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

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NATIONAL BUREAU OF STANDARDS

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Hationa! Bureau of Siaiwai MAY 4 «W

Laser Induced Damage

In Optical Materials: 1978

Proceeding 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

September 12-14, 1978

NBS Boulder, Colorado 80303

Edited by

Alexander J. Glass

Lawrence Livermore Laboratory

Livermore, California 94550

Arthur H. Guenther

Air Force Weapons Laboratory

Kirtland AFB, New Mexico 87117

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued December 1978

and

0CX OP
Library of Congress Catalog Card Number 79-600050

National Bureau of Standards Special Publication 541

Nat. Bur. Stand. (U.S.), Spec. Publ. 541, 362 pages (Dec. 1978) CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1979

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

Stock No. 003-003-02052-4 Price $5.50

(Add 25 percent additional for other than U.S. mailing).
Foreword

The Proceedings contain the papers presented at the Tenth Annual Symposium on Optical Materials

for High Power Lasers held at the National Bureau of Standards in Boulder, Colorado, on September 12-14. 1978. The Symposium was jointly sponsored by the National Bureau of Standards, the American Society for Testing and Materials, the Office of Naval Research, the Defense Advanced

Research Projects Agency, and the Department of Energy. The Symposium was attended by about 175 scientists from the United States, the United Kingdom, France, Canada, Japan, West Germany, and the

Soviet Union. It was divided into sessions devoted to the following topics: the Measurement of Absorption Characteristics, Bulk Material Properties, Mirrors and Surfaces, Thin Film Damage,

Coating Materials and Design, and Breakdown Phenomena. The Symposium Co-chairpersons were

Dr. Alexander J. Glass of the Lawrence Livermore Laboratory and Dr. Arthur H. Guenther of the Air

Force Weapons Laboratory, who also served as editors of this report.

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

contained in the report, and for the summaries of discussion found at the end of each paper. The manuscripts of the papers presented at the Symposium have been prepared by the designated authors,

and questions pertaining to their content should be addressed to those authors. The interested

reader is referred to the bibliography at the end of the summary article for general references to the literature of laser damage studies. The Eleventh Anuual Symposium on this topic will be held in Boulder, Colorado, from October 30 to 31, 1979. A concerted effort is being 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 1979 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 damage problems is anticipated.

Fabrication and test procedures will continue to advance, particularly in the micro-machined 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 Eleventh Annual Symposium.

Certain commercial equipment, instruments, or materials are identified in this publication in order to adequately specify the experimental procedure. In no case does such identification imply

recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material or equipment identified is necessarily the best available for the purpose.

A. H. Guenther and A. J. Glass

Co-chai rpersons

DISCLAIMER

iii
CONTENTS

Page

Section

Foreword iii A. J. Glass and A. H. Guenther

Summary and Conclusions vi i A. H. Guenther and A. J. Glass

S. I. Conversion Units xxv

INTRODUCTORY REMARKS

DOE Welcome--Presentati on of Awards to Drs. Guenther and Glass 1 C. M. Stickley

Defense ARPA Welcome 3 H. V. Winsor

Symposium Welcome 5 A. J. Glass

MEASUREMENT OF ABSORPTION CHARACTERISTICS

Discussion of a Theory of Analysis of Rate Calorimetery which Includes Coating Absorption 7 N. C. Fernelius and G. T. Johnston

Absorption Coefficient of NaF by Attenuated Total Reflection Spectroscopy 13

D. L. Burdick

Laser Calorimetric Measurement of Two Photon Absorption 19 M. Bass, E. W. Van Stryland, and A. F. Stewart

A Comparison of Bulk and Surface Absorptions in NaC£ and KCji between 9.2 and 10.8 ym 24

H. Vora, M. C. Ohmer, and T. G. Stoebe

A 1.06 ym Laser Absorption Calorimeter for Optical Coatings 33

T. H. Allen, J. H. Apfel , and C. K. Carniglia

Measured Thin Film Absorption at the Air-film Interface, the Film Bulk,

and the Film-substrate Interface 37

P. A. Temple, D. L. Decker, T. M. Donovan, and J. W. Bethke

Photoacoustic Spectroscopy Studies of Thin Film Coatings on Laser Windows 43

N. C. Fernelius and D. A. Walsh

BULK MATERIAL PROPERTIES

Piezo-optical Coefficients of Some Meodymium Doped Laser Glasses

and Single Crystals of CaF2, BaF2, and SrF£ 50

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

Refractive Index of Strontium Fluoride 55

M. J. Dodge

The Development of Fluorides for High Power Laser Optics 59

J. F. Ready, H. Vora, R. A. Skogman, K. M. Leung, and E. Bernal G.

Optical Properties of KCz Forged between Optically Polished Dies 65

R. H. Anderson and J. M. Bennett

Bulk Optical Properties of Fine Grained Forged Calcium Fluoride 70

R. H. Anderson, R. A. Skogman, J. F. Ready, and J. M. Bennett

CW Laser Damage in AR Coated Alkaline Earth Flourides at 3.8 ym 78

J. A. Detrio and R. D. Petty

Lattice Absorption, Phonon Assignments, and Image Spoiling Properties of CVD ZnS

in the Infrared.... 86

C. Klein, B. Di Benedetto, R. Donadio, T. Kohane, and J. Pappis

i v
Page

Laser Induced Damage in Fluoride Glasses: A Status Report 99

$. E. Stokowski, D. Milam, and M. J. Weber

Liquids for High Repetition Rate Glass Laser Systems

J. M. Rinefierd, S. D. Jacobs, D. C. Brown, J. A. Abate,

0. Lewis, and H. Applebaum 109

MIRRORS AND SURFACES

Physical and Optical Properties of Surfaces Generated by Diamond-Turning on an Advanced Machine

D. L. Decker and D. J. Grandjean 122

Optical and Metallurgical Characterization of Molybdenum Laser Mirrors

S. M. Wong, G. Krauss, and J. M. Bennett 132

1064 nm Laser Damage Thresholds of Polished Glass Surfaces as a Function of Pulse Duration and Surface Roughness

D. Milam 164

Large Giant and Free-running Laser Pulse Energy and Power Densities through Optical Fibers

M. J. Landry 168

THIN FILM DAMAGE

Ultraviolet Damage Resistance of Laser Coatings

B. E. Newnam and D. H. Gill 190

Multithreshold Evaluation of 100-nsec Pulsed Laser Damage to Coating Materials at 2.7 and 3.8 ym Wavelengths

J. 0. Porteus, T. M. Donovan, J. L. Jernigan, and W. N. Faith 202

Multithreshold Damage Measurements on AS2S3, As2Se3, and NaF at HF and DF Wavelengths

T. M. Donovan, J. 0. Porteus, J. L. Jernigan, and E. J. Ashley 212

TEM Investigation of Effects of a Barrier Layer on Damage to 1.064 pm AR Coatings

C. K. Carniglia, J. H. Apfel , G. B. Carrier, and D. Milam 218

Optical Techniques for the Determination of Pulsed Laser Damage in Thin Films

T. W. Walker, A. H. Guenther, and P. E. Nielsen 226

A Statistical Analysis of Absorptive Laser Damage in Dielectric Thin Films

A. B. Budgor and K. F. Luria-Budgor 235

COATING MATERIALS AND DESIGN

New Coating Materials for IR Laser Optical Components

R. C. Pastor, J. A. Harrington, L. E. Gorre, and R. K. Chew 249

Improved PbF2 Coatings for the Infrared

P. Baumeister, G. P. Arnold, and D. F. Edwards 257

Graded Index Coatings of Cubic Thallium Iodide (Til) and Lead Fluoride (PbF2)

T. J. Moravec and R. A. Skogman 259

Simple Expressions for Calculating the Effect of Volume or Interface Absorption in Thin Films

on the Performance of High Reflectance or Antireflectance Multilayer Coatings

H. E. Bennett and D. K. Burge 266

Simplified .Description of Dielectric Reflectors

M. Sparks and M. Flannery 278

Electric Fields near Coated Surfaces: Application to Damage Protection

H. B. Rosenstock 288

BREAKDOWN PHENOMENA

Computer Simulation of Laser Damage Morphology in the Alkali Hal ides

P. Kelly, D. Ritchie, P. Braunlich, and A. Schmid 296

The Relative Role of the Impact and Multiphoton Ionization Mechanisms in Laser Induced Damage

of Transparent Dielectrics

B. G. Gorshkov, A. S. Epifanov, and A. A. Manenkov 299

v
Page

Theory of Laser Damage in Dielectric Solids

S. Brawer and W. L. Smith 303

Laser Induced Damage in Semiconductors

Yu. K. Danileiko, A. A. Manenkov, and A. V. Sidorin 305

Frequency Dependence of Breakdown Fields in Single-Crystal Had and KCji M. J. Soileau, M. Bass, and E. W. Van Stryland 309

Investigation of the Surface Breakdown Mechanism in IR-optical Materials

V. I. Kovalev and F. S. Faizullov 318

APPENDIX I

Invited Review Papers 328

APPENDIX II

Participants 329

vi
Laser Induced Damage in Optical Materials

Tenth ASTM Symposium

September 12-14, 1978

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

Symposium) was held at the National Bureau of Standards in Boulder, Colorado, from 12-14 September 1978. The Symposium was held under the auspices of ASTM Committee F-l,

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

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

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

France, Canada, Japan, West Germany, and the Soviet Union. The Symposium was divided into

sessions concerning the Measurement of Absorption Characteristics, Bulk Material Properties,

Mirrors and Surfaces, Thin Film Damage, Coating Materials and Design, and Breakdown

Phenomena. 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 use from 10.6 urn to the uv region. Highlights included surface

characterization, thin film-substrate boundaries, and advances in fundamental laser-matter

threshold interactions and mechanisms. The scaling of damage thresholds with pulse duration,

focal area, and wavelength was also discussed. In commemoration of the tenth symposium in this series, a number of comprehensive review papers were presented to assess the state of the art in various facets of laser induced damage in optical materials. Alexander J. Glass

of Lawrence Livermore Laboratory and Arthur H. Guenther of the Air Force Weapons Laboratory were co-chairpersons of the Symposium. The Eleventh Annual Symposium is scheduled for 30-31 October 1979 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 Tenth 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, from 12 to 14 September 1978. The Symposium was held under the auspices of the ASTM Committee F-l, Subcommittee on Laser Standards, with the joint sponsorship of NBS, the Defense Advanced Research Projects Agency, the Department of Energy, and the Office of Naval Research. Working sessions of the Committee F-l Subcommittee on Lasers were held on Monday 11 September. About 175 scientists attended the Symposium,

including representatives of the United Kingdom, France, Canada, Japan, West Germany, and the Soviet

Union. The Symposium was divided into sessions concerning the Measurement of Absorption

Characteristics, Bulk Material Properties, Mirrors and Surfaces, Thin Film Damage, Coating Materials

and Design, and Breakdown Phenomena. As in previous years, several poster sessions were held. The

general consensus of those presenting poster papers and those viewing them was highly favorable. In all, over fifty technical presentations were made. Alexander J. Glass of Lawrence Livermore Laboratory

and Arthur H. Guenther of the Air Force Weapons Laboratory were co-chairpersons of the Symposium. At this, our Tenth Annual Symposium, we paused to reflect upon the past ten years' activities in laser

induced damage to optical materials. We enlisted the services of fourteen distinguished speakers who

are acknowledged to be the principal contributors in the many facets of the laser damage field. They

summarized and quantified as well as possible the state of the art and the state of the understanding

in their specialized areas. These review or tutorial lectures are being assembled and published as a separate volume on the subject of laser induced damage in optical materials. In addition, round-table

discussions were held on those specific subjects in which there is still some question as to the correctness of our understanding to give the participants an opportunity to make recommendations for

future research. The purpose of these symposia is to exchange information about optical materials for

high power lasers. The authors will welcome comments and criticism from all interested readers

relevant to this purpose and particularly relative to our plans for the Eleventh Annual Symposium,

scheduled for 30-31 October 1979, at the National Bureau of Standards, Boulder, Colorado.

2. Principal Conclusions

Advances in the field of laser materials are incremental rather than revolutionary. Each year's

progress represents a small step forward from the preceding year's position. The advances are also

sporadic, unfortunately. New problems or new approaches are often identified in one year's symposium,

only to be ignored in succeeding years, possibly to resurface a few years later, or not at all. One

reason for this is the reluctance of investigators to report on those methods which failed. We hear

only of the new approaches which have succeeded. Because of the incremental nature of advances in this

field, the results reported in this summary must be viewed in the context of ten years of laser damage

research; thus the significance of many of these results will not be clear for several years to come.

In the realm of operative damage phenomena, thin film damage remains the least well understood.

This is to be expected, considering the complexity of the problem and the number of variables required

vii
to characterize fully a thin film system. Most of the advances to date have been empirical, and even for film systems that usually exhibit high damage thresholds, control of the process variables is uncertain. Thus, a coating which performs excellently in one production run may well exhibit a lower threshold on the next run. The identification of the critical process variables has not been made.

There is abundant evidence that damage in thin films is not intrinsic, but arises from defects,

impurities, or absorbed materials. The question is, to what degree can multilayer dielectric films be made free of these defects and impurities. The morphology of the film--its grain size, porosity, and crystalline orientation--all depend on a multiplicity of process variables, including deposition

method, background pressure, substrate temperature, rate of deposition, and many others.

It is not surprising then that attempts to find simple solutions to thin film damage have not yielded consistent results. Techniques like barrier layer deposition work sometimes, but not always.

At pulse durations of several ns and beyond, damage seems to be thermal, originating at highly

localized sites. The identity and origin of these sites are still unclear. One does not expect this

picture to improve greatly in the near future.

Regarding surface damage, the picture is substantially clearer. Chemical contamination remains

the single greatest concern to surface damage. The contaminant can be H2O, hydroxy! ions, or polishing residue. Surface roughness plays a role in determining the damage threshold, more so at long

pulse durations, although the reason for the observed dependence remains open to conjecture. Regarding

surface damage, the number of process variables is significantly smaller than for thin films, so the

connection between process control and material performance should be much more straightforward.

Thermally induced bulk damage is an area in which great progess has been made. Note that

"damage," in this context, includes distortion, birefringence, and other thermal mechanisms, in addition to catastrophic failure. By the sufficient reduction of impurity absorption, bulk absorptions

can be reduced below 10~4cm~l for specific materials such as Si O2 and the alkali halides.

The intrinsic limit due to multiphonon absorption can be reached at infrared frequencies, and the

relevant phonon assignments identified. Thus, both empirical improvement and basic understanding have

been achieved for this particular scenario.

The case is quite the opposite for bulk breakdown. There is no consensus regarding the relative

role of the several mechanisms which can contribute to dielectric breakdown. These include multiphoton

absorption, impurity absorption, electron avalanche, and others. Attempts to link first-principle

models to observed damage thresholds are probably futile, given the complexity of the intervening

phenomena. Experiments are beginning to be reported in which precursors to damage, like photoconductivity or photocapacitance, are observed. These seem to promise improved correlation with theoretical

models.

It is, however, safe to say that dielectric breakdown in bulk materials is not a critical problem

for any application currently under investigation. It is important to understand the process and to understand electron behavior in dielectrics, because both surface damage and thin film damage are often accompanied by plasma formation, and so involve related, if not identical, phenomena. However, bulk dielectric breakdown is not on the "critical path" for system development.

Scaling laws provide a general guide to the behavior of damage thresholds as experimental

conditions —such as pulse duration, spot size, or surface roughness —are changed. Departures from

expected scaling often indicate the onset of some new phenomenon, or the introduction of extrinsic

factors. Thus, the dependence, which is widely observed to apply to surface damage, is often

found to describe thin film damage only at very short pulse durations. For pulses larger than a few hundred psec, extrinsic factors may enter, which can result in a departure from the simple scaling

1 aw.

The simple scaling relationships which have been reported are empirical and lack full theoretical

justification. Nevertheless, they provide a useful guide for exploratory damage results to new conditions and provide a test of consistency for measured threshold values.

Whatever its origin, catastrophic laser damage is a statistical phenomenon, like any other mode of material failure. Failure does not depend on the average properties of the material, but on the strength of the weakest point. The "damage threshold" reported by experimentalists is a statistical

concept, representing the intuitive dividing line between power densities at which failure is likely to occur and those at which failure is unlikely. This year, the Weibull statistics, which are widely

employed to model failure of electrical components, were applied to laser damage data with gratifying

results. Note that this analysis does not presuppose a particular physical model for damage, although

the data can be examined, using this analysis, to see if the results are consistent with a particular

model. The general approach to laser damage as a generic failure mode appears very promising.

The capability to measure, localize, and identify the dominant source of failure has been greatly

advanced in recent years. This year, we heard of greatly increased sensitivity in the measurement of weak absorptions in optical materials. Bulk absorptions of less than 10"^cm"l have been measured calorimetrical ly. both on long bar samples of KCi and coated samples of fused silica. Surface

absorptions as low as 10 - ° have also been measured.

vi i i
Using techniques like long bar calorimetry, the scanning adiabatic film calorimeter and photoacoustic spectroscopy, the contributions of absorption in bulk materials at the surface, at thin

film interfaces, or in thin film layers can be separated. This enables one to say where, if not why,

the damaging absorption is occurring. Present indications are that surface absorption at the surface-film interface is a common source of absorption-induced damage.

As the pulse length varies, the dominant mechanism changes. Multithreshol d analysis allows one to

determine which damage process dominates as the pulse length is varied. Again, the root cause of the

damage is not identified, but rather the consequence.

Scattering, as a sensitive indicator of damage, has been investigated for some time. This year, a quantitative, sensitive scattering technique was discussed, which should provide a useful addition to the arsenal of sensitive measurement techniques.

Several advances were reported in the realm of materials preparation and fabrication. One such advance is in the isostatic forging of optical elements. The forging of Li F optics, reported this

year, is of great interest for short wavelength optics, since very few materials other than Li F are available for high power use at 250 nm or shorter wavelengths. Similarly, forged optics made from

other alkali halides and CaF2 will find use in systems operating in both the uv and IR. At present,

forged optics show higher scattering losses than the original crystalline material, but the approach is very promising.

Metal mirrors continue to be attractive, especially where diamond-turning can be used in the

fabrication. Molybdenum is a substrate material of considerable interest. As interest shifts to the

uv the problem of dielectric coatings for metal mirrors commands increasing attention. The

compatibility of coating and substrate will add another dimension to the already complicated problem of

mirror design.

Short wavelength systems are beginning to yield data on component damage; and, as expected, laser damage is a limiting factor for uv systems. Two-photon absorption is seen to play a role in damage in both coatings and bulk material, and techniques have been advanced to measure nonlinear absorption.

Nonlinear absorption coefficients have only been measured in a few materials, perhaps a dozen in all. The purity of these materials and the sensitivity of the two-photon absorption at various wavelengths

to the presence of impurities have not been determined. Clearly, a substantial amount of research is required to bring the state of uv optics up to the required level.

As a consequence of a decade of damage studies, system designers are aware of the limitations

imposed on system performance at high power levels, by laser damage. This heightened consciousness of damage problems within the community will undoubtedly spur the improvement of uv optics for high power

systems. Additionally, at all wavelengths, techniques for damage avoidance provide a way around the

limitations of materials. The term "damage avoidance" refers to methods whereby the total power from

an aperture can be increased, while keeping the peak power below the damage-imposed limit. These

include spatial filtering, optical relaying, plasma filtering, and phase conjugation. Other techniques

undoubtedly remain to be discovered.

Fiber optics and semiconductor lasers seem out of place at a symposium on high power optics, but the power densities achieved in these devices can be high. Because of the commercial importance of these devices, it is important that the optical materials community be aware of the specific problems

encountered in their use. This year, in a paper presented on damage in optical fibers, the speaker

concluded that surface damage at the edge of the core was the decisive failure mode. Considering the remarkable purity of the core material in low-loss fibers, this is to be expected. This surface damage

in fibers thus may be a problem strongly similar to other surface damage phenomena, and knowledge

gleaned in the high power laser field may be applicable to its solution.

It is becoming clear that the materials of choice, from the infrared to the ultraviolet, are the

light fluorides and oxides, along with NaCje. and KCl. Other materials are under development, especially

for use as high index coating materials; but, wherever possible, it seems preferable to use fluoride or fluorite crystalline materials: sapphire, and oxide and fluoride glasses. Except for sapphire, these

are all low index materials, which have been developed for optical use over a long period of time.

Purified starting materials are available for these materials, and they are chemically stable, which

reduces surface contamination.

Given the base of materials identified above, the situation for new optical materials is comparable to that of laser materials in the 60's. At that time, ruby had been refined and developed

to the point where it was the standard material for crystalline lasers. There was widespread interest

in finding a better material, one with a lower threshold and with comparable, if not better, optical

and mechanical properties. Literally hundreds of other crystalline hosts were examined, in small samples, with a "shotgun" approach. It was not until Bell Laboratories undertook the systematic

development of YAG that a replacement for ruby was obtained.

i x
The lesson to be learned is twofold. One must identify what characteristics are desirable for a material, and pursue development of those which offer significant promise of exceeding the performance

of what is already available. Choices must be made, and a long process of development undertaken,

involving purification of starting materials, optimization of the manufacturing process, and development of fabrication and finishing techniques. If these choices are made from materials which

provide a reasonable promise of the same or better chemical, mechanical, and optical properties- than those materials already available and if a real commitment is made to their development, significant

advances will be made.

3. Summary of Papers

The subject matter of the Tenth Symposium broadly consists of six topics, and these proceedings

are organized accordingly. The topics are as follows: (1) Measurement of Absorption Characteristics,

(2) Bulk Material Properties, (3) Mirrors and Surfaces, (4) Thin Film Damage, (5) Coating Material and Design, and (6) Breakdown Phenomena. In this section, a concise summary of each paper is provided.

Closely related papers are discussed together, wherever possible. The interested reader is referred to the complete manuscript of the papers for detail; our intention here is to provide the reader with an overview of the Symposium, and to identify the topics of discussion and the authors of the papers.

Each topic is introduced with a brief statement of the underlying problems and the status of understanding within that area of interest.

3.1 Measurement of Absorption Characteristics

Without question, the optical property of ultimate concern in any high average power laser system

is absorption. This property, whether present within the bulk of an optical window or at the surface

of coated or reflecting elements, can lead to unacceptable thermally induced damage. Thus, it is not surprising that several papers heard at this year's meeting dealt with the measurement and identification of absorption. The papers ranged from a theoretical analysis of rate calorimetry to the comparison of several novel techniques such as total reflection and photoacoustic spectroscopy, with

standard calorimetric procedures. Several authors dealt with the particularly pernicious problem of surface absorption in thin films and its separation from bulk absorption. One paper addressed the growing concern for two-photon absorption as interest moves to shorter wavelengths.

A general method for analyzing window characteristics by rate cal orimetery, which includes coating

absorption, was the subject of a presentation by N. C. Fernelius and G. T. Johnston of the University

of Dayton Research Institute. Previous treatments of this procedure were extended to include samples

with dissimilar levels of absorption on their faces. The method requires the measurement of temperature rise and reflected power in both orientations to three significant figures. Unfortunately,

it is difficult to make reflected power measurements at the levels encountered with AR coated windows

to better than two significant figures at the present time. Many other investigators are resorting to

long bar substrates or variable thickness coatings to overcome this limitation. However, these approaches require the preparation of special samples to allow one to separate the surface and bulk absorption contributions to the total window absorption.

The increased development of high power, ultraviolet lasers is stimulating the search for and characterization of optical materials, for use both as window materials and as coating components.

One promising candidate for use as a low index film material from the uv to the ir is NaF. When assessing a material's utility as a film component, it is necessary to test it in thin film form. To this end, D. L. Burdick of the Naval Weapons Center, employing the powerful technique of Attenuated

Total Reflection Spectroscopy, has studied the absorption coefficient of 0.56-ym thick NaF films on ZnSe substrates from the visible to ~3 pm. The spectrum revealed a strong water band at 2.95 urn of very high absorption relative to the substrate. An estimate of the thickness of an adsorbed water

layer of 7A was given. The contribution of scattering to absorption was also investigated in this report. It was observed that coated portions of the test plate tended to scatter less than the uncoated portion at any given wavelength. The scattering difference was about 16 percent of the normal -i nci dence value observed for the uncoated surface.

With increased emphasis on short wavelength lasers, there is developing a growing interest in techniques for measuring multi-photon absorption coefficients. M. Bass, E. Van Stryland, and

A. Stewart of the University of Southern California reported on a calorimetric method for measuring

two-photon absorption. In their experiments, a repetitively pulsed, Q-switched Nd:YAG laser was used

to illuminate samples of CdSe and CdTe at 1.06 urn. The sample was isolated in a vacuum chamber, and

its temperature measured with a thermocouple. The temperature rise was measured as a function of the incident intensity.

The resulting curves were analyzed assuming that there were both an incident wave and a refelcted wave in the sample. Thus, the reflection at the back surface was taken into account. Some deviation

from the theoretical prediction was observed, which was ascribed to the presence of free carriers

generated in the absorption process (either linear or nonlinear).

This approach to measuring two-photon absorption looks very promising, especially when combined

with more sensitive methods of detection, such as photoacoustic spectroscopy. This research also shows

x
the importance of carrying out a careful and exact analysis of reflected wave effects in optical

samples, especially in the presence of nonlinear interactions.

The level of specific absorption is extremely important in assessing the suitability of an optical

material for use as a laser window over a given spectral range. H. Vora, M. C. Ohmer, and T. G. Stoebe

of the Air Force Materials Laboratory have made a comparison of the bulk and surface absorption in both

NaCil and KCl over the spectral range from 9.2 to 10.8 pm using the long bar technique. The measured

bulk absorption in the best RAP-grown UaCl was apparently intrinsic at 9 x 10"^cm"l at 10.6 um and 1.4 x 10 _4cm_1 at 9.2 um. By comparison, the best KCi showed bulk absorption of

9 x 10"5 at 10.6 pm and 4 x 10~5cm~l at 9.2 pm. These values are probably limited by the presence of some residual CIO3 or CT O4 impurities.

A variety of samples of MaCz and KCi from various sources were evaluated together with an investigation of the efficacy of different cleaning and etching procedures to reduce surface

absorption. This study has led to a proposed new multiphonon limit for the intrinsic absorption in

NaCJl, lower than Deutsch's reported value of 1.17 x lO-^cm-1

.

As optical component quality improves, it becomes increasingly imperative that diagnostic

instrumentation for the characterization of these elements keeps pace. No where is this more necessary

than in the measurement of the absorption in thin films and at interfaces. Thus, it was quite

appropriate that T. H. Allen, J. H. Apfel , and C. K. Carniglia of the Optical Coating Laboratory

reported on the construction and testing of a 1.06-pm laser absorption calorimeter for thin film

coatings. This instrument is quite suitable for routine measurements of coatings deposited on thin

discs. Samples, irradiated by a 6 W cw Nd:YAG laser, were usually 2.54 cm in diameter and ranged in thickness between 0.02 and 0.13 cm. The temperature of the coated disc is compared to a similar

uncoated element using thermistors, arranged in an AC Wheatstone Bridge, in direct contact with each

disc. An ambient pressure of 100 torr is maintained in the sample chamber to reduce thermal variations. The precision of the calorimeter was determined over an absorptance range of 9 x 10"6 to 3.8 x 10"4 and was found to be essentially constant at 2 x 10"6. Sensitivity for a 0.04-cm

thick substrate was 3.8 x 10^ pv/watt. Tests on uncoated fused silica discs with thicknesses from

0.04 to 0.13 cm show a surface absorptance of 6 x 10~6 and a bulk value of 5 x 10~5/cm.

A very elegant technique for isolating the contributions due to interface, bulk, and substrate

absorptions in single-layer films was described by P. Temple, D. Decker, T. Donovan, and J. Bethke of the Michelson Laboratory. They prepared samples in the form of a wedge of coating material deposited

on a substrate, leaving part of the substrate uncoated. The absorption of this sample was then measured, using the scanning adiabatic calorimeter the authors described in the 1977 Symposium

Proceedings. The sample was scanned with a 0.5 W laser, in a spot 500 pm in diameter. A plot of energy absorbed versus film thickness was then generated. The intercept of this plot, corresponding to zero thickness, was generally different from the absorption value obtained on the uncoated portion of the sample. The difference was assumed to represent the sum of the contributions from both the outer

and inner interfaces.

The slope of the absorption plot yields the bulk absorption coefficient. By measuring the absorption at the X/2 and X/4 thicknesses, and taking into account the difference in field

configurations, the authors can extract the relative contributions of the air-film and the film-substrate interfaces. Data are presented for NaF and As2Se3 films on CaF2 substrates,

measured at 2.87 pm and 2.72 pm. The former wavelength lies within a water band, while the latter does

not. For the NaF film, bulk absorption due to water was detected, while for the A.S2Se3 film, the

principal effect was due to water absorption at the film-substrate interface.

Photoacoustic spectroscopy (PAS) is a technique for measuring weak absorptions that dates back to

Lord Rayleigh and the spectrophone, but is enjoying a revival in current physics. N. Fernelius and

D. Walsh of the University of Dayton Research Institute applied this method to the measurement of the absorption spectra of several thin films and bare samples, including samples coated using the discrete

graded-index method described in the paper by Moravec and Skogman (see below). PAS provides

information concerning the spectral absorptance of the sample and can discriminate between bulk and surface absorption by the variation in the phase of the signal with chopping frequency. It is thus a valuable method for the analysis of optical samples.

In this work, it was found that thin film materials showed different absorption spectra from the

same materials in bulk, due either to different crystal forms or to impurities in the coating. The theory of Rosencwaig and Gersho for the chopping frequency dependence of the PAS signal was verified.

The PAS method should find wide applicability for investigations of optical materials.

3.2 Bulk Material Properties

Absorption characteristics, discussed in the preceding section, represent only one of the many

parameters of interest for the optical materials used in laser systems. Other optical, thermal, and mechanical properties of laser materials are also of importance in determining system performance. At high power, stress-optical and thermo-optical properties can be of great significance for laser window

materials, lens materials, substrates, and, in glass lasers, the laser medium itself.

xi
For many years, the National Bureau of Standards has carried out a program of measurement and computation of the optical properties of materials suitable for use in various laser systems. The evaluation of stress-induced birefringence is an important consideration in the mechanical design and thermal performance of high-power laser systems. These changes in refractive index can lead to significant wavefront distortion in actual operation. This year, R. M. Waxier, A. Feldman, and

D. Horowitz reported extensive measurements of the piezo-optical coefficients of a wide variety of increasingly important optical materials, including four neodymium phosphate and f luoro-phosphate laser glasses, as well as CaF2, BaF2, and SrF2- Polarimetric and i nterferometric techniques were

employed at the HeNe wavelengths of 0.6328 pm, 1.15 pm, and 3.39 pm. The laser glasses obviously were

not measured at 3.39 pm.

Another aspect of the optical materials characterization program at the National Bureau of Standards concerns the measurement of refractive index and dispersion in transparent window materials.

M. J. Dodge, of the above organization, reported on the refractive index of fusion cast SrF2 at 20°C over the spectral range 0.2138 pm to 11.475 pm. SrF2 and other alkaline earth fluorides,

BaF2 and CaF2, are candidates for the 2-pm to 6-pm range because of their low index of refraction

and dn/dT values as well as their good mechanical stability. These factors, along with others, play an important role in determining optical distortion.

Using a minimum deviation method, 53 data points were fitted to a three-term Sellmeier dispersion

equation of the form:

2 , r» 2, 2 2,-1,

n - 1 = I [A . X (X - X .) ].

j

J J

The resulting constants for SrF2 were as follows:

Aj = 0.67805894 Xi = 0.05628989

A2

= 0.37140533 X 2

= 0.10801027

A3 = 3.3465284 X3 = 39.906666

Similar results are presented for for BaF2 and CaF2, as well.

Wide band-gap materials, especially monovalent and divalent fluorides and cholorides, are of great

interest as materials for use in windows and lenses, over the entire range of wavelengths from 10.6 pm

to 250 nm. Si ngle-cyrstal materials can be fabricated in large sizes, but these materials exhibit poorer mechanical properties than polycrystall i ne material. A technique for combining the optical

homogeneity and chemical purity of single-crystal materials with the mechanical strength of polycrystall ine materials is isostatic forging. Several authors contributed to a status report on this

promising fabrication technique.

Addressing the need for improved optical materials with both good ultraviolet transparency and low linear and nonlinear refractive indices, J. F. Ready, H. Vora, R. A. Skogman, K. M. Leung, and

E. Bernal G. reported on the development of Li F windows by isostatic forging of single crystal starting

material. While the mechanical properties of yield strength and fracture energy were seen to improve,

it was found that the ultraviolet transmission of the final product was not appreciably degraded. This

is an important observation, since Li F has the widest band gap of any known optical material and can now be made into relatively large, strong, high quality optical elements. The forgings are usually

made along the <100> axis at a strain rate of 5 x 10 - 3/min under a 2000 ps isostatic helium

constraint with temperatures ranging from 300° to 600°C. Resultant grain sizes are in the 10- to 40-pm range. Long-term stability appears to be acceptable. However, whereas the index inhomogeneity

of the single crystal starting material was in the range of 4.4 to 8.8 x 10 -

6 over 2.5 cm diameter

samples, the forged materials exhibited An values of 17.4 to 22.3 x 10 _ 6.

In a continuing development of forged optics, R. H. Anderson of the Honeywell Corporation and

J. M. Bennett of the Naval Weapons Center have investigated the optical properties of plano-plano and plano-concave KCl optical elements i sostati cal ly forged at 4000 psi between optically polished dies.

In this study, the elements were prepared in a two-step forging process. The KCl was forged between

teflon sheets first, and then water polished, prior to final forging, between fused quartz or pyrex

dies at 200 to 275°C under a helium atmosphere. The optical figure, homogeneity, internal strain,

scattering, and surface character of these two-step samples were compared with both single-crystal and one-step, press-forged KCfc samples. It was readily apparent that the two-step process produced a superior element in terms of surface figure, homogeneity, surface roughness, and scattering. The water

polishing greatly reduced the occurrence of grain boundaries and thus surface scatter. While the surface topography (roughness) of the die was not replicated exactly, the forged surface compared

favorably with state-of-the-art, mechanically polished samples and exhibited lower scatter values.

In a somewhat related paper, R. H. Anderson, R. A. Skogman, and J. F. Ready of the Honeywell

Corporation and J. M. Bennett of the Naval Weapons Center presented results of an initial investigation

xi i
of the forging of CaF2« However, in this case, the isostatic forging resulted in considerable

veiling (cloudiness) in the polycrystal 1 i ne CaF£ windows. Microscopic evaluation suggests that the

veiling may result from microvoids formed by the aggregation of vacancies produced by dislocation

intersections during plastic deformation, which also results in measurable internal stress. The

CaF2 samples were forged at 750° to 800°C in a helium atmosphere at a pressure of 14.82 MN/m2 . Single crystals having <111>, <100>, <112>, and <113> orientation were deformed approximately

60 percent to ensure a uniform grain size of less than 15 urn. Obviously, much more work needs to be accomplished to assess fully the suitability of isostatic forging for materials such as CaF2,

particularly if their intended use is at shorter wavelengths where scattering is of major concern as a loss mechanism.

In choosing a window material for a high power cw laser, both the power level at which failure

occurs and the nature of the damage process are of interest. J. Detrio and R. Petty of the University

of Dayton Research Institute conducted an investigation of thermally induced failure in SrF2 and

CaF2 windows, using the multi-kilowatt DF laser at the Army Chemical Laser Facility. Polycrystal! i ne samples of Raytheon, fusion-cast SrF2 were irradiated at power levels up to 8.4 kw on a spot size of

0.3 cm2 area. Failure inevitably occurred at power densities greater than 22 kw/cm2 . All samples were AR-coated at 3.8 ym.

Failure was noncatastrophic and was ascribed to compressive loading in the center of the irradiated area and consequent strain. Plastic deformation was seen to occur in the form of slip along

the (100) plane. At the point of failure, the compressive stress was estimated to be 15 Kpsi, and the

peak temperature 98°C.

A feature of this investigation was the observation of the sample before, during, and after

irradiation using i nterferometry and a polariscope simultaneously. Comparison of the polariscope

patterns with those observed in oriented single crystal specimens of CaF2 made it possible to identify the crystalline orientation of the polycrystall i ne samples.

Thermal damage in bulk materials which are used in the region of transparency arises from

multiphonon absorption and residual impurities. The latter effect can be reduced by controlling the

purity of the raw material, and preventing contamination in the manufacturing process. The chemical

vapor deposition process is one means of maintaining the purity of bulk materials during fabrication.

However, even in the purest materials, multiphonon absorption must be measured, to obtain the limit of transparency.

As an indication of our progress in achieving an understanding of the properties of one particular

material, C. Klein, B. diBenedetto, R. Donadio, T. Kahone, and J. Pappis of the Raytheon Company

reported a detailed listing of phonon assignments leading to an excellent prediction of experimental

lattice absorption for CVD (chemical vapor deposition) ZnS, a potential window material for use at 2.7 and 3.8 urn. Through an analysis of spectrophotometri c data at wavelengths to 25 ym, it was concluded

that multiphonon processes dominate the absorptive spectrum at frequencies below 1000 cm"l. As an adjunct to this study, the absorption coefficient was measured for CVD ZnS at laser wavelengths in ym

of 2.7 (HF), 3.8 (DF), 5.25 (CO), 9.27 C C02 )

, and 10.6 (CO2). The absorption coefficients in cm" 1 being (7.2 + 0.5) x 10" 3

, (2.3 + 0.5) x 10" 2

, (4.5 + 1.1) x 10" 2, (7.9 + 1.3) x lO^2 , and (2.4 + 0.1) x 10" 1

, respectively. Additional data relating to the

utility of CVD ZnS as a window material for infrared imaging applications were also presented.

In the laser fusion program, emphasis is being placed on the construction of very large

facilities. CO2 and Nd glass lasers operating at 100 kJ or more are under construction. The capital

cost of these facilities is very large, and the output obtained must be maximized by careful optimization of all components of the systems. Two papers were presented relevant to the choice and development of materials for large, Nd glass lasers.

In an effort to reduce the nonlinear index coefficient of laser glasses, the Lawrence Livermore

Laboratory, working with the major laser glass manufacturers, has undertaken the development of

f luorophosphate and f luoroberyllate laser glass compositions. S. Stokowski, D. Milam, and M. Weber

reported on the status of the damage properties of these fluoride-based laser glasses. Damage measurements were made in both f 1 uorophosphate and fluoroberyllate glasses at 1.06 ym, using pulse

durations from 0.1 nsec to 1 nsec. Both bulk and surface damage were observed.

FK-51 is a fl uorophosphate composition which has been commercially available for some time. It exhibits a surface damage threshold close to that of BK-7 or fused silica, i.e., 20-25 J/cm2 at one

nsec. Bulk thresholds are greater than 20 J/cm2 in FK-51. Newer compositions, both fl uorophosphate

and fluoroberyllate, show surface damage at values of 12-16 J/cm2

, and bulk damage thresholds in excess of 25 J/cm2 . The glass manufacturers are still developing the melting process for these

glasses, so further improvements are expected. The newer glasses are still observed to contain bubbles

and crystallites, which may be the locus of damage. Development of polishing techniques for these

glasses is still under way.

Thus, although low index laser glasses are still in the state of development, preliminary results

indicate that surface and bulk damgage thresholds comparable to those achieved in borosilicate glasses

should be attainable.

xi i i
Optical liquids find a variety of uses in glass laser systems operated at moderate or high

repetition rates: as coolants, for index matching, and as electrodes. The physical properties of these liquids are of great importance to the laser system designer. A series of measurements in the

physical and optical properties of a number of liquids of interest has been carried out by J. Rinefierd, S. Jacobs, D. Brown, J. Abate, 0. Lewis, and H. Applebaum of the University of Rochester.

A variety of index matching and coolant liquids was examined. The properties measured were refractive index, dispersion, optical absorption, dn/dT, density, thermal conductivity, and viscosity.

An estimate of the electronic portion of the nonlinear index coefficient, n2, was obtained from the

measured values of the index and dispersion. Values of these optical and physical parameters are tabulated for twelve liquids. In general, where comparisons exist with tabulated values, the agreement

is satisfactory. Among the liquids examined, H2O exhibits an attractive combination of properties

for a coolant: high thermal conductivity, low dn/dT, low viscosity, and low n2»

3.3 Mirrors and Surfaces

In most situations, optical surfaces, whether they be on reflecting or transmissive elements, are

the bridge between the laser system and the real world and as such are often subjected to hostile

environmental conditions. In the heirarchy of damage sensitivity, surfaces fall midway between bulk materials and coatings, and become the critical element in systems forced to use uncoated elements.

Thus, there is considerable import associated with raising the damage threshold of uncoated surfaces.

One obvious solution, at least for reflecting components, is to use diamond-turned metal mirrors.

Papers were presented not only on the physical and optical properties of micro-machined elements but also on the correlation between optical and metallurgical properties.

It seems that the subjects of surface roughness and its influence on damage resistance are still with us. A paper was presented concerning the pulse duration dependence of damage on surfaces of varying roughness. In addition, for the first time, a paper on damage to optical fibers with both nsec

and msec pulses was given. We are sure that both of these subjects will be favorite topics for many years to come.

Many diamond-turned metal surfaces are adequate for 10.6 ym applications, but significant improvement is necessary in their quality if they are to be used at shorter wavelengths without additional

polishing. D. Decker and D. Grand jean of the Michel son Laboratory reported on their progress in establishing a high precision diamond-turning mechanism. The machine differs from conventional

diamond-turning devices in that all components are mounted on a black granite surface plate, isolated

from the environment through air suspension. The planar layout minimizes the effects of vertical

temperature gradients, and provides a convenient work surface with great flexibility of configuration.

The machine is operational with open-loop control. Surfaces show rms roughness of lOA* (OFHC copper), with slope errors a factor of 2 less than those observed on optics diamond-turned on the

Battelle or Y-12 machines. The addition of closed-loop, i nterferometric controls of slide position is expected to increase the machine precision even further. A contour figure accuracy of 100 A peak-to-vall ey over a 0.4-m diameter is expected, when the full servo system is implemented. The authors discuss the design of the machine and the relative roles of tool and machine errors in determining the surface character, and show surface profiles and statistics for pieces turned on this

machine as well as for those turned on the Battelle and Y-12 machines.

Molybdenum is a mirror element of potentially great utility because of its good heat conductivity,

low thermal expansion, stiffness, and relatively high damage threshold. Unfortunately, it is difficult

to fabricate with low-scatter surfaces. In a rather comprehensive study of Mo mirrors, an extensive

optical and metallurgical characterization was accomplished by S. M. Wong of Rockwell International,

G. Krauss of the Colorado School of Mines, and J. M. Bennett of the Naval Weapons Center. The study

attempted to correlate metallurgical process variables and resulting microstructure and surface finish

of several samples from different sources. It was determined that there is a close correlation between

grain and subgrain structure in the material and structure appearing on polished surfaces, resulting

evidently from variations in hardness. As might be expected, little correlation was seen on ground

surfaces. Inhomogeneity of the Mo is sufficient to produce measurable variations in surface finish

within a given lot, particularly when i nhomogeneities are associated with dispersion hardening agents

such as Ti or Zr. Guidelines for the selection of optimum raw Mo stock for use as mirror substrates

are given.

It is well established that at long pulse durations (-40 ns), there is a strong correlation

between surface damage threshold and surface smoothness. At pulse durations of a few ns or less,

however, this correlation is not consistently observed, especially as the surface roughness is reduced below 40 A. It is important to know what the influence of surface roughness on damage threshold

is, since this information can influence the procedure used for polishing optical surfaces and the surface quality required for laser components.

xi v
D. Milam of the Lawrence Livermore Laboratory investigated the pulse duration dependence of 1.06-pm surface damage on three BK-7 samples and one fused silica sample, with different surface

roughnesses. Among the BK-7 samples, one was conventionally polished, grade A material, one was PH-3

material, carefully ground with alumina grits of descending size, and finally polished with commercial

CeC>3, and the third had one bowl-feed polished surface. The fused silica sample was conventionally

pol ished.

In general, entrance surface thresholds were observed to be higher than exit surface thresholds,

but not as high as predicted from simple Fresnel theory. Cleaning raised the threshold on the

conventionally polished BK-7 surface, but not on the others, indicating they were fairly clean

initially. The expected scaling with pulse duration (/T) was generally observed. No correspondence

between surface smoothness and damage threshold was observed in this range of pulse durations,

indicating that other factors are dominant. The damage threshold observed at the entrance surfaces were typically 20-25 J/cm^ in one ns.

Both from the value observed and from the adherence to the root-T scaling law, it appears that the surface damage seen in these experiments may be considered "intrinsic," or at least not dominated by

absorbing impurities on the surface. The implication of the research is that super-polished surfaces

(roughness -10 K) offer no advantages over conventionally polished surfaces for pulse durations

of a few nsec, as long as the surfaces are carefully cleaned. It is not obvious why the sensitivities

to surface roughness for very smooth surfaces should be so much greater at longer pulse durations.

A welcome addition to this year's Symposium was the report of M. J. Landry of Sandia Laboratories

on the subject of laser induced damage to fiber optics including both single and multiple fibers. To our knowledge, this is the first time this subject has been treated at our Symposium and is an indication of both the breadth of the meeting and the coming importance of fiber optics as power transmitting optical elements. In this study, fibers of five manufacturers were subjected to 1.06-um, 30-nsec pulsed laser exposures as well as unpolarized free-running exposures of a spiked nature for

~ 100-vis duration. The highest surface damage levels for single and multiple fibers exposed to the

Q-switched pulse were 48.2 and 31.6 J/cm^, respectively. In the case of long duration exposures,

single laser exposure levels of 4.66 KJ/cm2 were the highest achieved. Multifiber bundles initially

exhibited damage in the cladding or matrix material. A decrease in the fiber coupling efficiency

(ratio of output to input energy) was attributed solely to surface damage. Damage to gamma-irradiated

fibers exposed to -10° rad(Si) from a Co^O source were also reported. In general little effect

was noted on coupling efficiency or damage level due to the irradiation. This result is to be expected

since most of the interaction took place at the entrance surface, and thus the remainder of the fiber

played little role in these experiments.

3.4 Thi n Fi lm Damage

Even though optical materials in thin film form remain the most damage sensitive component of high power laser systems, our efforts to improve their performance are largely empirical, based primarily on parametric studies and go/no go testing of commercially supplied elements, frequently with a short-gain

approach characterized by new candidate materials, designs, etc. This situation is made manifest by the exigences of program deadlines and the need for rapid system maintenance. What is needed is a broad-based, fundamental program founded upon the growing wealth of parametric studies and some good sound science and materials engineering. This will be discussed further in the recommendations.

This year's thin film session was again generally a mix of parametric studies and an assessment of the state of the art in coating technology. The first paper was concerned with the current trend to shorter wavelengths (266 nm and 353 nm). Immediately following papers dealt with multithreshold

analysis of coating damage at 2.7 urn and 3.8 ym and the role that adsorbed water layers play at the former wavelength.

Continuing their series of short wavelength studies of optical damage in thin film coatings,

B. Newman and D. Gill of the Los Alamos Scientific Laboratory measured the damage threshold of fourteen

different thin film materials at both 355 nm and 266 nm. The incident light was generated by

doubling and tripling the frequency output of an Nd:YAG laser, Q-switched to produce a pulse of 35-ns

duration. The 355-nm pulse width was 27 ns, and the 266-nm pulse width was 22 ns.

The film materials were deposited as quarter-wave layers on fused silica substrates, and, in some

cases, as binary multilayer dielectric coatings designed for high reflectance. Damage thresholds were

obtained as a function of wavelength, coating design, and, in cases where picosecond damage data had previously been taken, pulse duration. Both fluoride and oxide materials were tested.

The single-layer coating with the highest threshold was NaF, the material with the greatest band

gap, which exhibited a threshold of 11 J/cm? at 355 nm. Fogging of the film due to exposure to a humid atmosphere apparently had no effect on the damage threshold. Of the multilayer reflectors, the NaF/Al203 film showed the highest thresholds: 3.6 J/cnv? at 266 nm and 12 J/cm^ at 355 nm.

Al 2O3 in a single layer showed a lower threshold, presumably because damage was occurring at the coating-substrate interface in the latter case. The wavelength dependence observed was consistent with

the linear absorption increase at short wavelengths, for several oxide materials, although the

xv
evidence was not entirely conclusive. Two-photon absorption did not seem to be playing a major role in damage at 20 ns, whereas it was significant for 20-ps pulses. The damage threshold was found to increase with pulse duration more slowly than This would be expected if the damage were mediated by absorbing impurities. Other scaling laws were also investigated.

This work, providing as it does a broad survey of materials and parameters under wellcharacterized test conditions, is very valuable in developing a picture of the relevant factors for

short wavelength damage in thin films. It also provides a practical guide as to what materials appear

most promising.

Pulsed laser damage in NaF, SiO x M2O3, ZnS, AS2S3, and Si coatings at 2.7 and 3.8 pm was evaluated via a multithreshold analysis by J. 0. Porteus, T. M. Donovan, J. L. Jernigan, and

W. N. Faith of the Naval Weapons Center. This type of evaluation is most appropriate at the intermediate pulse length of 100 nsec employed in this study to allow for a greater delineation of damage mechanisms between thermal failure and dielectric breakdown. This type of analysis aids in understanding the role of impurities such as water, which absorbs at one of the two wavelengths

empl oyed.

After discussing in some detail the deposition conditions and substrate characteristics, the authors described the experimental variables and multithreshold diagnostic instrumentation including

ion and light emission, the onset of delami nation, cracking, flow, erosion, or perforation as well as other selective, as opposed to uniform, damage criteria. When appropriate correction factors were

applied to convert from incident to internal energy/area values, NaF and M2O3 were seen best at both the DF and HF wavelengths with Si0 x also useful at 3.8 pm. ZnS, AS2S3 and Si, all higher

index materials and SiO x at 2.7 pm were markedly lower in damage resistance.

Continuing their work on multithreshold damage to films of AS2S3, As2Se3, and NaF,

T. M. Donovan, J. 0. Porteus, J. L. Jernigan, and E. J. Ashley of the Naval Weapons Center extended

their measurements this year to 2.8 pm and 3.8 pm from those reported last year at 10.6 pm. Two

principal damage processes were identified. The first is a "uniform" mode associated with the

amorphous chalgogenide matrix while the second is a more "selective" phenomena associated with

micron-sized crystalline defects. Each type of damage is easily discernable by inspection of the

damage morphology of the irradiated area. It was noted that single-layer film (N on 1) conditioning

led to an improvement in damage resistance, most probably through gently removing by desorption any adsorbed water layer, which would absorb at 2.7 pm. However, for multilayers of As2Se3 and NaF, (N on 1) conditioning led to enhanced crystallite growth, which resulted in a decrease in damage

thresholds.

Important results of this study indicated that AS2S3 first damages at a fluence (areal energy

density) of -50 J/cm^ while NaF, with a 30 times higher absorption, damaged at -400 J/cm^. Thus,

absorption alone does not determine the damage sensitivity. For comparison purposes, multilayer films

of alternate As2Se3 and NaF layers in an enhanced reflection design on polished Mo exhibited a higher threshold than Au-coated, diamond-turned Cu mirrors.

The remaining papers in this session dealt with three diverse areas of coatings, which have been with us a long time. They are a morphological investigation of damage in barrier layers, which emerged

last year as a potential means of increasing the damage threshold of antireflection coatings; a significant improvement in the utility of scattering as a quantifiable indication of the onset of damage; and finally, an in-depth analysis of the statistics of absorptive laser damage and its usefulness for identifying the appropriate damage mechanism and as an engineering design tool.

The relatively low damage threshold of antireflection (AR) coatings makes it a limiting factor in the design and achievement of high-power laser systems. Last year the utility of barrier layer

coatings to enhance the damage resistance of AR coatings by as much as 50 percent was revealed.

C. K. Carniglia and J. H. Apfel of Optical Coating Laboratory, Inc., G. B. Carrier of the Corning Glass

Works, and 0. Milam of the Lawrence Livermore Laboratory have continued to investigate the role barrier

layers play on the damage sensitivity of 1.064-pm AR coatings. The basic coating employed was a standard four-layer silica/titania design on BK-7 glass substrates. Samples were prepared with and without half-wave silica barrier layers. Detailed morphological investigation employing transmission

electron microscopy revealed that damage sites were characterized by a 1-pm nuclei which could lead to 3-pm craters. The titania layer seems to fracture while the silica layers appear to melt or tear with

evidence of heating in the substrate. The results of this study seem to indicate that damage

originates at the interface between the coating and the substrate and may arise as a result of thermal expansion of the material at, or close to, this boundary, most probably resulting from polishing or cleaning residue rather than from absorption in the bulk of the materials themselves.

For many years people have felt that scattering should be both a good indicator of surface or film

quality and a useful diagnostic technique to indicate the onset of damage. Unfortunately, to date,

proposed laser scattering techniques, while sensitive, have been rather subjective. To increase the

utility of scattering procedures and to place them on a quantifiable basis, T. W. Walker and

A. H. Guenther of the Air Force Weapons Laboratory and P. E. Nielsen of the Air Force Institute of

xvi
Technology reported on new optical techniques for the determination of pulsed, laser-induced damage in thin films. Briefly, this scheme incorporates two fast photo-diodes. The first diode records a reference signal directly proportional to the intensity of the incident laser pulse, as well as a signal due to that fraction of the incident pulse nearly (-5°) back scattered from the target, and a probe signal generated from the incident pulse but delayed up to 55 nsec after the incident exposure of

the test film. The delayed probe pulse is reflected from the exposure site into the photo-diode. By

appropriate use of delay cables, all th-ee signals can be displayed on a high time resolution transient

digitizer. A second photo-diode records the transmitted incident laser pulse through the film and

substrate.

By this technique, the relative amplitudes of the signals are compared first at an energy below

the damage threshold, second at the damage threshold, and finally at an energy comparable to the first

laser exposure by another exposure on the same site. The ratioing of the incident, delayed probe and

back scattered signal affords a positive indication of change which can be arrived at by an objective

evaluation of the signal amplitudes. The changes in the signal ratios offer, as well, a potential

insight into the changes experienced by the thin film-substrate system upon irradiation by pulsed laser

radiation. The use of the variable delay probe combined with re-exposure after damage indicates when

modification of the film structure takes place and whether or not it is permanent. Films of MgF2,

ThF4, and Zr02 were tested at 0.53 pm with 18-nsec pulses. Interestingly, this technique has indicated damage in films without the observance of either a visible spark or a modification of the

transmitted laser pulse. This technique should gain wide acceptance, since it now places sensitive

scattering techniques on an objective, but relative, quantitative basis.

It has long been suspected that an appropriate statistical treatment of pulsed laser damage

experiments would afford an insight into the physical mechanism of damage and thus allow one to choose

between competing theories. To this end, A. B. Budgor and K. F. Luria-Budgor of Lawrence Livermore

Laboratory have performed a statistical analysis of absorptive laser damage in dielectric thin films

using data supplied by D. Milam of the same organization. They applied a Wei bull distribution, which

is normally employed to describe both time-to-failure of electrical components and the DC breakdown of insulating materials. The Weibull distribution arises from the theory of extreme values. In this

study, the time-to-damage and intensity-to-damage statistics were treated for thin films of Zr02,

Si02, and Al 2O3 . In all cases, correlation greater than 90 percent was achieved. Excluding

"over-simplified" statistical theories such as the lucky electron model, it was concluded, based on the

pulse length dependence of breakdown and an experimentally nonobserved >2 multiphoton absorption, that

the avalanche mechanism is the most likely initiator of the plasma requisite for lattice melting. It was pointed out that statistical confidence bands for material survivability as a function of laser intensity and pulse length can now be constructed, leading to a practical utility of Weibull distributions as engineering design and diagnostic tools.

3.5 Coating Materials and Design

Since thin film coatings remain the Achilles heel of high power laser systems, there is continuing

interest in improving their performance. New coating materials continue to be investigated, often in connection with a particular wavelength, or a specialized application. It is well established that not only the coating material but also the design and details of deposition influence the characteristics

and performance of a thin film coating.

Among the materials investigated for use as the low-index component of coating systems, fluorides

are the most common, since they usually exhibit wide band gaps, and correspondingly low absorption in bulk form.

R. C. Pastor, J. A. Harrington, L. E. Gorre, and R. K. Chew of the Hughes Research Laboratories

prepared and evaluated six compounds as candidates for 3.3-ym and 9.3-ym dielectric coatings. They

were the rare earth fluorides LaF3, PrF3, CeF3; the tri-halides of Bi , B i F3 , and B i

1 3 ; and the binary salt KGaF4. Using a purification procedure involving RAP (Reactive Atmosphere Process),

an intrinsic absorption coefficient of < 0.1 cm

_ l was sought. Unfortunately, the bulk absorption

of the rare earth fluorides ranged between 0.6 and 0.9 cm~l.

Optical transmission studies of single crystals of the rare eartl, fluorides indicated that only

LaF3 does not exhibit absorption bands in the 3- to 10-ym region. Both CeF3 and PrF3 exhibit

absorption between 3.5 and 6 ym due to low lying Stark levels. While it was not possible to grow large

enough single crystals of Bi F3 or KGaF4 to permit accurate measurements, these materials appeared

to have low absorption at 9.2 yrn and 3.8 ym. Bi 1 3 looked the most attractive, due to its featureless

spectra between 2.5 and 20 ym.

For a rather specific application, P. Baumeister of Aerojet-General and G. P. Arnold and

D. F. Edwards of Los Alamos Scientific Laboratory investigated the deposition and damage thresholds of single-layer PbF2 films employed as anti reflection coatings for a CdSe 0P0 (optical parametric

oscillator) pumped at 2.8 ym with 120-nsec pulses. Reported damage thresholds were as follows: uncoated CdSe, 68.5 MW/cm2

; x/4 PbF 2 on CdSe, 50.6 MW/cm2

; X/l PbF 2 A/8 ZnS, 50 MW/cm2 (ZnS, used as an adhesive layer, exhibited no detrimental effects on the damage threshold); commercial PbF2 coatings on CdSe, 6 MW/cm2

; and finally X/4 PbF2 on Ge at 3 urn, 22 MW/cm2 . Explicit and

xvi i
complete details as to selection of starting materials, polishing, and deposition conditions are given.

The presence of PbOH in the starting material or hydrolized from PbFg leads to the production of metalic Pb upon heating, resulting in inferior films.

Thin film coatings are recognized to be the most easily damaged optical elements in a high power

laser system. Damage occurs in regions of high electric field and at film-substrate interfaces, where

impurities aggregate. Recently, strong interest has developed in ways to eliminate conventional

antireflection coatings. One such way is by use of graded-index coatings. In graded-index coatings,

two materials are co-deposited on the surface simultaneously, with varying rates, so that the refractive index varies continuously in the desired fashion during the deposition. The control of the

graded-index coating process is often difficult. An alternative means to achieve the same end has been proposed by T. Moravec and R. Skogman of the Honeywell Corporate Material Sciences Center. They

constructed discrete graded-index films, by alternatively depositing layers of high index material

{111, n = 2.4 at 10.6 \im) and low index material (PbF2, n = 1.63 at 10.6 pm) on KCl substrates. The

thickness of each pair of layers was determined to give a certain value of the average refractive index

at that layer. By building up a coating from as many as 50 to 100 layers, a discrete approximation to

a continually varying index can be constructed. Individual layers as thin as 100 A were employed.

Til normally grows in an orthohombric crystal. On KCl, two crystalline orientations develop in Til, resulting in optical scattering in the film. By growing only thin layers, the Til is kept in a cubic crystalline form and scattering is reduced. Discrete graded-index films are also free from

birefringence, further evidence of cubic crystal structure.

The discrete film method provides a very flexible means of constructing graded-index films.

Discrete graded-index films exhibited absorption values similar to those seen in conventional films of the same material. Damage tests have not yet been done, so comparisons with conventional, multilayer

films and continuous graded-index films are not available.

The standing-wave pattern formed in a multilayer, dielectric coating creates local maxima and minima of electric field within the coating. The placement of these extrema is an important factor in determining the peak performance of the coating. Thus, the analysis of coating performance is a valuable tool in evaluation of coating designs for high power use.

Although exact calculations of thin film performance are possible using large computers, it is often useful to be able to carry out approximate evaluations of thin film reflectance, transmission,

and absorption, using analytic expressions. H. E. Bennett and D. K. Burge of the Michelson Laboratory

discussed the general method of obtaining simple analytic expressions for evaluating weakly absorbing,

antireflection or high-reflectance coatings. In this age of programmable pocket calculators, the utility of the expressions will be immediately apparent.

In order to employ these expressions, one must know the volume absorption coefficient of the coating materials as deposited, the substrate absorption, and the absorptance at each interface. The indices of all materials must be known. For metal substrates, the effect of surface roughness can be included. All of these parameters can, in principle, be determined experimentally. Their measurement

has been discussed in several papers presented at the Symposium this year and in past years. Additionally, these analytic formulae can be used to estimate the effect of absorption, in the volume

of the coating materials or at interfaces, on the performance of thin film coatings. In this mode,

they provide a useful tool for coating design analysis and for determination of coating specifications.

The analysis of the reflectivity and absorption of a complex dielectric multilayer film, as a function of frequency, generally requires a significant computational effort. M. Sparks and

M. Flannery of Xonics, Inc., showed that simple analytical approximations can be obtained in the case

of a quarter-wave stack, a design commonly used for high-reflectance mirrors. The authors showed that,

to a good approximation, the phase shift can be represented by a linear function of oj-^ c , where wc is the center frequency for the stack (zero phase shift), and that the absorption can be represented by a quadratic function of -^c ), plus a substrate contribution. The coefficients of the quadratic form can be obtained from analyzing the infinite, quarter-wave stack, and depend only on the absorption constants and refractive indices of the low and high index coating materials.

By varying the operating frequency with respect to oj c , the field maxima can be shifted into

either the high or low index media. Thus, depending on which of these is more susceptible to damage,

the coating should be designed to operate off the center frequency, to minimize the field in the more vulnerable material

Herbert B. Rosenstock of the Naval Research Laboratory reviewed the situation relative to electric

fields produced at or near surfaces and in coated elements. He pointed out quite correctly that the

use of a coated element at other than design wavelength can either increase or decrease the total electric field depending on the optical constants, thicknesses and specific film design, as well as the

wavelength. He also pointed out that it may be useful to coat the exit surface of a transmission

element to reduce the field at that point, but that it may introduce a weakness due to increased

absorption in the film or at the film/substrate boundary.

xvi i i
3.6 Breakdown Phenomena

The phenomenon of the bulk failure of a transparent dielectric, in the presence of an intense

light wave, associated with the formation of a luminous plasma, may be the oldest, and least well understood of the various effects observed under the heading "laser damage." A number of theoretical

models have been advanced, each usually stressing one or another aspect of what is ultimately a very complicated process. Indeed, it may not be prudent or even possible to model the breakdown process

completely.

Among the competing effects are multiphoton ionization, direct excitation from impurity levels or defect sites, avalanche ionization, thermal ionization of highly absorbing impurities, and phonon-assisted processes. At present, research is directed toward identifying the relative importance

of each of these competing effects, as a function of pulse duration, wavelength of illumination, and experimental conditions. It is safe to say that, as yet, no wholly satisfactory model has been developed for dielectric breakdown.

A very preliminary account was given of an attempt to simulate, numerically, the propagation of

light through a damage site in a transparent dielectric. P. Kelly and D. Ritchie of the National

Research Council and P. Braunlich and A. Schmid of Washington State University are in the process of adapting a "particle-in-a-cell " (PIC) code, originated by D. Henderson of the Los Alamos Scientific

Laboratory, to model light propagation in the dielectric in the presence of polarons. The generation

of the polarons is assumed to proceed via a four-photon process for NaC£, the material initially

studied.

Such a computation is necessarily limited by the extent of the physics phenomena which can be included. In reality, the nonlinear optics of the medium, as well as the generation of polarons, must

be included; and impurity effects will have to be modeled. The applicability of the PIC method to a strongly convergent focus, especially in the presence of self-focusing, is also open to question.

There has been extensive discussion in the laser damage literature of the relative roles of impact

and multiphoton ionization in determining the bulk damage threshold of transparent dielectrics.

B. G. Gorshkov, A. S. Epifanov and A. A. Manenkov of the Lebedev Physical Institute in Moscow presented

a theoretical analysis of the relative dependence of these two phenomena on optical frequency and laser

pulse duration. The dependence of multiphoton ionization on frequency is simply given by the ratio of the ionization potential to the photon energy, for all pulse lengths of interest. For avalanche

(impact) ionization, the authors conclude that two different dependencies on photon energy can be observed. In the subnanosecond region, the breakdown field is always expected to show a monotonic

increase with frequency, while for pulses several nanoseconds wide, if the photon energy approaches the

gap energy, the breakdown field can show an oscillatory dependence on photon energy. Consequently, as the pulse width is varied at fixed frequency, in some cases, the two processes can compete over a wide

range of pulse durations.

In a short presentation, S. Brawer and W. Smith of the Lawrence Li vermore Laboratory offered a conjecture regarding the modeling of bulk damage in transparent dielectrics. They proposed that

electrons near the bottom of the conduction band can be considered as lying in localized states. Transitions between these states are assumed to be dipole allowed. When a conduction electron acquires

an energy equal to the band gap in the material, it can excite a valence electron to the bottom of the conduction band by an Auger process. In this way, a population of conduction electrons is built up. Electron energy loss occurs via collisions with the lattice (phonon emission). This process heats the

lattice and leads to damage. Avalanche may or may not accompany damage.

The new aspect of the theory is that it assumes localized states in the conduction band and does

not require an avalanche for damage to occur. Only preliminary conclusions have been drawn from the

theory, and detailed predictions will await further calculation.

A series of investigations of laser induced damage in semiconductor materials was reported by Yu.

K. Danileiko, A. A. Manenkov, and A. V. Sidorin of the Lebedev Institute of Moscow. Using three laser

probes, a 10.6-pm, CO2 laser with a 60-ns pulse width, a 2.76-ym, Er:CaF2 laser with a 90-ns pulse

width, and an Er:YAG laser operating at 2.94 urn with a 100-ns pulse width, they measured the dc and microwave photoconductivity induced in Si, Ge, and GaAs. In Ge, no damage was observed, although

significant photoconductivity was measured. At the shorter wavelengths, the photoconductivity was attributed to two-photon absorption, with a measured two-photon absorption coefficient of

7.5 x 10 _ 2 cm/MW. At 10.6 11m, the mechanism giving rise to the photoconductivity was unclear.

Neither avalanche ionization nor thermal effects could account for the observed dependence of carrier

concentration on incident intensity. The absence of damage was attributed to sel f-defocusi ng of the

incident light by the free carriers generated in the focal region. However, damage was observed in GaAs and Si. The observed damage levels were poorly correlated with measured bulk absorption

coefficients. This was probably due to absorbing defects in the material. Defect type of morphology

was observed in all cases of damage.

xix
Scaling laws have been useful guides in damage studies, providing estimates of system performance

over a range of operating parameters. Additionally, departure from the expected scaling is often

evidence of the influence of extrinsic factors. M. J. Soileau of the Michelson Laboratory and M. Bass

and E. Van Stryland of the University of Southern California tested the bulk damage threshold of KCl

and Had as a function of spot size and of laser frequency. Significant departures from the empirical

scaling law proposed by Bettis, et al., for spot-size dependence were observed in both materials.

Furthermore, the frequency dependence of the damage threshold, measured at a Hxed-spot diameter,

contradicted the values obtained earlier for Had in the infrared, yielding values more in line with

those reported last year by Manenkov.

Experiments were carried out at 2.7 ym, in a 140-ns pulse, at 3.8 pm, in a 70- to 100-ns pulse,

and at 10.6 ym, in a 60- to 110-ns pulse. Spot sizes varied from 20 pm to 60 pm. In general, the threshold field decreased monotonical ly with increasing spot size, but no consistent pattern or functional form for the dependence on spot size was evident. In several cases, the spot-size

dependence was different at different frequencies.

The authors attribute the lack of a simple set of scaling relations to the effect of impurities,

defects, and other extrinsic factors in the materials examined. As Manenkov pointed out in his paper

this year, however-, the frequency dependence of damage can be quite complicated, even for ideal materials, without invoking any impurity effects. As to the departures of the spot-size dependence

from the conjecture of Bettis, et al., uncertainties in the data and the absence of a firm foundation

for the scaling law make it difficult to draw any unequivocal conclusion from the result.

' E A careful study of the surface breakdown mechanism for infrared optical materials was presented by

V. I. Kovalev and F. S. Faizullov of the Lebedev Physical Institute in Moscow. They propose as the

dominant mechanism in IR material damage a two-step process, consisting first of the boiling off of adsorbed water from the surface, followed by gas breakdown in the evolved water vapor. They carried

out experimental tests of five critical predictions of this model. The tests were as follows:

a) The breakdown threshold depends on the ambient gas pressure, but not the gas species, as long

as the ambient gas is transparent to the incident light.

b) Since the electron loss in the breakdown region is limited by the depth of the evolved gas rather than the width of the illuminated area, no spot-size dependence is expected.

c) Surface treatment to reduce surface absorption should increase the observed damage

threshol d.

d) At optical frequencies, the threshold intensity should decrease with increasing wavelength as

e) For long pulses (> 1 us), the threshold should increase with increasing pulse rise time, due

to gas-dynamic expansion of the water vapor during the breakdown process.

Good agreement was observed between the predictions of this model and the experimental results.

The materials tested included Had, ZnSe, KRS-5, KRS-6, and Ge.

c 4.0 Recommendations

As a result of this year's symposium format, it is quite straightforward to cull from the invited

papers and round-table discussion an objective summary of the state of the art and understanding in the

general area of optical materials exposed to high power lasers. Resulting recommendations are, of necessity, less detailed and specific than normal, but rather are general in nature as appropriate for

a ten-year assessment of the field.

Following the order of topics discussed in the section on principal conclusions, we single out those areas which need further attention and note why they are important, whether due to a lack of consensus, inadequate capability or just incomplete data.

The first area appropriately concerns the general subject of laser induced damage to materials,

in thin film form, at their surfaces and within the bulk proper. Of interest is not only demonstrated

damage resistance but also our understanding of the damage process or mechanism itself. Thin films are

at present the most damage sensitive optical feature in any optimally designed system. Optical

materials in this form, as compared to surfaces and bulk materials, exhibit their lowest damage

resistance. This undoubtedly arises from the presence of absorbing impurities or imperfections in film structure or at interfaces.

The existing data base, correlating the effects of starting material purity (of both films and substrate) and deposition conditions with the resultant structural and optical properties of the prepared optical coating, is grossly inadequate. To obtain a true understanding of why films fail and

how to improve their performance, one must follow the road religiously, from describing the selection,
character, and purity of starting materials and substrate, through the fully documented deposition

process, to a comprehensive physical and structural characterization of the thin film. Following this,

one must determine the film's optical properties and correlate them, if possible, with the early steps.

Damage testing and careful morphological investigation to elucidate performance and failure mode can

then hopefully lead, after analysis, to an assessment of why films fail and suggest material and

process changes which can be recommended to the fabricator. Comprehensive and careful multithreshold

analysis will be invaluable at this stage.

The lack of a sufficient capability in this area is leading laser specialists to consider

seriously the elimination of coatings, accepting the resultant losses. This is really a very inefficient type of damage avoidance. Other solutions recently rediscovered involve the use of graded-index surfaces which can be achieved by ion implantation. In this case, there is really no film

per se but rather a hopefully appropriate index variation from air to bulk. Certainly more work is required in this area to answer the question, "Do these structures exhibit bulk damage levels, damage

levels of a reduced index material, or levels typical of thin films." In addition, achievable

uniformities, index gradients, optical performance (high and low reflectivity, filtering action), and damage resistance should be determined. One should note that the index change is not only due to the concentration of ions implanted in the base structure but also to the concentration of defects produced

as well. Questions on the choice of ions, their energies, rates, angles of incidence, etc., and the utility of annealing, etc., still remain to be answered.

Two years ago, a way around the sensitivity of film/substrate interfaces was proposed. The barrier layer came on the scene with a great hope for materially improving the damage resistance of

anti ref 1 ecti on coated elements. Up to 50 percent improvement in the damage threshold was reported, and

the mechanism for this improvement seemed to be understood. Subsequent studies, however, indicated

that improved performance was not always obtained and that the morphology observed was not always

consistent with the understanding of the role of barrier layers. Further work is required to reduce

the concept to practice reliably. One last principal recommendation in the thin film area concerns the need for improvements in reflective dielectric coatings for metal mirrors, particularly in the uv. Unstable resonator systems

require large aperture mirrors. Metal mirrors must be coated, to obtain a high reflectivity in the uv

and to prevent unacceptable levels of thermal distortion. Also, large aperture focusing lenses for uv use will be difficult to fabricate and will be expensive, since the choice of materials is severely

constrained by two-photon absorption. There is no question that, at short wavelengths, coated-mi rror

damage resistance levels will define system design and size. There will be a greater importance

attached to scattering in thin films, as a loss mechanism, at these shorter wavelengths.

Even if one either eliminates coatings or can raise their damage resistance to levels comparable

to the intermediate values typical of surfaces, there will still be several important areas which will

need to be addressed. Certainly, the most important is the further development of metal mirrors and their figuring and finishing to uv required tolerances whether by diamond turning or more conventional

methods. The two most important concerns are the reduction of surface absorption (equally true for

window surfaces), no matter what its cause, be it plasmon excitation or just plain dirt, and concurrently cleaning, which is particularly important prior to coating. There is no question but that

the future bodes well for the further development and employment of metal optics in high power

applications from the ir to the uv, if further attention leads to improved surfaces. As metallic

optics continues to invade the commercial field, increased attention to environmental degradation will

be required to extend the utility and range of application of these components.

This brings us to the last area associated specifically with the laser induced damage process:

breakdown mechanisms and phenomena. It has been amply stated that there is no consensus on an analytical or phenomenalogical description of laser induced breakdown in dielectric materials. Thus,

scaling relation studies, statistical analysis, and multithreshold investigation are necessary as bench

marks to validate and bound proposed theoretical descriptions. For example, roughness scaling may

indicate whether an experimental result is indicative of intrinsic behavior or an impurity dominated

extrinsic result. The same can be said for the use of statistical analysis as an indicator of probabilistic or deterministic behavior, another measure of intrinsity or extrinsity, similarly so for

multithreshold experiments coupled with careful morphological i nvestitgati ons

Unfortunately, most morphological assessments are more indicative of the secondary effects of breakdown or of a breakdown plasma interacting with the specimen than the initiation process itself.

Thus there was a strong call this year for threshold interaction experiments, to quantify the phenomena

of potential precursors before a catastrophic response. Scattering and conductivity, e.g.,

el 1 i psometry , were immediately suggested as candidate diagnostics of these pre-breakdown experiments as both are very sensitive indicators of change. This is an excellent area for basic research, and should

be closely coupled with scaling studies to insure that mechanisms are not changing, therefore allowing

one to bound the applicability of the relation.

The necessity of determining the structural and optical properties of materials, particularly . thin films and surfaces, resulting from process variables was mentioned earlier. In order to accomplish this task adequately, improvements in measurement and characterization techniques are

xxi
dictated. In concert with developments in these areas, calibration and standardization methods must

keep pace to take advantage of the numerous advances in instrumentation. This requirement is best met

by the National Bureau of Standards which not only has prime responsibility in the standards area, but should increase its tabulation and documentation efforts as well. To accomplish this task effectively, 1 we must all support the continuation of NBS's activities and encourage their expansion through the acquisition of the latest advances in instrumentation. The desirability and necessity of this course

of action is evident to those working in the field of optical materials for high power lasers.

An especially fruitful area of endeavor is the refinement of techniques for measuring nonlinear

absorption. One would like to have accurate measurements of two-photon absorption, as a function of frequency, together with a comparison of the spectral dependence of one- and two-photon absorption, as

a measure of the density of states accessible by odd and even parity transitions.

In the area of materials and fabrication, there are two principal recommendations, one in each category. Based upon a careful consideration of the numerous and diverse reports presented on thin

films over the years during this series of symposia and upon a realization of the almost unlimited

number of potential coating materials, it seems particularly propitious to recommend a narrowing of our attention to a reduced set of candidates. We tentatively propose the oxides of aluminum and silicon as ; well as fluorine containing compounds such as the fluorides and higher fluorine analogues (e.g.,

PbF£, CaF2, CeF3, ThF4, ...). It appears that this group provides a reasonable range of refractive indices for the designer (perhaps augmented by a few selected high index materials) and is sufficiently durable and environmentally stable, due to the strength of the chemical bonds in these

materials. The thin film community needs to concentrate on perfecting the deposition of these

materials. This recommendation will surely meet with some opposition, for everyone has his pet

material or design. However, in spite of this expected reluctance, it appears that a concentration on | these materials can lead to real improvements in the damage resistance and utility of coated elements. >

We are still unsure as to how the hardness of alkali halides contributes to the differences and difficulty of grinding and polishing optical elements fabricated of these materials to required

tolerances and figure. Forging may be a solution to this problem. Each year, we see continuted

improvements in both understanding and demonstrated quality of forging technology, this year

particularly evident with CaF2- Additional research and develoment are desirable in this area to more fully appreciate the capability and limitations of this technique for fabrication of optical

components from crystalline dielectrics. This is an area where additional material characterization is particularly necessary.

The shibboleth of the high power laser systems designer is damage avoidance. He now looks at the

overall system performance and asks himself where the critical problems are. For example, can one really expect to achieve diffraction-limited system performance in the uv. Probably not, but then

again, it may be necessary. In this uv case, the limit will probably be due to damage levels,

unmanageable tolerance requirements, or just plain saturation. The designer instead looks to damage

avoidance techniques such as phase conjugation to achieve desired system quality. Other damage

avoidance techniques will undoubtedly surface now that our attention has been acutely drawn to this

concept.

\i Phase conjugation is only one of a growing list of new endeavors in the area of laser induced

damage to optical materials. Potential solutions usually bring potential problems as well. In this case, phase conjugation devices are subjected to potentially increased damage sensitivity due to the

requirement for overlapping high power beams. This is somewhat reminiscent of the different damage

sensitivities of intra- and extra-cavity second harmonic generation crystals reported many years ago.

In that case, while frequency conversion efficiency was improved for the intra-cavity case, the resistance to damage was much less than for low efficiency extra-cavity elements.

As has been pointed out, this year saw the first report of laser induced damage to optical fibers.

It is anticipated that interest in this area will grow as applications for power transmission through

fibers increase and the frequency domain of usage extends into the infrared where new materials promise 1 extremely low transmission losses. It is also anticipated that new problems will arise in connection

with the continued development of free electron lasers over a wide range of wavelengths. To this we

must obviously add the continued movement to shorter wavelengths, which promises increased concern

about scattering, more stringent figures and surface tolerances and nonlinear processes.

The revelation that copper optics degrades in industrial C0£ laser environments is one indication of the important role surface chemistry plays in optical damage phenomena. Environmental

stability in general, along with the problems of surface contamination in the preparation of optical

elements, needs to be addressed using the analytical tools and diagnostic techniques of surface

chemistry. Many diagnostic techniques, such as ATR spectroscopy, and ion, electron, and optical

microprobes, are available for this area of investigation.

The laser fusion community is moving to slightly longer pulse durations, in the few to 10 nsec

regime. Thus, one would expect more emphasis on thermal mechanisms, especially in thin films, most certainly if systems are repetitively pulsed. It is at these longer pulse durations, compared with the subnanosecond domain, that certain scaling relations, such as surface roughness, become more manifest.

xxi i
There are two remaining recommendations, both of which we have touched upon earlier. We have matured enough in this field that there should be considerably less excuse for repeating the errors and inefficient approaches characteristic of our earlier years. Through these proceedings, we have tried

to transfer information and effectively document in one series the principal efforts and thoughts on laser induced damage to optical materials.

We should now be able to advance in technology by paying careful attention to our history, our tortured and occasionally wandering travels, capitalizing on past successes, and avoiding past

pitfalls. This is an appropriate time to assemble our understanding, and to tabulate the results of the past decade's program of measurement of relevant properties of materials. The former will be accomplished through publishing the invited papers given at this, the Tenth Symposium. The latter will probably be best accomplished by the National Bureau of Standards. NBS has maintained a measurements

program for optical materials for several years. We should all support the assumption, by NBS, of the responsibility for assembling and cataloguing the parameters of optical materials.

The variety of factors which influence the properties and performance of optical thin films is only matched in number by the fragmented, mostly uncoordinated efforts involving thin films. Unlike

the thin film activities of the electronics industry, there is no single incentive for an optical thin

film application of comparable magnitude. Rather, there is large diversity of applications, no one of which can justify a comprehensive and fundamental approach to understanding and improving thin films.

When this situation arises and is recognized, there is no more appropriate solution than for the

problem to be addressed by a government agency or group of agencies collectively. The first

alternative is difficult because the problem is so large and multifaceted that, like industry, no one

agency can go it alone either. Therefore, the only recourse is a collective attack by interested

parties. The group should most logically include the Office of Basic Energy Sciences of the Department

of Energy, the various research arms of the Department of Defense, including the Army Research Office,

the Office of Naval Research, the Air Force Office of Scientific Research, and, of course, Defense

Advanced Research Projects Agency. To these we add the National Bureau of Standards of the Department

of Commerce and the National Science Foundation. Industry and education must of necessity play key

roles along with government. We must revitalize our depleted thin film physics capability in the academic community. Thin film optics is still largely an art. The development of centers of excellence would not only lead to improvements in understanding and performance but would more importantly establish an open and thus widely available technology base. A concentrated, coordinated,

and sound program in optical thin films can lead to tremendous improvements in commerce, communication,

defense and energy, as well as many other endeavors. Your support for this approach is solicited.

5. Acknowledgment

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

Dr. Aaron Sanders, Ms. Marilee Hood, Ms. Margaret Wooley, Ms. Phyllis O'Rourke, and the other involved

staff members of the National Bureau of Standards in Boulder, Colorado, for their interest, support,

and untiring efforts in the professional operation of the Symposium and in the preparation and publication of the Proceedings. The continued success of the Damage Symposia would not have been possible without the enthusiastic support of those named above.

xxi i i
6. Bibliography

[1] "Damage in Laser Glass," A. J. Glass and

A. H. Guenther, Editors, ASTM Special

Technical Publication 469, ASTM,

Philadelphia, PA (1969).

[2] "Damage in Laser Materials," A. J. Glass and

A. H . Guenther, Editors, NBS Special

Publication 341, U.S. Government Printing

Office, Washington, D.C. (1970).

[3] "Fundamentals of Damage in Laser Glass,"

N. Bloembergen, National Materials Advisory

Board Publication NMAB-271, National Academy

of Sciences, Washington, D.C. (1970).

[4] "High Power Infrared Laser Windows,"

N. Bloembergen, National Materials Advisory

Board Publication NMAB-292, National Academy

of Sciences, Washington, D.C. (1972).

[5] "Damage in Laser Materials: 1971,"

A. J. Glass and A. H. Guenther, Editors, NBS Special Publication 356, U.S. Goverment Printing Office, Washington, D.C. (1971).

[6] "Laser Induced Damage of Optical Materials:

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

NBS Special Publication 372, U.S. Government

Printing Office Washington, D.C. (1972).

[7] "Laser Induced Damage of Optical Elements, A Status Report," A. J. Glass and A. H. Guenther,

Applied Optics _12, pp. 637-649 (1973).

[8] "Laser Induced Damage in Optical Material

1973, " A. J. Glass and A. H. Guenther,

Editors, NBS Special Publication 387, U.S Government Printing Office, Washington,

D.C. (1973).

[9] Laser Induced Damage in Optical Materials

1973: A Conference Report," A. J. Glass and

A. H. Guenther, Applied Optics 14, pp. 74-88

(1974).

[10] "Laser Induced Damage in Optical Material

1974, " A. J. Glass and A. H. Guenther,

Editors, NBS Special Publication 414, U.S Government Printing Office, Washington, D.C (1974).

[11] "Laser Induced Damage in Optical Material

6th ASTM Symposium," A. J. Glass and

A. H. Guenther, Applied Optics 14, pp. 698-715 (1975).

[12] "Laser Induced Damage in Optical Materials:

1975, " A. J. Glass and A. H. Guenther,

Editors, NBS Special Publication 435, U.S Government Printing Office, Washington,

D.C. (1975).

[13] "Laser Induced Damage in Optical Materials:

7th ASTM Symposium," A. J. Glass and

A. H. Guenther, Applied Optics 15^, No. 6, pp. 1510-1529 (1976).

[14] "Laser Induced Damage in Optical Materials:

1976, " A. J. Glass and A. H. Guentehr,

Editors, NBS Special Publication 462, U.S. Government Printing Office, Washington, D.C

(1976) .

[15] "Laser Induced Damage in Optical Materials:

8th ASTM Symposium," A. J. Glass and

A. H. Guenther, Applied Optics _16, No. 5, pp. 1214-1231 (1977).

[16] "Laser Induced Damage in Optical Materials:

1977, " A. J. Glass and A. H. Guenther,

Editors, NBS Special Publication 509, U.S. Government Printing Office, Washington, D.C

(1977) .

[17] "Laser Induced Damage in Optical Materials:

9th ASTM Symposium," A. J. Glass and

A. H. Guenther, Applied Optics 17, No. 15, pp. 2386-2411 (1978).

xxi v
S.I. CONVERSION UNITS

In view of present accepted practice in this technological area, U.S. customary units of measurement have been used throughout this report. It should be noted that the United States is a signatory to the General Conference on Weights and Measures which gave official status to the metric

S.I. system of S.I. units in 1960. Readers interested in making use of the coherent system of S.I.

units will find conversion factors in ASTM Standard Metric Practice Guide, ASTM Designation E 380-76

(available from American Society for Testing and Materials, 1916 Race Street, Philadelphia,

Pennsylvania 19103). Conversion factors for units used in this paper are:

Length

1 in = 0.0254* meter

1 ft = 0.3048* meter

1 microinch = 2.5400* x 10~8 meter

Area

1 in 2 = 6.4516* x 10"4 meter2

1 ft 2 = 9.2903 x 10-2 meter2

Force

1 lb (Ibf) = 4.448 newton

1 kip = 4448 newton

P ressure Stress

1 psi = 6895 pascal

1 psf = 47.88 pascal

Energy

1 ft-lbf = 1.3558 joule

Moment

1 lbf-ft = 1.3558 newtonmeter

Temperature

To C

= 5/9 (Top - 32)

Heat

Thermal conductivity,

1 cal (thermochemical ) cm.s. °C = 418.40* watt/meter kelvin

specific heat, C

1 cal (Thermochemical )/gm.°C = 4184.00* joule/kilogram kelvin

*Exact value

xxv
It

!

f
DOE WELCOME PRESENTATION OF AWARDS TO DRS. GUENTHER AND GLASS

C. Martin Stickley

Director, Office of Laser Fusion

Department of Energy

Washington, DC 2054S

It is both an honor and a privilege to be invited to speak at this tenth anniversary meeting. This

symposium has become an established fixture in the laser community, for whom its only title will always

be "The Laser Damage Meeting," even though I recommended changing it. As a participant and attendee of

many of these meetings over the years, and even as an editor with Art Guenther and Alex Glass for the

first one, I must admit that ten years ago few of us expected that our trip to Boulder would become an annual pilgrimage. On the other hand, I doubt that any of us could have appreciated the tenacity with

which this group would hold onto the opportunity to spend a week in the Rockies each year!

While preparing these remarks, I was looking back through the proceedings from years past, and I was immediately struck by the progressive size growth of that yearly NBS publication. From barely a hundred pages and a dozen papers in the first years, the proceedings have grown to last year's S61 page

tome with some fifty-odd papers.

Another thing I noticed in reviewing the proceedings was the significant maturation of the damage

field as evidenced by the kinds of papers submitted. The early meetings, as one might expect, were dominated by empirical papers presenting evidence of damage, principally using glass laser systems. The

papers tended to be rather general in nature and more times than not the data in papers by different

authors at different laboratories tended to disagree. In recent years the scope of the meeting has broadened significantly to include a large variety of laser sources and optical materials. The basic

issues have not changed but the damage thresholds now discussed are appreciably higher and the laser pulses somewhat shorter and more powerful than most were at the early meetings. The titles of the papers tend to sound a bit more erudite than they used to but occasionally one still notices aspects

that recall the old days -- things like "scotch tape tests" and "calibrated cheesecloth wipes."

The laser damage field has, in fact, matured to the extent that it is really two rather distinct

subfields. On one hand there is the technology community, the laser builders and users (including much

of the DOD effort) , for whom laser damage is a serious problem which needs to be made less serious but

does not necessarily have to be fully understood. On the other hand, many in the research community

look upon laser damage as a physical process which ought to be understood in its own right; one which

should be well characterized before attempts are made to solve technological issues. This dichotomy in perspective leads at times to an interesting juxtaposition of papers at these damage symposia. Neither

of these approaches is totally right or wrong, each has its place in a field as broad as ours. What

is necessary is to be able to decide which perspective is appropriate with regard to the situation one

has at hand. The desire to find fundamental solutions to problems must be balanced against the needs

of technology programs to achieve short term results.

In the laser fusion program most of our present efforts in laser damage are directed at finding

short term solutions to improve the performance of our operational systems. However, we are well aware

that we must soon begin to address the fundamental issues in the damage field if we ever are going to be able to construct megajoule lasers for power plants of the future. Achieving the proper ratio of technology to fundamental research is always a difficult problem for an R§D program manager and it is not any less so in the laser damage field. As many of us are painfully aware, it is quite possible to throw good money after bad in pursuit of "easy" empirical solutions to damage problems, and, likewise,

it is possible to believe that one is investing in a black hole in looking for fundamental solutions to some questions. There is no question but that we are making progress in understanding laser damage

mechanisms and improving thresholds, but it is comforting to know that ten years from now we will surely

be here celebrating the twentieth anniversary of this symposium.

One other thing that is comforting is the knowledge that when we return here ten years from now, Art Guenther and Alex Glass will still be running this symposium. We all owe a large debt of gratitude

to these two young men who not only originated the idea of this meeting but who also have worked each

year to put it together and to insure that its deliberations are preserved for posterity. It is probably safe to say that Art and Alex run the most unique conference in the laser business if not in all of

physics. This is the only meeting I can think of that is run by a junta, and I can only say that I could wish more meetings were run that way if it would insure as smoothly run a meeting as this con- ference has always been.

1
As a symbol of our esteem for this pair of hard working meeting organizers, I would like to take

this opportunity to present to them tokens of our appreciation of their efforts. First, on behalf of

the ASTM and speaking for Haynes Lee, who could not be here for this meeting, I want to present Alex

and Art with this award from the ASTM. The citation on the certificate of appreciation is: "The officers and members of Committee F-l on electronics express their deep appreciation and gratitude in recognition of the outstanding contributions you have made to the laser industry by serving as Co- Chairman of the Laser Damage Symposium for 10 years." Next on behalf of the Department of Energy,

and of the Defense Advanced Research Projects Agency, who is represented here by Captain Harry Winsor,

I want to present them with these plaques. The inscription is, "This plaque expresses the appreciation

of the Department of Energy and the Defense Advanced Research Projects Agency for leadership in the development of new and improved optical materials over the past decade. Presented to (Alex and Art),

Co-Chairman "10th Symposium on Optical Materials for High Power Lasers." Again, my congratulations and thanks to both of them for their outstanding efforts over the past years.
REMARKS TO THE TENTH ANNUAL

BOULDER OPTICAL MATERIALS

FOR HIGH POWER LASERS SYMPOSIUM

Captain Harry Winsor

Material Science Office

Defense Advanced Research Projects Agency

Washington, D.C.

Let me add DARPA's and my voices to Martin Stickley's welcome to the tenth annual gathering of the Boulder Optical Materials for High Power Lasers Conference. On my meeting papers this has appeared

variously as "Boulder Damage Symposium" and "Laser Seminar." You'd think by the tenth time around we

would have found a name for this meeting.

The subject of this meeting is components for power optics systems. I say power optics in the

same sense that electrical engineers say electrical power handling equipment —the technology,

materials, and concerns of optics are fundamentally different, yet strangely the same as those of signal optics. Power optics breaks down into two categories, which are also analogous to the

electrical case: pulse optics and high flux (current) optics. Components in both areas have the basic

problem of handling large amounts of energy without incurring momentary or permanent loss of function.

Let us examine this situation more closely.

The first problem encountered was laser materials. Specifically, I mean ruby and various other

laser host materials. This falls under the pulse optics rubric, since the problem was the thermal damage due to inclusions or electric field induced damage in laser rods. The damage occurring in these

materials was the subject of many an early session. To a large degree, the problems have been

al 1 evi ated , as amplified by Dr. Stickley and other contributors at these meetings. The primary

remaining problem from DARPA's viewpoint is the generation of materials for miniature lasers for designators and rangefi nders

Other early topics were the investigation of pulse damage of materials introduced in the focus of

pulse lasers and the examination of such topical problems as the onset of avalanche breakdown. All research was done in conjunction with the ASTM, since there was a critical need for new standards in this area. You have also heard about this from Dr. Stickley, and you will hear more duing the rest of this Symposium.

Aside from a gain medium, lasers require mirrors and, at times, windows. Both mirrors and windows

may use thin films for reflection control. The primary differences between signal and power optics

mirrors are the use of water-cooled substrates and the emphasis on absorption as opposed to reflectivity. These concerns emphasize the fact that the parasitic losses are large enough to melt,

deform, or fracture the components. Windows are required to separate incompatible environments

internal and external to the laser system. (More often, it seems that laboratory high power CW laser

systems operate without windows.) The problems of developing window materials with low absorption have

been solved at several wavelengths, and can be solved operationally at any other desired wavelengths.

Speaking of wavelengths brings to mind a situation I mentioned last year--every time we solve a material absorption or film deposition problem at one wavelength, the system designers change the wavelength at which they want to operate a system. This has led to a restructuring of DARPA's Laser

Window and Coating programs to develop a generic range of experience at a variety of wavelengths rather

than to derive solutions for specific wavelengths. We are therefore leaving the problems of optimization of materials and films at most wavelengths to the systems' projects. We have provided

documented examples of how the problems may be solved and have performed the necessary survey studies

to list the most likely impurity problems that will be encountered at all probable wavelengths.

I would like to reiterate another theme from my remarks last year--the system designers will always want to increase the power handling capacity of the components, so reduced absorption research

will be with us until we can show that we have reached intrinsic levels of performance using the best possible materials for the specific job. It is thus very important to list in the technical reports,

simply and clearly, whether there is room for further development of the material and what directions

that development might take.

In addition to this, there is one other thing the the industrial and commercial concerns

represented here should remember: the governmental support available for long-range, fundamental

research in power optics materials will be in direct proportion to the requests for this support. For

instance, the National Bureau of Standards (NBS) currently wants to hear from industrial and commercial

enterprises concerning their needs for basic optical measurements such as index of refraction, optical constants of bulk and thin-film materials, such as strain birefringence and temperature dependence of index of refraction, as well as thermal conductivities and thermal expansion coefficients of optical

3
materials. Word of new research you want the NBS to conduct is also solicited. Please communicate

this information to Bruce Steiner at NBS, Washington, D.C., or even better, call him and follow up with

a letter. NBS is trying to restructure its activities, but much of the work of the sort being done by

Al Feldman and Marilyn Dodge may be discontinued unless it serves a documented "commercial" need. This

documentation may not come directly from the DoD. It must come from prospective DoD contractors and subcontractors. Commercial product needs may also justify such research.

Returning to the power optics problems remaining to be solved, two stand out prominently:

(1) anomolously high absorption levels in thin films as compared to the bulk material used for deposition of the films, and (2) survival of power handling components in space and other operating

environments. The coating absorption problem appears to be yielding rather rapidly at this time, as will be apparent from Marshall Spark's papers, and the NWC efforts, which were, unfortunately, not cleared in time for presentation here. The space and environmental problems are under attack on a number of fronts and hopefully should yield in the near future. Both these problems combine together

for thin films for spaceborne HF laser optics and for mirror coatings for duty in rare gas-halide

eximer laser cavities. In the years ahead, they will also be problems for the tunable, free-electron

laser. With this latter system, designers have finally devised a system which maximizes the rubber

wavelength problem referred to earlier. If this system is developed, as appears nearly certain, we

will encounter almost all conceivable adverse internal environmental circumstances in operating power optics components--al 1 on one laser system.

The free-electron laser represents an opportunity as well. Greater than 50 percent wall plug ! efficiency in photon production opens the possibility of photo-purification of optical materials. The 1 free-electron laser may thus lead to a dramatic improvment in laser component materials. This is lifting oneself up by one's bootstraps. The free-electron laser will also provide i sotopti cal ly pure materials for construction of refining machinery and systems. This will allow us to tailor the ' isotopic mix of the trace impurities left in our optical materials. This may become a major way of improving not only power optics materials but also optical waveguide materials. Other promising

isotopic engineering opportunities may well appear as these avenues are explored.

Passing on to some more fundamental problems, the improved scatter required of components for uv

' power optics systems leads to requirements for improved surface roughness in substrates and films. : This means that the long-neglected areas of the polishing of mirror substrates and the nucleation and growth of thin films will require renewed interest. These problem areas lead optical scientists into

the unfamiliar (to them) areas of molybdenum metallurgy, wear theory, and surface topographic

characterization. Add the problems that arise as one seeks optical coatings which resist impurity

absorption, radiation damage, and erosion in adverse environments, and the optical scientists are entering the areas of polymer chemistry, atomic physics, and corrosion rate theories. The point of this is that the area is astonishingly multidisciplinary. Add the problems encountered with

repetitively pulsed lasers: material fatigue, thermal conductivity, and interface adhesion theory,

and.. ..The problems and interconnections never stop. How will we be able to document this astonishing

array of interdisciplinary problems and techniques in a manner that will facilitate their being

understood by people who haven't enough time to assimilate all the details. More and more, the

} complexity of this field inhibits our ability to determine whether more research is needed, and if so, what research. Who will write the book which brings perspective to this field, and how long will it take. How much of this complexity must be mastered by systems designers. How soon. I am sure that - the proceedings of this Symposium will play a significant role in answering this need for readable ' documentati on.
SYMPOSIUM WELCOME Alexander J. Glass Lawrence Livermore Laboratory

P.O. Box 5508, Livermore, CA 94550

It is my annual hope that each year's Symposium will be the last. Not because I dislike the

subject, or because I find Boulder inhospitable, but because I'd like to think that we can solve the

problems of laser damage, and move on to new challenges.

Like other speakers addressing you at the beginning of this conference, I asked myself what we

; have learned in ten years, and what remains to be done. What we have learned in ten years, very simply,

j stated, is how to build high power lasers. Ten years ago we were discussing high power lasers. Now,

j high power lasers are a reality. According to Aviation Week, there exist high power, cw lasers

! operating in excess of 100 kilowatts. At the Lawrence Livermore Laboratory, and elsewhere, there exist

j multi-terawatt systems operating on a relatively routine basis. Gigawatt lasers have become

commonplace, and hardly worthy of mention in a high power laser meeting. Lasers with design figures of

100 terawatts or greater are under construction in at least two locations in the United States. The

I technology of high power lasers has not waited for us to solve the damage problems, and that is the

theme I want to discuss.

One of the things we have learned in the last ten years is how important laser damage and optical

materials considerations are in the design of high power lasers. We have three papers at this meeting

addressed to the influence of materials problems on the design of high power laser systems: one on cw

lasers, one on pulsed IR lasers, and-one on pulsed neodymium glass lasers.

The failure of optical materials, either transient or permanent, remains the major limit to the performance of high power laser systems. Although there have been great advances, it remains as one of

j the highest leverage areas for improvement. System designers are now well aware of damage problems,

which in itself represents a major achievement. The overall awareness of damage problems on the part

| of everyone involved, from the people who make the materials to begin with, to the people who apply the materials in the end use, has increased tremendously. We have learned, as we were taught at our mother's knee, that cleanliness and purity are virtues. The cleanliness and purity of the basic

materials from which optical components are made are very important. It has been as a consequence of

the work of the people attending this symposium that the importance of cleanliness and care in

j maintenance of operating systems has also come to be realized. Captain Winsor alluded to the importance of keeping in mind the fact that the system must operate in a certain environment. In some

| of the environments that we anticipate, the maintenance of the necessary cleanliness in the system may

be very difficult.

But all of that comes under the heading of consciousness raising. It is not sufficient simply to

' make people aware of the problems. One must also provide solutions. In the course of the ten years in which this symposium has been going on, we have made significant advances in the quality of optical

materials for high power lasers. There is good evidence to believe that for any wavelength desired,

; low loss window materials can be provided. In the neodymium glass area, the advent of materials with

low nonlinear index materials is a reality. We shall be hearing about some of those advances at this

' symposium. These two things have been very real accomplishments and lead directly to improved system

performance. In the coating area, Captain Winsor mentioned that damage thresholds have been significantly increased, to which I would only add the word "occasionally." What are we going to have

to learn to avoid having this meeting in ten years? The first thing is to translate research advances

into production terms. I am somewhat biased by recent experiences at the Lawrence Livermore

Laboratory; but I am sure it is true generally that it is one thing to obtain a research coating which

is of extremely good quality and exhibits a very high damage threshold, but it's another thing to get

it on a production basis over a large aperture, when you need it. In the materials area, and particularly regarding thin films, we need good materials and components to be available on a regular

commercial basis, at reasonable cost, and in reasonable quantity. There is a strong incentive to the

manufacturers to pursue this, because there are very large systems under design and on which

construction will soon start. These very large systems will carry with them significant orders. It is not only the orders for the first round of components, but for a continuing supply of replacement parts

as well. Large lasers are inevitably driven beyond the limit imposed by the onset of damage. The importance of generic studies was mentioned. We clearly have to learn to transfer knowledge from one

pulse length to another, and from one wavelength to another. In the laser fusion program, for example,

as the lasers get larger, the pulse lengths get longer.

For a while, we were reveling in the relative simplicity of sub-nanosecond damage phenomena. Now

we are working with longer pulses. The pulse duration is moving steadily out into the one-to-ten

nanosecond regime, where a host of new and fascinating effects enter. It is not sufficient to say we

have solved the problem; just operate your system at 100 picoseconds. The end use requirement dictates

that we must continue to come to terms with damage phenomena over a range of pulse durations.

Some years ago, Martin Stickley suggested changing the name of the Symposium. He felt that "Laser

Damage" was too restrictive and had a negative connotation. We now officially refer to the Symposium

j on Optical Materials for High Power Lasers. I should like to advocate broadening the scope of the

5
Symposium a little further, to place increased emphasis on "damage avoidance."

What do I mean by damage avoidance? I mean the following: There is always some level at which

the system fails, but in designing the system we try to maintain the minimum ratio of peak-to- average power density. Almost all the phenomena we talk about are either sensitive to the maximum of power density or to the time integrated maximum of power density. If we can reduce the modulation on the beam temporally and spatially, then we can transmit the maximum average power subject to local damage

constraints. Examples of the techniques to do this, of damage avoidance techniques, include the following. Spatial filters have been introduced in neodymium glass systems to suppress the growth of spatial modulation. Optical relaying is an application of the imaging character of the spatial filter,

which enables us to suppress diffraction effects, and therefore make more efficient use of the aperture

to again raise the total power through the system while holding the peak power density below some limiting value. A technique which is showing great promise, especially at 10.6 microns, is phase

conjugation, double passing through an amplifier and using a nonlinear element to correct out any phase

distortion, which is a very promising technique for maintaining a low peak-to-average ratio. There also is the plasma spatial filter, invented at KMS, in which dithering of the beam provides spatial

averaging during the course of the pulse as a way of reducing peak-to-average energy density on any particular spot. Design techniques are also damage avoidance techniques. The best example is designing thin film coatings to minimize the local field maximum. I am sure there are other good ideas

for damage avoidance, and I invite you all to extend your thinking beyond the particular problem of a given material or component to this area as well. I think that it is an area of great promise. There are many good ideas which have not been exploited, and we need new approaches quite urgently. We are currently building large systems which must perform well, and the leverage is very great. Improvement

in the ability to transmit high power or high energy through the aperture translates directly into more kilojoules or more terawatts per megabuck. In the real world, that is the figure of merit. I know I speak for the preceding speakers when I say there are funds available for good ideas in the area of damage avoidance.

It is fairly certain that we will meet here next fall for the Eleventh Symposium on Optical

Materials for High Power Lasers. We still have not solved the problems of laser damage. I sincerely

hope, however, that we can broaden the scope of our activity to include not only improved understanding

of damage phenomena, but also new ways to avoid damage as well.

In closing, I'd like to acknowledge the continuing support of the Defense Advanced Research

Projects Agency, the Office of Naval Research, and the Department of Energy. Both Dr. Guenther and I are personally grateful to Dr. Stickley and Captain Winsor for their kind words on the occasion of the Tenth Symposium. We want to express our gratitude also to Haynes Lee of the American Society of Testing and Materials and to Dr. Boyne and the staff of the National Bureau of Standards for their

hospitality and their continuing efforts to make these symposia both pleasant and productive.

6
DISCUSSION OF A THEORY OF ANALYSIS OF RATE CALORIMETRY WHICH INCLUDES COATING ABSORPTION

Nils C. Fernelius and George T. Johnston

University of Dayton Research Institute

300 College Park. Avenue

Dayton, Ohio 45469

A simplified method of analyzing laser rate calorimetry on samples with absorbing

coatings is presented. Special cases of rate calorimetry are shown to agree with

previously derived cases. Results are given for samples with dissimilar faces in which the temperature rise and the reflected power in both orientations to the laser

beam are measured. A computer program was written to reduce data. A sample case

shows that the output requires the reflected and transmitted powers to be measured

to three significant figures for meaningful results.

Key words: Coating optical absorption; laser calorimetry theory; optical calorimetry

theory; surface optical absorption.

1. Introduction

The problem of how to treat rate, or slope, calorimetry in samples with absorbing coatings has appeared from time to time during the course of our work. This paper gives a simplified method to handle more general cases of rate calorimetry and is shown to agree with the results of previous

analyses of some special cases. We will first derive basic calorimetric equations for a window sample

which includes surface absorption for the case of absorption independent of incident medium and for incident medium dependence. Later these results will be applied to coatings in the approximation that coherence effects may be ignored.

2. Basic Equation Derivation

The basic calorimetric problem is illustrated in figure 1. Each surface of the sample is described by an absorbance, reflectance, and transmittance coefficient subject to these restrictions: A

l +" R

l + T

l = 1 atld A

2 + R2 + T2 = 1 ' The bulk absorbance, A = 1 - exp(-g-£), where for small fit, A

q

= g£. Likewise, the unit incident power

has this restriction: P + P^ + P^ + P + P = 1,

, I K a L £, where . , P = power transmitted,

P = power reflected,

R

P = power absorbed in bulk,

B P^ = power absorbed in front surface,

= power absorbed in back surface. Coherent effects in the substrate are neglected, which is acceptable in thick samples with non-optically

parallel surfaces. They are also ignored on the surfaces which may be seriously incorrect on coated

samples. However, even with these severe assumptions, the resulting simplified treatment is barely

tractable. Multiple reflection effects are handled by the use of the binomial expansion,

2 3 4 5 1/(1 - x) = 1 + x + x + x + x + x + . . . valid for x^<l. For unit incident power, the power transmitted at the exit surface is

PT

= Cl-A

o

) TlT

2 + (l-A/T^R^ + (l-A^ 5 ^^ + .... (1-A )T T

P = 2_L_2 (1) T 2 1 - (1-A ) R R0 O 12

The power reflected at the front surface, 1, is

P R

- R

l + (1-VV? + ^-A

o )4r

2 T

1R1 + a-V6^2R

l +

(1-A

o

) 2 T2r

2 R1+ R2 (1-Ao

) 2 (tJ -rJ) (2) PR

= \ + 2 _ -

K 1 2 1 - (1-A ) R,R„ 1 - R,

0 (1-A ) O I 2 LAO

The power absorbed at the front surface,l, is

P = A, + A.. (1-A ) 2 T,R, + A^l-A ) 4T R^R + ....

1 1 1 ol2 1 o 1 I I . A T (1-A )\ kJl + (1-A ) R (T -R )] P=A+^ ^A-=^ °- Lll (3)

1 1 - (1-A ) R.R. 1 - R1 R_ (1-A ) o 1 2 i I o

7
The power absorbed at the back surface, 2, is

P

2 = d-A

o

) TlA

2 + (l-A^T^^^ 4- (1-A

o

) 5 TiA

2 r2r2 + .... T A (1-A ) p

2 = —°—r- (4)

1 - R^d-V

The power absorbed in the bulk per pass is A

q times the intensity at the beginning of the pass. Thus,

P

B

= A

o

[ Tl 4- (l-A

o

) 2 W2 + (l-A

o )VlR

2 + (1 -A

o )6t

i R1 R2

3 + 3 +

A

o [(l-A

o

) Tl R

2

4- (1-a/t^R24- (1-a/t^R2 +....]

P = A

oTl +

A

o

(1 -VT

l R

2 _ Vl[l + d-Ao )R

2

3 B 1-R^.d-A ) 2 1 - R.R-U-A ) 2 1 - R_R„(1-A ) 2

I I o 12o 1 2 o The power v absorbed in the sample is P, = P, 4- P_ + P A 1 2 B Wh6re

A

1 [l + (l-Ao

) 2 R

2 (T

1 -R

1 )] + (l-A^T^ 4- A

qT1 [1 4- (l-A^]

p =

2 r (6)

1 - R^d-V

By going through the algebra, we find that PA

4- PR4- P^ = 1, as it should for conservation of energy. Another approach to this problem is presented in Heavens [l]"*". He treats the optical properties

by the use of the complex index of refraction, n = n - ik. The optical absorption coefficient can be expressed as 3 = 4uk/A , where A is the vacuum wavelength. This yields a reflection coefficient of R = [(n-1) 2 4- k

2 ]/[(n4-l) 2

4- k ] . Note that for a 6= 10 cm at 10 urn wavelength, we get k = \ B/(4ir) = -7 0 10 , an insignificantly small number in most cases. Heavens treats the case of an absorbing film on an absorbing substrate and includes coherence effects in the film (pp. 76-77). He comments that the reflectance at the film-substrate interface differs according to the direction from which the interface

is approached but that the transmittance remains the same. Very complicated formulae are given for the

two reflectances. They are identical except for the interchange of two brackets. Unless one is dealing with heavily absorbing films, the two reflectance values are essentially the same in this treatment. Gibbs and Butterfield [2] have used this approach for laser window calorimetry.

Workers in the area of laser damage have consisently observed that a transparent dielectric

material traversed by a collimated light beam, always damages on the exit surface at a lower power level

than that at which the entrance surface damages. Crisp et al [3,4,5] have proposed that this is explained mainly as an effect of Fresnel reflection. All of their analysis assumes phase coherence. A way of incorporating this effect into laser rate calorimetry has been suggested by Miles [6] and Apfel

[7]. It will be given in the following paragraphs.

At each surface as shown in figure 1, the absorbance, reflection and transmission for light

approaching from air is A^,R^ and T^ ; A2> R

2 and T

2 as before. However when light approaches the surface

from inside the substrate, the absorbance and reflectance are different and denoted as A^,R^,A

2 and R

2

. This leads to additional conservation conditions, viz. A^ 4- Rj 4- T^ = 1 and A^ 4- R^ 4- T2 = 1 where A^ = xA^ and A^ = xA2> Thus R^ = R^ 4- A^(l - x) and R

2 = R2 + A2 (l - x) . When coherence is not assumed [6,7] x = n, when it is assumed[3 ,4 , 5] x = 4n

2

/ (n 4- l)

2

. Going through a similar analysis as before, various results are changed. The analogous equations

will be numbered as before only primed.

(1-A )T T

PT

= °-^~

2 d')

1 - (1 -a ) r'r:

o i I Rn + Rl(l-Ao

) 2 (T

2 -R1 R')

p

1 ? o 1 1 1 (2 ')

1 - (1 -a ) r:r;

o LI

A 4- (1-A ) 2 R!(A'T -A R')

P

1

0 2 11 11 (3')

1 - (1 -a ) r:r;

O i L T AI(1-A ) P„=-JL? 2 (4-)

2 1 - (1 -a ) 2 r'r;

o 12

Figures in brackets indicate the literature references at the end of this paper.

8
Vl[l + (1 "A

o )R2

] P

B

= —— 2 (5')

1 - (1-A ) Z R'RI

o 1 I An + (1-A ) 2 R' (A'T -A R' ) + (1-A )T 1 Al + A T [l + (1-A )R;]

p

1 o l 11 11 O 1 2 O 1 O 2 CAM

A 2 ^

' i - (i-A ) r;r:

o 12

Again with the additional conservation equations, conservation of energy is maintained with these new

results

3. Applications of Basic Equations

Now we shall apply these results to some special cases. A few have been obtained previously and

are used as a check of the validity of this method. These applications fall into two general classifications —those allowing for large absorptance and those regarding absorptance as a small quantity. The

cases treated are summarized as follows:

A. A

q may be large

Case 1- no restrictions on A's or R's

2- R2=0, i.e. side away from laser beam (back side) an AR coating

3- A^=0, assumes no surface absorption on front side

4- A^=A

2 =0, bulk absorption only

B. A ,A, ,A„ less than 0.1

o 1' 2 Case 5- A^=A

2 =0, get the often used formula which has the correction for reflection at the rear surface.

6- R^=R

2 =0, AR coatings on both sides

7- Measure reflected power as well as transmitted power with window in both orientations with respect to incoming light beam.

8- If both surfaces identical, no new information from second orientation. Find

surface and bulk absorption inseparably linked.

9- Before and after measurements when applying AR coatings. —RZ CASE 1: If A = 1 - e with no restrictions on the magnitude of the A, R, and T's, then

PT

= T^e'^/Cl - R[R

2 e~

6£

]

P R

= l\ + R

2 (T^ - R^e^Ml - R'R2 e~ m]

A

1 + R

2 (T

1A|-A1 Rpe"2S£ + A^e"6^ + T^l + R'e~ B£

) (1 - e~

S£

) ?A=

1 - R;R

2 e- 2e£

This yields /T"? j S£ = MiT^U + PA

/P

T

) + 'T^d + PA

/P

T

) + *(Aj_ + T

1 )[R

2 (A

1R| - TjAp + T^] }/2(A1 + T

]_)] The - / solution is unphysical since the argument of the log would be negative. Removing the primes

on the above quantities yields the results obtained in the original simple treatment.

CASE 2: If the rear surface is an AR coating, then R_ = 0 and a = MT^U + pa

/p

t )/(a1 + Tx )] In the primed case, this equation is true if also A2 = 0. CASE 3: If we assume that A^ = 0, (i.e., the front surface is bare and not absorbing with the rear side

coated) , then r-z ^ B£ = £n[{T

2 (l + PA

/PT

) + ^2 (1 + W + 4R

2 }/2]

Again, removing the prime yields the original result.

CASE A: If A

L = A

2 = 0, Rx + T1 = 1, and R

2 + T2 = 1, then U = In [{(1-R

2 )(1+PA

/PT

) + /(1-R

2

) 2 (1 + PA

/PT

) 2 f 4R

2 }/2]

This formula has been obtained previously.

In all of the following cases, we shall assume that A

q

= &l < 0.1 and that A^ , A^, , A^ <0.1 ; that is, the substrate and coatings are all at least moderately transparent. To first order in the A's,

we obtain the following expressions, where the transmittances have been eliminated: R + R

2 [l - 2(A1+R1

) + (x+DA^ - 2A

q (1-2R1 )] - (l^R^ (x-l)A

2 PR

=

1 - R

1 R2 (1 - 2A

q

) + (x - 1)(A

1 R

2 + A^)

9
(l-R^CL-R^ (1-A

Q

) - A

1 (l-R

2

) - A

2 (l-R

1

)

1 - R

1 R

2 (1-2A

Q

) + (x-IHAjRj + A

2 R

1

) A

1 [l+R2 {x-(x+l)R

1

}] + xA

2 (l-R

1

) + A

q (1-R ) (1+R ) 1 - R

1 R

2 (1-2A

q

) + (x-DCA^ + A

2 R

1

) When x = 1, we obtain the results of the original treatment. Again in this approximation,

P Thus, R + PT + PA

" 1

A

1 [l+R2 {x-(x+l)R

1

}] + xA

2 (l-R

1

) + A

o (l-R

1

) (1+R ) (l-R

1 )(l-R2 )(l-A

o

) - A

1 (l-R2

) (1-^)

The calorimetric analysis of optical absorption data used in our laboratory follows the method

described in the appendix of M.Hass et al [8]. The method considers the time situations before the laser is turned on, while it is on, and after it is turned off. Before the laser is turned on, while it

is on, and after it is turned off. Before the laser is turned on, the sample is at temperature T . o When the laser is on, the power absorbed is

P = m C (dT,/dt) + p(T,-T ). A p Z Z O After the laser is turned off, no power is absorbed, so

0 = m C (dT./dt) + p(T -T )

.

p J 3 o By measuring slopes where T

2 = T^, we obtain

PA= m C [(dT

2 /dt) (dT./dt)] = m C (AT/At).

P Since we measure the power transmitted by the sample, P this equation must be converted to use the expression P^/P^ derived above: m C£ AI

At

PA

A

o (l-R1 )(l+R2

) + A

1 [l+R

2 {x-(x+l)R

1

)] + xA

2 (1-R ) (1-R,)I/O (1-R,) (1-A ) A

1 (l-R

2

) A

2 (l-R

1

)

Now we will treat some additional special cases, all of which assume that CASE 5:

CASE 6:

CASE 7:

' , A , A' and A ' < 0 . 1

.

o 1 2 If there is no surface absorption with reflections at each surface, i.e., A,12= A. =0, A W o A

o (l+R2 )/[(l-A

0 )(l-R2 )] P /P = A T

then

ergo, we may get a different

:(i+r

2 )/(i-r

2

) (1+R^) / (l-R^) where R^ is the reflectivity at the back surface. Note that when we reverse the orientation of the window with respect to the

o, laser beam. If the window is uniform with no coatings, R = [ (n-1) / (n+1) ] , where n is the

b

9 index of refraction of the substrate and (1+R^O/ (1-R^) = (1+n )/2n. This correction factor

is used for uncoated samples.

If a window has AR coatings, i.e., R = R. = 0, then P A /P„ = (A,+xA„+A )/[l-(A +A1 +A„ ) ] ==A, +A+xA0 1 z AT 12o Olzloz the absorbance of each surface plus the bulk contribution. If in addition A^ = A

2 = 0, then we

get PA/PT A /(1-A ) = u/a-BD m C (1/PT ) (AT/At)

P With all the parameters in the above theory, additional experimental information is needed to evaluate them all. One way to obtain additional data is to measure the power reflected by the sample and the transmitted power. Alternatively, a beamsplitter could be used to measure the incident powers or the sample could be measured with each surface facing the beam. The reflection from each surface can be measured with a spectrophotometer. With the one per cent accuracy obtainable on the Perkin-Elmer 180, obtaining values from it for A. and A

2 is probably out of the question, since they may be considerably smaller. The front side of the window is defined as that one facing the incoming laser beam. The following equations are useful when there are unequal surfaces, e.g. one side is AR coated. The general first order solution for small A's is: rPo = A [l+R

2 {x-(x+l)R }] + xA2 (l-R ) + A (1-R )(1+R2

)

T' froi

2 in

(1-R..) (1-R,) (1-A ) 1 z o A^l-R^ - A

2 (l-R

1

) xA-d-R,) + A„[l+R.{x-(l+x)R1 }] + A (1-R,) (1+R,)

1 Z 21 1 o z I

front

1 in front

(1-R,) (1-R,) (1-A ) I 2 O A

1 (l-R

2

) - A

2 (l-R1

)

R + R

2 [l-2(A

1+R

1

) + A^d+x) - 2A

o (l-2R

1 )] - (1-2R

1

) (x-l)A

2 (l-R

1 )(l-R

2 )(l-Ao

) A

1 (l-R

2

) - A

2 (l-R1

)

10
R

2 + R

1 [l-2(A

2+R2 )+A2 R

2 (l+x) - 2A

Q (1-2R

2 )] - (1-2R2

) (x-DAj

2 in front (1-R.)(1-R,)(1-A ) 1 2 o A

1 (l-R

2

) A

2 (l-R

1

)

The above equations can be rearranged as linear equations in the A's. A computer program was written to solve them when x = 1. As a test of the computer program, some trial answers were

assumed: k

Q

= 0.003, k = 0.0005, A

2 = 0.001, and an incident power of 25 watts. The quantities

PR1'?T1' ^l^Tl' P R2'

PT2' and PA2 /

' PT2 were calculated by hand to about 5 significant figures.

This simulates an ideal case. The results were P R1

= 4.29206 watts, PT1

= 20.6125 watts, pA1 / pT1

= 0.0046325, PD0 = 4.25803 watts, P„,- 20.61245 watts, and PA2

/PT2

= 0.0062832. The substrate sample

consisted of a ZnSe window with a PbF

2 coating. The measured reflectances on a Perkin-Elmer 180 were R = 0.171 and R„ = 0.001. The output results were A, = 0.00050055, A„ = 0.00110937, and A =

J- ^ 1 2 o 0.00289008, in good agreement with the assumed values. Thus, we are satisfied that the program is working properly. As a check on how sensitive parameters in the program are, various inputs were

singly changed by a small amount and the new results were obtained.

inputs used and their corresponding outputs.

Table 1 lists some of the

Table 1. Results of parameter sensitivity study of computer program

Input

quantity

changed Newvalue A

l

Outputs

A

2 A

o

P R1 4.30 .00143 .19246 -.18939

PR1 4.28 -.00091 -.29006 .29547

PR2 4.26 .00050 .00111 .00289

PR2 4.25 .00050 .00111 .00289

PR2 4.20 .00050 .00111 .00289

PA1/PT1 .0050 .00136 -.03639 .03968

P /P A2' T2 .0068 -.00053 .00111 .00413

The results of this exercise show that the power absorbed with the AR coating in both orientations shold be measured to three significant figures for reasonable results. The same can be said

with the AR coating in the rear. The latter is much more difficult to achieve experimentally.

Perhaps with improvements in the precision of power measurements, this treatment will be helpful in using laser rate calorimetry to handle windows with absorbing coatings.

In one experimental test no reflected power was measured when the AR coating faced the beam. Putting numbers in the formulae we find that the power reflected with the AR coating in front and with it in back should be identical to within a few per cent. Apparently the reflected beam was not centered in the power meter entrance window.

CASE 8: An uncoated window with identical front and back surfaces is a commonly encountered case. Here we have A = A

2 = A, R = R2 = R, T T =T.2 This case yields the same equations whether side 1 or side 2 is in front; thus, no additional

information is obtained by taking data in both positions.

PA/P

T

= [(l+x)A + A ](1+R)/[(1-R) (1-A

Q

) - 2A] - [U+x)A + A

q

] (1+R) / (1-R) We see that the surface and bulk contributions are inseparably linked together as in the AR coat- ing case.

CASE 9: One experimental situation has been found to be tractable. In this case the uncoated substrate

with assumed identical surfaces was measured. Thus, A^=A

2 =A^ and R^=R

2 =R, therefore,

( Wo = [A

o + (1+*>A

U](1+R)/(1-R).

After applying an AR coating on one side, say 1, we have R^=0, A^=A , R

2 =R, A

2

=

coating facing the beam,

(PA/PT

) 1 =

t A

c

( 1+xR) + + A

0

( 1+R) -! /(1 ~ R)- With the coating away from the incident beam,

(PA

/PT

) 2 = [^d-1*) + A

u + Ao (l-R)]/(l-R).

Thus AP = (P A /PT ).-(P./PT)„ = {A [l-x+2xR] + (x-l)A + 2RA }/ (1-R)

When the substrate is AR coated on both sides, ( VV2c = A

o + (1+X)A

CWith the

1 1
These equations yield,

A

q = [(1-R) (l+x)AP

c - (l-x+2xR)(PA

/PT

) 2c

- (x-l)(PA

/PT

) o (l-R)/(l+R)]/2R

A

c = [{2R(l+x) - ( x

- 1 )XpA

/ pT

) 2c

" (x-l)(PA/PT

) o (l-R)/(l+R) - (l+x)(l-R)AP

c ]/[2R(l+x)]

A

u

= [(l-x+2xR)(P

A

/P

T

) 2c

- (x+1) (l-R)AP

c + (2R-l+x) (PA

/PT

) o (l-R)/(l+R)]/[2R(l+x)] . For the case of x = 1, these results have been applied to PbF. coatings on ZnSe, see Fernelius et

al [9].

2

4. Conclusion

When surface absorption occurs, most experimental laser calorimetry situations do not yield data where surface and bulk absorption are solvable. Unequal faces are needed for a solution. The trans- mitted and reflected power can be measured in both window orientations (three measurements are needed)

.

For a meaningful solution, the reflected power must be measured to better than two significant figures

which is currently not viable. Another possibility is to measure uncoated, coated on one side (both

ways), and coated on both sides. With equal faces an alternative is to use a long bar sample and/or

multi-thickness samples.

5. References

[l] 0 . S . Heavens

,

Optical Properties of Thin Solid

Films , (Butterworths, London, 1955)

[2] W.E.K.Gibbs and A. W. Butterf ield , Appl. Optics

14, 3043 (1975)

[3] M.D.Crisp, N.L.Boling and G.Dube', Appl.

Phys.Lett. 21, 364 (1972)

[4] N.L.Boling, M.D.Crisp and G.Dube, Appl.

Optics, 12_, 650 (1973)

[5] M.D.Crisp, IEEE J. Quantum Electronics,

QE-10 ,57 (1974)

[6] P. A. Miles, Appl . Optics ,16 , 2897 (1977)

[7] Joseph A. Apfel, private communication

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

[9] N.C. Fernelius, J. A. Fox, D.A.Walsh and

G.S. Coble, Proc. High Power Laser Window

Conference, Denver, 1978

Figure 1. Diagram of Calorimetry Sample

12
ABSORPTION COEFFICIENT OF NaF BY ATTENUATED TOTAL REFLECTION SPECTROSCOPY*

David L. Burdick

Michelson Laboratory, Physics Division

Naval Weapons Center, China Lake, CA 93555

Attenuated total reflection (ATR) spectroscopy has been employed to measure

the absorption coefficient of NaF films from the visible to 3 um. A 1-mm-thick

ZnSe ATR plate (50-mm long) was coated on two sides with NaF (0.55- and 0.57-umthick films). By coating half the width of the ATR plate, "sample-in/sampleout" measurements could be performed to reduce substrate effects. The spectra

reveal a strong water band in NaF peaking at 2.95 um with a peak absorption

coefficient at 165 cm

--'- relative to the substrate. o Scattering effects are noted

at shorter wavelengths, and it is estimated that 7 A of water are adsorbed onto

the bare ATR plate.

Key words: Absorption coefficient; sodium fluoride film; total internal reflection; 0.65 to 3.0 microns.

Introduction

While attenuated total reflection (ATR) or, more appropriately, internal reflection spectroscopy

has been used for some time by chemists to study interfaces [l] 1

, it has only been very recently that ATR has been seriously applied to the study of thin films for optical technology [2-5]. The advantages

of the method are rooted in the fact that specially designed film substrates, i.e., the ATR plate, can

be either fabricated or purchased to produce many reflections of the probing light beam, thereby increas

ing the total interaction between film and photon.

We have assembled an optical system for performing ATR measurements using commercially available ATR plates. This system has been tested using NaF films on a ZnSe plate. This paper presents the results of our ATR measurements from 0.8 to 3.0 um on NaF films, and discusses briefly a scattering

phenomenon in the region of 1 um. Our measurements also permit us to calculate the thickness of the adsorbed water layer on the uncoated ZnSe plate.

Experimental Details

The optical layout of the ATR system is shown in figure 1 and is rather conventional. The light

beam exiting the monochromator is split by a mirror splitter (1) into a sample beam and a reference

beam. The reference beam is focussed by a mirror (4) onto the detector; this beam is chopped at 104 Hz by a tuning-fork chopper (2). The sample beam is focussed onto the entrance aperture of the ATR plate

by mirrors; it is chopped at 150 Hz by the tuning-fork chopper (3). Finally, the beam leaving the ATR

plate is focussed by mirrors onto the same detector. The system uses a Perkin-Elmer 98 monochromator,

a 100-W tungsten filament source, and a room-temperature PbS photoconductive detector.

The signal is processed by two Princeton Applied Research 124 lock-in amplifiers. The resulting

sample and reference signals are ratioed electronically and the ratio is applied to the y-axis of an x-y

recorder. The x-axis is driven by the monochromator wavelength drum. The spectra so obtained contain

only ATR data; all source and detector factors are eliminated.

A single ATR plate was coated on half of each broad side with NaF deposited in an ultrahigh vacuum

system. The films were 0.5523- and 0.5708-um thick as measured by a Talystep stylus. A diagram of the plate is shown in figure 2. The reason for coating just half of each side is to enable us to make

"sample-in/sample-out" measurements. The ATR plate was attached to a vertical translation stage so that

it could be raised and lowered relative to the focussed sample beam. As a result, we could scan both

the coated and uncoated portions with no changes in the system other than displacing the plate by about

1 cm.

Results

Figure 3 shows the ATR spectra obtained in our measurements. Two spectra, from about 0.8 to 3.0 um are shown; the upper line with the large dip near 2.9 um is for the NaF films, and the lower line with

the smaller dip at 2.9 um is the uncoated ZnSe ATR plate. Each spectrum shows a monotonic increase in

* Work supported by the Defense Advanced Research Projects Agency.

1. Figures in brackets indicate the literature references at the end of this paper.

13
signal as X increases, with no structure until the dip at 2.9 um is reached. Note that the coated portion of the plate is transmitting more energy to the detector at the shorter wavelengths than is the uncoated portion, a seemingly anomalous result. We will discuss each of these spectra in the following

sections.

Analysis

We will follow the treatment presented by Harrick [1]. In normal fashion, the reflectance in total internal reflection is reduced by both absorption and scatter so that the intensity of the sample beam is

I = IQ (1 - A - S) N

(1 - R)

2

, (1)

where I is the incident intensity at the ATR plate entrance aperture, R is the reflectivity of the entrance and exit apertures, A and S are the absorption and scatter for one internal reflection, and N

is the number of reflections. Under our experimental conditions the ratio of the coated I and uncoated

. . c

I intensities is

. N

I / 1 - A - S\

r-(i -*"-.') • (2 > U x U U'

We will examine this relationship for three cases.

1) A = A = 0:

c u In a spectral region where there is no absorption, or the absorption is small compared to the scattering, we have

.1/N

i - s Vi } u \ u

(3) x °u

Assuming that S << 1,

it)

I x 1/N

' - 1 . (4)

This is the condition which applies in the 0.8- to 2.3-um region in our NaF films.

2) S ,S << A ,A : c u c u In this case,

I

I

1/N

(5)

This condition holds near the absorption band at 2.9 um in our NaF films. To obtain a value for the absorption coefficient we shall use the effective penetration depth of Harrick [1] and write

A = a d (6)

c c

where a is the absorption coefficient of the NaF film (assumed to be the same for both films) and the

c effective penetration depth d is approximated as

2 2 2 2

/ \ A-,n„,cos6 3sin 6 + n 01 sin 9 - 2n,.

d = i(d + d ) = /

21

, =-r Y~— , (7)

l \

e

\\ el/ 2^(1-11^) (sin 6-n^) 2 (l+n2

1 )sin 9-n 21

where X, = A/n„ „ , n. = n., „/n„ „ , 6 = angle of incidence on interface (45°), and d , d are the

1 ZnSe 21 HaF ZnSe e

y e

j_ effective penetration depths for the two polarizations of light.

Equation (7) is valid for internal reflection at an interface between two infinite media. Since

we are dealing with films about 0.5-um thick, we must estimate the validity of eq. (7). Thus we have

calculated the true penetration depth of the power in the evanescent wave at the 1/e point. This is given by

14
p 2 2 1 4ir(sin 6 - n,^)

(8)

This depth, which is essentially linear in X , is plotted in figure 4 together with the film thicknesses.

It. is clear that the infinite approximation should be valid over the range of our experiments. Our calculations were further modified to include the finite film thickness by introducing a factor of l-exp(-L/d ) into the effective penetration depth for each film (L is the film thickness).

Two additional factors were included in our calculations. The first, and simplest to handle, is the fact that the two films on the ATR plate were of different thicknesses. The second is the fact that absorption measurements on the coated portions actually measure a ZnSe surface which has water adsorbed

onto it, coated subsequently by NaF. We have attempted to introduce these facts by finally solving this

equation

2/ 1 \

2/N

1- (A +Au))(l- (A +AJ = (1-a/UM . (9)

1 2

x u

Here A^ , A

c are the absorptions in the two different films and A^ is the absorption in the ZnSe surface with adsorbed water. Pursuant to our discussion, A = ad , and the factor 1 - A is obtained

below. Our final result is °1,2 ' u

2/N) d

1 + d2 -^ (d 1 + d

2

) 2 -4dld2 (i- (77^

a = (1 - A

u

) — . (10)

3) Thin layer of water:

This is a special case for the uncoated ZnSe ATR plate and corresponds to the small dip at

2.95 urn in figure 3. This spectrum is assumed to be a very thin layer of water adsorbed onto the ATR

plate. It is described by the equation

where I is represented by the level dashed line (the "bleached" film)

This equation provides direct access to 1 - A as required in eq. (10) (assuming that the adsorbed

water layer is unaffected by the NaF evaporation). A further result is that, by assuming the optical

constants of bulk water [6], we can use eq. (11) to estimate the thickness of the adsorbed water layer.

Using the thin film approximation of Harrick [1], we have

w \ o'

(1 - n )[(1+ n )sm 6 - n ] — — —— . (12)

2 4 2 2n

21 cosG[(2 + n

31 + n^) - 2n31 ]

Here a is the absorption coefficient of bulk water, n_, = l/n„ „ , n„. = n /n„ „ , n._ = 1/n , and the w 31 ZnSe 21 w ZnSe 32 w

other parameters have been defined earlier.

Since the ATR plate was 50-mm long and 1-mm wide and the angle of incidence 8 onto the totally

reflecting face is 45°, the number N of bounces is 50. In our calculation we used the refractive index

of ZnSe given by Dodge and Malitson [7] and the refractive index of NaF taken from the American Institute of Physics Handbook [8].

Discussion

First consider case 1), that of low absorption but some scattering. In figure 3 this region lies

roughly between 0.8 and 2.3 um. We have some confidence that this region is dominated by surface scattering because of a series of normal-incidence scattering measurements on the uncoated portion of the -3 -3 plate at 0.6471 um and 1.15 um. The scattering was 8.2 x 10 and 2.6 x 10 , respectively, at these

two wavelengths, and the ratio of these numbers is 3.2; this is also equal to (1.15/0.6471)2, the square

of the wavelength ratio. Thus, we feel that surface scattering is most important here. Applying eq.

(4) at 1.15 um, we find that
S - S = 4 x 10 u c

This scattering difference is about 16% of the normal-incidence scattering from the uncoated surface. As one moves toward longer wavelengths the effect persists, but diminishes until absorption in the NaF

film finally causes the two spectra to overlap at about 2.4 pm. One can see that the scattering difference is significant by noting figure 5 which shows scattering measurements obtained by placing the detector close to the exit aperture of the ATR plate, without intercepting the exiting beam. Although

these rough measurements do not give a measure of the absolute scatter, they do show that the coated

portion scatters less than the uncoated portion over the same spectral range as the ATR measurement.

The ratio of the scattering curves in figure 5 is nominally independent of X , a result consistent with

^ —m . -, a A scattering law.

Turning now to the absorption coefficient of NaF, we note immediately that there is a large absorption peak in figure 3. When analyzed by means of eq. (10), we obtain the absorption coefficient as a function of A; this is shown in figure 6. It is believed that the peak is due to water incorporated

into the NaF film, both during the evaporation process and from the atmosphere prior to the measurement.

The peak absorption coefficient is 165 cm~l. This peak is located at 2.923 um as compared with the bulk water absorption peak at 2.950 um [6]. The arrow in figure 6 indicates the bulk water absorption peak

location. We do not know yet whether this difference in spectral location is real or illusory. Our absorption coefficient is high compared to all the calorimetry data taken at the Naval Weapons Center on NaF films. This is not surprising in view of the fact that the sample has been exposed to the atmosphere

for an extended period prior to measurement. Calorimetry samples, on the other hand, are usually measured shortly after removal from the vacuum system. As a check on these ATR measurements, our first, the sample was measured near 3.0 um on a Perkin-Elmer 180 spectrophotometer. While it is not particularly

easy to see a dip on the order of 2%, our measurements at normal incidence through the two NaF films showed a dip which is entirely consistent with about 1.1 um of material absorbing at 165 cm

--'-. Thus,

although the sample used here cannot be considered a state-of-the-art film, it does seem to verify the techniques and interpretation.

Finally, let us examine the dip at 2.95 pm in the uncoated spectrum. We assume that it is due to adsorbed water, and by using optical constants of bulk water and eq. (12) we have estimated the layer

thickness to be about 7 A. This is about twice the thickness of the adsorbed water layer on CaF^ ATR

plates reported by Palik et al. [5]. These calculations, while interesting, must be viewed with caution

because of the assumed optical constants of the adsorbed specie.

Conclusions

Attenuated total reflectance measurements of NaF films on a ZnSe plate allow us to determine the absorption coefficient of the film at 2.92 pm (165 cm~l) , to estimate the thickness of the adsorbed water layer (7 A), and to observe a difference in the scattering between a NaF-ZnSe interface and an air-ZnSe interface.

References

[1] See the book by Harrick, N. J., Internal

Reflection Spectroscopy, Interscience (1967) and the review article by Hansen, Wilford N.

,

in Advances in Electrochemistry and Electrochemical Engineering, Vol. 9, John Wiley and

Sons, Inc. (1973) for good reviews of the subject.

[2] Holm, R. T., and Palik, E. D. , Appl. Opt. 17_, 394 (1978).

[3] Willingham, C. , Bua, D. , Statz, H. , and Harrigan, F. , "Laser Window Studies," Final

Technical Report on Contract DAAHOI-74-C-0719

,

Raytheon Research Division, Waltham, Mass. (August 1975).

[4] Deutsch, T. F. , J. Electron Mater. 4_, 663

(1975).

[5] Palik, E. D. , Gibson, J. W. , Holm, R. T.

,

Hass, M. , Braunstein, M. , and Garcia, B.

,

Appl. Opt. 17_, 1776 (1978).

[6] Downing, H. D. , and Williams, D. , J. Geophys.

Research 80, 1656 (1975).

[7] Dodge, M. J., and Malitson, I. H. , in Laser Induced Damage in Optical Materials: 1975,

A. J. Glass and A. H. Guenther, eds. Washington, D.C., National Bureau of Standards, 1975

(NBS Spec. Publ. 435), p. 170.

[8] American Institute of Physics Handbook, 3rd Edition. (McGraw-Hill Book Co.), 1972, pp. 6-45.

16
MONOCHROMATOR

BEAM

LIGHT BEAM

Figure 1. Diagram of ATR optical arrangement.

(1) Mirror beam splitter; (2) 104 Hz tuning-fork

chopper; (3) 150 Hz tuning-fork chopper;

(4) focussing mirrors; (5) plane mirrors.

Figure 2. Diagram of ZnSe ATR plate coated on two

sides with NaF. The plate is translated vertically

in the system to illuminate the coated and uncoated

portions in succession.

1 1 -1 1 1

1

— \ ^"V**^

\ UNCOATED ZnSe

\

s/ sy

\

\\

\ NaF—COATED ZnSe \ i sy

// //

//

s/

i i i i

Figure 3. Tracing of ATR spectra. The solid line is the transmission of the uncoated ZnSe plate.

The dashed line is the transmission of the NaF-coated poriton of the ZnSe plate; note that it is greater than the uncoated portion from 0.8 to 2.5 ym. The short dashed section from 2.7 to 3.1 ym

is the estimated transmission of the "bleached" plate.

FILM THICKNESS

Figure 4. Penetration of power into the NaF films. The solid line shows the 1/e depth of power as caluclated from eq. (8). The dashed .lines indicate the physical thickness of the films. At these

depths the approximations used in the analysis are valid.

17
1.993 2 406

X(/im)

Figure 5. Scattered radiation from both the coated and uncoated portions of ATR plate. The

scattering from the uncoated portion is greater than that of the coated portion, a result which

tends to explain the "anomalous" difference in the two ATR spectra from 0.8 to 2.3 urn.

ABSORPTION COEFFICIENT NaF (ATR MEASUREMENT)

Figure 6. Calculated values of the NaF absorption coefficient. The peak is assumed to be due to water in the films. The arrow indicates the location of the absorption peak in bulk water.

13
LASER CALORIMETRIC MEASUREMENT OF TWO PHOTON ABSORPTION

Michael Bass, Eric W. Van Stryland and A.F. Stewart

Center for Laser Studies

University of Southern California

Los Angeles, California 90007

A laser calorimeter has been used to measure two photon

absorption in solids. This new technique provides greater

sensitivity than is possible in nonlinear transmission measurements. The two photon absorption coefficients of CdTe and CdSe at

1.06 um using ^ 16 nsec (FWHM) pulses are 0.13 + 0.04 cm/MW and

0.050 + 0.014 cm/MW respectively.

Key words: CdSe; CdTe; internal reflections; laser calorimeter;

linear absorption; Nd : YAG laser; two photon absorption.

1. Introduction

The laser calorimeter is a device that is widely used to measure small linear

absorptions at wavelengths where laser sources are available. Such an instrument-^ can measure absorptions which are less than one part in 10 5 using a 1 W laser, [l]

The calorimeter measures the total absorption due to all processes including the

intensity dependent phenomenon of two photon absorption (TPA) . Thus the experimental

technique described in this paper is based on a measurement of the total absorption

as a function of the laser intensity. Calorimetric measurements of the total absorption in CdTe and CdSe as functions of 1.06 um laser intensity were used to obtain

both linear and TPA coefficients of these materials. The results can be understood

by using a simple model for the attenuation with distance in the presence of TPA and

by properly accounting for multiple reflections in. the sample.

In 1931, M. Goppert-Mayer presented a theoretical description of TPA in which the

transition probability was shown to be proportional to the square of the light intensity [2]. Thus a material having both linear and two photon absorption will transmit light according to the expression

^(x,y,z,t) = + 6I(x,y,x,t) I (x,y,z,t) , (1)

(See fig. 1) . 2 Here I(x,y,z,t) is the light intensity in MW/cm and is a function of the space

coordinates and time; z is the direction of propagation; a is the linear absorption

coefficient in cm-1 , and 3 is the two photon absorption coefficient in cm/MW.

-2 -1 -2

In many materials of interest a will be small, e. - 10 cm , and 6 = 10 cm/MW

according to transmission measurements at high intensities [3]

Thus a modest intensity of 1 MW/cm^ will result in a two photon contribution to the

total absorption which is equal to the linear term (See eq. (1)). On the other hand,

this intensity will result in a change in transmission for samples shorter than 1 cm

of less than 1% which is too small to be measured accurately. A comparison of the

two techniques can be made by noting that in a transmission experiment one measures

a small change (the reduction of transmission due to TPA) in a large quantity (transmitted intensities) , whereas using calorimetry one measures a large change (the added

absorption due to TPA) in a small quantity (total absorption)

2 . Experimental

The experimental procedure is to measure the total absorption as a function of

intensity and evaluate the data according to eq. (1) , taking into account reflections

at the rear surface. A flashlamp pumped, Q-switched, TEMQ0 mode, Nd:YAG laser was the

source for the experiments as shown in figure 2. The laser was operated at <v5 Hz with

an output of % 1 mj/pulse [4] . The pulse temporal waveform was a Gaussian function

with full width at 1/e points in intensity of 19 nsec (See fig. 3) . A beam scan was

used to determine the spatial profile which was Gaussian having a 1/e full width in

intensity of 1.2 mm at the focusing lens (See fig. 5). An 8 cm focal length lens was

1. Figures in brackets indicate the literature references at the end of this paper.

19
used to increase the intensity at the samples, which were placed 4 cm behind the focus.

The attenuators and optics in front of the sample limited the maximum peak on axis

2 intensity at the sample to ^30 MW/cm with an average power of approximately ^5 mw. The energy per pulse was measured by averaging the output of over 100 pulses with a pyroelectric joulemeter. The accuracy of the TPA measurements is sensitive to pulse

to pulse energy variations which must therefore be kept as low as possible (^5%) (See

fig. 4) . The beam intensity at the sample was adjusted using a two polarizer attenuator described in references [5] and [6]

.

A thermocouple attached to the otherwise thermally isolated sample recorded the

sample's temperature change due to the absorption of the laser light. When amplified

and recorded, the thermocouple voltage appeared as sketched on the chart recorder in

figure 2 and shown in figure 6. While the laser is on the temperature rises. The

gentle slopes while the laser is off are due to small thermal losses that are seen with thin samples. The time constant of the calorimeter was long enough so that a

5 Hz laser beam caused no uneveness in the recorded temperature change. Thus the

calorimeter responded to the total average power absorbed. The calorimeter was of

conventional design and when evacuated had a measured sensitivity to absorption of one part in 10"* with a 1 W source.

3. Results and Discussion

Data such as in figure 2 was obtained at several laser intensities for room temperature samples of CdTe and CdSe. The samples were single crystals and the CdSe was oriented with its optic axis normal to the entrance (exit) surface. This orientation

eliminated the possibility of second harmonic generation at 0.53 ym which could be

directly absorbed. The CdTe sample had its optic axis approximately 22° away from the

normal to the entrance surface. Figures 7 and 8 show the intensity dependence of the

total absorption of these two materials measured calorimetrically . For a small total

absorption (i.e. (a + 81) z << 1), eq. (1) predicts a straight line for absorption

versus intensity. The vertical intercept of this line gives a and its slope determines

8. In these experiments which employ a Gaussian beam profile and a Gaussian temporal

shape the TPA coefficient would be 2/ 2 times the slope in such a plot. The data

fits a straight line quite well but the total absorption at the highest intensities

used for CdTe was over 20%. Thus eq. (1) must be solved exactly. This was done

previously including the effects of Gaussian spatial and temporal profiles but the

effects of multiple reflections were neglected [3] . For high index materials the

intensity of the reflected beam from the rear surface can significantly affect the

computed absorption coefficients. Since TPA results from the imaginary part of the

third order optical nonlinear susceptibility, x [3] , the field equations for both the

incident and reflected beams were used to obtain the following coupled differential

equations for the intensities:

dl dI R g - -<« + 3d + I R

))I; = +(a + 6( i + i r))

i r

( 2)

where I is the intensity of the incoming beam and I is the intensity of the reflected

beam. (See fig. 9) . Subsequent reflections have been neglected because they contribute still less to the total absorption. Noting that the product I(z)I (z) is a constant everywhere in the sample, these equations can be solved in closed form with

the constant as a parameter. A rapidly converging numerical iterative procedure for

determining the constant from the boundary conditions was used to determine the total

absorption integrated over the spatial and temporal beam profiles.

The theoretical fits along with the data are shown in figures 7 and 8. These

fits were used to obtain the values for 3 listed in Table 1. The values given for a were obtained using a cw Nd:YAG laser and the usual calorimetric techniques. These

Table 1. Linear absorption coefficient, a, and two

photon absorption coefficient, 8 at 1.06 urn

Material a (cm

1

) 8 (cm/MW)

CdSe .062 + .06 .050 + .014

CdTe .335 + .03 .130 + .036

measured values of a are quite accurate and as such were used in the calculations of 8. This means that the values of 8 given in the table are the result of a one parameter

fit. The deviation of the fit from the data at high intensities particularly for

CdSe may be due to a contribution from direct absorption by two photon created free

carriers as discussed in references [3] and [7]. However, at this time the data is

20
insufficient and the error bars too large to make any conclusions concerning the

possible role of free carriers. Work is continuing in this area seeking a predicted

pulse width dependence to TPA if free carrier absorption is significant.

4. Conclusions

The experimental error bars shown in figures 7 and 8 were determined from scatter

in the data and uncertainties in determining the heating rate from curves such as shown in figure 6. The laser intensity fluctuations, the accuracy of the laser spot

size measurement and temporal profile measurement along with the uncertainties mentioned above combine to give the quoted accuracies in Table 1. The accuracy of TPA

measurements using laser calorimetry has the potential for considerable improvement.

The initial work presented here has, however, demonstrated the feasibility of the

technique

The inclusion of the reflected beam in the analysis of TPA measurements is essential in high index materials where it has been ignored previously, [3] but may be

eliminated in future experiments (thus greatly simplifying the calculations) by using

AR coated samples. It should be pointed out that in certain materials the two photon

excited level can decay through a combination of radiative and nonradiative processes.

Then a part of the total TPA may not register as heat generated in the sample. This means that a simultaneous transmission measurement may make it possible to determine

the quantum efficiency of such radiative transitions. This type of quantum efficiency

study and measurements of TPA using two different lasers are in progress.

5. Acknowledgments

The partial support of Air Force Office of Scientific Research under Contract

No. F49620-78-0065 is gratefully acknowledged. Valuable help in calorimetric

measurement techniques was provided by Dr. Susan D. Allen and Dr. Jean-Claude Diels

provided a turbine driven Q-switch for the NdtYAG laser as well as valuable theoretical

help.

6. References

[l] A. Hordvik, Appl. Opt. 16^, 2827 (1977).

[2] M. Goppert-Mayer , Ann. Phys. (Leipz.)

9, 273 (1931).

[3] J.H. Bechtel, W.L. Smith, Phy. Rev. B,

13, 3515 (1976) .

[4] For CdTe the prf was 6.7 Hz and for

CdSe the prf was 7 Hz. The CdTe

sample was 2.0 3 mm thick and the CdSe

sample was 0.89 mm thick.

[5] Laser-Induced Damage to Nonlinear

Crystalline Materials, M. Bass, NBS

Special Publication 341, Proc. of

1970, Damage in Laser Materials

Conference

[6] Damage to 10.6 ym Window Materials

Due to C0

2 TEA Laser Pulses, K.M.

Leung, M. Bass, and A.G.J. BalbinVillaverde, NBS Special Publication

435, Proc. of 1975, Laser Induced

Materials Conference.

[7] F. Brynkner, V.S. Dneprovskii , V.U.

Khattatov, Sov. J. Quant. Electron;

Vol. 4 , No. 6 , (12-74)

21
dlo dZ

» I^l-R)

= -(a + B,^) I2

I^l-R)-

WAVEFORM

MONITOR

I PULSED ]_ !/ V~

j LASER \ --/

9 y\~

VARIABLE ATTENUATOR as.

CHART RECORDER

TIME

POWER

METER

!

VACUUM CHAMBER THERMOCOUPLES

SAMPLE

THERMAL

REFERENCE

NANOVOLTMETER

WiNDO'f

Figure 1. Absorption in a two photon absorbing

medium.

jo *>s/dL<-

Figure 3. Ten successive laser pulses as detected

with a fast photodiode.

Figure 2. Experimental apparatus for TPA measurements using laser cal orimetery

.

Figure 4. Pulse to pulse energy reproducibility measurement with a slow pyroelectric detector.

Figure 5. Ream spatial profile generated with a .15 mm slit scanned continuously at 14.6 sec/nm,

1 aser prf ~ 7 Hz

.

Figure 6. Typical plot of thermocouple voltage vs. time. Laser prf ~7 Hz. Calorimeter not evacuated.

22
Figure 7. Intensity dependent absorption in CdTe: high intensity data (O's), low intensity (cw)'data (X's), theory ( —).

Figure 8. Intensity dependent absorption in CdSe: high intensity data (O's), low intensity (cw) data (X's), theory ( —).

38 32 34

INTERNAL '.ERECTION 91HANCFS TPA

I 2 > I^l-R)

Ij » hR(l-R)

I 8 , lid-R)^

THESE COUPLED EQUATIONS HAY BE SOLVED WEB I CALL

THE TOTAL .ABSORPTION IS THEN GIVEN 8Y KU * l:

a/a

I 3 -RI2

1 P

2 ABS

"

L h

1 P3abs

"

L p 3

> AfiS •

P2abs

•v 2 L P

2

Figure 9. Absorption with internal reflection in a two photon absorbing nedium.

23
A COMPARISON OF BULK AND SURFACE ABSORPTIONS

IN NaCl AND KC1 BETWEEN 9.2 AND 10.8 ym*

H. Vora, M.C. Ohmer and T.G. Stoebe

Air Force Materials Laboratory/LPO

Wright-Patterson AFB, Ohio 45433

Tunable CO^ laser calorimetry has been used to study the wavelength dependence of optical absorption at 16 different wavelengths

between 9.2 and 10.85 ym in NaCl and KC1 . Samples prepared on a small scale using the reactive atmosphere process (RAP) growth, as well as samples cut from large diameter ingots prepared commercially

for high-power CO^ laser windows, have been investigated. A differentiation between the bulk and surface absorption has been made in

several cases using long bar samples. The measured bulk absorption

in the best RAP-grown NaCl crystal is intrinsic over the entire

wavelength range from 9.2 to 10.85 ym, showing bulk absorption

-4 -1 -4 -1 values near 9x10 cm at 10.6 ym and 1.4x10 cm at 9.2 ym.

-5 -1 The best KC1 shows bulk absorption values near 9x10 cm at 10.6

-5 -1 ym and 4x10 cm at 9.2 ym; these values are above the expected

intrinsic limit due to residual absorption caused by the presence

of at least two bulk impurities, possibly CI and CIO,-, -

.

Key words: Absorption; KC1 ; NaCl; 9.2 to 10.6 ym; tuneable laser

calorimetry.

1. Introduction

Infrared absorption in the 9.2 to 10.8 ym region is critical in determining

those materials applicable for use as laser windows in this spectral region.

Considerable work has been carried out in both NaCl and in KC1 , which have pre-

-3 -5

dieted multiphonon absorption limits at 10.6 ym of 1 x 10 and 7 x 10 ,

[II

3 respectively . Comparing these intrinsic values of absorption, KC1 appears

to have an advantage over NaCl in the laser window application with a lower

intrinsic absorption coefficient. Further, for equal absorption coefficient

* Research supported by the National Research Council (NRC) and conducted durinc

the tenure of H. Vora as an NRC Associate at the Air Force Materials Laboratory.

1. Present address: Honeywell, Inc., Corporate Research Center, 10701 Lvndale

Avenue South, Bloomington, MN 55420.

2. Permanent address: Division' of Metal 1 urgical Engineering, University of

Washington, Seattle, WA 98195.

3. Figures in brackets indicate the literature references at the end of this

paper.

24
values, the figure of merit for the peak stress (the product of Young's modulus

and the thermal expansion coefficient) for KC1 is approximately one-half that

[21

of NaCl . However, NaCl has other advantages in being less soluble and less

moisture sensitive than KC1 , having a lower intrinsic solubility for absorbing

impurities and being apparently harder than KC1 ^ 3-6

^

. Small-grained polycrystalline windows of NaCl are more stable than KC1 , and no alloying or doping is

required to maintain small grain sizes^. NaCl has a higher dielectric

strength than KC1 , a point of further interest for pulsed applications.

Laser calorimetric measurements of bulk absorption at 10.61um in RAP-grown

KC1

[8 ' 9] and total absorption in RAP-grown NaCl and Harshaw NaCl [10] have

yielded values in good agreement with those expected on the basis of intrinsic

-5 -1 -3 -1 multiphonon processes, %8xl0 cm for KC1 and ^1x10 cm for NaCl. Allen

T91 -5 -1 and Harrington 1 J recently reported a value of 2x10 cm for the bulk absorption coefficient at 9.27ym of a RAP-grown KC1 . While this is the lowest

absorption level reported to date at 9.27ym in KC1 , it is significantly higher

- 6 -

than the expected intrinsic value of %5xl0~ cm~ at this wavelength, suggesting

that an impurity absorption band is possibly present near 9.27ym even in such

high purity KC1 . Apparently this absorption band, and/or some other bands, are present at a significantly higher level in KC1 prepared using proprietary processes; these materials typically yield total absorption coefficient values in

-4 -3 -1 -4

the range of 9x10 to 1.3x10 cm at 9.27 ym and in the range 5x10 to

9x1

0" 4

cm"

1 at 10.61ym.

Many molecular impurities, when present in alkali halides, give rise to

strong infrared absorptions, as demonstrated by the calculation of Duthl er ^

This calculation indicates that impurities like S0^"~ or HCO^

-

, when present

in KC1 concentration levels in the range 0.01 - 0.03 ppm, will give rise to

-4 -1 absorption coefficient levels of %lxl0 cm . Studies of absorption in KC1 due

[12 131

to CI 0^ and CI 0^ impurities L

' J indicate that these impurities have

stronger absorption near 10.61ym than the impurities considered by Duhler; an assumption of incorporation of only 0.001 ppm of chlorate (ClO^ ) in the KC1

-4 -1 lattice is required to account for the residual absorption of 1x10 cm at

10.61ym.

Of the techniques available for the measurements of low absorption coefficient values, the laser calorimetric method has found the widest application

[8,14-17]^ ^ n ^tractive feature of this method is that it can be used to

differentiate between bulk and surface absorption by employing a long bar sample

[8,14]^ If the ha -| f i^gth 0 f the sample is greater than its radius, the time

required for the heat generated by absorption at the two end surfaces to reach

the thermocouple will be greater than the corresponding time for the heat generated by bulk absorption near the specimen axis; a time- temperature plot obtained

during laser irradiation of a bar sample length L thus exhibits two linear

reginns. Here the slope of the first linear region is proportional to the value

25
of 3 gL , the fraction absorbed by the bulk, where 3

g is the bulk absorption

coefficient, and the slope of the second linear region is proportional to the

value of total absorption, 3

g

L + 2S is the fraction absorbed by the two end

surfaces'-

14 -'. From the known values of specimen mass, length, specific heat

and refractive index, the measured transmitted power and the slopes of two linear

regions, the values of 3g and 2S can be determined.

In the present paper, results obtained on absorption coefficients in the

9.2 to 10.8pm region are reported, using the technique of tunable CO^ laser

calorimetry, are presented on several NaCl and KC1 samples. RAP-grown materials

and the long bar technique were used, when samples were available, to determine

more information on intrinsic absorption limits and the extent of extrinsic

absorption caused by bulk and surface defects in this wavelength region.

2. Experimental

A total of five long bar single-crystal samples were employed in the present

study. The sources, materials and dimensions of -these samples are indicated in

table 1. Absorption measurements were made initially on each of these samples

after mechanically polishing their surfaces to remove visible scratches. The

RAP-grown NRL (Naval Research Laboratory) NaCl and Hughes KC1 crystals were then

etched to determine absorption values at the etched surfaces of NaCl and KC1

.

Etching was done in concentrated HC1 (2 min) followed by successive rinsing in

ethanol (10 sec), 2-propanol (10 sec) and 2-propanol (10 sec). After final

rinsing, each sample was dried in warm air as it was gently pulled from a bath

of 2-propanol, then immediately transferred to the calorimetry chamber for

absorption measurements. All absorption measurements were made in vacuum.

Table 1. Sources, materials and dimensions of various long bar samples

Source Material Dimensions (cm)

Hughes Research Laboratories RAP-grown KC1 13.0 x 2.5 (dia.)

Naval Research Laboratory (NRL) RAP-grown KC1 8.2 x 3.3 (dia.)

Naval Research Laboratory (NRL) RAP-grown KC1 8.3 x 2.5 (dia.)

Harshaw Chemical Company KCl:Eu 6.6 x 2.5 x 1.6

Harshaw Chemical Company KC1 : Rb , Eu 12.6 x 3.2 x 3.2

Several additional samples of single and poly-crystalline NaCl, 1 cm in

thickness and 2.8 cm in diameter, cut from large diameter ingots, were provided

by Harshaw. The samples were polished by Harshaw and the absorption measurements

at 9.27 and 10.61ym were made both in the as-received condition and after etching.

The procedure used for etching was the same as above.

All our laser calorimetric experiments on long bar samples were carried out

by placing two copper-constantan foil thermocouples (connected in series) diametrically opposite to one another on the periphery of the sample at the center of

its length. Rubber bands were used to keep the thermocouple junctions in intimate

contact with the specimen surface. The duration of the scattering region in our

26
experiments varied from about one-half to less than one-fourth of the time

required for the surface heat to reach the temperature sensor. The approximate

o magnitude of this time is obtained from the relation t = (L/2) /6k, where L and

k are the length and thermal diffusivity of the specimen; for a 6/. 6 cm long sampl

of KCl (k % 0.06 cm /sec), t is 30 sees. Since the time required for a long bar

sample to reach equilibrium with its surrounding is inconveniently long, the

r 1 8

three slope formula , which takes into account the cooling rates of the sampl

both before and after laser irradiation, was used to calculate s

R and 2S from the

t-T plots obtained in our study.

3. Results and Discussion

The wavelength dependence of the bulk absorption coefficient for the 9.2-

10.8 urn region in four long bar samples of KCl, and the total absorption in an etched 6 mm thick Harshaw KCl : Rb sample, is shown in figure 1. The long bar

samples have been characterized in table 1; the Harshaw KCl : Rb sample was selected for the present study because it showed unusually high absorption near 9.3

urn. The magnitude of the error bars represent the standard deviation of absorption, determined from at least three calorimetric measurements at the indicated

wavelengths. These results were all obtained with mechanically polished surfaces

The solid line shown in figure 1 was obtained from Deutsch^. Absorption coefficient values for the same long bar samples in the etched condition at 9.27

and 10.6ym are reported in table 2. -4-1

Table 2. Measured bulk absorption in various KCl crystals (units 10 cm ) and ratio of absorption coefficient values at 9.27 to that at 10.6ym

X HRL

RAP KCl

NRL

RAP KCl

Harshaw

KCl :Eu

Harshaw

KCl :Rb,Eu Mul tiphonon

9.27 0.43 1.2 5.4 4.8 0.052

10.6 0.9 1.6 5.6 2.5 0.76

Ratio

e 9.27

B 10.6 0.48 0.75 0.96 1 .9 0.068

The ratio of the bulk absorption coefficient 3g at 9.27ym to that at 10.6um

varies randomly from sample to sample, as shown in table 2. This is also seen

in the relative inconsistent trends in figure 1 of the absorption coefficient

variation with wavelength. This indicates that the residual absorption in the

region 9.2-9.7um is independent of the residual absorption observed in the

range 10.l-10.8ym, suggesting that at least two impurities, one with absorption

predominating in the region 10.1-10.8um and the other with absorption predominating in the region 9.2-9.7ym are limiting the transparency of KCl crystals at

the C0^ laser frequencies. The variation in the relative concentrations of

these impurities from sample to sample would give rise to the observed variation

in the raio 3g at 9.27ym to 3g at 10.61ym.

27
Since tlie estimated strength of absorption at 10.61um per unit concentra-

("121 tion of CI 0^ impurity in KC1 L 1 is greater by at least an order of magnitude

than the corresponding absorption strengths of impurities considered earlier by

Duthler the CI 0^ ion would appear to be a likely impurity to limit the

transparency of KC1 near 10.61um.

The C10

3

~ impurity in alkali halides absorbs in the range 10-10. 9ym'-

6 ' 12 -'

compared to the spectral range of 9.2-10.85ym of the observed residual absorption in KC1 crystals used in the present work. This suggests that, in addition

to CI 0^ , an impurity with strong absorption in the range 9.2-9.7um is present

in these KC1 crystals. This impurity would appear to be C10^~, which has an absorption near 9.27ym which is as strong as the absorption of C 1 0^" near 10.61um [6]

.

Two surface absorption (2S) and bulk absorption data for the long bar

RAP-grown NRL NaCl and Hughes KC1 crystals are shown in figure 2. These

data are for chemically etched surfaces, and indicate that the wavelength

dependence of the surface absorption increases with decreasing wavelength in

both materials. The surface absorption of NaCl appears to be slightly lower

than that of KC1 , with the surface absorption of NaCl being too small to distinguish in the laser calorimetry results in the 10.8-10. lym region. The

surface absorption component in the Hughes RAP-grown KC1 is seen to be greater

than the bulk absorption at all wavelengths. This dominance of surface

absorption is assumed to be caused by the surface interaction with oxygen ions,

forming ClO^ and CI 0^ as discussed above.

Total absorption coefficient data at 9.27 and 10.61ym for single-crystal

and polycrystal 1 i ne Harshaw NaCl are shown in tables 3 and 4, respectively,

for both as-received and etched conditions. All twelve samples in table 3 were obtained from large diameter ingots; samples numbered 1 to 6 were cut

from near the seed end of Kyropoulos grown ingots, while those number 7 to 12 were cut from near the cone end of Bridgman grown ingots. The polycrystal 1 i ne samples in table 4 were obtained by forging slices cut from large diameter

NaCl crystals grown using the Bridgman technique. The slices were cut from

the middle, cone or heel ends of the ingots, as indicated. These data demonstrate the influence of surface preparation on observed absorption coefficient

values: The Harshaw single crystal samples (table 3) show a 25% reduction in

average absorption coefficient at 10.61ym after surface etching and a 30%

reduction at 9.27ym. Similar reductions are seen in the polycrystal 1 ine

Harshaw samples (table 4). High values of surface absorption also influence

the apparent bulk absorption coefficient in polished samples. Comparing

results from figures 1 and 2, the observed increase seen in polished vs. etched RAP-grown KC1 is 10% at 10.6ym, increasing to 30% at 9.27ym.

28
Table 3. Absorption coefficient data at 9.27 and 10.61ym

on Harshaw NaCl crystals. Sample thickness = 1 cm

3 T 9.27ym (xlO

_3cm~

1

) 6 T 10.6!ym (xlO"

3 cm

_1

) Sample No. As Rec'd. Etched As Rec'd. Etched

1 1.45 0.73 1 .76 1.12

2 1 .24 0.84 1 .69 1 .29

3 1 .53 0.92 1 .75 1 .35

4 1 .92 0.80 2.40 1 .45

5 0.87 0.83 1.28 1 .31

6 1 .23 0.75 1 .61 1.16

7 1.17 0.78 1 .55 1 .31

8 1 .34 1 .05 1 .81 1 .61

9 1 .08 1 .00 1 .53 1 .35

10 1 .43 1 .04 1 .69 1 .42

11 1.08 0.85 1 .21 1.19

12 1.29 1 .06 2.06 1 .94

Average 1 .30+0.26 0.89+0.12 1.70+0.32 1.37+0.21

Table 4. Absorption coefficient data at 9.27 and 10.61ym

on Harshaw NaCl polycrystal 1 i ne samples. Sample thickness = 1 cm

B T9.27ym(x 10" 3cm

-1

) Bjl0.61ym (x!0~

3 cm

_1

) Sample No. Location As Rec'd Etched As Rec'd Etched

1 Cone 1 .17 0.88 1 80 1 .64

2 Cone 1 .38 0.87 1 66 1 .57

3 Cone 1 .01 0.69 1 67 1 .34

4 Mid 1 .32 1.27 2 17 1 .94

5 Mid 1 .21 0.89 1 93 1 .38

6 Mid 1 .22 1 .27 1 90 1 .90

7 Heel 1 .69 1 .20 2 53 2.18

8 Heel 1 .46 1 .31 2 28 2.23

9 Heel 1 .81 1 .25 2 95 2.49

Taking into account absorption at the exterior surfaces only in the cone section samples of Harshaw polycrystal 1 i ne NaCl (table 4), bulk absorption

values similar to those in Harshaw single crystals are obtained. Higher

values in the mid-section and heel of the polycrystal! i ne sample are probably

due to higher bulk absorption due to higher impurity concentrations. These

results seem to indicate that the interior surfaces in the polycrystal! i ne sample do not contribute to surface absorption under these conditions.

29
In the cases shown here, the etched surface results in an absorption coef-

-4 -1 ficient of about 10 cm . Workers in this field have often used the absorption coefficient of an etched, 1 cm thick sample as the bulk absorption value.

The present results demonstrate that this assumption is not valid for samples

with low absorption coefficient.

The bulk absorption data for the NRL RAP-grown NaCl in the 9.2 to 10.6ym

range is shown in figure 3, in comparison with the least squares extrapolation

of Deutsch's earlier data'*-' indicated by the dashed line. The latter is norm- ally taken as the intrinsic multiphonon absorption limit in NaCl, but as can

be seen, the present data fall below this line. The total absorption coeffiat 10.6ym previously determined by Rowe and Harrington'- 19

^ for RAP-grown NaCl

also fell slightly below the extrapolated value.

Due to this observed discrepancy, the bulk absorption data for RAP-grown

NaCl given in figure 3 have been used to operationally define a new multiphonon

absorption limit. For an exponential dependence of the absorption coefficient

on frequency of the form 3=Aexp(-yo)/co 0 )

, a least square fit of the nine data

points near 10.6um yields A = 8999.8 cm""' and oj 0 /y = 58,495 cm, with a correlation coefficient of 0.9976. Using this new multiphonon limit, the intrinsic

absorption coefficient for 10.6ym is .0009 cm , compared to Deutach's value of

.00117 cm~^. The measured values for 3 near 9.3ym in the NRL RAP-grown NaCl

lies above this newly define multiphonon limit by a factor of about 4. This

may be due to higher scatter caused by the higher surface absorption in this

region, or it may be caused by impurities that are more likely to contribute

to bulk absorption in this low absorption region. Hence, only those data

points in the 10.1 to 10.8ym region are used in the definition of the multiphonon limit.

4. Conclusions

m The present study has established a new operationally defined multiphonon

limit for intrinsic absorption in NaCl, and has documented the importance of

surface absorptions, possibly related to CI 0^" and C10^", in determining total

absorption coefficients for NaCl and KC1 in the 9.27 - 10.8ym region. The

measured bulk absorption in RAP-grown NaCl is intrinsic in the 10.1 - 10.8ym

region but may be impurity dominated near 9.27ym. Bulk absorption of etched

Harshaw NaCl is nearly intrinsic at 10.61ym (table 3) but could be improved by

a factor of 4 at 9.27ym by added purification. The bulk absorption of the RAPgrown KC1 sample is the lowest measured; it is intrinsic at 10.6i_im but increases

to a factor of 10 above the extrapolated intrinsic value at 9.27ym. Compared

to available KC1 laser window material, the availability of RAP-grown KC1 laser

windows would reduce bulk absorption by an order of magnitude throughout this

wavelength range.

30
5. Acknowledgements

The authors would like to thank Dr. G.E. Kuhl of the Air Force Materials

Laboratory for helpful discussions, and Dr. P.H. Klein of the Naval Research

Laboratory, Dr. J. A. Harrington of the Hughes Research Laboratories, and Mr.

D.J. Krus of the Harshaw Chemical Company for providing the NaCl and KC1 samples

for the present study. The authors are indebted to Mr. J. A. Fox and Mr. P.

| Greason of the University of Dayton Research Institute for invaluable technical

assistance.

6. References

[11 T.F. Deutsch, J. Phys. Chem.

Solids 34, 2091 (1973).

[2] M. Sparks and H.C. Chow, J. Appl. Phys. 45, 1510 (1974).

[3] L.S. Combes, S.S. Ballard

and K.A. McCarthy, J. Opt.

Soc. Am. , 41 , 215 (1951 ).

[4] Harshaw Optical Crystals,

A. Smakula, et al , eds.

,

Harshaw Chemical Co., 1967.

[5] Handbook of Chemistry and

Physics, 56th ed. , R.C.

Weast, ed., CRC Press, 1975.

[6] G.N. Krynauw and C.J.H.

Schutte, Spectrochim Acta 21 , 1947 (1965).

[7] J. Fenter, AFML, private

communication

[8] H.B. Rosenstock, M. Hass,

D.A. Gregory and J. A. Harrington, Appl. Opt. J_6,

2837 (1977).

[9] S.D. Allen and J. A. Harrington,

in Proceedings of the High Power

Laser Optical Components and

Component Materials Meeting,

October 3-4, 1977, Boulder,

Colorado, compiled by J.S. Harris

and C.L. Strecker (Defense

Advanced Research Projects Agency : Arlington, VA, 1977), p. 438.

[10] J.M. Rowe and J. A. Harrington,

in Optical Properties of Highly

Transparent Solids, S.S. Mitra

and B. Bendow, eds. (Plenum,

New York, 1975), p. 109.

[11] C.J. Duthler, J. Appl. Phys.

45, 2668 (1974); Proceedings of

the Fourth Annual Conference on Laser Window Materials, November

18-20, 1974, Tucson, Arizona,

compiled by C.R. Andrews and C.L.

Strecker (Advanced Research

Projects Agency, Arlington, VA,

1975), p. 166.

[12] H.G. Lipson, J.J. Larkin, B. Bendow, and S.S. Mitra, J. Electron. Mater. 4, 1 (1975).

[13] D. Shim and C. Wittig, Appl. Opt.

1_5, 1896 (1976).

[14] M. Hass, J.W. Davisson, H.B. Rosenstock, and J. Babiskin, Appl. Opt.

14, 1128 (1975).

[15] L.H. Skolnik, in Optical Properties

of Highly Transparent Solids, S.S.

Mitra and B. Bendow, Eds. (Plenum,

New York, 1975), p. 405.

[16] S.D. Allen and J.E. Rudisill, Appl.

Opt. 16, 2914 (1977).

[17] A. Hordvik, Appl. Opt. 1_6, 2827

(1977).

[18] M. Hass, J.W. Davisson, P.H. Klein

and L.L. Boyer, J. Appl. Phys. 45,

3959 (1974).

[19] J.M. Rowe and J. A. Harrington, J. Appl . Phys. 47, 4926 (1976)

31
HARSHAW KC/ Rb,£ T KCI Eu.0 B

" " KCt Rb, Eu;#

e NRL RAP KG!, 0 e HUGHES RAP KCI , 0B

3 xlO3 927^xm 10.61/

2.

KG I o /3g (cm 1 ), • 2S

NoCI & /3 B

{ cm1 ) , A 2S

J I I L_

Figure 1. Wavelength dependence of absorption for four long bar samples and one thin KC£:Rb sample.

Bulk absorption eg separated from surface

contribution using laser calorimetry on long bar

samples; total absorption (3j reported in thin

sample. Extrapolation of Deutch's KCl data, taken

as the multiphonon limit, shown for comparison.

1100 1050 1000 950 900 WAVE NUMBER ( cm )

Figure 2. Comparison of bulk (0b) and two-surfac<

(2S) absorption for long bar NRL NaCji and Hughes

KCi. Chemically etched surfaces. Bulk absorptior

in cm

_ l; surface absorption in fraction absorbed

per surface.

OEUTSCH'S EXTRAPOLATION *»/

/3 24273 o»pl-»/56) / V

S999.8 e«p (-•»/ 58.51

Figure 3. Bulk absorption for NRL RAP-grown NaCi

with least squares line fit to data near 10.6 urn (solid line). Extrapolation of Deutch's data shown

for comparison (dashed line). These data define a new operational multiphonon limit line for NaCi.

1100 1050 1000 WAVE NUMBER (cm1 )

32

/
A 1.06um LASER ABSORPTION CALORIMETER FOR OPTICAL COATINGS

Thomas H. Allen, Joseph H. Apfel, and C.K. Carniglia

Optical Coating Laboratory, Inc. Santa Rosa, California 95402

We describe a laser calorimeter for measuring the absorption

of an optical coating deposited on a thin disc. The sample with

diameter 2.54cm and thickness between 0.02 to 0 . 1 3cm is irradiated

with 6 watt CW Nd:YAG laser. The temperature of the coated disc

is compared to that of a reference disc using thermistors in me- chanical contact with the samples. Measurements are made under vacuum (100 torr) to reduce temperature variations caused by convection. The calorimeter was evaluated by measuring coated and

uncoated discs of various thicknesses with absorptances from

— 6 — 3 9 x 10 to 5 x 10 . These measurements demonstrated a repeat-

-4 ability of ± 2% at an absorptance level of 1 * 10 and a sensitivity for a 0.04cm thick substrate of 3.8 x 10 5 p V/watt . The

^ lower absorption limit of the calorimeter is 2 x 10 . The re- sults of measurements on polished, uncoated fused silica discs

with thicknesses from 0.04 to 0.13cm show a surface absorptance

— 6 — 5 of 6 x io and a bulk value of 5 " 10 /cm.

Key words: Absorption; calor imetry ; optical coatings.

1. The Laser Calorimeter

A wide range of techniques has been developed for measuring the absorptance of optical materials [1,2]. Of these various techniques, laser calorimetry is the most

widely used because of its simplicity and high sensitivity [3]. Laser calorimetry inivolve measuring the increase in temperature of a sample that results from the irradiation by a known amount of laser energy. In most low level laser absorption calorimeIters, including the adiabatic calorimeter described by Decker and Temple [4], the samj pie and its mounting are assumed to be at essentailly uniform temperature throughout

I the experiment. For dielectric samples the uniform temperature requirement limits the

shape to thick discs or rods for axial illumination [5].

We have choosen to use thin discs for substrates in order to keep the absorption

of the disc smaller than that of the coating. This sample geometry results in a large

, thermal gradient between the central illuminated region and the edge where the temperature measurement is made. Therefore, a new measurement and calibration procedure and

thermal analysis are required for this calorimeter.

The optical arrangement of the calorimeter is shown in figure 1. The beam of a

6 watt Nd:YAG laser is transferred by a series of lenses and apertures to provide a well

defined spot approximately 5mm in diameter at the sample plane. A He-Ne laser is used

: for alignment purposes. The intensity of the laser beam is monitored with a silicon

detector prior to the all-dielectric turning mirror (M5) which reflects the beam into

the sample chamber. A small amount of the incident laser beam is transmitted by this

turning mirror (M5) to the beam position monitor. The entrance and exit windows of

the sample chamber have been positioned to transmit the laser radiation reflected by

the sample and the exit window. The laser power transmitted by the sample is measured

by a radiometer. All lenses and windows are multilayer coated to achieve a reflectance of -.5% at 1.064um.

The most critical design problems involved supporting the sample disc and measuring

its temperature. These problems were solved by the use of thermistors for temperature

measurement and the sample mounting arrangement shown in figure 2. The sample is supported by an arrangement of three thin-walled glass tubes to minimize thermal leakage

to the support structure. Two thermistor beads are mounted on the tops of the glass

tubes to contact the sample. The support was designed so that the temperature sensor

is not irradiated by laser radiation scattered by the sample or its support. An un- irradiated reference disc is mounted on an identical arrangement of three glass posts

and two thermistor beads. The four thermistors are electrically arranged in an AC

Wheatstone bridge with an input voltage of 1.600 volts at a frequency of 950Hz. The

33
output of the bridge is measured with a lock-in amplifier. This electrical arrangement

is capable of measuring changes in output voltage as small as luV.

Absorption measurements are made by measuring the transmitted laser power and the

increase in sample temperature 4 minutes after opening a shutter in the laser beam.

These signals are combined with the sensitivity to determine the total absorptance of

the sample as discussed in the evaluation section. All absorption measurements on coated samples were made with the coated side away from the incident laser beam. A

pressure of 100 microns was used to reduce convection cooling of the sample.

2. Thermal Analysis

We have carried out a computer analysis of the thermal response of our calorimeter

using standard numerical methods [6]. The sample was modeled as a thin absorbing disc

irradiated in the center by a laser beam. The heat conduction through the sample was assumed to be radially out from the center so that a one dimensional analysis could be

used. Radiation losses and gas conduction losses were also included in the model.

The results of the thermal analysis show that the temperature of the disc rises to a limiting value at which the absorbed power is balanced by the radiation and conduction losses. During the heating cycle and at equilibrium, a large thermal gradient

exists across the part. For a 0.25mm thick disc, the temperature in the center is

approximately 20 times the temperature at the edge. As the thickness increases, the

temperature at the center decreases and the temperature at the edge increases. Figure

3 illustrates the heating curves for several thicknesses of fused silica discs. The

temperature at the edge of the disc is plotted against time after the laser is turned

on. These curves also illustrate that the time required to reach equilibrium increases

as the thickness increases.

The major result of the thermal analysis is that the temperature at the edge of

the disc is a linear function of the total power absorbed by the sample, in spite of

the large thermal gradient across the part. Thus either the equilibrium temperature with constant irradiation of the part, or the maximum temperature after irradiation for a fixed time interval can be used to measure the absorption of the sample. The linearity

of the system has been confirmed experimentally as shown in figure 4.

3. Evaluation

The calorimeter sensitivity and linearity were determined by measurements on a sample coated with a slightly absorbing SiC^-Cr-SiC^ trilayer. The reflectance and

transmittance of the sample were measured with a precision laser photometer [7] whose

accuracy is reported to be ±.0005 (±.05%). Its absorptance at 1.06 micron was found to be .0048. The linearity was determined by measuring the signal level as a function of

transmitted laser power by inserting attenuators of known transmittance in the laser

beam incident on the sample. Figure 4 shows that the signal is a linear function of

absorbed power over more than three orders of magnitude. The sensitivity is the

slope of the linear relationship between signal and absorbed power , and is

3.8 x 10^yV/watt for this substrate thickness.

The precision was established by repeated measurements on a set of four uncoated

glass and fused silica samples choosen to cover an absorptance range 1 x 10 ^ to

4 x 10~ 4

. The results for 0.5mm thick BK7 are shown in figure 5 and are summarized for

all samples in table 1.Table 1. Summary of precision measurements.

Sample

Fused Silica

Fused Silica

BK7

BK7

Thickness

(mm)

.15

.41

. 25

.51

Number of

Measurements

10

1 3

12

12

Mean

Absorptance

,-6

9 x 10

x 10

-6

1.2 x io

3.8 x io

-4

Standard

Deviation

2 x 10" 6

2 x 10" 6

3 x io" 6

5 x io" 6

34
These results show that the standard deviation is essentially constant over this absorption range. It is felt that the standard deviation of 2 x 10 represents the

noise equivalent absorptance inherent to the calorimeter.

4. Total Absorptance of Uncoated Fused Silica

The absorptance of uncoated fused silica has been measured to determine its variation with thickness. Samples were fabricated from synthetic fused silica (Corning 7940)

and ranged in thickness from 0.4 to 1.27mm. The total absorptance of an uncoated substrate can be approximated by

A = 6d + A

1 + n A

2 where $ is the absorption coefficient (cm ^) of the material, d is the substrate thickness, A., and A

2 are the absorptances of the entrance and exit surfaces, and n is the

refractive index. The factor n results from the increased electric field strength at

the exit surface described by Crisp, Boling, and Dube [8].

The measured absorptance as a function of substrate thickness is shown in figure 6. -5 -1

These results indicate that the fused silica has 3 = 5 x 10 cm and a total surface

absorptance of 6 x 10 If the two surfaces have the same absorptance,

A

1 = A

2 = 2.5 x 10~ 6

.

5. Summary

A 1.06 micron laser calorimeter has been designed and built for measuring the absorptance of optical coatings. The sample consists of a thin 25.4mm diameter disc

whose central portion is irradiated by the laser beam. The temperature of the sample

disc is compared to that of an identical but unradiated disc by thermistors in physical

contact with both discs. The thermistors are electrically arranged in an AC Wheatstone

bridge. The precision of the calorimeter was determined over an absorptance range of

9 x 10 ^ to 3 . 8 x 10 ^ and found to be essentially constant at ~2 x 10 ^. The sensitivity was determined using a Si0

2 ~Cr-Si0

2 trilayer absorptance standard and found to

be 3.8 x 10 yV/watt for a 0.4mm thick substrate. The absorptance standard was also

used to show that the measured temperature rise is a linear function of absorbed power

I as predicted by thermal analysis. Measurements have been made on uncoated fused silica

to resolve the bulk and surfaces absorptances.

6. References

[1] Hordvik, A., Appl. Opt. 16,

2827 (1977).

[2] Moravec, T. J. and Bernal G. , E.

,

Appl. Opt. 17, 1938 (1978).

[3] Hass, M. , Optical Engineering

17, 525 (1978).

[4] Decker, D. L. and Temple, P. A.,

NBS Special Publication 509, 281

(1977)

[5] Bernal G. , E. , Appl. Opt. 14, 314

(1975) .

[6] Adams, J. A. and Roger, D. F.,

Computer-Aided Heat Transfer

Analysis , McGraw-Hill, 1973.

[7] Anthon, E. , Proceedings of the

Society of Photo-Optical Instrumen

tation Engineers, 140 (1978).

[8] Crisp, M. D., Boling, N. L. and

Dube, G., Appl. Phys. Lett. 2_1, 364 (1972).

35
OPT/CM AJ28AM5EMEA/TVCALOe/4A£T£G

SAMPLE SVPPO&T Fiqure 4. Measured siqnal as a function of absorbed pov/er used to determine linearity and sensitivity.

-THe/ZM/STO/Z. SEAQ

LAse/z eeA/v\ coppea. base

eAP/AT/o/v' s/JrELD

&/-AS5 S^«P«7-

Figure 2. Sample support showing the location of

thermistor heads and olass supports.

^ 300

+ + + +

- MEAN SIGNAL + + + + + + +

O STANDARD DEVIATION

TEMPEFATVeEASA FVMCT/OH

OF TEST TIME

NUMBEC OF M-EASUEEMBNTS

OOOOI

/// x

s— ,4mm

s— <S mm

^— 1 .Q mm

1 (4 MINUTE POINT)

I IO IOO TIME- (seconds)

Figure 5. Repeated measurements on BK7 used to determine precision.

ASAr/y*C7-/<WOr 7W/C*r*ESS

o.ao THICKNESS (mm)

Figure 3. Computed temperature as a function of test time for three different samnle thicknesses.

Figure 6. Measured absorntance of fused silica

showing the effects of bulk and surface abosrptio

36
MEASURED THIN FILM ABSORPTION AT THE AIR-FILM INTERFACE,

THE FILM BULK, AND THE FILM-SUBSTRATE INTERFACE*

P. A. Temple, D. L. Decker, T. M. Donovan, and J. W. Bethke

Michelson Laboratory, Physics Division

Naval Weapons Center, China Lake, CA 93555

A measurement technique is described where the absorption introduced by the addition of a single-layer thin film is separated into three parts: absorption

(a) within the bulk of the film; (b) at the air-film interface; and (c) at the film-substrate interface. The technique employs a scanning adiabatic calorimeter to measure substrates which are partially coated on the entrance surface

with films of continuously varying thickness. By measuring the absorption on the bare substrate and at various film thicknesses, we have unambiguously determined the film-bulk absorption coefficient. We have also measured the combined

absorption due to the two film interfaces. It will be shown that, in some

cases, it is possible to distinguish between air-film interface absorption and film-substrate interface absorption and determine the magnitude of each. Data will be presented for 2.9 and 2.7 um for films made of NaF and As2Se3 on CaF2> These films show the presence of impurities in the film and/or at the film

interfaces. The relative magnitude of absorption in the three regions will be

discussed.

Key words: Absorption coefficient; adiabatic calorimetry; As2Se3; interface

absorption; laser calorimetry; NaF; thin film absorption; water absorption;

wedged film.

Introduction

In this paper we report the measurement of interface absorptance and the bulk absorption coefficient

as a function of frequency of a material in thin film form. This was accomplished by adiabatic calorimeter measurements, using a tunable laser source on a film of varying thickness. This technique was

used to determine the presence of water in films as separate from water at film interfaces. The effects

of electric field modulation within thin films on the total loss due to film and interface absorptance

are also analyzed.

One commonly determines the absorption properties of a material in thin-film form [l]

1 by first

measuring the absorptance, A]_, of a suitable substrate, A film of known thickness is then deposited on

the substrate and the absorptance, A2 , is again measured. The increased absorptance is attributed to

j the film, and a film absorption coefficient, g = (A2~A^)/d, is calculated. It is generally recognized

j that the increased absorption is due not only to the bulk of the film, but also to the newly introduced

; film-substrate interface and the film-air interface.

In order to separate the interface contributions from the bulk contributions and to determine the bulk absorption coefficient of the film, at, we have used samples with a film of varying thickness

deposited on one face of the sample [2]. This has allowed calorimetric absorptance measurements to be made for various thickness films on the same substrate. As discussed below, this allows us to separate

! the absorption of a film into three parts: (a) that within the bulk of the film; (b) the absorption

[ at the air-film interface; and (c) the absorption at the film-substrate interface. This technique has been particularly useful in determining the presence of water in films as separate from water on films.

Experimental

The adiabatic calorimeter used to make these measurements has been described elsewhere [3,4]. In this instrument the sample is surrounded by a temperature-controlled box such that the surroundings are always at the same temperature as the sample. The sample and box are in a vacuum chamber. When the sample temperature is increased due to laser heating, the surrounding temperature is also increased.

This adiabatic nature allows us to use a large variety of laser sources since low irradiation laser

power is needed. The measurements reported here were made with a laser output power of less than 1/2

watt. The source was an HF chemical laser with a grating as the high reflector cavity element.

A second feature of this instrument is its ability to allow irradiation at various positions on

the sample. This is permitted since the absorptance is deduced from the total temperature rise of the sample and not the rate of temperature rise. The beam diameter used was ~ 0.5 mm as measured at the e_2 points, and any point on a specified diameter of the sample could be irradiated with spatial resolution

of 0.5 mm.

1.

Work supported by the Defense Advanced Research Projects Agency.

Figures in brackets indicate the literature references at the end of this. paper.

37
The sample configuration is shown in figure 1. A mask was used to produce the two edges. The wedge shape was produced by slowly obscuring the sample from the deposition source with a shutter. Thi was done so the film-substrate interface was produced at one instant in the deposition. The film thick ness profile was determined by scanning the film edge every 3 mm with a profilometer [5]. A significant advantage of using samples of this configuration is that one substrate and one deposition are

involved. Thus, while measurements of the type described here may be made with several depositions of various thicknesses on several substrates, we have eliminated a significant source of experimental

error by using one substrate and one deposition. A second advantage is the savings in time used to load and equilibrate one sample rather than several samples.

Calorimetric measurements were then made along the diameter which is indicated in figure 1. This

allowed absorptance measurements to be made on the uncoated substrate and at various film thicknesses.

Figure 2 shows data of a typical wedged film. This is a NaF film on a CaF2 substrate measured at 2.72 and 2.87 urn. The ordinate indicates the measured absorptance, A =

a^ so ^

ei^ power

absciss. incident power

indicates the film thickness at which the absorptance was measured. The measured zero thickness absorp

tance (bare substrate) is indicated by the two data points at zero thickness. The extrapolated zero thickness absorptance is indicated by the ordinate of the left end of the solid curves. The absorptanof

offset from the measured value is the absorptance due to the film-air interface and the substrate-film j interface. These two contributions are always present together in the substrate plus film measurements. We assume that, for the thicknesses measured, the two interface contributions are constant,

independent of film thickness.

Determination of ct^

The absorptance of a sample under normal incidence irradiation is given by the expression

2

« r L / \ l E ( x ) I , \ j A = J n(x) J—~—L a(x)-dx , o |e I 2 0

I E(x) I where n(x) J——~ is the time averaged relative power density p(x) at position x, and ct(x) is the

|e I

1 o1 absorption coefficient at x. |e

o

| is the time average of the incident electric field. We can then write the absorptance as

A = / L p(x)a(x)dx

o

For a single film at the incident face we have assumed that the absorptance at the interfaces can

be characterized by a specific absorptance a ^ = aa f^^a f> where the air-film interface is of thickness

AH a and has absorption coefficient a _. Similarly, the film-substrate interface is characterized by

af af

a r = ct_ Ait, . We also assumed that the bulk absorption coefficient of the film, a., is constant

fs fs fs r f throughout the thickness of the film. All the relative power densities were calculated from a thinfilm computer program [6]. Analytic expressions are given in reference [7]. In all cases we have

spoiled Fabry-Perot type interference in the substrate by slightly tilting the sample. This guaranteed

that the incident beam and the beam reflected back from the exit surface of the substrate did not intersect at the substrate entrance surface.

Under these conditions, the substrate contribution to the absorptance is very nearly given by

(T/T )A , where A is the uncoated substrate absorptance, T is the uncoated substrate first surface

o o o o transmittance , and T is the first surface transmittance with the film in place. This is an approximation which ignores the absorption of the back-reflected beam in the film. In the cases reported here,

the back-reflected beam is ~ 3% of the transmitted beam.

With the above approximations, the single-film plus substrate absorptance becomes

A = p .a , + p.a^-Cj. + p. a c + — A

af af r

f f f

r fs fs T o o See figure 3. p^ is the spatially averaged relative power density within the film.

In order to determine af, the bulk absorption coefficient within the film, we have measured the absorptance at film thicknesses which had the same relative power densities at the interfaces and

in the bulk of the film. In this case, the film absorption coefficient is given by

_ dA/d&
Two cases are represented in the data. These are for n

f = n

g and nf # n . The time-averaged (squared)

electric field pattern for these two cases is shown in figure 4. In case A, n

f — ns

> there is no reflection from the film-substrate interface. Interference from the back face of the substrate has been spoiled. Therefore, the relative power density is constant throughout the sample in the region

near the front surface. The data in figure 2 were taken in this way.

In case B, n

f ^r^, the constant relative power density conditions are met for integral half-wave

film thicknesses. The data for such a case are shown in figure 5, which is a film of As

2 Se

3 on CaF

2

. In this material the bulk absorption coefficient of the film is very small with nearly all the absorption being interface absorption.

Determination of a r and a„

af fs

Once a

f was determined, we measured the absorptance at 1/4A as well as 1/2A film thicknesses.

This changed the relative power densities at the various interfaces and within the bulk of the film. By writing the absorptance equations for these two cases (unprimed for A/2, primed for A/4), with all measured quantities on the right, we see that a and a c can be determined by these two measurements: af f s

T

a p + a p = A - p^Uj-l. - — A afaf fs r fs r f f f T o o

T' a ,p', + a, p' = A' - p'a,i' - — A

af af fs^fs *t f f T o

o

Only the case for n^ n

g is considered. The relative power density plots for this case is shown in figure 6. While it is possible to determine interface properties with an exit surface film, the simple

expressions above do not apply and will not be considered here.

Figure 7 shows the absorptance of an As2Se^ first surface film on CaF^ measured at film thicknesses

of A/4 and 3A/4, as well as A/2 and A, as shown in figure 5. An analysis as above indicates that all of the interface absorption takes place at the film-substrate interface.

Discussion of Data

Figure 2 shows the absorptance of a NaF film on CaF^ deposited in a U^-trapped oil-pumped system.

This is a front surface wedged film and was analyzed only for bulk absorption coefficient. The presence

of water in the film is evidenced by the higher absorption coefficient at 2.87 urn which is within the waterband than at 2.72 \m, just outside the waterband. Note that 1/2 \im of NaF on CaF^ results in equal

interface and film bulk absorption.

Figures 5 and 7 show the absorptance of an As2Se^ film deposited on CaF^. In this case, we have

determined the absorption coefficient of the film, a^, and the specific absorptances a^ and a^ at the

two interfaces. It can be seen that the presence of water in the film is minimal, with essentially all

the absorption taking place at the film-substrate interface. Such data are very helpful if one is attempting to reduce the absorption due to the presence of a film.

Concluding Remarks

We have presented a description of a method for determining the absorption coefficient of the bulk

of a film and the specific absorptance at the two film interfaces. In addition, we have shown representative data for a film which exhibits substantial bulk absorption within the film and a film which

exhibits almost exclusively interface absorption. While not discussed above, this method can be directly extended to investigate the absorption properties of film-film interfaces by depositing a wedged film on a previously deposited constant thickness film.

Finally, we would like to make the point that while much attention has been given to the electric

field plots for multilayer coatings in damage resistance discussions, it is the relative power density

which must be considered in discussing thermal absorption since the local absorptance is the product of the relative power density and the absorption coefficient at that point. The time-averaged electric

field squared and the relative power density in a three-layer AR stack are shown in figure 8.

The requirement that the tangential component of the electric field be continuous across a boundary forces the relative power density to be discontinuous across the boundaries. Once the interfacial

specific absorptances and the bulk absorption coefficients are determined, it is possible, from a plot

such as in figure 8, to determine the heat deposited within an AR or HR stack.

39
References

[1] See, for example, Harrington, J. A., Rudisill,

J. E. , and Braunstein, M. , "Thin-Film 2.8 um and 3.8 um Absorption in Single-Layer Film,"

Appl. Opt. 17, 2798 (1978).

[2] Burdick, D. L. , "Optical Constants of AS2S3

by a Wedged-Film Technique," in Laser Induced

Damage in Optical Materials: 1977, ed. by

A. J. Glass and A. H. Guenther. Washington,

D.C., National Bureau of Standards, 1977. (NBS Spec. Publ. 509.) P. 352.

[3] Decker, D. L. , and Temple, P. A., "The Design

and Operation of a Precise, High Sensitivity

Adiabatic Laser Calorimeter for Window and Mirror Material Evaluation," op. cit . , p. 281.

[4] Temple, P. A., and Decker, D. L. , to be published in Rev. Scl. Instrum.

[5] Elson, J. M. , and Bennett, J. M. , "Relation Between the Angular Dependence of Scattering

and the Statistical Properties of Optical

Surfaces," J. Opt. Soc. Am. (in press).

[6] Baumeister, P. W. , private communication.

[7] Temple, P. A., to be published.

Figures

Figure 1. Calorimeter sample configuration showing

a thin wedge of material which has been deposited on one face of the substrate. The bare region to the

left is used to measure A0 and the step in the foreground is used to determine the film thickness

prof i le.

Figure 3. The four absorbing regions of a front surface film. Shown is the incident beam of intensity Ig, the air-film interface, the film

bulk, the film-substrate interface, and the substrate whose absorption properties are characterized by a af , of, af s

, and

(T/T0 )A0 , respectively. The film is shown

separated from the substrate for clarity.

40

Figure 2. The absorptance of a NaF film on CaF£ measured at various film thicknesses. The two dat; points at 0.0 urn thickness are bare substrate

absorptances.

*, * *, « • wa. x. . .

.

OR. t - X/4. 3X/4, . . .

Figure 4. The relative power density pattern

(standing wave pattern) for a first surface film

used to determine a. The short bars indicate the film-substrate interface.
AC-130. S-50 V 2.87 i<m O 2.72 urn

050

THICKNESS, iim

a 0.4 cm 1

* n.. I! - X/4, \/2

Figure 5. The absorptance of an AS2S3 first

surface film on CaF2 measured at 2.72 and 2.87 ym wavelength and x/2 and X film thickness. The two

data points at 0.0 ym thickness are bare substrate

absorptances.

Figure 6. The relative power density pattern

(standing wave pattern) for a first surface film

used to determine a a f and af s. The short

bars indicate the film-substrate interface.

a = 0.4 cm

AC-130. S-50 a

a

, = 0 V 2.87 „m a

= 5 0 x 10-" O 2.72 iim

a = 0.0 cm

- '

a,, 3 -0.4 X 10-4

a,, 5 2.3 X 10~ 4

0.50 THICKNESS, (im

Figure 7. The absorptance of a first surface film

of As2Se3 on CaF2- This figure is identical

to figure 5 except that the absorptances at X/4 and

3x/4 are also shown along with the calculated

specific absorptances.

41
o - H . A

g |

Figure 8. The |E?| values and relative power densities for a three-layer AR coating. The |E2| values are continuous at the boundaries by virtue of

a boundary condition. This then requires that the relative power densities be discontinuous at all boundaries. The absorptance is the product of the relative power density and the absorption

coefficient. The relative power density plot is shown by the dashed line.

Discussion

Two suggestions were made to obtain more information from this procedure: one was to take measurements through the substrate , with the wedged film at the rear of the sample, to obtain an

additional measure of substrate surface absorption; and the other was to extend the film to "zero thickness ," rather than to end the wedge in a step discontinuity, to confirm the extrapolation to zero depth .

42
PHOTOACOUSTIC SPECTROSCOPY STUDIES OF THIN FILM COATINGS ON LASER WINDOWS

Nils C. Fernelius and David A. Walsh

University of Dayton Research Institute

300 College Park Avenue

Dayton, Ohio 45469

Photoacoustic spectroscopy (PAS) studies have been made of Till and graded PbF

2 -T2.I coatings on KCl. Wavelength dependent PAS

studies on the TO absorption edge indicate impurities in T£I in the

graded coating and/or the existence of cubic Til layers. PAS wave- length studies were made on a solid ZnSe window and various ZnSe

coatings on CaF2« Wavelength dependent studies show poorer stoichiometry as the coating gets thinner. Frequency dependent PAS studies

at 440 nm on the solid and thin film ZnSe all showed a signal dependence of S = Af . Theoretical calculations based on the RosencwaigGersho theory show that these results are consistent if (3 is greater

than 50 0 cm for ZnSe at 44 0 nm. Key words: Absorption edge; CaF2; KC£; optical absorption coefficient; optoacoustic spectroscopy; PbF2,* photoacoustic spectroscopy;

Til; ZnSe.

1. Introduction

Photoacoustic spectroscopy (PAS) is a recently developed technique for studying

solids [1-51. A periodically interrupted light beam impinges on a sample and heats it

through optical absorption and non-radiative decay. The periodically heated sample

heats the gas in contact with its surface creating a sound wave. If the gas volume is

enclosed with a sensitive microphone, the sound can be phase sensitively detected with

| a lock-in amplifier.

There are basically two types of experiments which can be performed. In the first

the wavelength is swept on a monochromator illuminated, for example, by a xenon lamp.

The variation in power output with wavelength is normalized by dividing the spectrum

of interest with that of carbon black. The second technique operates at a given wavelength and varies the chopping frequency. Elaborate theories have been developed to

calculate the magnitude and phase of the PAS signal generated [6-9]. According to

these theories in the optically transparent region, the amplitude of the PAS signal

from regions near the surface has a chopping frequency dependence of Af "*"; from bulk

-3/2 regions $f ' . At low chopping frequencies the bulk contribution should predominate

while at higher frequencies the surface contribution is larger. This agrees with the

physical intuition that slower chopping frequencies are needed to provide time for the

bulk contribution to diffuse to the surface. There is also a saturation effect. When

the 2 of the material is very high, light penetrating the sample is quickly attenuated

so only surface absorption contributes to the signal. Thus depending upon the 3 and f -1 -3/2 values encountered, a f dependence at low f may be seen tapering into a f at

higher frequencies.

A description of the equipment used has been given elsewhere [10] . Here we shall

present results from both types of experiments plus theoretical calculations for the

special cases studied experimentally.

2 . Experimental Results

2.1. TSl I and Related Coatings on KCl

One of the earliest samples studied by spectroscopy PAS in our laboratory was UDRI

Sample No. 1523 (Honeywell 806) which consisted of a 2.2 ym thick coating of Till on

KCl: 1.75% RbC£. A piece was cut to fit inside the PAS cavity and a wavelength sweep

was made. The results were measured with an Ithaco Model 391 lock-in amplifier. The

chopping frequency used was 740.4 Hz and the lock-in time constant was 4 s. The signal

amplitudes were measured and divided by the amplitude at the same wavelengths of a

1. Figures in brackets indicate the literature references at the end of this paper.

43
carbon black soot sample. Note in figure 1 that there is a rather sharp absorption

edge at around 435 nm and that the transparent region was reached by about 450 nm. Recently we received some samples from Honeywell consisting of Harshaw single

crystal KC£ coated with an alternate step graded index T£I-PbF2 coating on approximately half of the face of each sample. These samples were developed by Honeywell

under DARPA contract DAHC-15-73-C-0464 . More information on these samples is given

in [11] on pp. 54-65.

UDRI Sample No. 2722 (Honeywell No. 801262) had an AR minimum at 10.8 ym. PAS

wavelength dependent studies were made between 300-800 nm, which showed some interesting features of the absorption edge. Data was taken with a PARC 5204 Lock-in Analyzer

in the bandpass mode using an Ithaco 167 pre-amplif ier . The input was A, Float, and

xlO. The d.c. prefilter had a 1.1 s time constant. The output was xlOO with 1 s time constant. The lock-in phase on the coating was set at 400 nm and was 171.7°.

The sweep was on high blaze at 50 nm/min. The chopping frequency was 319.6 Hz. The

reference signal was obtained from 3M Nextel Velvet Black freshly sprayed on a 1/4 inch

thick quartz disc. The lock-in phase was set at 500 nm and was 157.1°. Otherwise the

experimental conditions were identical with that of the coating. The data was recorded

on a Hewlett-Packard Model 7046A X-Y^-Y2 Recorder and digitized on a Tektronix 4954

digitizing tablet. The normalized plot was displayed on a Tektronix 4014-1 graphics

terminal. It is shown in figure 2. Again an absorption edge occurred around 430 nm. The main difference from the previous results was the gradual falloff with wavelength

to the transparent region which was not reached until about 540 nm. Often a gradual

falloff is indicative of impurities in the material or a non-stoichiometric condition.

Another possible effect, which may be shown in the curve, is that some Til layers may

have been deposited in the cubic form. The absorption edge of the cubic form is

shifted toward the red end of the spectrum.

ESCA depth profiles of similarly prepared samples ( [11] , pp. 64-5) indicates that

there are alternate layers of PbF2 and T£I coating the KC£ substrate. Sputtering with

argon tends to pit a surface, thus the samples may have a more abrupt graduation than

shown in the figures. Yet there still were regions where the T£I had a considerable

amount of impurity.

Chopping frequency dependent studies were also made on this sample at 440 nm wavelength. The data between 50 and 400 Hz fell on a straight line in a log-log plot of

PAS signal versus chopping frequency. The data fit a signal dependence of S = Af n where n = 0.85. The thermal diffusion length in PAS is defined as L

fc ^

= /a/irf where

a is the thermal diffusivity and f is the chopping frequency. a = k/pC where k is the

thermal conductivity, p the density, and C the heat capacity of the sample. For TCI,

= 0.039/ f~E~ cm, using numbers from Ohmer [12] . At 50 Hz it is 55 urn and at 1 kHz

it is 12 ym. Thus a 2.5 ym film is a surface absorber at all frequencies studied.

For 6 - PbF2* L

fc ^

= 0.0425//T" cm, using numbers from Fernelius, et al. [13]. At

440 nm PbF2 is transparent so that the model is that of layers of absorbing material

interspersed with a thermally similar but inactive material. These nonabsorbing

regions contribute no heat generation but do have thermal resistance, thus they lower

the frequency dependence slope.

An earlier study on a T£I/KC£/T2,I sandwich on a KCl backing (UDRI 1745) gave a value of n = 0.97. The layer thicknesses were Till, 1.0 ym; KC£, 0.7 ym; and T£I,

0.5 ym. For KCl , L

th

= 0.090//F" cm, a somewhat larger value than T£I. This gave good

agreement with a surface absorption regime where n = 1.0. Perhaps the optically inactive regions acted as thermal shorts giving essentially the same results as solid

Til.

2.2. Study of Solid ZnSe and Thin Films on CaF

2 An uncoated ZnSe laser window (UDRI No. 1310) was mounted in PAS tip No. 1. The

spectra from the polished surface were noisier than those from a powder, but were

still obtainable. The lock-in phase was set at A = 450 nm. Spectra were run from

300-800 nm. All spectra were digitized on a Tektronix 4954 graphics terminal. Spectra from carbon black soot deposited on an A^O^ platelet were run at 111.6 Hz chopping frequency and digitized as the reference spectrum. The raw data spectrum was divided point-by-point to obtain a spectrum normalized for the variation in light

intensity with wavelength of the xenon lamp. Figure 3 and figure 4 show normalized

spectra for the solid ZnSe window (UDRI No. 1310) taken at chopping frequencies

316.9 Hz and 407 Hz. Note that there is a sharp absorption edge at 460 nm and that

the transparent regime is reached by 490 nm.

44
UDRI Sample No. 1797 consisted of a 21,300 A thick ZnSe film deposited on a CaF

2 substrate. A spectrum was run at 317 Hz chopping frequency. The normalized result is

shown in figure 5. The absorption edge is at about 470 nm. The absorption curve then

falls off more slowly than other solid samples, not reaching the transparent regime

until about 59 0 nm. UDRI Sample No. 1976 is a 2650 A thick ZnSe film on CaF

2

. Spectra were run at

318 Hz chopping frequency. The normalized result is shown in figure 6. Here the

absorption edge is blurred considerably and the transparent regime is reached by about

560 nm. The explanation for these results may be that the deposited thin films did

not have the correct stoichiometry or they may contain some argon gas in them.

The chopping frequency dependence studies in addition to studying various thicknesses of ZnSe also involved a comparison of different lock-in amplifiers and operating

modes. A typical plot is shown in figure 7. The results of various combinations of

experimental arrangements are given in table 1. Above 500 Hz coherent noise corrections were made following the prescription derived in the appendix. As can be seen

from inspection of the table, the solid ZnSe and the thin films all have essentially

the same chopping frequency dependence at 4 50 nm wavelength, viz., that of surface

absorption

Table 1. Chopping frequency studies.

Measurement

Samples Mode n Solid ZnSe P-B* 0.99

(UDRI 1310) P-FT 1.07

P-B 1.09, 1.09

P-F 1.12, 1.02

1-3** 1.02

21,300 A ZnSe on CaF ? P-B 0.99

(UDRI 1797) P-F 0.99

2650 A ZnSe on CaF

? P-B 1.06

(UDRI 1796) P-F 1.03

Note: All results except the first two use Ithaco

167 Pre-Amplif ier , the frequency range is 50Hz-lkHz,

signal = A/fn

, and X = 4 50 nm. *P-B = PARC 5204 Lock-in Crystal-Het Bandpass mode.

tP-F = PARC 5204 Flat mode.

**I-3 = Ithaco 393 Dynatrac 3. Rosencwaig and Gersho [6,7] give a very complicated complex expression for the

magnitude and phase angle of the PAS signal. A complex number computer program was written to calculate this expression and examples corresponding to the ZnSe samples

studied above were determined. Figure 8 shows a chopping frequency dependence plot

for the 1 cm thick sample. It shows that for 3 = 1000, there is a f dependence with

a 90° phase angle similar to surface absorption. Since the ZnSe signal in this region

-4 was about 10 , that of carbon black soot, it was hard to believe that g was so large.

This is probably an example of how much greater surface area soot has compared with

a polished flat sample. As 3 decreased, the slope got steeper until n = 1.5 was reached with a 45° phase angle. Results of calculations on the film coatings, figures

9 and 10, all showed an n = 1 throughout the B range indicating surface abosrption.

The results are summarized in table 2. Thus the apparent discrepancy between the

solid window and thin film results is reconciled for a 6 greater than 500.

3. Conclusion

Wavelength variation photoacoustic studies around the absorption edge of coating

materials can give information about the stoichiometry of the coating, its impurity

content, and crystalline phase. Chopping frequency dependent studies in the heavily

absorbing region seem to be consistent with the Rosencwaig-Gersho theory. Hopefully

the theory will also be valid in the transparent region where the ratio of bulk to

surface absorption will be attainable.

4 . Acknowledgements

One of us (NCF) wishes to thank the Air Force Office of Scientific Research for

financial support.

45
Table 2. Rosencwaig-Gersho Results for ZnSe.

Thickness Q Q = Af" n Q X 10" 6

h Backing B @1 Hz @1 Hz n @10 kHz (310 kHz

1 cm l a a A Au j y d 89 .7 1 . 00-1 . 06 2 3 5 69 . 7 100 0 376 86 .8 1.06-1.36 4 .41 49 .5 air 10 0 235 69 . 7 1.22-1.49 0 .476 45 .5

1 0 0441 49 . 5 1.467 0 .0479 45 .1 0.1 0 00476 c 0 . 0048 45 .0

21, 300A 1000 0 0948 89 .9 1.00 8 .57 84 .7 100 0 0104 89 .9 1.00 0 .940 84 .6 CaF

2 10 0 00105 89 .9 1.00 0 .0949 84 .6

1 0 0001052 89 .9 1.00 0 .0095 84 .6

0 2 6 5 OA T A A A 1UUU u AT OA 90 .0 1 A A 1 . U U I .278 89 . 3 100 0 00131 90 . 0 1.00 0 .129 89 . 3 CaF

2 10 0 000131 90 . 0 1.00 0 . 0129 89 . 3 1 0 0000131 90 . 0 1 . 00 0 .0013 89 . 3

5. Rei

[1] Rosencwaig, A. , Physics Today 28^(9) , 23 (September 1975) .

[2] Rosencwaig, A., Anal. Chem. 17(6)

,

592A (May 1975) .

[3] King, A. A. and Kirkbright, F. F.,

Laboratory Practice 25^ 377 (1976).

[4] Rosencwaig, A., Rev. Sci. Instrum.

48, 1133 (1977)

.

[5] Rosencwaig, A. Chapter 8 in "Optoacoustic Spectroscopy and Detection",

edited by Yoh Han Pao (Academic Press,

N. Y. , 1977) , pp. 193-239 .

[6] Rosencwaig, A. and Gersho, A., J. Appl. Optics £7(1), 64 (1976).

[7] Rosencwaig, A., J. Appl. Physics 49 , 2905 (1978)

.

[8] Bennett, H. S. and Forman, R. A.,

Appl. Optics 14, 3031 (1975); 15, 347,

1313, 2405 (1976); 16, 2834 (1977).

[9] Bennett, H. S. and Forman, R. A., J. Appl. Phys. 48, 1432 (1977).

[10] Fernelius, N., Appl. Spectrosc. 32 , Scheduled for Publication Nov.-

Dec. 1978.

[11] Bernal G. , F . , Anderson, R. H.,

Leung, K. M. , Moravec, T. J., Ready,

J. F., Skogman, R. A., and Wertman

,

D. , Honeywell, Inc., "A Study of

Polycrystalline Halides for High

Power Laser Applications", HR-78-

256: 2-59, (March 1978) . [12] Ohmer, M. , TS.I Coatings for KCi for

9.27 um, AFML-TR-77 , 157.

[13] Fernelius, N. C, Fox, J. A., Walsh,

D. A., and Coble, G. S., Proc . High

Power Laser Window Conference,

Denver, 1978.

46
6 . Appendix

Correction Due to Coherent Noise for Lock-In Signals

The input voltages to a lock-in amplifier consist of a reference signal to define

a phase angle zero and the signal of interest, which can be written as V^

n

= A exp(i(J>) + A

n expli^) ; where A is the amplitude of the signal and <j> is the phase angle with

respect to the reference signal. A^ and (j> are similar quantities for coherent noise.

The lock-in provides an internal phase shift, <j> , which can be applied to the incoming

signal before undergoing phase sensitive detection. Some lock-in's provide an inphase and quadrature output. The in-phase output is

I = A cos(d) - <J> ) + A cos((b - d> ) o

T r nT nT r and the quadrature output is

Q = A sin(d) - * ) + A sin(d) - d> ) . o

r r nT nT r When there is no coherent noise, A =0, and one varies <t> such that d> = <b . n r r This gives the maximum in-phase output. This is most readily done by varying $ so that QQ

= 0 , since it is easier to detect a precise zero crossing than to ascertain

the maximum of a cosine function.

When coherent noise is present, one still sets Q

q

= 0 and thus obtains a <f ro

» but

then corrections have to be made to obtain A. We have A sin(<j> - <b ro) - ~&

n

sin(<|> -

d>v ) and I = A cos (A - <t> ) + A cos (A - <b ) . Having done this, one can block off

ro o

r T ro nT nT ro 3 the light source, say with a shutter, i.e., A = 0, and measure I = A^ cos(<j> n

- <i> r

) and Q = A

n

sin(<J> - 4> ro

) • From these equations we have A sin(<J> - $ rQ ) = ~Q and

J = A cos(<() - <t> rQ ) + I- Squaring and adding, one obtains

r~2 2 A = /Q + (I - I) . Note that tan(d) - d> ) = Q/I . Once can also calculate the phase change due to con ro herent noise. When A = 0 , we have <f> = <b . n r

r T ro r

I = A cos6 + I oO = A sin6 + Q

tan 6 = -Q/(I " D

-I ~T~

Til COATED KCI LASER WINDOW

: 0.5

400 500 600 700 WAVELENGTH IN nm Figure 1. Till coated KCl laser window.

47

UDRI 2722^ Tl I - PbF

2 COATING ON KCI

1 1 1 38a <ee 503 600 703 S83

WAVELENGTH in nm Figure 2. T«.I-PbF2 coating on KCl.
X10 & e.aas

_i e.ns

^ e.ias

see eee

WAVELENGTH in nm

WAVELENGTH in nm

Figure 3. Photoacousti c signal vs. wavelength for Figure 4. Photoacoustic signal vs. wavelength for solid ZnSe laser window (UDRI 1310) f = 407 Hz. solid ZnSe laser window (UDRI 1310) f = 316.9 Hz.

~f".—

'

60O 700 200 360 400 500 600 700 300

WAVELENGTH in nm WAVELENGTH in nm

Figure 5. PAS signal fron 21,00 A thick

ZnSe on CaF2 substrate vs. wavelength (UDRI 1797)

f = 317 Hz.

Figure 6. PAS signal from 2650 A thick ZnSe

on CaF2 substrate vs. wavelength (UDRI 1796)

f = 318 Hz.

48
Figure 7. UDRI 1310 ZnSe window, Par 5204 lock-in, Figure 8. Rosencwai g-Gersho theory, 1 cm thick

crystal-het. bandpass mode, Ithaco 167 pre-amp., ZnSe air backing,

output X 100, input X 10, r =. 1 sec.

CHOPPING FREQUENCY IN Hz CHOPPING FREQUENCY IN Hi

Figure §. Rosencwai g-Gersho theory, Figure, 10. Rosencwai g-Gersho theory,

21,300 A, ZnSe on CaF 2. 2650 A, ZnSe on CaF 2

.

49
PIEZO-OPTIC COEFFICIENTS OF SOME NEODYMIUM DOPED LASER GLASSES AND SINGLE CRYSTALS OF CaF

2 , BaF

2 and SrF2

*

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

Ceramics, Glass and Solid State Science Division

Center for Materials Science

National Bureau of Standards

Washington, D. C. 20234

The piezo-birefringence coefficients of one neodymium doped phosphate

laser glass and three neodymium doped f luorophosphate laser glasses have been

measured at 0.6328 ym and 1.15 pm. All of the piezo-optic constants have been

obtained for two of the glasses. The piezo-optic coefficients, q,,, q,-, and q

11 12 of CaF

2

, BaF

2

, and SrF

2 have been measured at 0.6328 ym, 1.15 urn, and 3.39 urn. The coefficients were obtained by interferometric and polarimetric techniques.

Key words: BaF

2

; CaF

2

; laser glass; photoelasticity ; piezo-birefringence

;

piezo-optic constants; SrF

2

; stress-optical coefficient.

1. Introduction

Stress in an optical element will produce by way of the pi-ezo-optic effect a change in refractive

index that will distort the wavefront of an optical beam propagating through the element. In laser rods and discs stress can be produced by clamping or by thermal gradients produced by flashlamp pumping. In windows and lenses in high average-power laser systems, stress may result from thermal gradients induced

by absorption of laser radiation in the element. As part of a continuing program at the National Bureau of Standards to characterize the optical properties of materials, we have measured piezo-birefringence

coefficients of one neodymium-doped phosphate laser glass, designated Q-88, three neodymium-doped f luoro- phosphate glasses, designated LG-812, E-181, and LHG-10, and the alkaline earth fluoride crystalline

materials, CaF 2> BaF

2

, and SrF

2

. In addition the absolute piezo-optic constants of the crystalline

materials and glasses LH6-10 and Q-88 were also determined. The measurements on the glasses were per- formed at 0.6328 ym and 1.15 urn; the measurements on the alkaline earth fluorides were performed at 0.6328 Mm, 1.15 ym, and 3.39 ym. The glass specimens were obtained from the Lawrence Livermore Laboratory

and the alkaline-earth fluorides were obtained from a commercial source.

The piezo-optic constants have been treated in the literature [1]\ and, hence, will not be described

in detail here. The glasses, which are isotropic materials, have two independent piezo-optic constants q^ and q^ 2 [2], whereas the alkaline-earth fluorides, which belong to the cubic crystal class m3m, have

three piezo-optic constants q^i ^^2' an<^ ^44 t^'^]- ^ne va l ues °f q^j anc* q^ can '3e obtained from a measurement of the absolute change of refractive index with stress, and are found with interferometric

techniques. Values of q and (^^"l^-' can '3e ^oun<^ by measuring the stress-induced birefringence with

polarimetric techniques. In glasses, it is common to define another coefficient, C, called the stress- optical coefficient.

^ c = - \- (q ir q

i2 }

, (i)

where n is the zero stress refractive index,

o

2. Experimental Methods

All specimens were prepared in the form of rectangular parallelopideds with approximate dimensions

35 x 13 x 13 mm. One specimen of each glass material was prepared; two specimens of each crystalline

material were prepared, one specimen with the long dimension along the [001] crystal lographic direction

for obtaining q^j, q^ anc^ ^\\~^\2^ ' an<^ t '16 otner s P ec i men with the long dimension along the [111] crystal lographic direction, for obtaining q . The crystals were oriented by X-ray techniques and cut to

*This work was supported in part by the Defense Advanced Research Projects Agency and by the Air Force Office of Scientific Research, Air Force Systems Command, USAF, under Grant No. AFOSR-ISSA-78-0026, by

the Lawrence Livermore Laboratory of the Department of Energy, and by the National Bureau of Standards. The United State Government is authorized to reproduce and distribute reprints for govermental purposes

not withstanding any copyright notation herein.

1. Figures in brackets indicate the literature references at the end of this paper.

50
within one degree of the appropriate orientation. Two opposite rectangular faces of each prism were polished sufficiently flat and parallel so that about six localized Fizeau-type interference fringes

could be observed across the face when illuminated with collimated radiation from a He-Ne laser operating

at 0.6328 urn. The method for mounting and stressing the specimens has been described earlier [5].

The absolute coefficients have been measured principally by the method of Fizeau interferometry

This method requires a knowledge of the elastic compliance component, s^. in order to obtain q^ and q^_-

Values for can be obtained from the literature or from an independent measurement by the method of Twyman-Green interferometry. The use of Fizeau and Twyman-Green interferometry for obtaining q^, q^

and s=._ nas been discussed in the literature [6-9], and hence, will not be described here.

The coefficients (^n" 0^) I44 can be- determined directly by stress-birefringence measurements.

Light passes through a polarizer oriented to transmit radiation polarized at 45° with respect to the vertical axis. This polarized radiation propagates through the specimen, oriented with the stress axis

vertical, and is reflected back through the specimen and the polarizer. The output intensity will undergo a series of nulls as a function of applied stress and from these data one can calculate C, 1

_ ^ 1 2

or q . For the crystalline materials, (q-Q -0.-^-' is obtained from specimens stressed along the [001] direction and q is obtained from specimens stressed along the [111] direction. A more detailed analysis

of experimental techniques for measuring piezo-birefringence is discussed in the literature [9]

A problem arose in the measurements of q^ and q^ at 1.15 ym and 3.39 prn for the crystalline

materials. The use of the [001] prism was inadequate because of the small shift of interference fringes

with applied stress; instead the [111] prism was used. Measurements were made of the change of refractive

index for light polarized both parallel and perpendicular to the stress axis, q^ and q^ 9 were then eval- uated by solving simultaneously, the two equations:

An

l = "o

3 (q il

+ 2q !2+ 2 W J'

(2)

An

2 = "o

3

Cq ll

+ 2q 12 " q44 } V

(3)

where An^ and An^ are respectively the refractive index changes for light polarized vertically and horizontally and P is the applied stress.

3. Results and Discussion

In order to calculate the q.., n it is necessary to know n . Values of n for the glasses and crystals

ij ' ' o o

0 ' are presented in Table 1 together with the literature reference sources.

The results of the study of the neodymium doped glasses are presented in Table 2. It can be seen

that the phosphate glass, Q-88, has a value of C much larger than that of the three f luorophosphate

glasses. There is relatively little dispersion for the coefficients, but the values of C are consistently

higher at the shorter wavelength.

To evaluate the constants, q and q^, it is necessary to know the elastic compliance, s^. in order

to account for the change in thickness of the specimen when it is compressed. Of the four glasses, we have measured s 12 only for Q-88 and LHG-10 at present, and the q^ . are reported here for these two

glasses. There is little dispersion shown. From these values, it is possible to calculate the change of refractive index with applied hydrostatic pressure according to the equation,

§ = %_ (q ll

+ 2q l2 ]

> (4)

A and the change of refractive index with density, pj^-, may then be found by multiplying ^5- by the bulk

modulus. Because of uncertainty in the value for the bulk modulus, it is not possible to calculate

p dn/dp although its sign can be determined. In a poster session at the Symposium*, we indicated that

LHG-10 glass had a negative value for pdn/dp. This result was based on erroneous values for the elastic

constants, and, hence, is incorrect. Based on the value of s._ we have obtained from Fizeau and TwymanGreen interferometer measurements the values of q ^ and q^ have been changed and we find pdn/dp to be positive

*The Tenth Symposium on Materials for High Power Lasers (The Damage Symposium), National Bureau of Standards, Boulder, Colorado, September 12-14, 1978.

51
The data on the alkaline-earth fluorides are shown in Table 3. It can be seen that the errors are greatest at 3.39 pm. Within the experimental error, it appears that there is no significant wavelength

dependence in the coefficients.

All errors were calculated from the standard deviations of our data and the errors in the elastic constants obtained from the literature which were on the order of one percent [10,11]. However, there

is a question about the accuracy of some of the elastic constant data. For example, in preliminary mea- surements on SrF^, it was found that the coefficients and q^ obtained on the <001> specimen differed

from the values obtained on the < 1 1 1 > specimen. The discrepancy was resolved by making Twyman-Green

interferometer [6,8,9] measurements in addition to the Fizeau interferometer measurements. Both sets of measurements permitted us to calculate elastic compliance components, and these values differed from the values given in the literature. This suggests that additional Twyman-Green measurements on the CaF- and

BaF^ crystals should also be made.

In Table 4, our results at 0.6328 pm are compared with other determinations of q^. for the alkalineearth fluorides in the visible region. There is good agreement between the sets of data in some cases, but in several instances the disagreement is outside the experimental error. A possible cause for the discrepancies is that different investigators used different values of the elastic constants. In addition values of the elasto-optic constants p.. are frequently given in the literature, as was the case of

SrF.. In order to compare our values with the values in the literature, it was necessary to make the transformation

q.M

. = I p., s. . (5) ij 'ik kj v ' k

where k = 1 through 6. Any errors in the elastic constants would result in data disagreement.

4 . Summary

The piezo-birefringence coefficients , (q^-q^)and C, of one neodymium doped phosphate laser glass

and three neodymium doped f luorophosphate laser glasses have been measured at 0.6328 pm and 1.15 pm. The absolute piezo-optic constants q^ and

q ^ have also been obtained for two of the f luorophosphate glasses. The piezo-optic r f constants q, , , q,. and q„. of CaF.,, BaF., and SrF_ have been measured at 0.6328 pm, ~m< n

12 144 2' 2 2

' 1.15 pm and 3.39 pm. The coefficients were obtained by interferometric and polarimetric techniques. The results of the study bring out the importance of having reliable data on the elastic constants in order

to calculate the photoelastic constants from the optical measurements. It appears that it is not advisable to accept data on the elastic constants that are reported in the literature. The data on the photoelastic constants are presented in this report only tentatively, and it is planned for future work to make additional interferometric measurements that will permit the evaluation of both elastic and photoelastic

constants

.

5. References

[1] Nye, J. F., Physical Properties of Crystals

(Oxford University Press, London, 1957), pp. 243-

254.

[2] Vedam, K. , Proc . Ind. Acad. Sci., [A]

3J_, 450

(1950) .

[3] Rao, K. V., Narasimhamurty, T. S., J. Phys. Chem. Solids, 3_!> 876 (1970).

[4] Dickinson, S. K. , Infrared Laser Window

Materials Property Data for ZnSe , KC1, NaCl , CaF_

2

, SrF_

2

, BaF_2> (ARCRL-TR-75-0318) Physical

Sciences Research Papers, No. 635, Solid State Sciences Laboratory, Projects 5620, 3326, Air Force Cambridge Research Laboratories, L. G. Hanscom

Field, Bedford, Massachusetts 01730, June 6, 1975),

pp. 147-194.

[5] Feldman, A., McKean, W. J., Rev. Sci. Instrum.,

46, 1588 (1975).

[6] Feldman, A., Waxier, R. M. , Horowitz, D., Optical Properties of Highly Transparent Solids , Ed. by S. S. Mitra and B. Bendow (Plenum Publishing

Corp., New York, 1975), pp. 517-525.

[7] Waxier, R. M., Farabaugh, E. N. , J. Res. NBS, 74A , 215 (1970).

[8] Feldman, A., Electro-optical Systems Design,

8, 36 (1976).

[9] Feldman, A., Optical Eng. 17_, 453 (1978).

[10] Wong, C, Schuele, D. E . , J. Phys. Chem. Solids, 29, 1309 (1968)

.

[11] Gerlich, D., Phys. Rev., 136A , 1366 (1964).

52
Table 1. Refractive index data used in computing the piezo-optic coefficients.

Refractive Index, n

o

Material .6328 \im 1.15 urn 3.39 ym

Glasses

LG-812 1.4297 a 1.425 a E-181 1.4418

a 1.437a Q-88 1.5432 a 1.536

a LHG-10 1.458l

a 1.453 a

Crystals

CaF 1.433 1.427 1.416 SrF, 1.436*: 1 - 432 ^

J - 423

H BaF^ 1.473 1.468 d 1.459d

a Dr. Stanley Stok-owski, Lawrence Livermore Laboratory,

Livermore, California, Personal Communication.

b

I. H. Malitson, Appl. Opt. 2_, 1103 (1963).

c M. J. Dodge, National Bureau of Standards, Data to be published.

d

I. H. Malitson, J. Opt. Soc. Am., 54, 628 (1964).

Table 2. Piezo-optic coefficients and s of

-12 -1 neodymium doped laser glasses (10 Pa )

LG-812

(q n

-q

12 )

E-181

A = 0.6328 urn X = 1.15 \im

0.92 ±0.03 0.89 ±0.02

C 0.75 ±0.04 0.71 ±0.01

(q n

-q

12 )

Q-88

C 2.17 ±0.04 2.06 ±0.02

(q n

-q

12 ) -1.18 ±0.03 -1.14 ±0.02

q n

0.74 ±0.08 0.72 ±0.07

q 12 1.92 ±0.08 1.86 ±0.07

s = -4.27 ±0.08

LHG-10

C 0.63 +0.03 0.60 ±0.02

(q n

-q

12 ) -0.41 ±0.02 -0.39 ±0.02

q n

0.71 ±0.09 0.74 ±0.08

q 12 1.12 ±0.09 0.13 ±0.08

s,_ = -4.14 ±0.08

53
Table 3. Piezo-optic coefficients of alkaline-earth

fluorides (10~ 12 Pa"

1

)

A = 0.6378 urn A = 1.15 urn A = 3.39 um

CaF,

M ll q

i2

(q n

-q

12 ) q 44

SrF_

2 q ll

q

l2

(q n

-q

12 ) q 44 BaF

q ll

q i2

Cq n

-q

12 ) q44

-0 38 + 0 03 -0 •10 ±0 06 -0 52 ±0 1

1

1 08 ±0 03 1 09 ±0 06 1 00 ±0 11

-1 46 ±0 01 -1 49 ±0 02 -1 51 ±0 03

0 71 ±0 01 0 72 ±0 01 0 87 ±0 06

-0 64 ±0 04 -0 63 ±0 05 -d 83 ±0 09

1 4 5 ±0 04 1 50 ±0 06 i 23 ±0 07

-2 08 ±0 01 -2 13 ±0 04 -2 05 ±0 06

0 60 ±0 01 0 62 ±0 02 0 72 ±0 04

-0 99 ±0 03 -0 91 ±0 07 -0 75 ±0 07

2 07 ±0 04 2 13 ±0 07 2 11 ±0 05

-3 06 + 0 01 -3 03 ±0 02 -2 91 +0 08

0 95 ±0 01 0 95 ±0 01 0 99 ±0 07

Table 4. Comparison of data in the visible region on piezo-optic coefficients of alkaline-earth

fluorides (10" 12 Pa"

1

)

NBS Literature

CaF

1

1

(q n

-q

q 44

12

1

-0.38 1.08

-1.46 0.71

-0.29

1.16* -1.45 2 0.70

£

-0.41" 1.04? . -1.45° -1.48^ -1.44 6 0.89 0.81

SrF_

2q ll

q

l2

(q n

-q

12 ) q

44

-0.64 1.45

-2.08 0.60

-0.58

1.77

C -2.35 C 0.59

C

BaF.

M ll

(q n

-q

q44

12 J

-0.99 2.07

-3.06 0.95

-0.62^ 2.31*

-2.93b

, 1.06

a

F. Pockels, Lehrbuch der Kristallobtik (B.G. Teubner

,

Leipzig und Berlin, 1906). (X= 0.5893 um)

b

K. V. Rao, T. S. Narasimhamurty , J. Phys. Chem. Solids 3_1_, 876 (1970) . (X = 0.5893 Um)

C 0. V. Shakin, M. F. Bryzhina, V. V. Lemanov, Sov. Phys. Solid

State 13, 3141 (1972). (A= 0.6328 ym)

d

K. S. Iyengar, K. G. Bansigar, Current Science 27, 436 (1958).

(A= 0.5890 um)

6 A. J. Michael, J. Opt. Soc. Am. 58, 889 (1968). (A= 0.5461 ym)

f V. G. Krishna Murty, Ph.D. Thesis (Osmania University,

Hyderabad, India, 1964). (A = 0.5461 um)

54
REFRACTIVE INDEX OF STRONTIUM FLUORIDE*

Marilyn J. Dodge

Ceramics, Glass, and Solid State Science Division

National Bureau of Standards

Washington, D. C. 20234

The refractive index, relative to air, of fusion-cast strontium fluoride was determined from 0.2138um to 11. 475pm by means of the minimum-deviation method. Measurements were made on a precision spectrometer at a controlled room temperature near 20 °C. The experimental data were fitted to a Sellmeier-type dispersion

equation which permits refractive index interpolation within several parts in 10 -5

. The index of SrF2 is compared graphically with that of BaF2 and CaF2

. Key words: Alkaline earth fluorides; refractive index; strontium fluoride.

1. Introduction

The efficiency of a high-power laser system is dependent on the optical and mechanical stability of

the optical components within the system. Optical distortion caused by absorptive heating can render a component ineffectual at power levels below what would be required to cause fracture of the component.

Predictions of the optical distortion in laser system components depends on the knowledge of the refractive index, temperature coefficient of index, thermal expansion coefficient, and stress-optical constants

of the material under consideration. An optical materials characterization program [l] 1 is currently in progress at the National Bureau of Standards (NBS) to determine these pertinent properties.

The alkaline earth fluorides, BaF 2 , CaF 2 , and SrF 2 are excellent candidates for component materials

in the 2 to 6pm range because of their low index of refraction and dn/dT values as well as their good

mechanical stability. Both the index and dn/dT of CaF 2 and BaF2 have been well characterized. The dn/dT of SrF 2 has been determined [2,3], but reliable refractive index data over a wide wavelength

range is not readily available.

Dr. James Stanford of the Naval Weapons Center provided an optically finished prismatic sample of

SrF 2 for index measurements. The material was made by the technique of fusion casting by the Raytheon

Corp. 2 The prism had two polished faces 3.0 cm x 1.5 cm, and a refracting angle of approximately 65°.

2. Experimental Technique

The refractive index was determined relative to air by means of the minimum-deviation method on a precision spectrometer shown schematically in figure 1 [4] . Known emission wavelengths of mercury,

cadmium, helium, and zinc were used as sources for the measurements from 0.21 to 2pm. Beyond 2pm, a glo-bar was used for the radiant-energy source, and index determinations were made at known absorption

bands of water, carbon dioxide, polystyrene, 1,2-4 trichlorobenzene and methylcyclohexane . A series of narrow band interference filters was also used between 3.5 and 10.6pm. A photomultiplier tube was used

as the detector from the ultraviolet to 0.55pm in the visible. A thermocouple with a Csl window was

used for detection from 0.4 to 11.5pm. Several wavelengths in the visible were measured with both detectors in order to check the consistency in the system alignment. The scale of this spectrometer is considered accurate to within 1.0 second of arc. The refractive index of good optical quality material

can be determined to within several parts in 10" 5 over a wide wavelength range.

The refractive index of fusion cast SrF 2 was measured from 0.2138 to 11.475pm at a controlled room

temperature of 20 °C. Fifty-three experimental data points were fitted to a three-term Sellmeier-type

dispersion equation [5] of the form

n

2 -l = y[A.X 2 (X 2 -A. 2 )" 1

]

,

j

3

where n is the refractive index, A is the wavelength of interest, the ^j's are the calculated wavelength

of maximum absorption and the A

j

' s are the calculated oscil' iator strengths corresponding to the absorption wavelengths. The ^j's and A

j

1 s do not necessarily have any physical significance, but are chosen primarily to procure a mathematical fit of the measured data useful for interpolation.

* This work was supported in part by the Defense Advanced Research Projects Agency.

1. Figures in brackets indicate the literature references at the end of this paper.

2. The use of company and brand names in this paper are for identification purposes only and in no case

does it imply recommendation or endorsement by the National Bureau of Standards and it does not

imply that the material used in this study is necessarily the best available.

55
3. Index Data

The constants calculated for the dispersion equation are given in table 1.

Table 1. Constants for three-term Sellmeier dispersion equation to calculate

refractive index of SrF 2 at 20 °C.

Ax = 0.67805894 Xj = 0.05628989

A2 = 0.37140533 X 2 = 0.10801027

A 3 = 3.3485284 A 3 = 39.906666

These parameters refer specifically to one particular sample of fusion-case SrF 2 at 20 °C. Since the variation in index from one sample to another is unknown , care should be taken when applying these con- stants to another specimen of SrF 2

. This is especially true, if the wavelengths of interest are close

to the regions of strong infrared or ultraviolet absorption where the discrepancies in index from sample

to sample would be the greatest due to impurity absorption band differences. In any case, interpolations

using the parameters in table 1 should be limited to the range of experimental data or approximately

0.21 to 11.5um. The average absolute residual (the average absolute difference between the experimental

values and the calculated values) was 2.1 x 10 -5. This average residual is an indication of the overall accuracy of the experimental data.

The parameters in table 1 were used to calculate the refractive index at regular wavelength interval

Figure 2 shows a curve of the index of SrF 2 plotted as a function of wavelength. For comparison purposes

index curves of BaF2 [6] , CaF2 [7] , and SrF 2 are shown in figure 3. At 4ym the refractive index of SrF 2 is higher than the refractive index of CaF 2 by about 1 * 10~ 2 and lower than BaF 2 by approximately 4 x 10

4. Conclusions

The refractive index of fusion cast SrF 2 was measured over the wavelength range 0.21 to 11.5ym and

the data was fitted to a Sellmeier dispersion equation. Although the parameters for the Sellmeier

equation apply specifically to one specimen they can be used as a guide for calculating the index at any desired wavelength between 0.21pm and 11.5um of another sample of SrF2

. The refractive properties of strontium fluoride are similar to those of CaF 2

, with the index differing at most by 1 * 10~ 2 from the ultraviolet to about 5ym in the infrared. Because of its superior intrinsic hardness and low absorption [8] , SrF 2 is a very good candidate for high-power laser system components.

5. References

[1] Feldman, A., Malitson, I., Horowitz, D.

,

Waxier, R. M. , and Dodge, M. , Laser Induced

Damage in Optical Materials, 1974, NBS Special Pub. 414, 141, (1974).

[2] Feldman, A., Horowitz, D. , and Waxier, R.M.

,

Laser Induced Damage in Optical Materials:

1977, NBS Special Pub. 509, 74, (1977).

[3] Lipson, H. G., Isay, Y. F., Bendow, B., and

Ligor, P. A., Appl. Optics, 15_, 2352 (1976).

[4] Rodney, W. S. , and Spindler, R. J., J. Res. Nat'l. Bur. Stds. (US), 51_, 123, (1953).

[5] Sulton, L. E. and Stavroudis, 0. N. , J. Opt.

Soc. Am. 5^, 901 (1961)

.

[6] Malitson, I. H. , J. 0. S. A., 54_, 628 (1964).

[7] Dodge, M. J., Laser Induced Damage in Optics

Materials: 1976, NBS Special Pub. 462, 64,

(1976)

.

[S] Newberg, R. T. , Readey, D. W., Newborn, H. A. and Miles, P. A., Proceedings Fourth Annual

Conf . on Infrared Laser Window Materials,

p. 445, ARML (Jan. 1975)

.

56
DEVICE

Figure 1. Schematic diagram of the modified Gaertner precision spectrometer showing the optical path. The prism is rotated at one half the rotation rate of the telescope assembly by gear system, thus maintaining the condition of minimum-deviation for any wavelength. The scanning device

drives the assembly which scans the spectrum to identify lines or bands and determine their

approximate scale positions.

152 —

)—1—

50 0 WAVELENGTH ( M m)

Figure 2. Refractive index of fusion-cast SrF as a function of wavelength (logarithmic scale). Data

2 2 2 2-1

at 20°C were calculated from the dispersion equation of the form N - 1 = Z A .X (X -X . )

57
4

\

I%

J u

50.0 WAVELENGTH (urn)

Figure 3. Refractive index of BaF^, SrF^ an<^ ^a^2 aS a func *-i° n °f wavelength (logarithmic scale).

58
THE DEVELOPMENT OF FLUORIDES FOR HIGH POWER LASER OPTICS*

J. F. Ready, H. Vora, R. A. Skogman, K. M. Leung and E. Bernal G. Honeywell Corporate Material Sciences Center Bloomington, Minnesota 55420

The development of improved optical materials with low nonlinear refractive

index and good vacuum ultraviolet transparency is needed for future laser systems

to heat fusion targets. This program has produced lithium fluoride windows through

isostatic forging of single crystalline material. For reductions around 60%,

forging yields fine-grained material with considerably increased yield strength

and fracture energy. The ultraviolet transparency of the material is not degraded

by the forging.

This paper will describe the improvement of yield strength and fracture

energy as a function of forging parameters. It will discuss measurements of optical homogeneity, optical scattering and visible and ultraviolet transmission

for both the original material and the forged material.

Key Words: Fluroides, forging, laser windows, lithium fluoride, mechanical

properties, optical properties, ultraviolet transmission

Introduction

The development of improved optical materials with low nonlinear refractive index n2, and good

ultraviolet transparency would simplify the design of future laser systems to generate short intense

pulses at short wavelengths to heat targets for laser fusion. The program described in this paper

is intended to develop materials having both low nonlinear refractive index and good ultraviolet

transparency. In addition, the materials will have good mechanical stability, resistance to environmental degradation and optical uniformity.

The employment of lithium fluoride as a laser window material provides a material with excellent

optical transparency, broad spectral transmission, low refractive index so as to have high damage

threshold, and very low nonlinear refractive index. For the shorter wavelength lasers being developed,

ultraviolet transparency is an essential requirement. Lithium fluoride has excellent ultraviolet

transmission; in fact, it is the material with the largest energy band gap known. Fluorides in general

and in particular lithium fluoride, have very low nonlinear index of refraction, less than 0.5 x 10~^- esu [1] . This means that optical components of lithium fluoride will have relatively low self-focusing and beam breakup and therefore relatively high resistance to the damage associated with selffocusing. In addition, among the halide materials, lithium fluoride is relatively resistant to degradation by environmental water vapor. The combination of these properties makes lithium fluoride

an excellent candidate material for use as an optical material in high power, short pulse, short wavelength lasers.

Single crystal LiF, particularly single crystal LiF of the high purity compatible with good

ultraviolet transmission, has relatively low mechanical strength. Therefore it is not readily compatible with use as a structural element in laser systems. In our previous work on development of alkali

halides for use in high energy laser applications [2], [3], we have demonstrated the value of hot

forging for improving the mechanical properties of materials such as KC1, NaCl, KBr and CaF2- The hot

forging process produces fine-grained material with considerably increased fracture energy and yield

strength. At the same time, all the good optical properties that these materials possess inherently

are maintained.

In the program described in this paper we have been developing LiF for use specifically in high

intensity pulsed laser systems. We have placed emphasis on strengthening the LiF with high trans- parency by forging, so as to achieve a fine-grained poly crystalline material with much improved mechanical properties as compared to the single crystalline starting material. The optical homogeneity and

the transparency in the ultraviolet are preserved in the forging process.

The forging process begins with billets of single crystalline material which have been obtained

from three sources: Harshaw, Optovac, and the University of Utah. The material obtained from all these sources is of high purity, quoted to be in the parts per million regime for both monovalent

and divalent cations. Measurements of ionic conductivity, according to the method described by Stoebe

and Pratt [4] , indicate that the concentration of divalent cations (largely magnesium) is less than

*This work is supported by the U. S. Department of Energy under Contract No. ED-78-C-08-1555.

1. Figures in brackets indicate the literature references at the end of this paper.

59
one part per million for both the Harsh aw single crystal material and the University of Utah material

and is approximately 9 parts per million for the Optovac material. Attempts to measure the impurity

concentration of the single crystalline starting material by atomic absorption spectroscopy have been

unsuccessful because the impurity concentrations lie below the threshold for detection of cations in the presence of the lithium ion background.

The starting materials have also been tested for optical homogeneity and ultraviolet transmission. These measurements and their comparisons with the results for the similar properties in the forged

material will be described in the discussion of optical properties.

Forging

The isostatic forging process for LiF is identical to that described previously for CaF2[5]. The important feature of the process is the use of isostatic constraint by high pressure (2000 psi) helium

gas. This constraint prevents billet fracture and permits lower forging temperatures.

Figure 1 is a schematic representation of the forging chamber. The LiF billet rests between

graphite platens on top of the stationary lower ram. The upper ram transmits the forging pressure

applied by a hand-operated hydraulic press surrounding the forging chamber. Water cooled 0-rings seal

the rams against high pressure helium from a compressed gas cylinder. A valve near the inlet vents pressure build-up during the chamber heating. Deformation rates and percentages are monitored with a dial indicator mounted on the press. Forging temperature is monitored by thermocouples located directly in the forging chamber.

o All crystals to date have been forged along the LiF <100> axis at a strain rate of 5 x 10 /min

under 2000 psi isostatic helium constraint. The forging temperature has been varied from 300 to 600°C. Deformation reductions from 15 to 86% have been achieved. All forgings were cooled at approximately

50°C/hr.

Twenty-five single crystal LiF samples have been forged to date. Three of the forgings contained

one or two small cracks which occured sometime after the forging operation. All of the other forgings were crack-free.

Grain sizes range from approximitely 10pm for samples forged at 400°C and 74% reduction to approximately 40pm for samples forged at 500°C and 62% reduction. Figure 2 shows photomicrographs of the forged material, indicating grain sizes under these two sets of firging parameters. Large increases

in yield strength and fracture toughness were observed in all forged specimens when compared to the single crystal starting material.

The forged samples exhibit good stability against grain growth and recrystallization , even under conditions of high ambient relative humidity.

Optical Properties

Optical absorption measurements in the region 200-650 nm were made on single crystal and polycrystalline specimens using a Cary-14 spectrophotometer. The UV and visible transparency of LiF were

not adversely affected by the forging operation, as shown in figure 3. In this figure, curve (d) represents the spectrum of Harshaw LiF single crystal and curve (a) represents that of a forging made

using Harshaw material. Also shown in the figure are the spectrum of an Optovac LiF single crystal, curve (c) , and that of a forging made using University of Utah LiF starting material, curve (b). Ab- sorption bands in the region 200-400 nm seen in figure 3 (b) are believed to have been present in this starting material. Although the absorption in all LiF samples increases with decreasing wavelength

in the ultraviolet, the rate of increase is much greater in Optovac and University of Utah LiF than

in Harshaw LiF. Note that the losses due to reflection were taken into account in computing the ab- sorption coefficient (S) values shown in figure 3 for various LiF samples. Because of the high visible

transparency of the LiF samples, it was not possible to obtain quantitative information on their

relative transparency in the 400-650 nm region with the spectrophotometric technique used in our study

.

We also carried out measurements of the scattering produced by forged samples and the single

crystalline starting material in a helium-neon laser beam at 0.6328pm. The samples are immersed in index matching fluid (Cargille Laboratories) in order to minimize contributions from surface scattering.

The scattering from the fluid cell, which has single crystalline LiF windows, is measured with no sample in the cell. This background level is subtracted from the scattering observed with a sample

in the cell. This result gives the volume scattering produced by the presence of the sample. The scattered helium-neon laser radiation is detected by a photomultiplier tube which is rotated along a circular track with its center at the position of the sample. The smallest angle at which measurements

could be obtained was approximately 2° from the HeNe beam; at smaller angles the transmitted beam

struck the edge of the photomultiplier housing and yielded a spurious signal. The results indicate

that the forging process with its associated fine-grained structure does not increase the scattering

as compared to the original single crystalline starting material. For both the single crystalline

60
r

uaterial and the forged material, the fractional small angle scattering was <0.002/sterdian over the aneular range of 2° - 15°. Optical homogeneity was measured using the technqiue described by Bennett, et al. [6]. The measurements were carried out using coated quartz plates in contact with the sample surfaces to form a Fizeau interferometer. Index matching fluid was used in the contact regions. A 0.6328um HeNe laser

was used as the light source. The fringe pattern was detected with a TV camera and displayed on a TV

monitor screen. The interference patterns were photographed with a Polaroid camera.

Analysis of the interferograms was performed by digitizing the fringes and then converting the digitized interf erograms to contours of optical path difference through the use of the computer program

[ FRINGE, developed at the Optical Sciences Center, University of Arizona. This method of analyzing

| interf erograms has been described previously [ 7 ] . We characterize the inhomogeneity by the maximum

span of the optical path difference across the sample. The inhomogeneity An in index of refraction for samples of single crystalline material measured to date ranges from An = 4.4 x 10

-

^ to An = 8.8 x 10" , across samples about 2.5 cm in diameter. For samples of forged material measured to date, An ranges

from 17.4 x 10

-

^ to 22.3 x 10

_

6 . The results indicate that LiF samples can be forged without serious

degradation of the excellent optical homogeneity of the single crystalline starting material. Efforts

to correlate optical homogeneity with forging parameters are underway.

Mechanical Properties

The modified double-cantilever-beam technique of Freiman, et al. [8] was used to determine the

,j critical fracture energy of forged LiF. Specimens 2.5 cm long, 1 cm wide and 0.5 cm thick were cut from forging Ul (a University of Utah LiF crystal forged to 50% reduction at 500C) such that during

P testing, the crack propagated parallel to the top and bottom surfaces of the forging along a groove

j which was cut halfway through the thickness of the specimen to guide the crack. After polishing with

J 6pm diamond grit in oil, the specimens were chemically polished in f luoroboric acid and rinsed in meth-

. anol and ether. Sharp cracks were introduced in each specimen before testing by gently tapping with a blade. The tests were carried out at a crosshead speed of 0.025 cm/minute.

Yield strengths of single- crystal and forged LiF were determined by compression tests on specimens

cut to dimensions of 4 x 4 x 10 min. Single-crystal specimens were cut with surfaces parallel to

(100) cleavage planes. Samples were polished with 6um diamond paste. The single-crystal samples were

then annealed in a helium atmosphere at 700°C for 2 hours and furnace cooled and the forged samples

' were chemically polished as described above to remove any residual stresses that may have been introduced during mechanical polishing.

The results of compression and fracture - energy tests on samples from forging Ul are given in table

1, where they are compared with single-crystal data of Ahlquist [9], Compression tests on University

of Utah LiF single crystals yielded a value for yield stress of 1.9 MN/m2 at 0.2% offset. This value

is lower than that reported by Ahlquist, apparently because of the higher purity of this starting

material ..

The data in table 1 indicate that the yield strength and critical fracture energy of single-crystal

: LiF can be improved by at least an order of magnitude by the forging operation.

Reference

[8] This work

This work

I

Table 1. Mechanical Properties of Lithium Fluoride

Fracture Energy

(MN/m2

)

: Yield Stress , -, , 0\ „2^* Y„ <W>

Single - crystal 2.4 0.64

Single - crystal 1.9 Polycrystalline 36.0 6.75 (forging Ul, ^40um

subgrain size)

*0.2% offset

61
Summary

Isostatic forging has produced stable fine-grained poly crystalline material with both yield

strength and fracture energy increased by at least an order of magnitude as compared to the single

crystalline starting material. The optical properties of the single crystalline LiF, including high

transmission in the visible and ultraviolet, good homogeneity and low scattering, are not degraded by the forging operation.

Acknowledgements

The authors are happy to acknowledge the able technial assistance of Dave Woodward in this work. We also thank Dr. Thomas G. Stoebe of the Dept. of Metallurgical Engineering, University of Washington,

for the measurements of ionic conductivity.

References

[1] D. Milam, M. J. Weber, and A. J. Glass,

Appl. Phys. Lett 31, 822 (1977).

[2] E. Bernal G . , et al .

, A Study of Polycrystalline Halides for High Power Laser Applications,

Final Technical Report on Contract DAHC-15-

73-C-0464 (May, 1978).

[3] R. H. Anderson and E. Bernal G., Proceedings

of the High Power Laser Optical Components

and Component Materials Meeting, 3-4 October,

1977, Boulder, Colorado., J. S. Harris and

C. L. Strecker, eds . P. 429.

[4] T. G. Stoebe and P. L. Pratt, Proc. Brit.

Ceram. Soc. 9, 181 (]967)

.

[5] R. H. Anderson, B. G. Koepke and E. Bernal G.

,

in Laser Induced Damage in Optical Materials:

1976, A. J. Glass and A. H. Guenther, eds., NBS Special Publication #462.

[6] H. E. Bennett, J. W. Bethke, and W. P. Norris

,

in Proceedings of the High Power Laser Optical

Components and Component Materials meeting,

3-4 October, 1977, Boulder, Colorado, J. S. Harris and C. L. Strecker, eds. P. 328.

[7] E. Bernal G., K. M. Leung, and J. S. Loomis,

Interferometric Testing and Analysis of Windows

Exposed to CO2 Laser Irradiation, Final Technical Report for Navy Contract N605-30-77-W6911, 20 May ]977.

[8] S. W.Freiman, D. R. Mulville, and P. W. Mast,

J. Mater. Sci. 8, 1527 (1973).

[9] C. N. Ahlquist, Acta Met. 22, 1133 (1974).

62
0" RING SEAL

Figure 1. Schematic diagram of isostatic forging apparatus.

C3
62% Reduction

500°C

100/xm

i i

Figure 2 Photomicrographs of fine-grained LiF, forged under the indicated conditions.

0 2

t O.I z

0 = 4.1 x 10

200 300 400 500 600 WAVELENGTH ( nm

)

Figure 3 Optical absorption spectra of LiF (a) Forged Harshaw material, thickness t = 3.5 cm;

(b) Forged University of Utah material, t = 3.6 cm; (c) Single crystalline Optovac material,

t = 2.7 cm; (d) Single crystalline Harshaw material, t = 2.6 cm. Values of absorption

coefficient S in cm

- -'- are indicated.

64
OPTICAL PROPERTIES OF KC1 FORGED BETWEEN OPTICALLY POLISHED DIES*

R. H. Anderson

Honeywell

Corporate Material Sciences Center Bloomington, MN. 55420

and

Jean M. Bennett Michelson Laboratory, Physics Division

Naval Weapons Center, China Lake, CA 93555

Plano-plano and plano-concave KC1 optcial elements have been isostatically

forged using a two-step forging process in a closed die. No optical polishing

was required for the finished surfaces. The initial forging was performed between

teflon sheets, the sample was then water polished, and the final forging was done

between optically figured fused quartz or pyrex dies. Forgings were typically done

in the 200 to 275°C temperature range in a helium atmosphere at 29.65 MN/m^ (4000

psi) . Measurements on the finished samples include optical figure, homogeneity,

internal strain, total integrated forward and backscattering, surface texture

(Nomarski microscopy) , and surface roughness . The results of these measurements

are compared with similar measurements on optically polished single-crystal and press-forged KC1 samples.

Key Words: Alkali halides, backward scattering; forward scattering; grain

boundaries; isostatic forging; optical homogeneity; optical figure; single

crystal; Talystep traces.

Introduction

The unique combination of physical and optical properties possessed by the alkali halides make

them potential candidates for infrared windows and many other optical elements. Their broad spectral

transmission from the ultra-violet out to 12pm and beyond also contribures to their consideration for very broad band use. One of the physical properties which is hindering the potential development of the alkali halides is their extremely low hardness.

This low hardness contributes to the difficulties which are encountered when conventional optical

grinding and polishing techniques are used to produce a desired optical figure and geometric shape.

This paper discusses results obtained from a forging process which may be an alternative method for providing the desired optical figure and geometric shape. This technique uses die surfaces which have

been optically figured from quartz, pyrex, or metal. The die surface shape and figure is replicated

on the halide by forging with these die surfaces. This forging approach should be compatible with

spheric as well as aspheric surfaces. The ease with which diamond turning can provide radially

symetrical surfaces makes the eventual use of metal dies especially attractive.

The forging process is described and the result of varying a number of the forging parameters on surface roughness, optical figure, scattering and optical homogeneity are discussed.

The Forging Process

To provide mechanical strengthening and eliminate welding of the halide forging to the quartz,

pyrex, or metal die surface, the single crystal starting material is forged at a temperature at which the crystal is 'normally brittle. To prevent fracture during forging and enhance lubrication

lj between the halide forging and the die surface the forging operation takes place in a high pressure

I gas environment. The gas provides a constraining force which prevents fracture during deformation,

f This forging method has been named isostatic forging El J"*".

Presently the forging is done in helium at 29.65 MN/m^ at a temperature from 200° to 250°C. All

|| crystals are now <100> orientation. Quartz, pyrex, and diamond-turned electroless-nickel metal die surfaces have been used for replication. The closed die assembly as used is shown in place in figure 1 Basically, it will be noted, that this is a closed die forging operation done in a heated variable

volume, high pressure chamber.

li *The forging work was partially supported by Defense Advanced Research Projects Agency under Contract

No. DAHC-15-73-C-0 464 . The optical evaluation was supported by Naval Weapons Center Independent

Research Funds.

1. Figures in brackets indicate the literature references at the end of this paper.

65
Ideally, it would be desireable to forge the halide single crystal to its final shape in a single

processing step. The single crystal cylinder is preshaped with water to produce a dome on each end. This shape ensures that the forging operation originates at the center of the die surface and progresses radially outward thereby eliminating the entrapment of gas between the forging and the die surface.

An alternative forging process has also been explored. This is called the two step process.

The single crystal cylinder is initially deformed between teflon sheets to at least 60%. This step

provides a uniform strain distribution throughout the bulk of the forging while minimizing the development of internal stresses. The forging is then preshaped as in the one step process and re-forged

in the same way.

Surface and Optical Evaluation

The surface and optical evaluation of four forgings are discussed in this section and compared with

a press forged window that was optically figured using the conventional method and also a single

crystal window that was figured using the same method and then chemically polished. Chemical

polishing of polycrys talline window surfaces will preferrentially attack the grain boundaries,

consequently, is only useable on single crystal materials.

Figure 2 illustrates the optical figures and homogeneity of a one step forging on the left and

the press forged mechanically polished window on the right. The optical figure of the one step forging

is obviously related to the original crystal orientation and would be difficult to correct by altering

the die surfaces. The homogeneity fringes below indicate high internal stress and birefringence. The

window on the right is about A/4 flat and his low internal stress. This window was press forged and mechanically polished.

A plano-concave sample, shown in figure 3 illustrates the resultant optical figure and homogeneity

using the two step process. The piano surface is actually convex but the nearly circular fringes

suggest the possibility of improving the figure by making the die surface comparably convex. The fringes show the internal stress to be low. The curvature of the fringes is due to an index mismatch.

The method used to determine the homogeneity results utilized a Zygo interferometer and high reflectance quartz flats and index matching fluid [2].

The surface finish produced by the one step forging varies with location on the piano surface. The area on the piano surface represented by the initial single crystal diameter is much smoother

than the additional area produced during forging. Figure 4 shows Nomarski photographs illustrating

this difference. The Talystep traces vividly emphasize this surface roughness variation.

The piano surface of a plano-concave element produced by the two step process is shown in figure 5. The Taly step traces show a marked reduction in roughness compared to the previous figure. The substitution of a 1 pm radius stylus indicates the surface contains many small, narrow cracks.

The major excursions of the trace may be due to grain boundary etching resulting from the preparation

for the second forging step.

To determine if these grain boundaries were contributing to the surface roughness and scattering,

two plano-plano samples were examined. The element identified as #534 in figure 6 was forged without

any attempt to remove the grain boundaries. The other specimen #537 was rubbed gently with paper sat- urated with methyl alcohol to eliminate the grain boundaries.

One of the die surfaces used to produce both #534 and #537 had a pair of shallow fiduciary marks

ground into the forging surface. All of the scans shown in figure 6 were taken between these marks

to compare die roughness and the forged halide surface roughness. The trace shown in the lower center

is from one of these areas of the die.

Scattering measurements in forward and backward directions were made on all the forgings, as well as, the press forged and the single crystal specimen. These results are summarized in tabular form in figure 7. The identification of each sample is as follows: Sample History

Honeywell Window (1) One Step Forging

Honeywell Window (2) Two step forging

Grain Boundaries Reduced AFWL Polished Press Forged

Mechanically Polished

E-3 Etched Single Crystal

Mechanically Polished

Etched

66
Sample

Honeywell #534 Two Step Forging

Grain Boundaries Evident

History

Honeywell #537 Two Step Forging

Grain Boundaries Reduced

It is apparent that in almost all cases the forged materials exhibit lower scattering values.

The results in the forward direction were taken from several hundred points and the fiduciary marks

do contribute to these scattering results of both windows #534 and it 537

Figure 8 illustrates in tabular form rearrangement of the data shown in figure 7. All values

less than 1.0 show the Honeywell forged elements to be superior to the element identified in the denominator of the fraction.

It is apparent that the two step forging process produces a superior element from the standpoint

of surface figure, homogeneity, surface roughness, and scattering.

The preparation of the water polished surface prior to the second forging step to reduce or eliminate the grain boundaries also improves the forged element properties. The surface is smoother,

consequently the scattering values are lower.

The comparison of the die surface roughness to the halide forged surface does not replicate the

die surface exactly. However, the forged surface does compare favorably with the "state of the art" mechanically polished samples and the scattering values are lower than mechanically polished or etched

samples.

These forging results are encouraging and efforts are now directed to demonstrate the forging

of halide optical elements with satisfactory optical figures.

The authors would like to acknowledge the excellent technical assistance provided by Messrs.

W. J. Polzin and R. A. Skogman of this laboratory and Messrs. P. C. Archibald and J. W. Bethke

of the Michelson Laboratory, Naval Weapons Center, China Lake.

Conclusions

Acknowledgemen ts

References

[1] R. H. Anderson and J. M. Bennett, Proceedings

of the High Power Laser Optical Components

and Component Material Meeting, 3-4 October, 1977, Boulder Co. J. B. Harris

and C. L Strecker, eds. (Defense Advanced

Research Project Agency, Arlington, VA, 1977)

[2] H. E. Bennett, J. W. Bethke, and W. P. Norris

ibid. p327-338.

67
PRESS PLATEN

Figure 1. Sectional schematic of the isostatic Figure 2. Optical figure and homogeneity of a KC1 forging system for producing finished halide sample prepared by a one-step forging process

optical elements. (left) and a conventionally polished KC1 forging

(right)

.

HONEYWELL PRESSED KCI LENS

Figure 3. Optical figure and homogeneity of a sample prepared by a two-step forging process.

OPTICAL FIGURE HOMOGENEITY

(PLANO SIDE)

SURFACE FINISH ON HONEYWELL FLAT PRESSED KCI WINDOW

CENTER EDGE

Figure 4. Surface finish of center and edge areas

of a one-step forging (Momarski) and corresponding

Talystep traces of surface roughness.

1000X

126A RMS*

•TALYSTEP. 8pm RADIUS STYLUS,

610«m SCAN LENGTH

••TOTAL INTEGRATED SCATTERING

50/jm 273A

1 RMS*

288A

1 RMS*

68
Figure 5. Surface finish of a typical area of a two-step forging (Normarski ) and resultant Talystep

traces.

Fiqure 6. Surface finish of an area between the fiduciary marks (f'ornaski) with (right) and without

(left) surface treatment and Talystep traces of the

same area of both forgings and the quartz die.

0. 6471 Urn 3.39/im I0.6um 0.647lym 3. 39,um 10.6/i BACKSCATTER BACKSCATTER

Honeywell Window Honeywell (IWEtched 0.25 0.64 0. 18

Flat Pressed (II 7.11 x I0"4

5.20 x I0"5 1.80 x IO"

5

Honeywell Lens

4.05 x IO"3

* Honeywell (D/AFWL 0.07 0.21 0.32

Flat Pressed t2) 8. 18 x I0"5 3.33 x IO"5

AFWL 6" Diam. Honeywell (2>/Etched 1.43 1.00 0.32

Polished 9.98 x IO"3 2.49 x I0"4

5.58 x IO"

5

E-3 Etched 2.82 x IO"3 8.13 x IO"5 1.02 x IO"4 Honeywell (21/AFWL 0.41 0.33 0.60

•Wavelength 0.5682i/m FORWARD SCATTER FORWARD SCATTER

Honeywell Window

Flat Pressed (1) 4.21 x IO"3 2.19 x IO"4 6.45 x IO"5 Honeywell (l)/Etched 0.78 0.51 0.22

AFWL 6" Diam. Honeywell (l)'AFWL 0. 14 0.37 1.00

Polished 2.96 x I0"2

5.90 x IO"4 6.46 x IO"5

E-3 Etched 5.42 x IO"3 4.33 X IO"4 2.95 x IO"4 Honeywell (3)/Etched 1.09 0.54 0.47

Honeywell Window J534 5.91 x I0"3 2.33 x IO"4 1.38 x IO*4 Honeywell (31/AFWL 0.20 0.40 2.14

Flat Pressed 13) Honeywell Window #537

Flat Pressed 141

3.39 x IO"3 2.03 x IO"4

4.97 x

10-5 Honeywell (4)/Etched

Honeywell (41/AFWL

0.63

O.ll

0.47

0.40

0.17

0.77

Figure 7. Total integrated scattering results from

several KC1 samples.

Figure 8. A summary of the scattering results comparing isostatical ly forged samples to conventionally prepared samples.

69
BULK OPTICAL PROPERTIES OF FINE GRAINED FORGED CALCIUM FLUORIDE*

R. H. Anderson, R. A. Skogman, and J. F. Ready

Honeywell Corporate Material Sciences Center Bloomington. MN. 55420

and

Jean M. Bennett Michelson Laboratory, Physics Division

Naval Weapons Center, China Lake, CA 93555

Single crystals of CaF, having <111> . <100> . <112>. and <113> orientations

have been routinelv isostaticallv foreed in a helium atmosDhere at 750°C. The

crvstals were deformed 60 percent to ensure a uniform grain size of less than

15pm. A fine cloudiness of "veiline" occurs when the forging is done below the recrystallization temperature of about 800 °C for CaF2- Nomarski micrographs

show that voids are distributed within the bulk of the f oreines . PolariscoDe

photographs show internal stress. Total integrated forward scattering measurements

have been made on the sample immersed in a cell containing index matching fluid

to eliminate scattering from the sample surfaces. These measurements show variations cannot be definitely correlated with the positions of veils in the

CaF

2

. Key Words: Calcium fluoride, forging, laser windows, optical properties, scattering.

Introduction

Calcium fluoride is a prime candidate material for use as a laser window in the chemical laser band from 2.8 to 5.3pm. Its combination of physical and optical properties are most attractive.

Unfortunately, its combination of high Young's Modulus, large coefficient of expansion, and extremely

low thermal conductivity contribute to its sensitivity to thermal and mechanical shock when CaF9 is used as single crystal windows.

The strengthening methods that were successfully used with the alkali halides have been applied

to CaF2. [I]"*"- Forged CaF2 with a grain size less than 15 m can be routinely produced. Forged

CaF2 has shown an increase in the critical fracture energy in excess of five times that of single

crystals. After the forging operation has been completed, examination of the forging reveals cloudy

regions within the bulk of the forging, which is called "veiling".

Efforts to eliminate the veiling have been largely unsuccessful. Several single crystal orien- : tations were forged in an attempt to eliminate the veiling. Many of the forging parameters were varied1 such as strain rate, reduction, forging temperature and forging sequence. An alloy crystal of CaF2 with 90 ppm Eu was also forged. All these attempts to eliminate veiling had negative results.

Early efforts to isolate the source of the veiling by optical microscopy, scanning electron

microscopy or Auger analysis were unsuccessful. Impurity segregation, which had been observed in the past by Stockbarger [2] in material of much lower purity, was discounted because of the high

purity of currently available crystals. Vacancies due to dislocation interaction during deformation

appeared the most likely source. Efforts to detect voids by any method were unsuccessful until the development of techniques described later in this paper.

This paper discusses the forging system now used, presents recent birefringence photographs for

< 111 > and < 100> orientations, discusses scattering results obtained using a special cell, presents

scattering maps of the samples tested in the cell at 0.6471 and 3.39pm, and also discusses the angular j and wavelength dependence of the scattering. Some comments are also made about the origin of the veiling

The Isostatic Forging System

To minimize the development of internal stress within the CaF2 forging, it is vital to maintain

isothermal conditions in the region where the forging occurs. Figure 1 illustrates the present iso- static forging system used for forging fine grained CaF2 and designed to provide isothermal forging

*The forging work was partially supported by Defense Advanced Research Projects Agency under Contract

No. DAHC-15-73-C-0464. The optical evaluation was supported by Naval Weapons Center Independent

Research Funds and Honeywell Material Sciences research funds.

1. Figures in brackets indicate the literature references at the end of this paper.

70
Table 2 Data from Scattering Measurements

6471A 3.39pm

Test Cell Test Cell and <111> Crystal

Test Cell Test Cell and

<100> Crystal

5.192 + 0.583 x 10"

1.146 + 0. 652 x 10"

5.74 + 1.87 x 10~ 3

2.55 + 1.06 x 10~ 2

2.32 + 1.79 x 10

2.299 + 0.848 x 10 2.70 +0.51 x 10" 5

2.72 + 0.61 x 10" 4

-4

The scattering levels for the sample, cell, and index matching fluid were also measured as a function of position on the sample. The scattering from the cell without the <111> sample was approximately equal to the scattering from the sample in the visible. The scattering from the cell without the <100> sample was approximately 1/3 of the scattering from the sample in the visible. In

the infrared the <111> sample was two orders of magnitude lower and the <100> sample was one order of magnitude lower. On the average the bulk scattering from each sample is:

6.27 + 6.52 x 10~ 3 at 6471& bulk scattering <111> 2.30 + 0.85 x 10

-4

at 3.39 m bulk scattering <111>

19.76 + 10.6 x 10~ 3 at 6471& bulk scattering <100> 2.45 + 0.61 x 10~ 4 at 3.39 m bulk scattering <100>

From these results the bulk scattering of the <111> sample is nearly 30 times larger in the visible

and the <100> is nearly 80 times larger in.the visible, than in the infrared.

If we assume that the entire scattering levels of the cell plus index matching fluid are caused

by the two windows (2 surfaces plus bulk scattering for two windows) then we can compare bulk and surface scattering from single crystal CaF2 with bulk scattering from isos tatically forged <111> CaF2.

For the single crystal CaF2, the scattering in the visible is over 2000 times larger than in the infrared. The scattering levels for the two types of CaF2 are comparable in the visible (the press forged

sample scattering slightly more), but the press forged sample scatters about 100 times more than the single crystal windows at 3.39 m. It should be emphasized that the comparisons are not strictly valid

because the total thickness of the single crystal windows is only about two-thirds that of the pressforged sample. Also, there is scattering from 2 of the 4 surfaces of the single crystal windows and

this may be much larger than the bulk scattering in the material.

The spatial distribution of the scattering from the sample was plotted as a contour map with

interpolations made from the measured data. Actual data points were taken on concentric circles

whose radii differed by 0.05 inch, out to a maximum radius of 0.800 inch (1.6 inch diameter). On

each circle data points were spaced 0.05 inch apart. Thus there were a total of 817 points for each

case. Averages of these blocks of 817 data points are the values given above.

Figure 5 and figure 6 show the contour plots as a function of orientation and wavelength. All attempts to compare the veiled pattern of the forgings in the visible with the contour maps were fruitless. It is very difficult to compare the measured total integrated scattering from the sample with

the position of the veils in the sample. The veils can only be seen when the sample is viewed

obliquely, and tend to appear differently as the sample orientation is changed.

In addition measurements were performed on the angular dependence of the scattering. Measurements

were performed at four separate wavelengths: 0.4416pm, 0.6328pm, 2.8pm and 3.8pm. The measurements

were performed by directing a laser beam through a small area of the sample and observing the scattered

light with a photodetector . The photodetector was moved along the circumference of a circle with its center at the point where the laser beam emerged from the sample. The relative signal due to scattered

light was measured as a function of the angle between the incident laser beam and the line from the sample to the detector. Care was taken that only light scattered by the sample could reach the detector. The detectors were a S-l photomultiplier for the two shorter wavelengths and a PbTe detector

for the two longer wavelengths. We ensured that the detectors were operating in the linear portion of their response by inserting filters of known attenuation in front of the detector and observing the decrease in signal. In contrast to the measurements of total integrated scattering described above,

these measurements were carried out for only a few selected areas on the sample and do not lead to maps for the scattering. They do allow comparison with Mie scattering theory, however.

71
conditions. If CaF2 is to remain fine grained after forging, the deformation must occur below its recrystallization temperature, about 800°C. Since CaF2 behaves in a brittle manner below 800°C a hoop

stress or constraining force must be applied to the single crystal throughout the total forging operation. The high pressure helium environment provided by this forging technique provides this constraining force.

The present forgings are done at 750°C in helium at 14.82 MN/m2.

and cooled 25° C per hour. The forging is done sequentially

Figure 2 shows polariscopic photos of a <111> and a <100> oriented forging. The level of internal

stress is low but has not been quantitized.

Table 1 presents data on the critical fracture energy (F ) obtained from several samples and sources. These results have been described previously [1], but are included here to make the present paper self-contained. The strengthening achieved by the forgings done at 750°C is apparent.

An illustration of how "veiling" appears in visible light is shown in figure 3. This is a <111> orientation crystal and it appears that the veiling is related to the 3-fold symmetry of this crystal

orientat ion.

Table 1 Fracture Energies

Y c of Several CaF2 Samples

Condition Y

c (J/m }

Single Crys'tal 0.45, 0.51

(111) 110 Forged at 500°C 1.63 Forged at 51 5°

C

Annealed 1 Hr at 720°C 2.00 Forged at 650°C

Annealed 24 Hr at 720 °C 1.56

(Large grained)

Hot worked large

Grained (l-10mm) 1.6 Forged Isothermally at 750°C Annealed 24 Hrs . at 700° C 2.6 Kodak Irtran Hot Pressed Powder 3.6

(1)

(2)

(1)

(1) Freiman, et al . NRL

(2) Harshaw Polytran tested by Freiman et al .

, NRL

Scattering Results

A number of experiments were performed to determine what effect, i statically forged CaF2 have on the transmittance and forward scattering

scattering from the surface of the sample which tend to mask bulk scat

to immerse the test sample in a cell containing index matching fluid so scattering at the sample surface. The total integrated forward scatter

index matching fluid was measured. The measurements were performed in

at the Naval Weapons Center. The difference between these two measurem

veils or other bulk defects in the sample. Measurements were performed

(krypton laser red line) and at 3.39ym in the infrared (He-Ne laser),

test cell, which has windows of single crystalline CaF2.

f any, the veils in the iso- The problem was to eliminate

tering. This procedure was

that there would be no ing for the sample, cell, and

the Optical Evaluation Facility

ents gave the effect of the at 64717a in the visible

Figure 4 illustrates the

The data obtained from the scattering measurements are summarized in Table 2. The scattering

levels for the cell and the index fluid without the test sample were quite constant at 64718. The somewhat larger variations at 3.39ym are because the scattering is close to the noise level of the system, and one count on the digital voltmeter produces a proportionately larger change.

72
Results are shown in Figure 7 for scattering from a veiled area and in figure 8 for scattering

from a clear area. The results in each case are normalized to the on-axis power as unity. This allows

us to compare results for different values of laser power and different detector responsivity at the various wavelengths.

The results in figure 7 indicate that the angular dependence is independent of wavelength for 0.4416pm, 0.6328pm, and 2.8pm lasers, but at a given angle the fraction scattered into unit solid angle

is much lower at 3.9ym than at other wavelengths. The minimum value of the angle at which measurements

could be made (0.9 degrees for the two shorter wavelengths and 1.9 degrees for the two longer wavelengths) was set by the size of the detector housings. When the unseat tered beam struck the edge of the housing, reflection of that beam led to a spurious signal.

The results in figure 8 show that at a given angle, the scattering is such less when the beam

was incident on a visually clear area of the sample, as compared to a veiled area. This is to be expected, but the relative wavelength dependence is not. In particular, the reason for the relatively

large scattering at 2.8pm is not obvious. The measurement at 2.8pm was repeated on another clear

area, the clearest area that could be located on the sample. The same relative signals were found as for the first clear area. Figure 8 does not include data at 0.4416pm; the power of the HeCd laser was

low, and meaningful measurements could not be obtained for scattering from a clear area.

We attempted to interpret the results of Figure 7 on the basis of Mie scattering theory for transparent spheres, in order to determine whether the scattering arises from spherical voids within

the material. For voids with index of refraction 1 embedded in a material with index of refraction n, Mie scattering theory may be applied with an effective index of refraction m = 1/n. For CaF2 at 0.6328pm, m = 0.698. Tables of the Mie scattering functions are available for m = 0.7 [4]. It did not

prove possible to assign the scattering to Mie scattering by spherical voids within the CaF2- There

is no value which can be assumed for the radius of the void which will yield the relative magnitudes

of the scattering at 2.8pm and 3.8pm as shown in figure 7. If one assumes voids with a distribution of radii, it will not improve the situation. Thus, the wavelength dependence of the scattering is not a - menable to interpretation on this basis.

The results are useful for indicating the relative seriousness of the veiling at various wavelengths. The results of Figure 7 indicate that the scattering at 2.8pm is as great as in the visible;

therefore, veiling would presumably be a problem for CaF2 used with HF lasers. However, the reduced

scattering at 3,8pm indicates that veiling should be less of a problem for CaF2 used with DF lasers.

Both types of scattering measurements indicate a considerable reduction in scattering at infrared

wavelengths greater than 3pm, as compared with visible light scattering. They cannot be compared in

detail because the angular scattering measurements contain a contribution from surface scattering,

whereas the total integrated scattering measurements do not.

Studies of Veiling

CaF2 single crystals when forged under certain conditions of temperature and strain become visibly

"veiled". Microscopic examination identifies the source of the scattering as voids located along grain

boundaries in the forged CaF2. Scanning electron microscopy and EDAX (Energy Dispersive Analysis of X-rays) confirm the existance of voids and identify no detectable impurities. Recent work has shown

however, that the voids can be eliminated from forged material.

"Veiling" has been observed for all forging orientations i.e. <111> , <110> , and <100>, after die amount of deformation exceeds approximately 45 percent. The extent of veiling however, decreases with

increasing forging temperature but still exists in forgings done at 800°C, the temperature limit of the forging chamber. Early attempts to eliminate veiling while maintaining material strength were largely

unsuccessful.

Microscopic examination gives a better understanding of the nature and cause of the veiling in

CaF~. The individual scattering points comprising the veils are very small and extremely difficult

to locate microscopically. A veiled area intersecting the surface of a mechanically polished CaF2

forging can, however, be examined microscopically. See figure 9. Careful examination of the underlying

volume using a HeNe laser illuminating a scattering defect and a high magnification long working

distance Nomarski lens to focus on it enables one to find the scattering source. The laser is turned

off and the transmitted light source of the microscope is turned on. The scattering points are now

easily brought into focus at depths up to 4 millimeters below the CaF2 surface.

These defects appear very narrow and are typically only a few microns in length. The needle-like

defects lie parallel to one another in the bulk of the CaF2 . When the defects are brought to focus immediately below and on the sample surface, they are seen lying parallell to the boundaries of the elongated grains characteristic of the forging. These defects appear as voids located at the grain

boundaries

7 3
Fractured specimens containing veils show both by optical microscopy and SEM the existence of what appear to be voids. EDAX identifies no detectable impurities i.e. precipitates.

The cause of the voids is not precisely known but an explanation has been proposed in previous

work [5]. A high vacancy concentration can be expected at elevated temperatures and high deformations

because of non conservative dislocation movement and climb. When areas of high vacancy concentration

recrystallize the vacancies could be swept along and "precipitated" at the grain boundary.

This phenomenon is not likely to be avoided at temperatures and pressures attainable in conven- tional hot forging apparati. Recent encouraging work conducted by the Air Force Materials Lab, however,

has shown that veiling can be removed from previously veiled specimens [6].

Conclusion

Isostatic forging of several CaF2 crystal orientations did not succeed in eliminating veiling in poly crys talline windows. Scanning electron microscopic examinations and optical microscopy suggest

that veiling may be caused by microvoids formed by the coalescence of vacancies which were produced by

dislocation intersections and annihilations as a consequence of plastic deformation. Optical scattering

measurements indicate high values at 2.8pm, but reduced scattering at 3.8pm. Contour maps of the total

scattering show similar wavelength dependence, but do not show a correspondence to the visual appearance

of the windows

.

Acknowledgements

The authors wish to acknowledge technical assistance and helpful discussions with many people,

including E. Bernal G., K. M. Leung, M. L. Filzen, H. Vora, K. 0. Park, W. J. Polzin, D. W. Woodward

and R. I. George of the Honeywell Corporate Material Sciences Center and P. C. Archibald of the Naval Weapons Center, China Lake.

References

[1] R. H. Anderson and E. Bernal G. , in Proceedings

of the High Power Laser Optical Components

and Component Materials meeting, Boulder Co. Oct. 3-4, 1977, J. S. Harris and C. L. Strecker, eds.

[2] D. C. Stockbarger, J. Opt. Soc. Am. 39, 731

(1949) .

[3] H. E. Bennett, M. J. Soileau and P. C. Archibald, in Laser Induced Damage in Optical Materials: 1975, A. J. Glass and

A. H. Guenther, eds., NBS Special Publication

435.

[4] R. H. Boll, et al . , Tables of Light Scattering

Functions, the University of Michigan Press

(1958)

.

[5] E. Bernal G. , et al .

, A Study of Polycrystalline Halides for High Power Laser Applications

Final Technical Report on Contract DAHC-15-

73-C-0464, (1978) p. 30.

[6] A. K. Hopkins, AFML, private communication.

74

Figure 5. Contour maps of scattering from veiled

<111> oriented CaF?. Upper photo: 0.6471um

scattering. Lower photo: 3. 39um scattering.

Figure 6. Contour maps of scattering from veiled

<100> oriented CaF2» Upper photo: 0.6471pm

scattering. Lower photo: 3.39ym scattering.

76
oo

<

CD 3

or O

uj

o m

_l

<

oro

o

<

z

CD 3

ft O

LU hI-h-< O

O LU

CO Nl

O

01

0.01

10

-t 10

id"

+x

x+

0 442 firn 0.633 /i m

3.8/im

2.8

of

Figure 7. Relative scattering vs. scattering angle

for a veiled area on CaFo.

0 2 4 6 8 10 12

SCATTERING ANGLE (DEGREES)

I r

0.1

,6

3 -

-4

10 ^

fb

5

© 2.8/im

• 3.8/im + 2.8 /im (very clear areq) x 0633 /im

x*

Figure 8. Relative scattering vs. scattering angle

for a clear area on CaF2-

0 2 4 6 8 10 12 SCATTERING ANGLE ( DEGREES)

Figure 9. Photograph of voids in a veiled region

intersection mechanically polished CaF£ surface.

The width of the area covered by the photograph is about 0.2 mm.

'I

i

77
CW LASER DAMAGE IN AR COATED ALKALINE EARTH FLUORIDES AT 3.8 um*

John A. Detrio and Roger D. Petty

University of Dayton Research Institute

Dayton, Ohio 45469

We have measured the damage threshold of AR coated SrF^ and

oriented single crystals of CaF^ using a cw chemical laser operating at approximately 3.8 m. The samples were irradiated for

three seconds up to a total power of 8.4 kW. The specimens all

2 2 failed at a power density of 22 kW/cm over a 0.3 cm area. The

failure mode was not catastrophic and is believed to consist of

plastic deformation due to compressive loading in the center of

the irradiated region on the exit surface. The surface absorption is the dominant contributor to the failures. A novel

statistical approach to defining the damage threshold and assessing

the confidence level of the data was used.

Key words: Antiref lection coatings; calcium fluoride; cw laser

damage; DF laser; fusion cast; laser damage; laser window; single

crystal; strontium fluoride.

1. Introduction

There have been few material windows used in high power cw laser systems. One

factor contributing to the avoidance of material windows is the shortage of engineering data including survival potential, which would permit a systems engineer to design

around the materials power handling limitations. The problem of generating engineering quality survival data is complicated by the brittle mechanical behavior of most

candidate window materials and by the variability in antiref lection coating quality.

The objective of the test described in this paper is to demonstrate a method for

obtaining statistically significant data on the survival of coated window specimens.

Achieving this objective represents a first step in deriving engineering data on which

to base a rational window design.

As a test vehicle, we selected polycrystalline SrF

2 (fusion cast by Raytheon)

because of the availability of a large, homogeneous batch of substrate specimens. We

chose a commercial coating supplier who is not engaged in developing state-of-the-art

antireflection coatings for DF laser applications so that we could avoid the variability in quality associated with specimens from a coating research program. For

this reason the window performance discussed in this paper does not represent the best

available DF laser windows and coatings.

We also examined a systematic aspect of stress birefringence in alkaline earth

fluorides by studying the response of oriented single crystals of CaF

2 to DF laser

irradiation. The polariscopic images of these materials during irradiation exhibit

the predicted response to polarized laser radiation. These results allow one to assess the influence of texturing in the polycrystalline substrates on the propagation of polarized radiation.

2 . Apparatus and Experimental Details

2.1. Apparatus

The experimental apparatus is shown in figure 1. The incident power was measured

by reflecting the power from the beam switch onto a 0.7 percent rotating mirror chopper and through a CaF^ beamsplitter into a CRL Model 201 power meter and reflected a portion of the beam into a Laser Precision pyroelectric detector. The pyroelectric

detector is capable of very rapid time response which allows the temporal variations

in laser output to be monitored. The power transmitted by the specimens was measured

with a rapid response cone calorimeter. This device operates as a flow power meter

with a thermistor sensing the temperature rise of the water. The water flow rate and

temperature rise were measured and converted into a power reading. A series of beam

diagnostic test runs were made • to calibrate the CRL power meter, cone calorimeter, and

pyroelectric detector against one another. In order to maintain internal consistency,

all of the power readings are referenced to the CRL meter which had been calibrated

against an NBS standard.

*This work was supported by the Air Force Materials Laboratory

78
The laser beam handling system is also shown in figure 1. The first mirror is

mounted atop a concrete pedestal. ; The beam is then directed to a mirror on a table,

then back to a pedestal mounted mirror, and then to a second mirror on the table. By

combining the two focusing mirrors in this way the astigmatism in the optics is partially compensated and the required image quality and beam size at the sample is

achieved. Before striking the sample the beam passes through or is reflected from the

beam switch. When the switch is closed (the normal position) , the beam goes to the

diagnostic table where the 0.7 percent mirror chopper and CaF^ beamsplitter sample the

pyroelectric detector. When the shutter is open, the beam strikes the center of the

sample mounted on the carrousel wheel and, after passing through the sample, is

captured in the cone calorimeter.

The Agastat Controller was pre-programmed to control the timing and operation of

the various functions required during a test run. The test sequence and typical data

are shown in figure 2. The first action is the firing of the solenoid operated 35 mm

camera shutter; the beam switch is then activated under the control of a timer set for

three seconds. Approximately 2 seconds into the irradiation, the 35 mm camera is

again fired. After the beam switch is closed the carrousel clutch is energized and

the drive mechanism is engaged to advance the next sample into position. (The first

sample is in position at the start of the test.) The next command from the Agastat is to obtain a before-irradiation 35 mm photo. When the beam switch closes the sequence

begins again. This program is repeated three times during a given laser run and pro • vides for the test of three samples.

In order to measure the power density on the samples, it was also necessary to measure the beam size. The beam profile was obtained by irradiating Plexiglass specimens located at the sample position with various levels of power for 1/4 second.

Castings were made of these burn patterns using a silicon potting compound and the

resulting molded pattern was examined with an optical comparator to give the burn

depth and full-width at half -maximum depth.

The response of the samples to laser radiation was monitored by the use of an interferometer and a plane polariscope. Both kinds of information were captured on

35 mm film and on video tape.

The interferometer patterns are obtained by interference between HeNe laser light

reflected from the front and back of the samples. In most cases, the sample faces were sufficiently parallel so that several fringes could be observed.

The polariscopic view is produced in blue light using a Wratten No. 29 filter in

combination with an air-cooled light source (100 W) and a combination of diffuser

material and a Fresnel lens.

The recording of the polariscopic and inter ferometric data is achieved by com- bining the images in two ways. For video recording the images are formed side by

side on a screen that is viewed by a TV camera. The two images are also superimposed

and recorded by a 35 mm camera on Kodak Ecktachrome color film. The intensities are balanced by the use of neutral density filters and crossed polarizer attenuators to give an acceptable balance between the two images when a typical strain pattern is

present. The images may be color separated (using Wratten No. 29 (recj) and No. 46 (blue) filters) for study of either interference or strain pattern.

The Spectro Physics Model 155 HeNe laser source for the interferometer is mounted

with a Jodon pinhole spatial filter and beam expander. The light source for the

polariscope and the HeNe source are mounted on tripods for ease of adjustment and

freedom of placement. The data camera system and carrousel are attached to a 61 x 122 cm optical breadboard from NRC and the breadboard was fastened to a die table

which facilitated adjustment of the height of the table to match the laser focusing

optics

2.2. Samples

The samples are listed in table 1. Fusion cast SrF

2 substrates from Raytheon

were coated by Valtec (Optical Coating Division) with a SiO/ThF^ (A/4, A/4 ) AR design

adapted from one previously used at 5.3 pm on CaF

2

. These samples were procured to represent a homogeneous batch of equivalent samples. They were polished to a parallelism of less than twelve arc seconds. Not all of the samples were polished to the

same precision; thus, there are differences in the fringe count across the samples.

This difference should not affect the test results unless the difference in parallelism is related to differences in polishing and handling which in turn may influence

the^ surface quality and laser absorption. The measured values of absorption and

scattering were compared to the fringe count to see if there was any significant cor- relation between these quantitatives . No correlation was found between either

79
scattering and fringe count and absorption.

Table 1. Data summary for AR coated Sr?2 samples,

3.8 ym Scattering

Absorption at 6328 8

Sample -4 -1

(-J-U cm ) Fringe

Number (Ratio - 1 0 ) (zero

1883 44 9 1 19

1929 41 10 2 21

1930 43 10 8 18

1935 40 11 3 8 1937 42 12 4 9 1942 49 10 2 10

1943 46 8 9 6 1944 38 12 1 11 1949 44 9 4 7 1950 24 10 2 17

1951 37 9 2 9 1955 40 10 7 14 2006 39 9 1 23

2007 50 10 5 2008 41 9 8 2009 43 10 1 14 Three oriented single crystals of CaF2 grown by Harshaw Chemical Company were

coated at Northrop Research Technology Center by Dr. Sam Holmes. The coating is

a two-layer SiO/ThF^ (A/4, A/4) AR design for 3.8 ym. These specimens offered an interesting opportunity to test the influence of orientation on the damage threshold

of the coated optics. The stress birefringence of the three oriented samples will be

different and this difference can be observed in the polariscopic photographs. The

pre-test data available for the oriented substrate samples are listed in table 2. Table 2. Oriented single crystal CaF

2 samples coated by Northrop.

Total Absorption Scattering

_ 4 Coating* Ratio

Thickness (-^10 ) Absorption , Sample Orientation (cm) NRTC UDRI (2 Surfaces) (UO" ) 2409 (100) 0.689 5.28 10.6 4.14 8.1

2410 (110) 0.639 4.41 8.4 3.59 6.9

2411 (111) 0.529 3.57 9.2 2.65 9.3

*Coating: 2-layer SiO/ThF

4 (A/4, A/4).

3. Data Reduction and Analysis

3.1 Power Measurements

A series of eight calibration runs was performed to relate the power measurements

obtained with the three detectors and to correlate these with the burn patterns in

plastic discs.

The pyroelectric detector with its rapid time response showed that the laser

power was more nearly constant during the entire run than the power meter output

would indicate. This is a consequence of the transient reponse of the meter which

does not yield a faithful reproduction of the laser's temporal output profile.

The calibration curves for the cone calorimeter and the pyroelectric detector

are shown in figures 3 and 4. The CRL meter is used as the primary standard since

its calibration history was most complete. The fitted curves are lines contrained

to pass through the origin. The assumption was made that zero power output represents a zero reading in all meters. The uncertainties in the prediction of the power

from a cone calorimetry measurement or pyroelectric detector signal are no better

than 1.5 percent for the cone and 3.2 percent for the pyroelectric detector based on the uncertainty in the determination of the calibration constant. There are other

sources of error which can cause a greater uncertainty and which are not well con- trolled; these are changes in the alignment, principally for the pyroelectric device,

and effects due to varying water flow rates or beam displacement and attenuation

caused by the samples which influence the cone calorimetry signal. As a practical

matter, base line drift produced a noticeable effect on the cone calorimetry readings,

especially during the typical three-sample runs. The overall accuracy for a transmitted power measurement is estimated to be not better than ± 15 percent.

80
The slow time response and alignment problems associated with the CRL meter and

the sensitivity of the pyroelectric detector to alignment, made these two devices

unsuitable for measuring power. Since most of the samples had very low attenuation,

absorption, and reflection losses (less than 1 percent) and the samples were finished

to a high degree of parallelism, there was little difference between the incident and

transmitted power. Therefore, the cone calorimeter values are used to define the

measured power levels.

3.2. Beam Profile

The beam profile, including the cross sectional area, was obtained by measuring

silicon rubber castings of the burn pattern in plexiglass. The burn pattern dimensions were measured with an optical comparator. The full width at half-maximum was measured along a major and minor axis (the cross section is elliptical) and found to

2 be 0.58 and 0.46 cm (± 0.036 cm). This represents an area of 0.3 cm ± 13 percent at

2 the equivalent 1/e diameter.

3.3 Pre- and Post-Test Evaluations

Prior to testing, the samples were characterized with respect to their optical

quality by measurements of absorption at 3.8 pm (DF laser), scattering at 6328 8,

surface flatness, optical path difference, spectral reflectance, and transmittance

Visual inspections were also made using a Nomarski microscope, polariscope , and

transmission and oblique-incidence surface photographs. Many of these same observations were made after the tests for comparative purposes. A Schlierin optical

system was also used to visualize a very subtle change in transmission properties

induced by the laser. Schlierin photographs were not made prior to the tests.

4. Discussion and Conclusions

The results of the tests are summarized graphically in figure 5 in which the

transmitted power and irradiation history is shown for each sample and the damaged

specimens are shown as shaded squares. The interferometer and polariscope photographs are shown in figure 6. Damage is visible at the center of the specimens in

these photographs. The relationship between orientation and stress patterns for the

randomly oriented test specimens may be correlated by reference to the oriented CaF2

samples. For example, sample no. 1951 probably Jias a (111) orientation.

There were 14 specimens tested; four were tested repeatedly at five increasing

power levels, (5-on-l); while two samples were tested once at each of the power

levels, (1-on-l). All six of the samples tested at the highest power (about 8 kW)

suffered a subtle form of permanent damage. There is no "conditioning" effect

observable for these samples and the 1-on-l damage threshold equals the n-on-1 value.

The degradation was observed in the interferometer photographs and in transmission polariscopic or Schlierin observations. The defects were not visible in

Nomarski micrographs until the surface was aluminized. A typical damage spot is

shown in figure 7. With the relatively large number of equivalent samples, we can say with better

2 than 80 percent confidence that the damage threshold is 7 kW/0.3 cm for three second

irradiation. The uncertainty in this value is about ±15 percent. Since the samples

did not fail catastrophically , the damage threshold is conservatively stated.

It is not normally possible to define a single threshold for damage when all

possible failure mechanisms are included. It has been our custom to define mechanical

damage as the observation of permanent residual strain in the sample (or other physical damage such as fractures or gross failure) . The residual strain criteria is good

for detecting the onset of mechanical damage in materials which undergo plastic deformation. For brittle materials like SrF

2

, residual strains will normally not be

observed without some other failure occurring.

It is of questionable value to give a damage threshold in terms of the power or power density, especially when it is desirable to extrapolate the results to large

windows. The problem is compounded when one is using a brittle material such as SrF2

whose failure is described in statistical terms. We would rather specify that the

samples failed at a given value of temperature and stress. The failures we observed

appear to be due to compression in the center of the sample over such a small area

that crack propagation does not occur. The compressive stress at the center of the

window was calculated to be approximately 100 Mp; (15 kpsi) with a peak temperature

of approximately 98 C at the center.

We believe that the damage morphology is the result of plastic deformation in the

form of (100) slip. This conclusion is inferred from x-ray analysis of a CaF2 window

81
which had previously failed in service on the ACL. The slip planes, cleavage plane,

and crack orientation for the single crystal window CaF

2 are shown in figure 8. 5. Acknowledgements

It is a pleasure to acknowledge the help and support of the Army Chemical Laser

Facility personnel. Dr. Tom Barr was especially helpful in the administrative aspects

of the tests and in seeing that everything went smoothly. The operation of the laser

facility was supervised by Ben Wilson.

The laser operator and test conductor was Ed McCracken, with Tom Witset responsible for optics and laser alignment; Roy Yell was the computer operator and Tom

White was responsible for the mechanical details and gas handling. A very special

thanks to Ben for his competent and efficient help in every phase of the testing.

The oriented single crystal samples were prepared by Dr. Sam Holmes of Northrop

Corporate Research and Technology Center with data supplied by Dr. Paul Kraatz.

Dr. Richard Harmer of the University of Dayton provided the x-ray analysis. John

Fenter of the Air Force Materials Laboratory was instrumental in obtaining the test

time at Redstone and in supporting the conduct of these tests.

3.00

Figure 1. Experimental set-up for DF laser

damage tests at the Army chemical laser.

The HEL beam handling optical and beam

diagnostics and the polariscope and interferometer optics are shown approximately

to scale.

I 00

1

h'hhhhhhhhhhhh timing

SHUTTER

CLUTCH

PYROELECTRIC CAMERA

SHUTTER

000 12 00 16.00 TIME (SEC)

20.00

Figure 2. Graphical data output obtained

during a typical damage test run. The

chart record of cone calorimeter, and CRL

power meters output are shown with the

oscillographic record of the various

timing function and fast pyroelectric detector output. This composite summarizes

the quantitative data obtained during the

tests and shows the temporal sequence of

the significant events occurring during a typical test.

82
2 3 4 CRL POWER METER ( kW)

Figure 3. Cone calorimeter calibration

data. The power measurements for cone calorimeter are plotted against the calibrated CRL meter readings to permit the

arbitrary cone calorimeter signals to be

converted to power.

I l i10

>-

01<

\-

CD

tr<

QLo

LlI o

? 4

I 2 3 4 5 6 7 CRL POWER METER (kW)

Figure 4. Pyroelectric detector calibration data. The power measurements for the

pyroelectric detector are plotted against

the calibrated CRL meter to permit the

arbitrary pyroelectric detector signals to

be converted to power.

o

I I l_

„

U

_y i i_ 1943 1929 1955 1951 1937 2006 2009 2409 241 1949 1883 1944 1935 1936 1950 1942 2410 SAMPLE NUMBER

Figure 5. Summary of AR coated SrF 2 sample tests. The transmitted power levels

associated with every irradiation on each of the AFML supplied samples are shown as square symbols. The shaded squares represent the observation of permanent mechanical

deformation - plastic deformation of the surface for these samples.

83
n - ON - I TESTS I - ON -

1 TESTS

Figure 6a. Composite interferometer-polariscope photographs for the AR coated SrF2

samples. Permanent deformation near the center of the specimens irradiated at the

maximum power level is seen in each of the specimens irradiated at 8 kw. Samples

irradiated at lower power levels are not damaged. Although these samples did not

fracture - they have been permanently damaged. The third specimen from the right is

Sample No. 1951. Note the absence of a strain pattern.

CRYSTAL ORIENTATION

SAMPLE

NO.

CI 00)

2409

(110)

2410

(111)

2411

BEFORE

DURING t

AFTER 1*1 D

Exit Surface

Interferogram

0

Angstromeler

Photograph dim i Nomorski

Micrograph

Figure 6b. Composite interferometerpolariscope photographs for oriented single crystals of CaF2- The relationship

between the plane polariscope pattern and

the crystal orientation for CaF2 is clearly

shown in these photographs which permit the

reader to determine the orientation of the

other samples shown in Figure 6a. The

interferometer fringes are not visible in

these photographs.

Figure 7. Composite photograph showing

the results of several methods applied to

make the damage site visible. The scale

in the Nomarski micrograph is 1 mm. The

Rngstrometer photograph is taken with

green Hg light. The surface was aluminized in all but the Schlieren image.

84
Figure 8. The slip traces and crack orientation for a CaF2 window are shown superimposed on a photograph of the specimen. The orientations were determined by the

analysis of Laue back reflection X-ray photographs. These slip traces cover a larger

area than those shown in Figure 7. The specimen image diameter is about 1 cm.

85
LATTICE ABSORPTION, PHONON ASSIGNMENTS, AND IMAGE SPOILING PROPERTIES

OF CVD ZnS IN THE INFRARED

C. Klein, B. diBenedetto, R. Donadio, T. Kohane, and J. Pappis

Research Division, Raytheon Company

Waltham, Massachusetts 02154

Chemical-vapor-deposited zinc sulfide (CVD ZnS) has been established as a highly promising material for advanced infrared imaging applications and is attracting attention as a potential window material at HF/DF laser wavelengths. The

present contribution concerns work that has been carried out in the context of as- sessing the optical characteristics of CVD-ZnS blanks, particularly with regard to: (a) lattice absorption. Careful examination of transmission traces at wavelengths

up to 2 5 |_Lm indicates that multiphonon processes dominate at frequencies below

1000 cm~l; peak-absorption assignments in terms of zone-boundary phonons have

been made on the basis of best available inelastic neutron-scattering results for zincblende ZnS. (b) Temperature dependence. Absorption coefficients at temperatures up to 600 K have been investigated in light of spectral transmittance and

emittance measurements supplemented by calorimetric data at HF, DF, CO, and

CO2 -laser wavelengths. (c_) Image spoiling. Line-spread functions in conjunction

with index-inhomogeneity maps demonstrate that wavefront distortions resulting

from the presence of a CVD -ZnS window do not inject any detectable degradation in contrast transmittance.

Key words: Characteristic phonons; chemical -vapor deposition; image spoiling;

infrared imaging; lattice absorption; zinc sulfide.

1. Introduction

In recent years, work on chemical-vapor-deposited zinc sulfide (CVD ZnS) has resulted in the development of a material that shows much promise for applications involving windows and domes, in the area of passive infrared systems operating in the 8- to 12-p.m spectral passband fl] . * We note, in particular, that the capability to produce optically finished CVD-ZnS blanks in the sizes required for large-aperture/high-resolution thermal imagers has been demonstrated, and that cost-effective manufacturing techniques have been established. In addition, it has been shown that CVD-ZnS windows

capable of satisfying structural requirements stemming from the mechanics of high-speed flight maintain sufficient transmittance to insure that any window-induced degradation in system sensitivity will remain tolerable, over the whole range of anticipated thermal loads.

In a previous paper [2] , we have examined how aerodynamically heated windows may affect the thermal sensitivity as a result of gross attenuation of incident signal radiation in addition to enhanced

background radiation originating from the window emittance in the wavelength region of system operation.

In effect, we have shown that AR-coated CVD-ZnS windows can degrade the electrical signal-to-noise

ratio of operational IR systems via two mechanisms: (a) a first-order reduction in target signal reflecting

radiation absorption by the window, and (b) a second-order degradation in system detectivity reflecting

the additional quantum noise generated by window photons. It was also pointed out that, in order to avoid

injecting a significant loss of resolution in the optical image, the window must satisfy strict requirements

in terms of spatial -contrast transmittance characteristics.

At this point, it should be said that the degradation in signal-to-noise ratio can be evaluated in rather general terms, as specific features of the optical arrangement other than those relating to the spectral passband do not enter the calculation. In reference [2] , we have shown that, for systems

operating in a photon-noise limited mode, difficulties arising in evaluating the signal-to-noise racio

(SNR) in the presence of a "hot window" can be avoided by considering an SNR defined as the ratio of H C£. and NEI, where H represents an effective signal irradiance for extended targets, and NEI is eff ' eff r 6 fc> > the noise -equivalent irradiance at the peak-response wavelength of the detector. In this manner, we find that the degradation involves a reduction in target signal and an increase in system noise best expressed

as follows:

1 . Figures in brackets indicate the literature references at the end of this paper.
< H

eff>W . T(^Tw

)W

x (T B

)dX

(H

eff'0 oh

and

(NEI)W

[TM

2 E ^ Tw) Q K (TW) dK + £ *T (X, Tw) Q^(Tg) dx

(NEI)

o r

^

2 J Q

(2)

2 where T(X., T,,,) measures the transmittance of the window and E(X,T.lr ) its emittance. In the same vein, w w

and since the modulation-transfer function (MTF) of the system includes contributions originating from all the components [3], it is immediately seen that the MTF of a "bare" system must be multiplied by the transfer function of the window in order to arrive at an overall modulation-transfer function:

(MTF)W

= ?W

(MTF)

0

• (3)

Equation (3) implies that scattering and other image -spoiling properties of IR window material can be

characterized by means of a spatial transfer function, r^. , which establishes the precise effect of a window blank on the resolution performance of the optical system.

The importance of accurately measuring the optical properties of CVD ZnS becomes evident upon

considering eqs. (1, 2, 3) because they involve the emittance, the transmittance, and the transfer function

of the window material. With regard to emittance and transmittance, it will be kept in mind that, in the case of AR -coated windows of thickness t, the following holds: E= l-exp(-St) and T= exp ( - St) - R,

if R is indicative of the residual surface reflectance and /? is the bulk absorption coefficient. For this reason, it will be the primary purpose of this paper to consider information relating to 6 and to assess in detail some critical features of the absorption spectrum at the multiphonon edge of CVD (or cubic) ZnS.

In connection with the matter of transfer functions, we may emphasize that, in the process of creating

a new optical material, standard properties such as the spectral transmittance are usually the first to be investigated and form the basis upon which potential applications are to be developed. With highsensitivity/high-resolution systems, however, image spoiling caused by light scattering, index inhomogeneities, and other imperfections may seriously impact the usefulness of candidate IR transmitting

materials [4] and must be carefully assessed from the point of view of their significance relative to aperture -diffraction effects. This is best done by specifying the performance of the blank in terms of

a line -spread function (LSF) since LSF yields the transfer function simply by taking the Fourier

transform

In section 2, we first examine some general characteristics of room -temperature transmission

spectra for state-of-the-art CVD ZnS and derive absorption coefficients over a wave number range

extending from 400 to 1400 cm *

. An analysis of the spectrum is then carried out (sec. 3) on the basis

of the best available lattice-dynamics model, that is, on the basis of neutron-scattering data and densityof-states calculations by Vagelatos et al. [5] . Specifically, it will be shown that all significant features

of the two-phonon absorption regime can be assigned to summations at critical points in the Brillouin

zone, thus yielding accurate frequencies for the phonon modes at high-symmetry locations- In section

4, we present transmittance and emittance data obtained at elevated temperatures and discuss their implications in the light of la.ser-calorimetry results; it thus will be demonstrated that, beyond 10 ^m, structure in the absorption spectrum of CVD ZnS is indeed due to intrinsic multiphonon processes. Information

of significance in terms of image spoiling (sec. 5) will be considered from the point of view of IR systems

requirements as well as index-inhomogeneity measurements. Finally, section 6 includes the summary

and our conclusions.

2. This, of course, assumes that both window and background are at uniform temperatures TTM. and

Tg, respectively.

3. For an elementary discussion of these concepts, see reference [6] .

87
2 . Room -Temperature Features

2.1 Spectral Transmittance

Figure 1 shows the infrared transmission profile of three specimens of CVD ZnS ranging in thick- ness from 0.2 5 to 1. 30 cm; such traces are typical of serial production material deposited at a temperature of 675°C, a furnace pressure of 40 Torr, and a large excess of zinc vapor. These measurements,

which were carried out on a carefully calibrated Perkin-Elmer spectrophotometer (model no. 457)

operating in the double-beam mode, immediately reveal that there is absorption, or attenuation, at all wavelengths in the infrared. Most prominent is the broad band centered at 1600 cm~l, which is also

observed in chemical -vapor-deposited ZnSe, and has been tentatively attributed to vibrational modes

associated with residual zinc hydride radicals [7]' . In the wavelength region of interest for thermal

imaging (8 to 12 (j.m), we note that, beyond 10 \±m, there is a substantial increase in absorptance, which

is known to be caused by multiphonon processes [8] and, consequently, imposes intrinsic limitations on

the performance of ZnS windows. In effect, the "tailing off" that occurs between 10 and 15 ^m may well

reflect summations involving three phonons, considering that the high-frequency cutoff for two-phonon

-1 absorptions should be at 2 x LO (F ) = 704 cm if the Raman phonon is as indicated by Nilsen [91 In 1

that case, the structure that emerges in the 900 cm wavenumber region emphasizes the role of covalen

bonding in ZnS and should be amenable to further interpretation (sec. 3) in the context of a comprehensive

investigation of lattice absorption in this material.

For this purpose, we have obtained the transmission profiles of a set of progressively thinner samples in order to extend the range of available CVD-ZnS absorption data into the two-phonon region. Figui

2 displays the far-infrared transmittance trace of our thinnest specimen (0.018 cm), which illustrates

the dominance of structure in the absorption at wavelengths above 15 fj.m. In section 3, it will be shown

that this tracing yields appropriate information for initiating a critical point analysis of the phonon spectr

in cubic ZnS

.

2.2 Absorption Coefficients

For near-normal incidence, and in the absence of interference effects, the classic expression for

the transmittance of an optical blank is

T = (1-R) exp (- j3t) 1-R 2 exp(-2|3t)

which takes multiple internal reflections into account. The absorption coefficient j3,

(4)

(1-R)

~2T~ R

2 + ^

4T'

(5)

may thus be derived from the measured transmittance if the single-surface reflectivity R is available as

a function of wavenumber. Our work assumes that the reflectivity can be simply expressed in terms of the refractive index, R (n-l)

2 /(n+l)2

, with an index as given by Dodge [10]

i r 3 A . \

J (6)

where the A.'s and \.'s are empirically determined constants. In this connection, we note that the coefJ J 2 2 ficient of extinction does not enter the calculation because k remains much smaller than (n-1) at all relevant wavenumbers, as can be ascertained by referring to figure 3. Furthermore, we may point out

that CVD ZnS exhibits maximum transmittance between 8.0 and 8.5 (j.m, with thin specimens (t< 0.06 cm)'

having transmittances of 0.747± 0.001. On assuming zero absorptance, the theoretical transmittance,

2 T = 2n/(n +1), is 0 . 748 ± 0 . 00 1

, which indicates that the losses are almost exactly as expected from

Fresnel reflections, thus implying that any form of scattering should play an insignificant role in the wavelength range of interest.

In figure 3, the absorption coefficients are plotted against wavenumber over a frequency range

covering the two-, three-, and four-phonon regimes. This spectrum actually represents a "composite"

formed by joining the best segments of three data sets derived from eq. (5) on using the transmittances

88
j of three specimens ranging in thickness from 0.018 to 1.59 cm. This procedure has allowed us to map

the absorption with great accuracy, especially at the wavelengths of relevance in terms of thermal imaging

| technology. The spectrum displays a multiphonon-like behavior with structural features that are characteristic of semiconductors [11] , up to wavenumbers of about 1000 cm Beyond that point, there is

little change with frequency, presumably because of the presence of a weak absorption band at 1100 cm" , which can be attributed to ZnO impurities. Towards shorter wavelengths (\ < 8 (j.m), CVD ZnS invariably

shows evidence of increasing absorption, which reflects the dominance of the long-wavelength wing of the zinc hydride impurity.

2.3 Laser Calorimetry

The results of thermocouple-calorimetry measurements conducted at standard laser frequencies on

I a number of CVD-ZnS samples originating from three different deposition runs are listed in table 1 . At

j both CO, -laser wavelengths, the calorimetry data are in good agreement with the absorption coefficients

Table 1. Room -temperature laser calorimetry results for CVD ZnS.

Laser wavelength (am) Absorption coefficient (cm -1

)

2.7 (HF) (7.2 ± 0 5) X 10

3.8 (DF) (2. 3 ± 0 5) X 10

5.2 5 (CO) (4. 5 ± 1. 1) X 10

9.27 (C0

2

) (7.9 ± 1. 3) X 10

10.6 (CQ2

) (2.4 ± 0 1) X 10

j derived from transmission traces (see fig. 3) and, by the same token, they verify our procedure for interpreting spectrophotometer data. At chemical laser wavelengths, the present results point to substantially

J lower absorptions compared to those reported by Harrington [12] for an "early" specimen of RAYTRAN

I ZnS; still, they remain too high from the point of view of establishing CVD ZnS as a credible window -

I material candidate for high-energy lasers

At this point, we may emphasize that the absorption coefficients in figure 3 are indicative of state- of-the-art material, even though some non-uniformity has been observed in the impurity-dominated

region, for samples cut from similar deposits. It would appear, therefore, that the CVD-ZnS data of Bendow etal. [11] are not quite compatible with experimental evidence outlined here. In this regard,

it should be kept in mind that inaccurate assumptions on the wavenumber dependence of the refractive

index may lead to relatively large errors in deriving absorption coefficients, since eq. (5) becomes

essentially a function of the reflectivity R, for small values of )3 .

3. Critical -Point Analysis

Chemical-vapor-deposited ZnS crystallizes in the zincblende system. The recent phonon-dispersion

data of Vagelatos etal. [5] are therefore applicable and provide highly reliable guidelines from the point

of view of doing a critical-point analysis of the two-phonon absorption spectrum displayed in figure 2. Earlier attempts of this nature include the work of Johnson [13] and of Irwin [14] but these-authors both

rely on the low-temperature data of Deutsch [ 8] . First, it will be recalled that "critical points" are

located at zone-edge positions such as X (1, 0, 0), L(l/2, l/2, 1/2), and W (1, 1/2, 0), in addition to the position r (0, 0, 0) at the center of the Brillouin zone. If allowed by selection rules, frequencies corresponding to multiple -phonon combinations at these points should give rise to "features" in the infrared

absorption spectrum, such features reflecting singularities in the multiple -phonon density of states. Both

electric -dipole and electric -quadrupole allowed transitions in zincblende have been worked out by Birman

[15] ; the reader may consult this paper for clarification of concepts and notations used here.

The two tables on the next page (table 2 and table 3) summarize the results of our analysis of twophonon absorptions in cubic ZnS.

• The strong absorptions centered at 668, 6 36, and 596 cm are assigned to overtones of LO and

TO modes, keeping in mind that LO overtones are dipole -forbidden at point X.

• Electric -dipole allowed two-phonon summations at the highest-symmetry zone-edge points (X and

L) have all been identified and assigned in accordance with the lattice-dynamics model described

in reference [ 5] . • The features at 662, 612, and 602 cm 1 are assigned to quadruple-allowed optical phonon overtones occuring near point W, thus demonstrating the effect of non-linearities in the electric

moment on the IR absorption spectrum of ZnS.

89
Table 2. Critical-point analysis, two-phonon IR spectrum of cubic ZnS.

(k = kink, m = minimum, s = shoulder)

p t; & lux tf IVlcdoUI o d cn yonCU Phonon C ale ulated C omment

see fig. 2)

_ I position (cm ) assignment

_ j position (cm ) (see note)

1 (k) 704 2 LO (r

)

704

2 (m) 668 2 LO (L) 668 Raman active

3 (s) 662 2 0 1 (W) 662 Quadrupole

4 (s) 6 50 LO(X) + TO (X) 6 50

5 (m) 6 36 2 TO (X) 6 36 LO(L)+ TO(L)?

6 (s) 612 2 02 (W) 612 Quadrupole

7 (m) 602 20

3 (W) 602 Quadrupole

8 (m) 596 2TO(L) 596

9 (k) 544 LO (X) + LA (X) 544

10 (s) 530 TO (X) + LA (X) 530

11 (s) 526 LO(L) + LA (L) 526

12 (m) 488 TOIL) + LA (L) 490

13 (m) 4 50 O^W) + A

2 (W) 4 50 Raman active

14 (k) 420 LO (X) + TA (X) 420 Raman active

15 (s) 406 TO(X) + TA (X) 406 LO(L) + TA(L)

16 (s) 386 2 LA (L) 384 Raman active

Note: Features at 6 36 and 612 cm ^ are also Raman active.

Table 3. C ritical -point phonon frequencies (cm ) in cubic ZnS.

The lattice-dynamics data are those of reference 5.

Critical point Phonon branch Lattice dynamics IR spectrum

r LO 348 ± 5 352 TO 277 ± 3 X LO 330 ± 3 332

TO 316 ± 3 318

LA 2 12 ± 3 212

TA 90 + 1 88 L LO 337 ± 8 334 TO 289 ± 3 298

LA 195 + 3 192 TA 70 ± 1 72 K/W - °1 330 ± 5 331 °2 308 ± 5 306 °3 302 ± 5 301 A

l 194 ± 2 A 2 117 ± 2 119

A 3 87 + 1

90
• Some of the assignments listed in table 2 also apply to Nilsen's [9] second-order Raman spectrum

and do reflect the polarization situation.

• With the possible exception of the TO(L) mode, phonon energies derived from the IR spectrum

agree remarkably well with the results of neutron-diffraction work (see table 3) and yield more

accurate critical-point frequencies.

• The mode frequencies at point W have not yet been established on a "lattice dynamics" basis;

relevant "IR spectrum" assignments rely on point K frequencies as given by Vagelatos etal. [5],

In the three-phonon region (see fig. 4), structural features are seen to be much less prominent,

I which suggests that, initially, it should be more meaningful to analyze the absorption in terms of characteristic phonons [6] rather than relying on a complete set of critical phonons. For this purpose, we can make use of a recent phonon density-of-states calculation for ZnS [5] and postulate that structural features at the lower levels of absorption relate to four main peaks in the density-of-states curve.

In other words, we proceed on the basis that the gross features of the three-phonon absorption process

can be interpreted in terms of combinations of the four characteristic phonons listed in table 4. In this

Table 4. Characteristic zone-edge phonons in cubic ZnS.

Phonon branch: LO TO LA TA

Mode frequency (cm" 1

) : 330 295 193 89

approximation, and in light of some pertinent observations by Bendow et al .

[ 1 1 ]

, it appears reasonable

to entirely ignore selection rules and assign each possible combination strictly on the basis of summation energies. The procedure is illustrated in figure 4 and discloses a number of impressive correlations; specifically, we note that the prominent absorption band at 885 cm"' can be attributed to TO

overtones, which clearly identifies the phonon branch that is responsible for the loss in transmittance

experienced at around 11 p.m with CVD-ZnS windows. In addition, it is seen in figure 4 how impurityinduced absorption affects the level of transmission at wavelengths below 10 |j.m, since four-phonon

processes should not alter the transmittance in any significant way.

4. Elevated -Temperature Features

4.1 Spectral Transmittance

The temperature dependence of photon absorption at the multiphonon edge of cubic ZnS was first considered by Deutsch [8] and subsequently reinterpreted by Marshall and Mitra [16] in terms of tentative characteristic zone -edge phonons. From a practical point of view, the situation is best

summarized as in figure 5, which exhibits long -wavelength spectral transmittance traces for CVD

ZnS at temperatures ranging from 22° up to 200°C. These measurements were conducted in the usual

fashion, with a small furnace placed close to the focal point of the spectrophotometer; at wavenumbers

below 800 cm"', corrections were made for eliminating spurious contributions originating from the sample's self -emittance

The behavior of the absorption band at 885 cm 1 warrants further investigation: The band broadens

and shifts its position to lower wavenumbers as the temperature increases. Considering the results

of section 3, this band must be attributed to a three-phonon process, specifically, to a 3 x TO summation

with TO referring to a characteristic transverse optic mode of frequency w , at the zone boundary. The

-5-1

data presented in figure 5 then indicate that we have (1/w

q

) (dw^/dT)* -6 x 10 K , or essentially the

j same type of "mode softening" as reported for optical phonons in ZnSe.^ In this connection, we note that

I the temperature dependence of the mode frequency originates mainly from thermal expansion and is given

by [18] T

io(T) = u(0) exp (-37 J

a dTj (7a)

or

(l/u) (dw/dT) = -3yo , (7b)

where a is the linear coefficient of thermal expansion and y is the mode Grlineisen parameter. With

a= 7.85 x 10~6 K

_1

[1] and 7= 1.8 [19] , it follows that the calculated value of ( l/w ) (dw

Q /dT) should be

4. See, for instance, ref. [17]

91
on the order of -4 x 10 K and, thus, consistent with our experimental results. This assumes,

however, that the Griineisen parameter does not show much dispersion, as the work in reference [19] only concerns long-wavelength (k_~ 0) lattice vibrational modes.

4.2 Spectral Emittance

The use of emittance techniques for investigating optical properties of solids is now well established [20] and rests primarily on exploiting the basic emittance equation,

E = (1-R)[ l-exp(-ffl)]

^ (g)

1 - R exp(-/3t)

for spectroscopic purposes. An application to CVD ZnS in the 0° to 100°C temperature range is illustrated in figure 6. First, we note that the zinc hydride band at 6 (j.m exhibits little dependence

on temperature, which confirms that we are dealing here with a molecular impurity. Also of interest are the following observations:

(a) In the 8- to 10-(_im range, there is virtually no change in emittance with temperature, and the levels involved are sufficiently low to allow us to extract absorption coefficients simply from the E = jS t approximation. We thus obtain j3 's as listed in table 5, which are in satisfactory agreement with

values derived from room-temperature spectrophotometry as well as laser calorimetry (see sec. 2).

Table 5. CVD-ZnS absorption coefficients derived from emittance measurements.

IR wavelength ( ^m): 8.0 9.0 10

Absorption coefficient (cm" 1

) : 0.043 0.074 0.11

(b) The emittance peak at 1 1 |j.m reflects the multiphonon (3 x TO) absorption process described

earlier^ and is seen to shift towards longer wavelengths as the temperature increases, thus confirming

the mode -softening trend.

(c) Beyond 14 \±m, we note that the emittance reaches a plateau, which is characteristic of an opaque spectral region. In such situations, eq. (8) reduces to E = 1 -R and becomes a function of the index of refraction only. At 0°C, the average emittance of CVD-ZnS specimens originating from three

depositions turns out to be 0.87 at 15 ^m, which is precisely the amount anticipated for an index (n = 2.11) as obtained from eq. (6); at higher temperatures, however, the emittance appears to decrease

at a faster rate than predicted on the basis of available dn/dT data [10] .

4.3 Temperature Dependence

Around 10 p.m, the temperature dependence of the absorption coefficient of CVD ZnS can be estab- lished quite accurately by means of laser calorimetry. The results of such experiments, at 10.6 and

9.27 |am, are displayed in figure 7. In conjunction with the 10.6-|_litl data, we have also plotted some

absorption coefficients obtained from the transmittance and the emittance curves of figure 5 and figure

6; note that the IRTRAN-2 data point refers to hot-pressed ZnS and was derived from an emittance

measurement [2 1] by means of the Stierwalt-Potter equation [20] . Consider now a multiphonon-like absorption involving p emitted phonons: If hv is the energy of an incident photon, this means that the phonons must have an average energy hw =hv/p. Since the probability of a photon absorption resulting in the creation of p "average" phonons is proportional to

[ 1+N ("R u ) ] P whereas the probability of a photon emission induced by the destruction of £ phonons is proportional to [NOhu )] p

, it follows that, in terms of absorption coefficients we have [6]

/3o([ 1 + N(hw )] p - [N(fiu )] P

, (9)

where N represents the Bose-Einstein function. Furthermore, we take it that the matrix element

expressing the transition probability is nearly temperature independent, which leads to the conclusion

that eq. (9) can be utilized to specify the temperature dependence of an absorption involving a fixed

number of phonons. ^ In figure 7 we show the results of an evaluation of eq. (9) in the three-phonon

5. This approach ignores the temperature sensitivity of normal modes and may not be valid at excessively high temperatures.

02
regime (upper part), on assuming incident radiation of 10.6 to 12 ^m, and for the four-phonon regime

} (lower part) in the case of photons of 1079 cm . The comparison with experimental results confirms

that, in the three-phonon region, the absorption is near intrinsic and increases exponentially with

temperature as predicted by the model. At 9.27 jam, however, the absorption exhibits a much weaker

dependence than intrinsic mechanisms thus demonstrating that, below 10 (j.m, the bulk of the absorption

in CVD ZnS must be due to phenomena involving defects and/or impurities, which are not easily removed

i through improvements in the deposition procedure.

I

5. Modulation -Transfer Function

From an image -spoiling point of view, the performance of an optical blank is best specified in

j terms of a line -spread function because this information immediately yields the transfer function:

| r^-= '^'|LSFj . The procedure is illustrated in figure 8, which refers to early long -wavelength data

I for a thick (5/8 in.) slab of CVD ZnS, at near-normal incidence. Note that we are showing here single- pass bulk material data as obtained after removal of diffraction, detector, and target-aperture effects on the convolution. The width of the line-spread tracing at the 15 percent points is 47 ^rad and measures

j the size of the window-material blur spot in the sense that, on assuming Gaussian distributions, this spot

contains 84 percent of the transmitted energy [22] . Since this fraction corresponds to the diameter of

j the first dark ring of the Airy disk, 2p = 2.44(X/D), it follows that the "resolution" of CVD ZnS as

I displayed in figure 8 is equivalent to the theoretical diffraction blur of a 20-in. diameter aperture, at an average wavelength of 10 \±m. However, it should be emphasized that Gaussian fits such as

(10 a)

(10 b)

| do not provide an accurate description of CVD-ZnS image -spoiling features at high spatial frequencies

(f > 5 cy/mr)

Actually, recent progress in establishing process conditions leading to optimized CVD material

has resulted in the production of large blanks of ZnS, which exhibit essentially no spoiling. As evidenced

in figure 9, the presence of a 10-in. diameter window at the entrance aperture of an MTF test system

1 does not have any significant effect on the contrast transmittance, even at spatial frequencies as high as

20 cy/mr, which correspond to the optics cutoff of our system. This is in sharp contrast to the situation

j! experienced with large -diameter germanium components, which have been reported to inject additional

losses of some 40 percent in the MTF of long -wavelength thermal imaging devices [23] .

In this context, we have also examined some independent index-variation measurements involving

10.6-|j.m interferometry (see table 6). This typical set of data points to An's in general accord with the refractometry work done at NBS [10] , which indicates that two specimens of CVD ZnS prepared under

quite dissimilar conditions exhibit differences in An of at most 1 x 10"^, throughout the useful wavelength

range. Root-mean-square An's obtained from An "maps" show an interesting pattern in the sense that

the (An)^

v

: ( An) rms

ratios are always close to 4.5, as seems to be the case for many IR transmitting

materials [24] . Wavefront distortions caused by index inhomogeneities are best expressed in terms of

Table 6. Index homogeneity and wavefront distortion

(8-in. diameter, 5/8-in. thick CVD-ZnS disk ) •

Index homogeneity: (An) , p/v

= 1 3 x 10"

(An) rms

= 0 29 x 10 Wavefront distortion: (OPD) , p/v

= 0 20

(OPD) rms

= 0 * 044

Strehl ratio: i

= = 0 92

In wave units at a wavelength of 10.6 |j.m.

93

LSF= exp (-z

2 /2aw

2

) and

, „ 2 2,2.

r = exp (-2tt ^ f )
optical path differences (OPD) across the specimen and are most conveniently measured in wavelength

units, OPD = (t/\) An. In table 6 it is seen that our windows are in fact diffraction limited, both in the sense of Rayleigh [(OPD)^

v

< X./4 ] and in the sense of Marechal [ (OPD) rmg < \/ 14] . Furthermore,

since optical designers often assess the effect of solid windows on the system performance in terms of Strehl ratios,

i = 1 - [ 2ir (OPD) ] 2

1 , (11) rms J

'

v

we may point out that a Strehl of 0.92 (see table 6) should not degrade the MTF by more than a few

percent, at any resolution frequency, and in accord with direct evidence produced in figure 9.

Refractive index variations derived by means of interferometry represent An variations averaged

over the thickness of the optical blank. If one assumes that these "average" An's reflect local, in-plane

variations in density, the connection can be established via the Lorentz -Lorenz law, which states that

[24] the fractional change in density that causes an index inhomogeneity An is given by^

Ap 6 n

p (n-l)(n^+2)

In this light, it would appear that, for n = 2 . 19, we may anticipate a relationship such as Ap /p~ An/2

.

Preliminary indications on density fluctuations in CVD-ZnS material originating from three different

-3 - 5 deposits shows that p = (4 . 0833 5± 0 . 00020) g

* cm or Ap/p = 5xl0 . We conclude that index inhomogeneities should be on the order of 1 x 10" , which is definitely compatible with the bulk of available

evidence on An in CVD ZnS.

6. Conclusion

The optical properties of CVD ZnS are of considerable importance in contemporary IR technology,

and suitable test facilities have been established in order to monitor the production of large -size blanks.

For instance, the significant factors that determine the performance of CVD-ZnS windows in a thermalimaging application include the transmittance, emittance, and image-spoiling characteristics of the window material, in the 8- to 12-|j.m wavelength region. This paper gives an account of work that has

been carried out in this context, particularly with regard to assessing lattice -absorption processes.

• Room -temperature absorption coefficients derived from spectrophotometer tracings, emittance

spectroscopy, and laser calorimetry are in good agreement, throughout the wavelength range

of primary interest.

• Both wavenumber and temperature dependence of the absorption demonstrate that intrinsic multiphonon phenomena control the transmittance/emittance at wavelengths above 10 y.m.

• A total of 16 two-phonon summation bands has been identified at wavenumbers less than 2xLO(r)

= 704 cm"'1

; a critical-point analysis yields zone -boundary phonon frequencies in excellent

accord with dispersion data obtained by other methods.

• Three -phonon processes evidenced at the lattice-absorption edge of massive ZnS deposits evolve

from four (4) major singularities in the density-of -states spectrum; the strong absorption band

at 885 cm -1

, in particular, can be attributed to a summation of three (3) characteristic TO phonons

• Line-spread functions of state-of-the-art CVD-ZnS blanks demonstrate that scattering and other

potential image-spoiling properties of this material do not inject any detectable degradation in system MTF, at spatial frequencies of relevance in contemporary high-resolution thermal imaging

applications

.

• Index-homogeneity maps generated by CO2 -laser interferometry confirm that the transfer function

of full-size CVD-ZnS windows is indeed diffraction limited in the 8- to 12 -^m spectral region.

6. This assumes that the wavelength of interest is sufficiently removed from both absorption edges,

which is indeed the case for ZnS at 10.6 |j.m, and, furthermore, that the material is free of strain

and/or second-phase inhomogeneities

.

94
7. References

[1] diBenedetto, B., Pappis, J., and Kohane,

T., Proc. IRIS 20, 543 (1977). This paper

is unclassified

[2] Klein, C. A., in Proc. 1976 Infrared Imaging

Meeting (ERIM, Ann Arbor), p. 37. This

paper is unclassified.

[3] Lloyd, J. M., Thermal Imaging Systems

(Plenum, New York, 1975), chap. 5.

[4] Reynolds, L. H . , Infrared Phys . 8, 233

(1968). —

[5] Vagelatos, N., Wehe, D., and King, J., J. Chem. Phys._60, 3613 (1974).

[6] Spitzer, W. G., Semiconductors and Semimetals_3_, 17 ( 1967).

[7] Lipson, H.G., Appl. Optics 16, 2902 (1977).

[8] Deutsch, T., in Proc . 1962 Int. Conf. Physics

of Semiconductors (Institute of Physics, London), p. 505.

[9] Nilsen, W. G., Phys. Rev. 182, 838 ( 1969).

[10] Dodge, M. J., in Proc. 1977 Symp. LaserInduced Damage in Optical Materials (USGPO,

Washington), p. 83.

[11] Bendow, B., Lipson, H., and Yukon, S.,

Phys. Rev. B 16, 2684 (1977).

[12] Harrington, J. A., in Proc. 1976 Symp.

Laser-Induced Damage in Optical Materials

(USGPO, Washington), p. 45.

[13] Johnson, F.A., Progress in Semiconductors

9, 181 (1965).

[14] Irwin, J. C., Can. J. Phys. 48, 2477 (1970).

[15] Birman, J. L., Phys. Rev. 13 1, 1489 (1963).

[16] Marshall, R. and Mitra, S., Phys. Rev. 134 A 1019 (1964).

'

[17] Skolnik, L., Lipson, H., Bendow, B., and

Schott, J., Appl. Phys. Letters 25, 442

(1974).

[18] Barker, A. J., Wilkinson, G.R., Massa, N.

E., and Mitra, S.S., in Proc. 1975 Int. Conf

Optical Properties of Highly Transparent Solids

(Plenum, New York), p. 45.

[19] Mitra, S., Postmus, C, and Ferraro, J.,

Phys. Rev. Letters IS, 455 (1967).

[20] Stierwalt, D.L. and Potter, R.F., Semiconductors and Semimetals_3_, 71 ( 1967).

[21] Hatch, S.E., Appl. Optics 1^ 595 (1962).

[22] Dippel, C. E., Proc IRIS 18, 51 (1973). This

paper is unclassified.

[23] Lidwell, M. O., Opt. Actaj22_, 31 7 (1975).

[24] Bennett, H. E., Bethke, J. W., and Norris,

W. P., in Proc. 1977 High Power Laser Optical Components and Component Materials

Meeting (AFML, Dayton/Ohio), p. 328.

CHEMICAL - VAPOR - DEPOSITED ZnS

WAVELENGTH ( Mm)

2.5 3 4 5 6 7 8 10 12 16 25

Figure 1. Spectral transmittance profile for three specimens of chemical-vapor-deposited zinc sulfide

[of differing thickness. Cutoff markers for multiphonon absorption are based on a Raman-phonon

frequency of 352 cm=l.
PBN .78-562 WAVELENGTH ( ^m)

12 14 16 18 20 25 30 3540

CVD Zinc Sulfide t= 0 018 cm

800 600 400

WAVE NUMBER (cm')

Figure 2. Two-phonon absorption region in chemical-vapor-deposited zinc sulfide. Structural

features are identified and assigned to critical-point phonons, in table 2.

PBN- 78-543 WAVELENGTH (^.m) 25 20 15 10 9 8 7

1 1 1 •

1 >1

1 1 I I

CHEMICAL VAPOR DEPOSITED ZnS

* Ia a/

- -v \/y

i/

1 Specimen MT-4 (.007"

1

Z. Specimen MT-4 1.100"

)

3. Specimen DR-3 ! .625")

i :

i

i

3 —

" lul L0

t 1 , 1

2ui L0 3wL0 4<i<L0

1 1 1 T 1 1 1 400 600 800 1000 1200 1400 WAVENUMBER (cm" 1 ) Figure 3. Wavenumber dependence of the absorption

coefficient of chemical-vapor-deposited zinc sulfide

at frequencies corresponding to phonon

multiplicities of 2, 3, and 4. Data points are as obtained from transmittance tracings for three

specimens of increasing thickness.

WAVELENGTH ( jxm)

14 16 18

1400 1200 1000 800 WAVENUMBER (cm"' ) 600

70 as

£ 50

20

WAVELENGTH (^m)

10 II 12

CVD ZINC SULFIDE

thickness. O 762cm

(0.3in)

Figure 4. Three-phonon absorption edge in chemical-vapor-deposited zinc sulfide of engineering

thickness. Assignments reflect combinations of characteristic phonons as obtained from the density-of-states calculation in reference 5.

1300 1200 1100 1000 900 800 700 60i WAVENUMBER ( cm" 1 ) Figure 5. Spectral transmittance profile of chemical -vapor-deposited zinc sulfide at elevated

temperatures. Contributions originating from

sel f-emittance mask features occurring at far-infrared wavelengths.

96
PeH-TT-579

100 P6N- 78-540

g 0 _ CHEMICAL - VAPOR- DEPOSITED ZnS

Deposition Temperature: 650 °C

Motor Ratio" - 1/2

Spec«rien Trwchness 170cm

-i 1

1

1

1 1 r • CALORIMETRY I 10.6pm)

k EMITTANCE(ll.5/im) T TRANSMITTANCE (IZ/tml

~1

1 <~

WAVELENGTH ( f±m)

Figure 6. Sepctral emittance of chemi cal -vapordeposited zinc sulfide. Note the temperaturedependence behavior in the 8- to 12-um wavelength

region.

300 400 500 600 TEMPERATURE (K ) PBN-77-95

~\—i

1—i—

WINDOW TRANSFER FUNCTION

— Goussian Fit + As Recorded

0.8

-0.6

04

-0.2

Figure 7. Temperature dependence of the absorption coefficient of chemicalvapor-deposited zinc sulfide, in the 8- to 12-um region. Calculated dependencies

are as derived from a model involving a fixed number of "average" phonons.

30 o 30

IMAGE WIDTH (/ir) 0 2.5 5.0 7.5 10 SPATIAL FREQUENCY (cy/mr)

Figure 8. Image-spoiling characteristics of chemical-vapor-deposited zinc sulfide. The data

refer to long-wavelength measurements on a full-size

window and point to non-Gaussian distributions at high spatial frequencies.

PBN-78-542

MODULATION TRANSFER FUNCTION

Spectral Bond: 8- 12Mm

Test Apparatus Only (7- in aperture)

With 10-in. Diameter CVD-ZnS Window

$ 60

8 12 FREQUENCY (cy/mr)

Figure 9. Contrast transmittance as a function of spatial frequency in the presence and the absence of

a chemical -vapor-deposited zinc sulfide window at the entrance aperture of the test system. There is

no significant effect on the modulation-transfer function.

97
Discussion

It was pointed out that low temperature measurements of the absorption spectrum would be useful

in identifying which phonon processes were dominant in the absorption process.

The speaker was also asked if CVD samples of ZnS could be prepared which would be a free of

absorption in the visible ("water-white" ) as the best conventionally prepared samples. He indicated

that the preparation of such samples was planned but had not yet been carried out.

98
LASER INDUCED DAMAGE

IN FLUORIDE GLASSES: A STATUS REPORT*

S. E. Stokowski, D. Milam, M. J. Weber

University of California

Lawrence Livermore Laboratory

Livermore, California 94550

Bulk and surface damage thresholds of f 1 uorophosphate and f 1 uoroberyl 1 ate glasses

have been measured for 1-ns 1064-nm laser pulses. For crystal-free samples, threshold

values for fluoride glasses are comparable to those measured in optical quality fused silica and borosilicate glasses. However, for many samples investigated the presence

of solid inclusions results in low damage thresholds. Some examples of solid inclusions

are shown and discussed. Surface damage thresholds in FK-51 fl uorophosphate glass are uniformly high and exhibit an approximate square root dependence on pulse duration in the range of 0.1-1 .0 ns. Key words: Damage thresholds; fl uoroberyl 1 ate glass; fl uorophosphate glass; inclusions;

pulse duration.

1 . Introduction

Fluoride glasses have the smallest linear and nonlinear refractive indices of optical glasses. These glasses can therefore be used for optical components in high power laser systems, such as the proposed Nova laser at LLL, to minimize beam self-focusing [1]. Components include i) transmitting

elements such as lenses, windows, and substrates and ii) active elements such as rare-earth doped

amplifier glass and Faraday rotator materials. Because of their large transparency range—from

approximately 160 nm (BeF2) in the ultraviolet to 4000 nm in the infrared—fluoride glasses are potentially useful for many laser applications, particular in the UV.

Low-index fluoride glasses are also of interest to test empirical scaling laws for the dependence

of laser damage on refractive index [2]. Damage threshold data for a large class of transparent

dielectric materials indicate that damage thresholds are inversely proportional to the index of refraction [3,4]. Because of their low indices, fluoride glasses, pure BeF2 in particular, have promise as very damage resistant materials.

Fluoride glasses include i) pure fluoride glasses based upon BeF2 or ZrF^ as the glass network

former and ii) oxyfluoride glasses, such as fl uorophosphate, fl uorosi 1 icate, and fl uoroborates

Development of fluoride glasses has been rapid over the past two years. For example, Nd-doped

fl uorophosphate glass is now melted and cast routinely in sizes up to 40 X 80 X 8 cm, and fluoroberyllate glass has been cast in 20-cm diameter by 3-cm thick disks. Here we report the first measurements of the optical damage thresholds of these glasses for 1-ns 1064-nm laser pulses.

A primary motivation for developing fluoride glasses was to obtain materials with lower nonlinear

refractive indices. The intensity-dependent contribution to the refractive index of a material is given by yl, where y is the nonlinear refractive index coefficient. It has been found that low

index, low dispersion materials also have small y values [5]. An empirical expression [6] relating y

to the linear index n^ and Abbe number v[(n^ - l)/(np - n c

)] has been successfully employed in predicting y values. Computed lines of constant y are plotted as a function of n n and v in figure 1. 1 H

General regions of oxide and fluoride optical glasses are indicated in figure 1. Because the

principal contribution to the refractive index in common silicate glasses arises from oxygen and polarizable, high atomic number cations, the replacement of oxygen by fluorine results in lower

linear and nonlinear refractive indices. The lowest index glasses are those with beryllium fluoride

as the glass network former. The measured y value of pure BeF2 was, as predicted by the empirical

expression, the lowest yet found for any solid [7]. The glasses studied in this investigation are

located in the clear areas labeled fl uorophosphate and fl uoroberyl! ate.

Until recently, fl uorophosphates such as Schott's FK-51 and FK-52, and similar glasses made by other companies, had the lowest refractive index of commercially available optical glasses. These

are designated as Group I in Table 1 which gives manufacturers, linear indices, and Abbe numbers.

Optical and Nd-doped fl uorophosphates with smaller oxygen content and lower indices have now been

*Work performed under the auspices of the U.S. Department of Energy under contract No. W-7405-ENG-48

and the Materials Sciences Program.

99
produced. These new glasses, designated f 1 uorophosphates II, are also listed in Table 1. The y-values

of f 1 uorophosphate II glasses are 2-3 times less than for silicate laser glasses. For reference, the corresponding properties of two standard optical materials, BK-7 borosilicate glass and fused silica,

are included in Table 1.

The design of the Nova Nd:glass laser at LLL calls for damage thresholds of >25 J/cm2 on bare surfaces at normal incidence for 1-ns pulse durations. Operation at other pulse durations in the range 0.1-10 ns is also contemplated. Glasses such as FK-51 have demonstrated damage resistance

comparable to BK-7 and fused silica, which is adequate for the above design. Our interest is in the newer, lower index fl uorophosphate II glasses and the beryllium fluoride glasses.

2. Samples

Samples consisted of commercial BK-7 glass, optical grade fused silica, and FK-51 glass; the remaining glasses in Table 1 were from experimental melts by the sources indicated. The general

compositions of the fluoride glasses are given in Table 2. The principal differences between the Group I and Group II fl uorophosphate compositions are a reduction of the oxygen/fluoride ratio and

an increase in the proportion of low atomic number cations, both of which result in a lowering of n n and yThe fl uorophosphate samples in most cases were polished using cerium oxide and standard finishing

techniques. However, for surface damage measurements some samples were polished with ZnO, which

produces excellent surfaces on fl uorophosphate glasses [8]. The beryllium fluoride glasses were

rough polished with 0.5 urn diamond powder and ethylene glycol on a pellon pad. The final polish was achieved using tin oxide and ethylene glycol on a lap made of a 1:1 mixture of 64 and 73 Gulgoz

pitch. With good temperature control, satisfactory optical polish can be obtained. High-purity

cerium oxide was also tried, but was too hard and did not yield a good surface. Finished samples

had areas >1 cm

2

.

The fl uorophosphate samples were obtained from both small (<1 liter) and large (10-20 liter) melts

poured from platinum crucibles. The fl uoroberyll ate glass was melted in a graphite crucible-mold.

Beryllium fluoride glass from UCLA was vacuum distilled to improve purity and to remove water. Because

of the hygroscopic nature of BeF2, the surfaces of this material were susceptible to water contamination. The other fl uoroberyll ate glasses, however, have satisfactory chemical durability in normal atmospheres

.

3. Measurement Techniques

Damage measurements were made using procedures described in detail previously [9,10]. One-ns 1064-nm pulses were obtained by gating out a portion of a 30-ns Q-switched Nd:YAG laser pulse. Pulses were subsequently ampl ified in a series of Nd:glass rod amplifiers. A collimated beam of 2-3 mm

diameter (e" 1 ) was incident on the samples. The spatial profile of the beam at the sample was recorded

for each shot using a vidicon and video disk recorder. Absolute flux was computed by an on-line

data reduction system [11]. The waveform of each pulse was recorded to insure constancy of pulse

shape. All measurements were made at room temperature.

Bulk damage was detected by observation of induced light emission during irradiation, and by

permanent alteration of bulk scattering levels. Samples with high bulk damage thresholds were examined

by Nomarski microscopy to determine surface damage thresholds.

Samples were initially irradiated at flux levels of 4-6 J/cm 2

. If damage was induced on the first

firing at this level, the test was discontinued except in instances where a parameter study such as water content or scattering was involved. Therefore, thresholds were not actually measured for many

samples; it was simply determined that the threshold was less than some small value. If the sample

failed to damage at 4-6 J/cm2

, the test was continued until a threshold was determined.

4. Results

4.1. Damage Threshold

Threshold data for bulk and surface damage are summarized in Table 3. For BK-7, Si^, and

fl uorophosphate I the results represent typical values; for tne fl uorophosphate II, BeF^,, and

fl uoroberyll ate glasses, the best values measured to date are given. For the latter group of fluoride

glasses, large variations in bulk damage thresholds were seen. For example, the flux levels at which

bulk damage is observed in fl uorophosphate II glasses, plotted in figure 2, range over a factor of ten.

inn
Bulk damage in the fluoride glasses is evidenced by the presence of tracks in the region exposed to the laser beams. These tracks for a fl uorophosphate glass are illustrated in figure 3 and are caused

by scattering from a large number of spherical inclusions. A photomicrograph of a damaged center is shown in figure 4. The center is an =2 ym sphere with some evidence of surrounding glass fracture. Identification of the center has not been made as yet because of the difficulty in isolating it on

the surface for X-ray analysis. The refractive index of these centers is lower than the glass matrix,

however [12]. This observation suggests that they are gas or vacuum bubbles resulting from heating

of the glass by laser energy absorbed at microscopic centers.

In silicate laser glasses platinum inclusions were a severe damage problem [13]. However,

employing the techniques of X-ray fluorescence and emission spectroscopy, little evidence of Pt in fluorophosphate glass has been seen. Analyzed levels of Pt in the fl uorophosphate glasses are <1 ppm;

one analysis gave <70 ppb [14].

In early samples of fl uoroberyl late glasses the presence of seeds, solid inclusions (including

graphite particles), and high stress fields resulted in low damage thresholds of %7 J/cm 2. The use

of vitreous carbon crucibles and better melting techniques has yielded excellent fl uoroberyl 1 ate

glasses with no inclusions visible by optical microscopy and the high thresholds reported in Table 3.

In the case of glassy BeF2> the presence of transition metal impurities and poor surface quality due

to the hygroscopic nature of this material contributed to low damage threshold; this is not believed

to be indicative of the "intrinsic" limits for this material.

Several of the fluorophosphate and beryllium fluoride glasses were examined by optical and electron microscopy for possible damage inducing defects [15]. Some examples of observed seeds

' (bubbles), and crystals are shown in figure 5. Star-shaped crystallites were common and in some

cases were high in Ca and low in P. Variation in the composition of the crystallites has been

, observed, thus different crystals have grown in the glass. Some needle-shape voids are present in the vicinity of crystal growth; this can cause considerable enhancement of the optical field, thereby

leading to damage at low intensities. In addition, the crystal -gl ass interface is a region of compositional gradient or discontinuity. It may contain impurities, which along with associated

voids and stress fields, are deleterious for damage.

At this time we do not know what impurities and/or melt conditions nucleate the crystal growth.

The effect of water content in the glass, as monitored by IR absorption, on the bulk damage thresholds

values of fluorophosphate II glasses was checked but no correlation was found. Identification of the crystal composition would aid in finding approaches to eliminate bulk damage.

5. Pulse Width Dependence

The Nova laser will be operated in a pulse width range of 0.1 to 10 ns, therefore it is of interest

| to know the pulse dependence of surface damage thresholds in fluorophosphate glass. This data was obtained on FK-51 (fluorophosphate I) and is compared with data on fused silica [16] in figure 6. The

results indicate that in the region 0.1-1.0 ns, high quality fluorophosphate glass exhibits the approximate square root of pulse duration dependence of damage threshold found for the other optical

glasses [2].

6. Conclusions

1. Fluorophosphate and fl uoroberyllate glasses with high damage thresholds sufficient for current

fusion laser applications have been observed.

2. Reproducible preparation of large melts of inclusion-free damage-resistant glasses is needed

I to render these glasses useful for high power laser applications. A bulk damage problem was encountered

! in fluorophosphate I glasses and solved [17] by a suitable modification of melting techniques. A

similar effort is underway for the newer fluoride glasses.

3. The internal quality and surface finishes of the present fluoride glass samples are ini sufficient to test the validity of the empirical rule that low index materials should have high

damage thresholds.

4. Damage threshold data as a function of pulse duration data indicates that high-quality

fluorophosphate glasses exhibit an approximate square root of pulse duration dependence in the 0.1-1.0

ns time domain.

7. Acknowledgments

We thank the following people for providing test samples: Fluorophosphate glasses - K. Mader,

N. Neuroth - Schott Optical Glass, Inc.; G. Dube" and P. Vergano, - Owens Illinois, Inc., Inc.;

T. Izumitani, D. Segawa - Hoya Optics, U.S.A. Beryllium fluoride glasses - D. Morgan, Corning Glass

Works, Inc.; C. Baldwin - University of California, Los Angeles; C. Cline, D. Kingman - Lawrence

101
Liver-more Laboratory. We are grateful to R. Maney for polishing the beryllium fluoride glasses and to

R. Landingham for optical and electron microscopy. Helpful discussions with W. Haller of the National Bureau of Standards are also gratefully acknowledged.

8. References

[1] S. E. Stokowski, R. A. Saroyan, M. J. Weber, [8] and W. F. Hagen, Proc. Electro-Optics/Laser

77 Conference, Anaheim, CA, October 1977

(Industrial and Scientific Conference

Management, Inc., Chicago) p. 29. [9]

[2] J. R. Bettis and A. H. Guenther, "Scaling [10] Laws for Damage" in Optical Materials for

High Power Lasers (Academic Press) - to [11]

be published.

[3] J. R. Bettis, A. H. Guenther, and A. J. Glass, [12] "Laser Induced Damage in Optical Materials;

1974," NBS Special Publication 414, U. S. [13] Government Printing Office, Washington, D.C. (1974), A. J. Glass and A. H. Guenther,

editors, p. 214. [14]

[4] J. R. Bettis, R. A. House, A. H. Guenther, [15] and R. R. Austin, "Laser Induced Damage

Optical Materials. 1975," NBS Soecial [16]

Publication 435, U. S. Government Printing

Office, Washington, D. C. (1975), A. J. Glass

and A. H. Guenther, editors, p. 289. [17]

[5] For a review see M. J. Weber, D. Milam and [18]

W. L. Smith, Opt. Engineering, 1_7, 463 (1978) and references cited therein.

[6] N. Boling, A. J. Glass, and A. Owyoung, IEEE

J. Quantum Electronics, QE-14 , 601 (1978).

[7] M. J. Weber, C. F. Cline, W. L. Smith,

D. Milam, D. Heiman, R. W. Hellworth, Appl

.

Phys. Letter 32, 403 (1978).

Table 1. Refractive index and Abbe number of representative glass types included in this study.

N. J. Brown and R. T. Maney, "Optical Fab- rication of Fl uorophosphate Laser Glasses,"

LLL Report No. UCRL-81081 (1978).

D. Milam, Appl. Opt. 1_6, 1204 (1977).

D. Milam, Proc. SPIE Technical Symposium East

W. L. Smith, A. J. DeGroot and M. J. Weber,

Appl . Optics (in press)

.

W. Haller, private communication.

Hopper, R. W. and Uhlmann, D. R., J. Appl.

Physics, 4J_, 4023 (1970).

P. Vergano, private communication.

R. Landingham, private communication.

D. Milam, Laser Program Annual Report-1976, Lawrence Livermore Laboratory, UCRL-50021 -76

K. Mader, private communication.

0. Deutschblein, M. Faulstich, !!. -latin, G. Krolla, and N. Neuroth, Appl. Opt. 17, 228 (1978). —

Glass Type Source

D

Borosil icate

Fused silica

Fl uorophosphate

Group I

Group II

Beryllium Fluoride

BeF

2 K-Ca-Al-BeF

2 Al-Mg-Ca-Ba-BeF

2

BK-7

4000

FK-51 , LG-800

E-123

FK-54, LG-812 LHG-10

E-309

Schott

Dynasi

1

Schott

Owens Illinois

Schott

Hoya Owens-Ill inoi s

UCLA, LLL Corning

Corning

1 .517

1 .459

1 .487

1 .485

1 .431

1 .459

1 .436

1 .275

1 .341

1 .38

64

68

84

93

90

90

107 104

-100

102
Table 2. Approximate compositions of fluoride glasses in mol . %.

Compound Fl uorophosphate I Fl uorophosphate II F1 uoroberyllate

A1F

3 20 32 10 LiF 12 NaF 1 12

KFMgF

2

7

/ QO CaF

2 16 27 16 SrF

2 23 8 BaF

2 12 8 A1(P0

3

) 3

7 5

A1

2 0

3

2 BeF

2 47

Table 3. Observed damage thresholds for 1.0-ns 1.064-nm laser pulses incident at 10° from normal.

Damage Threshold - J/cm 2

Material Bulk Rear Surface Comments

Borosil icate BK-7 >40 20 PH-4 quality

Fused silica >40 22 Optical grade

Fluorophosphate I FK-51 >20 20 ± 3

Fl uorophosphate I LG-802 >20 13 40 J/cm2 at 40 ns (ref. 18)

Fluorophosphate II E-309 >25 16 + 2 Small melt

Glassy BeF

2

6 ± 1 Impurities; hygroscopic

Fl uoroberyl 1 ate >25 12 - 15 Free of solid inclusions; poor surface finish

103
Figure 1. Refractive index, Abbe number, and nonlinear refractive index coefficients of known

optical glasses. The fluoride glasses investigated have values in the clear areas indicated.

MEASURED BULK DAMAGE THRESHOLDS OF FLUOROPHOSPHATE

GLASSES FOR 1.0 ns 1064 nm LASER PULSES [

2

o

30

25

20 -

£ 15 -

E

"° 10

3

00

t—i—|—i—i

1—i

1—|—i1—|—

r

"i

1

1—i—

r

1

I

T

1 I I I I I I I I I I I I I I I I I I L

50-50-0978-3091

6 9 12

Sample number

15 18 21

Figure 2. Record of the bulk damage thresholds for Group II type fl uorophosphate measured with

1.0-ns 1064-nm laser pulses.

10A
Figure 3. Laser-induced damage tracks in f 1 uorophosphate glass.

OPTICAL MICROGRAPH OF A DAMAGED CENTER IN

FLUOROPHOSPHATE GLASS ig

2.5 jum

Figure 4. Photomicrograph of a damage center in fl uorophosphate glass. The center is -2.5 urn in diameter and shows evidence of glass fracture around it. The glass was subjected to a 5 J/cm2 1-ns 1064-nm pulse.

105
Figure 5. Optical and SEM micrographs of crystals in f 1 uorophosphate glass.

a. Interior of a 1-mm diameter polycrystall ine region.

b. Crystalline regions as detected by SEM on a freshly fractured surface.

106
2-3 ym sized crystal at the surface of a highly-scattering piece of fl uorophosphate

glass.

d. Ellipsoidal inclusion (-15 x 30 ym) examined by SEM; crystals are evident.

107
DAMAGE THRESHOLDS OF CONVENTIONALLY POLISHED BARE

SURFACES

1000 f 1—I

I I I

2

o

03

£

T3

100

10

1.0

"I I I Mill] 1

1 I II I 11

1

1

1 I I I I I I

I I I 1 1 1

1

^ Fused silica

FK-51

J i l I I I I ll J i i i i ml I i i i i i i 0.01

02-05-0378-0726

0.1 1.0 Pulse duration.10 100

ns

Figure 6. Damage thresholds of conventionally polished base surfaces as a function of 1064- laser pulse duration. The dashed line corresponds to a square root of pulse duration dependence

of the damage threshold.

Discussion

The speaker was asked if the polishing compound used on fluorophosphate glass led to a reduction in surface damage threshold . He replied that the observed thresholds were comparable

those seen in BK-7 surfaces .

103
LIQUIDS FOR HIGH REPETITION RATE GLASS LASER SYSTEMS

James M. Rinefierd, Stephen D. Jacobs, David C. Brown,

Joseph A. Abate, Owen Lewis, and Henry Appelbaum

Laboratory for Laser Energetics

University of Rochester

Rochester, New York 14623

We present physical properties data for a number of liquids currently being used for cooling and/or index matching of components in high peak power, low repetition rate Nd: glass laser systems. Liquids examined include saturated water solutions of ZnCl g and

ZnBrg, TCP, benzene, DMSO, glycerol, ethylene glycol, FC-104, Dow-Corning 710, and Cargille

5040 and 5610. Thermal properties (conductivity, diffusivity) , mechanical properties (viscosity, density), chemical properties (pH, materials compatibility), and optical properties

(refractive index, Abbe value, nonlinear index, dn/dT, optical transparency) are tabulated

and compared to projected liquid properties requirements for high repetition rate systems.

Key words: Abbe value; coolants, dn/dT; index matching liquids; Neodymium glass lasers; nonlinear refractive index; thermal conductivity; viscosity.

1. Introduction

A variety of liquids serve as important components of current moderate repetition rate glass laser

systems. Specifically, these liquids find application as coolants and as index matching mediums in amplifiers and electro-optic isolation systems. In addition, future systems may have other uses for liquids such as in liquid electrodes for electro-optic devices.

It is the intent of this paper to provide data on the physical properties which are important for the selection of the most appropriate liquid for a given application. In what follows we identify the materials currently in use or proposed for use in large glass laser systems, and the physical properties

of interest. We then give a description of the various measurment techniques utilized, followed by a tabulation of data. Finally, a discussion of results including advantages/disadvantages of certain

liquids in particular applications is presented.

2. Liquids

A dozen liquids were chosen for inclusion in this study,

facturer, grade, lot number, and use in table 1. These liquids are identified by manuLiquid

Distilled H

2 0 Ethylene glycol

Tricresyl phosphate

Glycerol

Dimethyl sulfoxide

Benzene

ZnBr 9 (aq)

DC-710

C5040

C5610

FC-104

Table 1. Liquids of interest

Manufacturer Grade/Lot#

Mai 1 inckrodt

J. T. Baker

Ma 11 i nckrodt

Eastman Fisher

Cargille

Dow-Corning

Cargi 1 1

Cargi 1 1

3M

AR/EGK

515303 AR/WRDN

Reagent/B44

AR/714259

optical/551

BFC345

"ND 1.495 (3 10,600R"

"ND 1.495 @ 10,600fl" 526

Descri ption

Coolant

Coolant

Index match

Index match

Index match

Index match

Index match

Index match

solid ZnCl 0

- from mixed cresol

- prepared @ LLE with Fisher

AR/746575

Index match - silicone oil Index match - immersion liquid, hydrogenated terphenyl and mineral oil Index match - immersion liquid,

silicone oil Fluorochemical flashtube coolant

ZnBr2 anc' ^n ^2 are both highly acidic and corrosive; they do however exhibit stability under intense

UV flashlamp irradiation. The Cargille liquids 5040 and 5610 can be obtained from the manufacturer

109
mixed to a desired refractive index of between 1.46 and 1.56. These Cargille liquids possess an extremely low water content, and are resistant to high voltage breakdown, making them compatable with

KD*P in Pockels cell devices. Liquids such as benzene, tricresyl phosphate, and dimethyl sulfoxide

pose health hazards [l]

1

.

3. Measurements and apparatus

The physical properties discussed in this paper fall into four categories: optical, thermal,

thermo-optic, and mechanical. Specific properties studied include the following: refractive index

at A = 1.064pm, 0.6328um, 0. 6563pm(n

c

)

, 0. 5876pm(n

d

) , 0.4861pm(n

F

)

; Abbe value v

d

; spectral trans- mittance from 0.2pm to 1.4pm; absorption coefficients at a variety of wavelengths; a calculated value

for r\r, the nonlinear refractive index; the temperature coefficient of refractive index dn/dT; thermal conductivity, density and viscosity. Table 2 gives a listing of measurement apparatus and experimental precision obtained for the above.

Measurement

Refractive index

Abbe value, v.

Table 2. Measurement apparatus and precision

Apparatus/Technique

Hi 1 ger-Chance Ref ractometer [2]

Nonlinear refractive index, n.

Absorption coefficient a

Spectral Transmi ttance

Temp, coeff. of ref. index, dn/dT

Thermal conductivity, k @ 25°C Density, p @ 38°C Viscosity, kinematic, v @ 21°C Viscosity, absolute, V 0 38°C

Cary 14 spectrophotometer

Minimum angle of deviation

Thermal comparator [3] Gay-Lussac pyncometers

Cannon-Fenske viscometers ASTM sizes 25, 100, 200, 450

Precision

±.0005 ±7% low Abbe values

±50% high Abbe values ±8% large values ±50% small values n£ ±.015 cm

-1

±5% ±3% ±.003 gm/ml

v, ±5%

V, ±3%

Refractive index is determined using a Hi 1 ger-Chance ref ractometer [2], constructed of BK-7 and calibrated with Hoya BSC-7 glass. The device has a range of 1.25-2.0. Yag, HeNe, and dye laser sources are used to provide the wavelengths necessary to generate the index data.

The Abbe value, is calculated from the standard formula,

n d

" 1

n

p

- n

c (1)

Figure 1 gives a plot of nd versus v^.

The electronic part of the nonlinear refractive index, n^ , which causes self-focusing of optical beams [4], is calculated using v^, n d and the following relationship:

e nn-13 , 68(n

d

- l)(n/ + 2)

2

{ n

? (10 esu) = = ^=i rp J (2) v

d { 1.517 + [(„/ + 2)(n

d + Dvd]/6n

d }

2

Figure 2 gives a plot of n

d versus r\2~

A Cary spectrophotometer provides the spectral transmi ttance of each liquid from 0.2pm to 1.4um. Absorption coefficient data is similarly obtained from this instrument. For spectral scans the samples are contained in a quartz cell with an optical pathlength of 3.7 cm. Measurements are made at

a scanning rate of 50A/sec. These scans are presented in figures 3-8. Absorption coefficients have been calculated for five wavelengths (1.34pm, 1.064pm, 1.054pm, 0.6328pm, 0.3550pm) using the relationship

Figures in brackets indicate the literature references at the end of this paper.

110
(optical density) + 2 log^d-Rj) + 2 log

1Q

(l-R

2

) a

cm

-i = (3) L(1og

1Q

e)

where L = length of cell = 1.0 cm

2

R, = reflectivity - quartz/air interface = (

c

1 uartz

+ J \ quartz /

2

R

2 = reflectivity - quartz/liquid interface =

( q uartz ~ liqmdj \

n quartz "liquid/

n = refractive index at appropriate wavelength.

The temperature coefficient of refractive index, dn/dT, is measured via a minimum angle of deviation technique at A = 1.06um near room temperature. The liquid is placed in a hollow glass prism

(vertex angle 30°50'); the deflection of a laser beam passing through the liquid at minimum angle of deviation is determined as the liquid temperature is changed from 29°C to 23°C. Temperature is moni- tored by a thermocouple placed within a capillary tube inside the prism. The value of dn/dT is calculated from the following equation

dn _ cos %{a + 6) d6_ - m dT- " 2 sin h a dT ^'

where a = calibrated angle of prism = 30°50'

9 = 2 sin" (h, g,^ sin a/ 2) - a

d8 _ (beam displacement in mm)

dT (path length in mm) (At) At = temperature change in °C

Beam displacements are typically 10-20 mm for path lengths of 15-18 meters.

Thermal conductivity, important for heat flow calculations, is measured at room temperature for each liquid with a thermal comparator [3]. This instrument records the rate of cooling of the tip of

a probe heated to 9°C above ambient upon contact with the teflon membrane of a liquid sample cell. A thermocouple pair responds to the temperature change by generating a microvolt signal which can be converted to thermal conductivity by calibration with reference standards. Figure 9 gives a thermal conductivity plot established with such standards. Thermal conductivity values for the samples are determined from this calibration curve with the microvolt signals obtained.

Density at 38°C is determined per ASTM D369-67 using Gay-Lussac type pyncometers. The filled

pyncometers are heated to 38°C±.1°C in a water bath whose temperature is controlled using a Thermomix

1460 [6]. Mass determinations are made with a Sartorius 2474 analytical balance [7]. The volume of the pyncometers is obtained using a standard value for the density of F^O at 38°C [8].

Viscosity at 21°C and 38°C is determined using Cannon-Fenske type viscometers. Each viscometer

comes with a factory calibration at 38°C and is experimentally calibrated at 21°C. Temperature is maintained at 38°C using the Thermomix and at 21°C by using a large water bath at room temperature.

The kinematic viscosity, v in centistokes, is found as v = Ct where C is the calibration constant of

the viscometer and t is the efflux time of the travel of the liquid meniscus under the pull of gravity

between two etched marks on a capillary tube. Absolute viscosity, V in centipoise, is expressed as

V = vp where p is the density of the liquid at the appropriate temperature. The absolute viscosity

is determined at 38°C using density values measured at 38°C and the kinematic viscosity found with the viscometer calibration at this temperature. Kinematic viscosity is determined at 21°C only, because

no density data was determined at this temperature.

4. Results

The data generated in this investigation are presented in three different forms. Table 3 collects

numerical data on the physical properties of interest. Figures 1, 2, and 9 present graphs of n d versus

v

d'

n

d versus n 2- anc' thermal conductivity versus thermal comparator signal respectively. Figures 3-8 give spectal transmittance scans of the liquids.

Measured values of refractive index range from approximately 1.56 (TCP) to 1.27 (FC-104). The

Hi 1 ger-Chance technique results in measured values of refractive index in close agreement with published values (generally ±.001). Table 4 lists a number of liquids and compares our data with

published refractive index values.

Ill
cm co ro lo M W W

lo lo lo ro lo

co lo lo

lt> lo Ln lo ro

lo LO

cn o lo lo lo ro

CJ> c

•i- o

I C i-l 3 S- l-» o o Q <_>

co cn ro

LO LO LO ro

cm CO ro CO >3- LO LO LO LO LO

co "—' cm lo r—

«

rO LO LO LO LO LO

LO LO LO LO LO rO

CO CM ro

cn O <—I <—

I

«3- LO LO CO

ro ro CO .—

I

i—

i

1cn CM CM CM ro o O o O OJ

CM

m rn r-~ -—

I

^~

(XI CM CM CM CM O O o O ro ro

ro o LO CO CO CM o o o O CM

CO 1—

1

CM o r-. CM o cn o CM o CM o O -—

i

ro

i—

i

CM 00 co r-- LO

LO CM CM .—

i

ro o O o o

LO

cn

1—

1

00

CTi 00 CO LO o LO CM LO

cn cn o O r—

i-H CM CO

CM

CM LO LO o CM 1—

1

LO CM <* 00 LO

cu > E —

•i- :3 Q OO

LO LO LO LO CO CM f-.

o LO CM CM

«3" O O LO >*

CM O O

o

1/1

o.

i— 0) >,-!-> lo to cui.O LO

•i- o

1— o-

<u o

r— O

>> >>

LO LO LO ro

cm ro ro ro ro cm

ro ro ro

CO ro ro

ro ro

LOro

LO LO LO

r< r< ^ .-<

*

LO LO CM --t O O

LO CM CM

(Tt CO CM

<D XJ i-H

-O .— T"

LO LO CM LO

CO LO LO O CM i-H CM

ro o LO

i ro

1—

1

CO

r-^ CM o CM 00

LO

CM O LO

1 i-H

1—

1

CO

1 1 CM LO cn cn 00

cn LO

CM

1 cn

cn

OIL0 CO o 0 E o_ o

>,ro +-> CM

-t-> 1 E c

0J o oi CU ca

5- o > (J o o >>

4-> o " o +j c

H- —

o o r_> o

L|- "3" rj 1 CO o to LO

01 1 "O o ro o cu LO o o c cu 00 u LO U i—

I

o lo <2j CO CO o

o 1 +j £ E <3j > CO to O- X O i— E E cu ro \ ro >> >> u 4-> ro E r— (J +-> +-> c S- 1— C i— s- ro -r- +-> OJ +-> •i- ro cu CO LO rO o

4-> E -sz ro c o E

-o S- +-> .c CU (_> (U cn \ cu o Q c cn C -£= cu A

T3 r— >z Q. > •5 *

112
Table 4. Refractive index measurements compared to literature values

Liquid Measured values Values from other sources

H

2 0 n r

= C

1 333 n,~C

1 331 [9]

n d

= 1 334 n D = 1 333 [9]

n

F = 1 338 n

F = 1 337 [9]

n

D = 1 333 [10]

Ethylene glycol n d

= 1 432 n D = 1 432 [10]

n D

= 1 429 [8]

Tricresyl phosphate nd

= 1 556 n D

1 556 [9]

Benzene- n d

= 1 502 n D

1 501 [9]

n D

1 501 [10]

n D

1 498 [8]

Glycerol n d

= 1 474 n D

1 473 [9]

FC-104 n 1.06

= L271

n

c

= 1.272

.6328 1.273

DC-710

n d

= 1.273

n , = 1.533

1.271, X unspecified [11]

1.533, A unspecified [12]

The Abbe value which was calculated using refractive index data requires some explanation. It is seen that large Abbe values have errors of approximately ±50% while small Abbe values have calculated

errors of ±7%. Because a high Abbe value indicates low dispersion while a low Abbe value indicates

large dispersion; it follows that for low dispersion as seen in FC-104 and water, the limit of np-n^ as np+n^ = 0. Hence, is large and any small change in np, n^, or n^ can have a large effect

'! on v

d

.

The absorption coefficients and the spectral transmi ttance scans indicate wavelength regions in which transmission through thick samples of certain liquids would not be advisable due to strong

absorption losses. Liquids exhibiting the strongest IR absorption are water, aqueous ZnC^ and ZnB^,

ethylene glycol and glycerol. All the test liquids are highly transparent in the visible. Differences

in absorption coefficients at .3550um can be attributed mainly to the effect of the UV edge for the different liquids as indicated in table 3. The importance of the UV edge in relation to flashlamp

degradation will be discussed later. It is noteworthy that F^O and ethylene glycol which absorb in

in the IR near 1.06um also have the lowest UV edges (with FC-104) of the liquids tested.

-4 The temperature coefficient of refractive index for water, 1.34 x 10 /°C is clearly substantially

lower than dn/dT for any other liquid tested. The measured dn/dT values for liquids are approximately

2 10 times larger in magnitude than dn/dT values for glasses at 1.06um [13]. All the liquids exhibit

negative dn/dT values in that the refractive index decreases as the liquid temperature increases. From

a product information sheet on Cargille immersion liquids [14], dn/dT values for 15-35°C average

-3.3 x 10" 4/°C which are close to measured values for C5040 and C5610 of -3.6 x 10 7°C and -4

-3.1 x 10 /°C respectively.

_3 Thermal conductivity values fall between 1.45 cal/cm-sec-°C x 10 for water to approximately

.25 cal/cm-sec-°C x 10" 3 for TCP, DC-710, C5040, C5610 and FC-104. The plot in figure 9 of absolute

signal versus thermal conductivity is a least squares fit of the data for calibration standards CC1.,

2 toluene, ethanol , methanol, and ^0 [3]. This plot has a coefficient of determination r = .96. Little

conductivity data is available for liquids other than the calibration standards. Product literature

gives conductivity data for FC-104 and DC-710. FC-104 has a reported conductivity of .15 cal/cm-sec-°C

-3 -3

x 10 [11] versus a measured value of .23 cal /cm-sec-°C x 10 . For DC-710 the published value is .35 cal/cm-sec-°C x 10" 3 [12]; the measured value is .24 cal/cm-sec-°C x 10" 3

. For these two liquids

113
the percentage difference between published and measured values is large. It should be noted however,

that ZnBr£ and ethylene glycol were measured at two different times using the same technique with

resultant conductivity values within the stated precision of ±3%.

Measured viscosity and density exhibit a wide range of values. Viscosity is low for ^0, benzene, FC-104 and DMSO; it is two orders of magnitude higher for DC-710 and glycerol. Density values range from approximately .9 gm/ml for benzene and C5040 to 2.4 for ZnB^. It is interesting to note that benzene and FC-104 have kinematic viscosities which are practically identical but density differences

result in the absolute viscosity of FC-104 being twice that of benzene at 38°C.

There is generally good agreement between measured and published values of viscosity and density. Table 5 compares those values for water, benzene, glycerol, ethylene glycol, FC-104 and DC-710.

Table 5. Measured values of density and viscosity compared to other sources

Liquid Measured values Data from other sources

Water V 38

= .69cp Poo = 38 .99299 gm/ml [8]

V 21

= .98cs V 38= .678cp [8]

V 38

= .681cp [9]

v * V 21

= .980cs [8]

V 21* = .983cs [9]

Benzene p 38.859 gm/ml V 40 .503cp [8]

V 38. 52cp V 30

= .561cp [9]

V 21

. / 4CS v * 21

= .742cs [8]

v 21

= .737cs [9]

Ethylene glycol p 38 1.101 gm/ml p 30

= 1.106 gm/ml [15]

V 38 10.23cp v

* V 25 approx. = 15. 6cs [9]

V 21 15.8cs v 20* = 19.0cs [15]

Glycerol p 38 1.250 v 20* = 1182cs [8]

V 38 320cp v 20* = 848cs [9]

V 21 915cs

DC-710 p 38 1.089 gm/ml n **

p 25

= 1.107 gm/ml [12]

V 38 248cp v 25

= 500cs [12]

V 21 566cs

FC-104 p 38 1.727 gm/ml p 25

= 1.75 gm/ml [11]

V 38 1.02cp v 25

= .8cs [11]

v 21 .77cs

* Kinematic viscosity computed from reported absolute viscosity by: v = V/p.

Density determined by multiplying specific gravity values [11] by density of water

at reference temperature.

** In product literature specific gravity given at 25°C. Density determined by

p DC-710 = (spec grav?l-) x (p H

? 0 ?I-).

5. Considerations regarding n,.

In this paper we have considered liquids which may be applicable to high repetition rate Nd: glass

lasers. Accordingly, one parameter of great interest is the nonlinear refractive index, ng, important

for the propagation of high peak power laser pulses. The electronic part n^ of has been calculated

according to the method of Boling, Glass and Owyoung [5], and can be written

114
S 13 K(n. - l)(n

2

d + 2)

2 4 (10

13esu) = —t— d d (5) v

d { 1.517 + [(n/ + 2)(n

d + l)vd]/6nd }

2

where K is an adjustable parameter (taken to be K = 68 here). The results for liquids in table 1 are

given in table 3 and figure 2, where n£ has been plotted versus the linear refractive index n^. As

is the case for glasses, a low nonlinear index is generally correlated with a low linear index of refraction. In figure 1 we show nd plotted versus v^, and it is apparent that large Abbe values also correlate with liquids having low calculated values of n|.

Although there is a paucity of data concerning experimentally determined ^2 values in liquids, we

have been able to compare our calculated values of n 9 for benzene with those found in the literature.

(3) T The third order susceptibility Xi-mi for a liquid or glass is related to the total n 9 , through the relationship

where

J - 12tt (3) ic\

n 2-H7 xllll (6)

n

2 = n2 + n2 ^

Here n^ is the nuclear contribution to n^, and is usually in the range of 20-30% of n^ in glasses, but

in liguids can be much larger. In this paper we concentrate strictly on n~; the nuclear contributions

T (3) to n

9 for liquids will be the subject of a separate publication. If we write Xim as a sum of electronic (xi^j) an d nuclear (xj^) parts

e . then ^2 is given by

(3) _ (3e)

+ v (3n) (R) x llll " x llll+ x llll (8)

e _ 12tt (3e)

n

2

" n— Mill (9)

(3e) Levenson and Bloembergen [16] have reported a value for X^n > as has Owyoung [17]. Using eq. 9 to calculate n| we arrive at the results shown in table 6 below.

Table 6. Calculated vs. experimental values for n,^ in Benzene

n| (10" 13esu) n^ (10" 13esu) n^ (10" 13esu)

(calculated) [16] TTT]-

3.1 4.62 5.02

p J

T Owyoung [18] has determined that n,^ is approximately 20% of ^2 in benzene, and that 02 = 14 x 10

* 3esu; hence the nuclear part is n", = 10 x 10

* 3 esu. Our value for n^ is in fair agreement with

both of the reported values, and will probably show better agreement once a data base has been established and the adjustable constant K in eq. 5 has been recalculated for liquids. Thus the data given

in table 3 and figure 2 should be treated in a first order sense at present.

This is, to our knowledge, the first application of the Boling, Glass, Owyoung theory [5] to the liquid state, and gives the electronic contribution to the total nonlinear refractive index. Extension of this work to other liquids for which the total nonlinear refractive index is known will allow the separate contributions of electronic and nuclear susceptibility to be determined.

6. Discussion

Optimum characteristics for an index matching liquid or a coolant may be considered to be the following: low viscosity, high thermal conductivity, low absorption in important spectral regions,

low dn/dT and low n 9

.

115
Low viscosity is an important property where liquid flows are considered. For example, glycerol

exhibits relatively good values for n^, thermal conductivity and dn/dT which might lead one to consider

glycerol as a coolant. However, the extremely large viscosity of glycerol would put a severe strain

on any pumping system.

Heat removal considerations for cooling applications optimally require liquids with high thermal conductivities. A comparison of H^O and FC-104 will serve as an example of this factor. Both liquids

have low viscosities, low n£ values, similar UV edges and FC-104 has lower absorption than H2O,

especially in the near-IR region. However, on the basis of thermal properties alone, H2O would be the superior choice for a coolant as its thermal conductivity is 6 times greater than FC-104 and dn/dT

for H2O is approximately 1/3 lower. Another important thermal property is thermal diffusivity, defined

as K/(pCp) where K = thermal conductivity, p = density, and Cp = specific heat at constant pressure. The specific heat of these liquids, not yet measured, is required to calculate this quantity. Another

thermal measurement to be performed on these and other liquids at a later date is an examination of

the coefficient of thermal expansion in the region around room temperature.

Characteristics other than those specifically mentioned above may come into play in an application

decision. For example, silicone oils are quite difficult to dissolve with normal solvents. Hence, any cleanup of these materials can be a monumental task. Also silicones and TCP are examples of liquids

which degrade in UV radiation. The UV edge of these liquids shifts towards the visible with exposure

to intense UV from flashlamps. Care must also be used in choosing coolant/index matching liquids that

are chemically compatible with materials used to contain them. ZnC^ (aq) is corrosive to aluminum

and stainless steel, but does not react with 70/30 brass [19].

7. Conclusion

A wide range of values for refractive index, n^, dn/dT, thermal conductivity, density, viscosity,

pH, and spectral transmittance is observed for the liquids examined in this work. The data generated

in this investigation should aid in a system designer's decisions in choosing a particular liquid for

a given application. Further investigation is planned of other useful properties including: thermal expansion, specific heat, thermal diffusivity, and n^ i^m

- This work will be expanded to consider

additional liquids such as deuterated water, toluene, ethyl alcohol, cyclohexane, CCl^, and acetone.

8. Acknowledgements

This work was partially supported by the Laser Fusion Feasibility Project, which is sponsored by: Exxon Research and Engineering Company, General Electric Company, Northeast Utilities Company, Empire State Electric Energy Research Company, and New York State Energy Research and Development

Administration

.

9. References

[1] N.I. Sax, Dangerous Properties of Industrial

Ma terial

s

, 4th ed. VanNostrand Reinhold Co.,

1975.

[2] J.V. Hughes, J. Sci. Inst., 18, 234 (1941).

[3] Lafayette Instrument Co., Lafayette, Indiana.

[4] R.W. Hellwarth, Phys. Rev., 152, 156 (1966).

[5] N.L. Boling, A.J. Glass, A. Owyoung, L 3 report UCRL 75628 (1974).

[6] B. Braun Melsungen A.G., San Mateo, Calif.

[7] Sartorius-Werke GMBM, Gottingen, Germany.

[8] R.C. Weast ed., Handbook of Chemistry and Physics , 52nd ed., Chemical Rubber Co.,

(1971).

[9] J. Dean ed., Lange's Handbook of Chemistry , McGraw-Hill Book Co., 11th ed. , 1973. [10] L. Meites, ed. Handbook of Analytical

Chemistry , McGraw-Hill Book Co., 1st ed.,

1963.

[11] Product information on Fluorinert liquids,

3M Co. , St. Paul , Minn.

[12] Bulletin #22-281 (6/1974), Dow-Corning Corp.,

Midland, Mich.

[13] D. Brown, S. Jacobs, J. Abate, 0. Lewis,

J. Rinefierd, Laser Induced Damage in Optical

Materials: 1977 , NBS special pub., 509, 416. [14] Cargille Laboratories, Cedar Grove, NJ. [15] M. Windholz ed., The Merck Index , 9th ed.,

Merck & Co. Inc., 1977.

[16] M.D. Levenson and N. Bloembergen, Phys. Rev.

B, 10, 4447 (1974).

[17] A. Owyoung, Optics Comm., 16, 266 (1976).

[18] A. Owyoung, App. Phys. Lett. 26, 168 (1975).

[19] LLE Annual Report 1977.

116
to

1.58

l.5fa

1.54

1.52

I.JO

1.46

N.44-

x 1.40

£

1,38

CI 1. 34 ^ 132

I.Jo

1.28

1.26

FC-104

TCP

C 504O _ C 56 \0

GLYCEROL

ETHYLENE GLYCOL

H 2 0

J L no no ioo <\> go 70 foo *?o 4o $0 lo <3 Ae>&E' VALUE Vti

Figure 1. Abbe value versus refractive index.

to

"a

if5

I

5b —

54

,52

50

46

4fo

.4^>

42

,40

.58

.36

,54

.30 .28

26

TOP

C5040 C5foiO PC 7IO

0EWZENE

ETMYtEUE GLYCOL

_ fl 2 0

F<-'-|04

NOW LIN)EAti INDEX ( P2 X|0 'csu

Figure 2. Calculated nonlinear refractive index versus linear refractive index.

117
Figures 3-8 Spectral transmi ttance of liquids under study. All liquids

scanned in 3.7 cm quartz cells at 50 A/sec.

I LJ 1 1 1 I | | | | | | | | | | | | | ,V> .25 K> .15 AO .-»5 .50 .55 .60 tS ,70 .75 .SO .05 ,<K> f> I OO |05 110 |.I5 1.20 I If I K> | W iAO

Figure 3. Spectral transmi ttance of FC-104 and Benzene.

I I I I I I I I I I I I I I I L_l I I I I

I I I I .20 .25 .K> ,V> .40 AS SO .55 .bo .ich JO .75 .60 .$» .to m 100 105 no 1.15 t.lo UK l io t )9 l%> A , WAVELENGTH Jlmt^

Figure 4. Spectral transmittance of H

2 0 and ZnCl

113
.20 .25 .30 .}5 AO .45 50 -55 .c>5 .70 .75 .5*3 .05 .90 .95 I 00 l « 1 10 1.15 I ZO 1.25 I Jo (?* 1.40

Figure 5. Spectral transmi ttance of ZnB^ and DC- 710.

I I I I

I I I I I I I I I I I I I I I I ! I I I .20 .15 .*> .Vy AO .45 .50 .55 MS .<<& JO .75 .*> .£5 .9o .9 5 I -00 |.05 ( to (.« (10 ( 25 I *> {.35 1-4*

Figure 6. Spectral transmi ttance of Ethylene glycol and C5040.

119
Figure 7. Spectral transmi ttance of glycerol and C5610.

I I

I I I I lili I LJ I L_J I I LJ I I I L_l

.20 .15 .30 35 40 ,45 .50 .55 .foO .fc5 .70 .75 ,80 4)5 .**> .95 I.OO 1.05 MO M5 | 20 1.2? I ?o | 40 A, WAVELENGTH ^^fj^

Figure 8. Spectral transmi ttance of Dimethyl sulfoxide and Tricresyl phosphate.

120
Figure 9. Thermal comparator signal versus thermal conductivity

121
PHYSICAL AND OPTICAL PROPERTIES OF SURFACES GENERATED BY DIAMOND-TURNING ON AN ADVANCED MACHINE

D. L. Decker and D. J. Grandjean

Michelson Laboratory, Physics Division

Naval Weapons Center, China Lake, CA 93555

A superprecision, two-axis, air-static bearing, diamondturning machine of advanced design has been previously described at this Conference and is now operational. The microtopography and other physical characteristics of surfaces turned by this machine will

be examined in detail. Some effects of machining parameters will

be briefly discussed. The ability of this machine to turn surfaces

with very small slope errors (< 10 ) and small rms roughness

(~ 10 A) is discussed with relation to the resulting optical absorption, scattering, and laser damage resistance. The characteristics

of the machine which permit these high quality surfaces are identified,

and some projections are given to the future application of diamondturned optics at near infrared or visible wavelengths.

Key words: Absorption; diamond-turning; scattered light; surface

roughness.

This paper describes some significant advances in the technology of diamond-turning of precision optics with specific emphasis on those results which bear on laser component

applications, especially at wavelengths shorter than 10.6 urn. Although state-of-the-art

diamond-turned surfaces are adequate for many demanding applications in the 8- to 14-um

wavelength region, the use of such surfaces at shorter wavelengths is hampered by limitations of both finish and figure. A paper by the first author of this paper was given at this Conference in 1976 which described details of the design of an advanced all airbearing, two-axis lathe at the Naval Weapons Center (NWC) [I] 1

. This machine is completed

and is now mechanically operational. Optical and physical characteristics of plane surfaces

turned on this machine are presented as examples of present capability.

The work at NWC has not only included systematic study and optimization of the effects

of the machining process on the optical properties of turned surfaces, but also significant

advances in machine technology. Slides, slide drives, and servo controls have received

particular attention. The results at this point are significant and very encouraging.

Surface roughness has been a first concern and, by a somewhat empirical, iterative approach,

has been reduced to ~ 10 A rms on bare OFHC copper measured by total integrated scatter

(TIS) of visible light. This finish is characterized by maximum slope errors a factor of two to three smaller than previously measured on surfaces from other machines. The measured

infrared reflectance of this surface is identical to that of the best evaporated films and

is 0.992 at 10.6 urn wavelength. These values have been achieved without the benefit of interf erometr ic servo control, relying on the intrinsic truth of motion of slide and spindle,

and by an optimized machining procedure. The NWC machine, which was manufactured by Pneumo

Precision, Inc., Keene, New Hampshire, provides slide motion which is much smoother and more accurate than available from any previous machine. A 6-inch-diameter copper mirror

has been turned flat to A/10 [633 A peak-to-valley (p-v) ] with no servo correction. With

the addition of a modern high-speed inter ferometric servo loop all residual slide errors

can, in principle, be corrected to ~ 100 A p-v, over the full range of motion of the machine.

Figure 1 is an isometric schematic of the NWC machine, showing the arrangement of slides and spindle. The machine base is an air-suspension isolated, granite surface plate.

* Work supported by Navy Independent Research and Independent Exploratory Development funds, and the Air Force Weapons Laboratory.

1. Figures in brackets indicate the literature references at the end of this paper.

122
This arrangement has several advantages over the most common machine configuration, namely,

the Moore #3 conversions [2]. All critical components are tied thermally and mechanically

to the base in a similar manner, with all critical parts at essentially the same height

from the floor. Vertical air temperature gradients are thus less important. Also, the granite surface plate represents a very versatile two-dimensional optical/mechanical bench,

allowing changes in components or configuration to take place without extensive redesign or rework. For example, the incorporation of an interferometer to measure slide position is easily accomplished, as is shown in figure 1. It is relatively easy to have the interferometer beam at spindle centerline height, thus minimizing Abbe offset errors. Figure 2 is

a photo of the machine showing tool and spindle slides, tool post assembly, chip/coolant

guard, and spindle housing. A 6-inch copper flat is being turned. Table 1 is an outline

of the machine performance as actually measured. Of particular interest is the maximum

slope error associated with slide motion. Figures 3 and 4 give, respectively, examples of the long- and short-period departures from ideal straight line motion in the yaw plane of the tool slide. Similar motion is observed in the pitch plane and for pitch and yaw of the spindle slide. Note, in particular, the magnitude of the maximum slope observed in the short-period motion (1.6 x 10 ).

Table 1. Actual measured performance data of the NWC diamond- turning machine.

Hydrostatic Air Spindle

Axial and radial runout . . . < 3 yin. total indi- cator reading

Angular (wobble) error . . . < 0.1 arc sec Axial stiffness 0.5 x 106 lb/in.

Radial stiffness at nose . . 0.3 x 10 6 lb/ in. Load capacity at nose .... 300 lb Swing 14 in. over tool carriage

Hydrostatic Air Slides (both x and y)

Straightness (yaw) and flatness (pitch) . . 3.2 yin. per in. max short range

8.2 yin. in 8-in. max long range Stiffness in pitch 0.3 x 10 6 lb/in.

Stiffness in yaw 0.5 x 10^ lb/in.

Axial stiffness 0.4 x 10^ lb/in.

The finish achievable on a single-point diamond-turned surface is determined by both

the quality of the tool and of the machine, and by the machining technique employed.

However, it appears that virtually all of the surface finish imperfection can be ascribed

to unwanted machine motion or to the machining process itself rather than the tool, when

tools of the highest quality are employed. The ideal surface generated by turning with a radiused nose tool is shown in figure 5. The peak-to-valley roughness 6 is given approx2 Pv imately by the expression 6 = f /8r, valid if the ratio of the feed/revolution of the spindle, f, to the tool radius, r, is small [3]. Other useful expressions are given in this figure and are computed similarly. A high quality diamond-turned surface has superposed, on the ideal structure just discussed, a wealth of fine scale roughness as a consequence of unwanted tool/workpiece motion or interaction, including tool build-up or chip

interaction with the work. Figure 6 is an intercomparison of the surface profile perpendicular and parallel to the machining grooves for surfaces turned at NWC, Battelle Northwest,

and Y-12 Union Carbide Laboratories. Of particular interest is the reduced fine scale structure displayed especially by the NWC perpendicular scan but also by the parallel scan. The parallel scans cover a distance of 50 ym, which was cut in a time on the order of 50 ysec Obviously, there is much structure in all cases with a period of 1 ysec or even less. This

time interval is % 4 orders of magnitude smaller than the period of the fundamental resonant frequencies of the spindle axial or slide yaw oscillation. This fine structure and the fine structure displayed in the perpendicular scans is similar in all examples of this figure and is almost certainly a consequence of microscopic variations in the machinability

123
of the work material or to variations in the flow of the chip away from the work surface. Assuming that this structure was generated by harmonic motion, and assuming an amplitude of

o 4 2 10 A, one computes a maximum acceleration of 4 x 10 m/sec or 4,000 g. It is clear that since the measured machining tool forces are typically a fraction of a gram weight, only

extremely small masses can follow this motion if the reaction forces act on this tool or workpiece. A 1-cm length of copper chip removed with a 10 ^ inch/revolution feed rate with

a 1/8-inch nose radius tool has a computed mass of only 4 x 10 g. Hence, it is perfectly

reasonable that a small section of chip respond with the amplitude and frequencies observed

associated with the very fine scale surface structure. This motion may represent torsional

or longitudinal acoustic vibration of a very small length of chip still attached to the work surface. It has been observed empirically that by guiding the chip with a small,

vigorous, liquid jet from a hypodermic needle, much of the large scale chip motion observed

easily with a low power microscope can be greatly reduced. Subsequently, it was discovered

that the effect of this procedure is to produce a considerable reduction in fine scale surface structure. This improvement has a dramatic effect on light scatter especially at short wavelengths. The finish improvement may come about from a chip motion damping effect

or from improved lubrication of the chip on the tool. The latter explanation seems most

likely since the chip oscillation is possibly driven by variations in tool/ chip friction.

The perpendicular scan profiles are superpositions of "snapshots." The cutting profile

at the tip of the tool is preserved and is overlayed by the similar profile with successive

adjacent passes. Even for the best of the surfaces turned at NWC, significant departure

from the ideal surface exists, as is indicated in table 2. This discrepancy has been

almost universally observed. Control of the chip or chip/tool interaction is the key to producing very smooth turned surfaces. The measured surface parameters in table 2 were

obtained on a 6- inch-diameter copper flat; similar data are compared to the previously

discussed Battelle and Y-12 samples in table 3. The NWC sample is characterized by a very

small roughness in the parallel direction, and an rms slope in the perpendicular direction

a factor of two to three smaller than the Battelle and Y-12 samples'. The surface roughness

values obtained by TIS must be interpreted with caution. The scattered light originates at structure on the sample surface associated with the turning process or at defects in the basic material exposed by turning. Scattered light is, however, limited by a spatial cutoff length equal to the incident wavelength, so that structure with a correlation length less than the wavelength does not contribute to the scattered light intensity. This is the primary reason for the discrepancy between TIS and profilometer roughness values [4].

Table 2. Comparison of calculated and measured surface

finish for a feed rate of 10.9 ym (~ 500 pin.) per revolution and a tool radius of 3175 ym (1/8 in.).

Calculated Measured

6 = 46.8 A 200 A

PV -3 -3

a = 1.7 x 10 10 x 10 max

Table 3. Quantitative summary of roughness values measured by TIS

and by profilometer and slopes measured by profilometer

for the surfaces intercompared in figure 6. rms Roughness rms Slope

rms (Talystep, (Talystep,

Roughness parallel to parallel to (TIS), A grooves), A grooves), A/um

NWC, 6-in. dia. 19.0 7.7 43 Battelle, 2-in. dia. 12.3 12.9 85 Y-12, 1.5-in. dia. 20.1 9.7

124
Inclusions and voids occur with surprising concentration even in "high purity" materials. Easily visible in either dark field or Nomarski microscopy, these defects contribute

significantly to the light scatter of the surface. For example, the 6-inch NWC copper disc

"roughness" of 19 A rms is substantially due to this origin. If the 90 points at which

scatter was measured are ranked, and the highest scatter 50% discarded, the apparent roughness drops to under 10 A rms. The concentration of scattering centers (primarily voids and microcracks) is on the order of 500/mm . A copper substrate with a much lower concentration

of defects was turned with similar machining parameters which had a measured TIS roughness

of 10.1 A, in corroboration of the defect origin of much of the scattered light. Mechanical

polishing appears to smear over the microscopic defect structure just described, and so is not discovered by routine examination. Much of the spatial variation of pulsed laser damage thresholds on apparently uniform polished bare metal surfaces is believed to be due

to subsurface defects of this sort [5].

The NWC diamond-turned surface in figure 7 displays the very uniform residual turning structure resulting from precise machine geometry and chip control, and also the very low defect concentration of this surface. It is instructive to qualitatively compare the surface structure of turned surfaces coming from several different laboratories in detail.

Figure 8 is the autocovariance function for a perpendicular scan of the 6-inch-diameter

flat already discussed [6]. The long-period structure displayed here has the effect of producing scatter very near the specular direction. Figure 9 shows the height and slope

distribution function for this same surface. Neither the height nor slope distributions

are well represented by Gaussian profiles. A Y-12 diamond-turned bare copper surface appears in figure 10, showing a typical concentration of localized defects. Figures 11 and

12 show, respectively, the autocovariance function and height and slope distribution functions for the same Y-12 sample. These distributions are very similar to those of the NWC

sample, with slightly larger rms roughness and an rms slope a factor of two larger. The Battelle diamond-turned characteristics shown in figures 13, 14, and 15 are quite different. The surface structure is of much higher spatial frequency, as is indicated by the autoco- variance function in figure 14. The rms roughness is intermediate between the NWC and Y-12

results, but the rms slope is significantly larger than the Y-12 case. The Battelle surface possesses a wealth of low amplitude but short-period, high slope structure. The effects of such a surface profile on laser damage remain unknown although the infrared reflectance of this surface is not notably degraded [5].

Much of the larger scale nonideal structure has been demonstrated to be reproducible

on a scale of a few hundred angstrom units in the case of modified Moore //3-type lathes [7]. In principle, by calibrating the slides of the machine, and by incorporating this information in the program or tape commanding the machine motion, the systematic unwanted motion

can be controlled. However, the servo bandwidth of controllers in current use are typically

only a few hertz and, consequently, much high frequency spatial irregularity remains uncorrected. This approach can make significant improvements in gross figure, but has little

impact on small scale figure or finish.

To effect small scale figure improvements, two areas must be attacked simultaneously:

the spatial frequency of unwanted slide and spindle motion must be reduced, and the servo

loop must incorporate a minimum Abbe offset interferometric position/velocity sense and high bandwidth. Nonreproducible slide motion in plain hydrodynamic way lead screw-driven

carriages is primarily a consequence of spatial variations in friction. Hydrostatic ways, either air or o il , can result in very significant improvement, since the motion is completely

free of stick-slip, and has a friction component only as a consequence of shearing an air or oil film. The drive problem can be addressed by several approaches: 1) superf inished

recirculating ball lead screw, 2) a traction drive of some sort, 3) linear induction motor. All three have been implemented and can work. In the present case, a ball screw directly

coupled to a dc torque motor provides the carriage motion.

The Hewlett Packard 5501A doppler interferometer system provides a well-documented

yardstick for gaging slide motion to a position resolution of A/80 (.311 min) and with an absolute accuracy of 0.5 parts in 10^ and a stability of one part in 10^. A thoroughly

modern phase lock servo loop can make optimum use of the very high bandwidth of the Hewlett

Packard system, and being a digital/analog hybrid can possess the very high velocity reso- lution and stability required to produce a two-axis contouring cut limited primarily by the resolution of the interferometer. These authors hope to report success in this regard

125
at this Conference a year from now. Since the very high frequency structure, which is a

' primary contribution to surface roughness, is not a direct consequence of slide motion it should be possible to maintain surface roughness — 10 A rms under these conditions for parts up to the full capacity (0.4-m diameter) of the machine. This machine will thus be capable of producing optical surfaces useful for many demanding visible wavelength applications.

References

[1] Decker, D. L. , and Cram, R. E. , "Naval

Weapons Center Diamond Turned Optics

Facility," in Laser Induced Damage in Optical Materials: 1976, ed. by A. J. Glass and A. H. Guenther, Washington,

D.C., National Bureau of Standards (NBS

Spec. Publ. 462), pp. 145-48.

[2] Arnold, J. B. , Steger, P. J., and Burleson, R. R. , "Slide Position Errors Degrade Machined Optical Component

Quality," in Laser Induced Damage in Optical Materials : 1975, ed. by A. J. Glass and A. H. Guenther, Washington,

D.C., National Bureau of Standards (NBS

Spec. Publ. 435), pp. 75-89.

[4] Decker, D. L. , and Bennett, J. M.

,

"Surface Evaluation Techniques for Optical Components," in Proceedings of the Society of Photo -Optical Instrumentation

Engineers, Vol. 140, June 1978 (in press

[5] Decker, D. L. , Bennett, J. M. , Soileau,

M. J., Porteus, J. 0., and Bennett, H. E "Surface and Optical Studies of DiamondTurned and Other Metal Mirrors," Opt.

Engr. 17, #2, 160 (1978).

[6] Bennett, J. M. , "Surface Evaluation Tech

niques for Optical Components," this

Conference (to be published by Academic

Press)

.

[3] Donaldson, R. R. , "Nomograms for Theoretical Finish Calculations With Round Nose Tools," presented at the Society

of Manufacturing Engineers Conference,

San Francisco, Calif., November 1974.

[7] Bryan, J. B. , presented in a panel discussion at the Society of Manufacturing

Engineers "Precision Machining Workshop"

in Williamsburg, Va. , June 1978.

Figures

Figure 1. Shown in this isometric schematic of the NWC two-axis diamondturning machine are the placement of the spindle and tool slides in a "T" base

configuration. Also shown is a simple interferometer layout for measuring the primary motion of the slides.

126
Figure 2. This photograph of the NWC machine shows the tool and spindle slides

the tool post assembly including precision height adjuster, chip/coolant guard, and spindle housing. A 6-inch-diameter copper flat is being machined.

SLOPE 3.5 X 10" 6

SLIDE TRAVEL

Figure 3. Measured long-period departure from ideal straight line motion in the yaw plane of the tool slide. An electronic gage was run against an optical

flat for this measurement.

Figure 4. Measured short-period departure from ideal straight line motion in the yaw plane of the tool slide.

127
5 a f 2

/8t ASSUMING THAT f/r « 1

amax a f/2r OR RMS VALUES:

i r|I1 a 6 p ./f5"

armi - »nii.'^

Figure 5. Ideal finish achieved by machining with a tool of nose radius r, and

a feed rate f. Both peak-to-valley (pv) and rms values are given.

SCANS PARALLEL TO GROOVES SCANS PERPENDICULAR TO GROOVES NWC

BATTELLE

Y-12

50 inn 50 >im

Figure 6. An intercomparison of the surface profiles of several diamond- turned

surfaces obtained with a Talystep profilometer with a 0.1- urn radius stylus.

Scans both parallel and perpendicular to the residual machining structure are

shown. The NWC surface is a 6-inch sample turned at 800 revolutions/minute

with a feed rate of 3 x 10- 4 inch/revolution.

Figure 7. This Nomarski micrograph shows the very uniform residual turning structure on the NWC diamond-turned surface shown in figure 6.

AUTOCOVARIANCE FUNCTION SCAN PERPENDICULAR TO GROOVES

Figure 8. The autocovariance function plotted in this figure is for a scan made perpendicular to th( machining grooves of the 6-inch NWC turned copper disc shown in figures 6 and 7.
SCAN PERPENDICULAR TO GROOVES

Figure 9. This figure shows the height and slope distributions corresponding

to the autocovariance function plotted in figure 8.

1000 tim

Figure 10. This Nomarski micrograph is of the Y-12 diamond-turned copper surface displayed in figure 6, showing some irregularity of the residual struc- ture and a typical concentration of localized surface defects.

LENGTH p(mkran)

Figure 1J . This figure is a plot of the autocovariance function of the Y-12 sample shown in figure 6.

129
Figure 12. This figure is a plot of the height and slope distribution functions of the Y-12 sample

shown in figure 11.

Figure 13. This Nomarski micrograph is of a Batelle Northwest diamond-turned copper surface

shown in figure 6, displaying a large variation in the noticeably higher spatial frequency structure

of this surface.

G(p)t/?2

AUTOCOVARIANCE FUNCTION SCAN PERPENDICULAR TO GROOVES

1.8 |im

"0 10 20 30 LENGTH p(micron) Figure 14. The autocovariance function for the Battelle sample of figure 13. This surface is characterized by very strongly correlated short-period structure.

SCAN PERPENDICULAR TO GROOVES

HEIGHT

DISTRIBUTION FUNCTION

SLOPE

DISTRIBUTION FUNCTION

-100 —50 0 50 100 DISTANCE FROM MEAN SURFACE LEVEL (X)

0 250 500 SLOPE (AVmicron) Figure 15. The height and slope distribution functions of the Battelle surface

shown in figure 14 have a roughness value intermediate between the NWC and Y-12

values, but with an rms slope considerably larger than the Y-12 value.

130
Discussion

It was pointed out, that for the present studies, the machine is not temperature stabilized but performs satisfactorily due to its large thermal mass and the relatively short duration (minutes) of

the cut . Ultimately , as more precise surfaces are required, cuts will be of longer duration; and

thermal stabilization or temperature control of the machine will be required

It was pointed out from the audience that diamond-turned copper mirrors degrade more rapidly

than polished copper in the gas environment of industrial CO2 lasers . This result is somewhat at variance with past exper ience with diamond-turned aluminum, which oxidized more slowly than polished

aluminum . It should be noted that both the quality of the copper and the cleanliness of the mirror

would affect the rate at which it degrades. The effect noted was significant, however, and warrants further investigation .

131
OPTICAL AND METALLURGICAL CHARACTERIZATION OF MOLYBDENUM LASER MIRRORS*

Shew M. Wong

Rockwell International, Rocky Flats Plant, Golden, CO 80401

George Krauss Department of Metallurgical Engineering, Colorado School of Mines

Golden, CO 80401

and

Jean M. Bennett Michelson Laboratory, Physics Division

Naval Weapons Center, China Lake, CA 93555

A study has been performed to determine the correlation between

the microstructure , metallurgical processing, and surface finish of eight specially selected molybdenum (Mo) specimens from various

sources. Samples of bar and plate stock produced from pure Mo

either arc-cast or pressed and sintered, and TZM (Ti-Zr-Mo) Mo alloy were examined in the form of optically polished 1 1/2-inch- (3.86-

cm-) diameter discs. Metallurgical characterization included

hardness measurements, grain size and shape determinations from polished and etched sections, scanning electron microscopy, and Auger electron spectroscopy. Optical characterization consisted of measurements of total integrated scattering, rms roughness, height

distribution functions, and autocovariance. In some cases, the surface roughness of as-polished mirror surfaces could be directly

related to grain structure and substructure as revealed by etching.

These observations and their implications for the selection of Mo starting stock to be used in the production of low-scatter laser mirrors will be discussed.

Key words: Laser mirror; metallurgical characterization; microstructures; molybdenum; optical characterization.

Introduction

Molybdenum (Mo) is a material of interest for high power laser mirrors because it has

good heat conductivity, low thermal expansion, a reasonably high damage threshold, and is sufficiently stiff that thin mirror faceplates can be fabricated and attached to watercooled backs. The primary drawback to the use of Mo is that it is difficult to obtain low scatter surfaces. Since Mo cannot be diamond- turned using present techniques [l] 1

, the only way to obtain optical surfaces is by polishing. Either the bulk material can be polished, or a layer of Mo can be sputtered onto a polished Mo surface and the sputtered

layer polished. This latter method has the advantage that the sputtered layer can have a very fine grain structure, and hence can be polished to a very smooth finish, of the order

of 15 A rms. However, the sputtered layer may be porous, and the damage threshold for polished sputtered Mo on Mo is lower than that of rougher conventionally polished Mo surfaces. The reason for the lowered damage threshold may be related to porosity in the sputtered layer which can affect the heat transfer and make the film unable to support stresses developed during heating and subsequent thermal expansion. There may also be a contamination film at the interface between the sputtered layer and the bulk material which

can adversely affect the damage threshold. More work needs to be done in this area.

* Work supported by Defense Advanced Research Projects Agency and NWC Independent Research

funds

.

1. Figures in brackets indicate the literature references at the end of this paper.

132
The purpose of this study was to determine the relation, if any, between the grain structure in the Mo and the structure appearing on well polished Mo surfaces. Eight samples, which represented commercially available types of material including arc-cast and powder metallurgy products fabricated into both bar and plate stock, were chosen for the

study. Metallurgical analyses were performed on all eight samples and included investigation of the as-polished mirror surfaces, etched mirror surfaces, and polished and etched

surfaces cut normal to the mirror surfaces. The microstructures , i.e., grain size, sub- structure and second-phase particles, and hardness were correlated with the processing of the various samples, as supplied by the manufacturer. Chemical analyses of the various Mo specimens were obtained, and the grinding and polishing compounds used to fabricate the optical surfaces were chemically analyzed for impurity content. The optical characterization consisted of absolute normal incidence reflectance, total integrated scattering (TIS)

,

and surface profile measurements. From the scattering and profile data, rms roughness, rms slope, and autocovariance length information were obtained. In addition, Auger scans were

made on one sample to determine surface impurities and to see how these penetrated into the depth of the material. Finally, the metallurgical and optical data were compared, and it was found that there is a direct relation between the grain and subgrain structure in the bulk material and the structure appearing on well polished Mo surfaces.

Mirror Materials

Table 1 lists the identification number, shape, method of preparation, final mechanical

and thermal processing, and chemical analysis, where available, of the eight Mo mirrors

examined in this investigation. The materials selected represent a good mixture of processing histories that might be expected to affect microstructure and polishing behavior.

There are essentially three groups of materials: (1) low carbon, arc-cast Mo, (2) powder

metallurgy Mo, and (3) arc-cast Mo TZM alloy. The first type of Mo is produced by vacuumarc melting of powdered Mo in a water-cooled copper mold, and is expected to be of high

purity with a minimum of second-phase particles. The second group of Mo samples is produced

from compacted and sintered Mo powder, and is expected to contain some voids and/or oxide

particles as a result of this processing approach. The TZM alloy is produced by vacuum-arc melting of carbon-deoxidized Mo with the alloying elements titanium and zirconium in a water-cooled copper mold. The titanium and zirconium additions are designed to produce a dispersion of fine carbide particles that improve strength and high temperature structural

stability. The types of Mo described above may be wrought into either bar or plate shapes

after the ingot or initial compacted bar is produced. Also included in table 1 is vacuumarc-cast plate sample 50194, similar to specimen NC7720, that was metallurgically characterized but not optically polished.

Polishing

The first four of the eight Mo samples listed in table 1 were polished by Abe Klugman

of the Northrop Corporation, according to the grinding and polishing processes documented

in the literature [2]. The surface preparation consisted of (1) blocking all four substrates

on a flat aluminum plate, (2) rough grinding with a 100-grit silicon carbide wheel on a surface grinder with water and coolant to bring all four substrates into a plane and to remove rough machining marks from the surface, (3) medium grinding on a conventional optical

grinding and polishing machine with 22 . 5-um-diameter A^O^ grit in water and a cast iron

flat grinding tool to remove all 100-grit marks from the surface, (4) fine grinding with

9. 5-um-diameter A^O^ grit on the same machine, and (5) optical polishing, as described

below. For the first 10 hours all four substrates were polished on the same block on a conventional optical polishing machine using a medium-hard pitch lap with 0 . 3-um-diameter

Al^O^ powder in water. The brush feed method (applying the alumina to the lap with a brush) was used. After 10 hours of polishing, the substrates were removed from the block

and were polished separately for 13 more hours, still with the same pitch and polishing

procedure. Nomarski micrographs were taken at various stages of the grinding and polishing

process to show how the surface texture developed.

Chemical analyses of the 22.5- and 9 . 5-um-diameter A^O^ grinding compounds and the

0 . 3-um-diameter A^O^ polishing compound revealed the impurities shown in table 2. The table shows that the grinding compounds are full of many impurities, the largest amounts being titanium, iron, magnesium, calcium, silicon, cerium, and zirconium. The polishing

133
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134
Table 2. Impurities in grinding and polishing compounds obtained

from spark source mass spectroscopy.

Numbers are parts per million by weight.

Materials

Impurities 22.5 Mm AI2O3 9.2 um AI2O3 0.3 pm A1 2 03 As 1 1 -

B 2 - 2 Ba 160 99 - Ca 1100 2000 2 Ce 920 1700 — CI 70 42 - Cr 190 230 - Cu 29 23 - Fe 3940* 2950* 9 K 320 1600 62 La 180 100 - Mg 2100 2600 6 Mn 400 490 - Na 520 640 53 Ni 47 17 -

P 15 18 8 Pb i

-L Pr 82 87 -

S 110 120 67 Si 1100 1400 70 Sr 160 300 Ti 20,000 11,000

V 75 91

Y 8 14 Zn 130 40

Zr 470 1900

* Results obtained by atomic absorption

compound is very much cleaner, and has appreciable amounts of only silicon, sulfur, potassium, and sodium. There are only 9 ppm of iron in the polishing compound, although iron is the only impurity other than sodium that appeared on the polished surface of one of the Mo samples

Surface Chemistry

In order to obtain information about what materials are present on Mo surfaces, Auger

scans were made on the low carbon, arc-cast plate sample NC7720. The results are shown in figures 1 and 2. Figure 1 shows that on the as-polished surface there are detectable

amounts of carbon, oxygen, nitrogen, iron, and sodium. Of these, carbon, oxygen, and nitrogen are present on most surfaces because of adsorption of air molecules. The iron and sodium may have come from the polishing compound, since both are present there in small

amounts. After 3 minutes of sputter-etching with argon, during which some material was

removed, no iron, sodium, or nitrogen could be detected, even though the gain on the high energy part of the scale was boosted by a factor of 3. The amounts of oxygen and carbon were considerably reduced and the Mo peaks were much more prominent. In order to show how

the amounts of contaminants varied in the depth of the material, an Auger depth profile was

made, as shown in figure 2. The two major contaminants, oxygen and carbon, dropped after

the sputter-etching began, the oxygen decreasing more slowly; the small amount of iron

135
became an undetectable amount. The proportionate amount of Mo increased as the contaminants

were removed. After the sputter-etch stopped, the amount of oxygen on the surface started

to increase. The oxygen seems to be tightly bound on the surface layers and, once removed,

the fresh Mo surface seems to absorb oxygen quite rapidly.

Optical Measurements and Surface Roughness Characterization

The normal incidence reflectances of the first four Mo samples listed in table 1 were measured shortly after the polishing was completed, and the results are shown in table 3. The precision of the measurements is about ±0.0004 or better and the accuracy is ±0.001.

It is seen that the reflectances of the various samples at a given wavelength are within a few tenths of a percent of each other, even the alloy TZM. Thus, the small amounts of titanium and zirconium apparently do not affect the reflectance. The maximum reflectance

differences are at the shortest wavelengths in the ultraviolet where the effects of surface

films, scattering, etc. would be the largest. Since all the samples scattered about the same amount of light, the constancy of the reflectance indicates that the reflectance is insensitive to the metallurgical form of the Mo, i.e., the grain and subgrain structure.

Surface scans made by a Talystep surface-profiling instrument [3,4] are shown in figures 3-12. These correlate very well with the Nomarski micrographs of the polished

surfaces shown in figures 14(a)-21(a). For example, the coarse surface structure of specimens NC10280 and S279 are clearly shown in both the surface scans (figs. 3 and 10) and the Nomarski micrographs [figs. 14(a) and 19(a)], The scratches on the surface of specimen

AP041 shown in the Nomarski micrograph [fig. 21(a)] also, appear in the surface scan (fig.

12), although the largest scratches have been avoided. Since this material is similar to that from which specimens NC10280 and N243 were made, it is clear that the polishing tech- nique plays an important role in the surface finish which can be obtained. The Nor\arski

micrographs do not show the particulates which project 100 to 200 A above -the surface level

in the NTZM10395 and N243 specimens. However, these particulates have diameters of 1 to 2 urn, as judged from the surface scans, so they would be close to the limit of resolution of the optical microscope used for the Nomarski pictures. The particles may be inclusions, or, in the case of the TZM, particles of titanium or zirconium carbide.

Table 4 summarizes the optical measurements made on all eight polished Mo specimens.

The first column contains rms roughness values for the golished surfaces derived from measurements of TIS made at a laser wavelength of 5682 A or 6328 A. A relation predicted

by the scalar scattering theory is used: TIS = (4tt6/ A ) , where 6 is the rms roughness and

A the wavelength [5]. This relation defines an "effective rms roughness," since implicit

in the derivation of the relation is the assumption that the distribution of heights of surface features about the mean surface level is Gaussian. This assumption is not satisfied

for several of the Mo specimens, as has been determined from measurements of the height

distribution function from digitized Talystep surface height data [6]. Figure 13 shows two such non-Gaussian height distributions. Specimen NC10280 had proportionately too many

small bumps and too few shallow holes, while specimen NTZM10395 had the only bimodal height

distribution yet measured. This distribution may be caused by small carbide particles

which either project slightly above the mean surface level or tear out of the surface

leaving shallow holes. The remainder of the columns in table 4 contain data obtained from

Talystep surface scans. The digitized surface height data are used to calculate the rms roughness, rms slope, and the autocovariance function. The autocovariance length is obtained from the 1/e value of the autocovariance function, and can be thought of as a measure of the separation of surface features. Short correlation distances mean that the features are closely spaced, and, conversely, larger correlation distances mean that they are farther apart. Light scattering from a surface depends not only on the heights of surface features but also on their slopes. A surface with a small rms roughness but large

rms slope may scatter as much as a more wavy surface with a much larger rms roughness and very small rms slope. Thus, both light scattering data and surface profile data are necessary to understand the optical behavior of the Mo surfaces.

136
Table 3. Normal incidence reflectances of polished molybdenum mirrors

Low carbon arc— Low carbon arc- Powder met

iz.ii arc cas Wavelength cast bar cast plate bar bar

(um) NC10280 Mr*7 ML 7 on / / ZU NsLy Ij Z \it7mi none N1ZM10 jy

0.2600 0 6712 0.6731 0.6772 0.6722 0.2800 0 6564 0.3000 A

uOloU 0.6190 0.6209 0.6188

0.3200 A

ujoZi 0.3300 0 5740 _ 0.3400 u joy i _ 0.3500 nU

joyz 0.5705 0.5699 0.3600 a

U.

c; c l. Q JD40 0.3700 0

5633 _ _ 0.3800 0

5652 _ .0.3900 0. 5671 _ 0.4000 0 5 / Do 0. 5711 0. 5695 0.5701

0.4200 05761 0.4400 0

JOOO 0.4600 0

6001 _ _ _ 0.4800 0

_ _ _ 0.5000 a

u0.6039 0.6051 0.6043 0.5200 a

U

0.5400 A

U . oyy d _ _ 0.5461 A

(J . CQQ 1 jyoi 0.5964 0.5978 0.5970 0.5500 0

5971 0.5959 0.5966 0.5962 0.5600 AU . jy j j 0.5682 0 . 5936 0.5934 0.5931 0.5909 0.5800 0

5934 0.6000 0 5931 0.5930 0.5928 0.5922 0.6250 0

5955 0.6500 0 5982 _ _ 0.6750 0

6000 _ _ _ 0.7000 0. 5984 0.5977 0.5976 0.5971

0.7250 0. 5963 0.7500 0 5948 _ _ _ 0.7750 0 5930 _ _ 0.8000 0 5915 0.5921 0.5911 0.5906 0.8250 0 5926 0.8500 0 5919 _ _ _ 0.9000 A

u OULD _ _ _ 0.9500 0 6313 _ _ _ 1.0 0 6938 0.6912 0.6922 0.6920

1.2 0

7 Q c 0

/ y jz _ _ 1.5 A

u

_ _ _ 2.0 0 9399 _ _ _ 3.0 A

u y / uy 0.9692 0.9700 0.9674

4.0 Au

Q 7 9 ft y / zo _ _ - 5.0 0

n 7 / £

y / 40 _ _ _ 6.0 A

u

0.7 A A y / dh - _ _ 7.0 0 9780 0.9767 0.9778 0.9808

8.0 Au Q7Q7 y / y / _ _ - 9.0 0 9800 _ _ - 10.0 0 9816 _ _ 10.6 0 9820 0.9802 0.9818 0.9831

12.0 0 9823

14.0 0 9833 16.0 0 9845 0.9841 0.9847 0.9852

18.0 0 9850

20.0 0 9858 0.9857 0.9863 0.9872

137
Table 4. Surface statistics of polished molybdenum mirrors.

Sample

rms Roughness

(A)*

rms Roughness

(A)**

rms Slope

(A/um)**

Auto—

covar iance

length (urn)

NC10280 15. 4 40.2 47. 0 24.4 NC7720 17.0 71.4 43.6 14.4 NSC9152 20.7 37.8 59.0 5.3 NTZM10395 21.7 60.6 156.2 2.4 N243 16.3 56.9 131.1 3.0 S279 20. 7 64 .

8

62.3 7.0 H218 23.2 31.0 51.3 3.3 AP041 63.4 31.8*** 79. 3*** 1. 9***

* Calculated from total integrated scattering measurements.

** Measured with Talystep surface profiling instrument.

*** Region without deep scratches.

Microstructural Characterization

The results of the metallurgical characterization are summarized in table 5, and micrographs of each of the polished specimens are shown in figures 14-21. In each figure

the Nomarski micrograph of the polished mirror surface is designated by (a) , while the balance of the pictures are standard light micrographs of metallographically polished or polished and etched surfaces. A few scanning electron microscope micrographs are included

in some of the series and are marked as such. In some cases, only the microstructure of normal surfaces could be etched because the mirror surfaces had been coated for high

reflectance in the ultraviolet wavelength region subsequent to the optical measurements

described in the previous section. The coated specimens are identified in table 5.

The results of the metallurgical characterization show that, in general, elements of

the grain structure or the substructure within the grain were revealed on the polished

mirror surfaces if good polishing techniques were used [2]. The grain structure in each

case was strongly affected by the nature of the mechanical processing. In the bar stock

the grains were elongated parallel to the longitudinal axis of the bar, and hence the mirror surface intersects the cross sections of the deformed grains. Figures 18(d) and

21(d) show the elongation of the grains on surfaces normal to the mirror surface, while

figures 18(b) and 21(b) show the grain cross sections on the mirror surfaces. For the most

part, the grains in bar stock have equiaxed cross sections, but some grain boundaries show

a curvature that has developed to accommodate the cylindrical shape of the bar stock. In plate stock, on the other hand, the grains have been flattened by rolling to a plate or pancake shape with the flat, broad grain surfaces intersected by the mirror surfaces [figs.

15(b) and (c)] and the reduced, elongated grain sections parallel to the mirror surface

[figs. 15(d) and (e)]. The pronounced elongation and deformation of the grains in all of the specimens is a consequence of the large amounts of cold work introduced during the final reduction stages, 67% to 85%, as noted in table 1, and the fact that the stress

relief treatments did not produce recrystallization or a return to an equiaxed grain structure. The stress relief treatment did, however, produce a fine substructure within the grains, as shown best in figures 14(c) and 15(c). Frequently, the scale of the surface

relief revealed in the Nomarski micrographs could be directly related to the substructures

developed by the stress relief treatment.

An important effect of grain orientation is the difference in the rates of polishing

of adjacent grains having different orientations. Evidently, when certain crystal planes

are parallel to the plane of polish, they polish much more slowly than do their neighbors,

thus producing a noticeable surface relief or roughness directly related to the grain

138
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139
structure. This phenomenon is demonstrated on most of the specimens, and figures 18(c) and

21(c) are good examples. The differential polishing effect related to grain orientation is probably the major cause of the surface relief seen in the Nomarski micrographs and the Talystep profiles.

In addition to grain structure and substructure, the metallographic examination showed

second-phase particles of various forms consistent with the processing history of the Mo mirror stock. Fine carbides were found in the TZM specimen [figs. 17(c) and (d) ]

, in accord with the alloy carbide dispersion expected in this alloy. The powder metallurgy

fabricated specimens showed evidence of elongated oxides and/or porosity. Figures 16(c) and (d) show good examples of the latter features. The source of the oxides is frequently

due to the oxides on the Mo powder which is pressed and sintered, and frequently porosity

results when theoretical density is not achieved during the pressing, sintering, and subsequent reduction of the powder metallurgy product.

Laser Damage Measurements

Multithreshold laser damage measurements were made at a wavelength of 10.6 urn (100 nsec

pulse length) on two of the polished samples, NC10280 and NC7720, using the laser damage

facility described previously [7]. The damage profile of vacuum-arc-cast Mo plate specimen

NC7720 is shown in figure 22. The threshold for craters is greater than 59.8 J/cm , the

0 maximum energy in the laser pulses. The threshold for melting, 15.1 ± 0.4 J/cm , is a convenient parameter to use for comparing the performance of different samples. For eight

bare Mo samples damage tested, this parameter varied from a maximum of 17.5 ± 2.0 to a

2 minimum of 9.2 ± .0.2 J/cm . The lowest value was for a polished Mo specimen onto which a layer of Mo had been sputtered and then polished. The lowest threshold for a polished

2 sample was 11.0 ±0.3 J/cm ; this sample was the roughest of those tested, having a rough- ness of 47.3 A rms, as measured by TIS. The rest of the samples, which were smoother, also had higher damage thresholds, and sample NC7720 was among the smoothest, and was also among

the most damage resistant. Although the surface microstructure was not characterized for the other specimens (except for NC10280) , it is probable that a variety of types of structures were represented. Thus, we can conclude that for well polished Mo specimens the damage threshold for melting for pulsed 10.6 pm laser radiation is probably independent of microstructure.

The damage morphology for sample NC7720, shown in figure 23, is typical of that for a well polished Mo surface. Figure 24 shows a portion of the damaged area and a Talystep

surface scan made across this area. The white areas in the Nomarski micrograph where the material has melted and recrystallized are appreciably rougher than the undamaged surface.

The damage morphology for film-covered sample NC10280 is shown in figure 25. Since

the multilayer (Hf 02/MgF2) ^ , designed to enhance the reflectance of the Mo in the ultraviolet, is absorbing at 10.6 pm, the morphology shown in this figure may not be representative

of morphology obtained in a region where the films are transparent. However, as the surface

scan in figure 26 shows, the films are apparently separating at their interfaces, and there

is also melting of the inner MgF£ layers. Melting and recrystallization greatly increase

the roughness of these materials. Failure of film-covered substrates generally occurs at cracks and defects in the films, and occurs at much lower energies than that for the film materials in bulk form or for the substrate material.

Conclusions

In conclusion, Mo samples having metallurgical structures representative of the commercially available types of material have been optically and metallurgically characterized.

It has been found that, on well polished samples, there is a close correlation between the grain and subgrain structure in the material and the structure appearing on the polished

surface. A major cause of this effect is believed to be associated with variations in hardness, i.e., polishing behavior, of different crystal planes lying parallel to the mirror surface in adjacent grains. On ground or poorly polished samples the grinding

and/or polishing marks obscure the grain structure. Different lots of nominally the same material may not have the same metallurgical structure and optical surface structure. Dispersion strengthening elements such as titanium and zirconium that form small particles of

140
alloy carbides and inclusion particles detract from the polishability of the material and produce small hard particles that project above the surface or pull out, leaving holes. Both holes and bumps produce proportionately much more scattering than the rest of the grain structure of the Mo. More work should be done to determine if it is possible to process Mo in order to get an easily polishable, uniform grain structure which will make

possible consistently high quality, low scatter mirrors.

Acknowledgments

The authors would like to thank Russell W. Burman of AMAX Specialty Metals Corporation

for providing some of the specimens and for helpful discussions and information about the processing of the specimens, James Capes of Rockwell International, Rocky Flats Plant, for metallographic assistance, and James 0. Porteus of the Naval Weapons Center for the laser damage measurements.

References

[1] Arnold, J. B. , Morris, T. 0., Sladky,

R. E. , and Steger, P. J., "Machinability

Studies of Infrared Window Materials and Metals," Opt. Engr. 16, 324 (1977).

[2] Klugman, A., and Bennett, J., "Polishing

of Molybdenum Substrates," in Proceedings of the High Power Laser Optical

Components and Component Materials Meeting , 3-4 October 1977, Boulder, Colorado,

Defense Advanced Research Projects

Agency, Arlington, Virginia, December

1977, pp. 163-173.

[3] Elson, J. M. , and Bennett, J. M. , "Relation Between the Angular Dependence of Scatterxng and the Statistical Properties of Optical Surfaces," J. Opt. Soc.

Am. , in press.

[4] Bennett, J. M. , "Surface Evaluation Tech- niques for Optical Components," this conference.

[5] Bennett, H. E. , and Porteus, J. 0.,

"Relation Between Surface Roughness and Specular Reflectance at Normal Incidence,

J. Opt. Soc. Am. 51, 123 (1961).

[6] Bennett, J. M. , and Elson, J. Merle,

"Surface Statistics of Selected Optical

Materials," in Laser Induced Damage in Optical Materials: 1977, A. J. Glass

and A. H. Guenther, eds. , National Bureau

of Standards, Washington, D.C. (NBS Spec. Publ. 509) , pp. 142-156.

[7] Porteus, J. 0., Jernigan, J. L. , and Faith, W. N. , "Multithreshold Measurement and Analysis of Pulsed Laser Damage

on Optical Surfaces," in Laser Induced

Damage in Optical Materials: 1977,

A. J. Glass and A. H. Guenther, eds.,

National Bureau of Standards, Washington,

D.C. (NBS Spec. Publ. 509), pp. 507-515.

141
Figure 1. Auger electron spectroscopy scans of the surface of arc-cast Mo

plate, specimen NC7720, before and after a 3-minute sputter-etch with argon.

142
AUGER DEPTH PROFILE OF MOLYBDENUM NC7720 SURFACE

ELAPSED TIME (MIN)

Figure 2. Auger depth profile of the surface of the same Mo specimen as in figure 1.

143
SURFACE SCANS - NORTHROP MOLYBDENUM NC 10260 7/20/77

Figure 3. Surface scans of vacuum-arc-cast Mo bar, specimen NC10280.

SURFACE SCANS - NORTHROP MOLYBDENUM NC 7720 1/30/78

Figure 4. Surface scans of vacuum-arc-cast Mo plate, specimen NC7720.

144
SURFACE SCANS - NORTHROP MOLYBDENUM NSC 01S2 1f2im

Figure 5. Surface scans of powder metallurgy Mo bar, specimen NSC9152.

Figure 6. Surface scan of vacuum-arc-cast TZM alloy bar, specimen NTZM10395.

145
SURFACE SCAN - NORTHROP TZM, MOLYBDENUM ALLOY TZM 10395 7/22/77

Figure 7. Surface scan of vacuum-arc-cast TZM alloy bar, specimen NTZM10395.

SURFACE SCAN NORTHROP MOI YBDEMUM s ?43

_ __„^__^__ 1/3 ITS

G u m RADIUS STYLUS

j 2 mg LOADING

Figure 8. Surface scan of vacuum-arc-cast Mo bar, specimen N243.

146
Figure 9. Surface scan of vacuum-arc-cast Mo bar, specimen N243.

Figure 10. Surface scans of powder metallurgy Mo bar, specimen S279.

147
HEIGHT DISTRIBUTION FUNCTIONS FOR POLISHED MOLY AND TZM

Figure 13. Surface scans and height distribution functions for vacuum-arccast bar, specimen NC10280, and vacuum-arc-cast TZM alloy bar, specimen NTZM

10395.

143
Figure 11. Surface scans of powder metallurgy Mo plate, specimen H218.

Figure 12. Surface scans of vacuum-arc-cast Mo bar, specimen AP041.

149
Figure 14. Microstructures of vacuum-arc-cast Mo bar, specimen NC10280:

(a) Nomarski micrograph of optically polished mirror surface, (b) micrograph

of metallurgically polished normal surface, and (c) scanning electron micrograph- of metallurgically polished normal surface.
100 um

Figure 15. Microstructures of vacuum-arc-cast Mo plate, (a) to (e) specimen

NC7720; (f) and (g) , specimen NC50194; (a) Nomarski micrograph of optically

polished mirror surface, (b) and (c) micrographs of metallurgically polished

and etched mirror surface, the latter with emphasis on substructure, (d)

,

(f), and (g) micrographs of metallurgically polished normal surface, and (e) micrograph of metallurgically polished and etched normal surface.

151

Figure 16. Microstructures of powder metallurgy Mo bar, specimen NSC9152:

(a) Nomarski micrograph of optically polished mirror surface, (b) and (c) micrographs of metallurgically polished and etched normal surface, and (d) scanning electron micrograph of etched normal surface.

153
Figure 17. Microstructures of vacuum-arc-cast TZM alloy bar, specimen

NTZM10395: (a) Nomarski micrograph of optically polished mirror surface,

(b) micrograph of metallurgically polished normal surface, (c) and (d) micrographs of metallurgically polished and etched normal surface.

154
100 pra

* .1 1

- \ 100 urn

Figure 18. Microstructures of vacuum-arc-cast Mo bar, specimen N243: (a) Nomarski micrograph of optically polished mirror surface, (b) micrograph of metallurgically polished and etched mirror surface, micrographs of metallurgically polished (c) and polished and etched (d) normal surface.

15 r3
-a

100 um 100 pm

Figure 19. Microstructures of powder metallurgy Mo bar, specimen S279:

(a) Nomarski micrograph of optically polished mirror surface, (b) and (c) micrographs of metallurgically polished and etched mirror surface, (d) micrograph of metallurgically polished and etched normal surface.

156
Figure 20. Microstructures of powder metallurgy Mo plate, specimen H218: (a) Nomarski micrograph of optically polished mirror surface, (b) and (c) micrographs of metallurgically polished and etched normal surface.

157
Figure 21. Microstructures of vacuum-arc-cast Mo bar, specimen AP041: (a) Normarskl micrograph of optically polished mirror surface, (b) micrograph of metallurgically polished and etched mirror

surface, micrographs of metallurgically polished (c) and polished and etched (d) normal surface.

158
BULK Mo. OPTICAL POLISH CNORTHROP C-7720}

SLIP BANDS

MELTING

SELECTIVE DAMAGE

CRATERS

ION EMISSION

LIGHT EMISSION

WRK . FN. CHG

... —

[-

h

1 > J

rr

]

i f

• 0.0 10.0 20.0 30.0

INC. THRESHOLD CJ/cm2 @j@0ns>

Figure 22. Multithreshold laser damage profile (10.6 Mm wavelength, 100 nsec pulse length) for vacuum-arc-cast Mo plate, specimen NC7720.

15°
200 fim

I [ Figure 23. 10.6 pm laser damaged area on specimen NC7720.

lf>0
DAMAGE CRATER ON NORTHROP MOLYBDENUM NC 7720

200 pim

Figure 24. Enlarged portion of damaged area from figure 23, and Talystep surface scan approximately o the line indicated in the photograph.

161

DAMAGE CRATER ON NORTHROP MOLYBDENUM NC 10280

163
1064 - NM LASER DAMAGE THRESHOLDS OF POLISHED GLASS SURFACES

AS A FUNCTION OF PULSE DURATION AND SURFACE ROUGHNESS*

D. Milam

University of California

Lawrence Livermore Laboratory

Livermore, California 94550

Laser damage thresholds were measured for four polished glass surfaces, using

linearly polarized 1064-nm pulses with durations of 0.17 ns, 1.0 ns, 1.6 ns, and

3.2 ns. Thresholds scaled approximately as the square root of pulse duration, but were insensitive to variations in surface roughness when the roughness was <25 A rms. Careful cleaning increased the damage threshold at 3.2 ns by removing

particulates.

1

Key words: BK-7 glass; fused silica; laser damage; polished surfaces; pulse duration

dependence of damage; surface roughness.

1 . Introduction

The relationship between surface roughness and laser damage thresholds of polished glass surfaces

is not well understood. Extreme roughness, rms roughness >100 A, caused reductions in damage threshold for 1064-nm pulses of 40 ns [1] and 0.15 ns [2] durations. For relatively smooth surfaces, rms roughness <40 A, the correlation between roughness and thresholds held at 40 ns [1], but not at .15

ns [2]. This is but one example of what seems to be a systematic difference in damage induced by

short-duration and long-duration pulses [3]. If such systematic variations are real, theories of laser damage must be compatible. The question of surface roughness and its influence on damage

thresholds is also of practical importance. If smoother surfaces have higher thresholds, additional

polishing will be an easy fix for some laser damage problems. We report here additional measurements

of damage thresholds on polished surfaces of varied roughness.

Entrance-surface and exit-surface thresholds for laser damage on four glass windows were measured

using linearly polarized 1064-nm pulses with durations of 0.17 ns, 1.0 ns, 1.6 ns, and 3.2 ns, incident

at 10° from the normal. Two lasers were used to make the measurements. A passively mode-locked

Nd:YAG oscillator provided the 0.17-ns pulses. The waveform of each pulse was not recorded except by

a fast diode/oscilloscope combination which was adequate to detect improper mode-locking. The assigned

pulse width 0.17 ns is the mean pulse width most recently recorded for this laser. This mean is stable. It has varied from 0.15 to 0.17 ns during the last two years. However, the shot to shot variation in pulse widths from passively mode-locked lasers is large, typically + 30% of the mean. Intensity or optical electric field strength cannot be accurately measured for mode-locked oscillators

unless the waveform of each pulse is recorded. Pulses at 1.0 ns, 1.6 ns, and 3.2 ns were obtained

by gating a portion from a 30-ns pulse produced by a Nd:YAG oscillator operating in a single cavity

mode. The waveform for each pulse was recorded to verify pulse duration.

With both lasers, the beam profile in the sample plane was recorded on each firing, and absolute

flux computed to within + 7%. Beam diameter at the sample was 2-3 mm. Details of this measurement

procedure are recorded elsewhere [4].

Three samples, designated A, 081, and C18, were BK-7 glass. Sample A was a A/20, 2"-diameter,

3°-wedged window of grade-A material purchased for use in the damage experiment. It had been used

for over two years prior to these tests, and frequently cleaned with acetone or alcohol. Details of

its preparation are unknown, except that "absorbing" abrassives, i.e., iron oxide, were not used. Sample 081 was carefully prepared from PH-3 grade material for damage testing. It was ground using

progressively smaller alumina grit size; at each size the surface was ground to a depth equal to 3 times the previous grit diameter. Final polish was done with commercially available CeOg. The surface roughness was 10 & rms [5], which is extremely smooth for conventionally polished BK-7. Sample CI

8

had one conventionally polished surface, and one surface finished by bowl-feed polishing. Companion

samples to CI 8 exhibited rms roughness of 7-9 ft on the bowl -feed surfaces [5]. Ce0

? was used to polish

*Work performed under the auspices of the Material Sciences Program of the U.S. Department of Energy

and Contract No. W-7405-ENG-48

.

2. Threshold Measurements

3. Damage Samples

C18.

164
One sample of optical grade fused silica was tested. It was a 2-inch diameter window prepared for

jse in the damage facility. Polishing history is unknown except that iron-oxide was not used as a Dolishing agent.

All four samples had surfaces which meet or exceed specifications for usable optical components.

Two were typical of conventional polishing, with roughnesses of 20-25 K; two had carefully prepared

surfaces and were somewhat smoother.

4. Results and Conclusions

Damage thresholds for the four samples are shown in figures 1-4. oe drawn. The following conclusions can

1. The smoothest BK-7 surfaces did not have the highest thresholds. Samples C18 and 081 were

arefully prepared yet had thresholds less than that of the routinely polished surfaces on sample A. Some other variable, probably surface absorption, is more important than surface roughness in setting

short-pulse thresholds on surfaces with roughness <30-40 A.

2. Contamination by particulates can lower thresholds at 3.2 ns by as much as 50%. The first

experiments on sample A yielded low thresholds. The sample was scrubbed with soap and water, alcohol,

and acetone, and retested. Surface scatter was reduced and thresholds increased as shown in figure 1. Scrubbing did not change thresholds on new samples C18 and 081.

3. Thresholds scaled approximately as the square root of pulse duration [6]. Departure from this caling law was greatest on the surface with the lowest threshold, the rear surface of C18. For that

sample thresholds at 1.6 and 3.2 ns were nominally the same.

4. Exit-surface thresholds were less than entrance-surface thresholds, although the ratio was not

always as predicted [7] by the ratio of intensities at exit and entrance surfaces,

L exit

[ ent.

4n 2

n + 1

5. Acknowledgments

The author gratefully acknowledges the assistance of G. E. Murphy and S. E. Peluso in maintaining

and operating the damage facility, the assistance by W. L. Smith and W. H. Lowdermilk in collecting

some of the data, frequent useful discussions with M. J. Weber, and the efforts of Lou Beggio in preparing the manuscript.

References

fl]

:2]

House, R. A., Bettis, J. R. ; Guenther, A. H., and Austin, R., "Correlation of laser-induced

damage with surface structure and preparation

techniques of several optical glasses at 1.06 um," in Laser-Induced Damage in Optical

Materials: 1975, A. J. Glass and A. H. Guenther, eds., Nat. Bur. Stand. (U.S.) Spec.

Publ. 435, pp. 305-320 (1975); also R. A. House, "The effect of surface structural properties on laser-induced damage at 1.06

um," Ph.D. dissertation Air Force Institute

of Technology, Wright-Patterson AFB, Ohio, 1975 (now available as report AFWL-TR-76-62,

Kirtland AFB, NM 87117); also R. A. House,

J. R. Bettis and A. H. Guenther, "Surface

roughness and laser damage threshold," IEEE

J. Qu. Elect. QE-5, pp. 361-363 (1977).

Milam, D., Smith, W. L., Weber, M. J., Guenther, A. H., House, R. A., and Bettis,

J. R. "Effects of Surface Roughness on 1064-

nm 150-ps Laser Damage," in Laser Induced Damage in Optical Material

; 1977, A. J. Glass and A. H. Guenther, eds., Nat. Bur. Stand. (U.S.) Spec. Publ., 509, pp. 166-173

(1977).

[3] Milam, D. , "Can a Model Which Describes Gas Breakdown Also Describe Laser Damage to the Bulk and Surface of Solid Dielectrics," in Laser-Induced Damage i n Opti cal Material : 1976; eds., A. J. Glass and A. H. Guenther,

Nat. Bur. Stand. (U.S.) Spec. Publ. 462,

pp. 350-356 (1976).

[4] Milam, D., Laser-Induced Damage at 1064-nm

125-ps; Appl. Opt. 1_6; 1204 (1977).

[5] J. M. Bennett, private correspondence.

[6] Bettis, J. R., and Guenther, A. H., "Scaling

Laws for Damage," elsewhere in this

publ ication.

[7] Crisp, M. D., Boling, N. L., and Dube\ G., "Some Aspects of Surface Damage That Can

be Explained With Linear Optics," Appl.

Phys. Lett. 21; 364 (1972).

165
BARE BK-7 SAMPLE A

Pulse duration, ns

50^)0-0678 2362

Fig. 1. Surface damage threshold vs. pulse duration, BK-7 sample A. Data indicated by squares

was taken prior to a vigorous scrub cleaning. This sample had been used as a splitter prior to these

tests. The accumulated particulates could not be removed by less than scrubbing. Curves are best-fit

square-root functions.

BARE BK-7 SAMPLE 081 Lj

50 00-0678 2360

Fig. 2. Surface damage threshold vs. pulse duration, BK-7 sample 081. Curves are best-fit square root functions.

166
BARE BK-7 SAMPLE C18

Fig. 3. Surface damage thresholds vs. pulse duration, BK-7 sample C18. This sample was not large

enough to allow measurement at all four pulse durations. Curves are best-fit square root functions.

FUSED SILICA .10

2

o

-C

ts> OJ

l_ -CICBO)

CDE

CO Q

50

40

30

20

10

o Entrance Surface Conventional Polish • Exit Surface Conventional Polish

1 2

Pulse Duration, ns

50-00-0678-2358

Fig. 4. Surface damage threshold vs. pulse duration, polished fused silica. Curve is a best-fit

(to exit surface data) square root function. The error in entrance surface threshold at 3.2 ns is due to site to site variations in the surface, and exceeds the + 7% uncertainty in flux measurements.

167
LARGE GIANT AND FREE-RUNNING LASER PULSE ENERGY

AND POWER DENSITIES THROUGH OPTICAL FIBERS*

M. J. Landry

Sandia Laboratories

Albuquerque, N. M. 8 718 5

The surface threshold damage levels (STDL) and coupling efficiencies, e, (ratio of output-to-input energy) have been determined

for single and multiple optical fibers of five manufacturers. For

giant laser pulses of 30 nsec duration, the largest STDL's were observed in American Optical Company fibers for single and multiple

laser shots with values of 48.2 and 31.6 J/cm 2

, respectively. For

free-running laser pulses of 100 usee duration, the largest STDL was observed in Corning Glass Works fibers for single laser shots with

a value of 4.66 kJ/cm 2. For multiple laser shots increased

energy caused the initial damage sites in the binding or fiber

cladding materials to increase in size and cause new sites in these

materials at lower STDL's than for single laser shots. The giant

laser pulses caused the most severe surface damage. There is no evidence to suggest that the observed decrease in e was caused by

other than the increase in damage to the input surface of the

fibers

.

Key words: Free-running laser pulses; giant laser pulse; optical

fiber damage; pulsed Nd laser damage; single and multiple element fibers; surface threshold damage.

1 . Introduction

Many optical fiber applications being considered by investigators include transmitting multimode giant and free-running laser pulses containing high energy and power

One such application requires propagating short duration, high energy giant laser

pulses through optical fibers to initiate secondary explosives [l] 1

, another requires

propagating longer duration free-running laser pulses through optical fibers to initiate primary explosives [2]. The output energy and power handling capacity of

optical fibers are limited by optical damage to the fibers [3-8] and by losses in the

fibers, which include absorption in the bulk material, scattering at the core-cladding

interface, Rayleigh scattering and backward stimulated Raman and Brillouin scattering

[9-13] .

It is the purpose of this paper to report measurements of the coupling efficiency

(output energy/input energy E 2 /E 1

) , input and output energy and power densities, the

surface threshold damage levels (STDL's) and the optical damage in optical fibers

caused by ^30 ns and 100 ys duration Nd:glass laser pulses.

2. Experimental Set-Up

2.1 Mechanical and Optical

The mechanical and optical set-up used in studying the threshold damage level of

the maximum energy transmitted through optical fibers is shown in figure 1. For giant

pulse operation the oscillator (OSC) consists of a 75% reflective (R) output coupler

(M 2 ), Nd 3+ laser rod (LR) pumped by a double elliptical cavity, a stack glass polarizer

(P 2 ) for horizontal polarization, a 3.87 mm diameter aperture (Ai ) , and the Q-switch

(QS) . The QS consists of a Pockels cell (PC) operated at A/2 voltage, a stack glass

polarizer (Pi) for vertical polarization, and a chip-free knife-edge right angle prism

maximum R (Mj ) . For the free-running operation Mj replaces PC, eliminating PC and Pi from the 100 cm long laser cavity. The free-running laser pulse is unpolarized.

The output of OSC is directed through the laser amplified (AMP) and to the input

end of the optical fiber by three mirrors (M 3 , Mi, , and M 5 ). The AMP built at Sandia

Laboratories, consists of a 61 cm long by 1.27 cm diameter, 5% Nd + ED-2 glass rod,

four 50.8 cm by 1.3 cm diameter linear Xenon flashlamps in a close couple pumping

* Work supported by the Department of Energy.

1. Figures in brackets indicate the literature reference at the end of this paper.

168
aeometry, and a power supply. The ends of the rod are cut parallel to each other with

their normals 6° relative to the geometric axis of the laser rod to eliminate reflec-

| tion feedback into the OSC . The power supply charges four 275 yF capacitors to 1.7 kJ.

The OSC beam is sampled at station ( STA) #0 by a beamsplitter (BS 0 ). The AMP beam

is sampled at STA #1 and STA #2 by two beamsplitters (BS! and BS 2 ), with STA #1 sampling before the aperture (A 2 ) and STA #2 after. Each STA contains two uncoated quartz

beamsplitters (BS 0 , BS 1 , or BS 2

, and BS 3 ); a 50-50 coated BK-7 beamsplitter (BS,,); a KD1 photodiode (D 0 , D lf or D 2

) manufactured by Korad; and two linear semiconductor

diode arrays (RA) manufactured by Reticon. Beamsplitters BS 0 , BS 1

, and BS 2 each sample

M% of the incident laser beam. Approximately 3.7% of the incident laser beam is directed to a pressed magnesium oxide diffuser (DF) . The response of D 0 , Di , and D 2 is

calibrated using energy meter (mfg. Quantronic) EM X, EM 2 with A 2 removed, and EM2 with

A 2 in the beam, respectively. EM 3 measures the energy transmitted through the fibers.

Approximately 0.11% of the incident beam is directed toward the Reticon arrays

Model EC-256, EC-384, and EC-512 diodes, which are 1.27, 1.91, and 254 cm long and

contained 256, 383, and 512 diodes, respectively. The diodes, each 0.00254 cm on

edge, are fixed 0.005 cm on line center. At each station one array displays energy

contained in this 0.00254 cm slit height across the laser beam in a horizontal direction (H) and another in a vertical direction (V) . Neutral density filters keep the

arrays and photodiodes operating in their linear regimes.

Aperture A2, beamsplitter BS2, and microscope M are mounted on adapter plates

which are kinematically mounted to plate #1. The microscope is also kinematically

mounted to plate #2. This mounting technique allows the microscope, which is used to

photograph the ends of the fibers (magnification of 35.6 X), and the other components

to be removed and repositioned without refocusing or realigning.

The microscope is a Zeiss Model FS-45 trinocular polarizing tube with a Nikon

photomicrographic attachment Model EFM containing a relay lens, an eyepiece, and a polaroid adapter.

A telescope (T) , aperture (A3), mirrors (M6 and M7) , and beamsplitter (BS5) are

( used with the Spectra Physics Model 124B laser (He-Ne) to align the OSC, AMP, optical

fiber, and other components. Components BS5 and M7 are positioned on kinematic mounts

for ease of removing and precise repositioning during alignment. All the mirrors

which reflect both the He-Ne and the Nd 3+ laser beams are coated for maximum reflectivity at both 632.8 and 1060 nm.

2.2 Electrical

Figure 2 shows the block diagram of the electrical connections of the laser OSC

and AMP, Pockels cell (PC) power supply, oscilloscopes (SCOPE), photodiodes (KD1)

,

Reticon array (H and V) , and associated equipment (Generators) . The timing sequence

for the giant laser pulse experiment is shown in the lower left portion of the figure.

A switch closure in the operator's hand initiates the triggers of the Rutherford

Generator at T 0 = 0. The trigger Ti, to section B of SCOPES 1, 2, and 3 is adjusted to the giant pulse

emission time. Times T6 and T5 are adjusted so the arrays are triggered at the beginning of the first graticule of the oscilloscope. The oscilloscope sweep speeds are adjusted so that each array is read out in ten divisions making each scale division

0.127, 0.191, and 0.254 cm for Model EC-256, EC-384 and EC-512 arrays, respectively.

For the free-running laser pulse experiment trigger T3 is eliminated. Trigger T,, is adjusted to trigger section B of SCOPES 1, 2, and 3 before the laser free lases.

2.3 Beam Profile

The typical responses of the EC-256 Reticon linear diode arrays at STA #2 for

j 0.279, 0.439, and 0.508 cm diameter aperture A 2 are illustrated in figures 3A, 3B,

1 and 3C and for a 0.508 cm diameter hole in figure 3D. The upper traces are the re- sponse of the arrays which are oriented in the vertical direction (V) relative to the

table top and. the lower traces in the horizontal direction (H) . Each major division

(time axis) is calibrated to correspond to 0.127 cm. Figures 3A-3C illustrates the laser beams centered on A2 yielding symmetrical

\ response. The shoulders on the RA response in figure 3A-3D are caused by the Sandia

AMP operating in its saturation regime [14,15]. The RA response in figure 3D contains

concentric rings caused by over filling the AMP rod with the OSC beam. These diode

169
responses show that the assumption of a plane wave incident onto the optical fibers for

the giant pulse experiment is reasonably good for the 0.508 cm diameter aperture.

3 . Theory

For a highly collimated laser light source with no area mismatch to the fiber the

entire energy is coupled into the fiber except for Fresnel reflection and fiber packing fraction losses (neglecting scattering from the 'input face), since numerical aperture (NA) is larger than the laser divergence. The coupling efficiency which accounts

for losses due to transmission (T) , scattering (a) , absorption (a) , and fiber packing

fraction (f x

) is given as

e = TfjCTCt = E 2 /E! , (1) where E 2 and E 1 are the output and input energy of the fiber, respectively. The transmission is given as

T = (1-R) 2 = (1-p 2

) 2

= [l - (n 0 -n 2 ) 2 /(n 0 +n 2 ) 2

]

2

, (2) where n 0 and n 2 are the refractive indices of the air and core material, respectively.

For long fibers (> several meters) the packing fraction, which consists of the packir-ij density (y) and the core fraction (f 2 ), is given by f : = y/f 2

. The maximum packing density is 0.79 or 0.91 for square or hexagonal packed fiber array [16], respectively. If the fibers are loosely packed, these values will be smaller. The core fraction is f 2 = (a/b) 2

, where a and b are the core and core plus clad diameters,

respectively.

For the giant laser pulse tests the length of the fibers used were 1.5 ± 0.1 m and

for the free-running laser pulse test they were 0.3 ± 0.01 m. Cladding modes were not

removed for these short fibers. For these short, low loss fibers (<20 db/Km)

scattering and absorption losses are negligible within ± 1.5%.

4. Experimental Results

In this study we tested optical fibers manufactured by American Optical Co. (AOC)

,

Corning Glass Works (CGW) , Fiber Optic Cable Corp. (FOC) , Incom Inc. (II), and Valtec

(V) . The physical and optical characteristics of these fibers are listed in table 1. The fiber bundles were in single and multiple fiber forms. The individual fibers in

the II and AOC bundles were hexagonal in shape due to the method used to fuse nearest

neighbors. An epoxy binder was used to hold the fibers in the CGW bundles. A silicon

type binder was used to hold the fibers in the FOC and V bundles. Ref loss % in table

1 refers to Fresnel reflection losses.

4.1 Giant Laser Pulse Experiment

Figure 4 illustrates the coupling efficiency (e) as functions of input energy

density and power density (p!) for AOC fibers FN5-FN8 which sustained the largest

STDL and maximum d and pj for giant laser pulses. Multiple laser shots (^20 shots) were incident onto the input end of the fibers with each laser shot containing progressively higher energy. The cross-sectional area of the laser beam A [17] was

1.17 x 10

2 cm 2 for FN 5 and 4.58 x 10

2 cm2 for FN6-FN8. We have d = Ei/A, p x =

P x /A, £ 2 = E 2 /A, and p 2 = P 2 /A where Ej, E 2 , Pj, and P 2 are input and output energy

and input and output power, respectively. The power P is E/x , where E is energy and

t is the laser pulse duration at FWHM.

Both FN 5 and FN6 exhibited an e ^ 0.50 at the onset of the test. For FN 5 and FN6

as the laser energy density was increased e remained approximately constant until the

STDL's of d = 10 and 31.6 J/cm 2 and p x = 0.334 and 1.05 GW/cm 2 were reached, res- pectively. The coupling efficiency continued to decrease with increasing energy

density until at the end of the test an = 19 and 44.1 J/cm 2 and p x = 0.634 and 1.47

GW/cm 2 resulted in an e = 0.369 and 0.368, and maximum energy and power densities out

of the fiber of 17.0 and 39.5 J/cm 2 and 0.568 and 1.32 GW/cm 2

, respectively. The £ 2 and p 2 were derived assuming absorption and scattering losses were negligible with £ 2 = T? ! and p 2 = Tp lf where p 1 = £>1 /t.

Fiber bundles FN 7 and FN 8 were tested differently in that initially each fiber

received a laser pulse containing low energy, then FN7 and FN8 received two and one high energy laser pulses, respectively. The measured coupling efficiency was slightly

less for the smaller laser pulse. For FN 7 , the third laser pulse with Ci = 40.8 J/cm

170
gave a calculated e = 0.44. This ^ value failed to exceed the STDL of FN7 . A laser

pulse with a higher £ x = 48.2 J/cm 2 and p 1 = 1.61 GW/cm 2 gave a calculated z = 0.468

for FN8. This exceeded the STDL for FN8 . The multiple laser shots (LS's) in FN6

Table 1. Physical and optical characteristics of fibers

Fiber rirg C03TG no. or Core Corel Ref T NA

NO. Mat Fibers dia (ym) n Loss % Giant Pulse Laser Exp.

FN1, FN2 II Flint* 1 0.635 (cm) 1 .602 5. 35 0 .896 0 .66+ FN3 , FN4 II Flint** >600 200 1 . 608 5. 43 0 .894 0 .66

+ FN5 , FN6,

FN7, FN8

AOC Flint* >600 84-90 1 .602 5. 35 0 . 896 0 .66

+

FN9 CGW Silica 61 87-96 1 467 3 . 62 0 .929 J

0 .18 + + Free-Running Laser Exp.

FN10-FN17,

FN22

V Silica***

*** Silica

1,7

1 100-140

250

1 449 3 . 36 0 .934 0 .30 FN18-FN21 FOC 7 210-240 1 449 3. 36 0 .934 0 30

FN23-FN27 CGW Silica++

7 87-96 1. 467 3. 62 0 . 929 0 18 + +

t Measured at 1060 run + Calculated at X = 589.3 nm, n = 1-62 (core) and 1.48 (cladding)

** F-7 Schott glass

++ Ge-doped baron doped silica cladding

tt Calculated at X = 1.060 nm, n = 1.467 (core) and 1.459 (cladding)

*** Cladding plastic

caused damage at lower levels than for single LS's, in agreement with observed surface

damage in bulk material [18] . The photographs in figure 5 illustrate the increase in surface damage observed at

the input end of FN5 (liqht spots) with increasing £i. Prior to each photograph £i = 10, 11.3. 14.5, 14.4, 15.6, 16.5, and 19 J/cm2 was incident on FN5 for figures 5A-5G,

respectively. Although a decrease in e was observed at 10 J/cm 2 in figure 4 for FN5, no apparent optical surface damage is visible in figure 5A which is the photograph of the input end of the fiber after that LS. It is evident that either damage

must be occurring in the core even though the damage is not visible or cladding modes

are excited thus causing the decrease in c. The first visible optical surface damage

to the input end of FN5 appears between two fibers after the next LS with an input

energy of Ei = 0.132 J, which is a STDL of C i = 11.3 J/cm 2 and a pi = 0.377 GW/cm 2 (fig. 5B).

It is believed that the points where damage initiated have the largest stress be- cause of the difference between cladding and fusing materials. As the energy in the

laser beam was increased new damage sites appeared and the old ones became larger.

This is as expected if the STDL for the entire end of the fiber is constant. As the

energy in the laser beam increased its energy density at the center also increased

causing more damage at the old sites. As the energy density at the periphery of the

beam reached STDL new sites were formed. At the end of the test, the first damage

site was the largest.

The photograph in figure 5B shows that initially the STDL was exceeded over an area approximately equal to the diameter of a single fiber (7.2 x 10 -3 cm) which is

comparable to the diameter of the most intense portion of the triangularly shaped

laser beam (5.1 x 10~ 3 cm shown in fig. 3B) . Over this area E, x = 90 J/cm . The dimension of the smallest damage structure in figure 5G is (<0.1 fiber core

diameter) <7.2 x 10

-1

* cm. The maximum input energy of Ei = 0.222 J caused damage

over a maximum linear dimension of 1.61 cm, which was a slightly larger dimension

than that of the laser beam FWHM (0.123 cm), but less than the dimension of the laser

beam containing 90% of the energy (0.254 cm). The implication is that the fibers at

the periphery damaged at a lower STDL than those in the center.

171
Figure 5D compared to figure 5C shows that a laser pulse containing the same energy

as the previous pulse caused additional surface damage. This is in agreement with observed STDL trends in bulk material which has seen previous laser energy [18]

.

Fibers FN1-FN4 were tested similar to FN5 and FN6 . Within experimental error the

e measured for FN2 (0.895) was equal to T (table 1) , since f i = 1. The lower e of

0.81 observed for FN1 was attributed to either increases in internal scattering or external scattering at the ends of the fiber due to end fabrication. FN1 and FN2

broke 18 and 17.8 cm from their input ends when 5 i = 4.76 and 5.63 J/cm 2 and pi = 0.159 and 0.188 GW/cm 2 were incident on the fibers, respectively. A = 0.1829 cm 2 (as

determined using the FWHM method) and the maximum input energy was 1.07 J. The output

energy densities £2 = 3.76 and 4.67 J/cm 2 and output power densities P2 = 0.125 and

0.156 GW/cm 2 for FN1 and FN2 , respectively. No damage was observed on the input faces

of either fiber.

The values of £ measured for FN3 and FN4 were less than those observed for FN1 and

FN2 because f 1 ^ 1. The hexagonal packing density was close to optimum so f2 ^ 0.93.

Neglecting absorption and scattering losses in the fiber, a calculated value for the

coupling efficiency is e (C) = Ty

f

2 = ( 0 . 894 ) (0 . 91 ) (0 . 93 ) = 0. 75 ; however, the cou- pling efficiency measured was between 0.5 to 0.6 and 0.5 for FN3 and FN4 , respectively.

The hexagonal shape of the individual fibers in fiber bundle FN3 and FN4 could have

caused increased losses which were not accounted for in the calculation. The observed

increase in 90° off-axis scattering in FN 3 and FN 4 compared to FN1 and FN 2 is suspected

as being caused by core to cladding interface scattering. The STDL's of FN3 and FN4 were measured at 4.02 and 4.62 J/cm 2

, respectively. A maximum value of £ 1 = 6.78 J/cm 2 and pi = 0.226 GW/cm 2 was observed for both FN3 and FN4 . The corresponding values of

£2 and P2 will be less by a factor act, which if taken to be e (C) is between 0.20 to 0.33 for FN 3 and 0.33 for FN4

.

As the energy in the laser beam was increased, new damage sites appeared and the

old ones became larger. At the end of the test, the first damage site was the largest.

Prior to testing CGW fibers, we attempted to produce a more spatially uniform laser

beam and still retain fairly high energy density by changing aperture A2 from a 0.279 cm to a 0.439 cm diameter hole. This failed and produced a large laser-to-fiber mismatch for FN9.

In order to reduce the potential future damage to the fibers in determining e at

low input energy, the first e values was measured using a laser pulse of long duration

('vlOO us pulse length). The pi is 3 x 10~ 3 times smaller for the free-running laser

pulse compared to the giant laser pulse. Input energies of Ei = 0.050, 0.092, and

0.127 J resulted in E 2 = 0.0020, 0.0038, and 0.005 J and P 2 = 20.0, 38.0, and 50.0 W

with corresponding £,2 = 0.46 , 0. 88 , and 1.15 J/cm 2 and p 2 = 4 . 60 , 8 . 80 , 11. 5 kW/cm 2 produced e = 0.040, 0.420 and 0.042, respectively. The low e values were caused by the

laser beam-to-fiber bundle mismatch.

Two relatively high energy pulses (giant 30 ns pulse duration) of Ei = 0.790 and

0.775 J produced E2 = 0.036 and 0.0336 J, respectively. The corresponding £ 1 = 6.48

and 6.35 J/cm 2 and pi = 0.216 and 0.212 GW/cni 2 and £2 = T£ x = 6.02 and 5.90 J/cm 2 and

P2 = Tpi = 0.216 and 0.212 GW/cm 2

, if the laser beam area (A = 0.122 cm 2

) used in the

density calculations is that determined by the FWHM area method. If we consider the core cross-sectional area A(f) of the 60 good fibers the corresponding £ 2 = 8.29 and

7.74 J/cm 2 and p 2 = 0.276 and 0.258 GW/cm 2 and d = 5?/T = 8.92 and 8.33 J/cm 2 and p 1 =

p 2 /T = 0.297 and 0.278 GW/cm 2

. The E, and p values calculated using A(f) are ^25%

higher than those calculated using A. This large difference illustrates the inaccuracy

involved in assuming the laser beam to be spatially uniform in calculating E, and p values and in using the FWHM area method in determining the cross-sectional area of the

laser beam.

No visual optical surface damage was evident at the input end of FN9 after both

high energy laser pulses, nor was there any decrease in e. It must be concluded that

the STDL for single laser shot for FN9 was not reached. Our E, 1 values were limited by

the aperture diameter, the size of the OSC and AMP, and the fact that we wanted to use unfocused optical radiation at the input of the fiber during the giant laser pulse

experimer. t.

4.2 Free-Running Laser Pulse Experiment

In this portion of the experiment free-running laser pulses were transmitted

through optical fibers before and after they were exposed to >10 6 rad(Si) of gamma

radiation. The optical fibers were irradiated at the Sandia Laboratories Gamma

172
Irradiation Facility using a CO 60 source. Tests duration were from 9.7 to 11.6 hs

Table 2 lists the 18 fibers tested using the free-running laser pulses. The maxi- mum calculated output energy density £ 2 (M) = E 2 (M)/A(f), where E 2 (M) is the maximum

output energy and A(f) is the total area of the fiber core or cores and the maximum

calculated input energy density C i (M) = £ 2 (M)/T. The total number of laser shots

transmitted through the fiber at both low and high energy density before or after

gamma irradiation (irr) is recorded under No. Laser Shots.

Table 2. Free-running laser experiment energy

densities and coupling efficiencies in fibers

Fiber

No

Mfg No. Fibers

Max imum e No. Laser

Shots

Irr

Mr ad* c^ 2

, kJ/cm 2 ? 2 kJ/cm 2 (Max.

)

FN10 V 7 0. 061 0.065 0.160 3 FN11 V 7 0. 093 0.100 0.107 4 FN12 V 1 +

0. 014 0. 015 0.017 5 FN13 V 1 + 0.739 0.791 0.896 3 FN14 V 7 2.766 0.962 0.881 7 10 6 FN15 V 1+ 2.450 2.623 0.708 3 FN16 V 1

+

3 . 554 3 .806 0.887 3 FN17 V 1

+ 0.286 0.306 0.800 3 FN18 FOC 7 1. 022 1. 094 0.788 12 - FN19 FOC 7 0.135 0.145 0.814 7 10 6 FN2 0 FOC 7 0. 907 0. 971 0.852 11 10 6 FN21 FOC 7 0.269 0.288 0.858 1 10 6 FN2 2 V 1 +

0. 079 0.085 0.908 5 FN23 CGW 7 2.680 2.895 0.792 11 10 6 FN24 CGW 7 0.278 0.300 0.744 4 10 6 FN25 CGW 7 3.978 4.282 0.688 5 10 6 FN26 CGW 7 1.950 4. 500 0.295 2 10 6 FN27 CGW 7 0.281 0.303 0.905 6 10 6

* Background 3-5.7 x 10 s rad(Si)

+ PC10

+ PC 5 Fibers FN10-FN16 were aligned relative to the free-running laser pulse using a pointing system, while FN17-FN27 were aligned using a Hadron trinocular viewing system.

In the first method a laser burn was made on Hadron "fingerprint" paper placed at the

input end of the fiber approximately at the focal point of an 8.09 cm focal length lens

(Li). A pointer was placed at the center of the laser burn. The paper was removed.

The input end of the fiber was center to the pointer. A 10X eyepiece was used in positioning the above.

The Hadron trinocular viewing optical system contained a flip-flop mirror which

was used in viewing the input end of the fiber and the burn. With the flip-flop

mirror up a burn was made on "fingerprint" paper placed at the input end of the fiber

approximately at the focal point of the 9.01 cm focal length lens (L 2 ) of the Hadron

trinocular viewing optical system. With the flip-flop mirror down the crosshair of

the viewing eyepiece was centered over the image of the burn. The image of the input

end of the fiber was centered to the crosshair by moving the input end of the fiber.

Initially the input ends of FN10 and FN11 were placed 0.87 cm before the focal

point of Lj . The laser pulse was incident on all 7 fibers of these fiber bundles.

The total area A(f) of the 7 fiber cores of FN10 was A(f) = 6.4 x lO"" cm 2

. E 2 =

0. 038 J resulted in E, 2 = 0. 059 kJ/cm 2

, C i = 0. 063 kJ/cm 2

, and a low e = 0.16. From

the E x = 0.242 J for the same shot and £i value above, the area of the laser beam

A = E 1 /E, 1 = 0.242/0.063 = 3.75 x 10" 3 cm2 was calculated. The area of the furrule

173
containing all the fibers was 2.89 x 10" 3 cm

2

. No visual damage resulted at these_low

input power levels. A similar calculation for FNll resulted in a A(f) = 5,58 x 10 11 cm 2 and a slightly smaller A = 4.86 x 10 " 3 cm 2

.

For both fibers, photographs after each laser shot (LS) show that the binding and

cladding materials vaporized and recollected onto the ends of the fiber. This accounts

for the observed decrease in e with increasing number of LS 1 s , as well as the physical

change to the input face of the fibers.

A single PC10 V fiber (FN12) was placed 0.38 cm past the focal point of Li . The

area of the fiber core A(f) = 5.27 x 10~" cm 2 while A = = 0.415/14.5 = 2.86 x 10" 2 cm 2

. Photograpl.s of FN12 in figures 6A-6E, taken after each LS , show that the

left side of the cor« is increasingly overlayed with what appears to be recondensed

vapors of the vaporised plastic cladding. This accounts for the observed decrease in

e from 0.017 to 0.0108 with increasing number of LS ' s . Only the left side of the

cladding shows deterioration because the other portion was broken at a slant range

away from the plane of the paper such that the laser beam was not incident onto it with

as high an £i. The low e values indicate a large mismatch between the laser beam diameter and the core diameter.

Another single PC10 V fiber (FN13) was used in order to decrease the laser beam to

fiber core mismatch. The Li-to-fiber dimension was increased by another 0.15 cm thus

positioning FN13 0.5? cm past the focal point of Li , as was determined by burn _ dimensions on "fingerprint" paper as a function of distance from Li. The 0.53 cm dimension

resulted in the smallest burn diameter. The maximum e value for this fiber position

was 0. 896. The area of the fiber core A(f) = 5.03 x 10 ~" cm 2 while A = Ei/£i = 0.415/791 = 5.25 x 10"" cm 2

, which was slightly larger than A(f). The photographs

after the LS * s do not show any apparent damage to the fiber core yet e decreased from

0.896, 0.876, to 0.723 for approximate constant £i values.

The central fiber of FN14 was tested with the same laser beam diameter as used for

testing FN13. The £ 2 and e values are listed in the upper (kJ/cm 2

) and lower numbers

in each photograph, in figure 7. The appearance of the input end of the fibers changed

gradually until the 4th LS (figure 7), where an abrupt change occurred. More LS '

s

continued to change the physical characteristics of the input face of the fibers.

The high values of e calculated for FN14 in figure 7 indicate that the laser energy

was transmitted through only the central fiber. The central core area A(f) = 1.37 x

10 ~" cm 2

, while the calculated A = 1.45 x 10"" cm 2, which is slightly larger than A(f)

.

For the lower value of £ = 0. 426 the calculated A = 3.00 x 10 ~" cm 2 is slightly larger;

however, the laser beam area was not changed for this shot. The larger calculated A

indicates that the £i value was smaller due to the decrease in transmission at the face

of the fiber due to vapors recondensing over the entrance or core of the central fiber

of FN14. The ratio of the laser areas 1.45 x 10~"/3.00 x 10'" is equal to the ratio

of e's. The conclusion is that all the energy went through the central fiber core. We placed FN15-FN17 into the test system to determine whether we could couple the

laser energy into single fibers of 1.21 to 1.26 x 10" 2 cm diameter (d) . The variations

in the observed e values for these fibers (table 2) are attributed to the uneven breaks

in the ends of the fibers. The £ 2 of 3.554 kJ/cm 2 for FN16 was the highest record

for V fibers.

The photographic sequence for FN16 (d = 1.21 x 10" 2 cm) in figure 8 shows that after

the 1st LS containing large input energy density, (£1 = 3.806 kJ/cm 2

) figure 8A, slight

damage appeared at the input face. This was the 2nd LS into the fiber, the 1st was at

£1 = 0.307 kJ/cm 2

. The next LS , £1 = 3.768 kJ/cm 2 (figure 8B) increased the damage.

At lower £1 values in FN17 (d = 1.21 x 10

" 2 cm) more severe damage occurred after the

3rd LS into the fiber (compare photographs in figure 8C taken after the 1st LS £ 1 = 0.131 kJ/cm 2

, to figure 8D taken after the 3rd LS , £1 = 0.306 kJ/cm 2 ). The calculated

A for FN15, FN16, and FN17 was 1.65, 1.21, and 1.35 x 10"" cm 2

, respectively. The A

for FN15 and FN16 compare favorably to that calculated for FN14 , when the same test

configuration was used. The Hadron viewing system was used for FN17 with the fiber

at the focus of L 2 (f = 9.01 cm).

The 10 6 rad(Si) of gamma irradiation did not reduce the £ 2 or z values observed

for V fibers as seen by comparing FN13 to FN14 (table 2)

.

Both low and high £ 2 values were calculated for the FOC fibers FN18-FN21. The

cross-sectional areas of the central fiber core tested varied from 3.49, 3.49, 4.17,

to 4.49 x 10"" cm 2

, respectively. The calculated £ 2 values in table 2 are low since

the area used was that of the core of the fiber tested. The laser beam cross-sectional

174
area calculated from FN17 data was A = 1.34 x 10' k cm 2 or 2.60, 2.60, 3.11, and 3.35

times smaller than the core area of fibers FN18-FN21, respectively. The variations

in e from fiber to fiber is attributed to variations in surface finish of the ends of

the fiber.

As a means of determining the area of the laser beam for FN18-FN21 more accurately, a V fiber FN22 with a 1.13 x 10~ 2 cm core diameter (A(f) = 9.94 x 10" 5 cm 2

) was

tested. As shown in table 2, the maximum c value observed was 0.908, but the variation in its value was from 0.881, 0.908, 0.895, to 0.885 for an average of 0.892. If

the fiber losses consisted of only Fresnel reflection losses at its ends e = T = 0.929

(table 1). It can be concluded that the cross-sectional area of the laser beam at the

input end of FN22 was equal to the cross-sectional area of FN22 core to ±2%. The A

values varied from 1. 02, 1. 02, 1. 04 , to 1.08 x lO" 1* cm 2 for an average value of 1.03 x

10

~ k cm 2

. Using this as the A for FN18-FN21 the calculated £ 2 and Ci values changed

as shown in table 3. These values are A(f)/A = 3.4, 3.4, 4.1 and 4.4 times larger

than those listed in table 2 for FN18-FN21, respectively.

Table 3. Free-running laser experiment

calculations of £ 2 and £i using

the average A from FN2 2 Fiber

No.

A(f] /A Maximum ^ 2 kJ/cm z kJ/cm 2

FN18 3 4 3.490 3.675

FN19 3 4 0.453 0.487

FN20 4 1 3.640 3.897

FN21 4 4 1.162 1. 245

Figure 9 illustrates two things: (1) the variations in the surface finish of the

ends of FOC fibers represented by the speckle appearance or white spots on the ends of

the fibers and (2) the optical damage to the central fiber of FN20 after £i = 0, 0.068,

4.090, 3.974, 4.090, 4.283, 4.100, 4.042, 4.023, 4.071, 4.139, and 4.234 kJ/cm 2 for

figures 9A-9L was incident on the fiber for the 1st LS before irr and Ist-lOth LS after

irr, respectively. There is no drastic change from one photograph to another, but rather, a gradual change takes place in the immediate surroundings of the central fiber

between the 12 o'clock and 4 o'clock position, a blistering on the periphery of its core between the 11 o'clock and 1 o'clock position, and an over layer of recondensed

binding or fusing material located over the central core radially inward from the

12 o'clock position. This small change in appearance from start to finish is in agreement with the observed change in e of about 5.7%.

Again the 10 6 rad(Si) of gamma irradiation did not reduce the £ 2 ° r £ values for

the FOC fibers (compare FN18 to FN20) . Comparable or higher e values were obtained for

before and after irradiation.

The CGW fibers (FN23-FN27) were the final group of fibers tested. Both low and

high energy densities were propagated through these fibers. The cross-sectional area

of FN23-FN27 A(f) varied from 5.97, 5.97, 6.36, 5.59, to 6.76 x 10" s cm 2

, respectively.

These fibers had the smallest core diameter of all the fibers tested resulting in the

largest £ 2 values (FN25) as shown in table 2. A total of five LS ' s were incident on FN25, three had an £ 2 of 0.107, 0.107, and

0.121 kJ/cm 2 before irr and the remaining two LS ' s had £2 of 3.978 and 0.173 kJ/cm 2 after irr. The corresponding £ 1 values were 0.132, 0.135, 0.135, 4.282, and 4.477

kJ/cm 2. The last K\ value was calculated using A = 8.22 x 10" 5 cm

2

, which was obtained

from the Ei and the £1 values of the previous LS. The low 5 2 value for the last LS

indicates that the fiber was either not aligned or it broke during the shot. Figure

10C shows that after the last LS the central fiber core was broken. Figures 10A-10C

are photographs of the input end of FN25 before the 1st LS , after the 1st LS (E, 1 = 4.282 kJ/cm 2

) after irr, and after the 2nd LS (E^ = 4.477 kJ/cm 2

) after irr,

respectively.

Figures 10D-10F are photographs of the input end of FN26 before the 1st LS , after

the 1st LS (?! = 0.089 kJ/cm 2 ), and after the 1st LS (£1 = 4.500 kJ/cm 2

) after irr,

respectively. The area of the laser beam was the same as for FN25. There is evidence

in the last photograph (fig. 10F) that the beam could have been larger than the central

core since not only it changed in appearance. The cross-sectional area of the damaged

area was 5.84 x 10

~ k cm 2

. If this is taken as the area of the laser beam instead of

175
the cross-sectional area of the central fiber core then, £ 2 = 0.187 kJ/cm 2

. Earlier

test of FN22 established an upper limit on A = 1.03 x 10

5 cm 2 setting a lower limit of

£2 = 1.05 kJ/cm 2 for FN26. What looks like burned areas around the central fiber could

be recollected blow off of binding material.

At lower £1 and £ 2 values the input ends of the CGW fibers damaged less severely

as is illustrated in figures 11A-11G for the top fiber of FN23. A total of ULS's were incident on this fiber. Figure 11A is a photograph before any LS ' s and figures 11B-11G

are for lst-5th, and 7th LS 1 s after irr with values of 2.750, 2.262, 2.406, 2.895,

0.887, and 2.804 kJ/cm2

, respectively. The low e calculated for figure 11F indicates

that the laser beam was misaligned and causing damage to the surroundings of the top

fiber. The inhomogeneous material around the top fiber in figure 11F and 11G appears

to be recondensed vaporized binding material.

The e values were observed to decrease with increasing number of LS ' s for all

fibers whose input ends were damaged. Figure 12 illustrates the decrease in e values

for large E, 1 (AVG) values for FN18 , FN20, and FN23 as functions of laser shot No. The

decrease in e values from start to finish were 0.071, 0.059, and 0.173 for FN18, FN20,

and FN23, respectively. The erratic behavior of the FN23 e values is attributed to the

laser beam diameter being approximately equal to the fiber core diameter and some varying malalignment between the two.

5. Discussions and Conclusions

5.1 Giant Laser Pulse Experiment

For all nine fibers tested with the giant laser pulse, table 4 lists core material,

£j levels required to produce optical damage in bulk material, £j and p x levels required to produce optical damage in fiber material, and the maximum energy and power densities applied and transmitted through the fibers. In the case of FN1 and FN2 the damage

was inside, not on the input surfaces. The STDL for the bulk F2 glass was slightly

less than for the F7, while the STDL's for these glasses in fiber form were reversed

[19]. The F2 or F7 glass used in the core was not the material that damaged initially,

but rather, the binding material used between the fibers damaged. For FN3 and FN4 the

binding material was the 10ym boron silicate cladding. For FN5-FN8 the binding material was the ASG67 AOC glass around the En-2 glass cladding. The bulk STDL's for these

materials are not known. At higher energy levels the STDL of the F2 and F7 fiber core material was exceeded. It is not known whether the STDL of F2 and F7 core material

would have been exceeded if the II fibers had stood alone without being bonded to neighbors or if the AOC fibers had stood alone without the ASG67 binding material. The

stress in the cladding of the II fibers could have reduced the STDL of the boron silicate cladding material just as stress might also have been involved in the damage of

the ASG67 binding glass.

Table 4. Giant laser pulse experiment energy

and power densities in bulk and fibers

Fiber

No. CoreMat

STDL

Bulk Maximum in Fiber + Fiber

?! kJ/cm2 J/cm 2 Pl

2 GW/cm 2 J/cm 2 J/cm 2 Pi GW/cm 2 GW/cm 2

FN1+ F-2* 2.8 4.76 0.159 4.76 3.76 0.159 0.125

FN2"1

" F-2 2.8 5.63 0.188 5.63 4 .67 0.188 0.156

FN3 F-7* 3 . 5 4 . 02 0.134 6.78 6.06 0.226 0.202

FN4 F-7 3 . 5 4.62 0.154 6.78 6.06 0.226 0.202

FN5 F-2 2.8 11. 3 0.377 19.0 17.0 0 . 634 0.568

FN6 F-2 2.8 31.6 1.05 44.1 39 . 5 1.47 1.32

FN7 F-2 2.8 40.8 1.35 40.8 36 . 5 1.36 1.22

FN 8 F-2 2.8 48.2 1.61 48.2 43.2 1.61 1.44

FN9 Q++ 8 . 92 8.29 0.297 # 0.276

+ [19]

t Volume damage occurred

* Schott glass

++ Ge-doped fused quartz

# Calculated from £ 2 and P 2 values

176
The AOC fibers, followed by the CGW fibers, exhibited the largest STDL for single

(FN8) and multiple (FN6) LS ' s and the largest maximum E,

1 , £ 2 , pj , and p 2

. For fibers

receiving multiple LS's the AOC fiber exhibited the greatest decrease in coupling

efficiency.

In all cases, the damage at the input end of the fibers was initiated in the bond

between the fused fibers. Increased input energy caused the initial damage site to increase in size and caused new sites to be formed at lower STDL with increasing radial

dimension relative to the initial site. The fibers receiving single LS's containing

large energy densities (FN7 and FN8) exhibited as large or larger coupling efficiencies

with less optical damage than those fibers receiving comparable £1 and p lf but had seen previous laser energy (FN5 and FN6). For multiple LS's into the fibers once optical

damage occurred the e for the fiber decreased with equal or increasing laser energy.

5.2 Free-Running Laser Pulses

As for the giant laser pulse experiment, fibers tested with free-running laser

pulses damaged initially at the cladding or binding material used in holding nearest

neighbors. Table 5 lists the STDL for 7 of the 18 fibers tested. For multiple LS's

two FOC fibers FN18 and FN20 exhibited the largest STDL's followed by two V fibers

FN16 and FN14 then by a CGW fiber FN23 . For single LS's two CGW fibers FN25 and FN26

exhibited the largest STDL's followed by a V fiber FN15. The STDL for a single LS was

not determined for FOC fibers.

Table 5. Free-running laser experiment

STDL in fibers

Fiber

No. Mfg STDL No. of

Laser

kJ/cm Shots 2 Pi MW/cm 2

FN14 V 2.962 29.62 3 FN15 V >2.623 26.23 1 FN16 V >3 .806 38 . 06 2 FN18 FOC 3 . 675 36.75 2

4 . 660* 46.60 2 FN2 0 FOC 3. 897 38.97 8 4.320* 43.20 8 FN23 CGW 2.895 28 .95 6 FN25 CGW 4 . 282 42.82 1 FN26 CGW 4 . 500+ 45.00 1

* A = 1.03 x 10 "* cm 2 determined from FN22 test

+ Damaged during laser shot

At lower than STDL many free-running laser pulses can be transmitted through V,

FOC, and CGW fibers with only slight decreases in e (Ae) as shown in table 6. Table 6. Free-running laser experiment

decrease in e for

multiple laser shots in fibers

Fiber

No. Mfg Ae No. of

Laser Shots

Avg £, kJ/cm 2

FN 14 V 0.185 4 2.873

FN18 FOC 0.071 9 1 . 052

FN20 FOC 0. 059 10 0.929

FN2 3 CGW 0. 173* 7 2.605

* Variation, last shot e largest

177
The 10 s rad(Si) of gamma irradiation did not reduce the E, 2 or e in V fibers (compare FN14 to FN16 in table 2) , in FOC fibers (compare FN18 to FN20) , or in CGW fibers.

The e values were approximately the same for both low or high E, 2 values after gamma

irradiation of the fibers as seen for FOC fibers (compare FN19 and FN21 to FN20) and

for CGW fibers (compare FN24 and FN27 to FN23, FN25, and FN26).

In general the damage caused by free-running laser pulses was much less severe

than that caused by giant laser pulses (compare fig. 5 to fig. 6-11).

For all fibers in which STDL's were reached there was no evidence that the observed decrease in e was caused by nonlinear optical behavior. It is believed that the

decrease in e was due to increases in observed optical damage at the input face of the

fibers

.

6. Acknowledgments

I would like to thank H. P. Davis for his competent and diligent assistance in

setting up this experiment and, with the help of R. J. Haushalter, acquiring the data.

The financial support of AFWL/ELP for the free-running laser experiment is also

acknowledged.

7. References

1 Landry, M. J., SLA Report No. SAND78-0602 (1978).

2 Partial support by AFWL/ELP,

P077-134.

3 Brown, R. G. , Appl. Optics 6_, 1269

(1967)

.

4 Veinberg, V. B. , Kuz'mina, N. P.

,

and Frolev, D. M. , Sov. J. Optics

Tech. 43, 282 (1976) .

5 Davis, F. W. and Shrader, J. E.

,

Boeing Co. Report No. 'D18 0-1923 5-1

(1976) .

6 Crow, J. H. , Appl. Optics 13_, 467

(1974) .

7 The method for determining energy

density in reference 6, as determined by the energy divided by the area of the damaged area, results

in a threshold damage level which

is high relative to the threshold

level determined by energy divided

by the area of the laser beam

containing one-half the beam energy.

8 Goldman, J. A. and Meyer, R.

,

Nature _205, 892 (1965) .

9 Smith, R. G. , Appl. Optics 11 , 2489 (1972).

10 Stolen, R. H. , Ippen, E. P., and

Tynes, A. R. , Appl. Phys. Lett.

20, 62 (1972).

11 Ippen, E. P. and Stolen, R. H.,

Appl. Phys. Lett. 21, 539 (1972).

12 Stolen, R. H. and Ippen, E. P.,

Appl. Phys. Lett. 22, 276 (1973).

13 Stone, J., Appl. Phys. Lett. 2_6, 163 (1975)

.

14 Brannon, P. J., SLA Report No. SC-TM-72-0558A (1972).

15 Palmer, R. E. and Brannon, P. J.,

SLA Report No. SLA-73-0157 (1973).

16 Holimer, J. P., SLA Report No. SAND75-0610 (1976).

17 The area used in determining the

densities is that of a circle whose

diameter is equal to the dimension

at FWHM of the horizontal Reticon

array response at STA #2, see figure

3, which is the polarization direction of the laser beam, hereafter

called the FWHM method. The

horizontal dimension of the laser

beam is about 3% greater than its

vertical dimension.

18 Olness, D., J. Appl. Phys. 3_9, 6 (1968).

19 Neuroth, N., Hasse, R. , and Knecht,

A., in Proc . 3rd Symp. Damage in

Laser Mat's., ed. by Glass, A. J. and Guenther, A. H. , (NBS Spec.

Pub. No. 356, 1972), pp 3-14.

178
Figure 1. The mechanical and optical set-up used in studying surface threshold damage

levels of optical fibers is illustrated. The components are the Nd 3+ laser oscillator

(OSC) and amplifier (AMP), alignment gas laser (He-Ne) , beam sampling stations (STA #0,

STA #1, and STA #2), energy meters (EM X , EM 2

, and EM 3 ), microscope (M) , optical fiber,

directing mirrors (M 3

, , M 5 , M 6

, and M 7 ), beamsplitters (BS 5 , BS 0 , BS lf and BS 2 ),

and restricting apertures (Aj , A 2

, and A 3 ). Each beam sampling station has two beamsplitters (BS 3 and BS„), a diffuser (DF) , a photodiode (D„, D x

, or D 2 ), and two linear

diode arrays (RA)

179
RUTHERFORD

GENERATOR

b

2-

PC POWER

SUPPLY

LASER OSC. POWER

SUPPLY

SYS Fl

'0

H P 214

3EN. # 1 ^

LASER AMP

POWER

SUPPLY

To © q

SCOPE

INT RT

T/i Tn Ct\ Ja In CD T„

KDI

T

0 © T

4 SCOPE

INT RT

KDI

0©

SCOPE

INT RT

DC POWER

SUPPLY

KDI

XT

h P 214 GEN. #

2

'5®

SCOPE

V H

RC 100

Or

5 ©

SCOPE

V H

RC 100

US

IN HP 214 GEN. # 3 la

h

T

5 ©

SCOPE

V H

RC 100

RETICON POWER

SUPPLY

T 1= 40

T

2 = 180 /iS

T

3 = 530 ^S

T

4 = 5 3 1

T

5 = 570 VS

T

6 = 571/JS

T0 T

1

jj a T3W6

Figure 2. This block diagram illustrates the electrical connections of the laser OSC

and AMP, Pockels (PC) power supply, oscilloscopes (SCOPE) , photodiodes (KDI) , Reticon

arrays (H and V), and associated equipment (HP 214 GEN. #1, #2, and #3, Rutherford

generator, DC power supply, and Reticon power supply) . The relative time sequence is

also illustrated.

130
-H-H

-H-f-tFigure 3. The laser beam profile as observed by Model No. EC-256 Reticon arrays at

Sta #2 for apertures with diameters of (A) 0.508 cm, (B) 0.279 cm, (C) 0.439 cm, and

(D) 0.508 cm. Full scale deflection in amplitude (vertical) is 5 divisions. Each

major horizontal division is 0.127 cm.

INPUT POWER DENSITY, P. (GW/cm^)

in

INPUT ENERGY DENSITY, ( J/cm ) in

Figure 4. The coupling efficiency (e) is illustrated as functions of input energy

density and input power density (pi) for four AOC fibers (FN5-FN8)

131
A B

Figure 5. Photographs A-G show the increase in surface damage to the input end of

FN 5 (AOC fiber) for increase in input energy density (£1) = 10, 11.3, 14.5, 14.4,

15.6, 16.5, and 19 J/cm2

, respectively.

132
Figure 6. Photographs of the input end of FN12 illustrate its surface damage and

decrease in e (lower no.) for low £2 values (upper no. in kJ/cm2

) with increasing laser

shot (LS) no. Photographs A-E were taken after the lst-5th LS , respectively.

183
Figure 7. Photographs of the input end of FN14 illustrate the increase in its surface

damage and corresponding decrease in e (lower no.) for high £2 values (upper no. in

kJ/cm2

) with increasing laser shot (LS) no. Photograph A was taken after the 1st LS

.

Photographs B-F and H were taken after the lst-6th LS after irr. Photographs F and G were taken after the 5th LS after irr and H and I after the 6th LS after irr. Photograph J was taken before the 1st LS.

184
______

Figure 8. Photographs A and B of the input end of FN16 illustrate the increase in its

surface damage after two high £2 value (upper no. in kJ/cm2

) laser shots (LS's).

Photographs C and D of the input end of FN17 illustrate the increase in its surface

damage after the 1st and 3rd LS for low £2 values.

185
Figure 9. Photographs of the input end of FN20 illustrate the gradual increase in the

surface damage to the central fiber and decrease in e (lower no.) for large £2 values

(upper no. in kJ/cm2

) . Photograph A is before any laser shot (LS) ; B is for after the

1st LS; and C-L are for after the Ist-lOth LS after irr, respectively.

186
Figure 10. Photographs A-C of the input end of FN25 illustrate the damage to the

central fiber during the 2nd LS after irr (C) . Photograph B is after the 1st LS

after irr. Photographs D-F, are of the input of FN26 before the 1st LS , after the 1st

LS, and after the 1st LS after irr, respectively. The £2 values are the upper no. in

kJ/cm2 and the e are the lower no.

137
2. 547

St

0.738

2. 228

0.655

2. 680

0.792

0.821

•5*

0.242

2. 597

i

0.787

Figure 11. Photographs A-G of the input end of FN23 (CGW fiber) show little changes

to the top fiber for large £2 values (upper no. in kJ/cm2

) . Photograph A is before

any LS and B -G are after the lst-5th and 7th LS after irr , respectively. The e values are the lower no.
1.0

U)

>-' o

3 0.5

o

*FN18 £

jn

= CONST. ± 3.9%, £

jr|

(AVG) = 1. 052 kj cm

:

FN20 £. = CONST. ±3.7%, C. (AVG) 0. 929 kj cm

in in

*FN23 €. CONST. ±10.9%, €

jn

(AVG) - 2. 618 kJ cm

2

x

4 6

SHOT NO.

10

Figure 12. The decrease in e values is illustrated for three fibers FN18 , FN20, and

FN2 3 as functions of laser shot no.

139
ULTRAVIOLET DAMAGE RESISTANCE OF LASER COATINGS*

Brian E. Newnam and Dennis H. Gill

Los Alamos Scientific Laboratory

Los Alamos, New Mexico 87545

The damage resistance of several thin-film materials used in ultraviolet laser optics was measured at 266 nm and 355 nm. The coatings included single, quarter- wave (QW) layers of NaF, LaF3 , MgF2 , Th02 , A1 2 0 3 , Hf

0

2 , Zr02 , Y2 0 3 and Si02 , plus

multilayer reflectors composed of some of these materials. The substrates were uv-grade fused silica. Single-shot thresholds were obtained with 22-ns and 27-ns

(FWHM) pulses at 266 and 355 nm, respectively. One of the samples had previously

been tested using 20-ps pulses, providing a pulsewidth comparison.

At 266 nm the coating with the highest damage threshold was a QW layer of NaF

at 10.8 J/cm2 (450 MW/cm2 ), whereas for a maximum reflector of Al 2 03/NaF the value

was 3.6 J/cm2 (154 MW/cm2 ). At 355 nm the QW layer of NaF could not be damaged at

38 J/cm2 (1390 MW/cm 2 ), and the threshold of the maximum reflector was 12.2 J/cm 2 (470 MW/cm 2

)

.

The results were analyzed to determine correlations with standing-wave electric

fields and linear and two-photon absorption. Scaling relationships for wavelength,

refractive index and atomic density, and pulsewidth were found. Key words: damage thresholds; electric fields; laser damage; nanosecond pulses;

pulsewidth dependence; standing waves; thin-film coatings; two-photon absorption;

ultraviolet wavelength scaling.

1. Introduction

The recent growth in popularity of rare gas-halogen lasers has pointed up the lack of damage thres- hold data in the ultraviolet. This paper examines a variety of coating materials useful for uv laser

optics. Single QW layers and multilayer total reflectors were irradiated at 266 nm (22 ns) and 355 nm

(27 ns). Materials evaluated at both 355 nm and 266 nm were NaF, Th02 , Al 2 03 and Zr02

. In addition, LaF 3 , MgF2 , Hf

0

2 , Y2 0 3 and Si02 were tested at 266 nm. Total reflectors of Al 2 0 3 /NaF and Th02 /Si02 were tested at both wavelengths, and reflectors of Th02 /MgF2 and PbF2 /cryolite were tested at 266 nm. The film and substrate characteristics are listed in table 1. All coatings were deposited by use of an electron gun, except those by rf-sputtering as noted.

Table 1. Coating and substrate characteristics.

Coating Substrate Substrate Substrate Coating Index of Refraction Coating

Material Roughness A rms Deposition

Temperature

Absorption

Edge

3

0 C 266 nm 355 nm nm

NaF Suprasil 2 12 30 1 34 1.33 > 130 Si02 (sputtered) Suprasil 2 12 165 1 52 1.51 > 160 MgF2 Suprasil 2 12 30 1 39 1.38 > 115 LaF 3 Suprasil 2 12 30 1 61 1.60 > 135 Th02 Dynasil 1000 20 200 2 0 1.95 250 A1 2 0 3 Suprasil 2 12 30 1 67 1.65 > 140 Zr0 2 Optosil 1 20 200 2 17 1.95 225 Zr02 (sputtered) Suprasil 2 12 175 2 4 2.27 260 Hf02 Ultrasil 20 250 2 15 2.08 220

Y2 0 3 (sputtered) Suprasil 2 12 200 2 0 1.79 215 Al 2 0 3 /NaF Suprasil 2 12 30 Th02 /Si02 Dynasil 1000 20 200 Th02 /MgF 2 Dynasil 1000 20 200 PbF2 /Cryolite Suprasil 2 12 30 2 17/1.37 230/>130

Coating absorption edge was defined where measured absorption was 10%. Values preceded by > are for bulk materials [l] 1

; negligible absorption was measured for these coatings down to 200 nm.

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

1. Figures in brackets indicate the literature references at the end of this paper.

100
Most of the samples were on identical substrates of Suprasil 2 with approximately a 12 A rms surface roughness. A few samples were on other types of uv-grade fused silica and had an approximate

surface roughness of 20 A rms. Spectrophotometric traces for the single QW layers are shown in figure

1. Note that some of the layers were not exactly A/4 thick at 266 nm. The actual wavelength at which

each coating was A/4 thick is tabulated in tables 2 and 4.

The NaF single-layer samples were fogged in appearance. This was probably due to the high relative

humidity in the coating company's plant (reported to be about 65%). However, A^Oo/NaF reflectors

were not fogged, probably because the outer layer of AI2O3 protected the NaF. The gradual decline in transmission at shorter wavelengths for NaF (fig. 1) was due to scattering losses from the fog. Special

note is also made of the short-wavelength transmission of the Th02 which did not extend to the expected

absorption edge.

2. Experimental Procedure

The experimental arrangement is shown in figure 2. A Nd : YAG laser was Q-switched using Bis (4-dimethylamino-dithiobenzil) nickel dye and 3 etalons inside the cavity. The combination of slow-relaxation-type dye and etalons produced a 35 ns pulse at 1064 nm. This beam was then frequency doubled and subsequently tripled or quadrupled. The 266 nm pulsewidth was nominally 22 ns and the 355 nm pulsewidth

was nominally 27 ns. Two dispersive prisms were used to separate out the desired wavelength. Beam pickoffs reflected energy into a fast photodiode (ITT F4014) and a Laser Precision Energy Meter. The photodiode signal was sent to a Tektronix R7912 Transient Digitizer. Another beam pickoff was focused

by a 500 mm fl lens, identical to that in the main beam path, onto a Reticon linear diode array, as shown in figure 2, or onto an aperture in front of another Laser Precision Energy Meter, as shown in figure 3. The Reticon array was used at 355 nm to determine focal spot size; it could not be used at 266 nm due to fluorescence of the array window.

The arrangement shown in figure 3 was used at both wavelengths to determine the peak energy density

on the sample. The two apertures were 70 Mm in diameter. Aperture 1 and Energy Meter 1 were located at the sample position, which was 450 mm from the 500 mm fl lens. After the calibration, these were removed and the samples placed at position 1 for the tests. Aperture 2 and Energy Meter 2 remained in position tor the entire test. By determing the ratio of the transmitted energy in the two paths, path 2 could be used urmg the test to establish the energy density at the sample. Since the beam spot size w was 0.15 mm (radius at 1/e intensity), the beam profile was not completely flat across the aperture. A

small correction factor (6.3%) was used to calculate the peak energy density.

Energies from the energy meters, time profiles from the R7912 and (at 355 nm) the spatial profile

from the diode array were fed to an on-line computer and data acquisition system for immediate analysis

[2]. The calculated peak energy- and power-densities were printed out after each shot for use in plotting the data on a scatter-plot as the test proceeded.

Each sample was irradiated with an average of about 40 shots. Each site was irradiated only once. Damage was determined by visual observation of the irradiated site through a 40X stereo microscope.

Two types of illumination were used. A bright white -light source illuminated the entire sample. A He-Ne laser illuminated only the actual irradiated zone. The laser-induced scattering of both light sources was used to determine the occurrence of damage.

3. Results

Table 2 lists the damage thresholds of the various single QW layer materials tested at 266 nm. The

ranges quoted in the damage threshold columns are from the lowest values that did cause damage to the highest values that did not cause damage [3]. For each laser shot the computer integrated the photodiode signal and calculated an effective pulsewidth which was then used to calculate the peak power

density. Thus the apparent pulsewidth one obtains by dividing energy density by power density is not a constant. The previously quoted pulsewidths of 22 ns for 266 nm and 27 ns for 355 nm are average values

for all of the tests.

The peak internal electric field was calculated using a LASL code that computes the E-field distribution within a dielectric stack. The actual thicknesses of each layer were taken into account. The various layers were not exactly a quarter wave at 266 nm, as shown in table 2. Figure 4 shows the relative E-f ield-squared distribution within two materials, one whose index of refraction is less than

the substrate and one whose index is greater than the substrate. The peak E-fields are substantially

different for the two cases. Also shown in figure 4 is the effect of testing at 355 nm a coating that

is QW thick at 266 nm.

The low index material with the highest damage threshold was NaF, in spite of its fogged appearance. The high index material with the highest damage threshold was LaF3

191
Material

Table 2. Comparison of materials at 266 nm

Thickness

X/4 at (nm)

Damage Threshold

Energy Density

J /cm2 Power Density Peak Internal E-Field

(Front surface)

MW/c MV/cm

NaF 266 10 8 1 7 450 + 70 . 37 + .03

01.U2 (sputtered

)

^19 Q LH + U j/.in + 1 n . JZ MgF2 266 6 8

+ 0 1 310 + 30 .30 + .02 LaF 3 250 6 7 + 0 6 280 + 30 .26 + .01 Th02 290 2 8 + 0 4 122 + 17 .16 + .01 A12 03 266 2 6

+ 0 2 106 + 10 .16 + .01 Zr02 300 1 5 i 0 1 67 + 4 .11 + .01 Zt02 (sputtered) 505 1 5 + 0 1 66 + 4 .13 + .01 Hf02 278 1 3 + 0 2 68 + 10 .11 + .01 Y2 03 (sputtered) 374 0 6

+ 0 2 31 + 10 .08 + .01 Si02 (Suprasil 2) 8 0 + 0 3 350 + 10 .29 + .01

Material

Table 3. Comparison of total reflectors at 266 nm.

5£E£S£_Threshold_

Power Density

MW/cm2

Energy Density

J /cm2

Peak Internal E-Field

MV/cm

Al2 0 3 /NaF

Th02 /Si02 Th02 /MgF2 PbF2 /Cryolite

3.6 ± .4 1.3 ± .1 1.1 ± .3 0.5 ± .1154 ± 17 71 ± 5 55 ± 15 19 ± 4

.29 ± .02

.16 ± .01 .14 ± .02 .11 ± .01

Table 3 lists the four reflectors tested at 266 nm. The Al 2 03/NaF reflector was the best by a considerable margin. The threshold of the PbF2 /Cryolite reflector was quite low.

The single-layer materials tested at 355 nm are listed in table 4. It should be emphasized that these are the same., identical samples as those tested at 266 nm. At 38 J/cm2 (1.39 GW/cm2

) we were

unable to damage the NaF thin film. There were only small differences between the other three samples.

The two reflectors tested at 355 nm are listed in table 5. Again, the Al 2 03/NaF was best by more

than a factor of 2. The reflectors tested at 355 nm were not the same as those tested at 266, but rather were maximum reflectors at 355 nm made by the same company on the same day as the maximum re- flectors at 266 nm.

All of the materials and reflectors that were tested at both wavelengths are brought together in table 6 for easier comparison. This data is then plotted in figure 5 in terms of energy density and in figure 6 in terms of electric field. Figure 7 is a plot of the data obtained previously [3] on some of the same materials at 1064, 532 and 355 nm. Pulsewidths for this data were 30 ps , 20 ps and 17 ps, res- pectively. Spot sizes were similar to the present data. The higher damage thresholds at 532 nm

compared to those at 1064 nm were observed to be in accordance with the frequency dependence of the electron avalanche mechanism.

Table 4. Comparison of materials at 355 nm.

Damage_Threshold

Material Thickness

X/4 at (nm)

Energy Density

J/cm2 Power Density

MW/cm2 Peak Internal E-Field

MV/cm

NaF

Th02 Zr02 A1 2 0 3

266 290

300

266

> 38 9.1 ± .2 7.1 ± .2 5.9 ± .5

> 1390

340 ± 10 250 ± 10 210 ± 20

> .65

.27 ± .01 .23 ± .01

.22 + .01

192
Material

Al2 0 3 /NaF Th02 /Si02

Table 5. Comparison of total reflectors at 355 nm. _Damage_Threshold

Energy Density Power Density

J /cm2 MW/cm2

12.2 ± .9 4.9 ± .1 470 ± 40 180 ± 10

Peak Internal E-Field

MV/cm

.51 ± .27 ± .02

.01

Material

Table 6. Spectral dependence of damage thresholds.

Damage Thresholds

Energy Density (J/cm2

) 266 nm 355 nm

Peak E-Field (MV/cm) 266 nm 355 nm

NaF 10 8 + 1 7 > 38 .37 i .03 > .65 A12 0 3 2. 6 + 0. 2 5. 9 + 0.5 .16 + .01 .22 ± .01 Al2 0 3 /NaF Reflector 3. 6 + 0 4 12 2 + 0.9 .29 + .02 .51 ± .02 Th02 2 8 + 0 4 9 1 + 0.2 .16 + .01 .27 ± .01 Th02 /Si02 Reflector 1 3 + 0 1 4 9 + 0.

.16 + .01 .27 + .01 Zr0 2 1 5 + 0 1 7 1 + 0.2 .11 + .01 .23 ± .01

4. Discussion

4.1. Electric-Field Correlations

Several experimental studies with short laser pulses (30 ps-lOns) have determined that coating

damage occurs first at the locations of internal electric-field maxima [4,5]. Accordingly, the threshold electric fields listed in tables 2-6 have also been computed at the standing-wave (SW) maximaHowever, at our relatively longer (20-30 ns) pulses, a complete analysis should include thermal conduction during the pulse which reduces temperature extremes at SW peaks. This is more applicable for coatings which damage by absorption than by avalanche breakdown.

An exact correlation of peak electric fields for 22-ns pulses at 266 nm is found with Th02 coatings. A breakdown field of 0.16 MV/cm was measured for Th02 both as a single-layer and in the reflector

configuration of Th02 /Si02 (table 6). (Damage occurs first in the high-index layer of the reflector).

A correlation of peak fields was not observed in Al 2 0.j coatings, however. The respective threshold

fields in single-layer and reflector configurations (Al2 0 3 /NaF) were 0.16 and 0.29 MV/cm. There are two apparent explanations for the difference. One is that the Al2 03 single layer had greater absorption or was weaker, a fact which would be difficult to specifically measure. The second possibility is that the material adjacent to the A1 2 03 was influential.

To discuss the latter, we first note that the peak fields in both coating configurations occurred

at the interfaces with adjacent material, the fused silica substrate for the QW case and the first NaF

layer in the reflector. If absorption is the damage mechanism, then thermal conduction away from the interfaces during the 22-ns pulse must be considered. Since the thermal diffusivity of NaF at 0.070

cm2 /sec (bulk, 300°K) is an order of magnitude greater than that of fused silica (0.0083 cm2 /sec) [6],

conduction losses at the interfaces could well have been significantly different. Additionally, coating

damage could have resulted from heating of inclusions in the polished surface layer of the silica substrate. Further experiments would be required to determine which is the correct explanation.

4.2. Spectral Dependence

4.2.1. Linear and Two- Photon Absorption

The decline in damage resistance at 266 nm when compared to that at 355 nm (shown in figures 5 and

6) was anticipated. Linear absorption increases rapidly as the absorption band edge is approached.

Besides linear absorption, multiphoton absorption also becomes a potential damage mechanism. As can be deduced from the values of absorption edge listed in table 1, two-photon absorption (TPA) is energetically possible for all of the coating materials for 266-nm irradiation. At 355 nm, TPA is also possible

for Th02 , Zr02 , Hf

2 , Y2 0 3 and PbF2 , but 3 photons are required for resonant absorption in the remaining

coatings listed.

193
In a previous study with picosecond pulses at 355 nm, Newnam and Gill [3] suggested that TPA was a plausible damage mechanism for Zr02 , HfC>2 , and Si02 . This possibility could not be confirmed because

TPA coefficients (S) were not known, nor were accurate linear absorption coefficients (a) measured by

calorimetric techniques available. Derivation of a from spectrophotometry curves is subject to error when coating inhomogeneity (variation of refractive index with thickness) and scattering losses are not well quantified. Unfortunately, a similar deficiency in film characterization existed in this study.

However, recent measurements of TPA coefficients of laser crystals and windows at 266 and 355 nm re- ported by Liu, et al. [7], allow estimation of the relative role of TPA for a few of the coatings in this study.

In absorption processes, the power absorbed per unit volume, dl/dz, is the quantity of interest.

For both first- and second-order absorption,

f = - al - 6 I

2 dz When the plane-wave expressions for I (= y \J ^1 e |F.| 2

) and I (incident) are substituted, we obtain

(1)

-5— = - nl

dz c E(z)

(a + y), (2)

where (?nl E(z)

(3)

and |E(z)/E 1 2 is the standing-wave distribution normalized to the incident field. To heat a material

to its melting point (a well-defined damage threshold) , the minimum total absorption coefficient

(cm 1

) is given by

(Tmp

300°K) peak (4)

where p is density, c^ is specific heat, and ^ is the incident energy-density threshold.

From calculations of a^, and y using experimentally measured damage thresholds, it is possible to determine the relative role of 1- and 2-photon absorption in the damage process, ignoring other possible

competing mechanisms. The reported values of 6 (cm/MW) at 266 and_355 nm for AI2O3 are 2.7 x 10 ** and

< 1.6 x 10 6

. and for fused silica are <_ 4.5 x 10 5 and < 1.3 x 10 s

, respectively [7]. Values for other coating materials_tested in this study are not available except for Zr02 for which 6 at 694 nm was reported to be 2.4 x 10 3 cm/MW [8]. Corresponding values at uv wavelengths would, of course, be much

larger. Computed values of a,^ and y are shown in table 7. For purposes of comparison, typical values

of a for thick commercially deposited coatings are presented in table 8.

The primary observation is that for nanosecond pulses 2-photon absorption is not an important da- mage mechanism for these three materials. Only for picosecond-pulse irradiation of Zr02 is it significant. Secondly, nanosecond-pulse damage to Zr02 coatings at 266 nm occurred via linear absorption. The

value of to cause melting is within the value listed in table

cm 1 for Zr02 exceeds that in table 8 (210 cm

1

) by a considerable amount, defects were damaged at a lower laser intensity.

At 355 nm, the computed of 900 Possibly, absorbing coating

Table 7. Total and two-photon absorption contributions to uv laser coating damage.

Coating

Material

(Pulsewidth)

Zr02 (E-gun) Zr02 (sputtered)

A12 0 3 AI2O3 (in reflector)

S±0Z (sputtered)

Si02 (substrate)

266 nm 355 nm

a

T

(cm l

) y(cm L

) a

T (cm 1

) Y (cm 1) (22 ns) (27 ns) 3400 > 1 900 > 1 2900 > 1 2300 0.03 1000 0.001

700 0.10 200 0.002

250 0.02 350 0.02

(17 ps) 2800 > 300

1000 < 0.2 Zr02 (E-gun)

Si02 (E-gun)

194
Table 8. Typical absorption coefficients for thick uv coatings deposited by electron gun [9]

Coating

Material 250 nm

Zr0 2 6000 Hf0 2 950 A1 2 0 3 190 LaF 3 850 PbF2 900

a(cm ) 350 nm

210

270 150 430

100

Computed values of for A1 2 0 3 are much in excess of typical values. As these coatings were deposited at ambient temperature and with no reactive oxygen atmosphere, the oxidation of the Al probably was

not complete, resulting in high absorption.

Finally, linear absorption at 266 nm was probably the operative damage mechanism for the other

oxides, Y 2 0 3 , Hf0 2 and Th0 2 , although the available evidence is not conclusive.

4.2.2. Wavelength Scaling

The spectral dependence of laser damage in figure 6 suggests an interesting relationship between the electric-field thresholds at 355 and 266 nm. For the coating materials, NaF, A1 2 0 3 , Th02 and Zr02 , which were tested at both wavelengths, the ratio of the threshold fields is

E (355)

I

-m) = 1.73 ± 0.23 (5)

2 This is surprisingly close to the ratio of the laser wavelengths squared, i.e. (355/266) = 1.78!

It is probably permissable to use this wavelength-squared ratio to estimate the 355-nm thresholds

of the additional coating materials that were only tested at 266 nm (see tables 1 and 4) since the wavelength range is narrow. However, the relationship E t ^ A

2 observed here is not expected to be a general

scaling law for laser damage at ultraviolet wavelengths. Such a dependence is not generally derivable

from the theory of optical absorption in dielectrics. That theory predicts that the absorption coefficient at a frequency v near the center frequency of an absorption band V

q is of the form

2 2

where y is a material constant acounting for the breadth of the absorption band [10]. The subject of a wavelength scaling law for uv laser damage merits further consideration.

4.3. Refractive-Index and Atomic-Density Scaling

In a number of laser damage studies of thin film materials used in the visible and near-ir regions,

high-index coatings have been found to damage more easily than low-index coatings [11, 12, 13, 14, 15].

Bettis, et al, [14]. have derived a first-principles equation for the macroscopic breakdown electric

field in terms of material parameters, — x yio^ (7) Eo cr

_ index, q is electronic charge and Xcr

is the critical

separation required to free an optical electron from its atom. The threshold fields for a number of coating materials, when irradiated with 40-ns, 1.06 )jm pulses, were shown to follow this relationship

fairly well. [14]

The present coating thresholds for 266 nm have been plotted versus N/(n 2 -l) (the dominant term in

eq. (7)) in figure 8. A straight line provides a reasonable fit to the data with certain exceptions.

The threshold for NaF was considered low, due to fogging, MgF 2 is generally an inhomogeneous film with

substantial H 2 0 content, and the single-layer A1 2 0 3 had an anomalously low threshold, probably due to high absorption. A least-squares straight-line fit of the data in figure 8, ignoring the points for lossy films of NaF, MgF 2 , and single- layer A1 2 0 3 , provides the equation

E . (MV/cm)^ 0.54 - (8) th n -1 Certain corrections could be applied to improve the observed correlation. First, the atomic density, N, should be multiplied by the coating packing density since coatings are not as dense as bulk crystals. For example, p = 0.72 for MgF 2 deposited at 30°C before exposure to the atmosphere [16]. Secondly,

Hf02 and Th0 2 coatings were deposited on slightly rougher substrates than the other films (^ 20 A rms

as compared to ^ 12 A rms). House, et al . [17 ]

, _have demonstrated that increased substrate roughness

decreases the damage threshold field as E^ ^ CT °" 5

, so the thresholds for Hf0 2 and Th02 could be

195
adjusted upward about 25% for comparison purposes.

4.4. Pulsewidth Dependence

A pulsewidth dependence of laser damage at 355 ran for a single-layer coating of Zr02 on Optosil 1 is shown in figure 9 in which the present measurements at 27 ns are compared to a previous measurement

at ^ 20 ps on the same sample [3]. The laser spot-size radii were slightly different, 120 \lm versus

150 Mm, but this does not prevent a direct comparison. (For 30-ps pulses, no significant threshold

dependence on spot size was measured [3].) The observed t

1/6 dependence for energy-density threshold

is much more gradual than the 1/2-power dependence observed for the bulk and surfaces of many window

materials [18].

In previous measurements at 694 nm over a pulsewidth range of 12 to 34 ns, a T dependence was measured for a single half-wave coating of Zr02 only when the occurrence of a photometrically detectable

spark was used as the criterion for damage [12]. Using the more sensitive technique of laser-induced

scatter as the damage criterion, only a T ^3 dependence was measured for the same sample. In another

study at 694 nm at 20 ps and 23 ns, Bliss, et al. [19], measured the thresholds (J/cm2

) of Zr0 2 /Si0 2 reflectors to increase as T

1/3 to T

1/2

. In their tests, a spark was visible on most shots causing

detectable damage.

From the above and other observations, one may conclude that the energy-density damage threshold

of coatings generally increases with pulsewidth at a rate less than or equal to t

1' 2

. For the Zr0 2 coating tested in this series at 355 nm, a much weaker dependence was measured.

5. Summary

The damage thresholds of a variety of single-layer coatings and multilayer reflectors were measured

at 266 nm and 355 nm using 22- and 27-ns pulses, respectively. The coating materials included NaF, cryolite, MgF 2 , Si0 2 , LaF 3 , A1 2 0 3 , Th0 2 , Hf0 2 , Y 2 0 3 , Zr0 2 and PbF 2

. The principal findings of this

study were:

a. Reflectors comprised of A1 2 0 3 and NaF layers had the highest thresholds (3.6 J/cm2 at 266 nm and 12 J/cm2 at 355 nm) . In a fluorine-gas environment a LaF 3 /NaF multilayer would be suggested.

b. The single-layer coating with the highest threshold was NaF (11 J/cm2 or 0.37 MV/cm

at 266 nm and 38 J/cm2 or 0.65 MV/cm at 355 nm) , although the coating was fogged by exposure to a humid atmosphere.

c. The electric-field thresholds at the SW maxima in a Th0 2 single layer and in a Th0 2 /Si0 2 reflector were equal. A correlation of peak fields for similar A1 2 0 3 coatings was not observed, presumably because the coatings had unequal linear absorption or because of the differing thermal dif f usivities of the adjacent materials.

d. The electric-field threshold (internal peak value) is related to the atomic density N and the refractive index in the form N/(n 2 -l). At 266 nm the relationship was

E ^ 0.54 N/(n 2 -l).

e. Two-photon absorption was not an important damage mechanism for 20-ns pulse damage at 266 and 355 nm for coatings of Zr0 2 , A1 2 0 3 and Si0 2 . (For 20-ps pulses, however, TPA

is substantial in Zr0 2 at 355 nm.) For the other coating materials, measurements of TPA coefficients are required before conclusions are possible.

f. Linear absorption was responsible for damage of Zr0 2 at 266 nm; failure at 355 nm may have occurred at sites of absorbing inclusions. Damage of Hf0 2 , Th0 2 , Y 2 0 3 and A1 2 0 3 was probably caused by the same mechanisms, although the available evidence was insufficient .

g. A wavelength scaling relationship of E (MV/cm) ^ A

2 was discovered for several coatings

between 266 and 355 nm. Though not derivable from basic theory, it can be useful for estimating relative thresholds in this restricted spectral range.

h. Increased damage resistance at longer pulsewidths was measured at 355 nm for a Zr0 2 coating. From data points at 20 ps and 27 ns, a threshold relationship of <^(J/cm2

) ^ t

1/6 was apparent. This is a much more gradual pulsewidth dependence than T

1 2

, which has been observed for uncontaminated laser window materials.

6. Acknowledgements

The authors gratefully acknowledge the technical assistance of John Meier in conducting the damage

tests. The following persons and companies supplied thin-film coatings that were used in the tests and provided technical information about the coatings: Joe Latore, Lambda/Airtron; Verne Costich, Design

Optics; Alex Shimkunas, Optical Coating Laboratory, Inc.; Jim Larin, Laser Optics; Manfred Grindel,

Continental Optical; John Hartmann, Battelle Pacific Northwest; and Philip Baumeister, University of Rochester. The substrates were polished by Laser Optics.

196
7. References

[1]. Sparks, M. , Sen, P.N., Flannery, M. , and Loh,

Jr., E tJ Theoretical Studies of High-Power

Ultraviolet and Infrared Materials, Final

Tech. Report, Contract DAHC15-73-C-012 7, p. 225, March 31, 1978.

[2]. Kuckertz, T. H. and Gill, D. H. , Proceedings

of Eleventh Hawaii Int'l. Conf. on System

Sciences, p. 162 (19 78).

[3]. Newnam, B. E. , and Gill, D. H. , NBS Spec.

Pub. 462, 292 (19 76).

[4]. Apfel, J. H., Matteucci, J. S., Newnam, B. E. and Gill, D. H. , NBS Spec. Pub. 462,301 (1976).

[5]. Newnam, B. E. , "Laser Induced Damage Pheno- mena in Dielectric Films, Solids, and Inor- ganic Liquids," Ph.D. dissertation No. 73-

09, 317, University Microfilm, Ann Arbor,

Mich.

[6]. Moses, A. J., Handbook of Electronic Materials, Vol. 1: Optical Materials Properties , (IFI/Plenum, New York, 1971), pp. 68,82.

[7]. Liu, P., Smith, W. L. , Lotem, H. , Bechtel,

J. H. , Bloembergen, N. , and Adhav, R. S., NBS Spec. Pub. 509, 489 (1977).

[8]. Picard, R. H., Milam, D. , Bradbury, R. A., and Fan, J. C. C., NBS Spec. Pub. 435, 272

(19 76).

[9]. Costich, Verne R. , Design Optics, private

communication

[10]. Seitz, F., The Modern Theory of Solids,

(McGraw-Hill, New York, 1940), p. 634.

[11]. Turner, A. F. , NBS Spec. Pub. 356, 119 (1971).

[12]. Newnam, B. E., and DeShazer, L. G. , NBS Spec.

Pub. 372,123 (1972).

[13]. Newnam, B. E. , and Gill, D. H. , NBS Spec. Pub.

435, 254 (1976).

[14]. Bettis, J. R. , House, R. A., Guenther, A. H.

,

and Austin, R., NBS Spec. Pub. 435, 289 (19 76) [15]. Smith, W. L., Milam, D. , Weber, M. J., Guenther, A. H., Bettis, J. R. , and House,

R. A., NBS Spec. Pub. 509, 244 (1977).

[16]. Ritter, E., App. Opt. 15, 2318 (1976).

[17]. House, R. A., Bettis, J. R. , Guenther, A. H.

,

and Austin, R. , NBS Spec. Pub. 435, 305 (1975)

[18]. Bettis, J. R. , House, R. A., Guenther, A. H.

,

NBS Spec. Pub. 462, 338 (1976).

[19]. Bliss, E. S., and Milam, D. ,NBS Spec. Pub.

372, 108 (1972)

Figure 1. Spectral transmittance for single QW coatings used in laser damage tests.

197
Figure 3. Experimental arrangement for calibration of axial energy density.

198
40

39 - s

E

,10-

r

-+ NaF

cnj 12 - • NaF/AloO

2 3

, Reflector

/

/

/ / ,D Th0

2 /

'

I / A Zr0 2 ^ \ / //> Al

2

0

3 / /

/ , w Th0

2 /Si02 / ' / Reflector

200 250 300 350 400 450

Wavelength (nm)

Figure 5. Spectral dependence of energy- density threshold for damage of coating materials and reflectors

Pulsewidths were 22 ns and 27 ns for 266 nm and 355 nm, respectively.

199
0. 7

0. 6

-

0. 5 Eo>

0. 4

0. 3

0. 2

« 0. 1

/ / + /

/

IDO-

+ N a F

• NaF/AI,0,

Reflector

Th02/Si0 2 Reflector ^Th02 '"Jo-AZr02

Al

2 0

3

0_

200 2TTT 300

Wavelength (n m

3^f) rtU 410

Figure 6. Spectral dependence of electric-field threshold for damage of coating materials and reflectors. Pulsewidths were same as for figure 5.

200 400 600 800 1000 WAVELENGTH (nm)

1200

Figure 7. Spectral dependence of electric-field threshold for damage of four coating materials. Pulse- widths were 30 ps, 20 ps, and 17 ps for 1064 nm, 532 nm, and 355 nm, respectively (from Ref.

3).

2no
0. 4

0. 3

^ 0.

0.

S i 0 2 (film) o

Si 0

2 o

(substrate) LaF

3 o

Zr0

2 o

bF

2 o y

6 f O Y

203

O A I ? 0

3

in reflector)

o N a F (foqqed

O Mq F 2

O A I 2 0

3 (sinqle layer)

Laser Wavelength: 266 nm

Pulsewidth: 22 ns

0. 2 0.4 0.6 0. N/(n 2 -l ) <1 0 23 atoms/cm 3

)

1. 0

Figure 8. Electric-field threshold versus N/(n 2 -l) for optical coatings irradiated at 266

10

9 E

<_> 8 ^ 7

e 6

a> o

>^ 5

en

i_

CD 4 c

LxJ o 3

o

sz 2

a>

i_ x:i— 1

..50

..16

Zr0 2 X/4 at 355 n m

Tested at 355 nm

JL 01 0.1 1.0

Pulsewidth (nsec

10 100

Figure 9. Pulsewidth dependence of threshold energy density for a single QW layer of ZrC>2 at 355 nm

Picosecond data from Ref. 3. 201
MULTITHRESHOLD EVALUATION OF 100-nsec PULSED LASER DAMAGE TO COATING MATERIALS AT 2.7- AND 3.8-um WAVELENGTHS*

J. 0. Porteus, T. M. Donovan, J. L. Jernigan, and W. N. Faith

Michelson Laboratory, Physics Division

Naval Weapons Center, China Lake, CA 93555

In this time and wavelength regime optical coatings typically fail from

thermal damage. Unlike catastrophic damage resulting from dielectric breakdown,

one finds disparity among the thresholds for one or more of the following damage

characteristics: delamination, cracking, flow, erosion, perforation, ion emission, and light emission. Comparison of the different thresholds provides

useful information on how damage proceeds, and how the absorbed energy is distributed in the coating layer. Comparison of multithreshold damage data at the two different wavelengths permits evaluation of the importance of impurities

which absorb preferentially at one of the wavelengths, such as water. Comparing

thresholds at the two wavelengths after N/1 conditioning provides detection of preferentially absorbing impurities which are laser-desorbed. Complementary

information may be obtained from laser calorimetry. The following materials,

prepared as single-layer, antiref lection coatings, were evaluated: NaF, SiO^,

Al^O^y ZnS, As^S^, and Si. Samples of the first three were prepared in ultrahigh vacuum, in addition to the usual standard vacuum preparation. Thresholds

are expressed in terms of the theoretical energy per unit area in the coating

layer in order to permit a practical intercomparison of materials for multilayer

coating design.

Key words: Absorption; antiref lection coatings; damage thresholds; impurities;

laser- induced damage; thermal damage.

Introduction

The pulsed damage threshold of a transparent optical coating is usually well below that of bulk

specimens of the same nominal material. Since absorption coefficients are typically several orders of magnitude greater than corresponding bulk absorption coefficients, coatings are more susceptible to heating and thermal damage. As a result, the thresholds for such damage characteristics as delamination,

cracking, flow, erosion, perforation, etc. of a given coating often differ, according to the physical

properties of coating and substrate, and the conditions of laser irradiation. This contrasts with

the catastrophic damage associated with bulk dielectric breakdown, where all characteristics typically

share a common threshold. As in the case of metals which was reported last year [l] 1

, a study of the relative magnitude of the various thresholds for coating damage can reveal the immediate cause of failure. This in turn provides useful information for improving damage thresholds. For example, a susceptibility to thermal stress-related damage is revealed by relatively low thresholds for delamination and cracking. Similarly, spatially nonuniform damage resulting from a highly absorbing interface

or an inhomogeneity in composition can be inferred from other threshold relationships, as will be demonstrated. The comparison of multithreshold results at two different wavelengths adds another

dimension to the information obtained. Damage resulting from preferentially absorbing impurities,

e.g., water, which has an absorption band at 2.7 um but not at 3.8 um, becomes evident. Volatile

impurities can also be detected by comparing 1/1 with N/1 (conditioned) damage thresholds. Supporting

evidence of preferentially absorbing impurities and excessive interface absorption is provided by

multiwavelength laser calorimetry [2].

We have used the above methods to evaluate the damage resistance of the following materials: NaF, SiO , Al^O^, ZnS, A^S^, and Si, prepared as single-layer antireflection coatings on sapphire, fused

silica, and CaF2 substrates. A complete discussion of the first three materials is presented here,

including inferences concerning damage- related material properties. The AS2S., results are more complex

and are presented in detail in a separate paper at this conference [3].

Coating Samples

Samples were prepared in two different coating systems: an unbaked "standard" vacuum system and a baked "ultrahigh" vacuum system [3]. The former is pumped with a liquid ^-trapped 6000 l/sec oil

diffusion pump, using an oil which was specially prepared for low backstreaming. Base pressure is

* Work supported by the Defense Advanced Research Projects Agency and Navy Independent Research funds.

1. Figures in brackets indicate the literature references at the end of this paper.

202
10 ' Torr. The second system is ion-pumped during bakeout and has a base pressure of 10~9 Torr. During deposition a turbomolecular pump is used and the ion pump is switched off.

Al^O^ coatings were electron-beam deposited from pulverized sapphire window material with the substrate at room temperature. NaF coatings were evaporated from a Mo boat containing react ively

purified starting material with the substrate at 150°C. sio

x and si02 were evaporated with the sub- strate at room temperature, using 4-nines pure SiO in a Knudsen cell and dry oxygen admitted in varying

amounts. The SiO films (yellow) were produced in 2 x 10 ^ and 1 x 10 ^ Torr 0„ in the standard and ultrahigh vacuum

Xsystems, respectively. The ultrahigh vacuum Si02 (clear) was produced in 10-4-5 to 10 Torr As^S^, ZnS, and Si were each evaporated from starting materials exceeding 4-nines in purity. All coatings were half-wavelength at 2.7 pm in thickness, except for NaF, which was one quarter-wave

thick. Spectral scans revealed that the Si02 film, which was initially water-free, absorbed water from

the atmosphere over a period of hours. Also, multiwavelength laser calorimetry revealed evidence of water in the NaF and SiO films [21. x Damage Measurements

Damage measurements were made in a previously described damage facility [4], which is shown schematically in figure 1. The source is a modified Lumonics TE laser [5]. This provides single-mode,

multiline radiation centered at either 2.7 or 3.8 ym, depending on the use of either or D2 , in combination with SF. , to produce HF or DF lasing. A 1.7-mm spatial filter A, eliminates off-axis

o 1 modes. Pulse energy is controlled by a pair of ZnSe plates reflecting at the Brewster angle. The first plate rotates as a unit with Cu steering mirrors M^-M^, which are carefully aligned to prevent

off-axis beam walking. This attenuator, when used with aperture A2 , offers the advantage over one previously described [4,5] in eliminating near-axis multiple reflections. A GaP photon drag detector

is used to monitor pulse temporal characteristics. The pulse energy is monitored by a calibrated

pyroelectric energy meter. The beam is focussed on target by a 12-cm-focal-length ZnSe lens.

Pinhole beam scans performed at the focal point of the lens indicate a highly Gaussian spatial

2 profile with 1/e full widths of 32.8 pm and 47.5 ym at HF and DF wavelengths, respectively. Thresholds

reported here refer to the peak energy density of the Gaussian spatial distribution in each case. The temporal pulse width is nominally 100 nsec. The ratio of peak energy density to peak power density is 126 nsec and 89 nsec at HF and DF wavelengths, respectively.

Emission of light and ions was monitored by a 20-power microscope and impedance-matched ion collector (20-mA/sr detector sensitivity), respectively, mounted in the ultrahigh vacuum test chamber. The Auger analyzer and associated electron imager were used sparingly in order to avoid electron-beam

damage, to which many of these coatings are highly susceptible. Postdamage analysis was performed with

Nomarski and dark-field optical microscopy at magnifications up to 900.

Multithreshold measurement and analysis procedures have been reported in detail [6]. For 1/1

data, the sample to be analyzed is exposed to one pulse at each of approximately 30 sites, spaced at 1- mm intervals along its surface. Pulse energies, which are measured for each pulse, are uniformly

distributed over a range which includes the thresholds for each of the damage characteristics of interest. For N/1 data, the procedure is the same, except that the potentially damaging test pulse is preceded by N - 1 = 5 nondamaging conditioning pulses of equal or lower energy at each site. The energy of all conditioning pulses is nominally the same, and is chosen to be slightly below the 1/1

damage thresholds. The damage observed at each site issummarized by a set of code numbers, each number

representing a specific damage characteristic. Damage summaries and corresponding energy densities are entered and the data are computer-reduced, using the threshold algorithm previously described [6]. The result is an average threshold for each damage characteristic, together with its standard deviation,

the latter being a measure of the threshold uniformity over the surface. Results are presented in the form of a bar chart called a damage profile. Examples are given below.

A.F. and M.F. Thresholds

In order to make meaningful comparisons of performance between different coating materials,

thresholds must relate to energy flow inside the sample coating. The total energy per unit area ij>(x) inside a semi-infinite medium (one interface) is related to the energy per unit area incident at x = 0 by

9 4>(x) = n lE-Lkl * , (1) <E2

> °

o

2 2 where <E ) and (E ) are time averages of the internal and incident electric field squares, respectively,

The relationship of i)> to the irradiance I is

203
k = / I (t)dt

o „ o

(2)

where the integral extends over the pulse length t . The absorbed energy per unit area per unit length

is

d<j>(x) dx

>(x) (3)

where g is the linear absorption coefficient. Since 6 is much smaller than the reciprocal wavelength

A~l in any material of practical interest, the attenuation due to absorption is negligible to a depth o the order of a coating thickness, and the damage threshold is expressed unambiguously in terms of

4>(x) ~ const.

2 In an actual coating, eq. (1) still applies, but <E > (x) and <)>(x) may vary strongly within the coating layer as a result of standing waves resulting from reflections at the second interface. Here

the threshold should be expressed in terms of the value of <|>(x) which is physically most appropriate.

With a very short pulse, which permits negligible thermal equilibration before damage, <() max

corresponding to the maximum field (E ) in eq. (1) (M.F. threshold) is most appropriate. However, for a long pulse, where thermal damage is preceded by a considerable degree of thermal equilibration within

the coating, <(> , corresponding to the average field (E^> (n.F. threshold) is more appropriate. In av av quantitative terms the conditions of applicability to thermal damage are /at << I for the M.F. and

vat >> I for the A.F. threshold, where a is the thermal diffusivity and I is the coating thickness.

P Unfortunately, under present experimental conditions, /at~ ~ I, so that neither condition is satisfied. We adopt the A.F. threshold arbitrarily since it leads to more conservative damage thresholds.

Field distributions for applying eq. (1) were computed using a program due to Baumeister [7]. Examples are given in figures 2-4. Conversion factors obtained from such distributions are given in table 1. Only the conversion factors in the first two columns were used to calculate the A.F. thresholds presented in this paper. The second two columns provide the information required to convert to M.F. thresholds.

Table 1. Conversion factors from incident to internal energy/area.

Coating /Substrate HF

> / y aV ° DF HF maX 0 DF NaF/sapphire 1 026 0.932 1 288 1 192 SiO /quartz

X

0 956 0.932 1 025 1 013 A^O^/quartz 0 974 0.903 1 127 1 082 ZnS/CaF

2

J. 053 0.762 1 544 1 254

As

2 S

3 /CaF

2

1 073 0.742 1 639 1 274 Si/sapphire 0 638 1. 273 0 950 2 086

Damage Characteristics

Damage is classified into two general categories, uniform or selective. The former refers to damage whose spatial distribution is determined strictly by that of the laser intensity, and not by

spatial nonuniformities of the coating. The latter category refers to damage spatially "selected" by

defects or inhomogeneities in the coating. Specific damage characteristics may fall in either category

Examples of the characteristics of uniform damage for which thresholds are reported are shown in figure 5. At the damage site on ZnS/CaF^, massive delamination and cracking of the film has occurred

in the outermost damage zone. Sharp regularly spaced interference fringes can be seen here under

visual observation. Inside cf this zone the substrate is exposed, indicating film perforation. The innermost dark circle represents substrate damage, which is excluded from present considerations.

Drop-like formations on the inner edges of the delaminated film are indicative of flow. The damage

site on the SiO^ coating shows thickness nonuniformities in the outermost damage zone. Since the surface is not roughened, we infer that flow rather than erosion has occurred. Although interference

fringes are apparent, they do not have the sharp, regular character associated with delamination. Between this zone and the dark inner zone of perforation lies a zone of roughening and thinning which

exemplifies erosion.

204
The SiO

x damage site of figure 5 also illustrates selective damage. Just outside the zone of erosion, particularly at the lower left and to the right of center, lie a few isolated damage areas with dimensions ~ 3 um. Because of the smooth spatial profile of the laser beam, these can be attributed to localized weakness of the coating rather than to intensity "hot spots." No subclassif ication

of selective damage into specific damage characteristics will be made here.

In addition to the characteristics of permanent damage morphology just described, thresholds for the transient characteristics, ion and light emission, are also reported. Because of inadequate spatial

resolution of the damage-chamber microscope and ion collector, it is difficult or impossible to cate- gorize these characteristics definitely as either uniform or selective when both categories of permanent

damage are present.

Except for delamination and perforation, analogous damage characteristics are observed on bare

substrate surfaces. Also, an electron image of the damage site can be obtained on those materials not subject to damage by the 8 keV electron probe beam. The contrast for electron imaging is provided by

spatial variations in work function, which is governed by surface composition and topography.

Bare Sapphire Versus Al^O^ Coating Material

Thresholds for the onset of the damage characteristics are displayed for comparison in the form of damage-profile bar charts, as previously described. An HF damage profile of a polished bare sapphire

surface is shown in figure 6. The equivalence of the thresholds is characteristic of dielectric breakdown, where catastrophic failure is precipitated by the electric field, with no significant thermal

damage at lower levels of intensity.

Comparing the damage profile of an A^O^ coating (fig. 7(a)) with figure 6 shows considerably

reduced thresholds overall. Also, we find that the threshold for delamination, and possibly for cracking, are smaller than for the remaining damage characteristics. The disparity in this example is evidently related to stress due to differential expansion, compounded by differential heating. It follows from eqs. (1,3) that differential heating can result from differences in both absorption coefficient and refractive index between coating and substrate, although the absorption coefficient usually

dominates. In the N/1 profile (fig. 7(b)) the thresholds are somewhat larger and more nearly equal than

in figure 7(a). This indicates that the conditioning pulses have done one or more of the following:

(1) relieved residual stress, (2) improved the mechanical bond, (3) improved the thermal bond between

coating and substrate. DF results (not shown) are qualitatively similar to those shown in figures 7(a) and 7(b).

NaF Coating

From the field plot of the NaF coating (fig. 3) one would expect relatively greater heating at the vacuum-coating interface, as compared to either A^O^ or SiO^ (figs. 2 and 4, respectively). We find

evidence of this in the NaF damage profile shown in figure 8(a), where erosion at this interface occurs well below the thresholds for the other damage characteristics. Notice particularly that the energy required for perforation is over three times that required to produce erosion. Another contributing

factor to damage, particularly at HF wavelengths, is water, whose presence in the coating has been

verified by laser calorimetry [2]. At HF wavelengths one finds higher N/1 (fig. 8(b)) than 1/1 thresholds (fig. 8(a)); whereas, at DF the 1/1 and N/1 results are nearly identical (figs. 9(a,b)). We conclude that water is desorbed by the conditioning pulses, resulting in reduced absorption at HF, which lies in the water band. Percentagewise, the effect is largest for flow, which appears to be superficial, and for erosion, indicating that water is desorbed mainly from the vacuum-coating interface, as one would expect in view of the field distribution.

S10 and Si0 o Coatings

x 2 Figure 10(a) shows the HF damage profile of a yellow SiO^ coating, deposited from SiO in the presence of insufficient oxygen to produce typically clear SiC^. The flatness of the field inside the coating, shown in figure 4, predicts uniform absorption. The equivalence of the thresholds for erosion

and perforation agrees with this, in contrast to the case of the NaF coating, discussed above. However,

the presence of selective damage (fig. 5) indicates that the absorption is not homogeneous. Another

indication of inhomogeneity is the high threshold for ion, as compared to light emission. Light from a plasma formed at an absorbing inclusion is transmitted by the transparent matrix, but the ions are occluded unless the matrix has been sufficiently dispersed, which generally requires more energy. 2

2. An alternative explanation of the observed difference in emission thresholds is plasma formation at the substrate interface as a result of abnormally high interface absorption. Results of 2.7-um

laser calorimetry show relatively little absorption at this interface on a similar sample. Also,

the absence of delamination (blistering) supports the interpretation given in the text.

205
The cooler matrix may also inhibit flow initiated by inclusion heating, an effect which probably

explains the relatively high flow threshold. One result, therefore, is that the SiO is of inhomogeneous composition extending through the coating.

The DF profile for SiO

x

(fig. 10(b)) is strikingly different, showing equivalently high thresholds

for all characteristics, including selective damage. This provides a second result; namely, that SiO

x contains material with an absorption band at HF, probably water. SiO,, coating material, which is discussed below, has a strong affinity for water, as verified by time-lapsed spectral scans made immediately after removal from the coating chamber. This, together with the above evidence, strongly

suggests that the SiO coating is actually an inhomogeneous mixture of various silicon oxides, including

Si02- The selective damage is believed to be a result of high absorption in small Si07 -rich regions,

which are also rich in water.

Surprisingly, N/1 thresholds for SiO^ at HF and DF are only slightly higher than 1/1 thresholds at both wavelengths ,' with little evidence for the water desorption effect observed in NaF. The explanation

for this may lie in the difference in film structure as determined by transmission electron microscopy.

The NaF has an open structure consisting of an aggregate of NaF crystallites, while SiO is amorphous

with no apparent porosity.

The damage profile of a clear SiO 2 coating is shown in figure 11. The deposition was performed in

a baked system using dry oxygen in an effort to minimize water content. In spite of this precaution,

however, the coating had incorporated a considerable amount of atmospheric water before the damage

experiment. The coating showed no selective damage or other evidence of inhomogeneity . The predominant

failure mode was a swelling of the film, classified as flow, followed at slightly higher energy by delamination, erosion, and perforation. The equal thresholds for the last two characteristics indicate that

the water is incorporated throughout the coating, rather than being confined to a surface layer. DF thresholds are almost an order of magnitude higher than at HF, except for ion and light emission which

are essentially unchanged. Results are consistent with the conclusion that water-rich Si02 is incorporated in the SiO film. x Standard Versus Ultrahigh Vacuum

To demonstrate that the different vacuum system was not in some way responsible for the differences

between the Si02 and SiO^ films we prepared a yellow SiO^ film in the baked system also. The damage

profile for this film, which is shown in figure 12, is nearly identical to that of the film prepared in standard vacuum (fig. 10(a)), except for slightly lower thresholds overall. This result is typical of similar comparisons made for the A^O^ and NaF coatings. We conclude that films produced in clean,

fast-pumped standard vacuum are at least as damage resistant as those produced in a baked system.

Inter comparison of Materials

A comparison of the damage resistance of the various coating materials is shown in figure 13 in terms of the damage characteristic with the lowest threshold in each case. For the materials discussed

above, this characteristic can be identified from the damage profiles. The dominating damage characteristic for As2S.j/CaF2 is selective damage in the form of delamination at random defects [3]. ZnS/CaF2

failed primarily as a result of cracking, apparently due to stress, and secondarily, as a result of selective damage at isolated defects. Si/sapphire perforated, apparently as a result of melting and vaporization. As would be expected, materials with higher refractive index tend to have lower damage

thresholds. Of the materials evaluated NaF and Al^O^ are outstanding performers. SiO

x can also be very damage resistant outside of the water absorption band.

Summary

It has been demonstrated that multithreshold evaluation of optical coating damage at two wavelengths

can provide insight regarding properties related to laser damage and other aspects of coating performance. A disparity in the thresholds for the various observable damage characteristics permits a distinction

between dielectric breakdown and thermal damage resulting from excessive absorption. Relatively low

thresholds for delamination and cracking are indicative of thermal stress and/or adherence failure,

which may result from high absorption at the coating-substrate interface. The relationship between

thresholds for erosion and perforation indicates the importance of absorption at the vacuum-coating

interface. Absorption inhomogeneity, which cannot be ascertained from calorimetric studies, becomes

apparent from a disparity in the thresholds for ion and light emission. Comparison of thresholds at different wavelengths is helpful in detecting impurities which display absorption bands, such as water. Comparison of 1/1 and N/1 thresholds at different wavelengths can be used to determine the presence of desorbable impurities. The combined information from these studies can be used to evaluate and improve

coating materials. Of the materials evaluated here NaF and Al 0 appear to be the most promising for

206
damage resistance at 2.7 and 3.8 ym. SiO

x also shows promise at 3.8 urn. The performance of NaF and SiC^ coatings could be significantly improved at 2.7 ym by eliminating water content. A^O^ coatings

can be improved by minimizing residual stress at the substrate interface.

Acknowledgments

The authors are grateful to Dennis K. Burge for figures 2-4, and to Marshall Sparks for contributing

to the discussion of threshold definitions. Helpful comments from David Milam and Brian Newnam are also acknowledged.

References

[1] Porteus, J. 0., Fountain, C. W. , Jernigan,

J. L. , Faith, W. N. , and Bennett, H. E.

,

"Pulsed-Laser Stress Phenomena on Highly

Reflecting Metal and Alloy Surfaces," in Laser Induced Damage in Optical Materials

1977, A. J. Glass and A. H. Guenther, eds. Washington, D.C., National Bureau of Standards, 1977 (NBS Spec. Publ. 509), p. 204.

[2] Temple, P. A., Decker, D. L. , and Donovan,

T. M. , "Measured Thin Film Absorption at the Air-Film Interface, the Film Bulk, and the Film-Substrate Interface," this conference.

[3] Donovan, T. M. , Porteus, J. 0., Jernigan,

J. L. , and Ashley, E. J., "Multithreshold

Damage Measurements on ks^S^, ks^S^-^t and NaF at HF and DF Wavelengths," this

conference.

[4] Porteus, J. 0., Soileau, M. J., Bennett,

H. E. , and Bass, M. , "Laser Damage Measurements at CO2 and DF Wavelengths," in Laser

Induced Damage in Optical Materials: 1975,

A. J. Glass and A. H. Guenther, eds. Washington, D.C., National Bureau of Standards,

1975 (NBS Spec. Publ. 435), p. 207.

[5] Van Stryland, E. W. , Bass, M. , Soileau,

M. J., and Tang, C. C. , "Pulsed HF/DF Laser

Damage in Window Materials," in Laser Induced Damage in Optical Materials: 1977,

A. J. Glass and A. H. Guenther, eds. Washington, D.C., National Bureau of Standards,

1977 (NBS Spec. Publ. 509), p. 118.

[6] Porteus, J. 0., Jernigan, J. L. , and Faith,

W. N., "Multithreshold Measurement and Analysis of Pulsed Laser Damage on Optical

Surfaces," op. cit . , p. 507.

[7] Baumeister, P.

Figures

W. , private communication.

Figure 1. Layout of laser damage facility. M1-M5 are beam-turning mirrors, A} and A2 are apertures, S1-S3 are beam splitters, and D is a beam dump. The function of the attenuator, which

incorporates S\, S2, M4-M6, A£, and D, is described in the text.

207
~i 1 r VACUUM

n-1

_l I I I I l_

~\ 1 1 r

MAX./AV. - 1.16

-1 I I U I

QUARTZ

n=1.42

-1.2 -0.8 -0.4 0 0.4 0.8 THICKNESS (MICROMETERS)

Figure 2. Field plot of A42O3 coating (x/2 at 2.7 ym) on quartz at HF wavelength. The plot at DF wavelength is similar with max/av = 1.20.

1.5

<E2 >/<E0

2 >

1.0

t 1 r

VACUUM

n=l

-I I l_

NaF

n=1.32

MAX./AV. = 1.26

-0.8 -0.4 0 THICKNESS (MICROMETERS)

SAPPHIRE

n=1.70

Figure 3. Field plot for NaF coating (A/4 at 2.7 ytn) on sapphire at HF wavelength. The plot at DF wavelength is similar with max/av = 1.28.

<E2 >/<E 2 >

1 1 1 1 1

1 VACUUM

I I I I

SiO„

1QUARTZ

n=1.00 n=1.50 r>-1.42

'

MAX./AV. = 1.07

1 1 1 1 _l

-1.2 -0.8 -0.4 0 0.4 0.8 THICKNESS (MICROMETERS)

Figure 4. Field plot for SiO x coating (x/2 at 2.7 u m) on quartz at HF wavelength. The plot at DF wavelength is similar with max/av = 1.09.
POLISHED SAPPHIRE

DE LAMINATION CRACKING FLOW

PERFORATION

SiO.'OUARTZ

FLOW

SELECTIVE OMG

EROSION PERFORATION

Figure 5. Typical coating damage characteristics

observed with Nomarski microscopy. The energies

were well above threshold for all illustrated

characteristics in each case.

LIGHT EMISS.

ELECTRON IMAGE H

0 8 16

A.F. THRESHOLD (kj/cm2 AT 100 ns)

Figure 6. Damage profile of bare sapphire surface:

1/1 at HF wavelength.

DELAMINATION

CRACKING

FLOW

EROSION

PERFORATION

LIGHT EMISS.

DELAMINATION

CRACKING

FLOW

EROSION

PERFORATION

ION EMISS.

LIGHT EMISS.

0 8 1.6 A.F. THRESHOLD (kj/cm2 AT 100 n«) 0.8 16

A.F THRESHOLD (kJ/cm2 AT 100 ml

Figure 7(a). Damage profile of Aji2°3 coating Figure 7(b). N/1 damage profile corresponding to

(A/2 at 2.7 ym) prepared in standard vacuum on fused figure (7a).

quartz: 1/1 at HF wavelength.

FLOW

EROSION

PERFOflATION

ION EMISS.

LIGHT EMISS.

0.8 1.6 2.4

A.F. THRESHOLD IkJ/cm2 AT 100 ni)

FLOW

EROSION

PERFORATION

ION EMISS.

LIGHT EMISS.

N/1

0.8 1.6 2.4

A.F. THRESHOLD (kj/cm2 AT 100 n«) Figure 8(a). Damage profile of NaF coating (A/4 at 2.7 pm) prepared in standard vacuum on fused quartz: Figure 8(b). N/1 damage profile corresDondi na to

1/1 at HF wavelength. figure 8(a).

209
1/1 N/1

PERFORATION

LIGHT EMISS.

0.8 1.6

A.F. THRESHOLD (kj/cm2 AT 100 ns)

Figure 9(a). Damage profile of NaF coating (a/4 at 2.7 \im) prepared in standard vacuum on fused quartz:

1/1 at DF wavelength.

PERFORATION

LIGHT EMISS.

0.8 1.6 A.F. THRESHOLD (kj/cm2 AT 100 ns)

Figure 9(b). N/1 damage profile corresponding to figure 9(a).

FLOW

EROSION

PERFORATION

ION EMISS.

LIGHT EMISS

SELECTIVE DMG.

0.2 0.4 0.6 0.8 A.F. THRESHOLD (kj/cm2 AT 100 ml

FLOW

EROSION

PERFORATION

ION EMISS.

LIGHT EMISS

SELECTIVE DMG.

0.2 0.4 0.6 0.8

A.F. THRESHOLD (kj/cm2 AT 100 ns)

Figure 10(a). Damage profile of SiO x , x ^ 2, coating (a/2 at 2.7 um) prepared on fused quartz in Figure 10(b). DF damage profile corresponding to standard vacuum: 1/1 at HF wavelength. figure 10(a).

SiO. (X < 2)

DELAMI NATION

FLOW

EROSION

PERFORATION

ION EMISS.

LIGHT EMISS.

FLOW

EROSION

PERFORATION

ION EMISS.

LIGHT EMISS.

SELECTIVE DMG.

0.2 0.4 0.6 0.8 A.F. THRESHOLD (kj/cm2 AT 100 ns)

0.2 0.4 0.6 0.8 A.F. THRESHOLD (kJ/cm2 AT 100 ns)

Figure 11. Damage profile of Si0 2 coating (a/2 at n 12> D e fi]e of siQ x < 2> coating

2.7 urn) prepared on fused quartz in a baked vacuum (x/2 at 2/7 ym) pre p ared on fused quartz in a baked

system: 1/1 at HF wavelength. vacuum system: i/i at H F wavelength.

210
DF

MTL. n

N»F 1.32

SiO, 1.50

Al

2 03 1.86

ZnS 2.26

A^Sj 2.40

Si 3.80 ED

0.2 0.4 0.6 0.8 A.F. THRESHOLD (kJ/cm2 AT 100 ra)

MTL. n

NaF 1.32

SiO, 1.60

AljOj 1.6S

ZnS 2.26

As

2 S3 2.40

Si 3.80

ZED

1

0.2 0.4 0.6 0.8 A.F. THRESHOLD IkJ/cm2 AT 100 m)

Figure 13(a). Comparison of thresholds for all materials evaluated in terms of damage

characteristic with the lowest threshold in each

case: 1/1 at HF wavelength. Nominal values of the refractive index n are given for the coating

material in each case.

Figure 13(b). Threshold comparison corresponding to figure 13(a) at DF wavelength.

211
MULTITHRESHOLD DAMAGE MEASUREMENTS ON AS2S3, As 2 Se3

, AND

NaF AT HF AND DF WAVELENGTHS*

T. M. Donovan, J. 0. Porteus, J. L. Jernigan, and E. J. Ashley

Michelson Laboratory, Physics Division

Naval Weapons Center, China Lake, CA 93555

We previously reported on the deposition, absorption, and CC^-laser damage

threshold for AS2S2, As2Se^, and NaF coatings. It was shown that the damage

threshold was much lower for coated than uncoated substrates and that the mechanism for damage was different in the coatings than in the substrates. We specu- lated that the damage in the coatings was related to the presence of micronsized crystalline defects distributed in the amorphous chalcogenide matrix. In this paper we report the results of damage measurements on these materials at 2.8 and 3.8 um and identify two operating damage processes: "uniform" damage,

which we associate with the amorphous chalcogenide matrix, and "selective"

damage, which we associate with the crystalline defects. For single-layer

films, we find that N/ 1 conditioning generally produces an increase in the damage thresholds. The wavelength dependence of multithreshold results in single-layer coatings of As

2

S

3 ana NaF suggests that improvement in threshold

results from the thermal desorption of contaminating layers of water at 2.7 urn. However, for a dielectrically enhanced reflector containing quarter-wave layers

of As2Se^ and NaF, N/1 conditioning promoted crystallite growth which resulted

in a definite deterioration of N/1 thresholds.

Key words: Absorption; arsenic selenide; arsenic trisulfide; defects; laser

damage; sodium fluoride.

Introduction

As^S^ and As2Se^ films have been evaluated [1] for use in three-layer antireflection designs at 10.6 um. Recent [2] absorption measurements at 2.8 and 3.8 pm indicate that these materials should perform optically well in this wavelength region also. However, relatively low pulsed damage thresholds were observed in these materials and we speculated that the damage was associated with micron-sized

crystalline defects imbedded in the amorphous matrix of these films. We have completed preliminary

multithreshold damage measurements at HF and DF wavelengths and report observations on possible damage

mechanisms

.

We observe selective damage on single-layer films which we associate with the crystalline defects. N/1 conditioning with pulses below threshold for actual damage results in an improvement in thresholds

for both selective and uniform damage processes, both at HF and DF wavelengths. The improvement appears

to be associated with the desorption of surface contamination. Finally, we have fabricated a dielectrically enhanced reflectance multilayer on polished Mo mirrors using As2Se.j and NaF as the high and low index layers, respectively. The damage threshold was two times that of a Au-coated, diamond-turned

Cu mirror. N/1 conditioning, in this case, leads to defect formation and degradation of the coating

resulting in a significant reduction in damage threshold.

Multithreshold Measurements

The details of the multithreshold analysis technique have been described by Porteus [3] and the results of measurements at 2.8 and 3.8 um on a number of coating materials including NaF are reported

[4] elsewhere in this conference. The results of comparing first visible damage on six different

materials from the study [4] are shown in figure 1. It is seen that As 9 S» first damages at an effective

2 2 fluence of about 50 J/cm while NaF, with a much higher (30X) absorption, damages at about 400 J /cm , indicating that absorption alone does not determine the optimum damage resistance.

As further background, figure 2 shows electron micrographs of AS2S.J an^ &S2 Se 3 films revealing the size and morphology of typical crystalline defects found in these films. The formation, morphology,

numbers, and chemical analysis of these defects are described in reference 1.

* Work supported by the Defense Advanced Research Projects Agency and Navy Independent Research funds.

1. Figures in brackets indicate the literature references at the end of this paper.

212
Selective and Uniform Damage

The initial or first observed damage to As^ films typically occurs in off-axis regions of smaller

dimension than the beam diameter; this we refer to as selective damage, or damage which is spatially

selected by defects or inhomogeneities in the coating. Figure 3 shows damage resulting from a pulsed

C0

2 beam. There is delamination of the film around the damage site and perforation of the film. Figure 4 shows multiple damage sites, interference or ripple patterns, associated with enhanced fields

resulting from interference between the incident and scattered beams at a defect [5], and "scrubbing",

where the region of the film in the beam is swept free of crystalline defects, which are shown here as small bright spots. This damage we associate with the presence of the type of micron-sized crystalline

defect shown in figure 2.

Uniform damage, which includes erosion as well as ion and light emission, occurs uniformly over

the area of the incident beam. The spatial distribution is determined then by laser intensity and not

by specific inhomogeneities in the film, as in selective damage.

Comparison of HF/DF Damage Thresholds

Selective damage occurs in As^S^ at about one half the energy of uniform damage. This is shown

in figure 5(a) where the damage for selective processes is compared with uniform processes including

erosion, perforation, ion, and light emission. At DF wavelengths [fig. 5(b)] the thresholds are reduced

by about a factor of two, except for the threshold for perforation which is reduced slightly. This is different behavior than for films such as SiO^ which contain t^O impurity [4]. In this case, the thresholds increase on-going from 2.8 to 3.8 ym, as ^0 absorbs strongly at HF (2.8 um) . As

2

S 3 and

As2Se,j, on the other hand, have extremely low absorption (6 < 0.5 cm "*") in the ^0 absorption band at 2.8 urn [2]. The decrease in damage threshold observed here is possibly related to the increase in spot

size in going from 2.8 to 3.8 um.

Conditioning (N/1) Behavior

Figures 6(a) and (b) show multithreshold results for HF damage, both as single-shot and after preconditioning the surface with pulses (typically N = 5, with energy just below that needed for damage)

prior to the damaging pulse. It is interesting to note that uniform damage processes show significant

improvement while that due to selective damage is increased only slightly. This suggests that conditioning pulses have sufficient energy to thermally remove surface contamination (observed by calorimetry

15 1 1 [2], N analysis [6], ESCA [1]) but are not effective in reducing defect sites. Defects are "scrubbed"

free, as shown in figure 4, but this process occurs only when there is sufficient energy to damage.

Detail of the selective damage process and N/1 behavior is shown for DF damage in figures 7(a) and (b)

It is of interest to note that ripple patterns or melting is only associated with selective damage

processes or crystalline defects. Within the amorphous matrix the film erodes or vaporizes.

(As

2 Se

3 /NaF)

2 /Ag/Mo

As^S^ and NaF were used to fabricate an enhanced reflection design on Ag-coated Mo (R = 0.997 @ 2.8 um) . The damage measurements are compared to those of a Au-coated, diamond-turned, Cu mirror. As

2 can be seen in figure 8(a), the thresholds are slightly greater than 300 J/cm at 100 nsec incident energy and are the same for uniform damage as selective damage. The interesting point is that the threshold is two times that of an uncoated mirror [fig. 8(b)].

N/1 conditioning leads to the creation of crystalline defects and degradation of the surface

resulting in "selective" damage at an energy less than damage of the Au-coated Cu mirror [figs. 9(a) and

(b)]. The intense HF/DF laser radiation possibly induces local heating which results in the nucleation

and growth of recrystallized regions within the coating. Possibly a more uniform and controlled heating

process would result in more uniform, defect-free stable coatings.

Conclusions

Preliminary 2.8- and 3.8-um multithreshold damage measurements have been completed on a singlelayer AS2S2 film deposited on CaV^ and a multilayer film consisting of quarter-wave stacks of As2Se.j and NaF.

1. Two types of damage are observed with As^S^: damage that occurs within selected regions of the incident spot which we associate with the presence of micron-sized crystalline defects, and damage

that occurs uniformly over the area of the spot and is associated with erosion and other processes within the amorphous matrix.

213
2. Multilayer films consisting of alternate layers of As2Se, and NaF in an enhanced reflection

design on polished Mo have a higher threshold than Au-coated, diamond-turned Cu mirrors. The As

2 S

3 /NaFcoated mirror degrades significantly during irradiation at energies less than threshold (N/1) at both

HF and DF wavelengths. We associate this with the production of micron-sized crystalline defects

during the irradiation process.

References

[1] Donovan, T. M. , Baer, A. D. , Dancy, J. H.

,

and Porteus, J. 0. , "Defects and Impurities

in As

2

S 3> ks^Se^, and NaF Coatings," in Laser Induced Damage in Optical Materials:

1977, ed. by A. J. Glass and A. H. Guenther. Washington, D.C., National Bureau of Standards (NBS Spec. Publ. 509), p. 342.

[2] Temple, P. A., Decker, D. L, and Donovan,

T. M. , "Measured Thin Film Absorption at the

Air-Film Interface, the Film Bulk, and the Film-Substrate Interface," this conference.

[3] Porteus, J. 0., Jernigan, J. L. , and Faith,

W. N. , "Multithreshold Measurement and Analysis of Pulsed Laser Damage on Optical Sur- faces," in Laser Induced Damage in Optical

Materials: 1977, ed. by A. J. Glass and

A. H. Guenther. Washington, D.C., National

Bureau of Standards (NBS Spec. Publ. 509),

p. 507.

[4] Porteus, J. 0., Donovan, T. M. , Jernigan,

J. L., and Faith, W. N., "Multithreshold

Evaluation of 175-nsec Pulsed Laser Damage to Coating Materials at 2.7- and 3.8-um Wavelengths," this conference.

[5] Temple, P. A., and Soileau, J. M. , "Resonant

Defect Enhancement of the Laser Electric

Field," in Laser Induced Damage in Optical

Materials: 1976, ed. by A. J. Glass and

A. H. Guenther. Washington, D.C., National

Bureau of Standards (NBS Spec. Publ. 462),

p. 371.

T6] Tombrello, T. , private communication.

Figures

Diem- 1

) • A - 2.87 urn

SiOJCLR) 1.5

SiO„(YLLW) 1.6

A1,0, 1.7

ZnS

Si

2.3

2.4

3.8

Figure 1. HF first visible laser damage of various

single-layer coatings.

ED

0.2 0.4

A. F. THRESHOLD AT 100 m IkJ/om2

)

Figure 2. Defects formed on AS2S3 and As2Se3 films.

r

0

214
100pir.

Figure 3. CO^ laser damage sites showing del ami nati on and perforation.

Figure 4. Multiple CO2 laser damage sites showing ripple formation and scrubbing effect

215
HF, 1/1

EROSION

PERFORATION

ION EMISSION

LIGHT EMISSION

SELECTIVE DAMAGE

0.1 0.2 A.F. THRESHOLD (kj/cm2 AT 100 nil

PERFORATION

ION EMISSION

LIGHT EMISSION

SELECTIVE DAMAGE

0.1 0.2 A.F. THRESHOLD IkJ/cm2 AT 100 ns)

Figure 5 (a). HF laser damage of single-crystal

CaF2 coated with a half-wave of AS2S3. Figure 5 (b). DF laser damage of a half-wave

coating of AS2S3 on single-crystal CaF2.

HF. 1/1

EROSION

PERFORATION

ION EMISSION

LIGHT EMISSION

SELECTIVE DAMAGE

0.1 0.2

A.F. THRESHOLD (kj/cm2 AT 100 ns)

Figure 6 (a). HF laser damage of AS2S3 film on CaF 2

.

EROSION

PERFORATION

ION EMISSION

LIGHT EMISSION

SELECTIVE DAMAGE

N/1

0.1 0.2 A.F THRESHOLD (kj/cm2 AT 100 nsl

Figure 6 (b). HF laser damage of AS2S3 film on CaF after N/1 conditioning.

PERFORATION

ION EMISSION

LIGHT EMISSION

SELECTIVE DAMAGE

0.1 0.2 A.F. THRESHOLD (kj/cm2 AT 100 ns)

PERFORATION

ION EMISSION

LIGHT EMISSION

SELECTIVE DAMAGE

0.1 0.2

A.F. THRESHOLD (kj/cm2 AT 100 nsl

Figure 7 (a). DF laser damage of AS2S3 film on single-crystal CaF2- Figure 7 (b). DF laser damage of AS2S3 film on single-crystal CaF2 after N/1 conditioning.

216
DELAMINATION/

CRACKING

FLOW

EROSION

PERFORATION/

ION EMISSION

LIGHT EMISSION

SELECTIVE DAMAGE

DELAMI NAT ION/SLIP

CRACKING

FLOW

EROSION

PERFORATION/CRATER

ION EMISSION

LIGHT EMISSION

SELECTIVE DAMAGE

0.2 0.4 INC. THRESHOLD (kj/cm2 AT 100 nl)

0.2 0.4 INC. THRESHOLD (kj/cm2 AT 100 ns)

Figure 8 (a). HF laser damage of Ag/Mo mirror

(reflectance enhanced with quarter-wave layers of As2Se3 and NaF).

Figure 8 (b). HF laser damage of diamond-turned

Au/Cu mirror.

HF. N/1

DELAMINATION/

EROSION

PERFORATION/

ION EMISSION

LIGHT EMISSION

SELECTIVE DAMAGE

DELAMINATION/

EROSION

PERFORATION/

ION EMISSION

LIGHT EMISSION

SELECTIVE DAMAGE

02 0.4 INC. THRESHOLD (kj/cm2 AT 100 n)

0.2 0.4 INC. THRESHOLD (kj/cm2 AT 100 ni)

Figure 9 (a). HF laser damage of Ag/Mo enhanced

reflectance mirror after N/1 conditioning.

Figure 9 (b). HF laser damage of diamond-turned

Au/Cu mirror.

217
TEM INVESTIGATION OF EFFECTS OF A BARRIER LAYER

ON DAMAGE TO1.064yAR COATINGS

C. K. Carniglia and Joseph H. Apfel

Optical Coating Laboratory, Inc. Santa Rosa, CA 95402

and

Gerald B. Carrier

Cornincf Glass Works

Corning, NY 148 3 0 and

D. Milam

Lawrence Livermore Laboratory

University of California

Livermore, CA 94550

Silica/titania 4 layer AR coatings on BK-7 glass, with and without a half-wave silica barrier layer, have been damaged at 1.064y by

150 ps laser pulses. The damage sites have been examined by TEM replica

techniques at a magnification of 70,000. The size of the damage sites

appears to be "quantized." Most sites consist of a lu nucleus which leads

to a 3p crater in a fully developed site. The mechanical properties of

the films appear to play a role in the morphology of the damage sites.

The titania layers seem to fracture while the silica layers seem to melt or tear. A series of micrographs of near threshold damage sites

shows the development of the sites from a rupture which appears to occur at or near the substrate surface. Evidence of heating in the

substrate is also present, although it is difficult to pinpoint the

exact starting point of the damage mechanism.

Key words: Antiref lection coatings; electron microscopy; laser damage;

optical coatings; thin films.

The low damage threshold of antiref lection coatings makes them a limiting factor

in the achievement of large laser powers at 1.064ym for laser fusion. The damage

threshold for AR coatings is roughly half that of high reflectors in spite of the fact

that the electric field intensities in the two types of coatings indicate that the AR

coatings should have a higher damage threshold [l]. The purpose of this study is to

determine the location of the origin of laser damage in AR coatings using transmission

electron microscopy (TEM) to provide detailed pictures of the damage sites. While the

results are not conclusive, there are many indications that the damage originates at

the interface between the coating and the substrate. However, this conclusion does

not answer the question regarding the cause of the damage.

1. Coating Design

The AR coating used for this study is similar to the design used on the larger

components of the SHIVA laser system and is referred to here as the SHIVA AR. It is

a four layer design consisting of silica and titania. The basic design and electric

field intensity are shown in the upper field plot in figure 1. The field is shown for

a wave incident from the left in air. The titania layers are shaded. The lower

E-field plot in figure 1 shows the same design with an additional half wave barrier

layer of silica adjacent to the substrate. Notice that the electric field strength in

the four layers of the AR coatings is the same in both cases and also that the field

at the substrate interface is the same with or without the barrier layer. Figure 1 shows the spectral performance of the SHIVA AR on both BK7 and fused silica substrate.

These curves are similar with or without the barrier layer.

A series of damage tests on the SHIVA AR on BK7 with and without the half wave silica barrier layer has been undertaken previously [2]. The results indicate that

for a 0.15ns laser pulse, the threshold for the AR without the barrier layer is be2 tween 2.7 and 4.2 joules/cm . The presence of a half wave of silica under the same

2 coatings raises the threshold to the range 4.8 to 6.3 joules/cm with a typical improvement in excess of 50%.

218
There are several possible locations for the origin of the damage. The most likely

possibilities are: 1) in the substrate just below the surface, 2) between the substrate and the coating, 3) in the first (thin) titania layer and 4) in the second

(thick) titania layer. It is generally accepted that silica has a lower absorption

than titania at 1.064ym and thus the damage is not expected to originate in the silica

layers. This conclusion is strengthened by the fact that the addition of a thick

silica layer raises the threshold.

The large effect of the barrier layer seems to indicate that absorption by surface

contamination or by the titania are not sufficient explanations of the damage mechanism. In fact, the success of the barrier layer is difficult to understand in terms of

any one damage mechanism taken by itself. It suggests that the titania in contact with

the substrate is more damage prone that silica in contact with the substrate. This

may be due to an increased absorption caused by an interaction between the titania and

the substrate, or due to a better adhesion of the silica. Possible causes of the damI age are as follows: a) weakening of the substrate near the surface due to the grinding

and polishing process; b) the presence of polishing compound at or below the glass

surface; c) contamination left on the surface after cleaning but before coating, resulting either in poor adhesion of the films or in absorption by the contaminants

themselves; d) bulk absorption in the titania films.

Further factors that may be relevant to this study are the mechanical and structural properties of the layers. The silica films have a compressive stress while the

i titania films have a tensile stress. The net stress of the coating is slightly compressive in the case of the SHIVA AR and highly compressive with the barrier layer.

j The silica layers are amorphous and the titania layers are crystalline. This is verified in the TEM pictures discussed in section 3.

2. Damage Configuration

For this study coatings of the above designs were prepared on 1.5" dia. bowlfeed polished BK7 substrates. The damage testing was done at the Lawrence Livermore

Laboratory facility. These samples were subjected to 0.15ns laser pulses at powers

1 well above damage threshold. Sixteen laser pulses were spaced evenly around a 1cm dia.

ring in such a way that they did not quite overlap. Generally the damage occurred

without the emission of light indicating that the mechanism was linear absorption

rather than avalanche breakdown.

Optical microscopy of the damaged areas reveals in most cases a heavily damaged

i region several hundred micrometers across surrounded by many smaller damage sites.

These small damage sites appear to be uniform in size and to decrease in density with

distance from the heavily damaged region. It has been previously observed [4] that

: the density of the individual damage sites is related to the intensity profile of the

laser pulse, much as in a photographic image. Thus the sites around the edge of the

pulse are expected to be representative of threshold damage. For the same laser pulse

I energy, the damage to the AR without the barrier layer appears to be more severe, with

large regions where the coating is removed completely. The sample with the barrier

layer shows a lower level of damage. These observations are consistent with previous

findings on other samples regarding the relative damage thresholds of the two coatings.

3. Electron Microscopy

I The original intent of this study was to use "over-the-edge" replica techniques

to provide a cross section of a near threshold damage site. This technique involves

fracturing the sample to be studied and then depositing the replica materials on the

top surface of the coating as well as over the fractured cross section of the sample

] [3]. The replica is then removed and "flattened out" and studied using conventional

TEM techniques. Examples of the results of this technique are shown in figures 2 and

3. These micrographs are over-the-edge replicas of the two coatings studied. Figure

2 shows the cross section of the SHIVA AR illustrating the amorphous silica top layer

I: (between single black arrows) , the oriented crystalline titania layer (between double

white arrows) , the thin silica layer (between double black arrows) and the thin

crystalline titania layer (between the white-black pair) . The single white arrow at

the top of the figure indicates the top edge of the sample. The white bar at the

bottom of the figure is lum in the length. Figure 3 is a similar cross section of the

SHIVA AR with barrier layer. The five layers are indicated by pairs of black arrows

with the top of the sample to the right as before. A slight etch after fracturing was

required to make the boundary between the half wave silica layer and the substrate

visible

21Q
The arrangement of the laser shots in a closely spaced ring was intended to improve the chances of fracturing the sample through a damage site. A technique was de- i veloped for fracturing the samples radially through the damaged areas. However, away

from the heavily damaged region, the density of near threshold damage sites was so low 1 that it was nearly impossible to fracture through one and obtain a cross section.

Thus most of the micrographs were taken using standard replication techniques.

All of the electron micrographs are taken from platinum-preshadowed carbon replicas. The platinum-palladium shadowing metal is evaporated from a point source and

remains on the sample at nearly the position of impact. The carbon which forms the

support film is nearly uniformly thick over the sample surface. The replica is then

removed and investigated using TEM. The micrographs are all negative prints. The

dark shadowed regions are due to the platinum-preshadowing (corresponding to regions

without platinum) . The white regions correspond to abrupt steps in the surface contour and are due to the carbon support film. These features are illustrated in figure

4 which is an annotated micrograph of a damage site from the SHIVA AR plus barrier

layer. This micrograph illustrates most of the important features of the damage sites"

Our interpretation of the features on figure 4 is as follows:

(1) This pair of arrows shows the cross section of the outer silica layer.

(2) This pair of arrows shows the top surface of the outer titania layer

where the outer silica layer has separated from the titania layer.

(3) This pair of arrows shows the location of the abrupt step where the

outer titania layer cleaved normal to the film surface.

(4) This pair of arrows shows the cross section of the second silica

layer

.

(5) This pair of arrows shows the surface of the inner titania layer

where the second silica layer has separated from it.

(6) This pair of arrows shows the location of the abrupt step where the

inner titania layer cleaved normal to the film surface.

(7) This pair of arrows shows the cross section of the innermost silica

layer

.

(8) This pair of arrows shows a region where the underlying glass has

erupted through a portion of the inner silica layer and flowed outward over the silica surface.

(9) This pair of arrows shows the glass substrate. Note that there are

a few craters extending down into the glass and that there has been

flow of the glass around these craters. Obviously then the glass has

bF 3n quite hot in the regions of these craters.

(10) Hackle marks - a characteristic feature frequently observed on fractured glass surfaces.

Figures 5a-5 are micrographs of a series of different damage sites from the AR without bar" _ayer showing a progression in damage level from near threshold to fully

damaged 5_i.ce. The fracturing in the center suggests that the damage originates as an n explosion beneath the surface which forces the outer layers up and off. The combination of the fracturing in the center and the presence of the large crater suggests

that the damage is caused by thermal expansion of the coating beneath the surface

according to the model of Duthler and Sparks [5]. It is interesting that the outer

edae of the craters have a similar diameter in all cases. This suggests that the

. ^e originates at the same depth within the coating.

Similar micrographs of partially developed damage sites from the sample with the

barrier layer are shown in figure 6a and 6b. These suggest a similar damage mechanism

namely, an expansion of the material from within the coating.' However in this case,

there are several different sized craters present. The fractured pattern in figure 6a

is larger than those in figures 5a-g indicating that it originated at a greater depth

and thus probably at the substrate surface under the barrier layer. The sites in

figure 6b are smaller suggesting that they originated within the titania layers. An

alternate explanation for the small sites is that they represent the central fracture

due to damage originating beneath the barrier layer.

220
Figure 5g has many of the same features as figure 4, demonstrating that the damage mechanism has many similarities with and without the barrier layer. The substrate

appears to have been molten in both cases, suggesting that the hottest area was near

the substrate. Figure 7 is an over- the-edge micrograph of the AR without a barrier

which is one of the few cases where the fracture goes through one of the damage sites.

(The black arrow indicates the fracture edge. ) This figure illustrates clearly that

the damage extends into the substrate.

The hackle marks which appear in several of the micrographs give further support

to the thesis that the damage originates at the substrate interface. Figure 8 is a micrograph of a severely damaged region from the sample with the barrier layer. Near

the center bottom of the micrograph are two arrows enclosing a series of hackle marks

which occur in the silica barrier layer. These hackle marks indicate that the silica

was brittle while the glass substrate was obviously quite fluid. The steps that produce these lines are generally normal to the direction of propagation of the fracture.

Thus the lines themselves lie in the direction of propagation. Note that the lines

are not parallel, but focus toward a point to the left. It is likely that this focal

point is the approximate location of the origin of the fracture.

Electron micrographs of laser damage sites on the SHIVA AR with and without a silica barrier layer strongly suggest that the damage is due to thermal expansion

of the material at or close to the substrate-film boundary. It appears that the

damage is caused by polishing or cleaning residue rather than by absorption in the

coating materials themselves. However, this does not explain why the silica barrier

layer is so effective in raising the damage threshold. Thus the controlling factors

in producing a high damage threshold are not clear from this study. Further studies

which could help clarify this point might include 1) using fused silica as a substrate

material and 2) using barrier layers of different thicknesses.

4 . Conclusion

5. References

[1] Apfel, J 509, 251

H. , NBS Spec. Pub.

,

(1977) . [4] Milam, D(1977) . Appl. Opt. 16, 1204

[2] Apfel, J Milam, D Weber, M

509, 255

H. , Enemark, E. A.,

, Smith, W. L. , and

J. , NBS Spec. Pub.

(1977)

[5] Duthler

,

NBS Spec

C. J. and Sparks, M.

,

Pub. 414, 219 (1974).

[3] Carrier, G. B. , J. Am. Ceram.

Soc. , (submitted for publication)

221
SH IVA AR

A

A

BK7 J

//FS

0.9 WAVELEN6TH 1.4

Figure 1. Reflectance versus wavelength of the SHIVA AR on BK7 and fused silica substrate. The electric field profiles for the AR without, barrier

(upper inset) and with barrier layer (lower inset) are shown.

Figure ?.. Over-the-edge replica nicrograph of th> SHIVA AR showing the four layers. The top surfaa

of the coating is to the right of single white

arrow. White bar indicates 1 micrometer.

Figure 3. Over-the-edge replica nicrograph of the SHIVA AR with silica barrier layer. The top

surface of the coating is to the right. Single arrow indicates substrate interface.

Figure 4. Damage site from the SHIVA AR with

barrier noting the important features as discussed

in the text.

222

Figures 5a-g. Various damage sites from the SHIVA

AR illustrating a progression in damage level from

a slightly damaged site (5a) to a fully developed

damage site (Cg)

.

22A
Figures 6a, b. Near threshold damage sites from SHIVA AR with barrier.

Figure 7. Over-the-edge micrograph of a damage

site showing that the damage extends into the

substrate. Arrow indicates edge of substrate.

Figure 8. Heavily damaged area from the SHIVA AR with barrier layer illustrating hackle marks

between arrows.

225
OPTICAL TECHNIQUES FOR THE DETERMINATION OF PULSED LASER DAMAGE IN THIN FILMS

T. W. Walker and A. H. Guenther

Air Force Weapons Laboratory

Kirtland AFB, NM 87117

and

P. E. Nielsen

Air Force Institute of Technology

Wright-Patterson AFB, OH 45433

Two techniques, to determine the occurrence of pulsed laser induced damage

in thin film dielectric coatings, were compared and evaluated against normally

employed methods. One of the new techniques utilized near back scattered radia- ' tion from the damaging pulse and proved to be as sensitive as the best existing

optical probe method while being experimentally facile. The second new method

recorded a time delayed probe pulse which was reflected from the thin film damage

site. This method appears to be more sensitive to a local index of refraction

change in the thin film rather than increased reflectance/scattering from the damage site. Preliminary test results show that most of the morphological damage

occurs after the damaging pulse has passed.

Key words: Laser damage; optical probe technique; radiation scattering; thin

films

.

1. Introduction

Over the last several years optical probe techniques have been developed to detect the onset of

laser induced damage in thin films (1-5). This paper describes a variation of the laser induced scatter (LIS) method (1) which offers several advantages over other proposed techniques. The optical

probe arrangement of Alyassini and Parks (2-5) requires specially shaped samples to allow the proper angle of incidence of the probe beam at the film-substrate interface. Furthermore this tech- nique loses sensitivity when the film's index of refraction is higher than that of the substrate.

This original LIS method was found not as sensitive for some types of films as the present technique

and does not offer a recordable quantitative indication of the damage event.

In making this comparison films of ThF^, MgF^ and ZrO^ were tested. Analysis of the damage to MgF^ agrees with the result stated in (5); that is, damage in this material can occur after the damaging pulse has passed. This situation also appears to be the case with the ZvO^ films.

2. Experimental Method

The experimental approach was to compare the intensity of near back scattered radiation from the damaging beam, produced in the thin film, to a reference pulse. The reference pulse was also compared t to a time delayed probe pulse which was coincident spacially with the damaging pulse at the thin film/

substrate interface. The experimental arrangement is shown in figure 1. The reference pulse was de- layed 55 nsec from the damaging pulse, while the probe had an adjustable time delay of 0 to 50 nsec. The three separate pulses of light were collected in one ITT S-20 FW 114A (A) detector. The detector

output was dc coupled to a Tektronic R7912 transient digitizer. A separate S-20 (B) detector was used to monitor the transmitted damaging pulse after it exited the sample. A He-Ne probe beam was coincident with the damaging beam and was used for visual LIS detection of damage.

The damaging pulse had a FWHM 18 nsec and a wavelength of 0.53 pm. The beam diameter on target

was calculated to be 200 ym at the e"^ points and the probe beam was focused to have an equivalent spot;

size at the target site. Three different thin film materials were tested: MgF^, ThF^ and Zr02- All films were deposited on fused silica substrates and were of A/2 thickness at 1.06 ym.

5

3. Results and Discussion

Figures 2, 3 and 4 depict the test results for ThF^ films. Figure 2 exhibits the scattered, probe

and reference pulses at an energy well below the film breakdown threshold. The important feature in each test sequence is the ratio of the three peak amplitudes. At low incident laser energies, well below the damage threshold, the ratio of the reference to near back scattered intensity varied somewhat

226
between different test films, but was essentially constant for different sites on the same films.

This low energy pulse ratio for the reference :probe : scattered signals is 1.0 : 0.86 : 0.75. With occurrence of damage the ratio becomes 1.0 : 1.07 : 1.11 which is shown in figure 3. A subsequent

return to sub-threshold low energy level is shown in figure 4. The ratio is 1.0 : 1.07 : 1.29. The

fact that the relative scattered intensity has increased slightly after the return to low energy level

will be commented on later. The morphology of the damaged ThF^ site is shown in figure 5. The diameter of the site is 1.3 urn and the edge is smooth. There was no detection of damage by the visual

LIS method using HeNe illumination. The morphological observations are similar to those described by

Alyassini and Parks (5) when they observed damage with their probe technique, but did not detect damage

by the visual LIS method.

The results for the MgF^ films are shown in figures 6 through 9. The initial low energy ratio was

1.0 : 0.69 : 0.69; the ratio at damage was 1.0 : 0.7 : 0.79; the ratio upon a return to low energy

level was 1.0 : 0.7 : 1.11. The damage site is shown in figure 9. This damage event was detected by

the visual LIS method. This observation is in agreement with Alyassini and Parks (5) , probably due to

the larger damage site size and its more irregular edge.

Damage tests on ZrCb produced the respective low, high, low energy ratios of 1.0 : 1.29 : 0.81,

1.0 : 1.48 : 1.04 and l.u : 1.63 : *. The (*) indicates that the scattered signal was so large that

the peak was off scale relative to the probe and reference beams. This is shown in figure 12. The relatively large damage site for the ZrC^ films is shown in figure 13.

The striking difference of the ThF^ and ZrO- film behavior compared with that of ^g^2 film concerns

the variation in the probe beam intensity. For the first two film materials there is a definite indication of damage while with MgF. the probe beam shows essentially no change before, during or after damage.

One possible explanation of this is that the probe beam is a reflected beam. As such it would be more

sensitive to a small change in optical thickness, resulting from either a change in refractive index

or thickness of the thin film, in a fairly large region around the damage site than it would be to a totally reflective 2-3 um damage site. In the case of ThF^ and ZrO^ films, the reflectance is at a minimum and any local change in the optical thickness of the film would produce an increase in reflectance. The MgF^ film, on the other hand, is at a maximum of reflectance and any change would effect

a decrease in reflectivity. The fact that this does not occur with MgF^ films may indicate that a local index change does not take place in MgF^ or that the effect is compensated by increased reflectance from the damage site. The differences in morphology of the three sites can aid in the interpretation. The drastic increase in scattering for ZrC^ indicates that the near back scattered signal is probably a strong function of the damage site area. Whereas the similar changes in the probe signals

for ZrC^ and ThF.- films require that the probe signal be at best a very weak function of the damage

site size. Thus it would seem that the probe signal increase in ThF film is not due to the damage

site itself. Since the MgF^ film and ThF^ site sizes are similar in size, whatever is occurring in ThF^ is not occurring in MgF2 film. Finally, the ThF^ damage site has a melted nature while that of

the MgF^ more of a cratered nature, indicating that subtle changes may have occurred around the first

site and that changes were more localized at the second site. The degree to which the signal ratio's

change for the various films is undoubtedly also a function of the refractive index of the individual

films and their difference from the substrate's value.

Another interesting feature is the difference between the ratios at the damaging energy level

and at the return to low energy. For ThF^ film most of the change has occurred during the damaging

pulse and is over by the time the probe is incident on the site. The increase in the scattered

light signal at the low energy level indicates that some damage has occurred after the damaging pulse

but prior to the probe pulse. The MgF2 and ThF^ films exhibited a large increase from the high to the

low energy levels. This is particularly noticeable in ZrO^ film and was in evidence in all of the tests

on these three materials. Substantial damage evolves in all materials after the damaging pulse has passed through the samples.

The time delay was decreased for the probe in several tests. During this variation there was essentially a constant behavior observed when MgF^ films were tested. However, there was a small variation in the ThF. probe data when the time delay was reduced to 25 nsec, indicating that structural

changes may still be occurring to that time. However, the variations were too small O 10%) to be conclusive

Finally, it was noted that there was no visible spark observed nor any distortion to the transmitted damaging pulse in any of the tests discussed. When a visible spark occurred the scattered

227
light signal was several orders of magnitude greater than the reference signal. The damage morphology showed several damage sites with diamaters in excess of 10 urn.

4. Summary and Conclusions

We have found that two optical techniques, different from those previously employed, provide a viable technique for determining the occurrence of damage in thin films. Collection and detection

of the near back scattered (^ 5°) light from the damaging pulse and its subsequent ratioing to the incident pulse intensity was the most reliable measure of damage. In the case of MgF 2 and Zr02 films, best confirmation of damage is obtained by comparison of the ratio after re-exposure at a low level rather than from ratios obtained during the damaging event. Since most damage experiments are accomplished using a single site-single shot format, this should provide no obstacle other than requiring two additional low energy exposures. The low energy events can be such a level to exclude

any conditioning of the film by the laser pulse.

It is not clear why the probe gives a successful indication of damage for ThF^ but not MgF

? films.

An answer to this point may shed light on why MgF

2 fluoride films do not fit the Bettis, et al. (6) predicted index of refraction dependence on breakdown.

We find that the back scatter method can be more sensitive than the visual LIS method. However,

it is only an electronic variation of the same effect. In comparison with the Alyasinni and Parks

(S) optical probe, it has more versatility and is much simpler experimentally. Furthermore, it can be made in an objective manner affording a quantitative measure for comparison.

The results found in the backscatter experiment for damage site morphology versus damage detection

by visual LIS agree with those of Alyassini and Parks (5)

.

TABLE I Relative Changes in the Intensity of Probe and Back Scattered Beams Below At Return to Damage Damage Below Damage

MgF (p) 0.69 0.7 0.7

(s) 0.69 0.79 1.11

ThF (p) 0.86 1.07 1.07

(s) 0.75 1.11 1.29

ZrO (p) 1.29 1.48 1.63

(s) 0.81 1.04 *

Reference (incident) pulse normalized to 1 p = probe beam

s = scattered beam

* = scattered beam intensity off scale being orders of magnitude greater than level at damage

5. References

(1) B. E. Newnam and L. G. DeShazer, 4th ASTM Symp. , Damage in Laser Materials, Edited by A. J. Glass

and A. H Guenther, NBS Spec. Pub. 372, 123 (1972).

(2) J. H. Parks and N. Alyassini, 4th ASTM Symp., Damage in Laser Materials., Edited by A. J. Glass and

A. H. Guenther, NBS Spec. Pub. 372, 104 (1972).

(3) N. Alyassini, J. H. Parks, and L. G. DeShazer, 5th ASTM Symp., Damage in Laser Materials., Edited

by A. J. Glass and A. H. Guenther, NBS Spec. Pub. 387, 133 (1973).

(4) N. Alyassini, Ph.D. Thesis, University of Souther California, 1975 (University Microfilm, Ann Arbor,

Mich. )

.

(5) N. Alyassini and J. H. Parks, Laser Induced Damage in Optical Materials Symp., Edited by A. J. Glass

and A. H. Guenther, NBS Spec. Pub. 435, 284 (1975).

(6) J. R. Bettis, R. A. House, A. H. Guenther, and R. Austin, Laser Induced Damage in Optical Materials

Symp., Edited by A. J. Glass and A. H. Guenther, NBS Spec. Pub. 435, 289 (1975).

220
TEK R7912

He-Ne PROBE

LASER

SAMPLE t

ITT FW 114A

PROBE BEAM

0-50 nsec DELAY

55 nsec DELAYED REFERENCE BEAM

-y

FIGURE EXPERIMENTAL SCHEMATIC

Figure 1. Experimental arrangement for damage experiments employing optical diagnostic techniques.

(i, is the incident damaging beam; r, the reference beam; s, the back scattered beam; t, the transmitted beam; and p, the delayed probe beam. The photodiode A records the s, p, and r beams while photodiode B records the t beam.

)

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

-0.2

ThF

4 JIM 1 I 1 1 MM MM 1 1 1 1 1 M 1 1 I 1 1 1 1 M

Re

l ii i Ference MIL

S mattered

P robe A p

z

"I 1 1 1

I I I l MM 1 1 1 1 MM MM Mil 1 1 l 1 1 1 1 l i i I r

a

a>

0 10 20 30 40 50 60 70 80 Nanosecond

Figure 2. Three pulses at an energy level below the ThF^ film breakdown threshold. The ratio of

peak amplitudes from right to left is 1.0:0.86:0.75.

229
ThF,

0 5 n 1 1

1 1 11 1 1 1 i i1 1 1i i i 1 1 i 1 11 1 i i i 1 1 11 1 1 1i 1 1 1

1 i 1 1 1 1

1 1 1

r

0 10 20 30 40 50 60 70 Nanosecond

Figure 3. The same three pulses at film breakdown. The right-to-left ratio is 1.0:1.07:1.11.

ThF,

10 20 30 40 50 60 70 Nanosecond

Figure 4. The three pulses upon return to low energy. The ratio is 1.0:1.07:1.29.

230
Figure 5. Nomarski micrograph of the damage sit (in brackets) for the ThFi, film.
MgF,

30

25 t 2.0 c

C

1.5

£ 1.0

Jill

1

1 II 1 1 1 1 1 MM Mil 1 1 II MM Mil

F

i i i 1 ieferenc MIL

e -

1

< Scottered i 1

= TX Probe

1 r

\ _

\ -

1

1 J-

\_

_

"III 1 1 1 1 1 1

M

MM 1 1 1 1 MM i i i i MM MM i 1 1 r

05

0 10 20 30 40 50 60 70 Nanosecond

Figure 7. At film breakdown the ratio becomes 1.0:0.7:0.79.

500

400

MgF?

3

cr

Jill MM

Scott

i i i i

ered MM Ml MM MM MM

Refer

Mil

ence

1 1 1 l_

Prob e

"mm i i i i i i i i l l 1 l l ll l i i ii ii i i l ii 1 Mil 1 1 1 f

0 0 20 30 4 0 50 60 70 80 90 10 Nanosecond

Figure 8. Upon return to low energy the ratio's become 1.0:0.7:1.11.

232
hhhhhhhhhhihhhbhbi

Figure 9. A Nomarski micrograph of the damage site (in brackets) for the MgF 2 film.

Figure 10. Initial low energy exposure yields ratio's of 1.0:1.29:0.81 for Zr0 2 films.

233
ZrO,

v 0 10 20 30 40 50 60 70 80 90 100 Nonosecond

Figure 11. The ratio at film breakdown becomes 1.0:1.48:1.04.

400

350

300

^ 250

2 200

_C

CD^ !50 o

a) tr 100

50

Zr02

-50

Jill 1 1 1 M 1 1 1

|

1 1 Scattered

i i i I MM 1

l 1 1 Ml! 1 II 1 III 1 L

1 :

—n —

\

1 Probe

,—

,

r Referer ce :

1 i

"l 1 1 1 III : ; i i i i i i i i i llll"

0 10 20 30 40 50 60 70 Nanosecond80 90

Figure 12. Upon return to low energy the ratio's become 1.0:1.63:*. The (*) indicates that the scattered signal is several orders of magnitude larger than the reference for these Zr0 2 films

.

Figure 13. A Normarski micrograph of the damage site in Zr02 shown in (brackets).

234
A STATISTICAL ANALYSIS OF ABSORPTIVE

LASER DAMAGE IN DIELECTRIC THIN FILMS*

Aaron B. Budqor and Karen F. Luria-Budgor

University of California

Lawrence Livermore Laboratory

P. 0. Box 808 Livermore CA 94550

The Wei bull distribution arises as an example of the theory of extreme

events. It is commonly used to fit statistical data arising in the failure

analysis of electrical components and in DC breakdown of materials.

This distribution is employed to analyze time-to-damage and intensityto-damage statistics obtained when irradiating thin film coated samples of

Si02 , Zr0£, and A^O^ with tightly focused laser beams. The data used is furnished by Milam. The fit to the data is excellent; we often obtain least squared correlation coefficients greater than 0.9.

It is found almost universally that statistical models of breakdown,

such as the lucky electron theory, oversimplify the damage process by

neglecting nonlinear interactions and anisotropics induced by impurities.

Thus, the fundamental intensity I relation on pulse length t often deviates

-1/2 from the classical t dependence resulting from 2-photon absorption

without diffusion, or from linear absorption with diffusion, to dependencies

-0 22 -0 44

as high as t ' for the former to t ' for the latter. This fact,

coupled with the experimental nonobservability of higher than 2-photon

absorption seems to imply that the avalanche mechanism is the most likely

initiator of the plasma requisite for lattice meltdown.

Statistical confidence bands for material survivability as a function

of laser intensity and pulse length can be constructed; this lends high

practical utility to the Weibull distribution as an engineering diagnostic

tool

Key words: Laser damage; thin films; probability; statistical analysis;

Weibull distribution.

*Work performed under the auspices of the U.S. DeDartment of Enerqy by the Lawrence Livermore Laboratory under contract number W-7405-ENG-48.

235
1 . INTRODUCTION

Laser induced breakdown of solid dielectric films and bulk materials has been a subject of onqoing

interest and concern at these symposia. Although the complete theory leading to breakdown will most

likely never be understood, i.e., aspects pertaining to imperfections intrinsic and extrinsic to the material, much effort is spent to evaluate two possible physical mechanisms initiating the breakdown

process -- avalanche and multi-photon ionization. Both of these theories have been sugqested as candidates to provide the requisite number of electrons precursory to lattice melting [I] 1

.

The avalanche theory propounds the existence of loosely bound electrons derived from shallow traps

arising from such structural imperfections as F-centers. These electrons interact with the laser field, become excited, and multiply upon impact ionization with the lattice.

When the laser photon energy is >^ — times the bandgap energy n-photon ionization becomes an alternate mechanism for the creation of conduction electrons, but with rapidly diminishing probability

as n increases. Thus, the longer the wavelength the laser light is the less effective multi-photon

ionization is for excitinq valence electrons to the conduction band. For example, a 10.6 cm laser

light source requires a 21-photon process to elevate electrons through the NaCS. F-center bandgap of 2.4 eV.

In reality there is probably some auqmentation between multiphoton and avalanche ionization which

is dependent on photon energy Tfw and E . As the photon energy attains a value > E q /2 two photon

absorption will quite likely assist the avalanche process, for E^/3 <_ ffw < E^/2 three photon absorption may occur, but with reduced probability, etc. Liu et al have set up experiments to detect 3-photon

absorption at 355 nm in NaC£ by direct absorption [2]. Not only were they unsuccessful, but all studies

including those by photoconductivity have never been able to discover absorption higher than 5-th order

in solids. Apparently the major pitfall is that at the intensity levels of observation breakdown

usually intervenes. How to deconvolute this presumed joint dependence of breakdown on both mechanisms

is a subject of this paper.

As the total number of such conductive electrons generated depends on the 1) laser pulse duration,

2) photon energy of the laser, 3) energy gap and impurity levels, and 4) ambient conditions there is' a great deal of lattitude for interpreting the damage event statistically. Such a point of view has been corroborated by the experiments of Milam et al [3] for dielectric breakdown times of bulk and thin film

materials when irradiated at small focal volumes by tightly focused laser beams.

In the following two sections we shall discuss the role and application of the Weibull distribution to the analysis of damage data. This distribution is then fit to data compiled by Picard et al and Milam for various thin film coated samples of Si^, AL^O^, and [4,5]. Estimates concerning

characteristic lifetimes and breakdown intensities are given as well as a scheme for determining the reliability of those estimates. It is also shown that the dependence of breakdown intensity on laser

pulse duration, for the most part, does not deviate markedly from linear or 2-photon absorption. A strong case is then made for the avalanche process.

2. STATISTICS OF FAILURE: THE WEIBULL DISTRIBUTION

The use of statistical failure models to make predictions and deductions about material breakdown

in the presence of external forces can be traced to the pioneering work of Griffith in his study of

flaws and their correlation to fracture [6]. Griffith theorized that the reason for the difference

between calculated and observed values of material strengths resides in the fact that there exist flaws

in the body which weaken it. If the flaws are distributed inhomogeneously they will provide a distribution of strengths as a function of material position, each position requiring a different degree of force to produce fracture. When the actual performance of the material is then predicated on its weakest "link" the use of extreme value statistics is in order, that is, the statistics of maxima and minima [7]. Some of the rudiments of this theory are presented in the Appendix.

To pursue the analogy of the foregoing to laser breakdown of optical materials we postulate that a damage event is due to a number of underlying causes. Not only is it a function of laser intensity,

impurity concentration, and structural defects, but interposed with these causes is the unifying factor

that damage may be the result of cumulative laser pulses. This implies that damage is not necessarily

an all or none event but most probably depends on the material's previous history; this includes

weakest "links". Inhomogeneities in the material and external fluctuations subsumes the use of extreme

value statistics.

1. Figures in brackets indicate the literature references at the end of this paper.

236
An extreme event distribution which has been previously found to fit the failure data of electrical

components and of D.C. breakdown of materials is the Weibull distribution [8,9]. Asymptotically it is given by the distribution function

F(y) = l-e-^ 3 y>o (1) a, 6 > o where y denotes the random variable

y = 1 im y

n = lim min ( X. , --, X p I (2)

comprised of the minimum of a larqe number of independently distributed damage random variables whose

distributions are given by (A10). The parameters a and 3 are respectively defined as scale and shape

parameters, with a being a characteristic failure coefficient since it is the 100 x (1-e ^) = 63.2 percentile of the distribution for any value of 3.

In an attempt to understand the meaning of (1) we examine plots of the density function 6(y)

6(y) = BMy/a) 6

- 1 (3)

in figure 1. For values of 3 in the range 0 < £> <_ 1 6(y) is an amodal function of y with mean u and

2 o/o — 1/2 2 variance a (eqs. A2) ranges, respectively, <*xy<l and (2 /p - 3~ ) < o <_ 1 . At 3 = 1 (1) is exponentially distributed. For values of 8<1 , 6(y) is unimodal , resembling a Gaussian at 3 ~3.6. The

2 limiting values of y and a are

lim u = 1

a

2 - 0

(*)

In plotting failure data y^ i = 1 , . . , N , to (1 ) we rank the y. 's from smallest to largest and assign values to F(y^) as ^ - m (5)

By placing these points on Weibull graph paoer (fig. 2) and drawing a straight line through them rough

estimates of a and 3 can be obtained. The a value is found by entering the probability scale at the 63.2% point and then reading its projection on the abscissa. 3 is found by drawing a line parallel to the fitted straight line through the point marked origin; the value of its intersection with the

vertical shape parameter scale at the left equals 3. An alternate and more accurate procedure for determining a and 3 is to least squares fit a straight line to

inln [1 - F(y)]" 1 = 3&ny - 3£na ( 6 )

One of the drawbacks to utilizing (1) occurs when failure events pile up at the small values of y. This tends to shift the weight of the distribution to the left, away from the true mode which varies as

_1 In N resulting in correspondingly poorer fits. This drawback can be remedied by choosing as a variant

of (1) the distribution

F(z) = e" (a7z)6 (7)

z is the random variable defined as

z = lim z = lim max ( X,,--, X I , g

,

the Xj's being independent and identically distributed according to (A18b). a' and 3' are the counterparts to the a and 3 of (1). This distribution is also known as the Pareto distribution after

the economist Pareto [10]. Thus, instead of asking what the minimum value of y for which failure

occurs is, we now invert the question to the maximum value of z. Pileups of for small values of z

237
will better the fit to (7). The parameters a' and £T are found by least squares fitting a straight line

to

2.n£n F(z)"'

1 = 6'S.na' - 6' dn z (9)

3. DAMAGE STATISTICS OF THIN FILM DIELECTRICS

In this section we apply the formulae of section 2 to damage data compiled by Milam for various

sputtered thin film dielectric samples of S.^, and Ai^O-^. These films were prepared on CG7059

electronic substrates with various percentages of sputter bias and sputtering gas (Og or A ) concentrations. The materials were irradiated at a number of separate sites with 20-30 nsec pulses focused to approximately 6 ym focal spots. The pulses were obtained by switching out the central portion of the output from a single longitudinal mode, single transverse mode Q-switched laser by means of a Pockels

cell shutter. For a given laser Dulse intensity, I, the time delay to damage, or survival time t^ was measured from the width of the transmitted pulse, and beam power from the height of a reference pulse.*

These values were obtained to a ±2.5% accuracy. As this I was varied these measurements were equivalent

to those of breakdown threshold energy density I g versus t .

A statistical ensemble to which equations (1) and (7) are fit is obtained by ranking all survival

times for specific values of Ig, or alternatively by ranking all Ig for specific values of t^. The results of least squares fitting this data to these distributions are given in tables I - 1 1 1 . Parts A of these tables correspond to distributions in t^, F(t^). The first and second columns rowise pair

individual breakdown intensities I g with the N values of t^ (given in parentheses) to be fit to F(t^) and their respective t^ ranges. The ct's denote characteristic sample lifetimes, Tj. The entries in the parts B tables correspond to distributions in Ig, F(Ig). Now the first and second columns rowise pair

individual lifetimes t , with the N values of I D to be fit to F(I D ). The ct's denote characteristic

breakdown intensities, T„. For both parts 6, r , u, and a are shape parameter, correlation coefficient

squared, mean, and variance. The entries in the a and 6 columns are not to be literally interpreted as

2 the a's and 3's from (1). They may be a' and 3^ values, dependinq on whether r was larger for the fit

to (9) than it was to (6). This statement is correspondingly true for the calculated mean u and var2 lance a values.

The deductions to be drawn from these tables are best summarized as follows:

i) The least squares correlation coefficients r indicate that the fits of thin film damage

data to the Weibull distribution are excellent.

ii) Parts A imply that increasingly larger values of I D lead to decreasinqly smaller values

of T

d

. iii) Parts B imply that increasingly larger values of t^ lead to decreasingly smaller values

of Ig-, this is true since the sample experiences the damaging pulse for longer times.

iv) The 6 coefficients from parts B will in general lead to much narrower distributions for

F(Ig) than for F(t

d

). v) In some cases the variances are not sufficiently broad to distinguish statistics

from experimental error.

*The relation between laser pulse enerqy E, intensity I, and power P, is formally given as

dt I(x, y, t) // dxd

'J

where x and y are coordinates in the plane normal to the light propagation axis. For a constant

intensity pulse this reduces to

where tp is pulse duration and A is focal spot area.

233
vi ) There may be a stabilization on the range of 6 values as a function of sputter bias and sputtering gas concentration. More data is needed to investigate this point.

If we now plot for each sample category the median In values as a function of (T^y. and least sguares fit the result to ^ '

In I = q In T. + C

( 10 ) ^ do

2 the ensuing results for q and r are presented in table IV. By arquing that breakdown occurs when a fixed amount of heat Q R is deposited per unit volume of the sample, the relevance of these q values can

be discerned from the heuristic expression

Q

B

= k i B g(t

d

) (11)

where K is an effective multiphoton absorption coefficient and the function g(t.) may be expressed as[4]

absence of thermal diffusion

for thin films or long laser pulse durations

in the presence of thermal diffusion

Thus, for the samples studied, in the absence of thermal diffusion breakdown incurred by pure multiphoton ionization leads to a maximal 5-photon process. This upper limit is reduced in the presence of diffusion to a 2-photon process. Since the true picture is probably somewhat less than these upper

limits due to the availability of conduction electrons from avalanche processes or from other excited

states (excitons, for example) we feel that multi-photon events greater than 2 probably play an insignificant role in the damage process.

4. DISCUSSION

As demonstrated in this paper the Wei bull distribution is perfectly suited as a statistical tool for the analysis of dielectric breakdown of thin film optical coatings. The authors have performed a similar analysis on gas breakdown for Ar at 500 psi [3,5]. The same excellence of fit was attained, but with the major difference that 3 was considerably less than those values reported here, consequently

2 leading to much larger values of a . It is the contention of these authors that the Weibull distribution can be utilized to characteristically fit laser induced breakdown of any material. We must wait

for experimental verification of this conjecture.

We have shown in section III that characteristic lifetimes and breakdown intensity TR are de- terminable from a (or a'). This has high practical significance since with their respective confidence

bands delimits on the reliability of the material can be established [11]*. This, of course, enables

computation of confidence bands around and I R for any material under investigation.

In regard to ascertaining the correctness of previous physical or statistical models of breakdown

it seems fairly clear that coupled with experimental nonobservabi 1 i ty of higher than 2-photon absorption [1] the I R (T

d

')" q fits seem to imply that an avalanche mechanism is the most likely initiator of

the plasma requisite for lattice meltdown. These fits also imply that the only other statistical

avalanche mechanism for plasma creation, the lucky electron theory, is wrong since this model predicts

*A confidence band around a distribution function F(x) provides an interval by interval estimate of

F(xT! The width of the interval depends upon the confidence level (1 - e) x 100%, 0<e<l . In terms of

the confidence level one can be (1 - c) x 100% sure that F(x) lies between some upper and lower

boundary curves, L(x) and U(x), respectively. For the Weibull distribution these curves are usually

found in terms of the relations

L(x) = IT

1 (e/2); U(x) = H"

1 (1^) (12)

where H (y) is the probability distribution function of the beta distribution [12].

239
a failure distribution $ which varies on t and the electric field £. = I^[3, 13] as

• - 1 - e" rt/Tc (13)

where

r = e- kI_1/2 (W)

k beina a constant qreater than zero. At damaqe threshold this imDlies for a constant value of I D a

D linear dependence on t^, while for a constant value of a Gumble-like dependence on I„

_ e "lg

(i.e., e ) [7]. It seems fairly clear that the linear deoendence on t^ is incorrect. The exponential deDendence of (11) of Ig, however, often seems to fit our Weibull description for the large values

of 3 obtained. This can be seen upon recasting r into the form

r = (i/y) s

, (15)

with y > I. Then g must be

6

= BTyTT)

kl" 1/2

By defining y as a characteristic damage intensity, y = a = I_, then as I + L, B + °°. Suitable values

of I can be found to match the 6 values we found.

The strong power law dependencies exhibited here may be due to highly nonlinear microscopic

mechanisms which in the macroscopic limit are averaaed out, once thermal diffusion is included, to produce near linear absorption. Unfortunately it is impossible from the analysis here performed to make any definitive predictions as to the exact microscopic mechanism, other than the improbability of

high order multi-photon ionizations.

240
APPENDIX

The Wei bull distribution has frequently been employed as a time to failure model of electrical comDonents and of DC breakdown of materials. It is one of a whole class of distributions arising from

the theory of extreme values and is qiven by the distribution function A(y)

A(y) = 1 -e

- (y/a)B

y > 0

< A1 )

a, 6 > 0

o A(y) has density function <5(y), mean y, and variance a

6(y) = 6/a (y/a) M e _(y/a)6

y > 0 (A2)

= 0 , elsewhere

u = ar (1/6 +1),

a

2

= a

2 -jr(2/3+l) - r2 (1/3+1)

|

The theory of extreme events poses the following problem:

Given n samples

j

, X^ drawn randomly and independently from a population with density

function f (x) [distribution function F(x)] let us define y such that

y = min 1 X, . . , X >

n n (

1 n f

(A3)

The distribution function for y

n is formally

G

n (y) = Prob (yn <y) (M)

= 1 - P^b (y

n

> y)

From (A3) and the independence of all the X . , the second equation in (A4) may be rewritten as

G

n (y) = 1 - Prob [ U X. > y| ^ ' (A5)

= 1 - [1-F(y)] n

For relatively simple distribution functions F(y) such as uniform and exponential distributions, G (y) has a rather simple structure. In qeneral , however, it is a fairly complicated function. In the limit

as n becomes large it is therefore much more convenient to utilize an asymptotic relation for G

n (y)- This is derivable by defininq the new random variable

C

n = nF(y

n

) (A6)

such that for any fixed u, o <_ u <_ n

Prob (s

n

< u) = Prob (nF(y

n

) £ uj

= Prob (yn

< F"

1 (u/n)) (A7)

= A

n

(u:

By virtue of (A4) and (A5) A (u) is

241
A

n (u) = 1 - (l -

u /n) (A8)

which in the limit as n + <» converges in distribution to A,

lim A

n (u) = A(u) = 1 - e"

u (A9)

By now choosinq the underlying distribution of the X^'s as a power law

F(Xj) =0 Xj < 0 (A10)

0 < X. < a - J -

then

n n (y n /af

-1/6 1/6

- n

s n

(All)

and A becomes

A (y) = l-e"

{y/a) y > o (A12)

= 0, elsewhere

One should note that when 6 = 1, F(X

1.) is uniformly distributed and A reduces to the Poisson distribution characteristic of the lucky electron theory (as a function of time).

As (Al) is derivable from an asymptotic analysis of minima, a similar procedure given the random

variable

2 n max |X

1 Xn |

(A13)

with distribution function

H (z) = Prob (z„ < z) (A14)

will upon defining ?

n as

E

n - n (l - F(z

n )) (A15)

result in an asymptotic distribution function A (v), 0 <_ v <_ r\, of maxima

A

n (v) = Prob (c

n < v) < A16 >

= 1 - (l-v/n)

n

As n * °° A

n also converges in distribution to

A(v) = lim A (v) = 1 - e"

v (A17)

n-xx>

Corresponding to the Wei bull distribution, if as our underlying density for the X. we choose

242
then

and

f(X.) =0 X. < 0

.1 J -

= 37«' (a'/X-r +1

F(X.) = 1 - (a'/X.f (A18b)

A(z) = e" (a7z)

(A19)

ACKNOWLEDGEMENTS

We authors would like to thank R. Mensinq, W. L. Smith, and D. Milam for their helpful discussions.

Special thanks go to D. Milam for supplying us with the damage data here analyzed.

REFERENCES

[1] Reviews of laser-induced breakdown in optical

materials are given in: N. Bloembergen, IEEE

J. Quant. Elect. QE-10 , 375 (1974); W.L. Smith

Opt. Engr. 1_7, 489, 1978.

[2] P. Liu, W.L. Smith, H. Lotem, J.H. Bechtel

,

N. Bloembergen, and R.S. Adhav, Phys. Rev. B 1978 (in press).

[3] D. Milam, R. A. Bradbury, and R. H. Picard,

in Laser Induced Damage in Optical Materials:

1975 (NBS Special Publication 435), p. 347 (1975).

[4] R. H. Picard, D. Milam, R. A. Bradbury, and

J. C. C. Fan, in Laser Induced Damage in Optical Materials: 1975 (NBS Special

Publication 435), p. 272 (1975).

[5] Data supplied by D. Milam, private

communication.

[6] A. A. Griffith, Phil. Trans. Roy. Soc. 221A,

163 (1920).

[7] E. Gumbel , "Statistical Theory of Extreme

Values and Some Practical Applications,

Appl . Math. Series 33, NBS (1954).

[8] J. H. K. Kao, Technometrics ]_, 389 (1959).

[91 P. H. H. Fischer and K. W. Nissen, IEEE Trans. Elect. Insul . EI-11 , 37 (1976).

[10] H. T. Davis, "The Theory of Econometrics

(Bloomington, Ind. 1941), p. 23.

[11] G. J. Hahn and S. S. Shapiro, "Statistical

Models in Engineering", (John Wiley and

Sons, New York, 1968).

[12] J. H. K. Kao, Proc. 6th Nat. Symp.

Reliability Quality Control in Electron.,

pp. 190-201, 1960.

[13] M. Bass and H. H. Barrett, IEEE J. Ouant. Electron. QE-8 , 338 (1972).

243
At.,0-5 TABLE IA

GW/cm?

N (nsec) 8 a

*d

2

M

2 a

NO BIAS, NO 0

2

7 <

1 B < 7.4 6.8 <_ tj <_ 10.2 (12) 7.46 7 84 0.878 7.36 1 .36 7-7 < IB

< 8.1 6 ' 2 <-V- 8 (17) 15.64 6 74 0.864 6.52 0.26

8.1 < I B < 8.S 6 i*dl. 7 (11) 15.96 6 88 0.885 6.66 0.26

9.3 <I„ < 9.8 4.6 <_t

d

< 7.2 (10) 7.15 6 17 0.966 5.78 0.91

10 <Ig < 10.5 7.4 ( 9) 5.73 5 37 0.939 4.97 1 .01 10.7 < I B

< 11.3 <-?<.t

d

< 6 (10) 14.05 5.01 0.894 4.83 0.18

10% BIAS, NO 0

2

•

6.5 < I 8

<_ 6.9 12 < t . < — d

1 9 (10) 5.61 17.54 0.946 16.21 11.17

7 <-h < 7.4 19 (10) 6.63 12. 68 0.980 11 .83 4.37

7.6 <I

B < 8 9 i 16 (15) 6.50 13 60 0.972 12.67 5.20

8.6 < I B < 9 8 «td i 11.2 (12) 9.47 8 80 0.978 8.35 1.12 < 9.7 6 " 2 I'd i

12 (10) 5.14 . 7 41 0.952 6.81 2.32 10.8 < I B

< 11.3 5 <td i7.3 ( 9) 7.48 5 72 0 .948 5.37 0.72

11.7 < I„ < 12.4 4-3 <td i 7.3 ( 8) 5.21 4 62 0.913 4.25 0.98 — 0

10X BIAS 1 ox o

2

8 lh < 8.4 10.8 < td < 15.5 (12) 5.73 12 69 0-971 16.37 10.95

9 - 3 < h

< 9.8 7-2 < t

d < 15.7 (12) 5.03 9 01 0.876 8.28 3.55 11.6 <_ I B

< 12.2 '• 6 i'd i 7.6 (10) 4.93 5 28 0.914 4.04 1 .26 12.3 < I B < 12.9 3.6 < t d < 7 (12) 5.03 5 92 0.984 5.44 1.53

13 < I B < 13.7 3,4 <td

< 7.7 (U) 3.68 4 .20 0.913 3.79 1 .31

Table I. Thin films of alumina prepared under various sputter bias and sputtering gas conditions.

A. Distributions in t^: The 1st and 2nd columns rowise pair individual breakdown

intensities Ig with the N values of t^ (given in parentheses) to be fit to F (

) and their respective t^ ranges. The a's denote characteristic sample lifetimes, t^.

B. Distributions in 1^: The 1st and 2nd columns rowise oair individual lifetimes with

the N values of I D to be fit to F(I D ). The a's denote characteristic breakdown - 2 2 intensities, Ig. 3, r , u, and a are, respectively, shape parameter, correlation

coefficient squared, mean, and variance.

Table II. Thin films of zirconia . Legend description same as table I.

Table III. Thin films of silica . Legend description same as table I.

Table IV. Scaling law for breakdown intensity as a function of laser pulse length.

244
A«. 2 0

3 TABLE IB

NO BIAS, NO 0

?

5.3

5.4 _< t

rf <_5.7

6 < t

d

< 6.3

6.7 < t d< 7 7.2 < td £ 7.6

10% BIAS, NO 0

2

7.2 < t 7.6

14

N (GU/cm^)

8.7 < I B i 11.3 (13) 8.8 < I B

< 11.2 ( 9) 7.9 < Ib < 11 0°)

7.4 < I B < 10.2 (22) 6.3 < Ib £ 10.3 ( 9)

9.2 < I <11.7 (11) 6.5 < 1^ < 8 ( 9)

11.90

9.62 8.70

13.20 6.18

13.21 14.35

10.68

9.12 8.49

7.89 7.29

9.65

7.73

0.969

0.905 0.949

0.900

0.984

0.969 0.919

10.23

8.66 8.03

7.59 6.77

9.28

7.45

1.09

1.17

1.21 0.49 1.63

0.73 0.40

10X BIAS, 10X 0.

5.5 < td< 5.£

10.5 < t

rf < 11

11.3 < I 8< 14.2

8.2 < I n < 10.5

( 8) (ID

12.26

11 .20

12.22

9.88

0.963

0.967

11.72

9.44

1.35

1.04

Zr0

2 TABLE IIA

GW/crn N (nsec)

7.4

7.5 1 I n i 7.9

8.3 < I B

< 8.7 8.8 < I < 9.2 - B -

9.4 < I B

< 9.9

9.8 < I < 10.3

30% BIAS, NO 0

2

8.8 < Ig < 9.2

9.3 <_ Ig < 9.8

10.1 1Ig <_ 10.6

10.7 < Ig < 11.2

11.3 < Ig < 11.9

12.1 < I D

< 12.7

< t d -

9.8

8.8

7.7

5.8

5.1 4.3

(14)

(10) (10)

(13)

( 9) (11)

10.5 <_ t

rf < 17

< t

d < 10.3

< t

< t.

d i

30% BIAS, + 0,

9.3 <_ I B< 9.8

9.9 < I B< 10.4

10.7 < Ig < 11 .2 11.3 < I D < 11.9

(10)

(12)

(23) (16)

(20)

(11)

8.14

1.99

5.67 10.06

7.63

1.43

6.62

7.33

5.29 4.04

3.10

1.15

8.89 8.19

5.12 4.70

4.09

3.72

1213

10

1

1

(12)

(14)

(13)

(11)

(13)

7.95 10.57 4.32

0.79

1 .02

15.34

14.94

9.63 6.08

6.26

4.14

17.09

14.17

9.94 7.92

8.35

0.933

0.732 0.993

0.938

0.739

0.775

0.956

0.974

0.961 0.959 0.966

0.818

0.919

0.953 0.984

0.817 0.803

8.38

7.26

4.73

4.47

3.84 3.38

14.31

14.01 8.87

5.51 5.60

3.94

16.09

13.51 9.05 9.06 8.28

1.49

14.52 0.93

0.29 0.36

5.75

6.41 5.08

3.72 2.35

3.90 11.80

5.76

2.38

5.61 133.9

65.94

245
Zr0

2 TABLE I IB

nsec N (GH/cm

2

) S a - Ig r

2 M o

2

NO BIAS. ONLY

4.1 <. t

d < 4.4 8 6 < h<- 9.9 ( 9) 20.77 9.63 0.976 9.38 0.31

4.8 < t

rf < 5.1 8 1 < h<- 9.6 ( 9) 19.96 9.07 0.950 8.83 0.30

30X BIAS, NO 0.

*d< 1 10 8 < 16.7 (16) 10.49 12.29 0.901 11.72 1.81

30JS BIAS, + 0,

t d< 1 11 2 < 16.3 (28) 11.10 14.30 0.967 13.66 2.22

Si0

2 TABLE 1 1 IA

GW/cm

2

N (nsec) 8 a = F

d

r

2 2 a

NO BIAS, NO 0

2

7.4 < I 0< 7.8 7 < t d

< 19 (15) 9.27 17.93 0.938 17.00 4.83

9.6 < I B

< 10 6.2 < *di 11.2 (15) 6.28 9. 36 0.983 8.70 2.61 10 < I B < 10.4 2.7 < 9 (13) 2.74 7.78 0.865 6.92 7.44

11.8 < I B

< 12.2 3.3 < 4.8 (13) 8.90 4.40 0.977 4.16 0.31

BIAS SPUTTERED

6.4 < I D< 6.8 7.7 < 10 ( 7) 9.19 8.10 0.922 7.68 1.00

6.7 < I D < 7.1 7.4 < 9 (11) 17.89 7.81 0.915 7.58 0.27

9.4 < I 0

< 10.2 3.4 < *d£ 4.5 ( 8) 12.20 4.18 0.935 4.01 0.15

BIAS SPUTTERED, + 0

?

6.8 < I B

< 7.2 8.8 < *dl 12 (11) 10.20 9.69 0.925 9.23 1.19

9.8 < I B < 10.2 4.3 < kdi 5.8 (11) 9.96 5.36 0.927 5.10 0.38

10.9 < Ig < 11.3 3.2 < l d± 4.3 (10) ,10.45 4.12 0.919 3.93 0.20

11 i Ig 1 H.4 3.2 < ldi 4.3 ( 9) 10.14 4.09 0.936 3.89 0.21

11.7 i Ig < 12.1 2.8 < 'di 4 (11) 10.37 3.56 0.930 3.39 0.16

2^6
Si0 9 TABLE 1 1 IB

NO BIAS, NO 0

2

3 < t d

<_ 3.3

3 - 6 < td < 4

7.8 < t

rf < 8.2

BIAS SPUTTERED

3.8 <:t

d

< 4.2

7.7 < t

rf < 8.4

N (GH/cm2

)

11.3 < I B< 13.3 (10)

11.8 <. Ig < 12.4 ( 8)

'7.2 < Ig < 10.7 ( 8)

9.4 < Ig <_ 10.7

6 < I„ < 7.2

( 7)

19.32

45.68

25.51

20.24

(10) 18.26

BIAS ^PUTTERED, + 0.

2.8 < t

d < 3.2

3.3 < t

d

< 3.7

4 < t A < 4.3

4.5 < t 5

11 < Ig < 13.2 (11)

11 < Ig < 12 ( 8) 10.2 <Ig< 11.7 (10) 8.6 < I n < 10.8 ( 9)

19.76

30.37 20.50

13.00

12.89

12.24

9.92

0.912

0.895

0.951

9.55

6.97

12.56

11.81 11.31 10.48

0.834

0.737

0.966

0.874 0.853

0.921

TABLE IV

12.54

12.09

9.71

9.30

6.77

12.22

11.60

11 .02 10.07

0.64

0.11

0.22

0.32

0.21

0.59

0.23

0.44 0.89

I!

9(tJ = v2q

no diffusion

diffusion

Maximal

multiphoton,

Maximal

multiphoton

q_= no diffusion diffusi

Si0

2 no bias, no 02 -0.325 0.992 3 1-2

Si0

2 bias sputtered, no 02 -0.582 0.997 2 1 Si0

2 bias sputtered, +02 -0.532 0.993 2 1 Zr0

2 no bias, Ar only -0.350 0.966 3 1-2

Zr0

2 30% bias, no 02 -0.221 0.950 5 2

Zr0

2 30% bias, +02 -0.252 0.740 4 2

Al

2 03 no bias, no 02 -0.992 0.940 1 0

Al

2 03 10% bias, no 02 -0.442 0.973 2 1 Al

2 03 10% bias, 10% 02 -0.408 0.956 2-3 1

50-90-0878-3073

2A7
WEIBULL DENSITY FUNCTIONS -a = 1 :13 My) = (/3/a)(y/a)'3 - 1 e

(v/or

3.5 -

3.0 -

2.5

2.0

1.5

1.0

0.5

i i i i. i . i i i

/j = aH 1/0 +1)

a

2 =a2

{ T(2/(3 + 1) - T

2

(1/(3 + 1)}

J3 = 10

1 "-.2

IrfS-l

l\ /f K /

>

V 0 = 3

i \ i*^ »i -i 1 l i

50 02-0878-3068 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 y

1 o Figure 1. Graphs of the Weibull density function 6(y) for a = 1 and & = 1, 3, and 10. /u and a

are, respectively, the mean and variance for this distribution.

Origin

0

0.5

1.0

1.5 2.0 « 2.5 8 3.0

3.5 4.0

4.5

5.0 5.5

6.0

Ln (time/stress) 1.0 1.5 2.0

1.0

T

Si02 no bias, no O

I B = 8.4, N = 23

cv= 18.8,j3= 1.13

I I l

50-02-0878-3069 4 5 6 7 8 9 10 12 14 16 18 Time/stress axis, t

.

1.5

1.0 0.5 0.0 -0.5

-1.0

-1.5

-2.0

-2.5

-3.0

-3.5

-4.0

-4.5

20

Figure 2. A plot on Weibull graph paoer of the cumulative distribution of t^ (in nanoseconds), from smallest to largest, at a fixed breakdown intensity I R of ^ 8.4 GW/crn^. The samDle irradiated is a Si0

2 thin film deposited without sputter bias or 0^ sputtering gas on CG 7059 substrate. N is the total number of spots irradiated, of which 16 damaged, a is the characteristic sample lifetime, and g the shape fitting parameter.

248
NEW COATING MATERIALS FOR IR LASER OPTICAL COMPONENTS*

R.C. Pastor, J. A. Harrington, L.E. Gorre, and R.K. Chew

Hughes Research Laboratories

Malibu, California 90265

Six materials (LaF^ , PrF

3

» CeF

3

, BiF

3

, KGaF^ , and Bil-j) were

prepared and optical evaluated for use as thin film coating ma- terials at 3.8 and 9.27 pm. Key words: Infrared laser windows; materials purification;

Reactive Atmosphere Process (RAP) chemistry; thin film coating

materials

1. Introduction

An optical coating materials purification program was initiated to study new materials which have the potential of producing infrared coatings with improved

mechanical and optical properties. The materials studied included three rare-earth

fluorides, LaF

3

/ CeF^, and PrF-j along with BiF

3

, Bil^, and KGaF^ . Using a purification procedure involving Reactive Atmosphere Process (RAP) , it was hoped that an intrinsic bulk absorption coefficient near 0.1 cm could be achieved. As will be

shown, the bulk absorption of the rare-earth fluorides ranged between 0.6 and

0.9 cm

-1

.

2. Materials synthesis and purification

2.1 Rare-earth fluorides; CeF^, PrF^, and LaF^

Rare-earth fluorides have long been used as durable coating materials at visible

wavelengths. Since their transmission range in the infrared extends to approximately

10 ym, it was felt that highly purified material may be a good thin film material

from 3 to 9.5 ym. Each of the three fluorides, CeF^, PrF^, and LaF^ were readily

prepared from four- to five-nines pure sesquioxide, R

2°3 where R = La, Ce and Pr.

The oxide was first wet-converted to RF^ by treatment with an excess of HF(aq) which

contained 49% by weight of HF. Teflon wares were used. Since wet conversion is

usually only 95% complete, anion purification was completed by dry conversion. The

latter step consisted of treating the residue from wet-conversion with a dry mixture

of HF/He at 700°C.

These RF^ powders were compacted by melting (carbon crucible) under HF/He and

cooling slowly. Transparent castings, 5-cm diameter, were obtained reproducibly

with this technique.

2.2. Bismuth Trifluoride, BiF^

The starting material was Bi20

3

, five-nines pure in the cation. The first step

in processing was wet conversion, following the same procedure as in the case of RF^,

above. Characterization of the product by x-ray powder pattern shows that the highest

phase purity is achieved in the wet-converted material. The material is very white.

All attempts to process further under HF/He at a higher temperature led to the gray

powder. Hence, DTA characterization of the melting point was not feasible due to

the progressive decomposition of the material with each thermal cycling. An open-cup

run with the material under a flow of He shows one broad endotherm which starts at

6 30° or 6 50°C depending on the conditions of outgas of the apparatus. Such a run

is irreversible because the sample volatilizes in the first heatup. With the material

bottled inside a platinum capsule, cold-welded in vacuum after a thorough outgassing

This work was carried out under Contract No. F33615-77-C-5049 , under the sponsorship of the Air Force Materials Laboratory, Wright-Patterson Air Force Base

(Dayton, Ohio)

249
[l] l

, two unresolved endotherms at 680° and 745°C result. These results were

reproduced after ten consecutive thermal cyclings. It is possible that melting

occurs at 680°C and overlaps with boiling at 745°C.

It appears that the published thermal characterization of this material is in

an unsatisfactory state. Cubicciotti reports melting at 649 + 2°C [2], von Wartenberg

at 725-730°C [3], and Darnell and McCollum at 755 + 5oc [4]. Cubicciotti suspects

that von Wartenberg 1 s high result stems from oxide contamination. He also points

out that Darnell and McCollum used a boron nitride container which is corroded by

BiF^. Our criticism of Cubicciotti ' s work is that he used a gray powder and makes

no mention of a reproducible melting behavior with one sampling. As mentioned above,

the DTA run which yields one endotherm in this thermal region is irreversible and

is complicated by the overlap of another er.dothermic process, volatilization.

The problems in materials processing stem from the following behaviors:

1. Corrosive action on metal-oxide ceramics can arise from HF released by the

hydrolytic action of outgas H^O:

BiF

3 + H

2 0(g) -» BiOF + 2HF (g) . (1) 2. Corrosive action on metals is due to free Bi resulting from four possible

causes

:

a. Dissociation of the oxide impurity at high T,

3 BiOF + BiF

3 + 2Bi + | O . (2) b. Reduction of the oxide impurity by trace organic matter,

4BiF

3 + 3C - 4Bi + 3 CF

4 (g), DOES NOT OCCUR. (3) 3BiOF + 3C - BiF

3 + 2 Bi + 3 CO(g). (4) c. Free Bi, being below H in the emf series, is not reconverted

to BiF

3 by HF,

Bi + 3 HF(G) * BiF

3 + | H (g) . (5) d. Free Bi may be formed by disproportionation at high T,

5BiF

3 * 2 Bi + 3 BiFj- . (6) We have not found thermodynamic data in BiF,- to make an estimate of the lower limit

of T for Eq. (6)

.

These problems of further processing at higher T led us to resort to a procedure

for roon- temperature compaction of the wet-converted powder. One-inch diameter

billets are conveniently produced by pressing under N_ at 38,200 psi/30m at 25°C.

-3 The density of the compact is 5.36 + 0.02 g cm , which is somewhat higher than

the value in the handbook, 5.32 g cm

-3

[5].

2.3. Potassium tetraf luogallate, KGaF^

This is the only binary system in the group of materials studied. The synthesis

recipe which yields the highest anion purity is based on using an equimolar mixture

of the nitrates of potassium and gallium. The mixture is gradually heated (decomposed)

to 700°C, under HF/He, in a platinum- lined carbon boat.

Depending upon the atom ratio of K:Ga, various stoichiometries (compounds) are possible in the KF-GaF system. These compounds and the melting behavior are

as follows:

K

3 GaF

6 — Reported to melt congruently at 1012 C [6]. We observe

one sharp endotherm at 9 86°C. The material appears to be

quite hygroscopic. Solid-solid transitions are reported

at 172° and 239°C [6].

1. Figures in brackets indicate the literature references at the end of this paper.

250
K

2 GaF5 — Melts incongruently at 600°C (ref. 6).

KGaF

4 — A solid-solid transition at 530°C is followed by incongruent

melting at 588°C [6 J. We observe two separate endotherms

at 534° and 566°C.

Our temperature assignments were based on the following melting-point calibrations of the thermocouple: Au, 1063°C; Ag, 960. 8°C; Sb, 630. 5°C; Zn, 419. 5°C; Cd,

320. 9°C; and Sn, 231. 9°C.

These compounds are linked to each other by the ease of dissociating of KF. For instance, three consecutive thermal cyclings under Ar yielded a progressive

change in the endotherm [7]. This led to the speculation of dissociation,

K 0 GaF . K~ GaF c + KF. (7)

J o z b The interpretation appears to be in error, probably due to unsuspected low-level

hydrolysis. Our sample of K^GaF^ was synthesized under HF/He and the phase purity

was substantiated by x-ray powder pattern. Such a sample, bottled in a sealed

platinum capsule that has been thoroughly outgassed (cf. ref. 1) shows only one sharp endotherm, which corresponds to melting. This endotherm pattern persisted

after seven consecutive thermal cyclings.

Regarding the dissociation linking I^GaF,. to KGaF^

,

K_GaF c

-» KGaF. + KF, (8)

2 5 4 we observed the appearance of an extraneous phase, identified by x-ray to be K^GaF^,

in certain preparations of KGaF^. However, in these cases, we also knew that the

anion purity of the product was not the highest achievable. When a more rigid

control of the HF/He atmosphere and exclusion of oxide impurity was exercised in

the preparation of KGaF^, no evidence of K^GaF^ was obtained. We still have to resolve the question on whether ^GaF is a legitimate phase under reactive atmosphere processing.

The preceding discussion indicates that x-ray shows that phase purity in

the preparation of KGaF^ is sensitive to anion purity. Under HF/He, we have verified

by x-ray that KGaF^ sublimes without decomposition, a feature of interest to film

deposition of binary compounds.

Using the platinum-capsule method (cf. ref. 1), we obtained the endograms shown

in figure 1. The two endotherms are reproduced after the eighth thermal cycling

(not shown) . The solid-solid transition at 534°C precludes single-crystal growth.

Thus, KGaF^ was compacted as a polycrystalline ingot by melting and crystallizing

under HF/He.

2.4. Bismuth Tri-iodide, Bil

3 The starting material is five-nines pure Bi metal. The metal is freed of

low- level oxide contamination by trickling through a bed of spectroscopic-grade

carbon, using the apparatus scheme shown in figure 2a. The compound (Bil^) was synthesized by reacting the purified elements in a vitreous-silica ampoule that

was sealed off in vacuum. The details of the apparatus are given in figure 2b.

The reported melting point of this material has steadily decreased with the

passage of time; presumably, the trend correlates with the improvement in the purity

of the material. The usual handbooks up to 1967, report a melting point of 439°C [8].

Cubicciotti and Keneshea report 408. 6°C [9]. A more recent handbook (cf. ref. 5,

p. B-182) lists 408°C. Darnell and McCollum (cf. ref. 4), report 407.5 + 1.0°C.

These determinations do not comment on the question of reproducibility on one

sampling. Again, this issue is pertinent because of hydrolysis 'jy outgas H2O and

the fact that the solid and the melt are volatile. Our DTA measurement, using the

sealed platinum-capsule (cf. ref. 1), yields a sharp endotherm corresponding to

a melting point at 404°C. This behavior remained unchanged after three thermal

cyclings. We realize that our method is a determination of the triple point and

251
not the melting point. For the present, we do not distinguish between the two

characterizations because the pressure difference is less than one atmosphere.

The effect of pressure on the fusion temperature is 0.02 to 0.03°C atm 1 [4].

Because of volatility problems, single-crystal growth (2-cm diameter) by a vertical-Bridgman mode was carried out in a sealed vitreous-silica crucible. The

charge consisted of a polycrystalline ingot, compacted by melting under I^/He.

The crucible was then sealed off at 10 2mm Hg . The melt was soaked at 475°C/lh.

Crystal growth was carried out at a constant lowering rate of the crucible. A

foliated texture, characteristic of a layer-lattice crystal, resulted at a lowering

rate of 7.5 mm/h and at 1.6 + 0.1 mm/h. To obtain a more coherent texture, the

2-cm diameter crystal cylinder was forged to a 2.5-cm diameter billet at 210°C/

9400 lbs.

-3 -3 The room- temperature crystal density is 5.79 g cm . A value of 5.64 g cm

is reported [10]. The latter value, also seen in a 1967 compilation (cf. ref. 8),

is too low. A value of 5.8 g cm ^ is reported in the eleventh edition of reference

8, and 5.78 g cm ^ on p. B-182 of reference 5. Our measurement is in good agreement with the later compilation.

3. Optical evaluation of bulk materials

3.1. Rare-earth fluorides

The optical transmission of the three rare-earth fluoride castings is shown

in figure 3. From the figure it can be seen that only LaF^ exhibits no absorption

bands between 3 and 10 pm, thus making it the only material of the three that would

be a good coating material between 2 to 4 pm. CeF^ and PrF^ suffer optically from

intense electronic absorption bands between 3.5 to 6 pm due to low lying Stark levels

of the Ce^ + and Pr^ + ions [ll] . The multiphonon edges in figure 3 are the high

energy side of the reststrahl peak and may be used to estimate the ultimate limiting

absorption in the more transparent regions of the solid [12].

To estimate the multiphonon or intrinsic absorption at 9.27 pm, the log of the

absorption coefficient 8 was plotted versus frequency oj. Since B ae

Aa) [12], interpolation or extrapolation of the data yields the intrinsic 8 at the wavelength of

interest. Figures 4 to 6 are plots of the multiphonon edges for CeF^, LaF^, and

PrF . From these curves, the intrinsic 3 at 9.27 pm was determined to be CeF , 0.84

-1 -1 -1 cm , LaF , 0.66 cm ; and PrF , 0 . 9 cm . These values are all greater than the

3 _ ^3 desired goal of 0.1 cm nevertheless, these materials should still produce low

loss thin films at 9.27 pm. No evaluation was made on the LaF^ at 3.8 pm but this

material should be a good film material at the DF laser wavelength owing to the

lack of structure in the ir transmission in this region (cf . figure 3)

.

3.2. BiF

3

, KGaF

4

, and Bil

3 It was not possible to evaluate single crystal BiF^ because large single crystals

could not be fabricated. Therefore, pellets of KBrrBiF^ were pressed and ir transmission data were taken. The BiF^ pellets showed generally a featureless spectrum

except for one pellet which had an absorption band at 13.5 pm. KGaF^ was evaluated

as a translucent solidified mass. There were small absorption bands at 8.9 pm and

10.3 pm. Pellets of KGaF^ exhibited even more structure beyond 16 pm. This structure

is somehow associated with the GaF^ as it shows up in pure GaF^ pellets too. A

thin section of single crystal Bil^ yields a flat ir transmission from 2.5 to 20 pm.

4. Conclusions

We have synthesized and purified three rare-earth fluorides, LaF^, CeF3, and

PrF^, BiF

3 , Bil^., and KGaF^ for use as coating materials at infrared laser wavelengths.

The rare-earth fluorides have absorption coefficients between 0.6 and 0.9 cm

at 9.2 7 pm and only LaF would be a good material of the three rare-earths at 3.8 Urn.

252
While it was not possible to grow large single crystals of BiF^ of KGaF^, these

materials appear to have low absorption at 9.27 and 3.8 urn. Bil^ looks particularly

attractive due to its flat response between 2.5 and 20 pm.

5 . Acknowledgments

The authors would like to thank the following for their assistance: M. Robinson

for crystal casting the rare-earth fluorides; R. Turk and N. Ramirez for compaction

of BiF^ and Bil^; A. Timper for x-ray analysis of BiF^ and KGaF^; K. Arita for casting

KGaF^; A. W. Robertson for pellet pressing and ir spectra; and M. Braunstein for overall program management.

6. References

[1]

[2]

[3]

[«]

[5]

M

For details of the procedure, see

R. C. Pastor and M. Robinson, Mat.

Res. Bull. 9, 569 (1974)

D. Cubicciotti; J. Electrochem.

Soc. 115 (No. 11), 1138 (1968).

H. von Wartenberg, Z. Anorg. U. Allgem. Chem. , 244, 337 (1940).

A. J. Darnell and W. A. McCollum,

J. Phys. Chem., 72 (no. 4), 1327

(1968)

"Handbook of Chemistry and Physics,

by R. C. Weast (editor) , 49th

Edition, page B-182 (The Chemical

Rubber Co., 1968-1969).

J. Chassang, Rev. Chem. Min.

1115 (1968) . 5,

[7] I. V. Tananaev and T. B. Vorotilina,

Iz. Akad. Nauk SSSR, Neorg. Mater.,

5 (No. 4) , 752 (1969) . [8] For instance, see "Lange's Handbook

of Chemistry," by N. A. Lange

(editor) , Tenth Edition, pp. 244-245

(McGraw Hill, 1967)

[9] D. Cubicciotti and F. J. Keneshea,

J. Phys. Chem. 6_3, 295 (1959).

[10] "Comprehensive Inorganic Chemistry,"

by A. F. Trotman-Dickenson (Executive

Editor) , Vol. 2, pp. 589-590 (Perg- amon Press, 1973).

[ll] R. A. Buchanan, H. E. Rast, and

H. H. Caspers, J. Chem. Phys. 44 , 4063 (1966).

[l2] For a review of multiphonon absorption see M. Hass and B. Bendow,

Appl. Opt. 16, 2882 (1977).

Fi gures

J I I I L

Figure 1. Heatup curves for KGaF4 cold-welded in a platinum capsule at 10 -6 mm Hg (Ref. 1). The satellite endotherm shows a soiid-solid transition; the main endotherm is claimed to be incongruent

melting (Ref. 6).

253
Bi PELLETS

PYREX WOOL

OUTGASSED CARBON

CHIPS

PYREX WOOL

(a) PREPARATION OF OXIDE-FREE BISMUTH

VACUUM

t

Bi 630 - 650 °C - 180 C

(b) VAC-SYNTHESIS OF 0.3 KG OF Bi 1 3

Figure 2. Purification and synthesis of Bil 3 . (a) Preparation of oxide-free bismuth,

(b) Vac-synthesis of 0,3 kg of Bil 3

.

8 00

WAVELENGTH,

3.5 4 5 6 7 8 9 10

CO

GO2

<

3400 3000 2600 2200 1900 1700 1500 1300 1100 900

WAVENUMBER, CM 1

Figure 3. Infrared transmission spectra of rare earth trif luorides

.

254
10

0.001

- i i i 1 1 1

- Ite

CeF3

_

- =

a

-

9.27 (im ^flc, a

Q _ - (1079 CM" 1

) *

/3 (9.27) = 0.84 CM" 1

)

-

_

O CAST 99.9 CeO (FIRST SAMPLE)

-

-=

-

- CAST 6/6/77 (SECOND SAMPLE)

A CAST 7/11/77 (99.999 Ce02

)

1 1 1 1 1 1 900 1100 1300 1500

u) , CM" 1

Figure 4. Multiphonon curve of CeF3 for interpolation of B at CO2 laser frequencies

10

0.1

0.01

0.001

10"

o 99.99 PURE NO. 3 99.9 PURE NO. 1

-4 l l I J L 800 1000 1200 1400

cu , CM 1

Figure 5. Multiphonon curve of LaF3 for interpolation of 6 at COo laser frequencies

255
10 pr

7331-12

1 -

IU

Q3L

0.1

0.01

0.001

PrF 3

- CAST

£(9.27)

= 0.9 CM

o

O

D °o0 o°

=" OCAST 6/12/77 LARGE PIECE

CAST 6/13/77 SMALL PIECE

J L J L 900 1100 1300

u) , CM" 1

1500

Figure 6. Multiphonon curve of PrF3 for interpolation of e at CO2 laser frequencies.

256
* IMPROVED PbF„ COATINGS FOR THE INFRARED

P. Baumeister

Aerojet Electrosystems Co.,

Div. of Aerojet-General Corp.

Azuza, CA 91702

and

George P. Arnold and David F. Edwards

Los Alamos Scientific Laboratory

Los Alamos, NM 87545

A technique is described for reproducibly depositing single layer PbF2 antiref lection coatings for CdSe that have a damage threshold of about 50 MW/cm

for 2.87 ym laser radiation. This is an 8-fold increase in damage resistance over coatings made by other methods. These same coatings applied to Ge have a damage

threshold of about 22 MW/cm2

. The coated Ge value is less probably due to the index mismatch between the PbF2 and the Ge. Key words: antiref lection coatings, CdSe, coating substrate, damage resistant, Ge, PbF2 , pulsed HF laser.

A technique has been developed for reproducibly depositing PbF2 antiref lection coatings having

increased damage resistance. These PbF2 coatings on CdSe substrates have damage threshold values of about 50 MW/cm2 for 2.87 ym laser radiation. This is an 8-fold increase over coatings made by previous

deposition methods. These damage resistant coatings have also been deposited on Ge test substrates. The technique is also applicable to other substrate materials provided the index matching problems can be surmounted.

The PbF2 material yielding the highest damage threshold values has been single crystals obtained

from Optovac, Inc., N. Brookfield, MA. PbF2 powder and vacuum fused PbF2 pellets, with no exceptions

gave inferior damage resistant coatings.

Prior to deposition, the CdSe and Ge substrates were polished using a diamond paste on cloth lap

resulting in a 20/10 or better scratch and dig surface [l]. 1 No attempt was made for surface flatness.

The residue of the polishing step was removed by gently scrubbing with a lens tissue and a weak Neodol**

solution. The samples are then rinsed with tap water followed by a spectra grade isopropyl alcohol

rinse and dried with dry nitrogen. The substrates were given a final cleaning with a glow discharge

just prior to cold trapping the evaporation chamber. For this discharge cleaning, argon gas at a pressure of 0.05 to 0.1 torr was bled into the system and a glow discharge (300 ma) was maintained for 10-20 minutes

Platinum was found to be the best boat material for evaporating the PbF2 . A few depositions were

made using a vitreous carbon boat that had a higher outgassing rate than the Pt boat. For this reason

and since there was no evidence of chemical reaction between the Pt boat and the crystalline PbF2 , the

Pt boat was used for most depositions. A Balzer BA-500 system with thermal evaporation was used for all depositions. With this system pressures of 1 to 3 x 10 6 torr are routinely achieved.

The source and substrate holder are located close to the center line of the chamber. The evaporated material impinged on the substrates 4 to 8 degrees from normal.

The PbF2 was deposited at a rate of about one quarter wave per minute at 632.8 nm which corresponds

to a physical thickness rate of about 100 nm/min. To maintain the constant deposition rate the electrical power to the boat required continuous adjustment because the PbF2 sublimes rather than melts at the deposition temperature. With the Pt boat the chamber pressure remained approximately constant during

the PbF2 evaporation.

The coating thickness was monitored during deposition by measuring the substrate reflectance at 550

nm. Single layer coating of PbF2 one quarter wave thick at 3 ym was applied to several CdSe and Ge substrates. A few CdSe substrates had thin (about 25 nm) ZnS adhesor layers deposited prior to the PbF2 layer.

The damage threshold measurements were made at 2.87 um using the HF oscillator-amplifier system

previously described [2]. The samples were placed near the focal plane of the output lens of this laser

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

** Neodol - Shell Oil Company trade name for ethoxysulfate alcohol detergent.

1. Figures in brackets indicate the literature references at the end of this paper.

I i

257

L
system. This lens is used as a 6:1 magnifier to enlarge the beam waist. The laser pulse is near Gaussian with a 120 nsec width (FWHM) . The spatial profile has some structure. The beam spot size was measured from a burn pattern on darkened photographic paper. The burn patterns were approximately cir- cular and were from 1.3 to 3.5 mm in diameter depending on the position relative to the focal plane.

For the damage measurements the samples were progressively positioned nearer the focal plane for increased power densities in the samples. The laser spot size changes, but the damage threshold is expected to be spot size independent [3] for 2.87 ym radiation. Both N on 1 and 1 on 1 measurements

were made with no apparent difference in the threshold value. Threshold was determined when there was visual evidence of a disturbed layer as viewed by a magnifying glass.

The damage threshold values are shown in table 1. These values indicate that the ZnS adhesor layer

had no influence on the damage threshold level. The commercial antiref lection coatings on CdSe were multilayers of ZnS and ThFi, or single layers of PbF2 . The best coatings produced from powdered PbF2 had damage values as high as 12 MW/cm2

. The best PbF2 coating on Ge had a damage level about half that of CdSe/PbF^. The reason for this difference is believed to be due to the index mismatch in Ge/PbF2 resulting in higher electric fields inside the coating. From these data one can see that coatings from single

crystal PbF2 consistently have higher threshold values. Powdered PbF2 has always formed coatings with

inferior damage properties. The presence of the hydroxide, PbOH, is believed to be the cause of inferior

coatings made from the powdered PbF2. The powdered PbF2 hydrolizes easily to form PbOH which decomposes

to form metallic Pb upon heating prior to deposition. Some metallic Pb is then passed to the substrate

during deposition. These metallic particles act as heat centers for the laser irradiation. Laser damage

nucleates from the microscopic metallic inclusion. Examination of the damage sites using a microscope

indicates that metallic inclusions could be the damage mechanism. Crystalline PbF2 has negligible

hydroxide as indicated by the lack of spectral absorption in the 2.7 ym OH band for PbF2 crystals 2-3 cm

in thickness. During the heating of powdered PbF2 before deposition, small spherical masses are seen in

the deposition boat. These are believed to be the metallic Pb particles. These spheres are not formed when crystalline PbF2 is used as the starting material.

Table 1. Laser damage threshold values for single layer VbT^ antiref lection coatings on CdSe for a laser pulse at 2.87 ym and 120 nsec pulse width.

CdSe uncoated surface 68.5 MW/cm

CdSe/PbF

2 A/4 at 3 ym 50.6 MW/cm2

CdSe/PbF

2 /l/8A ZnS ^ 50 MW/cm

2 CdSe/PbF2 commercial coatings 6 MW/cm

Ge/PbF, A/4 at 3 ym 22 MW/cm2

References

[1] Edwards, D. F Physics E:Sci

and Lazazzera, V. , J. of

Instr. , 3, 156 (1970).

[3] M. J. Soileau, private communication.

[2] Wenzel, R. G. and Arnold, G. P., Appl. Opt.

15, 1322 (1976); Arnold, G. P. and Wenzel,

R. G. , Appl. Opt. 16, 809 (1977).

258
GRADED INDEX COATINGS OF CUBIC THALLIUM

IODIDE (Til) AND LEAD FLUORIDE (PbF 2 )*

T. J. Moravec and R. A. Skogman

Honeywell Corporate Material Sciences Center Bloomington, Minnesota 55420

We present a new technique for producing graded index films from alternating

very thin layers of two materials. During deposition the thickness of the layers

is adjusted so that the resulting average index matches the index of the profile

for that total thickness. This method shows great promise for making films with

any desired complicated index profile in a very straightforward manner. Results

are presented for graded index AR films near 10pm made from Til and PbF2- The

10.6|jm absorption of these films was similar to that of discrete, multilayer AR

films composed of the same materials.

The alternating layer method also takes advantage of the polymorphism of Til in

thin layers to produce films in the cubic phase. We show by X-ray and optical measurements that each Til layer possesses the CsCl structure at room temperature.

It thus appears that each layer of Till is ignorant of the existence of the previous

layer of Til due to the layer of buffer material (PbF2)

Key Words: Thin films, Thallium Iodide, Lead Fluoride, graded index, polymorphism.

1. Introduction

Inhomogeneous films with the refractive index graded with a known and prescribed way could have

improved optical properties over discrete multilayer optical coatings. By grading the index, the film/

film interfaces are eliminated. Laser induced damage to thin film coatings has beem shown to occur at the interface of the films and at the substrate/film interface L~l»2,3 H."*" The reason is that the effective electric field strength of the light beam is a maximum at interfaces in some multilayer film stack

designs. In fact, there is a definite correlation between internal field maxima and damage thresholdH3]]. One way to increase the damage theshold is to design coatings that minimize electric fields at the interfaces and in the materials that have larger absorption coefficients [14,5]. However, this technique has practical limitations C5D.

We report here graded index AR films of Til mixed with PbF2 on KCl. Initial results of graded index

films of this system were reported previously These two materials were chosen as representing low absorption 10.6ym high and low index, respectively, coating materials for KCl. Laser damage testing of these films has not been done. However, calorimetry measurements showed that the graded index films

did not increase 3 over discrete AR films.

To our knowledge, grading the index from high to low for laser applications has been tried only

once before. Austin et al. did it for an order suppression type filter. They did not see an increase in laser damage threshold for the inhomogeneous versus the homogeneous filter. However,

they did not analyze the films to see if they were truly inhomogeneous, as we have done and detailed

below. We feel that the question of laser damage threshold for inhomogeneous versus homogeneous films

is still unresolved.

2. Preparation of Graded Index Films

Jacobsson L~8] has introduced a useful method for describing the film grading of two materials,

A and B. He defines a parameter a by

v°> a vA

(T) (1)

where v^(o) = 0, Vg(T) = 0, and where vj(t) is the rate of deposition of material j at time t in 8/sec

varying linearly with time from t = 0 to t = T. In our case, A is PbF2 and B is Til. This assumes a

*This work was supported by Defense Advanced Research Projects Agency under Contract No. DAHC15-73-C-0464

ARPA Order No. A02416.

1. Figures in brackets indicate the literature references at the end of this paper.

259
two source evaporation with changing rates to produce the index gradient. Jacobson used simple linear

mixing theory and the Clausius Mossotti Lorentz - Lorentz electric field theory to calculate the di- electric constant for the mixture in the inhomogeneous film. From this, he obtains the following

expression for the refractive index as a function of distance x from the air interface for a film of cotal thickness H: 2,,

nA

2- B V +B(l-a) (n/ - n

B

) n < x > = —7~a + ; o z—vr (2) (1-6) ^<cH3) - (1-6) [1 + (a

2 -l)^

9 — 1 where a = 6 a^l&2- The parameters a^ and &2 are given by a. = (n^ +2) (i = A,B)

.

Jacob ss on goes on to develop the electromagnetic theory for the calculation of the reflection and transmission coefficients of inhomogeneous films . He also develops approximate methods and solutions

for special cases, such as hyperbolic index profiles. An interesting result of this is that for some values of a the AR minimum is not very sensitive" to changes in a. Since this is a difficult parameter

to control experimentally, it should not be too difficult to deposit AR inhomogeneous films.

In figure 1 we show the refractive index profiles calculated from equation (2) for the Till-Pt^

system for some values of o. The indices used represent those at 10.6pm; 2. AO for Til and 1.63 for

PbF2« The total thickness is H and the distance measured from the air interface is X. We can also calculate a mean refractive index versus a from these curves. This is shown in figure 2 for the TS.I— PbF2 system.

We have developed a method that will produce graded index films without rate variations. The procedure is as follows. We desire that an inhomogeneous film be made with its refractive index

gradient some n(x) , where x is the distance measured from the air interface as shown in figure 3. The total thickness of the film is H and the film should have an index of njg(e.g., = 2.4) next to the substrate and ng (e.g., = 1.6) next to the air/ film interface. To make this film by our method, we

divide up the film into steps, for example of width H/10, with each step having an average index de- termined by the particular form of the gradient, n(x), such as exponential, hyperbolic, etc. The gradient could be very complicated and could possess oscillatory behavior extending over several periods

The average index for step 10 in our example is 2.2. We produce this average index for thickness of increment width H/10 by evaporating first one layer that is d^Q thick of material with index n-^g (=2.4).

This is followed by one layer of thickness (H/10 - d^g) of material with index n„ (=1.6) such that

d

io n

io + ( To " d

io> n

o = ^io* ( T5°

(3)

or d,„ = I (777) (« 10 4

v 10'

This is shown schematically in figure 4.

Thus for all <n^> , i = 10 to 1, the average indices, the thicknesses required of the discrete

layers of the two materials are given by

d

i n

10 + ( T^- d

i

} n

o =1 <n

i >; i - 10 to 1 (5)

This can be generalized for M increments in H as

d

i no + (l" d

i

} ' (l

<n

i>; 1 = M t0 1 (6)

Thus the sequences of high and low index materials are repeated from substrate to air interface

each time the appropriate thickness of the two layers is adjusted, so that the average index is equal

to n(x) for that value of x in the index profile. Obviously, a smoothly varying index profile can be

approximated very well by small increments (say H/1000 or M08 for typical optical coatings). Most

films discussed in this paper were produced with M = 50 to 100.

The advantage of this method over ones that vary the rates of the two sources, or mix the materials

together before evaporation, is that very complicated index profiles n(x) can be easily reproduced.

This is so because accurate control of the two rates is not required as it would be for methods that vary the rates to obtain inhomogeneous coevaporated films. This method also lends itself easily to automated control via feedback from a thickness monitor coupled with an automated shutter. We have

found that it is not necessary that the individual layer thicknesses be very accurate. Because each step is much smaller than the wavelength of light used, the effect is that of an inhomogeneous gradient

index film.

260
3. ESCA Depth Profiling

Films prepared by the above method were subject to ESCA depth profiling to insure that the films were graded in composition. ESCA was used instead of Auger spectroscopy because of severe charging

effects with dielectrics.

Figure 5 shows a profile of a typical graded index film 0.2ym thick made by the step method. It can be seen that the films appear smoothly graded. This result is reassuring, for it means that our step method does make graded composition films within the accuracy of the ESCA method. Some films showed 0 at the film/substrate interface. It is not known if 0 is a contaminant here, or if it was

adsorbed on the surface during measurement.

4. Experimental Results

An optical monitor was used' to determine the end of the deposition run. A 5 watt continuous COlaser operating at 10.6pm was used for the light source. When the transmission maximum was reached

the deposition was stopped. A considerable improvement in accuracy of AR minimum could be obtained by

development of an optical monitor that could operate at shorter wavelengths than the design wavelength.

Figure 6 shows the near infrared reflectivity for two graded index TlI-PbF2 films on KC1 taken with

a prism on the back surface to remove that reflection. Curve 1 is a 2.5pm thick film with a ^1.0 made

by the step method and curve 2 is a 2.6)Jm film made by varying the rates with a mechanical shutterf_6l]

The curves are fairly similar with their minima displaced due to different thicknesses and possibly to variation in the gradation. The interesting feature is that every minimum besides the one near 10pm

is very antireflecting. This agrees with theory f_9]. All the graded index films had vurves similar to

these. This spectra can be compared to that of a two layer AR coating that has only one AR minimum as can be seen from figure 7. Curve 2 is the two layer, and curve 3 (film produced by mechanical shutter)

the graded index reflectivity in the near infrared.

Following Jacobsson fj9 D , we can make the rough assumption that the minima of curve 2 in figure 6 are separated by nH/2, where H is the thickness, and n is the average index. The n is calculated to be about 2.11 for this film. From figure 2, this gives a value of a = 1.4, which is plotted in figure

8. This value of a is about what we might expect from a varying rate mechanical shutter arrangement

for making the graded index films

With the step method, however, profiles with different ot's can easily be made. Figure 9 compares

the reflectivity of sample 801191 (a ^1.0) with sample 802041 where we attempted to make a = 0.2.

From the minima positions, ri is calcualted to be 1.91, making a = 0.5. The agreement is considered to be good, since the n calculation is an approximation and the run is usually terminated when the optical

monitor reaches a peak in transmission regardless of where this occurs in the step sequence.

As was pointed out previously, it was difficult to determine the cutoff of the deposition using the

10.6|jm optical monitor. This resulted In a range of minima between 8pm and 13pm in the films that were

produced. In order to compare the width of the minima, figure 10 shows the experimentally measured

minimum near 10pm of several films in which the minima were all shifted to reference to the same point

and values of a. a = 0.2 (2) is graded Til to PbF2 to air, while a = 0.2 (3) is graded Til to PbF 2 to Til to air, which mimics a three layer design. We note the interesting result that the lower a values

i have broader AR minima.

Many samples were made of two layer, three layer, and graded index films of Til and PbF2 to determine the effect, if any, upon 6 of the various AR coatings. The results of these measurements are listed

in ref. 10. The 10.6pm average absorption for 2 and 3 layer films was 0 . 717%/surf ace and that for the graded index was 0 . 548%/surf ace . Overall, the values are fairly high and we believe this to be due to impurities in the PbF2 part. The source material for PbF2 was cerac optical grade powder without further treatment. Some transmission spectra did show inorganic absorptions, but no clear pattern could

be found. It would be interesting to have samples of homogeneous and inhomogeneous film design tested

i for laser damage threshold.

5. Physical Characteristics of Til Films

Til films on KC1 and other alkali halides scatter visible light. The exact nature and peculiarities

of this phenomenon have been explained previously Cll]. Briefly, the mechanism causing the scattering

is inherent in the way Til forms on KC1. Til nucleates and grows epitaxially on KC1 . On and near the

KC1 (100) plane the Til (004) plane grows parallel to: the KC1 surface. The Til <b > direction then lies

parallel to the KC1 <110> direction lying nearest the KC1 substrate surface. On ?he exact KC1 (100)

plane two KC1 <110> directions lie directly in the surface. The orthorhombic Til nucleates along both

of these KC1 <110> directions. The result is two orientations of Til grains whose <b 0

> directions are parallel to the two orthogonal KC1 <110> directions. Optical scattering results from the refraction

of light between adjacent birefringent Til grains whose optic axes are at right angles from one grain

to the next.

261
These characteristics of Til on KC1 have always been observed in films of interest for infrared wavelength applications, i.e. with films of physical thickness between 0.5ym to 2.0um. Blackman and Khan

observed that thin layers of Til <^100-200fi had the cubic CsCl structure temperature Ll2]. . Furthermore,

Khan reported that thin layers of Til always possessed the CsCl structure when deposited onto cleavage

faces of alkali halides at room temperature Cl3] and at -180 C when deposited onto amorphous substrates

(Formvar) C 1 2 H - Work reported by M. Shiojui et al . may offer an explanation for this phenomenon ClAH

.

They grew Til crystals in silicon hydrogels and found that Til crystals always nucleated in the cubic

phase. After growing to a critical size, the crystals transformed to the stable orthorhombic phase.

Since Che cubic orthohombic transformation in Til proceeds by simple shear, they concluded that the gels may prevent the cubic crystals from changing to the orthohombic by shear. The same mechanism may hold

for thin films of Til held from shearing by the alternating layers of a different material.

Samples of TlI-PbF2 step graded films were examined in our x-ray diffTactometer . Table 1 shows results for a typical film. No orthorhombic Til lines are observed. The two diffraction lines that could not be attributed to g-PbF2 indicate the presence of cubic 8-T1I with a0 = 4.218 agrees well with

that reported by Khan El3] for cubic Til.

Figure 11 shows two photos taken in transmitted light between parallel polarizers. The left hand

side represents a two-layer TlI-PbF2 AR coating on a KC1 substrate. The right hand side represents a 180 alternating layer TlI-PbF2 graded index AR coating on KC1. The two-layer film scatters light while

the laternating layer film does not.

Figure 12 shows photos taken with the polarizers crossed to extinguish the light. The left side

is the two layer film and is very biref ringent . The right side is the alternate layer film and is not biref ringent . None of the graded index films exhibited any birefringence.

6 . Conclusions

A unique technique for graded index films has been developed that shows great promise for making

films with any desired index profile. The technique was used to make several films AR near lOym.

The absorption of these films was similar to absorption of discrete multilayer AR films composed of the same materials.

The alternating layer method produces films of Til in the cubic phase. This is confirmed by x-ray analysis. Since the films are cubic they should not be birefringent . Polariscopic examination shows

that they are, indeed, not biref ringent . Finally, since the Til is not biref ringent , it cannot scatter

light by the mechanism mentioned previously. Films of Til prepared by this new method do not scatter

light and are very clear.

Table 1. Identification of observed x-rays diffraction

lines from graded index sample 801253

Observed e--PbF? Cubic Til

d(X) I* hkl hkl

3.4 m 3 428 111 2.95 m 2 97 200 2 97 110 2.43 f 2 428 111

2.10 s 2 10 220 2 10 3 200 1.78 s 1 791 311 1.72 m 1 715 222 1 717 211 1.485 m 1 485 400 1 487 220 1.36 m 1 362 331 1.327 m 1 328 420 i 33 310 1.215 m 1 212 422 1 214 222 1.14 m 1 143 511 1.12 w forbidden line for 3PbFT 1 12 321

1.05 m 1.050 440 1 052 400 1.005 m 1 0043 531 0.99 m 0 9903 600 0 991 330 0.93 in 0.9 39 4 6 20 0 94 420 0.906 f 0.9059 533

*Intensity: s = strong, m = medium, w - weak f = faint.

262
6. References

1. A. J. Glass and A. H. Guenther, "Laser Induced

Damage o f Optical Elements - A status Report,"

Appl. Optics 12, 637 (1973).

2. D. Milam, "Laser- Induced Damage at l064nm, 125 psec," Appl. Optics 16, 1204 (1977).

3. B. E. Newnam, D. H. Gill, and G. Faulkner,

"Influence of Standing-wave Fields on the Laser

Damage Resistance of Dielectric Films," in Laser Induced Damage in Optical Materials, NBS Special Publication 435 (1975).

4. J. H. Apfel, J. S. Matteucci, B. F. Newnam, and

D. H. Gill, "The Role of Electric Field Strength

in Laser Damage of Dielectric Multilayers" in Laser Induced Damage in Optical Materials, NBS Special Publication 462, (19 76).

5. J. H. Apfel, "Optical Coating Design with Re- duced Electric Field Intensity," Appl. Optics

_16, 1880 (1977).

6. T. J. Moravec and R. A. Skogman, "Heavy Metal

Halide Optical Coatings on KC1: BiF3 , BII3,

PbF2 and Pbl2", Proc. High Power Laser Optical

Components and Components Material Meeting,

1977.

7. R. R. Austin, R. Michawd, A. H. Guenther, and

J. Putnam, "Effects of Structure Composition

and Stress on the Laser Damage Threshold of Homogeneous and and Inhomogeneous Single Films

and Multilayers," Appl. Optics 12, 665 (1973).

8. R. Jacobsson, in Physics of Thin Films, ed.

G. Hass , M. H. Francombe, and R. W. Hoffman,

8_, 51-98 (1975).

9. R. Jacobsson, "Optical Properties of a Class

of Inhomogeneous Thin Films," Opt. Acta. 10 , 309 (1963)

10. E. Bernal G. , R. H. Anderson, K. M. Leung,

T. J. Moravec, J. F. Ready, R. A. Skogman,

D. Wertman, Honeywell Final Technical Report

on ARPA Contract DAHC15-73-C-0464, 31 March,

1978.

11. R. A. Skogman, Laser Induced Damage in Optical Materials , NBS Special Publication 462 , (1976) .

12. M. Blackman, and I. H. Khan, "The polymorphism

of Thallium and Other Halides at Low Temperatures," Proc. Phys . Soc. London, 77 , 471

(1961)

13. I. H. Khan, "On the Polymorphism of the Thallium Halides", Proc. Phys. Soc. London, 76 . 507 (1960) .

14. M. Shiojir, C. Kaito, Y. Saito, M. Muraham,

and J. Kawanota, "Growth of Thallium Iodide

Crystals in Silicon Hydrogels," J. Cryytal

Growth 43, 61 (19 78)

7 . Addendum

Subsequent to the preparation of this manuscript, an article was noted on the preparation of gra- dient index films from small steps of homogeneous films by V. N. Yadova, Shiv. K. Sharma, and K. L. Chopra, Thin solid Films 21, 297 (1974) . Their approach is similar to ours in that alternating thin

layers of two materials (in their case, ZnS and MgF2) are deposited sequentially to produce a film

that performs as a gradient index film. However, their technique is different in that each small

increment is composed of symmetrical thickness pqp layers (p=MgF2> q = ZnS). Our technique utilizes

increments of pq layers (p = Til, q = PbF2) and the concept of an average refractive index to produce

the gradient. Although the resultant optical properties of the multilayer gradient index film are similar, the theoretical techniques for calculating the individual increments are different.

"I I I I I I T

TL[-PBF 2

HLPHR = RRT E UL I ) RHTE(PaF 2

)

~I

J I L 1 I I

Figure 1. Refractive index profiles for the Figure 2. Average refractive index versus a for the TU-PbF2 system as a function of a noted in the TJtI-PbF2 system, center of each curve.

263
SUBSTRATE

Figure 3. n(x) versus x for graded index film

divided up into ten equal increments.

ave. n for

x = H to x-'(9/IO)H -

2.4

2.2

I 6

I— 3/4(H/IO>* e(9/IO)H x-H

Figure 4. Schematic of first step layer to produce

index profile of figure 3 by sequential evaporation

of material n^g of thickness 3/4 (H/10) followed

by material ng of thickness 1/4 (H/10).

SAMPLE 711282 ~ 0.2,, THICK

MINUTES SPUTTERING

Figure 5. ESCA Profile of Graded Index Film

2000 A Thick. The following peak heights were used: Tl 4fy/2, I 3d5/2, Pb 4f7/2, F Is,

h

Z' TLI/PBFH

3' TLI-PBF2/ GRRDEO

4000 32BS 2SE0 I ana 1120 PHDTQN FREQUENCY CI /CM)

Figure 7. Near infrared reflectivity of graded

index film (curve 3) compared with two-layer AR (curve 2)

.

32BB 2SG0 1840 PHOTON FREQUENCY (I /CM)

Figure 6. Near infrared reflectivity for graded

index T«.I -PbF2 films on KCl. Curve 1: step

graded with a ~ 1, 2: Varying rates with a ~ 1.

Figure 8. Refractive index profile for T2I-PDF2 film graded index film with a = 1.4.

264
32B0 2SE0 I BH0 1120 PHOTON FREQUENCY C I /CM

)

Figure 9. Near infrared reflectivity of two graded

index T£l-PbF2 films made with a ~ 1 (curve 1) and

a ~ 0 .5 (curve 2)

ARBITRARY WAVE LENGTH UNITS Figure 10. Comparison of experimentally measured AR minima of two-layer film with several graded index

films for several a. All minima have been adjusted

to go through zero percent reflectivity and zero wavelength units.

Figure 11. Left-hand side: Two-layer UI-PbF 2 AR film

on half coated KCi. shown in transmitted white light. Coated part is on right side

of sample and scatters light

to give dark appearance. Right-hand side: Step graded

TiI-PbF 2 AR film on half

coated KCi shown in transmitted white light. Coated area is on bottom part

of sample.

Figure 12. Left-hand side: Corresponding sample of figure 11 shown through

crossed polarizers with same sample orientation. Note birefringence of coated

(right side) part.

Right-hand side: Corresponding sample of figure 11 shown through

crossed polarizers. Note no birefringence.

265
SIMPLE EXPRESSIONS FOR CALCULATING THE EFFECT OF VOLUME OR INTERFACE ABSORPTION

IN THIN FILMS ON THE PERFORMANCE OF HIGH REFLECTANCE

OR ANTIREFLECTANCE MULTILAYER COATINGS*

H. E. Bennett and D. K. Burge

Michelson Laboratory, Physics Division

Naval Weapons Center, China Lake, CA 93555

Approximate closed-form expressions are developed for predicting the effect

of volume and interface absorption in thin films on the performance of quarter- wave multilayer coatings. For highly reflecting coatings on dielectric sub- strates the volume absorption expressions are equivalent to those obtained by

Sparks using a perturbation approach. The results are extended to metal sub- strates by introducing the concept of a generalized substrate index. An approximate expression for scattering losses for high reflectance coatings is included.

Multiple quarter-wave antiref lectance coatings are also analyzed using admittance

theory. A technique for experimentally separating and determining interface and volume absorption in single-layer films has recently been reported. The expressions developed in this paper make it possible to predict the performance of high reflectance and antiref lectance multilayer stacks from these single-film measurements without recourse to a computer.

Key words: Absorption; antiref lectance coatings; high reflectance coatings;

interface absorption; multilayer films; scattering.

Introduction

The development of very high reflectance, low absorption multilayer coatings on dielectric or metal substrates is a continuing problem in optical coating technology. If the individual films were

truly nonabsorbing, as assumed in typical multilayer calculations, reflectances arbitrarily close to zero or unity could be achieved by simply using enough layers. In practice, however, the individual

layers absorb slightly and may also scatter light, some of which is absorbed. In addition, absorption

and scattering may occur at film-film or film-substrate interfaces. Although the apparent absorption

observed for such thin films is very small, it is typically orders of magnitude higher than would be

predicted from the absorption coefficient of the bulk filming material. In only a few cases is the absorption coefficient of the material in thin film form equal to that of the bulk material [1,2] 1

. Why additional absorption occurs when materials are deposited as thin films is one of the most in- teresting questions before the thin film physics community. Whatever its source, it clearly limits

the reflectance or transmittance achievable in multilayer film coatings.

A straightforward attack on the optical effects of single or multilayer films on a surface as a boundary value problem results in immensely complicated expressions [3]. The solution to this problem

has been to make iterative calculations using impedance theory and matrix mechanics. Although the films are usually assumed to be nonabsorbing, the general theory allows absorption to be present in the films and even at interfaces. The multilayer film problem has therefore been solved numerically.

However, the functional relationships between absorption in one or more layers or interfaces and the resultant multilayer performance remains obscure. Fortunately, in the special cases in which the films

have optical thicknesses which are exactly a quarter or half wavelength and in which the multilayer

film either has a high reflectance or is an antiref lection film, a considerable simplification occurs

and it is possible to obtain simple closed-form expressions. These allow one to predict, without the aid of a computer, what the approximate maximum or minimum reflectance of a multilayer coating on a dielectric or absorbing substrate will be, and how that reflectance is affected by absorption in the coatings or at the interfaces. Developing these expressions is the subject of this paper.

Generalized Index of Refraction

The complex index of refraction of a medium is chosen to be n = n - ik, where both n and k are

real. If the medium is nonabsorbing then k = 0. If the substrate is absorbing, the generalized index

of a substrate-single film combination may be made real at a given wavelength by choosing the optical

thickness of the film so that the phase change induced by the film adds to that of the substrate to produce a resultant phase change of 0 or tt. The normal incidence intensity reflectance R for a single

interface is R = rr" and

n - n

r = -2 (1)

l + n

o

* Work supported by the Department of Energy and the Defense Advanced Research Projects Agency.

1. Figures in brackets indicate the literature references at the end of this paper.

266
where the asterisk indicates the complex conjugate and n and n are the indices of refraction of the incident medium and the medium upon which the light is incident, respectively. The admittance is defined as H

Y = T~^T > (2) k x Ea

where H

a

is the magnetic field, E is the electric field, and k is the unit vector in the direction of propagation, assumed here to be normal to the surface. From Maxwell's equations H/E = n in Gaussian

units [4], so that the admittance is equivalent to a generalized single medium index of refraction.

By using symmetrical multilayer films it is possible to simulate, as far as phase change on reflection goes, the index of refraction of a single film at a given wavelength [5]. If the multilayer

films are nonabsorbing , this simulated or generalized index of refraction, often called a Herpin index,

will be real if the optical thicknesses of all films are a quarter or half wavelength or multiples

thereof. It may also be real if they are not quarter-wave films but are symmetrical and are chosen so that the resultant phase change on reflection from the film stack adds to zero or tt. Only the quarterwave case will be considered in this paper. The admittance of a multilayer stack on a substrate of index n^ is given by Y = b/a, where [6]

N

n r=l l n sine

(i sin 6 )/n

r r cos6

(3)

The outermost film has r ated with each film of index n and thickness

r

1, the film next to the substrate has r and for wavelength A

,

N, the phase factor 6 is associr

2im SL r r

(4)

If the optical thicknesses of the coatings are each a quarter- or half-wave, then respectively. The characteristic matrix for a quarter-wave film is then

= tt/2 or tt

,

i/n

0

and for a half-wave film,

t :]

Antiref lectance Coatings

If there is no reflection from a surface coated with an antiref lection film and n 1 (air) , then

from eq. (1) Y = 1. For a single antiref lection quarter-wave film of index of refraction n^, then

l/nT

' in

s /nLl

. inT J

(5)

2 1/2 and Y = b/a = n /n = 1 giving the well-known result that n = n

Li J Li O

2 ing film. If the transmitted power equals the incident power, i.e., if n E = E Y, then when n

2 1/2 00 ° the field at the air interface is E = E (n /n ) . Similarly, the field at the substrate interface is

for a single perfectly antireflect1

E /n

1/2 and the average power in the film is (nL

E

Q /2ng) (n^ + 1)

For a two-layer, quarter-wave film stack, N = 2 in eq. (3), and if the outer layer has index n^

and the inner one has index n , then

n ~-Vn

L

0

" 1

" "-nH

/nL

0 -n

L

/nH

. L-n

s nL

/rv

(6)

267
so that the requirement for the film to be antiref lecting is

2 2 1/2 ? 1 /? The field at the air-film interface is E0 /(n n /n ) , at the film-film interface is E / (n„/n ) 1/2 o H S and at the substrate interface is E

Q

/n

s

• A three-layer all quarter-wave antiref lecting film can be worked out in a similar way. A more common three-layer coating, however, is the quarter-half-quarter

coating, which has low reflectance over a broader wavelength region than does the equivalent quarterquarter-quarter coating [7]. Since a half-wave coating has a unit matrix, it does not affect the fields at either boundary. However, the field at the center of the half-wave coating of index n will

2 2 1/2 h

be E / ("g1^/11 ]^ ' as may be seen by considering the half-wave coating to be two quarter-wave coatings

back to back. The field distributions inside the various layers are sinusoidal, as shown in figure 1.

Reflectances of the quarter-wave antiref lecting films are obtained by substituting Y for n in eq.

(1). For a single quarter-wave antiref lection (AR) coating in air the reflectance is then

2.2

(8) K + n

T./

and for a double quarter-wave AR coating or a three-layer coating the middle layer of which is a halfwave in optical thickness

2 2.2

= /

nH

" n

SnlA

R

2 = I— o I (9)

These expressions are exact, provided absorption is negligible.

To obtain the absorption in a film, one utilizes Poynting's theorem, which states that the power dissipated in a medium is proportional to nE , where n is the index of refraction of the medium and E

is the local field strength. The volume absorption per unit of incident power in a film is then

A

v

= (Bf>) (nE

2 /E^) , (10)

2 where (3 is the absorption coefficient in the film of thickness I and nE is the average power in the

film. The interface absorption should in principle be handled in the same way, except that the power

at the interface would be used instead of the average power in the film. In fact, interface absorption must occur in a narrow region on either side of the interface. The absorption coefficient is decreasing rapidly on both sides of the interface and the experimentally measured absorption is the integral

of the product of the absorption coefficient and the depth on both sides of the interface. If we redefine the absorption coefficient at the interface to be the absorption per unit of optical rather

than physical thickness, then n in eq. (10) is absorbed into the interface term and the difficult

problem of assessing the fractional contribution to the absorption made by each side of the interface

is avoided. We then may express the resultant absorption at an interface between two materials

designated 1 and 2 as

A = A.(E/E ) 2

, (11) where

I i

v

do

A. = J° nJ/L + f n„B„d£„ (12)

l ^111 o 222

is the interface absorption per unit field and is a parameter which can be determined experimentally

from calorimetry [8].

2 _2 . The approximate absorption in an AR film is then obtained by substituting for E or E m eqs

.

(10, 11). Since we have assumed the films to be a quarter wavelength in optical thickness, the absorption A^ in a single-layer quarter-wave AR film becomes

a

r x N s Al= A

0L + 8n; (n

S + 1)+ n7 " (13)

268
For a two-layer quarter-wave AR film the average power in the outer (low index) film is

2 v „2

E nT

\ nH

/ 2n

H

and in the inner (high index) film is

E

2

2 nH /

E

on

S . Eo\ -o ^2 2,

The absorption A2 in a double-layer quarter-wave AR film is then

A

2 " OL + —2 + \H ~ + —~2 + n— " (16) 8nH

nH Siy^ S

In a three-layer film in which the middle layer is a half wavelength in thickness the absorption

will be A2 minus the interface contribution A^ plus the contribution from the half-wave film. Proceeding as before we find the absorption of such a three-layer antiref lection coating to be

A

3 = A

2 + 4n-(- + -T) + T^ + AhH" V > < 17 > S \n

H 1^/ nH

where the subscript h represents the half-wave coating.

The approximate performance of antiref lectance coatings consisting of quarter-or half-wave films

can now be simply determined. The reflectance of a single-layer antireflectance coating is given by

eq. (8) and its absorption by eq. (13). Its transmission is unity minus the sum of the reflectance and absorption losses. Similarly, for a two-layer antiref lectance coating the reflectance is given by eq.

(9) and the absorption by eq. (16). For a three-layer quarter-half-quarter coating the reflectance is given by eq. (9) and the absorption by eq. (17). A comparison of results using these expressions with

exact computer results is given in table 1. (No interface absorptions are included.) The theoretical

performance of coatings made using various coating materials may thus be compared using experimentally

measured single-film values of volume and interface absorption for the various filming materials.

The above equations are also useful as a starting point for developing optimum nonquarter-wave

antiref lection coating designs. These designs often make possible more perfect matching of impedances

using available index materials than do quarter-wave designs, and may sometimes also result in thinner

coatings. Two-layer coating designs, of which two exist in most cases for each appropriate pair of materials, are facilitated by using a Schuster diagram [9]. Simple relationships governing the design

of three-layer coatings, which can result in significant reductions in absorption relative to two-layer

coatings when one of the coating materials has a large absorption coefficient, were derived by Baer [10].

High Reflectance Quarter-Wave Multilayers

The concept of a generalized index can also be applied to high reflectance, nonabsorbing quarterwavelength multilayer films. The characteristic matrix for each high-low (HL) index pair is

i/iu

0 _

L/nT — 0

(18)

so that the characteristic matrix for N such multilayer pairs is this matrix raised to the N

The generalized index of this multilayer stack is then

th power.

(nH/nL

) Z1 n

s (19)

and the reflectance of the combination is

269
1 - (nH /n

L) n

s

1 + (nH

/nL

} n

S

(20)

which approaches unity as N becomes large.

In deriving eq. (20) we have assumed that the outermost film has a high index and that the film next to the substrate has a low index. If the multilayer stack is symmetric so that high index films are next to the substrate as well as being the outermost layer, the characteristic matrix becomes

(-nL/nR

) N

0 0 L/n,.

L

inH ^

l/ "» < " ^ (21)

and the generalized index of the multilayer-substrate combination is

2N+2 , , 2N . > H /(n

L

n

s

) (22)

A simple way to visualize the performance of a multilayer high reflectance stack, each of whose

layers is a quarter of a wavelength in optical thickness, was pointed out by Sparks [11]. Assume for the moment that the substrate is a dielectric. A standing wave is formed for a perfect reflector

having the characteristics shown in figure 2. If the reflectance is unity, at the outer film interface

of each HL index pair the field is zero, whereas at the interface between the high index film and the low index film it is a maximum. Inside each film the field is one-quarter of a sinusoidal standing

wave. This standing wave is composed of two traveling waves, one from the left having a field strength

E+

, and the other from the right having a field strength E . At the outer film-pair interfaces E+

+ E

0. On the air side of the film-air interface, the incident field strength E =1. Since the mirror

is a perfect reflector, E -1 and E+ + E 0. However, E

q is made up of two parts, one reflected

from the air-film interface and one transmitted back through it from the multilayer film stack. Letting

the subscript H represent the outermost film, the amplitude reflection coefficient r on the air side

On

IS

"OH

1 - n.

1 + n,. (23)

The amplitude transmission coefficient from medium 1 to 0 is

"10

2n,

1 + n. (24)

Then the reflected amplitude E^, which is equal to (-1), is given by

-1 = 1 x

1 "

nH x

- 2nH + E, x

1 +

J l 1 + n. (25)

where E^ is the amplitude of the wave traveling to the left in the outermost film. Solving gives

(26)

The wave traveling from left to right in medium 1 is E_ . It is composed of the transmitted component + 1 of E and the reflected component of E . Then

270
1 X C 01 + V 10 2_ l /

nH

~ 1 \

1 + n

H

" + V

°H

(27)

The amplitude of this wave traveling from left to right is unchanged at the HL interface as a consequence of the boundary conditions on the tangential component of E. At the LH interface the average value of E is zero, the amplitude in the next HL pair is determined by the transmission at the LH interface and will be lower than that of the first HL pair by a factor of n^/n^, and so on. In each

HL pair the amplitude is decreased by a factor n /n .

. L H

Absorbing Substrate

If the multilayer film is deposited on a metal substrate and the thickness of the layer next to the substrate is not adjusted to compensate for the phase change on reflection introduced by the metal,

the reflectance of the metal/film combination can be lower after the first two layers have been deposited than it was for the bare metal. To compensate for this phase change, the first dielectric layer

is made thinner (or thicker) than a quarter wavelength in optical thickness. The phase change on reflection introduced by the metal substrate is 6 where

2nk„

Thus, when n the standard conventions

<4*

tan6 = —

^ n -

6 is in the first quadrant.

12,13].

(28)

Otherwise, 6 is in the second quadrant using

The optical thickness n^S, of the first dielectric film on the metal substrate in the high index

case would be chosen to be V 4 TT (29)

More commonly the low index film is deposited on the metal substrate since a smaller optical thickness

is required and hence the coating design is more efficient. In the low index case, the optical thickness n^i is given by

nr l = t L 4 TT (30)

In either case, these values are substituted in eq. (3). The iterative relationship for the admittance

of the substrate-initial film relation in matrix notation gives the characteristic matrix of the generalized substrate as

2lT

o cos r nJt A

• 2tt

„ l n sxn —— nl

(i sin nJl) /n

A

cos — n£

A n -ik„

S S J

where the index of the substrate is now n^ - 1kg. The characteristic admittance of this substratefilm combination is then

(n c - ik )cos(^f nl) + i n sin(-^- nil) Y = —§ § 2-, — ^— , (31)

0 „ (n - ik ) _•

,2tt „. , . S S ,2tt . cos(— n£) + i sm(— nil) A n A where n is either n or nT

, which makes the imaginary part of the admittance zero when the right-hand H L

side of eq. (29) or (30) is substituted. The generalized substrate consisting of the metal-film combination as seen by the multilayer film is thus effectively transformed into a dielectric. The generalized index of refraction of this dielectric, n is given by eq. (31), which reduces to

271
(32)

where 6 is the phase change on reflection given by eq. (28) and the upper or lower sign is chosen for

the low or high index case, respectively.

Volume Absorption in a High Reflectance Multilayer Film

The standing waves shown in figure 2 can be utilized to calculate the absorption of the multilayer

stack when the films that compose it are slightly absorbing and there is .also a small amount of interfacial absorption. Consider first the contribution from volume absorption. If we assume that the films are homogeneous with a volume absorption coefficient 6 T for the low index film and 6 for the

L th " high index film, the energy absorbed in the low index film in the i film pair from volume absorption

is A , where

4

W 5 T * (n

T E?/E^)

L L L 1 o (33)

The average

A similar expression holds

and the optical film thickness n I = A/4, where A is the wavelength of the incident light.

— j— L L field E. = E. //2, since within each layer the fields are sinusoidal

l l max

for absorption in the high index layers. But from figure 2, E. = (l/n„) (nT /n„) 2E so that the

th (i) (i) (i) l max H L H o absorption in the i film pair A = A^ + A^ is given by

A

(i) = (B L + 44nH\V

2 (PL +

/ \ 2i 4 \ v

(34)

The total volume absorption in a multilayer film stack is then A^ where

,2i \ " \

(SL + (35)

Making use of the binomial expansion gives for N °°,

\max= 2 (6L + W 2 (36)

Equation (34) was first deduced in an equivalent form by Koppelmann [14] and more recently by Sparks

[11] using an argument similar to that presented here. It allows us to predict, from the volume absorption coefficients of the filming materials, what the limiting reflectance will be if interface absorption (and scattering leading to absorption) can be neglected and if enough layers are used to make the contribution of the substrate negligible. If the absorption or volume scattering of the films is too high, it is easy to see that a desired reflectance may be unobtainable.

If a smaller number of pairs is used, absorption and scattering in the film decreases, but the substrate contribution increases. Although eq. (35) can be used to calculate the multilayer film

volume absorption in this case, a Sparks' paper, is more convenient form, obtained using the expansion in Section V of

2

r / v2N+l-,

]

(37)

Interface Absorption

An important source of absorption in multilayer film stacks is related to the HL film interfaces.

Reference to figure 2 shows that since the fields are very low at the outer interface and at each LH

272
interface, contaminants or scattering at these interfaces will not to a first approximation affect film

performance. However, contaminants at the HL interfaces, where the field is a maximum, will be important.

The absorption arising from the HL interfaces can be calculated in a manner exactly analogous to that used to calculate volume absorption except that the field involved is E . , not E.. The contri1 max 1 bution of interface absorption from the dielectric pairs is then A , where

IF

A„ = 4A, IF 12 2 fe) ]

2N+1-

(38)

and Aj. is the interface absorption in an optical thickness at the interface which is small enough so that the field is effectively constant. What little data we have on direct interfacial absorption [15] suggest that the interfacial thicknesses involved are ~ 0.1 ym. Scattering leading to absorption

should also be most important at the interfaces.

Another interfacial absorption that may have to be considered is A^g, that at the film/substrate

interface. It is typically most important when the substrate is a metal. In the dielectric case, it is probably negligible for most highly reflecting films, but for completeness it should be mentioned. If n < n<, , the film dielectric substrate interface occurs at a node for the 0(HL)^S case, and surface

absorption and scattering at this interface can to a first approximation be neglected. However, for a N

0(HL) HS film where n, > n , the substrate interface occurs at an antinode. In this case, where the

H s high index film is next to the dielectric substrate, the interface absorption is

/ \ 2N A

IS " 4A

s "

where A^ is the absorption by the high index dielectric/dielectric substrate interface.

In addition to absorption in the multilayer film or at the interfaces, some light may also be transmitted into a dielectric substrate. If the low index film is next to the substrate, the power transmitted into the substrate is

k 2N

n

S V

nH/

TL S^l^J . (40)

Alternately, if the high index film is next to the substrate, the transmitted power is

. • (41) H

Equation (41) is also useful for calculating the energy absorbed by a metal substrate coated with a matched low index film. Substituting n

g for n

g

, this absorption is then

/ + / \l2N2

H

which may be compared to the interface absorption at a dielectric substrate [eq. (39)]. Equation (40) could be similarly modified to represent absorption of a metal substrate coated with a matched high

index film.

The absorption of a multilayer coated metal mirror having a matched low index first dielectric

layer is then

. 2N+1-,

(1-R) |W+ v+^i]!}-^) ](

nH-^)

_1

n

2

, W

k2N -) (43)

273
If the substrate is a dielectric with both the outer film and the film next to the substrate high

index, the expression is the same except n is substituted for and A [eq. (39)] is added. If a

S S IS 2n low index film is next to the dielectric substrate, the last term in eq. (43) becomes (4/n ) (n /n ) S L H

We thus have a general simple closed-form expression which allows us to calculate approximately what

the absorption will be in a multilayer coated metal or dielectric mirror. A comparison of calculations

made using eq. (43) with exact calculations is shown in table 2. No interface absorption has been

included in these calculations. Also note that for the metal substrate case in the table, the number

of layer pairs N is one less than it would be if we had not included the final film into the generalized

substrate.

Scattering

Light is scattered at interfaces between multilayer films, at the film-substrate interface, and at

the film-air interface. In addition, it may be scattered by defects in the volume of the film itself

or in the volume of a dielectric substrate. What little evidence there is suggests that interfacial

scattering is usually the most important [16,17], except possibly very near the specular direction [18].

It arises both from microirregularities on the substrate surface, which deposited films contour remarkably well, and irregularities resulting from film formation itself. For example, the print-through of

the substrate surface was sufficiently exact in the case of silver films deposited on topaz substrates

to permit Koehler to determine interf erometr ically the 7 A lattice spacing of topaz [19], On the other

hand, a reliable technique for producing surfaces having varying microroughnesses is to deposit various

thicknesses of calcium fluoride at room temperature on superpolished quartz surfaces [20]. In this

case, the large crystallites formed by the calcium fluoride result in roughness which is uncorrelated

to that on the substrate surface. In the limiting case of microirregularities which are correlated

with the substrate, so that each film layer replicates the surface irregularities of the layer below

it, Eastman [21] showed that to a good approximation (scalar scattering theory) the ratio of scattered

to specularly reflected light from the film-covered substrate is the same as that from the uncoated

substrate. The light scattered into all directions by a multilayer film whose microirregularities are correlated with those of the substrate is then approximately AR, where [22]

AR = R (4tt6/A)

2 (44)

o and R

q is the calculated reflectance of the multilayer film in the absence of scattering. For very high reflectance coatings R

q — 1. Here 6 is the rms microroughness of the substrate surface, preferably

determined by light scattering experiments [23] and A is the wavelength.

Recent calculations utilizing vector scattering theory suggest that although in most cases the scattering from a correlated multilayer-coated surface will track that from the uncoated substrate,

anomalies can occur [24] . Much larger anomalies are expected when the microirregularities at the various interfaces are uncorrelated. If optical thicknesses are close to a quarter wavelength, scattering will be smaller than for the case of correlated irregularities; elsewhere it may be larger [24]. Also, phenomena other than microirregularity scattering are expected to contribute to the observed

scattering, particularly in the infrared [23]. Equation (44) thus gives only a first approximation to the scattering loss from the multilayer film.

Summary

Approximate expressions have been developed to predict, from experimentally measurable parameters

characterizing the filming materials and substrate, what the performance of a high reflectance or AR quarter-wave film coating will be. These expressions are simple enough so that no computer is required

even for multilayer problems. They form part of the basis for choice among various thin film materials

and also give a starting point for film design efforts aimed at optimization of reflectance, absorption,

transmittance, or laser damage threshold. They may be summarized as follows:

AR coatings

Single layer

x n + n ' L

s+1> *«OL ^n

s

(45)

Double layer

T

2 - 1

H

8n,

+ AnT + A. u —- +

OL LH 2 n^

S

8nHn

S

(46)

274
Three-layer QHQ

3, X /n n \ n 4tH + -| )"-| (ALh + AhH- V (47)

High reflectance coatings (R — 1)

Low index next to metal substrate

r "ir/ \2N+1i r t

1

High index next to dielectric substrate

RH S 2 n

H Low index next to dielectric substrate

(48)

(49)

(50)

where n^ for a metal is given by eq. (32) and the low index layer next to the metal has a thickness

given by eq. (30). The various symbols have the following meanings:

Symbols Subscripts

R Reflectance L Low index layer

T Transmittance H High index layer A Absorptance h Half-wave layer

3 Volume absorption coefficient I Interface

X Wavelength F Film

n Index of refraction S Substrate

N Number of layer pairs 0 Incident medium

6 rms roughness

4

Admittance (real) of substrate/

matching film combination

To use these equations it is necessary to know the volume absorption coefficients of the materials

as thin films, the interface absorptions, and the indices of refraction. The surface microroughness of

the substrates should also be known for high reflectance coatings. These parameters may all be determined experimentally for individual films or surfaces. Knowing them the approximate performance of AR

or high reflectance multilayer film combinations can then be simply predicted.

Refer

[1] Donovan, T. M. , and Baer, A. D. , in Proceedings of the Fifth. Annual Conference on Infrared Laser Window Materials, C. R. Andrews

and C. L. Strecker, eds. (Defense Advanced

Research Projects Agency, 1976), pp. 291-300.

[2] Harrington, J. A., Rudisill, E. J., and Braunstein, M. , Appl. Opt. 17, 2798 (1978).

[3] Bennett, J. M. , J. Opt. Soc. Am. 54, 612

(1964).

[4] Macleod, H. A., Thin Film Optical Filters

(American Elsevier Publ. Co., Inc., New

York, 1969) , p. 10.

[5] Macleod, H. A., op. ait., pp. 114-115.

es

[6] Macleod, H. A., Thin Film Optical Filters

(American Elsevier Publ. Co. , Inc. , New York,

1969) , pp. 17-20.

[7] Cox, J. T. , Hass, G. , and Thelen, A., J. Opt.

Soc. Am. _5_2, 965 (1962).

[8] Temple, P. A., Decker, D. L. , and Donovan,

T. M. , "Measured Thin Film Absorption at the Air-Film Interface, the Film Bulk, and the Film-Substrate Interface," this conference.

[9] Cox, J. T. , and Hass, G. , in Physics of Thin

Films, G. Hass and R. E. Thun, eds., Vol. 2 (Academic Press, New York, 1964), pp. 249-265.

275
[10] Baer, A. D. , in Laser Induced Damage in

Optical Materials: 1976, A. J. Glass and

A. H. Guenther, eds. , National Bureau of Standards, Washington, D.C. (NBS Spec. Publ.

462), pp. 221-229.

[11] Sparks, M. , J. Opt. Soc. Am. 67_, 1590 (1977).

[12] Bennett, H. E. , and Bennett, J. M. , in Physics

of Thin Films, G. Hass and R. E. Thun, eds., Vol. 4 (Academic Press, New York, 1967),

pp. 79-85.

[13] Muller, R. H. , Surf. Sci. 16, 14 (1969).

[14] Koppelmann, G. , Ann. Phys. (Leip. ) ]_, 388

(1960).

[15] Soileau, M. J., Bennett, H. E. , Porteus,

J. 0., Temple, P. A., and Bass, M. , in Pro- ceedings of the Fifth Annual Conference on Infrared Laser Window Materials , C. R. Andrews and C. L. Strecker, eds. (Defense

Advanced Research Projects Agency, 1976),

pp. 392-417.

Table

Single-layer MgF

2 /BK7

Two-layer

MgF2 /Al2 0

3 /BK7

Three-layer (QHQ)

MgF

2 /Ti0

2 /Al20

3 /BK7

MgF„ A1 2°3 Ti02 n = 1.38 n = 1.65 n = 2.25

B = 0.7 cm B = 1.0 cm B = 30 cm'

[16] Kozawa, S. , Proceedings Conference Opt. Instrs. Tech., London 1961, K. J. Habell, ed. (Chapman and Hall, London, 1962), p. 410.

[17] Tien, P. K. , Appl. Opt. 10, 2395 (1971).

[18] Guenther, K. H. , Gruber, H. L. , and Pulker

,

H. K. , Thin Solid Films 34, 363 (1976).

[19] Koehler, W. F. , and Eberstein, A., J. Opt.

Soc. Am. 43, 747 (1953).

[20] Bennett, H. E. , Bennett, J. M. , Ashley, E. J. and Motyka, R. J., Phys. Rev. 165, 755 (1968)

[21] Eastman, J. M. , PhD Thesis (unpubl.), University of Rochester, New York, 1974.

[22] Bennett, H. E., and Porteus, J. 0., J. Opt.

Soc. Am. 51, 123 (1961).

[23] Bennett, H. E. , Opt. Engr. 480 (1978).

[24] Elson, J. M. , "Diffraction and Diffuse Scat- tering from Dielectric Multilayers," J. Opt.

Soc. Am. (in press).

Transmittance

Approx. Exact

0.987384 0.987386

0.999029 0.999030

0.998304 0.998311

BK7 A = 1.06 ym

n = 1.52

1. AR coatings: comparison with exact calculations.

Absorptance Reflectance

Approx. Exact Approx. Exact

1.5 x 10~5 1.3 x 10~5 0.012601 0.012601

3.1 x 10~5 3.0 x 10

-5 0.000940 0.000940

7.56 x 10~ 4 7.54 x 10~ 4 0.000940 0.000935

Table 2. Mirrors: comparison with exact calculations.

Infrared, metal substrate (ZnSe/ThF ) NZnSe(ThF, /Mo) N A

film A . substrate R

* Approx. Exact Approx. Exact Approx. Exact

\ =

0 0.0006 0.0010 0.0062 0.0061 0.9932 0.9929 ZnSe

1 0.0012 0.0013 0.0019 0.0019 0.9969 0.9968 n =

2 0.0014 0.0014 0.0006 0.0006 0.9980 0.9980 B =

3 0.0014 0.0014 0.0002 0.0002 0.9984 0.9984 ThF4

4 0.0015 0.0015 0.0001 0.0001 0.9985 0.9985 n =

10.6 ym

B = 10 cm

Mo

n = 10-44j

Ultraviolet

,

dielectric substrate (Hf0

2 /Si02

) NHf0

2 Si02 N A _ T R film substrate

Approx. Exact Approx. Exact Approx. Exact

1 0.0029 0.0025 0. 7231 0.5171 0.2740 0.4803 \ = 0.35 ym

3 0.0042 0.0040 0.1973 0.1783 0.7985 0.8177 Hf02

5 0.0045 0.0045 0.0538 0.0521 0.9417 0.9434 n = 2.088

7 0.0046 0.0046 0.0147 0.0145 0. 9807 0.9809 B = 276 cm

9 0.0046 0.0046 0.0040 0.0040 0.9914 0.9914 Si02 11 0.0046 0.0046 0.0011 0.0011 0.9943 0.9943 n = 1.509

13 0.0046 0.0046 0.0003 0.0003 0.9951 0.9951 B = 276 cm

1

1

276
277
SIMPLIFIED DESCRIPTION OF DIELECTRIC REFLECTORS

M. Sparks and M. Flannery

Xonlcs, Incorporated

Santa Monica, California 90401

The spectral reflectance, R(to) , of a quarter-wave reflector is quite asymmetric

in general as a result of absorption. The maximum electric field E in an infinite

stack is in the center of the high-index (low-index) layer at the low-frequency

(high-frequency) end of the high-reflection band. Thus, the absorptance minimum of

a stack having much greater absorption in the high-index material occurs at to > to c (band center). Operation at to > to c could increase the damage resistance if the' high-index material is easily damaged. At to c

, E decays rapidly. As to departs from

u) c in an infinite reflector, E penetrates deeper into the coating, with no decay at the band edges. The peak-to-peak distance in an infinite dielectric stack is constant for all frequencies in the band. By using the continuity of E and dE/dz, E can be obtained from E = 0 at the metallic substrate or, in an infinite stack, from

E = 0 at the reflector surface for (o = to c or E = 0 in the center of the high- or low- index layer at the band edges. Simple closed-form approximations for the spectral

absorptance and the phase of E are accurate and sufficient for present applications.

Key Words: Dielectric reflectors; spectral absorptance; reflectance, electricfield distribution; theory; asymmetric absorptance; high-power; damage thresholds.

1. Introduction

Multilayer-dielectric-coated metallic reflectors are required in most high-power applications at all wavelengths since the intrinsic absorptance of metals is too great. As shown in figure 1, the lowest available absorptance in the ultraviolet and vacuum-ultraviolet regions is approximately eight to ten percent, and the values of absorptance at the hydrogen-fluoride-laser wavelength, X=2.75 ym, are

0.6, 0.7 and 0.8 percent for silver, gold, and copper, respectively [1]. All of these values are much

greater than required values, which are at least as low as 0.1 to 0.01 percent, for high-power systems.

In considering high-power reflectors, it is convenient to consider two scales of reflectance — a fine scale from 0.997 to 1.000 and a large scale from 0 to 1.0. The high-power laser sees the fine- scale reflectance, so to speak. For example, doubling the absorptance from 10 -

^ to 2 x 10

-

^ doubles the heating, but the difference in the reflectance seen on the large scale is insignificant. Scattering is neglected; thus, the absorptance A is equal to 1 - R, where R is the reflectance, for the infinite-stack

reflectors and dielectric-coated-metal reflectors (with zero transmittance)

.

The large-scale spectral reflectance R(to) at normal incidence is shown in figure 2(a) for

(A£

2 0

3 /Si)°° and (As^/GeSe) 00 lossless infinite quarter-wave stacks [2,3]. The notation Mo (Ai^O-^''''"'") indicates three pairs (six layers) of Ai^O^ - Si quarter-wave films, with low- index material A^O^ and high-index material Si, deposited on a molybdenum substrate with an Ai^O^ layer adjacent to the substrate. The perfect-reflectance bands (R=l) of these stacks have full bandwidths 2to A centered at to r \ C C [4,5]. The values of 2A in figure 2 are 2A= 0.428 for the (AJl^/Si) 00 reflector and 2A= 0.157 for the

(As 0 S 0 /GeSe) C0 reflector. As the index ratio n /nT of the layers increases, the bandwidth increases

until the perfect-reflectance band extends from to = 0 to to = 2a> c as u^

n-^ approaches infinity.

Simple forms of the electric fields in dielectric reflectors will be discussed and used to explain

important properties of the reflectors. Apfel and co-workers [6], Apfel [7], Newnam and co-workers [8],

Gill and co-workers [9], Sparks [10], and Sparks and Flannery [11], have also investigated effects of the electric-field distributions in dielectric reflectors and other structures. The contents of the paper are as follows: 1. Introduction; 2. Electric Fields; 3. Absorptance and Phase; 4. Summary of Results; and 5. Acknowledgments.

2. Electric Fields

The key to the explanation of the central results for the spectral absorptance and phase shift is the simple form of the electric field in a perfect reflector (R=l) composed of an infinite number of quarter-wave layers. First consider the center frequency to c of the high-reflectance band shown in figure 2. In each layer the electric field is a quarter of a sinusoidal standing wave, as seen in the center curve of figure 3. The amplitude is constant in each pair of HL layers. The electric field E and its normal derivative (slope) dE/dz are continuous everywhere, including the boundaries. Since the slope is continuous at the 0-H interface and the wavelength in the high- index (n ) medium is ^/ n

H

>

278
where A is the wavelength in vacuum, the amplitude of the field is reduced by a factor of n./n at the

U H 0-H interface. Similarly, at the nodes at the L-H interfaces, the amplitude is reduced by a factor of n^/n^. The phase (j)^, must be equal to zero at to = cj in order to satisfy the boundary condition at infinity — that the electric field remains finite. If <j> were not zero, then the term ~ cos kz in the expansion sin (kz -<()) = sin kz cosc{> - sin<f> „ cos kz would grow to infinity since the nodes would occur at H-L interfaces (and the magnitude of E would increase by a factor of n /n at every H-L interface).

H L As |gj-W

c

| approaches the band edges, oj/w =1±A, the field penetrates deeper into the coating

until there is no decay at the edges, as shown in figure 3. At the high-frequency edge, to/w

c =l + A, the field has peaks at the centers of the low-index layers and nodes at the centers of the high-index

layers, as illustrated in the bottom curve in figure 3. At the low-frequency edge w/o)c =l-A, the field has peaks at the centers of the high-index layers and nodes at the centers of the low-index

layers, as illustrated in the top curve in figure 3. The stack wavelength A, defined as twice the distance between adjacent nodes, remains constant for all frequencies in the band for the infinite stack.

The major differences between the electric fields in the infinite stack (fig. 3) and the finite

stack (fig. 4) result from the near-node in the electric field at the metallic surface. Since the fields at the rear of the sixth layer of the infinite stack at lo/u) =1±A are far from nodes, while the

c corresponding fields in the coated-metal reflector have near-nodes at the metal, the metal produces ex- tensive modifications in the fields near the band edges, as shown in figure 4. Other features of the electric-field distribution are summarized in section 4.

3. Absorptance and Phase

In order to develop an approximation to the spectral absorptance A(w) , consider three cases.

First, the absorptance at the center frequency of an infinite quarter-wave stack is [12,13,10]

|(BH +SL

)X n A = ± S- = 1.66 x 10

J

, (1)

c ,2 2, (nH

~ V

where = 2 it c/to^ is the center wavelength and the numerical value [2] is for (Al^O^/Sl) 00

. Second,

the absorptance A., at the center frequency of a lossless N-pair dielectric coating on an absorbing

Nc 2N substrate in the limit A^ << 1 is closely approximated by the simple result [10,14] A^

c ~ (n^/n^) A<, , 2 2,-1 , i 1

where A is the absorptance of the uncoated substrate, A = 2n [ (n +1) + K q

] ~ 2n /

| n | . The sub- strate index is n^ = n + i K<, . The approximate equality is satisfied when Ag << 1 is satisfied.

Third, the perturbation-expansion for the absorptance of an infinite quarter-wave stack for fre- quencies near the center frequency is [13]

A (to) = A + A (to/to - 1) + A„(w/w - l)

2

, (2)

op C 1 C 2C

where A is given by eq. (1) and

(BL

n

H

- W (3)

(nH

- n

L

) (nH

- n^)

A

2 = af 2°

"

C

2 ,

I (

g

H + V [nH +

"H "L + 4 ~ 4

(nH + V' ] 8(n

H

- n

L

)

+ eL

nH[nH(n

2 + 2n2) + 2n

L (2n2 + nJ) ] + eH

nHnL(4n2 + 5nJ)] . (4)

Combining these expressions for Aqo (oj) and A^ gives a useful approximation for the spectral absorptance of a dielectric-coated-metal reflector:

A(w) = AjuOU - (nL/nH

) 2N

] + (nL

/n

H

) 2N A

s

. (5)

The linear perturbation-theory result [13] for the standing-wave electric field phase (j) (which is one half the phase (J) of the Fresnel reflection coefficient r= |r| exp i<j>) in an infinite stack,

279
*E

= 1

77 n

0

(U)/u)c " 1)/(nH

" n

L

)

' (6)

is quite accurate near the center of the high-reflectance band. The phase for an infinite quarter-wave

stack reaches its maximum (or minimum) at the high-frequency (or low-frequency) band edge, where the standing-wave electric-field 'phase is

^Ee

= ±tan n

0 (nH

n

L

) -1/2

(7)

These results for the edge phases can be found quite easily by using the continuity of E and dE/dz at the 0-H interface and the fact that E has a maximum or a node at the center of the first high- index

layer. They can also be derived from eqs. (6) through (14) of reference 13 with somewhat more effort.

The high accuracy of the quadratic approximations to A

a> (o)) from eq. (2) is shown [15] in figure 5 for three (ThF^/ZnS)°° reflectors which have their losses localized in the low-index layers, uniformly

distributed, and localized in the high-index layers. The dashed curves are the exact infinite quarterwave stack reflectances computed numerically from eqs. (6) through (14) of reference 13. The quadratic

approximation provides a good estimate of the absorptance throughout the central part of the highreflectance band for all three loss distributions.

The asymmetry of the spectral reflectance can be understood by considering the electric fields in the reflector in figure 3. For cu c < to the fields peak in the low- index layers, while for w < oi the fields peak in the high- index layers. Thus the reflector in figure 5(a), with gu

= 0 and gT =23, has high absorptance for u)^ < oo since the peak fields are located in its lossy low-index layers. The re- flector in figure 5(c), with B = 2$ and 3T =0, has high absorptance for co < w„ since the peak fields are located in its lossy high- index layers.

The high accuracy of the central results (5) and (2) for the spectral absorptance is shown in figure 6 for a practical reflector. The exact theoretical absorptance (heavy solid curve) was calculated by a standard matrix method using eqs. (4) and (9) of Flannery, Sparks, and Loh [13]. The general methods are presented by Lissberger [4] and Heavens [5]. The approximation (5), shown as heavy

dots, is in excellent agreement with the exact result. This approximate absorptance is the sum of the approximate infinite-stack absorptance Aoo (w) (dash-double-dot curve) and the approximate substrate

absorptance (light dots). Notice that the substrate absorptance A^

c ~1.5><10_

^ ± s much smaller

than the absorptance Ag - 10 ^ of the uncoated substrate. Also, the term -(n^/n^) ^(oo) in (5) is negligible in figure 6. Similar results are shown in figure 7 for Ag^s^S^/GeSe) reflectors. The reflectance peak occurs at u < Co since the high-index layer is lossier, $ /n < 3,,/n . C L L rl h

4. Summary of Results

Asymmetric spectral-reflectance . The spectral-reflectance curves, R(co) , of dielectric quarterwave-stack reflectors are quite asymmetric in general as a result of the absorption in the coating

materials. The position of the peak in R(u)) is shifted from the absorption-free center frequency co c by

a large amount, typically ten percent, as illustrated in figures 5 through 7. This dependence of R(to) on the coating absorptance should be taken into account in designing coatings. The asymmetric spectral

reflectance and the other results discussed below are explained in terms of the simple form of the electric field E(z) in a high-reflectance (1-R<<1) coating at normal incidence, as illustrated in figures 3 and 4.

Electric-field distribution . The maximum of E(z) in an infinite-stack reflector occurs in the center of the high-index [or low-index] material at the low-frequency end of the band [or the highfrequency end], as seen in figure 3. Thus, the absorptance minimum of a stack having greater absorption

in the low-index material occurs at a)<co

c

, as seen in figures 5(a) and 6.

At the center frequency coc the electric field decays rapidly, reducing in amplitude by a factor of n

i/ nH

at every n°de (at the L-H interfaces), as seen in the center curves in figures 3 and 4. As to de- parts from the center frequency in an infinite reflector, the field penetrates deeper into the coating

until it approaches a constant amplitude as to approaches the edges of the reflectance band, at which

frequencies the peaks of E are in the low-index and high-index material as discussed above. As seen in figure 3, the stack wavelength A, which is the peak-to-peak distance in an infinite dielectric stack,

is constant for all frequencies in the band.

The electric fields in N-pair dielectric-coated metal reflectors that satisfy the condition

(n /n )^«1 are similar to those in the corresponding infinite quarter-wave stack for frequencies

L H near co c

. But the stack wavelength A is not constant as a function of frequency, and the fields decay

quite rapidly even near the band edges, as seen in figure 4. The electric fields are insensitive to

280
changes in the small values of the absorptance of the coating materials, in contrast to the large

changes in the fine-scale spectral reflectance.

It should be possible in general to improve the performance of a dielectric reflector by changing

the layer thicknesses to change the relative values of the nominal center frequency to c and the laser

frequency. For example, the value of w could be chosen to center the laser band in the lowest absorptance region (at U)=0.91 U) c in figure 6, for example). An alternate choice of co c could be used in reflectors that are limited by laser damage to one of the layers — usually the high-index layer [16]. For example, by choosing co c less than the laser frequency u), the electric field in the high- index

material can be reduced, as seen in the bottom two curves of figures 3 and 4.

Obtaining the electric-field distribution . The electric-field distribution is easily obtained from

the continuity of E and dE/dz when the phase <|)g of E is known. The electric-field distribution at any frequency in the high-reflectance band of a dielectric-coated metal reflector with metallic absorptance

Ag « 1 can be obtained to excellent accuracy by starting with E = 0 at the metallic substrate and using

the continuity of E and dE/dz at the interfaces and boundaries. Simple relations that are useful in matching E and dE/dz at the surface and interfaces are given by Sparks and Flannery [11]. For an infinite-stack reflector, the electric field is easily obtained for U) - uj c

< A U)c /3 , typically, and for

0) at the band edges, a)=u) (1 ± A) , since the phases are known for these frequencies. Specifically, the fields can be plotted by starting at E = 0 at the surface or in the center of the low- index or high- index

layer for U)=wc , (0 C (1-A), or u (1 + A), respectively, and using the continuity of E and dE/dz.

Simple closed-form approximation to the absorptance . The simple approximation (5), with A^Cw)

given by (2), developed for the spectral absorptance A(oj) of an N-pair absorbing stack on an absorbing

substrate is sufficiently accurate for present applications when A(w) is small. In (5), A^Cio) is the spectral absorptance of the corresponding infinite absorbing stack, and Ac is the absorptance of the

2N uncoated substrate. The term (n /n ) Ac is the absorptance of the substrate with the incident

2N 2N irradiance reduced by the dielectric coating by the factor (n /n ) . The term -(n /n ) A (to) L H L H

00 corrects for the absorptance in the missing layers from 2N to infinity.

The expression for Ag and a simple approximation for the case of Ag << 1 are given under eq. (1). The infinite-stack spectral absorptance A^fu) is well approximated by the quadratic perturbation expansion (2). The accuracy of (2), which is sufficient for present applications, is good for all fre- quencies in the band that are not near the band edges, as seen in figures 5, 6, and 7. The infinitestack absorptance AOT (a)) is approached as more pairs are added. Values of A^Cw) can be calculated

exactly, with a moderate amount of computation, from eqs. (6) through (14) of Flannery, Sparks, and Loh

[13]. However, using the exact value of A00 (co) in the approximation (5) for the dielectric-coated-metalreflector absorptance may not give a great improvement over the simpler approximation (5) with Aoo

((jj) given by eq. (2), as seen in figures 5, 6, and 7. Finally, the exact value of A(w) can be obtained

numerically, with a considerably greater amount of computation, by the matrix method, using eqs. (4) and (9) of Flannery, Sparks, and Loh [13] or the general results as presented by Lissberger [4] or Heavens [5]

Simple closed-form approximation to the phase . The standing-wave electric-field phase (j)^ in an infinite stack is zero at the center frequency U) c

. The simple expression (7) for the band-edge phases

is easily obtained by using th= continuity of E and dE/dz at the reflector surface and using the zeros

of the electric field in the center of the high- index and low- index materials. The perturbationexpansion approximation to the phase <J> E given in (6) is accurate for frequencies near 0) = U) c

. Extrapolating between (6) for 0) near u> c and (7) for w=io (1±A) gives sufficiently accurate results for infinite stacks for many applications. Numerical calculations of <f> E using eqs. (6) through (14) of Flannery, Sparks, and Loh [13] can be used if greater accuracy is required.

5. Acknowledgments

This research was supported by the Defense Advanced Research Projects Agency with technical administration by the Naval Weapons Center, the Air Force Office of Scientific Research, and the Air Force Weapons Laboratory with technical administration by the University of Dayton Research Institute.

281
6. References

The data from which figure 1 was plotted is from the following references: Aluminum and

gold: Jean M. Bennett and E. J. Ashley,

Appl. Opt. 4, 223 (1965), and H. E. Bennett,

M. Silver, and E. J. Ashley, J. Opt. Soc

.

Am. 53, 1093 (1963); copper: High Energy

Laser Mirrors and Windows , Semiannual Report

No's. 7 and 8, Naval Weapons Center, China

Lake, Ca. , July 1976, p. 59, and H. E. Bennett,

M. J. Soileau, and P. C. Archibald in Proceedings of a Symposium on Laser Induced

Damage in Optical Materials , NBS Spec. Publ.

435, edited by A. J. Glass and A. H. Guenther, Boulder, Colo., July 29-31, 1975; silver: Jean M. Bennett and E. J. Ashley,

op. cit., and High Energy Laser Mirrors and Windows , Semiannual Report No. 3, Naval Weapons Center, China Lake, Ca. , Mar. -Sept.

1963, p. 79; gallium, mercury, and sodium:

American Institute of Physics Handbook , third edition (McGraw-Hill, New York, 1972);

molybdenum: High Energy Laser Mirrors and Windows , Semiannual Report No. 6, Naval

Weapons Center, China Lake, Ca. , Sept. 1974 - Mar. 1975, p. 9.

Values of parameters for A^O^/Si in figure

2 are as follows: n (A£9 0,) = 1 . 733,

3L

(AJl 2 0

3

) = 100 cm

x

, n^Si) = 3.436, and

3u

(Si) = 8.1 cm

-1 at X=2.75 Mm; A =99.1

n o percent for silver at 2.7 to 3.8 ym. Values of n and Ag are from the American

Institute of Physics Handbook , third edition, Dwight E. Gray, editor (McGraw-Hill, New York, 1972), pp. 6-40, 6-41, and 6-150. Absorption coefficients $ are from P. A. Miles, J. E. Hopson, and W. T. Beauchamp in High Power Laser Optical Components and Component Materials , compiled by James S. Harris and Charles L. Strecker, Boulder,

Colo., Oct. 3-4, 1977, p. 257.

Values of parameters for As2S

3 /GeSe at 2.75 ym in figure 2 are as follows: n^(As2S

3

) = 2.42, from American Institute of Physics

Handbook , third edition, Dwight E. Gray,

editor (McGraw-Hill, New York, 1972), pp. 6-54; n

H

(GeSe) = 3.1 is an approximate value;

3(As2S-j) = 0.27 cm , which is a very rough

estimate at 2.75 ym, is the X= 5.3 ym value

from C. Willingham, D. Bua, H. Statz, and

F. Horrigan, Raytheon Research Division

Final Technical Report, Contract DAAH01-74-

C-0719, Aug. 1975; and B(As 2 S

3

) = 31 cm

-1

, which is a very rough estimate at 2.75 ym,

is from the value 3(As2S

3

) = 16 cm ^ at X= 10.6 ym, from John F. Lewis and Melvin C. Ohmer in Proceedings of a Symposium on Laser Induced Damage in Optical Materials: 1976 , NBS Spec. Publ. 462, edited by A. J. Glass

and A. H. Guenther, Boulder, Colo., July

13-15, 1976, p. 279.

P. H. Lissberger, Rep. Prog. Phy. 33, 197

(1970)

.

[5] 0. S. Heavens, Optical Properties of Thin

Solid Films (Dover, New York, 1965),

chapter 4.

[6] Joseph H. Apfel, John S. Matteucci, Brian E. Newnam, and Dennis H. Gill in Proceedings of a Symposium on Laser Induced Damage in Optical Materials: 1976 , NBS Spec. Publ. 462,

edited by A. J. Glass and A. H. Guenther,

Boulder, Colo., July 13-15, 1976, p. 301.

[7] Joseph H. Apfel, App. Opt. 1_6, 1880 (1977);

Appl. Opt. 15, 2339 (1976); in Proceedings

of a Symposium on Laser Induced Damage in Optical Materials: 1977 , NBS Spec. Publ. 509,

edited by A. J. Glass and A. H. Guenther,

Boulder, Colo., Oct. 4-6, 1977, p. 251.

[8] Brian E. Newnam, Dennis H. Gill, and George

Faulkner in Proceedings of a Symposium on Laser Induced Damage in Optical Materials:

1975 , NBS Spec. Publ. 435, edited by A. J. Glass and A. H. Guenther, Boulder, Colo.,

July 29-31, 1975, p. 254.

[9] Dennis H. Gill, Brian E. Newnam, and John

McLeod in Proceedings of a Symposium on Laser Induced Damage in Optical Materials:

1977 , NBS Spec. Publ. 509, edited by A. J. Glass and A. H. Guenther, Boulder, Colo.,

Oct. 4-6, 1977, p. 260.

[10] M. Sparks, J. Opt. Soc, Am. 67, 1590 (1977).

[11] M. Sparks and M. Flannery, to be published.

[12] G. Koppelmann, Ann. Phys. (Leip.) ?, 388

(1960)

.

[13] M. Flannery, M. Sparks, and E. Loh, Jr., Appl. Opt., in press.

[14] M. Sparks, Theoretical Studies of High-Power

Ultraviolet and Infrared Materials , Xonics

Eighth Technical Report, Contract DAHC15-73-

C-0127, Dec. 31, 1976; M. Sparks and E. Loh, Jr . , Theoretical Studies of High-Power Ultraviolet and Infrared Materials , Xonics Final

(Eleventh) Technical Report, Contract DAHC15-,

73-C-0127, March 31, 1978.

[15] The values of nL

(ThF

4

) = 1 . 527 and n

H

(ZnS)

= 2.24 at X= 2.75 ym are from M. Sparks,

Theoretical Studies of Materials for HighPower Infrared Coatings , Xonics Sixth Tech- nical Report, Contract DAHC15-73-C-012 7

,

Dec. 31, 1975, p. 139; and P. Kraatz in Proceedings of the High Power Laser Optical

Components and Component Materials Meeting , compiled by James S. Harris and Charles L. Strecker, Boulder, Colo., Oct. 3-4, 1977,

p. 401.

[16] J. R. Bettis, R. A. House, A. H. Guenther,

and R. Austin in Proceedings of a Symposium

on Laser Induced Damage in Optical Materials:

1975 , NBS Spec. Publ. 435, edited by A. J. Glass and A. H. Guenther, Boulder, Colo.,

July 29-31, 1975, p. 289.

282
2t ~ -J

24 • 1 -

—

(St

? 03 /$i)* (ASjSj/CeSe)"

/ /

I \ \ \ / J V \ ^-^^

— 1 1 1

*

0 0.5 1.0 1.5 2.0 Normalized frequency, u/ut (dimens ionless)

(a)

1 1 (Al,0, /Si)"

(

1 1

A ' X

/ " -

~~

~ - ~\ /

1

^~(As,S,/Ge$el*

2 3

1 1 0.5 1.0 1.5 2.0 Normalized frequency, w/i« (dimens ionl ess)

(b) Figure 2. (a) Large-scale spectral reflectance of lossless infinite quarter-wave reflectors showing

the perfect reflectance in the high-reflectance band, 1 - A<w/wc

< 1 + a, the rapid decrease in reflectance outside the band, and the even symmetry of the curves about the center frequency ^c

= 2itc(2.75 um)" 1

. (b) Phase of the electric field for the reflectors in (a) showing the zero

phase at the center frequency wc , the maximum phase amplitudes at the band edges, and the odd symmetry of the curves about wc

. Data are from references 2 and 3.

283
Figure 3. Standing-wave electric fields E in the infinite lossless quarter-wave reflector

(Aj^C^/Si)" showing: the peak fields at the H-L interfaces for and the peak fields in the centers of the low-index (unshaded) or high-index (shaded) layers at the band edges ^/^c

= 1+A,

respectively; the rapidly decreasing field amplitude for^>=^c ; the constant field amplitudes for

u^/oj = 1 +a; and the constant stack wavelength A = ~x(n ^+ n for all frequencies in the perfectc 2 H L reflectance band. Data are from reference 2.

Figure 4. Standing-wave electric fields E in a three-pair quarter-wave coating deposited on a silver

substrate showing: the relatively rapidly decreasing field amplitudes even near the band edges, in contrast to the infinite-stack constant-amplitude fields at oj/i>c =l+A; the change in the stack wavelength A with u, in contrast to the constant A for the infinite stack; and the approximate node in the fields near the silver surface at all frequencies. Data are from reference 2. 284
Figure 5. Fine-scale spectral reflectance for (ThF^ZnS)". In (a), for loss in L only, A is greater for uic

<co since E max occurs in L for w c

<lo. In (b), for 6h

= 2i» A still is greater

for ^

c <w. The center slope is zero for 3L/ n L = &H/ nH> f rom (3). In (c), for loss in H only, A

is greater foroj<u)C

. Data are from references 14 and 15.

(Th F

4 /2nS)"

P„ = 0

, A^ (w) : 1 inear approximation \ from (2)

Ml

(ThF

4

/ Zn S) , loss

in coating, exact

Vy^\ M '• quadratic \ approximation (2) \ ww

\ \

\ \

I \i

lir-laser emiss ion band

~i

0.9

Normalized frequency, uj/li c (dimensionless)

(a)

Fi gure 5 (a)

(Th F. / Zn S)"

A^fw): linear approximation

from (2)

I

// // //

/

/

I

I

A^ (u>) : quadra ti

approximation (2)

I (ThF./ZnS) ,

\ \

HMaser

emission

| <

ba " d >|

Normalized frequency, w/w

c (dimensionless)

(b)

Figure 5 (b),

285
1 00 . 0- 0.0

A^fw) : 1 inear approximation

from (2)

A {w) quadratic approximation {2)

I

(ThF

4 /Zn$)

loss in coating, |r exact

! H

/

' 1/ /

-M- h—

Normalized frequency, u/w (dimension! ess)

(c)

Figure 5 (c)

.

Figure 6. Fine-scale spectral reflectance of a lossy three-pair dielectric stack on silver,

Ag(A£2<33/Si

)

3

, showing: the approximate absortance (ni/nH) 6 As of the silver substrate

with a lossless three-pair coating (light dotted curve); the good agreement between the exact reflectance for the infinite stack (A^C^/Si )°° (dashed curve) and the quadratic approximation (2) (dashed-double-dot curve); and the excellent approximations (5) and (2) (heavy dots) to the exact A(u>) for the Ag(A£203/Si )3 reflector (heavy solid curve). Data are from reference 2.

286
Ag (As

2

S

3 / Ge Se)

Normal ized frequency, u/u> (dfnens(onless)

Figure 7. Fine-scale spectral reflectance of a lossy seven-pair dielectric stack on silver,

Ag(As2S3/GeSe) 7

, showing: the reflectance of an Ag (As2S3/GeSe) 7 reflector with no loss in the coatings; the asymmetric reflectance of a lossy infinite stack (As2S3/GeSe) a>

; the agreement

between the quadratic approximation (2) and the infinite-stack reflectance; the approximations (5)

(2) (solid dots) and (5) with A„(^) calculated exactly (plus signs) to the exact A(^) for the

Ag(As2S3/GeSe) 7 reflector with loss in the coating; and the lossy infinite-stack approximation

(double-dot dashed curve). Data are from reference 3.

and

Discussion

If this analysis were applied to ultraviolet coatings , the effects of scattering would have to

be considered. It was pointed out that uv scattering in multilayer dielectric films is a complicated function of wavelength, due to standing-wave resonances . On another subject, the speaker indicated that in well-designed films, the optimal frequency shift depends only weakly on

the number of layers, since the multilayer stack approximates the infinite stack very closely .

287
ELECTRIC FIELDS NEAR COATED SURFACES:

APPLICATION TO DAMAGE PROTECTION

Herbert B. Rosenstock

U. S. Naval Research Laboratory

Washington, D. C. 20375

Damage to transparent materials from intense light pulses

usually begins at the exit surface. This is usually explained by

"constructive" interference between incident and reflected waves: The total electric field may exceed the threshold for damage only

upon reflection from the exit surface. However, in case of a surface coated with a thin film, interference may either increase

or decrease the total electric field. Details depend on the

optical constants, thickness, and wavelengths. A film designed

to provide protection against excessive electric fields at one wavelength may not do so at other wavelengths. Also, attempts to minimize the field in the material proper may entail novel problems

in the film, in which the damage mechanism may be an absorptive one. Key words

:

Damage threshold; Electric fields; Internal reflection; Protective coatings

INTRODUCTION AND SUMMARY

The most common use of optical coatings is to eliminate reflection and resulting

losses; the body of literature on such "AR-coatings" is large. Here we consider

briefly another use of coatings -- to avoid damage in systems exposed to pulsed laser

beams

.

Damage to transparent materials exposed to short, intense pulses usually begins

at the exit surface [lj^ (not in the bulk or the entrance surface). This is usually

explaiu-.d by the "interference" (more simply put, the addition) of the beams incident

on and reflected by the exit surface. For an uncoated material (provided its index

of refraction is greater than 1, as is almost always true) elementary electromagnetic

theory predicts that the "total" electric field (incident plust reflected) is larger

than the incident alone; since it is high electric fields that are believed responsible for damage to transparent material [2] (by avalanche production or a similar

process), the damage threshold is first exceeded just above the exit surface.

The main purpose of this note is to show that this argument is not necessarily

valid if the exit surface is coated. In that case, the "total" field can, in fact,

be smaller than the incident field alone (i.e., coatings can make the interference

"destructive" instead of "constructive"). Coating the exit surface of a material can

thus protect it against damage from intense pulsed beams.

A related problem that must be considered is the possible damage in the coating

itself. The electric field there is not hard to compute theoretically (it is, in

fact, the same on the two sides of the material-coating interface), but the damage

mechanism in the coating may be different (viz. absorptive), and possibly severe. TRANSMITTED AND REFLECTED ELECTRIC FIELDS AT A SINGLE INTERFACE

Here, we outline the derivation of needed results for the behavior of an electromagnetic wave at an interface. Our purpose is to provide clear definitions and assurance that the assumptions are few and fundamental. More details can be found in textbooks [3].

1. Figures in brackets indicate the literature reference at the end of this paper.

288
The boundary conditions for a light beam at an interface are the continuity of

the tangential component of both E and H. For normal incidence, the tangential com- ponents of E and H are simply E and H themselves. So, following the notation of

figure 1, we have E

Q

+ Ergfl = Etr and H

Q

+ Hrefl = H. . Also, E and H are related

The - sign indicates

that either E or H must undergo a phase change of 180 degrees upon reflection since

propagation is now in the opposite direction. Eliminating the H's from these expressions gives Ere j^ and E^. r in terms of Eq,

and similarly

p

^ U rt |,

' - l^'V ^[{W).

Here A and B are the complex indices of refraction,

ft

"= ~ L K ft

in the usual notation.

Division of eq. (1) by Eq now gives the conventional transmission and reflection

coefficients for electric fields and similarly, from eq. (2) for magnetic fields:

(1)

(2)

r

'; e , (A -13) | Cfl + 2;

V - Y - - T

(3)

(4)

(5)

(6)

The energy transported in a beam is given by the mean value of the Poynting

vector E x H; in case of a plane wave this becomes [4] I = |Re (E x H*) or, since E and H are perpendicular, I = |Re(EH*). Substitution of eqs. (1) and (2) into

this expression then gives the transmitted and reflected energies. The transmission

and reflection coefficients for energy are then obtained by dividing by the incident

energy

'tie.

h% ^- 'I (7)

(8)

289
EFFECTIVE COEFFICIENTS IN THE PRESENCE OF A FILM

We next calculate the effective transmission and reflection coefficients for

both fields and energies of a thin film F between two media A and B, as shown in

fig. 2. Since the film is thin and reflections therefore coherent, we must calculate

the total electric and magnetic fields due to multiple reflections first and then,

having added these separately, calculate the energy from the two sums. So let us first go through the calculation for the electric fields. Proceeding step by step in

fig. 3 from E^ to Eg to Eg . .

. , we find

and so forth. In words, , where A is the wavelength in vacuum (in cm)

and ^ the thickness (in cm), appears as a factor whenever the film is traversed once, and the appropriate reflection coefficient (known from eqs . (6) and (4)) appears as a factor upon reflection. Now adding Eg + Eg + E^q . . . and summing the resulting geometric series gives

\r- i,G>, to - ..

(9)

similarly, we find

j<L)g-|t with A ^ I— ^fl^PB^ (io)

The total transmitted field is, from fig. 2, seen to be

where we used eq. (9), or again looking at fig. 2

Similarly for the reflected electric field we find

(11)

(12)

The calculation for magnetic instead of electric fields is quite the same; merely

replace E by H both in the field quantities and superscripts, and obtain

b frP L

/ ^ (13)

instead of eq. (11), and

(14) H H H 2 instead of eq. (12). Here A = 1 - is analogous to eq. (10), and from eq. (6),

xj r d iA

therefore A = A . (15)

Let us next get rid of the r.H. and t.H

. coefficients in favor of r.EE. and t. ..

290
With eqs. (5), (6) and (15), eq. (13) becomes

HJHo =5 t

e

ff1 c /a

and eq. (14), using eq. (15) for A, eq. (5) for t, and eq. (6) for r, becomes

Hvc|t/ \\ -= - V»£ - f^O^r^^ (17)

Knowing the total reflected and transmitted electric and magnetic fields, we can now use eq. (13) to get the reflected and transmitted energies, and define the coefficients tfro .= Xir ji, ^/hj- Iha I7' These then turn out to be

(19)

Br3 I > '

I (V~ ra r A (18)

TOTAL FIELD ABOVE AN INTERFACE

We now show that the total electric field may be increased or decreased by

reflection at a surface, while the net energy flow is always decreased.

It is easy to show from our formalism that when the surface is bare (no film on

it), the total field increases. Consider eqs (la) and (2a)

when both A and B are real (no absorption) and A is greater than B (back surface).

Then is i n same direction as Eq and

v*t r (20)

is greater than E

Q

. The opposite is true for the front surface where A < B, and the

opposite again for H. The total energy (3) is, with (1) and (2),

and can be written

Since the last ( ) is purely imaginary, we find simply

which, since rr* is positive, is always smaller than I Q

. The situation is less simple when a film is on the surface. We develop a general expression for this situation, and show that can be either greater

or smaller than E

Q

, depending on circumstances. The reflected electric and magnetic

fields are now given by (12) and (17) which we write as

291
where

f IE ^ - ^ ? VL ^T ^Ja " (21)

We need the total field (incident plus reflected - see fig. 3),

^ +2>

tot

(22)

and find after a little algebra, that we can write

or even simpler

(23)

(24)

where

(25)

Again the fact that ^ foi ^ '

0 can be established at once from (3) and (23):

« L £kUi-^; fi-s)*J

I- 9^ 1

which is smaller than Iq always.

Evaluation of \ ^~h>\ \ for all values of the parameters is a little more involved.

We will later show some numerically calculated examples, but first consider some special cases analytically to show directly that this quantity can be either greater

or smaller than Eq. We consider three cases: CASE I. ^ - 1 or ' -J (j = integer)

with kp = 0, nonabsorbing film.

CASE II. T - - ) or t)\ - h 0 J 1

with kp = 0, nonabsorbing film.

CASE- III. \^ " D

, fully absorbing film, Kp ^/^ ^

CASE I. (21) becomes, after a little algebra,

(26)

which, by (l),we recognize as the^ref lection coefficient in absence of a film. So

if the thickness is such that \ - -+ I -.the film has no effect at all. If A > B, E f^is in phase with E

Q

and Etot therefore greater than E

Q

.

292
CASE II.

In this case, (22) becomes, again with a little straightforward algebra,

(27)

which can be negative, zero, or positive depending on the values of A, B=, or F. The

value zero is attained then F = VA~B, the familiar result for an anti-reflection

coating [5] The total electric field is, by (23)^ given by

Em - £ o T-An-jQn-iF-)

<28)

which can be either less or more than E„.

u CASE III. 0

? fully absorbing film

We see from (21) that g = (A-F)/(A+F) and

with F now purely imaginary. So again Lt.fc can be either larger or smaller than

Eq, depending on the values of A and F. Turning next to magnetic field, we see from sec. Ill that

\o~ ~ E*fcl£* (29) so that, in the notation of (21),

yi„r - n.a

(30)

and so analogous to (23)

5) (31)

So for Case I it again follows (from (24) and (6)) that the film has no effect at

all. For Case II, (31)i) givesves ,

(32)

which again can be ^ 0 depending on whether E. Qt >

Eq.

Some graphs depicting special cases and showing how ]£f»t/ / can vary

as a function of film thickness and optical constants are shown in fig. 4. In each

case shown, total fields below the incident are attainable for some film thickness.

(For k = 0, continuation of the plot would be periodic in B/X. ) Finally, we should note explicitly that the total electric field as plotted is

the same just above and just below the material-film interface. (This follows

directly from the boundary conditions on which sec. Ill is based.) A film of optical

constant and thickness appropriate to the optical constant of the material and the

expected wavelength of the impinging radiation can therefore not only reduce the

total field in the material, it will also produce a smaller field in the film. However, since the damage mechanism in the film may be of a different nature (i.e.,

absorptive), a new set of problems may arise from its presence.

293
ACKNOWLEDGMENTS

Some of the calculations reported here were begun in connection with experimental work of Drs . E. D. Palik, N. Ginsburg and R. T. Holm [6]. I am grateful to

Dr. M. Hass for many discussions.

REFERENCES

[1] M. D. Crisp, N. D. Boling and

G. Dube', Appl. Physics Letters 21 , 364 (1972).

[2] M. Sparks, Proc . Symposium (Boulder,

Colorado 1975) NBS Special Publication #435 (A. Guenther and A. J. Glass, eds., April 1976).

[3] 0. S. Heavens, Optical Properties of

Thin Films (Dover, New York, 1965);

A. Vasicek, Optics of Thin Films

(NorTTh Holland, Amsterdam, 1960).

[4] M. Born and E. Wolf, Principles of

Optics , fourth edition (Pergamon,

Oxford, 1970), page 33.

[5] Reference [4], page 64.

[6] E. D. Palik , N. Ginsburg, H. B. Rosenstock and R. T. Holm, Applied

Optics (in print, 1978).

Figure 1. Field at one interface.

Figure ?. Multiple reflections in a fill

Figure 3. Sunnary of figure ?..

294
295
COMPUTER SIMULATION OF LASER DAMAGE MORPHOLOGY IN THE ALKALI -HAL IDES

P. Kelly and D. Ritchie

Physics Division, National Research Council

Ottawa, Ontario K1A OS!

and

P. Braunlich and A. Schmid

Washington State University

Pullman, Washington 99163

A numerical model of the spatio-temporal behaviour of a pico-second laser pulse has been developed which allows the study of the interaction of macroscopic photons with electrons in an alkal halide lattice. Trajectories of the photons are computed. Gradients in the refractive index change

the direction of the photons and this can lead to focusing or defocusing of the pulse. Rotational

symmetry is assumed for both the pulse and background. Results obtained to date will be presented.

On behalf of my colleagues, I present our progress to date on the investigation of the spatial extent of damage produced by pico-second

pulses in the Alkali-Halides. I refer first to our work presented at this symposium last year (1), and subsequently published (2), wherein we showed that spatio-temporal analysis was warranted and that the inclusion

of the refractive index appeared as an important element in analysis.

I also call to your attention the excellent work of Smith

et al . (3), who presented interesting measurements of the damage morphology.

They showed, that with a 21 psec (FWHM) doubled Neodynium pulse,

damage in NaCl consisted of spatially distinct damage vestiges along the propagation axis of =ly in diameter and defined the flux, F, as the threshold F-^. With F ^ 1.5 F-TM the damage vestige extends 'vlOOp by <v!p.

Can the polaron model of damage explain such behaviour? This

is the' question to which we now address ourselves. The non-linear

behaviour of the interaction of such a monopulse with NaCl dictates

computer simulation of the problem (while, nevertheless, we still look for analytic solutions).

COMPUTER MODEL

The choice of computer model is based on a new computer

technique developed by Elliott and Henderson (4). Therein, the theoretical background has been supplied as well as applications to various physical problems. Their corpuscle - in-medium treatment is more fully described by D'udder and Henderson (5) in a macroscopic photon

in cell (MPIC) computer code.

To date a modified version of the MPIC method has been used

to simulate the experimental conditions of Smith et al . (3), where the interaction is between a monopulse and a polaron gas. Starting then with the MPIC program, the following changes have been made:

1. the refractive index, n(w), is given by photon-optical phonon-pol aron

i nteracti on ( 2)

,

2. the group velocity is calculated from

where Vp = c/n (and is equivalent to that of Elliott and Henderson (3) when e << e )

.

296
3. the equation of motion of a macroscopic photon is given by

^Hiit) = vn

so that,

At = (ij. • vn) tx

AS

with AS = VgAt/n

4. the ionization fraction is defined as

9 - n

p /N

v (0)

where n

p is the generated polaron concentration and N

v (0) is the initial valence band concentration and g = (l-g)R where R = (instead of a Keldysh mechanism).

5. The grid was 50x50 with 8096 particles, i.e., 4 per active cell.

2 6. A full Gaussian pulse was employed with clipping at the 1/e points.

7. Shrinking and stretching of the grid in the radial direction was allowed but no particles were allowed to be "lost".

8. A 21 psec FWHM Gaussian pulse of 2.33 eV photons was focused by

a 2.54 cm focal length lens to vacuum focal spot located 0.2 cm inside a NaCl crystal.

9. Rotational symmetry of the pulse and medium was assumed.

10. The initial energy of the pulse was 157x that of Smith et al . (3), to produce maximum fluxes of the same order as FTU (in our

cells). TH

A series of slides was presented illustrating the behaviour

of the power density in cells located above the propagation direction

and the corresponding profile of the ionization fraction. The focal volume is in front of the vacuum focal spot. Figures 1-6 are representative of the series. The MPIC program (presently) places a grid over the pulse travelling through the medium, which for this case produces a severe lack of resolution.

From the work to date it is clear that it will be practical

to consider placement of the grid over an "interaction volume" in the

NaCl sample with a spacing which should yield the morphology to micron

size. Of course, refraction at the interface, as well as, absorption

by the beam will then be included in the analysis.

(1) A. Schmid, P. Kelly and

P. Braunlich, 465 (1977) "Laser Damage in Optical Materials:

1977", NBS special publication

509, US Government Printing

Office, Washington, D.C.

(2) A. Schmid, P. Kelly and

P. Braunlich, Phys. Rev. B, -16, 4569 (1977).

(3) W.L. Smith, J.H. Bechtel and

N. Bloembergen, "Dielectric

Breakdown Induced by Picosecond

Laser Pulses", Tec. Report 685,

Harvard University, Cambridge,

Massachusetts (1976).

(4) C.J. Elliott and D.B. Henderson,

J. Appl . Phys. 46, 354 (1975)

(5) H.D. D'udder and D.B. Henderson,

Comp. Phys. Comm. 10_, 155 (1975)

297
Log <XX> 1

LCK3 «X>

-2 6f| i

Log F*X>

30.3. MSTEP - lie

Log «x>

-2 3

Fiqures 1-6 Top- Plots of the intensity profile from cells along the propagation axis, as a function

of actual penetration of a 21-psec (FWHM) monopulse into NaCi.

Bottom: Corresponding ionization fraction profiles, f - Locates geometric focus (in cms)

Discussion

The speaker indicated that the model currently does not conserve energy in the presence of

absorption and that, as presented here, it does not model avalanche but could be modified to do so. The assumed four-photon absorption is intended to model experiments carried out with doubled

neodymium laser irradiation.

298
THE RELATIVE ROLE OF IMPACT AND MULTI PHOTON IONIZATION MECHANISMS IN LASER INDUCED DAMAGE OF TRANSPARENT DIELECTRICS

B. G. Gorshkov, A. S. Epifanov, and A. A. Manenkov

Lebedev Physical Institute

Moscow, USSR

The results of a theoretical analysis of the electron avalanche process at high frequencies of the electromagnetic field are presented. The dependence of the avalanche rate on radiation intensity and of the critical field on frequency are obtained for nanosecond and picosecond pulse durations. The relative role of electron avalanche and multiphoton ionization in laser damage

of transparent dielectrics is discussed and it is shown that it is strongly

influenced by the pulse widths.

Key words: Damage threshold; electron avalanche; frequency and pulse width

dependencies of the critical field; multiphoton ionization.

1. Introduction

Impact and multiphoton ionization are expected to be the main fundamental processes limiting the resistance of transparent dielectrics to high intensity laser radiation. The role of these processes

in laser induced damage in real optical materials of different types, ionic crystals in particular,

has been extensively studied during the last decade. This especially refers to the impact ionization process (usually referred to as the electron avalanche mechanism.) Recently rather detailed

theoretical and experimental investigations have been carried out in the Lebedev Physical Institute

[1-5]. (The results of these investigations have been discussed by Manenkov at the IX Damage

Symposium [6].) It has been found that the frequency and temperature dependencies of the damage

threshold observed in some highly resistant samples of alkali halide crystals are in rather a good

agreement with the theoretical predictions, giving a reasonable estimate of the electron-phonon

collision frequency (v - 6 x 10 '4 sec"! f or N a Cl). However, some experimental results concerning, particularly,- the temperature dependence of the damage thresholds observed at low and high radiation frequencies have not been explained by the avalanche ionization theory. It has been supposed that the disagreement between the observed and theoretical temperature dependences at high

frequencies (X = 0.53 urn) might be attributed to multiphoton ionization as an effective mechanism for generation of free carriers.

These facts motivated us to investigate theoretically the peculiarities of the avalanche

ionization process in more detail for high frequency radiation, and to analyze the relative role of the impact and mult oton ionization mechanisms in the laser damage of transparent dielectrics. The principal results ot this work are briefly described in this paper.

2. Results of Theoretical Analysis

The relative contribution of these two processes to the free carrier generation rate, which is characterized by the avalanche and multiphoton rate parameters, y and Wn respectively, has to be strongly influenced by the dependence of y and Wn upon the intensity (E^) and frequency, ft , of the electromagnetic radiation. From consideration of the multiphoton transitions, it follows that in the pulse duration range of t = 10~7 - 10 - ^ sec the rate Wn is proportional to E^n if n £ 10. As far as the intensity dependence of the avalanche rate is concerned, it can be considerably

stronger. As has been shown in previous papers [1-3], for electromagnetic breakdown in the nanosecond pulse duration range, the coefficient y depends on radiation intensity exponentially. For breakdown in the picosecond range of pulse widths, the y{E) dependence is expected to be considerably weaker. Therefore, in order to elucidate the question of which of the tw"> free-carrier

generation mechanisms, impact ionization or multiphoton ionization, is predominant, one has, first of

all, to study in detail the intensity dependence of the avalanche rate over a wide range of pulse

durations and frequencies of the electromagnetic field. In order to find the y{E) and E

c

(fi ) dependences (E c is the breakdown critical field strength) the quantum kinetic equation for the electron distribution function [5] which satisfactorily describes the avalanche process at relatively low radiation frequencies f/fe << I, where I is the effective ionization potential [1-3],)

has been solved without the diffusional approximation.

The solution has shown that in the nanosecond pulse duration range, for relatively large

radiation quanta (I/tin = n, n < 4), y ^ e2(" + U, whereas for the smaller quanta (n > 4), y becomes

an exponential function of E [1-3]. To illustrate such behavior, the y(E) dependences calculated

for various photon energies are shown in Figure 1. y{E) dependence has also been calculated for the picosecond range of radiation pulse durations. From the data for Y(E), and using a damage

299
criterion connecting the avalanche rate and the radiation pulse duration [l], we have obtained the frequency dependence of the critical field, which in practice is one of the most important damage

characteristics. It has been found that the character of the E c

(fi) dependence is rather strongly

influenced by the pulse duration. This dependence calculated for two values of x is shown in Figure

2. One can see that at relatively low frequencies (hfi/I < 0.15) the diffusional relationship

El * (fi2 + Veff2 ) (1)

satisfactorily decribes the E c(fi) dependence in for both picosecond and nanosecond pulses, whereas

in a high frequency region (rTQ/I > 0.2) the behavior of Ec

(fi) is very different for the two cases:

for picosecond pulses the E C (Q) dependence remains similar to the diffusional one whereas for nanosecond pulses, Ec

(fi) has a nonmonotonic, oscillatory behavior. The latter is due to the oscillatory form of the energy distribution function for hot electrons in high frequency

electromagnetic fields.

For elucidation of the relative role of electron avalanche and multiphoton ionization in laser damage, the kinetics of lattice heating due to absorption of laser radiation by free electrons

generated in both processes has to be considered. A change of the free carrier density, N, in the conduction band of a crystal is described by the equation ^ = YN + W

n

- R(N) , (2)

where R(N) is the term describing recombination of the carriers.

Solving Eq. (2) together with the coupled equation for the lattice temperature 9 = T/T0

:

^-=36K N, (3)

where T0

is the initial temperature of the lattice. k and 3 are parameters depending on the electron-phonon interaction [3]. One can obtain a relationship between the pulse width and ionization rates y and Wn required

for a catastrophic temperature rise during a radiation pulse. The final step of this procedure gives

the critical field values or the damage threshold intensities.

Figure 3 shows the dependence for the critical field parameter q c

, related to E

c , upon the pulse width for the two mechanisms of free carrier generation, avalanche and multiphoton ionization.

One can see that two significantly different cases are possible, depending on the multiphoton

generation rate; either the q c ("0 curves corresponding to the avalanche and multiphoton ionization

are intercepted or not. It means that in the first case there is a competition between the avalanche

and multiphoton ionization processes at some pulse widths which can, in general, lie in both the picosecond and nanosecond regions. In other words, in this case the electron avalanche can be the dominant mechanism of laser damage over a wide range of pulse durations, including both the picosecond and nanosecond regions.

In the second case, where the avalanche and multiphoton curves do not intersect each other at any pulse widths, the dominant mechanism of intrinsic damage is associated with multiphoton

ionization.

3. Conclusion

The features established in the theoretical study of avalanche and multiphoton ionization

concerning, in particular, the frequency and pulse width dependences of the critical field, can be used in experimental investigation to elucidate the dominant mechanism of laser induced damage in optical materials.

4. References

[1] Epifanov, A. S. JETP, 67, 1805 (1974)

Soviet Physics JETP 40, 897 (1975).

[2] Epifanov, A. S., Manenkov, A. A. and Prokhorov, A. M. , JETP Pis'ma, 21, 483

( 1975) . [JETP Letters, 2]_, 223"Tl975)].

[3] Epifanov, A. S., Manenkov, A. A. and Prokhorov, A. M. JETP, 70, 728 (1976).

[Soviet Physics, JETP, 43, 377 (1976)].

[4] Gorshkov, B. G. Danileiko, Yu, K.

,

Epifanov, A. S. Lobachev, V. A. Manenkov,

A. A. and Sidorin, A. V., JETP Letters, 2, 284 (1976).

300
[5] Gorshkov, B. G. Danileiko, Yu, K., Epifanov, A. S. Lobachev, V. A. Manenkov,

A. A. and Sidorin, A. V., JETP, 72, 1171 (1977).

[6] Manenkov, A. A. NBS Spec. Publ. 509,

p. 455 (1977).

Figure 1. Dependence of the electron avalanche rate Figure 2. Frequency dependence of the critical

on the radiation electric field amplitude at various field for picosecond and nanosecond pulse durafrequencies ( 1/ = n). Notations in field tions. The values used in calculation refer to parameter q: e and m are the electron change and Had.

effective mass, respectively, v s is the velocity

of sound.

oaoi o.oi ii i io m

Pulse width, r (ns)

Figure 3. Critical field versus pulse width for the avalanche and multiphoton ionization damage mechan

isms. The values used in calculations refer to NaCjt.

301
Discussion

The work of Manenkov, et al. , was challenged on two points, both relating to scaling laws. First, it was pointed out that previously reported data taken at Harvard showed the familiar

t^^ dependence for breakdown intensity for pulse durations from 30 psec to 10 nsec. Manenkov'

s

theory would predict a much weaker dependence, but his analysis does not include either electron or

thermal diffusion phenomena. Secondly , neither Manenkov's theory nor his data show strong spot-size

dependence, although other data reported here and elsewhere do show a threshold decrease with

increasing spot size. the speaker agreed that if his experimental results (and theory) refer to intrinsic damage, then a strong spot-size dependence would not be expected.

302
THEORY OF LASER DAMAGE IN DIELECTRIC SOLIDS*

Steven Brawer and W. Lee Smith

University of California

Lawrence Livermore Laboratory

Livermore, California 94550

In this theory a new approach to the interaction of conduction band (CB) electrons

with damaging radiation is considered. We assume band gaps greater than 4 ev and radiation frequencies greater than 1 ev. Two processes are required for damage. First,

the incident radiation rapidly excites a few "primer" electrons from trap levels to the bottom of the CB. Next these primer electrons can either cause enough energy to be absorbed to cause damage without avalanche, or they can trigger an electron avalanche

and subsequent damage. In either case a mechanism for the excited-state absorption

process is required.

Recent work [1] - [6] on excited-state absorption in dielectrics shows that the low-lying CB states are localized. The applied field can induce di pol e-al 1 owed transitions among these levels. Dipole-allowed interband transitions can also occur

among more extended states. If avalanche is to occur, the CB electron must gain at least an amount of energy equal to the band gap energy. Since a large number of electronic states are involved, we model the situation by assuming a continuum of possible states and an average transition probability among them. Transitions up this continuum take place by a sequence of one-photon absorptions.

The electron can also "backslide", losing energy by phonon emission. A minimum

value of the electron-phonon relaxation time must be inverse (angular) phonon frequency

for the dominant phonon, about 5 x 10" 15 sees for a 1000 cm

-1 phonon. This is the average time required to emit one phonon.

Assuming an oscillator strength of unity, the average electron cross section for absorption is about 10" 15 cm

2

. The probability/time that an electron can absorb one

phonon, for a field strength of 1 MV/cm and frequency of 1 micron, is about 10 13 /sec,

comparable to the rate of generation of ten phonons. (It requires 10 phonons to backslide an amount of energy equal to one absorption step in this example.)

It is now assumed that when a CB electron gains energy equal to the bandgap

energy, it excites an additional electron from the valence band to the CB. After

this excitation, both electrons are in the low levels of the CB. In this way, an avalanche is triggered.

These assumptions are sufficient to write equations describing the energy and number of CB electrons as a function of time, and of the heat delivered to the lattice.

Damage is presumed to occur when the lattice is heated to the melting point. We come to the following conclusions. (We assume for this discussion that the electronphonon relaxation time is 5 x 10" 15 sec and the absorption cross section is 10~ 15 cm

2

. These parameters can vary for each case, so the numbers below are only an indication

of the trends and should not be given a rigid, absolute significance.)

1. For a field of 1 MV/cm and pulse duration about 10 ns, about 10 16 electrons/cc

in the CB will absorb enough energy to cause damage. No avalanche is needed.

2. If avalanche occurs, we predict a relation between the pulse duration x and the threshold electric field E . For small fields < .25 MV/'-.m the

o %

quantity

^ ^— is predicted to be about A/hw where A is the band gap

o energy, for fields - 10 MV/cm the slope becomes smaller than 2.

3. For given pulse duration, we predict that the threshold decreases markedly

with increasing frequency for small fields and is nearly independent of

frequency for large fields.

These predictions are in qualitative agreement with experiment.

*Work performed under the auspices of the U.S. Department of Energy under Contract No. W-7405-EN6-48

and the Materials Sciences Program.

303
References

[1] R. T. Williams and M. N. Kabler, Phys. Rev.

B9, 1897 (1974).

[2] R. T. Williams, M. N. Kabler, W. Hayes,

and J. P. Stott, Phys. Rev. BM, 725

(1976).

[3] R. T. Williams, C. L. Marquardt, J. W. Williams, and M. N. Kabler, Phys. Rev. B15 , 5003 (1977).

[4] J. W. Appel in Solid State Physics, vol. (edited by F. Seitz, D. Turnbull, and

H. Ehrenreich, Acad. Press, N.Y., 1968).

[5] F. DeMartini, D. Frigione, G. Guiliani,

P. Mataloni, and F. Simoni in Nonl inear

Spectroscopy (edited by N. Bloembergen,

North Holland, Amsterdam, 1977).

[6] R. T. Williams, J. N. Bradford and W. L. Faust, in Optical Properties of Highly

Transparent Solids (edited by S. S. Mitra

and B. Bendow, Plenum, N.Y., 1975), p. 233

Discussion

It was pointed out that the theory proposed by the speaker is similar to the "cold-electron"

model of damage proposed by R. Hellworth at this Symposium in 1972. In order for damage to occur without avalanche, in this theory, a concentration of 10^ to 10^-^ impurity or defect

centers, capable of single-photon ionization, would be required. It is assumed that these centers are weakly absorbing in the ground state (otherwise the material would be strongly absorbing ) but

that, when electrons are liberated from these centers, the cross section for absorption is as large

as 10~15cm2 m Tne speaker noted that, except in the purest semiconductors , impurity

concentrations of parts per billion are commonly found. Dr. Peter Braunlich of Washington State University pointed out that one could test the model using a material with a large number of

impurity or defect states, at low temperature , by comparing its conductivity, or damage threshold

,

with and without the liberation of electrons from the impurity centers via uv radiation. One can also look for different statistics of damage onset in the two cases.

304
LASER- INDUCED DAMAGE IN SEMICONDUCTORS

Yu.K. Danileiko, A. A. Manenkov, and A. V. Sidorin

Lebedev Physical Institute

Moscow, USSR

The laser-induced bulk breakdown in Ge, Si and GaAs crystals at the

10.6 urn, 2.76 ym, and 2.94 ym radiation wavelengths of the pulsed CO2,

Ca 2 F:Er 3+ and YAG:Er3+ lasers is investigated. Generation of free carriers has been observed and their kinetics studied by the dc and microwave

photoconductivity measurement techniques. In Si and GaAs, the laser-induced

bulk damage has been observed and the damage threshold measured. In Ge, the damage was not observed due to a self -defocusing effect associated with the negative contribution of nonequi 1 ibrium electrons in the refractive index. Mechanisms of free carrier generation and laser-induced damage are discussed.

Key words: Laser induced breakdown; laser-induced damage threshold;

nonequil ibrium carriers; semiconductors, Ge, Si, GaAs.

1. Introduction

Electric breakdown in semiconductors has been extensively investigated in dc and microwave

fields, but has not been completely studied in laser fields. However, investigation of laser produced breakdown may be of considerable interest for both the fundamental physics of semiconductors, and the application of these materials in the high power lasers. This paper is concerned with some first results of our experimental investigation of bulk breakdown in Ge, Si, and GaAs crystals, under high power, IR-laser radiation. In order to elucidate

the role of free carriers in laser damage, along with the standard damage threshold measurements, we have studied the kinetics of dc and microwave photoconductivity. The data obtained in the photoconductivity experiments can also be of more general interest for understanding the excitation

mechanisms of nonequilibrium carriers in semiconductors.

2. Experimental Conditions

In the experiments we have used pulsed CO2, Ca2:Er3+ and YAG:Er3+ lasers operating in single, TEM00 mode. The radiation wavelengths and pulse widths were 10.6 ym, 2.76 ym, 2.94 ym, and

60 ns, 90 ns, and 10 ysec, for the CO2, CaF2:Er3+ and YAG:Er3+ lasers, respectively.

Excitation of samples was produced in both parallel and focused beam geometries. For beam focusing,

a 10 mm focal length Si lens was used. Special attention was paid to lens aberrations, which are very important in order to obtain correct measurements of beam intensities inside the sample.

For detection of dc and microwave photoconductivity kinetics, due to generation of nonequilibrium carriers, a conventional experimental arrangement with a microwave oscillator

(klystron), operating at the 3.2 cm wavelength, was used. Identification of laser-induced damage inside the sample, and the damage morphology studies,

were done with an IR-microscope. The typical characteristics of the samples investigated were the following. Si: Dislocation free, with room-temperature resistance, p = lO^fi - cm; GaAs:

1) Undoped, with 10.6 ym absorption coefficient a - 1 cm

- ''. 2) Semi -isolati ng, « = 10"^ cm~\

Ge: Dislocation-free, p

~ 40ft - cm.

3. Results

In Ge, we have observed generation of nonequilibrium carriers, under excitation of samples

by radiation, with all the lasers used. The characteristic decay times of the microwave

photoconductivity kinetics signal varied over the range of 0.1 to 1.0 msec, depending on the excitation geometry and sample size. In the case of tight focusing of the laser beam inside large

samples, the decay time is longest, and determined by the bulk recombination of the carriers. Because of the higher mobility, the observed photoconductivity was mainly attributed to the electrons (the contribution of the holes is expected to be negligible).

With 2.76 ym and 2.94 ym excitation, the carrier generation is due to two-photon ionization. We

have measured the two-photon absorption coefficient, 3, at 2.76 ym, to be equal to 7.5 x 10-2 cm /MW.

With 10.6 ym excitation, the nature of the carrier generation mechanism is no longer clear. In order to elucidate this question, we have measured the carrier concentration at various excitation

levels using dc photoconductivity techniques. It was found that generation becomes detectable at a relatively low laser radiation intensity, I = 10^ W/cm^, and the electron-hole pair concentration

reaches a value of N - 10 16 cm

-3 at I - 10 7 W/cm2 .

305
The relatively weak N(I) dependence observed (at high intensities N % I^* , as is seen in Figure 1) calls into question the interpretation of carrier generation in Ge by 10.6 ym radiation, on the basis of the impact ionization mechanism, because the theory of this process [1] predicts the exponential growth of N with intensity. We have considered other, alternative possibilities to explain the carrier generation in Ge under 10.6 ym excitation. Among them are thermal ionization,

connected with lattice heating by the laser pulse, and photoionization by a hot inclusion existing in the sample. The estimates have shown that neither mechanism can contribute sufficiently to the carrier generation process, so that the question of the nature of this process is open, and requires

further investigation.

An interesting result of our studies of Ge at 10.6 ym is that we did not succeed in obtaining

laser damage, even at very high incident beam powers, and with tight focusing of the beam inside the

Ge samples. We have explained this result as due to self -def ocusing of the beam, arising from the negative contribution of nonequ i 1 ibrium electrons to the refractive index of the sample.

Consideration of self -defocusi ng of tightly focused beams in the solid state electron plasma, of the type which exists in our experimental conditions in Ge, has shown that the intensity in the focal region is limited to a definite value, depending on the focusing geometry and the carrier generation

rate. For our particular experimental conditions, we estimate that I - 100 MW/cm^, and Nmax = 3

x 10^ crrr^. Note that if the self -defocusing effect is not taken into consideration, the intensity in our experiments would reach very high levels of about 10^ W/cm^, at which damage

should probably occur.

In Si and GaAs, damage has been observed at all three wavelengths, with rather high thresholds

(see Tables 1 and 2) especially at 10.6 ym.

Table 1 Bulk laser damage threshold intensities Jd (GW/cm^) and effective

critical fields Eeff (MV/cm) in Si and GaAs

Material Wave Length, ym

(10.6) (2.76) (2.94)

Jd EeffJd Eeff Jd E eff

Si 5.0 0.75 2.3 0.5 0.15 0.13

GaAs 16.5 1.4 0.8 0.3 0.08 0.1

Table 2 Bulk laser damage threshold intensity Jj (GW/cm^) and effective

critical field Ee ff (MV/cm) for different GaAs samples at 10.6 ym

Absorption coefficient

Sample cm~l Jd E eff

Semi -isolating

1 0.03 1.3 0.4

2 0.035 14.5 1.3

3 0.03 12.4 1.2

4 0.02 10.3 1.1 Undoped

1 0.9 16.5 1.4

2 0.9 53.6 2.5

In order to understand the mechanism of laser- induced damage in Si and GaAs, we have compared

the critical fields Ee ff, calculated from the laser damage threshold intensities measured, with the field E 0 for dc electric breakdown, which is close to 2 x 10^ v/cm [1, 2]. Since E eff > E 0 , especially in the undoped GaAs samples, one can suppose that electron avalanche is the mechanism of

laser damage in Si and GaAs at 10.6 ym. Using the relationship

Eeff - Eo( fi2 + v

eff2) 1/2

, O)

which avalanche theory predicts [3, 4], we have estimated the value of the effective electron-phonon

collision frequency ve ff. The estimated value is \> e ff = 5 x 10^ sec~l for Si, which is close to the well -determi ned, cold electron collision frequency. Although the reasonable correlation

between the laser-induced damage and the dc breakdown data makes it attractive to attribute the laser damage in Si and GaAs at 10.6 ym to the electron avalanche mechanism, we think that additional data

are needed to confirm that conclusion. At least, the role of defects of different types in laser

306
damage has to be cleared up, because our studies have shown "inclusion-type" damage morphology, even

in the high threshold samples. We have observed simialr morphology in both Si and GaAs samples at 2.76 pm and 2.94 ym (see Figure 2) which indicates that defects undoubtedly affect the damage process

Notice also, that in contrast to Ge, the nonequil ibrium carriers in Si and GaAs have been detected only at intensities at which damage occurs.

3. References

[1] Aladinsky, V. K. , FTT (Fizika tverdogo

tela), 7, 1813 (1965).

[2] Aladinsky, V. K. , Salomikova, M. V., Electron technics, serija II, No 2, 45 (1973).

[3] Epifanov, A. S., Manenkov, A. A. Prokhorov,

A. M. JETP Pis'ma 21_, 483 (1975). Soviet

Physics JETP Letters 2T_, 223 ( 1975).

[4] Epifanov, A. S. Manenkov, A. A., Prokhorov,

A. M. JETP 70, 728 (1976) Soviet Physics

JETP 43, 377 (1976).

307
is

5

8

5

too

to

WO/4^1

Figure 2. Bulk damage produced by CaF2.Er3+ laser in GaAs ( = 2.76 m) at the intensity of

1000 times above threshold, = .8GW/cm2 . Arrow shows the direction of incident laser beam

propagati on.

2 4 € S& Intenvty MW/rm*

Figure 1. Concentration of nonequi 1 ibri urn carriers

in Ge versus CO2 laser intensity.

Discussion

The speaker was questioned on several aspects of this work relating to the competition of intrinsic and extrinsic damage. The observed damage thresholds in GaAs did not show any correlation with

measured bulk absorption coefficients. Also, the observed threshold damage morphology consisted of

small damage sites of 1 to 2 Mm in diameter . The speaker was asked about the work of Austin and

Shank, previously reported, in which a strong decrease in carrier lifetime was observed in Ge at high

concentrations . He replied that, in his work, this concentration- induced decrease in carrier lifetime was not observed. He felt that near the surface, where high carrier concentrations can be created,

carrier lifetimes are already shortened by surface defects and impurities .

308
FREQUENCY DEPENDENCE OF BREAKDOWN FIELDS IN SINGLE-CRYSTAL NaCl AND KC1*

M. J. Soileau

Michelson Laboratory, Physics Division

Naval Weapons Center, China Lake, CA 93555

and

M. Bass and E. W. Van Stryland

Center for Laser Studies

University of Southern California, Los Angeles, CA 90007

In this paper new observations of the breakdown fields in single-crystal

NaCl and KC1 are reported. Measurements were made of the wavelength and focal

spot radius dependence of breakdown fields in NaCl and KC1 at optical frequencies. The measurements at different focal spot radii were used to establish

empirical scaling laws for these two materials over the range of focal spot

radii studied. Correct application of these scaling laws gives directly com- parable fields at all frequencies. The results indicate little dispersion in breakdown fields over the wavelength range of 10.6 to 0.53 pm. The ratio of the NaCl and KC1 breakdown fields are found to depend on frequency and this implies

that extrinsic phenomena were being probed by these experiments.

Key words: Breakdown fields; KC1; NaCl; spot-size dependence.

Introduction

Measurements were made of the wavelength and focal spot radius dependence of the pulsed laser

damage fields in NaCl and KC1. The damage irradiations were performed with pulsed CO2 , HF, and DF lasers with total powers less than 1/18 of the critical powers for self-focusing by tightly focusing

the Gaussian laser beams. At 10.6 and 2.7 p, the CO2 and HF wavelengths, a single triangular pulse

was used; however, at 3.8 um, the DF wavelength, a double pulse was obtained because the laser oscillated on several lines simultaneously. The measurements at different focal spot radii were used to establish empirical scaling laws for these two materials over the range of focal spot radii studied.

These scaling laws are then applied to both current data and previous data to determine the damage

threshold as a function of frequency for a common focal spot radius. The results indicate little

dispersion in damage threshold over the wavelength range of 10.6 to 0.53 pm.

Background

The objective of this work was to investigate the mechanisms for laser-induced breakdown in highly

transparent solids. Emphasis is placed on determining the frequency dependence of the breakdown thresh

olds of selected alkali hallde crystals. Although this problem has been explored by many workers, the detailed mechanism for breakdown is not well understood [l] 1 and considerable discrepancies in experimental data exist. Early measurements of breakdown fields in the alkali halides by Yoshiyoki et al.

[2], indicated that the thresholds for 0.69 um radiation were nearly the same as those for ordinary

impulse breakdown and were affected little by the band gap energy of the crystals. This evidence led the authors to suggest that breakdown was an avalanche process initiated by electrons excited to the conduction band from traps within the gap.

Yablonovitch found that the rms breakdown fields for various alkali halides at 10.6 pm were approx

imately equal to the DC breakdown fields and had the same systematic variation with band gap [3]

Fradin and Bass [4] found that this systematic variation of thresholds extended to 1.06 and 0.6943 pm. Such evidence led many workers to conclude that the alkali halide breakdown thresholds were due to an intrinsic propriety of the lattice [1]. However, subsequent measurements on very pure KC1 crystals at 10.6 um showed that the systematic differences in the thresholds observed by Yablonovitch at 10.6 pm

did not always hold [5,6]. Additional measurements by Yasaj ima et al. [7] for various alkali halide

breakdown fields at 1.06 and 0.6943 pm showed that the thresholds were not dependent on the band gap energy. Their measurements of temperature dependence of laser- induced breakdown and photoconduction at prebreakdown optical fields provided additional evidence that laser-induced breakdown was due to extrinsic processes.

Figure 1 shows the results of attempts by Sparks [8] to model the results of Yablonovitch, Fradin,

and Bass [3,4]. Recent results by Manenkov [9] are also given in figure 1. The top curve in this

* Work supported by ONR Project No. RR02202, Task RR0220202, Dr. A. M. Diness, Program Manager.

1. Figures in brackets indicate the literature references at the end of this paper.

309
figure is obtained from a modified version of the Yablonovitch dielectric breakdown model. The Sparks

model differs from the Yablonovitch model in that it is assumed that (a) starting electrons are provided

by multiphoton excitation of defect states, (b) losses to acoustical phonons are significant, and (c) the value of tp, the electron collision time with optical phonons, is much larger than that assumed by

Yablonovitch. The top curve was adjusted to fit the 10.6 um data and so predicts a strong frequency

dependence over the 10.6 to 1.06 urn wavelength range. The bottom curve in figure 1 results from the Sparks-Holstein model which assumes tight binding of electrons in the conduction band. This model is based on energy transfer from the laser field to the crystal via the Holstein photon-electron-phonon

interaction. When first postulated, the Sparks-Holstein model had the attractive features that it fit the then available data quite well and used no adjustable parameters. The Sparks-Holetein model pre- dicted little frequency dependence over the 10.6 to 1.06 um range. To the problem of selecting data

and models, one must now add the possibility that the measured threshold may depend on focal spot

radius, w, as was pointed out by Bettis et al. [10]. Measurements in the 1.06 to 0.53 um region are usually made with very small w (on the order of 10 um radius), whereas, the 10.6 um measurements are made with larger w (on the order of 50 um radius). In the present paper, variables such as the focal

spot radius are removed from measurements of damage threshold and a clearer determination of the breakdown fields in alkali halides is reported.

Experiment

In the damage measurements a transversely excited HF (or DF) pulsed chemical laser and a TEA CO^ laser were used as irradiation soruces. The output of the HF laser was multiline and centered about

2.7 um; the multiline DF output was centered about 3.8 um. The only change made in the system to go from HF to DF lasing was to switch H2 to D2 gas. The power supply, laser cavity, and all other optics

remained the same. The HF/DF and CO2 lasers are described in detail in previous work [6,11,12,13].

The occurrence of laser damage was determined by (1) viewing a change in scattering of a coaxial HeNe beam in a twenty power microscope, (2) observing a visible flash and/or fracture in the sample,

and (3) monitoring the transmitted waveform distortion. In this study, it was found that all three of these effects occurred simultaneously, i.e., when damage occurred the HeNe laser scatter increased, a visible flash and fracture were observed, and the transmitted waveform was abruptly truncated. In this

research the threshold was defined to be that flux which produced damage in 50% of the sites when each site was irradiated only once.

The laser output was attenuated by either a pair of Brewster angle ZnSe plate polarizers used in reflection [4], or by two pairs of Brewster angle ZnSe plate polarizers used in transmission [11]. The orientation of the ZnSe polarizers were such that the polarization in the beam reaching the target

remained constant. Unless otherwise noted, all data reported here were taken using the reflection

attenuators

.

A narrow slit was used to scan the focal planes of the three uncoated, meniscus, ZnSe lenses used

in these experiments at each of the three laser wavelengths investigated. The measured focal spot

radii were then corrected for the finite slit width [11]. The results of these measurements are sum- marized in table 1. The radii of the unfocused beams at the position of the focusing lens were 0.55 cm for C0 o and 1.01 cm for HF and DF.

Table 1. Focal spot radii.

Focal length Lens Lens at 10.6 um diameter

No. (cm) (cm)

1 2.54 2.54

2 6.35 2.54

3 12.70 5.08

Focal spot radius (1/e in intensity) (um) HF DF C02 9.55 10.60 20.50

12.55 13.55 45.00

26.50 28.50 85.00

The temporal pulse waveform was monitored at two points, one before the focusing lens and one after transmission through the sample. The CO2 laser produced a train of partially mode-locked pulses

with an envelope having full width at half maximum (FWHM) of 60 to 110 nsec (fig. 2). The HF laser produced a triangular pulse with some irregular spiking which had a FWHM of 100 to 140 nsec (fig. 3). The DF laser produced the double pulse waveform shown in figure 4. Damage occurred during the second

part of the DF pulse as observed by monitoring the waveform distortion upon transmission through the sample. The ratio of the area under the second DF peak (fig. 4) to the total area of the waveform is equal to the fraction of the total energy in the second part of the pulse. The damaging peak power is then the energy in the second part of the pulse divided by the FWHM of the second part of the pulse. An equivalent pulse width is then defined as the FWHM of a triangular pulse containing total pulse energy but having the same peak power as the second part of the pulse. Equivalent pulse widths of 70

to 100 nsec were found for these experiments. The pulse shapes from these lasers are sensitive func- tions of various operating parameters (i.e., gas mix, pressure, repetition rate, and discharge voltage) and so pulse width was carefully determined for each damage threshold measurement. The range of pulse widths given above are for different sets of measurements. The variation of pulse widths for a given threshold measurement was less than 10%. 310
The total energy of each pulse was monitored after the attenuator with a pyroelectric energy

meter. After each set of threshold measurements (i.e., a given specimen at a given wavelength and focal spot radius), the energy monitor was calibrated using a thermopile calorimeter. The calibration

was performed at the threshold value to avoid any questions concerning nonlinear response of the monitor detector.

The specimens used in this work were single-crystal NaCl and KC1 from Harshaw Chemical Company

[14]. The specimens used were free from visible (0.6328 um) scattering and were not limited by linear

absorption.

Data Reduction

The intensity thresholds were determined from the measured energy which caused damage in the following way. The total energy Q is given in terms of the energy density, S(r) , at the focal plane by

Q = / &(r)27rrdr . (1) o For the Gaussian beams used in these experiments

TTWO

2 & Of

o is the peak-on-axis energy density and wQ is the radius at which the intensity has fallen to l/e o its peak value on-axis. This gives the damage threshold in J/cm^ . The intensity damage threshold I i watts/cm^ is given by

29- . (3)

2

TTW t O p where tp is the laser pulse width (FWHM) . The rms electric field corresponding to the peak-on-axis

intensity is given by [6],

r T

. , 2, -.1/2 w ia. i \ i n i -i I (watts/cm ) E(volts/cm) = 19.41 — , (4)

where I is the peak on-axis intensity and n is the index of refraction of the specimen.

The total energy Q used in the above equations is related to the total energy incident on the focusing lens by

Q = Q. T T , , (5) input L s

where is the transmission of the focusing lens and T

g is the transmission of the entrance surface of the specimen.

Data

The following tables summarize the data from this work. The numbers quoted are the so-called 1- on-1 thresholds (one irradiation at each site). The thresholds given here are for what has been called

intrinsic damage, i.e., damage which occurred at the focal position and near the peak of the laser pulse when irradiated with flux levels which caused damage in 50% of the sites tested.

The bulk thresholds could not be measured at 10.6 ym using lens 3. The samples always failed at the surface even though the laser was focussed in the bulk of the material. This was due to the relative low damage thresholds of the surfaces and the relatively high f-number (f/12) of lens 3 at 10.6 um. The numbers listed for lens 3 at 10.6 ym in tables 2-5 are the highest values for which front surface

damage (or bulk damage) did not occur. Thus, these numbers represent a lower bound for the bulk damage

threshold

The energy threshold divided by the intensity threshold gives the pulse width for the particular

laser and focal spot radius.

Since the pulse widths are not the same for all the data points, one might question whether the data are pulse-width dependent for this pulse-width region. There is some dispute about the pulsewidth dependence for pulse widths of the order of several to hundreds of nanoseconds. Some workers

claim there is a dependence [10] and others claim no dependence on pulse width [15]. We tested speci- men 78-NC-6 at 10.6 ym using lens 2 with tp = 60 nsec and tp = 107 nsec. The resulting thresholds were

the same to within ±3.5%, which was well within experimental error. Thus, we found no pulse-width

dependence for these experimental conditions.

311
Six specimens of KC1 were tested. Specimen 78-KC-15 had visible scattering centers when viewed

with a 20X microscope with a focused HeNe laser for illumination. The data for this specimen are included for completeness, but are not used in the analysis of breakdown fields or for the development

of scaling laws.

Specimen 78-KC-15 exhibited damage characteristics at 10.6 urn, similar to what is usually referred

to as "extrinsic" damage. Near threshold the specimens would damage at one or more of the scattering

centers which were often off-axis and not in the lens focal plane. The damage sites were very localized,

and the transmitted pulse showed only slight distortion or no distortion at all. The 10.6 um threshold was approximately 10 times smaller than for the other five KC1 specimens tested. However, at HF and DF wavelengths the specimen had "intrinsic"-like behavior, i.e., damage at threshold occurred in the lens focal plane and at the peak of the pulse, the transmitted pulse was abruptly truncated, and a large

fracture and flash were observed. Thus, the visible scattering centers which initiated damage at 10.6

pm had little or no effect at 2.7 and 3.8 um.

Analysis

The focal spot radius dependence of the peak on-axis intensity damage threshold of NaCl and KC1 were found from the data given in tables 3 and 5 (excluding specimen 78-KC-15) . The data for each

specimen was normalized to the threshold at the minimum focal spot radius and the results were plotted

as a function of the inverse focal spot radius. Figure 5 is such a plot for NaCl at 2.7 and 3.8 um. The straight lines in these plots are the inverse radius dependence proposed by Bettis et al. [10], Note that some of the samples fall on this line while others deviate considerably and some samples have

a different dependence at different wavelengths. Points above the line derived from the Bettis model

have a radius dependence of (l/w)^, where p is < 1.0; whereas, for points below the line p is > 1.0. The stars within the black circles are the average values of normalized damage threshold intensity for the five specimens investigated. The average values are below the Bettis model line indicating a stronger dependence than 1/w. Note that the average values show about the same dependence at the two wavelengths.

In figure 6 we plot the spot-size dependence of the threshold averaged over the five specimens of NaCl and KC1. An attempt was made to fit a linear and quadratic dependence of intensity threshold on inverse focal spot radius. The quadratic dependence gave the best fit. We emphasize that the upperright corner of these plots are the points of normalization and thus must be included as data points in any attempts to fit the data to an analytical function.

2 The origin of the (1/w) dependence is not understood. Such a dependence would seem to indicate

that the damage was dependent on total power and that self-focusing was responsible for the damage

[16]. However, this is not the case since for the worst case (highest total power at 2.7 um) the total power was a factor of 18 or more below the critical power for self-focusing.

A possible explanation for the focal spot radius dependence is that the thresholds are determined

by defects within the focal volume. A strict dependence on focal volume leads to an intensity threshold proportional to (1/w) 4. However, for an avalanche process initiated by electrons excited to the conduction band from defect states the threshold should be only weakly dependent on the number of defects present, as long as at. least one defect is in the focal volume.

Let Vf equal the focal volume for which the probability of finding a single defect approaches

one. For focal volumes smaller than Vf a strong dependence on focal spot radius is expected since the probability of finding a defect is porportional to the focal volume. As Vf is approached little or no dependence on focal volume (and thus focal spot radius) is expected. Figure 5 is suggestive of this

since the focal spot radius dependence in the small focal spot region seems steepest.

The scaling laws shown in figure 6 were applied to our data and to that presented by Manenkov in 1977 [9] to scale all the data to a common spot size. Manenkov's results were for so-called "high

optical resistance samples," the 20% or so of samples which he reported to have had high thresholds. The average thresholds for our samples at 10.6 um are as high as those quoted by Manenkov for the so-called

high damage threshold samples. While we realize the danger of applying empirical scaling laws derived

from our samples to the Manenkov results, the agreement at 10.6 um (the only point for which direct

comparison can be made) lends some confidence for using the scaling laws on his data as well as ours.

In figures 7 and 8 we plot the frequency dependence of the breakdown field (rms field corresponding to the peak on-axis intensity) for NaCl and KC1. In these plots we have scaled our results and Manenkov's results to a common focal spot diameter. The top curves involve the least scaling. The 2.7 and 3.8 um points were measured at this spot size. Manenkov's data for

2 1.06, 0.69, and 0.53 um were

taken with focal spot diameters of 17, 14, and 16 um, respectively (l/e full width in intensity). Our data at 10.6 um was scaled from measurements with a 41 um focal spot diameter.

The most significant feature on the information displayed in figures 7 and 8 is the very small dispersion of breakdown field over the 10.6 to 0.53 um wavelength range when all data are scaled to the same focal spot diameter. In figure 7 there appears to be a negative slope for the data scaled to the

41 um diameter spot. This is probably not significant since if one puts on the error bars of ±15%,

312
laDie z . duJ-K. energy density 2 thresholds for NaCl (J/cm )

Specimen 78-NC-S1 78-NC-S2 78-NC-4 78-NC-5 78-NC-6 Lens 1 HF 9527 7479 7444 3828 8163 DF 11520 9802 7218 4410 5439

co„

2 3613 5170 3596 1444 2561

Lens 2 HF '5174 5531 4911 1844 3340

DF 8043 5398 5712 2203 3282

ce„

2 592 626 630 511 434

Lens 3 HF 2060 2490 1952 916 1851

DF 1945 2427 1785 1057 1848

CO >257 >138 >238 >213 >282

Table 3. Intensity thresholds 9 tor iNaui vxiu wat t s / cm )

Specimen 78-NC-S1 78-NC-S2 78-NC-4 78-NC-5 78-NC-6 Lens 1 HF 73.3 53.3 51.4 32 .

65.4

DF 121.0 125.0 83.

53.8 53.6

co

2 33.5 87.7 46.7 19.7 30.5

Lens 2 HF 39.3 47.8 44.5 15 .

28.9

DF 92.9 64.2 63.4 19 .0 29.0

co

2 7.40 7.73 6.62 6.13 7. 11

Lens 3 HF 17.4 22.8 17.2 7.87 15.0 DF 19.1 33.9 17.9 11.1 30.9

CO >3.1 >1.9 >2.4 >2.4 >3.4

Table 4. Bulk energy density thresholds for KC1 (J/cm ).

Specimen 78-KC-S1 78-KC-S2 78-KC-S3 78-KC-14 78-KC-15 78-KC-16

Lens 1 HF 3283 2865 3417 3783 3787 3749

DF 3306 5802 3173 4579 3980 4439

co

2 1071 1366 1226 976 108 1136

Lens 2 HF 1841 2309 2141 2182 2000 2229

DF 1639 1774 2260 2167 1516 2178

co

2 283 412 370 379 13.6 369

Lens 3 HF 1660 1521 1540 1437 1236 1559 DF 1227 1362 1362 1772 1086 1371

CO - >84 >225

1

>253 >176 0.25 >217

Table 5. Intensity density thresholds for KC1 (xlO

9 watts/ 2. cm )

Specimen 78-KC-S1 78-KC-S2 78-KC-S3 78-KC-14 78-KC-15 78-KC-16

Lens 1 HF 31.3 27. 3 30.5 26.3 30.8 32.4 DF 32.2 70.2 31.1 54.1 43.5 48.8

co

2 13.6 14.5 16.1 12.1 1.50 15.0

Lens 2 HF 15.6 19.8 16.1 18.2 18.3 17.4

DF 16.4 18.5 22.2 27.8 19.4 25.7

C°2__ 4.72 4.96 5.07 4.25 0.21 5.39

Lens 3 HF 12.3 11.8 12.8 12.8 9.30 12.8

DF 14.0 16.9 14.4 30.3 11.4 14.8

co

2 >1.2 >2.8 >3.1 >1.9 >0.003 >2.6

313
one could fit any one of the three curves within the resulting band. Note that the Sparks-Holstein

model would fit the NaCl data (fig. 3) for the 72 ym spot diameter quite well, but fails to fit the data for the 19 ym spot diameter. The modified version of the simple avalanche model proposed 'by Sparks would fit the 19 ym spot size data quite well at 1.06 ym, but would fit the 10.6, 3.8, and 2.7

Vim data only if much smaller values of electron relaxation time are used. The multiphoton absorption

by polarons model proposed by Schmid, Kelley, and Braunlich [17] would nearly fit the data for the 72 um spot diameter at 1.06, 0.69, and 0.53 ym, but predicts a much stronger wavelength dependence than

observed here and thus would fail to fit the 10.6 ym data for any of the spot sizes shown here.

Note that the threshold field at 2.7 ym for NaCl and KC1 is consistently below the best fit fre- quency dependence shown in figures 5 and 6. This could be associated with increased absorption at 2.7

Vim due to (OH) in the crystal. Corresponding increases in linear absorption have been reported in hygroscopic crystals such as NaCl and KC1 [18].

One can fit a linear dependence of breakdown field with laser frequency and extrapolate the results

to get an estimate of the DC breakdown field. Figure 9 is a plot of such a linear fit for the data

using the 71 ym spot diameter. The agreement of the predicted values with the measured DC values [19] is reasonable. As can be seen in figure 9, the ratio of NaCl to KC1 threshold is not constant with

frequency.

Conclusions

Measurements of the focal spot radius dependence of the pulsed laser damage thresholds of NaCl and KC1 have shown considerable variation from sample to sample as well as variation with laser wavelength.

The average intensity thresholds of the specimens studied had a (l/w)2 dependence. The 10.6 um average

thresholds measured in this work are in good agreement with those reported by Manenkov for his "high

optical resistance samples." This agreement lends confidence for using the empirical scaling laws derived in this work to scale our results and Manenkov' s results to common focal spot radii. The results indicate little dispersion in damage threshold over the wavelength range of 10.6 to 0.53 ym. The lack of strong frequency dependence is suggestive of avalanche breakdown as the mechanism of failure

in these materials. However, the ratio of the thresholds of the two materials studied is not constant with frequency. This implies that an extrinsic property of the material is being measured.

Acknowledgments

The authors acknowledge the support of the staff at the Naval Weapons Center in conducting these measurements, especially J. 0. Porteus, J. L. Jernigan, W. N. Faith, and J. W. Bethke. We are thankful

for the help in data-taking given by S. T. Wu and Kassem Moravvej of USC.

References

[1] Reviews of the proposed mechanisms for laser- induced damage in transparent materials can

be found in the following references: Bloembergen, N. , "Laser- Induced Electric Breakdown

in Solids," IEEE J. Quantum Electron. QE-10,

375 (1974); Smith, L. , "Fundamental Mechanisms

of Ionization," these proceedings.

[2] Yasajima, Y. , et al. , Japan. J. Appl. Phys.

1_, 552 (1968).

[3] Yablonovitch, E. , Appl. Phys. Lett. 19, 495 (1971).

[4] Fradin, D. W. , and Bass, M. , Appl. Phys. Lett .

2_2, 206 (1973).

[5] Allen, S. D. , et al. , in Laser' Induced Damage

in Optical Materials: 1974, A. J. Glass and

A. H. Guenther, eds. , National Bureau of Standards, Washington, DC (NBS Spec. Publ.

414, 1974), p. 66.

[6] Soileau, M. J. , et al. , in Proceedings of

the Fifth Annual Conference on Infrared Laser Window Materials, 1975, C. R. Andrews and

C. L. Strecker, eds. (Defense Advanced

Research Projects Agency, Arlington, VA, 1976), p. 391.

[7] Yasajima, Y. , et al. , Japan. J. Appl. Phys.

14, 815 (1975).

[8] Sparks, M. , in Laser Induced Damage in Optical Materials: 1975, A. J. Glass and

A. H. Guenther, eds., National Bureau of Standards, Washington, DC (NBS Spec. Publ.

435) , p. 331.

[9] Manenkov, A. A. , in Laser Induced Damage

in Optical Materials: 1977, A. J. Glass and

A. H. Guenther, eds., National Bureau of Standards, Washington, DC (NBS Spec. Publ.

509) , p. 455.

[10] Bettis, J. R. , et al. , in Laser Induced

Damage in Optical Materials: 1976, A. J. Glass and A. H. Guenther, eds., National

Bureau of Standards, Washington, DC (NBS

Spec. Publ. 462) , p. 338.

[11] Van Stryland, E. W. , et al. , in Laser Induced Damage in Optical Materials: 1977

,

A. J. Glass and A. H. Guenther, eds.,

National Bureau of Standards, Washington,

DC (NBS Spec. Publ. 509), p. 118.

314
[12] Soileau, M. J., et al. , in Laser Induced

Damage in Optical Materials: 1976,

A. J. Glass and A. H. Guenther, eds.,

National Bureau of Standards, Washington,

DC (NBS Spec. Publ. 462), p. 149.

[13] Porteus, J. 0., et al. , IEEE J. Quantum

Electron. QE-13 , 84D (1978).

[14] Harshaw Chemical Co., 6801 Cochran Rd. , Solon, OH 44139.

[15] Manenkov, A. A., private communication,

Sept. 1978.

[16] Fradin, D. W. , IEEE J. Quantum Electron. QE-9 , 954 (1973).

[17] Schmid, A., et al. in Laser Induced Damage

in Optical Materials: 1977, A. J. Glass

and A. H. Guenther, eds., National Bureau

of Standards, Washington, DC (NBS Spec. Publ. 509) , p. 465.

[18] Hass, M. , and Bendow, B. , Appl. Opt. 16, 2882

(1977).

[19] Von Hippel, A., J. Appl. Phys. 8, 815 (1937);

Phys. Rev. _54, 1096 (1938).

Figures

0.1 10 10.0 PHOTON ENERGY («V)

Figure 2. Laser waveforms related to the occurrence

of bulk damage in NaCi at 10.6 urn. The top trace is the reference and the bottom trace is the truncated

transmitted beam observed when damage occurs.

Figure 1. Frequency dependence of breakdown field

in NaC£. Solid lines are fron Sparks [8]; open circles are from Manenkov [9]; solid circles are from Fradin and Bass [4].

Figures 3a and 3b. Laser waveforms related to the occurrence of bulk damage in NaC«. at HF wavelengths. The left trace is the transmitted pulse which did not cause damage. The right trace is the

transmitted pulse when damage occurred near threshold.

315
DF LASER (X -3.8 urn)

Figure 4. Laser waveforms related to the occurrence of bulk damage in NaCi. at DF laser wavelengths. The top trace is the reference pulse and the bottom trace is the transmitted pulse. In this case, the damage is near the threshold value and occurs at the peak of the second pulse;

Figure 5. Spot-size dependence in UaCl. The white stars

in the dark background are the average threshold values of five separate specimens. The other symbols correspond to the thresholds of the individual specimens. The large

triangles are points of normalization and must be included

in any atempt to fit these data to an analytical function.

Figure 6. Spot-size dependence in NaCi and KCl. The above curves are the best fit of the spot-size

dependence of the average thresholds for five HaCl and five KCl specimens. Note that the upper-right

corners of the plots are the points of normalization and thus must be included as data points in any attempt to fit the data to an analytical function.

1 A •THIS WORK

1 1 1 1 1 1 1 1 1 1 II I

D A 0MANENKOV, 1977

5 0 Q

a ^_q-_ 2W-18«m~

4.0 o

3 0 2W - 41 (im _

0

a o

2 0 ® 2W-72nni~

•

1 0

i i i

E - (i) tn (h*) + b

i i i i i i

i ill

0.1 0.5 1.0 PHOTON ENERGY («V)

Figure 7. Frequency dependence of NaCU breakdown field. The fields given here are the rms electric

fields corresponding to the peak on-axis intensity thresholds.

316
"1 1

1 1—I—I—I I I

A • THIS WORK AO MANENKOV. 1977

1 1 1—

E - {) «n (tu-l + b

2W - 41 |/m ~2W-72;im

-l 1 I I I I' '

i 1_ J L

PHOTON ENERGY («VI

Figure 8. Frequency dependence of KCi breakdown field. The electric fields given here are the rms fields corresponding to the peak on-axis intensity thresholds.

~i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—

• NaC« EpQ - 1.71 x 106 V/cm; tpr • 1.50 « 106 V/cm DC O KCe Eqc - 1.46 x 106 V/cm, Enr ' 1.07 x 106 V/cm DC VON HIPPEL

J I I I I l_

PHOTON ENERGY |»V|

Figure 9. Linear fit to frequency dependence (2w = 72 ym). The DC data given here are from reference

[19].

317
INVESTIGATION OF SURFACE BREAKDOWN MECHANISM

IN IR-OPTICAL MATERIALS

V. I. Kovalev, F. S. Faizullov

USSR Academy of Sciences

Leninsky Prospect 53 Moscow, USSR

It has been found that with a decrease in ambient gas pressure, there

takes place a decrease of surface breakdown threshold in IR-optical materials

under TEA C02-laser radiation. To explain this fact, a new mechanism of the breakdown has been proposed, which is connected with evaporation of adsorbed

water. The breakdown stage has been experimentally investigated and a theoretical analysis has been performed. The dependence of the surface

breakdown in NaCl on ambient gas pressure, spot size, frequency and time have

been studied. The dependence on the quantity of adsorbed water (surface

absorption) has been studied as well. The breakdown thresholds have been measured with a calibrated, photon drag detector by the amplitude of the transmitted pulse. The results obtained are in good agreement with the proposed model

.

Key words: Adsorbed water; breakdown mechanism; IR window materials; NaCl; pulsed TEA CO2 laser surface breakdown; pressure; spot size; surface

absorption; frequency and time dependence of breakdown thresholds; transmitted

pulse.

1. Introduction

Surface breakdown threshold in IR-optical materials is determined by adsorbed water, as was found earlier in spectroscopic and mass spectroscopic investigations [1]. To avoid the influence of processes taking place in the ambient gas on the breakdown plasma luminescence at the surface, the investigations have been performed with the samples placed in vacuum. Breakdown threshold measurements in vacuum have shown that the threshold values are 1.5 to 2 times smaller in comparison with those observed at atmospheric pressure of the ambient gas. The results of these measurements are listed in Table 1.

Table 1 Surface breakdown thresholds (MW/cm^) in some IR-optical materials

in air and vacuum, and ratio between them.

Material Air Vacuum Air/Vacuum

NaCl 65 32 2.0 ZnSe 55 35 1.6 KRS-5 60 32 1.9 KRS-6 40 28 1.5 Ge 50 33 1.5

The method of measuring the threshold is described in detail in paper [2], and is based on measuring

the pulse amplitude of the TEA CO2 laser radiation transmitted through the sample, at the moment

when breakdown appears (moment of nontransparency) . The pulse amplitude measurement was performed

with a photon drag detector calibrated on power. Pulse risetime was 150 ns. Spot size of the incident radiation on the samples surface was measured by the autocal ibration method and reached

5.0 mm

.

The observed influence of ambient gas pressure on breakdown threshold may be explained by making

the assumption that the evaporation of adsorbed water from the surface takes place before the breakdown.

Theoretical analysis of the water evaporation mechanism shows that, in our case, water evaporation is the result of volume boiling of the adsorbed water layer. In this process, a certain

part of the threshold energy which is close to the evaporation heat is absorbed (this energy corresponds to an incident energy density Wm j n ~ 2.5 J/cm^). The water is evaporated under conditions close to critical (pcr

= 0.329 g/cm3

; Tcr

= 647 K°; P cr= 225 atm). As the result of gas-dynamical expansion, the vapor density decreases, and the breakdown conditions in this vapor are fulfilled, according to the Paschen curve (see Figure 1). This, briefly, is the breakdown

mechanism. We next present a number of experimental results confirming the proposed model.

318
2. Pre-breakdown Stage Study

An experimental study of the pre-breakdown stage was performed with the help of schlieren

photography in ruby laser light (Figure 2). The spot size of the incident beam of CO2 laser on the samples was ^ 6 mm; the flux density was about half of the threshold. Typical schlieren photos are shown in Figure 3. Exposure time is 10 nsec. The surface of the sample is horizontal; the radiation is falling from above. The photos were taken with 1.3, 3.6, and 6 ysec delay of the probing pulse in relation to the beginning of the CO2 laser pulse. It is seen that as a result of water vapor expansion at pre-breakdown intensity, a plane shock wave is formed which propagates in the air from the surface. Figure 4 shows a typical schlieren photochronogram of the shock wave motion, which was taken with a streak camera in the light of a ruby laser (free oscillation regime), and the corresponding x, t-diagram. During the first microsecond the velocity of the shock wave is 700 m/sec. Thus, the evaporated water may be considered to be the plunger, leading to compression of the ambient gas. In its turn, the ambient gas influences both the density distribution in the expanding vapor, and the vapor expansion velocity.

Figure 5 demonstrates experimental results which show the influence of nitrogen density on the shock wave velocity, when the free water and NaCl surfaces were under the action of the laser pulse.

The results of theoretical calculation of the dependence of the shock wave velocity on the ambient

gas density is presented here as well. These calculations were carried out using a model of vapor expansion in a centered expansion wave, taking into account the back pressure of the ambient

gas [3,4]:

D = (Yn + 1) U(Pg)/4 + V1 + [(Yo + 1) U(PQ )/4C Q ]2 1/2

(1)

P 0 D " (YV

- 1) U/2C v ]2y v/(V1)

= (3Yg + 1) Pg/(Yg + 1 ) + ( Yg + 1 ) U

2 P„g/2 , (2)

where Yg =

Cg and Cv

are adiabatic indices of the ambient air and vapor

Yv = 1.33

sound velocities

P 0

= initial vapor pressure,

U = velocity contact discontinuity.

In the case of free water, the experimental results are in good agreement with theory. In the case of NaCl, the difference between the absolute value of the velocity and the theoretical

prediction is due to a small amount of adsorbed water, but the character of the dependence of the velocity on the ambient gas density remains the same.

3. Breakdown in Water Vapor

Due to expansion, the vapor near the surface has a density which is determined by the formula

Pv(pq) = P0

[l - 0.167U(Pg )/C v]6

, (3)

where U(pg) is obtained from Eq. (2), and P0

is the initial vapor density.

In this vapor region, breakdown may occur. The condition for breakdown development in the water

vapor corresponds to the requirement that the rate increase of the energy of the electrons in the field of laser radiation with intensity J must exceed the maximum rate of energy loss by electrons,

due to elastic collisions with neutral molecules. This condition may be written as [3]:

8e

3t

4Tre 2m

2 . 2

CO + V vA > 0 (4a)

319
or J > -

where m = mass of the electron

e = electron charge

M = mass of the molecule

C = sound velocity

A = ionization potential

w = the laser frequency

2irMe 2.65 x 10' M + ^G7 (4b)

P

v

u ga/M = frequency of electron collisions with neutral particles,

where u g

= mean electron velocity

a = cross section of electron collisions with molecules

It can be seen that the dependence of the threshold intensity goes as ^ % Pv.

Figure 6 demonstrates the J(pv ) dependence calculated from Eq. (4). This formula usually

describes the dependence of the gas breakdown threshold well at high pressures, when v > w, but it is not valid for low pressures. In gases, when v < w, the threshold increases with a decrease in pressure, because electron losses due to diffusion from the interaction volume become significant.

To check the validity of these estimates, we have performed measurements of the breakdown threshold

of water vapor as a function of pressure. The results of the measurements are shown in the diagram

by dots. As seen from the figure, breakdown in water vapor may actually take place at intensities

which lead to surface breakdown.

4. Surface Breakdown Mechanism in IR-Optical Materials

By substituting Pv (pg) in Eq. (4), we obtain the dependence of the breakdown threshold on the ambient gas density in the following form:

J(p

g

) > 2.65 x 107 |l + [v<# u

2 ][l - 0.167U(P

g )/C v ] 12

J

(5)

where v

Q

= v(p 0 )

Figure 7 illustrates the results of calculation by this formula in relative units. Experimental

results combined with the theoretical dependence at the point corresponding to atmospheric pressure

are presented here. The agreement of calculation and experiment is satisfactory.

It should be pointed that Eq. (5) is necessary for breakdown, but it is not sufficient.

Actually, Eq. (5) will, at the same time, be sufficient when the ionization frequency in the radiation field v-j = A

- l(9e/9t) exceeds the probability of diffusion drift of the electrons out of the interaction zone, vp = D/A2

, where D is the diffusion coefficient, A

-2 = 23d -2 + 10£v -2

, d is the spot diameter in the lens focus, and £

v is the dimension of the vapor

region along the direction of incident radiation, i.e., it expresses the dimensions of the steady

-2 rm >>

10 _ 3 cm . hence, A

2 = 0 . 1 2-v2

, i.e., diffusion losses, and hence, the breakdown threshold, do

(4), the avalanche condition (vj >vq) may be written as follows:

not depend on spot size

Taking into account Eq

4 Tie mcA 2 2

i) + v

10D V

(6)

320
For example at ^co? = v ' i- e -» in the region of minimum dependence of the vapor breakdown

threshold on the density, Eq. (6) will take the form:

J » (mcADu)C02 )/(0.47re2 !l v2 ) . (7)

Inserting values of the constants, we have: J » 3 x 10^ w/cm2

. In fact, the threshold intensity for the water vapor which is found from Eq. (5) significantly

exceeds the value in the right-hand part of the inequality. Thus, for our experiments, Eq. (5) is not only necessary, but also sufficient.

On the other hand, the right-hand side of Eq. (7) is proportional to £

v ~2 , where

^v ^ ^w» 1S the equivalent thickness of the water film adsorbed at the surface. Reducing the film thickness should result in the violation of Eq. (7), at intensities defined by Eq. (5), and

hence, in an increase of the breakdown threshold. This explains the breakdown threshold increase at the surface of IR-optical materials under the action of pulsed CO2 laser radiation as a result of laser cleaning, chemical etching and improving the technology of mechanical treatment, which has been reported [2,6,7].

5. Some Specific Features of the Breakdown Due to Absorbed Water

The proposed mechanism results in some specific properties of the breakdown threshold, when it

is determined by adsorbed water:

(a) Dependence of the breakdown threshold on the ambient gas pressure (almost irrespective of the identity of the gas);

(b) Breakdown threshold does not depend on the spot size of the incident radiation;

(c) The relation between surface absorption and breakdown threshold;

(d) The dependence of the breakdown threshold on the wavelength of the incident radiation;

(e) Specific dependence of the breakdown threshold on pulse duration.

5.1 Pressure Dependence

The results of our study of pressure dependence were shown above (see Figure 7). They show good

agreement between the calculations and experiment.

5.2 Spot Size Dependence

As follows from the analysis of the breakdown mechanism (see above), the threshold, determined

by the breakdown in adsorbed water vapor, must not depend on the spot size of the incident

radiation. Experimental verification of this conclusion has been performed using a pulsed CO2

laser, with the radiation focused on a NaCl sample surface with a lens having 12 cm focal length. The spot diameter in the lens focus was 0.5 mm. The spot was enlarged up to 2.5 mm, by means of an increase in the distance between the lens and the surface. Here, to avoid breakdown of the ambient

gas at the focal point, the sample was placed in an evacuated chamber, so that the lens focus was in vacuum. We have also measured the breakdown threshold when the spot was i> 200 ym (lens focus, 5 cm).

Table 2 summarizes our experiments.

Table 2 The dependence of surface breakdown on the spot-size of the incident

radiation (in vacuum).

Spot Diameter, mm Breakdown Threshold, MW/cm2

0 23 36

0 50 35 0 65 36

0 92 36

1 24 35

1 84 37

2 25 36

It can easily be seen that, as was expected, the experimentally measured breakdown thresholds do not depend on spot size. In this case, the irradiated zone morphology corresponded to breakdown near the surface, without catastrophic damage.

321
5.3 Surface Absorption

As shown above, the damage threshold determined by adsorbed water is inversely proportional to the square value of the equivalent water film thickness £w , i.e., to the quantity on the surface.

Since the absorption constant at 10.6 ym wavelength is ^ 10^ cm"', naturally, the presence of a considerable quantity of water at the suface must influence the surface absorption. For this reason,

we have measured the surface breakdown threshold and the surface absorption on the same sample.

Surface absorption was measured by a calorimetric method. Initial measurements were performed on a NaCl surface, which had been prepared by mechanical abrasive polishing, and stored in the open air for a long time. The sample was then exposed for 5 min. to concentrated HC1, then washed in alcohol

and dried. We then measured the breakdown threshold and surface absorption and determined the dependence of the breakdown threshold on the duration of exposure of the sample to open air. The results are described in Table 3.

Table 3 Breakdown thresholds and surface absorption in NaCl after chemical

etching versus the exposure time in the open area

Air Exposure Time Breakdown Threshold, MW/cm^ Surface Absorption, %

Before Chem. Etching 36 1.6 5.0 min. After 220 0.24

20.0 min. 100 0.26

1.5 hrs. 113 0.28

20.0 hrs. 105 0.31 3.0 months 105

As seen from the table, there is a direct correlation between the surface breakdown threshold and surface absorption (the quantity of adsorbed water). It should be pointed out that on a surface free

of adsorbed water (cleavage and measuring of threshold in vacuum) the surface damage threshold

appears to be equal to the bulk breakdown threshold. The surface damage morphology corresponds to damage at impurities.

5.4 Wavelength

It follows from Eq. (4) that the frequency dependence of the breakdown threshold in water vapor

is Jth ^ An experimental study of the frequency dependence was carried out at 10.6 ym,

3.8 ym, and 2.7 ym by means of CO2, DF and HF-TEA lasers. The pulse risetime in all these cases

was 100 ns. The results are shown in Figure 8. The figure corresponds to the dependence

j'th ^ X"2 when normalized to the breakdown threshold at 10.6 ym. The agreement between the calculation and experiment is satisfactory in this case as well.

5.5 Pulse Risetime

The proposed breakdown mechanism is characterized by two parameters: (1) the minimum energy

density Wm -j n of the incident radiation which is necessary to boil off the surface adsorbed water

layer, and (2) the threshold intensity of the incident radiation (dependent on water vapor density)

which is necessary for electron avalanche in the vapor. As follows from a theoretical analysis of the water evaporation mechanism and from experimental results that Wmi

- n

- 2.5 J/cm^. It is clear

that with reduced pulse duration, the threshold energy density must tend to Wmi

- n

, and the threshold

intensity must increase approximately inversely, since the water boiling off is a necessary condition

for breakdown. On the other hand, there also must take place an increase of the threshold intensity

with increasing pulse risetime. The following considerations make it possible to understand this fact. In this case when Wm1

- n is absorbed, the laser radiation intensity will not be sufficient

enough for the development of the breakdown in the whole density region of the expanding vapor. When

the vapor density near the surface decreases sufficiently, due to gas dynamical expansion, so that

one is below the vapor minimum breakdown threshold, (but breakdown does not occur), further expansion

leads to an increase of the breakdown threshold, according to the results shown in Figure 6. We have performed an experimental study of the dependence of the breakdown threshold on pulse

risetime. Pulse duration was varied from 75 to 150 ns by changing the working mixture in a CO2, TEA laser. Due to specific features of the measurement method, the form and duration of pulse "tail" were not significant. We have produced a pulse with a 1500 ns risetime from an ordinary CO2, TEA

laser by means of a cell containing saturable absorber [8] (SF5 gas). Figure 9 shows typical

oscillograms of the incident pulse, with a 1500 ns risetime, and the pulse traversing the sample at surface breakdown. The results of measuring the breakdown thresholds (see Table 4) are in agreement

with the above mentioned analysis, and it confirms the proposed model as well.

322
Table 4 Threshold intensity and energy density at NaCl surface versus the pulse rise time

Threshold Threshold

Pulse Rise Time, ns Intensity, MW/cm^ EnergyDensity, J/cnv

1.2 2500 75.0 100 100.0 150 150.0 80 1500.0 200

3.0 [7]

6. Conclusion

The, mechanism for surface breakdown in IR-optical materials, under the action of pulsed lasers operating in the mid-IR wavelength region, is proposed and experimentally validated in the present

paper.

7. References

[1] V. I. Kovalev, F. S. Faizullov, Kvantovaya

Elektr., 4, 587 (1977); Sov. J. of Quantum

Electronics 7, N3 (1977).

[5] Yu. P. Raizer, Lazernaya iskra i rasprostenie razryadov, Moscow, Naukay

(1973).

[2] V. I. Kovalev, V. V. Moreozov, F. S. Faizullov, Kvantovaya Electr., 2172 (1974); Sov. J. of Quantum Electronics 4, N10 (1975).

[3] A. N. Tikhonov, A. A. Samarasky,

Uravnenniya Matemati cheskoy Fiziki,

Moscow, Nauka, (1977).

[4] Ya. B. Zel'dovich, Yu P. Raizer,

Fizikaudarnykn voln i vysokotemperaturnykh

gasodinamicheskikh yavleniy, Moscow, Nauka

(1966).

[6] S. D. Allen, M. Braunstein, C. Giuliano,

V. Wang, Laser Induced Damage in Optical

Materials, 1974 NBS Spec. Publ. 414, p. 66.

[7] W. H. Reichelt, E. E. Stark. Laser Induced Damage in Optical Materials, NBS Spec.

Publ. 387, p. 175 (1973).

[8] H. K. Kleiman, S. Marcus, J. Apply. Phys.,

44, 1646 (1973).

323
t = o

shock waves

aw p

ambient

gas densities

I 3

contacr discontinuity

£ J! * -4->

density

Figure 1. Surface breakdown model,

RUBY

LASER BEAM

EXPANDER

TEA CO,

LASER ' V--E3—

V

Cl

MIRROR

LENS /r—

r no n i Ms

I PHOTO-

&=WL==«-

I I DETECTORS

2#

_ 1 _ APERTURE

VACUUM

•h CHAMBER

SAMPLE

CM

HTGH

SPEED ) CAMERA 1 /

LENS

-\ PHOTO

FILM

Figure 2. Experimental installation for

investigation of surface phenomena in IR-optical

materials at the intensity of the incident radiation

less than the breakdown threshold.

CO LASER BEAM

>///// ////// SURFACE

TIME DELAY

OF CO„ AND RUBY PULSES

1,3 yvis

3,6

Figure 3. Schl i eren-photo of the shock wave near

NaCfc surface. CO2 and ruby laser pulse time delay

is 1.3, 3.6, and 6.0 usee.

Figure 4. (a) Typical schl ieren-photogram of the shock wave motion in the air in the light of ruby

laser (true oscillation regime); (b) X,t -diagram.

324
Figure 5. Shock wave velocity versus ambient gas density.

0

o

s

m I 2_Q l—i i—i i i i i—i 1 1 1

l_L

10* 10' 10

s

WATER VAPOR PRESSURE P torr

Figure 6. Breakdown threshold in water vapor versus the pressure. Dense curve is calculated by

formula (4).

325
450

100 ao

m

50

0

i

1 r a

AIR |Ne

4 N 0

1 r

AMBIENT

OASES

NaCl SURFACE

THEORY

10' iO

l 10* 10

A

AMBIENT GAS PRESSURE P_ torr

S

Figure 7. Surface breakdown threshold in NaCs. versus the ambient gas pressure. Dense curve is calculated by formula (5) in arbitrary units.

Figure 8. Surface breakdown threshold in NaCfc versus the wavelength of the incident radiation.

INPUT

PULSE

OUTPUT

PULSE

1 ^8 / div . Figure 9. Typical shape of CO? laser input (upper) and output (lower) pulses at surface breakdown

NaCt.

325
Discussion

The speaker was asked if any other species besides water evolved from the sample surfaces . He said that only water was seen below the damage threshold but that, when damage occurred, chemical

decomposition of the sample was observed . The speaker indicated that the wavelength dependence

experiments were carried out with pulses of the same rise time and comparable spot sizes. Finally,

the speaker was asked why the observed threshold at 2 .7. urn (Fig. 8) was not lowered since the absorption in water vapor is greater at 2.7 \im than at 3.8 ^m . The observed breakdown follows the prediction of plasma absorption (Eq . 4) and does not seem to be dependent on molecular absorption

for its initiation.

327
APPENDIX I.

INVITED REVIEW PAPERS

Several invited reviews were presented at the Tenth Annual Symposium on Laser Induced Damage in Optical Materials. These reviews given by the major contributors to this field summarized the state of the art and understanding in the principal sub-areas of optical materials for high power lasers. It is our intention to publish through the auspices of Academic Press, these and other appropriate reviews in

a separate volume. For completeness, we list those presented reviews.

The Design of Damage Experiments

Surface and Thin Film Damage

Scaling Laws for Laser Induced Damage

State of the Art of IR Window Materials

UV Materials for High Power Lasers

Thin Film Materials for the IR and UV

Design Consideration in Thin Film Damage

Surface Evaluation Techniques for Optical Components

Metal Mirrors and Diamond Turning

Factors limiting the Energy Transmitted by an Optical Train

Systems Consideration for Short Pulse, Glass Lasers

Fundamental mechanisms of Ionization

Laser Induced Semiconductor Damage

M. Bass University of Southern California

B . Newnam

Los Alamos Scientific Laboratory

J. R. Bettis

U.S. Naval Academy

A. H. Guenther

Air Force Weapons Laboratory

J. Harrington

Hughes Aircraft Corp.

M. Sparks

Xonics Corporation

M. Braunstein

Hughes Aircraft Corporation

J. Apfel

Optical Coating Laboratory, Inc.

J. Bennett

Naval Weapons Center

D. Decker

J. Porteus

Naval Weapons Center

H. Bennett

Naval Weapons Center

J. Trenholme Lawrence Livermore Laboratory

L. Smith Lawrence Livermore Laboratory

L. Esterowitz

Naval Research Laboratory

328
APPENDIX I I--PARTICIPANTS

S. R. Aiken

Rocketdyne Division

Rockwell International

6633 Canoga Ave. Canoga Park, CA 91304 (213)884-3346

Susan D . Al 1 en Center for Laser Studies

University of Southern California

Los Angeles, CA 90007 (213)741-6705

Thomas H. Allen

Optical Coating Laboratory, Inc.

P. 0. Box 1599 Santa Rosa, CA 95402

Rogers H. Anderson

Honeywell Corporate Material

Sciences Center

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S-l 14A (REV. 5-78)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS SP 541

2. Gov't Accession

No.

3. Recipient's Accession No.

TITLE AND SUBTITLE

iser Induced Damage in Optical Materials: 1978

5. Publication Date December 1978

6. Performing Organization Code

AUTHOR(S)

lited by Alexander J. Glass and Arthur H.

8. Performing Organ. Report No. Guenther

PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 7240580

11. Contract/Grant No.

Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Lectromagnetic Technology Division, National Bur. of Stand.,

julder, CO 80303, American Society for Testing & Materials,

jila. , PA 19103, Dept. of the Navy, Office of Naval Research,

lington, VA 22217, Defense Advanced Research Project Agency,

13. Type of Report & Period Covered

14. Sponsoring Agency Code

SUPPLEMENTARY NOTES Washington, D.C. 20234, Dept. of Energy,

Washington, D.C. 20545.

ABSTRACT (A 200-word or less factual summary oi most significant information. If document includes a significant

bibliography or literature survey, mention it here.) The Tenth Annual Symposium On Optical Materials for

Lgh Power Lasers (Boulder Damage Symposium) was held at the National Bureau of Standard;;

l Boulder, Colorado, from 12-14 September 1978. The Symposium was held under the auspices

I ASTM Committee F-l, Subcommittee on Laser Standards, with the joint sponsorship of

3S, the Defense Advanced Research Project Agency, the Department of Energy, and the

Ifice of Naval Research. About 175 scientists attended the Symposium, including repre-

;ntatives of the United Kingdom, France, Canada, Japan, West Germany, and the Soviet

Tion. The Symposium was divided into sessions concerning the Measurement of Absorption

laracteristics, Bulk Material Properties, Mirrors and Surfaces, Thin Film Damage,

(bating Materials and Design and Breakdown Phenomena. As in previous years, the emphasi:

f the papers presented at the Symposium was directed toward new frontiers and new

velopments. Particular emphasis was given to materials for use from 10.6 urn to the uv

2gion. Highlights included surface characterization, thin film-substrate boundaries,

nd advances in fundamental laser-matter threshold interactions and mechanisms. The

:aling of damage thresholds with pulse duration, focal area, and wavelength were also

Lscussed. In commemoration of the Tenth Symposium in this series, a number of comprepnsive review papers were presented to assess the state of the art in the various

pcets of laser induced damage in optical materials. Alexander J. Glass of Lawrence

(Lvermore Laboratory and Arthur H. Guenther of the Air Force Weapons Laboratory were cotiairpersons of the Symposium. The Eleventh Annual Symposium is scheduled for 30-31

:tober 1979 at the National Bureau of Standards, Boulder. Colorado.

KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

iser damage; laser interaction; optical components; optical fabrication; optical

iterials and properties; thin film coatings.

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

21. NO. OF PAGES

362

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

22. Price $5.50

. AVAILABILITY Unlimited

| ! For Official Distribution. Do Not Release to NTIS _

! Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20402, SD Stock No. SN003-OQ3 -02052-4

| | Order From National Technical Information Service (NTIS)

Springfield, Virginia 22161 I'SCOMM-DC 86680-P78 W U.S. GOVERNMENT PRINTING OFFICE: 1979 O—281-067 (94)
IIIII;
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Superconducting Devices and Materials. A literature survey

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