NBS

PUBLICATIONS

AlllQE fi7M37M

NATL INST OF STANDARDS & TECH R.I.C.

A111 02874374

NBS SPECIAL PUBLICATION 746

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

Laser induced Damage

in Opticai i\/iaterials:

1985
TM he National Bureau of Standards' was established by an act of Congress on March 3, 1901. The Bureau's overall Jg^ goal is to strengthen and advance the nation's science and technology and facilitate their effective appUcation for public benefit. To this end, the Bureau conducts research to assure international competitiveness and leadership of U.S.

industry, science arid technology. NBS work involves development and transfer of measurements, standards and related science and technology, in support of continually improving U.S. productivity, product quality and reUability, innovation

and underlying science and engineering. 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.

The National Measurement Laboratory

Provides the national system of physical and chemical measurement;

coordinates the system with measurement systems of other nations and

furnishes essential services leading to accurate and uniform physical and

chemical measurement throughout the Nation's scientific community,

industry, and commerce; provides advisory and research services to other Government agencies; conducts physical and chemical research; develops,

produces, and distributes Standard Reference Materials; provides

calibration services; and manages the National Standard Reference Data

System. The Laboratory consists of the following centers:

• Basic Standards^

• Radiation Research

• Chemical Physics

• Analytical Chemistry

The National Engineering Laboratory

Provides technology and technical services to the public and private sectors

to address national needs and to 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

capabilities; provides engineering measurement traceability services; develops test methods and proposes engineering standards and code

changes; develops and proposes new engineering practices; and develops

and improves mechanisms to transfer results of its research to the ultimate

user. TTie Laboratory consists of the following centers:

Applied Mathematics

Electronics and Electrical Engineering^

Manufacturing Engineering

Building Technology

Fire Research

Chemical Engineering^

The Institute for Computer Sciences and Technology

Conducts research and provides scientific and technical services to aid Federal agencies in the selection, acquisition, application, and use of

computer technology to improve effectiveness and economy in Government

operations in accordance with Public Law 89-306 (40 U.S.C. 759),

relevant Executive Orders, and other directives; carries out this mission by

managing the Federal Information Processing Standards Program,

developing Federal ADP standards guidelines, and managing Federal

participation in ADP voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal agencies; and

provides the technical foundation for computer-related policies of the Federal Government. The Institute consists of the following divisions:

Information Systems Engineering

Systems and Software

Technology

Computer Security

System and Network

i^chitecture

Advanced Systems

The Institute for Materials Science and Engineering

Conducts research and provides measurements, data, standards, reference

materials, quantitative understanding and other technical information

fundamental to the processing, structure, properties and performance of

materials; addresses the scientific basis for new advanced materials technologies; plans research around cross-cutting scientific themes such as nondestructive evaluation and phase diagram development; oversees Bureau-wide technical programs in nuclear reactor radiation research and

nondestructive evaluation; and broadly disseminates generic technical information resulting from its programs. The Institute consists of the following Divisions:

• Ceramics

• Fracture and Deformation^

• Polymers

• Metallurgy

• Reactor Radiation

'Headq'jaiters and Laboratories at Gaithersburg, MD, unless otherwise noted; mailing address

Gaithersburg, MD 20899. ^Some divisions within the center are located at Boulder, CO 80303.

'Located at Boulder, CO, with some elements at Gaithersburg, MD
Laser Induced Damage

in Optical Materials: 1985

RESEARCH INFORMATION CENTER

National Institute of

Standards and Technology

Gai thersburgi MD 2089?

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 28-30, 1985

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

BOULDER DAMAGE SYMPDSIUM

U.S. DEPARTMENT OF COMMERCE, C. William Verity, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued July 1 988
Library of Congress Catalog Card Number: 88-600557

National Bureau of Standards Special Publication 746

Natl. Bur. Stand. (U.S.), Spec. Publ. 746, 576 pages (July 1988)

CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1988

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325
Foreword

The Proceedings contain the papers presented at the Seventeenth Symposium on Optical Materials for High-Power Lasers held at the National Bureau of Standards in

Boulder, Colorado, on October 28-30, 1985. 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 Project Agency, the Department

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

by over 215 scientists from the United States, Israel, India, the United Kingdom,

Japan, Federal Republic of Germany, and the Soviet Union. 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.

The editors assume full responsibility for the summary article which contains an overview of the Symposium. 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 Eighteenth Annual Symposium on this topic will be held in

Boulder, Colorado, November 3-5, 1986. 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 1986 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 thin film areas.

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

for high-power lasers. The editors will welcome comment and criticism from all

interested readers relevant to this purpose, and particularly relative to our plans for

the Eighteenth Annual Symposium.

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

D. Milam, and B. E. Newnam

Co-Chairmen

i i i
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 responsibility for comments or recommendations contained

therein.

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

publication in order to explain the experimental procedure adequately. Such

identification in no way implies approval, 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.

i V
CONTENTS

Page

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

Disclaimer iv

Symposium Welcome xii

H.E. Bennett

Summary of Meeting 1

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

I. 0 Introduction 1

2 . 0 Overview 2

3 . 0 Acknowledgments 5

4.0 References 6

Materials and Measurements

A Versatile Laser Glass Inspection and Damage Testing Facility 8 J.E. Marion, G.J. Greiner, J.H. Campbell, F.H. Chaffee,

J.S. Hildum, J.Z. Grens , C.L. Weinzapfel,

S.M. Winfree, and D.Milam

Computer-Controlled Facility for Laser Switching and

Damage Testing 12 Jay S. Chivian, W.D. Gotten, D.F. Fuller,

R. B. Hemphill, and M.W. Scott

Damage Testing in Optics Production and Procurement 33 S.C. Seitel and E.A. Teppo

A Computer Automated Laser Damage Laboratory 34

Martin R. Sweet, L.J. Jolin, and S.R. Foltyn

U.V. Laser Beam Diagnostics Using a Computer Controlled

Video Frame Store System 35 S.E. Clark, I. Laidler, D.C. Emmony,

B . A . Omar , and M.J. Shaw

V
Beam Profiling Characteristics of a Sensitivity-Enhanced

Silicon Vidicon System at 1.06 Microns 44

R.M. O'Connell, R.A. Vogel

A.F. Stewart and D.A. Smith

Digital Fluence Profiling System and Software 53 S.C. Seitel and R.L. Rypma

Simple Accurate Inversion of Knife-Edge Data from Radially

Symmetric Laser Beams 54 R.M. O'Connell and R.A. Vogel

Investigation of Optical Damage via Resonance Ionization

Mass Spectrometry 61 R.C. Estler and N.S. Nogar

Laser Mirror Profilometry by High-Q Interferometry 68 J.R.Rotge'

Design and Construction of a Single Polarizer Infrared

Ellipsometer/Reflectometer for Characterization of Highly

Curved Surfaces 72 Jack B. Stubbs, John S. Loomis, and Thomas A. Leonard

Direct n2 Measurement in Yttria Stabilized Cubic Zirconia 80 Shekhar Guha, Nastaran Mansour, and M.J. Soileau

Measurement of Optical Damage Threshold of Wire Grid Polarizers at 10.6 Microns 84

M. Mohebi, A. Said, and M.J. Soileau

Laser Damage Studies of Several Acrylic Polymers 92 A.B. Romberger, B.W. Mullins, A. A. Shaffer, J.F. Hilbing,

K.E. Siegenthaler , T.F. Deaton, and Y. Chang

Improvement of the Bulk Laser Damage Threshold of Potassium

Dihydrogen Phosphate Crystals by Ultraviolet Irradiation 101

A. Yokotani, T. Sasaki, K. Yoshida,

T . Yamanaka , and C . Yamanaka

Laser-Induced Damage in Doped and Undoped Fluorozirconate

Glass 108

Nastaran Mansour, Shekhar Guha, M.J. Soileau,

Bernard Bendow, and Diane Martin
Platinum Particles in the Nd: doped Disks of Phosphate Glass

in the Nova Laser 120

D. Milam, C.W. Hatcher, andJ.H. Campbell

Evolution during Multiple- Shot Irradiation of Damage Surrounding

Isolated Platinum Inclusions in Phosphate Laser Glasses 128 R.P. Gonzales and D. Milam

Multiple Pulse Damage Studies of BK-7, KCl, and SiOj at 532 nm 138

D. Kitriotis, L.D. Merkle , and A. Dodson

Picosecond Nonlinear Investigation of Accumulated Damage in Molecular

Crystals 146

Thomas J. Kosic, Jeffrey R. Hill, and Dana D. Dlott

Laser Photoconductivity in Alkali Halides 152

Alexander Epifanov

Mirrors and Surfaces

The Scratch Standard is only a Cosmetic Standard 153

Matt Young

Ion Exchange Strengthening of Nd: Doped Phosphate Laser Glass 159

Kathleen A. Cerqua, Stephen Jacobs,

Brian L. Mclntyre, and William Zhong

Infra-Red Absorption Spectroscopy of Nd:YAG and Nd:GSGG

Surface Contaminants 170 M.A. Acharekar

Excimer Radiation Colouration Processes and Surface Damage to Single

Crystal CaF2 Substrates 182

I. Laidler and D.C. Emmony

Analysis of Optical Surfaces by Laser Ionization Mass Spectrometry:

Spatial Variation of the Laser Damage Threshold and its Relationship

to Polishing Compound Incorporation 190

Wigbert J. Siekhaus

Laser Induced Damage and Ion Emission of GaAs at 1.06 fim 191 A.L. Huang, M.F. Becker, and R.M. Walser

vi i
CO2 Laser Induced Damage to Germanium: Relation to Surface

Preparation 203 D.R. Gibson, CM. MacDonald, and A.D. Wilson

Study of Mo, Au, and Ni Implanted Molybdenum Laser Mirrors by

Spectroscopic Ellipsometry 215

Paul G. Snyder, George H. Bu-Abbud, Jae Oh, John A. Woollam,

David Poker, D.E. Aspnes, David Ingram, and Peter Pronko

Glancing Incidence Absorption Measurements of Metal Mirrors 227 W.D. Kimura and D.H. Ford

N-on-1 Testing of Single Crystal Metal Surfaces at 1.06 nm 236

Yong Jee, Michael F. Becker, and Rodger M. Walser

Laser Induced Mo Mirror Damage for High Power CO2 Laser 248

H. Okamoto, N, Matsusue , K. Kitazima, K. Yoshida,

Y. Ichikawa, M. Yamanaka, T. Yamanaka, and Y. Tsunawaki

Thin Films

Multifacet, Metal Mirror Design for Extreme -Ultraviolet and Soft

X-Ray Free-Electron Laser Resonators 261

Brian E. Newnam

The Design of Optimum Reflectivity Coatings for Grazing Angles

of Incidence 270

Jeffrey B. Shellan

Phase Control Coating Design Analysis 282

M. McGuirk and R.D. Poirier

Thermal Conductivity of Enhanced High Reflectors 283

M. McGuirk and R.D. Poirier

Yttrium Oxide Coatings for High Power Lasers at 351 nm 284

D.J. Smith, C.J. Hayden, B.U. Krakauer,

A.W. Schmid, and M.J. Guardalben

Self -Consistent Dependence of Porosity and Refractive Index on Composition in Co-Evaporated Films 299

A. Feldman, E.N. Farabaugh, and R.A. Stempniak

V i i 1
The Formation and Development of Nodular Defects in Optical

Coatings 305

Bangjun Liao, Douglas J. Smith, and Brian Mclntyre

Interfacial Structure of Ion-Beam Sputter Deposited

Si02/Ti02 Coatings 319 J.R. Sites and H. Demiryont

Ion Beam Deposited Oxide Coatings 325

T. Raj, J.S. Price, and C.K. Carniglia

Properties of IAD Al203/Si02 and Ta205/Si02 Thin Film

AR Structures 333 J.J. McNally, F.L. Williams, S.R. Wilson, and J.R. McNeil

Ion Assisted Deposition of Optical Films on Heavy Metal Fluoride

Glass Substrates 342 J.J. McNally, G.A. Al-Jumaily, and J.R. McNeil

Highly Damage Resistant Porous Dielectric Coating for High Power

Lasers in the Ultraviolet to Near-Infrared Wavelength Region 350

K. Yoshida, H. Yoshida, Y. Kato, and C. Yamanaka

Ion Beam Characterization of Multi-Layer Dielectric Reflectors 355 J.G. Beery, M.G. Hollander, C.J. Maggiore , A. Redondo

,

R.T. Westervelt, and T.N. Taylor

Raman Scattering Analysis of Thin Film Coatings Using Guided Waves 352

Alan F. Stewart, David R. Tallant, and Karen L. Higgins

Raman Microprobe of Laser- Induced Surface Damage Regions

in Ti02 and Zr02 Coatings 374

Donald M. Friedrich and Gregory J. Exarhos

Probing Structural Properties of Amorphous Films with Raman

Spectroscopy 383

C.Y. She and L.S. Hsu

Detection of Laser Damage by Raman Microscopy 388

P.M. Fauchet, I.H. Campbell, and F. Adar

Time-Resolved Temperature Determinations from Raman Scattering

of Ti02 Coatings During Pulsed Laser Irradiation 395

Gregory J . Exarhos

ix
Effect of Molecular Fluorine on the Performance of Excimer

Laser Optics

S.R. Foltyn, L.J. Jolin, and G. Lindholm

403

Thresholds Measured with 350-nm Pulses at 25-100 Hz for Bare Polished Crystals of

CaF2 and for Silica Sol-gel AR Coatings on Silica Substrates 404

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

Study of Temporal and Spatial Intensity Variations and Intracavity

Damage Thresholds of Alexandrite Lasers 412

D.J. Harter, H. Rainnee , and A.J. Heiney

Investigation of Metal Coatings for the Free Electron Laser 419

Marion L. Scott, Paul N. Arendt, Robert W. Springer,

Richard C. Cordi , and William J. McCreary

Influence of Deposition and Post Deposition Conditions on Laser Induced

Damage in Electron Beam Evaporated Ti02 and SiOj Films 429

K. Narasimha Rao, M. Adinarayana Murthy,

S. Mohan, and M. Ramakrishna Rao

Dependence of Damage Threshold of Anti-Reflection Coatings

on Substrate Surface Roughness 438

Y. Nose, Y. Kato, K. Yoshida, and C. Yamanaka

Correlation between Local HeNe Scatter and Defect-Initiated Laser

Damage at 2.7 pm 449

J.O. Porteus, C.J. Spiker, and J.B. Franck

CO2 Laser Induced Damage in a Model Thin Film

System: ZnS on Ge 460

D.R. Gibson and A.D. Wilson

Laser Induced Damage in Dense Optical Thin Films 472

K.L. Lewis, A.M. Pitt, A.G. Cullis

N.G. Chew, and L. Charlwood

Fundamental Mechanisms

Transient Absorption of Light Induced by Radiation 480

R. Godwin, F.B. Harrison, and A. Larson

Effects of Radiation Induced Defects on Laser-Induced

Breakdown in Si02 ^86

M.J. Soileau, Nastaran Mansour, Edesly Canto, and D.L. Griscom

X
Fundamental Mechanisms of Optical Damage in Short -Wave length

High-Power Lasers 497

R.F. Haglund, Jr., N.H. Tolk, and G.W. York

Temporal Distribution of Electron and Ion Emission Caused by Laser

Excitation of Optical Surfaces in Ultra-High Vacuum 509 W.J. Siekhaus, L.L. Chase, and D. Milam

, Anomalous Absorption in Optical Coatings 515 M.R. Lange , J.K. Mclver, and A.H. Guenther

Time of Damage and the Effect of the Pulse Repetition Frequency

on the Laser Induced Damage Threshold 529 R.M. Wood, S.K. Sharma, P. Waite, E.K. Gorton, and K. Lewis

Modeling Laser Damage Caused by Platinum Inclusions in Laser Glass 537

John H. Pitts

CO2 Laser-Induced Air Breakdown near Metal Mirror Surfaces 547

Vitaly Konov

APPENDIX I. List of Attendees 548

xi
WELCOME

H. E. Bennett

Michelson Laboratory, Naval Weapons Center

China Lake, CA 93555

On behalf of co-chairmen Art Guenther, Brian Newnam, Dave Milam,

and myself, I welcome you to the Seventeenth Annual Symposium on Optical Materials for

High- Power Lasers. We have a record number of papers this year and a record attendance

also; it promises to be an excellent conference. We especially want to welcome our many foreign guests, most of whom are contributing papers to the meeting. Many of them

have come a long way and it is a commentary on the quality of this meeting that

scientists from around the world will arrange to come here and share with us their

insights into the difficult questions surrounding laser damage to materials. Some of

their contributions have been quite significant too. For example, Professor Manenkov

of the Soviet Union started a whole new line of investigation with his paper on NaCi

damage limits a few years ago. Professor Yoshiharu Namba of Japan started us to thinking about float polishing. Dr. Achim Bubenzer and his colleagues from West Germany

helped to clarify the science behind hard, diamondlike carbon coatings, and last year.

Dr. Keith Lewis from Great Britain encouraged us to think that, using MBE techniques,

thin films can be prepared which exhibit bulk properties instead of having several

orders -of -magnitude more absorption than bulk material and so on. I could easily name many others from outside the United States who have contributed greatly to the welldeserved reputation of this conference for being a must for anyone who is interested in

laser damage to optical components. We are proud of the international cooperation

shown here in attacking problems in fundamental science, which knows no boundaries. In this spirit let me caution those of you who are engaged in applied research and

development as well as more fundamental studies to avoid embarrassing our foreign

guests and jeopardizing the excellent reputation of this conference by engaging in

inappropriate discussions in the halls. These are not cleared areas. Let us concentrate our efforts in the next three days on understanding the fundamental

problems associated with optical materials and laser damage to them. In this we can

all work together.

Optics is a fashionable field to work in now. I remember in the fifties that it was suggested to me that research on optical properties of materials had no future,

everything had been discovered which was of any useful interest, and only adding more decimal points to the optical constants remained to be done. The laser came along

about then and was regarded as a new and interesting curiosity; a solution looking for

a problem. Since then it has been used to solve many problems. Surgery,

communications, data recording, laser welding, surface modification, and space

applications are a few that come to mind. In all of them we are dealing with optical

trains which, although they may not transmit high energies, do have to handle high

powers. Lasers have introduced a new dimension into optical systems which we may call

power optics. Electrical engineers have an analogous field, power engineering, where

Xil
they study how to handle the output of hydroelectric dams and other sources of power.

Electrical engineering also encompasses small-signal processing in the radio and

microwave fields. The analogous field in optical design deals with telescopes,

microscopes, cameras, etc. and is often referred to as classical optics. With the

advent of power optics, for which this conference is one of the main forums, we enter a new kind of optical design, one in which the underlying physics is not well understood.

At first glance the problem seems simple enough. A high enough intensity of laser

energy causes either reversible or irreversible damage to the optical components

reflecting or transmitting the laser energy. But why do components damage and why do

damage thresholds of similar samples vary by as much as one or two orders-of -magnitude?

The absorption coefficients of the component substrate and coating obviously should be

related to the damage threshold, but frequently they are not. Even so, what makes the

materials absorb? Figure 1 shows some of the mechanisms which have been studied as causes of intrinsic absorption in low absorption dielectrics. Other mechanisms have

also been proposed, but these will suffice to demonstrate that even intrinsic

absorption is not necessarily simple. On the other hand, there is considerable

evidence that extrinsic effects dominate laser damage in low absorption regions, and

these may be even more complicated. Worse yet, absorption from extrinsic or intrinsic

effects may be completely unimportant. Structural effects may be the key, or the highfield strengths involved in laser beams may cause dielectric breakdowns or non- linear

effects. Sub-picosecond pulses get us down to the time intervals characteristic of

atomic processes, with consequent changes in the rules, and one concludes that laser

damage is not so simple after all.

We have made many important advances over the years in our understanding of laser

damage. Most have been reported here first, and the back issues of the Conference

Proceedings are regarded by most of the community as a gold mine of information on the

subject. Many of the back issues can be obtained from the American Society for Testing

Materials, the professional sponsor for this conference. Some advances which come to mind are the understanding of self focussing (which is not yet perfect) , the effect of

scratches and voids on damage thresholds (also not complete) , the asymmetry of laser

damage thresholds for the front and back surfaces of windows caused by standing-wave

fields, the development of experimental techniques to measure various key parameters.

We still do not understand the origin of surface absorption, but at least we can measure it and scale damage thresholds for small spot-sizes to larger beam diameters

for some types of surfaces. To understand laser damage phenomena, diverse disciplines

such as crystal growth, thin film technology, surface finishing technology and others

are required, and some notable advances have been reported here in these fields. One I remember was the achievement of absorption coefficients of 10"'' cm" range at 10.6

wavelength for both KCl and ZnSe, orders-of -magnitude better than had been possible

previously. Many other examples of progress could be mentioned and some of these

fields are among the most well-represented at this year's Symposium. Thin film

technology, for example, is now consistently the subject of more than one quarter of

all the papers presented in these Symposia.

xiii
Plenty of technical problems remain to be solved. One of the most puzzling is the reason for the decrease in damage threshold seen for multiple pulses, and the general

behavior of components to rep-rate pulses. The subject of short-wavelength laser

components is a fascinating one and brings in all the physics of multiphoton processes

and the happy discovery that things are not as hopeless as they seemed at first. Many

other topics could be mentioned, and indeed are in the Conference Program which you all

have. This is the largest conference we have ever had, let us hope that it will also

be seen in future years as one of the best. Enjoy and profit by the 2 1/2 days that we

are about to embark on.

Let me close by mentioning some items of which you all should be aware. First, save Tuesday evening. There will be a Wine and Cheese Tasting extravaganza that

evening, and I hope that you all can come, get to know each other on a more personal

basis, exchange ideas, and enjoy some fine wine and unusual cheeses and goodies.

Let us take this opportunity to thank our professional sponsor of this conference,

the American Society for Testing Materials, often familiarly known as the ASTM, and

also our financial sponsors. They are (1) the Air Force Office of Scientific Research,

(2) the Office of Naval Research through the Naval Weapons Center, (3) Lawrence

Livermore National Laboratory, (4) Los Alamos National Laboratory, (5) The Defense

Advanced Research Projects Agency, and (6) the National Bureau of Standards. The

Bureau does a great deal to make this meeting possible, and we particularly acknowledge

the support of Robert A. Kamper, Director of the Boulder Laboratories of NBS , and Aaron

A. Sanders, Group Leader of the Electromagnetic Technology Division, who performs the

functions of Conference Coordinator and Fundraiser. His secretary, Ms. Susie Rivera

continues to perform an outstanding job in managing the conference manuscripts, sending

out notices, typing editors comments, and generally keeping the show on the road. She

is aided by several other secretaries here at the Bureau. Ann Mannos here at the

Bureau handles the finances. No set of acknowledgments would be complete without

reference to the outstanding work done by Pat Whited, Art Guenther's secretary, in

making this Symposium happen. Finally, and most importantly, my cochairmen Brian

Newnam, Dave Milam and I want you to join with us in thanking Art Guenther, who with

Alex Glass started these Symposia and who, in spite of his impossibly busy schedule as Chief Scientist of the Air Force Weapons Laboratory, is still supplying much of the

motive force to keep it going. We hear that he is to be awarded the Distinguished

Senior Executive Award in Washington, D.C. and that it will be presented personally to him by President Reagan. Let us all stand and give a round of applause.

xiv
AVALANCHE

IONIZATION MULTIPHOTON-POLARON ABSORPTION

INVERSE

BREMSSTRAHLUNG

CONDUCTION

BAND

DRESSED ELECTRONPHONON SCAHERING

IMPURITY

ABSORPTION

F CENTER OR

SELF-TRAPPED

EXCITON FORMED

WHEN V,^ CENTER

TRAPS FREE

ELECTRON

VALANCE

BAND HOLE

GENERATION

HOLE ABSORPTION

Fig. 1 Some suggested nechanlsma for laser damage in solids.

XV

Laser-Induced Damage in Optical Materials

Seventeenth ASTM Symposium

October 28-30, 1985

The Seventeenth Annual Symposium on Optical Materials for High

Power Lasers (Boulder Damage Symposium) was held at the National

Bureau of Standards in Boulder, Colorado, October 28-30, 1985. The

Symposium was held under the auspices of ASTM Committee F-1,

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. Over 215 scientists attended the Symposium,

including representatives from the United States, the United Kingdom,

Israel, India, Japan, the Federal Republic of Germany, and the Soviet

Union. 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 highpower apparatus. The wavelength range of prime interest was from

10.6 /jm 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

Eighteenth Annual Symposium is scheduled for November 3-5, 1986, 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 Seventeenth 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 28-30, 1985. The Symposium was held under the

auspices of the ASTM Committee F-1, 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.

Over 215 scientists attended the Symposium, including representatives from the United

States, the United Kingdom, Israel, India, Japan, the Federal Republic of Germany, and

the Soviet Union. The Symposium was divided into sessions concerning Materials and

Measurements, Mirrors and Surfaces, Thin Films, and finally. Fundamental Mechanisms.

In all, approximately 70 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.

1
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

Eighteenth Annual Symposium, scheduled for November 3-5, 1986, at the National Bureau

of Standards, Boulder, Colorado.

2 . Overview

Our objective in this section is to give the reader a sense of the emphases and

general concerns of this year's symposium. Although we do not review all of the

excellent work presented, some of the highlights are touched upon briefly.

Materials and Measurements

A sizeable fraction of this 21-paper session described damage testing facilities

with significant automation and computer manipulation of the test parameters.

Automated beam profiling received the most attention. For example, one paper reported

computer-assisted profiling of the laser fluence from 193 nm to 10.6 fim using

fluorescence in a thin slab of BK glass for the uv, solid-state video camera direct

response for 0.45 to 1.06 pm, and laser- induced emission from graphite films for

10.6 fim. Another paper described highly automated, computer-controlled operation of

CO2 and HF lasers with pulse durations from 50 /is to fractional cw times, collection of

laser power readings and beam profiles (automated pinhole scan) , control of sample

position (x,y,z), and sample temperature ranging from 77 to 370° K.

A second group of three papers was stimulated by recent Lawrence Livermore

National Laboratory experience of reduced damage thresholds in laser glass discs caused

by platinum inclusions. These concerned characterization of the damage and its growth

under repeated irradiation at 1064 nm, dependence on the laser pulsewidth, and

destructive inspection by rastering a laser beam over the 46-cm discs.

Of great interest to researchers in this field was the diagnostic system that

featured two synchronized, picosecond dye lasers as multiwavelength probes to identify

causes of multiple-pulse damage. The lasers were used both to cause damage and sample

accumulation of defects. Another experimental study identified bacterial growth in

starting aqueous solutions as the cause of increased absorption and lower bulk damage

thresholds for KDP crystals. Irradiation with a uv lamp during crystal growth killed

the bacteria and resulted in dramatically higher damage resistance material.

Surfaces and Mirrors

Of the eleven papers in this session, three addressed methods of surface

preparation to maximize the damage resistance. One study determined, not surprisingly,

2
that 355-nin damage to surfaces of fused silica is very dependent on the polishing

compound. Highest thresholds were attained by using yttrium oxide, followed by silicon

carbide, and least with iron oxide. Spectral analysis of the surface plasmas always

revealed elements of the polishing compound. In a systematic study of the role of

surface preparation on damage to Ge , alumina polish followed by an argon- ion sputter

etch generally yielded the highest threshold. Diamond- turning was second-best, but

this caused higher absorption in the top Ge layer due to mechanically induced changes

in the work function.

Implantation of 150-keV Mo ions into polished Mo laser mirrors was found to increase the reflectivity by surface smoothing. With an appropriate Mo"*" fluence, a nearly atomically smooth surface was attained. Other ions, such as 1-MeV Au and 3-MeV

Ni, caused some surface roughening. A second paper reported successful use of an ionexchange process to toughen phosphate laser glass for high-average power applications.

This process produced a compressive surface layer that raised the thermal shock

resistance by a factor of five.

Several papers examined multiple-pulse effects. An important study of N-on-1

irradiation of single-crystal surfaces of Cu, Ni , and Al using a l-/jm laser revealed

two damage mechanisms: 1) defect-related local melting at low pulse numbers and 2) plastic deformation at high pulse numbers. A second paper discovered multiple-shot

accumulation damage in GaAs irradiated by a l-nm laser at levels 25% to 50% of the

single-shot threshold. No predamage correlation with ion emission was found.

The need for grazing- incidence , intracavity mirrors in free-electron laser

resonators motivated a study of the optical properties of a variety of metal surfaces.

Absorption was measured directly at 0.5 urn and 10.5 ^m for angles up to 89°. Except

for the highest angles, where complications due to surface topography were manifest,

the measured absorption of freshly coated mirrors agreed with that predicted by

Fresnel's equations.

Thin Films

Thin film coatings are generally the weakest components of high-power laser

systems and, because of their complex nature, they have been the most difficult to improve. In recent years, the amount of effort directed toward coating development has

steadily increased, and at this year's Symposium just over 40% of the total number of

papers presented concerned optical coatings. These 28 papers can be categorized in seven groups: 1) advanced coating diagnostics, 2) defect analysis and detection, 3) methods to improve coating structure, 4) optimization of coating deposition parameters,

5) survival in hostile environments, 6) performance of porous coatings, and 7) new reflector designs.

As an indication of the popularity of Raman spectroscopy as a diagnostic in thin

film analysis, four papers described its application to measurements of laser- induced

3
stress and strain, crystallinity , and phase changes in real-time with l-/jm spatial

resolution (better than Nomarsky microscopy) . A fifth paper described how Raman

scattering can detect temperature increases on Si02 and glass substrates and in Ti02

coatings by simultaneous measurement of Stokes and anti-Stokes intensities; higher

anti-Stokes intensities were seen above threshold; no effect occurred below threshold.

Yet another paper demonstrated how to increase the sensitivity of Raman analysis by

launching the probe laser as a guided wave within the thin film under investigation,

with the additional benefit of eliminating substrate interference.

X-ray photoelectron spectroscopy (XPS) was exploited in the analysis of the

interfacial structure of ion-beam-deposited Ti02/Si02 coatings. Intermixing of these

materials was detected over a 7-nm range at the film boundaries, oxygen deficiency was evident for films in contact with the Si substrate, and the outer free surface

apparently contained water vapor. Another paper described the use of MeV-ion beams

(alpha particles) to characterize multilayer dielectric reflectors. These were employed in Rutherford Backscattering (RBS) to measure the composition as a function of

depth, detect laser damage- induced structural changes, and by particle- induced

x-ray emission (PIXE) , to measure the presence of low- concentration impurities.

Under the topic of coating-defect detection and analysis, one paper described a positive correlation between local He-Ne laser scatter and defect- initiated laser

damage at 2.7 fim for certain sample types. Another group described microscopic

observation of damage- initiating sites of ~ 1 nm diameter at the boundary between an antiref lection coating and the substrate and a good correlation of substrate roughness

with the number of initiator sites.

Seven papers were concerned with coating microstructure , e.g. how to eliminate

columnar structure and decrease porosity. One group reported used of metal/dielectric

laminates of silver and thin Ti02 coatings to prevent columnar microstructure. This

had the positive result of increased laser damage thresholds at 351 nm. Use of

stratified films was also found effective in controlling the microstructure of

(ZnS/ZnSe)2 films deposited by molecular-beam epitaxy. Remarkably, these films had the same damage threshold as the uncoated substrates.

Co-evaporation of zirconia and silica films was reported to decrease porosity and

columnar structure. Furthermore, a second paper reported that the porosity of ZnS films is correlated with the damage threshold at 10.6 //m: denser films attained by

deposition at high substrate (Ge) temperature (200° C) had substrate-limited

thresholds

!

Confirming the report by Decker, et al. at the 1984 Symposium, a second group

determined that the thermal conductivity of coatings can be much less than the bulk

value. The measured conductivity of an enhanced high reflector was found to be a factor of 30 lower than predicted by use of bulk values. Especially for cw-laser

4
applications, methods to alter coating structure to more nearly match that of bulk

material will greatly improve the thermal performance.

Five papers reported experimental studies to optimize the deposition parameters

for particular coating processes and film materials. For example, ion-assisted

deposition (IAD) was uniquely able to produce durable films of MgF2 , Si02 , and

AI2 03/3102 near room temperature on heavy-metal fluoride glasses.

Valuable data on the effect of molecular fluoride on optics for excimer lasers, in

particular at 351 nm, was presented. (Regrettably, the manuscript of this paper was not supplied for this proceedings.) Coatings were exposed to a XeF excimer gas mix

containing 0.5% dry F2 at 760 Torr. All coatings with Si02 layers failed quickly and

catastrophically , and after 48-hours exposure, the transmittance of bare Suprasil fused

silica decreased by 7%. Multilayer reflectors of AI2 O3 /cryolite on Si02 exhibited good

resistance for F attack and had high damage resistance; similar performance was observed for Al2 03/MgF antiref lection coatings on CaF2

.

The properties and obvious advantages of graded- index AR coatings made of porous

m silica were presented by two groups. A new fabrication method developed in Japan

involves electron-beam codeposition of NaF and Si02 followed by dissolving the NaF with

water. These coatings feature 0.17% reflectance at 355 nm, surface roughness no greater than that of the substrate (11 A rms) , and very high damage thresholds. A second paper by Lawrence Livermore Laboratory scientists presented results of multipleshot tests at 351 nm at 25 to 100 Hz. Porous-silica AR coatings on fused silica had

damage thresholds double those reported for films prepared by physical vapor deposition

processes

.

Finally, three papers presented theoretical designs for advanced coatings. One

analyzed designs for multilayer dielectric coatings for phase control, while a second

maximized the theoretical reflectance of dielectric coatings set at grazing incidence

for possible use in free-electron laser resonators. Extending even further into the

future was a proposed reflector design for free-electron lasers that will operate in

the extreme ultraviolet below 100 nm. Total external reflection at multiple, flatmetal surfaces, e.g. Al and Si, was shown to be a feasible way to turn the light beam

by 180° with broadband reflectance exceeding 40% for wavelengths greater than 35 nm.

Fundamental Mechanisms

Of the eight papers in this session, two were experimental studies of the effects

of high-energy radiation on the absorption and laser damage of fused silica. First,

irradiation with 2-MeV electrons caused transient absorption at 250 nm which had two recovery paths as indicated by two time constants, 50 ns and 1 fxs . Second, defects

induced by 1-MeV gamma rays reduced the pulsed- laser damage threshold at 532 nm by as much as 40% due to two -photon absorption, but little change in thresholds was measured

for 1064-nm laser pulses.
Complementing the aforementioned experimental studies on the same topic, the

fracture mechanics of glass due to absorption at Pt inclusions were analyzed

theoretically. The author considered crack formation based on shock-wave propagation

when the temperature of the Pt exceeds the boiling point.

Observation of lower damage thresholds at moderate pulse-repetition frequencies

(PRF) is not yet well understood. In pursuit of an explanation, one experimental paper

reported the effect of <100 Hz prf on pulsed-COj laser damage to a number of infraredtransmitting materials. For some of the materials, including Si, ZnS , and ZnSe, the

damage thresholds dropped monotonically for prf's from 1 to 10 Hz and then remained

constant from 10 to 100 Hz. The authors believed that such behavior might be explained

by anomalously low thermal conductivity at the surface due to polishing- induced

structure changes or grain size.

For further information on the topics reported at this Symposium, the interested

reader should consult the individual manuscripts contained in this Proceedings.

3 . Acknowledgments

The editors would like to acknowledge the invaluable assistance of Mr. Aaron A. Sanders and the other involved staff members of the National Bureau of Standards in

Boulder, Colorado, for their interest, support, and untiring efforts in the

professional operation of the Sjrmposium. Particular thanks to Ms. Susie Rivera for her

lead in the preparation and publication of the proceedings as well as to Ms. Edit

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

coordination

.

4 . References

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

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-292; 1972.

[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: A

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

6
[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. , eds. Laser Induced Damage in Optical Materials:

1978, Nat. Bur. Stand. (U.S.) Spec. Publ. 541; 1978.

Glass, A.J.; Guenther, A.H., eds. Laser Induced Damage in Optical Materials: 10th

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

;i9] Bennett, H.E.; Glass, A.J.; Guenther, A.H. ; Newnam, B.E., eds. Laser Induced

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

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.

21] Bennett, H. E.; Glass. A.J.; Guenther, A. H. ; Newnam, B.E., eds. Laser Induced

Damage in Optical Materials: 1980, Nat. Bur. Stand. (U.S.) Spec. Publ. 620; 1981.

'22] Bennett, H.E.; Glass, A.J.; Guenther, A.H.; Newnam, B.E. Laser Induced Damage in

Optical Materials: 12th ASTM Symposium, Appl. Opt. 20 (17): 3003-3019; 1981.

[23] Bennett, H.E.; Guenther, A.H. ; Milam, D.; Newnam, B.E., eds. Laser Induced Damage

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

[24] Bennett, H.E.; Guenther, A.H. ; Milam, D. ; Newnam, B.E. Laser Induced Damage in

Optical Materials: 13th ASTM Symposium, Appl. Opt. 22 (20): 3276-3296; 1983.

[25] Bennett, H.E.; Guenther, A.H. ; Milam, D.; Newnam, B.E., eds. Laser Induced Damage

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

[26] Bennett, H.E.; Guenther, A.H.; Milam, D. ; Newnam, B.E. Laser Induced Damage in

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

[27] Bennett, H.E.; Guenther, A.H. ; Milam, D. ; Newnam, B.E., eds. Laser Induced Damage

in Optical Materials: 1983, Nat. Bur. Stand. (U.S.) Spec. Publ. 688; 1985.

[28] Bennett, H.E.; Guenther, A.H. ; Milam, D. ; Newnam, B.E. Laser Induced Damage in

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

[29] Bennett, H.E.; Guenther, A.H. ; Milam, D. ; Newnam, B.E., eds. Laser Induced Damage

in Optical Materials: 1984, Nat. Bur. Stand. (U.S.) Spec. Publ. 272; 1986.

[30] Bennett, H.E.; Guenther, A.H. ; Milam, D. ; Newnam, B.E. Laser Induced Damage in

Optical Materials: 16th ASTM Symposium, Appl. Opt. 26 (5): 813-827; 1987.

7
Manuscript received

2-31-86 SAR

A Versatile Laser Glass Inspection and Damage Testing Facility*

J. E. Marion, G. J. Greiner, J, H, Campbell, P. H. Chaffee, J, S. Hildum,

J. Z. Grens, C. L. Weinzapfel, S. M. Winfree and D. Milam

Lawrence Livermore National Laboratory

University of California

P.O. Box 5508, L-490

Livermore, California 94550

Most glass optical components for high power lasers are melted and homogenized in

platinum-lined crucibles leading to the potential presence of microscopic platinum

inclusions. In situations where large optics are exposed to high laser fluences, a significant problem can be created by an extremely low density of inclusions, as low as one

per component. Previously, direct visual examination or optical microscopy was used to inspect for these particles, limiting reliable detection to inclusions greater than lOym

diameter. Unfortunately, much smaller inclusions can initiate damage in modern lasers

operated at high fluences.

A test facility is described which detects small opaque inclusions in large transparent

components by using a commercial laser which delivers high energy pulses to the test sample

at moderate frequency in a small diameter beam. The sample is automatically scanned such

that each point in the volume is irradiated with ten pulses at twice the inclusion damage

threshold--an amount sufficient to cause visible damage at inclusion sites. This approach

permits detection of opaque inclusions in the parts per trillion and lower concentration

range. The specifics of the device design, and its performance are discussed in the context

of automatic inclusion inspection and mapping in large laser optics.

Key words: inspection techniques, laser glass, laser-induced damage, metallic inclusions,

platinum

1. Introduction

Modern fusion research lasers and proposed defensive laser weapons use very large optics

which are exposed to high laser fluences.' If present, opaque microscopic inclusions cause unacceptable damage. A simple device for performing large scale inspection of optical components

for the presence of inclusions is described. Our present application requires a system capable

of rapid and reliable inspection of Nd:phosphate glass samples for inclusions which absorb at 1.06 urn, such as platinum and carbon. The samples include finished amplifier disks, ranging in

volume up to about 10 liters, as well as numerous test samples. The size of the inclusions which

must be located extend into the sub-micron region--a territory in which conventional optical

microscopy is ineffective. Light scattering methods are unwieldy because bubbles, surface

imperfections and dust are indistinguishable from inclusions even when the sample is of high

optical quality. Signals from these imperfections can swamp the signals of interest because the

inclusion number density is quite low, in the lO-^/cm^ range.

We adopt a destructive technique for locating inclusions as the only reliable method available.

A commercial 0-switched YAG laser (JK Model HY 750) gives a focussed beam 2 mm diameter on the

sample which is scanned in X and Z directions by a commercial stage drive (Daedal Models 47215 LH and MLH, Fig. 1). In order to assure adequately large damage sites at each inclusion (to

facilitate subsequent optical inspection) each site in the sample is irradiated at normal

incidence with ten shots; each above 10 J/cm^, This fluence corresponds to twice the damage

threshold for platinum inclusions in glass at the 8 ns pulsewidths used here. 2 in practice, the

device is capable of scanning a 46 cm aperture phosphate glass amplifier disk (7000 cm-^ volume)

in about four hours using ten shots per site. Reducing the number of shots proportionately

reduces the scan time. The optical examination of the damaged disk takes an additional hour or

so depending on the number of damage sites which must be characterized and mapped.

*Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore

National Laboratory under Contract No. W-7405-ENG-48.

8
Laboratory

computer

Printer Interface

f = 5m

Mirror

Commerical Q-switched YAG laser

Commercial X-Z scanning

stage

: Optical path

Information path

Test sample

50 mm lens

Figure 1. Schematic of the inspection test device, as described in the text.

2. Device Description

The device is located in a class 1000 clean room in order to avoid dust contamination and

subsequent damage to the device optics as well as the test sample. The laser is quite stable

with better than 99X of the shots within 1% of the mean shot energy. The beam profile is

adequately uniform to insure that the sample is irradiated at the desired fluence (Fiq. 2), The

output energy is 800 mJ at 8 ns pulsewidth and 20 Hz PRF. The slightly diverging 8 mm diameter

output beam is approximately collimated by a five meter lens and directed toward the sample by

high power dielectric-coated mirrors. It is then focussed by a two meter positive lens into a mildly converging beam. A collinear HeNe alignment beam is introduced before the positive lens

to aid in sample positioning. A beam splitter (with the front surface anti-reflection coated) is

located after the two meter lens, and a calorimeter monitors the second surface reflection as a continuous check of laser performance. After the beam splitter, the laser beam, now 2 mm in

diameter, strikes the sample which is mounted on a X-Z motor driven stage.

The laser and the stage drive are controlled by a laboratory computer (Hewlett Packard

981 6S), interfaced through a general purpose data acquisition control unit (HP 3421A). If dust

or damage occurs upstream, leading to a degradation in the laser energy, the test is interrupted

by the computer monitoring the calorimeter. Automatic shut off of the laser and stage drive can

also occur if the sample reaches position limit triggers, or if there is a high laser cooling

water temperature.

The use of the computer also offers the possibility of real time inclusion mapping. When

irradiated, the damaging inclusions give a distinct optical signal associated with plasma

emission. This signal can be readily detected by focussing onto a photodiode array. This

information is combined with X-Z position coordinates to give an inclusion map. Although the

detection system has been demonstrated,-^ it is not yet sufficiently reliable to replace optical

inspection. Incorporation of this mapping option will reduce the inspection time by an order of

magnitude because only one shot at each site will be required. The post mortem optical

inspection will also be eliminated along with its inherent operator subjectivity.

9
X Direction beam profile Y Direction beam profile

Microns Microns

Figure 2. Beam profiles obtained from microdensitometer traces of the photographed beam.

The beam shape is reasonably Gaussian.

3. Device Applications

We believe that this facility offers a unique capability in materials damage assessment.

Prior damage testing has generally fallen into one of two categories. Previously, small (mm^ or smaller) beams at high fluences were used to test several spots on the sample, in conjunction

with a statistical treatment of the data.4 The limitations of that method in situations where

the damaging entities have a very low spatial density are obvious. A second approach uses a research tool such as the Nova laser to give high fluences over an entire area in single shots,

but at very low repetition rate.^ Aside from the expense of the latter method, our new device

is faster and less cumbersome. Large optics can now be quickly and reliably inspected for

inclusion volume concentrations in the oarts per quadrillion range (lO^S) (i.e., several one micron diameter inclusions in a 7000 cm-^ optical component.)

A further advantage of this testing device is its ability to rapidly perform multiple shot

damage characterization on inclusions that have been carefully analyzed prior to testing. Damage

growth rate data on single inclusions subjected to ten thousand shots or more can be obtained

readily, helping to relieve the paucity of information in that area.

Finally, it has not escaped the attention of the authors that this device has useful

applications in other fields of materials science and chemistry, for example, in any application

in which small concentrations of microscopic contaminants must be detected. In principle, any

laser wavelength may be used where the samples are reasonably transparent. The primary

requirement is that the microscopic inclusions or defects have a higher absorption than the

surrounding matrix at the laser wavelength.

4. Summary

Our requirement for rapid inspection of large optics for the presence of microscopic

inclusions has led us to construct a simple damage device using a Q-switched YAG laser and a computer controlled X-Z motor driven stage. Each point in the volume of the test sample receives

ten pulses at twice the damage threshold, permitting straightforward post mortem visual

determination of inclusion spatial density. An automatic detection scheme, in development, will

permit automatic mapping.

In addition to automated inspection, straightforward damage growth characterization to high

shot numbers is being pursued. This information is of interest for assessing optical component

reliability both for fusion research lasers as well as defensive laser weapon systems. '

10
References

Laser Program Annual Report, Lawrence Livermore National Laboratory, Livermore, CA, Eds. M.

L. Rufer, P. W. Murphy, 1984.

R. P. Gonzales and D. Milam, "Evolution during multiple-shot irradiation of damage

surrounding isolated platinum inclusions in phosphate laser glass", elsewhere in these

proceedings.

J. E. Marion, unpublished.

Statistical aspects of laser-induced damage were first treated in: M. Bass and H. H. Barrett, "The probability and dynamics of damaging optical materials with lasers".

Proceedings of Damage in Laser Materials: 1971, NBS Spec. Pub. 356, pp. 76, issued Nov. 1971. The first treatment of the probability of encountering isolated defects with a small

test beam was: L. G. deShazer, B. E. Newnam and K. M. Leung, "The role of coating defects

in laser-induced damage to thin films". Proceedings of Laser-Induced Damage in Optical

Materials: 1973, NBS Spec. Pub. 387, pp. 114, issued Dec. 1973.

D. Milam, C. W. Hatcher and J. H. Campbell, "Platinum particles in the Nd:doped disks of

phosphate glass in the Nova laser", elsewhere in these proceedings.

OIS( 1 AIMKR

This ducumeni »as prepared as an account of work sponsored by an agency of

the I niled Slates (.(nernmenl. Neither the I nited States doiernment nor the

I ni\ersity of C alifornia nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the ac- curacy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would nut infringe privately owned

rights. Reference herein to any specific commercial products, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily

constitute or imply its endorsement, recommendation, or favoring by the I nited States Government or the t niversity of ( alifornia. The views and opinions of authors expressed herein do not necessarily state or reflect those of the I niled States Ciovernment thereof, and shall not be used for advertising or product en- dorsement purposes.

11
Manuscript Received

11-8-85

Computer-Controlled Facility for Laser Switching and Damage Testing

Jay S. Chivian, W. D. Cotten, D. F. Fuller,

R. B. Hemphill and M. W. Scott

LTV Aerospace and Defense Company

Dallas, Texas 75265-0003

Apparatus has been assembled for mostly-automatic laser switching and damage

measurements in which dynamic events are followed in real time. Lasers, optical

elements, and detectors are available for operation at 2.8 \jsn and at 10.6 ^m. Sample

exposure time is variable from 50 \is to cw. Sample positioning in X:Y:Z is controllable

to 0.2 |im. Testing can be done with static beam position or with a scanning beam.

Samples can be tested at ambient temperature, or from near 77°K to 370°K.

Software provides for test control, data processing, and graphic, hard copy output.

Beam profiling can be done automatically at the sample position, and the profile fit to

a Gaussian of the same l/e^ width. Sample data acquisition is sequenced by the computer

after an enable pulse is given at the discretion of the experimenter. The computer

records and processes the incoming data, presents intermediate data display, completes

data manipulation, and compares processed measurement data with internally available

prediction models in graphic form. Representative output for a thin film nonlinear

optical material is shown.

Key words: automated testing; dynamic real time testing; laser-induced damage; laserinduced switching.

1. Introduction

It has been our objective to assemble versatile apparatus to make accurate and reproducible

measurements in real time of fast, dynamic processes occurring under intense laser irradiation. We wished to examine laser-induced switching and damage in nonlinear materials, and in a more general

sense, do damage characterization of any thin film material/substrate pair. For these purposes, we define "nonlinear material" as one for which its optical properties change abruptly at some fixed

condition, such as at a phase transition occurring at a given temperature.

Present and projected capabilities are summarized in tables 1 and 2. Hardware is reviewed in section 2, first in a generalized form, after which specific embodiments are discussed. The

procedural sequence, during which the experimenter interacts with the coordinating software, is covered in section 3. Data samples are shown at various stages of processing, as is representative

output for switching and damage events.

2. Hardware

The apparatus and instrumentation are shown in general form in figures 1 and 2. The block

labeled Beam Handling in figure 1 is a convenient way of describing a collection of items

distributed throughout the optical path which direct and shape the beam. These typically include

mirrors, lenses, windows, beamsplitters, and apertures. These optical components are usually

positioned during setup and alignment, and do not interface with the computer. The Pulse Selection

components are also distributed at convenient locations; included are a shutter, light chopper, and scan mirror (if used). The chopper is often placed at the internal focal plane of a beam expander,

which thus allows for fast rise times and short pulse lengths. The power meter shown in two positions is used during the calibration of the detectors. A pinhole (of appropriate size) can be

12
placed at the position of the sample for beam profiling of a stationary beam. Two pinholes

separated by a known distance are used to measure scan speed.

The available hardware is listed in more detail in table 3. Schematics for a stationary spot

experiment at 2.8 p,m, and for a scanned spot test at 10.6 |jm are shown in figures 3 and 4 respectively. Figures 5 and 6 are photographs of the actual apparatus, in the same order. Figure

7 shows the overall test instrumentation configured for a single short pulse; the lock in amplifiers are used only for repetitive pulses, as in long exposures. Figure 8 and 9 show the

timing electronics for long exposures (50% duty cycle at 20 kHz) and for the scanning test,

respectively. The timing sequence for a scanning laser pulse is given in figure 10 as an example.

The timing sequence for a single short pulse is similar to figure 10, but without the scanner or variable delay sequence (pulse trains at locations 3-7 only). The timing sequence for long

exposure is even simpler; the chopper is free running and the manually triggered shutter is open

long enough to include many cycles of the 50% duty cycle chopper. The laser isolator feature has

proved to be essential to prevent feedback causing modulation of the laser output power when a scanning beam is involved; its effect is shown in figure 11.

3. Experimental procedure and representative results

An outline of the experimental sequence is shown in figure 12. The mathematical quantities

will be defined in context.

The spatial distribution of the laser beam at the location of the sample is determined by

stepping an appropriately sized pinhole across the beam in a number of steps while measuring the

power transmitted by the pinhole. This is done automatically by the controller at the same time

that a graph of relative power versus position is plotted on the printer. Figure 13 is a typical

laser beam profile map with a theoretical Gaussian fit to the same l/e^ diameter (also generated by

the computer). Circular symmetry is assumed, although symmetry is easily monitored with an IR image plate or by additional profiles.

The apparatus is calibrated with no sample in the system using power meter and programmable

digitizer. Calibration is done at or near_operating power conditions. The ratio of the average

power to the transmitted detector system Pj(power at meter position 2) to the average power to the

incident detector Pj (power at meter position 1) is measured at the beginning of the experiment by

recording the power in front of the incident detector system and then in front of the transmitted

detector system several times to arrive at an average of the beamsplitter ratio

B-Pj/ Pi.

The error on this measurement does not affect the final results, as it will be absorbed into the

responsivity calculation.

The incident detector is calibrated with the power meter in front of the transmitted detector

system (position 2); the chopper is turned off and held open to read the transmitted cw power.

Next, the chopper is turned on and 32 pulses from the incident detector _are averaged by the

programmable digitizer. Using the digital cursors, the average voltage Vj from the incident

detector differential amplifier is measured. This is repeated several times. The responsivity (in

V/W) of the incident detector system is then calculated from

Rj = ViB/(GiPj),

where G = gain on the detector differential amplifier.

13
The transmitted detector system is calibrated from the incident detector:

Gy Pj Gj B Vj

To calibrate for long exposures, the procedure is the same except the 50% duty cycle chopper

is not stopped for power measurements. Also the outputs of the lock-in amplifiers are treated as the detector outputs.

Data is acquired after a sample has been inserted into the system. It is assumed that pulse

length, digitize rate, power level, and amplifier gain setting have been selected. An enable pulse

from the computer starts the sequence and the sample is irradiated. Data is taken into the

digitizer and voltages from the incident detector amplifier \/j(t) and the transmitted detector

amplifier VT(t) are displayed versus time, giving a dynamic record of the sample characteristic

during irradiation. At this point, the experimenter can repeat the test, or accept the data and

instruct the computer to continue. The computer next locates the beginning and end of the

exposure, and corrects for dc shift and ac coupled signal droop. Options exist to display voltage

or power versus time, and to print or not print the data in tabular form.

The power to the transmitted detector system Pj(t) is calculated by the computer from

and now includes the transmission of the sample T(t). The power incident on the sample Pj(t) is predicted by the incident detector from

G,R,

T(t) can then be calculated from

T(t) =

p;(t)

The peak irradiance Iglt) on the sample is determined from

14
0

where Aq = transmission of the optical elements between the sample and the transmitted detector

system. Aq « 0.96 (typical). The peak fluence incident on the sample from pulse initiation time

t^- to time t is calculated by the computer from

F(t) = / So(t)dt.

t,-

The computer then displays the percent transmission and integrated peak fluence versus time.

Sample plots of VWt), Vj(t), T(t), and F(t) for a switching event and for a damage event are shown in figure 14 for two different samples. For the switching event, the abrupt change in the

transmitted signal occurs when the nonlinear material switches from a high transmission state to high reflectance. In the damage event, the sample switches early in the pulse (high transmission

to low transmission); the transmission is seen to increase roughly midway in the irradiance pulse

due to film removal from the substrate. The operator sets the vertical (time) cursor on the T(t)

plot at the time the event starts. The computer sets the horizontal cursor where the time cursor

crosses the fluence curve, displays the indicated lapsed time to the event, and calculates and

displays the integrated fluence up to event initiation (threshold fluence).

The experimenter can now repeat the sequence for different power levels or pulse widths

(gather more data), or proceed with computation.

When enough data has been acquired, the computer is instructed to calculate values of

threshold fluence versus time for specified conditions from a stored predictive thermal model. The

thermal model is based on solutions of the diffusion equation for a thin film on a semi-infinite

substrate; solutions of arbitrary sophistication can be used. The conditions involve material

constants and test conditions including switch or damage temperature, bias temperature, beam spot

size at the sample, and scale ranges. As a final step, the computer plots the experimental data

points on the same sheet as the theoretical curve. Examples of a switching curve and a damage

curve (for the same sample) are shown in figures 15 and 15. Note that the damage data point from

figure 14 is the solid data point in figure 16.

4. Summary

The apparatus and techniques described are obviously not limited to application with nonlinear

material films, but can be used with any material which damages when subjected to intense laser

radiation. Samples can be examined under dynamic conditions, and before and after maps can be made

of damage regions without losing the ability to do structural studies after removal from the test apparatus. Thus the goal stated earlier has been achieved.

We wish to thank M. 0 Gracey and J. H. Nickelson for valuable technical assistance. J. D. Ward contributed greatly to apparatus development and assembly. D. S. Smith helped considerably

with procedural and software development, as did cooperative student employees L. E. Godwin,

C. D. Murphy, and L. Valdez.

15
Table 1. Present Capability

0 Multiple wavelength - ~50W cw at 2.8 |im(HF) and 10.6 (imCCOj)

0 Gaussian irradiance distribution

0 Diffraction-limited focusing - ~ 50 M,m l/e^ diameter spot

0 Peak irradiance range - up to ~ 10^ W/cm^

0 Variable exposure - single pulse at 50 \iS to cw (50% duty cycle at 20 kHz) - b[is minimum rise

time

0 Stationary beam position or swept beam (multiple scan option)

0 Sample positioning in X:Y:Z - 1 in. travel in 0.2 pjn steps - computer interfaced

0 Sample temperature control - ambient, or 77°K to 370°K

0 Calibration for absolute measurement of fluence

0 Dynamic, real time record of sample characteristic during irradiation

0 Automated data collection, analysis, plotting

0 Comparison of results with theory for switching and damage thresholds

Table 2. Projected additions to capability

Lasers available

0 TEA CO2 (pulsed) - 0.5J/pulse, pulse width ~ 50 ns

0 Transverse flow C02(cw) — 800 W

0 Nd:YAG (pulsed) - 1.2J/pulse, pulse width ~ 10 ns

Table 3. Hardware

Sources

0 2.8 |im - 50W cw, TEMqq (grating tuning available)

0 10.6 \im - 50W cw, TEM^q (500W pulsed capability)

- amplitude stabilized

- grating tuned capability

Detectors

0 InSb photovoltaic cooled to LNp

0 HgCdTe photoconductive cooled to LNp

0 Integrating spheres - adjustable attenuation factor by variable iris

Optical Elements

0 Mirrors and beam directors - high reflectance metallic

0 Lenses - CaPo, Si(AR/AR), ZnSe(AR/AR)

0 Beamsplitters - Al202(bare/AR) , ZnSe(coated/AR)

0 Isolator-attenuator for 10.6 ^m - pol ar i zer/anal yzer (ZnSe Brewster angle plates),

ZnSe Fresnel rhomb (AR coated)

Sample inspection

0 Fiber optic probe and photomul ti pi i er for flash detection - can be used to verify and time

catastrophic failure

0 Microscope and video monitor - before and after snapshot capability

16
CL<

17
OTTER

INTER

cc

18

20
8i-9£zzsaw

21
sz-ieoegdiAi

I

22
= <

O oc

HP

7225B

LOTTE

a.

3; in a. c

— ^ Q. t- u 5 o z

a. o -J I s S;

o

o z 2 O if

155

s 5

' U (J

'

I "31

9 2

S 5

z 2

I 5II

I "31

z o

5 < ir o S I - ^ S

5 t

5 X o

lu O o I- o

IT

23
oc

LU

Q.Q.o,X

a

o

cc 2 ^ UJ O

-SoQ.

Q.O 2

X o

o o

z— oc ^ UJ o-I Q. O 5

I- <

CM

24
111

o

z

>

(O

QC

UJzz

<o

to

o

u

s

o

oc

UJ< _l tC UJ < Q

>

ocUJ

Q.

CLO

X

o

oc

UJ3

X

CO

25
><

UJ

(Q<<>

>-

GDH

UJ(A 1 Q

UJ I UJ ' Q.V>

tcUJ

>- Q. CO ^

UJ

X

Q.soo

>-

ffi H

UJ

(0 zg

I- <

>- OC

ffi UJ

UJ UJ (A O

__J_I_

UJT

Q UJ L— u — UJ " >- «

UJ _i z

v)<n < ax

0)cOc

•S « oIs

<0

NZ cc

re o « _ <fl 3 * 3O '

>»

_re Q

re

re >3

Q.3o 3auc

>. v>

0)a

a.o

jiO 3aEoOEo3a

rec

UJ coU

isV) re E « S »

(0 c 3 .2 cV

a. O

(A to 2 2 S I

re , I 2 «3

Q.3o coO*3£Wc3o E

re

a>

ffi co

'35 o

Q.c

0)Q. oa

Q.ocO «cc

reu

(A3O£ a.

o> c

!S «=

re (0

26
_l I I

I I I I I I I L

o o q o o o

o (d csi 00 ^ o CM f 1- _i OC (O < LU I27
28
29
r- 5

UJ '- ? (O 7 O CM Z S

5 Si

<D lU

f» ce ^ <o = =

T. Z

lu <

z ^

toz z

< •» Q Q " UJ < N

z .! <> z

« s s

H X

I-t M Z

ill ± 0. u

(%) 33N3nid NOiivznvnuoN

(%) NOISSINSNVUl

(%) aoNamd NOixvznvwuoN

(%) NOISSIWSNVUl

30

CM

inD

<

E

=5. CO

CO

CO

lU

in

I lU

CO+

lU

CM+

UJ +

UJ

CO

UJ +

UJ

(zi/yo/r) 30N3md

o

UJw

UJ

UJ

(£D

(0o

Q.X

UJ

E

oo

-> in

32
- Manuscript Not Received -

DAMAGE TESTING IN OPTICS PRODUCTION AND PROCUREMENT

S. C. Seitel and E. A. Teppo

Montana Laser Optics, Inc.

Bozetnan, MT 59715

ABSTRACT

Laser-induced damage studies reported at these symposia have emphasized

basic understanding of the light-matter interaction physics, and

development of new, more durable materials and coatings. This paper

reports a parallel trend toward routine use of damage testing in the

procurement and production of reliable optics for low energy, high peak

power military and commercial laser systems. Materials and coating

designs are conventional, and in practice, in-service failures are as often due to system design effects and contamination as to improper

finishing or coating design errors.

The emerging roles of damage testing as a production process-control

tool, in acceptance testing, and in failure diagnostics, are discussed

and illustrated with specific examples and color photomicrographs.

Particular attention is paid to the problems of (1) meeting widely

varying test requirements, often on deliverable part of complex

geometry, in timely and cost-effective fashion; and (2) relating

calibrated single-mode damage test results to optics durability in

highly multimode military systems. A brief review of a new repetitively pulsed dual -wavel ength damage test station with

exceptional conf igurational flexibility and advanced digital fluence

profiling features is included.

Key Words: computer control; laser damage; laser beam diagnostics;

laser fluence measurement.

33
- Manuscript Not Received -

A COMPUTER AUTOMATED LASER DAMAGE LABORATORY

M. R. Sweet, L. J. Jolin, S. R. Foltyn

Los Alamos National Laboratory

Chemistry Division

Los Alamos, New Mexico 87545

ABSTRACT

The UV Laser Damage Program has been in existence at Los Alamos for over five years. Recently, the damage laboratories have been upgraded

by using a computer system to control and monitor testing parameters,

and to display and store the damage data.

The system was designed around already existing testing equipment

merged with an IBM-XT for cost effectiveness. The damage fluence is set by a programmable beam attenuator configured in a closed-loop

control system with the computer and an energy meter. A matrix of

damage sites is set up at the test plane such that the operator can move the sample sequentially between sites or pick sites at random. To

aid the operator in choosing test fluence values, a damage plot may be

generated on the monitor or on a high resolution plotter.

By automating the Laser Damage Laboratory with a computer, consistent

results can be achieved over a short testing time.

Key Words: computer-control; laser damage; laser diagnostics ultraviolet
Maniisari-pt received

1-27-86 SAB

U.V. Laser Beam Diagnostics using a Computer

Controlled Video Frame Store System

S.E. Clark, I. Laidler and D.C. Emmony

Department of Physics, University of Technology

Loughborough, Leics., England

and

B. A. Omar and M.J . Shaw

Rutherford Appleton Laboratory

Chilton, Didcot, Oxon, England

In the past the spatial intensity profile of a laser beam has been measured using

such techniques as scanning pinholes, detector arrays and photography.

This new system converts the Excimer beam into visible light using a glass fluorescer

with a very small penetration depth giving a good 2D image of the beam. The fluorescence

has been found to be linear over a wide range of input energy and is imaged by a CCD camera linked to the computer controlled video frame store.

The digitised information is then computer processed to yield such results as the

intensity profile along an arbitary line, peak fluence, the difference between two images

and various statistical values.

This new technique has been applied to beams with widely varying diameters including

analysis in the focal region and in the early detection of damage in materials.

Keywords:- Video frame store; computer controlled; 2D image; linearity; calibrated.

1. Introduction

The problem of beam orofiling is as old as the laser itself. Previous techniques have

included direct photography, scanning pinholes and detector arrays, all of which suffer from

significant problems.

Photography has the two main problems of chemical (wet) processing and image linearity. It

is not a trivial task to obtain a truely linear intensity representation of an object on film. If

the image is not linear calibration becomes not only far more tedious but subject to greater

error. Even with a linear image it is a time consuming problem to obtain the data from the film

by such instruments as micro- densitometers. The slightly redeeming feature is that a 2D image is recorded at one go.

The main problem with a pinhole is that it provides information about a point, thus a time

consuming rastering process must be used to obtain a 2D profile. In order to allow for shot to

s(iot variation an average of several shots must be taken at each point.

Detector arrays are a logical step forward but have often suffered from poor spatial

resolution and require complex driver electronics.

Using a video camera allows a 2D linear image to be obtained in one shot with good spatial

resolution due to the high packing of detectors on the camera chip. Typically without external

optics resolution of around 25ym and with external optics of lym is routinely obtainable.

35
2. The System

Figure 1 shows how the various components of the system are inter-connected and figure 2 a practical arrangement for imaging the beam.

Since the camera does not respond to light with a wavelength below 400nm a fluorescer is used

to convert the U.V. (usually krF at 249nm) to visible fluorescence which the camera can then image.

A boro-silicate glass is used as the fluorescer. To avoid problems of image distortion the

glass in the form of a thin slab (less than 150ym thick) with a U.V. penetration depth of less than

lym. The response of the glass to incident radiation was measured and found to be linear over a range of fluences from lOpJ/cm^ to above lOmJ/cm^. The normal operating fluence is in the 2mJ/cm2

range so as to use the full dynamic range of the frame store.

The camera used was a commercially available solid state device which was checked for linearity

of response and for the effects of automatic gain control. The device was found to have essentially

no automatic gain control and to be linear over a dynamic range of approximately 100 at the

operating fluences.

The fluorescence is imaged by the camera and the video output goes both to the frame store

and a specially developed trigger box.

The trigger box is required to ensure that the image is always captured by the system for

without it between 10 - 90% of the shots, depending upon the type of camera being used, would not

be recorded. The box works by sychronizing the triggering of the frame store and the firing of

the laser to the video frame (field) sync pulses produced by the camera so that the laser is fired

at the correct point in the frame that the store is set to capture.

The frame store is a commercially available unit, the Eltime Image III with slight hardware

modifications so that both the controlling computer and an output from the trigger box can initiate

a frame snatch.

The store is set up to hold two pictures each 256 x 512 pixels simultaneously in memory each

with 64 levels of intensity resolution (greylevels) . The incoming data is digitised and stored

in the frame store memory in a grid format so that any point in the picture can be addressed

using cartesian type co-ordinate addressing.

An Apple He computer is used to control the system and it is this which makes the system

particularly powerful and versatile.

A suite of programs was written to perform the following tasks.

a) Aquisition, storage and display of data.

b) Image processing.

c) Intensity profiling in X, Y and e directions.

d) Radially averaged profiling.

e) Numerical integration.

f) Statistical analysis.

By using options e and f it is possible to calibrate an image in absolute units and so obtain

the fluence (J/cm^) at any point in the image. Option f will also give such parameters as average fluence, fluence per intensity level recorded and standard deviation of fluence.

The profiling program allows the user to select the location of the profile and whether it

is a line profile or a radial profile and produces graphs of recorded intensity versus position

d^ong the profile.

The image processing routines include removal of background (dark level) noise, image

enhancement and a routine to display the difference (in intensity levels) between two frames.

There are also routines to store an image on disc for future reference and processing.

36
3. Experimental Considerations

The main drawback of the system is the limited dynamic range of the frame store and care must

be used to ensure that the fluence incident on the fluorescer is sufficient to-jfully use the

available dynamic range. The 64 levels are more than enough to determine the /e^ points which are down by a factor of 7.4 on the maximum.

The arrangement of the fluorescer and camera in figure 2 is not practically the most convenient as it is impossible to both profile and damage at the same time since the fluorescer

is sitting where the damage sample should be. A more convenient set-up would be to insert a beam splitter between the lens and target plane and profile the beam away from the damage plane.

However care is needed, since the beam is a converging one unless the beam splitter is very thin

it will introduce significant distortion in the transmitted beam.

For calibration the following data is required.

1. Beam energy per shot as measured for example by a calorimeter and

2. the relation between pixel size on screen and real object size. This can be measured by

observing calibrated graticules at the place where the fluorescer is to be placed.

4. Applications and Uses

The main application of the system is in profiling the beam from the Excimer laser here at Loughborough University of Technology which is operated at wavelengths between 193 and 308nm. By

suitable fluorescers the system can work in the infra red, e.g. graphite for 10.6ym CO2 radiation.

Between 450 and 1060nm (Nd.YAG) no fluorescer is needed as the camera will respond directly.

The system has also been used in conjunction with other techniques to look at the early

detection of laser damage where it enables an actual image of an area to be recorded rather than

giving a spatially averaged result as a photodiode would do.

5. Results

Figures 3 to 11 represent typical data from the system. Figures 3 to 5 are from a 5mW He-Ne laser operating at 633nm. Figures 6 to 11 are for a nominal IJ 30ns krF excimer laser.

Figures 10 and 11 show how use of the statistical analysis package can enable the system to be calibrated yielding several important parameters.

Comparisons between profiles taken from this system and those by using a scanning pinhole are

in near perfect agreement.

6. Summary

The system can provide a two-dimensional beam profile much faster than previous methods

and can process the data to yield much more information about the profile than with current methods.

In short this is a highly versatile system capable of providing high quality 2D beam profiles with

associated data in near real time.

S.E. Clark and I. Laidler would like to acknowledge the finanical support given by the United

Kingdom Science and Engineering Research Council.

37
Laser Came ra

Apple computer Frame store

Fi gure 1

.

Video monitor

Figure 2. With camera and lens in position 1 the thin beamsplitter is not required. This avoids the

problems of distortion associated with the beam splitter but means that both damage and

profiling cannot be done together. In position 2, both damage and profiling can be done

together but as the thin beam splitter has to be used there are problems of distortion on the transmitted beam.

38
Figure 4. He-Ne Laser Horizontal profile

39
40
Figure 7. Excimer Laser- Horizontal profile

41
Figure 8. KrF laser 243nm vertical profile

Figure 9. KrF laser 249nm horizontal profile

42
cr

<u 4-

O)

Figure 10. KrF laser 249nni unprocessed histogram

>)u

c

cr

O)

i.

XJ

Starting level = 0 Excimer prof i le Sampling started at level = 5 Background level subtracted off = 5 Fluence per greylevel = 1.60860 J/sq.cm

Average fluence = 2.13154E1 J/sq.cm

Standard deviation of fluence = 2.40314E1 J/sq.cm

Highest fluence = 8.36470E1 J/sq.cm

Most frequent fluence = 8.04298 J/sq.cm

[lllttftTTmTf^hni<Tm^^

Greylevel

Figure 11. KrF laser 249nm

Processed histogram: background levels removed

43
Manuscript received

4-10-86 SAR

Beam Profiling Characteristics

of a Sensitivity-Enhanced Silicon Vidicon System

at 1.06 Microns

R. M. O'Connell* and R. A. Vogel*

Electrical and Computer Engineering Department

University of Missouri

Columbia, MO 65211

A. F. Stewart and D. A. Smith

Air Force Weapons Laboratory

Kirtland AFB, MM 87117

A beam profiling system consisting of a silicon vidicon detector and an image

processing computer is described, and results of measurements to characterize the system

are presented. For the transmission and display of geometrically accurate images, a unique relationship was found to exist between pixel clock frequency, channel time and

channel length. The vidicon target's dark current and 1.06 pm detection sensitivity were

found to have peak-to-peak variations of 5.6% and 3.75%, respectively, throughout the

target's central 3.75 x 3.13 mm region. Signal-to-noise levels were optimum at a target

temperature near 0° C. Without being optimized, cathode voltage switching improved the

pulsed-mode dynamic range of the system to approximately A.

Key Words: beam profiling; dark current; detector target uniformity; dynamic range; transient

response.

1. Introduction

Two-dimensional electronic imaging devices are a potentially powerful beam profiling tool for

laser damage experiments. For beams that are neither too tightly focused (diameters less than

approximately 1 mm) nor too rapidly pulsed (pulse repetition period less than approximately 30 seconds) these devices can provide real-time profile information on a pulse-to-pulse basis[l].

Since no averaging is required, it is unnecessary to make the often questionable assumption that

the laser is absolutely stable from pulse-to-pulse. Such a requirement is necessary when masking

methods such as the knife-edge are used to profile a beam[2]. With these advantages in mind, a beam-profiling system, which combines various two-dimensional imaging devices with an image processing computer, has been developed and incorporated into a laser damage facility at the Air Force

Weapons Laboratory (AFWL).

In this paper, the results of a study to characterize the use of a silicon vidicon detector in

the AFWL beam-profiling system are presented. After a brief description of the entire system in

section 2, a discussion of those system parameters that control the reproduction of vidicon images

on the computer display monitor follows in section 3. Results of measurements made at various

temperatures to characterize the system's ability to profile fast (10 nsec) 1.06 micron pulses are then presented and discussed in sections A and 5. Included are measurements of the spatial variations of vidicon dark current and detection sensitivity in Section A and system linearity and

dynamic range in section 5.

2. Silicon Vidicon Beam Profiling System

The AFWL beam profiling system is shown schematically in figure 1. The PAR 1252E silicon

vidicon [3] was chosen for the study because its target offers several attractive features: it is

thicker to provide for enhanced sensitivity at 1.06 it is wedged to suppress Moire' interference; and its area (12.5 mm x 12.5 mm) is large enough to permit profiling of a wide range of laser

beam sizes.

The PAR 1216 vidicon controller shown in figure 1 performs several functions. First of all,

it is the interface for operator control of vidicon scanning conditions, including channel length

in pixels, channel time, and the number of channels per frame. ThuiL, the vidicon reads images in an

44
operator -determined non-standard video format. The "video" output of this system is approximately

a voltage- inverted RS 170 format with non-standard timing.

_Lisllt_^

N.D. Filters

PAR 1252E

Silicon Vidicon

(Cooled)

Scan

Conditions

Display

Monitor

Video

PAR 1216 Vidicon

Controller

Video

Exp

.

Start

Control

Conditioning

Exp. Start

and ^ Control

RCI Trapix 5500

Image Processing

System

Interf ace

jfExp. Start

To Laser Keyboard

Figure 1. Silicon vidicon beam profiling system.

The second function of the vidicon controller is to transmit images from the vidicon to the

Trapix computer [4] . For the computer to properly accept a non-standard video image, the image

must be transmitted via a set of control signals which serve as a pixel clock and channel and frame

synchronization flags. These signals are provided by the controller in the form of its 4.0 MHz master clock, its clock-out signal, and its trigger-1 signal, respectively. In considering various

clock sources, it was found that unless the 4.0 MHz master clock from the controller was used, an interference signal was always present in the displayed image. Use of the 4.0 MHz master clock

eliminated the interference signal, evidently because the clock-out and trigger-1 signals are derived from, and thereby phase-locked to it. The three control signals, illustrated in figure 2,

must reach the computer in TTL-compatible, differential form. Thus they are first processed at the

signal conditioning interface shown symbolically in figure 1 and detailed in figure 2.

4.0 MHz Master Clock.

• +

5- -O >

CI

Trigger 1

ock Out J I I Lq ^

Exp. Start

Pixel Clock

Channel Sync.

Frame Sync.

A.M26LS31

Differential

Line Driver

Figure 2. Control and timing signal conditioning interface: signals and conditioning circuitry.

45

T = channel time, T_ = frame time.

L» J?
The vidicon controller also provides a timing signal for frame synchronization, allowing

measurements with single laser pulses. When the system is used to characterize a pulsed laser, the

vidicon target is allowed to pre-equilibrate for 15 preparatory (prep) frames at the programmed

scanning conditions. The laser is then fired at the start of the next frame, which is transmitted

to the Trapix computer. Since the computer is programmed to capture and store the first frame

transmitted to it, image transmission from the controller to the computer must be delayed until the

laser is fired. This is accomplished with the controller-generated experiment-start signal, which

automatically turns on at the completion of the prep frames. As shown in figure 2, after some compatibility conditioning, the experiment-start signal simultaneously fires the laser and enables

the frame synchronization control signal (trigger-1). Since all three control signals must be

present for data transmission, a single laser pulse image is thus captured by the image processor.

The RCI Trapix 5500 image processing system shown in figure 1 completes the AFWL beam

profiling system. It is used to digitize, store, and display the non-standard video image from the

vidicon, and to perform image analysis and/or processing operations such as pixel grey level

averaging, contrast inversion, production of grey level histograms, and image profiling.

3. Transmission and Display of Geometrically Undistorted Images.

Since all image analysis and processing is performed on the image stored in the Trapix

computer memory and displayed on the display monitor, it is essential that this image be as true a reproduction of the vidicon image as possible. During the course of the study it was found that

this would be ensured if three conditions were satisfied.

First, it was essential to adjust the display monitor for square pixels. The Trapix software

program "ROZ" allows the operator to magnify the display up to eight times, enabling the physical

measurement of the dimensions of a 40 x AO pixel region. Adjustments were made via the display

monitor pixel-width potentiometer.

Next, the controller must be programmed for symmetric image sampling, so that channels and

pixels on the display monitor correspond to channels and pixels on the vidicon target. The corres- pondence of channels is assured by the channel synchronization control signal, but pixel correspondence requires more care. Since one pixel of vidicon information is transmitted per period of the

pixel clock, it was observed that pixel correspondence is assured if the programmed channel length

in pixels N^^ is equal to the transmitted number of pixels per channel N^j,, which in turn is set by

the programmed channel time T according to

= (T„ - 10 Msec) f^..

(1) ^

^

The 10 Msec in equation (1) represents the non-scanning retrace time during each channel, and f

^ is the pixel clock period, which was kept at 4,0 MHz throughout the study. Setting = in

equation (1) gives

(2) N = (T - 10 Msec) f „, c c ell

which is the condition that the programmable quantities N and T must satisfy for symmetric image

sampling.

The third condition to be satisfied for the optimum reproduction of a vidicon image on the

display monitor is that the vidicon channel deflection voltage must be set for symmetric image

transmission. If necessary, adjustment can be accomplished by first satisfying the previous two conditions, and then by displaying the image of a circular or square object and adjusting the

vidicon channel deflection voltage for a circular or square image. The Trapix software program

"CUE" can be used to surround the image with a rectangular cursor and count the number of pixels in

the X and y directions. When the two numbers are equal, adjustment is complete.

If the display monitor pixel-width potentiometer and the vidicon channel deflection voltage

are properly set, any (T ,N^) pair that satisfies equation (2) will result in the transmission and

display of a geometrically undistorted image. The PAR 1216 vidicon controller provides a choice of seven channel times. They are listed in table 1 along with the corresponding programmed channel

lengths that satisfy equation (2). The vidicon has a maximum of 512 pixels per channel; thus, the

140 Msec channel time, which requires 520 pixels per channel (see table 1), can not be used for

symmetric sampling. All the other channel times can be used.

46
Table 1. Channel time/channel length pairs that result in symmetric image sampling

T (usee) 20 40 60 80 100 120 lAO

c N (pixels) 40 120 200 280 360 440 520

c

The photos in figure 3, which show the circular image of a laser diode transmitted through an optical fiber onto the vidicon target, illustrate the effects of various choices of T^^ and N^^ on sampled image symmetry. The circular geometry of the object was verified by rotating it 90° (at

any T ,N pair) and observing no change in image dimensions. Note that in each photo the Y/X ratio

of image dimensions is equal to the ratio Nj^/N„, as expected. Also, the X dimension of the uniform

background of each image is proportional to w . This is as expected, also, because the x and y axes of the vidicon are transposed on the Trapix display monitor.

A) Symmetric sampling: T = 80 usee, B) Symmetric Sampling: T = 100 psec, N = N = 280 pixels. N = N = 360 pixels.

Figure 3. Effects of controller programming on image symmetry.

47
•4.1 Dark Current and 1.06 [im Detection Sensitivity Measurements

A.l Spatial Uniformity

To be useful in two-dimensional beam profiling, the vidicon target's dark current and

detection sensitivity at the wavelength of interest should be reasonably uniform throughout the

region of interaction with the beam. To measure these quantities, the vidicon was programmed for a 100 psec channel time and a 512 pixel channel length. While this (T ,N ) pair does not satisfy the

symmetric sampling criterion of equation (2), it is preferred in this measurement because it allows

a uniform survey of the entire target rather than the lower 70%, which would result if N were programmed for symmetric sampling, i.e., 360 pixels. Approximately uniform illumination of the

target at 1.06 ^im was provided by illuminating it with a fiber-optic illuminator through a frosted

glass window and a 1.06 nm band-pass filter. Since measurements were to be made with a fixed

illumination of the target at three different temperatures (24° C, 0° C, and -30° C), it was necessary to ascertain that the chosen illumination level did not cause target saturation at any of

the three temperatures. At each temperature, single frames of the illuminated and unilluminated

target were stored in Trapix memory. The unilluminated frames provided dark current information.

To analyze the data, the mean gray level (ranging from 0 to 83) was calculated in each of 63

equally spaced 27 x 27 pixel cells throughout each frame. The dark current value was then

subtracted from the corresponding illuminated value at each of the 63 cells to give backgroundcorrected signal levels. This procedure resulted in a pair of maps such as those shown in figure 4 for the case of a target temperature of 0° C. Note that the target response is maximum in the

center, which illustrates the greater efficiency of the reading electron beam in that portion of

the target.

14 3 14 9 15 3 15 0 15 1 14 b ]5 0

14 5 15 1 15 5 15 2 15 3 14 8 15 2 14 5 15 1 rT5 4 15 1 15 T] 14 7 15 2

14 6 15 3 15 6 15 3 15 4 14 9 15 3

15 0 15 6 16 0 15 7 15 9 15 4 15 7

14 7 15 3 7 15 4 15 u15 2 15 5

15 4 16 0 16 2 15 9 16 2 15 8 16 0

15 0 15 4 15 6 15 4 15 6 15 2 15 6 14 6 15 7 15 7 15 4 15 7 15 4 15 7

27 6 35 1 37 0 37 4 37 0 35 9 34 5

27 2 34 9 36 8 37 0 36 7 35 7 34 2 27 3 34 9 ^6 9 37 2 36 71 35 8 34 3 Background- 27 3 34 7 36 7 37 1 36 7 35 6 34 1 subtracted 27 1 34 6 36 7 37 0 36 2 35 6 34 0 signal 26 8 34 6 9 36 5 36 U 35 2 33 2 25 9 33 0 34 4 35 1 34 5 33 0 30 2 26 3 32 8 34 5 34 8 33 7 31 3 29 0 25 7 32 3 32 9 32 1 30 3 29 0 28 3

Figure 4. Mappings of vidicon target dark current and background - corrected signal in trapix grey

levels (0-83) at T=0° C. The bracketed 3x4 cell region corresponds approximately to

the central 3,75 x 3.13 ram region of the target.

48
Table 2 contains summaries of the peak-to-peak variation of the data in figure A for both the

entire target and the bracketed 3x4 cell region shown in figure A. This region corresponds to a 3.75 X 3.13 mm region near the center of the target.

Table 2. Peak-to-peak variation of target response at T = 0° C

Dark Current Background-Subtracted Signal

Whole Target 11.7% 31.3%

Central 3.75 x 3.13 mm 5.6% 3.75%

The numbers in table 2 suggest that while the whole-target variations are rather excessive, those

in the central 3.75 x 3.13 mm region, an area large enough to profile many typical laser beams, are quite acceptable. Results similar to these were obtained at the other two measurement

temperatures

.

A. 2 Temperature Dependence

The temperature dependence of target sensitivity was studied using data from figure A and the

corresponding mappings (not shown) for T = 2A° C and -30° C. In each case, dark current and

corrected-signal values from three adjacent 27 x 27 pixel cells near the center of the target were

averaged and plotted as shown in figure 5. The results show that while the target's signal

response decreases quite linearly with cooling over the entire 3-temperature range, the dark

current improves very little below 0° C. The widest gap between the two curves is in the vicinity

of 0° C, which indicates that the signal-to-noise ratio is optimum at that temperature.

24° 0° -30O

Temperature (°C)

Figure 5. Temperature dependence of target response using mean data from three adjacet 27 x 27 pixel cells near the center of the target.

49
5. Pulsed Measurements at 1.06 \Am

5.1 Linearity Improvement by Cathode Voltage Switching

The main drawback to the use of vidicons in pulsed applications is their nonlinear response

[5]. This is known to be caused by the capacitive "lag" that is inherent in all electron beam

readout devices [6]. To combat the problem, the method of frame summations has proven very useful

in the past [5], More recently, however, a cathode voltage switching method was developed and

shown to be superior to frame summation [7,8]. Thus, the effects of cathode voltage switching on the pulsed-mode linearity of the PAR 1252E vidicon were studied.

To incorporate cathode voltage switching into the beam profiling system, the vidicon

controller was modified to include a switching circuit [9] to raise the vidicon target voltage

immediately after the prep frames and before the read frames. The idea is to pre-equilibrate the

target at a low voltage, which establishes a relatively low background level, and to read the

signal at a higher voltage, which increases the saturation level of the vidicon photodiodes [7,8].

The net effect should be a greater dynamic range, i.e., the ratio between the optical signal level

that initiates vidicon saturation and that at which the vidicon response is just discerniible from

the background.

For the measurements, the vidicon was programmed for symmetric image sampling with T = 100

ysec and N = 360 pixels. It was illuminated with 1.06 pm, 5 nsec pulses from a Q-switchecP Nd:YAg

laser. The beam was focused to approximately 1 mm diameter on the vidicon target and adjusted for

various intensities with a wave plate attenuator. The vidicon controller was programmed to preequilibrate the target at 5 volts for 15 prep frames, fire the laser at the start of the next

frame, and transmit that frame to the Trapix computer for storage and analysis. With the switching

circuit [7] set for cathode voltage switching, the target voltage increased to 6.5 volts on the

16th (illumination) frame. Otherwise it remained at 5.0 volts.

To extract vidicon response data from the stored pulse images, a software program was written

to calculate the average gray level of all pixels with gray level values greater than 90% of the

maximum value. This is illustrated in the inset of figure 6. The two curves shown there are the

Figure 6. Comparison of linearity of vidicon response with cathode voltage switching (6.5 volt

curve) and without it (5.0 volt curve).

50
result of averaging approximately 10 such values at each pulse energy level, both with cathode

voltage switching (V^ = 6.5 volt curve) and without it = 5.0 volt curve). The greater

linearity and increased saturation level of the = 6.5 volt curve show that significant

improvement is possible with cathode voltage switching. The dynamic range in figure 6 is improved

by more than a factor of 2 to approximately A.

It appears from figure 6 that greater V_ cathode voltages will improve the linearity even

further. Investigation of this was not possible for the following reason. In generating the curves of figure 6, laser pulses were not used in the statistics if they caused saturation of even

one pixel on the display monitor. Thus the saturation obvious on the = 5.0 volt curve and

possibly just beginning on the = 6.5 volt curve are evidence of vidicon target saturation. The

saturation level of the display pixels was not much higher, however, because with relative pulse

energies higher than the upper level shown in figure 6, significant numbers of display pixels did

saturate. This is why the upper end of the = 6.5 volt curve is not extended further, and why

higher cathode voltages were not investigated. To do so would require raising the display

monitor pixel saturation level, which was beyond the scope of the study. The results in figure 6 were obtained with the vidicon target cooled to 0° C. Similar results (not shown) were obtained at

24° C and -30° C.

5.2 1.06 Min Laser Pulse Images and Analysis

Figure 7 shows the image of a typical pulse from the laser used in this study, including some ways of processing and analyzing the image. The results of the image inversion (figure 7b) and

profile smoothing software routines (figure 7d) are available approximately 30 seconds after firing

the laser. Thus, the system can provide real-time feedback in many laser damage experiments.

A. Pulse image. Negative video causes B. Pulse image inverted so Trapix grey

Trapix pixels to saturate at zero levels vary from zero at low energy to grey level. 83 at saturation.

51
6. Summary and Conclusions

A PAR 1252E vidicon and 1216 vidicon controller were successfully interfaced with a Trapix

5500 image processing system for the acquisition, display, and analysis of fast 1.06 nm laser

pulses. Procedures were established for minimizing geometric distortion of displayed images. It was found that each channel time has a unique channel length in pixels which must be programmed via

the vidicon controller. Several useful image analysis routines require less than 30 seconds,

thereby providing real-time feedback in some types of laser damage experiments.

The peak-to-peak variations of dark current and 1.06 ym detection-sensitivity are 5.6% and

3.75%, respectively, in the central 3.75 x 3.13 mm region of the target. These variations are

small enough and the area large enough for the system to be useful in profiling many laser beams.

Temperature-dependence measurements of dark current and detection sensitivity show that the vidicon

target has an optimum signal-to-noise ratio near 0° C.

Cathode voltage switching improves pulsed-mode linearity and raises the vidicon saturation

level. A factor of 2 improvement (to 4) in dynamic range was shown here, but the system was not

optimized because display monitor pixel saturation prevented investigation of higher cathode

voltages. It is doubtful whether an ultimate dynamic range greater than 20, measured previously

with an RCA C23250-NTD vidicon [1] will be achievable for this system in its present form.

''Research sponsored by the Air Force Office of Scientific Research/ AFSC, United States Air

Force, under Contract F49620-85-C-0013 . The United States Government is authorized to reproduce and distribute reprints for governmental purposes notwithstanding any copyright

notation hereon.

7. References

[1] Smith, W.L.; DeGroot, A.J.; Weber, M.J. Silicon Vidicon System for Measuring Laser

Intensity Profiles. Appl. Opt. 17(24): 3938; 1978.

[2] O'Connell, R.M.; Deaton, T.F.; Saito, T.T. Single- and Multiple-Shot Laser-Damage Properties of Commercial Grade PMMA. Appl. Opt. 23(5): 682; 1984.

[3] EG and G Princeton Applied Research, P.O. Box 2566, Princeton, NJ 08540.

[4] Recognition Concepts, Inc., 924 Incline Way, P.O. Box 8510, Incline Village, NV 89450.

[5] Fleming, G.R. ; Morris, J.M. ; Robinson, G.W. Picosecond Fluorescence Spectroscopy with a Streak Camera. Aust. J. Chem. 30: 2337; 1977.

[6] Redington, R.W. The Transient Response of Photoconductive Camera Tubes Employing Low-Velocity

Scanning. IRE Trans. Electron Devices Ed-4: 220; 1957.

[7] Liesegang, G.W.; Smith, P.D. Improving Vidicon Linearity in the Pulsed Illumination Mode.

Appl. Opt. 20(15): 2604; 1981.

[8] Liesegang, G.W.; Smith, P.D. Vidicon Characteristics under Continuous and Pulsed Illumination. Appl. Opt. 21(8): 1437; 1982.

[9] Cascio, H.E.; Smith, P.D. ; Liesegang, G.W. Simple Switching Circuit to Improve Vidicon

(0MA2) Linearity. Rev. Sci. Instrum. 53(7): 967; 1982.

52
- Manuscript Not Received -

DIGITAL FLUENCE PROFILING SYSTEM AND SOFTWARE

S. C. Seitel

Montana Laser Optics, Inc. Bozeman, MY 59715

and

R. L. Rypma

Big Sky Software Corp.

Bozeman, MT 59715

ABSTRACT

An advanced digital fluence profiling system based on a windowless GE TN2502 or TN2509 silicon array camera and Poynting Products 505RM frame

grabber is described. The camera array is located precisely in the

target plane or equivalent, and synchronized to the pulsed laser source with a flashlamp radiation sensor circuit. Data acquired by the frame

grabber are transferred to an HP Series 200 computer for analysis.

Analysis is performed with BEAMCODE 2.0 software which permits near

real time extraction of the peak fluence to total energy ratio in a single pulse to better than 20 percent. When corrected to camera non- uniformity and compared to large-aperture calorimetry, accuracy exceeds

10 percent. Other functions include measurement of pointing stability

(beam wander) and divergence, fit-to-gaussian, and graphics data

displays in contour and pseudo-3D formats.

Key Words: computer control; laser beam profile; laser diagnostics;

laser fluence measurement.
Manuscript received

4-10-86 SAB

SIMPLE ACCURATE INVERSION OF KNIFE-EDGE DATA

FROM RADIALLY SYMMETRIC LASER BEAMS

R. M. O'Connell and R. A. Vogel

Electrical and Computer Engineering Department

University of Missouri

Columbia, Missouri 65211

The intensity profile of a radially symmetric beam is shown to be related to the

inverse Abel transform of knife-edge-scan data. Two inversion algorithms, previously

applied to problems in plasma diagnostics, are described and applied to knife-edge data

from a confocal unstable resonator laser. The resulting profiles are shown to be in good

agreement with each other but significantly different from the profile obtained by assiim- ing the beam to be approximately Gaussian. As a beneficial consequence, the results are shown to imply a significantly higher laser damage threshold for polymethylmethacrylate

than previously reported.

Key Words: beam profiles; inverse Abel transform; knife-edge scan; laser damage threshold;

polymethylmethacrylate

;

1. Introduction

Because it is experigientally simple to implement and offers excellent spatial resolution (< 2 Mm) and dynamic range (10 ), the knife-edge method is frequently ijsed to obtain beam profile information of narrowly focused (< 100 [im) high intensity (> 1 GW/cm ) laser beams, which are used in some laser damage studies. Knife-edge data does not replicate a beam profile; it must subsequently

be inverted. The proper data- inversion method to be used depends on the functional form of the

beam. For a Gaussian beam, simple differentiation of a knife-edge scan is sufficient to produce

the required beam profile [1,2]. For this reason, non-Gaussian beams are also often approximated

as Gaussian, a simplification which, as we will show, can lead to erroneous beam profiles.

In this paper we consider the problem of knife-edge data inversion for the class of beams that are radially symmetric. This important group includes the Airy pattern, the focused annulus, and even the Gaussian beam as a special case. In section 2 we show that the knife-edge scan of a radially symmetric beam has a simple relationship to the Abel transform of the beam profile. Thus,

in principle, exact data inversion is possible. However, because the knife-edge scan consists of

discrete data, the inversion must be done numerically. In section 3, we describe two numerical

inversion algorithms, previously applied to problems in plasma diagnostics, which we applied to an appropriate set of knife edge data. We then present the resulting beam profiles and compare them

to the result obtained using the Gaussian approximation. Finally, in section 4 we discuss the

implications of these results for the single-shot bulk laser damage threshold of polymethylmethacrylate (PMMA).

2. Knife-Edge Data Inversion

2.1 Edge and Line Response Functions

In knife-edge scanning, illustrated in figure la, a straight-edge such as a razor blade is

stepped across the focal plane of the beam and the total uneclipsed energy or power is measured at each position of the knife-edge. In the case of pulsed beams, the knife-edge is maintained at each

position for several pulses. The average pulse energy at each position is then related to either

the average measured power (through the pulse repetition frequency) or the total measured energy

(through the number of pulses integrated). The result is a monotonically decreasing function such

as that illustrated in figure lb. If the knife-edge step increment is very small compared to fluctuations in the beam's spatial energy density (fluence), the measured data will closely

approximate the mathematical edge response function E(x)[3], the negative first derivative of which

is the line response function L(x)[3], illustrated in figure Ic. That is,

L(x) = - E'(x). (1)

54
The line response function would be approximated by the measured "slit-scan" that would result if

the knife-edge were replaced with a thin slit having a much narrower width than the beam's fluence

fluctuations

.

Figure 1. Relationship between the knife-edge scan(a), the edge-response function E(x)(b), and the

line-response function L(x)(c).

2.2 The Abel Transform Pair for Radially Symmetric Beams

The relationship between the line response function and the profile of a radially symmetric beam

can be obtained with the aid of figure 2. The circle of radius R represents the beam cross-section

Figure 2. Geometry used to determine the relationships between the line response

function L(x) and the profile of a radially symmetric beam F(r).

in the measurement plane at either a specific instant in time or under the assumption of time independence. Thus, it is a mapping of radially varying fluence, F(r). The line response function

L(x) is just a line integral at each position x. In terms of the geometry shown in figure 2,

(R^ - x^)

1/2

L(x) = 2 F(r)dy. (2)

55
To express dy and the limits of integration in terms of r, note that

2^2 2 X + y = r , (3)

from which

rdr

dy =

( 2 2. (r - X ) 1/2

(A)

and the lower and upper limits of integration become x and R, respectively. Thus, eq (2) becomes

L(x) = 2 F(r)rdr

< 2 2. (r - X )

1/2

(5)

which can be recognized as the Abel transform of F(r)[A]

inverse Abel transform relation [A], is

The solution of eq (5), known as the

F(r) = - i

TI

L'(x)

dx

( 2 2. (x - r ) 1/2

(6)

In eq (6), L' (x) is the first derivative of the line response function. Using eq (l), eq (6)

becomes

F(r) = i

TI

E"(x)

( 2 2. (x - r ) 1/2

dx , (7)

which is the desired relationship between the radially symmetric beam profile and the knife-edge

scan

.

2.3 Gaussian Beams: A Special Case

Gaussian beams are radially symmetric and therefore invertable from knife-edge data via eq

(7). However, because the Gaussian function is also separable in the x and y coordinate variables,

a much simpler result can be obtained by evaluating eq (2) specifically for the Gaussian case. In the plane of its waist, the fluence function of a Gaussian beam is expressible as

F(r) = Fq exp(-2r^/wQ^). (8)

where Fq is the peak fluence in the beam and Wq is the 1/e energy radius, i.e., the beam waist or spot size. Since the Gaussian beam has an infinite radial extent, eq (2) can be written as

56
oo

L(x) = F(r)dy . (9)

— oo

Substituting eq (8) into eq (9) and using eq (3) gives

oo

2 2 L(x) = F„exp(-2x /w. ) exp(-2y /w ) dy. (10)

—00

The integral in eq (10) is the constant Fp|W|-^iT 1 2. Therefore

L(x) = FqWqTt

1/2

/2 exp(-2x^/w^). (11)

which is just a scaled replica of the Gaussian function in eq (8). Thus, the line response

function L(x) gives the profile of a Gaussian beam directly. Recalling eq (1), it is then seen

that the edge response function E(x), i.e., the knife-edge scan, need only be differentiated to produce the desired profile. Because this procedure is so simple, non-Gaussian beams are often

approximated as Gaussian and inverted accordingly.

3. Application to Experimental Knife-Edge Data

3.1 Inverse Abel Transform Algorithms

Returning to the more general case of radially symmetric but non-Gaussian beams, the correct

inversion rule is given by eq (7). Since the edge response function E(x) in the equation is approximated by the discrete knife-edge scan, the equation must be evaluated numerically. Several methods have been developed to do this, and complete summaries can be found in references [5] and [6].

Here, we briefly describe two of these methods, the popular data inversion algorithms of Nestor and

Olsen [7] and Cremers and Birkebak [8], both of which we applied to a measured knife-edge scan. The

measured data and the results of the inversion process are presented and discussed subsequently.

In the Nestor/Olsen algorithm, a transformation of variables is made and the x axis is divided

into small segments, in each of which E"(x) is assumed to be constant. This reduces eq (7) to a series of easily solved and summed integrals. One problem with the algorithm is that the second

derivative function E"(x) is formed by taking second differences of the discrete knife-edge data

values. Nestor and Olsen showed that this step amplifies errors. Thus, for data with appreciable

scatter, the resulting profile can be seriously flawed.

To make the data less sensitive to fluctuations, the Cremers/Birkebak algorithm divides slit- scan data (rather than knife-edge data) into a small number of zones and least-squares-fits a fourth order polynomial to the data in each zone. This provides an expression for L(x) that can be

differentiated analytically to give L' (x) in eq (6), which can then be integrated exactly. The

weakness of this algorithm is that the optimum size and number of zones for a given set of data

need to be found empirically.

Despite their shortcomings, both of these algorithms were found to give good results when

applied to test data produced by deliberately introducing round-off error to line and edge response

function values generated with a polynomial profile function [7,8].

3.2 Experimental Data

Because they can give good results with artificial data, we applied both of the above described algorithms to previously measured knife-edge data from a narrowly focused 1.06 \xm wavelength,

Q-switched Nd:YAG laser [9]. The resonator in the Nd:YAG laser is of the confocal unstable variety.

57
Thus, its ideal output is a uniformly illuminated annulus [10] which focuses to a radially symmetric pattern similar to the Airy function [11]. The main difference between the two functions is

that the secondary lobes of the focused annulus may contain a much larger fraction of the total en- ergy than those of the Airy function. For example, whereas the main lobe of the Airy function con- tains approximately 84% of the total energy, the main lobe of a typical focused annulus may contain

less than half of the total energy. Thus, the secondary lobes of the focused annulus should not

generally be neglected, which is done when such a beam is assvimed to be approximately Gaussian.

The knife-edge data, shown in figure 3, were obtained as described in section 2.1 with the

laser free-running at 10 pps. The knife-edge, a 1/16-inch thick steel plate, was stepped across

the beam in 2.54 \iia increments and maintained at each position for at least 100 pulses of the beam. An S-1 photodiode recorded relative pulse energies and showed that the pulse-to-pulse variation was

<5% rms. Note that because the two data inversion algorithms assume radial symmetry, they can only

be applied to half-beam data, i.e., from 50% occlusion (at the r=0 point) to 100% occlusion (at the r=R point). Therefore figure 3 shows only that portion of the full knife-edge scan needed by the

two algorithms. Furthermore, since the Cremers/Birkebak algorithm requires line response or slit-scan data, the knife-edge data of figure 3 had to be differentiated first, according to eq

(1). Since the impetus behind the Cremers/Birkebak algorithm was to eliminate differentiation by

finite differences, we performed the operation by least-squares-fitting a quadratic equation to the

five point segment centered on each data point [9] and evaluating the derivative at each point

analytically. The right half of the resulting curve was then divided into five zones and used to approximate the slit-scan required by the Cremers/Birkebak algorithm. No attempt was made to optimize the number of zones.

(0

O

>

h-D

Q.

I-ZDO

DCO

I-o

hiLJQ

KNIFE-EDGE POSITION X CMICRONS!)

Figure 3. Measured knife-edge data from a 1.06 iim, Q-switched NdtYAG laser.

3.3 Results

Figures 4a and 4b show the beam profiles that result from applying the Nestor/Olsen and

Cremers/Birkebak algorithms, respectively, to the knife-edge data of figure 3. Except for the

thr^e nulls present in figure 4a but missing in figure 4b, the two profiles are very similar; the

1/e radii of their main lobes are 12 pm and 14 pm, respectively; and they both have significant

and similar energies outside the main lobe, extending to the same 46 pm radius. The lack of detail

outside the main lobe in figure 4b is probably due to excessive data smoothing introduced by the

58
quadratic f it/dif f erentiation, which we used to convert the knife-edge data to slit-scan data, and

the zonal polynomial fit/differentiation done in the Cremers/Birkebak algorithm itself.

RADIAL DISTANCE CMICRONS^

Figure 4. Beam profiles obtained by inverting the knife-edge data of figure 3 with the Nestor/Olsen (a) and Cremers/Birkebak (b) algorithms.

We also inverted the knife-edge scan of figure 3 using the Gaussian approximation. Since in

that case the knife-edge data needs only to be differentiated to produce the beam profile, the

result was already available in the^form of the quadratic f it/dif ferentiated data used with the

Cremers/Birkebak algorithm. The 1/e radius of the resulting profile (not shown here) is approximately 23 |jm, roughly twice as large as those in figure 4. Also, the profile has significantly

secondary lobe energy extending beyond a 50 nm radius. Thus, the beam profiles produced using the

59
two algorithms for implementing the correct inversion procedure are both similar to each other and

quite different from the profile obtained by assuming the beam to be approximately Gaussian.

Although it would be desirable to compare the profiles in figure 4 to an expected, calculated

profile, an accurate calculation would be very difficult to perform. First of all, film burns of

the laser output showed that it was not an ideal uniformly illuminated annulus, but only

approximately so. Secondly, in taking the knife-edge data, it was difficult to locate the focal

plane precisely. Thus, the standard Fraunhofer (focal plane) diffraction pattern of a uniformly

illuminated annulus can not be relied upon for accurate comparison. A more general and difficult

diffraction calculation would be needed.

4. Implications for the Laser Damage Threshold of PMMA

Pulsed laser damage thresholds are usually obtained from plots of measured damage probability

versus laser pulse energy. The damage threshold is then defined as the peak fluence associated

with the pulse energy causing a certain probability of damage; the onset (or 0%) and 50% levels are frequently used [12,13]. Accurate beam profiles are needed in this because they provide a calibration constant for relating peak fluences to pulse energies [9]. The calibration constant is found

by rotating the profile F(r) about its relative fluence axis, integrating with respect to area to obtain a total relative energy E^p» and then normalizing to the peak relative fluence Fpp in the

profile. The resulting calibration constant A has the units of area, i.e..

2it F(r)rdr

A = (12) F F PP PP

If the laser is reasonably stable from pulse to pulse (see Sec. 3.2), the measured beam profile

will be a scaled replica of any pulse from the laser. The peak fluence Fp in each pulse is then

just the ratio of the measured energy E^j, in the pulse to A, i.e..

(13)

If in eq (11) is the damage threshold pulse energy, Fp is the required threshold peak fluence.

Since the calibration constant A is determined from a beam profile, it is clear from eq (13)

that different beam profiles obtained from the same set of knife-edge data will result in different

values of the peak fluence damage threshold being associated with a single threshold pulse energy.

Alternate threshold values Fp^ and Fp^^ simply scale with their respective beam profile calibration

constants A^^ and A^ via the relation

F = F —

p2 %1 ^

• ^1

(14)

v;hich follows from eq (13). In the study in which the knife-edge scan of figure 3 was measured [9],

the beam was assumed to be approximately Gaussian and the data was inverted accordingly. From the

resulting beam profile and the threshold pulse energy, the single-shot bulk damage threshold of

polymethylmethacrylate (PMMA) was reported as 41 j/cm . Using eq (14) and the calibration constants

obtained as described above from figures 4a and 4b, the laser damage threshold of PMMA becomes 187

j/cm and 136 j/cm , respectively, according to the Nestor/Olsen and Cremers/Birkebak algorithms.

60
Manusaript received

2-14-86 SAR

Investigation of Optical Damage via Resonance Ionization Mass Spectrometry

R. C. Estler* and N. S. Nogar

Chemistry Division

Los Alamos National Laboratory

Los Alamos, New Mexico 87545

Resonance ionization mass spectrometry (RIMS) has been used as a diagnostic for

interrogating optical damage events. RIMS involves multistep laser photoionization,

generally through real intermediate states, followed by mass separation and detection.

It is a sensitive and selective means of detection, and has the potential for detection

of ultratrace inclusions in substrates and/or coatings.

In initial experiments, damage on uncoated CaF2 substrates was initiated by pulses

of 1.06-pm light from a Q-switched Nd"'"^:YAG laser. Interrogation of the spalled plume

revealed CaF radicals only when damage events occurred. No Ca atoms were observed, and

no attempt was made to detect F atoms or CaF2. In subsequent experiments conducted at laser intensities below the damage threshold, we observed the presence of surface adsorbents and a correlation between the presence of the adsorbents and the occurrence of

optical damage.

Xey Words: calcium fluoride; laser-induced damage; optical damage; photoionization; resonance ionization mass spectrometry (RIMS)

.

1. Introduction

Laser-induced desorption and laser-based detection of ablated or sputtered species are both

becoming widely used techniques in a variety of studies, including surface chemistry, sputtering,

and optical damage. Much work has concerned laser-desorption of adsorbed species from substrate

surfaces as detected by a variety of mass spectroscopic techniques [1-10]. Other experiments have

concentrated on probing the basic processes related to laser-ablation of the substrate material

itself [11-16]. Laser detection of desorbed particles via fluorescence or ionization is also becoming a widely used technique in more traditional particle-induced sputtering experiments [17-20].

Few investigations have, however, coupled the advantages found in laser-induced desorption to those of the sensitive and specific laser-based schemes now available for the detection of the

desorbed material [21-24]. We report here, the use of resonance ionization mass spectrometry

(RIMS) as a diagnostic for interrogating laser-induced optical damage events. RIMS is a sensitive

and selective means of detection, and has the potential for detection of ultratrace inclusions in

substrates and/or coatings as well as the detection of active sites on the surface where subsequent

damage is likely to occur.

2. Experimental

The damage/desorption experiments described below took place in the source region of a timeof-flight mass spectrometer described in detail previously [25]. Figure la depicts a schematic of

this region, while figure lb describes a typical timing sequence used. Briefly, the Q-switch

synch-out from the Nd^:YAG damage/desorption laser (Quanta Ray/Spectra Physics Model OCR lA) was

used to master the timing sequence. The laser output, initially frequency doubled to provide a visible beam, was focused onto the substrate surface with the aid of beam-steering prisms and a 250-mm focal length lens. Once alignment was completed, the frequency doubler was detuned, the

residual 532-nm light removed with a dichroic mirror, leaving only (>99%) the fundamental 1.06-pm

beam. The 10-ns pulses from this laser typically arrived at the substrate ~100 ns after the Qswitch synch-out trigger. Beam size at the substrate was approximately 0.47 mm^ , and pulse energies ^25 mJ were used producing fluences ^5.3 J cm""^. These values must be regarded as approximations, since the "doughnut" shaped output of the 1.06-pm laser produces a nonuniform intensity

distribution. No attempt was made in this preliminary study to modify the spatial distribution of

this laser.

^Permanent address: Department of Chemistry, Fort Lewis College, Durango, Colorado 81301

61
c

SUBSTRATE SAMPLE

Q-SWITCH

DYE LASER PROBE

100 ns I

-1 I 06^m PULSE ARRIVAL

-VARIABLE

.

(fDYE LASER

PULSE ARRIVAL

1.06 urn ~ 8 fiS n BOXCAR

GATE

a b Figure 1. Schematic of the experimental apparatus. The 1.06-pm damage pulse is typically focused

to a beam diameter of 0.7 mm at the substrate surface. The delay time to the dye laser probe

pulse was ~30-50 ps.

The CaF2 substrates (Janos Optical) were polished flats, 1.9 cm in diameter and 3 mm thick.

Each was cemented to a 15-cm stainless steel rod for mounting within the mass spectrometer source

region. The substrates could be translated and rotated while in the source region via a high vacuum feedthrough (Varian Model 1371). The rotation axis of the substrate was parallel but not coincident with the 1.06-pm laser beam axis. A displacement of 5 mm between these two axes allowed

the interrogation of different substrate sites via substrate rotation. Rotation rates varied from

1 revolution/15 minutes to 1 revolution/60 minutes. The long substrate mounting rods were required

to prevent metal ion formation near the source region from the focused 1.06-pm laser striking the

surfaces of the vacuum chamber.

A gas inlet system facilitated gas-phase RIMS studies and provided a means of introducing

adsorbents onto the surface of the substrate. A stainless steel nozzle opposite the surface being

interrogated directed the adsorbents to this surface. The inlet system was also constructed in

such a manner as to allow adsorbent purification via distillation prior to deposition. All adsorbents were triply distilled prior to use.

Pulses from an excimer-pumped dye laser (Lambda Physik, Models 101/2002) were used to effect

ionization. Ions were generated from ground electronic states of the detected species in a variety

of multiphoton transition sequences. The excimer laser typically produced 100-mJ, 15-ns pulses and

was triggered after a variable delay. Dye laser pulses (2 mJ, 10 ns) were spatially filtered and

loosely focused (0.07 cm diam) through the ionization region. The laser beam propagated parallel

to the substrate surface, at a distance of 2.5 cm. The extraction field was approximately

110 V/cm, perpendicular to the paths of both laser beams. This was followed by a drift tube at -1000 V; the flight path was 0.4 m. A pair of deflection plates between the extractor and flight

tube maximized the transmission of ions to the detector and minimized any transmission variation

due to ion velocity components perpendicular to the flight tube. Detection electronics consisted

of a channel electron multiplier, a preamplifier, an amplifier, and a boxcar averager whose gate

delay was set to the flight time of the ion of interest. The boxcar gate width was adjusted so as to encompass only a few mass units about that of the detected specie. An oscilloscope was used to observe simultaneously the arrival of the damage/desorption laser pulses, the ion signal, and the

boxcar gate.

3. Results and Discussion

In initial experiments, damage on uncoated, polished substrates was initiated by pulses of the

1.06-pm laser. Dielectric breakdown was observed at thresholds fundamentally consistent with previous measurements on CaF2 [26]. However, the breakdown threshold was surface site dependent, possiblly due to defects or surface contamination. Neither Ca atoms nor CaF radicals could be detected prior to breakdown. Upon breakdown, however, CaF radicals w^re easily detected via a two- photon transition to the F^n excited state (37548 cm"^) from the X^I ground state. Absorption of

a third photon of the same color results in ionization (ionization potential ~6.0 eV) [27]. Figure

2 shows the resulting excitation spectrum for this 2+1 ionization of CaF through the multibranched Av = 0 transition^ of the F state at flight time delay of 50 ps. In addition to the major

features of the F^n^-^- X^i ^2 + 1 ionization process, smaller features due to a 1 + 2 ionization

process via the B^i <- X^i Av = 0 transitions are also observed. We believe that this is the

first reported multiphoton photoionization spectrum of CaF.

62
CO

CO O

F^n - x2 2

+

(1.1) (2 + 1)

534 533 532 531 530 529

WAVELENGTH (nm)

528

Figure 2. Optical MPI spectrum of desorbed CaF, obtained at a 50-|js delay after the desorption

pulse. Bandhead positions are indicated for the Av = 0 sequences for the electronic transitions

observed.

Time-of-fl ight spectra of the CaF radicals could not be recorded due to a large nonresonant

signal from ions of many different masses at shorter flight times. The source of this contaminating signal is the intense UV-light pulse generated at the substrate surface during breakdown.

The source of the interference was confirmed by placing electron/ion deflectors in the flight path

of the spalled material. All attempts to reduce this signal by particle deflection failed, although ions are certainly generated in such a breakdown event. The breakdown-generated light pulse

was easily visible to the naked eye and sometimes generated overloads in the electronics due to the

formation of many ions as it struck the optic elements of the ion spectrometer. Such lightgenerated signals have been observed previously [2]. Direct inversion of spectral intensities to state populations is not currently possible (as in much laser-induced fluorescence work) due to the

lack of knowledge of the two-photon line strengths of the transitions involved [28]. However, it

is clear that several low-lying vibrational states of the ground electronic state have been populated in the damage event. No Ca atoms could be detected after the onset of breakdown, even though

detection sensitivity for desorbed alkaline earth atoms is high [29]. This suggests that the

mechanism for breakdown at 1.06 pm is thermal, or hydrodynamic in nature, since previous observations [20,30] suggest that an electronic or photophysical interaction leads to the production of

free metal atoms. No attempt was made to observe either F atoms or CaF2.

In an attempt to correlate surface contamination with low breakdown threshold, a series of

adsorption/desorption experiments were performed. In previous work [5,9], water aggregation ap- peared correlated to low damage thresholds at a particular site. However, water is not a good RIMS

candidate due to its high ionization potential and sparse electronic structure. Only recently has

the multiphoton ionization spectrum of water been obtained at the relatively high pressure of

100 torr [31]. In the experiments reported below, nitrogen dioxide (NO2) was chosen for the adsorbent for several reasons. First, at wavelengths at and below 380 nm, NO2 dissociates efficiently

to NO + 0, the NO being easily detected via multiphoton ionization due to its low ionization potential [32]. Second, if the NO2 reacts with surface water, several of the possible reaction products, like NO2, have dissociating UV absorptions at the NO resonance wavelengths. Lastly, NO2

,

like water, is polar and seeks the same high potential sites where damage is likely to occur [33].

In addition, at the wavelength chosen for this preliminary study, 365 nm (corresponding to the

large Franck-Condon factor transition C^n (v = 1) X (v = 0) in NO, i.e., the 6 (1,0) transition, high efficiency laser dyes are available. This technique of dissociation and fragment probe

within a single laser pulse has been used previously in several dynamic studies [31,34].

63
Following the substrate mounting, samples were placed within the mass spectrometer and evacu- ated to 10"^ torr for several hours. The substrates were then exposed to NO2 at low pressure (10~^

torr) for sufficient time to form several monolayers. Substrates were then again returned to 10*"^ torr. The RIMS sensitivity for detecting NO from NO2 is very high, requiring another several-hour

period of pumping to eliminate background nitric oxide RIMS signals prior to a desorption experiment.

Each area on the substrate was then sequentially exposed to low intensity and high intensity

laser pulses. At 1.06-pm intensities below the damage threshold (typically 0.4-0.6 J cm"^) the ion

signal at the bandhead wavelength of the NO 6 (1,0) was recorded as a function of substrate rota- tion angle. Such records are shown for two samples in figure 3b. On a second pass, at intensities

near or above the damage threshold (approximately 5 J cm"^), a signal due to breakdown was recorded

as a function of substrate rotation angle. These spectra are shown for the same two samples in

figure 3a. Only a single pass below the damage threshold was made, since laser annealing was observed for sites receiving multiple exposures below these thresholds. This annealing effect and

the low signal-to-noise ratio in these experiments prevented time-of-f 1 ight spectra from being

recorded for the adsorbent. The NO signals of figure 3 were recorded at a 50-ps flight time

45 90 135 180 225

CaF2 ROTATION (degrees)

Figure 3. Correlation between breakdown and adsorbent concentration for two different damage intensities (10^ W/cm^ ~ 10 J/cm^). (a) Breakdown (damage) signal as a function of substrate

rotation following the nondestructive desorption of adsorbent (Pass 2). (b) NO desorption

signal as a function of substrate rotation (Pass 1).

64
between desorption and ionization. The correlations observed between damage and desorbent con- centration for the two samples are representative of the correlations observed for all samples

interrogated. Not all sites showing higher concentrations of absorbent produce breakdown; however,

most all breakdown sites exhibited higher adsorbent concentrations .

The efficiency of detecting the active sites by adsorbent aggregation could be improved over that observed above by using a desorbing wavelength in resonance with the adsorbent [5,9]. This is not the case for the experiments described above. From our static gas phase NO2 experiments, we estimate (incorporating observed signal-to-noise, detection volume, etc.) a lower detection limit

of ~6 X 10^ molecules of NO2. Using this number, a rough calculation of our desorption efficiency

yields ~10%, assuming a temporal and spatial overlap of 10"^, a partition function of 10~^ , and a dissociation and ionization efficiency of 10"^. Although this desorption efficiency certainly

represents an upper bound for this nonresonant process, it could be dramatically improved via vibrational resonant frequencies. RIMS is inherently more sensitive for such studies than the traditional residual gas analyzers due to its order of magnitude better ionization efficiency over electron bombardment. Further improvement in the present apparatus can be made by providing a

1 ine-of-sight path between substrate surface and detectors. Such improvements will be incorporated

into future instrumentation.

Since the neutral/ion production ratio ranges from 10 ^ to 10^^ in laser desorption of adsorbents and substrates [11,16,21,22], efficient detection of the neutral major product channel is imperative to our understanding of such processes. RIMS provides such a sensitive, specific detection scheme. It shows potential for monitoring ultratrace inclusions in substrates and/or coatings, as well as for nondestructi vely locating active surface sites where subsequent damage is probable.

Note added in proof:

Since recording this data, beam-filling optics have been installed into the Nd"''^:YAG laser,

allowing for smaller spot sizes with smoother spatial profiles. With these optics installed, Ca atoms have been detected after breakdown in addition to the CaF molecules. These results, as well

as those observed at 266 nm and 355 nm desorbing wavelengths will be discussed elsewhere.

4. References

[1] Levine, L. P.; Ready, J. F. ; Bernal G. , E. J. Appl. Phys. 38, 331 (1967).

[2] Levine, L. P.; Ready, J. F. ; Bernal G. , E. IEEE J. Quant. Electron. QE-4 , 18 (1968).

[3] Antonov, V. S. ; Letokhov, V. S. ; Shibanov, A. N. Appl. Phys. 25, 71 (1981).

[4] Chuang, T. J. J. Chem. Phys. 76, 3828 (1982).

[5] Allen, S. D. , Porteus, J. 0.; Faith, W. N. Appl. Phys. Lett. 41, 417 (1982).

[6] Egorov, S. E. ; Letokhov, V. S. ; Shibanov, A. N. Chem. Phys. 85, 349 (1984).

[7] Hussla, I.; Chuang, T. J. Mat. Res. Soc. Symp. Proc. 29, 341 (1984).

[8] (a) Burgess, Jr., D. R. ; Hussla, I.; Stair, P. C. ; Viswanathan, R. ; Weitz, E. J. Chem.

Phys. 79, 5200 (1983); (b) , Rev. Sci. \nstrum. 55, 1771 (1984).

[9] Allen, S. D. ; Porteus, J. 0.; Faith, W. N. ; Franck, J. B. Appl. Phys. Lett. 45, 997 (1984).

[10] Sherman, M. G. ; Kingsley, J. R. ; Dahlgren, D. A.; Hemminger, J. C. ; Mclver, Jr., R. T. Surface Sci. 148, L25 (1985).

[11] Olstad, R. A.; Olander, D. R. J. Appl. Phys. 46, 1499 (1975).

[12] For a review of much work see Smalley, R. E. Laser Chem. 2, 167 (1983).

[13] Davis, G. M. ; Gower, M. C. ; Fotakis, C. ; Efthimiopoulos, T. ; Argyrakis, P. Appl. Phys. A36,

27 (1985).

[14] Kelly, R. ; Cuomo, J. T. ; Leary, P. A.; Rothenberg, J. E. ; Braren, B. E. ; Aliotta, C. F. Nucl. Instr. and Meth. B9, 329 (1985).

65
(a) Rothenberg, J. E. ; Kelly, R. Nucl. Instr. and Meth. Bl, 291 (1984); (b) Nucl. Instr. and Meth. B7/8 , 755 (1985).

Kang, H. ; Beauchamp, J. L. J. Phys. Chem. 89, 3364 (1985).

Pellin, M. J.; Wright, R. B. ; Gruen, D. M. J. Chem. Phys. 74, 6448 (1981).

Kimock, F. M. ; Baxter, J. P.; Winograd, N. Instr. and Meth. in Phys. Res. 218, 287 (1983).

Kimock, F. M. ; Baxter, J. P.; Pappas, D. L. ; Kobrin, P. H. ; Winograd, N. Anal. Chem. 56,

2782 (1984).

Tolk, N. J. Luminescence 31/32 , 913 (1984).

Beekman, D. W. ; Callcott, T. A.; Kramer, S. D. ; Arakawa, E. T. ; Hurst, G. S. ; Nussbaum, E. Int. J. Mass. Spectrom. Ion Phys. 34, 89 (1980).

Selter, K. P.; Kunze, H. -J. Physica Scripta. 25, 929 (1982).

Leismann, P.; Henc-Bartol ic, V.; Rebhan, U. ; Kunze, H. -J. Physica Scripta. 30, 186 (1984).

Nogar, N. S. ; Estler, R. C; Miller, C. M. Anal. Chem. 57, 2441 (1985).

Downey, S. W. ; Nogar, N. S. ; Miller, C. M. Int. J. Mass Spec. Ion Proc. 61, 337 (1984).

Smith, W. L. ; Bechtel , J. H. ; and Bloembergen, N. Phys. Rev. B12, 706 (1975).

Hildenbrand, D. L. ; Murad, E. J. Chem. Phys. 43, 1400 (1965).

Zare, R. N. ; Dagdigian, P. J. Science 185, 739 (1974).

Estler, R. C. ; Nogar, N. S. to be published.

Stof fel , N. G. ; Riedel, R. ; Colavits, E. ; Margaritondo, G. ; Haglund, R. F. ; Taglaner, E.

;

Tolk, N. H. Phys. Rev. B32, 6805 (1985).

Ashfold, M. N. R. ; Bayley, J. M. ; Dixon, R. N. Chem. Phys. 84, 35 (1984).

Radhakrishnan, G. ; Ng, D. ; Estler, R. C. Chem. Phys. Lett. 84, 260 (1981).

Radhakrishnan, G. ; Estler, R. C. Chem. Phys. Lett. 100, 403 (1983).

Bloembergen, N. Appl. Opt. 12, 661 (1973).

66
5. Siunmary and Conclusions

We have shown that knife-edge data from a radially symmetric but non-Gaussian beam is properly

inverted by an inverse Abel transform of the derivative of the knife-edge data. The frequently

used method of simple differentiation is strictly correct only with Gaussian beams.

The Nestor/Olsen and Cremers/Birkebak algorithms for implementing the correct data inversion

procedure gave similar and reasonable beam profiles when applied to the measured knife-edge scan of

a focused annulus beam. Thus, despite some shortcomings, both algorithms appear to be useful for

inverting knife-edge data from those radially symmetric beams with pulse-to-pulse stability similar

to that of the laser used to generate the data of figure 3. For data with greater scatter, certain

newer algorithms [5] may give better results. In contrast with the beam profiles produced by the

Nestor/Olsen and Cremers/Birkebak algorithms, the one produced by the quadratic fit/differentiation

procedure was significantly different, suggesting that the focused output of lasers based on confocal unstable resonators should not be approximated as Gaussian.

Values for the 1.06 ^jm single-shot bulk laser damage threshold of PMMA implied by the beam

profiles produced by the Nestor/Ols^n, Cremers/Birkebak, an^ quadratic fit/differentiation

algorithms are, r^pectively, 187 j/cm , 136 j/cm , and 41 j/cm . The average of the first two values is 161 j/cm . This fact and the apparent improvement in accuracy of the first two profiles

over the third lead to the conclusion that the laser damage threshold of PMMA is approximately four

times as large as previously reported [9].

6. References

[I] Arnaud, J. A.; Hubbard, W.M. ; Mandeville, G.D. ; de la Claviere, B. ; Franke, E.A. ; Franke,

J.M. Technique for fast measurement of Gaussian laser beam parameters. Appl. Opt. 10(12):

2775; 1971.

[2] Mauck, M. Knife-edge profiling of Q-switched Nd:YAG laser beam and waist. Appl. Opt. 18(5):

599; 1979.

[3] Gaskill, J.D. Linear systems, Fourier transform, and optics. New York: Wiley; 1978.

[4] Bracewell, R.N. The Fourier transform and its applications. New York: McGraw-Hill; 1965.

[5] Hansen, E.W.; Law, P. Recursive methods for computing the Abel transform and its inverse.

J. Opt. Soc. Am. A. 2(4): 510; 1985.

[6] Minerbo, G.N.; Levy, M.E. Inversion of Abel's integral equation by means of orthogonal

polynomials. SIAM J. Numer. Anal. 6(4): 598; 1969.

[7] Nestor, O.H.; Olsen, H.N. Numerical methods for reducing line and surface probe data. SIAM

Rev. 2(3): 200; 1960.

[8] Cremers, C.J; Birkebak, R.C. Application of the Abel integral equation to spectrographic

data. Appl. Opt. 5(6): 1057; 1966.

[9] O'Connell, R.M. ; Deaton, T.F.; Saito, T.T. Single- and multiple-shot laser damage

properties of commercial grade PMMA. Appl. Opt. 23(5): 682; 1984.

[10] Krupke, W.F.; Sooy, W.R. Properties of an unstable confocal resonator C0„ laser system. IEEE J. Quantum Electron. 5(12): 575; 1969.

[II] Born, M.; Wolf, E. Principles of optics. New York: MacMillan; 1964.

[12] Foltyn, S.R. Spotsize effects in laser damage testing. Nat. Bur. Stand. (US) Spec. Publ.

619; 1982. p. 368.

[13] Porteus, J.O.; Seitel, S.C. Absolute onset of optical surface damage using defect ensembles.

Appl. Opt. 23(21): 3796; 1984.

67
Manusaript received

9-2^86 SAR

Laser Mirror Profilometry by High-0 Interferonetry

J.R. Rotge'

Frank J. Seiler research Laboratory

USAF Academy

Colorado Springs, Colorado 80840-6528

A novel interferometric technique is described which exploits the relationship

between an optical resonator's length and resonant frequency. This concept has

potential for measuring length variations of an order of « 1 A . This paper

briefly describes the concept, the experiment performed to demonstrate the concept,

and the experimental, results. Results obtained indicate a repeatability in the

measured data to ^5 K rms.

Key Words: heterodyne interferometer; interferometry ; laser mirror profilometry; mirror

figure; profilometry; surface figure.

1. INTRODUCTION

Traditional optical interferometry involves splitting a beam of light and then recombining

the beam after the tv/o partial beams have traversed different optical paths. The length of one

path acts as a reference against which the other (signal) path is compared. This comparison is

usually done by means of measuring the relative phase difference (i.e., optical fringe shift)

between the two paths. There are at least two major characteristics which may be associated with our idea of "traditional" interferometry: (1) a single light source (arc lamp or laser) is used

to insure proper phase coherence and (2) the concept is based upon the precise measurements of

relative phase. In practice this is done by measuring relative changes in intensity as measured

by some photosensitive medium (film, photodiode, photocathode, etc.). This concept works well

for most applications, but when very small displacements \ ) are to be measured the

experimentalist must be very careful to account for errors and/or ambiguities which may arise.

These may be due to such things as low fringe contrast, time variations of fringe contrast,

detector/film non-linear response and finally the limit of detectabil ity (i.e., how precisely can fringe positions/spacings be determined?).

2. BACKGROUND

The concept discussed in this paper is based upon the relationship betv^/een the output

frequency of a laser and its effective cavity length.^ This relationship results from the

boundary conditions applying to a resonant cavity, namely an integral number (H) of v/avelengths

ix) must equal twice the cavity length (i).

NA = 2 £ (1)

In a HeNe laser the (Doppler broadened) gain curve has a width of /^1500 Mhz. From equation one

we derive a longitudinal inter-mode spacing (A)

That is, by holding Z constant and varying N (M—»M + 1), the laser wavelength (frequency)

changes to maintain the boundary conditions of equation one.

Conversely, if we consider a given longitudinal mode (N) and let the cavity length vary

slightly (if the variation in £ is excessive the longitudinal mode corresponding to N will drift

from under the gain curve, i.e., the laser "mode hops" as discussed above) the output frequency

variation (Sf) goes according to the relationship

Sf = f ^ . (3)

The potential sensitivity of this concept to small variations in cavity length becomes

obvious when we plug typical numbers into equation three. Assuming a cavity length of one meter,

nominal frequency of 5 x 10^^ hz, and a displacement (6£) of one Angstrom (lO-^^ m), the

resulting shift in the laser's output frequency is about five kilohertz.

To detect this frequency shift a reference (fixed) frequency source can be used. By

employing another HeMe laser oscillator as this reference we can be assured that the reference

frequency (f^,) will lie within a few hundred megahertz of the signal laser frequency (f). This

68
difference frequency (fo-f) can be electronically nonitored by interfering the beams from the tv/o lasers on a high-speed photodetector (Figure 1) and feeding the detector output into an electronic spectrum analyzer.

reference laser

sl^al laser detector —=v- b.s.

61

fr.-f

spectrum

analyser

Figure 1. Heterodyne interferometry

A recording of the spectrum analyzer output as a function of time will, after proper scaling

according to equation three, produce a time history of the variations in effective signal laser

cavity length.

3. EXPERIMENT

The surface profilometry experiment is relatively simple in concept if not in practice. The

mirror under test serves as the flat mirror of a folded spherical (hal f -symmetric) Fabry-Perot

resonator cavity. This cavity acts as the stable resonator for a heliun-neon (.633 ym) laser.

The flat mirror in question is mounted on an air bearing and is rotated in a plane perpendicular

to the laser optical axis (Figure 2). As the bearing rotates the laser spot traverses a circle

on the surface of the test mirror. The radius of this circle is adjustable by translating the

mirror-bearing assembly in a direction transverse to the optic axis. The output frequency from

this laser is compared to that of a reference laser and the difference frequency is monitored as

a function of bearing rotation phase angle. As the mirror rotates, deviations of its surface

from a plane cause cavity length excursions in the "signal" laser. These length changes are manifest as changes in the inter-laser beat freauency. By taking data over many revolutions any

random frequency excursions can be averaged out, leaving a relief map of the mirror surface at that radius. This surface nap is completed by compiling data at several discrete values of

radius.

air bearing detector

spectrum

analyser

frequency

-voltage

computei'

Figure 2. Differential cavity profilometer

69
4. APPARATUS

The first version of the apparatus consisted of the signal laser as described above and a connercial (Spectra-Physics node! 133) HeMe laser as the reference frequency source. The noise

levels (random excursions in the inter-laser beat frequency) were excessive with this

configuration and it was replaced with a differential cavity arrangement (Figure 2). The

experiment v/as mounted atop a granite slab which was supported by inflated inner-tubes to provide

some degree of vibration isolation.

In this configuration the reference laser is an open cavity as is the signal laser. The

advantage is that by having the cavities in a common mounting arrangement, common mode vibrations

will affect both lasers similarly and thereby have a diminished effect on the inter-laser beat

signal. This is especially important for the air-bearing mounted mirror since significant levels

of vibration originate in the rotating bearing assembly. The beat signal developed by combining

the beams from the reference and signal lasers on a high-speed (bandwidth /u 1 Hhz) photodiode is

displayed on the CRT of a spectrum analyzer and stored in digital fom on an Apple lie computer.

Details of the apparatus, techniques and circuitry developed to carry out these steps are described elsewhere.^

Figure 3 shov/s a mirror surface "profile" constructed by taking data at five radial distances

from the center of a mirror under test. Run-to-run repeatability in the data was shown to be

approximately 5 A rns, for averages of at least 50 samples per data point. "Profile" is set in

quotes since the data does not support the registering of one radial data set to the next. The

scans are azimuthal at constant radius so the data is insensitive to spherical aberrations. In this instance the data is arbitrarily registered at a fixed azimuthal position. This deficiency

could be removed by replacing the micrometer stage with a (linear) air bearing to provide radial

data scans.

5. RESULTS

a b

c d

Figure 3. Surface contour generated from data taken at five radii and

displayed at different aspect angles. The total surface relief is

303 Angstroms, (b and d are rotated 90°.)

70
6. CONCLUSIONS

We have demonstrated the sensitivity of the high-finesse (high-0) interferonetric techniaue,

having made surface contour measurements repeatable to 5 A ms with a relatively crude

experimental apparatus. Further improvenents to the experiment should provide a non-contact

technique to measure surface height variations to levels approaching 0.01 A , with lateral

resolution approaching 10 ]im. The requirement for highly reflective surfaces can be reduced

somewhat by passive cavity techniques (i.e., the test surface would constitute an element of a passive cavity pumped by a laser locked to the passive cavity resonance).

7. ACKNOWLEDGEMENT

I would like to thank Dr. Jim Small for his valuable insights and suggestions during the course of this work. This experiment was accomplished as a part of PhD dissertation research at the University of New Mexico.

8. REFERENCES

1. First proposed by Dr. J. Small in an unpublished white paper at the University of New

Mexico. J. Small, "High-Q Interferometry."

2. J.R. Rotge', "Laser Heterodyne Interferometry for the Measurement of Snail (Angstrom Level)

Displacements," PhD Dissertation, University of New Mexico, Department of Physics and Astronomy.

Publication pending.

71
Manuscript Received

12-19-S6

Design and Construction of a Single Polarizer Infrared

Ell ipsometer/Ref lectometer for Characterization

of Highly Curved Surfaces

Jack B. Stubbs, John S. Loomis, and Thomas A. Leonard*

University of Dayton Research Institute

KL-464

Dayton, OH 45469

(513)229-3724

ABSTRACT

The University of Dayton has designed and built an Infrared El 1 ipsometer/ Ref lectometer

(E/R) to characterize curved surfaces in support of the ALPHA Laser Program. The instrument is

tunable in wavelength and angle of incidence, is remotely controlled by a dedicated microcomputer, is positioned and scanned by a six-degree-of-f reedom translation stage, and makes

simultaneous measurements of Rp, Rs, ^, and A.

A return path configuration was chosen to provide the versatility necessary for the

variety of curved surfaces to be used in the ALPHA laser. This E/R is capable of measuring Rp

and Rs to 0.1% accuracy and Delta to better than 0.1° accuracy on curved surfaces to 1.0 cen- timeter radius of curvature.

The optical design and theory of operation as well as automatic alignment and focusing

features are discussed. Data taken with this instrument are presented.

Key Words: ellipsometer; infrared ell ipsometry; infrared ref lectometry ; reflectometer

.

1. Introduction - Design Considerations

The design specifications for the combined El 1 ipsometer/Ref lectometer were to measure the

reflectance of both P and S polarized light and the differential phase shift (A) to an accuracy

of 0.1% and 0.1 degree respectively. These measurements are to be made at discrete wavelengths

between 2.5 and 3.1 microns at angles of incidence between 20 and 55 degrees. The optics to be

measured by this instrument are, in general, thick circular disks, cones, and rings varying

from 28 to 142cm diameter. Reflectivities on the order of 96% to 99.7% are to be expected from

these surfaces, implying a value of ^ near 45°. Delta values will vary from + 45° to - 45°.

The small radii of curvatures near the truncated tips of the reflaxicon (0.76cm) and waxicon

(0.36cm) are the most difficult areas to measure.

2. Instrument Configuration

The combined E/R instrument is configured as a return path, single rotating polarizer

instrument capable of simultaneously measuring parallel and perpendicular reflectances (Rp and

Rs) as well as the ellipsometric parameters tj» and A. The Spinning Polarizer Infrared

Ref lectometer/El 1 ipsometer (SPIRE) is configured to measure both flat and highly curved sur- faces with focused light but may also be configured to measure flat surfaces with collimated

light. SPIRE has been designed with a broad-band light source and narrow-bandpass, wavelengthselection filters. SPIRE can be used to make reflectance and ellipsometric measurements at 1.0

to 5.0 microns wavelength by substitution of the appropriate optica i filter and can also be

used to make measurements in the visible spectrum by substitution of an appropriate polarizer

and detector.

SPIRE operates remotely and is controlled by a Hewlett-Packard 9836 microcomputer. Video cameras and monitors allow for the precise positioning and alignment of the instrument with

respect to the optics to be characterized. Wavelength selection, angle of incidence positions

(defined by the retro-arm position), data taking, and analysis are all performed by the dedicated computer.

*Thomas A. Leonard is currently at Analytic Decisions Inc., 5100/211 Springfield Pike, Dayton

OH 45431 (513)258-1192

72
Figure 1. The optical layout for the return path, double bounce ell ipsometer/ref lectometer

.

The retro-arm is shown in the calibrate position. The alignment level provides auto- alignment and focusing.

Figure 1 is a view of the optical layout of the E/R and alignment shelf. The Helium-Neon

Laser is used for visible alignment of all the internal optics. All optics are reflective

except for the output lens, the Nernst glower lens, and the lens directly before the detector,

so the only offset between the visible and infrared focal planes appears at these optics.

The light source is a Nernst glower shown in the lower right corner of figure 1. The

unpolarized light from the glower is focused onto a 1-millimeter pinhole by an A/R coated,

CaF2, 4-inch focal length lens. The light from the pin hole is collimated by a six-inch focal

length off-axis parabolic mirror. The hot mirror directs the infrared light from the Nernst t the main optical axis on the center of the table. The hot mirror also allows the He-Ne beam

to be transmitted from the back side, making it co-aligned with the Nernst light. The two apertures define the optical axis which passes through the measurement point (MEPO), which is defined by the retro-arm. A beamsplitter divides the incoming source beam into a reference

beam and sample beam. The beam splitter is a coated pellicle, approximately seven microns

thick, which eliminates secondary reflections. Both beams are directed through a rotating

chopper wheel with a single aperture. The geometry is such that the reference beam and the

sample beam alternately pass through the chopper. Background radiance measurements are made

when neither beam is passing through the chopper.

After the reference beam passes through the chopper, it is reflected back along its origi

nal path. At the beamsplitter, it is again divided into two components. The reflected com- ponent is discarded while the transmitted component proceeds to the detector. The function of

the reference beam is to provide a reference measurement of the source intensity.

After the sample beam passes through the chopper, it is converted to linearly polarized

light by a polarizer. The 10-inch focal length Calcium Fluoride (CaF2) lens at the front

of the table focuses the infrared light to a 1.6 millimeter spot at the MEPO, from which it is collected by the 1.25 inch focal length spherical retro mirror (one inch diameter). The light

then returns back through the same optics to the pellicle beamsplitter where it is reflected

toward the detector. As the return beam passes through the polarizer, it is again converted t linearly polarized light. The path lengths of the reference arm and sample arm are equal.

After returning to the beamsplitter both components follow an identical path to the detector,

although not at the same time. From the beamsplitter the light travels to a second hot mirror

where the visible light transmits to the upper table with the alignment cameras. The infrared

light reflects from the hot mirror to a 4-inch focal length CaF2 lens. The light is focused t the detector surface through the wavelength selection filters.

The reference beam remains unpolarized for its entire path. The sample beam is linearly

polarized by the polarizer before the second pass through the beamsplitter. The fraction of the sample beam reflected to the detector is senstive to its polarization state and in particular will depend on the orientation angle of the linearly polarized light.

73
The signal measured by the detector is repetitive,

with its fundamental frequency determined by the rotation

speed of the chopper wheel. The waveform, as it might be

observed on an oscilloscope, is shown in Figure 2. The

signal is AC coupled and inverted by the detector

amplifier. Amplification is adjusted to cover the range

-10 to +10 volts. A -full period, or one chopper cycle, -

is divided into four parts, labeled B1 , PR, B2, and PS. ^ The chopper passes the sample beam during PS and the § reference beam during PR. During B1 and 82, the chopper

~ blocks both the sample and the reference beam. The width g of the transition region between constant levels is m determined by the time needed for a chopper edge to sweep

across the corresponding beam. An A/0 converter within

the H-P computer system is used to sample the detector

waveform. One channel on the encoder has a single pulse

per revolution (PPR) which is used in the synchronization

of the polarizer and chopper rotations. A second channel

on the encoder contains 128 PPR. The 128 PPR are equally p^gup, 2 Exaiipla of the wavefora

spaced and correspond to 2.8 degrees rotation of the cowleta chopper cycle, polarizer. As shown in the waveform plot there are 8 ^h. chopper rotation rate is 2.5 Hz complete cycles of the polarizer for each cycle of the rotating pol.rizar rate is chopper. This corresponds to 1024 pulses per wavefora (1 go Hz chopper cycle)

.

RP CYCLE NTJMBER

The background levels are averaged and subtracted froa both the reference and sample

levels to produce intensity values. The ratio of sanple intensity to reference intensity is

taken to cancel the effects of source intensity variation. A calibration measurement is required at each polarizer orientation to factor out the effects of the beamsplitter and retro mirror.

In actual operation, the polarizer rotates at a rate of 20 Hz while the chopper rotates

synchronously at exactly one-eighth of the polarizer frequency. During each sample period PS,

measurements are obtained for a series of polarizer positions. An angle encoder on the

rotating polarizer ensures that the same polarizer positions are measured for any cycle. The

encoder clocks each A/0 conversion so that there are 128 measurements per cycle. This allows a Fourier decomposition using a Fast Fourier Transform. The amplitudes of the constant term and

the second and fourth harmonics are used to calculate reflectance, ^ and A.

Two mechanisms worthy of further comment are the retro arm and the phase lock loop circuit

which synchronizes the rotations of the polarizer and the light chopper.

The retro-arm is necessary to return the

reflected light from the sample to the E/R

pallet where the detector is mounted. The

retro-arm must rotate about a point in space

(MEPO) and allow the retro-mirror axis to always pass through that point. This is necessary to measure a defined location at any

angle of incidence. The retro-arm design is shown in Figure 3. It incorporates an Aerotech rotary stage with 0.01 degrees rotation per step. This acts as a pivot for the

lever arm action. A belt between the two pivot points allows for rotation of the retromirror about the pivot as well as about the

measurement point.

The chopper phase lock loop (shown in Figure 4) requires two input signals and produces an analog voltage to drive the chopper

motor to the correct speed and angular position relative to the rotating polarizer. The

one pulse per revolution (1 PPR) input coming

from the rotating polarizer is connected to the 4040 counter. This divides the 1 PPR input by

2 to the N power where N is between 1 and 12 (1 ,2,4,8. . .4096)

.

Figure 3. The retro-arm is shown in the calibrate position and at 20*

angle of incidence. The retro-mirror

may be positioned between 25° and

90" with 0.01° accuracy.

The output of the divider is compared with the chopper input by the 4046 phase-locked

loop- integrated circuit which generates two outputs. The first output generates a train

pulse with a duty cycle proportional to the frequency difference and phase difference of

the input signals. If the frequency at pin 14 is higher than at pin 3, then the output at pin

13 goes high or, conversely, if pin 14 is lower frequency than pin 13 then pin 3 goes low. When pin 14 and pin 3 are equal in frequency but differ in phase then the duty cycle is

74
proportional to the phase difference. A filter on pin 13 removes the AC component of the

signal leaving the average (DC) to control the chopper motor speed. The amplifier is used

to buffer this signal to prevent loading by the chopper controller. The voltage divider

on the output of the buffer is necessary to allow the 4046 to operate in its linear

region. It will not operate close to the negative supply (ground in this case).

The second output from the 4046 produces a train of pulses with an on time proportional to the phase error of the input signals. This output is filtered to produce a DC voltage proportional to the phase error. This signal is compared with a reference level (set by a pot) by

the LF356. When the signal is greater than the reference the chopper is not phase locked to the rotating polarizer. The LF356 drives a transistor to convert the output to TTL levels.

This signal drives an input to the computer so that the computer program will know the output

to the computer will be high. An LED is mounted on the panel to show when the system is out of

lock. The out-of-lock indicator was selected because it is easier to see small phase errors

when the LED is off and blinks on for a short duration than when the LED is on and blinks off.

Figure 4. The chopper phase lock loop circuit controls the synchronization of the chopper and

rotating polarizer to allow for data acquisition.

3. Instrument Theory

An ellipsometric measurement is concerned with the ratio of the principal amplitude

reflectances and p^:

P = Pp/Pg = tarolf e-^^

"

The angles ^ and A are called the ellipsometric angles.

Unpolarized light incident on the rotating polarizer is transmitted as a linearly

polarized beam oriented at an angle 6 perpendicular to the plane of incidence. The Jones vector E^ after the polarizer is given by

sin(e)

cos (8)

The angle 0 is the instantaneous polarizer orientation given by

9 = ut + 0

where u is the angular rotation rate and 0 is the orientation at time zero.

The polarized beam reflects from the sample to a spherical reference mirror which returns

the beam back to the sample. The beam is reflected at zero angel of incidence from the

reference mirror, which introduces no change to the polarization state, ^he effect of the

double reflection from the sample can be expressed in the following matrix equation

75
where E, is the Jones vector after the second reflection.

The return beam can be projected onto the position of the polarizer to find the magnitude

E of the Jones vector after the second pass through the polarizer.

E - [sin(e) cos(e)]E2 .

This expression can be simplified to

2 2 2 2 E = p sin (0) + Pg cos (9)

The intensity I measured by the detector is given by

I = EE*

The following equation is obtained by substituting for E to obtain

I = R ^sin*{9) + R ^cos*(9) + 1|R R cos(29)sin^(29)

P s P s

Reflectances and phase shift A are based on a harmonic analysis of the intensity I of the

form:

I = b + b cos (29) + b cos (49)

where

b« = 3(R^^+ /R + J<R R cos(29)

o s p p s

^^2 =

b, = (R ^+R ^)/8 - J<R R cos(2A).

4 * s p

' P s

In the harmonic analysis, the offset angle 9 between the zero position of the rotating

polarizer and the plane of incidence is contained in the phase terms. The magnitude of

the harmonic coefficients is independent of the offset angle. Each harmonic analysis,

therefore, implicitly includes a determination of the plane of incidence. The plane of

incidence is then equal to the difference between the zero angle and half of the phase of

the second harmonic.

The principal reflectances and the el 1 ipsometric angles ij» and A can be obtained from

the following equations:

R

^ = b + b + b,

s o 4 2

76
cos(2A) = (b^ - 3b^)/(RpRg)

tan^i|> = R /R . P s

The harmonic coefficients are non-negative because they represent magnitudes. Thus, the

above equations explicitly assume that Rs will be greater than Rp. If this is not the

case, the phase of the second harmonic will be 90° shifted from its normal value, indicating that Rp is greater than Rs. The second harmonic becomes zero when Rp is equal to R^.

In el 1 ipsometers without compensators, the phase shift A appears as cos(A). Taking

the inverse cosine leads to an ambiguity in determining A. Angles in the lower half plane

have the same cosine as those in the upper half plane. In a double-bounce el 1 ipsometer

,

the phase shift appears as cos (2A). The sensitivity in measuring phase shift is

increased because the sample is traversed twice, but the ambiguity also increases. The

cosine function has a principal range of 0° to 180°. Removing the factor of two reduces

the principal range to 0° to 90°. Thus, all phase shifts are folded into the first

quadrant. This folding process must be kept in nind when interpreting the results of a SPIRE measurement.

Another implication of the cosine dependence on A is that the measurement varies

quadradical ly for angles near 0° to 180°. The lack of a linear term means that the grecision of the measurement decreases as the cosine argument approaches either 0° or 180 .

After some trigonometric manipulation, it is possible to write the equations for the

harmonic coefficients in a form where the ellipsometric angles appear explicitly:

"o

"

'*(''s^*''p^^ * 1<RpRg[cos^(2A) - csc^(2»|>)]

b2 = RpR5[cot^(.<f) - tan^{(|>)]

^4 ^ 'iRpRs[sin^(2A) + cot^(24r)]

In this form, the ellipsometric dependence of the second harmonic is only on ^. The other

harmonics depend on a mixture of ij/ and A. There is some additional simplification for the case R=Rp=Rs, where the angle i|; is 45* and the harmonic equations reduce further to the

form

b = liR^ + 1iR^cos^(2A)

^2 = °

b. = 34R^sin^(2A)

4

bo, b2 and b4 are the coefficients for the modulus, and the second and fourth harmonic

terms of the harmonic series which describes the E/R waveform. Now the fourth harmonic

depends only on A. Measurements on metal reflectors, where \ii is approximately 45 , essentially satisfies these conditions. Then the second harmonic measures the departure of ^

from 45" and the fourth harmonic measures the value of A.

The equations used to obtain the ellipsometric angles involve ratios of the harmonic

coefficients. It is possible to obtain accurate ellipsometric values under conditions

where the reflectance measurements are not accurate because of light lost to the measuring

77
instrument. Ellipsometric angles are obtained as the ratio of reflectances. If a fixed

fraction of the reflected beam is blocked or otherwise removed from the measurement, an ellipsometric measurement is still possible. This vignetting affects each polarization

component equally. Therefore, the ratio is not affected.

4. Edge Detection

HZASUUKCrT SVOT

fCXS DiJtzcrioii

•try (or cd^* datBctign.

When the SPIRE instrument is coupled with a manipulator system, the combined system offers a mechanism for alignment. The basic measurement is that of edge detection. During edge detection, the

measurement spot is stepped across the edge and the

reflectance is measured as a function of position.

The reflectance is obtained by averaging the ratio

measurements for all polarizer positions. The

averaging operation is equivalent to using the zero harmonic as a measure of reflectance, but does not require as much time as an explicit Fourier transform.

To make an edge measurement, one endpoint of

the scan must be entirely on the mirror. This is

the position of maximum reflectance. The other endpoint must be entirely off the mirror. The test geometry is shown in Figure 5. This is the position

of minimum reflectance. Between these two positions, as shown in Figure 5, the reflectance remains

constant except for the transition positions where

the measurement spot overlaps the edge. In the

transition region there will be two shoulders and a middle linear portion. The linear portion is cen- tered on the inflection point of the edge scan. This inflection point represents the edge

of the mirror. For a symmetric measurement situation, the inflection points occur when

the measurement spot is half on the mirror so that the reflectance is halfway between the

maximum and minimum values. In addition, the shape of the curve will be symmetric about

the inflection point.

Figure 5, Test geometry and components

of an adg* scan.

In analyzing an edge scan, the endpoints of a measurement sequence are used to determine

the reflectance span (Rspan) between minimum and maximum reflectance. The target reflectance

(Re) is the average of the endpoints. The limits of the linear portion are assumed to be a fixed fraction of the reflectance span. The default fraction is 0.5. The linear region is then defined as all points within the range

|R-R I ^ 0.5 R span

where R^ is the target reflectance, and R„„,,

e ^ spar

In finding the edge, the software first

Then it locates the point along the scan whe

current point is below (above) Rg. These po polation of the edge position. However, the

the transition point and uses those within t of the edge position. This process ensures

points are unequally spaced or spaced too fa

region. Maximum precision is obtained when

linear region and the endpoints are sufficie

the edge.

is the reflectance span.

determines whether the scan starts high or low. re the previous point is above (below) Rg and the

ints are sufficient to provide a linear intersoftware examines the points in the vicinity of

he linear limit to make a least-squares linear fit

a successful edge measurement even if the scan

r apart compared to the extent of the linear

a number of measurement points lie within the

ntly away from the edge to eliminate the effects of

A single edge measurement determines both the position of the edge and the width of the

measurement spot. The location of the point R along the scan determines the edge location.

The slope of the reflectance scan at the edge determines the beam width. A working definition

of beam diameter d is given by

d = ^pan/^l°P^

An explicit definition depends on a model of the beam profile. A Gaussian profile, for

example, may be an adequate representation of the beam profile, in which case the measured beam

diameter can be related to the e"2 energy point.

This edge scanning routine is used with various modifications to perform system focus

scans, detector centering, sample positioning, and various internal alignments.

78
5. Measurements and Results

There are no other instruments to date capable of measuring reflectance and el 1 ipsometric

data on highly curved surfaces, so determining the accuracy of the E/R became a very detailed

task. Confirmation of accuracy on flat surfaces was accomplished by measuring bare metal flats

and comparing to theoretical value, and other reported measurements.

The reflectance accuracy on flat surfaces was determined by measuring copper samples, com- paring to theoretical values, and also making absorption measurements on the same samples. The

el 1 ipsometric accuracy was determined by comparing measurements with those made on a four-zone

null el 1 ipsometer

.

The accuracy for reflectance and ell ipsometric measurements on flats is +0.1% in reflectance and 0.1 degrees in differential phase. The accuracy in reflectance and ell ipsometric

parameters on curved surfaces was determined by manufacturing a conical mirror with a flat surface on approximately 5% of the surface. This permits the measurement of flat surfaces and

curved surfaces on one uniformly gold-coated sample. The accuracy for reflectances measurements is 0.2% on conical surfaces with curvature down to 20 millimeters radius. Ellipsometric

accuracy is 0.1°. The ellipsometric data taken in a photometric mode is not sensitive to light

losses as are reflectance measurements. Phase is determined from a ratio and the ratio is unaffected by some light loss. A summary of the instrument performance is shown in Table 1.

Table 1. El lipsometer/Ref lectometer Performance Summary

PARAMETER PERFORMANCE

Measurement wavelength

Spectral Bandwidth

Reflectance

Range in R

Accuracy in R

Range in A

Accuracy in A

2.622 urn

^2 2.745

^3 2.820

2.995

^5 3.129

FWHM 2% ± . 1% center X

measure Rp & Rg separately

40 + % to 100%

+0.1% flats; +0.2% conies

+ AS" (|>g - <<)p +0.1°

Angle of incidence range 20° - 55°

Accuracy in setting angle

^ of incidence < + 0.

1

Autocoll imation accuracy ~ 0.01°

Edge locating accuracy ~ + 10 urn

79
Manuscript Received

11-5-86

Direct 02 Measurement in Yttria Stabilized Cubic Zirconia

Shekhar Guha,* Nastaran Mansour, and M. J. Soileau

Center for Applied Quantum Electronics

Department of Physics

North Texas State University

Denton, TX 76203

We present here the results of direct measurement of the nonlinear refractive

index, of yttria doped cubic zirconia at 1.06 ym using external self-focusing. The

value of n2 was found to be independent of the yttria concentration. The measured value

(6 X lO"'^^ esu) compares well with that calculated from the breakdown measurements [1].

Key words: external self-action; laser induced damage; nonlinear refractive index; psec

pulses; yttria stabilized cubic zirconia; 1.06 ym.

1. Introduction

Cubic zirconia is an attractive candidate for window material in high power laser systems

because of its transparency, mechanical strength, and stability when doped with yttria (¥203). A knowledge of the nonlinear refractive index n2 is therefore necessary in the design of such laser

systems to properly account for the nonlinear beam propagation effects as well as in evaluating the

role of self-focusing in laser- induced damage process. n2 of yttria stabilized zirconia (YSZ) has

previously been inferred from breakdown critical power. We present here a description of an analysis and experiments used to directly determine n2 in the YSZ using picosecond pulses of

different states of polarization.

2. Analysis

The mechanism used to measure n2 is that of external self-focusing [2] in which a collimated

laser beam of Gaussian temporal and spatial profile is incident on a thin sample. (The sample

thickness is much less than the confocal parameter of the laser beam.) In this case the diffraction of the laser beam inside the nonlinear medium can be ignored and the irradiance I and phase

of the electric field

4

E =J_iL_ e^* (1) n e c o o

of frequency to follow the propagation equations;

dl

dz

= - al (2)

where a is the linear absorption coefficient and is related to n2 through

40tt6

' 1 2 2 c n

o

n (esu) (4)

*Present address: Martin Marietta Laboratories, Baltimore, MD 21227

80
The laser beam is assumed to be Gaussian in temporal and spatial profile and incident on the sample

at z=0 with irradiance

1(0, r,t) = exp [-(r/r^)2 - (t/t^)^] (5)

For a medium of length 1 and reflectivity R, the transmitted irradiance is given by

1(1, r,t) = (1-R)^ exp(-al) 1(0, r,t) (6)

and the phase change on transmission through the sample is given by

.(l,r,t) - (J.(0,r,t)

1 - (1-R)2 e-"^^

1(0, r,t) (7)

The irradiance and the phase obtained from equations 6 and 7 completely determine the electric

field E(l,r,t) at the exit plane of the sample from which the field at any point (l+z,r,t) can be

determined using the Huygens-Fresnel propagation formalism [3]

2 IT 2 E(l+z,r,t) =^-7— exp(iTrr /Az) / E(l,r',t - z/c)

1 A Z /q

X exp(i r'^Az) (2Trrr'/Az) r'dr' (8)

The experimentally measured quantity is the fluence at the observation plane at a distance z from

the sample. The fluence is

fj

ce . F = ^

/ I E(l+z,r,t)| dt (9)

3. Experiment

The apparatus used is described in greater detail elsewhere [4]. Single pulses of measured

Gaussian spatial and temporal profiles were obtained from a microprocessor-controlled, passively

mode-locked Nd:YAG laser system operated at 0.5 Hz. Single pulses were switched out of the mode

locked pulse train and amplified. About 1 mJ of energy in the infrared was routinely available in

pulses of 30 ps to 200 ps width. The pulsewidth was varied by using etalons of different thickness

as the output coupler of the laser. Attenuation of the energy in the pulse was achieved by using a half-wave plate polarizer combination which produced no measurable aberrations on the spatial

profile of the beam. The shot-to-shot variations of both the energy and the pulsewidth were

monitored. The change in the spatial profile of the laser beam transmitted through the sample with

increased incident irradiance was determined by using a vidicon tube interfaced with a microprocessor controlled optical multichannel analyzer (PARC 1215). By using a pinhole (25 pm diameter)

mounted on a photodetector , the on-axis fluence of the transmitted signal was also measured as a function of the incident irradiance.

4. Results

Figures 1 and 2 show the result of measuring the transmitted on-axis fluence in the near field

as a function of irradiance for 30 psec, 1.06 ym pulses of linearly and circularly polarized light

incident on 1 cm long samples of YSZ. Up to the highest irradiance used, no nonlinear absorption

was observed in the sample. Three samples of YSZ were studied, with yttria concentrations of 9.4%,

12%, and 21%, respectively. The measured n2 was found to be independent of the yttria concentration. The solid lines in figures 1 and 2 are the theoretical fit obtained from equations 6 and 7, with the value of n2 as the only free parameter. The best fit to figure 1 was obtained for

81
n2=6 X 10" esu. For comparison, in figure 3 we show the results of the same experiment performed

on a sample of sodium chloride. The value of n2 in YSZ was found to be 9.5 times more than that of

sodium chloride. The ratio of n2 (linear)/n2(circular) was found to be close to 1.5 for each

sample. The value of n2 obtained by this method compares well with the value of 8 x 10"^-^ esu

estimated from the measurement of the breakdown power (Pg) of 400 kW.

5. Summary

External self-action provides a relatively simple procedure for estimating the nonlinear

refractive index, n2. This method was used to determine n2 for yttria doped cubic zirconla. The

value obtained compares well with that determined by other methods.

6. References

[1] Mansour, N.; Soileau, M. J.; Van Stryland, E. W. "Picosecond damage in Y2O3 stabilized cubic

zirconia." Natl. Bur. Stand. (U.S.) Spec. Pub.; 1984.

[2] Weaire, D.; Wherrett, B.S.; Miller, D. A. B. ; Smith, S. D. "Effect of low-power nonlinear

refraction on laser-beam propagation in InSb." Opt. Lett. 4(10): 331-333; 1979.

[3] Born, M. ; Wolf, E. "Principles of optics," 5th ed. Oxford, U.K.; Pergamon Press; 383; 1975.

[4] 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.

INCIDENT IRRADIANCE (GW/cm^j

82
Figure 3. The transmitted on-axis fluence

measured as a function of incident

irradiance for zirconia, using

1.06 ym, 30 psec, linearly polarized

radiation. The measurement was performed in the near field. The solid

line is a theoretical fit with

no=4.5 X 10~12 gg^^

10 20 30 40

INCIDENT IRRADIANCE (GW/cm^j

83
Manuscript received

2r-19-r-86 SAR

Measurement of Optical Damage Threshold of Wire Grid Polarizers at 10.6 \im

M. Mohebi, A. Said, and M. J. Soileau

Center for Applied Quantum Electronics

Department of Physics

North Texas State University

Denton, Texas 76203

We report the results of measurement of laser induced surface damage threshold of

wire grid polarizers at 10.6 ym as a function of orientation of the wires and polarization of light. We also measured the absorption of wire grids at 10.5 ym, 1.06, and

0.53pm. The results of surface damage thresholds are consistent with the results of

absorption measurements and theory.

Key words: absorption; damage threshold; photoacoustic ; polarization; wire grid.

1. Introduction

Microscopic wire grids deposited on a dielectric grating at grazing angle are useful

polarizers. They are compact, easily rotatable, can be used with converging and diverging beams,

and work with all wavelengths at least four times smaller than the period of the grid, as long as the substrate's transmission permits. Gratings with a period as small as 0.4 ym are commercially

available. (Wire grid polarizers used in this work were acquired from PTR Optics Corporation, 145

Newton, Waltham, MA 02454.) With proper choice of the substrate, wire grid polarizers can be

fabricated for use at any wavelength for 2 ym to far infrared. However, their principal limitation for use with high power lasers is their damage properties. In this work we have measured

laser induced surface damage threshold of wire grid polarizers for pulsed radiation at 10.6 ym as

a function of orientation of wires with respect to the polarization of laser beam. In addition,

we measured absorption vs. x and orientation.

2. Experimental Results

The polarizers used in our experiment were two wire grids of aluminum deposited on ZnSe

substrates, one of the samples was overcoated with an anti ref 1 ection (AR) coating on both

surfaces. The period of wire grids was 0.84 y m with a symmetric 90° sawtooth groove shape. The

measurements were performed using linearly polarized, normal incident pulses from a hybrid COo TEA

laser. The laser was operated in the TEMqq spatial mode and single longitudinal mode with 130 ns (FWHM) pulses. Figure 1 shows a pinhole beam scan of the spatial profile of the beam as well as a temporal profile detected by a photon drag detector (500 MHz response). The beam scan was performed just before a lens (F = 25.4 cm) which focused the beam on the wire grid to a beam size of

0.36 mm (FWl/e^M). Each site was irradiated only once (one-on-one measurement). Figure 2a shows

the experimental results for the sample with no AR coating for orientation of the wires parallel

and perpendicular to the incident electric field. The damage threshold for this sample was

2 J/cm^ (corresponding to a peak-on-axis irradiance of 15 MW/cm^) when the wires were parallel to the incident electric field. Here we have defined the damage threshold as the lowest incident

fluence (or irradiance) for which damage occurred. The relatively large overlap region for this

sample was because of nonuni formities of the surface structure. Figure 2b shows the results for

the sample with AR coating on both sides. The damage threshold for this sample was 0.8 J/cm^

(6 MW/cm^) when the wires were parallel to the incident polarization. The damage threshold of

both samples increased by more than a factor of two when the wires were oriented perpendicular to the incident electric field (i.e., the orientation for maximum transmission).

In order to study the effects of orientation of wires on the absorption of the wire grids we monitored the photoacoustic (PA) signal generated in the wire grid by coupling a transducer to the

side of the sample. The beam size on the front surface of the sample was kept large

(3.2 mm FWl/e^M), in order to avoid surface damage. Figure 3 shows the PA signal as a function of

incident power for both orientations. The absorption of wire grid was greater by a factor of -3.6 when the polarization was parallel to the wires. This is consistent with the measurement of

damage threshold in the parallel and perpendicular orientation. It is well known [1,2] that the

optical properties of wire grids are strongly dependent on the ratio of wavelength to the period

84
of wires (x/d). The wire grids are effective polarizers only when X/d>>l. Both polarizations tend

to be equally transmitted or reflected for x/d close to or smaller than one. We used 30 ns pulses

from YAG laser at 1.06 y m and its second harmonic (0.53 ym) to change x/d in our experiment.

Figure 4a shows the PA signal versus incident power at 1.06 vm for both orientations. The absorption is smaller by a factor of 0.6 when the wires are parallel to the electric field. Figure 4b

is the result of the same measurement at 0.53 ym. It shows that absorption is smaller by a factor

of 0.8 when the wires are parallel to electric field.

3. Theory and Discussion

The theory of operation of wire grid polarizers has been the subject of investigations by

many researchers [3-5] in the past. All theoretical treatments start by assuming an ideal cross section for the wires. Flat strips [3] and cylindrical wires [4-5] have been considered. The

effect of nonuni formities in the wires cannot be easily treated. Only the plane wave approximation has been considered. The problem of incident electric field with polarizations other than

parallel to the wires, including ohmic loss has not been considered before. The reflected wave is regarded as originating from the currents induced in the wires, while the transmitted wave is the

superposition of the induced and incident fields. Absorption is considered as the difference

between incident and sum of transmitted and reflected waves. The current in wires all have the same magnitude (for plane wave illumination), but differ in phase from wire to wire by Kdsina,

where K=2-n/x and a is the angle of incidence. When the electric field is parallel to the wires

the current has the same phase everywhere along any given wire. The effect of adding ohmic loss

of conducting wires to this picture is to decrease the amplitude of reflection coefficient and the

phase of the transmission coefficient. Cassey's [5] calculations show that the loss in the wires

depends directly on the square of reflection coefficient, that is, the absorption of wire grid is greater when its reflection is greater. This is consistent with the results of measurements of

absorption vs. orientation in this work. Bird and Parrish [1] investigated the wire grids experimentally and found that the transmission curves for parallel and perpendicular polarization cross one another when x/d is decreased to a value of one. Their result was in agreement with the

theoretical work of Baldwin [3]. This suggests that reflection, and therefore absorption, curves for parallel and perpendicular polarization cross over when x/d is close to one. This is also

consistent with the measurement of absorption at three different wavelengths in this work. This

qualitative agreement is striking, considering that in this experiment we have a Gaussian profile

beam, and the fact that wire cross section is neither a flat strip nor a cylinder.

4. Summary

We have measured the surface damage thresholds of wire grid polarizers at 10.6 ym. Damage

thresholds were maximum when the polarization of light was perpendicular to the wires, and the

damage thresholds were greater for the uncoated wire grid. The ratio of absorption of parallel

polarized light to the absorption of perpendicular polarized light depends on X/d. The ratio of

these two absorption is greater than one for x/d>>l and it is less than one for x/d close to or smaller than one. Measured absorptions at three different wavelengths are in agreement with the

theory.

5. References

[1] G. Bird and M. Parish, J. Opt. Soc. Am. 50(9), 886-891 (1960).

[2] M. Hass and M. O'Hara, App. Opt. 4(8), 1027-1031 (1965).

[3] G. Baldwin and A. E. Heins, Math. Scand. 2, 103-118 (1954).

[4] E. A. Lewis and J. P. Casey, J. Appl . Phys. 23(6), 605-608 (1952).

[5] J. P. Casey and E. A. Lewis, J. Opt. Soc. Am. 42(12), 971-977 (1951).

85
1.2

-0.2 I I I I I I I =12 -8 -4 0 4 8 12

POSITION (mm)

Figure 1. The 200 ym pinhole beam scan performed just before the focusing lens (F=25.4 cm). The

inset shows the temporal profile of the pulse.

86
Damaged

Not Damaged

J.

_<$fa Sig o SS. o m nftn<%f^ty<& (9)

8

ENERGY DENSITY (J/cm^

Figure 2a. Damage data for the wire grid without AR coating. Each point represents a single

shot. Open circles are for polarization perpendicular to the wires. Closed circles

are for polarization parallel to the wires.

1

87
Damaged

Not Damaged

1 2 3

ENERGY DENSITY (J/cm2)

Figure 2b. Damage data for the wire grid with AR coating. The damage threshold for this sample

was less than the sample without AR coating by more than a factor of two.

88
INCIDENT POWER (KWj

Figure 3. Absorption of the wire grid at 10.6 um. The photoacousti c signal indicates greater

absorption when the wires are parallel to the polarization of the incident electric

field.

89
10

INCIDENT POWER (KW)

Figure 4a

90
INCIDENT POWER |KW]

Figure 4b

Figure 4. Absorption of the wire grid at 1.06 ym (4a) and 0.53 pm (4b). The photoacousti c signal indicates greater absorption when the wires are perpendicular to the electric

field.

91
Manuscript Received

10-30-85

Laser Damage Studies of Several Acrylic Polyners

A.R. Ronberqer

Pennsylvania State University, Berks Canpus,

P.O. Box 2150, Reading, PA 19608

B.W. Mull ins

Department of Physics, USAF Academy, Colorado Springs, CO R0840

A. A. Shaffer

Department of Chemistry, USAF Academy, Colorado Springs, CO 80840

J.F. Hilbinq, K.E. Siegenthaler, T.F. Deaton and Y. Chanq

Frank J. Seiler Research Laboratory, USAF Academy

Colorado Springs, Colorado 80840-6528

The bulk laser damage resistance of several acrylic copolymers and of pure

poly (ethyl )methacryl ate (PEfIA) were investigated v/ith a 1.06/im wavelength pulsed MdrYAR

laser having a pulse length of 8 + 2 ns and an e"^ radius spot size of 30 + 0.7 ^m.

These materials were synthesized in the bulk after extensive filtration or distillation

to remove absorbing dust particles. A correlation is made between the damage thresholds

and the number of scattering centers per volume observed in a HeHe beam. The HeMe laser

beam scattering in the sample was used as an indicator of the impurity content of the

sample. In the course of this experiment a spatial filter vjas installed in order to make the focused YAG laser beam more nearly Gaussian. Comparison of results obtained

from the filtered beam and the unfiltered beam (which is mathematically described as a sombrero function) indicated that previous characterization of the non-Gaussian beam

provided an incorrectly low damage threshold value.

Key Words: beam profile; bulk laser damage; damage threshold; laser damage;

multiple-shot; plastics; single-shot

1. Introduction

It has been suggested^ that transparent organic glasses (plastics) would be an attractive

optical material for use in laser systems if the laser-damage resistance of plastics could be

improved relative to that of silicate glass. Besides being lightweight, low cost, diamond

turnable, and castable, plastics are dye-impregnable and therefore potentially useful as simple,

passive 0-switches or as dye-laser hosts. As recent measurements have shown, hov/ever, the

laser-damage resistance of commercially available silicate glass^ is significantly better than

that of several commercially available transparent plastics, including polymethylmethacrylate

(PMMA),^ bisphenol-A polycarbonate (PC),^ and cellulose acetate butyrate (CAR).^

Various copolymers have been proposed as candidates for laser optics.^ '^»^ Modification

of a chemically pure polymer by adding another polymerizing material to improve the material

properties of the sample has been used in industry for a long time. One principal advantage of

a copolymer is that a more elastic material can be obtained compared to a chemically pure

92
sanple. For optical applications,

properties has been proposed J

copolyners. Both Emel'yanova, et. copolymers they studied. Emel

inprovement of a plastic by

One way of improving these

al.^ and Dyumaev, et. al ^ et. al

improving its viscoelastic

plastics is by preparing

, -.- report improvement for the

. 'yanova, et. al.^ report single-shot improvement in a polymethylmethacrylate (PflflA)-polybutylacrylate (PRA) copolymer. Dyumaev, et. al.^ report

minor single-shot improvement and significant multiple-shot improvement in some of the

copolymers they studied.

As a continuation of the laser damage in plastics research at the Frank J. Seiler Research

Laboratory, several copolymers v/ere synthesized and their damage thresholds were studied.

2. The Damage Experiment

The laser and optical system used to perform the damage studies has been previously

described and is shov/n in figures 1 and 2.^'^ The laser used was a Ouanta-Pay nCP-2A

0-switched Md:YAR system with an unstable confocal resonator having a linearly-polarized l.n6/im

wavelenth output, and producing an 8 + 2 ns FV/HM, 300 mJ maximum pulse with a variable

repetition rate of up to 10 pulses per second (pps). The laser was pumped with a constant

flashlamp energy and rate to foster thermal stability. The output was annular in shape and

attenuated with optical density filters.

BS Sample BS

\

PM

Screen \

Strehl

Detector

Filters

C3 Polarizer

Nd:YAG

t

He-Ne

Figure 1. Optical layout for damage studies done with a sombrero-shaped beam profile

Figure 1 shows the optical system used for the damage studies using the sombrero-shaped beam

from the unstable resonator. A bare silica wedge was used as a beam splitter with the 4% front

surface reflection focused into the sample by a 24.7 cm focal length lens. The radiation

transmitted by the wedge v/as collected by a Scientech power meter for monitoring the laser power.

In our previously-reported work^'^»3 characterized the beam using

technique described by Mauck^, and analyzed as described by O'Connell, et. resulted in an e"^ beam v/aist radius of 33 However, an annular beam

waist profile described by a sombrero function, and the differentiation of a

the knife-edge

. al.4 This test

should focus to a knife-edge scan is

inaccurate for this type of beam since the Bessel function is not separable into x and y

coordinates as is a Gaussian function. The peak intensity calculated from this test is probably

low, but the knife-edge technique was the only one available for much of our earlier work. We eventually performed checks of the validity of this technique by employing other beam profiling

methods. A more accurate determination of the peak intensity of the sombrero beam is the

measurement of the transmittance through a small pinhole (compared to the beam diameter)

93
centered on the bean. This test was performed using a 5 diameter pinhole, and the average

fluence (or approximately the peak fluence) was calculated by multiplying the transmittance

times the beam's total energy per pulse and then dividing by the pinhole area. Comparison of

the data from these two methods indicates a discrepancy of a factor of approximately 2.3, as discussed below.

In order to avoid the beam profile problems described above, a spatial filter was placed in

the beam so as to transmit only the central disk of the sombrero profile. The output from the

spatial filter is thus approximately described by a Raussian function. Figure ? shows the

optical layout for the damage studies done with this Gaussian beam. The output from the Md:YAR

was focused into a vacuum spatial filter by a 30 cm focal length lens. The filtered beam was

recoil imated by a second 30 cm focal length lens and attenuated by neutral density filters. The

beam splitter transmitted a portion of the beam to the Scientech power meter to monitor the

beam's power. The reflected beam was focused into the sample by the 24.7 cm focal length lens.

I \

PM

Vacuum

5 Filters Pinhole

BS

Sample

\

BS M

\

\

L3 \

\

\

\

\ Aperture

Strehl

Detector ^

I

Polarizer 123 t

He-Ne

Nd:YAG

BS

Screen

Figure 2. Optical layout for damage studies done with a spatially filtered beam

The spatially filtered Md:YAG beam viss determined to have a focused waist radius of 30

+0 .7 fir\. The characterization was carried out by again using the knife-edge technique

mentioned above.

The samples used for both experiments were approximately 1.5 cm thick, with the waist of the

Hd:YAG laser beam located nominally in the center of the sample. The HdrYAO beam entered the

sample surface at near normal incidence.

A 2 mVI HeMe laser v/as used as a source for the Strehl detector^ ^ and was aligned to be

collinear with the 1.06//m beam. Since the 24.7 cm focal length lens would focus the HeNe beam

to a different point in space than the 1.06/im beam, two lenses preconditioned the HeHe beam to bring the tv/o laser beams to a focus at approximately the same point. A knife edge test of the

94
HeMe beam yielded an e"'^ radius of 36 located approximately 6.5 ran in front of the MdrYAb

beam's waist. A dielectrically coated mirror, having a reflectivity of 70% at 632.8 nm and a transmissi vity of 99% at 1.06 //m, was used to separate the two beams, and to allow observation

of the diffraction pattern from the damage site on the screen.

The reflected HeMe beam v/as then sent through the optics comprising the Strehl detector.

The Strehl detector consisted of a 5 mm aperture centered on the HeMe beam, a 2.5 cm focal

length lens, a KG-3 crystal to block any stray 1 .06 fin radiation, and an unbiased pin photodiode

detector. The photodiode output voltage was then displayed on a digital voltmeter and recorded

on a strip chart recorder.

The Strehl detection system was used exclusively to determine the presence of damage. Past work has shown excellent agreement (>95%) between the Strehl detector and a post-irradiation

examination under a lOOX microscope. ^"^'^^ The microscope examination was therefore deemed

unnecessary. A post-irradiation Strehl intensity of less than 0.995 v/as used to determine if

damage had occured. This corresponds to the formation of a 2 fin diameter, or larger, damage

site. 10

Prior to performing the damage experiment, each sample was examined to determine the

relative number of inclusions. The use of the scattered light from a HeMe beam (other than as a Strehl detector) has been previously used successfully as an indication of the number of

inclusions in various samples. ^»1? In order to attempt to compare our current samples with

each other and to compare these results with the previous work done at Seiler, the current

samples were also studied to determine the average number of scattering centers/volume. A 5 mw

HeMe beam v/as focused into the sample by a 30 cm lens. The depolarized scattered light v/as observed through a 30X microscope. The average number of micron-size particles (bright

scattering centers) v/ithin the 0.4 mm-^ volume of the HeMe track v/as observed with the

microscope. These results are incorporated in table 1.

Table 1. Summary of the Damage Results (peak fluences at 50?^ probability of damage)

Sample Content # Scatt

Centers /mm^

fth

F (10 pps, Mcr=1000)

f^/FTH

S-61 MMA +5.0% nRA 13 67 J/cm2 2.4 J/cm2 0.036

S-621 MMA +5.0% nBA 23 69 1.1 0.016

S-50 MMA +11% nBA n 642 1.22 0.019

S-58 MMA +5.2% I DM 4 120 10.2 0.085

S-53 MMA +10% I DM 16 932

S-52 MMA +20% I DM 7 101 2 10.72 0.106

S-56 MMA +5.3% EA 6 103

S-571 MMA +5.1% EA 14 73 1 .6 0.022

S-63 MMA +5.1% CHM 23 56 3.1 0.055

S-543 PEMA 3.4 1052

S-554 PEMA 52 562 3.42 0.061

NOTES: 1. uv polymerized. Unless noted sample was thermally polymerized.

2. old beam profile used; values converted (see text).

3. filtered and polymerized thermally over 2 1/2 months

4. filtered with AlBM initiator added.

95
3. Sanple Preparation

The several copolymers studied were prepared fron the dried (with CaH^) bulk nonomer. All the copolymer samples were fractionally or vacuum distilled to remove the hydroquinone inhibitor

and then filtered with a 0.22 porosity membrane filter to remove as much dust as possible.

The copolymer components were then placed in the polymerizinq ampoule. The v/eight of each

component v/as recorded. The copolymers were then sealed in their cylindrical ampoules (17 mm

dia X 6 cm long) and either polymerized in a 40f>C thermal bath (most samples) or polymerized

by the uv (300-400 nm) radiation (Samples S-57 Samples S-62) from a laboratory mercury lamp.

All these samples polymerized in approximately three days. The PEMA samples v/ere only filtered

and placed in the temperature bath. S-55 had 4.03 mg azoisobutyronitrile added as an initiator;

S-54 did not. S-55 polymerized quickly (one day) v/hile S-54 took two and one half months to polymerize, presumbably due to the presence of the inhibitor and lack of initiator.

4. Results of the Damage Experiment

The damage results reported in table 1 were analyzed by the following methods which have

been described previously. 2 j^g single-shot damage at five or more sites for each peak

fluence was reduced to a probability of damage for that fluence. The peak fluence Fjh ^ corresponding to 50% probability of damage was determined by generatinq a linear least squares

fit (LSF) to the data (figure 3).

Figure 3. Single-shot damage data for sample 56; the + indicates the threshold fluence level

The 10 pulse per second (pps) multiple-shot data was similarly treated according to the

method reported at the 16th Laser Damage Symposium.^ ^ The probability of damage at a given

fluence for M=10, 25, 75, 100, 200 and 1000 shots was first determined. Each probability set for a given M was analyzed for the 50% probability peak fluence according to the single shot

method described above. A plot of M vs. peak fluence level was then made (figure 4).

96
All the reported damage thresholds have been converted to the values expected for a spatially filtered Gaussian beam (S-50 through S-55 and previously published PriMA data in table

2), or were actually determined using the filtered, Gaussian beam (S-56 through S-63). A factor

of 2.27 + 0.07 was used to convert the data obtained with the non-Gaussian beam to compare them

to the Gaussian bean data. This factor was obtained by comparing threshold peak fluence levels

for the same sample in both types of beams, and it was checked against the pinhole data ^or the

non-Gaussian beam.

1000 e500 -

200

0 10 20 30

PEAK FLUENCE, J/cm2

40 50

Figure 4. Multiple-shot, 10 pps data for sample 62

97
Table 2. Comparison of Copolymer Samples and PEMA with Previously

Reported^^ pf^|^;\ of Similar Inclusion Density

Sample # Scatt.

Centers/mm*^

Single-Shot

rj\] (J/cm'^) PHMA

Sampl

e

# Scatt.

Centers /mm-^ Single-Shot

FjH*( J/cm'^)

S-61 13 67 E-20 7.1 66

S-62 23 69 RO-1 20 73

S-50 n 64 E-20 7.1 66

S-58 4 120 S-43 1.7 111

S-53 16 93 RO-1 20 73

S-52 7 101 E-20 7.1 66

S-56 6 103 E-20 7.1 66

S-57 14 73 E-20 7.1 66

S-63 23 56 RO-l 20 73

S-54 3.4 105 S-43 1.7 111

S-55 52 56 RO-3 31 73

Previously reported data'^ converted to equivalent values for filtered Gaussian beam profile.

It is apparent that our ability to reduce the impurity content (as indicated by the number

of scattering centers observed) was not good for the 11 copolymers and PEMA shovm here. We have

shown previously^ '^'^ ^ that there is a strong correlation between scattering centers and the

damage thresholds. However, within the above restriction it also seems that varying the

copolymer content does little to influence the damage resistance of the sample. S-61 and S-50

have increasing amounts of n-butylacrylate (nBA) {5% and ^^% by weight, respectively), and S-58,

S-53 and S-52 have increasing amounts of isodecyl methacrylate (IDH) (5.2%, 10% and 20% by

weight, respectively), but neither copolymer conclusively demonstrates an increasing damage

threshold. In fact, the opposite is true for the nBA copolymers and nothing can be concluded

for the IDM copolymers.

IDM does have the largest and heaviest side group, thus it will spread apart the polymer

chains and make the entire polymer more elastic. This is a desired effect, because it will

improve the viscoelastic properties of the sample and thus should increase the damage threshold

of the material as indicated by Dyumaev, et. al.^ The same comparison should be true for the

nBA (S-61 or S-62) copolymers vis-a-vis the ethylacrylate (EA) copolymers (S-56 or S-57) but

does not hold.

It is probable that the inclusion content is so large that only a big change (i.e., nBA or

EA to IDM) in the copolymer will have a significant effect.

A comparison of polymerization methods was attempted with S-56 (thermal) and S-57 (uv) and

also with S-61 (thermal) and S-62 (uv) with the expected result that S-57 and S-62 would exhibit

improved thresholds based on our previous work with PMMA.^^ This is not true, but it should

also be noted that the particulate content of S-57 and S-62 is higher than their thermally

polymerized counterparts, even though all samples were prepared under conditions as identical as possible. It is clear that the inclusion content has the greater influence and no trend can be

established for these samples.

98
The two samples (S-54 and S-55) of PEMA recorded were the initial attempts at making these

samples. The inhibitor was not removed, and initiator was added to S-55. It is clear that

adding the initiator added inclusions and decreased the damage thresholds of S-55. This was

expected. PEMA is interesting however, because it is still relatively hard, but is softer than

PMMA. PEMA should therefore have improved viscollastic properties and greater damage resistance.

S-63 is a copolymer of MMA and cyclohexyl methacrylate (CHMA). Both materials alone make a hard polymer and PCHMA additionally tends to crystallize and become milky in appearance. The

copolymer sample studied was slightly cloudy, and there is little in its results to indicate any

improvement over the other samples. In fact, its single-shot behavior is relatively worse than

the other samples, while its multiple-shot behavior is not much better than several other

samples.

Finally, a comparison of the samples with PMMA of similar inclusion content is shown in

table 2. It appears that the copolymer material, or PEMA, has a somewhat higher single-shot

damge threshold. However, the differences are not very great, and certainly no sample had

damage thresholds greater than some of the PMMA samples previously reported.^

It is thus unclear whether copolymers or a different methacrylate polymer will have a significantly improved laser damage threshold.

5. Conclusions

The apparent improper characterization of the sombrero function beam's peak fluence yields a significantly lower damage threshold than those obtained with a properly determined Gaussian

peak fluence. The high inclusion density present in our samples probably masks the bulk

material properties that would influence the damage threshold. The copolymers and PEMA shows a modest improvement in single-shot threshold level over PMMA, v/hich agrees with the Soviet

literature.^'' The multi-shot threshold improvement of copolymers over that of PMMA is

inconclusive for our samples. We did not observe the improved multi-shot threshold performance

of the copolymers as was reported by Oyumaev, et. al.-^ Finally, due to the large number of

inclusions in our samples, work in this area needs to be continued with better prepared samples.

6. References

[1 ] K.M. Dyumaev, A. A. Manenkov, A. P. Maslyukov, G. A. Matyushin, V. S. Mechitailo, and

A. M. Prokhorov, "Transparent polymers: A Hew Class of Optical Materials for

Lasers," Sov. J. Quantum Electron , 1_3, (4), 503 (1983).

[2] D. A. Gromov, K. M. Dyumaev, A. A. Manenkov, A. P. Maslyukov, G. A. Matyushin, V. S. Nechitailo, and A. M. Prokhorov, "Efficient Plastic-Host Dye Lasers," J. Opt. Soc.

Am. B, 1, 431 (A) (1984).

[3] L. n. Merkle, M. Bass, and R. T. Swimm, "Multiple Pulse Laser-Induced Bulk Damage in

Crystalline and Fused Quartz at 1.064 and 0.532 urn," Opt. Eng. , 22, 405 (1983).

[4] R. M. O'Connell, T. F. Deaton, and T. T. Saito, "Single- and Multiple-Shot

Laser-Damage Properties of Commercial Grade PPMA," Appl . Opt. , 23, 682 (1984).

[5] R. M. O'Connell, T. T. Saito, T. F. Deaton, K. E. Siegenthaler, J. J. McMally, and A.

A. Shaffer, "Laser Damage in Plastics at the Frank J. Seiler Research Laboratory,"

presented at the Fifteenth Annual Symposium on Optical Materials for high power

lasers. Boulder, Colorado, Movember 14-16, 1983 MBS Spec. Pub. (to be published).

[6] B. M. Emel'yanova, T. F. Ivanova, M. P. Votinov, V. f1. Ovchinnikov, V. D. Piterkin,

and Z. A. Smirnova, "Optical Strength of Copolymers of Methyl Methacrylate and Butyl

Acrylate," Sov. Tech. Phys. Lett. , 3 (7), 280 (1977).

[7] V. I. Bezrodnyy, 0. V. Przhonshaya, Ye. A. Tikhonov, M. V. Bondar, M. T. Shpak,

"Polymer Active and Passive Laser Elements Based on Organic Dyes," Kvantovaya

Electronika, 9 (12), 2455 (1982).

[8] R.M. O'Connell, A.B. Romberger, A. A. Shaffer, T.T. Saito, T.F. Deaton, and K.E. Siegenthaler, "Improved Laser-Damage-Resistant Polymethylmethacrylate," JOSA B , J_, 853. (1984)

99
[9] M. Mauck, "Knife-Edge Profiling of a 0-Switched Md:YAG Laser Bean and Waist," Applied

Optic s, 18, 599. (1977)

[10] B.W. Mullins and A.B. Ronberger, "Real Time Laser Damage Detection in Bulk Materials

by Strehl Intensities," Proc. Southv/est Conference P" Jj^Ptics: 1985 , R.S. McDov/ell

and J.B. Gerardo, editors, accepted for publication, (1985).

[n ] B.W. Mullins and B.A. Richert, "Strehl Ratio Measurements of Laser Damaged Plastics,"

Proc. 15th Annual Symposium on Laser Induced Damage in Optical Materials , A. Guenther

and H. Bennet, editors, accepted for publication.

[12] R.M. O'Connell, R.V. Ellis, A.B. Ronberger, T.F. Deaton, K.E. Siegenthaler, A. A. Shaffer, B.W. flullins, and T.T. Saito, "Laser Danage Studies of Several Methacrylate

Polymeric Materials," in Proceedings of the 16th Annual Synposium on Optical

Materials for High Pov/er Lasers, Boulder, CO, 15-17 Oct 84.

100
Manuscript Received

12-19-85

Improvement of The Bulk Laser Damage Threshold of

Potassium Dihydrogen Phosphate Crystals by Ultraviolet irradiation

A. Yokotani. T. Sasaki. K. Yoshida. T. Yamanaka and C. Yamanaka

Institute of Laser Engineering. Osaka University.

Suita. Osaka. 565. Japan.

Potassium dihydrogen phosphate (KDP)crystals were grown under the irradiation of

ultraviolet light. The bulk laser damage threshold was improved two to three times (15 - 20 J/cm ) compared to the case of crystals grown by conventional methods. Microbes such

as germs and bacteria are frequently generated in the KDP solution with the usual growth

method, causing a low damage threshold. A main reason of the low damage threshold is due

to the organic materials such as microbes or their carcass which are incorporated m the

growing KDP crystals.

Key words : potassium dihydrogen phosphate; crystal: harmonic generation; high power

laser; laser fusion; damage threshold; ultraviolet light: microbes: germs: bacteria:

pasteurization; dissociation.

1. Introduction

^Potassium dihydrogen phosphate (KDP) crystals which can withstand the irradiation over 10 J/cm with a 1 ns pulse at 1.053 jam laser wavelength are necessary for the harmonic generation of

fusion lasers. Typically as-grown crystals have the bulk laser damage threshold of 6-9 J/cm .[1.21

Swain showed that the baking and pulsed laser irradiation of KDP crystals improved the laser

damage threshold without describing the reason. [31

The KDP crystals are usually grown from the aqueous solution at the temperature range between

60°C and 20°C. Microbes are sometimes observed in the KDP solution during the long growth term in spite of stirring of the solution. If these microbes or their carcass are incorporated into the

growing crystals, the damage threshold will show the lower values than the intrinsic value,

because the damage threshold of the organic material is much lower compared to the inorganic

material. To prevent the creation of the microbes in the KDP solution we have grown KDP crystals

under the irradiation of ultraviolet ( UV ) light.

The KDP crystals which have been growiuby this method showed the increase of the damage

threshold two to three times ( 15-20 J/cm ). We have also performed the experiment of the

effect of the ultraviolet irradiation on the bulk laser damage threshold of KDP crystals after

growth.

2. Damage threshold of KDP crystals grown under the UV Irradiation

2.2. Crystal growth unit

Figure 1 shows the crystal growth unit. The inner vessel was made of glass which was durable

against the UV radiation. Low pressure mercury lamps with Suprasil quartz tubes of which input

power were 10 and 20 W were used to pasteurize the KDP solution. The mother liquor was 9 liters

and the temperature falling method was adopted to grow crystals. The starting temperature was 45

C. The solution was filtered with 0.2 urn membrane before starting of crystal growth. Z-cut seed

crystals of which cross section was 5cm x 5cm were used. The growth rate of Z-axis was 3-8

mm/day. The technique of crystal habit control by metal impurities such as Cr and Al was

used. [4] The growth term was approximately one month.

101
2.2. Investigation of microbes in KDP solution

The microbes in KDP solution were studied by using Water Sampler from Millipore Limited ( SPC

Total-Count Sampler ). This is a standard method for total bacteria count in water. In the case

of solution unpasteurized by the UV lamps, a number of colonies of gel-like germs appeared on the

sampler after one or two days of incubation. Figure 2 illustrates a typical photograph of such

germs. The gathering of white strings are certain kind of mold which generated in the air

atmosphere from the gel-like germs.

We could not observe such germs from the solution pasteurized with the UV lamps even after

four or five days of incubation.

2.3. Laser damage test facility

Figure 3 shows the experimental setup of glass laser damage test facility. The measurement of

the bulk laser damage threshold was performed using a Q-switched YLF laser (1.053 jum) of

transverse and longitudinal single mode with a 1 nsec pulse. The pulse was focused into Z-cut KDP samples by a lens of which focus length was 3.5 cm. The surfaces of KDP crystals were finished

with combination of diamond turning and wet polishing with 0.05 jjm alluminun oxide powder. The

damage was observed by eyes. A He-Ne laser was also used to find the small damage spots by

utilizing the scattering from the damage spots. The focus spots was moved shot by shot.

2.4. Results of damage test

The results of the damage threshold measurement are shown in figure 4. The abscissa is damage

threshold and the ordinate shows number of samples. The empty squares illustrate the samples

grown without the UV irradiation. The shaded squares shows the samples with the UV irradiatioi).

In the case without the UV irradiation, most samples showed the damage thresholds of 6-10 J/cm".

whereas^for the samples with the UV irradiation showed considerably higher damage thresholds ( 10- 22 J/cm" ). There is a tendency that larger the intensity of UV irradiation is. higher the damage

threshold becomes.

In a few cases without the UV irradiation, very high damage thresholds ( 20 J/cm^ ) were also

observed. We consider that one reason of the wide spread of the damage thresholds may be due to the differences of the initial conditions of the KDP powders. We did not investigate the quantity

of microbes or other organic impurities in the KDP powder before dissolving in pure water.

3. Effect of the UV irradiation on KDP crystals after growth

We have investigated the effect of the UV irr-adiation on KDP crystals after growth. Lowrence

Liverriiore Laboratory reported that the repetitive Xenon flashes were not effective. [21

We used the UV light from continuous high pressure Xenon lamp of which input power was 500 W. The experimental setup is shown in figure 5. The KDP sample whose dimension was 1.5 cm x 1.5 ciri x

4 cm was set at the distance of 5 cm from the Xenon lamp. This sample was cut from the crystal

grown at the growth rate of 8 mm/day with addition of Cr and Al ions to the solution. The bulk

damage threshold were measured as a function of the distance from the UV irradiated surface, which

IS illustrated in figure 6. Exposure time of the Xe lamp was 3 hours. The damage threshold

decreased almost exponentially with the distance. The temperature of the sample became almost 50**C due to the irradiation of Xe lamp but there was no temperature distribution in the sample.

Therefore we can say that the increase of the damage threshold was induced mainly by the UV light

and not by the thermal effect. Figure 7 shows the damage threshold as a function of the UV exposure tune. The data were taken at a distance of 5mm from the UV irradiated surface.

Although we tried to measure the difference of the absorption spectra of KDP crystals grown

with arid without the UV lamps, it was too small to measure. Instead, we have measured the change

of the absorption spectra of a KDP solution which was kept for a long term ( approximately one

month ) at room temperature and had much microbes. The results are shown in Figure 8. A continuous high pressure Xe lamp whose input power was 500 W again used for the UV irradiation.

The tratismi ttance increased only by filteration of the solution with 0.22 pm membrane and more

102
increased by the further UV irradiation. The germs and bacteria generally have the absorption in UV regions! 3000 A -2000 A ). So it can be said that a dissociation in these organic material

induced due to the UV light and the absorption decreased. Although the quantity of these organic

material in KDP crystal is fewer than this KDP solution, the same dissociation may be induced in KDP crystal by the UV irradiation and as a result the damage threshold may increase.

We could not obtain this improvement of the damage threshold in every grown crystal. The

samples from other runs, at most 20 % improvement or not at all. At this moment we don't know the reason.

4. Conclusions

Two types of experiments were performed to improve the bulk laser damage threshold of KDP

crystals. One was to grow KDP crystals under the UV irradiation. The other was to irradiate KDP

crystals which had finished growing by the UV light. The present works are summerized as follows;

1) The damage threshold of the KDP crystal improved two to three times (15-22 J/cm'^) by growing

crystals under the irradiation of the UV light compared to the case of the crystals grown by

conventional methods,

2) A main reason of the low bulk laser damage thresholds of as-grown KDP crystal is due to the

incorporation of organic materials such as germs and their carcass which were generated in the

mother solution during the growth.

3) In some cases, the UV irradiation on KDP crystal after growth was effective. In this case, the

damage threshold decreased almost exponentially with the distance from the UV irradiated surface.

This effect was induced by the dissociation of the UV light and not by the heat.

4) Enough attention must be paid for the KDP crystals growth not to incorporate the organic

materials such as germs, bacteria and their carcass in the KDP solution.

This technique can be adopted for other crystal growth from solution for high power lasers.

5. References

[11 Swain. J. E.: Stokowski , S. E. ; MiUam. D.; Rainer, F. Improving the bulk laser damage

resistance of potassium dihydrogen phosphate crystals by pulsed laser irradiation. Appl.

Phys. Lett. 40; 1982 February. 350 p.

[21 Newkirk, H.; Swain, J. E.; Stokowski. S. E.; Millam. D. X-ray topography of lasr-induced

damage in potassium dihydrogen phosphate crystals. J. crystal growth 65; 1983. 651 p.

[3] Swain, J. E.; Stokowski. S. E.; Millam. D.; Kennedy. G. C. The effect of baking and pulsed

laser irradiation on the bulk laser damage threshold of potassium dihydrogen phosphate

crystals. Appl. Phys. Lett. 41; 1982 July. 12 p.

[4] Yokotani, A.; Koide, H.; Yamamuro, K.; Sasaki. T.; Yamanaka. T.; Yamanaka. C. Solution growth

of large KDP crystals by rotationg fluid method. Technol. Repts. Osaka University 33; 1983

October. 301 p.

103
UV lamp with suprasil tube

Temperature

sensor Electric drive

Stirrer

Outer r~l vessel

Heater

Temperature

sensor

Water

Inner vessel

(Growth Chamber)

KDP

Crystal

Figure 1. Schematic diagram of crystal growth unit with the UV lamp.

(b) 1 mm

Figure 2. Typical colony of the gel-like germs on the water sampler after two days of incubation

of KDP solution without pasteurization by the UV light. (a) with optical microscope

(b) with scanning electron microscope.

104
Single-Mode Q-sw.

YLF OSC Selector

A=1.053^101

Pulse width ins

X/2 Plate

Pol. iw

Monitor

AMP II

15'/*

AMP I

Lens

f=3.5cm

Sample

X-Y Stage

Figure 3. Experimental setup of the glass laser damage test facility.

(0

"5. E

(0 0)

Grown without UV Irradiation

S^Jl Grown with UV Irradiation 10W

LTIj Grown with UV Irradiation 20W

mm

> ——1- 10 20 30

.2 Damage Threshold ( j/cm )

Figure 4. Histogram of the bulk laser damage threshold measured for KDP crystals which were grown

in the solution with and without the UV irradiation (laser: ins, 1.053 )m)

.

105
KDP Crystal

500 W

5cm 4cm

2^.5cm

1.5cm

Figure 5.

High Pressure

Xe lamp

Experimental setup for the UV irradiation to KDP crystal,

whose input power was 500W was used.

A high pressure Xenon lamp

Figure 6 Changes of damage threshold as a function of the distance from the UV irradiated

surface. Exposure time of the Xe lamp was 3 hours.

106
(A

a.E

CO

As-grown

mm UV-Irradlated

after growth

I I 10m

3H 6H

1H 3H 1H

10 20

Damage Threshold (j/cm^)

Figure 7. Damage threshold of KDP crystals which was irradiated by the UV light after the growth.

The data indicated here include the results from crystals as-grown, irradiated 10 min..

1 hour. 3 hours and 6 hours.

100

o

c

(0E

(AC

<0

50

Solution left -as- it- is

0.22pm filtered

UV irradiated 30min.

UV irradiated Ihr

r -I—I i_ X

0.2 0.3 0.4

Wavelength (pm)

Figure 8. Changes of absorption spectra of KDP solution. Curves include the

solution -as-it-is. a solution filtered with a 0.22 jam membrane,

irradiated for 30 min. vad. a solution UV-irradiated for one hour.

results irom a

a solution UV107
Manuscript Reaeived

6-27-86 SAE

Laser-Induced Damage in Doped and Undoped Fluorozirconate Glass

Nastaran Mansour, Shekhar Guha, and M. J. Soileau

Center for Applied Quantum Electronics

Department of Physics

North Texas State University

Denton, Texas 76203

Bernard Bendow

BDM Corporation

1801 Randolph Road, SE Albuquerque, NM 87106

and

Diane Martin

Air Force Weapons Laboratory

Kirtland AFB, NM

Results of laser-induced damage studies in doped and undoped fluorozirconate (FZ)

glass in the system of ZrF4-BaF2-LaF3-Al F3 are reported. Measurements were made at 1.06 and 0.53 ym for pulsewidths of 45 psec and 20 nsec (FWHM), respectively. The breakdown

thresholds are significantly reduced for material doped with Or, Mn, Ni , and Nd. The

decrease in breakdown thresholds at the fundamental wavelength (x ) for the doped material was found to be roughly proportional to the linear absorption of the specimen at the

second harmonic {x/2). This suggests that two-photon absorption (2PA) is instrumental in initiating damage in the doped specimens.

Key words: avalanche breakdown; fluorozirconate glasses: laser-induced damage;

nanosecohd pulses, picosecond pulses; two-photon absorption.

1. Introduction

Heavy-metal fluoride (HMF) glasses [1-3] have attracted considerable interest in recent years

as potential mul ti spectral materials for a broad range of components such as laser windows, IR domes, laser hosts, lenses, prisms, filters, and optical fibers. While many HMF glasses have been

synthesized, probably the most attention to date has been directed towards three types of HMF

glasses, namely f 1 uorozi rconate-type (FZ), rare-earth (RE), and transition-metal (TM) fluoride

glasses. FZ-type glasses are composed primarily of ZrF4 or HfF^, in combination with BaF2 and a variety of tertiary components such as LaF3, ThF4, and alkali-fluorides. RE glasses [4,5], which are usually based on ThF^ and BaF2, in combination with ZnF2, YbF3, YF3, AIF3 and/or LiFo, offer

extended IR transparency [6] and improved chemical durability relative to FZ glasses. Another less

studied group of glasses are the TM fluorides [7,8], with typical compositions consisting of

combinations of the fluorides of bivalent metals such as Mn, Co, Ni , or Zn; trivalent metals such as Cr, Ga, or In; and other constituents such as PbF2 and AIF3. From a practical standpoint, the

fabrication technology for FZ glasses is the most advanced of these, and some fairly large

(> 20 cm) size components with good optical quality have been produced. FZ-type glasses can be

continuously transparent from about 0.3 \im in the near-UV (fig. 1) to about 7 ym in the mid-IR

(fig. 4). They are readily doped with a variety of RE and TM ions for active applications.

The objective of this investigation is to obtain information on laser-induced damage in

fluorozirconate glasses (FZ). Measurements were conducted using picosecond pulses at 1.06 ym and

nanosecond pulses at 0.53 pm. To date, there have been no reports in the literature on these

measurements. FZ glasses were chosen for this study because of their good optical quality and i their technological importance.

j

! For the present study, doped and undoped FZ glasses with the compositions indicated in table 1

| were acquired from Le Verre Fluore, St. Erblon, France. These samples were prepared by standard

melting techniques starting from oxides and/or fluoride constituents [2]. Polished samples in the

form of discs (diameter=2.5 cm and thickness=3.5 mm) were used in the experiments.

108 '
2. Experiment

The laser source used in the picosecond regime was a passively mode-locked, microprocessorcontrolled, neodymium:yttrium aluminum garnet (Nd:YAG) oscillator-amplifier laser system operating

at 1.06 ym. A single pulse of measured Gaussian spatial 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 [9]. The ratio was calibrated by measuring the pulsewidth using an autocorrelation scan. For the nanosecond pulses the laser source was an actively Q-switched Nd:YAG laser system operating

at fundamental wavelength (1.06 ym) with an oscillator and one amplifier. Single pulses of

measured Gaussian spatial profile were produced by the oscillator and amplified by a single pass through the amplifier. A KDP* crystal was used with the Nd:YAG laser to produce pulses at 0.53 ym. The crystal was temperature controlled for long-term energy stability at the second harmonic.

Residual 1.06 ym radiation was eliminated by reflection off three dichroic mirrors. The temporal

pulsewidth was approximately 20 nsec (FWHM) at 0.53 ym. The width of each pulse was monitored by a PIN photodiode detector and fast Tektronix storage oscilloscope (Model 7834).

The experimental setup at 1.06 ym is shown in figure 5. At 0.53 ym the experiment was performed in a similar setup as given elsewhere [10]. The spatial beam spot size (of the unfocused

beam) was determined by pinhole scans. A 40 mm focal length lens, designed for minimum spherical

aberration, was used to focus the light into the bulk material. 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 to a Gentec (ED-100) pyroelectric energy monitor. The incident beam polarization on the

sample was changed from linear to circular by adding a quarter-wave plate in front of the focusing

lens. Each site was irradiated only once, and damage was determined by the observation of

scattered light from a coaxial HeNe laser as viewed through a lOx microscope.

3. Results and Discussion

Tables 2 and 3 summarize the results of measurements of the laser-induced breakdown threshold

for fluorozirconate (FZ) glasses, undoped and doped with Cr, Mn, Ni , and Nd. The doped and undoped

FZ glasses were studied with linearly and circularly polarized light at 1.06 ym. The uncertainties

listed in the tables of data are relative error obtained by the method used in reference 11. All the data presented in tables 1 and 2 were taken using 45 psec (FWHM) pulses and a calculated 6.2 ym

focused spot size (HWl/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 table of data, the breakdown

fields decrease significantly for all the doped FZ glasses. However, the most reduction in laserinduced breakdown at 1.06 ym is observed in the FZ samples B, C, and D (fig. 6) which are doped

with the same molar percent (table 1) of trivalent metal Cr. In these three samples (B, C, and D) the breakdown field decreases roughly by a factor of 2 compared to undoped FZ sample A. From this, we conclude that doping with Cr significantly reduces the optical breakdown of FZ glasses at

1.06 ym.

We carried out the same measurements for circularly as well as linearly polarized light in order to confirm the absence of self-focusing in these measurements. The breakdown field for

linearly and circularly polarized light are displayed in figure 7. As clearly represented by bar

graphs, the breakdown field remains unchanged (to within ±5%) for undoped and doped FZ glasses.

Thus, we conclude that sel f -f ocusi ng was not a major contributor to the observed laser-induced

damage [10].

In the nanosecond regime, results of the laser induced breakdown measurements at 0.53 ym for

the undoped and doped FZ glasses are given in table 4. For all data summarized in this table (4) we used 20 nanosecond (FWHM) pulses and a calculated focused spot size equal to 2.7 ym (HWl/e'^M in irradiance). As shown in figure 8 the optical breakdown thresholds decrease for doped FZ glasses

with Cr, Mn, Ni , and Nd. At this wavelength (0.53 ym) the decrease in thresholds was larger in

samples doped with Cr, Mn, and Nd.

Figures 1, 2, and 3 are the linear spectra for the undoped and doped FZ glasses for wavelengths ranging between 0.2 and 1.2 ym. These linear spectra were acquired using a Perkin-Elmer

spectrophotometer (Model 330). The existence of impurity states within the energy gap for FZ glasses doped with Cr, Mn, Ni , and Nd can result in two-photon absorption which in turn can

initiate laser induced breakdown in these materials. Our experimental results were found to be in good agreement with a two-photon assisted electron avalanche breakdown model as given in reference

10. In this model it is assumed that laser-induced breakdown occurs when the density of free

carriers generated by a combination of two-photon and avalanche processes reaches some critical

109
value, Ng. In the avalanche process the buildup of free carrier density is given by

n nt

where ng = net ionization rate

N = initial free carrier

0

The net ionization rate for laser-induced damage is a function of the breakdown field, Eg. Some

authors have proposed models [13-15] which predict an ionization rate given by:

^B ^B

Then eq. (1) can be written as

Where "a" for our experiment is a constant related to many factors such as: frequency of light,

pulsewidth of laser light, the momentum transfer collision time, effective mass and electron

charge. The model proposed in reference 10 assumed that the initial free carriers are generated by

two-photon absorption from impurity states within the bandgap. Therefore, Nq can be written as:

N ^ n 0

where n = density of two-photon allowed impurity states.

Taking the natural log of eq. (2) yields:

2

aEp, = -In n + const (3)

The final assumption is that the density of two-photon allowed states at the fundamental wavelength

(a) is proportional to the linear absorption at the second harmonic {x/2). For that assumption

eq. (3) gi ves :

• In (al)

at A at x/2

(4)

where a= linear absorption coefficient at the second harmonic wavelength

1 = length of sample

This analysis [eq. (4)] predicts that the breakdown field at 1.06 y m should be proportional to the natural log of the absorption at 0.53 pm. Figure 9 is a plot of the square of the breakdown

field at 1.06 ym [(MV/cm)2] versus the negative natural logarithm of the linear absorption at 0.53 ym for the undoped and doped FZ glasses. A similar plot, for the breakdown field squared at 0.532 ym versus the -In of the linear absorption at 0.266 ym is given in figure 10. The linear

absorption was calculated from the linear spectra of these material (figs. 1, 2, and 3) using the

assumption that the change in index of refraction is negligible in the wavelength range between

0.532 and 0.266 ym. The agreement with the prediction of eq. (4) is quite good considering the

relative simplicity of the model and the assumptions made. Note that we measured the laser induced

damage for the undoped and doped FZ glasses at 1.06 ym in the picosecond regime; however, the

110
measurements at 0.53 ym were performed in the nanosecond regime. In spite of the complexity

associated with laser-induced damage mechanisms for dielectric materials, our experimental results

are quite consistent with two-photon-assisted electron avalanche processes.

4. Summary

Laser-induced breakdown was studied for undoped and doped fl uorozirconate glasses at 1.06 and

0.53 ym. Samples doped with Cr, Mn, Ni , and Nd were investigated. Laser-induced damage thresholds

decrease significantly for doped FZ glasses. The breakdown threshold field for sample A is approximately 25% lower than that of SiOp in similar conditions for 1064 nm. However, all samples have

substantially lower threshold fields (on the order of a factor of 5 lower) at 532 nm [9,15]. The

reduction of the damage thresholds at 1.06 and 0.53 ym was found to be related to the increase in linear absorption at the second harmonic of the light used to produce the damage. The experimental

results are consistent with a model that assumes that damage is caused by two-photon absorption

initiated avalanche breakdown. The two-photon absorption is assumed to be associated with states within the bandgap which are due to the added dopants.

The authors from North Texas State University acknowledge support of the Office of Naval

Research, the National Science Foundation (ECS #8310625), and the NTSU Faculty Research Fund.

Research of B. Bendow was supported by the U.S. Air Force Weapons Laboratory under Contract

#F29601-81-C-0012.

5. References

[I] Tran, D.; Sigel , G.; Bendow, B. J. Lightwave Technol. LT-2( 5) : 566-586 ; 1984.

[2] Drexhage, M. in Glass IV, Tomozawa, M.; Doremus, R., eds. New York; Academic Press; 1985.

[3] Digest of Extended Abstracts of the First International Symposium on Halide and Other Non- Oxide Glasses, Churchill College, Cambridge, U.K., 1982; of the Second International Symposium

on Halide Glasses, RPI, Troy, NY, 1983; and of the Third International Symposium on Halide

Glasses, Rennes, France, 1985.

[4] Poulain, M.; Poulain, M.; Matecki , M. Materials Research Bulletin 16(5): 555-564; 1981.

[5] Drexhage, M.G.; El-Bayoumi, 0.; Moynihan, C. T.; Bruce, A. J.; Chung, K.-H.; Gavin, D. L.; Lorentz, T. J. J. Amer. Ceram. Soc. 65( 10) :C 168-C 171 ; 1982.

[6] Drexhage, M.G.; Bendow, B.; Brown, R.N.; Banerjee, P.K.; Lipson, H.G.; Fonteneau, G.; Lucas,

J.; Moynihan, C.T. Rapid Communication. Appl. Opt. 21(6):971-972; 1982.

[7] Miranday,'j.P.; Jacoboni, C; de Pape, R. Rev. Chim. Min. 16(4):277-282; 1979.

[8] Miranday, J. P.; Jacoboni, C; de Pape, R. J. Non-Cryst. Solids 43:393-401; 1981.

[9] 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.

[10] Soileau, M. J.; Mansour, N.; Canto, E.; Griscom, D. L. "Effect of radiation induced defects

on laser-induced breakdown in Si02." To be published in the Proceedings of the 1985 MRS

Meeti ng

.

[II] Porteus, J. 0.; Jernigan, J. L.; Faith, W. N. "Mul ti threshol d measurement and analysis of

pulsed laser damage on optical surface." Natl. Bur. Stand. (U.S.) Spec. Pub. 568; 507-515;

1979.

[12] 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.

[13] Yabl onovitch, E.; Bloembergen, N. "Avalanche ionization and the limiting diameter of filaments induced by light pulses in transparent media." Phys. Rev. Lett. 29( 14): 907-910; 1972.

[14] Sparks, M.; Holstein, T.; Warren, R.; Mills, D. L.; Maradudin, A. A.; Sham, E. Loh, Jr.; King,

F. "Theory of avalanche breakdown in solids." Natl. Bur. Stand. (U.S.) Spec. Pub. 568; 467-

111
478; 1979.

[15] Soileau, M. J.; Williams, W. E.; Van Stryland, E. W.; Boggess, T. F.; Smirl, A. L. "Picosecond damage studies at 0.5 and 1 ym." Opt. Eng. 22(4):424-430; 1983.

0.266 0.532 1.064

O.ljLtm

Figure 1. Linear transmission spectra for FZ glass samples A and B for a wavelength range between

0.2 to 1.2 ym.

112
0.266 0.532 1.064

Figure 2. Linear transmission spectra for FZ glass samples C and D for a wavelength range between

0.2 to 1.2 ym.

100 p

90 -

80 -

70 - zo

C/) 60 - COS

50

<

1— 40

< 1 Z

_J 30

20

10

0

-0.266

HA

0.532

Sample E Sample F

1.064

0.1 jum

Figure 3. Linear transmission spectra for FZ glass samples E and F for a wavelength range between

0.2 to 1.2 vm.

113
100

CO

CO

1

'~

10 12

Figure 4. Infrared transmission spectra of f 1 uorozi rconate glass (sample A). Similar infrared

spectra were acquired for the other FZ glass samples.

Nd : YAG

LASER SYSTEM

BS AMP

PC OSC

PULSE WIDTH

MONITOR

50 PSEC

y- V

ATTENUATOR

HeNe U

TEM00;

2mm

VIEWER

ENERGY

MONITOR

Figure 5. experimental setup used for the picosecond pulse 1.06 pm damage measurements.

114
'

I

1

A= 1.064yum

tp = 45 PSEC

A

B

C

D

E

F m 1 1-:-- T-':';'J 1 1 0.5 1.0 1.5

\V |TW/cm21

Figure 6. Experimentally determined breakdown irradiance (peak-on-axi s irradiance) for undoped

and doped FZ glasses at 1.06 ym for a laser pulsewidth of 45 ps.

1 1

1 K = 1.064 jum

tp = 45 PSEC

A

B

c

D

E

F

1 0.5 1.0 1.5

Ebc/Ebl

Figure 7. Experimentally determined ratio of breakdown electric field for circularly and linearly

polarized light, EgQ/EgL, for undoped and doped f 1 uorozi rconate glass samples (EgQ is the breakdown field for circularly polarized light and Eg^ is the breakdown field for

linearly polarized light). Note that EgQ - Eg-]. The lack of polarization dependence

indicates effects of sel f -f ocusi ng are negligible for the observed laser-induced

damage

.

115
«

T ' '- X=0.532/uni

tp= 20nsec

B

C

D

E U

F

1 tlil^ 10 20 30

InlGW/cm^l

Figure 8. Experimentally determined breakdown irradiance (peak-on-axi s irradiance) for undoped

and doped f 1 uorozi rconate glass samples at 0.53 ym for a lasesr pulsewidth of 20 ns.

3.

CO

E

u

500

400

300 -

200 -

CM CO 100 - V

X=1.064)um

tp=45 psec ^

1

-fn (af) at .532^m

Figure 9. Square of breakdown electric field (MV/cm)^ at 1.06 ym versus the negative natural

logarithm of absorption at 0.53 ym.

116
E

a.

CMn

CME

CM OO 9

1 1 1 *-

k= 0.532 jum ^

tp = 20 nsec

1.0 1.5 2

-fn (af) at .266/um

Figure 10. Square of breakdown electric field (MV/cm)2 at 0.532 pm versus the negative natural

logarithm of the linear absorption at 0.266 ym. Note that sample E has sharp absorption bands near to 0.532 pm (fig. 3).

Table 1. Compositions of f 1 uorozi rconate glass samples.

ZrF4 BaF2 LaFs AIF3 CrFs MnF2 NiF2 Ndf^

(molar %) (molar %) (molar %) (molar %) (molar %) (molar %) (molar %) (molar %)

A Clear 57 34.5 4.5 4

B Green 56.75 34.25 4.5 4 0.5

C Green 56.5 34 4,5 4 0.5 0.5

D Green 56.5 34 4.5 4 0.5 0.5

E Violet 55.75 33.75 4.5 4 1 1

F Yellow 56.75 34.25 4.5 4 0.5

117
Table 2. Laser-induced damage data for undoped and doped Fl uorozi rconate glass at 1.06 y m using

linearly polarized light. The laser pulsewidth was 45 ± 15 picosecond (FWHM).

'BL Ebl Pbl Fbl

Sample GW/cm2 MV/cm KW J/cm2

A 1418 + 173 18.68+1.14 847 ±103 68.02 ±8.31

B 384± 12 9.72 + 0.15 229 + 7 18.42 ±0.57

C 398 ± 45 9.90 + 0.56 238 ± 27 19.10±2.16

D 226 ± 48 7.47 ±0.79 135 ± 29 10.87 ±2.32

E 612± 75 12.28±0.76 366 ± 45 29.37 ±3.61

F 767 ±109 13.74 ±0.98 458 ± 65 36.80 ±5.25

'BL ~ breakdown irradiance (peak on-axis irradiance)

EgL = breakdown field PgL = breakdown power

FgL = breakdown fluence

Table 3. Laser-induced damage data for undoped and doped Fl uorozi rconate glass at 1.06pm using

circularly polarized light. The laser pulsewidth was 45 ± 15 picosecond (FWHM).

'bc Ebc Pbc •"bc Sample GW/cm2 MV/cm KW J/cm2

A 1611 ±271 19.92 ±1.67 962 ±162 77.28 ±13.00

B 416± 52 10.12±0.63 248 ± 31 19.96± 2.49 c 380 ± 32 9.67 ±0.41 227 ± 19 18.23± 1.54 D 269 ± 38 8.13 ±0.57 161 ± 22 12.89 ± 1.81 E 627 ± 43 12.43 ±0.42 375 ± 26 30.09 ± 2.06

F 786± 54 13.91 ±0.48 470 ± 32 37.73 ± 2.59

'bc breakdown irradiance (peak on-axis irradiance)

EgQ = breakdown field Pbc = breakdown power

FgQ = breakdown fluence

118
Table 4. Laser-induced damage data for undoped Fl uorozi rconate glass at 0.53 ym for a pulsewidth

of 20 nanosecond (FWHM).

D Eq Pr Fr

SAMPLE GW/cm2 MV/cm kw J/cm2

A 20.1 ± 3.40 2.26 ±0.19 2.33 ± 0.40 427 ± 72

B 13.4 ± 1.83 1.86 ±0.12 1.55 ±0.21 284 ± 39

C 10.6 ± 1.38 1.65 ±0.11 1.23 ± 0.16 225 ± 29

D 7.31 ±0.28 1.36 ± 0.03 0.85 ± 0.03 155 ± 6

E 9.60 ± 0.96 1 .57 ± 0.08 1.11 ±0.11 204 ± 21

F 18.1 ± 1.83 2.15 ±0.07 2.09 ± 0.21 384 ± 39

Ig = breakdown irradiance (peak on-axis irradiance)

Eg = breakdown field Pg = breakdown power

Fg = breakdown fluence

119
Manuscript received

3-31-86 SAE

Platinum Particles in the Ndrdoped Disks of Phosphate Glass

in the Nova Laser*

D. Milam, C. W. Hatcher and J. H. Campbell

Lawrence Livermore National Laboratory

P.O. Box 5508, L-490

Livermore, California 94550

The disks of Nd:doped phosphate glass in the amplifiers of the Nova laser contain platinum

particles with sizes ranging from < 5um (detection limit) to about lOOum, The particle

density varies from about 0.01 to 1.0 cm-3. These particles cause fractures when irradiated

at fluences > 2.5 J/cm^ delivered in 1-ns, 1064-nm pulses. Under repeated irradiation at 5-7 J/cm2, damage from small (< 5ijm) particles asymptotically approaches a limiting size, but

damage surrounding the larger particles grows steadily. The damage threshold fluence,

2.5 J/cm2, corresponds to operation of Nova at one-half the desired output for pulse

durations longer than 1 nsec. Operation at higher fluences causes accumulation of damage in

the output amplifiers and requires replacement of the disks in those amplifiers on an accelerated schedule.

Large-area damage tests have been used to characterize samples from recent melts made to reduce the number of platinum particles. The 500-J, 1-ns pulses available at the midpoint of

an arm were used to produce fluences of 7-10 J/cm^ over test areas 6-9 cm in diameter.

Similar irradiations of areas 25-30 cm in diameter were accomplished using 5-kJ, 1-ns pulses

available at the output of an arm. Preliminary results from these experiments indicate that a significant reduction of the inclusion density can be obtained through changes in the vendor's

glass melting conditions.

Key words: platinum inclusions, phosphate glass, borosilicate glass, large-area damage tests.

1. Introduction

Isolated bulk damage sites caused by laser irradiation of platinum inclusions have been

observed in the components of the Nova laser and its predecessor, Novette. These damages occurred

in amplifier disks made of Nd-doped phosphate laser glass and in lenses made of borosilicate

glass. This problem was first encountered in operation of Novette, a laser with two beam lines

(each arm with 10 kJ, 1 ns output), each identical to one of the ten beams of Nova.

The output stage of a beam line arm of either of these lasers contains groups of Ndrglass

amplifiers with apertures of 20.8, 31.5 or 46 cm (Fig. ^).^>^ Each group of amplifiers is

followed by a vacuum spatial filter. A laser pulse passing along the beam line is amplified at one beam diameter to the desired pulse energy, and then focussed into a spatial filter which

removes small-scale spatial ripple and enlarges the beam diameter to properly match that of the

next group of amplifiers. A spatial filter at the output of the 46-cm amplifiers provides a final cleaning of the beam and increases its diameter to 74 cm which is maintained over the final

path to the target chamber.

As a result of this series of amplifications and beam expansions, the variation of pulse

energy or beam fluence with position along the chain is an approximately sawtoothed function

(Fig. 2), with the largest fluence occurring at the output of the last 46-cm amplifier. During

generation of 10 kJ, 1-ns pulses, the average fluence over the effective area of the 46-cm beam

(1330 cm2) is 7.5 J/cm^. Because this large fluence exists at the input to the final spatial

filter, the fluence at the maxima of beam ripples may be more than 1.5 times the spatially

averaged fluence. The components in this region of high fluence are the output 46-cm amplifier

disks and the 46-cm input lens to the final filter. The lens is positioned normal to the beam

and experiences the full beam fluence. Note, however that the fluence is lower in the 46-cm

amplifier disks because they are positioned at Brewsters angle.

*Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore

National Laboratory under Contract No. W-7405-ENG-48.

120
Rotator Spatial filter

31.5 cm amps 4lQ/|Eai3Eini[w

4 5 6 7 8

Input to 20.8 amp

Spatial

filter

20.8 amps

Figure 1: Schematic diagram of the final 3 amplifier stages (20.8 cm, 31,5 cm and 46 cm)

and spatial filters that are a part of each one of the 10 beam lines on Nova.

Average

fluence

(J/cm^)

at normal

incidence

16

14

12

10

8

6

4

Calculated fluence 20 kJ; 5 ns

5 ns threshold

1 ns threshold

344 5 6 7 8|9 10 11 12

Amplifier number

46.0 cm

Spatial 31.5 cm Spatial

filter filter

Figure 2: Beam fluence at normal incidence as a function of position

along the final 3 amplifier stages of a Nova beam line.

121
Internal damage in a 46-cm spatial filter lens occurred during the initial full power pulses

generated by Novette. Several small volumes each containing an internal damage were cored from

the lens and broken to expose the centers of the damages. When these locations were examined by

scanning electron microscopy, each was found to contain a platinum particle.

Because the glass was purported to be platinum free, this failure was initially believed to be an isolated incident caused by an identified degradation of a platinum stirrer used in

production of the glass. However, replacement lenses made from glass purchased from two other

vendors also damaged, indicating that platinum was probably present in most of the available

homogeneous glass, all of which had been produced in platinum crucibles. The problem of damage

to borosilicate glass was circumvented by fabricating the heavily loaded lenses from high purity

fused silica. To date, bulk damage has not been detected in these silica lenses.

A more serious problem arose when bulk damage was observed in the phosphate glass amplifier

disks in Novette. This problem could not be solved by substitution of materials because all presently available laser glasses are melted in platinum. A group of chemists, physicists and

engineers was assembled to address the task of eliminating platinum. There were five major goals

of the group:

(1) measure the thresholds for damage to selected individual platinum inclusions and the

rate of growth of the damaged volume surrounding the inclusions,

(2) develop an accurate model for the damage caused by laser heating of Pt inclusions,

(3) assess the damage on Nova and determine a safe operating range during the period of

amplifier repair,

(4) develop a reliable quality assurance technique for inspecting large volumes of glass for

the presence of Pt inclusions, and

(5) work with the glass vendors to quantify the mechanisms for platinum introduction and

develop processing conditions to eliminate or minimize it.

The first four of these tasks are completed and significant progress has been made on item 5 to make us confident that Pt particles can be eliminated from the laser glass. The primary intent

of this paper is to review the currently operating status of Nova and progress on understanding

the Pt damage mechanism. The results from other aspects of the work will be briefly summarized

to provide necessary background information.

Platinum inclusion damage in laser and optical glasses has been a recurrent problem throughout

history of high power lasers. In particular, platinum damage in silicate laser glass has been

well known for many years (see, for example, the review in Ref. 3), In some laser systems, the

platinum inclusions were not a problem. Our previous high power laser, Shiva, used a Nd:silicate

glass and we observed no damage in that system. However, results from current studies of the

Nova glass indicate that Shiva operated just below the Pt damage threshold.

It was anticipated that use of a phosphate based laser glass would cause a reduction of the

problem with platinum inclusions, because the solubility of Pt is much greater in phosphate

glasses than in silicates. When phosphate glass was selected for use in Nova, five 15 cm x 15 cm

X 2 cm samples were tested with 1-ns, 1064-nm pulses at 12 J/cm^ using a 6-cm diameter beam

produced by an arm of the Argus laser. These experiments indicated that the glass might typically

contain one small (< 10 ym) inclusion in each 10 cm^. This estimate of inclusion density appears

to be correct for all but a few of Nova's disks. However, these qualification studies did not correctly predict the large sizes (up to 250 ym) of some of the inclusions that have been found

in glass supplied for Nova, or the growth of the damaged volume that would result from repeated

irradiation of those large inclusions.

2. Results from Platinum Inclusion Damage Studies

2.1 Experimental Studies of Individual Inclusions

Individual inclusions with major dimensions ranging from 4 to 75 um were tested with 1064-nm

pulses having durations of 1.3, 9 or 50 ns. Thresholds for initiation of damage were 2.5 ± .3,

4 ± .4 and 8 ± 1 J/cm^, respectively, at these three pulse durations. These thresholds scaled

with pulse duration (t) as t0.31 (Fig. 3) and, as expected for particles greater than one micron, the thresholds were independent of particle size. Damage surrounding small (< 5 um)

122
inclusions generally grew to stable dimensions of < 250 m, while damage surrounding larger

inclusions C> 5 ym) grew to dimensions of 400-1000 urn durinq irradiation by 5-10, 1-ns pulses

at 5-9 J/cm2.4

cvj lOOpE

u

o

10 r v

CDE

(0 o

0)u

c«

3

"1

1 I I I I I I T 1—I I I I l U

o

J I I I 1 1 II 10

Pulse length, ns

100

Theory (Pitts, 1985 Boulder

damage symposium)

Gonzales, R. & Milam, D.

(1985 Boulder damage

symposium)

Avizonis, P.V. & Farrington, T.

(Appl. Phys. Ltrs. 7, 1965)

Yamanaka, C. et al. (NBS

Special Publ. 356, 1971)

National Matls. Advisory

Board (publication NMAB-271,

1970)

Figure 3: Measured damage threshold due to Pt° inclusions for laser pulses of several

durations. The open circles represent the results from our study and the line

represents the threshold calculated by Pitts.

2.2 Modeling Inclusion Damage

The temperature of a laser-irradiated platinum particle was calculated using a numerical heat

transport code (Topaz^.^) that allowed heat conduction into the interior of the irradiated

particle and into the surrounding glass, and cooling of the particle by radiation. Literature

values were used for the absorption coefficient of platinum and for the heats of fusion and

vaporization. Temperature dependence of the absorption coefficient was not treated. The Pt vapor state was described using the equation of state employed in the LASEX laser fusion computer

code.^ Details of these calculations are given in the paper by Pitts. ^ , .

For pulses with duration of 9 or 50 ns, the calculated values of the fluences necessary to

heat the surface of the particle to the vaporization temperature of platinum agreed with

experimentally measured thresholds for initiation of damage. For 1-ns pulses, the calculated

vaporization fluence was about one half of the measured threshold (Fig. 3). Calculated

vaporization fluences were independent of particle size (for particles with dimensions > 1.0 un)

and varied with pulse duration as t^-S. while the discrepancy between calculated and measured

1-ns threshold has not been resolved, there is close agreement between the results of Pitt's

calculations^ and the experimental results obtained both at LLNL and in other studies (Fig.

3). The model calculation suggests that the damage surrounding a platinum particle occurs as a result of fracture driven by the pressure of Pt vaporized from the particle surface. Efforts are underway to model the growth of the damage during repeated irradiation.

2.3 Status of Nova

A schematic diagram of the components in an arm of fiova is aiven in Fig. 1. The 12 disk- amplifiers are identified by numerals. Figure 2 gives the spatially averaged fluences present in

each amplifier during generation of nominal 10-kJ, 1-ns and 20-kJ, 5-ns pulses. Fluence values

are those in a plane normal to the beam. Fluences inside a given disk are lower by a factor of

n~^ , where n is the refractive index of the glass (n = 1.53). Also shown are bands representing

the measured thresholds for damage to platinum inclusions induced by pulses with durations of

either 1 or 5 ns. The displayed thresholds are larger by a factor of 1.53 than the actual

measured thresholds (Fig. 3). Recall that the thresholds were measured in samples oriented

123
normal to the test beam. We assume that it v^ould be necessary to increase the beam fluence by a factor of n to damage inclusions inside the Brewster oriented disks.

At the time we began evaluating Nova, each of the ten arms had been used to produce 1-ns

pulses with energies in excess of 8 kJ. If the experimental data (Fig. 3) correctly represented

the damage thresholds for the glass in Nova, damage should have occurred in each arm in the last

three 46-cm amplifiers (numbers 10, 11, 12) and the last 31 .5-cm amplifier. Because the local

fluence values exceed spatially averaged fluences, some damage might have occurred in the input

46-cm amplifier (#9) and the fourth 31. 5-cm amplifier (#7). The 20.8-cm amplifiers and the first

three 31. 5-cm amplifiers should not have damaged.

Several disks were removed from Nova and inspected. For each of these disks a map was constructed giving the locations and approximate sizes of the damage sites. Similar inspections were made of disks removed from the two-arm laser, Novette, v;hen it was disassembled and moved

into place as the final two arms of Nova. Almost all of the 46-cm disks and many 31-cm disks

contained damage. The volume density of damages ranged from about 1 to 0.01 cm"-^, with typical

densities being 0.05 to 0.1 cm"^. None of the 20.8-cm disks were damaged. The sizes of the

damage sites depended on the fluences that had been experienced by the disks. This is illustrated

in Fig. 4 which gives (for four 46-cm disks) normalized plots of the fraction of sites exceeding

a given dimension and in Fig. 5 which gives the median size of damages in seven disks as a function of fluence.

The size of the damages is the parameter that most strongly effects the operation of Nova. Codes that model Nova operation indicate that the beam quality is not seriously degraded if the

components contain a low volume density of beam obscurations with sizes < 250 ym. Larger

obscurations produce beam ripple that can damage optical elements lying downstream, and very

large (» 1 mm) obscurations block a significant fraction of the beam. Data indicate that

disks used at fluences near threshold contained damage smaller than the 250 m limit. In disks

subjected to fluences well above the damage threshold, the median size was greater than 250 \m,

and the largest damages had diameters > 10 mm (Fig. 4)

.

200 400 600 800

Damage size (/Ltm) in maximum dimension

Figure 4: Normalized plot of fraction of damage sites (in Nova 46cm disks) that exceed

the dimension shown on the abscissa. The disk amplifier positions are indicated by the

numbers 9, 10, 11 and 12 corresponding to the locations shown in Fig. 1.

124
Figure 5: Median damage size found upon inspection of Mova disks vs. the

average fluence at that disk location.

The correlation between the level of damage found in several of the Mova disks and the

experimental data that we have recently generated can be used to predict which disks are likely

to be damaged by a particular use of the laser. For safe operation at 1-ns and 10 kJ per beam

(100 kJ total output), replacement of the eight 46-cm disks and of at least eight 31.5 cm disks

in each arm is required. If each arm is to safely produce 20 kJ, 5-ns pulses, replacement of

almost all 46 and 31.5 disks and the final 20,8-cm disks will be required. Until replacement

glass is available, the laser can be safely operated at 5 kJ/arm (50 kJ total) with the existing

glass. Currently we have about 2 years of experiments that require less than the 50 kJ output.

This is sufficient time for us to replace the current glass with platinum free glass.

2.4 Inspection of Glass

Certification that Pt" inclusion-free glass has been obtained will require an improved

inspection procedure. Previously, the most effective technique for finding a low volume density

of inclusions in a large volume of glass was to illuminate the sample from the back or through

its edges and visually search for scattering centers. Even after the inclusion problem had

become apparent and the visual inspections were being executed with great care, less than 50% of

the inclusions were identified in most samples. Detailed microscopic examinations required too much time and were sometimes no more successful. Automated light scattering experiments are probably capable of detecting inclusions, but do not readily distinguish between inclusions and

other bulk defects such as bubbles or surface defects. The only procedure known to be completely

successful is a destructive damage test. Irradiating an inclusion with 5-10 pulses having

fluences well above threshold produces a damaged volume that is readily visible.

125
To guide the initial experiments aimed at improving glass, two large-area damage experiments were attached to the Nova laser. One was positioned at the midpoint of an arm and allowed

irradiation at fluences of 7-10 J/cm2 of areas with diameters of 6-9 cm. The second was at the

output of an arm and allowed similar experiments over areas 25-30 cm in diameter. These

experiments have been very successful in determining the damage susceptibility of large glass

samples, but use of Nova as a damage tester is highly impractical. To minimize shot time required

from Nova, a separate system was built to allow rapid scanning of a glass sample with the beam of

a commercially available 20-Hz Q-switched Nd:YAG laser. The beam from the laser is slightly

focussed so that the fluence at the sample is about twice the damage threshold for the 8-ns

pulses. Successful use of this system to locate platinum inclusions in glass is described in a separate paper by Marion et. al.°

2.5 Progress on Improved Glass Melting

Staff members at LLNL are working with scientists at the two Nova laser glass vendors (Hoya

Optic, Inc. and Schott Glass Technologies, Inc.) to eliminate the Pt inclusion problem in

phosphate laser glass. Details of glass manufacture at both these facilities are proprietary, so progress can only be discussed in general terms.

In brief, both companies have sought to control the Pt problem by fa) reducing or eliminating

possible outside sources of Pt contamination and (b) adjusting process conditions to put or keep

all Pt in solution, i.e. as ionic Pt (which does not damage) rather than as metallic inclusions

(v/hich do damage).

Considerable progress has been made in both areas. Recently we have tested 7 large pieces of

glass having a total volume of about 17-185, and found an average of less than 0.20 inclusions

per liter of glass. (Four pieces of glass were completely Pt° free).^ These new pieces were

prepared under conditions designed to convert any metallic Pt° to its ionic form. The samples were tested using both Nova and our new QA inspection tool at 2-4 times the damage fluence. A series of further process changes are in progress that should reduce the Pt inclusion level to less than 0.1 per liter^ thus meeting our design specifications. For comparison, the current

Nova disks have Pt° inclusion concentrations of between 10 to 1000 per liter, thus the current

new glass represents a 50 to 5000 fold improvement.

We have begun procurement procedures necessary for the replacement of the Nova glass. Our

present plans show construction of production melting facilities beginning in January of 1986 and

being completed by about May. Production melting should begin in May or June, and the first

replacement glass is expected to arrive at LLNL in Sept. or Oct. The complete replacement

process will take about 18 to 24 months.

The large-area damage test facilities at Nova were designed and installed by the mechanical

engineering staff of the fusion laser program. We are especially grateful to Brigitte Gim and

Daniel Clifton for their major roles in that installation. Large-area experiments required the

assistance of the Nova operations crews (shift leaders are Glenn Hermes, Gary Ross, and Timothy

Wieland), and of two individuals, Janice Lawson and Carolyn Weinzapfel, who conducted some of the

large-area testing. Inspections of disks removed from lasers for evaluation were done by Gary

Edwards and Donald Gemmell of the Nova clean room staff. The authors also benefited from the

council of Stanley Stokowski , Perry Wallerstein, Ian Thomas, Howard Patton, John Marion, John

Pitts and Frederick Ryerson, who are members of the group working on this problem.

3. References

[1] K. Manes and W. W. Simmons, "Statistical Optics Applied to High-Power Glass Lasers." J. Opt. Soc. Am. A 2, 1985, pg. 528.

[2] J. T, Hunt and R. J. Foley "Laser Systems and Operations" in Laser Program Annual Rept,

UCRL-50021-84, LLNL, (1985).

[3] "Fundamentals of Damage in Laser Glass" National Materials Advisory Board, Division of

Engineering, National Research Council; report NMAB-271 , Washington, P. C. July 1970.

126
[4] Ray Gonzales and Dave Milam "Evolution During Multiple-shot Irradiation of Damage Surrounding

Isolated Pt Inclusions in Phosphate Laser Glass." 17th Boulder Damage Symposium, Oct. 1985.

[5] A. B. Shapiro "Topaz - A finite element heat conduction code for analyzing 2-0 solids"

UCID-20045, LLNL, March, 1984.

[6] J. H. Pitts "Modeling laser damage caused by platinum inclusions in laser glass", 17th

Boulder Damage Symposium, Oct. 1985, Boulder, CO.

[7] C. Orth, Lawrence Livermore National Laboratory, Livermore, CA, Private Communication, July

1985.

[8] J. E. Marion, P. H. Chaffee, G. J. Greiner and J. H. Campbell "A versatile laser glass

inspection and damage testing facility", 17th Boulder Damage Symposium, Oct. 1985, Boulder,

CO.

[9] C. W. Hatcher, LLNL, 1985, unpublished results.

DISCI.AIMKR

This document »as prepared as an accouni of Hork sponsored b\ an agenc> of

(he I nited States (Joiernment. Neither the I nited Stales (Joiernment nor the

I nl»ersity of ( aiifornia nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the ac- curacy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use Hould not infringe priiately owned

rights. Reference herein to any specific commercial products, process, or ser>ice

by trade name, trademark, manufacturer, or otherwise, does not necessarily

constitute or imply its endorsement, recommendation, or fatoring by the I nited

Stales Go>ernment or the I niiersily of C aiifornia. The views and opinions of

authors expressed herein do not necessarily stale or reflect those of the I nited

States Government thereof, and shall not be used for advertising or product en- dorsement purposes.

127
Manuscript received

3.^32-86 SAR

Evolution During Multiple-Shot Irradiation of Damage Surrounding

Isolated Platinum Inclusions in Phosphate Laser Glasses*

R. P. Gonzales and D. Milam

Lawrence Livermore National Laboratory

University of California

P.O. Box 5508, L-490

Livermore, California 94550

Platinum inclusions in samples of laser glass were placed in the center of a 2-mm-diameter

beam and repeatedly irradiated with 1064-nm laser pulses having durations of 1.3, 9 or

50 ns. The initial major dimensions of the particles ranged from 4-75 ym. The thresholds

for initiation of damage were 2.5 ± 0.3, 4 ± 0.4 and 8 ± 0.8 J/cm^, respectively, for

the three pulse durations used in the tests. Thresholds were independent of particle size.

Three particles with initial dimensions of 4, 5 and 8 ym caused damage smaller than

250 um. One 6 um particle and all particles larger than 10 gm caused larger damage.

Key words: platinum inclusions, bulk laser damage, phosphate laser glass

1. Introduction

Although it is desirable that glasses used in large lasers should be free of inclusions, the

presence of a few small inclusions might be acceptable. For example, the codes that model the

NOVA laser predict that the quality of this laser's output beam would not be seriously effected

if the laser's components contained a low volume density of beam obscurations with sizes below

% 250 um.^ All of the platinum inclusions that we have located in samples of Md:phosphate

glass initially had sizes well below this limit. However, when these inclusions were repeatedly

irradiated by laser pulses, the sizes of the damaged volumes surrounding some inclusions were much larger than 250 ym. This growth of damage complicates the development of adequate

specifications for the inclusion content of laser glass. The preferred option is to insist on complete elimination of metallic inclusions, but this may be difficult to achieve in large

melts. Another option is to eliminate only those inclusions that will cause unacceptably large

damages, but exercising this option requires an understanding of the relationship between the

initial size of an inclusion and the ultimate size of the damage volume produced by repeated

irradiation of the inclusion. The primary purpose of the experiments described in this paper was

investigation of that relationship. Additionally, we needed to measure the dependence of the

threshold for initiation of damage on both the duration of the laser pulse and the size of the

i ncl usion.

2. Samples

John Marion and Susan Winfree of Lawrence Livermore National Laboratory provided small glass

samples each containing one or more platinum inclusions. In their inspection of several liters

of Nd:doped phosphate glass, they located three types of inclusions: spherical bubbles,

irregularly shaped transparent objects believed to be microcrystal s , and opaque inclusions. In each instance that an opaque inclusion was exposed and analyzed, it v/as found to be metallic

platinum. Because it was necessary to leave inclusions encased in glass during damage tests,

opacity was used as a criterion for selecting inclusions for testing. After the damage tests were completed some of the inclusions that had been tested were exposed, analyzed, and found to

be platinum. We are reasonably certain, therefore, that all inclusions studied were metallic

pi atinum.

*Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore

National Laboratory under Contract No. W-7405-ENG-48.

128
Sizes of the inclusions were determined by optical microscopy. Pecause the inclusions were

opaque, this measurement gave only the linear dimensions in a plane normal to the direction of

observation, and the thicknesses of the inclusions were unknown. The major dimensions of the

tested inclusions ranged from 4-70 ym. Attempts to locate inclusions smaller than 4 un were

unsuccessful

.

3. Damage Testing

The samples were mounted in the microscope station on our 1064-nm laser damage experiment,

with the polished surface above the inclusion oriented normal to the incident beam.? The

inclusions were centered in the beam which had almost Gaussian fluence distribution and was about

2 mm in diameter at the e-^ level. In such a beam, fluence over a central area with diameter

of .65 mm is >^ 90% of the peak on-axis fluence. The beam was, therefore, large compared to the

250 urn size of interest. Peak on-axis fluences were determined to within ± 7%.

Each inclusion received a sequence of single-shot irradiations usually beginning at a fluence

level near 1 J/cm^. Fluence was sequentially increased until the damage threshold was

exceeded, and then increased additionally to a selected level which was sustained in additional

irradiations until the size of the damage either stabilized or exceeded 250 ym. For tests with

1.3-ns pulses, the fluences used during these repetitive irradiations were 5-7 J/cm^, which are

identical to the spatially averaged fluences existent inside the 46-cm amplifiers of an arm of

Nova during production of pulses with output energies of 10-15 kJ. For experiments with either

9-ns or 50-ns pulses, the selected level for sustained irradiation v/as chosen to be about 2-2.5

times the threshold fluence.

Because threshold level damage was usually observed during the first few pulses of each

sequence, these experiments did not determine whether damage could have been produced by a large

number of irradiations at lower fluences. However, most of the inclusions tested were taken from

a 20.8-cm amplifier disk that had operated at fluences of 0.5-1.0 J/cm? during more than 500

shots in the Novette laser, and this series of irradiations by pulses with durations < 1-ns had

not caused damage at any of the inclusions in the disk. We infer from this that the multi-shot

1-ns threshold is not significantly below the typical 1-ns threshold 2.5 J/cm?) determined

by use of a few irradiations.

The occurrence and growth of damage at an inclusion was observed by using a microscope to inspect the inclusion after each irradiation. Most of the larger inclusions had irregular

shapes, and damage at threshold consisted of one or more small fractures extending a few microns

into the glass surrounding the inclusion. These initial fractures usually had shapes that were peculiar to individual inclusions. During a few irradiations at fluences above threshold, the

damage evolved into a circular planar fracture oriented normal to the incident beam. This is

believed to be the typical geometry of damage surrounding an inclusion, although additional or more intense irradiations sometimes produce complex damage consisting of intersecting planar

fractures and lobes of damage lying along the direction of the incident beam.

4. Data

In most of the experiments, repeated irradiation of the inclusion caused damage larger than

250 um. Data obtained in the test of the largest inclusion studied (major dimension of

75 ym) are given in Figs. 1 and 2. The inclusion survived three irradiations at fluences up to 2.4 J/cm2, and damaged during the fourth shot at 3.8 J/cm?. Two additional irradiations at higher fluences caused rapid growth of the fracture surrounding the inclusion, but the growth

ceased when the fluence was reduced to < 2.5 J/cm^-. Four additional irradiations at higher

fluences caused damage much larger than 250 ym. Photographs of the damage are shown in Fig. 2.

For three of the inclusions, the damage grew to a size that remained stable under repeated

irradiation. Data obtained in the test of an 4-ym particle are given in Figs 3 and 4. The

particle survived 5 irradiations at fluences as large as 2.5 J/cm?. The damage stabilized

after the fourteenth shot at 12 J/cm?, and did not grow during the final 27 shots.

All of the data generated in these experiments are summarized in Table 1.

129
1000 IT

0 S

.2 0) (0 Q)

C (U

eS

E.2

1 ^

100 -

E

o

0

u

c

3

O

CO

2 3 4 5 6 7 8

Shot sequence

9 10 11 12

Figure 1: Evolution of damage surrounding a bar-shaped inclusion with initial major

dimension of 75 um. Damage was initiated during shot 4 at 3.8 J/cm^, and grew

rapidly during each shot at fluences > 4 J/cm^. See associated photographs in Fig. 2.

130
initial appearance

After shot 4 at 3.8 J/cm2 After shot 6 at 5.5 J/cm^

After shot 10 at 4.3 J/cm^ After shot 12 at 7 J/cm^

Figure 2: Photographs of damage surrounding an inclusion with initial maior dimension of 75 um. Physical dimensions are given in the associated data, see' Fig.

131
200

100

10 20 30

Shot number

Figure 3: Evolution of damage surrounding an inclusion with initial dimension

of 4 urn. Damage was initiated durinq shot 6 at 3,7 J/cm^ and reached its stable

size during shot 14 at 12.3 J/cm^^. See associated photographs in Fig. 4.

132
initial appearance

After shot 6 at 3.7 J/cm^ After shot 9 at 6.1 J/cm^ After shot 11 at 7.6 J/cm^

After shot 13 at 9.1 J/cm^ After shot 14 at 12 J/cm^ After shot 41

Figure 4: Photographs of damage surrounding an inclusion with initial dimension of 4 m. The

size remained stable after shot 14. Major dimensions of damage sites are given in the associated

data, see Fig. 3.

133
Table 1 Size of Damage Surrounding Particles with Varied Initial Dimensions.

lac^v^ LCI be r Pill r U 1 oCOt: duration,

ns

Tpcf

{

a

)

Fluence

J/cm2

Miimhpr ( b

)

of shots

T ni t T 1 1 II 1 u 1 a 1 Particle

size.ym

Final Cttq r»f namana i im ( D ) r 1 lid 1 o 1 Zc UT Ualildge , UIm ^

' stabilized unstabilized

6-10 36(9) 4 176

1 5-7 20(14) 6 240

5-7 49(6) 8 75

, 5-7 9 18 410

1 3-9 8 25 750

1 4-7 7 70 1000

9 5-9 9 17 380

9 5-9 8 18 440

55 8-20 6 6 340

55 9-20 5 18 390

55 8-20 6 19 500

(a) Range of fluences for which growth of damage was observed.

(b) Number of shots applied at fluences within the stated range. Bracketed values

are numbers of shots during which damage grew prior to stabilization.

5. Discussion of Results

5.1 Maximum Allowable Size for Platinum Inclusions

We did not find a monotonic relationship between the initial size of an inclusion and the

stable size for the damaged volume surrounding the inclusion. One particle with initial

dimension of 6 urn and all the particles larger than 10 ym caused damage larger than

250 um. Three particles with initial dimensions of 4, 5, and 8 un caused damage that ; remained smaller than 250 ym. The fluence at which growth of damage ceased was 5-7 J/crn^- delivered in 1.3-ns 1064-nm pulses. Therefore, the limited data indicates that repeated

irradiations at 5-7 J/cm^ of an inclusion with initial size of 4-6 ym might not cause damage

larger than 250 ym. We believe that larger damage might be produced at larger fluences.

134
Several mechanisms have been proposed to describe the termination of the growth. It is known

that platinum is distributed onto the fractured surfaces surrounding an inclusion. 3 During

multiple shots, the platinum might eventually become so thinly distributed that the net absorption of light was reduced below that required to produce additional fracturing. This

mechanism could explain why the damage surrounding an 8-um inclusion was smaller than that

surrounding a 4-ym particle. It is possible that the 8-um particle was a thin platelet

whereas the apparent 4-vim size was the projected view of a particle with relatively large

volume.

The calculations by Pitts^ suggest that the damage is driven by high pressure vapor boiled

from the heated surface of the particle. Under this model, the fractured volume becomes a more

effective vent for the vapor, and the crack tips to which force must be applied become more

distant, as the damage grows. While this allows limitation of growth, the quantity of vapor to

be vented should be proportional to the irradiated area of the particle, so this model has some difficulty accounting for the nonmonotonic relationship between the sizes of particles and damage.

5.2 Threshold vs. Particle Size

The 1.3-ns thresholds were 2.5 ± 0.3 J/cm^ for inclusions with initial major dimensions

ranging from 4-75 ym, and the 50-ns thresholds were 8 ± 0.8 J/cm^ for inclusions with initial

sizes ranging from 6 to 19 ym. Therefore, thresholds were independent of particle size. All models for inclusion damage, beginning with the earliest major treatment of the subject,

predict this result which is caused by the limited dimension for heat flow during short time

intervals. In a material with thermal diffusivity D, the characteristic dimension for heat flow

in time t is x =/Dt. For platinum, D = 0.24 cm^ sec-1, and x = 0.15 ym for t = 1 ns and 1.1 ym for t = 50 ns. The heat deposited in 1-50 ns cannot be distributed across the surface of a particle with size > 4 ym, so the heating at each point on the surface of such large particles

is almost independent of heat deposition at other points on the remainder of the surface.

Analytical models also predict that inclusions with diameters of about 0,1 ym would be most

effectively heated by 1-ns pulses, and we initially suspected that laser glass containing large,

visible particles might contain many particles with sizes below 1 ym. With a single exception,

results of damage tests of large volumes of laser glass indicate that about 50% of the bulk

damages occurred at sites containing rather large inclusions that had been located during pre-test

inspections. A large fraction of the additional 50% contained large (> 5 ym) inclusions that had not been detected before they were made more visible by damage. Therefore, we must assume that

the existing laser glass is either free of very small inclusions, or that repeated irradiation of

very small inclusions fails to cause damaged volumes large enough to he readily detected.

5.3 Variation of Threshold with Pulse Duration

Thresholds measured at the three pulse durations, 1,3, 9 and 50 ns, were 2,5 ± 0,3, 4 ± 0,4

and 8 ± 0.8 J/cm^. As shown in Fig. 5, these thresholds scale with pulse duration t as with a = 0,3, "However, all theoretical models^>5 predict the scaling should be tO.5.

If the smaller scaling rate of the experimental data is due to measurement error, then the

actual 1.3-ns threshold must be less than the assigned value of 2,5 J/cm^, or the 50-ns threshold

must be greater than 8 J/cm^, The latter possibility is readily dismissed. Damage was observed

at 8 J/cm2 in the 50-ns tests, and fluence measurements in that test were accurate to within

± 7%. It is possible that the assigned 1,3-ns threshold is too large. Pitts estimates that the

damage produced by threshold level 1.3-ns irradiation might consist only of cracks extending ^ 1 ym away from the inclusion. One could argue that our assigned value, 2.5 J/cm^, is the

fluence necessary to produce a detectable level of damage, and not a true threshold.

There are additional facts that assist in bounding the 1.3-ns threshold. The inclusions

tested in these experiments were taken from a Movette amplifier that had survived many

irradiations at 1 J/cm^, so the 1.3-ns threshold is not less than this value. Secondly, the

level of damage found during examination of selected components in Nova can be explained if the

1.3-ns threshold is near 2.5 J/cm^. It is also generally true that inclusion damage has not been a problem in large glass lasers operating at fluences below 2 J/cm^. The Shiva and Argus

laser, for example, were free of inclusion damage. Therefore, all evidence available suggests

that the 1.3-ns threshold is 2,5 J/cm^, and additional experiments and modeling will be

required to understand the discrepancy between experimental and theoretical values of the

threshold scaling parameter for 1,3-ns pulses.

135
1.0 10.0 100.0

Laser pulse duration, ns

Figure 5: Variation of damage threshold of platinum inclusions in laser glass

as a function of pulse duration. Data are average values for the inclusions

tested. The curve is pulse duration raised to the 0.3 power (tO-3).

6. Summary

Particles with initial dimensions greater than 4-6 urn will cause damage larger than

250 urn when the particles are irradiated by 1-ns pulses at fluences of 5-7 J/cm^. Damage

around smaller particles may remain acceptably small. The size range of acceptable particles is

based on a few measurements, so it is somewhat uncertain and it also probably depends on the

fluence level at which the particles are irradiated.

For the large particles, the threshold for initiation of damage was independent of particle

size, and for pulse durations between 1.3 and 50 ns, varied with pulse duration as

We are grateful to several co-workers for assistance during this study. John Marion and

Susan Winfree prepared test samples containing inclusions. Numerous helpful discussions were

held with members of the sroup (headed by Jack Campbell) formed to study elimination of platinum

from glass.

7. References

[1] Calculation of the maximum size for beam obscurations in Nova was done by W. E. Warren of

LLNL using the code MALAPROP.

[2] D. Milam, "Measurement and identification of laser-damage thresholds in thin films", SPIE

Vol. 140, Optical Coatings-Applications and Utilization II, 52-61, 1978,

[3] The observation of the distribution of platinum into cracks surrounding an inclusion was attributed by Hopper and Uhlmann (see Ref. 5) to R. W. Woodcock of American Optical Company;

recent work by J. E. Marion of LLNL confirms this observation.

136
J. H. Pitts, "Modeling laser damage caused by platinum inclusions in laser glass", elsewhere

in these proceedings.

The extensive literature on laser heating of inclusions appears to stem primarily from two early papers: R. W. Hopper and D. R. Uhlmann, "Mechanism of inclusion damage in laser

glass", J. Appl. Phys., 41_(10), 4023, Sept. 1970; Yu. K. Danileiko, A. A. Manenkov, A. M. Prokhorov and V. Ya. Khaimov-Malkov, "Surface damage of ruby crystals by laser radiation",

Sov. Phys. JETP, 3]_{^), 18, July 1970.

DISC I WWYR

This document was prepared as an accouni of HOrk sponsored by an agencv of

the I nited States (.oiernment. Neither the I nited States fiovernment nor the

I ni>ersity of ( alifornia nor any of their employees, makes any «arranty. express or implied, or assumes any legal liability or responsibility for the ac- curacy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned

rights. Reference herein to any specific commercial products, process, or service

by trade name, trademark, manufacturer, or otherwise, does not necessarily

constitute or imply its endorsement, recommendation, or favoring by the I nited States (.overnment or the I niversity of ( alifornia. The views and opinions of authors expressed herein do not necessarily state or reflect those of the I nited States (.ovemment thereof, and shall not be used for advertising or product en- dorsement purposes.

137
Manusoript Received

12-12-85

Multiple Pulse Damage Studies of BK-7, KCl and Si02 at 532 nm

D. Kitriotis, L. D. Merkle and A. Dodson

Department of Physics

University of Arkansas

Fayetteville, AR 72701

We report the results of damage studies at 532 nm on BK-7 glass, KCl, fused silica

and two grades of crystalline quartz using a frequency doubled, Q-switched YAG laser.

This work represents the initial stages of a program to investigate the mechanisms of

multiple pulse laser-induced bulk damage in transparent solids. Comparison of BK-7

damage behavior with that of fused silica shows little effect due to the color center

generation which occurs in BK-7 due to its small band gap. Commercially grown KCl exhibits multiple pulse catastrophic damage at fluences significantly below single pulse

"threshold", probably due to grown-in defects. Laser-induced damage behavior has been

compared between two grades of crystalline quartz, with the surprising result that the

nominally less perfect material is the more resistant to damage. The presence of "etch

tunnels" may be involved, but considerably more study is required.

Key words: borosilicate glass; crystalline quartz; fused silica; multiple pulse damage;

potassium chloride; pulse repetition frequency.

1. Introduction

In this paper we report the first results of a research program being undertaken at the

University of Arkansas to study multiple pulse bulk damage and the spectroscopic properties of

laser-irradiated transparent materials. The purpose of this program is to investigate the influence of microscopic defects, both pre-existing and laser-induced, on catastrophic damage properties of solids. It is believed that such investigations will contribute to understanding of the

mechanisms of multiple pulse damage, which several recent studies have shown to involve gradual,

perhaps transient, and very elusive laser-induced property modifications prior to catastrophic

damage [1-5].

This paper contains early results of a study using 532 nm light to damage BK-7 borosilicate

glass, single crystalline KCl and both fused and crystalline SiOo. These materials were chosen to permit comparisons between substances with band gaps far larger than twice the energy of the incident photons and one with a gap of less than two 532 nm photons (BK-7). In addition, two grades

of crystalline quartz were studied, permitting investigation of the influence of pre-irradiation

defects and impurities on damage resistance.

In section 2 the experimental technique and apparatus are described. Section 3 presents and

discusses damage data on crystalline quartz, including the most striking observation of this study:

the dramatic difference in damage behavior between two samples of quartz differing only modestly

in concentrations of the most common impurities. In section 4 damage data on fused silica and

BK-7 are compared for evidence of the influence of their very different band gaps, and in section 5 the observation of multiple pulse damage in KCl is reported. In the final section the interpretations of all these results are discussed, as are the needed directions of future work.

2. Experiment

The experiments reported in this paper are straightforward multiple pulse damage experiments

in which a sample is irradiated with a train of nominally identical laser pulses and the number of

pulses required to induce catastrophic damage is noted. The irradiating laser beam is focused

tightly into the bulk of the sample so that only bulk damage is obtained. In this work "cata- strophic damage", or simply "damage", is defined as the event in which a flash of light, a sudden

increase in light scattering as observed by eye, a sudden decrease in transmission and the generation of a bubble-like damage track occur. These indicators of "damage" are all found to occur on the same pulse, so that the above definition can be applied unambiguously.

The experimental apparatus is diagrammed in figure 1. Irradiating pulses were produced at a

138
pulse repetition frequency of 10 Hz by a Q-switched, unstable cavity Quanta Ray DCR-1 Nd:YAG laser.

In these experiments the output was frequency doubled and the 532 ran light separated from the re- sidual 1064 nm light by a prism. The annular cross section of the output beam was converted to a smooth central spot with only several percent of the light remaining in concentric diffraction

rings by means of a spatial filter and subsequent apertures. The spatial filter was assembled using two positive and two negative lenses to produce a very long focal depth and large enough focal

spot radius that air breakdown could not occur. The beam was focused tightly into the sample using

a 25 mm focal length plano-convex lens. „The resulting focal spot was found by knife edge scans to

be adequately fit by a Gaussian with 1/e radius in the range 5.7 microns to 11 .7 microns, depending on configuration of the spatial filter and apertures.

The temporal profile of the laser pulse is not well characterized. The laser operates in multiple modes with a band width of about one inverse centimeter, so that the pulse of nominally 10 nsec duration contains large spikes unresolvable by available detectors. For this reason all re- sults are presented in terms of fluence rather than intensity.

The scatter from a coaxial He-Ne laser beam was employed as a convenient real-time indicator

of catastrophic damage.

Several aspects of the experiment were controlled or monitored by a microcomputer (LSI-11) by means of a CAMAC crate. Pulses were counted and the incident pulse energy was monitored using a calibrated Hamamatsu S1723-02 photodiode. Due to the small focal size, energies of only 10 pJ to 300 pj were required. The energy stability during multiple pulse experiments was typically ±15%,

and experiments in which variability exceeded ±20% were rejected. A Hewlett-Packard 5082-4220

photodiode monitored the transmitted energy, and a computer controlled shutter was used to start

and stop each experiment and to provide pulse repetition frequencies lower than 10 Hz.

3. Crystalline Quartz

A comparison of multiple pulse damage in two grades of single crystal quartz has been undertaken to investigate the influence of pre-existing impurities and defects on damage in Si02. Samples of "Premium Q" grade and "Electronic" grade quartz from Thermo Dynamics, Inc., were cut and

polished for beam propagation along the z-axis. Premium Q quartz has a lower concentration of OH ions than Electronic grade, and typically has lower concentrations of Al and the dislocations which

give rise to "etch tunnels" as well [6]. Such impurities and defect structures may well affect the

damage behavior of the material.

The results of damage tests on these materials due to 532 nm irradiation are shown in figures

2 and 3. In each case each data point represents an experiment in which a previously unirradiated

section of the sample was exposed to the number of laser pulses indicated on the logarithmic vertical scale at the average on-axis fluence indicated on the horizontal scale. Different symbols indicate experiments terminated by catastrophic damage and those ended without such damage, as indicated in the captions. The focal spot radii employed are also noted.

The most striking feature of the data is the dramatic difference in damage resistance between

the two samples. The Electronic grade sample was found to be considerably stronger against laser

damage, both single and multiple pulse, than the Premium Q grade at 532 nm. The scatter in the

data is too large to yield well-defined single pulse damage thresholds, particularly for the

Electronic grade sample, but the approximate "threshold" for Electronic grade is clearly about

twice that of the Premium Q grade.

Two other observations may be made regarding the Premium Q damage. The dependence upon focal

spot size evident in figure 2 is qualitatively similar to that commonly observed in many materials

and widely attributed to randomly distributed damage-promoting impurities or defects. Data were

taken with pulse repetition frequencies of both 1 Hz and 10 Hz with indistinguishable results, so the laser-induced property changes which lead to damage after many pulses must live much longer

than one second.

In an attempt to identify the pre- irradiation properties responsible for the very different

damage resistances of the Premium Q and Electronic grade samples, comparisons of their concentrations of OH, Al and etch tunnels have been made. Estimates of the relative OH concentrations were made by comparison-, of the absorption coefficients of the two samples at two OH absorption wave

numbers: 3410 cm" and 3500 cm . Aluminum concentrations were measured by Larry Halliburton of

Oklahoma State University using electron spin resonance on small samples cut from the material adjacent to each damage sample. Adjacent pieces were also cut and polished, then subjected to an etch

in saturated ammonium bifluoride solution by a technique similar to that reported by Vig, et al. [6]

All measurements were made on z-growth material, as were the damage tests themselves. The results

139
are summarized in table 1. Note that the approximate densities of pits on the etched surfaces are

listed. The density and size of the etch pits made it difficult to see etch tunnel

s

extending into

the bulk with a standard microscope, but according to Vig and coworkers most etch pits should be

the surface terminators of etch tunnels [6]. Note that the Premium Q sample has the lower concentrations of OH and Al , as expected. However, these subtly different concentrations appear unlikely

to account for the large difference in damage resistance between the two materials.

Table 1. Comparison of OH, Al , and etch pit densities in Thermo Dynamics 11-7-83-85 (Premium Q)

and Thermo Dynamics 11-14-83-29 (Electronic grade) quartz samples.

Impurity Absorption Coefficient or Concentration

in Premium Q in Electronic

C( Premium Q)

C( Electronic)

OH

Al

etch pits

on surface

*^3410 ~ ^-^^^ 1

"3500

=

3.7 ppm atomic

10-^-10^ cm"^

-1 "3410 ^ ^'^^ ^"^"^

"3500 =

''"'^

4.4 ppm atomic

2 3-2 lO'^-lO-^ cm

^

0.94-0.97

0.84

-vlO

The very different damage behaviors may be related to the large difference in etch pit con- centrations. The dislocations which give rise to etch tunnels contain broken or strained bonds

and are thought to contain high concentrations of alkali impurities. Thus they may contain electronic states which could couple to the irradiating light to promote damage. The low density of

such tunnels implied by table 1 dictates that damage dependent upon irradiation of such structures

would be initiated at coarsely distributed locations, particularly in the Electronic grade sample.

However, microscopic inspection of the damage tracks indicates that 85-90% of the damage events initiate within significantly less than 100 ym of the focal plane. Note also that microscopy pro- vides no evidence of inclusion-controlled damage. Thus it remains unclear what defect or impurity

gives rise to the dramatic difference in damage behavior between these two quartz samples.

4. Fused Silica and BK-7 Glass

It is known that BK-7 borosilicate glass, due to its small band gap, undergoes two photon absorption when irradiated by 532 nm laser light and that the resulting excitations form color cen- ters which absorb linearly in the visible [7J. Irradiation in the green thus has channels for the

excitation of electrons and the deposition of energy in BK-7 not available in fused silica, a glass

with a far larger band gap. For this reason it is interesting to compare the single and multiple

pulse damage properties of these glasses.

Damage data taken on these materials are summarized in figures 4 and 5. The single pulse

damage threshold in BK-7 is considerably lower than in fused silica: about 65 j/cm'^ versus 175

jVcm^, respectively, when equal spot sizes are compared. This may reflect the easier generation of

electron-hole pairs in BK-7 than in fused silica. The shapes of the pulse number versus fluence

curves are very similar, however. For example, the fluence required to cause catastrophic damage

in 1000 pulses is about 45% of single pulse threshold in both materials. This is perhaps surprisingin view of the generation of color centers in BK-7, which should let successive pulses encounter

a progressively less transparent material.

White, et al . , found that the linear absorption at 532 nm induced by two photon color center

generation in BK-7 saturated at a value proportional to the irradiating fluence [7]. Their data

extend to 26 j/cm^, at which we observe damage after several thousand pulses. To estimate the heat

deposition due to color center absorption at that fluence, assume that the absorption coefficient

equals a constant, a, within the focal volume of the laser beam and is zero outside that volume.

If it is also assumed that all absorb ed energy is converted to heat, then noting that 1/a is large

compared to the focal depth and that /4Dtp^-]^g « Wg for a spot radius, Wg, of several microns and

thermal diffusity, D, about 0.005 cm^/sec, the maximum temperature rise due to a single pulse is

140
Here Q is the heat absorbed, E is the incident pulse energy, V is the focal volume, p is the mass

density and Cp the specific heat. Using standard room temperature values for the BK-7 properties

and a=0.08 cm ' for a fluence of 26 j/cm^ from White, et al. [7], the single pulse temperature rise

is only 0.5 K. Buildup of temperature from pulse to pulse is insignificant with a focal spot size

of several microns, since /4D-0.1 sec >> Wq.

This estimate may be too conservative due to the spikes in the temporal waveform of the laser

used in this work. Comparison of figures 2, 3 and 4 with the data of Merkle, et al. [2] suggests

that the peak intensities obtained in the present work may be an order of magnitude above those

which would obtain at the same fluence with a smooth pulse of equal duration. Using the linear

intensity dependence of the saturated absorption coefficient observed by White, et al . [7], this

still gives a temperature rise of only several degrees. Thus heating due to color center absorption cannot significantly affect the damage process.

It is likely that absorption of a photon by an excited color center would produce a free con- duction band electron. Thus, the presence of color centers generated on earlier pulses should

promote the production of conduction band electrons to aid electron avalanche. For this reason

the fact that multiple pulse damage occurs in about the same number of pulses at a given fraction

of single pulse threshold fluence in BK-7 as in fused silica may indicate that the damage process

in BK-7 is not sensitive to the concentration of easily freed electrons.

5, Potassium Chloride

Multiple pulse damage has also been investigated in KCl from Harshaw. Catastrophic damage of

this material by repeated 10.5 ym pulses was reported by Wu, et al., who attributed the effect to the growth of damage from microscopic inclusions [8]. It is of interest to study damage at other

wavelengths where different mechanisms may be operative.

Figure 6 summarizes damage experiments performed at 532 nm using both 1 Hz and 10 Hz pulse

repetition frequencies. It is clear that multiple pulse damage does occur at this wavelength. The

scatter in the data make it evident that the damage mechanism involves interaction with inhomogeneously distributed defects. This is supported by microscopic inspection of damage tracks, many

of which initiate more than 100 im from the focal plane. In addition, although the majority of

each damaged volume is spindle-shaped as in the other materials, in KCl the damage tracks frequently begin with a line of "bubbles" extending up to several hundred microns. Another possible cause

of scatter in the damage data would be surface roughness. However, comparison of data taken on samples cleaved and etched in HCl with data on samples only cleaved tend to rule out this possibility. Thus, although routine microscopy does not reveal inclusions in the unirradiated material,

it is probable that very small inclusions or other defects control the damage behavior, much as was found to be true in the 10.6 jam data.

6. Discussion and Future Directions

It has been known since the early years of the Boulder Damage Symposium that microscopic defects such as inclusions are important causes of laser-induced damage [9,10j. The subtleties of

multiple pulse damage observed in various studies [1-5] and the importance of color centers to damage in at least one study [11] indicate that submicroscopic defects and impurities, even at the

individual -atom scale, must also be considered. The studies reported here provide mixed evidence

regarding the importance of such point defects, but may give indications as to the directions which

further work should take.

In commercially grown potassium chloride it appears evident that inhomogeneously distributed

defects or impurities such as inclusions control laser damage at 532 nm. It is worth noting, however, that F-centers in this material absorb strongly at 532 nm. It may thus be possible to investigate their effect on damage properties in highly pure samples.

The comparison of multiple pulse damage behavior in BK-7 and fused silica provides indirect

evidence that the presence of color centers in BK-7 is not important to the bulk damage problem.

Further work will be needed to confirm this conclusion. If it is true, the reason may be that the

two photon absorption possible in this material at 532 nm provides sufficient absorption and con- duction band electron density to initiate damage without the participation of color centers.

141
The most striking evidence of the influence of pre-existing defects on laser-induced damage

is seen in the comparison of two crystalline quartz grades. The greatest difference in known pro- perties which may be correlated with the damage behaviors is found in the density of etch pits (and

thus presumably of etch tunnels), but the observed damage morphologies do not seem to support such

a mechanism. It is possible that differing concentrations of more densely distributed point defects may control the damage behavior of quartz. This could be consistent with the higher damage

"threshold" of the Electronic grade sample if impurities such as Al or OH impede electron multiplication. In addition, numerous less common impurities may play a role.

The quartz study is being extended by the acquisition of samples varying more widely in the

concentrations of known impurities and defects. In addition, certain of these defects can be modified by ionizing irradiation, further extending the range over which the defect parameters can be

varied. Systematic damage studies on quartz with differing defect content should reveal which

species or structures strongly affect laser damage, and may lead to an understanding of the

mechanism.

In addition, spectroscopic measurements are being prepared to detect the absorption, lumi- nescence and radiative recombination of laser-induced point defects. These studies should probe

the influence of point defects upon catastrophic damage much more directly.

This work was supported in part by a grant from Research Corporation. The authors wish to thank Aaron Murray and Thermo Dynamics, Inc. for providing the quartz samples, and Larry

Halliburton of Oklahoma State University for measurements of aluminum content in the quartz samples

as well as for helpful discussions.

7. References

[I] Merkle, Larry D. ; Bass, Michael; Swimm, Randall T. Nat. Bur. Stand. (U.S.) Spec. Publ . 669;

50-59; 1982.

[2] Merkle, Larry D.; Koumvakalis, N.; Bass, N. J. Appl . Phys. 55(3); 772-775; 1984.

[3] O'Connell, Robert M. ; Deaton, Terrence F.; Saito, Theodore T. Appl. Opt. 23(5); 682-688; 1984.

[4] Wood, R. M.; Sharma, S. K. ; Waite, P. Nat. Bur. Stand. (U.S.) Spec. Publ. 669; 44-49; 1982.

[5] Jhee, Y. K. ; Becker, M. F.; Walser, R. M. J. Opt. Soc. Am. B2(10); 1626-1633; 1985.

[6] Vig, John R. ; LeBus, John W. ; Filler, Raymond L. Proc. of the 31st Ann. Sym. on Freq. Control;

1977. 131-143.

[7] White III, W. T.; Henesian, M. A.; Weber, M. J. J. Opt. Soc. Am. B2(9); 1402-1408; 1985.

[8] Wu, S.-T.; Bass, M.; Stone, J. P. Nat. Bur. Stand. (U.S.) Spec. Publ. 638; 152-159; 1981.

[9J Glass, A. J.; Guenther, A. H. Damage in Laser Glass. Am. Soc. for Test, and Mater. Spec.

Publ. 469; 1969.

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

341; 1970.

[II] Soileau, M. J.; Mansour, Nastaran; Canto, Edesly; Griscom, D. L. Proc. of Seventeenth Symp.

on Opt. Mater, for High Power Lasers; to be published by Nat. Bur. Stand. (U.S.).

142
He-Ne

laser

25 mm sample

Figure 1. Experimental apparatus.

X

100 0 L

CDZ

UJ

in

00

1 0

—

X • •

PREMIUM Q

QUARTZ

UJe-2=5.7

UJ^-2±9.9

>1K X •

• X*

X«6<X

1000

q:

I

100

UJ

100 200

F ON AXIS (J/CM^)

T ^ 1

^ xELECTRONIC

GRADE

V QUARTZ _

UJgr2=5.7

X

X

XX.

X

X

X

X X

X

X

XX X ' X

100 200 300

F ON AXIS Cj/Cm2)

Figure 2. 532 nm multiple pulse damage data, on "Premium

Q" quartz (Thermo Dynamic 11-7-83-85). The data are indistinguishable for pulse repetition frequencies of 1 Hz and 10 Hz. Filled circles and crosses represent experi- ments in which catastrophic damage was observed at the

indicated number of pulses, open circles and triangles

representing experiments terminated without catastrophic

damage. The vertical arrows on the latter indicate the

obvious fact that the pulse numbers at which these experi- ments were terminated represent lower limits on the survival time of the material in those experiments.

Figure 3. 532 nm multiple pulse damage data on "Electronic Grade" quartz

(Thermo Dynamics 11-14-83-29). All data were taken at a pulse repetition

frequency of 10 Hz. Experiments in which catastrophic damage was observed

are indicated by crosses, experiments

without such damage by triangles.

143
T

1000 -

00

0 _

X

FUSED

SILICA

UU^-2=11.7

X

jjm

X X. X X X X

xx

-> >«l>ft<'X 100 200

F ON AXIS (J/CM^)

I

1

X

x><xx BK-7

1000 X

X

>^ UU^-2±9.8

UJg 100

3

Ui

to

a.

1

X

1

Xx X

... i }4<;^.

0 25 50 75

F ON AXIS (J/CM^)

Figure 4. 532 nm multiple pulse damage data on fused silica (Corning 7940) at a pulse repetition

frequency of 10 Hz. Each cross represents an experiment in which catastrophic damage was observed at the indicated number of pulses.

Figure 5. 532 nm multiple pulse damage data on BK-7 borosilicate glass at a pulse repetition

frequency of 10 Hz. The data for a PRF of

1 Hz are indistinguishable from these. Data taken with a spot size of 11.7 urn give the same single pulse threshold (65 j/crrr) and give

N=1000 pulses for approximately the same fluence

Crosses represent catastrophic damage events,

the triangle an experiment terminated without

catastrophic damage.

144
6 HARSHAW KCI

lOOOL A , • ^

LlI CO

2 101

• •• • • 5100L . i,.

• 6 PRF =10H2

in .Tt. xAPRF=lHz 4*«

*

-»« 9^

10 20 30

F ON AXIS Cj/cm^)

Figure 6. 532 nm multiple pulse damage data on Harshaw

KCI. Experiments in which catastrophic damage was observed are indicated by filled circles and crosses, ex- periments without such damage by open circles and triangl es.

145
Manusaript received

2-26-86 SAB

Picosecond Nonlinear Investigation of Accumulated

Damage in Molecular Crystals

Thomas J. Kosic*, Jeffrey R. Hill and Dana D. Dlott*

School of Chemical Sciences

University of Illinois

Urbana, Illinois 61801

Accumulated damage in aromatic molecular crystals at low temperature is studied by

a variety of novel picosecond laser techniques. Damage in these materials is qualitatively different than in fused quartz, where dielectric breakdown is the predominant

mechanism. In the aromatic crystals, multiphoton ionization creates reactive chemical

fragments which are stabilized by the low temperature crystal matrix. These species

accumulate until a high concentration is reached. At that point, phonon and photon

assisted interactions release enough energy to damage the crystal. In a sense the

process can be viewed as a chemical explosion.

The picosecond YAG—dual dye laser system used for these experiments is described.

With this system we have demonstrated new methods to study damage. Subnanosecond damage

is time-resolved with a sampling technique. The mechanism of defect production and

crystal destruction is probed using damage detected spectroscopy. Scattering of the

optical phonons by the accumulating defects is detected via picosecond time-delayed

coherent Raman scattering.

Key words: accumulated damage; molecular crystals; nonlinear optics; picosecond lasers;

time-resolved coherent Raman scattering.

1 . Introduction

We are a group of solid-state chemists who have recently begun investigating optical damage

in chemical materials. Most solid state chemical reactions are initiated at nucleation centers,

and the extreme conditions of optical damage in chemical materials simulates the initiation of

solid explosives. Our initial experiments were performed on acetanilide crystals at low tem- perature [1,2]. Acetanilide crystals consist of hydrogen bonded chains of aromatic phenyl

groups. The phenyl group can be ionized to produce several reactive chemical species which are stabilized in the low temperature crystal matrix. When these species accumulate to high concentration, they begin to interact in photon and phonon assisted reactions, which produce damage in

the crystal.

In this work, we contrast the behavior of cold organic systems with optical damage in inoi

ganic glass at ambient temperature. We also describe our advanced picosecond spectrometer and some novel techniques for studying the fundamental mechanisms of accumulated damage.

2. Optical damage: organic versus inorganic

When a nonabsorbing glass is irradiated with intense pulses, bulk optical damage is caused

by dielectric breakdown [3]. The single pulse damage threshold is typically comparable to the

static breakdown voltage [3]. In accumulated damage, a material is irradiated with pulses below

the single pulse damage threshold. These pulses create defects which lower the single pulse

damage threshold. When this threshold is low enough, the material is damaged by the pulses

[4,5]. In most cases, only a small concentration of defects is created before the damage is

observed [^4,5].

Current address: Hughes Aircraft Corporation, El Segundo, California 902^15. Author to whom correspondence should be addressed.

146
In low temperature acetanilide crystals, multiple pulse optical damage is more like a chemical explosion. The defects in acetanilide are very high energy centers (E = 3-9 eV) in the

vicinity of radicals or ions created by multiphoton ionization. These defects are stabilized by

the low temperature crystal matrix and tend to reach very high concentrations (several percent)

before damage occurs. At a critical concentration, the metastable crystal is destroyed by phonon

and photon assisted processes which release the stored energy [1,2].

In our experiments, we used a 25 u laser spot size. Microscopic examination of damaged

crystals showed an inclusion of roughly the same diameter. To create a cavity of volume -10~

cm^, about 20 yj of irreversible work must be performed on the crystal. Thus the damage process

releases about 2000 J/cm-^ of translational energy [2]. This is comparable to a TNT explosion; the

heat of combustion of TNT is 22,000 J/cm^ of which about ^0% is released as translational excitation. This "solid state explosion" is confined to a small volume within a cold crystal, and

typically occurs on the time scale of about 1 ns.

3. Time-resolved laser experiments

3.1 Laser system

Figure 1 is a block diagram of the laser system we used for our experiments. The Nd:YAG

laser, shown in the inset, is continuously pumped and is pulsed with an acoustooptic Q-switch and

mode locker. It produces a very stable pulse burst of about 50 pulses, each 120 ps in duration,

and the largest about 80 yJ. The repetition rate can be varied from single shot to 1000 Hz. The

pulses are gaussian in time and in spatial profile, and are transform limited. This system

provides a particularly clean, reproducible source for damage measurements, especially multiple

pulse measurements. The gaussian beam can be focussed to a spot size of 5 y. This corresponds

to a fluence level of about 100 J/cm^, and a power density of lO^^W/cm^, values which are above

the damage threshold of most materials.

YAG LASER

500 HZ

PULSE

SELECTOR

SINGLE PULSE

80 M

I I20ps

..III lllh. DOUBLERS CD«A

PHOTO

DIODE TRIGGERtrn

PULSE TRAIN 2mJ

DYE PULSES

15/iJ l5ps(R6G)

5fnJ Green

tDYE#2h

- 0.5 mJ Green

[TUNE EXCITATION PULSES ^2 1 —

GREEN

PULSE

100ps

SCANNING OPTICAL DELAY

LINE PROBE

PULSE

OS W. M2 \HIDM| POL /PC ETALDNS DYE

CRYSTAL SAMPLE

IN DEVWR

-ASI

= 2cj|-a;2 '

SPECTROMETER AND PHOTOTUBE

COMPUTER

DISK PLOTTER

Figure 1. High repetition rate high

power picosecond laser system for optical

damage experiments. The inset shows the

details of the YAG and dye lasers. In the experimental arrangement shown, a crystal in a cryostat is damaged by the

green pulses. The crystal phonon

dynamics are studied by time-resolved

CARS. In this technique, a phonon is

excited by simultaneous pulses at oj^ and

0J21 and the intensity of anti-Stokes

emission produced by a delayed probe

pulse is detected versus delay. Key: M = mirror, Pol = polarizer, PC = pockels

cell, QS = acoustooptic Q-switch, ML = acoustooptic mode locker.

The frequency doubled laser is used to pump two synchronous cavity dumped dye lasers shown in

the inset. The dye laser pulses are transform limited and spatially gaussian. A single pulse

from the YAG pulse burst is selected with a pockels cell at the same time that the cavity dumpers

in the dye lasers are triggered. The dye lasers and the fundamental and harmonics of the YAG

laser can be mixed in nonlinear crystals to provide a tuneable pump and probe pulse in the

ultraviolent, visible, and near IR. Although this laser system was home built in 1980, similar

lasers are now commercially available.

147
3.2. Time resolving optical damage

Using a sampling technique, we have time resolved the damage process [1,2]. The sample is

irradiated with successive pulses of 85 ps FWHM, and the rate of damage is determined by counting

pulses. There is no fundamental limitation to extending this technique to the 10 fs time scale.

The apparatus we used is shown in figure 2. Photodiodes and sample-and-hold circuits detect and

normalize the incident and transmitted pulses. In a typical run, thousands of pulses are trans- mitted through the crystal before damage occurs. The computer acquires the time-dependent optical

density of the crystal during the accumulated damage process.

Figure 2. Apparatus used for

time-resolved damage studies and

"damage-detected spectroscopy". The

optical damage is time-resolved by

measuring the optical density change of

the damage pulses with photodiodes (PD)

before and after the crystal. Sampleand-hold circuits (SH) and a computer

(PDP11) acquire the data. Each pulse

irradiates the sample for 85 ps. The crystal may be damaged by timesequenced multicolor excitations which

are designed to pump a particular

vibrational or electronic level. The rate of damage for each pulse sequence

is used to investigate the coupling

between light and mechanical

excitations of the crystal.

Two such runs are shown in figure 3. Only the final 100 pulses of each run are shown in the

figure, corresponding to 8.5 ns of irradiation. In the upper run, there is no apparent change in

optical density for several ys (not shown). About H ns before destruction, some small fluctuations are observed. The catastrophic damage event takes 5 pulses, or roughly HOO ps of irradiation. In the lower trace, an oscillatory damage event is seen. Under the conditions we used,

this mode occurred on 10? of the runs. We believe that the oscillations in the optical density

of the metastable crystal are driven by fluctuations in the defect pumping rate.

MOTORIZED

OPTICAL

DELAY

HARD

DISK

HARD

COPY —1 PDP11

CRYOSTAT

PD

TRIG SHI

SH2

Figure 3. The time development of two damage events. Only the final 100 pulses of 85 ps duration are shown. Each event was preceded by several thousand pulses (not shown).

The left figure shows the abrupt damage mode and the right figure shows an oscillating

damage mode.

148
Time resolution of the optical damage introduces a new dimension to our measurements. Since

defect accumulation Is the rate limiting step in accumulated damage, conventional N-on-1 testing

determines only the rate of defect producing processes. With our method, we determine the rate

of crystal destruction processes as well. We can distinguish between models where these two processes occur by different mechanisms.

3.3. Damage detected spectroscopy

"Damage detected spectroscopy" involves setting up a time sequenced multicolor pulse train

which pumps a specific electronic or vibrational level, and determining the effect of this pumping

on the rate of defect production and damage. The apparatus in figure 2 is used. Usually we damage

a number of identical volumes in a single crystal and average the result. The goal of these ex- periments is to determine how the radiation field couples into destructive mechanical excitations

of the crystal.

In a series of damage detected spectroscopy experiments, we have shown that the defects in

acetanilide are created by a 4-photon ionization process involving two sequential 2-photon absorptions [2]. The energy level diagram for acetanilide is shown in figure 4. Using the combinations

shown in the figure, we determined that the first step In defect accumulate was excitation of the

S-| state. We also determined that the ionization process was stepwise, rather than simultaneous,

by delaying the ionizing pulses behind the exciting pulses,

scheme can be investigated in this manner,

In general, any proposed pumping

2X

532

nm

IONIZED STATES

2X

656

nm

^=1 2200 cm602

nm

+532

nm

• \

2X \

612

nm

: ^1

2X

565

nm

So

DEFECTS Figure H. Energy level diagram for

acetanilide. By using the one and

two-color combinations shown in the

figure, we have determined that the

defects are created by 2-photon

absorption to the state, and subse

quent 2-photon ionization.

Since the crystal damage process occurs on a time scale which is faster than many vibrational

relaxation rates in this material [6], nonequilibri um vibrational populations must play an important role. In a preliminary experiment, we have demonstrated the feasibility of studying the

optical damage produced by pumping a specific molecular vibration or phonon with wavevector

aligned along a particular crystallographic axis [1,2]. Using simultaneous pulses at 532.0 and

561.9 nm, the 1000 cm~^ vibration of the phenyl group was excited via stimulated Raman scattering.

Compared to a control experiment where this mode was not excited, vibrational excitation reduced

the damage rate by one half. Most likely there are other modes which increase the rate of damage.

This will depend on the degree of coupling of a vibration with the translat ional degrees of

freedom and the defects.

S.'*. Phonon dynamics and optical damage

The Raman active phonons in a crystal are sensitive probes of changes in crystal structure

during damage. Fauchet et al have used Raman spectroscopy to study structural damage in silicon

crystals [7]. The damage caused a shift and broadening of the phonon spectrum. With a conventional Raman spectrometer, the resolution is typically 1 cm~^ . In silicon, a 1 cm~^ shift can be

brought about by pressure gradients of order of a few kilobars [?].

149
In our lab, we use ps CARS spectroscopy [6] to measure the decay of phonons excited by

short laser pulses. At low temperature, this decay is related to the width of the phonon

Raman spectrum by (ps) = 5.3/(width in cm~^). With our spectrometer, the resolution

is better than 0.001 cm" . In this case, pressure gradients on the order of a few bars are detectable [6].

Figure 5 shows some ps CARS data on low temperature acetanilide crystals [1]. The phonon

lifetimes constantly decrease as the crystal is irradiated. This crystal was damaged at 29 ys of irradiation. During the first 5 ys, one phonon lifetime decreased from 67^ to 540 ps, corresponding to a broadening of the Raman spectrum of 0.00791 cm~^ . This broadening is caused

by phonon scattering from the accumulating defects and strain in the crystal [2].

nCETRNILIDE PHONONS DURING

INCUBATION PERIOD

O 5 10 15 20 25

IRRADIATION TIME (|JS>

Figure 5. As defects accumulate in

the lattice, the optical phonon

lifetimes measured by ps CARS decrease.

The phonon lifetime is a very sensitive

probe of the predamage environment.

The open squares are for the 38 cm~^

phonon and the filled squares are for the

39 cm~^ phonon. The crystal was damaged

at 29 ps of irradiation.

This very sensitive probe of the predamage environment can be used to determine the

efficiency of various damaging or annealing processes. Using psCARS as a noninvasive,

nondestructive probe of the damage, the crystal can be irradiated until it is about to damage

(the "brink of destruction"). At this point the crystal is a very interesting metastable

system. It may prove possible to anneal these metastable crystals, either through optical or thermal processes. Annealing would be detected by an increase in the phonon lifetimes.

M. Conclusions

In a series of experiments made possible by our development of an advanced picosecond laser

system, we have characterized damage in low temperature molecular crystals and demonstrated novel

time-resolved diagnostic techniques to study this "solid-state explosion". Our work includes 1) First detailed study of optical damage in aromatic molecular crystals, 2) First picosecond time

resolution of the destruction process in an accumulated damage system, 3) First use of phonon

dynamics and ps CARS to detect the onset of damaging states, '4) First use of picosecond multicolor

pulse sequences to investigate the mechanism of damage production, and 5) First demonstrate of the

effect of non-equilibrium vibrational populations on optical damage.

This research was supported by the National Science Foundation, Solid State Chemistry through

grant NSF DMR 8^4-15070. Dana D. Dlott is an Alfred P. Sloan fellow.

150
5. References

[1 ] Hill, J.R., Kosic, T.J., Chronister, E.L., Dlott, D.D. Picosecond vibrational dynamics of

hydrogen bonded solids: phonons and optical damage. Springer Ser. Phys. 4: 107-111; 1985.

[2] Kosic, T.J., Hill, J.R., Dlott, D.D. Phonons, defects and optical damage in crystalline

acetanilide. Chemical Physics (in press).

[3] Bloembergen, N. Laser-induced electric breakdown in solids. IEEE J. Q. E. QE-10:

375-386; 1974.

[4] Manekov, A. A., Matyushin, G.A., Nechitailo, V.S., Prokhorov, A.M., Tsaprilov, A.S. On the

nature of the accumulation effect in laser-induced damage to optical materials. Opt. Eng. 22: 400-404, 1983.

[5] Merkle, L.D., Bass, M. , Swimm, R.T. Multiple pulse laser-induced bulk damage in

crystalline and fused quartz at 1.064 and 0.532 ym. Opt. Eng. 22: 405-410; 1983-

[6] Kosic, T.J., Cline, R.E., Jr., Dlott, D.D. Picosecond coherent Raman investigation of the

relaxation of low frequency vibrational modes in amino acids and peptides. J. Chem. Phys. 81

14932-4949 (1984).

[7] Fauchet, P.M., Campbell, I.H., Adar, F. Long range material relaxation after localized

laser damage. Appl. Phys. Lett. 47: 479-481; 1985.

151
- Manuscript Not Received -

LASER PHOTOCONDUCTIVITY IN ALKALI HALIDES

Alexander Epifanov

Institute of General Physics

Moscow, USSR

Key Words: alkali halides; laser damage; laser-induced

photoconductivity; photoconductivity.

152
Manuscript Received

12-5-85

THE SCRATCH STANDARD IS ONLY A COSMETIC STANDARD

Matt Young

National Bureau of Standards

Boulder, Colorado

In this paper, I present a history of the scratch and dig standard for optical surface quality and show that this standard has since its inception been

recognized as a cosmetic standard and not as an objective or performance standard.

In addition, I attempt to dispel the myth that the scratch standard was changed

during the 1960's and show that scratch number cannot be related to scratch width.

Finally, I describe a preliminary aging experiment that suggests that the scratch

standards have not aged with time and are, in fact, extremely stable.

Key Words: optical polishing; scratch-and-dig standards; surface figure.

Since learning that I was to give this paper at 5:30, I have had the recurring fantasy that

there would be only one person left in the audience. I would give my presentation and conclude,

"Thank you very much for listening," and she would reply, "Quite all right; I am the next speaker."

This paper is about the scratch and dig standard, mostly about its history. Those who do not remember this history are perhaps not condemned to repeat it, but rather to misunderstand or misuse the standard.

The scratch and dig standard was proposed in the mid forties by McLeod and Sherwood, who made

clear that the standard is cosmetic. They prepared comparison standards numbered 10 through 120

and commented, "if the scratch is made in a certain manner the numbers are the widths of the

sample scratches in microns." They went on to note "that there is little correlation between the

appearance or visibility of a scratch and its measured width. The shape of a scratch has a lot to

do with its visibility .... The samples . . . have been selected to look alike, not necessarily

to have the same measured widths." Likewise, in 1945, Frankford Arsenal stated unequivocally,

"These numbers are arbitrary, and are not to be assumed as denoting the width of the scratch."

(All italics mine.)

The standard we use today, MIL-0-13830A, is based on a set of glass artifacts that may have

been prepared as early as 1954. These artifacts are the primary or master standards against which

secondary or submaster standards are calibrated and sent into the field for inspection of many

kinds of optical element, whether transmitting or reflecting, transparent or metallic. The

scratch and dig standard is the most widely used indicator of optical quality in the United

States.

The dig standard is based on the diameter of the dig and so presents no problem. The secondary scratch standards, however, are certified by a subjective brightness comparison with the master standards. The standards are backlighted and viewed with the naked eye, by dark field

illumination, and usually within a few degrees of the incident beam. Primary standards with a certain designation or scratch number (such as S-20) come in pairs with high (S-20H) and low

(S-20L) visual brightness. A specimen that falls between the high and low primary standards is given their scratch number (in this example, S-20). A certified set of submasters is then boxed

but not hermetically sealed. (It must therefore be resubmitted periodically for recertification.)

This set is in turn used for naked eye comparisons with scratches on optical surfaces.

The light scattered by an isolated scratch no more than a few micrometers wide is a small

fraction of the total light scattered by any surface. In part for that reason, the scratch standard can (with minor exception, such as inspection of reticle blanks) never be more than cosmetic.

In addition, the scratches are almost invariably much narrower than the limit of resolution of the

eye; since they can have almost any cross section, the naked eye can never, never, never! estimate

the width or depth of any scratch by a simple brightness comparison.

The standards document, MIL-0-13830A, was adopted in 1954 and revised in 1963. It contains

no specifications but rather refers to a 1945 drawing whose earliest versions apparently have been

lost. Originally, this drawing showed only how the sets were to be boxed. The earliest version I could locate. Revision E (1956), has no relevant notes, but in 1960 a note was added saying,

"Scratch numbers do not denote width of scratch. The numbers indicate that the scratch has the

153
same . . . visual appearance as the master scratch . . . ." This is perfectly clear. Confusion began

with Revision H (1974), which states that the number of a scratch is equal to its width in micrometers. This revision probably resulted from a desire to define the standard so that the user could manufacture his own masters, but within two years another revision proclaimed that the

scratch width was one tenth the scratch number and that, in any case, "Manufacturer-generated

standards must be calibrated by Frankford Arsenal." This calibration was effected by the same old

visual brightness comparison.

During all this time, absolutely nothing in the certification procedure had changed, nor has

it changed to this day. The current revision. Revision L (1979), uses the parenthetical and enigmatic remark, "Dimensions are for reference only."

I argue that all reference to scratch width should be removed from the drawing. At best, a width specification can provide a starting point for the manufacture of artifacts; at worst, it can cause hopeless confusion. In any case, to be legitimate secondary standards these artifacts

must still be compared visually with the primary standards located at Picatinny Arsenal. These

primary standards were sent there when Frankford Arsenal closed in 1976 and are believed to be the

set used since the inception of the standard.

The effort to correlate the scratch's number with its width ended with the closing of

Frankford Arsenal but was probably doomed to failure anyway. Unfortunately, it contributed to three distinct misconceptions: (a) that the number of the scratch is somehow related to its width

(or that it can be), (b) that the widths of the standard scratches somehow decreased by a factor

of lU and the standard had therefore become more stringent over the years, and (c) that width may

be measured instead of appearance (Fig. 1).

Myth-conceptions

(a) Scratch number is related to scratch width

(i) Width may therefore be inferred from number

(b) Scratch widths have decreased over the years

(i) The standard has therefore become more stringent

(c) Width may be measured instead of appearance

Figure 1. The most common misconceptions about the scratch standard.

Additionally, the startling factor-of-10 change between Revisions H and L has caused some users to speculate that the primary standards may have healed with the passage of time. Certainly

the glass around a scratch cannot flow, but chemical weathering of the surface due to water vapor

in the air is a possibility that could cause the primary standard either to heal or to worsen. There is evidence, however, that most weathering would take place and stabilize within a few hours

or days. In any case, a change has to be quite substantial before it is apparent to the naked eye

and, I think, would be picked up by the trained inspectors when they make repeated comparisons

with secondary standards. More probably, it seems to me, small scratches that are reported to have "disappeared" have been contaminated by the volatile constituents of their wooden boxes and

merely required a good degreasing.

Nevertheless, I performed a "preliminary" experiment on aging. Figure 2 shows polar scans of the scattering by a scratch that I made in a microscope slide. This scratch was rejected because

of the asymmetry in the scan. The sharp peak is the unscattered or undiffracted light, and the

rest of the curve is the logarithm of the power scattered by the scratch as a function of angle.

The interesting part, as far as the scratch standards are concerned, is the region between about 5 and 10 degrees. This scratch has a side crack that gives rise to a two beam interference pattern

beyond -20 degrees. Before placing some good samples into high humidity environments, I dumped

this one into a beaker of de-ionized water for a bit over one day. If we make the naive assumption that concentration of water molecules is the only important factor, then this corresponds to aging over a period of about 150 years, assuming that each month is July and the location is New Jersey. Despite the dramatic change at the larger angles, the scattering at 5 degrees -- where it matters — remains very nearly constant. With drying, even the large angle scatter recovers somewhat . 154
Figure 2. Power scattered by a scratch as a function of angle, on semi -logarithmic scale.

Scratch has been immersed in de-ionized water, as stated. Periodic pattern beyond -10

degrees is two beam interference pattern resulting from undesirable side crack.

155
Figure 3. As in Figure 2. Scratch has been aged for 12 days in high humidity environment at 70 degrees Celsius. Scattering between 5 and 10 degrees is not measurably affected by

aging. Lower right shows scattering by substrate alone and demonstrates that increased

scattering at higher angles is due to etching of the substrate.

156
I subjected many other scratches (which had acceptable scattering profiles) to various stresses such as high humidity at 5, 20, and 70 degrees Celsius, and dry heat at 70 degrees for 12 days, and immersion in water for one or two nights. Only the scratches exposed to moist heat for

12 days showed measurably changed scattering patterns.

Figure 3 shows one such scratch, roughly an S-30, that was aged in a high humidity environment at approximately 70 degrees Celsius for 12 days. The substrate was severely etched and

showed comparatively high scattering (lower right); nevertheless, the power scattered by the

scratch itself is not measurably affected at angles less than 10 degrees. This suggests that the

scratches must be extremely stable over long periods of time.

In recent years, I have heard speculation that laser damage threshold is related to the width

of surface defects; this quite plausible speculation has unfortunately led to the rebirth of misconception (a). I therefore recommend (Fig. 4) that all those introduced to the standard after

October 29, 1985 be required to write 50 times, "The scratch standard is only a cosmetic standard." All the rest of us should write it 100 times.

on/y

01 iy

on/y TU

rUon /y

a- CcS>*^^t\c ctcx.neia.rd.. Scfc^^Jk stanJarJ is o*^ /y

cey<^itie.'f"tc sta.»c/ar<iSc-fa.'tcJL s'ta.icla.rJ is ffhly

Figure 4. And it isn't a width standard, either!

The history portions of this paper were published roughly concurrently in Laser Focus,

November, 1985, pp. 138, 140. Based in part on a paper presented to the Workshop on Optical

Fabrication and Testing, June 12-14, 1985. I am indebted to Dick Goldgraben, Jack Feeney, Jack

Stateler, Jim Shean, and Hans Guttwein for helping me with the history of the standard, and to George McLellan and Brian Lawn for their explanations of crack formation and surface weathering.

Keith Masterson encouraged the aging tests, and Edie DeWeese prepared the manuscript with her

usual care. Any errors in this paper are, unhappily, my own. Contribution of the National Bureau

of Standards, not subject to copyright.

Bibliography

1. J. H. McLeod and W. T. Sherwood, "A Proposed Method of Specifying Appearance Defects of Optical Parts," J. Opt. Soc. Amer. 35, 136-138 (1945).

2. M. Young, "Scratch Standards Should Not Be Used to Predict Damage Threshold," Proc. Conf. Laser Induced Damage in Optical Materials, 1982, Nat. Bur. Stand. (U.S.) Spec. Publ . 669,

151-155 (1982).

3. M. v. Swain, "Microfracture about Scratches in Brittle Solids," Proc. Roy. Soc. London A366,

575-597 (1979).

157
4. For a summary of our work relating the appearance of the scratches to their physical characteristics, see M. Young and E. G. Johnson, Jr., "Redefining the Scratch Standards," Nat. Bur. Stand. (U.S.) Tech. Note 1080 (1985); M. Young, E. G. Johnson, Jr., and R. Goldgraben, "Tun- able Scratch Standards," Proc. Soc. Photo-Opt. Instr. Engrs. 525, 70-77 (1985); M. Young,

"Objective Measurement and Characterization of Scratch Standards," Proc. Soc. Photo-Opt.

Instr. Engrs. 362, 94-100 (1983).

5. For additional background material on optical surface quality measurements, see M. Young,

"Can You Describe Optical Surface Quality with One or Two Numbers?" Proc. Soc. Photo-Opt.

Instr. Engrs. 406, 12-22 (1983); see also pp. 119-131.

Table 1. An abbreviated history of the scratch and dig standard.

McLeod & Sherwood, 1945

FED 1304, 1945(?)

MIL-0-13830, 1954 and

MIL-0-13830A, 1963

Drawing C7641866,

Revision E, 1956

Revision E-3, 1960

Revision F, 1965

Revision H, 1974

Revision J, 1976

"Samples . . . have been selected to look alike, not necessarily to have

the same measured widths."

. the width of the scratch."

Revision L, 1980

"These numbers are arbitrary, and are not [Refer to Drawing C7641866]

[No statement]

Added Note 2 : "Scratch numbers do not denote width of scratch. The

numbers indicate that the scratch has the same weight or visual appearance as the master scratch bearing the same weight number . . . ." Added to Note 2 : "(Master scratch samples are maintained by the

Frankf ord Arsenal .

)

Added Note 6 : "Only the item listed in this drawing . . . has been

approved by Frankford Arsenal .... Substitute item shall not be used

without prior approval."

Changed Note 2 : "Scratch number denotes width of scratch in microns."

Changed Note 6 : "Manufacturer has option to measure scratch width . . . or to utilize comparison standards identical to those illustrated

herein. (Master scratch standards are maintained by the Frankford

Arsenal .)"

Changed Note 2 : "Scratch standards width are (sic): (Dimensions are for

calibration purposes only.)

"#10 Scratch 1 mm ± .1 "#20 Scratch 2 mm + .2 . . . " Changed Note 6 : "Manufacturer has option to generate visual comparison

standards ... or to utilize Frankford Arsenal visual comparison stan- dards. Manufacturer generated standards must be calibrated by Franford

Arsenal ..." (my italics).

Changed Note 2 : "(Dimensions are for reference only.)" (My italics.)

Note 6 unchanged except for replacement of Frankford Arsenal by ARRADCOM.

158
Manuscript received

2-27-86 SAB

Ion Exchange Strengthening of Nd Doped Phosphate Laser Glass

Kathleen A. Cerqua and Stephen Jacobs

Laboratory for Laser Energetics

and

Brian L. Mclntyre

New York State Center for Advanced Optical Technology of the Institute of Optics

University of Rochester

250 East River Road

Rochester, New York 14623-1299

716-275-5101

and

William Zhong

Kigre, Inc. 5333 Secor Road • "

Toledo, Ohio 43623

Recent advancements in high repetition rate and high average power laser systems

have put increasing demands on the development of solid state laser materials with high

thermal loading capabilities.

We have investigated strengthening of commercially available Nd doped phosphate

glass via an ion-exchange process. Introduction of a compressive layer to the glass'

exterior surfaces through this exchange, results in dramatically improved

(approximately 4-5x) thermal shock resistance for specific sample geometries. This

resulting increase in strength was directly measured through thermal shock resistance

tests.

Our thermal shock test data showed extremely small (approximately ± 5%) standard

deviations compared with data acquired through conventional mechanical fracture

techniques. Additional topics investigated were strengthened layer thickness and

effectiveness as a function of treatment time and the relationship of strengthening to the overall optical quality of the strengthened sample's surface.

Key Words: fracture strength; high-average power lasers; ion-exchange surface treatment;

phosphate glass; solid-state laser glass; surface compression; surface

strengthening; thermal shock resistance.

159
1. Introduction

Recent advances in high repetition rate and high-average-power (HAP) laser systems have put

increasing demands on the thermal-loading capabilities of solid-state laser materials. In

addition to the material's thermal properties, mechanical and optical limitations must also be

identified if a reliable, high-performance system is to be maintained. After considerable

effort in materials research at LLE and elsewhere, the physical limitations of various solidstate laser materials have become better understood. This article describes specific efforts

to increase the thermal -shock resistance of a commercially available, Nd-doped phosphate laser

glass. Improvements in this physical property permit a glass to be pumped at levels that may

previously have led to its fracture, resulting in a capability for higher-average output

powers.

A recently completed HAP laser system at LLE, which serves as a high repetition rate source

for the in-house damage-testing facility, consists of a Nd:glass oscillator, rod preamp, and a phosphate-glass slab through which a beam propagates in a series of zig-zag, total internal

reflections. Total internal reflection (TIR) makes efficient use of the entire gain medium.

The slab is pumped and cooled on the two TIR sides, which reduces thermal beam distortion often

present in alternative geometries. The slab is operated in a multi-pass configuration to additionally amplify the oscillator output. These features illustrate how the slab geometry

lends itself well to HAP applications.

With renewed interest in HAP systems, emphasis needs to be placed on those properties of

glass that make it a better HAP lasing gain medium. It is essential that the gain of the glass

be high if a compact system is to be capable of high average powers. The requirement for a low

nonlinear index is not as relevant for these systems, since longer pulses (nanosecond vs picoseconds) are frequently used. However, the issues of chemical compatibility and ease of

fabrication become increasingly important. Slabs in most HAP systems utilize continuous

cooling provided by a liquid flowing at a relatively high velocity across the pump faces of the

glass. The gain medium must be durable, as chemically induced degradation to the precisionpolished TIR surfaces would adversely affect output performance.

Fabrication issues become exceedingly important with HAP systems. Slab geometries, in particular, require very tight tolerances. Large polished faces with sharp edges are required

to be extremely parallel, flat, and of excellent surface (scratch/dig) quality. Polished

Brewster-angle end faces need to be parallel to within arc seconds and of comparable, if not better, quality than the pump faces. For high-performance slabs these stringent fabrication

requirements frequently lead to high costs and long fabrication times.

In attempting to optimize the physical properties of a material to be used in a HAP system,

it is necessary to define those properties which limit its performance. The primary limitation

of glass is its thermal-loading capability, or more specifically, its relatively low thermalshock resistance. Byer [1] expresses the average laser power a HAP solid-state gain medium can withstand as the product of the material's thermal-shock resistance and the ratio of plate area

to plate thickness. A material's thermal-shock resistance is directly related to its strength,

via its modulus of elasticity, thermal expansion, and thermal conductivity. These physical

constants determine the stress limit, the fracture stress, at which a material will fail.

160
The thermal -shock resistance of a material, T^j^, is the temperature difference (AT) at which

the fracture stress (commonly called the tensile yield-stress) is reached and failure occurs.

This relationship is illustrated in Eq. 1, [2]

'SH

(1 - y)k

(1)

where is the fracture stress, y , k, E, and a are the material's Poisson ratio, thermal

conductivity. Young's modulus, and linear coefficient of thermal expansion, respectively.

Krupke [3] examined the (1 - y)k/a E portion of the term as a material figure of merit (FOM).

Data compiled for a number of crystalline and noncrystalline materials show that the FOM is much

larger for crystals than for glasses. The FOM, coupled with each material's known tensile yieldstress, indicate that crystals are several times more thermal shock resistant than glasses.

While this is a plus, crystalline materials of high optical quality and homogeneity are considerably more expensive than their glass counterparts and are in some cases simply

unavailable. Because of these limitations, we have chosen to examine the potential of external

treatments for improving the strength of commercially available high-performance laser glass.

Here we confine the discussion to Q-89, a high gain phosphate-composition glass manufactured by

Kigre, Inc. [4]

2. Ion Exchange Strengthening

The goal of any external strengthening treatment is to introduce a compressive stress at the

glass' surface, so as to increase the stress required to fracture the glass in tension.

Marion [5] has shown that by reducing surface and subsurface damage, present in the form of

microcracks (point sources where failure begins) via an acid etch, one can increase the fracture

strength of a material. This benefit comes, however, at the expense of a thickness of ~ 200 ym

of material and leaves a surface of relatively poor optical quality. Our alternative approach is to chemically treat the surface by way of an ion-exchange process (cracks included), in which the

surface becomes stronger through the introduction of a layer of compressive stress to the glass

surface. In principle, ion-exchange strengthening can be performed without sacrificing the

optical surface quality of the glass part.

Ion-exchange processing is routinely utilized by the glass industry for strengthening

various products. The mechanism involves the exchange of a mobile alkali-ion that is present

throughout the bulk material with a slightly larger ion of identical valence. The exchange is carried out in a molten-salt bath at an elevated temperature to accelerate the exchange of

diffusing ions. The mobile ions readily diffuse out of the bulk, leaving vacancies behind.

The larger B^ ions migrate from the salt bath into the glass by occupying the small A^ vacancies.

The important feature of this "crowding" mechanism, as outlined by Kistler, [6] is that it is performed at temperatures sufficiently below the strain point of the glass. An optimized

161
treatment schedule prohibits structural relaxation from occurring and the surface, as a result of

the larger ions, is "expanded" into compression. A number of factors will control the net compressive stress that results from the process: [7]

0 the radius ratio of the exchanging ions

0 the degree to which the exchange occurs

0 stress relaxation due to structural rearrangement of the surface

0 any change in thermal expansion coefficient

0 localized densi fication

0 the depth of the compressive layer.

Of these factors, the depth of the compressive layer is the most important. The layer can

be thought of as a physical barrier that impedes the propagation of cracks. A thick layer will

be more resistant to crack initiation and thus failure, since any flaw would have to penetrate

past this depth to initiate fracture.

Considerable work has been done on the strengthening of silicate glass. To our knowledge,

the process has not previously been extended to phosphate laser glass. While the basic mechanism

remains the same, the composition of the base glass network determines the efficacy of the

treatment. Silicates lend themselves well to ion exchange, because their network consists of

openly spaced SiO^ tetrahedra. This open network creates channels through which alkali ions can easily migrate.

The structure of phosphate glass is more complex. Whereas SiO^ tetrahedra are able to bond

exclusively to each other to form a network, phosphate glasses are composed of PO^ tetrahedra,

which are limited in that they can only bond to three like structures during network

formation. [8] For this reason, Al^O^ is frequently added to assist as a network former in a phosphate-glass matrix. This, coupled with relatively large amounts of another network modifier,

BaO, leads to a much more closed structure. This tighter matrix impedes the diffusion of alkali

ions deep into the glass. Silicate glass systems strengthened via ion exchange are known to develop thicker compressive layers than phosphates.

The diffusion process that controls the exchange mechanism is very sensitive to changes in the exchange bath. Physical variations due to particulate, moisture, ion concentration, and

other forms of bath contamination severely decrease the effectiveness of the process. In this

work the primary A^ alkali ion is Li^, a major constituent in Q-89 in the form of LigO. The

+ + + exchanging B ions are Na and K , present in the bath as nitrate salts. Figure 1 depicts the

ion-exchange mechanism in Q-89 and lists the other major constituents of this glass. The

stoichiometry of the nitrate salts used in the bath is important; a large Na^/K^ ratio would be

desired since Na^ is the predominant exchanging ion. However, Na^ is corrosive, and in applications such as these, where post-treatment surface quality is critical, a tradeoff must be

made.

3. Characterization of Exchange Process and Experimental Results

Several experiments have been conducted to examine methods for optimizing the strengthening

process. The various test geometries and results are described as follows:

162
compressive

layer

G1665

Fig. 1 Ion-exchange mechanism in Kigre Q-89. Mobile Li^ ions diffuse out of the glass,

leaving vacancies behind. Large Na^ and ions preferentially diffuse from the

bath into the small Li^ vacancies, creating a compressive layer on the glass

surface.

3.1. Thermal -Shock Tests

The improved strength through ion-exchange treatment was determined by a series of thermalshock-resistance tests. Samples of Q-89 were fabricated into right-circular cylinders, in diameter by h" in length with fine ground surfaces (#400 grit) and acid etched in NH^HF^ for

-6 minutes prior to exchange treatment. The cylinders were then evaluated for their thermalshock resistance through a soak/quench test. Treated cylinders were placed in an oven, raised

to a prescribed temperature, and "soaked" at that temperature for ~ 30 minutes to guarantee

sample-to-sample temperature uniformity. The samples were then removed and plunged into ice

water t"5-7°C). Upon removal from the ice bath, each sample was visually inspected for

fracture. If no fracture occurred, the AT(T - T . ) was recorded and the process

oven water' ^ repeated at a soak temperature increased by +10° C. This continued until all samples had

fractured. Results from these tests are shown in Fig. 2. Cylinders were treated in the

exchange bath for 0, 2, 4, 6, and 10 days. The treatment time (in (hours) ^) is plotted in Fig. 2 versus measured thermal-shock resistance, in degrees °C. The treatment was carried out

163
[Treatment Time (h)]^'^

( ) number of samples fractured

I

first standard deviation

G1669

Fig. 2 Fivefold increase in thermal shock resistance in cylindrical, Q-89 samples.

Excellent statistics (±5%) can be attributed to a clean, contamination-free bath.

at an exchange-bath temperature of 320°C, with samples treated statically (no agitation) and

removed and rinsed with deionized water every other day to remove any residual material buildup

on the sample surfaces.

Introduction of a compressive layer through ion exchange resulted in a fivefold increase

in thermal -shock resistance. Our thermal-shock test data show extremely small (approximately

±5%) standard deviations compared with the 50%-100% variation observed in data acquired through

conventional mechanical-fracture techniques.

The widening spread in data for days 1-6 are a result of an increase in the level of bath

contaminants. Depletion of diffusing ions and increased moisture content of the treatment bath

are believed to have led to a larger standard deviation and poorer sample surface quality. The

treatment of 15 samples for 10 days in a new, presumably dry bath restored the low standard

deviation and improved sample surface quality. Although samples treated longer than 10 days were not tested, it is important to note in Fig. 2 that we did not observe any leveling off in the strength improvement data. Thus, the ultimate levels of strength improvement are not known

at this point and deserve further investigation.

Ion penetration depths of approximately 60 \m were measured via electron microprobe

analysis. [9] A depth profile for the light, highly mobile Na"*" ion that preferentially

164
Fig. 3 (a) Ion-exchange depth profile for six-day treated sample, as determined by

electron microprobe analysis,

(b) Edge profile of alkali (Na"*") ion concentration as a function of treatment time.

Rollover after six days may be due to stress relaxation mechanism in glass

network, which allows migration of Na"*" ions away from surface and further into

the bulk.

diffuses into the glass is shown for a six-day treated sample in Fig. 3a. Similar profiles for

one-through four-day samples showed an alkali-ion dropoff to the detection limit at depths of

35 to 40 iJTi. However, six and ten-day samples remained at ~3%-5% levels considerably further

into the bulk. This observation is useful in explaining alkali-ion edge concentration data

shown in Fig. 3b. Here, we plot the edge concentration of alkali (Na^) ions as a function of

days of treatment. The rollover that occurs after six days may be due to stress-relaxation

processes occurring in the glass network at the elevated bath temperature of 320°C. This stress relaxation could allow ions in high concentrations at the material's edge to migrate

further into the bulk, thus decreasing the concentration at the edge.

Current work is focused on answering a number of questions relating to the details of the

exchange mechanism. Reproducible profiles of light-ion concentrations are difficult to obtain,

and can be improved by fabricating samples with very clean edges. Supplemental analytical

techniques will assist in confirming the details of the ion exchange, specifically in the

analysis of the compressive layer.

165
3.2. Refractive-Index Variation in the Surface Layer

In examining the optical effects of the strengthening process, experiments were designed

to investigate the "new" compressive surface layer. The surface refractive-index variations were measured as a function of days of treatment. A polished half cylinder was measured

unstrengthened and then remeasured following additional one-day increments of treatment. These

measurements were made on an Abbt refractometer , at X = 632.8 nm and at a temperature of 22°C.

The results of these tests are plotted in Fig. 4. The observed index decrease in the third

1

\ 1 1 \

0 12 3 4

Days of Exchange Treatment

G1666

Fig. 4 Refractive-index variation in surface layer as a function of days of treatment. Measured index decreases as LizO/R 0 ratio decreases.

decimal place is consistent with Kigre bulk-glass data for Q-89, which indicate a decrease in

refractive index with decreasing LiaO/RaO ratio. This decrease continues to occur in the

strengthened layer with additional days of treatment. It is not expected that this small

reduction in the surface index causes any deleterious effect on the TIR propagation of the beam

through a strengthened slab.

3.3. TIR Transmission Tests

This experiment focused on treatment-induced degradation to optically polished surfaces.

Thin slabs of Q-89, 15 x 75 x 3 mm, were fabricated for TIR transmission measurements, at

166

C\J

I O-JO

1.557

1.556

1.555

1.554
X= 1064 nm at a glass-air interface. The objective was to examine surface degradation and

increased scatter as well as possible limitations to TIR as a function of exchange-treatment

time. Measurements on treated slabs were compared with the transmission through an untreated

control sample. A YAG (x= 1064 nm) laser was injected at -45° (> critical angle, = 40°) into

the slab through a BK-7 prism, which was index-matched to the Q-89 surface, as shown in Fig. 5a.

1 .5489

I I \ \ 0 12 3

Days of Exchange Treatment

G1667

Fig. 5 (a) TIR experiment on thin slabs of Q-89, 9=45°, X = 1064 nm. Measurements were made in air.

(b) Reflectance per bounce decreases as a function of days of treatment. Gentle buff of

treated surface restores TIR reflectance to the >99% level.

After a series of TIR bounces the beam exited the sample and the total transmission drop was monitored. From this observation the reflectance per bounce was calculated. The result is shown

as a solid line in Fig. 5b. Surface films may become apparent after as little as three days of treatment. Visible

inspection of the samples after treatment showed a very irregular, mottled film, which gradually

worsened with treatment time. This film caused the steady drop in reflectance that was observed

as treatment time was lengthened. A gentle buff removing the surface film was found adequate to restore TIR reflectance to the 99% level (dotted line). Minimal { 0.5 jjh for three-day sample)

material removal was required to achieve this improvement. We believe the results of this test

to be based on a worst-case experiment, due to the use of an old, contaminated salt-bath. We

167
suspect that the observed surface film is the result of moisture contamination in the bath, [10]

which can be eliminated with proper preventive measures.

4. Summary

The details of the process as outlined here support the claim that phosphate laser glass

can be strengthened effectively. The demonstration of a fivefold increase in thermal -shock

resistance of Q-89 illustrates the potential of this technique. A first step in assessing the

impact of the ion-exchange process on the optical surface quality of fabricated parts is completed.

Ion-exchange strengthened phosphate glass is not yet available commercially. A number of

materials scale-up issues are yet unresolved. Work is continuing to address the issues of bath

contamination, optical surface degradation due to moisture, the effect of exchange treatment on the interferometric quality of fabricated parts (optical surface deformation due to compressive

stress), and the compositional gradients of the compressi ve-stress layer. The scalability of the

technology to larger, full size slabs will ultimately prove the feasibility of the process, and serve as a basis for increasing the capabilities of high average power laser systems.

This work was supported by the General Electric Company Advanced Laser Technology Group,

Binghamton, New York under contract #A25-E-0580BS, the New York State Center for Advanced Optical

Technology of the Institute of Optics, and the sponsors of the Laser Fusion Feasibility Project

at the Laboratory for Laser Energetics whose sponsors are as follows: Empire State Electric

Energy Research Corporation, General Electric Company, New York State Energy Research and

Development Authority, Ontario Hydro, Southern California Edison Company, and the University of

Rochester. Such support does not imply endorsement of the content by any of the above parties.

5. References

[1] R. Byer, Slab Geometry Lasers presented at International Lasers and E-0 Exhibition, Tokyo,

Japan, January 31, 1985.

[2] W.D. Kingery et al . , Introduction to Ceramics, Wiley, NY (1975), pg.823.

[3] W.F. Krupke, Insulator Materials in High Power Lasers for Inertial Fusion: Present and

Future, UCRL-89439 (1983).

168
[4] Kigre, Inc., 5333 Secor Road, Toledo, Ohio 43623

[5] J. Marion, Strengthened Solid State Laser Materials, UCRL-92680 (1985).

[6] S.S. Kistler, J. Am. Ceram. Soc. 45, 59 (1962).

[7] s.D. Stookey, High Strength Materials, pg. 569, V.F. Zackay, editor, Wiley, NY (1964).

[8] W.D. Kingery et al.. Introduction to Ceramics, Wiley, NY (1975), pg. 110.

[9] Analysis performed by Xerox Analytical Laboratory, Jos. C. Wilson Center for Technology,

Webster, NY.

[10] Private communication. Dr. W. LaCourse, Institute for Glass Science and Engineering,

Alfred University, Alfred, NY.

169
INFRA-RED ABSORPTION SPECTROSCOPY OF Nd:YAG AND Manusaryt Reoezved

Nd:GSGG SURFACE CONTAMINANTS

BY

DR. M.A. ACHAREKAR

LITTON SYSTEMS, INC. LASER SYSTEMS DIVISION

2787 SOUTH ORANGE BLOSSOM TRAIL APOPKA, FL 32703

Neodymium doped yttrium aluminum garnet (Nd:YAG) is a commonly used laser

material for solid-state high power, high PRE lasers. Gadolinium scandium gallium

garnett doubled doped with neodymium and chromium (Nd:Cr:GSGG) shows higher

efficiency than the Nd:YAG laser. We previously reported the bulk and surface damage

thresholds of the Nd:Cr:GSGG crystal. It was shown that the bulk damage threshold of

the Nd:Cr:GSGG was as high as the Nd:YAG crystal.

The surface damage threshold measurements for the Nd:YAG and the Nd:Cr:GSGG are reported in this paper. Infrared absorption spectroscopy was used to analyze surface

contaminants. The data show that the organic contaminants, CH^, derived from

hydrocarbon (skin oil) associated with improper parts handling, signifantly reduce

the surface damage threshold.

Key Words: co-doped GSGG; infrared absorption spectroscopy; laser induced damage;

Nd:Cr:GSGG; NdrYAG; optical surface contaminants; solid-state laser; surface 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 NdrYAG is limited to 2-3 percent. Gadolinium scandium gallium garnet doubled

doped with neodymium and chromium (Nd:Cr:GSGG) shows higher efficiency than the

NdrYAG laser. We previously reported ^^-^ the bulk damage thresholds of NdrCrrGSGG

crystal. It was shown that the bulk damage threshold of the NdrCrrGSGG was as high

as the NdrYAG crystal. In table 1, NdrYAG and NdrGSGG material properties I^^^'l^^^

are summarized.

The surface damage threshold data collected for NdrCrrGSGG material are reported

in this paper. For the optical damage measurements, a stable, TEM^^, Q-switched,

NdrCrrGSGG laser oscillator was set up. The unusual laser damage morphology is associated with NdrCrrGSGG samples. This statement follows from microscopic

examination of hundreds of damage sites. Therefore, the samples were studied using

infrared spectroscopy. The results of the infrared spectroscopy of the test specimen contaminants are also provided in this paper.

170
TABLE 1 GSGG and Nd:YAG Material Properties

PROPERTY Cr:Nd:GSGG Nd:YAG

Thermal Conductivity, < 7-9 W/m°C (Cone. Dep.) -13 W/m°C

Thermal Expansion, a 7.8 X 10"^/°C 7.8 X 10"^/°C

Young's Modulus, E -21,000 kg/mm^ -31,700 kg/mm^

Breaking Strengtht (4-pt bend), Sj 15-20 kgW -20 kg/mm^

Poisson's Ratio, v 0.11-0.18 Not Reported

Thermal Shock*, Rj -700 W/m -800 W/m

Index of Refraction, 1060 nm 1.925 1.815

Density 6.46 4.55

Nd-Segregation Coeff. 0.65 ± 0.04 0.18

10 ns Damage Threshold (Bulk) >28 J/cm^ 20 J/cm^

* R^ = (1 - v) KSy/aE

+ Depends on Fabrication

171
2. SURFACE DAMAGE MEASUREMENTS

2.1 Test Specimens

For measuring the surface damage threshold of Nd:Cr:GSGG and Nd:YAG, optically

polished discs of 0.38 cm diameter by 0.1 cm thick were prepared. The discs were prepolished prior to final polishing by the various methods. In the prepolish, the

discs were diamond lapped to a specular but sleek finish. The following three

methods were used for the final polishing.

Test samples in Group "A" were fine polished on a lapping surface of medium hard

polishing pitch on a planetary lap. The polishing agent was 0.3 ym alpha alumina

mixed thin in water. Test samples in Group "B" were fine polished on a lap of Pyrex (Tm Corning

Glass). The lap was plane-ground with 30 pm alumina to an even roughness and

cleaned. The polishing was done with 0.3 m alpha alumina applied to the Pyrex lap. For this method the solid alumina content was high and a substantial part of the

vehicle was a commercial 1 igand-surfactant. The polishing was done by hand in the

traditional figure 8 stroke. Test samples in Group "C" were polished by a hybrid of the other two polishing

methods. The discs were substantially polished-out on the pitch polisher and then

touched up to improve flatness on the Pyrex polisher. All the samples were flat

within A/5 at 632.8 nm.

2.2 Experimental Techniques

For the optical damage measurements, a stable, TEM^^, Q-switched, Nd:Cr:GSGG

laser oscillator was set up. The experimental set up is shown in figure 1. In the

pump cavity, 0.5 cm diameter by 6.50 cm long, the Nd:Cr:GSGG laser rod was pumped

using a -Xenon gas filled at 400 torr pressure in a cerium doped fused silica envelope

flashlamp. The laser resonator consisted of a 5 meter radius mirror coated for

maximum reflectivity, spaced 40 cm apart with an output coupling mirror of 40 percent

reflectivity. In the resonator, a lithium niobate Pockels cell with a polarizer was used as the Q-switch. Q-switched output energy of 135 mJ was obtained in a beam

divergence of 2.18 mR with 8 J/P input energy at 10 pps. The pulsewidth of the

Q-switched pulses was measured using a EG&G 580 radiometer and a Tektronix Model 454A

oscilloscope. The typical full width at half amplitude (FWHA) pulsewidth for the

laser was 17.0 ns.

172
173
The Q-switched, Nd:Cr:GSGG laser system described above was used for the surface

damage threshold measurements. 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 photosensitive film; however, calculated spot

size was used in obtaining the damage threshold.

The surface damage threshold measurements were conducted using single and

multiple shots. In this work, "damage" is defined by the appearance of a flash of

light and the attenuation of a transmitted HeNe beam through the sample. This

definition and method are widely used in the study of single and multiple shot

damage, and the details of the methods can be found in reference [4].

3. EXPERIMENTAL RESULTS

3.1

The Nd:YAG and Nd:Cr:GSGG Surface Damage Threshold was initially measured using

the sample discs in Group "A" (pitch lap polished). The damage threshold for single

and multiple shots was measured. It was observed that the Nd:YAG crystal single shot

2 surface damage threshold was 13.51 ± 1.26 J/cm , while the Nd:Cr:GSGG crystal single

2 shot surface damage threshold was 16.38 ± 3.11 J/cm . However, the multiple shot

surface damage threshold for the pitch lap polished samples shows the NdrYAG crystal

at 9.74 ± 0.95 J/cm^ while the Nd:Cr:GSGG crystal was 11.03 ± 1.07 J/cm^.

The significant increase in mean deviation for the single shot damage threshold

as compared to the multiple shot damage threshold is associated with the stability of

the laser output. Generally, higher pulse-to-pulse stability is observed at 10 Hz rsi repetition rate due to the thermal compensation in the resonator '- . However, the mean deviation difference for the single shot surface damage threshold between the

Nd:Cr:GSGG (±3.11) and the Nd:YAG (±1.26) led us to believe that the surface quality

for the Nd:Cr:GSGG may not be as good as the Nd:YAG samples. Therefore, Nd:Cr:GSGG

discs were Pyrex lap polished (Group "B" samples). The surface damage threshold of

2 2 15.62 ± 0.72 J/cm was observed for single shot and 10.58 ± 0.72 J/cm for multiple

shot. It is clear that the single shot surface damage threshold for the Pyrex lap

polished samples did not improve the higher mean deviation (±3.01). Hence, a hybrid

polishing technique of pitch and Pyrex lap was used (Group "C" samples). The data

collected for all the samples are summarized in table 2. As seen in the table, the

polishing technique did not seem to change the higher mean deviation observed for the

single shot surface damage threshold of the Nd:Cr:GSGG crystal.

174
Table 2. Surface Damage Threshold in J/cm for Nd:Cr:GSG6

Group "A" Group "B" Group "C" PITCH LAP POLISHED PYREX LAP POLISHED HYBRID PITCH + PYREX

SINGLE MULTIPLE SINGLE MULTIPLE SINGLE MULTIPLE

SHOT SHOT SHOT SHOT SHOT SHOT

24.76 10 .51 20.51 9.75 18.14 9.82

12.88 8 .85 15.72 12.99 15.19 10.86

12.78 9 .64 12.02 9.04 13.73 12.36

16.96 13 .47 21.69 9.57 18.31 10.39

12.70 11 .55 12.34 10.33 11.44 12.14

15.78 13 .38 11.02 10.60 16.79 10.36

21.60 11 .41 18.71 10.89 19.02 11.24

14.40 10 .84 15.86 10.94 19.72 10.01

19.70 11 .05 11.67 11.21 14.29 9.00

12.01 11 .38 12.99 11.21 13.59 11.05

15.78 9 .45 20.69 10.46 15.00 11.72

17.30 12 .02 14.30 10.05 15.45 10.39

16.31 ± 3. 11 11.03 ± 1.07 15.62 ± 3 .01 10.58 ± 0.72 15.89 ± 2. 09 10.80 ± 0.7c

3.2 Damage Morphology

All the samples were examined with a microscope and scanned to find any

outstanding surface defects at lOX magnification. Any areas that became suspect were

then examined at 120X. Based on the microscope scan, less than two percent of the

samples were rejected for the damage threshold measurements.

In figure 2A, a microscopic photograph for the Nd:YAG front surface damage with

single shot at 1.061 ym (Nd:Cr:GSGG laser output wavelength) is shown. When this

damage site is compared with the typical damage site on the Nd:Cr:GSGG, as seen in figure 2B, several differences are observed. It may be noted that the Nd:YAG crystal

is pink-violet in color while the Nd:Cr:GSGG is green in color. The most notable

difference in the damage sites was that the Nd:Cr:GSGG surface damage always was associated with dark color rings around the damage site. This fact led us to believe

that the surface was contaminated with organic material.

175
A, Ncl:YAG Front Surface Damage with Single Shot 1.061 ym (120X)
4. INFRARED SPECTROSCOPY

4.1 Typical Surface Contaminants

Typical surface contaminants of optical elements can be analyzed using infrared

absorption spectroscopy. Using a tatanium blade Micro-Spatula wetted with

chloroform, the damage site is gently scraped, and if any contamination is present

the chloroform will dissolve or soften it to adhere to the Micro-Spatula. The

Micro-Spatula is then dipped into the 50 cc beaker which contains -20 cc of Burdick & Jackson infrared spectroscopy grade chloroform. The contaminants are then dissolved

into the beaker. This procedure is repeated until, using a lOX illuminated

magnifier, as much of the contamination as possible is removed. The solvent in the

beaker then is gently evaporated, using a warm hot plate, until approximately 200 y£

remains. This quantity then is transferred while still warm to a potassium bromide

infrared sampling plate using a 100 u£ Micro-Pipet. Thus, a micro-cast film of the

contaminants is prepared onto the plate.

This plate is now analyzed using a Perkin-Elmer Model 783 infrared

spectrophotometer. The infrared spectrum from 4000-500 cm"''' is obtained from a six scan signal -average run, typically using a 5.4 cm'''' slit and noise suppression factor

4. The spectrum is analyzed using a Perkin-Elmer Model 7500 infrared data system.

The spectrum is Computer Library searched for identification of all possible

molecular structural units and spectral matching. In figure 3, the typical infrared

spectra for organic contaminents , viz. skin oil, is shown. Using a similar method,

infrared spectrum features were identified for -CH vibrational modes associated with

organic contaminants by Lewis and Arthur '- at the Royal Signal and Radar

establishment, Malvern, UK.

4.2 Test Specimens Contaminants

The method described above was used to identify surface contaminants on one disc

of Nd:Cr:GSGG from Group "A" and one disc of Nd:YAG crystal. These samples were

selected by a lOX microscope examination of the damage sites. The discs were

selected particularly for the appearance of dark rings around the damage site. The

infrared spectra collected from the contaminants is shown in figure 4. The infrared

spectra collected for Nd:YAG and Nd:Cr:GSGG did not show a significant difference,

indicating that organic contaminants were not the cause of the higher mean deviation

observed in the Nd:Cr:GSGG surface damage threshold.

177
Figure 3. Typical Infrared Spectra for Organic Contaminant

178
Figure 4. Infrared Spectra for Nd:Cr:GSGG Surface Contaminants

179
5. CONCLUSIONS

a. The surface damage threshold for Nd:YAG, pitch lap polished, using 0.3 ym

2 alpha alumina mixed thin in water, was measured to be 13.5 ± 1.26 J/cm for

2 single shot and 9.74 ± 0.95 J/cm for multiple shots. For the damage

measurements, a stable, TEM^^, Q-switched Nd:Cr:GSGG laser operating at

1 .061 pm was used.

b. The surface damage threshold for Nd:Cr:GSGG, pitch lap polished, using 0.3 ym

2 alpha alumina mixed thin in water, was measured to be 16.38 ± 3.11 J/cm for

2 single shot and 11.03 ± 1.07 J/cm for multiple shots at 1.061 ym.

c. The higher mean deviation for the single shot surface damage threshold as compared to the multiple shot surface damage threshold is associated with

poor shot-to-shot stability in the single shot operation of the laser.

d. The mean deviation for the single shot surface damage threshold observed for

Nd:Cr:GSG6 (±3.11) is substantially higher than the mean deviation for the

single shot surface damage threshold observed for Nd:YAG (±1.26).

e. Three different methods of polishing used did not seem to make a difference

in the single shot surface damage of the Nd:Cr:GSGG or the high mean deviation. The single shot surface damage threshold for pitch lap polished,

Pyrex lap polished, and hybrid pitch plus Pyrex polished was measured at 16.31 ± 3.11 J/cm^, 15.62 ± 3.01 J/cm^, and 15.89 ± 2.09 J/cm^, respectively.

f. The damage site comparison shows that the Nd:Cr:GSGG surface damage always

was associated with dark color rings around the damage site.

g. Infrared absorption spectroscopy of the damaged Nd:Cr:GSGG and Nd:YAG did not show significant difference in organic contaminants on the surface.

h. Finally, the Nd:Cr:GSGG surface damage threshold is as high as the surface

damage threshold of the Nd:YAG, but the mean deviation in the Nd:Cr:GSGG

surface damage threshold is higher and the damage sites are surrounded by

dark color rings.

6. ACKNOWLEDGEMENTS

This work is a part of an internal R&D program conducted at and supported by

Litton Laser Systems, Orlando, FL. Thanks are due to Marvin Kaplan for assistance

with Infrared Spectroscopy and Julie Trudell, Phyllis Milligan, and Jacqueline

Wartell for preparation of this manuscript.

180
7. REFERENCES

[1] Acharekar, M.A., McCarthy D.P.; Blackman R. Pulsed laser induced optical damage

threshold in Nd:Cr:GSGG crystal, Laser induced damage in optical material.

Boulder Damage Symposium, 1984 Sept., (Proceedings to be Published).

[2] Zharikov, N.N. Laptev, V.V.; et.al., Sov. J. Quantum Electron. 13 (1), 82-85,

1983 Jan.

[3] Krupke, W.F.; Modern solid state laser materials, Lawrence Livermore National

Laboratory, Tech. Note UClD-20119, 1984 June.

[4] Merkle, L.D.; Bass, M. and Swimm, R.T.; Multiple pulse laser induced damage in crystalline and fused quartz at 1.064 and 0.532 ym, Nat. Bur. Stand. (U.S.)

Spec. Publ. 669, 1982 Nov., 50 p.

[5] Teppo E.A.; Nd:YAG laser technology, Naval Weapons Center, Tech. Note 4051-2,

1972.

[6] Lewis, K.L.; Arthur, G.S.; Surface and free carrier absorption processes in CVD

zinc selenide, Nat. Bur. Stand. (U.S.) Spec. Publ. 669, 1982 Nov., 86 p.

181
Manusaript received

2-28-86 SAB

Excimer Radiation Colouration Processes and

Surface Damage to Single Crystal

CaF2 Substrates

I. Laidler and D.C. Emmony

Department of Physics

Loughborough University of Technology

Loughborough, Leicestershire, LEll STL)

This paper considers the effects of a 249nm excimer pulse on CaF2 substrates and is concerned with both surface damage and bulk colouration processes.

The effect of different polishing procedures on the surface damage threshold of

CaF2 has been investigated.

CaF2 has been shown to develop colour centres under prolonged U.V. irradiation.

A technique has been developed to monitor the transient excited states created in the bulk

material when irradiated with a 25ns excimer pulse. This involves a simultaneously pumped

dye laser pulse probe operating at the absorption wavelengths of the excited species.

From the results information about the photon absorption cross sections can be derived,

the concentration of defect states at any time during the excimer pulse may be estimated

and a knowledge of the maximum safe fluence levels established.

Key Words:- CaF^ single crystal; surface finish; laser damage; colour centres; transient

de fects

.

1. Introduction

Calcium fluoride is an important material for those working at the excimer wavelengths, both

within and external to, the laser cavity. Damage in CaF2 optics may manifest itself in one of two ways. Bulk and surface mechanical damage, or the long term degradation of the material's optical

properties due to the irradiation induced generation of colour centres. Both these effects have

been investi gated using a 249nm, 30ns excimer laser with an output pulse energy of around 700mJ.

The experimental configuration and beam characteristics can be seen in figures 1 and 2.

2. Sample Preparation

In order to study surface effects eight sample surfaces were prepared and the damage threshold

and damage morphology of each studied. The surfaces were variations on a standardised polishing

technique consisting of an alignment and cutting stage, an intermediate and then final polishing

stage. Each surface was damage tested, then chemically etched and retested. The variations

enabled conclusions to be drawn as to the effect of sample purity, surface stress induced during

the cutting stage, the effect of surface scratches and chemical etching. The surfaces can be classified as follows and are given with their damage thresholds.

i

)

Standard Material and Standard Polish 6Jcm" 2

i i

)

As in i) plus a chemical etch 9Jcm- 2

i i i

)

Optipur Material and Standard Polish 6Jcm" 2

i v) As in iii) plus a chemical etch. 9Jcm- 2

v) Standard Material but stress reduced Standard Polish 5Jcm" 2

vi

)

As in v) plus a chemical etch 11 Jem" 2

vi i

)

Standard Material plus ultra fine Surface Polish 7Jcm" 2

vi i i

)

As in vi i ) plus a chemical etch 12Jcm' 2

182
All figures have a ± 0.5J tolerance.

Damage has been defined as the fluence required to produce any visible damage (using a 1000 x Nomarski microscope) in 50% of the shots. The figures quoted are single shot thresholds and the

spot size is large enough to remove any statistical dependence of damage threshold.

3. Discussion

As well as Nomarski microscope a Transmission Electron Microscope (T.E.M.) was used to look at the surfaces, both before and after damage testing. Prior to damage the surfaces looked identical

except for samples vii) and viii) which were much more scratch free. No differences could be

detected on the chemically etched surfaces. Visually they looked the same as the un-etched

substrates. Practically, however the chemical etch raised the damage threshold by at least 50% and

up to 100%. Figures 3a and 3b contrast a Standard Polish with an Ultra Fine Polish.

Observations of the damage sites has revealed the following points;-

a) cleavage occurs along scratch lines or grooves. This was true on all surfaces.

b) Sample v) with the lowest damage threshold has embedded impurities causing localised absorption

and circular damage sites and cleavage centering on the impurity. See figure 4. The

surounding area was untouched by the radiation. The chemical etch appeared to have removed

these impurities.

c) Although cleavage was the most obvious form of damage in all samples it did not appear to be

the first form except in sample v). All the other surfaces showed, without exception, that

the onset of damage was light pitting and melting. The presence of scratches were irrelevant

to this mechanism.

d) All cleaved areas show that the actual cleaved surface has flowed, i.e. melted, again

indicating that melting is the first form of damage.

e) Samples vi ) and viii) had a large difference in surface roughness as seen in figures 3a and

3b and yet only a IJ difference in damage threshold.

Firstly it would appear from these results that the purity of the bulk material is not significant in surface damage. Mass spectrographi c analysis of the two grown sample materials

show trace impurities of the rare earths in the ppm level of the naturally occuring fluorspar

which is not present in the optipur material.

The cause of cleavage in CaF2 has previously been discussed |1,2| and attributed to the

plastic deformation of the material during heating. This causes slip planes to appear along the

(ill) faces and specifically orientated (111) surfaces to be pushed up out of the surface,

figure 5, |3|. On cool down the plastic deformation results in a residual stress which may cleave

the crystal, figure 6 . We can assume that at room temperature CaF2 can withstand a specific

degree ot internal stress but above this value it must yield and cleave. (Remember it can plastically deform at high temperatures). Around scratches and grooves the intrinsic stress is enhanced by a factor of the order ( -jh where i is the length of the crack and r is the radius of

the crack tip. For surface featured characteristic to these polished samples an enhancement of

30x is typical. Thus cleavage, a stress related phenomena, first manifests itself around such

features.

The results, however, would indicate that it is necessary for the surface first to melt before

there is enough thermal energy to cause significant plastic deformation giving rise to residual stresses large enough to cause cleavage. Even around typical scratches the cleaved material has

first flowed.

The exact role played by surface scratches is still uncertain, but does not seem all that

great in CaF2, especially in the light of vi ) and viii). Perhaps their most important role is as

a trap for absorbing impurities. The scratches themselves would not appear to play a part in the

damage initiating mechanism. The chemical etch has greatly increased the thresholds without

significantly changing the volume of the surface features and it is suggested that it has acted

purely as a thorough cleanser. Damage may be initiated by surface dirt or grease etc, heating the

substrate to a sufficient temperature for some instrinsic temperature dependent effect to take over

i.e. avalanche ionization. This mechanism may then take the whole surface to the melting point.

183
4. Colouration Processes

The role of colour centres in bulk damage processes is continually being debated |4|. With

this in mind an experiment has been devised to monitor, during the damaging radiation pulse, the

concentrations of primary defects produced by this pulse. This work has two aspects of interest

Firstly to try and understand if the laser induced, transient defects play any role in bulk damage

phenomena. Then secondly to investigate the processes that lead to radiation induced colour

centres in single crystal CaF2. With the increasing interest in large single crystal CaF2 as an excimer window material the long term degradation of the window's transmission through colour

centre generation is a frustrating and expensive problem.

5. Experimental Arrangement

The experimental arrangement can be seen in figure 7. It uses an identical damage facility

as employed for the surface work. The samples are now, however, cubic in geometry. Immediately

on leaving the laser the excimer beam is split, 15% of the 249nm light being used to pump a dye

laser. The dye beam is T.E.M. with a beam diameter of 2mm and a temporal profile corresponding

to that of the excimer pumping pulse, though clipped in the wings. Figure 7 shows the dye beam

split into two arms, a probe and a reference arm. The probe arm interrogates the crystal, entering

perpendicularly to the excimer and passing concurrently in time through the excimer's focus. The

reference arm traverses a 35ns delay line. Both beams are then incident upon a vacuum photodiode

and the signal recorded by a fast oscilloscope capable of resolving their temporal profiles. Thus,

direct comparison of the two pulses shown on the oscilloscope, the time dependent absorption of

the dye beam can be ascertained. This beam is tuned to the absorption band of the excited species

and primary defect of interest. The focussing optics of the probe beam is such that refractive

index changes in the sample will not result in a loss of light and hence an apparent absorption.

The first excited state to be considered was that of the V, centre or "self-trapped hole". In

CaFj^ it is this state that leads to the production of metastable F and H centres. The V|^ centre

is formed every time an electron can be excited out of the valance band to the conduction band or to impurity traps. The hole relaxes to its lowest energy configuration, i.e. it forms a covalent X" molecule with the nearest neighbour halogen ion. The V, centre, providing a highly efficient

electron trap, does not, in CaF2, directly revert back to the perfect lattice by straight forward

recombination. Rather the centres rotates into the (111) direction and a close F-H pair is the

result |5|. The close F-H pair may then radiatively recombine or separate, depending upon the

environment, forming the stable F centre and H centre and colouring the crystal. It is hoped that

the experiment described will give us a better insight into the conditions for colour centre

generation and ways of avoiding them.

The e>lperiment was initially performed using two different grades of CaF2 . Crystals were

grown using optipur material and also a naturally occuring fluorspar. Mass spectrographic analysis

of these materials showed the naturally occuring fluorspar to contain between 0.1 and lOppm of

the rare-earths whilst the optipur material, within the detection^capabi 1 i ty was free of these

elements. The rare-earths will enter the fluorite lattice as a 3 ion sittj^ng above the valance

band and accessible to U.V. photons |6|. On trapping an electron the new 2 state now lies within

a photon's reach of the conduction band. A method of V, centre generation is thus available,

and it can be seen that a single impurity may give rise to more than one centre due to the

cyclic nature of its production. The lifetimes of such states and formation rates need to be

understood.

6. Results and Discussion

Figure 8 shows the increasing optical absorption caused by an increasing V, centre concentration as theexciting pump fl uence is raised. Trace A is that of the naturally occuring

fluorspar, 15 ns into the excimer pulse, A2 10ns later. Trace B which indicates no optical

absorption is that for the optipur material. Simultaneous fluorescence due to the first excited

state of the Vj^ centre was observed in the fluorspar material but not in the optipur.

Analysis of the results using a knowledge of the excimer's spatial and temporal profile

energy content and crystal impurity level enables an estimation of the V, centre creation cross section (lO'^^cm'^). The continuous lines on the graph represent a first order theory from which

is derived the V. centre creation cross section. It will be noted that the graphs tend to saturate

at a level defined by the rare earth impurity concentration and exciting photon fluence. After

saturation a gradual linear increase is experienced itj concentration. This is di|ie to the cyclic

nature of the rare-earth trapping process where the 2 state is ionized becoming a 3 impurity

available for valence electron trapping. It would appear that the time for one cycle to be

completed is of the approximate order of the excimer oulse.

184
7. Summary

Two forms of damage have been investigated. Work on surface finish of CaFa surfaces indicates

that the damage threshold of the surfaces is not dependent upon ionic impurities, and not as dependent upon surface topography as believed. Surface cleanliness is by far the greatest factor

affecting damage thresholds.

Colouration processes have been investigated and a sensitive technique developed to iponitor

the concentration of primary defects. So far work indicates the presence of rare earth 3 ions

is crucial in the formation of both transient and metastable colour centres. It is hoped to continue the work to see how these states affect the bulk damage phenomena.

The authors would like to thank B.D.H. Chemicals, England^for their help and sponsorship of

this work, through an SERC CASE award, for providing and preparing the crystals and most

importantly, the initial impetus for the work.

8. References

|1| Detrio, J. A.; Graves, G.A. ; Wimmer, J.M. Failure criteria for laser window materials. Nat.

Bur. Stand. (U.S.) Spec. Publ . 568; 1979. p.p. 151-158.

|2| Loomis, J.S. Fourth conference on high power infra-red laser window materials. 1974

13| Porteus, J.O. ; Soileau, M.J.; Fountain, C.W. Character of pulsed laser damage to Al at 10,6pm

infra-red from single-crystal targets in vacuum. Nat. Bur. Stand. (U.S.) Spec. Publ. 462;

1976. p.p. 165-172.

|4| Brallnlich, P.; Brost, G. ; Schmid, A.; Kelly, P. The role of laser induced primary defect

formation in optical breakdown of NaCl . Nat. Bur. Stand. (U.S.) Spec. Publ. 620; 1980,

p.p. 406-415.

|5| Williams, R.T. Photochemistry of F-center halide crystals. Semiconductors and Insulators.

Vol. 3 p.p. 251-283; 1978

|6| Kiss, Z.J. Photochromatic materials for quantum electronics. I.E.E.E. Journal of Quantum

Electronics, Vol. QE-5, No. 1, p.p. 12-17; 1969.

185
Calorimeter

Video profiling/monitoring

system

Figure 1. Damage configuration

Horizontal profile

Vertical profile Temporal profile

Figure 2. Focused beam characteristics

186
Figure 4. Impurity centre damage

187
Figure 5. CaF^ (HI) surface and slip planes
Reference arm

35ns delay line

Spatial filter

<-4-s

Probe arm

Sample

/Calorimeter

"h Excimer

laser

Attenuator

Fi 1 ter

I

V^rijum

_Jfl^ photodiode

Figure 7. Experimental arrangement for monitoring transient absorption

40

Figure 8. Absorption (V^^ centre concentration) growth as a function of energy density

189
- Manuscript Not Received -

ANALYSIS OF OPTICAL SURFACES BY LASER IONIZATION MASS

SPECTROMETRY: SPATIAL VARIATION OF THE LASER DAMAGE

THRESHOLD AND ITS RELATIONSHIP TO POLISHING

COMPOUND INCORPORATION

Wigbert J. Siekhaus

Lawrence Livermore National Laboratory

Livermore, CA 94550

190
Manuscript received

3-S-86 SAJR

Laser Induced Damage and Ion Emission of GaAs at 1.06 ym

A. L. Huang, M. F. Becker, and R. M. Walser

Electrical and Computer Engineering Department and

Electronics Research Center

The University of Texas at Austin

Austin, Texas 78712

The experiments in this study were focused on the N-on-1 laser damage and subdamage threshold ion emission of gallium arsenide. The initial goals were (1) to determine the N dependent damage threshold due to N-on-1 laser irradiance and (2) to correlate ion emission with surface damage events. A Q-switched Nd:YAG laser (1.064 ym,

45 nsec pulse, 580 ym spot diameter) was used to irradiate the <100> single crystal

gallium arsenide samples.

Using values of N from 1 to 100, we obtained accumulation curves based on 50%

damage probability. Corresponding damage threshold fluences were 0.4 - 0.8 J/cnr for

N>1 and approximately 1.5 J/cm^ for N=l . We observed large site-to-site fluctuations

in ion emission, and emission occurred before surface damage was initiated. We found

the onset of ion emission at a fluence of 0.2 J/cwr for all cases, whereas the damage

threshold fluence was two to five times larger and dependent on N. Experimental mea- surements of linear and nonlinear absorption coefficients were made to check for

anomalous absorption. The measured values were in agreement with those previously

reported. Once surface damage occurred, ion emission was greatly increased. This type

of behavior seems to support a surface cleaning model for ion emission which precedes

surface damage.

Key words: gallium arsenide; charged particle emission; N-on-1 laser damage threshold.

1. Introduction

The interaction between laser radiation and solids has been a perplexing problem for many

years. Often the lifetime of an optical device is determined by its susceptibility to optical

damage. There has been much controversy and unexplained phenomena associated with the energy

transfer mechanism of normal laser damage as well as the physical nature of surface damage.

The objectives of the experiments reported here were to characterize the statistical nature

of surface damage for gallium arsenide, to observe the relationship of charged particle emission

to surface damage, and to observe the damage morphology of gallium arsenide. In silicon it has

been reported that charged particle emission is coincident with surface damage [1]. A relation

between charged particle emission and surface damage has yet to be reported for gallium arsenide.

To explore the statistical nature of surface damage of gallium arsenide, we performed various single shot and N-on-1 sub-threshold laser tests. We measured the positive charged particles emitted

during each laser pulse to correlate the charge emission events with surface damage. Theoretical

calculations for a thermal model, as well as an electron-hole production model, have been carried

out to determine the mechanism responsible for the surface damage. .

2. Experimental Setup and Apparatus

2.1. Samples

The gallium arsenide samples used in the experiment were supplied by the microwave integrated

circuit production group at Texas Instruments in Dallas, Texas. The samples had a <100> orientation and were very lightly doped with chromium (;^lxl0^6 cm"^) , but otherwise undoped and unannealed. These samples were characterized by a resistivity of greater than 1x10' fi-cm and an etch

pit density of 40-60 x 10^ cm"^. Only the front face of the wafer was polished to optical quality.

To prepare the sample for the vacuum tank we sequentially cleaned the gallium arsenide wafers

in boiling solvents of trichlorethylene, acetone, and methanol to assure that all contaminants

had been removed. While in each solvent, the wafer was ul trasonical ly cleaned before proceeding.

191
After the final cleaning in methanol, the wafer was rinsed in deionized water (less than 5 minutes at room temperature) and placed into the vacuum tank.

To measure the linear and non-linear absorption coefficients in the transmission tests, the

gallium arsenide wafers were given an optical quality finish on both sides. The backside of the

wafer was mechanically polished in a two step process. Rough polishing with a grinding pad with

six micron diamond ferrous paste was used to initially buff the wafer. The finishing pad used a liquid, 0.05 micron alumina, non-ferrous, suspension as a grinding media. After polishing, the

wafer was chemically cleaned as described above.

2.2. Experimental Setup

The experimental system is shown schematically in figure 1. It utilized a Q-switched NdrYAG

laser with a full width half maximum (FWHM) pulse length of 45 nanoseconds, TEMqq mode, and wavelength of 1.064 pm. A knife edge scanning technique was used to measure the focused spot diameter

on the sample surface which was found to be 580 pm. The laser stability fluctuated from pulse to pulse resulting in a 2-5 percent standard deviation in energy for each data run. The laser was operated at a 15 Hz repetition rate and the incident energy was attenuated by rotating a half-wave

plate in front of a polarizer.

A computer was utilized to control and count the laser pulses striking the gallium arsenide

sample and to record the charge collected. The computer had a Z80 microprocessor and supported

several data acquisition ports via sample and hold amplifiers and a mul ti -channel A-to-D converter. The delay for the sample and hold amplifier was set to 130 psec in hardware. This assured

that the integrated values read for the charge emission were near the peak value, but were collected well after the laser noise burst. The electronics were triggered by a vacuum photodiode.

A shutter, controlled by one of the computer's digital output ports, was used to select pulses for

sample irradiation. To allow time for data computation and storage between pulses, the shutter

selected every third pulse from the laser train.

A 760 mm focal length lens focused the beam onto the sample. The long focal length gave a region of constant spot size approximately 3 mm deep at the focal plane and permitted greater error in the placement of the sample. A system of 90° prisms was placed after the lens to scan

the beam across the sample in the vacuum tank.

To detect charged particle emission the sample and detection device were placed into the vacuum environment. The system was dry pumped to achieve experimental pressures of less than

1x10"° Torr. A Hamamatsu electron multiplier tube model R596 served as the ion detector and was placed inside the vacuum system at a 36° angle to the laser beam path. The sample surface was

normal to the optical path. The output of the detector was connected to a capacitive voltage

divider and amplifier. The dynamic range of the data acquisition system was increased by making

one channel 100 times less sensitive than the other. The electron multiplier current gain was estimated to be approximately 0.75x10° at a voltage of -2kV. A data value of 1000 in the more sensitive A-to-D converter channel corresponded to a charge of approximately 1.5 x 10"'' Coulombs.

3. Data and Analysis

Three experiments were conducted on the cleaned samples: 1) an emission scan of the gallium

arsenide sample at a fixed laser fluence, 2) an N-on-1 emission/damage threshold test, and 3) a transmission test. All of the tests were performed in vacuum with the exception of the transmission test. The calibration and beam profiles were checked at the start and end of each experimental session to assure accurate beam fluence measurements.

The emission scan of the gallium arsenide sample checked the uniformity of charge emission

under constant laser fluence. The scan spots were separated by 0.8 millimeters and 42 samples were taken. The sample was irradiated with a constant fluence of 0.63 J/cm^ ( ± 7%). The

results in figure 2 show a bi-level emission contour. A different pattern of emission variations

was observed on each sample. Site-to-site fluctuations of the defect density in gallium arsenide

have been previously noted [2] and could disrupt the statistics of charged particle emission and,

possibly, the emission threshold. To minimize the effects of these site-to-site variations, we conducted the emission/damage tests over a small area of the wafer.

Other experiments were concerned with the multi-pulse and single pulse irradiance of gallium

arsenide. In the multi-pulse experiments, fluences below the single pulse damage threshold were

used to search for accumulation effects associated with either emission or damage. The objectives

of these experiments were (1) to identify accumulation effects associated with surface damage, (2) measure a damage threshold based on 50% probability statistics, (3) search for correlations

192
inhomogeneous processes is made apparent by examining the laser absorption in the sample. From

the slope and intercept of the linear fit of the inverse transmission versus intensity shown in figure 10, we extracted the nonlinear absorption coefficient, Pq, and the linear absorption coefficient, Uq. First ctQ was found from the equation:

= (1-R)^ expC-a^L) (1)

where is the zero fluence transmission intercept, R is the reflectivity at the air-gallium

arsenide wafer interface, and L is the thickness of the gallium arsenide sample. In our case Tq was 0.447 (from figure 10), R was calculated to be 0.306, and L = 0.0635 cm. Substituting these

values into eq. (1) and solving yields a =1.18 cm"^. With known, the nonlinear absorption coefficient, 6q, can be calculated from the following equation:

m = eotexp(ctQL)-l]/[ao(l-R)] (2)

where m is the slope of the linear fit to the inverse transmission versus incident intensity data.

Equation (2) was derived from eq. (3) as described in reference [6]. From figure 10, we obtained

m = 40.2. Solving for 6q in eq. (2) yields 3q=0.42 cm/MW. When the pulse length is accounted for,

the values of and e are in agreement with those previously reported by Bechtel and Smith [7]

and Lee and Fan [8]. Therefore there is no evidence of anomalous homogeneous absorption.

4.1. Thermal Model

To determine the mechanism responsible for the laser induced damage, we examine the temperature change of the surface. The linear and nonlinear absorption must be included in the heating

model because the laser energy is absorbed throughout the gallium arsenide sample. For uniform

illumination, the intensity relative to that at the surface is given by the differential equation:

dl/dx = - a^I - 3ol2 (3)

where I = I(x) is the intensity at a depth x into the absorbing medium, and 3q are the linear

and nonlinear absorption coefficients, respectively. Assuming that all the absorbed energy is thermalized without diffusion, the peak surface temperature rise can be written

AT = (dl/dx)^^^f tp / P

where t is the pulse length, is the heat capacity, and p is the mass density. Substituting

eq. (3) into eq. (4), and taking the surface reflectivity into account gives

AT = [a^ + 6^ (1 - R)](l - R) tp /C^ p (5)

where Iq is the incident intensity. For gallium arsenide at room temperature, we have = 0.327

J/g-°K and p = 5.32 g/cm^. Using a typical damage intensity for the single shot case, Iq = 19 MW/cm2, we obtain a maximum temperature rise of 2.3°C. This dif fusionless model assumes homogeneous energy absorption and temperature independent reflection and linear and nonlinear absorption

coefficients.

From the results of the previous calculation we conclude that uniform heating is not sufficient to explain the melting effects observed on the gallium arsenide surface. A similar conclusion has been reported for silicon by Merkle, Uebbing, Baumgart, and Phillipp [9]. However, this

does not exclude the possibility of an abnormal absorption, or point absorption in defects at, or

near, the gallium arsenide surface.

193
between surface damage and charged particle emission, and (4) determine the damage morphology as a function of fluence and number of pulses.

For each value of the number of pulses N used (N = 1, 3, 10, 30, and 100) we irradiated 150

to 200 different sites in order to obtain the statistics of surface damage. Surface damage on the

front surface of the sample was identified by searching for surface changes with a Normarski optical microscope at 200X. The damage data for N =1, 10, and 100 are plotted in figures 3-5, respectively. In each plot we used a linear curve fitting program to obtain the damage probability.

From the linear fit we obtained the 50% probability fluence for each value of N, and used this

value in the accumulation plot in figure 6.

For all values of N, we observed the onset of charged particle emission at an average

fluence of 0.2 J/cm^ ( ± 0.069 J/cm^). The values for the onset of emission and the 50% damage

threshold are given in Table 1.

Table 1. Onset of Emission and 50% Damage Threshold

Fluence of N-on-1 Experiments.

H Charged Particle 50% Damage

Emission Onset Fluence [J/cm^]

Fluence [J/cm^]

1 0 18 1 518

3 0 15 0 7751

10 0 17 0 6357

30 0 32 0 7907

100 0 17 0 6781

From these damage fluence values and their plot in figure 6, it is apparent that an accumulation

effect was present. As the number of laser pulses incident on the wafer increased, the 50% damage fluence decreased. Similar curves to the one shown in figure 6 have been reported for metals

[3], polymethylmethacryl ate and modified polymethylmethacrylate [4].

The positive charged particle emission data was collected on a pulse-by-pulse basis. A typical profile for N = 30 is shown in figure 7. The site from which this emission profile was obtained was damaged. From these data we note that the first few pulses (from 2 up to 10 depending on the case) induced a charge emission that decreased as the pulse number increased. After

this, the emission increased greatly. We interpret the decrease in the first few pulses as a sur- face clea"ning effect. These first pulses cleaned any residue left from either the chemical cleaning process or that had been ejected from previously laser irradiated sites. This emission could

also be due in part to the depletion of the more volatile specie. As, from the surface atomic

layers. Figure 7 also shows that, once the site experiences surface damage, charge emission is greatly increased. The emission at non-damaged sites always decreased with increasing number of

pul ses

.

Figures 8 and 9 are SEM micrographs showing the development of laser induced surface damage

morphologies. The initial surface was featureless and without contrast. In the Normarski microscope the initial surface change appeared as a depression with an area equal to that of the laser

spot. Within this area there were several very small pitted regions (similar to those shown in

figure 8 but not as well defined). As the fluence increased, or the number of pulses increased,

the initial surface damage evolved into the melt pits shown in figure 8. The damage evolution

ends with severe cratering as shown in figure 9. The theory of Fauchet and Siegman [5] would

seem to apply in this case. They postulate that laser initiated ripple patterns on the surface

of silicon and gallium arsenide are formed from the interaction of the incident laser wavefront

with scattered optical surface waves. At 1.064 ym, however, with a pulse length of 45 nsec and

spot diameter of 580 ym, no ripple patterns were seen in our experiments. Accordingly, the

longer pulse lengths may inhibit ripple formation.

4. Discussion •

Thermal models have been used in the past to explain both laser annealing processes and

laser damage phenomena. These models assumed uniform heating, and the possibility of

194
4.2. Electron-hole Plasma Model

An alternative process that might lead to damage is heating caused by the electron-hole

plasma produced during the laser-solid interaction. The relation of this process to silicon

annealing has been reported previously [10-11]. To account for the heating by the plasma, one first calculates the peak number of electron-hole pairs produced by the inter-band absorption of

photons. Sample heating results from the thermal ization of the excess energy absorbed by the

electrons raised to the conduction band.

The peak electron-hole pair density produced is calculated from the following differential

equation describing the generation rate of electron-hole pairs,

dN/dt = (a^I + e'^I^) / (hf) - N/t^ (6)

In eq. (5) f is the frequency of the laser, h is Planck's constant, t^ is the free carrier lifetime and e'o is the non-linear absorption coefficient reported by Bechtel and Smith [7]

(3'o = 0.0265 cm/MW). In eq. (6) Auger recombination was neglected due to the low expected

carrier density involved, and ambipolar diffusion was neglected because of the short pulse length.

A steady-state solution to eq. (6) gives the maximum electron-hole pair density produced and is given by:

N(I) = (a^I + e'ol2)t^ / (hf) (7)

To obtain an upper bound for the electron-hole pair density, we used the incident intensity

of a typical 1-on-l damage pulse, 1 = 19 MW/cm^, aQ=1.18 cm-1 , t^, = 100 psec (as given in [12] for

similar conditions), and e'o=0.0265 cm/MW. Accounting for the reflectivity, we obtained a peak

electron-hole density of N=1.7xl0l^ cm"3. For comparison, the compensated intrinsic carrier con- centrations were calculated. Accounting for the chromium dopant concentration (^ 1x10^^ cm"3) and

the resistivity of the sample (>^ 1x10^ -cm), we calculated an electron concentration of

7.35x10' cm"-^ and a hole concentration of 1.56x10^ cm"-^. Mobilities of 5000 cm^/V-sec and

300 cm^V-sec were used for the electrons and holes, respectively. Although the photogenerated

carrier density is much greater than the compensated intrinsic density, it is still quite small

compared to the densities where plasma effects become important, 10 19 . io20 cm"-^. Therefore, we

can disregard the plasma model from further consideration as a primary damage mechanism in gallium

arsenide.

4.3. Other Models

To check for anomalous absorption, we compared the measured non-linear absorption coefficient,

Bq, to the reported value, b'q, with the following equation that accounts for the linear and non- linear absorption coefficient, and for free carrier absorption.

dl/dx = -(qq + b'qI + Nfi) I (8)

In eq. (8), Q is the free carrier absorption cross section. Substituting eq. (7) into eq. (8) for

N yields

Since the transmission curve (figure 10) is linear, we can neglect the I-^ term in eq. (9). Therefore the 3o determined from the transmission test measurements corresponds to the quadratic coefficient in eq. (9). Setting these terms equal to each other yields:

= + (%t^) / (hf). (10)

195
Solving for Q and substituting the corresponding numerical values into eq. (10) yields a free

carrier absorption cross section of = 6.3 x 10"^^ cm^. This value agrees with those previously

reported for gallium arsenide [13]. From this, we conclude that we have included the major homogeneous absorption mechanisms in our calculations and that no gross anomalous absorption was involved.

The charged particle detector system monitored the pulse-by-pulse charge emitted from the

gallium arsenide sample. The magnitude of the emitted charge has been plotted in figure 7, on a pulse-by-pulse basis, for a site which exhibited surface damage. By examining the emission profiles, the emitted charge was observed to decrease after the first few pulses, and later increased

at least two orders of magnitude. The relatively small, decreasing, emission profile of the initial pulses appears to be a surface cleaning effect. The later increase of charge emission, after

the 5^^ pulse in this case, is attributable to surface damage. For gallium arsenide, only two types of charge emission profile were observed: either a cleaning effect without damage or a cleaning effect with damage. Damage in the absence of a cleaning effect was never observed,

although at very high intensities the two profiles merged and became indistinguishable. This differs greatly from the emission characteristics of silicon as reported in [1]. For silicon, emission of charged particles coincided with surface damage initiation and no cleaning emission was observed. Furthermore, the emission of charge prior to damage in gallium arsenide did not affect

the observed damage behavior in any detectable way.

It appears that neither the uniform heating model nor the electron-hole pair production model

can adequately describe the damage mechanism of GaAs. In addition, it does not seem possible to relate the nature of the damage to surface cleaning, although the latter appears to be a necessary

and possibly essential, precursor to damage. From these results we are led to speculate on other

possible influences that may cause, or enhance, surface damage. Lattice defects near the surface

of the gallium arsenide wafer are possible energy absorption sites for nucleating damage. These

lattice defects include anomalous vacancies, interstitials , and dislocations introduced during the

crystal growth or surface preparation processes. A single point defect seems an improbable physical area for absorbing sufficient energy to cause a melt spot. It is more probable that a clus- ter of point defects will act as an efficient absorption site.' This would result in a surface dam

age morphology of random melt points within the beam diameter of the laser.

Other possible sites for the nucleation of damage are physical surface defects caused by

processing, or physical handling, of the wafer that scratch the surface. Surface defects on an optical surface will increase absorption and scattering, possibly creating absorption nuclei which

could lead to damage.

5. Conclusion and Comments

We observed accumulation effects in gallium arsenide with multiple pulse laser irradiance

below the 1-on-l damage threshold intensity. These experiments also yielded various N-on-1 laser

damage thresholds derived from the statistics of the surface damage. In the experiments we also

measured the linear and nonlinear optical absorption constants for the gallium arsenide sample.

With this information, calculations were performed to determine the mechanism responsible for the

surface damage. The results for the uniform heating and electron-hole production models clearly

show that these mechanisms could not be responsible for surface damage. There remains the possibility that non-uniform, localized heating and carrier generation may be involved in the damage

process. An inhomogeneous heating model is supported by the observed damage morphologies. It is also possible that the sample contained absorbing defects, or defect clusters, that could contribute to the damage process, or surface defects (e.g., pits and scratches) that might cause a melt

spot to nucleate.

By investigating the pulse-by-pulse history of the charged particle emission, a cleaning type

profile was discovered. This cleaning emission proved to have no observable influence on the subsequent damage. Apparently, adsorbates remaining on the wafer surface after cleaning did not act

as absorbing sites to cause surface damage. The charge emission proved to be very noisy from

site-to-site. Although the emission did not strongly correlate with the event of surface damage,

once surface damage commenced, an increase in charge emission was observed. This behavior is con- sistent with the idea of a hot, partially ionized, vapor being released from melt pits as damage

proceeds.

Further investigation is needed to understand the mechanism of energy transfer that causes damage and emission. The relationship of the accumulation effect to lattice defects could prove

to be a fundamental key in understanding the damage process. Another parameter which might influ- ence the damage threshold is the dopant level of the semiconductor material.

196
We gratefully acknowledge support given by the DoD Joint Services Electronics Program under

Research Contract AFOSR F49620-82-C-0033. We would also thank Dr. John Beall at Texas Instruments

for providing the GaAs samples.

6. Bibl iography

[I] 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 Laser Damage Symposium, NBS

Special Publication #669, Boulder, CO, 1983; and Y. K. Jhee, M. F. Becker, and R. M. Walser,

"Charge Emission and Precursor Accumulation in the Multi-pulse Damage Regime of Silicon,"

Journal of the Optical Society of America B 2, October 1985, pp. 1626-1633.

[2] S. Dannefaer, B. Hogg, and D. Kerr, "Defect Characterization in Gallium Arsenide by Positron

Annihilation," Thirteenth International Conference on Defects in Semiconductors, Eds. L. C. Kimerling and J. M. Parsey, Jr., pp. 1029-1033.

[3] C. S. Lee, N. Koumvakalis, and M. Bass, "A Theoretical Model for Multiple-pulse Laser-induced

Damage to Metal Mirrors," Journal of Applied Physics 54 (10), October 1983, pp. 5727-5731.

[4] A. A. Manenkov, G. A. Matyushin, V. S. Nechitailo, A. M. Prokhorov, and A. S. Tsaprilov, "On the Nature of Accumulation Effect in the Laser-Induced Damage to Optical Materials," 14th

ASTM Laser Damage Symposium, NBS Special Publication #669, Boulder, CO, 1983, pp. 436-447.

[5] P. M. Fauchet and A. E. Siegman, "Surface Ripples on Silicon and Gallium Arsenide under Picosecond Laser Illumination," Applied Physics Letters 40 (9), May 1, 1982, pp. 824-826.

[6] E. W. Van Stryland, H. Vanherzeele, M. A. Woodall, M. J. Soileau, A. L. Smi rl , S. Guha, and

T. E. Boggess, "Two Photon Absorption, Nonlinear Refraction, and Optical Limiting in Semiconductors," Optical Engineering 24 (4), July/August 1985, pp. 613-623.

[7] J. H. Bechtel and W. L. Smith, "Two-Photon Absorption in Semiconductors with Picosecond

Laser Pulses," Physical Review B 13 (8), April 15, 1976, pp. 3515-3522.

[8] C. C. Lee and H. Y. Fan, "Second-Harmonic Generation in InSb, InP, and AlSb," Physical

Review B 10 (2), July 15, 1974, pp. 703-709.

[9] K. L. Merkle, R. H. Uebbing, H. Baumgart, and F. Phillipp, "Picosecond Laser Pulse Induced

Damage in Crystalline Silicon," Laser and Electron-Beam Interactions with Solids, Eds. B. R. Appleton and G. K. Celler, 1982, pp. 337-342.

[10] J. A. van Vechten and A. D. Compaan, "Plasma Annealing of Semiconductors; Plasmon Condensation to a Superconductivity Like State at 1000 K?" Solid State Communication 39 (2), (8),

August 1981 , pp. 867-873.

[II] D. M. Kim, R. R. Shah, D. Von der Linde, and D. L. Crosthwait, "Picosecond Dynamics of Laser

Annealing," Laser and Electron-Beam Interactions with Solids, Eds. B. R. Appleton and G. K. Celler, 1982, pp. 85-90.

[12] V. K. Mathur and S. Rogers, "Relaxation of Photoexcited Carriers in GaAs," Applied Physics

Letters 31 (11), December 1, 1977, pp. 765-768.

[13] R. K. Willardson and A. C. Beer, Semiconductors and Semimetals: Optical Properties of III-V Compounds 3^, New York: Academic Press, 1957, p. 409.

197
Attenuator Shutter ) 0 ^

I ^ Lena

*

To Computer

Energy Meter

To Computer

Photodetector

To Computer

Lena 7eOmm

local length Vacuum Tank

Sample

0-SwHched Nd:YAO Later

|l.0e4^m, 46nt«c pula«.680/xm apol diametar at the sample \

Figure 1. Experimental set-up

Scanning Emission Profile

t

o

(D T- D) X

(0

•D 3

E 2

uj .tr

10

98

7-6-

5-:

4-: 3-

2

1 H 0 _j

10

•• —• "'

I 20

Position (mm)

30

Figure 2. Charged particle emission versus position on wafer for a constant laser fluence of

0.63 J/cm^.

198
1 Shot GaAs

1 -y

0.9-

0.8-

Fluence (J/cm^)

Figure 3. Damage and emission probabilities versus fluence for N=l

10 Shot GaAs

Fluence (J/cm^)

Figure 4. Damage and emission probabilities versus fluence for N=10

199
100 Shot GaAs

Fluence (J/cm^)

Figure 5. Damage and emission probabilities versus fluence for N=100.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

FLUENCE (J/cm2)

Figure 6. Number of pulses required to reach 50% damage probability versus fluence. The line was hand drawn to aid the viewer.

200
100000

10000

1 — 1000-

100-

15

Pulse Number

Figure 7. Pulse-by-pulse charged particle emission for N=30 at an average fluence of 0.46 J/cm .

Figure 8. SEM micrograph of damage due to 3 pulses at an average fluence of 0.51 J/cm^.

201
Figure 9. SEM micrograph of damage due to 30 pulses at an average fluence of 0.38 J/cm^.

TRANSMISSION TEST

3.4 1

3.2 -

3.0 -

2.8 -

2.6 -

2.4 -

2.2 -

2.0 -

. ••••

... • * •

• •

• • •

• • •

• •

1.8 -

1.6 -

1.4 -

1.2 - 1 r r 1 I 1 "T-' 1 1 1 0 2 4 6 8 10 12 14 16 18 20

Intensity (MW/cm^)

Figure 10. Inverse transmission versus intensity for a GaAs wafer polished on both sides. The

solid line represents a least squares linear fit to the data.

202
Manuscript received

1-24-86 SAR

CDs LASER INDUCED DAMAGE TO GERMANIUM : RELATION

TO SURFACE PREPARATION

D.R. Gibson, C.M. MacDonald and A.D. Wilson

Barr and Stroud Ltd.,

Applied Physics Group,

Caxton Street, Anniesland,

Glasgow G13 IHZ, U.K.

The laser damage behaviour of germanium is examined in relation to surface

preparation methods including alumina polishing, Syton polishing, diamond machining

and plasma etching. Data on laser damage threshold (LDT), damage morphology, surface

absorption, surface roughness and the transmittance of the laser pulse (TLP) are used

in conjunction with a thermal model in an attempt to identify the various mechanisms

responsible for surface damage in Ge. Alumina and Syton polishing result in

significantly different LDTs, morphologies and TLPs which can be directly correlated

with the predictions of a thermal model based on laser heating of embedded polishing

particles. Plasma etching removes polishing debris and thus increases the LDT. Diamond machining gives a wide range of LDTs related to variations in surface

topography and work damage. A practical ranking of the effect of surface preparation

method on LDT is presented.

KEY WORDS: CO2 Laser Damage; Diamond Machining; Germanium; Plasma Etching; Polishing.

1. INTRODUCTION

In a previous paper [ij it was shown that CO2 laser induced damage to alumina polished

germanium occurred through a defect mechanism. The damage morphology consisted of localised

sites, within the beam area, which showed clear evidence of gross melting of the Ge surface.

Many sites followed the loci of surface or subsurface scratches suggesting that polishing

debris may have been incorporated in the surface during the polishing process. Confirmation of

the defect nature of the damage process was obtained by measuring the pulse length dependence

of the laser damage threshold: a t? dependence was obtained [l,2].

For a standard 10.59/xm CO2 TEA laser pulse (100ns FWHM gain switched spike + c^2jti,s relaxation

tail) the front surface laser damage threshold (LDT) of alumina polished Ge was found to be —35Dcm~^ which is considerably lower than many of the "new" materials such as ZnS and ZnSe

which "typically have LDTs in the range 60 to 70 3cm"^ . Since Ge is still preferred over ZnS and

ZnSe for many IR optics applications due to its higher refractive index, lower dispersion, lower

scatter (and, not least, its lower price) and since its CO2 laser damage resistance is limited

by extrinsic defects, a study of methods of laser—hardening Ge has been undertaken. Specifically,

we have investigated the effect on the LDT of the surface preparation method since this

directly affects the inclusion of defects in or the extent of work damage to the surface. The

aim of the present investigation is to further understand the mechanisms responsible for CO2 laser induced damage in Ge and to identify surface preparation methods which will lead to an increased LDT, hopefully, approaching those of ZnS and ZnSe.

Three alternative surface preparation methods have been investigated. Firstly, the effects

of alumina and "Syton" polishing are directly compared. "Syton ' [sj is a colloidal suspension

of silica particles with typical diameters of =il25nm [4]. In 1982, Hutchinson et al. [sj

reported that Syton polished Ge had a lower surface absorptance than alumina polished Ge and

thus the prospect of reduced thermal runaway. Presumably, an improved laser damage resistance

could also be anticipated. Secondly, the effects of diamond machining are investigated. Diamond

machining does not lead to particulate inclusion [6] but can give a highly structured surface

and may give rise to significant levels of work damage. Finally, the possibility of removing

the surface layer of alumina polished Ge is further investigated. In reference 1 we reported

that ion beam planing increased the LDT, In the present work an argon ion RF sputter etch

process is investigated.

203
2. EXPERinENTAL

2.1 CO2 TEA Laser Damage Test Facility

The CDs laser damage test facility has been described in detail previously fl] . For this

work a standard spike (100ns FWHM) and tail {^2fis) 10.59jU,m pulse was used. Injection locking

and cauity length tuning ensured a smooth temporal profile without longitudinal mode beating.

The spatial profile at the sample was a near-Gaussian with a l/e^ diameter of > 0.9mm. The

pulse energies were measured using Scientech surface and volume calorimeters. The reported

laser damage thresholds correspond to the incident peak energy densities required to cause an increase in the scatter of a He/Ne laser coincident with the damaging beam.

2.2 CO2 Laser Calorimeter

A CQe laser rate calorimeter incorporating a grating-tunable CW rf waveguide laser giving

an output from 9.2 to lO.S^m was used to measure sample absorptances. The sample was held in

a blackened chamber evacuated to Bmtorr to eliminate convection and conduction effects. Laser power is monitored in the incident and transmitted beams. The system is capable of measuring

absorptances of the order of 0.05^.

2.3 Surface Profilometry

A stylus profilometry (Tencor, Ql-step) is used to characterise the topography of surfaces.

The stylus tip radius is ^2jU,m giving a lateral resolution of <2^m and the vertical resolution

is —1 A RMS. The raw data (vertical displacement vs horizontal position) are transferred to an HP85 computer for analysis. Standard output comprises the RNS roughness over the total scan length (^l.Smm) and a frequency analysis (using the FFT technique) to obtain information on the

spatial wavelength content of the surface. This latter output can be in the form of the power

spectral density function [7 J or a fjlot of equivalent sine wave amplitude vs spatial wavelength

which we find allows a rnore direct picture of the surface. The range of spatial wavelengths

obtained extends from 4^ to >500yxm.

2.4 Angle Resolved Scatter (ARS)

ARS provides a convenient means of examining surfaces for spatial wavelengths below the

lower limit of the stylus profilometer . The scatter angle, 9, is related to spatial wavelength,

A~ , by the Bragg equation ?/As = sin 6. For this study a He/We laser provided the illumination

(A = 0.6328Lim).

3. COnPARISON OF ALUMINA AND "sYTON" POLISHED Ge ' . •.

'

3.1 Absorptance Data

The absorptance of samples of Barr and Stroud IQtPcm n-type Ge of 4 thicknesses (1.5, 3.5,

6.5 and lOmm) cut from the same boule and polished with either alumina or Syton (liJ30 grade) were measured by laser calorimetry at 10.59/xm. The data were analysed to give the bulk absorption

coefficient and the surface absorptance, Ag. The bulk absorption coefficient was estimated at 0.019 cm ^ in good agreement with other studies [5, sj. For both polish types, Ag was ^0.07^

per surface. The value for Syton . is in good agreement with the best values measured by

Hutchinson et al. [5] but the value for alumina is lower than the average value reported in

that study (0.47^ + 0.28^). The alumina polishing used in the present work was the standard

production process at Barr and Stroud which uses, in the final polish stage, O.S/^im diameter

alumina and the value obtained for A 5 is typical of those obtained for a large number of batches.

3.2 Laser Damage: Experimental Results

Front and rear surface laser damage thresholds of 1mm thick Ge samples are reported in

Table 1. Several important points emerge :-

(i)The "Syton" polished Ge has a lower front surface LDT value than the alumina polished

material (E ( Syton )/E (alumina) ^0.62) ; (see Footnote)

Footnote: E denotes energy density not electric field.

204
(ii)The rear surface LDT is higher than the front surface value for alumina polished Ge

(EE,ear/'^front> 1»66);

(iii)The front and rear surface LDTs are similar for "Syton" polished material

(Erear/Efront -0.9 to l.l);

(iu)Considering the electric field build up within the sample [sj the Eyear/^front ratio

should be -0.4 in Ge (n = 4) . .

The relative ease of front surface damage with both polishing types indicates the

existence of a mechanism which reduces the energy density at the rear surface. This effect has

been observed but not explained in previous work ^1, lo]

.

A further difference between jDolishing types is encountered in the form of the damage

morphology, Figure 1. With Syton polishing the damage is localised predominantly along

scratches, Figure la. At higher magnifications, these extended features are resolved into a number of discrete damage initiation sites from which extend surface ripple patterns, Figure lb,

of the type often associated with defect scattered waves [ll, 12]. Although surface melting is

obviously involved in the generation of the ripples, this morphology is quite distinct from

that found for alumina polished material [ij , which shows a gross melting of the surface,

Figure Ic.

3.3 Laser Damage: Thermal Model

A simple thermal model is used in an attempt to explain the experimental data. It is

assumed that polishing particles embedded in the Ge surface are responsible for the damage.

The energy density needed to cause damage can be calculated using the theory of Goldenberg

and Tranter [l3] in which an absorbing, spherical particle is heated in a uniform radiation

field: the non—absorbing host material is heated by thermal conduction from the hot particle.

Damage is deemed to occur when either the particle temperature or host temperature, taken to

be the temperature at the particle/host interface, exceed their respective melting points

(Tmpj Tnih)t The ratio of the laser damage thresholds of the particle (Ep) and the host (E^,^) is given by (see Appendix)

^p Tmp

•=r2

Trnh 2kp

Using the property data of Table 2, this ratio is 1.13, indicating damage to the Ge host,

for alumina polished Ge and is 0.06, indicating particle damage, for Syton polished material.

It is clear that a low particle thermal conductivity results in particle damage. Based on this result, the ratio of the laser damage thresholds for Syton and alumina polished material

is given by

E ("Syton") TmSiQs ksiOg SfMzOs ^Als 03

E (alumina) T^Ge '^Ge ^5102 QsiOs

where a is the particle radius and Q is the absorption efficiency factor obtained from Mie

theory [l4]. The calculated ratio is found to be 0.12 indicating that Syton polished

material has the lower damage threshold, in qualitative agreement with experiment. The total

energy transmitted through the Ge samples, measured as a function of the incident energy

density. Figure 2, provides experimental corroboration of the results of the model. The

constant transmittance observed for Syton polished material, until the LDT is exceeded, is

expected if the particles are heated but the Ge remains relatively cold. The heating of the

Ge predicted for alumina polished Ge is consistent with the observed gradual decrease in

transmittance before damage occurs. The data for the best diamond machined (DFl) Ge sample

(see Section 4 for a detailed description) shows that Ge can be heated in the absence of

particulate defects. In this case, the absorption initiators may be regions of work damage.

Furthermore, since the relative decrease in transmittance (—40^) for alumina polished Ge is

larger than the fraction of the surface area damaged (^10%) this further suggests that the Ge between the particulate defects is being heated, again^ presumably through the effects of

work damage. The constant transmittance observed for Syton polished Ge may indicate that

205
work damage is not significant for this polishing technique. We note that in addition to polishing debris the nature and effects of work damage may need to be considered in a more detailed model,

3,4 Discussion

The differences in damage morphology are consistent uith the model. In alumina polished

material, the Ge surrounding embedded polishing particles is heated and results in localised

regions of gross melting. With Syton polished Ge, the SiGs particle damages and the Ge remains

relatively cold. No gross melting is expected or observed. The ripple damage morphology

observed for Syton polished Ge is of the type associated with defect scattered uiav/es ^11, 12],

A ripple pattern is often observed in a small region at the periphery of the gross melt damage

site in alumina polished Ge but is never observed on its own. This suggests that gross damage

and the ripple pattern are generated simultaneously (in energy or time) e.g. at the point where

Ge adjacent to an alumina particle melts or the SiOa particle melts or otherwise damages.

It remains to explain the problem of the difficulty of rear surface damage. As noted

previously, there must exist a mechanism which jDrotects the rear surface of the germanium and

which is more efficient with alumina than with Syton polished material. Firstly the effect of

the Ge heating in alumina polished material is considered. The temperature dependence of the

optical properties of Ge, as calculated using free carrier theory. Figure 3, reveals a significant increase in k and decrease in n for T > 7G0K, These changes will have significant

effects on the electric field distributions within the sample, A tentative explanation of these

effects is given below. Initially, the rear surface region (R) is heated :^2,5 times faster than

the front surface region (F) due to electric field enhancement at R [s]. When the R temperature

exceeds —700K the field, and thence the heating rate at R, is reduced by the effect of the

lower n value at R: since F is at a much lower temperature its heating rate is unaffected. When

F exceeds 700K the interface transmission and F heating rate are increased. At higher

temperatures the increase in the k value of F further increases the F heating rate but decreases

the energy reaching R and thus the heating rate. Eventually the k value of F may become so large that reflection becomes significant reducing both the F and R heating rates. This

behaviour is sketched in Figure 4 to illustrate how the temperature at F may exceed that at R and

thus how damage to F may occur first.

With "Syton" polished material the lack of Ge heating means that the above mechanism does

not apply and thus the E front/E rear ratio should be lower as is indeed observed. However the

ratio is still larger than expected from theory [s]. To check whether defocussing was responsible for rear surface protection the transmitted spatial profile was measured but no significant broadening was observed. Thus the observed effect cannot be explained fully at this

time.

4, DIAnOND MACHINED Ge

The samples were diamond machined on one surface, the other surface being alumina polished.

The RMS surface roughness of all but one sample was in the range 78 + 18 8 measured perpendicular

to the grooves: one sample had an RMS value of 446 8, The absorptance at 10.59^m was measured

with the radiation incident on the DM face with the electric field vector of the radiation both

perpendicular and parallel to the diamond machining grooves. The bulk absorptance and rear surface absorptance were then subtracted to obtain the surface absorptance (Ag) of the diamond

machined face. The lowest Ag value was 0,08% which is similar to the values for alumina and

Syton polished material. More typical values ranged from 0.2% to 0,8%; a value of 4,2% was obtained for the very rough sample.

The LDT was also measured with the electric field vector perpendicular and parallel to the

grooves. These data are presented as a plot of LDT against reciprocal surface absorptance.

Figure 5, which reveals a good correlation between these parameters. Only the sample with the

very low surface absorptance has a higher LDT than alumina polished material. Most samples are significantly poorer. The damage morphology consists of a single uniform melt site corresponding

to the beam's spatial maximum. Figure Id: there is no evidence of a localised defect-type damage

morphology

,

The variation of LDT with i/a^ is not unexpected if the surface absorption is spatially

uniform. The surface absorption may be related to the extent of work damage an d/ or the roughness

and structure of the surface. The RMS roughnesses of all but one sample vary by only + 25%,

These variations cannot account for the absorption and laser damage data. However, the spatial

wavelength distributions contributing to the roughness can vary significantly between samples.

The differences in spatial wavelength distributions were investigated, in detail, for both the

206
"best" sample (lowest Ag, highest LDT) and for a typical sample with an LDT lower than for

alumina polished material. Figure 6 shows profilometer derived spatial wavelength spectra and

Figure 7 shows angle resolved scatter data. Both samples have significant roughnesses at long

spatial wavelengths (>200 microns) corresponding to errors in micro-figure and at short spatial

wavelengths (< 1.6 microns) which correlate with the tool feed rate. The best sample has no significant structure at spatial wavelengths between these limits whereas the typical sample shows

a significant peak at a spatial wavelength of about 20 microns. Thus there are significant

differences, between the two samples, in the content of spatial wavelengths close to the laser

wavelength. Consequently, different radiation coupling efficiencies [l5, 16], and hence laser

damage thresholds, can be anticipated. That the best sample has the highest LDT is consistent

with the lack of a suitable coupling surface structure.

The values of Ag and LDT were found to depend on the orientation of the electric vector and

the diamond machining grooves: for 5 samples the LDT was higher (and Ag lower) for the parallel

orientation; for 2 samples the reverse was true; 2 samples, including the best sample, were independent of the orientation used. For a grooved surface it is expected that the LDT will be

reduced when the electric field vector is perpendicular to the grooves [l?]. This is only

observed for 5 out of the 9 samples. That the best sample does not show any dependence of the

LDT on the electric vector/groove orientation can be explained by noting a lack of surface

structure with a wavelength close to that of the radiation. However the remaining 3 samples

show behaviour contrary to expectations. This may indicate that the effects of surface structure

are more complex than envisaged or that the effects of work damage have a masking effect. A fuller examination of the spatial wavelength content of these diamond machined surfaces is

underway in an effort to explain the apparently anomalous behaviour.

5. SPUTTER ETCHED Ge

An argon ion RF sputter etch (13.56 MHz, lOOkl, 5 mtorr) was used to remove approximately 2 microns from both surfaces of alumina polished Ge samples of varying thickness. The effect on the 10.59 micron absorptance is shown in Figure 8. The surface absorptance (intercept at zero thickness by 2) is reduced to negligible levels (^i^O.Ol^ per surface) suggesting that the layer

containing embedded alumina particles (and any work damage) has been removed. Profilometr^

showed that the etch process did not alter the surface microroughness or the micro-figure . The

front surface CO2 laser damage threshold of the etched surfaces was —40 D cm ^ which is an increase of ^2B% over the value for alumina polished material.

6. SUmARY AND CONCLUSIONS

The effects on the laser damage behaviour of Ge of a range of surface preparation methods

have been assessed.

Particulate polishes, alumina and "Syton", show significantly different behaviour: (i) the

front surface LDT is lower for "Syton" polished material (20 3 cm~^ cf 32 2 cm~^ ) ; (ii) both

result in defect type damage, comprising localised sites within the beam area, but with distinct

local—scale morphologies (local melting for alumina, surface ripple for Syton ); and (iii) the

front surface/rear surface LDT ratio is higher for alumina than Syton polishing and is higher

than predicted by simple theory [9j, A thermal model based on the assumption that polishing

particles embedded in the Ge surface are responsible for laser heating predicts that the Si02

particles damage in the case of Syton polished material but that the Ge host damages in the case of alumina polishing. These predictions, which are supported by data on transmission of

the laser pulse, allow a qualitative explanation of the differences in front surface LDTs and

damage morphologies. (Conversely, the agreement between experiment and theoretical prediction

is evidence that damage is initiated at embedded polishing particles.) In addition, an attempt

is made to explain the front/rear LDT ratio anomaly for alumina polished material. It is

suggested that the field distribution, which initially favours rear surface damage, is modified

by the heated Ge surfaces, which are expected to show a significant decrease in n arid increase in

k, such that heating of the front surface is enhanced. However when the Ge host is not heated

significantly, as with "Syton" polished material, the fact that the front/rear anomaly, persists,

albeit to a lesser numerical extent, cannot yet be explained. Heasurements suggest that

defocussing does not lead to a significant reduction in rear surface energy density and thus cannot explain the results. Clearly, a more comprehensive theoretical model is required perhaps

also taking into account the effects of work damage in introducing absorbing centres and the

temperature dependent growth of absorbing regions. Diamond machining (DM), although not introducing particulate contamination, does lead to significant levels of surface absorption.

Generally, front surface LDTs are lower than for alumina polished material. For one sample,

only, the surface absorptance was low and an LDT AUfo higher than for alumina polished material

207
was obtained. The reasons for the typical low LDTs could not be determined unambiguously but

probably inv/olue both the effects of the periodic structure produced by the DM (i.e. grooues) in

coupling the laser radiation to the sample when periodicities are close to the laser wavelength

and also the effects of work damage. A more detailed study of the surface topographies of the

samples is planned to elucidate which effect predominates.

Argon ion sputter etching has been used to remov/e the surface layer of alumina polished

samples. As expected, removal of defects led to a decrease in surface absorptance and a significant increase in laser damage threshold.

From a practical viewpoint, the various surface preparation methods can be ranked as follows:

highest LDT diamond machined (exceptional)

argon ion sputter etched (alumina pre—polish)

alumina polished

diamond machined (typical)

Syton polished

lowest LDT diamond machined (poor)

The authors thank the (Management of Barr and Stroud for permission to publish this paper.

REFERENCES

[l] D.R, Gibson and A.D, Wilson, "studies of CO2 laser induced damage to infrared optical

materials and coatings , paper presented at the 16th Annual Symposium on Optical Materials

for High Power Lasers, Boulder, Col., Oct. 1984.

[2] D.R. Gibson and A.D. Wilson, Appl. Phys. Lett., 47, 914 (1985).

[3] Monsanto Technical Bulletin, "Syton Colloidal Silica", 53-3 (E) M-E-4 (1979).

[4] We have deposited the particles of a "Syton" suspension onto a Ge disc and measured the

infrared transmission spectrum. The major absorption peak at 9.15 microns with a shoulder

at—8.4 microns and a subsidiary peak at 12.5 microns indicate that the particles are very

close in composition to Si02

.

|_5] C.J. Hutchinson, C. Lewis, 3. A. Savage and A. Pitt, Appl. Opt., 21_, 1490 (1982).

[sj D.L. Decker, in proceedings of "Workshop on Optical Fabrication and Testing", paper FEl

(1981).

[7] E.L. Church and H.C. Berry, Wear, 83, 189 (1982).

[8] P. 3. Bishop and A.F. Gibson, Appl. Opt., 12_, 2549 (1973).

[9] N.L. Boling, M.D. Crisp and G. Dube, Appl. Opt., 12, 650 (1973).

[10] B.E. Newnam and D.H. Gill, NBS Spec. Publ., 509, 298 (1977).

[11] C.T. Walters, Appl. Phys. Lett., 25_, 696 (1974).

[12] n.3. Soileau, IEEE 3. Quantum Electron., QE-20 , 464 (1984).

[13] H. Goldenberg and C. Tranter, Brit. 3. Appl. Phys., 3, 296 (1952).

[14] H. van de Hulst , "tight Scattering by Small Particles", Chapman Hall (London), p270 (1957).

[15] M. Bass and L. Liou, 3. Appl. Phys., 5_6, 184 (1984).

[I6] I. Ursu, I.IM. Mihailesou, Al. Popa, A.M. Prokhorov, U.I. Konov, A. P. Ageev and U.N. Tokarev,

Appl. Phys. Lett., 45, 365 (1984).

[17] M.3. Soileau, 3.0. Porteus and D.L. Decker, NBS Spec. Publ., 568, 195 (1979).

[I8] M.L. Lang and W.L. Wolfe, Appl. Opt., 22, 1267 (1983).

208
APPENDIX

Using the expressions of Goldenberg and Tranter l^isj the energy, E , required to melt the centre of the absorbing particle is giuen by ^

E = mp p h Al

P aQ (2k + k^ - 6k^Ii

)

" n

and the energy required to melt the host adjacent to the particle, E|_^, is giuen by

4T , k k, t E^ = mh p h A2

h aQ (Kp - 3k^l2)

luhere

CO

Ii = 1^/F(y)dy

o

T 2n r F(y) sin y , I2 = -J

_yL_ L dy

P(y) =

exp (-y^t/7)(sin y - y cosy)

^ y (c sin y - y cosy) + b sin y

12 c = 1- kVk 7= a^/D

h p

kj is the thermal conductivity, Di is the thermal diffusiuity, is the melting point of

material i, a is the radius of the absorbing particle, and t is the pulse length. Q is the

absorption efficiency factor obtained from Hie theory [l4] and for Ge (n = 4) at 10.59 microns is

giuen by

^^91^nj^ ^ ^3

(n^ + 32)^

where n and k are, respectively, the refractive index and extinction coefficient of the particle.

For the pulse length (t > 100ns) and material properties (Table 2) under consideration

the integrals (Ii, I2 ) may be neglected to a first approximation (< b% error). Equations Al and

A2 then reduce to

8T k k, t

E ~ mp p h

^ aQ T2k + k, ) P h'

and

4T , k, t

E. _ mh h h —

aQ

and thus the ratio of particle to host LDTs is given by

E T k _£-. 2

P E^~ T ^ 2k + k h mh p n

The ratio for particle damage for "Syton" polished Ge to host damage in alumina polished

material is given by

E(" Syton") "'"mSiOa '^mSiOg ^Alg Og "^Alg Og

E (alumina) ~ T _ k a . Q . ^ mGe Ge SiOg SiOg

209
1. CDs LASER DAMAGE THRESHOLDS FOR ALUMINA AND SYTON POLISHED Ge

Front Surface Rear Surface

Polishing

LDT LDT Material

(3 cm"^) (3 cm"^)

Alumina

32 >53

(Linde "a")

Syton

20 ^ 20

(liJ30)

2. MATERIAL PROPERTIES USED IN THERMAL MODEL

Materials

Ge SiOs Als Os Property

^

Refractive index at 10.6^ 2.0 1.0

Extinction coefficient at

10.6|im

1.3 X lO"* 0.14 0.08

Thermal conductivity

at 300K (li)m"''K"''

)

50 1 40

Melting Point (K) 1260 1990 2320

Particle radius (jx^) 0.063 0.15

IM.B. 5i02 and AI2O3 optical data are taken from reference ([iS"].

210
Nomarski photomicrographs of COg laser induced damage sites on Ge: a) and b) , "Syton" polished;

c) , alumina polished; d) , diamond machined. Scale bar corresponds to 100 microns.

0 5 10 15 20 25 30 35 40 45 50

ENERGY DENSITY , Jcm^

Total energy transmission as a function of incident peak energy density for Ge: ^ , alumina

polished; , Syton" polished; 0 , diamond machined. The arrowed symbols indicate damage

thresholds

.

211
300 500 700 900 1100 1300

TEMPERATURE , K

3. Variation of refractiue index (n) and extinction coefficient (k) of Ge with temperature as calculated using free carrier theory.

ENERGY (£ TIME INTO PULSE)

4. Schematic diagram showing variation of front (F) and rear (R) surface temperatures with energy

( =to time into a constant intensity pulse): dotted lines drawn assuming field build-up for a dielectric medium with n = 4; full lines for modified field distribution as discussed in text.

212
L DT

(Jcm^) 30 1-

4 5 6 7 8 9

l/(ABSORPTANCE,X)

10 11 12 13

Relationship between laser damage threshold (LOT) and the reciprocal of the surface

absorptance for diamond machined (DM) Ge; open symbols, electric field vector parallel to grooves; closed symbols, electric field vector perpendicular to grooves; 0 , the "best" DM sample for which both electric field/groove orientations gave same LDT and surface absorptance

values

.

SFflTIftL llfiVELEHGTH urn S P H T I fl L I'l fl V E L E H G T H urn DFC1 DFC2

Comparison of the spatial wavelength spectra obtained from stylus profilometer data for a typical (DFCl) and the best (DFC2) diamond machined Ge sample .

213
1

10

21 10 5

ALPHA-STEP

RESOLUTION LIMIT

I 3 1.6

V-i—

NO SIGNIFICANT

STRUCTURE ABOVE

NOISE LEVEL

12 15

1.4 —I

Xs(Mm)

1.3 1.2 —r r DFC1

DFC2

/^j 1 1 23 27 29 32

SCATTER ANGLE (deg)

Angle resolued scatter data for a typical (DFCl) and the best (DFC2) diamond machined Ge sample

.

214
Manuscript Received

12-22-86

Study of Mo, Au, and Ni Implanted Molybdenum Laser Mirrors

By Spectroscopic El 1 ipsometry*

Paul 6. Snyder, George H. Bu-Abbud,"*" Jae Oh, and John A. Woollam

University of Nebraska, Lincoln, NE 68588-0511

David Poker

Oak Ridge National Laboratory, Oak Ridge, TN 37830

D. E. Aspnes

Bell Communications Research, Inc., Murray Hill, NJ 07974

David Ingram and Peter Pronko

Universal Energy Systems, Dayton, OH 45432

The implantation of 150 kev molybdenum ions into polished molybdenum laser

mirrors is found to increase the complex dielectric constant in the visible

spectrum. Analysis using the Bruggeman effective medium approximation demonstrates

that the increase is due to surface smoothing, and that the surface is made nearly

atomically smooth by a fluence of 5 x 10 /cnr. Implantation of Au at IMev caused

considerable microscopic roughening, as well as a change in the bulk optical

properties. 3 MeV Ni ion implantation caused only a slight surface roughening. A thin dielectric film (probably a hydrocarbon) is found to condense in a laboratory

atmosphere, reducing the reflectivity, and is removable by rinsing with methanol and

distilled water.

Key Words: Molybdenum laser mirrors; spectroscopic ellipsometry ; ion implantation;

surface smoothing.

1. Introduction

Molybdenum is of interest as a laser mirror material because it possesses desirable thermal

properties [1]. Various techniques for producing very smooth, highly reflecting surfaces have

been investigated, including mechanical and electrochemical polishing [1,2], chemical vapor

deposition [3], and ion implantation[4,5]. The optical properties of Mo prepared by bulk and thin

film techniques have been studied using several methods [6], among them ellipsometry [5,6,7],

reflectance [6,8], and reflectance and transmittance [9] measurements. Roughness and

microstructure of molybdenum surfaces have been studied by Nomarski imaging, prof i 1 ometry , and

total integrated scattering [1,2,5,10,11]. In this paper we present the results of single and

multiple angle of incidence spectroscopic el 1 i psometric measurements in the visible to near ultraviolet on polished Mo mirrors which have been implanted with 150 keV Mo, 1 MeV Au, or 3 MeV

Ni ions. We describe the effect of implantation on the Mo mirror optical properties for a wide

range of fluences.

2. Samples and Experiment

Three commercially prepared vacuum arc cast [1] molybdenum mirrors, about 4 cm in diameter,

were mechanically polished with alumina to a final grit size of 0.3 micron. Chemical analysis of

Research supported in part by the U.S. Department of Energy under contract DE-AC05-840R21400 at ORNL, NSF/SBIR Grant PHY-8318798, and the College of Engineering, UNL.

"''Present Address: Reliance Comm/TEC, Advanced Development Lab, Richardson, TX 75081

215
the starting molybdenum surfaces showed there to be less than the following percentages of

impurities: 0.005 carbon, 0.008 iron, 0.002 nickel, 0.008 silicon, 0.0015 oxygen, 0.0005

hydrogen, 0.002 nitrogen. Six 1x3 cm regions on each mirror were implanted at room temperature

with fluences from 1 x 10 to 5 x 10 cm" , the first mirror with 150 keV Mo ions, the second

with IMeV Au ions, and the third with 3 MeV Ni ions. All implantations were made at room temperature, because it was found in previous work that heating the substrate during implantation

caused an increase in the surface roughness [5], The Mo implantation was performed at Oak Ridge

National Laboratories on a 200 keV Varian "Extrion", and Au and Ni were implanted on a 6 MeV

General lonex "Tandetron" at Universal Energy Systems. Reflection of a helium-neon laser beam

from all of the implanted surfaces was specular, with no distortion of the beam.

Two automatic rotating analyzer el 1 ipsometers were used. One, at Bell Communications

Research Inc., had an angle of incidence <^ fixed at 67.08° and a photon energy scanning range of

1.5 to 6 eV [12]. It was used to measure the el 1 i psomet ric parameters tanij; and cosA for the

various regions on the Mo implanted mirror. Contamination effects were minimized by optically

prealigning the mirror in a windowless cell, cleaning its surface by rinsing in sequence with

distilled water, methanol, and distilled water, and then maintaining its surface in dry filtered

N2 during measurement.

The other el 1

i psometer , at the University of Nebraska-Lincoln, was similar in design to the

first, but had a variable angle of incidence [5]. It was used to collect all of the data for the

Au and Ni implanted mirrors, and some data for the Mo implanted mirror. Measurements were made at angles of incidence from 71 to 79°, and at photon energies from 1.8 to 3.8 eV. The Au implanted

mirror was prealigned and rinsed as described above, then blown dry with high purity Argon. Data were obtained within several minutes of rinsing. The Ni implanted mirror was not rinsed at all before measuring. When the rinsing procedure was performed on the Mo implanted mirror, the two

el 1

i psometers produced almost identical pseudodielectric function data for this carefully cleaned

sample in the overlapping energy range, as shown in figure 1, despite the fact that data were obtained at different angles of incidence. This demonstrates that data from the Bellcore and UNL

el 1 i psometers agree with each other to better than one percent when proper surface preparation

procedures are followed. Multiple angle of incidence data provided additional information about

the surface and bulk optical properties, as described below.

3. Mo Implanted Mirror

The pseudodielectric function is the effective, or apparent, dielectric response calculated

from the measured \|; and A values at any given photon energy and angle of incidence, ^, by

assuming a two-phase (smooth substrate-ambient) model [13]:

<e> = <E.>-j<e„> = sin2.j,[l+ iziiniei^ ^ tan^<i>] (1)

^ ^ l+tani|;eJ^

Pseudodielectric functions <e> for the various regions on the Mo implanted mirror surfaces, for

fluences from 1 x lO-'-^cm"^ to 5 x lO^^cm"^, are shown in figure 2. There is little apparent

change in <e> as the fluence is increased from 1 x 10 cm" to 1 x 10 cm" , but at 2 x 10 cm" both <£,> and <Cr,> are significantly changed over the entire spectrum. Increasing the fluence to

5 xlO^^cm" causes a slight further increase in <e^>.

The changes in <e> described above could be due to thickness changes of a dielectric

overlayer, or to changes in the substrate mi crostructure (as in a loss of crystal 1 i nity )

, or to changes in the surface microscopic roughness. While we have observed thin films to physisorb onto

Mo surfaces, such films are completely removable by the rinsing procedure described above. A very thin passivating layer of natural oxide [14] is also known to grow on Mo. In a previous study [5]

electron spectroscopy for chemical analysis (ESCA) measurements detected less than 10 A of M0O2 on

Mo surfaces. No attempt was made to remove the oxides from our samples, because the necessary

reactants would also have attacked the metal and roughened the surface. Even a very thin

(monolayer) oxide will have a small effect on the <e> data, but this effect should be essentially

the same for all samples. Thus physisorbed films or natural oxides cannot explain the differences

in figure 2. Secondly, there is no broadening or shifting of the <e> spectral features with

increasing fluence, which indicates that there is no implantation-induced change in crystal 1 i nity

or grain size near the surface [15]. We conclude that the changes in <e> with implantation

fluence are due to changes in microscopic roughness.

We call attention to the fact that we are using the terminology "mi crostructure" in the

standard materials science convention, that is, to refer to i nhomogeneiti es on a length scale of

216
the order of 1 to 100 nm [16]. In the optics literature, "mi crostructure" is commonly used to refer to what we would call macros tructural i nhomogeneities, with length scales greater than 1000

nm, i.e., to inhomogeneities that can be resolved with an optical microscope (see, e.g., ref.

1). The distinction is important because quite different physical properties are involved:

mi crostructure determines the dielectric discontinuity at the materi al -ambi ent interface, i.e.,

the reflectance properties or light loss from transmission into the substrate material, whereas

macrostructure determines light loss from scattering. El 1

i psometri c data are essentially

unaffected by macrostructure, while scattering data are essentially unaffected by mi crostructure.

The increase in magnitude of <e„> with increasing fluence indicates microscopic smoothing of

the surface by Mo ion implantation. To verify this and to quantify the changes in microscopic

roughness, a single-parameter model was fit to the measured (ip,A) data using the Marquardt

minimization algorithm [17,18]. The three phases of the model were the molybdenum substrate, a "roughness" layer consisting of equal volume fractions of Mo and voids (absence of any material),

and the air ambient. The size of the void regions is assumed to be much less than the wavelength

of light. The Bruggeman effective medium approximation [19] (EMA) was used to model the

effective dielectric response of the roughness layer, and the layer thickness was the model

parameter.

The reference e spectrum for Mo was taken to be the <e> spectrum for the Mo implanted region

with a fluence of 5 x 10 cm"^, which is seen in figure 2 to have the highest values of <Zy>.

This implies that the 5 x lO-'- cm" region comes closest to realizing the sharp interface between

bulk and ambient that is assumed with the two-phase model.

The roughness layer thicknesses obtained in the analysis are given in table I. The sharp

drop in thickness above 1 x lO^^cm"^ correlates with the change in <e> seen in figure 2. In addition, the calculations show that Mo ion implantation at fluences higher than 2 x 10 cm" does

not significantly further decrease the surface roughness. These results are similar to those

described in reference 5, in which the microscopic surface roughness of mechanically polished

samples of molybdenum rolled sheet stock decreased when they were implanted at room temperature

with 150 keV Mo ions at a fluence of 2 x lO-'-^cm"^, but did not decrease further when higher

fluences were used.

A two phase (substrate, ambient) model was also investigated, in which the substrate was an effective medium containing voids, and the void fraction was the single variable parameter. As before, the <e> from the 5 x 10 cm" fluence region was used as the substrate e. This model,

which excludes the possibility of surface roughness, produced a poor fit to the data.

Furthermore, when the two models were combined to allow both substrate voids and surface

roughness to be treated together, the calculated substrate void fraction converged to zero in all cases. This further indicates that the changes in <e> with fluence are due to changes in the

surface rather than in the bulk mi crostructure.

4. Film Growth on Mirrors

When measurements were made with the sample surfaces in air or in unfiltered nitrogen from a high pressure gas cylinder, the el lipsometric data revealed the presence of a film growing on Mo surfaces. A small (<1%) decrease in <e„> due to the growing film was often detectable within a few minutes after rinsing, and a film several days old lowered both <ep> and <e,> by almost 20 percent as shown in figure 3. Complete film removal by the distilled R20-nie'thanol -di sti 1 1 ed H2O

rinse was confirmed by analysis of data taken immediately after rinsing. The film was not an oxide, which would not be removed by the simple rinse procedure. The film is probably due to hydrocarbon contaminants from the nitrogen cylinder and/or in the laboratory air.

It was also inadvertantly discovered that the film was not easily removed after the mirror

had been left exposed to air in the lab for several months. This film was only slightly reduced

in thickness by soaking for several hours in acetone and then rinsing with methanol and distilled

HpO. In contrast, the film on another polished Mo mirror which was kept in a desiccator jar over

the same period of time was easily removed by rinsing. We suggest that the unremovable films are

the result of catalytic decomposition of hydrocarbons on the Mo (or Mo-oxide) surface to form

polymeric compounds, a process known to lead to contact failure in mechanical relays with

transition metal contacts [20].

5. Au Implanted Mirror

The pseudodielectric functions of the Au implanted molybdenum mirror are shown in figure 4. These <e> spectra were derived from multiple angle of incidence measurements of tani|) and cosA. At

217
each photon energy measurements were made at two angles of incidence near the principle angle.

Each <e> spectrum of figure 4 (and also the UNL spectrum in fig. 1) is the average of the two spectra obtained by applying eq. 1 at each angle of incidence. The <e> spectra of the lowest

fluence region are almost identical to those of the unimplanted region, and to those of the lowest

fluence region on the Mo implanted mirror. Therefore Au or Mo implantation at a fluence of 1 x 10 era" or less has practically no effect on the polished mirrors. Au ion fluences above

5x10 cm" caused a monotonic lowering of <ej,>, and also caused the structure in both

<ep> and <e,> to be washed out. As discussea in the preceeding section the former effect is due

to microscopic surface roughening, while the latter is due to changes in the bulk dielectric

function of the mirror material near the surface.

In addition to calculating the pseudodielectric functions, we used the multiple angle of

incidence ellipsometry data to simultaneously solve for the wavelength dependent Mo optical

constants, and the thicknesses and void fractions of the surface roughness layers. This was possible because the system configuration was overdetermined, there being four data points

and A at two angles of incidence) at each wavelength, and only two unknowns (e, and e^) plus

the wavelength independent roughness layer thickness and void fraction. Thus by using multiple

angle of incidence, multiple wavelength data, we were able to eliminate the effect of surface

roughness and to calculate the "true" dielectric function of the substrate material. It was assumed that the optical constants of the molybdenum metal in the substrate are identical to those

of the Mo in the roughness layer. The optical constants (e, ,ep) were allowed to vary, as well as the roughness layer thickness and void fraction, to obtain the best fit to the data. The

Bruggeman EMA was again used to model the surface roughness layer. The thickness and void

fraction solutions were 12A and 0.5^ respectively. In figure 5 the calculated substrate

dielectric function of the IxlO-'-^cm" region is compared directly to the pseudodielectric function

of the same region, and also to the pseudodielectric function of the 5x10 cm" region on the Mo implanted mirror. If the 5 x 10 cm" Mo implantation actually yields a nearly atomically smooth

surface, and if the cleaning procedure really removes all surface overlayers, then the "substrate"

and Mo implanted spectra should agree. Figure 5 shows that this is indeed the case. Thus the

validity of the above assumptions is demonstrated.

Model "substrate" solutions for the higher fluence regions are shown in figure 6, and the

corresponding thicknesses and void fractions of the roughness layers are given in table II. As with the Mo implanted mirror, the primary effect of roughness is to lower <e„>. Note that even after correcting for surface roughness, the spectral features become broadened with increasing

fluence. In addition, the e, is shifted to more negative values. Both indicate a disruption of

the electronic structure of the Mo grains, that is, amorphization of the near-surface material.

However, electron microscopy analyses on similar samples have shown that there is little or no change in grain size in Mo-implanted Mo surfaces [21]. A more likely reason for the change in spectral structure is the formation of dense dislocation loops near the surface. In Rutherford

backscatteri ng measurements less than 0.1% Au was detected near the mirror surface, therefore the

"washing out" of spectral features is not due to alloying. These points are discussed in more

detail below.

6. Ni Implanted Mirror

The pseudodielectric functions of the Ni implanted surfaces are shown in figure 7. The

mirror was not rinsed before measurements, and thus was covered by a thin dielectric film similar

to that described above. The effect of the film is similar to that of roughness, namely, it lowers <e> (see fig. 3). However, because the optical constants of the film are different from

those of Mo and from air, the thin dielectric film and roughness can be distinguished. Using the

dielectric function of smooth Mo obtained in the previous section, we determined the thickness and

dielectric function of the dielectric film and the thickness of the roughness layer as a function

of fluence. A four-phase model was used, consisting of: a Mo substrate, a roughness layer

(containing equal volumes of Mo and hydrocarbon material and modeled in the EMA), the hydrocarbon

film, and the air ambient. The roughness corrected dielectric function in figure 5 was used to represent Mo in the model. The thickness of the roughness layer, as well as the thickness and

dielectric function of the hydrocarbon films, were variables in the model. The hydrocarbon index

of refraction was found to be 2.58 + 0.1 over the entire photon energy range, and the results for

the thicknesses are shown in table III. From the analysis we find that the Mo surface roughness

decreases monotoni cal ly with increasing fluence to 1 x lO-'-^cm" , at which point the trend reverses. In addition, a slight lowering and loss of optical structure was found for the Mo dielectric function at the highest fluence, indicating some disordering of the bulk crystalline

structure near the surface. It is noteworthy that the initial smoothing trend with increasing

fluence was not evident from the pseudodielectric functions. In contrast, for the Mo implanted

mirror which had no film, the surface smoothing trend was observable in the pseudodielectric

218
function data of figure 2 as well as in the results of the analysis. Thus the effect of surface

smoothing on <e> is masked by the presence of the film. The increase in hydrocarbon film

thickness with fluence seen in table III is probably an artifact resulting from the time span of

several days over which the mirror was exposed to air while measurements were made.

7. Discussion

The dielectric function data show that 150 keV Mo implantation improves the surface

microstructure of the Mo mirrors, while 3 MeV Ni initially improves it, and 1 MeV Au or further Ni implantation degrades it. The fact that both improvement and degradation are seen for the same target material provides an opportunity to gain information on the mechanisms involved.

To proceed further we need the implantation £arameters for each of the three systems. These

are given in table IV. The values of the ranges x and straggle lengths Ax, and the deposited

energies in the near-surface region for both electronic and nuclear loss mechanisms were calculated with the TRIM simulation code [22] using the nominal implantation energies and standard

bulk density data for Ni , Mo, and Au. TRIM uses a linear binary collision approximation and assumes that the energy lost by a moving ion may be separated into electronic energy loss and

nuclear energy loss. The former is rapidly dissipated in metals by plasmons and causes no permanent damage. ''^

Nuclear energy loss may result in permanent damage if the kinetic energy transferred to a lattice atom in a collision exceeds a threshold value. For metals a reasonable value is 25 eV but

it is strongly dependent (within a factor of two) on the direction of displacement within the

crystal lattice and the elemental nature of the target. Not all atoms which are displaced from a lattice site to an interstitial position during the adiabatic phase of each ion cascade (<100 ps)

remain displaced; some return to their original site as the lattice relaxes. The remaining

i nterstiti al s will diffuse away until they are trapped by another vacancy, dislocation, or grain

boundary (including the boundary at the vacuum surface), if the temperature of the lattice is high

enough. The preferred trapping locations for these diffusing interstitials at the surface will be

those which will reduce the free energy of the surface, i.e., jogs, steps, etc. The surface

should be smoothed on a microscopic scale by interstitials diffusing to the surface if no additional roughening takes place because of some other property of the bombardment.

Because the optical data are only sensitive to the material properties of the first 20 nm (approximately) of the solid, it is important to examine the spatial properties of the ion

cascades generated by the three ion species in molybdenum. The range parameters in table IV indicate that at the highest doses employed, the nickel content of the surface will be less than 1 ppm, and the gold content about 200 ppm. Although there are no data in the literature on the

effect of small amounts of these impurities on the optical properties of molybdenum, one may assume that the levels of gold or nickel present in the molybdenum mirrors should have a negligible effect on the reflectance. Therefore the broadening seen in the bulk optical structure

is not likely to be due to alloying effects.

Another effect which may cause broadening of <e> spectra involves a change in grain size

[15]. In a previous study [21], transmission electron microscopy was used to determine the grain

size in molybdenum samples which had been polished mechanically, ending with a 1 ym diamond paste

polish. No difference was found between unirradiated samples and samples irradiated with 150 keV

molybdenum ions to a dose of 1x10 cm"^ at room temperature or 500° C. The mean grain size was

250 nm with some evidence of plastic deformation due to the polishing. Thus a change in grain

size is also not likely to be the cause of the spectral broadening. The broadening may be due to the formation of dense dislocation loops near the surface, caused by the collapse of small voids

generated during the ion cascade.

Sputtering, or the loss of target atoms from the first one or two atomic layers which receive

sufficient energy during the ion cascade to overcome the surface barrier energy, can in principle

roughen an originally flat surface [23]. The ion cascade and hence the sputtering, which ensues

when the cascade intersects the surface, is a localized event and results in material loss in the

form of microscopic pits if the cascade is sufficiently dense [24]. This roughening process can compete with the smoothing process described above.

If the depth at which the majority of interstitials are created is much greater than the mean

free path between trapping sites then the flux of interstials at the surface will be much less per

incident ion for an ion which on average deposits its energy further below the surface than for an ion which on average deposits its energy nearer the surface. Thus the ion penetration depth will

influence both the smoothing and roughening processes.

219
As an additional complicating effect, the sputtering rate can be greatly enhanced if the

energy density of the ion cascades cannot be described by the "linear binary collision

approximation". This can happen if the majority of atoms enveloped in each cascade are in motion

and the cascade becomes nonlinear. This situation is referred to as an "energy spike".

For the ion target combinations examined here it is reasonable that the 150 keV molybdenum

ions produce cascades which are "dilute" or "linear", and are sufficiently near the surface for

the diffusing interstitials to smooth the surface faster than sputtering can roughen it. For 1 MeV gold the sputter rate is much larger than that for 150 keV molybdenum, which we speculate to

be due to spike effects too large to be offset by the production of interstitials near the surface

since the gold ion energy density is about twice that of the molybdenum ions. We postulate that

the 3 MeV nickel ions in Mo produce deep diffuse cascades which generate interstitials which are trapped before reaching the surface. The sputtering that is generated causes the surface to roughen at a low rate compared to that of gold, and the sputtering rate for Mo is much lower than

for both Au and Ni implants.

Thus the experimental data for Mo, Au, and Ni implants into Mo can be explained in terms of

the ion energetics and competing smoothing and roughening processes.

8. Conclusion

The effects of ion implantation on the optical properties of highly polished molybdenum

mirrors were determined using single and multiple angle of incidence spectroscopic ellipsometry.

These measurements allowed us to obtain the bulk dielectric function of Mo from 1.5 to 6.0 eV as well as the thicknesses of surface roughness layers. The smoothest surfaces were obtained by

implanting 150 keV Mo ions at a fluence of 5 x 10-^^cm'^, which produced a microscopically smooth

surface. In contrast to reflectance measurements, the el 1 ipsometric measurements are found to be

very sensitive to microscopic roughness. A thin dielectric overlayer (probably a hydrocarbon)

condensed on mirror surfaces exposed to air, considerably lowering the apparent dielectric

function and also reducing the reflectance.

9. References

[1] J.M. Bennett, S.M. Wong and G. Krauss, Appl . Opt. 3562 (1980).

[2] J.M. Bennett, P.C. Archibald, J. P. Rohr and A. Klugman, Appl. Opt. 4048 (1983).

[3] G.E. Carver, Thin Solid Films 169 (1979).

[4] R.A. Hoffman and W.J. Lange, "Ion Polishing of Optical Surfaces", Final Report to Air Force

Weapons Laboratory from Westinghouse Research Laboratories AFWL-TR-76-155, September 1976.

[5] G.H. Bu-Abbud, D.L. Mathine, P.G. Snyder, J. A. Woollam, D. Poker, J.M. Bennett, D. Ingram

and P.P. Pronko, J. Appl. Phys. _59, 257, (1986).

[6] J.H. Weaver, C. Krafka, D.W. Lynch and E.E. Koch, in Physics Data : Optical Properties of

Metals I (Fachinformationszentrum Energie, Physik, Mathematik GmbH, Karlsruhe, 1981), p. 139.

[7] M.M. Kirillova, L.V. Nomerovannoya and M.M. Noskov, Sov. Phys. JETP 23, 1210 (1971).

[8] B.W. Veal and A. P. Paulikas, Phys. Rev. BIO, 1280 (1974).

[9] J.E. Nestell, Jr. and R.W. Christy, Phys. Rev. B21^, 3173 (1980).

[10] J.M. Bennett and J.H. Dancy, Appl. Opt. 1785 (1981).

[11] J.M. Bennet, Opt. Eng. _24, 380 (1985).

[12] D.E. Aspnes and A. A. Studna, Appl. Opt. _14, 22 (1975).

[13] R.M.A. Azzam and N.M. Bashara, Ellipsometry and Polarized Light , North Holland Publishing

Co., Amsterdam 1977.

220
[14] E. Deltombe, N. deZoubov, and M. Pourbaix, in Atlas of Electrochemical Equilibria 1n Aqueous

Solutions, ed. by M. Pourbaix (National Association of Corrosion Engineers, Houston, 1974),

p. 272.

[15] U. Kreibig, J. Phys. _R, 999 (1974).

[16] D.E. Aspnes, J.B. Theeten and F. Hottier, Phys. Rev. B20, 3292 (1979).

[17] D.W. Marquardt, J. Soc. Indus. Appl . Math. JA, 431 (1963).

[18] G.H. Bu-Abbud, N.M. Bashara, and J. A. Woollam, Thin Solid Films 138. 27 (1986).

[19] D.A.G. Bruggeman, Ann. Phys. _24, 636 (1935).

[20] H.W. Hermance and T.F. Egan, Bell System Tech. J. 739 (1958).

[21] P. Pronko and D. Ingram, Phase 1 Report, "Ion beam processing for laser mirrors," NSF Award

#PHY8260333, 1983.

[22] J. P. Biersack, J. Nucl . Instrum. Meth. 174, 257 (1980).

[23] J.E. Westmoreland and P. Sigmund, Radiation Effects 6, 187 (1971).

[24] W. Jager and K.L. Merkle, 9th International Congress on Electron Microscopy, Vol. 1, p. 378

(1978).

Table 1. Mo Implanted Mo.

Fl uence Roughness Layer

13A ± lA

13. 5A ± lA 16A ± lA 14A ± lA 0.5A ± 0.5A

1 X lO^W^

2 X 10^\m"2

5 X 10^%-2

1 X lO^W^

2 X lO^^cm'^

Table 2. Au Implanted Mo.

Fl uence Roughness Layer Void Fraction

1 X lO^^cm"^ 12A ± lA 0.5

1 X lO^^cm"^ 22A ± 3A 0.43

2 X lO^^cm"^ 62A ± lOA 0.39

5 X lO^^cm'^ 130A ± 20A 0.6

221
Table 3. Ni Implanted Mo.

F1 uence Roughness Layer Film Thickness

2 X lO^^cm"^ 9A ± 2A 23A ± 2A

5 X lO^^cm'^ 7A + 2A 23A ± 2A

1 X lO^^cm"^ 5A ± 2A 26A ± 2A

2 X lO^^cm"^ 6A ± 2A 29A ± 2A

5 X lO^^cm"^ 12A ± 3A 33A ± 3A

Table 4. Implantation Parameters

Parameter Ni Mo Au

Implantation energy (keV) 3000 150 1000

Range, Y (A) 7450 295 1037

Straggle, "ax (A) 1800 140 388

Estimated surface fraction <0.001% <0.1%

Deposited surface energy:

electronic (eV/A) 250 83 195

nuclear (eV/A) 26 255 367

Integrated deposited surface energy:

at 1 X lO-'-^ions/cm^:

electronic (eV/atom) 390 129 304

nuclear (eV/atom) 41 397 572

222
u

c

LlT3O

T3

CO

35 0

30.0

25.0

20.0

15.0

10.0

5.0

0.0

-5.0

-10.0 h

-15.0

Mo Implanted

BellCore UNL

_j L_ -1 L. 15 20 2 5 3.0 35 4.0 4.5 5 0 5 5 60

Photon Energy, eV

14 -2 Figure 1. Pseudodielectn c functions of the 5x10 cm fluence region of the Mo implanted mirror,

measured by the two different el 1 ipsometers used in this investigation, as described in the text.

Photon Energy, eV

Figure 2. Pseudodielectric functions of the Mo implanted mirror as a function of fluence.

(Bellcore el 1 ipsometer)

223
35 0

30 0

Mo implanted

-10.0 -

-15 0 I 1 ' 1 1 L- 15 2 0 25 30 3.5 4.0

Photon Energy, eV

Figure 3, Pseudodielectric functions of an Mo surface after cleaning (solid lines), and of the same surface after several days in air (dashed lines).

Figu^^ 4. 2 Pseudodielectric functions of the Au implanted mirror as^a function of fluence. The

2x10 cm and 5x10 cm" regions were almost identical to the 1x10 cm' region (UNL

ellipsometer).

224
35.0

300

25 0

c 20.0 oo

c

3

15 0

Lj- lU u o

5 0

b 0.0

-5 0

- 10 0

-15.0

Mo+ voids Mo

2.0 2.5 3.0 3.5 4.0

Photon Energy, eV

14 -2 Figure 5. Dielectric function of the 1 x 10 cm fluence region of the Au-implanted mirror,

calculated by correcting the data for 12A roughness (curve 1); pseudodi^Ject^ic function data of

the same region (curve 2); pseudodielectric function data of the 5 x 10 cm fluence region of

the Mo-implanted mirror (curve 3). Inset: 3 phase model used to correct data for surface

microscopic roughness.

Photon Energy , eV

Figure 6. Dielectric functions of the Au implanted mirror, calculated by correcting the

corresponding pseudodielectric function for microscopic roughness. The thicknesses and void

fractions obtained from the calculations are given in Table II.

225
-100 -

-15.0 I

, , , ,

1.5 2.0 2.5 3.0 3.5 4.0

Photon Energy, eV

Figure 7. Pseudodielectric functions of the Ni implanted mirror, as a function of fluence (UNL

ellipsometer).

226
Manuscript Reaeived

12-9-85

Glancing Incidence Absorption Measurements of Metal Mirrors

W. D. Kimura and D. H. Ford

Spectra Technology, Inc. . Bellevue, WA 98004

(formerly Mathematical Sciences Northwest)

Glancing incidence absorption measurements are performed on bare Cu, Mo, Au.

Al, and Ag mirrors, and on Al and Ag mirrors with a protective dielectric overcoat. Data

is obtained for angles of incidence from 0° to >89° . Wavelengths studied are 0.5 /im

and 10.6 p.rr\ at both s and p polarizations. The absorption is measured using a photoacoustic calorimetry technique in which the transducer is attached directly to the back

of the test mirror. Absolute calibration of the absorption data is obtained by using a ratiometer to independently measure the absorption at near normal incidence.

The results are compared with the absorption predicted by Fresnel's equations for bare metals using values of n and k of the complex index of refraction measured by other sources. The absorption of a mirror at glancing incidence appears to be a sensitive

function of its surface condition. New mirrors tend to agree with Fresnel's equations;

while "old" mirrors have much higher absorption probably due to a layer of tarnish. The

overcoated Al data deviates greatly from Fresnel's equations indicating that bare metal

theory cannot be used and a thin film theory should be used instead. Representative

results are given.

Key words: absorption; Fresnel equations; glancing incidence; metal mirrors; overcoats: photoacoustic calorimetry; polarization;

1. Introduction

Various concepts for laser particle accelerators (LPA) [1] require lasers with high average power

(to provide high particle luminosity) and small beam diameters (for optimum mode matching).

Thermal distortion and damage of the reflective optics used in such systems can become a critical issue. Glancing incidence mirrors can be used to overcome this problem; however, data on their performance is sparse. Previous experiments have primarily measured optical damage and were limited

to <80° angle of incidence [2,3].

In the work described in this paper, the thermal absorption characteristics of various mirrors are measured from 0° to >89° angle of incidence for both s and p polarized laser light (perpendicular

and parallel to the plane of incidence, respectively). Measurements are taken at two laser wavelengths: 0.5 and 10.6 fim. Tested are commercially available bare metal mirrors (Cu, Mo, Au,

Al, and Ag) and protected Al and Ag mirrors with thin overcoats on their surfaces.

In the next section, Fresnel's equations for the reflectivity of a metal surface is briefly

reviewed. The experimental setup is described in Section 3 and sample results are given in Section 4. Section 5 concludes with a discussion of the results obtained thus far in this program.

2. Theory

The complex index of refraction, n, of a material is given by,

n(X) = n(X) + ik(X) II]

where n is the index of refraction and k is the extinction coefficient. The values of n and k are in general a function of the wavelength of the light.

227
Beginning with Maxwell's equations and assuming a plane electromagnetic wave impinging from a

dielectric medium (such as air) onto a conducting metal surface at an angle, 6. measured from the normal, one can solve for the amplitude, and p^, of the reflected wave whose polarization is

perpendicular and parallel to the plane of incidence, respectively. These are referred to as the

Fresnel equations 14]. Assuming unity for the dielectric refractive index, Fresnel's equations are,

2 2

2 (cose - U) + V Pi = 2 2

12] (cos0 + u) + V

2

2 [(n^ - k^jcose - u] + [2nkcos0 - v]^

p = _ 13] [(n^ - k^] cos© + uJ + [2nkcos© + v]

2 r2 21 2 \ f [ 2 21 21^ 22]

u = In - k J - sin © + 1 L In - k J - sin ©J + 4n k J

2 f2 21 2 I \ ( 2 21 21^ 22}

= -In - k J + sin © +llln - k J - sin ©J + 4n k J

where

1/2

1/2

2v

2 During this program, a computer code solves for the absorption at the mirror surface, i.e. 1 -

2 and 1 ~ using Equations 2 and 3.

When comparing our data to theory, values of n and k at the wavelength of interest must be

chosen to substitute into Equations 2 and 3. A number of different sources [51-[8] are used for values

of n and k. It is important to realize that often these sources do not agree with one another on the values of n and k for the same material. This is not totally surprising when one considers that

different forms of the same material may have been tested (e.g., films versus bulk), the condition of the surfaces may vary from author to author, and the various measurement techniques are susceptible

to different systematic errors.

Fortunately, the overall shape of the absorption curves as a function of the angle of incidence

for a material do not change significantly for small changes in n and k. The magnitude of the

absorption at normal incidence, however, does change significantly for small changes in n and k. Hence, when comparing our data with theory, it is important to compare both the overall shape of the curves and their absolute magnitudes. As will be shown, there are times when the overall shape

agreement between the data and theory is excellent, but there may be a disagreement with the

absolute magnitude of the absorption.

As will be explained in the next section, during the experiment the absolute magnitude of the

absorption at near normal incidence is measured independently with a ratiometer energy meter.

Therefore, any disagreement between the data and theory in regards to the absolute magnitude of the

absorption is probably due to differences in the mirror surface condition rather than disagreement with the theory itself. Viewed with this perspective, it is more important that the overall shapes agree

rather than the absolute magnitudes.

Finally, in regards to the practical usage of these mirrors at glancing incidence, it is the

magnitude of their absorption at glancing incidence that is important. Since our data, once it is calibrated with the ratiometer measurements, provides the absolute magnitude of the absorption for ail angles, our experimental results are still valid despite any disagreements with theory.

3. Description of Experiment

Photoacoustic calorimetry is used to measure the very small absorptance expected at glancing

incidence. The basic technique is well established (91; a modified version [10] is being used in this experiment where the acoustic transducer is attached directly to the back of the mirror rather than on

the test chamber wall. This modification greatly enhances the sensitivity of the technique: absorption

-4 -5

coefficients (absorbed light/incident light) of 10 to 10 are detectable.

228
Briefly, in photoacoustic calorimetry. energy from a laser beam incident upon a mirror is absorbed at the mirror surface. This absorbed energy generates an elastic strain wave (acoustic

wave) which propagates through the mirror substrate. This strain wave passes through the piezoelectric transducer rigidly attached to the back of the mirror substrate. The strain induced in the piezoelectric causes a voltage to be generated across the transducer electrodes which is directly

proportional to the magnitude of the strain wave. Thus, the voltage gives a direct indication of the amount of energy absorbed at the mirror surface.

Figure 1 is a schematic of the glancing incidence measurement system. Two different cw.

vertically polarized lasers are used: an argon ion laser (0.5145 nm) and a CO^ laser (10.6 urn).

The output from the lasers is sent through a chopper, a collimating telescope, a half-wave plate to rotate the polarization, and a "clean-up" polarizer which rejects all light not rotated by the half-wave

plate. The laser light then reflects off the test mirror mounted on a rotation stage. A phase sensitive

lock-in amplifier measures the piezoelectric transducer output which may be as low as 10-100 nV at glancing angles.

CHOPPER COLLIMATING

TELESCOPE

HALF -WAVE

PLATE

LASER

POLARIZER

MIRROR

PIEZOELECTRIC

TRANSDUCER

LOCK IN

REF SIGNAL

9 9

85 09482

Figure 1. Schematic of glancing incidence measurement apparatus.

There is a small dependence of the transducer output on the shape of the laser footprint on the mirror surface. At glancing incidence, the laser footprint on the mirror becomes very elongated and

much longer than the diameter of the transducer. This footprint dependence can be easily determined

by directing the laser beam at normal incidence to the test mirror surface and using a cylindrical lens

to create an elongated beam spot on the mirror. The absorption at the mirror surface is relatively constant as the footprint size is varied because the angle of incidence of the laser beam never exceeds 10° as the beam expands from the cylindrical lens. Knowing the length of the footprint and

the diameter of the laser beam, the equivalent angle that corresponds to the footprint can be

calculated.

The correction for footprint dependence is found to be negligible for angles less than about

80°. Even at 89° the correction is typically only a factor of two which is small when compared with the orders of magnitude changes in reflectivity that occur at glancing incidence. The footprint

calibration curve is obtained for every mirror tested and at the two wavelengths investigated.

Absolute calibration of the photoacoustic calorimeter is obtained by using a dual-head laser energy ratiometer. Using the same laser as used for the angle dependence measurements, the

calibration is performed by measuring the amount of energy absorbed on the mirror surface at near normal incidence. A beamsplitter reflects a portion of the incident laser light into one of the

ratiometer sensing heads which is used as a reference source in order to eliminate fluctuations in the

229
laser output energy. The other ratiometer head is positioned alternately before and after the mirror to measure the incident and reflected light, respectively. The ratiometer automatically averages over hundreds of light pulses.

4. Experimental Results

Figure 2 shows the absorptance characteristics of bare silver at 0.5145 fim. To minimize

tarnishing of the bare silver mirror, the mirror was shipped to Spectra Technology from the coating

vendor in a leak-tight container filled with an inert gas and was tested in a nitrogen purged

environment. Error bars on the data indicate the reproducibility of the data. The calibration at normal incidence is uncertain by approximately ±1-2% absolute absorptance. The solid curve is Fresnel's equations (eq. 2 and 3) using the values of n and k given in reference 5. There is a

slight disagreement between the data and theory in regards to the absolute value of the absorptance;

however, there is excellent agreement in overall curve shapes.

PERPENDICULAR

POLARIZATION

PARALLEL

POLARIZATION

0.01

0 10 20 30 40 50 60 70 80 90

ANGLE OF INCIDENCE (DEGREE)

100 _

0.1

0 10 20 30 40 50 60 70 80 90

ANGLE OF INCIDENCE (DEGREE)

8 5 0 9 8 6 6

Figure 2. Absorptance curves for bare silver at 0.5145 /irr\ for both s and p

polarizations. The solid curve is Fresnel's equations using the values of n and

k given in reference 5.

The effects of a protective overcoat ("1000 angstrom thick layer of ThF^) on the silver surface

is shown in figure 3. The protected silver mirror appears to have slightly higher absorptance at glancing angles than the bare silver mirror. This is true for both polarizations and may be related to a "trapping" of the laser radiation between the dielectric layer and the metal surface at high angles of incidence.

2 2

In general, for the metals and wavelengths tested, either (sin©/n) «1 and/or (sin9/k) «1.

Then, for the perpendicular polarization case, eq. (2) simplifies to first order to a simple cos©

dependence [2]. Hence, a simple comparison of the experimental absorptance curve shape can be

made with the theoretical one by calculating the deviation of the data from the cos© dependence.

Figure 4 is a plot of the percentage deviation from this cos© dependence of the data for the bare and protected Ag mirrors. If the data followed exactly the cos0 dependence, the points would lie on a straight line at zero deviation. The bare silver (fig. 4a) does exhibit a low deviation until approximately 85°, at that point the data actually indicates lower absorption than theory predicts.

230
However, since the error bars are also relatively large for these points, it is more likely that the absorption is very close to that predicted by theory.

PERPENDICULAR PARALLEL POLARIZATION POLARIZATION

100.0 ^

ANGLE OF INCIDENCE (DEGREE) ANGLE OF INCIDENCE (DEGREE)

Figure 3. Absorptance curves for protected silver at 0.5145 for both s and p

polarizations. The overcoat is an ^lOOO angstrom thick layer of ThF^. The

solid curve is Fresnel's equations using the values of n and k given in reference

5 for bare silver.

The protected silver mirror (fig. 4b) displays clear deviations away from the cos© dependence.

At approximately 40° it has lower absorptance than predicted, and for angles greater than 70° it has

significantly higher absorptance.

Absorptance results for bare gold and copper mirrors at 10.6 fim are given in figures 5 and 6. Values of n and k for the theoretical curve are from references 5 and 6 for the bare gold and copper

cases, respectively. The bare copper mirror is an example of the results when testing a "old" mirror;

that is. a mirror which, although undamaged, has a tarnished surface as a natural result of aging.

The copper data displays a curious increase in absorption at around 82°-86°; this may be related to the effects of the tarnish layer.

Figure 7 shows the deviation of the bare gold and copper data from the cos© dependence. The

bare gold follows the dependence quite well until approximately 80°; at higher angles the absorptance

tends to be higher than that predicted by theory. At glancing angles, the absorptance of the

tarnishea copper mirror Is significantly higher than theory predicts. The tarnish layer is probably

largely responsible for this higher absorptance.

Finally, figure 8 gives an example of how an overcoat can dramatically alter the absorption

characteristics of a mirror. The results are for a protected aluminum mirror with a MgF^ overcoat measured at 0.5145 fim. It is clear the data does not follow theory (7,81; however, this simply

implies a theory which incorporates the effects of a thin dielectric film needs to be used.

231
X = 0.5145 pm PERPENDICULAR POLARIZATION

BARE SILVER

10 20 30 40 50 ANGLE OF INCIDENCE

(a)

60 70 (DEGREE)80 90

50

40 -

30

20 -

10

PROTECTED SILVER

10 _

-20

-30 -

-40 -

-50 10 20 30 40 50 60 70 ANGLE OF INCIDENCE (DEGREE)

(b)

—t— 80 90

Figure 4. Deviation of data shown in figures 2 and 3 from the first order cos0 dependence

as predicted from Fresnel's equations.

PERPENDICULAR

POLARIZATION

PARALLEL

POLARIZATION

1.0 . 100.0

0. 001

10 . 0

0. 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 ANGLE OF INCIDENCE (DEGREE) ANGLE OF INCIDENCE (DEGREE)

Figure 5. Absorptance curves for bare gold at 10.6 fim for both s and p polarizations.

The solid curve is Fresnel's equations using the values of n and k given in reference 5.

232
PERPENDICULAR

POLARIZATION PARALLEL

POLARIZATION

10.0 ^

1.0 -

0.1 _

0.01

100

10

0.1

BARE METAL THEORY

10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 ANGLE OF INCIDENCE (DEGREE) ANGLE OF INCIDENCE (DEGREE)

Figure 6. Absorptance curves for a bare but tarnished copper mirror at 10.6 Mm for both

s and p polarizations. The solid curve is Fresnel's equations using the values

of n and k given in reference 6.

X = 10.6 ym PERPENDICULAR POLARIZATION

90

BARE GOLD BARE COPPER (USED)

80 .

60

40 -

20

-20

i

-40

-60

-80

400

300

200 _

100

-100

>

g -200

-300 -

-400 60 70 (DEGREE)

80 90 10 20 30 40 50 60 70 80 ANGLE OF INCIDENCE (DEGREE)

0 10 20 30 40 50 ANGLE OF INCIDENCE

(a) (b) Figure 7. Deviation of data shown in figures 5 and 6 from the first order cos© dependence

as predicted from Fresnel's equations.

90

233
PERPENDICULAR

POLARIZATION

100 -, 100 -1

0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 ANGLE OF INCIDENCE (DEGREE) ANGLE OF INCIDENCE (DEGREE)

Figure 8. Absorptance curves for protected aluminum at 0.5145 nm for both s and p

polarizations. The overcoat is a thin layer of MgF^. The solid curve is Fresnel's equations using the values of n and k given in references 7 and 8 for bare aluminum.

PARALLEL

POLARIZATION

5. Conclusion

Glancing incidence absorption measurements have been performed at angles >89° on a number

of different bare metal and overcoated metal mirrors. Absorptions as low/ as 0.04% have been

measured at 89°. While agreement between the data and Fresnel's equations for a bare metal

surface is generally good for new bare metal mirrors, there is occasionally great disagreement when

the mirror is either tarnished or has a protective overcoat.

Codes are available which predict the absorptance of mirrors with thin films [11]. Efforts are under way to compare the data obtained during this program with some of those code predictions.

Work is also continuing on further measurements and reduction of the data.

6. Acknowledgments

The authors wish to thank Drs. J. M. Slater and D. D. Lowenthal for their technical advice during

this program. They also wish to acknowledge J.S. Demboski for his assistance during some of the

measurements. This work was sponsored by the Department of Energy, Contract No. DE-AC06-

83ER40128.

7. References

1. Channell, Paul J., ed. Laser acceleration of particles. AlP conference proceedings. No. 91. New

York: American Institute of Physics; 1982.

2. Figueira, J. F. and Thomas, S.J. Damage thresholds at metal surfaces for short pulse IR lasers. IEEE J. Quan. Elec. QE-18 (9): 1381-1386; 1982.

3. Mumola. P. and Jordan, D. Glancing incidence optics for high power lasers. Proceedings of the

Society of Photo-Optical Instrumentation Engineers; 228: 54; 1981.

234
4. Born, Max and Wolf. Emil. Principles of optics. Oxford: Pergamon Press; 1975. 628-630.

5. Weaver, J.H.. Krafka, C. . Lynch, D. W. , and Koch, E. E. , ed. Physics data: optical properties

of metals, pts. I and II. Federal Republic of Germany: Fachinformationszentrum; 1981.

6. Hagemann, H.J.. Gudat. W. , Kunz. C. Optical constants from the far infrared to the x-ray

region: Mg. Al. Cu. Ag, Au, Bi, C. and AI^O^. J. Opt. Soc. Am. 65(6): 742-744; 1975.

7. Schuiz, L. G. The optical constants of silver, gold, copper, and aluminum. I. The absorption

coefficient k. J. Opt. Soc. Am. 44(5): 357-362; 1954.

8. Schuiz. L. G. and Tangherlini, F. R. The optical constants of silver, gold, copper, and

aluminum. II. The index of refraction n. J. Opt. Soc. Am. 44(5): 362-368; 1954.

9. Parker, J.G. Optical absorption in glass: investigation using an acoustic

technique. Appl. Optics. 12(12): 2974-2977; 1973.

10. Hordvik, A. and Schlossberg. H. Photoacoustic technique for determining optical absorption

coefficients in solids. Appl. Optics. 16(1): 101-107; 1977.

11. Stanford. J.L. , Michelson Laboratory, China Lake, OA. Woodberry, F.J., Rocketdyne Division,

Canoga Park, CA, and Stubbs, J. B. , University of Dayton, Dayton, OH. private communication.

235
Manuscript reeeivea

7-18-86 SAR

N-on-1 Damage Testing of Single Crystal Metal Surfaces at 1.06 ym

Yong Jee, Michael F. Becker and Rodger M. Walser

Electronics Research Center

and

The Department of Electrical and Computer Engineering

The University of Texas at Austin

Austin, Texas 78712

We examined the accumulation dependence of laser damage on single crystal

metal surfaces under Q-switched NdrYAG laser irradiation (l,064ym, 20 nsec pulses

with a 200pm spot diameter). The samples were chemically polished, electropolished, and single point diamond machined AT, Cu and Ni single crystal surfaces

of several crystallographic orientations. The single point diamond machined

samples did not have single crystal surfaces as detected by electron channeling

patterns, whereas the other samples had good single crystal surfaces.

Damage probability curves and accumulation curves were plotted to investigate

N-on-1 damage behavior. The slope of the damage probability curve increased with

the number of pulses required for damage. Accumulation curves, derived from 50%

damage fluences, often had two regions showing different slopes. We interpret

these measurements to indicate two different damage mechanisms, a defect related

local melting at low pulse numbers and plastic deformation damage at high pulse

numbers. These observations correlate with typical damage morphologies obtained by

a Nomarski optical microscope and by SEM.

Key Words: accumulation; aluminum; copper; nickel; damage probability; single

crystal metal surfaces

1. Introduction

The interaction of high power laser beams with metal surfaces has a variety of damage phenomena such as slip band formation, melting and evaporation. The difficulty of making the determination of a unique damage phenomenon has led to the study of the effect of spot-size [1,2] and pulse

width [3] on the single shot damage of metal mirrors.

Several models were proposed to analyze these damage mechanisms. Sparks and Loh's thermodynamic model has been successfully applied to analyze 1-on-l damage testing data [3]. But the

accumulation effect has not been studied fully on metal surfaces, particularly at 1.06pm. Lee,

Koumvakalis and Bass have studied the accumulation effect on diamond machined Cu at 10.6pm and

proposed a model based on plastic deformation damage. Musal's model [4] was another, similar

explanation which related damage to the thermal stress field induced by the laser pulse at the

metal surface. Figueira et al . [5,6] have also investigated N-on-1 damage to metal surfaces at 10.6pm and have observed anomalous, reversible fluence dependent damage.

The purpose of the present work is to measure single and multiple pulse damage thresholds for

metal surfaces of single crystal Cu, Al and Ni under 1.06pm irradiation. Because of the close

relation of damage to surface preparation, we have also studied the effect of different polishing

methods on the damage of single crystal surfaces. We have examined the slope of damage probability curves with several different polishing conditions, and have studied the accumulation effect

by varying the incident number of pulses. Damage morphologies near threshold with various incident number of pulses will be discussed. Finally, the correlation between the cumulative effect

of laser damage on metal surfaces and the linear slope regions of the accumulation curves will be

investigated. The accumulation curves are plots of log (E»,N) vs. log N where is N-pulse damage

threshold fluence and N is the incident number of pulses. From accumulation curves and damage

morphologies, N-pulse damage can be explained in terms of a cumulative process which is dependent

on the incident number of pulses. The metal samples were found to have a storage cycle dependent

on preceding pulses if the incident energy was larger than the plastic yield threshold.

236
2. Experimental Procedures

The laser source used in these experiments was a Q-switched Nd:YAG laser with a 20 nsec pulse

width and with pulse energy fluctuations of less than 3%. Incident pulses were attenuated by a rotating half-wave plate followed by a polarizer, and monitored by an energy meter whose output

value was recorded by the computer. This energy meter was calibrated using a second energy meter

before and after each testing session. The incident number of pulses was selected by an electromechanical shutter whose open and close operation was also automatically controlled.

The laser was operated at a pulse repetition frequency of 10-20 Hz. Output pulses were monitored by a vacuum photodiode and the pulse shape was displayed on a Tectronix 519 oscilloscope. Test samples were placed at the actual focal plane of a 50 cm focal length lens.

The beam spatial profile was measured with the knife-edge scanning method at the beginning

and at the end of each session [7], In this measurement, we calculated an equivalent Gaussian

FWHM (Full Width at Half Maximum) from the detected error function data and then obtained the beam

spot size using the equation 2W = FWHM//ln2/2 , where W is the beam radius of l/e^ in intensity.

The actual operating beam spot 8iameter was around 200pm° with a measurement error less than 3%.

In the damage testing process, a predetermined number of laser pulses were incident on the

sample surface, the energy of each pulse was recorded, and the laser beam was blocked by the

shutter. A new site was then selected by the step-motor stage controlled by the computer system.

Testing sites were separated by 0.7-1 mm in order to avoid overlap. As it was tested, each site

was viewed under an 80X optical microscope mounted on the experimental apparatus. Finally the

sample was removed from the sample holder and examined under a 200X optical Nomarski microscope to determine whether there was a permanent change on the metal surface. The damage features were

also examined using a scanning electron microscope (SEM) and a photograph was taken if needed.

All damage thresholds presented here correspond to the energy fluence on-axis of a focused beam

and represent 50% damage probability thresholds as determined by examination under the 200X

optical Nomarski microscope.

The prepared samples were single point diamond machined, chemically polished and electropolished single crystal Al , Cu and Ni surfaces. The single point diamond machined samples were prepared at the Los Alamos National Laboratory. Chemically polished and electro-polished samples were prepared using the solutions shown in table 1. The samples were then placed in the sample

holder, blown off with a freon dust chaser, and damage tested.

Table 1. Sample Preparation Techniques

Single Point

Diamond Turned - Chemically Polished Electro-polished Surface

Sample Surface Solution Temp Solution Volt. Time

Al Los Alamos

National Lab

H2SO4 25ml

H3PO4 70ml

HNO3 5ml

293K

Ethanol(95%)800ml

Water 140ml

HCLO4(60%) 60ml

35V 55s

Cu Los Alamos

National Lab

HNO3 66ml

H3PO4 66ml

CH3COOH66ml

293K Water

H3P04(85%)

175ml

825ml

1.6V 30m.

Ni

Los Alamos

National Lab

Water

H2SO4

250ml

600ml 6V 5m.

237
In order to investigate the surface crystal quality, electron channeling patterns (ECP), a Laue camera and an X-ray diffractometer were used. Since the electron channeling pattern can provide crystal orientation and crystal perfection information from a surface layer less than 50nm

thick, sample surfaces can be checked effectively with this instrument [8]. The X-ray beam of the

diffractometer and Laue camera penetrates more deeply into the material, up to 20vim. An X-ray

diffractometer or a Laue camera is recommended for examining bulk crystals. Chemically polished

or electro-polished surfaces exhibited good single crystal quality for either the ECP, X-ray

diffractometer or Laue camera. However, when we examined single point diamond machined surfaces

with ECP, they did not exhibit good single crystal qualities within a O.lym depth. Only with

X-ray techniques could we see their single crystal quality.

3. Experimental Data

3.1 Damage Probability Curves

When each sample was damage tested, we determined the damage probability curve which plotted

damage probability versus incident fluence as shown in figure 1. This sample was single point

diamond machined <110> Cu tested with N = 3 pulses. Damaged and undamaged data points are over- lapped within the interval between the maximum nondamaged and minimum damaged fluences. The

linear curve is obtained from these overlapped data points by a least square curve fit. From this

linear fit, we obtain the 50% damage threshold (Fj^,) for N pulses, and the damage onset, which is the zero intercept fluence for the linear fit. We also extracted the slope of the linear fit for

later correlation with the surface defect density [1].

3.2 Accumulation Curves

We have repeatedly tested samples at different incident numbers of pulses, taken similar

damage probability curves, and put the 50% probability threshold data on one graph as shown for

<110> Al in figure 2. This damage fluence curve indicates a decreasing damage threshold for

increasing incident number of pulses. The error bars indicate the minimum damaging fluence and

the maximum nondamaging fluence.

If we redraw the damage fluence curve in another way, we can get a damage accumulation curve. This accumulation curve is used to examine the effect of cumulative laser action on metal surfaces. The y-axis is the log of the product of the N-pulse damage threshold, Fj^, and the incident

number of pulses, N, [log(F^, N)]; and the x-axis is the log of the incident number of pulses, [log

N]. This plot, figure 3, shows two straight lines for the <110> Al surface shown previously in figure 2. Below N = 10 pulses, the curve has a smaller slope of S = 0.558; and above N = 10, the curve has a steeper slope of S = 0.902. Correspondingly, in figure 2, the curve below N = 10 has

a steep decline, which means that the additional energy required to damage the surface is increasing rapidly as N decreases. Above N = 10, the curve becomes nearly flat, indicating that there is no great change in the additional energy to damage the surface with decreasing N. In figure 3,

the slope for large N, S = 0.902, approaches the limit of S = 1. A slope of one means that the

damage threshold does not change at all with increasing pulse number and that there is no accumulation effect.

From the linear segments of the damage accumulation curve, we can derive the following

N-pulse damage threshold equation:

For this Al example, when N s 10, S = = 0.0558, and F^^^ = F^ represents single shot damage

threshold. When N 5 10, S = Sp = 0.902, and F^^^ is replaced with Fp^ which is the threshold for

plastic deformation damage. F . is obtained from the N = 1 intercept of the N S 10 line and is given by ^

F, th

S-1

(1)

N

''pd ""th 10 (Sm-Sp)logN^

(2)

238
where is the turning point. Using = 10 and the above values for and Sp, F^^ = 0.45F^ =

0.8 J/cm^. This means that the extrapolated single shot damage threshold for plastic deformation

is approximately half of single shot damage threshold for this diamond machined Al sample. For the case of Cu, a similar accumulation curve is obtained, but there is just a single straight line

as shown in figure 6. In this case = Sp and Fp^ = F^^. The interpretation of these linear re- gions on the accumulation curves will be discussed in a subsequent section.

3.3 Damage Morphology

For small N, higher fluences are required for damage, and surface defects are damaged easily.

Figure 7 shows single shot damage and local melting pits on diamond machined Cu at a fluence of

9.06 J/cm2. This picture was taken by a 500X optical Nomarski microscope. This morphology near

the damage threshold illustrates the form of defect related local melting. However, when the

threshold for slip formation is significantly lower than that for defect damage, visible slip

lines appear on the surface prior to catastrophic damage or melting. Figure 4 shows slip lines on an Al surface produced by the thermal stress field induced by 1-on-l laser tests. The photograph

was taken by a optical Nomarski microscope from a single point diamond machined <111> Al sample

irradiated at a fluence of 1.5 J/cm^ with a beam spot diameter of 460Mm. These slip lines are not ripple patterns and are not periodic. Similar slip lines on Ni are shown in figure 5. These slip

lines represent slip along <111> direction surfaces. This picture was taken by SEM from an electro-polished <111> Ni sample.

We observed that in the low pulse number regime, defect damage and plastic deformation damage

are competitive. If the defect damage threshold is lower than, or of the same order as, the

plastic deformation threshold, then defect damage appears first on the surface, and slip is minimal and invisible. If the plastic deformation threshold is significantly lower than that for

surface defect damage, then visible slip lines will appear first. In the high pulse number regime

where damage fluences are lower, plastic deformation is dominant near threshold as shown in figure

8. This typical picture, taken from a 3000 pulse damage site on diamond machined Cu, suggests

damage by plastic deformation. This is strong evidence for the cumulative effect of thermomechanical stress induced by repetitive laser pulses on the sample. Prior to catastrophic failure

(i.e., before a flash or a spark was observed) [2], the irradiated sites became roughened and the

degree of roughness increased with the incident number of pulses. Visible surface roughness was observed after a large number of pulses without the emission of light.

3.4 Damage Tests Results

For different materials and polishing methods, the slope of the accumulation curves and

single shot threshold data are summarized in table 2. For a single point diamond machined Al sample, the single shot damage threshold was 1.77 J/cm^, damage onset was 1.62 J/cm^, and the

slope of the accumulation curve was 0.71 for N s 10 and 0.88 for N > 10. For Cu samples, the

single shot damage threshold was about 10 J/cm^. Although the damage threshold of Cu surfaces

changed with the polishing method, there was only one slope to the accumulation curves.

4. Discussion

When we drew the various damage probability curves, the slope was found to increase with the

incident number of pulses and decrease with increasing numbers of defect damage spots in the beam.

This is in agreement with Foltyn's findings [1] that the slope is dependent on the beam spot size

and surface defect density, and that surface defects can be probed using damage probability

curves

.

Theoretical melt thresholds and measured 1-on-l damage thresholds are compared for a pulse

width of 20 nsec in table 3. In the second column, <A> is the average optical absorptance which

is obtained from the integral equation of Sparks and Loh [3].

(3) <A> =

239
Table 2. Test Data Summary

Al Cu Ni

oAlVll I^ii U. I

.

8/5/84

Sr. 84151

5/22/85

T-v nr

L*. 1

.

8/12/85

Sr. 85055

P D li.r

.

6/23/84

JJ. 1 . 5/22/85

Sr. 84156

1 on 1 50% Prob.

f J/cm2

)

1 .77 ( ,15 i.r>r>s.') 9.40 ( .79 10,2 2.72

)-on-l onset

(J/cm2)

1.62 6.84 8,24 1.81

Slope of

1 on-] damage

Probability Curve

330.3 46.1 (%cm2/J)

Slope of 0.709 0.732 0.980 0.942 0.75

Accumulation N-10 N<30 N^IO 1<N<10'^ N-5

Curve 0.878 0.960 0.880 s N^IO N>]0 N>1

Table 3. 7'heoretical Melt Thresholds vs. 1-on-l Damage Thresholds

Theoretical values Measured values

A Melting threshold Diamond turned surface

Sample (average) ('20nsec, 1.06um) 20nsec 47nscc

(J/cm2) (J/cm2) (J/cm2)

(with spot size as noted)

Al 0,0704 2.74 1.77±.0.]5 3.83±.0.06

(200nm) (68.5|irn

;

Cu 0.0253 19.52 9.40±.0.79

(208|im)

Ni 0.3175 ^ 0.77 0.93i_0.2 2.72^0.07

(68.7p,m) (80.2^m)

240
where A is the surface optical absorptance at room temperature, a is the coefficient of temperature dependent absorption, T is the melting temperature, and T is the ambient temperature (°K).

When an intense laser beam rnteracts with a normal metal surfa°ce, a fraction of incident energy

penetrates the metal to a skin depth and is absorbed by the free carriers in the metal. This

absorbed energy is converted to a thermal distribution which generates a thermal stress field. It

is the stress field which contributes to the cumulative effect of deformation and slip in multiple

pulse damage. If the incident radiation is intense enough, the absorbed energy can raise the

surface temperature to melting. The threshold for single shot surface melting is given by [3].

Fm=

(T -T ) (TtKCt 1 ^ m o' ^ p'

2<A>

(4)

where is the fluence (J/cm^) required to melt the surface, tp(sec) is the pulse duration, C is the volumetric specific heat capacity in (J/cm3°K), and K is the thermal conductivity (W/cm°K).

In the case of Al and Ni , the theoretical melt threshold and measured single pulse damage

threshold values are nearly identical. The agreement is not as good for Cu where the theoretical

melt threshold is near 20 J/cm^, but the measured damage threshold is only 10 J/cm^.

For the case of N-on-1 damage, we will consider cyclic thermomechanical stress induced by the

laser pulse as a possible explanation of the accumulation effect. The thermal expansion of metal

surfaces resulting from the transient temperature rise generates compressive stress, followed by

tension during the cool-down period. If the temperature rise and the coefficient of thermal

expansion are large enough, a permanent irreversible plastic strain will remain due to the com- pression and tension process. The fluence corresponding to the plastic yield (slip) threshold

(F^) is given by [4]

(TTKCt^)* (l-v)Y

2<A> BE

where v is Posson's ratio, Y is the yield stress, 6 is the coefficient of thermal expansion and E is Young's modulus of the metal for an idealized case of strain being linearly dependent on the stress. The experimental threshold for visible plastic deformation, F ,, identified in equation

(2) was Fpj = 0.8 J/cm^ for <110> diamond machined Al . The calculated^yield threshold at the free

surface [4] for the same material is 0.12 J/cm^. When we compared these two values, the experimental threshold for visible plastic deformation was 6.7 times larger than the theoretically

predicted yield threshold. According to Musal's model [4], the predicted free surface elastic

displacement is approximately 12 nm per incident J/cm^ for the case of high quality Al mirror

surfaces. If we use the same argument for our diamond machined Al surfaces, the predicted free

surface displacement will be approximately 1.4 nm at the fluence of 0.12 J/cm^, and about 6.7

times larger for the experimental case or approximately 9.6 nm. The distinction between these two cases appears consistent with the expected vertical detectivity threshold in Nomarski microscopy.

In other words, the threshold for visibility and for subsequent damage due to plastic deformation

might be larger than the threshold for the initiation of plastic deformation. Surface deformation

above this threshold is distinguished by an increase in absorptance sufficient to lead to cata- strophic surface damage to microscopically visible surface roughening. This cyclic thermomechanical effect can lead to the accumulation observed in N-on-1 damage.

In the previous section, we have shown that the accumulation curves and damage morphologies

suggest two damage regimes for low N and high N, where in both cases the N pulse damage threshold,

Fuj, is in the form of equation (1). The N-on-1 damage threshold decreased with increasing N suggesting an energy storage cycle produced by the thermomechanical stress-strain field of the

absorbed laser beam.

241
For cases such as Cu having a single slope in the accumulation curve, these results may be

combined with the equation for the theoretical melt threshold. The result is an equation for

N-on-1 damage threshold in terms of material constants, as shown below.

(T -T ) (^KCt^) mo p

,1-S 2<A;

(6)

If S is 0.9 and N is 1000, then the 1000 pulse damage threshold is reduced by a half. Thus the

N-pulse damage threshold may be predicted using equation (6) when a typical value of S for the

material is known.

5. Conclusions

Single and multiple pulse damage threshold measurements have been performed on single crystal

Cu, Al , and Ni single point diamond machined, chemically polished, and electro-polished surfaces

with a 1.06 ym laser beam in air. Our findings imply that plastic deformation is an important

contributor to the nanosecond pulse laser damage of these metal surfaces even at fluences well

below their melting or their defect damage thresholds.

The crystal quality of the samples was checked with electron channelling patterns and X-ray

diffraction. Diamond machined surfaces had a disordered layer at least 0.1 pm in depth, but the

electro-polished and chemically polished surfaces were single crystals. It is well known that the

diamond machining process involves high stresses at the surface, so this disorder is not unexpected.

The slope of the damage probability curve is related to the surface damaged defect density

and the incident number of pulses. We found that in addition to the expected decrease in slope

with increasing defect damage densities, the slope increased with increasing N in N-on-1 experiments .

Accumulation was observed in all samples and the accumulation curves indicated that the

damage mechanism had two different regimes -- a lew pulse number regime and a high pulse number

regime. In the low pulse number regime, the morphology suggests defect dominated local melting;

and in the high pulse number regime, the morphology suggests plastic deformation mediated by

energy storage cycles from previous pulses. In some cases where the threshold for plastic deformation or yield was extremely low, plastic deformation damage was dominant even for N = 1.

The accumulation effect for all the samples could be described by the experimentally derived

equation (1) which indicates decreasing N-pulse damage thresholds with increasing pulse numbers.

The thermal stress field produced by the incident laser pulse appears to drive cyclic thermomechanical behavior involving energy storage by nonconservati ve circuits around the stress-strain

curve. Slip lines form when the absorbed energy exceeds the yield (slip) stress threshold.

Under repeated laser irradiation this is manifested in surface roughness, and eventually the

surface damages.

This research was supported by the DoD Joint Services Electronics Program under research

contract AFOSR F49620-82-C-0033. We" also are grateful to Dick Rohrer at the Los Alamos National

Laboratory for his assistance with the diamond machined samples.

6. References

ri] S.R. Foltyn, "Spotsize Effects in Laser Damage Testing." Nat. Bur. Stand. (US) Spec. Publ

.

#669; 368, 1982.

[2] C.S. Lee, N. Koumvakalis, M. Bass, "Spot-size Dependence of Laser-Induced Damaqe to Diamond

Turned Cu Mirors," Appl . Phys. Lett. 48(7); 625 (1982); "A Theoretical Model for

Multiple-pulse Laser Induced Damage to Metal Mirrors." J. Appl. Phys. 54(10); 5727 (1983).

242
[3] M. Sparks, E. Loh, Jr., "Temperature Dependence of Absorptance in Laser Damage of Metallic

Mirrors: I. Melting." J. Opt. Soc. Am.; 69(6), 847 (1979).

[4] H.M. Musal , Jr., "Thermomechani cal Stress Degradation of Metal Surfaces under PULsed Laser

Irradiation." Nat. Bur. Stand. (US) Spec. Publ . #568; 159 (1980).

[5] J.F. Figueira, S.J. Thomas, "Damage Thresholds at Metal Surfaces for Short Pulse IR Lasers."

IEEE J. Quantum Electron; QE-13 , 1381 (1982).

[6] S.J. Thomas, R.F. Harrison, J.F. Figueira, "Observations of the Morphology of Laser-induced

Damage in Copper Mirrors." Appl . Phys. Lett.; 40(3), 200 (1982).

[7] D.C. Cohen, B. Little, F.S. Lueche, "Techniques for Measuring l-m Diameter Gaussian Beams."

Appl. Opt.; 23(4), 637 (1984).

[8] J.I. Goldstein, H. Yakowitz, Practical Scanning Electron Microscopy . Plenum Press, New

York; 149-210 (1975).

DIAMOND-TURNED CU(llO)

PULSE WIDTH- 47 nsec

SPOT SIZE- 101. ium SHOT NUMBER= 3

*^.«0 3.(0 4.40 S. 20 «. 00 6.80 7.60 INCIDENT FLUENCE(J/CM2)

Figure 1. Damage probability curve for single point diamond machined <111> Cu, N=3.

243
DIAMOND-TURNED AL(llO) PULSE WIDTH=19nsec

SPOT SIZE=68. 7um

1 {

f f f

f

rT| 11—I I I I 1 1

1 1 1—[ I I I I r

I 10' 102 10' NUMBER OF PULSE (N)

10O 10*

Figure 2. Damage fluence versus N for single point diamond machined <110> Al

.

Figure 3. Accumulation curve (replotted data) for the same Al sample as figure 2.

244
4-00 Mm

Figure 4. Optical Nomarski micrograph of the damage morphology on a diamond machined <111> Al surface. All the spots have been single shot damage tested at 1.5 J/cm^.

''to.

Figure 5. SEM micrograph of slip line formation on an electro-polished <111> Ni surface, N - 245 at 1.3 J/cm2.

245
10' ip2 ip3 10*

J 1 I I 1 ml L 1 1 I I Lill 1' I I I ml I 1 1 I I I III ELECTRO-POLISHED CU(IIO) PULSE WIDTH=13nsec

SPOT SIZE = 108. urn "

I,- N'

c

1

' I I IIIP| 1

1 I I

1

r I I I 1—I I 1 M ill 1

1 I I I II! —°

10° lO' 10^ lo' 10* 105 NUMBER OF PULSE (N)

Figure 6. Accumulation curve for electro-polished <110> Cu.

\0OMm

Figure 7. Optical Nomarski micrograph of damage on single point diamond machined <100> Cu which

shows defect related local melting pits, N = 1 at 9.06 J/cm^.

246
Figure 8. SEM micrograph of plastic deformation on a single point diamond machined <110> Cu surface, N = 3000 at 3.28 J/cm^.

247
Manuscript received

12-22-85 SAE

LASER INDUCED MO MIRROR DAMAGE FOR HIGH POWER CO2 LASER

H. Okamoto, N. Matsusue, K. Kitazima

Central Research Labolatory, Nippon Mining Co., Ltd.

3-17-35 Niizominami , Toda-shi, Saitama, 335, Japan

K. Yoshida, Y. Ichikawa, M. Yamanaka, T. Yamanaka

Institute of Laser Engineering, Osaka University

2-6 Yamadaoka, Suita-shi, Osaka, 565, Japan

and

Y. Tsunawaki

Osaka Industrial University

3 Nakagaito, Daito-shi, Osaka, 574, Japan

ABSTRACT

We have developed a high-purity molybdenum (Mo) mirror by electronbeam melting method (e.b.m. Mo mirror). Its high laser-damage threshold

has been demonstrated for the high-power TEA CO2 laser system. The e.b.m. Mo mirror has a damage threshold of 2 to 4 times higher than that of an Au coated glass mirror and had a life time of 3 times longer than that of

a powder metallurgy Mo mirror (p.m. Mo mirror) when the laser energy density

of higher than 60 J/cm^ was irradiated. We found that on the laser-damage

threshold the difference between the e.b.m. Mo mirror and the p.m. Mo mirror was due to the fine surface without voids and the small amount of

impurities

.

Key words: Laser mirror; molybdenum; CO2 laser; laser damage; polished

surfaces; surface roughness; optical characterization.

I. Introduction

We have been developing an optically pumped high power far infrared (FIR) laser system for

plasma diagnostics. ^' ^ The maximum output power of this laser is limited mainly by low laserdamage threshold of optical components. Although Au coated glass mirrors are usually used in

a high power TEA CO2 laser system for pumping the FIR lasing molecules, metallic mirrors are more Suitable for such a laser system because of the higher laser-damage threshold. Especially,

molybdenum (Mo) is a material of interest as high power laser mirrors because it has a good

heat conductivity, a low thermal expansion, a high melting point, a high reflectivity, and a high hardness of the surface.^ Conventional Mo mirror produced by sintering method has many

voids on the surface, which are considered to cause the damage threshold lower than the expected

value. We had newly developed Mo materials by electron-beam melting method to obtain a voidless

mirror.

In this paper, a laser induced damage on electron-beam melting (e.b.m.) Mo mirror was studied comparing with powder metallurgy (p.m.) Mo mirror and Au coated glass mirror by using

the TEA CO2 laser (100 mJ, 50 ns)

.

II. Experimental Method

The e.b.m. Mo mirrors with high purity of 99.999% used in this test were prepared for target

materials in semiconductor industry. The surface of the e.b.m. and p.m. Mo mirrors were polished

by an oscillation type machine. The surface roughness was measured by a Talystep (Rank Taylor

Hobson Limited) which had a conical 12.5 ym tip radius for height measurement. The Au coated

glass mirror consisted of Au and Cr layers with few micrometers of thickness on glass substrate.

o The reflectivity of the Mo mirror was mesured using 6328-A He-Ne laser, 1.06-lJm YAG laser

and an infrared spectrometer.

248
The laser damages against a high power TEA CO2 laser were studied for three kinds of mirrors.

The 9.26-lini R(22) beam with pulse width of 50 nsec from TEA CO2 laser oscillator (Lumonics

K-921S)'* was focused with a 100 mm foacal length ZnSe lens onto the mirror set on an XYZ-stage.

The incident energy of TEA CO2 laser was varied from 10 to 65 mJ by changing the charging voltage

and the mixing ratio of CO2 laser gas components of CO2 , He and N2 • The beam spot size was estimated by Nomarski microscopic observation of damaged pattern on the plastic film and silicon

wafer. The damage threshold was defined through microscopic observation as the minimum laser

energy density when the surface states changed slightly. The plasma light produced by the laser

irradiation on the mirror surface was observed by eyes. The plasma-production threshold was defined as the minimum laser energy density when plasma light was observed. The damage threshold

of Mo mirror surface was, furthermore, studied for the multi-shots irradiation onto the same

site of the sample.

III. Experimental Results and Discussion

A. Surface Characteristics of Mo Mirrors

The surface roughness of the p.m. and the e.b.m. Mo mirrors was measured by the Talystep as shown in Figs. 1(a) and (b^. The sujface roughness (peak to valley value) of the p.m. and e.b.m. Mo mirrors were about 250 A and 120 A respectively. Although the tip diameter was 25 \m, a difference of the roughness of these two kinds of Mo mirrors was well distinguished. The surface

flatness of both Mo mirrors was measured by Zygo interferometer to be less than A/5 (X=6328 A)

.

Table I shows the chemical compositions of the p.m. and e.b.m. Mo, respectively. It can be seem that the impurities of Na, K, Al, Si, Fe and Ni included in the e.b.m. Mo. is considerably

less than that of the p.m. Mo. From this analysis, we can expect that the laser induced damage

threshold of the e.b.m. Mo mirror has higher value than that of the p.m. Mo mirror.

The reflectivity of the Mo mirror at three wavelengths is shown in Table 2. The reflectivity

for He-Ne laser and YAG laser was obtained with nearly normal incident angle. The e.b.m. Mo mirror had a reflectivity higher than the p.m. Mo one. At the wavelength of 9.26 ym, the reflectivity of both Mo mirrors was comparable to that of the Au coated glass mirror.

B. Anger Electron Spectroscopy Scan of the Mo Mirror

We have measured the contamination of the polished Mo mirror surface, and the impurities

segregation of the Mo fracture surface by using the Anger electron spectroscopy method to investigate the corelativity with the laser induced damage. Fig 2 shows the Anger scan of the

as-polished surface of the p.m. Mo mirror and the e.b.m. Mo mirror, respectively.

The Anger scan of the Mo mirrors after 10 mins sputter etch with argon is shown in Fig. 3. By using the argon sputter etch, the Na contamination on the surface is completely removed, but

the C and 0 is considerably decreased. Anger scan for the fracture surface of the void in p.m. Mo and the grain boundary of the e.b.m. Mo is shown in Fig. 4. We found that the alkali metals

of K and Na, and C, N, 0 was deposited of the void in p.m. Mo, and N and traces of 0 was deposited in grain boundary of the e.b.m. Mo.

C. One-Shot Laser Damage

The micrographs of the mirror surface damaged by one shot irradiation of the TEA CO2 laser

are shown in Figs. 5, 6 and 7, for the Au coated glass, the p.m. Mo and the e.b.m. Mo mirrors,

respectively

.

In Fig. 5, the Au coated glass mirror was irradiated by TEA CO2 laser energy of 53.3 J/cm^

and it is seen that the two coated layers of Au and Cr on the glass substrate is broken by the

laser beam. The Au coated glass mirror with the irradiated energy density of larger than 17.6 - 21.6 J/cm^ was always damaged.

Figures 6 and 7 show the micrograph of the p.m. and e.b.m. Mo mirrors damaged by 146.3 and

J/cm^, respectively. It is seen that the p.m. Mo mirror melted inhomogeneously ; on the contrary

e.b.m. Mo mirror melted homogeneously. For the less fluence of TEA CO2 laser, the e.b.m. Mo mirror

was melted along the grain boundary, and the grain was clearly observed in the damaged spot as

249
shown in Fig. 8.

The surface damage threshold and the plasma production threshold for three kinds of mirrors

under the one shot irradiation of TEA CO2 laser are summarized in Table 3. The surface damage

threshold of the e.b.m. Mo mirror was 2 to 4 times higher than that of the Au coated glass mirror.

The damage threshold of the p.m. Mo mirror decided by the surface observation using a microscope

is higher than that of the e.b.m. Mo mirror. However, the threshold of energy density that the

laser light reflected by the e.b.m. Mo mirror abruptly decreased due to the absorption of radiation by strong plasma on the surface is about 30% higher than that of the p.m. Mo mirror. This

fact means that the e.b.m. Mo mirror is more resistant against high power TEA CO2 laser irradiation than p.m. Mo mirror. The lasma-production threshold of the e.b.m. Mo mirror was, however,

about 3 times and about 5 times higher than that of the p.m. Mo mirror and the Au coated glass

mirror, respectively. It seems that the difference of these thresholds between the e.b.m. and

p.m. Mo mirrors is due to the surface condition with and without voids which causes the plasma

production.

D. Multi-Shots Laser-Damage for Mo Mirrors

For practical use of the Mo mirror, the study on the damage due to multi-shot irradiation of TEA CO2 laser is very important.

In this study, the focused TEA CO2 laser beam was irradiated with 0.5 pps at an incident

angle of about 15°, and the energy of the reflected laser beam was measured by a pyroelectric

detector. The reflected laser beam from the ZnSe lens surface was monitored by another pyroelectric detector.

Figure 9 shows the variation of the reflectivity to the shot number of TEA CO2 laser with

various incident laser energy densities for the e.b.m. Mo mirror. The similar tendency was also

observed for the p.m. Mo mirror. At a certain shot number depending on the incident energy

density, the strong plasma produced with a sound on the mirror surface and then the reflectivity

decreased abruptly to about half value due to the absorption of radiation by plasma. After that,

bright plasma was always observed and the reflectivity kept constant value. The shot number

where the reflectivity reduced suddenly with plasma is plotted in Fig. 10 as a function of the

incident laser energy density. It was found that the e.b.m. Mo mirror had a longer life time

than the p.m. Mo mirror. Especially, for the incident laser energy density higher than 60 J/cm^,

the life time of the e.b.m. Mo mirror was about 3 times longer than that of the p.m. Mo mirror.

The micrograph of the p.m. Mo mirror surface, for example, after 480 shots irradiation of

60.7 J/cm^ is shown in Fig. 11. The microscopic observation of a series of damaged p.m. Mo mirror surface with multi-shot irradiation of TEA CO2 laser suggested that the voids on the

mirror surface gradually began to melt by weak plasma and the melting area extended until the

reflectivity decreased suddenly. Figure 12 shows the micrograph of the e.b.m. Mo mirror surface

damaged after 26 shots irradiation of 72 J/cm^. In this case, it is seen that the smaller grain

is formed in the damaged surface due to the recrystallization based on thermal effect with

repetitive laser irradiation. We are now pursuing the production mechanism of this phenomenon.

This phenomena would be due to the relative lower melting point at the grain boundaries because

the impurity is generally easy to gather at the grain as can be seen from the Auger scan of

Fig. 4 and results in making the melting point lower.

IV. Conclusion

We showed the availability of the e.b.m. Mo mirror for the high power TEA CO2 laser system. For wavelength at 9.26 ym, the Mo mirror had almost same reflectivity as the Au coated glass

mirror. The surface damage threshold of the e.b.m. Mo mirror for one-shot irradiation of TEA

CO2 laser was 2 to 4 times higher than that of the Au coated glass mirror. For multi-shots

irradiation of TEA CO2 laser, the e.b.m. Mo mirror had a life time of 3 times longer than that

of the p.m. Mo mirror for the laser energy density of higher than 60 J/cm^. The laser damages

on the Mo mirrors mainly depends on the surface condition; on the p.m. Mo mirror, a number of

voids on the mirror surface originates and extends the damage, while on the e.b.m. Mo mirror,

the impurities at the grain boundaries of polycrystalline mirror surface are the cause of the

laser damage. Therefore we would be able to expect much higher damage threshold and longer life

time with a single crystal Mo mirror.

250
Acknowledgements

The authors would like to thank S. Suzuki and K. Abico of the Institute of Metals, Tohoku

Univ. for testing of Auger electron spectroscopy scans in ultra high vacuum of the Mo fracture

surfacies and helpful discussions.

References

[1] Y. Tsunawaki, M. Yamanaka, Y. Ichikawa, [3] S.M. Wong, G. Krauss, and J.M. Benett, "Laser

L. Wu, S. Okajima, T. Iwasaki, A. Induced Damage in Optical Materials", A.J. Mitsuishi, T. Yamanaka and C. Yamanaka, Glass and A.H. Guenther, Eds. (NBS. Spec.

9th Int. Conf. on Infrared Millimeter Publ., 1978), pp. 132-163.

Waves (Takarazuka, Japan, 1984) Th7-4. '

[4] Y. Ichikawa, Y. Tsunawaki, M. Yamanaka,

[2] Y. Ichikawa, L. Wu, Y. Tsunawaki, M. T. Yamanaka, A. Mitsuishi and C. Yamanaka,

Yamanaka, S. Okajima, T. Iwasaki, Infrerad Phyc. 35, No. 4, 633 (1985).

T. Yamanaka and C. Yamanaka, Technol.

Repts. Osaka Univ. 35, No. 2 (1985)

(in press)

.

251
1 —

50 — (a) p.m. Mo _

_

11''

' —— 3

—J

—

^±

—

—

— /is: —4

-40-

-3C.

-20

—1

-!r

I

1 : —

1 -f— i 1

—l-n ^-

—!- --

-

-10

rSO CK^ e.b.m. Mo

'*' —

h

f—1 — - II

1 30

!

^

- r2C- .. . 20

- 1 0 0 A ——

1

.. , .

: : i—

-10

1 —

5 0 /i -

-10

:

—1 —^—L —J 1 —1—

1 -0- rr: [12-5 iCtm r-aicijLias stylu s)

;

Figure 1. Surface scans of the Mo mirror; (a) ,p.in. Mo mirror and (b) e.b.m. Mo mirror.

Table 1. Chemical compositions of the p.m. and e.b.m. Mo mirrors.

(ppiii) C 0 N Na K Al Si Fe Ni

p.m. Mo 30 30 3 12 46 7 15 50 24

e . b . DI . Mo 10 30 10 0.001 0.005 0.004 0.4 <0.5 0.2

252
Table 2. The reflectivity of g-m. and e.b.in. Mo mirrors for

wavelengths of 6328 A (He-Ne laser), 1.06 ym

(YAG laser) and 9.26 ym (infrared spectrometer).

WAVELENGTH

Reflectivity ( % ) p.m. Mo e.b.in. Mo

6328 A 58 65.8

i:06 61.2 70

10 pm almost same as

Au coated mirror

Table 3. The threshold of surface-damage and plasma-production

against 9.26-ym R(22) TEA CO2 laser beam (50 ns) for the

e.b.m. Mo mirror, p.m. Mo mirror and Au coated glass

mirrors

.

mi rror

threshold energy density ( J/cm^ )

damage plasma

e.b.m. Mo

( 1

) . mirror

^5.5 - 59 94

, . , ^ p.m. Mo

(11)

mirror

87 30

Au coated

(jjj) on glass

mirror

17,5 - 21,6 17,5 - 21 ,5

253
Figure 3. Auger electron spectroscopy scans of the after 10 mlns

.

sputterd surface of the p.m. and e.b.m. Mo mirrors.

254
500 1000 1500

Electron Energy (eV)

The e.b.m. Mo Mirror

_ N 1

Na

C

1 0 500 1000 1500

Electron Energy (eV)

Figure 2. Auger electron spectroscopy scans of the as-polished

surface of tlie p.m. and e.b.m. Mo mirrors.

255
220 420 620

Electron Energy (eV)

820 1020

3oundary in the e.b.m. Mo Fracture Surface

Mo

20 220 420 620

Electron Energy (eV)

820 1020

Figure 4. Auger electron spectroscopy scans surface of the p.m. and e.b.ni. Mo vacuum. (Tested by Tohoku Univ.)

of

in

the fracture

ultra high
200 fj- m

Figure 5. Microstructure of Au coated glass mirror damaged

by TEA CO2 laser of 53.3 J/cm^. The mirror

consisted of the two coated layers Au and Cr on the glass substrate.

100 n\

Interference Contrast Image

Figure 6. Microstructure of the p.m. Mo mirror damaged by TEA CO2 laser of 146.3 J/cm^

.

257
Interference Contrast Image lOO At m

Figure 7. Microstructure of the e.b.m. Mo mirror damaged by TEA

CO2 laser of 152 J/cm^

Interference Contrast Image lOO fj.

Figure 8. Microstructure of the e.b.m. Mo mirror damaged by TEA CO2 laser of 95.6 J/cm^,

258
100 C98. 3)

>->

u

LU

_J

u_

LjJ cr

80

60

^0

1

1 r 1 1 1 1 1 1

8^.9 72 66 59.5 J/cm2 -

1

I 1 1 1 1 r

1 10 100 .1000

SHOT NUMBER

Figure 9. The dependency of the reflectivity on the number

of TEA CO2 laser shots for the e.b.m. Mo mirror.

1500

CC 1000

LU

CD

5 500

to

HB— e.b.m. Mo mirror,

p.m. Mo mirror.

60 70 ENERGY DENSITY ( J /cm^

Figure 10. The relation between the shot number and the threshold

of energy density that the laser light reflected by Mo mirror abruptly decreased due to the absorption of radiation by plasma on the mirror surface.

259
20 0 m

Figure 11. Damaged microstructure of the p.m. Mo mirror surface after the repetitive irradiation of 480 shots.

200 m

Interference Contrast Image

Figure 12. Damaged microstructure of fhp o k v.

260
Manuscript Beaeived

2-9-87

MuHifacet, Metal Mirror Design for Extreme-Ultraviolet and Soft

X-Ray Free-Electron Laser Resonators*

Brian E. Newnam

Los Alamos National Laboratory

Chemistry and Laser Sciences Division

Los Alamos, New Mexico 87545

Resonator mirrors with reflectance greater than ~40% are required if freeelectron laser (PEL) oscillators are to be extended to wavelengths below 100 nm. The

principle of total -external reflection has been used to design multifacet mirrors of

several metal films that surpass this requirement between 10 and 100 nm. These

reflectors have high reflectance over a relatively broad band which suits well the

inherent PEL wavelength tunability. The practical limitations to achieving and maintaining high reflectance include oxide- and carbon-contamination, scattering, and

surface-pl asmon absorption.

Key words: aluminum films; carbon contamination; extreme-ultraviolet reflectors; freeelectron lasers; metal reflectors; multifacet reflectors; oxide contamination; rhodium

films; silicon carbon; silicon films; soft x-ray reflectors.

1. Introduction

The recent successful operation of several free-electron laser (PEL) oscillators in the

visible and near-infrared spectral ranges [1,2] has encouraged researchers to examine the possible short-wavelength limits for these devices [3,4]. One of the major technological con- straints presently blocking their extension into the extreme-ultraviolet (XUV) is the inherently

low reflectance of available resonator mirrors, of the order of 10 to 20%. Unless this limitation is overcome, very high values of single-pass optical gain will be needed. Por example, if

each resonator mirror were to have a reflectance of only 25%, the small-signal gain would have

to exceed 1600% just to begin oscillation, a value which may be difficult to achieve. Development of resonator mirrors with reflectance ^40% appears to be a prerequisite for future operation of PEL oscillators at wavelengths below 100 nm.

2. Present Status of Reflectors for the XUV

There are four types of normal-incidence reflectors under development for the XUV. Pirst,

smooth surfaces of chemically vapor-deposited (CVD), single-crystal, silicon carbide (SiC) have

been produced and used successfully in synchrotron radiation beam lines. The highest measured

values of the normal -i nci dence reflectance for SiC have varied between 40 to 50% for wavelengths

between 60 and 220 nm [5,6]. Below 60 nm, SiC reflectance drops rapidly to less than 10%, as shown in figure 1.

Simple metallic films represent a second type of reflector, and the reflectance properties

of two examples, platinum and gold, are compared in figure 1. Only for wavelengths longer than

250 nm does their reflectance exceed our minimum 40% requirement. The notable exception are aluminum films freshly deposited on smooth substrates in ultra-high vacuum (10-^ to 10-^° torr).

With care, the normal-incidence reflectance can exceed 40% for wavelengths as short as 80 nm

[7,9]. However, even in high vacuum the reflectance gradually decreases with time as an oxide

forms on the surface. Overcoating with a layer of Mgp2 does prevent the oxidation, but high

reflectance (^80%) is then limited to wavelengths longer than 120 nm. We will revisit the use

of Al films in following sections.

The third class of reflector includes multilayer thin-film structures which operate on the

principle of standing-wave interference of multiple reflections from the film interfaces. Since

all dielectric materials are absorbing for wavelengths shorter than 110 nm, alternating metal

layers having differing absorption coefficients are used. Por the soft x-ray and XUV spectral

regions from 10 nm to 110 nm, this technology generally has yielded reflectances less than 50%

*Work performed under the auspices of the U.S. Deparment of Energy.

261
for near-normal incidence over a bandwidth limited to ^10% [10]. The status of normal-incidence

reflectors as of 1983 was summarized by Attwood et al.[ll], which is reproduced here as figure 2.

Multifacet reflectors, the fourth type of XUV reflector and the subject of this paper,

involve multiple reflections from a series of metal mirrors. These make use of the principle of

total external reflectance (TER) which is possible only when the refractive index is less than

unity. We have determined that semi-TER (R < 100%) is possible for a few metals over certain

spectral ranges in the XUV. Surprisingly high values of retroref lectance (redirection of the

optical beam by 180°) are possible, especially with S-plane reflections at large angles of

incidence. In addition, the high reflectance extends over a relatively broad range, a feature

that well suits the inherent tunability of FEL oscillators. Since the reflectance at each facet can exceed 95% for large angles of incidence, such as 80°, these reflectors offer another advantage: relatively high resistance to laser damage and thermal distortion.

In the following sections, we review the potential for the multifacet, vacuum-deposited

metal films, the risks due to practical fabrication limitations and environmental contamination,

and how they would be used in a ring-resonator configuration.

3. Requirements for Total -External Reflectance Mirrors

We first describe the fundamentals of total external reflection. When light travels across

the boundary from one medium of refractive index nj into a second medium of index n2 in which

the velocity is greater, total reflection occurs when the angle of incidence exceeds the critical angle (J) defined by sine}) = n2/ni. If the first medium is vacuum for which n^ = 1.0, TER

can occur only if n2 < 1.0. For metals, there are many spectral regions where the real part n of the refractive index N = n + ik is less than unity. This is less common for dielectrics,

however. A second requirement for total (100%) reflection is that the extinction coefficient k of the media be absolutely zero (which is never attained). Still, if the absorption is small

(k << 1.0 while n2/r\i < 1.0), a very large reflection can occur. Finally, higher values of

reflectance are achieved with S-polarized light rather than with P-polarization. Figure 3 visualizes the conditions for TER.

Soviet researcher A. V. Vinogradov and colleagues [11-13] were the first to analyze the

potential for high-retroref lectance in the soft x-ray regime using multiple, grazing-incidence

reflections beyond the critical angle. They considered a single, cylindrical mirror configuration of a variety of metals and used optical constants derived from the theoretical tables of

Henke et al.[14]. For use as resonator mirrors, however, we suggest a multifacet, flat mirror

configuration as indicated in figure 4. Flat mirrors will cause much less astigmatism which is essential in an FEL resonator to obtain maximum intensity within the magnetic undulator. Also,

flats are easiest to fabricate with the required surface figure. (The final facet should be an off-axis paraboloid for collimating the beam.)

4. Examples for the XUV: Aluminum and Silicon Retroref lectors

We have scrutinized the available tabulations of optical constants [14,16] for the metallic

elements and various compounds, searching for wavelengths for which both k and n are sufficiently less than unity. In the XUV spectral region ^25 nm, we initially identified three candidates: Al , crystalline Si, and Mg. The spectral dependence of the optical constants of Al is shown in figure 5 for reference. Vacuum-deposited films of Al are especially attractive since

they have been characterized by a number of researchers over the last 35 years. Also, since the

films are transparent in this region, interference effects with the substrate, e.g. SiC, can enhance the reflectance slightly. Less information (but still adequate) is available for

crystalline Si films in the XUV. Practical use of Mg films is doubtful since an absorbing oxide

forms rapidly at the surface.

The reflecting properties of a smooth, uncontami nated Al film on SiC at the wavelength of

50 nm are shown in figure 6 as a function of the number of mirror facets. Optical constants

from the critical review and tabulation of Smith et al. [8,9] were used in the computations.

According to the analysis of Goldstein et al. [17] for an FEL conservatively designed for 50 nm, the predicted 60% retroref 1 ectance indicated in figure 6 is above the level needed to reach

laser oscillation. Interestingly, only six facets are needed to achieve this value. Figure 7 shows the calculated retroref lectance over the entire spectral range (25 to 130 nm) where TER is oeprative for various numbers of mirror facets. It is obvious that the reflectance of a multifaceted aluminum mirror greatly exceeds that of a single, normal-incidence Al mirror over most

of this region.

262
Examination of the optical constants of crystalline-Si films [18] reveals that TER occurs over the same 25- to 80-nm spectral range as for Al . Figure 8 illustrates the spectral characteristics of the retroref lectance of multifacet reflectors of this element.

5. Example for Soft X-Ray Wavelengths - Rhodium Retroref lector

In the soft x-ray region below 14 nm, a variety of metal coatings may be useful as multifacet retrorairrors in laser resonators. These and the approximate wavelengths for which maximum

reflectance enhancement occurs include Rh (12 nm) , Os (12 nm) , Mo (12 nm), Ag (10 nm), Au

(10 nm). In (7.5 nm). La (6.5 nm) , and Ba (5.5 nm) [14,19]. The calculated retroref 1 ectance (60 to 65%) of a multifaceted rhodium reflector at 12 nm (figure 9) is very encouraging as is the

30% spectral band (10 to 13.5 nm) over which the reflectance exceeds 40%. A corresponding plot

of reflectance versus number of mirror facets at 12 nm is shown in figure 10. Rhodium films are especially interesting since the available tabulation of optical constants [19] is for films

deposited in modest vacuum (10-^ torr). From this fact we expect that oxide growth may not be a serious problem.

6. Risks of Contamination and Surface Roughness

Unless great care is taken, the TER enhancement in retroref lectance of a multifacet mirror

will not be realized. The hazards to be avoided are surface contamination by oxide and carbon

films and surface roughness which represent potential losses by absorption or scattering. We now discuss the magnitudes of these hazards and the conditions necessary to minimize their

effects.

Oxide Contamination . In general, the metal films must be deposited and maintained in high vacuum to minimize the formation of oxide layers that absorb strongly at all wavelengths below

150 nm. This is evident for aluminum in figure 11 in which Smith et al . [9] have summarized the

normal -incidence reflection for a variety of Al surface conditions. In particular, our computations of the potential degradation of an Al retroref lector (figure 12) show that very little

oxide formation can be allowed before the reflectance drops below 40%. Several researchers have

determined that ultra-high vacuum (10-^ to 10-^° torr) and fast deposition rates are necessary

to preclude oxide contamination [20,21]. At these vacuum pressures over the period of two days,

H. E. Bennett [21] was not able to detect (via el 1 ipsometry) the growth of an oxide layer on Al films. Oxide growth on Si and Rh is expected to be much less of a threat.

Carbon Film Contamination . Surface contamination by carbon compounds, which are normally

present in all environments, must be prevented to minimize optical absorption. As shown in

figure 13, a carbon layer of only a few angstroms thickness will seriously degrade the TER con- dition required for multifaceted Al retromi rrors. We have also calculated the potential for degraded performance of a multifaceted Rh mirror at 12 nm due to growth of a carbon epifilm (see

figure 14). From comparison of figures 13 and 14, we conclude that carbon contamination should

have less serious consequences near 10 nm than near 50 nm. The presence of high-energy photons, such as produced by synchrotrons, has been observed to greatly increase carbon film growth

(to ~10 nm thickness) on optical surfaces [22]. Synchrotron users have been able to control the rate of carbon accumulation on optical surfaces by use of ultra-high vacuums (10-^^ to 10-^^ torr). Of course, a superior solution is to eliminate all carbonaceous compounds from the vacuum environment. Recently, an rf plasma discharge of O2 or H2O vapor has been used to effectively remove all traces of such contamination on metallic diffraction gratings [23,24].

Scattering Losses . The third requirement is that the reflecting films be deposited on smooth substrates to minimize scattering losses. For wavelengths much greater than the rms surface roughness a, that is k » a, the effect of scattering on the specular reflectance, R((l)), at some angle of incidence, is approximately [25,27]

R = R^(i|))exp[-(47ta cos(t)A)^] , (1)

where R is the reflectance of a perfectly smooth surface. For XUV wavelengths ^30 nm, mirror

substrates with rms surface roughness of ^5 K are sufficiently smooth to limit scattering losses

to ^4% for normal incidence. When used at 80° incidence, the losses should decrease to ^0.1%

according to equation (1).

Although the derivation of equation (1) should make its application at soft x-ray wave- lengths less valid, we use it here for estimation purposes for relatively smooth surfaces at

12 nm (120 A). For example, incidence at 80° on Rh films with 5 A rms surface roughness would

result in 0.8% scattering loss per reflector facet. The net drop in retroref lectance (nine

facets) would be about 5% (from 65% to 60%).

263
Surf ace-Pl asmon Absorption . Polished substrates with the above smooth surfaces are attained in practice on flat and spherical substrates of fused silica, glass, Si, and occasionally

SiC. For large angles of incidence, such as 80°, and relatively long wavelengths ^30 nm, still

rougher surfaces, e.g. with a equal to 10 to 15 A, could be tolerated. However, at wavelengths

near a surface plasmon resonance, surface roughness can produce absorption loss within the metal. For Al , such a resonance occurs at 10.54 eV (118 nm) [28], and the effect on reflectance is shown in figure 15. This motivates a tighter limit on the tolerable roughness when using Al reflectors in the vicinity of this wavelengths.

7. Practical Implementation

It is obvious that the practical application of multifacet metal reflectors will require

deposition of the films in ultra-high vacuum with the reflector substrates mounted in place in

the laser resonator. This is convenient for an FEL oscillator in which the reflectors are usually mounted in vacuum anyway. With this provision, it would be possible to periodically

overcoat the metallic films as required to offset the effects of any gradual deterioration that

may occur. Of course, the total thickness of the films must not become too great or else sur- face roughness will become a problem. It is conceivable, too, that an ion gun mounted in the vacuum chamber could be used to periodically sputter away aged films and then deposit fresh

layers. Certainly, initial removal of carbonaceous compounds from the vacuum environment by use

of a rf plasma discharge, cited above, will also be beneficial.

8. Plans for Implementation

Los Alamos researchers Scott, Arendt, and Newnam have begun a project to implement the

multifaceted metal-mirror concept. Included in the research plans are el 1 ipsometric measure- ments of the oxide- and carbon-contamination growth rates as a function of O2 , H2O, and CO partial pressures and reflectance measurements of single- and multiple-facet reflectors for vacuum environments from 10-^ to 10-^° torr. If the results are encouraging, the multifacet

retroref 1 ectors will be eventually used in an XUV FEL oscillator as shown in figure 15. Recent three-dimensional calculations of the single-pass, small-signal gain for a soft x-ray

FEL at 12 nm have shown that the 60% net reflectance possible for Rh retromirrors should be

sufficient to achieve laser oscillation. This is indeed an exciting prospect!

9. References

[1] Newnam, B. E. Symposium welcome and perceptions of future research. Natl. Bur. Stand.

(U.S.) Spec. Publ. 727; (to be published 1986). vii-x.

[2] Newnam, B. E. Evolution of free-electron laser development. High Power and Solid State

Lasers, Proc. SPIE 622; to be published 1986. 84-92.

[3] Madey, J. M. J.; Pellegrini, C. ; Eds. Free Electron Generation of Extreme Ultraviolet

Coherent Radiation . AIP Conf. Proc. No. 118 (Amer. Inst, of Phys. , New York); 1984.

[4] Harris, S. E. ; Lucatorto, T. B. ; Eds. Laser Techniques in the Extreme Ultraviolet . AIP

Conf. Proc. No. 119 (Amer. Inst, of Phys., New York); 1984.

[5] Rehn, V.; Choyke, W. J. SiC mirrors for synchrotron radiation. Nucl . Instr. and Methods

177, 173-178 (1980).

[6] Choyke, W. J.; Palik, E. D. Silicon carbide (SiC). Handbook of Optical Constants of

Solids , E. D. Palik, Ed. (Academic Press, New York); 1985. 587-596.

[7] Shiles, E; Sasaki, T. ; Inokuti, M. ; Smith, D. Y. Self-consistency and sumrule tests in the Kramers-Kronig analysis of optical data: applications to aluminum. Phys. Rev. B 22,

1612-1628 (1980).

[8] Smith, D. Y. ; Shiles, E. ; Inokuti, M. The optical propeties and complex dielectric function of metallic aluminum from 0.04 to 10"* eV. Argonne National Laboratory Report

ANL-83-24; March 1983.

[9] Smith. D. Y. ; Shiles, E. ; Inokuti, M. The optical properties of metallic aluminum. Handbook of Optical Constants of Solids , E. D. Palik, Ed. (Academic Press, New York); 1985.

369-406.

264
[10] Barbee, Jr., T. W. ; Mrowka, S; Hettrick, M. C. Molybdenum-silicon multilayer mirrors for

the extreme ultraviolet. Appl. Opt. 24, 883-886 (1985).

[11] Vinogradov, A. V.; Konoplev, N. A.; Popov, A. V. Broad-band mirrors for vacuum ultraviolet and soft x-ray radiation. Sov. Phys. Dokl . 27, 741-742 (1982).

[12] Vinogradov, A. V.; Kozhevnikov, I. V.; Popov, A. V. On wide-band mirrors for soft x-ray

range. Opt. Commun. 47, 361-363 (1983).

[13] Vinogradov, A. V.; Sagitov, S. I. New types of mirrors for the soft x-ray range (review).

Sov. J. Quantum Electron. 13, 1439-1447 (1983).

[14] Henke, B. L. ; Lee, P.; Tanaka, T. J.; Simabukuro, R. L. ; Fujikawa, B. K. Low-energy x-ray

interaction coefficients: photoabsorption , scattering, and reflection (E = 100-2000 eV, Z = 1-94). Atomic Data and Nuclear Data Tables 27, Nl, 1-144 (1982).

[15] Hagemann, H.-J.; Gudat, W. ; Kunz, C. Optical constants from the far infrared to the x-ray

region: Mg, Al , Cu, Ag, Au, Bi , C, and AI2O3. J. Op. Soc. Am. 65, 742-744 (1975); also

DESY Report SR-74-7; May 1974.

[16] Palik, E. D. , Ed. Handbook of Optica! Constants of Solids , (Academic Press, New York);

1985.

[17] Goldstein, J. C. ; Newnam, B. E. ; Cooper, R. K. ; Comly, Jr., J. C. An xuv/vuv freeelectron laser oscillator. Laser Techniques in the Extreme Ultraviolet . AIP Conf. Proc.

No. 119, (Amer. Inst, of Physics, New York); 1984. 293-303.

[18] Edwards, D. F. Silicon (Si). Handbook of Optical Constants of Solids . E. D. Palik, Ed. (Academic Press, New York); 1985. 547-569.

[19] Lynch, D. W. ; Hunter, W. R. Comments on the optical constants of metals and an introduction to the data for several metals. Handbook of Optical Constants of Solids , E. D. Palik,

Ed. (Academic Press, New York); 1985. 275-367.

[20] Feuerbacher, B. P. Steinmann, W. Reflectance of evaporated aluminum films in the 1050

-1600 S region, and the influence of the surface plasmon," Opt. Commun. 1, 81-85 (1969).

[21] Bennett, H. E. Naval Weapons Center, private communication. July 1985.

[22] Boiler, K. ; Haelbich, R.-P.; Hogrefe, H. ; Jark, W. ; Kunz, C. Investigation of carbon

contamination of mirror surfaces exposed to synchrotron radiation. Nucl. Instr. and

Methods in Physics Res. 208, 273-279 (1983).

[23] Samson, J. A. R. , University of Nebraska, private communication, 1986.

[24] Johnson, E. D. ; Garrett, R. F. ; Williams, G. P.; Knotek, M. L. In-situ reactive glow discharge cleaning of x-ray optical surfaces. Presented at the VUV8 Vacuum Ultraviolet Radiation Physics Conf., Lund, Sweden, August 1985; to be published in Rev. Sci. Instr., 1987.

[25] Bennett, H. E. ; Porteus, J. 0. Relation between surface roughness and specular reflec- tance at normal incidence. J. Opt. Soc. A. 51, 123-129 (1961).

[26] Bennett, H. E. Specular reflectance of aluminized ground glass and the height distribution of surface irregularities. J. Opt. Soc. Am. 53, 1389-1394 (1963).

[27] Haelbich, R.-P.; Segmuller, A.; Spiller, E. Smooth multilayer films suitable for x-ray

mirrors. Appl. Phys. Lett. 34, 184-186 (1979).

[28] Endriz, J. G. ; Spicer, W. Study of aluminum films. I. Optical studies of reflectance

drops and surface oscillations on control 1 ed-roughness films. Phys. Rev. B 4, 4144-4159

(1971).

265
I—I—1—I—I—I—I—I—r—1—I—I—I—I—(—I—I—

r

NORMAL INCIDENCE

0.8 fii

0.6

eO.4

0.2

10 20 30 PHOTON ENERGY (tV) 40

>

o

0)o

1.0

Wavelength

1,000 A 100 A 10 A

0.5

Optical

coatings

Projected

multilayers

Difficult void

10 eV 100 eV 1 keV

Photon energy

1 A

Natural

crystals

lOkeV

Figure 1. Reflectance of CVD-SiC at normalincidence with comparison to platinum and

gold films. After Rehn and Choyke [5].

Figure 2. Present and projected reflectance

of normal-incidence mirror configurations as compiled by Attwood, et al . [11]. Recent

data points are those for Mo/Si multilayer

reflectors produced by Barbee, et al. [10].

SIN<t)c= n„ AT CRITICAL ANGLE

IF (t) >(|)c ANDk=0 R = 100%

(t) > t>c ANDk/^0 > R « 100%

i}> >»t)c AND k<<1.0 — R large^

Figure 3. Reflection of an optical beam from

a vacuum-metal interface for angles larger

than the critical angle (}) = sin-^n/n . If

k = 0 when n < 1, total external reflection

occurs; with k sufficiently small, very

large reflection can occur.

CASE 1 ( e = 90°) CASE 2(9 = 45°)

V /

CASE 4 (e = 80°) CASE 3 ( e = 60°) 9 FLAT FACETS

Figure 4. Multifacet retroref lector composed

of N flat metallic reflectors. Totalexternal reflection is possible only when

the number of mirror facets N is large

enough so that the angle of incidence (j) = 90° X(N - 1)/N > (t)^.

266
100 60 40 ?0

K3 \ I'l' I

WAVELENGTH (i^m)

2 I 06 04 02 01 006 0 04 0 02

ENERGY (eV)

Figure 5. Spectral dependence of the complex

refractive index of aluminum films deposited

in high vacuum (after Smith, et al. [8,9]) Semi-total external reflectance is possible

from 25 to 80 nm where both n and k are simultaneously less than unity.

100

llJ 80 o

2

iS 60

O

UJ

_l 40

u.UJ

q: 20

r 1 1 1 1 1 1 1 1

ASYMPTOTE -64.4%

ii

ALUMINUM (vacuum

deposited at IO"'°torrl WAVELENGTH • 50 nm

if(60-] <72-) (80-) <-i 1 1 1

1

(84*)

1 1 1 1 1 5 7 9 II 13 15 17 NUMBER OF FACETS (N)

19 21

Figure 6. Calculated retroref lectance (4)=180°)

and single-facet reflectance for S-polarized

XUV radiation at 50 nm for multifacet aluminum

films deposited on smooth SiC substrates in

ultra-high vacuum. Calculations made use of

the tabulation of optical constants by Smith,

et al. [8,9]. Notably, only six mirror facets

are needed to reach 60% reflectance with 64.4%

being the maximum achievable.

a. 100

"1 1 1 1 1 1

1 I r

ALUMINUM (vacuum deposited at 10"'°torr)

20 30 40 50 60 70 80 90 100 110 120 WAVELENGTH (nm)

Figure 7. Total retroref lectance ((})=180°) versus XUV wavelength for multifacet aluminum mirrors deposited on smooth SiC in

ultra-high vacuum. Calculations were made

for_ S-polarization using the tabulation of

optical constants by Smith, et al. [8 9] Below 90 nm, the reflectance attainable with

multifacet mirrors is much greater than that

for a single (N=l), normal-incidence Al

mi rror.

"1 '

\

'

1 ' r

SINGLE-CRYSTAL SILICON

60 80 WAVELENGTH (nm)

100 120

Figure 8. Total retroref 1 ectance ((1)=180°) versus XUV wavelength for multifacet, crystalline Si films deposited on smooth Si in ultrahigh vacuum. Calculations were made for S- polarization using the optical constant com- pilation by Edwards [18]. Below 90 nm, the

reflectance attainable with multifacet mirrors

is much greater than that for a single (N=l)

normal -incidence Si mirror.

267
100

liJ 80 o

z

TA 60 o

UJ

_l

u. 40

LU

tr

1 1 1 1 1 1 1 1

'

ASYMPTOTE = 64.9%

RHODIUM

(Vocuum- Deposited at 10"' torr) - WAVELENGTH = ll,9nm

// 172°) / 1

(80')

1 1 1

184')

1 1 1 1 10 II 12 13 WAVELENGTH (nm)

3 5 7 9 II 13 15 17 19 21 NUMBER OF FACETS (N)

Figure 9. Retroref 1 ectance of muHifaceted

rhodium coatings on smooth SiC substrates in

the soft x-ray region for S-polarized radiation. Optical constants for Rh films deposited in a moderate vacuum (10-^ torr) were

used in the calculations because oxidation is not serious for Rh at ambient temperatures

[19]. Retroref lectance greater than 60% is calculated at 12 nm.

Figure 10. Calculated retroref 1 ectance and

single-surface reflectance of rhodium films on smooth SiC for S-polarized radiation at 11.9 nm as a function of the number of mirror

facets. Only seven mirror facets are needed

to reach 50% net reflectance with 64.9% maxi- mum ach ievable. Optical constant tabulations

by Lynch and Hunter [19] were used.

WAVELENGTH (mictors) 40 20 10 5 2 1 05 02 01 005

n I I1 I1 I1 I1 I1 I1 I1 -l^i— 1 J_ J 1 1 -1 L 1 LLLi JJ_L

High Vacuum ====»^ Ulltahiqh Vacuum Films in UHV^

Films in Ai >\ ^ \ \b \V'

Mechanically Polished Bulk -

Mechonicolly Polished Bulk.^ \ and Spullered Films \ \

Sputtered Films -A \

1 1 t 1 1 [ r 1 I 01I T t r 1 1 1 1 r 1 r I 1 1 m' '

i

1 10 ENERGY (eVI

Figure 11. Reflectance of various types of

aluminum surfaces at room temperature (after

Smith, et al. [9]). Highest reflectance

(curve a) is achieved with Al films deposited

on substrates in ultra-high vacuum (10-^ to 10-1° torr) with high deposition rates. The

effect of light scattering and surface plas- mon absorption (at 10.6 eV) is shown by curves b and c. Curve b is for Al on a clean

microscope slide and curve c is for a uhv

film with rms roughness of 18 A [15].

100

AI2O3 EPIFILM THICKNESS (A)

Figure 12. Influence of oxide thickness on the calculated retroref lectance (S-polarization) of a nine-facet aluminum mirror at XUV wavelengths of 50 and 100 nm. Optical con- stant tabulations for Al by Smith, et al. [8,9] and AI2O3 by Hagemann et al . [15] were

used.

268
Figure 13. Influence of carbon film contamination on the calculated retroref 1 ectance

(S-polarization) of a nine-facet aluminumcoated mirror (smooth SiC substrate) at XUV wavelengths of 50 and 100 nm. Optical con- stant tabulations for Al by Smith, et al

.

[8,9] and C by Hagemann, et al. [15] were

used.

Figure 14. Influence of carbon film contamination on the calculated retroref lectance

(S-polarization) of a nine-facet, rhodiumcoated mirror (smooth SiC substrate) at soft

x-ray wavelengths of 10.5 and 11.9 nm. Optical constant tabulations for Rh by Lynch and

Hunter [19] and C by Hagemann, et al . [15] were used.

Figure 15. Degradation of the normalincidence reflectance of uhv aluminum

coatings on rough surfaces caused by the

surface plasmon absorption resonance at 10.5 eV. Use of a fire-polished microscope slide (Feuerbacher and Steinmann

[20]) or a bowl-feed polished glass sub- strate minimized plasmon absorption

(after Endriz and Spicer [28]).

Figure 16. A free-electron laser ring-resonator

design for extreme-ultraviolet and soft x-ray

wavelengths has two unique features: (1) intracavity grazing- incidence mirrors (hyperboloids)

to expand the beam incident on (2) multifaceted retroref lectors (N-1 flats and one off-axis paraboloid for beam col 1 imation).

Below 100 nm the multifaceted mirrors exhibiting semi-total eternal reflectance, e.g. Al

,

Si, Rh, Ag, Ru, Os are used to attain a retro- reflectance ^40% in various XUV wavelength

regions.

269
Manuscript Received

12-11-85

The Design of Optimum High Reflectivity Coatings

for Grazing Angles of Incidence

Jeffrey B. She! Ian

W. J. Schafer Assoc., 22222 Sherman Way, #205

Canoga Park, CA 91303

Presented by Title only

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 angles. 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

Ti/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, ^'^ 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 w when radiation incident from a low index medium is reflected from a high

270
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 (j) is less than the Brewster angle, (tan<t)g

= n^^/nQ, where n^ and are the indices of the high index layer and the superstrate

respectively). For ij> greater than (})g, 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 it>><}ig.

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 (iJ>>iJ)b). 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. Shell an° 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 sub- strate 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 n^, high-index layer with index n^, low-index layer of index ni , and

the substrate with index n^. The E field with incident amplitude E^ (normal to the plane of

incidence) is shown by the solid line and the amplitude attenuation factor S^j was shown

to be given by , .

n./rij

' (1)

where n^- = (n?-n^sin2(),) . (2)

(f> = 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^,- was shown to be equal to

= (n?nj)/(n|n,) (3)

271
The fields are sinusoidal in each layer, and the thickness of layer i in the quarter-wave

stack is

I. = X/(4n.) (4)

where X is the wavelength in free space.

From figure 3, the electric field penetrating into the substrate for the S polarization is given by Sg^S^ " •'• ^ o

' where 2N is the total number of layers and Eq

is the incident field. The incident flux is proportional to n^E. ^cos ^ij), whereas the flux

penetrating into the substrate is proportional to "s^^OH^LH^^LS^^o^^*

coS(ti5, where ij) and 4)5 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

As(2N)

= ns(SoHSLH^-^SLs2Eo)2cos4^/(noE2cos 4-)

= 4S S

(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 Plm'^^Bq. The flux is proportional to (l/n)B2 coS()), and the P-polarization substrate absorption for a coating with 2N layers is found

to be

% (2N) = 4PQ3P2N . (6)

If we consider the case of grazing incident angle mirrors (GIAMs) and neglect second order

terms in the grazing angle a {a=Ti/2-^), then equations 5 and 6 can be rewritten as

A3(2N)

4 is[,f' . (7)

A {2N)

P

0

4 pO OL j. ,g^

^ ^^IH^ ' ^LS a

where S?., nP, P? . are equal to S.., n , and P. . evaluated at ((> = 7f/2(a=0),

272
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 implicitly 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^-, 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'Z'. 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*" 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^'

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^*^ low index layer (n is measured from the superstrate) is + A^"^,

measured in phase units. The actual thickness of the perturbed layer is thus

where I-,,, 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 ti/2), is the same width as

all the other unperturbed L layers. The distance A|_^"' from the vertically dashed line

to the HL interface is the perturbation. If we start from the substrate where the penetrating

field has amplitude 2Eg and work backward to the n*!? L layer, the amplitude of the sine wave in the perturbed layer is 2E5S3|_S^|_^""(-1 )'^""'*'-^ for S-polarized

1 iqht.

The equation for the field in the n^*^ L layer in a coordinate system whose origin is at the front surface of the layer (see fig. 4) is

1/4 (9)

D cos[x - a["^] = D cosa["^cos x + D sinA|^"^sin x, (10)

where

D _ 2E5Sj^_S^L N-n

(-1)

N-n+1

(11)

273
The cos X and sin x field components of cos[x-A|^(")] have been drawn in dashed lines

in figure 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) = (-1)"d[Shl""^ShO^°^^l"^^^" ^

(12)

+ S^h" sinA^") cos y].

This can be expressed as the sum of a forward-propagating wave and backward-propagating

wave

C[e"^"y + (G-iF)/(G+iF)e"'^] , (13)

where

^41^ [i s?,:h

HL "HO

cos A|^"'+ S|^^ sin (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|^, the absorption is proportional to with the constant of proportionality quickly found by comparing equation 5 with

= 0. The s polarization absorption for the t low index layer is perturbed by A[_^"' is found

|E,/C|2

1 E^/C I ^ for the case > wave stack where the n for the case of quarterto be

A5(2N.a;;'

4 S -2N OS -^LH cos A|^ Is^ S OH

sin

(15)

In

:2N,

a similar manner, an expression can be fpund for the s polarization absorption,

,Al|'"^), a quarter-wave stack with the n''" high index layer shifted by

phase units.

A3(2N,aJ,' cos

4 S .2N OS -"LH

LH 'OH sin A, (16)

H

274
The corresponding expressions for p polarization are

4 P p2N

^ ^OS LH '^'''-'^

' - cos^ 4"^^ [^-^^^OH Sin ^l^^f

(17)

(n), _ ^ ''oS ^LH

A (2N,

) 2 ,(n)^ rD2(n-l)n ^ TnTT? cos ^+ [P^;^ ^Pq^ sin a;^ m

(18)

Equation 8 indicated that the p polarization absorption varies as 1/a for small grazing

angles and quarter-wave stacks,. .Since Pg^^ ^s well .as Pgw vary as 1/a, equations 17 and

18 indicate that Ap(2N, A[_("^) and ip(2N,A^^"M will vary as a, not 1/a, for

sufficiently small a, as long a^s , sin A^" ' # 0. Equations 15 through 18 also indicate that

the absorption is periodic in A^"^ 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 fj

be the fraction of s polarized light incident on the MLDC, then

A^.=f3A3(2N)+(l-f3)Ap(2N) . (19)

where equation 15 or 16 is used for l^^{2H) and equation 17 or 18 is used for Ap(2N), de- pending on where a high or low index layer is perturbed.

If we differentiate equation 19 with respect to. sinA^"^, we find two solutions for

the coating design giving lowest total absorption, A^^"^^"). The result is

sin'-A 2A^-(n) = 0 (20)

or

sin^Af."):

[(1-^3) POS LH s"0S "LH

i, M U Sq^ )] 1/2

(1-s

OH^"^')[(l-fs)PosPL[!(''o^"^-l)]^^'^(P^'"^'^^"

^ (21)

'OH '-l)[f3So3S,^S(l-s'OH )]

1/2

275
where

i = L or H, n = 1,2,...N

and

(Hn) _ <-2(n-l)<-

^OH - ^LH ^OH (22)

(Hn) _ p2(n-l)

PQH - ^LH '^OH (23)

(Ln) _ ^.2n-l ^OH ^LH ^OH (24)

(Ln) _ p2n-l

p PqH =

'^LH '^OH (25)

Equation 21 will give 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

(in)%

'OH (26)

For the case of thickness adjustments to the outermost high-index layer, Pqh

= Pqh and the condition Pqh'^ 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 numera- tor be positive,

polarized light.

This in effect defines a critical value for f„ e (l-f<.), the fraction of p

If equations 15. through 19 and 21 are combined, the absorption can be found for the optimum

MLDC for GIAMs, A^.('"^"^ The result is

276
r f s s^M V2

+

1 /2

L i Sq^ J

^OH os^lh^Pqh ''] ^Pqh s^os\h^^ ^OH '\

2 ll/2i

in,

PqH " ^OH

in)

(27)

i = L or H, n = 1, 2 N

It can be shown that the best layer, to adjust, tp achieve the lowest absorption is the out- ermost high index layer. Thus Squ^^"^ and Poh should be replaced

with Sq^ and Pg^ in equations 21 and 27.

Section 4. wi 1 1 . cpmpare the predictions of equations 15, 16, 17, 18, and 27 with numerical

results. The A^^"'^"^ 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. ^'^"^ 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 Ap versus outer-layer thickness for the coating

Si (Al203/ZnS)'' comparing the analytic results (equations 16 and 18) with exact results.

The curves are indistinguishable for A and only differ for A^ in regions of high absorption. All layers, other than the top Znb layer, are quarter-wave thick at the incident angle of 88°. There is almost a four order of magciitude change in the s absorption and more than a two order of magnitude variation in Ap as ' 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 (Si02/Zr02)'*' "substrate compensated" quarter-wave stack at incident angle 89°

with one adjustable layer was used (x=1.06ym, nu=2.1, n|_=1.4, n5=0.18+7.05i ). The absorption was calculated using equation 27 for N = 1, 2, 3, 4, 5 and n = 1, 2,...N for f^ = 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. ^ ^' 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.

277
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 agree 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. ^0. 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. Shellan, "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. 6_7, 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).

278
Figure 1. Phase shifts at each dielectric Interface

for s polarized light for all Incident

angles and for p polarized light If

<j) < ((ig, where (}>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.)

K y////////////77777? .

" y///////////////// :

L

Phase shifts

at Interface

ref lecttons

Figure 2. Phase shifts at each dielectric interface

for p polarized light Incident at (J) > ^r,. D

279
Figure 3. E-field distribution for S-polarlzed light and B-fleld distribution for P-polarlzed light In

a h1gh-reflect1v1ty quarter-wave stack.

Figure 4, Geometry and field dlstrlbutton for calculating

the reflectivity of a quarter-wave stack with one layer of arbitrary thickness.

280
T I I 1 [- 1 r

S and P polarization absorption as a function of top layer thickness 4,^^^ as calculated

exactly and 1n the standing wave approximation for SI (AlgOj/ZnS)^. All layers, other

than the top one, are quarter-wave thick at the Incident angle of 88*. (X«2.8um)

.990 .992 .3Sk .996 .998 1.000

f fraction of s polarized light

QW stack (substrate compensated design

for p polarization)

Optimum design as found analytical ly (top layer adjusted)

• Optimum design as found numerically with computer synthesis

code

"ZrOj

"SIO,

2.1

- \.lt

- 0.18+ 7.051

X - 1 .06 pm

Figure 6. AgCSIOj/ZrO^) absorption as a function of the amount

of s polarized light Incident at 89°.

281
Manuscript Not Received -

PHASE CONTROL DESIGN ANALYSIS

M. McGuirk and R. D. Poirier

The Perkin-Elmer Corporation

761 Main Avenue

Norwalk, CT 06859-0420

ABSTRACT

A novel graphical design technique was developed for two -wave length

achromatization of phase control coatings. The technique makes it

possible to rapidly identify regions of approximate solutions for exact

computer optimization. The technique gives an indication of the

sensitivity of the design to thickness errors. The technique has been

applied to design coatings yielding an average phase error of 0.3 phase

degrees per mirror pair over a ±5% bandwidth.

Key Words: optical coating design; phase -control coatings;

thin film coatings.

282
- Manuscript Not Received -

THERMAL CONDUCTIVITY OF ENHANCED HIGH REFLECTORS

M. McGuirk and R. D. Poirier

The Perkin- Elmer Corporation

761 Main Avenue

Norwalk, CT 06859-0420

ABSTRACT

Perkin-Elmer has designed and fabricated a simple thermal conductivity

tester for thin film coatings. The work is motivated by the results

Don Decker, et al . , reported at this conference last year.l Our

measurements confirm his finding that the thermal conductivity of thin

films is substantially less than the thermal conductivity of the

corresponding bulk material. In one case the measured thermal

conductivity of an enhanced high reflectance coating is a factor of 30

less than the conductivity predicted from bulk values.

Perkin-Elmer has considered the theoretical reasons for the low

conductivity, and methods for increasing the conductivity. Increased

thermal conductivity should increase the laser damage threshold for CW

appl ications

.

Key Words: laser-damage resistance; thermal conductivity; thin film

coatings.

1 Decker, D.L., Ashley, E.J., and Koshigoe, L.G., "Thermal Properties

of Optical Thin Film Materials" Poster Session 1984 Symposium on Laser Induced Damage in Optical Materials.
Yttrium Oxide Coatings for High Power Lasers at 351 nm Manuscript Reaeived

4-S-87

D. J. Smith, C. J. Hayden , B. U. Krakauer, A. W. Schmid, and M. J. Guardalben

LABORATORY FOR LASER ENERGETICS

and NEW YORK STATE CENTER FOR ADVANCED OPTICAL TECHNOLOGY

THE INSTITUTE OF OPTICS

University of Rochester

250 East River Road

Rochester, New York 14623-1299

Yttrium oxide has proven to be a useful coating material for high power UV lasers. In combination with silicon dioxide it produces an effective dual wavelength anti refl ection

coating at 351 and 1054 nanometers which has good resistance to damage at both wavelengths.

While a durable coating material, yttrium oxide may be removed safely from a lightly damaged

optic and reapplied. Index inhomogeniety of the material is investigated as well as modification of the film structure with ion-assisted deposition.

KEYWORDS: Anti refl ection coatings, thin film materials, i nhomogenei ty , coating damage,

ion-assisted deposition.

1. Introduction

Yttrium oxide is useful for damage-resistant coatings in the ultraviolet primarily due to its low absorption edge and its moderately high refractive index. The material has been

investigated by a number of groups for its damage resistance in various regions of the

ultraviolet spectrum. [ 1-3 J Coatings of yttrium oxide and silicon dioxide were used for the

upgrade of the OMEGA laser at the Laboratory for Laser Energetics from 1054 nanometers to the

frequency-tripled 351 nanometers. Yttria lends itself to simple two and three-layer designs

which have a minimum of interfaces. These designs tend to reduce damage which propagates from

impurities at the interfaces and from the intrinsic stress inherent in more complicated

designs. While yttrium oxide forms hard and durable coatings, it is also removable under the

appropriate conditions.

During production of the coatings the yttrium oxide films showed a significant degree of

i nhomogenei ty as has been observed by Borgogno et al . [4 ] for other materials deposited by

evaporation. The films also suffered from problems of reproducibility and, as a result, had to be made under stringent process conditions for the production of the OMEGA laser optics. These

problems led to the study of the structure of yttrium oxide films.

Harris et al

.

[ 5 ] have developed a model which suggests the inhomogenei ty in thin films

is caused by a gradient in packing density of the columnar thin film. Klinger and

Carniglia [ 11 J have shown that inhomogenei ty in zirconium dioxide thin films may also be the

result of different crystalline phases becoming dominant during thin film growth. In this

study, the origin of inhomogenei ty in yttrium oxide appears to follow the Harris model as determined by electron diffraction.

Ion-assisted deposition techniques have been shown to favorably affect the structure of

thin films with regard to stability, moisture absorption, stoi chiometry and index

homogeneity. [ 6-10 ] Preliminary experiments performed with Argon ion assist on yttrium oxide

films indicate an improvement in homogeneity and film stability. Markedly different structure

is seen in the ion-assist films.

284
2.1 Design of Coatings for the OMEGA Laser .

The OMEGA laser at the Laboratory for Laser Energetics is used primarily for experiments

in inertial fusion. It is a 24 beam Nd:glass laser with an output aperture of 180 mm in each

beamline. The primary 1054 nm radiation is converted to the third harmonic at 351 m using a pair of KDP crystals. The system is capable of producing in excess of 2400 joules at 351 nm in one nanosecond pulses. Since the system uses the primary 1054 nm radiation for alignment

purposes, all optics beyond the frequency conversion cells must perform at both the first and

third harmonic.

The anti -refl ection coatings for the OMEGA laser operate predominantly at normal

incidence. The most straightforward designs used were those which consisted of quarter-wave

optical thickness layers. These types of designs had several benefits: (1) a design which

performs at the first harmonic will also perform at the third harmonic, (2) the designs have a minimal number of layers and interfaces, and (3) the coatings may be monitored during

deposition by both optical and physical thickness methods.

Selection of the final designs was determined by the results of damage tests. Abate

et a1 . [ 1 ] had performed tests on similar coatings using Yttria at the appropriate wavelength

and pulsewidth and found in general that a lower number of interfaces was beneficial for damage

resistance. The improvement of damage threshold with the incorporation of an undercoat in the

design as reported by Lowdermilk [ 15 ] was not observed by Abate, and was not used in this

study.

The two design options were a two-layer design of yttrium oxide and silicon dioxide

quarter-wave and a three-layer design consisting of a quarter-wave of yttrium oxide (next to the substrate), a half-wave of tantalum pentoxide, followed by a quarter-wave of silicon

dioxide. The advantage of the three layer design is its broader low reflectance region, and

the disadvantage is the higher absorption at 355 nm of the high index tantalum pentoxide layer

(fig. 1). The wider AR band allows less stringent requirements on uniformity and is desirable

due to the significant narrowing of the band which is characteristic of 3rd order interference

effects. Despite this advantage, the two layer design was chosen on the basis of N-on-1 and

1-on-l damage tests which favored the two layer design (fig. 2).

Dual-wavelength requirements

R < 0.5% at 351 nm

R < 0.5% at 1054 nm

¥203/73205/5102

Two-layer

YjOa/SiOa--/

design / /

Three-layer / j

/

300 320 340 360 380 Wavelength (nm)

400

CME

2

o

M

0)

0)o>

reE

re O

351 nm

1 ns 2-mm spot size

T

n on 1 • 1 on 1

_L D-270 D-271 D-274 D-275

two-layer three-layer

G1647 G1648

Figure 1: Two designs for the 351 nm/1054 nm Figure 2: Damage tests at 351 nm

AR were considered; a two layer coating favored the two layer coating,

similar to a V-coat, and a three layer

quarter-half-quarter design.

285
There are five optics with a clear aperture of 200 mm and seven surfaces in each beamline

of the OMEGA laser which were coated with the dual wavelength AR. Eighteen of the twenty-four

beamlines were coated at the University of Rochester coating facility along with the highreflector coatings for beam transport to the target chamber.

2.2 Fabrication and Performance of the Coatings

The optics were coated using conventional electron-beam evaporation techniques. The

conditions of evaporation for the yttrium oxide are given in table 1. All substrates were

prepared by a mechanical scrub followed by a wash in an aqueous ultrasonic bath. The parts were then rinsed in a series of water sprays and ultrasonic baths and finally dried with a solvent drier. The substrates were placed in the chamber as soon as possible after cleaning

and it was immediately pumped.

The majority of the coatings had good optical performance, although there was some variation in the index of the Yttria and the wavelength shift experienced upon exposure to atmosphere. These points are discussed in a subsequent section. Performance within

specifications is essential in high power laser systems due to the danger of ghost image

formation from residual reflection from curved surfaces.

As of November, 1985, the coatings in the OMEGA system had survived more than 450 system

shots. The peak fluences seen by the coatings was 1.57 ± 0.31 J/cm , at 351 nm, 1 ns. The

coatings were carefully inspected after an extended shut-down for further upgrade. Few of the

coatings showed signs of damage. Those that were damaged were located near regions of bulk

damage in the fused silicon dioxide substrates. Since the damage threshold of the bulk fused

silicon dioxide exceeds 9 J/cm , [ 12 ] . we assume most of these regions were formed by spurious

ghost images. Figure 3A shows the highest level of damage seen in the OMEGA laser AR coatings

to date. The two photomicrographs show the damage was not isolated to the coating but exists

also in the substrate. The damage seen in Figure 3B extends 20 microns into the surface, which

is a good deal further than the thickness of the coating (approximately 0.33 microns). Coating

damage is seen only in the vicinity of the high fluence, as indicated by the bulk damage. The

damage was caused by temporary misalignment of one beam with respect to the target. VJith the

exception of this one optic, all the transmissive optics have remained intact in the system.

Focus Lens

G1649

Figure 3: Coating damage in the system was typically accompanied by bulk damage in the

substrate indicating an extremely high local fluence due to ghost images.

286
Table 1: Evaporation Conditions for Yttrium Oxide

54" System 28" System

Source of material

Rotation

Source to substrate

Base pressure for

consistant index

Pa. (Torr)

E. Merck

Planetary

66 cm

9.31 X 10" (7.0 X 10"

Single

50 cm

2.66 X 10""

(2.0 X 10"^)

Temperature

Rate

Oxygen Backfill

Pa. (Torr)

Sweep

Yttria

pre-heat

Electron Beam Voltage

Ion Beam Gun Parameters:

Beam Voltage

Beam Current

Gas

Ion Impingment Ratio

200 degrees

0.22 nm/sec

6.7 X 10"^

(5 X 10"^)

200 degrees

0.15 nm/sec

1.1 X 10"^

(8 X 10"^)

1 X 2 cm htz sweep manually moved over pot.

2 min. burn-in at low beam power (0.05 A)

followed by 3 min. of high beam power (0.2 A)

6 kV 5 kV

600 V

10 ma

Argon

1 Ar+/Yttria molecule

Table 2: Removal of Yttrium Oxide Coating by Various Solutions

Test Result

100% humidity no observed effect

50/50 2-propanol

and water deterioration/37 days

R.O. water deterioration/6 days

NaCI solution deterioration/5 days

5% micro complete removal/2 days

1% HCI complete removal/1 day

287
2.3 Removal of the Yttrium Oxide/Silica Coatings

In the process of fabricating the coatings we discovered an interesting aspect of yttrium

oxide coatings; although it is a hard and durable coating, it is easily removed under certain

conditions. Yttrium oxide is slightly soluble in an acid, and as a result the coatings will come off quite readily when soaked in an acid solution. An experiment was performed to test

the durability of the coatings in various environments. Glass substrates were coated with

150 nm of yttrium oxide. The coated substrates were submerged in a variety of acidic solutions

and solvents as well as kept in a high humidity environment.

The results are shown in table 2. The coatings kept in a humid atmosphere showed no deterioration after five weeks. Mild environments such as immersion in water, water/alcohol,

and salt water produced slight deterioration of the coatings. Solutions of water and Micro (a commercial laboratory detergent) and weak acid solutions removed the coating completely in as little as four hours. In addition, coatings based on yttrium oxide in use on the OMEGA laser

have proven to be durable in typical laboratory conditions.

intercept length = 75 /im ¥203/8102 DET

G1651

Figure 4: Both (a) and (b) show the three stages of coating removal in the yttium oxide

coatings: (1) Initial penetration through silica overlayer, (2) Dissolution of the

yttrium oxide underneath the silica, (3) Fracture of the silica overlayer.

Figure 4 shows the mechanisms for the removal of a yttrium oxide/silicon dioxide antireflection coating. The yttrium oxide coating is next to the substrate and underneath the

silicon dioxide layer. Three distinct stages of deterioration can be observed. (1) The acid

solution penetrates through the pinhole and begins to corrode an area of yttrium oxide

approximately 5 microns in diameter. (2) The yttrium oxide continues to dissolve underneath

the silicon dioxide to an area as large as 35 microns. (3) The silicon dioxide layer finally

breaks away and leaves the bare substrate exposed, and the solution continues to dissolve the

yttrium oxide at the edges.

288
The removal process was used to strip both the coating and the target debris from the

OMEGA laser target chamber blast shields. The contamination produced by the residual target

materials has been difficult to remove from uncoated optics and impossible to remove from

coated optics. Now the blast shields may be regularly removed, the coatings stripped,

recoated, and replaced. Only bulk damage to the substrate will limit the life of the blast

shiel ds

.

3.1 Properties of Yttria Films and Modification of Films with IAD

During the production of the coatings for the OMEGA laser, we observed some difficulties

in the deposition of the yttrium oxide films. The first problem encountered was a significant

variation of index of refraction for the first layer of yttrium oxide from different deposition

runs. By compiling data from test and production runs, a correlation of index to base pressure

of the system was noted. The data in figure 5 show that generally a lower index and a greater

spread in index is obtained when the base pressure is above 1.0 x 10""* Pa (7.7 x 10"^ Torr).

The base pressure is the degree of high vacuum which a chamber is allowed to pump prior to backfilling the chamber with oxygen. No amount of careful substrate preparation could reduce

the index variation.

c

o

o

(0

X

0) o

1.8 -

1.7

1.610

J L

O2 backfill: 4 X 10-5 T

Source to substrate:

45 cm

Tsub- 200°C

^mop: 575 nm

I I

I I I I I

-7 10

-6 10'

log Base Pressure (Torr)

G1654

Figure 5: Tests in a 54" chamber show the the index of refraction of the first quarter-wave is somewhat dependant on the base chamber pressure. The threshold for obtaining

repeatable performance is <7xl0"^ Torr.

289
Index variations in the first layer of a deposited material have been observed by

Netterfield [ 13 j as well as Behrndt and Doughty [ 14 ] for zinc sulphide. They perceived the

variation as differences in nucleation of the first few layers of the film on the substrate.

The problem was alleviated by first depositing a thin layer of another material with an index

of refraction close to that of the substrate. The thin "Barrier" layer apparently produced

evenly distributed nucleation sites and increased the sticking coefficient of the zinc

sulphide. The "barrier" layer is reminiscent of the undercoat which has been used in various

AR designs to increase damage thresholds. [ 16 ] In a similar manner uneven nucleation of the

yttrium oxide, due to background gas contamination of the substrate may have caused the large

spread in refractive index we observed. Generally, coatings made at lower pressures would see less contamination at the substrate, thereby promoting even nucleation of the film and the

resulting higher index. Undercoats have not produced favorable damage results with this

material and were not considered in this study although the topic merits further

investigation. [ 1 ]

The second detrimental effect seen in the yttrium oxide films was inhomogeneity of the

index of refraction of the films with thickness. Inhomogeneity in Yttria films was first

observed by Borgogno et al . [ 4 ] They determined their yttrium oxide films showed an inhomogeneity An/n = -1.5%. The yttrium oxide films deposited in this study had a much greater

inhomogeneity with typical values being -5%. The index variation was determined in situ using

a technique similar to Netterfield [ 13 ] and described in detail in the appendix. The results

shown for some typical evaporation on hot and cool substrates are shown in figure 6. Each

layer is marked by a steep drop in index in the first quarter-wave optical thickness layer

(~50 nm) followed by a steady and gradual increase after the third quarter wave. These results

are quite typical, although on occasion the initial drop was absent from a coating. The

calculation of the index variation assumes that absorption in the films is negligible. A gradual increase in absorption as the layer is deposited could account for the gradual increase

in index seen for the thicker coating, but would not affect the steep initial drop greatly.

This is discussed in more detail in the appendix.

co

u

(0

0)

X

0) o

c

1.7 -

i.

1.6

1.5

[A]

J.

Tsub = 20° C, No. 67T

Tsub = 20° C, No. 63T • Tsub = 250° C, No. 69T

2 3 4

No. of Quarter-Waves at 340 nm

Figure 6: The index of refraction for conventional depositions changes with deposited

thickness. Generally this index decreases sharply in the 1st quarter wave of optical

thickness (~500 Angstroms).

290
The steep drop in the index in the first 50 nanometers of the film is a surprising but

unfortunate result. A more gradual inhomogenei ty was expected. The steeper drop (which is difficult to control) generally degrades the performance of an anti-reflection coating. The

effect of the inhomogeniety on the damage threshold of a coating is not known. To understand

the effect of inhomogeniety, a study was initiated to relate the physical structure to the

optical characteristics. In addition, control of the inhomogeniety was attempted with ion

assisted deposition techniques.

3.2 Experiment

A 28" vacuum coater was fitted with a series of shutters on its single rotation to allow a deposition of four discrete thicknesses on different sets of substrate without breaking vacuum.

The chamber was fitted with a 10" diffusion pump topped by a liquid-nitrogen cooled baffle.

All coating were deposited using reactive electron beam evaporation according to parameters

given in table 1.

One fused silicon dioxide substrate in the center of the shutter assembly was used for

optical monitoring of the films at 340 nm as they were being deposited. An Inficon quartz

crystal head was mounted directly above the electron-beam source for monitoring rate and

physical thickness of the film. The crystal head was outside the ion beam when the ion beam

was used. Special considerations taken with the optical monitor are described in Appendix 2.

The ion source was a 3 cm, Kaufman source with a hot cathode and dual graphite grid

optics. The operating parameters for the source are given in table 1. The power supply for

the source was operated in the manual mode since electrons from the electron beam gun would

interact with the beam and cause problems with the neutral izer feedback circuits in the ion source power supply. Argon gas was fed to the gun while Oxygen was let into the chamber in a 50/50 ratio with the argon. The profile of the ion source at the substrate was not characterized. The substrates were not exposed to the ion beam until the crystal monitor

recorded a thickness of 2.0 nanometers deposited on the substrates. This was to allow for easy

removal of the coating for analysis afterward.

The substrates for each thickness were two pieces of fused silicon dioxide for optical

measurements and several pieces of mica. The freshly-cleaved mica was coated with ~5 nanometers of carbon prior to being mounted in the chamber. All substrates were within a 7 centimeter radius of the center of the holder. The substrate holder v;as rotated about its center during all depositions.

Three coatings under the above conditions v;ere deposited under varying conditions of

substrate temperature and ion impingement.

1. Cool substrate (~20° C), no ion assist.

2. Hot substrate (250° C), no ion assist.

3. Cool substrate (~20°C), with ion assist.

Except for temperature and ion assist, all substrate preparation and process procedures were as identical as possible. Coatings of 1, 2, 3, and 7 quarter-wave optical thickness at 340 nanometers were deposited on each set of substrates. Between each thickness the electron

gun and the ion source were shut down while the respective shutter was positioned over the

substrate set

.

3.3 Analysis

Data from the optical monitor during the deposition was used to determine the index

profile of the coating on the monitor piece. The index profile for the three runs is shown in figure 7. The index for the coating deposited on the cool substrate tended to be the lowest

while the coating with ion assist had the highest index. In this particular data the initial

sharp drop in index is missing from the conventionally deposited films. Based on experience,

this is an infrequent exception to the behavior of the yttrium oxide films. However, the IAD runs always had a higher index anc always drifted less in index throughout the deposition. An increase in absorption of the ion assisted film may account for some of the index drift seen in

291
the inuex (see appendix.) The hot substrate coating index starts at 1.7 and gradually drifts

up until the fourth quarter-wave where data was lost from the optical monitor. The coating on the cold substrates drops only about 0.25 initially and gradually declines afterward to fairly

low values. The ion assisted coating started at a much higher index of 1.85 and was fairly

stable throughout the run dropping only a small amount in the last quarter waves.

2 3 4 5 6

No. Quarter-Waves at 340 nm

Tsub = 20° C, with ion assist,

No. 72T

« Tsub = 250°C, no IAD, No. 71T

Tsub = 20°C, no IAD, No. 69T

G1658

Figure 7: The IAD run had a significantly higher index as well being somewhat more stable than

those films deposited by conventional E-Beam method on cool and hot (200°C)

substrates

.

The spectrum of each of the coatings for the 3 quarter-wave thicknesses is given in figure 8. Confirmation of some of the index measurements in figure 7 are seen in here. The

transmission minimum of the ion-assist run is the lowest, indicating the highest index,

followed by the heated run, and finally the cool run has the lowest index. Also evident is the

reduced shift in wavelength for the ion-assisted coatings which is similar to the findings of

Martin et a1 . [ C ] All three depositions were stopped at the same optical thickness. The

measurements, made in atmosphere, indicate the shift in wavelength each coating experienced

when water entered the porous coatings and increased the effective optical thickness. The cold

deposition, with the lowest packing density, shifted the greatest amount followed by the heated

substrate, and finally by the ion-assisted coating with a shift of only 10 nanometers. The

last bit of information from this figure is the longer wavelength UV cut-off exhibited for the

ion-assist film. This result was consistent with the test runs made previous to the

experiment. Also, the monitor piece, which was in the center and experienced a slightly higher

ion flux, showed significantly higher absorption throughout the spectrum.

3.4 Structural Characteristics

The films deposited on the mica were floated off, placed onto TEM grids, and observed

under bright field and dark field conditions in a transmission electron microscope. The same films were anlayzed with wavelength dispersive spectroscopy in a Cambridge Instruments scanning

electron microscope. Fractures of the coatings were also observed in the SEM but the detail in the coatings was too fine for this method. Using Pearson's [ 17 1 technique, replica cross- sections of the coating were generated and observed in the TEM.

292
105

Transmission (%) vs Wavelength (nm)

G1659

Figure 8: Spectral scans of the coating indicate highest packing densities for the IAD coating

as indicated by the small shift from the monitor wavelength of 350 nm. However, IAD coating also showed the highest absorption.

The bright field transmission electron micrographs and electron diffraction patterns for

the single quarter-wave coatings are shown in figure 9. The instrument was operated at 100 kV with a tungston filament. Three thicknesses (~50, 100, and 150 nm) of each coating were observed. While no changes in the structure were observed for the various thickness films in each deposition, there are striking differences between the micrographs of the three deposition

conditions. The diffraction pattern of the hot substrate matches the powder diffraction files

for cubic phase of yttrium oxide. This is a highly stable phase of this compound and no variation from it was seen. The diffraction pattern for the hot substrate is clearly defined

indicating a well developed mi crocrystal 1 i ne structure. The cold and IAD depositions both

exhibit a pseudo-amorphous structure. The bright field micrographs show further differences.

The grains in the hot sample are fairly uniform in size and are about 10-20 nm. Voids are observed in the coating. The grains in the cold deposition are also uniform in size but are smaller, less than 10 nm and voids also are observed. The IAD coating appears to have a background of a very fine structure with very large grains 30-50 nm superimposed on the finer structure. The lighter area appears to be depleted regions where a nucleation site has

attracted a large radius of adatoms due to the increased mobility. This phenomena was observed

by Marinov [ 18 ] for aluminum films. This work, although observed on thinner films, bears a strong resemblance to the IAD coatings. It is interesting to note that the finest and the

largest grain structures are seen simultaneously on the IAD film.

Figures 9 also shows TFM micrograph cross-sections of the thickest films (~350 nm) for the

three coatings. The hot films snow a clear columnar growth and the interface with the

substrate is clearly identified. The cold film shows only a faint columnar structure, although

the interface is once again clearly identified. The IAD case shows some very peculiar

features. There are very large "outcrops" in the cross-section which shadow the deeper regions

of the cross-section. Secondly, no clear interface with the substrate is seen. Although the

structures are larger in the film, they do not appear to affect the surface layer.

293
Figure 9: Structural analysis using cross-sectional replicas of a 350 nm film viewed with a TEM ^Izr: ^""^9^* ^^^^'^ TM

images (center) and electron diffraction patterns (right) of the 50 nm films. The films deposited on the cool substrates showed a fine grain structure (iO nm) and had a pseudo-amorphous structure, while those deposited on hot

substrates (200C) showed a larc^er grain size (10-2C nm) and a distinct cubic structure.^ Films deposited witli ion-assisted deposition (IAD) had the largest grain size ^30-50 nm) yet, they are dominated by an amorphous structure. Also note the uisapearance of a distinct film/substrate interface in the IAD sample indicating ennanced adhesion, ^

294
The results from the wavelength dispersive spectroscopy are qualitative and preliminary.

To use the data quantitatively requires a set of references of Yttria with the same thickness

as the films. Currently some ul trami crotomy techniques are being investigated to produce these

references. The data indicates that the films become Oxygen depleted as they become thicker.

The films on the cold substrates showed the greatest tendency toward oxygen depletion.

4. Discussion

The steep index drop in the first fifty nanometers of the yttrium oxide coatings might be explained by the modeling techniques of Liao and Macleod. [ 20 ] The first layers of the model

indicate a high packing density which decreases as the columnar structure formation begins. It

is unusual that in yttrium oxide films the region where the packing density is high is thick

enough to significantly affect the refractive index of the first quarter-wave at 351

nanometers. The model predicts the formation of the columnar growth within tens of monolayers

of the surface. Experiments with undercoats of the same refractive index as the substrate may

provide insight into the nucleation behavior of yttrium oxide. A relationship between index

and stress as a function of coating thickness may provide additional insight into the film structure. Stress measurements are now being planned using the stress induced birefringence

apparatus [ 21 ] at the Laboratory for Laser Energetics.

Our preliminary study confirmed that yttrium oxide inhomogeneity could be controlled

through ion-assisted deposition. The physical microstructure of the IAD films was distinctly

different than the films grown using conventional deposition techniques. Effects of reduced

ion energy, oxygen ions, and beam intensity will be considered in the next phase of the study.

Finally, ion-beam assisted coatings will be tested for damage susceptibility at 351 nm.

Verification of the steep index drop will be facilitated by a rapid scanning

monochromator . This would also provide direct useful information on the shifts of the coating

due to water absorption which relates to the packing density of the film. The data from the

scanning monochromator would also confirm the cause of the apparent index increase for the

thicker coatings. If absorption in the coatings is not the cause of the increase, then

temperature fluctuations of the substrate during deposition must be considered.

Despite the many problems associated with the deposition of yttrium oxide, it remains an important material for high laser fluence coating applications. It is the intent of the

authors to continue investigations of yttrium oxide to understand the effects of stress,

inhomogeneity, absorption, and stoichiometry on the damage threshold of the film. Attempts

will be made to improve the damage susceptibility of the films through ion-assisted deposition.

Acknowledgment

The work performed at Rochester was supported in part by the New York State Center for Advanced Optical Technology of The Institute of Optics, the U.S. Department of Energy Office

of Inertial 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, Ontario Hydro, Southern California Edison Company, and the

University of Rochester.
V. References

1. J. A. Abate, R, Roides, S.D, Jacobs, W. Piskorcwski, and T. Chipp, "Laser Damage of Optical

Coatings at 351 nm," NBS Special Publication 669, 282 (January 1984).

2. F. Rainer, W.H. Lowdermilk, D. Milam, C. Carniglia, T.T. Hart, and T.L. Lichtenstei n

,

"Materials for Optical Coatings in the Ultraviolet," App. Opt. Vol . 24 , No. 4, 496 (1985).

3. IJ.M. Lowdermilk, D, Milam, "Review of Ultraviolet Damage Threstiold Measurements at Lawrence Livermore National Laboratory," Presented at SPIE Technical Symposium East '84,

Arlington, Virginia, April 29 - May 4, 1934.

4. J. P. Borgogno, B. Lazarides, and E. Pelletier, "Automatic Determination of the Optical

constants of Inhomogeneous Thin Films," Appl . Opt., Vol . 21 , No. 22, 4020 (1982).

5. i'i. Harris, H.A. Macleod, S. Ogura, E. Pelletier, and B. Vidal , "The Relationship Between

Optical Inhomogeneity and Thin Film Structure," Thin Solid Films, 57, p. 173, 178 (1979).

6. R.P. Netterfield, W.G. Sainty, P.J. Martin, and S.H. Sie, "Properties of CeO Thin Films

Prepared by Oxygen- lon-Assi sted Deposition," Appl. Opt., Vol. 24 , No. 14, 2257 (1985).

7. P.J. Martin, R.P. Netterfield, W.G. Sainty, G.J. Clark, U.A. Lanford, and S.H. Sie, "Ion- Assisted Deposition of Bulklike ZrO Films," Appl. Phys. Lett. 43 (8), 711, (1983).

3. P.J. Martin, H.A. Macleod, R.P. iletterf i el d , C.G. Pacey, anH W.G. Saint>, "lon-BeamAssisted Deposition of Thin Films," Appl. Opt., Vol. 22 , No. i, 178 (1983).

9. J.R. i'lcNeil, A.C. Sarron, S.R. Wilson, and W.C. Herrmann, Jr., "Ion-Assisted Deposition of

Optical Thin Films: Low Energy vs. High Energy Bombardment," Appl. Opt., Vol . 23 , No. 4,

552 (1984).

10. J.R. McNeil, G.A. Al-Jumaily, K.C. Jungling, and A.C. Barron, "Properties of TiOa, and

SiOz Thin Films Deposited Using Ion Assisted Deposition," Appl. Dpt., Vol . 24 , No. 4, 486

(1985).

11. R.E. Klinger, and C.K. Carniglia, "Optical and Crystalline Inhomogeneity in Evaporated

Zirconia Films," Appl. Opt., Vol . 24 , No. 19 (1985).

12. W.H. Lowdermilk, Laser Research and Development, LLNL Laser Progress and Annual Report , 1982, pp. 7-28.

13. R.P. Netterfield, "Refractive Indices of Zinc Sulfide and Cryolite in Multilayer Stacks,"

Appl. Opt., Vol. 15 , No. 8, 1969 (1975).

14. K.H. Behrndt and D.W. Doughty, "Fabrication of Multilayer Dielectric Films," J Vac. Sci.

Technol., Vol. 3 , 264 (1966).

15. W.H, Lowdermilk, D. Milam, "Laser Induced Surface and Coating Damage," IEEE J. Quantum

Electron., 17, 1888 (1981),

16. T, Hart, T. Lichtenstei n , C. Carniglia, F. Rainer, "Effects of Undercoats and Overcoats on Damage Thresholds of 248 nm," NBS Special Publication No. 683 (September 1983).

17. J.M. Pearson, "Electron Microscopy of Multilayer Thin Films," Thin Solid Films, 6(1970)

349-358.

18. M, Marinov, "Effect of Ion Bombardment on the Initial Stages of Thin Film Growth," Thin

Solid Films, 46 (1977) 267,

19. H,A, Macleod, Thin Film Optical Filters , Adam Higler Ltd, London (1969),

20. B. Liao and H.A. Macleod, "Thin Film Mi crostructure Modeling," presented at Southwest

Conference in Optics, 4-8 March 1985, Albuquerque.

21. 5. Jacobs, "Anisotropic Stress Around Defects in Thin Films," LLE Review, Vol . 10 , 3 (March 1982).

296
Appendix - In-Situ Optical Monitoring of Refractive Index vs Coating Thickness

The characteristic matrix for a single film is given by Macleod, L 19 J

cos 9 (i sin 9)/nj

i n^ sin 0 cos 6

where n_. = complex refractive index for the jth layer

2tt nd g and 6 = phase thickness of the layer = \

If we let 0 = 7t/2 (quarter-wave optical thickness) and assume zero absorption (i.e., is real) in the films, then the matrix reduces to,

0 yn.

1 n . 0

The characteristic matrix is multiplied by the column vector from which the admittance, Y, is found

.

->- -»•-» Y = H / kxE , ns = substrate index.

The value for the admittance in this simple layer case is,

Y = "jVns (Eq. 1)

Additional layers require only building up the characteristic matrix, multiplying the matrices,

and solving for the admittance. So, for two and three layer, non-absorbing quarter waves we have

,

/A ( kxE\

)

1 IS 1

n/ n^

^ 2 n.

(Eq. 2)

Y

= (Eq. 3) n2 ns

The admittance may be found in this manner for any number of layers. The admittance may be found directly from reflectance measurements since,

R = where n = incident index.

(n + Y)^

°

^ 0

'

Taking the square root of the reflectance will yield two solutions, but only one has any

physical significance and is quite obvious.

Equation (1) is easily solved for n(l) if we know the reflectance at the quarter-wave

thickness. Likewise, if n(l) has been found, eq. (2) may be solved for n(2) assuming R is known at the appropriate quarter-wave position. The process can continue for any number of

layers as long as the reflectance is measured only at the quarter-wave points, and there is negligible absorption in the films.

297
The index profiles of the yttrium oxide films in this study were determined using the

above method. Needless to say, it is imperative that the optical monitor for determining R be

accurate (in this case transmission was measured and R was found using R = 1 - T).

Considerable effort was made to insure that the optical monitor gave stable and accurate data.

An absorption filter was used in the input slit of the monochromator to reduce the effect of

scattered stray light from the grating. The monitor wavelength, 340 nm is close to the

absorption edge of standard viewport materials and can be seen in the optical monitor as the

absorption band of the glass moved up and back down the spectrum. So, fused silicon dioxide

viewports were substituted for the original glass viewports. The viewports were carefully

shielded to prevent an accumulation of coating which also would lead to inaccuracy.

There is a question of the validity of allowing the absorption to be zero for these

measurements. In fact, post-deposition measurements did show that some of the films had

significant absorption in the UV. A series of simulated depositions were made using existing

programs, and the "optical monitor" data was extracted from the simulations. The model

predicted the index profile of the nonabsorbing films as expected. However, in absorbing

films, after the steep drop in index, there was often a gradual increase in index. The model

with absorption also displayed this increase in index. Since a drop in index can only be caused by i nhomogenei ty , the initial drop is likely to be real, but the gradual increase may

well be an artifact caused by absorption in the layer.

298
Manuscript received

1-24^86 SAB

Self-Consistent Dependence of Porosity and Refractive Index on Composition in

Co-Evaporated Films

A. Feldman, E.N. Farabaugh, and R.A. Stempniak

Ceramics Division

Institute for Materials Science and Engineering

National Bureau of Standards

Gaithersburg, MD 20899

Zirconia : silica films grown by e-beam coevaporation show a decreased porosity

compared to pure zirconia films. This decreased porosity causes an anomalous dependence

of refractive index on composition. A maximum in refractive index occurs at about 20

percent by volume of silica, the concentration at which the films appear to become

amorphous. A simple model which derives the porosity as a function of composition from

the deposition parameters predicts the general compositional behavior of the refractive

index. The results agree best with the Drude model of refractive index for mixed

component systems.

Key words: coevaporation; porosity; refractive index; thin films; Zr02; SiO

1 . Introduction

One approach suggested for obtaining optical coatings resistant to laser damage is to produce

films that possess a bulk-like glassy structure. E-beam deposited films generally are not bulk-like in that they exhibit a columnar structure and a significant porosity. Because of this

porosity, water is adsorbed and desorbed from the film depending upon the ambient atmospheric

humidity. The net effect is an unstable refractive index. In addition water can seep between

film layers leading to delamination. One method shown to decrease porosity is ion-assisted

e-beam deposition [1]. In this work we show a similar decrease in porosity by e-beam coevaporation and without the need for an ion gun. This result is important because manufacturers of

thin film coatings can use the current e-beam technology to produce bulk-like films without

investing in the newer ion-beam technology.

Recent work has shown that mixed zirconia : silica films have varying structures depending on composition [2], Pure zirconia films are polycrystalline with two crystalline phases present.

Films with a silica content of up to 20 % by volume have a single crystalline phase. At volume

fractions of silica >20 % the films show an amorphous structure as determined by x-ray diffraction.

In this work we show that some of these films appear to exhibit bulk-like densities. A simple model derives the porosity of the films as a function of composition. The model also is

consistent with the observed anomalous compositional dependence of the refractive indices of the

films. We find that the addition of silica to zirconia can produce films with larger refractive

indices than pure zirconia films. The compositional dependence of the refractive index agrees

best with the Drude model.

This work was supported in part by the Air Force Office of Scientific Research under Grant Nos. ^AFOSR-84-00060 and AFOSR-ISSA-85-0006

.

Guest worker from Institute Tecnologico de Aeronautica, Brazil with partial support from CNPq.

299
2. Experimental Methods

Films in the system ( ZrOp),^: (SiOp) (i _x) were produced by e-beam coevaporation from two independent sources onto fused silica substrates at a substrate temperature of 300 °C. The

deposition system has been described elsewhere [3]. Each source was monitored by a separate

quartz-crystal thickness monitor. Films were produced varying in composition from pure Zr02 to pure SiOp. The thickness monitors were calibrated on the basis of stylus measurements on the pure

films. The target deposition rate for the sum of the constituents was 0.5 nm/s, however, a higher

rate was used on the deposition of some of the films with a low fraction of one constituent

because the monitors had to operate near the lower limit of their sensitivity range. Thicknesses

were in the range 0.7 to 1.1 ym. Two sets of films were produced. In the first set, the deposition rates were controlled

manually; in the second set, the rates were controlled by a commercial controller. The results

reported were similar for both sets of films.

The thickness of each film was measured by stylus profiling at several points around the film

border. The reported results are based on the average value. The refractive index was obtained

from a combination of channel spectra measurements [4] and the thickness measurements. Thickness

and refractive index were also obtained by the method of m-line spectroscopy [5]. Although there

appears to be some discrepancy between the two methods of obtaining refractive index, the agreement is sufficient to sustain our principal conclusions.

3. Symbols and Nomenclature

In order to expedite the discussion we introduce the following symbols and nomenclature:

t == measured film thickness.

t1 == monitored thickness of silica,

t2 == monitored thickness of zirconia,

Dn == (t-i + t2)/t, nominal densif ication.

fl == packing fraction of silica in film.

^2 == packing fraction of zirconia in film

f == (f^ + f2), packing fraction of film,

P

== pore fraction in film.

Pi == pore fraction in pure silica film.

P2 == pore fraction in pure zirconia film.

n = refractive index of film.

"1 == refractive index of silica, and

"2 == refractive index of zirconia.

The following relationships hold:

f = 1 - p (1)

1 = f^ + f2 + p (2)

4. Results and Discussion

Figure 1 shows a plot of refractive index as a function of the thickness fraction of silica,

ti/(ti+t2). for film set one. If it is assumed that the volume fraction is proportional to the

thickness fraction, then the behavior of the refractive index with composition is anomalous. On the basis of all theories of mixed component systems, the refractive index of the composite should

have a value that falls between the values of the pure constituents. Thus, the addition of

silica, a low index material, to zirconia, a high index material, would be expected to lower n. Instead, we find that initially n increases. A decrease in film porosity would account for this

behavior

.

300
In order to test this hypothesis, we have compared t-i + t2 with t for each film. In every case we find t-^ + t2 > t. This is clearly demonstrated in figure 2 where we observe > 1 for all mixed films. On the basis of a simple model, we are able to calculate the pore fraction

of the coevaporated films as a function of composition. We can also model the compositional dependence of the refractive index.

5. Porosity Model

We make the following assumptions: 1) The sticking coefficient is independent of composition. This implies that the quantity of material deposited is proportional to the monitored

thickness. 2) A given volume is associated with each type of molecular unit [6]. This implies

that the deviations of t^ + t2 from t are due to porosity.

From these assumptions several relationships can be derived:

t-, (l-p^ ) + t2(l-P2)

f

= (3)

t

Dn =

(1-p) (t^+t2)

t^ (1-p^ ) + t2(l-P2)

From eq (3) we see that the packing fraction of a film can be obtained from the measured

quantities t, t-] , t2 and assumed values for the pore fractions in pure films p-] and P2. A theoretical expression for D^^ can be obtained if we can assume a relationship between the pore

fraction in the film and the fractions of the two constituents, f-i and f2. The following

expression was chosen as an empirical fit to simulate the experimental data:

p = P2 exp[-(fT/A^ )]2 + p^ exp[-(f2/A2) ]2 (5)

The parameters A-j and A2 are chosen so that the porosities of the pure films are obtained, that is

A^ << 1 , A2 <<1 . The expression implies that a small increasing fraction of one constituent acts

to reduce the porosity. If we solve equations (1), (2), and (4) in a self-consistent manner we can obtain a curve for comparison with one set of experimental data. Figure 3 shows such a comparison with the experimental data of film set two. The values for p-] , P2, A-] , and A2 are chosen so that the packing fraction did not exceed unity. We see that a reasonable fit is obtained with p^=0.10, P2=0.25, A.,=0.15, and A2=0.25.

These results were used to calculate values of refractive index as a function of film

composition on the basis of three different models, the Drude model, the Lorentz-Lorenz model, and

the linear model. The Bruggeman effective media model agrees approximately with the linear model

so it has not been used. Each of the models average the refractive indices in a different

manner

,

Drude

n2 => f, n,-^ (6)

Lorentz-Lorenz

;

n^-l n,.2-i -- =2:^i

-----

n2+2 i n^2+2

(7)

301
Linear

:

i

In Figure 4, the results for each of the models are superposed on the experimental data of

film set two. All of the models show the same general compositional behavior as the experimental

data. Two data points are shown for each specimen; one point is obtained by m'-line spectroscopy

(x's), the other point by the method of channel spectra and stylus thickness measurements (dots).

We believe the discrepancies are due to the different sensitivities of the measurement methods to compositional inhomogeneity . However, in spite of this difference, the two sets of data still

display the same behavior and do not alter our conclusions. The best quantitative agreement

appears to be provided by the Drude model.

6. Conclusions

The pore fractions in e-beam coevaporated zirconia:silica films are significantly smaller

than in pure zirconia films. This results in an anomalous dependence of refractive index on composition. A porosity model, which requires estimates of porosity in pure films only, predicts

the compositional dependence of the pore fractions in the films. This result is In reasonable

agreement with the experimentally determined nominal densif ication of the films. Calculations of

the compositional dependence of the refractive index agree with the behavior of the experimental

data. Of the different models of refractive index in mixed component systems examined, the Drude

model appears to agree best with the experimental data.

We thank T. Vorburger and C. Giauque of the Micro and Optical Technology Group for the

diamond stylus measurements of specimen thicknesses.

7. References

[1] Martin, P.J., Macleod, H.A., Netterfield, R.P., Pacey, C.G., and Sainty, W.G., Appl. Opt. 22,

178 (1983).

[2] Farabaugh, E.N. and Sanders, D.M., J. Vac. Sci. Technol. A1_, 356 (1983).

[3] Sanders, D.M., Farabaugh, E.N., Hurst, W.S., and Haller, W.K., J. Vac. Sci. Technol. _18_, 1 308

(1981 ).

[4] Feldman, A., Appl. Opt. 23, 1193 (1984).

[5] Tien, P.K., Ulrich, R., and Martin, R.J., Appl. Phys. Lett. 291 (1969).

[6] Jacobsson, R., in Physics of Thin Films, _8, Bass, G., Francombe, M.H., and Hoffman, R.W.,

editors (Academic Press, New York, NY, 1975).

302
2.20

2.00

-1.80

1.60

0.2 5 0.50 0.7 5 THICKNESS FRACTION SILICA1.00

Figure 1. Refractive index of zi rconia : sil ica films vs. thickness fraction

of silica for film set one. The dots were obtained from channel spectra and

stylus thickness measurements. The x was obtained by m-line spectroscopy.

zO 1 . 2 5

Z 1.00

0.7 5

0 . 5 OL0.00 0.25 0.50 0.75

THICKNESS FRACTION SILICA

1.00

Figure 2. Nominal densif ication of zirconia silica films vs. thickness

fraction of silica. The x's are for film set one, the dots for film

set two.

303
1.50

1.25

^ 1.00

£

O 0. z7 5

0.50 " I I I II 0.00 0.2 5 0.50 0.7 5 1.00

THICKNESS FRACTION SILICA

Figure 3. Nominal densif ication of zirconia:si1 ica films vs. thickness

fraction of silica. The curve is derived from the porosity model calculation

with p,=0.10, p^=0.25, A,=0.15, and A^=0.25. The data are from film set two.

2.20

2.00

1.80

1.40

0.00

N

• N

. \ N

0.25 0.50 0.75

THICKNESS FRACTION SILICA

Figure 4. Refractive index of zirconia : si 1 ica films vs. thickness fraction

of silica. The three curves shown were derived by insertion of the results

of the porosity model into models of refractive index for mixed component

systems. The topmost curve is for the Drude model, the middle curve is for

the linear model, and the lowermost curve is for the Lorentz-Lorenz model.

The data are from film set two. The x's were obtained by m-line spectroscopy,

the dots from channel spectra and stylus measurements.

304
Manuscript Eeaeived

3-27-87

The Formation and Development of Nodular Defects in Optical Coatings

Bangjun Liao, Douglas J. Smith and Brian Mclntyre

LABORATORY FOR LASER ENERGETICS

and NEW YORK STATE CENTER FOR ADVANCED OPTICAL TECHNOLOGY

THE INSTITUTE OF OPTICS

University of Rochester

250 East River Road

Rochester, New York 14623-1299

We have investigated the nodular defect using computer simulations and experimental

techniques. A modified version of the hard-disk model, in which the migration and attraction

of atoms are introduced in simplified ways, is used to simulate the formation and development

of nodules. This simulation reveals the self-extending nature of nodule growth and the

dependence of the nodule shape and size upon the conditions of the coating's formation,

including deposition geometry, migration, and capture length of atoms. By planting artificial

seeds on substrate surfaces and overcoating with single and multilayer stacks, we have

encourged nodular formation under controlled and variable deposition conditions. Fractured

cross-sections of these coatings have been examined by scanning microscopy (SEM). A comparison

between our experimental observations and the modeling results help explain the mechanisms for

nodule growth.

KEY WORDS: Coating defects; nodules; hard-disk model; laser damage; thin films; thin film structure.

1. Introduction

The nodule is a characteristic defect in optical coatings. It causes scattering and

moisture penetration; in high energy laser coatings, the localized stress, [1 ] localized

heat, [2 ] and micro-lens effects [3 ] generated by nodules are assumed to trigger laserinduced damage. Nodular defects have been found in various metal and dielectric films which

are fabricated by sputtering, chemical vapor deposition, electroplating, thermal evaporation

(both electron beam and thermal resistive) and ion plating. A number of reports have focused

on this microstructure during the last 15 years. K. Guenther [4 ] outlined the importance of

the nodule in optical coatings. Nodular defects [5 ] were thought to be formed by the same self-shadowing effects which cause columnar structure, [4,5 ] However, this effect alone can

305
explain neither the nodular growth in the films deposited at normal incidence nor the peculiar

shapes of nodules. This paper investigates the character, formation, and development of

nodular defects in thin films. Computer simulation and experimental verification are emphasized.

By using the modified version of the hard-disk model, the simulation maintains the

simplicity which has made it possible to model the growth of a film on a large scale (up to 120,000 deposited disks). The modified model can accommodate different deposition conditions

and material properties and thereby link the growth of nodules to these conditions.

2. Hard Disks with Multi-Jumping and Variable Capture Length

The starting point of our calculation is the two-dimensional hard disk model of Dirks and

Leamy [5 ] in which disks fall randomly onto the surface and then stick where they land or roll into the nearest pocket, where each disk is supported by two other disks which had arrived

earlier. This model describes the general appearance of the columnar structure and the

behavior of columnar angle in a thin film, but it can only grow low density films and cannot be used to provide more information about film features and formation conditions. During actual

condensation, an adatom or admolecule moves some distance over the surface after impact due to transverse momentum and thermal diffusion. [7 ] This migration of adatoms and admolecules

depends on substructure temperature, kinetic energy of incident flux, residual atmosphere,

surface topography and the adatom's and admolecule's activation energy.

The modified model (Fig. 1) [8,9 ] includes the migration process such that an incident

disk can move over the surface by jumping from one pocket to another after impingement until an eligible site is reached.

The model quantitatively represents the mobility by X , which is the average distance

traveled by the disk divided by the diameter of the disks. This mobility, X, is derived from

the product of the distance traveled and the probability of jumping 1, 2, 3 . . . or N times:

X = 0.6Rj + a R

n n (1)

where f\

, R2

'^n ^ probability distribution function for 1, 2, ... N jumps. These

sati sfy

,

n R. = 1 J (2)

j

= l

306
a = vapor incidence angle A = average distance jumped by hard disks

G1625

Figure 1 The modified hard disk model allows

multiple jumps of an adatomafter it impinges on

the substrate. In this example the adatom

was allowed to jump three times before coming to rest.

2D X extending factor G1755

Figure 2 An approximate potential is by implemented extending the capture

length of an adatom. In the hard

disk model the capture length is just

2x the diameter of the adatom. In our

model we extend this length by a factor ranging from 1 to 1.75.

The coefficients 0.6, 1.6, 2.6 . . . are the average distance traveled by a disk for 1, 2, 3 jumps and are determined by another simulation. The number of jumps taken by a disk is determined randomly, but is constrained to follow the probability distribution given by the

values for , R2 , R3 which are input during the simulations. The actual value for X is output

after the simulation is complete.

The interaction between atoms or molecules can be generalized by the Lennard-Jones

potential. One expression for this potential used by Leamy et al

.

[10 ] in their dynamic

simulation is,

^ 12 ^ 6

<t> (r) = e - 2(^) (3)

where r is the distance between disks, r^ is the equilibrium value of r for ^ = 0 and e is the

value of -(J)(rg). Conventionally the negative sign denotes the attractive forces, and the

positive sign indicates the repulsive forces so the first and second term in the square bracket

correspond to attraction and repulsion, respectively. Figure 3(a) is the curve for the force

of the potential function, f = - d^ /dr. The maximum of the attractive forces occurs at

r = l.lr^, and it still has a value of one-fourth of the maximum at r = i.Sr^j. This reflects

307
the behavior of the long ran^e attractive forces. A simplified representation of this

interaction can be included in the hard disk model by extending the capture length. The

capture length is the diameter of a circle which vjill cause a passing disk model to be "captured" by the fixed disk (Fig. 2). In the common hard disk model the capture length is two times the diameter of the disk, i.e., the disks must collide in order to be captured. By

extending the capture length we create an approximation of the attractive forces. A disk

passing within the extended capture length will be attracted to the fixed atom. The force of

this simplified potential is shown in Fig. 3(b). In our model we multiply the capture length

by an extending factor ranging from 1 to 1.75.

f A

2.7 -ff A h +00

r = 1.1 r.

-oo

(a)

G1754

(b)

Figure 3 The force resulting from the Lennard- Jones potential is shown in Figure (a). The

approximation to this force as it is used in the simulation program is shown in Figure (b).

Simulation programs have been implemented on IBM-XT and HP9000/200 microcomputers. By

choosing various parameters one can create a coating with a nodule in "controlled conditions."

One of the programs can trace the dynamic process of a "coating" growth on a computer screen.

Experiments in support of the simulation activities used Pi\ grains, whose diameters were larger than 0.05 ym, as seeds for nodule growth on polished glass surfaces. The seeded

308
substrates were overcoated with either a single layer of ZnS or multilayer Ta205-Si02 stacks.

Both normal incidence on stationary substrates and various oblique-incidence angles for

rotating substrates were explored. The samples were fractured, and coating cross sections were examined by scanning electron microscopy.

3. The Self-Extending Effects of Clusters and Nodules

Almost all nodule shapes found in thin films are characterized by an inverted cone with a dome or elliptical top. Their diameter increases monotonical ly with the film thickness, which

we describe as a self-extending effect of nodules and clusters. This formation of the nodule

can be visualized by examining the effect of the extended capture length on the growth process.

We find that nodule-like growth occurs even when the effects of the surrounding film are ignored in structures we call clusters.

A deposited disk. A, has a capture length greater than its diameter, as shown in Fig. 2. New disks arriving randomly within this capture length will impinge upon A so that the cluster

starting at disk A extends its diameter during the growth and forms a cone regardless of seed

size or adatom mobility. The cone angle of clusters which form on an individual disk is 36° + 2° at X = 0 as shown in (Fig. 4a), and is almost the same result as P. Ramanlals' disk

fan. [11 ] The cone angles vary from 30° - 40° if clusters initiate at a larger circle seed

(Fig. 4b).

The effects of extending the capture length, when adatom mobility is present, are seen in Figs. 4(c) and 4(d). An increase in mobility results in a decrease of the cone angle of the

cluster when the capture length is not extended. However, if the capture length of the disks

is extended the cone angle of the cluster increases. This suggests that longer range

attractive forces between atoms produce larger cone angles on the nodules.

Clusters forming on a smooth substrate will not initiate the self-extending process in the

computer simulations because of the competition among adjacent clusters. Nodules will not form

spontaneously from homogeneous nucleations on substrate surfaces for similar reasons. Only

those clusters which start at protrusive seeds have chances to develop the shapes of nodules.

4. The Geometry of Nodule Growth

Nodules take two distinctly different forms depending on whether or not the substrate is rotated in its own plane during deposition. Substrate rotation with an offset vapor source is commonly used to provide thickness uniformity on the substrates.

309
(a)

G1626

(C)

G1627

nucleus: single disk

\ - 0, 0 = 36° ±2°

A = 1,9, 0 = 20°

extending factor = 1 .0

(b) nucleus: circle A = 0, 0 = 30°~40^

(d) A = 2.0, e = 45°

extending factor = 1 .5

Figure 4 Development of the model for simulating cluster growths in "free space." Growths

with low adatom mobility are seen in (a) and (b) where the nucleus is a single adatom and a larger circle. Repeated trials of (a) gave consistent values for the cone angle. Allowing

some adatom mobility (c) results in a lower cone angle. Extension of the capture length (d)

increases the cone angle.

310
A growth simulation of a nodule on a non-rotating substrate with normal vapor incidence is

shown in Fig. 5(a). The structure is characterized by straight sides similar to those of

Fig. 5(b) (single layer of ZnS) and Fig. 5(c) (multilayer of Ta205-Si02). The cone angle of

the nodule in Fig. 5(c) is 42°, close to the cluster with an extending factor of 1.5 in

Fig. 4d. The thickness of the simulated film in Fig. 5(a) is about 1700 ^ if a disk represents

an atom of diameter 5 K.

Figure 6(a) is a simplified case of the initial stage of cluster growth on a rotating

substrate with oblique vapor incidence (a = 50°). Any point on the nodule side is exposed to

the incident flux during half of a rotation period so that the seed grows with a cone angle

2 Y . If J is the intensity of incident flux, and a is the incident flux angle, the evolved

angle, y > satisfies:

TAN Y

=

J sin Y) d *

(4)

J cos a d

For small Y, the approximation holds:

TAN Y -^j^ tan a

Equation (4) establishes the relation between a and y in a cluster forming on a rotating

substrate. An increase in the incident angle a results in a corresponding increase in the cone angle y A cluster enlarges its diameter rapidly (Fig. 6(b)) during the early growth stage.

In fact, Eq. (3) holds only at the initial stage of a coating growth. After the step around

the seed disappears and the dome on the top of the nodule forms, the increase of the nodule's

diameter slows down. As a result, a bowl-like bottom or parabolic sides evolve in contrast

with the straight sides of the nodules grown under normal incident flux on stationary

substrates (Fig. 7). Nodules in a coating deposited by wide source (for instance, sputtering)

exhibit similar shapes due to the spread in angle of incident particles. Figures 8(a) and 9(a) are two nodule simulations growing on 3-dimension rotating substrates (on axis). Note that the

diameter

311
G1630

Figure 5 The characteristic shape of a nodule formed on a fixed substrate with normal vapor

incidence is an inverted cone. The model (a) is supported by experiment with single (b) and

multiple (c) layer films

312
(b) I . I

G1638

Figure 5 Rotation of the substrate and an oblique vapor incidence results in an increase in the nodule cone angle. The simulation in (b) shows this effect for a nodule without the

restrictive influence of the surrounding film. This is typical for the initial stages of

nodule growth.

G1632

Figure 7 There are two characteristic shapes of nodules in vapor-deposited films. The shape

on the left is typical of films deposited with a fixed incident flux. The shape on the right

corresponds to a nodule formed on a rotating substrate with an oblique incidence source.

of the nodule grown on incident angle a = 70° is much larger than that on a = 50°. In Figs. 8(b) and 8(c) as well as Figs. 9(b) and 9(c) experimental test specimens are shown for

comparison. Although the scale lengths are orders of magnitude different between simulations

313
(Figs, 9(a) and 10(a)) and the corresponding micrographs ((b) and (c) of both figures), the

visual agreement in geometry is quite good.

a = 50°, A = 3,5, extending factor = 1 .5 Figure 8 Nodule simulations and

experiments on rotating

substrates at 45 degrees vapor

incidence.

Simulation

G1635

45° vapor incidence/single ZnS film

G1636

A seed is a necessary condition for the nodule to form. However, while larger seeds tend

to generate larger nodules in simulations, not all seeds will grow into nodules. We find that

a seed of three disk diameter merges in a film during simulated growth at normal incidence. At

314
sufficient adatom mobility (a = 3.4), disks migrate longer distances along the surface and fill

the volume around the small seed. Larger imperfections, such as dust, polishing residue, and

rough protrusions on substrate surfaces all act as the starting point for nodules. Smaller

defects on substrates, such as impurities, scratches, steps and ledges are the favorable sites

for nucleation and may develop into large seeds for nodules under suitable condensation

circumstances

.

Figure 9 Nodule simulations and

experiments at 70 degrees vapor

incidence. The size of the

nodules varied significantly at

this vapor angle.

a = 70°, K = 3.5, extending factor = 1 .5

a fe: Simulation

G1639

70° vapor incidence/single ZnS film Small Seed Large Seed

2 fim —

H

G1641

315
Figure 10 shows the simulation for a three-layer film containing a nodule initiated at a spatter site in first layer. It also shows the nodule growth through the layer interfaces.

There are three nodules in Fig. II. They grew on the same substrate, but apparently have

different sizes and shapes, which may originate from the various size seeds, since the

simulation has shown that significant differences in nodule sizes follow the various sizes of

seeds even if the other conditions are almost the same. A seed significantly higher than the

substrate will exhibit growth similar to a cluster. In this sense, the shape of the nodule

relates to the size of the seed and the thickness of the film.

The shadowing effect of a nodule produces an empty area around it during growth that causes a gap if addisks do not have enough migration to fill the area. These gaps, which

amount to 50 ^ to 100 ?i in corresponding simulations, depending on mobility, can be clearly seen around large nodules in the SEM micrographs of the film cross sections (Fig. 9). These gaps represent a significant break in a film's continuity. They also impede thermal conductance

away from the nodule if, under high-power laser irradiation, the nodule's absorption should cause local heating.

a = 50°, A = 3.7, extending factor = 1.25, rotation

G1642 G1637

Figure 10 Particles thrown up from the Figure 11 Nodule size is related to the

evaporant can act as the nucleus for size of the seed. The three adjacent nodules

nodules. The nodule seen here also in this picture are the result of varying

propagates up through a multilayer. sizes of the seed or agglomerated particles.

316
5. Conclusion -

We have shown that the growth of nodules in coatings is a physical process which depends on

the geometry of the coating process. The increasing diameter of the nodules with thickness is

due in part to the extension in capture length of the disks. Attractive forces between adatoms

are responsible for the extended capture length. Only hard disk models which include this

extended capture length will correctly predict nodule shape.

In our study nodule growths only occur at seeds which protrude above the surface of the

film. The seeds may take many forms, including dust, preferential nucleation sites, or spatter

from the vapor source. The nodules take two distinct shapes: those produced on rotating

substrates form a rounded bowl-like bottom, while nodules on stationary substrates form an inverted cone with straight sides.

As one might expect in the rotating substrate case, an increase in the vapor incidence angle

results in a large nodule. The self-shadowing effect in this case also generates a gap

separating the nodule from the surrounding film. This gap may be seen in both the simulations

and experiments and may be responsible for local heating within the nodule. The cone angle is also a function of adatom mobility and decreases as the adatoms become more mobile. Further work

will examine techniques to reduce the formation of nodules and modeling defects in different

deposition schemes.

Acknowl edgment

The work performed at Rochester was supported in part by the New York State Center for

Advanced Optical Technology of The Institute of Optics, the U.S. Department of Energy Office of

Inertial 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, Ontario Hydro, Southern California Edison Company, and the University of

Rochester.

References

1. T. Spalvins and W.A. Brainard, J. Vac. Sci . Technol., Vol . 11 , 1186 (1974).

2. K.H. Guenther, Appl . Opt., Vol . 23 , 3806 (1934).

3. J. Murphy, Contemporary Infra-Red Sensors and Instruments, Bellingham, WA, SPIE, 1980, p. 64 (SPIE Vol. 246 ). 4. K.H. Guenther, Appl. Opt., Vol. 20 , 1034 (1981).

5. J.W. Patten, Thin Solid Films, Vol . 63 , 121 (1979).

6. A.G. Dirk and H.J. Leamy, Thin Solid Films, Vol . 47 , 219 (1977).

7. K.L. Chopra, Thin Film Phenomena, Chapter 4, Robt. E. Krieger, Publ., Huntington, NY (1969).

317
8. Bangjun Liao and H.A. Macleod. Proceedings of the Southwest Conference on Optics of SPIE

Vol. 540 , 4-8 March 1985, #TA 118.

'

9. Bangjun Liao and H.A. Macleod. Thin-Film Microstructure Modeling, Proc. SPIE, Vol 540 150

(1985).

' '

10. H.J. Leamy, G.H. Gilmer, and A.G. Dirks, Current Topics in Material Science, North-Holland

Publishing Company, 6 (1980), p. 309.

11. P. Ramanlal and L.M. Sander, Physical Review Letters, Vol. 54 , 1828 (1985).

318
Manuscript Received

6-24-86

Interfacial Structure of Ion-Beam Sputter Deposited SiO^/TiO^ Coatings ... J. R. Sites and H. Demiryont

Physics Department, Colorado State University

Fort Collins, CO 80523

X-ray photoelectron spectroscopy (XPS) was used to evaluate the interfaces between

ion-beam sputter deposited Si02 and TiO^ layers, their free surface, and their interface

with a silicon substrate. XPS depth profiles of nominally abrupt Si02/TiO interfaces

showed mixing over an 7 nm range, but no valence states other than Si+^ and Ti+^ . Thus

the interfacial region consists of Si, Ti 0„ where x varies from 0 to 1 . The free

1 - X X Z surface of either SiO^ or TiO^ extends over 1.5 nm and shows evidence of OH bonds,

presumably due to absorbed water. Interfaces between SiO^, or Ti02, and an optically

smooth silicon substrate are about 3 nm wide and exhibit +2 and 0 bonding states for Si,

and +2 for Ti, states characteristic of oxygen deficient films. The TiO^/Si interface

also contains Ti-Si bonds, or partial silicide formation.

Key words: optical coatings; interfaces; ion beams; TiO^; SiO^; x-ray photoelectron

spectroscopy

.

1. Introduction

Multilayer Si02/Ti02 coatings have long been used to minimize or maximize reflection from

optical components. The optical characteristics of such coatings are determined by both the bulk

properties of the individual films and those of the interfaces between films. This report

examines such interfaces in ion-beam sputter deposited coatings using x-ray photoelectron

spectroscopy (XPS).

2. Ion-Beam Sputter Deposition

Silicon and titanium oxide films were deposited by ion-beam sputter deposition techniques,

which are described in more detail elsewhere [1-2]. A mixed argon-oxygen beam, 1100 eV in energy

and 1 mA/ cm^ in current density, is neutralized by an equal density of electrons. It strikes

either a titanium or a silicon oxide target, and sputters material onto a nearby substrate. The

two oxide targets can be switched in situ to produce layered coatings. They can also be partially

overlapped to produce Si^

x'^'''x^2

^^"'"'"^ with any value of x from 0 to 1

.

Figure 1 shows schematically the types of structures used for test purposes. The substrates

were polished silicon wafers intended for semiconductor fabrication. For the XPS studies,

thicknesses were deliberately kept small (10-20 nm/ layer) so that interfaces could be reached

without excessive etching. The two layer structures of la were used for interfacial measurements,

the single layers (lb) for bulk reference as well as for free surface and substrate interface

studies, and the composite films (Ic) for comparison with the interfacial region of la.

3. X-ray Photoelectron Spectroscopy

X-rays are used to expel core electrons from the material under study. As seen in figure 2a,

the x-ray energy used is 1254 eV, and the measured kinetic energy (KE) of the electrons is simply

1254 eV minus their original binding energy (BE). The escape depth of electrons varies with

energy but is typically 1-2 nm, meaning that the measurement technique is very surface sensitive.

Energy analysis of the electrons, plus the appropriate sensitivity factors, reveals the relative

concentrations of the constituent elements. On a finer energy scale, one can deduce the type and

number of neighboring atoms. In many cases, the splitting of peaks will reveal the relative

amounts of different bonding states [2-4] and the width of peaks, the distribution of bond

angles [ 5 ]

.

319
TiOa

SiOg

Silicon

SiOg

TiOg

Silicon (a)

SiO,

Silicon

TiO,

Silicon (b)

Si,., Ti,02

Silicon (c)

Figure 1. Schematic of test structures used.

1254 eV

Argon Ion Beam (1000 eV)

Figure 2. (a) Schematic drawing of the x-ray photoelectron process. (b) Depth profiling using an argon ion beam.

320
To measure changes in composition as a function of depth, the sample is etched with an argon

ion beam (figure 2b). To avoid atomic rearrangement induced by the ion beam, a relatively low

energy (1000 eV) is used, and the measured spectra are monitored for signs of peak broadening as a function of etch time. In practice the etching and the XPS measurement are alternated so that the

result is a stepwise profile as a function of depth. There is some concern about charging the

surface of insulating materials, and hence shifting the observed electron energies. In the

present work, however, the thin layers seem to allow a weak, but sufficient, electrical contact to the silicon substrate.

4. SiO^/TiO^ Interface

Figure 3 shows representative spectra taken at different points during an etch through the

TiO^-SiO„ structure of figure la. Within the nominally TiO„ top layer (3a), only the Ti 2p

Titanium Silicon Oxygen

I I I _j I : 1

465 460 455 105 100 535 530

Binding Energy (eV)

Figure 3. XPS response from (a) TiO^ top layer, (b) TiO^/SiO^ interface, and (c) SiO^ lower

layer

.

321
doublet and the 0 Is single line are seen. Furthermore, the titanium spectrum shows only the Ti

bonding state (four oxygen neighbors), with no evidence of Ti+2 or Ti^ as seen in non- stoichiometr ic titanium oxides [3].

In the interfacial region (figure 3b), the titanium response is reduced in magnitude, but

otherwise unchanged. The Si 2p line appears, and like the titanium, only shows the +A valance

state. The oxygen line increases in width and shifts to higher energies, as expected if some of

the titanium neighbors are replaced by silicon. The total area of the oxygen line is unchanged.

Quite likely, the oxygen peak is actually a superposition of three bonding energies: oxygen

bonded to two silicon atoms, oxygen bonded to one silicon and one titanivim, and oxygen bonded to two titaniums [2]. This spectrum, midway through the interface, is virtually identical to that

from a composite Si, Ti 0^ film (figure Ic) where x = 0.5. ^ 1-x X 2 °

Finally in the nominally SiO„ layer (figure 3c), the titanium line has disappeared, the

silicon line is larger, but still shows only one valence state, and the oxygen line has narrowed

again, its energy corresponding to silicon neighbors only. Figure A shows the normalized areas of

the titanixim, silicon and oxygen peaks as a function of depth into the coating. The depth

calibration is made by measuring the time to etch through single layers (figure lb) of known

thickness, deposited simultaneously with the double layers. The transition from pure Ti02 to pure

Si02 takes place over a distance of approximately 7 nm implying considerable mixing of the sputter

deposited constituents.

5. Free Surface and Substrate Interfaces

The XPS information for the free surfaces of Si02 and Ti02, and their interfaces with the

silicon substrate are summarized in figure 5. For both oxides, the free surface shows (5a) a high

energy shoulder on the oxygen line due to (OH) from absorbed water vapor. This shoulder quickly

disappears when the depth profile is begun, the distance being about 1.5 nm. In the bulk of the

single layers the spectra are identical to those of the double layers discussed above. At the

substrate interface, however, the oxygen line is broadened. In the TiO„/Si case the broadening is

due once again to oxygen having both titanium and silicon neighbors; for SiO^/Si the bond angles

have presumably become more distorted.

At the SiO„/Si interface (figure 5b), the silicon line is split into the three valence states

seen in non-stoichiometr ic silicon. Farther into the substrate, the Si*-* peak becomes dominant.

Width of the transition is approximately 3 nm. At the Ti02/Si interface, there are two features

to note. First (figure 5c), the titanium oxide becomes oxygen deficient, and the characteristic

+2 sub-oxide valence peak is seen, possibly meaning that the stronger tendency of oxygen to bond

with silicon is drawing it away from the titanium. However, no evidence of metallic titanium

(TiO) or of silicon oxides is seen. Instead the residual titanium forms a Ti-Si bond, or a silicide structure then diminishes to zero, again over a 3 nm distance.

322
o

c

o> CO

(/) X

Oxygen

(a)

Si02 Surface TiOg Surface

Silicon in SiOg/Si

Interfoce

105

Si02 Bulk TiOs Bulk

Si0 2 /Si InterfaceTiOg/Si

Interface

100

Titanium in TiOg/Si

Interface

(b)

95

(c)

535 530 470

Binding Energy (eV)

460 450

Figure 5. (a) Oxygen XPS response at free surface, bulk and substrate interface for Si02 and Ti02

single layers. (b) Silicon XPS spectrum at SiO^/Si interface. (c) Titanium XPS

spectra just above and just below TiO^/Si interface.

6. Conclusions

extended over approximately

7 nm, indication of sub-oxide bonding in either the oxide layers or in the interface between them. The

free surface, however, did

silicide formation.

The interfaces between layers of ion-beam sputtered SiO^ and TiO^

the free surface over 1.5 nm, and the silicon substrate interface over 3 nm. There was no

Ln e: show substantial water vapor, and the TiO^/Si interface evidence of

We gratefully acknowledge assistance with the XPS apparatus and from Kent Geib. This work

was supported by the Air Force Weapons Laboratory under Contract F29601-83-K-0079.

323
References

Sites, J.R. ; Gilstrap, P.; Rujkorakarn, R. Ion beam sputter deposition of optical coatings.

Optical Engr. 22(A): 4A7-AA9; 1983 July-August.

Rossnagel, S.M.; Sites, J. R. X-ray photoelectron spectroscopy of ion-beam sputter deposited

SiO^, TiO^, and Ta^O^. J. Vac. Sci. Technol. A2(2): 376-379; 1984 April-June.

Demiryont, H. ; Sites, J.R. Effects of oxygen in ion-beam sputter deposition of titanium

oxides. J. Vac. Sci. Technol. A2(4): 1A57-1A60; 1984 October-December.

Demiryont, H.; Sites, J.R.; Geib, K. Effects of oxygen content on the optical properties of

tantaliiim oxide films deposited by ion-beam sputtering. Appl. Optics 24(4): 490-495; 1985

15 February.

Nucho, R.N.; Madhukar, A. in The Physics of SiO^ and Its Interfaces, edited by S.T. Pantelides. New York: Pergamon; 1978. 60-75.

324
Manuscript received

Ion Beam Deposited Oxide Coatings 4-18-86 SAB

T. Raj, J.S. Price, and C.K. Carniglia

Martin Marietta Aerospace, Laser Technology Systems

Kirtland Air Force Base, Albuquerque, NM 87119

Single-layer coatings of Si02, AI2O3 and Zr02 were deposited by ion beam

deposition (IBD). IBO is carried out by directing an ion source at a target made of the desired coating material. The material sputtered from the target is directed

toward the substrate being coated. The targets chosen for this work were commercially

available hot-pressed oxides. To further modify the coating properties, a secondary

ion gun was directed toward the substrate to assist the deposition. Thus, ion assisted

deposition (IAD) can be combined with IBD. The following coating deposition parameters

were varied: composition of the sputter deposition gas mixture, composition of the ion

assist gas mixture, and beam voltage and beam current for the ion assist beam. No external heat was applied to the substrates.

Single-layer Zr02 coatings were also deposited from a Zr metal target by bombarding this target with an 02:Ar ion beam (Reactive Ion Beam Deposition [RIBD]).

These depositions were performed with ion assistance (RIBD/IAD) and without. However,

filament life of the ion guns was found to be a problem with these coatings because of

the presence of oxygen in the ion beam.

The refractive indices for all the coatings were determined by ellipsometry at 633 nm. Representative coatings were analyzed by Auger Electron Spectroscopy (AES) to determine stoichiometry and to provide information on the nature and level of impurities. The RMS surface roughness was determined by TIS (Total Integrated Scatter)

measurements. In addition, absorption was determined for a few selected coatings from

spectral transmission measurements. The influences of ion beam deposition parameters

and ion assist-beam parameters on thin film properties of Si02, AI2O3 and Zr02

coatings are discussed.

High reflector coatings at 351 nm were deposited on super-polished substrates

utilizing the optimum deposition parameters obtained from the single-layer studies.

The reflectors were made from Zr02 and Si02 targets. A peak reflectance greater

than 99% was achieved.

Key Words: A1203; ion-beam deposition; ion-beam-assisted deposition; Si02; thin film coatings;

The process of ion beam deposition (IBD) of optical coatings involves directing an ion source

at a target made of the desired coating material. The material sputtered from the target is directed toward the substrate being coated. Coatings made using IBD are generally denser than

those made using standard evaporation techniques.

In recent research on thin film deposition processes, substantial evidence has been collected

that the film growth and the film properties may be significantly influenced if energy and

momentum are supplied to the growing films by additional impinging ionized and accelerated

species [1-3].^ The process of direct impingement of the growing film by ions from an ion beam

is termed ion assisted deposition (IAD). This technique has been successfully applied in modifying the film properties of evaporated optical coatings (especially dielectric oxides) by a number

of researchers [4-9]. Ion assisted coatings are generally denser, less porous and have higher

refractive indices than similar films made without IAD. However, the effect of ion assistance on IBD coatings has not been studied in detail. This effect is equivalent to the effect of biassputtering on a conventional sputter deposited coating. The technique of using two ion beams

(one for deposition and one for ion-assitance) is termed dual-ion beam deposition by several

groups [3]. A survey of published literature shows that a group of researchers at Colorado State

*This work was sponsored by the Air Force Weapons Laboratory, Air Force Systems Command, United

States Air Force, Kirtland AFB, New Mexico 87117.

^Numbers in brackets indicate the literature references at the end of the paper.

INTRODUCTION

325
University has used the dual ion beam technique to deposit Si02, Ta205 and Ti02 coatings

[10-14].

In the present study, single-layer coatings of AI2O3, Si02 and Zr02 were deposited by

IBD. The targets chosen for this work were commercially available hot-pressed oxides. The ion

assistance was performed with a second ion gun. The following coating deposition parameters were

varied: composition of the sputter depostion gas mixture, composition of the ion assist gas mixture, and beam voltage and beam current for the ion assist beam. No external heat was applied

to the substrates.

Single-layer Zr02 coatings were also deposited from a Zr metal target by bombarding this

target with an 02:Ar ion beam (Reactive Ion Beam Deposition [RIBD]). These depostions were performed with ion assistance (RIBD/IAD) and without. However, filament life of the ion guns was found to be a problem with these coatings because of the presence of oxygen in the ion beam.

The refractive indices and thicknesses for all the coatings were determined by ellipsometry

at 633 nm with a Gaertner Null El 1 ipsometer. The precision for the ellipsometry was about

1 percent.

Representative coatings were analyzed by Auger Electron Spectroscopy (AES) to determine

stoichiometry and to provide information on the nature and level of impurities. The Auger

spectrometer used was Physical Electronics' Model 590. The detection limits for AES are 0.1 atomic percent for metalic contamination and 0.5-1 atomic percent for carbon contamination.

The RMS surface roughness was determined by TIS (Total Integrated Scatter) measurements. The

spatial frequency bandwidth of the TIS measurements is 0.5 ym to 25 ym. The lower surface

roughness detection limit of the TIS measurements is about 5A.

The ellipsometry. Auger analysis and TIS measurements were all performed on films deposited

on Si wafers. In addition, absorption was determined for a few selected coatings on UV grade

fused silica substrate from spectral transmission measurements performed with a Varian 2300

spectrophotometer. The precision is about 1% for the refractive index n and 4 x 10"^ for the

absorption coefficient k.

SINGLE-LAYER COATINGS

One ion gun (deposition gun) was directed at a target at an incident angle of 45 to 60

degrees. The ion beam parameters for the deposition gun were as follows: ion beam voltage

(Vt)) = 1000 eV, ion beam current (i^) = 50 ma, and sputtering gas pressure =; 2 x 10"4

torr. A second ion gun (assist gun) was directed at the substrate stage at an incident angle of 60°-70°. The beam voltage for the ion assist gun was in the range of 100 to 200 eV.

The sputtering gas in the deposition gun for an IBD coating deposited from an oxide target

was either 100% Ar or a mixture of Ar:02. Only 100% Ar was used as a sputtering gas in the

deposition gun for ion assisted IBD coatings. The discharge gas mixture, Ar:02, for the ion

assist gun had one of the following compositions: 100:0, 70:30, 30:70 or 0:100.

AI2O3 Coatings

Single layers of AI2O3 were deposited on 5.0 cm semiconductor grade Si wafers by assisted

as well as unassisted ion beam deposition from a hot pressed A12O3 target. The thickness of the films was in the range of 50 to 90 nm. The RMS surface roughness, as measured by the TIS

technique, for assisted and unassisted AI2O3 coatings, was less than 5 A.

Assistance with a 100 eV/5 ma ion beam (with pure Ar or 10% O2 in Ar) lowers the refractive

index of AI2O3 films. The effect of increased beam current is rather insignificant. Assistance with a 300 eV/5 ma ion beam (with pure Ar or 10% O2 in Ar) , lowers the refractive index

further. This lowering of the refractive index (away from that of bulk AI2O3) could be

attributed to the impregnation of the assist gas in the coating. Changing the composition of the

deposition gun gas (from pure Ar to 70% O2 in Ar) produces no significant change in the

refractive index of the unassisted AI2O3 coatings.

A 100 eV/5 ma assist beam using 70% 02/30% Ar produces no significant effect on the

refractive index of AI2O3. However, increasing the beam voltage of the assist beam from

100 eV to 300 eV produces a decrease in the refractive index of the AI2O3 coatings.

326
One unassisted (IBD) A1203 coating and one assisted (300 eV, 5 ma, 90% Ar/10% O2)

AI2O3 coating were analyzed by AES. Important observations are as follows:

1) Auger and ESCA analyses indicate that the metallic contamination levels in these coatings

were below the detection limits of these techniques.

2) These sputter-deposited AI2O3 coatings are very stable when exposed to an Auger E-beam.

Oxygen-deficient AI2O3 could not be detected at the surface or inside of these coatings.

3) A significant amount of Ar was detected at the surface of the assisted coating, although it was below the detection limit in unassisted coating. The presence of Ar in the assisted coating

could be attributed to the gas embedded by the 300 eV ion assist beam.

AES analysis of these two AI2O3 coatings is illustrated with the help of the following

figures. Figures 1 and 2 indicate AES surveys taken at the surfaces of the unassisted and the

assisted coating, respectively. Notice the presence of the Ar Auger peak (212 eV) in the surface

spectrum of the assisted coating while it is at noise level in the spectrum for the unassisted

coating. Since these spectra were taken without any sputter cleaning, any surface contaminants

like carbon should be ignored. Figures 3 and 4 indicate the Auger surveys taken after sputter

etching with 2 keV Ar ions about 10 nm of the coatings. These surveys show that carbon and

metallic contaminations inside the unassisted, as well as assisted coatings, are below the

detection limit of AES.

t

N r*CT0(MS.9 3

r^-

i

L- Al (/ilO,)

: 1—

.J : 0

)

in w iM m m iiM im 104 its tm

incite Doci, n Figure 1. AES survey taken at the surface

of unassisted AI2O3 coating.

EIPANSldN rACTOR'19.33

i

A.l_<*.iPx}

IN m tM rn M« iin i3w im im im

IIICTIC CMKT. a

Figure 2. AES survey taken at the surface

of ion beam assisted AI2O3.

Figure 3. AES survey taken after sputter

etching 10 nm of unassisted coating.

lu m M

IIICTIC pon, n Figure 4. AES survey taken after sputter

etching of 10 nm of ion assisted coating.

327
Si02 Coatings

The deposition procedure for Si02 coatings was the same as for AI2O3 coatings. The

coating thicknesses were in the range of 40 nm to 90 nm. The RMS surface roughness, as measured

by the TIS technique, for assisted and unassisted Si02 coatings was low (less than 5 A).

The effects of ion assist beam voltage and beam current on the refractive index of Si02

coatings are as follows:

1) The refractive index of unassisted Si02 coatings is higher than the refractive index for

bulk Si02 by about 0.02.

2) Ion assistance with a 100 eV beam lowers the index value to 1.475. This decrease in the

refractive index is independent of the beam current at 100 eV.

3) An increase in assist beam voltage from 100 to 300 eV does not decrease the refractive index

any further.

AES analysis of one unassisted (IBD) and one assisted (100 eV, 10 ma, 100%) deposition

provided the following information:

The Si02 coatings are very clean; the contamination level is below the detection limit of AES. These coatings are not as stable as the AI2O3 coatings when exposed to the Auger

E-beam. In particular some oxygen can be desorbed leading to oxygen deficient Si02.

In summary, the Si02 coatings deposited by IBD are clean. However, they are probably non- stoichiometric (oxygen deficient), and their refractive index is higher than that for bulk Si02.

Zr02 Coatings from an Oxide Target

The deposition procedure for the Zr02 coatings was similar to that of the AI2O3 coatings. The coating thicknesses were in the range of 50 nm to 100 nm. The surface roughness of

Zr02 coatings on Si wafers, as measured by the TIS technique was less than 5.3 A. This is significantly lower than that of Zr02 coatings deposited by E-beam evaporation.

The effect of ion assistance on the refractive index at 633 nm of Zr02 coatings deposited

from a Zr02 target was found to be as follows:

1) Unassisted IBD Zr02 made from a Zr02 target with 100% Ar gas has a refractive index which

approaches that of the Zr02 bulk value (2.15).

2) Unassisted IBD Zr02 made from a Zr02 target with 10% O2 in Ar gas has a refractive

index of 2.11

.

3) The effect of 100 eV/2 ma ion assistance on the refractive index of IBD Zr02 is insignificant.

4) As the beam current of the 100 eV ion assist beam is increased from 2 ma to 5 ma, there is a slight decrease in the refractive index of IBD Zr02 (2.12).

In summary, ion beam deposited Zr02 from a Zr02 target without assist or with a small

assist can provide Zr02 coatings with a refractive index approaching the Zr02 bulk value of 2.15.

A typical AES analysis of the IBD Zr02 coatings obtained from a Zr02 target provided the

following information:

1) Argon was detected in assisted as well as unassited Zr02 coatings. Though it is not possible to quantify by AES the amount of Ar in these coatings, it can be stated that the amount

of Ar trapped in IBD Zr02 coatings is significantly larger than that trapped in IBD AI2O3

or Si02 coatings.

2) Carbon was detected below the surface of the Zr02 coatings. The amount of carbon was in the range of 2-4 At %. Since Zr is a very reactive element, the presence of carbon could imply

nonstoichiometric Zr02 coatings.

328
Refractive Index and Absorption Coefficient for ZrO g Coatings

The values for the refractive indices and absorption coefficients of Zr02 at 351 nm were determined using thick single-layer depositions and the following methods:

The transmission of a thick Zr02 single-layer coating was measured with a Varian 2300 dual

beam spectrophotometer in the wavelength range of 200 to 2000 nm. The index values in Figure 5 were obtained by using the value Uq = 2.13 at 633 nm, measured previously by el 1 i psometry . The

indices at the wavelength x^i of "the turning points (maxima and minima) were then determined

using

n(Xni) = no"'W("'o^o)

where m is the order number (number of quarter-waves) at X^i- "(Nn) the index at that

wavelength, and mg and X^q ^""^ "the order number and wavelength of the turning point nearest

to 632.8 nm.

n

250

240

2.30

220

2J0

200

Dispersion

Zr02

500

Wavelength (nm)

1000

Figure 5. Dispersion curve for an IBD Zr02 coating.

The absorption values were also derived from the spectrophotometric scans. The absorption at the wavelength where m is an even number, (transmission maximum) was determined using

k(Xn,) = n(Xm)A(\n,)/(4irm)

where m is the order number (number of quarter-waves) at X^i' "\m the index of coating

material at that wavelength and A(\g,) is the absorption at that wavelength attributed to the coating. The error in determining k by this method is ± 4 x lO"'^.

329
Thick Zr02 coatings were deposited from a Zr02 target on 2.5 cm fused silica substrates

using three different deposition techniques: IBD, IBD/IAD and IBD with O2 backfill. Table 1 lists the deposition parameters along with the refractive indices and absorption values for these

coatings at 351 nm. The O2 backfill pressure (column 3) is the partial pressure of O2 in the

chamber, regardless of whether O2 was introduced into the chamber through the ion gun or directly into the chamber. For run number IC 445, 10% O2 sputter gas was introduced through

the deposition gun to achieve a chamber pressure of 2 x 10~^ torr. This corresponds to an O2 partial pressure of 2 x 10""^ torr. In the case of IC 446, pure O2 was introduced through the

etch gun to achieve a chamber pressure of 6 x 10"^ torr. Pure O2 was introduced directly

into the chamber for IC 447 and IC 450 to achieve the respective partial pressures of 2 x 10~5

torr and 6 x 10~^ torr.

Table 1. Effect of O2 Partial Pressure on Absorption of IBD Zr02 Coatings.

Run

No. Depo Gas (02:Ar)

Backfill

Gas (100% 0.)

k (9 335nm

(10"^

Comments

IC 445 10: 90 2x10"^ torr 34 Oxygen introduced through the

deposition gun.

IC 446 0:100

-5 6x10 torr 42 Oxygen introduced through the

ion assist gun.

IC 447 0:100 2x10"^ torr 34 Oxygen introduced directly

into the chamber.

IC 450 0:100

-5 6x10 torr 42 Oxygen introduced through the

ion assist gun.

The values in this table clearly show that the value of k = 0.0034 obtained using IBD with an

O2 partial pressure of 2 x 10^5 -^orr was the lowest absorption obtained. The absorption does not depend on the method of introducing O2, but only on the partial pressure of O2.

Zr0 2 Coating from a Zr Metal Target

Single-layer Zr02 coatings were also deposited from a Zr metal target by bombarding this

target with an 02:Ar ion beam [Reactive Ion Beam Deposition (RIBD)]. These depositions were performed with ion assistance (RIBD/IAD) and without. The position of the Zr metal target with

respect to the deposition gun and the deposition ion beam parameters was similar to the one used

for oxide targets, the only exception being that the discharge gas was varied from 70% to 20%

O2 mixed with Ar.

There were a number of experimental problems with the reactive sputter deposition technique.

Ihe lifetime of the cathode filaments varied from 15 to 38 min with 70% O2 discharge gas in the

deposition gun. Consequently, some Zr02 coatings turned out to be so thin that el lipsometric

measurements of the refractive index were not very accurate. Though the average lifetime of the

cathode filament was longer with 50% O2 discharge gas than with with 70% O2, the cathode

filament appeared to be thin, and was therefore replaced at the end of each run. In addition, a yellow deposit appeared on the cathode supports near the filament connections as well as on the

inside surface of the grids. This yellow deposit was determined (by AES) to be tungsten oxide.

Ihe cathode supports were cleaned before each run. Due to the fragility of the pyrolytic graphite grids and the critical nature of the grid/pair alignment, no attempt was made to clean the

grids. These problems were reduced by using 30% and 20% O2 discharge gas, but they still

remained significant when compared to those associated with the use of pure Ar discharge gas.

A study of the refractive indices for unassisted Zr02 coatings from a Zr metal target

plotted as a function of the deposition gas mixture produced the following conclusions:

1) The index is independent of the O2 concentration in the deposition gas from 20% to 40% O2.

2) The index decreases as the O2 concentration increases above 40%.

The effect of ion assistance on Zr02 coatings obtained from a Zr metal target is summarized

below:

330
1) The refractive index increases slightly by ion assistance with a AR:02 mixture.

2) The increase in index is larger when the ion assistance is performed with pure O2 or a high

O2 content Ar:02 mixture. This implies that ion assistance with O2 improves the

stoichiometry of the unassisted Zr02 coatings.

HIGH REFLECTOR COATINGS AT 351 nm

A 13 layer Zr02/Si02 quarter wave reflector coating at 351 nm was deposited on superpolished substrates. No external heat was applied to the substrates. The quarter waves were monitored by a reflection monitor (used at 45°) at 320 nm. Each quarter wave layer was either

deposited from the Zr02 or the Si02 target using an ion source output of 1000 eV beam voltage

and 50 ma beam current. One hundred percent Ar was used in the ion sources. No ion assistance

was performed, but an O2 backfill pressure of 2 x 10"^ torr was used. The reflectivity of these coatings was measured to be greater than 99% at 351 nm.

SUMMARY AND CONCLUSIONS

1 ) AI 2 Q3 Coatings

a. IBD produces AI2O3 coatings of a high index (1.65 at 632.8 nm)

.

b. These coatings are very clean (free of carbon and metallic contamination).

c. They are stoichiometric (absence of Al inside the coating).

d. The amount of Ar trapped in the IBD coating is low.

e. The effect of an ion assist beam (IAD) is to lower the index of IBD AI2O3.

f. The amount of Ar trapped within an Ar ion beam assisted AI2O3 coating is significantly higher than the amount of Ar in an IBD coating. This implies that the ion assist beam

embeds Ar into the coating.

g. The RMS surface roughnesses for IBD AI2O3 coatings deposited with and without IAD are low.

IBD AI2O3 coatings are the most promising oxide coatings resulting from the IC program.

2) Si02 Coatings

a. IBD produces Si02 coatings of an unusually high index. This could be due to a density

higher in IBD Si02 coatings than in the bulk Si02 or due to a lack of stoichiometry.

b. These coatings are very clean.

c. They could be slightly oxygen-deficient.

d. The effect of an ion assist beam is to lower the index and to increase the amount of trapped sputter gas in the coating.

e. The RMS surface roughness of IBD Si02 coatings deposited with and without IAD is low.

3) Zr02 Coatings

a. IBD produces Zr02 coatings having index values of almost bulk index value.

b. These IBD coatings contain significantly larger amounts of trapped Ar gas than do IBD Si02 and AI2O3 coatings.

c. These coatings are non-stoichiometric

.

d. The absorption of IBD Zr02 coatings at 351 nm is significant.

331
e. The effect of an O2 ion assistance at 100 eV is to lower the index slightly and to reduce the absorption.

f. The lowest absorption (k = 0.0034 at 335 nm) is obtained using IBD with an O2 backfill

pressure of 2 x 10"^ torr. This low absorption does not depend on the method of introducing

O2 (i.e., whether it is introduced into the sputter gun or directly into the chamber).

g. The surface roughness of IBD Zr02 with or without IAD is low - much lower than the

roughness for E-beam evaporated Zr02 coatings.

4 . Reactive Ion Beam Deposition of Zr0 2 for Ar Targets

a. This technique did not provide any particular advantage over IBD using a Zr02 target.

b. The highest index (2.13) for Zr02 is obtained by RIBD from a Zr metal target with a mixture of 40% O2 in Ar.

c. For higher fractions of oxygen in the assist gun, the index increased slightly.

REFERENCES

[I] Gautherin, G.; Weissmantel, C, "Some Trends in Preparing Film Structures by Ion Beam

Methods", Thin solid Films, 50, 135 (1978).

[2] Weissmantel, C, "Ion Beam Deposition of Special Film Structures", J. Vac. Sci. Tech. 18

(2), 179 (1981).

[3] Harper, J.M.E., "Thin Film Processes", Vossen, J.L. and Kern, W. (Eds), Academic Press, New

York, 1978, Chapter II-5.

[4] Martin, P.J.; Macleod, H.A.; Netterfield, R.P.; Pacey, C.G.; Sainty, W.G., "Ion Beam

Assisted Deposition of Thin Films", Appl. Opt. 22, 178 (1983).

[5] Martin, P.J.; Netterfield, R.P.; Pacey, C.G.; Sainty, W.6., Clark, 6. J.; Lanford, W.A.;

Sie, S.H., "Ion Assisted Deposition of Bulklike Zr02 Films", Appl. Phys. Lett. 43, 711 (1983) .

[6] Sainty, W.G.; Netterfield, R.P.; Martin, P.J., "Protective Dielectric Coatings Produced by

Ion-Assisted Deposition", Appl. Opt. 23, 1116 (1984).

[7] Martin, P.J.; Netterfield, R.P.; Sainty, W.G., "Modifications of the Optical and Structural

Properties of Dielectric Zr02 Films by Ion Assisted Deposition", J. Appl. Phys. 55, 235

(1984) .

[8] McNeil, J.R.; Barron, A.C.; Wilson, S.R.; Hermann, W.C. Jr., "lonAssisted Deposition of Optical Thin Films: Low Ion Energy vs High Ion Energy", Appl. Opt. 23, 552 (1984).

[9] McNeil, J.R.; Al-Jumaily, G.A.; Jungling, K.C.; Barron, A.C., "Properties of Ti02 and

Si02 Thin Films Deposited using Ion-Assisted Deposition", Appl. Opt. 24, 486 (1985).

[10] Demiryont, H.; Kerwin, D.B.; Sites, J.R., "Optical Properties of Ion Beam Sputtered Ti02

Films", Nat. Bur. Stand. (U.S.) Spec. Publ. 688, 311 (1983).

[II] Sites, J.R.; Gilstrap, P.; Rujkorakarn, "Ion Beam Sputter Deposition of Optical Coatings",

Opt. Engr. 22, 449 (1983).

[12] Rossnagel, S.H.; Sites, J.R., "X-ray Photo-electron Spectroscopy of Ion Beam Sputter

Deposited Si02, Ti02 and Ta205", J. Vac. Sci. Tech. A2(2) . 376 (1984).

[13] Demiryont, H.; Sites, J.R., "Effects of Oxygen in Ion Beam Sputter Deposition of Titanium

Oxide", J. Vac. Sci. Technol . A2(4) . 1457 (1984).

[14] Sites, J.R.; Demiryont, H.; Kerwin, D.B., "Ion Beam Sputter Deposition of Oxide Films", J. Vac. Sci. Tech. A3(3) . 656 (1985).

332
Manuscript Received

Properties of IAD KV^^/S,iQ^ and Ta20^/Si02 Thin Film AR Structures 10-17-86

J.J. McNally

Department of Physics

USAFA/DFP

United States Air Force Acadanriy, CO 80840-5701

and

F.L. Williams, S.R. Wilson and J.R. McNeil

University of New Mexico

Albuquerque, NM 87131

We have investigated the properties of Al^O^/SiO- and Ta20^/Si02 anti-reflection

coatings which have been fabricated using ion-assisted deposition flAD) techniques.

The coatings were deposited with simultaneous 0~ bombardment during film growth.

Preliminary laser damage results are presented. We describe the importance of

oxygen-ion energy and flux in applying IAD to dielectric materials. Results

illustrating increased values of film refractive index, reduction in optical scatter

and improved environmental stability are reported.

Key Words: coatings; laser-damage; oxygen-ion assisted deposition.

1. INTRODUCTION

A number of novel deposition techniques have been applied in attempts to improve the

properties of thin films. In these techniques, energetic processes are employed to supply

sufficient activation energies to increase adatom mobility and eliminate the formation of

columnar microstructure. One of these techniques is ion-assisted deposition (IAD). Ion-assisted

deposition employs a separate ion source to direct a beam of ions at the growing film during

deposition. Thin films deposited using IAD have exhibited increased packing density, improved

1—8 stability and durability, modifications in stress and improved stoichiometry. Recently, IAD

has been applied in the production of optical coatings. Allen applied IAD to obtain increased

4 values of refractive index in TiO„ films. Martin et. al. have studied the effects on the

7 9 properties of ZrO_ and CeO- films bombarded with 0„ and Ar ions during deposition. ' McNeil et.

6 8 z z ^

z. al. ' examined the effects of 30 and 500 eV O2 bombardment on the properties of Ti02 and Si02

films deposited using TiO and SiO starting materials. Al-Jumaily et. al. examined the effects of

10 ion energy and flux on optical scatter and crystalline phase m Cu, T1O2 and S1O2 films.

In this study Kaufman ion sources were used; they are attractive because it is possible to independently control and accurately measure the important experimental parameters involved. The

ion beam energy, current density and ion angle of incidence can be varied over a wide range of

values independent of the deposition process. The purpose of this work is to systematically

investigate the effects of IAD parameters on the properties of Ta20^ and Al^^ thin films. The

work is divided into two phases: an initial single-layer parameter optimization study, followed

by a multi-layer laser damage study. The optical constants, optical scatter and environmental

stability of single-layer coatings are reported. We also report preliminary results frcsn laser

damage tests on Al202/Si02 anti-reflection coatings.

333
2. EXPERIMENTAL ARRANGEMENT

The coatings were deposited in a 90-an diameter, cryogenic-pumped bell jar vacuum syston. A schematic of the deposition arrangonent is illustrated in Figure 1. The source-to-substrate

separation was 35 cm. The 'ra20^ and ^12^3 costings were electron-beam evaporated at a rate of

—1 . . —4

0.30 - 0.40 nm sec with oxygen backfill pressure of 1.0 x 10 Torr. The coatings were deposited onto heated substrates (275°C) , and were banbarded with oxygen ions during deposition.

A 10-cm Kaufman ion source provided a monoenergetic, neutralized ion beam independent of the

material evaporation process. This source has a number of advantages over conventional plasma

configurations. These advantages are derived from the basic operating principle of the ion source. This principle is the isolation of ion production and acceleration from the substrates.

Briefly, the ion source operates as follows. Gas is introduced into the discharge chamber;

electrons emitted from the cathode impact-ionize the gas molecules. The discharge-chamber

contains a conducting plasma canposed of approximately equal numbers of ions and electrons. The

plasma potential is essentially equal to the anode potential, therefore the ions originate at

approximately the anode potential. The screen grid aligns the ions that are accelerated from the source by the electric field established in the region between the screen and accelerator grids.

The ions travel from the source to the substrates which are held at ground potential. The source provides a low-energy (300-1000 eV) beam of Ar or 0^ ions incident on the film surface at an angle of 45°. A Faraday probe was used to monitor the ion current density at the substrates.

Prior to deposition the substrates were cleaned with 300 eV Ar^.

The coatings produced in this study were deposited onto a number of different substrates.

One inch diameter, 1/4" thick UV-grade fused silica substrates were used for the single-layer

coatings. The substrates used for the laser damage coatings were provided by Los Alamos National

Laboratory (LANL) and were 2" dia., 3/8" thick fused silica. The substrates were cleaned as follows. Lint and dust particles were removed using dry nitrogen gas. A horse hair brush was

used in a circular motion to scrub the substrate surface with a 50-50 mixture of Liquinox soap

and deionized water (ISilM -cm). The substrate was rinsed with de-ionized water (DI) , and the

scrubbing/rinsing process repeated. The substrate was dried with nitrogen. Kodak lens tissue

with ethyl alcohol (200 proof) was used to drag wipe each substrate surface. Two inch diameter

polished silicon wafers were coated for optical scatter measurements. The wafers were cleaned

using a standard semiconductor industry technique as follows. The wafers were rinsed in DI and

degreased in a 3:2 solution of H2SO^/H202 for 10 minutes. A DI rinse was repeated, followed by a 10 minute etch in a 5% HF solution. After a final DI rinse the wafers were blown-dry with

nitrogen.

The coatings transmittance and reflectance spectra were measured over the wavelength range

0.185 - 1.2ym using a dual-beam spectrophotometer. The transmittance and reflectance spectra were used to calculate the refractive index (n) , the extinction coefficient (k) and the thickness

(t) for each coating. They were also used to determine the possible presence of index

inhomogeneity in the coatings. A technique due to Manifacier, et. al. was used to determine

the values of n, k and t. The optical scatter characteristics of the coatings were examined

using an angle-resolved scatterometer which is described in detail in Reference 10. When

evaluating the scatter characteristics of a dielectric coating it is important to know the

334
optical thickness of the film. For a film which is an integral number of half-waves in optical

thickness, scatter is primarily due to volume effects in the film. The films were deposited onto

polished silicon wafers and were multiple half-wave optical thickness at 633 nm. A He-Ne laser

illuminated the coated Si wafers; scattered light was collected by a photo-multiplier detector.

The detector rotated about the scatter point on the sample in the plane of incidence at a radius

of 40 cm. Humidity cycle tests were conducted in a Vapor-tsnp controlled humidity chamber to examine

film stability properties. The spectral characteristics of conventional coatings are generally

unstable, primarily due to porosity and their tendency to adsorb moisture. The coatings were

exposed to 97% relative humidity at 35°C for extended periods of time. The film transmittance

spectra were examined before and after the humidity exposure to determine the amount of spectral

shift.

A number of anti-ref lecting (AR) coatings were laser damage tested at Los Alamos National

Eiaboratory by Stephoi Foltyn and L. John Jolin. The damage thresholds were measured at 351 nm with 9 nsec pulses. The pulse repetition frequency was 35 pps. The laser beam was focused to

2 give a beam with a mean spot diameter (1/e ) of 0.46 mm at the sample surface. All the testing was n-on-m, where m sites were irradiated at each fluence for a maximum of n pulses. Ten

discrete sites were irradiated at each test fluence, and each site was exposed to the laser for 4 sec (140 pulses) . Damage was defined as any visible change in the coating observed under 60X

microscope examination (microscopic damage) . The coatings were tested at several fluence levels

between the value at which 0 of 10 sites damaged and the value at which 10 of 10 sites damaged.

_2 The fraction of damage sites was plotted as a function of laser fluence, m units of Joules cm A linear regression fit was performed on the data resulting in a sloped, fitted line from a fluence at which all ten sites damaged to a lower fluence at which no damage occurred. The LIDT

value was defined as the zero-percent crossing of the fitted line (zero-probability intercept) . Note that this is different from another conventional definition of LIDT value which is the

average value of the minimum fluence that always causes damage and the maximum fluence at which

no damage occurs. The AR coatings in this study were deposited onto Corning 7940 fused silica substrates with

_2 bare surface LIDT values of approximately 8 J cm . The coating design was SLLHL where S was the

substrate, L was quarter-wave thick SiO^ and H was quarter-wave thick ^120^ or Ta20^.

3. RESULTS

The transmittance spectra for two

'^^2''^s given in Figure 2. The curve labeled

J=0 is for a coating deposited with no ion bombardment; the curve labeled J=5 is for a coating

bombarded during deposition with 500 eV 0^ at a current density of 5 uA cm . The curve for the

ion assisted coating contains larger differences in trananittance extrema than the curve for the

unbombarded coating. These larger differences indicated a larger value of refractive index for

the ion assisted coating. Good film stoichiometry was obtained for the ion bombardment

conditions employed as indicated by an absence of measurable absorption at wavelengths for which

the film is multiple half-wave in optical thickness down to 340 nm. Examination of the

reflectance spectra for these coatings indicated the absence of any refractive index

inhomogenei ty

.

335
The refractive indices of Ta20^ films deposited at 0.3 nm sec"''' are plotted in Figure 3 as a function of oxygen- ion current density. The values increase from 2.16 for the unborabarded case

(J=0) to 2.25 and 2.28 for films bombarded with 500 and 300 eV 0_^, respectively. The increase

+ in the values of n with increasing 0^ current density indicated that ion bombardment during

deposition modified the growth of film columnar microstructure resulting in film densif ication.

Observe for the 500 eV case that the film index values decreased for ion current densities larger

than the value at which the maximum index occurred. The decrease in index may be explained as a result of degradation in film stoichiometry , creation of closed isolated voids or oxygen

incorporation into the films. A similar dependence of refractive index on bombarding ion flux

7 9 has been reported for ion assisted "Z-r^O^ and Ce02 films. Work is on-going at 300 eV and 200 eV

to extend the behavior illustrated in Figure 3. The coatings banbarded during deposition at oxygen ion current densities up to approximately

the value for maximum index exhibited good optical characteristics. For higher levels of

bombardment, the optical absorption of the coatings increased. In Figure 4, values of extinction

coefficient (k) for Ta20^ coatings bombarded with 500 eV 0^ during deposition are shown for

different levels of ion current density. The values of k were calculated at X = 400 nm. As illustrated in Figure 4, film optical absorption increased with higher levels of oxygen ion

bombardment. The most probable mechanism for this is the preferential sputtering of oxygen in

the Ta20^ molecule. Preferential sputtering would result in oxygen-deficient layers continuously

integrated into the coatings as deposition occurs. This mechanism has been observed in other IAD films.

The refractive indices of ^12*^3 f^^TM^ deposited at 0.4 nm sec"''" are plotted in Figure 5 as a function of oxygen-ion density. The values increase from 1.64 for J=0 to 1.71 and 1.68 for films

banbarded with 1000 and 500 eV 0^, respectively. As illustrated in the case of Ta20^ films

(Figure 3), the values increase and then subsequently decrease for increasing level of ion

bcmbardment. The likely mechanism for this behavior is oxygen incorporation in the films. These

results illustrate that the effects of ion bombardment on the values of n are material dependent.

Work is on-going at 300 eV and 1000 eV to extend the behavior illustrated in Figure 5. Attempts to measure the values of extinction coefficient for the Al^O^ coatings (400 nm

thick) were limited due to the minimum sensitivity of the measurement equipment. All computed

-4

values of k were less than 2.0 x 10 . The low values of k calculated for the ^^2'^3 costings

indicate that for the conditions examined, preferential oxygen sputtering is not a dominant

mechanism for ^"^2^3 case for Ta20^ (Figure 4). This result again illustrates that the

effects of ion bombardment on film properties are material dependent.

Measuranent of the optical scatter characteristic of coatings is a method used to determine

+ 10 . . the effects of 0^ bombardment on film microstructure. The optical scatter characteristics of

the coatings were examined using an angle-resolved scatterometer . Figure 6 illustrates the

scatter characteristics of two Ta-O^^ coatings; one deposited with no ion bombardment (J=0) , and

+ -2

the other bombarded during deposition with 500 eV 0^ at a current density of 12 yA cm . The

vertical axis is the bidirectional reflectance distribution function (BRDF) of the coating, in

units of inverse steradians. The BRDF is proportional to the normalized intensity of scattered

light (logarithmic scale). The horizontal axis is the angle from the sample normal, in degrees,

at which the scattered light is collected. The scatter characteristic for an uncoated silicon

wafer is illustrated for comparison.

336
As illustrated in Figure 6, the optical scatter from the IAD coating (J = 12) was less than

that from the coating deposited without bombardment. All IAD coatings were compared with

coatings deposited without ion bombardment (J=0) , and a comparable reduction in scatter was

observed for other IAD ^°3*^i""3s relative to unbombarded coatings except for coatings

bombarded with very high levels of current density (>90 yA aii~^)

.

Figure 7 illustrates the scatter characteristics of two ^^2'^3 coatings; one deposited without

ion bombardment (J=0) , and the other bombarded during deposition with 500 eV at a current

-2 . . density of 35 yA cm . As illustrated in Figure 7, the optical scatter from the IAD coating

(J=35) was less than that from the coating deposited without bombardment. Comparable reduction

in scatter was observed for other IAD ^^2'^3 coatings relative to unbombarded coatings except for

coatings botibarded with very high levels of current density.

These results indicate that moderate levels of ion bombardment during deposition modifies the

growth of film columnar microstructure producing a more homogeneous film morphology. These

results are similar to the reductions in scatter reported for IAD Ti02 coatings. ''"^ Total

integrated scatter (TIS) measurenents reported for IAD CeO„ coatings also indicated less optical

9 scatter in coatings deposited under certain conditions of ion bombardment.

The environmental stability of optical coatings is in large part limited by the porosity of

the film microstructure. A number of Ta20^ coatings were exposed to humidity testing as described in the previous section to examine the effects of ion bcmbardment on film stability.

Figures 8 and 9 are the transmittance curves for two

'^^2'~^s coatings exposed to humidity testing.

The spectra in Figure 8 are for a coating deposited onto a heated silica substrate with no ion

bombardment (J=0) . The spectra in Figure 9 are for a coating bombarded during deposition with

+ . -2

300 eV O2 at a current density of 20 yA cm . The curve labeled AIR is the spectrum measured

after the coating has been removed fron the vacuum chamber and exposed to the ambient atmosphere.

The curve labeled POST HUMIDITY is the spectrum for the same coating remeasured after exposure to

97% relative humidity at 35°C for six hours.

The curves in Figure 8 illustrate a spectral shift to longer wavelengths of 1% for the

coating deposited without bombardment. The shift was most likely due to water adsorption into

the microvoids in the film microstructure. This increased the effective index value of the

coating and, in turn, increased the optical thickness (nt) . Figure 9 is typical of the results

obtained for all IAD coatings examined except one. No spectral shifts within the measurenent

precision (+^0.5 nm) of the spectrophotometer were observed. The coating bombarded with 300 eV

at a current density below the critical value exhibited a spectral shift of 0.3%. Similar

7 3 results have been reported for IAD Zr02 and Ti02 coatings. ' The '^^2'^5 J^^sults are summarized

in Table I. The spectral shift for the J = 0 coating indicated the likely existence of columnar

microstructure. The shift for the coating bombarded with 300 eV at a current density of 10

_2 yA cm indicated some columna'' microstructure existed although the coating was more stable and

had a larger value of n (see Figure 3) than the J=0 coating. These humidity test results, as well as the observation of reduced optical scatter and increased refractive index, are consistent

with IAD

'^^2'^S coatings having an increased packing density and being less susceptible to water

adsorption.

337
TABLE I Spectral Shifts for Ta-O- Coatings After Humidity Exposure

E (eV) J (yA an ) Spectral Shift (%)

0 0 1.0%

300 10 0.3%

300 20 0.0%

300 40 0.0%

500 10 0.0%

500 20 0.0%

A small number of anti-reflection (AR) coatings were laser damage tested at LANL. The

coatings were Al202/Si02 and were designed for 351 nm. The damage tests were performed as described in Section 2. The AR coatings deposited with no ion bcmbardment (J=0) had an average

-2 + -2 damage threshold of 4 J cm . The coatings bombarded with 300 eV 0_ at J=10 yA cm had an

-2

average damage threshold of 6 J cm . For the other ion bcmbardment conditions examined (300 eV,

_2 J=20 and J=45; 500 eV, J=20 and J=40) the average damage threshold was 3 J on or less. It is

pronature to draw any firm conclusions fron the limited data presented. Further laser damage

tests are being conducted on more Al202/Si02 AR coatings and on Ta20^/Si02 AR coatings. A report

on the completed test results will be made next year.

4. SUMMARY

Oxygen ion bombardment during deposition produced '^^2^5 ^""^ ^'"2^3 with increased

values of refractive index. The increase in n was dependent on ion energy and current density.

An increase in optical absorption was observed for IAD Ta20^ coatings. The values of optical

absorption increased for bombardment at increasing values of ion current density for a constant

ion energy. The increase in optical absorption was attributed to preferential sputtering of

oxygen in the Ta20^ molecule. Increases in optical absorption were not observed for ^120^

coatings except for coatings bombarded at very high current densities. The effects on the

optical constants of IAD coatings were found to be material dependent. The optical scatter

characteristics for IAD coatings were measured. The optical scatter for IAD coatings was lower

than the scatter for unbcxnbarded coatings. Improved environmental stability was obtained for IAD Ta20^ coatings exposed to humidity testing.

Preliminary laser damage results indicate ccsnparable, and in certain cases, higher threshold

levels for IAD AR coatings. Further damage tests are being conducted and results from these will

be reported next year.

REFERENCES

1. J.J. McNally, G.A. Al-Jumaily, S.R. Wilson and J.R. McNeil, "lon-Beam-Assisted Deposition of

Optical Thin Films - Recent Results," Proc. Soc. Photo-Opt. Instrum. Eng. 540 , 479 (1985).

2. W.C. Herrmann, Jr. and J.R. McNeil, "Ion Beam Applications for Optical Coating," Proc. Soc.

Photo-Opt. Instrum. Eng. 325 , 101 (1982).

338
3. P.J. Martin, H.A. Macleod, R.P.Netterf ield, C.G. Pacey and W.G. Sainty, "lon-Beam-Assisted

Deposition of Thin Films," Appl. Opt. 22, 178 (1983).

4. T. Allen, "Properties of Ion Assisted Deposited Silica and Titania Films," Proc. Soc. Photo-Opt. Instrum. Eng. 325 , 93 (1982).

5. J.J. Cuomo, J. M.S. Harper, C.R. Guarnieri, D.S. Yee, L.S. Attanasio, J. Angilello, C.T. Wu

and R.H. Hammond, "Modification of Nb Film Stress by Low-Energy Ion Bombardment During

Deposition," J. Vac. Sci. Technol. 20, 349 (1982).

6. J.R. McNeil, A.C. Barron, S.R. Wilson and W.C. Herrmann, Jr., "Ion-Assisted Deposition of

Optical Thin FiLms: Low Energy vs High Energy Bombardment," Appl. Opt. 23^, 552 (1984).

7. P.J. Martin, R.P. Netterfield and W.G. Sainty, "Modification of the Optical and Structural

Properties of Dielectric Zr02 Films by Ion-Assisted Deposition," J. Appl. Phys. 55 , 235 (1984).

8. J.R. McNeil, G.A. Al-Jumaily, K.C. Jungling and A.C. Barron, "Properties of Ti02 and Si02

Thin Films Deposited Using Ion-Assisted Deposition," Appl. Opt. 24, 486 (1985).

9. R.P. Netterfield, W.G. Sainty, P.J. Martin and S.H. Sie, "Properties of Ce02 Thin Films

Prepared by Oxygen-Ion Assisted Deposition," Appl. Opt. 24^, 2267 (1985).

10. G.A. Al-Jumaily, J.J. McNally, J.R. McNeil and W.C. Herrmann, Jr., "Effects of Ion-Assisted

Deposition on Optical Scatter and Surface Microstructure of Thin Films," J. Vac. Sci. Technol.

A3, 651 (1985).

11. J.C. Manifacier, J. Gasiot and J. P. Fillard, "A Simple Method for the Determination of the

Optical Constants n,k and the Thickness of a Weakly Absorbing Thin Film," J. Phys. E9, 1002

(1976) .

•

SUBSTRATE

THERMAL

SOURCE

ION SOURCE

Figure 1. Deposition Arrangement.

339
1 mnv,—

1

1 —

1

—

unc

i- Dated subs rate r——

90«

80% -- =0 -

70%

=j=

---1

15—

- —

-

60* =

50% — - - -

40%

30% —

20%

: -TaaO;

- - SOOeV O2* bombardment

'~ -Current density in jihcm-'^

: -Heated substrates

_

|10%

OV

200nm 400nm 600nm SOOnm

Figure 2. Transmittance spectra for two

Ta205 coatings.

2. lot- \ 1 ^

15 30 45 60 75 90 Oi CURRENT DENSITY (pAcm')

Figure 3. Refractive index vs ion current

density for Ta^Og coatings.

500.V

2 1 45 90 9B Oj CURRENT DENSITY (pAcrn^)

Figure 4. Extinction coefficient vs ion

current density for Ta^Oc coatings.

50 75 100 125 ISO Oj CURRENT DENSITY (pAcm')

Figure 5. Refractive index vs ion current

density for Al coatings.

340
300nm 400nm 500nm 600nm Figure 8. Transmittance spectra for a Ta20^ coating deposited with no ion Figure 9. bombardment subjected to humidity test. Transmittance spectra for a Ta-O^ coating bombarded during

deposition subjected to humidity test.

341
Manuscript received

2^20-86 SAB

Ion Assisted Deposition of Optical Films on Heavy Metal Fluoride Glass Substrates

J. J. McNally, G. A. Al-Jumail,y and J. R. McNeil

University of New Mexico

Albuquerque, NM 87131

and

6. Bendow

The 8DM Corporation

Albuquerque, NM 87106

Heavy metal fluoride glass materials are attractive for optical applications in

the near ultraviolet through infrared wavelength regions. However, many compositions

are relatively soft and hygroscopic, and possess low softening temperature (250°C-300°C)

.

We have applied ion assisted deposition (IAD) techniques to deposit MgFj , SiOj and

Al^Oj/SiO^ thin film structures on fluoride glass substrates at ambient substrate

temperature (~100°C). The coatings deposited using IAD improve the environmental

durability of the fluoride glass and appear to have reasonably good optical charateristics; without application of IAD, the deposited coatings are not durable and have poor

adhesion.

Key Words: coatings; durability; fluoride glass; hermetic; ion-assisted

1. INTRODUCTION

Heavy metal fluoride (HMF) glasses have attracted considerable interest as multispectral

transmissive optical components, laser host materials and optical fibers. HMF glasses possess

desirable optical properties, including a broad transmi ttance range from the mid-IR (~7 ^m) to near-UV (-0.3 jim), low absorption and scatter losses, low dispersion and index of refraction, and

low thermal distortion [1]. However, despite their promising optical characteristics, many compositions are relatively soft and hygroscopic. For example, it has been shown that the solubility of

typical f 1 uorozi rconates, a category of HMF glass. Is many orders of magnitude greater than that

of high silicates [Z]. Also, measurements of fiber strength reveal that while initial strengths are high, HMF optical fibers degrade rapidly when exposed to high humidity, and that protective

Teflon coatings do not serve as effective barriers against the degradation. This susceptibility

to attack by moisture indicates that hermetic coatings are required to protect HMF glasses.

Moreover, because HMF glasses are chemically sensitive, an additional requirement is that optical

coatings must be applied In a non-damaging way.

Ion assisted deposition (IAD) Involves the simultaneous bombardment of the growing film with

low energy Ions during deposition. Ion bombardment can also be used prior to deposition to clean

the substrate. IAD has been Investigated as a technique for improving the optical and mechanical

properties of many materials [3,4]. Thin films deposited using IAD have exhibited increased

packing density, Improved stability, and reduction in stress [5-7]. In previous publications,

[8-12] we described the effects of ion energy and flux on film optical constants, optical scatter

characteristics, s tol ch 1 ometry , stability. Impurity content and adhesion for a number of

materials. Kaufman ion sources were used in these investigations; they are attractive because it

Is possible to Independently control and accurately measure the important experimental parameters

342
involved. Reactive or inert gas ions can be employed; ion beam energy, current density and ion

angle of incidence can be varied over a wide range of values.

The purpose of this work is to investigate the use of lAO to deposit protective and antireflection (AR) coatings on HMF glass substrates, in order to obtain good quality coatings at reduced temperatures. The results are preliminary, as only a limited number of samples and

conditions have been investigated. The coating materials selected for this investigation were

MgFj , SiO^ and Al^Oj. Single-layer coatings of MqF^ and SiO^ were deposited onto substrates of

Hf -8aF2 -LaFj -Al F3 (HBLA) glass, and an AljOj/SiOj AR coating was deposited onto a substrate of ZrF^ -BaF2-LaF3-AlF3 (ZBLA) glass. Tests were performed to determine film and

substrate environmental durability, scratch-resistance and adhesion. The optica! properties of the

coatings were also characterized.

2. EXPERIMENTAL ARRANGEMENT

Details of the film deposition arrangement are given in Reference 12; a schematic of the

arrangement is shown in figure 1. A Kaufman ion source is used to provide a monoenergetic, neu- tralized beam of ions independent of the material evaporation process. It provides a low-energy

(300-1500 eV) beam of Ar or O2 ions incident on the film surface with variable angle of 0-45''. A Faraday probe was used to monitor the ion current density at the substrate. Prior to deposition

the substrates were cleaned with 300 eV Ar ions. The MgF^ film material was evaporated at 0.3

nm/sec using a resistively heated boat, and the growing film was bombarded with 300 eV Ar ions at

a current density of 5 ^lA/cm during deposition. The temperature was approximately lOO^C due to heating from the boat. The SiOj and AI2O3 film materials were electron-beam evaporated at rates

-4 of 0.3 nm/sec with oxygen backfill pressure of 1.0 10 Torr. The growing films were bombarded

with 300 eV 0^ ions at a current density of 30 \J\/cm during deposition. The substrate temperature

was approximately 150°C.

3. RESULTS

3. 1 .Environmental Stability

An HBLA glass sample was coated with 0.5 vim IAD MgF^ film, and another sample was left

uncoated. A drop of water was placed on each sample for approximately 16 hours. The samples were then examined using a Nomarski microscope. The two micrographs are shown in figure 2. The

results for the uncoated sample illustrate the known susceptibility to aqueous corrosion of an unprotected fluoride glass surface. The result for the coated sample shows that it did not suffer

any noticeable degradation, and indicates that an IAD MgF2 coating might serve as an effective

barrier against moisture penetration. Figure 3 is a micrograph of an HBLA glass sample coated with

0.5 pm IAD SIO2 and subjected to a water drop test. There 1s no observable difference in the film

exposed to the water drop relative to the portion not exposed. We also examined a ZBLA glass

sample coated with a two-layer AI2O3/SIO2 AR coating. The sample was exposed to a water drop test

as described above. Again, there is no noticeable degradation In the film exposed to the water

drop; it appears exactly as the portion not exposed. We have also conducted humidity cycle tests to examine film stability properties. Films were exposed to 97% humidity at 35"C for six hours

343
after removal from the coating chamber. Spectral shifts in film transmi ttance curves were observed for films deposited without IAD, indicating that the films adsorbed water. However, for

all IAD films examined, no spectral shifts were observed in the spectrophotometer trdces[13].

These results, as well as previous observations of reduced optical scatter and increased

refractive index [8-10], are consistent with IAD films having an increased packing density and

being less susceptible to water adsorption. Thus, overcoating a fluoride glass with an lAO

coating might protect it from moisture attack and allow the use of HMF glasses in non-ideal

envi ronments

.

3. 2. Abrasion Resistance

We investigated the effect of a coating on the abrasion resistance of HMF glass substrates.

Samples were subjected to an eraser-rub test and examined using a Nomarski microscope. A sample was divided into two sections; one section was coated with 0.5 ]im IAD MgFj , and the other section

was left uncoated. The sample was then rubbed a few times with an eraser. Figure 4 is a micrograph

of the uncoated H6LA glass section and the IAD coated section. The figure shows that the unprotected HMF glass is soft and subject to abrasion, while the IAD coated section is hard and not subject to abrasion. We investigated the abrasion resistance for two MgF2 coatings on glass

substrates. One sample was coated with 0.5 pm IAD MgF^ , and the other was coated with 0.5 ym MgF^

deposited without Ar ion bombardment. The samples were then rubbed a few times with an eraser. Results are shown in figure 5. The unbombarded (J=0) sample is scratched, whereas the bombarded

sample is much less scratched. This illustates that the IAD coated sample is more durable than

the sample coated without bombardment. We divided another HMF sample into two sections. One

section was coated with 0.5 pm IAD SiO^, and the other section was left uncoated. Results

similar to those obtained for the IAD MgF^ coated samples were obtained. The coated section is much less scratched than the uncoated section. These results indicate that overcoating a fluoride

glass with an IAD coating can enhance the durability of HMF glasses.

3. 3. Fi Im Adhesion

All samples were subjected to a tape-pull test to examine the effect of IAD on film

adhesion. Coatings deposited without IAD had poor adhesion to the glass substrates. None of the

samples coated with ion bombardment during deposition experienced delamination.

3.4. Optical Properties

Transmittance and reflectance spectra for all samples were obtained using a dual-beam spectrophotometer. One important parameter that can be qualitatively characterized from spectrophotometric measurements is film stoichiometry. Poor film stoichiometry leads to a higher UV cutoff

wavelength in optical transmittance. The transmittance spectra of the coated and uncoated HMF glass samples were examined, and in no case was the UV cutoff wavelength higher for an IAD coated

sample relative to the uncoated substrate. This indicates that ion bombardment at these

conditions is not detrimental to film stoichiometry for low temperature coating. Another film

property that can be obtained from spectrophotometric measurements is refractive index inhomo344
geneity. The presence of an i nhomogenei ty can be deduced from the transnii ttance and reflectance

data at wavelengths for which the film is multiple half-wave in optical thickness. Index

i nhomogenei ty can often occur in low temperature coatings. No detectable index i nhomogenei ty was present in any IAD coating. The transmi ttance spectra for an uncoated ZBLA glass substrate and a two-layer A1^03/Si02 AR coating are shown in figure 6. Note that the spectra for the AR coating

touches that of the uncoated glass at approximately 500 nm, and that the film optical characteristics are good above 400 nm; note that below 400 nm the film becomes absorbing.

4. CONCLUSIONS

We have successfully applied lAO techniques to deposit MgF^ , SiO_; and AljO^/SiO^ thin film

structures on HMF glass surfaces at low substrate temperatures (100 - 150"C). The coatings

deposited using IAD were hard and dense, and Improve the durability of the fluoride glass.

Environmental stability and corrosion resistance of the optical surfaces were significantly

improved; abrasion resistance was Improved; excellent film adhesion was obtained; film optical

properties were good and shown to be compatible with the HMF glass material. The demonstrated

properties enhance the prospects for the use of HMF glass materials in a variety of applications.

Optimization of the coating process should enable further improvements in fluoride glass durability for use in harsh environments.

* This work partially supported by the Air Force Weapons Laboratory under contract no.

F 29601-81-C-0012.

5. REFERENCES

[1] Tran, 0. C, Sigel, 6. H., Bendow, B. "Heavy metal fluoride glasses and fibers: A review,

" IEEE J. Lightwave Tech. 5, 566, 1984.

[2] Simmons, C. J., Azali, S. A., Simmons, J. H. "Chemical durability studies of heavy metal

fluoride glasses," Second Int. Symposium on Halide Glasses, Troy, NY, 1983.

[3] Herrmann, Jr., W. C, McNeil, J. R. "Ion beam applications for optical coatings," Proc.

Soc. Photo-Opt. Instrum. Eng. 325, 101, 1982.

[4] Martin, P. J., Netterfield, R. P, Sainty, W. G., Clark, G. J., Lanford, W. A., Sie, S. H. "Ion-assisted deposition of bulklike ZrO^ films," Appl. Phys. Lett. 43, 711, 1983.

[5] Martin P. J., Macleod, H. A., Netterfield, R. P., Pacey, C. G. , Sainty, W. G. "lon-beam-

-asslsted deposition of thin films," Appl. Opt. 22, 178, 1983.

345
[6] Allen, T. H. "Ion assisted deposition of titania and silica films," Proc, Soc. Photo-Opt.

Instrum. Eng. 325, 93, 1982.

[7] Bland, R. 0., Kominiak, G. J., Mattox, 0. M. "Effect of ion bombardment during deposition

of thick metal and ceramic deposits", J. Vac. Sci. Technol. U, 671, 1974.

[8] McNally, J. J., Al-Jumaily, 6. A., McNeil, J. R. "Ion-assisted deposition of 13^05 and

Al^Oj thin films," J. Vac. Sci. Technol. A, to be published, May/June 1986.

[9] McNally, J. J., Al-Jumaily, G. A., Wilson, S. R. , McNeil, J. R. "lon-beam-assisted

deposition of optical thin films - recent results," Proc. Soc. Photo-Opt. Instrum. Eng.

540, 479, 1985.

[10] Al-Jumaily, G. A., McNally, J. J., McNeil, J. R., "Effect of ion assisted deposition on optical scatter and surface microstructure of thin films," J. Vac. Sci. Technol. A3,

651, 1985.

[11] McNeil, J. R., Al-Jumaily, 6. A., Jungling, K. C, Barron, A. C, "Properties of TiO^ and

SiO^ thin films deposited using ion assisted deposition," Appl. Opt. 24, 486, 1985.

[12] McNeil, J. R., Barron, A. C, Wilson, S. R., Herrmann, Jr., W. C. "Ion assisted deposition

of optical thin films: low energy vs high energy bombardment," Appl. Opt. 23, 552, 1984.

[13] McNally, J. J., Al-Jumaily, G. A., McNeil, J. R. , "Properties of Ta^O^ and Al^O^ thin

films deposited using ion assisted deposition", to be published.

346
EXPERIMENTAL ARRANGEMENT

SUBSTRATE

\,//

THERMAL

SOURCE

ION SOURCE

Schematic of the experimental arrangement.

Nomarski micrographs of a) an uncoated HBLA glass sample, and b) a HBLA

sample coated with 0.5 jim IAD MgF^ film after water drop test.

347
Nomarski micrograph of a H8LA sampie coated with u,b pn km i>iu. after water drop test.

'

Nomarski micrograph of a HBLA sample after eraser rub test. The left

side of the figure is the uncoated section, and the right side is the

section coated with 0.5 lAO MgF^.

348
t

^ b

5. Noinarski micrograph of two coatings after erM«r n»b test.

a) Sample coated with 0.5 \m MgF^ deposited without ion bowbardraent,

and b) s^le coated with 0.5 iim IAD MgF2.

6. Transmittance spectra for a two-layer Al203/Si02 AR coating

deposited on a ZBLA s£«iple.

349
Manuscript Reaeived

10-29-85

Highly Damage Resistant Porous Dielectric Coating for High Power Lasers in the Ultraviolet to Near-infrared Wavelength Region

K. Yoshida, H. Yoshida, Y. Kato, and C. Yamanaka

Osaka University

Osaka, Japan

Mechanically hard and broadband gradient refractive index artireflection coating

was developed. It has a high damage threshold of >6 J/cm^ at 355 nm and 12-13 J/cm^ in the visible to near-infrared region. It will be applicable to high power pulsed lasers

as well as to industrial uses.

Key words: gradient refractive index, porous dielectric AR coating, damage threshold,

quartz, mixed thin film, surface roughness.

We have developed a new technique for producing antireflection (AR) coatings on various

substrate materials including fused and synthetic quartz and optical glasses. This coating

applied on a quartz has a damage threshold of over 6 J/cm^ at 355 nm. Since the coating is very

broadband and mechanically stable, it will be applicable not only to high power lasers, but also

to other areas such as optical communications.

Development of high power lasers require damage-resistant AR surfaces. Present multilayer

dielectric AR coatings do not have sufficient damage thresholds to pulsed lasers, particularly in the short wavelength region. Graded-index AR surfaces produced by neutral solution processing[l

]

on bare polished borosilicate crown glass BK-7 have a damage threshold of 12 J/cm^ at a laser

wavelength of 1064 nm with the pulse width (xp) of 1 ns[2]. However, the neutral solution

processing is not applicable to the surface of silica substrate which is required for ultraviolet

lasers. The AR surface produced by this method is susceptible to mechanical damages. Porous silica AR coating was developed for high power ultraviolet lasers[3]. In this method,

porosity-graded Si02 coating is deposited on a substrate from a polymer solution derived from

silicon ethoxide. This method is applicable to a wide variety of substrates including silica and

KDP crystals. This porous silica AR coating has an excellent damage threshold of 6-9 J/cm^ at

351/)m (t =1 ns). However, this AR surface is also susceptible to mechanical damages and thus

requi res'^careful handling. The AR coating which we report in this letter might exceed the

above-mentioned AR coatings in terms of laser damage, mechanical stability, and easiness of

coating production.

In the present technique, two dielectric materials A and B having different refractive

indices n^ and nn, where n/\>nD are simultaneously deposited in vacuum on a substrate such as quartz or optical glasses. Then the mixed thin film is chemically processed in a liquid solution

which preferentially dissolves the material B. These processing result in a porous thin film

which has gradient refractive index and thus has the AR property over broad bandwidth. Hereafter

we call the AR coating produced by this method as porous dielectric AR coating.

In actual production, two dielectric materials Si02 and NaF were evaporated from independent

evaporation sources and simultaneously deposited in vacuum on one side of a heated quartz

substrate. The quartz substrates used in the following tests were standard fused quartz plates

for photolithography in semiconductor industry and the synthetic quartz for optical fiber

industry, respectively. The porous dielectric AR coating was produced by dissolving NaF in ultrapure water. The wavelength of the minimum reflectivity is controlled by the coating

thi ckness

.

The closed circles in Fig. 1 shows the measured reflectivity versus wavelength of the

coating after chemical processing. The minimum reflectivity of the porous dielectric AR coating

is 0.17% at the optimum wavelength, which is 355 nm in Fig. 1. It has a very broadband

characteristic with the reflectance of <0.5% over 200-nm and <1% over 300-nm wavelength range. Let us assume that the coating has the refractive indices of nf(a)^nf(g) on the air side and on the substrate side, respectively. Then the reflectivity versus wavelength at normal incidence is approximately given by

4n.(a)n.(g)n

R. ^ 1 - 1 I H [nf(a)ng+n^(g)]2 - [n^(a)2-l ][ng2.n^(g)2]sin2(6/2) (1)

350
4iT rdr 217 • 6 = — M n(Z)dZ — [n.(a)+n.(g)]d., (2)

where nq is the refractive index of the substrate, and df is the coating thickness[4] . The

dashed Tine in Fig. 1. shows the calculated reflectivities using Eqs. (1) and (2) with the

parameters of nf{a)=1.220, nf(g)=1.303, and df=70 nm, respectively. The solid Ine is the

calculated reflectivity with the homogeneous thin film of nf(a)=nf(g)=l .220 and df=73 nm. Thus

it is reasonable to assume that the AR coating produced in this method has the gradient

refractive index. This refractive index indicates that this AR coating has the density '^^46% that

of the standard Si02 coating.

Figure 2 shows the Nomarski micrographs of the porous dielectric AR coating before and after

chemical processing. The roughness of the coated surface after evaporation disapeares after

chemical processing, and the resultant surface is completely smooth under optical microscope. We have measured the surface roughness of the quartz substrate and the porous dielectric AR coating

with digital optical profilometer Wyko Optical Inc. (Type NCP-1000). The results are shown in

Fig. 3(a) and (b), respectively. The surface roughness of the substrate and the AR coating were

almost the same value of llA rms. , .

The damage threshold of the porous dielectric AR coating was measured at the laser

wavelengths of 1053,527, and 355 nm, respectively. The output from oscillator (a Q-switched Nd: YLF laser or an actively mode-locked Nd: YAG laser) was amplified by two Nd: glass amplifires of

15-mm aperture and frequency up converted by type II KDP crystals. The amplified laser pulse was focused with a 1-m focal length lens. On the test sample, the laser beam had a smooth Gaussian

shape with a spot diameter at l/e^ peak intensity of 400 ym. The laser energy was adjusted with

the combination of a half-wave plate and a dielectric polarizer without changing the pumping

energies of the amplifiers so as not to change the beam shape on the sample. The laser energy

and the pulse shape was monitored with biplanar phototubes which were calibrated with a standard

calorimeter. The coating damage was detected by observing before and after laser irradiation

with a Nomarski microscope at 200x magnification. The test sample was moved after each shot

irrespective of the presence or absence of the damage. Therefore the damage thresholds quoted

Table 1. Comparison of the damage threshold between the porous dielectric AR coating, the standard AR coating, and the quartz at x=355 nm (t =0.4 ns),

527 nm (t =1 ns), and 1053 nm (t =1 ns), respectively. ^

r r

AR coatings Damage Laser coating materials and layers test Threshold:

(J/cm2)

Multilayer 3-4 1053 nm No undercoat; Ti02/Si02, 1 -on-1

2-6 layers

Multilayer 4-8 1 ns X/2 Si02 undercoat; Ti02/Si02 1 -on-1

3-7, layers

Porous dielectric 12-13 1 -on-1

Multilayer 2.5-3.5 527 nm No undercoat; Ti02/Si02, 1 -on-1

2-6 layers

Multilayer 4-5 1 ns x/2 Si02 undercoat; Ti02/Si02 1-on-l

3-7, layers

Porous dielectric 12 1-on-l

Monolayer 2-4 X/4 Si02 1 -on-1

4.4-6.4 x/4 SIO2 N-on-1

Mixed thin film 2.5-3.2 355 nm NaF+Si02 1 -on-1

Multilayer 1-3 0.4 ns x/2 Si02 undercoat; Al 203/8102, 1 -on-1

SC203/Si02, 3-7 layers

Porous dielectric 6-9.5 1 -on-1

10.5-11 .5 N-on-1

Quartz 9.5-16 1-on-l

351
hereafter are those for "one-shot damage" (1-on-l tests). More than 20 samples were tested for

the damage measurements.

Table I summarizes the results of the damage thresholds of mixed thin film and the porous

dielectric AR coating together with the data of the bare quartz, single layer Si02 coating and

the standard AR coatings measured at the same experimental conditions. The porous dielectric AR coating has the damage threshold of 6-9.5 J/cm^ at X=355 nm (Tp=0.4 ns) and 12-13 J/cm^ at both

A=527 and 1053 nm (Tp=l ns), respectively. Compared with the standard commercial AR coatings,

the porous dielectric AR coating has more than twice higher damage threshold at all the tested

wavelengths. When same coated site was repeatedly irradiated (N-on-1 tests) at fluence levels

below the 1-on-l threshold, the damage threshold of porous dielectric AR coating was increased

from 8 to 11.5 J/cm^ which is almostly the same value of the bare quartz surface. It is

interesting to note that the porous dielectric coating has a significantly higher damage

threshold compared with the single layer Si02 coating.

In Fig. 4 we show the representative pictures of the phase contrast photomicrographs of the

damage morphology seen in the porous dielectric AR coatings at different laser fluences at 355

nm. In general, the damaged pattern of this AR coating consists of small pits whose diameters

are less than '^3 pm. However, at high laser fluences, large damages are produced such as the

picture in Fig. 4 at 15.2 J/cm^.

The porous dielectric AR coating which we have developed is mechanically hard, since it is physically a single layer hard dielectric coating deposited in vacuum. The AR surfface is strong

against abration by tape peeling test. The surface can be cleaned with, for exampTe, lens

tissues to remove dusts and contaminations without causing mechanical damages. Due to the

excellent optical and mechanical properties together with the easiness of fabrication, the porous

dielectric AR coating might find many applications in various fields including industrial uses.

We have appl ied the porous dielectric AR coatings on fused quartz optics (aspheric lenses,

blast shield glasses, and KDP window glasses) used for third harmonic conversion of our high

power glass laser having a 18-cm beam diameter[5]. No damage on the coatings has been produced

under the 3 wavelengths irradiation (1052 nm, 526 nm, and 351 nm) with the total fluence of <5 J/cm^ for the extended period of over 10 months.

In summary, we have developed a new, porous dielectric AR coating. This coating is

applicable in various substrates including fused and synthetic quartz. It has gradient

refractive index and the broadband AR characteristics. The measured damage threshold of this

coating is 6-9.5 J/cm^ at 355 nm (Tp=0.4 ns) and 12-13 J/cm^ at 527 nm and 1053 nm (Tp=l ns),

respectively. Since the AR surface is mechanically hard, it does not require particular

precautions for handling. The reliability has been tested in a high power frequency converted

glass laser system.

The authors with to acknowledge valuable contributions to this work by Dr. Y. Namba of Osaka

University and Mr. T. Izawa of Showa Optical Co., Ltd.

References

[1] L.M. Cook, S. Liolek, and K.H. Mader, American Ceramic Soc. ^, 152 (1982).

[2] W.H. Lowdermilk and D. Milam, Appl. Phys. Lett. 36, 891 (1980).

[3] D. Miliam, W.H. Lowdermilk, J.G. Wilder and I.M. Thomas, CLEO '84 Technical Digest THB3, 136

(1984).

[4] H. Schroder, Ann. Physik 39, 55 (1941).

[5] Y. Kato, K. Yoshida, J. Kuroda, and C. Yamanaka, Appl. Phys. Lett. 38, 72 (1981).

352
4.0

200 400 600 800 1000 1200 1400

Wavelength(nm)

Figure 1. Reflectivity vs wavelength of the porous dielectric AR coating

after chemical processing. The closed circles shov/ the measured

reflectivities. The dashed line is the calculated reflectivity

using Eqs. (1) and (2) with n.(a)=1.220, n.(g)=1.303, and d.=70

nm. The solid line is the calculated reflectivity for the

homogeneous monolayer coating with n^(a)=nj:(g)=l .220 and dr.=73nm.

I 1 10pm

(a) before processing (b) after processing

Figure 2. Nomarski micrograph of the porous dielectric AR coating before and

after chemical processing.

353
RM5= 1 1 H. P-v= S5 fl.

0 200 400 G01 801 1001 Distance on surface in microns

(a) Synthetic quartz

RMS= I 1 fl. P-V- 5? R.

40iii 681 B01 1001

Distance on surface in microns

(b) Porous dielectric coating

Figure 3. Surface profile by digital

optical prof ilometer.

3D <J9 SO' 60 70

50 pm

10.6 J/CM2

13.2 J/CM2

15.2 J/CM^

Figure 4. Representative pictures

of the phase contrast photomicrograph of the damage

morphology at various laser

fluences.

354
Manuscript Received

10-31-85

Ion Beam Characterization of Multi-Layer Dielectric Reflectors

J. G. Beery, M. G. Hollander, C. J. Maggiore,

A. Redondo, R. T. Westervelt, and T. M. Taylor

Los Alamos National Laboratory

Los Alamos, MM 87545

Energetic ion beams were used to characterize multilayer dielectric reflectors.

Alpha-particle beams with beam spot sizes between 10 microns and a few millimeters were

scattered from reflectors consisting of 32-layer Si02/Hf02 and 38-layer MgF2/ThF4. The

RBS spectra reveal the nature of the laser damage processes by providing information on diffusion, mixing, and loss of material in the coatings. The particle-induced x-ray

emission (FIXE) technique gave complimentary results on low-concentration impurities in

the coatings.

Key Words: dielectric reflectors, nuclear microprobe, particle-induced x-ray emission,

Rutherford backscattering.

1. Introduction

Considerable work in the damage mechanisms of multilayer dielectric reflectors has previously

been reported, using a variety of experimental techniques. The fundamental damage mechanisms are

not completely understood because of the complexity of the systems. We have carried out Rutherford backscattering [1] (RBS) and particle-induced x-ray emission [2] (PIXE) experiments using

energetic ion beams to characterize high-reflection coatings on laser mirrors before and after

damage. These experiments are particularly suitable for measuring the composition as a function

of depth, the presence of low concentration impurities, and changes in the structure of the films

as a result of the damage processes. The measurements have been made on undamaged coatings with

spot sizes of a few square millimeters. Localized structure in the region of individual damage

pits has been studied using a nuclear microprobe [3,4], which produces spot sizes as small as ten microns in diameter. The RBS spectra obtained provide information about damage profiles, mixing,

diffusion, and loss of material in the coatings. The PIXE technique has been used to measure low

concentration levels of impurities in the films.

2. Experimental Method

Rutherford backscattering is a well-established quantitative technique for interface and thin

film analysis. It is an analytical technique based on known kinematics and energy loss that can determine the composition and structure in materials to a depth of a few microns.

Figure 1 is a schematic drawing of the general geometry of a backscattering experiment. A

beam of light ions from a Van de Graaff accelerator strikes a target, and the scattered particles

are detected with a surface barrier detector at a fixed angle. Figure la shows a thin film of a heavy element on top of a lighter substrate. The observed energy of the backscattered particles

is determined by the kinematics of the elastic scattering and the depth of the scattering event

below the surface. Figure lb shows a typical RBS spectrum. The highest energy, KE^^' corresponds to elastic scattering at the surface of the film. The energy Ef is due to elastic scattering from the film at the film-substrate interface and the energy loss of the particles traversing the film. The width of the peak, flE, is related to the areal density (atoms/cm^) of

the film. The relative masses of the incident and target particles and the scattering geometry

determine the kinematic factor, K. The backscattering energy increases with the mass of the target atoms. Thus, RBS is particularly well-suited for measuring the distribution of heavy elements

in a lighter matrix.

355
DETECTOR

COoo

tr

UJ

CD

f KE in •in

Figure la. General backscattering geometry

for Rutherford backscattering measurement.

Figure lb. RBS spectrum obtained from a heavy thin film on a light substrate.

The RBS experiments reported here were carried out with 2.3 and 2.5 MeV a-particles with

®in = ^ ®out = 20 degrees. We studied 32-layer Si02/Hf02 and 38-layer MgF2/ThF4 coatings on fused silica. Since the kinematic factor for the lighter elements, Si, 0, Mg and F, is much less

than that for the Hf or Th, the high energy part of the spectrum characterizes the depth distribution of the heavy elements in the film.

The technique favors high Z elements because the Rutherford backscattering cross section is

proportional to Z^. If the elemental masses are too close, the kinematic constraints limit the mass resolution and result in overlapping spectra for compound samples. Consequently, the analysis of Al203/Si02 coatings is somewhat more difficult. These factors, together with finite

detector resolution, limit our depth sensitivity at the surface to approximately 50 angstroms.

Energy straggling degrades depth resolution below the surface.

When an energetic ion looses energy in the sample, it induces x-ray emission characteristic

of the atoms in the target. Energy analysis of these characteristic lines permits elemental

identification at low concentration levels. The PIXE technique is generally applicable to atoms

with Z > 10. The sensitivity depends on the matrix and the atomic number and is usually in the

range of 10 ppm. For the work reported here, the incident beam was 2.0 MeV protons.

These techniques can be applied to the study of localized damage by focusing the incident ion

beam into small spatial dimensions. We have studied individual damage pits on multilayer dielectric reflectors using these focused ion beams with both the RBS and PIXE techniques. A nuclear

microprobe with spot sizes of a few microns in diameter was used.

We studied Si02/Hf02 and MgF2/ThF4 multilayer coatings on fused silica substrates. The

structure of these films is given in Table 1. Table 2 describes the dimensions of the beam spots

and the resulting current densities. For the two larger spot sizes, the RBS and PIXE techniques

are non-destructive. For the smallest spot size of ten microns, the current density is high

enough to create color centers.

Table 1. Structure of multilayer

dielectric reflectors.

Table 2. a-particle beam parameters.

HR351-166-SPP 351 nm

(H. LpO H |L(1.429), H(0.705)l5 L(2) H; HIO2

L; S1O2

HR24B-213-OCL1

(HL)19 H

H: ThF4

L: M9F2

248 nm

CURRENT CURRENT

SPOT SIZE (nA) DENSITY A/m^

5 mm X 5 mm 20 8 X 10 '*

r = 100 M 5 6 X 10 1

r = 10m 2 3x IQI

356
3. Results

Figure 2 shows the experimental

RBS energy spectrum from the Si02/Hf02

coating. One can clearly see 16 separate hafnium peaks. Also evident is

the change in layer thickness for the

topmost five layers of Hf02. The silicon contribution overlaps the lower

energy portion of the hafnium peaks

extending up to 1.4 MeV, but it does

not significantly alter the spectrum

due to its lower scattering cross section. The low energy cutoff at 200 keV

is due to the discriminator setting in

the electronics.

Similar data were also taken on

an undamaged MgF2/ThF4 coating. These

results are shown in Figure 3. The

first 15 layers of this spectrum have

been simulated using the model parameters listed in Table 3. The simulation

was performed using computer programs

based on first principles for the scattering and parameterized energy loss

equations. The input consists of average stoichiometric ratios for the individual elements in each layer. The

program uses them to calculate the

corresponding spectra which are then

compared with experiment. The best fit was achieved with a decreasing ratio

of thorium to fluorine as one moves

toward the substrate (e.g. , 1 to 4 for

the top layer and 1 to 5 for the eighth

layer). In addition, the thorium that

is missing from a given ThF4 layer appears uniformly distributed in the adjacent MgF2 layer. This can be interpreted as evidence for interlayer mixing

during the fabrication process. The

thorium fluoride layers are uniform to within 5%. The magnesium fluoride layers

appear to be fairly uniform; however,

the simulation is not very sensitive to the low mass layers.

Figure 4 is a Nomarski micrograph

of a laser damage spot in the silica/

hafnia coating. The large oval feature

at the center is the visibly damaged re- gion that penetrates to the substrate.

The striations to the left and right were made visible during or after the

damage process. A relatively clear area

0.5 1.0 1.5 2.0 2.5 3.0 ENERGY (MeV)

Figure 2. RBS energy spectrum for undamaged region

of Si02/Hf02 reflector.

Energy (MeV)

1.5 2.0 2 5

400

1

' r

600 800

Channel

1000

Figure 3. RBS energy spectriim for undamaged

region of MgF2/ThF4 reflector (points).

Simulated spectrum for fifteen layers

nearest surface (solid line).

357
Table 3. Simulated structure for fifteen layers of MgF2/ThF4. Layer 1 is at the surface.

Thickness

Layer Mo. (10^5 atoms/

1 405

2 430

3 410

4 430

5 410

6 430

7 410

8 428

9 405

10 428

11 387

12 428

13 405

14 428

15 410

Atomic Concentration (%)

Th F

20 0 80 0 66 7 33 3 18 9 81 1

1 0 66 0 33 0 18 5 81 5

1 0 66 0 33 0 18 2 81 8

1 6 65 6 32 8 17 5 82 5 2 6 64 9 32 5 17 4 82 6 3 2 64 5 32 3 16 1 83 9

4 2 63 9 31 9 16 1 83 9

immediately adjacent to the damage pit suggests that these striations were annealed by thermal

processes. To study in detail the damage at a single pit, we focused the ion beam to a spot 10

microns in diameter using the nuclear microprobe. The line of small spots across the lower part

of the oval feature is due the microprobe ion beam. We believe that the radiation damage from the

ion beam created color centers.

Figure 4. Momarski micrograph of laser damaged spot and surrounding region.

358
For the line scan across the laser damage area we obtained RBS spectra, one spectrum at each

spot. Figure 5 is a composite of five spectra taken at various positions near or on the laserdamaged area of figure 4. The widest spectrum corresponds to the aicroprobe analysis at the edge

of the damaged region and is similar to that of figure 2 except that the statistics are poorer.

In addition, there is a small surface peak corresponding to hafnium that has been ejected from

the pit and is on top of the Si02 overcoat. As one crosses the crater, the spectra change

significantly due to mixing and loss of material. The spectrum corresponding to a spot inside

the edge of the crater shows the loss of the overcoat and 5 or 6 pairs of silica/hafnia layers.

It also shows that these are the outer layers of the coating. The next spot in the direction

toward the bottom of the crater shows additional loss of hafnia and silica. The layered

structure is absent indicating interlayer mixing. The last two spectra show successively greater

loss of coating material toward the center of the pit.

ENERGY (MeV)

Figure 5. Composite of five RBS energy spectra taken near and on damaged spot.

Reflector is Si02/Hf02.

The RBS technique is sensitive to thin overlayers such as the half-wave

overcoat of low-index material commonly

applied to these mirrors. For example,

figure 6 consists of a spectmm from the

undamaged region of the Si02/Hf02

mirror over-layed with a spectrum near

the damaged area. The small shift in the

energy of the upper edge of the hafnium

corresponds to a 160 A loss of the Si02

overcoat. In addition, the valleys of

the undamaged spectrum are deeper than

those of the damaged one. This indicates

that sub-surface diffusion occurred near

the damaged spot. These results suggest

that in these haf nia/silica coatings, the

damage mechanism proceeds from the top

toward the substrate.

50 D

_JUJ>o

z

<o

COo

<

m

1.0

DAMAGED (B) UNDAMAGED (A)

16 nm LOSS OF

Si02 OVERCOAT

I 1.5 2.0 ENERGY (MeV)

2.5

Figure 6 . RBS spectra taken from undamaged and

slightly damaged regions of Si02/Hf02 showing

partial loss of overcoat and diffusion.

359
Figure 7 shows the RBS spectrum of an undamaged MgF2/ThF4 coating using a 100-micron spot size. The 19 layers

of thorium fluoride are clearly visible. Note the similarity of this spectrum and that of figure 2. As one approaches the laser-damaged area, the

differences between the damage behavior

of the Si02/Hf02 and that of the MgF/ThF^

become evident. These differences are shown in figure 8 for line scan data

taken using a 10-micron spot size. The

spectrum taken near the edge of the

damage region exhibits a loss of layered structure near the substrate

whereas the thorium peak is enhanced

for the top layer. As we move toward

the center of the damage region we observe complete loss of structure and

50% reduction in the amount of thorium.

In addition, the surface thorium peak

gradually disappears. Changes in the

slope of the spectra at 0.9 and 1.2 MeV

show the presence of flourine and magnesium in the damage area. We believe

that this indicates mixing of the magnesium fluoride and thorium fluoride

films. The behavior of the surface

thorium peak in these data, coupled

with other spectra from regions which

are not visibly altered, indicate that

a significant amount of fluorine

leaves the surface region. These

spectra suggest that the damage

mechanism starts from the substrate

and progresses outward through the

coating.

We have also used PIXE to search

for low concentration impurities.

Figure 9 shows a spectrum obtained by

bombarding the Si02/Hf02 sample

with a beam of 2.0 MeV protons. In addition to the hafnium L lines, K-a

lines from iron and zirconium impurities can be seen. The Fe is present

at a concentration of 0.14 and the Zr

at 0.29 atomic percent relative to the Hf.

Energy (MeV)

1,5 2,0 2.5r

1000

Figure 7. RBS energy spectrum from undamaged

region of MgF2/ThF4 reflector.

Energy (MeV)

1.0 1.5 2.0

400 600

Channel

800 1000

Figure 8. Composite of three energy spectra

taken near and on damaged spot.

Reflector is MgF2/ThF4.

4. Conclusions

Spatially resolved RBS provides detailed information about laser damage mechanisms in multilayer dielectric coatings. Our results show that the fabrication process can result in some interlayer mixing, particularly for those layers closest to the substrate. In addition, the data

allow us to identify different damage processes for the two coatings under study. In the case of

Si02/Hf02, the interlayer mixing progresses from the surface toward the substrate. For the

MgF2/ThF4, coating the process is more complex, involving subsurface mixing and loss of near-surface fluorine.

360
X-RAY SPECTRUM OF HAFNIUM

DIOXIDE/SILICON DIOXIDE MULTILAYER COATING

0 5 10 15 20 25

ENERGY (keV)

Figure 9. PIXE spectrum from Si02/Hf02 reflector.

We thank Larry Doolittle for providing us with an RBS simulation code and for guidance in

operating the code. We are grateful to Lev Borodovsky for helping with the experimental runs and

the computer simulations. The experiment would not have been possible without the assistance of Joe Tesmer and the operating staff of the Van de Graaff facility at Los Alamos National Laboratory.

*This work is supported by the Department of Defense.

5. References

[1] W-K. Chu, J. W. Mayer, and M-A. Nicolet, Backscattering Spectrometry . (Academic Press, New

York, 1978).

[2] T. B. Johnson, R. Akelsson, and S. A. E. Johansson, Nucl. Instr. Meth. 8A, 141 (1970).

[3] J. A. Cookson, "The Production and Use of A Nuclear Microprobe of Ions at MeV Energies,"

Nuclear Inst. Methods, 65, 477 (1979).

[4] C. Maggiore, "Materials Analysis with a Nuclear Microprobe," Scanning Elect. Microsc, 1,

439 (1980).

361
Manuscript received

c 4.4. • « 1 • r -n.- r--i 3-17-86 SAH

Raman Scattering Analysis of Thin Film

Coatings Using Guided Waves

Alan F. Stewaft

Air Force Weapons Laboratory

Kir-tland AFB, New Mexico 87117

and

David R. Tallant and Kar-en L.Higgins

Sandia National Laborator-ies

Albuquerque, New Mexico 87185

Selected refractory oxide, thin film coatings have been analyzed using Raman spectr-oscopy

.

Diamond and r-utile prisms were used to couple the pr-obe beam into the thin film coatings which

served as waveguides. Propagation of the probe beam as a guided wave in the thin film increases

the probability for- Raman scattering by species in the thin film, compared to techniques which

depend on a single pass through or- probe beam reflection at the film. Because the probe beam is

confined entirely within the thin film coating, Raman spectra are obtained which are generally

free of vibrational bands due to the substrate. The combination of mor-e efficient coupling be- tween the pr-obe beam and the thin film and the elimination of substrate interferences r-esults in

improved sensitivity and the ability to record recognizable Raman spectra of thin films composed

even of materials that are r-elatively inefficient Raman scatterer-s.

Key words: Thin film coatings, Raman spectroscopy, optical waveguide, prism coupling

1 . Introduction

Raman spectr-oscopy has been used extensively for- the deter-mi nation of the crystalline str-uc- ture of thin film coatings .^""^ As an optical technique, Raman scatter-ing offer-s some significant

advantages over- other- analysis methods. Since the Raman effect is the result of atomic motions,

local atomic structure in disordered as well as crystalline materials can yield distinctive Raman

signatur-es. However-, Raman scattering is intrinsically a weak effect with conver-sion

efficiencies on the order- of 10"'. The study of optical thin film coatings compounds the problem

of detecting a weak effect, since the interaction volume of the (micrometer- or- less) coating and

a focused laser- beam is very small. The supporting substrate may also contribute its own Raman

bands, which frequently overwhelm the film's contribution. To improve sensitivity, a variety of

experimental techniques including glancing incidence illumination, polarization selectivity and

specially designed coating structures have been implemented to enable the spectroscopi st to study

the structure of thin film coatings.

In this study, we have sucessfully demonstrated that, by using a thin film coating as a waveguide for- the probe beam, significant improvements in sensitivity can be achieved. The

interaction between the probe beam and the film is enhanced because the entir-e probe beam travels

along the film rather than passing through it a single time or- reflecting at the film's

surface. Because the probe beam is confined entirely w'thin the thin film coating, Raman spectra

are obtained which are free of Raman bands from the substrate. With the enhanced coupling be- tween the probe beam and the thin film and the elimination of substrate interfer-ences, it is pos- sible to record the Raman spectra of thin films composed even of materials that are relatively

inefficient Raman scatterers.

2 . Experiment

Raman spectra of thin film coatings were recorded using guided waves as well as glancing

incidence illumination. The experimental configuration, as detailed in figure 1, consisted of

both argon and krypton ion lasers, collection optics, a double monochromator- and cooled

photomultipl ier-. For this study, the spectral resolution of the system was 5 cm" . Typically,

the entrance slit height on the monochromator- was set at 1 cm. The collection optics provided a 6X magnification factor- so that the system "saw" only 1 .5 mm of the guided wave as it propagated

through the film.

362
The p»-obe beam at either- 514 or 647 ran was coupled into var-ious thin film coatings using

either diamond or rutile prisms. Both prisms wer-e cut at 40, 50, and 90 degree angles to provide

the appr-opiate range of coupling angles. The sample mount appears in figure 2 with a low-loss

film waveguiding less than a milliwatt of 633 nm light. Dur-ing these exper-iments, incident beam

power- levels at 514 nm and 647 nm were typically 300 and 120 milliwatts r-espectively . The laserpower- pr-opagating through the film is believed to have been much lower-, because of coupling inefficiencies and scatter-ing losses in the films.

The thin film coatings used in this wor-k wer-e of Ta^O^, Zr Oo, Y2O3, HfOo and Th02 . These

coatings wer-e deposited on super-polished fused silica substr-a-fes. The tnickness of these

coatings r-anged from 1/2 to 3/4 wave at 1.06 micr-ons. The Ta^O^ thin film coating was fabricated

using ion beam sputter- deposition. Electr-on beam deposition in an ultrahigh vacuum (UHV) chamber- was used to fabricate the Zr-02 film under- study. The Y2O3, Hf02 and Th02 films were fabricated

using conventional electron beam technology.

The ability to couple light into a film and propagate that light over- an observable distance

depends on a number- of factor-s. The coupling process itself r-elies on total inter-nal reflection

inside the pr-ism with evanescent wave propagation into the waveguide. In practice, it is difficult to achieve coupling efficiencies as high as the 90% levels predicted by theory. Propagation losses in the waveguide depend on the roughness of the substr-ate-film and air-film inter-- faces and the film microstr-ucture. The type of substr-ate, sur-face quality of the substrate, tem- perature and other- deposition parameters, in addition to the thermodynamic properties of the

coating mater-ial itsel-f, all ser-ve to deter-mine the film micr-ostr-ucture. Amorphous films such as those produced by ion sputter- deposition typically exhibit the lowest scatter-ing losses and

usually make the best waveguides. Thus the films which ar-e the most difficult to study using

other- analysis techniques are best suited for- waveguide studies.

It was obser-ved in the course of this study that film pr-opagation losses wer-e str-onqly

dependent on wavelength as well. Waveguide pr-opagation losses at 514 nm wer-e much higher- than at

647 nm. Even though Raman scattering efficiencies decrease as A

" at longer- wavelengths and

available laser- power- was reduced to 30 % of that available at 514 nm, comparable or- better- spectra could be obtained in some cases using 647 nm light due to reduced propagation losses.

3 . Results and Discussion

Initial attempts to use Raman scattering to study thin film structure were not very encour-a- ging. A spectr-um obtained on an amorphous Ta^Og coating using glancing incidence illumination

appears in figur-e 3. The spectr-um is completely dominated by the Raman signature of the fused

silica substrate. After- subtraction of the silica bands the remaining spectr-um is essentially

featur-eless.

The concept of using guided waves to study thin films had or-iginated much earl ier-.^ However-, the technique had not been applied to the study of optical thin film coatings. As in

earlier- work in integr-ated optics, a r-utile (Ti02) prism was initially used to couple 514 nm light into the same amorphous Ta20c; coating. The spectrum obtained appears in figure 4a. The

rutile prism produced its own Raman bands superimposed on those of the Ta20^. The rutile bands were subtracted out to produce the difference spectrum in figure 4b which resembles the reference

standard oowder- spectrum in figure 4d.

A diamond prism was found to be pr-eferable to the rutile prism as the coupling element.

Diamond has a high refractive index (2.41 at 556 nm) for- efficient coupling and no Raman bands

below 1300 cm"-^ . The spectrum of diamond has been compared to that of r-utile in figure 5. The

diamond prism allowed the collection of spectr-a without the need for- digital subtr-action of an interfering background spectrum. As demonstrated in figure 4c, the Ta205 film spectrum obtained

with a diamond coupling prism has more clearly defined featur-es and lower- noise levels. Peaks in

the film spectrum are either- missing or- br-oadened compared to the powder- spectrum. These ar-e typical differences observed when comparing spectra of crystalline and glassy for-ms of the same

mater ial .

A spectr-um obtained on a Zr02 film guiding 647 nm light appear-s in figure 6. Tliis film pro- pagated 547 nm light considerably better than 514 nm light. Weak, but distinct Raman bands due

to Zr-02 ^" ^^^^ ^^'^ shown to corr-elate with the Raman bands present in both monoclinic

and tetragonal zir-conia. Thus, both monoclinic and tetragonal local symmetries are found to be

present in the film. These observations are confirmed by the x-ray diffraction data plotted in figure 7. The x-ray diffraction data seems to indicate that the film structure is pr-imarily te- tragonal with peaks correla ting to the 111, 200, and 220 orientations. Secondary maxima appear

correlating to the 111 and 111 orientations of the monoclinic str-ucture. Although the Raman

353
bands of the two forms of Zr02 are approximately eaual in intensity in Figure 6, the r-elative

concentration of the monoclinic form is much less than that of the tetragonal form. The Raman bands of the two forms have similar- intensities because the scattering cross sections of the

major- monoclinic bands are nearly ten times the scattering cross sections of the major- tetragonal

bands. Thus, the Raman data also indicates that the film is primarily composed of the tetragonal form of Zr-02

.

Two of the lowest loss waveguides wer e f-jlms of Y2O3 and Th02 • The Y2O2 spectr um appear s in figure 8 with the principal band at 378 cm"^ clearly evident. However-, this band rests on a broad background with a drop-off at 500 cm"^ corresponding to the silica substrate spectrum.

This is the only spectra we have recorded using the guided wave technique which clear-ly shows the

influence of the substrate. We believe this results from propagation in a higher- order- waveguide

mode, which couples more strongly to the substrate than a lower- order- mode. The 378 cm"^ band

obtained from the film matches, within experimental error-, the most intense Raman band obtained

from cubic Y2O3 reference material.

In figure 9, the spectrum obtained on a Th02 film is compared with a reference spectrum of

cubic Th02. Clearly, the film spectrum does not resemble that of cubic Th02 - the only stable

form known for- Th02 . The thin film Th02 spectrum in figure 9 and the monoclinic Zr-02 spectrum in

figure 6 have similar- patterns, although the bands are shifted in frequency. This correspondence

in spectra suggests that there is also a correspondence in structure between monoclinic 7r02 and

the Th02 in the film.

Hafnia (Hf02), in bulk and in thin film form, was determined to be a weak Raman scatterer-.

In figur-e 10, the thin film spectrum is reminiscent of that observed on Zr-02 in terms of peak intensities. Both the HfOp and Zr-02 ^^^^^ relatively low loss waveguides, but neither- yielded

strong Raman bands. The HfOp reference spectrum peaks do match up well with those observed in the

thin film spectrum However-, the thin film peaks rest on a broad feature extending out beyond

1000 cm"-^ which is believed to be due to fluorescence (source unknown).

4 . Conclusion

We have demonstr-ated that Raman analysis of thin film coatings using guided waves can result

in significant improvements in sensitivity. Through the use of a diamond prism to couple light

into the film, spectr-a have been obtained which originate solely from the film mater-ial. From

the Raman spectra, it is possible to determine the local structure associated with the material

in the film. Thin film coatings which are amorphous, as determined by x-ray analysis, typically

ser-ve as excellent waveauides and thus are ideally suited for- study using this technique.

364
5. Refet-ences

[1] Pawlewicz, W.T.; Exat-hos, G.J.; Conaway, W,E.; Appl Opt 22, 1837 (1983).

[2] Hsu, Long-Sheng; She, Chiao-Yao; Exathos, Gregory J.; Appl Opt 23, 3049 (1984).

[3] Rujkorakarn, R.; Hsu, L.S.; She, C.Y.; Mat Bur- Stand (U.S.) Spec Publ # , pp (1984). Proceedings of the Symposium on Optical Materials for High Power Laser. (To be

publ i shed) .

[4] Sarid, Dror; Appl. Opt. 18, 2921 (1979).

[5] Swalen, J.D.; Rabolt, J.F.; Schlotter, N.E.; Knoll, W.; Gir-lando, A.; Philpott, M.R.; Proc

Soc Phot Opt Inst Eng 362, 31 (1982).

[6] Hether-ington III, W.M.; Van Wyck , N.E.; Koenig, E.W.; Stegeman, G.I.; Fortenbet-ry , R.M.; Opt

Lett 9, 88 (1984) .

[7] Tall ant, D.R., unpublished data

365
GUIDED WAVE RAMAN EXPERIMENT

n n

PRISM COUPLER AND

COATED SAMPLE

Kr LASER

I I

PM

I

Ar LASER

DOUBLE

MONOCHROMATOR

COLLECTION OPTICS

Fig. 1 Experimental arrangement

g. 2 633 nm light propagation through a Y2O3 thin film coating waveguide

366
1500

RAMAN SHIFT CM(-l)

Fig. 3 Raman spectra obtained on an amorphous Ta205 coating using the

glancinn incidence technique

367
GUIDED WAVE SPECTRA OF AMORPHOUS

TazOs COATING

1 1 1 I—I—1—I—1—I—I—

1

RAMAN SHIFT CM(-l)

Fig. 4 Raman spectra obtained on an amorphous Ta205 coating using

the guided wave technique: a) Spectrum obtained using a rutile prism for coupling, showing both rutile and Ta205

bands; b) Difference spectrum obtained after subtraction

of rutile bands; c) Spectrum obtained using a diamond

prism for coupling; d) Reference spectrum of Ta205 powder

368
1—1—I—

r

1 I I I I 1 1 I i r

TI02 PRISM

DIAMOND PRISM

I I I I I I I L

500 1000 1500

RAMAN SHIFT CM(-l)

2000

Fig. 5 Raman spectra of diamond and rutile

369
GUIDED WAVE SPECTRA OF Zr02 COATING

RAMAN SHIFT CM(-l)

Fig. 6 Spectrum obtained from a Zr02 thin film coating showing

correlation to monoclinic and tetragonal phases

370
X-RAY DIFFRACTOMETER DATA

ZrOj COATING

371
TH02 ON SI 02

CUBIC TH02

500 1020

RAMAN SHIFT CM(-l)

Fig. 9 Raman spectra of a Th02 thin-film coating and of

cubic Th02

372
1 1 1 r

HF02 ON SI02

HF02 - POLYCRYSTALLINE

J I I 500

RAMAN SHIFT CM(-l)

1000

Fig. 10 Raman spectra of a Hf02 thin-film coating and of

polycrystal 1 ine Hf02

373
Manuscript Received

20-31-85

Raman Microprobe of Laser-Induced Surface Damage Regions

in Ti02 and Zr02 Coatings

Donald M. Friedrich and Gregory J. Exarhos

Pacific Northwest Laboratory*

Richland, Washington 99352

The anatase-to-ruti le phase transition, which attends pulsed laser-induced damage

of anatase Ti02 thin films, is studied by simultaneous microphotography and Raman microprobe. The damage is distributed across the damage area in regions of distinct

morphology and spectra. In multilayer coatings, a mechanical surface-fracture zone surrounds a set of thin, layered flat rings of distinct but irregular boundaries.

These rings surround a rough, pitted transparent central crater in the quartz

substrate. Raman difference spectroscopy is used to reveal damage regions containing

the rutile phase and regions devoid of Ti02> Similarly, Raman microprobe spectra

reveal the monoclinic crystalline form of Zr02 in damaged regions of amorphous Zr02

coatings. Finally, frequency shifts of the anatase line (nominally 143 cm"l)

are used to identify stress in laser-pulse irradiated anatse coatings.

Key words: laser damage; microprobe; Raman spectroscopy; Ti02 optical coatings;

Zr02 optical coatings

1. Introduction

Previous time-resolved Raman studies have shown time-dependent reversible and irreversible

structural changes in pulse-laser irradiated Ti02 and Zr02 dielectric optical coatings. [1] In single and multilayer anatase Ti02 coatings, sufficiently intense pulsed laser irradiation at 532 nm led to observation of a partial loss of intensity from anatase Raman lines and concurrent

production of an irreversible phase transition from the anatase to the rutile crystalline forms

seen in the Raman spectrum. Similarly, Raman spectra from amorphous Zr02 coatings revealed

phase changes from amorphous to monoclinic crystalline form in the irradiated region of the

coating. These measurements and observations are described in references [l] and [2].

In order to investigate in greater detail the nature and distribution of pulsed laser-induced

morphological changes in these dielectric optical coatings, we have explored the damage regions

with a Raman microprobe. Damage regions range in size from several microns to nearly a millimeter

in diameter, depending upon the laser energy, and they vary widely in shape and depth. The Raman microprobe can collect spectra from coating surface areas down to a few microns diameter. The

damage topology then can be correlated with observed Raman spectra, which can reveal changes in the morphology of the coating materials. The microprobe also reveals stress conditions of the

anatase crystalline form. The stress is observed through frequency changes in the sharp

143 cm"l (v5(Eg)) line of anatase.

Pacific Northwest Laboratory is operated by Battel le Memorial Institute for

the U. S. Department of Energy under contract DE-AC06-76RL0 1830.

374
2. Experimental

The damage areas studied were previously induced in the Ti02 and Zr02 dielectric coatings

as described in [l]. Briefly, the coatings were prepared by reactive sputtering of Ti or Zr in an Ar/02 atmosphere on fused silica substrates [3,4]. These coatings were irradiated with a range

of 532 nm pulse energies (0 - 10 mJ) and focused to spot sizes 1 mm or less. The equilibrium and

time-resolved Raman measurements from these samples were reported in reference [l]. Selected

damage regions from these same samples were studied in detail by the Raman microprobe technique.

The Raman microprobe apparatus consists of a Spex Micramate attached to a Spex 1403 double

monochromator with the Datamate photon-counting system. The laser excitation was focused on the

sample by a 40X (.60 NA) objective, which also collected the back-scattered Raman light for

imaging on the entrance slit of the monochromator. The images, in reflected or transmitted white

light, were also relayed to a video camera. The sample damage regions were photographed from the

video screen. Good spectral resolution of the sharp 143 cm-^ anatase line was achieved with

spectral bandpass of 4 cm-1 (400y slits). Spectral line shifts of 0.5 cm"! could be observed

reproducibly. Spectral line shifts were further studied at higher resolution (0.5 cm"^

bandpass) and confirmed the lower resolution findings.

CW Ar"*" laser lines of 514.5 nm and 488.0 nm were used in these studies with power levels up to 220 mW into the Micramate optics. With regard to effective spatial resolution of surface

features, it should be noted that, for a given signal-to-noise ratio, the surface area resolved

will increase with increasing laser-probe power. Usable Raman signals could be obtained from the

damage regions with powers as low as 30 mW into the Micramate. Less than 40% of the input power

finally reaches the sample surface. The high cw-laser powers never damaged the refractory surface

of an undamaged coating region. However, the cw probe-laser did occasionally induce damage to darker damage features or coating cracks at probe powers in excess of 100 mW into the Micramate

optics. These observations will be described below.

3. Damage Topology and Raman Spectra

Damage spots produced by the higher energy pulses revealed layered topological features in the thin (<ly) single-layer and multilayer coatings. In the thicker anatase coating, the damage

features were qualitatively different and did not show distinct boundaries. Representative damage

features in the thin and thick coatings are shown in the photomicrographs (figures 1,3,6) and are now discussed.

3.1. Thick (4y) Anatase Coating (Sample No. IIIJ)

A thick, diffuse reflecting aflatase coating was studied in order to provide a reference

damage behavior most like bulk Ti02 anatase. In the undamaged state, this coating consisted of

a close-packed array of irregular islands of anatase separated by thin cracks. The pattern

resembled the cracking pattern appearing in drying mud-flats. The cracks appear to be less than

ly wide. The anatase islands varied in size, between 10 and 20 microns across, and were

designated as "mud-cakes" (figure la). The Raman spectrum from these was identical to that of

pure Ti02 powder. In the center of most damage craters, a similar more finely-grained cracking

pattern was observed (figure lb) with mud-cake islands ranging 5 to lOy across. The Raman spectrum from each of these smaller cakes in the damage center showed no shifting and no evidence

for rutile bands, i.e., the spectrum from the damage region was also identical to Ti02 powder.

Surrounding the center damage region is a rippled region resembling sand dunes; and surrounding

the dunes region is an irregular topology which appears to consist of melted splashes of coating

material (the "melt region", figure Ic). Neither of these regions revealed rutile features in the Raman spectra.

The cw Ar"*" laser caused damage to the coating when the probe laser spot was positioned in one of the cracks, either inside or outside the pulsed laser-induced damage region, and the cw

laser power exceeded 100 mW. The coating developed a single black spot (neither reflecting nor transmitting light in the microscope); and concurrently, the anatase was converted to rutile as indicated by loss of anatase lines and strong increase in rutile lines in the Raman spectrum

(figure 2). This cw-induced damage is probably thermal in origin and indicates the necessity to use lower cw powers when probing the damage regions even in refractory materials.

375
Figure 1. Photomicrographs of a 4]j-thick anatase sample (#111J).

(a) Undamaged 4y-thick anatase layer showing the larger "mud-cake"

topology;

(b) Center of a large (ca. 1 mm diameter) damage spot showing smaller

"mud-cake" forms;

(c) Transition from melt region to dunes region to central crater in damaged 4y-thick anatase (lOX).

376
Figure 2. Raman spectrum of cw-laser-induced thermal damage to the 4y-thick

anatase coating (#111J). The 488 nm laser was positioned at a crack

between undamaged anatase cakes (see figure la). The laser power was

220 mW into the Micramate optics, Rutile shoulders appear at 422 cm-1

and 595 cm-1. The anatase V5 line (not shown) appears at 148 cm"l.

(b) Reference spectrum from undamaged anatase coating, (c) Reference

spectrum from undamaged rutile coating.

3.2. Thin and Multilayer Anatase Coatings

Two anatase mirrors were investigated: a single thin layer (0.96y, sample #111F) and a 17-layer Si02/Ti02 three-cavity transmission filter for 514.5 nm (sample #115C). The

multilayer coating showed remarkably regular damage topology (figure 3). The damage areas consisted of a central dark spot, often rough in texture, frequently with one or more flat,

greenish-colored islands in the dark crater. Raman spectra from central damage regions generally

showed only two broad, weak bands from amorphous Si02 and no Ti02 spectral features. The dark crater is surrounded by a series of layered shelves, which show little or no Ti02 content. The

shelves tend to be narrow (5 to 20ij wide). The shelf region is surrounded by a severely

pockmarked coating surface, which is then surrounded by a region of surface fracturing. The

surface fracturing is remarkable in forming an intricate array of angular geometric patterns of

high aspect ratio resembling a map of streets and boulevards. The extent of the fracture region

varies, but it seems to increase with increasing pulse energy and crater size. The microprobe

spectra from selected microscopic areas within these regions show that the anatase is partially or completely depleted and that the anatase vg is often shifted one or two wavenumbers to higher

frequency. Evidence for rutile concentrated in microscopic areas of the multilayer coating was below the signal/noise limit of these experiments.

377
SiOa/TiOz Multilayer

a. 10x Damage Edge

100/7

8 7 6 5 4 3 2 1

5

4 3 2 1

c. 40x Fracture Zone

Figure 3. (a) Damage topology in the Si02/Ti02 anatase multilayer (#115C;

lOX objective, 0.22 NA), The damage crater is the dark region on the

right (1). A series of irregular but distinctly bounded shelves

surround the darkened damage crater (2 through 5). These regions are depleted in Ti02. The transition to the undamaged coating is made

through a deeply pitted ring (regions 6-7) and a broad zone of surface

fractures (8).

(b) Shelf regions (1-5) shown at higher magnification (40X objective,

0.60 NA).

(c) Surface fracture zone (8) at higher magnification (40X) showing the

angular narrow street-like fracturing of the mirror surface.

378
The thin-layer anatase mirror (#111F) did not show the regular damage topology observed in the multilayer coating. There was no sharp transition from damaged to undamaged regions. The

damage craters had a rough and dark surface of irregular shape and no sharp boundaries. At sufficiently low pulse energies, the coating did not form craters but simply darkened by forming a dense array of dark microcraters (<lp dia.). As in the multilayer microprobe studies, the rutile

Raman lines from microscopic probe areas from the single coating were extremely weak--being at the

signal/noise limit.

The anatase-to-rutile conversion following laser-induced damage of coating #111F was recorded

earlier [l] by using a cw-probe laser spot, which integrated a much larger surface area (SOy to lOOy probe diameter at focus). Since strong rutile lines are absent from the microprobe spectra,

the observed rutile may be distributed at low concentrations over a wide area of the damage

region. The microprobe results show that the dominant mechanical damage effect is ablation of

Ti02 from the surface.

3.3. Amorphous Zr02

This sample (l.lSy thick, sample #1) shows a featureless Raman spectrum from the undamaged

coating. All damaged regions show either the growth of Raman lines from the recrystal 1 ized

monoclinic form of Zr02 (figure 4) or else amorphous fused Si02 bands (in strongly damaged

craters). These findings are consistent with the spatially unresolved studies in [1]. The Zr02

damage topology is similar to that of the anatase mirrors. The crater is surrounded by regions of

surface fracture and pockmarks.

I Amorphous Zr02 1 .1 8 //

300 500 700

Wavenumbers

Figure 4. Raman spectrum of amorphous Zr02 (l.lSy, sample #1) showing lines

from (a) monoclinic crystalline form produced in the damage region;

(b) undamaged amorphous coating.

379
0.96 fj Anatase

100 150 200

Wavenumbers

Figure 5. Expanded trace of the vg line in anatase showing (a) unstressed

form (143 cm-1) and (b) stressed form (146 cm"l)

4. Laser-Induced Stress in Anatase Coatings

It is well-known that the process of coating a substrate can result in stressed coating and

substrate. The stress is often estimated by measurement of thin substrate distortion. In a recent study Fauci^et, Campbell, and Adar [5] report laser-induced relaxation of stress in thin

silicon films on quartz substrates. By contrast, observation of the 143 cm"l line of anatase

coatings suggests that high-energy laser pulses induce a compressional stress in dielectric

coatings, Ohsaka, et al., [6] report a specific spectral shift of 0.312 cm"l kbar"^ for the

143 cm"l V5 line of bulk anatase [5]. We always observe an increase in this vibrational

frequency in the vicinity of damage in an anatase coating (figure 5). The stress-induced spectral

shift is not uniformly or smoothly distributed. In figure 6 the anatase multilayer coating

(sample #1150) surrounding a single-shot damage crater reveals two responses to the laser-pulse.

On the right-hand side of the crater, the coating has cracked into islands of apparently higher

reflectivity. The anatase in these islands shows no compressional stress (Vg = 143 cm"l).

Around the remainder of the periphery, the undamaged coating shows no cracks. Instead, irregular

fingers of lower reflectivity fan out into the coating for about I6O1J. Figure 7 is a graph of the

Vg frequency versus distance from the edge of the damage crater along one of these darker fingers.

Far from the edge, the anatase is unstressed (Vg = 143 cm"l). As the edge is approached, the stress suddenly increases about lOOp away from the edge. At the crater edge, the frequency shift

is highest (146-147 cm"-^). This shift corresponds to a bulk compressional stress increase of

about 10 kbar. The largest such stress-induced frequency shift was observed inside a small damage

crater created by an intense cw-probe laser (514.5 nm) in this coating. The observed Vg of

149 cm"l results in a computed bulk compressional stress of 19 kbar.

380
Si02/Ti02 Multilayer

10 X Objective

Figure 6. Photomicrograph of (a) stressed and uncracked and (b) unstressed

but cracked anatase multilayer coating (#115C) around a damage

region.

5. Conclusions

The process of material phase changes, discovered in the time-resolved Raman spectra of

pulse-damaged coatings [1], is confirmed by the Raman microprobe technique. The morphology

changes are distributed non-unif ormly over the damage region. Severe damage spots show complete

ablation of Ti02 as indicated by loss of Ti02 features in the Raman spectra from the crater

regions. The combination of macroscopic probe [l] and microprobe studies indicates that the

rutile, produced by pulse damage in anatase coatings, may be distributed at low concentrations

throughout the damage regions. Amorphous Zr02 shows conversion to the monoclinic crystalline

form in the damage zones. Monoclinic damage in amorphous Zr02 is easier to detect because the

spectrum of undamaged amorphous material is relatively featureless.

Surface fracturing in angular, geometrical patterns and deeply pitted or pocked areas are observed around the larger damage craters in the mirrors. Anatase stress, which may be a precursor to the fracturing, is measured by the frequency shift of the anatase vg line. The

typical compressional stress shift observed in these studies was approximately 3 cm-1 or 10 kbar

381
146(

145

(cm'')

144

143

142

50 100 150

Microns from Edge

of Damage Crater

200

Figure 7. Raman frequency of vg on a track from the edge of the damage

crater out to 160ij away from the edge, along a visible stress

line in the coating.

This work was supported by the Air Force Weapons Laboratory, Kirtland, AFB, NM, under

Contract PO-85-037. The optical coatings were provided by Dr. W. T. Pawlewicz. The authors also

thank Mr. Clark H. Nguyen and Captain P. L. Morse for their assistance and advice during this work.

6. References

[l] Exarhos, G.J., and Morse, P.L. Raman studies of laser-damaged single- and multi-layer

optical coatings. Proceedings of SPIE Southwest Conference on Optics; 1985 March 4-8;

Albuquerque, NM.

[2] Exarhos, G.J., and Pawlewicz, W.T. Raman characterization of al 1 -dielectric multilayer

Si02/Ti02 optical coating. Applied Optics 23(12) :1986-1988; 1984.

[3] 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 January 26-27; Los Angeles, CA. Proc. Soc. Photo-Opt. Instrument.

Eng. 325:105-116; 1982.

[4] Pawlewicz, W.T., Hays, D.D., and Martin, P.M. High-band-gap oxide optical coatings for 0.25

and 1.05 micron fusion lasers. Thin Solid Films 73:169-175; 1980.

[5] Fauchet, P.M., Campbell, I.H., and Adar, F. Long-range material relaxation after localized

laser damage. Applied Physics Letters 47:479; 1985.

[6] Ohsaka, T,, Yamaoka, S., and Shimomura, 0. Effect of hydrostatic pressure on the Raman spectrum of anatase (Ti02). Solid State Communications 30:345-347; 1979.

382
Manuscript received

2-11-86 SAB

Probing Structural Properties of Amorphous Films

With Raman Spectroscopy

C. Y. She and L. S. Hsu*

Physics Department, Colorado State University

Fort Collins, Colorado 80523

Using an oblique geometry, we have previously demostrated that Raman spectroscopy

be used to characterize submicron-thick optical films. With this technique, we have

discovered and studied amorphous-crystalline and anatase-rutile transformations in various types of titania films. These studies were however done with treatment and diagnostics carried out in a sequential manner. One objective of this paper is therefore to establish the feasibility of using in-situ Raman spectroscopy to monitor the process of

crystallization and structural transformation of optical thin-films in real time. A second objective is to demonstrate that Raman spectroscopy can be used to detect microscopic differences existed in amorphous thin films. We show that the microscopic differences in three otherwise similar types of amorphous titania films may be correlated with their obvious structural differences upon thermal annealing.

Key words: Raman spectroscopy; optical coatings, Ti02; amorphous types; in-situ study;

structural transformation.

1. Introduction

We have previously reported the use of Raman spectroscopy for characterization of micronthick optical films [1]. Raman spectroscopy has not been used much until recently, due perhaps to

an obvious fact (or misconception) that a film thinner than one optical wavelength may not scatter

enough Raman signal for structure characterization. Recently, using Raman spectroscopy in a 180°

backscattering geometry, Pawlewicz et al . [2] were able to characterize crystalline single-layer

optical coatings with a measurement time a factor of 100 less than that required for the conventional method of x-ray diffraction, demonstrating Raman spectroscopy as a rapid method for film

diagnostics. One of the major factors which limits the sensitivity of their single-layer, Raman technique is the interference resulting from the Raman signal of the substrate. This substrate

interference is probably also the reason which renders a commercial Raman microprobe ineffective

in characterizing a submicron film, in particular an amorphous film on a transparent substrate.

We have devised techniques to improve detection sensitivity by using (1) an oblique-incident

geometry to reduce laser light in the substrate and (2) a different collection system, with which

the scattered light is collimated before refocussing into a spectrometer, to limit the depth of

view mostly to the film of interest. For example, when the substrate interference is suppressed

by more than 50-fold in this manner £3], we obtained Raman spectra of titania coating as shown in

Fig. 1. The fact that the spectra of three titania films with different structures, but the same stoichiometry , are so dramatically different demonstrates the effectiveness of Raman spectroscopy

for structural characterization of thin films. The spectrum of the amorphous film represents its

characterization for the first time by any method. Even the time consuming method of x-ray diffraction fails to characterize an amorphous coating.

With our Raman technique, we have discovered and investigated several phase transformations

in submicron titania films induced by laser [4] and thermal [5] annealing. The similarity between

laser and thermal annealing processes may be seen in Fig. 2 where the spectra of Ti02 coatings

under different laser and thermal annealing conditions are compared. From the thermal annealing

results, we have found that all amorphous titania films studied crystallize at around 350°C upon

heating. Our Raman data suggest that the exact transformation temperature and the transformed

crystalline structure depend upon fabrication method and deposition rate.

*Present Address: Control Laser Corp., 11222 Astronaut Blvd. Orlando, FL 32821

383
2. Microscopic Differences in Amorphous Titania

What is perhaps more significant is that the outcome of the transformed crystalline structure

seems to correlate with the minute differences existed in Raman spectra of the as-deposited amorphous films. Figure 3 shows Raman spectra of three types of amorphous titania films A, B, and C. A and B were prepared by ion-beam sputtering and C by e-beam evaporation. Besides the broad lowfrequency shoulder, indicative of amorphous structure, weak rutile peaks of different strengths and

widths at 440 and 612 cm"l can be seen in these spectra. By means of Raman spectroscopy, we have

established the fact [5J that an amorphous film with weak (broad linewidth) rutile information

(Type C) crystallizes at a lower temperature into the anatase structure, while a film with relatively strong (narrower linewidth) rutile formation crystallizes into microcrystal 1 ine rutile

structure at a higher temperature (Type B). The intermediate Type A films result in a mixed (rutile and anatase) microcrystal 1 ine structure. We think that since the anatase structure is ener- getically more favorable around 350°C, the outcome of the transformation depends upon the competition between the thermodynamical ly favored anatase and the minute rutile seeds existing in the as- deposited film.

3. In-situ Raman Study

These initial experiments have clearly demonstrated the potential of using spontaneous Raman spectroscopy for optical and electronic film characterization and studies. These studies were done

with treatment (by laser or thermal annealing) and diagnostics (with Raman scattering) carried out alternatively in a sequential manner. There is an obvious need for in-situ diagnostics in which

Raman spectroscopy is recorded simultaneously as the process of crystallization and/or structural

formation is taking place in the film. This way, possible changes or contamination on the film

between treatment steps can be avoided; the rate of crystallization and transformation processes

can also be determined.

A home-made oven with optical access was constructed to demonstrate the feasibility of in-situ

Raman study. As an example, the growth of anatase in a titania film at 400°C is investigated on real-time with this oven. The spectrometer was scanned back and forth around the strongest anatase

peak at 150 cm"^ and the peak height was monitored as a function of time. This procedure was in

fact carried out for a Type C titania film coated on a Si wafer, and the resulting growth of

anatase peaks was recorded in Fig. 4(a). For the setting corresponding to annealing at 400°C,

anatase growth begins after 20 min. of heating. The process of anatase crystallization is approximately completed after 45 min. of heating at this temperature; its Raman intensity signifies 100%

of anatase formation. After the crystallization process is completed, the whole Raman spectrum is also recorded for reference; as shown in Fig. 4(b), the film indeed consists of anatase structure

without trace of rutile formation. The peak height may be converted to the percentage of anatase

formation giving rise to the experimental points labeled 400°C in Fig. 5. Alternatively, realtime observation of the crystallization process may be made by setting the spectrometer at the

peak of the strongest Raman mode, i.e., 150 cm~l for 400°C etc., and recording the Raman intensity

as a function of time. This was in fact carried out for annealing at 325°C and 600°C. A different piece of identical Type C film was used for each annealing temperature giving rise to respective anatase growth shown as solid curves in Fig. 5. The effect of annealing temperature on growth rate is also evident. In-situ Raman study of Type A titania films has been published

elsewhere [6].

4. Conclusion

We have demonstrated that Raman spectroscopy is a simple and effective diagnostic tool for

thin-film studies. Among other things, our Raman technique is capable of distinguishing microscopic differences in amorphous films, analyzing the effect of laser and thermal annealing, and

predicting the outcome and exploring the physics of structural phase transitions. The in-situ

capability demonstrated here suggests that Raman spectroscopy can be used as a real-time probe

for film deposition process as well. Although only titania films are investigated in this report,

the approach should be of general interest and can be applied to other thin-film materials [7].

We thank G. Al-Jumaily and J. R. McNeil of UNM, and R. Rujkorakarn and J. R. Sites of CSU for

supplying the films for this study. This work was supported by the U.S. Air Force Weapon Laboratory under contract F29601 -83-K0079.

384
5. References .

[1] R. Rujkorakarn, L. S. Hsu and C. Y. She, "Crystallization of titania films by thermal heating," Proc. 16th Annual Symposium on Optical Materials for High Power Lasers, NBS Special

Publication (1985).

[2] W. T. Pawlewicz, G. J. Exarhos and W. E. Conaway, "Structural characterization of Ti02 optical

coatings by Raman spectroscopy," Appl . Opt. 22^, 1837 (1983).

[3] L. S. Hsu, C. Y. She and G. J. Exarhos, "Reduction of substrate interference in Raman spectroscopy of submicron titania coatings," Appl. Opt. 2^, 3049 (1984).

[4] L. S. Hsu, R. Solanki, G. J. Collins and C. Y. She, "Raman study of structural transformations

of titania coatings induced by laser annealing," Appl. Phys. Lett. 45^, 1065 (1984).

[5] L. S. Hsu, R. Rujkorakarn, J. R. Sites and C. Y. She, "Thermally induced crystallization of

amorphous titania films," J. Appl. Phys. (accepted for publication).

[6] L. S. Hsu and C. Y. She, "Real-time monitoring of crystallization and structural transformation of titania films with Raman spectroscopy," Opt. Lett. 1_0, 638 (1985).

[7] R. Rujkorakarn and J. R. Sites, "Crystallization of zirconia films by thermal annealing,"

J. Vac. Sci. Techno!, (to be published. May 1986).

TlOi/SlOt ANATASE

2.0

500 WAVENUHBERS

900 100

TlOt/SlDi RUTILE

500 WAVENUMBERS

2.5

900 100 500 WAVENUHBERS

900

Figure 1. Raman spectra of stoichiometric Ti02 films with different

structures demonstrating the effectiveness of using Raman spectroscopy

for thin film characterization. Film thickness is 500 nm.

385
0

- 3.0

BEFORE ANNEALING

(a)

0

3.[>

500C ISM IN

(b)

90dc 15MIN A

(0

550 WAVENUMBERS

1000-

1000 300 500 WAVENUMBERS

700 100 500

WAVENUMBERS

900

Figure 2. Raman spectra comparing thermal annealing of a titania on silica

film and laser annealing of titania

on silcon film. Both ion-sputtered

films are Type A amorphous as- deposited, and the annealing pro- cesses vjere carried out with fast

heating

.

Figure 3. Raman spectra of as- deposited amorphous Ti02 films

(a) Type A, (b) Type B and (c) Type C. All spectra were obtained with

obi i que- incident geometry along with

substraction technique. Film thickness is again 500 nm.

386
20 30

MINUTES

0 300 600

RAMAN SHIFT (cm"

Figure 4. Typical in-situ Raman record of a Type C titania film annealed at 40Q°C as a function of time showing the amorphous-anatase transformation,

(a) Raman intensity of the Eg mode of anatase at 150 cm"^ . (b) Entire

Raman spectrum scanned after completion of crystallization.

MINUTES

Figure 5. Growth curves of anatase crystallization of Type C titania films. Curves for the annealing temperatures at 325°C

and 600°C are derived from the direct records of Raman peak

intensity of the Eg anatase mode, while the 400''C curve is deduced from the intensity scans of Fig. 4(a).

387
Manuscript received

2^18-86 SAB

Detection of laser damage by Raman microscopy

P.M. Fauchet, I.H. Campbell

Department of Electrical Engineering

Princeton University .

F. Adar

Instruments S.A.

ABSTRACT

We demonstrate that Raman microscopy is a sensitive and quantitative tool to detect and

characterize laser-induced damage in solids. After damage is induced with single or multiple high power laser pulses, a Raman microprobe maps the surface of the sample with one micron spatial

resolution. By performing accurate measurements of the Stokes line, we have been able to measure

stress, strain and crystallinity in various samples which had been exposed to high intensity pulses. These results are compared to those obtained using conventional tools such as Nomarski microscopy. Major advantages of Raman microscopy include sensitivity to subtle structural modifications and the fact that it gives quantitative measurements.

Key Words: laser damage; Raman microscopy; silicon.

INTRODUCTION

Many questions regarding the nature of single and multiple pulse laser damage remain unanswered, in large part because of the lack of adequate probes. Detection and characterization of localized laser damage should be performed in

situ, with nondestructive and sensitive probes having good

spatial resolution. Furthermore, quantitative interpretation should be possible and the data acquisition time reasonably

short. Here, we demonstrate that Raman microscopy meets

these criteria by investigating the structural modifications that are produced in thin films by high power near infrared and visible pulses. The power of Raman scattering resides in

its sensitivity to structural properties such as stress, strain, and microcrystaUinity. The use of a microprobe is essential because dramatic changes do occur on a scale of a few

microns. RAMAN MICROSCOPY

To demonstrate the power of Raman microscopy, we

elected [l] to first investigate in some detail a well-known

material. Silicon was chosen because good quality bulk or

thin film samples are conveniently available, and because its Raman line is well characterized. Thin (less than one micron)

silicon films on insulator (SOI) grown by low pressure chemical vapor deposition (LPCVD) at 900K have a columnar

structure with an average grain size <50 nm. Damage is pro- duced by pulses at 1064 nm from a Nd.YAG laser, which are weakly absorbed by silicon. The model case under study here

is thus relevant for the study of laser damage in thin films and optical coatings. Very recent results obtained with

titania films damaged at 532 nm are also presented later and

can be discussed in the light of our silicon results.

The Raman Stokes line is characterized by three major

parameters:

a: the peak frequency

brTtheFWHM

c: ^^/^b t'he asymmetry factor

For this study, the influence of stress or strain, and the

influence of microcrystaUinity on the line must be considered. Uniform stress shifts the peak frequency [2] but leaves the FWHM and the asymmetry factor unchanged. In microcrystalline material, the phonons have a finite spatial extent pro- portional to the crystallite size [3]. Phonons with wavevector

diff'erent from 0 thus contribute to the line. In many materials, such as silicon, the phonon frequency decreases with

increasing wavevector. Thus, as the crystallite size decreases, the Raman line shifts toward lower frequency, broadens, and

becomes asymmetric [4]. Figure 1 compares the line of unstressed single crystal silicon with that of microcrystalline SOI. Figure 2 is a block diagram of the Raman microprobe

[5j. With this apparatus, the spectra consist of points spaced

by 0.1 or 0.2 wavenumbers taken with a 1 to 4 seconds exposure time (for Si). The spectral resolution is set at 3 wavenumbers, a convenient compromise between accuracy and data acquisition time. The laser power is kept around 3 mW in a one micron spot on the sample (for Si). The beam

is positioned with the help of translation stages and a high

resolution display of the surface. A polarizer/analyzer pair is used to take advantage of the polarization selection rules. The various wavelengths available from a krypton laser and

an argon laser allow depth profiling. The entire experiment is under computer control.

388
RESULTS AND DISCUSSION

In a first experiment [l], we exposed SOI to single 40 ps/1064 nm high power pulses. At damage threshold, high

resolution Nomarski microscopy reveals a few randomly dis- tributed "wormlike" features 1 to 2 microns in size. The six spectra of Figure 3 were taken, from top to bottom, at the

center of one isolated damage site, on the edge, 2, 5, and 9 microns away, and on virgin material. After damage, the material displays good crystalline properties (narrow line =

good homogeneity, symmetric line = large grain size) but it

is under considerable tensile stress (10^'^dyne/cm The

amorphoushke shoulder at 490 wavenumbers is not present, indicating the absence of grain boundary material. The

Raman line does not become identical to that of virgin SOI

even 10 microns away from the damage site where the surface appears untouched. This long range material relaxation is rather surprising but has been verified in a variety of situa- tions [6]. In a second experiment [l], large (100 microns) hetero- geneously damaged regions have been produced by 600 shots

at one-half single shot damage. The morphology of the sur- face appears to be a dense packing of "wormlike" features. At

the edge of the damaged area, we observe a smoother annular

region. In figure 4, the boundary between virgin and

transformed material is at 0. The focused probe beam is scanned from well inside to well outside the damaged area. In the center, where damage is heavy, the surface is very dis- torted and large variations in stress are observed. The stress becomes uniform in the smooth annular region (from -15 to 0) but the FWHM gradually increases. At the visible boundary,

the line is abruptly altered, but takes 20 microns to fully recover. K the probe wavelength is changed, depth profiling can be achieved. We have repeated these experiments at three different wavelengths and obtained similar results. This

indicates that a large fraction, if not all of the one micron

thick film has been modified.

In figure 5, we plot T vs ^^/^^ and OJ^ vs F for vir- gin SOI (0), and at various locations in and around single- shot () and multishot (•) damage sites. Closer to the damage site, r, cJq and F^/F^^ become smaller. The full line results from a calculation which includes the effect of crystal

size on the Raman line but neglects the influence of stress. Obviously, the laser irradiated material is now under considerable stress. We also note that F, u)^ and F^/Fi, tend to become smaller on areas damaged by single shots then on

areas damaged after multipulse illumination. This difference may come from the fact that complete melting takes place in the former case, leading to regrowth with larger grain size under large tensile stress, whereas in the latter case, the

material fails heterogeneously and does not undergo thorough

melting. This point deserves further study.

The experiments performed on SOI are now being

repeated on titania, zirconia, and other materials used in high power AR and HR coatings. Preliminary results obtained with 662 nm thick titania films on silica indicate that the Raman microscope is also a powerful tool in transparent media. A film was damaged with a several high power 532 nm/30 ps pulses from a Nd:YAG laser system. We took spec- tra on the edge of the damaged area and compared them with

the spectrum of the virgin film. In figure 6, we show the transformation of the intense 142 wavenumbers line of anatase. The data acquisition rate is approximately 10,000 counts per second at the line center with 10 m W at 514.5 nm

focused to 1 micron. Our present interpretation of the data is that there is a stress variation under damage as in silicon. More detailed experiments are now under way. CONCLUSION

In conclusion, we have shown that Raman microscopy is an excellent probe of the structural modifications produced

during laser damage. Material alterations not detected by

high resolution Nomarski microscopy or probe light scatter

are clearly observed and mapped with one micron resolution. For the first time, significant laser-induced damage was

detected up to 20 microns away from the visible damage

sites. This finding, which is also important for laser processing, highlights the necessity of using a microprobe. Future

studies win be concerned with the role of stress and solid phase transformations in single and multiple pulse damage.

ACKNOWLEDGEMENTS

This research was supported in part by an NSF grant and an equipment loan from LLNL. We thank Dr. N. Johnson (Xerox PARC) and Dr. G. Exarhos (Pacific Northwest

Laboratory) for providing the SOI and titania samples respectively. REFERENCES

1. P.M. Fauchet, I.H. Campbell and F. Adar, "Long range

material relaxation after localized laser damage," Appl. Phys.

Lett. 47, 479-481 (1985).

2. E. Anastassakis, A. Pinczuk, E.Burstein, F.H. Pollak and M. Cardona, "Effect of static uniaxial stress on the Raman

spectrum of sihcon," Solid State Commun. 8, 133-138

(1970).

3. H. Richter, Z.P. Wang and L. Ley, "The one phonon

Raman spectrum in microcrystalline silicon," Solid State Commun. 39, 625-629 (1981).

4. I.H. Campbell and P.M. Fauchet, "The effects of microcrystal size and shape on the one phonon Raman spectra of

crystalline semiconductore," Submitted for publication.

5. P.M. Fauchet, "The Raman microprobe: a quantitative

analytical tool to characterize laser-processed semiconductors," IEEE Circuits and Devices Magazine, 2, 37-43 (1986).

6. I.H. Campbell, F. Adar and P.M. Fauchet, "Properties of

thin films after focused beam processing," presented at, the VVnnual Fall Meeting of the Materials Research Society (Boston, December 1985), and to be published in Semiconductor

on Insulator and Thin Film Transistor Technology, MRS

Press - 1986.

389
WAVE NUMBERS

Figure 1

Optic phonon Stokes line of single crystal silicon (fine line) and of polycrystalline silicon thin film (thick line). With the standard operating parameters of our instrument, CJ^ = 520 cm~\

r ~ 3.8 cm~^, and TJT^ ~ 1 for crystalline silicon, and u)^ ~ 518.5 cm"'^ F ~ 9 cm~\ and

T^/Ty^ ~ 1.4 for the polycrystalline silicon thin films. In these films, the exact parameters of the Raman line are not perfectly identical everywhere (for example, varies from 518.5 to 519 cm~^ over large distances). These variations reflect stress and homogeneity changes in the

film itself.

390
DISPLAY

Figure 2

Block diagram of the Raman microprobe. The probe laser beam is focused through the objective

O to a one micron spot on the sample surface. The backscattered Raman signal is collimated by

the same objective and analyzed by a double-1 meter spectrometer. The standard optical microscopic image of the surface appears on the display.

391
J I 1

' ' I 480 490 500 5KD 520 530 540

WAVENUMBERS

Figure 3

Spectra taken around an isolated ~1 micron damage-site formed on a polycrystalline silicon film by a single 40 ps/1064 nm laser pulse at damage threshold. The spectra are taken (from top to bottom) on top of the damage site, on its edge, 2, 5, and 9 microns away, and on virgin, unirradi- ated material. High resolution Nomarski microscopy suggests that the surface remains unaltered

except right at the ~1 micron damage site. The amorphous-like shoulder at 495 cm~\ quite ap- parent in the virgin material, reappears starting at 5 microns away. This shoulder indicates the presence of a relatively large fractional volume of highly distorted material, presumably the grain

boundary material. Closer to the damage site, the average grain size has increased, perhaps follow- ing melting and recrystallization, and the material is under considerable stress. This extra stress re- laxes away from the damage site but has not recovered to the value in the virgin film even after 9 microns.

392
-40 -20 0 20 40

DISTANCE {fJ.tT\)

Figure 4

Parameters of the Raman line around a large heterogeneously damaged area formed by repeated il- lumination (600 shots) at one-half single shot damage. The dashed line indicates the boundary

between damaged (left) and undamaged (right) regions as defined by high resolution Nomarski microscopy. The full lines on the right show the parameters of the virgin polycrystalline film. Beyond the visible boundary, it takes ^ 20 microns for the Raman line to fully recover. (Note however that F seems to recover immediately). Inside the damaged region, we observe different parameters depending upon the smoothness of the surface. In the center, the surface is rough and ~ 517 cm~^ and F ~ 5 cm~^ Around the edge, the surface is smoother and ^ 516.5 cm~^ and F ~ 5.5 cm~^'

393
r(cm"')

Figure 5

Parameters of the Raman line at various locations in and around single-shot (•) and multishot (•) damage sites, and in virgin polycrystalline silicon films (o). The full line, which accounts for vari- ations in the grain size only, does not agree with the data. The u)^ vs T data follow a line that

actually curves in the opposite direction from that predicted by theory. The stress variations dominate the changes in the Raman line. When OJ^, T and Tj^/rj, are small, the material is made of large grains, and is under large and homogeneous tensile stress. Note that the results on single- shot damage sites tend to show a larger and more homogeneous stress.

5

WAVENUMBERS

Figure 6

Anatase (Ti02j before (fine line) and after (thick line) damage with several 30 ps 532 nm pulses, showing a stress variation similar to that observed with silicon.

394
Manusaript received

10-28-85 SAR

Time-Resolved Temperature Determinations From Raman Scattering of

Ti02 Coatings During Pulsed Laser Irradiation

Gregory J. Exarhos

Pacific Northwest Laboratory*

Richland, Washington 99352

Simultaneous measurement of Stokes/anti-Stokes Raman intensitities for a particular

vibrational mode allows instantaneous surface temperatures to be determined.

Time-resolved Raman spectra were measured on a specially constructed spectrometer using

pulsed laser excitation and gated detection techniques. Measurements on equilibrated

coatings and uncoated substrates at several temperatures served to confirm the utility of

the technique for temperature determination as well as calibrate the intensity response

of the instrument.

Pulsed irradiated Si02 and Y(P03)3 glass substrates yield no temperature

increase at laser energies where surface damage is not apparent. At higher pulse

energies, an increase in anti-Stokes scattered radiation is observed, signifying a temperature rise. Analogous results are seen in Raman spectra of low energy

pulse-irradiated Ti02 coatings recorded at delays of 0 and 72 nsec from the excitation

pulse. At higher energies, increased coating temperatures are observed at these times as well as at short times preceding the peak maximum of the excitation pulse, Raman results

are used to model the thermal response of pulsed laser-irradiated dielectric coatings.

Key words: laser heating; molecular vibrational studies; thermal transients;

time-resolved Raman spectroscopy; Ti02 optical coatings

1. Introduction

Pulsed laser irradiation of insulating solids results in rapid heating of localized areas via

single or multiphoton absorption processes and subsequent energy transfer to lattice vibrational

modes. Irradiated substrates and dielectric coatings often suffer irreversible changes ranging

from subtle microstructural modification to catastrophic structural change. Knowledge of the

transient thermal response of the substrate or coating during irradiation is paramount to developing a model for the damage process in a given material.

Conventional methods for measuring temperatures, which include thermal and optical pyrometric

techniques, are limited in their application to relatively long time determinations over large

sample areas. A viable alternative for temperature determination on ultrashort timescales involves

vibrational band intensity analysis of spontaneous Stokes and anti-Stokes Raman scattered

radiation, which is excited by the damaging laser pulse or a different wavelength probe laser.

Vibrational band intensity ratios are related to the Boltzmann factor by equation (1).

I (a))/(w + oj)^

exp [-hcu/kt] = A(ca)—2^ 5 . (1)

I (0)) 7(03 - 0))'^

Pacific Northwest Laboratory is operated by Battel le Memorial Institute for

the U. S. Department of Energy under contract DE-AC06-76RL0 1830.

395
la(to) and are respectively the anti-Stokes and Stokes band intensitites measured at the

vibrational frequency w in wavenumbers . T is the absolute temperature; Wq is the laser

frequency; and the other constants have their usual meaning. [1 ,2] The factor A(w), a spectrometer response function, takes into account the nonlinearity of spectrometer throughput

and detector sensitivity as a function of frequency. Methods to adequately determine this

function have been extensively discussed [2] and were used to calibrate the spectrometer used in these studies.

The Raman technique for determining lattice temperatures in pulsed laser annealed silicon

has been reported by Compaan, et al., [3] and critically reviewed by Wood and coworkers. [4] This

work involved Raman band intensity measurements at the Stokes frequency for the strong silicon

mode using a gated photomultiplier detector and then measurement at the anti-Stokes frequency

under presumably identical conditions. Relatively low lattice temperatures were determined to

ca. ±30%. An improvement in the uncertainty of the band intensity ratio is expected if all the Raman data could be collected at the same instant in time. Accurate bandshapes and baselines

could also be determined. The presence of spurious fluorescence features, which might accompany

the laser pulse, may also be detected and the vibrational band intensities corrected accordingly.

A special spectrometer has been constructed for simultaneous measurement of Stokes and

anti-Stokes Raman spectra excited by a single laser pulse. The instrument includes an array

detector for parallel signal detection and a special intermediate slit aperture for rejection of

the laser excitation line and associated Rayleigh scattering. Spectra of several materials

recorded under equilibrium conditions at various temperatures are used to determine the

spectrometer response function for band intensity correction. Time-Resolved Raman Spectroscopy

(TRRS) has been used to determine surface temperatures during pulsed laser irradiation of high

purity bulk anatase (Ti02) and vitreous silica, and single-layer sputter-deposited anatase

coatings. Spectra recorded during the damage pulse and at long delays are used to evaluate the

thermal response of these materials.

2. Experimental

All Ti02 coatings (15 nm grain size and ly thick) were made by reactively sputtering Ti in Ar/Op atmospheres onto fused silica substrates in an rf diode system. [5] Other samples used

for investigation included high purity fused silica substrates and pressure compacted high purity

Ti02. Fused silica and amorphous Y(P03)3 were used as intensity standards according to Griffiths. [2]

Raman measurements were recorded from samples mounted in a 90° forward scattering

geometry. Scattered radiation was collected by a Graflex f/5.6 camera lens and imaged onto the

slits of a triple monochromator (.25m focal length). A notch filter served to effectively reject

the excitation laser wavelength. The triple monochromator consisted of a subtractive double

monochromator with a special intermediate slit as depicted in figure 1 in tandem with a single

monochromator, which dispersed the filtered radiation onto the elements of an intensified gated

array detector. Interchangeable ruled gratings with different grove densities were used to isolate particular spectral regions on the array detector. This design is essential for

simultaneous recording of both Stokes and anti-Stokes Raman spectra due to the six orders of

magnitude difference in intensity between the pulsed laser emission and the associated weak Raman scattering.

A pulsed NdrYAG laser (10 nsec pulse bandwidth at half maximum) operating at '532 nm and from

between 1 and 50 mj/pulse was used for sample irradiation. The focused spot diameter on the

sample was approximately 1 mm. Raman spectra were acquired at short time periods preceding the

laser pulse maximum, at the laser pulse maximum, and at long delays using ancillary lower energy

probe pulses from a pulse train created by the pellicle beamsplitter /delay network. Spectra

acquired at detector delays that did not overlap the laser pulse were used to distinguish between

induced fluorescence and true vibrational Raman features. Raman spectra were measured as a function of pulse laser energy and detector delay. Static measurements of samples equilibrated

in an oven at various temperatures were recorded for spectrometer throughput calculations. For this particular instrument, the A(co) factor in equation (1) can vary from .5 to 1 over the

spectral bandpass of ±2000 cm-1 centered at the probe laser frequency. Instrumental slitwidth

was maintained at 250y for all measurements.

396
AJtenuolor

I SplittSf s.

Nd YAG Laser

Da1a

System

Sample

Sample Stage

F 56

Nanosecond

Delay Generator

Notch Camera Lens Double Monochromator

TRRS

SYSTEM

Pulse Generator

Gated

Detector

Single Monoctvomotor

Figure 1. Experimental set-up for measurement of time-resolved Raman spectra (TRRS)

3. Results

Raman measurements were used to determine lattice temperatures of pulsed laser irradiated

bulk Ti02 (anatase) and single-layer anatase coatings on silica. Five prominent vibrational

features at 143 cm'l (VS), 199 cm"! (u), 397 cm-1 (S), 515 cm-1 (S), and 636 cm-1 (S)

have been assigned in the bulk crystal [6] and extensive Raman characterization work on single

and multilayer anatase coatings has been reported [7,8,9,10]. Figure 2 shows several spectra of

an anatase coating measured under thermal equilibrium conditions at several temperatures. The

three highest frequency bands are observed in both the Stokes and anti-Stokes spectra in addition

to the probe laser line separating them. The strong 143 cm"^ band is extinguished since it falls within the rejection band of the notch filter. At a temperature of 7420C, the

Stokes/anti -Stokes features are comparable in intensity. Calculated temperatures using a predetermined spectral response function from the standard reference glasses, as described in reference (2), agree to ±2% with measured ambient temperatures of the oven housing the sample.

The overall uncertainty in the temperature calculation is principally determined by the

uncertainty in the band intensities. These measurements serve to demonstrate the utility of the

Raman technique for measuring surface temperatures under equilibrium conditions where the

excitation laser produces negligible surface heating.

At higher pulse excitation energies, an appreciable rise in surface temperature is evident

from measured Raman spectra. Figure 3 exhibits several representative spectra recorded with the

detector gate fully open at the peak of the incident laser pulse. (The gate bandwidth at half

maximum was set at 8 nsec.) Samples were irradiated at room temperature, and laser pulse

energies were measured for each acquired single-shot spectrum. Unirradiated sample regions were

used for each successive pulse, and the spectra recorded in figure 3 are averages of 50 scans each differing in pulse energy by less than ±4%.

Spectra depicted in figure 3 were obtained using finer ruled gratings in the monochromator

and with the notch filter removed so that the 143 cm"^ anatase line appears. At higher pulse

energies, extra features appear on either the Stokes or anti-Stokes side of the spectrum and are assigned to laser-induced fluorescence or emission lines. These features are detailed in the top

spectrum of figure 3 taken at a detector delay of 31 nsec. No Raman lines are observed at this

time, which is longer than the duration of the laser pulse.

Similar results are observed in spectra acquired at a delay of 72 nsec from the primary

pulse maximum. Surface temperataures calculated from band intensity ratios average about 600C

lower at this delay when compa-^ed with calculated temperatures at the pulse maximum (0 delay).

397
For such measurements, the primary pulse is split into a pulse train with each separate pulse

delayed by the transit time around a fixed delay path (figure 1). The detector is locked onto one

of the delayed pulses, which has ca. ten percent of the primary pulse energy. Long-lived emission

features induced by the primary puTse can be removed from the spectrum by subtracting the

normalized emission spectrum obtained with the delay beam blocked.

ANATASE

EQUILIBRIUM TEMPERATURE

MEASUREMENT A

1 1 1 1 r -1200 -600 0 600 I2C WAVE NUMBERS

Figure 2. Simultaneous Stokes, anti-Stokes Raman spectra of a ly anatase

coating equilibrated at several temperatures (600 g/mm grating

with notch rejection filter)

ANATASE TRRS

pulse energy (J) delay (nsec)

_y ^"31^

c1 L_

-600 -300 0 300 600 WAVENUMBERS

Figure 3. Raman spectra of pulsed-irradiated anatase showing appearance of

laser-induced fluorescence features at high pulse energies. The top

spectrum was recorded at a detector delay of 31 nsec from the pulse

maximum and exhibits pure fluorescence (1200 g/mm grating without

notch rejection filter)

398
The time dependence of the surface temperature rise at the highest pulse energy shown in figure 3 was also studied for irradiated anatase coatings. In this experiment, the detector gate

is opened at times preceding the laser pulse maximum as illustrated in figure 4. The hatched

regions in figure 4 represent the total integrated laser energy contributing to scattered Raman radiation at two windows prior to the pulse maximum (-15 and -10 nsec). Measured Raman spectra

corresponding to these time periods are shown in figure 5. Calculated temperatures from measured

Raman intensities are listed in table 1.

Table 1. Calculated Surface Temperatures at Time Periods Preceding

the Primary Pulse Maximum

Sample Calculated Temperature

ly Anatase coating 22°C

-15 nsec delay 109°C

-10 nsec delay 134°C

Marked baseline changes accompany the temperature rise, which may result from laser-induced

photoluminescence. Previous work [11] concerning pulsed laser-irradiated Ti02 (rutile) has

demonstrated that such emission is induced by multiphoton absorption processes observed in an energy regime where catastrophic damage to the crystal did not occur.

Similar measurements were performed on vitreous silica and bulk anatase samples. Results for

bulk anatase were comparable to those seen from 4y anatase films. Since the bulk anatase was of

extremely high purity, observed results are indicative of the thermal response of the intrinsic

material and should not be ascribed to impurities. Stokes/anti-Stokes ratios for vitreous silica

exhibited little change as a function of laser pulse energy, indicating that at these energy levels

no appreciable rise in temperature had occurred.

4. Discussion

Surface temperatures of pulsed laser-irradiated anatase coatings have been computed from

Stokes/anti-Stokes band intensity ratios at zero time delay as a function of pulse energy and are shown in figure 6. Qualitative characterization of three regions are based upon observation of

induced surface damage and the functional dependence of temperature on pulse energy. Region I is associated with no observable surface damage and a slight linear increase in temperature with pulse

energy. Region II is characterized by "footprints" (darkening) of the coating, the appearance of

other non-vibrational features in the spectrum, and a variation of temperature with the square of

pulse energy.

Extensive surface damage and cratering is indicative of region III damage, which is also

characterized by a nearly complete loss of Raman lines and appearance of intense fluorescence. The

temperature dependence on pulse energy follows a higher power law than that exhibited in region II.

Region III is also characterized by a nearly complete loss of intensity at the excitation

laser frequency, which was a prominent feature in regions I and II. Self-absorption of the pulse

laser wavelength was not observed in regions I or II and probably occurs when available laser

energy is sufficient to create a plasma near the surface. Spontaneous Raman scattering is also

absorbed by the plasma discharge under these conditions. This plasma results from breakdown of the

Ti02 coating since pulse irradiation of other materials (silica, Zr02, aluminum, zirconium)

under the same conditions yields markedly different emission features.

The highest surface temperature measured for the anatase coating under high energy pulsed

irradiation is ca. 970OC. For higher energy pulses, no vibrational lines ascribable to anatase

399
CHARACTERISTIC LASER PULSE

• 10 nsec Bandwidth

Gate Pulse

-20 TIME (nsec)

Figure 4. Laser pulse profile in time showing detector gate widths and total

integrated laser power delivered at delay times of -15 and -10 nsec.

ANATASE TRRS 532 nm

-I0 nsec delay

-1200 -600 0 600

WAVENUMBERS

1200

Figure 5. Raman spectra acquired at detector delay settings of -15 and -10 nsec

(600 g/mm grating with notch rejection filter).

400
1000 SURFACE TEMPERATURE

VARIATION WITH

PULSE ENERGY

200

5 10 PULSE ENERGY (mJ)

Figure 6. Calculated surface temperature as a function of laser pulse energy for an irradiated ly anatase coating. The three indicated regions correspond to increasing damage to the coating.

are observed. This is reasonable since at these temperatures, anatase irreversibly crystallizes to the rutile modification of Ti02 with concomitant increase in density. [12] The phase change may

be responsible for plasma formation at high pulse energies.

Transient Raman spectra obtained at detector delays preceding the damage pulse maximum

(figure 5) indicate a gradual rise in surface temperature with integrated laser pulse energy.

Temperature increase results from a combination of linear and nonlinear absorption phenomena and

accumulates with deposited laser energy. Thermal diffusion away from the damage region is apparently slower than energy deposition as shown by transient Raman measurements at a detector

delay of 72 nsec from the pulse maximum. Surface temperatures are uniformly lower than

temperatures determined at zero detector delay in regions I and II.

5. Conclusions

Transient Raman measurements recorded during pulsed laser irradiation of dielectric coatings

allow surface temperatures to be calculated from measured intensity data. A unique spectrometer

has been constructed that simultaneously measures both Stokes and anti-Stokes vibrational features

excited by a single laser pulse at variable times preceding the pulse maximum and at longer times

using a lower energy probe pulse. Measurements on anatase coatings demonstrate the viability of

the technique for obtaining time-resolved surface temperature information. Laser-induced heating

of anatase coatings results from single and multiphoton absorption phenomena and an irreversible

phase transition to the rutile phase when the lattice reaches a critical transformation

temperature. Additional data is needed from these and similar coatings in order to understand the

thermal response of dielectric coatings to laser irradiation. Future work will be directed toward

using transient Raman measurements to determine thermal parameters of the coating during

irradiation.

This work was supported by the Air Force Weapons Laboratory under Contract PO-85-037.

Sputter-deposited coatings have been graciously supplied by Dr. W. T. Pawlewicz. The assistance of

Mr. T. C. Nguyen with experimental measurements is gratefully acknowledged.

401
References

Herzberg, G., Molecular Spectra and Structure. II. Infrared and Raman Spectra of Polyatomic

Molecules. Princeton; Van Nostrand Press; 1945; p. 261.

Malyj, M., and Griffiths, J.E. Stokes/anti-Stokes Raman vibrational temperatures:

reference materials, standard lamps, and spectro- photometric calibrations. Applied

Spectroscopy 37(4) :315-333; 1983.

Lo, H.W., and Compaan, A. Raman measurement of lattice temperature during pulsed laser

heating of silicon. Phys. Rev. Lett. 44(24) :1604-1607; 1980.

Wood, R.F., Lowndes, D.H., Jellison, G.E. Jr., and Modine, F.A. Melting model and Raman scattering during pulsed laser annealing of ion-implanted silicon. Appl. Phys. Lett. 41(3):287-290; 1982.

Pawlewicz, W.T., Martin, P.M., Hays, D.Dk. 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 January 26-27; Los Angeles, CA. Proc. Soc. Photo-Opt. Instrum. Eng. 325:105-116; 1982.

Capwell, R.J., Spagnolo, K., and DeSesa, M.A. A rapid determination of low concentrations

of anatase in rutile Ti02 pigments by Raman spectroscopy. Appl. Spectrosc. 26:537-539;

1972.

Pawlewicz, W.T., Exarhos, G.J., and Conaway, W.E. Structural characterization of Ti02

optical coatings by Raman spectroscopy. Applied Optics 22(12):1837-1840; 1983.

Exarhos, G.J., and Pawlewicz, W.T. Raman characterization of all-dielectric multilayer

Si02/Ti02 optical coatings. Applied Optics 23(12) : 1986-1988; 1984.

Long, S.H., She, C.Y., and Exarhos, G.J. Reduction of substrate interference in Raman

spectroscopy of submicron titania coatings. Applied Optics 23( 18) :3049-3051 ; 1984.

Exarhos, G. J. Substrate signal suppression in Raman spectra of sputter-deposited Ti02

films. J. Chem. Phys. 81(11):5211-5213; 1984.

Royce, G.A., and Kay, R. B. Multiphoton interactions in rutile. Applied Optics

23(12):1975-1979; 1984.

Parkes, G.D., Mellor's Modern Inorganic Chemistry, Revised Edition. London; Longrans

Publishing Co.; 1961; 803 p.

402

Manuscript received

3-31-86 SAB

Thresholds Measured With 350-nni Pulses at 25-100 Hz for Pare Polished

Crystals of CaF2 and for Silica Sol-gel AR Coatings on Silica Substrates*

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

Lawrence Livermore National Laboratory

University of California

P.O. Box 5508, L-490

Livermore, California 94550

An XeF laser-damage experiment was used to measure damage thresholds for bare

polished crystals of CaF2 and porous silica antireflection coatings on fused silica

substrates. Damage was induced by applying 1000 350-nm, 25-ns pulses at repetition

rates of 25-100 Hz to small (typically 0.12 x .03 cm) sites on the surfaces of the

sample. Eight samples of fluorescence-free CaF2 exhibited both surface and bulk

damage at fluences as low as 10 J/cm^. Three samples of llV-grade CaFp also had

surface damage thresholds of 10 J/cm^, and bulk damage v^as not observed in this

material. Thresholds for the porous silica coatings were 17-23 J/cm^.

Key words: calcium fluoride, multishot damage testing, porous silica antireflection

coatings, ultraviolet laser damage.

1. Introduction

We are evaluating solution deposited optical coatings for use at the 350 nm wavelength of XeF

lasers. Discharge pumped XeF lasers can operate at repetition rates in excess of 100 Hz, and

they emit pulses of short duration, typically 10-30 ns. In available commercial XeF lasers,

output single-pulse fluences are less than 0.5 J/cm^ and laser-induced damage to the optical

components in these lasers is not a serious problem. However, many applications of these lasers

require concentration of the beam and produce both high peak intensities and moderate average

power loadings that damage available UV coatings.

Porous silica antireflection (AR) coatings^ capable of handling single pulses at fluence of

5-9 J/cm^ delivered in 350-nm, 0.6-ns pulses were developed for the Nova laser. ^ Because of

their high single-pulse thresholds, they were obvious candidates as AR coatings for XeF laser

components made of IJV transmitting materials such as fused silica and CaF2. Twelve porous

silica AP coatings were deposited on fused silica substrates and tested with a recently assembled

XeF laser damage apparatus. We have also tested eleven bare polished raF2 crystals to provide

a data base that will assist in interpretation of the results of planned tests of porous silica

AR coatings on this material.

2. XeF Laser Damage Experiment

The arrangement of our XeF laser-damage experiment is shown in Fig. 1. The laser was manufactured by Lambda Physik (Model EMG 202 MSC). It emits 25-ns, 150 mO pulses at repetition

rates up to 150 Hz. The pulse energy decreases over the life of a gas fill, but is stable to better than 5% over short intervals (> 10^ shots). The output beam is rectangular in

cross-section, with approximate dimensions of 0.8 x 2.6 cm, and the divergences in these two dimensions are respectively, 1 mr and 3 mr (full angle at half power). In planes near

the output mirror the fluence distribution in the center of the beam is uniform to within a few

percent over a rectangular area with dimensions of 0.4 x 2.0 cm.

Image relaying was used to shape the beam incident on the damage samples. We placed an aperture in the beam that transmitted only the central uniform fraction, and used a single

plano-convex lens to form a dov/n magnified image of the aperture at the sample surface. Fluence

*Work performed by the Lawrence Livermore National Laboratory under the joint auspices of the Air

Force Weapons Laboratory, Kirtland AFP, Wew Mexico 87117-6008, under P.O. #85-060, and the U.S. Department of Energy under Contract Mo, W-7405-EMG-48.

404
Sample plane

XeFI laser image plane

1 50 pps, 1 50 mJ/pulse

25 ns pulse

Laser, XeF

Pulse energy; 1 50 mJ, stable within 5%

Pulse duration; 25 ns

Repetition rate; up to 1 50 HZ

Beam shape; rectangular (.8 x 2.8 cm); uniform to 10% over cental .4 x 2.0 cm

Beam divergence. Full angle at half power:

Long dimension 3 mr

Short dimension ~ 1 mr

Figure 1: XeF laser damage experiment.

at the sample's surface was controned by varying the size of the image. Each site on a sample was irradiated by a sequence of pulses. The number of pulses in the sequence and the pulse

repetition rate could be preselected by use of a programmable laser controller. The irradiation

sequence began from a single command and could be manually interrupted.

Computation of the single-pulse fluences incident on the sample required measurement of the

pulse energy and the beam shape. The energy in single pulses in the sequence was computed by

dividing the average power delivered to the sample plane by the pulse repetition rate. Average

power was measured by placing a power meter in the sample position. To allow measurement of

pulse energies during a damage experiment, a photodiode was placed in a diagnostic beam and the

amplitude of the signals from the photodiode was calibrated against the single pulse energies

measured by the power meter. We believe the measurements of single pulse energies were accurate

to within 3% which was the total difference in power measurements obtained in side-by-side

comparisons of three independently calibrated power meters.

The greatest uncertainty in measurement of fluence was that in determining the exact shape of

the images relayed to the sample plane. Because the divergence was large in the plane containing

the long dimension of the beam, the beam width exceeded the aperture of the available lens. The

combination of excluding some edge rays and spherical aberration in the plano-convex lens

prevented formation of uniformly illuminated images of the beam limiting aperture. The solution

to this problem is use of corrected optics with larger aperture, and an aspheric imaging system

with zoom control of magnification has been designed and will soon be available.

Rather than invest a significant effort in learning to precisely measure the shape of

aberrated beams that will not be a permanent aspect of this experiment, we are computing fluences

by dividing single-pulse energies by the areas of beam imprints recorded on sections of

ultraviolet sensitive paper placed in the sample plane. Imprints of intense 350-nm beams are

readily recorded on photocopy papers, a variety of common white papers and highly absorbing

optical thin films. An example of an imprint recorded in an impure highly absorbing Ta205

thin film is shown in Fig. 2. The imaging aberration is apparent by the greater severity of

damage in the center of the beam as compared to that at the distantly separated ends of the burn.

Some of the relayed images were also recorded on Kodak I-Z emulsions using a camera with

magnification of 17. The magnification was obtained by use of a single plano-convex lens.

405
Figure 2: Imprint of the beam in the sample plane, recorded in a highly absorbing

Ta205 film. Beam dimensions are 1.3 x .25 mm.

Analysis of these photographs indicated that the fluence profile was constant across the lesser

dimension of the rectangular images, but smoothly rounded across the longer dimension. Peak

fluence values computed from these data were 30-40°/ greater than those computed assuming that the

fluence was uniform across the rectangular area recorded in beam imprints. However, photography

of magnified images of small objects placed in the sample plane indicated that the camera lens was responsible for some of the aberration in photographic records of the beam, so our reported

fluence values are systematically conservative by an amount less than the 30-40% difference in

the two fluence measurements.

The final aspect of the experiment is detection of damage, and we have continued our use of

three techniques: observation of laser-induced plasma during irradiation, post-irradiation visual

inspection with intense white-light backlighting, and Nomarski microscopy. Damage was defined to be a permanent disruption of the sample detectable by one of the two inspections. We do not yet

know whether laser-induced plasmas will be a reliable indicator of damage induced by sequences of

350-nm 1 aser pul ses.

Our initial experience with this configuration indicates that image relaying is the optimum

technique for irradiating samples with XeF laser beams that are highly divergent and weakly

coherent. Use of corrected optics will eliminate the image aberration in our present system, and

allow both formation of uniformly intense beams and use of the full power of the laser in all experiments since fluence at the sample can be controlled by varying the size of the image.

Corrected optics for photography of beams have also been selected.

3. Test Samples

3.1 Porous Silica AR Coatings

Porous silica films were deposited on fused silica substrates by immersing the substrate in

the coating solution and then withdrawing the substrate using a pull rate calibrated to deposit

coatings 88 nm in thickness, the quarterwave thickness for the 350-nm wavelength. The coating

solutions were either ethanol or methanol containing silica particles with diameters of 10-20 nm. To investigate the effect of aging of the coating solutions, four substrates v/ere coated with an ethanol solution that was prepared 11 months before it was used and four substrates were coated

with a freshly prepared methanol solution. The coatings were dried in air at room temperature

and required no additional processing.

3.2 CaF2 Substrates

Bare polished crystals of CaF2 were purchased from three vendors. One vendor supplied

material advertised as UV grade. Two supplied crystals advertised as "fluorescence-free" when

irradiated at the ArF wavelength of 190 nm. The polishing processes of two vendors are proprietary; the third used conventional lapping with diamond and ethylene glycol. All crystals were either 2 inch in diameter and 6 mm in thickness or 1,5 inch in diameter and 10 mm in

thickness.

406
4. Damage Thresholds

4.1 Porous Silica AR Coatings

Damage thresholds measured for the porous silica AR coatings on silica substrates are given

in Table 1. With 1000 shots applied at 25 Hz to each test site, the thresholds were about

20 J/cm2. Within experimental error, thresholds were sharply defined for each sample; fluences

below threshold did not cause damage and those above threshold did. Damage consisted of small

pits randomly distributed in the irradiated area. Only one or two pits were produced at threshold, and the spatial density of pits increased as the fluence was increased above

threshold. Since the area irradiated at 20 J/cm^ was 0.226 mm^, this indicates that the spatial

density of the defects responsible for damage at 20 J/cm^ was 4.4 mm-2.

Because only a few small areas on each sample were tested, one cannot determine from this data

whether the coatings contained other widely spaced defects with lower thresholds. The performance

of these porous coatings in the Nova laser suggest, however, that the thresholds of coatings

measured with mm-diameter beams are probably the same as would be measured with much larger beams.

The 46-cm diameter input lenses in the final spatial filter in each arm of Nova are made of fused

silica and coated with porous silica films. These lenses are surviving irradiation by 1-ns

1064-nm pulses with spatially averaged fluences of 5-7 J/cm^ and local fluence variations of

30-50%. The irradiated area is 1330 cm^. In tests with 2-mm diameter 1064-nm beams, we found

the median threshold to be 11 J/cm^. This correlation between small -spot and large-spot

1064-nm thresholds for these coatings and the consistency of the results observed in our 350-nm

tests of several samples give reason to believe that the porous silica coating can withstand

about 20 J/cm2 applied to areas larger than that of our test beam. j

Table 1. Damage thresholds of porous silica coatings on fused silica substrates.

Thresholds were measured with 25-ns, 351-nm pulses at 25 Hz.

Coating Single-Pulse

Sample Solution Damage Thresholds, J/cm^

3823 Ethanol, aged 11 months 22 + 3.5

4019 II 24 + 4.0

3818 II 23 + 4.0

3433 II 17 + 3.0

3328 methanol , freshly prepared 18 + 3.0

3368 It 23 + 4.0

3369 II 23 + 4.0

3330 II 18 + 3.0

Our results for solution deposited coatings can be compared with thresholds for coatings made

by physical vapor deposition (PVD). Tests at 350-nm of PVD coatings have been made for several

years, although single laser pulses were used in most of these studies. Single-shot thresholds

of 2-3 J/cm2 were observed in 17-ps tests of single layers of Zr02, SiO^ and Hf02.^ Measurements

made with single 27-ns 355-nm pulses gave thresholds of 5.9, 7.1 and 9.1 J/crr^ for single-layer

films of respectively, AI2O3, Zr02 and ThF4, a value > 38 J/cm^ for a film of MaF, and thresholds

of 4.9 and 12.2 J/cm^ for multilayer reflectors made of Th02/Si02 and Al203/NaF.5 in single-layer

PVD films with optical thicknesses of 250 nm and 125 nm, respectively, thresholds measured with

355-nm 15-ns pulses were 5 and 19 J/cm^ for CaF2, 12 and 15 J/cm^ in Si02, 9 and 11 J/cm^ in

ThF4, 5 and 6 J/cm^ in MgF2, 6 and 6 J/cm^ in AI2O3, 4 and 4 J/cm^ in MgO, 4 and 3 J/cm^ in

Hf02, and 2 and 3 J/cm^ in Zr02.^ Single-shot thresholds that typically were 2-3 J/cm^ were measured in several experiments using 350-nm pulses with durations of 0.6-1.0 ns.'"'^ Coatings

tested in those studies v^ere PVD multilayer AR and HR coatings made using a total of 16 different

pairings of high-index and low-index materials which included both oxides and fluorides. The

largest thresholds reported were 4-5 J/cm^.

407
Fewer multi-shot thresholds measured with repetitively pulsed lasers are available. An Al203/Si02 multilayer reflector^^ tested with 10-ns, 351-nm XeF pulses at 35 Hz damaged at 8.5 J/cm^

and thresholds ranging up to 8 J/cm^ have been measured under the same test conditions on PVD antireflective coatings. jhe antireflective coating with threshold of 8 J/cm^ was made of

Al203/Na3AlF6.

Our recently measured thresholds of 20 J/cm^ for porous silica coatings are, therefore,

comparable to the largest thresholds reported for single-layer PVD films and considerably larger

than thresholds reported for PVD antireflective coatings. It is important to test these porous

silica coatings at higher repetition rates and for many shots to insure that they are capable of

sustained operation at high fluences.

4.2 Bare Polished CaF2

In tests of either two or three samples from each of the three vendors, we applied 1000 shots

at 25 Hz to each test site. An additional sample from each vendor was tested at 100 Hz. Although

damage was usually more severe at 100 Hz, thresholds did not depend on repetition rate, and did not vary significantly from sample to sample. Therefore, all data taken on samples from a particular vendor were combined in a single graph, see Figs. 3-5. We believe that the severity

of damage induced at 100 Hz was caused by applying many additional shots to a damaged area before

the experiment could be manually interrupted. Further, the equivalence of thresholds at either

25 or 100 Hz indicates that damage resistance was limited by the single-pulse threshold and not

by average power loading. For tests at 100 Hz, the average intensity at threshold v/as about

1 KW/cm^ while peak power in the individual pulse was about 4x10^ W/cm^. A dependence of damage

thresholds on average power must eventually appear if the average loading of the crystals is

increased by either improving single-pulse thresholds or increasing the repetition rate.

D

>

tnO

0}E

3

I 4

D

CO^ 2

in c c ro

Q) "O

O H-3

CO

E

(0O

Bulk damage

0 sites

rm

Surface damage

23 sites

£}

No damage

21 sites

EL

10 12 14 16

Test fluence, J/cm^

18 20

Figure 3: Events observed in tests (25-ns, 350-nm, 25 or 100 Hz)

of 44 sites on three bare, polished, UV-grade, CaF2 crystals.

408
As is apparent from Fig. 3, the three UV-grade samples had sharply defined surface damage

thresholds of about 10 J/cm^. At fluences of 10-14 J/cm^, damage to these samples consisted of a disturbed area having the rectangular shape of the beam. These disturbed areas had little if any

vertical structure, and were more readily visible by visual inspection with intense white-light

backlighting than by Nomarski microscopy. This suggests that the damage was an alteration of a surface gel layer that could be eliminated by use of an alternate polishing procedure or removed

by etching. Bulk damage was not observed in the llV-grade material.

Thresholds were not sharply defined in the six samples of fluorescence-free CaF2, and bulk

damage occurred at several test sites (see Figs. 4 and 5). The lack of definition in both

surface and bulk thresholds indicates that damages were caused by defects separated by distances

comparable to the beam dimension. If tested with a larger beam, the thresholds of the

fluorescence-free samples v^ould not exceed 10 J/cm^, the least fluence at which damage was

observed with the small test beam.

Surface damage in the fluorescence-free samples always consisted of cleavage-like fractures

that sometimes covered an area larger than that of the test beam. The shallow beam imprint type

of damage was not observed.

From the study of eleven samples we conclude that the threshold for commercially available CaF2

is about 10 J/cm^. Similar tests of UV-grade CaF? with 10-ns 355-nm pulses incident at 35 Hz produced surface damage thresholds of 5-12 0/cm^.'-^ The fluorescence-free CaF2 material that

we tested was not superior to the UV-grade material for 350-nm applications. However, the

observation of bulk damage only in the fluorescence-free crystals could be a misleading result

obtained by testing only eleven samples. Elimination of fluorescence induced by UV irradiation

was supposedly obtained by improving the purity of the starting materials and grov/th process. We

see no obvious reason why that should lead to production of material with reduced bulk damage

thresholds, and believe that testing of many samples from several boules would be required to clearly establish a difference in the bulk quality of the two types of CaF2 crystals.

D

>

0)

(A SIOc

>

a;

0)^ 6

E

a ^ 3 2

CQ

0, 0

o>E 6

(0V

u

M- «5 2

0 t: ^

OJ 03 0

1= fi o) 6

§ 4

-aI

2

Bulk damage

12 sites _ a. n n m

n

Surface damage

1 3 sites

n mo

No damage

40 sites

m rr n

8 10 12 14 16 18 20

Test fluence, J/cm^

Figure 4: Events observed in tests (25-ns, 350-nm, 25 or 100 Hz)

of 65 sites on four bare, polished, fluorescence-free CaF2 crystals from vendor A.

409
0)>

(A SIO

0)E

3

0)

£

jse: 3 2

OQ

a, 0

£

6

(0 ° 4

0)o

*r 45 2

o w3

(/>

0)

E

(0 T3

T

Bulk damage

1 0 sites

n

Surface damage

1 5 sites

n rihlyUkfT

No damage

49 sites

8 10 12 14 16

Test fluence, J/cm^

18 20

Figure 5: Events observed in tests (25-ns, 350-nm, 25 or 100 Hz)

of 74 sites on four bare, polished, fluorescence-free CaF2 crystals from vendor B.

5. Summary

Porous silica AR films deposited on fused silica substrates had thresholds of 17-23 J/cm^

when irradiated by 1000 shot sequences of 350-nm, 25-ns pulses at 25 Hz. These thresholds are about twice as large as the best values previously reported. Because the 1064-nm thresholds

measured with 2-m\ diameter beams are comparable to the fluence levels at which the coatings

survive in large area 1064-nm beams, we believe that 350-nm thresholds of about 20 J/cm^ can be

realized on areas much larger than our 350-nm test beam.

Bare polished surfaces of commercially available CaF2 crystals damaged at about 10 J/cm^, and

the test results indicated that surface damage thresholds might be improved by at least 50%

through use of polishing procedures that eliminate gel layers. However, the bulk thresholds of some crystals does not exceed 10 J/cm^, so improvements of surface quality may not improve the

capability of this material to operate at high fluence. Unless bare CaF2 can be improved,

thresholds of porous silica coatings on CaF2 substrates will be limited by the performance of

the substrate.

We are grateful to several employees of LLNL for assistance in establishing the new XeF

damage laboratory. Design and installation of electronic controls for the experiment were

supervised by S. E. Peluso and the mechanical aspects of upgrading the building and installing

the experiment were supervised by R. E. Wilder. Much of the installation of electronic and

mechanical apparatus was done by L. R. Truelsen and G. VonVille.

410
6. References

[I] I, M, Thomas, J. G. Wilder, W. H. Lowdermilk and M. C. Staggs, "High damage threshold porous

silica anti reflective coating", Lawrence Livermore National Laboratory report liCRL-91428,

April 25, 1985; to be published in the Proceedings of the 1984 Conference on Laser-Induced

Damage in Optical Materials.

[2] J. F, Holzrichter, "High-power solid-state lasers". Nature, Vol, 316, No. 6026, 25-31, July

1985.

[3] The zoom magnification system with aspheric lenses was designed by Lynn Seppala of LLML.

[4] B. E. Mewnam and D. H. Gill, "Spectral dependence of damage resistance of refractory oxide

optical coatings", in Laser-Induced Damage in Optical Coatings: 1976, MPS Spec. Publ . 462,

pp. 292, issued December 1976,

[5] B. E. Newnam and D. H. Gill, "Ultraviolet damage resistance of laser coatings", in

Laser-Induced Damage in Optical Materials: 1978, MPS Spec. Publ. 541, pp. 190, issued

December 1978.

[6] T. W. Walker, A. H. Giienther, C. G. Fry and P. Nielson, "Pulsed damage thresholds of

fluoride and oxide thin films from 0,26 m to 1.06 m" , in Laser-Induced Damage in

Optical Materials: 1979, NPS Spec. Publ. 568, pp. 405, issued July 1980.

[7] J, A. Abate and R. Roides, "Laser damage thresholds of optical coatings at 351 nm" , in

Laser-Induced Damage in Optical Materials: 1982, NBS Spec. Publ. 669', pp. 282, issued

January 1984.

[8] C. K. Carniglia, T. Tuttle Hart, F. Rainer and M. C. Staggs, "Recent damage results on high

reflector coatings at 355 nm" , to be published in the Proceedings of the 1983 conference on Laser-Induced Damage in Optical Materials.

[9] T. Tuttle Hart, C. K. Carniglia, F. Rainer and M. C. Staggs, "Recent damage results for

anti reflection coatings at 355 nm", to be published in the Proceedings of the 1984

conference on Laser-Induced Damage in Optical Materials.

[10] T. Tuttle Hart, C. K. Carniglia and M. C. Staggs, "Effect of overcoats on 351 nm reflectors", to be published in the Proceedings of the 1984 conference on Laser-Induced

Damage in Optical Materials.

[II] S. R. Foltyn, L. J. Jolin and P. E. Newnam, "Progress in ultraviolet damage testing at Los Alamos", in Laser-Induced Damage in Optical Materials: 1982, NPS Spec. Publ. 669, pp. 266,

issued January 1984.

[12] S. R. Foltyn, Los Alamos National Laboratory, private correspondence.

[13] S. R. Foltyn, L. J. Jolin, "Catastrophic versus microscopic damage: applicability of

laboratory measurements to real systems", to be published in the Proceedings of the 1983

conference on Laser-Induced Damage in Optical Materials.

I)IS( l.AIMKR

I'his document was prepared as an account of work sponsored b> an ageno of

the I nited Stales (iovernment. Neither the I niled Stales Cjo\ernment nor the

I niversity of ( alifornia nor an\ of their emplo\ees, makes an\ «arrant\, express or implied, or assumes an> legal liahiiily or responsibility for the ac- curacy, completeness, or usefulness of any information, apparatus, pr()duct. or process disclosed, or represents that its use would not infringe privately (jwned

rights. Reference herein to any specific commercial products, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily

constitute or imply its endorsement, recommendation, or favoring bv the I nited States fiovernment or the I niversity of ( alifornia. I he views and opinions of authors expressed herein do not necessarily state or reflect those of the I niled

.States Government thereof, and shall not be used for advertising or product en- dorsement purposes.

411
Manuscript Received

Study of Temporal and Spatial Intensity Variations and 12-9-86

Intracavity Damage Thresholds of Alexandrite Lasers

D.J. Harter, H. Rainnee*, and A.J. Heiney

Allied-Signal Corporation

7 Powder Horn Drive

Mt. Bethel , NJ 07060

The energy densities which can be obtained in a laser resonator before optical damage

occurs are usually much lower than the optical damage thresholds measured in extracavity

laser experiments. We have measured intracavity optical damage thresholds in an alexandrite laser operating in both single- and mul ti -transverse mode and found the multitransverse mode laser has a much lower damage threshold. We studied the temporal and

spatial variations of a multi -transverse mode laser and found intensity hot spots which

may be an important factor in the difference in the intracavity damage thresholds.

Key Words: alexandrite; antireflection coatings; intracavity optics; laser damage; multimode

laser; surface damage.

1. Introduction

Alexandrite is a tunable solid state laser materia1[l] which has a relatively low emission

cross section and therefore a large saturation energy. Thus, to obtain high extraction efficiency

from alexandrite, high intracavity fluences are required. Also, because of the low emission cross section and the long excited state lifetime, alexandrite lasers do not experience significant loss

from amplified spontaneous emission (ASE). Thus, optical damage is one of the most important

limitations on the performance of alexandrite lasers.

Single shot damage measurements of alexandrite have shown that the crystal has a very high

bulk damage threshold (1100-4300 J/cm2). With an acid assisted surface polish, the surface damage

threshold can approach the damage threshold of the bulk material while mechanically polished sur- faces with AR coating have damage thresholds between 50 and 1300 J/cm2[2]. However, in practice,

intracavity damage thresholds are much lower than measured damage thresholds and seem to vary widely for different resonator designs. In this paper we describe experiments in which intracavity energy densities were measured accurately just before optical damage. In addition, experiments which show the temporal variations of the intensity profile for a multi -transverse mode

laser are described. The temporal variations in the intensity profile appears to contribute to the variation of the intracavity damage threshold for different resonator designs. In addition to knowing the intracavity damage threshold for a specific laser design for a few laser shots, it is necessary to know the damage threshold for longer term operation. The experimental setup for mea- suring laser parameters preceding optical damage for long operation times will be described.

2. Intracavity Energy Density Measurements

Figure 1 illustrates the experimental setup for measuring the intracavity energy density distribution of the laser beam before optical damage. The energy density distribution is measured at the thin film polarizer since this is the weakest element in the cavity and damages first. Since we are imaging the beam, the energy density at the location where damage occurs can be measured.

We find that the beam profiles vary significantly with the direction of passage through the reso- nator. This is caused by an aperture for limiting the laser to TEMqo operation. The laser beam

which has exitted from the aperture has near-field Fresnel rings while the beam which has propagated a longer distance appears to be the far field diffraction pattern of an aperture (Airy pattern). The profiles of the beam propagating in the two directions are shown in figure 2. It is interesting to note that in spite of the fact that the total energy in the beam reflected from the

output coupler is 37% less than the energy of the other beam, it has a higher peak energy due to

its profile. The images of the beam profiles from the two directions are superimposed in order to measure the intracavity energy density at the polarizer. Figure 3 shows the energy density profile of the laser operating in a single transverse mode just before damage. The peak intracavity

energy density at the thin film polarizer before damage occurred was measured to be 35 J/cm2. The

laser resonator is designed so that the beam is smaller on the laser rod. Therefore, the energy

density is greater on the rod surface than the polarizer. This measurement was also performed

with the laser running in multi -transverse mode; the energy density profile is shown in figure 4. The peak intracavity energy density was 18 J/cm2 before damage. We find that the damage threshold

* Present address: Philon Corp. 541 6th Ave. NY, NY 10011

412
is lower for multi -transverse mode operation compared to single transverse mode. We believe this

is due to temporal variations in the beam profile. Measurements of the temporal variation in the

intracavity energy profile will now be discussed.

Figure 2: The beam profile at the polarizer

from the two directions. The upper beam profile is right out of the aperture and has the near field diffraction pattern of the aperture

(Fresnel pattern). The lower pattern has propagated farther and is the far field diffraction of the aperture (Airy pattern).

413
Figure 3: Photograph and three-dimensional plot of the beam profile at the thin film polarizer

for a single transverse mode laser. The peak intracavity energy density before damage of the thin

film polarizer was 35 J/cm2 with a 40 ns long pulse.

Figure 4: Photograph and three-dimensional plot of the beam profile at the thin film polarizer

for a multi -transverse mode laser. The peak intracavity energy density before damage of the thin

film polarizer was 18 J/cm2 with a 40 ns long pulse.

3. Temporal Intensity Variations in Mul ti -transverse Mode Beam Profiles

The energy density profile for a mul ti -transverse mode beam as shown in figure 4 is relatively

smooth, however, this profile is integrated in time. In this experiment we will study the change

in the intensity profile during the temporal evolution of the laser pulse. Figure 6 shows the

experimental setup. The beam profile at the output coupler of a pulsed, Q-switched alexandrite

laser is imaged onto a camera. In the optical path we place a Pockels cell which transmits light

for a 4 nanosecond duration. This device is triggered by the leading edge of the Q-switched pulse

and can be delayed with respect to the laser pulse. The temporal profile of a 4 nanosecond slice

414
from the laser pulse is shown in figure 5b. Figure 5c shows the 4 nanosecond pulse and its temporal relationship with the laser pulse. This profile was obtained by slightly misaligning the

electro-optical device to allow the transmission of a portion of the laser pulse. In figure 6 a series of images of the beam profile is shown for various times in the laser pulse; the integrated

pulse profile is shown as well. As the laser pulse develops, it starts lasing in a low order mode

which has the lowest diffraction loss. At the end of the pulse, the laser is lasing in higher

order modes. The intensity profile has hot spots which are not observed in the smooth energy profile shown in figure 4. Qualitatively, we find that the extent of the irregularity in the intensity profile is dependent on the curvature of the resonator mirrors. The temporal variation of

the intensity profile most likely contributes to the lower energy damage threshold for multitransverse mode operation compared to single transverse mode operation and also the variation of

the energy damage threshold for different resonator designs for alexandrite lasers. This hypothesis is further supported from experience with a resonator which favored single transverse mode

operation. When this resonator was operated in multi -transverse modes, a double Q-switching technique was needed to prevent damage from fast buildup of low order modes. [3]

The previously described experiment raises some additional questions. Does the temporal

variation of the intensity profile differ from shot to shot? Another question is how much greater

is the intensity in one of the hot spots compared to the average intensity of the pulse? Another

experimental method to measure the temporal variation of the intensity at a single point in the

laser beam profile is shown in figure 7a. A pinhole is placed in the image plane of the beam profile at the output coupler. A fast photodiode is placed behind the pinhole and measures the temporal variation at that location in the beam. Figure 7b shows the temporal profile of the entire

beam. Figure 7c shows the temporal behavior of the pulse at two different spots in the beam. At these two locations the temporal behavior is quite different and the peak intensity is greater

than what is expected with the entire pulse profile. The temporal scales are the same in figures

7b-c. We found that the temporal profile at a specific spot in the beam was quite reproducable

from shot to shot. The intensity in a hot spot was only about 2X greater than what the average

intensity would be at that spot. However, during these experiments the laser was operated at a power much lower than powers when damage occurred. Further experiments close to the damage

threshold of the laser would be of interest.

Monitor

a)

! i 1 fl

Alexandrite Laser

Figure 5a: Experimental apparatus for measuring the temporal evolution of the spatial mode pat- tern of a multi -transverse mode, pulsed alexandrite laser, b) 4 ns temporal sample from a laser

pulse, c) short temporal sample shown with 50 ns Q-switched pulse which transmitted through the

pulse slice Q-switch by misaligning this electro-optic device.

415
Beginning Of Laser Pulse 1/4 Through Pulse

Peak Of The Pulse 3 4 Through Pulse

Spatial Profile Integrated Over Entire Laser Pulse

Figure 6: Series of 4 ns temporal samples from the 50 ns Q-switched pulse. At the beginning of

the pulse lasing starts in a low order mode. Near the end of the pulse lasing occurs in higher

order modes. Structure in the pulse is not observed in the pulse energy profile.

416
Mb

Figuce 7a: Experimental apparatus for measuring the temporal behavior of different spatial areas

of the laser beam, b) Entire pulse profile, c) and d) Temporal behavior from two different areas

of the beam.

4. Computer Controlled Life Time Experiments

Thus far we have discussed our results in measuring intracavity damage thresholds for a few

repetitive shots. What is of greater interest is the intracavity energy densities at which the

laser can run reliably. Since this will require the accumulation of significant amounts of data, we have set up the computer controller laser lifetime experiment shown in figure 8. The temporal

pulse width, spectral linewidth, the wavelength, the pulse energy, and the spatial beam profile

are calculated and stored in real time at 10 Hz. The waveforms are displayed every third shot.

Figure 9 shows a typical output of the screen.

Pulsed Q-Switched

Alexandrite Laser

Figure 8: Experimental apparatus for computer controlled lifetime experiments.

417
Figure 9: Computer display of laser parameters during lifetime experiment.

6. Conclusion

In conclusion, we have measured peak intracavity energy densities where the damage threshold

of the AR coated rod surface is greater than 35 J/cm2 for a single transverse mode pulse. This was the damage threshold for the intracavity thin film polarizer and during these experiments rod

damage did not occur. This fluence is still a factor of 3-lOX lower than the damage thresholds

measured in single shot measurements on coated surfaces. We find the damage threshold for multitransverse mode operation to be about a factor of two (18 J/cm2) lower than for single transverse

mode operation. We attribute the lower damage threshold for multi -transverse mode operation partially to the temporal fluctuations in the intensity profile. Even in multi -transverse mode

operation the optical damage threshold is greater than the saturation energy of alexandrite at elevated temperatures. At 150° C the saturation energy is between 11-15 J/cm^ for the 710-785 nm wavelength region.

Optical damage has been the chief limitation of the extraction efficiency for alexandrite

lasers. However, as important as having a good correlation between single shot damage thresholds

and intracavity damage thresholds, is to have good correlation between intracavity damage thresholds and threshold for long term operation of the laser. We have described the experimental con- figuration for these measurements.

We wish to thank J. Barrett, H. Samelson, and J. Walling for encouragement and helpful discussions and also K. Whittle and G. Shipp for their technical assistance.

6. References

1. J.C. Walling, O.T. Peterson, H.P. Jenssen, R.C. Morris, and E.W. O'Dell, "Tunable Alexandrite

Lasers," IEEE J. of Quantum Electron., QE-16 , 1302, (1980).

2. Report to Allied Corporation (unpublished) by S. Seital, Naval Weapons Laboratory, Unpublished

results of J.C. Walling and of D.J. Harter, and S.A. Zamor. Some of the uncertainty in the

data is due to differences in temporal pulse lengths, damage site spot size, and the wavelength of the laser used in the three different damage threshold experiments.

3. R.C. Sam, R.W. Rappoport, and M.L. Shand, "Design and Performance of a 125 Hz, 50 W Alexandrite Laser," in Proc. Southwest Conf. Opt., Albuquerque, NM, Mar. 4-8, 1985.

418
Manuscript received

1-27-86 SAB

Investigation of Metal Coatings for the Free Electron Laser*

Marion L. Scott, Paul N. Arendt, Robert W. Springer,

Richard C. Cordi, and William J. McCreary

Materials Science & Technology DiVvision Los Alamos National Laboratory

Los Alamos, NM 87545

We are investigating the deposition and characteristics of metal coatings for use

in environments such as the Free Electron Laser where the radiation resistance of metal

coatings could prove to be of great benefit. We have concentrated our initial efforts

on silver laminate coatings due to the high reflectance of silver at 1 micron wavelength. Our initial laminate coatings have utilized thin layers of titanium oxide to break up the columnar structure of the silver during electron-beam deposition on fused

silica substrates. Our initial results on equal coating thickness samples indicate an improvement in damage threshold that ranges from 1.07 to 1.71 at 351 nm.

Key Words: metal coatings; silver coatings; coating structure; laminate

coatings; laser damage

1. Introduction

The Free Electron Laser is a hostile environment for the optical coatings typically used in

a laser cavity. Radiation, such as x-rays, gamma rays, neutrons, and scattered electrons are present in the laser cavity due to collisions of the high energy electrons from the accelerator

with magnets, walls, etc. Dielectric materials used in multilayer reflectors are sensitive to this radiation environment and the performance of a multilayer dielectric coating as well as its

laser damage threshold can be quickly degraded in response to these radiations.

Previous work at Los Alamos on aluminum films indicate that the physical properties of

aluminum films could be improved by interrupting the growth of columns in the aluminum films with

pulses of oxygen forming thin oxide layers periodically during the aluminum deposition [1,2,3,4].

Similar work was conducted at Los Alamos with tantalum films and pulsed acetylene gas [5] . A TEM photomicrograph of one of these aluminum laminate structures is shown in figure 1. The yield

strength of these aluminum films increased 20 fold form 20 MPa to 400 MPa. In addition, unbacked

cylindrical aluminum films only 2,000 Angstroms thick were deposited for the Inertial Confinement

Fusion Program [6].

This previous work on aluminum films encouraged us to look for a similar technique to use with the silver films. One difficulty in extending this technique to silver is that the si].ver

film does not readily oxidize in the presence of oxygen. Thus, we were led to depositing a thin

layer of another material to interrupt the structure of the silver films. The material we chose

to use in this laminate structure is titanium oxide. A thin layer (50 Angstroms) of

non-stoichiometric titanium oxide was deposited periodically during the deposition of silver

films on fused silica substrates to investigate its effectiveness in improving laser damage

threshold (see figure 2)

.

2. Film Analysis

A.n Auger depth profile of one of our first silver laminate films is shown in figure 3. The

titanium oxide layers are well defined in this structure, though the oxide layers are only 50 to

85 Angstroms thick. A SEM photomicrograph of the cross-section through this laminate structure

lacked sufficient resolution to see structure effects in these films. TEM photos have not yet

been obtained. SEM photos of the surfaces of silver laminate films and typical silver films

indicate that the laminate surfaces are smoother (see figures 4 and 5).

* This work was performed under DOE contract W-7405-ENG-36

.

419
3. Film Fabrication

We prepared our silver films using electron beam deposition in a high vacuum chamber. The

deposition rate of the silver layers was relatively high at 25 Angstroms/sec. Thin layers of

titanium metal were evaporated periodically and this thin layer was immediately oxidized by

introducing a pulse of oxygen into the chamber. All of the samples to be reported here were deposited on highly polished fused silica substrates, 5.08 cm in diameter and 0.995 cm thick.

Two samples each of four coating designs were deposited for this study. The first coating

design is a 2,000 Angstrom thick silver film reference, deposited on a 100 Angstrom layer of

chromium. The second design is a four layer laminate beginning with a 50 Angstrom titanium oxide

layer, followed by a 1,500 Angstrom silver layer. This layer pair is repeated to complete the

four layers. The third coating design is similar to the second, except that there are 12 layers,

or six pair of titanium oxide and silver layers, instead of two pair. The fourth coating design

is a 9,200 Angstrom thick silver reference film deposited on a 100 Angstrom chromium binder

layer. This fourth design is a silver reference of equal thickness to the six pair laminate of

the third design. Comparison of equal thickness samples is required due to the thermal insulation of the substrate.

4. Surface Roughness Measurements

Surface roughness measurements were performed on these silver samples using a Wyco optical

heterodyne prof ilometer . This non-contact method of determining the microroughness of an optical

surface will not affect the subsequent laser damage threshold measurements. The results of these

measurements on our eight samples are given in table 1. The thin samples appear to be somewhat

rougher than the thicker samples, however, all of the samples are less than nine Angstroms rms along measurement lengths of 1.2 mm.

Table 1. Silver film surface roughness measurements

Sample // Design RMS Roughness (A)

1 lOOA Cr/2,000A Ag 8. 4

2 lOOA Cr/2,000A Ag 7 9

3 [50A TiOx/l,500A Ag]*2 7 5

4 [50A TiOx/l,500A Ag]*2 7 7 5 [50A TiOx/l,500A Ag]*6 6 3

6 [50A TiOx/l,500A Ag]*6 5 1

7 lOOA Cr/9,200A Ag 5 9 8 lOOA Cr/9,200A Ag 6 7

5. Reflectance Measurements at 351 nm

Reflectance measurements were made on the eight samples using a W-V attachment on a spectrophotometer from 300 nm to 500 nm. These spectrophotometer measurements are shown in fig- ures 6 to 9. These spectral traces show the emergence of the well known surface plasmon reso- nance in silver (at 345 nm) , particularly in the thicker samples. Since the wavelength of the

laser performing the damage tests conducted on these samples is 351 nm, the reflectance values at this wavelength have been summarized in table 2. The silver film absorption (1-R) varies between

18% and 36% in these samples due to the variation in the surface plasmon coupling with film

thickness. Surface plasmon coupling variation with surface roughness is well known, but the

variation in coupling with thickness is not explainable on this basis.

420
Table 2. Silver film reflectance measurements at 351 nm

Sample # Design - Reflectance (%)

1 lOOA Cr/2,000A Ag 77.8

2 lOOA Cr/2,000A Ag 77,8

3 [50A TiOx/l,500A Ag]*2 76,

C

4 [50A TiOx/l,500A Ag]*2 82,2

5 - [50A TiOx/l,500A Ag]*6 66.6

6 [50A TiOx/l,500A Ag]*6 72,5

7 lOOA Cr/9,200A Ag 68,2

8 lOOA Cr/9,200A Ag 64,2

6. Laser Damage Threshold Measurements at 351 nm

Laser damage tests of these samples were conducted at Los Alamos in Steve Foltyn's XeF

(351 nm) laser damage laboratory by John Jolin. The absolute laser damage thresholds determined

in these tests are not important in our investigation. Thus, the high absorption in the coatings

and the substrate being a thermal insulator do not alter the conclusions of our tests. The

relative damage performance of the laminate coatings when compared to the reference silver

coatings of the same thickness illustrates any improvement due to the laminate design. The

results of these laser damage tests are given in table 3.

Table 3. Silver film laser damage thresholds at 351 nm

Laser Damage

^ Sample # Design Threshold (mJ/cm )

1 lOOA Cr/2,000A Ag 50

2 lOOA Cr/2,000A Ag 50

3 [50A TiOx/l,500A Ag]*2 80

4 [50A TiOx/l,500A Ag]*2 70

5 [50A TiOx/l,500A Ag]*6 150

6 [50A TiOx/l,500A Ag]*6 240

7 lOOA Cr/9,200A Ag 140

8 lOOA Cr/9.200A Ag 140

The effect of thin film thickness is clearly evident when one compares the damage thresholds

of the last two samples to the first two samples. These samples are identical silver films

except for thickness. The first two samples have a total metal film thickness of 2,100 Angstroms

each and the last two samples have 9,300 Angstroms each. This increase of 4,5 times in coating

thickness results in a factor of three increase in laser damage threshold of the silver reference

films.

The most significant results of these laser damage tests are seen in comparing the thresholds measured for the six pair laminate structures (samples 5&6) and the equal thickness reference silver films (samples 7&8) , The two ^hick referenc^ samples each measured 140 mJ/cm , whereas the laminate films measured 150 mJ/cm and 240 mJ/cm for samples 5 and 6, respectively.

Thus, there is an improvement in the damage threshold of a factor of 1.07 to 1,71 for the laminates over the silver reference films. We cannot account for the large discrepancy between the

damage thresholds of samples 5 and 6, except to note that these coatings were deposited in two separate coating runs using the same prescription. Perhaps further investigation of these

samples will reveal some difference in the coatings.

421
7. Conclusions

We have concluded form this preliminary investigation of silver films that laminate structures such as the ones used in this study may prove to be useful in increasing the laser damage

thresholds of metal films. This technique could also be extended to dielectric thin films for use in multilayer coatings.

The authors would like to express their appreciation to Steve Foltyn and John Jolin of Los Alamos for performing the laser damage measurements quoted in this paper. We would also like to acknowledge Glen Lindholm and Kathleen Padgett of Los Alamos for performing the spectrophotometer

reflectance measurements.

8. References

[1] Springer, R. W. ; Catlett, D. S. Rate and pressure dependence of contaminants in vacuumdeposited aluminum films. J. Vac. Sci. Tech. 15(2): 210-214; 1978 March-April.

[2] Springer, R. W. ; Catlett, D. S. Structure and mechanical properties of Al/AlxOy vacuum

deposited laminates. Thin Solid Films 54: 197-205; 1978.

[3] Springer, R. W. ; Barthell, B. L. ; Rohr, D. Quantitative characterization of high

strength aluminum foils vapor deposited on curved surfaces. J. Vac. Sci. Tech. 17(1):

437-439; 1980 January-February.

[4] Springer, R. W. ; Hosford, C. D. Characterization of aluminum- aluminum nitride coatings

sputter deposited using the pulsed gas process. J. Vac. Sci. Tech. 20(3): 462-465;

1982 March.

[5] Springer, R. W. ; Ott, N. L.; Catlett, D. S. Effect of periodic chemical variation on the mechanical properties of Ta foils. J. Vac. Sci. Tech. 16(3): 878-881; 1979

May-June

.

[6] Duchane, D. V.; Barthell, B. L. Unbacked cylindrical metal foils of submicron thickness. Thin Solid Films 107: 373-378; 1983.

422
Figure 1. TEM photomicrograph of an aluminum/aluminum oxide laminate film in

cross-section. Note the thin oxide layers separating the thick

aluminum layers.

Ag

Ag

TiO 2

^lucleation Layer

TiO 2

^u^^l^B^ion Layer

substrate

Figure 2. A schematic view of a silver/titanium oxide laminate film with

50 Angstrom titanium oxide layers and 1500 Angstrom silver layers

423
Time

Figure 3. Auger depth profile of a silver/titanium oxide laminate film.

424
Figure 4. SEM photo of the surface of a typical silver film.

425
426
300 400 500

WAVELENGTH (nm)

Figure 6. Spectrophotometer reflectance traces of samples 1 S 2 with a reference curve calculated from published optical constants of silver.

WAVELENGTH (nm)

Figure 7. Spectrophotometer reflectance traces of samples 3 8 4 with

calculated silver curve.

427
q4 L 1 1 1» 1

» I 1 1 300 400 500

WAVELENGTH (nm)

Figure 8. Spectrophotometer reflectance traces of samples 5 and 6 with

calculated silver curve.

q4 I I I L_ I

I L__ _i L 300 400 500

WAVELENGTH (nm)

Figure 9. Spectrophotometer reflectance traces of samples 7 and 8 with

calculated silver curve.

428
Manusoript Received

12-29-86

Influence of Deposition and Post Deposition

Conditions on Laser Induced Damage in Electron

Beam Evaporated Ti02 and SiO^ Films

K. Narasimha Rao, M. Adinarayana Murthy, S. Mohan

and M. Ramakrishna Rao

Instrumentation and Services Unit

Indian Institute of Science

Bangalore, Karnataka 560012, India.

Composition and structure of oxide films vary with different deposition techniques, conditions and starting materials, resulting in different damage thresholds. In this

investigation, single and multi layer films of TiOo and SiOp

were deposited with TiO and Si02 as starting materials. The

substrate temperatures were maintained 25, 75, 125 and 175 C

and^post heat treatment was carried out at 75, 125, 175 and

225 C in air. These films were subjected to laser damage

threshold studies, using Q-switched NdrYAG Laser of 8 to 9ns.

pulse width.

2 Maximum threshold values obtained were 19 J/cm for ^ Ti02 films deposited at 25 C and post heated at 75 C; 44j/cm

for Si02 film deposited at 25 C and 18j/cm^ for multilayer

stacks of Ti02/Si02 deposited at 25 C. Variation in damage

threshold values is explained on the basis of film composition, structure and optical properties.

Key words : electron beam evaporation; laser induced damage;

multilayers; optical properties; post deposition heat treatment; Ti02 and Si02 films.

1. Introduction

With the increase in application of high power lasers in areas like fusion, microelectronics etc., the necessity of developing high

power lasers and their accessories has been seriously felt. The

development of lasers is very much interlinked with the laser coatings. New trends in the coating technology are resulting in better,

more reliable and high damage threshold coatings.

Though the damage resistance of the coatings depends on laser

parameters such as wavelength, spot diameter, pulse width and mode

of operation, it has been estaoxished that impurities and absorption

in coatings which in turn are decided by the deposition techniques

and parameters influence the damage resistance of the films [l-5].

Selection of materials is also very important in improving the damage

resistance.

Oxides of Titanium and Silicon have been widely used for preparing multi-layer optical coatings for visible and near infrared applications due to their exceptional stability against adverse envi*-

429
ronmental conditions. To obtain films with the required stoichiometry, adhesion and optical properties, a wide variety of techniques

such as electron beam evaporation, R.F. Sputtering, Ion assisted

deposition, Ion beam sputtering have been used [10-17]. The films

have also been subjected to different types of post deposition treatment such as heat treatment in air and vacuum. Ion bombardment,

laser annealing, etc.

In this paper, the authors present the results on the studies

made on electron beam evaporated films deposited at different ambient

substrate temperatures and subjected to post deposition heat treatment

in oxygen. The influence of this treatment on the optical properties

and damage thresholds of single layer coatings of Ti02 and Si02»

5-layer partially reflecting and 21-layer fully reflecting laser

mirrors is discussed.

2. Experimental Techniques

Single and multilayer films of Ti02 and Si02 were deposited by

reactive electron beam evaporation from a four hearth crucible of a 6 KW electron beam gun (Leybold Heraeus). The bell jar (19" dia and

24" height) was pumped by a cheveron baffle - diffusion pump - rotary

pump combination to get an ultimate vacuum of 10~° torr. Quartz crystal

and optical monitors were used to monitor the film thickness during

deposition. The substrates were well polished fused quartz plates of

25mm dia. and glass slides of 2 to 3mm thickness. Substrate rotation

was provided to get uniform coatings. The substrates were cleaned with

'Balzers' cleaner 1, rinsed in running water, cleaned with detergent

and tap water and then again rinsed in double distilled water. Finally

the substrates were subjected to vapour degreasing with iso-propyl

alchohol. The substrates were subjected to heat treatment in vacuum

prior to depositing the films.

Ti02 films were prepared by evaporating TiO (Balzers) in a partial

oxygen pressure of 2 x 10"^ torr at a rate of lOoA/min. whereas Si02

films were prepared by evaporating Si02 (Merck) in a partial oxygen

pressure of 5 x 10"^ torr at a rate of 240A/min. The thickness of the

single layer films of Ti02 and Si02 was corresponding to the quarter

wave thickness for 1.06|im. Partial [G(HL)^HA] and fully reflecting

[g(HL)-'-'-'hA] laser mirrors also have been prepared with alternate quarter

wave layers of Ti02 and Si02 as high (H) and low (L) refractive index

layers. The films were subjected to heat treatment in air at different

temperatures upto 225 C after deposition. The transmittance and reflectance of the films before and after heat treatment at different temperatures were recorded with HITACHI 330 AAodel LP/- Vis. and near I.R. double

beam spectrophotometer with an accuracy of 0+0.2/.. The refractive

index and extinction coefficient of the films were calculated by the

envelop technique [18] from the transmission measurements.

Damage threshold measurements on these coatings were carried out

with a Quanta Ray DCR-2 K'd:YAG laser with a pulse width of 8 to 9 ns. spot diameter 6.4mm and operating at 1.06|im. The set up used for these

measurements is shown in Figure 1. The laser energy density on the

sample was varied not only by using neutral density filters but also

430
by moving the sample towards or away from the focal point of the condensing lens. In this process the spot diameter varied between 0.5mm to

3mm. The influence of the spot diameter on damage threshold was neglected since the spot diameter is well above the minimum diameter [2 ].

The energy of the laser beam was measured with 'Scientech' calorimeter.

The spot size of the laser beam on the test sample was recorded each time

on an exposed photographic paper. The damage threshold (j/cm^) is the

ratio between the energy of the laser 6.n Joules) and the area of the

beam (in sq.cm.). A probe beam from a low power He-Ne laser aligned with

a switched-Nd: YAG laser has been used to observe the damage by measuring

the scattered light. The upper end of the damage threshold is the maximum pulse energy density for which there was no damage.

3. Results and Discussion

3.1 Single Layer Ti02 Films

The damage thresholds and the extinction coefficient of the single

layer TiOo films for the substrate temperatures 25, 75, 125 and 175 C

and^for trie post heat treated films in air at temperatures 75, 125,

175 C and 225 C have been presented in table 1. It has been observed

that the damgge threshold increases with increase in substrate temperature upto 75 C and then starts decreasing. The extinction coefficient

measured on these films show that the films with the highest damage

threshold has the lowest absorption. It appears that the films deposited

at 25 C are not complgte oxides and hence have higher absorption. The

films deposited at 75 C seems to be the optimum ones with the improved

oxidation and hence minimum absorption. This particular substrate temperature seems to provide the necessary activation energy for the oxidatign of the material. The films deposited at temperatures higher than

75 C again appear to be incomplete oxides, may be because of the preferential desorption of oxygen molecules during deposition at those temperatures. Hence the extinction coefficient of these films also is more,

resulting in decrease in damage thresholds. This explanation on the

basis of absorption in films is supported by the insitu optical transmittance studies of these filmg during deposition shown in Figure 2. in

which the film deposited at 75 C shows the highest transmittance for

half wave thickness.

The damage thresholds and the extinction coefficient of the films

as a function of the post deposition heat treatment have also been presented in the same table. The damage thresholds show an improvement

upto a particular temperature which dependii on the deposition temperatures and then shows a decrease. The extinction coefficient also shows

variation but the variation is exactly opposite in nature but the transition occuring at the same temperature. This shows a direct dependence

of the damage threshold on the film absorption. The initial heat treatment upto the transition temperature appears to help in further oxidation,

thereby minimising the absorption and improving the damage threshold.

Heating above the transition temperature may once again result in desorption of oxygen leading to a slight increase in absorption or scattering

due to increase in grain size and decrease in damage threshold. However

the films deposited at 75 C show deterioration on further heat treatment.

As mentioned already, these films might have been oxidised to the limit

431
during deposition, further heating resulting only in desorption of oxygen . 3.2 Single Layer Si02 Films

The damage thresholds of the single layer Si02 films deposited at

25, 75, 125 C and heated^to 75, 175, and 225 C are presented in table 2.

As deposited films at 25 C showed the maximum damage threshold (44j/cm2j.

The optical transmission curves showed practically negligible absorption.

It appears that the deposition rates and the reactive oxygen pressure

during deposition is quite optimum to give the absorption free Si02 films

at room temperature. The damage threshold of the films deposited at

higher temperatures as well as heat treated films were considerably less.

3.3 Multi Layer Films

2 The damage thresholds of the 5 layer [G(HL) HA] partial mirrors and

21 layer [G(HL)-'-*-'hA] fully reflecting mirrors are presented in tables 3

and 4 respectively. The variation in the damage threshold both as a function of substrate temperature during deposition and due to heat treatment is very much similar to that observed in the case of single layer

Ti02 films. These observations show that the damage threshold variation

can be explained on the basis of absorption in the films and the room temperature deposited films heated to 75 C in the atmosphere have the

required oxidation exhibiting the maximum damage threshold to the Nd:YAG

laser.

4. Summary and Conclusions

Single layer Ti02 and Si02 and 5 layers and 21 layers of Ti02/Si02

have been prepared by reactive electron beam evaporation technique. The

laser damage thresholds (Nd:YAG laser) and extinction coefficient of

these films measured on films deposited at temperatures 25, 75, 125 and

175 C and heat treated in air at temperatures 75, 125, 175 and 225 C

showed that damage thresholds are dependent on the absorption films.

The films deposited at 25 C and heat treated at 75 C in exhibited minimum absorption and maximum damage threshold.

The authors wish to thank Prof. Y.V.G.S. Murthy and his colleagues,

Indian Institute of Technology (IIT), Madras; for their help in making

damage threshold measurements. They also thank Prof. E.S. Raja Gopal,

Chairman, ISU for his encouragement and keen interest in this study.

5. References

[l] Desharzer, L.G. ; Newnam, B.E.; Leuing, K.M., The Role of Coating

Defects in Laser Induced Damage to Dielectric Thin Films. Nat.

Bur. Stand. (US) Spec. Publ. 387; 1973, 27-32.

[2] Newnam, B.E.; Gill, D.H., Spectral Dependence of Damage Resistance

of Refractory Oxide Optical Coatings. Nat. Bur. Stand. (US) Spec.

Publ. 362; 1976, 292-300.

432
Lowder Milk, W.H.; Milam, D. ; Rainer, F., Optical Coatings for

Laser Fusion Applications. Thin Solid Films, Vol.73; 155-166,

1980.

Walker, T.W. ; Gunther, A.H.; Nielsen, P., Pulsed Laser Damage

to Thin Film Optical Coatings Part I-Experimental . IEEE J.

Quant. Elect. QE-17, 2041-52, 1981.

Guenther, K.H.; Humpherys, T.W.; Balmer, J.; Bettis, J.R.;

Casparis, E. ; Ebert, J.; Eichner, M.? Guenther, A.H.; Kiesel,E.

Kuehnel, R. ; Milam, D.; Ryseck, i/V. ; Seitel, S.C.; Stewart, A.;

Weber, H.; Weber, H.P.; Wirtenson, G. ; Wood, R.M.; 1.06 Micron

Laser Damage on Thin Film Optical Coatings - a Round Robin

Experiment Involving Various Pulse Lengths and Beam Diameters.

Dundenhausen , B. ; Mollenstedt, G. ; Reactive Vacuum Evaporation

of TiO under Stimulated Oxidation of TiO^. Z. Angew. Phys.,

Vol.27; 191, 1969.

Ritter, E.; SiO and Si203 Phases in Thin Films, Optica Acta,

Vol.9; 197-201, 1962.

Awarter, M.A., Process for the Manufacture of Thin Films.

U.S. Patent, 2920, 002, 1960.

Pulker, H.K.; Paesold, G. ; Ritter, E.; Refractive Indices of

Ti02 Films Produced by Reactive Evaooration of Various Titanium

Oxygen Phases, Appl. Opt., Vol.15; 2986-91, 1976.

Heitman, W. , Reactive Evaporation in an Ionized Gases. Appl.

Opt., Vol.10; 1971, 2414-18.

Kuster, H.; Ebert, J., Activated Reactive Evaporation of Ti02

Layers and their Absorption Indices. Thin Solid Films,

Vol.70; 43-47, 1980.

Martin, P.J.; Macleod, H.A.; Netterfield, R.P.; Pacey, C.G.

Sainty, W.G., Ion Beam Assisted Deposition of Thin Films, Appl.

Opt., Vol.22, 178-184, 1983.

McNeil, J.R.; Al-Jumaily, G.A. ; Jungling, K.C.; Barron, A.C.,

Properties of Ti02 and SiOo Thin Films Deposited using Ion

Assisted Deposition. Appl. Opt., Vol.24; 1985, 486-489.

Bunshah, R.F.; Deshpandey, C.V. , Plasma Assisted Physical

Vaoour Deposition: A review, J. Vac. Sci. Technol. A3(3)

553-560, 1985.

Sites, J.R.; Gilstrap, P.; Rujkorakarn, R., Ion Beam Sputter

Deposition of Optical Coatings. Opt. Eng. Vol.22; 447-9, 1983.

Narasimha Rao, K. ; Mohan, S. ; Ramakrishna Rao, M. , Effect of

Post Deposition Heat Treatment on Multilayer Hard Coatings for

He-Ne Laser. Proceedings of the 3rd Symposium on Lasers and

Applications, 1983, Dec. 16-12, Kanpur, India, 81-82.

433
[17] Narasimha Rao, K. ; Mohan, S, ; Ramakrishna Rao, M., Hard Laser

Mirrors for 6328A, Efficiency Changes on Heat Treatment.

Proceedings of 12th Symposium on Optics and Opto-elec tronics

,

1984, Feb. 1-4, Bangalore, India, 64-68.

[18] Swane Poel , R. , Determination of thickness and Optical Constants of ct - Si:H. J. Phys. E: Sci. Instrum. Vol.9; 1214-22,

1983.

434
Table 1. Damage thresholds in j/cm of TiO^ films as a function of substrate temperature (Tc) and post deposition heat

treatment temperature (Tl,).

25°C 75°C 125°C 175°C 225°C

9.0 19.1 12.5 10.1 8.1

(0.002) (0.001) (0.002) (0.003) (0.003)

75°C 10.1 8.8 8.0 7.9 7.9

(0.001) (0.001

)

(0.004) (0.004) (0.004)

125°C 2.1 7.0 9.2 7.9 6.8

(0.010) (0.005) (0.004) (0.005) (0.005)

175°C 1.2 10.8 12.9 8.8 7.2

(0.020) (0.010) (0.003) (0.004) (0.005)

Values

ent of

within the

the films.

brackets indicate the extinction coef f iciTable 2. Damage thresholds in j/cm of

5i02 films as a function of substrate

temperature (T^) and post deposition

heat treatment temperature (T^,).

25°C 75°C 175°C 225*^0

25°C 44.1 16.5 15.2 15.2

75°C 26.0 18.0 16.0 15.0

125°C 25.4 16.0 15.4 12.5

435
Table 3. Damage thresholds in j/cm of partial

mirrors [8(HL)2ha] as a function of substrate

temperature (T3) post deposition heat treatment

temperature (!„)•

25°C 75°C 125°C 175°C 225°C

25°C 12.5 14.0 10.0 9.7 19.0

75°C 12.5 11.0 10.0 9.5 9.2

125°C 11.0 11.6 10.0 8.7 6.0

Table 4, Damage thresholds in j/cm of high re- flecting mirrors [S(HL)iOHA] as a function of

substrate temperature (T^) and post deposition

heat treatment temperature (Tu)*

25°C 75°C 125°C 175°C 225°C

25°C 18.2 18.0 14.1 12.0 10.6

75°C 11.7 8.3 7.8 8.0 8.0

125°C 3.5 9.9 9.4 9.2 6.9

436
Q-5WITCHED

Nd: YAG LASER

B.5

LENS

He-Ne LASER

N.D

FILTER SPECIMEN

ENERGY METER

Figure 1. LASER DAMAGE THRESHOLD MEASUREMENT SETUP.

6 8 10 12 U

DEPOSITION TIME IN MINUTES

16 18

Figure 2. IN5ITU OPTICAL TRANSM ITTANCE WITH THICKNESS

FOR DIFFERENT SUBSTRATE TEMPERATURES.

437
Manuscript Received

11-3-86

Dependence of Damage Threshold of Anti-Reflection

Coatings on Substrate Surface Roughness

Y. Nose, Y. Kato, K. Yoshida

and C. Yamanaka

Institute of Laser Engineering, Osaka University

Suita, Osaka 555, Japan

Geometrical shapes of the damage sites in the 5-layer SiO^/ZrO^ anti-reflection

(AR) coatings generated by 1pm laser irradiation have been accuratefy determined.

The damages can be classified into tvio types; deep damage and shallow damage, the

former having the lower threshold. Dependence of the damage threshold of the deep

damages on substrate surface roughness is given. It is suggested that presence of

absorption centers embedded in the substrate is responsible for determining the damage

threshold. Possible approaches that will lead to stronger AR coatings are proposed.

Keywords: Anti-reflection coating; laser-induced damage; polishing process; surface

roughness

.

1. Introduction

Previous observations show that transmitting coatings such as anti-reflection (AR) coatings

and polarizing beam splitters have lower damage threshold than high-reflectance (HR)

coatings [1]. The damage to AR coatings originate at the interface between the first coating

layer and the substrate [2]. However the exact process which leads to the laser-induced damage

in the AR coating has not been clarified so far.

We report here the results of the detailed observations of the laser-induced damages in the AR coatings. In particular we have studied the correlation between the damages and the surface

roughness of the substrate. Based on these observations the most probable mechanism for damage

formation in the AR coatings is discussed. Proposals are made which will lead to the

improvement of the damage threshold of the AR coatings.

2. Experimental Conditions

2-1 .Preparations of the substrates

In order to obtain consistent results in the damage properties of the AR coatings, careful

attentions were paid to fabrication of the substrates and the coatings used in the following

experiments

.

The substrates were high optical quality BK-7 glasses of 30 mm in diameter and 10 mm in

thickness. They were polished either by conventional polish or by bowl-feed polish process,

respectively [3]. In the latter the substrates were immersed during polishing in the liquid

containing the polishing compounds, resulting in smooth surfaces. Ceria compounds were used as the polishing material.

The surface roughness of the polished substrates were measured with a Wyko NCP-IOOOM

optical profilometer. The typical rms (peak-to-valley) surface roughness was 30-35 A (^200 A) for conventional polish and 5-9 A (30-60 A) for bowl-feed polish, respectively. Figure

1 shows the surface profiles of the substrates #1 and #2 which were prepared by conventional and

bowl-feed polishes, respectively. As shown in Figure 1, we define two parameters which

characterize the lateral scale length of the surface profile: the average distance between the

peaks (Ad) whose height exceed the rms roughness and the typical width of these peaks (Aw).

Table 1 shows these parameters for the samples #1 and #2. We note that deep and wide rough

peaks produced by conventional polish (#1) are smoothed by bowl-feed polish (#2) resulting in

smaller value of Aw.

438
2-2. Coating

Five layer Si02/Zr02 AR coatings of quarter-wave design were vacuum deposited on the

substrates, with the first SiOp layer as the undercoat to increase the damage threshold [4].

The substrates after polish and before coating (typically week) were kept in dry atmosphere

to prevent chemical changes on the fresh surface. All the substrates with different surface

roughnesses were coated at the same time in order not to introduce uncertainty due to the

coating process.

2-3. Laser irradiation

Each sample was irradiated with a laser pulse of 1.064 wavelength from a Q-switched

Nd:YAG laser. The pulse width was 1.5 ns , generated by slicing a Q-switched pulse with a fast

Pockels cell switch. After two-stage amplification with Nd: glass amplifiers of 15 mm diameter,

the laser pulse was focused with a 500 mm focal length lens. The laser beam^on the sample had a smooth Gaussian spatial distribution with the spot diameter of 400 ym at 1/e point. The laser

energy was measured with a calibrated photodiode.

The irradiation site was observed in site with a Nomarski microscope before and after each

laser irradiation to detect the laser-induced damage. The sample was moved after each laser

shot irrespective of the presence or absence of the damage.

2- 4. Characterization of the laser-induced damages

The following techniques were used in order to definitely determine the geometrical shapes

of the damage sites generated at different experimental conditions.

1) Optical microscopy: A Nomarski microscope was used to inspect and record the morphology

of the damages.

2) Electron microscopy: Enlarged views of the damages were obtained with a scanning

electron microscope.

3) Surface profi lometry: A Talystep surface profilometer was used to determine the surface

profile and the depths of the damage sites, since these parameters are difficult to determine with the optical and electron microscopes.

4) Material analysis: An electron-probe x-ray mi croanalyzer (XMA) was used to identify the

materials at the damage sites.

3. Experimental results

We have studied the dependences of the laser-induced damages on the surface roughness of

the substrates. First we show the results regarding the geometrical shapes of the damage sites

generated at typical experimental conditions. Then more quantitative relations between the

damages and the surface roughness of the substrates are presented.

3- 1. Geometrical shapes of the damage sites

1) Optical microscopy

Morphology of the damages obtained with a Nomarski microscope are shown in Figure 2. The

two samples are AR coatings fabricated using the subrates #1 and #2 which have different

roughness as shown in Table 1. The damages were produced at the laser fluences (E^) near the

damage threshold (E, '^^E ) and far above the threshold (E.'^^3E ). Note that the damage threshold

depends on the surface*^ roughness as will be described later! therefore E^ and Ej^ are quite

different in Figure 2 for the two samples.

Near the threshold, damages which have small diameter (which we call "deep damages"

hereafter) are distributed over the irradiated area. When the laser fluence is increased, the

number density of the deep damages increases significantly, but the size changes very little.

Futhermore when the fluence exceeds approximately twice the threshold of the deep damages,

damages covering the large area (which we call "shallow damages" hereafter) are generated.

2) Electron microscopy

439
Magnified views of the damages obtained with an electron scanning microscope are shown in

Figure 3. The deep damages are mostly circular with the diameters of 3-4 ym. The microscope

picture for #2 near B.'^E^-j-, which was taken at a tilted angle of 15°, reveals each coating layer

at the wall of the crater. It shows that the damage wall has two steps: SiO^, ZrOp/SiO^ and

ZrO-j/SiOp counted from the outer surface to the substrate. The deep damages have the depth

which probably reaches to the substrate, although it is difficult to definitely conclude only

with the electron microscopy.

At the higher fluence of E|'^3Ep-p, shallow damages are produced over the wide area covering

the deep damages. In the shallow damages, the top coating layer (Si02) is removed from the ZrO^

layer. However it appears that the top layer remains around the deep damages with the diameter

of 20-30 ym even with high fluence irradiation.

3) Surface profilometry

A Talystep was used to measure the depths of the damages. Figure 4 shows the Talystep

scans of the samples having the shallow damages. It shows that the depth of the shallow damage

corresponds approximately to the thickness of the top coating layer, and the depth of the deep

damage is close to the total thickness of the 5 coating layers. However the latter is not accurate since the stylus is not small enough to reach to the bottom of the deep damage which

has '^1 ym diameter at the bottom. Also this measurement confirms that the top coating layer

remains around the deep damage within the shallow damage.

4) Material analysis

Figure 5 shows the results of the XMA analysis at four different sites outside and within

the damages. Although XMA does not provide good depth resolution since the probe electrons (25

kV) have 1^2 ym peneration depths, the result supports the above observations regarding the

profiles of the damage sites.

Figure 6 gives the schematic picture of the damage sites based on our experimental

observations. At E.'^E , deep damages which extend to the substrate are generated. At far above

threshold like E,'^3E , the top coating layer is blown-off resulting in the shallow damage,

whereas the top layer remains around the deep damages.

3-2. Dependence of the deep damages on the laser fluence and the surface roughness

Since the deep damage has a lower threshold than the shallow damage, we study here

dependences of the deep damage on the two parameters; laser fluence and surface roughness.

1) Dependence on the laser fluence

Here we determine the exact damage threshold of the deep damage using the sample #2 whose

substrate has been bowl -feed polished.

The laser beam has the Gaussian spatial distribution on the sample given by

E(r) = Epe-2^'/^' , (1)

where a=200 ym and E is the peak fluence at r=0. The damage occurs within the area of radius

r where E(r) exceed^ the threshold fluence E ; E(r )=E^. We have examined the laser damages

w^th the Nomarski microscope, and determined the crTtical radius r and derived the threshold

fluence E . For the damages generated at different laser energies up to the peak fluence on the

sample of''3Q J/cm , the value of E thus determined was approximately constant having the value

of 7-5 J/cm . Therefore we may conclude that the actual threshold for the deep damages is 7.5

J/cm for the particular sample that we have studied.

The area within r^ is given by

S = TTr^^ = (7Ta^/2)Jln (Ep/E^) , (2)

440
If all the deep damages have the same threshold of E , then the number of the deep damages

within the area S should increase logarithmically with E . Figure 7 shows the experimental

results of the total number of the damages and the average distance between the damages plotted

against the peak fluence Ep. Since the number of the damages increases with Ep more strongly

than logarithmic, ^this shows that the deep damages have variation of the thresnolds whose lowest

value is 7.5 J/cm .

2) Dependence on the surface roughness

It is reasonably well established [1] and our present results support that the damages in the AR coating are caused by some imperfections at or near the boundary between the coating and

the substrate. Also there is a set of experimental data that higher damage threshold is obtained with the bowl -feed polish compared with the conventional polish [2].

Figure 8 shows our experimental data of the damage threshold plotted against the rms surface roughness and the peak-to-valley surface roughness. Three bowl-feed polished and one conventional polished samples were used. The damage threshold was determined as the peak

fluence at which the damage was detected in situ with the Nomarski microscope. The error bars

in the damage threshold show the range of data obtained from many shots on each sample. These

data clearly show that the AR coating deposition on the smooth substrate results in higher

damage threshold. However it should be noted that the threshold of E/^-9J/cm is lower

than that obtained with good HR coatings in which the value of E >12 J/cm has been

achieved [1 ,5].

^

We have measured the diameters and the average distances of the deep damages produced at high laser fluence of E, '^^3E , and compared them with the lateral scale lengths of the surface

roughness of the substrates that we have defined in Figure 1. The result is shown in Table 2. Here we find close correlation between the diameter of the deep damage and the width of the

surface roughness (Aw), and also between the average distance between the damages and the

average distance between the peaks of the surface roughness (Ad). The implications of this

results are discussed in the next section.

4. Discussion

First we discuss on the deep damages that give the lower bound of the damage threshold of

the AR coating. Lowdermilk and Milam suggested [1] that the damage begins with the generation

of very hot spot near the interface with the substrate. The intense heat melts the glass and

generates high pressure which lift the coating layers. When the crack reaches to the surface,

molten glass is ejected from the center and solidifies quickly.

Our observation of the geometrical shape of the deep damages shows that the damage is extending to the substrate. The damage threshold is related to the surface roughness of the

substrate. Also we found close correlation between the geometrical size of the damage and the

typical width of the surface roughness, and the correlation between the distance between the

damages and the distance between the peaks of the surface roughness. In addition we have other

data with the AR coating on a conventional-polished substrate in which deep damages are aligned

on a straight line. Close examination of the substrate before coating with the Nomarski

microscope reveals the presence of sharp straight scratches on the surface.

Considering these observations, we may propose that the deep damages are generated due to the presence of absorption centers in the substrate. With the bowl-feed polish compared with

the conventional polish, the size and also the number density of these absorption centers become

less, resulting in higher damage threshold. The nature of the absorption centers is not clear;

however they may well be the polishing compounds that are embedded in the valley of the

scratches or in the subsurface structure.

It is an intriguing question that how much we can improve the damage threshold when we keep

reducing the surface roughness using special techniques. If these new techniques could reduce

the absorption centers in the substrate during polishing, then they will result in higher damage

threshold. Otherwise it will be difficult to attain significantly better result. We may

chemically remove the absorption centers, however it might result in rough surface with high

scattering level. Laser annealing of AR coating by repetitive irradiation with low power laser

is known to be an effective process to increase the damage threshold [6]. In this case, absorption centers probably spread due to laser heating, resulting in less absorption and thus

441
higher threshold.

We have another viewpoint which we have derived from our recent work on porous dielectric

coating [7]. In this coating, SiO^ and NaF are coated at the same time on the substrate and NaF

is dissolved after coating, leaving porous layer of SiOg. This coating has very high damage

threshold of 12-13 J/cm at 1 ym even when we use rough substrate. According to our interpretation, the reason for the high damage threshold of the porous coating is that the

pressure generated at the absorption centers is not accumulated under the coating layer, but it

is quickly released due to the porous nature of the coating [8].

The shallow damages have higher threshold than the deep damages. Although we have not tried to accurately determine the threshold of the shallow damage, it is 20^^-25 J/cm for the

5-layer SiO^/ZrOp AR coating on the bowl-feed polished substrate. Therefore if we could

completely eliminate the deep damages, there is a possibility that the AR coating could have the

damage threshold exceeding 20 J/cm .

It is an interesting observation that the top layer remains around the deep damages located

within the shallow damage. One of the possible interpretations is that the plasma ejected from

the deep damage refracts and absorbs the laser beam, reducing the laser fluence and thus

preventing the damage around the deep damage. There may be interference of the refracted beam

with the direct beam near the damage affecting the damage threshold of the shallow damage.

Further study will be necessary to clarify various aspects related to the shallow damages.

5. Conclusion

Based on our experimental observations, we have suggested that the presence of absorption

centers in the substrate is responsible for determining the damage threshold of the deep damages

which give lower bound of the threshold of the AR coating. If this interpretation is correct^

then there are two posspilities that we could take to improve the threshold to above 10 J/cm

and possibly to 20 J/cm . The first is to reduce the size (and the number) of the absorption

centers. Smaller absorption center will result in the reduction of pressure caused by the

heating of the absorption center and thus increase in the damage threshold. Further

quantitative experimental and theoretical studies are necessary to clarify these relationship.

Another possibility is to develop porous coatings which releases the pressure generated at the

substrate. The porous AR coatings have already been developed [7,9,10]. However this concept

could be extended to other transmitting coatings such as the polarizing beam splitter.

The above discussions are pertinent to the AR coating at 1 pm where the damage is determined by the substrate property. At shorter wavelengths such as in the ultraviolet, the

damage is mostly determined by the absorption within the coating layer. In this case selection

of the coating materials becomes more important. However porous nature of the coating might

help improving the damage threshold also in this wavelength region.

5. Acknowledgement

We appreciate T. Izawa of Showa Optical Company for preparation of the samples used in this

study.

7. References

[1] W.H. Lowdermilk and D. Milam, IEEE J. Quant. Electr. QE-17, 1888 (1981), and references

sited therein.

[2] C.K. Carniglia, J.H. Apfel , G.B. Carrier and D. Milam, NBS Special Pub. 541, 218 (1978).

[3] R.W. Dietz and J.M. Bennett, Appl . Opt. 5, 881 (1966).

[4] J.H. Apfel, E.A. Enemark, D. Milam, W.L. Smith and M.J. Weber, NBS Special Pub. 509, 255

(1977).

[5] T. Izawa, I. Hashimoto and H. Shikakura, Rev. Laser Eng. IJ^, 662 (1983) (in Japanese).

[5] J.E. Swain, W.H. Lowdermilk and D. Milam, Appl. Phys . Lett. 41, 782 (1982).

442
[7] K. Yoshida, H. Yoshida, Y. Kato and C. Yamanaka, Appl . Phys. Lett. 47, 911 (1985).

[8] K. Yoshida, T. Yabe, H. Yoshida and C. Yamanaka, J. Appl. Phys. 60, 1545 (1986).

[9] L.M. Cook, W.H. Lowdermilk, D. Milam and J.W. Swain, Appl. Opt. 21, 1482 (1982).

[10] I.M. Thomas, Appl. Opt. 25^, 1481 (1986).

RMS 29A P-V 208A

110 T

RMS 8.6A P-V 54A

30 1

-30 J 1 ^ < ^ 1 0 200 400 600 800 1000

Distance on surface in microns

Figure 1 Surface profiles of the substrates prepared with different polishing

techniques; conventional polish (#1) and bowl-feed polish (#2). Shown in the figure are definitions of Aw and Ad which characterize lateral

scale-lengths of the surface roughness.

443
' 50pm

El~Ec

El~ 3Ec

#1 #2

Figure 2 Morphology of the damages of the AR coatings on the substrates #1 and #2

having different surface roughness. Damages near the threshold (E.'^E ) and far above the threshold {E.'^3E ) are shown. ^

El-Ec El-3Ec

1 lam 5 tjm

Figure 3 Scanning electron microscope pictures of the deep damages of the AR coatings on the substrates #1 and #2 having different surface roughness.

Damages near the threshold (E^'^E ) and far above the threshold (E^^'^-SE ) are shown. For #2 the substrates were tileted at 15° for better observation

of the crater wal 1 s

.

444
#1

0.2nm

50pm

#2

lOOMm

aoB 200nm

Figure 4 Talystep scans of the damages of the AR coatings on the substrates #1 and #2 having different surface roughness. The scales of the deep damage

and the shallow damage are shown in the scan for #1 and #2, respectively.

Total S 1 Z r K Bs J u d g erne n t

1 6690 2606

0. 1 67 34960.211

937

0. 066

792

0. 048

S 1 Oj

(1) 2 1 6989

1 2328 0.137

3999

0. 236

97 1 0. 067740

0. 044

Z r Oj

(2)

3 17316

1 282 1 0. 163 3826

0. 209

900

0. 062

766

0. 044

S i Oj

(1) a 8 196

1

1 268 0.166

4 1 4 0.06 1 780

0. 096

662

0. 069 Substrate

Upper column:Count number Lower columnlPercentage

Figure 5 XMA analysis of the sample #2 at four different sites; 1: undamages site, 2 in the shallow damage, 3: near the deep damage, and 4: in the deep damage.

445
Deep damage Shallow damage

i i

Zt£k.

SiCb Zr02 SiOT

Substrate

1pm

rinure 6 Schematic picture of the damage site of the 5-layer Si02/Zr02 AR coating

S> 20

E

03 Q

0)E

3

10

0

/

o

k—

1 0 20 30

E

100 ^

o

c

CO

(A Q

<DO)

50 2

>

<

Peak Fluence (J/cm^)

Figure 7 Total number of the deep damages (o) and the average distance between them

() generated within the laser-irradiated area at different laser fluences.

446
RMS Roghness (A)

10 20 30 40

—1

1 1 1

100 200 300 400

Peak to Valley Roughness (A)

Figure 8 Dependence of the damage threshold of the AR coating on the surface roughness

of the substrate.

447
Table 1. Surface roughness and lateral scale lengths of the

two samples prepared by different polishing

processes

.

Sampl

e

Polish

Surface roughness Scale length

rms peak-to-valley Aw Ad

#1 Conventi onal 29A 208A 11pm 39ym

#2 Bowl - feed 8.6A 54A 4ym 25ijm

Table 2. Diameters of and average distances between the

deep damages produced at Ei'^SE . Also shown are lateral scale lengths of the surface roughness of the

substrates

.

Sampl

e

Pol i sh

Deep damage Scale length

Diameter Distance Aw Ad

#1 Conventional 4-15ym 32ym 1 1 ym 39ym

#2 Bowl - feed 3-5ym 26ym 4ym 25ym

448
Manusaript received

3-31-86 SAB

Correlation Between Local HeNe Scatter and Defect-Initiated

Laser Damage at 2.7 pm

J. 0. Porteus, C. J. Spiker, and J. B. Franck

Michelson Laboratory, Physics Division

Naval Weapons Center, China Lake, California 93555-6001

Evidence of a correlation between defect-initiated pulsed laser damage and local predamage scatter in multilayer infrared mirror coatings was reported last year at this conference. Analysis of a much larger data base confirms this result on certain sample types over a substantial range of energy densities above the damage onset. Scatter signals from

undamaged test sites were obtained using an HeNe scatter-probe damage monitor with a focal

spot that nearly coincides with the L50-ym-diameter (Dj^/g^) focal spot of the damaging

beam. Subsequent damage frequency measurements (1-on-l) were made near normal or at 45-deg

incidence with 100-ns pulses at 2.7-ym wavelength. The correlation is characterized by an increase in damage frequency with increasing predamage scatter signal. Characteristics of

the correlation are compared with a simple model based on focal spot intensity profiles.

Conditions that limit correlation are discussed, including variable scatter from defects

and background scatter from diamond-turned substrates. Results have implication for

nondestructive defect detection and coating quality control.

Key words: damage correlation; defect scattering; laser-induced damage; multilayer dielectric coatings; nondestructive quality control; optical defects; scatter probe.

Introduction

The importance of defects as initiators of laser-induced damage in optical coatings is well

recognized. Defect detection and characterization, particularly when related to laser damage, is therefore of considerable interest to coating development efforts and for quality control. Attempts

to correlate damage with photothermal , photoluminescent , and other types of defect signatures have

been described in numerous papers at this conference and elsewhere. Our efforts in this area have

been based largely on individual defect scattering, which provides relatively simple and rapid detection that can be combined readily with laser damage testing. Association of pulsed visible-wavelength

damage with discrete scatterers of the test source radiation has been established using a video

microscopy system [1,2].^ Also, limited evidence of correlation between infrared laser damage and

visible wavelength scattering by defects has been reported [3]. The scattered-light detector in this case was a HeNe scatter probe designed to detect damage events automatically. As a result of high

sensitivity and spatial resolution better than the mean defect separation, it was possible to detect

scattering from individual defects before the damage-probe laser pulse occurred. Close coincidence of

the sampling field of the damage probe with that of the scatter probe permitted correlation of damage

with the observed defect scattering. The correlation appears as a rise in damage frequency vs predamage scatter as shown in figure 11 of [3]. Limitations on the generality of this result must be

established before it can be applied to nondestructive quality control. Therefore, a more extensive

examination of the available data and its interpretation was performed, as described in this paper.

Scatter Probe

The design and operation of the automated damage test facility used in this work have been

described in detail [3]. Essentially, the facility consists of a pulsed laser source, conditioning

optics and diagnostics for the laser beam, and a damage test station that includes the sample translation stage and scatter-probe damage monitor. Test sequencing, data acquisition, and maintenance of constant energy density on the sample surface are under computer control. This arrangement provides a convenient means for measuring damage frequency defined as the damaged fraction of a large number of

identically tested sites. Such measurements are made for each of several fixed energy densities on a

Work supported by the Strategic Defense Initiative Organization and Navy Independent Research Funds.

Numbers in brackets indicate the literature references at the end of the paper.

449
given sample, and damage onsets (0% thresholds) are inferred by extrapolating to zero damage frequency

using a defect damage model developed for this purpose [4].

A schematic of the scatter probe and its relationship to the pulsed damage probe beam for testing

mirror samples at normal incidence is shown in figure 1. For the scatter probe, a high-stability HeNe

laser provides a visible plane-polarized CW beam which is brought to a focus by Lens LI and Mirror Ml at a "negative pinhole" just beyond the reflection at the sample Mirror SM. The position of the HeNe beam and its focus are carefully adjusted to ensure close coincidence at SM with the damage probe

beam, which is focused by Lens L3. If the sample surface within the scatter probe field is optically

perfect, all of the HeNe beam is specularly reflected and deflected into a dump (not shown) by a tiny

diamond-machined mirror on the negative pinhole. However, if the field includes a damaged area, some HeNe radiation is scattered past the negative pinhole, is collected by Lens L2, and is reflected by M2

into the detector, thus indicating a damage event. In a 1-on-l damage test, the scattered signal is measured at each test site immediately before and immediately after each damage probe pulse, and a damage determination is made when the difference exceeds the combined HeNe source and detector noise

fluctuations. One thus discriminates against existing defects that cause the absolute scatter level

to vary from site to site across the undamaged sample surface. In performing the damage measurements

on certain samples, it was observed that damaged sites often have exhibited an unusually large

predamage scatter signal, indicating the presence of a defect. This observation provided the

motivation for the present study.

Relationship of Damage Frequency to Localized Scatter

In order to understand the relationship of the predamage scatter signal to damage frequency, one must first understand its relationship to the damage threshold. We treat here the case of a normally

incident damage-probe beam with the scatter-probe beam at incidence angle 9 > 0, as shown in

figure 2. The probe beams are both assumed to be Gaussian, so that the energy density of the damage

probe beam at radius r in the sample plane is

I(r) = I^ exp[-(J)2] , (1)

where Ig is the axial (r=0) energy density. Similarly, the intensity of the scatter-probe beam at position (r,(j)) is

J(r,<f),9) = exp[-(^)2 (1 - cos2(t) sin^e)] , (2)

where <^ is the azimuth angle measured from the plane of incidence. The scatter probe signal S from a defect with scattering efficiency a located at (r,(fi) is

S(r,(|>,e) = aJ(r,(t,,9) (3)

over and above the background signal S^,. Ideally, there is a threshold scattering level Sq that

corresponds to the onset of damage Iq, so that the distribution of damage frequency vs scatter is a unity step function at S = Sq. This relationship is illustrated in figure 3. From the definition

of Iq, only defects at radial positions r <^ rQ, where the damage-probe spatial profile rises above

Iq, will initiate damage [4]. Because of the intensity of the scatter-probe spatial profile in this

region [eq. (2)], such defects will scatter with intensity S >^ Sq, as shown in the lower part of the

figure. However, as a result of the asymmetry when 9 > 0, Sq varies with the azimuth of the defect,

ranging from a minimum Sq at <}> = it/2 to a maximum Sq ' at 41 = 0. Combining eqs. (1) through (3)

for the conditions r = r^, 1 = Iq, and S = Sq, one obtains an explicit expression for Sq in terms of ^ and Iq:

-r -(-)^ (1 - cos^cj) sin^g)

I V

S (4,) = aJ (^) . (4)

o a i o

Since the defects are random in azimuth, the rise in damage frequency at Sq is not abrupt, but has a finite slope unless 9 = 0. The mid-point Sq of the rise can be obtained approximately from eq. (4)

by replacing cos ^4) by its mean value, giving

I "^v^^ " i

^^""^^^

S - a J (7^) .

~ (5)

o a I o

450
Observe that Sq decreases as becomes larger. Thus, a downward shift of the distribution to lower scatter levels is to be expected as the incident energy density increases.

The degree of correlation between damage and scatter can be quantified in terms of either the

width ASq or the slope of the rising portion of the distribution. In any practical situation, various sources of broadening can contribute to the width and degrade the correlation. The relative

broadening from azimuthal asymmetry associated with nonnormal incidence of the scatter probe can be obtained from eqs. (4) and (5) as

2

I \ j

(^)' sin2e

a \ 2 V

- 1

. (6)

Poor axial coalignment of the probe beams can introduce additional azimuthal asymmetry and

consequent broadening. Broadening also accrues from pulse-to-pulse fluctuations in axial energy

density. If Alp represents the maximum deviation from the nominal I;,, the associated full-width

broadening contribution averaged over the actual Ip's is

1 9

2" sin'^e)

2AI

(7)

Similiarly, significant broadening can result from excessive fluctuations in the scatter probe laser

or detector noise.

Broadening associated with inherent sample properties generally is less straightforward to quantify. Such broadening can arise if all defects do not have the same damage threshold 1q and the same scattering efficiency a as assumed in the above analysis. The variation of individual defect

damage thresholds can be inferred from the dependence of damage frequency on energy density by a method developed in [4], Variation of scatter efficiencies can be inferred in a similar way from the

frequency distribution f(S) of measured scatter levels. For n randomly sited scatterers per unit area excited at 9 = 0 with a Gaussian beam.

f(S)

miv^S-^ S < aJ

S > aJ (8)

In the case of nonidentical a's represented by the distribution function n(a), eq. (8) can be generalized to

f(S) = TTV^S"^ / n(a) do

S/J

(9)

Here the discontinuity in Sf(S) at S = aJg, which characterizes identical scatterers, is replaced by

a more gradual decrease to zero as S increases. The range of the decrease provides an indication of

the range of a variation in terms of S. Because of the standing wave field within a multilayer

coating, a depends on defect depth as well as on inherent defect scattering properties [5]. Since a can be strongly wavelength dependent as demonstrated in reference [2], correlation between scatter and

damage measured at different wavelengths tends to be weakened by variations in defect depth. However,

unless the local standing wave field is distorted severely by the defects, depth variations should be

apparent from variation of individual defect damage thresholds [6].

Measurement Conditions and Procedure

The damage measurements used for this study were performed to evaluate dielectric-enhanced mirror

coatings developed in proprietary efforts, where mirror characteristics and measurement conditions were determined by program requirements. Two distinctly different configurations with respect to damage-probe incidence angle and substrate finish are represented: (1) normal incidence on polished

substrates and (2) 45-deg incidence on single-point diamond-turned substrates. Three samples of the

first group and six of the second were analyzed for correlation of damage frequency with predaraage

451
scatter. Only three samples are discussed in detail here, two from the first group and one from the

second. Significant information on these three samples in their test configurations is given in

table 1. The multilayer dielectric coatings were designed for maximum reflectance at 2.7-pm

wavelength and at the damage-probe incidence angle indicated. Mirrors A and C were made from the same coating materials, which differ from those of Mirror B. In the case of Mirror C, a polarizing filter

was required in front of the scatter-probe detector to reject a large background component scattered

from the diamond-turning tool marks on the substrate. To minimize scattering and damage from dust

particles the samples were dusted with boil-off from a liquid N2 container before mounting in the vacuum test chamber. The 1/e radii of the damage-probe and scatter-probe beams were w = 53 and v =

71 pm, respectively, normal to the beam axes at the sample position. The former dimension was measured by pinhole scanning, while the latter was estimated from microscopic inspection of the focal

spot. Coalignment of the probe beams was verified on each sample before damage testing by adjusting

the scatter-probe beam position for maximum signal from well-developed (uniform) damage produced by

the damage probe.

Table 1. Principal samples and test configurations

Mirror Substrate

Damage

Incidence

probe

Polarization

Scatter

Incidence, 6 probe

Polarization

A Au/Mo^ 0° 45° P

B Ag/U^E^ 0° 45° P C Cu 45° c P 30° P Optically polished before application of the reflective metal overlayer. ULE is ultra low

expansion glass.

^Diamond-turned, with tool marks nominally normal to incidence plane of probe beams,

c Scatter viewed in s-polarization only.

As described in [3], samples were tested for damage in a matrix of sites. Each site was subjected to a single 100-ns (nominal) pulse of multiline 2.7-ym radiation having one of several

selected energy densities within the limits AI3/I3 = ±0.2. The number of sites tested was governed by the requirements for statistical precision in the damage onset determination [4] . Data

blocks corresponding to a given energy density include data from ~10 to ~100 sites, a greater

number being required to maintain adequate precision at low damage frequencies. Recorded information

for each site includes site coordinates, actual energy density, pre- and post-test scatter, and a binary damage/no damage digit used to compute the damage frequency of the entire block. Damage

morphology, as later verified by Normarski microscopy, consisted of spatially selective pitting with

uniform erosion occurring at the higher energy densities.

Damage frequency vs scatter distributions were obtained at each energy density selected for the

damage tests. This was done by sorting the corresponding data blocks into subblocks representing

scatter intervals at each level of predamage scatter. The damage frequency for each subblock then was used to construct a histogram of data points representing the distribution at each of the various

scatter levels. In order to facilitate comparison with eq. (5) and to evaluate distribution widths, a straight line was fit to the data points in the rising portion of the histogram using a weighted least

squares algorithm similar to that described in [4]. The fit was refined by one-by-one exclusion of

data points determined to be outside the rising portion. To qualify for exclusion from the fit, a data point must (a) represent a damage frequency of either 0 or 1, (b) represent an extreme scatter

level of the group that includes the data point in question, (c) degrade the average fit when included

based on the standard error of estimate, and (d) reduce the slope of the fitted line when included^.

The scatter level corresponding to damage frequency = 0.5 on the fitted line is used to define Sq

after subtracting the background scatter Sjj. The difference in scatter between the intersections of the line with damage frequencies 0 and 1 is used to define the width ASq.

Results and Discussion

Generally, all three mirrors of the normal-incidence, polished group showed excellent correlation

of damage with scatter, while little or no correlation was found in any of the six diamond-turned

mirrors tested at 45 deg. The specific examples of table 1 are discussed below.

Mirror A

Predamage data obtained from Mirror A with the scatter probe is summarized in table 2. Scatter

levels S above background given in the top row define the midpoints of half-unit scatter intervals

(AS = 0.5) comprising the range of observed scatter signals. The second row gives the relative

452
frequency f(S) AS of scatter signals falling within each interval. Corresponding values of Sf(S)

given in the third row gradually decrease to zero between S = 1.35 and S = 3.85, indicating

significant variability in a [see discussion of eq. (9)]. The relatively low value of Sf(S) at S = 0.35 apparently is caused by the actual profile being lower in the wings than the assumed Gaussian

form. In spite of these complications n can be obtained approximately if Sf(S) is averaged to emphasize intermediate values of S. Appropriate weighting factors are provided by the tabulated f(S) AS values. Application of eq. (8) to the weighted average yields n//2 = 2.4 x 10^ cm~^ for the

density of scatterers, where the division by /2 corrects for the 45° incidence angle.

Table 2. Mirror A scatter frequency data

S^ 0.35 0.95 1.35 1.85 2.35 2.85 3.35 3.85 4.35

f(S)AS 0.20 0.34 0.30 0.12 0.02 0.01 0.01 0 0.00

Sf(S) 0.14 0.58 0.82 0.45 0.11 0.06 0.03 0 0.02

Excludes background S^ = 12.4 (arbitrary units)

Results of the damage tests on Mirror A are shown in figure 4. Damage frequencies measured at eight different energy densities are Indicated together with standard deviations computed from

eq. (26) of [4]. The solid curve shows the power-law defect damage model providing the best representation of the data based on a weighted least-squares fit [4]. The zero-damage intercept of this

three-parameter curve gives the damage onset, the slope provides the defect density, and the power

(curvature) defines the ensemble of defect failure thresholds. In this case, the power p equals -1,

indicating the degenerate ensemble where all defects initiate damage at the same local energy density.

The density of damage-initiating defects is 2 x 10~^ cm~^, in excellent agreement with the density of

scatterers obtained with the scatter probe.

Figures 5(a) and (b) show examples of damage frequency vs predamage scatter distributions for two

of the eight energy densities of figure 4. Damage frequencies and standard deviations are based on damage data subblocks representing half-unit scatter intervals.* The indicated background S^, = 12.4

represents the lowest predamage scatter reading recorded on any site. Parameters obtained as described above from Mirror A distributions such as figure 5 are summarized in table 3. Column 1 gives the relative energy density for each distribution, column 2 gives the slope of the fitted line,

column 3 gives the mean threshold scatter level Sq above S^, and column 4 gives the width ASq.

Table 3. Mirror A distribution parameters

AS

0 / o

I /I

a 0 Slope

S ^

o AS

0 Observed

b Computed

1.38 0.204 3.28 4.90 1.5 0.3

1.75 0. 112 4.95 8.93 1.8 0.3

2.50 0.343 1.58 2.92 1.9 0.4

3.00 0.234 2.14 4.27 2.0 0.5

3.50 0.486 1.69 2.05 1.2 0.5

4.00 0.484 1.71 2.07 1.2 0.6

4.50 1.16 1.25 0.86 0.7 0.6

5.00 0.216 2.50 4 . 60 1.8 0.6

^ith background, S(j = 12.4 subtracted.

^Sum of contributions from eqs. (6) and (7)

Standard deviations for experimental damage frequencies exactly equal to 0 or 1 are given as l/(s+l)

in the figures, where s is the number of test sites represented in the data subblock. This is based on the uncertainty of the result of testing s+1 sites. Otherwise, standard deviations were computed

from eq. (26) of [4], which gives correct results only at intermediate damage frequencies.

453
Columns 2 through 4 are expressed in arbitrary scatter units. Column 5 provides the relative width

ASq/Sq for comparison with the total known contribution computed from eqs. (6) and (7) given in

the last column.

Correlation of damage with predamage scatter is quite evident in all eight distributions of this

sample as indicated by the uniformly positive slopes in table 3. Generally, the correlation improves

with increasing Ia/Io» is apparent from the correspondingly increasing slopes and decreasing

widths ASq. Comparison of the observed and computed relative widths show the former to be larger in

all cases, the difference becoming larger as la/^o decreases toward unity. Damage with =: Iq involves defects close to the damage-probe optic axis, where the defect scatter signal is

strongest and the asymmetry broadening is minimized, as can be seen from figure 3. This suggests

variation in a as a more important source of broadening than either azimuthal asymmetry or variation

in the scatter background. The suggestion is confirmed by the evidence for a variablility from

table 2, where the range of S over which Sf(S) goes to zero corresponds roughly to the average ASq

given in table 3. Significant broadening from scatter-probe source instability and detector noise can

be eliminated on the basis of excellent reproducibility of the scatter-probe signal from a given

site. Also, broadening from defect damage thresholds above Iq can be largely neglected, since the

damage ensemble is degenerate, as discussed in connection with figure 4.

Examination of column 3 of table 3 shows that Sq generally decreases with increasing Iq/Iq,

as predicted by eq. (5). A graphical comparison is given in figure 6, where linear least-squares fits

representing the observed and computed shifts in Sq are indicated. Although the observed shift is larger, the difference is within the experimental uncertainty based on distribution widths.

Mirror B

Predamage scatter data for Mirror B is summarized in table 4, which is of the same form as table 2. Variations in Sf(S) are similar to those observed with Mirror A, but decrease more gradually

to zero indicating a wider range of a variability. The density of scatterers obtained by applying

eq. (8) to the weighted average of Sf(S) is n//2 = 2.0 x 10^ cm"^, nearly the same as for Mirror A.

Table 4. Mirror B scatter frequency data

S^ 0. 15 0. 65 1. 15 1.65 2.15 2.65 3.15 3.65 4.15 5.15

f(S)AS 0.06 0. 26 0.29 0.19 0.09 0.05 0.04 0.00 0.02 0.00

Sf(S) 0.02 0. 34 0.66 0.64 0.39 0.25 0.22 0.02 0.13 0.03

Excludes background S^ = 11.1

Damage test results from Mirror B, shown in figure 7, are very similar to those from Mirror A in

spite of different coating and substrate materials. The best-fit power law model based on the solid

data points is again degenerate and gives the same defect density as obtained from the scatter probe.

The open data point at la/^o ~ -^"^ appears to be nonconforming, suggesting intrusion of a secondary damage mechanism at higher energy densities and therefore was excluded from the fit.

Examples of damage frequency vs scatter distributions are shown in figures 8(a) and (b) for two

of the eight energy densities of figure 7. Damage frequencies are again based on half-unit scatter

intervals, and S]^ = 11.1. Parameters obtained from these and the other six distributions are sum- marized in table 5, which is similar to table 3. Although slopes are generally smaller and widths

greater than for Mirror A, a definite correlation exists in each case, except for la/Io ~ 2.88,

where the distribution is almost flat. Here, the data indicate several sites with scattering centers unassociated with damage, suggesting excessive substrate scattering in a limited area. A trend to improved correlation with increased la/^o less apparent than in Mirror A because of larger

fluctuations in the distribution parameters. Observed relative widths are much larger than computed

values and do not decrease at small la/^o predicted, again pointing to variations in defect

scattering as the chief contributor to broadening. This explanation is reconfirmed by the evidence

from table 4, where the wider range of a variability (than for Mirror A) agrees qualitatively with the

larger average ASq.

Observed and computed dependences of Sq on 1^/1q are compared in figure 9. Except for the

anomalous data point at la/^o ~ 2.88, which was excluded from the least-squares fit, the observed

shift is again greater than that computed from eq. (5). However, the difference does not exceed the

experimental uncertainty when the widths of the distribution are considered.

454
Table 5. Mirror B distribution parameters

AS ,S 0/ o

I /I

3 0

CIbiope S ^

0 AS

0 Observed

b Computed

1.63 0.091 5.82 11.0 1.9 0.3

1.80 0.053 8.04 18.6 2.3 0.3

2.00 0.376 1.73 2.65 1.5 0.4

2.38 0. 169 3.42 5.93 1.7 n 4 2.70 0.267 2.19 3.74 1.7 0.4

2.38 0.009 46.2 115.0 2.5 0.5

3.00 0.256 1.63 3.92 2.4 0.5

3.88 0.183 2.53 5.47 2.2 0.6

^ith

Sum

background , S^

of contributions

= 11.1

from

subtracted,

eqs. (6) and (7).

Mirror C

Figure 10 shows damage test results from Mirror C, which has a somewhat nondegenerate damage

ensemble (p = -0.45) and a defect density of the same order of magnitude as Samples A and B. Typical

scatter distributions are shown in figures 11(a) and (b). Because of the different test configuration

and introduction of the polarizing filter, the scatter units are significantly different from those of

Samples A and B. Consequently, slopes of the Mirror C distributions cannot be compared numerically

with those from the other two samples. However, it is obvious that the present distributions as indicated by the straight-line fits do not rise appreciably over the full range of scatter signals,

indicating that the correlation is poor. Consequently, no attempt was made to infer distribution

widths or threshold scatter levels for damage on Mirror C.

Evidently, the poor correlation on Mirror C and the other diamond-turned samples is attributable

to the polarizing filter required to suppress the strong visible scattering from the tool marks. In another study it was found that visible light scattered from metal surfaces is indeed more highly

polarized than that from coating defects [2]. However, the defects scatter a substantial polarized

component, which the filter removes, thus reducing the scatter-probe sensitivity. Also, on some of

these samples, a systematic dependence on sample position was observed suggesting incomplete suppression of tool-mark scattering. In retrospect, it is unfortunate that the arc-shaped tool marks were essentially normal to the plane of incidence, which maximized the resulting background scatter.

Conclusions

Nine multilayer dielectric mirrors have been studied quantitatively for correlation between the HeNe scatter signal from individual defects and 2.7 ym pulsed-laser damage. After examining over 50

examples of damage frequency vs scatter data, we conclude that a strong correlation can be observed

provided certain conditions are satisfied. These include near coincidence of the scatter- and damageprobe spatial profiles, profile dimensions that are smaller than the mean defect separation, adequate

stability of the laser sources, and noninterference from scattering unrelated to damage. Also, the

correlation is degraded if defects have unequal individual damage thresholds or unequal scattering

efficiencies. Variable defect scattering efficiency is the dominant limitation in samples evaluated

here, based on evidence from scatter frequency distributions and evaluation of other sources of degradation. A poor correlation observed on diamond-machined dielectric-mirror substrates is attributed

indirectly to background scatter from tool marks aligned normal to the incidence plane. Rotation of the sample by 90 deg is expected to alleviate this problem.

For mirrors with good correlation, the mean scatter threshold for damage decreases with

increasing energy density above the damage onset. This is consistent with a simple mathematical model

that accounts for the different dimensions of the spatial profiles of the two probes, including

asymmetry of the scatter-probe profile due to nonnormal incidence. More data would be useful in order

to test the model with greater statistical precision.

Overall, the scatter probe shows promise as a simple nondestructive indicator of local laser

damage probability when properly implemented and calibrated against damage on representative highquality mirror samples. Once correlation has been established, the probe could be used to scan large

coated areas rapidly to assess the density of damage-initiating defects. Also, defect sites can be identified for more extensive materials analysis or for megashot survivability tests, both of which

are inherently time-consuming.
References

[1] Marrs, C. D. ; Porteus, J. 0.; Palmer, J. R. "Defect damage precursors in visible-Wavelength

mirrors," 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 Nov. Pp. 378-384.

[2] Marrs, C. D.; Porteus, J. 0.; Palmer, J. R. "Nondestructive defect detection in laser optical

coatings." J. Appl. Phys. _5Z.; ^^^^J l^^^.

[3] Franck, J. B.; Seitel, S. C. ; Hodgkin, V. A.; Faith, W. N. ; Porteus, J. 0. "Automated pulsed

testing using a scatter-probe damage monitor," presented at the 16th annual symposium on optical

material for high power lasers; 1984 October 15-17; Boulder, CO (proceedings in process).

[4] Porteus, J. 0.; Seitel, S. C. "Absolute onset of surface damage using distributed defect

ensembles." Appl. Opt. 23; 3796; 1984.

[5] Newnam, B. E. "Damage resistance of dielectric reflectors for picosecond pulses," in Proceedings

of the 6th annual symposium on optical materials for high power lasers . Glass, A. J.; Guenther,

A. H., ed. 1974 May 22-23 Boulder, CO. Nat. Bur. Stand. (U.S.) Spec. Publ. 414; 1974 December.

Pp. 39-47.

[6] Newnam, B. E.; Gill, D. H. ; Faulkner, G. "Influence of standing-wave fields on the laser damage

resistance of thin films," in Proceedings of the 7th annual symposium on optical materials for

high power lasers . Glass, A. J.; Guenther, A. H. , ed. 1975 July 29-31; Boulder, CO. Nat. Bur. Stand. (U.S.) Spec. Publ 435; 1976 April. Pp. 254-271.

NaCl BREWSTER WINDOW

HELIUM-NEON

LASER

Figure 1. Schematic of scatter probe

and damage probe optics in the evacuated test chamber. The scatter-probe

optical train runs from the HeNe laser

to the detector, incorporating optics

L1-M2. The 2.7-ym damage probe beam

enters through the NaCl Brewster window

and is focused on the sample Mirror SM

by Lens L3

.

Figure 2. Geometrical relationship of

scatter- and damage-probe beams shown

for the case of normal incidence damage

testing. The scatter-probe beam is incident at angle 6. Defect positions

in the sample plane x,y are designated

by polar coordinates r,(().

456
DAMAGE PROBE

SPATIAL PROFILE

SCATTER SIGNAL

SPATIAL PROFILE

0 ""o DEFECT RADIAL POSITION r ^

Figure 3. Relationship between the

spending scatter threshold for damage

Sq, as governed by the damage- and

scatter-probe spatial profiles. The

shaded area between the solid and

dashed curves indicates the asymmetry

of the latter profile in the sample

plane, which leads to the shaded variation in S{- in the text,

Symbols are identified

1 1

BEST 3-PARAMETER

1

FIT: 1

1 1

=-1.0 (DEGENERATE)

- 2 X 103 DEFECTS/cm2

_

/ 1 1 1 1 0 1 2 3 4 5 6 ENERGY DENSITY da/lol Figure 4. Damage test results for

Mirror A. Data points with

uncertainties give damage frequencies

measured at the energy densities

indicated in units of the damage

onset. The solid curve represents the

best-fit power-law model obtained as described in [4].

II 1 1

laAo = 1.38

1

SLOPE = 0.204 -

Sb —1

I 1 Ir —1— 1 i

1

1 13 14 15 16 PREDAMAGE SCATTER (ARBITRARY UNITS)

Figure 5(a). Distribution of damage

frequency vs predamage scatter for

Mirror A at la/Io = 1.28. Data

points indicate damage frequencies

at half-unit scatter intervals,

solid data points are those included

in fitting the straight line

representing the rising portion of he distribution.

13 14 15 16 PREDAMAGE SCATTER (ARBITRARY UNITS! Figure 5(b). Distribution similar to figure 5(a) for Mirror A at la/Ic 3.50,

457
OBSERVED

+ COMPUTED

(REF. So AT I3/I0 = 3,5) J I I I

1 2 3 4 ENERGY DENSITY (lo/U Figure 6. Threshold scatter level

Sq vs energy density for Mirror A. Squares represent experimental data

and crosses represent values computed

from eq. (5). Dashed and solid lines

are the respective linear leastsquares fits (regression lines).

Plotted scatter levels are relative

values with Sq at la/^o ~ 3.5 as an arbitrary reference.

BEST 3-PARAMETER FIT: P = -t o (DEGENERATE!

2 X 103 DEFECTS/cm2

ENERGY DENSITY 234 (Ij/IqI Figure 7. Damage test results for

Mirror B presented in the manner of

figure 4. Only the seven solid data

points were used in fitting the

power-law model.

458
4.0

(REF. So AT Vlo = 2.7) >

0.0 I 1 i 1 0 12 3 4 ENERGY DENSITY (I3/I0) Figure 9. Threshold scatter level

Sq vs energy density for Mirror B presented in the manner of figure 6. The value of Sq at T-JIq = 2.7

is used as a reference for plotting

relative scatter levels.

BEST 3-PARAMETER FIT:

' 0 10 20 30 40 50 ENERGY DENSITY (Ig/Io) Figure 10. Damage test results for

Mirror C presented in the manner of

figure 4. The defect density includes

all defects that initiate damage at I3 = 2Iq in the nondegenerate

ensemble (see [4]).

2.0 2.2 2A PREDAMAGE SCATTER (ARBITRARY UNITS) Figure 11(a). Distribution of damage

frequency vs predamage scatter for

Mirror C at la/^o = ^'^ presented

in the manner of figure 5. Data

points represent damage frequencies of

tenth-unit scatter intervals. The

scatter scale differs significantly

from that of Samples A and B because

of the different test configuration.

PREDAMAGE SCATTER (ARBITRARY UNITS) Figure 11(b). Distribution of damage

frequency similar to figure 11(a) for

Mirror C at IJIq = 11.7.

459
Manuscript received

1-24-86 SAE

CO2 LASER INDUCED DAMAGE A MODEL THIN FILM SYSTEM : ZnS ON Ge

D.R. Gibson and A.D. Wilson

Barr and Stroud Ltd.,

Applied Physics Group,

Caxton Street, Anniesland,

Glasgow G13 IHZ, U.K.

The CG2 laser absorption and damage characteristics of thermal and electron beam

evaporated ZnS films on Ge substrates haue been investigated. The substrate temperature

during deposition is shown to be an important parameter controlling film porosity and

hence water ingress. The role of water is probed by a variety of techniques including

laser induced desorption (mass) spectroscopy. Water is shown to lead to high

film-substrate and bulk-film absorptances and to low laser damage thresholds (LDTs) with

damage originating in the film. Physically dense films, which do not allow water

ingress, haue high LDTs limited by damage to the Ge substrates. Examination of ZnS films on Si and ZnS substrates indicates that the film absorptances and LDTs are dependent on the substrate material.

KEY WORDS: CO2 laser damage; film porosity; laser calorimetry; laser desorption

spectroscopy; water sorption; ZnS films

1. INTRODUCTION

Thin film coatings are often the parts of infra-red optical elements most susceptible to CO2 laser induced damage [l]. Currently, there is much interest in obtaining coatings with a high

laser damage resistance and to this end, new coating techniques such as molecular beam epitaxy

^2] and ion assisted deposition [3j are being investigated. Surprisingly, however, there is only

limited information on films produced using conventional vacuum technologies and thus there is an insufficient data base for quantifying improvements effected by the new technologies.

The susceptibility of coatings to laser damage is in the main an artefact of the deposition

methodology, which can give rise to coatings with structural defects, impurity inclusions and

film structures which are susceptible to atmospheric contamination (e.g. water ingress). In addition, substrate surface quality and pre-coating preparation may also be a contributing factor

in lowering or limiting the laser damage resistance of the coating.

Previously, we have examined the laser damage behaviour of ZnS thin films, produced by

conventional deposition methods, on Ge substrates. Although in many samples the LOT is limited

by the Ge substrate, a significant fraction of the samples are film limited. To resolve the reasons for this behaviour and to increase our general understanding of the damage characteristics

of conventionally produced coatings, the CO2 laser induced damage of this relatively simple thin

film system (ZnS films on germanium) has been investigated. The films have been produced using

thermal evaporation (at various substrate temperatures) and electron beam evaporation. In addition ZnS films (thermally evaporated) on high laser damage threshold (LDT) substrates (ZnS

and Si) haue been assessed.

The principal aims of the investigation were to identify the sources of damage within the

films in relation to surface and bulk film properties, and to relate these to the deposition

conditions. In addition, methods of increasing the laser damage resistance of the films, by

optimising the deposition conditions are reported.

The study shows that films deposited at high substrate temperatures have high LDTs and that

performance is limited by damage to the Ge substrate. The use of low substrate temperatures can result in films with low LDT values. This is traced to the increase in film porosity which

allows water ingress. The water absorbs strongly at 10.59yum and is the cause of the damage in

these poor films. The use of pulsed laser induced desorption (mass) spectrometry (PLIDs),

pre—pulsed laser calorimetry and pre-pulsed laser damage measurements provide unambiguous evidence

of the important role of sorbed water.

460
2. EXPERinENTAL

2.1 Samples

ZnS films of various optical thickness ( ?/4, ?/2, 3?/4 and A, where A = 10.6^) on Ge substrates, have been produced using thermal and electron beam evaporation. The Ge substrates

were polished using alumina (Linde A, 0.3 micron). Three sets of samples were produced,

corresponding to two sets of thermally evaporated ZnS films deposited onto "cold" (— 120 C) and

hot (— 200 C) substrates. Since the ZnS sticking coefficient relative to 40 C is 0.93 at 120 C and 0.83 at 200 C ^Sj, it is practicable to produce films at the higher temperature. The third

set of samples was produced using electron beam (E.B.) evaporation. In addition, hot thermally

evaporated ZnS films ( a/4 optical thickness) on ZnS and Si substrates were produced.

In order that coherence effects within the Ge substrates were defeated, each sample had the

exit substrate face coated (using the same deposition methodology as the entrance face) with a a/4 optical thickness (A = 10.6^) ZnS anti-reflection coating. Coherence effects would

complicate the results of several of the techniques (laser calorimetry, laser damage assessment)

employed to characterise the films.

2.2 CO2 TEA Laser Damage Test Facility

The CO2 TEA laser damage facility has been described in detail previously ^1, 4

J. For this

work, a standard spike (100ns FUJHn) and tail (2yLis) 10.59|i(,m pulse was used. The 1/ e^ spot size

was > 0.9mm. The probability method of Foltyn [6 ] was used to obtain the LDT value which is

defined herein as the peak energy density corresponding to the zero probability of damage.

2.3 CO2 Laser Calorimeter

The basic rate calorimeter has been described previously ^4^. For certain measurements

involving irradiation of the sample with CO2 TEA laser pulses prior to measurement (i.e.

pre—pulsing) the arrangement sketched in Figure 1 was used.

2.4 CO2 Pulsed Laser Induced Desorption Spectrometer

Pulsed laser induced desorption (PLID) has been reported by Allen and co-workers fv]]. A simplified adaptation of that technique, Figure 2, consists of a (Balzers QMA 064) quadrupole mass analyser (QMA) and positionable sample holder mounted in a UHU chamber fitted with KCl windows,

and a CO2 TEA laser. Before carrying out PLID analysis of the films, the sample was positioned

out of the beam and the windows irradiated to drive off any desorbable contaminants. This

process was carried out until no increase in system partial pressure was monitored. Unfortunately

quantitative information on the number of desorbable molecules cannot be obtained sinee the

collection volume of the QMA is not known and the rise time of the desorption signal is limited

by that of the QMA. However, useful qualitative information can be obtained.

3. ZINC SULPHIDE FILMS ON GERMANIUM SUBSTRATES

3.1 Absorptance Data

To determine the interfacial and bulk film contributions to the total absorptance, the

absorptance of the various ZnS film thicknesses were determined using CO2 laser rate calorimetry.

Figure 3 shows representative absorptance data for the cold thermally deposited ZnS, measured

at one week and one month after deposition. For each sample the absorption was determined

at three separate sites, so that variation across the sample could be assessed. For the samples

measured this variation was typically + 10% of the mean absorptance value.

The difference between the measured and extrapolated zero-thickness absorptances , Figure 3,

is due to absorption at the film-air and substrate-film interfaces. The two contributions to the interface absorption were separated using a method, due to Temple fsj, which was originally

developed for single film coated substrates but has been modified to account for the

anti-reflection layer on the exit face of the substrate. It is assumed that the bulk, air-film

and film-substrate absorptance within the exit face anti-reflection layer are the same as the

front surface film values. This is a reasonable assumption since both films were deposited using

the same deposition conditions.

The bulk film absorption coefficient (oc^) , and the air-film (a^^) and film substrate (a^^)

absorptances are given in Table 1 for the films produced by the three deposition conditions.

Results are indicated for absorptance data, taken one week and one month after deposition of the

461
films. The small negatiue values of agf are considered to be zero uithin experimental error. The

film-substrate absorptance of the hot thermally and electron beam evaporated films is comparable

to (but, of course, additional to) the surface absorptance of the uncoated alumina polished Ge. The cold thermally evaporated films have a significantly higher film-substrate interface

absorptance which, as will be shown in section 3.4, is due to water ingress. Ue note that

Donovan et al. [ 9 J detected water, using a nuclear resonant technique at the ZnS film-substrate

interface (the substrate material was not specified). The bulk absorption coefficient of the

cold films exhibited an 8-fold increase after a 3 week exposure to a laboratory environment

(due to water ingress) but thereafter remained stable (suggesting the film bulk was saturated with

water) , The hot thermally and electron beam evaporated films showed no such increases in their

bulk absorption coefficients. These were, however, an order of magnitude greater than for bulk

ZnS at lD.59/iim.

3.2 C02 TEA Laser Induced Damage

Figures 4a, b and c show laser damage probability plots (LDPPs) for the "cold" and "hot"

thermally evaporated and the electron beam evaporated films. Included in each figure is the LDPP for an uncoated Ge substrate.

The fluence (E) within the substrate surface depends on the interface transmittance (T) of

the surface. The ratio of the fluence values for uncoated (u) and coated (c) substrates is

E^/e^ = T^/T|j . Thus an AR coating has the effect of coupling more energy into the substrate

surface. It follows that the ratio of the laser damage thresholds will be LDT^./LDT^J = T^j/t^. For Ge, T^^ = 0.64, For ZnS on Ge, a/4 and 3?/4 films have T^. s; 0.99 whereas V2 and A films have

T^ = 0.64. Consequently if the LDT of ZnS films on Ge is limited by damage to the Ge, the LDT ratio, LDTVLDT,, , will be = 0.64 for V' 4 and 3?/4 films and will be ~1.0 for iJ 2 and A films.

Inspection of Table 2 shows that hot thermally and electron beam evaporated films are substrate

limited but that the cold thermally evaporated films are film limited. Comparison of Nomarski

photomicrographs of the damage sites on the coated and uncoated samples revealed film-like

damage morphology for the cold films and substrate-like damage for the other samples. Both

cold and hot films showed defect-type damage morphologies with isolated damage sites

occuring within the beam area.

3.3 The Effect of Pre-Pulsing on the Laser Damage Threshold

Irradiating at an energy density below the LDT (hereafter called pre-pulsing) has been

shown to increase the subsequently measured LDT fvj. This effect is ascribed to desorption of

absorbing contaminants leading to a reduction in film absorption.

"Cold" thermally evaporated 'h/ h ZnS films were pre—pulsed at several energy densities up to the LDT value (~9 o/cm^ ) . No increase in LDT value was found, most probably due to the low

initial LDT value precluding contaminant desorption before film damage. For the a/2, 3a/4 and

A cold thermally evaporated films, the effect on the film LDT as a function of pre-pulse energy

density (five pulses were used at each energy density), is indicated in Figure 5 where it can be seen that pre—pulse energy densities in excess of =10 Ocm~^ are required to cause an increase in LDT. The LDPPs for these films after pre-pulsing at =11 3cm~^ are shown in Figure 4d. The films

now exhibit similar LDPP characteristics and LDTs to the hot thermally and electron beam

evaporated films. Neither these latter nor the Ge substrates showed any pre-pulsing effects.

This behaviour suggests that the "cold" films contain a desorbable contaminant which limits

their LDTs.

3.4 Pre-Pulsed Laser Calorimetry

The effect of pre-pulsing on the absorptance of the "cold" films was investigated using the

calorimeter described in Section 2.3. All measurements were made on films four months after

their deposition, at which time their absorption properties were not significantly different from

those as assessed one month after deposition (see Table l) . Indicated in Figure 6 is the

absorption reduction for the cold thermally deposited A thick film as a function of number of

pulses, for incident energy densities both below and above the threshold value obtained from

pre-pulsed laser damage measurements. The a/2 and 3a/4 films gave similar results. Significant

variations in the absorptance reduction for different areas of the film at the same energy

density were observed. To eliminate this variation the absorptance reductions shown in Figure 6 are the average values over five separate areas of the film (errors shown in Figure 6 are absolute

values)

.

462
It can be seen that below =:lo j/cm^ (peak incident energy density) there is no significant

change in the absorptance value. For energy densities aboue 10 j/cm^ the total absorptance

decrease (over four pulses) is 50%, reducing the absorptance to a ualue similar to the hot

thermally and electron beam deposited films (which did not exhibit absorptance reduction on pre—pulsing) , Pre—pulsing with more than four pulses led to no further changes in absorptance.

These results suggest that all of the desorbable contaminant within the bulk and at the

interfaces of the films is removed.

3.5 Pulsed Laser Induced Desorption (PLID) Analysis

To unambiguously identify the contaminant, PLID analysis was carried out as described in

Section 2.4. Irradiating "cold' films at ^11 Jcm~^ led to the increase in partial pressure (QP1A

tuned to mass 18 i.e. Hg ) shown in Figure 7. Irradiation at~6.5 Jcm~^ caused no measureable

increase in hfe 0 partial pressure. The number of (11 3cm~^) pulses required to desorb all of the

(desorbable) water is —4 which is exactly the same as is required to decrease the sample

absorptance to the hot film value. This demonstrates a very convincing correlation between

anomalous film absorptance and the desorbable water content of the film.

4. ZnS FILMS ON HIGH LOT SUBSTRATES

In order that the ultimate laser damage resistance of the best ZnS films could be assessed,

?/4 ZnS films were thermally deposited onto both faces of high damage resistant substrates (Si

and CUD ZnS) at 200 C (i.e. equivalent to the hot films discussed previously). Table 3 gives

the LOT values and the experimental and theoretical ratios of the coated to uncoated substrate

LDT values. Comparison of the experimental and theoretical LDT ratios suggest the ZnS on the Si

substrate is film limited and on the ZnS substrate it is substrate limited. This is also

evidenced by the damage morphology: for the coated Si substrate, the damage occurs at the

substrate— film interface; for the coated ZnS substrates damage occurs on the back coated

substrate face, and is a deep crater indicative of an electron avalanche ionisation damage

mechanism, as is the case for uncoated ZnS [ij. The important observation is that ZnS films

produced conventionally can be as good as bulk ZnS.

A preliminary analysis of the thin film absorptance data indicates that for films on Si, the

film-substrate interface absorptance is very large (greater than for good films on Ge) and thus

laser damage is most probably initiated at the film—substrate interface. The large interface

absorption may indicate reaction between the ZnS and Si. For ZnS films on ZnS, film-substrate

interface absorptance cannot be detected. These data indicate that the film growth and hence LDT may be dependent upon the substrate type,

5. SUmARY AND CONCLUSION

The optical absorptance and laser damage properties at 10,59jum of ZnS films on Ge have been

related to the deposition conditions. Films deposited on cold substrates (120 C) show much

higher substrate-film interface absorptances than films deposited on hot substrates (200 C) and

also have bulk absorption coefficients which increase on exposure to the laboratory atmosphere.

Such data indicate that "cold" films are subject to water sorption. The presence of water has

been proved unambiguously using pulsed CO2 laser induced desorption spectroscopy and has been

correlated with the anomalous absorptance of cold films (cf hot films). The water present in

"cold" films results in low 10.59^ LDT values but can be driven off temporarily by pre-pulsing

with a CO2 TEA laser at energy densities > 10 3cm~^ . "Hot" films on Ge have stable bulk

absorption coefficients and give LDTs which are limited by the substrate rather than the film.

The nature of the substrate defects responsible for damage is discussed elsewhere [4].

These observations can be related to the film structure, which is dependent on the energy

(i.e. the mobility) of the evaporant adaton.o on the substrate surface. The higher the mobility,

the greater the degree of structural order and hence film density [lO]. For the cold thermally

evaporated films, there is a relatively low surface mobility, producing a polycr^stalline/

amorphous structure, which is porous and susceptible to water ingress. For hot thermally and

electron beam evaporated material, the "hot' substrate surface supplies the energy for

crystalline growth and hence higher density films, which preclude water ingress into the film

structure. In addition, water adsorbed onto the substrate surface before coating will be significantly reduced by heating the substrate (as in the case for hot thermal evaporation).

For "cold" thermally evaporated material there may be insufficient energy to desorb substrate

surface adsorbed water, leading to a high concentration at the film-substrate interface.

tl II Supporting evidence for the higher porosity of the cold films has been obtained recently from

463
two studies; the "cold" films shou higher leuels of CO2 laser light scattering from the film bulk

confirming their greater degree of inhomogeneity ; an electron microprobe analysis of fracture cross—sections of the films indicates a greater decrease in density (as indicated by a decrease in

element count rate) for cold films as the film thickness is increased [ll]. As far as we are aware, this is the first report in which the effect of water sorption in ZnS, caused by film

porosity, has been linked unambiguously to anomalously low CDs laser damage thresholds.

In addition to the dependence of film properties on the substrate temperature there is also

a dependence on the substrate type. For example ZnS films thermally evaporated onto hot CUD

ZnS haue undetectable leuels of film-substrate interface absorption and haue LDTs equal to the

uncoated ZnS euen though the ZnS film absorption coefficient is 10 x that of the ZnS substrate.

The mechanism of damage in this case is electron aualanche ionisation which is relatively

independent of the bulk absorption coefficient e.g. cf ZnS and ZnSe [l]. ZnS films on Si show

very large values for film-substrate interface absorptance and it is likely that damage is then

initiated at the film-substrate interface.

In summary, we haue demonstrated that ZnS films produced by conventional thermal

euaporation can result in substrate (Ge, ZnS) limited laser damage behaviour. For this to be so, the film density must be sufficiently high to eliminate water ingress. In the present work this

has been ensured by using substrate temperatures of —200 C, which value is higher than is

conventionally used for ZnS. Ion assisted deposition allows higher density films to be

produced at lower substrate temperatures []l2j. Epitaxial growth processes are also known to produce void-free high density films. For example, I^BE films of ZnS on Ge substrates and ZnSe on ZnSe substrates [21 have been shown to be substrate limited in their CO2 laser damage behaviour.

The authors wish to thank the Management of Barr and Stroud for permission to publish this

paper

.

REFERENCES

^ij D.R. Gibson and A.D. Wilson, "studies of CO2 Laser Induced Damage to Infrared Optical

Materials and Coatings , paper presented at the 16th Annual Symposium on Optical Materials

for High Power Lasers, Boulder, Colo., Oct., 1984,

f 2] K.L. Lewis, J. A. Savage, A.G. Cullis, l\I.G. Chew, L. Charlwood and D.kl. Craig, "Assessment of

Optical Coatings Prepared by Molecular Beam Techniques , paper presented at the 15th Annual

Symposium on Optical Materials for High Power Lasers, Boulder, Colo., Oct., 1984.

[3] 3.R. Sites, P. Gilstrap and R. Rujkorakarn, IMBS Spec. Publ., 669 , 243 (1982).

[4] D.R. Gibson, CM. MacDonald and A.D. Wilson, "CO2 Laser Induced Damage to Ge : Relation to Surface Preparation , paper presented at the 17th Annual Symposium on Optical Materials for

High Power Lasers, Boulder, Colo,, Oct., 1985.

[5] E. Ritter and R. Hoffmann, 3. Mac. Sci. Technol., 16, 733 (1969).

[e] S.R. Foltyn, NBS Spec. Publ., 669, 368 (1982").

[7] 3.0. Porteus, W.N. Faith and S.D. Allen, NBS Spec. Publ., 638, 273 (1981).

[8] P. A. Temple, Appl. Phys. Lett., 3^, 677 (1979).

[9] T.M. Donovan, P. A. Temple, S.C, Wu and T,A, Tombrello, NBS Spec. Publ., 568, 237 (1979").

[10] K.H. Guenther, Appl. Opt., 23, 3806 (1984).

[11] K. Clarke, D.R. Gibson, CM. MacDonald and A.D, Wilson, unpublished data.

[12] S.G. Saxe, M.3. Messerly, B. Bovard, L. De Sandre, F.J. van Milligen and H.A. MacLeod, Appl.

Opt. , 23, 3633 (1984)

,

464
1. ABSORPTANCE OF ZnS FILMS ON Ge

EUAPORATION

METHOD

(cm ^) ^fs' ^fa' ^=

1 week 1 month 1 week 1 month 1 week 1 month

Thermal

cold

1 8 0.54 0.55 -0.07 -0.07

Thermal

hot

2 2 0.11 0.13 -0.04 -0.01

Electron

beam 2 2 0.18 0.15 -0.05 -0.05

Oi ^, bulk film absorption coefficient,

^fg?

f ilfn-substrate absorptance,

a^^, film— air absorptance.

Times are those after deposition.

2. RATIO OF FILM TO SUBSTRATE LASER DAMAGE THRESHOLDS FOR ZnS ON Ge

FILM THICKNESS

(NO. OF 1/4

WAVES)

LDT (FILM)/lDT (SUBSTRATE)

THERMAL

"cold" THERMAL"hot"

ELECTRON

BEAM

1 0.26 0.63 0.60

3 0.48 0.65 0.63

2 0.63 0.92 0.99

4 0.66 0.94 1.05

Probable

Damage

Source

Film Substrate Substrate

3. ZnS FILMS ON HIGH LDT SUBSTRATES

SUBSTRATE

TYPE

LDT (3cm~^) LOT RATIO

SUBSTRATE ONLY WITH \/4 ZnS FILM

ON BOTH SIDES EXPERIMENTAL THEORETICAL IF SUBSTRATE LIMITED

Si 66 38 0.58 0.7

ZnS 70 69 0.99 1.00

465
Sample

Manipulator

Sampte

Beam

Diagnostics

KCI

windows

Quadrupote Mass [ Spectrometer

0!.

Turbomolecular

Attenuator

CO2

TEA Laser

Pump

Figure 2. Schematic of CO2 Pulsed Laser Desorption Mass Spectrometer

466
12

10

9 8

K

UJu

i 6

o

M

CD<

4 H

2

4

• uncoatftd Ge

e ZnS on Ge :1 week after deposition

X ZnS on Ge:1 month after deposition

2 3

OPTICAL THICKNESS ( X A/4)

Figure 3. Dependence of absorptance on film thickness for ZnS thermally euaporated onto "cold' Ge substrates

i

467
INCIDENT ENERGY DENSITY (Jcm^)

Figure 4. Laser damage probability plots for

a. T^'ermal evaporated ZnS on "cold" Ge substrates

b. Thermal evaporated ZnS on hot" Ge substrates

468
O

<

<

o

u.O

CD<

COo

Q.

C3L5-

-^-^-^ - V4 ZnS film _&_o-o_ X/2 film -K--)^-M- 3A/4ZnS film -it—m~-m- A ZnS film

-s^——4- uncoated Ge

—r- 10 20 30

INCIDENT ENERGY DENSITY ( Jem"'

)

10

Uio

<

s

Iko

0

<

COO

(C0.

05

V2 ZnS film

ZnS film A ZnS film + t « uncoated Ge

10

-T

20 30 40

INCIDENT ENERGY DENSITY (Jem')

ure 4 cont'd. Laser damage probability plots for

c. Electron beam evaporated ZnS

d. Thermally evaporated ZnS on "cold'

5 pulses at llJcm

-2 Ge substrates after pre-pulsing with

469
35r

30

EoQ 25

O

z

(A

lij I I 20F

m

(3

<

o

GC

III (/) <

10

-A/j ZnS FILM

.3^4 ZnS FILM

. A ZnS FILM

(

J2_

5 10

INCIDENT PRE-PULSE ENERGY DENSITY, Jem'

15

Figure 5. Effect of pre-pulse energy density on subsequently measured laser damage threshold

for "cold" ZnS films

12t.

10

"o 8-

o

z

|S 6

Q.o

V)

CO< 4

2-

PfiE- PULSE ENERGY

DENSITY (Jem'*) .H-M--K— 6.5

a a 11.0

—I 1— 2 3

PULSE NUMBER

Figure 6. Effect of pre-pulsing on the absorptance of a A-optical thickness "cold" ZnS filr

470
2 3

NUMBER OF PULSES

Figure 7. Effect of pre-pulsing at llJcm

thickness cold ZnS film

on the water desorption signal from a A-optical

471
Manuscript Received

Laser Induced Damage in Dense Cptical Thin Films 10-30-85

K.L.Lewis, A.M.Pitt, A.G.Cullis, N.G.Chew and L.Charlwood

Royal Signals and Radar Establishment, St. Andrews Road, Malvern, England WR14 3PS

Molecular beam deposition techniques have been used to generate dense cptical thin films

and film structures for studies which allcw the assessment of the onset of laser damage

effects, and which explore the control of microstrue ture. In the first study, ZnS films

have been examined on germanium substrates by electron microscopy and the evidence

available suggests that damage is initiated by laser beam interactions at dislocation

loop clusters in the Ge surface. In the second study the feasibility of using

stratified films to control microstructure has been assessed. Damage thresholds in

(ZnS/ZnSe)j^ are equivalent to those of uncoated substrates despite the incorporation of

100 or so separate interfaces. A wide range of coatings can be synthesized utilising

such structures, which offer the potential of enhanced damage thresholds and broadband

optical properties.

Key Words: laser damage; molecular beam deposition; thin films; zinc selenide; zinc

1. Introduction sulfide.

Previous papers-'-'^ presented at this meeting have highlighted the potential afforded by

molecular beam techniques in producing cptical thin films with a high resistance to laser induced

damage. Much of the improvement produced as a result of growth of polycrystaline films in ultrahigh vacuum (UHV) appears to be associated with the avoidance of the presence of intergranular

voids which renders the film impervious to the ingress of water and other impurities when exposed

to the atmosphere. Examples of high resolution transmission micrographs, taken in cross section

(XTEM) were presented in reference 2 showing the microstructure achievable. The adoption of UHV

environments together with in-situ availability of ion-beam cleaning techniques ensures that the

polycrystal 1 ine films are of very high purity and essentially free from impurity segregation

effects at grain boundaries. In addition the achievement of clean film/substrate and film/film

interfaces is routine.

The availability of dense optical thin films allows an assessment of various aspects of

damage initiation phenomena. In particular it is possible to explore damage onset in the case

v^ere polishing damage is present at the substrate/film interface. This has been assessed for ZnS

films on Ge substrates where significant reductions in damage threshold have previously been

observed^

.

This paper also explores the conditions necessary for the modification of the columnar

character of thin films by the synthesis of stratified layer configurations in which the thickness

of each layer is negligible compared with the wavelength under consideration. In such a film the

refractive index of the film is simply a function of the indices (n-j^, n2) and thicknesses {t-^,t2i

of the coTponent materials selected. Specifically, for each layer pair-^

2 "l^ti + n22t2

tl + t2

In the example to be described, ZnSe and ZnS are used as the two component materials with

refractive indices 2.5 and 2.3 respectively, and are successively deposited as discrete layers 50

to 300A thick. Since ZnSe and ZnS have different lattice constants (5.6686A and 5.4093A

respectively), epitaxial growth of successive layers onto each other should not be possible and

propagation of columnar grains should therefore be suppressed at each interface.

2. Experimental

The coatings described in this paper were fabricated in a 3-chambered UHV facility

incorporating Knudsen sources, a small RF plasma source, a molecular beam sanpling spectrometer.

Auger and X-ray photoelectron spectrometers and ion guns for substrate cleaning^. iVie morphology

of the coatings was examined in detail by XTEM at 120kV and 400kV using specimens prepared in

cross-section by mechanical polishing and subsequent low voltage Ar"*" ion milling. In some

instances the Ar ion milling was supplemented by a final polish using an iodine ion beam at low

energy. This had been shown in other studies to be useful for the reduction of ion beam damage

artifacts in ZnS produced during thinning.

472
Laser damage t±ireshclds were determined using a short cavity TEA laser at 10.6um (33nsec FWHM

with tail extending to IVOOnsec) as described previously^. The point of damage onset was assessed

using a miniature vidicon TV camera mounted on a xlO microscope objective situated close to the

sample. The criterion for damage was the threshold production of microscopic surface defects when

viewed by the TV camera under oblique tungsten lanp illumination. Beam diameters of lOOum 1/e were measured by the scanning pinhole technique.

3. Micros trueture and Optical Properties of Deposited Films

ZnS films deposited on germanium substrates at 50°C have been found to have a micros true ture essentially similar to that previously found for ZnSe films'^. The coatings are dense (figure 1) and no pores are detected, even on the scale of the resolution of the 400kV microscope available

for sane of the work. Hcwever, unlike in the case of ZnSe, the columnar structure of the ZnS does

not propagate throughout the thickness of the film and the stacking fault contrast within the

columnar grains is no longer perpendicular to the direction of growth. The polycrystal 1 ine

structure is less ordered and the degree of fibre texture somewhat reduced compared to ZnSe.

A thin amorphous region is present at the surface of the Ge substrate arising from damage

induced during ion team cleaning of the substrate. However, other artifacts sane 0.1 to 0.2um in

width are also present, which might appear at first sight to be particulate inclusions or polishing rubble. However, close examination of the substrate surface (figure 2) under the

correct diffraction conditions reveals that the artifacts are in fact clusters of dislocation

loops which have been punched into the surface by the action of abrasive particles and indeed the

substrate surface is slightly depressed at the site of these dislocation loops. The electronic

structure of the germanium will be considerably modified in the region of such dislocations and

may well give rise to regions of enhanced optical absorption, possibly as a result of impurity

segregation as has indeed been found at grain boundaries in polycrystal line germanium^.

The microstructure of an alloyed ZnSe^^S^.j^ film deposited on a glass cover slip (figure 3) is

very similar to that of ZnSe, Here again, columnar grains dominate the microstructure.

Surprisingly, in the case of the stratified (ZnS/ZnSe)^^ film the columnar character of the deposit

is unchanged (figure 4). The inclusion of 80A thick sublayers of ZnS is insufficient to prevent

propagation of the columnar structure of the film. This may be due to the fact that the lattice

parameter mismatch between the ZnSe and ZnS ( 5%) is too small and the strain imposed by the

mismatch is accomnodated by a higher density of stacking faults in the region of the interface.

The fact that the two sub-layers share a common chemical element (ie Zn) may be a contributing

factor in allowing this to happen. At higher resolution, it is apparent that stacking faults are

indeed present near the interface and by lattice imaging (figure 5) it is possible to examine the

extent to which the strain is being accommodated. Figure 5 shows the otherwise high degree of

interface perfection achieved by the absence of voids and inclusions. Close examination of the

grain boundary regions shows a similar lack of porosity.

In principle it should be possible to prevent the propagation of columnar grains using

mismatched interlayers but it is apparent from the above that a much greater degree of mismatch

than 5% is required. A good exarrple would be provided by a BaFVZnS structure where the lattice

parameter of BaF2 (a =6.2001A) is some 15% greater than ZnS. In addition, the material

crystallises in the cuoic fluorite structure canpared with the cubic zinc blende lattice of ZnS. Should a closer match be required, then it would be possible to examine BaF2/ZnSe (mismatch 9%) or SrF2/ZnS (7%).

As a general principle the potential of stratified designs is far reaching. For example,

index grading can easily be achieved simply by changing the mark/space ratio of the successive

materials being deposited. This has recently been explored by Southwell^. However, graded index

designs can also be used to avoid high electric field levels in multilayer dielectrics. For

exairple, Lowdermilk amd Mukherjee^ found that graded index f ilne prepared by a sol-gel process had

laser induced damage thresholds (LIDT) four times greater than those of multilayer A/R films at

1.07um for Insec pulses.

4 . Effect of Coating M icros trueture on Laser Damage Threshold

The results of the assessment of the various combinations of films and substrates examined in this

work are summarised in table 1.

4.1 ZnS/germanium specimens

The origin of the low thresholds found in the case of ZnS/Ge have been explored by crosssectioning the film at near-threshold damage sites and examining the microstructure by TEM. It

473
has been found that at such threshold f luences, damage only occurs in the central part of the area

irradiated by the gaussian beam (figure 6). Since the energy densities at the extremities of the

focussed beam waist are significantly lower than those at the peak, it is possible to explore

threshold effects by studying the microstructure immediately adjacent to the damage site. The

only problem in undertaking such a study is that secondary effects such as film ablation may have

occurred and a section of the film in the desired region may no longer be present. In addition

the problems of sectioning the specimen near the centre of a laser damage spot some lOOum in

diameter to produce a TEM foil lum less in thickness are by no means trivial. Sane success has

been achieved, however, as witnessed by the micrograph shown in figure 7. The centre of the

damage site is away to the left hand side of this micrograph, and the coating has been completely

removed in that region, with extensive melt back and cratering of the substrate. The most

^parent change in the coating is the degree of grain growth that has occurred in the film nearest

the air interface. This is clearly a result of localised melting and r^id resolidif ication as found in the laser annealing of polycrystalline silicon films. TVin bands nucleated during melt

recrystallisation are clearly visible in the large grain just to the left of the centre of the

micrograph. The average grain size in the recrystalised region is approximately 2500A and the

depth of recrystallisation approximately 4000A. Although the lower section of the film appears to have been unaffected by the laser pulse, the interface region has and the film is clearly

separated from the substrate. The surface of the substrate appears to have melted to a depth of some 24 OA.

On the basis of the evidence available, it is argued that the damage process is initiated at

the regions of high absorption in the germanium surface at the dislocation loop clusters referred

to in section 3. Certainly it has already been suggested by Foley et al^ that a direct

relationship exists between the LIDT of uncoated germanium and the nature of the damaged subsurface region. At other damage sites in our specimens where little melt back of the ZnS film has

occurred, there are indications of sane disturbance at the Ge interface (figure 8) although it is not possible to determine conclusively what this is due to. Once heating has been initiated,

absorption levels are rapidly increased and a process of thermal runaway ensues, resulting in

ablation of the film above the substrate and plasma formation. It is as a result of this plasma

heating that the surface melting of the ZnS film occurs at a position away from the centre of the

damage site. ZnS generaly sublimes at atmospheric pressure to give zinc and sulphur v^x)ur and it

is necessary to impose an overpressure of ISObar to stablise the liquid phase at the melting point

of 1850°C. The plasmas generated through such vapour plumes can have front velocities in the

region of 600m/s' and shock wave pressures many orders in excess of ISObar can be generated.

4.2 Zinc Sulpho-selenide Films

The purpose of the study of zinc sulpho-selenide deposition was to assess the conditions

necessary for the use of stratified configurations to determine the microstructure of thin films,

and to examine the effect that this may have on the laser damage threshold. However, an equally

important factor to be determined in such a film is the effect of having introduced a large number

of layer interfaces. In the coating considered here, thin films of ZnS 80A thick were deposited

between alternating layers of ZnSe 240A thick and the configuration repeated to make up a coating

2um in total thickness. The number of interfaces contained within the coating is therefore 124. Since deposition was entirely carried out in UHV by alternate shuttering of ZnS and ZnSe sources

it was hoped that the degree of interface absorption would be low. Indeed transmission values at

interference minima both in the stratified design and the alloyed equivalent on ZnSe substrates

are very similar with the stratified design being, if anything, 0.8% higher.

As can be seen from Table 1, the laser damage thresholds of both (ZnS/ZnSe)p| and its alloyed

equivalent are identical within the limits of experimental error, and are within the range

measured for the uncoated substrate. The reason for the slight reduction in LIDT of both sulphoselenide films conpared with that measured for pure ZnSe or pure ZnS films on ZnSe is unclear and

further work is underway to assess whether this is typical of other compositions. Since the

damage threshold is substrate limited, future work will also explore deposition of stratified

films on other substrates of higher LIDT, eg ZnS, KCl. This will allow a more realistic

assessment to be made of any enhancement brought about by the stratified concept.

5. Conclusions

The origin of laser damage in dense ZnS films on germanium substrates has been explored by

cross-sectional transmission electron microscopy. The evidence available suggests that the seeds

for initial absorption of laser radiation are clusters of dislocation loops at the germanium that

have been punched into the surface by the action of abrasive particles.

The conditions necessary for the control of microstructure in thin films by the synthesis of

474
stratified structures are discussed. Despite containing more than 100 film/film interfaces,

coatings of (ZnS/ZnSe)^^ have negligible levels of optical absorption and laser damage thresholds

identical to the alloyed equivalent. Such film structures have considerable potential for

incorporation into gradient index designs where electric field strengths are reduced.

References

1. K.L.Lewis and JJ^Savage: Proc 1983 Boulder Damage Symposium

2. K.L.Lewis, J.A.Savage, A.G.Cullis, N.G.Chew, L.Charlwood and D.W. Craig: Proc 1984 Boulder

Damage Syirpos ium

3. W.H.Southwell: Appl Optics , 24, 457 (1985)

4. C.Lewis, R.H.Runalls, G.N.Turner and S.T.Davis: Adv in Optical Production Technology, SPIE , 163, 1 (1979)

5. W.H.Lowdermilk and S.P.Mukherjee: Proc 1981 Boulder Damage Symposium , NBS Special Publ, 638,

432 (1983)

6. J.Foley, S.K.Sharma and R.M.Wood: Proc 1980 Boulder Damage Syrrposium, Special Publ, 620, 248

(1981)

7. J.E.Field: University of Cambridge, private conmunication

Table 1 Laser Damage Itiresholds of Various UHV-produced Films

Film Substrate LIDT/J cm"2

Uncoated Ge* 25

ZnS Ge* 15 - 16

Uncoated ZnSe 50 - 60

ZnSe ZnSe 60 - 73

ZnS ZnSe 79

ZnSj^Sei_5, ZnSe 56 - 57

(ZnS/ZnSe)^ ZnSe 54 - 60

All LIDT values determined for pulse lengths of 32nsec FVJHM and l/e'^ beam

diameters of lOOum

*Denotes substrates affected by surface work damage - see text

Copyright (c) Controller HMSO, London, I985

475
I 1

0.1;jni

Figure 2 Dislocation loop clusters at surface of germanium

substrate arising from action of abrasive particles

476
477
10nm

Figure 5 High resolution lattice image of section of stratified

(ZnS/Znoe)j^ film. Four ZnS/ZnSe interfaces are within

the field of view. The direction of growth is vertical

I 100 umI

Figure 6 Laser Damage Site at just above threshold fluence in Zno film grown by MBE on Ge substrate, showing damage

artifacts induced by micro-defects at periphery of

central crater.

478
Figure 7 Cross sectional TEM image taken of ZnS film at periphery

of laser damage site. Degree of melt back of ZnS induced

by heating effect of laser driven plasma is clearly shown

imifMiiii

0.2}m

Figure 8 Micrograph showing disturbance of damage initiators at germanium substrate surface at threshold laser fluences

479
Manuscript received

4-15-86 SAB

Transient Absorption of Light

Induced by Radiation

R. Godwin

F. B. Harrison

A. Larson

Los Alamos National Laboratory

Los Alamos, New Mexico, 87454

We have measured two components in the radiation-induced

absorption of 250-nm light in fused silica. For the first, the

time constant is 48.3 ± 1.3 ns and the absorption coefficent is 6.68 ± 0.37 cm-1 per Mrad. For the second, the time constant is long compared to 3 ys, and the absorption coefficient is 1.65 ± 0.36 cm-1 per Mrad. The errors quoted are purely statistical.

Key Words: electron irradiation; fused silica; radiation damage;

transient absorption.

480
1. Introduction

We have measured the absorption of 250-nm light in Corning 7940 fused silica, under

irradiation by 2-MeV electrons from a Febetron 705 accelerator. The samples were 0.5 mm thick,

held at 45° to the electron beam. The light source was a xenon flashlamp excited by a current of

10 kA to an estimated brightness temperature of 25,000° K. The light passed through the sample

at an angle of incidence of 45°. The detector was an ITT planar photodiode. In front of the

photodiode was an interference filter with a 30 nm wide passband centered at 250 nm.

The radiation dose, as measured by radiochromic film, was 760 krad at the front face of the

sample and 410 krad at the back face, giving an average dose of 585 krad. The radiation pulse

length was 30 ns.

Because of the 45° angle of incidence, the path length of the light in the sample is 0.565 mm. The data show a radiation-induced absorption with two time constants. One is long

compared to the 3 ys duration of the measurement (but short compared to the time between shots,

which was a few minutes). The other time constant is short. An analysis of the data is presented below.

2. Derivation of Equations

The fraction of light transmitted

T = e . e (1)

IS nft) = number density of absorption centers fcm-3),

a = absorption cross-section (cm^), and

6 = window thickness (cm).

Subscript (1) refers to centers that recover, subscript (2) to "permanent" damage centers, i.e.,

with time constants long compared to measurement times.

The rate of recovery is given by

n^(t) = K^D(t) - Ln^(t) (2)

where

D(t) = dose rate (rads/s)^

K = production constant (centers/rad) and,

L = decay rate (s-^) •

For the "permanent" term

n2(t) = K2D(t) (3)

Equation (2) has the solution

n^(t) = K^e'^^y* ^dt'D(t')e'-^' (4)

481
Let t* be the time when D goes to zero and stays there. Define

t* . Lt DfL) =/" dfD(t)e

J - 00

Then

-a^K,D(oo )6 t(oo) = e

^

From Eqs. (8) and (9)

(5)

ni(t) = KiD(L)e-Lt, t >t* . (6)

Equation (3) has the solution

n2(t) = K^y^ dt' D(t') = K^Dft) . (7)

From Eqs. f n, (6), and (7)

Tnx = -a^K^DfDe ^^-a^K^Dfco), t>t* (8)

For t » 00, we have

(9)

^ -'ih'^^^^

^"''^ t > t* . (10)

482
and

In r = -a2K2D(oo )5, t > t* (11)

Data Analysis

Set

f(t) = In = A - Lt, (12)

where the zero of time is set at the peak of the dose rate curve, and

A = In faiKiDs)

Examination of the Febetron pulse shape and the absorption data indicates that the formulas

outlined above should apply from about 30 ns after the peak of the radiation pulse, and that 100

ns later the transient absorption is no longer detectable. On this basis, we have performed a least squares fit to the data.

Averaging data for 21 points in 10 ns increments between t=: 150 and 350 ns, we find

Using this value of r in equation (11), we made a least squares fit to the data from t = 30 to 140 ns in 10 ns increments, and found

L = (2.07 ± .05)xl07 s-1

r = 0.947 ± 0.011 (13)

or L-1 = 48.3 ± 1.3 ns (14)

for the attentuation decay constant, and

A = -(1.17 ± 0.05) (15)

The excellent nature of the fit to the data from t = 30 to 130 ns is shown in figure 1.

483
The errors quoted in equations (13), (14) and (15) are those arising from the statistical

analysis; these take no account of systematic errors, which will be discussed qualitatively.

Carrying the statistical errors forward, we find from equation (11)

a2K2 = (1.65 ± 0.36) /(Mrad cm); (16)

taking the total effective dose to be Dq = 0.585 Mrad.

From equation (12)

1 a K = — exp A

1 1 D6 (17)

D6 contains any uncertainties in Dq6 plus knowledge of the shape and time of the

radiation pulse. We estimate that

D = 1.405 Do = 0.822 Mrad

and that errors due to inaccurate information on the pulse shape and time are less than 10

percent. With this D and equations (15) and (17), we find

aiKi=(6.68 ± 0.37) /(Mrad. cm) (18)

Measurements at Sandial at 257 nm gave an attenuation constant 3 = 0.045 cm"l with a radiation dose of 30.3 krad in a pulse about 300 ys long. Using our value for 3 = (3'^2 (equation 16), we predict for the Sandia measurement.

e = (0.050 ± 0.011) cm-1

in good agreement with their result. This suggests that the recovery time of the "permanent"

darkening is at least several hundred microseconds.

1.65 ± 0.36 cm~l per Mrad

R eference

[1] J. Brannon, private communication

484

Manuscript Eeoeived

12-5-86

EFFECTS OF RADIATION INDUCED DEFECTS ON LASER-INDUCED BREAKDOWN IN Si02

M. J. Soileau, Nastaran Mansour, Edesly Canto, and D. L. Griscom*

Center for Applied Quantum Electronics

Department of Physics

North Texas State University

Denton, Texas 76203

The effects of radiation damage on bulk laser-induced breakdown in Si02 were

investigated. Samples studied included Spectrasil A, B, and WF (water free). Measurements of laser-induced breakdown were conducted with 532 and 1064 nm laser pulses of

approximately 20 nsec duration. Reductions of up to 40% in the laser-induced breakdown

threshold were observed at 532 nm for samples exposed to 10^ rads of y-radiation. The

decrease in breakdown threshold for irradiated Si02 samples at 532 nm was found to be

proportional to the linear absorption of the specimen at 266 nm. These results are in

good agreement with a proposed model which suggests that two-photon absorption initiated

avalanche process is responsible for laser-induced breakdown for these materials.

Key words: laser-induced breakdown; nsec pulses; two-photon absorption; Y-irradiated

Si02; 1064 nm; 532 nm.

1. Introduction

The effects of radiation damage on the linear optical properties of materials have been

extensively studied in many materials and under a variety of conditions [1]. In this paper the

effects of Y-radiation damage on nonlinear optical properties, e.g., laser-induced breakdown, of

materials are reported.

Laser-induced breakdown in highly transparent materials is known to be an extremely nonlinear

process. A detailed quantitative description of the breakdown process (or processes) is not yet

available. Recent work [2,3] has indicated that bulk laser-induced damage in wide bandgap dielectrics is due to multiphoton initiated, electron avalanche breakdown. Sample-to-sample variations

and focal spot size dependence of the damage [2-5] lead to the conclusion that this optical damage

is due to extrinsic factors such as material defects and impurities. However, researchers have had

little success in correlating the optical damage with specific impurities and/or defects in good

quality optical materials.

The motivation for this work was to study optical damage in samples with known types and

densities of defects. The basic idea was to produce various concentrations of E'-centers in fused

silica by Y -rad iation. The radiation damaged samples were then laser-damage tested at 532 and

1064 nm. This paper is a progress report of this ongoing investigation.

2. Experimental

2. 1. Sample Preparation

Single specimens of three types of fused silica were investigated: Spectrasil A, B, and WF

(water free). A single sample of each material was sliced into four parts. One quadrant was kept

as a control, i.e., was not subjected to any ionizing radiation. The remaining three quadrants were subjected to as much as 10^ rads of Y-radiation from the Naval Research Laboratory cobolt

source.

Figures la and lb are the linear transmission spectra for the witness samples (no radiation)

and the samples exposed to 10^ rads. Note that there is no apparent change in the transmission of

the radiation damaged samples at the two laser wavelengths that were used for damage testing (i.e.,

532 and 1064 nm). However, there is a substantial increase in the absorption at the 266 nm, the

fourth harmonic of the 1064 nm radiation.

*Naval Research Laboratory, Washington, DC 20375-5000

486
2.2. Laser-Damage Setup

The laser source used in this study was an actively q-switched neodymium :y ttr ium aluminum

garnet (Nd:YAG) oscillator-amplifier laser system operating at 1064 nm. A single pulse of measured

Gaussian spatial profile was produced by the oscillator and amplified by a single pass through the

amplifier. A KD*P crystal was used with the Nd:YAG laser to produce pulses at 532 nm. The crystal was temperature stabilized for long-term energy stability at the second harmonic. Residual 1064 nm radiation was eliminated by reflection off two dichroic mirrors.

The experiment was performed in similar setups at 1064 and 532 nm as shown in Fig. 2. The

temporal pulsewidths were approximately 30 and 18 nsec (FWHM) for 1064 and 532 nm, respectively.

The width of each pulse was monitored by a PIN photodiode detector and fast Tektronix storage

oscilloscope (Model 7834). The spatial beam spot size (of the unfocused beam) was determined by

pinhole scans. A 40 mm focal length lens, designed for minimum spherical aberration, was used to focus the light into the bulk of the materials. 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 pyroelectric (Gentec ED-100) detector, and was calibrated with respect to a Gentec

(ED-200) pyroelectric energy monitor. Each site was irradiated only once, and damage was determined by the observation of scattered light from a coaxial HeNe laser as viewed through a lOx

microscope

.

3. Results and Discussion

Table 1 summarizes the results of measurements of the laser-induced breakdown threshold at 532 nm for non-irradiated Spectrasil A, B, WF, and the samples irradiated with Y source of 10^,

10'', and 10^ rads. All the data presented in table 1 were taken using linearly polarized laser

pulses with a duration of 20 nsec (FWHM) and a calculated 2.2 y m focused spot size (HWl/e^M in

irradiance). The uncertainties listed in the table of data are relative error obtained by the

method used in reference 6. The value given for the electric field are rms fields corresponding to the peak-on-axis irradiance. As indicated in the table of data, the breakdown irradiance decreases

significantly for all the irradiated samples. For example, a 40% reduction in breakdown irradiance

was measured for 10^ rads irradiated Spectrasil A compared to the non-irradiated samples.

Table 2 summarizes the results of similar measurement for the same samples at 1064 nm. For this wavelength the pulse duration was 30 nsec (FWHM) and the calculated focused spot size was

3.1 Pm (HWl/e^M in irradiance). At this wavelength (1064 nm), the laser induced breakdown remains

the same for non-irradiated and irradiated Spectrasil A, B, and WF samples.

The effect of self-focusing in our measurements is negligible because the breakdown power is

significantly less than its critical power for self-focusing. The critical power for self-focusing

for a focused Gaussian beam is obtained from the numerical solution of the nonlinear wave equation

and is given by ref. [7]

= 3.77 cA^/32TT^n2

where n2 is the nonlinear refractive index in esu, X is the laser wavelength, and c is the speed of

light in vacuum. For Si02 samples, the values of critical power for self-focusing are ~1.7 MW

and ~6.8 MW at 532 nm and 1064 nm, respectively. These values are obtained using a measured n2

value of 0.6 x 10"^^ esu [8]. In our measurements the breakdown power is two orders of magnitude

smaller than the calculated critical power for self-focusing (tables 1 and 2). We performed

polarization dependence tests of the breakdown power for these samples [9]. For all samples, the

breakdown threshold for linearly polarized and circularly polarized light was equal, confirming

that self-focusing was not a major contributor to the observed damage.

Figure 3 is a plot of the results of the laser-induced breakdown measurements at 1064 nm using

30 nsec (FWHM) linearly polarized pulses. Results for the witness samples and the samples exposed

to 10^ rads are shown. The shaded areas are the relative uncertainties in the damage thresholds,

i.e., the irradiance region for which the probability of damage was greater than 0 and less than 1. The absolute error bars are estimated to be ±15%. The conclusions drawn from this figure are (1)

the 1064 nm damage thresholds for the three samples are essentially the same and (2) the damage

threshold irradiance at 1064 nm is uneffected by y-radiation doses up to 10^ rads.

Figure 4 is a similar plot for the results of the 532 nm measurements. As in the previous

figure, the top bars are the results for the unirradiated samples and the bottom bars are the

results for the samples exposed to 10^ rads. The two conclusions drawn from this plot are:

487
(1) The breakdown irradiance for the witness samples are lower at 532 nm than at 1064 nm. This

implies that the laser damage process is not simply avalanche breakdown. (2) The laser damage

threshold irradiance at 532 nm is substantially reduced for the samples exposed to 10^ rads.

Recall that figure lb showed no observable change in transmission at 532 nm for the Y- irradiated samples. It is not unusual for samples with the same linear transmission to have

drastically different breakdown thresholds [5]. To date all attempts to correlate the laserinduced damage threshold to linear absorption have been unsuccessful. Also note that the linear

absorption at 266 nm is substantially affected by the Y-radiation. This suggests that the decrease

in laser damage threshold for irradiated Si02 samples at 532 nm must be related to the linear

absorption at 266 nm (the second harmonic of 532 nm). This correlation will be explained by the

two-photon assisted electron avalanche breakdown model [10,11] in next paragraph.

Assume that laser-induced breakdown occurs when the density of free carriers generated by a combination of multiphoton and avalanche processes reaches some critical value, Ng (this critical

density is assumed to be approximately equal to the density required for the plasma frequency to equal the laser frequency).

In the avalanche process the buildup of free carrier density is given by

N = N e (1)

D O

where n = net ionization rate

D

N = initial free carrier

o

The net ionization rate for laser-induced damage is a function of the breakdown field. Eg. Some

authors have proposed models [12,13] which predict an ionization rate given by:

„2

n a: E

'b b

Then eq. (1) can be written as:

N„ = N e (2)

D O

where "a" for our experiment is a constant related to many factors such as: frequency of the

light, laser pulsewidth, the momentum transfer collision time, effective mass and electron charge.

The model proposed is that free carriers are initially generated by two-photon absorption from

defect state within the band gap. Therefore, can be written as:

N « n

o J

where n = density of two-photon allowed defect states. Taking the natural log of eq. (2) yields:

2 aE„ = -Ixi n + const (3)

The final assumption is that the density of two-photon allowed states (for 532 nm incident radia488
tion) is proportional to the linear absorption at 266 nm (i.e., the second harmonic of the 532 nm

radiation). For that assumption eq. (3) gives

(4) at 266 nm

where a = linear absorption coefficient at the 266 nm

t= length of sample

This analysis [eq. (4)] predicts that the breakdown field at 532 nm should be proportional to the natural log of the absorption at 266 nm. Figures 5, 6, and 7 are the plots of the square of

the breakdown field at 532 nm [(MV/cm)^] versus the negative natural logarithm of the linear

absorption at 532 nm for the Spectrasil A, B, and WF samples exposed to various Y-ray doses up to 10^ rads. The linear absorption was calculated from the linear spectra of these materials using

the assumption that the change in index of refraction is negligible in the wavelength range between

532 and 266 nm. The agreement with the prediction of eq. (4) is quite good considering the relative simplicity of the model and the assumption made.

4. Summary

The effects of Y-radiation on laser-induced breakdown was studied in fused silica at 1064 and

532 nm. The results show no change in the 1064 nm damage thresholds for doses up to 10^ rads. The

532 nm laser damage threshold irradiance was found to decrease by as much as 40% for samples

exposed to 10^ rads. The reduction of the damage thresholds at 532 nm was found to be related to the increase in linear absorption at 266 nm. These results are consistent with a model that assumes that damage is caused by two-photon absorption initiated avalanche breakdown. The twophoton absorption is assumed to be associated with the defects created by the Y-radiation.

These results indicate that studies of the effects of radiation induced defects can yield

important information about the initiation of laser-induced damage in wide bandgap solids. There are practical implications as well. Optical components exposed to ionizing radiation could

experience substantial reduction in laser breakdown thresholds, particularly for lasers operating

in the visible and ultraviolet spectral regions. This problem could be an important consideration

in choosing the optimum laser wavelength for various applications. For example, free electron

lasers by their nature produce ionizing radiation which can effect the optical components of the

lasers. In that case one may want to choose to operate the laser in the near infrared (e.g.,

1000 nm) rather than the visible (e.g., 500 nm) so that the problem of radiation induced defects

could be minimized.

The North Texas State University authors acknowledge the support for this work from the Office

of Naval Research and the North Texas State University Faculty Research Fund.

5. References

[1] Friebele, E. J.; Griscom, D. L. "Radiation effects in glass." Glass II. 17; 257-351; eds. New

York; Academic Press; 1979.

[2] Soileau, M. J.; Williams, W. E. ; Van Stryland, E. W.; Boggess, T. F. ; Smirl, A. L. "Picosecond

damage studies at 0.5 and 1 ym." Opt. Eng. 22(4): 424-430; 1983.

[3] Soileau, M. J.; Williams, W. E. ; Van Stryland, E. W.; Boggess, T. F. ; Smirl, A. L. "Temporal

dependence of laser-induced breakdown in NaCl." Natl. Bur. Stand. (U.S.) Spec. Pub. 669; 387-

405; 1982.

[4] 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.

[5] Soileau, M. J. "Frequency and focal volume dependence of laser-induced breakdown in wide band

gap insulators." Ph.D. Dissertation; University of Southern California; 1979.

at 532 nm

489
[6] Porteus, J. 0.; Jernigan, J. L. ; Faith, W. N. " Mul t i- 1 hr esho Id measurement and analysis of

pulsed laser damage on optical surface." Natl. Bur. Stand. (U.S.) Spec. Pub. 568; 507-515;

1979.

[7] Marburger, J. H. Self-focusing: Theory. Progress in Quantum Electronics, edited by Sanders,

J. H.; Stenhold, S. Pergamon. Oxford. 4(1):35-110; 1975.

[8] Williams, W. E.; Soileau, M. J.; Van Stryland, E. W. "Simple direct measurements of n2." Natl. Bur. Stand. (U.S.) Spec. Pub. 688; 1983.

[9] 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. 688; 1983.

[10] Soileau, M. J.; Mansour, N. ; Canto, E. ; Griscom, D. L. "Effect of radiation induced defects on

laser-induced breakdown in Si02«" To be published in the Proceedings of the 1985 MRS Meeting.

[11] Mansour, N.; Guha, S.; Soileau, M. J. "Laser-induced damage in doped and undoped fluorozirconate glass." Natl. Bur. Stand. (U.S.) Spec. Pub.; 1985.

[12] Yablonovitch, E. ; Bloembergen, N. "Avalanche ionization and limiting diameter of filaments

induced by light pulses in transparent media." Phys. Rev. Lett. 29(14): 907-910; 1972.

[13] Sparks, M. ; Holstein, T. ; Warren, R. ; Mills, D. L. ; Maradudin, A. A.; Sham, E. L. , Jr.; King,

F. "Theory of avalanche breakdown in solids." Natl. Bur. Stand. (U.S.) Spec. Pub. 568; 467-

478; 1979.

OO

oo

100

90-

80

70

60- ^ 50

40

30

20-

10

0

WF

A

B

266 532 1064

A(nni)

I: ;/

1/

1/

Figure la. Linear transmission of unirradiated Spectrasil A, B, and WF used in this work.

490
00

00

100

90

80

70

60

50

40

30

20

10

tfH===^^

WF

A

B

266 532 1064

A (nm)

1

Figure lb. Linear transmission of samples exposed to 10° rads of y-^adiation. Note that there is no detectable change in transmission at 1064 and 532 nm and a substantial reduction in

transmission at 266 nm.

Figure 2. Schematic of the laser-induced damage setup.

491
A=1.06yum

tp=30nsec

rads

5]

A+108 rads

WF

WF+108 rads

200 400 600 800 1000 1200 1400

Figure 3. Laser-induced damage results for 30 nsec 1064 nm pulses. The thresholds for damage for

the three different specimens are approximately equal and there is no change in the

laser damage thresholds for the samples exposed to 10^ rads.

A= .53^ m

tp = IBnsec

B

B + lO^rads CD

A

A + lO^rads

^

WF CD

WF + lO^rads

200 400 600

'B

Figure 4. Laser-induced damage results at 532 nm. Note that the laser-induced damage thresholds

of the witness samples (i.e., the unirradiated samples) are less at 532 then at 1064.

At this wavelength there is a substantial reduction in the optical damage thresholds

for the samples exposed to 10^ rads.

492
CM

CO

ir»

CM

180

160

140 - E

^

120 -

UJ

100 -

80

• Spectrasil A

• X = 532nm

• tp = ISnsec

A

Figure 5.

0 1 2 3 4 5

-In \ai\ at 266nni

Square of breakdown electric field (MW/cm)^ at 532 nm versus the negative natural

logarithm of the linear absorption at 266 nm for non-irradiated and irradiated spectrasil A samples.

180

180 -

CM

CO

ID

:;: ho

fa

estE

o

CM

00

120

100

80

• Spectrasil B

• X = 532nm

• tp = 18nsec

-ln(af) at 266nni

Figure 6. Square of breakdown electric field (MV/cm)'^ at 532 nm versus negative natural logarithm

of the linear absorption at 266 nm for non-irradiated and irradiated Spectrasil B samples. Recall that for irradiated Spectrasil B sample with the 10^ rads there is a substantial increase in the absorption at the 266 nm. However, the samples irradiated

with 10^ and 10^ rads are uneffected and linear transmission spectra remains the same

as the non-irradiated sample.

493
CO

ITS

CM

180

160

t- 140 -

120 -

100 -

80

! 1

• Spectrasil WF

I 1

_ • X = 532nm r ^

_

-

- /

9 1 2 3 4 5

-In (af 1 at 286nm

Figure 7. Square of breakdown electric field (MV/cm)^ at 532 nm versus the negative natural

logarithm of the linear absorption at 266 nm for non-irradiated and irradiated spectrasil WF samples.

494
Table 1

Laser induced damage data for non-irradiated and irradiated Spectrasil A, B, and WF samples at

532 nm. The errors shown are relative uncertainties as discussed in the text. The absolute error

bars are estimated to be ±15% of the irradiance.

Radiation Ig Eg Pq Fq

Sample rads GW/cm2 MV/cm KW KJ/cm2

A 0 614±30 12 4±0 3 42 8+2 1 11 7 + 0 6 A 106 532±30 11 6+0 3 37 1±2 1 10 1±0 5 A 107 517+22 11 4±0 2 36 1+1 6 9 9±0 4 A 108 382+37 9 .8±0 5 26 6±2 6 7 3+0 7

B 0 554±45 11 8±0 5 38 7±3 1 10 6+0 9

B 1 0^ 562±22 1

1

9±0 2 39 2±1 6 1 0 7±0 4 B 107 577+30 12 0±0 3 40 2±2 1 11 1+0 5 B 108 480±22 10 9±0 2 33 4+1 6 9 2±0 4

WF 0 554±22 11 8±0 2 38 7±1 6 10 6+0 4 WF 106 592+60 12 2+0 6 41 3+4 2 11 3 + 1 1 WF 107 532+60 11 5±0 6 37 1±4 2 10 2+1 1 WF 108 442122 10 3±0 2 30 8±1 6 8 4+0 4

Ib = breakdown irradiance (peak on-axis irradiance)

Eb = breakdown field Pb = breakdown power

Fb = breakdown fluence

495
Table 2

Laser induced damage data for non-irradiated and irradiated Spectrasil A, B, and WF samples at

1064 nm. The errors shown are relative uncertainties as discussed in the text. The absolute error

bars are estimated to be ±15% of the irradiance.

Radiation Ig Eq Pq Fq

Sample rads GW/cm2 MV/cm KW KJ/cm2

A 0 1212+50 17.4+0.4 180.0+7.5 38.9+1.6

A 108 1110+72 16.7+0.5 164.0 + 10.5 35.6+2.2

B 0 1198136 17.4 + 0.3 177.0+5.3 38.5 + 1.1

B 108 1120+46 16.8+0.3 166.0+6.9 35.9+1.5

WF 0 1176+23 17.2+0.2 174.0+3.4 37.8 + 0.7

WF 108 1170+52 17.1+0.4 173.0+7.7 37.6 + 1.7

\q = breakdown irradiance (peak on-axis irradiance)

Eg = breakdown field Pq = breakdown power

Fb = breakdown fluence

496
• " . Manuscript Received

9—2i2i~uG BAR

Fundamental Mechanisms Of Optical Damage In Short-Wavelength High-Power Lasers

R.F. Haglund, Jr. and N.H. Tolk

Department of Physics and Astronomy, Vanderbilt University

Nashville, TN 37235

G.W. York

Physics Division, Los Alamos National Laboratory

Los Alamos, NM 875^5

Evidence has been accumulating for many years that the physical mechanisms responsible

for damage to optical materials in and from high-power, short-wave-length lasers (SWLs)

differ in fundamental ways from the thermal processes identified in infrared and

visible-wavelength laser damage problems. We propose that this difference stems primarily from the electronic nature of the absorption and excitation processes which

occur when SWL photons strike an optical surface, and that electrons, ions and uv photons generated in the laser excitation cycle also contribute to optical damage. In this paper, we present recent experimental results which have pinpointed specific

electronic excitation mechanisms which can operate in the high-power laser environment.

In many optical materials of interest for SWLs, the deposition of electronic energy

creates self-trapped excitons which decay through the energetic expulsion of atoms and

molecules from the surface of the material. This erosion process is accompanied by

the creation of permanent electronic defects which become nucleation sites for further

damage. The relationship between these microscopic mechanisms and observed macroscopic

damage phenomenology is discussed, along with evidence for the existence of a surface

overlayer which may point the way to radically new techniques for protecting SWL optical elements from laser damage.

Key Words: alkali halides; desorption induced by electronic transitions (DIET)

laser desorption; surface damage; surface erosion.

1 . Introduction

Early studies of optical damage to laser components at wavelengths of 1 and 10 microns

showed that there were two primary causes for catastrophic failure of large optical elements in

high-power laser systems: first, the heating of submicron inclusions left from polishing or other manufacturing processes [l]; second, avalanche break-down or field emission at microcracks or pores in thin optical coatings [2]. These modes of laser damage have been studied for

many years, and this work has brought significant improvements in the performance of optical

materials, largely through improved quality control in the fabrication process. However, the

very success of these explanations has also created a predisposition to ascribe all optical

damage to thermal effects, even in the face of experimental evidence that short-wavelength laser

photons can induce physical and chemical transitions in pure, structurally-perfect materials by

multiphoton or other non-thermal mechanisms [3].

We have been working to identify the fundamental damage limits for short-wavelength lasers

set by physics and chemistry of the laser-materials interaction rather than by deficiencies in

optical manufacturing practice. Our experimental techniques allow a significant degree of

control over and characterization of the incident primary radiation, the substrates and overlayers

chosen as model systems, and the dynamical state of atoms, ions and electrons ejected from the

target materials following irradiation. Experiments based on these techniques are yielding an increasingly detailed picture of the ways in which microscopic absorption, localization and

redistribution of absorbed photon and particle energy triggers macroscopic destructive events

such as desorption, erosion, and ablation of optical surfaces.

Building on this evidence, we have argued elsewhere [4] that the primary damage mechanisms

in short-wavelength lasers are electronic and that they are triggered not only by the laser

light itself, but also by electrons and UV photons created in the pumping process and by excited

ions and atoms in the laser gas plasma. This picture is consistent with recent experiments

indicating the onset of a major change in the morphology of damage to selected dielectrics

occurring somewhere in the wavelength region around 250 nanometers [5]. It is our contention

that this damage mechanism can be identified with the onset of Desorption Induced by Electronic

Transitions (henceforth DIET), a generic category of phenomena implicated in erosion and radiation

497
damage caused by low-energy electrons, photons and heavy particles (atoms, ions or molecules).

Thus, we propose that the catastrophic failure modes of excimer laser optics are precipitated by DIET processes caused by ions, electrons and ultraviolet photons from the laser environment as well as single- and multiple-photon desorption events induced by the laser photons. However, as we shall show, these electronically-induced desorption events also have thermal consequences,

particular at laser flux levels above the plasma-formation limit.

In this paper, we first present experimental data from the Large-Aperture laser amplifier

Module (LAM) at the Los Alamos National Laboratory, indicating the intensity of the various

categories of radiation striking the optical surfaces. We next consider the ways in which DIET processes changes the surface composition and structure of model optical materials, keying our remarks both to specific electronic mechanisms and to the solid-state properties of alkali

halides. We then discuss a model for damage-initiating events in a typical large excimer laser,

in which not only the laser photons but also uv photons, electrons, ions and neutral atoms

created by the laser pumping process can contribute to the destruction of optical surfaces. We

shall make a plausible argument, supported by these data, showing how initial low-fluence events

induced by an electronic pre-cursors can lead to changes in surface composition which result in

catastrophic thermal breakdown when an intense laser pulse impinges on an optical surface.

Finally, we adduce experimental evidence for the existence of a protective surface overlayer, the

understanding of which may lead to a new class of damage-resistant coatings for short-wavelength

lasers.

2. Sources Of Optical Damage In High-Power Excimer Lasers

Figure 1 shows a schematic of the Large-Aperture KrF laser amplifier Module (LAM) now operating at the Los Alamos National Laboratory, a device typical of current and planned large

excimer lasers for fusion applications [&]. The LAM has an active volume of some 2 m', and

optical surfaces (resonator mirror and windows) exceeding 1 m^ in size. A population inversion

in an Ar-Kr-F2 mixture is created through electronic excitation of the laser gas by two 400 kA

beams of 750 KeV electrons from a cold cathode discharge. The electron trajectories in the gas are constrained to a direction transverse to the optical axis by a 4 kG magnetic field produced

by a pair of large Helmholtz coils. The mirror and window have f used-silica or Pyrex substrates,

typically coated with multi-layer dielectric thin films of Si02, AI2O3, or other metal halides

or metal oxides.

The design fluence of the LAM is 1 J/cm^ in a 500-ns pulse; large UV lasers now under

design will produce laser fluences ranging up to several J/cm^. The significance of these flux

levels can be gauged by observing that, for a KrF laser (248 nm), 1 J/cm^ corresponds to some

lO" photons per surface site on the irradiated optical elements. In electron-beam-pumped lasers,

Bremsstrahlung X-rays produced by the deceleration of near-megavolt pump-beam electrons, as well

as scattered or diffusing low-energy electrons, will also irradiate nearby optical surfaces.

Each of these sources of radiation efficiently produces neutral atoms and molecules through DIET

processes. Moreover, many low-energy ion species in the laser plasma—including oxygen, argon

and fluorine—are known to be efficient at producing reactive-ion etching on insulators and

semiconductors, particularly in the presence of uv radiation [7]. Thus, in the precursor phase

of optical damage, radiation both from the laser environment and the laser photons themselves

can play important roles.

498
Gross measurements of the radiation flux in the plane of the LAM exit window show that, in

spite of the large magnetic guide field, there is substantial leakage of the pump-beam electrons

from the active laser volume to the optical surfaces of the chamber. In these experiments, a Faraday cup was placed in the center of the output window, a distance of approximately MO centimeters

from either electron beam cathode and the current was measured during repeated shots from one cathode. It was found that at this center position that the electron current density ranged

from 30 to 60 milliamperes per square centimeter, yielding an electron dose on the order of 10 to 100 electrons per surface site per shot. We also measured the total radiation dose at the

window at several locations, using lithium fluoride thermal luminescence detectors, and discovered

that the average is approximately 2 rads total dose per square centimeter per shot. Thus the laser

environment contributes in a major way to optical damage, consistent with anecdotal observations

of dramatic reductions in damage threshold for optical materials "inside" high-power lasers.

3. Desorption Induced By Electronic Transitions in Optical Materials

The key to a fundamental understanding of optical damage in optical materials for SWLs is to find the link or links between the microscopic ( i.e ., atomic-scale) desorption of individual

atoms or molecules from the optical material, and macroscopic erosion and large-scale surface

damage. This requires dealing both with the detailed mechanisms of desorption and with the

surface and solid-state properties of particular optical materials. The key questions revolve

around the existence of certain decay channels through which incident energy may be transformed

and redistributed to create defects and particle desorption. Hence, in contrast to thermal

desorption processes—which depend only on the bulk thermodynamic properties of the' optical

material—any serious investigation of electronic desorption mechanisms must concern itself with

the details of the optical material properties, such as band structure. In this section, we shall discuss DIET processes in a general way, and present representative experimental data used

to obtain detailed dynamical information about them.

3.1. Electronically Stimulated Desorption of Neutral Atoms

DIET processes occur when low energy photons, electronics, ions or neutral atoms interact

with the atoms in the surface and the near surface layer of bulk material. The DIET process as shown in Figure 2 is characterized by three stages: (1) an excitation stage in which the energy

of the incoming particle or photon is deposited in the near-surface bulk; (2) the localization

phase, lasting from 10"'^ to 10"^' second, during which one or more atoms or molecules participating

in the excitation arrives at an energy state which is on the repulsive part of the relevant

potential energy surface; and (3) desorption, as the particle is actually expelled from the

surface. During this last phase the particle may also undergo charge exchange or other electronic

particle-surface interactions.

Fig. 2. Illustration of the three stages of a DIET process: (left) excitation; (center)

localization of energy leading to desorption; and (right) particle-surface interactions.

DIET spectroscopy of ions has been used for many years to obtain structural and compositional

information on a variety of surfaces. Several mechanisms for ion DIET have been proposed, the

first of which was the Menzel-Gomer-Redhead model [S], devised to explain the desorption of

absorbed gases on metal surfaces. This model, characterized by the assumption that two-body

gas-phase potentials can be used to explain the repulsive energy state, accounts in a qualitative

way for the desorption of neutral adsorbates such as hydrogen from transition-metal surfaces. In

the 1970's, Knotek and Feibelman propounded a different mechanism to explain the desorption of

ions from maximally valent covalent solids such as titanium dioxide [9]. In their model, the

creation of long-lived hole pairs produced desorption via a Coulomb explosion. By demonstrating

explicitly that the formation of hole pairs could increase the stability and lifetime of an

499
electronic excitation leading to desorption, they pointed to the role of specific solid-state

properties of a select class of oxides in desorption phenomena [lo]. The link between bond angles

and desorption probabilities has been exploited by Madey, Yates and their coworkers to obtain

detailed structural and compositional information about adsorbed species on both metal and

insulator surfaces [ll].

However, neither the Menzel-Gomer-Redhead nor Knotek-Felbelman models are able to explain

recent experiments showing conclusively that the DIET phenomenon in insulators (such as the alkali

halides and alkaline earth halides) is overwhelmingly dominated by neutral particle production

[12]. The predominance of neutral particle yields in electronically stimulated desorption means

that in these materials ion desorption is largely irrelevant to studies of the fundamental

dynamical mechanisms of desorption, simply because the largest fraction of absorbed energy goes

into the production of neutral particles. We now consider representative results from neutral

desorption spectroscopy in alkali crystals.

3.2. DIET Experiments on Single-Crystal Alkali Halides

The detection of neutral particles and the study of the desorption mechanism in DIET processes

occurs primarily via optical techniques [13]. Figure 3 shows the schematic of a typical DIET experiment in which neutral particles either in the ground state or in excited states desorbing

from a surface are observed. The radiation source — low energy photons, electrons or heavy

particles — is shown as incident from the left. The target surface, generally an insulator, is

kept in a sample chamber under ultra-high vacuum, so that the formation of impurity adsorbates

on the surface can be controlled or minimized. The target surface is mounted on a micromanipulator

stage which can be heated or cooled to study temperature-dependent effects ( e.g . , defect migration)

on desorption dynamics. When the radiation impinges on the target surface, desorption events

occur and the desorbing particles begin to move away from the surface. Excited particles among

the desorbing atoms and molecules radiate as free particles as they leave the surface. This

radiation is detected by a spectrometer whose focal volume is arranged to intersect the volume

of the exciting beam immediately in front of the target surface. It is this radiation which is

detected by the spectrometer and which gives a characteristic optical signature of particle

identity, internal energy state, velocity distribution and dependence on the state of the surface

(e.g., temperature).

RADIATION SOURCE • ELECTRONS • UV OR LASER PHOTONS • HEAVY PARTICLES

A A A A

munm

¥ V V V

DE-EXCITATION RADIATION

GRATING SPECTROMETER

Fig. 3. Schematic of a DIET experiment using optical

techniques to detect

neutral atoms and molecules.

Ground-state particles leaving the surface, on the other hand, are detected by means of laserinduced fluorescence using a tunable dye laser beam, which is brought to a window in the ultra

high vacuum chamber via a fiber-optic link. The light from the laser is collinear with and

anti-parallel to the exciting beam so that the particles leaving the surface are excited by the

laser into a state from which they can radiate via characteristic spectral lines.

As examples, we consider the results of PSD and ESD experiments on LiF single-crystal

surfaces. The excitation source for the PSD experiments was the Tantalus storage ring at the

University of Wisconsin Synchrotron Radiation Center, with photons in the energy range from 40-

200 eV. In the ESD experiments, the electron source was a high-current, low energy (0-100 eV)

gun specially designed for this application.

500
80 100 120 140 160 PHOTON ENERGY (ev)

Figure M shows the optical emission

intensity of (a) desorbed Li* (excited

Li atoms) as a function of the

primary-beam photon energy, compared

to the similar curve for (b) Li"*" (ions)

and (c) the LiF photon absorption

coefficient. In all three curves,

the main excitonic structure of the

Li K-edge is evident. Also, the

high yield over a wide range of

photon energies suggests that DIET processes are likely to remain

efficient even down to relatively

low energies, and certainly at energies

characteristic of the electron-beamproduced Bremsstrahlung in a device

such as the LAM. However, the most important feature of these curves is

that the Li* yield exceeds that of

the Li"*" by five orders of magnitude.

More recent measurements of the PSD yield of Li° (ground-state neutral

lithium) showed that these yields

exceed those of Li* by at least two orders of magnitude.

Fig. 4. (a) PSD yield of Li from LIF.

(b) PSD yield of Li* from LIF.

(c) Optical absorption for LIF.

60

40

20

In the alkali halides following electron or photon impact, the halogen neutral atoms are ejected

rom the surface with a suprathermal energy, while the alkali neutrals are desorbed thermally.

Figure 5 shows the temperature

dependence of the ESD yield of

30 ground-state and excited-state

lithium metal atoms from singlecrystal lithium fluoride. The

desorption yield curve for the

ground state neutrals follows

the classic Arrhenius desorption

shape, increasing as a function

of temperature until the yield

of lithium metal atoms from the

surface is equal to the rate at which they are being produced

by the irradiating electrons.

The excited state neutrals, on the other hand, are not desorbed

thermally; there is other

evidence suggesting that the

Li* are in fact not in thermal

equilibrium with the surface at the moment of desorption. At temperatures below those required

for desorption of either Li° or

Li , then, there are microscopic

agglomerations of free alkali

metal on the surface of the

irradiated alkali halide crystals

- a situation fraught with

Fluorescence yields of Li° and Li* desorbed from significant implications for

single-crystal LiF, as a function of temperature. optical damage.

o

o

Q

—IUJ>

lUo

z

UJO

</) o

3

A

Li*(x5)

-

ESD YIELD OF Li» - AND Li*NEUTRALS

FROM LiF CRYSTAL

200 300 400 TEMPERATURE (°C)

500

Fig. 5.

501
Collateral evidence for this picture of ESD/PSD in lithium fluoride comes from measurements

on sodium halides showing that the velocities of desorbing ground-state sodium atoms have a Maxwellian distribution at the temperature of the surface. The number versus velocity distribution

of the desorbing atoms is given by

dn/dv = Cv^ exp(-mv^/2kT) , (1)

where the velocity v is related to the measured frequency v of the emitted photons by

the first-order Doppler formula

V = Vq[i + (v/c) cos e]. -. (2)

The parameter Vq is the central frequency for stationary atoms and 6 is 0 in the geometry of

these experiments. A frequency scan of the tunable dye laser around the characteristic groundstate absorption lines allows desorbing atoms to resonantly absorb laser light at the Dopplershifted wavelength appropriate to their rest frame. The graph of fluorescence yield vs. frequency

is thus equivalent to a scan of dn/dv.

0 1000 2000 3000

„-^^(GHZ) VELOCITY (m/sec)

Fig. 6. (Left) Fluorescence yield as a function of laser frequency for ground-state sodium

desorbing from various sodium halides. (Right) Velocity distribution derived from the

best-fit temperature for the measured Doppler-shif ted frequency spectrum.

Figure 6 (Left) shows the measured data in the Doppler scanning configuration of the experiments

on a Na-halide crystals, running over the two hyperfine peaks of the Na D2 line. Figure 6 (Right)

compares the fitted Maxwell-Boltzmann temperatures with the measured surface temperatures over

the range covered in one set of experiments. The measured surface temperature and the temperature

inferred from the fitted velocity distributions differ by a constant amount, prob-ably due to the

gradient between the thermocouple probe and the surface. The measured surface and best-fit

temperatures were completely consistent with neutral ground-state alkalis desorbing in thermal

equilibrium with the surface [is].

Some further clues to the character of the DIET mechanism in alkali halides come from timeresolved ESD measurements carried out on LiF. In these experiments, a chopped electron beam and

a time-to-amplitude converter coupled to a multichannel analyzer were employed to measure the

desorption yield in the "afterglow" period after the electron beam was turned off. Typical data

for ground-state and excited-state neutrals at two different sample temperatures are shown in

Figure 7. When the electron beam is turned off, the ground-state fluorescence yield decays

slowly, indicating that some kind of diffusive process brings defects to the surface, where they

reduce their free energy by participating in desorption events. The same kind of experiment has

been carried out looking at the excited-state neutrals desorbed from lithium fluoride. But in

this case, the yield of excited state neutral lithium exactly tracks the on-off pattern of the

irradiating electron beam, indicating that there is no near-surface bulk contribution to the

desorption yield; instead, the neutral excited lithium comes off the surface immediately as it

is being irradiated. This shows that we are dealing with quite a different mechanism than that

responsible for the desorption of ground state neutral lithium.

502
BEAM ON

Fig. 7. Time dependence of

yield for ground-state

Li in ESD from LiF.

Channel (0.01 ms/ch)

3. 3. Mechanism for Ground-State Alkali Desorption

Summing up, one can say that electronically-stimulated desorption of ground-state alkalis

from alkali halides exhibits features consistent with thermal desorption from the surface and

near-surface bulk. In contrast, although the evidence is not so detailed as the results cited

here for the alkalis, it has been shown that halogens desorbed from alkali halides under lowenergy electron bombardment have suprathermal energies and are emitted in preferred directions

along the halogen "strings" in the crystal. This suggests that the formation and relaxation of

F-centers, as orginally proposed by Pooley and Hersh [l6], is a likely mechanism for the ejection

of the halogens from the surface and near-surface layer of the bulk. The Pooley-Hersh model

involves a specific electronic excitation leading to preferential and energetic ejection of

halogens along the <110> directions (Fig. 9). Simultaneously, by leaving behind an electron to bind to the now undercoordinated alkali atoms near the surface, it provides a mechanism for the

generation of neutral alkalis to undergo thermal desorption. Moreover, because the Pooley-Hersh

mechanism is a simple one-step process — involving only the formation and relaxation of an exciton — the creation of neutral alkalis can occur relatively efficiently.

hv.

OoOoOoQo

oOoOsOoO

QoOoOoOo oOoO^oO

CJoOoOoOo

oOoOoOoO

O

OoOo Oo o

oOoOoOoO OoOoQoOo

oOoOoQoO

IRRADIATION

AND EXCITON

FORMATION

FORMATION

OF DIHALIDE

DISSOCIATION

AND EJECTION

Fig. 8. Schematic of the Pooley-Hersh mechanism. The initial stage localizes the incident

electronic energy through the creation of a self-trapped exciton. The transient

dihalide molecular ion, formed by the overlap of the excited and a neighboring (normal)

halogen ion, is in a predissociati ve state. When a hc.logen atom or ion is ejected

preferentially along the halogen string directions, it leaves behind an F-center and a defect (halogen vacancy), allowing the undercoordinated alkali ion to be neutralized

and desorb thermally.

503
With respect to optical damage in SWLs, the significance of the Pooley-Hersh mechanism lies

in the existence of free alkali metal atoms on the surface following electron or photon irradiation,

as long as the surface remains sufficiently cold that thermal desorption does not occur. This

implies, as we now discuss, a variety of undesirable consequences for optical materials generically

related to the alkali halides.

^. Electronically-Induced Damage in Optical Dielectric Materials

From the microscopic point of view, electronically-induced damage in short wavelength

optical materials arises from the ability of these dielectrics to store energy in a highly localized

way and then to release that energy through destructive channels. The problem may be seen more

clearly by contrasting the situation with that of a pure metal, in which the highly non-localized,

conduction-band electrons are able to drain away incident energy from the absorption site and

disperse it throughout the metal. In an insulator, such a dispersal of the incident energy is not easily possible, because of the tight binding of the valence-band electrons, reflected in

the large size of band-gap energies for typical optical dielectrics. Moreover, in dielectrics

there exist a variety of material dependent mechanisms — including exciton formation and the

population of surface states — through which incident electronic energy can be localized in space

and time sufficiently to trigger defect formation, desorption and other destructive events. In

the typical laser environment, all materials, including metal mirrors and metal layered structures

,

are covered either with an oxide or with some other coating due to the presence of air or the

laser environment, which renders the surface an insulator or, at best, a semiconductor.

In this section, we discuss one possible scheme for analyzing the susceptibility of optical

materials to damage based on generic materials properties. We also discuss evidence for the

existence of an as yet uncharacterized overlayer on the surface' of most materials, which appears

to retard substrate desorption. Finally, we discuss a model of optical damage at short wavelengths,

in which an electronic precursor mechanism changes the material composition and structure in

ways which lead ultimately to catastrophic damage and failure of the material.

4.1. The Role of Specific Mechanisms

Early thermal models of optical damage were guided by an understanding of the interaction

of infrared photons with the phonon spectrum of metals and insulators. Our picture of short

wavelength optical damage is likewise guided by a recent characterization of electronic interactions

in insulators and semiconductors due to Itoh [l?]. This model of different classes of electronic

materials allows us to relate experimentally observed desorption dynamics to specific materials

properties. By concentrating on specific excitation mechanisms and on the dynamics of energy

flow leading to desorption and, ultimately, erosion and material ablation, it is possible to correlate the properties of the incident radiation with the properties of specific optical

materials to produce a microscopically detailed picture of the damage process. This understanding

of the desorption dynamics leads to a valid description of damage from well below the plasma

threshold up to and beyond that limit, and allows the identification of critical rate constants.

The Itoh classification of semiconductors and insulators hinges on an understanding of the

ability of materials to support exciton formation, on the one hand, and relaxation to mobile,

permanent electronic defects, on the other. The categories of materials together with representative

substances which have these various features, are shown in Table 1.

Table 1

Category Self-Trapped Excitons Permanent Defects Materials

I No No Compound semiconductors

Some metal oxides

II Yes No a-Quartz

III Yes Yes Alkali halides

Alkaline-earth halides

Fused silica

Other metal oxides

(alumina, titania)

It is interesting to note that virtually all optical materials of current interest in short

wavelength laser designs fall into either category 2 or 3, forming self-trapped excitons or permanent defects or both.

504
We have seen that the alkali halides exhibit efficient ESD and PSD through the Pooley-Hersh

defect-induced desorption mechanism. Materials in classes II and III of the Itch taxonomy — which

include virtually all interesting SWL optical materials — are particulary vulnerable to optical

damage because as a class, they can participate in a variety of similar processes. In theory,

materials in class 1 which forms neither self-trapped excitons, nor permanent defects, should be

relatively invulnerable to electronical stimulated desorption and erosion of the type we have

been considering. However, there is evidence [l8] that the compound semiconductors are in fact

etched or eroded to a significant degree by electrons and photons or by reactive ions, particularly

in the presence of ultraviolet light. This probably means that the Itoh classifications scheme,

while helpful, is not yet sufficiently differentiated to account for all the factors which

contribute to optical damage. For example, since the Itoh scheme is based on bulk properties of

materials, it fails to account for surface states, which are increasing being implicated in

optical damage processes [l8]. Therefore, while we shall consider the Itoh taxonomy as a guide

to the general phenomenology of electronically-induced optical damage, it is only that at present.

In particular, it has little predictive power for extracting rate constants.

4.2. The Protective Overlayer

In early electron-stimulated-desorption experiments on alkali halides, it was found that

samples prepared in air and then inserted into the ultra-high vacuum system only exhibited

neutral alkali desorption after lengthy irradiation by the electron beam, even when the target

was heated to very high temperatures in an ultra-high vacuum environment. Even sputtering by

modest doses of noble gas ions failed to produce a virgin surface yielding neutral alkali atoms

under electron stimulated desorption. It was not until detailed spectral scans throughout the

visible region of the spectrum were taken that it became clear that desorption was in fact

occurring, but not desorption of the alkali halides until some significant radiation damage had

been done to the surface.

NaD

e-ON

NaCI Ha

^^-^J-^-X^^^ OH

1 1 1 1 T"-^

e-ON NaF Ha

NaO

lOH

1

1 700 600 500 400 300 WAVELENGTH (nm)

200

Figure 9 shows a pair of pictures showing

electron stimulated desorption on sodium

fluoride and sodium chloride. In this experiment

the electron beam energy was held fixed while

the visible spectrometer (Figure 3) was scanned

through the range of wavelengths indicated on

the horizontal axis. The peaks in the desorption

spectrum were identified from the atomic lines

of various elements. In addition to the

sodium line, there appears a virtually complete

hydrogen spectrum (Balmer series), as well as lines from OH radicals and from a quasi-molecular

structure whose exact nature is still under

investigation [l9]. The crucial item from the

standpoint of optical damage is that, in the case of sodium fluoride, the relative yield of

sodium is smaller than that of the hydrogen

and other contaminant peaks, while in the case

of sodium chloride, the sodium yield exceeds

that of the contaminant species.

Fig. 9. ESD of Na from NaF and NaCl in a hydrogen atmosphere. The electron

energy was approximately 500 eV.

This implies that the surface overlayer and the binding of the overlayer to the substrate

determine the rate at which substrate material will be ejected by electron irradiation. Similar

results have been achieved in preliminary experiments in photon stimulated desorption at the

Synchrotron Radiation Center of the University of Wisconsin. One of the tantalizing questions

for further research is the extent to which this surface overlayer can be deliberately deposited

on an optical surface in such a way that it does not affect the optical properties of the surface,

but does influence the rate at which substrate material can be eroded through electron or photon

bombardment

.

505
4,3. The Transition from Electronically to Thermally Induced Optical Damage

Using the DIET mechanisms in alkali halides as a model, and considering the effect of the

surface overlayer, we now present a model of damage to short wavelength optical materials which

takes account of electronically-induced changes in the structure and the composition of the

material surface, and of the thermal effects which arise for laser intensities well above the

plasma formation limit. In this model, we assume that a Pooley-Hersh type mechanism is operative

in a substrate material of Type III, and assume that there is a protective overlayer which

erodes at a rate significantly lower than the substrate material. The general scheme for this

model is shown in Figures 10a, b, and c.

In Figure 10a, short wavelength laser or other irradiation is incident on an optical element

with a protective overlayer and a substrate material forming both self-trapped excitons and

permanent electronic defects. In Type III materials, defects produced in the substrate bulk are "gettered" to the interface between the overlayer and the substrate, forming a metallic

interlayer. In the early phases of irradiation this metal-rich layer has no effect other than

to present a thermal absorption site slightly below the surface of the optical element. It should be noted however, that in multilayer dielectric films, especially where the materials contain

metals, it is likely that the existence of these interlayers may lead to serious thermal consequences

in the catastrophic phase of laser damage.

In Figure 10b the irradiation has gone on sufficiently long or at sufficient intensities that

the protective overlayer has begun to erode, exposing to subsequent irradiation the metal rich

interlayer created during the precursor phase. Efficient creation of additional defects in the near surface bulk, and diffusion-dominated migration to the breach in the protective overlayer,

begin to occur. It should be noted that the precise location of the breaches or gaps in the

protective overlayer will be determined by a number of random events, including, but not limited

to: (1) the statistical migration of defects formed in the surface overlayer creating vacancies

and under coordinated lattice in the overlayer; (2) surface rearrangements and reconstruction

induced by desorption of materials at the surface of the overlayer and (3) the migration of

metals from the interlayer into the surface overlayer, producing defects and under-coordinated

sites in the lattice structure of the overlayer. Such processes can occur both in crystalline

and in amorphous materials; in amorphous materials, the irregularities in the local electron

densities simply means that it becomes more difficult to predict the sites at which defects or vacancies will eventually or temporarily come to rest.

o©o®o^o©

o96§o969

@o©o©o©o

©0©OfA>0

hv, e- hv,eOsO<>©0©

696?6969

©o©o©o©o

Fig. 10. Phases of electronically induced laser damage. From left to right:

overlayer. (b) Overlayer erosion, (c) Catastrophic damage.

(a) Crystal with

By far the richest phenomenology and the most damaging events occur with the arrival of a laser pulse on a surface which has gone through the evolutionary phases indicated in Figures 10(a)

and 10(b). When the density of metallic atoms is sufficiently high that small agglomerations of

metal have actually formed in the breached areas of the overlayer, it is possible for an intense

laser pulse to create a metallic plasma, since clusters of even only a f ew tens of atoms already

have essentially metallic properties [20 ]. The electrons accelerated in the laser field can reach energies sufficient for electron-stimulated-desorption with near unit efficiencies, ejecting

additional atoms or molecules from the optical material. The ions accelerated in the plasma by

the laser field are even more damaging since, at relatively low energies, they have sputtering

506
efficiencies greater than unity and produce significant desorption, erosion and cratering of the

surface. The acceleration of ions and electrons also produces ultraviolet radiation which can cause photon stimulated desorption. And, finally, the absorption of light in the metal-rich

overlayer will also produce thermal cratering and damage typical of the phenomenology already

well-known at infrared laser wavelengths.

Thus beginning with a picture of individual desorption events initiated by the deposition of

electronic energy, we have come full circle to the situation where catastrophic damage is produced

collective phenomena, such as plasma formation and thermal absorption at the surface. It is

important to understand, however, that while the ultimate damage effects may be thermal in

nature, and could perhaps at this stage even be described by a thermal model, the correct description

of the microscopic physics (including the determination of the critical rate constants) hinges

on understanding electroni cally stimulated processes induced by short wavelength laser photons,

and by hard ultraviolet photons and energetic electrons from the laser pumping process. Moreover,

while we have not mentioned surface chemistry explicitly, we should not overlook the fact that

reactive ions in laser plasmas, together with metastable neutral species and superthermal atoms from the laser plasma, will also work destructive mischief at the surface.

5. Conclusions

We have argued that DIET processes are likely to be the primary cause of damage to optical

materials at short laser wavelengths, say, below 400 nm. The evidence for this point of view rests on a growing body of experimental data from which we are rapidly building a detailed

atomic-scale picture of the dynamics of desorption processes. While the materials studied (the

alkali halides) are not the specific choices -in most SWL's, they are representative of a generic

class of wide band-gap insulators which support the creation of self-trapped excitons and relaxation

to permanent defects. Because so many SWL optical materials fall into this class, the predictions

of our damage model should generally hold true, although the desorption rates (or efficiencies)

may well be different in, say, the alkal ine- earth halides or metal oxides than in the alkali

halides we have studied.

The key steps in the damage process, based on the experimental evidence, are:

(1) the formation of defects on the surface and in the near-surface bulk;

(2) changes in surface and near-surface composition (formation of a metal rich overlayer)

due to the thermally-driven migration of these defects; and

(3) the occurrence of catastrophic plasma formation leading to self-reinforcing desorption

and sputtering processes.

While we have concentrated on processes occurring on the near-surface bulk induced by relatively

high-energy electrons and photons, there is also evidence implicating long-lived surface states

in the desorption of excited neutral atoms. Should the photon-energy thresholds for reaching

these states prove to be sufficiently low, it would make possible the triggering of DIET processes

by low-energy laser photons in efficient, single-photon excitations.

Acknowledgement

The Vanderbilt contribution to this work was partially supported by a research contract from the Los Alamos National Laboratory. The experiment on the KrF LAM was made possible by the

diligence and technical skill of the Aurora laser system operating team.

References

[l ] Lowdermilk, W.H. ; Milam, D. "Laser-Induced Surface and Coating Damage," J. Quantum Electronics

QE-17 (9); 1888-1902; 1981 September.

[2] Bloembergen, N. "Role of Cracks, Pores, and Absorbing Inclusions on Laser Induced Damage

Threshold at Surfaces of Transparent Dielectrics," Appl. Opt. ]2 (4); 661-664; 1973 April.

See also Bloembergen, N. "Laser-Induced Electric Breakdown in Solids," J. Quantum Electronics

QE-10 (3) ; 1974 March.

[3] Schmid, A.; Braunlich, P.; Rol , P.K. "Multiphoton-Induced Directional Emission of Halogen

Atoms from Alkali Halides," Fhys. Rev. Lett. 35; 1382-1385; 1975 November 17. BrSunlich,

P.- Schmid, A.; Kelly, Paul. "Contributions of Multiphoton Absorption to Laser-Induced

Intrinsic Damage in NaCl," Appl. Phys. Lett. 26 (4); 150-153; 1975 February 15.

507
[4] Haglund, Richard; Tolk, Norman. "Stimulated Desorption as a Potential Damage Mechanism in

Ultraviolet Laser Optical Components," in Desorption Induced by Electronic TransitionsDIET II , Brenig, W. ; Menzel , D.,eds. Heidelberg, Federal Republic of Germany: Springer;

1985. 277-280.

[5] Rothenberg, J.E.; Kelly, Roger. "Laser Sputtering. Part II. The Mechanism of the Sputtering

of AI2O3," Nucl. Instrum. Methods in Phys. Research B1_, (2,3); 291-300; 1984 February.

[6] Rososcha, L.A.; Bowling, P.S.; Burrows, M.D.; Kang, M.; Hanlon, J. A.; McLeod, J.; York,

G.W., Jr. "An Overview of Aurora: A Multikilojoule KrF Laser System for Inertial Confinement

Fusion," Lasers and Part. Beams 4_; 55-70; 1986 February.

[7] Chuang, T.J. "Laser-Induced Gas-Surface Interactions," Surf. Sci . Rep. 3; 1-105; 1983.

[8] Menzel, D.; Gomer, R. "Desorption from Metal Surfaces by Low-Energy Electrons,"

J. Chem. Phys. 4l_ (11); 331 1-3328; 1964 December 1, Redhead, P. A. "Interaction of Slow

Electrons with Chemisorbed Oxygen," Can. J. Phys. 42; 886-905; 1964.

[9] Knotek, M.L.; Feibelman, P.J. "Ion Desorption by Core-Hole Auger Decay,"

Phys. Rev. Lett. 40; 964-967; 1978.

[10] Knotek, M.L. "Stimulated Desoption from Surfaces," Phys. Today 37 (9); 24-32; 1984 September.

[11] Czyzewski, J.J.; Madey, T.E.; Yates, J.M. "Angular Distributions of Electron-StimulatedDesoption Ions: Oxygen on W(100), "Phys. Rev. Lett, 32 (14); 777-780; 1974 April 8.

[12] Tolk, N.H.; Feldman, L.C.; Kraus , J.S.; Morris, R.J.; Traum, M.M.; Tully, J.C. "Optical

Radiation from Electron-Stimulated Desorption of Excited Particles," Phys. Rev.

Lett. 46 (2); 134-137; 1981 January 12.

[13] Tolk, N.H.; Haglund, R.F., Jr.; Mendenhall, M.H. ; Taglauer, E. ; Stoffel, N.G. "Electronically

Induced Desorption of Neutral Atoms Observed by Optical Techniques," in Desorption Induced

by Electronic Transitions: DIET II ; 152-159.

[14] Tolk, N.H.; Traum, M.M.; Kraus, J.S.; Pian, T.R.; Collins, W.E. ; Stoffel , N.G. ; Margaritondo,

G. "Optical Radiation from Photon-Stimulated Desorption of Excited Atoms, "Phys. Rev. Lett. 49

(11); 812-815; 1982 September 13-

[15] Stoffel, N.G.; Riedel, R. ; Colavita, E. ; Margaritondo, G.; Haglund, R.F., Jr.; Taglauer,

E. ; Tolk, N.H. "Photon-Stimulated Desorption of Neutral Sodium from Alkali Halides Observed

by Laser-Induced Fluorescence," Phys. Rev. B 32: (10); 6805-6808; 1985 November 15.

[16] Pooley, D. "[110] Anion Replacement Sequences in Alkali Halides and Their Relation to F- Centre Production by Electron-Hole Recombination," Proc. Phys. Soc. 87_; 257-262; 1966. Hersh,

H. N. "Proposed Excitonic Mechanism of Color-Center Formation," Phys. Rev. 2ii8 (2); 928-932;

1966 August 12.

[17] Itoh, N. ; Nakayama, T. "Electronic Excitation Mechanism of Sputtering and Track Formation

by Energetic Ions in the Electronic Stopping Regime," Nucl. Instrum. Methods in Phys.

Research B1^: 550-555; 1986.

18] Nakayama, T. ; Itoh, N. "Non-Thermal Laser-Induced Desorption of Compound Semicounductors,"

in Desorption Induced by Electronic Transitions: DIET II ; 237-244.

[19] Cherry, D. ; Mendenhall, M. ; Albridge, R. ; Cole, R; Haglund, R. ; Heiland, W.; Hudson, L.

;

Peatman, W. ; Pois, H. ; Savundararaj , P.; Shea, M. ; Tellinghuisen, J.; Tolk, N. ; Ye, J. "Broadband Luminescence from Particle Bombardment of Alkali Halides," Nucl. Instrum. Methods

in Phys. Research B1^: 533-536; 1986.

[20] Wimmer, E. "All-Electron Local Density Functional Study of Metallic Monolayers: I. Alkali

Metals," J, Phys. F: Met. Phys. 1_3; 231 3-2321; 1983.

508
Manuscript Received

6^24^86

Temporal Distribution of Electron and Ion Emission Caused by

Laser Excitation of Optical Surfaces in Ultra-high Vacuum*

W, J. Siekhaus, L. L. Chase and D. Milam

Lawrence Livermore National Laboratory

Livermore, California 94550

We have measured electron and ion emission under high vacuum conditions from several

semiconductor and insulator surfaces excited by the first and third harmonics of a 1.06 urn Nd laser. The dependence of this emission on laser fluence and wavelength is

consistent with a multiphoton excitation process. The dependence on laser pulse length

implies, hov/ever, that other processes, either diffusive, thermal or thermomecbanical

,

are also involved. Additional information is provided by the temporal and spatial

distribution of the charge emission. The most extensive results have been obtained with

single crystals of ZnS, for which nearly equal magnitudes of negative and positive charge

emission are observed. The results suggest the possibility that the observed charge

emission results primarily from the ejection of neutral atoms and molecules from the

surface, followed by selective multiphoton ionization of one or more of the neutral

species. The irreversible surface modification implied by the observed emission may be

the precursor to observable surface damage. Detailed studies of the properties of the

emitted particles may be very useful in developing a basic understanding of the damage

mechani sms.

Key words: damage precursors; electron and ion emission, multiphoton ionization.

We have measured electron and ion emission resulting from laser excitation of polished sur- faces of several amorphous and crystalline optical materials. The objective of this research is to identify the basic mechanisms of optical damage at photon energies much less than the optical

bandgap and to identify surface regions that are most susceptible to damage so that these regions

can be characterized by surface analytical techniques. The emission of electrons, ions and

neutrals resulting from surface bombardment by ions and UV photons has been an active field of

research for over two decades [1]. There has, however, been little work of this type involving

surface excitation by photons in the transparency region, where the mechanisms for electronic

excitation are not generally known. The significance of the information that can be obtained

through studies of surface emission is well illustrated by work on alkali helide surfaces

reported a decade ago by Schmid, Braunlich and Pol [2], who observed a very efficient surface

ablation process caused by multiphoton excitation of trapped excitons by a ruby laser beam at intensities far below the single-shot damage threshold. Those results suggest that surface

emission processes may not only contribute to an understanding of surface damage mechanisms, but

they may actually be a common initiating factor in optical surface damage. It is therefore

important to investigate laser-induced surface emission for a wide range of optical materials.

In order to study intrinsic particle emission processes on clean surfaces, it is necessary to work in an ultra-high-vacuum environment. The present work was performed in a UHV chamber at a residual pressure of 10~° to 10-^0 torr. The apparatus and experimental procedure have been

discussed previously and will not be described here in detail [3]. The polished sample surfaces

v/ere cleaned in the evacuated chamber by several laser shots at fluences well below the damage

threshold. Large quantitites of emitted charge were observed following the first one or two shots

due to desorption of contaminants from the surfaces. Subsequent shots gave fairly reproducible

charge emission as a function of peak laser fluence. The peak fluence in the 3 mm diameter

irradiated area was measured with a vidicon and calorimeter [4]. Laser pulses with nearly rect- angular temporal orofiles and variable widths from 1 ns to 50 ns were employed. Data were obtained for several materials at both the fundamental and third harmonic of the Nd laser. A two-stage microchannel electron multiplier was used with a fast oscilloscope to record the

temporal dependence of the electron and ion emission with a subnanosecond time resolution.

*Work performed under the auspices of the Division of Materials Sciences of the Office of Basic

Energy Sciences, U. S. Department of Energy and the Lawrence Livermore National Laboratory under

Contract Mo. W-7405-Eng-48.

509
The total emitted negative charge as a function of peak laser fluence for several of the

materials investigated is shown in figure 1 for 1.06 um and 0.355 um laser wavelengths. These

data fit power law dependences on the fluence, with power law exponents approximately equal to the order of the multiphoton absorption required to excite electrons across the bandgap of the

materials [3]. Although it may be possible to fit the data with other dependences on the

fluence, such as an exponentially activated thermal process with a temperature proportional to

the fluence [5], it would be difficult to account for the change in slope of the data with laser

wavelength with such a model. One possible mechanism that could account for these results is

photoelectric emission resulting from multiphoton production of electron-hole pairs, followed by

further excitation of the electrons up to the vacuum level, after which they may leave the sample

surface. If that is the case, the total charge emission would be expected to depend on the laser

fluence F and temporal pulsewidth tp as

0 = KF^/t^-l (1

)

Then, the ratio of fluences required to produce a given charge emission Q for two pulse

durations, tp] and tp2, is given by

F(Q.tpT)

'p2

1-1 /N

(2)

Data for CdTe, 7nS and MaCl at several pulse durations is shown in figure 2. The logarithm of the

ratio in eq (2) is given by the horizontal displacements of the lines for the different pulse

lengths. For all three materials this ratio scales approximately as tl^^. This agrees with

eq (2) only for CdS, where N=2. For ZnS (N=4-5) and NaCl (M=7) the behavior expected for the

multiphoton photoemission model is clearly not observed [3].

A second indication that a more complex process than photoemission is occurring is that for

at least two of the materials investigated, Si02 and ZnS, comparable amounts of positive and

510
negative charge emission were observed. We have investigated the nature of these two emission

components for ZnS by measuring the temporal dependence of the charge emission, A two-stage

microchannel electron multiplier was placed about 10 cm from the sample in a direction at right

angles to the laser beam, so that emission from both the front and back surfaces was detected.

Different time delays for detection of the emission from the two surfaces could be obtained by

rotating the sample in order to make the path lengths to the detector unequal. The front surface

of the detector was biased to potentials of +700 volts or -1800 volts in order to detect the

negative and positive emission, respectively.

The negative emission from ZnS is shown in figure 3 for laser pulse lengths of 1 ns and 50 ns. The time delay and duration of this emission are too short for anything but electrons to reach the

detector. Measurements on a much longer time scale, shown in the upper part of Figure 4, indicate

that there is no identifiable negative ion component, although there is a weak, diffuse emission

persisting for several microseconds following the laser pulse. The rise time of the laser pulses

is comparable to the 0.5 ns time resolution of the detection system. The response to the 1 ns pulse represents the "impulse response" of the electron emission, which consists of a rapid risetime of several nanoseconds, followed by a decay time of about 30 ns. This short-pulse response

possibly involves geometric time-of-fl ight factors in addition to any temporal dependence of the

emission process itself. We can therefore conclude from these results only that the electron

emission occurs within about 30 ns of the laser excitation. If only transit time effects cause

the temporal spread of this impulse response, the response to a much longer pulse should be well

approximated by a risetime equal to the decay time for the short pulse. The data for a 50 ns pulse are close to these expectations, except for a delay of about 5 ns after the leading edge of

the optical pulse, during which the charge emission gradually develops. This is followed by a period, lasting for the duration of the optical pulse, during which the charge emission rises in

the expected manner. The initial delay period, which could be caused by cumulative effects, such

as heating, may be the cause of the unexpected dependence of the integrated charge emission on the square root of the pulse length.

511
ZnS electron emission

(bl t„ = 50 nsec

40 80 100 Time (nanoseconds!

Figure 3. Time dependence of electron emissionfrom a 7nS crystal surface for two laser

pulse widths. a)tp = 1 ns, F = 0.76 J/cm^; b) tp = 50 ns, F = 1.1 J/cm^

Further evidence of the nature of the emission process is provided by the time-of-fl ight data

obtained for emitted positive ions. Typical data for ZnS are shown in figure 4. The ion mass associated with a particular transit time from sample to detector can be obtained from the

calibration based on the time delay of the sharp leading edge of the electron emission. The two strongest peaks in figure 4 result from emission of a single mass species from the front and back

surfaces of the sample, which was rotated so that the back surface emission had a shorter path to the detector. Assuming a singly charged species, its molecular or atomic weight is about 100.

This is approximately the mass of either (ZnS)''' or (S3)''', which would be the only reasonably

expected products with this mass from a ZnS surface. It should be noted that the only other peaks

in figure 4 are two very weak signals from front and back surfaces at a mass corresponding to some hydrocarbon desorption from the surface.

The absence of any peaks in the emission spectra corresponding to ionized Zn or S is notable.

Two possible explanations for this behavior are apparent. First, some electronic desorption

process may be operative that is rather specific to the release of one of these ionized molecules

after the production of initial electrons and holes by multiphoton absorption. It is not clear,

however, how to account for the dependence of the emission on laser pulsewidth with such a mechanism. A more plausible hypothesis is that neutral atoms and molecules are emitted from the

surface, possibly due to heating by the laser beam, and either ZnS or Sp is preferentially

photoionized by the laser after ejection from the surface. It is, in fact, well established for

a number of insulating materials that neutral atom and molecule emission is far more probable than

ion emission for surface bombardment by ions or UV photons [1]. This mechanism would also account

for the observation that only positive ions and electrons are observed.

The evaporation of surface material caused by heating is a possible cause of the observed

photoemission. In that case, the composition of the emitted vapor would consist primarily of

atomic Zn and S, with ZnS and S3 present at the one percent level or lower if the available

vapor composition data obtained from Knudsen Cell measurements [6] adequately represents the

situation of surface heating or melting of the crystalline solid. Since the ionization potentials

of Zn and S are on the order of 10 eV, photoionization of this vapor may well favor the production

of the molecular ions by a large margin. One problem with a thermal emission model is that a much

more complicated dependence of the total emitted charge on the excitation fluence would be

expected since the emission from the surface should follow an Arrhenius dependence [5] on the

512
ZnS — Negative emission k

J \ 1 I I I 0 4 8 10 12 14

ZnS — Positive emission

(ZnS)* or (Sj)*

I

0 4 8 10 12 14 Time (microseconds)

Figure 4. Negative and positive charge emission from ZnS following a 1 ns laser pulse

of wavelength 1.06 ym.

surface temperature, which is determined by the laser fluence and pulse duration. As we pointed

out earlier, such a behavior is difficult to reconcile with the dependence of the slope of the curves in figure 1 on the laser wavelength. Measurements are planned incorporating an ionizing

mass spectrometer in place of the microchannel detector in order to measure the relative abundance

of neutral Zn and S atoms and the molecular products. The dependence of this neutral emission on excitation fluence may be of great utility in assessing the roles of thermal evaporation and of

multiphoton absorption processes in the solid and in the emitted vapor.

Absorption of laser energy by the plasma resulting from the electron and ion emission is a possible source of energy deposition near the sample surfaces that could lead to optical damage.

We have established that, for ZnS, the electron emission occurs within 30 ns of a 1 ns laser

pulse. Since the total emitted charge is also knov.'n from the data of figure 1, the plasma density

near the sample surface can be estimated if the mean velocity and spatial distribution of the

emission over the sample surface are known. We have made such estimates assuming a reasonable

thermal ion velocity to estimate the plasma expansion rate and find that plasma densities on the

order of 10^^ cm"-^ or greater, which would be required for significant surface absorption,

can only result if the charge emission that we are measuring is localized in regions much smaller

than the 3 mm diameter laser spot employed in this investigation. It is therefore important to determine the spatial distribution of the charge emission from optically excited surfaces. We are developing techniques to accomplish this.

In conclusion, these results demonstrate that microscopic damage in the form of surface ablation is occurring at fluences well below the damage threshold for ZnS. This may also be true of

the other materials for which we have measured electron and ion emission. [3] The charged and

neutral particle emission would appear to be a sensitive way to locate damage susceptible surface

regions and to study the mechanisms causing the emission, which may be the initial stage of the

processes leading to observable damage.

References

[1] See, for instance T. E. Madey, J. T. Yates, Jr., J. Vac. Sci. Technol. 8, 525 (1971);

D. Menzel, Surf. Sci. 47, 370 (1975).

513
[2] A Schmid P Braunlicb and P. K. Rol . in Laser Induced Damage in Optical Materials- 1975

Itu/r l'J-n T\''''^ i-

^' G"^"ther, National Bureau of Standards Vcial Pub ication

' Number 435, (U.S. Department of Commerce, 1976), p. 366.

ruuncazion

[3] W. J. Siekhaus, J. H. Kinney, D. Milam, and L. L. Chase, Appl . Phys. A39, 163 (1986).

[4] W. L. Smith, A. J. DeGroot and M. J. Weber, Appl. Opt. 17, 3938 (1978).

[5] Y. K. Jhee, M. F. Becker, and R. M. Walser, J. Opt. Soc. Am. B_2, 1626 (1985).

[6] G.^DeMaria, P. Goldfinger. L. Malaspina and V. Piacente, Trans. Far. Soc. (GB) 6^, 2146

DISCLAIMER

This document was prepared as an account of work sponsored by an agency of

the I nited States (iovernment. Neither the L nited States Government nor the

I'niversity of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the ac- curacy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned

rights. Reference herein to any specific commercial products, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily

constitute or imply its endorsement, recommendation, or favoring by the L'nited States (lovernmenl or the I 'niversity of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the I nited

-Slates Government thereof, and shall not be used for advertising or product en- dorsement purposes.

514
Manuscript veaeived

1-15-86 SAR

Anomalous Absorption in Optical Coatings

M. R. Lange and J. K. Mclver

University of New Mexico

Albuquerque, m, 87131 , USA

A. H. Guenther

Air Force Weapons Laboratory

Kirtland AFB, NM, 87117, USA

The observation of isolated damage sites in optical thin films sub3ected to pulsed

laser irradiation has lead to a model based upon localized absorbing regions. This

"inclusion initiated" model does not adequately treat several details of the radiation

absorption process, but relies principally on thermal conduction as the sole energy

transport process. Despite these rather severe shortcomings, qualitative predictions of

this theory agree remarkably well with much of the quality experimental data. On the

other hand, quantitative agreement can only be achieved if absorption coefficients sev- eral orders of magnitude higher than those measured for todays thin films are employed,

which thanselves are already very high compared to the materials value in bulk form.

In order to explain this apparent discrepancy a detailed theory of the spatial and

taniporal evolution of the highly absorbing regions must be conceived. In this paper we present our initial steps in this direction. The absorption/thermal diffusion process

that leads to damage is modeled by two equations; the evolution equation for the free

carrier density and the thermal diffusion equation. In practice these equations are strongly coupled. We decouple the free carrier equation by assuming that all rate con- stants are independent of the lattice temperature. The solutions of these simplified

equations are presented along with their sensitivity to the magnitude of several of the

unknown rate constants. Those regions of parameter space in which the original thermal

diffusion model is valid are pointed out, as well as the potential implications for more

appropriate modeling of the absorption process as functions of experimental and material

variables.

Key Words: Anomalous Absorption; Avalanche Ionization; Coatings; Color Centers; Electron

Diffusion; Laser Damage; Thermal Diffusion.

1. Introduction

In work set forth to date this paper's authors have stated, with significant morphological

evidence [Ij , their concepts of a localized absorbing region within an optical thin film. These

localized absorbing regions are assumed to be due to anomalies of some sort. Heat transfer

equations were solved indicating trends in damage thresholds as a function of material and

experimental parameters. Thermal diffusion was assumed as the dominant controlling mechanism in

the damage process and experimental data has correlated quite well with this model [2] . The

thermal diffusion equations incorporate a nonhomogeneous thermal source, which is due to a localized electron density interacting with both the incident field and the lattice structure.

In actuality, there are two coupled nonhomogeneous diffusion processes involved. They are thermal diffusion due to the aforementioned thermal source, and electron diffusion due to a localized electron source. The electron source properties depend, in the main, upon the local

optical field and local temperature. These coupled equations are generally untractable and as such are usually decoupled through assumptions which can apply in certain limits.

Assumptions that have been used in previous works are that the operative coefficients and the source for thermal diffusion are independent of temperature, and the electron density has been

assumed constant. That is, the electron density is modeled as a step function in time and space

such that it evolves instantaneously and does not diffuse. These assumptions are obviously not valid in all cases of interest. Certain refractory oxide film damage [1] , for example, exhibits

morphology, which grows spatially during irradiation to a size comparable to the incident beam

diameter

.

515
In addition to the short comings of these previously enployed assumptions, the use of an instantaneous electron density must be justified. Thus, mechanisms for the evolution of an electron density had to be investigated.

2 . Theory

In an attempt to refine this model in the direction of more comprehensive theory, the

concentration of effort is placed upon the evolution of the localized electron density and the

absorption processes involved. The approach taken to this end is to write down the classical

equation of electron diffusion and solve it for a generalized electron source. Conservation of

electromagnetic and thermal energy is then onployed with the principal purpose of ascertaining

useful scaling laws. The comparison of these scaling laws with experimental data implies certain

preferred absorption mechanisms. These mechanisms are then further investigated via the

appropriate expressions governing electron diffusion.

The principal findings are that; i) local metallic concentrations supplemented by color

centers may be a viable mechanism for the initial electron production in certain optical thin

films and ii) very small regions of these anomalies (^200 K) can possibly lead to damage when

supplanented by an avalanche ionization. Avalanche ionization is only assumed to occur in optical

coatings and serves to expand the region that is absorbing at the anomalously high rate.

The classical equation of electron diffusion is given as:

dn

dt (rD n) e

+ g(n,T,lE|) - (n - n.)Y (n,T) (1)

where = electron diffusivity (cm /S) , |E| = electric field strength (V/cm) , g = electron

carrier generation (an^S -^), n = electron carrier density {cm ^ ), n^ _= initial carrier density

(an 3 ), T = temperature ( K) , and Y (n,T) = free carrier decay rate (s ) .

The assumptions made in solution of the above equation are that Dg is independent of n and T (which is not true in general) , and g is independent of T. The dependence of g on n and E are

incorporated iteratively and the other assumptions will be addressed at a later time.

The solution of the above equation in all space is found via integral transform [2], n^ is

absorbed in n so that n becones n = n - n^. The general solution is found to be

n(r,t) =— I dg sin(3r)e-(De6^+Y)t dt' r'dr' sin(6r' )g(r' ,t')

e

(d3 +Y)t'

(2)

In order to decide upon a reasonable choice for the electron source generation term g(r,t)

,

attention is diverted to a consideration of the conservation of energy. The morphology of damage

suggests a localized source of electron generation, however, it does not imply a mechanism.

Scaling laws derived from the conservation of energy consideration may, though.

The Poynting theorem may be written as

Where w is the field energy, S is the Poynting vector, J is the electron current flux, P is the

material polarization and M is the magnetization vector. If the field is in approximate steady

state with the absorption process and magnetic transitions are neglected, then

516
The J • E term. contains the energy stored in electron acceleration along with electron-phonon

collisions and the t t term contains the energy stored in polarization along with electronic

transitions. The S t term is the dominant term in avalanche- ionization and inverse

brehmsstrahlung processes while the ^ • E term is assumed dominant in single and multiphoton

transitions. All of these ar^ possible mechanisms of absorption. In order to avoid restriction

to any certain mechanism the V- 2 term is used to represent absorption.

For simplicity an incident plane wave is assumed, which gives

In -aZ

In Vo

r -oz

e C5)

where I, n^ and v are the incident field intensity, real index of refraction and magnetic

permeability. a is the generalized absorption coefficient and a is the generalized absorption

cross-section with V the volume that is absorbing.

From the first law of thermodynamics the energy balance of an absorbing region is

V- (- V(kT)) + (Pc^T) = - V - t , (6)

where k = thermal conductivity, c^ = specific heat at constant volume, and p = mass density, which

leads to

i^(J/an ) Vn ^

{4- (PC T) -

a ot V

V • (V (kT) ) } (7)

where Ep, is the energy density of the incident pulse at which the region reaches a critical

temperature which causes damage.

The energy depletion from the incident field occurs over a region represented by the length

which includes absorption by electrons that immediately thermalize along with those that

thermalize after electron diffusion of length 6^. As pointed out by Meyer et al. [3], the thermal

diffusion may be incorporated in the same manner. That is, the energy that is deposited in the

lattice is deposited in a region resulting from absorption and thermal ization of electrons (both

with and without diffusion) along with the thermal diffusion 6^ that occurs after deposition

during the pulse. The thermal diffusion length is given by 6

= Dt . The two parallel processes

may be added to give

eff

X 1

la + 6

- _ +

X 2

la.

"'" + 6 + e d J

-1

(8)

Here is the fraction of laser energy deposited directly into the Lattice and X2 is the

fraction of energy that diffuses before being deposited.

517
Thus we can write

r J 0

(9)

The importance of this procedure is the demonstration of useful scaling laws, fc'or example,

when > 6^/ a"''"^

)C kt

V p

X 1 -1

1

6 T

X 2 -1

1 + £5— +

6 T 6 T

(10)

and when { , 6,„ <°' e T -1

dTpC^a X 1 X 2

I"! T

] P "^";'5e "^a 6t]

-1

(11)

The first case exhibits the same scaling law derived for a spherical absorbing region where

thermal diffusion dominates [4] . Here, it is much more general in that no specific geonetry has

been assumed and temperature variation of the constants is incorporated.

The second case demonstrates an absorption dominated process and implies that Ep °^ a ^. Thus

the wavelength dependence of a( X) should exhibit itself in the damage threshold data roughly as

'D In addition to the exhibition of wave length dependence in certain cases, an important point

of the above scalings is the demonstration of the perspective which they lend to the all important

energy transport process. The scales over over which energy transformation and transport occur

are solely responsible for whether the damage is absorption or diffusion dominated.

The wavelength dependence for damage from reference [IJ may be found by averaging over films

( X and X/2) for five and fifteen nanoseconds. When this is done it is found that between

X= 0.26 and 0.53 yM the fluorides damage asx'^ " while oxides damage as X^ ' '^^and between

X= 0.53 pM and 1.06 pM fluorides damages as X

0 24 and oxides X There is significant

statistical deviation from film to film (i.e. for the various coating materials)

In all of the theories investigated, this increasing power of frequency in the absorption

functon with increasing photon energy was only noted in photo- ionization processes. However, the

films and photon energies investigated LI] should not be near band edge transitions. It should be

noted that a large density of impurity states can exhibit this same behavior. In addition, the

transition between different, but related, processes of absorption at different frequencies is a possibility (i.e., different sorts of impurities). These may occur in the same, or even separate

discrete regions.

The a is constituted as a n n «n "^"FC where

absorption and ap^ is the absorption coefficient for free carriers

that n-photon absorption produces free carriers.

is the absorption coefficient for n-photon

n «n is related to a„„ in

rL.

Although these processes are generally considered intrinsic they also may apply to the

obviously extrinsic damage processes which occur in optical coatings due to localized shallow or impurity states in the coating. These shallow impurity states support photo-ionization and impact

518
ionization at threshold levels far below those expected of a pure material. These states may

occur due to impurity atons, ions, non-stoichictnetric mixtures leading to metallic colloids, color

centers or simply because of local disorder. Shallow states due to disorder have been confirmed

for D.C. fields by conductivity measurements [5]

.

Thus, due to the above and the observed morphology it is assumed that there exists the

possibility of very small localized regions of impurities.

One obvious choice for an electron source function is a uniform spherical region, of

impurities or shallow states, of radius rg . The origin of such a region may be a coagulation of

centers resulting from dislocations, as discussed below.

This source term is given by

g(r',t') = f(t'){0(r') -q (r'-r^)} (12)

Where for single and multiphoton transitions of shallow states

f(t') = hv 2h\ Q(t'

)

(13)

Here I is the incident intensity (ev/om^-s) , is the one photon coefficient (1/cm) , B is the two photon coefficient (cm-s/ev) , riQ is the quantum efficiency (1) , hv is the photon energy (ev)

,

and the Q 's are step functions. This integral (Eq. (12) into Lq. (2)) is very similar to one done

previously [2] and affords a solution of

n(r,t) =

D r e

- (D y + Y ) t vr e 0 dy sin — {siny - y cosy) (1 - e )

y2(y^ + r^Y/Dg)

(14)

For the region of parameter space of interest here, the above integral provides numerical

problsns in its evaluation. However the first term of the integral may be done analytically

giving a solution of

= .rD

dy sin(yr/ry) (siny - y cosy)e

-(D//r2 + Y)t

2, 2 ^ 2 , y (y + r^Y/D^)

(15)

where

A = (rDg7r/(2rQY) ) {1 - (exp[- (l+r/r^) c] + exp[- (l-r/r^) c]

)

- (r^/(2cr) ) (l+(l-r/g )c) exp [- (l-r/r^) c] - (1+ (l+r/r^) c) exp [- (l+r/r^) c]

}

(16)

2 2 and where c = r^Y/D^

519
This solution is numerically convergent, but only valid for the region of 0 < r/r^ < 1. A

numerical plot fran this solution is given in the next section.

In the far field (r » r^) the source may be described as

g(r-,f) . -(^)y-)V

^^^3

4Trr'

Where 6(r') is a delta function and V is the characteristic volume of absorption. This gives for

the electron density.

n(r,t) =

t 2 Vf(t') -r df ^

'- exp . , . exp(-Y(t-f)) . (ig)

0 8(7tD (t-t'))-^/ e^ ' e

f(t') is the same as before and and 3 are conputed from the stimulated transition rates which

can be found in terms of the oscillator strengths of the impurity states.

In alkali-halides the shallow impurity states may be provided by a distribution of metallic

colloids resulting from poor stoichiometry and various aggregates of color centers. Colloids may

be produced in the process of heating to 200 to 350 C [6,7] or even by insufficient heating during

film deposition. In addition, mild heating causes the coagulation of F-centers to colloidal

particles [6]. With specific reference to alkaline earth fluorides, "F-centres in

additively-colored alkaline earth fluoride crystals readily aggregate forming more complex

structures" [7]. An example of the sort of distribution that may occur is shown in figure 1.

Where, in optical coatings, the possible localized concentrations of absorption centers such

as these, come from is not being assumed here. Dislocation clusters and poor stoichiometry are quite likely in optical coatings. The assumption made is that the conditions of deposition and

processing of coatings allow this to be true.

As an illustrative example of one of the materials of interest [IJ , CaFo has an absorption

peak at 521 nm in what is called the g band predominantly due to M-centers. It should be pointed

out that in CaF^ the F-centers peak at 376 nm and certain colloids peak at 550 nm. In addition to these are R centers, Mp centers, M'*' centers and many other cotiplex combinations of centers [7] . The definitions of these may be found in references [6,7 and 8].

The oscillator strength may be assumed to be ~ 1 [9,10] and the transition rate for single

photon transitions is given as [11,12a]

I g(v)e^f

W.. = — , (19) ik mchv

where g(v) is the normalized lineshape, f^^^ is the oscillator strength and I is the laser

intensity. g(v) [11,12] can be expressed as follows

2 g(v) = exp[-4(ln2) \-] (20)

One of the wavelengths of interest is 530 nm [2] , which is slightly off center of the

transition line. Due to the large width of the lines [6, 7, 8, 12b, 13] , we find g(v) - 0.9/Av,

however to be conservative a line shape factor of 0.1/Av will be used.

520
COLLOID ABSORPTION BANDS M.Mp.M+. R ECT CENTERS

F CENTERS

ev

A POSSIBLE ABSORPTION STRUCTURE DUE TO COLLOIDS AND CENTERS.

SEE, FOR EXAMPLE, pg 259 AND pg 265 OF REFERENCE 6

Figure 1. A possible absorption distribution vs. photon energy

for a complex aggregate of absorption centers.

From this ccmputation it is found that W^^ = 1.4x10 s fron ground state to the first

excited state. This first excited state is long lived [9,10] and will easily ionize deeply into

the conduction band from here.

3. Results

Assuming an absorption center density of n = 10-^'' cm"^ (which may be conservative for a thin

film [13]) ^

JLl. . ^= 7x10^7 cm-^ s-^ ^21)

hv 2

Here a factor of 1/2 was included to account for the final ionization step, sane of which may

occur thermally. This factor may be placed into the quantum efficiency t]q = 1/2 and can be

considered conservative due to the ionization transition rate approximated frcxn equations in

references [11,14 and 15].

Several important points related to table 1 should be mentioned. The results that follow are based upon classical diffusion which should not apply where small numbers of electrons are

involved. If one multiplies the electron densities by the impurity region volume it is clear that

rather small numbers of electrons are indeed involved. There is no attempt made here to model

this situation rigorously. This aspect of the study is used to demonstrate relative orders of

magnitudes, and the feasibility of very small local anomaly regions producing large electron

densities in the conduction band.

521
Sane of the numerical results from the integration of eq. (14) are shown in Table 1.

a 1

6 HQ

D

ehi

hv

4.91 (cm •')

, (for n. 10''"^cm ^ , a = 2.1x10 "'"'')

Y

=

n(50A, 0.1ns)

A t

0.

1/2

0.01 (on^/s)

1.0 (oscillator strength)

O.l/Av (line shape factor)

2.34 ev

10"^ s . '

.

radius of region of impurities

conduction electron recombination rate (s ''")

2 . : . absorption cross section (cm ) probability of electron production within volume of interest

o electron density at 0.1ns and r = 50 A

time at which probability of electron production exceeds 1

Table 1. Electron densities achieved at 50 A within an absorbing region due to photoionization of absorption centers.

I GWJ

2^

L cm J rQ(cm) a (cm^) Y(S ^) p n(50A, 0.1ns) At

0.1 10-^ 2.1x10"-^^ 10^2 <1

10^^ <1

10^0 <1

1.0 10^2

10^1

<1

<1

10^0 >1

17 4.2x10 40 ps

2x10"^

-17

1.6x10 10^2 <1

10^1

10^0

>1

>1

6.5x10^^

17 4.2x10

10

8

ps

ps

10-^ 1.6x10"^^ 10^2 <1

10^1 >1

17 5.2x10 ' 6 ps

10^0 >1 3.4x10-^^ 4 ps

522
For example, the initial color center density for the table above is chosen to be 10^^ arf^ , however the final electron densities produced fran this exceed this number. This number is not

chosen to be realistic as much as it is chosen to be conservative. In a region where trap centers

are generated by polishing damage or seme other process, the trap centers would probably be orders

of magnitude greater than 10^^ cm~3 . Thus, lO^'' is chosen to provide a conservative transition

rate, though it is not allowed to limit the number of electrons which transition. Conduction

electrons are limited, however, by recombination, or decay. It may further be noted that if

10- on" 3 color centers existed in a region of 100 A in radius, at best one half of an available

electron would be in this region. Clearly its only purpose is to give conservative transition

rates. As such, the numbers in tables 1 and 2 should not be taken as accurate. They are presented to demonstrate relative orders of magnitudes of conduction electrons that could be

produced under the conditions of radiation, decay and diffusion. One primary point of table 1 is

the denonstration of the fact that the size of a region and the intensity of radiation dictate a threshold condition due to competing probabilities of transition and decay in the region. What

constitutes this region is not assumed here, other than tlie fact that it contains shallow centers.

It may be an inclusion at the base of a nodule [16] or at a grain boundary. It may be a dislocation cluster [17] . It should be noted here that colloidal particles deposit selectively at dislocations [6] . In addition, dislocations are the source of color centers.

This threshold is not, however the damage threshold. The important parameter in damage is the rate at which lattice energy builds in a local region. EVen if the electron transition

threshold is exceeded, the energy may be harmlessly conducted away by electron and phonon

transport. Table 1 also serves to danonstrate that if sufficient numbers of electron trap centers

(H-centers, in this case) are present, the transition is quick (<<t ) . The partitioning of such

regions where the volume may be smaller than the volume between trap centers is very artificial.

It is intended to generalize the table to weaker transitions at higher densities, or perhaps

colloidal transitions. This is why the cross section is listed in table 1. Whatever the source

of electrons, if the cross section is as given, the table should remain valid in the region of the

transitions.

Although the electron densities represented in these small regions are impressively high,

they could not alone transfer sufficient energy to the lattice to cause damage. Densities of this

sort are required over regions approaching 0.1 to 1. micron in diameter [4]. Since anomaly

regions of this size are not observed in optical coatings (except for nodules) , these absorbing

regions must evolve during irradiation. Since nodules are of the same material as the surrounding

film the absorption in and around than must similarly evolve. Of the various mechanisms studied

for this process, the only reasonable explanation found is the expansion of the absorbing region

via impact ionization. That is, these small local high densities of conduction electrons ionize

the surrounding off resonant electron trap centers and impurities. This is commonly termed

avalanche ionization, though it usually applies to purely intrinsic processes (ionizing valence

electrons) . It should be noted that if the absorption center region is on the order of 0.1 to 1. micron, no avalanche is required to produce the appropriate region of high electron densities.

This, however, is probably unlikely. A solution of this case is shown in figure 2 which

emphasizes the effect of y

An avalanche due to impact ionization of impurity, colloidal or trap, states would explain

the observation of lower damage thresholds in doped materials regardless of whether or not the

doping impurities are in resonance with the field. The photoionization of centers to initiate the

process would explain qualitatively the wavelength dependence of damage thresholds in optical

coatings.

Although specific numbers are used for a given case in a certain material, this is done for

conputational purposes. It is felt that the calculation of a specific example is desirable. The

concepts are intended to be applied to more general cases through the use of absorption cross

sections.

A detailed study of avalanche ionization will not be conducted here. That has been the topic

of considerable research by such authors such as [13], [18], [19], and [2u] . As reference [19]

implies, avalanche-ionization has been confirmed in solids, and as reference [13] indicates, a conputational ccxnpetency has been achieved with the processes involved. It will simply be assumed

here that due to shallow trap states, densities m excess of those required [13] to initiate an avalanche have been provided. An exponential growth then follows, which is subsequently limited

by ionization of a large fraction of the available atoms, carrier decay and the dynamic Burstein

shift [21]. A simple calculation with eq. (22) shows that at optical frequencies (%500 nm) and

damage intensities ( 1-2 Gw/on^) m fluoride films a conduction electron can gain energies of one half to two and a half eV in 10 ^° seconds.

523
Figure 2. The growth of the electron density at a radius of 500 A within an absorbing region of

1000 & radius for various y's and the parameters given. All unlisted parameters

are those given for Table 1.

de

dt

2 ^ \

m (1+w Tj^ )

(22)

(In eq. (22) e is the energy transferred to the electron and is the time between collisions) [13].

The spread in values is due to uncertainties in collision frequencies. These energies are sufficient to ionize many electron trap centers and some impurities [22]

.

The simple model used herein will produce a conservative estimate of the growth of an electron density initiated by a small region of absorption centers. This relates to previous

works [2] which denonstrated a requirement of isolated regions on the order of the film thickness

with an average electron density on the oraer of 10^^ cm"^.

For this case, the form of g(r,t) chosen is

g(r',f) = f(t')e

2 2 -P (f )r'^

(23)

where p(t') represents a spatial expansion of

It is assumed from previous studies [19] that

conservative estimate of the region's size at

the electron source due to the avalanche process,

an avalanche regi on'

s

growth is nonlinear, however a

a time t is 6 = /D^t. That is, the distance

524
electrons have travelled by diffusion from their initiation point. Thus; p(t)

used as a trial parameter.

= 1//Dgt can be

Subsequent integration of eq. (2) affords

,3/2

n(r,t) = dt'f (t')(Det' )

'

^ , ,^

'-^ ^2 exp(-Y (t-f) ) exp

-r

° Dg((t - f) + )) 4D^p^

4(D (t-f) + —

^ 4D p^

e"^

(24)

with

f(f) = n.a e

a t' a

(25)

where is the avalanche coefficient. Equation (25) is simply a means of generating electrons

initially at an exponential rate. This growth is terminated at some point due to processes such

as the dynamic Burstein shift and ionization of a large fraction of the available electrons.

Table 2

Electron Densities achieved at 12.xl0 ^ cm from color center initiation due to an electron

avalanche for various parameters. The avalanche is assumed to spread via diffusion.

^ik

g(v)

hv

tP

* n^

10l0(s-^

0.01 (on-^/s)

1.0

0.1/Av

2.34

io-«

20

= 10

Avalanche coefficient

Electron diffusion coefficient

Oscillator strength

Line shape factor

Photon energy

Pulse length

Limiting electron density

* The limiting electron density n^ was chosen as a reasonable value based upon [21j

.

[gw 1 ^ 2 Icm j

r (cm) Y (s h « ^(s ") d n(r ,1ns)

1.0 2.1 12.0x10"^ 10^^

lO^O

12 4.7x10

S.xlO

16.4 10^1

10^0

10^

ioi«

10^

10^0

2x10

1.2xl0-'-^ 12 6.3xlU

e.xio^^

1 525
A rigorous justification of eqs. (24) and (25) above is not possible. These expressions are simply a reasonable choice for the purpose of a Gedanken study of a postulated process. It should

also be noted that as shown m Ref. [19] avalanche is not a symmetric process. It in fact tends

to propagate in the direction of the beam source due to an attenuation of the field. In optical thin films this propagation is modified by encounter of the film surface.

Seme of the numerical results are presented in figures 3 and 4 and table 2.

Figure 3. The growth of the electron density

at a radius of lOOA for various

parameters. The parameters chosen

are considered reasonable. Electron

densities become very high at this

radius both with and without avalanche

ionization.

0.01 cmVs.

Dg is assumed to be

Figure 4. The growth of the electron ^ density at a radius of 1200 A

(1300 A is the largest radius

possible in a half wave film at 0.53 yM). Densities of the

IT order of 10 " can be achieved

by diffusion in the case considered if a sufficient number

of electrons are available for

photoionization. Otherswise an electron avalanche is required to reach and certainly to exceed

this. The assumed Ug=0.01cm'^/s.

4. Discussion

The numerical results presented in the previous section indicate the feasibility of

microdamage sites initiated by a localized absorbing anonally. An avalanche of electrons can produce large, densities of electrons expanding out to diameters on the order of the film

thickness.

526
It should be noted that the field thresholds are lower than those reported previously for

avalanche processes in similar materials. However, those were reported in bulk solids. The

starting mechanism in thin films should provide lower thresholds for starting the process

(colloids and aggregates of high density centers) . The large density of shallow states (centers)

which may coagulate about colloids, or coagulate about thanselves, should provide lower levels

required for a sustained avalanche process (i.e., the ionization of the off resonant centers

requires significantly less energy than valence electrons)

.

The avalanche coefficients (10^- 10^°) were chosen by scaling data from ref. [18] using

relations of Epifanov [20] for 2.0 to 2.5 ev impurity states. These numbers were further

supported by eq. (22) . It should be noted here that eq. (22) is overly simplistic and may not be

sufficiently conservative. In addition Lpifanov [23] believes that the coefficients determined in

ref. [18] may be too high.

The approach talten here may not be considered rigorous. This is intended to be a feasibility

study for which more sophistication may be incorporated. The feasibility of this process or a similar one is considered sufficiently dsnonstraced. The exact processes involved are still

considered unknown.

5. Conclusions

It has been shown that due to the non-ideal structure of an optical thin film, large local

electron densities may be generated by non-stoichiometric colloids or aggregates of color centers.

These are likely to exist in optical systons due to the conditions of deposition, substrate

polishing and subsequent treatment. If avalanche ionization does occur in these films, regions as small as 100 %. in radius of colloids and centers may initiate the process. Otherwise regions

closer to the thickness of the film 10^ - 10^ A of colloids and centers may be required.

Since these studies are initial, more investigation will be required to support these

theories. It should be pointed out that colloids in the fluoride films would probably be oxidized

due to ever present atmospheric oxygen. The process is thus further complicated. Band edge sort

of transitions may actually be occuring in oxidized metallic colloids in fluoride films. In addition to this, correct absorption cross sections of metal colloids in the optical region have

yet to be conputed. Errors or inappropriate assumptions have been found in those reported in the

literature.

It is clear that much work ronains to be done in this area. Measurements of colloidal

absorption bands indicated that it is likely that colloidal absorption at y = 1.06 ym is a strong

ccmpetitor for the two-photon process which would "be required for M-centers in Car^ • The

two-photon conputation produces transition rates significantly less than the one photon at 0.53

yh. However, correct colloidal cross sections need to be computed for comparison. Also, more

rigorous avalanche-ccmputations need to be performed for this specialized case, if possible. It may not be possible due to the lack of knowledge of the thin film structure. In addition,

complications such as oxidation, tanperature effects and electron density variation of Dg and

need to be accounted for.

Finally, in order to label this a theory, computation of electron densities would have to be

performed in a more rigorous fashion. This list of work to be done can become quite lengthy. It

is believed, however, that feasibility has been shown.

This research was suponsored by the Air Force Weapons Laboratory under Contract

F29601-84-K-0092.

6 . References

[1] Walker, T. W. ; Guenther, A. H; Nielsen, P. "Pulsed Laser-Induced Damage to Thin-Film Optical

Coatings-Part I: Experimental," IEEE J. Quant. Electron, QE-17 (10); 2041; 1981.

[2] Lange, M. R.; Mclver, J. K.; Guenther, A. H. "Pulsed Laser Damage in Thin Film coatings:

527
Fluorides and Oxides," Thin Solid Films, 125 (1-2); 143; 1985.

[3] Meyer, J. R.; Bartoli, F. J.; Kruer, M. R. "Optical Heating in Sanrii conductors" Physical

Review B. 21 (4); 1559; 1980.

[4] Lange, M. R.; Mclver, J. K.; Guenther, A. H. "Pulsed Laser Induced Damage of An Optical

Material With a Spherical Inclusion: Influence of the Thermal Properties of the Materials"

in NBS (U.S.) Spec. Publ. 669. Eds. H. E. Bennett, Guenther, A. H., B. E. Newnam, D. Milam,

1982.

[5] Ovshinsky, S. R. "Reversible electrical Switching Phenomena in Disordered Structures,"

Physical Review Letters, 21; (20) 1450; 1968.

[6] Schulman, J. H.; Canpton, W. D. "Color Centers in Solids," Pergamon Press, 1963.

[7] Hayes, W. "Crystals with Fluorites Structures", Oxford: Clarendon Press, 1974.

[8] Fowler, W. B. "Physics of Color Centers", Academic Press, N.Y., 1968.

[9J Beaumont, J. H.; Hayes, W. "M Centres in Alkaline Earth Fluorides, Proc. Roy. Soc. A. 309 ; 41; 1969.

[10] Fowler, W. B. "Relaxation of the Excited F Center," Phys. Rev. 135 (6A) ; 1725; 1964.

[llj Yariv, A., Quantum Electronics , John Wiley, 1967.

[12] (a) A. I. P. Handbook, Sec. 7; 3rd ed.; Ed. Gray, D. E.; McGraw Hill 1973.

(b) A. I. P. Handbook, Sec. 9; 3rd ed.; Bd. Gray, D. b.; McGraw Hill 1973.

[13] Sparks, M.; Duthler, C. J. "Theoretical Studies of High-Power Ultra Violet and Infrared

Materials," Fifth Tech. Rep. Xonics, Corp. Contract DAHC15-73-C-0127 , 1975.

[14] Vaidyanathan, A.; Mitra, S.; Narducci, L.; Shatras, R. "One-Photon Absorption in Direct Gap

Semiconductors," Sol. Stat. Com. 21; 405-407; 1977.

[15] Vaidyanathan, A.; Walker, T.; Guenther, A.; Mitra, S.; Narducci, L. "Ccsnparison Keldysh and

Perturbation Formulas for One-Photon Absorption," Phys. REv. B20(8); 3526-3527; 1979.

[16] Lao, B.; Smith, D. "The Development of Nodular Defects in Optical Coatings", NBS Spec. Pub.

(this edition) ; 1985.

[17] Lewis, K.; Pitt, A.; Cullis, A.; Chew, N.; Charlwood, L. "Laser Induced Damage in Dense

Optical Thin Films", NBS Spec. Pub. (this edition); 1985.

[18] Yablonovitch, E.; Bloembergen, N. "Avalanche Ionization and the Limiting Diameter of

Filaments Induced by Light Pulse in Transparent Media," Phys. Rev. Let. 29 (14); 907; 1972.

[19]. Fradin, D. W.; Yablonovitch, E.; Bass, M. "Confirmation of an Electron Avalanche Causing

Laser-Induced Bulk Damage at 1.06 pm." Applieo Optics, 12 (4) 1973.

[20] Epifanov, A. S. "Avalanche Ionization Induced in Solid Transparent Dielectrics by Strong

Laser Pulses", Sov. Phys. JETP, 40 (5); 1974.

[21] Burstein, E. "Anomalous Optical Absorption Limit m In Sb," Phys. Rev. _93; 632; 1954.

[22] Sparks, M.; Duthler, C. J. "9-10 Tech. Rep. Xonics, Corp. Contract DAHC15-73-C-0127; 1977.

[23] Epifanov, A. S.; Private Communications, Nov. 1985.

528
Manuscript received

11-30-85 SAR

TIME OF DAMAGE AND THE EFFECT OF THE PULSE REPETITION FREQUENCY ON THE LASER INDUCED DAMAGE THRESHOLD

R M Wood, S K Sharma, P Waite, E K Gorton* and K Lewis*

GEC Research Ltd., Hirst Research Centre, East Lane, Wembley, Middlesex, United Kingdom

Royal Signals Research Laboratories, Malvern, Worcestershire, United Kingdom

ABSTRACT

The time of damage has been measured as a function of pulse input energy for a range of i-r windows

and mi rrors

.

More data and understanding of the mechanisms involved has been gained of the effect of laser pulse

repetition frequency on the laser induced damage thresholds of infra-red transmitting window

materials. Results on Ge, Si, GaAs , ZnS, ZnSe, NaCl and KCl are presented together with comments

on the mechanisms which lower the damage threshold from the single shot damage level.

Key Words: CO2 laser; GaAs; Ge; KCl laser damage; NaCl; repetitive-pulse damages; ZnS; ZnSe.

1 INTRODUCTION

In the search for a reliable measurement of the minimum laser induced damage threshold of a particular component we have investigated the time of damage against input energy for a range of

materials. This has resulted in a better understanding of the mechanisms involved.

At previous damage symposia [1,2] the preliminary results of investigations into the effect of

pulse repetition frequency on the laser induced damage threshold of infra-red transmitting window

materials were presented. This paper reports the results of more experimental measurements on a wider range of infra-red window materials and looks again at the mechanisms which lower the damage

threshold from the single shot damage level.

2 APPARATUS

The results in this paper were mainly gained using the LAL hybrid TEA CO2 pulsed laser. Figure 1. This laser has a Gaussian TEMoo mode, (Figure 2a) has a gain switched spike plus tail (Figure 2b)

and can be operated at pulse repetition frequencies from single shot to 100 Hz.

3 MEASUREMENT OF DAMAGE THRESHOLD

Laser induced damage thresholds can be due to a variety of mechanisms:

Dielectric Breakdown

Bulk Effects

Surface Effects

Enhancement by scratches

Bri 1 1 oui n Scatteri ng Self-focussing

Thermal Absorption

Bulk effects '

- _ : Surface effects

Localised absorption

Conduction and free-carrier electron absorption

Transient absorption

and is dependent on both the material parameters and the laser pulse wavelength, and spatial and

temporal characteristics. Damage may correlate with either:

Peak power density (spatial and temporal) in the laser pulse.

Maximum energy density (spatial) of the laser pulse.

Average energy density in the pulse spike.

Average energy density in a pulse with both a spike and a tail.

529
Totals energy density or average power density in a pulse train.

Cw power density (energy in a given time)

It is therefore commonly found that each component or surface may exhibit a range of laser induced

damage thresholds (LIDT). A typical graphical record of the percentage of the pulses resulting in damage versus irradiation level is shown in Figure 3. Both the shape of this graph and its

absolute position in space is an intimate function of wavelength, spot size and pulse repetition

frequency.

4 TIME OF DAMAGE

The damage threshold of a transmitting material such as KCL or NaCl is caused by dielectric

breakdown and is expected to occur at or before the peak intensity of the laser pulse. In an absorbing material however the time of damage will be dependent on the energy absorbed and on the

thermal properties of the material testunder. Figure 4 shows in schematic form the (a) incident

and (b) transmitted pulse shapes of a typical TEA laser. 4c shows damage at the peak irradiance of

the pulse and 4d damage at some earlier time. It will be noticed that the plasma is assumed to be

formed instantaneously and that consequent transmission (or reflection) is negligible. It will

also be realised that these graphs could be gained at just above damage threshold for dielectric

breakdown or well above it, in the case of thermal mechanisms. Figures 4e and 4f indicate the onset of damage due to thermal mechanisms alone.

The time of damage has been monitored both in transmission and in reflection for a range of i-r

transmitting window materials. The following photographs of the time of damage have been gained

using Iwatsu digital storage scopes monitoring both the incident and either the transmitted and/or

the reflected beam. It is possible to superimpose both beams on each other and in the cases where

this has been done the tranmi ssi on/ref 1 ected beam has been normalised to the incident beam.

Figure 5a shows damage occurring to a Cu mirror in the tail of the TEA laser pulse. This is the

minimum damage threshold for this mirror. Figure 5b shows the difference between an unattenuated

pulse (lower trace with tail) to an attenuated (damage) pulse which has cut the tail off. Figure

5c shows the normalised incident and reflected beams at higher laser pulse energy showing that the

reflection/absorption properties of the mirror change slightly before the main plasma attenuation

takes place.

LIDT to a Germanium substrate on the other hand cannot be induced to occur as late in the pulse as late as that to a Cu mirror. Figure 5d shows the latest time of damage to be 20 ns after the peak

of the pulse and 5e shows damage at the peak of the pulse. Figure 5f shows the 'latest' time of damage to a Silicon sample (40 ns after the pulsepeak). This photograph indicates that the change

of transmission/reflection just before catastrophic breakdown is not so pronounced in Si as in Ge

or Cu . Figure 5g indicates the length of time the plasma absorbs the incident radiation.

It will be realised that the time of damage can be used to give guidance both as to the minimum

damage threshold which applies and as to the precise mechanisms involved in the damage.

5 PULSE REPETITION FREQUENCY AND DAMAGE THRESHOLD

The preliminary results of our investigations into the effect of pulse repetition frequency on the

laser induced damage threshold of infra-red transmitting window materials were presented in 1982 at this conference [1]. In presenting these results we made the comment that the only consistent

physical mechanism we could offer to explain the shape of the curve was that of an anomolously low

thermal conductivity. L D Merkyl [3] questioned this explanation but even so did not find another.

We have undertaken more work using the same hybrid TEA CO2 laser (Figure 1) and have been very

careful to ensure that the result is not a system phenomenon. For instance possible changes in

sensitivity of the original photon-drag detector have been ruled out by use of a room temperature

CMT detector.

A summary of these results is shown in graphical form in Figure 6. The p.r.f. LIDT's are normalised to the single shot value to facilitate comparison between materials. Ge shows a decreasing p.r.f. LIDT with increasing p.r.f. to over 100 Hz. GaAs shows the same general shape

but flattens out at lower p.r.f.. ZnSe and ZnS both flatten out earlier and higher (some more evidence will be shown on a range of ZnS samples). Si on the other hand only showed a 10% drop

with increasing p.r.f. and a correlation has be found between damage level and number of shots at constant p.r.f.. This behaviour is not unexpected as Si has higher absorption coefficients than

530
the other i.r. window materials tested and the thermal model is therefore three dimensional rather

than the two dimensional model which approximates to the low absorption case. The dielectric

substrates KCl and NaCl were found to have steady damage thresholds irrespective of p.r.f..

In all cases damage occurred within 20 shots and it was found that if damage did not occur at a particular pulse energy and p.r.f. within this period then it did not occur within 15 seconds.

In order to check that these prf damage thresholds were not just a short term cumul ati ve^ffeet

some long irradiation runs (10^ shots) were made on all samples at about 70% of the 100 Hz prf

threshold value and 10° shots at 50% of the 25 Hz threshold value were made using the Ge sample.

It should be noted that these graphs have been found to be consistent over a number of commercially

obtained, well polished substrates.

Investigation has been made of the mechanisms which this change in LIDT with p.r.f.. In 1982 we invoked the theory of an anomolously low thermal conducti vity/diffusi vity to explain the shape of

the Ge curve. This could be of bulk or surface origin. The theory of surface origin is favourite

at the present time.

Irradiation of a \/4 ZnSe overcoated Ge substrate showed that the prf behaviour of the component

was changed to be more like a bare ZnSe substrate. Irradiation of the uncoated side showed typical

Ge behaviour.

Figure 7a shows the laser damage morphology of a GaAs polycrystal 1 ine substrate. This photograph

shows line damage around the central damage spot. This line damage was produced consistently

during the last two or three shots prior to the catastrophic damage at the centre. It is thought

that the lines correspond to cracked grain boundaries. If this is so then the temperature increase

during the prf run might be contained within the grain being irradiated. Eventually the grain

boundary cracks and the grain raises its temperature up to damage level. Figure 7b shows two damaged areas. The lower damaged area is a grain which was literally blown out of the surface

after line damage was produced. Figure 7c shows the central damage spot showing melted material at the edges and typical grain boundary structure within the spot itself.

Once the linedamage appeared on the front surface the central area damaged within two or three

shots. This damage occurred all the way through the crystal and damage has been observed on both

front and back surfaces simultaneously.

A further series of tests have been made on a series of CVD grown polycrystal 1 i ne ZnS with

distinctive, narrow ranges of crystal ite sizes, from RARE Malvern. These varied from 2-s-5, 35^40,

73, to 148 \xm. Samples 2 and 3 reproduced the ZnSe/ZnS band whilst the other two more nearly

reproduced the GaAs curve. It was hoped that relationship could be gained between the LIDT's and

the p.r.f. behaviour and the grain sizes. This has not been gained unambiguously although

variations in behaviour have been observed. These samples are in process of being repolished to a higher standard so as to eliminate differences other than grain size.

Time of damage measurements have been shown to be useful both to determine the minimum damage

thresholds and to allow further determination of the mechanisms involved in damage.

Further measurements of the relationship between damage threshold and the laser prf have been made

for a range of single crystal and polycrystal i-r transmitting substrates. The favourite mechanism

for the observed effect is that of anomolously low thermal conductivity which has been linked to surface condition and grain size.

This work has been supported by the Procurement Executive, Ministry of Defence (Directorate of

Components, Valves and Devices) and sponsored from the RSRE Establishment.

6 CONCLUSIONS

ACKNOWLEDGEMENTS

REFERENCES

1

2

R M Wood, S K Sharma and P Waite, NBS Spec. Pub. 669, 1984, p 44

R M Wood, S K Sharma and P Waite, BDS Symp. 1983

3 L D Merkyl , 1985

531
Figure 1: Photograph of damage apparatus
P**** tp - time of main pulse peak

Figure 4: Schematic of the time of damage

533
Figure 5a: Cu: showing damage in tail of TEA laser pulse

Figure 5b: Cu: showing difference in reflectance with increasing

energy in laser pul se
Figure 5g

Figure 5: Time of damage

I I I I I I I I

I I I 0 20 AO 60 80 100

PRF (Hz)

Figure 6: Normalised prf LIDT values as a function of prf for a selection of substrate material

535
Figure 7a: Laser damage showing line damage Figure 7b: Two damage areas

Figure 7c: Central damage spot

Figure 7: prf laser damage morphology in GaAs

536
Manuscript received

1-24-86 SAR

Modeling Laser Damage Caused by Platinum

Inclusions in Laser Glass*

John H. Pitts

Lawrence Livermore National Laboratory

P. 0. Box 8808, L-A80

Livermore, California 94550

A fracture-mechanic theory suggests that crack-propagation rates are proportional to the square root of the inclusion radius and to the three-quarters power

of the fluence. The theory does not yet account for pressure loss or convection heat

transfer as platinum vapor flows into the crack, and needs to be developed further

before accurate propagation rates can be predicted. However, a simple estimate obtained

by multiplying the time that the platinum is in the vapor state by the sonic velocity in

glass gives a conservative but reasonable result. At fluence levels of 2 J/cm2, crack

propagation of h ym per shot are predicted.

A crack-development hypothesis, based on shock-wave propagation, successfully

predicts the shape of typical damage sites. The sites have an annular crack extending

from the equator of the inclusion and planar crack lobes extending from the poles. The

lobe at the pole exposed to laser light is larger than the lobe at the unexposed pole.

A numerical heat-transport model predicts temperature profiles in and around a metallic platinum inclusion imbedded in a phosphate laser glass matrix when illuminated

with l-pm laser light. The thermal model predicts that glass damage will occur if the

platinum temperature exceeds the boilina point. Predicted fluence-damage limits of

l.h J/cm2 for a 1-ns pulse and 4.3 J/cm^ for a 10-ns pulse agree with experimental

data. The damage limit is independent of inclusion size above about 1 ym in diameter.

Key words: crack propagation rates; damage sites; glass fracture; fluence damage

limits; Nova laser; platinum inclusions; thermal modeling.

1. Introduction .

Platinum inclusions in neodymium-doped phosphate laser glass on Nova and Novette have been

the cause of glass damage when fluence levels greater than about 2 J/cm2 were reached. Laser

light is absorbed by the opaque platinum, causing a thin skin of platinum on the front surface of

the inclusion to vaporize. A shock wave propagates through the inclusion and is transmitted to the glass. Glass, being a brittle material, cracks because of the shock wave and the pressure of

the vaporized platinum. These cracks propagate until the pressure of the vaporized platinum

decreases below that required for further crack propagation.

The phenomena was studied as early as 1970 by Hopper and Uhlmann [1]. They showed

photographs of damaged glass and calculated temperatures exceeding 10 000 K when fluence levels were 20 J/cm^ over a 30-ns period. They theorized that very small inclusions were safe because

heat losses to the glass kept the temperatures and, hence, stresses within allowable bounds.

However, they argued that for sizes above 1000 A (0.1 ym), the strength of the glass was exceeded and the glass could fracture. Their thermal modeling suggested that large inclusions

were less damaging than intermediate-sized ones (over 1000 A) because the surface-to-volume

ratio of the inclusion decreased as the radius increased. Therefore, the larger inclusions had more thermal capacity and hence lower average temperatures and stresses.

Sparks and Duther [2] also examined the inclusion problem and showed that glass damage was greater with high-intensity, short-pulse lasers than with low-intensity, long-pulse lasers. A number of authors [3-5] have studied heating of a sphere illuminated from one side.

»Work performed under the auspices of the U. S. Department of Energy by the Lawrence Livermore

National Laboratory under contract number W-7405-ENG-48.

537
Their solutions all show that the high temperatures predicted by Hopper and Uhlmann will exist.

We discuss three items in this paper:

• A fracture-mechanics theory suggesting that glass damage will be worse with larger

inclusions;

• A hypothesis explaining the shape that typical cracks develop in the glass and that

matches many damage sites; and

t Thermal modeling results suggest a fluence threshold of 1.6 J/cm2 for a 1-ns

pulse or 4.3 J/cm^ for a 10-ns pulse in phosphate glass. This threshold is

independent of inclusion size above about 1 m in diameter. The thermal modeling

results compare favorably with experimental data.

2. Fracture-Mechanics Theory

Consider glass to be a brittle material initially with a penny-shaped crack that is the

diameter of the platinum inclusion. The criterion for crack propagation is based on a material

property, Kj^, called the fracture-toughness (stress-intensity) factor [6]

K, = a VffF (1)

where

Ki = material fracture toughness (Pa Vm)

a =. applied stress (or pressure), Pa

a = crack radius (which we take initially equal to the

inclusion radius), m.

If the value of exceeds a critical value, then the crack will propagate. This critical

value termed the critical-fracture-toughness or critical-stress-intensity factor, Kiq, is a material property, the magnitude of which is normally obtained experimentally.

The Mie-Gruneisen equation of state [7,8] is applicable for shock-wave/high-stress values,

where energy is deposited isochorically; it relates stress and specific energy as

a = Fpe (2)

where

r = Gruneisen constant (dimensionless)

p = density (Kg/m^)

e = specific energy deposition above that required for vaporization (J/Kg).

Following Hopper and Uhlmann 's article [1] for an inclusion with finite thermal diffusivity

and a lO-J/cm^, l-ns pulse, we have a spatial temperature distribution using their material

properties (see their Eqs. (6) and (7b)).

538
4 /Ins \l/2 1 10 3/cni

^1 ns ^ ^ 10 - V30 nsl ' \^ -, „ 2 10 J/cm^ ^ ^ J/cm

0.6 Z

• 1^^^^

V(l ns/30 ns)l/2

5 . ,

• :.. ~ 1 X 10 ierfc (3.286 z) . (3)

Here z is the distance into the inclusion from the surface in pm, and T is temperature in K. Values of ierfc, the integral of the complementary error function, are tabulated by Abramovitch

and Stegun [9]. The surface temperature drops below the vaporization temperature (4800 K) at

z = 0.307 ym. If we double the fluence to 20 J/cm2, the temperature drops below the

vaporization temperature at z = 0.371 ym. If we use these values to calculate the power

dependence of the fluence, J, on the specific energy deposition (recalling that specific energy

deposition is approximately proportional to temperature)

0/ 0 0

2\"' 2 aCLJ/cmf \ _ 20 J/cm /0.371 m

10 J/cm^y 10 J/cm^/0.307 pm

m = 3/A

Combining this result with equations (1) and (2) we have

The crack will propagate as long as Ki > K^q. Hence, this model suggests that the extent

of crack propagation in the glass per shot is proportional to the square root of the crack radius

and the three-quarters power of the fluence. Initially, the crack radius is equal to that of the

inclusion but it would increase after each shot. Hence, at constant fluence, the rate of crack

propagation would increase after each shot.

The model does not account for loss of pressure or convection heat transfer as the vaporized

platinum flows into the crack, both of which counteract the effect of increasing crack

propagation rate in later shots by making it harder to propagate a large crack. Both effects

oppose the square root dependence of the crack radius, and could partially account for some experimental data [10,11,12,13], where the amount of crack propagation first increases and then

decreases; or alternately propagates and then stops. Another reason cracks could stop

propagating in later shots is that all the platinum initially in the inclusion has been deposited

in the crack area with a thickness that is now transparent to laser light. Regardless, this

model suggests that the larger the inclusion, the worse the damage.

Additional development of this fracture-mechanics theory could lead to a better prediction of

how far a crack would propagate during a particular shot. In order to proceed further, we may

need to estimate the thickness (width) of a typical crack so that we can calculate the pressure

distribution in the crack as a function of time. Further, we may need to account for the change

in opacity below unity of platinum deposited in the crack region if the platinum thickness is in

the 100-A, or less, range.

539
3. Crack-Development Hypothesis

Many damage sites have an annular crack around the equator of the inclusion with an additional planar lobe-shaped crack at each pole [13,1^] (see Fig. la). The crack at the pole,

facing the laser light illumination, is always larger than the crack at the pole hidden from the

laser light illumination by the inclusion itself.

During discussions with Hovingh [15], he suggested the phenomena was a result of shock

propagation. As energy from laser light is deposited on the surface of the inclusion, a shock wave is developed that propagates through the inclusion (see Fig. lb), which deforms to an oval

shape, putting pressure at the sides of the inclusion. This initiates an annular crack site at the equator. The pressure over the annulus is uniform because of symmetry and a completely

symmetric annular crack site is formed.

Typical geometry of Deflection of an many fracture sites inclusion with time

Figure la Figure lb

Figure 1. Sketch of a typical damage site and a shock

propagation analogy of why this geometry occurs.

It has been asked how pressure can cause a crack site because the pressure forces are compressive. The pressure is applied not only perpendicular to the incoming laser light, but

also parallel to it, thereby tending to spread the glass material in tension parallel to the

incoming laser light. A similar phenomenon occurs in the walls of a spherical pressure vessel,

where the hoop stess is tension and yet there is pressure exerted on the inside wall surface.

Further, any compression generated at the crack site changes to tension as the shock passes

through the glass material itself.

The shock continues through the inclusion, dissipating energy and becoming weaker with time.

The inclusion returns to a circular shape, but then deforms to an oval shape with a major axis

parallel (rather than perpendicular) to the direction of the incoming laser light. However, the

oval formed has a much more concentrated deformation at the poles because of the concentration of

energy deposition there. Crack sites are initiated at the poles only and not as a complete

annulus.

540
An analogy to this type of deformation occurs when a golfer hits a good drive. The golf ball

has energy deposited at the surface, deforms to an oval with major axis perpendicular to the

direction of the golfer's swing. During flight, the golf ball returns to circular shape and then

deforms to an oval with the major axis parallel to the direction of the golfer's swing.

This crack-development hypothesis explains why the form of many crack sites are as shown in

Figure la. The hypothesis suggests the location of crack initiation sites. However, it is the

vaporized platinum and its associated pressure that actually propagate the cracks beyond the

initial site. Because the platinum vapor must flow further to reach the crack site at the pole

hidden from the laser light, pressure developed there is less than at the opposite pole crack

site. Hence, a smaller crack should develop at the hidden pole than at the exposed pole. This

is verified in observed glass damage sites.

4. Thermal Modeling

Our thermal model was a half-sphere of platinum surrounded by a half-spherical annulus of

glass as shown in Figure 2a. The model represents what could be a typical inclusion, shown in

Figure 2b. Vaporization of the platinum is concentrated at the top of the inclusion, which in

both cases is normal to the incident laser light. Hence, the spherical model is a good

approximation to the more generally shaped inclusion of Figure 2b. We use just a half-sphere in

the model because of symmetry. In the model, laser light was absorbed on the upper surface of

the platinum sphere with a cosine distribution used to account for the curvature of the surface.

^-^xm laser light

1-/Lim laser light

Platinum^ —

a, = 0.27

/

/ /

/ /

/

/ Coarse zoned

Medium zoned

\

Fine zoned:^

\^

Medium zoned

Phosphate glass

= 0

. Phosphate

: glass

Figure 2a Figure 2b

Figure 2. Hemispherical model of a platinum inclusion in glass illuminated from

the top with laser light, and what could be a typical non-spherical inclusion

Thermal properties used in the model for platinum and glass are shown in table 1. Platinum

properties were obtained from Touloukian, et. al., [16] and Hampel [17]. The boiling point shown

in Table 1 is higher than in some other references (e.g., Weast [18] shows AlOO K) but is used

for consistency with other properties taken from Hampel [17]. The key platinum properties are the heat of vaporization and the normal spectral absorptance, so that the difference in the

documented boiling point has an insignificant effect. The normal spectral absorptance shown is at room temperature. The absorptance does change with temperature, but it is more a function of

the surface itself. Changes of an order of magnitude are feasible depending on whether the

541
Table 1. Thermal Properties of Materials Used in the Analysis

Platinum LHG-8

inclusion phosphate glass

Density, kg/m

Specific heat, J/kg K

Thermal conductivity, W/m K

Melting point, K

Heat of fusion, J/kg

Boiling point, K

Heat of vaporization, J/kg

Normal spectral absorptance at 1 ym

21,400

146

80

2,040

101,000

4.800

2,260,000

0.27

2,850

263

0.52

surface is polished or not and if the surface is oxidized. We used the room temperature value

because no elevated temperature values were available. The equation of state for platinum above

the boiling point was that used in the LASNEX Code [19]. Properties of glass were obtained from

Stokowski, Saroyan, and Weber [20] for LHG-8 phosphate glass. The surface of the platinum was allowed to radiate to a 300-K surrounding. No absorption was permitted in the glass, but heat

was conducted between the glass and the platinum with negligible contact resistance.

Modeling was performed using Shapiro's TOPAZ [21] code with Halquist's MAZE [22] and

ORION [23] codes used for preparing the input and processing the output. TOPAZ is a heat

conduction code, which is applicable to this problem because inclusion sizes are so small that

natural convection is negligible. Problems were run on the CRAY computer with an average run time of about 30 minutes each.

Results are shown in Figure 3 where the fluence damage threshold is plotted against laser

pulse length. Comparison with experimental data is excellent. Avizonis and Farrington [24] and

Yamanaka [25] included error bars, which are also shown. The theory developed in this paper is

the solid line, which has a slope of 1/2 corresponding to a square root dependence on laser pulse

length. The slight departure of the theory from the data of Gonzales and Milam [26] at short

laser pulse lengths may result from the crack propagation per shot being so small in this region.

Hence, it takes a number of shots before any damage can be physically observed.

Temperature profiles at two different times are shown in Figure 4. Peak temperatures are high (electron volt range; 1 ev = 10 000 K) and so are the resulting pressures of the vaporized

platinum. Note that the temperature contours are all concentrated in the platinum at the exposed

surface. There is no change in the temperature profiles when the size of the inclusion is

changed above about 1 ym because the thin layer where the profiles are located is similar to a planar surface. Hence, results are independent of inclusion size above about 1 ym in

diameter. Table 2 lists maximum temperatures, maximum pressures, and the resulting crack

propagation per shot for various fluences. The maximum pressures were calculated using eq. (2).

We used a Griineisen constant, r, equal to unity; and a value of pe calculated from the TOPAZ

results at the time of the peak temperature by dividing the total energy in the vaporized

platinum at the exposed pole by the volume of the vaporized platinum.

542
100 c 1—I I I I M I

j

1—I r I r I m

1 jum wavelength

* 10 -

J I

I I 1 1 1 1

1 I I

I I 1 1 II 10 100

Laser pulse length, ns

— Present theory

A Avizonis, P. V. & Farrington, T.[24]

Yamanaka, C. et al. [25]

o Gonzales, R. & Milam, D. [26]

O National Matls. Advisory Board [27]

Figure 3. Effect of laser pulse length on the fluence damage threshold.

Theoretical results are independent of inclusion size above about 1 ym

in diameter and are in excellent agreement with experimental data.

Time, ns 1.0 10.0

Minimum contour, K 1500 570

Maximum contour, K 11000 2700

Fluence damage threshold is independent of inclusion size above

1 jLtm diameter

Figure A. Temperature contours for a 5-pm-radius platinum

inclusion surrounded by phosphate glass, and illuminated by 1-pm laser

light at an incident fluence of 3.7 J/cm^.

543
Table 2. Calculated Characteristics of Crack Propagation

Incident Absorbed Pulse Maximum Maximum Crack

Fluence Fluence Time Temperature Pressure Propagation

Per Shot

(J/cm2) (J/cm2) (ns) (K) (Pa) (ym)

37 10 1 1.3 X 105 2.8 x lOl-l- 500

18 5 1 7.6 X 10^ 1.5 X lOll 150

3.7 1 1 1.5 X 10^ 0.5 X lO^-l 8.4

1.8 0.5 1 6.5 X 10^ <0.5 X lO^l 3.5

1.5 0.4 1 5.3 X 10^ <0.5 X IQII 1.5

3.7 1 10 4.5 X 105

Fluence damage threshold is 1.4 J/cm2 for a 1-ns laser pulse length ,i

' 1

We have no doubt that the pressures are high enough to propagate a crack. In conjunction

with experimental results [10-14], this suggests that damage will occur if the temperature of the

platinum exceeds its boiling point . Experimental data (see Fig. 3) suggests a damage limit of

about 2 J/cm2 for a 1-ns pulse and 5 for a 10-ns pulse. This is in agreement with the

calculated values.

The fracture-mechanics theory is not developed enough to predict crack growth. However,

crack growth can be conservatively estimated by multiplying the time during which platinum is in a vaporized state by the sound speed in glass. Values of crack growth for fluences close to those of available experiments are reasonable (see Table 2).

The key conclusion of this thermal analysis is that damage will occur if the platinum

temperature exceeds the boiling point. Damage is independent of inclusion size (above about

1 pm) and is worse for shorter-duration laser pulses.

5. Conclusions

• Fracturp- mechanics theory suggests that crack propagation rates are proportional to the

square root of the inclusion radius and the 3/4 power of the fluence. Larger inclusions above

about 1 pm in diameter are worse than smaller inclusions.

• A crack-development hypothesis based on shock propagation successfully predicts the shape

of many fracture sites. The shape is that of an annular crack around the inclusion equator with

planar lobes at the poles. The lobe at the pole exposed to laser light is larger than the lobe

at the unexposed pole.

• Thermal modeling predicts that glass damage will occur if the platinum vaporizes and that

the fluence damage threshold is both proportional to the square root of the laser pulse length

and independent of the inclusion size above about 1 ym in diameter. Predicted fluence damage

limits of 1.4 J/cm2 for a 1-ns pulse and 4.3 J/cm^ for a 10-ns pulse agree with experimental

data.

544
6. Acknowledgement

Besides those people listed in the references, I would like to extend my appreciation to J. H. Campbell at the Lawrence Livermore National Laboratory who directed this effort and reviewed

the results.

7. References

[1] Hopper, R. W. and Uhlmann, D. R., "Mechanism of Inclusion Damage in Laser Glass",

Journal of Applied Physics . Vol. ^1, No. 10, Sept. 1970, pp. A023-A037.

[2] Sparks, M. and Duther, C. J,, "Theory of Infrared Absorption and Material Failure in

Crystals Containing Inclusions," Journal of Applied Physics . Vol. 44, No. 7, July 1973,

pp. 3038-3045.

[3] Cheung, J. B., Chen, T. S., and Thirumalai, K., "Transient Thermal Stresses in a Sphere

by Local Heating", Journal of Applied Mechanics . Vol. 41, Dec, 1974, pp. 930-954.

[4] Werley, K. A. and Gilligan, J. G., "The Temperature Distribution of a Sphere Placed in a Directed Uniform Heat Flux", Journal of Heat Transfer . Vol. 103, May 1981, pp. 399-401.

[5] Duffy, D., "The Temperature Distribution within a Sphere Placed in a Directed Uniform

Heat Flux and Allowed to Radiatively Cool", Journal of Heat Transfer . Vol. 107, February

1985, pp. 28-32.

[6] Rolfe, S. T. and Barsom, J. M., "Fracture and Fatigue Control in

Structures - Applications of Fracture Mechanics", Prentice-Hall, Englewood Cliffs, New

Jersey, 1977, p. 15ff.

[7] Baaken, L. H. and Anderson, P. D., "An Equation of State Handbook", Sandia Laboratories,

Livermore, Calif., Report SCL-DR-66-123, January 1969, p. 25.

[8] Zel'dovitch, Ya. B. and Raiser, Yu. P., "Physics of Shock Waves and High-Temperature

Hydrodynamic Phenomena", Academic Press, New York, 1967, p. 697ff, and specifically

Eq. 11.13.

[9] Abramovitch, M. and Stegun, I. A., "Handbook of Mathematical Functions," Dover, New

York, 1965, p. 317.

[10] Milam, D. and Gonzales, R., "Damage Tests of Inclusions in a 20.8-cm Disk Taken from

Novette", Lawrence Livermore National Laboratory, Livermore, Calif., Internal Memorandum

LDG85-34/4080D, April 16, 1985.

[11] Milam, D. and Gonzales, R., "Additional Tests of Platinum Inclusions," Lawrence

Livermore National Laboratory, Livermore, Calif., Internal Memorandum LDG85-56/4667T,

July 8, 1985.

[12] Hatcher, C. W. and Milam, D., "Status of Nova-Hoya Glass Damage Tests," Lawrence

Livermore National Laboratory, Livermore, Calif., Internal Memorandum TF85-246,

July 23, 1985.

[13] Milam, D. and Hatcher, C. W., Private communication, Lawrence Livermore National

Laboratory, Livermore, Calif., June and July 1985.

[14] Hopper, R. W., Private communication, Lawrence Livermore National Laboratory, Livermore,

Calif., June 1985.

[15] Hovingh, J., Private cormnunication, Lawrence Livermore National Laboratory, Livermore,

Calif., July 1985.

[16] Touloukian, Y. S. at. al., "Thermophysical Properties of Matter - Thermal Radiative

Properties, Metallic Elements and Alloys," IFI/Plemem, Press, New York, 1970.

[17] Hampel, C. A., "Rare Metals Handbook" Reinhold, 1961, p 319-320.

545
[18] Weast, R, C, "CRC Handbook of Chemistry and Physics" CRC Press, Cleveland, Ohio, 65th

Edition, 198^.

[19] Orth, C, Lawrence Livermore National Laboratory, Livermore, Calif., Private

communication, July 1985.

[20] Stokowski, E. E., Saroyan, R. A. and Weber, M. J., "ND-Doped Laser Glass Spectroscopic

and Physical Properties," Lawrence Livermore National Laboratory, Livermore, Calif.,

M-95, Rev. 2, Nov. 1981, Glass #509^.

[21] Shapiro, A. B., "TOPAZ - A Finite Element Heat Conduction Code for Analyzing 2-D

Solids," Lawrence Livermore National Laboratory, Livermore, Calif., UCID 20045, March

1984.

[22] Hallquist, J. 0., "MAZE - A Input Generator for DYNA2D and NIKE20," Lawrence Livermore

National Laboratory, Livermore, Calif., UCID 19029, Rev. 2, June 1983.

[23] Hallquist, J. 0., "ORION: An Interactive Post-Processor for the Analysis Codes NIKE2D,

DYNA2D and TAC02D," Lawrence Livermore National Laboratory, Livermore, Calif.,

UCID 19310, Jan. 1982.

[24] Avizonis, P. V. and Farrington, T., "Internal Self-Damage of Ruby and Nd-Glass Lasers",

Appl. Phys. Ltrs. 7, 1965.

[25] Yamanaka, C. et. al., "Investigation of Damage in Laser Glass" NBS Special Publication

356, 1971.

[26] Gonzales, R. P. and Milam, D. "Evolution During Multiple-Shot Irradiation of Damage

Surrounding Isolated Platinum Inclusions in Phosphate Laser Glasses" 17th Annual

Symposium on Optical Materials for High Power Lasers, Boulder, Colorado, October 28-30,

1985.

[27] National Materials Advisory Board, "Fundamentals of Damage in Laser Glass". Publication

NMAB-271, 1970.

546
- Manuscript Not Received -

CO2 LASER- INDUCED AIR BREAKDOWN

NEAR METAL MIRROR SURFACES

Vitaly Konov

Institute of General Physics

Moscow, USSR

Key Words: air breakdown; CO2 laser; laser- induced damage;

metal mirrors.
BOULDER DAMAGE SYMPOSIUM

Attendees

October 28-30, 1985

Joseph Abate

Hampshire Instruments, Inc. P.O. Box 10159

250 East River Road

Rochester, NY 14610

(716) 482-4070

Byong H. Ahn

NV & EOL USA CECOM

DELNV-L

Ft. Belvoir, VA 22050-5577

(703) 664-5364

Jonathan W. Arenberg

Hughes Aircraft Co. P.O. Box 902

Bldg. Ea M/SD125

El Segundo, CA 90245

(213) 615-3090

Tsuyoshi Asaeda

Canon U.S.A.

3191 Red Hill Ave. Costa Mesa, CA 92525

(714) 556-1035

Bernard Bendow

The BDM Corporation

1801 Randolph Road, SE Albuquerque, NM 87105

(505) 848-5129

Ralph R. Berggren

Los Alamos National Laboratory

Los Alamos, NM 87545

(505) 567-0593

V. Warren Bircik

Northrop Research & Tech. Center

One Research Park

Palos Verdes Pensinsula, CA 90274

(213) 544-5306

James Boyer

Los Alamos National Laboratory

MS J555

P.O. Box 1663

Los Alamos, NM 87544

(505) 557-0041/9222

M. A. Acharekar

Litton Laser Systems

P.O. Box 7300

Orlando, FL 32854-7300

(305) 297-4450

Philip C. Archibald

Naval Weapons Center

Code 3818

China Lake, CA 93555

Paul Arendt

Los Alamos National Laboratory

P.O. Box 1553

MS E-549

Los Alamos, NM 87545

(505) 557-7851

Michael F. Becker

University of Texas at Austin

Electrical Engineering Dept.

133 Engineering Science Bldg.

Austin, TX 78712

(512) 471-3628

Harold E. Bennett

Michel son Laboratory

Naval Weapons Center

Code 38101

China Lake, CA 93555

(714) 939-2869

Jerry Ray Bettis

Rockwell International

Rocketdyne Division

Dept. 531-111 FB33

5533 Canoga Avenue

Canoga Park, CA 91304

(818) 700-5165

Lynn Bonsai 1 Rockwell International

Rocketdyne Division

6533 Canoga Avenue

Canoga Park, CA 91304

(818) 700-4848

Ernie Braunschweig

RDA ATO 9377

Albuquerque, NM 87119

(505) 844-8446

548
Morris Braunstein

Laser Power Corp.

11211 Sorrento Valley Road

San Diego, CA 92121-1325

(619) 455-0751

Dennis Burge

Naval Weapons Center

Code 3816

China Lake, CA 93555

(619) 939-1427

John H. Campbell

Lawrence Livermore National Lab

P.O. Box 5508 L-482

Livermore, CA 94550

(415) 422-6497

Robert Brown

Los Alamos National Lab

P.O. Box 1663

Group MP-7, MS H840

Los Alamos, NM 87545

(505) 557-5515

Larry W. Burggraf

AF Office of Scientific Research

AFOSR-NC

Boiling AFB Washington, DC 20332-6448

(202) 767-4960

Charles K. Carniglia

Martin Marietta Aerospace

P.O. Box 9316

International Airport

Albuquerque, NM 87119

(505) 844-1064

Susan D. Carson

BDM Corporation

1801 Randolph Road

Albuquerque, NM 87106

(505) 848-5696

Kathleen A. Cerqua

Laboratory for Laser Energetics

University of Rochester

250 East River Road

Rochester, NY 14623

(716) 275-5101

Jay S. Chivian

LTV Aerospace & Defense Co. Vought Missiles & Adv. Prog. Div.

P.O. Box 650003

MS TH-67

Dallas, TX 75265-0003

(214) 266-7808

Carlos Casteleiro

Kaman Instruments

1500 Garden of the Gods Road

Colorado Springs, CO 80933

(303) 599-1916

Lloyd L. Chase

Lawrence Livermore National Lab

MS/L-490

P.O. Box 808

Livermore, CA 94550

(415) 422-5270

Don A. Dalton

U.T.R.C.

5301 Central Ave. Suite 1517

Albuquerque, NM 87111

(505) 266-5828

John G. Daly

Litton Laser Systems

P.O. Box 7300

Orlando, FL 32954

(305) 297-4457

Donald L. Decker

Code 3816

Naval Weapons Center

China Lake, CA 93555

(519) 939-3247

Hulya Demiryont

Colorado State Univ.

Physics Dept. Fort Collins, CO 80523

(303) 491-8452

John A. Detrio

University of Dayton

Research Institue

300 College Avenue

Dayton, OH 45469

(513) 229-3453

Dana D. Dlott

School of Chemical Sciences

Noyes Laboratory, Box 37 505 S. Mathews Ave. Urbana, IL 61801

(217) 333-3574

Hermann J. Donnert

Kansas State University & Frank J. Seiler Research Lab.

Ward Hall

Manhatten, KS 66505-7-39

(913) 532-5960

549
Richard W. Dyer

W. J. Schafer Associates, Inc. Corporate Place 128

Bldg. 2, Suite 300

Wakefield, MA 01880-1258

(617) 246-0450

David F. Edwards

Lawrence Livermore National Labs

L-332

Livermore, CA 94550

(415) 422-0747

Alexander Epifanov

Institute of General Physics

Acad. Sc. USSR

Vasilov Str. 38

Moscow

USSR

Gregory J. Exarhos

Pacific Northwest Laboratory

P.O. Box 999

Richland, WA 99352

(509) 375-2440

Ron C. Estler

Fort Lewis College

Durango, CO 81301

(303) 247-7238

Phi 1

i p M. Fauchet

Princeton University

Dept. of Electrical Engineering

and Computer Science

Princeton, NJ 08544

Albert Feldman

National Bureau of Standards

A257 Materials Bldg.

Gaithersburg, MD 20899

(301) 921-2899

Joseph F. Figueira

Los Alamos National Laboratory

P.O. Box 1563

Los Alamos, NM 87545

(505) 557-5448

Diane M. Fischer

USAF

AFWL/ARAO

Kirtland AFB, NM

(505) 844-1275

87117

Stephen R. Foltyn

Los Alamos National Laboratory

MS-J 555, CHM-5

P.O. Box 1553

Los Alamos, NM 87545

(505) 567-1123

Alan M. Frank

Lawrence Livermore National Lab

Box 808, L-35

Livermore, CA 94550

(415) 422-7271

David Gallant

Rocketdyne Div.

Rockwell International

P.O. Box 5570 KAFB Albuquerque, NM 87185

(505) 844-5595

Donald M. Friedrich

Battel le-Pacific Northwest Labs.

Richland, WA 99352

(509) 375-2440

Gerald A. Gallegos

Los Alamos National Laboratory

MS J566

P.O. Box 1663

Los Alamos, NM 87545

(505) 567-6448

Ann Genco

Hughes Aircraft

P.O. Box 902 Bldg E4 MS/M133

El Segundo, CA 90245

(213) 515-9182

Adolf Giesen

DFVLR, Institute fur Tech.

Pfaffenwaldring 38-40

7000 Stuttgart 80

FED. REP. OF GERMANY

0711-6862-223

Physi

Dennis H. Gill Los Alamos National Laboratory

Chem. Div. J553

P.O. Box 1563

Los Alamos, NM 87544

(505) 567-8580

Ray Gonzales

Lawrence Livermore National Lab

P.O. Box 5508, L-490

Livermore, CA 94550

(415) 422-6059

550
George A. Graves

University of Dayton

RM 165 Kettering Labs,

300 College Park

Dayton, OH 45469

(513) 229-4415

Arthur H. Guenther

AFWL/CCN

Kirtland AFB, NM 87117

(505) 844-9856

George Greiner

Lawrence Livermore National Lab

P.O. Box 5508, L-474

Livermore, CA 94550

Shekhar Guha

Martin Marietta Laboratories

1450 South Rolling Road

Baltimore, MD 21227

(301) 247-0700

Gerald A. Gurtman

S-CUBED, Div. of Maxwell Labs, Inc. P.O. Box 1620

La Jolla, CA 92038-1620

(619) 453-0060 Ext 443

Richard F. Haglund, Jr. Vanderbilt University

Department of Physics & Astronomy

Nashville, TN 37235

(615) 322-2828

Robert E. Hahn

Optical Coating Laboratory, Inc. 2789 Northpoint Parkway

Santa Rosa, CA 95401-7397

(707) 525-7791

Donald J. Harter

Al 1 ied Corporation

7 Powderhorn Dr. Mt. Bethel, NJ 07060

(201) 560-1750

Wil 1 i am Hatcher

Lawrence Livermore National Lab

P.O. Box 5508, L-482

Livermore, CA 94550

(415) 422-4375

Allan Heiney

Allied Corporation

7 Powderhorn Drive

Mt. Bethel, NJ 07050

(201) 560-1750

R. B. Hemphill

LTV Aerospace & Defense Co. P.O. Box 550003

MS/TH-67

Dallas, TX 75265-0003

(214) 266-7105

Robert L. Hengehold

Air Force Inst, of Technology

Wright-Patterson AFB, OH

(513) 255-2012

Lowell M. Hobrock

Hughes Aircraft Company

2000 E. El Segundo Blvd.

P.O. Box 902

El Segundo, CA 90245

(213) 616-9620

Lonnie Holder

McDonnell Douglas Astronautics Co. MS-E804/101A/2/230

P.O. Box 515

St. Louis, MO 53165

(314) 234-8436

Alan K. Hopkins

AhWAL/Materials Lab.

Wright-Patterson AFB, OH 45459

Sandra L. Holder

Department of the Navy

Naval Sea Systems Command (PMS-405)

CM2 Room 113

Washington, DC 20362-5101

(202) 692-8641

Samuel J. Holmes

Northrop Research & Technology Centr

One Research Park

Palos Verdes Peninsula, CA 90274

(213) 544-5314

Long-Sheng Hsu Central Laser Corporation

11222 Astronaut Blvd.

Orlando, FL 32819

(305) 851-2540

551
Austin C. Huang

Electrical & Computer Eng. Dept.

University of Texas at Austin

Austin, TX 78712

(512) 471-3628

Mark Imus Optical Coating Laboratory, Inc. 2789 Northpoint Parkway

Santa Rosa, CA 95401-7397

(707) 525-7742

Stephen D. Jacobs

Laboratory for Laser Energetics

University of Rochester

250 East River Road

Rochester, NY 14623

(716) 275-5101

Linda Johnson

Naval Weapons Center

Physics Division, Code 3818

China Lake, CA 93555

(619) 939-1422

L. John Jol in Los Alamos National Laboratory

P.O. Box 1663

CHM-5 MS-J565

Los Alamos, NM 87544

(505) 667-7314/2151

Kenneth Jungl i ng Univ. of New Mexico, EECE

Albuquerque, NM 87131

(505) 277-3317

Austin Kalb

Rockwell International

3370 Miraloma Ave. MS/GE 14 Anaheim CA 92803/4921

(415) 632-5008

John A. Kardach

U.S. Air Force

AFWL/CA

Kirtland AFB, NM 87109

(505) 844-7358

Geza L. Keller

Los Alamos National Laboratory

M/S E523

P.O. Box 1663

Los Alamos, NM 87545

(505) 667-1537

Marshall N. Huberman

TRW, Bldg. 01, Room 1051

One Space Park

Redondo Beach, CA 90278

(213) 536-4413

Takao Izawa Showa Optical Co. , LTD

5-3 Shinmachi, 3-Chome

Setagaya-Ko

Tokyo 154

JAPAN

03-429-4131

Yong Jee Electrical & Computer Eng. Dept.

University of Texas at Austin

Austin, TX 778712

(512) 471-3528

George T. Johnston

Optical Coating Laboratory, Inc. 2789 Northpoint Parkway

Santa Rosa, CA 95401-7397

(707) 525-7148

Michael P. Joseph

Dynavac Corporation

30 Woodrock Road

Weymouth, MO 02189

(517) 337-4111

Doug Jungwirth

Northrop Electronics Div.

2301 West 120th Street

Hawthorne, CA 90250

(213) 505-5744

Marvin M. Kaplan

Litton Laser Systems

P.O. Box 7300

Orlando, FL 32854

(301) 297-4528

Kenneth Kawabata

Hughes Aircraft

P.O. Box 902 E1-F185

El Segundo, CA 90245

Wayne D. Kimura

Spectra Technology

2755 Northup Way

Bellevue, WA 98004-1495

(206) 827-3517

552
Tim F. Kish Laser Power Optics

11211 Sorrento Valley Road

San Diego, CA 92121

(619) 455-0751

Christopher Klein

The Aerospace Corporation

MY 980,

P. 0. Box 92957

Los Angeles, CA 90009

(213) 548-6226

Will i am Koldewyn

Scientech, Inc. 5649 Arapahoe

Boulder, CO 80303

(303) 444-1351

Vital iy Konov Institute of General Physics

Acad. Sc. USSR

Vasilov Street 38

Moscow

USSR

Paul Korbin Nessim Lagnado

Rockwell International Hughes Santa Barbara Research Cent

1049 Camino Dos Rios 75 Coromar Drive

Thausand Oaks, CA 91360 Goleta, CA 93117

(805) 373-4485 (805) 958-3511 Ext 2615

Ian Laidler

Loughborough University of Technology

Physics Department

Loughborough

Leicestershire, LEll 3TU

ENGLAND

(0509) 2563171 Ext 5044

Michael R. Lange

University of New Mexico

Inst, for Modern Optics

Albuquerque, NM 87131

Ramin Lalezari

PMS Electro Optics

1855 S. 57th Ct. Boulder, CO 80301

(303) 443-7100

Joseph R. Latore Airtron Div. of Litton Systems Inc 200 E. Hanover Avenue

Morris Plains, NJ 07950

(201) 539-5500

G. David Lawrence

Hughes Aircraft

211 Yacht Club Way #221

Redondo Beach, CA 90277

(213) 516-5237

Paul S. Laybourne

W. J. Schafer Associates, Inc. 1500 Randolph Ct. Albuquerque, NM 87106

Wil 1 i am A. Leamon

Los Alamos National Laboratory

P.O. Box 1663

Los Alamos, NM 87545

(505) 557-7314

Donald H. Leeds

Hughes Aircraft

P.O. Box 902

El Segundo, CA 90245

(213) 515-7695

Thomas A. Leonard

Analytic Decisions Inc. 337 N. Broad Street

Fairborn, OH 45324

(513) 879-4571

Daniel H. Leslie

W.J. Schafer Associates

1901 N. Fort Myer Dr. Arlington, VA 22209

(703) 558-7900

Keith L. Lewis

Royal Signals & Radar

Establ i shment

St. Andrews Rd. Malvern, Worcs. WR14 3PS

UNITED KINGDOM

(05845) 2733 Ext 3062

Hui H. Li CINDAS

Purdue University

2595 Yeager Rd. West Lafayette, IN 47905

(317) 494-6300

553
Bangjun Liao Jacques F. Linder

New York State Center for Advanced Hughes Aircraft Co. Optical Technology 2000 E. El Segundo Blvd.

University of Rochester El Segundo, CA 90274

Institute of Optics, Wilmot Bldg. (213) 616-6173

Rochester, NY 14527

Bo Ljung

Si nger-Kearfott Div.

1225 McBride Ave. Little Falls, NJ 07474

(201) 785-2761

Tony Lowderback

Ojai Research Corporation

11554 Ventura Ave. Ojai, CA 93023

W. Howard Lowdermilk

Lawrence Livermore National Lab

P.O. Box 5508, L-490

Livermore, CA 94550

(415) 422-5498

Samuel Lu Litton Industries

GCS

5500 Canoga Avenue

Woodland Hills, CA 91365

(818) 715-4565

Donald S. Luczak

Rome Air Dev. Ctr./USAF

Griffiss AFB, NY 13441-5700

(315) 330-3145

David Lunt Lunt & Co. P.O. Box 368

Richland, WA 99352

(509) 375-4792

Michael Lunt Technical Optics Ltd.

Second Avenue,

Onchan, Isle of Man UNITED KINGDOM 0624 4443

0624 74443

H. Angus MacLeod

Optical Sciences Center

University of Arizona

Tucson, AZ 85721

(602) 621-2449

Taaro Mandre

TRW Space & Technology Group

One Space Park

01/1251

Redondo Beach, CA 90278

(213) 535-7139

Diane J. Martin

USAF

AFWL/ARAO

Kirtland AFB, NM 87117-5008

(505) 844-1776

Carl S. Masser Debra A. May

Northrop Research & Technology Center KIGRE, Inc. One Research Park 5335 Secor Road

Palos Verdes Peninsula, CA 90274 Toledo, OH 43523

(213) 544-5470 (419) 473-3157

Will iam J. McCarty

Perkin-Elmer Corp., AOO

7421 Orangewood Avenue

P.O. Box 3115

Garden Grove, CA 92642

(714) 895-1557

Rick L. McGann

Naval Weapons Center

China Lake, CA 93555

(619) 939-3247

Fred McClung

Hughes Aircraft Co. 1500 Randolph Ct. S.E. Albuquerque, NM 87106

(505) 242-9818

James F. McNally

United States Air Force 8808 Camino del Sol Albuquerque, NM 87111

(505) 277-5502

554
John R. McNeil

University of New Mexico

Electrical Engineering Dept.

Albuquerque, NM 87131

David Milam

Lawrence Livermore National Lab

P. 0. Box 5508, L-490

Livermore, CA 94550

(415) 422-5499

S. Mohan

Indian Institute of Science

Instrumentation & Services Unit

Bangalore-550 012 INDIA

Mark B. Moran

Naval Weapons Center-Physics Div.

Code 3817

China Lake, CA 93555

(619) 939-3827

Dennis Morelli

Optical Coating Laboratory, Inc. 2789 Northpoint Parkway

Dept. 428-1

Santa Rosa, CA 95407-7397

(707) 525-7024

John D. Myers

Kigre Inc. 5333 Secor Road #10

Toledo, OH 43523

(419) 473-3157

Yoshiharu Namba

Dept. of Precision Engineering

Osaka University

2-1 Yamadaoka, Suita

Osaka 565

JAPAN

05-877-5111 ext. 4507

Brian E. Newnam

Los Alamos National Laboratory

MS-J554

Los Alamos, NM 87545

(505) 557-7979

Nicholas S. Nogar

Los Alamos National Labs.

CHM2 G-738

Los Alamos, NM 87545

(505) 557-93-5

Larry D. Merkle

Dept. of Physics

Univ. of Arkansas

Fayetteville, AR 72701

(501) 575-5425

Mark Mildebrath

Rockwell International

3370 Miraloma

MS GE 14 Anaheim, CA 92803

(714) 752-3411

N. Kent Moncur

KMS Fusion, Inc. P.O. Box 1557 Ann Arbor, MI 48105

(313) 759-8500

David Mordaunt

Hughes Aircraft Company

Bldg. El . Mail Station E142

P.O. Box 902

El Segundo, CA 90243

(213) 515-5422

Patricia L. Morse

AFWL/ARBE

Optical Components Branch

Kirtland ARB, NM 87117

(505) 844-1776

George Nado Texas Instruments

Box 225015

Mail Stop 3196

Dallas, TX 75265

(214) 480-6412

Ontario H. Nestor

Harshaw Chemical Company

5801 Cochran Road

Solon, OH 44122

(215) 248-7400

Davis B. Nichols

Boeing Aerospace Co. Mail Stop 8H-18

P.O. Box 3999

Seattle, WA 98124

(205) 773-8938

Robert M. O'Connell

Univ. of Missouri - Columbia

Elect. Engineering Dept.

College of Engineering

Columbia, MO 55201

(314) 882-8373

555
Takayuki Okamoto

Okamoto Optics Work, Inc. 8-34 Haramachi, Isogo-Ku

Yokohama 235

JAPAN

(045) 752-2233

Yoshiaki Okamoto

Okamoto Optics Work, Inc. 8-34 Haramachi, Isogo-Ku

Yakohama 235

JAPAN

(045) 752-2233

Harumichi Okamoto

Ni ppon Mi ni ng Co . , Ltd

.

3-17-35 Ni izomminami , Toda-shi

Saitama 335

JAPAN

454-42-1811

Richard Owen

Spiricon Inc. 2600 N. Main

Logan, VT 84327

(801) 753-3729

Kathleen M. Padgett

Los Alamos National Laboratory

P.O. Box 1563, MS J565

Los Alamos, NM 87545

(505) 657-7677

Roger A. Paquin

Perkin Elmer Corporation

Electro-Optical Div.

100 Wooster Heights Road

M/S 955

Danbury, CT 06810

(203) 797-6130

James R. Palmer

COMCARO,Inc.

1201 N. China Lake Blvd.

Ridgecrest, CA 93555

(619) 446-7571

Walter T. Pawlewicz

Metallurgy & Coating Section

Battelle Northwest Lab.

2955 George Washington Way

Richland, WA 99352

(509) 375-2074

Samuel F. Pellicori

Santa Barbara Research Center

75 Coromar Drive

Goleta, CA 93117

(805) 958-3511 Ext 2379

Ralph Perkins

Corning

SP-DV-01-1

Corning, NY 14831

(507) 974-3018

John H. Pitts

Lawrence Livermore National

P.O. Box 5508, L-480

Livermore, CA 94550

(415) 422-5327

Robert D. Poirier

Laboratory Perkin Elmer Corp.

751 Main Avenue

Norwalk, CT 05859-0420

(203) 834-4934

Bradley Pond

Martin Marietta

P.O. Box 9316

Albuquerque, NM 87119

(505) 844-1064

James 0. Porteus

Naval Weapons Center

Code 3817

China Lake, CA 93555

(519) 999-3827

Shari Powel 1 -Pi sher

Optical Coating Laboratory, Inc. 2789 Northpoint Parkway

Santa Rosa, CA 95401-7397

(707) 525-7778

Thomas Price MIT Lincoln Laboratory

P.O. Box 73 L-218

Lexington, MA 02173-0073

(517) 853-5500

Kopel Rabinovitch

Electro Optics Industries, Ltd. P.O. Box 1165

Rehovot 76110

ISRAEL

(78) 486253

Tilak Raj

Martin Marietta Denver Aerosp

P.O. Box 9316

International Airport

Albuquerque, NM 87119

(505) 844-1064

556
John Ranson

Pilkington P.E. Glascoed Road

St. Asath

Clwyd LL170LL

United Kingdom

Theresa M. Roberts

USAF

RADC/ESMG

Hanscom AFB

Redford, MA 01731-5000

(617) 851-3593

Andrew Romberger

Pennsylvania State University

Berks Campus

P.O. Box 2150

Reading, PA 19508

(215) 320-4817

Carlos Roundy

Spiricon, Inc. 2500 N. Main

Logan, UT 84321

(801) 753-3729

Joseph Sanchez

Optical Coating Laboratory, Inc. 2789 Northpoint Parkway

Santa Rosa, CA 955407-7397

(707) 525-7728

Marion L. Scott

Los Alamos National Laboratory

P.O. Box 1553, MS E549

Los Alamos, NM 877545

(505) 557-4258

Steven C. Seitel

Montana Laser Optics, Inc.

P. 0. Box 4151

Bozeman, MT 59772-4151

(406) 585-5100

Keith R. Shillito

AFWAL/MLPJ

Wright-Patterson AFB

Dayton, OH 45433

(415) 423-3508

Dennis R. Sigler

Burleigh Northwest Optical, Inc. Burleigh Park

Fishers, NY 14453

(715) 924-9355

Herman E. Reedy

Two-Six Inc. Saxonburg Blvd.

Saxonburg, PA 16056

(412) 352-4455

Raymond S. Robinson

Colorado State University

Ft. Collins, CO 80523

(303) 491-6202

James R. Rotge

Frank J. Seiler Research Laboratory

FJSRL/NH

USAF Academy

Colorado Springs, CO 880840

(303) 472-3122

Theodore T. Saito

Lawrence Livermore National Lab.

P.O. Box 808, L-791

Livermore, CA 944550

(425) 442-7600

Paul Schall

The Aerospace Corporation

M2/321

P.O. Box 920957

Los Angeles, CA 90009

(213) 648-7502

David W. Seegmiller

W.J. Schafer Associates, Inc. 1500 Randolph Ct. S.E. Suite #202

Albuquerque, NM 87106

(505) 242-9992

C. Y. She

Physics Dept.

Colorado State University

Fort Collins, CO 80523

(303) 491-6261

Wigbert Siekhaus

Lawrence Livermore

National Laboratory

P.O. Box 8081 MS-L311

Livermore, CA 94550

(415) 422-6884

Philip A. Simpson

724.02

National Bureau of Standards

325 Broadway

Boulder, CO 80303

(303) 497-3789

557
James R. Sites

Physics Department

Colorado State University

Fort Collins, CO 80523

(303) 491-5850

Thomas Smith TRW

One Space Park

Redondo Beach, CA 90278

(415) 532-8269

Paul G. Snyder

University of Nebraska

Department of Electrical Engineering

W194 Nebraska Hall

Lincoln, NE 68588-0511

(402) 472-3771

Will iam H. Southwell

Rockwell Science Center

1049 Camino Dos Rios

Thousand Oaks, CA 91360

(805) 373-4485

James L. Stanford

Naval Weapons Center, Code 3818

China Lake, CA 93555

(619) 939-1418

Alan F. Stewart

Air Force Weapons Lab.

AFWL/ARBE

Kirtland AFB, NM 87117-5008

(505) 844-7358

W. Kent Stowell

AFWAL/AADO-2

Wright-Patterson AFB

Dayton, OH 45433

(513) 255-5147

Martin R. Sweet

Los Alamos National Laboratory

Los Alamos, NM 87545

(505) 657-7100

H. Takahashi

Nippon Mining Co. , LTD 1858 Dalehurst Ave.

Los Altos, CA 94022

(415) 854-7948

James G. SI iney

Rockwell International

Rocketdyne Division

6633 Canoga Avenue

D/531, 055, FA03

Canoga Park, CA 91304

(818) 700-4900

Douglas J. Smith

Institute of Optics

University of Rochester

Wilmot Building

Rochester, NY 14527

(716) 275-4135

M. J. Soileau

North Texas State University

Dept. of Physics

Denton, TX 75303

(817) 565-3263

Mike C. Staggs

Lawrence Livermore National Lab

P.O. Box 5508, L-490

Livermore, CA 94550

(415) 422-5050

James L. Stapp

Rocketdyne

P.O. Box 5570 - M/S RNMl

Kirtland AFB., NM 87185

(505) 844-9013

C. Stonecyper

Litton Laser Systems

P.O. Box 7300

Orlando, FL 32854

(305) 297-4458

Jack B. Stubbs

University of Dayton

Research Institute

KL-464

Dayton, OH 45469

(513) 229-3724

Randall T. Swimm

University of Southern Californ

University Park

Los Angeles, CA

(213) 743-4370

Ian Thomas

Lawrence Livermore National Lab

P.O. Box 5808, L-483

Livermaore, CA 94550

(415) 433-3896

558
David A. Thompson

Corning Glass Works

Sullivan Park DV-25

Corning, NY 14831

(607) 974-3311

Therese Tillmann

USAF - AFWL/ARBE

Kirtland AFB, NM 87117

(505) 844-1776

Norman Tolk

Vanderbilt University

P.O. Box 1807 Station B Dept. of Physics & Astronomy

Nashville, TN 37235

(615) 322-2828

H. Toratani

Hoya Optics Inc. Can. LTD 3400 Edison Way

Fremont, CA 94538

(415) 490-1880

John R. Triplett

S-Cubed, A Div. of Maxwell Labs, Inc. P.O. Box 1520

La Jolla, CA 92038-1620

(619) 453-0060 Ext 407

Trudy Tuttle Hart

OCLI Optical Coating Laboratory, Inc. 2789 Northpoint Parkway,

MS/121-1

Santa Rosa, CA 955407-7397

(707) 525-7181

Willard R. Wadt

Los Alamos National Laboratory

MS J569

P.O. Box 1663

Los Alamos, NM 87545

(505) 557-7763/2097

Stephen Tuenge

Martin Marietta Denver Aerospace

P.O. Box 9315

International Airport

Albuquerque, NM 87119

(505) 844-1054

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

Anetta J. Weber

Air Force Weapons Lab/ARAO

Kirtland AFB, NM 87117

Will i am J. Welker

USAFA

AFWL/ARBE

Kirtland AFB, NM 87117-5008

(505) 844-1776

Stan Whitcomb

Northrop Electronics Division

2301 West 120th Street

Hawthorne, CA 90250

(213) 606-5758

Frederic H. White III Keith Wier

Rocketdyne Division Scientech Inc. Rockwell International 5549 Arapahoe

6533 Canoga Ave. Boulder, CO 80303

Canoga Park, CA 91304 (303) 444-1351

John G. Wilder

Lawrence Livermore National Laboratory

P.O. Box 5508, L-490

Livermore, CA 94550

(415) 423-3896

Ronald A. Wilklow

Perkin-Elmer

77 Danbury Road

Wilton, CY 05897-420

(203) 834-4450

Jeffrey T. Will iams

COMARCO

1201 N. China Lake Blvd.

Ridgecrest, CA 93555

(519) 446-7571

A. D. Wilson

Barr & Stroud Ltd. Caxton Street

Annies! and

Glasgow, 9131HZ

SCOTLAND, UK

(041) 954-9501

559
G. Richard Wirtenson

Lawrence Livermore National Laboratory

P.O. Box 5508, L-491

Livermore, CA 94550

(415) 422-1332

Frank Woodberry

Rocketdyne FA 42 6533 Canoga Ave. Canoga Park, CA 91304

(818) 700-4931

Matt Young

724.02

National Bureau of Standards

325 Broadway

Boulder, CO 80303

(303) 497-3223

Thomas G. Zocco

Los Alamos National Laboratory

P.O. Box 1563, MS G730

Los Alamos, NM 87545

(505) 657-4481

Roger M. Wood

G. E. C. Research Laboratories

East Lane Wembley, Middx

UNITED KINGDOM

01-904-1262-205

Kunio Yoshida

Osaka University-Inst. for Laser Eng.

2-6 Yamada-oka

Suita Osaka, 565

JAPAN

(05) 877-5111

Rick Zoborowski

OCLI

2789 Northpoint Parkway

Dept. 428-1

Santa Rosa, CA 95407-7397

(707) 525-7010

560
NBS-n4A >REv. 2-8C) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

PUBLICATION OR

REPORT NO.

NBS/SP-746

2. Performing Organ. Report No 3. Publ ication Date July 1988

4. TlYtE AND SUBTITLE

Laser Induced Damage in Optical Materials: 1985

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 N6S, see ;n struct/on sj

NATIONAL BUREAU OF STANDARDS

U.S. DEPARTMENT OF COMMERCE

BOULDER, CO 80303

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS fStreet. 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 88-600557

I I Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most si gnificant information. If document includes a si gnificant

bi bl irigraphy or literature survey, mention it here) The Seventeenth Annual Symposium on Optical Materials for High^Power Lasers (Boulder

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

October 28-30, 1985. The Symposium was held under the auspices of ASTM Committee F-1,

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

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

Air Force Office of Scientific Research. Over 215 scientists attended the Symposium,

including representatives from the United States, the United Kingdom, Israel, India,

Japan, the Federal Republic of Germany, and the Soviet Union. The Symposium was dividec

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 prime

interest was from 10.6 ym 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. Guenth|er

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 Eighteenth Annual Symposium is scheduled for Nov. 3-5, 1986, at

the National Bureau of Standards, Boulder, Colorado.

12. KEY WORDS (Six to twelve entries; alplnabetical order; capitalize only proper names; and separate key words by semicolons)

laser damage; laser interaction; optical components; optical fabrication; optical

materials and properties; thin film coatings.

13. AVAILABILITY

rXI Unlimited

• For Official Distribution. Do Not Release to NTIS

rX| Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402. Q Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

576

15. Price

USCOMM-DC 6043-P80
INSTRUCTIONS

FORM NBS>114A: BIBLIOGRAPHIC DATA SHEET. This bibliographic data sheet meets the standards adopted for use by all U.S. Government agencies. It is needed forNTIS processing and must accompany all NBS papers, those cM^pearing in nongovernmental media as well as those in N BS series, since all reports of NBS technical work are normally entered into the NTIS system. For all GPO publications, it becomes an

integral part of the document and is widely used by librarians and abstractors.

Items 1, 2 — Complete if information is available; otherwise Publ ications Office will complete later. If non-NBS publication, state "see item 10" (Enter other agency sponsor's report number if requested to do so, and enter NBSIR number under item 2).

Item 3 — Complete if known; otherwise Publications Office will complete.

Items 4, 5 —Complete as shown on manuscript. When NBS-II4A is resubmitted along with NBS-266,

following publication of non-NBS media papers, these items must agree with published paper.

Item 6 — If not NBS, blank out and enter Grantee/Contractor name and address, or if performed

jointly, show both.

Item 7 — Complete when applicable.

Item 8 — Enter "Interim," "Final," or period covered.

Item 9 — Enter all sponsors' names and addresses. Include NBS if also a sponsor.

Item 10 —Enter other relevant information, i.e., related or superseded documents. Also used by

Publications Office for Library of Congress catalog number, and entry of non-N BS media

citation upon receipt of Form NBS-266 from author. Check block if appropriate and attach SFI85.

Item 11, 12 — Prepare abstract and key words with special care. These are published separately by NBS, NTIS, and other bibliographic services, and are vital elements in guiding readers to your paper. The key words will be used as entries in a subject index. See NBS Communications Manual

for additional guidance.

Item 13 — Indicate "Ualimited" — for open-literature documents cleared under NBS editorial procedures,

or "For official distribution. Do not release to NTIS" — for limited, restricted, or need-to- know material (See Communications Manual). Publications Office will mark appropriate

"order" box and complete Stock Number when known.

Items 14, 15 -Leave blank. NBSIR's.

To be completed by Publications Office or call Printing and Duplicating for

NBS-l 14A (REV. 2-80) U SCOMM-O C e043-P 80

«U. S.COWEHNinENT PRINTING OFF I C£ : 1 988 - 2 1 6-8 25


Technical Publications

Periodical

Journal of Research—The Journal of Research of the National Bureau of Standards reports NBS research and development in those disciplines of the physical and engineering sciences in which the Bureau is aaive. These include physics, chemistry, engineering, mathematics, and computer sciences. Papers cover a broad

range of subjects, with major emphasis on measurement methodology and the basic technology underlying

standardization. Also included from time to time are survey articles on topics closely related to the Bureau's

technical and scientific programs. Issued six times a year.

Nonperiodicals

Monographs—Major contributions to the technical literature on various subjects related to the Bureau's scientific and technical activities. Handbooks—Recommended codes of engineering and industrial practice (including safety codes) developed in cooperation wath interested industries, professional organizations, and regulatory bodies.

Special Publications—Include proceedings of conferences sponsored by NBS, NBS annual reports, and other

special publications appropriate to this grouping such as wall charts, pocket cards, and bibliographies.

Applied Mathematics Series—Mathematical tables, manuals, zmd studies of special interest to physicists,

engineers, chemists, biologists, mathematicians, computer programmers, and others engaged in scientific and

technical work.

National Standard Reference Data Series—Provides quantitative data on the physical and chemical properties

of materials, compiled from the world's literature and critically evaluated. Developed under a worldwide pro- gram coordinated by NBS under the authority of the National Standard Data Act (Public Law 90-396).

NOTE: The Journal of Physical and Chemical Reference Data (JPCRD) is published quarterly for NBS by

the American Chemical Society (ACS) and the American Institute of Physics (AIP). Subscriptions, reprints,

and supplements are available from ACS, 1155 Sixteenth St., NW, Washington, DC 20056.

Building Science Series—Disseminates technical information developed at the Bureau on building materials,

components, systems, and whole structures. The series presents research results, test methods, and perfor- mance criteria related to the structural and environmental functions and the durability and safety charaaeristics of building elements and systems.

Technical Notes—Studies or reports which are complete in themselves but restrictive in their treatment of a

subject. Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject

area. Often serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under procedures published by the Department of Commerce in Part 10, Tide 15, of the Code of Federal Regulations. The standards establish nationally recognized re- quirements for products, and provide all concerned interests with a basis for common understanding of the

characteristics of the products. NBS administers this program as a supplement to the activities of the private

sector standardizing organizations.

Consumer Information Series—Practical information, based on NBS research and experience, covering areas of interest to the consumer. Easily understandable language and iUustrations provide useful background

knowledge for shopping in today's technological marketplace.

Order the above NBS publications from: Superintendent of Documents, Government Printing Office,

Washington, DC 20402. Order the following NBS publications—FIPS and NBSIR's—from the National Technical Information Service, Springfield, VA 22161.

Federal Information Processing Standards Publications (FIPS PUB)—Publications in this series collectively constitute the Federal Information Processing Standards Register. The Register serves as the official source of

information in the Federal Government regarding standards issued by NBS pursuant to the Federal Property

and Administrative Services Act of 1949 as amended. Public Law 89-306 (79 Stat. 1 127), and as implemented

by Executive Order 1 1717 (38 FR 12315, dated May 11, 1973) and Part 6 of Title 15 CFR (Code of Federal

Regulations).

NBS Interagency Reports (NBSIR)—A special series of interim or final reports on work performed by NBS

for outside sponsors (both government and non-government). In general, initial distribution is handled by the sponsor; public distribution is by the National Technical Information Service, Springfield, VA 22161, in paper copy or microfiche form.
U.S. Department of Commerce

National Bureau of Standards

Galtherstxjrg, MD 20899

Official Business

Penalty for Private Use $300

Sllmulillng Anwhca's Progress 1913-198S