the

plotting. However, the LSI 11/23, or other minicomputers (e.g. HP 9000 series) could be used for

the contouring depending on the availability of the appropriate software.

All the components of the camera system are available "off the shelf" and are made to be

compatable owing to the conformance to the CAMAC standards. Similar components can be obtained from

other vendors and the selection is up to the individual user. In addition it should be mentioned

that the system as described is quite versatile and is far more than a dedicated camera system.

The total cost of the system is less than 18,000 dollars (without the VAX of course), and could be

78
less depending on the features selected.

4. Data Processing

Two forms of image presentation are routinely employed, both of which are illustrated in

figure 3. The bottom portion is a 3-D plot of the relative pixel response in an 80 x 80 subset of

the array. This particular spatial profile is of the 200 \is pulse train. For this shot the beam

was focussed to a 0.75 inch diameter. Using the procedure described below, the pixel response can be

converted to a fluence value in the target plane which is used to label the vertical axis in the

figure.

Directly above the 3-D plot is a fluence contours derived from the 3-D data. The contours

are labelled by the fluence in the target plane. The multi-spatial mode of the beam is quite

apparent from these images.

To calculate the fluences in the target plane the following steps are followed. First,

normalize the area under the 3-D image to the measured beam energy. This gives the fraction of

energy represented by each pixel response (E^.). Second, the magnification of the camera is

measured. This determines the area represented by a pixel on the screen, (A

g

) . Third, the linear

magnification (M) of the focussing lens is known and so the area in the target plane represented by

a pixel (Aj.) can be obtained by:

A = M

2 A

t s

Last, the fluence in the target plane is then:

F . . = E. . /A . il iJ t

The array F„ is then used as the input of the routine used to generate the fluence contour.

5. Application

The application of the fluence contour map in the determination of surface damage thresholds is

straightforward as figure 4 illustrates. Here, the appropriately scaled map is superposed onto an enlarged photo of a damaged sample. By correlating the visible surface damage with the nearest

contour permits the damage threshold to be rapidly determined. The example shown here is of an aluminum alloy (7075) clad with 98% pure aluminum which has been electropolished . The. 200 ysec

pulse train was used to irradiate the sample with a total energy of 80 Joules over an area of

1.7 cm

2

. Although the sample is not of optical quality, the application to optical surfaces and for

shorter laser pulses is obvious.

6. Discussion

The technique described here was prompted by the requirement that large target areas of at least

several square centimeters be irradiated and that surface damage thresholds be obtained. Currently

materials testing is often done with small lasers which are focussed to a spot diameter of ^ 100 ym

or so. Because the beams usually have gaussian spatial profiles, the intensity gradient across the

spot is relatively large. (For a gaussian spot, 6l/l is approximately 1 over the spot diamter) . The

use of larger area irradiation allows a more gradual intensity profile to be obtained across the

sample thus reducing the complication of thermally induced mechanical stresses in damage threshold

determination. In fact, by the proper tuning and apodizing of the incident beam the intensity

gradient can be varied systematically enabling the quantitative study of the intensity gradient

effects. In addition, large area irradiation also allows the study of surface and coating uniformity

79
and how it relates to the failure of optical components. The use of the beam imaging system

described here is an indispensable and necessary tool for these studies.

7. References

[1] Knudson, J. Thomas; Ratzlaff, Kenneth L. Laser beam profile analysis using a two dimensional

photodiode array. Rev. Sci. Instrum. 54(7), July 1983.

80
u

OJ

ao • eg m

rH CJ ft

-a

u

0) to

is

o

iHC 3 00

D. CO •H O

tu

an

-O Q) m

QJ

um

0J CO _Q ,-1 E

ccj <u

<Do

U 4J

CO

c

•U OJS X o

4-1 CO

bO D. C

-H 0) O -U JZ

en -ac U CO

COB

<D

C

03

0)

a)3

81
SCREEN

ATTENUATION

FILTERS 1.06 [im

LENS3 RETICON

ARRAY

RETICON

RC 502A

READOUT

BOARD

LECROY

8800A/12

MEMORY

MODULE

LECROY

82 12A/4

ADC STOP TRIGGER

KINETIC

SYSTEMS

3923

CONTROLLER

VAX 11/780 TEKTRONIX

COMPUTER LSI- 11/23 4025

FOR FLUENCE COMPUTER DISPLAY

CONTOUR PLOTS TERMINAL

DISK

STORAGE

TEKTRONIX

4632

HARDCOPY

UNIT

Figure 2. Block diagram showing the details of the data acquisition

system used to record, store and process the image data.

82
10 20 30 40 50 60 70 80

X

Figure 3. A computer-generated illustration of the two types of display

used to characterize the laser beam spatial profile.

83
Figure 4. Illustration of the application of the contours to the determination of surface damage thresholds. Top is a photo of a damaged sample; note the damage in the ring produced by the

pulse in figure 3. Bottom is the same photo with the contour

overlay

.

84
Manuscript Received

10-23-85 SAR

High-Precision Damage-Resistant Multiple-Pass Ultraviolet Ref lectometer

L. John Jolin and S. R. Foltyn

University of California, Los Alamos National Laboratory, Los Alamos, NM 87545

A multiple-pass cell was reported by John White in 1942 [1]. Since then, it has been

adapted for use as a high-precision ref lectometer . The multiple-pass ref lectometer has

been studied and reported by Arnon and Baumeister [2].

Here, a reflectometer which is similar is described. It utilizes a uv laser operating

at X = 351 nm as the source and the White-cell mirrors are high-reflection dielectric

coatings designed for that wavelength. Because of the low-loss reflectors used in the

cell, a high number of traversals, reflections, can be achieved; R > 239. The use of

dielectric mirrors also improves the damage resistance of the apparatus which is important

when a uv laser beam is used.

The results of reflectance measurements performed on several ultraviolet high reflectors are also reported. These include conventional dielectric coatings as well as a hybrid coating consisting of A^O^, HfC^, and Si0

2 layers.

Key words: dielectric coatings; high reflectance; multiple-pass reflectometer ; White

cell

1 . Introduction

The Los Alamos optical damage laboratory has observed and defined several types of laserinduced damage to optical materials [4], In the case of high reflection, multi-layer, dielectric

coatings, damage may be defined as any change in the coating layers or at the substrate interface

which causes a measurable change in reflectance at its design wavelength. Since most dielectric

high reflectors have reflectances approaching unity, and a change caused by laser irradiation can be

minute, a precise method for measuring high reflectance must be utilized. A multiple-pass reflectometer based on the White cell was selected because of its demonstrated precision and high accuracy. It utilizes a laser as the source for reasons described later.

2. Ref lectometer

2.1. The ref lectometer configuration is similar to that described by Edwards and Baumeister [3]

with the exceptions of dimensions, number of traversals, and light source. Figure 1 shows the optical layout for the ref lectometer described here. Although figure 1 shows both a folded and an un- folded cavity about M2 , the reflectometer discussed here is only used in the folded configuration.

The cell mirrors are M3, M5 , M2, and M4 and have a radius of curvature of 50 cm, with the exception

of M2 which is flat. All four of these mirrors are dielectrics designed for 351 nm which is the

laser wavelength. They were all coated in a single run and all have the same reflectance. In practice, M3 and M5 are positioned directly above and in close proximity to M4 (figs. 2 and 3). This

allows for near normal angle of incidence on M2 which is the location of the mirror under test, and

alternately, the calibration mirror. M6 is positioned so that it intercepts the output laser energy

and directs it toward the output detector. M1 is the injection mirror and has a radius of curvature

of 25 cm which mode matches the collimated laser beam to the cell modes.

The laser used for this apparatus is a Lumonics excimer laser, Model 861, operating at 35 Hz,

with a 10-ns pulse duration. As is typical with excimer lasers, the output decreases slightly with

increasing age of the static fill gas so a small portion of the beam is diverted to detector B as a reference. The detectors are Laser Precision, Model RjP 734 and Model RjP 735 for A and B, respectively. The detector outputs are compared by a ratiometer, also Laser Precision, Model Rj 7200.

2.2. General Considerations

As discussed in much detail by Arnon and Baumeister, and Edwards and Baumeister, the multipass

reflectometer relies on the ability to vary the number of reflections on an unknown and to achieve a high number of reflections. To this end, high-reflection dielectric coatings are utilized in this

cell to not only minimize losses but also to withstand the higher energy of the laser.

85
There are several advantages to using a laser as the source. These include an increased

signal-to-noise ratio, an increased number of traversals within the reflectometer , and relatively

easy mode matching of the source to the cell. The higher energy of the laser also allows for meas- urements to be performed under room light conditions.

2.3. Reflectance Measurements

The key to operation of the ref lectometer is to keep track of the number of reflections, or traversals, within the cell at any given time. Each traversal consists of eight reflections—one on

each of mirrors M3 and M5 , two on M4, and four on M2. This is true for all traversals except the

last since the energy will escape one bounce on M4 and proceed to the output detector via M6. Thus,

the last traversal will have only seven reflections. This reflectometer routinely utilizes up to 30

traversals (239 reflections), so keeping track of them can seem a formidable task. In reality,

however, all that is necessary is to align for one traversal and from then on keep track of each

additional traversal. Successive traversals can be achieved by horizontal adjustment of M3, and

counted as they swing into position on the output detector.

To perform a reflectance measurement on an unknown, the reflectance of the cell mirrors must

first be determined. To accomplish this a cell mirror is used in position M2. The output is

recorded for each set of traversals from 1 to 30. The number of traversals is converted to number

of reflections, (N), and the negative log of the ratio of B to A is plotted vs N. Using a linearregression fit, a straight line is fit to the data. The average reflectance of all of the mirrors

within the cell is related to the slope of this line. Since all reflectances are equal, the equation for the cell mirror reflectance is:

where: m is the slope.

Now that the cell mirror R has been determined, M2 can be replaced by an unknown and measurements are performed in the same manner as for the cell. Now, however, there will be a change in

slope caused by the different R value of the unknown.

Since the laser energy enters and exits from M2 , there is always one more reflection on M2 than

all three cell mirrors combined. This can be seen in a simplified layout for one traversal,

figure 4. A and B are detectors. R

c represents the three cell mirrors, and Ru represents the

unknown (M2 )

.

If the extra bounce on the unknown mirror in the ref lectometer is ignored there is, of course,

an equal number of reflections on the cell mirrors as on the unknown. If the total number of re- flections within the cell is N then the reflections on M2 = the reflections on the cell mirrors = N/2. The following equation can be generated:

R = log

-1 m (1)

(2)

where

,

A energy at detector A

B energy at detector B

and K

1 n

B

It can be seen that this is an equation to a straight line with

86
as the slope, (m). Minor manipulation of eq (3) and insertion into eq (1) yields the equation for

the unknown reflectance value.

r = log 1 2m

u R

c

(4)

where: R u

= the reflectance of the unknown

and: R

c is the reflectance of the cell mirrors.

To disregard the extra reflection on the test sample causes no significant error since the

number of bounces is in N in eq (2) and not in the slope.

3. Results and Conclusions

Figure 5 shows data plots and fits for several reflectors tested. Notice that reflectors with

higher R's than the cell itself can be measured. The measured R for a number of reflectors available for test is shown in table 1. (The coating materials are listed for reference only and not intended to indicate general ranking of these materials.) Repeated measurements and multiple users

yielded a precision for this ref lectometer of ±0.0007.

Table 1 . Measured Reflectance of Various Mirrors

COATING MATERIALS SUBSTRATE

0.9976±0.0007 Ta

2 0

5 /Si0

2 F.S.

0.9948 HF0

2 /Si02 DYN1000

0.9948 HF0

2 /Si0

2 S.C. Si

0.9938 Al

2 0

3

/Si02 PC. Si

0.9938 HF0

2 /Si02 DYN1 000

0.9934 Zr0

2 /Si02 S.C. Si

0.9933 Al

2 0

3

/Si02 PC. Si

0.9924 10(HF0

2 /Si02 ) 7(A12 0

3

/Si02

) DYN1000

0.9906 UNAVAILABLE

0.9906 UNAVAILABLE

0.9902 Al 2 0o/Si02 S.C. Si

0.9898 (Zr0

2 /Si02

) (Al 2 0

3

/Si0

2

) S.C. Si

0.9875 UNAVAILABLE

0.9863 Al 2 0

3

/Si02 F.S.

0.9844 UNAVAILABLE

0.8859 Al

2 0

3

/Si02 S.C. Si

0.8498 Al

2 0

3

/Si0

2 S.C Si

0.8095 ALUMINUM MIRROR WITH MgF

2 O.C. (NRC MI

The alignment necessary to achieve the number of reflections reported here was relatively easy.

With care, more traversals could be obtained; however, it is easy enough now that an inexperienced

operator can learn to operate it within minutes and obtain results within the reported precision.

The reflectometer will probably prove most useful in a range from about 0.9990 down to 0.8000.

From figure 5 it can be seen that for a reflector of 0.8498 the attenuation was so high that only 23

reflections were possible. The laser energy could have been increased but mirror damage would have

been possible. The cell mirrors have damage thresholds much below state of the art so this could be

improved with better coatings.

87
4. References

[1] White, John U. Long optical paths of large aperture. J. Opt. Soc. Am. Vol 32, No. 285; 1942-

[2] Arnon, 0.; Baumeister, P. Versatile high-precision multiple-pass ref lectometer . Appl. Opt. Vol.

17, No. 18; 1978.

[3] Edwards, David F. ; Baumeister, Philip. High-precision multipass reflectometer , SPIE Vol. 288,

Los Alamos Conf . on Opt.; 1981.

[4] Foltyn, S. R. ; Jolin, John. Catastrophic versus microscopic damage: applicability of laboratory

measurements to real systems, Nat. Bur. Stand., Boulder Damage Symposium; 1983. (To be published )

APERTURE

Figure 1. Ref lectometer optical layout. Here, 15 reflections (2 traversals), is illustrated. This

ref lectometer utilizes only the folded cavity as represented by M^, M

2 , M^, and . M2 is the

position of the unknown.

88
Figure 2. NU, (top), and (bottom) are shown. The separation of the irradiated areas in the

vertical direction limits the angle of incidence at which one can test. Here, it is 7 degrees

.

89
90
Figure 4. Simplified schematic for a reflectometer . R

c represents the cell mirrors; R y represents

the unknown reflector, n can assume values of 3,7,11 ,.. .for the ref lectometer described.

91
1400

50 100 150 200

N (total number of reflections)

250

Figure 5. Data plot for several reflectors and the cell, itself, are shown. The reflectances are: M

3

= 0.8*184, M

2

= 0.9863, M

1 = 0.9976, and the cell mirror = 0.9906.

92
Manus oript Received

A Low Loss, High Power Variable Laser Attenuator 4-7-86 SAR

S. R. Foltyn, J. E. Griggs, L. J. Jolin,

J. H. Roberts, and D. Keaton

Los Alamos National Laboratory

Los Alamos, NM 875^5

The need for improved means of beam attenuation in uv optical damage experiments has led to the

development of a new reflective attenuator using conventional multilayer dielectric optics. The

device has low insertion loss even for unpolarized beams, high damage resistance, continuously variable attenuation, and a large clear aperture. Also discussed is a transmissive version of this device

which uses standard reflectors or edge filters. Either version lends itself well to computer automation.

Key words: angle tuning; excimer lasers; laser attenuation; optical interference filters

1 . Introduction

A perennial problem faced in most experiments using lasers is intensity control, this being

typically accomplished by attenuation of the output beam. The problem of laser attenuation is especially important in measurement of optical damage thresholds where repeated variations of fluence are

an integral part of the testing. In response to specific needs of the ultraviolet damage projects at

Los Alamos, a simple and effective beam attenuator that is particularly useful with high power un- polarized lasers (eg. commercial excimer devices) has been developed. Attenuation is derived from

angle tuning of multilayer dielectric interference filters.

2. Existing Attenuation Schemes

The two most commonly used elements for attenuating beams are polarizers and neutral density

filters. A third type of attenuator involves Fresnel reflection losses from a set of wedged plates.

While each type provides effective attenuation under certain conditions, none was found to be especially well suited for use in excimer-based optical damage experiments. A brief description of these

attenuation schemes - and why yet another approach was needed - follows.

Polarizers - Polarizers are available in a variety of configurations and provide, by rotation

with respect to the linear input polarization, a simple and efficient means of attenuation. Problems

arise however for excimer lasers, which usually produce unpolarized and highly divergent output

beams; collimating adequately for small polarizer acceptance angles can be problematic, and producing

polarized excimer output can reduce its energy by half.

Neutral-Density Filters - These elements, typically inconel coated glass or fused silica, do not

require polarized or collimated beams and are also available in a variety of configurations. However, because they operate by absorbing part of the incident radiation (part is reflected), they have

very low ultraviolet damage resistance: 248nm and 351 nm damage thresholds of less than 0.05 J/cm2

have been measured [1]. Furthermore, at even lower fluence levels, metallic filters are subject to

changing density values during long exposures.

Wedged-Plate Attenuator - As an alternative to these conventional methods, Bennett and Byer [2,

3] developed a novel attenuator using two pairs of counterrotating quartz wedges. While offering

numerous advantages over polarizers and filters, this design is somewhat lossy for unpolarized light

and has a limited clear aperture even for long wedges. In addition, there is some beam expansion in

the plane of incidence that may be unacceptable, for example, in optical damage testing.

3. Attenuation with Interference Filters

93
3.1. Principle of Operation

The spectral response of an interference filter is a function of angle of incidence and polarization of the input radiation. For example, as a normal-incidence reflector is tilted, the reflectance band shifts toward shorter wavelengths and transmittance at the design wavelength increases,

first for p-and then for s-polarization. This is shown graphically in figure 1 for a reflector of narrow bandwidth - in this case a uv design employing ^4 9 quarterwave layers of AlpO^ and SiC^. For

polarized input and ideal non-absorbing films, transmittance at fixed wavelength varies from essen- tially 100? to 0? over a 12° angular range. Figure 2 shows the result for an unpolarized incident

beam: The reflectance (approximately 1-T) varies from 100? at 20° to less than 20? at 33°. This

variable reflectance is the basis for laser attenuation.

The use of a narrow bandwidth coating is important. Figures 3 and 4 show that a reflector of

moderate bandwidth (23 layers of SC2O2 and MgF2) exhibits problems such as a large angular tuning

range and strong oscillatory behavior. If an attenuator is to be computer driven via an active feedback loop, oscillations at the band edge - artifacts of polarization splitting and reflector sideband

structure - are unacceptable. Fortunately, actual Al202/Si02 reflectors were found to give a relatively smooth, monotonic functional response as shown in the next section.

The above discussion pertains to the use of standard quarterwave dielectric reflectors as attenuating elements. It should be noted that improved performance can be achieved with different filter

designs such as long- or short-pass filters, or non-polarizing edge filters. An example is presented

in a later section.

3.2. Reflective Attenuator

Figure 5 is a photograph of a reflective attenuator showing the beam path for an incident angle

of 26°. Each mirror rides on a carriage that allows translation in the input/output beam direction.

The carriage fixtures also allow mirror rotation about an axis perpendicular to the plane of incidence .

Simultaneous translation of the mirror carriages in opposite directions is accomplished by a double rack and pinion driven by a stepper motor. A cam, visible at center, provides coordinated

rotation of the reflectors. The cam faces were contoured on a numerically-controlled milling machine

to provide the following relationship between angle of incidence 9, and the translational distance x:

x = D/(2tan29)

,

where D is the distance between translational axes and x = 0 when the beam segment between mirrors is

vertical (6 = 45°). The design shown is operable for incident angles from 25° to 50°.

One problem with the prototype shown in figure 5 - and a key concern for tunable attenuators in

general - is beam deviation. The design is particularly sensitive to contour errors in the cam and

tracking errors in the carriages. Nonetheless, this initial version of a reflective attenuator ex- hibited less than 0.5 mrad of angular beam deviation and no measureable beam displacement. A more

robust version with linear bearings and a precision cam has been fabricated but has not yet been

evaluated. Also, computer-coordinated translational and rotational hardware could eliminate entirely

the need for a cam, but this variation has not yet been pursued. To date, no simple linkage arrangement has been found that can provide the required mirror motion.

Optical performance of the reflective attenuator at 248nm is shown in figure 6 - a plot of

transmitted energy fraction versus incident angle. The reflectors were well-centered at 248nm and

employed 49 layers of Al 20o and Si0 2 giving a FWHM bandwidth of only 24nm. Energy throughput could

be varied from approximately 99? to 3? over a 10° tuning range. The small ripples in the response

function for these off-the-shelf reflectors are not problematic in most applications; however, as will be shown in the next section, they can be eliminated by use of an alternate coating design. The

small difference between the results of figure 6 and the prediction of figure 2 is a result of using,

for the calculations of figures 1 and 2, a value of 1.70 for the index of Al

2 0g instead of the more realistic value of 1.65. Also, the use of two reflectors in the actual attenuator effectively

squares the response function of a single element.

94
3.3. Transmissive Attenuator

Figure 7 is a photograph of an attenuator for which the output is the transmitted beam fraction.

Two elements are used to compensate for beam displacement. The fixture is much less complicated than

a cam-based reflective attenuator, and has been further simplified by elimination of the orthogonal

adjustments on each optic holder. The same reflectors described in the previous section can be used;

however, maximum throughput for two reflective surfaces is limited to about 7056.

Improved response is obtained by using edge filters as shown in figure 8. Here, attenuator

throughput is calculated for two identical elements, each with a filter on one side and a matched AR

coating on the other [4]. Actual transmitted energy for such filters was found to be variable from

0.3% to 88% over a 28° tuning range.

4. Discussion

Thus far, several advantages and disadvantages of this type of attenuator have been presented;

these are summarized in table 1

.

Table 1 . Advantages and Disadvantages of Tunable Filter Attenuators

Advantages

* High Damage Resistance3

* Low Loss Even for Unpolarized Beams

* Large Clear Aperture (Typically 70% of Component Diameter)

* Continuously Variable Attenuation

* Suitable for Automation via Active Feedback

Disadvantages

* Output Polarization Varies if Input is Unpolarized3

* Attenuator Coatings Must Be Designed for a Specific Wavelength

* Stray Reflections Must Be Blocked

3See discussion below

Damage resistance of optical coatings depends strongly on both the coating materials and the

vendor who deposits them. In addition, run-to-run variations can be quite large, especially for

non-sequential coating runs. What can be said, however, is that both detuned reflectors and edge

filters have damage thresholds that are comparable to those for standard reflectors using the same materials. Based on this argument, thresholds of up to H J/cm2 (248nm) and 8 J/cm2 (351 nm) can be

obtained at 10-20 ns pulselengths for an appropriate choice of materials and vendor.

Regarding the output polarization, potential users should be cautioned that, in some cases (eg.

when conventional reflectors are used), the output polarization state varies as the attenuator is

tuned. This can be alleviated to a large extent by the use of non-polarizing edge filters. If, however, the attenuator optics are illuminated by s- or p-polarized light, then input polarization will

be preserved at the attenuator output.

95
5. Conclusions

A new type of variable laser attenuator has been presented. While several implementations of

this attenuator have been constructed and described, all are based on angle-tuning to produce a change in reflectance (or transmittance) of multilayer dielectric filters. The achieved ranges of

attenuation are useful for most applications and can be further extended by use of additional filter

elements. Several features of this design make it particularly well-suited for use with high-power,

unpolarized excimer lasers.

6. References

[1] L.J. Jolin, S.R. Foltyn, Damage Thresholds of Ultraviolet Neutral Density Filters, Los Alamos

Memorandum CHM-5-83: 1 19 (1983).

[2] K. Bennett, R.L. Byer, Computer-Controllable Wedged-Plate Optical Variable Attenuator, Appl. Opt.

19(14), 2408 (1980).

[3] K. Bennett, R.L. Byer, Variable Laser Attenuators - Old and New, Laser Focus, April, 1983.

[4] Filter design data courtesy J. Latore, Airtron, Morris Plains, New Jersey.

96
100 100

10 20 30 40 SO

ANGLE OF INCIDENCE (degrees)

10 20 30 40 60

ANGLE OF INCIDENCE (degree*)

Figure 1 . Calculated change in transmittance

versus incident angle for either p- or s-polarized irradiation of a narrow band

reflector. In this and the next three figures,

reflectors are designed for use at normal

incidence and the wavelength is fixed.

Figure. 2. Response of the Fig. 1 reflector

to unpolarized light. The small oscillation

on the edge of the reflectance band is

problematic and was not observed experimentally

(see text).

ANGLE OF INCIDENCE (degrees) ANGLE OF INCIDENCE (degrees)

Figure 3. Calculated response for a reflector

of moderate bandwidth showing alarger angular

range and a greater degree of polarization

splitting.

Figure 4. For the moderate-bandwidth case

with an unpolarized incident beam, the range

of attenuation values is limited and response

is strongly oscillatory.

97
Figure 5. Reflective attenuator prototype.

Mirror rotation is coordinated with translation by the camvisible at center; this allows

incident angle to be continuously varied from

25° to 50° without deviation in the output beam.

Beam path for an incident angle of 26° is shown.

i i i i i i 30 32 34 36 38 40 42 44 46

ANGLE OF INCIDENCE

Figure 6. Measured throughput of the reflective

attenuator for unpolarized illumination at 248 nm.

ft 10° change in incident angle gave a monotonic

decrease in output energy fraction from over

98? to 3%.
Figure 7. Transmissive attenuator employing

two counterrotating elements. Interference

filters can be used on from one to four

surfaces depending upon the required attenuation range. Nonactive surfaces must be

antiref lection coated to minimize losses.

1 1 1 r

1 \

1 ^

J \ _

/ \

/ \

/

1

10 20 30

ANGLE OF INCIDENCE

40

Figure 8. Calculated response of a transmissive

attenuator at 351 nm (unpolarized) using two

Al

202/Si02 edge filters with AR-coated back

surfaces. (Edge filter data courtesy J. Latore,

Airtron)

.

99
Manuscript Received

12-28-84 SAB

STUDIES OF C02 LASER INDUCED DAMAGE TO INFRARED OPTICAL MATERIALS AND COATINGS

D.R. Gibson and A.D. Wilson

Barr & Stroud Limited

Physics Department

Caxton Street, Anniesland

Glasgow G13 1HZ, U.K.

A new versatile C02 laser damage facility which uses a TEA laser giving a single

mode output of up to 15 Joules per pulse is described. The temporal characteristics of

the pulse can be readily varied. A method of producing pulses with durations less than

that of the gain switched spike using plasma-based temporal attenuators is described.

Data is presented on a range of infrared optical materials as well as commercial and

experimental AR coatings and dielectric reflectors. Detailed examination of the damage

morphologies coupled with the dependence of the damage threshold on pulse length is used

to determine the mechanisms of laser induced damage.

KEY WORDS: C02 Laser Damage; Coatings; IR Materials; Plasma; Pulse-Length Variation.

1. INTRODUCTION

The interaction of C02 laser radiation with optical materials and thin films continues to be of

considerable technological and scientific importance to those engaged in the design of optical

systems for handling high energy laser pulses.

Inspection of literature data on transmissive components reveals that a wide range of laser

damage threshold (LDT) values have been reported for any given material. This is not too surprising

in view of the reported LDT dependence on spot size [l, 2 ] , pulse length [3, 4], method of surface

preparation [5], sample purity [6], and the type of coating [7, a]. It is also noted that little

data is available on the performance of the commercial coatings which most system designers are constrained to use. Consequently, we have initiated a study programme which will examine the interaction of C02 TEA laser pulses with substrate materials and commercial and experimental coatings as

a function of the laser parameters (pulse temporal profile, spot size, pulse repetition rate, etc.)

and the methods of preparation of components (substrate polishing, cleaning, film deposition

conditions, film materials, etc.). The aim is to provide a comprehensive and consistent data base

which can be used by system designers.

In this initial paper we describe the C02 TEA laser damage facility including methods of

varying the pulse temporal characteristics to allow the pulse length dependence of the LDT to be

investigated. This aspect is not only of practical interest to the system designer, but, as will be

demonstrated, can also provide useful insights into the mechanism of the damage process.

The laser damage characteristics of a range of infrared optical materials (Ge, ZnS, ZnSe, KC1,

and chalcogenide glasses) as well as commercial and experimental antireflection coatings and multilayer dielectric reflectors are presented. Data on damage morphology and the pulse length dependence

of the LDT are used to comment on damage mechanisms in certain materials and coatings.

2. THE C02 LASER DAMAGE FACILITY

The C02 single transverse mode TEA laser (Laser Applications, Hull, U.K.) is housed in a large

enclosed optical table assembly, as shown in figure 1. The TEA cavity has an unstable resonator

configuration, the output pulse being extracted using a scraper mirror. A 3W cw rf waveguide,

wavelength tunable C02 laser can be used to injection lock the output of the TEA cavity to the

normally dominant 10P20 transition (10.59l|-im). Cavity tuning both the TEA and injection lasers

enables operation in a single longitudinal mode [9] thereby eliminating mode beating. The maximum

energy output is 153 for single shot operation, 103 at 5Hz repetition rate and is reproducible to + %.

100
As a consequence of the TEA cavity configuration, the output beam is annular (50mm o.d., 25mm

i.d.). This is reshaped using an axicon to minimise the central obscuration (28mm o.d., 3mm i.d.)

before being focussed by an AR coated Ge lens to give the effective far field profile shown in figure

2a. Use of a beam profiling detector array during set—up eliminates significant alignment errors and

thus beam irregularities which would be undetectable by examination of burn marks.

The temporal profile can be varied by changing the TEA cavity gas composition. Normal operation

(C02 /N2 /He = 1/1/8) gives an initial gain switched spike of FWHM 100ns followed by a 2y.s relaxation

tail, figure 2b. Increasing the 1\1 2 content allows the tail to be extended to ^ 4|is which is on the

limit for a system with an effective output reflectivity of ^ 50$ [10], The tail can be extended to

6(is by injection locking on the 10P16 transition (10.55lijm). It is likely that the 10P16 transition

predominates during the initial part of the pulse but that the higher gain 10P20 transition predominates during the later stages. The small change in wavelength (0.04|-im) is not considered

important for damage work. Running without N2 gives a high intensity spike of FWHN — 90ns (injection

locking used) without a relaxation tail, figure 2b.

Pulses with durations less than that of the gain switched spike are produced using a plasmabased temporal attenuator. Using the arrangement sketched in figure 3a, pulse durations as short as 20ns FWHfl can be reliably obtained, figure 3b. (The incident pulse had the normal spike and tail

form).

The use of a plasma to attenuate a C02 laser beam is well established [ll—I4]. In all cases

the beam is tightly focussed in order to cause electric field induced breakdown in a gas. With air

breakdown, we find the spatial profile of the transmitted pulse is poor, with very pronounced sidelobes, figure 4a, and is also highly variable making accurate damage studies impossible. Air

breakdown is initiated at random dust particles which provide the initial electrons for avalanche [12],

The ensuing plasma, which is irreproducible and of non—uniform density, acts as a lens of spatially

variable power deflecting radiation out of the direct beam. It is also well established [15-19 ] that

surface plasma formation at transparent solids leads to pulse truncation. We have examined a number

of materials (KC1, Ge, AR coated Ge) and find that the spatial profile of the transmitted pulse is

regular and without significant side-lobes, figure 4b, reflecting the near uniform plasma density

formed at high incident/damage threshold fluence ratios: typically > 20 in this work. A fresh area

of the solid is exposed on each shot. The superimposed spatial profiles from three consecutive

shots, figure 4c, show that the reproducibility of the transmitted pulse is excellent, and thus a surface plasma pulse truncator can be used for laser damage work.

Two further advantages accrue from the use of a surface plasma initiator: (i) The energy

density required for pulse truncation (1003cm" 2 for AR coated Ge) is lower than that required using

air breakdown 20000cm-2

); (ii) The transmitted pulse length (20-90ns) can be altered by varying

the incident fluence. It can be shown that the broadening of the spatial profile of surface plasma

truncated pulses (cf. figures 2a and 4b) is due to the reduced attenuation experienced by the beam

wings. This effect is eliminated from damage threshold evaluations by 2D spatial integration to

obtain the peak energy density.

The optical elements and the beam diagnostic equipment, figure 5, are housed in a laminar flow

cabinet to minimise dust contamination. Pulse energy is monitored using calibrated volume and

surface calorimeters (Scientech). The temporal profile of the pulse is measured using Ge photon drag

detectors (Rofin). The spatial profile of the beam is measured using a 64 element pyroelectric array

(Spiricon). In all measurements reported herein, the spot size is > 0.9mm and is defined as the l/e2 diameter of an equivalent Gaussian fitted to the beam profile. The onset of damage is assessed by

viewing, with a x 6 telescope, the scatter from a HeNe beam co—incident with the area irradiated by

the high energy pulse. The damage thresholds are single shot values and are reported as the peak

energy density at which an increase in scatter is first observed from either the front or back

sample surfaces.

3. RESULTS AND DISCUSSION

3.1 IR Substrate Materials

Figure 6 summarises the damage threshold, measured with a spike + 2[is tail pulse, and absorption

coefficient data for a range of ir substrate materials. Clearly there is no correlation between LDT

and absorption coefficient. This is not surprising since the materials also exhibit a wide range of

thermal behaviour and, as we shall discuss, are subject to various damage mechanisms.

The two samples of n-type Ge have very similar damage thresholds. The Barr and Stroud material

is a standard ir grade, polished using alumina. The Laser Power Optics material is designated laser

101
quality. Both samples have similar surface roughnesses (around 208 RMS) but the LPO material has

fewer residual polishing defects, such as scratches and digs, as determined by Nomarski microscopy.

Both samples show very similar damage morphologies comprising localised sites distributed

within the irradiated area and occurring on the entrance face only, figure 7. Since the dimensions of

these sites are generally in the range 25 to 100(am with an inter-site separation of up to 20D(jm,

studies carried out with small diameter beams (< 200|im) will measure a wide range of LDT values

depending on whether a defect is within the irradiated area. In the present work, use of a large

diameter beam (> 0.9mm) allows examination of the distribution and frequency of the damage sites and

gives the defect dominated damage threshold which should be independent of spot size [ 20] . Host

often the damage sites are located along residual polishing scratches but are also found to follow a smooth locus which does not correspond to a surface feature (see top photograph in figure 7).

Individual damage sites show signs of melting. This is most clearly demonstrated by surface

profiling which reveals a central raised peak and a raised rim, figure 7. This morphology is

consistent with the melting/resolidification process described in detail by Willis and Emmony [21.].

At the periphery of the damage site there is a ripple pattern which may be due to interference of the

incident radiation with a defect scattered wave as suggested for many other materials [22, 23], We

suggest that damage occurs at absorbing regions (probably polishing debris or physical damage) which

are located in surf ace or sub-surf ace defects (scratches or grain boundaries). The obvious surface

scratches have sub-micron widths and thus are in the regime where enhanced field and absorption

effects are most likely for 10.6|im radiation [24],

In a preliminary study of ion beam planing of conventionally polished Ge we have observed

significant increases in LDT (up to 30$). The ion planing process removes some of the layer of

physical damage produced by conventional polishing, widens and reduces the slope of surface scratches

and also reduces the surface roughness at low spatial wavelengths, i.e. below 50(jm. Each effect is

expected to increase the LDT value. This work is continuing.

In a high refractive index material, such as Ge, exit surface damage should predominate [28].

However, as will be discussed in a further paper [25], the non—linear nature of the defect induced

thermal damage mechanism modifies the back surface field distribution and only front surface damage

is incurred.

The infrared glasses (As2 S3 , As2 Se3 and GeAsSe) show very low damage thresholds. The damage

morphology of GeAsSe glass consists of localised sites occurring primarily on the exit face but also

occasionally on the entrance face. Unlike Ge, the damage sites are not particularly associated with

obvious surface defects. Profilometry reveals that these sites are pits, figure 8. In view of the

relative softness of this class of material it is likely that localised defects will be produced

during the polishing process. The existence of a defect surface is supported by laser calorimetry

data for a range of sample thicknesses which show the bulk absorption coefficient to be 0.015cm" 1 with a typical surface contribution of ^ 0.06$ per surface. Such defects are most probably located

in a sub-surface layer. The low damage thresholds may be understood by noting that for impurity

dominated damage [26] the LDT is independent of the thermal properties of the impurity but scales as TDhVkh j where TDh is the damage temperature and kh is the thermal conductivity of the host material.

The very low value of kh for GeAsSe (2.5rnW cm*"1 K"*1 [27] cf. 600mW crrr1 K

-1 for Ge) coupled with its

low "melt" temperature would combine to give a low LDT value. That pitting damage occurs preferentially on the exit face is consistent with the enhanced electric field near that surface leading to

preferential heating of exit surface defects.

Although having absorption coefficients which differ by an order of magnitude, ZnS and ZnSe,

both produced by chemical vapour deposition, have very similar damage thresholds and morphologies.

Damage occurs at the exit face often having the form of a single deep pit, as in the lower surface

profile of figure 9, and is always accompanied by intense plasma formation. This suggests an electron avalanche ionisation mechanism. The upper photomicrographs of figure 9 illustrate a damage

site in which material removal has not occurred. Rather a large area, sub-surface damage event has

fractured the sample surface. The profile obtained normal to the fracture shows the surface has been

displaced upwards by ^ 1pm. This is a fortuitous "snapshot" of the first stage in the damage event.

If slightly more energy had been input the deformed surface would have been further stressed and the more normally observed pit would have formed. The lower photograph of figure 9 shows an instance

where only part of the surface has been lost. The clear evidence of sub-surface initiation, confirming the prediction of Boling et.al. [28] that standing wave patterns should be largest just inside the

exit face, is further evidence for field induced avalanche ionisation.

KC1 also shows damage at and near the exit face with extensive fracturing occurring just within

the sample. This is again consistent with an avalanche ionisation mechanism.

102
3.2 Coatings on Ge : Location and Nature of Damage Sites

An examination of laser damage sites on coated Ge, using Nomarski microscopy, reveals a variety

of morphologies which can be broadly classified into two groups:

(i) damage primarily in the film with secondary effect on the substrate;

(ii) damage primarily on the substrate with secondary effect on the film.

Examples of both types are shown in figure 10. A typical A/4 layer of ZnS (LDT ^ 0.5 x LDT of

uncoated Ge) shows film damage in the form of localised pitting. With a A/4 layer of ThF4 (LDT ^0.2

x LDT of uncoated Ge) the damage sites are located on the substrate and appear similar to those found

on uncoated Ge. Note especially the peripheral ripple pattern which is characteristic of the Ge meltdamage event, see figure 7, and is not observed with film dominated damage. The ThF4 /Ge and uncoated

Ge surface profiles are also similar exhibiting a raised hump in each case. A "substrate-like" damage

morphology is also observed, figure 11, for a Ge/(LH)n reflector with n = 1, where L = ThF4 and H =

ZnS, (LDT ^ 0.75 x LDT of uncoated Ge). When n is increased to 4, virtually no radiation reaches the

Ge surface and the damage sites are clearly located in the films (LDT 1.25 x LDT of uncoated Ge).

It is noted that "substrate-like" damage only occurs when ThF4 films are contiguous to the Ge

substrate and never with ZnS films.

At this stage we offer a tentative explanation of these observations and attempt to determine

whether these particular systems are limited by the films or by the substrate.

In comparing samples containing ThF4 contiguous to the Ge which show "substrate-like" damage,

the effect of differences in fluence at the substrate-film interface can be examined by normalising

the laser damage threshold, LDT, with respect to the interface transmittance, Ti, i.e. with respect

to the square of the electric field in the substrate. The product LDT x T^ is similar for the single

layer and the Ge/(LH) partial reflector but significantly lower than obtained for uncoated Ge. This

suggests that in addition to the fluence at the film-substrate interface an additional factor operates

to control the damage threshold.

It is well known that the substrate-film interface can show anomalously high absorption [29, 30],

In the present case, C02 laser calorimetry has revealed absorption coefficients of 12cm -1 for A/4

ZnS and 40cm" 1 for A/4 ThF4 films when deposited onto Ge. The average absorption coefficient of a (ThF -ZnS) LH-pair within a Ge/(LH)4 reflector is estimated from laser calorimetric data, as ^ 6cm~1

. Since the absorption in the Ge/(LH)4 reflector is dominated by the top two (LH) periods appreciable

substrate-film interface absorption is present in our samples. This enhanced absorption is most

likely associated with localised defects. When these defects are heated by absorption of radiation

both the substrate and film are expected to damage and the damage morphology would be expected to

mimic uncoated substrate damage since the new interface defects and the "intrinsic" substrate

defects are both located close to the substrate-film interface. As can be seen in figure 10 the

damage sites in ThF4 /Ge are often located at substrate scratches where the field enhancement effect

[ 24 ] will still be significant since ThF4 has a low refractive index, n ^ 1.35.

Thus we conclude that, in the cases examined, defects introduced during film deposition are responsible for damage in coated Ge optics.

3.3 Diamond-like Carbon AR Coatings on Ge

Diamond-like carbon (DLC), made by plasma assisted decomposition of hydrocarbons, has received

considerable attention due to its extreme hardness and physical/chemical durability [31-36]. The

description "diamond-like carbon" is a misnomer since the films consist of crystalline regions in a matrix of amorphous hydrogenated carbon [31], By varying the composition and structure through

processing parameters [33] the refractive index can be varied between ^ 1.8 and 2.3 and thus DLC is

suitable for use as an antireflection coating for high index substrate materials. An absorption

minimum between 10 and 11 microns has led to its suggested use as an AR coating for Ge in C02 laser

systems [34].

We have measured the absorption and damage characteristics at 10. 6pm of Ge substrates coated on both faces with DLC AR coatings produced at Barr and Stroud using rf plasma assisted decomposition of

butane. The production version of this coating is marketed as ARG4 [35], A sample produced by STC

(UK) using a similar process has also been studied. The data are presented as a plot of LDT (spike +

2|is tail pulse) against the reciprocal of the single film absorptance, figure 12, and show the

proportionality expected for film dominated damage,, The range of absorptance values is much wider

than would be found in commercially available coatings. The lowest absorptance films (3.2% per film)

produced for this study are comparable to the best reported elsewhere [36] and define the minimum

103
absorption coefficient (250-cm^1 ) obtainable for DLC films with n ^ 2. Normal production runs typically produce films with between 4$ and 6% absorption and are represented by two points in

figure 12. The STC sample also corresponds to this case. The typical LDT values of ^ 63cm" 2 are within a factor of two of the values reported by Dischler et.al.[36] for films prepared from benzene

and measured using a smaller spot size.

At or just above the LDT, the damage morphology consists of a region of film ablation at the

centre of the irradiated area and occurs on both exit and entrance faces. This has an irregular

"petal-like" boundary which may indicate that damage is initiated at a series of closely spaced

defects.

3.4 High Efficiency AR Coatings on Ge

A number of commercial and experimental high efficiency AR coatings on Ge have also been

investigated. These include:

(i) ARG3 produced by Barr and Stroud [37],

(ii) the Infralin series (211, 213, 214/216) produced by Balzers [38],

(iii) NLAR coatings on Ge (6040001, 6040008) produced by 0CLI [39],

(iv) a "laser durable" coating produced by STC (Ge0023N) [40],

(v) single layer ARCs of ZnS and ZnSe.

Measurements were made with the coatings on both faces of the Ge substrates. Transmission spectra,

figure 13, reveal the differences between these coatings. ARG3 is a broadband coating for the 8-l2(jm

region. The Balzers and 0CLI coatings are similar to ARG3. The STC coating has a V-type characteristic with a transmission region just slightly narrower than for a single layer ARC of ZnS and is

specifically designed for use at 10.6|am. The 10.6(jm reflectance of most of the commercial coatings

is low being typically 0.1$. The ZnS coating has a reflectance of ^ 1.5%.

A plot of LDT (spike + 2|is tail pulse) against reciprocal film absorptance, figure 14, shows

that even for this disparate set of coatings which vary in design, thickness and film materials the

trend of increasing damage threshold with lower absorptance is followed. In particular the three

examples of the ARG3 design produced with different absorptances emphasise the need to minimise the

coating absorptance. That the broadband coatings show higher absorptances and have lower damage

thresholds than either the single films or U-type coating is not surprising since, in addition to

relatively non-absorbing ZnS and metal fluoride layers, they may contain Ge and thin layers of metal

oxides. The STC V-coat, specifically designed for use at 10.6|jm, has a slightly lower damage threshold than a single layer ARC of ZnS but has the advantage of a minimised reflectance which may be

relevant if feedback to a laser cavity must be avoided.

The damage morphologies of all of these coatings show signs of localised damage, most often

resembling the sites shown in figure 10 for ZnS films. This again suggests defect controlled damage

with the defects, perhaps, occurring in clusters. The ratio of the LDT of ZnS coated and uncoated

Ge is ^ 0.6 for the best coatings examined. This value is close to the ratio of interface transmitt—

ances (=^ 0.65) and suggests that damage may be influenced by the substrate as well as the film

defects. For all other films and coatings the ratio is less than 0.5 indicating that film damage

predominates.

3.5 Dependence of LDT on Pulse Temporal Characteristics

For selected materials and coatings the LDT has been measured for a range of pulse temporal

characteristics. Both variation of the relaxation tail, in which the pulse shape changes, and also

pulses with durations less than or equal to that of the gain switched spike, in which the pulse shape

is similar, have been used. To test the validity of using plasma attenuated pulses for laser damage

studies, a pulse of 90ns was generated from a spike and tail input pulse and used to damage uncoated

Ge. The peak fluence LDT was 223cm~ 2 which is within 10% of the LDT measured using a 90ns pulse

produced by running with no nitrogen in the TEA cavity (LDT = 20. 23cm-2

). A further check was carried

out using freshly diamond fly-cut 0FHC copper which being a surface absorber should show a t? dependence [4l]. The peak fluence LDT values determined at 50ns and 90ns were 693cm-2 and 953cm-2 respectively

which do indeed conform to a t£ law within the measurement accuracy. These data demonstrate the

validity of the use of plasma attenuated pulses.

Damage thresholds are shown in figure 15 for materials and figure 16 for AR coatings on Ge.

The data are also plotted against pulse length in figure 17 for the pulses of similar shape i.e. 20,

50 and 90 ns FWHM. These latter plots can be used to provide further information on the mechanism of

damage. UJe note that impurity dominated damage leads to a dependence on the square root of the pulse

104
length [ 26, 42 1. The time dependence of avalanche ionisation is more complex. At short pulse lengths

the energy density must exceed a critical value for creation of electrons whereas at longer pulse

lengths there is sufficient energy for electron creation but the intensity (i.e. the rate of energy

input) must exceed a critical value to overcome electron energy losses. Consequently the damaging

fluence is expected to be independent of pulse length at short pulses and approximately proportional

to pulse length for longer pulses [42-45], The transition between these dependencies may occur in

the nanosecond regime [44].

The present data have been fitted to E = At

n where n is equal to either \ (defect) or 1

(breakdown). For Ge and GeAsSe glass substrates and for the AR coatings on Ge the data can be fitted

adequately by using n = \ y confirming a defect mechanism. Using 1.15ns and 70ns C02 pulses, a near relationship was observed by Newnam and Gill [3] for both coated and uncoated Ge. For ZnS, ZnSe

and KC1 the data are adequately fitted using n = 1 suggesting an electron

i avalanche mechanism.

Newnam et.al. [46], using pulses of 1.15ns and 65ns duration, observed a t5" dependence for damaging

fluence which apparently contradicts the present data. A possible explanation of this discrepancy

may involve the time dependence argument noted previously. Thus we conclude that the measured

temporal dependencies corroborate our previous assertions regarding damage mechanisms.

Perhaps of more practical importance to the systems designer is the variation of LDT with

changes in the extent of the relaxation tail. The materials which damage through avalanche ionisation

exhibit a damage threshold fluence independent (or nearly so) of the tail extent: a result we are, as yet, unable to explain. The defect dominated materials, Ge and GeAsSe glass, and the coatings show a pronounced dependence with the LDT fluence increasing as the tail is extended. An analysis of this

effect using a thermal defect model is presently underway. These data confirm the importance of

adequately characterising the pulse temporal profile including, the tail duration, in C02 damage

experiments.

For all substrate materials and coatings investigated the damage morphology was found to be

invariant over the range of pulse temporal characteristics used.

A. SUMMARY AND CONCLUSIONS

A technique of producing pulses with durations less than that of the gain switched spike has

been demonstrated and used successfully in C02 laser damage studies of materials and coatings.

Coupled with detailed morphological examination of samples damaged with a large diameter beam

(> 0.9mm) the time dependence of the LDT allows a ready identification of the mechanisms of laser

damage.

High purity samples of ZnS, ZnSe and KC1 exhibit an electron avalanche breakdown process

characterised by an LDT varying linearly with pulse length. GeAsSe glass and n-type Ge substrates

show evidence of an impurity damage mechanism with the characteristic dependence of LDT on the

square root of the pulse length.

Analysis of coated Ge indicates that substrate-film interface and/or film defects introduced

during deposition are predominantly responsible for damage. The measured time dependence for these

coatings conforms to LDT = htz which is consistent with defect dominated damage. In both the

"absorbing" films (diamond-like carbon) and the "less" absorbing coatings (ZnS, commercial ARCs, etc)

the LDT increases with decreasing film absorption. This emphasises the importance of minimising the

absorption in coated optics.

With diamond-like carbon (largely hydrogenated carbon), which has use as an AR coating on external surfaces, the scope for reducing the absorption is limited by the material's intrinsic

absorption due to C-C stretching vibrations. The best films have an absorption coefficient of

250cm-1 at 10.6(jm and, for spike + 2|J.s tail C02 TEA pulses, peak fluence damage thresholds of ^

6Hcm~z

. Standard production coatings will probably show higher absorption and hence a slightly

lower damage threshold.

Many commercial AR coatings are designed for broadband use in the 8-l2|_un region and often contain

relatively absorbing materials, e.g. metal oxides, to optimise mechanical and/or spectral properties.

Clearly there may be scope to improve the C02 laser damage characteristics of such coatings by

re-design and selection of less absorbing materials. It should not be assumed, however, that use of

metal oxide films will necessarily lead to a lower LDT. For example we have recently studied some single films of refractory metal oxides on Ge which show similar absorption to typical DLC films but

which have LDT values increased by a factor of two. Clearly this relates to the temperature

excursion which a material can tolerate i.e. the material's damage temperature, Tq: the implication

105
being that these oxide layers have a higher Tq than DLC. Similarly DLC has a higher Tq than ZnS,

ThF4 , etc. as judged by examining the product of LDT and film absorptance. In a multilayer stack

with contiguous low and high Tq materials, enhanced absorption in the high Tq layer may be

insufficient to cause damage to that layer but may lead to damage in the adjacent low Tq layers.

In this respect the effect is similar to damage to a matrix by heating of a defect.

For the single layer coatings of ZnS and ZnSe the damage morphology indicates a film dominated

defect mechanism. Defects within the substrate surface may contribute to the damage process in

certain cases e.g. the best ZnS films. These films have absorption coefficients of ^ 10cm-1 part of

which may be due to an enhanced contribution from the substrate-film interface. These values are orders of magnitude higher than the bulk absorption coefficients but are similar to values quoted

for conventional thin films by other workers [47, 48]. Most of the absorption is due to defects. A significant change in absorption coefficient will only be obtained by use of either UHV/MBE growth

[49] to eliminate defects formed by contamination (e.g. oxide inclusions in the ZnS, etc.) or energy

assisted deposition [501 which promotes adatom mobility in the growing film and thus minimises

physical defect structures. Recently ZnSe films with an absorption coefficient < 1cm-1 have been

reported [51], Such films are expected to lead to improvements in damage threshold and may produce

optics in which performance is limited by the substrate. At present, damage is mainly film dominated

and thus most effort should be applied to improving film quality. The substrate surface may also

influence the initial growth of the films and thus may affect the substrate-film interface absorption

but as far as we are aware this aspect has not received significant attention.

The authors would like to express their appreciation of the assistance given by many of their

colleagues and of the Management of Barr and Stroud for permission to publish this paper.

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R.M. wood, S.K. Sharma and P. Uaite, GEO 3. Sci. Technol., _48, 141 (1982).

A.D. Baer, T.M. Donovan, A.K. Green and G. Turner, NBS Spec. Publ., 462 , 214 (1976).

H. Tashiro, T. Shimada, K. Toyoda and S. Namba, IEEE D. Quantum Electron., QE-20 , 159 (1984).

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G. McLelland and S.D. Smith, Optics Comm., 27, 101 (1978).

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S.D. Czuchlewski and F.3. Figueira, Appl. Phys. Lett., 38, 325 (1981).

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H. Dupont, A. Donzel and 3. Ernest, Appl. Phys. Lett., 11_, 271 (1967).

L.D. Merkle, M. Bass and R.T. Swimm, Opt. Eng., 22., 405 (1983).

M.D. Solieau, M. Bass and P.H. Klein, NBS Spec. Publ., 568 , 497 (1979).

T.W. Sheheen et.al., 3. Appl. Phys., 53, 4652 (1982).

S.R. Foltyn, NBS Spec. Publ., 669 . 368 (1982).

L.D. Willis and D.C. Emmony, Opt. Laser Technol., 7., 222 (1975).

P. A. Temple and M.D. Solieau, IEEE D. Quantum Electron., QE-17 , 2067 (1981).

106
23) P. A. Temple and M.J". Soileau, NBS Spec. Publ., 462 , 331 (1976).

24) N. Bloembergen, Appl. Opt., 12, 661 (1973).

25) A.D. Wilson and D.R. Gibson, to be published.

26) 1*1. R. Lange, CJ.K. Mcluer, A.H. Guenther and T.W. Walker, NBS Spec. Publ., 669 , 380 (1982).

27) "Chalcogenide Glass Type 1", Barr and Stroud, Information Sheet 2060 (1983).

28) N.L. Boling, 1*1. D. Crisp and G. Dube, Appl. Opt., 12, 650 (1973).

29) P. A. Temple, Opt. Eng., 23, 326 (1984).

30) P. A. Temple, Appl. Phys. Lett., 34, 667 (1979).

31) T. Mori and Y. Namba, 3. Appl. Phys., 55, 3276 (1984).

32) U.S. Patent No. 4,444,805 (24.4.84) "Optical Coating" by Barr and Stroud Limited.

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April 1982.

34) A. Bubenzer, B. Dischler, G. Brandt and P. Koidl, Opt. Eng., 23, 153 (1984).

35) "ARG4 Ultra-Hard Anti-Reflection Coating for Germanium", Barr and Stroud, Information Sheet

0379 (1979).

36) B. Dischler, A. Bubenzer, P. Koidl, G. Brandt and O.F. Schirmer, NBS Spec. Publ., 669 , 249 (1982).

37) "Anti-Reflection Coatings for Germanium Optics", Barr and Stroud Information Sheet 1501 (1982).

38) "Infralin. Anti-Reflection Coatings on Germanium", Balzers, Data Sheet BD800 011 NE(8304), (1982).

39) "Infrared Stock Filter Catalogue", 0CLI (1982).

40) "Infrared Coatings and Filters", STC Components, Data Sheets, (1984).

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42) T.W. Walker, A.H. Guenther and P. Nielsen, IEEE 3. Quantum Electron., QE-17 . 2053 (1981).

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44) E.S. Bliss, D. Milam and R.A. Bradbury, Appl. Opt., 12, 677 (1973).

45) A.G. Molchanou, Sou. Phys. - Sol. St., 12, 749 (1970).

46) B.E. Neunam, A.V. Nouiak and D.H. Gill, NBS Spec. Publ., 568 , 209 (1979).

47) W.E.K. Gibbs and A.W. Butterfield, Appl. Opt., 14, 3043 (1975).

48) A.M. Leger, Appl. Opt., 18., 2979 (1979).

49) 1*1. Rona and P.W. Sulliuan, NBS Spec. Publ., 669 . 234 (1982).

50) P.3. Martin, H.A. Macleod, R.P. Netterfield, C.G. Pacey and W.G. Saintly, Appl. Opt., 22, 178

(1983).

51) K.L. Lewis and 3. A. Sauage, Priuate Communication.

107
CM, cavity mirror. BW, KCI Brewster window. SM . scraper mirror. G . grating . P, piezo-positioner.

Fig. I SCHEMATIC OF CARBON DIOXIDE TEA LASER

Fig. 2 SPATIAL &

TEMPORAL

PROFILES

0.75mm

a- SPATIAL PROFILE

COj.1 Nj.1 He.

8

500ns

. —^

C02

. 1.5 He. 8 5

100ns A

b- TEMPORAL PROFILES

108
AR coated Ge lenses

.1 Ln input beam

location of sampli under test location of surface plasma initiator

a- Experimental Arrangement

i

i\ 20ns f\

j 50ns

4 J\

b- Temporal Profiles

Fig.3 PRODUCTION OF SHORTENED TEA PULSES

3- Air Breakdown

mm 1

b- Surface Induced Plasma

Fig.4 SPATIAL PROFILES OF SHORTENED PULSES

Fig.5 SCHEMATIC OF C02 LASER DAMAGE MEASUREMENT FACILITY

109
LDT

(Jcm2

) 5 o

o

3

-log(cx,cnfi1

)

I

GeAsSe

glass As.S^b

• Barr& Stroud ACVD Inc. * Laser Power Optics

f Harshaw

Shaded Areas Indicate Range Of LDT Values

Fig. 6 LDTs AND ABSORPTION COEFICIENTS OF

INFRA-RED MATERIALS

100pm

50mti

A

25>jm

Ge

Surface Profile

Nomarski Micrographs

Fig. 7 DAMAGE MORPHOLOGY ON GERMANIUM: ENTRANCE FACE

110
5

1

io

4 A

GeAsSe glass

...Ji ItI I

250 pm

Surface Profile

Nomarski Micrographs

Fig. 8 DAMAGE MORPHOLOGY OF GeAsSe GLASS: EXIT FACE

Nomarski Micrographs Surface Profiles

Fig.9 DAMAGE MORPHOLOGY IN ZINC SULPHIDE = EXIT FACE

111
unetched film part-removed unetched

by etching

ThF ZnS 4

Fig .10 DAMAGE MORPHOLOGY OF GERMANIUM COATED WITH SINGLE

FILMS OF Thfj & ZnS

Fig .11 DAMAGE MORPHOLOGY OF REFLECTORS

Ge/(LH)

n

L=ThF, H=ZnS

112
8

LDT

(Jem 2

) • Barr& Stroud (ARG4) O STC(Ge0021NI

(Absorptance , %)

Fig.12 DIAMOND-LIKE CARBON AR COATINGS ON Ge

R(10.6um)

1 Barr & Stroud ARC 3 .1 %

2 STCIGe0023NI .1 *. 3 ZnS 1.5 %

Fig.13 AR COATINGS ON Ge BOTH FACES COATED

LDT

(Jem 2

)

( Absorptance, %)

Fig .14 AR COATINGS ON Ge

OCLIIMLAR Series)

• Barr4Stroud(ARG3) OSTCIGe0023N)

o Balzers { Infralin Series) x ZnS * ZnSe

113
a b c d e

I

GeAsSe

glass

b c d e

b c d e

ZnS

ft

b c d e b c d e

Pulse Characteristics: a , 20ns. b , 50ns. c ,90ns . spike only

d , spike + 2 jis tail . e . spike + 5\is tail

Shaded Areas Indicate Range Of LDT Values

Fig. 15 DEPENDENCE OF LDT(E, Jem2) ON LASER

PULSE CHARACTERISTICS SUBSTRATES

I

I

abed

DLC(lowoc) I

I

b c d e

STC(Ge0023N)

I

I

I

a b c (F

DLC(higha) m

in

a b c d e

Pulse Characteristics^, 2ons. b.sons. c, 90ns i spike only

d, spike* 2jistail. e.spike* 5ns tail

Shaded Areas Indicate Range Of LDT Values

Fig. 16 DEPENDENCE OF LDT(E, Jem"2

) ON LASER

PULSE CHARACTERISTICS : COATINGS ON Ge

Fig.|7 DEPENDENCE OF LDT(E , Jem'2

) ON LASER PULSE LENGTH(t,ns).

114
A questioner pointed out that tests reported at this conference in 1977 on damage thresholds of

antireflection coatings on germanium showed no correlation between coating absorption and the

damage threshold. It depended entirely on the surface of the germanium substrate. Coatings

from 13 different vendors and various surface treatments were tested. The speaker replied

that the damage thresholds reported in the 1977 paper were somewhat lower than those reported

in this paper, which would lead one to infer that perhaps the germanium surface was better in

the later work. In response to the surprise expressed about the linear pulse length dependence

reported, since the pulse length dependence is often 1/3 and never greater than 1/2, the

speaker replied that Bass, as he recalled, reported pulse length dependences as high as zero. The author had checked his plasma chopping by chopping up a spike and tail teapot to give a

90 ns FWHM pulse and compared that to the 90 ns gain switched spike running without nitrogen.

They were within about 10%. Also, copper has been checked and found to be about t

2 at SO ns and 90 ns. The author feels that the data he reports is OK. Another questioner reported

that they had been making diamondlike carbon films and had found that the properties of the

films depend strongly on how they are made. The optical gap of the films, for example, can

range from 1 eV to 3 eV. How are the films reported here made? The author replied that his

films are made by rf plasma assisted deposition from butane. Butane, propane, benzene

or an unsaturated hydrocarbon give the same results. The refractive index is exactly 2 and

reflectances are 0.1% or less. Absorption and damage thresholds may vary considerably in

production films. The films damage may be defect dominated, but the author feels that they

have reached the practical limit of about 3% per quarter wave or 250 cm-1 absorption coefficient.

There they are seeing about 7 J/cm axial flux, which is probably as well as one can do. A

member of the audience commented that if one believed the impurity dominated damage mechanism,

one can get pulse length dependence anywhere from 1/2 to linear depending on the distribution

of the particles, their properties and the variables in the experiment.

115
Manuscript Received

2-28-85 SAB

Surface Potential as a Laser Damage Diagnostic

M. F. Becker

Electrical and Computer Engineering Department

The University of Texas

Austin, TX 78712

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

Air Force Weapons Laboratory

Kirtland, NM 87117

We investigated the relationship between surface potential changes and N-on-1 laser

surface damage on a wide range of materials. The surface potential or work function difference was measured as a function of position by a small non-contacting Kelvin type probe.

This design and operation of the probe is described. Using this probe, the change in

surface potential due to laser irradiation was mapped with a mm resolution. Although

no consistent pre-damage changes in potential were observed, all larger damage features

had surface potential changes associated with them. The insulating materials studied,

fluoride and oxide thin films, bare fused silica and magnesium fluoride substrates, all

showed the accumulation of negative charge in areas more than ten times larger in diameter

than the laser beam spot or damage area. This initial charge was observed to decay on the time scale of hours to a lower fixed value of potential associated with permanent

damage to the surface.

Key words: laser damage, N-on-1 damage, surface potential, contact potential, work function,

surface charge, charge decay.

1. Introduction

This is the first reported study of the relationship between surface potential and laser induced

damage of insulating and semiconducting optical materials, and the first such study for metals damaged

at wavelengths shorter than 10.6 microns [1]. By surface potential we simply mean the difference

in work functions or the contact potential between two materials. Usually one material is employed

as a reference; stainless steel was used in this study. Surface potential is related to a number

of material surface properties which may be of interest in the study of laser damage. For metals

and semiconductors, surface potential is sensitive to band bending at the surface which can be related

to surface preparation procedures, fixed surface states, or adsorbates. For dielectric materials,

surface potential is sensitive to these same effects as well as to fixed charge either in the form

of surface or volume charge distributions or even permanent electric dipole states.

Cur interest in surface potential was aroused by our previous experiments utilizing charge

emission into vacuum as a diagnostic for the onset of laser damage or more importantly as a precursor

to laser induced damage [2-4]. Although no charge emission was observed prior to damage in N-on-1

tests for silicon and ThF4 thin films in these earlier experiments, all other materials, including

copper mirrors and several types of oxide thin films, showed charge emission at fluences as low

as 1/20 of the 1-on-l damage threshold. The copper mirrors exhibited a reduction of emission for

repeated shots to the same site (N-on-1) as one would expect in a conditioning or cleaning effect.

To further complicate the situation, all of these materials showed either accumulation or hardening

in N-on-1 tests. The idea of a non-contacting charge sensitive technique which could measure changes

in the surface state of a sample appeared an attractive method to study these N-on-1 effects.

Previously, Porteus, et al . [1] used Auger electron imaging as a qualitative measure of laser

induced changes in work function. In our experiments we have applied a different technique which

is capable of giving spatially resolved quantitative maps of surface potential over the region in

and around the laser interaction area. This technique utilizes what is known as a Kelvin probe

or the Kelvin method to measure surface potential without making physical contact with the surface,

and as such is also non-intrusive.

In this paper we first describe the Kelvin probe apparatus used in our experiments as well

as the other diagnostics. The sample set was chosen to include a wide variety of optical material

classes, including copper mirrors, silicon crystals, dielectric thin films and bare dielectric

116
substrates. After the experimental samples and cleaning procedures are described, data taken by

the Kelvin probe from a series of N-on-1 experiments will be presented. Finally, the implications

of these measurements for the understanding of laser induced damage will be discussed.

2. Experimental

2.1 Kelvin Probe

The Kelvin method for measuring surface potential is essentially one of adjusting the dc voltage

on the test capacitor formed by the sample surface and the reference electrode so as to null out

ac variations in the capacitor voltage caused by physically dithering the reference electrode. In these experiments, we used a feedback technique which would adjust the dc voltage for an ac null

automatically [5,6]. The probe assembly is shown in figure 1. The probe electrode tip is 1 mm

in diameter and is on a carrier which may be positioned over the laser beam axis and adjusted in

spacing from the sample or may be withdrawn when laser irradiation takes place. The sample location

is also mechanically controlled in order to position it to new irradiation sites and to scan the

sample under the probe in a raster pattern for measuring surface potential contours. Typical raster

scans were squares of either 5 mm or 10 mm on a side. Data points were taken at 0.25 mm intervals

on each row while the scan rows were separated by 0.25 mm for the small squares and 0.5mm for the

large squares. The scan rate of the stepper motors was the chief factor limiting data acquisition

speed. A magnetic drive is used to dither the probe at 88 Hz with a peak-to-peak amplitude of 0.1

mm. It requires a drive signal at 44 Hz of about 5 W.

In operation, the probe tip is placed so that its closest approach to the sample surface is

about 25 urn as viewed by a long working distance Questar microscope with a CCTV system. This view

is shown in figure 2. Since the capacitance between the probe and a grounded sample is about 0.1

pF and the capacitance with a dielectric sample 9.5 mm thick backed by a ground plane is considerably

less, eliminating stray capacitance was crucial [7,8]. To do this we mounted a low input capacitance

electrometer op-amp directly to the end of the probe arm. Other metallic objects were kept as far

away as possible from the oscillating probe tip. As a result, the probe sensitivity to surface

potential changes was less than 10 mV. This level is also of the same order as the noise level

and the reproducibility of the measurements.

The feedback circuit used to automatically adjust the probe dc voltage to be equal to the surface

potential is shown in figure 3. The preamp has an ac voltage gain of 11 while the lock-in amplifier

is adjusted for the maximum gain possible without oscillation with a 0.3 sec damping time. The

dc output of the lock-in is fed back through a high impedance path to supply the surface potential

to the probe. The dc output is also read by a digital voltmeter which was interfaced to the laboratory

mini-computer used for automatic data reduction. Only the critical adjustment of setting the probe

height above the sample surface was done manually. The probe was always scanned over the unirradiated

site to obtain a background potential map which was later subtracted from the data to obtain the

laser produced change in surface potential. The probe was next removed, the sample irradiated,

and the probe returned to scan for the data. Sequences of scans over time could also be programmed

in order to monitor the time decay of laser induced effects. The time needed to scan a 10 mm square

was 16 minutes, and about half that for the 5 mm square.

2.2 Diagnostics

The optical and diagnostic layout is shown in figure 4. The fundamental 1.06 urn wavelength

of a Molectron Q-switched Nd:YAG laser was used at a rep rate of 10 Hz. It was focused on the sample

with a 2 m focal length lens. Time and space profiles were checked regularly. The pulse length

was 18 ns FWHM, and the focused spot was typically 0.39 mm in diameter at the l/e^ points. The

beam was scanned in both the vertical and horizontal directions with a narrow slit at the focal

plane. An electromechanical shutter was used by the computer to control the irradiations. Pulse

energy for every shot was recorded and statistics were computed. The standard deviation in pulse

energies for 10 to 100 pulses was typically 1% or less and never greater than 3%.

After a sequence of sites had been tested on a sample it was examined under a Nomarski microscope

to determine the corresponding damage morphology. Although exact damage thresholds were not measured,

data was generally taken at fluences between 1/2 and 2 times threshold with an exposure of 10 or more pulses in order to attempt to observe pre-threshold as well as permanent damaging effects.

2-3 Samples

The sample set consisted of 0FHC diamond turned copper mirrors, single crystal [111] silicon

substrates, MgF2 half wave (at 1.06 urn) thin films on fused silica, Hf02 half wave thin films on fused silica, ThF4 half wave thin films on fused silica, bare oriented crystalline MgF2 substrates,

crystalline quartz and bare fused silica substrates. The silicon and fused silica substrates used

117
in this study were fabricated using the controlled grinding technique. Total integrated scattering

(TIS) measurements on witness samples indicated an average surface roughness of 5±2 A RMS for both

substrate types. TIS measurements were repeated on fused silica witness substrates after film deposition. 0 The measured surface roughness of half wave Hf02 films on fused silica was found to be

5±1.5 A RMS. In contrast, half wave MgF£ films on fused silica were found to have an average roughness

of 10±1.5 A RMS. In subsequent examination of these samples under the Nomarski microscope, only

the MgF2 films were observed to have a definite microstructure and a "parquet tiled" appearance

resulting presumably from columnar growth.

The cleaning procedure did not require touching the sample surfaces with any solid object.

The samples were cleaned in a photoresist spinner with deionized water and high purity acetone and

blown off with dry nitrogen. The dielectric samples were pre-cleaned by spinning on a collodion

layer and subsequently lifting it off to remove any tenaciously held particulates.

3. Experimental Data

In this section, we present a selection of typical data obtained by the Kelvin probe and correlations with microscopically observed damage morphology.

3.1 Dielectric thin films

Figures 5 and 6 illustrate the two graphic formats we used for data presentation. Figure 5 is a three dimensional projection plot of the potential change with a small, unsealed contour plot

below, and figure 6 is a full size contour plot. The large contour plots and the raw data arrays

were used to extract all numerical data since small changes in surface potential were readily apparent.

However, the three dimensional projections are more easily viewed, especially when the change in

potential is in an upward direction. For this reason, we show only projection plots in the remainder

of this paper with their potential axis polarities oriented such that the change in potential at

the damage site is always upwards.

The data in figures 5 and 6 is for an MgF2 thin film irradiated with 10 pulses at 69 J/cm?.

It is representative of all the thin films studied in these experiments. Note that the observed

potential change is negative for this sample. Subsequent microscopic examination showed large scale

damage covering the entire beam footprint. The profile of the change in surface potential was 4

to 6 mm in diameter with a magnitude of nearly half a volt. This diameter is distinctly larger

than both the laser spot and the resolution of the Kelvin probe. In addition, all large scale damaging events were detected by a similar potential change. No potential changes were detected when

laser damage was not observed. In these N-on-1 experiments, small damage pits were observed only

on the Hf02 film. About 50% of these small damage sites were detected by the Kelvin probe as small

changes in surface potential, while the remaining sites resulted in no observable change. The surface

potential change on these thin film samples was observed to decay with time. This effect and its

relation to surface charge will be discussed in a later section.

3.2 Conductors

The surface potential changes on damaged silicon and copper, although similar to each other

in diameter and magnitude, were opposite in polarity. In fact, copper was the only material that

showed a positive surface potential change when damaged. (Bare MgF2 substrates also showed a positive

potential change but only when bulk cracking was created by exit surface damage.) A typical surface

potential plot for single crystal silicon is shown in figure 7. The object to the left in the field

is the adjacent, previous damage site.

For silicon, not all microscopically observed surface damage could be detected by the Kelvin

probe. When pits were formed, indicating a more severe degree of damage, the potential changed

as shown in figure 7. However, when only melting and resol idification occurred with the accompanying

formation of ripples or ridges, no change in surface potential could be observed. No pre-damage

changes in surface potential were ever detected.

The OFHC diamond turned copper was always observed to damage by melt pit formation, and these

pits were detected by their accompanying changes in surface potential. As for the silicon, the

observed diameter was limited by the 1 mm resolution of the Kelvin probe tip. One case of pre-damage

change in the surface potential was observed for copper. In three other cases near the threshold

fluence where no observable surface damage occurred, no surface potential change could be detected.

The surface potential change for the conductive samples was found to be constant and reproducable over time. In this case, no decaying component was observed as was for the dielectric samples.

118
3.2 Bare dielectric substrates

The bare dielectric substrates seemed to be more unpredictable in their behavior. Experimental difficulty was experienced due to their tendency to damage on the exit surface. In N-on-1

experiments for large enough N, damage would propagate from the rear to the front surface before

surface damage was initiated on the front surface. Experiments were thus limited to less than 10-20

pulses per site.

A typical potential contour plot for bare Si O2 is shown in figure 8. Both fused silica and

the polycrystal 1 ine MgF2 substrates showed similar behavior. The surface potential change was 4

to 6 mm in diameter but smaller in magnitude than for the thin films. The Kelvin probe noise level

seemed to increase in the vicinity of the damage sites.

Microscopy of the damage sites on the bare substrates showed less distinct damage features

which resembled surface erosion. Larger diameter surface damage sites were detected by the Kelvin

probe while several smaller diameter damage sites and all undamaged sites showed no surface potential

change. These surface potential changes were observed to decay with time just like those for the

thin film samples, and will be discussed later.

The MgF2 substrates showed unusual behavior when a crack from the rear surface propagated to

the front. The arrival of the crack at the front surface would be accompanied by a sudden strong

positive change in the surface potential. This change may be associated with the piezoelectric

properties of the material or with the exoemission of electrons from the crack which leaves the

substrate positively charged [9].

4. Discussion

4.1 Surface charge density

One of the most interesting and unexpected findings in these experiments was the significance

of surface charge effects on the dielectric samples. First it will be necessary to relate the surface

potential measurements to surface charge density. In measuring contact potential as between two

conducting samples, the Kelvin probe separation from the surface does not affect the potential difference so long as the increase in distance can be compensated for by an increase in the gain in the

feedback loop. The case of free charge on the surface of a dielectric material is entirely different.

It resembles very closely the case of fixed charge in a Shottky or MOS device. The potential required

to place an equal and opposite charge on the probe tip is now dependent on the tip to surface distance

The surface potential is related to the surface charge density (ignoring fringing field effects)

by the parallel plate capacitor formula:

Q s /A = e 0 V/d (1)

where Qs is the total surface charge under the probe, V is the surface potential, and d is the mean

probe height over the surface. Typically, d was 70 urn so that a charge density of 1.26 x 10" ^ C/cm^

per volt of potential change was measured in these experiments. As an example, a spot 4 mm in diameter

with a potential change of 0.1 V would represent about 1.3 x 10^ negative charges. A rather large

amount of charge is spread from the 1/3 mm diameter laser damage site to a distance of several mm.

Closer analysis of the surface potential contour maps reveals that the effect is even more

widespread. The shape of the potential change peak is flat topped with a sharp drop at a diameter

of 4 to 6 mm. The drop is not to zero however, since there is about 10% remaining change in surface

potential which decreases slowly with distance for another several mm. This might lead an investigator to rethink the problem of site spacing for laser damage experiments on dielectric samples.

The charge related effects of a damaging event extend across the sample surface much further than

would be expected from either the observed damage morphology or even the incident beam diameter.

4.2 Charge decay

Detailed measurements of the surface potential decay as a function of time were made on the

Hf02 thin film and on the MgF2 bare substrate. The results for the two were similar and the thin

film data will be presented in detail.

Figures 9a-c show selected surface potential maps of the damage site at t=0, 1 hour, and 2 hours respectively. Note that each scan took about 16 min so that there was an initial delay of

8 min to scan to the beam center. Subsequent scans of the center were spaced by intervals equal

to the scan time plus a programmed inter-scan delay time.

119
In figure 9, the potential change is observed to decay without significant migration of the

charge. Presumably recombination, not diffusion, is responsible for the decay. It is not certain

whether the recombination charge comes from the air or the material; however, these sites could

be discharged artificially with airborne charge by using a static charge gun or by creating another

charge cloud from a nearby laser damage site.

The decay of the surface potential peak values obtained from the scans shown in figure 9 is

plotted in figure 10. From the simple linear graph, we infer a single exponential decay process

with a time constant of 62 min. The decay asymptote is not zero potential change. There is permanent

damage, and some fixed change is expected. In this case the fixed part of the potential change

is -25 mV as compared to the initial peak of -100 mV. As indicated previously, similar data was obtained for the MgF2 bare substrate for which a 30 min time constant was observed.

4.3 Conductors

Obviously no such free charge effects will be observed for conductive samples. An earlier

study using Auger analysis of damage sites on OFHC copper surfaces demonstrated the effects of surface

shape changes (pit formation) on the work function [1]. We also observed these effects on copper

with 1.06 urn illumination. There is no way of telling if the sign of the change observed by the

authors of reference [1] matches that measured by the Kelvin probe since they used a different method

which measured only qualitative potential changes. Similar potential changes were observed at damage

pits on silicon but of opposite polarity. There is no obvious reason why such a polarity difference

should exist.

4.4 Pre-damage effects

One of the objectives for undertaking these experiments was to observe sub-damage threshold

changes in the surface potential on those materials which emitted charge at 1/10 to 1/20 of the

threshold fluence, or showed accumulation or cleaning effects. In this respect we were unsuccessful.

One possible pre-threshold event was observed for copper out of four total observations. If a subthreshold surface potential effect exists, it is not large.

5. Conclusions

We observed distinct surface potential signatures associated with laser damage. For conductors, silicon and OFHC copper, small diameter surface potential changes were detected in conjunction with pit formation. No surface potential change was seen on silicon when only surface ripples

or ridges formed. Copper differed from silicon and all other materials in that the sign of the

surface potential change was positive.

All of the insulating materials showed surprisingly large diameter surface potential changes

around the laser damage spots. These potential changes were observed to extend over an area 4 to

6 mm in diameter as compared to the 1 mm diameter Kelvin probe resolution and the 1/3 mm laser beam

spot diameter. These charged areas contained as many as 10? negative charges. In light of this

large diameter charging effect, the spacing of adjacent sites in laser damage experiments on insulating substates and thin films should be carefully reexamined.

The charge on the insulators' surfaces was observed to decay with time constants on the order

of an hour to a constant level whose value is 1/4 or less of the initial value. We associated this

change with recombination, and the fixed change in potential with the effect of surface geometry

and damage morphology on surface potential.

No consistant pre-damage potential changes were observed indicating that the charge emission

and surface cleaning observed in previous experiments do not have a significant effect on surface

potential. Evidently these effects are not appropriate for study by surface potential methods.

This reseach was supported by an AF0SR/SCEE/RIP grant and by the DoD Joint Services Electronics

Program at The University of Texas.

6. References

[1] J.0. Porteus, D.L. Decker, D.J. Grandjean, S.C. Seitel, and W.N. Faith, "Defect-Damage Resistant

Copper Mirrors," in 11th ASTM Symposium on Optical Materials for High Power Lasers, NBS Special

Publication #568, Boulder CO (1979).

120
M.F. Becker, F.E. Domann, A.F. Stewart, and A.H. Guenther, "Charge Emission and Related Precursor

Events Associated with Laser Damage," 15th ASTM Symposium on Materials for High Power Lasers,

NBS Special Publication, Boulder CO (1984).

J.K. Jhee, M.F. Becker, and R.M. Walser, "Charge Emission and Accumulation in Multiple-Pulse

Damage of Silicon," 16th ASTM Symposium on Materials for High Power Lasers (elsewhere in these

proceedings), NBS Special Publication, Boulder CO (1985).

M.F. Becker, Y.K. Jhee, M. Bordelon, and R.M. Walser, "Charged Particle Exoemission from Silicon

During Multi-Pulse Laser Induced Damage," 14th ASTM Symposium on Optical Materials for High

Energy Lasers, NBS Special Publication #669, Boulder CO (1983).

Y. Petit-Clerc and J.D. Carette, "New Feedback Kelvin Probe," Rev. Sci. Inst. 39, 933 (1968).

J.C. Campuzano and R.G. Greenler, "Instrument for Combining Reflection-Absorption Infrared

Spectroscopy with other Surface-Sensitive Techniques," Rev. Sci. Inst. 5_2, 678 (1981).

R.J. D'Arcy and N.A. Surplice, "The Effects of Stray Capacitance on the Kelvin Method for Measuring Contact Potential Difference," J. Phys. D: Appl . Phys. 3, 482 (1970).

N.A. Surplice and R.J. D'Arcy, "A Critique of the Kelvin Method of Measuring Work Functions,"

J. Phys. E: Sci. Inst. 3, 477 (1970).

P. Braunlich, "Exoelectron Emission from Optical Surfaces," 2nd ASTM Symposium: Damage in

Laser Materials, NBS Special Publication #341, Washington, D.C. (1970).

MOTORIZED

PROBE >L MOTION Z MANUAL

LASER_

BEAM AXIS

CERAM I

1 1 1 1 j PREAMP CIRCUIT BOARD

ELECTROSTATIC

SHIELD

SOFT

IRON /

DRIVE

COIL

STAINLESS

PROBE ARM

SAMPLE

STAINLESS

PROBE TIP

1mm DIAMETER

X *

COORDINATE

SYSTEM

r

TIP VIBRATION:

100 pm PEAK-TO-PEAK

25 fi<r\ CLOSEST APPROACH

TO SAMPLE SURFACE

MOTORIZED

SAMPLE MOTION

SAMPLE AND MOUNT GROUNDED

Figure 1. Mechanical diagram of the Kelvin probe.

121
Figure 2. Photograph of the Kelvin probe over a silicon sample.

The probe is higher than its usual operating distance above

the sample surface.

ND FILTERS

ND: YAG LASER

0.4J -15 nsec.l .06/i.m

2 METER

FOCAL LENGTH WEDGE SHUTTER

SAMPLE

PYROELECTRIC

ENERGY METER

CAPACITANCE

PROBE

PDP II - 34a CONTROLLER POSITION

CONTROL

Figure 4. Optical layout and laser diagnostics.

122
Figure 5. Surface potential change contour plot for the MgF2 thin film

irradiated by 10 pulses at 69 J/cm2 . The format is discussed further

in the text.

CONTOUR PLOT

X-axis (cm)

Figure 6. Same subject as figure 5, but plotted in a different

format (see text).

123
CO Qo

UJ O

P

DC

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rsj o

U_

LU

>-

CO

CJ HH

4->re

4->c

QJE

3

S-

+->

QJ -Oo

s2

QJ

-l->

s-cn

re

•r" T3

(J I—

+-> re

E=

QJ

.£= o

I/O

QJ

S-cn

m O. O —

Q- <=> O VI CD > _l O — —

cc ac cd

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124
Figure 7. Surface potential change map for

single crystal silicon irradiated by 1 pulse at 3.7 J/cm2 . The feature at the

left is the adjacent previous damage site.

Figure 8. Surface potential change map for

bare fused silica irradiated by 10 pulses

at 140 J/cm 2 .

CO

0

i

0

0.102'

Figure 9. Surface potential maps for the Hf02 thin film irradiated by 10 pulses at

50 J/cm 2 showing the peak decay at times; a. t =0; b. t=l hour; c. t=2 hours;

(left to right).

125
t i i i i i r

1 1 i i i i i i

i i i 0 20 40 60 80 100 120 140 160 180 200

Time (min)

Figure 10. Surface potential peak height versus time for the same site on Hf02 as Figure 9. The constant -25mV has been subtracted since it is the

asymptotic value for the exponential decay.

One question was what effect the surface potential developed might have on scanning electron

microscope examinations done after the tests. The author replied that he could speculate only

since although he had had considerable experience with metallic and semiconducting substrates

but not on dielectrics. He would expect that charging would affect electron microscopy for

several hours after the tests were done. With SEM one would be charging the sample anyway, which

makes use of this technique difficult. In reply to the question of where the charging comes from in the case of laser damage, the author pointed out that when laser damage occurs, both

neutrals and charged particles are emitted. Presumably the electrical imbalance created by this

process is the origin of the observed charging. Another questioner pointed out that about

10 years ago the Russians put some electronegative gas on the surface of a material they were

trying to damage and saw no effect. Why? The author pointed out that the charge occurs after

damage. The electronegative gas would affect the decay time and what is seen after damage,

but would not be expected to affect damage initiation. Another question was why an effect

would be seen in a good conductor such as copper. The author replied that in good conductors

he did not believe that they were seeing surface charge but rather the effect of band bending

near the surface and changes in the band structure. Effects were only seen in connection with

melt pits for silicon and copper.

126
Manuscript Received

12-28-84 SAR

Charge Emission and Accumulation in Multiple-Pulse Damage of Silicon

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

Electronics Research Center and Electrical and Computer Engineering Department

The University of Texas at Austin

Austin, Texas 78712

To contribute to further understanding of damage mechanisms of silicon surfaces induced

by picosecond NdrYAG 1.06 urn laser pulses, damage at laser intensities below the 1-on-l

damage threshold has been studied. By using a biased charge collector or an electron multiplier tube, one can detect the charge emitted during small pit formation which may be considered as the initial damage morphology. The investigation of the accumulation period has been

conducted by observing the first charge emission which indicates its termination. This

accumulation time is analogous to the lifetime of the solids subject to a repeated mechanical

load and is characterized by processes that are irreversible for at least 3 seconds. This

suggests that the damage precursors are long-lived excitations or an accumulation of permanent

states. The heterogeneous nature of the nucleation of damage suggests that these precursor

states act as nucleation seeds to the laser induced damage. Charge emission after the incubation period was also studied. Positive and negative particles are emitted equally after

damage initiation, probably due to thermal evaporation of silicon from small regions. The

charge emission follows the Arrhenius relation. Experimentally, this result was independent

of pulse repetition period up to at least 10 seconds.

Key Words: accumulation; charge emission; N-on-1 damage; silicon.

1. Introduction

There have been several reports of undesirable laser-induced surface damage observed during the

illumination of semiconductors by intense laser pulses. The nature of this damage has been established

[1-3] by studying the nucleation and growth of damage near threshold intensities where the heterogeneous

nature of nucleation was confirmed. Furthermore, charge emission from a silicon surface induced by

picosecond laser pulses at 532 nm has been studied [4-6], but the energy transfer mechanism from the

radiation field to the semiconductor in the multi-pulse damage regime is not well understood.

Single crystal silicon, a covalent material, was chosen because it has an absence of polar optical

coupling, well characterized properties, high crystalline quality, and is technically important. Furthermore, Nd:YAG laser light (1.17eV) is ideally matched to the band gap of silicon (1.11 eV at room temperature). Near band gap excitation limits the heating of the sample by fast phonon decay of hot electrons

so that damage at fluences below the uniform melt threshold may be studied.

The objective of this study is to contribute to the understanding of the damage mechanism below

the 1-on-l damage threshold on silicon surfaces induced by picosecond Nd:YAG laser pulses at 1.06 u.m. Studies of "subthreshold" formation and evolution of damage under the action of laser irradiation may

be of considerable importance in understanding the energy transfer mechanism.

In this study, the detection of charged particle emission was used to investigate laser damage

on crystalline silicon surfaces at fluences below the 1-on-l damage threshold. We describe the results

of using this method to observe the accumulation period and dynamics of charge emission following the

accumulation period. The pulse repetition frequency dependence of these phenomena is also examined.

2. Experimental Method

Figure 1 shows the experimental arrangement. TEM00 mode pulses were obtained by the use cf a passively mode locked 1.06 nm Nd:YAG laser. The pulses had an average FWHM duration of 60 psec, determined by the use of an autocorrelation system utilizing sum frequency generation in KDP. From the

train of pulses, a single pulse was selected by an electrooptic cell which was positioned between the

output of the laser (with vertical polarization) and an output polarizer, and was activated by the

laser triggered spark gap.

A photodiode, PD-1, was used to trigger the microcomputer. For calibration, the Rj-7100 energy

meter from Laser Precision Corp. measured each pulse energy while the output of photodiode, PD-2, was

127
recorded by the microcomputer. These two quantities have a linear relationship for our experimental

energy range. Thus the carefully calibrated photodiode, PD-2, measured the energy incident on the

silicon sample. The standard deviation of the pulse-to-pulse laser energy stability was < 20%. Data

obtained with relatively stable sequences of pulses were selected and averaged. A 330 urn thick, high

resistivity, [100] silicon sample was place 8 mm behind the focus of 172 mm focal length lens where

the beam spot diameter was approximately 100 urn. The spot size at the sample was measured by scanning

with a narrow slit. Experiments were conducted on silicon samples cleaned chemically just prior to

their introduction into a chamber with a vacuum maintained < 10" 5 torr. The cleaned silicon sample

was used as one electrode while a spiral wire placed 2mm before the sample served as the other electrode.

A ±1300 V bias was applied between two electrodes. This potential was suficient to collect all emitted

charge and was above the saturation point of the collected charge vs. potential curve. At its highest

sensitivity, the detection limit of the charge collector was 10~"C. We also used an electron multiplier

tube, which magnified the charge signal ^ 10^ times, to investigate charge emission before damage initiation. The electron multiplier gave results identical to the wire charge collector.

3. The Accumulation Period

In this set of experiments, we investigated the behavior of silicon before damage was initiated.

The duration of the accumulation period at various pulse rates was studied as well as the absorption

properties of the material for laser fluences below the 1-on-l damage threshold.

3.1 Nonl inear Absorption

The absorption coefficient prior to damage initiation was determined from transmission measurements.

The result is plotted in figure 2, where the dots represent averaged values within a small interval

of laser energy. This data was adjusted for sample reflectivity before it was plotted, so intensity

is given within the silicon. The inverse energy transmission coefficient T"l for a Gaussian beam is

given by [7]:

>F I d-e-aL

)

2ae 9 " aL/f

Jo

£n[l + I (l-e"

aL

)I e"

x

] dx

(1)

where L is the thickness of sample, a is the one photon absorption coefficient, and 8 is the two photon

absorption coefficient. Equation (1) is used to derive 6 from the experimental results of Figure 2. A good least squares error fit is obtained with a value of a = 7.25 cm"* and 6 = 7cm/GW. We find from

this experiment that two photon absorption dominates during the incubation period. Direct two-photon

transitions and two-photon stepwise transitions may both be present in this experiment conducted at room temperature. Since pure two-photon absorption is small for this laser wavelength, two-photon

stepwise absorption, or free carrier absorption, presumably dominates in this case.

Though the silicon band gap is well matched with the photon energy of Nd:YAG laser, we cannot

neglect heating of the sample by fast phonon decay of hot electrons generated by free carrier absorption

during the accumulation period. We discuss this temperature rise in a later section. No anomalous

behavior or evidence of accumulation was observed in the nonlinear absorption measurements.

3.2 Intensity-Dependent Accumulation

In this section we describe measurements of the accumulation period dependence on laser fluence.

Though the conventional methods for defining damage have been applied widely, we used the method of

recording the first observable negative or positive charge emission. This emission was always observed

coincident with the first observable surface damage pits as seen by SEM. Many nondamaging experiments

were carried out to verify this fact. We note that a recent report of multiphoton photoemi ssion from

silicon at 1.06 urn [6] may be used to calculate the photoemi ssion electron fluence expected for our

geometry. The resulting maximum charge is two orders of magnitude below our reported detection limit.

In figure 3 we plot the number of laser pulses needed to initiate the first charged particle emission versus laser fluence at 1.06 urn. The experimental points were obtained by observing the first

positive charge emission or negative charge emission. Within experimental error, negative charge emission and positive charge emission gave identical results as shown in figure 3. The solid curve obtained

128
by the least squares fit of the given data can be represented by

i - (F-F

Q

) (2)

where F0 is the hypothetical multiple-pulse damage threshold fluence, or fatigue limit. Several thousand

shots were applied without observation of any charged particle emission at a fluence at or below 0.32

J/cm^. It is possible that no damage occurs even for N -> °° below this energy. This means that there

exists a multiple-pulse damage threshold or fatigue limit below which crystalline silicon is undamaged.

The sample surface did not undergo visible structural change during the incubation period as seen by

SEM or Nomarski microscopy. Surface changes were always observed coincident with negative and positive

charge emission even when we used an electron multiplier tube to magnify the charge emission signal

times. The first visible damage morphology was composed of small pits, about 0.3 urn in diameter

or larger. [8,9]

Experimentally, the 1-on-l damage probability P, determined by applying a single pulse to a number

of different sites, is not equal to the N-on-1 damage probability P', determined by applying a series

of pulses with the same intensity to a given damage site. That is, "spatial" statistics do not agree

with "temporal" statistics. This indicates that a purely random model does not adequately explain

the behavior in this case, and that the damage threshold decreases with an increasing number of laser

pulses. This suggests the irreversible accumulation of permanent material changes, or the reversible

accumulation of long-lived states.

The lucky electron model [10,11] of Bass and Barrett, which is an extension to the optical regime

of Shockley's experiments on dc breakdown in semiconductors, does not account for our experimental

results. In this model, the damage probability depends on the optical electric field but is independent

of the number of pulses. This conclusion leads to the concept of an accumulation of irreversible,

or perhaps, reversible changes that create the prerequisites for damage at subthreshold fluences.

To further determine the nature of the incubation period, the relation between the logarithm of

the laser fluence and the logarithm of the number of pulses needed to initiate damage was plotted in

figure 4. This plot was motivated by the work of Lee, Koumvakalis, and Bass [12] on the N-on-1 damage

of metal surfaces. The plot in figure 4 is the analog of a log(strain) vs. log(N) curve for mechanical

fatigue testing of materials. Although the two slope behavior resembles that of many metals and alloys,

there is no evidence for silicon that slip is accumulating as evidenced by changes in surface roughness.

No such changes were ever observed during the accumulation period for silicon.

Recognizing that the one-shot damage threshold is dependent on beam spot size, the experimental

threshold fluence can range widely for different focusing conditions. Some earlier experiments conducted

with large laser beam areas [13] reveal that the temperature rise at the end of the laser pulse is

only about several tens of degrees. The experiment reported here was done with a smaller beam area

and measured correspondingly higher laser damage fluences. This result suggests that excitations are accumulated due to laser irradiation in conjunction with the energy of local thermal fluctuations near

the peak thermal excursion, rather than due to the average temperature during the incubation period.

This result is based on the condition that the average temperature during the incubation period is

proportional to laser energy; but is very small in all cases, since the thermal relaxation time is

much shorter than the period between laser pulses.

These experiments conclusively establish that the number of pulses needed to initiate the first

visible damage on the silicon surface depends on the laser pulse energy. Reduced damage thresholds

due to repeated pulses must be explained by the cumulative nature of excitations rather than by probabilistic arguments devoid of time dependent correlation.

3.3 PRF Independent Incubation Period

To examine the cumulative damage effect more extensively, we measured the accumulation period

as a function of pulse repetition frequency (PRF). The same experimental configuration as in figure

1 was used. The laser beam was modulated by a mechanical shutter. Damage initiation was determined

by recording the positively charged particle emission which was coincident with it. The experimental

results are plotted in figure 5. Within experimental error, the accumulation period was independent

of pulse repetition frequency. This result rules out the possibility that a thermal accumulation of

absorbed energy occurred during the incubation period. It is possible, however, that there is a reversible process whose lifetime is over 3 seconds. The accumulation of long-lived excitations [14] or perma129
nent states caused by repeated laser irradiation is, therefore, a strong candidate. These states may

absorb more energy from the field like absorbing inclusions of small diameter in transparent materials.

The experimental temperature dependence of damage to silicon was reported by Sheng [13]. According

to his results, the total damage area was reduced as the temperature of the sample was increased. The

38 picosecond laser pulse length was much shorter than the 0.2 sec interval between laser pulses. Some

long-lived excitations may have decayed more rapidly during the interval between laser pulses due to

the elevated sample temperature. On the average then, fewer nuclei would have sufficient time to reach

a critical concentration prior to damage.

It is reasonable to assume that the accumulated long-lived or permanent states preferentially

absorb energy from the incident field, and act as nucleation seeds for damage initiation. This accumulation process is heterogeneous, and its heterogeneous nature is revealed by observing the surface damage

morphology as explained in Refs. [1-3].

4. Charged Particle Emission and Damage

In this section we report on our measurements of charged particle emission after damage initiation.

A dependence on laser intensity, but not on pulse repetition frequency was observed.

4.1 Intensity Dependent Charged Particle Emission

After the accumulation period, for which no visible damage was observed, positive and negative

charges were emitted equally from the silicon surface. The variation of such emission was so dispersive

at the early stages of damage evolution that it was very difficult to get information from each datum

of charge emission. However, when the data was averaged over small energy intervals, a reproducible

relationship between the laser pulse energy and total emitted charge was obtained. Negative and positive

charges gave the same information.

In a log-log plot the slope of the data fits a fifth power law quite well except in the higher

energy range where the slope is higher. This result suggests that five-photon processes might be responsible for initiating the emission of silicon ions. To investigate this possibility, we collected the

emitted particles on a microscope cover glass with the aid of a bias accelerating voltage. The collection pattern consisted of clearly visible spherical debris, similar to those seen near the surface damage

sites [8,9]. Individual particles were spherical in shape and might be re-solidified droplets of silicon. The thickness of the ejected material on the microscope cover glass could be clearly seen with

the naked eye as a darkening. The energy required to remove droplets composed of many atoms is certainly

higher than the energy of five-photons. Furthermore, the energy of the fastest particles, as measured

by time of flight, is much larger than the energy of five-photons. Thus lattice constituent emission,

due to multiple-photon process is unlikely, unless a larger energy release is triggered by a five-photon

process

.

We show this charge emission data on a semilog scale (see fig. 6) with the log of the average

charge emitted on the y-axis and the inverse laser fluence on the x-axis. This plot shows a linear

relation between the two quantities. This result suggests that the emission may be due to thermal

evaporation of silicon where the temperature of the emission site is linearly proportional to laser

pulse energy. In this case the emitted charge, N, follows an Arrhenius relation [5],

N = N exp

RT

N Q exp

k'F

(3)

where F is the laser fluence, N f and U 0 and k are constants is the energy of activation. This result

indicates that the laser energy^ absorbed first by free carriers, is transferred eventually to the

lattice and localized. The local temperature is raised high enough to evaporate silicon, and some

of the evaporated material becomes ionized. Thus charge emission would depend on local temperature

and follow an Arrhenius equation. Observations of the corresponding surface damage morphology [8,9]

show one or more pits of fairly uniform 0.3 micron diameter. Furthermore, the pits constitute a very

small percentage of the laser beam area at the initiation of damage.

For N-on-1 damage, we observed visible light emission coincident with surface damage initiation.

That no visible radiation is observed before damage is initiated confirms that the temperature at the

sample is much lower. When the vaporization temperature is reached locally and a plasma forms, there

will be considerable thermal radiation. These experimental results indicate that the final damage

130
event induced by a series of laser pulses is the result of local heating of the material at what are

now highly absorbing, submicron regions.

4.2 PRF Independent Charged Particle Emission

We discussed charged particle emission after the accumulation period in the previous section. To

find how the pulse repetition frequency affects this emission, we interrupted the incident beam in

the manner described previously.

The experimental results are shown in figure 7. This figure is a plot of positive charge emission

versus fluence for different PRFs. This experiment was conducted over a small range of energies near

the multiple-pulse damage threshold. Experimentally we could find little difference in charge emission

in spite of the large difference in PRFs. Furthermore, charge emission occurred at the expected pulse

number after a five minute interruption of the illumination.

This experiment leads to the conclusion that cumulative thermal effects, that result from stepwise

ratcheting increases of the temperature, are not the cause of this charge emission. Charge emission

is independent of PRF within the experimental error limit (laser energy fluctuations < 20%). Damage,

after the incubation period, is independent of the PRF and is apparently due to irreversible processes

or states having a lifetime of more than 10 sec.

4.3 Charge Emission Development

The iso-intensity variation of positive charge emission at 0.45 J/cm^ and 1.25 J/cm^ is plotted

in figure 8. These fluences are just above the multi-pulse threshold and just below the 1-on-l threshold, respectively. Note that the vertical scale on the lower curve has been magnified 10 times. Charge

emission varied rapidly for a small number of pulses in the early stage of damage evolution. At later

times, the charge emission became stabilized.

At higher fluence, the incubation period was very short and was followed by a rapid evolution

of damage. The initial damage morphology consisted of a few randomly distributed pit chains. After

the first emission event, charge emission increased very rapidly as seen in figure 8. A maximum emission

occurred approximately when the beam area was covered with a ripple pattern. This kind of large ripple

pattern is often observed for a wide variety of materials including semiconductors, metals, and dielectrics; and is well described by various authors. Our experiments revealed that this large ripple pattern

becomes well formed near the 1-on-l damage threshold. This is not the case for lower fluences near

the multiple-pulse damage threshold, because the center of the ripple pattern is destroyed before the

edges are fully formed.

5. Conclusions

An investigation of the accumulation period in the laser damage of silicon has been conducted

by observing the first charge emission event which indicates the termination of accumulation, and by

measuring optical transmission during this period. Negative and positive charges are emitted at the same time and give equivalent information. Nonlinear absorption is dominant before damage is initiated,

and it may be attributed mainly to two-photon stepwise absorption (free carrier absorption) rather

than simultaneous two-photon absorption. Thus carrier absorption may have an important role during

this period. No anomalous behavior was observed in the absorption measurements to indicate an accumulation mechanism. This accumulation effect in laser damage can be compared to the lifetime of solids

subject to a repeated mechanical load. Unlike the case of metal mirrors, no accumulating slip or surface

roughness was observed on silicon.

Our data suggests that long-lived excitations or permanent states may be accumulated due to laser

irradiation. The accumulation period was independent of the pulse repetition frequency and characterized

by irreversible processes for at least 3 seconds. This suggests that the precursors to laser damage

involve either long-lived excitations or permanent state accumulation. The heterogeneous nature of

the nucleation of damage suggests that these precursors may act as the nucleation seeds to laser induced

damage.

Positive and negative charge was emitted after the accumulation period in equal amounts. This

emission was probably due to thermal evaporation of silicon. The local temperature at the damage sites

appeared to increase linearly with laser energy, and the charge emission followed an Arrhenius relation.

Visible light was observed coincidently with the occurrence of surface damage and may be considered

evidence of high temperature thermal radiation. Experimentally these results were independent of pulse

repetition frequency and entirely exclude the possibility that thermal accumualtion occurred. Charge

emission increased as the damage extended to fill most of the beam area.

131
This reseach was supported by the DoD through the Joint Services Electronics Program, contract

F49620-82-C-0033.

6. References

[1] D.Y. Sheng, R.M. Walser, M.F. Becker, and J.G. Ambrose, Appl . Phys. Lett. 39, 99 (1981).

[2] R.M. Walser, M.F. Becker, D.Y. Sheng, and J.G. Ambrose, "Heterogeneous Nucleation of Spatially

Coherent Damage Structures in Crystalline Silicon with Picosecond 1.06 u.m and 0.533 urn Laser

Pulses," in Laser and Electron-Beam Solid Interactions and Materials Processing, T.J. Gibbons,

W. Hess, and T. Sigmon, eds. (Elsevier, New York, 1981) p. 177.

[3] M.F. Becker, R.M. Walser, Y.K. Jhee, and D.Y. Sheng, "Picosecond Laser Damage Mechanism

at Semiconductor Surfaces," in Picosecond Lasers and Applications, January 1982, Los Angeles,

CA, SPIE Vol. 322, p. 93.

[4] J.M. Liu, R. Yen, H. Kurz, and N. Boembergen, Appl. Phys. Lett. 39, 755 (1981).

[5] J. A. van Vechten, J. Appl. Phys. 53, 9202 (1982).

[6] T.L.F. Leung and H.M. van Driel , Appl. Phys. Lett. 45, 683 (1984).

[7] J.H. Bechtel and W.L. Smith, Phys. Rev. B13, 3515 (1976).

[8] R.M. Walser, M.F. Becker and D.Y. Sheng, "Laser Damage of Crystalline Silicon by Multiple

1.06 urn Picosecond Pulses," in 13th ASTM Symposium on Optical Materials for High Power Lasers,

NBS Special Publication #638, Boulder, CO, 1982.

[9] M.F. Becker, Y-K. Jhee, M. Bordelon, and R.M. Walser, "Charged Particle Exoemission from

Silicon During Multi-Pulse Laser Induced Damage," in 14th ASTM Symposium on Optical Materials

for High Power Lasers, NBS Special Publication #669, Boulder CO, 1983.

[10] M. Bass and H.H. Barrett, Appl. Opt. JL2, 690 (1973).

[11] M. Bass and H.H. Barrett, IEEE J. of Quant. Electron. QE-8, 338 (1972).

[12] C.S. Lee, N. Koumvakalis, and M. Bass, J. Appl. Phys. 54, 5727 (1983).

[13] D.Y. Sheng, Ph.D. Dissertation (The University of Texas at Austin, 1981).

[14] M. Bordelon, R.M. Walser, M.F. Becker, and Y.K. Jhee, "A Study of the PRF Dependence of

the Accumulation Effect in Multiple Pulse Laser Damage of Silicon," in Proceedings of the

14th ASTM Symposium on Optical Materials for High Power Lasers, NBS Special Publication

NBS, Boulder CO, 1984.

132
MICROCOMPUTER

Nd : YAG LASER

and

SINGLE PULSE

SELECTOR

PREAMP

RD. 1 RD.2

ATTENUATOR

7^

B.S.I B.S.2

BIAS

LENS

R

c —I hTo ION PUMP

VACUUM CHAMBER

Figure 1. Schematic of the experimental arrangement. The laser produced 60 ps, 250 u.J. pulses at a PRF of 13 Hz. The laser spot diameter was 100 urn, and the charge

detection limit was 10" ^c.

1.201 i i i i i i I I I I I 0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 20.00 22.00 24.00

Intensity (GW/cm2

)

Figure 2. Dependence of the inverse of transmission upon incident 1.06 urn laser flux for

[100] Si. The sample was 300 urn thick and the transmission has been adjusted

for reflection. Error bars show standard deviation of laser energy fluctuations.

(Pulse length = 60 psec.)

133
o

UJo

<

IU

UJH<

Q

LUQ

UJ

UJ

</> UJCO

_l

z>

u.o

or

2

120

100

80

60-

40

20

"NEGATIVE CHARGE

POSITIVE CHARGE

00 0.5 1.0 1.5

FLUENCE(J/cm2

)

2 0 2.5

Figure 3. Number of pulses to initiate exoemission and damage versus laser fluence. The

solid curve is inversely proportional to laser fluence. The curve diverges

at the multi-pulse damage threshold of 0.32 J/cm^.

0.2

0.1

n—

r

J L

S = -.13

J L

' 2 4 6 10 20 40 60 100 Number of Pulses Needed to Initiate Damage

Figure 4. Damage fluence versus number of pulses to damage in log-log format of a strain

vs. N curve. Slope S gives power law dependence for each segment of the curve.

134
0.60

0.00 40.00 80.00 120.00 160.00 200.00 240.00

Fluence (Arb. Unit)

Figure 5. Accumulation period at pulse repetition frequencies 1/3 Hz and 12 Hz. The line

is a least squares fit to the data.

FLUENCE" 1 (J /cm2

)

Figure 6. Charge emission versus the inverse of laser fluence for N-on-1 damage. The line is

a least squares fit to the data.

135
10°

<J i / 1U HZ O 00 0

(Arb.

Unil - • 13 Hz « O • •

0 •

•

O

•

:ed

Charge 10 2 — O 0

•

•

•

W

Emitl

10 1

9

0 0

10 15

I 20

III

25

Fluence (Arb. Unit)

Figure 7. PRF independent positive charge emission after damage at 13 Hz and 1/10 Hz.

1 x id

8 T

3

O -9 o 8 x 10

200 600 1000 1400

NUMBER OF LASER PULSES

Figure 8. Positive charge emission versus number of laser pulses at the same site (or

vs. time, at a 13 Hz PRF). The upper curve is for an energy density just below

the 1-on-l damage threshold, and the lower curve is at an energy density just

above the multi-pulse damage threshold (vertical scale lOx).

136
Manuscript Received

1-25-85 SAB

Laser Generated Ripple Patterns on Dielectrics and

Intermediate Band Gap Semiconductors

Nastaran Mansour, Giancarlo Reali, Patrizia Aiello, and M. J. Soileau

Center for Applied Quantum Electronics

Department of Physics

North Texas State University

Denton , Texas 76203

In this paper we report the results of studies of laser generated ripple patterns

on intermediate band gap semiconductors (e.g., ZnS, ZnSe) and dielectrics of various

indices of refraction by 10.6 ym laser pulses. The results confirm, only on a coarse scale, the scenario of the formation of coherent structures on the front or rear

surface of damaged materials. In particular, for a variety of transparent dielectrics, we carefully determined the ripple spacing through microscopic observation and by

diffraction pattern measurements. The ripple spacing and direction are consistent with

prior observation by other workers. Similar measurements on front and rear surfaces of

ZnS and ZnSe were also carried out revealing the presence of domains of regularly

spaced ripple formations and fine structures superimposed parallel to the main pattern.

Such fine structures are also present in the damaged dielectric samples, although they are perpendicular to ripple direction. Furthermore a "wrong" orientation, i.e.,

parallel to the incident field, of the induced gratings was observed. This points

toward a less naive interpretation of laser induced ripples than has been described in

the literature thus far.

Key words: laser-induced surface damage, dielectric materials, intermediate band gap

semiconductors, exit surface damage, ripple structures.

1. Introduction

Laser generated ripple patterns have been studied extensively for dielectrics [1-11], narrow

band gap semiconductors [12-22], and metals [3-5,20,22-28], For the most part these results are explained qualitatively by one or more of the models proposed for the generation of these

features [2,9,30,31,32]. Most of the research reported over the past two decades led to an explanation of ripple spacing and direction independent of the dynamics of pattern growth [29].

Laser generated ripple patterns observed in wide band gap dielectrics, small band gap semiconductors and metals are very similar in that they run normal to applied optical field, but they

differ in the ripple spacing for a given optical frequency. It has been observed that for small

band gap semiconductors and metals the ripple spacing is equal to the optical wavelength in vacuum (^ 0

)» while for wide band gap dielectrics the ripple spacing is approximately equal to the

optical wavelength in the material (* 0

/n). The different behavior of small band gap semiconductors and metals, and wide band gap dielectrics led us to investigate intermediate band gap

materials, such as ZnS and ZnSe. In addition we started a more detailed experimental study of a variety of dielectric materials. All the results reported here are for 10.6 ym laser induced

surface structures. The experimental setup will be described in Section 2. The results of the

laser induced surface structures in dielectrics and ZnS and ZnSe will be presented in Section 3. A summary of the results is contained in Section 4.

2. Experimental Setup

The laser source used in this study was a linearly polarized TEA CO2 pulsed laser operated

in the TEMqq spatial mode at 10.6 ym. A cw, inside-the-cavity injection locking section provided

single longitudinal mode operation. The resulting pulses had a smooth Gaussian spatial distribution and a smooth temporal profile as shown in Fig. 1. The temporal pulsewidth was approximately 150 nanoseconds [full width at half maximum (FWHM)], as monitored by a photon drag detector and a fast 7834 Tektronix oscilloscope (500 MHz bandwidth). A 5 inch focal length lens was

used to produce a focal spot of radius approximately equal to 100 y m (HW 1/e M of the irradiance)

on the surface of the samples. A rotating Brewster angle polarizer - wire grid polarizer combination was used to vary the irradiance on targets while keeping the polarization at the samples

137
constant. Shot-to-shot energy variation was continuously monitored by pyroel ectri c detector

which was calibrated with respect to a GenTec ED-200 energy monitor. A HeNe laser beam

(632.8 nm) was used to illuminate the irradiated site in order to provide a diffraction pattern

of the damaged regions. A Nomarski microscope with a maximum magnification of HOOx allowed the

surfaces to be viewed in real space.

3. Experimental Results

Figs. 2-10 are Nomarski micrographs of laser generated ripple patterns on surface of various

dielectrics (SrF£, BaF2> KC1 , NaCl , KBr) and intermediate band gap semiconductors (ZnS, ZnSe).

The laser irradiation for all the samples was linearly polarized and normally incident on the

surfaces. The arrows show the direction of the applied electric field. The results of the

measurements of ripple spacings are summarized in Table 1. The refractive indices are all at room temperature.

Previous studies of laser-induced ripple patterns on dielectric surfaces showed that the

spacing of the ripples produced in such a material is approximately A Q /n, and they have been

consistently interpreted by a simple electrostatic dipole model [2].

An alternative model, assuming surface excitations-laser coupling, has been suggested by

other authors [9,28]. This model requires that, during its thermodynami cal evolution, the

exposed material acquires a negative refractive index to satisfy momentum conservation between

incident light and surface excitations.

We have found that the spacing of the ripples, produced by single shot irradiation, is AQ /n on the exit surface of all the tested dielectric materials. This result was also confirmed by

diffraction pattern measurements. In the case of dielectric materials the ripples run perpendicular to the applied electric field. In the presence of defects (e.g., scratches, digs, etc.)

the ripple growth is in the privileged direction of the normal to the symmetry axis of the

defect. This clearly indicates the relevance of seeds in starting the coherent process [3]. We

notice that the characteristics of the ripple formation in SrF£, BaF2, KC1 , NaCl and KBr are very

similar, and that the maximum uncertainty in the Ag/n-ripple spacing is at most of 5%, as deduced

by the statistical analysis of the determinations of the spacings over several sample pictures.

These results are in agreement with the model of Temple and Soileau [2] as well as with a more sophisticated analysis by Sipe et al. [30]. Instead, the surface excitations-model [9,28] seems to be excluded by the fact that 10.6 ym radiation is very unlikely able to match surface

polariton dispersion relation of any of the 5 dielectrics examined. [For the dielectrics tested

the Reststrahl frequencies vary from 0.35 x 1014Hz (KBr) to 1.13 x 10 14Hz (SrF 2

], Figure 7 shows

laser generated ripple patterns on the front surface of ZnS. The ripples are formed perpendicular to incident field with spacing near to A0 (Table 1). On the exit surface of ZnS we found

ripple spacing of approximately AQ /n and also periodic surface structures with variably spaced

ripples and wrong direction (parallel to applied electric field). In this sample, ripple structures are generated by multiple shots near the surface damage threshold. The larger uncer- tainties in the measured spacing in this material and in ZnSe are due to the fact that the ripple

structures are not as well coordinated or as sharply defined as those observed for the dielectrics .

Figures 8 and 9 are Nomarski micrographs of ripple patterns on the front surface of ZnSe

which are induced by multiple shots just above surface damage threshold of this material. In Fig. 9 we moved the target parallel to direction of applied field. The ripples were formed by

multiple shots in the neighborhood of previously formed ripples. We have observed ripple

spacings at near Ag and A 0 /n on the front surface of ZnSe with direction perpendicular to laser

electric field. Similar damaged sites on the exit surface of ZnSe were also produced [Fig. 10].

The resulting ripples on the rear surface are in the "wrong" direction, i.e., parallel to the

applied optical field and have spacing of 2A0 /(n+l). The appearance of the surfaces of ZnS and

ZnSe as viewed by a Nomarski microscope suggest that the process of growth and formation of

ripples is quite different from that which produced the ripple in the dielectric materials. The

geometry of the peaks and valleys of the ripples, and the general appearance of the ripples on these semiconductors are clearly different than that observed in the dielectrics. The periodicity of patterns are disturbed in some regions of the irradiated site and finer structure is

superimposed on the main pattern.

4. Summary

We have presented results of a study of laser induced ripple formation at 10.6 ym on dielec138
tries of various index of refraction (SrF 2 , BaF 2

, KC1 , NaCl , KBr) and intermediate band gap

semiconductors (ZnS, ZnSe). Ripple patterns with spacing of A Q /n were formed on the exit sur- faces of the dielectric materials perpendicular to direction of applied field. We observed A spaced ripples on front surface and ripple patterns with spacing of approximately A Q /n on exit

surface of ZnS. We found ripple spacings of approximately \ Q and A /n on the front surface of

ZnSe. These ripples occurred perpendicular to the laser electric field at both surfaces of ZnS

and ZnSe. However, on the exit surface of ZnSe we observed periodic structure with "wrong"

orientation (ripples parallel to the applied electric field) having spacing of 2A 0 /(n+l).

Furthermore, we observed wrong direction, variably spaced ripples on the exit surface of ZnS and

ZnSe. All these observations taken together cannot be completely described by existing models in

the literature so far.

The authors acknowledge the support of the Office of Naval Research and the North Texas

State University Faculty Funds.

5. References

[I] P. A. Temple and M. J. Soileau, Nat. Bureau of Standards, Boulder, Colorado, Rep. 462, pp.

371-378, 1976.

[2] P. A. Temple and M. J. Soileau, IEEE J. Quantum Electronics, Vol. QE-17, No. 10, 1981.

[3] M. J. Soileau and E. W. Van Stryland, Nat. Bureau of Standards, Boulder, Colorado, Rep. 669,

pp. 406-414, 1982.

[4] M. J. Soileau and E. W. Van Stryland, J. Opt. Soc. Amer., Vol. 72, p. 1756, 1982.

[5] M. J. Soileau, IEEE J. Quantum Electronics, Vol. QE-20, No. 5, pp. 464-467, 1984.

[6] M. J. Soileau, J. M. Bennett, J. 0. Porteus, W. Faith, J. Jernigan, and T. T. Saito, Nat.

Bureau of Standards, Boulder, Colorado, Rep. 462, pp. 149-157, 1976.

[7] M. Bass, K. M. Leung, C. Tang, and M. J. Soileau, Nat. Bureau of Standards, Boulder,

Colorado, Rep. 462, pp. 136-144, 1976.

[8] M. J. Soileau, J. 0. Porteus, and D. L. Decker, Appl. Opt., Vol. 19, pp. 3043-3044, 1980.

[9] F. Keilmann and Y. H. Bai , Appl. Phys. A, Vol. 28, pp. 1-10, 1982.

[10] C. T. Walters, Appl. Phys. Lett., Vol. 25, pp. 696-698, 1974.

[II] M. Siegrist, G. Kaech, and F. K. Kneubuhl , Appl. Phys., Vol. 2, pp. 45-46, 1973.

[12] M. Birnbaum, J. Appl. Phys., Vol. 36, pp. 3688-3689,

[13] D. C. Emmony, R. P. Howson, and L. J. Willis, Appl. Phys. Lett., Vol. 23, pp. 598-600, 1973.

[14] G. N. Maracus, G. L. Harris, C. A. Lo, and R. A. McFarlane, Appl. Phys. Lett., Vol. 33, pp.

453-455, 1978.

[15] H. J. Leamy, G. A. Rozzonyi , T. T. Sheng, and G. K. Celler, Appl. Phys. Lett., Vol. 32, pp.

535-537, 1978.

[16] M. Oron and G. Sorensen, Appl. Phys. Lett., Vol. 35, pp. 782-784, 1979.

[17] P. M. Fauchet and A. E. Siegman, Appl. Phys. Lett., Vol. 40, pp. 824-826, 1982.

[18] J. F. Young, J. E. Sipe, J. S. Preston, and H. M. van Driel, Appl. Phys. Lett., Vol. 41, pp.

261-264, 1981.

[19] J. F. Young, J. E. Sipe, and H. M. van Driel, Phys. Rev. B, Vol. 30, pp. 2001-2015, 1984.

[20] J. F. Fiqueria and S. J. Thomas, Appl. Phys. B, Vol. 28, p. 267, 1982.

139
[21] N. Tsakada, S. Sugata, and Y. Mita, App] . Phys. Lett., Vol. 42, pp. 424-426, 1983.

[22] J. F. Young, J. S. Preston, H. M. van Driel, and J. E. Sipe, Phys. Rev. B, Vol 27 dd

1155-1172, 1983. '

*

[23] M. A. Cutter, P. Y. Key, and V. I. Little, Appl. Opt., Vol. 13, pp. 1399-1404, 1974.

[24] J. C. Koo and R. E. Dusker, Appl. Phys. Lett., Vol. 28, pp. 614-616, 1976.

[25] N. R. Isenor, Appl. Phys. Lett., Vol. 31, pp. 148-150, 1977.

[26] R. K. Jain, V. N. Kulkarmi, D. K. Lood, and J. S. Uppal , J. Appl. Phys., Vol. 52, pp. 4882-

4884, 1981.

[27] S. J. Thomas, R. F. Harrison, and J. F. Figueira, Appl. Phys. Lett., Vol. 40, pp. 200-202,

1 98 2 •

[28] S. R. J. Brueck and D. J. Ehrlich, Phys. Rev. Lett., Vol. 48, pp. 1678-1681, 1982.

[29] H. M. van Driel, Laser Focus, Vol. 19, p. 12, 1983.

'* 30

"' 1141 " 1154

6

'1983

F

* Y ° Ung ' J * S

' Preston ' and H « M - van Driel, Phys. Rev. B, Vol. 27, pp.

[31] Z. Guosheng, P. M. Fauchet, and A. E. Siegman, Phys. Rev. B, Vol. 26, pp. 5366-5381, 1982.

[32] R. Ritchie, Surface Sci., Vol. 34, pp. 1-19, 1973.

Index of

Refraction XQ

/n

Ripple Spacing

on Back Surface

Ripple Spacing

on Front Surface

Material n (um) (um) (um)

SrF 2

1.36 7.79 *7.76l0.13

BaF2 1.39 7.62 7.60l0.08

KCI 1.45 7.31 6.9810.06

NaCI 1.49 7.11 *7.09l0.06

KBr 1.52 6.95 *6.77lo.14

ZnS 2.20 4.82 5.2210.28 9.5210.46

ZnSe 2.40 4.42

** 6. 2410.1

**7.73l0.1

**9.29l0.1

4.2710.30

9.5010.39

* Checked by diffraction pattern measurements.

** Ripples not perpendicular to the incident field.

Note that 2X0

/(n+1) = 6.24 um

Table 1.

140
141
10UM

Figure 2. Laser generated ripple patterns on the exit surface of SrF? at 10.6 ym. The arrow shows the direction of the applied optical field. The ripple spacing is

7.76 ± .13 ym. Note that A Q /n=7.79 ym.

10 UM

Figure 3. Laser generated ripple patterns on exit surface of BaF2 at 10.6 ym. The laser was linearly polarized in the direction indicated by the arrow. The ripple spacing is

7.60 ± .08 M m. Note that AQ /n = 7.62 y m.

142
10 UM

Figure 4. Laser generated ripple patterns on the exit surface of KC1 at 10.6ym. The arrow

shows the direction of applied electric field. The ripple spacing is 6.98 ± .06 ym.

Figure 5. Laser generated ripple patterns on exit surface of NaCl at 10.6 ym. Note that

ripples formed parallel to existing scratch on the surface. The ripple spacing is

7.09 ± .06 ym and A0 /n = 7.11ym.

143
Figure 6.

10 UM

Laser generated ripple patterns on the exit surface of KBr at 10.6 ym. The arrow shows the direction of the applied optical field. The ripple spacing is

6.77 ± .14 ym.

Figure 7. Laser generated ripple patterns on front surface of ZnS at 10.6 ym. The arrow shows

the direction of the applied optical field. The ripple spacing is 9.52 ± .46 ym.

144
10 UM

Figure 8. Laser generated ripple patterns on front surface of ZnSe at 10.6 ym. The ripple

spacing is 9.50 ± .39 ym.

K

10 UM

Figure 9. Laser generated ripple patterns on the front surface of ZnSe at 10.6 ym. The ripple

spacing is 4.27 ± .30 y m.

145
10 UM

Figure 10. Laser generated ripple pattern on the exit surface of ZnSe at 10.6 ym. The ripple

formed is the "wrong" direction, i.e., ripple parallel to the applied optical field

as indicated in the above micrograph.

146
Manuscript Received

1-22-85 SAR

Surface Damage Mechanisms in Nontransparent Media

P.M. Fauchet* and A.E. Siegman*

*Department of Electrical Engineering and Computer Science, Princeton University

^Edward L. Ginzton Laboratory, Stanford University

Abstract

We review our experimental and theoretical work on

surface transformations produced on amorphous and crystalline semiconductors and metals by single or multiple

ultrashort laser pulses. In addition to melting and vaporization, two phenomena have attracted our attention: the

formation of surface ripples and transformations due to

repetitive subthreshold illumination.

A detailed theoretical model that explains successfully

the formation and the major properties of the surface ripples has been developed and we illustrate the agreement by

many examples. However, we also present experimental

results which suggest that to describe the interaction

between the incident beam and the surface diffracted

waves, a perturbative approach is not always sufficient. A

phenomenological explanation is presented for these non- standard ripples.

Repetitive subthreshold illumination can produce the same surface transformations as single pulse illumination

above threshold. We present universal growth curves and

determine a safe level for multiple shot exposure in various

materials. Our results are compared to several models,

including accumulation of plastic deformation, creation of

microcrystallites, and the early stages of ripple formation.

1. Introduction

We have performed our experimental work exclusively

in nontransparent media. Although transparent media are often of more interest to the high power laser community,

experimental results obtained on semiconductors or metals can be compared to theory and, at least for single shot

experiments, the agreement is quite good. The reason for

this is that the index of refraction n has an intrinsic nonzero imaginary component (n = n + jk). The intrinsic

absorption coefficient a = k is therefore finite and the X penetration depth 6 = l/a is small. In most of the experiments reported here, 6 1 fim, but in principle 8 may

vary from ~ 1 mm (photon energy at the bandgap) to ~ 100A (for metals).

In semiconductors, n ~ 3-4 and the reflection

coefficient R = |rc-l/n + l| 2 ~ 0.3; while for most

metals, R > 0.9. Thus, defects or impurities may play a

significant role in laser-induced damage of metallic surface,

by locally increasing the absorptance A = 1 - R. Semiconductors on the other hand are "perfect" materials. First,

they can be made very pure and defect-free. Second, their

intrinsic absorptance is quite large and not likely to be

altered by any remaining imperfections. We can also

predict that intrinsic thermal effects are likely to be important. Our experiments were performed on amorphous (a-) or crystalline (c-) Si, Ge, GaAs and Cu in bulk or thin film

form. The laser pulses were provided by modelocked

Nd:YAG laser systems (X = 1.06 /im or 532 nm) or by a Q-switched Nd:YAG laser (X = 1.06 /im).

2. Single Pulse Damage

2.1 Melting and Vaporization

The simplest form of damage is melting or vaporization. It has been known for years that intense laser pulses can melt and/or vaporize semiconductors. In some cases

(e.g., laser annealing of disordered layers), the laser action

is beneficial. Picosecond laser pulses have now been shown

to produce similar transformations. In fact, picosecond

laser-induced transformations include crystalline to amorphous, amorphous to crystalline and crystalline to crystalline (through a molten period) transitions [l]. A reliable theoretical description of these phenomena

is now available. Models have been developed to account

for the temporal and spatial evolution of the lattice temperature and the electron temperature and density. For

example, the melting threshold, the electron temperature

(that may be much greater than 2000 ° K) and carrier density (that may exceed 1021 /cm3

) predicted by theory agrees

with the experimental observations [2]. In fact, we have

been able to use this successful theory to predict some elec- tronic properties of highly excited disordered semiconductors [3]. The experimental set-up is described in figure 1. A

delay is introduced between two synchronized picosecond

pulses. The visible pulse (532 nm) creates a dense

(> 1020 /cm3

) electron-hole plasma at the surface of the

semiconductor. This plasma then absorbs the infrared

pulse (1.06 /im) by free carrier absorption (FCA). In the

absence of visible pulse, its intrinsic absorption of the sample in very low (no; ~ Egap for Si, two-photon absorption

for GaAs). In the presence of the plasma, the coupling of

the infrared pulse to the solid is greatly enhanced. The

plasma, heated by the infrared light cools down in a few

picoseconds, thus transferring energy to the lattice in the

form of phonons. Figure 2 shows some computer simulations for a-Si. For negative delays (infrared before visible),

heating occurs through the intrinsic absorption; for small

positive delays, FCA dominates; for larger delays, the

plasma density has decreased when the infrared pulse

arrives, thereby decreasing its coupling. Note that the

147
electron-hole plasma density is maximum for the largest

temperature rise: this is attributed to the increase of a

with temperature.

We have applied this idea to disordered Si and GaAs

(as well as c-Si). Some of our results are shown in figure 3. They are in good qualitative agreement with the model

outlined above. In fact, a quantitative agreement can be

obtained for small delays if one assumes a 0.1 ps relaxation

time for hot electrons, a 10 ps recombination time and a

cross-section for free carrier absorption equal to 10- 17(T/300)cm2

. A more complete discussion of these and

other results can be found elsewhere (4). We want to stress here that it is possible to have a very good description of

single-shot melting of semiconductors.

2.2 Surface Ripples

Another common damage mechanism involves the for- mation of ordered surface structures, commonly called sur- face ripples [5]. In this section, we concentrate on stan- dard ripples produced by single-shot illumination.

It is now well known that relatively intense c.w. or pulsed laser beams can produce ordered surface structures on most materials. Although such laser-induced surface

ripples had been observed for fifteen years, activity in the

field started to peak up in the early 80's, when various

groups published a detailed theoretical modelling of the

electromagnetic interactions that lead to ripple's formation.

We now describe and compare briefly the main features of some models and list their successes.

Experimentally, surface ripples are produced during

laser damage, laser annealing, laser- assisted deposition or

etching, etc. They almost always appear close to the melting or damage threshold and they appear in all materials

illuminated with light pulses at various wavelengths (from

10 ^m to the U.V.) and of various durations (from ps to

c.w.).

In all existing models, ripples result from interference

between the incident beam and surface waves. Although it

is believed that minute surface irregularities (of dimensions

smaller than X) are necessary for the production of the sur- face waves, there is a controversy on the exact nature of

these waves. Among the candidates are surface polaritons

[6] and radiation remnants [7]. In our theoretical work [8], we have described the interaction of the incident beam

with surface diffracted waves on a corrugated surface (with a sinusoidal profile) or on a flat surface (where the dielectric function e has a periodic sinusoidal variation).

The nature of the surface wave is revealed by applying the appropriate boundary conditions. After solving

Maxwell's equations by first-order perturbation theory, we

predict ripples with a periodicity identical to that given by

the surface polariton model, if Re(t) = e R is negative [lj. For e R > 0, however, we find that ripples can grow with a spacing A well described by X = A/n(l ± s'mu), where n is the refractive index of the incident medium (= 1 if air or vacuum) and v is the angle of incidence. The ripple

growth coefficient is then smaller than for metallic-like sur- faces (l).

In all models, first-order perturbation theory describes

the interaction (e.g., h/A « 1, where h is the height of

the corrugation) and the properties of the substrate (e.g.,

crystalline orientation) play an insignificant role. Furthermore, theory has not combined the periodic variation of e during illumination with the effect of surface corrugation.

This periodic variation of e is especially striking in the for- mation of aligned, coexisting liquid and solid lamellae during c.w. illumination [9]. Recently, permanent variations

of e have also been observed after U.V. illumination of

metal films [10). Thus, although we do not have a complete theoretical

description for the creation of surface ripples with singleshot illumination, we find a rather good agreement

between theory and experiments.

3. Multipulse Damage

3.1 Above Threshold

Repeated illumination with identical pulses of intensity above single pulse threshold leads to fascinating but

poorly understood nonlinear surface electromagnetic

interactions. What is observed is the formation of non- standard ripples, which cannot readily be explained by any

model. Although several recent reports are concerned with

such ripples, we concentrate on our results. During a multipulse sequence, the amplitude of the

surface corrugation increases (probably due to repeated

localized vaporization), so that the ripple's depth h

becomes a fraction of its period A. Once the aspect ratio ( = h/A) of standard ripples becomes comparable to unity,

additional illumination may lead to the formation of per- pendicular ripples (running at 90 ° to the standard ripples).

Figure 4 shows typical results after repeated illumination

of Ge above threshold for ripple formation. Here, the

periodicity appears equal for the two types of ripples and

the surface looks like a checkerboard. Often, the two spacings are different [12]. In some cases, instead of a regular

pattern, we have observed [5] perpendicular stripes with a

periodicity that is not very sharp and appears to vary with

the intensity. Figure 5 shows such ripples on GaAs. In another experiment, we have observed [ll] that repeated

pulsed illumination of <111> Ge by normal-incident,

circularly-polarized 1.06 /im light produced hexagonal patterns, reminiscent of the crystal orientation, as shown in Figure 6. Note that similar nonstandard ripples have been

observed by others [13].

It is quite clear that existing theories must fail for large aspect ratios. Our calculations indicate that the

modulation of absorbed power due to surface corrugation

reaches 100% before h/A = 1. Then regions where the

power flow is maximum alternate with regions where the

power flow is zero. Consequently, interactions between

different surface diffracted waves has to be considered.

Furthermore, the dielectric function is also expected to vary strongly. Theory will break down where the magnitude of the surface wave becomes comparable to that of

the incident beam as in the case of illumination of a sur- face having deep ripples. Since we have shown that the

ripple's growth coefficient is larger for metallic-like surfaces

(tp < 0), nonstandard ripples will also be produced more

easily during U.V. laser-assisted growth of metal films. We

have not developed our model beyond first-order perturbation theory, but we have recently considered various

mechanisms and attempted a phenomenological description

148
of the surface interactions [14]. For example, we have been

able to recover the observed diffraction pattern of the surface illuminated by a visible probe beam by considering

appropriate mixing among various surface diffracted waves. This approach unfortunately does not give us much insight

in the nature of the surface interactions. New surface waves can exist at a nonlinear interface [15]. As we discuss

elsewhere [14], it is plausible that by considering the complete nonlinear, inhomogeneous dielectric function of the

solid surface, we can predict the observed nonstandard ripples. However, this problem is very difficult and it is not

at all certain that a satisfactory solution can be reached.

We thus see a first example for the failure of singlepulse models when we try to use them to explain multipulse effects.

3.2 Below Threshold

It is well known that repeated pulsed laser illumination of solids with identical pulses of intensity below single

pulse threshold may eventually produce irreversible

transformations including pitting, melting and cracking.

Experiments have now been performed on insulators, semiconductors and metals, with very similar results. We

review our experimental finding and then discuss the

various models that have been proposed.

For these experiments, we have used an actively

modelocked, repetitively Q-switched Nd:YAG laser. Prelasing improved the energy stability. Histograms of the

pulse energy were obtained after selection of a single pulse.

Approximately 95% of the shots have an energy within 7%

of the mean energy; the remaining 5% have lower energy

and correspond to mistriggering of the Pockels cell. The

transverse beam profile is checked with a standard Reticon

array and the spot size is known accurately by varying the

beam energy and recording the damaged spot size. Figure

7 shows how the spot size and the single-shot threshold can be simultaneously measured. In all cases, the samples were simply cleaned and held at room temperature in air. To determine the onset of damage in a multipulse

sequence, we used optical microscopy (Nomarski), scanning

electron microscopy, and, occasionally, the probe beam

scatter technique or electrical measurements. Nomarski

microscopy proved to be the most reliable method.

Figure 8 illustrates some typical results [16]. We plot

the damaged or transformed area (as measured by optical

microscopy) versus number of laser shots. The single-shot

threshold is ~ 2.2 GW/cm2 for c-Si illuminated with 80 ps

pulses at 532 nm. Repeated illumination with pulses having one-third of this intensity produces visible damage

after ~ 300 pulses. Repeated illumination with pulses

having one-fifth of the single-shot threshold intensity pro- duces no visible damage even after 6000 shots. The optical

fatigue of the material is also clearly illustrated by the

inset of figure 8. The threshold for damage deduced from

these curves decreases with increasing number of pulses

and appears to tend asymptotically towards a safe level. This particular experiment was performed at 100 Hz

repetition rate. However, we find no repetition rate dependence of these curves within the experimental uncertainty

of the measurement [16]. To further illustrate this, we plot

in figure 9, the reduction of the damage threshold of ionimplanted GaAs with increasing number of shots. These

data represent an average of seven runs at three different

repetition rates (5, 10, and 40 Hz). We note that these curves are in qualitative agreement with those obtained on

other materials by other workers [17,18].

In addition, we note that our results are deterministic: two multipulse sequences lead to similar optical

fatigue curves. We observe no striking spot size dependence, although the limited output energy of our laser did

not allow us to vary this parameter over a wide range.

Finally, for crystalline materials, we usually find that the

safe level for multipulse exposure was limited to one-third

of the single pulse threshold; while in amorphous materials,

the safe level could become as low as one-tenth of the sin- gle pulse threshold.

In the light of these remarks, we have rejected the

hypothesis of a slow temperature increase of the surface

during the sequence. Two hypotheses are not inconsistent

with our findings.

Musal has proposed that cumulative plastic deformations produced by repeated thermal cycling can explain the

eventual failure of Cu mirrors under repeated illumination

[19]. The small amount of light absorbed by the metal tar- get produces a temperature increase and thus strain near

the surface. If the resulting stress exceeds the yield stress of the metal, plastic strain occurs. This plastic strain

accumulates during each thermal cycle (heating due to the

laser followed by cooling due to thermal diffusivity), leading to slip bands in Cu. Once the surface becomes rough,

absorption increases and a thermal runaway eventually

occurs. Musal has obtained analytical expressions for uniform irradiation assuming a linear elastic/perfectly plastic

stress-strain diagram for Cu. Bass and coworkers have

refined the analysis, including the finite laser beam spot

size [20]. Various observations, including those by Bass et

al., seem to support this interpretation. No attempt has

been made so far to adapt this model to nonmetals, especially semiconductors, which usually absorb at the

wavelength of interest. At low substrate temperature (e.g.,

300 0 K) crystalline silicon does not yield but rather frac- tures. At elevated temperature however, tetrahedral semiconductors do yield. Since the coupling of visible or

infrared light to semiconductors is at least one order of

magnitude larger than to most metals, the maximum temperature rise during each pulse can easily be as large as a few hundred degrees °C. This is very favorable for plastic

yield in Si (and other semiconductors) and after suitable

refinements Musal's model may be a very good candidate

for explaining the damage precursors.

Vitali's experimental results imply the formation and

growth of very small microcrystalline regions in an amor- phous material during repeated subthreshold illumination

[21]. Once several of these microcrystallites are present,

the effective recrystallization threshold may be lowered

under the action of strong band bending at these sites (enhancing locally the electron-hole plasma density by

confinement) or of concentration of stresses and strains at the interface. The effects due to the formation of microcrystallites and to the accumulation of plastic strain will effectively be additive. This is consistent with, but not

proven by, our observation that the safe level for multipulse laser-induced transformation is lower in disordered
than in crystalline materials. In Vitali's model, the formation of the first microcrystalline region is somewhat mysterious. It may be due to the inhomogeneity, on a microscopic scale, of disordered materials, or it could come from

micro-plastic deformations produced by a local plastic

strain.

These two models are also consistent with the production of surface ripples during a multipulse illumination

sequence with pulses below melting threshold. Local melting or recrystallization is possible in these models and it would occur preferentially where the power flow is max- imum. However, nothing is known of the possible changes

that occur prior to damage during the illumination

sequence. A variety of probes have been used to detect

such changes; they include SEM, TEM, RHEED, particle

emission, Kelvin microprobe, calorimetry, electrical measurements, probe beam scatter and optical microscopy. To

our knowledge, no correlation between the measurements

of these probes and structural changes prior to damage has

been obtained. No conclusive evidence in favor of or against these models exists and multipulse effects remain

misunderstood.

4. Conclusions

In non transparent media, we can explain successfully our single-pulse damage results. Multipulse effects however, remain unexplained and mysterious, due to the

failure of standard single-pulse models. We have presented

experimental and theoretical results for single-pulse effects. The agreement is very good. We then have shown selected

multipulse experimental results and we have outlined some

models that may help explain our findings.

Acknowledgments

This research was supported by AFOSR.

References

1. P.M. Fauchet, Zhou Guosheng and A.E. Siegman, in Laser-Solid Interactions and Transient Thermal Processing of Materials, Narayan et al. editors, NorthHolland (1983), pp. 205-210.

2. A. Lietoila and J.F. Gibbons, Appl. Phys. Lett. 40,

624 (1982).

3. P.M. Fauchet and A.E. Siegman, Appl. Phys. Lett. 43, 1043 (1983).

4. P.M. Fauchet and A.E. Siegman, "Picosecond

dynamics of electron-hole plasmas close to the melting phase transition in silicon and gallium arsenide,"

to be published in the Proceedings of the International Conference on Lasers '83 (1984); and

"Ultrafast spectroscopy of very dense and hot elec- tron hole plasmas in crystalline and amorphized semiconductors," to be published in the Proceedings of

the 17th International Conference on the Physics of

Semiconductors.

5. P.M. Fauchet and A.E. Siegman, Appl. Phys. Lett. 40, 824 (1982).

6. S.R.J. Bruech and D.J. Ehrlich, Phys. Rev. Lett. 48,

1678 (1982).

7. J.E. Sipe, J.F. Young, J.S. Preston and H.M. van

Driel, Phys. Rev. B 27, 1141 (1983).

8. Zhou Guosheng, P.M. Fauchet and A.E. Siegman,

Phys. Rev. B 26, 5366 (1982).

9. R.J. Nemanich, D.K. Biegelsen and W.G. Hawkins,

Phys. Rev. B 27, 7817 (1983).

10. R. Wilson, F.A. Houle and C.R. Jones, Materials

Research Society meeting, Symposium B, paper B4.5

(Boston, 1984).

11. P.M. Fauchet and A.E. Siegman, Appl. Phys. A 32,

135 (1983).

12. R.M. Osgood, Jr. and D.J. Ehrlich, Optics Lett. 7,

385 (1982).

13. A.M. Prokhorov, V.A. Sychugov, A.V. Tishchenko

and A.A. Khakimov, Sov. Tech. Phys. Lett. 9, 28

(1983).

14. P.M. Fauchet and A.E. Siegman, "Laser-induced surface ripples: what is understood and what is not," to be published in the Proceedings of the 1984 Materials

Research Society Meeting Symposium A (1985).

15. W.J. Tomlinson, Optics Lett. 5, 323 (1980).

16. P.M. Fauchet, Phys. Lett. A 93, 155 (1983).

17. N. Koumvakalis, C.S. Lee and M. Bass, Opt. Engin.

22, 419 (1983).

18. D.Y. Sheng, R.M. Walser, M.F. Becker and J.G. Ambrose, Appl. Phys. Lett. 39, 99 (1981).

19. H.M. Musal, in NBS Special Publication 568, 1979,

pp. 159-173.

20. C.S. Lee, N. Koumvakalis and M. Bass, J. Appl.

Phys. 54, 5727 (1983).

21. G. Vitali, Phys. Lett. 78A, 387 (1980).

Fig. 1 Experimental set-up for picosecond laser-solid

interactions. The 532 nm beam goes into a variable delay line (DL) and the 1064 and 532 nm

beams are focussed onto the sample. A storage

oscilloscope (SO) monitors the energy in each

beam (VA = variable attenuator; F = filter; PD = photodiode).
Fig. 2 Computer simulation of the lattice temperature

and plasma density as a function of time in amorphized silicon. The time delay of the

infrared pulse (0.3 J/cm2

, 42 ps) with respect to the visible pulse (54 mJ/cm2

, 30 ps) is -50 ps (1), 0 ps (2), 50 ps (3) and 100 ps (4).

-450 ps -50 ps

20

CME

E 80

C\J to

If) Ll. 40

K a\

>•

\

\

\

\\

\

\

0 0.4 0.8

F

l.06 ^ J/cm2 '

\

\ \\

N

i

\

s \

\ , v- V

\ P—\—

t>\Ut-

\ -\

,\ «E—

N X

+80 ps

\\

\ —>l

\

\\

k

1 *V

s

\\

\v

\

\

\ \

\

\

*\

\

\

,\*\iI

*

+4.5 and 10 ns *- +350 ps «- + 220 ps

Fig. 3 Combined energy densities just required to melt

the surface of a-Si, for various time delays.

Further details can be found in references 3 and

4.

151
SEM microphotograph of a Ge surface after Fig.

repeated illumination above threshold (X = 1064

nm). The electric field of the normal-incident

beam was perpendicular to the most prominent

ripple's direction.

5 Microphotograph of GaAs after repeated illumination above threshold (X = 1064 nm). The

electric field of the normal-incident beam was

vertical.

29KV X19499 TE) 819 51983 CMR

Fig. 6 SEM microphotograph of a <111> Ge surface

after repeated illumination above threshold (X =

1064 nm) with normal-incident, circularly polarized light.

100

50

20

o 10

(E

iii

<I00> Si (2 • I0 16 Si + @ 200 keV)

X = 532 nm

w = 48 /im

-F^ = 202 mJ/cm2

-F° = II5 mJ/cm 2

I0J 2-I03

r 2

( M m

2

)

Fig. 7 For a Gaussian beam, F(r) = (2E/7rw2

) exp (- 2r2/w2

), where F is the fluence and r is the

radius of the melted spot. By varying the

energy E and recording r, a simultaneous meas- urement of the laser spot size (w) and the melting threshold (F^) is possible. Here, one threshold is for the formation of amorphous material,

the other is for crystalline regrowth.

152
I Th (GW/cm2

)

1.7 GW/cm2

E 0.1 E<

UJ<Q

ft 0.5 o

li. if) Z<

UMl.3GW/cm2

1.5 I

MflX

= l.7GW/cm

0-5 -jr0.41 GW/cm2

500 I I 1000 3000

4'

-0.73 GW/cm2

z

0.41 GW/cm

I 500 2000 4000 6000

NUMBER OF SHOTS n

Fig. 8 As the number of identical laser shots increases,

damage occurs, which corresponds to a decrease

in the damage threshold. For c-Si illuminated

with 80 ps pulses at 532 nm, the safe level is ~

40 mJ/cm2.

Fig. 9 Results obtained at different repetition rates appear indistinguishable in this experiment per- formed on a-GaAs. Similar results have been

obtained for the other materials under investigation here.

In the sixfold symmetry pattern gotten from the nonlinear mixing case, is there any connection

between the sixfold symmetry and the crystal lattice structure of the sample being looked at?

The author replied that there was. The direction in which one sees the effect corresponds

within 5 deg to one of the crystallographic axes. For surfaces which do not show sixfold

symmetry one does not see the correlation. The authors have made no experiments as yet on transparent materials. 153
Manuscript Received

4-18-85 SAB

Picosecond-Pulse Damage Studies of Diffraction Gratings

Dennis H. Gill and Brian E. Newnam

University of California, Los Alamos National Laboratory

Los Alamos, NM 87545

Diffraction gratings are frequently used in dye-laser cavities as wavelength-tuning

elements. These gratings often limit the maximum laser output energy because of their low

damage thresholds. We have measured the damage characteristics of both ruled and holographically produced gratings, under a variety of conditions. Using the single-shot-persite mode, the samples were irradiated by 30-ps, 1.064-um pulses having a spot size of

0.5-mm radius.

It was found that holographic gratings have damage thresholds from 1.5 to 5.0 times

higher than similar ruled gratings. Thresholds for S-polarized light (E parallel to

grooves) were higher by factors of 1.5 to 6. For the same type grating, gold coatings

yielded higher thresholds than aluminum, although this is wavelength dependent. For holographic gratings, replicas have slightly higher thresholds than masters. Dependence upon

groove spacing was weak.

Data are presented to show a variety of comparisons between different types of gratings, including two different manufacturers and usage at higher orders of incidence.

Key words: laser-induced damage; holographic gratings; ruled gratings; picosecond pulses;

1064 nm; polarization effect; metallic overcoat.

1. Introduction

Diffraction gratings are frequently used in laser systems, e.g., dye-laser cavities, as wave- length-tuning elements. These gratings usually have the lowest damage threshold of any component in

the system and thus determine or limit the maximum energy output available from the laser. It is

therefore important to understand the damage characteristics of gratings and how they can be improved.

Damage-threshold characteristics have been measured for both conventionally ruled (replica)

gratings and for the newer holographical ly produced gratings. A comparison between aluminum and gold

as the surface coating has been made, and the effect of the number of lines per millimeter on the

damage threshold has been determined. Grating damage thresholds were measured for both S- and P- polarizations. The effect of using ruled gratings in higher orders was investigated. Comparisons

were made between master and replica holographic gratings and between replica ruled gratings from two

different manufacturers.

For these tests the gratings were mounted in Littrow condition, first order, as is common for

their use as tuning elements in dye lasers. Using the single-shot-per-site mode, the samples were irradiated by 30-ps, 1064-nm pulses having a spot size of 0.5-mm radius.

It was found that holographic gratings have damage thresholds from 1.5 to 5 times higher than

similar ruled gratings. For all but one of the gratings tested, thresholds for S-polarized light were substantially higher than for P-polarization, by factors ranging from 1.5 to 6.

For the same type grating, gold is a much better overcoat than aluminum at 1064-nm, as would be

expected. Thresholds for gold-coated gratings were approximately 1.5 to 10 times higher than the

threshold for aluminum-coated gratings.

154
2. Experimental Setup

The laser damage threshold measurements were made using a Nd:YAG oscillator-amplifier configuration. The laser was mode-locked, and a single 30-ps pulse was extracted from the mode-locked train.

Care was used throughout the system to maintain a single transverse spatial mode.

This beam then entered the interaction area shown in figure 1. A 2-m focal-length lens focused

the beam at a point beyond the sample (grating) so that the spot size radius at the sample was 0.5 mm (Gaussian parameter w). The spot size was measured on every shot using a Reticon linear-diode array.

Damage was detected in three ways: (1) visual observation during the shot; (2) photomultipl ier

observation of spark; and (3) comparative observation of the scattering of a He-Ne laser before and

after the shot. The interaction room was completely darkened during the tests and an observer

watched the sample through glasses highly shielded for the 1064-nm radiation. The observer saw a spark if substantial damage occurred, and was also able to detect very slight changes in the scattering from a coincident He-Ne beam. The primary damage detection was the signal from a photomultipl ier

viewing the spark at the interaction point. The photomultipl ier was heavily filtered so that it

could only see a narrow range of wavelengths centered at 4200 A.

After each shot the sample was translated so that no site was irradiated more than once. An

average of about 40 shots was used to establish the damage threshold for each test.

The gratings were mounted in Littrow condition, first order. Therefore gratings having different groove spacings were irradiated at different angles of incidence relative to the plane of the

gratings. A half-wave plate was used to change the polarization of the incident laser beam. In

order to avoid confusion over the terms S-plane and P-plane, we will explicitly refer to the electric

vector parallel to the grooves, E,,, and the electric vector perpendicular to the grooves, E,.

LASER

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LIGHT

TRAP

FILTERS

WALL

ENERGY

MEASUREMENT

FOCUSED

BEAM -DIAMETER

MEASUREMENT

DAMAGE

DETECTION

Figure 1. Experimental setup used to measure laser-induced damage thresholds.

155
3. Metallic Coatings

Four pairs of gratings were obtained such that, within a pair, the gratings were identical

except for coating. One grating of each pair was coated with aluminum and one with gold. The damage

threshold results are shown in table 1. In every case, the gold coated gratings had higher thresholds by a substantial ratio.

Table 1. Damage thresholds of gold and aluminum-coated gratings used at 1.06 microns.

Damage Threshold (J/cm2

) Threshold Ratio

E//Grooves E_[Grooves (Gold/Alum.)

Type Grooves/mm Gold Alum. Gold Alum. E//Grooves EJ_Grooves

Holog. 1800 2.6 0.32 1.2 0.14 8.1 8.6

Holog. 1800 1.0 0.28 0.87 0.09 3.6 9.7

Ruled 600 1.1 0.19 0.44 0.07 5.8 6.3

Ruled 300 0.49 0.32 0.07 0.05 1.5 1.4

This is reasonable at 1064 nm, since the reflectivity of gold is higher than that of aluminum,

as shown in figure 2. Below 600 nm, however, the situation is reversed and one would expect that

aluminum coatings would have higher damage thresholds than gold. Thresholds for both materials would

be lower at 600 nm than those thresholds reported here, due to the increased absorption at shorter

wavelengths.

Figure 2. Reflectance as a function of wavelength for aluminum and gold [1].

One might expect a relationship between the minimum coatings thickness that should be used and

the laser pulse length (in time). For replica gratings, both ruled and holographic, the layer underneath the outer metallic layer is some form of epoxy (fig. 3). For master holographic gratings, the

layer underneath the outer metallic layer is a photoresist material. These materials are thermal

barriers, because of their low thermal conductivity, thus keeping most of the heat in the metallic

layer. Since all of the absorption occurs in a thin (-200-300 A) layer at the surface, increasing

156
the thickness of the coating decreases the maximum temperature attained by the coating. This is only

true up to a point, however, since the heat must have time to diffuse into the metal. Once the laser

pulse has turned off, further heat diffusion cannot reduce the (already attained) maximum temperature. Although they were not tested in this experiment, ruled masters may have some advantage for

long pulse lengths in that they do not have the insulating epoxy , layer. The use of metallic substrates could also help for high average power applications. The substrates used here were glass.

RULED REPLICA GRATING HOLOGRAPHIC GRATING

Figure 3. Cross-sectional view of ruled replica and holographic gratings showing metal deposited on epoxy or photoresist material.

4. Ruled Versus Holographic Gratings

Table 2 compares the damage thresholds of machine ruled and holographic gratings (both replicas). It can be seen that holographic gratings hold a clear and substantial advantage over ruled

gratings.

Table 2. Damage thresholds of ruled and holographic gratings.

Damage Thres hold (J/cm*) Threshold Ratio

E//Grooves E^Grooves (Holog. /Ruled)

Grooves/mm Coating Ruled Holog. Ruled Holog. E//Grooves EJ_Grooves

1800

600

Gold

Gold

0.47 2.6

1.1 1.7

0.24 1.2

0.44 1.0

5.5 5.0

1.5 2.2

One possible explanation of the difference is the sharpness of the respective groove shapes, as demonstrated in figure 3. Ruled gratings are actually cut into the metal layer with a diamond tool,

producing sharp corners and whiskers of metal. Electric-field enhancement at these sharp corners

could account for lower thresholds. The ruled gratings used in these tests were actually replicas

rather than masters, but it is presumed that the sharp features replicate faithfully.

Holographic gratings are made by irradiating a photosensitive surface with an optical interference pattern. The pattern etched into the surface is some truncated form of a sinusoid and is,

therefore, smoother in shape than a ruled pattern. Figure 4 shows photomicrographs of two different

holographic gratings. The reflective metal layer is deposited on top of the exposed and developed

photoresist material.

All of the gratings in table 2 were tested in first order except the ruled 600 groove/mm grating, which was tested in second order. As will be seen in the next section, this may have had an adverse effect on the results. However, the results for the 1800 groove/mm gratings are clear and

val id.

157
Figure 4. Photomicrographs of two holographic gratings illustrating groove profiles. Groove spacing

on the left is 1800 £/mm and on the right is 1200 £/mm.

5. Higher Orders

One possible advantage of ruled gratings is their ability to go to higher orders efficiently.

From a laser damage threshold viewpoint, however, using higher orders is not advantageous, as can be seen in table 3. Here, a holographic grating in first order is compared to ruled gratings in second

and fourth orders. The groove spacing and order number are such that the Littrow angle is constant

for the three cases. Each of the ruled gratings was blazed for use in the order listed in table 3 at

1064 nm. The use of higher orders reduces the damage threshold.

Table 3. Damage thresholds of holographic gratings used in first order compared to ruled gratings

blazed for higher orders.

Damage Threshold (J/cm*)

Type Coating Grooves/mm Order E//Grooves E | Grooves

Holog

.

Gold 1200 1 1.9 1.4

Ruled Gold 600 2 1.1 0.44

Ruled Gold 300 4 0.49 0.07

6. Effects of Polarization

Polarization of the incident beam had a substantial effect on the damage threshold of a grating,

as seen in table 4. For all but one of the gratings tested, the threshold was higher for the electric vector E parallel to the grooves than for E perpendicular to the grooves. It is well known that

the efficiency of diffraction gratings is different for the two different polarizations, and that the curves of efficiency versus wavelength vary from one grating to another [2]. This would imply that

the details of the electric field distribution at the surface of the grating differ, perhaps accounting for the different damage thresholds. Typical grating efficiency curves are shown in figure 5. A

simplistic approach of correlating the efficiency of a grating with its damage threshold for the two

polarizations does not work at all. We have not attempted a detailed analysis of this area, but it can be seen from figure 6 that simple metal absorption theory would predict higher damage thresholds

for S polarization (E parallel to grooves), in qualitative agreement with the data. Specifying the

angle of incidence is more difficult, however, because of the angles involved in the grooves themselves. It is apparent that the higher efficiency configuration (E perpendicular to grooves) has the

lower damage threshold.

158
Table 4. Effect of polarization on damage thresholds of gratings.

Threshold

Damage Threshold (J/cm2

) Ratio

Type Coating Grooves/mm E//Grooves EJ_Grooves E/V /El

Holog. Gold 1800 2.6 1.2 2.2

Holog. Alum. 1800 0.32 0.14 2.3

Holog. Gold 600 1.7 1.0 1.7

Ruled Gold 1800 0.47 0.24 2.0

Ruled Alum. 830 0.26 0.35 0.7

Ruled Alum. 300 0.32 0.05 6.4

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ALUMINUM.

FOR S POLARIZATION,

E IS PARALLEL TO GROOVES

IN GRATING

GOLD

20 40 60 80

ANGLE OF INCIDENCE (degrees)

Figure 5. Typical grating efficiency curves. Actual curves vary widely from one grating to

another.

Figure 6. Ratio of

polarizations for Al

angle of incidence.

absorptions for the two

and Au as a function of

7. Groove Spacing

The dependence of damage threshold on groove spacing is not clear. In table 5, for holographic

gratings and E parallel to the grooves, the threshold increases with the number of grooves per millimeter. However, for the same gratings and E perpendicular to the grooves, threshold increases from

600 to 1200 grooves/mm, then decreases from 1200 to 1800 grooves/mm. For the ruled gratings shown,

the two different polarizations have opposite dependence on groove spacing. One can only conclude

that other factors appear to be more important than groove spacing.

159
Table 5. Damage thresholds of gratings as a function of groove spacing

(tested in first order).

Damage Threshold (J/cm*)

Type Coating Grooves/mm E//Grooves EJ_Grooves

Holog. Gold 1800 2.6 1.2

Holog. Gold 1200 1.9 1.4

Holog. Gold 600 1.7 1.0

Ruled Alum. 830 0.26 0.35

Ruled Alum. 300 0.32 0.05

8. Master versus Replica Gratings

Two master-replica pairs of holographic gratings were tested for relative damage thresholds.

For both aluminum and gold coatings and for both polarizations the replica gratings had higher

thresholds than the master gratings, as shown in table 6. One possible explanation is the different

types of material layers used under the reflective coating, as discussed in Section 3. Holographic

masters use a photoresist material so that the initial pattern can be "recorded." Replicas have no such requirement since they are merely taking the physical shape of the master by impression in an epoxy layer. Another possible explanation is that sharp points and corners may not replicate faithfully, but may come out more rounded. It should be noted that these replicas are actually secondgeneration or "positive" replicas so as to have the same phase as the master.

Similar tests on ruled gratings were not performed due to the considerable cost of ruled masters.

Table 6. Damage thresholds for master and replica holographic gratings.

"Damage Threshold (J/cm*)

E//Grooves EJ_Grooves

Grooves/mm Coating Master Repl ica Master Rep! ica

1800 Alum. 0.28 0.32 0.90 0.14

1800 Gold 1.0 2.6 0.87 1.2

9. Two Manufacturers of Ruled Gratings

Table 7 gives the damage thresholds for two ruled gratings made by two different manufacturers

but otherwise identical. Since the relative positions of the gratings changed between the two polarizations and the thresholds are close to one another, neither grating appears to be better than the

other. This test was not performed on holographic gratings.

Table 7. Damage thresholds of ruled gratings from two manufacturers.

Damage Threshold*(J/cm^)

Manufacturer Type Coating Grooves/mm E//Grooves EJ_Grooves Blaze k

JY Ruled

B&L Ruled

Alum. 600

Alum. 600

0.19 0.07

0.18 0.12

2 urn, 1st order

2.5 urn, 1st order

* Damage thresholds for 1.06 urn irradiation, 2nd order

160
10. Conclusions

The thresholds for laser- induced damage to holographic and ruled gratings have been measured and

compared for 30-ps pulses of 1.064 urn radiation. For these conditions, holographic gratings have

higher damage thresholds than ruled (replica) gratings. Ruled master gratings were not tested. For

both holographic and ruled gratings, gold coatings have higher damage thresholds than aluminum coatings, as would be expected at this wavelength. In general, the damage threshold was highest for the

electric vector parallel to the grooves, by a substantial factor. Unfortunately, this orientation

normally produces lower efficiency. Operating in lower orders gives the best damage resistance,

where that is practical. Ruling density does not appear to have a clear effect on damage threshold.

The holographic replica gratings tested in this series had higher damage thresholds than holographic

master gratings. This is not fully understood. No significant difference was found between ruled

replicas from two different vendors.

11. References

[1] Jobin Yvon Optical Systems, Laser Grating Handbook.

[2] Loewen, Erwin G. , Maystre, Daniel, McPhedran, Ross C. , and Wilson, Ian. Correlations between

efficiency of diffraction gratings and theoretical calculations over a wide range. Japan. J. Appl. Phys. 14 (1975) Suppl. 141: 143-152.

161
Manuscript Received

11-27-84 SAB

Assessment of Optical Coatings Prepared by Molecular Beam Techniques

K.L. Lewis, J. A. Savage, A.G. Cullis, N.G. Chew, L. Charlwood and D.W. Craig

Royal Signals and Radar Establishment

Malvern, Worcestershire

England WR14 3PS

Thin films of ZnSe and other materials have been produced by molecular beam techniques

in an ultra-high vacuum facility dedicated to the examination of fundamental aspects of thin

film technology as applied to optical coatings. The ZnSe films have negligible values of

absorptance at 10.6 urn and have laser damage thresholds in the range 35-38J/cm2, a significantly higher energy density than hitherto reported for ZnSe films. Examination of the films

in cross section by high resolution transmission electron microscopy has revealed a polycrystalline columnar morphology, with the crystallite size dependent on deposition temperature.

A complete absence of intergranular voids is noted which renders the film impervious to the

ingress of water and other impurities when exposed to the atmosphere. This feature, together

with the inherently high purity of the film is believed to be the factor responsible for the

enhancement in laser damage threshold.

The electron micrographs do however reveal sub-grain structure in the form of stacking

faults and twin planes which are potential sinks for residual impurities in the material.

Further improvements in damage threshold may be possible by elimination of this sub-grain

structure.

1. Introduction

The case for a fundamental approach to the solution of the problem of susceptibility of thin film

structures to laser induced damage was made at the 1983 Boulder Conference [1]. Here, the potential

afforded by molecular beam techniques was explored for the deposition of improved optical coatings. The

molecular beam technique allows a high degree of control over the deposition process and in situ assessment techniques, such as Auger and X-ray photoelectron spectroscopy allow the characterisation of the

surfaces produced. By combining Knudsen evaporation with RF sputtering processes in a dedicated UHV

facility, it is possible to fabricate coating structures containing a wide range of different materials

including sulphides, selenides, tellurides, arsenides, phosphides, fluorides, chlorides, oxides and

nitrides without necessarily having to break vacuum and risk the creation of contaminated interfaces.

Application of the equipment was described with reference to the production of ZnSe coatings of up to 3 Mm

in thickness which had negligible values of absorptance at 10.6 Mm.

The achievement of low absorption per se is not a necessary criterion for the realisation of a high

damage threshold, although it may play an important role particularly at longer pulse lengths (200 nsec +)

where the rate of heat generation by virtue of absorption is greater than its rate of transport to

surrounding areas. At short pulses (100 nsec or less) coatings fail either as a result of heating microscopic coating defects or by electron avalanche processes in the coating material itself. It may be

argued that the most serious coating defects are voids, from the point of view that they cannot be

controlled by attention to cleanliness, dust control etc. The presence of open porosity results in a high

susceptibility to water ingress and inevitably a lower damage threshold. Ultra high vacuum conditions

create an excellent environment for reducing particulate inclusion during coating deposition. This is

especially so for the Knudsen source where the configuration of the oven avoids the ejection or spattering

of discrete particles of evaporant.

It is likely that the morphology of thin films plays an important part in determining resistance to

laser induced damage. Films prepared by conventional evaporation techniques exhibit a polycrystal 1 ine

morphology with a characteristic columnar structure [2]. It has long been asserted that void formation is

enhanced at grain boundaries in thin films and that diffusion of impurities especially water readily proceeds along such paths. The presence of heterogeneous impurities at such grain boundaries

also results in the generation of compressive stresses in the film [3,4]. Lack of control of the deposition process - changing deposition rate, improvements to the vacuum environment as a result of gettering

processes potentially lead to gradation of stress levels within a film, increasing the likelihood of poor

adherence. An immediate solution would appear to be to avoid producing films with polycrystal 1 ine columnar

morphology, and to deposit single crystal films with an epitaxial relationship to the substrate. Unfortunately this requires a close match of the crystal 1 ographic parameters of the film and substrate and it

162
may be too much to hope that this can be achieved with simultaneous satisfaction of refractive index

criteria in any one film/substrate combination. The requirements of high interface perfection to ensure

good epitaxy cannot be met in other than a UHV system employing ion beam cleaning and annealing/surface

reconstruction techniques, or in chemical vapour deposition equipment where high substrate temperatures/

gaseous etchants can be employed.

The alternative solution, the adoption of amorphous morphologies may offer some potential since the

coating need not be lattice matched to the substrate. The behaviour of materials such as AS2S3 has

already been explored and their ability to withstand moderate levels of laser power demonstrated [5].

Variation of refractive index can be effected by controlled deposition of amorphous alloys typified by the

chalcogenide system [6].

As a preliminary stage of our fundamental studies, we have examined the limits to which coatings with

columnar polycrystall ine morphology may be taken. ZnSe and ZnS are two examples of such thin film

materials and are commonly used in various designs of multilayer coatings. The present work assesses the

microstructure of films produced by molecular beam techniques and the susceptibility of the coatings to

laser induced damage.

2. Experimental

The equipment used for film deposition has been described in a previous publication [1]. The

3-chambered facility, manufactured by Vacuum Generators Ltd., comprises a growth chamber (pumped by ion

and titanium sublimation pumps) which contains a molecular beam sampling mass spectrometer and Auger electron spectrometer. A preparation chamber immediately adjacent allows cleaning using a raster scanning ion

gun. Specimens are transferred between the separately gated chambers using a rack and pinion manipulator

system. A load-lock system is provided allowing rapid entry into the ultra high vacuum (10~11 mbar)

environment. The loading chamber is UHV cryopumped and contains a small (2" diameter) RF plasma source

for the deposition of refractory materials (eg oxides, nitrides). Three Knudsen cells are available for

simultaneous use in the growth chamber and for this work were charged with broken lumps of ZnSe (for

stoichiometric beams of Zn and Se2), ZnS (for beams of Zn and S2) and Ag2S (for S2 alone). Substrates for

coating (ZnSe, GaAs, Si, Ge) were carefully cleaned with trichloroethane and acetone before loading into

the equipment, and following evacuation to 5 x 10~7 mbar were further cleaned by Ar+ beam bombardment for

10 minutes at a density of approximately 200 uA/cm^. Coated specimens were examined by Nomarski interference microscopy, infra-red spectroscopy (Perkin Elmer model 983G) and adiabatic laser calorimetry. The

morphology of the coatings was examined in detail by cross-sectional transmission electron microscopy

(XTEM). Specimens were prepared in cross-sectional configuration by sequential mechanical polishing/low

voltage Ar+ ion milling, and were examined by direct imaging in a Jeol 120C instrument at 120 kV. The

milling beams were generated in saddle-field ion guns and were collimated with a cross-sectional area of

about 3 mm^. The ion energy was usually 5 kV and the ion current 20 uA.

Laser damage thresholds were determined using the equipment shown in figure 1. Two separate TEA CO2

lasers were used to examine the dependence of damage threshold on pulse length. A short cavity laser [7] was used for short pulse work. Its output consisted of a 33 nsec FWHM gain switched spike followed by a tail extending to 1700 nsec. The energy in this pulse could be approximated by a square pulse of width

120 nsec and a height equal to the intensity of the gain-switched spike. Longer pulse lengths were achieved using a grating tuned hybrid laser (Edinburgh Instruments type 222) operating with its CW section

below threshold. The pulse length, comprising solely of a single, slightly skewed gaussian peak (no tail) was 340 nsec FWHM. Temporal profiles were recorded using Labimex model R005 uncooled CMT detectors (rise

time 1 nsec) coupled to Tektronics 7912AD transient digitisers. Energy measurements were effected using a calibrated Gentec detector (Joulemeter ED200). Spot widths were varied by choice of focussing lens and

measured using a 25 urn scanning aperture. Groups of measurements were made for constant spot widths at

varying incident energies by interposing various combinations of ZnSe and CaF2 attenuators in the incident

beam.

3. Coating Morphology

The molecular beam produced coatings were mirror smooth and essentially replicated the topography of

the substrate used. In the case of ZnSe substrates a slight degree of surface relief was evident complementing that of the polycrystall ine substrate. This is largely a result of preferred etching of the

different oriented crystallites during ion beam cleaning. Some inclusions were evident in coating Ge

windows, but these were also present in uncoated regions of the samples and were attributed to polishing

debris.

Examples of XTEM micrographs are presented in figures 2-5. Figure 2 shows a 0.7 urn thick ZnSe grown

on GaAs at 50 C. The polygrained morphology is clearly evident with the columnar grains running throughout the thickness of the film. The mean grain width is approximately 250a. The crystalline structure

within the material is highly defective with a high density of stacking faults each running perpendicular

163
to the direction of growth. X-ray diffraction measurements taken on the film surface show that the

material is highly ordered with cubic (111) planes lying parallel to the surface. The fault orientation

is as expected since twinned regions in the zinc blende structure usually have a [111] twin axis and com- position surface parallel to ( 1 1 1

) . A few instances of Moire fringe contrast is also found in the micrographs. This arises from interference effects between surface grains and others lying immediately beneath

them.

Careful examination (figure 3) of the grain boundary regions reveals the complete absence of any

voids within the scale of resolution available (20A). This is clearly the case since voids are easily

distinguished in the TEM, providing contrast which alternates between light and dark when imaging

conditions are changed from under to over-focus.

Examination of the interface region at higher magnification (figure 4) reveals a transition region

some 1500a in height above the substrate, where growth of the columnar grains is clearly unstable and new grains are being nucleated. Also evident is the narrow (608) amorphous region at the surface of the substrate resulting from ion beam damage during wafer cleaning. This amorphous region would require re- crystallisation in a high temperature anneal (500°C) before epitaxial growth could occur.

The morphology of a ZnSe film produced at 192°C on GaAs is essentially similar to that of the low

temperature case with the exception of a greatly increased grain size of 1 000a (figure 5). The grain

boundaries are less well defined than in the film produced at 50°C and stacking faults in one grain tend

to merge into those of neighbouring grains. No voids were detected.

4. Damage Assessment and Discussion

4.1 Zinc Selenide Films

Molecular beam grown ZnSe films were assessed as 1 um thick layers on polycrystall ine ZnSe substrates.

Typical deposition conditions were as follows - ZnSe source temperature 950 C, substrate temperature 175°C,

growth rate 1.2 um/h, p^O) 2 x lO"^ mbar, p(C0) 2 x 1

0

—

1 0 mbar. Film absorptances were too small to be

measured with the laser calorimeter system available. The laser induced damage threshold (LIDT) of the

films for short pulses was in the range 60-73 J/cm^ (l/e^ diameter 102 um) . This was in excess of the

value of 50-60 J/cm2 measured on uncoated regions of the same sample. Although the difference between the

two values was slight, repeated measurements showed it to be real, with the uncoated sample always

damaging at a lower incident energy. This was attributed to the presence of residual surface impurities

on the substrate which had been removed in the coated region by the pre-deposition ion beam cleaning procedure. At longer pulse lengths (340 nsec), the LIDT value for the ZnSe coating was determined to be

31-38 J/cm^ (l/e^ diameter 180 um). Micrographs of the coating (figure 6) clearly show that damage was initiated at discrete microscopic inclusions resulting in craters some10 um in diameter. The uncoated

rear surface of the substrate always damaged at these energy densities.

Although no direct comparison with other measurements is possible, damage thresholds are significantly

in excess of those previously reported for ZnSe films on other substrates. For example, Golubovic etal [8]

report a value of 20 J/cm^ (100 nsec FWHM 180 um spot size) for sputtered ZnSe on KC1 substrates. The

high thresholds achieved in this work are attr ibuted to the near theoretical density of the film. Since

the films have been fabricated on ZnSe substrates, transmission measurements can be used as a sensitive

indication of whether the refractive index of the coating is significantly different from that of the substrate. An example of an expanded transmission spectrum is presented in figure 7. It is difficult to

assert whether any of the features suggest interference effects since there are clearly absorption bands

present, the most notable being those due to OH at 3420 cm"' and ZnH at 1620 cnH . From the remaining

spectral regions, we suggest that some optical resonance may be apparent but it represents at the most the

reflectivity change associated with a A/4 film of ZnSe with refractive index some 0.4% different from that

of the bulk ZnSe. This is a significant improvement on for example sputtered ZnSe [9] where refractive

index variations of up to 15% (albeit at 0.6328 um) have been reported.

These improvements have been obtained in films that exhibit a polycrystal 1 ine columnar morphology.

However as the TEM evidence indicates, the structure of each crystallite is highly imperfect with stacking

faults and twins running perpendicular to the direction of growth. Since these crystal faults are essen- tially interruptions in the elementary stacking sequence of planes in the zinc blende lattice they are able to give rise to strain and refractive index inhomogeneity on the atomistic scale particularly when

impurity species are present. This may perhaps lead to a reduction in laser damage threshold compared

with that achievable in an ideally structured film.

4.2 Zinc Sulphide Films

The molecular beam facility has also been used to study the deposition of ZnS films. The problems of

film stoichiometry were considered to be an additional problem in this case since the sticking coefficient

164
of sulphur from an S2 flux is likely to be less than that of Zn. Experiments have therefore been carried

out to assess the importance of this effect by supplying additional sulphur (specifically S2) from a second source. This was conveniently provided by a Knudsen oven containing silver sulphide which dissociates to produce S2 and Ag with partial pressures in the ratio of about 10^. Provided that this source

is purely used as a 'top up

1 measure, significant contamination of the ZnS with Ag is likely to be

negl igible.

Most ZnS films have so far been deposited at A./4 thicknesses on germanium substrates at 50°C and have

damage thresholds in the range 15-16 J/cm? (33 nsec 200 urn FWHM). This value is slightly lower than the

thresholds reported by Gibbs and Wood [10] for clear regions of single-layer AR ZnS/Ge of 16.8-24 J/cm2

(60 nsec 140 urn spot size). Examination of the samples revealed that the surfaces of the germanium substrates contained a great deal of polishing debris and other visible defects and themselves had low damage

thresholds of the order of 25 J/cm^. The results for the ZnS films cannot therefore be considered as representative of what can be achieved and further parametric studies are underway using different substrates and substrate materials.

Some important factors have however emerged. A direct comparison of film deposition in conventional vacuum environments and UHV has been carried out in the same equipment using otherwise identical coating

conditions (deposition rate, temperature, film thickness etc). A conventional coating plant environment

was simulated by opening the gate valves between the growth and loading chambers, switching off the growth

chamber pumps, running with all cryopannel ing at room temperature and arranging for the whole system to be

pumped using a small (80. 1/sec) diffusion pump. By ensuring that the loading chamber had had a recent

exposure to air, it was possible to arrange satisfactory 'dirty' conditions with total pressure of

4 x 10~6 mbar. Residual gas analysis revealed that the dominant species present had partial pressures as follows: p(H20) 2.5 x 10"6 mbar, p(H2) 7 x 10" 7 mbar, p (CO) 3 x 10"? mbar and p(H2S) 4 x 10-7 mbar. The

presence of H2S was unique to this particular experiment and clearly arises from reaction between H2O and

S2 in the growth chamber. Residual species for the UHV experiment were as for ZnSe above.

Figure 8 shows a comparison of the damage morphology of the ZnS/Ge samples produced in the two experiments. Although the damage threshold was the same in each case (16 J/cm^) due to the poor finish and

presence of inclusions on the substrate surfaces, there are clearly significant differences in the way

that the coatings have responded to the laser pulses. The film produced at 4 x 10

_6 mbar has delaminated

in a wide region around the central damage zone. In comparison, the UHV produced film strongly adheres to

the substrate. Debris from the damage spot is clearly visible well away from the irradiation zone. Infrared spectral analysis (figure 9) clearly shows the presence of OH species in the film produced in poor vacuum whilst the UHV-grown ZnS is essentially free from such impurities.

5. Conclusions

It has been demonstrated that the molecular beam technique is capable of producing optical coatings

of near theoretical density which have the potential of high resistance to laser induced damage. This

resistance has been demonstrated for ZnS films which have higher damage thresholds than bare ZnSe surfaces.

The films have been examined by cross-sectional transmission electron miscroscopy and have polycrystall ine

columnar morphology. No voids can be detected. A high density of microtwins and stacking faults is

evident however, suggesting that further improvements in damage threshold may be feasible.

Copyright © Controller HMSO, London 1984

References

[1] Lewis, K.L. and Savage, J. A. : Proc. 1983 Boulder Damage Symposium.

[2] Guenther, K.H. and Pulker, H.K.: Appl . Optics 2992 (1976).

[3] Pulker, H.K.: Thin Solid Films, 58, 371 (1979).

[4] Pulker, H.K.: Proc. SPIE 325, 84 (1982).

[5] Braunstein, A.I.; Wang, V; Braunstein, M.; Rudisill, J.E. and Wada, J.: Proc. Boulder Damage

Symposium 151 (1973).

[6] McLachlan, A.D. and Gibbs, W.E.K.: Boulder Damage Symposium 222 (1977).

[7] Hollings, R.C. and Jordan, D.L.: J.Phys.D.Appl .Phys. J5, 1881 (1982).

[8] Golubovic, A.; Fitzgerald, J.; Berman, I; Bruce, J.; Bradbury, R.; Comer, J.J.; Ewing, W.S. and

Lipson, H.G.: Proc. Boulder Damage Symposium 271 (1976).

165
[9] Walsh, D.A. and Detrio, J. A.: Proc. Boulder Damage Symposium 253 (1976).

[10] Gibbs, R and Wood, R.M. : Proc. Boulder Damage Symposium 181 (1976).

Ene rgf Meter

He-Ne laser

0

Variable

At tenuator

Ene re; Meter

Transient

! Digitiser

detector- Micro

Computer

Hydrid TEA laser

3*tO nsec

Short cavity TEA laser

33 nsec

Figure 1 Schematic layout of laser damage facility.

0-2|jm

Figure 2 Cross sectional TEM image of ZnSe film grown on GaAs at 50 C.

166
Figure 3 Detail of 50°C ZnSe film at higher magnification with particular

reference to grain boundary regions.
TS =192°C 0-1 pm

Figure 5 Detail of ZnSe film grown on GaAs at 192 C. Magnification is as figure (3).

50 urn

Figure 6 Damage morphology in ZnSe film on ZnSe substrate produced by

340 nsec TEA laser pulse. Spot size 180 urn.

168
72

71

co

'(/)

(/) E

i/> c

o

70

OH

I I CH ZnH

69

5000

—! I 1_ 4000 3000 2000 1000

Wavenumber /cm"

Figure 7 Expanded transmission spectrum of 1 urn ZnSe film on ZnSe substrate.

a

, 250um ,

Figure 8 Comparison of damage morphology in ZnS film on Ge substrate

produced by 33 nsec TEA laser pulse. Spot size 200 urn.

(a) Film produced under UHV conditions.

(b) Film produced under high vacuum 4 x 10 ^ mbar.

169
3800 3500 3200

Wavenumber /cm

-''

Figure 9 Comparison of infra-red absorption spectra of ZnS films on Ge

(a) Film produced under UHV conditions.

-6 c

(b) Film produced under high vacuum 4 x 10 mbar.bome

interference fringing remains in both traces following

the computer flattening and expansion routines.

Expansion of X30 in each case.

In response to a question the author pointed out that he couldn't tell if his MBE coatings of ZnS were

as low as the bulk ZnS because the absorption of the coatings was too low to measure. The coated windows showed higher damage resistance than the bulk. Possibly the damage on the uncoated surface was caused by polishing defects, which were covered over by the film. Their samples were 1" diameter, but

MBE systems are being planned which can take 11" samples. In their system they can have 3 different

materials to evaporate. Some systems go as high as 8. Growth rates are comparable to those obtained

using normal coating methods. Another questioner pointed out that a less restrictive term than twins

would be stacking fault defects. The speaker agreed. He stated that they would do about 8 sites per sample

and quoted the lowest fluence at which they got damage at the damage threshold.

170
Manuscript Received

12-13-84 SAR

Some Recent Observations on the Properties of UHV-Deposited Zr02

T. Raj, L. D. Weaver, S. R. Tuenge, J. S. Price and K. C. Jungling

Martin Marietta Aerospace, Albuquerque, New Mexico 87119

(Present affiliation of K. C. Jungling: University of New Mexico, Albuquerque, NM)

Single-layer Zr02 coatings were deposited in an ultrahigh vacuum system by two

techniques: laser evaporation (utilizing a 100-W, continuous wave, CO2 laser) and

electron-beam evaporation. Substrate temperatures were maintained at either 150°C or 250°C with backfill oxygen pressures between 0 and 5x1

0~5 torr. The coatings were analyzed by Auger Electron Spectroscopy to determine stoichiometry and to provide information

on the nature and level of impurities. The refractive indices for the coatings were determined by ellipsometry at 6328 A. Selected coatings were damage tested at 531 nm.

This report discusses the influence of backfill pressure, substrate temperature and

evaporation technique on thin film properties and compares the results obtained by Auger

and ellipsometric analyses. Also presented is a comparison between films prepared under

high vacuum and ultrahigh vacuum conditions.

Key Words: damage threshold; impurities; optical thin films; refractive index; stoichiometry; ultrahigh vacuum; Zr02-

1. Introduction

This paper describes some of the preliminary results of experiments conducted during early

phases of the coating development program at the Air Force Weapons Laboratory's (AFWL)

Developmental Optics Facility (DOF). Also discussed is Martin Marietta's approach to advancing the

science of deposition technology.

The overall approach of the DOF to deposition technology is to first develop the technology

required to produce superior single-layer coatings. This is a necessary step before multi-layer

designs, having much-improved damage thresholds, can be achieved. Therefore, a long-term objective

of this program is to characterize the effects on the optical and mechanical properties of optical

thin films (both singleand multi-layer) of what are considered the "controllable deposition parameters". This approach emphasizes the need for control over the following principal film parameters: impurities, stoichiometry and microstructure. Other desirable optical and mechanical properties of thin films (e.g., low absorption, low scatter, good adhesion, low intrinsic stress,

hardness and high damage threshold) are products of these controllable parameters. In this context, absorption is related to the stoichiometry, microstructure and purity of the deposited

layers; while scatter, internal stress and hardness are functions of microstructure and impurities. Likewise, adhesion is a function of chamber and substrate cleanliness, substrate temperature, method of deposition and choice of material. Finally, the property of laser damage threshold

could be related to stoichiometry, microstructure and choice of materials, as well as to defects

(including impurities, voids, particulates, etc.).

In order to minimize impurities in the coatings, an ultrahigh vacuum (UHV) system with a typical background pressure of 5xl0 _1 0 torr was employed for the depositions. Furthermore, the source materials, as well as the substrate, were handled with a UHV-type procedure rather than the

usual high-vacuum (HV) procedure of handling.

Two techniques of evaporation were employed for these depositions; continuous wave (CW) laser

and electron beam (E-beam). CW-laser evaporation was carried out with two objectives in mind: (1)

to reduce the contamination arising from the source container and (2) to determine if CW-laser

evaporated coatings were closer than E-beam evaporated coatings to the bulk in stoichiometry.

171
Oxygen backfill pressure was varied to determine its effect on the stoichiometry of coatings.

Finally, different substrate temperatures were employed to determine if a substrate temperature (of

approx. 150°C) lower than that commonly used (approx. 250°C) could produce coatings with good

mechanical properties. This latter consideration is desirable for several engineering aspects.

Refractory (high melting point) oxides have been selected as the top-priority class of materials to investigate. Most oxides have a spectral transmittance range spanning the ultraviolet to

the near-infrared, and this is the wavelength range of interest. Refractory oxides have such

desirable characteristics as: (1) being thermally, chemically, and thermodynamical ly stable materials (i.e., the most environmentally durable), (2) being the best diffusion barriers (i.e., providing the highest resistance to diffusion at interfaces), and (3) having been more thoroughly

studied and more understandable than any other compounds of interest.

2. Experimental and Analytical Procedures

2.1. Deposition Procedures

A UHV system (assembled by this laboratory), with a typical base pressure of less than

5xl0 _1 0 torr, was chosen as the coating chamber for this portion of the program. The evaporation

techniques were: (1) laser evaporation with a 100-watt, CW, CO2 laser (designed and constructed

by the DOF) and (2) E-beam evaporation with a UHV-compatible Ai rCo/Temeseal E-gun.

Three refractory oxides were chosen for this study: Zr02, SiC>2 and AI2O3. During the

initial phase of this program, only Zr02 was used. The Zr02 deposition rates for CW laser eva- poration were restricted to about 0.5 A/sec due to limited laser output. The E-beam deposition

rates of Zr02 were kept to the same level in order to compare the two techniques. The substrates

chosen for most of the depositions were 5-cm semiconductor-grade silicon wafers. The substrate

temperature during deposition was maintained at either 150°C or 250°C, while the backfill oxygen

pressure was varied from 0 to 5xl0~5 torr. The coating thickness was kept below 1000 A for most

of the depositions.

To monitor the progress associated with UHV conditions in general, some Zr02 coatings were E-beam deposited in an oil diffusion pumped HV system. The parameters chosen for the HV depositions

were, to a considerable degree, influenced by the findings from the UHV depositions, and represented a departure from the usual industry practice.

2.2. Analytical Procedures

The contaminant levels and the stoichiometry of the coatings were determined by Auger Electron

Spectroscopy (AES) with a Physical Electronics Spectrometer, Auger 590/ESCA 548/SIMS 2500.

The refractive indices of the coatings were determined by the method of ellipsometery using a Gaertner LI 1 7 Null Ellipsometer operating at 6328 A. The analyses were performed using a computer

program developed for the DOF by the University of Dayton Research Institute. The accuracy of the

measurements is estimated to be in the range of 1 to 1-1/2 percent.

A few coatings were deposited on superpolished fused silica substrates and were subjected to

damage threshold measurements by AFWL personnel.

3. Results and Interpretations

The analytical techniques used to evaluate the quality of the coatings included abrasion resistance, AES, ellipsometry and laser damage threshold; each being discussed below.

3.1. Abrasion Resistance

Representative coatings from each vacuum system were tested for abrasion resistance according

to the standards of MIL-C-675. All coatings passed the abrasion test regardless of method of

evaporation, substrate temperature or O2 backfill pressure.

172
3.2. AES Analysis

Although AES techniques are not generally suitable for quantitative analysis of dielectric

materials, procedures were established to minimize the effects of surface charging, O2 desorption

by the probe electron beam and Argon sputtering. The two primary objectives of AES analysis were

the determination of the impurity level and of the chemical composition of the coatings.

The AES peak-to-peak ratio of oxygen to metal of the source material was determined to be

4.8+0.2. AES data for UHV deposited coatings are summarized in tables 1 and 2. The fifth column

of each table shows the 0:Zr ratio measured at the surface of the coatings. The surface was sputter-cleaned for a few minutes, and the 0:Zr ratio was again measured. This near-surface 0:Zr

ratio is shown in column six. The seventh column indicates an average 0:Zr ratio for the depth

profile (inside the coating). The comments section indicates whether or not the coating was contaminated. The coating is considered to be contaminated if the carbon peak can be detected unambiguously. It should be pointed out that the carbon sensitivity of the present analysis scheme is

about two to three atomic percent.

Table 1 . 0:Zr Peak-to-Peak Ratios for Laser Evaporated Coatings

0:Zr P-P Ratios

Coating

Run * Substrate

Temperature

02 Pressure

Thickness

(in A)

Surface Near

Surface

Coating

Prof i le

Comments

LC 29 140°-160°C 3xl0~5 845 4.0 4.2 3.6 ± 0.1

LC 30 140°-1 60°C 2x10-5 822 4.6 4.1 4.1

LC 31 140°-160°C 5x1

-5 901 4.5 4.3 4.2

LC 32 140°-160°C 781 4.2 3.9

LC 33 140°-1 60°C 1x10-5 - 0 .0 .

LC 34 140°-160°C 0 784 4.5 4.4 3.8

LC 45

LC 48

240°-260°C

240°-260°C

5xl0-5 386

550

4.3

4.4

3.8

3.5 3.5 ± 0.1

Contaminated

Contaminated

LC 49

LC 51

240°-260°C

240 o -260°C

5x10-5 551

591

3.8

4.3

3.4

3.7

3.2 ± 0.1

3.8 + 0.1

Contaminated

LC 52 140°-160°C 533 4.4 4.2 4.2

LC 53

LC 56

240°-260°C

140°-160°C

5x10-5 573

532

4.5 3.8

4.03

3.7 ± 0.1

4.4 ± 0.1

LC 57

LC 58

LC 59

240°-260°C

240°-260°C

140°-1 60°C

534

687

564

4.2

4.5

4.7

3.5

3.6

3.4 ± 0.1

3.6

4.2 ± 0.1

* LC - laser evaporated coatings

173
Table 2. 0:Zr Peak-to-Peak Ratios for E-Beam Evaporated Coatings

u.^r r-r Katios

Coating Substrate 0 2 Thickness Surface Near Coating Comments

Dim jc Kun i eniperdxure pressure (in A) ourtace rrOT 1 1

e

EC 5 140°-160°C — 1182 3.5 3.5 3.2

EC 9 140°-160°C 741 3.8 3.4 3.3 High deposition

rate (10-12 A/s)

EC 14 140°-160°C 5x10-5 564 4.5 3.7 3.7

EC 17 140°-160°C 546 4.8 3.8 3.8

EC 18 240°-260°C 5x1

0"5 561 3.4 3.4 Contaminated

EC 21 240°-260°C 573 4.5 3.8 3.9

EC 22 140°-160°C 5xl0-5 604 4.3 3.3 3.4 Contaminated

EC 25 140°-160°C 558 4.5 3.6 3.7

* EC - E--beam evaporated coatings.

Analysis of the Zr02 coatings revealed several factors which were common to the majority of

UHV coated samples. These factors were:

1. A large number of the coatings contained insignificantly small amounts of carbon. This

implies that the UHV coatings were cleaner than similar coatings produced in conventional HV

systems

.

2. The 0:Zr ratio for every UHV coating was compared with the ratio associated with the

Zr02 source. In every case, the ratio for the coating was found to be lower than that for the

source.

3. The 0:Zr ratio inside the coating was lower than the ratio at the surface. This result

simply confirms that free Zr at the surface of the coating combines with oxygen.

4. Figures 1 and 2 illustrate the main Zr peak, MNN, of the Auger spectrum of a coating at

the surface and near-surface. The MNN Auger peak of Zr (the lowest negative excursion near 140 eV

in the survey spectrum) had a different shape and energy peak position at the surface than that

inside the coating. This was true even if only 15-20 A of the coating were removed. The location

of the Zr peak in the spectra indicates whether the Zr detected is in chemical combination with

other elements or not; the Zr peak observed indicated that some of the Zr within the coating

existed in the form of a free metal.

A survey of the data presented in tables 1 and 2 reveals correlations of the stoichiometric

properties of the deposited films with both the deposition techniques and the deposition conditions. These correlations are summarized below.

3.2.1 . Cw Laser UHV Coatings

The 0:Zr ratio for a high-temperature deposition is lower than that for a low-temperature deposition in coatings deposited with the same 0 2 backfill pressure. This indicates that there is more oxygen loss from coatings at elevated temperatures than at low temperatures.

Among those coatings deposited at the same temperature, there was no systematic dependence

between the 0:Zr ratio and the 0 2 backfill pressure. This implies that molecular oxygen does not

combine significantly with the oxygen-deficient oxide during the deposition process.

174
132 134 136 138 148 142 144 146 148 158 152

KINETIC ENERGY, EV

Figure 1. Zr MNN Auger Peak from a Zr02 Coating prior to Sputtering

_;ES FROFILE B9/BB/83 5,2311

KINETIC ENERGY,

Figure 2. Zr MNN Auger Peak from a ZrC<2 Coating after Sputtering

175
3.2.2. E-Beam UHV Coatings

The influences of temperature and O2 backfill pressure on the film stoichiometry were not

pronounced. However, two statements can be made with respect to the data collected: (1) the

E-beam evaporated coatings have lower 0:Zr ratios than the ratios measured for the CW laser

coatings; and (2) a larger fraction of the E-beam coatings have been found to be contaminated as compared to the CW laser coatings.

3.3. Refractive Index Analysis

The refractive indices of all the ZrC"2 coatings deposited in the UHV during this phase were determined from ellipsometric analysis at 6328 A. Figure 3 shows the refractive index dependence

on O2 backfill pressure for CW-laser-evaporated coatings at both low and high substrate temperatures. The data presented represent the average of several coating runs. Figure 4 shows the

results for E-beam -evaporated coatings.

2.10 T

1 2 3 4 5

02 Pressure (10~ 5 Torr)

Figure 3. Refractive Index of CW Laser Deposited Coatings vs

C<2 Backfill Pressure

3.3.1 Laser Evaporation

Three distinct features are observed in the refractive indices of the laser-evaporated Zr02

coatings. These are: (1) a monotonic decrease in the refractive index (away from the bulk index)

with increasing O2 backfill pressure, (2) lower values for the refractive index at high substrate

temperatures, and (3) the abnormally low refractive index (1.94) on high-temperature substrates

without oxygen backfill. Three film properties could be responsible for this behavior: stoichiometry, structure and purity. It has been pointed out that the 0:Zr ratio is independent of 0?

backfill pressure at a given substrate temperature. For low temperatures, the monotonic behavior

was observed on clean coatings. By process of elimination, it may be concluded that this monotonic

decrease in refractive index is due to the entrapment of molecular oxygen in the coating which, in

turn, results in reduced film density.

176
1.86 T"

3

„-5

4

*15

Q2 Pressure (10 Torr)

Figure 4. Refractive Index of E-Beam Deposited Coatings vs 02 Backfill Pressure

AES analysis indicated that the high-temperature coatings exhibited an even lower 0:Zr ratio

than the low-temperature coatings. This observation is in agreement with the observation that the

high-temperature coatings have a lower refractive index.

The abnormally low refractive index value observed at high substrate temperatures without backfill oxygen does not appear to be an anomaly since several coating runs yielded similar results.

Also it would not seem to be associated with contamination, since most of the coatings made under

these conditions were relatively clean. We offer no explanation for this behavior.

3.3.2. E-Beam Evaporation

For E-beam coatings, there is a general trend for the refractive index to decrease with

increased O2 backfill pressure.

The distinction between high- and low-temperature E-beam depositions, although real, is relatively minor. This lack of a significant distinction could be due to variations in the deposition

rates, to fluctuations in the O2 pressure, or to the level of contamination (AES analysis indicated that these coatings generally had a higher level of contamination than CW-laser-evaporated

coatings). Of more significance is the fact that AES analysis indicated that the stoichiometry was about the same for both high- and low-temperature coatings. This would account for the lack of distinction between the refractive indices.

Another observation associated with E-beam coatings is that the presence of an O2 backfill

does not result in a change in film stoichiometry. Thus, the monotonic decrease of refractive index

with an increase in O2 pressure is structure related. Like the CW-laser-evaporated coatings,

this is probably due to the entrapment of molecular oxygen, which causes a decrease in the density

of the film.

An important final observation is that the indices of refraction of the CW laser coatings are greater than those of the E-beam coatings. This is consistent with AES analyses which indicated

lower 0:Zr ratio in the E-beam coatings than in the CW laser coatings.

177
3.4. Miscellaneous UHV Coatings

Table 3 below lists the properties of several ZrC"2 coatings which have characteristics different from those summarized above, but which were deposited during the course of these investigations .

Tahl ex 1

1 3D i e O

.

un see 1 1 aneous 1 1 U W r r\n + i fine unv coaxings

Coating Substrate O2 Pressure Coating Refractive 0:Zr Peak to Peak Ratio

Run Temperature (10~5 torr) Thickness Index Surface Coating Profile

LC 27 140°-160°C 9 222A 1 .95 4.8 4.9 ± 0.1

LC 28 140°-160°C 5 167A 1 .97 4.8 5.0 ± 0.1

LC 35 140°-160°C 2 343A 2.03 4.5 4.1 ± 0.1

LC 30 140°-160°C 2 822A 1 .99 4.6 4.1 ± 0.1

For very thin coatings, LC27 and LC28 (thickness less than 250 A), the 0:Zr ratio, at the surface as well as within the coating, approaches the bulk value. However, the refractive indices for

these coatings (1.95 and 1.97, respectively) are lower than that of the bulk index. This implies

that stoichiometry alone is not the only factor in determining the refractive index; the structure

of the coating must also play an important role. It is perhaps of significance that these very

thin coatings were deposited at a high O2 backfill pressure.

Coating LC35 was deposited at an O2 backfill pressure of 2xl0 -5 torr. At the end of the

deposition, the substrate temperature was raised from 150°C to 235°C; the substrates were subjected

to a thermal soak in a 2xl0~5 torr O2 atmosphere for one hour. Subsequent AES analysis re- vealed that the 0:Zr ratio was the same for this coating as for another coating (LC30) which was

not thermally soaked. However, the refractive index had increased from 1.99 (LC30) to

2.03 (LC35). Thus, the thermal soak in the presence of O2 would seem to produce a rearrangment

of O2 within the Zr02 matrix and a corresponding change in refractive index.

3.5. HV Coatings

A coating deposited by E-beam evaporation in the HV system was analyzed using AES surface and

profile techniques. The deposition parameters and conditions were similar to those in the UHV

system. The carbon level was found to be ten atomic percent (ten atomic percent means there is one carbon atom for every ten zirconium dioxide molecules). The coating represented one of the highest

quality HV coatings produced in the D0F. However, the level of carbon indicates clearly that HV

coatings generally exhibit a higher degree of contamination than UHV coatings, a result which is to

be expected.

El 1 ipsometric methods reveal that the indices of refraction for HV coatings are lower than

those for coatings deposited in the UHV system (both laser coatings and E-beam coatings). It was also evident that the the repeatability of refractive indices of the HV coatings was not as good as

it was for coatings produced in the UHV system.

Also of note is the comparative homogeneity of HV and UHV coatings; the HV coatings being less

homogeneous than the UHV coatings. Coatings from the HV system that did exhibit good structure, as evidenced by good homogeneity, had very low indices of refraction, typical values ranging from 1.82

to 1 .85.

178
3.6. Damage Test Results

Coating samples for damage testing were prepared by depositing one-half wave of Zr02 on superpolished fused silica substrates. The coatings were deposited in the UHV system by both CW

laser and E-beam evaporation and in the HV system by E-beam evaporation.

The samples were damage tested by the AFWL at a wavelength of 531 nm. The damage thresholds (0%

probability) are summarized in table 4 below.

Table 4. Damage Threshold of ZrC>2 Coatings at 531 nm

UHV Laser UHV E-beam HV E-beam

4.3 J/cm2 4.3 J/cm2 4.1 J/cm2

4.5 J/cm2 1.3-4.4 J/cm2 6.8 J/cm2

2.75 J/cm2 6.5 J/cm2 3.8 J/cm2

Although the damage thresholds of the HV coatings are about the same as those for the UHV

coatings, the results are inconclusive. This is probably due to the fact that only a limited

number of samples were tested, resulting in a large statistical spread in the measurements.

Furthermore, the UHV laser coatings were deposited under unfavorable conditions (deposition rates were far below the rates normally used and the substrate temperature was much higher than

planned). Further work is planned under improved conditions.

4. Summary and Conclusions

Many aspects of this investigation are yet to be completed. In particular, extensive work

will be required to establish a correlation between the controllable deposition parameters and

damage threshold. However, from the work conducted to date the following conclusions may be drawn:

1. The stoichiometry of Zr02 films deposited via electron beam or laser evaporation is

not directly influenced by molecular oxygen at backfill pressures up to 5xl0 -5 torr.

2. The stoichiometry of ZrC<2 films deposited on substrates at 150°C is superior to that

obtained at higher substrate temperatures. Also, the refractive indices obtained at the lower substrate temperature are closer to that of the bulk material as compared to the refractive indices

obtained at higher substrate temperatures.

3. The optical isotropy of Zr02 coatings deposited by either electron beam or laser

evaporation methods in a UHV environment is superior to the isotropy of coatings deposited in a HV

environment.

4. The existence of free metal and/or suboxides of Zr02 has been established in all

coatings irrespective of the method of evaporation or vacuum environment.

5. The contamination level of ZrC»2 coatings deposited in a UHV environment is at least an order of magnitude less than the level found in coatings produced in a HV environment.

This work was supported by the Air Force Weapons Laboratory, Air Force Systems Command,

United States Air Force, Kirtland AFB, New Mexico 87117.

The authors wish to acknowledge Jerry Kienle and Art Westerfeld for providing machine shop

support and Graham Flint for helpful discussions.

179
Manuscript Received

7-24-85 SAR

Oxygen Threshold for Ion-Bean Sputter Deposited Oxide Coatings

H. Demiryont and J. R. Sites

Physics Department, Colorado State University

Fort Collins, CO 80523

Reactive ion-beam sputter deposition, using a mixed argon-oxygen beam, has been

demonstrated to be an effective technique for fabrication of dense, amorphous Ti O2

»

Ta205> and Si02 coatings.. For both elemental and oxide targets there is a critical

level of oxygen in the beam necessary to produce stoichiometric films. This threshold is seen in the films' composition, refractive index, and optical band gap. With

elemental targets there is a major reduction in deposition rate at the oxygen threshold, strongly implying that an oxide layer is formed on the target, and is subsequently

sputtered onto the substrate.

Key words: amorphous; ion beam; optical band gap; optical coatings; reactive sputtering;

refractive index; silicon oxide; tantalum oxide; titanium oxide.

1. Introduction

Thin oxide films for optical coating applications can be produced by bombarding a target of

the desired material with a beam of argon, or other inert gas, ions. In general, however, the

oxygen content of the deposited film is somewhat lower than that of the target. Stoichiometry can

be maintained by adding a small amount of oxygen to the bombardment beam, or by adding a second ionbeam directing oxygen at the deposition substrate. In fact it may be preferable from impurity con- siderations to use an elemental target, such as very pure titanium metal, and add sufficient oxygen

to the system to produce stoichiometric HO2 [1]. The practical question then becomes how much

oxygen is required, and what factors affect that amount.

2. Experimental

Films of silicon, tantalum, and titanium oxides were sputter deposited using a 5 cm diameter

bombardment beam from a Kaufman-type ion source [2]. The beam was a mixture of argon and oxygen

with ion energy 1100 eV and ion flux typically 1 mA/cm2. To minimize beam divergence and target

charging effects electrons were added to the beam with a hot wire filament. Films were deposited on glass, quartz, or silicon depending on the analysis to be done. Film thickness, as well as refractive index and extinction coefficient as a function of wavelength, were extracted from optical transmission data [3,4]. Thicknesses were generally 200-300 nm, and all films were dense and nearly

amorphous

.

Optical bandgap was deduced using amorphous semiconductor theory [5] from the extrapolated

intercept of (ahv)^ vs. hv, where a is the absorption coefficient and hv the photon energy. Composition was determined from x-ray photoelectron spectroscopy (XPS) by measuring the fraction of

Si, Ta, or Ti in each observed bonding state [6,7]. The composition parameter x is defined to be 1 under stoichiometric conditions, and in general the ratio of oxygen atoms present compared to the

number needed for stoichiometry. The XPS determination was in all cases consistent with the composition estimate made from the optical constants.

3. Results

Dispersion curves for tantalum oxide films are shown in figure 1. In each case a pure tantalum target was used, the total background pressure in the deposition chamber was 8x1 0"^ torr

[1x10-2 Pa], and the beam current density was 1.0 mA/cm?. As the oxygen fraction of the ion-beam,

and hence the background partial pressure is raised, the films approach stoichiometric Ta205- The

visible and infrared absorption becomes very small, the absorption edge converges to an optical

bandgap of 4.3 eV, and the refractive index at longer wavelengths goes to 2.1. The critical background pressure to produce a stoichiometric film is 40-50%.

180
12.5%

O

1 1 1 1 ' I I I 0.3 0.5 0.7 0.9

Wavelength (^m)

Figure 1. Dispersion curves for tantalum oxide films ion-beam sputter deposited with varying

amounts of oxygen in the primary beam. Percentages refer to the fractional amount

of oxygen background pressure.

A similar convergence of dispersion curves is found for oxides sputtered from elemental titanium and silicon targets. The critical oxygen background pressure for stoichiometric Ti O2 is some- what less than for Ta205; for Si O2 » it is greater. Figure 2a shows the degree of stoichiometry as

a function of oxygen background. As can be seen the silicon oxide did not become stoichiometric

Si O2 at the highest oxygen fraction used. In contrast, a Si02 target will yield an Si O2 film with

only a small amount of oxygen in the chamber.

There is a strong correlation between the composition of a sputtered film and deposition rate,

shown in fig. 2b. The drop in deposition rate with oxygen background corresponds to the difference

between a high sputter rate for an elemental target and a slower one for an oxide target. The silicon target rate in fact approaches that of our Si O2 target in the limit of large oxygen background.

The strong implication is that background oxygen is oxidizing the target, if the oxidation rate is

faster than the sputter rate, a stoichiometric oxide is sputtered. If it is less, than target oxidation is not complete and an oxygen deficient film is sputtered. The model we suggest is very

similar to that of Castellano [8] with slight differences noted below.

181
i r

0 I I I I I 0 20 40 60 80

BACKGROUND OXYGEN PERCENTAGE

Figure 2. Composition and deposition rates of ion-beam sputter deposited oxides as a function of

oxygen background. Total background pressure was 8xl0~5 torr. Ion beam flux was

1 .0 mA/cm2 .

There is (fig. 3a) a dramatic change in the optical band gap of the deposited layers with oxygen

background. However, as is suggested in fig. la, the convergence of the band gap is less sensitive

to the degree of stoichiometry than the longer wavelength absorption, known as the Urbach tail [9].

Furthermore, it is this longer wavelength absorption which is important to minimize in practical

optical coating applications. This point is illustrated for silicon oxides in fig. 3b where the

absorption, particularly in the 2-4 eV region, increases dramatically with oxygen deficiency.

4. Model

Returning now to the oxidized target model of deposition, the target should look like the sketch

in fig. 4 when a stoichiometric oxide is being sputter deposited. The oxide thickness z must of course be greater than the maximum depth from which material is sputtered. The graph shows the

competition between oxidation rate R ox and target sputter rate R5. In this case sufficient oxygen

is available to form a 60 A thick oxide layer, and for sputtering purposes one has an oxide target.

However, if one lowers the oxygen availability or raises the sputter rate, the curves will either

cross at a very low oxide thickness or not at all. In either case the target material sputtered

will be deficient in oxygen.

182
COMPOSITION PARAMETER x

2 4 6 PHOTON ENERGY CeV]

Fiqure 3 (a) Optical bandgap as a function of composition for silicon, tantalum, and titantium

oxides, (b) Optical absorption curves for silicon oxides of varying composition.

1000

Figure 4. Possible oxidation and sputter rates for an elemental target as a function of the oxide

thickness z.

183
The critical pressure for stoichiometry occurs when the solid and dashed curves intercept at

zero. Changes in target temperature will alter the diffusion limited part of the solid curve, but

should have little effect on the critical pressure. The only difference in Castellano's model [8]

is that he assumes that only a monolayer of oxygen coverage is necessary to create an oxide-like

target. From either viewpoint, Castellano's prediction of the critical oxygen pressure

-5 ^SM

P = 3x10

0 torr

OX AG

0

(1)

should be valid. Rs^ is the sputter rate of the metal, and aG 0

is the free energey of formation.

Using our values of sputter rates (approximately 4 times our deposition rates) and published free

energies [10], we find

P oxt Ti °

2

] 5 3.5x10 torr

P„ [Ta 90E ] - 4x10 torr

ox 2 5

P [Si0 9 ] * 5x1

0

5 torr

ox I

(2a)

(2b)

(2c)

With a total argon-oxygen pressures of 8x1

0

_ 5, these values correspond to oxygen percentages of 44%,

50%, and 63% respectively, in rough agreement with the data shown in figs. 1 and 2.

Equation (1) further predicts that if the sputter rate Rsm is increased, the critical oxygen

pressure will be proportionally increased. Figure 5 illustrates the point by showing the changes

UJ

hUJ<

<

Q_

O

I—

COo

Q_

o

o

0.5

O 20 40 60

BACKGROUND OXYGEN PERCENTAGE

Figure 5. Effect of sputter rate on the increase in composition parameter with background oxygen.

in stoichiometry for silicon oxide films when the ion-beam current density, and hence the sputter

rate, is decreased by 30% and increased by a factor of two. The roughly proportional shift in

background oxygen needed to maintain constant composition confirms that it is the oxygen background,

not the oxygen in the sputter beam, that controls the sputter rate. Thus, one may with some confidence construct the critical oxygen pressure vs. beam density curves of fig. 6. Use of the other

elemental targets will require different slopes, which may be related to one of these oxides through

the relative ratios of R$m to aG0.

184
8x10

Pox Ctorr]

2

1 1 1

Si / // - /Ta/ /

/ //Ti target

0 1 2

BEAM CURRENT DENSITY CmA/cm2

]

Figure 6. Critical oxygen background pressure P ox to produce stoichiometric oxides by ion-beam

sputter deposition of elemental targets.

This work was supported by the Air Force Weapons Laboratory under Contract F29601 -83-K-0079.

5. References

[1] Sites, J. R.; Gilstrap, P.; Rujkorakarn, R. Ion beam sputter deposition of optical coatings.

Optical Engr. 22(4): 447-449; 1983 July/August.

[2] Kaufman, H. R.; Cuomo, J. J.; Harper, J. M. E. Technology and applications of broad-beam ion sources used in sputtering. Part I. Ion source technology. J. Vac. Sci. Techno!. 21(3):

725-736; 1982 September/October.

[3] Manifacier, J. C. ; Gasiot, J.; Filland, J. P. A simple method for the determination of the

optical constants n, k and the thickness of a weakly absorbing thin film. J. Phys. E: Sci.

Instrum. 9: 1002-1004; 1976.

[4] Demiryont, H.; Kerwin, D. B.; Sites, J. R. Optical properties of ion-beam sputtered Ti02 fill

NBS Special Publication, 1983 Boulder Damage Symposium, to be published.

[5] Tauc, J. Amorphous and liquid semiconductors. New York: Plenum; 1974.

[6] Demiryont, H.; Sites, J. R. Effects of oxygen in ion-beam sputter deposition of titanium

oxides. J. Vac. Sci. Technol . A2(4): 1457-1460; 1984 October-December.

[7] Demiryont, H.; Sites, J. R. ; Gieb, K. Effects of oxygen content on the optical properties of

tantalum oxide films deposited by ion-beam sputtering. Appl. Opt. 24(4): 490-495; 1985

February 15.

[8] Castellano, R. N. The metal-dielectric transition pressure in reactive sputtering. Proceedings of the 7th international vacuum conference and 3rd international conference on solid

surfaces; 1977; Vienna; 1449-1452.

[9] Dow, J. D. Optical properties of highly transparent solids, S. S. Mitra and B. Bendov, ed.

New York: Plenum; 1979. 131-150.

[10] Handbook of chemistry and physics, 51st edition, R. C. Weast, ed. Cleveland: The Chemical

Rubber Company; 1971. D69-D71.

185
One questioner wondered if the oxygen pressure had an effect on the physical properties such

as viid density as well as on stoichiometry . The author stated that there were not many voids

for films prepared in this way and effect on film phusical properties was slight. The XPS

values shown were taken on the surface. After sputter etching the bulk of the film is re- duced a little bit. They have tried to approach the limit of low energies for sputter etching

but one is still not convinced that the bulk values are being obrained in XPS. On the other

handy these materials are chemically active so an oxide film will form on the surface and just

a little sputter etch may be optimum. Another question concerned the reflective medium

approximation used. Since only amorphous silicon data was used, were the films assumed to

be amorphous? The author replied that the films were amorphous with a band gap at 1.75 eV.

Approximately 1% off stoichiometry could be detected. The ion beams penetrated about 2 ran into

the surface. If there is sufficient oxygen, the oxide layer is much thicker than 2 nmj otherwise it may be thinner than 2 nm. In response to a question .of the perceived advantages of

ion beam sputtering over conventional reactive sputtering, the author named three; (1) more

nearly amorphous films of SiOx and TaOx, (2) better process control and (3) higher density

films because more energetic atoms strike the substrate. The main contaminant from the ion

gun which gets into the film is carbon. Using XPS they have looked for contaminants , particularly carbon and iron (because the chamber is stainless steel). They have found no iron.

There is a small amount of carbon, which apprears mainly on the surface of the films. The

interior of the films has a carbon level below 0.5% to 0.7%, the limit of detection using this

technique. How much absorption arises from impurities is not known, but it does not appear

to be a major drawback to the technique.

186
TOMORROWS COATINGS TODAY - THEY NEED HELP Manuscript Received

4-3-85 SAR

Alan F. Stewart

Arthur H. Guenther

Air Force Weapons Laboratory

Kirtland AFB, NM 87117

A representative set of dielectric coatings prepared using novel deposition

techniques have been evaluated. Single layer coatings of Zr02 and AI2O3 were deposited on super polished fused silica substrates by conventional electron beam,

electron beam in UHV, electron beam with ion beam assist, CW laser in UHV, and an advanced epitaxial technique. Characterization included total integrated scatter,

spectrophotometry, x-ray diffractometry and pulsed laser damage testing at 532 nm. Our results indicate that considerable additional work will be required to optimize

the optical performance of coatings produced by these novel deposition methods, let

alone understand the relationship between deposition process, structure, optical

characteristics and durability.

Key Words: Electron beam; epitaxial; ion assisted; laser damage; laser deposition;

novel deposition techniques; scattering; structure; thin film coatings; ultrahigh

vacuum

.

1 . Introduction

In response to the growing requirement for higher quality optical coatings, thin film process

development has undergone a dramatic surge in the past decade, primarily as a result of the

stringent demands relating to many laser applications. In most instances, novel deposition

techniques offer greater control over the coating growth process than was possible in earlier

conventional thermal evaporation systems. More control over film deposition along with carefully

planned and controlled experiments should be leading to a higher level of understanding and

performance with proponents of each technique promising higher purity films, more stoichiometric

films approaching bulk material packing densities, either completely amorphous or epitaxial films

and higher laser damage thresholds than heretofore. However, as the complexity of a deposition

process increases, the number of control parameters expands and it takes more time to optimize the

procedure in a trial or parametric manner. This may explain why the anticipated impressive

breakthroughs in thin film performance have not been evidenced to date.

The objective of this small study was to evaluate current performance levels for films

produced by several novel deposition techniques. In some cases, the films tested as part of this

survey were the first of their kind to reach this laboratory. No attempt has been made yet to

optimize coating performance by an iterative process. Data produced thus far will serve as a benchmark to gauge against future improvements.

Refractory oxide films of Zr02 and A1203 were deposited on super polished fused silica

substrates. These coatings were characterized using total integrated scatter (TIS),

spectrophotometry, x-ray diffractometry and pulsed laser damage testing at 532 nm. Nomarski

microscopy was used to study film structure and damage morphology. Overall, test results were

encouraging. These films were comparable to coatings deposited by more conventional and practical

techniques in most respects presaging an improvement in the future as these new processes evolve.

2 . Sample Fabrication

Thin film coatings deposited at five facilities were tested in this survey. Only single layer

coatings were tested. Each had an optical thickness of lx at 532 nm. All of the coatings tested

were deposited on super polished Suprasil T22 fused silica produced by the controlled grinding

technique. [1] Two substrates, SN 303 and 304, were polished and coated by one commerical

vendor. The remaining 23 substrates were supplied by a second vendor after they deposited

coatings on samples SN 37, 38, 84 and 85. All substrates were smooth with total integrated

scatter data indicating an average surface roughness for the entire set of 5.5 ± .7 A Rms. In

addition, the Bidirectional Reflectance Distribution Function (BRDF) of these bare substrates was

187
measured using a variable angle scatterometer . The average scattering level for these super

polished substrates was 0.265 PPM/steradian. [2] Both the TIS and BRDF data indicate that these

surfaces were of good but not superior quality. Approximately half of the substrates coated in

this survey were supplied with a centered sputtered molybdenum dot for film scattering

measurements.

Various deposition techniques were used to fabricate these single layer coatings. The

deposition methods and cleaning procedures used for each sample tested are summarized in table

1. For samples with ion beam assisted and the epitaxial coatings it was clear from normal visual

observation that the substrate cleaning procedures employed were inadequate. Cleaning procedures

can certainly be improved. Each deposition method is more thoroughly described in the following

paragraphs.

At the Developmental Optics Facility (DOF) at AFWL, Zr02 films were deposited in an ultrahigh

vacuum chamber (UHV) using either a standard electron beam source or a CW CO? laser to heat the

coating material. The coating chamber base pressure was typically 5 x 10~ 1U Torr following a 24

hour pumpdown and bakeout cycle. During deposition, substrates in the chamber were heated to

200° C using Quartzline lamps and the system pressure was raised to 2 x 10 Torr as oxygen was bled into the chamber. The deposition rate using the electron beam source was 1 A/sec. With an incident beam power of 70 to 100 watts, the C0£ laser deposition rate was comparable

at 0.5 A/sec. At all times a residual gas analyzer was used to monitor the chamber environment.

An additional set of ZrO? coatings were fabricated at the DOF in a conventional diffusion

pumped vacuum chamber outfitted with an LN^ cold trap. The system base pressure prior to

deposition was 2 x 10 Jorr. During deposition, oxygen was bled into the chamber and the system

pressure rose to 3 x 10" 5 Torr. The film growth rate during this coating run was 5 A/sec.

Ion beam assisted electron beam deposition was used to fabricate some Al films, in addition

to a fourth set of Zr02 thin film coatings. During electron beam deposition an ion beam of either

600 ev 02

+ or 100 ev Ar+ ions bombarded the growing film. The 0£ ion current density

was 60 y amp/cm2 . For the ZrOo films, the deposition rate was 3.5 A/sec. The rate was slightly

higher for the A^O-j films at 1 J A /sec.

An advanced epitaxial film deposition process was used to fabricate AI2O3 and Ti 0o coatings.

Details of the deposition process were not available. However these films were baked at 500°C for

10 hours after deposition.

Conventionally deposited commerical coatings of Z^, Al and Hf02 were included in this

survey to provide baseline data. Electron beam deposition was used for each of these coating

material s.

188
Table 1 Sample Preparation and Deposition Parameters

ZrC^ Films

SN 97, 99, 110

SN 98, 101, 111

SN 102, 107, 108

SN 25, 52

SN 26, 54

Deposition Method/Parameters

C.W. CO? laser, 70

UHV system, base pressure

Deposition pressure, 2x10

oxygen bleed

0.5 A/sec deposition rate

100 watts

xlO" 10 Torr,

Torr with

E beam, UHV system as above

Deposition pressure, 2xl0~ b Torr with

oxygen bleed

1 A /sec deposition rate

E beam, vacuum system base pressure

2x10";? Torr, Deposition pressure

3x10 Torr with oxygen bleed

5 A /sec deposition rate

Ion beam assist, 600 ev 02

+ ions,

ion current density 60 u amp/cm2 3.5 A /sec deposition rate

Ion beam assist, 100 ev Ar

+

ions,

3.5 A/sec deposition rate

Cleaning Procedure

Prior to Coating

cheese cloth rub with

isopropanol, isopropanol

rinse followed by

isopropanol /freon vapor

degreasing

cheese cloth rub with

isopropanol, isopropanol

rinse followed by

isopropanol /freon vapor

degreasing

cheese cloth rub with

isopropanol, isopropanol

rinse followed by

isopropanol /freon vapor

degreasing

Spin cleaner with

isopropyl alcohol

uncleaned

SN 303, 304

A1 2O3 Films

SN 30

SN 21, 49

SN 84, 85

E beam

Deposition pressure 6.5x10'

with oxygen bleed

Torr

Ion beam assist, 600 ev 02

+ ions,

ion current density 60 p amp/cm2

, 13 A /sec deposition rate

Epitaxial followed by baking

at 500° C for 10 hrs

E beam

liquid detergent

followed by deionized

water rinse

Spin cleaner

with isopropyl

alcohol

lens tissue

and ethanol

proprietary

Ti O2 Films

SN 22, 50 Epitaxial followed by baking lens tissue and

at 500°C for 10 hours ethanol

HfQ? Films

SN 37 , 38 E beam proprietary

189
3. Film Characterization

Coatings deposited by each technique were subjected to extensive characterization. The

transmission characteristics of these films near the UV cutoff was studied using a spectrophotometer.

a ' Total integrated scatter measurements provided data on the amount of

apparent surface roughness induced by the coating. X-ray diffractometry was used in an attempt to

study film structure. Pulsed laser damage testing was performed at 532 nm. After damage testing,

the surface and damage site morphology was examined using a Nomarski microscope.

Examination of spectrophotometer curves for these films showed that the optical thickness

varied considerably from sample to sample. Each thin film coating was nominally half wave at 1.06

microns. However, samples produced by the DOF had thicknesses varying from half wave at 0.89 to

1.39 microns. In addition, there were apparent differences between samples coated in the same run at DOF. These two problems were caused by the lack of calibration of a crystal thickness monitor

and no planetary drive. It is not known what problems were experienced at the other facilities

although the other coatings tested were generally closer to the design goal.

The transmission characteristics of these films near the UV cutoff were also compared. One

Zr02 film, SN 25, produced by ion beam assisted electron beam deposition showed lower overall

transmission, a shallower approach towards cutoff indicating higher absorption and a longer

wavelength UV cutoff. The spectrophotometer curves for SN 25 and a conventional electron beam

deposited Zr02 film, SN 303, are compared in figure 1. Similar observations hold for an Al 2O3

film, SN 30, produced by the same technique. All other films of each type had transmission curves

which, except for thickness variations, were indistinguishable.

(a) Cary 2300

190
Table 2

Damage Threshold and TIS Results

Deposition Method aTIS (Film 532 nm Damage

roughness in Threshold

A RMS) (J/cmZ

) ± 22%

Zr02 Films

SN 97

99

110

SN 98

101

111

SN 102

107

108

SN 25

52

26

54

SN 303

304

AI2O3 Films

CW laser, UHV

CW laser, UHV

CW laser, UHV

E beam, UHV

E beam, UHV

E beam, UHV

E beam, HV

E beam, HV

E beam, HV

Ion beam assist

(600 ev 0?+

)

Ion beam assist

(100 ev Ar)

E beam

~8T5~ ±5.8

7.4 + 3.3

UTS"! 1.8

6.4 ± 1.8

15.8 ± 2.4

14.5 ± 1.3

7.3 ± 0.9

4.3

4.5

2.8

4.3

1.3

6.5

4.1

6.8

3.8

0.8

3.1

2.0

3.7

4.4

SN 30

21

49

SN 84

85

Ion beam assist

(600 ev 0

2 +

)

Epitaxial

Epitaxial

E beam

E beam

18.6 ±2.5

37.1 ±4.9

10.1 ± 1.2

6.6 ± 0.8

4.0

1.3

2.0

4.8

2.5

Ti02 Films

SN 22

50

Epitaxial

Epitaxial 28.1 ±2.1

< 0.5

Hf0

2 Films

SN 37

38

E beam

E beam

5.5 ± 0.7

5.8 ± 0.7

6.8

6.0

Si O2 Bare Surface

SN 80 None 5.8 ± 0.8

(on Moly dot)

13.1

191
Total integrated scattering measurements were performed at AFWL using instruments operating at

632 and 514 nm wavelengths. Both instruments track closely with data from other laboratories as demonstrated in round robin testing. [3] Surface roughness calculations based on TIS measurements

are summarized in table 2. Total integrated scatter on samples SN 97-111 were inferred from data

on witness samples. In each case, the roughness quoted is the difference between measurements

performed on the coated and on the uncoated substrate. Overall, the smoothest films evaluated in

this study were those fabricated by a commercial vendor using electron beam deposition. Coatings

produced at the DOF using electron beam and laser deposition were comparable to those commercially

available. Films produced using ion beam assisted electron beam deposition were generally rougher

with the exception of SN 52 which was very smooth. The advanced epitaxial technique produced

films with consistently high scatter or roughness levels. Unless single crystal films can be

grown using this technique, higher scattering losses may prove to be a serious penalty. It should

be noted however, that these films were grown in a commercial facility for nonoptical thin film

appl ications.

A Siemen's x-ray diffractometer was used to study the structure of a limited number of thin

film coatings. This instrument is equipped with computer enhanced signal processing which

provides greater sensitivity to the inherently weak diffraction maxima produced by thin film

coatings. The majority of the films tested had a crystalline structure and were not amorphous as had been anticipated. Equally surprising was the fact that the observed structure varied

significantly and in a reproducible fashion as the deposition process was altered between coating

runs. This is observed particularly in data obtained on zirconia coatings deposited at the DOF.

Coatings from each of the three deposition processes used at the DOF were measured and these

diffractometer scans appear in figure 2. All three curves exhibit a large broadened peak centered

at 21.5 degrees which originates from the fused silica substrate. The lower curve is

representative of coatings deposited by DOF using an electron beam in a conventional diffusion

pumped vacuum system. Only one very small peak occurs at 34 degrees corresponding to a lattice

spacing of 2.57 Angstroms in the tetragonal form of zirconia. The middle curve is representative

of coatings produced again by electron beam deposition but this time in a UHV chamber. In sharp

contrast to the lower curve, these coatings exhibited many peaks corresponding to the tetragonal

structure at 2.94, 2.57 and 1.80 Angstroms. The monoclinic structure was also in evidence with a small peak at 2.84 Angstroms. The monoclinic structure is again absent in the upper curve which

is representative of zirconia films produced in the same UHV chamber but using CW laser

deposition. Very strong peaks at 2.94, 2.62, 2.54, 1.81 and 1.53 Angstroms all relate to various

orientations of the tetragonal structure.

In figure 3, additional data from zirconia films produced by ion beam assisted electron beam

deposition is plotted. The lower curve from sample SN 26 (100 ev Ar ions) shows no identifiable

peaks other than that due to the substrate. The upper curve from sample SN 25 (600 ev 0£+ ions)

shows a strong peak at 2.84 Angstroms from the monoclinic structure and secondary peaks at 2.94

and 2.61 Angstroms from the tetragonal structure.

The x-ray diffractometer data from the majority of these zirconia films demonstrates that

condensation of atoms or molecules from the vapor phase did not occur under equilibrium

conditions. The monoclinic structure is the only stable phase of pure zirconia at room temperature. The tetragonal and cubic phases occur only at temperatures above 1,020°C but can be

stabilized at lower temperatures by mixing with a few weight percent of alumina or yttria.

However, these stabilizing agents are not present in measurable amounts in the zirconia films

tested in this survey. The tetragonal form is clearly a metastable phase. It should also be

clear that an amorphous zirconia film is also a metastable form. By analyzing films and relating

this data back to conditions present during the deposition we may learn how a film grows.

The presence of metastable phases in thin film coatings may have serious implications. When

zirconia undergoes a phase transition from monoclinic to tetragonal, the bulk density decreases by

about 1.5%. However, optical properties such as refractive index have not been measured for the

high temperature phase. Large differences in optical absorption and scattering have been measured

between the rutile and anatase phases of titania in thin films. Recent work using Raman

spectroscopy to monitor the film structure has demonstrated that phase changes in titania and

zirconia films occur while annealing the films at temperatures substantially below the transition

temperature for the bulk material. [4] Phase transformations may also be induced by localized

heating following the absorption of laser radiation. Laser processing or annealing of thin films

might prove to be of considerable benefit by producing uniform films of a stable phase as opposed

to the unintentional catastrophic changes that occur during laser damage.

192
An alumina coating, SN 21, produced by an advanced epitaxial technique was also tested on the

x-ray diffractometer. There was no apparent structure in the film. This was very suprising since

even if the film had been deposited as amorphous, the long period of baking at elevated

temperature might have caused conversion from the metastable amorphous phase into the crystalline

phase. Titania films produced by this epitiaxial process have not been tested. However, even if

these films did exhibit crystalline structure it would not be clear whether the film nucleated and

grew in that form or if the film structure changed radically during the baking cycle.

Examination of both bare fused silica substrates and each coated substrate under a high

contrast Nomarski microscope demonstrated a frequently observed phenomenon. The physical

structure of some of the thin film coatings, as determined visually, appeared to correlate well

with the surface structure of the uncoated substrates. Surfaces coated with zirconia by the DOF

at AFWL appeared to be covered with a fine web of scratches as is shown in figure 4. The zirconia

films deposited by three different procedures at the DOF all have similar appearances under the

microscope. Total integrated scatter measurements on these coatings (table 2) indicated an average roughness of about 8 Angstroms which is excellent for the deposition methods . represented

in this study. However, these films have very different crystalline structures as is evidenced by

the x-ray diffraction data. It is apparent that the nucleation of the coating material on these

substrates and hence to some extent their scattering characteristics and physical appearance is

largely dependent on the substrate surface and how it was prepared. In contrast, the crystalline

structure of the film seems to be solely dependent on the deposition process used.

Thin film coatings produced by ion beam assisted electron beam deposition exhibited a different appearance under the microscope. Unlike the coatings produced at the DOF, these

coatings did not appear to replicate the structure of the substrate surface. The coated surfaces

appeared to be covered with randomly distributed small diameter ( ~ 1 um) pits. This deposition

process involves bombarding the surface with energetic and even reactive ions throughout the

coating run. This type of process continuously changes the growth surface through kinetic energy

transfer and sputtering with higher ion energies and it is thus logical that substrate

characteristics would be less influential in the final film topography - at least at the visually

observable level. The appearance of surface pitting is almost certainly related to sputtering by

the incident ion beam.

Under the microscope, aluminia and titania coatings produced by an advanced epitaxial

technique looked like a step backwards. These films were covered with scattering defects, perhaps

crystallites, which scattered considerably in the field of view. These observations confirmed the

TIS data which showed excessively high scattering levels. The overall roughness of these coatings

completely dominated any contribution from the substrate. It should be noted again that these

films were part of a production run for a nonoptical thin film application.

A frequency doubled Nd:YAG laser operating at 10 hz was used in damage testing. The laser was extremely stable with shot-to-shot energy fluctuations in the 1 or 2% range. The experiment

configuration is detailed in figure 5. Samples were tested in the focal plane of a 2 meter focal

length lens. The beam spatial profile was measured in the focal plane using the beam scan technique with a 40 micron wide slit and a sensitive pyroelectric detector. In this way the beam

profile was measured to have an ellipticity which ranged from 14 to 27% over the course of these

measurements. A representative beam scan appears in figure 6a. The average beam diameter (1/e

in intensity) was 520 microns. The temporal profile of the laser pulse was measured with a fast vacuum photodiode and a transient digitizer. As shown in figure 6b the waveform is three humped

with a central maximum. By adjusting the oscillator pump energy, the side lobes were made to be

of comparable height. The pulse duration is therefore not easily defined but was on the order of

15 nsec FWHM.

Damage testing was performed in the N-on-1 mode. The laser was used to irradiate each site

with up to 100 consecutive shots. Twelve equally spaced fluence levels were used with 10 sites

irradiated at each fluence level. During each irradiation sequence, the sample surface was observed with a long working distance microscope. With the onset of visually observable damage a shutter was closed to prevent splattering on nearby sites. After testing, each site was observed

under a Nomarski microscope to provide an absolute determination as to whether permanent damage

had occured.

The damage threshold for each sample is listed in table 2. Data was plotted up as shown in

figure 7 after reference [5]. The damage threshold was defined as the maximum energy fluence for

which there is zero probability of damage occuring. In some instances, the probability would rise

to say 20%, fall back again to zero and then increase sharply at still higher fluences. This is

193
an indication that not enough sites were tested at each fluence to sample a large enough area of

the coating.

Some distinct trends were observable in the data. The probability curves obtained on samples

SN 25 and SN 26 appear in figure 8. Both zirconia films were produced using ion beam assisted

electron beam deposition but with different ion species and different ion energies. The zero probability onset levels are quite different and the slope of the data obtained on sample SN 26 is

very shallow by comparison to the steep onset of data from SN 25.

A shallow slope in the probability curve has been attributed to a defect dominated damage

process [5] [6]. Sample SN 26 and each of the coatings produced at the DOF appear to have similar

defects or impurities which damage at about the same fluence level. However, the density of these

defects may be process dependent to some extent. The probability curve measured on coatings

produced at the DOF in the UHV chamber generally exhibited shallower slopes as for SN 26

indicating a lower surface density of defects. It should be pointed out that if a larger beam

diameter had been used in the test sequence those apparent differences might not have been

observed. A larger beam diameter increases the probability of hitting a defect with each shot.

Thus the measurement becomes more deterministic and the slope of the probability curve becomes

infinite. Using a smaller diameter beam does make the measurement of an absolute threshold more difficult since more sites must be irradiated to generate a smooth curve. However, our qualitative observation of shallower slopes in the data may be an indication that coatings

produced in the UHV chamber and using O2 bombardment are a step in the right direction.

The damage morphology of these films was quite different from that observed in previous 1-on-l

testing at this laboratory. Instead of the isolated pits observed previously, much larger area features evolve in N-on-1 testing. The irradiation sequence of 100 shots was halted by the

operator only if damage or an increase in light scattering was observed through the telescope.

Even under ideal conditions the damage morphology observed after testing is a result of 2 or 3

consecutive destructive shots. However, some general statements can be made. A distinctive

feature of damage sites in the zirconia films produced at the DOF appears in figure 9a. Damage

sites such as this are about 50 microns in diameter and are surrounded by ejected material. In marked contrast, a damage site on zirconia produced by ion beam assisted electron beam deposition

appears in figure 9b. These sites have a much "softer" appearance as the film looks as if it

bubbled up on the surface. During irradiation, these coatings were not observed to eject large p- lasma plumes and there was no audible indication of a violent breakdown. The only observed change was a slow increase in the scatter levels at a site. Epitaxial coatings fall somewhere in between

as is shown in figure 9c. The damage process appeared to result in a more radical or violent

change than that observed in the ion assisted coatings. Although much lower fluences were

required to produce damage, a pronounced halo surrounded each damage site. Normally, such a halo

is associated with plasma formation. However, the overall features of these sites cannot be

clearly studied since they scatter so much. This suggests that in figure 9c the disrupted

material inside the halo has undergone a phase change and is the rutile phase of the titania. The

observed damage morphology in the epitaxial films may then be the result of a completely different

damage process.

The morphology of laser damage sites in alumina films was quite different from that observed

in zirconia. As shown in figure 10a, alumina films fracture with the onset of damage. This site

on sample SN 84, which is a commercial electron beam coating, shows a large damage site with a thermal or plasma halo and extensive radial fracturing. Films produced by either ion beam

assisted electron beam or the epitaxial process exhibited a much different damage morphology. The

fracturing, cracking or selective etching tended to pattern itself after what might have been the

substrate surface structure as is shown in figure 10b. This site is from sample SN 21, an epitaxial coating with low damage threshold. The fractures are not radially oriented but seem to

follow what can only be patterns of polishing scratches on the substrate surface. As stated

previously, films produced by these deposition procedures do not appear to replicate the substrate

surface. However, the onset of damage nearby may provide the thermal "trigger" for rapid

crystallization of the amorphous or metastable film. Another possible mechanism involves the

interaction of the damage site plasma with the surrounding surface. Plasmas were not observed to

form with the onset of damage in these films.

194
4. Conclusions

The data collected after extensive film characterization has been summarized for each novel

film deposition technique in table 3. The performance levels of the films evaluated in this

limited survey in terms of film scattering and laser induced damage thresholds compare well with

conventionally deposited films. Thus we have produced an encouraging base line upon which

deposition improvements or perfection of these novel methods can ensue. The one exception was the

films produced using the advanced epitaxial technique. However, even films produced by this

method may perform much better if adequate cleaning procedures are utilized. Each of the

deposition procedures sampled by this survey is relatively new to the scene and much work needs to

be done to optimize the resultant films. The analysis and characterization work reported here is

only an initialization.

The study of film structure using x-ray diffraction or other techniques provides perhaps our most important insight as to how a film grows so we can relate it to how the film damages. The

presence of the metastable tetragonal phase of zirconia in laser deposited thin films does

indicate that the film formed under nonequil ibrium conditions. Comparison of film structure with

laser damage thresholds while varying substrate temperature during deposition or in a post

deposition annealing treatment is the next logical step. The sensitivity of the film structure to

temperature variation is a critical unknown in itself and is currently under study. [4]

There are several inconsistencies which are yet to be resolved. For example, the damage

morphology observed in films deposited by ion sputtering has been attributed to uniform film

absorption because the damage site appears to replicate the beam footprint. Similiar morphology

was observed in this study on thin films produced by ion assisted deposition. Yet it seems contradictory that thin films produced by the ion sputtering technique routinely demonstrate

extremely low losses due to scattering or absorption. Certainly these properties are dependent on the type of film defects and impurities and their distribution. The most likely mechanism for

damage in these films as in conventionally deposited films is impurity absorption. But the

difference lies in that the heated impurity in the ion sputtered or ion assisted film most

probably causes the surrounding material to undergo a phase change either from amorphous to

crystalline or from one crystalline phase to another. This type of laser induced damage may be more prevalent in conventionally deposited films than was previously thought.

The objectives of this study have been met in that films produced by novel deposition

techniques have been evaluated and compared with commerically available coatings. Even though no optimization has occured yet to maximize laser induced damage thresholds or minimize film

scattering, the performance levels observed to date are encouraging. Certainly, parameterization

of these deposition processes is in order followed by even more extensive film characterization.

It is important to realize that a film may perform at a very high level for one application, low

scatter films for laser gyros for instance, whereas these same films may not be damage resistant

in HEL use. Both performance levels may be attributable to one variable - the film structure - or there may be other "hidden" variables that as yet remain unrecognized. The interrelation of film

performance levels is difficult to study because there are so many variables that must be

controlled. Once the salient characteristics of a film have been identified and the deposition

processes completely parameterized we may reach a positon where the most suitable film can be

fabricated for a given application.

195
Table 3

Summary of Observations/ Character! sties

CW Laser, UHV Film scattering levels, < 10 A RMS for Zr02

.

Crystalline structure of metastable tetragonal phase of Z^.

Film surface replicates substrate surface topography.

Medium level damage thresholds.

Slopes of damage probability curves indicate lower defect densities than

found in conventional films.

Electron Beam, UHV Film scattering levels, < 10 A RMS for Zr02.

Crystalline structure of metastable tetragonal

and stable monoclinic phases of Zr02

.

Film surface replicates substrate surface topography.

Medium to high damage thresholds.

Slopes of damage probability curves indicate lower defect densities

than found in conventional films.

Ion Beam Assist Low (~ 6 A RMS) scattering levels are possible with 02+ bombardment but

with lower damage thresholds.

Crystalline structure of stable monoclinic phase

or amorphous in Zr02 and A1203.

Exterior film surface does not replicate substrate surface morphology,

but internal film structure may.

Medium level damage thresholds.

Slopes of damage probability curves indicate lower defect densities with

02

+ bombardment compared with Ar+ bombardment.

Advanced Epitaxial Very high scattering levels ( > 20 A RMS).

Low level damage thresholds (< 2 J/cm2 ).

196
A member of the audience commented that he believed that ion assisted deposition should be done

with ion beam -potentials lower than 100 eV to avoid damage to the growing film. 600 eV potentials

are too high. The author made the point that what is needed is a parametric study of deposition

conditions vs. damage threshold to settle such questions . Such a study is of particular importance now since new deposition conditions are being used.

197
5. References

[1] House, R.A.; Bettis, J.R.; Guenther, A.H.; "Correlation of Laser-Induced Damage with Surface

Structure and Preparation Techniques of Several Optical Glasses at 1.06 pm", Nat Bur. Stand.

(U.S.) Spec Publ. 435, 1975, pp. 305-320.

[2] Silva, Robert M.; Orazio Jr, Fred D.; Stowell, W. Kent; "Scatter Evaluation of Supersmooth

Surfaces," Musikant, Solomon, ed. Proceedings of SPIE Vol 362 : Scattering in Optical

Materials; 1982 Aug 25-27; San Diego, CA pp. 71-76.

[3] Bennett, Jean M; Stowell, W. Kent; "Round-Robin Testing of Low Scatter Optics", Nat. Bur.

Stand. (U.S.) Spec. Publ. 688, 1983, to be published.

[4] Rujkorakarn, R.; Hsu, L.S.; She, C.Y.; "Crystallization of Titania Films by Thermal

Annealing", Nat. Bur. Stand. (U.S.) Spec. Publ. (1984) to be published.

[5] Folytn, S.R.; "Spotsize Effects in Laser Damage Testing", Nat. Bur. Stand. (U.S.) Spec. Publ.

669; 1982, pp. 368-379.

[6] Seitel, S.C.; Porteus, J.O.; "Toward Improved Accuracy in Limited Scale Pulsed Laser Damage

Testing via the Onset Method", Nat. Bur. Stand. (U.S.) Spec. Publ. 688, 1983, to be published.

6 . Figure Captions

1) Comparison of spectrophotometer transmission curves of electron beam and

ion assisted electron beam deposited Zr02 films.

2) X-ray diffractometer scans obtained on ZrOg films fabricated using laser

and electron beam deposition. Films deposited in an ultrahigh vacuum system

exhibit the crystalline structure of the tetragonal phase.

3) X-ray diffractometer scans obtained on ZrO^ films fabricated using ion

beam assisted electron beam deposition. Films deposited using oxygen ion

bombardment exhibit the crystalline structure of the monoclinic phase.

4) Nomarski micrograph of Zr02 film surface texture showing replication of the

substrate surface.

5) Experimental layout used in 532 mm N-on-1 laser damage testing.

6) Representative beam a) spatial and b) temporal profiles.

7) Laser damage probability curve obtained on Zr02 film SN 111.

8) Laser damage probability curves obtained on ion assisted ZK^ films.

9) Representative laser damage site morphology on a) UHV electron beam

deposited ZK^ films; b) ion beam assisted electron beam deposited

Zr02 films; c) advanced epitaxial process Zr02 films.

10) Representative laser damage site morphology on a) electron beam deposited

AI2O3 films and b) ion beam assisted electron beam deposited Al films.

198
Al2 03 532 nm DAMAGE

E BEAM

200

200 Mm

Al 2 03 ION BEAM ASSISTED

Figure 10: Representative laser damage site morphology on

(a) electron beam deposited A^O, films and

(b) ion beam assisted electron beam deoosited

Al oC^ films.

199
Ti02 532 nm DAMAGE 100 m

ADVANCED EPITAXIAL TECHNIQUE

Figure 9: Representative laser damage site morphology on

(a) UHV electron beam deposited Zr0

2 films

(b) ion beam assisted electron beam deposited ZrC^ films

(c) advanced epitaxial process ZrO? films.

200
I.4n

I .2 - P

JOULES PER SQUARE CENTIMETER

Zr02 E BEAM, UHV SN# 111

Figure 7: Laser damage probability curve obtained on Zr0

2 film SN 111.

1.4-1

1.2-

JOULES PER SQUARE CENTIMETER

Zr02 ION ASSIST, E BEAM SN# 25, 26

Figure 8: Laser damage probability curves obtained on ion assisted ZrC^ films.

201
Zr02 E BEAM, UHV

200 Mm

Figure 4: Nomarski micrograph of ZrCL film surface texture showing replication of the

substrate surface.

0.3

- 0-2

20 25 30 NANOSECONDS

BEAM TEMPORAL PROFILE - 532 nm

50

(a)

0.2

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2

MILLIMETERS

BEAM SPATIAL PROFILE 532 nm

(b)

Figure 6: Representative beam

(a) spatial and (b) temporal profiles.

202
X-RAY DIFFRACTOMETER DATA

Zr02 FILMS -ION ASSIST

600 ev 02+

•v.

K

100evAr+

50 45 40 35 30

DEGREES

25 20 15 10

M'gure 3: X-ray defractometer scans obtained on ZrOo films fabricated using ion beam assisted

electron beam deposition. Films deposited usinn oxygen ion bonbardment exhibit tne

crystalline structure of the monoclinic phase.

ND FILTERS

ND: YAG LASER

0.4J -15 nsec, 532 nm

SAMPLE

Figure 6: Experimental layout used in 532 mm U-on-1 laser damage testing.

203
z

o

8

z

6?

100

80

60

40

20

200 300 400 500 600

NANOMETERS

700 800

Figure 1: Comparison of spectrophotometer transmission curves of electron beam and

ion assisted electron beam deposited Zr0

2 films.

X-RAY DIFFRACTOMETER DATA

Zr02 FILMS

CW LASER *. UHV

-» E BEAM

UHV

E BEAM

HV

55 50 45 40

DEGREES

35 30 25 20 15 10

Figure 2: X-ray defractometer scans obtained on ZrCn films fabricated usino laser anc electron beam deposition. Films depositee! in an ultrahiph vacuum system

exhibit the crystalline structure of the tetragonal phase.

204
Manuscript Received

3-18-85 SAR

HIGH DAMAGE THRESHOLD POROUS SILICA

ANTIREFLECTIVE COATING*

I. M. Thomas, J. G. Wilder, W. H. Lowdermi lk

,

and M. C. Staggs

Lawrence Livermore National Laboratory

Livermore, California

A quarterwave-thick, narrow-bandwidth, antiref lective coating

for fused silica optical components and KDP crystals has been

developed. The coating consists of porous silica prepared from

a silica sol in ethanol. It is applied by dip or spin from a

solution at room temperature and reauires no further treatment.

The damage threshold levels are about eaual to the surface damage

thresholds of the uncoated substrates.

Key Words: antiref lection coatings, laser damage, tetraethyl

orthosi 1 icate, silica sols, fused silica, potassium dihydrogen phosphate

1 . Introduction

For some time we have been investigating the use of porous silica as an antiref lective (AR)

coating for high power laser optics. The use of porous silica as an AR coating is well known;

acid leaching, well summarized by Cook and Mader (1), acid neutralized sodium silicate (2), and

silica sols (3) have all been used on glass substrates to produce films of varying degree of

effectiveness. All of these are aqueous systems and use commercially available materials. Some

work has also involved the use of organic silicates in organic solvents as the silica source (4,

5). These materials are readily hydrolyzed to silica and have been applied as acid-catalyzed,

partially hydrolyzed products in organic solution to give porous coatings after exposure to water

and heat:

Si(0R)

4 + 2H

20 > SiO

? + 4R0H

where R represents CH^ or C^H^.

Because of the special nature of coatings required for laser applications, particularly damage

resistance, our investigation has involved the use of organic silicates, specifically tetraethyl

orthosi 1 icate, Si (OC^Hg)^, as the silica source. This material is a volatile liquid and can readily be purified by fractional distillation; the silica obtained by hydrolysis retains the high

purity level and thus laser damage due to impurities is minimized.

*Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore

National Laboratory under Contract No. W-7405-ENG-48.

205
The hydrolysis of tetraethyl silicate requires either an acidic or basic catalyst and the

intermediate products are auite different in each case. The reactions are shown in Figure 1.

With an acid catalyst a soluble poly-ethoxysi loxane is first formed; at this stage it can be

applied to a substrate and subsequent heat treatment to 450 C, to decompose organic entities,

followed by a mild HF etch gives a porous silica coating. When a base catalyst is used, a colloidal suspension of silica particles, completely free of organic entities, is formed and when

this is applied to a substrate a porous silica coating of layers of silica particles is obtained

with no further treatment.

Si(0C2 H5 ) 4 + H20

(C2 H5 0) 3 SiOH + C2 H5 OH

acid / base

(C2 H5 0) 3

- Si - O - Si(OC2 H5 ) 3

80°C

OC2 H5

I

-I- Si — O

(C2 H5 0) 2 Si(OH) 2

R.T.

OC2 H5

450°C

Si02

Si(OH),

R.T.

n

SiO :

Fig. 1 Hydrolysis of tetraethyl silicate.

Initially, we investigated the acid-catalyzed system, but had problems in some cases with the

laser damage threshold being low, even though the AR properties were quite satisfactory. This

laser damage was thought to be due to carbonaceous residues left in the coating from incomplete

removal of organic material during the heat treatment. We then turned our attention to the basecatalyzed system, and this has given hiqh damaqe thresholds and quite satisfactory optical

performance. In addition, the coatinqs are much simpler to prepare and anply.

Fused silica focussing lenses of 80 cm diameter and 71 cm square potassium dihydroqen

phosphate (KDP) crystals are now coated routinely with the coating solution described below.

206
2. Experimental

Tetraethyl orthosi 1 icate was fractionally distilled using a 120 cm vacuum jacketed and

silvered Vigreux glass column fitted with an infinitely variable distillation head. The fraction

boiling at 166-167 C was collected for subsequent use.

The coating sol was prepared by the base catalyzed hydrolysis of the distilled product by a method similar to that described by Stober (6) as follows. Concentrated ammonium hydroxide

solution (57% analytical reagent, 9.6 g) was added to a solution of tetraethyl silicate (31.2 g)

in anhydrous ethyl alcohol (259 g) with stirrinq at room temperature. The reaction mixture was

then allowed to stand at room temperature for three days to allow hydrolysis and sol formation to

be completed. The final product consisted of a colloidal suspension of SiO,, particles in

substantially anhydrous ethanol at a concentration of 3.0%. Transmission electron microscopy

indicated the silica particles were approximately spherical with a diameter of about 20 nm.

Coating was carried out either by a spin or dip process. Samples were coated at room temperature and then air dried; no further treatment was required. For AR coatings optimized for

350 nm wave length light, a withdrawal rate of 5 cm/min was suitable for the dip process.

Spinning was carried out at 350 rpm and required ethanol dilution of the coating sol to 0.75%

silica. Thicker coatings for longer wavelength light were obtained by multidip or spin with air

drying in between.

The results described in the next section were obtained on 5 cm diameter by 1 cm thick

polished fused silica substrates and on 5 cm x 5 cm x 1.75 cm KDP crystals with diamond turned

surfaces. Larger samples are now routinely coated and their performance is similar to that

reported for the smaller samples.

3. Discussion of Results

Transmission spectra of our coatings on fused silica and KDP substrates are shown in Figures 2

and 3. These spectra are characteristic of quarterwave AR coatings of refractive index

corresponding to the relationship n£= v n-j n^ where nc is the index of the coating,

n-j is the index of air, and n£ is the index of substrate. Appropriate substitution indicates

that the index of the coating is approximately 1.22. Further calculation then shows that the

silica particles must be stacked to give an average porosity of about 50%.

It is interesting to note that the densest possible packing of uniform spheres gives a porosity of only 26%. We must therefore conclude that the slight variations in particles size and

shape and also possible particle porosity contribute to the increased porosity of coatings

obtained from them.

The effect of multicoats on fused silica substrates is also illustrated in Figure 2. The

transmission maximum of the first coat is masked by absorption of the substrate; the second and

third coats however show that thickness is additive, and in this case calculations indicate that

each coat is approximately 39 nm thick (46 nm optical thickness). As the particle size is only

about 20 nm, each coat is only two particle layers thick.

207
We have found that thicker coatings can also be obtained by increasing the silica content in

the coating sol, as might have been expected.

Figure 4 gives laser damage thresholds measured at three different laser wavelengths and pulse

durations for the sol coatings on fused silica substrates, and that measured at one laser

wavelength for the coatings on KDP substrates. The 248-nm threshold was measured by Foltyn at Los

Alamos National Laboratory. These thresholds were measured on coatings whose thicknesses were

arranged such that maximum transmission occurred at the measurement wavelengths. The thresholds

obtained approximate to the thresholds measured for the uncoated substrate surfaces.

4. Summary

A method has been developed to prepare porous silica AR coatings on silica or KDP substrates.

This involves the preparation of a silica sol in ethanol from a high purity organic silicate

starting material and application of this sol, by spin or dip, to substrates at room temperature

followed by an air dry. No further processing is reauired and coatings with high laser damage

thresholds and excellent optical performance are obtained.

Wavelength, nm

Fig. 2 Transni ttance of coatings Fig. 3 Transmi ttance of coatings on

on fused silica substrates. diamond-turned KDP substrates.

208
5. References

(1) L. M. Cook and K. H. Mader, Optical Engineering, 21(1), SR-008, 1982.

(2) E. M. Pastirik and M. C. Keeling, 13th IEEE Protov. Spec. Conf., Washington D.C., 5-8

June (1978).

(3) H. R. Moulton, U.S. 2601123 (1952).

(4) H. R. Moulton, U.S. 2474061 (1949).

(5) B. Yoldas and D. P. Partlow, "Wide spectrum antiref lective coatings for fused silica and

other glasses. Appl. Opt., 23, 1418 (1984), also

B. Yoldas, and D. P. Partlow, Formation of Broad Band Antiref lective Coatings on Fused

Silica for High Power Laser Applications, Thin Solid Films, (to be published).

(6) W. Stober, A. Fink, and E. Bohn. J. Colloid and Inter. Sci. 26, 62 (1968).

Laser

248 nm, 15 ns pulse

346 nm, 0.6 ns pulse

1064 nm, 1.0 ns pulse

Si02 substrate

4-5 J/cm2

8.5- 10 J/cm2

10 - 14 J/cm2

KDP substrate

> 4 - 5 J/cm2

Fig. 4 Laser damage thresholds.

209
A questioner wanted to know if these silica gel coatings could be used for calcium fluoride substrates. The author had not tried it but was confident that they could be. The durability of

these coatings has not ben studied, but coatings have been used for several months in air with

no -problems. There is certainly a water layer on the silica, but it has not caused a problem.

Also, sol gel coatings are notorious for leaving residual carbon. However, these coatings, which

are not heated, are OK. For acid hydrolized systems, which must be heated to 450 C, it was impossible to eliminate carbon residues, which caused very low damage thresholds , and that process

was abandoned. Work is just starting on depositing multilayer coatings by using organometallic

layers and decomposing to get the oxides. Nothing yet to report on this process.

210
Manusvript Received

Pulse Duration Dependence of 1064-nm Laser 3-18-85 SAR

Damage Thresholds of Porous Silica Antiref lection

Coatings on Fused Silica Substrates

D. Milam, I. M. Thomas, C. Weinzapfel, and J. G. Wilder

Lawrence Livermore National Laboratory

Livermore, CA 94550

We used 1064-nm pulses with durations of 1 , 5, 9 ns to measure

laser-damage thresholds of 10 porous silica antiref lection coatings

deposited from both methanol and ethanol solutions containing silica

particles with diameters of 10-20 nm. The median thresholds measured

at the three pulse durations, 10.8 J/cm 2

, 26.5 J/cm 2, and 38 J/cm 2

, scaled as pulse duration to the 0.56 power.

Key words: laser damage, antiref lection coatings, pulse duration

scaling, porous silica.

1 . Introduction

Porous silica coatings were developed at Livermore in response to the need for damage

resistant antiref lection films for use at ultraviolet wavelenqths, primarily 248 nm and 351 nm.

Because these coatings performed well in the ultraviolet, and because techniaues had been

developed for applying such films to optical elements with diameters up to 1 meter, it was

interesting to evaluate their potential for use at 1064 nm.

2. Fabrication of Test Coatings

The coatings were deposited from both ethanol and methanol solutions containing 3 weight

percent of colloidal silica particles 10-20 nm in diameter^. A substrate was lowered vertically

into the solution and then withdrawn at 4.0 cm/min. Build-up of coating thickness was accomplished by use of three depositions. The substrates were 2-inch diameter disks of Corning

7940 high purity fused silica that had been polished by Zygo, Inc. Immediately prior to

application of the coating solution, the surfaces of these samples were cleaned with lens tissue

and ethanol. Four of the substrates were also etched in dilute HF prior to being coated:

additional comments on this etching are given below.

The two-surface transmittance spectrum for a silica substrate bearing three-layer sol -gel

coatings is given in Fig. 1. The measured transmittance was .994 at 1064 nm, .996 at the spectral

maximum near 1000 nm, and equal to that of the bare substrate at the minimum near 500 nm. Using

the values of these transmittances and the positions of the spectral extrema, we estimate that the

coatings were homogeneous layers with optical thickness of 250 nm, refractive index % 1.26, and

silica content of 56%. The homogeniety of the coating sugqests that dried porous material from an

*Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National

Laboratory under Contract No. W-7405-ENG-48.

211
initial deposition was not filled during a subseauent deposition, and that the optical thickness

was increased by about 83 nm during each application of the coating. A loss believed to be

scattering caused transmittance at wavelengths below 250 nm to be less than expected.

3. Measurement of Laser Damage Thresholds

Damage tests were made with 1064-nm pulses in a weakly convergent beam that was about 2 mm in

diameter at the sample surface. For each pulse used, the pulse energy and the energy distribution

in the beam were measured, and the peak on-axis fluence was computed to within ± 7%.

Duration of the laser pulses was controlled by using a Pockels cell shutter to slice them

from a Gaussian waveform pulse with duration of 150 ns. At the minimum pulse duration, 1 ns, the

pulse waveform was governed by the rise and fall times for the shutter and was approximately

Gaussian. Pulses with duration 5.5 and 9 ns had rise and fall times identical to those of the

1-ns pulses. These longer pulses were sliced from the leading edges of the 150-ns pulses and had

waveforms that were trapezoidal with a slight upramp. Gain saturation in the laser amplifier

reshaped the pulses into trapezoids with trailing edges as much as 30% below the leadinq edges,

the saturation and reshaping being most significant in the pulses with greatest energy.

During test of a sample, each test site was irradiated only once. To detect damage, we observed for laser-induced light emission during a shot, visually inspected the test site under

intense white light illumination after the shot, and also used dark field microscopy to record 100

times magnified photographs of the center of the test site before and after the site was

irradiated. Damage was defined to he a permanent alteration of the sample detectable by these

examinations. The threshold-level damage usually consisted of very small volumes visible in the

Measured AR

Fig. 1 . Two-surface transmittance for a silica window with sol-gel AR coatings

on both surfaces. Dsahed curve is the

spectrum corrected for small measurement

errors.

0.2 0.4 0.6 0.8

Wavelength, microns

1.0 1.2

212
The scaling of median thresholds as t

" is within error comparable to the t"

2 scaling that has been observed in similar 1064-nm tests with bare polished glass surfaces and

3 5 with graded-index surfaces made by etching . However, since the rule x" can be attributed

to a wide class of physical effects, it does not uniquely identify the mechanism for damage.

These coatings will be useful at a few places in the Nova amplifier chains. Lenses and

windows that transmitted only the 1064-nm beams in Nova were made of BK-7 glass and had

antiref lecti ve surfaces made by neutral solution processing (NSP)^. During initial operation of

the Novette laser, transmittance of large diameter beams of high-intensity throuqh optical

elements made of BK-7 glass revealed the presence of a low volume density of platinum inclusions

with damage thresholds of 4-7 J/cm (1-ns, 1064-nm). Therefore, high purity fused silica may be

more suitable than BK-7 glass for fabrication of a few elements in NOVA that receive the greatest

fluence loading. In Fig. 3, we compare the 1-ns 1064-nm thresholds reported here for sol-gel AR

films with thresholds previously measured on NSP surfaces on Bk-7 glass. Thresholds for films

made by the two process are comparable, so if we are required to use fused silica for some 1064-nm

components in Nova, the sol-gel coatings should adequately serve as damage resistant AR films.

213
dark field presentation of the microscope. Except for the least damages, these areas were usually

seen in the simple visual inspection of the test site, but were not always apparent in inspection

by either white-light or Nomarski microscopy. Therefore we assume the damages were small

localized fractures in the coating that were most apparent by their ability to scatter light.

Laser-induced emission of light was observed on some samples, particularly those with the

greatest thresholds, during shots at fluences below those which caused permanent damage detectable

by the inspections described above. At these subthreshold-f luences, the light emission did not

occur as a bright spark. Instead, it appeared as a weakly luminous disk on the sample surface,

had the size and shape of the laser beam, and sometimes abated when a site was repeatedly

irradiated. It has not been determined whether this emission of light signals the existence of a

plasma with density sufficient to cause a serious perturbation of a beam.

4. Experimental Results

Measured thresholds are given in Table 1. For coatings in the first two preparation groups,

thresholds for those made from an ethanol solution were systematically above thresholds of those

made from a methanol solution, and thresholds of the methanol-solution coatings were almost

independent of pulse duration. This was difficult to understand. Coatings made from the ethanol

solution consist of layers of silica spheres having diameters of about 20 nm, whereas those made

from the methanol solution consist of smaller spheres with diameters of 6-10 nm. Since the sizes

of the spheres are all very small relative to the 1064 nm test wavelength, and since both of the

precursor solutions should have provided silica of comparable purity, it was believed that the

apparent difference in performance of the two types of coatings was due to an extrinsic effect.

Therefore, we fabricated a third set of coatings on the four substrates that had been used in

fabrication of the initial group of coatings. The substrates were etched in dilute HF acid to remove the existing coatings, cleaned in water and ethanol, and recoated using the same ethanol

and methanol solutions that had been used in preparation of coatings in sets 1 and 2.

Thresholds for the coatings in set 3 were independent of the type of precursor solution,

systematically larger than those obscured in tests of coatinqs in sets 1 and 2, and they all

scaled uniformly with pulse duration. However, for these latter coatinqs, thresholds for

laser-induced light emission were sometimes significantly below the larqe fluences reauired to

cause detectable permanent physical damage. The fluence for laser-induced emission of light may

be the practical upper bound for thresholds in these coatinqs.

In Fig. 2 we have plotted all the measured thresholds as a function of pulse duration,

indicated the median threshold at each pulse duration, and also plotted the function

T = 10. 8t"

56 which is a best fit for scaling of the median threshold T with ... pulse duration

t. Note that the median thresholds are generally greater than thresholds for the initial

methanol-sol films that may have been limited by substrate cleanliness, but well below the largest

thresholds measured for coatings in set 3, and even conservative relative to light emission

thresholds for coatings in set 3. Therefore, we believe it is possible by simple attention to

preparation to reproducibly prepare coatings with thresholds greater than or equal to the median

values.

214
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215
Acknowledgments

We are grateful to S. E. Peluso who has provided significant support in installation and

maintenance of our laser damage facility.

References :

1. Ian M. Thomas, John G. Wilder, W. Howard Lowdermilk, and M. C. Staggs, "High Damage Threshold

Porous Silica Antiref lection Coating," elsewhere in these proceedings.

2. David Milam, "1064 nm Laser-Damage Thresholds of Polished Glass Surfaces as a Function of

Pulse Duration and Surface Roughness", in Laser Damage in Optical Materials: 1978 , NBS Spec.

Publ. 541, pp. 164-167, issued Dec. 1978.

3. James E. Swain, W. Howard Lowdermilk, and David Milam, "Raising the Surface Damage Threshold

of Neutral Solution Processed BK-7 by Pulse Laser Irradiation," in Laser Induced Damage in

Optical Materials: 1982 , NBS Spec. Publ. 669, pp. 292-295, issued Jan. 1984.

4. Lee M. Cook, W. Howard Lowdermilk, David Milam, and James E. Swain, "Antiref lective Surfaces

for High-Energy Laser Optics formed by Neutral-Solution Processing," Appl. Optics, Vol. 21, p. 482-1485, April 15, 1982.

5 10 15

Damage threshold, J/cm2

20 5 10 15

Damage threshold, J/cm2

20

Fig. 3. Laser-damage thresholds measured with 1-ns 1064-nm pulses on sol-gel

AR coatings (left) and on surfaces made by neutral solution processing (right)

216
Manuscript Received

3-18-85 SAB

Variation of Transmittance Spectra of Porous Antiref lection

Coatings with Index Profile

David Milam, Giles G. Peterson

Lawrence Livermore National Laboratory

Livermore, CA 94550

Transmittance spectra were calculated for single-layer

coatings with (1) homogeneous refractive index, (2) index profiles

that increase monotonical ly with distance into the coating, and (3)

index profiles that rise in discrete steps. The calculations

predict that large transmittance at tuned wavelengths can be

obtained using any of these coating designs and known, porous silica

coating materials. The spectral bandwidth of transmittance maxima

increases when the index profile is altered from homogeneous to any

profile whose index increases monotonical ly with thickness in the

coating. Bandwidth is greatest for the linear-ramp coating whose

index rises linearly from that of air to that of the substrate, but

this design cannot currently be used because we are unable to

fabricate silica coating material with the extremely large porosity

needed to provide a coating index near unity at the air interface.

For currently attainable fractional silica contents, estimated to be

in the range 0.4-0.5, the optimum bandwidth is provided by a coating

with a single discrete step in its index profile.

Key words: transmittance spectra, single-layer coatings, porous

silica, antiref lection coatings.

1. Introduction

Porous silica coatings can be deposited from alcoholic solutions containing either silica

1 2 microspheres with diameters of 10-20 nm or smaller polymer silica molecules . A coating made

from a mat of polymer molecules has interconnected pores with an average diameter of 4 nm, but is

spatially homogeneous over any volume containing representative numbers of voids and molecules.

Partial etching of this material can enlarge the pores at the outer surface of the coating and

produce a gradient along the surface normal in both the mass density and the refractive index.

A coating made from microspheres bonded only at points of contact has larger pores, but is

also spatially homogeneous as initially deposited. The refractive index of such coatings is not

easily graded. Etchants readily penetrate the larger pores and produce a spatially homogeneous

reduction of the mass in the film. Further, only slight etching is required to destroy the

coating by breaking the relatively few bonds joining the microspheres.

Coatings of a third class have been fabricated at LLNL from mixtures of the solutions

containing smaller, polymer silica molecules and those containing microspheres. The polymer

material tends to fill the voids between the spheres, so the density of the dried film can be

controlled by the mixing ratio for the two solutions. Antiref lection coatings with stepped index

profiles have been made by depositing several layers with progressively lower indices.

*Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National

Laboratory under Contract No. W-7405-ENG-48.

217
The transmi ttance spectrum of a window bearing coatings made by one of these three

techniques is uniquely related to the refractive index profile of the coatings, and our model for

computing spectra was initially used only as a technique for determining index profiles in

experimental coatings. An assumed profile was adjusted by trial and error to obtain agreement

between calculated and measured spectra. Measured spectra could usually be explained by

postulating that the index profiles were slight variations of one of three generic profiles:

(1) spatially homogeneous index, (2) index increasing monotonical ly with distance into the coating

along a surface normal (linear-ramp or arctangent functions), and (3) index profiles with one or more abrupt steps. In this paper we discuss the basic differences in the spectra for coatings

with these index profiles.

Because codes for calculation of transmittance spectra are well known, and because the

mathematics describing multilayer coatings is cumbersome, it is tempting to immediately resort to

numerical analysis when considering a new type of film. In fact, many of the characteristics of

coatings can be estimated through use of much simpler procedures. This is especially true for

films such as those described in this paper, films in which the refractive index n^ rises

monotonical ly along a surface normal and spans at most the limited ranqe between the refractive

indices of air and the substrate. We have found simple models to be useful in development of

intuition and will use them to describe some coatings. Discussions based on intuitive models will

be supplemented by presentation of spectra computed by standard numerical techniques and by

analysis of closed form equations for transmittance that are known for coatings of some designs.

2. Calculation of Reflectance and Transmittance

In our calculations of spectra, a coating was described by stating the variation of its

fractional silica content, F(z), with distance z into the coating along a surface normal. The

physical boundaries of the coating were at z = 0 and z = z Q

. For z < 0, the medium was

assumed to be air with refractive index n = 1. For z > z the medium was fused silica with

o index n

g given by the Sellmeier equation,

„ .6961663 X

2 .4079426 X2 .8974794 X2 n . n (X) - 1 = ' *

' S X

2 -(.0684043)2 X

2 -(.1162414)2 X2 -(9. 8916161

)

2

where X is the wavelength in microns. At a position z inside the porous coatinq, the refractive

index of the film was assumed to be a weighted average of the indices of air (n = 1.0) and

3

si 1 ica,

n f (X,F,z) = F(z)n

s (X) + 1 - F(z) . (2)

This expression incorporates index dispersion through its dependence on n

g (X), is easily

understood and provides index values for porous silica that are accurate to within % 5% over the

whole porosity range, 0 _< F(z) <_ 100%. If desired, indices could be calculated with slightly

greater accuracy by use of the Clausius-Mossotti relationship. Finally, note that the silica

fraction profile, F(z), is independent of wavelength whereas the index profile n^(z) varies with

wavelength through dispersion. It is advantageous, therefore, to specify the coating by stating

F(z) and the physical thickness z 0.

218
In calculation of single-surface reflectance of a porous coating, the coating was assumed to

consist of m (typically 30-50) separate layers with index n m

(X,z) and optical thickness

n (X,z)z /m. Reflectance from this stack of coatings was calculated by a standard matrix

ffl 0 <^ procedure . Two-surface transmittance, T, of a window with coatings on both its surfaces was

calculated from the single-surface reflectance, R, by the equation,

T = [1-R]

2

. (3)

This neglects the possibility of coherent interaction between waves reflected from the two

surfaces of the window and is appropriate for comparison of calculated spectra with those recorded

by typical spectrometers, but will not always model spectra measured with narrowband laser sources

for coated windows with very parallel surfaces.

3. Spectra of Homogeneous Coatings

In this section transmittance spectra of single-layer homogeneous coatings computed by our

model which includes index dispersion are compared with those calculated from the well known

analytical expression for transmittance. For a homogeneous film with index n,. and thickness

4

z on a substrate with index n , the single-surface transmittance t is given by ,

t

=

8n

f

n

s

' S

, (4)

(1 + n

2.)^2

- + n

2

) + 4n

2n

s + (1 - n

2 )(n2

- n

2

) cos 26

6 = |£ nfzQ

C0S cp , (5)

where cp is the angle of incidence, the index of air was assumed to be unity, and index

dispersion was neglected. At normal incidence, cp = 0, and the transmittance varies between

maxima that occur at wavelengths specified by n^z

Q

= mX/4 with m an odd integer, and minima

that occur at n^.z o

= mX/4 with m even. At these extrema,

(n

2 rrg*

4n

s tmln = _

m even - ( 7 ) 0 +n

s)"

The second of these equations is simply the transmittance for the surface of the uncoated

substrate. Therefore, t . is equal to that of the substrate irrespective of the value of

nf

" ^max does depend on n^ and has value unity when n^ = vTfjT.

A calculated two-surface transmittance spectrum for a homogeneous porous coating that was designed to provide maximum transmittance at X = 1.0 ym is shown in Fig. 1. This positioning

219
of the maxima was accomplished by requiring that the optical thickness have the value of d = X/4

= 0.25 vim, and setting n^ = \/n^ with n

§ = 1.4504 at X = 1.0 um. The corresponding

physical thickness was z q

= 0.2075 um, and the silica content, computed from Ea. (2), was F

=

0.454. The two-surface transmittance spectrum for the bare polished silica substrate and a plot

of silica content vs. physical thickness in the coating are also given in the figure. As expected

from Eq. (7), transmittance at spectral minima is equal to that of the uncoated substrate.

However, since these spectra were calculated by the computer code that included index dispersion,

the substrate transmittance varys with wavelength and the secondary transmittance minima and

maxima lie at wavelengths slightly qreater than those specified by X = 4d/m.

At all of the spectral maxima in Fig. 1, peak transmittance is 100%. Although we precisely

adjusted n, and F to obtain best transmittance at maxima, large transmittance would have been

obtained over a wide range in those variables. Transmittance exceeds 99% for 1.123 <_ n^ <_

1 .295, which corresponds to the range 0.28 <_ F <_ 0.66, see Fig. 2. Coatings with silica

content in the range 0.5 <_ F _< 0.6 are readily prepared by known sol-gel coating techniques,

so it has been possible to fabricate efficient porous antiref lection coatings.

Transmittance at spectral minima is, however, extremely sensitive to the homogeniety of the

coating. Figure 3 is a comparison of transmittance spectra for a homogeneous coating with F =

0.454 and for an inhomogeneous coating with F = .445 at the air interface and F = .463 at the

substrate. This ±2% ramp in silica content produced a readily apparent change in T . , but no

Fig. 1. Calculated two-surface

transmittance for a silica window

with porous homogeneous coatings.

The transmittance for the bare

substrate, and a plot of silica

content as a function of physical

thickness of the coating are also

given.

0.6 0.8

Wavelength (microns)

220
Fractional silica content of the coating

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

1.1 1.2 1.3 1.4

Coating refractive index

1.5

100

99

98

97

96

95

94

93

92

91

Physical

thickness (fim)

0 0.1 0.

Homogeneous _ index

.FA = 0.454

-L_lj I L

Thickness: 0.20758 ~~

No. of surfaces: 30 _

No. of wavelengths: 50

1.0

0.5 £

COu

GO

0.30 0.40 0.50 0.60 0.70

Wavelength (microns)

Fig. 2. Two-surface transmittance of porous

anti reflection coatings as a function of

coating index and porosity.

Fig. 3. Variation of two-surface

transmittance at spectral minima with

coating inhomogeniety.

4. Coatings with Linear-Ramp Profiles

The sensitivity of Tm ^ n to coating homogeniety illustrates that index gradiation can be

used to increase the bandwidth over which transmittance is large. In this section we analyze

coatings with linear-ramp silica profiles to illustrate two primary characteristics of

graded-index coatings: (1) transmittance at spectral maxima is governed by the averaqe value of

the index in the profile and (2)transmittance at minima depends on the slope of the profile.

In an idealized linear-ramp coating, the index increases linearly with distance along a

surface normal from the value n^ = at the air interface to a value n^ = n£ at the

substrate. The coating whose index variation spans the entire range = 1 t° n^ = n

g is an excellent tutorial example. The calculated transmittance spectrum for such a coating with

physical thickness 0.2075 urn is shown in Fig. 4. Note that the fundamental transmittance

maximum for this graded coating with physical thickness 0.2075 urn and optical thickness

0.2542 urn lies at X = 0.51 urn, whereas the maximum for a homogeneous coating with the same

optical thickness was at X = 1.0 urn. In general, graded-index coatings must be about twice as thick as homogeneous coatings when both are designed to function at the same wavelength. The

transmittance spectrum of the graded coating is oscillatory, but two-surface transmittance exceeds

99.6% over the entire passband which includes all wavelengths X < 2d. Clearly d could be

tuned to position the passband to include any desired wavelength for which silica is transparent,

or to accommodate incidence angles greater than zero.

221
Fig. 4. Calculated transmittance

spectrum for linear-ramp coatings.

02 0.4 0.6 0.8 1.0 1.2 Wavelength (microns)

Considerable insight is gained by examining a very simple model which predicts all the

principal features of the spectrum in Fig. 4. The ramp profile of the coating can be approximated

by a series of small discrete steps as shown in Fig. 5. To determine the reflectance of this

coating at a single wavelength X

q

, we imagine that the coating is irradiated by a plane wave

and consider the relative phasing of wavelets reflected from interfaces between the steps. If the

optical thickness of the coating, d, is very small relative to X

Q

, all reflected waves will be

in phase and the surface reflectance will be equal to that of the bare substrate. When d is

increased to ^

0 /2, each wave reflected from a step lying in front of the midplane can be

paired with an out-of -phase wave reflected from a step lying behind the midplane, and the net

reflectance will approach zero. Increase of d from X Q

/2 to 3X

Q /4 produces a set of

partially phased waves and an increase in reflectance, but reflectance will again be small when d

is increased to 2X

Q /2.

The coating thickness corresponding to the complete set of reflectance minima (transmittance

maxima) are specified by the equation d = mX

o /2, with m a positive integer. Reflectance

maxima occur for the coating thickness d = (2m+l)X /4, and the single-surface reflectance at

maxima, R m

, decreases as m increases. The reflectance at a particular maximum arises from

summation of partially phased waves reflected from steps in the profile lying between

d = mX^/2 and d = (2m+l)X

Q

/4. The fraction of the total index discontinuity spanned by

steps in that interval, An^ = (n -l)/(2m+l) , decreases with m. At the first maximum,

m = 1, the greatest possible value for R is the reflectance from an interface between index

222
Air Coating I Substrate

1 0

\

\

\

\

\

0 d A/4 A/2 3A/4 A 5A/4

Optical thickness Net coating thickness (d)

Fig. 5. Approximation of a linear-ramp coating, and its

reflectance as a function of coating thickness.

values n x

= 1.3 and n £= 1.45; R ^003. Therefore, the amplitudes of secondary

f f m reflectance maxima are very small, and the practical requirement for obtaining large transmittance

at A

Q

is only that d j> *- Q

/2. This thickness condition would also be met for any

wavelength A<A

o .Therefore, transmittance is insensitive to wavelength and to incidence angle,

provided the coating is sufficiently thick to meet the d > A/2 condition for the greatest

effective wavelength of interest.

The numerically calculated spectrum shown in Fig. 4 is in remarkable agreement with the

predictions of the simple model. The transmissive maxima lie at wavelengths 0.51 and 0.26 ym,

very near the predicted positions (A = 2d/m) of reflectance minima for a graded coating with

d = 0.254 um. A minimum in transmittance is at A = 0.37 ym, near the predicted position

A = 4d/(2m+l) with m = 1. At this minimum, the two-surface transmittance is .996, corresponding

to a single-surface reflectance of .002, which is well below the estimated greatest value of

reflectance at a secondary maximum, R m

= .003.

While the linear ramp spanning the index range n£ = 1 to n£ = n

$

is of tutorial value,

currently known procedures do not readily produce coatings with F below 0.4-0.5, which corresponds

to refractive indices in the range 1.18 to 1.23, so the value n« = 1 at the air interface cannot

be attained. The impact of this limitation can be understood by examining Schroeder's approximate

equations for two-surface transmittance at maxima and minima in the spectrum of a linear-ramp

coating

5

(8)

(9)

223
At minima, greatest transmittance occurs for the maximum difference in n^ and n^, or for the

greatest ramp angle. This is apparent by inspection when Tm ^ n is written in terms of

o = n£ - n^;

"mm 1 -

n

2 (n

s -l) - a

n

2 (n

$

+l) + a,

n 2

(10)

At maxima the transmittance is greatest when n 2

and n

2 are chosen so that n2

n

s /n

2

, and for pairs (n

2 ,n2

) satisfying this condition, the average value (n

2 + n2 )/2 is

approximately egual to v

/rT7 the index needed to fabricate efficient homogeneous antireflecti

coatings. The simultaneous need for <n^> % y/n~ to optimize Tm„„ and for large ramp

t s max

angle to optimize T - n cannot be met with current coating techniaues, which do not provide

values of n

2 « VnZ. The design limitation is guantified in Figure 6 which gives plots of

T . and of T as a function of silica fraction and index at the air interface. The curve? mm max were plotted from Schroeder's approximate eauations and also plotted from values of Tmax

and

T

mi

- n measured from spectra calculated by the matrix technigue for coatings with varied n

2 and n£ all satisfying (n2+n2>)/2 = n s

. To produce linear-ramp coatings with two-surface

transmittance even as large as 99%, it will be necessary to produce coatings with F

2 = 0.15.

An alternative optimization, allowing <n^> to rise abovev/n^ so that the ramp angle

can be increased without reducing n

2

, allows production of some interesting coatings with large

bandwidth and moderately large transmittance. An example is given in Fig. 7. Transmittance

spectra in that figure were computed under the assumption that the minimum attainable value of

silica content at the air interface was F = 0.3. Best performance, T > 98.6% across the

passband, occurs when the ramp angle is steepest, i.e., when n^ = 1.0.

Fig. 6. Two-surface transmittance of a silica window bearing coatings whose average

silica content is 0.454, plotted as a function of silica fraction at the air

interface.

O O.I 0.2 0.3 0.4 0.5

Silica fraction at air interface

I

224
Wavelength (microns)

5. Step Function Silica-Content Profiles

Coating with linear-ramp profiles are one of the extreme types of graded-index coatings.

The opposite extreme is a coating with a single step AF in its silica content profile. The

details of spectra of coatings with step-function silica profiles depend on the magnitude of the

step, its position in the profile, and the average value of silica content in the coating.

Cumbersome exact solutions are known for the reflectance of such a two-layer coating. However,

for the purpose at hand, it is easier first to examine a special case which illustrates the

influence of a step in silica content on the bandwidth of transmittance maxima, and then to

generalize this example, than to attempt a complete discussion of two-layer coatings.

Figure 8 gives spectra for a homogeneous coating with F = 0.454 and for a coating with a

step, AF = 0.2, centered in the coating profile. The primary difference between the two spectra

is the improved transmittance at X = 0.5 urn for the step-function coating. For both coatings,

transmittance at the minimum near X = .25 urn is equal to that of the substrate and

transmittance at maxima is nearly 100%, although the rudimentary qradiation provided by the step

did cause a shift in the positions of the maxima for the step-function coating. Clearly, a

properly positioned step can be used to increase transmittance at a selected spectral minimum,

thereby broadening the transmittance bandwidth.

The selective increase of transmittance at one minimum can be understood by examining the

simple diagram shown in Fig. 9. The homogeneous coating was 0.25 urn in thickness and designed

to be ant i reflective at X = 1.0 urn. For an incident wave with wavelength X = 0.5 urn, the

coating thickness is X/2, and according to Eq. (7), the film has no effect on transmittance at

0.5 urn. Stated differently, all waves reflected from the boundaries of the coating are in phase

when X = 0.5 urn, and the sum of these waves is equal to that of a wave reflected from the bare

substrate. Placing a slight step in silica content at the center of the coating, indicated by a

dashed line in Fig. 9, generates a new family of reflected waves that are out of phase with the

225
1.0 i0.454 -

0

1.00

0.704

0.204

0

F

Fig. 9. Relative phase shifts of 0.5-ym wavelength waves reflected from boundaries of a coating

and from an index step centered in the coating. Left: for a coating with optical thickness of

.25 urn, the wave with ,05-um wavelength that is reflected from a hypothetical index step

centered in the coating is out of phase with waves reflected from the coating boundaries. Right:

dephasing of the wave reflected from the boundary is only .05 X when the magnitude of the step

is AF=0.5.

226
sum of waves reflected from the coating boundaries. If the discontinuity AF is sufficiently

large, the net sum of all the waves will be zero. While it might seem that placing a large step

at the midplane would significantly disturb the relative phasing of the reflected waves, this does

not occur because the possible excursion for F, 0 to 1.0, produces an index excursion of only 1.0

to 1.45. The right side of Fig. 9 gives refractive indices and optical thicknesses for the two

layers produced by placing a step AF = 0.5 at the coating midplane. The dephasing of the wave

reflected from the step is only .05 X and could have been avoided by placing the step slightly

more than 50% of the way across the coating.

The sketch in Fig. 9 also shows why a centered step did not influence transmittance at X =

0.25 urn in the calculated spectrum shown in Fig. 8. The thicknesses of the homogeneous coating,

d = 0.25 urn, and of either half of the coating are multiples of X/2 for X = 0.25 urn, so

all of these films are absentee layers. Note, however, that we could have positioned the step

about one-fourth of the way across the coating and made a significant change in transmittance at

X =0.25 urn, although it can be shown that this would have provided only a small effect at X =0.5 um. The step can in fact be thought of as a "transmittance helper" that can be positioned

to increase transmittance at a selected minimum, or the step can be divided to provide some

increase in transmittance at two or more minima. The linear-ramp coating discussed in the

previous section is the limiting case of this process, a discontinuity AF that was divided into

an infinite set of steps uniformly distributed acrossing the coating. However, nothing is free;

as the discontinuity AF is distributed its magnitude must be increased if the distributed step

is to produce large transmittance over an increased bandwidth. Values of AF -*• 1 were needed

for the linear ramp; AF ^ 0.5 is adequate for the discrete step.

That a smaller value of AF is needed for the step-function coating removes some of the

practical difficulty in fabrication of such films. In Fig. 10 are shown spectra calculated under

the assumption that the minimum achievable value of F is 0.3. A centered step of magnitude AF =

0.4, ranging from F = 0.3 to F = 0.7, can provide two-surface transmittance 98.5% for the first

three harmonics of the NdrYAG laser, X = .355 um, 0.532 um and 1.064 um, or the coating

thickness could be tuned to provide > 99.5% transmittance for two adjacent harmonics.

6. Arctangent-Profile Coatings

In this section we examine spectra of coatings with silica-content profiles described by the

functional form F(x) = arctangent x, for -5.0 < x <_ 5.0. The arctangent function is a better

representation of silica content in a coating whose porosity was graded by acid etching than is

either the linear ramp or the step function. Since the gradiation in an arctangent coating is

intermediate to that in the latter two coatings, the principal characteristics of spectra of

arctangent coatings are predictable from results of the previous sections.

As in homogeneous coatings, values of Tmax

in the spectra of an arctangent coating are greatest when <F> = .45, but T does not vary rapidly with <F>. This is illustrated

max

in Fig. 11. For these calculations, the thickness of the arctangent coating was held at z =

0.2075 ym, the value used in previous examples, and the arctangent function was "centered" in

the coating profile. The resulting spectra have significantly improved transmittance at the

minimum near X = 0.5 um, which mimics results obtained with a centered step, and also somewhat

227
100

1 _08

1 0.7

- \ 1 \

> Step

1

1

Fig. 10. Optimized spectral

bandwidth for step-function coatings,

0.6 0.8 Wavelength (microns)

Fig. 11. Variation of transmittance spectra

with the average silica content in arctangentprofile coatings.

c

o

E

C

CO

100

99

98

97

96

95

94

93

92

91

_

\ -*

Pi !y

s W */ •

I * ^»

•'c

c

P?3*L

A^>>S*-.-

.Vc"*^

* 1

*

B

A

I ;/; 0 0.1 0.2

Physical thickness

(/urn) Arctangent profiles

A. < F > = 0.354

B. < F > = 0.454

I

C. < F > = 0.554

1

1

I I I

1.0

0.5

0.2 0.4 0.6 0.8 1.0

Wavelength (microns)

1.2

improved transmittance at the minimum near X = 0.25 um, which indicates that the distribution

of gradiation in the arctangent function allows the coating to partially replicate the broadband

improvement of transmittance that is characteristic of linear-ramp coatings.

co

u

co

COo

00

Figure 12 gives spectra for four arctangent coatings that have varied excursions in silica

content, AF = 0.2 to 0.8, but the same average silica content, <F> = .454. The bandwidth in

the transmittance spectrum is greatest for silica excursions near AF = 0.6, but optimization of

that coating requires use of a very low value of F at the air interface, F 9

= 0.15. In the

region of greatest transmittance, .40 <_ X <_ .65 um, transmittance for this optimized

arctangent coating is only slightly better than that obtained with a simple step function spanning

a smaller range in silica content, AF = 0.4.

228
cg

C

CO

100

99

98

97

96

95

94

93

92

91

Phisical

thickness, (//m)

0 0.1 0.2 0.3

/

Arctangent profiles; < F > = 0.454

A. F

2 = 0.354, F2

' = 0.554

B. F

2 = 0.254, F

2

' = 0.654

s^C. F2 = 0.154, F

2

' = 0.754

D. F

2 = 0.054. F2

' = 0.854

I I I I I

1.0 co

u

0.5 £

COO

0.2 0.4 0.6 0.8 1.0

Wavelength (microns)

1.2 1.4

Fig. 12. Variation of transmittance

of arctangent-profile coatings with

range of silica fraction. The average

silica fraction was 0.454 for all

coatings.

Finally, if we again take F = 0.3 as a minimum attainable value, the bandwidth of the

arctangent coating is optimized for AF = 0.5, with F = 0.8 at the substrate interface, see Fig,

13. The optimized coating provides transmittance > 98.5% over the wavelength range from 0.35

pm to 1.064 ym, and > 99.5% transmittance across the visible spectrum.

Fig. 13. Transmittance spectra of arctangentprofile coatings with silica fraction F=0.3

at the air interface.

°-2 0.4 0.6 0.8 1.0 1.2 1.4

Wavelength (microns)

7. Conclusions

For homogeneous coatings, transmittance oscillates between values T

mgx

that depend on n^.

and are greatest when n^ = \/*fr^~and F = 0.454, and values T^ that equal the transmittance

of the substrate.

For coatings with a linear-ramp profile, transmittance spectra are also oscillatory, but

transmittance at all minima is greater than that of the substrate. The largest values of Tmax

229
I I

occur when <F> ^ 0.454, while largest values for Tm ^ n occur for the greatest ramp angle.

True optimization of the linear-ramp profile requires the attaining of F * 0 (n^ ^ 1 ) at the

air/film interface, which cannot be achieved by currently known sol-gel coating techniques.

Partially optimized linear-ramp coatings with F = 0.3 at the air/film interface provide T >

98.6% over the whole passband.

If the gradiation in silica content consists of a single abrupt step, the step can be

positioned in the coating to produce a large increase in the value of T . at a selected

minimum, but transmittance at other minima may not be improved. Alternatively, the step in silica

content can be distributed into several steps to obtain some improvement of transmittance at more

than one spectral minima. A single step from F = 0.3 to F = 0.7 placed near the midplane of the

coating with physical thickness 0.2075 urn provides transmittance > 98.5% for all wavelengths

between 355 um and 1.0 ym.

All other monotonical ly rising profiles are simply intermediate cases between the

linear-ramp profile and the step-function profile, and spectra of such intermediate coatings are a rather predictable mixture of the principal features of spectra of the limiting cases.

To graphically illustrate these conclusions, we show spectra for optimized linear-ramp,

centered-step and arctangent coatings with an assumed minimum achievable F of 0.4 (Fig. 14) and

with assumed minimum F of 0.3 (Fig. 15). If the lowest silica content attainable is 0.4, and the

intent is to fabricate antiref lection coatings with good transmittance over the spectrum

Fig. 14. Spectra for optimized linear-ramp,

arctangent and stop-function coatings with

F=0.4 at the air interface.

Fig. 15. Spectra for optimized linear-ramp,

arctangant and step-function coatings with

F=0.3 at the air interface.

230
encompassing the first three harmonics of the Nd:YAG laser, the step-function is the preferred

design. If F can be reduced to 0.3, coatings with some gradation (such as the arctangent) are

slightly advantageous. The greater potential of the linear-ramp coating can only be realized when

the value of F at the air interface can be reduced to a value approaching zero.

Therefore, mastering the art of fabricating highly porous silica is the key to production of

coatings with both large transmittance and large bandwidth.

References

1. Ian M. Thomas, "Antiref lecti ve Coating for KDP Crystals," in the Technical Digest of the

Conference on Lasers and Electro-Optics, held June 19-22, 1984 at Anaheim, CA., IEEE catalog

number 84CH1965-3, paper THB5, p. 138.

2. Bulent E. Yoldas, Deborah P. Partlow, and Harry D. Smith, "Polymer Oxide Antiref lection

Coating for Fused Silica," to be published in the proceedings of Laser Induced Damage in

Optical Materials, 1983, held in Boulder, CO., Nov. 14-16, 1983.

3. M. Born and E. Wolf, Principals of Optics , Pergamon Press, London, 1970 (Fourth Edition), p.

58.

4. 0. S. Heavens, Optical Properties of Thin Solid Films , Butterworths Scientific Publications,

London, 1955, p. 58.

5. H. Schroeder, "Remarks on the Theory of Light Transmission through Inhomogeneous Transparent

Layers," (in German), Ann. Phys., Vol. 39, p. 55, 1941.

231
A question was raised about the form of the function used and a Gaussian was suggested. The

author replied that the transmitted spectrum is not strongly affected by the shape that one chooses if it is monotonically rising. There will be some slight redistribution of the transmission at the minimum and you can always choose to put the maximum at 100%. There should thus

not be a very strong change if a Gaussian were used in place of the arctan function shown. In

response to a question about the antireflectance requirements at Livermore, the author replied

that a reflectance less than bare fused silica was an advance. They hoped to get a two surface

loss of 1% or less with a two surface loss of %% at the design wavelength, but they would

settle for less if they could find something that didn't blow off.

232
Manuscript Eeaeived

7-24-84 SAR

Characterization of Aluminum Nitride/ Aluminum Oxide Reactively Sputtered Antirefl ection Coatings

L. G. Koshigoe

Michelson Laboratory, Materials Engineering Branch,

Systems Engineering Division

L. F. Johnson, T. M. Donovan, and C. D. Marrs

Michelson Laboratory, Physics Division

Naval Weapons Center, China Lake, California 93555

Aluminum nitride/aluminum oxide multilayer protective anti refl ection coatings are being

developed for use on laser windows. These materials have been shown to be relatively stable and

scratch resistant, thus demonstrating their applicability for this use. Both two- and

three-layer coatings were designed at the wavelength 0.500ym. The design techniques utilized, as

well as the resulting transmission spectra, are discussed. Single-layer films have been

deposited on fused quartz and calcium fluoride substrates, and a three-layer film has been

deposited on a fused quartz substrate. Studies of the resistance of these films to various

environmental conditions including laser radiation, fluorine, electron beam irradiation, and

humidity are presented. Surface analysis techniques such as scanning Auger microscopy (SAM),

scanning electron microscopy (SEM), energy dispersive X-ray microanalysis (EDX), X-ray

photoelectron spectroscopy (XPS) , and Nomarski microscopy were used to examine coating

composition and stoichiometry. Preliminary measurements on the laser resistance of the materials

indicate that damage at defects is the predominant failure mode.

Key words: A1N/A1 2 0 3 multilayer; anti refl ection coatings; electron beam irradiation; fluorine

exposure; humidity; laser damage.

1. Introduction

New multilayer antireflective coatings are being developed for protective use on laser windows. These

coatings consist of alternating layers of aluminum nitride and aluminum oxide, which are stable and

scratch resistant. Both materials have large bandgaps and are highly transparent from the ultraviolet to

the infrared, thus making them ideal for use with many laser systems.

Both two- and three-layer designs have been developed for use at certain wavelengths. The two-layer

design was based upon the work of Cox and Hass [1], and uses aluminum oxide as the outer material. This

is particularly desirable since A1 2 0 3 is resistant to moisture as well as to fluorine. The three-layer

design was developed by applying the works of Baer [2] and Mouchart [3], who have determined techniques

which allow for much design flexibility. This flexibility is the result of an extra degree of freedom

which may be used to optimize the design with respect to low absorption, large bandwidth, good adherence,

or ease of fabrication [4].

Preparations of the films have been achieved by numerous techniques ([5] through [36]), including

reactive sputtering and evaporation, chemical vapor deposition, ion plating, activated decomposition of

organometal 1 ic compound vapors, aluminum anodization, and laser-assisted deposition during electron beam

evaporation. In this study, the methods of dc magnetron, ion beam, and rf diode sputtering have been used

to deposit single layers and multilayers. Films prepared by these techniques have typically been shown to

be stoichiometric and of high packing density.

233
Studies on the reactions of the multilayer materials with fluorine, electron beam irradiation, and

humidity as well as on their damage by laser radiation will be discussed. The main techniques used for } analysis and characterization of the films after exposure to the above environments are scanning Auger

microscopy (SAM), scanning electron microscopy (SEM), energy dispersive X-ray microanalysis (EDX), X-ray i photoelectron spectroscopy (XPS), and Nomarski microscopy.

2. Mul til ayer Designs

The choice of materials, A1N and A1 2 0 3

, necessitate the use of multiple-layer design techniques for

obtaining zero reflectance. Both materials have high transmittances at the wavelength of interest (i.e., < 0.500pm), but by themselves cannot accomplish the reflectance requirement. These materials have been

prepared with indices of refraction which allow for production of zero reflectance coatings.

Since quarter-wave designs which produce zero reflectance are not possible, general solutions to the

two- and three-layer cases must be used. These solutions allow flexibility in the choices of layer thicknesses, which is very desirable from the standpoints of absorption in the layers and film preparation.

Absorption in the layers is a problem which normally must be accounted for when designing multilayer

systems, but the design techniques utilized in this study do not include this factor. Thus, an indirect

approach should be used such as optimizing the solution through manipulations of layer thicknesses [2].

I

2.1 Two-Layer Design

As discussed by Cox and Hass [1], for zero reflectance, when the multilayer consists of two layers

whose thicknesses are not related by integral multiples of each other, one obtains:

tan^

tan<j>2

r 1 -r 3 +r 2 (l-r 1 r 3 ) ri-r3 -r2 (l-rir 3 )

(1)

and tan<f>i tan<j>2 r 1 +r3+r 2 (l+r 1 r 3 )

r 1 +r 3 -r 2 (l+r 1 r 3

) (2)

where = 2^1 j = 2ttXand

nj-r n

j n j-i

+n

j

Note that nj is the index of refraction of the jth layer, lj is its thickness, and X is the design

wavelength.

Equations (1) and (2) may be written in the form:

- tan"

2tt [*(

n, (n 3 -n 0

) (n

(r\i n 3 -n 2 n 0 ) (n 0 n 3 -n 1

)

+ li

2 (3)

234
and I tan" 1 [±(

"2

2 ("3- n o)( n o"3-"i

2

) )V2

] + %

2ir (n 1 2

n 3 -n 2

2 n 0 ) (n 2 2 -n 0 n 3

) 2

(4)

where = 0, ±1, ±2,

and m2 = 0, ±1, ±2,

Thus, one may obtain a series of X x 's and X 2

's and hence a series of physical thicknesses, 1

1 s and

l 2 's. Note that eqs (3) and (4) imply four possible pairs of solutions for each m x and m2 set, but in

actuality, only two of these pairs will satisfy eqs (1) and (2).

Applying eqs (3) and (4) to the present case of interest, where no=1.00, ni=1.60, n 2=2.00, n3=1.46

(these are the incident medium, outer aluminum oxide layer, inner aluminum nitride layer, and fused quartz

substrate indices of refraction, respectively), and x=0.500um, a zero reflectance choice of thicknesses is

then given by l^O.OeSlym and 1 2=0.0832vim. This configuration is shown in figure 1. From this

information, and assuming no absorption in the layers, normal light incidence, and constant indices of

refraction for the wavelength region 0.3 to 0.9ym, the transmission spectrum has been determined using a program developed by Loomis [37].

This spectrum is shown in figure 2 as a function of wavelength. As can be seen, there is 100%

transmission at 0.500ym and a bandwidth of approximately 0.06ym with a reflectance loss of 0.5% or less.

The normalized electric field intensity as a function of depth into the layers is shown in figure 3 and

was determined by El son [38]. It is apparent that the electric field minimum occurs in the thicker

aluminum nitride layer. This is advantageous from the standpoint that the aluminum nitride layer is the more absorbing material as shown by preliminary absorption measurements.

2.2 Three-Layer Design

Based upon the works of Baer [2] and Mouchart [3], an analytic solution for a general three-layer

antirefl ective coating has been utilized. According to Baer, Smith's theorem may be used when determining

what conditions must be applied to obtain zero reflectance. These conditions indicate that both the real

and imaginary components of the Fresnel coefficients must vanish. Then, as discussed by Baer, one obtains:

X x = ± - cos

r2

2+r 1 2 -|r

b

| 2 (l+r 2 2 r 1 2

) (5)

2r2 ri( |r b | 2 -D

and (6)

where _ r2 +r! exp(-4irX 1

i

)

1+r2 r i exp(-4irX 1i

)

(7)

235
r b

= r 3 +ri+ exp(-4iTX 3 i)

l+r 3 r 4 exp(-4TTX 3 i)

(8)

and (9)

A

nil = 0,±1,±2, ...

m 2 = 0,±1,±2,

Note that 5 a and 6b refer to the phases of r a and r^ respectively and that the Xj and rj are as defined earlier. Hence, for a choice of X 3

, and given the indices of refraction of the layers, one may obtain a series of solutions. Note that there are two series of solutions for each X 3

.

Figure 4a shows a plot of Xi and X 2 as functions of X 3 for A=0. 500pm, n 0 =1.00, n^.OO, n 2 =1.60,

n 3 =2.00, and n 4 =1.46. Figure 4b shows a similar plot with A=0. 500pm, n 0 =1.00, ni=2.00, n2=1.60, n 3 =2.00,

and n lt =1.44. These indices of refraction correspond to the incident medium, aluminum nitride layer,

aluminum oxide layer, aluminum nitride layer, and fused quartz substrate (fig. 4a) or calcium fluoride

substrate (fig. 4b), respectively. Note that the solid curves correspond to the first solution and the

dashed to the second.

A particular solution from the results shown in figure 4a is X 3 =0.250, X^O.447, and X2=0.326. Hence,

1 1=0 . 112pm, 1 2=0. 102pm, and 1 3 =0. 625pm. This configuration is shown in figure 5.

As in the two-layer case, a plot of the transmission spectrum as a function of wavelength is shown in

figure 6 (for the particular solution discussed above). Again, there is 100% transmission at 0.500pm as expected, but the bandwidth at a reflectance loss of 0.5% is reduced to approximately 0.02pm. Hence, from

the standpoint of bandwidth, this design is not as desirable. The normalized electric field intensity is

shown in figure 7. In this case, the electric field minimum again occurs in one of the aluminum nitride

layers, with the maximum occurring in the aluminum oxide layer. Thus, as in the two-layer case, the

design appears to be a favorable choice from the standpoint of absorption.

3. Sample Preparation

Several single-layer samples and one three-layer sample were deposited onto substrates of fused

quartz, calcium fluoride, or silicon. Substrate diameters were 1 inch and thicknesses were 0.063 inch,

0.058 inch, and 0.023 inch, respectively. Very thin layers of Ag/Cr/Mo, Au/Cr/Mo, or Au/Cr were deposited

onto some of the substrates prior to A1N or A1 2 0 3 deposition for the purpose of aiding in eliminating

charging problems during analysis. Tables 1 and 2 contain lists of the specimens prepared along with any

pertinent information relating to their depositions for depositions in the dc magnetron and rf diode

sputtering systems, respectively. Approximate deposition temperatures for the two systems were 160°C and

300°C, respectively. Note that samples on Si or on aiding layers were used as witnesses. Sample M-177 of

A1 2 0 3 , prepared in the ion beam sputtering system, was deposited on aiding layers of Au/Cr with a substrate of Si. The approximate gas pressure was 1.5 x 10

_l+ torr of Ar, deposition was at room temperature, and the estimated film thickness was 2800A. Table 3 contains information regarding the

purities of the materials used (for the various depositions) and their suppliers.

236
TABLE 1. Information on Samples Prepared in dc Magnetron System.

Sampl

e

Number

Film

Material

Ai di ng

Layers

Approximate

Gas Pressure

(torr) Substrate

Estimated

Fi lm Thickness

(A)

T-261S AIN — 3 x 10" 3 N 2 Fused Quartz 2030

T-265S AIN 1.5 x 1.5 x

10" 3 N 2 10~ 3 Ar

Si 2000

T-266S AIN Ag/Cr/Mo 3 x 10- 3 N 2 Fused Quartz 2000

T-268S AIN Ag/Cr/Mo 1.5 x 1.5 x

10" 3 N 2 10~ 3 Ar

Fused Quartz 2000

T-270S A1 2 0

3 3 x 2.7 x

lO-^Oo.

10" 3 N 2

Fused Quartz 2000

T-276S AIN 1.5 x 1.5 x

10~ 3 N 2 10" 3 Ar

CaF 2 2000

T-281S AIN 3 x 10" 3 N 2 CaF 2 4000

T-289S (a) AIN

(outer)

(a) 3 x 10" 3 N 2 Fused Quartz (a) 1120

(b) A1 2 0,

(middle)

(b) 3 x 2.7 x lO-^Oz

10" 3 N 2

(b) 1020

(c) AIN

h nside)

(c) 3 x 10" 3 N 2 (c) 625

T-290S AIN Ag/Cr/Mo 3 x 10" 3 N 2 Fused Quartz 2000

AIN An / f* w» /Ma AU/Ur/rlO 3 x 10" 3 N 2 rusea ijuariz £UUU

T-297S AIN Au/Cr 3 x 10~ 3 N 2 Si 2800

TABLE 2. Information on Samples Prepared in rf Diode System.

Sample

Number

Film

Material

Approximate

Gas Pressure

(torr) Substrate

Film Thickness

(A)

S-826 A1 2 0 3 8 x 10- 4 0 2 7.2 x 10" 3 N 2

Fused Quartz 2100

S-827 AIN 2 x 10~ 3 N 2 6 x 10_3 Ar

Fused Quartz 7000

S-833 A1 2 0

3 8 x 10^02

7.2 x 10~ 3 Ar

Fused Quartz 2100

S-834 AIN 2 x 10~ 3 N 2 6 x 10" 3 Ar

Fused Quartz 7000

S-835 AIN 8 x 10-'»N2

7.2 x 10- 3 Ar

Fused Quartz 7000

237
TABLE 3. Purities of Materials Used in Sample Depositions.

Ma terial Purity (%) Source

Al 99.999 Specialty Metals Division of Varian

Al ? 0o Unknown Microelectronics Branch, NAVWPNCEN

Au 99.999 American Smelting Company

Mo 99.999 American Smelting Company

Cr 99.996 Specialty Metals Division of Varian

Most of the samples were deposited using a dc magnetron sputtering system with a 5-inch Varian sputter

gun (samples listed with a "T" before their numbers; see table 1). Some samples were prepared in a Randex

model 2400 rf diode sputtering system (samples with an "S" before their numbers; see table 2). One sample

was prepared by ion beam deposition using an Ion Technology, Inc., 2.5mm ion gun (as indicated by an "M" before the sample number above).

The three-layer sample was prepared according to the 0.500ym design discussed previously (particular

solution). Indices were assumed to be 2.00 and 1.60 for A1N and A1203, respectively, and the materials

were deposited until the necessary thicknesses had been obtained. Although the indices of refraction have

not been accurately determined in this study, many researchers have previously determined them to range

from 1.70-2.15 for Al N and 1.42-1.79 for A1 2 0 3 [5], [6], [16], [23], [25], [26], [28], [29], [30], [35],

[36], implying that the assumed values are realistic. Furthermore, recent laboratory studies indicate

that the assumed indices are more likely.

A transmission spectrum of the three-layer sample coating over the wavelength region of 0.3 to 0.9ym

was measured using a Beckman DU-7 UV/VIS spectrophotometer. This spectrum is shown in figure 8. In

comparison to the theoretical spectrum shown in figure 6, it is apparent that the two are similar in curve

shape, but some additional tuning is necessary in the experimental sample preparation to more closely

approximate the theoretical curve. The experimental maximum in transmission of 98. 8% occurred at

0.553ym. By the use of a program written by Loomis [37], variations in the thicknesses and indices of the

layers were made to attempt to determine what changes would cause a calculated transmission spectrum to

approach the appearance of the experimental spectrum. It was found that the differences between the two

spectra is a combination of a need to determine the actual film indices and to measure more accurately the

deposition rates of the materials. At this time, efforts are being made to accomplish these tasks.

4. Film Structure

Study of the structures of both single and multilayer films were made using a scanning electron

microscope. The films were coated with thin layers of Au (approximately 150A) to avoid charging problems.

I

Shown in figures 9a and 9b are typical top and cross sectional views of an A1N single layer. This

film was sample number T-297S prepared in the dc magnetron system. Figures 10a and 10b show top and cross

sectional views of an A1 2 0 3 single layer prepared in the ion beam system (sample M-177). Figures 11a and

lib show top and cross sectional views of the A1N/A1 2 0 3 /AlN multilayer (sample T-289S) prepared in the dc

magnetron system. In the last case, the three layers can be distinguished.

Many defects are apparent on the surface of the A1N single and multilayer films. It is not known at

this time whether the defects are growth defects of a nodular type or whether they are bubbles or splatters on the surfaces only. These defects are typically on the order of 5-10ym in diameter. The AI2O3

specimen is of a much finer surface structure, although smaller defects (approximately lym in diameter)

are apparent on it also. Figures 12a through 12c show magnified views of the A1N, AI2O3, and multilayer

defects, respectively.

The cross sections or side views of the A1N layers display columnar growth type structures. It has

been determined by transmission electron diffraction that these films are polycrystalline. The side views

of the A1 2 0 3 layers show much smoother textures with no columnar growth structures apparent at the

magnifications achieved in these experiments. A pattern was not obtained from the diffraction studies for

the A1 2 0 3

, hence it is probably amorphous. Note that the A1 20 3 single layer and multilayer specimens were prepared in different sputtering systems, but the texture of the A1203 still remains similar in the

two cases.

238
From electron spectroscopy for chemical analysis (ESCA) studies, A1N materials have been determined to

be 95% stoichiometric with the remainder consisting mostly of A1 2 0 3 . A1 2 0 3 films have been shown to be

99% stoichiometric.

5. Results

Results have been obtained on sample behavior following exposure to laser radiation, fluorine, electron beam irradiation, and humidity. The samples studied were discussed earlier and will be referred to

by their designated sample numbers. Not all samples given earlier will be discussed since some were only

used for obtaining preliminary spectral information and since experimental difficulties were encountered

during analysis with others which were associated with film quality and charging problems.

5.1 Laser Radiation

The effects of laser radiation on the multilayer sample were studied with a Phi 600 scanning Auger

microscopy system (after damage). The sample surface was cleaned during analysis by sputtering with

Ar ions using an accelerating voltage of 4kV. For Si02 , an approximate removal rate has been determined

to be 40A/min.

A pulsed dye laser in a triaxial configuration was utilized for damaging the film at a wavelength of

0.497um. The first step in the laser damage study was to scratch crosshairs onto the film on opposite

sides of its center along a sample diagonal. These crosshairs were then used as index marks for locating

five equally spaced sites between them. Each site was then irradiated in air with a few laser pulses of

increasing energy density. The pulse shape was a 1mm diameter flat top (± 5% flatness) with a duration of

0.55ys. Film breakdown thresholds were roughly determined by observing the beginning of film changes with

a video microscopy system (VIMS). Note that no attempts were made in this study to determine actual

damage thresholds of the materials. The main purpose of this study was to determine the behavior of the

film after exposure to laser radiation.

Table 4 lists the energy density of each shot for the five sites. Shown in figures 13a through 13e

are portions of the centers of each of the five sites. These figures in comparison to a typical undamaged

site (such as that in fig. 11a), show an increasing number of damage craters with respect to defects for

an increasing energy density. Site 1 shows mostly defects plus some small damage craters. Sites 2 and 3

show more and larger damage craters with a reduced number of apparent defects. Site 4 shows the beginning

of damage crater agglomeration, and site 5 shows many small pit marks which appear to lay at the centers

of what were originally damage craters (the pits are at the centers of circular patterns which resemble

agglomerated craters). In addition, for site 5, it is apparent that there is a loss of much of the

outer film.

TABLE 4. Laser Energy Densities for the Five Damage Sites.

Energy Density (J/cm2

)

Site 1 Site 2 Site 3 Site 4 Si te 5

1st pulse 3.2 9.2 12. 20. 50.

2nd pulse 5.9 9.2

3rd pulse 9.4

The approximate number of defects for typical portions of the film where no damage was done is 70 (see

the sampling area of fig. 11a for an example). For sites 1 and 2, the number of damage craters plus

defects which did not damage are approximately 60 and 70 (in the same size sampling area), respectively.

These are similar to that of the undamaged site which implies that damage is initiating at defects. For

sites 3 through 5, the number of craters plus undamaged defects is large in comparison to the number of

defects on the undamaged site, increasing in number with increasing energy density (assuming the pit marks

of site 5 are indeed the centers of what were originally damage craters), although few apparent defects

remain on these sites. These results support damage initiating at the defect sites, with some other

possible mode(s) of damage (e.g., caused by non-observable defects) also contributing to the cratering.

239
Auger spectra were obtained for some of the damage sites. Thin layers of gold were deposited on the

films prior to the study and then removed by Ar ion sputtering (as discussed earlier) just until Auger

peaks corresponding to the underlying film were obtained. Figures 14a and 14b show spectra obtained at

site 2. Figure 14a illustrates a portion of the damage site which appeared to be unaffected by the laser

radiation (herein referred to as smooth film) and figure 14b was taken within a damage crater. It is

apparent that the smooth film consists mostly of A1N and the crater site consists of A1N plus A1 2 0 3

. Similar spectra were obtained for some of the other sites, where the spectra from the craters always

showed an increase in AI2O3 with respect to A1N. In addition, when comparing damage sites, it is apparent

that the amount of AI2O3 with respect to AIM increased with increasing energy density. Figure 15 shows

the spectrum obtained on a smooth portion of the film at site 4. A possible explanation for the growth of

the oxide peak is the dissociation of some of the surface A1N, and then combination of the Al with oxygen

to form AI2O3 (recall that these studies were performed in air). Thus, the amount of oxidation of A1N

would increase with energy density. In addition, the larger oxide peaks in the craters may be explained

by assuming that these are positions on the film where the energy was absorbed more, or not conducted away

as quickly, hence resulting in temperature buildup and more oxidation.

5.2 Fluorine Exposure

Single layer samples T-297S and M-177 of A1N and AI2O3, respectively, were exposed to 10% HF in Ar for

1 hour, and were then examined for fluorine penetration.

EDX was initially used to detect the fluorine. Since this technique examines the constituents of the

materials to a much greater depth (on the order of a micron), than a technique such as AES, any severe fluorine penetration should be detected. Fluorine was not detected in either the Al N or AI2O3. Hence,

either the fluorine had not penetrated the material to a very great depth, or the fluorine concentration

was low (low atomic number materials have low X-ray intensities due to low fluorescence yield and high

sample absorption; see [39]).

AES and XPS were then used to analyze the film makeup since the results of the EDX had indicated no great depth penetration of the fluorine in either film. With these techniques fluorine was observed.

Figures 16a and 16b display AES profiles of the A1N and AI2O3 films, respectively. Note that the AES

system used in this case is a Physical Electronics 548 Auger/ESCA system, which employed a lkV beam of Ar

ions for sputter removal. For Si0 2 , a removal rate of 15A/min. is typical. Assuming this rate for A1N

and AI2O3 yields approximate depth penetrations of 30-45A for both films, although the fluorine appears to

drop off (with depth) at a slightly higher rate in the AI2O3 film.

5.3 Electron Beam Irradiation

During surface studies such as SEM and SAM, physical changes of the A1N and AI2O3 films were

observed. Dark spots indicating changes in electron yields were observed at positions on the films where

the electron beams were concentrated. At this time it is uncertain whether these changes were temporary

(i.e., caused by contaminants being deposited on or removed from the film surfaces) or permanent. It is

important to note that, as discussed earlier, it was necessary to coat the films with thin layers of a conducting material such as Au to avoid charging problems.

5.4 Humidity

Single layer samples T-297S and M-177 of A1N and A1 20 3 , respectively (several sister samples were

produced resulting in the availability of a "fresh" sample for each study), were exposed to 95% relative

humidity at 60°C for 24 hours. With the use of a Nomarski microscope, no change in the AI2O3 sample was

observed although the A1N showed signs of degradation.

6. Conclusions

The theoretical aspects of the A1N/A1 20 3 multilayer designs indicate that perhaps the two-layer case

may be more useful because of its bandwidth. At a reflectance loss of 0.5%, it has a bandwidth three

times the size of that of the three-layer design. Although it has not been discussed in this study,

simple calculations show that a two-layer quarter wave design is possible with a low reflectance (less

than 0.005). This may be a more practical design. In this study, a film corresponding to the three-layer

design was prepared and transmission results indicated a need to determine more accurately the indices of

refraction and deposition rates of the materials.

-

240

I

1
Film structure studies indicate that defects are apparent on both the A1N, A1 20 3 , and Al N/Al 2O3/AI

multilayer samples. The defects on the A1 20 3 appear to be smaller (approximately 1pm in diameter as com- pared with 5-10pm for the AIM single and multilayer films). The growth structure of the A1N is columnar,

whereas that of the A1203 is much smoother (no structure is observed at the SEM magnifications used in

this study)

Laser damage studies indicate that multilayer damage initiates predominately at defect sites.

In addition, SAM studies indicate that the A1N outer layer of the multilayer oxidizes upon exposure to

laser radiation in air. The amount of oxidation increases with increasing laser energy density and is

also greater within damage craters than on portions of the film which appear to be unaffected by the

laser radiation.

Fluorine exposure studies show depth penetrations of approximately 30-45A (assuming the same sputter

removal rate of A1N and A1 20 3 as Si0 2

) in both films with the fluorine dropping off with depth at a slightly higher rate in the A1 20 3

.

Surface analysis techniques such as SEM and SAM indicate a sensitivity of the films to the electron

beams. It is not certain whether the film changes after electron beam exposure are permanent.

After exposure to humidity, a A1 2 0 3 single-layer film underwent no observable change, although an A1N

single layer film exposed to the same condition degraded.

Most importantly, it is apparent that the use of an A1N/A1 20 3 multilayer in the environments described

above dictates that A1 20 3 should be the outer material.

Overall, both films appear to be durable and relatively easy to prepare as a multilayer system. These

studies are continuing and clean room techniques are being applied to achieve films with fewer defects.

The authors would like to gratefully acknowledge the technical contributions of E. J. Ashley,

A. D. Baer, R. Z. Dalbey, W. N. Faith, A. K. Green, J. L. Stanford, and R. W. Woolever. The authors

would also like to thank P. J. Jaime for preparation of the manuscript. And lastly, the authors would

like to acknowledge the Defense Advanced Research Projects Association (DARPA) for their partial support

of this work.

7. References

[1] Cox, J. T.; Hass, G. Antireflection coating for optical and infrared optical materials, chapter 5 in

Physics of thin films, Vol. 2. Hass, G.; Thun, R. E., ed. New York: Academic Press; 1964. 239-304.

[2] Baer, A. D. Design of three-layer antireflection coatings. Glass, A. J.; Guenther, A. H. , ed.

Laser induced damage in optical materials: 1976; 1976 July 13-15; Boulder, CO. Nat. Bur. Stand.

(U.S.) Spec. Publ. 462; 1976 December. 221-229.

[3] Mouchart, J. Thin film optical coatings. 2: Three-layer antireflection coating theory.

Applied Optics 16(10): 2722-2728; 1977 October.

[4] Rudi sill, J. E. Exploratory development on antirefl ective coating for 2 to 6 micrometers for

fluorine windows. Hughes Research Laboratories; Malibu, CA. Report No. AFML-TR-78-19;

1978 March. 78 pages.

[5] Duchene, J. Radiofrequency reactive sputtering for deposition of aluminum nitride thin films.

Thin Solid Films 8: 69-79; 1971.

[6] Shuskus, A. J.; Quinn, D. J.; Paradis, E. L.; Berak, J. M.; Cullen, D. E.

Sputtered thin film research. United Aircraft Corporation Research Laboratories; East Hartford, CT.

Report No. M951337-9; AD-769972; 1973 October. 59 pages.

241
[7] Shuskus, A. J.; Quinn, D. J.; Paradis, E. L. ; Berak, J. M.; Cullen, D. E. ; Reeder, T. M.

Sputtered thin film research. United Aircraft Corporation Research Laboratories; East Hartford, CT.

Report No. N921337-15; AD/A-003005; 1974 November. 211 pages.

[8] Lieske, N.; Hezel , R. Formation of AT -nitride films at room temperature by nitrogen ion

implantation into aluminum. J. Appl . Phys. 52(9): 5806-5810; 1981 September.

[9] Hantzpergue, J. J.; Pauleau, Y.; Remy, J. C; Roptin, 0.; and Coiller, M. Electrical properties of

sputtered A1N films and interface analyses by Auger electron spectroscopy. Thin Solid Films 75:

167-176; 1981.

[10] Murayama, Y.; Kashiwagi, K.; Kikuchi, M. Aluminum nitride films by rf reactive ion plating.

J. Vac. Sci. Techno!. 17(4): 796-799; 1980 July/August.

[11] Duffy, M. T. ; Wang, C. C. Piezoelectric aluminum nitride films. RCA Laboratories; Princeton, NJ.

Report No. PRRL-74-CR-65; AD-A007657; 1974 October. 139 pages.

[12] Liu, J. K. ; Lakin, K. M.; Wang, K. L. Growth morphology and surface-acoustic-wave measurements

of Al N films on sapphire. J. Appl. Phys. 46(9): 3703-3706; 1975 September.

[13] Lakin, K. M.; Liu, J. K.; Wang, K. L. Abstract: Growth morphology and surface-acoustic-wave

measurements of Al N films on sapphire. J. Vac. Sci. Technol . 13(1): 37-38; 1976 January/February.

[14] Lakin, K. M. Aluminum nitride for surface acoustic waves. University of Southern California;

Electronic Sciences Laboratory; Los Angeles, CA. AD/A-002138; 1974 August. 37 pages.

[15] Takeda, F.; Mori, T.; Takahashi, T. Effect on hydrogen gas on C-axis oriented A1N films prepared by

reactive magnetron sputtering. Japanese J. Appl. Phys. 20(3): L169-L172; 1981 March.

[16] Gerova, E. V.; Ivanov, N. A.; Kirov, K. I. Deposition of A1N thin films by magnetron

reactive sputtering. Thin Solid Films 81: 201-206; 1981.

[17] Lakin, K. M.; Wang, J. S.; Landin, A. R. Aluminum nitride thin film and composite bulk wave resonators. 36th Annual Frequency Control Symposium; 1982. AD P001565. 517-524.

[18] Fathimulla, A.; Lakhani, A. A. Reactively rf magnetron sputtered A1N films as gate dielectric.

J. Appl. Phys. 54(8): 4586-4589; 1983 August.

[19] Rairden, J. R.; Aluminum nitride thin films deposited by reactive evaporation. Vranty, F., ed.

Thin Film Dielectrics Symposium; 1969. Electrochem. Soc, Inc.; New York, NY. 279-285.

[20] Yamashita, H.; Fukui, K.; Misawa, S.; Yoshida, S. Optical properties of A1N epitaxial thin films

in the vacuum ultraviolet region. J. Appl. Phys. 50(2): 896-898; 1979 February.

[21] Winsztal, S.; Wnuk, B.; Majewska-Mi nor, H.; Niemyski, T. Aluminum nitride thin films and

their properties. Thin Solid Films 32: 251-254; 1976.

[22] Mirsch, S.; Reimer, H. Preparation and electrical properties of Al-AIN-Si structures.

Phys. Stat. Sol. (a) 11: 631-635; 1972.

[23] Puychevrier, N.; Menoret, M. Synthesis of III-V semiconductor nitrides by reactive cathodic

sputtering. Thin Solid Films 36: 141-145; 1976.

[24] Noreika, A. J.; Francombe, M. H. Structural, optical, and dielectric properties of reactively

sputtered films in the system A1N-BN. J. Vac. Sci. Technol. 6(4): 722-726; 1969 July.

[25] Bauer, J.; Biste, L. ; Bolze, D. Optical properties of aluminum nitride prepared by chemical

and plasmachemical vapour deposition. Phys. Stat. Sol. (a) 39: 173-181; 1977.

[26] Duffy, M. T.; Kern, W. Chemical vapor deposition of aluminum oxide films from organo-aluminum

compounds. RCA Review 31(4): 754-770; 1970 December.

242
[27] Sil vestri , V. J.; Osburn, C. M.; Ormond, D. W. Properties of AT 2 0 3 films deposited from the AT CI 3

,

C0 2 , and H 2 system. J. Electrochem. Soc: Solid-State Science and Technology 125(6): 902-907;

1978 June.

[28] Ferrieu, E.; Pruniaux, B. Preliminary investigations of reactively evaporated aluminum oxide films

on silicon. J. Electrochem. Soc: Solid State Science 116(7): 1008-1013; 1969 July.

[29] Salama, C. A. T. RF sputtered aluminum oxide films on silicon. J. Electrochem. Soc: Solid State

Science 117(7): 913-917; 1970 July.

[30] Mier, M. G. ; Buvinger, E. A. A comparative study of anodized, evaporated, and sputtered aluminum

oxide thin films. J. Vac Sci. Technol. 6(4): 727-730; 1969.

[31] Kanstad, S. 0.; Nordal, P. E. Infrared photoacoustic spectroscopy of solids and liquids.

Infrared Physics 19(3-4): 413-422; 1979 August.

[32] Hart, R. R.; Reuter, F. W. Ill; Smith, H. P. Jr.; Khan, J. M. Oxygen K-shell X-ray production in

thin films of aluminum oxide by 20- to 100- keV photons. Physical Review 179(1): 4-9; 1969 March 5.

[33] Chen, C. H.; Silcox, J. Surface guided modes in an aluminum oxide thin film.

Solid State Communications 17: 273-275; 1975.

[34] Vyatskin, A. Ya. ; Trunev, V. V. The interaction of electrons with thin films of dielectrics.

Radiotekh. Electron 17(9): 1899-1905; 1972.

[35] Korzo, V. F. Preparation of thin layers of aluminum oxide by pyroacti vated decomposition of

organic compounds. Zhurnal Prikladnoi Khimii 49(1): 74-77; 1975 January.

[36] Rowe, J. M. Laser-assisted deposition coatings. Northrop Research and Technology Center;

Palos Verdes Peninsula, CA. Report No. AFWL-TR-83-34; 1983 September. 76 pages.

[37] Loomis, J. S. Computing the optical properties of multilayer coatings. Air Force Weapons

Laboratory; Kirtland Air Force Base, NM. Report No. AFWL-TR-75-202; 1975 September. 30 pages.

[38] The calculation of field dependence was performed by J. M. El son.

[39] Barbi, N. C. Electron probe microanalysis using energy dispersive X-ray spectroscopy.

Princeton Gamma-Technology, Inc.; Princeton, NJ.

243
100

AIR

Al 2 0

3

AIN

FUSED

SiO„

INCIDENT

RADIATION

0.063 jum

0.083 Mm

FIGURE 1. Two-layer Design Configuration.

<

<

0.30 0.40 0.50 0.60 0.70 0.80 0.90 WAVELENGTH, Mm

FIGURE 2. Theoretical Transmittance of Two-layer Design.

>•

i—

o

0.75 -

0.50 -

0.25 -

0.000 0.063 DISTANCE FROM AIR INTERFACE, Mm0.146

FIGURE 3. Normalized Electric Field Intensity of Two-layer Design.

244
X

3 (WAVELENGTHS) X3 (WAVELENGTHS)

FIGURE 4. Anti reflection Solutions for a Three-layer Design with Fused Quartz as Substrate,

(a)

FIGURE 4. Anti reflection Solutions for a Three-layer Design with Calcium Fluoride as Substrate.

(b)

100

AIR

INCIDENT

RADIATION

0.1 12 Mm

0.102 Mm

0.062 Mm

FIGURE 5. Three-layer Design Configuration

with Fused Quartz as Substrate.

o

z<

5<

cc

0.30 0.40 0.50 0.60 0.70 WAVELENGTH, Mm

0.80 0.90

FIGURE 6. Theoretical Transmi ttance of

Three-layer Design with Fused Quartz as Substrate,

245
0.000 0.112 0.214 DISTANCE FROM AIR INTERFACE, Mm

0.276

FIGURE 7. Normalized Electric Field Intensity of Three-layer Design with Fused Quartz as Substrate.

100

as

0.30 0.40 0.50 0.60 0.70 0.80 0.90

WAVELENGTH,

FIGURE 8. Experimental Transmittance of Three-layer Design with Fused Quartz as Substrate.

246
1 0Oju FIGURE 9. Topography of A1N Single Layer. FIGURE 9. Cross Section of A1N Single Layer.

(a) (b)

FIGURE 10. Topography of A1 20 3 Single Layer. FIGURE 10. Cross Section of A1 2 0 3 Single Layer.

(a) (b)

1 00,u 1 ft SURE 11. Topography of Al N/Al 20 3 /Al N Multilayer. FIGURE 11. Cross Section of Al N/Al oOa/Al N Multilayer.

(a) (b)

247
< >

10m FIGURE 12. Enlargened Top View of Al N/Al 2 0 3 /Al N Multilayer Defect.

(c)

248
100m IOOjj

FIGURE 13. Damage Site 1 Showing Small

Damage Craters Amongst Defects,

(a)

FIGURE 13. Damage Site 2 Showing More and Larger

Damage Craters Amongst Defects (than Site 1).

(b)

100m FIGURE 13. Damage Site 3 Showing a Reduced Number of

Apparent Defects and More Damage Craters (than Site 2)

(c)

100m FIGURE 13. Damage Site 4 Showing

Agglomeration of Damage Craters,

(d)

100m FIGURE 13. Damage Site 5 Showing Small Pitmarks and Loss of Much of Outer Film.

(e)

249
7

>% 4

CO

tr<

"Oz

3 —

2 —

1 —

Al

< N

_L

200 400 600 800 1000 1200 KINETIC ENERGY. EV1400 1600

FIGURE 14. AES Spectrum of Smooth Film Portion of Site 2

(a)

200 400 600 800 1000 1200

KINETIC ENERGY, EV

1400 1600

FIGURE 14. AES Spectrum Taken Within Damage Crater of Site 2.

(b)

250
7

-c1 2

3n

C N

Al

< O

200 400 600 800 1000 1200 1400 1600 KINETIC ENERGY, EV

FIGURE 15. AES Spectrum of Smooth Film Portion of Site 4.

251
SPUTTER TIME, MIN

FIGURE 16. AES Profile of A1N Single Layer Exposed to Fluorine.

(a)

0 1 2 3 4 5 6 7

SPUTTER TIME, MIN

FIGURE 16. AES Profile of A1 20 3 Single Layer Exposed to Fluorine.

252
Manuscript Received

12-17-84 SAR

Crystallization of Titania Films by Thermal Heating

R. Rujkorakarn, L. S. Hsu, and C. Y. She

Physics Department, Colorado State University

Fort Collins, CO 80523

Two types of low-loss, low-scatter submicron thick titania films, deposited by ionsputtering at room temperature, were thermally annealed in air. Upon heating to about

350°C, these amorphous films show evidence of crystallization. Using a sensitive Raman

spectroscopic technique, the crystalline structures are identified as anatase in one case (Type A) and rutile in the other (Type B). This amorphous-crystalline transformation coincides with a decrease in optical transmission and small changes in refractive

index and energy gap of the film, along with a dramatic increase (^100-fold) in elastic light scattering. Further heating of Type A films beyond 900°C shows a second

phase transformation to anatase-rutile mixtures.

Keywords: absorption; amorphous; anatase; annealing; crystallization; extinction

coefficient; ion beam; optical band gap; optical coatings; optical extinction;

Raman scattering; refractive index; rutile; titania; titanium dioxide.

1. Introduction

Recent interests in low-loss and low-scatter optical coatings have further promoted studies in

deposition and investigation of titania films which have high refractive index and low absorption.

Particularly dense titania films can be made by ion-beam sputtering. Sensitive optical diagnostic

techniques which are nonintrusive and require a relatively short measurement time are applied here

to elucidate the properties of these films.

Ion beam sputter deposition of amorphous titanium dioxide [1] may result in two types of

titania films. These films are optically identical but crystallize differently upon annealing to

about 350°C. In our study, we investigate the amorphous-polycrystal 1 ine transitions and the changes

in optical properties of the films upon thermal annealing over a range of temperatures from 200 to

600°C.

Raman spectra [2] show that Type A film crystallizes at ^ 350°C into the anatase phase, where

it remains until ^ 900°C, then transforms to anatase-rutile mixtures; Type B film crystallizes

directly to rutile phase at 400°C. The crystallization process coincides with a sharp rise in

optical extinction, a small increase in refractive index variation, and two orders of magnitude

increase in elastic light scattering at visible wavelengths. Film thickness remains constant

within experimental error (^ 5mm), but the optical band gap drops from 3.2 to 3.05 eV.

2. Sample Preparation

Ti02 films are fabricated by sputtering a metallic Ti target with 02 and Ar ions from a Kaufman-type ion source [3]. The ion ratio is set to be 1:1 at 7 x 10~5 torr chamber pressure.

The primary ion beam energy and the ion flux are 1100 eV and 1.0 mA/cm2 respectively. Subsequently,

the substrate holder can be water cooled if a low substrate temperature is desired.

Recent investigation indicates that Type A films are obtained with low ion beam current

(0.4 - 0.8 mA/cm2

) and relatively low substrate temperature, Type B with beam current > 1 mA/cm2 regardless of substrate temperature.

The titania is sputtered onto three different types of substrate: glass, quartz, and silicon.

The coating thickness is determined optically [4], and AES (Auger Electron Spectroscopy) is used to

confirm the films' stoichiometry. Two sets of titania films on glass substrates, each set deposited

under different deposition conditions, are used for Raman and other optical investigations. The

film types and thicknesses are given in table 1.

253
Table 1. Sample Characteristics

Sample Thickness Substrate Type

H30905 2233 & Corning 7059 A

RAM 11 3117 A Corning 7059 B

3. Experimental Methods

At each pre-selected temperature (less than 2°C uncertainty) the titania coatings are annealed

in an air furnace for 20 minutes. They are slowly cooled to room temperature before the Raman

scattering, as well as elastic light scattering and optical transmission measurements are made.

After preliminary investigation, the selected set of annealing temperatures are 200, 300, 350,

400, 450, 500 and 600°C. In addition, Raman spectra of the Type A film at 950°C was also taken.

The Raman spectra from titania coatings are obtained using the technique reported by Hsu,

et al. [2] The elastic scattering of laser light integrated over a solid angle of 0.8 sterradian

is measured with the same Raman spectrometer. The transmission is measured using a Beckman spectrophotometer with 0.3 to 2.0 ym wavelength range. Optical constants are deduced using the method

of Demiryont, et al. [5]

4. Light Scattering Measurements

Figure 1 shows the Raman spectra for films deposited on Corning 7059 glass substrates subjected to various annealing temperatures. Raman spectra of the as-deposited films (20°C) are obscured by the broad features of the substrate at 480 citH and 790 cm" 1 ; its broad scattering

shoulder is indicative of amorphous nature of the film [2]. The spectra of annealed films at 200°C

(not shown) and 300°C appear to be identical. For Type A films as shown in figure 1(a). Upon

reaching 350°C, there is a small bump at 612 cm"' without any hint of the 144 cm"' feature. This

indicates crystallization with some rutile microcrystals, but no anatase. At 400°C, evidence of

anatase is quite clear by the appearance of the peaks at 144 cm~l and 635 cm"l and additional

shoulders at 400 cnH and 515 cnH. The concurrent presence of rutile growth is evident by the

weak peaks at 400 cnH and 612 cm

-1

. Subsequent annealing at 450°C, 500°C (not shown) and 600°C

show that the anatase peaks becoming more prominent while the rutile peaks remain constant. At

600°C and higher temperatures, the Raman intensity tends to be unchanged signifying the completion

of the crystallization process.

If a similar Type A film is heated directly at 600°C, bypassing the relative low temperature

steps, it results in anatase crystallization without any detectable rutile as illustrated in

figure 2(a). If these Type A anatase films were further heated to 900°C or above, where rutile

structure is thermodynamically more favorable [6], the anatase films transform into anataserutile structure with rutile dominance, as shown in figure 2(b). The high background above 500 cm"'

is due to partial melting of the glass substrate at this temperature. The general behavior of the

amorphous-anatase-rutile transformation has been studied recently by laser annealing with titania

films on silicon [7].

As shown in figure 1(b), a Type B film retains its as-deposited amorphous features up to 350°C,

where it crystallizes into rutile structure without any anatase. Such evidence again comes from

the appearance of the 440 cm-1 and 612 cnH rutile peaks. When the annealing temperature is

increased further to 400, and 600°C, the rutile signal intensity increases along with the two-photon

Raman modes of rutile at 250 cm"l and 690 cnH

.

The elastic light scattering of both types of films at various annealing temperatures has also

been recorded simultaneously with the Raman spectra of figure 1. Using the scattering from MgO as

a calibration, the percentage scattering can be determined and the results are given in figure 3.

The dramatic two-order of magnitude increase in elastic scatter accompanying the film crystallization near 400°C has obvious implications for use of these films as coatings for high-power laser

mirrors.

254
5. Optical Transmission Measurements

A typical optical transmission scan is shown in figure 4. Using the published technique [5],

optical parameters, including film thickness, and energy gap, can be determined in a straightforward

manner. These parameters are given for the annealing temperatures used.

The film thickness was determined to be independent of annealing temperature, suggesting

negligible porosity in the films as-deposited. The film refractive indices (n) for four different

wavelengths are plotted in figure 5(a) and 6(a) as a function of annealing temperatures. The index

rises between 300°C and 400°C. Although single crystal rutile (n = 2.71, 2.75) has index higher

than that of anatase (n = 2.53, 2.56) [8,9], films of both crystalline phases are found to have

similar indices; n = 2.55 (rutile) and n = 2.50 (anatase) at A = 0.5 ym. Our films appear to be

somewhat more dense than those made by Pawlewicz, et al . [10] using RF sputtering (grain size

"» 20 nm); their titania indices are n = 2.4 to 2.5 (rutile) and 2.30 (anatase). Both Pawlewicz, et

al. [10] and Williams and Hess [6] found that RF sputtering at low power densities favors the

growth of anatase, consistent with our observations at low ion beam current. Figure 5(b) and

figure 6(b) show the annealing temperature dependence of film extinction coefficient (k) at various

wavelengths. It increases sharply when the crystallization temperature is reached, due most likely

to grain boundary scattering. Comparing the extinction coefficient data of this figure to the

scattering data of figure 3, some conjectures are in order. Crystallization and associated grain

boundaries cause a two-order of magnitude jump in elastic scattering. In addition, there is an increase in impurities absorption associated with the heating process. This absorption increases

further as temperature increases beyond crystallization in contrast to the leveling off of scattering at higher temperatures.

The optical absorption edge data have been plotted to reveal a ^ (E)-l(E-Efi) 2 dependence [11].

Extrapolation of the absorption beyond the Urbach edge [12] yields the optical band gap E2. As

shown in figure 7, E°j decreases slightly with annealing temperature. The band gap behavior is con- sistent with a drop in the conduction band energy minima, during the transition from amorphous to

crystalline states.

6. Conclusions

Raman scattering with the complementary optical transmission studies have been shown to be useful in identifying the crystallization of sputtered titania coatings. The amorphous-crystalline

transition occurs abruptly at a modest annealing temperature ^ 350°C.

Crystallization in Type A titania coatings reveals the existence of two competing structures

with the rate of anatase growth dominating at higher temperatures (< 900°C). On the other hand,

only rutile structure exists in Type B films. The nature of this competing growth mechanisms of

Type A films is interesting and is under further study.

Optical constants and optical bandgap are found to be temperature dependent. In particular,

both optical extinction and elastic scattering increase drastically at crystallization temperature,

suggesting that a moderate amount of heating may degrade the coating performance, and affect the

laser damage threshold.

The authors would like to thank Professor James R. Sites and Dr. H. Demiryont for their

helpful suggestions and stimulating discussions. This work is supported by the Air Force Weapon

Laboratory under Contract F29601-83-K-0079.

7. References

[1] Sites, J. R. , Gilstrap, P. Rujkorakarn, R. Ion beam sputter deposition of optical coatings.

Optical Engr. 22, 447(1983).

[2] Hsu, L. S. , She, C. Y., Exarhos, G. J. Reduction of substrate interference in Raman spectroscopy of submicron titania coatings. Appl . Opt. 2^3, 3049(1984).

[3] Kaufman, H. R. in Advance in Electronics and Electron Physics (ed. L. Morton, Vol. 36), New

York; Academic Press; 1974. 265 pp.

[4] Heaven, 0. S. Optical Properties of Thin Solid Films . London; Butterworth; 1950. 158 pp.

255
[5] Demiryont, H., Kerwin, D. B., Sites, J. R. Optical properties of ion-beam sputtered TiC>2 films.

NBS Special Publication, to be published.

[6] Williams, L. M. ; Hess, D. W. Structural properties of titanium dioxide films deposited in an rf glow discharge. J. Vac. Technol. 4, 1810(1983).

[7] Hsu, L. S., Solanki, R. , Collins, 6. J., She, C. Y. Raman study of structural transformations

of titantia coatings induced by laser annealing. Appl. Phys. Letts., in press.

[8] Hass, G. Preparation, properties and optical application of thin films of titanium dioxide.

Vacuum 2, 331(1952).

[9] Jarzebski, Z. M. Oxide Semiconductors . New York; Pergamon Press; 1973. 224 pp.

[10] Pawlewicz, W. T., Hays, D. D. , Martin, P. M., Laegreid, N. Reactively sputtered optical coatings

for use at 1064 nm. NBS Special Publication 568: 359-375; 1979.

[11] Tauc, J. Amorphous and Liquid Semiconductors . New York; Plenum Press; 1974. 159 pp.

[12] Urbach, F. The long-wavelength edge of photographic sensitivity and the electronic absorption

of solids. Phys. Rev. 92, 1324(1953).

256
TYPE A

(b)

100 500

WAVENUMBERS

f^=H20C

900 100 500 900

WAVENUMBERS

gure 1. Raman spectra for (a) type A/glass and (b) type B/glass annealed at various temperatures.

TYPE A

500

WAVENUMBERS

900

7.5

TYPE A

c_> tu

CO

to

ZDo

CJ

(b) 900C 15HRS

4.5i 1 1 1 1 1 1 1

100 500 900

WAVENUMBERS

Figure 2. Raman spectrum for type A/glass annealed directly at (a) 600°C to form anatase and

(b) 950°C to form anatase-rutile mixture.

257
o '.TYPE A DtTYPE B

\s

U)

V)

Q

_J

id

z

q:

LU

I-

<

(J

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001

200 400 600

TEMPERATURE Ct)

Figure 3. Scattering loss of Ti02 films as a function of annealing temperature.

o

.c

CO E

COc

80

60

40

20

0

03 ! 0.6 1.0 1.4 1.8 2.2

Figure 4. Typical transmission scan for a TiC^ film where Xcfis the cut off frequency.

258
3. 00

TYPE A/GLASS (a)

c 2.75

x

UJa

£ 2.50

<

u.

UJ* 2.25

2. 00

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0

^; A0.4jum

0. 5 Aim O 0. 65 /Uin O 2. 00 jum

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200 400

TEMPERATURE Ct)

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600

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0. 03

TYPE A/GLASS

u.

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(—

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}\'. a 0. 40 jam

0. 50 yum o 0. 65 /Um

0 2. 00 /jm

(b)

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--

A-'

—-

o-o200 400

TEMPERATURE (t)

600

Figure 5. (a) Film refractive index and (b) film extinction

coefficient vs. annealing temperature for type A/glass.

259
3. 00

c 2. 75

x

Ixl a

2. 50

lj<

u.UJ*

2. 25

TYPE B/GLASS (a)

2. 00

8

o

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200 400 600

TEMPERATURE <b

Figure 6. (a) Film refractive index and (b) film extinction

coefficient vs. annealing temperature for type B/glass.

260
3.3

3.2

>

ai

3.1

<

3.0

a TYPE A/GLASS

oTYPE B/GLASS

2 2

S

2

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200 400

TEMPERATURE (t)

600

Figure 7. Optical band gap of Ti02 films as a function

of annealing temperature.

Question: Is the light scattering observed coming from the bulk of the film or from the surface?

The author had no data on that question. It was pointed out that except for light losses the

conventional and the novel optical systems, which are both f/l 3 should have the same depth of focus. The question of adequate reference data for Raman spectra arose. The author believes that

this lack is not a serious limitation to use of this technique. Another member of the audience

stated that Raman spectra of both zirconia and tantalum has recently been taken at the Air

Force Weapons Lab and is available.

261
Manuscript Received

12-11-84 SAR

Time Resolved Raman Studies of Laser

Induced Damage in Ti0 2 Optical Coatings

G. J. Exarhos and P. L. Morse*

Battel le Pacific Northwest Laboratory**

Richland, Washington 99352

Molecular information available from Raman scattering measurements of sputter

deposited Ti0 2 on silica substrates has been used to characterize crystalline phases,

thickness, and surface homogeneity. A two laser technique is described for investigating

transient molecular changes in both coating and substrate which result from pulsed 532 nm laser irradiation. Single layer and multilayer coatings of both anatase and rutile phases

of Ti0 2 have been probed by Raman spectroscopy immediately following the damage pulse

(nanoseconds) and at longer times. Transient measurements are designed to follow surface

transformation/relaxation phenomena; measurements at longer times characterize the equilibrium damage state. The anatase coatings were found to be more susceptible to laser damage

than rutile coatings and exhibited irreversible changes during the damage process. Rutile

coatings exhibited a reversible phase transition to the anatase form followed by

recrystal 1 ization to the rutile phase (milliseconds). Loss of coating material from all

damaged surfaces is inferred from decreased Raman scattering intensity. Substrate damage

characterized by laser induced scrambling of polarized light was also observed. Raman

results and molecular structural information are used to formulate a possible damage

mechanism for Ti0 2 coatings.

Key words: laser damage mechanisms; laser heating; molecular vibrational studies; phase

transformations; time resolved Raman Spectroscopy; Ti02 optical coatings.

1. Introduction

Rapid characterization of thin optical coatings deposited on transparent or opaque

substrates is possible through state-of-the-art Raman spectroscopic techniques. This

non-destructive optical method which can be adopted for in situ analyses can provide detailed

chemical bonding information from measurement of molecular vibrational spectra excited by a low

energy probe laser. The technique is particularly well suited to studies concerned with phase

identity and homogeneity of thin films, as well as film stability studies under extreme

conditions.

Application of Raman spectroscopy to characterization of thin optical coatings has only

recently been explored. Conventional measurements of single layer Ti0 2 on silica have

unequivocally identified the anatase phase in submicron sputter deposited coatings and

demonstrated a linear correlation of band intensities with thickness from O.ly up to several microns. [1] The technique of interference enhancement has also been developed allowing Raman

spectra of extremely thin films to be measured. [2,3] High quality spectra have also been

acquired from al 1 -dielectric multilayer Si0 2 /TiOp coatings of general or arbitrary design

with no special specimen preparation required. [4J The Raman technique is also a sensitive probe

of microcrystallite grain orientation in sputtered coatings through polarization analysis of the

scattered light. Furthermore, suppression of substrate Raman scattering from coatings

consisting of unoriented grains can be achieved through measurement of the appropriate

polarization component of the scattered light enabling extremely thin coatings to be characterized by this technique. [6]

Raman spectroscopy has been used to evaluate laser damage to single layer Ti0 2 coatings on silica and more extensive measurements on multilayer coatings are currently in progress.

Conventional measurements are used to characterize the equilibrium damage state where two

Air Force Weapons Laboratory, Kirtland AFB, NM 87117.

Pacific Northwest Laboratory is operated by Battel le Memorial Institute for the U.S.

Department of Energy under Contract DE-AC06-76RL0 1830.

262
colinear laser beams are directed at normal incidence to the sample under investigation. (A low

energy CW visible laser serves as the Raman probe while a high energy pulsed laser causes damage

to the coatings.) Raman spectra measured before and after the damage pulse provide information

on the survivability of the coating and substrate with respect to induced phase transformation

and coating decomposition.

A second experiment has been designed to evaluate equilibrium damage in Ti0 2 optical

coatings caused by changes in sample temperature. Raman spectra are acquired while the sample

is heated by an auxilliary C0 2 laser from room temperature to _ca. 1500°C. Under these

conditions the sample is in thermal equilibrium and the vibrational levels are populated

according to the Boltzmann distribution. The Raman experiment probes the ground electronic

state. Equilibrium phase transitions (anatase+rutile) , should they occur, will be manifested in

measured Raman spectra.

To study the dynamics of laser induced damage in optical coatings, Time Resolved Raman

Spectroscopy (TRRS) was used. Spectra were acquired at various delay times following the damage

pulse. Under these non-equilibrium conditions the Raman experiment probes the excited

electronic states of the material. Resonance enhancement of vibrational lines is possible

leading to increased intensity of selective Raman modes. Interpretation of measured spectra can yield information concerning structure of the excited state.

The data obtained from these experiments has been used to develop a mechanistic model for

laser damage in Ti0 2 optical coatings. Both anatase and rutile coatings sputter deposited on silica substrates are the primary materials investigated by Raman spectroscopic methods.

2. Experimental

All Ti0 2 coatings were prepared by reactively sputtering Ti in Ar/0 2 atmospheres in an rf

diode system onto fused silica substrates. Deposition details appear in the 1 iterature.[6,7]

Microcrystal 1 ine grain sizes were ca. 50 nm and sample thicknesses varied from 0.1 micron to over 5 microns.

Raman spectra were excited at normal incidence using the 180° backscattering geometry

depicted in figure 1. The sample was held in a micropositioning stage having spatial resolution

of several microns. Scattered light was collected at f/1.4 arid imaged onto the slits of a SPEX

LENS (MOVABLE)

LENS

Nd > YAG

SAMPLE CHAMBER OPTICS

180° Backscattering

E Mirror

SCRAMBLER

Mirror Notch

Filter

Double Monochrometer

Adjustable

Slits

Figure 1. Backscattering geometry for Raman experiment with damage/heating and probe beams

colinear.

263
0.85m double monochromator. Dispersed radiation was detected by an RCA C31034-01 PMT and

signals were processed by conventional photon counting electronics. A gated intensified diode

array detector was used to record spectra in a time resolved mode. In this instance, only the

entrance slits of the monochromator were used; to reject Rayleigh scattered light under these

conditions a special notch filter tuned to reject the probe laser wavelength was inserted in

front of the entrance slits.

Three different laser beam excitation arrangements were used. Equilibrium laser damage

measurements at room temperature required col i near Ar+ Raman probe and Nd:YAG damage beams

focused onto the sample surface. The 488 nm CW Ar+ probe beam operating at 200 mW had a 50n

spot size while the 532 nm pulsed Nd:YAG damage beam operating from 1-50 mj/pulse had a 500y

spot size. (Measured pulse widths were 10 nsec.) The second arrangement required combining the

10. 6y CW C0 2 beam used to heat samples appreciably above room temperature with the 532 nm Nd:YAG

pulsed beam operated at a low pulse energy as the Raman probe. Sample temperatures were measured by means of a two color optical pyrometer. The final arrangement used the Nd:YAG laser

to both create damage and act as the Raman probe as shown in figure 2.

SIGNAL DELAY SCHEMATIC

m3

Vm4

Remote

Control

Nd. YAG Loser

Diode

Array Detector

NDG

To Sample Chamber

I

I

I

I

I

I

I

i

I

Beam Splitter

m5

\

18 nanosecond delay m2 8 nanosecond delay

(ml 8m2 turned 90°)

Damaging Beam

Probing Beam

-Circuit Lines

Pulse Width: 10 nsec Pmo« IOOmJ/cm2

A.-532nm (355nm)

40 Hz

(1064 nm)

Figure 2. Signal Delay Schematic showing optical delay path. (NDG refers to a nanosecond delay

generator.

)

A pellicle beamsplitter served to split individual pulses into a primary and secondary

pulse which contained ca. 10% of the primary pulse energy. Figure 2 also shows the electronic

delay schematic designed to open the array detector at selectable times following arrival of the

damage pulse at the sample. The detector bandwidth was maintained at 8 nsec for all

measurements

.

3. Structural Considerations

Titanium Dioxide occurs mainly in two crystalline phases, anatase and rutile. The general

structures of these phases are shown in figure 3. In rutile, the titanium atom is localized in an octahedral oxygen environment, while in the anatase phase, the oxygen atoms in the square

plane are puckered forming a tetrahedral -1 ike unit. The puckering of the oxygen atoms results

in a much longer bond length in the localized unit. Accordingly, rutile is the denser phase at

4.27 g/cc compared to anatase at 3.90 g/cc.[8] Due to marked structural differences in the

phases of Ti0 2

, individual Raman spectra are rather different and provide unambiguous

identification of each phase.

264
LOCALIZED STRUCTURE

AROUND TITANIUM

V)

\

'7

RUTILE

ANATASE

Figure 3. Localized molecular structures of rutile and anatase phases of Ti0 2

.

Thermodynamic data indicates that anatase undergoes a transformation to the rutile phase at

ca . 900°C.(9) These structural and thermodynamic considerations are very important in the

interpretation of the Raman spectra and subsequent modeling of the damage process.

Thin film Ti0 2 optical coatings have been characterized with regard to phase identity and

homogeneity by Raman Spectroscopy. The Raman spectrum of a thin anatase film on silica is

depicted in figure 4. By analyzing the Z(X Y)Z component of the scattered radiation, Raman

scattering from the silica substrate is markedly attenuated at frequencies below 800 cm" . The

broad feature at ca. 800 cm" is, however, assigned to Raman scattering from the glass

substrate. For the other scattering geometry Z(X X)2 substrate features overwhelm Raman lines

from the thin optical coatings. Therefore, in all subsequent measurements, the Z(X Y)Z _, scattering geometry was used. The major anatase lines at 145, 199, 398, 517, and 639 cm" and

rutile features at 244, 440 and 610 cm" previously assigned clearly distinguish the two

crystalline phases. [10, 11]

The equilibrium damage state was characterized by Raman measurements of both anatase and

rutile optical coatings subjected to 532 nm pulsed irradiation from a Nd:YAG laser. When the

damage threshold for rutile coatings was exceeded, measured Raman spectra at long times

following the damage pulse were qualitatively similar to the undamaged coatings; however, band

intensities were lower than for the undamaged samples. While no change in crystalline phase was detected, the lower band intensities indicate removal of Ti0 2 by the damage laser pulse. Since

Raman band intensities correlate with coating thickness, depth profiling of the damaged area is

possible and has been determined for several samples.

Subtle changes in band positions and bandwidths were observed in Raman spectra of anatase

coatings following the damage pulse. As seen in figure 5, all major features are significantly

broadened and reduced in intensity. The major-,145 cm" band appears at a higher frequency

following damage whjle features above 300 cm" appear at slightly lower frequencies. Following

damage, the 398 cm" band shows a weak shoulder at higher frequencies, and the 639 cm" band

appears broadened to the low frequency side. These are regions where Raman scattering

attributed to the rutile phase is expected.

Results from such measurements suggest that for a constant laser damage energy and similar

coating thickness, rutile coatings appear to have a significantly higher damage threshold than

4. Results

265
Anatase Coating

50 150 250 350 450 550 650 750 850

Wavenumbers

Figure 4. Raman spectra of a 1811 A anatase coating on Si0 2

. The standard scattering geometry

designation is based upon a coordinate system centered at the sample where Z and 2 are the

incident and scattered laser directions and X and Y define the particular component of the

electric field transmitted by the analyzer.

anatase coatings. In addition, the extent of coating removed depends on the laser pulse energy

and coating thickness. Multiple pulses on the same area remove successively less and less

coating eventually converging to a limit. The 488 nm Ar+ probe laser (1W CW) was found not to

damage these rutile or anatase coatings even when focused to a 50u spot size at the sample.

The effect of temperature on phase stability of sputtered TiOp optical coatings was investigated using 10. 6n radiation from a CW CCL laser as a heating source. For this

measurement, a col i near pulsed Nd:YAG laser operated at low pulse energy served as a Raman

probe. To suppress sample blackbody emission at high temperatures, a gated detection scheme was used and the detector opening was synchronized to the laser pulse. Since the normal Raman

effect is a spontaneous process, effective rejection of sample blackbody radiation at high

temperatures is achieved.

Rutile coatings exhibited no phase changes during heating to nearly 1500°C. However,

significant band shifts to lower frequency, were measured. For instance, the a, mode at 610

cm at room temperature shifts to 530 cm at ca. 1500°C. Upon cooling to 25 e, the initial room temperature spectrum is recovered.

Anatase coatings exhibited a rather different response to temperature as shown in figure 6.

An irreversible phase transition to the rutile crystalline form occurs at a temperature below

910 C C. This is consistent with equilibrium thermodynamics which predicts a phase transition in

this temperature region. However, when the coating is cooled down to room temperature, the

rutile phase persists. The phase change is accompanied by a significant change in density,

which leads to swelling and eventual microcrack formation in the coating.

The Raman experiments discussed previously were designed to characterize equilibrium damage

to optical coatings induced by pulsed high energy laser irradiation. A second group of

experiments was performed in order to characterize the non-equilibrium state immediately

following irradiation by a high energy laser pulse in a regime where equilibrium thermodynamics

does not apply. The first series of experiments involved measuring Raman spectra separated in

time by increments of 10 milliseconds following the Nd:YAG laser damage pulse. The 488 nm CW

Ar+ laser line was used as the Raman probe. Below damage threshold for any coating Raman

266
spectra acquired at 10 msec intervals are identical. When anatase coatings incur damage, a

rapid intensity rise near the exciting line is observed, however, the major anatase lines

LASER INDUCED DAMAGE

IN Ti02 (Anatase)

FREQUENCY (cm"1 ) PRISTINE DAMAGED

145 149

199 200

398 396

517 515 639 636

(air')

Damaged

(cm" 1 ) 450 550 650 750

Figure 5. Raman spectra of a 9587 A anatase film on silica before and after exposure to a single 532 nm high energy pulse.

Anatase C02 Low Heating 532 nm Nd'YAG Probe

500 700 WAVENUMBERS

Figure 6. Raman spectra taken during equilibrium C0 2 laser heating experiment showing

irreversible transformation of anatase to rutile at ca. 910°C.

267
are still evident albeit at lower intensity in all subsequent spectra. Results for rutile

coatings are shown in figure 7. At 10 msec following the damage pulse, the low frequency intenr

sity rise is evident. However, the rutile vibrational lines are not seen. Rather, the 640 cm"

e anatase line is observed. At longer times, the rutile spectrum recovers, again at a lower

intensity than the undamaged coating. An apparent reversible phase transition to an anatase-1 ike phase has occurred.

The existence of an anatase-like phase at short times following laser damage to rutile

coatings is further supported by data depicted in figure 8. Spectra were recorded as a function

of detector delay for a single Nd:YAG laser pulse that served to both induce damage and act as the Raman probe. Spectra were acquired for detector delays up to several hundred nanoseconds.

Evidence of an anatase-like phase persists even at detector delay times in excess of several

hundred nanoseconds. At lower pulse energy below damage threshold conditions only the rutile

spectrum is measured at zero detector delay. For longer detector delay times, no Raman signal can be detected. Similar results have been observed using optically delayed laser pulses of

high (damage) and low (probe) energy separated in time.

TRRS OF RUTILE

488 nm excitation

DELAY

300 500 700 900

WAVENUMBERS

TRRS OF RUTILE

532 nm excitation

DELAY \ undamaged

20 nsec JLa

XT

400 nsec

i

ly/V

300 500 700 900

WAVENUMBERS

Fig 7 Fig 8

Figure 7. Time resolved Raman spectra of a rutile coating subjected to a high energy laser

damage pulse showing reversible transformation to an anatase-like phase. Spectra were

acquired under CW Ar+ excitation on a millisecond time scale.

Figure 8. Time resolved Raman spectra of a rutile coating subjected to a single high energy

laser pulse acquired under 532 nm Nd:YAG excitation. Vibrational lines characteristic of the

anatase lines are seen at short times following the damage pulse.

5. Discussion

Measured Raman spectra under equilibrium and non-equilibrium conditions provide sufficient

structural information to formulate a model for the laser damage process in Ti02 optical

coatings. The relative instability of anatase coatings under laser damage conditions results

from the observed irreversible phase transformation to the rutile phase at temperatures below

910°C. Localized heating from the laser damage pulse stimulates the phase transition. Induced

swelling at the damage spot leads to lattice mismatch and localized fracturing of the surface.

Vesitages of the rutile phase are transported away from the damage center. The remaining

anatase, not having enough thermal energy to change phases, persists at long times exhibiting

lower intensity Raman lines than the undamaged starting material.

A slightly different mechanism is proposed to explain laser damage phenomena in rutile

coatings, which exhibit a higher damage threshold than the anatase coatings. The rutile phase

268
undergoes no detectable phase transition at temperatures below ca^. 1500°C. Raman observed band

shifts to lower frequencies indicate localized bond length increases which can result in bulk

swelling. These changes are reversible, however, and should not contribute to the damage

process. However, time resolved Raman measurements reveal features characteristic of the

anatase phase for spectra acquired soon after the damage laser pulse has encountered the

surface. Since this phase transition (rutile-»-anatase) does not occur under equilibrium

conditions, a non-equilibrium explanation is in order.

High energy laser irradiation of materials can give rise to electronic excitation through

multiphoton processes. [12] Vibrational levels may still be populated as in the ground state

since the excitation process is rapid. Time resolved measurements will then probe the excited

electronic state.

An elementary molecular orbital treatment of titanium localized in an octahedral oxygen

environment suggests that the filled sigma bonding molecular orbitals between titanium and

oxygen ligands are of a-, t, , and e symmetry. [13] Theoretical band structure calculations

for rutile conclude that^the Upper vatence band has a predominant 0-p orbital character whereas

the conduction band of Ti0 2 is derived mainly from Ti-d states. [14] Therefore, excitation of a bonding electron from the highest occupied e state to an antibonding t, * state which is

strongly allowed would tend to weaken the bonding between titanium and oxygen. Electron

repulsion effects would act to perturb the octahedral structure forcing the square planar oxygen

arrangement to distort. The resulting "puckered" structure in the excited state of rutile is

qualitatively similar to the structure of the anatase ground state. Therefore, the Raman

spectrum of the excited rutile state should exhibit similar features to the normal Raman

spectrum of anatase. When the excited electron relaxes back to the ground state, the rutile

structure is recovered. In this non-equilibrium case, the electronic transition causes a "phase change" as opposed to the equilibrium thermally induced phase change for anatase. The

higher damage threshold of rutile coatings may result from the fact that a significant amount of

the laser damage energy is stored in the excited electronic state, whereas in anatase, the

energy may rapidly be thermal ized leading to the irreversible phase transformation.

The persistence of the Raman spectrum of laser damaged rutile coatings at times approaching

several hundred nanoseconds following the damage pulse may be explained in terms of resonance enhancement of the vibrational band intensities. Rosseau and Williams[15] have demonstrated

that under resonance excitation of I 2

, Raman spectra can be recorded from a single laser pulse

at detector delay times approaching 1 microsecond. A similar mechanism is reasonable when

electron excitation is induced in solids by laser irradiation.

6. Conclusion

Raman studies of laser induced damage in Ti0 2 optical coatings were undertaken to provide

molecular structural information for understanding the damage process. Anatase coatings,

determined to have a lower damage threshold than rutile, fail by virtue of an irreversible phase

transition to the rutile structure at temperatures below 910°C. Rutile accommodates energy in an excited electronic state resulting in a "non-equilibrium" phase transformation to an "anatase-like" state. Both coatings apparently fail by selective removal of the anatase phase

from the coating.

In addition to structural considerations of the coating, substrate effects may also be

important in understanding the mechanism responsible for coating failure. Figure 9 shows

measureable damage to silica substrates as a function of the number of laser shots. The laser

induced polarization intensity changes result from microscopic surface damage (microcracks

,

density fluctuations) which act to scramble the polarization of the probe laser beam.

Substrate damage could lead to optical coating failure by other mechanisms and suggests

another topic for additional work in this area. Raman techniques will be applied to laser

damage studies of other coatings such as Zr0 2 and multilayer designs in future work where

attention to the coating-substrate interaction will also be focused.

This work has been supported by the Air Force Weapons Laboratory under contract PO-84-004.

Optical coatings have been supplied by Dr. W. T. Pawlewicz who is also thanked for helpful

discussions regarding this work. The authors also thank R. A. Swanson and C. H. Nguyen for

assistance with experimental measurements.

269
I -I

0.75- O

>-

UJ

0.25-

0.5-

0 0 500 1000 1500 2000 2500

LASER SHOTS (532 nm)

Figure 9. Induced polarization changes in a laser damaged Si

0

2 glass substate as a function of

number of pulses. The intensity ratio refers to transmitted laser light through crossed

polaroid filters relative to the parallel filter orientation.

7. References

[1] Pawlewicz, W. T., Exarhos, G. J., and Conaway, W. E. Structural characterization of Ti0 2 optical coatings by Raman spectroscopy. Applied Optics 22(12): 1837-1840; 1983.

[2] Nemanick, R. J. Connell , G. A. N., Hays, T. M., and Street. R. A. Thermally induced

effects in evaporated chalcogenide films. Phys. Rev. B. 18: 6900-6914; 1978.

[3] Newmanick, R. J. , Tsai, C. C, and Connell, G. A. N. Interference-enhanced Raman

scattering of very thin titanium and titanium oxide films. Phys. Rev. Lett. 44: 273-276;

[4] Exarhos, G. J. and Pawlewicz, W. T. Raman characterization of all-dielectric multilayer

Si0 2 /Ti0 2 optical coatings. Applied Optics 23(12): 1986-1988; 1984.

[5] Exarhos, G. J. Substrate signal suppression in Raman spectra of sputter deposited Ti0 2 films. J. Chem. Phys. 81(10): 000; 1984.

[6] Pawlewicz, W. T., Martin. P. M., Hays, D. D., and Mann, I. B. Recent developments in

reactively sputtered optical thin films. Seddon, R. I., Ed. Proceedings of the SPIE

conference on optical thin films; 1982 Jan. 26-27; Los Angeles, CA. Proc. Soc. Photo-Opt.

Instrum. Eng. 325: 105-116; 1982.

[7] Pawlewicz, W. T., Hays, D. D., and Martin, P. M., High-band-gap oxide optical coatings for

0.25 and 1.06 micron fusion lasers. Thin Solid Films 73: 169-175; 1980.

[8] Clarke, R. J. H., The chemistry of Titanium and Vanadaium, An Introduction to the chemistry

of the early transition elements. North Holland; Elsevier Publishing Co.; 1973. 265 p.

[9] Parkes, G. D., Mellor's Modern inorganic chemistry, Revised Edition. London; Longrans

Publishing Co.; 1961; 803 p.

1980.

270
[10] Capwell , R. J., Spagnolo, K. , and DeSesa, M. A. A rapid determination of low concentrations

of anatase in rutile Ti0 2 pigments by Raman spectroscopy. Appl . Spectrosc. 26: 537-539;

1972.

[11] Beattrie, I. R., and Gilson, T. R. Single crystal laser Raman spectroscopy. Proc. R. Soc.

London Sec. A.; 307: 407-429; 1968.

[12] Royce, G. A., and Kay, R. B. Multiphoton interactions in rutile. Applied Optics 23(12):

1975-1979; 1984.

[13] Figgis, B. N. Introduction to Ligand Fields. New York; Interscience Publishers; 1966.

193 p.

[14] Munnix, S and Schmeitz, M. Surface electronic structure of rutile-type semiconductors:

Sn0 2 (110) and Ti0 2 (110). Surface Science 126: 20-24; 1983.

[15] Rosseau, D. L. and Williams, P. F. Resonance Raman scattering of light from a diatomic

molecule. J. Chem. Phys. 64(9): 3519-3537; 1976.

A questioner wanted to know what temperature change occurred when going from one phase to

another. The author had not measured any thermal studies of the phase changes. In answer

to the question of how far below damage threshold the laser induced depolarization, presumably caused by laser induced damage, was seen, the author replied that the effect occurred

before any damage could be seen '.under a microscope, but that the laser fluence at which it

occurred has not yet been established. Another questioner suggested that the temperature

might be measured by determining the ratio of the Stokes to -the Anti-Stokes scattering

intensity. The author pointed out that this approach would be valid for conventional

equilibria Raman scattering. However, if a resonance exists caused by Raman scattering from

an excited electronic state, then the ratio may not give the true molecular temperature.

Measurement of the surface temperature of bare fused silica should be valid, although the

author had not done such measurements. He had used this technique successfully to determine

the temperature of the CO£ laser heated material and gotten good agreement with the results

of a two color pyrometer. In answer to the question of the magnitude of the peak pressure

of the laser induced pressure wave, the author replied that they measured a Raman shift of

4 cm~l . Osaka measured a shift of 18 cm~l for a 43 kbar pressure, which would suggest

that at least as a crude approximation the pressure may have been about 10 kbar.

271
Ultraviolet Thin Film Coating Characterization Manuscript Re,

3 3-20-85 SAB

Alan F. Stewart

AFWL/ARBE

Kirtland AFB NM 87117

David J. Gallant

Rockwell International

Rocketdyne Division

P.O. Box 5670

Kirtland AFB NM 87185

ABSTRACT

Multilayer dielectric high reflectance coatings designed for 351nm were fabricated using ion

beam sputtering and electron beam deposition on super polished silicon substrates. These optics were evaluated in terms of total integrated scatter levels, absorption, reflectance and laser

induced damage thresholds. Ion beam sputtering was found to produce optics with the lowest losses

while electron beam deposited coatings demonstrated higher laser damage thresholds.

Key words: Absorption, cal orimetry , laser damage, reflectance, roughness, silicon, total

integrated scattering, 351 nm

1. Introduction.

Recent advances in the development of high power lasers operating in the visible and ultraviol£i_have demonstrated that optical thin film coatings for this wavelength regime require con- siderable study and improvement. The visible free electron laser, operational for the first

time in May 1983, required laser cavity optics with extremely low losses. [1.1 These FEL experiments utilized surfaces and coatings developed by the ring laser gyro industry where "super

optics" are commonplace. Higher power applications require the development of different coating

materials than those used in the gyro industry as well as the incorporation of silicon substrates

for efficient heat removal. To support development programs for the high power excimer laser

and the FEL, a program was established at the Air Force Weapons Laboratory to define the stateof-the-art by characterizing low loss substrates and coatings. Our results to date confirm that

much additional work needs to be done to develop thin film coatings and surfaces for use in the

ultraviolet.

The objectives of this effort were twofold. The first objective was to design and construct

instrumentation for the characterization of low loss optical surfaces and thin film coatings.

Evaluation criteria included absorption, reflection, scattering losses and laser induced damage

thresholds. The second objective was to determine the performance levels of polished silicon

substrates and thin film multilayer high reflectance coatings designed for low total losses.

Three different polishing vendors and two deposition techniques from four coating houses formed

the sample matrix for this study. Overall, test results were encouraging in the sense that some substrate/coating design combinations performed very well in certain categories. Unfortunately,

thin films that had high damage thresholds exhibited higher losses due to scattering, absorption

and reflectance.

2. Sample Fabrication.

To define the state-of-the-art in silicon substrate polishing techniques and low loss optical

coatings, vendors were selected with proven past performance. Since the highest quality optical

surfaces available are those produced for the ring laser gyro industry, polishing vendors were selected for the study on the basis of their experience with silicon and silica based gyro

272
optics. Single crystal silicon was specified but the orientation was left to t he discretion of

the polisher. As indicated in Table 1, polishers "A" and "C" selected the 111 orientation

with polisher "B" using the 100 orientation. Polisher "0" had not attempted to polish silicon

before but was able to achieve remarkably good surfaces on polycrystal 1 i ne substrates.

Polishers A, B, and C fabricated 38 millimeter diameter substrates for scattering, reflectance

and damage threshold measurements. An additional set of 19 millimeter substrates were fabricated for calorirnetry measurements.

After Total Integrated Scatter surface roughness measurements had been completed, substrates

were placed in sealed containers and distributed to four coating vendors. Electron beam and ion

beam coating processes were used to fabricate the high reflectance mirrors characterized in this

study.

A variety of coating designs and materials were selected for this study as shown in Table 2. Coater "A" had considerable experience with ion beam sputtering of low loss Si02/Ta205 coatings

and success with this combination was easily achieved. With coater "B", we tried to take advantage of the measured high damage threshold of Si Og/Al 2O3 coatings 1 2 ] while achieving high reflectance with an underlying stack of Si02/Zr02. One limitation of Si O2/ Al 2O3 coating stacks is the narrow reflectance bandwidth and this may have affected our subsequent measurements. Coaters

"C" and "D" utilized Si O2/HFO2 , Si02/Zr02 and Si02/Sc203 multilayer designs which are well

presented in the literature.

Before any characterization measurements were performed, substrates and coatings were cleaned

carefully. A spinner system was used to dispense low residue electronic grade acetone or methanol

on the surface. Collodion was then used for the final cleaning of each part before each set of

characterization measurements was performed.

3. Experimental

Instrumentation was constructed to evaluate optical surfaces and thin film coatings designed

for low losses in the mid-visible and ultraviolet. The argon-ion laser was selected as the source for these measurements since it could provide 351nm and 514nm outputs to match the XeF

excimer line and possibly a visible FEL. Established and proven instrumentation designs were

used to construct a Total Integrated Scatter (TIS) device, a laser calorimeter and a cavity

ring-down reflectometer. Adaptation of these instruments for use in the ultraviolet was not a problem except with certain elements of the reflectometer. Cross calibration of these instruments

with other laboratories is currently in progress.

Total Integrated Scatter (TIS) is a well known technique used for surface characterization. ("3

] A schematic layout of the AFWL instrument is shown in Figure 1. The beam is focused to a diameter

of approximately 1 millimeter at the sample. Three separate detectors are used in the instrument.

The first detector monitors the laser power and this output is used to normalize the outputs of

the other two detectors. The second detector monitors both the incident and specular reflected

beam with a rotating mirror. The third detector is located at the specimen image plane inside

the hemispherical collector to monitor the diffuse reflectance or scatter.

TIS is the ratio of the diffuse reflectance to the total reflectance

or TIS = R diffuse jIS ~ R diffuse

R diffuse + R specular R specular

From scalar scattering theory the effective root-mean-square roughness of the surface, a, is

defined by

a - _A_ (TIS)V2

Because a single wavelength is used to measure the scatter and because of geometrical scatter

collector limitations, the device is sensitive to a narrow band of spatial frequency components.

The effective bandwidth of this instrument extends from 0.08 to 2.7^-1 at 351 nm.

In practice, each sample was measured at three sites on the TIS instrument. Sequential

measurements on the same site indicated a measurement precision of +0.5 Angstroms RMS. Data

obtained on uncoated super polished silicon substrates is summarize"? in Table 1. This data was taken using an identical TIS device operational at 633nm. The tolerances given are the standard

273
deviation over the set of forty pieces measured from each polisher. The smooth surfaces measured

on samples from polisher "D" were a surprise development during this effort. It should be noted

that because of bandwidth limitations, TIS is only marginally adequate for the evaluation of such

ultrasmooth surfaces. However, it should be pointed out that on our instruments, epitaxial grade

silicon wafers consistently measure out at 1 Angstrom RMS, so that the surfaces polished by vendor

"D" therefore may be nearly damage free and close to the optimized optical surface.

After coatings had been deposited on this set of super polished silicon substrates, each was again evaluated on the TIS instrument. Measurement data at the design wavelength of 351 nm for

each coating vendor and each polishing vendor is summarized in Table 2. The lack of correlation

between uncoated surface roughness and subsequent measurements on coated optics should be noted.

This is true in all cases except for ion beam sputtered films from vendor "A" which maintain the

relative ranking between substrates. These coatings were clearly superior to all others tested

in terms of low scatter or surface roughness levels. It is useful for comparison purposes to note that a TIS roughness figure of 7 Angstroms RMS correlates to losses due to scattering of

about 650 ppm.

A laser calorimeter was used to measure directly the optical absorption at 351nm. The instrument is designed as described in Figure 2 and can accommodate either reflective or transmissive

optics. The calorimeter consists of a vacuum chamber enclosure and an AC thermistor bridge with

lock-in detection. K J The thermistors themselves are unencapsul ated ceramic beads 250um in diameter with 50un diameter lead wires. Sensitivity was found to vary with chamber pressure and the

duration of time the thermistors were under vacuum, perhaps due to the absorption of water vapor

into the ceramic beads. Consequently, all measurements were performed with a chamber pressure

of 200mTorr. Calibration by electrical substitution indicated a bridge signal output of

1 Volt/°K at this pressure.

Measurements were performed on low thermal mass (approximately 1 gram) super polished silicon and fused silica substrates 19 and 25mm in diameter respectively. The sensitivity of the

calorimeter bridge was determined to be about 50 ppm using a laser power of 1 Watt. The beam was "p" polarized with respect to the sample and was incident on the sample at 2.5° off normal.

Absorption measurements for each deposition method and coating design are summarized in

Table 2. There was no apparent dependence of coating absorption on substrate polisher and the

data is an average value of a number of measurements of each design. Tolerances are indicative

of the ±10% estimated accuracy of the instrument. It is important to note that the absorption

measured is the sum of absorption in the dielectric stack and that absorbed in the substrate.

Measurements on identical coatings on both silicon and fused silica substrates for Si02/Ta205

from polisher "D" indicate that the effect of substrate absorption can be substantial. The fact

that the laser was operated broadband with output lines extending from 337 to 363nm may also

help to explain the high apparent absorption levels. Even though the coating designs incorporated high index materials and hence, the bandwidth of the coatings was typically 500 Angstroms,

spectrophotometer curves of selected mirrors showed -1% reflectance variations across the bandwidth of the laser. This was unexpected but is unquestionably due to the effect of dispersion

in the coating materials used.

A cavity ring-down ref 1 ectometer was used to measure the total losses of the test optics. [5]

The instrument is shown in Figure 3. The basic elements include the laser source, fast electroptic shutter, mode matching lens and test cavity. When the energy trapped in the cavity

reaches a preset level, the detector-amplifier-comparator network shuts off the incident laser

light with a 40nsec fall time Pockel 's cell. The detector then monitors the decay time of the

trapped light in the cavity as it is gradually lost due to cavity mirror transmission, absorption, scattering and diffraction. The reflectivity product of the cavity optics is then determined from the exponential decay time constant (t) ) of the trapped cavity radiation through the

where L is the round trip cavity length and c is the speed of light.

The detector output is recorded by a digitizing storage oscilloscope and the data is fit to

a computer generated exponential curve to determine the cavity decay time. A typical plot

appears in Figure 4. The buildup of the trapped cavity energy is plotted on the left side of

the figure. The Pockel 's cell "fires" at 2.5 microseconds and the cavity energy begins to decay. The solid curve is an exponential curve generated by at least a square's fit to the

rel ation

274
digitized data. For the optics tested in this survey at 351nm with "S" polarization, typical

decay time constants ranged from 1 to 3 microseconds and total cavity losses ranging from 2,000

to 10,000 ppm. A computer printout for 30 consecutive decay curves for one test optic appears

in Figure 5. Lower cavity losses lead to longer decay times and greater precision in the

reflectivity product. From the uncertainty in the reflectivity product, the absolute accuracy

ranged from 100 ppm to a worst case of 1,000 ppm.

The data obtained on the sample matrix using the reflectometer is summarized in Table 3.

The tolerances given in the table are the standard deviation from measurements made on each set

of optics and shows the sample-to-sample variation observed in a given coating run. Clearly,

ion sputter deposited films deposited by "A" are superior performers with an average reflectance

of 0.999. The lowest reflectances were measured on highly scattering Si02/Zr02 films from

coater "D".

Laser damage testing was performed at Los Alamos National Laboratory by Stephen Foltyn, John

J o 1 in and Billie Mauro. A XeF excimer laser was used in N-on-1 testing using standardized test

procedures. [2 J Damage thresholds were defined as the maximum fluence for which there is zero probability of damage occurring. The results of these measurements appear in Figure 6. Coatings on both fused silica and silicon substrates were tested as indicated in the figure.

Clearly, ion beam sputtered films from coaters "A" and "B" have lower damage thresholds by a factor of two or more in comparison to electron beam coatings from the other vendors. There are differences in the materials used by each coater and this prevents a direct comparison between

coating technologies. In other test series at LANL, Si O2/ Al 2O3 coatings have demonstrated

superior damage resistance over all other material combinations. [2 J However, there have been no iterative coating development and damage testing programs involving ion beam sputtered films to

date so there may be considerable room for improvement.

Ion beam sputtered coatings were measured to have lower laser damage thresholds overall and

these thresholds appear in Figure 6 to be independent of the substrate type. While electron

beam deposited films were superior overall in terms of laser damage thresholds, the measured

threshold of coatings on silicon substrates were generally lower than identical coatings on fused silica substrates. The calorimetry data in Table 3 for electron beam deposited films from

coater "D" indicates significant transmission through the coating followed by absorption in the

substrate. Substrate absorption may therefore account for the lower thresholds observed on silicon substrates with electron beam coatings.

4. Conclusions.

Low loss, high reflectivity, multilayer dielectric mirrors designed for use at 351nm have

been evaluated in terms of total losses and laser damage thresholds. Substrates of super

polished silicon and fused silica were utilized to minimize coating scatter levels. Our results

indicate that fabrication of superior substrate surfaces in terms of figure and roughness can be

easily and routinely achieved. Polishing techniques employed by various vendors were capable of

producing surfaces with 5 Angstroms RMS roughness. However, it is clear that improvements can

be made to produce even better surfaces on silicon approaching the epitaxial grade surfaces used

in the semiconductor industry.

Optical coatings pose the most significant development problem for continued advances in the

ultraviolet. It was clear from this study that some of the coatings fabricated by the new ion

sputtering technology can exhibt very low losses with high reflectivity, low absorption and low

scattering losses. However, these films have lower laser damage thresholds than those produced

by electron beam technology. While considerable effort has gone into the development of damage

resistant electron beam coatings over the past few year, it is hoped that the greater control over deposition parameters achievable with ion technology could result in comparable or superior

performance levels after a similar development program. Certainly continued work is necessary

for ultraviolet optics, from improvements in characterization methods to advances in polishing

and coating techniques.

Table 1. Silicon Crystal Orientation and Average Surface Roughness Determined from TIS Data at

6328 Angstroms.

Table 2. TIS Surface Roughness Data at 351nm on Sample Coating/Substrate Matrix.

Table 3. Laser Calorimetry and Reflectometer Data at 351nm on Sample Matrix.

Figure 1. Total Integrated Scatter Instrument Configurations.

275
Figure 2. Laser Calorimeter Optical Configuration and Circuit Layout.

Figure 3. Cavity Ring-down Ref1 ectometer Instrument Configuration.

Figure 4. Typical Decay Curve of a Trapped Cavity Mode in the Ref1 ectometer.

Figure 5. Printout of Refl ectometer Data Run Showing Typical Cavity Losses, Decay Times and

Reflectivity Products at 351nm.

Figure 6. Laser Damage Thresholds from N-on-1 Testing at 351nm of High Reflectance Stacks on Super Polished Silicon and Fused Silica.

References

.

1. Billardon, J.M. ; Elleaume, P.; Ortega, J.; Bazin, C. ; Velghe, M. ; Petroff, Y. ; Deacon, D.

;

Robinson, K. ; Madey, J.; Phys. Rev. Lett 51, p. 1652.

2. Foltyn, Stephen R. ; Jo 1 in, L.J. ; "Alumina/Silica Multilayer Coatings for Excimer Lasers";

Nat. Bur. Stand. (US) Spec. Publ . #669, 1983. Los Alamos Publication LA-UR-84-1831.

3. Bennett, Jean M. and Stowel , W. Kent; "Round Robin Testing of Low Scatter Optics"; Nat. Bur.

of Stand. (US); Spec. Publ. #669, 1983.

4. Allen, Thomas H. , Apfel , Joseph H. ; Carniglia, C. K. ; "A 1.06mh Laser Absorption Calorimeter

for Optical Coatings"; Nat. Bur. Stand. (US); Spec. Publ. 541; 1978 December, pp 33-36.

5. Anderson, Dana Z. ; Frisch, Josef, C. ; Masser, Carl S. ; "Mirror Refl ectometer Based on Optical Cavity Decay Time"; Appl . Opt. 23, p. 1238-1245 (1984).

276
TOTAL INTEGRATED SCATTER

SUPER POLISHED (UNCOATED) SILICON

6328 A (HeNe)

SURFACE ROUGHNESS

POLISHER "A"

SINGLE CRYSTAL

1-1-1 ORIENTATION

3.4 A RMS ±1.3

POLISHER "B"

SINGLE CRYSTAL

1-0-0 ORIENTATION

4.4 A RMS ± 1.6

POLISHER "C

SINGLE CRYSTAL

1-1-1 ORIENTATION

6.1 A RMS ±1.4

POLISHER "D"

POLYCRYSTALLINE

1.7 A RMS ±0.4

• BEST FUSED SILICA -0.5 A RMS

Silicon Crystal Orientation and Average Surface Roughness Determined

from TIS Data at 6328 Angstroms .

277
Table 2. TIS Surface Roughness Data at 351 nm on Sample Coating/Substrate Matrix.

278
1 |

REFLECTANCE

.9990

±

0.0005

.99881

0.0004

.8880

1

.0004 e

e• •

N

•••

.9877

1

.001

*

.9983

1

.0008

*

STD.

DEV.

FROM

10

MIRRORS

**

ABSOLUTE

ACCUR/

1.001

PER

MIRROR

LECTION

AT

35

Intr

ABSORPTION

i

* i *

o o

e ' o

o

1 e

i , +i • 1 •

« 1 »

e ' o

e o

• i •

.0076

±

.001

.0016

±.0002

N

i o

1 CO co s :««

e

1 o o

+1 . o o

« l + , ° o

.

I « «-

I o o

i ' o

ABSORPTION/REF

FILM

DESIGN

10 o

M

•H

N

o

5

*J ?

< N

*x >. « CC o ©

• •

N

o

If. X

M

o

•

« 1 o

O i en k o

N 1 CO

-v I ^ « , W

o ; ©

• 1 •

COATER

CO < 1 •

1

tff* CO o • • III

CO a i • Ul

Table 3. Laser Calorimeter and Reflectometer Data at 351 nm on Sample Matrix.

279
SAMPLE

SCATTER DETECTOR

/HEMISPHERICAL

DOME

INCIDENT/SPECULAR

DETECTOR

ARGON-ION

LASER

REFERENCE ^DETECTOR

LENS APERTURE

TOTAL INTEGRATED SCATTER (TIS) LAYOUT

a(RMS) = ^ (TIS)1/2

INSTRUMENT BANDWIDTH : X = 5 1 4 . 5 n m f _ - .05 - 1.8 nm~

1 s

X = 351nm fg - .08 - 2.7 pm" 1

Figure 1. Total Integrated Scatter Instrument Configurations.

280
CALORIMETER OPTICAL LAYOUT

ELECTRONIC LAYOUT

• INSTRUMENT ACCURACY ± 8%

• SENSITIVITY ^ SOppm

Figure 2. Laser Calorimeter Optical Configuration and Circuit Layout.

281
COMPARATOR

H V

A fth P

DIGITAL

STORAGE

SCOPE

COMPUTER

re \

1/4X

PLATE

0-

POCKELS V

CELL N

1 /2 X \ POCKELS

PLATE ^ CELL

C ALCITE

POLARIZER

ARGON-ION

LASER

C ALCITE

POLARIZER

MODE MATCHING

LENS

CAVITY

f

CAVITY REFLECTOMETER LAYOUT

LOSSES = ~-

CT

R = 1 -LOSSES PRODUCT

Figure 3. Cavity Ring-down Reflectometer Instrument Configuration

282
X-AXIS 2000. NS/DIV

Y-AXIS 200. MVVDIV

VERSION 007 CHECKSUM IS OK

CH2 DC

100 CM CAVITY DECAY

DATA RECEIVED FROM TEKTRONIX 468 SCOPE DECAY TIME = 0.1S601E-0S SECONDS REFLECTIVITY PRODUCT = 0.93642 CORRELATION COEFFICIENT = -0.33SS9

Figure 4. Typical Decay Curve of a Trapped Cavity Mode in the Reflectometer

OPERATOR: DAVE GALLANT 2 6- 5EP- 34

TEST MIRROR ID: OJAI LOT 2 121- 5 V CAVITY MIRROR #1 ID: o MTR #1

CAVITY MIRROR *2 ID: OJAI 11-1 3V

WAVELENGTH= 351.0 r.M CA' / 1 TY LENGTH= 200 .QO CM

MINIMUM CC = 0. 93000 CUTOFF = 3, 50

REFLECTI VI Ti- ( PRODUCT 1 OF CAVITY MIPPOR(S): UNI NOWN

THE CAVITY LOSS REFLECTI VIT DECAY THE

DATA IN PARTS PRODUCT OF TEST TIME IN CORRELATION

POINT PEP MILLION cavity mirpore SECONDS COEFFICIENT

NUMBER (CL) < pp > ; DT(CO

1 7906 . 60 0 . 99214 0 . ;6S~5E-05 -0. 99463

2 5699 . 33 0.99432 0. 2341 1E-05 -0.98035

3 7662. 10 0 . 99233 17414E-05 -0 . 93714

4 8203. 46 0 . 99 ! 34 0. 16255E-05 -0. 93657

5 6403. 68 0. 99 36 ? 0 . 20S36E-05 -0. 98330

6 744?. 68 0 • 9 9260 0. l~925E-05 -0.98454

7 ~597 . 24 0 . 992-5 n , 17=52E-05 -0.936 3 0 3 7523.26 0 . 99252 0. :~~ 35E-05 -0. 99489

9 3060. 50 0.9919? 0 . 1655BE-05 -0. 93322

10 8079 . 52 0.99197 0. 16514E-05 -0. 93376

1 1 72SC » T 9 0 . 99276 , 1S32SE-05 -0. 99438

12 7568.65 0 .93247 0 . l~629E-05 -0. 99414

13 548! . 56 C . 99 1 5"

015731E-05 -0. 98959

14 81-3.66 0.99190 0. 16374E-05 -0. 98669

15 7324. 54 0 . 992" 1 32 16E- 05 -0. 99155

16 3357.63 0.99169 0 . 1 5965E-05 -0. 99029

17 6010. 14 0.99402 22200E-05 -0. 98456

18 0 99227 17157E-05 -0.93963

19 6270 . ^4 0 . 99376 0

2 1 278E - 05 -0.99254

20 7903. 04 0.99214 0. 15883E-05 -0.98306

21 7199.71 0. 99284 0 . 18532E-05 -0.99139

22 7064. 47 0. 99297 0

188S7E-05 -0.98980

23 5S97 . 07 0. 994 13 0. 22625E-05 -0. 98133

24 7950. 33 0.99210 0. 167S2E-05 -0. 98605

25 7283. 84 0. 99276 0. 1S31SE-05 -0. 99277

26 5640.42 0.99438 0

23655E-05 -0. 98288

27 7566. 63 0. 99248 n

17633E-05 -0.99221

23 5960.55 0.99407 0. 22385E-05 -0.93354

29 7996. 13 0. 99205 0. 156S6E-05 -0. 93847

30 6886.30 0.99315 0. 19376E-05 -0.9B479

AVERAGE CL = 7304.93 PPM STANDARD DEVIATION = 0.838E+03

AVERAGE RP = 0.99274 STANDAPD DEVIATION = 0.B30E-03

AVERAGE DT = 0. 18524E-05 SEC STANDARD DEVIATION = 0.235E-06

AVERAGE CC = -0.98351 STANDARD DEVIATION = 0.394E-02

Figure 5. Printout of Reflectometer Data Run Showing Typical Cavity Losses, Decay Times and

Reflectivity Products at 351 nm.

283
5-

4-

<ME

<o

a>3O

°2-

I -

0-L 1-1 i

B C

Coater A

Si02 /Ta205

N-on-l Damage Thresholds at 351 nm ^

n

1

1

\ B

Coater B

Si02/Al203

SK)2/Zr02

1

!

I

I

B

Coater C

Si02/HF02

* LANL Data (Foltyn. Jolin)

T-IOnsec FWHM

CJ»0.58mm

35 pps

I

A B

Coater D

Si02/Zr02

- Fused Silica

rTl

!

11

B

Coater D

SlO2/Sc203

!

Figure 6. Laser Damage Thresholds from N-on-1 Testing at 351 nm of High Reflectance Stacks on Super Polished Silicon and Fused Silica.

I
Manuscript Received

12-27-84 SAR

Effect of Overcoats on 355-nm Reflectors

C.K. Carniglia and T. Tuttle Hart

Optical Coating Laboratory, Inc.

2789 Northpoint Parkway

Santa Rosa, CA 95407-7397

M.C. Staggs*

Lawrence Livermore National Laboratory

University of California

P.O. Box 5508, L-470

Livermore, CA 94550

Previous experiments have demonstrated that halfwave overcoats of a low-index

coating material are effective in increasing the damage thresholds of dielectric high

reflector coatings (HR's) at the wavelengths of 1064 nm and 248 nm. This overcoat

layer led to a 50% increase in damage threshold at 1064 nm and to a 100% increase in

threshold at 248 nm. This paper presents new results on the effect of halfwave overcoats at an intermediate wavelength: 355 nm.

Results reported at last year's Boulder Damage Symposium indicated that halfwave

silica overcoats were detrimental to the damage thresholds of HR's at 355 nm. New

results confirm that silica overcoats decrease thresholds and also show that magnesium

fluoride overcoats do not improve the thresholds at 355 nm. A series of eight reflectors

without overcoats had an average threshold of 2.4 J/sq. cm. The addition of silica

overcoats decreased the average threshold by 2%, while magnesium fluoride overcoats

decreased the threshold by an average of 7%.

Key words: electric field; high energy laser; laser damage; laser reflector; overcoat;

reflector; thin film.

1. Introduction

A high reflector (HR) for use with near infrared (IR), visible, or ultraviolet (UV) lasers

typically consists of alternate layers of two dielectric materials which have different refractive

indices. The design is such that the optical thickness of each layer (the refractive index times

the thickness of the layer) is one quarter of the laser wavelength.

An HR design which has proven to be successful for short-pulse UV lasers [1,2] is illustrated

in figure la. It consists of 21 quarterwave layers. Scandia (Sc^O^) is used for the high-index

layers, and silica (SiO^) and magnesium fluoride (MgFg) are used for the low-index layers. The

design starts and ends with high-index layers to achieve maximum reflectance. The use of two

different low-index layers, as illustrated here, has proven useful in reducing the net stress of

the coating.

*Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore

National Laboratory under Contract No. W-7405-ENG-48.

285
The standing wave electric field pattern for laser light incident on the HR from the left is

also shown in figure la. It is highest in the outer layers and drops to a negligible value at the

substrate. Note that the HR designs shown in both figures la and lb are specifically for use at

355 nm and the refractive indices of the coating materials at this wavelength are given in the

figure caption.

The addition of an overcoat of the low-index material is a design trick which has proven useful at wavelengths in the IR and UV [2-5]. The optical thickness of the overcoat is usually half

of the design wavelength. Such overcoats have made significant improvements in the damage thresholds of laser reflectors. These results are discussed in section 2. A 355-nm HR with a halfwave

magnesium fluoride overcoat is illustrated in figure lb, together with the resulting electric field

distribution. Note that the electric field within the 21-layer HR is unchanged by the addition of

I 1

'/,-/,0 1

y sub

i —

Figure la. Design and electric field distribution for a 21-layer, 355-nm

laser reflector. The coating design is indicated in the lower

part of the figure. The materials and refractive indices are

as follows: crasshatched layers - scandia, n = 1.97; unshaded

layers - silica, n = 1.47; dotted layers - magnesium fluoride,

n = 1.39. The intensity of the incident field is indicated by

the arrow at the left, and the curve represents the time average

of the square of the electric field within the coating.

Figure lb. Same as figure la for a 21-layer, 355-nm laser reflector with

a halfwave magnesium fluoride overcoat.

286
the halfwave overcoat. The peak electric field within the overcoat itself has a magnitude of 4/n

relative to the incident electric field, where n is the refractive index of the overcoat. Because

the overcoat has a low index, the electric field within the overcoat is usually higher than the

highest field in the reflector stack itself.

Recent damage results at 355 nm [l] have indicated that the addition of a halfwave overcoat

does not improve the damage threshold of 355-nm HR's. It is the purpose of the present study to

further investigate this finding. The results of the current work are reported in section 3.

2. Previous results at 1064 and 248 nm

The beneficial effects on laser damage resistance of adding a halfwave silica overcoat to

titania/silica quarterwave-stack HR's at 1064 nm has been conclusively demonstrated [3]. Twelve

HR's with overcoat had an average threshold of 14.4 J/sq. cm for 1-ns laser pulses. The standard

deviation was 3.9 J/sq. cm. Three of the HR's with overcoat had thresholds in excess of 20 J/sq. cm

- the highest thresholds observed for conventionally evaporated coatings with 1-ns, 1064-nm laser

pulses. The average threshold of 17 HR's without overcoat was 8.8 J/sq. cm, with a standard

deviation of 2.8 J/sq. cm. Thus the average threshold was improved by more than 60% by the addition

of the silica overcoat.

The reasons for the success of silica overcoats at 1064 nm have not been well understood. As

indicated by figures la and lb, electric field considerations do not explain the improvement: the

field within the HR is the same with or without the overcoat, and the field within the overcoat is

the highest anywhere in the coating. Because of damage morphology differences observed between HR's

with and without the overcoat, the improvement in damage threshold for coatings with overcoats has

been attributed to an increase in mechanical strength of the coating provided by the overcoat [4],

The titania layers have a columnar structure and are known to fracture easily. The silica layers

have a more homogeneous structure and are more fracture resistant. In addition, the compressive

stress of the silica layers is thought to be more damage resistant than the tensile stress of the

titania layers. Thus the model for the improvement in the thresholds of HR's at 1064 nm due to

halfwave overcoats is a mechanical one: the overcoat strengthens the outer surface of the coating,

making it more damage resistant.

An extensive study of 45 HR coatings for use at 248 nm [2,5] has demonstrated an even greater

improvement in damage threshold due to the addition of a halfwave overcoat. The study involved

scandia/silica/magnesium fluoride coatings with designs similar to those shown in figures la and lb.

The basic HR stacks had either 19 or 31 layers and were deposited onto either fused silica or BK-7

substrates. Two different coating temperatures were investigated, 150 C and 250 C. The results

showed that all films without overcoats had thresholds below 4 J/sq. cm for 20-ns, 248-nm laser

pulses, while all coatings with halfwave overcoats had thresholds above 4 J/sq. cm. The median

threshold for overcoated HR's was 6.3 J/sq. cm, which was more than double the median threshold of

3.1 J/sq. cm for non-overcoated HR's. [5]

A significant part of this earlier study at 248 nm was that two different low-index materials

were used for overcoats: silica and magnesium fluoride. Overall, magnesium fluoride overcoats

produced an average threshold 20% greater than silica overcoats. A close inspection of the data [5]

reveals that magnesium fluoride gave a greater improvement than silica at the lower coating temperature (150 C). At the higher temperature (250 C), both overcoat materials gave about the same improvement.

The significance of the success of magnesium fluoride as an overcoat material is that it

discounts the mechanical model developed to explain the results at 1064 nm. Magnesium fluoride

layers have large crystalline structures and high tensile stress. In addition, they are mechanically "soft" at coating temperatures as low as 150 C. Thus, the silica and magnesium fluoride

overcoats are mechanical antitheses. The fact that they both lead to large improvements in damage

thresholds poses a serious challenge to a mechanical model for explaining the improvement due to

the overcoat.

Even without a satisfactory explanation, these previous results make it clear that overcoats

are beneficial at two widely different wavelengths: 1064 and 248 nm. While we recognized the

necessity to test the effect of overcoats at intermediate wavelengths, there was little doubt that

overcoats would lead to an improvement. Surprisingly this was found not to be the case at 355 nm.

3. Results at 355 nm

The first indication that overcoats did not improve the damage thresholds of 355-nm HR's was reported at the 1983 Boulder Damage Symposium [1]. A survey of reflectors made with four different

high-index materials produced the following results: the average damage thresholds at 355 nm of

287
HR's containing zirconia (Zr0

2

) and tantala (Ta

2 0

5

) were 3.0 J/sq. cm and 2.4 J/sq. cm, respectively,

without overcoats, and about 20% lower with halfwave silica overcoats. HR's containing hafnia (Hf0

2

) had a very low average threshold (1 J/sq. cm) with or without silica overcoats. The highest average

threshold (3.5 J/sq. cm) was observed for scandia/silica/magnesium fluoride HR's with magnesium

fluoride overcoats. No HR's of this type without overcoat were tested, however, since from the

earlier work at 248 nm it was assumed that the scandia HR with magnesium fluoride overcoat would

have the highest threshold.

The conclusions relative to the effect of overcoats at 355 nm from this early work were unclear.

Silica overcoats seemed to be detrimental to the threshold, but the coating with the highest threshold had a magnesium fluoride overcoat. The question remained as to whether the high thresholds

observed for the scandia HR were due to the magnesium fluoride overcoat or the differences between

scandia and the other high-index coating materials. To answer this question, a set of 24 scandia/

silica/magnesium fluoride, 355-nm HR coatings with various overcoat options was made for damage

testing. The coatings were made at OCLI in a 1.2 meter box coating machine. All materials were deposited by evaporation using an electron gun source. The substrate for all coatings was BK-7

which had been polished by Zygo Corporation using a continuous-feed polishing technique. Six parts were coated in each of four runs. A 25-layer scandia/silica/magnesium fluoride HR with a design

similar to that shown in figure la was coated simultaneously onto all six parts. Then, with the

help of movable masks, a halfwave silica overcoat was added to two of the HR's and a halfwave

magnesium fluoride overcoat was added to two other HR's. The remaining two HR's received no overcoat. Two of these runs were made at 150 C and two were made at 250 C.

The damage testing of these parts was done by LLNL using a frequency tripled Nd-glass laser.

The 355-nm, 0.6-ns laser pulses were focused to give a beam with a 1-mm diameter at the coating

surface. Each test site was irradiated with one laser pulse and the presence of damage was determined visually using Nomarski microscopy. The precision of the damage threshold determination was ± 10% for each part.

The damage results for the two sets of coatings made at different temperatures are given in

figures 2 (T = 150 C) and 3 (T = 250 C). Each open circle indicates the damage threshold measurement for one part. The error bars indicate the precision of the measurement. The four parts for

each overcoat option (none, magnesium fluoride, and silica) and each temperature are grouped together in the figures. A horizontal line through the data points indicates the average threshold

of the four parts of each type. One immediate observation about the damage results presented in

figures 2 and 3 is that the average thresholds of all of the coatings, and especially of those with

magnesium fluoride overcoats, are lower than those of the previously reported study. The reasons

for this are not understood, but may be due to long-term variations in the coating process. This

result emphasizes the importance of making each coating experiment complete by itself, so that it

need not rely on earlier work for its validity and significance.

The major conclusion that can be drawn from the data is that neither silica nor magnesium

fluoride overcoats improve the damage threshold of these 355-nm HR's. The differences between the

average thresholds of the various types of coatings at each temperature are small compared to the

overall scatter in the data. However, one cannot help but notice that at both temperatures the

average thresholds of the overcoated HR's are lower than the average thresholds for the non-overcoated HR's.

The damage threshold data presented in figures 2 and 3 do demonstrate a clear advantage of

coating at the higher temperature. Ignoring overcoat differences, the average threshold of the

12 parts coated at 150 C is 1.85 J/sq. cm with a standard deviation of 0.35 J/sq. cm. The average

of the 12 parts coated at 250 C is 2.79 J/sq. cm with a 0.59 J/sq. cm standard deviation. Thus,

there is a 50% increase in threshold for coatings made at the higher temperature. This is consistent

with results reported previously for zirconia/sil ica HR's [1],

The reasons for the failure of overcoats at 355 nm are understood to an even lesser degree

than the reasons for the success of overcoats at 248 and 1064 nm. There are a number of differences

between the coatings that could be related to an explanation. The thickness of the layers is

proportional to the wavelength. Thus the layers for the 1064 nm coatings are approximately four

times thicker than those for the 248 nm coatings. The thicker layers may have better mechanical

integrity, but also higher net stress than the thinner layers. The shorter laser wavelengths are closer to the absorption edges of the coating materials than the longer wavelength is and this may

enhance the possibility of multiphoton effects. Also, the absorption of the materials is expected

to be higher at the shorter wavelengths. Another, possibly significant, difference in the measurements is the pulse length of the damage test laser. The pulses were on the order of 1 ns for the

1064- and 355-nm lasers (Nd-glass and frequency tripled Nd-glass), but were 20 ns long at 248 nm

(KrF). It may be possible that competing mechanisms involving these factors can explain the

288
Damage Thresholds -for 355nm HR's at 150C

5

X X 5

n

1 -jj 2 §

2

No Overcoat MgF*2 Overcoat SiOg Overcoat

Figure 2. Laser damage thresholds for 355-nm HR's with various overcoat

options coated at a temperature of 150 C. Each circle represents the damage threshold of one sample. The error bar for

each sample represents the range between the lowest fluence

which caused damage and the highest fluence which did not.

The average damage threshold for each overcoat option is

indicated by a horizontal bar.

Damage Thresholds -Tor 355nm HR's at 250C

No Overcoat MgFg Overcoat S102 Overcoat

Figure 3. Laser damage thresholds for 355-nm HR's with various overcoat

options, coated at a temperature of 250 C. Refer to caption

for figure 2.

289
observed results.

4. Conclusion

The current results confirm earlier findings that halfwave, low-index overcoats do not improve

the damage thresholds of HR coatings for use at 355 nm. This is an unexpected result in view of

previous damage tests which demonstrate a significant improvement due to overcoats at 1064 nm and

248 nm.

The authors wish to acknowledge the work of Bill Klapp in preparing the coatings for this study.

8. References

[1] Carniglia, C.K.; Tuttle Hart, T. ; Rainer, F. ; Staggs, M.C. Recent damage results on high

reflector coatings at 355 nm. Nat. Bur. Stand. (U.S.) Spec. Publ . ; 1983 (to be published).

[2] Tuttle Hart, T. ; Lichtenstein, T.L.; Carniglia, C.K.; Rainer, F. Effects of undercoats and

overcoats on damage thresholds of 248 nm coatings. Nat. Bur. Stand. (U.S.) Spec. Publ. 638;

1981. 344-349.

[3] Carniglia, C.K.; Apfel, J.H.; Allen, T.H.; Tuttle, T.A.; Lowdermilk, W.H.; Milam, D.

;

Rainer, F. Recent damage results on sil ica/titania reflectors at 1 micron. Nat. Bur. Stand.

(U.S.) Spec. Publ. 568; 1979. 377-390.

[4] Carniglia, C.K. Oxide coatings for one micrometer laser fusion systems. Thin Solid Films

77(3): 225-238; 1981 March 6.

[5] Rainer, F. ; Lowdermilk, W.H.; Milam, D; Tuttle Hart, T.; Lichtenstein, T.L.; Carniglia, C.K.

Scandium oxide coatings for high-power UV laser applications. Appl . Opt. 21(20): 3685-3688;

1982 October 15.

290
Nanus cript Received

5-15-86

Thermal Properties of Optical Thin Film Materials*

D. L. Decker, L. G. Koshigoe, and E. J. Ashley

Michelson Laboratory, Physics Division

Naval Weapons Center, China Lake, California 93555-6001

The performance of coatings for high power laser application depends not only on the

optical properties of the films, but the thermal properties as well. An examination of the

literature indicates that virtually all work has concerned the optical absorptance or reflectance, and that virtually no thermal property data are available for optical materials as thin films. Since it is known that the optical absorptance is often larger by

orders of magnitude for a material as a thin film compared to bulk, a similar change

(decrease) in thermal diffusivity might be expected. Of additional concern are the interfacial thermal barriers occurring at the boundaries between films. This paper presents

experimental data for thin-film materials including A1 20 3 and Si0 2 deposited in high

vacuum. A simple theoretical analysis is also presented as well as recommendation for

further work.

Key words: heat capacity; optical materials; optical properties; thermal conductivity;

thin films.

Introduction

Much effort has been expended in characterizing and analyzing the optical properties of thin-film

materials. It is obvious, however, that an understanding of the thermal properties is also essential

to a quantitative description of the laser-induced damage process. Even for heat loading levels below

that at which damage occurs, the thermal properties are essential to an engineering analysis of both

window and mirror performance. The thermal drop across the dielectric stack of a typical quarter-wave

enhanced reflector in the infrared is of the same order as the face sheet drop in a state-of-the-art

water-cooled mirror design. Unfortunately, an examination of the literature reveals very little thermal properties data are available for dielectric optical materials as thin films. Direct measurement

would be desirable but is very difficult, with potentially serious systematic errors associated with

some techniques [1-3]. 1 Indirect methods such as photoacoustic spectroscopy can provide thermal data

but are subject to potentially large uncertainty. Work has been reported, however, that indicates

that with proper analysis and calibration, quantitative results can be obtained by that method [4].

For theoretical reasons to be discussed, it is expected that the disorder typically found in vacuumdeposited thin films will produce a decrease in thermal conductivity of an order of magnitude or more below bulk values. Results presented in this paper corroborate this analysis. Similar results have

been reported for a wide variety of thin films deposited for nonoptical application [5,6]. The size

effects reported are caused by film thickness, grain size, and crystalline perfection limitations.

Results presented in this paper are intended to provide (1) validation of the methods employed and (2) a limited survey of useful optical thin-film materials. Thermal conductivity is shown to be sensitive

to deposition technique, so that a full catalogue of thin-film thermal properties data would require a substantial commitment of resources.

Theory

The heat capacity of dielectric materials is a consequence of energy stored in lattice vibrational excitation (phonons). The classical high-temperature molar heat capacity is 3Nk, where k is

Boltzman's constant and N is Avogadro's number [7]. This value is closely approached by most

materials even at room temperature. The heat capacity at high temperature (i.e., room temperature) is

sensitive primarily to short-range order. Hence specific heat C

p/p (heat capacity per unit mass) is

not sensitive to density p or to large-scale imperfection. A measurement of specific heat of thinfilm amorphous germanium was performed in support of this paper, which corroborates these

Work supported by Defense Advanced Research Projects Agency and Navy Independent Research Funds.

Numbers in brackets indicate the literature references at the end of the paper.

291
assertions. The density of thin films is usually lower than corresponding bulk material usually by no more than a few percent. Hence, purely density-related effects are not large [8]. Details of these

measurements are reported in the experimental section.

The rmal conductivity K, on the other hand, is very sensitive to local order, depending directly

on the phonon mean free path 1:

K - jC

p vi , (1)

where v is the average phonon velocity. This simple expression is obtained from elementary kinetic

theory [7]. The phonon mean free path is limited by scattering from other phonons and from lattice

boundaries and imperfections. For an evaporated film, order may exist on a scale no larger than a few

lattice units and, hence, may directly limit the conductivity.* As a consequence of the columnar

growth morphology, which is evident in many vapor-deposited thin films, it might be anticipated that

substantial anisotropy in thermal conductivity might be observed [8]. For such a film, the

conductivity would presumably be higher transverse to the film where longer range order would exist in

the branching structure of a column. Table 1 gives values of £ computed for various bulk materials

and assumes an average phonon velocity of 5 x 10 5 cm/sec. Very large differences in thin film and

bulk thermal conductivity are to be expected.

Table 1. Values of Cp and K taken from the literature for various bulk crystalline pure materials at room temperature, together with a value of I computed

from equation (1) in the text. It is assumed that

values of K for noncubic crystals are for conductivity

parallel to the c axis.

C

p

, K, I,

Material j/cm3 -°C w/cm-°C A

Si0 2 2.00 0.12 36

A1 2 0 3 3.18 0.25 47

ZnS 1.99 0.26 78

Si 1.81 0.84 278

Other limitations on thermal conductivity of multilayer coatings may arise from interlayer thermal resistance of the sort that is observed in mechanically clamped joints even when the fit is

essentially an "optical contact" [9,10]. In some coating designs, intimate chemical bonding almost

certainly exists, e.g., in a layer pair like Si/SiO. In other cases, the bond is presumably much

weaker, and the layers are connected by Van der Waal-type forces, e.g., ZnS/SiO. An additional

complication arises in the case of thin films which are transparent in the infrared. At high

temperatures, additional heat is transported by radiation. In fact, as has been shown, for very thin,

low absorption films, direct radiation transport dominates at temperatures above ~1100K [11]. This

is of no direct impact here, as this paper concerns thermal tranport at relatively low temperatures.

Experiment

In this section, sample preparation and experimental measurement techniques will be reported for

thin-film specific heat and thermal conductivity. For specific heat measurement, a straightforward

technique using microdif f erential calorimetry was employed using samples as small as 20 mg. For verification of the prediction made in the Theory section, germanium was chosen because the bulk thermal

properties have been so well documented. To obtain a suitable sample, 99.999% pure amorphous germanium was sputter-deposited using a commercial rf diode system on a polished sodium chloride substrate

[12]. Upon cooling, the germanium film would show negligible adhesion to the substrate and could be

easily removed mechanically. A 1-ym-thick film of approximately 40-cm2 coating area was required to

obtain the necessary 20 mg. A straightforward but very carefully executed measurement with a commercial differential scanning calorimeter [13] yielded an average value 0.082 cal/gm-°C. Over a temperature range of 50 to 150°C, no variation in specific heat was observed for the thin-film

sample. A 100-mg mass sample of 99.999% pure bulk amorphous germanium was also prepared and examined

* On such a scale, the very concept of lattice waves may be called to question, but this simple model

can serve the purposes of this paper.

292
in the differential calorimeter. The measured value of specific heat was temperature dependent, with

a value at 50°C of 0.0775 cal/gm rising linearly to 0.0825 at 225°C. The small discrepancy between

bulk and thin-film values was thus as expected [14].

To obtain insight into thin-film thermal transport, it was decided that for a first attempt, a straightforward measurement of thermal conductivity would be appropriate. Figure 1 schematically

shows the apparatus used. The thin-film sample is suitably vapor deposited on a 0.020-in.- (0.50-mm)

thick, 0.75-in.- (19-mm) diameter, single-crystal, polished sapphire disk. This disk in turn is

thermally contacted to source and sink blocks, which have an interface diameter of 0.444 in. (area A = 1.00 cm

2 ). The heater in the source block can dissipate a maximum of 100 W, so the maximum heat flux

through the sample can approach 100 W/cm 2

. Because the source and sink areas are identical and the

thickness of the substrate is small by comparison to their diameter, negligible radial flow occurs in

the substrate and the heat flow in the thin film or in the substrate is accurately one dimensional.

The source and sink surfaces are finished by lapping. Thermal connection to the sample is typically

enhanced with a thermal grease containing a high concentration of A1 20

3 particles. The entire

assembly was then clamped with a force of about 700 gm.

Type-K thermocouples ( chromel/alumel) were used for temperature measurement, since they have

high sensitivity but low thermal conductivity. Fine-wire thermocouples (0.003-in. diameter), TC j and

TC^, were placed in 0.006-in. -diameter holes drilled just under the interface surface of both source

and sink blocks. The configuration just described permits the simple calculation of thermal conductivity:

where the heater power Pnis measured electrically by measuring the voltage drop across the heater

and across a known series resistor to obtain the current flow (see fig. 2). The thermal gradient

dT/dx can be directly obtained from the temperature drop TCj-TC^ and the known sample thickness. If

the sample were homogeneous and if negligible temperature drop existed across the thermal grease con- nections to the sample, a satisfactory measurement of K would be obtained. However, unless great care

in construction and assembly is used, larger thermal drops occur across the grease interfaces than

across the sample. Also, the thermal drop across the substrate is typically significantly larger than

the film drop. To eliminate these error sources, a modification to the procedure described above was employed in which thin-film thermocouples were directly deposited on the sapphire substrate and on top

of the dielectric thin films. The dielectric thin film provided electrical isolation of the thin-film

thermocouples (TC 2 and TC3), and an additional 0 . 020-in. -thick sapphire disk provided electrical

isolation for TC 2 from the source block. TC 2-TC 3 now provides a direct measurement of the thermal

drop across the thin film and is thus independent of the substrate or thermal grease drop. The

thin-film thermocouples were deposited by direct e-gun evaporation of heavy (10 ga) thermocouple wire

segments. Sequential deposition of the chromel and alumel elements was made through suitable masks.

The actual thermocouple was formed in the center of the sample by crossing the 1-mm-wide x 1000-A-thick deposited elements. Electrical connection was made to the external switches and

microvoltmeter by soft soldering 0.003-in. chromel or alumel wire to thin-film pads deposited for the

purpose on the substrate. This scheme is similar to a thin-film thermocouple device reported by

Goldsmid and Paul [15]. Figure 3 shows the calibration of a thin-film thermocouple so constructed

with the temperature determined by a calibrated mercury-in-glass thermometer. Interestingly, the

measured sensitivity differs from published bulk-wire, type-K thermocouple results by less than 3% over the 20 to 160°C range [16].

Table 2 gives a summary of results for measurements made on a nominal 1-pm-thick Si0 2 e-gun evaporated film [17]. The physical thickness was measured by a Talystep stylus instrument [18]. All

K = (P h/A)(dT/dx) -1

(2)

Table 2. Measured values of source heater power and thermocouple

potentials for an e-gun evaporated 1 .05-ym-thick thin film. Data

are given as an example.

P h

, TCj, TC 2 , TC 3 , TC^, (TC 2-TC 3 )/Ph

,

19.9

39.8

62.1

970

1770

2660

660

1050

1520

620

980

1350

390

580

800

2.30

2.41

2.35

293

L
thermocouple measurements are referred to junctions held at ambient temperature. The thermocouple

potential A = TC 2 -TC 3 is caused by the thin-film temperature drop. The thermocouple electromotive

force per watt A/P^ is given in the last column. The average value of A/P^ can be obtained,

2.35 uV/W. The thermal conductivity is simply

K = S(t/A)(A/P

h

)" 1

. (3)

The appropriate sensitivity S, for type-K thermocouples, is 39 uV/°C; substituting the appropriate

thickness t, area A, and value of A/P^, one obtains a value K = 1.74 x 10-3 W/cm-°C. The thermocouple difference TCj-TC^ is produced by the total temperature drop across 0.50 mm of sapphire and two

thermal grease joints. TC^-TC^ is produced similarly, except that only one grease interface is

involved. These values are consistent with a sapphire bulk conductivity (parallel to the c-axis) of

0.25-W/cm-K average grease interfacial drop of 0.1°C/W.

For direct comparison, table 3 summarizes thin-film measurements made on this and other samples

and appropriate bulk values. Note that significantly higher conductivity was measured on an rf diode

Table 3. Summary of thin-film thermal conductivity values computed

from experimental data such as is summarized in table 2. Bulk handbook values are also given for reference.

Material Deposition method Thickness, pm K, W/cm-°C

Si0 2 e-gun evaporated 1.05 1.7 X 10-3

sputtered 0.50 2.8 X 10" 3 bulk (amorphous) 1.4 X 10- 2

A1 2 0

3 sputtered 1.00 3.6 X 10- 4 sputtered 1.00 2.5 X 10-3

bulk (polycrystalline) 2.0 X 10- 1

High visible absorption

reactively sputtered Si0 2 film [19] than on the e-gun evaporated film just discussed in detail. This

value is, however, a half-order of magnitude lower than reported in the literature for fused silica,

which was chosen to have a bulk structure most like the disordered structure of the thin film [20].

Similarly, data were obtained for a 1-ym-thick film of reactively ion-beam-sputtered A1 20 3 [21]. A

surprisingly low conductivity was measured, a half -order of magnitude lower even than the thin-film

Si0 2 result. However, this film showed considerable absorption in the visible caused by contamination

or lack of stoichiometry (a ~ 10 3 cm

-1

). Accordingly, a second sample was prepared in a nominally

identical manner and measured. The value obtained, 2.5 x 10

-3 W/cm-°C, is also shown in table 3.

This film displayed very low visible absorption and was in all respects a high-quality film. This

value is essentially identical to that obtained on sputtered Si0 2 but is a factor of 80 lower than

reported on sintered bulk (crystalline) A1 20 3

. No bulk A1 20 3 material with greater disorder was found

for comparison [22]. The estimated uncertainty in these measured results is ±5% based upon the

reproducibility of the necessary thermocouple potentials and from an estimate of heat lost unavoidably

from the source block down thermocouple and heater leads and otherwise not strictly accounted for.*

Table 4 is a summary of known potential sources of systematic error. Of the potential errors listed,

all have been carefully addressed in the design of the experiment and discussed in some detail in this

paper. The last listed is the most difficult to address. A simple experimental approach to directly

assessing the effect would consist of measurement on nominally identical films of differing

thickness. A solution to the resulting linear equations would yield values of thin-film and

interfacial resistances. However, resources precluded such measurements for this paper. It can be

seen, however, in comparing results for the two Si0 2 films reported, that the effect (if it exists)

must be small. If it were present in the measurements reported but not separately accounted for, the

effective thin-film conductivity value would be lower for the thinner film. In fact, a larger value

is inferred, presumably the result of real material property differences.

The entire apparatus was effectively insulated with glass fiber wool, and both thermocouple and

heater leads were 0.003-in. diameter. To directly verify that the losses were small, a simple calorimeter experiment was performed in which a fixed volume of water was continuously recirculated

through the sink block and the rate of rise in temperature measured and compared with the known

electrical input power. No heat loss was detectable at the 0.5-W level for an input power of 40 W.

294
Table 4. List of known sources of systematic error

that apply to the thermal conductivity measurements

reported in this paper.

1. Thermocouple calibration errors

2. Error in measurement of P^

3. Unknown heat loss or gain to source

4. Sample thickness error

5. Unknown thermal resistance between thin film

thermocouples and thin dielectric film

Discussion

For some time, it has been suggested that defects leading to localized laser damage can be thermal in origin as well as optical [23,24]. The thermal properties measured by the techniques described

in this paper yield values averaged over a significant area (1 cm

2 ). To obtain thermal conductivity

values as low as those reported here has very important implications, especially upon realizing that

at some defect sites even lower effective conductivity may prevail. It is instructive to consider a simple example. A typical multilayer enhanced reflector in the mid-infrared has a total coating

thickness of perhaps 4 pm. If we assume an overall effective conductivity of 2 x 10-3 W/cm-°C and at

a nominal steady-state heat flux of 100 W/cm 2

, a total temperature drop of about 20°C would occur if

all of the heat were to flow through the entire stack. The obvious significance is that, for severe

thermal loading in high-power CW applications, the coating temperature rise is much larger than

estimated from bulk thermal conductivity values and, in fact, can easily approach damaging levels on average . For the analysis of transient thermal effects, similar conclusions can be reached since the

diffusivity, which is the appropriate thermal variable in that case, is just the ratio K/pCp. Since

thin-film densities are typically very nearly equal to bulk values, large reductions in thin-f ilm-vsbulk-dif f usivity values are to be expected. It is tempting to conclude that proportionately higher

peak temperatures during pulsed irradiation will result. However, caution must be used in attempting

to apply these results to the analysis of pulsed laser-induced damage. In this case, very high peak

temperatures may occur, with additional heat transport by radiation [11]. This case is further

complicated by the necessity to take into account the temperature dependence of the transport and

optical properties. It is likely that only numerical solutions to the appropriate heat flow equation

will be possible.

Summary and Conclusions

The experimental results and predictions of the simple theory presented here are in qualitative

agreement. It is to be expected that the specific heat of a material is largely independent of

structural perfection and density. Measurements of thin-film and bulk Ge corroborate this assertion:

The thin-film and bulk specific heat values are in agreement. However, the thermal conductivity of

thin films has been argued to be significantly lower than bulk values as a result of thin-film

disorder. Measured values of Si0 2 thin-film conductivity are lower than bulk fused silica by a factor

of 5 to 8. In comparing a high-quality amorphous A1 20 3 film with bulk crystalline material, a reduction of ~80 was observed. It has been predicted and corroborated to a limited extent with Si0 2 films produced by thermal and sputter deposition that the thermal conductivity will depend strongly on deposition technique. A major area yet to be explored concerns the effect of interfacial

discontinuities on thermal resistance and the closely related problem of interlayer bonding and

adhesion. The data presented here are clearly to be regarded as tentative, and additional

corroborative data for the same and different materials must be obtained. However, these results and

predictions for dielectric thin-film materials are in agreement with a rather massive body of similar

data in the literature primarily relating to thin metallic and semiconducting films. Although

disturbing, the results disclosed in this paper are not surprising.

References

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[2] Kelemen, F. Thin Solid Films 36; 199-203; 1976.

[3] Nath, P.; Chopra, K. L. Thin Solid Films 18; 29-37; 1973.

[4] Swimm, R. T. Appl. Phys. Lett. 42; 11; 955-957; 1983.

[5] Nath, P.; Chopra, K. L. Thin Solid Films 20; 53-62; 1974.

[6] Boikov, Yu. A.; Gol'tsman, B. M. ; Kutasov, V. A. Sov. Phys. Solid State 20; 894-895; 1978.

[7] Kittel, C. Solid state physics , 5th ed. New York: Wiley; 1976.

[8] Macleod, V. A. In Proceedings of the SPIE Int. Soc. Opt. Eng. 325 ; 21-28; 1983.

[9] Clemens, P. G. Thermal conductivity , Chapter 1, Vol. 1. R. P. Tye, ed: New York: Academic

Press; 1969. 51-58.

295
Fried, E. Thermal conductivity , Chapter 5, Vol. 2. R. P. Tye, ed: New York: Academic Press;

1969.

Howlett, S. P.; Taylor, R. Thermal conductivity , Session TC-11B. J. G. Hust, ed: New York:

Plenum; 1982. 447-457.

The r.f. diode sputtering system used was manufactured by the Perkin-Elmer Corporation, Randex

Model 2400.

The differential scanning calorimeter, Model DSC-2, manufactured by the Perkin-Elmer Corporation

was employed.

Chen, H. S.; Turnbull, D. Harvard University Division of Engineering and Applied Physics

Technical Report No. L2, 1969.

Goldsraid, H. J.; Paul, G. L. Thin Solid Films 103; L47-L48; 1983.

Manual on the Use of Thermocouples in Temperature Measurement. Philadelphia, PA: American

Society for Testing and Materials, 1974. ASTM STP470A.

The Si0 2 films were deposited using a Varian e-gun at a deposition rate of about 10 A/sec at a background pressure of 5 x 10 - ^ Torr in a diffusion pumped vacuum system.

Bennett, J. M. ; Dancy, J. H. J. Appl. Opt. 20; 1785; 1981.

Reactive sputter deposition was performed in the Randex system (reference 12) at an 02 partial

pressure of 8 x 10-1+ Torr.

Wray, W. L. ; Connolly, T. J. J. Appl. Phys. 30; 1702; 1959.

The ion beam system used was custom made at Naval Weapons Center but employed an Ion Tech 2.5-cm

ion gun source. Kingery, W. C. J. Am. Ceram. Soc. 44; 302; 1961.

Decker, D. L. ; Franck, J. B. ; Faith, W. N. ; Porteus, J. 0. In Proceedings of the 14th annual

symposium on optical materials for high power lasers , Bennett, H. E.; Guenther, A. H.; Milam, D.

;

Newnam, B. E. , ed. 1982 November 15-17; Boulder CO. Nat. Bur. Stand. (U.S.) Spec. Publ. 669;

1984 January. Pp. 178-185.

[24] Walker, T. W. ; Guenther, A. H. ; Nielsen, P. IEEE J. Quantum Electron QE-17; 2053-2065; 1981.

THIN FILM (1 cm2

) SAMPLE

SUBSTRATE

(AC2 03

)

TC2/TC 3 THIN FILM THERMOCOUPLES

TCd

WATER

Figure 1. Mechanical schematic of thermal source,

sample, and thermal sink together with thermocouple placement. Additional construction and

operation details are furnished in the text.

296
<?"«<?

WW

25.0 a

HV 2512 Si VH

HEATER

PH= VM

V

H

/(25.0) (watts)

Figure 2. Schematic diagram of electrical measurement of heater power. High

voltage heater minimizes lead size and

hence heat loss.

5.0 -

BULK

THIN FILM

20 40 60 80

T, °C

100 120 140 160

Figure 3. Intercomparison of thin-film and bulkwire type-K thermocouple calibration over the

temperature range from 20 to 160°C.

297
Manus oript Received

5-17-85 SAR

Interface and Bulk Absorption of Oxide Layers and Correlation to Damage Threshold

at 1 . 06 um

D. Ristau, X.C. Dang* and J. Ebert

Institut fur Quantenoptik

Universitat Hannover

Federal Republic of Germany

We describe an infrared line scanning technique for measuring the temporal and spatial

development of a temperature field which is generated by a high power cw Nd-laser on the surface of a coated sample. A theoretical model based on the solution of the differential equation of heat transport for an instantaneos point source is discussed.

In this theory the optical and thermal properties of a laser heated single layer

are related to its temperature field. Investigations on a temperature calibration

technique are made in order to rule out the effect of different emissivities of the

coating materials under test. In the present study interface and bulk absorption of

single layers and multilayer stacks are measured and related to the laser damage

threshold attained from 15 ns pulses at 1.064 urn. The layers are produced by e-beam

evaporation and by ion-beam assisted deposition. Results will be given on Ti0 2

, Ta

2 0 , Hf02 , Al 2 03 and Si02

.

Key words: Bulk absorption measurement; infrared temperature field measurement; interface

absorption; Nd-YAG-laser; oxide layers ; Pb-Se detector

1 . Introduction

Oxides are still of current interest for the production of high power coatings in the range

of .2 to 3.5 microns [1]. As those materials improved interface and surface absorption have

become increasingly the dominant parts of the total absorption in a multilayer-stack. Since

there is a strong correlation between the absorption and the damage thresholds of oxide coatings

interface and surface absorption have to be regarded as an important cause for laser induced

damage [2].

We apply a pyrometric temperature recording technique in order to determine the absorption

in a thin film coating. Temperature profiles of laser-heated ln2 03 -layers were measured and

compared to a theory describing the temporal and spatial behavior of the generated temperature

field. The interface absorption of oxide-materials in conjunction with Si02 was measured

indirectly. Damage tests were carried out with a pulsed Nd-YAG laser.

* X.C. Dang is a Humboldt fellow from the Institute of Physics,

Academie of Sciences of Vietnam, Hanoi- S.R. Vietnam

298
2 Absorption measurement

Our absorption measurement facility consists of a high power cw laser and an infrared line

scanning device. If the laser beam is focussed on the coated sample a temperature field is built

up in the layers due to absorption. Thus it is possible to determine the total absorption by

recording this temperature profile with the use of an infrared line scanning device.

2.1 Generation of the temperature field

In order to determine thin film absorption by temperature measurement one has to know how

the absorption is related to the spatial und temporal development of the laser generated

temperature profile. The actual problem is illustrated in figure 1. The laser beam with a gaussian intensity distribution hits an absorbing layer coated on a substrate with thermal

properties distinct from thin film properties. Having a small spotsize 2s compared to the

dimensions of the substrate and the thin film area the substrate can be assumed as infinite. As

long as tire thin film absorption is low the temperature dependence of the absorption coefficient

and the other parameters, like the thermal conductivity K and the heat capacity c, can be

neglected. The differential equation of heat transport for our. problem

pct(r,t)=KAT(r,t)+A(r,t) (1)

contains the additional source term A(r,t) which describes the heat generation in the layer due

to absorption. It has a gaussian radial distribution and it is uniform inside the layer along

the z-direction. Instead of integrating eq (1) [3,4,5] we start with the solution of the

transport equation for an instantaneos point source located in an infinite medium with thermal

diffusivity [6]

T °( r - t)= 8Wk% exp(rV4k

m t) (2)

This equation describes how the heat quantity Q which is produced at time t=0 propagates into

the medium for t>0. A set of these point sources, radiating with different intensities and at

locations distributed like the incoming heat flux, can be superposed for solving the heat

transport equation (1) under consideration. Since the layer's surface is in contact to the

surrounding air and there is a film substrate interface two optional boundaries have to be

considered. Like for the corresponding optical model a reflection coefficient for thermal waves

can be attached to every interface. The coefficients are functions of the specific weights P , the heat capacities c and the thermal diffusivities k of the two materials as long as the

thermal contact is ideal. They are derived from the condition that the temperature field must be

contirrous at the boundary. (Subscript 1 depicts the coefficients of the air, subscript s depicts

the substrate's coefficients and the layer's coefficients have no subscript)

k

1 p1 c

1 /k--k P c^ 1 (3)

1 ~k j P j c j/K+kpcVk

j

kpcVk

s -k

s ps cs A

K

* = k P c7T<

s + ks ps cs

7Tc

299
R is the thermal reflection coefficient for the film air interface and R describes the film 1 '2 substrate interface.

With all the assumptions made above the final expression for the temperature field in the

layer can be found by adding the effects of all reflected point source fields in the beam area along the z-diretion.

T<r,,.tW<U l«P(TVP(t))

0 "pTtT

n 0

i=0

(5)

{ Kzo ,zj-d) + l(z; ,z;-d)+R1 Hz+.z^-dJ+R, I (

z

x" ,zf -d) J-I (z ,z+d

)

z£=z+2id z; = z-2id zf=z+(2i+1 )d zf=z- (2i+1 )d (6)

I(a,b)= / dv exp(-v*)

b/2vTTE

(7)

p(t)=4kt+2s (8)

, 2aI 0 S

2

rrpc (9)

Where I

0 is the incoming intensity and d is the film thickness, a is the absorption coefficient

of the layer, R t and R 2 are the thermal reflection coefficients and s is the beam radius as stated before. The sum regarding to the reflected parts of the heat flux is extended to a value

n

0 representing a number of reflections for which the reflected heat fluxes become negligable. n

£ is directly correlated to the exposure time.

n 0 =2/107d (10)

Cw-operation is allowed for by a time integration over the exposure time t. The radial

integration is already carried out, but the z as well as the time integration must be performed

numerically. A seperation of bulk and interface absorption can be made by inserting a z-dependend absorption factor a(z)> but for the real situation the thickness of the layer is

small compared to the thermal diffusion length leading to a non detectable difference in the

temperature profiles. For sligthly different thermal properties of substrate and coating

materials the temperature rise on the surface in the beam centre can be described by a simple

atn-function.

T(0,0,t)=L-atn(V2kT/s) (11)

. vT alpSd

L "

i P ck (2 + R, -tR'2 ) (12)

300
If the beam size is known the thermal diffusivity of the layer can be determined by this

dependence. If there is more than one layer the expression becomes more complex because more

interfaces are to be considered, so that we restricted ourselves to single layer calculations.

But having long exposure times the multilayer structure can be considered as a single layer with

an average absorption because diffusion length is big compared to the scale of the film

structure.

2.2 Experimental setup

The temperature profile recording set up is shown in figure 2. The pump beam is produced by

a 200 Watts cw Nd-YAG-laser consisting of two arc lamp pumped 1/4 3° Nd-YAG rods. The resonator

is fit to the thermal lenses of the rods so that the output beam profile is nearly gaussian. The

deviation of the diode array recorded spatial profile in the focus of the lens from the ideal

profile is less than 5% for the entire range. A gold coated curved mirror images the heated

sample on a lead selenide detector. Scanning is performed by a plane mirror which is driven with

the aid of a galvanometer scanner. The window in front of the detector is an antiref lection

coated germanium plate with high transmittance in the range of 2.2 to 6.5 microns. Thus the low

wavelength sensitivity limit for temperature radiation is 2.2 microns ruling out the effect of

scattered 1.064 microV) pump radiation. The upper limit of 6.5 microns is given by the detector

characteristic. The cooling device of the PbSe-detector is not shown. It consists of a liquid

nitrogen dewar and a temperature regulation system with a peltier element.

The detector signal is amplified by a low noise preamplifier and then fed via an analog to

digital converter into a CBM 3032 minicomputer. The computer is synchronized to the sine wave

motion of the scanner with the aid of the nonmaskable interrupt input of the 6502 microprocessor, so that the temperature profile can be displayed on the screen and stored away on a floppy disc for calculating the thermal diffusivity and the absorption.

Table 1 Scanning, system data

PbSe detector cooled with liquid nitrogen, temperature regulated

wavelength-response: 1 - 6.5 urn,, with window: 2.2 - 6.5 urn Scanning range line of 25 mm length

Scanning rate 60 lines per second

Spatial resolution 256 points/line according to .1 mm

Temperature resolution temperature 20 - 70 °C smaller than 1 °C

temperature higher than 70 °C smaller than .5 °C

2.3 Calibration

A severe problem related to the temperature measurement by radiation detection lies in the

different emittances of the coating anrd substrate materials. Since emittance is a function of

wavelength and temperature, and the surrounding atmosphere affects the measurement also the

301
apparatus has to be calibrated to every sample separately. Calibration is performed in two

steps: First a blackened substrate with defined temperature is measured. This gives a calibration curve of the detector voltage vs. "blackened substrate"-temperature if the used

paint is selected so that the detector voltage is the same for all other substrates and coatings

with equal temperature. For the second step the sample under test is one half black painted and

again heated up to a defined temperature . Now the temperature record shows a step according to

the difference of emittance of the coated and the blackened part of the sample. Since the

calibration curve exists for the blackened part the detector is now calibrated to the specific

sample under test.

In figure 3 calibration curves for BK 7 glass, some coating materials and the painted

substrate are shown. They are nearly linear in the depicted temperature range, but for

temperatures above 50 °C detector voltages are increasing faster with temperature due to the

nonlinear wavelength response of the detector and the increase of temperature radiation. . The

detector noise voltage is also indicated, it limits the smallest detectable black body

temperature to 13 °C.

2.4 Measurement of moderately absorbing films

Temperature profile recordings of BK 7 samples coated with In4 0, have been performed and

compared to the theoretically predicted behavior. Ih

2 03 was chooseh because it has a considerable absorption at 1.064 micron, so that clear temperature profiles are produced in the

films. The computer calculated data are in good agreement with the temperature distribution

measured (fig. 4). This can only be achieved by assuming the thermal conductivity of the ln2 03 -films being orders of magnitude smaller than that of the bulk. This effect of reduced thin film

thermal conductivity for oxide materials is also reported by D.L. DECKER et al. in this

publication. In this paper thin film thermal data were directly measured with the aid of

thermocouples giving very low conductivies for SiO

z and A1

2 03 layers. In figure 5 the

temperature rise in the beam centre is shown for the aforementioned sample. The curve for a film

of the same thickness produced with ion beam assistance is also depicted. The temperature rise

is in accordance with the predicted atn-function for either sample, although the thermal

conductivities of the substrates and the films are totally different. The superior properties of

the ion beam processed sample can be clearly seen: The e-beam deposited film has the higher

absorption and the lower thermal conductivity in both samples.

2.5 Measurement of slightly absorbing films

The absorption of e-beam deposited high power oxide coatings is too small for recording a temperature profile with the line scanning method. Even with films thicker than 15 quarterwaves

and a laser power of 200 Watts the maximum temperature in the beam centre amounts to less than

1 °C. Therefore the apparatus was changed to an absorption measurement facility.

The optical path of the new setup is the same as in the linescanning device, but the output

signal is now ac-coupled to a lock in amplifier. Since the temperature rise in the coating is

small, the second harmonic of the detector voltage is a direct measure for the heat quantity

deposited in the film by the pump laser. Thus the reading of the lock in amplifier can be

interpreted as the thin film absorption if the reference signal has the doubled frequency of the

scanning sine wave voltage. The sensitivity of this measurement is limited by the scattered

laser light as in the laser calorimeter, but since the sensitive element is located at a long

distance from the irradiated area sensitivity is much higher. By comparing the device with a laser calorimeter the sensitivity was found to be 2 ppm. The output voltage of the lock in

amplifier is only nonzero if there is a temperature gradient 'on the sample or on the imaging

302
components. Thus a change of room temperature does not affect the measurement. Scattered light

might be a source for errors because it leads to an inhomogenous heating of the imaging mirror.

Depending on the substrate material also temperature gradients in the back area can be detected

and influence the measurement. For shielding the backgroud we used a plate cooled down to a temperature well below the detection limit of the device. A hole is provided for the beam

outlet.

Suggestions for improving the sensitivity may be the use of a mercury-cadmium- tel luride

detector with a doping ratio aligned to a wavelength range of 8 to 12 microns. Although those

detectors have a lower detectivity they are superior to lead selenide detectors for temperature

recording due to the fact, that the temperature radiation maximum for room temperature falls

into their wavelength range. A resolution of .1 °C is common for infrared cameras using this

detector type. Sensitivity might be also increased with an optional in phase Q-switching

technique of the pumplaser. A big step towards better performance. can be achieved by keeping the

sample under vacuum because convection losses are avoided leading to higher temperature and a better accuracy [7]. With all these improvements the sensitivity for this absorption measurement

technique may be far below 1 ppm.

3. Samples

All layers are deposited in a high vacuum evaporation plant using electron beam evaporation

in a reactive oxygen atmosphere. The film thickness is monitored optically while the deposition

rate is controlled by a quartz crystal monitor.

In order to distinguish bulk and interface absorption two stacks for each material were

produced (fig. 6). One system consists of a four halfwaves block of high index material and a four halfwaves block of Si02

. The second stack with alternating halfwave layers was produced by

keeping all cleaning and evaporation parameters constant. Compared to the block sample it has

six additional interfaces but the same optical thickness and the same distribution of the

electric field strength for both materials. Assuming equal surface and bulk absorption for

corresponding samples it is now possible to calculate the interface absorption. Especially for

Ti0

2 other samples with gradual interfaces were produced by controlling the shutter position

during' simultaneous operation of both electron beam sources. In order to determine bulk

absorption for each material a single layer of eight halfwaves was used.

Table2 Evaporation process data

Substrate Suprasil I, 25mm diameter, 1mm thick

Substrate cleaning lens tissue rub with isopropanol, draw off with solidified collodium

Reactive evaporation

-4

oxygen into electron beam, partial pressure: 2-4-10 mbar

substrate temperature 200 - 350 °C, evaporation rate: 5-20 A/s

Ion-assisted evaporation Argon-ion arc discharge gun, substrates exposed to high current ion

flux during e-beam deposition

Electron gun 8 kV, rotating targets 60 mm diameter

4. Results

For high index materials, we tested Al

2 0

? ,Hf02 ,Ta2 05 and Ti02 in conjunction with Si02 because of the widespread use of these materials for high power coatings (fig. 9). In all cases

303
total absorption of the stacks is higher than that of the blocks. Hf02 has the smallest

absorption among those four materials followed by Ta2 05 and A1

2 03

. The absorption of the Ti0

2 films was found to be the highest. With the exception of Hf0 2 the contribution of the interface

absorption to the total absorption is small. But testing the single film for each material we found a very considerable increase of absorption when passing from the single films to the stack

designs with Si02

. Hf

0

2

, known as a good high power material with small bulk absorpion, has in

conjuntion with Si0

2 an interface absorption that dominates the other absorption sources. In order to rule out the effect of contamination at the interfaces we deposited an optional Hf0

2 stack consisting only of five halfwave layers , whereby the evaporation source was shut down for

five minutes after the completion of every halfwave layer before depositing the next one. The

absorption of this sample was only 20% higher than that of the bulk sample. The Ti0

2 /Si0

2 stack

with coevaporated boundaries has a total absorption value which is higher than that of the block

sample but lower than that of the sample with abrupt' index transitions. In order to compare our

results for the blocks to the single film absorption we produced for every material optional

single layers with eight quarterwaves of optical thickness. The absorption data of these samples

are illustrated in the backplane of figure 8 which also shows the absorption per interface for

the corresponding stacks in conjunction with Si02

. The extinction coefficients (losses due to

scattering are not included) in table 3 are also calculated from these single film samples.

Table 3 Absorption of oxide materials in conjunction with Si0 2

8 QWOT at 1.064 urn central wavelength

material total absorption absorption per interface extinctior coefficient of bulk

Hf0 2

Ta 2 05

A1 2 0 3 Ti0 2 Si0 2

194 ppm

274 ppm

316 ppm

812 ppm

32 ppm

5 ppm

5 ppm

42 ppm

2.5 E-6

2.0 E-5

1.9 E-5

5.0 E-5

9.0 E-7

4.2 Damage thresholds

Measurements are made with a single stage passivily Q-switched Nd-YAG-laser (fig. 7). TEM

00 operation is guarenteed by the use of a convex-concave resonator in conjunction with two

apertures and is controlled with a diode array. The spot size for damage tests is adjusted to

approximately 90 urn with the aid of a convex lens with a focal length of 160 mm. Q-switching is

achieved by two sheets of KODAK-dye No. 14015 in eel loluse-acetate as a saturable absorber

giving ripple-free pulses of 15 ns duration with energies up to 90 mJ. For every shot the energy

is measured by a diode (BPX 65) and an integrating amplifier, while the temporal profile is

monitored with a fast photodiode and a transient digitizer (Tektronix model R 7912). Damage is

detected by recording the scattered light of a HeNe-laser and by an optional plasma-detector.

After the tests every sample is visually inspected with a Nomarski-microscope. All damage data

in figure 10 refer to one on one experiments.

We tested the same set of specimens as for the absorption measurements (table 4). With the

exception of the HfO samples and the coevaporated TiO layers there is a strong correlation 2 2

304
between damage thresholds and absorption data. Materials with high absorption have a low damage

resistivity, and apart from TiO

a the stacks are damaged always with less energy than the blocks.

The experimental findings are the best for Hf02, but they indicate a discrepancy because the

damage thresholds for the stacks are, in contradiction to the absorption data, only slightly

smaller than those for the blocks. Looking at the damage morphology there is evidence of

inclusion dominated breakdown of these samples leading to thresholds not according to the

interface absorption of Hf

2

. Damage thresholds of the Ta2 05 and A1 2 03 films are nearly equal

like the corresponding absorption data. Most damage sites of the blocks reveal inclusion and

defect dominated damage while there is a totally different breakdown mechanism for the stacks.

Therefore interface absorption has to be regarded as one of the most important parameters for

increasing the damage thresholds of Ta2 05 and A1 2 03 layer designs. Ti02 blocks have the lowest

damage thresholds among all the tested samples. This may occur due to the high stress formation

in thick Ti0 2 layers. An interesting result is, that the damage thresholds of the Ti02 stacks

can be doubled by coevaporation. By switching from abrupt to graduel interfaces there is a drastic change in breakdown mechanism. The damage of coevaporated stacks is inclusion

dominated, but the stacks with abrupt interfaces are mainly destroyed due to melting of the

deposited material.

Table 4 Damage thresholds of oxide materials in conjunction with Si0 2 pulselength: 15 ns; spotsize: 90 um; wavelength: 1 .064 urn

materials design threshold (J/cm 2

)

Hf0 2 /Si0 2 HHHHHHHHLLLLLLLL block 192

Hf0 2 /Si0 2 HHLLHHLLHHLLHHLL stack 166

Al 2 0 3 /Si0 2 HHHHHHHHLLLLLLLL block 119

Al 2 0 3 /Si0 2 HHLLHHLLHHLLHHLL stack 78

Ta 2 05 /Si0 2 HHHHHHHHLLLLLLLL block 114

Ta 2 05 /Si0 2 HHLLHHLLHHLLHHLL stack 72

Ti0 2 /Si0 2 HHHHHHHHLLLLLLLL block 44

Ti0 2 /Si0 2 HHLLHHLLHHLLHHLL stack 35

Ti0 2 /Si0 2 HHLLHHLLHHLLHHLL stack coevaporated 75

5. Conclusions

In this paper we have presented a new high sensitive absorption measurement technique and

the corresponding theoretical model on the temperature field in a single layer structure. For

ln

2 0

3 the calculated results are in good agreement with recorded profiles. The interface

absorption and bulk absorption of Hf02

, Al

2 03

, Ta, 0, and Ti02 in conjunction with Si02 was measured and compared to the damage thresholds at 1.064 um. In all cases but Ti0 2 blocks damage

thresholds can be related to the total absorption and stacks have lower radiation resistivity

than blocks due to interface absorption. Regarding thresholds and absorption HfO appears to be

the best material among the tested samples. All Hf02 specimens show inclusion dominated

breakdown. The damage threshold of Ti0

2 stacks can be doubled by the use of a coevaporation

technique. These results show that interface absorption is an important parameter for high power

laser coatings.

305
6 References

1 Bennett, H.E.; Guenther, A.H.; Milam, D.; Newnam, B.E.; Laser induced damamge in

optical materieals: fourteenth ASTM symposium. J. Appl. Opt. 23(20): 3782-3795;

1984 November

2 Kuster, H,; Ebert, J.; Laser induced damage in optical materials. Nat. Bur. Stand.

(U.S. ) Spec. Publ. 568; 1979. 269p

3 Lax, M. ; Temperature rise induced by a laser beam. J. Appl. Phys. 48(9): 3919-3924;

1977 September

4 Calder, I.D.; Sue, R. ; Modeling of cw laser annealing multilayer structures. J. Appl.

Phys. 53(11): 7545-7550; 1982 November

5 Mansuripur, M.; Cornell, G. A. N. ; Laserinduced local heating of moving multilayer

media. Appl. Opt. 22(5): 66-670; 1983 March

6 Carslaw, H.S.; Jaeger J.C.; Conduction of heat in solids, Oxford, Great Britain:

Clarendon Press; 1959

7 Bernal.B.G.; Heat flow analysis of laser absorption calorimetry. Appl. Opt. 14(2):

314-321; 1975 Februar

306
figure 1: Statement of the

situation described by the

theoretical model: A beam

with gaussian intensity

distribution hits an arrangement consisting of

an absorbing single layer

with thermal diffusivity

K i on an infinite nonabsorbing substrate with

thermal diffusivity K 2

. Ri

and R 2 are the reflection

coefficients of the heat

flux.

incident laser beam

reflected heat flux

absorbing layer

nonabsorbing substrate

transmitted heat flux

.4. m h

/ ^^^^

200 W cw Nd-YAG laser

f=26 mn sample

figure 2: Experimental arrangement of the temperature line scanning device

307
40-

LU

cr< 30

LU

CL

LU

20

10

BLACK

BODY

50 100 150

DETECTOR-VOLTAGE (mV)

figure 3: Temperature calibration

curves for several materials

coated on BK 7. The antiref lection

coating AR II consists of ThF*

ZnS. The depicted detector noise

allows for temperature measurements below room temperature.

The calibration curve for the

blackened substrate is the same

as that for the black body.

ln,0.

550^

figure 4: The radial temperature

profile of a laser heated ln 2 0 3 layer post an exposure time of

32- seconds. The calculated temperature distribution is indicated by the solid line. Parameters are: k=4.5 E-5; s=150 urn

LU

DC

Z3

I-<

CC

LU

Cl

LUh275-

0

RADIUS r(mm)

K^i.S E-5 cnv's

K -7.0 E-5 cm/s

figure 5: Measured temperature

in the beam centre as a function

of time. The temperature rise

of both samples is nearly

ideally according to an atnfunction.

50 100

TIME (s)

150

308
BLOCKS

n = 2

Air

STACKS

figure 6: layouts of the samples. Both samples have the same optical thickness

but the stack has six additional interfaces. The distribution of the electric

field strength is illustrated by the dotted curves. For each material the

distribution in the stack is equal to that in the corresponding block.

>

HI

H? dye-sheet Nd-YAG rod aperture R=50% f=160 rnn

attenuator

scatter plasma

detector detector

fast photodiode

transient digitizer

figure 7: Experimental set up of the damage test facility. Damage detection is

achieved by a plasma detector and a detector for the scattered light of a HeNelaser beam which is superposed with the test beam of the Nd-YAG-laser. The Gewindow reflects the scattered light of the HeNe-laser onto the scatter-detector

meanwhile the infrared radiation for the plasma and temperature detector passes

the window.

309
figure 8: The absorption of single layers of eight quarterwaves optical

thickness compared to the absorption per interface for the corresponding

stacks. The interface absorption for the Ti0 2 -stacks can be decreased

with the aid of a coevaporation technique.

310
figure ,9: The absorption data of the block and stack samples.

The blocks have lower absorption than the corresponding stacks.

All block's consist of a HHHHHHHHLLLLLLLL-design. The stacks

have the same optical thickness as the blocks for each material

but the design is HHLLHHLLHHLLHHLL.

figure 10: Damage thresholds of the same set of samples as above. Pulse length is 15 ns, beam diameter is 90 urn at a wavelength of 1.064 urn.

311
It was pointed out that a necessary assumption in the paper was that the surface absorption

for the thin films and for the thick films was the same. The author agreed, but felt that

this assumption was reasonable in this case, because the same chambers and techniques were used for both and the chamber was not used for anything else. However, it was pointed out

that the stresses in a film are very thickness dependent. To the extent that the surface

absorption is caused by these stresses, the equality assumption may be questioned. The

author agreed and stated he would investigate this question in the next series of ex- periments.

312
Manuscript Received

10-28-85 SAB

Effect of Anisotropic Stress on Thin Flim Damage Thresholds

A.J. Heiney*, J. Eastman, and C.W. Gabel

The Institute of Optics and Laboratory for Laser Energetics

University of Rochester

Rochester, NY 14627

Damage thresholds for silica/titania multilayer reflectors are measured using single,

30 ps, Nd:YAG laser pulses to study the effect of anisotropic stress on thin film performance. Damage testing sites near the edges of coatings were used to spatially correlate

damage with reproducible anisotropic stress profiles taken parallel to the edge.

Anisotropic stress in a coating is stress which has a directional dependence in the

plane of the film. A microscope based modulated ellipsometer is used to measure anisotropic

stress via induced birefringence. The damage testing system centers around a vertical illumination microscope. The damage pulse is directed by a 45° high reflector in the microscope

tube section through a low power objective. The sample surface is viewed through the same objective used to focus the laser to 8 ym FWHM in the center of the viewing field. The goal

is to spatially relate this data with microscopic stress measurements.

In most cases, negative anisotropic stress areas are found to have lower damage

thresholds while positive anisotropic stress correlates well with high damage threshold.

Damage thresholds for uniform striplike areas of film are found to change dramatically com- pared to adjacent strips. In one case average damage threshold changed from 1.3 to 6.9

J/cm^ within 100 ym. The width of measured threshold bands in this work ranged from 0.5 to

2.5 J/cm^.

Key words: Anisotropic Stress; El lipsometry; Microscope-laser integration; Picosecond laser induced

damage; Thin films.

1. Introduction

Optical coatings have spatial variations in damage resistance, either due to inclusions or other

effects. Here we attempt to show a relationship between the spatially resolved damage resistance of

a coating and its anisotropic stress. Damage test areas were chosen near the edge of two multilayer

Si02/Ti02 high reflectors designed for normal incidence and 1.06 ym. These areas were chosen

because reproducible stress measurements over a wide area were possible. Anisotropic stress will occurr in the center of films that were coated at a high angle of incidence but not rotated, or highly stressed films which have or are about to craze. Bloom and Costitch[l] have indicated that

the passage of picosecond laser pulse in Si02/Ti02 films causes compressive stresses in the film

structure due to linear absorption and heating. Therefore, the stress integral of a film having

compressive ambient stress will increase with the passage of a picosecond pulse and therefore will

damage sooner. Tensile areas, however, have more "headroom" to absorb induced compressive stress

and will have a higher damage threshold. A calculation based on this premise is presented in

Section 3.

2. Apparatus

Single pulse damage thresholds were obtained with the damage testing system shown in figure 1.

It consists of a passively mode-locked/Q-switched Nd:YAG laser, a single pass amplifier when needed,

a microscope, and beam diagnostics. The oscillator, similar to those used for short pulse fusion

experiments, uses a flowing dye cell at the HR end and incorporates a single pulse switchout system.

This yields 20-50 picosecond pulses at 100 yJ. The laser is operated near threshold where repeatability is good and double pulsing is minimized. The beam waist is located at the output coupler and

is -850 ym in radius.

Pulses are incident on a modified metallurgical microscope as shown in figure 2. Viewing with

vertical illumination is possible through the permanently mounted 1.06 ym reflector so the surfaceof

the film could be examined using the same microscope objective used to focus the beam onto the film

surface. The Bausch & Lamb 4x doublet has a focal length of 32.75 mm at 1.06 ym. With a laser beam

spot the size of 3.3 mm at the objective, the system is -f/5.

* Present address: Allied Corporation, Corporate Technology, 7 Powder Horn Drive, Mt. Bethel, NJ

07060

313
Beam spatial profile was examined extensively with this stable output and energy density ratios were obtained. A separate beam profile for each damage shot was not obtained. We used an aluminized

mirror as very high contrast film by damaging it. By varying the incident energy, we were able to

map the focal energy distribution. A peak energy density factor of 1.3 J/cm^/pJ was obtained. This

number transforms the damage thresholds which were obtained in total energy to energy densities.

Since we are looking for a relative connection between stress and damage, the accuracy of this num- ber is not critical. Even so, some astigmatism was measured and we modeled the expected beam spatial

profile at the circle of least confusion on a beam propagation code and obtained a peak energy density factor of 1.5 J/cm^/jjJ.

A streak camera was used to measure pulsewidth and a photodiode was used to measure total energy.

The photodiode was calibrated using a Laser Precision ratiometer. Figure 9 shows a sample shot on the streak camera as well as a calibration curve in picoseconds per OMA channel. The pulse is

bounced through an etalon having 70% reflective surfaces thereby giving successive pulses reduced in

intensity by a factor of two. Pulsewidth data was also used to test for peak power damage dependence. No power dependence was found, as expected.

To measure anisotropic stress, a modulated el 1 ipsometer[2J was adapted for microscopic measurement and visual inspection/indexing. The system is shown in figure 3. A 45° polarized HeNe laser

beam is incident on a natural quartz wedge mounted on a common audio driver. An identical wedge is

rigidly mounted in compensation for the wedge angle. The beam is then focused onto the sample sur- face and recollimated by a pair of microscope objectives. A microscope cover slide directs incoherent illumination in the opposite direction using the collimating objective as a condenser. An

eyepeice views the image reflected by the first cover slide. A sample has anisotropic stress proportional to its birefringence which can be measured accurately using the lock-in amplifier:

HI A* A „ jq7 nm

360 C 0

h cm

where C 0 is the stress-optic coefficient and h is the film thickness, a is related to the birefringence and is derrived from Lichtenstein's[2] application of the generalized ellipsometry

equations[3]:

I u

2 Sin A Ji(B)

I dc Cos A J 0 (B) - 1

where Ioi is the signal at the fundamental lock-in frequency and Idc 1S measured independently. B is

the modulation phase retardance range covered and was set to be 360°. Therefore,

lu) _ 0.43 Sin a Idc 1 - 0-22 Cos A

and a must be solved for analytically,

It should be pointed out that a uniform stress level in the plane of the film would not give

rise to birefringence and is therefore unmeasureable by this method.

3. Experimental Results

Two stress scans were made perpendicular to the edge of a film on either side of the damage testing region and these regions were in narrow strips parallel to the edge spanning a large distance.

The damage testing conditions used closely follow those of Newnam, Gill, and Faulkner[4]:

- x = 1.064 ym - normal incidence irradiation

- single pulse, 30 picoseconds - energy controlled with optical density filters

- single irradiation per site - 40-50 shots to establish a threshold

314
A threshold as taken in film area A of figure 5 is shown in figure 4. Damage was detected by

microscopic inspection at 200x. Damage thresholds are given in total energy which can be related to

energy density by the data obtained on beam spatial profile (section 2.). Figures 5 & 6 shows the

relationship between an anisotropic film scan in two paths perpendicular to the film edge and the same damage thresholds measured near those scan regions. There is a good relationship between positive values of a and high damage thresholds and between negative values of o and low damage thresholds for areas A - D. Regions E and F show an apparent lack of correlation. This could be due to

less spatial selectivity exercised when damage threshold measurements (which were completed before

stress measurements) were made. Notice that within <50 ym between areas A and B, threshold changes

by more than a factor of four. The damage threshold increases again in area C. Another reflector was tested showing a similar result (figure 7). In this case, however, damage thresholds were obtained by extrapolating from damage site diameter data. This was done with the methods used to measure beam spatial profile at the focus of the mircroscope objective.

A scanning electron micrograph of a damage site occurring well above damage threshold in a low

threshold area in figure 8. The layers of the film peeled away cleanly and the deformation of the

substrate appears to have been caused by melting. Considering that the film/substrate interface is

particularly susceptible in a stressed coating because that is where the net stress changes sign, a thermal event at the interface is likely to be catastrophic for the film.

We will now calculate the stress induced by the passage of a picosecond pulse. The induced

stress caused by heating is compressive and is given by:

a

= - Y a AT

where Y is Young's modulus, a is the expansion coefficient of the film, and aT is the temperature

rise. Note that these quantities are always positive so a is always negative. Table 1 shows the

ambient anisotropic stress from figure 5 for areas B, C, and D. The average damage threshold for

those areas are given in the next line (an energy density factor of 1.3 J/cm2 /yJ was used).

Table 1. Calculations of the Stress Induced by a Short Laser Pulse

Damage Area

in Sample #1 B C D

Ambient

Anisotropic x 1CH kg/cm

Stress

-8.4 + 7.0 -2.4

Average

Damage J/cm2 Threshold

1.3 7. 1 2.5

Temperature

rise (°C)

Assuming: 15°

J

1

cm2 95°

37cm2

19.0

120

106 . 5

670

37\5^^

240

Induced

Stress x 10 3 kg/cm2 -0.47/

0

-2.7

-16.8

-0. 94

.-^6 . 0

Total

Stress x 10"5 kg/cm z -8.9/^

^-\\. 4

+ 4. 3/-"^ - 5 . \^

•^8. 4

Bloom & Costitch[l] assumed a temperature rise in the film of 15°C for a 1 J/cm2 pulse. In addition we chose to calculate the induced stress arbitrarily assuming 95°C/ J/cm2

. By multiplying the energy

density at damage threshold by these factors we get the temperature rise aT. We then calculate the

induced stress from the above equation and finally add the induced stress to the ambient anistropic

stress to get the total stress at threshold. We see that for the 95°C/ J/cm2 case, the total stress

in the three areas are within ±16% when damage occurs indicating that this is the point at which the

mechanical strength of the film is exceeded. This supports the assertions of Bloom & Costitch.

315
Conclusions

We have spatially resolved the varying damage thresholds of two samples and related them to the

measured anisotropic stress in those areas. We found that nominally compressive anisotropic stress areas have a lower damage threshold than tensile anisotropic stress areas. We also made a quantitative comparison based on a temperature rise assumption. In most cases we obtained reasonably narrow threshold bands indicating good distinction between adjacent stress areas, good damage detection sensitivity, and a stable focal energy distribution. The microscope based damage testing

facility was extremely useful for this work. It is particularly suitable for integration of the

ellipsometer and perhaps other measurement/detection schemes into a single unit.

A. Heiney would like to thank C.W. Gabel and J. Eastman for their assistance and support as master's thesis advisors. He would also like to thank John Agostinelli, George Harvey, John Whitaker

and Jack Kelley. We would like to acknowledge the financial support and facilities provided by the

Institute of Optics and the Laboratory for Laser Energetics.

References

[1] Bloom, A.L.; Costitch, V.R., Design for

High Power Resistance. Nat. Bur. Stand.

(U.S.) Special Publication 435; 1975.

248 p.

[3J Azzam, R.M.A.; Bashara, N.M., Ellipsometry

and Polarized Light. New York: NorthHolland; 1977.

[2] Lichtenstein, T.L., M.S. Thesis, Institute

of Optics, University of Rochester, 1980.

[4J Newnam, B.E.; Gill, D.H.; Faulkner, G.,

Influence of Standing-Wave Fields on the

Laser Damage Resistance of Dielectric

Film. Nat. Bus. Stand. (U.S.) Special

Publication 435; 1975. 254 p.

[5] Agostinelli, J. A., Ph.D. Dissertation,

Institute of Optics, University of

Rochester, 1980.

316
DAMAGE TESTING SYSTEM,

Oscilloscope

Laser

' i

all 1 1 1 ,,.

Switch Out

Flashlamp

Trigger

Microscope

Oscilloscope

Photo

Diode

CRT OMA

k

Streak Camera

Amplifier

J

Optical

Delay

Streak Trigger

Figure 1. Schematic of Nd:YAG laser damage testing facility.

8 8

Dial Gauge

Alignment

Laser 7

Optical

Density

Filters /J/ Damage

JTJ-— Pulse

Sample

Figure 2. Microscope used for irradiation and examination

of samples. Dial guage is used to return to IR focus.

317
He Ne

Laser

MODULATED ELLIPSOMETER SYSTEM

_ up ah

10- Microscope Incoherent

Objectives Illuminator

+ 45<> Modulator

Compensator

DC

Voltmeter

PAR 124A

Lock-In #116

Internal Pre-Amp

Oscillator

Audio

Amplifier

Ground

Glass

0.6328 Pass

Filter

Polarizer lQl Compensator LJ Sample

Analyzer

Eyepiece

UDT

PIN 10 DP

Density

Pre

Amplifier

WW

/\s\

Oscilloscope

Figure 3. Birefringence measuring AC modulated ellipsometer showing

quartz wedge modulator.

o

UJz

UJ

o

r9i—

-o-- Damage

0 o Threshold

Band

o o

cP o o

o

_L

• Damage

0N0 Damage

Error Bar I

J

10 20 30 40 50

SHOT NUMBER

Figure 4. Damage threshold of area A as shown in figure 5.

318
Figure 5. Anisotropic stress scan spatially aligned with damage

thresholds relative to the edge of the uniform film region. Energy

threshold scale on right.

Figure 6. Scan made on the other side of the damage testing region

shown with the same thresholds.

319
Eo

18

14

10

<T -2 o

-10

-14

-18

1

Damage

Threshold

Band

J i L J i I i I i I i I i_ 100 200 300 400 500 600 700 800 900

DISTANCE FROM VISIBLE EDGE (flm)

>

O „ CC CM "J E Z u

UJ -

0 t

1 V)

V) z

HI UJ

Figure 7. Anisotropic stress scan of the second sample.

I i

1 ym

Figure 8. Scanning electron micrograph of a damage site

occurring well above threshold.

320
(a)

Figure 9. a) Streak camera/Optical Multichannel Analyzer (OMA)

output of a 30 ps pulse bounced through a thick etalon having a 125 ps round trip time; b) Calibration curve for the streak camera obtained using the 125 ps bounce time vs. channel # midway

between peaks [5].

321
Manuscript Received

1-31-85 SAR

Design of High Power Laser Coatings for

Grazing Incidence Mirrors

W.H. Southwell

Rockwell International Science Center

Thousand Oaks, California 91360

Several interesting features occur for high reflectance (HR) coatings at near- grazing incidence: the existence of a Brewster's angle, a double-valued effective index

for the p-polarization (enabling a MacNei lie-type polarizer to be constructed in air),

and a half -wave retarder. It is shown that HR coatings can be designed which have high

reflectance and, as compared to normal -incidence designs, extremely low internal Efield standing waves for both s- and p-polarization components. Such mirrors should

exhibit considerably less thermal distortion due to decreased laser absorption.

Key words: coatings; damage thresholds; electric fields; grazing incidence; mirrors.

1. Introduction

The use of grazing incidence mirrors has been proposed for high power free electron lasers.

Mani and Hammond* cite the advantage of grazing incidence mirrors as being their decreased absorption due to grazing incidence, resulting in higher damage thresholds. They also cite a disadvantage, however, which is that only one polarization component allows this decreased absorption

which necessitates the use of a linear, rather than helical, wriggler configuration. Woodberry 2 has recently shown that multilayer enhanced reflectors at grazing incidence can have internal

standing wave E-field patterns which are over two orders of magnitude less than enhanced mirrors

at normal incidence. This E-field suppression is accompanied by high reflectance for s-polarized

beams. Such multilayer designs show no such suppression for p-polarized incident light, nor are they highly reflective for the p-component. The coating design presented by Woodberry should have

high damage thresholds, although one must guard against any p-polarization. This also places a constraint on the amount of curvature on the grazing mirror and in the incident beam.

In this paper we present a new dielectric multilayer design which has high reflectance for

both s- and p-polarization components and in addition has low internal standing wave E-fields,

which gives it the potential for high damage thresholds for any polarization.

We begin the discussion with a comparison of high reflectance coating performance at normal

incidence, 45° and 86° in order to illustrate some interesting features of grazing incidence

reflection. Throughout the paper we emphasize the role of the internal standing wave E-fields.

2. Tuned Quarter-Wave Stacks

The standard high reflectance coating consists of a quarter-wave stack alternating between a high index and a low index layer. In standard notation, an example is Air (HL) 10 H Sub, where H and L represent a quarter-wave thick layer of high or low index material. Tuning the stack for a particular angle of incidence means that the layer thickness, t, has the value,

where x is the wavelength, n is the layer index, n 0 is the incident medium index (usually 1.0),

and e0

is the angle of incidence in air. This thickness provides a quarter-wave optical thickness

for both s- and p-polarization components.

Figure 1 shows the reflectance for HR (high reflectance) coatings at normal incidence, 45°

and 86°. In each case the thicknesses are "tuned," that is, determined by eq. (1) so as to be

quarter-wave at each angle of incidence. (In these examples we chose 2 and 1.38 as the high and

low index values for the layers and a substrate index of 1.45.) At normal incidence s- and pcomponents coincide. At 45° the reflectance zone broadens and increases for the s-component and

t = 4(n 1/2(1)

322
narrows and decreases for the p-component . This is consistent with the concept of effective

index, which states that the layer index for s-polarization at an angle of incidence, e0

, behaves

as though it were at normal incidence with the same optical thickness but with an index

n

s = n cos e (2)

Likewise for p-polarization the layers have an effective index

n/cos e (3)

where 9 is the incident angle in that layer. The angle, 9, is also a function of the layer index,

n, and the angle of incidence in air, 9

Q

, through Snell's law,

cos 9

2 • 2 n >l/2

n sin 9l

(4)

Using these equations one can show that the high/low index ratio increases with angle of incidence

for s-polarization and decreases for p-polarization. Generally, it is this ratio that determines

the reflectivity and bandwidth of the high reflectance zone. However, we may apply the effective

indices (including the incident medium and substrate) to a normal incidence expression 3 for

reflectance

~l 2

:2 f _ ,_ si-l

'

R = Vs - n A^

n A/n B^

n n +

o s n A

(n

A

/n

B

)

(5)

where A is the number of layers (odd), n A

is the index of the outside layer (next to air), and n B is the index of the other layer material, and obtain the reflectance as a function of the angle of

incidence for tuned HR coatings. It turns out that for some combinations of materials the numerator in eq. (5) vanishes at a particular angle. This angle represents the Brewster's angle for the

HR coating substrate. For the materials chosen in figure 1 this angle is about 88°, which

explains the low p-reflectance at 86°.

idu

wrveUength

(b) wrveL

5ength

Ulu

z

(E1-u

U

_1

u.u

a.

' ( c ) wrve'lIngth

Figure 1. Reflectance versus wavelength for the coating design Air (HL) 10 H Sub tuned for

(a) normal incidence, (b) 45°, and (c) 86° angle of incidence. Dashed curves, in

all figures in this paper, are the p-polarization components.

323
In addition to the existence of a Brewster's angle, tuned HR coatings at high angles of incidence may also be used as a basis for a MacNeille polarizer 1* in air. That is, one not requiring

the coating to be placed on the back side of a prism. This becomes evident when the effective indices are plotted versus n, the material index, as is shown in figure 2. At 86° angle of incidence, we note that the effective index for p-polarized light is double valued. This means we may

pick a high and a low index material that has the same effective index for p-polarized light and

yet good contrast for s-polarized light. Each quarter-wave pair becomes a half -wave layer and,

hence, absentee for p-polarization. For an even number of layers, the reflectance becomes that of

the substrate for the p-component. If the appropriate anti reflection layer (or layers) is

applied, then the p-component is transmitted and the s-component reflected. Previously, it had

been supposed that this could only occur for values of n0 sin e0 greater than unity, hence, the

requirement to have the incident medium be a prism.

LRYER INDEX n

Figure 2. Effective index n„ (upper curves) and n

s (lower curves) as a function of actual index n.

3. Internal Standing Wave E-fields

Shown in figure 3 are the internal E-field standing wave patterns for the tuned quarter-wave

coatings described in figure 1. Also plotted are the E-field patterns outside the stack in a half-wave air "layer." A striking feature of this figure is the large width of the half-wave air

layer at 86°. Additional half-waves of the incident medium would show more fringes identical to

the one shown. In fact, that pattern represents the fringe pattern for a Lloyd's mirror experiment. 5 At near grazing incidence, the reflected beam interferes with the incident beam producing

a fringe spacing which increases as the angular separation of the two beams decreases.

We also note that at 45° angle of incidence the s field inside the stack is less than the p field and has a desirable null at the air-coating interface. At 86° the s field is too small to

be seen on the scale of figure 3. It is shown on an expanded scale in figure 4. As observed by

Woodberry, 2 we note that the peak value of the standing wave (calculated as |E 2

) is over 160

times smaller than at normal incidence. This is a dramatic result and should lead to increased

damage thresholds over whatever is observed at normal incidence.

The above prediction is based on the assumption that the damage mechanism involved has an associated E-field threshold. Such an assumption is a straightforward consequence of the

existence of an incident laser beam power threshold. The internal E-fields in the coating are directly proportional to the incident beam intensity. Since there is an E-field null at the

coating surface, the damage can only be due to the internal E-field.

Of course, we have assumed that we have infinite plane waves and perfectly plane layers of

homogeneous, nonabsorbing material. Although this won't occur in practice, we should expect,

through careful coating procedures, that the layer surface roughness and the size of the imperfections be small compared to the wavelength so as not to perturb the gross features of the standing

wave E-fields. It has been shown 6 that local fields near micropores and cracks could be enhanced

by as much as n

4 (if the size and shape were just right), which is a factor of 16 in our case. However, these effects are also present at normal incidence, so the improvement over normal incidence damage thresholds should still be observed. Even so, 16 is still small compared to the

theoretical 160.

324
hiU

a

CE

OL

a

THICKNESS

NORMAL

45°

86 c

Figure 3. Standing wave E-fields, |e| 2» f° r the configurations shown in

figure 1. Also shown are the standing waves in a half-wave

air layer just outside the coating surface.

u

u

z

cr

i-) a

cr

a.

THICKNESS

Figure 4. Standing waves for the s-polarized component inside the coating of

figure 1 at 86° of incidence. The number at the upper right, 0.00648,

is the peak value relative to an incident beam of unity amplitude.

So far, we have only discussed the decrease in the E-fields. Another factor which should

also improve damage thresholds is the decrease in energy density due to the large beam cross section inside the layers due to the large angle of incidence. The geometry of the situation is

shown in figure 5. As seen there, this effect is equal to cos 90 /cos 6, where 9 is the refracted

angle of the beam in the layer, and is given in eq. (4). A plot of this factor for a layer of

index 2 is shown in the upper curve in figure 6. It should be noted that this factor is related

to the width of the beam in the layer and not related to the width or footprint of the beam at the

coating surface. It is a smaller effect than would be predicted on the basis of the size of the

beam footprint. The middle curve in figure 6 is the peak value of |E

$ | 2 for tuned high-low HR

coatings. The E-field reduction is much more dominant than the geometrical effect shown in the

upper curve.

Absorption or heating at any point in the coating is proportional to the quantity,

kn E

9 cos 9~

Z o cos e (6)

325
Figure 5. Geometry of a beam entering a layer of index n. The energy

density of the beam decreases by the ratio W0 /W = cos en /cos e.

Figure 6. Internal coating flux reduction factors for s-polarized

light for tuned HR coatings of the type shown in figure 1.

The upper curve is the geometrical angle of incidence factor.

The middle curve is the peak value of |E p | 2

, and the lower

curve is the product of the upper two.

where k is the extinction coefficient. The lower curve in figure 6 is a plot of the peak value of

|E

S | 2 cos 0o/cos 0 as a function of angle of incidence for a tuned HR stack. This plot should be

a predictor of laser damage thresholds. We note three orders of magnitude less heating at 85°

than at normal incidence for the s-component for the incident beam.

4. High Reflectance Criteria Coatings

We have shown the extremely good performance that can be expected for the s-component of an HR coating near grazing incidence. We have also shown its poor performance for the p-polarization

component. Before addressing the problem of finding an HR coating with improved p-performance, we

ask: what configuration gives the highest reflectance for both s- and p-components, regardless of

the internal field strengths?

From an examinaton of eq. (5), we see that by placing a low index layer next to air, the

numerator will not vanish. Such a configuration will not exhibit a Brewster's angle and we can expect good reflectance for both s- and p-polarizations.

Figure 7 shows the reflectance of such a low-high quarter-wave stack. Also shown in figure 7 are the standing wave E-field patterns for the same coating. Several interesting features of this

design are worthy of comment. First of all, the design is probably optimum for high reflectivity

for both s- and p-polarizations. The p-component now has a standing wave null at the coating

surface and low levels inside the coating. The s-component, on the other hand, has a standing

wave peak at the coating surface and rather high levels in the stack. Since the coating surface

can pick up dust or other contaminants, the high E-field at the surface seems undesirable for high

power laser applications where damage is likely. However, for other applications, this HR coating

may be well suited. For example, unpolarized light would not exhibit fringes in the region above

the surface (near grazing incidence). This is because the s- and p-fringe patterns are out of

phase and produce a uniform sum.

326
The abo»e observation, as seen in figure 7, means that if one were to perform a Lloyd's

mirror experiment 5 with this mirror, fringes would not be seen. Furthermore, if incident s-polarized light were used s the opposite result would be obtained. This means that there is no pi phase

change on reflection for the s-component as would be expected for both p- and s-reflection from

dielectric substrates and metals. This means that this HR coating produces a pi retardation in

the reflected beam. Thus, it acts as a half-wave plate. This feature may be of interest in some

appl ications.

We would not expect this design, however, to have high damage thresholds because of the high

internal fields for the s-polarization 3

1 1-

u

z

CES .5

Ul

_J

_i i l_ wrve'length_1 1 1_

u

z

CEn

a

THICKNESS

Figure 7. Reflectance and standing wave patterns for a low-high

quarter-wave stack Air (LH) 11 Sub tuned for 86° angle

of incidence. The dashed curve is the p-component.

5. High Reflectance and Low E-field Criteria Coatings

Our goal is to obtain high reflectance for both s- and p-components and, in addition, have

low standing-wave E-fields internal to the coating and at the coating surface for near grazing

incidence radiation.

After a numerical optimization study, we have discovered that such objectives can be

achieved. A design using the example index values and number of layers used previously in this

paper is shown in figure 8. The standing wave patterns are also shown in figure 8. We note that

there is not a strong field at the coating surface for either polarization. This is a consequence

of adjusting the phase shift of the p-reflection. We note that there is now only a small phase

shift between the s- and p-components.

THICKNESS

Figure 8. 21 layer HR coating Air 0.35 H 0.988 (LH) 10 Sub tuned for 86°

angle of incidence with the high and low index being 2 and 1.38.

327
The lower plot in figure 8 shows that the peak internal standing wave is the same for both

components. Furthermore, this peak value is almost a factor of 40 less than the normal incidence

value. Although this is 4 times worse than the high-low quarter-wave stack for the s-component,

it is 40 times better for the p-component. When coupled with the cosine ratio effect, heating

losses show a theoretical reduction by a factor of 100 over normal incidence HR coatings for both

polarizations.

The design feature that predominates in achieving this performance is the reduced thickness

of the outer high index layer. In the design shown in figure 8, the outer high index layer is

0.35 of a quarter-wave in thickness. The rest of the stack is slightly detuned to account for the

small shift in peak reflectivity produced by the nonquarter-wave outer layer.

Finally, two things can be done to boost the p-reflectance; use a higher high index material

and increase the number of layers. This is illustrated in the design shown in figure 9.

\ f N

: J V V

1 \

i i i i 1 WAVELENGTH

THICKNESS

Figure 9. 33 layer HR coating Air 0.285 H 0.988 (LH) 16 Sub tuned for 86° angle

of incidence with the high and low index being 2.35 and 1.38.

6. Comments

We have shown that HR coatings can be designed for grazing incidence applications. Such

coatings can also be designed to sustain very low E-field standing waves for both polarization

components as compared to HR coatings for normal incidence. These dramatic decreases in E-field

strengths should result in higher damage thresholds according to current damage mechanism

theories. Perhaps more important is the decrease in thermal distortion that these mirrors should

provide when used in laser cavities.

We are aware of some experimental results which failed to show the predicted angular dependence on damage thresholds of coatings at 351 nm. Our only comment at this point is that the

assumptions regarding the standing waves and laser damage are better satisfied at longer wavelengths , since optical tolerances become tighter at ultraviolet wavelengths and electric fields are more sensitive to coating imperfections and surface roughness.

7. References

[1] S.A. Mani and J.H. Hammond, "Optics and resonator design issues for high-power free electron

layers," Proceedings of the International Conference on Lasers (1981) p. 586.

[2] F.J. Woodberry, "Electric fields in grazing incidence laser mirror coatings," Appl . Opt. 23,

3493 (1984).

[3] P.H. Berning, "Theory and Calculatins of Optical Thin Films," in Physics of Thin Films , vol.

1, G. Hass, editor, Academic Press, New York (1963) p. 96.

328
[4] S.M. MacNeille, "Beam Splitter," U.S. Patent no. 2,403,731, July 9, 1946.

[5] F.A. Jenkins and H.E. White, Fundamentals of Optics , McGraw Hill, New York (1957) p. 243.

[6] N. Bloembergen, "Role of Cracks, Pores, and Absorbing Inclusions on Laser Damage Threshold at

Surfaces of Transparent Dielectrics," Appl . Opt. _12_, 661-664 (1973).

[7] B.E. Newnam, S.R. Foltyn and D.H. Gill, "Angular Dependence of Multilayer Reflector Damage

Thresholds," Sixteenth Annual Symposium on Optical Materials for High Power Lasers, National

Bureau of Standards, 17 Oct. 1984.

The quest-ion was asked "How critical is your design to layer thickness control?" The author

replied that the outer layer was going to be sensitive because it changes the coating from a high-low stack to a low-high stack as one varies that outer layer. He has not yet calculated

the exact tolerances . Concern was expressed as to the number of layers. The author replied

that the limiting number of layers depends on the absorption per layer and 33 layer coatings

have been made successfully

.

329
Manuscript Received

12-11-84 SAB

The Design of Optimum High Reflectivity Coatings

for Grazing Angles of Incidence

Jeffrey B. Shell an

W. J. Schafer Assoc., 22222 Sherman Way, #205

Canoga Park, CA 91303

(Material was actually presented at the classified session)

Abstract

Although quarter-wave multilayer dielectric coatings (QW MLDCs) usually offer the

highest reflectivity possible at a single wavelength, this may not be the case at grazing angles of incidence. For incident angles greater than the Brewster angle, the p polarization absorption can easily be several orders of magnitude higher than the s polarization absorption due to the destructive interference between the reflection from

the superstrate - top layer interface and the rest of the coating. An analytic approach

is developed for designing optimum reflectivity coatings at grazing angles of incidence, once the fraction of s and p polarized light is given. These coatings can give at least

an order of magnitude reduction over QW MLDCs in total coating absorption, even for the case where less than 0.1% of the incident radiation is p polarized. The absorption of

the designs found from the analytic results compare favorably with designs generated

using numerical methods of non-linear optimization.

Key words: FEL optics; grazing angle of incidence coatings; optical coatings

1, Introduction

The purpose of this paper is to present analytic results that can be used to design

multilayer dielectric coatings (MLDCs) with maximum reflectivity at grazing angles of incidence. The designs found can have absorptions orders of magnitude less than quarter-wave

stack designs, since the p polarzation reflectivity of quarter-wave stacks can be quite low at grazing anqles. The closed form solutions found compare quite favorably with designs generated

using numerical non-linear optimization procedures.

Grazing angle of incidence optics are used in numerous applications where ultra-high re- flectivity, high damage thresholds, or reflectors in the far ultra-violet and soft x-ray are needed. For example, the high flux levels needed in the gain region of a free electron laser

(FEL) are several orders of magnitude greater than can be handled by conventional optics. A possible solution to this problem is to use a grazing incident angle mirror (GIAM) to expand the

beam geometrically before it strikes a mirror normally. If a is the grazing angle, defined as

it/2 minus the incident angle, then the beam foot print will be increased by a factor 1/a (for

small a). In addition the absorption will be shown to vary as a, giving an absorbed flux per area proportional to a , and damage threshold proportional to 1/a . It has been shown by S. A. Mani and J. H. Hammond that the optimum value of a, for minimizing the resonator length,

is on the order of one to two degrees for high power FELs invisioned for the near future.

Another area where grazing incident optics are needed is for the reflection of very short

wavelengths where the index of refraction of most dielectrics approaches 1 and reflection from

bare metal at normal incidence is quite low.

For most applications where maximum reflectivity at a single wavelength is desired a quarter-wave MLDC is used, since it usually allows the coherent addition of the reflections from

each dielectric interface. The advantages of deviating from a quarter-wave stack have been

discussed for the case of slightly absorbing dielectric layers, 2 ' 3 as an attempt to increase

the damage threshold by reducing the internal flux, and as a means of controlling the difference in s and p polarization phase shifts.

Figure 1 illustrates the phase shifts experienced by a beam reflection from each of a number of dielectric interfaces. The 0 superstrate is usually air, the S layer represents the

substrate, and H and L represent high and low index layers, respectively. In most cases the

phase is shifted by it when radiation incident from a low index medium is reflected from a high

330
index medium. There is no phase shift when the light incident from a high index material is

reflected by a low index material. Thus if the layer thicknesses in figure 1 are all quarter

wave at the incident wavelength, maximum reflectivity will result for both s and p polarized

light. The phase shifts shown in figure 1 hold for all incident angles $ in the case of s polarized light. They also hold for p polarized light if <|> is less than the Brewster angle, (tan^g

= n n/n o' where n^ and n Q are the indices of the high index layer and the superstrate

respectively). For <j> greater than ^3, the superstrate - high index layer phase shift is 0 for

p polarized light as shown in figure 2. Thus the quarter-wave stack will give poor p polarization reflectivity at grazing angles

From figures 1 and 2 it is clear that if only s polarization is present, then the quarterwave design is best, while if only p polarization is present the top layer should be half-wave

and the remaining layers quarter-wave thick (<j>><j)g). The purpose of this paper is to develop

expressions for the coating design that will give the minimum total absorption as a function of

the fraction of s and p light incident on the MLDC.

Section 2 will develop the theory used to find the optimum coating. A review will be given

of work by Shellan in which simple expressions were found for the reflectivity of quarter-wave

stacks at oblique angles of incidence. This analysis technique is then extended to the case of one layer being non-quarter wave thickness. Section 3 uses these results to find the coating

with maximum reflectivity. Section 4 compares the results of sections 2 and 3 with exact computer results based on the matrix method. Numerical non-linear optimization solutions are compared

with the analytic solutions and the reflectivities in the two cases are found to agree closely.

2. Analysis Approach

M. Sparks , H. E. Bennett and D. K. Burge , and Jeffrey B. Shellan" have found simple

but powerful techniques for calculating the optical properties of quarter-wave reflectors. The

approach by Shellan is the most general since it is valid for arbitrary angle of incidence.

The techniques will be reviewed and then extended to the case of a quarter-wave stack containing

one layer of arbitrary thickness. Expressions can be developed to include the effects of substrate, volume, and interface absorption, as was done in references 6, 7, and 8, but only substrate absorption will be considered here. For high quality coatings, especially ones deposited

with advanced deposition techniques such as ion beam sputtering, the substrate absorption will

usually dominate unless many layers are used. For the grazing angles of primary interest in

this paper, designs with 1, 2, or 3 layer pairs are probably sufficient.

Figure 3 illustrates the E field distribution and B field distribution for s and p polarized

light, respectively, within a quarter-wave stack. The quantities 0, H, L, and S designate the

superstrate with index nQ

, high-index layer with index n

H

, low-index layer of index ni , and

the substrate with index n

$

. The E field with incident amplitude E Q (normal to the plane of

incidence) is shown by the solid line and the amplitude attenuation factor S^.- was shown

to be given by

S„ - n/nj (1)

where n- = (n?-n^sin 2

<p) 1/2 (2)

<(> = angle of incidence from superstrate

The B field distribution, used to find the properties of p polarized light, is given by

the dashed curve in figure 3 and its attenuation factor P

n-j was shown to be equal to

331
The fields are sinusoidal in each layer, and the thickness of layer i in the quarter-wave

stack is

i. = X/(4 ni ) (4)

where A is the wavelength in free space.

From figure 3, the electric field penetrating into the substrate for the S polarization is

given by SqhSlH^ "^LS^o » where 2N is the total number of layers and E

n 0E Q

2cos2

<f>,

; 0H S LH

whereas

N-l

the

o

is the incident field. The incident flux is proportional to flux

penetrating into the substrate is proportional to n$ ( Sq^S^^ "^LS^o ) 2 *

cosc|>$, where <j> and are the angle of incidence and the propagation angle in the substrate,

respectively. The fraction of energy penetrating into the substrate, the absorption of the

substrate, for 2N layers and S-polarized light is

A

S (2N)

4S3 S 0S LH 2N

(5)

From the form of the B-field distribution for the P-polarized light in figure 3, the B field penetrating into the substrate is equal to P|_H^2B 0

. The flux is proportional to

(l/n)B 2

cos<j>, and the P-polarization substrate absorption for a coating with 2N layers is found

to be

A

p (2N) - 4P os p2|J (6)

If we consider the case of grazing incident angle mirrors (GIAMs) and neglect second order

terms in the grazing angle a ( a=Tr/2-ct>), then equations 5 and 6 can be rewritten as

A

S

(2N)

- 4 (S° ] 2N

S?r a LH- 'LS o

(7)

A (2N)

P

4 (P°

H

) 2N ,o n o\ 1 LS ~T (8)

where S?., n.°, P? . are equal to S.., n , and P.. evaluated at 4> = ir/2(a=0).

332
Equations 7 and 8 indicate that for a quarter-wave stack the s and p polarization are proportional to a and 1/a, respectively, for GIAMs. In deriving equations 5 and 6 it was assumed

that the absorption is small and thus equation 8 will breakdown as a approaches zero.

Up to this point we have implicitjly assumed that the substrate is a dielectric since we have taken n^, n-j, S^-, and P.,-.- to be real quantities. For a metal, these quantities are complex, but we can effectively make S-jj, and P^- real by covering the metal

substrate with a thin dielectric layer. The details of this are covered in Reference 6.

Although the s and p absorptions for bare metal can differ by almost two orders of magnitude

in the visible, the difference in the two absorptions is even more pronounced for MLDCs at grazing angles. The ratio of the MLDC absorptions can be 3 to 5 orders of magnitude at a~l°2°.

For this reason it is desirable to alter the quarter-wave stack slightly to improve the p polarization reflectivity. Expressions will now be derived for the s and p absorption for a modified

quarter wave stack.

Figure 4 illustrates the effects on the internal electric field of including one layer, the

n tn low index layer, of arbitrary thickness. The tangential component of the electric field

is fixed at zero at the substrate interface, for most high reflectivity coatings containing a metal substrate. This has been verified by exact computer runs based on the matrix method 9 ' 10

. The field distribution is the same as for a perfect quarter-wave stack between the substrate

interface and the perturbed layer.

The thickness of the n*"*

1 low index layer (n is measured from the superstrate) is ^ + A^n

^,

measured in phase units. The actual thickness of the perturbed layer is thus

4

n)

= (l+2A( n) /*)£1/4 (9)

where is the quarter wave thickness.

The perturbed layer has been expanded in the drawing for clarity, but its unperturbed width,

from the LH interface to the vertically dashed line (of phase width it/2), is the same width as

all the other unperturbed L layers. The distance b\}^> from the vertically dashed line

to the HL interface is the perturbation. If we start from the substrate where the penetrating

field has amplitude 2E

$

and work backward to the n !? L layer, the amplitude of the sine wave in the perturbed layer is ZE^S^S^^ -

n

(-l ) R-n+1 for S-polarized

1 ight.

The equation for the field in the ntn

L layer in a coordinate system whose origin is at the front surface of the layer (see fig. 4) is

D cos[x - a[")] = D cosa^cos x + D sinA^sin x, (10)

where

D = 2E

s

S

SL

S HL

N

- n (-l)

N

- n+1

(11)

333
The cos x and sin x field components of cos[x-A

L

( n

)] have been drawn in dashed lines

in fiqure 4. Equation (1) indicates that when the null of a sinusoidal wave is at an ij interface, the ratio of the wave amplitude on the i side to the j side is S--. On the other hand,

if the peak of the sinusoidal wave is at the interface, the amplitude is the same in the i and j

regions. Using this fact, we can easily find the amplitude of the sin x and cos x components at the mirror surface. At a y-coordinate system whose origin is at the mirror surface (see fig. 4),

the field is

E(y) = (-l)

n

D[S.

HL

n ^SH0GOS.4n, Sln y

l c n . A (n) i + S LH sinA^ cos yj.

(12)

This can be expressed as the sum of a forward-propagating wave and backward-propagating

wave

C[e" iy

+ (G-iF)/(G+iF)e iy

] (13)

where

[1 S

Sl" 1s

H0

C0S A

L

n)+

5 n (n)

(14)

and F and G are defined as the coefficients of sin y and cos y, respectively, in equation 12.

Since the incident intensity is equal to |C| 2

, the absorption is proportional to

|E

$ /C| 2

, with the constant of proportionality quickly found by comparing equation 5 with

j E J/C

|

2 for the case Ai = 0. The s polarization absorptjon for the case of quarter- wave stack where the n low index layer is perturbed by A|_' n ' is found to be

A

s (2N,A[n)

)

4 S .2N

OS \h

2~TF

COS A^ ' 1 \h 5 0H sin (nhZ \ ]

(15)

In a similar manner, an expression can be fpund for the s polarization absorption,

As (2N,Au( n )), a quarter-wave stack with the nth high index layer shifted by

Au' n ) phase units.

A

s (2N,A(

n)

) cos

4 S S 2N

^OS LH

2 (n) +

r s 2(n-l)

s sin A (n),2 (16)

OH

334
The corresponding expressions for p polarization are

4 P P 2N

V

2

^^ =

2 TnX J(n-l) p

< 17 > cos '+ [P^ ; P 0H

sin A^ '\

2N

Equation 8 indicated that the p polarization absorption varies as 1/a for small grazing

angles and quarter-wave stacks.. .Since Pqs as well .as Pqu vary as 1/a, equations 17 and

18 indicate that A

p (2N, A|_

n

' ) and A

p (2N,AH

^ n ^) will vary as a, not 1/a, for

sufficiently small a, as long as . sin A^ n ' * 0. Equations 15 through 18 also indicate that

the absorption is periodic in Av nv with period equal to tt (half wave thicknesses), as expected.

Equations 15 through 18 will now be used to find optimum GIAM reflectors.

3. Optimum Grazing Incidence Coatings

If we let A^ be the total substrate absorption due to both s and p absorption, and f

s be the fraction of s polarized light incident on the MLDC, then

A

t

=f

sAs (2N)+(l-fs )A

p

(2N) (19)

where equation 15 or 16 is used for A

$ (2N) and equation 17 or 18 is used for A (2N), depending on where a high or low index layer is perturbed.

If we differentiate equation 19 with respect to. sinA^ 11 ), we find two solutions for

the coating design giving lowest total absorption, A^.' mln '. The result is

sin 2 A

i

( n ) = 0 (20)

or

[fl-f ) P P 2N

( D

( 1n ) -1) 1 1/2- Ff S S

2N (l-s( in ) 111/2

. „

( n ) [[ s> 'OS LH ^ P0H

L)

> IVOS b

lH

[L S 0H

sin^A; '= (21)

(l-s

(in) HQ-f )P P 2N

fP (in) -l) l 1/2+fo (in)-l Wf S S 2N M J'")

2 U s ]]!^

qh T

s

,K0SKLH lK 0H

L) i tp0H

L} LV0Sb

LH U S 0H jJ

335
where

i = L or H, n = 1,2,...N

and

(Hn) .2(11-1).

S 0H - \H ^OH

(Hn) = p 2(n-l)

p

. . Pqh - LH

K OH

(Ln) = s 2n-l

s s OH " \H ^OH {C *>

(Ln)

p 2n-l

P

p (25)

0H - r LH ^OH

Equation 21 will qive the minimum absorption design if two conditions are met. Otherwise

equation 20, which defines the standard quarter-wave stack, will give the optimum coating. The

first of these two conditions is that

P^n)2

> 1 (26)

For the case of thickness adjustments to the outermost high-index layer, Poh^

1'^

= Pqh and the condition Pqh = 1 defines the Brewster angle discussed in the introduction.

The second condition that must hold for equation 21 to give the best coating is that the numerator be positive. This in effect defines a critical value for f

p

= (l-f

s

), the fraction of p polarized light.

If equations 15. through 19 and 21 are combined, the absorption can be found for the optimum

MLDC for GIAMs, A

t

( min )„ The result is

336
(min)

, <. <-2N VOS^LH

l/:

1-s

(in) : OH

• D

( in ) P 0H

p 2N

OS LH

1/2

-2N

J 0S°LH

(in) 2 i 1/2 ^ OH )]

4

n)2 -(in)

5 0H

(27)

i = L or H, n = 1, 2,

It can be shown that the best layer. to adjust, to achieve the lowest absorption is the outermost high index layer. Thus Sgu^ 1n ' and Poh should be replaced

with Sqh and Pq^ in equations 21 and 27.

Section 4. will, compare the predictions of equations 15, 16, 17, 18, and 27 with numerical

results. The A

t

^ m1n ' given in equation 27, which is based on a quarter-wave stack

with only the outer layer adjusted, will be shown to compare quite favorably with that obtained

using a numerical non-linear optimization procedure in which all layers can be adjusted.

4. Comparison Of Analytic And Exact Results

The accuracies of equations 15 through 19 can be verified by comparing them with numerical

results. The computer results were obtained by using 2x2 matrices to represent the effects of

dielectric discontinuities and propagation in a uniform dielectric layer. 9 ' 10 The relationship between left-and right-traveling light is established at each side of the dielectric discontinuities and at each end of uniform layers. The product of these matrices then gives a relationship between incident, reflected, and transmitted light for the entire system.

Figure 5 is a plot of A and A versus outer-layer thickness for the coating

Si (Al203/ZnS) y comparing the analytic results (equations 16 and 18) with exact results.

The curves are indistinguishable for A- and only differ for A

p

in regions of high absorption. All layers, other than the top ZriS layer, are quarter-wave thick at the incident angle of

88°. There is almost a four order of magnitude change in the s absorption and more than a two

order of maqnitude variation in Ap as A^p ' is varied from 0 to tt.

In order to assess the optimum MLDC design given in equation 21, the absorption given in

equation 27 was compared with that of a quarter-wave stack ("substrate compensated design") as well as that generated using a numerical non-linear optimization procedure.

A Ag (S^/Z^)^ "substrate compensated" quarter-wave stack at incident angle 89°

with one adjustable layer was used (X=1.06ym, nH =2.1, n

L =1.4, n$=0. 18+7 . 05 i

) . The absorption was calculated using equation 27 for N = 1, 2, 3, 4, 5 and n = 1, 2,...N for f

s = 0.990

to f

$

=0.999. It was found in all cases that varying the thickness of the top layer gave the

lowest absorptions and that the absorption was 6 to 100 times less than for the quarter-wave

reflector, depending on the number of layers used.

There are many techniques of non-linear optimization used in coating synthesis. 11 ' 12 ' 13 ' 11+

. The one first suggested by Dobrowolski was used with the merit function equal to the

total absorption, and compared with the analytic results.

Figure 6 compares the absorption for a quarter-wave design, the ideal coating found from

equations 21 and 27, and the numerically generated design. In all cases the absorption of the

analytic result is within 10% of that found using the non-linear optimziation computer program.

337
5. Summary and Conclusion

Equations were derived for the reflectivity of quarter-wave stacks containing one layer of

arbitrary thickness. The expressions were compared with exact numerical results and found to

aqree quite closely as long as the total absorption was small. These results were then used to

find the MLDC with highest reflectivity at angles of incidence greater than the Brewster angle, once the fraction of s and p polarized light was specified. The designs found can have absorptions at least an order of magnitude less than QW designs, even if only 0.1% of the incident

flux is p polarized. Good agreement was found between the absorptions of the analytically generated designs and that of coatings found using numerical methods.

References

[I] S. A. Mani and J. H. Hammond, "Optics and resonator design issues for high-power free

electron lasers," Proceedings of The International Conference on Lasers, New Orleans, La.,

December, 1981.

[2] C. K. Carniglia and Joseph H. Apfel, "Maximum reflectance of multilayer dielectric mirrors

in the presence of slight absorption," J. Opt. Soc. Am. 70_> 523-534 (1980).

[3] P. H. Lissberger, "The ultimate reflectance of multilayer dielectric mirrors, "Opt. Acta

25, 291-298 (1978).

[4] Joseph H. Apfel, "Optical coating design with reduced electric field intensity," Appl. Opt.

16, 1880-1885 (1977).

[5] W. H. Southwell, "Multilayer coatings producing 90° phase change," Appl. Opt. 18, 1875

(1979).

[6] Jeffrey B. Shell an, "Simple expressions for predicting substrate, volume, and interface

absorption and reflective phase shifts in high-reflectance quarter-wave stacks at oblique

angles of incidence," J. Opt. Soc. Am. 73, 1272-1281 (1983).

[7] M. Sparks, "A simple method for calculating the optical properties of multilayer-dielectric

reflectors," J. Opt. Soc. Am. 67, 1590-1594 (1977).

[8] H. E. Bennett and D. K. Burge, "Simple expressions for predicting the effect of volume and

interface absorption and of scattering in high-reflectance or anti -reflectance multilayer

coatings," J. Opt. Soc. Am. 70, 268-276 (1980).

[9] P. Yeh, A. Yariv, and Chi-Shain Hong, "Electromagnetic propagation in periodic stratified

media. I. General Theory," J. Opt. Soc. Am. 67, 423-438 (1977).

[10] M. Born and E. Wolf, Principles of Optics (MacMi 1 lan, New York, 1964).

[II] Arnold L. Bloom, "Refining and optimization in multilayers," Appl. Opt. 20, 66-73

(1981).

[12] William E. Case, "New systhesis method for optical thin-film coatings," Appl. Opt. 22^,

4111-4117 (1983).

[13] J. A. Dobrowolski, "Completely automatic synthesis of optical thin-film systems," Appl.

Opt. 4, 937-946 (1965).

[14] J. F. Tang and Q. Zheng, "Automatic design of optical thin-film systems - merit function

and numerical optimization method," J. Opt. Soc. Am. 72, 1522-1528 (1982).

338
szzzzzzzzzzzzzzzzz :

Phase shifts

at Interface

reflections

Figure 1. Phase shifts at each dielectric Interface

for s polarized light for all incident

angles and for p polarized light if

4> < p. where <J>g Is the Brewster angle.

(H and L designate the high and low index

layers, 0 represents a low Index superstrate,

and S Is a high index substrate.)

: S/////////////7777?

» 7ZZZZZZZZZZZZZZZZ

s Wm

Phase shifts

at Interface

reflections

Figure 2. Phase shifts at each dielectric interface

for p polarized light incident at <J> > <{>_.

339
Figure 3. E-f1eld distribution for S-polarlzed light and B-f1e1d distribution for P-polarlzed light 1n

a h1gh-reflect1v1ty quarter-wave stack.

Figure 4, Geometry and field distribution for calculating

the reflectivity of a quarter-wave stack with one layer of arbitrary thickness.

340
Figure 5. S and P polarization absorption as a function of top layer thickness as calculated

exactly and in the standing wave approximation for S1 (Al

203/ZnS) 9

. All layers, other

than tiie top one, are quarter-wave thick at the incident angle of 88*. (A=2.8um)

• 992 .99»t .996 .998 1.000 fraction of s polarized light

QW stack (substrate

compensated design

for p polarizatton)

Optimum design as found analytically

(top layer adjusted)

• Optimum destgn as found numerically

wtth computer synthesis

code

"Zr0„

SI0„

2.1

nAg

- 0.18+7.051

X » 1 .06 )im

Figure 6. Ag (S i

0

2 /Zr02

) absorption as a function of the amount

of s polarized light incident at 89°.

341
Manuscript Received

3-12-86 SAR

ANGULAR DEPENDENCE OF MULTILAYER-REFLECTOR DAMAGE THRESHOLDS*

Brian E. Newnam, Stephen R. Foltyn, Dennis H. Gill, and L. John Jolin

Los Alamos National Laboratory

Chemistry Division

Los Alamos, New Mexico 87545

The damage resistance of Hf02 /Si0 2 multilayer dielectric reflectors was measured as

a function of angle of incidence with 351-nm XeF-laser irradiation. The laser produced

nominal 10-ns pulses at a repetition rate of 35 pps . A series of reflectors designed for

0°, 30°, 45°, 60°, 75°, and 85° was tested with an S-plane polarized beam. To account

for variations in the separate coating depositions, some of the coating designs were tested at two angles of incidence. At large angles of incidence, we did not observe the

anticipated large increases in damage threshold predicted theoretically on the basis of

spatial dilution (l/cos6) of the intensity at the reflector surface and standing-wave

electric fields. For example, the threshold for a reflector designed and tested at 85° was only a factor of 2.5 larger than that of normal-incidence reflectors tested at 0°.

Several possible mechanisms to explain this discrepancy were considered.

Key words: coating defects; free-electron lasers; grazing-incidence reflection; hafnium

oxide, laser damage thresholds; multilayer reflectors; multiple-shot laser damage; silicon dioxide; standing-wave electric fields; thin films; ultraviolet reflectors; xenon

fluoride lasers.

1. Introduction

Free-electron laser (FEL) oscillators, driven by rf linear accelerators, are being designed

for high-average power applications in the ultraviolet to the near infrared from 400 to 1000 nm. The pulse format of these oscillators requires mirrors that will not damage or degrade under high

peak intensities and high repetition rates ranging from 10 7 to 10 8 Hz. To function without significant distortion, the resonator mirrors must not be subjected to excessive average-power loading.

Thus, very high mirror reflectance is required as well as low optical absorption to minimize the

heat flow into the water-cooled mirror substrates. Multilayer dielectric (MLD) reflectors are the

logical choice for the resonator mirrors since they have demonstrated the highest reflectance

values, e.g. ^99.99% for small-diameter, ion-beam deposited reflectors for 633-nm laser gyro

cavities

.

It is necessary to include FEL physics in establishing the resonator mirror design. Specifically, high intracavity intensity is required within the magnetic-undulator gain region to obtain

efficient conversion of the electron energy to coherent radiation. With undulator lengths ranging

from 1 to 10 meters, the optical beam should have both a small waist and low divergence i.e., a long Rayleigh range. FEL beams have exhibited the desirable characteristic of near-diffractionlimited quality [1]. As a result, very long distances are required between resonator end mirrors

to attain an acceptable intensity loading. However, if the mirror separation becomes too long, the

cavity becomes less stable and unavoidable mirror jitter can seriously decrease the overlap of the

optical and electron beams and thereby reduce the FEL efficiency.

Restriction of the FEL resonator length to minimize the effect of mirror jitter opposes the

need for large distances to allow diffraction to dilute the intensity on the mirrors. A solution

to this problem is to add a set of slightly curved intracavity mirrors oriented at large angles,

e.g. 85°, to diverge the optical beam to an acceptable intensity level on the end mirrors. Because

the FEL radiation is linearly polarized, these intracavity mirrors are oriented for S-polarized

reflection only. Theoretically, use at large angles should result in very high damage resistance

and less thermal distortion for a given beam intensity.

Since FEL oscillators are a relatively new development, we have given special attention to

their mirror needs in the above discussion. However, both excimer laser and FEL oscillator and

amplifier systems have the common additional requirement for beam directing mirrors used at non- normal incidence, and mirror damage data is urgently needed to permit realistic design of these

external systems.

*This work was performed under the auspices of the U.S. Department of Energy.

342
For metal mirrors, there have been a few measurements of laser damage resistance and optical

absorption at large incidence angles. These demonstrated the theoretically predicted [l/cos6] 2 threshold dependence for metals for S-polarization [2-4] . This quadratic dependence derives from

(1) the increased beam area on the surface as l/cos6, and (2) the angular dependence of S-plane

absorptance a (8) = a(0°)cos8. However, data regarding the angular dependence of MLD reflectors

are sparse ancf have been limited to incidence angles of 0° and 45°.

The present set of experiments was motivated to determine the angular dependence of multipleshot damage resistance of MLD reflectors for incidence angles from 0° to 85° and S-polarized beams.

Conducted at the low repetition rate of 35 pps, these tests did not address the issues of averagepower damage threshold or thermal distortion of mirrors both of which must be evaluated in further

test series with cw or long-pulse (>10 (Js) lasers.

Special consideration was given to the possible role of the standing-wave (SW) electric field

as a function of angle of incidence because of previous correlations with damage thresholds. From

multiple-shot tests using ~10-ns pulses at ultraviolet wavelengths of 248 nm [5], 308 nm [6], and

355 nm [7], we identified the peak SW electric field in the outermost high-index layer as setting

the threshold of damage. On this basis, our calculations of the SW fields led us to predict mono- tonically increasingly higher damage thresholds with angle of incidence, but only for S-polarized

laser beams. The results of these calculations for the specific reflectors tested in the present

experiments are shown in figure 1 for incidence angles of 0° and 85° for both S- and P-polarized

beams. The cosine intensity-dilution factor is inherently accounted for in the SW field calculations. The SW fields for angles less than 85° a^re intermediate to the curves shown. If optical

damage is correlated with the peak value of [E/E ] 2 in the first high-index layer, as would be the case with linear absorption, figure 1 indicates tRat we might expect the damage threshold at 85° to

be a factor of 102* larger than that at normal incidence.

2. Test Specimens

The test specimens were coated by Broomer Labs by electron-beam evaporation with ordinary

conditions as used in their commercial production. In addition to the general features listed in

table 1, particular details of the coating depositions included the following: 300°C substrate

temperature, deposition rate of 5 min/QW, 8 x 10 -5 Torr background pressure with 02 bleed, and 5 x

10 6 Torr initial vacuum. The reflectors were not post baked. For purposes of computing the film

thicknesses, Broomer used the refractive indices of 1.51 for Si0 2 and 2.08 for Hf02 , which are typical values at 351 nm.

Because the magnitudes of the SW fields can be strongly affected by film thickness errors, we evaluated the spectral transmittance curves from spectrometer measurements at normal incidence.

Comparisons of the design and measured center wavelengths are provided in table 2. We think that

the measured thickness discrepancies should not have influenced the damage thresholds to any great

degree

.

AIR L H L H L H L

OPTICAL THICKNESS

Figure 1. Standing-wave electric-field distributions in

+ Hf02 /Si0 2 reflectors designed for 0°- and

85°-incidence . Fields are plotted normalized to E , the incident electric field in air. At

large angles such as 85°, the internal fields for %-polarized radiation are much lower than

for normal incidence; modest advantage is attained with P-polarization. For angles smaller

than 85°, intermediate field distributions are calculated.

343
Table 1. Test samples

Coating materials: Hf02 and Si02 Design: S/(HL) 1X HL2 /A for S-polarization

Deposition process: Electron gun

Coating vendor: Broomer Labs

Substrates

:

Fused silica (5.1-

BK-7 glass (6.4 x

•cm diam)

12.7 cm)

Table 2. Coating thickness errors

Design Angle

Center Wavelength for Normal Incidence

Design (run) Measured (nm)" Deviation^

0° 351 354 + 1%

30° 367 384 +5%

45° 386 403 +4.5%

60° 407 405 -0.5%

75° 426 420 -1.5%

85° 434 425 -2%

Wavelength corresponding to the center frequency midway between 1%

transmittance points of the reflection band.

For 0° incidence on a reflector designed for 0°, a 3% thickness error

results in ^1% increase in the SW electric-field peaks. For 85° incidence

(S-polarized light) on a reflector designed for 85°, a 10% thickness error results in a 25% increase of the field peak in the uppermost high-index

layer and a 20% increase in the uppermost (HW overcoat) low-index layer.

3. Laser Damage Test Conditions

The laser damage test facility depicted in figure 2 and our standard multiple-shot measurement

procedures have been described previously [5,6]. One modification, not shown, was inclusion of

multilayer polarizers which we aligned very carefully to obtain a polarization purity of I /I

^ 1000 on the samples. In addition, the beam dimensions in the sample plane, which varied Swi?h

incidence angle, were measured directly with a Reticon silicon-diode linear array. As shown in

table 4, the measured diameters of the near-Gaussian beam did not deviate significantly from the

predictions. Naturally, the measured beam sizes were used in calculations of the damage

thresholds

.

VIDEO CAMERA (B&W)

ENERGY DETECTOR

Figure 2. Schematic of the Los Alamos excimer-laser , multiple-shot, laser damage test facility.

344
Table 3. Laser test parameters

Wavelength:

Polarization:

Pulsewidth:

2 Spot-size diameter (1/e ):

Repetition rate:

Shots per site:

351 nm (XeF)

S-plane with purity I s

/ I p

= 1000

10 ns (FWHM)

0.4 mm, mean value normal to the beam

0.4 mm/cos0, mean value on sample

35 pps

140 if no damage detected, or S140 if damage

Table 4. Angular dependence of the laser beam diameter (vertical plane) at the

reflector surface

Angle of Predicted Diameter Measured Diameter Deviation

Incidence 2w

o /cos8, (mm) (mm) from l/cos0

0° 0.128 0.128

30° 0.148 0.142 -4%

45° 0.181 0.173 -4%

60° 0.256 0.235 -8%

75° 0.495 0.470 -5%

85° 1.47

4. Experimental Results

Preliminary to damage tests over the entire range of angles of incidence, the possible threshold variations within a given coating run were determined for the 0° and 30° designs. These threshold values, given in table 5, indicate a range of ±10 to ±15%. Also, the 4 to 5 J/cm2 magnitude

was about the same as our previous measurements of other reflectors composed of these same coating

materials. Since each angle of incidence required a separate coating deposition, we attempted to

account for run-to-run variations by testing some of the reflectors at two angles of incidence. We

intended to obtain this two-angle registration for each coating design, but only 0° and 30° designs

were so evaluated due to decreased coating target area available at the larger angles. For these

two designs, the variations were within 10% as shown in table 6. Predicted differences on the

basis of the peak SW electric fields also were small.

Table 5. Comparison of damage thresholds of Hf02 /Si02 reflectors

fabricated in the same coating run

Design and Sample Damage Threshold

Test Angle Identification J/cm2

0° 0-1 4.1

0-2 4.7

0-3 5.2

30° (S-polar) 30-1 4.0

30-2 4.5

The results as a function of the full range of incidence angles are listed in table 7, and the

supporting data from which the threshold values were obtained are presented in the series of fig- ures 3. This extensive display of test data is given because of (1) the significance that might be

given these results in optical designs of FEL resonators and other beam directing elements, and (2)

to allow the reader the opportunity for a thorough examination of these results with the possibility that other interpretations might become evident. We note that the available laser energy was insufficient to reach the saturation fluence (damage at 10 out of 10 test sites) for the 85° case. Figure 4 allows direct comparison of the experimental results for all of the angles of incidence

used

.

345
Table 6. Comparison of damage thresholds for Hf02 /Si02 reflectors

tested at two angles of incidence.

Sample Design Test Damage Threshold Peak SW

Number Angle Angle J/ cm2 Electric Field

0-1 0° 0° 4.1 0.95

30° 4.5 0.89

30-1 30° 30° 4.5 0.85

45° 4.6 0.76

Calculated in the Hf02 films (for reflectors centered at 351 nm)

and normalized to the S-polarized r incident field E . o

Table 7. Damage thresholds versus angle of incidence for Hf02 /Si02

reflectors tested at the design angle.

Design and Damage Threshold Peak SW

Test Angle J/cm2 Electric Field

0° 4 6 + 0 5 0 95

30° 4 3 + 0 4 0 85

45° 5 2 0 72

60° 6 6 0 52

75° 6 9 0 28

85° 11 5 + 0 5 0 094

Calculated in the Hf02 films (for reflectors centered at 351 nm)

and normalized to the S-polarized incident field E . r o

We observed that the damage morphology at high angles of incidence differed from that for

near-normal incidence. At the large angles, the damage sites had an elongated triangular pattern,

different from the elliptical beam footprint, with the long dimension aligned along the direction

of the incident beam.

PEAK FLUENCE (J/cm2

) PEAK FLUENCE, (J/cm 2

)

Figure 3a. Figure 3b.

346
PEAK FLUENCE, (J/cm 2 ) PEAK FLUENCE (J/cm2

)

Figure 3e Figure 3f

Figures 3a-f. Multiple-shot laser damage test results for 351-nm Hf02 /Si0 2 multilayer reflectors

as a function of angle of incidence with S-polarization.

347
0 4 6 8 10 12 14 16 18 20

PEAK FLUENCE (J/cm2

)

Figure 4. Angle dependence of multiple-shot damage thresholds summarized from test results given in

figure 3.

5. Discussion

It is informative to compare the angular dependence of the damage thresholds against the predictions of several possible physical models. One obvious model is that the threshold is determined by the laser fluence incident on the outer surface of the reflector. Because the beam footprint becomes elongated as l/cos6, the fluence at the surface is diluted by the same l/cos6 factor.

Therefore, in terms of the laser fluence measured normal to the incident beam, which is the value

cited when measuring damage thresholds, this model would predict that the threshold should increase

as l/cos0. The second and third models predict that damage resistance is inversely proportional to some power of the peak SW electric fields in the high-index layers. In one case, the fields were computed for the case of a 100% S-plane polarized incident beam. In the other, the fields were computed as if there were a 100% conversion of the beam to P-polarization.

In figure 5, the angular dependence of the measured damage thresholds, normalized to the average result for 0° incidence, is compared to the predictions of the three models considered. Clearly, at large angles the measured damage thresholds fell far below the model predictions for both

the l/cos0 dilution and the S-polarized peak field-squared which assumes linear absorption. For

85° incidence, for example, the damage threshold was only 2.5 times larger than at normal incidence. However, the P-polarized peak-field-squared model underestimated the thresholds. It is

worth citing a similar result obtained at the U.K. Rutherford Appleton Laboratories for reflectors

designed for S-polarized laser radiation at 248 nm using ThF 4 /cryolite films [8]. In that case,

the single-shot (15-20 ns pulses) threshold of ~15 J/cm2 at 86° exceeded that for 0° incidence,

~3 J/cm2

, by only a factor of five.

Unfortunately, we have insufficient data by which to uniquely determine the cause of the un- expectedly low damage thresholds at angles of incidence greater than 30°. We have, however, speculated on a number of possible mechanisms. We list the most plausible ones for further consideration.

1. Larger surface areas exposed at large angles result in more coating defects being irradiated,

thereby increasing the probability of damage.

2. Reflectors designed for large angles of incidence are composed of thicker films which have

lower thresholds (more defects).

3. Nonuniform SW electric fields may occur near coating defects, and the near-field diffraction

field maxima directly behind opaque coating defects could be very large.

4. Scattered light trapped within the layers by total internal reflection could either increase

the effective film absorption or b« <-hannpled to absorbing coating defects. See figure 6.

348
13 - O

X

CO

iLl ocXT

uj2 O UJ Si3CD

_l —

U. UJ

11

>

6

Ul

a:

7 -

5 -

[Z 3

T

S- POLARIZATION TESTS

Hf0 2 /Si02 REFLECTORS

351-nm, 10-ns PULSES

AT 35 pps

MODELS

1/CO8 0

[ea m/^o')]

pl

[£p(fl>/£

p<0-)]

1

10° 30° 50° 70°

ANGLE OF INCIDENCE

90°

Figure 5. Measured damage thresholds normalized to the average result for normal incidence are compared to the predictions of three models: (1) l/cos0 dilution of the fluence at the reflector surface, (2) inverse of the normalized peak electric-field-squared in the top Hf02 layer

for S-polarized light, and (3) for P-polarization.

Figure 6. One physical model that may explain the anomalously low damage thresholds at large

incidence angles: Scattered light, trapped in the layers by total internal reflection,

could either sufficiently raise the effective absorption or channel radiation to absorbing defects.

349
5. Pinholes in the coatings could act like light tunnels. At large angles, incident radiation

could be funneled into the interior more or less independent of the angle of incidence.

6. The angular dependence of damage may have been only a measure of film defects at the air-film

interface. Protruding defects could have interacted with the SW electric field maxima in air,

2E

q

, which are much larger than those in the interior of the multilayer [9]. With increasing

angle, the location of the SW peaks moves away from the air-film interface.

7. The polarization purity of the incident beam (Ig/Ip 2:1000) might have been degraded upon scattering within the multilayers or rotated by coating birefringence. Depolarization of scattered light from multilayers has been observed previously [10]. (Almost total depolarization

(1^/1 = 1) of the entire beam would have been necessary to have produced the unexpectedly low

thresholds .

)

8. The coating depositions for the 60°-, 75°-, and 85°-reflectors may have produced more absorbing films.

Although we have no supporting evidence to absolutely distinguish between the above speculations, we suspect that items 1 to 5 are more plausible than items 6 to 8. Further clarification

will require auxiliary tests, e.g., calorimetric measurements of absorption which could detect any

unexpected (spatially averaged) increases in reflector absorption at large angles due, for example,

to trapped scattered light. If film scatter is responsible for restricted thresholds at large

angles, reflectors produced by coating deposition processes that produce films with less scatter

should be tested. These processes include rf sputtering, ion-beam sputtering, and ion-assisted

electron-beam deposition. According to S. Lu, however, the measured absorption of ion-beamdeposited reflectors for an S-polarized 633-nm beam was the same at 0° and 45° [11].

The experimental data as shown in figures 3 and 4 exhibited a second feature that is not

understood. For angles of incidence from 0° to 45°, the slopes (% damage/energy fluence) increased

as expected, since the spot area was increasing and more defects were irradiated. As first re- vealed by Foltyn [12], increased irradiation area should result in steeper slopes, but the threshold (0/10 sites damage) should remain essentially the same. However, for incidence angles from 45°

to 85°, the present experimental data produced slopes which declined with increasing angle. There

is an opportunity for some clever detective work to explain this.

6. Conclusions

The damage resistance of HfG^/SiC^ multilayer dielectric reflectors was measured as a function

of angle of incidence from 0° to 85° using S-polarized, 10-ns, 351-nm XeF laser pulses at 35 pps.

At large angles of incidence, we did not observe the anticipated large increases in damage threshold predicted theoretically on the basis of either spatial dilution (l/cos8) of the intensity at

the reflector surface or SW electric fields. For example, the threshold for a reflector designed

and tested at 85° was only a factor of 2.5 larger than that of normal-incidence reflectors tested

at 0°. The absence of correlation with the peak SW fields is contrary to previous experience with

non-quarter-wave reflector designs at UV wavelengths. We considered several possible mechanisms to

explain this discrepancy, but further testing is necessary to prove whether any of these are responsible

.

7. References

[1] Newnam, B. E. Capabilities of broadly tunable free-electron lasers. New Lasers for Ana - lytical and Industrial Chemistry , Proc. SPIE 461; 1984. 90-97.

[2] Goldstein, I.; Bua, D. ; Horrigan, F. A. Pulsed laser damage to uncoated metallic reflectors.

Nat. Bur. Stand. (U. S.) Spec. Publ. 435; 1976. 41-48.

[3] Figueira, J. F. ; Thomas, S. J.; Harrison, R. F. Damage thresholds to metal mirrors by shortpulse C0 2 laser radiation. Nat. Bur. Stand. (U.S.) Spec. Publ. 638; 1983. 229-238; also,

IEEE J. Quantum Electron. QE-18 , 1381-1386 (1982).

[4] Decker, D. L.; Porteus , J. 0. Laser damage to metal mirrors at nonnormal incidence. ibid.

239-245.

[5] Newnam, B. E. ; Foltyn, S. R. ; Jolin, L. J.; Carniglia, C. K. Multiple-shot ultraviolet laser

damage resistance of nonquarterwave reflector designs for 248 nm. Nat. Bur. Stand. (U.S.)

Spec. Publ. 638; 1983. 363-371.

350
[6] Foltyn, S. R. ; Jolin, L. J.; Newnam, B. E. Progress in ultraviolet damage testing at Los

Alamos. Nat. Bur. Stand. (U.S.) Spec. Publ. 669; 1984. 266-273.

[7] Carniglia, C. K. ; Hart, T. T. ; Rainer, F. ; Staggs, M. C. Recent damage results on high re- flector coatings at 355 nm. Nat. Bur. Stand. (U.S.) Spec. Publ. 688; 1985. 347-353.

[8] ft, Lunt, Technical Optics, Ltd., private communication on damage test results of their re- flectors as reported to him by F. O'Neill of Rutherford Appleton Laboratory, United Kingdom.

[9] P. A. Temple, Naval Weapons Laboratory, private communication; also Total internal reflection

microscopy: a surface inspection technique. Appl. Opt. 20, 2656 (1981).

[10] J. 0. Porteus, Naval Weapons Laboratory, private communication.

[11] S. Lu, Litton Industries, private communication.

[12] Foltyn, S. R. Spotsize effects in laser damage testing. Nat. Bur. Stand. (U.S.) Spec. Publ.

669; 1984. 368-379.

351
Manuscript Beeeived

7-24-84 SAM

SURVEY OF LASER DAMAGE THRESHOLDS FOR HIGH REFLECTOR

FILMS AT 1.315 MICRONS

Terrence F. Deaton

Frank J. Seiler Research Laboratory

United States Air Force Academy

Colorado Springs, CO 80840-6528

ABSTRACT

We present the results of a survey of laser-induced damage measurements for

high-reflector films at 1.315 microns. A pulsed atomic iodine laser with a maximum

output energy of 5 Joules in a pulsewidth of approximately 8 microseconds was used, with

a spot diameter at the sample of about 2.8 mm. All the thresholds reported are for

single shot per site illumination. The pulse energy, temporal profile, and spatial

profile information were monitored for each shot.

The samples tested are part of an extensive survey of currently available coatings

from several vendors and represent a wide variety of materials and designs. Each coating

was prepared on both silicon and molydbenum substrates. Damage thresholds were defined

by the presence of surface pitting visible under Momarski microscopy. Threshold fluence

levels ranged from 10 J/cm^ to 150 J/cm^ with uncertainties of typically + 15%.

1. Introduction

Recent interest in the applicability of high energy chemically driven atomic iodine lasers for

weapons applications has led to a survey of the current state of the art of optical coatings for

the laser wavelength of 1315 nm (ref. 1). This survey was performed by the Air Force Weapons

Laboratory and encompasses a wide variety of materials and designs. In this paper I will present

damage data for several high reflector coatings from this survey.

2. Laser Damage Experiment

The laser source used for this work is a pulsed photodissociation atomic iodine laser

currently on loan to the Frank J. Seiler Research Laboratory from Sandia National Laboratories in

Albuquerque. It has been significantly modified from its original configuration at Sandia (refs.

2,3) by the conversion of one of the original preamplifier stages into a long-pulse oscillator.

The laser head is a quartz tube of 22 mm ID and 1.22 meters length. The tube is filled to a pressure of approximately 50 Torr with n-heptafluoropropyl iodide (C3F7I) with no broadening

gas being used. The gas tube is surrounded by four linear high-pressure Xenon flash! amps

connected in two parallel circuits. Each pair of lamps is driven by a capacitance of 4 Mfd

charged to a maximum voltage of 38 KV. In the experiment the output energy of the laser is

controlled by varying the voltage and/or the fill pressure.

The laser is operated as a single-shot device, with the gas tube evacuated after each shot,

and in a gain-switched mode, with the pulse length determined by the flashlamp pulse-forming

network. For this work the pulse length (FWHM) is approximately 8 microseconds. A sample

temporal profile obtained with an ITT vacuum photodiode is shown in figure 1. The optical

resonator consists of a total reflector with 20 m radius of curvature and a flat, 30% reflective

output coupler. This geometry results in highly multimode beam, which is ultimately focussed to a waist diameter many times the diffraction limit. The spatial profile at the waist proved to be a relatively smoothly varying "blur," however.

The optical layout for the measurements is shown in figure 2. The output beam from the laser

is focussed onto the sample with lens of focal length 50 cm. A fused silica beamsplitter after

the lens diverts some of the energy to a photodiode for temporal profile measurements and to an aperture plate for spatial profile characterization. The spatial profile of the beam at the

sample was determined by measuring the transmission through a series of apertures ranging in size

352
0 4 8 12 16

TIME (psec)

Figure 1. Temporal profile of atomic iodine laser

CALORIMETER

INPUT

APERTURE

MIRROR

SAMPLE

CALORIMETER

Figure 2. Optical layout for laser damage experiment

down to 0.25 mm diameter. The transmission is plotted against aperture diameter in figure 3. The

solid line in the figure represents the expected results for a gaussian profile with a 1/e?

diameter of 2.8 mm, and it provides a reasonably good fit to the actual transmission data.

3. Results and Discussion

All the thresholds reported are for single shot per site illumination. Typically five shots

at a given fluence were done. The occurrence of damage was determined by either white light light

scattering or Nomarski microscopy. The microscopy was necessarily performed after a series of

shots and thus was employed only for those sites which were apparently near the threshold level.

The reported threshold is the average of the lowest fluence at which damage always occurred and

the highest at which damage never occurred in the five-shot sequence. The uncertainty values

reported in the table approximate the range between those two levels plus the uncertainty of the

peak fluence for a given shot as described above.

353
0

0 0.5 1.0 1.5

APERTURE RADIUS, mm

Figure 3. Aperture transmission for bean, taken at waist produced by lens of 50 cm focal length

The samples tested are all high-reflector film designs prepared by various vendors on substrates provided by the Air Force Weapons Laboratory, and they represent a small portion of an extensive industry-wide survey effort undertaken a few years ago. Each coating was prepared on both molybdenum and silicon substrates. The substrates as well as the finished samples were characterized at the University of Dayton Research Institute facility at AFWL (ref. 1). The

fractional power absorbed at a wavelength of 1.319 microns was determined for each mirror in the

set, and the values ranged from a low of 1.7 x 10~4 for the OCLI (Si02/Ti02)^ coating on silicon to a high of 4.4 x 10~3 for a (PbF^/ZnSe) 4 coating prepared by Laser Power Optics on

a molybdenum substrate. The rms roughness of the surfaces v/ere determined by a total integrated

scattering measurement at 633 nm both before and after coating (ref. 4). The silicon substrates

were determined to have an rms roughness of approximately 10 Angstroms, with the coated mirrors

having roughness values ranging up to 50 Angstroms. The molybdenum substrates typically had an

rms roughness of approximately 35 Angstroms, with the coating rms roughness varying 40 and 70

Angstroms (ref. 1). The complete tabulation of data is not reproduced in this report.

As shown in table 1, the observed damage thresholds for this set vary over quite a wide range,

with the titania/sil ica coatings typically at the highest values and the various lead fluoride

designs, the lowest. The films prepared on silicon substrates systematically showed higher

thresholds than those on molybdenum substrates, although the difference is most cases slight. In

virtually all cases the threshold level damage consisted of a few randomly distributed micro-pits

apparently originating in the first few layers of the coating.

4. Acknowledgements

I would like to thank Captain Mickey MacLin and Lieutenant Patricia Morse of the Air Force

Weapons Laboratory for supplying the samples as well as financial support for this work. I would

also like to thank Mr. Lee Burton for his assistance with the measurements.

354
Table 1. Iodine Laser Damage Survey

VENDOR

Laser Power Optics

Laser Power Optics

Laser Power Optics

Laser Power Optics

Laser Pov/er Optics

Laser Power Optics

Laser Pov/er Optics

Laser Power Optics

OCLI

OCLI

OCLI

OCLI

Spectra Physics

Spectra Physics

Spectra Physics

Spectra Physics

Northrop

Northrop

Northrop

Northrop

Litton

Litton

Litton

Litton

Coherent

Coherent

Coherent

Coherent

COATING DESIGN

(PbF

2

/ZnSr

(PbF

2 /ZnS)

4

(ThF

4/ZnS)

4

(ThF

4 /ZnS)

4

(PbF

2 /ZnSe)

4

(PbF

2

/ZnSe)

4

(ThF

4 /ZnSe)

4

(ThF

4 /ZnSe)

4

(Si0

2

/Ti0

2

) 8

(Si0

2

/Ti0

2

) 8

(ZnS/ThF

4

) 8

(ZnS/ThF

4

) 8

(Zr0

2

/Si0

2 )28 layers

(Zr0

2 /Si02 )28 layers

(Ti0

2

/Si0

2 )16 layers

(Ti0

2

/Si0

2 )16 layers

(ThF

4 /ZnS)

4

(ThF

4/ZnS)

4

(Al

2

0

3

/Hf0

2

) 5

(Al

2

0

3

/Hf0

2

) 5

A1(LH) 7 LL Run #2

A1(LH)

7 LL Run #2

Ti(LH) 10LL Run #1

Ti(LH)

10

LL Run #1

(G1777)

(G1777)

(G1752)

(G1752)

SUBSTRATE THRESHOLD

(J /cm2

)

Molybdenum 27 7/

Si 1 icon 1 71 / 0

Molybdenum 74 + 1 f)

Sil icon oD 4_T 1 0

Molybdenum C\> + c

Sil icon 1 11 1 4. )0

Molybdenum q + c.

Sil icon 1 0

4. 0J Molybdenum 1 f!

1 o

4. 50

Sil icon i t 7/ Molybdenum 117 4^ CO

Sil icon Of I oo Molybdenum 47 + 1

0

Sil icon l T QO

Molybdenum 117 + 1 71 /

Sil icon Oc c

Molybdenum 92 + 14

Sil icon 70 1 0

Molybdenum 35 + 7

Sil icon 35 + 5

Molybdenum 117 + 17

Sil icon 80 + 12

Molybdenum 50 + 8

Silicon 50 + 8

Molybdenum 95 + 15

Sil icon 144 + 20

Molybdenum 40 + 8

Sil icon 44 + 6

5. References

1. Detrio, J. A.; Fernelius, N. C; Harris, R. J.; Walsh, D. A. Optical Characterization of IR

Transmitting and Laser Window Materials. Semiannual Report No. 2, UDR-TR-81 -76, University of

Dayton Research Institute; Sep 1981.

2. Fisk, G. A.; Gusinow, M. A.; Hays, A. K.; Padrick, T. D.; Palmer, R. E.; Rice, J. K.; Riley,

M. E.; Truby, F. K. The Atomic Iodine Laser. Technical Report SAND78-1071, Sandia National

Laboratories; May 1978.

3. Jones, E. D.; Palmer, M. A.; Franklin, F. R. Subnanosecond high-pressure Iodine

Photodissociation Laser Oscillator. Opt. Quant. Electron. 8:231; 1976.

4. Detrio, J. A. Proceedings SPIE, 276:136-9; 1981.

355
Manuscript Received

5-16-84 SAB

Scanning Electron Microscopy Studies of Laser Damage Initiating

Defects in ZnSe/ThF^ and SiH/Si0 2 Multilayer Coatings*

L. F. Johnson, E. J. Ashley, T. M. Donovan, J. B. Franck

R. W. Woolever, and R. Z. Dalbey

Michelson Laboratory, Physics Division

Naval Weapons Center, China Lake, California 93555

Scanning electron microscopy (SEM) was used to identify four distinct laser damage

morphologies in ZnSe/ThF^ multilayer mirrors. There were three types of defect-initiated

damage morphologies. Oblong-shaped damage sites oriented perpendicular to the electric

field vector of the laser were associated with particulates on or near the surface of the

ZnSe/ThF^ multilayers. Circular-shaped damage sites were initiated by particulates embedded

beneath the top ZnSe layer. Selective laser damage at pinholes was identified as the third

defect-initiated damage morphology. In addition to defect-initiated damage, stress-related

damage was indicated by cracks near or within laser damage craters and erosion sites.

Selective laser damage at nodular growth defects in SiH/Si0 2 multilayers was also observed

using SEM. Samples with different numbers of nodules were prepared in-house using RF-diode,

reactive sputtering. The low -defect mirror had the highest laser damage onset, and the

mirror with the highest number of nodules had the lowest laser damage onset.

Key words: defect-initiated laser damage; laser damage; nodular growth defects; optical

thin films; particulate impurities; pinholes; scanning Auger microscopy; scanning electron

microscopy; SiH/Si0 2 multilayer mirrors; ZnSe/ThF^ multilayer mirrors.

1. Introduction

One of the first steps toward the fabrication of laser-damage-resistant optical coatings is to

identify the defects which initiate the damage. In addition, film properties which contribute to the

damage susceptibility can be inferred from the laser damage morphologies. One unwanted property which

can lead to inferior damage resistance is a high level of stress. Cracks in the coating near or within damage craters or within erosion patterns are an indication that high levels of stress existed

in the film at those sites before laser damage. In all of the laser damage sites shown here, the

laser was operated just above the fluence at which damage begins to occur. In other words, the laser

was operated just above the onset of damage for each of the multilayers. When the laser is operated

just above onset, selective sites associated with defects and sites associated with high levels of

stress begin to fail. These sites show nonuniform, irregular damage morphologies. By performing

careful beam scans, the spatial profiles of the lasers used in this study were determined to be

Gaussian. The irregular, nonuniform shapes of the damage sites are the result of defects in the

coatings, not irregularities in the spatial profiles of the laser beams.

Scanning electron microscopy (SEM) has been used to identify five laser damage morphologies in

two different optical coatings. Summaries of each of the five damage morphologies are outlined here

as an introduction.

1.1 (ZnSe/ThF^) 5 Multilayers

Orientation effects : Oblong-shaped, selective damage sites inside a larger erosion pattern. The

oblong sites are oriented perpendicular to the electric field vector of the laser and are initiated by

particulates at the film surface.

Embedded particulates : Selective erosion around particulate impurities located under the outer

ZnSe layer.

Pinhole defects : Selective laser damage initiated at pinhole defects.

*Work supported by Navy Independent Research Funding.

356
Stress and thermal effects : Selective crater formation inside a larger erosion pattern. The

craters exhibit stress-related cracking. Temperatures high enough to melt gold (1064°C) illustrate

the extreme thermal conditions that the multilayer experiences during laser damage.

1.2 (SiH/Si0 2 ) 5 Multilayers

Nodular growth defects : Selective laser damage at nodular growth defects.

2. (ZnSe/ThF 4 ) 5 Multilayers

2.1 Fabrication of the ZnSe/ThF 4 Multilayer Mirrors

The ZnSe/ThF^ multilayer mirrors are experimental coatings and were provided by Perkin-Elmer

.

The multilayers were evaporated onto molybdenum substrates overcoated with gold. The vapor stream was electrostatically filtered during the deposition in two of the three mirrors studied. Filtering the

vapor stream reduced the number of particulates in the coatings.

2.2 Experimental Parameters for Laser Damage Studies

The ZnSe/ThF^ multilayers were tuned for enhanced reflectivity at 3.8 ym. The HF/DF laser

operating in the DF mode (3.8 ym) did not produce sufficiently high fluences to consistently damage

each sample. For this reason, laser damage morphologies were produced using both the DF and the

stronger HF (2.7 ym) lasing modes. The HF/DF laser had a 1/e 2 spot size of 150 ym in almost all of

these experiments. For one of the damage sites, the 1/e 2 spot size was 500 ym. The pulse length is

nominally 1 ys.

2.3 Orientation Effects

The most dramatic damage morphology in the ZnSe/ThF 4 films is the selective, oblong damage sites

oriented perpendicular to the electric field of the laser. The oblong damage sites occur within a larger erosion pattern which is somewhat larger than the 1/e 2 laser spot size of 150 ym. Figure 1 shows the results of rotating the sample in the laser beam. In all three orientations, the oblong

sites are oriented perpendicular to the electric field of the laser.

The occurrence of these oblong sites was modulated by the density of particulates on the surfaces

of the mirrors. In areas where a high density of particulates existed, this damage morphology

frequently occurred. In areas where no particulates existed within the damage spot, this orientation

effect did not occur.

The undamaged surfaces of three ZnSe/ThF^ mirrors were examined using SEM (fig. 2). For samples

A and B, the vapor stream was filtered during the coating process. Consequently, a low number of

particulates appeared on the surfaces of samples A and B; sample C, unfiltered, produced many more particulates.

Fewer oblong damage sites were obtained on samples A and B than on sample C. On sample A, oblong

sites were obtained only when two or more particulates were located within the laser spot. No oblong

damage sites were obtained in particulate-f ree areas of sample A.

Different damage sites exhibit varying numbers of oblong sites within the larger erosion pattern

(fig. 3). Again, the number of oblong craters that occur within a single erosion pattern may be

modulated by the density of particulates.

Scanning Auger maps of the first damage site in figure 3 are shown in figure 4. The elongated

shape of the erosion pattern is due to the tilt of the sample in the microscope. Light areas indicate

regions of higher counts. The zinc and selenium maps show that the entire first layer of ZnSe was removed during the damage pulse. The thorium and fluorine maps confirm that the laser has eroded the

top layer of ZnSe and exposed the underlying ThF^ layer.

The oxygen map indicates that oxygen is associated with the ThF^ layer. Knowing that ThF^ is

very hygroscopic and considering the oxygen map, the presence of water in the ThF^ layer may be

deduced.

The depth of the oblong crater is revealed in close-up scanning electron micrographs (fig. 5).

The cratering has exposed at least an additional six layers (or three pair) of the ZnSe/ThF 4 stack.

The oblong-shaped craters may be due to an interference effect between the incident laser beam

and the scattered waves from the surface particulates. All of the sites shown here were obtained with

the incident laser beam at normal incidence. If the orientation effect is an interference phenomenon,

357
the shape of the craters should change as the angle of incidence of the laser beam is changed [1].

Similar orientation or ripple effects have been seen in bulk silicon [2-5] and other semiconductors

[6].

2.4 Embedded Particulates

Selective erosion at embedded particulates is shown in the scanning electron micrographs on the

left in figure 6. The laser spot for the damage site had a 1/e 2 diameter of 500 ym. A micrograph of an embedded particulate appears on the right. An Auger depth profile, not shown here, indicates that

the particulate is embedded beneath the ZnSe layer and is composed of calcium, carbon, and oxygen.

This combination suggests that it is calcium carbonate (CaC0 3 ). The bulk ThF^ may have been stored

with a desiccant containing calcium carbonate.

2.5 Pinhole Defects

Pinhole defects were found to initiate selective laser damage (fig. 7). Figure 8 shows a pinhole

before irradiation and another pinhole after laser damage. Pinholes are growth defects often associated with adherence problems. The melt pattern seen in figure 8 is caused by the poor thermal contact

of the jagged pinhole edges.

2.6 Stress and Thermal Effects

The preceding laser damage morphologies dealt with defect-initiated damage. The fourth type of

laser damage morphology in these ZnSe/ThF^ multilayers shows effects of a high level of stress in the

films. Figure 9 shows selective crater formation within a larger erosion pattern. The cracking at

the edges of the crater indicates that the film was highly stressed at this site. The irregular shape

of the erosion pattern also suggests a high level of stress.

An Auger line scan across the crater mid-line indicates that the 3-ym-size particle at the center

of the crater is gold (fig. 10). A probable explanation is that the underlying gold layer melted and

flowed up through the cracks in the ZnSe/ThF^ stack. To melt the gold, the multilayer temperature had

to reach 1064°C, illustrating the intense thermal conditions produced during the laser damage pulse.

3. (SiH/Si0 2 ) 5 Multilayers

3.1 Fabrication of the SiH/Si0 2 Multilayer Mirrors

The SiH/Si0 2 multilayers are in-house coatings made at the Naval Weapons Center. The multilayers

were deposited using RF diode, reactive sputtering. A Randex Model 2400-6J sputter system with a 6-in. high-purity silicon target was used for both the Si0 2 and SiH layers. The target-to-substrate

distance is 3.8 cm.

The SiH and Si0 2 in these multilayers are amorphous. Table 1 lists the deposition parameters for

the multilayers shown in the cross sections. Table 1 also summarizes the deposition parameters for

the mirrors in the laser damage tests.

The substrates in the cross sections of figures 11 and 12 are glass cover slips which are easily

fractured. The partial pressure of H 2 during the deposition of the multilayer showing the nodular

growth defect (fig. 11) was lower than the partial pressure of H 2 during the deposition of the

nodule-free multilayer (fig. 12). For both cross sections, the deposition parameters for the Si0 2 layers were identical.

Mirror substrates for the laser damage tests were 1-inch-diameter 0.020-inch-thick silicon

wafers with a standard polish on both sides. Mirror A had a low density of nodular growth defects and

mirror B had a high density. The deposition parameters for the Si0 2 layers in both mirrors again were

identical, whereas the forward powers for the SiH layers were changed. The forward power for the SiH

layers was 500 W in the low-defect mirror and 250 W in the high-defect mirror.

3.2 Experimental Parameters for Laser Damage Studies

The (SiH/Si0 2

) 5 multilayers were tuned for enhanced reflectivity at 2.7 ym. The in-band laser

damage onset data were obtained with an HF/DF laser operating in the HF mode (2.7 ym). Out-of-band

damage was obtained using a Nd:YAG laser at 1.06 ym. Selective laser damage at nodules was produced

by operating the Nd:YAG just above onset. The Nd:YAG laser had a 1/e 2 spot size of 50 ym and a pulse

length of 10 ns.

Numbers in brackets indicate the literature references at the end of the paper.

358
3.3 Nodular Growth Defects

A nodular growth defect in a SiH/Si0 2 multilayer is shown in figure 11. The SiH layers show

columnar structure at this magnification (20.000X), whereas the Si0 2 layers do not. The nodule

appears to originate at the third SiH layer and not at the substrate. This indicates that the nodules

are not a result of particulates on the substrate and are not caused by improper cleaning of the

substrate. Other cross sections, not shown here, of different multilayers clearly show the nodules

originating in the SiH layers and not in the Si0 2 layers.

The top surface and a cross section of a "nodule-free" film are shown in figure 12. The film may

not be entirely free of nodules but can be considered nodule-free in this discussion. As previously

stated, the SiH layers for the nodule-free film were deposited using a higher partial pressure of H2 than for the nodular film in figure 11. As can be seen in figures 11 and 12, the SiH layers have less

columnar structure in the nodule-free film, whereas the SiH layers have more columnar structure in the

nodular growth film. A possible conclusion is that the conditions which lead to increased columnar

structure may also contribute to the occurrence of nodular growth defects. Table 2 lists some possible causes for nodular growth defects [7].

Unequal growth competition, the result of inadequate adatom mobility, can be relieved by heating

the substrate during the film deposition. Careful attention must be paid to the possible loss of

hydrogen from the SiH layers at elevated substrate temperatures [8,9]. Similarly, it is possible that

the growing film will incorporate less hydrogen at elevated temperatures. In either case, loss of

hydrogen due to excessive substrate heating during the deposition could cause the optical properties

of the SiH films to change and ultimately deteriorate [10].

Table 1 indicates that the substrate bias was floating. According to Messier [11], a negative

substrate bias is induced by using low total pressures of reactive gas mixtures (Ar + H2 and Ar + 0 2 ).

The substrate bias was not measured in these depositions, but it is assumed that the pressures were

low enough to induce a negative bias on the substrate. A negative substrate bias means that the

substrate and growing film are bombarded by positive Ar ions. In principle, small defects are sputtered from the surface of the growing film by positive Ar-ion bombardment. The small defects are eliminated before growing into large growth defects.

Although further work needs to be done on the SiH to eliminate the nodular growth defects, the

Si02 film has superior properties. The lack of columnar structure in the Si0 2 was previously noted

(figs. 11 and 12). The x-ray photoemission spectroscopy data shown in figure 13 indicate that the

Si02 is fully oxidized. The water band is absent in the Fourier transform infrared transmission

spectrum shown in figure 14, indicating that very little water is present in the Si0 2 film. The

position of the Si-O-Si stretching frequency at 1075 cm

-1 is another indication that the Si0 2 is fully

oxidized [12].

Out-of-band laser damage at a fluence just above onset was obtained on the SiH/Si0 2 multilayers

using a Nd:YAG laser operating at 1.06 ym. SEM showed that selective laser damage occurred at nodular

growth defects (fig. 15).

Two SiH/Si0 2 multilayers with different numbers of nodules were prepared as described in

Table 1. SEM was used to determine the number of nodules per unit area using several fields of view.

Figure 16 shows representative fields of view for the two mirrors. No nodules were found in the

fields observed for the first multilayer, whereas about 5 x 10 7 /cm2 were observed for the second

mirror. Although no nodules were observed in the fields for the first sample, the sample may not have

been strictly nodule-free and should be considered a low-defect sample.

In-band laser damage using an HF laser was performed according to the method of Porteus and

Seitel [13]. The onset data for the low- and high-defect mirrors are shown in figures 17 and 18,

respectively. The HF laser at 2.7 pm had a pulse length of 1 ps. The spatial profile was near Gaussian with a 1/e 2 diameter of 150 pm. The test was performed using 1 pulse per site with a site

separation of 0.5 mm. The site array was rectangular. A parameter which is a function of the

uncertainty in the damage frequency is used to determine when an adequate number of pulses at a particular fluence has been taken. At low fluences and low damage frequencies, many more pulses are necessary than at high fluences and high damage frequencies. A total of 738 pulses were taken for the curves shown in figure 17, and a total of 1010 pulses were taken for the curves shown in figure 18.

At onset, the fluence was 0.27 arbitrary units for the low-defect mirror and 0.07 arbitrary units

for the high-defect mirror. In other words, the low-defect mirror has superior damage resistance.

359
4. Conclusion

This study has identified two categories of failure of optical coatings under pulsed laser

radiation. One mode of failure is defect-initiated damage caused by particulates, pinholes, and

nodular-growth defects. The second category of failure involves local areas of high stress.

Improvements in the laser damage resistance of optical coatings depend upon reducing the defects and

the stress, while at the same time maintaining the desired optical properties.

Two examples were described in which the deposition processes were modified and the resulting

defect densities were changed. The first example showed that electrostatically filtering the vapor

stream lowered the density of particulates in the evaporated ZnSe/ThF^ multilayers. In the second

example, the density of nodules increased in the RF-diode-sputtered SiH/Si0 2 multilayers when the

forward power for the SiH layers was decreased from 500 to 250 W. When low forward powers are used,

the temperature of the growing film does not increase as rapidly as when high forward powers are

used. This suggests that the increase in nodular growth was actually caused by a decrease in the

temperature of the growing film. It may be possible to eliminate the nodular growth which occurs at

low forward powers by using a substrate heater. Clearly, further work needs to be done to understand

the relationship between the deposition parameters and the nodular growth in RF-diode-sputtered SiH

films.

Table 1. Deposition Parameters for SiH/Si0 2 Multilayers

.

SiH layers

Partial

pressure

of H2 , mTorr

Total

pressure

of H2 +Ar,

mTorr

Forward

power

applied to

target, W

Reflected

power, W

Target

bias, V

Substrate

bias, V

Rate,

A/sec

Cross section A

"nodule free" 1 7 250 4 1100 floating 1.8

Cross section B nodular growth

defect

0.1 7 250 4 1100 floating 2.0

Mirror A

low defect 0.15 11 500 4 1600 floating 4.8

Mirror B high defect 0.15 11 250 4 1100 floating 2.3

Si02 layers

Partial

pressure

of 0 2 , mTorr

Total

pressure

of 0 2 +Ar,

mTorr

Forward

power

applied to

target, W

Reflected

power, W

Target

bias, V

Substrate

bias, V

Rate,

A/sec

Cross section A

"nodule free" 0.2 11 500 4 1300 floating 1.6

Cross section B nodular growth

defect

0.2 11 500 4 1300 floating 1.6

Mirror A

low defect 0.2 11 500 4 1300 floating 1.6

Mirror B high defect 0.2 11 500 4 1300 floating 1.6

360
Table 2. Possible Causes of Nodular Growth Defects.

1. Unequal growth competition

a. Low adatotn mobility - the surface temperature of the

growing film is too low to provide a narrow, highly peaked

thermal distribution.

b. Low substrate bombardment - insufficient local resputtering

of small growth defects.

2. Coating material spatters

a. Fluctuations in the power supply.

b. Arcing due to the buildup of material between the target

and the insulated target holder.

3. Microscopic dust particles sputtered from the inside of the

coating chamber.

4. Substrate defects such as cleaning residues and polishing marks.

References

[1] Ledger, T. Private communication.

[2] Bordelon, M.; Walser, R. M.; Becker, M. F.; Jhee, Y-K. "A study of the prf dependence of the

accumulation effect in multiple pulse laser damage of silicon," in Laser induced damage in

optical materials: 1982 , Bennett, H. E.; Guenther, A. H; Milam, D.; Newnam, B. E., ed.

1982 November 16-17; Boulder, CO. Nat. Bur. Stand. (U.S.) Spec. Publ. 669; 1984 January.

Pp. 427-435.

[3] Becker, M. F.; Jhee, Y-K; Bordelon, M.; Walser, R. M. "Charged particle exoemission from

silicon during multi-pulse laser induced damage," ibid . Pp. 415-426.

[4] Soileau, M. J.; Van Stryland, Eric W. "Ripple structures and enhanced absorption associated

with ordered surface defects," ibid . Pp. 406-414.

[5] Walser, R. M.; Becker, M. F.; Sheng, D. Y. "Laser damage of crystalline silicon by multiple

1.06 ym picosecond pulses," in Laser induced damage in optical materials: 1981 , Bennett,

H. E.; Guenther, A. H.; Milam, D.; Newnam, B. E. , ed. 1981 November 17-18; Boulder, CO.

Nat. Bur. Stand. (U.S.) Spec. Publ. 638; 1983 September. Pp. 103-113.

[6] Mansour, N. ; Reali, G. ; Soileau, M. J. "Laser generated ripple patterns on intermediate band

gap semiconductors," in Laser induced damage in optical materials: 1984 , Bennett, H. E.;

Guenther, A. H. ; Milam, D. ; Newnam, B. E. , ed. 1984 October 15-17; Boulder, CO. Nat. Bur.

Stand. (U.S.) in process.

[7] Guenther, Karl H. "Columnar and nodular growth of thin films," in Proceedings of SPIE:

thin film technologies and special applications , Hunter, William R. , ed. 1982 May 6-7,

Arlington, VA. Proc. SPIE Vol 346; 1982. Pp. 9-18.

[8] Oguz, S.; Collins, R. W.; Paesler, M. A.; Paul, W. "Effects of partial evolution of H from

a-Si:H on the infrared vibrational spectra and the photoluminescence , " J. Non-Cryst. Solids

35 and 36; 231-236; 1980 January and February.

[9] Zanzucchi, P. J.; Wronski, C. R. ; Carlson, D. E. "Optical and photoconductive properties of

discharge-produced amorphous silicon," J. Appl. Phys. 48( 12) ; 5227-5236; 1977 December.

[10] Ghosh, A. K. ; McMahon, T. ; Rock, E.; Wiesmann, H. "Optical and electrical properties of

evaporated amorphous silicon with hydrogen," J. Appl. Phys. 50(5) ; 3407-3413; 1979 May.

[11] Ross, R. C; Messier, R. "Microstructure and properties of rf-sputtered amorphous

hydrogenated silicon films," J. Appl. Phys. 52(8); 5329-5339; 1981 August.

[12] Lehman, A.; Schumann, L. ; Hiibner, K. "Optical phonons in amorphous silicon oxides (II),"

Phys. Status Solidi B 121(2); 505-511; 1984 February.

[13] Porteus, J. 0.; Seitel, S. C. "Absolute onset of optical surface damage using distributed

defect ensembles," Appl. Opt. 23(21); 3796-3805; 1984 November 1.

361
L J 100 um

Figure 3. Scanning electron micrographs (250X) of three laser damage sites. The different

numbers of oblong craters within the larger erosion pattern suggest that the occurrence of

oblong craters is modulated by the density of particulates.

(a) (b)

Figure 4. Scanning electron micrograph (a) and scanning Auger maps (b) zinc, (c) selenium, (d)

thorium, (e) fluorine, and (f) oxygen (570X) of the first damage site shown in figure 3.

362
(a) (b)

Figure 8. (a) Scanning electron micrograph showing a pinhole before laser irradiation (24000X)

and (b) another pinhole after laser damage (20000X).

363
I i 100 urn

Figure 1. Norraarski micrographs (360X) showing selective, oblong damage sites within a larger

erosion pattern. The sample was rotated in the laser beam. In all three orientations, the

oblong damage craters were oriented perpendicular to the electric field vector of the laser.

t 1 100 um

Figure 2. Scanning electron micrographs (250X) showing a low density of particulates on the

filtered samples (a and b) and a high density of particulates on the unfiltered sample (c).

364
365
(a) (b)

Figure 6. (a) Scanning electron micrograph (120X) showing selective erosion around embedded

particulates and (b) scanning electron micrograph (3100X) showing an embedded particulate that has

not been damaged.

Figure 7. Scanning electron micrographs of laser damage initiated by pinhole defects at (a) 250X and

(b) 1200X.

366
(a) (b)

Figure 9. Scanning electron micrographs (a) 680X and (b) 4500X of a laser damage site showing

selective crater formation within a larger erosion pattern. The site shows stress-related

cracking and loss of adherence within the erosion pattern.

7

DISTANCE (microns)

Figure 10. An Auger scan across the crater mid-line

indicating that the 3-ym-size particle at the center of

the crater in figure 9(b) is gold.

367
26KVX10000 1U038 NWC

(a) (b)

Figure 11. Scanning electron micrographs (a) 10000X and (b) 20000X showing a cross section of a nodular growth defect in a SiH/Si0 2 multilayer. The SiH layers show columnar structure whereas

the Si02 layers do not.

20KVX20000 1U 805 NWC

(a) (b)

Figure 12. Scanning electron micrographs (20000X) showing (a) the surface and (b) a cross section

of a nodule-free multilayer. The SiH layers have less columnar structure in this figure than in

figure 11.
(a) (b)

Figure 16. Scanning electron micrographs (5000X) showing representative fields of view of two

different multilayers. (a) No nodules were observed in the multilayer and (b) 5 x 10 7 /cm 2 were

observed in the multilayer.

0NSET=0.27+/-0.04

0NSET=0.70+/-0.04

1 . 0

0.9

0 . B

0.7

0 . B

0.5

0.4

0 . 3

0.2

0.1

0 . 0

0NSET=0 . 07+/-0 .01

0NSET=0.37+/-0.01 II

0.2 0.4 0.B 0.8 AXIAL FLUENCE (ARBITRARY UNITS)

1 . 0 0.0 0.2 0.4 0.6 0. AXIAL FLUENCE (ARBITRARY UNITS)

1.0

Figure 17. Single-shot laser damage frequency

data for the low-defect mirror shown in

figure 16.

Figure 18. Single-shot laser damage frequency

data for the high-defect mirror shown in

figure 16.

369
3 . 00

MICRONS

4 . 00 B . 00 25 . 00

4000 3600 3200 2800 2400 2000 1600 1200 800

WAVENUMBERS

400

Figure 14. The Fourier transform infrared transmission spectrum

shows no water band near 3 pm. The strong Si-O-Si stretching

frequency at 1075 cm

1 indicates that the Si0 2 is fully oxidized.

j 10 urn j 1 urn

(a) (b)

Figure 15. Scanning electron micrographs showing selective laser damage at nodular growth

defects at (a) 5000X and (b) 20000X.

370
Manuscript Received

12-19-84 SAR

Single Pulse Laser Induced Damage in IR Coatings at 10.6 ym

H. Deng, M. Bass and N. Koumvakalis

Center for Laser Studies, University of Southern California

University Park, Los Angeles, California 990089-1112

Results on single pulse laser damage to several reflective and anti-reflective

coatings designed for use at 10.6 ym are reported. Samples from two different

manufacturers were studied. Nomarski microscopy and TV vidicon detection were used

for the characterization of the samples before and after damage. Suggestions about

the future use of an improved TV system for the detection of scattering centers in

the visible are made.

Key words: damage thresholds; electric field; micrographs; reflective coatings;

scattering centers; substrate.

1. Introduction

Optical coatings, although studied for some time are still the weak points in high power laser

systems operating in the UV, visible or IR spectral ranges. In this paper experimental results are reported on the single pulse laser induced damage properties of several coatings designed for use at

10.6 ym. The coatings were reflective (100%, 85% and 65%) and anti-reflective designs involving

layers of ZnSe and ThF placed on Si, Ge and ZnSe substrates. No significant differences were found

in the damage thresholds of the reflective coatings. For the beam diameter used, the thresholds,

^90J/cm2, were independent of the reflectance, substrate and the defect density of coatings as determined by examination of Nomarski micrographs. The damage threshold of the anti-reflection

coatings was significantly lower than the threshold of the reflective coatings with an average value

of ^40 J/cm2. This result may be related to the design of the AR coatings.

The damage thresholds were measured by using well established techniques of beam characterization and damage detection [1]. The samples were examined with Nomarski optical microscopy before

and after irradiation. In addition, initial studies of visible light scattering using Ar ion laser

illumination and TV vidicon detection were performed to explore the possibility that scattering

centers could be detected whose presence would be correlated with the damage data.

2. Experiments

2.1 Damage Experiments

The laser damage experiments were carried out using the apparatus shown in Fig. 1. The CO2

laser was restricted to operate in the TEMqo mode by the intracavity cw discharge. The light energy

incident upon the samples was controlled by one fixed and one rotatable wire grid polarizer. This

energy was monitored for each pulse by the beam splitter-detector combination shown. The beam was focused on the sample with a ZnSe lens to a spot with beam radius at the 1/e point of intensity of

60 ym. The pulse duration was 230 nsec.

The damage threshold, as conventionally defined is the value of the incident energy or intensity lying midway between the lowest value at which damage always occur and the highest at which

damage never occurs. Table 1 lists the damage thresholds of all the coatings studied. The damage

thresholds for the reflective coatings are essentially independent of the reflectance, substrate and

the defect density. Due to the paucity of anti -reflective coated samples we could not examine

their damage threshold as a function„of such a range of parameters. The average value of threshold

for the reflective coating is 90J/cm or 370 MW/cm^ whereas the average value for the anti-reflection

coatings is 40J/cnr or 150 MW/cm^.

2.2 Coating Design

The variety of optics that were tested for laser damage included AR, partial reflectors and high

or total reflectors. These optics used the same material pair combination for all designs,

ZnSe/ThF, differing however in the number of pairs or the layer thickness. The AR design consists

of a standard two layer "V" coat with a relatively thin outer (air interface) layer thickness.

371
Table 1. Damage Thresholds of 10.6 ym Coatings

Nomarski

Substrate Reflectance Defect Density Energy Density Intensity

(mm

-2

) (J/cm

2 (MW/cm

2

)

Si 100% 37 91 + 18 371 + 74

Si 100% 54 84 + 22 359 + 100

Si 100% 80 + 31 342 + 132

Si 100% 39 100 + 21 410 + 86

Ge 99 . 3% 23 99 + 19 404 + 76

Ge 99 . 3% 18 96 + 21 392 + 84

ZnSe 60% 18 86 + 58 345 + 58

ZnSe 85% 25 88 + 12 361 + 49

ZnSe AR 27 41 + 12 169 + 48

ZnSe AR 32 30 + 9 123 + 37

Analysis of the electric field distribution in each layer for the various designs shows a twenty five percent increase in the peak electric field at the air interface for the AR coating

relative to the other designs [2]. This electric field value remains high through the next interface between layer 1 and layer 2. It is well known that all interfaces in thin film coating

structures have high potential for gathering contaminants, creating nucleation defects and material

mixing via diffusion across the interface. Asa result, coating designs having the peak electric

field at the air interface and maintaining a high value at the film/film interface is not optimum

for high damage threshold designs. Changing the electric field distribution by innovative coating

design is called for and may be an approach to increasing the damage threshold for the AR coatings.

3. Morphology

Nomarski micrographs were taken before and after the damage experiments. Figure 2 shows a characteristic picture for the undamaged surface of a 100% reflective coating on Si. One can see that there are rises or pits in the coating. These are 100 ym apart and could be encountered by the

small area beam used in the damage studies. The before damage morphology of all reflective

coatings was quite similar. The data of table 1 indicate that the defects seen by Nomarski microscopy could be responsible for the failure of the reflective coatings and consequently this

failure would then be independent of coating design, substrate and manufacturer. Figures 3 and 4

show Nomarksi micrographs of various damage sites in different coatings. The damage site at threshold is often smaller than the irradiated area. The role of the distributed defects in the damage

process can be demonstrated by those micrographs where a normally incident circular TEMqo beam

produces an asymmetric damage site.

4. Visible Light Scattering

The use of a TV vidicon system and a pulsed dye laser as a means of detection of scattering

from defects has been demonstrated recently [3]. A similar system shown in figure 5 was used by the

present authors to explore for the presence of scatterers in the coatings of interest. The

0.5145 ym, 0.200 W output of an Ar ion laser was used as the probe beam for this work. The output

of the Vidicon was displayed on a monitor adjusted in combination with the iris in the focusing lens

so that scatterers such as seen in figure 6 were observed against a black background. Figure 6 is

a photograph of the monitor screen showing scatterers in or on a 100% R coating on a Si substrate

before the damage tests. The magnification is about lOx. The important point is the presence of

discrete scattering sites spaced by about 0.5mm. The majority of the samples examined display the same behavior. Table 2 compares the scattering with the threshold damage. There is no correlation

but the scattering centers were not necessarily present in the small sites tested for laser damage,

so the measured low end of the damage range could not properly account for them. Obviously experi372
Table 2. Comparison of 0.5145 ym Scatter and 10.6 ym Damage Threshold

Type & Manufacturer Scatter Threshold

AR on ZnSe:

II-VI

L.P.O

Partial Reflectors:

II-VI

L.P.O.

100% R Coatings on Si:

II-VI

L.P.O.

Low

High

Low

Low

Average

Average

Comparable (30J/cm/cm ) Comparable (34J/cm )

Comparable (86J/cm ) p Comparable (86J/cm )

Comparable (91J/cm ) Comparable (lOOJ/cm )

ments must be performed in which the irradiated area is likely to include the discrete scatterers

and their role, if any, in determining the "onset" [4,5] of damage must be determined. In addition,

higher resolution TV vidicon-monitor combinations should be used to improve the sensitivity of this

technique to small, closely spaced scatterers.

5. Conclusion

The laser damage threshold of reflective and anti-reflective coatings on a variety of substances was examined. It was found that anti-reflective coatings of the design studied would

represent the weak link in a high power optical system. The damage thresholds of reflective coatings

on the other hand, for the beam sizes employed, are independent of the coating reflectance, substrate

material and defect density.

6. References

[11 Tang, C. C, Bass, M. , Soileau, M. J., and Van Stryland, E. W., "10.6 urn Laser Damage in

Coatings Containing As2Se3", Laser Induced Damage in Optical Materials: 1977, NBS Special

Pub. 509, 316 (1977).

[2] Rudisill, J. E., Laser Power Optics, Private Communications

[3] Marrs, C. D., Porteus, J. 0., and Palmer, J. R. , "Defect Damage Precursors in VisibleWavelength Mirrors" presented at the 1983 Symposium on Laser Induced Damage in Optical

Materials, Boulder, Colo., Nov. 1983 and to be published in the Proceedings of that meeting.

[4] Porteus, J. 0., "Determination of the Onset-Driven Pulsed Laser Damage in 2.7 ym Optical

Coatings", High Power Laser Optical Components Topical Meeting, Boulder, Colo., 18-19

November 1982. (Proceedings in process).

[5] Seitel, S. C, and Porteus, J. 0., "Toward Improved Accuracy in Limited-Scale Pulse Laser

Damage Testing Via the Onset Method", 15th Annual Sympsoium on Optical Materials for High

Power Lasers, Boulder, Colo., November 14-16, 1983. (Proceedings in process.)

373
REAR

REFLECTOR

TEST

COATING

C02 TEA

LASER

SECTION

CW C02 DISCHARGE

SECTION

MODE

APERTURE

1

ILLUMINATOR

CONDENSING

LENS

"\\MICROSCOPE

TOUTPUT

MIRROR ^

t

ENERGY

MONITOR

BEAM

SPLITTER

:5 BEAM

EXPANDER

HeNe

FIXED ROTATABLE

APERTURE WIRE GRID

POLARIZERS

Figure 1. Laser Damage Experiment.

•j >-.

__ (L.P. 0.)

(TX-2I

)

156^m

Figure 2. Nomarski micrographs of undamaged regions of 100% R, 10.6 ym, coatings on Si substrates.

The coater is indicated.

374
Nomarski Micrographs

Threshold Laser Damage

to 100% R on Si

(85j/cm 2

)

(L.P.O.)

< > 100/j.m

Figure 3. Nomarski micrograph of thereshold 10.6 ym laser damage to a 100% R, 10.6 ym, coating

on Si

85% R on ZnSe

(85J/cm2

)

(L.P.O.)

< >

Figure 4. Nomarski micrographs of threshold 10.6 ym laser damage to transmissive coatings on ZnSe.

375
SCATTERING EXPERIMENT

Figure 5. Visible scattering centers experiments.

0.5145 fj.m Scattering

from 100% R on Si

(~10x)

Figure 6. 0.5145 ym scattering centers in an undamaged 100% R, 10.6 pm coating on Si.

376
Manuscript Received

10-31-85 SAR

Light Stripping of UV Dielectric Coatings

John F. Asmus and Jerry R. Oldenettel

Maxwell Laboratories, Inc.

San Diego, CA 92123

High-performance ultraviolet lasers require optics with a tighter tolerance on

surface figure than comparable visible and infrared systems. Consequently, the costs

of large -aperture figured substrates can be quite high. When ultraviolet coatings become damaged through exposure to photons and/or halogen radicals, recoating is often

performed in order to salvage such substrates. If the coating is not strippable, this

entails the expense of regrinding and repolishing. We have found that the remains of

damaged coatings of Si02» LaF-j, MgF£, and HfOo can he removed with pulsed xenon

flashlamp radiation. A train of shots at 30 J/cm^ is sufficient to dislodge a coating

without damaging fused silica and Zerodur substrates. A pulselength of 600 microseconds was employed. Frequently, photochemical damage involves only the coating

and not the substrate. In these instances flashlamp stripping may be employed to

salvage the substrate without reworking the surface.

Key words: coating removal; coating stripping; dielectric coating; laser mirror;

mirror damage; xenon flashlamp

1 . Introduction

The harnessing of light energy for industrial and commercial cleaning applications has grown

in importance during the last decade. A recently developed industrial flashlamp cleaning system

has been undergoing evaluation for a number of applications [l] . Such systems have their origin

in the pioneering work of laser cleaning [2] . Use of the unique properties of laser radiation -- extremely fine focusing, directionality, high brightness, monochromaticity -- has allowed solutions to surface preparation problems once considered untenable by conventional cleaning methods.

The optimism that has met these impressive successes, however, has been tempered by the fact

that laser photons are expensive photons. Consequently, surface cleaning with lasers is presently

warranted for rare and costly items, or as a solution to "when all else fails" scenarios.

The use of flashlamp systems presents a cost effective alternative to laser light cleaning.

While lacking the sophisticated aspects of laser radiation, a flashlamp will emit very intense,

broad-band, short pulses of light. This radiation is of sufficient energy to physically and chemically alter and/or remove a variety of surface deposits. Additionally, the flashlamp cleaning process is often self-limiting in that once the unwanted surface layer has been removed, the exposed

substrate will be immune to further radiation treatment. Flashlamp induced divestment is thus an

effective technique particularly amenable to large surface areas. The uncollimated, diffuse characteristics of the radiation, however, diminish its usefulness on surfaces which are not flat.

Experimental studies of flashlamp cleaning effectiveness have involved a variety of materials. Successful applications include divestment of polyurethane and epoxy paints from metal,

removal of paint overcoats without destruction of the underlying primer, and etching of polymer

surfaces for coating purposes (Figure 1). The potential application presented here involves the

use of intense flashlamp radiation for the stripping of damaged large-area ultraviolet coatings

from laser optics. This opportunity is of particular interest as the figuring costs of larger ultraviolet optical surfaces are especially high due to the short wavelength. Thus, there is a strong

economic incentive in developing a method of stripping damaged 'hard" coatings without altering

the substrate. This would facilitate the reuse of such substrates.

377
378
2. Background

Inmost instances of intense xenon flashlamp radiation, stripping the surface film is absorbing and the temperature rise is substantial (500- 1 000 °C). Several divestment mechanisms come

into play in such instances. First, there is the possibility of superficial evaporation and sublimation of substances. Second, if the heating is sufficiently rapid, the expanding vapors are able to

scour the surrounding areas much like hydro or air blasting. Any surfaces that are coated with

water during the irradiation will produce a steam cleaning effect. Third, depending on the absorption properties of the material, a given layer may be broken down photochemically or photolytically

Fourth, differential thermal expansion of substances may lead to delamination of surface deposits.

Finally, for short laser pulses of a few tens of nanoseconds, a Shockwave is generated causing

material removal by ablation and spallation. Which of these mechanisms, or combination of mechanisms, is of predominant importance depends on the substance and conditions under treatment.

The materials employed in ultraviolet dielectric coatings are generally oxides and fluorides.

They are of course very low in optical absorption and have moderately high sublimation and decomposition temperatures. Consequently, light-induced divestment by the mechanisms cited above

would not seem particularly plausible. However, it is known that the thermal conductivities of thin

film materials are lower than those of bulk materials [3] . Consequently, it may be possible to

induce mechanism four mentioned above. As many thin film materials cutoff in either the ultraviolet or the infrared before reaching the fused silica cutoff limits of a flashlamp, it may be possible to produce some differential heating via the wings of the xenon radiation.

3. Results

By operating a linear xenon flashlamp at the upper end of its useful range we have been able

to illuminate surfaces at 30 J/cm^. The lamp employed in these tests had an 8 mm bore, a 5-inch

arc length, and a Suprasil envelop. The radiation output had a 600 Us FWHM and the input electrical energy was 4. 5 kJ.

The coating stripping tests were performed by placing the lamp and its reflector housing

above the coated surfaces much as indicated by Figure 1. Two damaged ultraviolet reflectors were

obtained from a 40 liter e-beam excited excimer laser employing unstable resonator internal

optics. The thin film layers on these two Zerodur substrates employed SiC^/LaF^ and MgF2/Hf02-

Figure 2 shows the output coupler after one end of it had been exposed to five shots of the xenon

flashlamp at 30 J/cm^. About 99% of the coating was removed from the exposed end. The substrate was free of observable damage.

The unstable resonator primary reflector was then scanned by the flashlamp and about 100

shots were applied sequentially by displacing the lamp laterally by a few mm after each five shots.

Whereas most of the thin film material has been ejected, a residue remains over about one half of

the surface. It appears that the low intensity fringes of the radiation footprint precondition the

coating so that complete ejection is not obtained when the high -intensity portion of the profile

reaches that zone. This radiation- stripped primary reflector is shown in Figure 3.

4. Conclusions

It appears that it may be technically feasible to employ intense xenon flashlamp radiation to

cleanly strip damaged hard ultraviolet coatings from substrates. However, it will probably be

necessary to employ a large array of lamps for total illumination of the surface. A fluence of

30 J/cm^ in a 600 us pulse is marginal for state of the art coatings. The utilization of a lamp

array may permit the attainment of a higher fluence which may yield better results. The results

reported here employed a low 0. 1 Hz pulse rate. Higher pulse repetition rates may also be more

favorable.

379
Figure 2. Partially stripped output coupler.

Figure 3. Flashlamp scanned primary reflector.

380
References

[l] "Light Cleaning, " Science Digest 89(3): 27; 1981 April.

[2] Asmus, John F. "Technology and Conservation, "

J3_ (3); 1 4- 18 ; 1 978 Fall.

[3] Decker, D. L. ; Ashley, E. J.; Koshigoe, L. G. "Thermal Properties of Thin Film

Materials, " in Program of the Sixteenth Annual Symposium Optical Materials for High Power

Lasers; 1984 October 15-17; Boulder, CO, 34.

381
An Initial Study of: Manuscript Received

The Inclusion Model for Repetitively Pulsed Laser Damage 1-9-84 SAR

M.R. Lange* and J.K. Mclver

Institute of Modern Optics, University of New Mexico, Albuquerque NM 87131

Arthur H. Guenther

Air Force Weapons Laboratory, Kirtland AFB, NM 87117

The inclusion model of laser initiated damage is extended to include an arbitrary

but repetitive and reproducible variation of the laser irradiation in time. The

controlling influence of the thermal properties of the film/substrate system and

inclusion, together with the duty cycle of the laser, i.e., the duration and energy

of a single pulse and of the repetition rate, on the damage threshold is discussed.

The predictions of this model are then compared to the results of the single-shot

model of inclusion dominated damage. Suggestions for experimental tests of this

model are given.

Key words: Absorbing inclusion, laser induced damage, repetitively pulsed,

thermal properties.

1 . Introduction

The problem of multiple pulse laser induced damage in optical components is a very important

and general one. It is the real situation that we wish to address when considering the

durability of optical components. This stems from the fact that multiple pulsed damage concerns

the use of optical elements that are subjected to more than one cycle regardless of the time

delay between pulses.

In a review of experimental results relating to this area, one encounters a large array of

seemingly contradictory data. For example in some instances there is no apparent effect of prior

pulses [1] (i.e., no memory), while in other cases there is unmistakable evidence of a build-up

or accumulation of damage. [2] Some situations demonstrate clear evidence of thermal build-up

effects [3,4] while others [5] show no discernable evidence of this effect. A pulse repetition

frequency dependence in some cases [3,4,6] exhibits reversible effects in some materials while

others [5] show no such evidence. It is also known that low level irradiation can condition

elements such that they can ultimately withstand higher fluences without damage. [7 ,8] A possible

clue to an understanding of this wide diversity of responses is the fact that damage is dependent

upon the configuration of the material (i.e. bulk, film, surface, crystalline, amorphous,

metallic, polymer, etc.)

The morphology of multi-pulsed damage is, as one might expect, just as varied, not only

between different materials, but between different configurations of the same material (i.e.,

anti-reflecting films and high-reflecting films). [9] One mode of damage for films, both with and

without some sort of accumulation mechanism, is for a small pit to form and no subsequent damage

to occur. [9] Another mode is for damage to grow with subsequent pulses to a diameter that is

consistent with points in the incident field that are ~ 25% of the single pulse damage

threshold. [9] Clearly there is no simple universal multipulse laser induced damage scenario.

There are many classes of multipulse damage morphology.

What we do herein is to once again exploit the thermal model of laser induced damage and

apply it to multipulsed laser irradiation The justification for this course of action is as follows. Absorption and diffusion must occur in all cases of laser damage. There is very

* Research partially sponsored by the Air Force of Scientific Research/AFSC, United States Air

Force, under contract F49620-82-C-0035 . The United States Government is authorized to reproduce

and distribute reprints for government purposes not withstanding any copyright notation hereon.

382
strong evidence that at least some cases of laser induced damage (certain optical films) are

thermally dominated for the single pulse case. [10] There is, in addition, evidence that the

mechanism for certain cases of laser induced damage is the same for the first pulse as for

subsequent pulses [6,11,12] (though, for other cases [3,13] this is evidently not true). Even

situations where damage is not thermally dominated a change of state necessarily accompanies a

change in temperature (e.g. a change in band gap) and thus, it is desirable to know the

temperature field. Finally, evidence of absorbing microinclusions [7] as damage initiators gives

further support for this model. One should note that beyond requiring a microinclusion or some

localized initiating mechanism, the physics involved is not restricted by this model, nor is it

determined. It simply implies that an isolated region absorbs energy in some manner and

subsequently diffuses the absorbed energy throughout the surrounding matrix. It is also not

required that this absorbing region remain fixed with time, pulse number, pulse repetition

frequency or any other experimental variable.

The model is generalized to handle any sort of general absorption that may occur in various

materials. However, at this stage of the model development the absorption function must be

determined for each different situation. The determination of the temporal and spatial

dependence of the absorption function for each situation, as for the single pulse case, is the

primary difficulty. For certain single pulse situations [10] a very simple absorption function

can prove adequate. However, due to irreversible changes (i.e. memory) of some sort, this is not

true for most multiple pulse interactions.

In the section that follows we derive the general theory of arbitrary multi -pulse absorption

for both a spherical inclusion in an infinite medium and a cylindrical inclusion in a thin

film. The arbitrary form of the absorption allows application to laser windows with macroscopic

gaussian beam absorption .[4] In addition, it allows application to an opaque material with

absorption of any general form (Eg. Ref. 3). The problem is then simplified to the case of a uniform, non-expanding absorption, without irreversible processes as an illustration of the

effects purely due to thermal build-up.

The results are as expected, in that thermal build-up in itself can not be responsible for

the observed decrease in damage threshold in most cases of isolated absorbing inclusion. Other

mechanisms must couple with the diffusion process to provide the observed behavior. The key to

understanding the many observations and mechanisms involved is to determine specific pulse to

pulse behavior of the absorption function. Any correct theory of the interaction involved must

predict the spatial, temporal pulse repetition frequency and pulse number dependence of the

absorption term in the heat diffusion equations.

383
2 . Theory

In previous works we have described models of absorbing regions which subsequently diffuse

their energy through the surrounding matrix. Both a spherical model [10], valid for the region

where the pulse length is not larger than the diffusion time to the boundaries of the film, and a cylindrical model [14], valid for much longer times, have been derived.

In general we use the diffusion equation to describe the distribution of temperature within

the system. In the absorbing region (the subscript i denotes the absorbing impurity),

and T, D, K are temperature, diffusivity (cm^/s) and conductivity

(J/°K-s-cm) respectively while A is the volume absorption function in (J/cm^-s)

1_ il = v

2 T (2)

D. at 1

'

corresponds to the heat material (subscript h) outside of the absorbing region. Here, perfect

thermal contact is assumed at the boundaries of the absorbing region and r and t are the radius

and time respectively. The two geometries give specific solutions within the impurity of

T. = — "/ -~ sin(ar) A (a,t) (3)

i r o N(a)

for the spherical case [10]

and

T. = 7 7TT~T a J (ar) ? COs(^) A (a,t) (4)

1 0

J N (a) 0 J = 0 £

for the cylindrical case. [14]

Here A is the transformed absorption function (defined below), N(a) is a normalization factor,

a

the radial eigenvalue and £ is the length of the cylinder (i.e., the thickness of the film). The

expression for the source term in the above equations is

384
A = exp iHt ] t

j dt' exp [?t'] J_dr3 Y.(r,a) A (r,f ) (5) o vol J

Here £ is the composite eigenvalue of the system, ¥j(?,a) is the eigenfunction of the composite

region and A(£,t) is the general absorption function with "Vol" refering to the volume of the

absorbing region. The subscript j on \|> applies to axial modes of the cylindrical geometry.

Expressions for these parameters are given in table 1 for specific geometries. The absorption

function written in the form

A(r,t) = Z A (r,t) (6)

n=l n

contains the complete evolution of the absorption including irreversible or long lived changes to

the material with each pulse from 1 to N. There is data [5] indicating that in some cases the

change in absorption occurs within a time that is small compared to the delay time ' t^' between

pulses. In some cases it may occur within a time that is small compared to the pulse length 'tp'

itself. For an idealized case where the absorption evolves early into the pulse and the

absorption that follows is proportional to the field, or the case where the evolution transient

occurs completely in between the pulses, we may write

A(r,t) = z A (r) {e(t-[(n-l)t + (n-l)t .])-e(t-[nt +(n-l)t .])} . (7)

n=l n p d p d

J

Here the nth pulse turns on at t = (n-l)t

p + (n-Dt^ and turns off at

t = ntp + (n-Dtj with t

p being the pulse length and t^ being the delay time between pulses

and e being the step function.

Thus, we need only concern ourselves with the spatial dependence of absorption from pulse to

pulse. In other cases [9] multipulse damage has been observed to occur after some number of

pulses, but not grow spatially. In fact, no further damage is observed after the initial

pitting. One may have totally vaporized the impurity. In this case

A(r,t) = f(r) z,A { e(t-[(n-l)t + (n-l)t ]) - e(t-[nt + (n-l)tj)} (8)

n=l n p d p d

'

so that only the magnitude of absorption varies with the pulse number.

In all of these cases the absorption evolution is unknown and is observed to be strongly

material and laser parameter dependent with an unspecified physical mechanism.

In at least one case [15] there is evidence, or at least it has been suggested that for the

385
material polymethylmethacrylate the absorption evolution is governed by irreversible mechanochemical processes driven by a temperature gradient induced stress. Whatever the driving source,

a measured empirical or theoretically derived form of A

n must be found before any specific

situation can be treated adequately.

Example case

An illuminating exercise as a first step, is to assume 'A

n

' is constant and develop the

solution. There are some cases where this seems to apply. This exercise also serves to

demonstrate purely thermal effects for repetitively pulsed lasers. It may be used to study

temperature profiles within optical windows and in absorbing inclusions at fluences below those

at which nonlinear absorption effects become important. For this case

A(r,t) = f{r) A !' {e(t-[(n-l)t + (n-l)t ]) - 9(t-[nt + (n-l)t.])} (9) o n=l p d p d

1

so that

A = Exp[-Ct ] */dt' ExpUt'lFU) A(t') (10)

where

A(t') = A

Q Jjj {e(t,(n-l)t ,(n-l)t

d

) - e(t,nt ,(n-l)t

d

)}

F(o) = v^ ] d

3 r ¥ (r,a) f(r) . (11)

Various F(a)'s are listed in Table 2 for different geometries and situations. The time

integration in equation (10) is carried out term by term and the resulting series is summed to a result of

I.aJM (l-e^p) ikL^l (12)

0 5 (l-ef' T

)

where x = tp + t

d

. This is a particularly simple and elegant result first realized by Walker

[16] in connection with the Goldenberg and Tranter [17] solution using the Laplace transform and

now proven here for that case and any general related case. Though this solution does not apply

in general to catastrophic repetitively pulsed laser induced damage, it is likely that it does

apply to some cases and may be informative for others for which it is not valid.

386
Results

Numbers are applied to the previous expression providing a sense of the effect of thermal

build-up in optical materials subjected to repetitive laser pulses. For example, for a one-half

micron thick thorium fluoride film using the isolated spherical absorbing impurity model with a

0.1 nanosecond pulse, if no growth of the absorbing region occurs we would need a repetition

frequency of greater than 10^ hz to see any significant decrease in damage threshold due to

thermal build-up. The criterion that we use for damage here is for the edge of the absorbing

region to reach the film melting temperature. Note on figure 1 the increase in radius most

likely to damage with increased number of pulses. So, if no growth of the inclusion occurs (i.e.

disc like growth into the film) and we are limited by the film thickness then the effect of

Q

thermal build-up is even less. Note also in Figure 2, of pulse repetition frequency equal to 10°

Hz, that essentially no thermal build-up occurs. Thus thermal build-up could not be a factor in

damage for this idealized case. This does not take into account a possibly dense distribution of

particles so that the diffused heat of nearby absorbing regions could build-up in the surrounded

region. Detailed experimental determination of the distribution and strength of absorbers would

be required for this calculation.

The largest possible effect of thermal build-up for a single isolated absorber would be for a

free standing film (allowing only two dimensional heat diffusion) of very low thermal

conductivity with a large strongly absorbing inclusion and a long pulse length. So, assume we

have a free standing film of SiC^ (k=0.014 J/cm-°K-s) ten microns thick with a 10 microsecond

incident pulse repeated at 1000 hz. The largest inclusion that could intially be embedded in the

film would be one with a diameter equal to the film thickness. Again, assuming no growth of the

absorbing region we find a decrease in damage threshold of only ten percent for 10° pulses due to

thermal build-up compared to the single exposure threshold. For a substrate of greater than zero conductivity this value would be further decreased. These extreme conditions should indicate

that thermal build-up by itself should not be a major factor in this case. However, there are conditions with and without the assistance of additional mechanisms where it is imaginable that

thermal build-up would be a factor in decreasing the damage threshold.

Suffice it to say, a dense distribution of absorbing impurities, irreversible changes or temperature dependent absorption that is initiated at an absorbing inclusion are a few such

cases. The latter suggestion is not new. Komolov [18] has analyzed non-linear temperature

dependent absorption of the form

-u/ T

X(T) = X

7

- + X Q

e ' (14)

Where X^ is the impurity absorption and X Qe~

u is an intrinsic film absorption, and u is a constant.

Also Manenkov et. al . [15] has suggested irreversible mechano-chemical reactions with a rate

387
constant of

K mcha e "(Ea -Ya)/k b T

(15)

where Ea is an activation energy and ya is the stress proportional factor driven by temperature

gradients assisting in the damage.

With one of these mechanisms the region that is absorbing expands into the film from its

origin at an inclusion or impurity. This inclusion may be as small as 100 A according to some theoretical work of Babadzhan et. al . [19]. Under these conditions a model, either theoretical

or empirical, of the absorbing function must be incorporated. This expansion to macroscopic

dimensions then necessarily involves thermal build-up. In fact according to the theories

mentioned above, the expansion in turn is driven by the thermal build-up.

Another situation involving thermal build-up is the case where absorption occurs on a macroscopic scale (on the order of the diffusion length between pulses). This occurs, for

example, in semiconductor grade windows [4] and on intrinsic absorbing metallic mirrors [3].

Table 2 lists the spatial part of the solutions for some of these cases. In particular the case

of a weakly absorbing window such that absorption is uniform with depth into the window and the

case of a standing wave in a thin film are shown. Many other similar solutions are possible

(surface absorption or absorption which has essentially any arbitrary depth dependence). These

are just mentioned to point out the tremendous generality of the integral transform technique.

As with microscopic absorption, most cases of observed macroscopic absorption are also

dependent on the number of pulses and/or the pulse repetition frequency. So once again a

function versus time, temperature, pulse number and pulse repetition frequency must be known or assumed to properly model this situation.

Some work by Nathan et. al . [20] has indicated that a multi-pulse damage decrease can occur

by thermal build-up and various relations involving multipulse parameters were realized.

Together these lead to

(16)

The terms above are defined as follows: E

d is the damage threshold in (J/crrr) and Nmi

- n is

the number of pulses at which damage is most likely to occur given some pulse length and

frequency. Like our previous works [10,21] this was found based upon the fact that the heat

balance gives some particle radius a mi

- n most likely to damage. Here ami

- n is the radius of the

particle most likely to damage at this pulse length and pulse repetition frequency. These

authors mention that relation 16 also applies to uniformly absorbing semi-infinite surfaces.

388
The numerical work of Nathan et. al . [20] was verified and the basic trends are shown in

figure 1. In addition, approximate relations for (K, p, C

p

, t

p

, t^, N) and

amin p ' ^p' *p' *d' ^ were derived exactly as done in a previous work [21] further

verifying the numerics. Unfortunately, as also indicated in figure 1, the minimum radius for

damage to occur quickly becomes very large. In fact it becomes several orders of magnitude

larger than most thin films for reasonable laser parameters. Nathan's relations therefore

primarily apply to bulk materials with extremely large inclusions, or more reasonably to surface

absorption on a semi-infinite absorber.

The derived relations mentioned above are not given because of their extreme complexity

yielding little additional insight into the problem. However, as with most real lasers, if one

assumes that t^ >> t

p and that impurity radii are small (limited to a thin film thickness) the

relations simplify to the ones given in reference [21] for the single pulse case. This simply

indicates that no thermal build-up can occur (i.e. in thin films, if inclusions damage thermally,

it must be on the first shot instead of the Nth). It also indicates that multipulse thermal

damage scales against the thermal parameters in a similar manner to the single pulse case. This

has not as yet been investigated for pulse dependent absorption.

The results of this work are certainly not conclusive. The problem as initially stated is

that there is no simple universal effect in multipulse damage. The aim of this work is more the

beginnings of an effort to isolate and classify multipulse damage, a baseline study, as it

were. It is believed that an understanding of the variation of the absorption function from

pulse to pulse will provide essential knowledge for understanding the mechanisms involved for

both single and multi-pulsed damage. Also, any theory of multi-pulse damage must predict the

appropriate behavior of this function. The primary direction of the research to follow will be

in the development of theories to predict this function. It is hoped that this summary of our efforts has stressed the importance of real time pulse to pulse observation of damage evolution

in all cases of multi-pulse laser induced damage. Along with this, pulse repetition rate

dependence, damage morphologies and laser hardening [7,8] or its inverse need to be studied for

all materials and classes of materials used in high energy laser optical components. The

statistics of damage along with the collection of data points above will provide essential

information for the isolation of the particular mechanisms involved in both single and multipulse

damage.

389
TABLE 1

Sphere Cylinder in a Film

Y

2 D

.

-jr1, Y = «a = D. ( a2 + (Jl)2)

, (r ,a) = sin {r

ar)

= J Q

(ar) cos (jp)

K . K N(a) = 2tt 2 ((CsinY - YcosY) 2 + b2Y2 sin2 Y)) = f

(A2 + B

2

) ^

b = tt^n i , tt3 K. / D

h „w

A = {K

i aJ 1 (aa)Y

0

(ea)-K

hpJ

0

(oa)Y 1 (ea)}

K

h . ' Di C=1-tt^ g = a

K. — D

h

B= {K

h

3J 1 (Ba)J

() (aa) - K.aJ 1 (aa)JQ (ea)}

390
Figure 1. This is a plot of one example of computed damage threshold in (J/cnr) versus

radius of the absorbing inclusion for various numbers of pulses. The pulse repetition

frequency is 10 Hz.

Figure 2. This is a plot of computed damage threshold in (J/cnr) versus radius of th: absorbing inclusion similar to Figure 1, but with a pulse repetition frequency of 10' Hz.

391
TABLE 2. F.(ct)

Sperical Absorption Cylindrical Absorption

Uniform

Absorption

4 ^4- (sinY - YcosY)

Y = aa

(aa) la

Standing Wave

pX/2

Uniform Depth

Gaussian Beam Cross

Section (Weakly

absorbing window)

, (aa)a £ r 5, „_ }

1 a 2

l "j,0

" j,2p

2

-a2 a

2 /16

/it ae a

2 a2

where a is the Gaussian spot size radius

and l± 1

, 2 is the exponentially increasing

modified Bessel function

Gaussian Cross

Section

Standing Wave

(Weakly absorbing

thin film)

-a

2 a2 /16

/tt ae a

2 _ £

(16/a2 )3/2 CI

- V2 (a 16-} _I

V2 (a2l6

)]

2 {6 j,o " ~f-}

392
REFERENCES

[I] M. Bass, H.H. Barrett, eds, A.J. Glass and A.H. Guenther, NBS Spec. Publ. 372: 58; 1972.

[2] L.D. Merkle, M. Bass, R.T. Swimm, eds., H.E. Bennett, A.H. Guenther, D. Milam, and

B. Newnam, NBS Special Publication 669: 50; 1982.

[3] N. Koumvakalis, E.S. Lee, M. Bass, eds., H.E. Bennett, A.H. Guenther, D. Milam, and

B. Newnam, NBS Special Publication 669: 186; 1982.

[4] R.M. Wood, S.K. Sharma, P. Wate, eds., H.E. Bennett, A.H. Guenther, D. Milam, and

B. Newnam, NBS Special Publication 669: 44; 1982.

[5] M. Bordelon, R.M. Walser, M.F. Becker, Y-K . Jhee, eds., H.E. Bennett, A.H. Guenther,

D. Milam, and B. Newnam, NBS Special Publication 669: 427; 1982.

[6] S.K. Balitskas and E.K. Maldutis, Sov. J. Quant. Elect. 11 (4): 541; 1981.

[7] B.G. Gorshkov, Yu. K. Danieliko, U.N. Nikolaev and A.V. Sidorin, Sov. J. Quant. Elect. _13

(3): 388; 1983.

[8] M.F. Becker, F.E. Domann, A.F. Stewart, A.H. Guenther, eds., H.E. Bennett, D. Milam

A.H. Guenther, and B. Newnam, NBS Special Publication (Boulder Proc); 1983.

[9] S.R. Foltyn and L.J. Jolin, eds., H.E. Bennett, D. Milam, A.H. Guenther, and B. Newnam, NBS

Spec. Publ. (Boulder Proc); 1983.

[10] M.R. Lange, J.K. Mclver, and A.H. Guenther, J. Thin Solid Films, Proc. ICTF-6 (to be

published); 1984.

[II] M. Bass and H.H. Barret, Appl Opt. 12_ : 690; 1973.

[12] A. A. Kovalev, B.I. Makshantsev, N.F. Dilipetskii, Tu V. Sidorin and O.G. Stonik, Sov. J.

Quant. Elect. _lp_: 736; 1980.

[13] E.K. Maldutis, Izvestiya Akademii Navk SSSR, Seriya Fizicheskaya, 4l_ (1) : 196; 1983.

[14] M.R. Lange, J.K. Mclver and A.H. Guenther, J. Thin Solid Films, 118 (1):47; 1984.

[15] A. A. Manenkov, G.A. Matylshin, V.S. Nechitailo, A.M. Prokhorov, and A. A. Tsaprikov, eds.,

H.E. Bennett, A.H. Guenther, D. Milam, and B. Newnam, NBS Spec. Publ. 669: 436; 1982.

[16] T.W. Walker, Air Force Weapons Laboratory, Private communication

[17] H. Goldenberg and J.C. Tranter, Brit. J. Appl. Phys., _3: 296; 1952.

[18] V.L. Komolov, Sov. Phys. Tech. Phys. _27 (3): 307; 1982.

[19] E.I. Babadzhan, V.V. Kosachev, Yu. N. Lokhov and M.I. Ryazanov, Fizika i Khimiya Obrabotki

Materialov, 17_ (1) : 12; 1983.

[20] V. Nathan, T.W. Walker, A.H. Guenther, unpublished

[21] M.R. Lange, J.K. Mclver, A.H. Guenther and T.W. Walker, eds., H.E. Bennett, A.H. Guenther,

D. Milam, and B. Newnam, NBS Spec. Publ. 669: 1982.

Diffusion barriers were suggested by Raj of Martin Marietta as a possible way to increase

damage threshold. Barriers as thin as 50 J? have been tried in the semiconductor industry and

are reasonably effective; 50 to 200 K are commonly tried for semiconductor coatings where

diffusion has been a -problem.

393
Manuscript Received

10-28-85 SAE

Self-Focusing in Damage Experiments Revisited

M. J. Soileau, William E. Williams,* and Eric W. Van Stryland

Center for Applied Quantum Electronics

Department of Physics

North Texas State University

Denton, Texas 76203

Last year we reported that the power of importance in self-focusing experiments is

the second critical power, P 2

, defined by P2=3.77 rather than the often quoted

P^=c XZ/(32Tr 2

n 2

). Here n 2 is the nonlinear refractive index, c the speed of light and

X the wavelength (both in vacuum). The factor of 3.77 is a numerical factor coming from

computer calculations as discussed by Marburger [1]. We also presented a method by

which we could obtain n 2 at irradiances very near to damage. In addition, we showed the

experimental conditions under which self-focusing is unimportant in damage experiments.

We have now extended these results so that in damage experiments where self-focusing is

important we can estimate the reduced spot size within the bulk at damage and, thus,

obtain the damaging electric field magnitude. This we do by observing the far field

time integrated spatial irradiance distribution. We present data for Si0 2

, NaCl , BK-7,

and CS 2.

Key Words: critical power; laser damage; picosecond pulses; self-focusing.

1. Introduction

The role of sel f -focus i ng in laser-induced breakdown was examined in detail in a paper

presented at the 1983 Boulder Damage Symposium [2]. In this paper we address questions raised in

response to that paper and present additional data that support the conclusions reached in reference 2. The conclusion of reference 2 is that the critical power of importance for laser-induced

damage experiments employing tightly focused Gaussian beams is the second critical power P

2

, where

P

2 = 3.77 P1 (1)

cx

2 and P

1 = CA? (2)

32 tt n 2

where X is the wavelength, c is the speed of light, and n

2 is the nonlinear refractive index.

(Equations (1) and (2) are taken from the classic work of Marburger [1].) We based this conclusion upon measurements of the polarization dependence of picosecond laser-induced breakdown and

upon measurements of beam distortions in the time-integrated spatial profile of beams transmitted

through the samples. In a related paper we reported the results of n 2 in NaCl and Si0 2 at 1064 nm

and 532 nm [3].

Much of the past bulk laser-induced damage research assumed that self-focusing corrections to

damage data are required for data taken with input power a fraction of P^ [4]. On the other hand, we have published work for which no self-focusing corrections were made even though the measured

breakdown power was a substantial fraction of Pi [5,6]. In references 5 and 6 we used the lack of

polarization dependence and lack of observable beam distortions to justify neglecting selffocusing in computing the laser-induced breakdown electric fields. It is critical that

researchers interested in this problem take into account these differences in interpretation and

data reduction when comparing the results and conclusions of the various bulk laser-induced

*Present Address: Texas Instruments, Inc., 8505 Forest Lane, P.O. Box 660246, M/S 3123, Dallas,

Texas 75266

394
breakdown experiments. One can question the sensitivity of the null test used in references 5 and

6 in ruling out self-focusing. For example, the beam distortion measurements involve the time

integrated spatial profile and are only sensitive to about A/4 peak distortion in the transmitted

beam. This problem emphasizes the importance of direct measurements of 02 in a m a nn er independent

of the damage test.

In the sections that follow we will review the theoretical treatment of self-focusing in bulk

damage experiments and present results of extensive studies we have conducted to verify several

key features of the theory. We then discuss in more detail the relevance of the results in

reference 2 to prior experiments.

2. Sel f -Focusing Theory

Marburger has addressed the problem of self-focusing for "pre-focused" beams in great detail

(see reference 1, pp. 66-67). By "pre-focused" we mean a beam focused into the medium by an external lens. The approximate expression for the irradiance, I(z), as a function of propagation

distance (z) in the nonlinear medium is given by:

i( Z )/i(o) =

2

1 nr ( 3 > u-z/Rr + (i-p/p

2 )(zVkV)

2 where V =

3 ' 77cX

(4)

c 32 /n 2

P = power in the beam

k = 2-ir/A

a = beam waist at the entrance of the sample

R = radius of curvature of the beam phase front due to the external lens

X = laser wavelength

n

2

= nonlinear refractive index

At the beam waist (focus) eq. (3) reduces to

*SF

s l o [r^] (5)

where I<-p = peak irradiance in the presence of self-focusing

I = peak irradiance without self-focusing

Equations (3) and (5) are approximations (sometimes referred to as the constant shape approximation) and are valid for

395
P < P

2 /4 (6)

When the input power is significantly greater than P 2 /4 one must use the more complete numerical

solution to compute the enhanced irradiance due to self-focusing. Table 1 is a comparison of the

approximation given by eq. (5) and the exact numerical solution given by Marburger (see reference

1, p. 66).

The factor of 3.77 in eqs. (1) and (4) comes from numerical solutions of the nonlinear wave

equation for an input beam with a Gaussian spatial profile. The power P2 is the least power for a singular self-focus to occur within the Rayleigh range, i.e., the beam confocal parameter, for

both pre-focused and unfocused geometries. This means that for samples thicker than the Rayleigh

range (as is the case for bulk laser-induced damage experiments) singular self-focus will occur within the sample and laser-induced damage will occur for input power equal to Note tnat ^or tightly focused beams, i.e., very small spot sizes, the breakdown field will be reached before P approaches Pp. If that is the case, then the results will be independent of sel f -f ocusi ng

effects. The focal spot size needed will depend on the material P 2 and the material breakdown

threshold.

Equation (4) can be verified by arranging an experiment in which the breakdown threshold is

very high and F>2 is very low. A classic example of such a material is CSg. This material is an excellent choice for model system studies since its nonlinear behavior has been studied for years

and is relatively well understood. The first step is to measure the r\£ in a manner independent of

the laser-induced breakdown measurements. Table 2 is a summary of such measurements for CS 2 and

other materials of interest using the technique described in reference [3], Values obtained by

other workers using various techniques are listed for comparison [7-10], With one exception the

agreement with other work is excellent. Note that as expected there is n£ dispersion in for

CS 2 over the 532 to 1064 nm wavelength range and little or no dispersion in no for Si O2 and NaCl —

contrary to the predictions of widely used theory for this phenomena [llj. BK-7 exhibits a negati ve dispersion in n

2 over this wavelength range.

We set up a bulk breakdown experiment in CS2> i.e., arranged the sample length to be much

longer than the beam confocal parameter, and measured the breakdown power [12,13]. We then used

eq. (4) to calculate n2 and compared this n2 to values obtained by beam distortion measurements

[3,7]. The results ot this comparison are shown in figure 1. In this experiment laser-induced

breakdown is totally dominated by sel f -focusi ng and the breakdown power is independent of the

focusing conditions [12], Note the excellent agreement between the n2 determined from the breakdown measurements and eq. (4) and those determined by beam distortion measurements. This verifies

that to the factor of 3.77 predicted by theory in reference 2 is correct to within the error bars

shown

.

Note the lack of dispersion in n

2 in CS 2 for the 532 nm and 1064 nm breakdown measurements.

The n2 values are computed from P2 measurements using eq. (4). The beam distortion measurements

confirm that there is no dispersion in n2 (the nonl inearity is due to nonresonant reorientation of

the CS2 molecules). This implies that the A 2 dependence of P2 in eq. (4) is correct. This A

2 dependence is a well known consequence of self-focusing theory, nowever, to our knowledge this is

the first direct confirmation of this wavelength dependence.

These results and those in reference 2 confirm is that P2 is the critical power of importance

in laser-induced breakdown experiments. For input powers much smaller than P2 sel f -focusi ng

effects will be minimal. As P approaches P 2 self-focusing effects will dominate. For P < Pp/4 one may use eq. (5) to correct for self-focusing effects and for P2 > P > P2/4 one must use the

full numerical solution to the nonlinear wave equation to calculate the irradiance at the focal

position

.

3. Relevance to Past Work

One conclusion of reference 2 was that past work in laser-induced breakdown needs to be re- examined in light of the new results regarding the role of self-focusing. This is what we attempt

in this section. Much of the early experimental work [14-16] used the scaling law proposed by

Zverev and Pashkov [17] in 1970. Fundamental to that technique is the assumption that the damage

threshold irradiance for highly transparent materials is an intrinsic [18] property of the

material and that any apparent dependence of the breakdown irradiance on focal spot radius is, w0

, in the material is due to sel f -focusi ng. The breakdown power for a given material was then

measured as a function of focal spot radius and the results were fit to the equation [15]:

396
where Pg is the power required for breakdown, Ig is the so-called intrinsic damage irradiance and

is the first critical power for self-focusing as defined by eq. (2). [Note that eq. (7) can be

derived from eq. (5) if Pp is replaced by Pj and the assumptions regarding intrinsic damage are made [15]. Such plots of 1/ Pg as a function of 1/area were assumed to yield Ig as a slope and

as the y-intercept. Figure 2 is an example of such a plot for two samples of NaCl from the

same vendor (Harshaw Chemical Co.). Note that the two plots yield values of P^ that differ by

about a factor of 4 and the Ig is approximately an order of magnitude larger than commonly

measured for this material. A similar plot of the results for sample 6 at 1064 nm yields

P^=0.70 MW. This result is unreasonable in that n 2 for NaCl has little if any dispersion in the

wavelength region and therefore Pi at 1064 nm should be about 4 times larger than that at 530 nm

(not equal as this result implies).

The data shown in figure 2 were taken in recent measurements in our laboratory and are

presented in this manner simply to illustrate the problems associated with using the procedure

discussed above. The main problems with the procedure are the assumption that Ig is intrinsic and

the use of P.j in eq. (7).

Our conclusion in reference 2 was that prior damage results be reexamined. Unfortunately,

much of the work using the Zverev and Pashkov [17] scaling is not recoverable from the literature

since the uncorrected thresholds are not reported and cannot be extracted due to insufficient

information. However Fradin [14], in anticipation of some problem with the Zverev and Pashkov

method did not scale the data reported in 1973 for self-focusing. Smith et al^ [16] in 1977 also

found some problem in using Pj to correct their 532 nm and 355 nm picoseconddamage thresholds for

self-focusing. In most cases their 355 nm thresholds were higher than Pp A scaling factor,

<}>(P), was proposed such that the equation for the beam intensification as a function of input

power is given by

I

1 = [1 - (PMP^)] *

Numerical values for $ (P) were proposed such that <I>(P) = 1 for P << P^ and<)>(P) = 3.7 for P -> Po.

However, since the exact functional dependence of <|>(P) was unknown at the time the thresholds

reported, reference 16 included the breakdown power and the uncorrected focal area so that

future workers could reexamine the data in the light of new measurements.

The results of this work indicate that <(>(P) is a constant with a numerical value probably

close to 3.77, the numerical value predicted by Marburger [1] for Gaussian beams. If <t>(P) exhibited the functional dependence predicted by Smith et al

.

[16] then our experimental data

should have indicated the presence of strong self-focusing for input powers below Pj. For example, if we take *(P)=1 and let our input power equal 0.9 P± then eq. (8) would predict a factor of

10 increase in the peak-on-axis irradiance. This increase in the peak-on-axis irradiance in the

material would result in an easily detectable change in the far-field transmitted beam profile.

The fact that beam distortions and a polarization dependence are only seen in our test materials

for beam powers approaching P 2 indicate that <j> (P) is independent of power.

We will now use eq. 8 with <J>(P) = 3.77 to reexamine the 532 nm breakdown data of Smith et al

.

[16]. In that work, the breakdown powers are all below P^ so the irradiance increase predicted by

eq. (8) with <|>(P) = 3.77 should be valid. In Table 3 we reproduce the 532 nm breakdown thresholds

reported in reference 16. Six materials were studied including fused quartz and NaCl. The second

and third columns contain the damage threshold power and the uncorrected focal area. The fourth

column contains the ratio of the breakdown power to the Pi critical power for the material. The

Pi values were calculated using n

2 values measured by techniques other than interferometry [15].

The fifth column contains the ratio of the breakdown power to the critical power P

2 calculated

using the interferometric measurements of n 2 of Weber et al. [18]. We use the n

2 values of Weber

et al

.

[18] to cal cul ate P 2 rather than scaling col urnn-Tour by the factor of 3.77, since the

measurements in reference 18 are more accurate and are conducted using picosecond pulses. Thus

the n 2 values should be a reflection of the electronic nonlinear pol ari zabi 1 ity only. In the

sixth column we list the R. M. S. breakdown fields in MV/cm uncorrected for self-focusing. Column

397
seven contains the data corrected for self-focusing using P^ as the critical power. These are the

breakdown thresholds for 532 nm light published in reference 16. The number in parentheses

represents the percentage increase of the corrected threshold over the uncorrected threshold.

Finally in column eight we list the R. M. S. breakdown threshold fields corrected for selffocusing using the ratio P/P 2 in column five. Again the numbers in parenthesis represent the

percentage increase over the uncorrected threshold.

In examining the data in column seven we see that the breakdown threshold fields increased as much as 50-60 percent using P^. However, when the thresholds are properly corrected using the

second critical power P

2 we see on the average only a 5 percent increase. This is well within the

±15 percent absolute uncertainty in the uncorrected breakdown fields reported by Smith et al. [16]

at 532 nm. In addition, the percentage increases in the breakdown threshold field for~Sl 0^~icould

well be smaller (assuming the fused quartz sample of Smith et al. [16] (Suprasil I) has the same

n 2 value at high irradiance as our own Corning 7940 fused quartz).

Smith et al

.

[16] also reported breakdown thresholds at 355 nm laser wavelength in thee

materials listed in Table 3. The results for this near uv study indicate that, in most cases, the

breakdown threshold powers for these materials were substantially higher than the critical

powers at this wavelength. The only exception was CaF2 where Pd was found to be 0.7 Pi. The Pi

critical powers were calculated using the n

2 values measured at 1064 nm under the assumption that

the dispersion in n

2 over this wavelength range was small [16]. In an attempt to correct their

data for the presence of self-focusing they scaled their breakdown threshold irradiance levels in

KH0PO4, NaF, and Li F using eq. (8) and <j>(P)=2.5 -* 3.7. The fused quartz data was scaled using the same equation and 4>( P

) = 2. 0 -* 3.0.

While they were on the right track, meaningful comparison of the 355 nm data with results at

other wavelengths is difficult due to the very poor spatial quality of the 355 nm beam used in the

measurements. The uncertainty in the 355 nm focal area and the uncertainty in the energy distribution within the focal area lead Smith et al. [16] to assign a factor of two range for the scaled

breakdown threshold fields at this wavelength. The actual breakdown thresholds fields may or may

not be within this range. A further complicating factor for interpreting Smith et al. [16] 355 nm

work is the recent result that the effective n 2 for a material may not be constant as a function

of wavelength for photon energies approaching a substantial fraction of the band gap energy.

Based on our measurements in BK-7 at 532 nm and those of White et al. [19] in BK-10 at 355 nm the

no values for these materials may well be substantially 1 ower at the third harmonic wavelength

when compared to the values at 1064 nm. This points to the need for accurate measurements of no

in these materials in the regime where multiphoton effects may be coming into play. As such we

will not attempt at this time to reexamine the 355 nm thresholds.

In our reevaluation of the breakdown results of other workers we have concentrated on the

work of Smith et al

.

[16] primarily for two reasons. The first reason is that the breakdown

measurements in that work were conducted for pulse durations comparable to our own. Therefore,

the self-focusing mechanisms in the test materials will be the same. The second reason is that,

of the workers who scaled their breakdown thresholds for the presumed presence of self-focusing,

Smith et al

.

[16] is the only work that contains sufficient data to deconvolve the true breakdown

thresholds. Other workers merely reported the scaled breakdown threshold irradiance levels without including the focal spot radii used in the measurements [14,15,17]. This makes it impossible

to recalculate the breakdown thresholds.

Several other breakdown studies have been conducted in these materials in which no selffocusing corrections were made. For example, Manenkov [20] reported breakdown measurements in the

alkali halides (including NaCl ) for nanosecond pulse durations at 10.6, 1.06, 0.69, and 0.53 ym

laser wavelengths. There is some uncertainty in the focal spot radius used in the measurements

since two values are reported without specifying which correspond to the breakdown irradiance

levels listed in Manenkov's [20] work. Therefore, we will examine both cases. First, we must

include the effects of electrostriction in our value for n 2 since this mechanism is believed to

play a strong role in the alkali halides for nanosecond pulse durations. We will use the value of

no=4.0 x 10" 13 esu calculated from nanosecond three wave mixing experiments [21]. With this in

mind we find that P B in NaCl is 0.5 P

2 at 532 nm and P B is 0.14 P

2 at 1064 nm if we use the larger

of the two focal radii cited in reference 20. If we use the smaller focal radius we find that Pg

for NaCl is 0.07 Po at 532 nm and 0.02 Po for 1064 nm. Thus self-focusing effects in Manenkov's

[20] work for NaCl are negligible except for the combination of largest spot size and shortest

wavel ength.

In a similar nanosecond study, Merkle et jfL [22] reported single shot damage thresholds for

Corning 7940 fused quartz for laser wavelengths ranging from 1064 nm to 355 nm. In fused quartz

electrostriction has been shown to play a small role in self-focusing effects for pulse durations

398
around 30 nsec [23], Therefore, we will use the n

2 value of 0.95 x 10" iJ esu reported by Feldman

et jfL [23] for nanosecond pulse durations which includes contributions from el ectrostriction,

IFermal and electronic effects. We find that the breakdown powers, P B , reported by Merkle et al

.

[22] for Corning 7940 are_< 0.08 Pp at 1064 nm and equal to 0.07 P

2 at 532 nm. Therefore, seTT^"

focusing effects in the work of Merkle et a_h [22] are negligible.

We have also reexamined [10] our own results published in references 5 and 6. In that work we used beam distortion and polarization dependence to verify that self-focusing was not the

dominant breakdown effect. However, these tests (i.e., polarization dependence and beam distortions) were not conducted for each experimental condition used. In addition, there is a limit in

the sensitivity of these techniques in detecting the presence of sel f -f ocusi ng , e.g., the time

integrated beam distortion measurements can only detect a quarter-wave peak distortion. For the

most part, little or no adjustment of the originally published numbers is needed. For the ultrashort pulse data (pulsewidth less than 5 psec) and largest spot sizes (14 micron 1/e 2 half-width)

for Si O2 at 1054 nm adjustments as high as a factor of two were needed. However, for most of the

Si

0

2 data only small adjustments were required. The corrected data showed essentially the same general dependences on pulsewidth and wavelength reported in references 5 and 6. For Si 0o and

NaCl the pulsewidth dependence was consistent with the predictions of electron avalanche breakdown

theory, while the wavelength dependence was not. The conclusion that bulk damage in these materials at 532 nm and 1064 nm is due to multiphoton initiated, electron avalanche breakdown is

consistent with the corrected data. Even with sel f-f ocusi ng absent, a spot size dependence

remains and sample to sample differences are observed. The latter results indicate the laserinduced breakdown is an extrinsic effect initiated by material defects.

4. Summary

The key conclusions of this paper and reference 2 is that P 2 is the critical power of

importance in laser-induced damage experiments. Equation (5) can be used to correct for selffocusing if P < 0.25 Po, otherwise the full numerical solution to the nonlinear wave equation must

be used. Beam distortion and polarization dependence of laser-induced breakdown can be used to

avoid experimental conditions for which self-focusing dominates the results.

The data reviewed in the last section of this paper is but a small sample of the laserinduced breakdown measurements available in the literature. We have examined a few specific

examples for the purpose of illustrating the importance of using Po instead of Pj in scaling

breakdown results for self-focusing effects. It is important to note in Marburger's landmark

review paper [1] he noted that Akhmanov's [24] constant shape approximation (which uses P^ and is

the basis of the Zverev and Pashkov procedure) gives correct results only if Pj is replaced with

P

2 and P < P 2/4. Our work simply confirms this early theoretical conclusion.

This work was sponsored by the Office of Naval Research; the National Science Foundation,

Grant #ECS-8310625; and the North Texas State University Faculty Research Fund.

5. References

[1] Marburger, J. H., in Progress in Quantum Electronics , J. H. Sanders and S. Stenholm, eds.,

New York, Pergamon Press, pp. 35-110, 1974.

[2] Williams, William E., M. J. Soileau, and Eric W. Van Stryland, "The Effects of Self-Focusing

on Laser-Induced Breakdown," Proceedings of the 1983 Boulder Damage Symposium, to be published.

[3] Williams, William E., M. J. Soileau, Eric W. Van Stryland, "Simple Direct Measurements of

n 2

," Proceedings of the 1983 Boulder Damage Symposium, to be published.

[4] Smith, W. L., Opt. Eng. 17, 489 (1978).

[5] Soileau, M. J., William E. Williams, Eric W. Van Stryland, Thomas F. Boggess, and Arthur L. Smirl, National Bureau of Standards (U.S.), Special Publication #669, 387 (1982).

[6] Soileau, M. J., William E. Williams, Eric W. Van Stryland, Thomas F. Boggess, and Arthur L. Smirl, Opt. Eng. 22(4), 424 (1983).

[7] Witte, K. J., M. Galanti, and R. Volk, Opt. Commun. 34(2), 278 (1980).

399
[8] Weber, M. J., D. Milam, and W. L. Smith, Opt. Eng. 17(5), 463 (1978).

[9] Owyoung, A., R. W. Hellwarth, and N. George, Phys. Rev. B _5, 628 (1972).

[10] Williams, William E. , Ph.D Thesis, North Texas State University, 1984.

[11] Boling, Norman L., Alexander J. Glass, and Adelbert Owyoung, IEEE J. Quantum Electron. QE- 14 , 601 (1978).

[12] Soileau, M. J., William E. Williams, and Eric W. Van Stryland, IEEE J. Quantum Electron. QE-

_19, 731 (1983).

[13] Williams, William E., M. J. Soileau, and Eric W. Van Stryland, Opt. Commun. _50, 256 (1984).

[14] Fradin, D. W., IEEE J. Quantum Electron. QE-9 , 954 (1973).

[15] Smith, W. L., J. H. Bechtel , and N. Bloembergen, Phys. Rev. B 12(2), 706 (1975).

[16] Smith, W. L. , J. H. Bechtel, and N. Bloembergen, Phys. Rev. B 15(8), 4039 (1977).

[17] Zverev, G. M. and V. A. Pashkov, Sov. Phys. JETP 30(4), 616 (1970).

[18] Bloembergen, N. , IEEE J. Quantum Electron. QE-10 (3), 375 (1974).

[19] White, W. T., Ill, W. L. Smith, and D. Milam, Appl . Opt. Lett. ^(1). 10 (1984).

[20] Manenkov, A. A., National Bureau of Standards (U.S.), Special Publication #509, 455 (1977).

[21] Levenson, M. D. and N. Bloembergen, Phys. Rev. B _10, 4447 (1974).

[22] Merkle, L. D., N. Koumvakal is , and M. Bass, J. Appl. Phys. _55(3), 772 (1984).

[23] Feldman, A., D. Horowitz, and R. M. Waxier, IEEE J. Quantum Electron. QE-9 , 1054 (1973).

[24] Akhmanov, S. A., A. P. Sukhorukov, and R. V. Khokhlov, Zn. Tekh. Fiz., 51, 296 (1966); JETP

24, 198 (1966).

Table 1. Comparison of Constant Shape Approximation to

Numerical Solutions for Focused Gaussian Beams

p/p 2 (I/In)

Constant Shape Approximation

(i/ip)

Numerical Solution

Numerical Solution/

Constant Shape Approximation

0.27 1.37 1.30 0.95

0.60 2.50 2.08 0.83

0.80 5.00 3.94 0.79

0.90 10.00 7.15 0.72

0.95 20.00 16.45 0.82

0.96 25.00 28.60 1.14

0.97 33.30 63.40 1.92

0.98 50.00 100.00 2.00

0.99 100.00 192.00 1.92

400
Table 2. n

2 Measurements in Liquids and Solids

Material Wavelength

(ym)

n

2

(x 10" 13 esu)

This Work

n

2

(x 10-13 esu)

Other Workers

cs 2 1.06

0.53

128 ± 10

123 ± 10

125 ± 30a

NaCl 1.06

0.53

1.37 ±0.15

1.38 ± 0.13

1.22 ±0.21b

Si 0

2 1.06

0.53

0.62 ±0.03

0.60 ±0.04

0.95 ±0.10b

BK-7 1.06

0.53

1.45 ±0.15

1.01 ±0.08

a

K. J. Witte, M. Galanti, and R. Volk, Opt. Commun.

ferometry at 1.32 ym.

34, 278-282 (1980); Time Integrated Interb M. J. Weber, D. Milam,

ferometry at 1.06 ym.

and W. L. Smith, Opt. Eng. 17_{5), 463-469 (1978); Time Resolved InterTable 3. 532 nm Breakdown Thresholds of Smith et al. [16]

Material (-V

(KW) (unsealed)

P/Pl P/P 2 (MV?cm)

(unseal ed) (scaled) (scaled)

KH 2 P04 151.0 16.0 0.57 0.15 15.3 23.4(53%) 16.6(8%)

Si0 2 129.0 15.9 0.46 0.12 14.5 19.0(31%) 15.4(6%)

NaCl 38.4 15.1 0.60 0.05 7.9 12.4(57%) 8.1(3%)

CaF 2 146.0 15.9 0.62 0.09 15.5 25.2(63%) 16.3(5%)

NaF 126.0 15.8 0.45 0.05 15.0 19.4(30%) 15.4(3%)

Li F 171.0 16.1 0.59 0.06 16.9 26.5(57%) 17.5(4%)

The last three columns contain the uncorrected breakdown fields and the values corrected for the

presence of sel f -f ocusi ng. Column seven (E

p

) represents the data reported in reference 16.

Column eight (E D

) contains the data scaled usfWg the correct critical power.

401
BEAM DISTORTION MEASUREMENTS

FROM LIB MEASUREMENTS

13

1.06 |im

12

BEAM DISTORTION MEASUREMENTS

FROM LIB MEASUREMENTS

\=1.32ju m TIME INTEGRATED INTERFEROMETRY

0.5 1.0

(n 2

»10" esu)

13

"WILLIAMS, ET AL, OPTICS COMM., 5J), 256, 1984.

12 SOILEAU, ET AL., QUANT. ELECT. QE-19 . 731, 1983.

7 WITTE, ETAL., OPTICS COMM. 3_£, 278,1980.

1.5

Figure 1. n 2 from laser-induced breakdown (LIB) measurements. The LIB results were conducted by measuring the power required for breakdown for a sample of CS 2 thick compared t^

,

tne beam confocal parameter. Equation (2) was then used to calculate n 2

(MWJ

A"' (urn"2

)

Figure 2. P- 1 vs. (Area)" 1 for NaCl. These data are for linear polarized, 532 nm, 35 + 10 Dsec

whir

e

h

S ;n

N m

that

K theHSe, data 3re Pl0Ued in this manner t0 show' that the procedure on which ec,. (7) is based does not work! The I B thusly determined are too large by an order of magnitude and TRe

factor of 4. Pys" for the two samples of the same material differ by a

402
In response to the quest-ion as to theoretical predictions of whether n% should go up with

optical frequency, the author replied that the change in dispersion for picosec pulses is

entirely because of electronic contributions and should go up with frequency, but only because

of local field considerations, which would be expected to be minor. Experimentally it goes up

significantly and the theory needs to be reexamined.

403
Manuscript Received

4-8-85 SAR

Two-Photon Absorption, Nonlinear Refraction and

Optical Limiting in Semiconductors

Eric W. Van Stryland, H. Vanherzeele, M. A. Woodall , M. J. Soileau,

Arthur L. Smirl, Shekhar Guha, and Thomas F. Boggess

Center for Applied Quantum Electronics

Department of Physics

North Texas State University

Denton, Texas 76203

Two-photon absorption coefficients go of eight different semiconductors with bandgap energy E„ varying between 1.4 and 3.7 eV are measured using 1.06 ym and 0.53 ym

picosecond piflses. ^ "" s found to scale as E" 3 as predicted by theory for the samples

measured. Extension of the empirical relationship between B

2 and Eg t0 In ^D w "" tn

E_=0.2 eV also provides agreement between previously measured values and the predicted

$

2

. In addition the absolute values of &2 are in excellent agreement (the average

difference being <26%) with recent theory which includes the effects of nonparabolic

bands. The nonlinear refraction induced in these materials is monitored and found to

agree well with the assumption that the self-refraction originates from the two-photon

generated free carriers. The observed sel f-defocusing yields an effective nonlinear

index as much as two orders of magnitude larger than CS 2 for comparable irradiances.

This sel f-defocusing in conjunction with two-photon absorption is used to construct a simple, effective optical limiter that has high transmission at low input irradiance

and low transmission at high input irradiance. The device is the optical analog of a Zener diode.

Key Words: nonlinear absorption; nonlinear refraction; no; optical limiting; semiconductors; two-photon absorption; CdTe; GaAs; CdS; CdSe; Znfe; ZnSe; ZnO.

1. Introduction

The ever increasing role of semiconductors in light-wave technology has created a pressing

demand for the characterization of the nonlinear optical properties of these materials. Semiconductors are attractive as elements in nonlinear-optical devices because of their large and

potentially extremely fast optical nonl inearities. A careful study of these macroscopic non- linearities should allow one to determine the dependence of these nonl inearities on fundamental

mi croscopic mechanical and electronic material properties (e.g., band gap, carrier lifetime,

carrier effective mass, etc.). The data base formed by this information would then allow one to

not only tabulate the materials that exhibit large nonl inearities but also to predict the

specific material parameters that give rise to these high nonl inearities. This predictive capability is extremely important from the standpoint of searching for materials with large non1 inearities.

A study of the nonlinear optical properties of several semiconductors is presented here, and

a relationship between the two-photon absorption coefficient ($2) an d other material properties

is verified. Eight different materials were experimentally studied for which the incident photon

energy fio) is less than the band-gap energy E q but greater than E q /2 so that two-photon absorption

(2PA) is allowed [1]. Both 1.06 and 0.53 ynf picosecond pulses are used in transmission experiments using semiconductors with Eg ranging from 1.4 to 3.7 eV. We find that the 2PA coefficient

&2 is given by

B

0 = K /E

p f(2fico/E

g )/n

2 E^ (1)

where K is a material independent constant, n is the linear refractive index, and E p is nearly

material independent for a wide variety of semiconductors [2]. The function f, whose exact form

depends on the assumed band structure, is a function only of the ratio of the photon energy 'fito to

E„ which determines the states that are optically coupled. The scaling given by eq (1) agrees

with the most recent theories for two-photon absorption [3-5] and allows for predictions of 2PA

404
coefficients for other materials at other wavelengths given minimal materials parameters. For

example, extension of this scaling to InSb (300°K) at 10.6 ym predicts a ^ °f 6.8 cm/MW which

is in excellent agreement with recent experiments. Specifically, Miller et al. [6] obtain a value of 8 cm/MW. Equation 1, is therefore valid over a range of 20 in band-gap energy from the

infrared to the visible. In addition we find that the proportionality constant K as calculated

by Weiler [4] for nonparabolic bands agrees with our experimentally determined K to within better

than 26%.

We use the experimentally determined 2PA coefficients along with a modified Drude theory

(modified to include effects of interband transitions and band filling) to model the nonlinear

refraction in these semiconductors. We quantitatively fit the predictions of this theory to beam

propagation data obtained for CdSe and obtain excellent agreement when all of the nonlinear

refraction is assumed to arise from the carrier generation [7]. That is, the contributions

proportional to the photogenerated carrier density dominate the usual bound electron nonlinear

refractive index changes. This has been previously shown to be the case for one-photon absorption in materials such as InSb at 5 vm [8] and Si at 1 ym [9], We find that the effective

nonlinear refraction can be two orders of magnitude larger than that for CS2 at comparable

irradiances.

Finally we utilize the combined effects of two-photon absorption and nonlinear refraction in

GaAs to make an irradiance (fluence) limiting device [10]. This device has high linear transmission at low irradiance (fluence) and low transmission at high irradiance (fluence). At very

high irradiances, laser induced melting is also involved in the limiting action. This device is

passive, has picosecond turn on time and is the optical equivalent of a Zener diode.

In Section 2 we describe the model used and derive the equations needed to describe both the

nonlinear transmission and nonlinear refraction observed in the semiconductors studied. In Section 3 we outline the experimental procedure used to determine the two-photon absorption

coefficients and present the experimentally determined 2PA coefficients. Section 4 presents a comparison of the two-photon absorption data to theory for parabolic and nonparabolic bands with

and without exciton corrections. In Section 5 we present the experiments and fits to the beam

propagation data using the results of Section 2. We describe a semiconductor optical limiter,

based on 2PA, its design, operation, and uses in Section 6.

2. Theory

The experimental configuration used throughout this work is one in which the sample was very

thin compared to the confocal beam parameter, and moreover, any self-induced beam phase changes were small enough that beam propagation effects in the sample were negligible (i.e., self action was "external" as described by Kaplan [11]). In this case the Maxwell wave equation for the

propagation of the electric field E can be written as

2ik 3E/8z = iaxm

o

E - (u) 2 /2c

2

) x^

3) |E|

2

E (2)

where denotes the third order nonlinear susceptibility, and

a = (a+a

gx

N) n ^Q /v

Q + i Ne

2

/(meh

u>) (3)

denotes the conductivity. Here, we have explicitly included the possibility of photogenerated

carrier absorption through the term ogx

N, where a gxis the total carrier cross section (holes + electrons) and N is the density of these carriers. Also a is the usual residual linear absorption (e.g., band tail absorption, impurity absorption, etc.), and m en

is the reduced electronhole effective mass. Writing the electric field as

E = Ae i$

(4)

with the irradiance given by I = (ne

Q

C/2)A

405
equation 2 can be separated giving

II = _ a i -3 t 2 -a N I dz 2 ex

(5)

and

dj>

dz

= 3j I - YjN (6)

where 32, tne two-photon absorption coefficient, is proportional to the imaginary part of ^ and 3j_ = uy/c is proportional to the real part of x' Y is related to the more usual no by

n^esu) = cn y/40tt where the right hand side of the equation is in MKS units, y^ in equation b is

given by

Y

l = ^ 0

e2cP/ ( 2nm

ehu ^ (7)

The parameter P is introduced here to account for contributions to the nonlinear refraction

proportional to Nex but not explained by the Drude model. An example of such a contribution is

that arising from interband transitions [12].

The equation governing the carrier generation is

showing that for every two absorbed photons one electron-hole pair is generated. This equation

is valid only for pulses short enough that recombination and diffusion can be neglected during

the pulse. We assume this to be the case for our picosecond pulses [13].

We note that eq (5) for the irradiance is independent of the phase eq (6). This is due to our assumption of a thin sample (i.e., no irradiance changes due to nonlinear refraction within

the material). We can, therefore, solve eq (5) simultaneously with eq (8) for the beam attenuation in a sample of thickness L. These equations must be solved numerically unless the contribution to the absorption from the photogenerated carriers is negligible. We can estimate under

what conditions this is true by finding the irradiance, denoted as I cr , for which the carrier

absorption is equal to the multiphoton absorption. An approximate relation is found in the limit

of small total absorption as

where R is the surface reflectivity and t Q is the HW 1/e M (half-width at 1/e of the maximum in

irradiance) of the assumed Gaussian temporal profile pulses. This result was first given by

Bechtel and Smith [13]. Note that this critical irradiance is independent of 32 since both the

transmission change and the photogenerated carriers result from 2PA. Thus, materials with small

3o, which require high incident irradiance to observe a transmission change, will be the most

likely materials to be affected by photogenerated carrier absorption.

The contribution to the change in transmission from these carriers is proportional to t~*.

Longer pulses of the same irradiance contain more energy and, therefore, produce more carriers.

We can determine if these carriers are contributing to the nonlinear absorption by measuring the

change in transmission for different pulsewidths. We show in Section 3 that the irradiances

used are well below I cr and that we can ignore photogenerated carrier absorption. This is the reason for using picosecond pulses as discussed in ref. 13. While we find the carrier absorption

to be negligible, the refractive index change proportional to the carrier density eq (6) is

(8)

I ~ 2 ^2. -ffo/[a t (1-R)]

cr

' L ex o

v /J (9)

406
definitely not negligible as discussed in Section 5.

The solution to eq (5) with this assumption is

I(z,r,t) = (1-R) j I(o,r,t) e^ z /[l+q(z,r ,t)] . (10)

_0t Z , where q(z,r,t) = 3

2 I(o,r,t) (1-R) (1-e" ')/« .

Inside the sample j=l, and behind the sample j=2, since there are two surface reflections. Also z is equal to L, the length of the sample. The effect of the rear surface reflection on the

absorption has been ignored but is not expected to lead to significant errors in the determination of for the samples used [14]. Equation 10 can be rearranged to give the instantaneous

transmission T

1 at each radial position of a sample of length L as

T'"

1 = [l+q(L,r,t)] eaL

/(l-R)

2

. (11)

Since q(z,r,t) is directly proportional to I(o,r,t) a plot of T'"* versus incident irradiance

should yield a straight line whose intercept determines a and whose slope determines 3o. Experimentally, pulses of Gaussian spatial and temporal profiles are used, which requires spatial and

temporal integration of eq (10). The resulting plot of T~* versus I has a slight downward

curvature caused by these integrations, since at the higher irradiances both the spatial and

temporal profiles are broadened toward the rear of the sample (i.e., there is more 2PA at the

middle, brightest part of the beam). Taking account of the temporal and spatial integrals we

find for the pulse transmission

T = 2a(l-R)/[I

Q

^B

2

(e

aL

-l)] fdx In [l+q(L,o,o) e"

x

] (12)

where we have taken

I(o, r,t) = I Q exp [-(r/r0 ) 2-(t/t Q ) 2

] (13)

Examples of plots of T" 1 from eq (12) as a function of I Q are shown in figures 5a and b of

Section 3.

In order to model the beam profile of the pulse and its propagation, we now integrate eq (6)

using eq (8) for N and eq (10) (j=l), for the irradiance to obtain an expression for the phase:

*(L,r,t) =*(o,r,t) +~ (1-R) In [1 + q(L,r,t)]

p 2

(i-r)

2yi r

l

+ -m^7j dt ' • < 14 > / */- CO

where

F l( t) = aln [1 + q(L.r.t)] - Sik^L [i -

; +V,r , t)

]

407
Equations 10 (j=2) and 14 together, completely describe the electric field at the exit plane of

the sample. From these solutions for I(L,r,t) and 4>(L,r,t), the field at any position outside

the sample (L+z,r,t) can then be determined using the Huygens-Fresnel propagation formalism as [15]

E(L+ z,r,t) = fi exp r'dr' E(L.r-.t - f)exp (^1) Jq (^Ll) . ( 15)

What we measure in our experiments, using short pulses, is the fluence given by

F(L+z,r) = / |E(L+z,r,t)| 2

dt (16)

Experimental results are compared with numerical evaluations of eq (16) and/or spatial integrals

of eq (16) as described in Section 5.

3. Experiment and Data

3.1 Experiment

In the first set of experiments we measured the transmission of several semiconductors as a function of incident irradiance to determine their nonlinear absorption coefficients. The experimental arrangement is shown in figure 1. The laser source used was a microprocessor controlled,

passively mode-locked, Nd:Yag laser that produced single amplified pulses of energy up to 7 mJ

per pulse at 1.06 ym when operated in the TEMqq mode [16]. The pulsewidth could be varied

between 40 and 150 psec (FWHM) by selecting etalons of varying thickness as the output coupler.

The width of each pulse was monitored by measuring the ratio, R, of the square of the energy of

the fundamental (1.06 pm) pulse to the energy of the second harmonic (0.53 ym), pulse that was

produced in a Li IO3 crystal [17]. This ratio is directly proportional to the laser pulsewidth

provided that the spatial profile remains unchanged. This ratio was calibrated by measuring the

pulsewidth using nearly background free second-harmonic autocorrelation scans while accepting

only pulses having a fixed ratio R within 15% of a preset value. To ensure that the ratio, R, was

proportional to the pulsewidth autocorrelation, scans were performed for three output coupler

etalons, and indeed, the ratio scaled properly. An example of such an autocorrelation scan is

shown in figure 2 along with the best Gaussian fit. The autocorrelation width of 54 psec (FWHM)

corresponds to a Gaussian pulsewidth of 38 psec (FWHM).

When 0.53 ym light was required, a temperature tuned CDA crystal was placed in the beam at

the position indicated by the arrow in figure 1. Light at 1.06 ym was blocked with a polarizer

and two 100% dielectric reflecting mirrors. Autocorrelation scans of the second harmonic beam

performed with an angle tuned KDP crystal showed that these pulses scaled as the 1.06 ym pulsewidth divided by /2 to within 10%, as expected for Gaussian shaped pulses. Again the ratio R was

held fixed, and the autocorrelation data was as clean as that shown for 1.06 ym in figure 2.

Two different pulsewidths of 40 and 150 psec were used in the transmission experiments on each sample at 1.06 ym. Since the output coupler etalons used to change the pulsewidth were

optically contacted to a flat rotatable quartz plate, the beam line as well as the measured beam

parameters remained fixed. (A few percent change in the beam spatial width probably caused by

slight self-focusing in the amplifier was taken into account.) The relative error bars between

one transmission experiment and the next, where only the pulsewidth was changed, were very small.

While at high irradiances (a few GW/cm 2

) we did see a small pulsewidth dependence of the transmission in some samples, this difference was consistent with values for the free carrier cross

sections (10 to 10~ 18cm 2

). No pulsewidth dependence was observed at the low irradiance levels

(0.5 GW/cm 2 at 1 pm) used to extract values of the 2PA coefficient. A calculation of I cr from

Section 2 for typical samples at 1 ym gives I cr ~5 GW/cm 2 for a =5xl0~ 18cm

2

. In fact, eq (9),

considerably underestimates I cr. From computer calculations, we find that I cr

is several times

larger, the difference arising mainly from the fact that the spatial irradiance averaging was

ignored in the approximate expression. In addition, at 0.5 ym the contribution of photogenerated

carrier absorption will be less than at 1 ym since hu increases and a gx decreases, both leading

to an increase in I cr. The maximum experimental irradiance used to extract 3^ from the 0.53 ym

data was, therefore, increased to 2 GW/cm 2

. The above experimental considerations justify

408
ignoring free carrier absorption in calculating the transmitted irradiance eq (5) [13].

The spatial beam profiles in both the horizontal and vertical direction were determined by

scanning a 25 ym pinhole at the position of the sample. The beam size was adjusted at the sample

by using pairs of collimating lenses. In all, four different spot sizes were used for the

1.06 urn data from 0.5 mm to 1.5 mm (FWHM). At 0.53 ym the beam size used was 0.5 mm. In addition the beam profiles were monitored on a vidicon to ensure that there were no hot spots,

spurious reflections, or shot-to-shot beam width fluctuations. Figure 3 shows a representative

pinhole scan for a 1.06 ym beam of FWHM 1.50 mm (FWHM).

The incident energy was continuously varied using a stepping motor controlled rotating half- wave plate in combination with a fixed polarizer. This apparatus kept the polarization on the

sample fixed and introduced no measurable beam walk with rotation angle. Previous experience

indicates that other alternatives, for example, rotating calcite polarizers, may cause the beam

to walk across the sample and across the energy monitoring detectors.

The choice of detectors, as well as the detection geometry, was also determined to be

critical. As discussed in Section 5 the phase aberrations introduced on the beam by the twophoton generated free carriers cause considerable defocusing so that the beam profile at the

detector varies with incident irradiance. Figure 4 shows an example of this defocusing as observed in the near field behind a sample of CdTe. As the irradiance is increased the beam

broadens and breaks up as is characteristic for sel f-defocusing [18]. Thus, any spatial nonuniformities in the detector response can lead to errors. Indeed care must be exercised to ensure

that "external" self action [19] does not result in overfilling the transmission detector--an

occurrence that could result in an overestimate of 3^ E 20 3 or result in optical limiting as discussed in Section 6. We found, however, that by using large area detectors (1 cm^) with a measured spatial uniformity of better than 10% and placing them as close as possible to the

sample (3 cm) that these effects were eliminated. The detectors were also determined to be

linear over their range of use and were absolutely calibrated with respect to a pyroelectric

energy monitor [21] which was in turn checked against two others. In addition, absorbing type

neutral density filters placed in front of these detectors were checked to have linear trasmission over a range at least a factor of ten greater than the range used in these experiments.

Filters were never used to attenuate the beam prior to the sample. In addition, spike filters

transmitting only 1.06 ym (0.53 ym) were placed directly in front of the detectors to reduce

optical noise from the flashlamps.

3.2 Data

Table I lists the 10 samples used in these experiments. In all, eight different materials

having either a zincblende or wurtzite structure were investigated. All of the samples were 1

1 -V

materials except for GaAs which is a 1 1 1 - V material. The thickest sample used (0.5 cm) was over

100 times thinner than the confocal beam parameter (the Rayleigh distance) used for this study.

Experiments were performed on each single crystal sample for two orthogonal directions of linear

polarization. Within our experimental accuracy, no anisotropy in the measured values for $2 was

observed. A 15 percent variation was reported in ref. 34 for room temperature CdTe; however, the

optical pulsewidth was not given. In addition the absolute values of the 2PA coefficients

reported there were an order of magnitude larger than we measure. This may indicate the

dominance of carrier absorption for long pulses. In ZnTe and the single crystal CdTe, the light

propagation direction k was in the (110) direction and in GaAs, k was in the (111) direction. In CdSe, CdS $Se^ and CdS^Se^, k was parallel to the c axis; while in CdS and ZnO, k was

perpendicular *to the c axis. "Examples of data used to extract the 2PA coefficient are shown in

figures 5a and 5b for 1.06 ym and 0.53 ym respectively. Each data point is the average of five

laser firings. The solid line in figure 5a is a fit for CdSe using a=0 and 32=18 cm/GW in eq

(12) of Section 2. The solid line in figure 5b is a similar fit for ZnSe using a=0.5 cm"* and

32=5.5 cm/GW at 0.53 ym. In all samples the linear absorption was small, and its value was

unimportant in the determination of 3£. In both samples of ZnS the scattering was significant

and this loss mechanism was included in the model as linear absorption. In the latter case, the

effect of the choice of a on 32 was less than 10%.

The results of these measurements of the two-photon absorption coefficients are given in the

next to the last column of Table I. The absolute error bars on the values of 3 2 are estimated to

be ±40%. The relative error bars of 32 determined from one sample to the next, are considerably

better as observed by measuring all the samples in sequence for each spot size and each pulsewidth. For example, ZnS(y) listed in Table I always had a larger 3

2 than ZnS(c). We conservatively estimate these relative error bars which are important in determining the parametric

dependence of 3 2 as discussed in the next section, to be ±25%.

409
In the second set of experiments (to be discussed in Section 5) we replaced the transmission

detector in figure 1 by either a vidicon tube interfaced with an optical multichannel analyzer

(PARC 1215) or by a a 25 vm pinhole placed at various radial positions and longitudinal distances

from the sample. We then monitored the spatial beam profile of the transmitted beam using the

vidicon, or we monitored the pinhole transmission as a function of incident irradiance. In Section 5, we describe the use of eq (16) to theoretically fit both of these results.

4. Comparison of $ 2's t0 Theory

4.1 2 PA Theory

We make three separate comparisons in this section. We first compare our experimentally

determined £ 2

's witn tne tneor\y of refs. 3, 4, and 5 using parabolic band structure. Excellent

agreement is found for all materials except ZnTe. We then use the nonparabolic theory and again

find good agreement for all materials except ZnTe. As shown by Weiler, [4] the differences

between the parabolic and nonparabolic theories are minor so that this fit is expected. We then

include exciton correction factors, as given by Lee and Fan [35] and as calculated by Weiler, [4]

and we find that when these are included, ZnTe nearly fits the dependencies shown by the other

material s.

As stated in ref. 5, the parametric dependence of 3 2 on n, E

q and E p was first explicitly

pointed out by Pidgeon et al., [3] although it was present in the calculations of Basov et al.

[36] The band structure and transition scheme used by Pidgeon et al. [3] is shown in figure 6. Calculations using this scheme have been performed for parabolic and nonparabolic bands. They

found

8 = £2lL f (^) = 53.8 -^P f (^A (17)

2 /2m c2

n 2

!

7 \e / ^E1 U /

g g g xg

'

where E

p and E q are in eV and 3? in cm/GW in the last expression. Here, m is the electron mass, e the electron charge, c the speed of light in vacuum, and n the refractive index. The values for

Eg, Ep and n for each material are listed in Table I. It is important to note that for both

parabolic and nonparabolic bands the parametric dependences on n, E„, and Ep predicted by the

theories are the same. The differences lie in the function f and the ratio of fiw to E q

(i.e.,

which states are optically coupled). Weiler [4] corrected an error in the calculation or f in

ref. 3 and obtained the following expression using parabolic bands.

f(x)=£(4^f) J<*4^. 96 .9 {F

2 (x)} (18)

where F

2

(x) is the same function defined in ref. 5. For nonparabolic bands, Weiler [4] finds

A«E

g 32 (x-1)

5 ~

3/2

3/x

( 2 x-1)

3/2

+ 1*±U_ (xW+6 ) 3x

5 (19)

As shown in ref. 4 the differences between f

nR for A<<E„ and A»E

g

, where A is the split-off

energy shown in figure 6 are small. The expression for A>^tg is

np w 10

3 3 ^ 2 45 5 H6X 2* D;

!

( 2 x-1)

(20)

Exciton corrections to values of e2 have been predicted by Lee and Fan [35], Weiler [4] has

evaluated these corrections in terms of the ratio e of the exciton binding energy E^ to the bandgap energy E

g

. These ratios are listed in Table I. We reproduce the results of these calcula410
tions at the excitonic enhancement g ex , of 3 2

, as a function of fi^/Eg in figure 7 for various

val ues of [4].

4.2 Comparison to Theory

Figure 8 shows a log-log plot of ^ 2 scaled by n

2

/(

v/E p Fp) (see eq (10) for F

2

) as a function

of E

Q

. The solid line is a least squares fit to the data (excluding ZnTe) for a line having a slope of -3 to account for the E~ 3 dependence of $ 2

. Clearly the parametric dependences using

this parabolic theory fit the data very well except for ZnTe. This single parameter fit yields a two-photon absorption coefficient given by the following equation:

S2 = (3., ± 0.5)103 PM^

(2i)

n E 9

where again E p and E q are in eV and e 2 1S in cm/GW. The values predicted from eq (21) are listed

in the last column of Table I for each material. The value of the constant (3,1 x 10 3 in eq

(21) predicted by theory from eqs. (17 and 18) is 5.21 x 10 3

, so that the absolute values of the

experimentally determined 3 2

's are, on the average, low by a factor of 1.7.

If we now compare the experimentally determined £ 2

's with the nonparabolic theory (A«E

q

, eq

(19) we obtain the results shown in figure 9. While the overall fit to the parametric dependence

is not quite as good as for the parabolic case (again the solid line is a least squares fit to a line of slope -3 excluding ZnTe), the absolute values are, on average, only 26% lower than

predicted by this theory. The dotted line is the curve predicted by eqs. (17 and 19).

A similar plot (not shown) using eq (20) for A>>E

q gives an almost identical fit except in

this case the difference between theory and a least squares fit of the data is reduced to only

3.5%. The actual case for these materials lies in between these two theories, although closer to

A «Eg, and may be partially responsible for scatter in the data (up to 23% change in 6 2

).

If we now include corrections to 3 2 due to excitons as given in figure 7 we obtain the curve of figure 10. Here, we have used the parabolic theory (i.e., F

2 ) except for the additional

scaling with gex

. This correction factor has only been calculated for parabolic bands. 35 What we

see in figure 10 is that most of the discrepancy between theory and experiment observed for ZnTe

(g ex

=4.3)^ has been removed by including excitonic enhancement. However, the overall fit to the

parametric dependence (again the straight line is a least squares fit) is somewhat worse. This

may be attributed to our lack of accuracy in calculating ge

„. Note that for most materials g ex

is of the order of 2 even when the coupled states are well above the gap (see fig. 7). Consequently, when excitonic enhancement is included for parabolic bands, the absolute 3 2 values

predicted by theory are a factor of 3.3 larger than those measured.

The reason that the exciton enhancement for ZnTe is considerably larger than for the other

materials is that at 1.06 ym, two photons couple states only 3.5% above the gap, where excitonic

effects should be greatest as shown in figure 7. We should point out, however, that we have only

one material where the coupling is this close to the gap, and the error bars for this material

are larger than for the other materials. ZnTe damaged easily allowing us to use a small spot

size, only and limited the range of irradiances for fitting 3 2

. On the other hand (as shown in

fig. 8), where there is overlap between the data taken here and data taken by Bechtel and Smith,

[13] the agreement is excellent. They also obtained a large 3

2 for ZnTe.

Two other data points listed in Table I and shown in figures 8, 9, and 10 require comment.

The polycrystal 1 i ne CdTe sample gave an experimental 3 2 lower than the single crystal CdTe

sample. It is designed as a 10.6 ym optical window and is doped with 10 17 cm" 3 indium [24]. It is not understood at present if or how these impurities lower the measured 6 2

. The ZnS(y) sample

is a chemical -vapor-desposition grown sample that has a yellow appearance caused by crystal

lattice imperfections that can be annealed out by a special heat treatment process [26]. The

ZnS(c) is the same starting material that has undergone this heat treatment. It appears water

clear, and its linear transmission cutoff is shifted into the UV. The indication from the data

presented here is that not only has the linear absorption in the visible and near UV been

reduced, but that the two-photon absorption at 0.5 ym has also been reduced by this heat treatment. That is, defects may be contributing to £2

. This may also be true for the ZnTe sample at

1 ym since it damaged easily.

411
5. Self-Refraction

In this section we present results for the beam propagation behind the sample given the

irradiance and phase distributions of equations 10 (j=2) and 14. Equation 16 (Section II)

describes the fluence at any point z,r behind the sample. We present data here using a beam of

width 1.70 mm (FWHM) incident on the CdSe sample listed in Table I, that is placed near the beam

waist. Thus $(o,r,t) is taken to be zero in eq (14). The transmitted signal was monitored at

distances of 0.5 m and 2 m behind the sample which are both near field regions. Figure 11

illustrates the change in the beam profile with irradiance at z=0.5 m (using 92 psec FWHM pulses)

as displayed on the vidicon. This scan is a narrow slice through the center of the beam equivalent to a pinhole scan. The dotted line is a fit to eq (16) as a function of r for 6 2=18 cm/GW,

P=3, and Y=0(i.e., nojO). We find that if present, the effects of bound electronic contributions

to the nonlinear refractive index are overshadowed by the photogenerated carrier effects. This

is confirmed by our other measurements which monitored the fluence transmitted by a pinhole as a function of irradiance.

Using a 25 ym diameter pinhole in front of a photodetector we measured the on-axis (r=0)

fluence of the transmitted signal as a function of the peak on axis input irradiance for two

pulsewidths and two distances. The results are shown in figures 12 and 13. To further verify

the validity of the theory, the irradiance dependence of the transmitted fluence at an off-axis

point was also measured. This is shown in figure 14.

The theoretical fits in figures 12, 13, and 14 were obtained from the numerical evaluation

of F given in eq (16). Other parameters (see Table I) used in the calculation are a =.2 cm , me ^=.104 m [30]. The total density N of charge carriers generated by two-photon absorption on axis can be calculated by integration of eq (8). At a peak input irradiance of 1 GW/cmS we

obtain a peak value of N evaluated at the input plane of the sample to be 2 x 10^°cm -3

. We find

that the best agreement between the theory and the experiments is obtained for P=3.5. The fits

shown in figures 12, 13, and 14, are for values of P=3.5 ± 1, with the exact value of P adjusted

between 4.5 and 2.5 to obtain the best fit. Using a two-level model, P has been calculated to be

Eq/(E^-h 2 w

2

) [37]. For CdSe, E

q =1.74 eV at room temperature, and this formula predicts P=2 at 1

urn. The peak phase change undergone by the beam, calculated from eq (14) of Section II is (for

I 0=l GW/cm 2

) A$ =-8.1, which is 1.3 wavelengths distortion. We have ignored the due t0 DOuncl

electronic effects in these calculations. Even if this nonlinearity for CdSe were as high as the

no of CS£ (i.e«> 10" 11 esu), the maximum contribution to the phase change would be 40-rrLIu)n2/n o which is 0.2 for I Q =1 GW/cm . This index change also would be a self-focusing effect and not a defocusing effect as observed. Thus the nonlinear refraction observed in CdSe is approximately

40 times larger than in CS2 at this irradiance. Higher irradiances give rapidly increasing

values of defocusing since the nonlinearity is induced by 2PA (i.e., a factor of two increase in

irradiance gives nearly a factor of four larger phase distortion).

6. Optical Li miter

In this section we describe a nonlinear optical device (an optical power limiter) [10] which

utilizes both two-photon absorption, as discussed in Sections III and IV, and the associated

nonlinear refraction discussed in Section V. This completely passive device has a high transmission for low input irradiance (fluence) but it clamps the output at a constant irradiance

(fluence) above a predetermined input. Such a device can be used as a protective element to

restrict the irradiance (fluence) of a pulse incident upon sensitive optical components or as a regulator to smooth optical transients. This device is the optical analog of a Zener diode.

Optical limiting by nonlinear absorption in semiconductors was proposed and demonstrated in

the late 1960's [38-40]. Moreover, nonlinear refraction combined with spatial filtering has been

used to demonstrate optical limiting in liquid [41,42] and gas filled cells [43]. Here we

demonstrate optical limiting in GaAs. Below the melting threshold, what we have done differently

is to use not only nonlinear absorption (2PA) in GaAs but nonlinear refraction together with

spatial filtering to construct a more effective device. Above the melting threshold, we also

take advantage of a solid-to-liquid phase transition, which affects the reflectivity and transmission.

The geometry we used for optical limiting is shown in figure 15. A single 40 ps (FWHM)

1.06 ym pulse was focused to a 100 ym (FWHM) spot at the surface of the GaAs with a 465-mm-focal

length lens Li. The transmitted beam was collected and collimated by a 381-mm-focal lens I2

placed one focal length behind the sample. The recollimated beam then passed through a 2-mmdiameter aperture placed one focal length beyond L2 and directly in front of a photodiode. The

sample used was nearly identical to the sample listed in Table I. The limiting capabilities of

the device are shown by the triangles in figure 16. At low input energies (< 0.5 yj), the device

412
response was linear and was consistent with the 45% linear transmission of the GaAs and the 73%

transmission of the pinhole. Above 10 yJ input, the output energy was essentially clamped at

1 yJ. The device continued to limit for input energies greater than 1 mJ. Over the full range

of operation, the system transmission was reduced from 33% to 0.1%. Notice that regulation

continued for input energies far above the GaAs single shot melting threshold of 0.9 J/cm 2 (indicated by the arrow in fig. 16). Above the melting threshold, the GaAs was translated before

each firing so that each pulse irradiated virgin material.

Below the irradiance required for melting, as the input energy was increased, the transmission of the GaAs was reduced by 2PA. In this regime, the amplitude of the spatial beam profile

transmitted by the GaAs was distorted solely by 2PA, and the phase distorted by index changes

associated with the 2PA-generated free carriers as discussed in Sections II and IV. The selfdiffraction associated with this phase and amplitude distortion reduced the effective pinhole

transmission. Above the melting threshold, the increase in the reflectivity and absorptivity of

the molten region further reduced the GaAs transmission and further distorted the amplitude

profile of the transmitted beam.

The contribution of 2PA and nonlinear reflection to the limiting action can be separated

from the contribution of nonlinear refraction by carefully collecting all of the transmitted

energy with the pinhole removed as shown in figure 16. These results are shown by the circles in

figure 16. Below E the nonlinear transmission is dominated by 2PA and can be fit by a 2PA

coefficient of 26 cm/GW [10]. The value of 26 cm/GW reported in ref. 10 for the 2PA coefficient

$2 of GaAs is nearly identical to the value of 23 cm/GW shown in Table I and indicates the con- fidence in the values of

fy. These measurements of 3 2 on GaAs were made independently with a nearly identical laser system. Above Ey^, the problem becomes much more complicated and is

outside the scope of this paper. In this regime, the central region of the pulse that arrives

after melting is initiated is heavily attenuated and reflected by the molten layer of GaAs. In addition, there is considerable evaporation of material for fluences more than -10% above the

melting threshold. It is clear, however, that the transition from below to above threshold is a smooth one. That is, there is no discontinuity in limiter response at threshold.

It is interesting to compare the contributions of TPA and self-diffraction just below the

melting threshold. At an input energy of 80 yJ, 2PA acting alone has reduced the transmission by

a factor of 5. On the other hand, the combined effects of TPA and self-diffraction have reduced

the transmission by a factor of 30. Thus, the present configuration is a considerable improvement over limiters that utilize 2PA exclusively.

We should also like to contrast the present switch with the Si device demonstrated in ref.

44 that utilizes indirect absorption, free-carrier absorption, self-diffraction and a solid-toliquid phase change to limit energetic pulses at 1 ym. The nonlinear absorption in Si at 1 urn is

strictly fluence dependent, and the device operation is independent of pulsewidth for pulsewidths

shorter than the carrier recombination time. The present device is considerably more complicated. The nonlinear absorption, which is dominated by 2PA, is irradiance dependent, while the

nonlinear refractive index that arises from the 2PA-generated free-carriers is a time integrated

effect that persists for the duration of the carrier lifetime. Nevertheless, the carriers cannot

be generated without a sufficiently intense pulse, which in practice restricts this device to

operation with short pulses. However, the limiting pulse energy can be varied considerably by

changing the geometry (e.g., using a very short focal length lens Lj in figure 15 will lower the

limiting energy). An advantage of the present device (and 2PA-based optical limiters in general) over the Si device is its higher linear transmission at 1 m. Another more important advantage

of 2PA-based limiters is the broader band-pass that they offer. For example, the GaAs device

should function for wavelengths between approximately 0.9 and 1.7 ym where 2PA is the dominant

absorption process.

7. Conclusion

The material parameter dependence found for a wide variety of semiconductors as discussed in

Section III allows us to predict, with reasonable confidence, the two-photon absorption coefficient of other materials at other wavelengths. This includes, for example, mixed ternary compounds. Thus the 2PA at a particular wavelength can be tailored for a specific application.

The fact that this scaling fits the data so well implies that all of the important materials

parameters have been included in the theory and that other contributions to 62 ( e '9«> higher

bands) cause small effects. One possible deviation from the predicted scaling is the effect of

excitons. We have one material (ZnTe) which indicates that such effects may be important. If we

attempt to extend this theory into the UV, we find that the predictions are in general con- siderably lower than experiments; however, a general trend can be found. If the coupled states

413
are well above the gap the deviations are relatively small, on the order of a factor of 4 or 5. As the coupled states get close to the gap, however, the deviation increases rapidly. For

example, in Rbl at 266 nm where 2tiu>/E =1.47 the experimentally measured 2PA coefficient of

2.49 cm/GW [45] is 3.7 times larger than predicted by eq (21). At a wavelength of 355 nm where

2t)w/E g=1.10, however, the measured 2PA coefficient of 5.08 cm/GW [45] is 14 times larger than

predicted. This trend is maintained for the limited data available [45]. Considering color

centers as excitons with large binding energies, such large correction factors may be accounted

for. The role of excitons and color centers in 2PA needs further study. A study of the wavelength dependence of 2PA near the gap using a continuously tunable laser should help clarify the

role of excitons.

It is interesting to look at the history of the measurement of 2PA coefficients. Figure 17

shows experimental values of e 2 reported for GaAs and CdSe for years beginning with 1966. This

figure illustrates the fairly steady decrease in reported values as both the lasers and the

experimental technique were refined. Much of the earlier data was obtained with nanosecond

pulsed lasers where photogenerated carrier absorption is expected to dominate. Additionally in some instances multimode lasers were used. An additional experimental problem not previously

recognized that can lead to an overestimate of 3 2 is that the extreme defocusing present may

allow some of the transmitted light to go undetected.

While this defocusing may be an experimental difficulty in determining 3 2 it can be

extremely useful for nonlinear optical devices such as the limiter discussed in Section VI. We

found that this defocusing was quantitatively explained by attributing all of the nonlinear

refraction to the build-up of excited carriers although the simple Drude theory had to be

modified to allow for interband transitions. These fits indicate that interband transitions may

actually dominate the refractive index change in these two-photon absorbers. In addition, we demonstrated a simple but effective optical limiter based on two-photon absorption, the asso- ciated sel f-defocusi ng, and spatial filtering. By choosing a substance with the proper materials

parameters and a specific geometry, this limiting can be tailored to suit a specific need.

Clearly further applications of the combined action of 2PA and self-refraction in nonlinear

optical devices have and will be made.

The authors are most grateful to B. S. Wherrett for many valuable discussions concerning the

theory of 2PA. This research was supported with funds from the National Science Foundation (ECS

#8310625), the Office of Naval Research, the Defense Advanced Research Projects Agency, and The

Robert A. Welch Foundation.

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Semiconductors Physics and Applications , ed. W. Zawadzki, Springer Verlag, New York, 1980,

p. 13, for values not listed in this reference the value of 21 eV was assumed.

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415
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416
Two-photon Absorption: X = 1.06/im 2hoj = 2.34 eV

ividici i d

i

fnrm' 3/ i \j 1 1 1 1

* n(X) En(eV) Eu/E„ p 2 GW'

otheor./^Hl)

p 2 *GW'

ZnTe<c > z 2.79< c> 2.26(i) 19.1 0.004<o) 4.5 0.89

CdSe(c) w 2.56< c) 1.74<j> 21 0.007<') 18 18.6

CdTed) z 2.84(c) 1.44(c) 20.7 0.003(o) 22 25.1

CdTe(d) Zp 2.84(c) 1.44c) 20.7 0.003(o) 15 25.1

CdS 5Se<^ w 2.45<» 1.93 (k) 21 0.010(l) 10 12.1

CdS 25Se w 2.51(D 1.78< k) 21 0.008(l) 15 17.7

GaAs(e) z 3.43(i) 1.42<'> 25.7 0.003(o) 23 19.7

Two-photon Absorption: A = 0.53;um 2fioj = 4.68 eV

Material form n(X) Eg(eV) Ep(eV) Eb/Eg Pt

e°r -1GW)

ZnS(f) Zp(c) 2.40<'> 3.66(i) 20.4 0.010(i) 2.0 2.10

ZnS<f> Zp(y) 2.40(i) 3.66(i) 20.4 0.010(i) 3.5 2.10

ZnSe^ Zp 2.70(' ) 2.67 (i) 24.2 0.008(o) 5.5 4.27

CdS(c) w 2.60(i) 2.42(c) 21 0.012(i) 5.5 4.87

ZnO(h) w 2.05 (i) 3.20(n) 21 0.020(i) 5.0 4.77

Table 1. Material parameters and two-photon absorption coefficients of the materials studied.

(a) z=zincblende, w=wurtzite p=polycrystall ine

(b) ref. 22

(c) ref. 23

(d) ref. 24

(e) ref. 25

(f) ref. 26

(g) ref. 27

(h) ref. 28

(i) ref. 29

(j) ref. 30

(k) ref. 31 (1) ref. 32

(m) ref. 33

(n) ref. 4

Nd:YAG

LASER SYSTEM

tp

MONITOR

ENERGY

MONITOR

1 . COLLIMATOR SAMPLE F

li-o-->- ——-•{]— -Q-^-i

ATTEN.

U

I ENERGY

MONITOR

VIEWER

HeNe

Figure 1. Experimental setup for measuring the two-photon absorption coefficients 6 ? at

1.06 um. The arrow before the beam splitter indicates the position of the second harmonic

crystal when 0.53 ym pulses were used.

417
3

Figure 2. An autocorrelation scan of pulses Figure 3. A pinhole beam scan showing a best

having a FWHM of 38 psec as calculated from the fit Gaussian (solid line) of FWHM 1.50 mm. best fit Gaussian (solid line) autocorrelation.

Figure 4. Vidicon scans (equivalent to a pinhole scan) in the near field of the 1.06 pm beam

transmitted through a polycrystal 1 ine sample of CdTe showing the defocusing for increasing

irradiance.

418
(a)

i- 1.6-

0.0 0.4 0.8 1.2 1.6

IRRADIANCE (6W/cm2)

IRRADIANCE (GW/cm<

Figure 5. Inverse transmission versus incident irradiance for (a) CdSe at 1.06 ym and (b) ZnSe

at 0.53 ym. The solid lines are fits using eq (12) of Section II.

PHOTON ENERGY/ENERGY GAP

Figure 6. Band structure used in refs. 3

and 4 calculate two-photon absorption

coefficients.

Figure 7. Exciton enhancement factor g ex

as a function of «ffto/E for various values of the excition

binding energy E b to the band-gap energy E q (reproduced with permission from ref. 4).

419
1 1 sr~]—

r

GaAs !

_i i i 1 1 1 —I

1.5 2.0 2.5 3.0 3.5 4.0

EJev)

Figure 8. A log-log plot of the scaled two-photon absorption coefficient versus energy gap

assuming parabolic band structure. The solid line is a least squares fit of the data to a line

of slope -3 (omitting ZnTe). The x's shown for GaAs, CdTe, CdSe and ZnTe are data from ref. 13. Data to the left of the vertical dotted line was taken with 1 ym light, and to the right with

0.5 ym 1 ight.

Figure 9. A log-log plot of the scaled two-photon absorption coefficients for nonparabolic band

structure (A<<E

q

) versus energy gap. The solid is a least squares fit of the data to a line of

slope -3 (omitting ZnTe). The dashed line is the theory of ref. 4. 420
Eg(ev)

Figure 10. A log-log plot of the scaled two-photon absorption coefficients including exciton

enhancement (g ex ) for parabolic band structure versus energy gap.

Figure 11. Vidicon scan of the beam

transmitted through the polycrystal 1 ine

sample of CdSe at (a) high irradiance

(1 G W / c m

2

i, and (b) low irradiance

(0.3 GW/cm 2

) at a distance of 0.5 m behind the sample (near field). The

pulsewidth used was 92 psec FWHM. The

beam profiles are normalized to have the same on axis fluence. The dashed line

is experimental and the solid line is

theoretical fit using eq (16) of Section

II.

a =

CO

1

1

'

j A

i

J

I \lb)

t

i

! w

r—^Mja^ia^aV'S 1 ^cq-~----s.--r""4— a--.

-4 -2 0 2

TRANSVERSE DISTANCE (mm)

421
0.0 0.1 0.2 0.3 0.4

INCIDENT IRRADIANCE (GW/cm?)

0.0 0.1 0.2 0.3 0.4 0.5

INCIDENT IRRADIANCE (GW/cm2)

Figure 12. Transmitted on axis fluence as a function of incident irradiance at a distance

of 0.5 m behind the sample, (a) 92 psec FWHM

pulses and (b) 43 psec FWHM pulses. The

solid lines are numerically calculated from

eq (16) of Section II.

Figure 13. Transmitted on axis fluence as a function of incident irradiance at a distance

of 2.0 m behind the sample, (a) 92 psec

FWHM pulses and (b) 43 psec FWHM pulses.

The solid lines are numerically calculated

from eq (16) of Section II.

5

Figure 14. Transmitted fluence measured at a point 1.1 mm off-axis as a function of

incident irradiance at a distance of 0.5 m behind the sample, (a) 92 psec FWHM pulses

and (b) 43 psec FWHM pulses. The solid

lines are numerically calculated from eq (16)

of Section II.

INCIDENT IRRADIANCE (GW/cm2

)

422
Figure 15. Schematic of GaAs optical limiter.

102

Figure 16. Device response with (triangles) and without (circles) the 2 mm

aperture in place. E th represents the

single-shot melting threshold.

10'

10l >-C9

i-2

1 i i i i

# •«% •••• •

• •

•

• •

•

•r

GaAs

• No Aperture

t i

E TH *2mm Aperture

i t i i i 40 80 120 160 200 240

ENERGY IN [fxl]

Figure 17. A semilogarithmic plot

of the reported two-photon

absorption coefficients for GaAs

(lower case letters) and CdSe (upper case letters) versus year.

10'

10° -

(a) ref. 36

(b) ref. 40

(c) ref. 39

(d) ref. 46

(e) refT 47

(f) ref. 48

(g) ref. 49

(h) ref. 35

(i) ref. 34

(k) ref. 13

(1) ref. 51

(m) ref. 52

(n) this work

(A) ref. 36

(B) ref. 39

(C) ref. 53

(D) ref. 13

(E) ref. 54

(F) this work

E

u

S3 10

CM

Q2.

10'

a

d

"® b

f h

® 6 ©

1 1 ®

n

c m ®

65 70 75

Year

80 85

423
APPENDIX I BOULDER DAMAGE SYMPOSIUM

Attendees

October 15-17, 1984

M. A. Acharekar

International Lasers Systems

3404 N. Orange Blossom Trail

Orlando, FL 32804

(305) 295-4010

Dennis P. Arndt

Naval Weapons Center

China Lake, CA 93555

(619) 939-3247

John F. Asmus

Maxwell Laboratories

9244 Balboa Avenue

San Diego, CA 92123

(619) 279-5100, ext 178

David S. Atlas

Eastman Kodak Company

Rochester, NY

William P. Barnes, Jr.

ITEK Optical Systems

Litton Industries

10 Maquire Road

Lexington, MA 02173

(617) 276-2246

W. T. Beauchamp

Optical Coating Laboratory, Inc.

2789 Northpoint Parkway

Santa Rosa, CA 95401-7397

(707) 525-7649

Michael F. Becker

University of Texas

Electrical Engineering Dept.

Austin, TX 78712

(512) 471-3628

Bernard Bendow

BDM Company

1801 Randolph Road, SE.

Albuquerque, NM 87106

(505) 848-5129

Harold E. Bennett

Naval Weapons Center

Code 38101

Michel son Laboratory

China Lake, CA 93555

(619) 939-2869

Steven L. Bernasek

Princeton University

Dept. of Chemistry

Erick Laboratory

Princeton, NJ 08544

(609) 452-4986

Lt. Col. Jerry R. Bettis

U.S. Air Force

Air Force Weapons Laboratory/CA

Kirtland AFB, NM 87117

Leonard Bordzol

Perkin Elmer

M/S 843

100 Wooster Heights Road

Danbury, CT 06810

Gordon Boultee

Optical Coating laboratory, Inc.

Dept. 441-2

2789 Northpoint Parkway

Santa Rosa, CA 95407-7397

(707) 525-7110

Ernie Braunschweig

Research & Developemnt Associates

P. 0. Box 9377

Albuquerque, NM 87119

(505) 243-5609

Morris Braunstein

Laser Power Corp.

4746 N. LaVilla Marina

Marina del Rey, CA 90292

Ronald Braustein

Corion Corporation

73 Jeffrey Avenue

Holliston, MA 01746

(617) 429-5065

George Brost

FJ Seiler Reserach Lab.

U.S. Air Force Academy

Colorado Springs, CO 80840

(303) 472-3122

Richard R. Brown

The Boeing Company

Mail Stop 8C-23

P.O. Box 3999

Seattle, WA 98124

(206) 746-4672

Dennis Burge

Naval Weapons Center

Code 3816

China Lake, CA 93555

(619) 939-3049

Charles K. Carniglia

Optical Coating Laboratory, Inc.

2789 Northpoint Parkway

Santa Rosa, CA 95401-7397

(707) 525-7191

424
Lloyd L. Chase

Indiana University

Physics Department

Bloomington, IN 47405

(812) 335-1485

Rod Clark

Teledyne Brown Engineering

Cummings Research Park

M/S 19

Huntsville, AL 35808

(205) 532-2355

Charles Cockrum

Laser Power Optics

11211-U Sorrento Valley Road

San Diego, CA 92121

(619) 455-0751

Edward L. Courtright

Battel le, Pacific Northwest Labs

Metallurgy & Coatings

2955 George Washington Way

Richland, WA 99352

(509) 375-6926

Terrence F. Deaton

Frank J. Seiler Research Lab.

U.S. Air Force Academy

Colorado Springs, CO 80840

(303) 472-3502

Hulya Demiryont

Colorado State University

Dept. of Physics

Fort Collins, CO 80523

(303) 491-7272

He Deng

Center for Laser Studies

Univ. of Southern California

University park

Los Angeles, CA 90089-1112

Hermann J. Donnert

Kansas State University &

Frank J. Seiler Research Lab.

Ward Hall

Manhattan, KS 66506-7039

(913) 532-5960

Fred W. Doss

Spectra Physics

Optics Division

1250 W. Middlefield Road

Mountain View, CA 94039

(415) 961-2550, ext 2145

Richard Dyer

W. J. Schafer Assoc.

Corporate Place 128

Wakefield, MA 01880

(617) 246-0450

Anne Easton

Optical Components Group

Ferranti PLC.

Dunsinane Ave.

Dundee, SCOTLAND

Dr. J. Ebert

Institut fur Ouantenoptik

Universitat Hannover

Welfengarten 1 D 3 Hannover 1 Federal Republic of Germany

0511-762-4894

David F. Edwards

Lawrence Livermore Natl. Labs.

L-432

Livermore, CA 94550

Osama H. El-Bayoumi

U.S. Air Force

RADC/ESM

Hanscom Field AFB, MA 01731

(617) 861-5832

Edward A. Enemark

Optical Coating Laboratory, Inc.

2789 Northpoint Parkway

Santa Rosa, CA 95401-7397

(707) 525-7108

Gregory J. Exarhos

Pacific Northwest Laboratory

P.O. Box 999

Richland, WA 99352

(509) 375-2440

Philip M. Fauchet

Princeton University

Dept. of Electrical Engineering

and Computer Science

Princeton, NJ 08544

Herbert W. Feinstein

MIT Lincoln Laboratory

244 Wood Street

Lexington, MA 02173

(617) 863-5500, ext 3671

Joseph Figueira

Los Alamos National Laboratory

P.O. Box 1663

Los Alamos, NM 87544

(505) 667-6448

Paul Filbert

Lockheed Palo Alto

Research Laboratory

0191-10, B/203

3251 Hanover Street

Palo Alto, CA 94304

(415) 424-2043

425
Dennis Fischer

Coherent Inc.

2301 Lindbergh Street

Auburn, CA

(916) 823-9550

Diane M. Fischer

Air Force Weapons Lab./ARBE

Kirtland AFB, NM 87117

(505) 844-0226

Steven E. Fisher

Rockwell International/

Rocketdyne Division

P.O. Box 5670

Kirtland AFB, NM 87108

(505) 846-7845

Steven Foltyn

Los Alamos National Laboratory

CHM-5, MS J566

Los Alamos, NM 87545

(505) 667-1123

Jerome Franck

Naval Weapons Center

China Lake, CA 93555

(619) 939-2470

Daniel Friart

Commissariat a'lEnergie Atomique

Centred' Etudes de Limeil -Valenton

B.P.n 27

g4190 Villeneuve-Saint-George

FRANCE

1-590-96 60 P0STE 6473

David Gallant

Rockwell International

Rocketdyne Division

P.O. Box 5670

Kirtland AFB, NM 87185

(505) 844-2052

Gerald A. Gal lager

Los Alamos National Labs.

Los Lamos, NM 87544

(505) 667-9926

Dennis H. Gill

Los Alamos National Labs.

Chemistry Division, J563

P.O. Box 1663

Los Alamos, NM 87544

(505) 667-6250

George A. Graves

University of Dayton

RM165 Kettering Labs.

300 College Parkway

Dayton, OH 45469

(513) 229-4341

Jeffrey W. Griffin

Batelle Pacific Northwest Labs

P.O. Box 999

Richland, WA 99352

(509) 375-2081

Ed Griggs

Los Alamos National Labs.

P.O. Box 1663

Los Alamos, NM 87544

(505) 667-6712

Arthur H. Guenther

U.S. Air Force

AFWL/CA

Kirtland AFB

New Mexico 87117

William J. Gunning

Rockwell International Science

1049 Camino Dos Rios

Thousand Oaks, CA 91360

(805) 498-4545

Lt. Michael Hackett, Jr.

AFWL/ARBE

Kirtland AFB, NM 87117-6008

(505) 844-1776

Steven G. Hadley

CVI Laser Corporation

P.O. Box 11308

Albuquerque, NM 87192

(505) 296-9541

James Harris

IITRI

1495 Bimini Drive

Dayton, OH 45459

(513) 435-6717

John S. Hartman

Pacific Northwest Laboratory

MS/2400 Bldg.

P.O. Box 999

Richland, WA 99352

(509) 375-2101

Allan Heiney

Allied Corporation

7 Powderhorn Drive

Mt. Bethel, NJ 07060

(201) 560-1750, ext 222

Paul D. Hillman

USAF

AFWL/ARBE

Kirtland AFB, NM 87117-6008

(505) 844-1776

Van A. Hodgkin

Naval Weapons Center

China Lake, CA 93555

(619) 939-3247

426
Samuel J. Holmes

Northrop Research & Technology Center

One Research Park

Palos Verdes Peninsula, CA 90274

(213) 377-4811

Alan K. Hopkins

AFWAL Materials Laboratory

(AFWAL/MLPJ)

WPAFB, OH 45433

(513) 255-6652

Marshall N. Huberman

TRW, Bldg 01, Rm 1051

One Space Park

Redondo Beach, CA 90278

(213) 536-4413

Thomas L. Hursman

TRW

One Space Park

Redondo Beach, CA 90278

(213) 535-8252

Hugh Hurt

Naval Weapons Center

Code 3816

China Lake, CA 93555

(619) 939-3049

Long S. Hsu

Colorado State University

Physics Dept.

Ft. Collins, CO 80521

(303) 491-5524

Thomas Hynes

Army Materials & Mechanics

Research Center

AMXMR-MCS

Watertown, MA 02172

Mark Imus

Optical Coating Laboratory, Inc.

2789 Northpoint Parkway

Santa Rosa, CA 95401-7397

(707) 525-7742

Stephen D. Jacobs

University of Rochester

Laboratory for Laser Energetics

250 East River Road

Rochester, NY 14623

(716) 275-4837

Linda Johnson

Naval Weapons Center

Physics Division, Code 3818

China Lake, CA 93555

(619) 939-2977

George T. Johnston

Optical Coating Laboratory, Inc.

2789 Northpoint Parkway

Santa Rosa, CA 95401-7397

(707) 525-7148

L. John Jolin

Los Alamos National Lab.

P.O. Box 1663

CHM-5 MS-J566

Los Lamos, NM 87544

(505) 667-7314

John A. Kardach

U.S. Air Force

AFWL/CA

Kirtland AFB, New Mexico 87109

(505) 844-7368

David Keaton

Los Alamos National Lab.

CHM-6, MS J564

Los Alamos, NM 87545

(505) 667-1123

Kent Kogler

ITT Research Institute

4130 Linden Avenue

Dayton, OH 45432

(513) 252-9969

W. M. Koldewyn

Scientech Inc.

5649 Arapahoe

Boulder, CO 80303

(303) 444-1361

Leslie G. Koshigoe

Naval Weapons Center

Code 3941

China Lake, CA 93555

(619) 939-3364

Munson Kwok

Aerospace Corporation

P.O. Box 92957

MS/747

Los Angeles, CA 90009

(213) 648-5441

Nessim Lagnado

Hughes Aircraft Company

Mail Station ED El D160)

2000 E. El Segundo Blvd.

El Segundo, CA 90245

(213) 616-6237

Michael R. Lange

Univ. of New Mexico

Albuquerque, NM 87131

(505) 277-6317

Joseph R. Latore

Airtron Div. of Litton Systems Inc.

200 E. Hanover Avenue

Morris Plains, NJ 07950

(201) 539-5500

Paul S. Laybourne

W. J. Schafer Associates, Inc.

303 San Mateo, NE Rte 103

Albuquerque, NM 87108

(505) 266-7701

427
R. J. L. Lerou

Physics Laboratory TNO

P.O. Box 96864

2509 JG The Hague

NETHERLANDS

970-264221 or 070-972925

Keith L. Lewis

Royal Signals & Radar Establishment

St. Andrews Road

Malvern

Worcss WR14 3PS

UNITED KINGDOM

06845 2733, ext 3062

Tony Lowderback

Ojai Research Corp.

11554 Ventura Ave.

Ojai, CA 93023

(805) 646-7198

W. Howard Lowdermilk

Lawrence Livermore Natl. Laboratory

P.O. Box 5508, L-490

Livermore, CA 94550

(415) 422-5498

Samuel Lu Litton Industries

GCS

5500 Canoga Avenue

Woodland Hills, CA 91365

(213) 715-3180

David L. J. Lunt

Lunt & Co.

P.O. Box 368

Richland, WA 99352

(509) 375-4792

Michael Lunt

Technical Optics Ltd.

Second Avenue

Onchan, Isle of Man

BRITISH ISLES

44 624 4443

Capt. Myron T. Maclin

USAF

AFWL/ARBE

Kirtland AFB, NM 87117-6008

(505) 844-1776

Jeffrey J. Malanify

USAF

AFWL/ARBE

Kirtland AFB, NM 87117-6008

(505) 844-7360

Tarro Mandre

TRW

One Space Park

Redondo Beach, CA 90278

Nastaran Mansour

North Texas State Univ.

Physics Dept., Box 77

Denton, TX 76203

(817) 565-3307

Frane Marcelja

Corion Corporation

73 Jeffrey Avenue

Holliston, MA 01746

(617) 429-5065

C. Denton Marrs

Naval Weapons Center

Code 3817

China Lake, CA 93555

(619) 939-2470

Diane J. Martin

USAF

AFWL/ARBE

Kirtland AFB, NM 87117-6008

(505) 844-1776

Robert L. Martin, Jr.

Optical Coating Laboratory, Inc.

2789 Northpoint Parkway

Santa Rosa, CA 95407-7397

(707) 525-7647

Billie R. Mauro

Los Alamos National Lab.

CHM-5

P.O. Box 1663, MS J566

Los Alamos, NM 87545

(505) 667-9679

James W. Mayo III USAF

AFWL/ARB

Kirtland AFB, NM 87117-6008

(505) 844-0721

Fred J. McClung

Hughes Aircraft Company

2650 Yale Blvd. SE.

Albuquerque, NM 87106

(505) 242-9818

Michael McGuirk

Perkin-Elmer

Main Ave., MS 420

Norwalk, CT 06856

(203) 834-4921

J. K. Mclver

Univ. of New Mexico

Dept. of Physics & Astron.

Albuquerque, NM 87131

(505) 277-8484

Jim McNally

U.S.A.F.

Univ. of New Mexico

9608 Avenida de La Luna

Albuquerque, NM 87111

(505) 822-8143

428
John R. McNeil

Elect. Eng. Dept.

Univ. of New Mexico

Albuquerque, NM 87131

(505) 277-5502

Charles Mellor

GTE Lighting Products

60 Boston St.

Salem, MA 01970

(617) 777-1900

Jean Francois Mengue

Commissariat a l'Energie Atomique

Centre d Etudes de Limeil-Valenton

B.P. no 27

94190 Villeneuve-Saint-Georges

FRANCE

-1-590 96 60 poste 6473

David Milam

Lawrence Livermore Lab.

7000 E. Ave.

Livermore, CA 94550

(415) 422-5499

Michael J. Monsler

KMS Fusion Inc.

P.O. Box 1567

3621 S. State Road

Ann Arbor, MI 48106

(313) 769-8500

David Mordaunt

Hughes Aircraft Company

Bldg El, Mail Station E142

P.O. Box 902

El Segundo, CA 90245

Dennis Morel li

Optical Coating Laboratory, Inc.

2789 Northpoint Parkway

Dept. 428-1

Santa Rosa, CA 95407-7397

(707 525-7011

Patricia L. Morse

AFWL/ARBE

Optical Components Branch

Kirtland AFB, NM 87117

(505) 844-0226

Bill W. Mullins

USAF Academy

Dept. of Physics

USAFA/Dep

USAF Academy, CO 80840-5821

(303) 472-4158

George F. Nado

Texas Instruments

Box 226015, M/S 3196

Dallas, Texas 75231

(214) 995-6412

Elizabeth Anne Nevis

Coherent Inc.

3270 W. Rayshore Rd.

Palo Alto, CA 94303

(415) 858-2250

Davis B. Nichols

Boeing Aerospace Co.

Physics Tech. Dept.

P.O. Box 3999, MS 8H-18

Seattle, WA 98124

(206) 773-8938

Andy Nowak

Los Alamos National Lab.

M/S J564

Group Chm 6

Los Alamos, NM 87545

Robert M. 0'Connell

Univ. of Missouri - Columbia

Elect. Eng. Dept.

Columbia, M0 65201

(314) 882-8373

James R. Palmer

Comarco, Inc.

1201 N. China Lake Blvd.

Ridgecrest, CA 93555

(619) 446-7671, ext 295

Roger A. Paquin

The Perkin Elmer Corp.

Electro-Optical Division

100 Wooster Heights Road

M/S 955

Danbury, CT 06810

(203) 797-6130

Charles Edgar Patty, Jr.

Teledyne Brown Engineering

Cummings Research Park

Mail Stop 19

Huntsville, AL 35807

(205) 532-1000, ext 3210

Walter T. Pawlewicz

Battel le, Pacific Northwest Labs

Metallurgy & Coatings Section

2955 George Washington Way

Richland, WA 99352

(509) 375-2074

Robert D. Poirier

Perkin Elmer Corporation

77 Danbury Road

M/S 420

Wilton, CT 06897

Dave Pollock

Teledyne Brown Engineering

Cummings Research Park

MS 19

"

Huntsville, AL 35807

(205) 532-1605

429
Jim Porteus

Naval Weapons Center

Code 3817

China Lake, CA 93555

(619) 939-3827

Scott Price

Martin Marietta Denver Aerospace

P.O. Box 9316

International Airport

Albuquerque, NM 87119

(505) 844-1064

Diane Quinn

Optical Coating Laboratory, Inc.

2789 Northpoint Parkway

Santa Rosa, CA 95401-7397

(707) 525-7652

John Rahn, MS- 12

Litton Guidance & Control

5500 Canoga Ave.

Woodland Hills, CA 91365

(818) 888-3415

Tilak Raj

Martin Marietta Denver Aerospace

P.O. Box 9316

International Airport

Albuquerque, NM 87119

(505) 844-1064

Herman E. Reedy

II-VI Inc.

Saxonburg Blvd.

Saxonburg, PA 16056

(412) 352-4455

David Reicher

Herron Optical Division

Laser Power Corp.

2035 E. 223rd Street

Long Beach, CA 90810

(213) 830-5404

David H. Rester

Texas Instruments

P.O. Box 660246

Dallas, TX 75266

(214) 995-1805

Detleu Ristau

Universitat Hannover

Weleengarten 1 Hannover 1 GERMANY

0511 762 4834

Andrew B. Romberger

Pennsylvania State Univ.

Berks Campus

P.O. Box 2150

Reading, PA 19608

(215) 375-4211

Carlos Roundy

Spiricon, Inc.

2600 N. Main

Logan, UT 84321

(801) 753-3729

J. Earl Rudisill

Laser Power Optics

11211-U Sorrento Valley Road

San Diego, CA 92121

(619) 455-0751

Rong Rujkorakarn

Dept. of Physics

Colo. State Univ.

Ft. Collins, CO 80523

(303) 491-1105

Joseph Sanchez

Optical Coating Laboratory, Inc.

2789 Northpoint Parkway

Santa Rosa, CA 95407-7397

Haluk Sankur

Rockwell Int. Science Center

1049 Camino Dos Rios

Thousand Oaks, CA 91360

(805) 498-4545, ext 263

Paul Schall

The Aerospace Corporation

M2/321

P.O. Box 92957

Los Angeles, CA 90009

(213) 648-7502

Gary L. Scheidegger

Martin Marietta Aerospace

Mail Point 185

P.O. Box 5837

Orlando, FL 32855

Erik A. Schwendeman

Corion Corporation

73 Jeffrey Avenue

Holliston, MA 01746

(617) 429-5065

Marion L. Scott

Research & Development Assoc.

P.O. Box 9377

Albuquerque, NM 87119

(505) 844-0434

S. C. Seitel

Naval Weapons Center

Code 3817

China Lake, CA 93555

(619) 939-3827

Robert R. Shannon

Optical Sciences Center

University of Arizona

Tucson, AZ 85721

(602) 621-6997

430
Chiao-Yao She

Colorado State University

Physics Dept.

Fort Collins, CO 80523

(303) 491-5524

Dennis R. Sigler

Burleigh Northwest Optical, Inc.

Burleigh Park

Fishers, NY 14453

(716) 924-9350

Robert M. Silva

VIT Inc.

4032 Linden Avenue

Dayton, OH 45432

(513) 254-6201

Meyer Silver

TRW

01/1240

One Space Park

Redondo Beach, CA 90278

(213) 535-8252

James R. Sites

Dept. of Physics

Colorado State University

Fort Collins, CO 80523

(303) 491-5850

Robert Soales

CVI Laser, Inc.

200 Dorado Place SE.

Albuquerque, NM 87123

(505) 296-9541

M. J. Soileau

North Texas State Univ.

Dept. of Physics

Denton, TX 76303

(817) 565-3263

William H. Southwell

Rockwell International Science Center

1049 Camino Dos Rios

Thousand Oaks, CA 91360

(805) 498-4545

Michael C. Staggs

Lawrence Livermore Labs.

East Ave.

Livermore, CA 94550

(415) 422-6060

James L. Stanford

Naval Weapons Center

Code 3818

China Lake, CA 93555

(619) 939-2443

James L. Stapp

Rocketdyne Div.

Rockwell Int.

P.O. Box 5670

Kirtland AFB, NM 87185

(505) 846-0204

Lawrence A. Stelmack

M/S 87

Litton Industries

Guidance and Control Divison

5500 Canoga Avenue

Woodland Hills, CA 91365

(818) 715-5184

Alan F. Stewart

Air Force Weapons Lab.

AFWL/ARBE

Kirtland AFB, NM 87117

(505) 844-7368

W. Kent Stowell

USAF

AFWAL/AAD0-2

Wright-Patterson AFB, OH 45433

(513) 255-5147

Joseph Tajani

Hewlett Packard

640 Page Mill Rd.

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Scott J. Thomas

Los Alamos National Laboratory

P. 0. Box 1663 /MSJ566

Los Alamos, NM 87545

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Optical Coating Laboratory, Inc.

2789 Northpoint Parkway

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Martin Marietta Denver Aerospace

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International Airport

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(505) 844-1064

Lt. John F. Turner

USAF

AFWL/ARBE

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431
Trudy Tuttle Hart

Optical Coating Laboratory, Inc.

2789 Northpoint Parkway

Santa Rosa, CA 95401-7397

(707) 525-7192

Eric W. Van Stryland

Center for Appl . Quantum Electronics

North Texas State Univ.

Dept. of Physics

P.O. Box 5368, NT Station

Denton, TX 76203

(817) 565-3263

Marc Von Gunten

Mail Stn. 4-30

Spectra-Physics Inc.

Optics Division

1250 W. Middlefield Road

Mountain View, CA 94039-7013

(415) 961-2550, ext 3008

Wil lard R. Wadt

Los Alamos National Laboratory

MS J 569

P.O. Box 1663

Los Alamos, NM 87545

(505) 667-7763

Lawrence Weaver

Martin Marietta Denver Aerospace

P.O. Box 9316

International Airport

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USAF

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Lawrence Livermore National Lab.

East Ave.

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Teledyne Brown Engineering

Mail stop 19

300 Sparkman Dr.

Huntsville, AL 35807

(205) 532-1592

Keith Wier

Scientech Inc.

5649 Arapahoe

Boulder, Co 80303

(303) 444-1361

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Bendix Field Engineering Corp.

Mail Stop L483

P.O. Box 5508

Livermore, CA 94550

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Mail Point 185

Martin Marietta

P.O. Box 5837

Orlando, FL 32855

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Caxton Street

Anniesland

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Lawrence Livermore Natl. Lab.

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Rockwell International Corp.

Science Center

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Dept. of Electrical Engineering

University of Nebraska

Lincoln, NB 68588

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GEC Research Laboratories

Wirst Research Centre

East Lane

Wembley, UNITED KINGDOM

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Rockwell Int.

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Rockwell International

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Shin-Tson Wu

Hughes Research Laboratory

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Malibu, CA 90265

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Rick Zoborowski

0CLI

2789 Northpoint Parkway, Dept. 428-1

Santa Rosa, CA 95407-7397

(707) 525-7011

432
NBS-1 14A (rev. 2-eo

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBS/SP-727

2. Performing Organ. Report No. 3. Publ ication Date

October 1986

4. TITLE AND SUBTITLE

Laser Induced Damage in Optical Materials: 1984

5. AUTHOR(S) Harold E. Bennett (NWC) , Arthur H. Guenther (AFWL)

David Milam (LLNL) and Brian E. Newnam (LANL)

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

Gaithersburg, MD 20899

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

National Bureau of Standards (NBS)

American Society for Testing and Materials (ASTM)

Office of Naval Research (ONR)

Department of Energy (DOE)

Defense Advanced Research

Project Agency (DARPA)

Air Force Office of Scientific

Research (AFOSR)

10. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 86-6005 87

3 Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

The Sixteenth Annual Symposium on Optical Materials for High Power Lasers (Boulder

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

October 15-17, 1984. The Symposium was held under the auspices of ASTM Committee F-l

Subcommittee on Laser Standards, with the joint sponsorship of NBS, the Defense

Advanced Research Project Agency, the Department of Energy, the Office of Naval

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

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

France, West Germany, and the Netherlands. The Symposium was divided intp sessions

concerning Materials and Measurements, Mirrors and Surfaces, Thin Films, (and Fundamental Mechanisms. As in previous years, the emphasis of the papers presented at

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

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

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

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

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

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

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

scheduled for October 28-30, 1985, at the National Bureau of Standards, Boulder, CO.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) laser damage; laser interactions; optical fabrication; optical materials; properties;

thin film coatings.

13. AVAILABILITY

fX] Unlimited

For Official Distribution. Do Not Release to NTIS

[X] Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

444

15. Price

USCOMM-DC 6O43-P80



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