A UNITED STATI DEPARTMENT ( COMMERC

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NATL INST. OF STAND & TECH

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OCT 1 7 1975

NBS SPECIAL PUBLICATION 372

U.S.

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National

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Laser Induced Damage

In Optical Materials:

197
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i mm ur tiimimm

Laser Induced Damage h1

In Optical Materials: 1972

Proceedings of a Symposium Sponsored by

The American Society for Testing and Materials

And by the National Bureau of Standards

June 14-15, 1972

NBS, Boulder, Colorado 80302

Edited by

Alexander J. Class

Wayne State University

Detroit, Michigan 48202

and

Arthur H. Guenther

Air Force Weapons Laboratory

Kirkland AFB, New Mexico 87117

U.S. DEPARTMENT OF COMMERCE, Peter G. Peterson, Secretary

{ NATIONAL BUREAU OF STANDARDS, Lawrence M. Kushner, Acting Director

Issued October 1972
Library of Congress Catalog Card Number: 72-600276

National Bureau of Standards Special Publication 372

Nat. Bur. Stand. (U.S.), Spec. Publ. 372, 215 pages (Oct. 1972) CODEN: XNBSAV

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. 03.10:372). Price $1.75
TonwonA.

These ptoceedingi tepott in detail the fonmal papens ptesented at the 4th Annual Symposium on Laszt

Damage, in Optical Materials held at the National Bateau of Standards, Boulder, Colorado on Jane 14 and

15, 7 972. This meeting was jointly i>ponsoted by the National Bateau of Standard* and the American Society

of Testing and Materials. The major topics coveted were bulk damage in transparent dielectrics , damage

at dielectric surfaces, damage due to t, elf- focusing, damage to dielectric coating*, as well as damage to

mitrons and window* in the infrared.

The co-chairmen, Vt. Alexander J. Glast, of Wayne State University, Vetnoit, Michigan, and Vr. A. H. Guenther of the Ait T-on.ce Weapon* Laboratory, Kirtland ARB, New Mexico, take full responsibility {on. the

nummary, conclusions, and recommendations contained in this report. Due to difficulties encou.nteM.ed in

the recording process, it was not poi&ible to include a nummary of the discussions at the conclusion of each pn.edentatX.on a& is out usual ptactice. It is suggested that individuals interested in the subject

of this meeting obtain copies of those publications referenced in the bibliography contained in the

Summary and conclusions

.

It is out intention to convene another symposium next yean, in Boulder during May to update and

document the state-of-the-art oh laser damage in Optical Materials at that time. This meeting witl

coven, the subject histonJically presented at these symposia, with additional emphasis on thin film damage,

the problem of the damage of materials and components at 10.6 ym, as well as address ounselves to the

subject of optical reliability as influenced by laser applications. We wish to encounage the reader to

contact us on matters pertinent to the intent of these conferences.

A. H. Guenther

Library of Congress Catalog Card Number: 72-600276

iii

Contents

Page

Foreword ..0.0...° « ..o . . . . . . . iii

A. H. GmntheA.

Summary and Conclusions . vii

A. J. Gtai,6 and A. H. Guznthvi

Opening Remarks 0 1

C. M. Stickley

Laser Damage of HOYA Laser Glass, LCG-11 . . . 3 Tetsuro Iaumitani , Kosaku Hosaka and Chiyoe Yamanaka

Interaction Gradients, Concurrent Light Scattering Experiments and Bulk Damage in

Solids. 11

C. Y. She and David F. Edwards

Optical Index Damage in Electrooptic Crystals 15

A. M. Glass, G. E. Peterson and T. J. Negran

Comparison of Laser Induced Bulk Damage in Alkali-Halides at 10.6, 1.06, and 0.69

Microns 27

D. W. Fradin and Eli Yablonovitch

Laser Induced Damage to Glass Surfaces 40

N. L. Boling and G. Dube 1

The Relation Between Surface Damage and Surface Plasma Formation 46 Concetto R. Giuliano

Ion Beam Polishing as a Means of Increasing Surface Damage Thresholds in Sapphire..... 55

Concetto R. Giuliano

Laser-Induced Damage Probability at 1.06 and 0.69 |-tm 58

Michael Bass and Harrison H. Barrett

Damage to GaAs Surfaces from Ruby- and Nd-Glass Laser Illumination.. ........ 70

J. Lynn Smith

Surface Science and Surface Damage ^

Jhan M. Khan

Self-Focusing with Elliptical Beams

John Marburger

Relative Contribution of Kerr Effect and Electrostriction to Self-Focusing 92

Albert Feldman, Deane Horowitz and Roy M. Waxier

Damage Measurements with Subnanosecond Pulses 100

John M. McMahon

Time Resolved Damage Studies of Thin Films and Substrate Surfaces...

J. H. Parks and N. Alyassini

Laser Induced Damage to Mirrors at Two Pulse Durations.....

E. S. Bliss and D. Milam

Study of Laser-Irradiated Thin Films 123

Brian E. Newnam and L. G. DeShazer

[nfluence of Structural Effects on Laser Damage Thresholds of Discrete and

Inhomogeneous Thin Films and Multilayers..... 135

R. Russel Austin, Raymond C. Michaud, Arthur H. Guenther, Joseph M. Putman

and Richard Harniman

rundamental Absorption Mechanisms in High-Power Laser Window Materials...... 165

Robert Hellwarth
Contents -continued

Page Recent Developments in High-Power Infrared-Window Research ....... 172

M. Sparks

Low Emittance and Absorptance Measurements of Windows and Mirrors . .... 176

Geert Wijntjes, Norman J. E. Johnson and J. Morris Weinberg

Investigations of Pulsed C02 Laser Damage of Metal and Dielectric-Coated Mirrors...... 183

V. Wang, A. I. Braunstein, M. Braunstein , and J. Y. Wada

Appendix

Participants 194

vi
Laser Induced Damage In Optical Materials

4th kSTM Symposium

June 14-15, 1912

kbstract

The fourth kSTM Symposium on Laser Induced damage in Optical Materials was held at the National Bureau of Standards in Boulder, Colorado on June 14-15 of this year. This symposium Is held as part oft the activity of Subcommittee II on

Lasers and Laser Materials, of the ASTM. Subcommittee II Is changed with the re- sponsibility of formulating standards for laser materials , components, and devices.

The chairman of Subcommittee II Is Waynes lee, of Owen-lllinols , Inc. Co-chairmen

for the damage symposia ate Dr. krthur H. Guenther, of the kin. T-on.ce Weapons Labo- ratory, and Professor klexanden J. Glass, Chairman of the Department of Electrical

Engineering at Wayne State University

.

kpproximately 125 attendees at the symposium heard 11 papers on topics re- lating to laser Induced damage In glass, crystalline materials , nonlinear optical

materials , thin film dielectric coatings, and Infrared components. Particular

attention was given to the processes of plasma formation at dielectric surfaces,

the role played by self- focusing In bulk damage In solids, damage morphology of thin film coatings, and the role of absorption In IR component failure.

The proceedings of these Symposia represent the major source of Information In the field of lasen induced damage In optical materials . The Symposia themselves, along with, the periodic meetings of Subcommittee II, provide a unique

forum for the exchange of Information regarding laser materials specifications

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

The Symposium also serves as a mechanism of Information gathering, to enable

the Subcommittee to write Infonmed and realistic specifications.

Key Words: IR windows and mirrors-, laser damage; laser materials; selffocusing; thin films.

1 . Principal Conclusions

In his opening remarks to the 1972 Symposium on Damage In Laser Materials, C. Martin Stickley, of the Defense kdvanced Research Projects kgency, summarized the progress which has been made to date In

the Identification, understanding, and remedy of damage mechanisms In laser materials , and the problems

remaining to be solved. It Is noteworthy that In the four-year span of these symposia, there has been

substantive progress In the understanding and improvements of the damage resistance of laser materials

,

progress which is documented in the series of symposium proceedings. In the study of damage In solid

laser hosts, there has been a healthy transition from Edlsonlan empiricism to carefully designed experimental Investigation, coupled with refinements In theory. There have also been real advances In materials. Bulk damage due to mlcrolnhomogeneitles Is now reduced to a problem In quality control.

Self- focusing damage can be avoided by design, once the material parameters are measured. Surface damage

has been unequivocally associated with plasma formation within the dielectric at the surface, and has been shown to be sensitive to the local electric field strength and to the surface cleanliness and

roughness, and Insensitive to the mechanical strength of the surface, or the composition or absence of background gas. Both surface and balk damage have been shown to be basically electron avalanche processes, with thresholds which depend on power density In the nanosecond region, and In^mrgij density In the picosecond region. Mew and sophisticated diagnostic techniques have been developed for the

detection and Investigation of damage processes, and a data base of relevant material parameters has begun to be catalogued.

In other areas of concern, such as thin films, nonlinear optical materials, and Infrared components, our understanding Is more tentative and our theory more speculative. The reader of this year's

proceedings wilt sense the somewhat empirical flavor, of the Investigations In these subject areas. It Is our hope and our expectation that subsequent volumes of these proceedings will see the same tran- sition from chance observation to careful experiment, and from conjecture to theory In these subjects.

This transition has almost been reached In the area of thin films, but Is more remote In the study of damage In nonlinear optical materials , and even further off regarding Infrared components.

vii
The speclfalc conclusions annlved at cu> a nesult ofa the neponts ofa this symposium one not stan- tlingly dlfafaenent fanom those advanced In last yuan'* proceedings. New nesults at a dlveAS-ity ofa wavelengths, and at pulse dotations -in the, picosecond Keg-ion have substantiated the conclusion that damage

-in tnanspanent dielectnlcs is essentially an electnon avalanche ph.oc.eA6. SuAfaace damage due to this „ pnocess Is the limiting faacton In glass and cnystalline lasens, and the pAesent levels [30 to 40 J/cm

faon 30 nanosecond pulses In lasen glass) can be expected to be noised by a faacton ofa 2 to 10 by sunfaace treatment, such a& canefaul cleaning, and -ion-beam polishing, but probably not much mone. As faon as

selfa-faocuslng Is concenned, thene seem to be tittle hope ofa changing the nonlinean -index, especially

the pnompt component, by vanylng the composition ofa laseA glass. The basic equations governing selfa- faocuslng ate novo well known, and, with the old ofa a digital computet, detailed design calculations

can be done faon any tune legion on. beam geometny, once the material panametens one known.

In mateAlals othex than glass and nuby, oua conclusions one {ok less sweeping and centaln. As stated last yean, theAe Is evidence that nonllnean absonption pnocesses play an Impontant nole In

damage In nonllnean optics mateAlals. No new Infaohmation negandlng this efafaect was faonthcomlng this

yean.

Sevenal Impontant faeatunes ofa damage In thin fa-Urns one beginning to emenge as a consequence ofa In- tensive Investigation, due, In genenal, to the poon homogeneity ofa thin fallms , both In single layeA

and multilayen faonm, It Is dlfafalcult to evaluate the nespective noles ofa absonption and electnon avalanche. Whatever, the openatlve mechanism, -it Is likely that It Is sensitive to the local faleld stnength

which, In a multilayen dlelectnlc, can exhibit pnofaound spatial vanlation. The use ofa vapon phase mixtunes enables {Urns with a gneat vanlation In nefanactlve Index to be laid down with a minimum ofa nesldual stness. This should pnovlde a valuable technique faon the study ofa fa-Urn damage. It seems clean that

by neduclng the stness In the {Urn, the damage thneshold can be noised. TheAe Is also evidence that

damage In fallms Is accompanied by an Incnease In light scattenlng, so that scattenlng measunements appew

to pnovlde a useful diagnostic tool.

(filth the tnemendous Incnease In Intenest In powenfaul cw and pulsed IR lasens, pnoblems ofa damage In the IR windows and mlnnons one becoming Incneaslngly Impontant. It -is genenally assumed that faon these systems, damage pnocesses one essentially thenmal In onlgln. Majon -unpontance Is attached to In- cneaslng the mechanical stnength and neduclng the absonption constant In Infananed window mateAlals to -Us lowest possible value. This Implies a theonectlcol ability to detenmlne what that Intrinsic value

Is, and an expenlmental ability to measune extnemely weak absonptlons. These capabilities one just be- ginning to emenge. The situation negandlng IR window mateAlals Is companable to the state ofa laseA

glass In the eanly Sixties; the limitations ofa these mateAlals one defined mone by Impunity content than

by Intnlnslc pnopeAties. This Is panltculanly tnue faon lange samples, which may exhibit, values ofa Impon- tant panametens significantly dlfafaenent fanom small samples. These dlfafaenences may have Impontant consequences faon system design.

We ate In a veny poon position to dnaw any conclusions negandlng damage at metallic sunfaaces, othen

than to assume -it -is thenmal In chanacten. VeAy little Infaohmation negandlng the monphology ofa the damage, and -its dependence on pulse donation and spot size has been obtained and made available to date.

2. Summany ofa Vapens

The twenty-one papens pnesented at the J 972 Symposium can be logically divided Into falve categohi.es,

These one: Bulk damage In MateAlals, Sunfaace Damage, Selfa-Tocuslng damage, dlelectnlc Coatings, and

Infananed Component Damage. Foun on falve papens wene pnesented In each categony. The following summonies one Intended to pnovlde the neaden with a bnlefa but cohenent nevlew ofa the mateAlal pnesented at

the Symposium, and to dined him to papens ofa specific Intenest.

2.1 Bulk Damage In MateAlals

The {Inst gnoup o fa pnesentations to be summanlzed one those concehned with damage occunnlng within

the bulk mateAAjol. It should be noted that neseanch efafaonts one declining In the study ofa bulk damage In lasen glass due to the geneAolly high quality ofa pnesent day laseA glass and quantitative undeAstandlng ofa the subject. Reponts wene heand on the pnopeAties and chanactenlstics ofa a new Japanese

lasen glass, the analysis ofa scattened nadlatlon as a diagnostic tool faon detecting the onset ofa damage,

the possible causes ofa Index damage In nonllnean optical cnystals, and falnally an excellent pnesentation

on Intnlnslc damage levels ofa alkali halides fanom 0.69 ym to 10.6 ym lasens.

Tetsuno Izumatanl, and Kosaku Hosaka, ofa the Hoya Glass Wonks , and Chlyoe yamaka, ofa Osaka UnlveAslty, neponted on the development and testing ofa a new Japanese lasen glass designated LCG-11 . This

mateAlal Is a Banlum Cnown glass, which In falnlshed faonm has a stimulated emission caoss section ofa 2 x ?0~2"cmz

, and a loss coefafalclent ofa 0.001 cm~'. Canefaul contnol ofa platinum Inclusion leads to a

glass slmllan In damage neslstance to melts In SIO^ on ceAamlc cnuclbles. The cenamlc-melted glass ex- hibited a hlghen losing thneshold, lowen slope efafa-tclency In the conventional mode, and lowen satunation

level In Q_-swltched openation than Pt-melted glass . T-nom extensive testing It Is concluded that the

viii
surface damage. is independent of glass strength, thermal expansion, or absorption of water on the glass

surface. The damage threshold shown by this glass is comparable to other available laser glasses i.e.

balk ~ 400 J/cm2 untreated surface ~ 28 J/cm2 and HF-etched surface - 40 J/cm2

, when employing a multimode laser of 30 nsec dotation.

In a short communication, C. Y. She and E. Edwards , of Colorado State University, discussed theiA

attempt at evaluating the damage sensitivity from an evaluation of the interaction gradients present in

the focal volume, in fused and crystalline materials , using both multimode and TEM ruby lasers.

Based on theiA observation of the intensity and appearance of toward and back scattered radiation in

relation to damage, they tentatively concluded that stimulated Brillouin scattering is responsible for laser-induced damage in both crystalline and fused quartz, while local heating is perhaps more impor- tant in the bulk damage of laser glass. Verification of these conclusions awaits detailed spectral

measurements of the scattered signals.

Turning our attention more towards purely nonlinear crystalline materials , Alistair Glass, G. E. Peterson, and T. J. Hegran of the Bell Laboratories reported on laser-induced optical index damage in

LiNbOv and LiTaO^. A sensitivity in An of 10'^ was claimed when evaluation was accomplished by measuring the diffraction efficiency of a holographic grating generated within the crystal, kt large fields, or for multidomain crystal*, where the above technique can not be used, they measured An directly from the birefringence^ By comparison, this technique has a sensitivity of ~ 10~$ in An. They have definitely

identified the Fe ion as the source of nonlinear index, damage probably proceeds by the reaction

Fe%* + hv = Fe^ +

+ e [cond) . The electrons then drift or diffuse out of the interaction region and are

trapped, kt high spatial frequencies, diffusion becomes the more important process. They have not been

able to ascertain why effect* due to other impurities such as manganese are not observed.

In an attempt to elucidate the intrinsic bulk damage threshold without complications due to selffocusing, multiphoton absorption or cascade ionization of impurities , Vavid Fraden, Eli Vablonovich

of Harvard, and Michael Bass of Raytheon carefully performed extremely welt-designed experiments to measure the absolute electric fields at breakdown in alkali halides, employing various laser sources

ranging from 0.69 \im to 10.6 ym. The experimental arrangement insured that the incident power never

approached the critical power ?„ for self- focusing by employing strongly focusing optical systems.

Sufficient attention was given to evaluation of the details of the incident baser pulses such that

electric field strengths could be accurately computed. Corrections for reflection losses and changes

in calculated beam diameter due to index nontinearity were made. As usual, damage was always associated with visual plasma formation. Results indicated a definite correlation between observed thresholds

for VC avalanche breakdown and the values of the optical field strength at the onset of laser-induced

(AC) breakdown.

There was, in addition, excellent agreement in the ratios of damaging field strengths, for different series of alkati halides. Additional support for the avalanche breakdown mechanism was afforded by considering the variation of damage thresholds when employing a short pulse (~<5ns) Nd:VAG

laser, or varying the rise time of the ruby laser pulse. Further measurements employing very short

optical pulses should conclusively demonstrate the applicability of the avalanche mechanism in the

absence of nonlinear effects.

2.2 Surface Damage

With the improved control of absorbing impurities in bulk material, the dominant mechanism for

damage of crystalline and glass materials is plasma formation at the dielectric surface. The investigations reported at the Symposium emphasized the role of plasma formation in the damage process, and the reasons why the observed threshold for damage at the entrance face is higher than that observed

at the exit face. Following a proposal made at a previous damage symposium, improvements in surface

damage threshold behavior were reported for the first time when sapphire elements were polished by ion

beam techniques.

Ho'cman Baling, of Owens-Illinois, described investigations carried out using amplified, Q_-switched

pulses from a Nd-glass system. The pulse duration was 30 nsec, and from 5 to 12 joules was obtained

in the TEM00 mode. The beam diameter at the sample surface was 1 mm. Samples of EV-2 laser glass were cleaned carefully, by washing in soap and water, rinsing in distilled water, and cleaning in a vapor of boiling isopropyl alcohol. With the beam normally incident on the sample face, entrance face

damage was seen at 120 to 200 J/cm2

, while exit face damage occurred at SO to 120 J/cm*. The average

ratio of entrance to exit threshold was, 1.5. An explanation of this ratio was advanced, based on the

work of M. Crisp. By considering the Fresnel ratios of incident, transmitted, and reflected waves at

the dielectric surfaces, it was predicted that in order to obtain the same energy density in the sample

at the entrance face as at the exit, face, the energy density incident would have to be 1.48 times as great. Experiment and theory seem to be in agreement. Further substantiation was provided by experiments in which samples were iAradiated with the beam incident at Brewster's angle. Since no reflection

occurs at this angle, the same energy density occurs within the material at the entrance and exit surfaces. The damage thresholds observed at: Brewster's angle incidence were essentially the same at

entrance and exit surfaces.

ix
As has been reported elsewhere, Boting obieAved that the moit volatile element of the glass, in

this case lithium, was depoiited in a ling on the. damaged iurfaces, just outside the legion of interne

illumination. This was ascribed to the depoiition of mateAial from the damage plasma in contact with

the cool iurface iurrounding the damage area.

Efforts to itrengthen the iurface by exchanging iodium far lithium ions, thereby cheating a iurface \ compnesiion layeA, led to no measurable increase in the iurface damage threshold.

Concetto Giuliano of Hughes Research Laboratory investigated the role of plasma formation in outpace damage. Using a ruby laden, emitting 20 miliijoules in a diffraction limited, single mode beam, in

a 20 nsec pulse, entrance and exit iunface plasma* famed on iapphlre samples were investigated. By

irradiating the iurface at an angle to the normal, Giuliano was able to resolve the ipatial component!, of the plasma. At the entrance iunface only, a component of the plasma was obieAved to propagate back

towards the laser during the time the laser pulse is present, vanishing thereafter. Kt both entrance

and exit iurfaces, another component of the plasma was obieAved to propagate away faom the iurface in a

1 perpendicular din.ecti.on, and to persist after the illuminating pulse was tanned off. When samples wetie 1 inAadiated in vacuum, the laseA-dniven plasma at the entAance iurface was not obieAved. Giuliano con- cluded that the lasex-dniven plasma was iupported by the backgnound gas, while the persistent plasma

was emitted faom the damaged mateAial. Both Giuliano and Boting indicated that, although the plasma

fanmed is iometimes very weak and difficult to detect, one can itiil iay quite confidently, that iunface

damage is alwayi accompanied by plasma fanmation.

Giuliano also examined the effect of ion-beam polishing on iunface damage. Sapphine iamples were

polished with an argon ion beam at 7 Kev. ¥nom 2.5 \im to 5 \im of mateAial were removed. X-ray topoj graphy indicated that at this depth, moit of the iunface icnatches wene nemoved, although iome residual

iunface featun.es wexe obieAved. At both entAance and exit faces, an improvement in damage thAeshold of a factor of 2 to 6 was obieAved. However, the ipKead in damage thAeshold values fox ion-polished iuAfaces was much greater than fan. abrasively polished iuAfaces, and the damage, when it occwmed on the

ion-polished iunface, was mone extensive. Both these features were attributed to the fact that the ion- | polishing removes regions of low threshold, which provide iites for failure in abrasively polished iuAfaces, leaving a iunface of uniformly higher resistance to damage. The greater uniformity of the iurface

enhances the itatiitical nature of the damage proceii.

Michael Baa and Harrison Barrett of Raytheon itudied iunface damage in a variety of transparent

dielectrics, using a Nd: vAG laser and Ruby laser of comparable output characteristics. In general, they

found that a mateAial is lesi likely to damage at a given power density level [or electric field itrength

when illuminated at the ihorter wavelength. In iome materials, this may be due to the presence of resonance effects, but far moit, they ascribe it to the fact that a nearly free electron gains lesi energy j in a higher frequency field at the iome field itrength. Thus, a higher field itrength is required at

this higher frequency to create electrons energetic enough to initiate an avalanche ionization procesi.

Baa and Barrett itudied the distribution of damage itanting times near threshold for two different , materials , at 1.06 ym. They found a iignificant ipread in damage onset times in relation to the laser

pulse maximum, although the distributions tended to be narrower at higher power densities. A probabilj istic explanation of the itanting time distribution was advanced.

The laser-induced damage to GaAs iemiconductive mateAial was the iubject of a itudy by J. Lynn

Smith of U.S. Anmy Hisiile Command. Both conventional and Q-iwitched lasers, operating at wavelengths

corresponding to photon energies below and above the band gap were employed. Modification of concen- tration uniformity was evident when iamples were inAadiated with long pulses, probably due primarily to thermal effects. Surface damage from Q_-iwitched laser pulses (i.e. high power densities) was fairly

iensitive to the iurface condition and the presence of mechanical flawi. damage thresholds were not ob-\

ierved to depend on the presence or type of doping [n-type, p-type) although doping changes affected

the photocurrent iignal amplitude. ObieAved thresholds were also insensitive to whether ruby or glaii

lasers were used, which was iomewhat iurprising. This may be the result of iome breakdown plasma ihielding of the iurface. damage which alwayi occurred at the entrance face was undoubtedly due to surface

imperfections, and not the intrimic itrength, or variations in compoiition of the GaAs iamples tested.

This obieAvatlon was iupported by photomicrographs of the damage areas.

Jhan Khan, of Lawrence Livermore Laboratories, made an impasiioned plea far application of very

fundamental and basic research methods to the iurface damage question. A review of applicable techniques, including iuch recently developed methods as thoie employing electron analyiis e.g., low energy electron diffraction ( LEEV) , inelastic electron icattering (IES), Auger electron ipectroicopy (AES) and

electron ipectroicopy for chemical analyiis (ESCA). The use of these and other analytical tools ihould

afford an improved characterization of the iurface, iuch that the detailed itructure, compoiition, and

electro -optical properties can be evaluated as to their influence on laser-induced iurface damage. By

analogy, this careful attention to basic material properties has paid a handsome dividend to the iemiconductor industry in termi of performance, reliabilty, reproducibility and econlmic advantage.
2.3 Self-Focusing damage.

Once the manufacture of laser material is brought under sufficient control that absonbing inhomogeneitieA axe absent, material failure due to the formation of interne filaments by self- focusing

become* the limiting <pn.oc.eMi in long samples. Although the basic featureA of self-trapping axe well

undeAStood, questions still remain regarding the actual valueA of nonlinear index coefficients for materials of intereAt, and the relatvie contributions of prompt nonlinearitieA [KeAn Effect) and delay&d

nonlinearitieA [ electrostriction) . In addition, the influence of variations, in beam geometries on the

meoAured threshold for trapping muAt be considered in interpreting experimental reAults

.

John Marburger, of the University oft SoutheAn California, treated the caAe of self- focusing of a

Gaussian beam of elliptical cross section. He used the self-simJLar, approximate solutions to the

paraxial equation, under steady-state conditions, . HiA analysis predicted that the critical power fan

A elf-focuiing iA a minimum fan circular beams, and that it it> increased for initially collimated,

elliptical beams in the ratio, (x + 1/x)/2, where x is the natio of the majon and minon axeA of the

beam cross section. Similar increoAeA over the critical power for circular beam* were predicted for

pie- focused elliptical beams, and far non-Gaussian beam profileA. ExpeAimental data were cited to iuppont theAe conclusions

.

The respective contributionA of electrostriction and Kern effect to the nonlinear index, n<i, of BSC glaAi, fused Ailica, and {lint glass were meoAured by Albert Feldman, Veane Honowitz, and Roy Waxier

of the National Bureau of Standard* in Gaithersburg . Their meaAunementA were carried out at 1 .06 \m,

uAing a laAer pulAe of 25 nsec duration using two different focal lengths [HI mm, 76 mm) fan sample

expo-sure. By comparing the facuAing thresholds fan circularly and linearly polarized light, they were

able to deduce the electro>strictive and Kerr components of the nonlinear index. Once theAe nespective

contributions are known, one can compute the expected threAhold fan any pulse length and facal volume,

oa long OA the material pnopertieA are known, and the Kern effect can be treated as prompt.

John HcMahon of the U.S. Haval Research Labonatory pneAented experimental meoAurementA of n^ in

faun commercial laser glasses, meoAured by observing the self-focusing length at 1 .06 vim fan pulse

durations fnom 20 psec to 250 pAec. Values obtained fan n^, in 10-14 zau, were oa fallows: BK-7 (6), EV-2 [l±d), CaAt MG-975 (7+3), and LG-56 (S±3) . Over the range of pulse duration from 20 to 250 pAec,

the value of n^ obtained woa sensibly constant.

2.4 Vamage to Optical Coatings

Aa expected, this year' a symposium included many mone papers on laser-induced damage to optical

coatings. Some additional infanmation on the damage threAhold of specific coating materialA woa pne- sented along with experimental evidence of the effect of film streAS and specific multilayer dejsign,

and developments of impnoved diagnostic methods. There was an underlying thread of possible theonetical explanation fan conrelation of damage values and monphology of damage by necourse to considerations

of the electric field strength distributions within the film system. Continuation along theAe UneA

will undoubtedly lead to a greater facility in the asseAsment of damage neAistance of various film

deAigns prion to fabrication. Specific papers dealt with the development of scattering measurements to asseAS the appnoach to damage prion to catastrophic failure, damage thresholds of various coatings at

both picosecond and nanosecond pulse durations, morphology oh film damage, the specific influence of film composition and deAign {including vapon phase mixtures and periodic inhomogeneous coatings) , and

measurements nelating film streAS to damage threAhold. There was a definite impnovement in film durability where stress compensation was utilized in vapor phase mixture* of opposite stress components.

J. H. forks and W. Alyassini of the University oh Southern California discussed the utility of employing a HeUe laser to measure variations in the scattering behavion of high power laser irradiated areas prion to, during, and after exposure to high power nadiation. This time-resolved measure- ment allowed them to look at precursors, concurrent neAponse, and long-term effects to the coated

surfaceA. They have demonstrated an ability to obseAve distnubances below oa well as above the threAhold

fan visually observed damage. They observed the ejects of surface pitting and plasma farmation; in

addition, pnopagating surface disturbanceA were tentatively identified as Rayleigh waveA. Monolayer

films of ZnS, CaFg, and Had on fused silica substrateA as well a* uncoated fused silica and Lit substrateA were studied.

The laser-induced damage to overcoated mirrors at two pulse duration woa the subject of a paper

by E. S. Bliss and V. Milam of the Air Fonce Cambridge ReAearch LabonatonieA . They employed both modelocked [20 psec) and Q_-switched [20 nsec) pulse durations fnom a ruby laser. Targets were 95% neflecting multilayer dielectric miArors. Considerable effort was expended in oAcertaining the specific

spatial and temponal characteristics of the damage-pnoducing lasers. Smooth layer nemoval over the

whole irradiation area was observed when both psec and nsec pulseA were employed, providing the intensity distribution was smoothly varying. However, if spatial inhomogeneities were evident [occaAionally

in the mode-locked case) nonuniform variation in damage sites was evidenced. A ragged pattern was normally pnoduced at the edge of the damage, area and was ascribed to coating inhomogentitieA , since the

laser output was determined to be highly homogeneous . Measured threAhold values ranged fnom 0.6 to

xi
2.9 J/cm &on. thz modz-Zockzd casz and- ^nom 16 to 58 3/arr ion. thz Q-iwltchzd zxpznlmznts dzpznding on

&lZm compoiition and numbzn. o& Zayzsu. Thz ipzcl^lc matzniaZi uizd wznz 9 to IS Zayzn. itacki o{, TlO^lSlQq]

13 to 25 Zayzn. itacki o& InD-ilSlO^ and a 21 Zayzn. itack o& ZnS and ThV^. Baszd upon zxpzAlmzntaZ obiZK-

| vation and mon.pkoZogtc.aZ examination, thzy concZudz that pZasma ^onmation Is thz opznatlvz damagz mzchanlim and that damagz ti bzit dzicxlbzd In tznmi o^ znzxgydzmltlzi at vzny ihont: puZszi and powzn. dznilty at Q_-iwltchzd puZiz Zzngths, conslstznt with an zZzctxon avaZanchz pn.oczii.

In a iznlzi o£ zxpznlmznts on thz Zaizn.-tndu.czd damagz ofa thin (>lZmi Bnlan Uzwnam and Lanxy

VzSkazzn. 0& thz UnlvznilAy OjJ Southzn.n CaZllon.nal invzitigatzd thz damagz oi mono-, bl-, and muZtlZayzuzd coatlngi o& T10„, Slfy, InQ^, MgF

2

, and 2nS on iubitxatzs ojj gZaii, n.ockiaZt and iplnzt, undzn. the. action o& a TEM0o Q_-iwlzchzd nuby Zaizn. In tznmi oi dzcnzaslng damagz izilstancz ofa quaAtzt-wavz ilZmi on BSC-2 gZaii, a hlznaxchy In on.dzn. \hgT- ^ $>IQ <j>ln.Q i^T10q>lnS> wai obiznvzd, which ^oZZowi an appKoxlmatzZy InvzAiz Kzlatlon to thz nz^Kactlvz indzx. By computing thz z^zctlvz zZzctnlc {IzZd itnzngth at

thz boundary in dl^znznt thlcknzii {IZmi a conxzZatlon bztwzzn damagz and z^zctlvz {IzZd wai iuggzitzd.

Ai ^llms bzcomz ilmpZ&n. in dzslgn, it wai iuggzitzd that an IncAzaiz In icattznlng, whzn iubizquzntZy

tZZuminatzd by a HzMz ZaszK, was a mon.z izniltivz tzit ion thz onszt oh damagz. In many ilngZz-Zayzx

^IZmi this opznatlon pn.zczzd.zd by a imaZZ amount thz itandaxd ipank tknzihoZd. Howzvzn., duz to thz ln- hzAzntZy highzn icattznlng manlfazit in mon.z compZzx muZtiZayzn. iyitzms thz ipank thnzshoZd was mon.z n.z- ZtabZz and thzfiz^on.z nzcommzndzd. Onz Zait intzAz&ting nziuZt o{, thz uion.li was a gznznaZ tzndzncy £on thz damagz thn.zih.oZd to pnoczzd in a mannzn. InvzmzZy with thz undamagzd falZm icattzn.

In an attempt to zvaZuatz thz InfiZuzncz oft boundanlzi on thz damagz tknzihoZd o& itandan.d dlscAztz

muZtiZayzn. faiZm iyitzmi, Ruii Auitin and Ray ULchaud o{> Vzniiin EZmzn. Con.p. and hit Guznthzn., Joe Putnam,

and Vick Hannnman ofa thz Kin Von.cz Wzaponi Lab, con&tnuctzd pzftiodlc inhomogznzoui ^iZm in which thz

n&6n.activz indzx vantzd in an appKoximatzZy iinuioidaZ mannzn. without ihanp opticaZ on. mzchanicaZ diicontinultLzA. Ihii wai achizvzd thxough thz iimuZtanzoui dzpoittton ofa two matznlaZi zmpZoying indzpzndzntly contnoZZzd ioun.cz naZz gznzn.atou. Thznz wai no iigni&icant di^zKzncz in damagz thkzihoZd

whzn companzd to thztn. diicAztz {iZm anaZoguz. TKii wai atAxibutzd to thz ovzwiiding infaZuzncz ofi thz ! thin {iZm, i.z. ThF^ and ZnS, iincz thziz matzniaZi in pun.z faonm onz gnoaZy dt^zKznt in damagz tkxzihoZd. In a izpanatz zxpznXmznt, vapon. mixtanzi oi thziz llZmi gznznaZZy zxhibttzd. a damagz thnzihoZd bziiozzn thz punz (^onrni which wai pnimaniZy dzpzndznt on thz ipzct^ic compoiition. EmpZoying matzniaZi o£ oppoiltz itxzii and ilmlZan damagz thn.zihoZd, \hgf<^ a|tcf 5^.02, a dz{,lnltz IncAzaiz In damagz thxzihoZd

ovzn. thz punz matzKiaJLi wai zvldznt duz to this ^onm o{, itkzii compzniation. By chooilng appncpnlatz

matzntaZi 0j$ oppoiltz itnzii and dl^zKznt KzhnxLctivz indlczi, thz ^IZm dzslgnzn. can havz a gnzat Zatltixdz in izZzcting {iZmi o^ IntznmzcLcatz nz{n.actlvz indlczi and Zow itAzii. VIZm iyitzmi can bz dzilgnzd

which zxhlbit a highzn. damagz thn.zihoZd and thus IncAzaiz utULity as opticaZ componznti . \lanljation In

damagz thAzihoZd duz to dif^znznt Incldznt coating angZzi was attnAhutzd to itnzii vanlationi as wzZZ. Thz pn.zizntjxtA.on conzZudzd with numz/ioui icanning zZzctAon mlcn.ognjx.ph and NomaKikl phasz contAast mlcAognaphs dzplctlng thz monphoZogy o^ damagz. ConAzZatlom baizd upon faiZm dzilgn, compoiition, and n.ziid- uaZ itnzii wznz madz.

2.5 Damagz to In&nanzd Wlndowi and IKlnxoKJS

With thz IncAzaslng zmphails on high powzn. pzK^onmancz In thz In^nxvizd, thz attzntlon o& thz Zas&n. matzniaZi community Is bzlng dnawn to thz pn.obZzmi ofa piovldlng windows and mlnAOKS capabZz OjJ handZing

high powzn. dzniltizi in thz InfiiaAzd, zipzzlaZZy ^n-om 2 ym to 10 ym. Although at vzxy ihonX. puZiz

duKatloni pZasma ^onmatlon and gas bn.zah.down can bzcomz Impontant, fion. cw ZaseAi In thli wavzZzngth

nzglon thz Zlmlting matzKiaZ pfiopznty Is thz KzilduaZ abionptlon o^ thz matznlaZ. At powzn. dzniltizi

zxczzding onz feW/cm , abion.ptloni which onz aZmoit unrnzoiunahZz at Zow Intznilty Zzad to iyitzm dzgnadation and componznt ^oIZukz. It Is antlclpatzd that thz Intzuzit o{ thz damagz iympoilum wlZZ bz con- czntnatzd on this pn.obZzm faon. izvznjxt yzanjs to comz.

Robznt HzZZwanth, ofi thz UnivzAilty ofa Soutkznn ZaZl^oKnla, pn.zizntzd a n.zvlzw ofi thz poalbZz ab- ioKption mzchanlsmi which might contnlbutz to thz nzslduaZ abionption In oitznilbZy tnanipanznt matzntaZi In thz IR. He divldzd thz pKoczazi Into two catzgonlzi , IntAlmlc and zxtsilmlc. IntAlmlc pKocziizi

onz thziz which occxoi In punz matzntats at thznmaZ zqulZlbnMxm, iuck as Zattlcz vlbnatlonaZ bands, f^Kzz- caMJLzn. abiOKption, and bound- zZzctxon zxcltatlon. Extxlnilc pKocziizi anJUsz duz to impuAltlzi , dislocations, on dz\jlationi {>n.om itolcklomztn.y . Hz pn.ovldzd a bnlzi n.z\)lzw o{ zach pnocza, and ikztchzd

thz kind oh wavzZzngth dzpzndzncz onz would zxpzct to obizKvz. Evzn though thzfiz an.z ilgnlfalcant gapi In thz ^undamzntaZ undzmtandlng o^ many oft thz pKocziizi InvoZvzd, zipzclaZZy zxtxlnilc pKoczazi

,

czntaln concZuilom can bz nzachzd nzgaxdlng thz pKognoili {on. matznlaZ lmpn.ovzmznt. Slncz moit matzniaZi zxhlbit fiundamzntaZ Zattlcz &n.zquznclzi In thz IR, thzsz pnocziizi wlZZ pZay an Impontant noZz In

dztzAmlnlng InlnaAzd abionption. Tn.zz-canAlzn. abion.ptlon li aZso unavoldabZz, but Its Impontancz as a

Zlmlting mzchanlsm can bz itAongZy In&luznczd by thz pKzizncz o{ Impunity ZzvzZi and o{ Intznband tnaniltlons. It Is zxpzctzd that thz iamz pKocziizi wlZZ bz Impontant In thin {Urn matzniaZi ai In bulk, once

thz {IZmi onz pnoduczd with iu&fiiclznt homogznzlty

.

MamhaZZ Spanki, o& Konlcs Inc., pnzizntzd a dlscuiilon o& how wzak abionption In window matzniaZi

can Zimlt iyitzm pznfaonmancz. Chooilng a ipzcl^lc izt ofa panamztzu {,on thz window [10 cm diamztzK,

7.3 pit pnziiuKZ dl^znzntiaZ) hz dz{lnzd a ilgunz o{ mznlt on thz bails ofi thz maximum Intznilty which

xii
could be transmitted through a given material without experiencing either thermal fracture or undue distortion. The latter can arise either through thermal distortion due to absorption heating ok pressure- induced distortion {bowing). Of the materXjals reviewed, ZnSe is identified as the most promising at

10.6 ym. Ton. the window dimensions assumed, a power density of 3.76 kW/cm^ could be transmitted, with

a safety factor of 4, limited by the pressure-induced fracture of the window. Using a value for the

absorption constant of KBr measured in laAge samples, (4 x 10~$cm~1 at 10.6 ym), that material is not

very promising, but if one assumes the estimated intrinsic absorption coefficient value fon. KBr of

5 x 10'^cm', then it compares favorably with ZnSe. In order to measure these low absorption value*,

Spank* recommended use of emissivity measurements.

An instrument designed to pnovide the measurements of veAy low absorption value* wo6 described by GeeAt Wijntjes and J. Morris Weinberg of Block EngineeAing. They have developed a cryogenic interferometAic spectrometer which can achieve a noise equivalent spectral radiance of ~ 1 x 10'" W/cm^-ster - cm

-

' at 10.6 ym, with a spectral resolution of ~ J cm". This pexfonmance requires about 10 sees of integration time. Absorption data is obtained from emittance measurements. The declared precision ap- proaches one paAt in 10^ . The high spectral resolution of this instrument permits examination of the

effects of absorption band structure from substrate and coatings as well as identification of contaminate mateAials. Measurement of bulk scattering effects in transmissive materials is also possible.

This technique should find wide application in selecting suitable new materials and in quality control.

A comparison of the performance of various types of high power mirrors at high heat loading was presented by Victor Wang, A. and M. Braunstein, and J. V. Wada from the Hughes Research Laboratories

.

They examined the limiting damage mechanism in metal mirrors, gold-coated refractories , and multilayer

dielectric coatings. Experimental data were presented showing that the failure mode of metal mirrors

is generally by melting or oxidation; that metal-coated refractories tend to fail by melting, oxidation,

bond separation, or cracking; and that multilayer dielectrics fail from melting, cracking, separation,

or pitting. Observed damage levels in & to 10 \isec pulses were 140 J/cm? for simple metal mirrors,

70 to 140 31 cm" for metal-coated metal mirrors, and as low as 10 J /cm" for multilayer dielectrics. The

last were admittedly of poor quality. Sputtered metal coatings on metal were identified as most prom- ising for high pulse power applications.

3. Recommendations

It has been pointed out in the section on principal conclusions that not much remains to be done

in the area of bulk damage in laser materials. The thresholds for damage due to bulk dielectric breakdown are so high that tlus phenomenon is clearly not a limiting factor in systems design. Similarly, in

the case of self- focusing, this phenomenon is now so well understood that, given the prompt and electrostrictive components of the nonlinear refractive index, avoidance of self- focusing is a matteA of straight

forward, albeit tedious, numeAical computation. One possible exception to this statement arises in the

design of systems operating with pulse lengths less than 10 pszc. Tor these very short pulses, the characteristic response time of the nonlinear index is still open to question. As reliable and reproducable

systems operating in the time region of one to ten psec become available, it is recommended that further

investigation of the transient response of optical nonlinearities be carried out.

There are some important recommendations to be made concerning laseA-induced surface damage. This

phenomenon is still the prime limitation in high power laser devices, especially if one is forced to short active elements or disk geometries to avoid self- focusing. We have heard in this symposium of

possible damage improvement through the use of ion polishing, and as previously reported, by chemical

etching with HF. Careful experiments on the characteristics of treated or polished surfaces Is certainly

warranted to evaluate the effect of the surface properties on damage threshold. The interrelation be- tween surface roughness and damage threshold remains to be elucidated. Surface roughness obviously can

be controlled by the appropriate choice of polishing technique.

The spectral dependence of surface damage has only been partially investigated. Bass's results

show that the surface damage threshold at 0.69 ym is lower than at 1.06 ym. It would be of great in- terest to extend these experiments to 0.53 ym, for example, by using frequency doubled Nd:VAG. As a

final step in verifying the electron avalanche nature of surface damage, threshold experiments in the

subnanosecond regime, with good mode control, should be carried out. The efficacy of surface treatment

for subnanosecond pulses remains to be verified. There is no reason to expect anomalies when ultrashort pulses are employed, but complete verification of the present model of surface damage is clearly

in order.

In regard to nonlinear optical materials, emphasis should be placed on improved material fabrication, including purification of starting materials, to afford better reliability and reproducibility.

Until this is accomplished, it will be difficult to evaluate properly the damage sensitivity of different candidate materials. Preliminary observations indicate a disastrous lowering of damage thresholds in nonlinear mateAials when multiple wavelengths are present, but no careful investigation of this

effect has been carried out. Clearly, it should be investigated.

As predicted, considerable research was undertaken in the area of damage to thin films since the

previous symposium. Based on the several excellent presentations at this year's symposium, several

xiii
positive recommendations can be made.. The experimental determination of damage threshold in various

thin film systems should be correlated with ike microscopic electric field strengths throughout the

structure. This obviously entails a. detailed modeling of the electric field distribution, energy deposition, and material response of the film system. These computations should be carried out dor both

normal and off-normal incidence. The sudden awareness tills year of the effect of the Fresnel relations at dielectric interfaces should provide a valuable lesson for those studying damage in multilayer

dielectrics

.

The effect of surface finish on thin film damage threshold has not been investigated. In this re- gard there could be valuable insight gained from the studies recommended of the surface damage of un- coated optical elements. While there novo exist various tabulations of the relative damage threshold oh commonly used coating materials, because of the lack of reproducibility in structure, method of preparation, impurity concentration, cleanliness and background coating pressure, no plausible hierarchy of damage resistance can be formulated which will be accepted with confidence by thin film manufacturers.

We still have not pinned down those material properties which will allow a judicious selection of materials for any given high power application. Therefore, carefully controlled experiments with de- tailed statements of significant material properties are necessary. Hot only should the damage resistance of specific materials be investigated, but attempts at elucidating the damage sensitivity in terms

of scattering, reflectivity, and absorption should be made. Finally, it: is suggested that to answer

questions concerning mechanical failure due to separation at discontinuities in discrete films, that

experiments on periodic inhomogeneous films be conducted using materials of similar damage threshold and

compared to their discrete layer analogues.

Our knowledge of damage processes for infrared windows and mirrors is in its infancy. Clearly,

this is an area to which the major research activity should be directed. Not much progress can be ex- pected; however, until fabrication and polishing techniques, especially for crystalline materials , and

purification methods for raw materials are improved. The characterization and measurement of weak ab- sorption is of fundamental importance in reducing thermally induced damage. Both theoretical and experimental advances are needed in this area, damage criteria for infrared components must reflect the

degradation of system performance, which may result from the distortion of the optical element rather

than from catastrophic damage, and must be so defined. Thus further study of system sensitivity to com- ponent degradation must accompany the investigation of material properties and damage phenomena. Im- proved diagnostic techniques will be required to detect and measure the thermomechanical response of optical elements, and correlate it with observations of reflectivity, scattering , absorption, depolarization, and. other purely optical parameters. Although at present the assumption is made that thermal

effects dominate, this conjecture remains to be substantiated.

Perhaps it is appropriate to conclude with some very general remarks concerning the future. With- out question, an area of future endeavor will be in extending these damage studies into the ultra-violet

and x-ray regions of the spectrum. Increasing interest in the high power Xe laser, for example, fore- warns us of impending requirements for damage resistant uv windows and mirrors. In addition, numerous suggestions for realizing x-ray laser action have been published. Here in pari specific crystalline

materials may effectively act as wave guides or even amplifiers.

Because of the tremendous increase in our understanding of many aspects of laser-induced damage,

it is probably appropriate to consider assembling this knowledge in a form suitable to the design

engineer. This would be an entirely appropriate activity for the ASTM Subcommittee on Laser Materials

to undertake. Such a compendium, including suggested approaches, material properties, and possible

pitfalls to be avoided, could be generated either as an ASTM document or as a supplement to existing

handbooks. This latter possibility is under consideration by the editors.

4 . Acknowledgment

We would like to acknowledge the Invaluable assistance of Dr. Harold Boyne, Mrs. Pauline Smith,

Mrs. Marjorle L. Wilson, and Mrs. Francis M. Stark of the National Bureau of Standards in Boulder,

Colorado for their interest, support, and untiring efforts in the operation of this symposium and in

the preparation and publication of the proceedings . The continued success of the damage symposium

would not have been possible without their support.

5 . Bibliography

[1] "Damage in laser Glass" , A. J. Glass and A. H. Guenther, Edilors, 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, V.C. [1970).

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

Publication NMAB-111, National Academy of Sciences, Washington, D.C. (1910).

xiv
[4] "Damage in LaieA MateAiali: 1971", A. J. Glaii and A. H. GuentheA, EditonA, W8S Speexa£

Publication 356, U.S. Government Pointing 0&&ic.e., Workington, V.C. ( 7 97/ )

.

[5] "High Power lnl>uui<Ld Laiex Window", N. BleembeAQ&n, National Material!, Advisory Board Publication

WMA8-292, National Academy Ojj Science*, Washington, V.C. ( 7 972).

[6] Pnoceedingi, ofa the Conference on High Power Infrared laser Window Material*, [October 1971). Ed. by C. S. Sahagian and C. A. PiMia (Air Pon.ce Cambridge Research laboratories , Special Report

No. 127, Vecember 1971).

A. J. Glass

A. H. Guenther

XV

"Opening Remarks"

C. Martin Stickley

Materials Sciences Office

Defense Advanced Research Projects Agency

Arlington, Virginia 22209

It is a pleasure to be here this afternoon attending this Fourth ASTM Conference on Laser Damage.

This is also the fourth one I have attended since I have felt for a long time that materials damage

places unnecessarily low limits on laser power output, and consequently have been pushing for financial

support to study this area in order to see what could be done to increase our understanding of these

nonlinear mechanisms and to raise the damage threshold.

Since work in this area has been underway for several years, it seems appropriate at this time to give a short assessment of the highlights of what we have learned, and then to point out some of what

remains to be done. Unfortunately (or fortunately if you happen to be earning your keep working in this field) it appears that more needs to be done than we have accomplished, but I feel that future progress ought to come rapidly over the next two or three years since we are getting fairly high up on

the learning curve, at least for transparent dielectrics.

To measure progress made, one must have a point of reference. At the start of most of the activity

that has been reported here at these Conferences , ruby was thought to be free from iridium but that re- mained to be verfied. Glass, on the other hand, still contained platinum which lead to low damage

thresholds and ruination of the laser rod. Some reports of damage thresholds for glass were in the lit- erature but their values spread over three orders of magnitude at 30 ns. The problem of damage to nonlinear optical materials was only beginning to be recognized, and mirror damage was known to all but understood by no one.

What have we accomplished? First, we learned iridium played no part in ruby damage, and we have

succeeded in removing platinum from glass. Second, our measuring techniques have improved vastly. We all now recognize the necessity of working with TEMqq mode beams in order to obtain reproducibility and understanding of what goes on. Our refined measuring techniques have also led us to distinguish between

the different kinds of damage - exit surface, self-focusing, inclusion, and entrance surface. Third, we have determined that self-focusing is the most important mechanism leading to damage in the material.

Although we still may not be absolutely certain what drives the self-focusing, we can, in principle,

design laser systems around it which can operate at hundreds of Joules /cm . Fourth, and this is perhaps

still somewhat subject to argument, it appears that the final breakdown, at least in the picosecond re- gime and perhaps for longer pulse lengths, is avalanche and that the track diameter is consistent with

the size necessary to trap sufficient power to breakdown the material by this mechanism. It turns out

that the breakdown field strength in alkali halides are equal to the dc values observed by von Hippie

some 35 years ago. Fifth, surface breakdown also appears to be initiated by avalanche and by treating

the surface so as to minimize the chances of an encounter of the laser beam with the right type of electrons, whatever they are, one can raise the surface damage threshold significantly. This total process,

however, is very poorly understood. With respect to nonlinear optical materials, damage thresholds have

been obtained but it is not clear why the large spread exists over the range of materials examined nor

if any of the thresholds represent a fundamental limit. That is, surface treatments and inclusion re- moval might be quite important in increasing the power levels at which they can be used successfully.

With respect to mirror and coating damage, the values are certainly higher than they were several years

ago, but this came about more through fine tuning of the art of making these than by any great depth of understanding of what goes on during the breakdown process.

Now what remains to be done? It seems to me that there are a number of areas, with varying degrees of importance, which it is necessary to pursue. First, in bulk damage, what mechanism leads to self-focusing in the different time regimes of interest? In the 30 ns or longer regime, it seems that

electrostriction is responsible, but for shorter pulses it is more debatable. Next knowing the mechanism, is it possible to design materials which have minimum susceptibility to damage? Second, now that we have a rough feel for what goes on in the bulk, one of the most fruitful areas to pursue is surface

damage. We need to increase our understanding of it and devise surface treatments to raise the threshold. Knowing this we need to translate these treatments into recipes which can be used by the optical

material firms which make these materials. This fits perfectly into the role of ASTM, our sponsoring

organization for this meeting. Regarding nonlinear optical materials, the success of surface treatments

needs to be demonstrated on them. Further , I feel that more work needs to be done to determine the

1
damage thresholds in a way which is more simply translatable to the device designer. Regarding material j inhomogeneity , that is a major problem; it will probably require a very important application to arise

before significant resources will be expended in that direction. With respect to mirror and coating damage, this has almost no base of understanding. As laser powers grow larger, this area will increase in importance since the cost of replacing the optics, as we have become used to doing in low average power

systems, will become prohibitive. In fact, our scientific knowledge bank of optical coatings and sur- faces is rather empty relative to that possessed by the electronics community, and thus, I see this as a 1 major area of interest and an opportunity for dramatic gains over the next several years.

One other topic which has not been addressed before until this meeting is that of high power infrared laser windows. Much needs to be done here in areas of interest to ASTM: techniques for measuring

bulk and surface scatter and absorption, techniques for specifying bulk and surface homogeneity, test procedures for specifying surface and coating damage thresholds, and others. Although there is a large ' amount of activity going on that is not being reported here I urge you to move in this direction since,

in the long run, ASTM can be of great benefit to the users of these components as well as the producers. |

In summary, we have learned a lot but there is a lot more to be done on laser damage to optical com- ponents. I think that our understanding of the processes that go on has increased tremendously, and that

it is now time to turn our attention more towards making this knowledge pay off where it can. I also feel that it would be appropriate to consider changing the name of this conference to the ASTM Conference on Optical Reliability. The title of "The Laser Damage Conference" to me described a group concerned with

how things damage. That is certainly important, but I feel it is but one aspect of the overall problem

of Optical Reliability, the other aspect being how do we increase the reliability of these components

once we know how and why they fail. Thus, this title change would tend to put our work in a more positive, useful light. It further recognized the fact that with the advent of high power lasers, optical

systems now have reliability problems - a situation which did not really exist before.

i I In closing, I wish to thank the National Bureau of Standards for the excellent job they do both as our host and in publishing the Proceedings of this meeting. We are indebted to them all, again, for their help in these ways.
Laser Damage of HOY-A Laser Glass, LCG-11

Tetsuro Izumitani & Kosaku Hosaka

HOYA Glass Works

Akishima, Tokyo, Japan

a ad Chiyoe Yamanaka'

Osaka University

Yamada-Kami , Suita

Osaka, Japan

In HOYA Glass Works we have developed a Barium Crown Laser

Glass, LCG-11. It has a stimulated emission cross section (tf) of

2.0 x 10~ cm and a loss coefficient (Y) of 0.001 cm

-1

.

In this report we show that, while platinum lowers the

damage threshold of laser glass, by controlling the number and

size of the platinum inclusion, the damage threshold of platinum

crucible melts of LCG-11 laser glass dges not go below 400 J/cm

in the passive test, nor below 28 j/cm in the active -test . This

compares favarably with the damage threshold of melts in Si02 and

ceramic crucibles. The ceramic melted glass showed a higher

lasing threshold, a lower slope efficiency in normal oscillation

and an output saturation tendency in y-switched oscillation.

We find that surface damage is independent of glass strength,

glass thermal expansion, and adsorption of water in glass surface.

We find that LCG-11 glass^ upon treatment with hydrofluoric acid, was improved from 28 j/cm to 40 J/cm , a higher damage threshold

in passive test, yet we were unable to find a change in glass

surface composition due to hydrofluoric acid treatment by using

electron microprobe. We assume that surface damage is dependent

upon the multi-photon ionization tendency of a glass surface.

This assumption is based upon our observation of silica glass

which has no single-bond oxygen and a high damage threshold.

Key Words: HF treatment, inclusion damage, passive and active

testing, platinum inclusion, surface damage, thin film coating

1. Introduction

New optical glass materials have been developed, in Hoya Glass Works, for

example, laser glass, LCG-11, acoustic-optical glass, AOT-5, and a photochromic glass

with rapid fading rate. In this paper the properties and damage of laser glass LCG-11

will be reported.

LCG-11 is a Barium Crown Glass. It has properties as shown in table 1. Its cross section for stimulated emission tf is 2.0 x 10" cm and its loss coefficient is

0.001 cm

1 . It has an internal damage threshold of 400 J/cm and a surface damage

threshold of 40 J/cm for a multimode laser pulse of 30 ns . in the passive test.

In the active test it showed a surface damage threshold of over 28 j/cm for the same

pulse. These damage thresholds were measured in Osaka University, using samples we made

.

The compositions of LCG—11 was determined on the bases of the relationships

between the life time and intensity of fluorescence versus glass composition of the

Ra0-R0-Si0a-Nda03 system. The NdaOa concentration was limited to 3.5 wt.J» by

3
c one entrati on quenching

.

LOG—11 is melted in platinum vessel using a special furnace to avoid platinum

contanination.

Table 1. Properties of LCG-11 laser glass

Glass type

Neodium doping

Neodium Concentration

Fluorescent life time

Fluorescent wave length

Fluorescent line width.

Cross section for stimulated emission

Specific gain

Loss Coefficient (at 1060 mji)

Lasing threshold

Max slope efficiency

Damage threshold

(multimode, 30 nsec.)

Internal damage threshold

Surface damage threshold

Barium Crown Glass

3.5 wt.?6 2Q 3.80 x 10 ions/cc glass

575 usee. 1.0624 H 290 A _20 a 2.0 x 10 0 cm*

0.101/cm/joule

0.001 cm 180 J (10^ x 160 mm rod, reflecting

mirror 60 "/<>, normal oscillation)

1.52 <fo

400 J/cm

40

28

j/cma

J/cm

a

(passive test)

,HF- treatment,}

^passive test ' /no HF- treatment,^

^active test i

2. Damage testing method

In damage testing laser glass, both passive and active tests were performed.

In the passive test a Q-switched laser beam was irradiated through a lens to the test

samples. A schematic diagram of the apparatus is shown in fig. 1A.

In the active test a Q-switched and amplified laser beam was irradiated to a pumped test sample. A schematic diagram of the apparatus is shown in fig. IB.

In the passive test, an oscillation glass rod with a- diameter of 10 mm and a length of 160 mm was pumped by a 1 kj linear flash tube. The output mirror reflectivity was 60 fo, A rotating prism was used for Q-switching and a beam splitter-photo

diode-tektronix-519 combination was used to measure the beam energy and pulse duration. Calibration was performed using TRG 107 calorimeter. The half pulse width was

30 nsec. The focal length of the lens used for focussing the laser beam was 10 cm. The size of the focal spot was determined from the diameter of the damage spot in

Al-foil.

In the active test, the amplification was accomplished in 4 stages. The 4th

amplifier las-er rod used was the test sample. In 1st and 2nd stage of amplification

amplifier glass rods with a diameter of 20 mm and length of 300 mm were pumped by 4

linear flash tubes with a total pumping power of 4 kjoules. In the 3rd amplification

stage, an amplifier glass rod with a diameter of 30 mm and a length of 300 mm was pumped by a pumping power of 6 k joules. In the 4th amplification stage, the glass

rod was 20 mm in diameter and 300 mm in length and the pumping power was 7 to 10

k joules. Two-element lens were placed between the 3rd amplifier and the 4th

amplifier to maintain a beam diameter of 15 mm and to increase the beam flux density.

Q-switched oscillation and the measurements of beam energy and pulse width were accomplished in the same manner as in the passive test.

3« Inclusion damage

It is well known (l)

1 (2) that colloidal state platinum inclusions deposited in a melt lower the damage threshold. But there js a question as to whether platinum

dissolved in the form of platinate ions PtOa ~ lowers the damage threshold, or whether

laser glass melted in a platinum crucible can be used as a high power laser glass.

Figures in parenthesis indicate literature references at the end of this paper.

A
3.1. Effect of Platinum Inclusion on Damage Threshold

The damage threshold was measured in passive test. The glass samples (15 x 15 x

50 mm), which were melted in SiOa crucible using platinum added glass batches. The

results are shown in Table 2.

As may be seen, the glass samples with platinum added showed a very low damage

threshold of 15 J/cm , while the glass sample without platinum added showed a very

high damage threshold. The glass samples with Pt added included metallic colloidal

platinum particles.

Table 2. Effect of Pt inclusions on the threshold damage in passive test

using multi-mode pulse of 30 ns. Sample Damage Threshold (j/cmz

) Barium Crown Glass

(100 ppm of Pt added) 15

Barium Crown Glass

(10 ppm of Pt added) 15

Barium Crown Glass

(no Pt added) 465

Barium Crown Glass

(3.5 fo of Nd203 doped)

(no Pt added)

351

3.2. Effect of crucible materials on damage threshold of LCG-11

Next, LCG—11 glass was melted in a platinum crucible, in a ceramic (electrically

fused zirconia) pot and in a clay poj as shown in Table 3. The LCG-11 internal damage

threshold showed to be over 400 J/cm , regardless of crucible materials. In the

platinum crucible -melting, a special melting apparatus was used to void colloidal

platinum particles. In this glass sample, ionized platinum might be dissolved or particles that are less than 0.1 \i might be included. The damage threshold, however,

was not affected by either the ionized platinum or the very small sizes of colloidal

platinum. It can therefore be concluded that laser glass melted in a platinum

crucible can be used as high power laser material if we can avoid the colloidal

platinum particles (3). It is interesting to note that glass samples containing

ceramic inclusions melted in ceramic or clay pots have a damage threshold of only

120 J/cm*.

Table 3. Damage threshold of LCG-11 melted in various crucibles.

(in passive test, using multi-mode pulse of 30 ns.)

Sample Damage Threshold (J/cm2

) LCG-11 (3.5 wt./o of Nd3+

) melted in Pt 400 ~ 560

LCG-11 glass (no dope)

melted in ceramic pot 480 ~ 540

120 (glass samples with

ceramic inclusions)

LCG-11 glass (no dope)

melted in clay pot 480 ~ 540

120 (glass samples with

ceramic inclusions)

5
3.3 Normal and Q-switched oscillation

Characteristics of LCG-ll glasses melted in platinum, ceramic and clay pots,

respectively

.

a) Normal oscillation characteristics

Normal oscillation was performed. The glass rod 10* x 160 mm was used. Mirror

reflectivity was 60 fo. As shown in fig. -2A the glasses melted in the ceramic and clay

pots showed higher lasing threshold energies, which are attributed to the high loss

coefficient. They also showed lower slope efficiencies, which are probably attributed

to the shorter fluorescent life time.

b) Q-switched oscillation characteristics

Q-switch oscillation was performed using a rotating prism. The pulse duration

was 30 nsec. As shown in fig. 2B f glasses melted in both ceramic and clay pots showed

a saturation tendency in output. This is perhaps attributable to the increase of

lasing threshold energy due to the increased loss coefficient.

4. Surface damage of LCG-ll

The surface damage threshold (20~40 j/cm2

) is always very low compared to the

internal damage threshold 400 j/cm when platinum free. The reason is not sufficiently clear. The surface damage mechanism of transparent dielectrics is considered as follows (4) (5).

The initial electrons produced by multi-photon ionization absorb laser photon

energy by Inverse Bremsstrahlung and then multiply. Once the plasma is produced, the

lattice is heated suddenly and a shock wave will be driven to the glass surface (6)

and thus both thermal and mechanical fracture is induced.

If the above is true, then the surface damage threshold relates to glass

properties, such as mechanical strength and thermal expansion, and it also depends

upon adsorbed impurities such as water vapor, and glass composition, because initial

electrons will parhaps come from non-bridging oxygen.

We therefore measured the surface damage thresholds of chemically strengthened

glass, of zero-expansion glass and of coated glass, of HF-treated glass, and of fused

silica glass.

4.1. Effect of mechanical strength on damage threshold

A plasma formation is found whenever surface damage is induced. We therefore

measured the threshold for plasma formation.

As shown in table 4, contrary to our expec|ation the plasma formation threshold

becomes, lower after the ion exchange between Na and K ions while the surface damage

threshold increased a little. Thus, a higher mechanical strength does not always

give a high surface damage threshold.

4.2. Effect of thermal expansion on the plasma formation threshold

The plasma formation threshold was measured on Hoya Crystron-0 glass ceramics

which has a thermal expansion coefficient of 0 + 1.5 x 10~ (-50 ~ 200 °C) . As shown

in table 4, the plasma formation threshold (15 Z. 19 + 2 J/cm ) was low. Thus, low

thermal expansion does not always give a high surface damage threshold.

6
Table 4. Effect of mechanical strength and thermal

expansion on surface damage

Glass Sample Chemical

treatment

Threshold for

plasma generation

(j/cm8

)

Surface damage

threshold

(j/cm2

)

Reference

Chemical

Strengthened

Glass

before

after

16 ~ 20 + 2

8 ~ 14 + 2 24+2

25 + 2

mechanical strength

1,000 kg/cma

6,300 kg/cma

Crystron-0 15 ~ 19 + 2 Thermal expansion

0 £ 1.5 x 10~7/C

4.3. Effect of thin film coating on surface damage threshold

On a glass surface, organic material such as oil, inorganic material such as polishing powder, and adsorbed gas such as water vapor will remain. After cleaning,

this film such as MgPa and Si02 was coated and then the plasma formation was observed.

The result is as shown in table 5. Note that the MgFa film coating does not improve

the plasma formation threshold. The Si02 film coating improves the threshold a little, but result needs further investigation. As a comparison, the silica glass

plasma formation threshold is given on the same table . It is very high.

The high surface damage ^threshold of fused silica seems to suggest that impurities on glass surface does not always determine surface damage threshold.

Table 5. Effect of thin film coating on plasma formation

Glass Sample Coating Plasma formation threshold (j/cm2

) LCG-11 no 23 ~ 28 + 5 LCG-11 MgF2 coating 9 ~ 10 + 5 LCG-11 SiOa coating 26 ~ 30 + 5 Fused Quartz 130 + 10

4.4. Effect of alkali or alkali earth oxide on plasma generation

Alkali ions may have an effect on the initial electron generation in plasma.

Then, alkali ions were replaced by alkali earth ions. The result is shown in table 6.

In this case, however a single mode pulse was used.

As shown in table 6, no difference between alkali ions and alkali earth ions can be seen on the plasma formation threshold. However, as mentioned above, the

threshold for fused SiOa is very high. These results seem to suggest that the damage

threshold is affected by the multi photon ionization tendency. Fused silica glass

has no non-bridging oxygen. To the contrary, alkali ions as well as alkali earth ions

gives non-bridging oxygen. Therefore initial electrons are easily produced in alkali

or alkali earth containing glasses by multi photon ionization.

7
Table 6. Effect of glass composition on plasma formation

threshold using single mode pulse of 30 nsec.

Glass Sample Alkali or alkali earth

Plasma formation

threshold (j/cm )

BaCD 11 (SK 11) Alkali

\RiO o.3 mol °/c) (RO 16.8 mol fo)

53-63 + 5

BaCD 5 (SK 5) Alkali earth

\ti<j mox JO J 54 ~ 66 + 5

BaP 8 Alkali

(RaO 4.5 mol ft) (RO 31.3 mol Jt)

62-94 + 8

BaP 9 Alkali earth

(RO 34.4 mol 1o) 67-90+5

4.5. Effect of Hydrofluoric acid treatment on surface plasma formation threshold

Swain (7) showed that HP treatment improves surface damage. We treated LCG-11

glass with a 10 /» hydrofluoric acid solution. The results are shown in table 7.

Note that the HP treatment substantially improv|s the plasma formation threshold.

The threshold in single mode pulse was over 140 J/cm . There is no doubt of the

effect HP-treatment has on surface damage. However, the reason is not clear. We

measured the change of glass composition on the glass surface, using an electron micro

probe x-ray analyzer (Shimadzu EMX-SM), because we expected the formation of a SiOa

rich layer or fluoride layer by HP-treatment. But we could not detect a composition

difference, as shown in Pig. 3A,B,C.

Table 7. Effect of HP-treatment on surface plasma formation

in multi-mode and single-mode pulse

Glass Sample HP-treatment Plasma formation thresholds

multi-mode

(J/cm*)

single-mode

(J/cm*)

LCG-11 no 28 + 5 88 + 5 LCG-11 10 io HP-treatment 40 + 5 > 140 + 5

5. Damage threshold of LCG-11 in active test

The damage threshold of LCG-11 was measured in an active test. The result is

shown in Table 8.

Table 8. Damage threshold in active test

Sample Damage threshold ( J/cm

8

) Reference

LCG-11 multi-mode

(20* x 300 mm) over 28 J/cm2 pulse of 30 ns.

LCG-11, which was not treated by hydrofluoric .acid solution, showed surface

damage threshold of over 28 J/cm2 without any internal damage. This surface damage

threshold is almost the same as in the passive test.

8
6. Discussion and Conclusion

Fused silica glass has only bridging oxygens, therefore its ionization potential

is expected to be very high. The high surface damage threshold of fused silica glass

seems to have a relationship with its multi-photon ionization tendency. But at

present, we don't know why the surface damage threshold is lower than the internal

damage threshold. Perhaps the ionization potential on the surface is lower than that

in the interior. Neither do, we know the reason why hydrofluoric acid treatment

improves the surface damage threshold.

The following conclusions are derived.

1) By controlling the number and size of the platinum inclusion, the internal

damage threshold of platinum crucible melts of LCG-11 laser glass does not go

below 400 joules/cm2 for a multi-mode pulse of 30 ns. in the passive test.

2) By hydrofluoric acid treatment, the surface damage threshold of LCG-11 does

not go below 40 joules/cm for the same pulse of in the passive test.

3) In the active test, LCG—11 showed the threshold of surface damage over 28 j/cm2

.

4) Surface damage threshold is independent of glass strength, glass thermal

expansion. It is not always determined by adsorbed impurities. It seems to be

dependent upon multi-photon ionization tendency.

7. References

(1) Young, C.G. & Woodcock, R.P., Damage

in Laser Glass, ASTM STP 469 84

(1969)

(2) Bliss, E.S., Damage in Laser Glass,

ASTM STP 469 9 (1969), Damage in

Laser Materials, NBS Special Publ

.

341 105 (1970)

(3) Hopper, R.W., Lee, C. and 'Uhlmann,

D.R., Damage in Laser Materials,

NBS Special Publ. 341 55 (1970)

(5) 48 II78 (1970)

(6) Boling, N.L. & Beck, R.W.,

Damage in Laser Materials, NBS

Special Publ. 356 15 (1971)

C7) Swain, J.E., Damage in Laser Glass,

ASTM STP 469 69 (1969)

(4) Sherma, B.S. & Rieckhoff, K.E.,

Can. J. Phys. 45 978l (1967)

8. Figures

LO»or «!••» 1

Foculing GIOM Ho-NoU.or

mpl.

1 LO»or J M 10* l«0»BI 6Collbrot.O TtklroKll 519

Photo Dlooo

OSC. Amp. I Tot somplt Btom Amp in | IV I Splitter

Lat'trGlato ztfxjOO 2O°X3O0 JffxSOO I 2fjx jooj. n „ j mm mm mm mm/ lOXieOmm f>30aiR$ Photo Didde

Ktwv Motor TR0I07 (q1

ToUronliSO

Figure 1A. Schematic diagram of the apparatus for laser damage testing.

Passive Test

Figure IB. Schematic diagram of the apparatus for laser damage testing.

Active Test

9
SOOa*

C*400|IF

L» 290 UH

- Mil Rod

I 0*t I60»»

X

HI*4

O

a.K3

3

a,H

§2

R.»«% R-60%

/melted in Platinum pot T575 US

melted in j ceramic" 9of

F«,0, contort 0.1- 02y

//

/ / / ^InlfioW

/' /cloy pot y X ^t0! content ,/V 0.9-10 %

0 500 1000 PUMPING POWER (J)

Figure 2A. Normal oscillation characteristics of

LCG-11 glasses melted in platinum, ceramic and clay pots.

I-S -

1-0

o

^J00% 60 %

X

Res

I •Own

molted in platinum pot iOki«o«« . 7«s79uS

CO«ltiy Ltngth 700 mm / =

C- 300JJF f

l*230uh / melted in ceramic pot 30 n i. Pult*

° /^ 7 - 549US

/.

/

/

# /molttd In / clay pot

V 7- 453 US

350 400 450 500 550 600 650

PUMPING POWER (J)

Figure 2B. Q-switched oscillation characteristics

of LCG-11 glasses melted in platinum, ceramic and clay pots.

10 %HF-Treatment (30 Min.) No Treatment

Figure 3A. Line analysis of K in HF-treated LCG-11

glass by electron micro probe x-ray analyzer.

Figure 3B. Line analysis of Ba in HF-treated LCG-11

glass by E.M.X.

LCG-H Glass Surface

Spot size : Zfi 96/i

h "N6n>Treatment

-iI

'

I Treatment

I I0%HF

30min.

Figure 3C. Line analysis of Si in HF-treated LCG-11

glass by E.M.X.

10
Interaction Gradients, Concurrent Light Scattering

Experiments and Bulk Laser Damage in Solids

C. Y. She1 and David F. Edwards 2

Quantum Electronics Laboratory

Colorado State University

Fort Collins, Colorado 80521

The concept of interaction gradient and its role on bulk laser damage is discussed. Results of light scattering experiments concurrent with damaging

laser pulses on crystalline quartz, fused quartz and laser glass samples are compared to substantiate the role of interaction gradient and stimulated

Brillouin scattering on bulk laser damage in solids.

Key Words: Bulk, crystalline quartz, fused quartz, interaction

gradient, laser glass, laser induced damage, light scattering,

stimulated Brillouin scattering.

1. Introduction

Our previous observations of the differences in damage properties of crystalline quartz for ruby

laser pulses having different mode structures [1,2]^ suggested that the onset of laser-produced damage

in solids depends, in a critical way, on the spatial and/or temperal gradient of the interaction

strength across the focal volume. The crystalline quartz in the scale of laser radiation and acoustic

phonon wavelengths appears uniform to the light beam and the interaction gradient could exist only

when a multi-mode laser is used. The existence and non-existence of interaction gradients when a multimode laser and a TEMqq laser is used, respectively, are also evident in the forward scattering data

presented. [2,3] The highly irregular data in figure 5 of reference 2 when a multimode laser was used

as opposed to the relatively smooth data in figure 8 of reference 3 when a TEM00 laser was used, is indicative of the presence of strong interaction gradient in one case (former), but not in the other. A piece of amorphous material, such as fused quartz and laser glass, however, is non-uniform in the same scale, and the interaction gradients (either spatial and/or temporal) may exist in the focal volume even when a uniform light beam from a TEMq,-, laser is incident upon it. One purpose of this paper

is to test the role of interaction gradient on laser damage by comparing the experimental results on crystalline quartz, fused quartz and laser glass samples.

By monitoring the light scattered, both backward and at 90°, concurrently with the damaging laser

pulses, information about interaction gradients on laser damage was obtained. In addition, the role

of stimulated Brillouin scattering as viewed from backward scattering, on laser damage of crystalline

quartz, fused quartz and laser glass are compared experimentally. A second purpose of this paper is thus to describe the concurrent light scattering experiments and to discuss the significance of the findings to bulk laser damage in solids.

Department of Physics . Department of Electrical Engineering and Department of Physics . 'Figures in brackets indicate the literature references at the end of this paper.

11
2 . Experimental

The experimental set-up is shown in figure 1. Using the proper optical delays, the incident,

transmitted, and backscattered laser pulses may all be recorded with a FW-114 photodiode on a Tektronix 519 oscilloscope. To minimize the confusion, the transmitted beam was monitored only in the forward scattering experiments as were reported in references 2 and 3. Otherwise, the transmitted beam1 was blocked by a stopper, and only the incident and backscattered pulses were detected by the FW-114

photodiode. In 90° scattering experiments, we monitored simultaneously both the incident and 90° scattered pulses. The weak 90° scattered light pulse was detected by a 1P22 photo-multiplier and a 6943 8 spike filter on a Tektronix 547 oscilloscope. In order to obtain desirable power (near the damage threshold) for the experiments, CuS2 attenuators and cut-and-try method were used.

The ruby rod of the TEMqq laser was cooled to 21°C before firing, and this way a sequence of con- secutive shots in an interval of about 2.5 min. were fired into the same spot in the crystal until bulk

damage was induced. The experiment was then repeated on a new spot in the solid.

3. Results and Discussions

Using the setup described above, the forward scattering intensity ratio on crystalline quartz and

the 90° scattering intensity ratio on crystalline quartz and fused quartz were recorded. These results

and their significances with respect to laser damage have been reported elsewhere [3] and will not be

repeated here.

In the process of carrying out these experiments, a significant amount of backward scattering was observed when the incident laser pulse was near the damage threshold. Also, the ratio of the incident

power of the damaging shot P2 to that of the preceding shot Pi for crystalline quartz, fused quartz and laser glass (Nd doped) samples have been evaluated. These new results and their significances in re- lation to bulk laser damage in solids will be reported, here.

Shown in figure 2 are a sequence of oscilloscope pictures of laser pulses scattered from a piece

of crystalline quartz. From the bottom up, the first four similar traces show the superposition of the incident laser pulse and the amplified backscattered pulses. The time intervals between peaks of the trace correlate well with the distances in the experimental set-up and they can be changed as the distance between the sample and the laser changes. In this instance, the bulk damage occurred in the fourth shot; no observable back-scattering was noted after the bulk damage was induced as seen in the fifth trace. The top trace in the figure is a typical 90° scattered pulse as recorded from the 1P22 photo-multiplier.

As far as back-scattering is concerned, the situation is nearly the same with fused quartz. This

is evident as one compares figure 2 with figure 3. However, when the ratio of the incident power of

the damaging shot P2 to that of the preceding shot Pi is calculated, the crystalline quartz and fused

quartz appear to behave quite differently as can be seen in figure 4, in which the ratio P2/P1 was

plotted for all experiments leading to bulk damage. This ratio P2/P1 for some experiments is less than unity in fused quartz. In fact, one can set the laser power so that a fused quartz sample could

be damaged in a few shots. This has been difficult for crystalline quartz, i.e. under normal conditions we either produced macroscopic damage in crystalline quartz in the first shot or produced no damage at

all. The P2/P1 ratio for the laser glass sample behaved similar to that of the fused quartz. This result is in agreement with the concept of interaction gradient which exists in the interaction between

amorphous material and a uniform light beam.

Due to the absorption centers existing in laser glass, this behavior mentioned above appears more

pronounced. Under the experimental conditions, we were able to control the laser power (by adjusting

the CuS2 attenuators) to produce bulk damage in laser glass in the second shot every time. Furthermore

no backward scattering was noticed before the bulk damage was produced in the laser glass. Instead, a considerable amount of backward scattering was observed after bulk damage was induced. This is shown

in figure 5 in which a sequence scope traces representing the laser pulses (from the bottom up) shot

into the same spot in the laser glass sample are compared. The bulk damage occurred in the second shot

in this figure, and the back-scattering was observed in the third and fourth shot after the bulk damage was produced. The backward scattered pulses in figure 2 and figure 3 were due to stimulated Brillouin

scattering. Comparing the differences between figures 2 and 3, and figure 5, we would tentatively con- clude that stimulated Brillouin scattering (catastrophic acoustic waves [1]) is responsible for laser induced damage in both crystalline and fused quartz, but local heating is perhaps more important in the bulk damage of laser glass samples.

12
4. Acknowledgements

We thank Dr. T. W. Broberg of Systems Research Laboratories, Inc. for technical assistance and helpful discussions. The initial phase of this work was sponsored in part by a contract with the

Air Force Cambridge Research Laboratories, Office of Aerospace Research, USAF.

5. References

[1] Draggoo, V. G., She, C. Y. , McAllister, G.L. and Edwards, D. F. , "Effects of Laser Mode

Structure on Damage in Quartz", IEEE Jour,

of Quantum Electronics QE-8 , 54 (1972).

[2] Edwards, D. F. , She, C. Y. , Draggoo, V. G.

,

Broberg, T. W. and McAllister, G. L.

,

"Investigation of Cumulative Effects in

Microscopically Damaged Quartz", in Damage

in Laser Materials: 1971 , Edited by Glass,

A. J. and Guenther, A. H. , NBS Special

Publication 356 , p. 24 (1971).

[3] Edwards, D. F. and She, C. Y. , "Laser Pro- duced Damage in Transparent Solids",

Technical Report AFCRL-72-0143 , Air Force Cambridge Research Laboratories, USAF (1972).

6„ Figures

tering experimental set-up.

I cm

Figure 2. Scope traces of the incident and backward and 90 scattered laser pulses and crystalline quartz. The time scale for the five bottom

traces is 10 ns/cm and for the top trace is 50 ns/cm.

13
I cm

Figure 3. Scope traces of the incident and backward and 90° scattered laser pulses from fused quartz.

The time scale for the five bottom traces is 10 ns/cm and for the top traces is 50 ns/cm.

Cryslollini Quortz

A

hi

I!

i

> ' \

I l\ " T I

| ) M / \ 1 / i-»

Figure 4. The ratio of the incident power of the damaging shot to that of the preceding shot 7^/7^

for crystalline quartz, fused quartz and laser glass samples.

cm

Figure 5. Scope traces of the incident and back- ward scattered laser pulses from a laser glass

sample. The time scale is 10 ns/cm.

14
Optical Index Damage in Electrooptic Crystals

A. M. Glass, G. E. Peterson and X. J. Negran

Bell Laboratories

Murray Hill, New Jersey 07974

Optically induced refractive index changes which occur at low incident power densities (<1 KW/crn^) in many electrooptic crystals have severely limited

the application of these materials in the visible spectrum. Following a brief

historical review of the subject, we discuss the recent advances in the understanding of the physical mechanisms involved, progress in reducing the damage

susceptibility, and in enhancing the effect for holographic applications.

Particular attention is paid to LiNbO-j and LiTaO^ crystals. Optical and EPR studies of these materials have demonstrated that Fe^+ impurity ions present in nominally pure material are responsible for the index damage. The effects of stoichiometry variation, heat treatment, impurity and color center

content, X-irradiation of crystals and the kinetics of the index damage process

are accounted for by Fe +-Fe3+ reactions

.

By careful control of crystal growth and stabilization of the Fe^+ impurity

ion state, LiNbO^ and LiTaO^ crystals with greatly improved damage resistance

have been prepared.

Key Words: Holographic measurements, laser induced index change, optical

memories, reversible low power damage.

1. Introduction

Laser induced refractive index changes occur in many electrooptic crystals. The index change distorts the wavefront of a transmitted optical beam and provides a serious limitation of the use of these

potentially useful materials for nonlinear optical applications. For this reason it is usually referred

to as laser damage, but it differs from the catastrophic damage which generally occurs at much higher

power densities in that the index change is reversible. Following a brief historical review of the subject," recent progress in understanding the origin and mechanism of the index change is described. While

index damage has been observed in several important materials including LiNbO, [1,2]1, LiTa03 [2], KTN [3], BaTi03 [4], Sr1_xBa

xNb 20

6 [5] and CdS [6] primary attention will be focused on LiNb03 and LiTa03 since we have studied these more extensively than the others. Methods of controlling the susceptibility

of crystals to the index damage, both from the point of view of minimizing the effect for nonlinear op- tical applications and maximizing the effect for optical memory applications are described.

2. Historical Review

In his early studies of laser induced index changes, Chen [7] accounted for his results by the drift of photoexcited electrons outside the illuminated region of the crystal where they are subsequently trapped. Chen proposed that the resulting space charge field gave rise to the index change

via the electrooptic effect. Bleaching of the index change is possible either by illuminating the entire crystal with the same light that created the index change or by heating the crystal, both of which

result in reactivation of the trapped electrons which return to their original sites. In the case of cubic materials such as KTN an external applied field is required to observe index damage, but in the pyroelectric crystals no external field is necessary.

Figures in brackets indicate the literature references at the end of this paper.

15
Chen's model had the following requirements:

15 -3

1. A source of photoexcited carriers of concentration > 10 cm . 2. Suitable trapping sites.

3. A permanent internal field of ~ 10^ V/cm.

4. A crystal resistivity sufficiently high that the space charge fields remained

for extended periods (weeks)

.

A considerable amount of work followed on the identification of the defects responsible for the damage

Reduction of LiNbOj and LiTaO-j was found to increase the effect, [7] while field annealing of these crystals at ~ 600° C reduced the effect [8], The latter effect was subsequently associated with 0H~ im- purity ions (or protons) drifting into the crystals from the atmosphere [9]. Defects associated with

of f-stoichiometry [10] of LiNbO^ and LiTaO,, such as oxygen vacancies [7,11] and stacking faults [12] were also suggested as possible trapping sites.

The large spontaneous internal field required by Chen's model was difficult to account for, since

it was of the wrong sign to be due to the pyroelectric effect and was too large to be due to surface

effects. However, Johnston [12] pointed out that no initial internal field is required for the damage

process. Optical excitation of a pyroelectric crystal gives rise to a macroscopic polarization change

in the illuminated region and the divergence of this polarization acts as a source for an electric fiel

Furthermore, the equilibrium requirement that the total electric field Etot = 0 means only that V.P = (e-l)p where p is the space charge density and e is the dielectric constant, so it is possible, in a pyroelectric crystal to have a spatial variation of polarization without a corresponding variation of

field. This polarization gives an index change via the electrooptic effect. In this way Johnston

showed that the distribution of trapped carriers resulting from the diffusion of conduction electrons,

skewed by electric field due to V.P was able to account for all the details of Chen's observations

(which Chen's model could not) without the introduction of any fields before or after illumination of

the crystal. This also removed requirement #4 of Chen's model.

The purpose of our work was to identify the defects involved so that some control over the optical

damage process may be possible. Early experiments on the effects of heat treatment, oxidation and re- duction, stoichiometry and field annealing were not reproducible and it was not until our discovery [13] of the importance of iron contamination of nominally undoped material in the damage process that the confusion was removed

.

3. Quantitative Measurement of Index Damage

There are two basic techniques for the measurement of index damage. One is to write in the index

change with a focused green or blue laser and to measure the birefringence change with a red probe beam which does not affect the damage. One such arrangement is shown in figure 1. The crystal polar axis is normal to the incident beams and 45 to the polariser axis. If the transmission of the Pockel's cell arrangement is initially set at zero with the compensator then the birefringence An induced by the 5145 A d T = sin2 (tt/V An__ dx (l) x

0

where d is the crystal thickness. Since most of the birefringence is induced over the near field distance] of the focal spot an approximate measure of An is obtained. The sensitivity of this technique is limited

by the crystals extinction ratio and typically An > 10 -5 can be measured. This technique gives a direct measure of the usefulness of a crystal for nonlinear applications and enables the use of high power den- ! sities in the write beam. It suffers from the disadvantage that thermally induced birefringence changes i may be larger than the laser damage and confuses some measurements. The holographic technique [4,14] provides a more accurate method for quantitative measurement of the index (not birefringence as before) change and the kinetics of the process. Two plane waves intersect in the crystal at an angle 9 as shown ! in figure 2 and the interference pattern forms an elementary hologram because of the induced index change. 1 The diffraction efficiency of the grating [15]

T] = sin TrAnd/Acos6 (2)

where here An is the modulation amplitude of the index change and d is the interaction length which in our case is the crystal thickness. The minimum index change which can be measured is limited by scatter- ing from defects and typically An > 10-7 can be measured. This greater apparent sensitivity is offset

by the fact that much lower power densities must be used. Thermal effects are unimportant, but vibration

effects become important. Thus the holographic technique was used for its greater flexibility and ac- curacy throughout most of this work, while the focused beam technique was used when higher power densities were necessary, or unpoled (multidomain) crystals were examined. For measurement of damage susceptibility

the 5145 A writing beams were a polarized with a power density ~ 1 W/cm2 while the 6328 A read beam was j rr polarized, the c axis of the crystal being normal to the grating planes. This measures An3

. Other

polarizations and crystal orientations were used to study the damage mechanism as discussed later.

16

I
4. Early Experiments

Crystals obtained from different sources grown from melts of the same stoichiometry varied widely

in their damage susceptibility. Even crystals from the same source, same stoichiometry and same heat treatment varied greatly from one to another. Variation of crystal stoichiometry from a Li/Nb ratio in the melt of 44% to 54% had no observable effect on index damage, which seemed to remove the possibility

of stoichiometric defects as the origin of damage. Studies of the thermal bleaching kinetics showed that

at each temperature the index decays were exponential (monomolecular) with a single activation energy of

1.2 ± 0.1 eV for all crystals regardless of damage susceptibility, although the entropy of activation was found to vary somewhat from one crystal to another and depended on heat treatment. The results are sum- marized in figure 3. Both the thermal and optical bleaching kinetics were independent of the spatial fre- quency of the index variation.

All crystals annealed in oxygen at 1000°C damaged less than crystals annealed in oxygen partial pressures of 100 mm Hg, but at lower oxygen partial pressures there was no further increase in damage susceptibility. A typical result is shown in figure 4. This seemed to eliminate oxygen vacancies as the origin

of damage and point to impurities, or other charge compensating defects. OH ion impurities were ob- served to enter crystals during field annealing in air, with a general reduction in damage [8], but these OH ions could be subsequently removed by annealing crystals in oxygen without any corresponding increase i, in damage. 0H~ ion content was in all cases measured by the absorption at ~ 3470 cm"^ . Crystals were analyzed and found to contain several impurities, but doping with these impurities gave no measurable

increase in damage. Long term field annealing and heat treatments often gave erratic and irreproducible

results. No optical luminescence associated with damage and no laser induced epr signals (10 KMc) were

found. Any photoconductivity was masked by the pyroelectric effect [16].

While all this data to some extent characterized the index damage process, and eliminated a few possible defects as the origin of the damage, little progress toward identification of the active centers

was achieved.

5. Iron Contamination

510 MHz epr equipment was set up to study impurities in LiNbO^ . This equipment was well suited

for the study of f erroelectrics both because of the large volume of crystal which could be contained in the cavity and because large fractional modulation of the magnetic field is possible. This is useful

for studying the broad paramagnetic resonances in these materials when g broadening is suspected.

In all nominally undcped crystals it was found that the Fe resonance line at g = 6 when Hl/c and

g = 2 when Hl/c dominated the spectrum [17], the impurity concentration varying from 10 to 100 parts per million in different crystals. A typical epr spectrum is shown in figure 5. Earlier spectroscopic analysis had not revealed the presence of these impurities due to interference from niobium lines but these

impurities have now been identified by more careful analysis [18] and results are consistent with the epr data.

The damageability of crystals was found to correlate well with the iron impurity concentration,

in fact, crystals doped with 0.05 wt% Y^Qy were found to be far more susceptible to index damage than

any other crystals examined. By analysis of LiNbO^ at each stage of the powder preparation and crystal

growth process it was found that most of the contamination of undoped crystals came from the ceramics

used as furnace tubes or crucible insulation [17] . By taking care to avoid any contact or proximity of

heavily contaminated materials with the LiNbO^ at high temperatures crystals as pure as the starting

materials (1 to 2 parts per million) were prepared. These crystals showed lower damage than any of the crystals we had studied. Comparison of the holographic diffraction efficiencies of newly poled crystals

with different iron content, rapidly cooled from 1100 C is shown in the first row of table 1. Valence

Change of iron in LiNbO^ occurs readily with suitable heat treatment. Almost complete conversion of Fez+ to Fe^+ is possible by heating crystals in oxygen at 600 C for two days. Even crystals doped with

0„05 wt% iron were perfectly clear after this treatment and the absorption spectrum is shown in figure 6. Rapid cooling of crystals following an anneal in oxygen for a few minutes at 1000°C always gave a high Fe concentration owing to the very small energy difference between the Fe^+ and Fe^+ states in the crystal. The crystal spectrum following this treatment is shown in figure 6 - crystals appeared quite

brown. Heating crystals in lower oxygen partial pressures gave increased Fe and almost complete

conversion to the Fe state was obtained by heating crystals in nitrogen - after which the crystals

appeared black.

While measurement of the Fe^+ content was possible with epr, identification of the Fe^ + state

was made on the basis of the optjcal spectra. The absorption band at 1.1 microns seen in the brown

crystals is characteristic of Fe ions in octahedral coordination [19] . Since there are no allowed

crystal field transitions of the Fe^"1" ion this spectrum is clear until the charge transfer band 0.4 \im.

17
The susceptibility of crystals to index damage was found to depend only on the Fe2+ concentration

of crystals. (The holographic diffraction efficiency depends on approximately the square of the Fe2+

concentration.) Diffraction efficiencies of oxidized crystals are shown in the second row of table 1 where it is seen that the damage is considerably lower than the quenched crystals in the first row.

In order to increase the damage resistance of crystals still further attempts were made to completely

stabilize the Fe^"1" by the addition of colorless impurities of different valence such as Ti^+, W*>+

, Mg2+

, and Al^"*" ions. None of these attempts were successful however. Since it is not known which sites the Fe ions occupy, nor what the charge compensating mechanisms are, the failure of these attempts has no special meaning.

Table 1. Diffraction efficiencies (%) of LiNb03 crystals, 0.2 cm thick, after various heat treatments.

The low Fe crystals are the highest purity crystals

obtained by careful crystal growth

Treatment Undoped 0.05% low Fe low Fe

Fe 2°3 #1 n

Poled from 1150°C

and quenched 0.20 95 a 0.019 0.010

Annealed for 75 hr not

at 600°C 0.021 1.0 0.0039 detected

Field annealed at 600°C for 15 min 0.007 b 0.0004 ii with 5 mA/cm2

a. This was the maximum diffraction efficiency observed. An efficiency of 50% was still obtained with

this crystal thinned down to 0.02 cm.

b. This crystal had very uneven Fe^+ 'distribution.

Crystals heated in oxygen pressures below 100 mTorr appear black. Thin wafers of these crystals

actually have a deep blue color. These reduced crystals have a much higher electrical conductivity

than clear crystals. A spectrum of a thin wafer is shown in figure 7. The absorption band centered at about 0.5 ym (20,000 cm

-1

) is characteristic of Nb^+ ions [19] and is not associated with the Fe content

of crystals. These ions act as donors (the conduction band of LINbO- being Nb^+ d

£ orbitals [20]) thereby

Increasing the crystal conductivity. Field Annealing of iron-doped crystals results in a visible con- centration gradient of Fe 2+ ions in the crystals , the higher concentration being near the negative

electrode. EPR measurements show a slightly higher Fe^+ concentration near this electrode also. The

results indicate that these impurities carry an effective positive charge (possibly substituting for

Li+ ions) and drift toward the negative electrode. 0H~ ions (or tf

1" ions) enter the crystal from the positive electrode to compensate for the Fe ion current. The corresponding decrease in damage ability of crystals is due to the decrease in Fe content (Fe ions being pulled right out of the crystal) and not

due to the increased OH

- ion content. If field annealing is performed in an atmosphere free of water

vapor the Fe ion drift is not as effective since there is no compensation mechanism. The effect of field annealing crystals of various iron concentrations on the holographic diffraction efficiency is shown in the third row of table 1. A further decrease of damageability Is seen. Extended field an- nealing gave no further decrease in damage, possibly because of Fe impurities entering the crystal from

the positive electrode (evaporated platinum) or because of iron impurities becoming immobile at crystal

defects. Although no diffraction was observed from one of the low iron crystals after the field annealing, index damage was still evident at higher power densities using the focused beam technique.

All these experiments show the correlation of index damage with Fe 2+ content of crystals and account

for all the previously observed effects. The importance of the cooling rate and the contamination of crystals from furnace ceramics during poling and heat treatments account for the erratic behavior of our

own early experiments. It Is of prime importance to monitor the Fe concentration of crystals to understand their damage characteristics. All heat treatments and poling of crystals for these results were

performed in furnaces sheathed with silica and high purity platinum electrodes and containers.

18
6. X-irradiation

X-irradiation of crystals also makes them more susceptible to index damage. This is again due to valence changes of iron impurities due to the ionizing radiation, as demonstrated by optical and epr

experiments. Crystals turn brown after irradiation, the absorption depth depending on the iron concentration and the spectrum is the same as the Fe^+ spectrum. (There is also some evidence of Nb reduction.)

Annealing of crystals at about 150°C restores the crystal to its initial state. The activation energy

for the thermal bleaching of the X-ray induced absorption is 1.3 ± 0.2 eV, much the same as that for the decay of the index damage, but the entropy of activation is smaller (decays occur nearer room temperature) and the decay appears to be bimolecular (retrapping of thermally activated carriers is important)

.

The effects of y irradiation on the damageability of crystals [21] also seem to be due to valence change

of Fe ions from the induced absorption spectrum.

7. Mechanism of Index Change

We now have strong evidence that Fe ions are the basis for the index damage. The Fe ions are the source of photoelectrons via the reaction

Fe2++ hv = Fe^+ + e (conduction). (3)

These electrons (e) drift or diffuse out of the illuminated region where they can be trapped, possibly

by Fe^+ ions via the reverse of reaction (3). This is indeed suggested by the similar wavelength de- pendence of the optical bleaching and writing processes. The index damage is then due to a spatial modulation of Fe^+ and Fe^+ ions. There is some assurance that the charge compensating defects of the iron

impurities do not play a primary role in the damage process since other divalent and trivalent impurities have no noticeable effect in index damage and one might expect the charge compensating mechanism

to be the same for some of these ions.

2+ The broad absorption band observed around 0.5 |_im in the Fe ion spectrum must be due to charge

transfer from Fe^+ ions to the conduction band [22] . Since comparison of figures 6 and 7 confirm that

it is not due to reduced niobium. The shape of this band accounts for the wavelength dependence of

optical damage.

There is no reason why other multivalent impurities such as Mn^ + - Mn^+ should not give rise to index damage by the same process. The apparent absence of any effect of these ions [23] in our experiments must either be due to the absence of a suitable charge transfer process at 5145 A or masking by

other optical absorption processes which do not give rise to conductivity. It appears that the set of

conditions required for index damage is energetically favorable for Fe ions and other impurities have

smaller effects which are not observable above the effects of residual iron impurities.

Once photoactivated electrons are in the conduction band they can move by drift or diffusion.

Johnston [12] pointed out that both effects are important to account for the index changes observed by

Chen [7], with diffusion being the more important for short exposures. In fact, one might expect that with increasing spatial frequency diffusion becomes more important since the concentration gradient of

conduction electrons increases for a given optical intensity. In figure 8 we show a photograph of the birefringence induced by a focused beam of 5145 A. radiation in a LiTaOg crystal doped with iron. Similar results were obtained with LiNbOg crystals. The birefringence pattern is symmetrical about the focal point which indicates that drift effects are small and the redistribution of carriers is by diffusion. The results of figure 8 are qualitatively the same as Chen's results on undoped LiNbO for short exposures. Recent work [24] suggests that diffusion is dominant for the higher spatial frequencies en- countered in holographic storage, using y irradiated LiNbOg. The birefringence pattern of figure 8 also shows that the index change does not result from a space charge field (this would give the birefringence change shown by the broken line in figure 8) but from the polarization change associated with

the redistribution of electrons. This result is expected since the conductivity of these LiNbOg crystals

(<1014 ohm cm) would relax the space charge fields in a few minutes or less. If higher resistivity

crystals are available then space charge fields could play an important role for extended periods [25]

.

Johnston's model for the spatial variation of polarization requires that there be a significant

difference between the dipole moment of unit cells containing Fe ions and Fe ions. The change in

dipole moment of each impurity is additive, owing to the polar nature of the host, giving a macroscopic

polarization change. Such changes of dipole moment £p. upon optical excitation have been measured ex- perimentally for the Cr^+ ion to be about 0.6 Debye in LiNbO [26] and preliminary results for Fe

ions suggest a change tM of about 1 Debye. Using this value we can obtain a rough estimate of the

quantum efficiency of the holographic writing process if other contributions to the macroscopic polarization change are neglected.

19
8. Quantum Efficiency

The quantum efficiency q of the writing process in the case of laser induced index damage is the probability of the electron from an excited Fe 2+ ion being trapped outside the laser beam and contributing to the index change. This will be a function of the spatial frequency written into the crystal

since for the lower spatial frequencies the electrons have to travel further and the probability of being

trapped within the illuminated region without contributing to the index change increases. Of course, in

the limit of dc (uniform crystal illumination) no index change is recorded. This effect could account

for the decreasing diffraction efficiency with increasing grating spacing observed in BaTiOo [A]. To estimate the quantum efficiency for a grating spacing of 1 ym we use the data of figure 9 where the diffraction efficiency n of 0.018 cm and 0.14 cm thick crystals of iron doped LiNbO^ have been plotted as a function of time for various incident power densities. These crystals had an extinction coefficient of a = 1.4 cm

-

-*- at 5145 A . The initial slope of the curve at short times for an incident power W of 0.9 W/cm2 is for the thinner crystal

gl) = 0.0011 sec" 1 or (0) = 1.2 X 10" 7 mV1

.

From eq. 2 we calculate for n << 1 'dn~\ 0 /dn

d^ ^ ~ 103 )

in - Since 1 Joule ~ 4.5 x 10- photons at 5145 A we have the index change per absorbed photon density N dn_ 1 dn_ ~. -dir = i -ar

= 1,3 x 10 ^/photon - ( 4 )

The same result is obtained for the thicker crystal in figure 9. Since the index change has electrooptic

origin, for the configuration used a

3 An

r AP n3 r W 3 = g(e

33 -D

= 2(e

33

-l) (5)

2+ 3+ where Ay is the change in dipole moment associated with the Fe to Fe change. Using published values

for n-j, £33, and r^ [28] and the preliminary value for Ay ~ 1 Debye we calculate from eqs. 4 and 5 a quantum efficiency of 1.3%. It has been demonstrated that larger values of q may be obtained with an external applied field to increase the drift velocity of the excited carriers [27].

It must be emphasized that this calculation assumes that space charge fields, and contributions to AP3 from sources other than the excited state dipole, were unimportant. This assumption is almost cer- tainly invalid - especially for short exposures, but the estimate does point out that this model provides

a reasonable interpretation of the data.

2+ Within these approximations it is also possible to estimate the Fe ion concentration necessary to obtain the maximum observed diffraction efficiency in figure 9. From this data and eq . 2 we calculate a maximum index change An^^ ~ 7 x 10-

^. Thus from eq . 5 we estimate the maximum number of trapped electrons

(using Ay ~ 1 Debye) to be about 7 x lO 2 ^ . This is some three times greater than the total Fe concentration of the crystal. It is doubtful that Ay can be increased to account for this discrepancy since

the dipole moment of a LiNbO^ unit is only 2.5 Debye, thus other contributions to the polarization - dipole induced polarization - are important. In view of the above assumptions this mechanism gives rea- sonable interpretation of the observations.

1 9. Photochromic Effects

Evidence that the laser induced index change has electrooptic origin is afforded by the polarization

dependence of the diffraction efficiency of an elementary hologram [14]. If the hologram is written with

the c axis normal to the grating planes, then the diffraction efficiencies of it and a polarized read beams at 6328 A are in the ratio 8.6:1 for n « 1. This is precisely what one expects from the ratio of electrooptic coefficients [28]

This result does not depend on the origin of the variation of P~ since both space charge fields and ex- cited state dipoles will both contribute to AP-j such that V.P^ f 0.

If, however, a hologram is written with the c axis parallel to the grating planes, then some information concerning the microscopic origin of V.P

3 can be obtained. The excited state dipole moment can

only, by symmetry, give a variation in P.,, while space charge fields will give a variation in P^ or P

2 only for this configuration. Thus in equilibrium V.P = 0 and no diffraction would be observed due to

20
the electrooptic effect. If the crystal is not in equilibrium and there are net space charge fields

due to trapped carriers (at times shorter than the relaxation time) , then diffraction will be observed. When the crystal b axis is normal to the grating planes

Hit = 0 and T\o = n

2 3r22AP2

For the same space charge fields r\o for this configuration should be about half of no for the previous

configuration. No diffraction is measured with this configuration at 6328 A in our experiments, suggesting that no space charge fields are present. However, when the 5145 A laser is used as the read beam,

then a diffraction efficiency of about 1% of r)a for the previous configuration with the same exposure was observed in an iron doped LiNbO., crystal 1.5 mm thick. This difference for the two wavelengths was too great to be due to dispersion of r22 . ^ possible explanation of the result is photochromism. Since the basis for the index variation is the photochromic Fe^"*" - Fe3+ reaction, an amplitude hologram is expected.

The diffraction efficiency of such an elementary hologram is [15]

„ /2 a d ,2 a d J f(r\

where a is the modulation amplitude of the absorption depth. To estimate the importance of this effect, we will assume that all 7e^+ ions possible contribute to the hologram, then for the 0.15 cm thick crystal, using a ~ 1.4 from figure 6 we find r\ = 0.009 with little dependence of the efficiency on the polarization of the read beam, due to the small dichroism. This estimate is considerably greater than the measured diffraction efficiency of 0.1% so the observed effect could indeed be due to photochromism.

10 . LiTa0

3 Qualitatively the behavior of LiNbOj and LiTaO^ is similar. There are, however, important quantitative differences. The index damage susceptibility of LiTaOj is much lower than LiNb03 for both undoped and iron-doped crystals by a factor of about 30. A comparison of the two materials is shown in table 2. The data in this table was obtained using the focused beam technique of figure 1 with a 7 cm

focal length lens. The incident power listed is the minimum power density necessary to just see trans- mission through the pockels cell due to the laser induced index change for the exposure times given. At these powers defocusing of the beam was also just observed. It was necessary to use this technique since

high temperature heat treatments dipole LiTaO-j (Curie temperature - 620°C) and repoling of course changes

the crystal characteristics. The results of table 2 give some semiquantitative idea of the' damage sus- ceptibility for comparing different crystals, but the absolute magnitudes depend on the spatial frequencies (focal : length of the lens) and the exposure time. The LiTa03 crystals obtained from Crystal

Technology Inc. had the highest damage resistance of all those studied after suitable heat treatments and

field annealing, even though the Fe^ content of these crystals was greater than 10 parts per million.

The iron is almost entirely stabilized in the Fe3+ state.

2 ° Table 2. Peak intensity in watts/cm at the focus of a 5145 A argon laser beam, necessary to just observe index damage during a 10 second exposure.

Crystal

Quenched

from

1000°C

Slow cooled

and field

annealed3

LiTaO annealed at 1200 °C for

3 25 hrs after growth 200 900

LiTa0

3 annealed at 1400°C for 25 hrs after growth 600 3000

LiTa0

3

: Crystal Technology 1500 8000

LiTa0

3 + 0.05% Fe

20

3 10 30

LiTa0

3 + 0.05% Fe 2 0

3 (reduced

15 minutes in N

2 at 1000 °C) in H

2 and annealed

25

LiNb0

3 low iron 40 300

LiNb0

3 + 0.05% Fe^ 0.5 6

a. Field annealed at 600°C for about 10 minutes with 5 mA/cm . This procedure poled

LiTaO,.

21
A clue to the reason for the difference in damageability of LiNbOj and LiTaOj may be given by the absorption spectrum of figure 10. LiTat^-Fe^"1" does not have the pronounced absorption band at about

0.5 ym which is present in LiNbOj. The excitation of an electron from an Fe2+ ion to the conduction

band may be less probable than excitation to a nonconducting state with 5145 A radiation. This may be related to the fact that the Ta5+ ion is a great deal more difficult to reduce to Ta^+ than the equivalent Nb 5+-Nb^+ reaction. Ta^+ can only be obtained in sufficient concentration to color the crystal if crystals are heated in hydrogen at 1000°C, while Nb^+ reduces with much less severe treatment. Because

of this property it is possible to obtain almost complete conversion of Fe^+ to Fe + in LiTa03 before

the crystals turn black. It is interesting to see from table 2 that the heavily reduced crystal seemed

to damage less than the quenched crystal. This implies that the presence of Fe3+ ions as traps are essential to maximize the damage. It is also interesting to see that annealing LiTa03 crystals in air at 1400°C for several hours results in lower damage than crystals annealed at 1200°C for the same time. This treatment increases the oxidation state of the LiTa03 thereby increasing the Fe3+/Fe 2+ ratio. The improvement of the Crystal Technology crystal is presumably due to a variation of the crystal growth

procedure.

11. Applications of Laser Induced Index Damage

Laser induced index changes have been used as the basis of an erasable pure phase holographic

memory [14] and may find future application in distributed feedback systems [29] and integrated optics. For these applications maximum writing sensitivity is required so that iron doped LiNbO^ is particularly

useful. By suitable doping, choice of crystal thickness and heat treatment, a compromise between optical

absorption loss and writing sensitivity may be achieved for any application. We have been able to achiev

holographic diffraction efficiencies of 50% using ~ 1 J/cm of incident energy with LiNbO + 0.05% Fe

crystals 0.2 cm thick with ~ 30% absorption loss at 5145 A. The angular selectivity

of this crystal was about 1 mR for a grating spacing of 1 ym. With thicker crystals higher angular

selectivity can be obtained, but without improvement in writing sensitivity since these crystals have to be oxidized somewhat to prevent excessive absorption loss. An advantage, in principle, of higher angular'

selectivity is that many holograms can be superimposed in the same volume of crystal and independently

read out. However, it is not possible to erase one hologram without erasing all the others in the same

volume. Furthermore, partial erasure occurs during readout, by optical bleaching since read and write

wavelengths must be the same to avoid distortion in the thick crystals. Fixing holograms eliminates the problem of optical bleaching during reading but it also removes the versatile feature of optical erasability. Fixing holograms may be accomplished by any polarization relaxation process which compensates

wholly or partially for the polarization variation in the crystal, then subsequent optical bleaching of the trapped electrons will leave a fixed hologram. Such procedures have been demonstrated for undoped

LiNbO^ [21,25] and BaTiO^ [30]. The polarization relaxation process could be local polarization reversal

[30], or other ionic rearrangement. For crystals not in equilibrium when there are space charge fields

present, fixing could be accomplished by ionic conductivity to relax these fields [25] . Polarization

relaxation requires only local changes while field relaxation requires ionic drift over macroscopic dis- tance. Since there is no dependence of the thermal bleaching associated with the fixing on the spatial

frequency of the index pattern in LiNbOo, the former mechanism seems to be operative, consistent with

earlier observations that space charge fields do not persist.

For some memory applications it may be advantageous to use thin crystals, heavily doped with

iron to maximize the writing sensitivity, so that the angular selectivity is poor. Then readout may be

performed with a longer wavelength radiation such that optical bleaching is not a problem. Second order

distortion effects may, be tolerable. We can see from eqs . 2 and 7 that for writing with 5145 A radiation

and reading with 6328 A we need A6 > 50 mR or crystal thicknesses < 4 x 10~2 cm. With this kind of system the holograms would not be superimposed but arranged in an array so that each can be spatially

addressed. In this way single holograms can be erased.

12. Diffusion Doping

To achieve such thin holograms of high sensitivity is relatively simple by diffusion doping of crystals. Heavily iron-doped layers about 25 ym (measured by the angular selectivity of an elementary

hologram) can be diffused into LiNbO^ in a few hours at 1100°C, while leaving the crystal in a single

domain state. 1% diffraction efficiency could be obtained with 10 J/cm incident energy or 3% with

60 J/cm2 and less than 10% absorption loss at 5145 A . This corresponds to a peak to peak index change

of about 0.0025 without even focusing. The measured angular selectivity A6 ~ 100 mR was sufficiently

great that both diffracted beams, each side of the normal to the crystal surface, could be observed

with normal incidence of the read beam.

22
13. Conclusions

The optically induced refractive changes in LiNbO^ and LiTaO^ have been reasonably well characterized

and the basic mechanisms understood. Our measurements are consistent with the interpretation that only

iron impurities contribute to the laser damage in these two materials, even though there is no fundamental reason why other impurities might not result in similar effects . The set of conditions necessary

for index damage appears particularly favorable for iron impuities in LiNbO-. It is known that the addition of iron increases the damageability of f^Na ^50^5 [21] and BaTlOg [30] but at the present time

it is not known whether this impurity is the only cause of damage in these materials . In the case of BaTiOg the situation is a little more complicated in that the addition of some impurities is accompanied

by an increase in the electrooptic coefficients at room temperature so that increased damage susceptibility could be due to this effect rather than the direct effect of charge transfer from the impurities.

Solely by varying the Fe^+ concentration of LiNbO., and LiTaOj it has been possible to vary the laser

induced index changes over about four orders of magnitude for the same exposure. It has only been possible to prepare LiNbOo and LiTa03 with resistance to index damage for incident powers less than

100 W/cm^ and 10 KW/cm^ respectively by careful control of crystal growth. Since the distribution coefficient of iron in LiNb03 is less than, but close to unity [16] zone refining is not expected to give

great improvement. However, any improvement at all increases the usefulness of crystals both as electooptic modulators, since increased powers can be used, and as parametric oscillators, since the termperature of operation can be decreased, thereby increasing the useful tuning range. Each order of magnitude

decrease in damage susceptibility decreases the lowest possible operating temperature by 20°C.

The study of iron in LiNb02 and I^Na Nb^O^j has also led to progress in the understanding of selffocusing by two step absorption in these materials at high power densities [31]. In this context it is worth pointing out that the excited state dipole effect provides a direct index change in addition to thermal effects which may contribute to self-focusing.

14 . Acknowledgments

Throughout this work we have had the assistance and interest of many of our colleagues. In particular we would like to thank P. M. Bridenbaugh and A. A. Ballman for growing the crystals used in this

work, Miss B. E. Prescott and Mrs. G. A. Pasteur for measuring the optical absorption spectra of our crystals and A. Carnevale for assistance with the epr measurements. Helpful discussions of unpublished work

with D. L. Nash and F. Micheron, M. Clark and L. K. Anderson are gratefully acknowledged.

15. References

[1] Peterson, G. E. , Ballman, A. A. Lenzo, P. V. and Bridenbaugh, P. M. , Appl. Phys . Letters,

5_, 62 (1964).

[2] Ashkin, A., Boyd, G. D., Dziedzic, J. M.

,

Smith, R. G., Ballman, A. A., Levinstein, H.

J. and Nassau, K. , Appl. Phys. Letters 9_, 72 (1966).

[3] Chen, F. S., J. Appl. Phys. 38, 3418 (1967).

[4] Townsend, R. L. and LaMacchia, J. T., J. Appl. Phys. 41, 5188 (1970).

[5] Thaxter, J. B., Appl. Phys. Letters 15_, 210

(1969).

[6] Ashkin, A., Tell, B. and Dziedzic, J. M., IEEE J. Quantum Electron. 3^, 400 (1967).

[7] Chen, F. S., J. Appl. Phys. 40, 3389 (1969).

[8] Levinstein, H. J., Ballman, A. A., Denton,

R. T., Ashkin, A. and Dziedzic, J. M. , J. Appl. Phys. 38, 3103 (1967).

[9] Smith, R. G., Fraser, D. B., Denton, R. T. and Rich, T. C, J. Appl. Phys. 39, 4600

(1968).

[10] Peterson, G. E. and Carruthers, J. R., Solid

State Chemistry 1, 98 (1969)

.

[11] Jorgensen, P. J. and Bartlett, R. W., Tech.

Rep. No. 0NRAD686721, Stanford Research

Institute (1969). Unpublished.

[12] Johnston, Jr., W. D., J. Appl. Phys. 41, 3279

(1970)

.

[13] Peterson, G. E., Glass, A. M. and Negran, T. J., Appl. Phys. Letters 19, 130 (1971).

[141 Chen, F. S., LaMacchia, J. T. and Fraser, D. B., Appl. Phys. Letters JL3, 223 (1968).

[15] Kogelnik, H., Bell System Tech. Journal 48, 2909 (1969).

[16] Phys. Stat. Sol. (a) 9_ 301 (1972). The optically excited current observed in this work

appears to have pyroelectric origin - not photoconductivity as reported.

[17] Peterson, G. E., Glass, A. M. and Carnevale,

A., (to be published).

[18] Nash, D. L., submitted to Applied Optics.

23

i
[19] See for instance C. J. Ballhausen "Intro- duction to Ligand Field Theory" McGraw Hill

Book Co., Inc., NY (1962).

[20] DiDomenico, M. , private communication

.

[21] Amodei, J. J., Phillips, W. and Staebler, D.

L. , Appl. Optics 11, 390 (1972).

[22] Clark, M. and Glass, A. M. , to be published.

[23] Ref. 21 reports small effects due to Mn and

Cu impurities, but the Fe content of these crystals is not reported.

[24] Staebler, D. L. and Amodei, J. J., J. Appl.

Phys. 43, 1042 (1972).

[25] Resistivites of 10 ohm cm have been re- ported for LiNb0

3 by Staebler, D. L. and Amodei, J. J., Ferroelectrics _3, 107 (1972).

[26] Glass, A. M. and Auston, D. H. , Optics Com- munication 5_, 45 (1972) .

[27] Staebler, D. L. and Amodei, J. J., 1972 Int. Quantum El. Conference, Montreal.

[28] Kaminow, I. P. and Turner, E. H., "Tables of Electrooptic Coefficients," Handbook of Lasers Chemical Rubber Co. Press (1971).

[29] Kogelnik, H. and Shank, C. V., J. Appl. Phys. , 43, 2327 (1972). |<

[30] Micheron, F. and Bismuth, G., Int. Electrooptics Conference (1972) Brighton, England.

[31] Shenke, D. P. and Smith, R. G., to be

published

.

16. Figures

Figure 1. Apparatus for measuring and photograph- Figure 2. Holographic technique for measuring

ing the birefringence change induced at the focus laser induced index changes. B is a beam splitter

of a 5145 A argon laser beam. and the are mirrors.

24
Figure 3. A plot of the thermal decay constant y Figure 4. Relative diffraction efficiency of a defined by An = An

Q exp-yt, against the reciprocal typical LiNbO-j crystal after heat treatments in absolute temperature for 1) undoped LiNbO, heated various oxygen partial pressures. Crystals were

in 0.04 atm. 0 2 at 1000° C 2) as grown LiNb03 + quenched from 1000° C. 0.05% Fe

20

3

3) as grown LiNb0

3 + 0.0057o Fe?^

4) undoped LiNbO, heated at 1 atm 0

2 at 100(jC.

DOPED LITHIUM NIOBATE

10 4 2 1 EFFECTIVE g EFFECTIVE g

Figure 5. E.P.R. spectrum of an undoped liNbO^ crystal (ground to a powder) compared with LiNb03 powder doped with Cr-^+

, Mn2+

, and Fe 3+ impurities.

25
1. OUENCHEO FROMIIOO'C

2. SLOW COOLED AND ANNEALED AT 600"C

- 3 UNDOPED LiNbO, -

-

TT SPECTRA

_

a SPECTRA

1 V 10,000 i f (cm-')

Figure 6. Optical absorption spectra of iron

doped LiNb03 (0.05% Fe

2 03) , 1) after quenching

from 1100° C 2) after slow cooling and annealing

at 600° C compared with 3) undoped LiNbO^ for both tt and (T polarizations.

70

60

40

Eua 30

20

Figure 8. Typical laser induced birefringence

patterns in LiTaO,:Fe (0.05%). The line drawing

is a plot of the birefringence change atong the c axis. The broken line is the birefringence change

which would occur due to space charge fields due

to diffused carriers.

V (cm-') Figure 7. Optical absorption spectra of LiNb03

after reduction in a nitrogen atmosphere at 1000°

C

for 10 minutes. The crystal appeared deep blue.

EXPOSURE ISEC0N0S)

Figure 9, Formation of elementary holograms as a function of exposure to 5145A radiation! a) is from a 0.018 cm thick crystal and b) is from a | 0.14 cm thick crystal.

j 10

I

1 1 . 1 AS GROWN

9 - 2.ANNEALED@I000''|N N2 - \ 3 ANNEALED IN H

2@ 1000'C IN Mj FOLLOWED BY ANNEAL IN N2 4 UNDOPED LpToOj

30,000 20,000 lOiOOO 0 30,000 20,000 10,000 0

V (cm-'l Figure 10. Optical absorption spectra of iron

doped LiTa03 (0.05% Fe 203 ) 1) as grown 2) after annealing at 1000° C N

? 3) after annealing in H2 at 1000° C, compared with 4) undoped LiTaO^, for

tt and a polarizations.

26
Comparison of Laser Induced Bulk Damage in Alkali-Halides

at 10.6, 1.06, and 0.69 Microns

*

D. W. Fradin and Eli Yablonovitch

Gordon McKay Laboratory, Harvard University

Cambridge, Mass. 02138

and

Michael Bass^

Raytheon Research Division

Waltham, Mass. 02154

It has long been recognized that catastrophic self -focusing cannot occur

below a critical power P and that perturbations from self-focusing become

progressively less important as the power of a probe optical beam is lowered

below P . Using this fact, we have designed and conducted a number of ex- periments to study bulk damage in alkali-halides in which self-focusing was eliminated and unequivocal measurements of damaging fields obtained. Strongly

focusing optical systems were used so that damage could be achieved while

probe powers could be restricted to between one and two orders-of-magnitude

below theoretical critical powers for electrostrictive self-focusing. Ex- perimental evidence confirms the absence of self-focusing.

By observing the absolute magnitude of breakdown strengths and relative

values among the alkali-halides, striking similarities between 10.6, 1.06 and 0.69 |J.m and d.c. avalanche breakdown were found. The results also showed no frequency dispersion over the wavelength range of 10.6 to 0.69 microns. The

implications of this work for surface damage studies are explored, and, in addition, the effects of inclusions on bulk optical strength are considered'.

Key Words: Alkali halides, avalanche breakdown, bulk damage, selffocusing, surface damage.

1. Introduction

Laser induced breakdown [1]^ in transparent dielectrics has been studied since high intensity

optical fields have been available. Nevertheless, it has been difficult to ascertain the intrinsic bulk

damage mechanism for these materials because of complications arising from self-focusing [2] , multiphoton

absorption and cascade ionization of impurities [3].

Recent work at 10.6 |im [4], however, indicates that intrinsic damage can be isolated. By choosing

pure wide bandgap insulators, multiphoton ionization or its low frequency limit [5], tunnel ionization,

can be 'eliminated . In addition, by confining laser powers to well below critical powers for self- focusing [6] catastrophic beam collapse becomes impossible and corrections from the index nonlinearity

become quite small.

Supported by the Joint Services Electronics Program at Harvard University under Contract No. N00014-67-

A-0298-0006.

Supported by the Advanced Research Projects Agency of the Department of Defense and was monitored by the Air Force Cambridge Research Laboratories under Contract No. F19628- 70-0223.

Figures in brackets indicate the literature references at the end of this paper.

27
The alkali-hallde family is a natural choice for breakdown studies. Besides being of practical

importance for their application as optical materials in CO^ lasers, they have large bandgap (about 7 ev

or more) and have been studied extensively for both their d.c. dielectric strength [7] and their 10,6 |_im characteristics [4],

The experimental studies reported here were conducted at 1.06 and 0.69 (am using Q-switched lasers. Since self-focusing may reach threshold before breakdown at these frequencies [8] , it is important to understand both the conditions under which catastrophic self-focusing occurs and the corrections from

the index nonlinearity when a catastrophic focus is prevented. For this reason an analysis of self- focusing is outlined in the appendices, and the general results relevant to our measurements are discussed

in Section 3. Confirming theoretical predictions that catastrophic self-focusing is absent, two important

experimental checks are then reported. We present in Section 4 the results of a series of carefully con- trolled experiments in which intrinsic bulk damage in alkali halides was measured. A well-characterized

TEMqq mode laser beam with total power more than one order of magnitude below calculated critical powers

for self-focusing was tightly focused within the samples in order to obtain the high intensities needed

for damage. Then, in Section 5 we show how the experimental results can be fully explained in terms of avalanche breakdown.

2. The Lasers and Beam-Handling Optics

Figure 1 shows schematically the principal features of the laser damage source. The experiments were

performed using a pulse pumped, electro-optically Q switched, Nd:YAG, or ruby laser. Some important prop- erties of these devices are summarized in table 1. Figure 2 shows that the Nd:YAG laser output was in the lowest order Gaussian or TEMqq mode.

The time structure of the pulses from both lasers appears reasonably smooth when viewed with a fast photodlode oscilloscope combination having a measured risetime of 0.5 nsec. We have conducted FabryPerot studies on the ruby output and have found that normally fewer than four adjacent modes are oscillating simultaneously, so that at least with the ruby, the time structure is effectively fully resolved.

Longitudinal mode selection is accomplished by aligning the faces of plane parallel laser rods parallel

to the resonator mirror. Despite the low reflectivity of the anti-reflection coated rod surfaces, the high gain of the rod and the high reflectivity of the rear mirror create an effective resonant reflector.

To verify that the transverse mode structure on axis is constant with time, the centers of the beams were sampled with a 25-micron pinhole and found to have the same time structure as the entire beam. The

stability on axis was found to be superior to stability of the spatially integrated power.

The breakdown data was taken by focusing through a %-inch focal length lens to approximately 2 mm

inside the samples. Care was taken to insure that spherical aberrations from both the lens and the plane entrance surface of the sample being tested were unimportant. A fast photodiode was used to monitor

the transmitted light, and an energy monitor recorded the energy in each laser pulse.

i Table 1. Laser parameters

Wavelength

Energy TEMqq Mode

Beam Diameter at Output

Mirror

TEMqq Mode

Polarization

Pulse Repetition Rate

Pulse Duration in TEMqq Mode

Pulse to Pulse Energy

Reproducibility

Nd:YAG

1.06 Um

1.5 mJ

0.8 mm

Linear

1 pps

4.7 nsec (FWHP)

± 77.

Ruby

0.694 Urn

2.0 mJ

0 . 7 mm

Linear

1 pulse/5 sec

14 nsec (FWHP)

± 10%

28
The combination of one rotatable and one fixed polarizer resulted in a variable light attenuator

which was highly sensitive, quite reproducible, and which did not affect the laser pulse's polarization,

spatial distribution, or duration. If the fixed polarizer is oriented to transmit the laser polarization and if fc) = 0° is the angle of the rotating polarizer which gives maximum transmission through

this attenuator, then the transmitted intensity at any other angle of rotation about the beam axis is

1(6) = bl cos

4

B . o

I is the incident light intensity and b is the fraction transmitted when 9=0°. Calibrated neutral

density filters were often used in conjunction with the variable Glan attenuator.

3. Self-Focusing

A laser beam propagating in a transparent medium induces an increase in the index of refraction by an amount proportional to the laser intensity. At powers in excess of some critical power this non- linearity causes the intensity distribution to become unstable, and a catastrophic beam collapse results.

Self-focusing may occur as the result of a number of nonlinearities . In solids for Q-switched laser pulses the process which normally leads to the smallest value of critical power and hence dominates self- focusing is electrostriction [10]. This is the case for the alkali halides where thermal and electronic

contributions to self-focusing are much smaller than the electrostrictive effect [11] and can be neglected.

An intense light wave whose power lies below a critical power P

c will not experience a catastrophic

collapse in a nonlinear medium, because although focusing by self-action will always be present, diffraction acts in the opposite sense to cause divergence and dominates at such powers [10]. At powers sufficiently far below P , therefore, the intensity distortion due to the index nonlinearity can be treated as a constant perturbation on diffraction effects and usually neglected. These observations allow us to ef- fectively eliminate self-focusing by restricting probe powers to well below calculated critical powers while focusing strongly by external optics to reach the field intensities necessary to cause optical

damage

.

Theoretical self-focusing parameters are defined and derived in the appendices where quantitative

corrections from the index nonlinearity at powers below P c are discussed. Table 2 summarizes the nu- merical results. The probe power is the experimental peak power on axis and is more than one order of

magnitude below P . From a purely theoretical viewpoint, therefore catastrophic self-focusing is im- possible, and it can be shown that beam distortion from the index nonlinearity introduces at most a few percent correction in the measured electric field strengths. If catastrophic self -focusing does occur,

then the breakdown damage data is a measure of the critical powers rather than intrinsic breakdown field. The measured threshold intensity will then scale with the square of the calculated focal diameter if the process is steady-state and will depend on the pulse-width if the process is transient. (The diameter

dependence in the steady-state results from the existence of a constant critical power P which does not vary with beam diameter.)

Table 2. Calculated steady-state, self-focusing

experimental values of pulse-width and parameters and peak power

Wavelength

(microns)

T

(10 -9

sec)

22

t n^XlO

P 2 -9

(10 sec) (mks)

P

cr

3 (10 watts)

P

c

3 (10 watts)

Pprobe

- , v 3.. (10 watts) *

10.6 11.0 200 48,000 175,000 120 NaCl 1.06 5.3 4.7 2.3 480 1,750 37.8

0.69 3.94 14 204 746 20 * 10.6 22.3 200 13,200 50,000 20 Rbl 1.06 10.7 4.7 8.4 132 500 6.1 0.69 8.0 14 56 203

For P . <

probe P catastropicc

r self-focusing will not occur.

* The 10.6 [M data is taken from E. Yablonovitch , Appl. Phys,, Letters 19, 495 (1971).

29
To test our belief that self-focusing was absent we conducted two experiments. In the first the relative field strength threshold for damage in NcCl was measured with three different focusing lenses,

corrected for spherical aberrations, and having focal lengths of 0.5, 1.0, and 1.5 inches. The experi- ment was conducted at 1.06 Urn. If steady-state self-focusing were present, the observed damage threshold

would have scaled with the inverse of the focal length. It did not, and, in fact, to within 5 percent

the field strength was independent of focal length. This effectively eliminated the possibility of steady-state self-focusing. Since t /t was just under unity, however, self-focusing could be transient,

and equation (A15) predicts the resufts observed. For this reason it was necessary to measure the damage

threshold as a function of pulse width with the beam diameter held essentially constant.

By changing the pumping level for the YAG laser, we were able to extend the pulse width by a factor

of 2.3 to 10.8 nsec. In addition, the breakdown strength at 0.69 \Ma was measured with ruby laser pulses

of 14-nanosecond duration and a focused diameter 25 percent smaller than that obtained with the YAG laser, the same %-inch focal length lens was used in all three measurements, and to compute the ruby value, we assumed the same transverse intensity variation as that present at 1.06 |_im. To within 15 percent no change was noted in the threshold field despite the pulse-width dependence in eq (16). The agreement for the ruby pulses was especially reassuring, because the critical power varies with wavelength squared. If transient self-focusing were present, we would have seen a change by a factor of 4.2 in the field strength

or a factor of 18 in the measured intensity—an effect which would have been quite dramatic. A factor of

9 comes from the pulse-width dependence of the transient critical power and a factor of 2 from the wavelength dependence.

Perhaps the best experimental check for self- focusing is the actual measurement of breakdown

strengths. Self-focusing theory appears to be totally unable to account for the experimental results

given below in which both relative and absolute values of breakdown strengths show striking similarities

to 10.6 Hm values. We thus conclude that prior to the onset of material damage, self-focusing has been

effectively eliminated as a competing nonlinearity

.

The possibility may exist that self-focusing by a mechanism other than electrostriction will be ef- fective after a sufficient number of electrons have been generated to cause intense local heating of the sample. Evidence for such an effect has been observed with gas breakdown [12], but the results are not entirely unambiguous. While theoretical arguments can be given against such an effect, it is sufficient

to note that our experimental findings indicate that any late developing nonlinearity is unimportant in our measurements.

4. Experimental Measurements of Breakdown

4.1 Damage Measurements at 1.06 Microns

To measure the breakdown strengths of the alkali halides, we focused the laser beam approximately

2mm into each sample and recorded the number of laser pulses necessary to produce internal damage at various power levels. In every case where damage occurred, a white spark was produced, and the damage

was later carefully inspected with a microscope. Because of the small volume damaged by our highly

focused 1.06 |Jm pulses (less than 2 X 10 mm ), a large number of data points could be taken with each sample (40 to 100).

Defining threshold as that value of incident power necessary to produce intrinsic damage in a single

shot for 50 percent of the positions probed [4], we calculated the r.m.s., on-axis electric field at the measured threshold in NaCl. Corrections were made for reflections from various surfaces and the changes

in the beam diameter due to the effect of the index nonlinearity. This was the basic calibration, and all other values of threshold were measured relative to E^ ^. In order to avoid errors from daily power fluctuations and possible alignment changes, a single sample of NaCl was tested with each alkali halide.

It was readily determined that a slight misalignment of the focusing lens (%-inch focal length) had no measurable effect on the relative breakdown strengths.

Visual inspection and the breakdown statistics suggested that spatial inhomogeneities from inclusions

were not affecting the results except in the single case of RbCl. Damage which we regarded as intrinsic

consisted at each damage position of a single pointed region which began at the geometrical focus and ex- tended a very short distance back toward the laser, increasing in cross-section to give a tear-drop ap- pearance. A typical example is indicated in figure 3. In RbCl , on the other hand, regions with low breakdown thresholds consisted typically of one or more spherical voids randomly distributed about the

focus (figure 4). A number of points, however, did appear visually to have intrinsic damage and were consistently more difficult to breakdown. These data points were used for the RbCl results.

Finally, a fast photodiode detector system with a 0.5 nsec risetime monitored the transmitted light

as shown in figure 5 and was used to confirm threshold levels in NaCl and KC1 as well as to establish the approximate time structure and stability of the laser output.

30
Values for the breakdown field obtained at 1.06 |im are summarized in figure 6 and in table 3 along

with both the 10.6 |-lm data collected by Yablonovitch [4] and accepted d.c. results [9], These results

are normalized to the respective values of field necessary to damage NaCl. The breakdown fields for NaCl are listed in table 4. This allows the striking similarity in trend of breakdown field to be easily

observed and the possible systematic deviations at 1.06 |0m to be easily recognized. The quoted errors at 10.6 \Jm are ± 10 percent, and our random experimental errors in relative fields are estimated to be no more than ± 10 percent with possible errors due to microscopic strains adding another ± 5 percent. For NaCl and KBr two different samples from two different manufacturers gave nearly identical results.

Table 3. Relative breakdown fields - normalized to ENaCl 2 X 10 V/cm

DC

10.6 nm

1.06 |Jm

Nal

0.460

0.405

(0.293)'

NaBr NaCl

1

1

1

NaF

1.60

1.78

KI KBr KC1 KF

DC

10.6 |_im 1.06 |im

0.380

0.369

0.272

0.460

0.482

0.375

0.667

0.713

0.568

1.27

1.23

DC

10.6 Mm

1.06 |Jm

Rbl

0.327

0.323

0.400

RbBr

0.387

0.400

0.550

RbCl

0.553

0.472

0.670

Crystal was extremely hydroscopic and no final check was made with the microscope to determine if inclusions were responsible for the damage

observed.

Table 4. Absolute breakdown strength of NaCl

E

Peak (dc >

E (10.6 microns)

rms

E (1.06 microns)

rms *

E (0.69 microns)

rms

v '

1.50 X 10 V/cm

1.95 X 10

6 V/cm

2.3 X 10 V/cm

2.2 X 10

6 V/cm

± 10 Percent

± 20 Percent

± 20 Percent

Gaussian profile assumed.

A 4 percent correction is incorporated into the 0.69 micron value because

of equation (A12) in the text. The data at 1.06 microns, however, is transient as seen from table 2. Equation (A15) is therefore used to evaluate 7^ and a 1.8 percent correction determined from (A12).

Careful statistics for variations in the breakdown strength were collected on NaCl because of the high quality of the two samples we obtained. It was found that, accounting approximately for experimental

uncertainties, the process appeared threshold-like. For the other samples larger fluctuations were

noticed, but the sample qualities were somewhat inferior to that of NaCl. Experimental uncertainties are considered to result from surface imperfections, internal strains, and laser fluctuations. No measurement

of any probabilistic nature to breakdown was made. Some evidence for intrinsic fluctuations, however, was found by monitoring the light transmitted

through the samples. On several occasions a laser pulse produced damage in the same position where a more intense pulse one second before had been focused without damaging the sample. While these observations may have resulted from unresolved time-structure in the second laser pulses, the same observations

31
were made at ruby wavelength where Fabry-Perot studies indicated that the pulses were normally free of

such fluctuations.

4.2 Damage Measurements at 0.69 |lm

The breakdown strength of NaCl was also measured with a ruby laser. Table 4 records the average of about 50 damage measurements. Although the laser was normally operating in a single longitudinal mode as indicated by Fabry-Perot and photodiode studies, each laser shot during the measurement was monitored

with a fast photodiode and recorded.

In figure 7 time-resolved photographs of transmitted light indicate the sudden attenuation normally

seen for laser pulses which caused damage. Because of the smooth time-structure of most of the pulses,

we are able to record a few cases in which the instant of first attenuation, considered to be the onset

of material damage, occurred after the peak of the laser pulse had passed. An example is given in figure 7c. The same effect was observed at 1.06 |Jm. This may be explained both by invoking a statistical

model for breakdown [13] or by the considerations of a time-dependent avalanche discussed in the next

section.

5. Discussion of Results

5 . 1 Bulk Damage

The experiments reported here were performed under carefully controlled conditions using stable,

well-characterized lasers and optical systems for which aberrations were unimportant. Because we were

able to probe each sample in many different positions, random fluctuations in breakdown strength were

averaged out. It was possible to distinguish between inclusion and intrinsic damage by inspection of the residual damage and to correct for the effects of inclusions in the one material for which they were im- portant. In addition, experimental tests showed that catastrophic self-focusing was absent and, consistent

with theory, that the index nonlinearity did not affect the results to within experimental error. It is therefore concluded that the results of the 1.06 and 0.69 iam study as summarized in figure 6 and table 4 represent accurate measurements of intrinsic bulk damage.

Because the techniques of this study are virtually identical to those of reference 4, direct com- parison can be made to breakdown strengths at 10.6 [lm. It has already been observed that the damage

thresholds for the alkali-halides at 1.06 |_lm follow a trend nearly identical to that observed with the

laser and, in fact, to the d.c. measurements of reference 9. It thus appears that the intrinsic

process of laser-induced damage for the alkali-halides has the same fundamental character as both a.c. damage in the infrared and d.c. avalanche breakdown. Moreover, the consistency of the absolute break-

down strengths at 0.69 (Jm suggests that this same process dominates at frequencies as high as 4.3 X 10 hertz

.

Additional support for an avalanche mechanism comes from three experimental observations concerning

the time-structure of the laser probe pulses. The first is that increasing the pulse-width of the YAG

laser output by a factor of 2.3 resulted in a 14 percent average drop in threshold intensity for NaCl,

Averages were taken at about 20 shots at each pulse width. This change, though small, is probably real,

because the test was made on a single sample of high-quality NaCl and thereby avoided a major source of experimental uncertainties arising from material variations. The second observation, noted at both 1.06 and 0.69 pm, is that high-frequency time-structure on the pulse has little measurable effect on the break- 1 down strength. And finally, after adjusting the power level so that damage occured regularly near the

top of the laser pulses, the probe intensities were increased by a factor of about three by changing the beam attenuation. When this was done, the intensity at which the transmitted light dropped (cf. figure 7) I was higher by 15 percent or more than it had been with the lower intensity pulses. This was determined

to mean that increasing the effective risetime of the optical field raises the measured breakdown

strength. To understand both this set of observations and the results from table 4, some discussion of existing electron avalanche theories [7, 14] is given.

i rapid multiplication of conduction-band electrons in which an interacts with an intense electric field in the presence of

a critical value, the number of electrons increases with time

N(t) = N

q exp [ I a(E)dt] .

-'o

An electron avalanche in solids is a initially low density N of free carriers

phonons. When the electric field reaches

approximately as

The gain coefficient a(E) is a strongly varying function whose value can be inferred from d.c. measure- ments of breakdown strength as a function of sample thickness for extremely thin specimens [15]. Lattice disruption then results from joule heating.

32
Two important conclusions develop from such an analysis. The first is that the entire process of

avalanche and damage involves energy exchange between the field and the material which is approximately

described by the well-known formula for a.c. conductivity

dW

dt

2 Ne T

2 2 m(l + (1) t )

(1)

where N is the time-dependent electron density, uu the angular frequency, and t the characteristic relaxation-time determined principally from phonon collisions. While this precise form of the conductivity may

not be correct for polar materials such as the alkali-halides , we will use it to qualitatively describe

breakdown for high frequencies.

2 2 2 Because of eq (1) , energy input to the material scales with frequency and field as E /(l + uu T )

,

and because the details of energy input determine the electron distribution function and hence N(t) , the threshold for damage will scale in the same manner. This justifies the use of root-mean-square fields in table 4. It also indicates that the a.c. breakdown strength will increase for frequencies near 1/t.

Calculation of T for NaCl [15] indicates that frequency dispersion should begin to occur somewhere near

that of the ruby laser.

The second relevant conclusion from an analysis of avalanche breakdown is that if insufficient time

exists for the electron density to reach values necessary to damage, then damage will not occur even

though electron recombination losses have been exceeded and an electron avalanche is underway. Damage

will only be produced when the field is subsequently raised above the steady-state breakdown strength and

the gain coefficient a is correspondingly increased. Such an effect has been observed in d.c. experiments

by varying the duration of constant applied fields [16] and by measuring threshold fields for samples of

thickness less than about 100 pm [17] , where transit effects limit the build-up time. When the time

available for the build-up of the avalanche is reduced below about 50 ns , larger fields are needed to damage.

The exerpiments of reference 16 give a useful comparison to our work. It was found that when the^ applied d.c. field duration was reduced to 10 ns , the breakdown strength of NaCl increased to 2.1 X 10 volts /cm. Because our laser pulses had durations of about 10 ns , the discrepancy in absolute field

strengths between optical and d.c. measurements may be fully explained. In addition, the field dependence of Oi(E) in NaCl is qualitatively correct to explain our observations of the pulse-width dependence

to breakdown, the insensitivity of threshold to fast-time structure, and the increase in breakdown

strength for rapidly rising pulses. It also indicates that when approximate corrections from pulsewidth considerations are made and experimental uncertainties ignored, the ruby laser threshold is a few percent greater than the threshold at 1.06 |Jm.

The essential details of the breakdown studies reported here are thus fully explained by the process

of avalanche breakdown.

5.2 Implications For Surface Damage Studies

Surface damage is often a practical problem in the operation of high-power lasers. For this reason

a number of investigations of surface breakdown [13, 18], have been made with the aim of elucidating the conditions and mechanisms of surface damage. The techniques of the studies reported here may provide a valuable tool for understanding surface damage by allowing direct comparison to bulk damage thresholds.

This comparison can be made by focusing a low-power laser beam first on a surface and then about 2 mm

into the bulk. Because focusing problems are much less severe in the bulk and damage from inclusions can apparently be distinguished by visual observation, a stable and repeatable reference exists for surface

studies. Careful investigation should help elucidate, in particular, the mechanisms responsible for surface damage under various conditions of surface preparation.

6. Conclusions

Careful measurements of laser-induced bulk damage have been made in ten alkali-halides without the confusing effects of self-focusing. Comparison of the results to studies at d.c. and 10.6 (Jm indicated

that the process of a.c. avalanche breakdown, similar in fundamental character to d.c. avalanche breakdown, is responsible for the damage observed. Analysis of time-related observations confirm this con- clusion.

7. Acknowledgments

The skillful assistance and advice of D. Bua and S. Maurici are gratefully acknowledged. We are also indebted to Prof. N. Bloembergen for valuable discussions of breakdown theory.

33
8. Appendix A

To demonstrate the claim that the relative balance between diffraction and self-focusing ef- fects is set at the entrance plane for a general steady state nonlinearity, we calculate the curvature

of the ray path from a modified, eikonal equation formalism that incorporates both diffraction ef- fects and the steady- state nonlinearity r 1 9] . Its applicability is restricted to beams with diameters 2a much less than a wavelength - -a condition fulfilled in our experiments.

Writing the electric field vector E(r) as A(r) exp i rk d>(r) - wtl where k = w/c and the index nonlinearity is n2 ^ ancl ignoring terms of order n

2 A2 /n

o> we can write Maxwell's equations

in the simplified form

[(nD

+ n

2 A

2

) 2

- (grad <t>) 2

] A + v^A

k

If the scalar product of this equation is taken with A, a term containing the factor l/k 4 dropped and

a cross term with n^A neglected, this leads directly to an effective eikonal equation

- (grad <t») 2

= 0 (Ai)

where

,2 1 ni = n

0

+ n

2 A + -_- . (A2)

2k n A

o o

In the limits of zero nonlinearity, and infinitesimal wavelength, this is just the basic equation of ge- ometrical optics. Results derived from the usual eikonal equation [201 -,£.fn ^ow be used with the index of refraction replaced by eq (A2). In particular, the curvature d

2 r/dp 2 for a pencil of rays

with position vector r" and with p the coordinate along the ray path is given by

? l dn. d

2 r = -[grad ni - ^ ) (A3)

Equation (A3) can be simplified by restricting the treatment to cylindrically symmetric beams

and by assuming that the maximum ray slope is small compared to unity. (In our experiments the maximum slope inside the sample and before the focus is less than 0. 05. ) Both the second term on

the right in eq (A3) and the longitudinal component of the Laplacian in eq (A2) are negligible. The

curvature is now expressed in a form first derived by Talanov r Z 1] . 2- ,2- , d r ^ d r _ 1 , — „

- IT, grad

l ni

• dp dz

This result is important to the study of self-focusing effects because the sign of d r/dz

indicates whether or not the beam is converging and its magnitude is a quantitative measure of that convergence or divergence. A positive curvature results in an increase in the slope of the ray

path with respect to the propagation direction and thus represents a divergence from the axis.

Diffraction alone will produce a positive curvature in an isotropic medium. A negative curvature, on the other hand, will cause convergence of the beam towards the axis and indicates the dominance

of the self-focusing nonlinearity.

A

2

(r) is proportional to the light intensity, and where the beam propagates with little ^r no change in shape, the intensity is equal to the power in the beam divided by the beam area n a (z). Let p be an effective power which absorbs these proportionality constants including the factor «

in the beam area. This effective power has, in fact, a functional form --it may be Gaussian

(exp [ - 2r /a ] ) for example -- and it is this functional form which describes the beam shape. We

can therefore consider p to be a function of a radial variable which is independent of beam size.

Defining the coordinate x as x = r/a(z), we can write

A

2

( r) = P(x ) = effective power

(A5) a (z) beam area

34
Equation (A5) is normally assumed in numerical calculations and has been referred to as the "constant shape approximation" f 22] . Along with eq (A4) it provides the basic relationships to calculate

the critical power, the self-focusing length, and the quantitative influence of the index nonlinearity

when diffraction dominates.

By expanding in cylindrical coordinates r and z it is easily seen that ^^A/A is also

proportional to a • We can then write eq (A2) as

n i = n

o

+ Vr f(x) a (z)

where f(x) is the sum of contributions from both diffraction and the index nonlinearity. Since grad^

is just a(z)~ [3/9x] , and since in the denominator of eq (4) can be replaced by n , the derivative

of f(x), which contains no dependence on z, determines the relative importance of diffraction and self- focusing. Once this relative importance is determined for one value of z, such as z = 0 at the en- trance plane, it is determined for all z. For completeness we write eq (A4) in the final form

where, using (A5),

dz o a (z)

f(x) = n2 (aA)

2

+ *

[ d-H> + 1

d(aA)_ (A8)

6 dx^ x dx

The importance of neglected terms can be determined for a particular beam shape and has been shown

to be negligible f 1SJ except near a catastrophic self-focus or under experimental condition where ex- treme external focusing is used.

The critical power Pc

is in principle calculated from the requirement that the derivative f(x) vanish for all x, leading to a detailed balance of self-focusing and diffraction. Since f(x) contains no dependence on beam diameter, the critical power will not be dependent on beam diameter for a steadystate nonlinearity.

Appendix B

The index nonlinearity leads to intensity distortions even below the critical power for catas- trophic self-focusing. Using the results of Appendix A quantitative corrections from the nonlinearity can be derived.

where

In eq (A7) r is replaced by x a(z) and a new function g(x) defined. This gives

= g(x) (A9)

dz a (z)

g(x)

= iHF dx" f W- < A10> o

If eq (A9) is multiplied by dz(da/dz) and integrated, we find

C>2

= - *Sf + c. (All)

a

Considerable simplifications results from expanding A and therefore g(x) about small x and retaining terms to order x

2 - In reference 2 this expansion is carried out for a Gaussian beam. We

can investigate the geometrical focus at low powers by setting the derivative in eq (All) equal to zero f 6] . After some manipulation the focal diameter d is evaluated in terms of d , the diameter

in absence of a nonlinearity. In particular,

d = dQ

(l - P/P

cr

)^ (A12)

35
where P is the full power in the beam and P£r

is given in cgs units by

P

cr = if"!! < A1 3) 32 n n The result (A12) is useful for P/P less than about 0.5. The calculated on-axis intensity

at breakdown must be multiplied by a factor (dQ/d ) 2 to approximately correct for the effects of the nonlinearity.

More extensive analysis shows that Pcr

is the critical power for self-focusing near the center of a Gaussian beam [22] . For input powers greater than Pcr

but less than P

c> diffraction

dominates everywhere except near the beam center. A collimated beam will initially intensify at such powers until the diffraction of the wings causes the on-axis intensity to drop. Ppr

differs from

Pc because the latter is a quantity averaged over the entire beam while P is determined by the behavior near the center. In fact, P„ is not a precisely defined quantity because it is not possible

to exactly balance diffraction and self- focusing over the entire beam cross- section. At an input

power of Pc

» therefore, a propagating beam will not change its size measurably and so not experience a catastrophic self-focus, but its intensity distribution will be distorted. P has the same

functional form as P„„ and differs by just a numerical factor as P = 0. 273 P for Gaussian

t t-i c r c beams.

Appendix C

The analysis of Appendix A and the results derived from it are correct only in the steady

state. In solids the dominant nonlinearity is normally electrostriction, and if the process is transient, it is no longer true that the relative balance between diffraction and self-focusing is independent of propagation distance and that the critical powers are independent of beam diameter. The

changes occur because electrostriction becomes non-local in both a temporal and a spatial sense. Although a susceptibility approach such as we have used is no longer strictly correct, it is nonetheless useful for establishing functional dependences for self-focusing parameters and approximate

quantitative values.

For our experiments two results from a transient analysis are important f 23] . The first is that transient effects decrease the effective nonlinear index n2 and thus make self-focusing more

difficult. If we wish to avoid self-focusing by restricting our powers to well below the critical

power, the steady- state analysis gives us a lower bound on Pc

• Being in a transient regime can, therefore, only increase our margin of safety and improve the accuracy of our experiment by making the corrections indicated by eq (A12) less important.

The second important result involves the dependence of the critical power on laser pulsewidth and on beam diameters. In the steady state the nonlinear index n

2

is given by [ 10]

9n 2 n

o<p yr } n- = tL

2 4«pv

where n is the index of refraction in the absence of the nonlinearity, p is the material density, and v is the acoustical sound velocity. The quantity (p 9n /a p) for cubic materials such as the alkali-halides may be found approximately by differentiating the Clausius-Mosotti equation. The re- maining constants are tabulated in handbooks.

When the laser pulse width tp

is shorter than the electrostrictive response time r = a/v, n

2

is decreased in value, thereby increasing the critical power. For a triangular pulse, Kerr [ 13] has shown that

^transient = (n

2 Steady- state [ 1 " ¥E

D ( ¥ )] (Al4

)

where D(-^£) ) is Dawson's integral with

D(|) = exp (-£) \ exp jf&ri

36
Vhent ~ t/2,

P 2.2

v t

^ n

2 ^transient ~ ^n

Z ^steady state 2

This result is valid for more general and realistic pulse- shapes with the numerical constant k being

of order unity and having a value dependent on the precise time- structure of the pulse. When this result is inserted into eq (A13), the critical power becomes

P

c j = (P

c

> • (A15)

t /t < steady state p

For short laser pulses, therefore, there exists more properly a critical intensity rather

than a critical power, and in this transient regime fairly small changes in pulse-width will have a significant effect on the critical power.

9. References

[1] Zverev, G. M. et al. , Zh. Eksp. Teor. Fiz.

53, 1849 (1967) [Sov. Phys. JETP 26, 1053

(1968)].

[2] Akhmanov, S. A. et al. , Usp Fiz Nauk. 93, 19 (1967) [Sov. Phys. Uspekhi 1_0, 609(1968)].

[3] Hellwarth, R. W. , in Damage in Laser Materials , edited by A. J. Glass and A. H. Guenther, NBS Spec. Publ. 341 (U.S. Dept.

of Commerce, Wash., D. C. , 1970), p. 67.

[4] Yablonovitch, E. Appl. Phys. Lett. 1_9, 495

(1971).

[5] Keldysh, L. V., Zh. Eksp. Teor. Fiz. 47,

1945 (1964) [Sov. Phys. JETP 20, 1307

(1965)].

[6] Zverev, R. M. and Pashkov, V. A., Zh. Eksp. Theor. Fiz 57, 1 128 (1 969) C Sov. Phys.

JETP 30, 616 (1970)].

[7] For a review, see J. J. O' Dwyer, The

Theory of the Dielectric Breakdown of Solids

(Oxford University Press, London, 1964).

[8] Guiliano, C. R. and Marburger, J. H. , Phys. Rev. Lett. 27, 905 (1971).

[9] Von Hippel, A., J. Appl. Phys. 8, 815

(1937).

[10] Akhmanov, S. A. , Sukhorukov, A. P. , and

Khokhlov, R. V., Sov. Phys. Usp. J_0, 609

(1968).

[11] Bloembergen, N. , private communication.

[12] Alcock, A. J., DeMichelis, Claudio, and

Richardson, M. C. , IEEE J. Quantum Elec.

QE-6, 622 (1970).

[13]Bass, M. and Barrett, H. H. , IEEE J. of Quantum Elec. QE-8 , 338 (1972).

[14] Zverev, G. M. , Mikhailova, T. N. , Pashkov,

V. A. , and Solov'na, N. M. , Zh. Eksp.

Theor. Fiz. 53, 1849 (1967). [Sov. Phys.

JETP 26, 1053 (1968). ]

[15] Yablonovitch, E. , Thesis, Harvard University (1972).

[16] Kuchin, V. D. , Rep. Akad. Sci. USSR, 114 , 301 (1957); Sov. Phys. Solid State, 1_, 405

(1959).

[17] Watson, D. W. , Heyes, W. , Kao, K. C. , and Calderwood, J. H , IEEE Trans Elec.

lus. El-1, 30 (1965). Also, Vorob'ev, G. A. Lebedeva, N. I. , and Naderova, G. S. , Fizika Tverdogo Tela, 1_3, 890 (1971), [Sov.

Phys. Solid State, 13, 736 (1971)].

[18] Giuliano, C. R. , to be published.

[19] Wang, Chen-Show, Phys. Rev. 173 , 908

(1968).

[20]Born, M. and Wolf, E. , Principles of Optics,

(Pergamon Press, Inc. , New York, 1959).

[21] Talanov, V. I., Zh. Eksperim, i. Theor.

Fiz. Pis'ma Redaktsiyu 2, 218 (1965)

[Sov. Phys. JETP Letters 2, 138 (1965)].

[22] Dawes, E. L. , and Marburger, J. H. , Phys.

Rev. 179 , 862 (1969).

[23] Kerr, E. K. , IEEE J. Quantum Elec. QE-6

616 (1970).

37
10. Figures

100% R

Mirror

NdYAG or ruby laser rod in single ellipse pump cavity

Intracavity apperture

to restrict oscillation

to TEM00 mode. A

"Mendenhall wedge"

I(0) = flo cos

4

(0)

Polarizer I Rotatable

Polarizer 2

Fixed Output mirror

dielectric for NdYAGiSopphire

etalon for ruby

Figure 1. Laser and variable attenuator configuration for damage studies.

Figure 2. Intensity disbribution of the YAG laser as a function of radial distance from the beam center

at the position of the focusing lens. (a) The data was taken by sweeping a 1-mil aperture through the beam with each data point corresponding to a different laser shot. A small dip near the center could not

be reproduced and indicates the experimental errors in the measurement. (b) By plotting the log of the intensity as a function of the radial distance squared, it was found that the output beam has a Gaussian profile [-2r^/c^] where a is measured to be 0.485 ± 0.005 .millimeters.

100 (im Direction of Propagation

100 jim Direction of Propagation

Geometric Focal

Plane Figure 3. Intrinsic damage in RbCl. Damage which

we have considered to be intrinsic had this basic

shape in all the materials tested.

Geometric Focal

Plane

Figure 4. Inclusion damage in RbCl.

38
5 ns Figure 5. Nd:YAG laser pulse transmitted through

the sample. (a) No damage produced. (b) Damage

produced at or just before the pulse reached its

maximum intensity. These were two successive

pulses in the same volume. Note that there is no evidence that the second pulse, the one which

produced damage, is any different from the rest.

1.8

1.4 G

uf 10

i? 0.6(-

0 20

© D C. + 10.6 microns X 1.06 microns X Questionable data point

at 1.06 microns

(See Table 3 )

1.8

1.4

(JoZ

UJ 1.0

X

UJ 0.6

0.20

1.8

1.4 6J 10

o m 06

0 20

Nal NoBr NaCI NaF

KBr KCI KF

Rbl RbBr RbCI

a)

b)

C)

d)

Damaged when the peak

laser field was reached E = 1 Damage ^ E Peak

Damaged before the peak

laser field was reached EDamage /E Peak = 0.896

Damaged after the peak

laser field was reached EDamage /E Peak

= 0.954

3 successive pulses no damage

E Peak = I (Arbitrary units)

•Figure 6. Comparison of breakdown strengths for various alkali halides studied at dc , 10.6 microns,

and 1.06 microns. Values for 10.6 and 1.06 microns are root-mean-square field strengths and not peak fields. The data at dc is taken from reference 9 and the 10.6 micron data is taken from ref- erence 4. (a) Relative breakdown fields for Na salts normalized to E, aCl'

(b) Relative break- down fields for K salts normalized to ^. (c) Relative breakdown fields for Rb salts nor- malized to E.NaCI*

Figure 7. Ruby laser pulses transmitted through NaCI sample.

39
Laser Induced Damage to Glass Surfaces

N. L. Boling and G. Dube'

Owens-Illinois , Inc

.

Toledo, Ohio 43651

Entrance and exit surface damage thresholds for Owens-Illinois ED-2 laser glass

are measured at Brewster's angle and near normal incidence. A theory that accounts

for the observed asymmetry in exit/entrance face damage thresholds is presented.

The nature of the damage at both surfaces is described for both untreated and chemically strengthened ED-2 laser glass.

Key Words: Chemically strengthened glass, glass damage, glass lasers, laser damage, Q-switched, surface damage.

1. Introduction

This paper describes investigations of damage to glass surfaces by high power laser beams. A glass

oscillator-amplifier system operating in the TEMoo mode and emitting several joules in a 30 nanosecond

pulse is used to damage Owens-Illinois ED-2 laser glass. Exit and entrance damage thresholds of untreated glass and of glass subjected to a surface strengthening process are reported. The reason for the difference between the entrance and exit thresholds is briefly explained.

2. Experimental Arrangement

The single transverse mode laser used in this study consists of an oscillator and four amplifiers.

For the purposes of this paper the essential characteristics of the laser are listed in table 1:

Table 1. Laser characteristics

Transverse Mode

Longitudinal Modes

Pulse Width

Polarization

Time Interval Between Pulses

Energy Output Beam Diameter at Position of Glass Sample

Repeatability of Energy Output

Figure 1 shows the arrangement used to calibrate the energy density of the sample. In order to minimize

any self-focusing effects the sample position was chosen to be at the waist of the focused laser beam. A specially designed 1 mm diameter aperture was placed at this chosen position in front of a thermopile.

The aperture was moved across the beam until a maximum reading was obtained from the thermopile.

Filters were added in the path of the beam until no plasma was detected around the aperture. This was

ascertained by photographing the aperture with an open shuttered camera during the laser shot. That

there was no plasma which might have interfered with the passage of the laser pulse through the aperture

was further verified by changing the transmission of the filters by a given factor and noting that the energy passing through the aperture changed by the same factor. Prior to this the filters were checked

TEMoo Many (No Control)

30 ns FWHM

Linear

7-10 minutes

5-12 Joules ^ 1.8 mm

± 10%

40
for bleaching during the laser pulse and no such bleaching was noted. Several thermopiles were checked

for use in these measurements. Readings among these varied by ± 10%. Even recent calibration did not insure readings more consistent than this.

In summation, the possible sources of error, other than the thermopile used, in the energy density

calibration described above are as follows:

1. Undetected bleaching of the neutral density filters during the laser pulse.

2. Undetected absorption of the pulse by a plasma around the aperture.

3. Positioning of the aperture off beam center.

Note that any of these errors would result in an underestimation of the energy density.

The arrangement of figure 1 was also used to monitor and control energy on the samples during

damage tests, the only difference being that the aperture and thermopile were replaced by the sample.

In all shots the pulse width, transverse mode, and energy out the laser were monitored. The laser was

always pumped at the same level, the energy on the sample being controlled by changing filters. Because

the transmission of the filters was changed in nominal steps of 10% and the repeatability of the laser output was ± 10%, the thresholds measured are relatively accurate to within 20%. All damage thresholds

reported in this paper are based on the average energy density across the 1 mm aperture used for calibration. Calculations based on the approximately Gaussian cross section of the beam indicate peak energy densities 25% higher than these average numbers.

The occurrence of damage is defined in this report as the appearance of a visible plasma at the sample surface. Although the sample was carefully examined after each shot, no change in the surface

was noted without the appearance of a plasma. Conversely, a visible change in the surface was observed

with every occurrence of a plasma. This change was sometimes very minute and difficult to see, but it was always present. Thus we find the occurrence of a visible plasma is a necessary and sufficient

condition for surface damage in glass.

3. Damage of Untreated Samples

One-half inch glass cubes of ED-2 laser glass were prepared for damage testing by polishing two opposite faces to normal commercial specifications. Several cleaning procedures were tried before

settling on one which seemed to give satisfactory results. This final procedure consisted of ultrasonic

cleaning in a soap and water solution, a rinse in distilled water, and cleaning in the vapors of boiling

isopropyl alcohol. Final examination for cleanliness consisted of a careful visual examination under a bright microscope light. Great care had to be exercised in the cleaning process before the sample could

pass this inspection. It is worth noting that these samples were much cleaner than the components of most laser systems. Nevertheless, inspection of the samples in a HeNe laser beam showed obvious non- uniformity of scattering over the cleaned surfaces. Immediately after cleaning of a particular sample

damage tests were begun. For a given sample these tests typically required between 1.5 and 2 hours, the sample being subjected to pulses of gradually increasing power densities. Thus the condition of the sample was representative of surfaces in a carefully cleaned laser system.

The exit and entrance damage thresholds of 5 typical samples are given in table 2:

Table 2. Entrance and exit damage threshold for normal incidence beam

Entrance Damage Exit Damage Entrance Threshold .'Entrance Threshold

Sample Threshold J/cm2 Threshold J/cm 2 Exit Threshold Exit Threshold

15 195 - 230 111 - 120 1.6 - 2.1 1.9

16 187 - 204 86 - 124 1.5 - 2.4 2.0

17 125 - 130 90 - 105 1.2 - 1.4 1.3

18 160 - 209 120 - 146 1.3 - 1.7 1.5

21 150 - 179 100 - 110 1.4 - 1.8 1.6

41
The thresholds of both surfaces vary over a wide range, the exit threshold generally lying between 80 and 120 J/cm2 (2.7 - 4 GW/cm2

) and the entrance between 120 and 200 J/cm2 (4 - 6.7 GW/cm2

) . There are

two plausible reasons for this wide range. First, the process which triggers the damage might be

inherently probablistic.Cl] 1 In connection with this it should be remembered that the beam diameter

used here is much larger than that used in the experiments described in [l]. Second, the condition of the surf ace undoubtedlt plays a role in determining the damage threshold. By purposely scratching the entrance surface it is possible to cause it to damage at much lower levels than it normally would.

Damage near threshold on both the entrance and exit surfaces typically takes the form of a ring

(figures 2 and 3). The diameter of this ring varies but is usually larger than the damaging laser beam,

3 mm being a nominal value. When viewed under a microscope the ring appears to be a layer of material.

This material cannot be removed by hard rubbing with water or acetone. On the entrance surface the material inside the ring generally exhibits no change in appearance after damage. On the exit face a slight ripple is often seen inside the ring. Pitting is rarely seen near threshold.

Since the damage ring is larger than the laser beam, it is obvious that it is due to the plasma

which accompanies damage and not to the laser beam. This plasma normally spreads over the surface to a distance of several millimeters from the beam center. Aside from the ring structure, a slight permanent

darkening of the glass surface is often seen several mm from the area irradiated by the laser beam. This again is due to the plasma.

Events leading to the ring deposition were theorized to be as follows: A plasma is formed in the area exposed to the laser beam. The temperature of this plasma in its incipient stage is 10 1* - 10 5 °K.[2] The plasma expands, remaining in close contact with the surface, until it becomes cool

enough for its components to condense on the cooler surface. Thus the ring is a condensation of certain components of the plasma. If we assume this explanation is correct, the ring should be rich in

the most volatile component in the glass surface, whether this be from the polishing compound or the glass per se.[3] In the case of ED-2 laser glass, the most volatile compound is Li20. In order to test

this hypothesis an elemental analysis of the ring was made using Auger spectrometry. Compared with

undamaged glass, the only element detected in excess in the ring was lithium.

4. Ion Exchanged Samples

Since some theories of surface damage involve mechanical breakage, a strengthening of the surface

presents itself as a possible method of increasing resistance to damage. We have used an ion exchange process on ED-2 glass to examine this approach. In this process lithium atoms in the surface are replaced by sodium atoms. This results in a "crowding" of atoms on the surface, leading to a surface

compression layer. In the work described here this layer is about 200 microns deep. Glass so treated

exhibits greatly increased mechanical strength.

No experimentally measurable increase in the damage threshold was noted for these ion exchanged

samples. However, the characteristics of the damage are quite different from that seen on untreated

glass. The damage is initially much like that described above for the untreated samples. In particular, the ring structure is present Immediately after the damaging shot. Within a few minutes after the shot no damage can be seen inside this ring. However, over an extended period of time, ranging up to two days, the surface inside the ring often gradually develops a tessellated pattern. Figures 2 and 4 show

respectively exit damage a few minutes and one day after the occurrence of damage. Out of fifteen

damage sites examined on ion exchanged samples, eight exhibited this cracking phenomenon. The same thing is sometimes seen on untreated samples but much less often and only after exposure to very high

power levels. For example, out of thirty damage sites on untreated samples only two showed tessellation,

It is of interest to note that cracking of the surface is due to the hot plasma and not the laser

pulse. These cracks often fill the interior of the several millimeter diameter ring, but they never

extend outside the ring. This is consistent with the theory presented above explaining the origin of the ring. The plasma inside the ring is hot enough to cause cracking of the surface, while outside the ring it has cooled to the point where it no longer causes damage. This cooling might in part be

attributable to the partial condensation of the plasma as the ring forms.

Figures in brackets indicate the literature references at the end of this paper.

42
A possible reason why ion exchanged glass exhibits delayed failure to a greater extent than does un- treated glass is that the thermal expansion coefficient of a glass is a strong function of the quality

and quantity of its alkali content. A sodium containing glass has a greater coefficient than the same

glass with lithium. Thus heating the Na rich surface of an ion exchanged sample results in much greater

stresses than in the case of an untreated Li containing surface. Still another and perhaps more plausible reason for the delayed failure in strengthened glass is the difference between stress in the surface

layer and stress in the glass immediately below the surface. This stress difference could be increased

greatly through the heating by the plasma and subsequent cooling.

5. Relation Between Entrance and Exit Damage Thresholds

In the 4th column of table 2 are listed the ranges of possible ratios of entrance to exit damage

thresholds based upon the numbers in columns 2 & 3 . In the 5th column the averages of these ranges are

given. Although these averages vary, because of the variation of the thresholds themselves, they are balanced about a ratio of approximately 1.5.

The reason for the asymmetry in the exit and entrance damage thresholds has been a subject of spec- ulation for several years, asymmetric plasma growth, [4] and stimulated Brillouin scattering being ex- amples of hypotheses which have been presented in explanation. This asymmetry can, however, be

quantitatively and qualitatively explained in terms of Fresnel reflections from the surfaces. [5] At the entrance face the reflected wave is it radians out of phase with the incident wave, resulting in electric

field at the entrance surface lower than the electric field of the incident laser beam. At the exit

surface the reflected wave is in phase with the incident wave, thereby leading to an increased electric vector at the surface. The ratio of the powers at the two surfaces is given by:

t Eexitl

2 = «

[F^]2 Tn-TTT*

where n is the index of refraction of the glass. For ED-2, n = 1.55 and the ratio of eq (1) is 1.48, in

good agreement with the data of table 2.

If the above explanation of exit and entrance damage asymmetry is correct, then there should be no difference between the entrance and exit damage thresholds of samples whose faces are at Brewster's

angle to an appropriately polarized laser beam, since no reflection waves occur in this case. Several

glass samples were subjected to this test and no experimentally determinable difference between entrance

and exit thresholds was found. Table 3 shows this:

Table 3. Entrance and exit damage threshold for beam incident at Brewster's angle

Entrance Damage Exit Damage Entrance Threshold ^^Entrance Thresholdv.

Sample Threshold J/cm2 Threshold J/cm2 Exit Threshold \ Exit Threshold

^

B 105 - 120 92 - 117 0.9 - 1.0 1.0

E* 52 - 57 57 - 60 0.9 - 1.0 1.0

F 83 - 88 83 - 88 0.9 - 1.0 1.0 G 112 - 130 112 - 130 0.9 - 1.2 1.1 *This sample was observed to have poor surface quality.

This hypothesis is also satisfying from a phenomenological point of view. It is reasonable to assume that the strength of the electric field at a surface is related to damage at that surface.

Further, these results emphasize the importance of paying attention to what is happening at the surface

of interest in measurements of damage threshold. The true damage threshold is not necessarily given by

the energy density in the incident laser beam, but rather must be calculated for the surface under

investigation. In the case presented here the calculation is straightforward. In the case of dielectric coatings, for example, the calculations can be more involved.

43
Finally, it should be pointed out that Fresnel reflections explain only the difference in damage

threshold between the entrance and exit surfaces. The development of damage after the threshold is reached is itself asymmetrical, the exit surface being prone to pitting and the entrance surface to less extensive damage. Further work is needed to explain this asymmetry but is quite possible that it can be

explained by an extension of the reflection theory presented above.

6. Conclusions and Discussions

The surface damage thresholds given in this paper for untreated glass are generally higher than

reported by other investigators working with glass. Care has been taken to insure that these thresholds

are not falsely high. Any source of error in the measurements tends to make the actual values larger

than those given herein. These relatively high thresholds could be due to the laser used in their measurement. For example, it has been reported that the surface damage threshold in some instances

appears to be a function of the beam diameter. [6] However, the most probable reason for the large

threshold values is the glass itself. Previous measurements on the same glass with a multimode laser

emitting several tens of joules yielded thresholds consistent with those reported here. [7] It is our intention to measure the damage threshold of ED-2 with a laser having a much smaller beam than was used

here. Perhaps in this way it can be learned whether damage thresholds depend on parameters such as beam

size.

From a practical point of view, the experiments described here were performed under conditions

which can, with care, be reasonably duplicated in practical laser systems. The samples were cleaned in

a straightforward manner and tested over long periods of time in the laboratory environment. The laser beam was free of "hot spots" and had a relatively large diameter. Consequently, in working systems power densities of 2 GW/cm2 (in 30 ns) can easily be achieved without concern for surface damage, and with care 3 GW/cm2 should be safely attainable on a normal operation basis. These numbers apply to the local energy density at the exit surface. Self-focusing of the beam, for example, can of course raise

the surface energy density in an uncontrollable manner.

Ion exchange strengthening of glass does not appreciably affect the damage threshold. In fact,

damage when it does occur, is more likely to spread than in the case of untreated glass. Thi6 process,

then, shows no promise of alleviating the surface damage problem. However, this negative result does

suggest that shock waves of any sort are not primarily responsible for surface damage near threshold.

In this sense the results are instructive.

The asymmetry of exit and entrance surface damage thresholds can be readily explained in terms of Fresnel reflections at the surface. This result suggests that it is particularly important to consider

the strength of the local electric field in damage studies of coatings, mirrors, etc.

This work was supported by the Advanced Research Projects Agency of the Department of Defense under

Contract No. DAHC 15-69-C-0303.

7. References

[l] Bass, Mi., Barrett, H. H., and Holway,

L. H. , Jr., "Experimentation and Investigation of Optical-Irradiation-Induced Surface

Damage in Optically Nonlinear Materials,"

Scientific Report No. 1, Feb. 1972, ARPA

Order No. 1434AMD#1.

[2] Boling, N. L. , and Beck, R. W. , "Damage in Laser Materials,": 1971, Glass, A. J., and Guenther, A. H., Eds., National Bureau of Standards Spec. Pub. 356 November 1971.

[3] Haller, W. , and Winogradoff, N. N., "Explosive Vapor Release as Cause of LaserInduced Surface Damage of Glass," Journal of

The American Ceramic Society, 54_, No. 6, 314, June 1971.

[4] Fersman, I. A., and Khazov, L. D., Sov. Phys.- Tech. Phys. 15, 834, 1970.

[5] Crisp, M. D., Boling, N. L. , and Dube', G.

,

"Importance of Fresnel Reflections in Laser i Surface Damage of Transparent Dielectrics,"

Submitted June 1972 to Applied Physics

Letters.

[6] Newman, B. E., and DeShazer, L. G. , "SpotSize Dependence of Laser-Induced Damage

Thresholds of Dielectric Thin Films,"

Digest of Technical Papers, VII International Quantum Electronics Conference,

Montreal, Canada, May 1972.

[7] Beck, R. W. , "Laser Glass Damage Testing at Owens-Illinois" 1970, Glass, A. J., and Guenther, A. H. , Eds. National Bureau of Standards Spec. Pub. 341, December 1970.

44
8. Figures

o

Figure 1. This figure shows the apparatus used

for the damage threshold measurements. The com- ponents are: (1) laser oscillator; (2) polaroid

film; (3) laser amplifier chain; (4) thermopile;

(5) photodiode; (6) Tektronix 519 oscilloscope;

(7) neutral density filters; (8) 1.5. m focal length lens; (9) camera; (10) sample.

Figure 2. Typical damage ring new threshold.

40X magnification. Ring is 4mm in length. This

particular ring is on a chemically strengthened

sample immediately after damaging shot.

Figure 3. Section of damage ring near threshold

100X magnification.

Figure 4. Damage on chemically strengthened

sample one day after damaging shot.

45
The Relation between Surface Damage and

Surface Plasma Formation*

Concetto R. Giuliano

Hughes Research Laboratories

Malibu, California 90265

A number of experiments are described in which entrance and

exit surface laser-induced damage on sapphire samples are studied.

Damage thresholds are measured as a function of beam size and

divergence at the surfaces. Temporal evolution of surface plasmas

is studied using a streaking camera, and spatial differentiation

of surface plasmas is accomplished by allowing the light to strike

the surfaces away from normal incidence. It is found that the

entrance surface has two plasma components, an air plasma which is

directed along the light beam and an "explosion" plasma which is directed normal to the surface. The exit surface has only the

explosion plasma. It is concluded that surface plasmas are a result of rather than a cause of surface damage.

Key Words: Entrance and exit surface damage, sapphire,

spatial and temporal plasma resolution, streak camera experiments, surface plasmas.

1. Introduction

It has been known for some time that laser-induced surface damage has distinctly

different physi cal .. character!- sti cs , depending on whether the entrance or exit surface

is damaged. [1-4] To a certain extent the conditions under which the damage is generated (e.g., focusing, surface finish) are important in determining some of the details

of the appearance of the damage, but as a general rule entrance surface damage is less

extensive and more difficult to detect visually than exit surface damage, especially

when generated under conditions of gentle focusing. Under such conditions for sapphire

the distinguishing features are that exit surface damage can be most easily described

as relatively deep fractured areas, while entrance surface damage is relatively shallow

crazed areas. Scanning electron micrographs of both types of damage are presented in

Figures 1 and 2.

The occurrence of laser-induced surface damage is known to be accompanied by a bright flash of light which comes from a spark or plasma at the surface. [1-5] (See

fig. 3) In fact, the occurrence of a visible spark has been held to be a criterion for

surface damage. [4] Spectra taken of exit surface plasmas show that the light contains

spectral lines characteristic of the material being irradiated. [1] The connection

between the surface plasma and the damage was not established in the early work. Whethe

or not the plasma is a result of or a cause of the surface damage or whether it is an independently occurring phenomenon that simply accompanies the damage phenomenon was not

determi ned

.

* Work supported by the Advanced Research Projects Agency under ARPA Order No. 1434 with

Air Force Cambridge Research Laboratories.

Figures in brackets indicate the literature references at the end of this paper.

46
Fersman and Khazov [3] have suggested that the surface damage is caused by a plasma-surface interaction and have explained the difference between entrance and

exit surface damage using a model in which both the plasmas grow in an upstream

direction as they develop in time, an extension of the observations for air breakdown

plasmas. [6] The exit damage would then be expected to be more extensive than entrance

damage because the exit plasma grows into the surface, while the entrance plasma grows

away from the surface. Fersman and Khazov observe a compress i onal wave which propagates

inwardly from the exit surface. Compressi onal waves arising from damage sites in solids

have been observed by others using both schlieren and holographic techniques. [7-8]

While the above picture can predict a difference between the entrance and exit damage,

it was not experimentally confirmed.

Another possible explanation for the difference between exit and entrance damage

is that exit damage might be somehow influenced by self-focusing of the beam in the

material, while the beam at the entrance would not be so affected.

We have carried out a number of experiments in an attempt to determine more explicitly the connection between the surface damage and the plasmas and to clarify the

nature of the damage mechanism or mechanisms.

All the experiments described in this paper were carried out using a single longitudinal and transverse (TEM ) mode Q-switched ruby laser. The laser and associated

monitoring apparatus are described in detail elsewhere. [2] Typical laser pulse width

and peak power are 20 nsec (FWHM) and 1 MW , respectively.

2. Damage Threshold as a Function of Beam Size and Divergence

One of the questions which arises in the consideration of exit surface damage is

concerned with the possibility that the damage at the exit surface might be influenced

by what the beam does in the bulk of the material. For example, if some sel f -focus i ng or other nonlinear phenomenon takes place, it might affect the beam so that the intensity at the exit face will be different from that expected on the basis of linear propagation theory. Also, some phenomenon which might cause the generation of acoustic

waves directed toward the exit surface would be affected by how the beam propagates

inside the sample before reaching the exit surface. A series of experiments on sapphire

was carried out in which the sample was moved relative to the focusing lens, so that

the spot size at the surface would vary over a wide range and also so that the beam

could be either converging or diverging at the surface, depending on whether the beam

waist lay downstream or upstream from the surface.

The result of these experiments is shown in fig. 4. In these experiments a 19 cm

lens for focusing the light from the mode-controlled ruby laser was used. The sapphire

samples were moved in 5 mm increments relative to the lens, and surface damage threshold

was measured for each location. The data on the left half of the figure correspond to the case of rays converging at the exit surface and the data on the right half to diverging rays. The beam area at the surface is also plotted as a function of distance in the

f i gure

.

Figure 4 shows that the results for exit surface damage threshold power in sapphire

sample L134 fall very close to the beam area curve. The fact that they fall so closely

is a coincidence of the scale and the units chosen, but the fact that they have the same functional dependence on distance is significant in that it strongly indicates that the

damage threshold is a power density phenomenon. Also, the fact that the threshold is

independent of whether the beam is converging or diverging at the exit surface indicates

that the damage seems to be strictly a surface or near-surface phenomenon. The exit

surface damage threshold for this sample is about 1 GW/cm2

. The data for entrance surface damage for sapphire sample C129 are shown in fig. 5. A similar dependence on lensto-sample distance is seen in the plot but in this case the scatter in the points is

somewhat greater. Here the beam area curve is plotted on a different scale than in

fig. 4 to show that the dependence of threshold power with distance is essentially the

same as the dependence of beam area with distance, again indicating that damage

threshold is a powf r density phenomenon. The entrance damage threshold for this case

i s about 1 . 5 GW/cm .

3. Surface Damage without Plasma Formation

We have carried out a few experiments in which surface damage was generated without observing an accompanying plasma. These experiments were conducted on sapphire

samples, and the observations apply only to the exit surface (entrance damage was not

47
pursued in this series of experiments). In these experiments we observed the exit

surface during laser irradiation both visually and photographically under different

conditions of optical filtering and exposure. It was found that when the sample

was exposed to focused radiation very close to the threshold, damage could be generated in the form of a series of micropits without the simultaneous accompaniment of

a detectable plasma or spark.

A series of experiments was initiated in which the sample was irradiated well

above damage threshold, and the plasma observed as the power was gradually reduced,

each time exposing a fresh area of the surface. During this series, the accompanying

plasma was seen to decrease in size and brightness as the laser power was reduced,

until a power point was reached at which the spark was no longer visible. On a number

of occasions after laser irradiation at a level just below that for generating a visible plasma, it was possible to detect damage in the form of a number of micropits

when the sample was carefully examined through a microscope with properly chosen illumination. This mi cropi tti ng , which is seen only very close to threshold, is less extensive than the single fracture crater that occurs more often at slightly higher powers

when the plasma is clearly visible. It should be noted that at the times when no plasma is visible, it is possible to see small amounts of laser light scattered from

the tiny surface pits which are being formed.

The results of these observations suggest that it is possible to obtain exit

surface damage without plasma formation. Of course, the nonobservabi 1 i ty of a plasma

does not necessarily imply that it is not present, and these results alone do not

allow us to form any definite conclusions regarding the connection between the damage

and the plasma.

4. Time Evolution of Surface Plasmas

One reason for the interest in studying the temporal evolution of the surface

plasmas has been to determine the direction of growth of the plasmas relative to the

crystal surface, which would lead to a possible explanation of the differences between

exit and entrance damage and to test the mechanism proposed by Fersman and Khazov.

We have employed an STL image converter camera operating in the streaking mode for

observing the growth of the entrance and exit surface plasmas (see ig. 6). For these

experiments, the output from our single-mode ruby laser was focused with a 19 cm lens

on the desired surface of the sapphire sample being examined. Typical laser output is about 20 mJ in a 20 nsec pulse, which gives a power density at the focus of approximately 10 GW/cm2

, about 10 times above damage threshold. For the streak camera experiments, a portion of the incident laser light was allowed to enter the camera directly

to serve as a marker streak for synchronization purposes.

Streak photographs showing the temporal development of the surface plasmas are shown in figs. 7 and 8. Because the photographic setup is such that the camera views

the surface at an angle less than 90 to the normal (see fig. 6), both the plasma and

its reflection in the surface are often seen. The fact that there are indeed reflections was confirmed by placing an appropriately oriented Polaroid filter in front of

the camera, in which case the reflection from the dielectric surface disappears as expected

.

Essential features of these streak photographs to be noted are

• The plasmas grow away from the surfaces in both cases, the entrance

plasma growing in the upstream direction and the exit plasma in the

downstream direction.

• The entrance plasma has two temporal components, whereas the exit

plasma has one temporal component.

• The extent of growth and luminosity of the faster growing component

of the entrance plasma depends on the incident laser pulse in that it ceases to expand and to emit light when the laser terminates.

e The slower growing, longer lived component of the entrance plasma

expands at about the same rate as the exit plasma. These plasmas

continue to expand outward and emit light long after the laser has

termi nated

.

• Typi cal g vel oc i ti es of expansion of the luminous plasma fronts are r ^2 x 10 cm/sec for the short-lived entrance component and ^1 x 10 cm/sec for the long-lived entrance component and for the exit plasma.

48
Several ideas are suggested by these observations. First, the fact that both

entrance and exit plasmas grow away from the surfaces suggests that the mechanism

proposed by Fersman and Khazov is incorrect. From the time dependences of the plasmas

relative to the incident laser pulse it can be inferred that the short-lived entrance

component is a plasma supported in the ambient atmosphere and sustained directly by

the laser beam itself, while the longer-lived components at both surfaces result from a kind of mi c ro -expl os i on caused by the deposition and subsequent release of energy at or near the s urf ace

5. Spatial Differentiation of Surface Plasmas

To test these hypotheses further, we devised additional experiments that were designed to allow a spatial differentiation of the plasmas. The reasoning behing these

experiments is as follows. A plasma sustained directly by the light beam would be

expected to exist only where the light is present and to be directed along the light

beam independent of the angle at which the light enters the material. A plasma which

results from a surface explosion caused by a deposition of energy at the surface would

be expected to extend in a direction normal to the surface no matter at what angle the

light should enter the material.

Figure 9 shows time-integrated photographs of the entrance plasma formed when the

light is allowed to strike the surface at angles other than normal incidence. The

photographs show the separation of the plasma into different spatial components. One

component extends along the direction of the light beam; this apparently is the shortlived air plasma. Another component that extends normal to the surface can be seen;

this is the component resulting from the surface explosion. A third more diffusely

defined plasma component can be seen, extending in the direction of the light reflected

from the surface and is presumably additional air plasma sustained by the reflected

light. Figure 10 shows a time-integrated photograph of the exit plasma formed when the

light leaves at an angle of 45 to the surface. This photograph shows a single plasma

component directed normal to the surface, as expected if a surface explosion were taking

pi ace

As a final confirmation of assignments for the different plasma components, we looked at the entrance plasma in a vacuum. Figure 11 shows photographs of the entrance

plasma for a sample tilted 26 from the incident light both at 1 atm pressure and at about 1 Torr pressure. Because of the shape and size of the container, it was impossible

to perform the experiment at angles larger than 26 . As a result, the two spatial com- ponents are not distinguishable in figure 11(a) and (b). However, note than when the

air is removed from the system (fig. 11(c) and (d)), the plasma component which extends

along the light beam is no longer seen; the plasma visible in vacuum is directed normal

to the surface, i.e., only the explosion plasma remains.

6. Conclusions

As a result of these experiments, the following conclusions are reached. First,

the surface plasmas are a result rather than a cause of the surface damage. Second,

two kinds of plasmas occur at the entrance surface, one which is sustained by the light

beam in the ambient atmosphere and another which arises from a mi cro -expl os i on at or near the surface. The exit surface plasma is of the explosion type. While it may be true that some of the features seen on the surface of a material that has been subjected

to damaging laser radiation might be caused by the interaction of the plasma with the

surface, the primary cause of the damage is not the plasma. Rather, the surface plasmas

are a result of the process or processes, whatever they may be, that give rise to the

surface damage.

It is still not clear why the exit damage is physically so different in appearance

than entrance damage. If the mechanism of energy deposition were simply one of surface

absorption, it would be expected that the damage would be very similar on both surfaces.

For some reason, there seems to be a distinct directionality to the surface damage

mechanism. Since the exit surface damage threshold appears to be power density phenomenon and since it is not apparently dependent on whether the beam is converging or diverging before it reaches the surface, one would be inclined to conclude that the

damage mechanism does not depend strongly, if at all, on phenomena which take place in the bulk of the material. It appears to be strictly a surface or near-surface phenomenon. The possibility that a directed intense acoustic wave [1] is involved in the

damage mechanism is still open, although Stimulated Brillouin Scattering (SBS) is not

likely since there seems to be no correlation between SBS thresholds and damage

thresholds for a number of different solids. A mechanism of the type proposed by

Hellwarth for bulk damage [9] might be a valid surface damage mechanism. The end result

of this mechanism is the generation of intense acoustic phonons, but whether one would

expect the acoustic wave to be directed in any particular way is not known at this time.

49
7. Acknowledgement

The author wishes to acknowledge the skilled technical assistance of G. R. Rickel.

8. References

[1] Giuliano, Concetto R. , "Laser-induced damage

to transparent dielectric materials," Appl.

Phys. Letters 5_, 137-139 (1964).

[2] Giuliano, C. R. and L. D. Hess, "Damage

threshold studies in ruby and sapphire," in Damage in Laser Materials , edited by A. J. Glass and A. H. Guenther , U. S. National

Bureau of Standards Special Publication

No. 341 (U.S. GPO, Washington, D.C. 1970),

pp. 76-89

{3] Fersman, I. A. and L. D. Khazov, "Damage of transparent dielectric surfaces by a laser beam," Sov. Phys. -Tech. Phys. 15, 834-838

(1970).

[4] Bass, Michael, "Nd:YAG laser-irradiationinduced damage to LiNb0

3 and KDP , " IEEE J. Quantum Electron. QE-7 , 350-359 (1971).

[5] Khazov, L.D. , A. N. Shestov, and G. P. Tikhomirov, "Luminous discharge on non- absorbing surfaces produced by a single pulse

laser," Sov. Phys. -Tech. Phys. 13, 1112-1114

(1969).

[6] Ramsden, S.A. and W. E. R. Davies , "Radiation i scattered from the plasma produced by a focuse ruby laser beam," Phys. Rev. Letters 13 , 227-229 (1964).

| [7] Belikova, T. P., A. N. Savchenko , and E. A. 1 Sviridenkov, "Optic breakdown in ruby and re- lated effects," Sov. Phys . -JETP 27 , 19-23 (196

;.'«j'j|v3B

[8] Boling, N.L. and R. W. Beck, "Laser glass

damage threshold studies at Owens-Illinois,"

in Damage in Laser Materials , edited by A. J. Glass and A. H. Guenther, U. S. National Bures of Standards Special Publication No. 356 (U. i, GPO, Washington, D. C. , 1971), pp. 15-23.

[9] Hellwarth, "Role of photo-electrons in optica'

damage," in Damage in Laser Materials . edited i A. J. Glass and A. H. Guenther, U.S. Nationali Bureau of Standards Special Publication No. 3< (U.S. GPO, Washington, D. C. , 1970), pp 6 7 - 75 1 j!

I

I

50
9. Figures
Figure 2. Scanning electron micrographs of entrance damage for sapphire. Pulse energy increases from

a-c.
1.2

S 1.0

S 0.8 o

O 0.6

0 4

0 2 -

1 1

1

-

1 1 1 A SAPPHIRE LI34 _

MEASURED

.

BEAM AREA

CONVERGING . DIVERGING RAYS - \

RAYS

/ -

1 1 1 i i i

xlO"

10

I 6

1.4

1.2

o l.O

0.8

0.6

0.4

0.2

-3 -2-10 I 2 3 DISTANCE OF SURFACE FROM BEAM WAIST, cm

1 1 1

1 1 1

A SAPPHIRE CI29 MEASURED BEAM AREA

CONVERGING RAYS-^

DIVERGING «»• RAYS — \ A

/a - \ A if

1 1 1

I I i -3-2-1 0 I 2 3 DISTANCE OF SURFACE FROM BEAM WAIST, cm

xlO"

Figure 4. Exit surface damage threshold power for sapphire sample as a function of surface distance

from beam waist for 19 cm lens. Beam area is also

plotted

.

Figure 5„ Beam area and entrance surface damage

threshold as a function of surface distance from

beam waist for 19 cm lens.

I9cm LENS SAMPLE

LIGHT DIRECTION

Figure

for

6. Schematic diagram showing setup used Figure 7. Streak photograph of entrance plasma c photographing

and streak

surface plasmas both for still sapphire surface showing two temporal c^eSs' photography, fuhcul.:,,

53
20nsec

T

-| lmm h

LIGHT DIRECTION

Figure 8. Streak photograph of exit plasma on sapphire surface.

Figure 9. Time-integrated photographs of entrance

plasma at sapphire surface tilted with respect to incident beam direction. Angle between light

direction and surface normal (a) 4y ; (b) 60

Figure 10, Time-integrated photograph of exit Figure 11. Time- integrated photographs of entranc plasma at sapphire surface tilted 45 with respect plasma at sapphire surface tilted 26 from beam

to light propagation direction. direction for different optical attenuations at camera, (a) and (b) taken at 1 atm pressure in air, (c) and (d) taken at 1 Torr pressure. Opt] cal attenuation at camera (a) N.D. 1.0, (b) N.D.

1.7, (c) N.D. 0.7, (d) N.D. 1.7. 54
Ion Beam Polishing as a Means of Increasing Surface

Damage Thresholds in Sapphire*

Concetto R . Gi ul i ano Hughes Research Laboratories

Malibu, California 90265

Polishing of sapphire surfaces with energetic Ar ion beams

is shown to result in a substantial increase in laser damage

threshold over that for conventionally polished surfaces. Data

for both entrance and exit damage are presented. The results

are interpreted in terms of an increase in surface strength with

ion beam polishing.

Key Words: Ion beam polishing, sapphire, surface damage,

threshold increase, threshold measurement.

1. Introduction

It is a generally accepted fact that the threshold for laser induced surface

damage of transparent dielectrics is a function of the condition of the surface being

irradiated. Any foreign material on a surface subjected to high optical fluxes can cause catastrophic damage to occur at levels well below those which can be sustained by

clean surfaces. The quality of surface fabrication has also been recognized as being

important in influencing the damage resistance of materials. For example, it is known

that a frosted surface damages more easily than a surface with a good optical polish, [l] Various chemical surface treatments have been tried on different materials [2-3] , some

of which have shown improvement but often the effect is only temporary.

We have found that polishing of sapphire surfaces with ion beams results in a marked increase of surface damage threshold compared with that for abrasively polished

samples

.

Experiments were carried out using a single transverse and longitudinal mode ruby

laser whose output is focused on either the entrance or exit surface of a sapphire

sample. The samples are sapphire bars 3 in long by 0.25 in square cross section. The

laser and associated monitoring apparatus are described in detail elsewhere. [4] Typical laser pulse width and peak power are 20 nsec (FWHM) and 1 MW, respectively. The area of the focused^spot has been measured using a modified multiple lens camera technique [5] to be 10" cm

2 at the 1/e 2 intensity points. The beam profile was measured

to be gaussian down to 8% of the peak. The power incident on the sample is varied by

rotating the first of a pair of air-spaced polarizers, the second being held at a fixed orientation.

A typical series of measurements is carried out as follows. The laser is fired at

a particular spot on the desired surface at a power below damage threshold. The sample

is then examined through a traveling microscope without disturbing the sample between

shots. If no damage is observed the incident power is increased (by ^10%-20%) and again

the sample is examined. The procedure continues until damage is observed; then the

sample is moved and the process repreated on an undamaged spot. Typically 10-20 damage

thresholds are measured in this way for a given surface.

Abrasively polished samples were used as obtained from the manufacturer (Union

Carbide). They were fabricated with the "laser finish" specifications typical for ]puby

laser rods. Ion polished samples were exposed after mechanical processing to an Ar beam (7 kV, 300 yA/cm2

) for periods from 2 hrs to 4 hrs. The beam strikes the surface

at an angle of 20 from the surface plane. An estimated 200 A/min is removed from the

sapphire surface under these conditions giving a total of from about 2.5 to 5 pm of

material removed in a given exposure.

*Work supported by the Advanced Research Projects Agency under ARPA Order No. 1434 with

Air Force Cambridge Research Laboratories.

^Figures in brackets indicate the literature references at the end of this paper.

55
2, Experimental Results

The results of a series of measurements on abrasively polished and ion beam

polished samples are shown in Fig. 1. In Fig. 1(a) and (b) the results for entrance

surface damage are shown. For the abrasively polished sample most of the damage

thresholds occur between 1 GW/cm 2 and 2.5 GW/cm 2 with a few higher values occurring.

For the^ion polished case (Fig. 1(b)) we see a few damage thresholds ranging from

2 GW/cm to 9 GW/cm 2 with a large fraction (>50%) above 10 GW/cm 2

. (The dashed portion

of Fig. 1(b) indicates that we were unable to reach damage threshold with the maximum

output from our laser under the focusing conditions of our experiment.) Fig. 1(c) shows

the exit damage thresholds for the abrasively polished sample. Here the bulk of the

points are clustered between 1 GW/cm 2 and 2 GW/cm2. For the,,ion polished case (Fig. 1(d)

the values are again spread over a wide range (up to 9 GW/cm ) but for this case no damage thresholds lower than 2 GW/cm2 are observed. The data in Fig. 1(d) were obtained

from samples which were ion polished for 2 hrs and 4 hrs. No noticeable change in the

distribution was observed between the 2 hr and 4 hr case and the data are presented

together in the figure.

Fig. 2 shows examples of x-ray reflection topographs for sapphire samples which

were abrasively polished (Fig. 2(a) and ion beam polished (Fig. 2(b), (c)). The depth

of the region being probed in these topographs depends on the angle of incidence and

energy of the x-rays as well as other experimental conditions and sample properties.

An estimated region 10-20y thick is being probed in the above topographs. We see in

Fig. 2(a) a number of deviations from crystal 1 i ni ty which appear to follow scratch

lines from the mechanical polishing process. In Fig. 2(b) the topograph for a different sample shows that some of the mechanical damage appears to have been "erased" after

2 hrs of ion beam polishing. Fig. 2(c) shows a topograph taken after 4 hrs of exposure

to the ion beam and, although evidence of damage still is seen, there is a marked difference between this and the previous topographs. We present these topographs to point

out that a distinct change in surface topography is brought about by ion beam polishing. A detailed correlation between the changes in the topographs and the damage

thresholds is not possible at this time.

3. Conclusions

The main conclusion reached by these preliminary results is that a distinct

increase (^2-6x) in surface damage threshold can be realized by ion beam polishing.

Although there is a wide scatter of results in measured thresholds for ion polished

samples, we see that even the low values for the ion polished samples are generally

higher than those for the abrasively polished samples. We interpret the scatter in the threshold data for the ion polished samples to indicate a variation of surface

strength from place to place over the sample. It is very likely that the influence

of mechanical damage due to abrasive polishing, although somewhat lessened, has not been

completely removed under the conditions of our experiments. This is suggested by the

x-ray topographs, although as yet no definite correlation between topograph details

and threshold variations has been established. We choose to interpret the data as reflecting a variation of threshold from spot to spot on the surface rather than in a probabilistic manner as described by Bass and Barrett. [6] It is apparent from the

topographs that the surface crystal 1 i ni ty varies considerably from place to place over

the sample and that the surface is not uniform all over. A possible interpretation of

the difference between the abrasive polish and ion polish results is that in the former

case the surface is "uniformly weak" and the damage th'resholds are found to have a relatively narrow range of values whereas in the latter case a distinct toughening of

the surface has taken place but the influence of mechanical damage from the abrasive

polishing still exists.

A final point worthy of mention is that the damage when it occurs at a high

threshold is generally much more extensive than when it occurs at a low threshold.

For example, exit surface damage in abrasively polished samples is generally characterized by a small fractured crater or pit about 10 pm in diameter when it occurs near threshold. On the other hand, the high threshold damage seen in the ion polished

samples is more severe; the damage craters are usually several times larger and more extensively fractured. This fact suggests that the effect of ion polishing is to toughen or strengthen the surface in some way rather than to change the basic damage

mechanism whatever it may be. It seems that the surface is able to sustain a larger

deposition of energy before catastrophic breakdown occurs but when the surface does let

go it goes with a bigger bang.

56
4. Acknowledgement

The author wishes to acknowledge critical suggestions by V. Evtuhov, as well as the aid of

H. L. Garvin who performed the ion beam polishing and K. T. Miller who provided the x-ray topographs.

5. References

[1] Khazov, L. p., A. N. Shestov, and G. P. Tikhomirov, Sov. Phys.-Tech. Phys. 13, 1112 (1969).

[2] Swain, J. E. , IEEE J. Quantum Electron. 4, 362 (1968).

[3] Davit, J. , in Damage in Laser Glass , edited

by A. Glass, A. Guenther , C. Stickely, and J. Myers, American Society for Testing and Materials Technical Publication No. 469 (ASTM,

Philadelphia, PA, 1969), p. 100.

[4] Giuliano,C. R. and L. D. Hess, in Damage in Laser Materials , edited by A. J. Glass and

A. H. Guenther, U. S. National Bureau of

Standards Special Publication No. 341 (U. S. GPO, Washington, D. C. , 1970) p. 76.

[5] Winer, I. M. , Appl. Opt. 5, 1437 (1966).

[6] Bass, M. and H. H. Barrett, IEEE J. Quantum

Electron. 8. 338 (1972).

6. Figures

5

- 01

ENTRANCE DAMAGE

ABRASIVE POLISH

m m J I I i_

ENTRANCE DAMAGE

ION POLISH UNABLE TO REACH I THRESHOLD AT

THIS POWER LEVEL

EXIT DAMAGE

ABRASIVE POLISH

likl

EXIT DAMAGE

ION POLISH

0 I 23456789 10 THRESHOLD POWER DENSITY, GW/cm2 Figure 1. Number of damage sites as a function of threshold power density at entrance and exit surfaces

of sapphire crystals for abrasively polished and ion beam polished samples.

Figure 2. Examples of x-ray diffraction topographs taken for (a) abrasively polished sample, (b) ion polished for 2 hours (7kV @ 300 HA/cm ), (c) same conditions as (b) , but for 4 hours. Ar

57
it Laser-Induced Damage Probability at 1.06 and 0.69 |Jm

Michael Bass and Harrison H. Barrett

Raytheon Research Division

Waltham, Massachusetts 02154

Measurements of the laser-induced surface damage process at both ruby and Nd :YAG laser wavelengths are reported in which the laser pulse and beam parameters were carefully held constant for both sources. This resulted in the first directly comparable measurements of laser damage at two different wavelengths. The most striking feature

of the data is that all the materials studied are harder to damage at 0.69 |_im than at 1.06 pm. In addition, the relationship between the damage probability and the optical

electric field strength at 0.69 pm, though similar to that at 1.06 l-im, suggests that more than one damage mechanism may be operative.

The probabilistic nature of the laser-induced damage process at 1.06 pm was ex- plored further by measuring the distribution of breakdown starting times with the image-converter streak camera. For a particular laser pulse a threshold-like damage process would result in a very sharp distribution of breakdown starting times. In- stead, a large spread is found, with the most likely starting time occuring before

the laser intensity reaches its maximum. The observed distribution is described by

the compound probability that breakdown occurs at a particular time, given that it has not occurred before that time.

In addition, several possible connections between the probabilistic and thresholdlike interpretations of laser-induced damage are discussed. It is shown that these points of view are not totally incompatible.

Key Words: Breakdown starting time, electron avalanche, probabilistic

nature of damage, surface damage, wavelength dependence of damage.

1. Introduction

The first direct comparison of damage in transparent media produced by ruby and Nd:YAG lasers was performed using experimental techniques described previously, [l]^ These studies have shown that a material is less likely to damage when subjected to a particular optical field at 0.69 pm than at 1.06 |_tm. Comparison of the 0.69 pm data with that recorded at 1.06 pm also shows that the relationship between the ( damage probability, p^, and the applied optical electric field strength, E, though still generally of the form p-^ oc exp (-K/E) , has changed to a greater degree than predicted by a very simplified avalanche

breakdown model. [1,2] In fact, for at least two materials studied ; k is so large at 0.69 pm that the damage process at this wavelength might properly be characterized by a well defined threshold field. The inadequacies of the simplified optical-frequency avalanche-breakdown model [1,2] are discussed and the possibility that more than one damage mechanism may be operative is examined in view of the experimental

data

.

A very fast image-converter streak camera was used to study the distribution of starting times for 1.06 pm laser-induced surface breakdowns. If the breakdown or damage process were completely described

by a well defined threshold, then this distribution should be very narrow and no breakdowns should occur

after the laser field reached its maximum. The data, however, shows broad distributions, particularly

when the applied field is such that < 1. In all cases the most probable time for breakdown is before

the time of maximum field, though some breakdowns do occur later. The properties of the measured distributions are described by the compound probability that breakdown occurs at a particular instant,

given that it has not occurred before that time and, as such, are added evidence for the probabilistic

interpretation of the 1.06 pm damage process.

This research was supported by the Advanced Research Projects Agency of the Department of Defense and was monitored by the Air Force Cambridge Research Laboratories under Contract No. F19628-70-C-0223.

Figures in brackets indicate the literature references at the end of this paper.

58
Several possible connections between the probabilistic and threshold-like interpretations of laser induced damage are also discussed. It is shown that these two points of view are not totally compatible.

The possibility that fluctuations in the laser pulse which were not detected experimentally caused the statistical nature of the damage data is considered. We show experimentally, however, that such fluc- tuations were not present.

2. Comparison of Ruby and Nd :YAG Laser Induced Damage

2.1 Experiments

An examination of the available literature reveals that due to variations in experimental conditions

there are no directly comparable published results for 0.69 and 1.06 (Jm laser induced damage. In the ex- periments described below the same sample was studied at both wavelengths. Self-focusing was eliminated

by studying surface damage only and the critical laser beam parameters were carefully held constant as shown in table 1. Under these conditions and using a slightly modified version of the experimental techniques described previously [1] directly comparable laser damage data was obtained.

Table 1. Laser parameters

Wavelength Nd :YAG

1 . 06^lm Ruby

0.694 |Jm Max. Energy TEM

0Q

Mode 1.8 mj 5.0 mj

Longitudinal Mode 1-5 1-5 Beam Diameter at Output

TEMqq Mode

Mirror

1 mm 0.75 mm Polarization Linear Linear

Pulse Repetition Rate 1 pps 1 pulse /5 sec Pulse Duration in TEMqq Mode 12 nsec (FWHP) 12 nsec (FWHP)

Pulse to Pulse Energy

Reproducibility ± TU ± 7% Focal Spot Size 0.003 cm 0.003 cm

The general experimental arrangement of reference 1 was modified as shown in figure 1. The laser focusing optics was unchanged but an automated pulse monitoring and damage detection system was installed.

One pen of the dual-pen chart recorder was driven with a signal proportional to the laser pulse energy

and the other pen monitored the intensity of a beam of light transmitted through the region which was exposed to the laser pulses. This arrangement was sensitive to the appearance of surface damage craters ~0.003 cm in diameter and ~2000 A deep, the smallest damage observed by earlier monitoring techniques [^] . A typical recording, derived from this arrangement, is shown in figure 2 where a ruby laser at « 9GW/cm

was used to irradiate an x-cut crystalline quartz sample. Note the ± 10 percent laser pulse energy variation. The first pulse to strike the sample is marked with an arrow and the 99th pulse, marked with a star, is the one which produced damage. This is indicated by the sharp reduction in the transmitted

monitor intensity. The laser trigger signal was picked up by the photomultiplier and so resulted in a fiducial marker on the record of the transmitted light intensity.

The data in figure 2 shows that at this level of irradiation the sample does not have to damage when

exposed to any particular pulse. In fact, preceding the damaging pulse (no. 99), there were several more energetic pulses which, according to the "threshold point of view," should have been the ones to cause

damage.

The data in figure 3, while similar to that in figure 2, show the improved pulse-to-pulse energy reproducibility of the ruby laser achieved by more careful tuning of the laser cavity components. Alignment of the flat, parallel rod faces with the cavity mirrors was found to be critical. Proper rod align- ment was always accompanied by a dramatic smoothing of the pulse waveform as recorded by a high-speed

photodiode and a 519 oscilloscope. Figure 4 shows this effect for the TEMqq ruby laser. Similar results

were obtained by aligning the Nd:YAG rod in its laser cavity.

The detection of a smooth pulse by a limited bandwidth detector may be the result of either singlefrequency output or the simultaneous oscillation of many randomly phased modes. In order to resolve this

question the lasers' output spectra have been studied with Fabry-Perot interferometers. The lasing

59
bandwidth narrowed significantly as the rod was aligned to achieve smoother pulses. Unfortunately, our interferometers lacked sufficient finesse to resolve individual laser cavity modes (At* « 300 MHz). The longitudinal mode content of a perfectly smooth laser pulse can be inferred, however, by combining the interferometer and pulse waveform data. The observed interferometer limited bandwidth of a smooth ruby

laser pulse was «s 1.5 GHz, implying that a maximum of ~ 5 modes could have been oscillating. Since the photodiode-oscilloscope combination could detect frequencies as high as ~ 3 GHz, the presence of five oscillating modes would have been detected through the presence of mode beating as in figure 4b. Therefore, our perfectly smooth ruby laser pulses, as shown in figure 4c, were caused by single longitudinal

mode lasing. In support of this conclusion we note that Giuliano et al. [4] reported that perfectly

smooth pulses correspond to single mode oscillation while non-smooth waveforms were always associated

with two or more oscillating modes.

Single longitudinal mode oscillation is generally obtained by placing an intracavity resonator in the laser with sufficient finesse to limit the number of modes [4]. In the present experiments intracavity resonators were created by aligning the plane parallel faces of the laser rods with the plane

parallel cavity mirrors as shown in figure 5. The finesse of each of the indicated resonators is very

low because the rod faces were antiref lection coated for less than 0.25 percent reflectivity at the laser wavelength. However, when lasing occurs, resonators M^-A, B-A, and A-M£ contain a medium with gain. Thus the effective reflectivity of the rod face in these resonators is increased and so is the effective

finesse. Resonator M;l-A which includes the 100 percent reflecting laser cavity mirror has the highest

finesse and is felt to be responsible for limiting the number of oscillating modes. For example, in the ruby laser, we used a 10 cm long rod with a gain coefficient of as 0.25 cm"''' [5] resulting in an effective

finesse of ~ 6

.

The possibility that subnanosecond fluctuations in the laser field during a single pulse which were

not detected by the photodiode-oscilloscope combination caused the statistical nature of the damage data had been suggested [6], However, the study of the frequency content of the smooth laser pulses showed

that the experimental data given here and in references 1 and 2 was obtained with a single longitudinal

mode laser pulse or, at worst, with one containing only a few oscillating modes. Therefore, such fluc- tuations could not have been present and could not have been the source of the form of the damage data.

In addition, the data for SrTi03 in reference 1 covers a range in power of 36 to 1 and fluctuations of

this magnitude in a laser pulse would certainly have been noticed.

Using the smooth-pulse, reproducible lasers the damage data for 0.69 and 1.06 (am summarized in table 2 was obtained. As shown therein the lowest power density required to always produce damage in a single pulse, , is generally higher at 0.69 |_lm than at 1.06 Hm. In addition, the quantity K in the relation

Pl « exp (-K/E) (1)

is larger for ruby irradiation.

Table 2. Comparison of damage properties at 1.0645 [dm and 0.6943 Urn.

K(10

8V/M) P (GW/cm

2

)

Laser Wavelength (yim) 1.0645 0.6943 1.0645 0.6943

X-Cut Crystal Quartz 33.0 60.5 5.4 20.2 KDP 28.5 92.8 16.6 27.6 LiNb0

3 16.6 > 200 11.1 11.1

LilO., 7.1 16.3 2.8 3.9

* SrTiO, 8.7 > 500 0.8 15.3

This SrTiO^ was sample No. 2 and was cut from a different boule than the one used

in references 1 and 2.

2.2 Discussion

In references 1 and 2 a model for the damage mechanism based on electron avalanche breakdown was proposed for two main reasons: 1) It gave the experimentally observed relation between damage probability and optical electric field; and 2) It did not require material properties inconsistent with transparent, insulating dielectrics. This model was fashioned on the assumptions that the statistics of the

60
avalanche breakdown process were largely determined by the first one or two ionizations and that the fraction of collisions which the accelerating electron could make and remain acceptable in phase with

the optical field, f, was independent of electron energy „ It was also assumed that the collision rate,

T coll was independent of electron energy. In this very simplified model, in contrast to the data, K has

a weak dependence on the optical frequency and large changes in P^ are not predicted. However, because

of the successes of the avalanche breakdown mechanism mentioned above a re-examination of its dependence

on optical frequency is in order.

If, as in our simplified model, the steps in the avalanche are all highly likely after the first one

or two, then the statistics of these early steps will dominate and the process will appear probabilistic.

If, on the other hand, the probability for each step in the avalanche is the same, damaging avalanches

will not be possible until a field is applied which is large enough to make the probability for each step

near unity. In this case the process will appear threshold-like.

Consider the sequence of events whereby an electron in an optical field is accelerated to ionizing

energies. Since the field direction reverses periodically the electron must undergo collisions at the proper times to be able to efficiently extract energy from the field. At low energies inelastic optical

phonon collisions dominate the electron-lattice interaction [7] and are, in effect, a frictional force on

the electron's motion. This means that there is some "friction barrier'' to the electron's energy which is determined by the energy range over which optical phonon interactions dominate. At higher energies elastic collisions with acoustic phonons dominate and it is much easier for the electron to accelerate further

to ionizing energies. Therefore electrons with initial energies greater than this "friction barrier" can dominate the avalanche process.

Now assuming an electron reaches the ionization energy, , it cannot undergo an ionizing interaction until it gains some additional energy since the ionization corss section is zero at e^. The probability for ionization, however, increases rapidly with electron energy [8]. Ionization is most likely

to occur, then, when there is a substantial excess energy over , with this extra energy divided between

the two ionization products. If the energy of these products exceeds the "friction barrier" then they

can be more easily accelerated to produce the next step in the avalanche and so on for each succeeding

step until damage occurs. The ionization products dominate the avalanche. On the other hand, if the excess energy is small, the ionization products may have energies less than the "friction barrier" and

be virtually indistinguishable from the rest of the free electrons. Each step in the avalanche then must follow the same course as the first, requiring large fields to produce damage.

In figures 6 and 7 the 1.06 and 0.69 \dm electric field required to accelerate an electron to a par- ticular energy is plotted. The curves are numbered according to the number of half cycles of the field

required to accelerate the electron to that energy assuming an optimally lucky collision occurs each time

the field reverses. This number is also the number of the collision which must occur if the electron is to continue to accelerate. For example, 12 collisions are required to reach the curve labeled 13. Per- fectly lucky collisions are those which exactly reverse the electron momentum. The results show that in

a Nd:YAG laser field an electron will gain more energy through fewer collisions than if the same field were due to a ruby laser. Both the collision rate, t ^;q> atlc' f decrease with increasing electron energy.

In fact, above 2-4 eV the cross section for acoustic pfionon scattering is strongly peaked in the forward

direction which means that "lucky" collisions at high energies are very unlikely [7]. Thus, since fewer collisions are required, the energy of an electron in a 1.06 \im field which causes ionization will, in general be further in excess of than when ionization is caused by a 0.69 \im field. In the 1.06 |_tm field then the ionization products have less of a "friction barrier" to clear than in the latter and an avalanche can proceed with high probability. In the 0.69 (im case the ionization products are less distinguishable from other free electrons and an avalanche may not develop. Larger fields should then be necessary to produce ionization at 0.69 Urn and the mechanism of producing a damaging avalanche should

exhibit higher K value or be more threshold-like than at 1.06 Mm. This very speculative interpretation

is presented only because it agrees with the most important experimental facts and in order to stimulate

interest in further theoretical study of the avalanche breakdown process in optical frequency fields.

Figures 6 and 7 also show that between 3 and 5 more "lucky" collisions are required to reach a par- ticular energy in a ruby laser field than in a Nd :YAG field. Thus if f is a constant, say 0.1, [1,2] the probability for damage at 0.69 Mm is between 10"^ and 10" times less than at 1.06 fim. However, as stated

above, f decreases with increasing electron energy and so the probability for ionization in a given field

at 0.69 p.m is even smaller. We have therefore an additional reason for the fact that damage with the ruby laser always requires higher fields.

It appears reasonable from the preceding discussion that the electron avalanche breakdown process,

which gives a qualitatively good picture of 1.06 |Jm laser induced damage, will be observed only at much

higher optical fields at 0.69 |Jm. However, as the ruby laser field is increased to find avalanche breakdown damage, it is possible that some materials may display damage due to another mechansim. In LiNbOo

and SrTi03

, both of which absorb strongly at 0.35 (Jm and have well defined experimental damage thresholds

for 0.69 !_lm irradiation, this mechanism may be two-photon absorption.

61
Both the 1.06 and 0.69 |Jm data In table 2 for SrT103 were taken for sample No. 2 so that direct com parlson is possible. In references 1 and 2, the value of K for SrTi0

3 at 1.06 |Jm, obtained using a sam pie cut from a different boule, was about one fourth that in table 1. This is the first instance yet

encountered where so large a sample-to-sample variation in K was observed. On checking with the manufac

turer [9], however, we found that sample No. 2 was doped with tungsten while sample No. 1 was not. Tung

sten had been added to make the material visually clearer and satisfy our order for "optical quality

SrTi03." The two samples therefore were not the same material and so differences can be expected. More

experimental data will be obtained on other samples as soon as possible.

3. Distribution of Breakdown Starting Times

Since the observation of a visible spark upon laser irradiation is always followed by the detectlo

of damage in the Irradiated area, a study of the temporal development of such sparks was initiated over

one year ago. The initial observations of this study were concerned with self-focusing and its relation

to the internal damage process [1], More recent work has been concerned with measuring the distribution

of surface breakdown starting times and the interpretation of such data.

Using the high speed image converter streak camera technique described previously [1] and the TEM^

mode Nd:YAG laser, the distributions shown in figures 8 and 9 were obtained. Twenty- five different mea urements of starting times were made to obtain each distribution. Plate glass and SrTiO^ were studied

because they have very different K values and because at convenient optical field strengths p]_ < 1 coulc be obtained.

The most obvious feature of these distributions is that, except for the highest field applied to plate glass, a high K material, they are not very sharp. Assuming constant pulse shape and material

properties, as implied by the excellent fit of the measured distribution of N (the number of pulses re- quired to damage) to the binomial distribution assuming a constant p-^ , [1] a process described by a wel defined threshold would yield distributions of breakdown starting times which ranged over an interval o time much less than the laser pulse duration. In addition, such a process would not permit any break- downs to occur after the time of maximum field strength: If breakdown did not occur in the most intens

field it would certainly not occur when the field was weaker. Both of these requirements for the dis- tribution of breakdown starting times of a threshold like process are violated by the data and so an interpretation based on the probabilistic point of view is sought.

Consider an ensemble of samples, each subjected to the same laser pulse, E(t) , such that E(t) = 0 for t < 0. Let V(t) be the fractional number which have not damaged at time t and let g(t) be the fractional rate with which the surviving samples breakdown. Note then that, within a scale factor, g(t

is the distribution of breakdown starting times. The applied laser field results in a probability per unit time that breakdown occurs, h(t), and so

g(t) dV(t)

dt U(t) h(t) (2);

Solving the second part of this equation gives

V(t) = exp (

t

J

h(t' ) dt') (3)

which satisfies the obvious initial condition that V (0) = 1.

Thus we have for the form of the distribution of breakdown starting times

g(t) = h(t) exp (

t

f h(t')dt'). (4-

Initially g(t) grows as h(t) but the influence of the exponential term rapidly becomes dominant and eventually g(t) decreases towards zero. The time interval over which g(t) is not negligibly small (th< time width of the distribution) depends on the precise form of h(t) for the particular material and la pulse. This interval, however, need not be very small compared to the laser pulse duration and can in elude times after the peak field is reached.

The most probable time for breakdown to begin is the time, t , when g(t) is maximum. Thus we consider

62
d(g(t)) | dt t.

d(h(t )) \

dt " O-'V < r )dt') = 0. (5)

Since exp ( - 1 h(t')dt') f 0, eq. (5) holds when

d(h(t )) air- = <h V> (6)

The solution for t., is then found during the interval when

M

M£i > o . (7)

Since the applied field, E(t) , determines the time dependence of h(t), we have

dh dh dE

dt

= dE dt (8)

It is physically reasonable to require that the damage rate increase as E increases and decrease as E decreases so that

^| > 0 for all t . (9) dE

Thus g(t) reaches its maximum at a time when

§ > 0 (10)

or before the peak field strength is reached. This result agrees with the experimental data in figures 8 and 9 wljere the most probable time for breakdown to occur is always found before the time at which the optical field strength is maximum,,

Further evidence for a probabilistic interpretation of the damage process is shown in figure 10. In this case a TEMqq mode ruby laser pulse was focused inside a NaCl crystal and the transmitted laser intensity monitored with a fast photodiode-519 oscilloscope combination. As will be discussed in a later

paper [10] the laser power was such that no self-focusing was likely. The photos in figure 10 are rep- resentative samples of the many that were taken and show that damage does not have to occur for every

pulse. In addition, they show that when ruby laser induced internal damage does occur, it can begin

before, at, or after the peak of the laser pulse.

The use of the breakdown or damage starting time to study the probabilistic nature of laser damage

is especially important for materials with large K's. In such cases one may not have sufficiently fine

control over the laser pulse parameters to confidently resolve the region of intermediate damage probabilities. Thus data such as that in figure 10 can be used to assist in identifying such important

characteristics of the damage mechanism as its statistics.

4. Comments on the Probabilistic and Threshold Points of View

The last paragraph of the preceding section indicates just one of the problems involved in distinguishing between probabilistic and threshold -like phenomena. For all practical purposes, materials

having large K's can be considered to have a damage process which is governed by a well-defined threshold. It is possible however, as shown above, to perform studies of the damage dynamics which reveal

some probabilistic properties.

The experiments described in this paper and in references 1 and 2 employed the techniques of irradiating a particular region of the sample until either damage occurred or 500 pulses spaced by a 1 or 5 second interval had been tried. A new region was then studied in the same manner at the same level of

irradiation. Having done this at least 25 different times an estimate of the damage probability was ob- tained by dividing the number of damages by the total number of laser pulses. We then always examined

the set of values obtained for the number of pulses required to damage at each region, N, to see if this

set is distributed according to

63
If good agreement is found, then it is fair to say that the probability for damage in each region

and for each pulse was always p-^ even though damage did not always occur. Data of this sort, obtained

from sets of ~ 100 values of N for two different materials, fused quartz and SrTi03 , are shown in figures 11 and 12 respectively.

If the fluctuations in N values which led to the probabilistic interpretation of damage represented

local variations in the threshold for damage due to dirt, stresses or other material imperfections then

the distribution of N values would not be given by eq (11). Instead, fN would be determined by the den- sity of the regions which had damage thresholds less than or equal to the maximum optical field strength.

Suppose, for example, that the maximum optical field strength is such that 20 percent of the material

could be damaged. Our experimental procedure would then generate approximately 5 places where N = 1 and ~ 20 where N^= 500 pulses with no damage, were recorded. We would then obtain a damage probability of

p^ 5 X 10" . Since the probability that 25 samples of numbers distributed according to eq (11) with

Pl = 5 X 10"^ contain five l's and twenty 500 's is very small, the set of measured N values and the resulting p-^ would not be accepted. This type of disagreement for the N value distribution would indicat

material variations or laser fluctuations. We feel that these are minimal in view of figures 11 and 12. However, as in the case of the LiNb03 sample discussed in reference 1, instances where such disagreements

occurred would be reported separately from more reliable measurements of p-^.

Following a suggestion of E. Bliss we now consider the effect of varying the irradiated area on the rate of change of damage probability with applied optical power density. If the optical field is kept constant and the irradiated area is increased, then the number of electrons in the sample subjected to the optical field is increased. Thus if p = p(E) is the probability for breakdown when one initial elec- tron is present , then

p'(E) = Np(E) (12)

| is the probability for breakdown when the area is increased and N initial electrons are now sampled. Here we assume that N t N(E). If Np(E) > 1 we must obviously set p'(E) = 1. From our damage data [1,2]'

dflnp(E) _ d(l/E)

" K (13)

and so, from eq (12),

d(W(E)) _ „ d(l/E) "

" K

Thus the two cases will appear in a log of probability vs 1/E graph as sketched in figure 13. From this we find that

A (|) =A (J.)

where E and E' are defined in figure 13. Now, in arbitrary units

P = E

2

where P is the power density. By differentiation we find

A ( A ) = - — -1

E P 2E

(14)

(15)

(16)

(17)

and

A (fr) = -AP'

P'

12E'

(18)

Therefore, the required fractional changes in power density necessary to achieve a change in damage

probability from say p^ to p^ are related by

AP ' . E'. AP.^.

i L-*2. P.

i (19)

But E ' . /E.< 1 and so

l l AP'

.

£21 APin (20)

64
or

(21)

In words, by increasing the irradiated area while maintaining a constant field one is performing

many experiments at once and so to the extent that different regions are independent, the probability for

at least one breakdown must increase with area. As a result, for example, if the area is large enough so that the damage probability changes from say 10""^ to 1 over a small range in power density then it is possible therefore to perform experiments where the probabilistic nature of the damage process could be overlooked. (See figure 14).

The preceding consideration of the case of increasing the area irradiated while maintaining the electric field strength shows that the probability of at least one breakdown occurring also increases.

However, the probability for damage per unit area is independent of the irradiated area and would be a useful quantity for describing a material's damage properties.

If the area of the beam is increased while the total pulse power is kept constant, the electric

field, E, decreases and the damage probability decreases.

We see from this treatment that it is essential to maintain constant irradiated area or volume when

comparing laser damage properties with different lasers or when comparing different materials. It is also

critical to take this into account when comparing the work of different researchers.

In the probabilistic view a report of a damage threshold could correspond to 1) a very high value

of K in eq (1) , 2) a study of a much larger volume of material than that which is small enough to reveal

the probabilistic properties of damage, 3) an experiment in which the intermediate range of power den- sities (power densities where 0 < p-^ < 1) was missed due to experimental procedure or 4) an experiment

in self-focusing always produced an intensity so high that the intermediate range was never studied. It is also possible that there is more than one damage mechanism and that some mechanisms, such as avalanche

breakdown, are probabilistic while others are threshold-like.

5 . Summary

The first direct comparison of damage in transparent media produced by ruby and Nd :YAG lasers re- vealed that: 1) A material is less likely to damage when subjected to a particular optical field at 0.69 |J.m than at 1.06 |Jm; and 2) The relationship between damage probability, p^ , and applied optical electric field strength, E, though still generally of the form pi a exp (-K/E) , has a value of K at 0.69 |Jm

which is substantially greater than at 1.06 |Jm. A tentative interpretation of these changes based on a modified optical frequency avalanche breakdown model was proposed. In addition the possibility that in some materials more than one damage mechanism may be operative was discussed.

The distribution of damage starting times was studied experimentally for two different materials.

In contrast to the very small spread expected for a threshold process, broad distributions were found. Each distribution showed, in addition, that the most likely starting time for damage is before the time of

maximum field. The properties of the measured distributions were described by the compound probability

that breakdown occurs at a particular time, given that it has not occurred before that time and, as such,

provided added evidence for the probabilistic interpretation of the 1.06 Urn damage process.

Several possible connections between the probabilistic and threshold-like interpretations of laser- induced damage were considered and the two points of view were shown not to be totally incompatible. Un- noticed, subnanosecond fluctuations in the laser pulse as the source of the statistics in the experiments

were ruled out by evidence showing that only one or at most a very few longitudinal modes could oscillate

in our lasers. This meant that shortest possible duration of any fluctuation was still long enough to have been detected by our fast photodiode-519 oscilloscope monitor system.

6. Acknowledgements

The authors wish to acknowledge the invaluable experimental assistance of D. Bua. The work of

D. Bua in automating the data recording and along with D. Fradin in establishing and interpreting the procedure which results in smooth laser pulses is especially gratefully acknowledged. Valuable discussions were held with E. Bliss, D. Milam, D» Fradin and F. Horrigan.

65
7. Ref

[1] Bass, M. and Barrett, H. H., NBS Special

Publication 356 (1971) p. 76 and IEEE J. Quant. Elect. QE-8 , 338 (1972).

[2] Bass, M. , Barrett, H.H. and Holway,Jr., L.H.

,

Scientific Report No. 1 for Contract No. F19628-70-C-0223 (February 1972).

[3] Bass, M., IEEE J. Quant. Elect. QE-7 , 350 (1971).

[4] Guiliano, C. R. , Hellwarth, R„ W. , Hess, L.D. and Rickel, G. R. , Semiannual Report No. 2 for Contract No. F19628-69-C-0277 (July 1970).

[5] Evtuhov, V. and Neeland , J. K. , "Pulsed Ruby

Lasers," Lasers, Vol. 1, ed. A. K. Levine

(M. Dekker, Inc., New York, N.Y.) (1966).

[6] Yablonovitch , E., Ph.D. Thesis, Harvard University (June 1972) .

[7] Seitz, F., Phys. Rev. 76, 1376 (1949).

i

I [8] Kane, E. 0., Phys. Rev. 159 624 (1967).

[9] Merker, L. and Cobb, J., N. L. Industries,

private communication.

[10] Fradin, D. W. , Yablonovitch, E. and Bas9, M. "Comparison of Laser Induced Bulk Damage in Alkali-Halides ," 4th ASTM-NBS Symp. on Damage

in Laser Materials (1972).

8. Figures

Figure 1. Schematic diagram of automatic pulse

and damage monitoring apparatus.

Figure 2. Record of exposures of X-cut crystalli

quartz to pulses of ruby laser light ~ 9 GW/cm^.

66
Damage produced by —

pulse no 20 is indicated by the sudden drop in sample monitor signal

Maximum pulse-to-pulse ~T energy variation was ±2%

of mean More typically

it was ±5%

b)

c)

10ns

Figure 3„ Ruby laser energy recording showing

best pulse-to-pulse stability.

Figure 4„ Effects of rod alignment on output

pulses for Q-switched ruby, a) rod badly misaligned, b) rod coarsely aligned, c) cavity finely

tuned .

Introcovity resonators

M, -A — M,-B B - A

• A-M,

B-M,

M,

100 % R Laser rod M 2 output mirror ~20%R or single saphire etalon Figure 5. Schematic diagram of a laser cavity

showing potential mode selecting intracavity resonators

.

Power density (GW/cm2) Figure 6„ Electric Field at 1=06 Ltm required to accelerate an electron to a particular energy. Each curve is numbered according to the number of

half cycles of the field required to accelerate

the electron assuming only perfectly lucky col- lisions occur. The number then is also the number

of the collision which must occur for the electron

to continue to gain energy from the field.

67
Figure 7. Electric field at 0.69 [±m required to Figure 8. Distribution of surface breakdown

accelerate an electron to a particular energy. starting times for plate glass for three different

Each curve is numbered according to the number of damage probabilities,

half cycles of the field required to accelerate

the electron assuming only perfectly lucky col- lisions occur. The number then is also the number

of the collision which must occur for the electron

to continue to gain energy from the field.

a) Damaged when the peak

laser field was reached

Typical pulse waveform .NdiYAG laser

( smoothed slightly )

i 1.00 H 0 75

£ 0 50 |

0 25

£ I 00

e 0 75

Z 050

£ 0 25

Moterol : Sr Ti 0 3 K = l8*IO B V/m

E 0

= 4I.8«I07 V/m

p = l.0

E„=24.2x IO? V/m

p =0.85 1,1.1 5 10 15 20

Breakdown starting time , f (nsec )

Figure 9. Distribution of surface breakdown

starting times for SrTiO^ for two different damage

probabilities

.

b)

c)

d)

Damaged before the peak

laser field was reached EDamage /E Peak =0.896

Damaged after the peak

laser field was reached ^Damage /E Peak =0.954

3 successive pulses no damage

E Peak = 1 (Arbitrary units)

10 nsec Figure 10. The occurrence of internal damage in NaCl due to ruby laser irradiation. The laser in- tensity transmitted through the sample is shown in these photos.

68
0.20r

Bars = Measured distribution —•— = Calculated using measured

P, in P

N

=P, (

I -P, >

f><_

' for fused quartz at 179 GW/cm2 at 1.06/x.

f N from ~ 100 measurements of N at 0.49 GW/cm 2

Material : Sr Ti 0

3

Sample NO. 2

Hn n n

10 15 20 25 30

N

Figure 12. Probability that damage occurs on the Nth pulse versus N for SrTi0

3

» A 0.5 GW/cm2 TEM^

mode 1.06 |Jm beam was used.

Figure 13. Sketch of a plot of the log of damage

probability versus the inverse optical electric

field for two different irradiated areas. p'(E)

is the result for an irradiated area larger than

used to obtain p(E)„

Figure 11. Probability that damage occurs on the Figure 14. Sketch of a plot of the log of damage

Nth pulse versus N for fused quartz. A 17.9 GW/cm probability versus power density for the same two TEMnn mode 1.064 |Jm beam was used. cases as in figure 13.

69
Damage to GaAs Surfaces from Ruby- and Nd-Glass Laser Illumination

J. Lynn Smith

Physical Sciences Directorate

U. S. Army Missile Command

Redstone Arsenal, Alabama 35809

Surface damage of GaAs due to 0.694- and 1.06-micron beams from ruby and Nd- glass lasers has been investigated. The wavelengths employed represent photon

energies above and below the bandgap of GaAs. For laser operation in the conventional mode, Ga-rich material developed on the surface to a greater extent than

for Q-switched operation. Surface damage from Q-switched beams did not appear to depend on the absence, presence or type of doping, although the nature of the surface preparation was important. The damage threshold for Nd-glass laser illumination was approximately 10^ W/cm^, only a little higher than that for ruby laser illumination. For Q-switched, Nd-glass laser beam exposure, the damage occurred

in highly localized regions and appears to be connected with material flaws near

the surface.

Key Words: GaAs, doping, laser induced damage, photoconductivity, surface

surface finish.

1. Introduction

Damage induced in GaAs by both ruby and Nd-glass laser light has been investigated by Grasyuk and Zubarev [1] • However, they did not look for a possible correlation between the type of damage or damage

threshold and the type of dopant in the GaAs or the nature of the GaAs surface. Furthermore, their criterion for damage was cracking of the sample which they attributed to thermal shock, but experience shows

that other types of damage occur before cracking which have important implications concerning the damage

mechanism.

This paper summarizes a study [2]^ in which GaAs was exposed to intense ruby and Nd laser light,

both in the conventional and Q-switched modes. For the conventional mode work, a photoconductivity sig- nal was obtained from n-type samples for correlations with damage, and an electron microprobe scan of regions which acquired high reflectivity was made. For the Q-switched mode operation, n-type, p-type, and undoped samples were exposed for surface damage studies. The effect of surface type on the damage

threshold and the appearance of damage were studied using mechanically polished, chemically polished, and cleaved surfaces.

2. Experimental

2.1. Conventional Mode Experiments

The photoconductivity (PC) response to laser exposure in GaAs decreases dramatically if microcracks

occur. Thus, the PC response was recorded to confirm that surface damage is not necessarily accompanied

by bulk fracture. Raw data for the PC response to conventional mode ruby laser radiation (0.694 (i.m) were recorded on dual trace oscillograms. One trace displayed laser power, and the other displayed the corresponding PC signal. Before and during laser exposure, current through the GaAs sample was kept

practically constant. Thus, the voltage change was proportional to the change of inverse effective con- ductivity. The conventional mode pulse contains laser spikes of various heights. Therefore, only a few selected oscillograms yielded sufficient data for a plot of PC response vs laser power for a given range

of total energy deposited. Curves were then plotted for different ranges of exposure. Visual examinations of the samples were performed throughout the experiment for correlation of PC response and sample

Figures in brackets indicate the literature references at the end of this paper.

i Although some results discussed have not been submitted for journal publication, most will appear in greater detail in reference [2]

.

70
damage. Later, each sample was placed under a microscope and photographed. Analysis for Ga and As non- stoichiometry with an electron microprobe was performed on one degraded sample.

The Si-doped, n-type samples used for PC response measurements were about 0.23 cm wide, 0^009 cm

thick, and 1 cm long. Electrical contacts were attached 0.45 cm apart by melting on In at 400 C for 5 min in a vacuum. The laser beam was incident upon the middle of the sample and focused to a 0.18-cm

diam half-power circle.

No PC signal could be detected when Zn-doped, p-type GaAs was exposed to ruby laser light. For Nd- glass laser radiation (1.06 |im), a two-photon process is probably necessary to cause direct band-to-band

excitation; therefore, the PC signal was too weak even with n-type GaAs for detection in this experiment

2.2. Q-Switched Mode Experiments

Damage thresholds for surface damage of n-type, p-type, and undoped GaAs samples were measured by

both Q-switched ruby and Nd-glass laser pulses of 20- and 60-nsec duration, respectively. Bulk material

characteristics are shown in table 1. Little, if any, external beam focusing was necessary with these

Table 1. Bulk properties of GaAs used

Carrier density Mobility

Type and dopant (cm" 3

) (cm /V-sec) Preparation technique

n, Si 1 X 10

18 3000 Czochralski

P. Zn

19 2.5 x 101 * 74.3 Czochralski

n, Te 2 X 10

18 > 2800 Czochralski

n, undoped < 5 X 10

16 > 4500 Czochralski

a

n, undoped 7 x 10

12 2910 Gradient freeze

Etch pit count on 100 surface of all materials approximately 1C1 /cm -

a0btained from Monsanto Chemical Company . Other materials obtained from Bell and Howell Electronic

Materials Division.

high power pulses (~ 20 MW) . The beam area was several times that of the samples, and often two or more

samples of different type were mounted together and simultaneously exposed to ascertain the relative

merit of each type material . Threshold values obtained were averaged over several measurements for each

material. Photomicrographs of surface damage were made for comparison. Q-switched operation was attained with passive dye cells for both ruby and Nd-glass lasers. Photographs of the sample during

exposure to laser light were made to confirm the existence of a luminescent plasma or spark.

3. Results

3.1. Damage from Conventional Mode Operation

The PC-laser power data showed that cumulative surface damage reduces the PC response gradually.

Most samples were not exposed sufficiently to produce the cracking which causes large, abrupt decreases

of PC. The conventional mode pulse of ~ 1/2-msec duration was adequate for dissociation of GaAs, regardless of the dopant. For both 0.694- and 1.06-|i,m laser flux, the damage appearance was similar on all samples and exhibited high! y reflecting regions (figure la) which were investigated with the electron

microprobe. The highly retlecting regions were found to be rich in Ga and depleted in As. Cracking

occurred along cleavage planes for samples repeatedly subjected to high energy pulses (^ 5 J/cm for the ruby laser).

3.2. Damage from Q- Switched Laser Light

The average power density at which surface damage is just observed (slight diffuseness in reflection, no cracking generally) for the Q-switched ruby or Nd-glass laser pulse is shown for various sam- ples in table 2. 71
Table 2. Damage threshold for GaAs samples

Threshold

Type and dopant

Surface orientation Surface finish3 0.694 urn (MW/cm2

)

1 .06 |j.m (MW/cm

n, Si 100 Chemical polish\

p, Zn 100 Chemical polish

8 ± 2 13 ± 5

n, Te 100 Chemical polish

n, undoped 100 Chemical polish/

n, undoped 110 Cleaved 13 ± 5

n, undoped 100 Mechanical polish 10 ± 5

aAll except cleaved surfaces prepared by supplier. Chemical polish utilizes Bromine methanol etchant

Of importance is the fact that the damage threshold for 1.06 urn laser flux is not much greater than

that for 0.694 nm laser flux. Yet the former has photon energy below the ~ 1.45 eV bandgap and the latter has photon energy above the bandgap

.

It is very significant that damage to the best GaAs surfaces for 1.06-p,m radiation occurs in highly

localized regions (figure lc). These regions are often only 2 or 3 p,m in diameter and appear to have even smaller origins. Inhomogeneity of the laser beam cannot account for this degree of localization

since, for geometry used, diffraction and interference patterns at the sample from inhomogeneous near- field laser emission could have spacial periods no smaller than about 50 |j,m.

It is also significant that the damage is independent of whether the GaAs is n-doped, p-doped, or the purest undoped material available. Noteworthy, too, is the fact that the damage is strictly an input

surface effect. No internal damage was seen upon inspection with an optical microscope and an infrared converter

.

Major variations in the appearance of surface damage shown in figure 1 are as follows: (1) Unlike

the case for conventional mode pulsing (figure la), Q-switched pulsing does not produce any great amount

of the highly reflecting, Ga-rich phase during 20-60 nsec exposure time; however mechanical deformation

appears to extend deeper beneath the surface. (2) Surface damage features are smoother (suggesting

overall melting) for 0.694-^m radiation than for 1.06-fj,m radiation (figures lb and lc). (3) Although the damage threshold is not much lower, damage from 1.06-|j,m radiation is more extensive on mechanically

polished surfaces (figure Id) than on cleaved or chemically polished surfaces. Whereas damage runs along

mechanical scratch lines on the mechanically polished surface, it occurs as highly localized craters on both cleaved and chemically polished surfaces.

4. Discussion

No damage threshold is calculated for the conventional mode (as is for the Q-switched mode) since

the laser radiation is spiky and damage is cumulative over the ~ 1/2-msec duration. The gradual decrease

of PC signal with repeated laser beam exposure of that n-type GaAs which did not acquire microcracks is interpreted as shielding by the Ga-rich phase found to develop on the surface.

4.1. Theories for Catastrophic Damage

The 0.9-)-im internal emission from GaAs laser diodes lies between the 0.694- and 1.06-M.m wavelengths

employed in this study. During lasing, population inversion in the active region of the GaAs diode

allows very little absorption of internal emission. Similarly, absorption at 1.06 nm is slight, although

the reason is that the 1.06-p,m photon energy is slightly below the ~ 1.45-eV bandgap of GaAs. The obvious similarities between the physical situation presently studied and that of GaAs laser junctions

are of practical importance. Therefore, the two main theories for catastrophic surface damage of GaAs

laser diode junctions are of relevance here. These theories are briefly presented and discussed in what follows

.

Dobson and Keeble [3] have proposed that optical energy is absorbed by inhomogeneities near the surface which causes heating. According to their view, resulting hot spots cause local decreases of bandgap which further increase absorption. Thus, local heating increases until Ga and As dissociate,

72
leaving a Ga-rlch phase on the surface. They suggest the Initial inhomogenieties may be local doping

variations. On the other hand, Kressel and Mierop [4] propose that the optical flux is absorbed through

stimulated Brillouin scattering (SBS) which results in the formation of destructive acoustical waves.

There are several reasons why catastrophic surface damage in GaAs is unlikely to result from stimulated Brillouin scattering: (1) Laser damage thresholds for quartz, sapphire, and glass which show

evidence of SBS [5,6] are two or more orders of magnitude higher than that for GaAs. (2) Grasyuk and

Zubarev [1] designed an experiment to observe the existence of laser induced SBS in GaAs and could find

no such effect, although the material degraded. (3) Damage observed by Giuliano [6] in glass was mostly

on the output face, whereas damage in the present study always occurred on the input face. (4) For 1.06-

|j,m laser radiation, the present study shows damage craters on GaAs which are much smaller than regions

where the optical flux is coherent and reasonably uniform. (5) Although general melting appears to occur between craters with 0.694-tAm laser radiation, the craters themselves no doubt originate the same way as when 1.06-nm radiation is used. (The damage thresholds are not greatly different.) Yet the penetration

for 0.694-fim light Is only on the order of 1 |j,m, hardly enough for SBS to occur.

Evidence from the present study lends general support to the theory of Dobson and Keeble, but differs by minimizing the role of doping inhomogeneities . As previously mentioned, the 1.06-|im radiation

damage thresholds for n-type, p-type, and the purest undoped GaAs available were about the same, and the appearance of damage (excluding mechanically polished surfaces) was the same. As seen in figure Id, the mechanical flaws associated with polishing greatly increase susceptibility to damage. No assertion is made that the presence of minor doping inhomogeneities cannot play a role, but simply that it is not the most influential factor in damage from ~ 10? W/cm^ laser pulses. Calculations for the upper-limit energy absorption of a free electron during a half cycle of the electric field associated with 10^ w/cm^

gives only ~ 10"4 eV, much less than the GaAs bandgap. Thus, the breakdown cannot result from an electron avalanche in the classical sense. This in itself, however, does not imply that collision ionization due to hot conduction electrons excited by inverse Bremsstrahlung does not take place. Yet each

free electron in GaAs excited to ionization energy with 1.06 y,m light by inverse Bremsstrahlung would

have to reach this energy through more than one event . The reason for this is that the photon energy is less than the bandgap, momentum must be conserved, and the electron also undergoes lossy collisions.

The lack of detectable influence on the damage threshold by degenerate n-type or p-type doping behooves any damage theory involving free electrons to explain the insensitivity of the threshold to initial free electron concentration and to the possible influence of hole density on free electron lifetime.

The high localization of damage sites with Q-switched laser light indicates inhomogeneity of material susceptibility. It cannot be concluded from this study whether the intrinsic threshold is much

higher. Generally speaking, however, the results of this study clearly suggest that for pulsed, near- bandgap photon flux, where gross, direct electron-hole pair creation and recombination are minimal, the flux limit (~ 10? W/cm2) is due to material flaws made more susceptible to damage by properties inherent

to a surface.

5 . Acknowledgments

Appreciation is expressed to P. P. Budenstein of Auburn University who acted as consultant, to C. W. Austin for electron microprobe work, and to R. A. Shatas for comments on writing the manuscript.

6. References

[1] Grasyuk, A. Z. and Zubarev, I. G., Sov. Phys.-

Semicond. 3 576 (1969).

[2] Smith, J. L., J. Appl. Phys. (scheduled for August 1972).

[3] Dobson, C. D. and Keeble, F. S., Proc. Internet. Symp. GaAs, Reading, 1966, p. 68.

[4] Kressel, H. and Mierop, H., J. Appl. Phys. 38 5419 (1967).

[5] Chiao, R. Y., Townes, C. H., and Stoicheff,

B. P., Phys. Rev. Letters 12 592 (1964).

[6] Giuliano, C. R., Appl. Phys. Letters 5 137 (1964).

I

73
7. Figures

a b

Figure 1. Photomicrographs of different types of laser damage on GaAs surfaces. (The scale in each figure is 2.5 |Jm /division. Damage is from (a) conventional mode pulses, (b) 0.694-|J.m Q-switched

pulses, (c) 1.06-Um Q-switched pulses, and (d) 1.06-Hm Q-switched pulses on mechanically finished

surface.

)

74
Surface Science and Surface Damage

Jhan M. Khan

Lawrence Livermore Laboratory

Livermore, California 9^550

The relatively new field of Surface Science is concerned with the measurement

of physical and chemical properties of well characterized surfaces. The ability

to characterize these surfaces has been steadily increasing due to the development

of new tools, or the application of older techniques to surface interrogation.

The overwhelming result of surface investigation is to draw attention to the

difficulty of obtaining such well-characterized surfaces. With this realization

in mind, it is highly instructive to review the conventional methods of creating

surface conditions. The assumptions involving the conditions at prepared sur- faces or interfaces must be reexamined. New procedures may have to be evolved

to control conditions to even assure reproducibility. Only when this has been

accomplished can measurements of surface damage thresholds be taken as intrinsic characteristics of prepared surfaces.

Key Words: Laser damage, surface science, thin films

1. Introduction

The purpose of this survey is to show how the tools of surface analysis might provide some answers to our current concerns of predictability and reliability of optical elements in high power laser appli

cations. The lack of predictability of the index of refraction and growth characteristics of coated

optics is well established and accepted in thin films. Although not as well established, reliability

seems also to be affected by the surface condition. This paper will attempt to convey the diagnostic

techniques necessary to characterize the near surface of solids.

In section 2, selected examples are employed to establish the tentative relevance of surface

effects to reflectivity of metallic surfaces and the index of refraction of disordered surfaces. In section 3 is presented a brief development of materials from the simplest form, the monocrystalline

solid, to impurity effects in polycrystalline films.

Section 4 contains a review of the commonly employed surface diagnostic methods and the basis of their operation. Section 5 attempts to point out the advantages and disadvantages of the common surfac preparation methods, including thin film deposition. These considerations only concern themselves with

characterization, not application.

The conclusion that is drawn by the author is that in almost all practical situations, there is only the crudest knowledge of the structure and composition of the surfaces employed in optical element

If this is a problem, then surface analytical methods and instruments exist that can be used to supply

such knowledge. Such exercises in characterization are generally time-consuming and require expensive,

sophisticated technology. The semiconductor industry has felt this has been justified. In addition

to analysis, certain of the tools have been incorporated into the production procedures as on-line

monitors. Perhaps this may also become the case in the area of high power laser optics.

2. Optical Predictability and Reliability

Predictability and reliability are both necessary when considering energy absorption in optical

elements. These both imply a knowledge of the surface characteristics of the system in terms of physical, chemical and optical properties. Failure due to excessive energy absorption may take place as a result of the departure of the system from initial design parameters or a change in the system after

initial preparation. These general comments apply to both simple terminations of a bulk material and

*Work performed under the auspices of the U. S. Atomic Energy Commission

75
structures resulting from a coating operation. The effects of the surface condition will vary from ma- terial to material. It would be unwise to generalize extensively.

A question that remains to be answered is "Are variations in surface characteristics responsible

for low reliability or low damage thresholds of surfaces?". The reported results of Swain (1)1 and

Divit (2) in altering surface thresholds by mechanical/chemical treatments suggest a possible importance.

These treatments are being reexamined by various experimenters.

The importance of the surface condition in influencing the reflectivity of metallic surfaces has

been established by Bennet.(3) In addition, there is ample evidence that the complex index of refraction of solids can be strongly effected by the surface structure. (4).

2.1 Reflectivity and Index of Refraction

Bennet (3) has studied the reflectivity of surfaces of silver, gold, and aluminum. In this work

the effect of overcoats, tarnish, and surface roughness were investigated. Bennet has shown that there

are changes in the fundamental absorption characteristics of the metal surface, due to the changes in the electronic states at the surface. In addition, surface roughness in the A range were observable in its effect upon the scattering characteristics of the surface for 10.6 um light.

In a recent ellipsometric study of the surface of silicon, Vedam (4) has observed the effect of

surface preparation methods on the index of refraction. He was able to determine the cause of variations

in the index of refraction from bulk values and connect this cause with structural disorder of um depth

for mechanically polished, ion -bombarded, and cleaved surfaces. The index resulting from this disorder was comparable with that observed for some amorphous surfaces produced by vapor deposition. These latter

values, however, show variations that indicate there is no unique "amorphous" index of refraction.

2.2 Surface Integrity

The observations noted in 2.1 clearly indicate that if structure and surface chemistry can effect

the optical properties, then changes in these two quantities will also alter the system characteristics.

In practical applications, optical surfaces are exposed to varying and potentially hostile environments.

Further study should be made of methods creating stable surfaces of the desired optical properties.

3. Solid Surfaces

The term "surface" has no unique meaning when applied in the field of surface science. The depth

of influence of a surface very strongly depends upon the particular phenomena that is being studied. It

is fair to say, however, that most influences subside in dimensions greater than 10 urn. To attempt to clarify this somewhat, the following portions of this section will consider a few cases.

3»1 Monocrystalline Solids

This is the purest and simplest case we can choose. Even here there exists some complexity. Let us consider several examples of different depth dependences.

3.1.1 Influence of Surfaces on Electron Motion

The characteristic distance of unimpeded motion is termed the mean free path (MFP) . The MFP is a function of the energy of the electron and the solid in which it resides, as well as the temperature of

the solid. When the electron is farther than several MFP's, it should not be influenced by the boundary

condition imposed by the surface. This would typically be of the order of 500 A. (5) Therefore, coupling of energy to electrons less than this depth should show some effect on the surface.

3.1.2 Atomic Lattice Constant

Distortions of the atomic lattice result from the lac.k of symmetry in the crystalline field at the

surface. There will be effects upon the vibration amplitude near the surface. In addition, there may be

the appearance of two-dimensional structure on the surface that does not reflect the bulk structure. These general effects may evidence themselves 0 in altered chemical reactivity on the surface. The dimenstion of these effects may be as much as 100 A.

^Figures in parenthesis indicate literature references at the end of this paper.

76
3-1.3 Surface Optical Reflection

This refers to the attenuation of the field of a reflected electromagnetic wave (photon) at the surface of a metal or under total external reflection from a0 dielectric

.

0 These dimensions may range from

the skin depth to the wavelength of the light, i.e., 500 A to 10,000 A, depending upon the conductivity

of the solid. (3)

3«2 Polycrystalline Solids

The statements made in 3»1 must be modified to include the effect of grain boundaries and variation

in crystal orientation of the grains. Many crystalline materials are not isotropic with respect to the

properties noted in 3«1« Therefore, these properties will be effectively averaged over all angles. In addition, the grain boundaries will represent scattering sites for any conduction electrons, and may well

influence energy absorption characteristics.

3-3 Thin Films

Typical thin films are comparable to or thinner than grain dimensions. These, then, may be looked upon as a special case of the polycrystalline solid. The thin film has structural characteristics that

are a result of the method of fabrication. The conditions of deposition will strongly influence the

crystalline structure, void size and shape, and surface topography. This is a result of nucleation and growth phenomena that is well documented. Up to this point we have assumed all of the ideal conditions,

i.e., purity and stoichiometry. When these conditions are relaxed, matters become less predictable.

3«4 Role of Impurities

Two classes of impurities exist: surface and bulk. There is a liberal interchange between these,

in practice. Burton (6) modeled the equilibrium concentration of impurities on crystal surfaces. In his

model he generated the following expression:

S = B exp (-£P/kr)

where S and B are the surface and bulk impurity concentrations, respectively, and AF is the change in free energy of the crystal when the impurity atom is moved to the surface. By considering surface sites, and

substitutional and interstitial sites he concluded that at temperatures below the melting point, impurity

atoms that are strongly bound to the solvent atoms will move to the bulk, while impurity atoms for which

the impurity-solvent bond is weaker than the solvent -solvent bond will concentrate on the surface. Rate

constants will, of course, be determined by diffusion constants. For the surface concentrating impurity,

the bulk may represent a semi-infinite reservoir just waiting for conditions to become favorable for mass

transport, such as temperature rise or radiation damage (radiation enhanced diffusion). This temperature

could be due to the repeated exposure of the surface to high power laser radiation.

Bulk impurities may supply low lying donor electron levels in the case of dielectrics. This phenomena is exploited in the semiconductor industry. These impurities may also influence mechanical properties, as they can affect defect motion. These impurities may be important for some damage mechansims.

At surfaces of grains or on the air-solid interface, impurities may significantly influence physical,

chemical, and mechanical properties in what may appear to be unpredictable ways.

3«5 Nonstoichiometry in Chemical Compounds

We are specifically interested in deviations from the Law of Definite Proportions. These deviations

occur most predominatly in solids. The observations of variable composition were put on a firm theoretical basis by Wagner and Schottky (7) in 1931 in a statistical thermodynamic analysis which established

the relation between lattice defects and nonstoichiometry, and showed that all crystalline inorganic compounds are inherently nonstoichiometric . (8)-(12) In any real crystal above absolute zero, there is a

finite concentration of point defects. Let us consider one case to illustrate.

For a binary compound MX, there are six simple types of point defects ( assuming no impurities): a) M vacancies

b) X vacancies

c) Interstitial M atoms or ions

d) Interstitial X atoms or ions

e) M atoms on X lattice sites

f) X atoms on M lattice sites

77
In any given crystal, all six types may be present. Of these types b, c, and e will lead to an excess

of M atoms in the crystal, while types a, d, and f will lead to an excess of X atoms . The concentrations

of these defects will not in general be equal, and the crystal will be nonstoichiometric

.

The above observations apply to the "perfect" single crystal. In practice, the materials that are commonly available have additional complicating factors. These include impurity concentrations, grain

boundaries (precipitation sites), and for thin films a potential dominance of purely surface effects

(surface states, structural irregularities, etc.)

The importance of deviations from stoichiometry lies in the changes in optical absorption that may

accompany them. This has been observed in ASgS.., where excess sulphur produces absorption bands in the

10 urn wavelength range (13)

4. Surface Characterization Methods

In this section a variety of techniques will be presented that are commonly used to determine the properties of solids in the first few nm or less. No single measurement is able to characterize the sur- face. The current trend in surface analysis is to combine more and more of the methods into one experimental system to allow complimentary characterizations.

4.1 Techniques Based upon Scattered Primary Electrons

An incident beam of electrons may interact with a solid in two ways reflecting the wave-particle

duality of quantum mechanics. As a wave the electron may be diffracted by an orderly array of atoms to

give information related to atomic arrangement. In addition the electron may lose energy to the solid

by collisions. In resonant processes the energy loss may be associated with excitation of specific

levels within the atoms or excitation of collective motion of electrons in the conduction band of metals

(plasmons). A special case of the collision case is elastic scattering where the electron looses no significant portion of its initial energy.

4.1.1 Low Energy Electron Diffraction (LEED) (I4)(l5)

This technique observes the two-dimensional structure on the surface of monocrystalline solids. It employs a primary beam of monoenergetic electrons of energy less than several hundred electron volts

energy. LEED is used to study nucleation and growth phenomena for adsorbed or deposited species on crystal surfaces.

This technique is limited in applicability by the need for preparing clean, well-ordered surfaces. Many materials undergo phase transitions during the cleaning process that leave the surface in a disordered state when averaged over beam dimensions of the order of a millimeter.

4.1.2 Inelastic Electron Scattering (IES) (l6)

By measuring the energy loss spectrum of these scattered electrons information may be obtained re- garding electronic excitations. These excitations play an important role in the absorption of laser

photons in metallic reflectors. The technique may be combined with LEED. Detailed information may be

obtained for collisions involving specific energy and momentum loss within the solid.

4.2 Techniques Based on Secondary Electron Emission

For purposes of surface analysis, attention is focused upon the characteristic portion of the second ary electron spectron. These characteristic electrons emitted are produced by specific transitions within the atoms of the solid. They are somewhat analagous to x-ray flourescence. The characteristic elec- trons studied here are generally quite low in energy and originate in the first 100 A of the surface or

less. Surface contamination is a very serious problem for these measurements.

4.2.1 Auger Electron Spectroscopy (AES) (I7)(l8)

The AES technique employs an electron beam to excite the atoms that emit the auger electrons. Under

most circumstances AES gives an elemental analysis of the first several atom layers of the surface with a, sensitivity of l/lO of an atom layer (i.e., about 10^ atoms/cm^). In general the results are qualitativ' or at most semi -quantitative.

78

i
4.2.2 Electron Spectroscopy for Chemical Analysis (ESCA) (19)

ESCA also looks at characteristic electrons, but these are photo electrons. An intense source of

characteristic x-rays is employed. The principle difference between AES and ESCA lies in the sharpness

of the electron spectrum observed. For AES the full width at half maximum (FWHM) is of the order of 1 eV, while for ESCA it may be as low as 0.1 eV. This latter value of the FWHM allows ESCA to observe

chemical shifts in a number of cases. With information of this sort, it is possible to identify the

binding state of an atom on the surface, thereby shedding light upon the surface chemistry of the system.

4.3 Techniques Based on Scattered Positive Ions (20)

Ions incident upon a solid sample lose energy by a variety of interactions: electronic excitation

and ionization and nuclear scattering. The latter may be either elastic or inelastic. Elastic scattering implies a conservation of momentum and kinetic energy. Three quantities are of interest: l) energy

of recoil of the incident particle, 2) the cross section or probability of scattering the incident particle into a specific element of solid angle, and 3) the probability that the incident particle is charged

after the scattering event. We will wish to look at two regimes of scattering, i.e., high energy

(E 10 keV) or low energy (E 10 keV) . In both cases the recoil energy, E^, from a two-particle

ev§nt is given by:

(M

]

_ + Mg)' cos 9 +

2 sin 9 1/2

(2)

where M. and M„ are the masses of the primary ion and the target atom respectively, and 9 is the scattering angle in laboratory coordinates.

4.3-1 High Energy Scattering (21) (22)

+ Tvpically+the incident energies are of the order of 1 MeV and the incident ions are low mass (H+

, He,C,W,0). The detection systems are usually only energy sensitive, so the charge state is irre- levant. The application of this technique yields:

1. Mass analysis. o 2. Depth distribution to the order of 0.5 pm with a resolution of 300 A or greater.

3. Spatial distribution with typical resolution of 1 mm.

4. Sensitivity: Mg> M

g (bulk masses): 1014 atoms/cm2

Mg< M

g

: 10$ of M

g

o From the above it is clear that the mass analysis is averaged over a minimum depth of 300 A or so.

4.3.2 Low Energy Ion Scattering (23) (26)

Typical incident energies are of the order of several keV. Under these conditions only incident

ions that are scattered from the outermost atoms of the solid remain charged. If the detection system

uses electrostatic or magnetic analysis (as is .usual) then only these charged species are detected.

Therefore, this technique gives a mass distribution of those atoms in the first atom layer of the solid.

4.4 Technique Based on Optical Reflection (Ellipsometry)

This optical technique is based upon the measurement of the reflectivity of the components of light

with planes of polarization perpendicular and parallel to the plane of incidence (4)(27)(3l)« The power

of this technique is illustrated by its application to a reflecting silicon substrate having a damaged

surface layer covered by a transparent film. The method was employed to characterize surfaces subjected

to a variety of treatments: mechanical polishing, chemical etching, sputtering by argon ions, cleavage

and annealing. The resulting reflectivities and indices of refraction obtained were very sensitive to treatment. This sensitivity indicates that ellipsometry is a technique that may have a valuable place

in the laboratory which studies optical surfaces.

4.5 Scanning Electron Microscopy (SEM)

The SEM conventionally displays the intensity of the low energy secondary electron spectrum and the

high energy elastically scattered scattered electrons. In the former case surface topography is observed

79
due to changes in efficiency of emission. The latter case utilizes the increase in the probability or cross section for scattering with charge on the scattering atom. Thus a display is presented of a rela- tive distribution of "light" versus "heavy" atoms on the surface.

The SEM may be upgraded to allow incorporation of auger analysis (SEM-AES) (32). The changes are most associated with improvement in the vacuum environment. The partial pressures of condensible, re- active species should be very much below 10 torr. Under these conditions, the AES-SEM systems allowso the study of surface composition with a spatial resolution comparable to the beam dimensions, i.e, 100 A.

Table 1.

Analysis

method

Qualit

.

Quant

.

Elemental Chemical Structural Depth Depth

resoc

.

Surface

resoc

.

Comment

LEED^ X X

0 3 A mm 2-D surface structure IES^ X X X

__

4 3 A — mm Collective

electron

oscillation

AES^ X X

0 10 A ... mm Difficult to apply to dielectrics

ESCA^ X X X

0 100 A cm Difficult to apply to dielectrics

HEBS ' X X

0 5000 A

O 300 A mm

LEBS ' X X

0 3 A mm

ELl/ g ) X X

O 100 A mm Determine inde

of refractionfilms on sub- strates SEM

(h) X X X

B 100 Ao 100 A

SEM-AES X X X

o 100 AO 100 A

(a) Low Energy Electron Diffraction (e) High Energy Ion Backscattering

(b) Inelastic Electron Scattering (f) Low Energy Ion Backscattering

(c) Auger Electron Spectroscopy (g) Ellipsometry

(d) Electron Spectroscopy for Chemical Analysis (h) Scanning Electron Miscroscopy

4.6 Summary

Table I shows a comparison of the surface characterization methods currently available. Many of

these exist as commercially available tools, some as purely experimental research instruments.

5. Surface Preparation Methods

Although surface preparation is usually associated with cleaning, the term will be expanded in its

meaning here to include the creation of surfaces or interfaces by deposition methods.

As a general statement all surfaces must be considered reactive. That is, the surface is prone to

the accumulation of impurity atoms, whether they be chemisorbed or physisorbed. The origin of the impuri

ties may be internal or external to the bulk.

80
As a result of the foregoing, all operations involving the surface should be accompanied with some experimental test of the surface condition. In the extreme, as many of the operations as possible

should be performed in a controlled environment so that the full history of the surface is known. In practical circumstances, this may not be possible on a routine basis. Under these conditions the systems

employed should be fully understood prior to their use, so that limits may be placed on the contributions

of different sources of imperfections.

5.1 Preparation of Solid Surfaces

The starting point for preparation of a surface is the bulk material. In view of the discussion in

section 3*4, the chemical composition of the bulk should be ascertained prior to any surface preparation.

This is crucial.' In addition, the completeness of the chemical analysis must also be assured. Given con- fidence in the bulk composition, then a variety of surface preparation methods may be applied.

Each method or sequence of methods of surface preparation has its own set of advantages and disadvantages, so that the techniques to be used for a given application should, in principle, be the result

of systematic study of the different preparation methods on the desired physical and chemical properties

of the finished surface.

5.1.1 Mechanical Operations

The act of cutting, grinding, or polishing will inherently cause disorder in mono or polycrystalline

solids. In addition, the polishing agents will be present in or on the surface. The depth of these

effects will vary but may extend many microns in general.

The principal purpose of mechanical operation is to give the sample an overall shape and smoothness.

Typically, after cutting and grinding of hard material, polishing is employed. The last stages of polish

ing usually employ a succession of finer and finer abrasive powders in a suspension. The last particle

size may be a fraction of a micron. Disruption and contamin tion may be expected to be found to depths

of 3 to 5 times the particle diameter. It will be necessary to remove this portion of the surface to remove such effects, if they are found to be detrimental.

5.1.2 Chemical Etching

To remove surface material, a chemical etch is frequently employed. The particular etch employed

will vary with the solid. Again, care must be taken. Some of the problems encountered are:

1) Concentration of less soluble impurities at the surface,

2) Impurities in the etchant remaining bound to the surface,

3) The etchant may partially decompose on the surface, leaving reactive elements behind (such as flourine)

,

4) Selective action by the etchant that will partially destroy the smoothness left by the final stage

of polishing. As in all cases of surface preparation, the surface should be characterized following

the etching operation.

5.1.3 Ion Bombardment

A recent technique that has been used

+to clean surfaces is inert ion bombardment at low energies

( 10 keV) . Typical ions are Ar+

, Ne , Xe . Material removal may be effected with reasonable efficiency.

Two potential problems arise: l) implantation of the ion into the surface and 2) production of a highly

reactive surface in the case of dielectrics. Disorder produced in the surface by low energy ion bombardment is well documented. Frequently, annealing may be used to remove disorder and drive off residual

inert gas atoms from the surface region.

5.1.4 Annealing

Annealing to remove disorder and impurities is well known. However, impurities may diffuse to the

surface as a result of this operation, as discussed in section 3«4. In cases such as the preparation of

atomically clean ordered silicon surfaces, many cycles of ion bombardment and annealing in an ultra-high vacuum environment may be required to reduce the impurity level to an acceptable value.

5.1.5 Cleaving

This technique is deemed the best for obtaining an atomically clean surface. Recent experiments (4) show that stress-strain fields may exist in the cleared surfaces up to microns in depth. If it is neces- sary to obtain a strain-free surface, annealing may be employed. The final surface should be characterized, however, to ascertain the impurity level.

81
5.2 Surfaces Prepared by Coating

The process of preparing a thin film coating on a solid is one of the more complex surface processes For a film of 1 urn or less, it should be looked upon as two surfaces, back-to-back. More rigorously,

there is no region of the film that does not feel the influence of an interface. As a result, the prop- erties of a film are not readily comparable with bulk characteristics of the same material. For optical

applications, this disparity with bulk properties requires the measurement of indices of refraction for

each film and are highly desireable.

It would be inappropriate in a survey of this sort to discuss in detail vaporization, condensation,

nucleation and growth of thin films. It is reasonable to identify the basic components in an evaporation

system. These componenets are coupled in such a way that the vacuum coater and sample must be viewed as

a system.

5.2.1 Coating Environment

Sources of contamination are l) residual gases (at their respective pressures) , 2) residual coating

materials from previous processes, 3) condensible material supplied by the vacuum envelope. To illustrat

the importance of contamination, it requires approximately 2 x 10"° torr-min exposure to a typical vacuum

environment to saturate an aluminum film. (33)

5.2.2 Substrate

The topography, surface chemistry, impurity concentration and temperature all affect the growth of

the film, and its physical, chemical and optical properties.

5.2.3 Vapor Source

The vapor source does much more than supply the coating material. A compound may be vaporized in

elemental ratios different from the stoichiometric ratio of the ideal compound. In addition, the vapor

source is also a source of radiant energy. The influence of this on the substrate surface should be

ascertained. Impurities and ionized species may also be present.

6. Conclusion

There is a reasonable indication that knowledge of the composition, structure and the electronic

and chemical character of surfaces may be necessary before the desired level of predictability and reliability in laser optics may be achieved. The careful application of surface science techniques may be abl to isolate the factors most influential in reducing predictability in coated optics and reliability in

both coated and uncoated optics. Given a knowledge of these factors, it should be possible, then, to modify production practice in a way to mitigate or eliminate their effects. Much work remains to be

done in these areas if the requisite understanding is to be acquired.

e in Laser Glass

,

(1) Swain, J. E. , ASTM STP 469, American Society for Testing and Materials, I969, pp. 69-78.

(2) Davit, J., J. Appl. Fhys. 39, 6052 (1968).

(3) Bennet, H. E., Damage in Laser Materials , ASTM STP 356, American Society for Testing

and Materials, 1971, pp. 153-167

.

(4) Vedam, K. and So, S. S., Surface Science 2£,

2 (1972).

(5) Kanter, H. , Fhys. Rev. B 1, 522 (1969).

(6) Burton, J. J., Phys. Rev. lJJ, 1346 (1969).

(7) Wagner, C. and Schottky, W., Z. Physik.

Chem. Bll , 163 (1931)-

(8) Anderson, J. S., Natl. Physical Lab, Symposium No. 9, Paper 7A (1959).

References

(9) Ehrlich, P., Z. Elektrochem. 45, 362 (1939)-

(10) Hauffe, K., Reaktionen in und an Festen

Stoffen, Springer-Verlag, Berlin (1955)

.

(11) Kroger, F. A. and Vink, H. J., Solid State Physics, Vol. 3, Academic Press, New York,

pp. 307-4=35 (1956).

(12) Libowitz, G. G., Progress in Solid State Chemistry, Vol. 2, Pergamon Press (1965).

(13), Frerichs, R., U. S. Patent 2,979,382 (1961).

(14) Haas, T. W., Dooley, III, G. J., and Grant,

J. T., Prog, in Surface Sci. 1, Part 2 (1971)

(15) MacRae, A. U., Surface Sci. 13, 130-3 (1969)-

(16) Porteus, J. 0. and Faith, W. N., Phys. Rev.

B, 2, 1532 (1970).

82
(17) Jenkins, L. H. and Chung, M. F. , Surface Sci.

28, 2 (1971).

(18) Taylor, N. J., J. Vac. Sci. Technol. 6, 241-5

(1969).

(19) Siegbahn, K., et al, Nova Acta Regiae

Societatis Scientiarum Upsaliensis TV, 20,

(1967). ~

(20) Rutherford, Sir Ernest, Geiger, H. and

Marsden, E. , Phil. Mag. 21, 669 (1911).

(21) Davies, J. A., Denhartog, J., Eriksson, L. and Mayer, J. W., Can. J. Phys. 45, 4053

(1967). ~"

(22) Rubin, S., Nucl. Inst. Methods 5, 177 (1959).

(23) Ball, D. J., Buck, T. M. , MacNair, D. and

Wheatley, G. H., Surface Sci. 30, 1 (1972).

(24) Goff, R. F. and Smith, D. P., J. Vacuum Sci.

Technolg. J, 72 (1970).

(25) Smith, D. P. J. Appl. Phys. 38, 340 (1967)-

(26) Smith, D. P., Surface Sci. 25,(1971).

(27) Fehlner, F. P. and Mott, N. F., Oxidation of

Metals 2, 59, (1970).

(28) Frankl, D. R., J. Appl. Phys. 34, 3514 (I963).

(29) Heavens, 0. S., Optical Properties of Thin

Solid Films , Academic Press, New York (1955)

.

(30) Kruger, J., in: Ellipsometry in the Measure - ment of Surfaces and Thin Films , Symposium

Proceedings, Eds. E. Passaglia, R. R. Stromberg

and J. Kruger (Natl. Bur. Std., Misc. Publ.

256, Washington, D. C.,1964).

(31) Paik, W. and Bockris, J.O.M. , Surface Science

28, 1 (1971).

(32) MacDonald, N. C . , Appl. Phys. Lett. 16, 76

(1969).

(33) Kruger, W. H. and Pollack, S. R. Pollack,

Surface Sci. 30, 2 (1972).

83
Self-Focusing with Elliptical Beams

John Marburger

Departments of Physics and Electrical Engineering

University of Southern California

Los Angeles, California 90007

The time independent self -focusing of beams whose constant intensity contours are ellipses has been studied in the paraxial ray-constant shape approximation. This

approximation leads to equations for the principal 1/G intensity diameters versus

axial distance which have been analysed previously by Vorob'yev by a different

method. Our method allows the treatment of non-gaussian intensity profiles. The

solutions are employed to find expressions for the critical powers and self-focal

lengths of nonaxially symmetric beams under a variety of initial conditions. The

critical power can be greatly enhanced by astigmatic focusing, even if the beam

shape is initially axially symmetric. This theory implies that bulk damage thresholds arising from self -focusing depend strongly on beam shape, in agreement with

experimental results of Giuliano.

Key Words: Bulk damage, damage threshold, elliptical beams, enhancement,

self-focusing.

1. Introduction

Most studies of self-focusing of intense optical beams assume that the incident beam has circular,

or axial, symmetry. However, many important laser systems produce a beam which is more nearly elliptical

than circular in cross section, and it is therefore important to understand how the performance of the system depends upon distortions of the beam shape. We show below that self-focusing is rather sensitive ; to such distortions, and that the critical power for self-focusing, and thus to some extent the bulk

damage threshold, can be enhanced by employing elliptical beams. This conclusion was reached experimentally by C. Giuliano [1]* independently of the theoretical analyses of Vorob'yev [2], Shvartsburg [3] and Vlasov, et al. , [4]. In this paper we sketch a simple theory of the self-focusing of elliptical

beams, and use it to derive formulas for the behavior of the self-focal point under a variety of incident

beam conditions. Some of our results have been obtained previously in [2]-[4], but this work is not yet

available in English translation, and we feel that it is worthwhile to repeat some of it here.

f In addition to departures from axial symmetry, we shall also briefly consider the influence of non-gaussian and acentric intensity profiles on self-focusing. The experimental verification of theory

developed here is by no means complete, but what data exist are consistent with our results [1].

2. The Motion of the Principal Axes of Elliptical Self-Focusing Beams

We shall develop our theory only for linearly polarized beams propagating in a lossless medium

with nonlinear dielectric constant

e = £ + e„ <E

2

> (1) ! °

2

where <...> denotes a time average over many optical cycles, and E is the peak instantaneous electric

field. This form is appropriate for a nonlinear polarization which responds instantly to changes in the

Research sponsored by the Joint Services Electronics Program through the Air Force Office of Scientific

Research under Contract F 44620-71-C-0067

.

Figures in brackets indicate the literature references at the end of this paper.

84

I

|
applied field, and therefore we are ignoring the slowly responding mechanism of electrostriction. We

use gaussian cgs units.

Writing the electric field strength as

E = -| E' exp i(Kz-Wt) + c.c. (2)

(all vectors will be assumed parallel to E which is approximately parallel to the x axis) , we may derive

the following equation for E'

2iK + V

2 E' + -| K

2 (e

2 /eo

) |E' I 2 E' = 0 . (3)

This may be obtained from the usual wave equation in the presence of matter, with the approximation

|8 2E*/3z

2

| « |K 3E'/3z| (A)

which means that most of the z dependence of E is in the factor exp iKz, and therefore E' = E'(x,y,z) is

a slowly varying "envelope" function of z. The time dependence has been suppressed here. As long as we

assume instantaneous response, the time dependence can be inserted a-posteriori into the solutions by

allowing the "initial" conditions at z = 0 to depend upon time. (See reference [6] for an account of this approach.)

If we let

E' - E

q exp i<f> (5)

we find from the real and imaginary parts of (3)

K 3E

2 /3z + V„ • (E2 VT

<)>) = 0 (6) o 1 ol

K 3<|>/3z + \

(VT

<}>) 2 - \

(V^E

o )/Eq

- \

(K

2 e

2 /eo )E

2 = 0 . (7)

Here 22222 = 3 z /3x + 3 222

/3y and e <o = c K . 1 o Equation (7) is the Hamilton-Jacobi equation for a particle of unit mass moving under the influence of the potential

- \ (V?E )/k\ - \

(e,/e )E

2 (8)

z io o 4 Loo

if the quantity Kz is interpreted as the time, and Kx, Ky as the particle coordinates. The equations of motion for these coordinates can be cast into a convenient form if we assume that

E

o

= E

ffi (z) «>-T 1*2+^1 <9)

where a and b are both functions of z. This is the constant shape approximation which forces the intensity contours of the beam to be elliptical at any axial distance z, although the ellipticity and size of the ellipse may vary with z. If we use (9) to evaluate (8) for the particle (ray) passing through the paraxial point (aa, 6b), where ot,6 « 1, then we find the components of "force" in the x and y directions:

K a o

85
B 2 2 8

„2. 3 2e m b Kb o (11)

2 2 The function E may be eliminated from these formulas by using the relation between E and the total power in the beam

E = (8P/nc ab) (12)

where n = eQ

. This leads finally to the following equations for the principal axes of the ellipse

formed by the e~l intensity points in the transverse plane:

2K

,2

2 d a dz

2 2n

a

2b

(13)

2K

2 d^b

dz

2 JUL

ab

2 (14)

3 2 where n = P/P^, and P-^ = n c/bz^ . These equations were first derived by Vorob'yev [2] who obtained

them using a variational method. The quantity P-^ is the power required in a circularly symmetric beam

in a self -focusing medium to cause the initial curvature of the on-axis intensity versus distance curve

to vanish. It is less by the factor 0.273 then the critical power P2 above which a "strong" or catas- trophic self-focus forms at z = 00 . (See reference [7].) In our approximate theory, however, ?± plays

the role of a critical power for circular beams, as we shall see below.

Equations (13) and (14) are formally equivalent to the equations of motion of a particle of mass

2k2 moving in the conservative two-dimensional potential well

u = VV 2 ^iba b

(15)

The equipotential curves of (15) are shown for several values of n in figures 1-5. Notice that as n increases from 0 to 1, the potential well remains repulsive for all positive values of a and b (corresponding to diffractive spreading of the beam), but develops a "trough" along the line a = b which

finally acquires a flat bottom at r| = 1. For n > 1, there are two straight lines U = 0 between which

the potential surface drops steeply to -°° at the origin.

Obviously the path of a particle moving in such a potential can be complicated, but the form of equations (13) and (14) allows a simple expression for the quantity a2(z) + b2(z), as shown first by

Vorob'yev. The reader may easily verify that

2 2 (a + b

Z

) dz

- (16)

and therefore

K

2 (a2 + b

2

) = K

2 (a2 + b

2

) + -\- (r, - ri) o o a b c

o o

z + 2K

2 (a a + b b )z + K

2 (a

2 + b2

) 00 00 (17)

Here aQ , bD are the axes of the ellipse formed by the e- -*- points of the transverse intensity profile at

z = 0, and

n = (a

2 + b

2 )/2a b

c o o 00 (18)

The quantities aQ , b 0 are the initial rates of change of these axes, and are simply related to the principal radii of curvature Ra , of the incident phase front:

86
a /a

o o b

= b /b

o o (19)

Equation (17) provides the starting point for the derivation of many interesting formulas regarding the self -focusing of elliptical beams.

3. Useful Equations for Self -Focusing Elliptical Beams

If the phase fronts of the wave at z = 0 are plane, then a

Q

= b0 = 0, and (17) becomes

a + b a + b

o o

1 + (1 n/n ) c Ka b

o o (20)

where n = P/P^. If n > n c » this function decreases to zero at the point

Ka b

o o

'fo , . ,.1/2

(n/n - i)

c

(21)

2 12 2 At this point the intensity ncE^/Sir = P/irab becomes infinite because ab £ j (a + b ). A glance at the potential curves shows that a and b must vanish together so that whenever ab = 0, we also have

a2 + b 2 = 0. Thus the quantity

p = n p, = k\Tr + — I c cl 2\b all \ o of

(22)

must be regarded as the critical power for self-focusing for this case, and zf 0 is the self-focal length

for P > P c

.

2 2 When P = P c

, then a + b remains constant (for plane incident phase fronts). Vorob'yev has

pointed out that in this case the equations of motion can be integrated in a closed form involving

elliptic integrals [2]. The trajectory of the analogous particle in the (a,b) plane is the arc of a circle, concentric with the origin, of radius (a2 + b

2 )-*-/ 2 and contained between the lines U = 0 as indicated in figure 4. The beam shape changes from an ellipse to a circle with lower axial intensity,

and back to an ellipse with interchanged axes, then repeats the cycle. This is the closest possible

analogue to a self -trapped beam for an incident elliptical cross section.

If the initial phase fronts have finite curvature, then equation (17) implies that when n exceeds

a certain value ncc» tne beam intensity becomes infinite at

1 +

Ka b \ n

o o \ c

1/2 (23)

where

n /n

cc c

1 +

Ka b . o o / 1 2n

(24)

and z Q is the distance at which the self-focus first appears as the power is increased to P = Tj ccPj_. It satisfies *

2 2 2 (a + b ) a (25)

When n is less than n cc > there is still an on-axis intensity maximum near the real part of Zf given by (23):

87
Re zf

; (26)

1 + (

Ka

I

bo) ( \

")

2 2 We say

" near Re Zf " because we can only compute the minimum of a + b versus z from (17) , whereas the intensity maximum occurs at the minimum of ab.

Notice that z 0 given by (25) is negative for diverging beams (Ra , Rj, are positive for diverging

beams) and therefore a self-focus first appears at z » 00 for such beams only at the critical power

P = n c<i Pi, where

n + i Ka b ( + ) cc2oo\bRaR,/ • (27)

\ o a o d /

'cd 'cc 2 o o lb R

\ °

The critical power enhancement predicted by (24) for astigmatically focused beams (Ra

i* Rjj) can

be quite large. Thus in Guiliano's recent experiment [1], a0

= 0.179 mm, b0= 0.232 mm, Ra

A. 05 cm, Rb = 7.04 cm, and Ka0

b0 = 66.1 cm. For these values, n c 1.03, but n cc 12.7. This is consistent

with the observed increase in threshold.

4. Formulas for Non-Gaussian Intensity Profiles

Our starting assumption (9) for the shape of the incident beam is rather restrictive, and it is useful to know how the results of the preceding section must be modified to account for deviations from

this form. If we start with the more general form

E

q

= Em

(z) gl(x/a) g

2 (y/b) , (28)

we may still find simple results for the self -focusing parameters. The functions g^ and g2 must be even,

and therefore we may write

IlW.,.,-iv(^)v... on

We shall also require the "normalization" integrals

c/a) d(x/a) = r (31)

f

> -co

r

g^y/b) d(y/b) = s . (32)

For gaussian beams, p=o=l, r=s=v^r. Beams which are flatter and broader than gaussians have

larger values of these parameters.

If the derivation of the equations of motion for a and b outlined in Section 2 is repeated for

the more general shape of equation (28), one finds equations exactly analogous to (13) and (14), but with new forces derivable from the potential

y- „£- + iI__ 2n . i_ , (33 ) a b

where

88
, TT I P

rs rs

(34)

The quantity d

2 (a2 + b2 )/dz 2 is still a constant of the motion, and a2 + b

2 may be written as before

with t| c replaced by

(35)

Thus all previous formulas can be corrected for non-gaussian beams by replacing ri c everywhere with n^-

One might object that (28) is still a rather restrictive assumption, but unfortunately more

general forms lead to systems of differential equations for the beam parameters which have no simple

solutions such as those given here. The value of our treatment is that it gives simple formulas for

the important self -focusing parameters which might be expected to give at least their qualitative

dependence on incident beam conditions. Since our theory correctly describes only rays very near the

axis, we cannot rely on its results too heavily for quantitative information.

5. Acentric Beams

In this section we simply wish to draw attention to the work of Kaplan [8] which shows that acentric beams will follow a curved path in a self-focusing medium. This bending of the path occurs because the rays comprising the beam see an induced prism proportional to the nonlinear index and the degree of acentricity in the beam.

A ray propagating in a linear inhomogeneous medium will possess a local radius of curvature, R, given by

-1 A Vn R = N • —n

where N is the principal normal of the ray, and n the refractive index. In our case, the inhomogeneity

in n is induced by intensity variations within the incident beam. The simplest variation leading to beam bending in the x,z plane is obviously

„2 _2 /,'_,_ 2A \ „ W

E = E [l+rr-x)--=-<x<-r

o m V W / j 2 — — 2 », '!« w

The corresponding intensity profile along the x axis is shown in figure 6. For this shape, the initial radius of curvature of the beam is

2 3 2 2n W n c W a

o o R

'2 m

where we have assumed that the total beam power is UE2 4 A £

2 P

n c E

2

This radius of curvature gives an angle of deflection of <(>/s = R

-

^ radians per cm of path length s in

), P = 10 10 W/cm2 = 10 17 erg/cm2 sec, nQ cgs units. For ££ = 10" (esu x 1.5, and a = W = 0.1 mm, we

find

<f>/s = 0.4 A radians per cm

This is a remarkably large angle for these not unreasonable beam parameters, and one might expect to be

able to see such a deflection experimentally. Notice that self-focusing causes the beam dimensions to decrease with path length which implies that R also decreases with path length. Thus we expect the path

of an acentric beam in a self -focusing medium to be a spiral terminating at the self-focus.

89
6. References

[1] Giuliano, C. , Marburger , J., Yariv, A., Appl.

Phys. Letters, July 1972, "Enhancement of Self -Focusing Threshold in Sapphire with

Elliptical Beams."

[2] Vorob'yev, V. V., IVUZ Radiofiz. 13, 1905

(1970), "Self- Focusing of Non-Axisymmetric

Optical Beams."

[3] Shvartsburg, A. B. , IVUZ Radiofiz. 13, 1775 (1970), "Self-Focusing of a Beam of Electromagnetic Waves with an Ellipsoidal Phase Front." (In Russian.)

[4] Vlosov, Petrichen and Tallanov, J. Radio

Fizika, 14, 1354 (1971). (In Russian -- reference incomplete.)

[5] Reference not used in text

.

[6] Marburger, J. "Theory of Self-Focusing for Fast Nonlinear Response," in Damage in Laser Materials: 1971 , A. J. Glass, A. H. Guenther,

eds. NBS Special Publication 356, 51 (1971).

[7] Dawes, E. and Marburger, J., Phys. Rev. 179 , 862 (1969), "Computer Studies of Self- Focusing. "

[8] Kaplan, A. E. , JETP Letters 33 (1969),

"Bending of Trajectories of Asymmetrical

Light Beams in Nonlinear Media."

7. Figures

Figure 1. Equipotential curves in the a, b plane Figure 2. Same as figure 1, but T| = .85. for the potential of Equation (15) in the text,

where T] = P/P^ is negligible.

90
Figure 5. Same as figure 1 but T) = 2 Figure 6. Notation for intensity distribution of

simple acentric beam.

91
Relative Contribution of Kerr Effect and Electrostriction

to Self-Focusing*

Albert Feldman, Deane Horowitz, and Roy M. Waxier

National Bureau of Standards

Washington, D. C. 20234, U. S. A.

The ratio of damage thresholds for circularly polarized radiation to linearly

polarized radiation was found to be > 1 in borosilicate crown glass, fused silica, and dense flint glass. A Q-switched Nd: glass laser operating in the TEMqq mode was

used. The damage was assumed to result from self- focusing. The fractional contributions

of electrostriction and the Kerr effect to the nonlinear index ^ are estimated from the threshold ratios calculated for each mechanism in the absence of the other and assuming

no other self-focusing mechanism. The high damage threshold of fused silica is attributed

to its relatively small Kerr effect. Measurements of self-focusing lengths as a function

of peak power give qualitatively good agreement with theory.

Key Words: Damage threshold, electrostriction, Kerr-effect, laser damage in

glasses, optical glasses, self-focusing.

1. Introduction

The mechanism responsible for the self-focusing of laser beams with pulse widths of 25 ns has

not been uniquely identified. Three mechanisms have been proposed to explain the effect [1]*:

electrostriction [2-6], Kerr effect [2-4,7], and thermal self-focusing [8-10]. Up to the present time

there has been insufficient data to judge the comparative importance of the proposed mechanisms.

Self- focusing in liquids is known to be caused by the Kerr effect because of the large contribution of

freely rotatable molecules or ions to the nonlinear index coefficient n£. Because solids lack freely

rotatable molecules, it was thought that electrostriction [2-4] was the dominant mechanism. Duguay and Hanson have calculated the nonlinear index of refraction due to the Kerr effect ^(K) from measurements

of induced birefringence in borosilicate crown glass using picosecond pulses [11,12]. They find that

the value obtained is of sufficient magnitude to cause self-focusing and are of the opinion that the Kerr effect is the predominant mechanism for all pulse widths.

E. L. Kerr [6] has developed the theory of electrostrictive self-focusing to explain the data of

Steinberg [13], who used 55-ns pulses from a ruby laser to measure damage thresholds in several optical

glasses. Kerr is of the view that electrostriction is the principal self-focusing mechanism for pulse

widths > 10 ns. For pulses less than 10 ns, on the other hand, the electrostrictive mechanism has insufficient time to take .effect.

For the glasses such as we are considering, thermal self-focusing is not thought to be important.

Quelle [10] in a sample calculation showed that the threshold for thermal self-focusing would be two orders of magnitude greater than for electrostriction.

2 We have made self-focusing measurements in borosilicate crown glass (BSC 517), fused silica, and dense flint glass (SF 55 formerly SFS 5) . Self- focusing lengths were measured as a function of beam

power [14] and compared with the theory of Dawes and Marburger [15] which assumes an instantaneous

self-focusing mechanism. Additional measurements were made which showed that the damage threshold is higher for circular polarization than for linear polarization. From these measurements we estimate

the relative importance of the proposed mechanisms to self-focusing. In liquids, similar measurements helped to confirm that the Kerr effect is the predominant mechanism [16,17].

* Research supported in part by the Advanced Research Projects Agency of the Department of Defense.

'''Figures in brackets indicate the literature references at the end of this paper.

2 Certain commercial materials are identified to specify adequately the experimental procedure. In no

case does such identification imply recommendation or endorsement by the National Bureau of Standards.

92
2. Experimental Procedure

The experimental apparatus is shown in figure 1. The oscillator consists of a Nd: glass rod

pumped by a helical flash lamp. It is Q-switched by means of a KD*P Pockels cell. There is an aperture in the cavity 2.44 mm in diameter for producing a beam in the TEMQo mode. The output from the

oscillator is a pulse whose width at half the maximum power is 25 ns as measured by a high-speed

photodiode.

The beam profile of the laser beam at the focusing lens is obtained from burn patterns on developed unexposed black (Polaroid)^ film [14]. The output of the laser is maintained at a constant pulse energy but is attenuated from pulse to pulse by interposing calibrated attenuation filters

between the oscillator and the film. Figure 2 shows a set of bum patterns used for determining the

beam profile. The radius of the outermost burn ring for each pajtern is plotted as a function of beam energy. The result is fitted to a Gaussian curve H = H0

e-r l & • The constant H0 is obtained in units of J/cm2 from

00 E = 2ir HQ 0 exp (-r2 /a2

) r dr, (1)

where E is the total pulse energy (99mJ-in this case). Figure 3 shows a plot of the beam profile

together with the Gaussian which is fitted to the data by eye.

The energy output at a given pumping level was constant to within ± 2% as measured by the thermopile. During the damage tests each shot was monitored by the photodiode. The energy of the beam

was varied in 20% increments with calibrated attenuation filters. Finer increments of energy were

obtained by varying the oscillator pump energy.

3. Self-Focusing Length Measurements

Recent experiments [18-21] have shown that the filamentary damage observed when solids are exposed to high-intensity laser radiation is due to self- focusing of the laser beam and that the self-focused spot moves toward the laser source as the pulse power rises. Dawes and Marburger [15]

have made a detailed computer calculation which predicts the self-focusing length as a function of beam power assuming that the nonlinear index coefficient ^ responds instantaneously to the laser

pulse shape.

We have produced damage filaments in optical glasses by focusing the laser beam at different power levels P into samples using a 181-mm focal length lens. A length was measured from the upstream end of the damage track to the sample entrance face. The self-focusing length z was

calculated from the formula [22]

z"

1 = z"

1 - R

_1

(2)

where R is the radius of curvature of the laser wavefront at the sample- entrance face. R is calculated from the measured focal spot of the lens and the propagation characteristics of Gaussian

beams. The critical power for self-focusing P.t is taken to be the lowestpower which produced damage in

the sample. The normalized inverse self- focusing length (z*)

-l is plotted as a function of (P/Pt )l/2

in figure 4 together with the theoretically calculated curve of Dawes and Marburger (our P

fc is equivalent to their P2) , where z* = z/2ka2 and k = 2ir nQA; n0 is the refractive index of the medium;

X is the wavelength of the laser beam in air; a is the 1/e intensity point at the sample face. There

is good qualitative agreement between the data and the theory. The deviations are attributed to electrostrictive self-focusing. Because of the low threshold in the SF 55, we can compare experiment

with theory over a large range of values. The fact that the slope of the data matches the experimental

slope confirms that the laser beam had a Gaussian profile. We believe the data demonstrate that the theory provides a good description of self-focusing in solids.

^Certain commercial instruments and materials are identified in this paper in order to specify adequately 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 instrument or material identified is necessarily the best available for the purpose.

93
4. Threshold for Linear and Circular Polarization

4.1 Experimental

Damage threshold measurements for linearly and circularly polarized light were made by focusing

the laser beam into the samples using lenses of 76 mm and 181 mm focal length. The oscillator output

was linearly polarized in the horizontal plane with the ratio of horizontal to vertical polarization

being greater than 200:1. Circularly polarized light was produced by inserting a quarter-wave plate

into the beam. The quarter-wave plate consisted of a plate of fused silica under uniaxial compression

[23] . The linear polarization damage measurements were made by relaxing the stress in the plate. Measurements were also made with the plate stressed to a half-wave retardation; the results were

consistent with the unstressed plate measurements.

Figures 5 and 6 show the damage threshold data obtained. The solid points represent pulses that produced damage; the open points represent pulses that did not produce damage. The scatter in the data

is due to sample variability, laser energy variations, and the statistical nature of the actual damage

process [20]. In all cases, the threshold for circularly polarized light P£ is higher than for linear

polarization, P t . (Unprlmed symbols refer to linear polarization and primed symbols to circular

polarization.) Table 1 lists the ratios P£/Pt

. The error limits represent the full scatter in the

data.

Table 1. Parameters for calculating electrostriction and Kerr effect

in optical glasses.

n0 (1.06 urn) qll (10~13 cm2 /dyn)

112 (10"13 cm2 /dyn) Pll

llles) (lb""

n|(es) (lO-13

n

2 (es) /n^(es)

n,(K)/n$ (K) P

t /Pt = n2 /n2 Reference d Reference e

esu)

esu)

(theory)

(theory)

(experimental)

Reference d Reference e

BSC 517 Fused Silica Dense Flint

1.507 1.450 1. 733

a0.315 b 0.418 c1.7

a1.92 b 2.71 c2.2 aO.HS b0.120 c0.21

a0.221 b0.270 c0.23

0.62 0.81 2.5 0.56 0.72 2.5 1.11 1.13 1.00

1.50 1.50 1.50

1.18 ± 0.07 1.09 ± 0.09 1.25 ± 0.08

1.26 ± 0.12 1.24 ± 0.11 1.32 ± 0.07 0.8 ± 0.2 1.15 + 0.35 0.40 ± 0.13

0.6 ± 0.3 0.7 ± 0.3 0.27 ± 0.13

aR. M. Waxier and C. E. Weir, J. Res. Natl. Bur. Std. (U. S.) 69A, 325 (1965) , X = 587.6 nm.

bW. Primak and D. Post, J. Appl. Phys. 30, 779 (1959), X = 546.1 nm.

cThis work, X = 632.8 nm.

<*Data obtained with 76-mm focal length lens , full scatter in the data.

eData obtained with 181 -mm focal length lens. Error limits represent

full scatter in the data.

Error limits represent

94
4.2 Relation of Experiment to Theory

The mechanisms of electrostriction and the Kerr effect are associated with the quadratic

dependence of refractive index on electric field E: n = n + %n£ E

2

; n2 = n2<es) + ^(K).

[es = electrostriction, K = Kerr effect.] The self-focusing threshold P t is inversely proportional to n£. The relative contributions of the above mechanisms to the self-focusing can be

calculated from the experimental data if we make several assumptions: (1) The damage is assumed to result from self-focusing; (2) thermal self-focusing can be ignored; (3) the ratios n2 (es) /n2 (es) and n2 (K)/n2

(K) can be calculated.

In calculating n2 (es)/n2 (es) , we assume the material responds to a D. C. field obtained from a time average of the optical electrical field tensor E.E.. It is assumed that the E field is uniform

within a cylinder and zero outside. The result will be the same on the axis of a Gaussian beam.

Detailed dynamical calculations are not expected to change the ratio n

2 (es) /n^ (es) even though they

would affect the absolute index changes. Steady-state conditions are assumed.

In the presence of an intense uniform electric field all solids undergo a strain [2A]

From thermodynamics we can show that

Y

ijki. =

"

K imIC jnqmnk£ /Alr

' (4)

where k is the dielectric tensor and q^jj^ is the stress -optical coefficient. If we assume a freely

expanding cylinder, we obtain the expressions

n2 (es) = n

Q

7 (p 11q 11 + 2p 12 q12 )/8ir ( 5)

n

2 (es) = n

Q

7 (p11 q 11 + 3p 12 q12 + + Pll^)

167T » < 6 >

where the p^. are the elasto-optic coefficients; the Voigt. notation is used. Values for the above

constants are given in table 1. Although the numbers quoted were obtained at visible wavelengths, we

do not expect them to differ significantly at 1.06 um.

For the Kerr effect we assume only an electronic contribution [11,12,25-27]. Recent experimental

work suggests this is indeed the case [28]. For an isotropic material the nonlinear polarization will

be [29]

?

NL

= 3CU22 (1.1)1. (7)

If we insert the field for a linearly polarized wave E = [E0 cos (wt-kz), 0,0'] and a circularly polarized wave = [EQ cos(wt-kz), EQ sin (tot-kz) , 0]/t^2 into eq (7) and calculate the Fourier component of

at frequency to, we obtain a ratio of 1.5 for n2(K) /n2 (K)

It is now possible to estimate the fractional contribution of electrostriction, a = n2(es)/n2»

and the Kerr effect, (1-a) = n2(K)/n2, to the nonlinear index coefficient. Calculated values for a are summarized in table 1, v

One cannot attribute any significance to one value of a being greater than unity for Si02

. This

value was obtained with a lens of short focal length. Because of the high self- focusing threshold

for fused silica, it is probable that only a slight amount of self-focusing took place and the threshold measured was essentially the intrinsic damage threshold for which one might expect no difference between circular and linear polarization.

95
The data indicate that the Kerr effect is larger in BSC 517 than in SI02

. This conclusion

would explain why fused silica has a higher damage threshold than BSC 517 since, on the basis of electrostriction alone, BSC 517 would have the higher damage threshold. In dense flint glass the Kerr effect appears to make a greater contribution than electrostriction; the observed difference

in damage threshold between circular and linear polarization must be ascribed to the Kerr effect since no difference is expected from electrostriction alone.

In all cases the value of a is larger for the longer focal length data. This is reasonable since

electrostrictive effects are expected to decrease with increase of the focal spot size while the Kerr

effect is expected to remain unchanged. It is interesting to note that electrostriction makes a relatively smaller contribution in SF 55 vs the other glasses. This is to be expected since

electrostriction depends on the parameter X = a

Q /vT where t is the pulse width and v the velocity of sound in the medium [6]. As X increases, electrostrictive effects decrease. For SFS 55, v = 3000 m/s,

while for Si02 and BSC 517, v = 6000 m/s. Thus, X is twice as large for SF 55 as it is for the other

glasses

.

Figure 7 compares our data with the data of Steinberg [13] and E. L. Kerr's theory of electrostrictive self-focusing [6]. The figures were obtained from Kerr's work. The X's represent

values we measured. In order to compare the data, it was necessary to scale the power with X 2 and

the beam radius with 1/t. We find relatively good agreement for the fused silica and borosilicate

crown data with that of Steinberg. The data for SF 55 do not agree, however. Our data seem

reasonable in view of the previous results. In SF 55 the relative contribution of electrostriction is small, and hence one expects a small change in damage threshold for the different lenses used. In fused silica and BSC 517, where electrostriction appears to be more important, we observe a definite

increase in damage threshold with a longer focal length lens.

The data presented suggest that the Kerr effect increases with refractive index. A large re- fractive index implies large charge displacements, and large charge displacements are more likely to lead to a nonlinear response to the electric field and, therefore, to a large Kerr effect. It is known

that flint glasses exhibit increasing polarizability and refractive index with increasing lead content

[30], The data obtained are consistent with axis rotation measurements of elliptically polarized light

[28],

5. Conclusions

Our self- focusing length measurements show that the theory of Dawes and Marburger provides a good

description of self- focusing.

From threshold measurements using linearly and circularly polarized radiation we conclude that the Kerr effect and electrostriction are of comparable magnitude for the beam geometry used.

6. Reference

[1] Akhmanov, S. A., Sukhorukov, A. P., and [10] Khokhlov, R. V., Usp. Fiz. Nauk 93, 19 (1967)

[Sov. Fhys.--Usp. 93, 609 (1968)].

[2] Chiao, R. Y. , Garmire , E. , and Townes, C. H.

,

Phys. Rev. Letters 13, 479 (1964). [11]

[3] Zel'dovich, Ya. B, and Raizer, Yu. P. ZhETF

Pis. Red. 3, 137 (1966) [JETP Lett. 3, 86 (1966)].

[4] Shen, Y. R. , Phys. Rev. Letters 20, 378 (1966), [12]

[5] Chaban, A. A., ZhETF Pis. Red. 6, 487 (1967)

[JETP Lett. 6, 20 (1967)]

.

[13]

[6] Kerr, E. L. , Phys. Rev. A 4, 1195 (1971).

[14]

[7] Chaban, A. A., ZhETF Pis. Red. 5, 61 (1967)

[JETP Lett. 5. 48 (1967)].

[8] Litvak, A. G. , ZhETF Pis. Red. 4, 341 (1966)

[JETP Lett. 4, 230 (1966)].

[9] Raizer, Yu. P., ZhETF Pis. Red. 4, 124 (1966) [15] [JETP Lett. 4, 88 (1966)]

.

Quelle, F. W. , "Self-Focusing in Glass," in Damage in Laser Materials , ASTM STP 469 , American Society for Testing and Materials,

1969, pp. 110-116.

Duguay, M. A. and Hansen, John W. , "Measure- ment of the Nonlinear Index n~ of Glass

Using Picosecond Pulses," in Damage in Laser Materials (NBS Special Publication 341),

p. 45.

Duguay, M. A., Hansen, J. W, , and Shapiro,

S. L. , IEEE J. Quant. Electron, QE-6 , 725 (1970).

Steinberg, G. N. , Phys. Rev. A 4, 1182 (1971).

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

,

"Laser Induced Damage Studies," in NBS Technical Note 703 , ARPA-NBS Program of Research on High Temperature Materials and Laser Materials , 1971, edited by A. D. Franklin and H. S. Bennett, p. 12.

Dawes, El. L. and Marburger, J. H., Phys. Rev. 179 , 862 (1969).

96
[16] Wang, C. C. , Phys. Rev. 152, 149 (1966).

[17] Close, D. H., Giullano, C. R. , Hellwarth,

R. W. , Hess, L. D. , McClung, F. J., and Wagner, W. G. , IEEE J. Quant. Electron.

QE-2 , 553 (1966).

[18] Zverev, G. M. , Maldutis, E. K. , and Pashkov,

V. A., ZhETF Pis. Red. 9, 108 (1969) [JETP Lett. 9. 61 (1969)].

[19] Giullano, C. R. , "Time Evolution of Damage

Tracks in Sapphire and Ruby," in Damage in Laser Materials, 1971 (NBS Special

Publication 356), p. 44.

[20] Bass, M. and Barrett, H.H., "The Probability

and Dynamics of Damaging Optical Wterials

with Lasers," in Damage in Laser Materials , 1971 (NBS Special Publication 356), p. 76.

[21] Giuliano, C. R. and Marburger, J. H., Phys.

Rev. Letters 27, 905 (1971).

[22] Marburger, J. H. , "Theory of Self-Focusing

for Fast Nonlinear Response," in Damage in Laser Materials, 1971 (NBS Special

Publication 356), p. 51.

[23] Waxier, R. M. and Farabaugh , E. N. , J. Res. Natl. Bur. Std. (U. S.) 74A , 215 (1970).

[ 24 ] Nye , J . F .

, in Physical Properties of

Crystals (Oxford University Press, London,

1957), p. 257.

[25] Buckingham, A. D. and Pople, T. A., Proc.

Phys. Soc (London) A68, 905 (1955).

[26] Buckingham, A. D. and Orr , B. J., Quart. Rev. (London) 21, 195 (1967).

[27] Buckingham, A. D. , and Orr, B. J., Trans. Faraday Soc. 65, 673 (1969).

[28] Owyoung, A. Hellwarth, R. W. , and George, N. Phys. Rev. B _5, 628 (1972).

[29] Maker, P. D. and Terhune , R. W. , Phys. Rev.

137 , A801 (1965).

[30] Fajans, K. and Keidl, N. J., J. Am. Ceram.

Soc. 31, 105 (1948).

7. Figures

O O o o • •

o o o

Figure 1. Apparatus for damage testing: OSC = oscillator; B = beam-splitter; P = high speed

photodiode; Q = quarter-wave plate for circular

polarization, zero or one-half-wave plate for linear polarization; T - thermopile energy meter movable into the beam; F = attenuation filters; L = focusing lens; S = sample.

Figure 2. Burn patterns on unexposed (Polaroid)

film used for determing beam profile.

97
2.0

1.0 - >«*-

(c) J L J I I I

I I 8

\/z*

12 16 20

Figure 4. Self-focusing length as a function of

the square root of normalized beam power. A lens of 181-mm focal length was used in obtaining the data. Samples used: (a) dense flint glass, (b) borosilicate crown glass, (c) fused silica. The

solid curves are from the work of Dawes and Marburger [15],

98
1000

600

400

200

150

BSC 517 FUSED SILICA DENSE FLINT CIRCULAR LINEAR CIRCULAR LINEAR CIRCULAR LINEAR 2000 _ _ - - -

BSC 517 DENSE FLINT

... -

CIRCULAR LINEAR CIRCULAR LINEAR CIRCULAR LINEAR

ooo

Ill'

00 - -

1000 ncoooo

-gS»»88aBo

0 ;:«Be°ooo "a

-

-

- -

£> - -

_-

-

- J 600 - - -

_-

cc

hi

- - - S

o

a- 400 - - - - -

PEAK - - - - -

200 - - -

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100 Figure 5. Damage threshold measurements in borosilicate crown glass (BSC 517), fused silica, and

dense flint glass (SF 55) for linearly and circularly polarized radiation; wavelength - 1.06 Lim; pulse width = 25 ns. The solid points represent

shots which produced damage; the open points rep- resent shots which did not produce damage. Data obtained using 76-mm focal length lens.

Figure 6, Damage threshold measurements in borosilicate crown glass (BSC 517), fused silica, and dense flint glass (SF 55) for linearly and circularly polarized radiation; wavelength = 1.06 |_lm; pulse width = 25 ns. The solid points represent

shots which produced damage; the open points rep- resent shots which did not produce damage. Data obtained using 181-mm focal length lens.

Figure 7. Track formation thresholds in borosilicate crown glass, fused silica and dense flint glass from the work of E. L. Kerr [6] and G. N. Steinberg [13]. Also shown are data from the work depicted by X. It was necessary to scale our data as the wavelength and pulse width used differed

from Steinberg's.

99
Damage Measurements with Subnanosecond Pulses

John M. McMahon

Naval Research Laboratory

Washington, D.C. 20390

The realization of high energy subnanosecond pulse lasers has been limited by

self- focusing damage in laser rods. In this paper measurements on ^ for laser glasses

with subnanosecond pulses are described. The systems aspects of using such glasses

are also discussed since in practice this can have a dominant effect on the performance

of a high energy short pulse device.

Key Words: Damage mechanisms in laser systems, disc amplifiers, Nd : YAG laser,

self-focusing, subnanosecond pulses.

1. Introduction

There is currently a great deal of interest in generating intense subnanosecond pulses for laser CTR applications as well as for other purposes. Success in construction of relatively efficient

solid state lasers which generate terawatt (or higher) level pulses and which operate reliably for many shots will require either that the pulse shape and intensity be tailored in time and spatial extent such that non-linear effects do not predominate or that geometries be chosen in which

these effects can reasonably be expected not to occur.

Disc amplifiers, for example, appear to be relatively immune to self focusing (if only in that

the surfaces will damage first). They however are somewhat unattractive from an efficiency and convenience standpoint compared to a solid rod system.

We have investigated the self focusing problem for three laser glasses in common use (OwensIllinois ED-2; Soveril-MG 915 and Schott LG-56) . Measurements of N2 are reported on unpumped rods

for pulse durations between 20 psec and 250 psec. Measurement techniques for subnanosecond pulses

are not presently as quantitative as might be desired. Pulse duration measurements by non-linear

optical techniques may lead to erroneous results unless great care is taken. Energy measurements

are complicated by plasma formation in the calorimeter at very low levels compared to q switched

pulses (=" 50 mJ/cm^ on carbon) which reduces the signal to noise ratio to (typically) less than

20:1 and there are difficulties in measuring near and far field intensity distributions with a high precision and wide dynamic range at 1 jum.

2. Measurement of in Laser Glasses

In the 1970 ASTM symposium, J. Davitfl] 1 of CGE and M. Duguay and J. Hansen[2] of BTL reported

on measurements on N2 for several glasses by various methods. Duguay and Hansen reported some measurements at ~ 8 psec on BK-7 and LASF-7 glasses and Davit on tests done at 2 nsec on LG-56

and MG-915 glasses. While BK-7 might be expected to behave in a similar fashion to the laser glasses Davit tested, the value of N2 was about an order of magnitude larger than the BTL value. This suggests the possibility that for very short pulses N2 might be smaller due to mechanisms

important on the nanosecond time scale not being able to respond to short pulses. Another

possibility was that since neither author used a laser which produced a time band width limited

pulse, both measurements could be consistent with a model in which some of the pulse energy is in very short temporal spikes.

Figures in brackets indicate the literature references at the end of this paper.

100
For the measurements reported here we used a mode locked Nd:YAG oscillator- amplifier which has been described elsewhere. [3] This system produces a TEMQ0 spatial mode output single pulse with an energy adjustable from 1 - 200 mJ in a pulse whose width can be adjusted from 20 psec to 1.3 nanoseconds by the use of transmission etalons in the cavity. Our measurements and those of Carmen and co-workers at LLL indicate that the pulse is essentially time-bandwidth limited[4] with a smooth

Gaussian spectral and temporal output. Figure 1 shows the geometrical arrangement of this device.

For the trapping measurements the beam was propagated ~ 3 meters to get rid of near field

Fresnel fringes and the pattern checked and found to be a Gaussian down to below the e"3 points.

The beam was then run through a sample rod and the trapping length noted from the side during

10-20 shots for each sample. In all cases the start of the filament was characterized by the emission of blue light (presumably from plasma breakdown in the track) . This was somewhat enhanced

in the ED-2, presumably due to the ultraviolet pumping the cerium which then fluoresced in the region of 4000 - 5000 A.

Table 1. Samples Tested

Glass Type -L(cm)

ED2

MG 915 LG 56 BK-7(cast) 303050

6

3 x 10 7.5 x 10

3 x 10-3

3 x 10-3

,-3

Table 1 lists the geometries and absorption coefficients of the samples tested. The Soveril

glass tested was a rod made by the old process and had a significantly higher absorption coefficient

than MG-915 made by the continuous flow process. The LG 56 rod was supplied by CGE with the NRL

VD640 and had a low absorption coefficient for LG-56. We initially questioned whether this was

LG-56 or LG-630 and were assured by CGE that it was LG-56.

In the actual sequence of experiments the laser was tuned to run at a given pulsewidth and

then the samples were each irradiated to 10-20 shots and the mean trapping length, peak energy

density, pulsewidth and spatial distribution determined. The laser was then tuned to the next

pulsewidth and the sequence repeated. Table shows the results for pulse durations of 20 psec and

250 psec. The pulse peak powers and beam parameters were PQ

= 8 x 10^ W/cm^, b = 1.1 mm and

P0 = 1 x 10^ W/cm^, b = 1.5 mm, respectively.

Table 2. Relative Data on Trapping Length

Type Z(250 psec) Z(20 psec)

ED224 ± 1 6.6 ± 1 MG-915 24 ± 1.5 6.1 ± 1 LG-56 21 ± 1.5 5.6 ± 1 BK-7 6 + *

* The 6 cm sample only trapped occasionally (=" 30% of

shots) and within 1 cm of the surface.

The difference in beam parameter was caused by the beginning of focusing in the YAG preamplifiers

which was not severe enough to noticably perturb the temporal or spectral structure of the pulse.

The cone angle of the self-phase modulated light from the samples was =-0.1 radian at 250 psec and

="0.25 radian at 20 psec. Typically 3-10 filaments would be formed which would traverse the length

of the rod.

101
Table 3- Trapping Data ED2

2 t(psec) P^(W/cm ) b(mm) Z(cm)

20 8 x 10

10 1.1 6.6 ± .5 125 1.2 x 10 10 1.2 18 ± 2 250 1 x 10 10 1.5 24 ± 1 125* 4.8 x 10 10 1.2 9.2 ± 1.6

* Etalon misaligned caused temporal beats with a four- fold enhancement of peak power.

Table 3 gives trapping data on ED-2 for a somewhat more complete set of experiments. Data sets were run for 20 psec, 125 psec and 250 psec "clean" pulses and also for a pulse 125 psec long with

the etalon misaligned to give two spectral components of equal intensity. These interferred giving

a peak intensity ~ four times the intensity for a non- structured pulse. This caused a reduction in

trapping length of a factor of two.

Table 4. N Values (esu)

BK-7 ED-2 MG-915

LG-56

6 x 10

(7 ± 3) x 10" 14

(7 ± 3) x 10-W

x 10" i4

(8 ± 3)

Table 4 gives the calculated value of N„ for the glasses tested with an estimated absolute accuracy of ± 407.. The value of BK-7 should be taken as an upper limit since I , =- Z, sample foe

3. Discussion of the Results

The data all showed an excellent agreement with Kelley's formula for the parametric variation

of self focusing length[5]

.1 (\\\_

4 \*2 )

E

Z foc = 7. ITT) $ • (1)

Additionally, over the range of temporal values tested for a particular glass was found to be sensibly constant.

It was additionally shown that will follow temporal structure on the pulse. If we had assumed our structured pulse to be smooth, an ^ value four times the observed value would have been obtained.

Under a given set of conditions there did appear to be small but experimentally significant

differences in self focusing length. That there is a difference between ED-2 and LG-56 is not too surprising since the two are dissimilar glasses, but the difference between the LG-56 and MG-915 is surprising, since they are essentially the same glass. These may be related to the difference in absorption coefficient of the two glasses.

4. Systems Considerations

Based upon the values of reported here there appear to be no fundamental difficulties in amplifying pulses to high energies if the trapping length is long compared to the amplifier length

and the pulse is temporarily and spatially smooth. Unfortunately, these conditions are rarely met

(for more than a few shots).

102
Dust particles on surfaces can provide nuclei for wavefront disturbances which can then cause

trapping in the next stage. Shot by shot this damage can then propagate through the system. This

problem can be minimized if the amplifiers are spaced in the far field of any filaments generated in the previous amplifier.

It is also generally very difficult to avoid Fresnel fringes in the beam pattern. Based on the experiments of Guiliani [6] , a Fresnel fringe itself may be expedted to have a high threshold for trapping (since it would be a one dimensional process). However, in a circularly symmetric beam, the Fresnel fringes may accentuate damage by moving energy into the beam center where it may tend to form

an intensity spike. We have been such a spike forming at times and we understand that Bettinger [7] at CGE has seen such a spike form and then self-focus. This effect seems to be somewhat diminished

if any apertures in the system are non-concentric.

Operating the CGE VD-640 amplifier train with 250 psec pulses outputs of 50-60 joules have been

achieved with little or no degradation over =• 100 shots. A limited number of shots have been run

at levels of 90-100 joules. No degradation has been ovserved to date.

5. References

[1]. Davit, J, NBS Special Publication 341, (1970 pp 37-44.

[2]. Duguay, M.A. and J. W. Hansen, NBS Special

Publication 341, (1970) pp 45-50.

[3] Emmett, J. et al, "Disc Lasers", IEEE Int. Quantum Electronics Conference, Montreal,

Canada, May 1972.

[4] Carmen, R. , IEEE Int. Quantum Electronics

Conference, Montreal, Canada, May 1972.

[5] Kelley, P.L. , Phys. Rev. Letters, 15, 26 (1965) , p. 1005.

[6] Guiliani, IEEE Int. Quantum Electronics

Conference, Montreal, Canada, May 1972.

[7] Bettinger, private communication.

6. Figures

Nd YAG SHORT PULSE GENERATOR

Figure 1. Nd :YAG Short Pulse Generator

103
Time Resolved Damage Studies of Thin Films and Substrate Surfaces*

J. H. Parks and N. Alyassini

University of Southern California

Los Angeles, California 90007

The time evolution of damage to thin films and substrate surfaces was observed

by monitoring the intensity variation of a He-Ne beam reflected from the film or surface during irradiation by a Q-switched ruby laser. Monolayered films including

ZnS, CaF

2 and NaCl on fused silica substrates were studied, as well as uncoated fused silica and LiF substrates. When the probe beam is reflected from the exit surface of

a substrate sample, the reflected intensity was observed to decrease within the timescale range of 20-200 nanoseconds to a final value significantly less than that prior

to damage. When the probe is spatially offset from the damage region, the reflected

intensity decreases and then returns to approximately the predamage value within

0.2-0.3(i sec. This reversible effect was observed to be associated with a propagating

disturbance on the substrate surface moving with approximately the speed characteristic

of a Rayleigh surface wave. The risetime and duration of the plasma which accompanies

surface damage was also measured.

The reflected intensity from thin films was observed to increase or decrease

depending on the film index of refraction. The monolayer film samples exhibited re- flectance changes on the timescale of the ruby pulse, however an offset probe position

indicates this change can occur after a time delay of ~ 0.1-0.2|J sec.

Key Words: Damage induced reflectivity change, laser induced damage, laser induced surface acoustic waves, surface, surface breakdown plasma, thin film,

time resolved damage.

1. Introduction

Studies of surface damage in laser glasses, crystals and thin films usually characterize the damage

in terms of the laser source used to induce the material breakdown. There has been considerable effort

to correlate damage mechanisms with laser power threshold values, radiative pulse width, and the morphol ogy of the damaged region. This work has led to a description of the conditions under which damage will happen and a clearer expectation of the resulting structural deformation. However, the present experimental techniques may not be particularly appropriate to reach a fundamental understanding of the primar

physical processes, intrinsic to the material, which originate the sequence leading to irreversible,

macroscopic damage.

We have developed an optical probe technique which allows the continuous time evolution of laser induced damage to be monitored on a timescale spanning 3 to 4 orders of magnitude. The time resolution

can include effects occurring during the damage pulse and also be extended to observe phenomena existing well after the laser pulse has past. This experimental technique places emphasis on the study of physic processes in the time domain in an attempt to unravel the damage sequence and perhaps isolate a precurso time event which signals the onset of material breakdown.

A description of these time resolved damage measurements is presented in Section 2 and a summary of initial results observed in the damage of thin dielectric films and substrate surfaces in Sections 3 and This presentation is intended as a progress report of a continuing research program rather than final results of a specific experiment. In the concluding Section 5, we summarize our findings and plans to extend these measurements.

*This research was supported by Air Force Cambridge Research Laboratories and the Advanced Research

Projects Agency of the Department of Defence under Contract No. 19628-71-C-0220.

^Departments of Physics and Electrical Engineering.

104
2. Time Resolved Damage Measurements

Figure 1 shows the experimental arrangement of the ruby laser, damage sample, and He-Ne probe laser. The passively Q-switched ruby laser is internally apertured to 6 mm and forms a uniform beam profile

indicated by unexposed Polaroid dilm. Although the beam quality is not clean enough for detailed threshold

measurements, it is adequate for the purposes of developing these techniques. The 15 cm focal length lens focuses the ruby beam at a point 2 cm past the sample exit surface and the ruby spot size is 1 mm diameter

at this surface. The substrate sample position was adjusted slightly for thin film studies to produce a spot size of 0.6 mm diameter at the^film surface^ The laser power density used to damage substrate exit

surfaces and thin films was ~2GW/cm and ~lGW/cm respectively. The ruby laser pulsewidth is 20 nsec.

During a damage pulse, the He-Ne probe beam is reflected from a point on the surface or thin film

which is aligned with the ruby laser. As shown in figure la and lb, the probe beam can be reflected

either internally or externally from the surface to be studied. In the experiments described in Sections 3 and 4, the exit surface was observed via an internally reflected probe beam to avoid absorption

of the probe radiation by the surface plasma. The He-Ne beam is focused on the damage region to a spot

size of less than 0.5 mm diameter. A system of apodized lenses and aperatures define the optical path of reflected probe and reduces plasma scattered light pickup. An RCA 7265 photomultiplier tube shielded by

two 6328 a narrow band filters detects the reflected probe intensity. The electronics risetime was 16 nsec which limited the time resolution in these experiments to the ruby pulsewidth. The He-Ne beam was polarized horizontally to allow near Brewster angle incidence on the substrate surface in order to avoid

photomultiplier saturation by the reflected probe beam.

3. Substrate Surface Damage

Time resolved surface damage was observed in samples of fused silica (T13 Optical-2 ,T20 Suprasil-2

grades) and LiF ground and polished disks of 3 mm thickness. The probe focal spot could be spatially

positioned with respect to the ruby spot by referencing to a second He-Ne laser beam used to align the ruby laser optics.

At the power densities used, the damage at the fused silica surface was in the form of pits 0.1 mm

to 0.5 mm in diameter. Damage was generally weaker for LiF; leaving pits at most 0.2 mm in diameter.

Using an optical telescope damage pits of 10 microns could easily be resolved visually. Figure 2a shows

an example of the time response of the reflected probe intensity when the probe beam was positioned on

the damage area. The general characteristics of the probe response under these conditions are shown in figure 3. When the damage pits are greater than 0.1 mm, a rapid decrease in intensity is observed during

the ruby pulse timescale. However, for smaller pits corresponding to weaker damage, the intensity decrease

begins within the ruby pulse width but only gradually reaches steady state long after the damage pulse has

ended. These results are observed independent of probe polarization and appear to result from diffuse

scattering of the probe beam by the damage pit.

When the probe beam is spatially displaced from the damage spot, a reversible intensity decrease is observed similar to the traces shown in figure 2b. When the spatial separation between damage and probe

positions is increased a distance Ax, the intensity change is observed to occur at a delayed time At given

by Ax = v -At where v

g —4 X 10 cm/sec for fused silica as shown in figure 3. The spewed of this propa- gating effect is close to the speed of a Rayleigh acoustic surface wave, v = 3.4 x 10 cm/sec on fused

silica. Such an effect has been observed previously [1]^ to be generated by transient surface heating in metallic films. Holographic techniques [2] have also shown the presence of bulk and surface waves excited

by laser induced damage in glass. In fused silica these propagating effects were observed when damage

pits greater than 0.1 mm resulted. In LiF substrates, propagating intensity variations of comparable

strength to fused silica were noted even when dama-ge pits were less than 10 microns.

These techniques were also applied to time resolve the exit surface plasma. Passing the probe beam

parallel to the exit surface at a distance of ~ 1 mm from the substrate allows the plasma electron density

to attenuate the probe beam by absorption. Figure 4 shows the probe intensity decreases for approximately

a microsecond duration indicating the order of magnitude of the plasma timescale. It is important to note

that the timescales of the propagating disturbance and the plasma duration vary by a factor of 10 which

indicates that the propagator is not simply generated by the expanding plasma.

4. Thin Film Damage

Monolayer films of ZnS , CaF2 , and NaCl were deposited on fused silica substrates in thickness of

X Ik and \ 12 at 6943 A. These films were damaged by the ruby beam on the exit surface of the substrate

and the probe beam was internally incident on the damage area. At the power densities used for the Zns

Figures in brackets indicate the literature references at the end of this paper.

105
and NaCl films, the fused silica surface showed no visible signs of damage. Figure 5a shows examples of

the probeintensity changes resulting from thin film damage. In all cases the rapid intensity change oc- curred within the ruby pulse width. This transition was consistent with a reflectivity change from the relative index of the thin film boundary to the relative index of a fused silica-air interface. The film

damage spot size was ~ 0.5 mm diameter and the probe beam is measuring the timescale during which the film

is leaving the substrate surface. When the probe is spatially offset from the damage area in NaCl films,

a time delay has been observed. It is likely that this is a measure of the rate of an expanding heated

region which is vaporizing the film. The CaF2 films were found to resist damage up to power densities for which the fused silica substrated became pitted.

Figure 5b indicates that the probe signal change can either increase or decrease consistent with the above interpretation of the probe time traces. Since the internal incidence angle was close to 20°, the Fresnel reflectivity is approximately given by the normal incidence dependence on relative refractive

index n = n2/n-^. The relative indices for fused silica to air, NaCl and ZnS boundaries are 0.69, 1.06 and 1.64 respectively which yield corresponding reflectivities of 0.034, 0.059 and 0.001 assuming normal

incidence. This shows an increasing reflectivity of 0.001 ~* 0.034 for NaCl film samples and a decrease

0.059 -* 0.034 for ZnS film samples, assuming the film is removed during damage leaving an uncoated fused silica surface to reflect the probe beam.

5. Concluding Remarks

The following points summarize conclusions drawn from these time resolved measurements:

1. Diffuse scattering at substrate exit surfaces, which arises from structural damage, can occur within the timescale of the ruby pulsewidth or continue well after the pulse , depending on the severity of the damage.

2. Damage at substrate surfaces can excite a propagating disturbance moving on the surface with speeds comparable to Rayleigh acoustic waves. This propagator can be detected by optical techniques;

however , it has not been determined whether the fundamental process leading to changes in the reflected

probe intensity are caused by a reversible diffuse scattering, absorption, or index variations.

3. LiF substrate surface damage appears to excite propagators with extremely small damage on

the scale of 5-10 microns. This material offers the possibility of isolating a precursor mechanism for detailed studies of the damage physics.

4. Thin films exhibit damage primarily within the duration of the ruby pulse but it is pos- sible to detect this rapid film destruction moving outward radially. No evidence of propagators has been detected optically in monolayer film structures.

It may be useful to indicate our immediate research plans to improve these time resolved damage meas-i urements. In addition to increasing the time resolution of the probe beam detection electronics, and improving the ruby pulse optical homogeneity, the following experiments are being pursued:

1. The correlation of surface plasma formation and the initiation of structural damage by

simultaneous optical probes.

2. A more detailed study of propagators on substrate surfaces and thin films as a possible precursor

.

3. The use of acoustic surface wave techniques to detect damage phenomena with a greater

sensitivity and time resolution than the optical methods provide.

4. These measurements will be applied to a wider variety of substrates and film materials;

particularly those which have been carefully characterized by damage threshold studies.

6. Acknowledgements

We would like to acknowledge discussions with Prof. J. Marburger and Prof. R. W. Hellwarth. The

film depositions were prepared by Mr. H. R. Owen.

7. References

[1] Lee, R. E. and White, R. M. , Appl. Phys . [2] Boling, N.L. and Beck, R. W. , NBS Special Letters _12, 12 (1968). Publication 356, Damage in Laser Materials:

(1971).

106
8. Figures

ruby pulse

"| |* too wsec

AX=0

AX -O

Figure 3. Characteristic time dependence of sub- strate exit surface damage observed by optical

probe techniques. The timescales indicate an av- erage range of many measurements. The ruby pulse

position is indicated by t .

Figure 4,

//U.Sec

Exit surface plasma duration measured

by optical probe absorption.

AX = -5 •mm

AX = /vrt-m

THIN FILM DAMAGE

Figure 2. Oscilloscope traces showing: (a) de- creases in reflected probe intensity during sur- face damage (b) reversible reflectivity changes associated with propagating surface waves observed when probe is offset a distance Ax from damage

ZnS

/VoCl

zonsH IjL

(A) Figure 5. Film damage characteristics showing-

(a) time dependence of reflected probe intensity for NaCl and ZnS films (b) analysis of reflect tivity R which indicates an intensity decrease

for ZnS and an increase for NaCl after film is removed during damage leaving n =1.

107
Laser Induced Damage to Mirrors at Two Pulse Durations

E. S. Bliss and D. Milam

Air Force Cambridge Research Laboratories

Hanscomb Field

Bedford, Massachusetts 01730

The damage thresholds of eight, multiple layer dielectric, 95% reflecting

mirrors have been measured for single pulses of 20 nanoseconds and 20 picoseconds duration. In both sets of measurements the pulse energy, an oscilloscope trace of the pulse, and the beam's transverse energy density profile at the surface being damaged are recorded for each shot. The variation of the threshold

from mirror to mirror and as a function of pulse duration, beam radius, and

appearance under microscopic examination is discussed in the context of possible

damage mechanisms.

Key Words: Dielectric mirror, laser-induced damage, laser monitoring,

laser damage mechanisms, electron avalanche, plasma production,

scattering sites.

1. Introduction

For several years an effort has been underway at the Air Force Cambridge Research Labs to perfect a mode-locked ruby laser system capable of reliably producing single subnanosecond pulses

with a smooth and radially symmetric energy density profile and enough energy to damage dielectric

materials with spot sizes larger than 0. 1 millimeter. Such a system is now in use and is described

in detail in earlier publications. [l,2]l To complement the mode-locked system and to facilitate experimental identification of the laser-induced-damge mechanisms by determination of their pulse

duration dependence [3], a standard Q-switched ruby laser system has also been constructed which

meets the same reliability radial energy distribution, and total energy requirements and operates in a single longitudinal mode as well. Figure 1 contains block diagrams of both systems. This paper

describes laser-induced-damage measurements performed on nine commercially produced dielectric

mirrors using these two lasers.

2. Laser Damage Monitoring Apparatus

Meaningful laser-induced-damage experiments demand precise characterization of the laser

pulse causing the damage. Accordingly, each pulse is scrutinized in unprecedented detail.

This research was supported jointly by the Air Force and the Advanced Research Projects Agency

of the Department of Defense.

Figures in brackets indicate the literature references at the end of this paper.

108
2. 1 Q-Switched System Monitoring

Figure 2 is a schematic diagram of the monitoring apparatus for experiments using Q-switched

pulses. The beam enters from the left and passes through two beam splitters. The first one deflects

a portion of the beam to a fast diode which drives a Tektronix 454 oscilloscope and provides a filmed

record of the temporal development of each pulse. The second beam splitter directs a known fraction

of the beam to an EG&G radiometer, which is used to determine the energy of each pulse. Energy

values measured by this instrument exhibit a random error of only a few percent. However, past

experience and comparison with other radiometers and a Quantronix 500/504 calorimeter/energymeter suggest that the absolute accuracy may be limited to 10 or 20 percent.

The beam then passes through a 50 cm focal length lens and, except for a small portion, is focused

onto the damage sample. When the test sample is a high reflectivity mirror, the path shown beyond

the sample is not used, and the mirror is tipped very slightly to avoid direct reflections back to the amplifier. The small reference beam which is split off after the focusing lens is identical to the damaging beam except for its energy content and it can be utilized for monitoring the focused radial

energy distribution as follows: A calibrated reticle is inserted in the reference beam in the plane

corresponding to the mirror surface in the main beam. An imaging lens is then placed such that the reticle is imaged onto the film in the multiple exposure-level camera. [4] For ruby lasers Polaroid

film Type 55 is ideal since it provides a positive for visual examination and a negative for densitometer

analysis. The magnification of the imaging system is determined by photographing the reticle with

0. 69/Ltm light. Then the reticle is removed and the camera now records the radial energy density

profile of each pulse in the damage plane. Examples of such photographs will be discussed in section

3. 1. b.

2. 2 Mode -Locked System Monitoring [l]

Figure 3 is a schematic diagram of the monitoring apparatus for experiments using mode-locked

pulses. The principal features are the same as for the Q-switch pulse monitoring system, but there are some differences. The focusing is done by a lens pair whose effective focal length can be varied

by changing the lens spacing (an effective focal length of approximately lm was used for all the measurements reported in this paper). Both an EG&G radiometer and a Quantronix 500/504 calorimeter/

energy-meter are used for energy measurement. Although the calorimeter is blocked when a high

reflectivity mirror is used for a sample, measurements with both instruments and no sample show

that the two devices, operating on entirely different physical principles, agree within 20% on the absolute value of the energy reaching the sample.

The fast diode drives a Tektronix 519 oscilloscope and examines light arriving along several

paths simultaneously. A small portion of the oscillator output is picked off at the pulse selector [l]

and enters the diode along the dotted path in figure 3. One percent of the amplified selected pulse is split off just ahead of the focusing optics and directed onto the diode with a delay appropriate to display

it between pulses in the oscillator's mode-locked train. The remaining path to the diode is for monitoring the pulse after passage through a transparent damage sample and is not used for mirror studies.

The radial energy distribution in the damage plane is imaged into the multiple exposure camera

by a lens pair instead of single lens. This allows the first lens to be well removed from the sample

area and gives more flexibility for imaging different planes and changing magnification. As in the Q-switched monitoring system the energy density profile in the main beam is not monitored for mirror

damage experiments and so no reference sample is required.

3. Output Properties of the Lasers

3. 1 Q-Switched System

a. Temporal Profile

It is known that smooth, apparently single longitudinal mode oscilloscope traces obtained by

spacially integrating over the whole beam on a diode do not guarantee that the temporal development

109
of the pulse is the same over the whole beam. [5] In order to determine whether deviations from

ideal temporal behavior off of the beam axis might be related to deviations from the ideal radial energy

distribution, a sequence of shots was fired in which the output mirror of the Q-switched oscillator was

rotated by 4 sec. of arc about the vertical axis after each shot. After appropriate attenuation the oscillator output was directed into the multiple exposure-level camera without focusing, and each

shot was photographed in turn. Figure 4a shows the results. Each shot produces a vertical row of successively less exposed energy density profiles, and the shots on the left show a slightly elongated pattern with the top tipped to the right, while the shots on

the right show a similar elongation with a tip to the left. When the temporal profile is examined

through a pinhole on axis ( fig. 4b, right) and through a vertically displaced pinhole ( fig. 4b, left) for

the mirror position giving no detectable tip (5th from the right), the off-axis temporal profile is found

to be considerably different from that on axis. However, if the process is now repeated for mirror

rotations about the horizontal axis, with 4 sec. rotations being made until the spot is no longer elongated but round as for the far right mirror position of fig. 5a, then the same temporal profile is found at both pinholes.

We conclude that a genuinely round output from the Q-switched oscillator implies uniform temporal

behavior over the whole beam. Typically the preparations for a series of damage tests included an examination of the oscillator output, but it was not always done with the thoroughness demonstrated in figs. 4 and 5. Consequently, it is quite possible that some of the measurements to be described in sections 4. and 5. were performed with pulses whose off-axis temporal behavior was less than ideal. The effect of small temporal deviations off-axis on the results of our damage tests is assumed to be

negligible.

In fig. 6 the spacially integrated temporal pulse profile as recorded by the monitoring apparatus

in fig. 2 has been compared for three randomly selected pulses out of a series of some 220 shots. Pulse no. 217 is indicated by a solid curve and points measured from the scope traces of shots 11 (O)

and 89 (Q) are plotted on a normalized scale which forces the pulses to coincide at their peak power

points. The fact that all three pulses fall on essentially the same curve demonstrates the high degree

of repeatability obtained in the temporal shape of the pulse.

b. Spacial Profile

Figure 7a shows two examples of the Q-switched pulse's radial energy distribution in the damage

plane as photographed by the imaging system described in section 2. 1. Each pulse produces a horizontal line of spots in which adjacent exposures differ by a factor of two. The microdensitometer

traces of three consecutive shots shown in fig. 7b verify the impression gained by visual examination

of the photographs that the energy density varies smoothly with radius. Furthermore, we have

determined by constructing energy density versus radius curves from the densitometer traces [6] that the radial dependence of the energy distribution is Gaussian.

3. 2 Mode-Locked System

a. Temporal Characteristics

The mode-locked oscillator produces a train of short pulses separated by approximatley 7 nsec. from which a single pulse is selected for amplification. In fig. 8 oscilloscope traces of four consecutive shots show the mode -locked train with the selected pulse missing and the amplified selected

pulse displayed between the first two recorded pulses of the train. The extra little bump appearing

just before the amplifier pulse in the bottom three traces is a stray reflection onto the diode and may

be ignored.

The duration of the mode-locked pulses generated by the oscillator has been estimated from timeintegrated two-photon fluorescence photographs to be approximately 20 psec. To check for the possibility that the pulse duration changes during amplification or that pulses from different parts of the pulse train have different durations, two-photon measurements of single amplifier pulses are planned,

but have not yet been accomplished. Even without such measurements, however, it is known from

the impulse response of the diode-oscilloscope combination that the pulse duration of the amplified

pulses is less than 100 psec. For the case of pulses selected from properly locked trains, the amplified pulse always gives the same impulse shape scope response as the oscillator output.

110
b. Spacial Profile

The examples of focused radial energy density profiles for single mode-locked pulses shown in

ig. 9 are similar to those shown for Q-switched pulses in fig. 7. Since the pronounced spike appearing on the left side of the first exposure in the bottom trace does not appear in the adjacent

exposure, it is not an intensity spike in the beam and must be identified as a spot or a scratch on the

film. Construction of energy density versus radius curves from these densitometer traces reveals some small departures from a true Gaussian profile, however the errors in damage threshold resulting

from treating the profile as Gaussian are small compared to the uncertainty already existing in total energy measurement.

4. Mirror Damage Experiments

Using the lasers and monitoring techniques described above, experiments have been conducted on

nine mirrors from five manufacturers. Basic information about the mirrors is given in table 1.

Table 1. Mirrors used for damage tests

Designation

Coating

Composition

No. of layers

Reflectivity

Density of scattering centers

A-1

A-4

A-6

C-1

D-1

Ti02 /Si0

2

it . j

ii

1

1

9

7

79

18

94. 3%

92. 5%

92. 5%

94. 3%

96. 8%

Moderate

Low

Low

Low

Low

C-4

C-7

E-1

Zr02/Si02

1

1

1

13

13 25

95. 6%

96. 0%

95. 7%

High

Low

High

B-l ZnS/ThF 21 96. 2% Moderate

4. 1 Observable Features Of The Damage Sites

When viewed under an optical microscope at a magnification of thirty, the most frequently

observed form of damage has the appearance of one or more bright rings having the same shape as

the beam's radial energy density profile. Sometimes one or more randomly located bright spots are observed within this overall pattern. Examination of such sites in a scanning electron microscope

reveals considerably more detail. In figs. 10 and 11 the ring appearance is seen to be caused by a step inside of which one or more of the quarter wave layers have been removed. Since the sample is viewed from an angle in a scanning electron microscope, the round damage site appears to be compressed along the vertical axis. fig. 12 is a close-up view of the bright spot seen below and to the

left of center in fig. 10. Since the position of this spot does not correspond with the position of maximum intensity in the beam, it is probably the result of a local absorbing center or other small

scale mirror defect. The photographs just described show damage sites caused by Q-switched pulses, but the physical

characteristics of sites damaged by single mode-locked pulses are similar. In figs. 13, 14, and 15, for example, the same overall pattern of well defined rings with a randomly located little crater is observed. In figs. 16 and 17 a tendancy toward smaller damage sites and removal of fewer layers is observed for less energetic pulses, fig. 16 also illustrates the fact that the transition from one layer

to the next is not always a well defined step. Since the energy density profile of this pulse in the plane of the mirror is observed to be a smooth function of radius on the multiple exposure-level

photograph taken during the experiment, the irregular pattern of material removal in the central

region of this site suggests a degree of inhomogeneity in the coating itself.

Ill
In contrast to the damage site characteristics described above, the affected areas on some mirrors

appear under the optical microscope to be made up of a large number of closely packed scattering

sites. Two examples of this type of damage are given in the lower right portion of fig. 18. This

mirror (E-l) has a generally high density of scattering centers even in areas not exposed to laser

radiation, but it is not yet established whether they contribute to the characteristics of the damage

sites themselves. The sites just described are produced by 20 psec. pulses and the ring type sites on the same mirror in the upper half of the figure are produced by 20 nsec. pulses. For mirror B-l

just the opposite is found: 20 psec. pulses produce rings and 20 nsec. pulses produce scattering centers

4. 2 Fogging Effects

Even when examination under the optical microscope reveals no obvious damage on a mirror,

fogging the coating with one's breath often reveals clear evidence that the radiation has altered the surface in some way. The microscope photos a and b in fig. 19 illustrate this point. A striking

feature of this effect is that an area much larger than the otherwise detectable damage region is af- fected. In c and d of fig. 19, for example, a region many times larger than the beam itself has differ- ent fogging properties. To some extent the effect is temporary as evidenced by the difference in appear- ance of the 20 hour old sites and the 1 hour old sites.

4. 3 Data Reduction

For each set of Q-switched damage measurements randomly selected pulses are quantitatively

checked to verify the Gaussian radial dependence of the energy density and the reproducibility of the temporal pulse shape. These two important properties are also checked qualitatively for each shot

by visual examination of the monitoring photographs. Any shots showing irregularities of either temporal or spacial profile are rejected. The Gaussian radius is measured for each remaining shot and

combined with the pulse energy measurement to give a value of energy density on axis for each pulse.

As will be shown, the diameter of each damage site is also of interest, and it is measured from optical

microscope photographs of the damaged coatings. Data reduction for the mode-locked pulse experiments is not quite so straightforward. Complications arise because of the limitations placed on energy measuring devices at this pulse duration. The

calorimeter, for example, reads 100 mj full scale on the most sensitive scale and its thermal sensitivity of 3J/°C makes compensation for ambient temperature changes very difficult. This leads to background variations which in the worst cases are on the order of the several mj pulse energies being

measured. The radiometer, which monitors a small fraction of the pulse energy on each shot, must

also be used on its most sensitive scale (0. 35 mj full scale) where background noise is a problem. The

seemingly expedient solution of putting a larger percentage of the laser output into the radiometer is risky since at such short pulse durations very little energy is required to cause saturation effects in the photodiode invalidating its calibration.

To circumvent these difficulties a series of shots is fired with no sample in place so that simul- ! taneous calorimeter and radiometer readings as well as 519 scope traces are obtained. In a plot of calorimeter readings versus radiometer readings, the noise in both energy detectors scatters the points

about a best straight line relating the absolute calibration of the two devices. It is found that the radiometer predicts 1. 18 times the energy arriving at the sample location as that predicted by the calorimeter. Since neither measurement is known to be intrinsically more accurate than the other, the average of 0. 924 times the radiometer reading is taken as the energy of a given pulse. Plotting the radiometer reading versus the height of the amplified pulse in the 519 scope trace provides a similar

calibration curve of pulse energy versus 519 pulse height. The noise on the scope trace is typically

much less than the noise on the radiometer; so the pulse height on the 519 scope makes a more consistent indicator of amplified pulse energy and when calibrated as described above, it is used as the primary energy measuring instrument. Radiometer readings are still taken throughout the run, how- j ever, to provide a constant check on the calibration curve. An obvious difference between the Q-switched and mode -locked damage measurements is that no

detailed analysis can be made of the temporal profile of the 20 psec. pulse. Any pulses longer than

about 100 psec. can be rejected on the basis of the 519 trace, however, as can shots giving a double

pulse or a poorly locked train.

112
4. 4 Variation of Damage Site Diameter With Energy Density

A convenient way to display the mirror damage data is on a plot of damage site diameter versus

the pulse energy density on axis. Fig. 20 shows such a plot for the Q-switch pulse damage tests on mirror A-4. For low enough energy densities no evidence of damage is found (O)- For energy densities above 30 or 35 j/cm some randomly located small scattering centers are created, but there is no well defined pattern to which a diameter can be assigned (A ). Finally for energy densities above

about 45 J/cm^ well defined damage sites develop with diameters which are larger for more energetic

pulses.

From such data two kinds of thresholds can be defined, the "practical damage threshold" below

which no damage is caused by a single shot and a higher idealized threshold, a "perfect coating threshold", below which damage seems to be associated with local flaws of some sort rather than with

inherent coating limitations. In fig. 20 the curve through the data is drawn to intersect the density

axis at a point corresponding to the practical threshold. A curve drawn through the perfect coating

threshold would intersect the axis at about 45 J/cm^ and rise more steeply to pass through the remaining data points.

The simplest assumption which predicts an increase of site diameter with energy density on axis is that a coating damages only out to the radius at which the energy density drops below some critical value. The Gaussian energy distribution function can be solved for the diameter at which a given

critical energy density is reached as a function of the on-axis energy density. The resulting curve is included in fig. 20 for a threshold value of 30 j/cm^. This curve does pass through two of the measurable - site data points but falls well below the data at higher energies. A similar curve for a threshold

of 45 j/cm^ would completely miss the data. Plots of measured site diameter versus Q-switched on-axis energy density for six of the nine mirrors

are shown (without data points) in fig. 21. The curves are drawn to cross the axis at the practical

damage threshold, and in general the perfect coating threshold is higher as seen in table 2. All of these curves show damage sites which are larger at high energy than predicted by the simple constant energy density assumption discussed above.

Table 2. Measured threshold values

Mirror a Threshold at 20 psec. Threshold at Practical

23 nsec. Perfect coating

2 J/cm on axis2 J/ cm on axis T/ 2 J7 cm on axis

A-l 1. 6 43 46

A-4 2. 2 30 46

A-6 26 C-l 1. 8 30 30 D-l 1. 6 33 38

C-4 1. 9 23 31 C-7 1. 8 58 60

E-l 2. 9 47 47

B-l 0. 6 16 24

a. For the mode-locked pulse the beam diameter at half the on-axis energy

density is 197 pm.

b. For the Q-switched pulse the beam diameter at half the on-axis energy

density is 130 jj,m.

Plotting the data from damage tests with single mode-locked pulses gives similar curves. There

are some notable differences, however. First, all of the detected damage sites have measurable

diameters, eliminating the distinction between the two threshold definitions discussed above. Second,

113
a sharp rise in the curve immediately above threshold is clearly established by the data for most of the mirrors. Finally, for all but two of the mirrors, the damage sites are smaller at high energy than

predicted by the constant energy density assumption. The first two differences are evident in fig. 22,

and the comparison^with a constant energy density model may be made by sketching a curve through

0. 0mm at 1.6 j/cm , 0. 2 mm at 3 J/cm , and 0. 3mm at 8 J/cm^. This gives the diameter at which

the local energy density is 1. 6 J/cm^.

Plots of measured site diameter versus the on-axis energy density of the 20 psec. pulse for seven

of the mirrors are shown (without data points) in fig. 23. Mirrors B-l and C-7 (not plotted) are exceptions in that they do fit a constant energy density curve quite well. Table 2 summarizes the results.

4. 5 Dependence Of Damage Threshold On Beam Diameter

To determine whether the observed damage threshold for 20 psec. pulses measured with a 197 (jm

beam diameter can be meaningfully compared with the threshold for 23 nsec. pulses measured with a

130 jim beam diameter, it is necessary to perform damage tests over a range of beam diameters at a fixed pulse duration. The results of such an investigation for mirror A-6 with 23 nsec. pulses are shown in fig. 24. As in fig. 20 two kinds of thresholds are evident, at least for the large diameter

beams, and so two curves are drawn, the solid curve for the perfect coating threshold and the dashed

curve for the practical threshold. These curves are simple fits to the data and do not represent any

theoretically derived functional form. The decrease between 130 and 197 jj,m should be taken into account when comparing the mode-locked and Q-switched data.

4. 6 Pulse Duration Dependence Of Mirror Damage Thresholds

Even allowing for possible 25% reductions of the Q-switched damage thresholds in table 2 if the measurements had been made with a 197 jxm beam diameter as the 20 psec. measurements were, the 23 nsec. values are from ten to thirty times higher than the 20 psec. energy density thresholds. Figure

25 illustrates this general observation with the results from mirrors A-l and A-4. In this plot no adjustment for beam diameter dependence has been made since the size of such a correction could

only be inferred from measurements on a different mirror. It is likely, however, that the Q-switched

values should be recorded at a somewhat lower energy density. In this figure the functional form of the curves is suggested by theoretical considerations discussed in section 5.4.

4. 7 Accuracy And Reproducibility

a. Experimental Errors

For the Q-switched pulse data random errors in energy measurement and beam size determination can lead to uncertainties of plus or minus 10% in on-axis energy density measurements for single data

points. As seen in ig. 20, however, either type of threshold can be determined to within about 5%

for some of the mirrors.

For the mode-locked experiments, the uncertainties are somewhat larger because energy measure- ment is more difficult and there are small deviations from a true Gaussian energy density profile.

Random errors in on-axis energy density for a single pulse may be as high as plus or minus 25% in this case. Again, however, threshold values are generally more accurately determined, as in fig. 22 where a reasonable estimate of uncertainty in threshold determination is plus or minus 10%.

In addition to the types of random error noted above, systematic error due to miscalibration of the

j energy measuring instruments must be expected. Comparison of the calorimeter and radiometer

measurements as described in section 2. 2 suggests that this error may be up to but probably does not exceed 20% for a given radiometer or calorimeter of the type used in this work.

I

b. Mirror Variations 'j

It is important to observe that ordering ten mirrors coated with specified dielectric materials from

a particular manufacturer does not guarantee that ten identical mirrors will be delivered. The thresh-^

olds recorded in table 2 for mirrors A-l, A-4, and A-6 and C-4 and C-7 illustrate the point. Decision',

concerning the relative capabilities of several manufacturers should not be based on the performance

of a single mirror.

114
5. Possible Damage Mechanisms

5. 1 Linear Absorption

Sufficient linear absorption in a coating can heat the film to its melting point. However, the thermal

time constants for conduction losses from the irradiated spot on the film, either radially or into the substrate, are expected to be long compared to both pulse durations. Therefore a linear absorption

model predicts the same energy density threshold over the whole pulse duration range and over the range of beam diameters tested as well. Neither prediction agrees with the experimental results. An

additional observation is that the damage looks more like cracking than melting. Of course, thermal

expansion resulting from linear absorption heating can cause fractures too, but the dependence on pulse

duration and beam diameter is still wrong.

5. 2 Local Defects

There is no doubt that small scale local defects can account for some of the damage features

reported here. Figures 12 and 14 are examples of pits believed to be caused by the explosion of small

absorption centers. It is likely that for mirrors exhibiting a large difference between the practical

damage threshold and the perfect coating threshold, small absorbing centers are causing the low

level damage. Possible defects include dirt, incompletely oxidized coating material, or some other

form of non-uniform deposition. The decrease of practical threshold with increasing beam diameter

may be related to the increased likelihood of finding a defect with a large beam.

5. 3 Surface Plasma

On most shots which cause detectable damage a spark can be seen at the damage site. When a spark is not seen on damaging shots, the damage is small and the spark may just be too weak to detect

by an observer looking through protective goggles. In any event, we believe the spark accompanies the principal damage event [7 ] rather than causing it, although the large area which is seen to have been

affected when the coating is fogged may be an indication of some secondary plasma effect [8 ] .

5. 4 Electron Avalanche

Two conditions must be satisfied to cause damage by electron avalanche [ 3 ] . First, the intensity

must be high enough that conduction electrons gain energy from the laser radiation faster than they

lose it to the lattice. Second, the laser energy density must be high enough to create a critical number

of conduction electrons within the duration of the pulse. Therefore, for very short duration pulses

the limiting quantity is energy density while for long pulses it is intensity. The straight line segments

of the curves in fig. 25 are drawn through the data points with the appropriate pulse duration dependence

to give constant energy density and constant intensity thresholds.

If this mechanism is operating for the 20 psec. pulse damage, the intensity criterion is exceeded by

a factor of about fifty, meaning that the electrons gain energy from the field much faster than they can transfer it to the lattice leaving many of the electrons to lose their energy to the lattice after the laser pulse has ceased. While they are returning to equilibrium with the lattice, however, there may

be some tendancy for electrons to drift away from the most intensely irradiated region and deposit their

energy beyond the radius where damage would be expected on the basis of a constant energy density

threshold. This does not lead to a larger damage site than expected, because the energy deposited at larger radii is not sufficient to cause damage. Rather, the fact that this energy was lost from the expected damage region will lead to a smaller than predicted damage site, as is observed for the modelocked measurements for all but two mirrors.

If electron avalanche is causing the 20 nsec. pulse damage, intensity is the important variable, and

the number of electrons grows more slowly since the field is smaller. As with the shorter pulse,

electrons tend to drift to larger radii before they reach equilibrium, but in this case the laser pulse is still hitting the sample, and the presence of electrons at large radii makes damage there easier than

expected. As a result the damage site is larger than predicted on the basis of a local energy density

threshold. The dependence of threshold on beam diameter is qualitatively what is predicted by a probability formulation of electron avalanche theory [9 ] . namely that the probability for finding "lucky

115
electrons" increases and the intensity needed to achieve a given probability for damage decreases for larger beams.

Some of the arguments in section 5. 4 are admittedly speculative . Information about electron decay

times and reasonable diffusion distances in thin dielectric films would help put the discussion on firmer

ground.

6. Acknowledgment

The authors thank R. A. Bradbury for suggesting and obtaining the scanning electron microscope

photographs used in this study. Technical and analytical assistance from E. E. Hoell, H. Miller, R. A. Clark, and C. C. Gallagher is also gratefully acknowledged.

7. References

Ll] Bliss, E. S. and Milam, D. , AFCRL Report

No. 72-0233 (1972). Available from the

Defense Documentation Center, the National

Technical Information Center, or the authors.

[2] Milam, D. , IEEE J. Quantum Electron.

QE-7, 319 (1971).

[3] Bliss, E. S. , Proc. 2nd ASTM Symp. Damage

in Laser Materials, NBS Spec. Pub. 341,105

(1970); also Optoelectronics 3, 99 (1971).

[4] Burnham, D. C. , Applied Optics 9, 1482

(1970).

[5] DeShazer, L. G. and Parks, J. H. , Proc.

3rd ASTM Symp. Damage in Laser Materials,

NBS Spec. Pub. 356, 124 (1971).

[6] Winer, I. M. , Applied Optics 5, 1437 (1966).

[7] Giuliano, C. R. , IEEE J. Quantum Electron. , to be published (1972); also elsewhere in these proceedings (1972).

[8] Boling, N. L. , elsewhere in these proceedings

(1972).

[9] Bass, M. and Barrett, H. H. , elsewhere in these proceedings (1972).

8. Figures

Q- SWITCHED LASER SYSTEM

OSCILLATOR AMPLIFIER MONITORING APPARATUS

MODE- LOCKED LASER SYSTEM

OSCILLATOR PULSE SELECTOR

AMPLIFIERS

MONITORING APPARATUS

.ocusing ""G ' N0

LENS SAMP<-£ LEN5

MULTIPLE EXPOSURE LEVEL CAMERA

Figure 1. Block diagrams of the laser systems. Figure 2. Monitoring apparatus for damage

experiments with Q-switched pulses.

116
Figure 3. Monitoring apparatus for damage experi- ments with sub-nanosecond pulses.

Figure 4. (a) Multiple exposure-level photographs

of the oscillator output for a range of mirror ro- tations about the vertical axis. The output mirror

was rotated by 4 sec of arc between successive exposures, (b) Temporal profile detected through a pinhole on axis (right) and through a vertically

displaced pinhole (left) for the shot producing the fifth exposure from the right in (a)

.

Figure 5. (a) Multiple exposure-level photographs

of the oscillator output for a range of mirror ro- tations about the horizontal axis. The output mir- ror was rotated by 4 sec of arc between successive

exposures. (b) Temporal profile detected through

a pinhole on axis (right) and a vertically displaced pinhole (left) for the shot producing the

far right exposure in (a)

.

Nanoseconds Figure 6. Spacially integrated temporal profile of three Q-switched pulses from a series of 220 shots: (O) shot 11, (0) shot 89, (—) shot 217.

117
Figure 7. (a) Multiple exposure-level photographs

showing the radial energy distribution in the damage plane for two Q-switched pulses. The dimension

of the scale marker is 100 ym. (b) Microdensitometer traces of multiple exposure-level photographs

for three consecutive Q-switched pulses. Adjacent

exposures on each trace differ by a factor of two.

Figure 8. Oscilloscope traces for four consecutive firings of the mode-locked ruby laser. The selected pulse is missing from the train just to the right of center, and after amplification it

is displayed between two early pulses of the train at the left.

Figure 9. (a) Multiple exposure-level photographs

showing the radial energy distribution in the damage

plane for three 20 psec pulses. The dimension of the scale marker is 1 mm. (b) Microdensitometer

traces of multiple exposure-level photographs for three consecutive 20 psec pulses. Adjacent exposures on each trace differ by a factor of 1.83.

Figure 10. Scanning electron microscope (SEM)

photograph of a damage site produced by a Q-switched

pulse. The site dimension is 250 ym from left to right.

118
Figure 11. SEM photograph of the edge of a damage

site. The entire photo is 57 ym wide.

Figure 12. SEM photograph of the bright spot seen below and to the left of center in figure 10. The width of the photo is 11 ym.

119
Figure 16. SEM photograph of a site damaged by a less energetic 20 psec pulse. An irregular pattern of material removal is evident in the central

portion. The width of the photo is 456 ym.

Figure 15. SEM photograph of the bright spot to the right of center in figure 13. The width of the photo is 18 ym.

Figure 17. SEM photograph of a damage site produced by a 20 psec pulse having an energy density

only slightly above threshold. The width of the* photo is 98 ym.

Figure 18. Optical microscope photograph of scattering site type of damage (lower row) caused by

20 psec pulse on mirror E-l, for which Q-switched

pulses produced mostly ring type damage (upper row)

.

The spacing between the two sites in the right side

of the lower row is 1 mm.

120
INTENSITY ON AXIS ( GW/cm'l

.5 10 1.5 2 0

Figure 19. Optical microscope photographs of (a) a damaged unfogged mirror, (b) the same mirror

after fogging with breath, (c) higher magnification

view of some sites which were damaged twenty hours

before the fogged photograph and (d) one hour before the photograph.

20 Ml 60 ENERGY DENSITY on AXIS ( J / cm

2

I

Figure 20. Damage site diameter versus Q-switched

pulse energy density on axis for mirror A-4. Shots causing no detectable damage are indicated by (O)

those causing only randomly located small scattering

centers by (A) , and sites with well-defined dia- meters by (O) at finite diameter values. A curve

is drawn through the data points to intersect the axis at the practical damage threshold. The second

curve gives the damage site diameter predicted on

the assumption that any place on the coating will

damage when subjected to an energy density of

30 J/cm2

.

INTENSITY ON AXIS IGW/cm'l 10 1.5 2 0

20 ( 10 60 80

F NTRGY DENSITY on AXIS I J ' cm ) ENERGY DENSITY on AXIS I J I cm I

Figure 21. Damage site diameter versus Q-switched

pulse energy density on axis for six mirrors.

Curves with faded labels may be identified by re- ference to the practical threshold values recorded

in table 2.

Figure 22. Damage site diameter versus 20 pec pulse energy density on axis for mirror A-l. Shots

causing no detectable damage are indicated by (X)

and those causing damage by (Q) . The error bars

shown are typical.

121
ENERGY DENSITY on AXIS

Figure 23. Damage site diameter versus 20 psec pulse energy density on axis for seven mirrors

.

LASER SPOT SIZE

.3

< mm } Figure 24. Q-switched damage threshold versus the beam diameter at half the on-axis energy density

for mirror A-6 . Shots causing no detectable damage are indicated by (O) , those causing only ran- domly located small scattering centers by (A),

and sites with well-defined diameters by (iQ) . The 1 solid curve indicates the best estimate for the variation of the perfect coating threshold, and the dashed curve the best estimate for the variation ofl the practical threshold.

PULSE DURATION (Seconds)

Figure 25. Single pulse damage threshold versus pulse duration for mirrors A-l and A-4. The curves are drawn to fit the functional form suggested by theoretical considerations discussed in section 5.4.

122
Study of Laser-Irradiated Thin Films

Brian E. Newnam and L. G. DeShazer

Departments of Physics and Electrical Engineering

University of Southern California

Los Angeles, California 90024

Laser damage to thin-film coatings was studied using a TEM Q-switched ruby

laser. Our study included mono-, bl- and multi-layered coatings of the materials

Ti0

2

, Si02, Zr0

2

, MgF2 and ZnS on substrates of glass, rocksalt and spinel. The

samples included coatings for antireflection and reflection at the ruby wavelength,

and were produced either at USC or by a commercial vendor. Damage threshold energy

and power densities were measured with attention paid to dependences on laser beam

spot-size, film material and thickness, substrate condition and incipient scattering

of the film. Particular attention was given to the establishment of a threshold

criterion for laser damage to thin films. Several effects attendant to damage were

investigated, with the conclusion that film scattering induced by the laser was the most sensitive detector of film breakup. The laser induced scatter was observed

prior to or at the threshold of spark formation at the film, depending on the film

material

.

Key Words: Damage morphology, damage thresholds, laser-induced scatter,

ruby, spark thresholds, thin films, truncated Gaussian optics.

1. Introduction

During the past year we have been investigating laser-induced structural damage to dielectric thin

films using a TEMQ0 Q-switched ruby laser. The investigation concentrated on the measurement of damage

threshold energy and power densities studying their dependence on film, substrate and laser parameters.

The coatings tested were mono-, bi- and multi-layered films composed of various materials and thicknesses

having differing initial film scatter. These coatings were on substrates of glass', rocksalt, spinel and YAG and were damaged while immersed in various liquids as well as air. The damage was studied as a function of laser beam spot-size, temporal pulsewidth and incidence direction. Also, several effects

accompanying damage were investigated, with the conclusion that film scattering induced by the laser was

the most sensitive indicator of film breakup. In addition, the damage thresholds, as determined from the laser-induced scatter, were correlated to the spark formation at the film, and the thin-film areas damaged

at laser powers above and below the film's spark threshold were studied by optical microscopy.

In this report the study of the dependence of damage on the film parameters will be emphasized,

leaving a fuller discussion of the laser and substrate dependences to a later time. Also, since the research effort up to the present time has been chiefly experimental, we will avoid speculating in theoretical models of damage mechanisms.

2. Experimental Configuration

A high-quality giant-pulse ruby oscillator-amplifier system, shown in figure 1, was the source of

the laser beam used in the damage studies. This laser was Q-switched by cryptocyanine in methanol and operated in a single longitudinal and transverse TEM mode. The single mode operation was important

because a temporally and spatially smooth pulse is necessary for an accurate measurement of the damage

thresholds. The time contour of the pulse was nearly Gaussian with pulsewldths (FWHM) from 9 to 35 nsec,

and peak powers up to 10 MW when using the amplifier. For such Gaussian pulses, the peak power PD

is related to the total energy E by P

Q = (0.941E)/T, where T is the full width of the pulse at half maximum.

Absolute energy calibration was performed using a ballistic thermopile (TRG 100) and agreed to within 5%

of the calibration of another thermopile used by C. R. Giuliano at Hughes Research Laboratories. In the

far field of the laser oscillator, the spatial intensity profile of the beam was close to a Gaussian

123
distribution as determined by a pinhole scan. At the distance of 100 cm (1.1 Rayleigh distance) from

the oscillator, the intensity distribution at the temporal peak of the pulse is shown in figure 2. The amplified profile also had a similar near-Gaussian shape. The detailed operation of this apparatus was discussed at the Third ASTM-NBS Laser Damage Symposium [1].^

3. Truncated Gaussian Optics

The spatial intensity distribution of the laser beam cannot be exactly Gaussian since the single

mode beam is coupled out of the laser through a circular aperture, resulting in the truncation of the aperture intensity illumination. This truncation produces rings in the far-field pattern and structure within the central disk in the near-field. Figure 3 shows the diffraction pattern of our laser in the far-field with three different exposures. The central disk is nearly Gaussian and in the overexposed

pictures, the presence of the rings around the central disk due to truncation is strikingly evident.

Now, the importance of recognizing the existence of truncation effects lies in the fact that if attention

is not paid to these effects, calculations of the axial intensity may be in error by as much as 1007o . Such discrepancies are particularly possible when truncated Gaussian beams are focused.

When a laser beam is focused by a lens the focused intensities can be simply predicted [2] only if the spatial profile of the beam has the ideal Gaussian distribution. Since an exactly Gaussian beam

remains Gaussian throughout an optical system, only the calculation of the spot-size of the Gaussian

beam with respect to the lens is required. The spot-size is the 1/e^ radius of the intensity profile of

a beam. The variation of the spot-size as the beam propagates along the axis of the lens is controlled

by the spot-size WQ at the waist of the input beam and the distance d-^ of the input waist from the lens ai well as the geometric focal length F of the lens. The geometry of the input beam to the lens is illustrated in figure 4, and the value of the spot-size W„ at a distance d. from the lens is determined from

eq (1).

The parameter b is defined as rrW0 A where X is the laser wavelength. This eq (1) is the general

expression giving as a function of W , d^ and F, and can be used to determine the peak power and energy densities anywhere in a focused Seam provided the beam had an ideal Gaussian profile. Many ex- perimenters using single transverse-mode lasers generally assume that this Gaussian description of focused beams applies to their laser setup. Truncation of the Gaussian beam can severely alter this calculation, though, and prevent a proper analysis of an experiment.

Truncation of the Gaussian distribution, which is infinite in extent, can never, of course, be avoided but it is generally unimportant in low-power lasers. In high-power Q-switched lasers, though,

due to their resonator design requirements the truncation is usually not negligible. Figure 5 shows several possible intensity distributions in the aperture plane of a laser oscillator having a Fresnel

number of 0.4, the Fresnel number of our oscillator. The profile for the fundamental stationary (Fox-Li)

mode is a truncated Gaussian having a truncation parameter of 0.84. The truncation parameter is the ratio of the radius of the truncating aperture a to the spot-size W. This Fox-Li profile is flattened

by saturation of the gain [3], which usually occurs in high-power pulsed lasers. Three examples are shown for initial numerical gains of 20, 12.2 and 7.4, all calculated assuming a uniform initial gain

distribution. When the initial gain distribution is peaked symmetrically about the resonator axis due

to the design of a particular pumping scheme the Fox-Li profile is sharpened, increasing the truncation

parameter. A typical example of axisymmetric pumping is the elliptical focusing of pump light into a cylindrical laser rod. For the laser oscillator used in our investigations, the truncation parameter

was near 0.6.

Since Campbell and DeShazer [4]have shown that diffraction effects due to trancation cannot be neglected when the truncation parameter is less than 2.0, the usual Gaussian description of focused beams does not apply to our laser experiment. Therefore, a calculation of focused truncated Gaussian

beams is required for the analysis. A formula has been published [5] for the intensity variation along

the optical axis for a special case of a truncated Gaussian beam where the lens is also the aperturing

element. U.O. Farrukh [6] has calculated the diffraction effects in focused truncated Gaussian beams 4

Figures in brackets indicate the literature references at the end of this paper.

124
for the more usual situation shown in figure 6. The source laser is represented by a collimated ideal

Gaussian beam of spot-size W apertured by a diverging lens of geometric focal length fi and radius a. A

lens of geometric focal length £2 i s placed a distance z from the lens f-, and the beam is viewed a dis- tance y away from the lens. The axial intensity I at the distance y is determined from eq (2).

Even though this is a rather complex expression, the axial intensities can be easily plotted by

computer and the plots have predicted the experimental results.

In our experiment, a lens of 20.7 cm focal length was located 109.5 cm from the laser. Since the equivalent Gaussian and truncated Gaussian calculations of the axial intensity differ greatly near the focus, the axial intensity was directly measured and compared to the two calculations. The axial intensity of such small spots can be accurately determined by measurements of the damage threshold of Polaroid

film. A microscope is required to examine the film. Fresh undeveloped Polaroid film (Type 410) was calibrated to have a damage threshold at 50 mj/cm^, which was independent of the laser beam spot-size.

Figure 7 shows the measured axial intensity distribution after the lens and its comparison to both the equivalent Gaussian and truncated Gaussian laser beam calculations. The measured intensity value near

the focus is about twice that predicted by the equivalent Gaussian calculation. The difference between

the measured distribution and the truncated Gaussian calculation is not as large as indicated because

the actual truncation parameter is near 0.6 and not the 0.5 as plotted.

Observation of the intensity profile of a truncated Gaussian beam after the focus reveals axial

maxima and minima, unlike the smooth distribution before the focus. This is explained by recognizing

that near-field axial extrema are present before the lens, which when the lens is used in the far-field

(z > 2a^A), are transformed to locations after the focus. Figure 8 illustrates that the intensity

distribution is not symmetrical about the focal plane for truncation parameters less than 2. Therefore,

it is quite important to place the test samples before the focus in order to know the laser power densities incident to the samples.

4. Laser Damage Detection

One of the important aspects of these current investigations has been the determination of a sensitive and reliable criterion for the onset of damage. Many investigators of laser-induced damage to sur- faces of transparent dielectrics have observed a luminous surface spark whenever physical damage occurred.

Some researchers have proposed that the spark is responsible for the observed damage. It has thus become

a convenient practice to identify the spark threshold as the damage threshold. We have used three methods

to monitor the onset of damage:

1) spark detection using a sensitive photomultiplier with a narrow band interference

filter (4358 A)

>

2) laser-induced increase in the scattering of lmW He-Ne laser beam (visually observed)

,

and ,

3) optical microscopic observation of film breakup.

The interesting result which we wish to report is, that for monolayer films, the laser-induced

scatter (LIS) of a He-Ne gas laser indicated that damage occurs before or at the detected spark threshold, depending on the film material. Multilayer coatings had an initial scatter level that was too high

to obtain thresholds with the LIS technique. If the initial scatter levels were low enough, we would

expect the same result for multilayer films also. The materials investigated, the energy density thresholds and the spark-to-LIS ratios are listed in table 1. It appears that laser-induced scatter occurs before spark initiation for films with high indices of refraction.

125
The film damage for all the tested samples was investigated by optical microscopy, and a certain

pattern in the damage morphology was observed. The damage areas below spark threshold were much more irregular in shape than those considerable above spark threshold. Far above threshold the damage was circular, nearly following the intensity profile of the incident laser beam (figure 9). However, below

threshold many small circular damage spots occurred in random clusters (figure 10) which may indicate

localized points of absorption, optical breakdown or film inhomogeneities.

The damage in single and double-layer films was sufficient to remove the entire coating from the substrate. For multilayer reflectors the damage did not penetrate to the substrate. This is expected

since the laser energy density decreases progressively due to the reflections at the multiple interfaces.

At threshold only the top few layers were damaged.

5. Monolayer Dielectric Coatings

The damage thresholds for quarter-wave films of various materials are listed in table 1. The

thresholds were determined by the onset of LIS using a He-Ne laser as a monitor of the scatter. In this paper, the thresholds are given as a range of values instead of a single number. This method of listing

thresholds reflects more accurately the uncertainties in the threshold measurement. Ten to thirty laser shots were taken for each entry of a range in the tables. The damage, as observed by a certain film

characteristic, is more or less identical within the reported range of values. Occasionally, a laser

shot having an energy density higher than the determined threshold range will produce little or no damage;

such shots were not included in the tables. It can be noted from table 1 that the damage thresholds were nearly inversely proportional to the refractive index of the film material. One possible explanation of

the variation of damage with material involves the standing wave pattern of the electric field by the re- I flection at the film interfaces (figure 11). For most light-matter interactions, the absorption of energy is required. In terms of the electric field intensities the power absorbed per unit volume

P

a

= an |E/E0 |^I 0 where a = 4tm A ; n and K are the real and imaginary parts of the film's refractive

index and IQ is the incident intensity (watts/cm ). Figure 11 illustrates that the absorption in a quarter-wave film is spatially dependent as shown by the variations of |E/E+

|^. By examining the field

variations in films of various thicknesses and for obverse and reverse laser incidence, and by correlating

these with the respective damage thresholds, a determination of the appropriate causes of damage may be possible.

Table 2 gives the damage thresholds for various film thicknesses. In particular, note the variation

of the thresholds for the three Ti02 films. The damage threshold of the half-wave film was about onethird that of the quarter- and three-quarter-wave films. Figure 12 shows the calculated relative intensity distribution for these three film thicknesses of Ti02« For a high index film material, such as T102,

between two lower index materials of air and glass, the net electric field has an antinode at the rear or glass-film interface. This rear reflected wave has an antinode (node) at the front or air-film interface

in the case of a half- (quarter-) wave film. The higher net electric field at the front surface of the half-wave film may be the origin of the lower damage threshold of the half-wave film. It is interesting

to note that the ratio of the intensities for the half- and quarter-wave films at the air-film interface

is about three.

6. Multilayer Dielectric Coatings

We have studied damage to commercial antiref lection coatings and reflectors. The antireflection

coatings investigated were two-layer, non-quarter-wave films called "vee" coatings [7]. The damage

thresholds measured for these coatings are listed in table 3. Like single layer films, LIS is the most

sensitive method of detecting damage. For multilayer reflecting films, for which the initial He-Ne

scatter level is quite high, spark detection is more sensitive. Table 4 lists the damage thresholds for sixteen reflectors. Samples 0103 and 0104 have the highest damage thresholds of any reflector tested in the program so far; both of these reflectors have spark threshold values near 120 J/cm^.

Another recent aspect of this study has been the investigation of a correlation between scattering

(before damage) from thin-film coatings and their damage thresholds. The results from this experiment

are shown in table 5. The scattering was measured by the intensity of light scattered at about 135° from

the direction of the incident beam. Since there is a strong wavelength dependence for the diffuse re- flectance from a dielectric film, we used a severely attenuated output from the single-mode Q-switched

ruby laser as the measuring beam. This beam had a power density of 40 MW/cm and did not produce any changes in the films. The scatter intensities listed in table 5 are averages of three measurements on

each film and are normalized to the value obtained from the coating (0104) having the lowest scatter. From this preliminary study, there appears to be a good correlation between film scatter and damage

threshold

.

I

126
Table 1. Damage thresholds for quarter-wave single coatings

(nominal pulsewidth, 11-13 nsec).

Sample Material Refractive Spot size Peak energy density Peak power density Ratio of Index spark

(6943 a) by spark by LIS by spark by LIS to LIS

mm J/cm^ j/cm^ GW/cm 2 GW/cm

0133 MgF

2

1, 38 0. 055 111-131 111-131 8. 2-9.

3

8. 2-9.

3

1. 0 0125 SiO

2 ZrQ

2

1. 456 0. 055 > 125 117 >10. 5 10. 5 > 1. 1 0129 1. 975 0. 072 61-68 35-44 4. 3-4.

9

2.4-3. 2 1. 6 0121 TiO

z

2. 28 0. 072 57-71 35-42 4. 2-5. 7 3. 0-3.

9

1. 7 U203 ZnS 2. 35 0. 072 63 5. 5-12.

5

4. 7 0. 4-0. 85 >5

All substrates are BSC2 glass

Table 2. Damage thresholds for various film thicknesses .

Sample Film Spot size Peak energy density Peak power density

mm 2 J/ cm

2 GW/cm

0133 MgF

2 ,x/4 0. 055 111-131 8. 2-9.

3

0135 MgF

2 ,x/2 0. 065 65-102 5. 3-8. 1

0125 Si0

2 ,x/4 0. 055 117 9. 8 0127 Si0

2 ,x/2 0. 055 >111 >9. 9

0129 ZrO

z

, X/4 0. 072 36-44 2. 4-3.

2

0131 Zr0

2 ,x/2 0. 072 12-13 0. 85-0. 90

0121 Ti02> X/4 0. 072 36-42 3. 0-3.

9

0124 Ti0

2 ,x/2 0. 097 7-8 0. 55-0. 65

0137 Ti0

2

, 3X/4 0. 072 17. 5-25 1.4-2. 2

0. 122

0138 TiQ2> 3>/4 0. 072 20.23 1.4-1. 5

Damage detected by laser -induced scatter

127
Table 3. Damage thresholds for bilayer antireflection coatings

Sample Description Peak energy density Peak power density Ratio of spark

by spark by LIS by spark by LIS to LIS

2 J/cm J/cm 2 GW/cm 2 GW/cm

0143 Zr0

2 /Si0

2 118-147 97-118 8. 6-11.

7

7. 6-9.

5

1 2 0147 Ti0

2/MgF

2 84-85 60-65 5. 8-6. 1 4. 0-4.

3

1 4 0145 ZrO /MgF 64-92 54-57 5. 1-6.

8

3. 5-4.

0

1 5 0142 Ti0

2/Si0

2 74-98 35-42 6. 5-6.

8

2. 4-3. 1 2 3

Sample configuration is GHLA, where G is BSC2 glass (0. 5mm thick), H is high index film (< \/4 thick), L is low index film (>X/4 thick), and A is air. Laser spot size is 0. 062mm, and nominal pulsewidth is 11-14 nsec.

Table 4. Thresholds for multilayer reflectors

(nominal pulsewidth: 12-15 nsec)

Sample Description Spot size Peak energy density Peak power density

mm 2 J/cm GW/cm2 O103 TiO /SiO 0. 056 121-126 9. 1-9.

3

O104 0. 056 107-127 6. 1-8.

9

aO104 0. 056 97-118 7. 6-8. 7 O101 1

1

0. 062 83-103 5. 5-8.

4

O102 1

1

0. 060 98-110 6. 8-7. 2 S101 1

1

0. 062 43-56 3. 0-4.

0

S102 0. 062 50 3. 7 O107 Zr0

2 /MgF

2

0. 058 81-113 4. 7-7.

8

O108 0. 062 90 6. 1 O105 ZrO /SiO 0. 13 18-20 1.2-1. 6 O106 2, 4

0. 13 18. 5 1. 3 S103 0. 12 7. 5* 0. 52 SS103 CeO /SiO 0. 14 17-19 1.4

SS102 0. 14 14. 5-17 1.1-1.2

SSI 01 1

1

0. 14 11. 5 1. 1 SSI 01 1

1

0. 072 14. 5-19 1.0-1.4

Ulll ZnS/ThF 0. 084 25-26 2. 0-2. 1 4

Second test after 2-|- months

128
Table 5. Comparison of weak- signal scatter and spark threshold

for multilayer reflectors

Sample Reflector Normalized scatter

intensities at 6943 \

Peak energy density

(spark)

2 J/ cm

O104 Ti0

2 /Si0

2

1. 0 107-127

O102 1 1 2. 3 98-110

aum ZnS/ThF,

4

2. 8 25-26

O101 Ti0

2 /Si02

3. 5 83-103

O103 I 1 3. 7 121-126

SI 0

1

6. 5 44-56

Ol 05 Zr0

2 /Si0

2

9. 3 18-20

SS103 Ce0

2 /Si02 14. 3 17-19

O106 Zr0

2 /Si02 16. 7 18. 5

SS101 Ce0

2 /Si02 24. 5 11. 5 aO108 ZrO_/MgF_ 24. 5 90

aO107 1

1

27. 0 81-113

SSI 02 Ce0

2 /Si02 37. 5 14. 5-17

S103 Zr0

2 /Si0

2 41. 5 7. 5

Notable exceptions to the correlation

Figures 13 and 14 are microphotographs of two undamaged multilayer reflectors, showing the incipient

film inhomogeneities . Figure 13 is a photograph of a Ti02/Si02 reflector which had low scatter and high

damage threshold. Figure 14 is a photograph of a CeC>2/Si02 reflector which had high scatter and low damage threshold. These pictures were taken with a Bausch & Lomb metalograph under dark field illumination.

Figures 15 and 16 are microphotographs of two laser damaged multilayer reflectors demonstrating the differing types of damage morphology with respect to initial film scatter. Figure 15 shows the damage

area on a Ti02/Si02 reflector which had low scatter. The damaged area is approximately circular with a diameter near 0.7 mm. The depth of damage is uniform, probably at one of the initial layer interfaces.

There are bubbles in the damage center which is indicative of heating, but otherwise the morphology shows

a lack of microscopic inhomogeneities. Figure 16 is a photograph of the damage area on a Zr02/Si02 re- flector having high scatter. The damaged region is a granular area about 0.2 mm in extent. The granularity indicates that multiple weak points were present in the film and that damage nucleated on these

points

.

129
7. Damage Dependence on Laser Parameters

In a previous paper [8] we reported that the laser damage to three samples was dependent on the spot-size of the laser beam. To verify that result we continued to study spot-size dependence by using

the laser output from the ruby oscillator focused by a 20.7 cm focal length lens. By placing the sample

at various distances from the lens we could vary the spot-size at the film from 0.1 to 0.2 mm, never putting the test sample closer than 5 mm to the focus. In this manner we could accurately measure the spot-size by a pinhole scan. In addition, we amplified the oscillator output and used a 30 cm FL lens

to obtain an even weaker converging beam. With this arrangement we were able to damage films at larger

spot sizes (0.32 mm). The results are shown in figure 17, which demonstrates that the damage thresholds

decreased with increasing spot-size. We also investigated the pulsewidth dependence of the energy density

thresholds and these results for a single layer Zr02 film are shown in figure 18.

8. Acknowledgements

The authors are grateful to the following companies and their thin-film experts who supplied test specimens for this study: 1) H.R. Owen of the University of Southern California, 2) L. Mott of Optical

Coating Laboratory, Inc., 3) A. Zook of Herron Optical Company, 4) R. Mortensen of Spectra-Physics Cor- poration, and 5) G. Ash of Spectrum Systems Division of Barnes Engineering. Others receiving our gratitudf are E. A. Maunders for microscopic photographs, C. R. Guiliano for discussions regarding beam focusing,

U. 0. Farrukh for theoretical analysis of focusing of truncated Gaussian beams, and A. G. J. Balbin Villaverde for spark detection design.

This work was supported by the Advanced Research Projects Agency, ARPA Order No. 1738 and monitored

by the Air Force Cambridge Research Laboratories under Contract No. 19628-71-C-0220 with the cooperation

of R. A. Bradbury, contract monitor.

References

[1] DeShazer, L.G. and Parks, J.H. , in Damage in Laser Materials: 1971 , edited by A.J. Glass

and A.H. Guenther (NBS Special Publication

356, November 1971), pp. 124-136.

[2] Dickson, L.D. , Appl. Opt. 9, 1854 (1970).

[3] McAllister, G.L. , Mann, M.M. and DeShazer,

L.G., IEEE J. Quan. Elect. QE-6 , 44 (1970).

[4] Campbell, J. P. and DeShazer, L.G., J. Opt.

Soc. Am. 5_9, 1427 (1969).

[5] Holmes, D. A., Korka , J.E. and Avizonis, P.V.

,

Appl. Opt. 11, 565 (1972).

[6] Farrukh, U.O. , USC, unpublished.

[7] Catalan, L.A. , J. Opt. Soc. Am. 52, 437 (1962).

[8] Newnam, B.E. and DeShazer, L.G. , IEEE J. Quan.

Elect. QE-8 , 611 (1972).

10. Figures

GIANT PULSE RUBY VARIABLE

OSCILLATOR ATTENUATOR

LONG FOCAL

LENGTH LENS TARGET

(20cm) SAMPLE

I.O

N = 0.4

1.12 mm 0IA. APERTURE / 0.8 PINHOLE (63/x) SCAN

AT 100cm (MR) HORIZONTAL PLANE / 0.6

0.4 ——* S = A mm

\

0.2

i I I 1 1 1 1 . .25 0 .25 .50 RADIAL DISTANCE (mm)

.75

Figure 1. Schematic of Q-switched ruby laser system and spark detection electronics.

Figure 2. Normalized oscillator intensity distribution in the far field (100 cm) of the mode selec- ting aperture (1.12 mm dia.) for a cavity Fresnel

number of 0.4. The profile was measured with a pinhole detector (64 |_im)

.

130
Figure 3. Photographs of the diffraction pattern of the giant-pulse ruby laser oscillator (Fresnel No. 0.4) at 3 Rayleigh distances (far field) from the laser. The three photographs were exposed under

three different beam attenuations.

Figure 4. Focusing of a Gaussian bei

aperture

Figure 5 Aperture intensity profiles at the tem- poral peak of a passively Q-switched ruby laser oscillator with planar mirrors and Fresnel No. 0.4.

APERTURE INTENSITY PROFILES

UNIFORM PUMPING

0.2 0.4 0.6 0.8 RADIAL DISTANCE, r/a

1.0

Figure 6. Focusing of a truncated Gaussian beam,

described by the truncation parameter a/w and the initial divergence represented by a lens in the aperture lens f .

.

300 200

100-

Colculated Truncated

.

Gaussiann(o /w°.5) 7 I

Measured

Calculated Equivalent Gaussian

20.7cm lens placed

at 109.5 cm from

1.12 mm dia. operture

4 8 12 16 20 24 DISTANCE AFTER LENS (cm) 28

Figure 7. Axial intensities before the focus of a lens (F - 20.7 cm) comparing the measured intensities to calculated equivalent and truncated

Gaussian distributions.

131
DISTANCE AFTER LENS (cm) Figure 8. Axial intensity after the focus of a lens (F = 20.7 cm) for truncated Gaussian beams with truncated parameters 0.5, 1.25 and 2.0.

M 0.28 mm !

Figure 9. Laser-induced damage above the spark

threshold for a 22 layer TiCL/SiO, reflector of

configuration G(HL) 10HL2A.

Figure 10. Laser-induced damage occurring before a Figure H. Relative intensity distribution for detectable spark in a single quarter-wave film of quarter-wave film on BSC-2 glass

TiO„.

132
-0.6 mmFigure 14. Surface of a CeO^/SiC^ multilayer re- flector before Irradiation which had a very low threshold for damage. Note the great number of

pinholes

.

Figure 15. Laser-induced damage far above the spark threshold for TiC^/SiO? reflector Note central bubbles.

133
Figure 16. Laser induced damage above the spark threshold for a Zr0

2 /Si0

2 multilayer reflector. Note granular structure of the damage and the gradual decrease of damage penetration with radius.

i40r

<CT 120 E

100

80

60

DAMAGE THRESHOLD VERSUS

LASER SPOT SIZE

12 nsec pulses

Mg F2

, X/4

35 1 1

1 I I I I /Constant Power Density

for Spork

Spark Threshold

'Constant Power Density

"or LIS

Laser -Induced Scattering (LIS Threshold

Zr02 Single x/2 Thick Layer

on BSC-2 Glass Ruby laser spot size: 0.07mm

.1 .2 LASER SPOT SIZE (mm)

20 30 40 60 80 100 PULSEWIDTH (FWHM) ,(nsec)

Figure 17. Damage threshold as a function of laser beam spot-size for a Ti02 /Si02 multilayer reflector

and a single quarter-wave thick MgF

2 antiref lection

coating on BSC-2 glass.

Figure 18. Laser pulse-width dependence of the damage thresholds for a single half-wave thick

film of Zr0

2 on BSC-2 glass. Damage was detected

by both laser-induced scattering and spark formation. Deviation from constant power density

thresholds is indicated by dashed curves.

134
Influence of Structural Effects on Laser Damage Thresholds of Discrete and Inhomogeneous

Thin Films and Multilayers

R. Russel Austin and Raymond C. Michaud

Perkin Elmer Corporation

Norwalk, Conn. 06852

and

Arthur H. Guenther, Joseph M. Putman and Richard Harniman

Air Force Weapons Laboratory

Kirtland Air Force Base Albuquerque, N.Mex. 87117

The dependence of single shot damage threshold on certain film properties

is discussed. Variable stress films are produced by mixing components with approximately equal damage thresholds but opposite stress characteristics a definite stress dependence is shown. Films formed from mixtures of high and low threshold components

are investigated and found to have damage thresholds between the high and low

component values. The damage threshold and damage mode of multilayer and periodic

inhomogeneous film systems is compared. The results confirm that the damage

threshold is almost entirely material dependent.

Key Words: Laser damage, optical coatings, inhomogeneous films, microscopy,

thin film stress.

1. Introduction

A first examination of failure modes in various single and multiple layer anti-reflection coatings

was undertaken by Austin and Guenther. [1]^ In that work an attempt was made to relate damage levels

and damage modes to some properties and structural parameters of the films and multilayers. Particularly

interesting was the film stress effect and the conclusion was drawn that there were indeed some failure

modes which could be explained on that basis. There were however insufficient experiments performed to clearly identify a relationship between stress and damage threshold.

Several instances of failure through delamination were evidenced by the study. These were sufficient to indicate the possibility of damage threshold effects due to film structure. The study also

identified a weakness in anti-reflection coating design capability due to lack of medium index

material with a high damage threshold. Additional studies were proposed which would allow property

and parametric effects to be considered in more detail. Many of those studies are covered by this

report.

2. Stress Effects on Laser Damage Threshold

In order to best examine this effect it was necessary to produce sample films in which the inherent

stress was the only major variable. As in many other experiments with thin films it was not possible

to exclude all other variables but by careful choice of film system these were reduced to levels which

should be small compared with the gross effect of film stress.

2.1 Choice of Films

The stress in some film materials can be influenced by deposition conditions, and in other

materials (particularly metals) the stress is a strong function of thickness. Aluminum for example

forms films which begin as tensile stress films and become compressive with increasing thickness. A

Figures in brackets indicate the literature references at the end of this paper.

135
condition of zero stress exists at a thickness of approximately 400 A. Even though we are then able to produce a stress spectrum using a film of a single material many other properties of the film might be widely different. In the case of a luminum films the thickness would vary from a few angstroms to say 1000 A, the reflectivity would vary from 5 percent to 90 percent; and the absorption would vary from

30 percent to 10 percent. Clearly then it would not be possible to isolate those effects purely due to stress in the presence of so many other variables.

Turner [2] studied the damage threshold of a number of common thin film materials and lists a hierarchy of resistance to laser damage as shown in table 1. Of the materials listed Magnesium Fluoride and Silicon Dioxide have damage threshold levels which are fairly similar. The refractive indices of the two materials are also fairly close. The stress properties of each material however are entirely different.

Magnesium Fluoride forming films with a high tensile stress and Silicon Dioxide forming films with a high

compressive stress. It was decided to use films which were a vapor phase mixture of these two materials

for our investigation. By altering the relative proportions of the two materials in the mixture, films

of different internal stress could be produced. Stresses could range from a very high tensile value

through intermediate values and zero to a very high compressive value. Total film thickness could be maintained constant and there would be only small difference in refractive index from one mixture film to another.

Table 1. Observed Damage Thresholds, Single QW Films at Focus of f=42 mm Lens Glass Substrate Thickness 2 . 3 mm

!

Q-Switched Ruby Lasers Material Threshold E+ in J/CM' ThF4 470 Si02 360 - 420 Si02 250 MgF

2 300 - 360 MgF2 250 - 420 A12 0

3 220 - 400 CaF2 50 - 300 Zr02 130 - 160 Zr0

2 200 - 280 Zr0

2 115 - 240 5NaF .3A1F 3 100 - 210 Ti02 115 - 180 SiO 115 - 150 LiF 100 MgO 100 Ce0

2 40 - 100 ZnS 30 ZnS 23

i

2.2 Film Preparation

The films were prepared by simultaneous evaporation of MgF^ and S1O2 from separate electron beam

sources. Each source was provided with a separate rate of evaporation control, and the mixture proportions in the final film were obtained by control of the rate of evaporation of each source. The apparatu

for the film deposition is described in detail in Appendix A. It was used for the generation of other ex

,

perimental samples described later in this report.

Substrates for the films were 1" square fused silica pieces all of which were cut from a common wind: blank. This ensured that the surface condition of the substrates was as similar as possible. The subj strates were cleaned simultaneously in boiling methyl alcohol and in a methyl alcohol vapor degreaser

prior to coating. It

I

'

1

1

i 136 ' ' I
2.3 Experimental Results

2.3.1 General Film Properties

The general properties of the films are documented in table 2. The thickness of the films was held

to minus 0 plus 4 percent of the intended thickness. Monitoring accuracy is not as high with mixture

films as it is with films of single material because of the tendency to form inhomogeneous films. An estimate of this inhomogeneity is given in table 2. Inhomogeneity in thin films does not mean variation

of composition but relates to variation in refractive index.

Table 2. Vapor Phase Stress Mixtures

of MgF2 and Si02

Sample Percentage Scatter Wavelength - Stress Refractive InhomogeNumber Composition (Percent) at which films (Measured) Index neity

MgF2 :Si02 are */ 2 °' T ' (K

g/cm2

) (Mean Value) @ 1.06pm

(by weight) TMU m ((? 2 x X Q /2)

1 100 : 0 0.0030 1.08 2950 (T) 1.38 0

6 82 : 18 0.0031 1.06 520 (T) 1.39 0

5 76 : 24 0.0035 1.11 162 (T) 1.40 0

4 66 : 34 0.0032 1.08 312 (C) 1.41 + 0.57.

3 50 : 50 0.0034 1.06 870 (C) 1.43 + OA'A

2 0 : 100 0.0030 1.10 2370 (C) 1.45 0

NOTES: 1. Scatter measurements are taken directly from the samples without aluminum overcoating. Measurements are taken @ 6328 X and are not normalized to account for the diffuse

reflection component or scattering due to the influence of substrate quality.

2. Stress values (T) indicates tensile stress (C) indicates compressive stress. Measured

using a stress interferometer [7].

3. The inhomogeneity value is the reflectivity difference (in percent) of the reflection

from the film from the reflection of an uncoated substrate. A completely homogeneous

film has an inhomogeneity value of zero. A positive value indicates an increase in refractive index in a direction from the substrate through the coating.

Figure 1 shows graphically the stress variation in the mixture films of MgF2 and Si02 as a function

of percentage composition. It can be seen that the relationship is not a simple one. Thirty percent

of Si02 in the mixture is sufficient to reduce the stress of the films to zero even though the single

film of Si02 in the same thickness as a MgF2 film is stressed less. This is indicative of some interaction possibly taking place between the Si02 and MgFj , in the deposition process.

There is no significant difference between the scatter values recorded for the mixture filmsubstrate combinations. The refractive index and Inhomogeneity measurements for the films were made

on substrates with n = 1.52. It was not possible to obtain good measurements of the homogeneity from

the fused silica witness pieces because of the similarity of the film and substrate indices.

2.3.2 Experimental Procedure

The experimental configuration utilized in this investigation is in general similar to that reported last year. [1] For the laser damage threshold measurements, a high brightness 1.06pm

glass laser was employed. The oscillator-amplifier system emitted in excess of 10 joules in a single 30 nsec FWHM pulse. The total beam intensity distribution is essentially flat topped with an rms intensity fluctuation of ^ 8% over > 90% of the total energy output cone.

An experimental arrangement as shown in Fig. 2 was used for all sample irradiation. The laser

system was operated at a constant pump level to insure maximum stability in experimental exposure

conditions. Under this restraint the laser output and beam quality as measured by its total

| divergence ( ^ 5 mr) [3] and intensity distribution was reproducible to between 2 and 3%. The

residual or aspheric portion of the beam divergence was ^ 0.1 mr as determined by use of a Hartman

diaphgragm. [3] To vary the real power density, homogeneous Schott filters were inserted before the combination 4 m and 6 m focusing lenses

.

137
In order to afford good sampling of surface Imperfections, an irradiation area 2.85 + 0.1 mm in diameter was selected. Thus the irradiation area was 0.061 + 0.002 cm at FWHM. This would insure

that results would be representative of large area damage thresholds with a sufficient probability

of occasionally encountering pinholes, dust particles or localized high stress regions. The 2.54 cm x 2.54 cm x 0.64 cm fused silica substrates with coating were inserted at an angle of 5° off normal

to the incident laser beam to eliminate feedback into the laser. These dimensional conditions

permitted nine (9) noninterfering exposures on each sample. Samples were only coated on one side and all exposures were accomplished with the laser radiation incident on the coated surface.

The intensity distribution from the rotating mirror Q-switched laser system is shown in Figure. From this known intensity distribution an average power density is calculated. To complete the experimental arrangement as shown in Fig. 2 a beam splitter was used to direct a portion of the beam

to facilitate recording of the intensity distribution on each exposure. Since no aperatures were

used to limit the area as employed last year, a slightly different intensity distribution was obtained.

This procedure eliminated to a large extent the occurrence of undesirable diffraction rings. All sample exposures were visually observed through a 1.06pm blocking filter to detect the formation of any visible plasma. In addition a visual spot check for damage was accomplished after each irradiation

before microscopic inspection.

Before insertion in an adjustable x-y mount for exposure each sample was cleaned by pulling a Kodak lens tissue wet with spectral grade ethyl alcohol across each face once. No detectable water

droplets or surface impurities were noted upon observing the evaporation of the residual alcohol film.

2.3.3 Damage Threshold Values

The evaluation of the single pulse damage threshold in this and later discussed experiments proved

to be primarily qualitative and relative as opposed to quantitative and absolute. As in the case of last year's report, in general two exposures were recorded for the highest level without damage and two exposures at the lowest level with damage. The presented errors are due primarily to the insufficient

selection of filters to further narrow down the uncertainty in the threshold value. Limited sample exposure areas prevented any systematic attempt at ascertaining the reproducibility of the damage

threshold. Three criteria were used to determine the onset of damage and two opinions were sought to determine whether or not the criteria were satisfied. These criteria were:

1. The appearance of a visible breakdown during irradiation.

2. Microscopic evidence of damage.

3. Immediate visual observation of damage.

As usual there was always a plasma formed when damage was observable. However, when testing very

damage resistant films plasmas would be formed at the surface which did not necessarily produce visual

damage. In some cases there appeared to be a self-annealing taking place in which the immediately

evident visual damage area would appear to lessen in a time period ranging from minutes to hours.

Figure 4 shows the damage threshold of the samples of varied film stress as determined by the spark criterion. This observation produced the least uncertainty in the overall results. It can be seen that all of the films that are mixtures show a higher damage threshold than the films of the pure materials, even though the trend of the measurements takes no immediately obvious form.

The threshold values for the discrete values were almost equal confirming the Turner results for

the same materials. The actual levels for these materials however are lower than one might expect

when compared with the levels for Zinc Sulphide and Thorium Fluoride generated in the mixture experiment discussed later. The damage levels for the mixture films are closer to the values that one might

expect for discrete films in Turner's comparison.

The morphology of the observed damage areas is discussed later. It is more appropriate to present

that analysis in conjunction with the observation of the damage evident in the other experiments where

comparison between material and film properties can be made manifest.

2.4 General Comments on the Stress Investigation

Although it is difficult to ascribe any order to the damage threshold values compared with film

stress the high magnification SEM pictures shows a progressive pattern of damage appearance and then an abrupt change in this appearance to another characteristic pattern as the Si0

2 content of the films

reaches a certain level. It is significant however that the samples showing the highest threshold do not show the granularity and fail in the complete fusion mode without any evidence of cracking at the edge of the irradiated area.

138
3. Damage Threshold Value for Mixtures of High and Low Threshold Components

3.1 Requirement for the Study

The data in section 2.1 shows that there is a tendency for the damage thresholds of low index

materials to be higher than medium and high index materials. The obvious exceptions to this rule are Cryolite (5NaF • 3 AlF^) with a refractive index n, on the order of 1.35 and is fairly low on Turner's

scale; and Alumina (A^Oo) which is fairly high on Turner's scale, yet having an n^ in thin film form

ranging from 1.65 to 1.75. It is noteworthy that films exhibiting high index values usually do so in the presence of some absorption and would therefore be more susceptible to laser damage.

In the design of multilayer anti-reflection coatings for either single wavelengths or broad bands

of wavelengths there is frequently a requirement for a material with a refractive index lying in the range 1.80 to 2.0. Two materials in Turner's tabulation satisfy this requirement MgO (with an n^~1.86)

and SiO (with an np~1.90). Both materials would rate fairly low for damage threshold with values a factor of 3 to 4 less than good low index materials. There are two design devices which would allow the thin film designers to avoid the use of medium index films. One device is the Herpin equivalent multilayer approach wherein a symmetrical multilayer system of high and low index materials is substituted for

the normally discrete layers in a multilayer film system. This would still pose a problem from the damage

resistance standpoint because of the relatively poor damage resistance of the high index films.

The other device would be to use films which were formed by vapor phase mixture of the high

and low index films as were the films prepared for the stress examination in Section 2. A second

experiment was designed to investigate films of that type.

3.2 Selection of Materials

It was decided that in order to obtain the best experimental results we should chose materials

with widely differing damage thresholds. Thorium Fluoride and Zinc Sulphide occupy the top and bottom of the Turner scale, their damage thresholds differing by well over an order of magnitude.

They are commonly used together in discrete films to form high efficiency reflectors and bandpass

filters. The purpose of this second experiment was to determine mixture effects. This material pair

was preferable to say Zr02 and Si02 where there is some overlap of the reported ranges of damage

thresholds and the interpretation of the mixture effect would as a result be more difficult. It is probable that mixtures of Zr02 and Si02 would have produced a more damage resistant film for a given

index than the ZnS and ThF^ mixtures.

The only problem anticipated was the modification of the result due to the stress range of the mixture films. ZnS is strongly compressive, ThF4 is strongly tensile and mixture films exhibit intermediate stress values. It was hoped that the stress effects would be sufficiently isolated by the previous experiments to make the appropriate allowances in the formulation of the results. Furthermore,

the stress—composition behavior for this pair of materials is considerably different than MgF2 - S102 previously studied. It is less composition sensitive to stress changes over a relatively large range

of compositions resulting in low stress values for considerably different compositions.

3.3 Sample Preparation

The samples were prepared by simultaneous evaporation, from two separate sources, of Zinc Sulphide

and Thorium Fluoride. The apparatus was basically the same apparatus used for the preparation of the stress samples in the previous section with the exception that radiant heated thermal sources were

used rather than electron sources.

All substrates were cut from the same window blank. The substrates were cleaned in boiling methyl

alcohol and then in a vapor degreaser containing methyl alcohol. Each substrate was subjected to a glow discharge cycle at 2.5 KV/100 mA for a duration of 10 minutes prior to each evaporation.

It was attempted to maintain each film at a thickness (optical) of one half wavelength at 1.06 v m

in order that the reflection from the film system would be independent of the refractive index value.

The films were difficult to monitor accurately because of inhomogeneity introduced by variations in evaporation rate from each source. This inhomogeneity does introduce some slight differences in the reflection values. In certain cases it was impossible to obtain any good information on the film

properties because of gross inhomogeneity in the film systems.

139
3.4 Experimental Results

3.4.1 General Film Properties

The general properties of the experimental films are listed in table 3. Samples #2,3, and 6 were

grossly inhomogeneous to the extent where it was not possible to acquire any meaningful information on their refractive index values.

The apparatus used for the deposition of the films did not contain a film stress measuring interferometer so that it was not possible to measure the stress of the samples tested. It was necessary to generate a separate estimate of the stress versus composition of the films by supplementary measurements

and by reference to existing data on these two materials. Figure 5 shows the estimated stress behavior

of the mixture films. Typical response curves for a homogeneous and an inhomogeneous mixture film is shown in figures 6 and 7.

While the variation of refractive index of ZnS-ThF, mixture with composition is essentially linear

(see table 3) the variation of refractive index with film stress takes the form of figure 8. Obviously

this mixture film system allows great selectability of refractive index between 1.45 and 2.35 while ex- hibiting relatively low residual film stress over that range.

Figure 9 shows the dependence of damage threshold on the composition of the mixture. As expected a range of damage threshold values spread between the results for the discrete materials was obtained.

The spread is such that it is possible to conclude at least tentatively a threshold versus mixture

behavior. The data presented indicates the trend could either be a general increase in damage threshold

as the ZnS7o is decreased or once the mixture becomes at least 50% ThF, there is a step increase in damage

threshold. Both types of behavior could be explained phenomenologicaily but additional data is warranted

to elucidate the appropriate trend.

The results for the discrete material films indicate the possibility of over an order of magnitude difference in the thresholds of ThF^ and ZnS which is in agreement with Turner's findings.

It can also be seen that if these values represent the extreme of the damage threshold range, as they

do in the Turner data then the values for the highly stressed discrete films of Magnesium Fluoride

and Silicon Dioxide do not agree except in relation to each other. It is of note however that the actual values for the stress relieved mixture films of MgF2 and SiOj do fall in the correct relative

position on the damage threshold scale.

The morphology of damage will be presented later in which differences between the failure of homogeneous, inhomogeneous and various compositions will be discussed with the other experimental

results

.

Table 3. Vapor Phase Mixtures of ZnS and ThF^

Sample Mixing Ratio Scatter Wavelength of Refractive* Inhomogeneity Stress Number ZnS:ThF4 (Percent) Xc /2 O.T. Index % (Estimated)

in u m kg/ cm2

1 100 0 0.024 1.05 2.36 + 0.6 2000 (C) 2

3

0

13

100

87

0.010

0.008 nominal 0.2 1500

670

(T)

(T) 4 23 77 0.010 1.15 1.59 0.6 500 (T)

5 31 69 0.024 0.8 1.72 + 3.3 450 (T)

6 15 85 0.012 640 (T)

7 90 10 0.036 1.06 2.33 + 1.0 540 (C)

8 80 20 0.040 1.06 2.15 + 1.5 100 (C)

9 50 50 0.036 1.06 1.91 + 5.0 100 (T)

*Refractive index measured at the 2 x ^ c /2 position.

O.T. = Optical Thickness = n x d Films // VPM 2,3,6 are inhomogeneous in nature.

!

140

]

!
3.5 General Comments on the Mixture Experiment

The micrographs reveal no evidence to indicate the mechanism by which the threshold of a low

threshold material is raised by adding a higher threshold component other than the progressive change

in the appearance of the damaged areas from characteristics of one material to the other. Some pictures

do reveal that the films may not have been as homogeneous as the spectral response data might indicate.

4. Damage Resistance Properties of the Periodic

Inhomogeneous Film Systems

4.1 Description of the Periodic Inhomogeneous Film

The Periodic Inhomogeneous Film (PIHF) is a single film whose refractive index varies periodically

with thickness according to some predetermined function [A]. By suitable choice of the function a film

can be made to produce any spectral response characteristic that is possible with a discrete film

system and some response characteristics that are difficult to produce in practice using solely discrete

film systems.

The significant feature of the PIHF is that it has no discrete interfaces other than the interface

with the surrounding media. There are no optical or mechanical discontinuities throughout the entire

thickness of the film. The discrete film system by comparison does contain many interfaces where there

are abrupt changes of mechanical stress or refractive index. The difference between the PIHF and the discrete film system is shown schematically on Figure 10.

4.2 Damage Patterns Occurring in a Discrete Film System

In a reflective film system comprising discrete quarter wavelength optical thickness films the standing wave pattern is such that electric field concentrations occur at every second interface in

the system, as these interfaces are generally^ lines of abrupt change of mechanical weakness.

Turner [2] observed in damage areas on multilayer reflections that delamination in certain instances

occurred in layer pairs as shown schematically in Figure 11. The standing wave pattern in the multilayer system is shown schematically in Figure 12.

4.3 Philosophy of the Experiment Choice of Film System

It was decided that as part of this experiment that not only should we investigate the damage

mechanism and damage resistance properties of the PIHF but also compare the same properties of the closest equivalent multilayer system.

The filming materials chosen were Zinc Sulphide and Thorium Fluoride. This choice was made not

only because of the availability of design information for these systems from previous Perkin Elmer

development programs but because of the availability of information on the damage resistance versus mixture data from the experiment described previously. This information was considered essential in

the analysis of the experimental results.

The PIHF chosen for the experiment was an order supression type filter optimized for transmission

on the short wavelength side of the fundamental reflection band. The closest approach to this using

discrete films of only two materials would be a short wavelength pass filter with a fundamental

reflection band at the test wavelength. This design type does not have the order supression property

of the PIHF but this property has no particular significance from the damage standpoint, more

important being the matching of total optical thickness in the discrete and PIHF designs.

Figure 13 shows the refractive index profile of one cycle of the PIHF chosen for the experiment.

The profile of the discrete film equivalent cycle is shown on the same arbitrary thickness scale. In practice the mechanical thickness of the cycles is not exactly the same.

4.4 Preparation of Samples

Two coating facilities were employed in the generation of samples for this experiment. One of. the plants was the dual source facility described in Appendix A. This system was used for the preparation of the PIHF samples. The other plant used for the deposition of the discrete film systems

was a C.V.C. 18" Glass Bell Gar system equipped with a 6" oil diffusion pump system. As in the other

experiments substrates 1' x 1' cut from a common window blank were used to eliminate the possibility

of experimental variables associated with substrate surface conditions.

141
A total of 7 P1HF samples were prepared for the test, the maximum thickness samples having 4 cycle

repetitions. Five discrete film reflectors having between one and four repetition groups were generated

for comparison purposes. The design of these systems was of the general form.

(L/2 H L/2)

n

where L = One quarter wave optical thickness of ThF^. H = One quarter wave optical thickness of ZnS.

n = An integer.

Table 4 describes the general properties of the experimental designs. It can be seen that the reflectivity of the discrete film reflectors is slightly higher than the equivalent PIHF reflector.

There are two reasons for this: the basic constructional difference and the fact that the high and low

index values in the PIHF never quite reach the value equal to 100 percent of either basic component.

The controls being set for 5 percent of the lesser component at the extremes. To maintain control of

the PIHF process, it is necessary to always have both sources at a finite evaporation rate, that is one

not completely shut off allowing the film to contain 100 percent of the other component. Figures 14 and

15 show the spectral response curves of a 4 cycle PIHF and the equivalent discrete film reflector. One other point of note is the slightly larger spread of scatter values for the PIHF systems compared with

the discrete films.

5. Discussion of Damage Site Investigations by Optical

and Electron Microscopic Techniques

Optical microscopy of the laser induced damage to the thin film coatings was performed on a Zeiss

microscope employing Nomarski Differential Interference Phase Microscopy. This interference-contrast

technique greatly enhances the contrast on a structured surface so that brilliant relief images can be

obtained, as well as displaying reflective and transmissive anomalies manifest through interference

effects. The photomicrographs obtained were taken using a variable intensity halogen source and a Polaroid film pack unit. Magnifications of 64 to 640 diameters were used in this particular analysis.

The ultimate limit of this unit is i 2500 diameters [5 ].

Table 4. Periodic Inhomogeneous and Discrete Film

Systems of ZnS and ThF.

Sample Wavelength of Peak Reflectivity @ Scatter Number Design Reflection (u) 1.06 um Percent

PIHF 4 1 cycle 1.20(R-0.29) 0.28 0.016 PIHF 5 2 cycles 1.02 (R-0.69) 0.48 0.008 PIHF 6 2 cycles 0.80(R-0.53) 0.04 0.016 PIHF 7 3 cycles 0.97(R-0.68) 0.64 0.012 PIHF 8 4 cycles 0.95(R-0.80) 0.68 0.008 PIHF 9 4 cycles 1.16(R-0.82) 0.70 0.012

PIHF 10 4 cycles 1.08(R-0.81) 0.81 0.032

DF 6 <T H

T> 1.26(R-0^32) 0.30 0.010

DF 2 (7 H £)

2 1.02(R-0.59) 0.59 0.008

DF 3 4 . p 0.98(R-0.80) 0.78 0.010

DF 8 4 » 4»

4 1.13(R-0.89) 0.86 0.012

DF 4

(.J H y) 1.03(R-0.89) 0.89 0.012

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The Scanning Electron Microsopic (SEM) investigation of the samples was performed on an Advanced

Metals Research Model 900 (SEM) . The system was operated in the secondary electron emission mode using

an acceleration potential 15 KV and a filament current of 100 |aa. The samples were vacuum coated with

a 500 A conductive layer of palladium-gold before analysis. The depth of focus or three dimensional

aspect of the photomicrographs evident is a direct function of the secondary electron emission. Magnifications of 75 to 7500 diameters were used in this analysis while the ultimate limits of the system

are 100,000 diameters magnification and 150 A resolution. Polariod photographs were obtained from an ancillary display cathode ray oscilloscope and records primarily surface cosmetics.

5.1 Comparison of Discrete Film Systems with Equivalent

Periodic Inhomogeneous Film Systems

In the following section, the morphology of laser-induced damage in discrete, vapor phase mixture

and periodic inhomogeneous film systems as elucidated from optical electron microscopy is presented.

The notation employed in the following description is related to the assignment of specific samples

of each set of tested film systems as given in the previous tables and denotes the method of observation.

The notation gives the sample designation, magnification, and microscopic technique employed, e.g. D6-64X(N) indicated discrete film system D6; the next designator 64X gives the magnification for a standard Polaroid print while (N) refers to the use of a Normarski Phase Contract Interference optical

microscope.

5.2 General Comments on the Discrete Film Series

The low magnification Normarski pictures clearly show the increasing complexity of the damage sites

with increasing number of layers. From the Scanning Electron Microscope pictures we are able to see that

as the number of layers in the system increases it is clearly the weak component films that cause failure

The plots of damage threshold values versus number of layers, figure 16, tend to confirm this hypothesis,

one can see that the failure threshold of the nine-layer systems are very close to the values for single

films of Zinc Sulphide. It is intended to produce and test more multilayer systems, containing up to

22 layers, to further confirm these findings.

5.3 General Comments on the Periodic Inhomogeneous Film Samples

The PIHF systems did show a characteristic damage pattern which is significantly different from

that observed in multilayer structures. The complexity of the pattern did increase with increasing

thickness (number of function cycles).

The splitting of the films at certain levels was totally unexpected and a clear explanation is not readily available. If it were possible to estimate the level in the systems at which the failure

occurred, then the composition of the film at that level could be estimated.

5.4 Damage Threshold Results for Multilayer and Periodic Inhomogeneous

Thin Film Systems

The damage thresholds for the multilayer film systems are shown in figure 16. The results clearly

show that the damage threshold does decrease as the total number of layers in the stack increases. The

two different sets of results are for films deposited at near normal incidence on stationary substrates

(DF6,DF7,DF8) and for films deposited at ~35 degrees vapor incidence angle in a rotating planetary fix- ture (DF1 , DF2 , DF3 , and DF4)

The samples deposited at the higher incidence angle have a lower threshold overall than the samples

deposited at near normal incidence. No attempt was made to observe the possibility of differences in the crystal structure of the two sets of films and in microcsopic examination it was not possible to detect

any significant difference in the damage patterns. A possible explanation for the difference may be that

the stress in the films deposited at the high incidence angles is greater than the stress in the samples

deposited at near normal incidence. The angle of incidence problem is certainly worthy of further study.

Both sets of results seem to form a trend to a leveling of the damage threshold at some periodic

repetition number greater than 4. The actual level of the threshold for these more complex multilayers

being much closer to the value for the low threshold component than the high threshold component.

Figure 17 shows the threshold results for the Periodic Inhomogeneous film systems. These systems

were generated in the same chamber configuration as the discrete systems DF6, DF7, and DF8. There is no great difference between the discrete film and the periodic film results . The same trend towards

the low threshold component level with increasing thickness was evident. One point worthy of note is the tendency for the damage threshold to be lower on consecutive evaporations of the same design.

Examples are PIHF5 and PIHF6; and PIHF8 , PIHF9, and PIHF10. This may possibly be related to an increase

143
in the finite size particle content of the films as the coating chamber becomes progressively more

loaded with deposits from previous runs. Scatter measurements of the films which might have confirmed

this hypothesis were however inconclusive.

In the three sets of Damage vs Thickness curves the most conclusive trend was the approach to the weak component threshold value with increasing complexity. Microscopic evaluation of the films confirmed

this hypothesis as it was only possible to positively identify the damage origin in the more complex

systems i.e. 3 or 4 groups.

6 . Summary

There are numerous results that can be stated from the previously described experimental observations. Some results are quite conclusive, others indicate trends, while the remainder suggest the need

for further work before laser induced damage effects can be elucidated. A few of the more positive re- sults are:

1. In accord with Turner's remarks, the thicker, thin films are, the more damage sensitivity they

exhibit. This is particularly evident as the number of cycles in the case of PIHF is increased as well

as the number of periods in discrete multilayers. There are several possible explanations for this be- havior including increased absorption, increased probability of encountering foreign particles, or, specifically in the case of reflecting multilayers, that an increasing percentage of the pulse is stopped

by the film system as it's reflectivity increases.

2. Damage complexity increases as well with the number of periods or layers.

3. A most obvious conclusion is that laser induced damage resistance is film stress sensitive, and

that vapor phase mixtures to relieve stress leads to improved film performance from a damage standpoint.

In this light VPM films offer the optical design engineer greater opportunities for the design of low stress improved performance coatings.

4. Thermally induced damage can be observed outside the region of direct irradiation.

5. The angle of incidence of deposition is related to damage sensitivity. This is not surprising

since it is also known to be related to the residual stress in the deposited film.

6. From the observation of the morphology of damage, pinholes are not necessarily low threshold

initiating sites. The microscopic records show physical evidence of thermal and melting effects, surface

tension stress relief, delamination of discrete films, and most surprising, the splitting of PIHF at regions of high concentrations of the low-damage component of such a film system.

7. The high magnification SEM records exhibits the vertical rod-like structure of the optical

films which do not seem to vary periodically in PIHF or discrete films.

Concerning the less quantitative results which suggest trends and possible areas of future study

specifically uncovered by this work,

1. The damage resistance of VPM films consisting of a high and low damage resistance material is definitely a function of the composition, whether there is a direct relation based upon composition

or there are in addition structural effects (e.g. crystallinity , etc.) was not ascertained.

2. As a result of the several experimental observations, one could suggest that PIHF could be produced that were possibly more damage resistant than their discrete film analogs - particularly if these were made of two similar damage resistant materials. This certainly should be investigated

further. However, one must realize that from an optical performance standpoint there are areas of both decreased as well as improved performance. The choice would have to be made upon specific

requirements

.

3. There are positive indications relative to the importance of coating geometry and procedures.

The exact influence of substrate temperature, pressure, angle of incidence, chamber and substrate

cleanliness, as well as the coating history of the deposition apparatus should be investigated further,

particularly as these factors affect the residual film stress.

4. The degree of film homogeneity may have an effect on damage resistance as a result of the threshold values evidenced by these few observations of inhomogeneous VPM film behavior.

5. There is insufficient evidence to draw any conclusions or indicate trends based on the re- flectivity or scattering of the film systems studied at the present time.

144
7. Suggested Areas of Future Study

Those areas which may have a profound influence on the damage threshold, besides the obvious one

of material selection, and should be studied in specifically designed experiments are the effect of

scatter, reflectivity or absorption, substrate finish, deposition method (i.e. R.F. sputtering, resistive

evaporation or electron beam evaporation)

8. Acknowledgment

The authors would like to express their appreciation to Dr. A. Saxman, Air Force Weapons Laboratory,

for his expert advice and assistance in the actual damage testing and laser operation.

9. Appendix A

1. A standard 18 inch VEECO coating plant with a Pyrex bell jar and a 4 inch oil diffusion pump, having

a pumping speed of 400 liters/second was employed in this study. The system has a 0.8 liter capacity

liquid nitrogen cold trap and is capable of reaching an ultimate pressure of 10" Torr. To assist the

4 inch diffusion pump, the system contains a liquid nitrogen cryopanel with a surface area of approximately 270 square inches. The cryopanel is used primarily to reduce the water vapor content of the residual atmosphere.

Two Sloan Model BXV-6, 6KW, 180 degree bend electron beam guns with large capacity, water cooled

hearths can be utilized for evaporating the individual materials. Each electron beam gun is equipped

with electromagnetic beam positioning and programmed beam sweep controls. Both electron beam guns are powered by a single Sloan Model Six/Ten electron beam power supply. The Model Six/Ten is a constant voltage (10 KV), variable emission (0.600 ma) type power supply that incorporates a gun filament source

and a two-gun magnet supply.

The power input and, therefore, the evaporation rate of each gun, is controlled by Sloan OMNI-II, Rate Controllers. Both OMNl-II's have low-rate measurement modification (0-30 cps) and can control the power input manually or automatically. A Lauda Model K-2 Constant Temperature Circulator is used to maintain both oscillator heads at 30 C, which is where the temperature coefficient of oscillation for a

5 MHz, AT cut quartz crystal is essentially zero. Optimum crystal performance is realized by minimizing

the thermal effects associated with radiant heating from the electron beam source.

The two electron beam sources and the quartz crystal sensor heads are integrated with the rate control and electron beam gain electronics to perform coevaporation. The resistance source can be used

as a direct or indirect source, automatically controlled by an OMNI-II or manually controlled by a Variac

Therefore, coevaporation can be accomplished with the two electron beam guns or with a resistance source

and one gun. Shields are required in front of the crystals to ensure that they only "see" the evaporated

vapor from the source being controlled. They are movable so that the different evaporation techniques

can be used with little or no problems. In the VEECO 18 inch bell jar coating plant, the separation be- tween the center of the gun hearths is 4.5 inches, while the vertical source to substrate distance is

17 inches. This particular geometry results in a maximum deposition incidence angle of 17.2 degrees

over a centrally positioned 6 inch diameter flat, located at substrate level. The resulting film thickness variation over a 6 inch diameter, from a single displaced source, is 14 percent. A combination of the Granville-Phillips' Series 236 Ionization Gauge Controller and the Series

213 Automatic Pressure Controller with servodriven bleed valve can be used to control the vacuum level

at any desired value above the base pressure.

2. Equipment for Inhomogeneous Film Deposition

2.1 Vernistat Speed Control Section

Since the program function is provided by the output of a Vernistat interpolating potentiometer,

it is necessary that the input shaft rotation be controlled with some degree of precision. A variable

speed DC motor equipped with a tachometer is used in series with a multiratio speed reducer. This com- bination allows the Vernistat input shaft rotation to be accurately controlled at very low RMP's. An

electric clutch coupling is used to start and stop the function interpolation cycle, while the motor

drive maintains a uniform speed.

2.2 Function Generator Section

Perkin-Elmer Vernistat interpolating potentiometer and adjustable function generator are used to program the deposition rate controllers. The Vernistat requires eleven turns of its input shaft to go through one complete function cycle. The function generator has 34 adjustable sliders that are set to the desired function voltages. By rotating the shaft of the interpolating Vernistat, the output voltage

curve conforms to a series of straight-line interpolations between voltages set up by the sliders.

145
2.3 Control Circuit

The control circuit is merely the necessary switch gear to stop and start the process, A timer is located in this section to monitor the elapsed time during the deposition. The timer also contains a 740 watt auxiliary relay, which can be used to activate a recorder-timer-marker circuit or energize a shutter in the vacuum chamber.

2.4 Deposition Rate Control Section

This section comprises two Sloan OMNI-II deposition rate controllers and thickness monitors. The

signal from the Vernistat is interpreted as a rate, which signals an SCR module to increase or decrease

filament power in order to maintain that rate.

10. References

[1] Austin, R. R., and Guenther, A. H. , Damage

in Laser Materials , NBS Spec. Pub. 356

(Nov. 1971) pp. 137-152; Austin, R. R. and Guenther, A. H., App. Opts. _11 pp. 695-697

(1972).

[2] Turner, A. Francis, Damage in Laser Materials , NBS Spec. Pub. 356 (Nov. 1971).

[3] Wick, R. V. and Guenther, A. H., Method of Test of Beam Divergence for Optically Pumped

Lasers, ASTM Committee F-l.

[4] Jacobson, R. , Progress in Optics, E. Wolf, 5_, pp. 247-286.

[5] Grifkins, R. C, Optical Microscopy of Metals

American Elsenir, New York (1970), pp. 94-97.

[6] Pearson, J.M., Thin Solid Films 6_, pp. 349-

350 (1970).

J7] Ennos, A. E., App. Opts. 5_, p. 551 (1966).

146
11. Figures

20 40 60 eo 100 Si02 (% , BY WEIGHT ) Variation of stress in mixture films of MgF2

" Si0

2

.

SAMPLE

AT 5" TO NORMAL

IN ADJUSTABLE K.Y MOUNT

PH0T00I00E »2

Figure 2 . Experimental arrangement for damage

threshold evaluation.

Figure 3. (a) Two dimensional intensity distribution at the entrance surface of the target as presented by an isodensitometer map. Increasing intensity is indicated by white-gray-black sequence,

(b) Microdensitometer trace through center of focal area as recorded in 3 (a).

147
100% ZnS

0 7 0 6 0-9 1.0 I.I 12 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 WAVE LENGTH IUICR0NS)

0 90

0 80

0.20

0.10

15% ZnS IN TBF,

SINOLE SURFACE REFLECTION

0.6 0.9 1.0 I.I 1.2 1.3 1.4 1.5 1.6 1.7 IB 19 2.0 2.1 2.2 2.3 2.4 2.5 WAVELENGTH ( MICRONS)

Figure 6. Typical spectral behavior (reflectivity Figure 7. Typical spectral behavior (reflectivity

vs wavelength) for a homogeneous film system. vs wavelength) for an inhomogeneous film system.

148
VARIATION OF REFRACTIVE INDEX WITH STRESS IN VAPOR PHASE

MIXTURES OF ZnS AND ThF„

\-„

DAMAGE THRESHOLD FOR MIXTURE FILMS OF ZINC SULPHIDE 8 THORIUM FLUORIDE ALL FILMS ARE APPROXIMATELY ONE-HALF WAVE OPTICAL THICKNESS

AT THE LASER WAVELENGTH (1.06 m)

PERCENTAGE OF ZnS IN MIXTURE

Figure 8. Variation of refractive index versus Figure 9. Damage threshold versus composition in stress for mixture films of ZnS - ThF^ . mixture films of ZnS - ThF .

Figure 10 Comparison of refractive index-thick- Figure n Schematic representatlon of „ d ness profile or a discrete multilayer system and a slte in a dlscrete multlla s tem indicatin! periodic inhomogeneous single film. delamination occurring in layer pairs

149
HIGH INDEX -LOW INDEX

STANDING WAVE

( WAVE LENGTH = X„ )

Figure 12. Schematic of the standing wave pattern

in a multilayer system.

2.3

2.2

x 2.1

LlI Q

r-< 1.8 (E

1.7

1 - -i DISCRETE

. ! FILM

\ ' PROFILE

\ PIHF

1

. L_. . . 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 THICKNESS (ARBITRARY UNITS)

Figure 13. Refractive index profile for one cycle

of a periodic inhomogeneous film system compared

to one period of a discrete film system.

06 as 1.0 I.I 1.2 1.3 1.4 I.S 1.6 1.7 I WAVELENGTH (MICRONS)

.8 1.9 2.0 2.1 2.2 2.3 2.4 2.S

Figure 15. Spectral behavior of a four cycle

discrete film multilayer system.

Xli. \

L = X 0 /4 OPTICAL THICKNESS ThF4 H = x 0 /4 OPTICAL THICKNESS ZnS

n = GROUP REPETITION NUMBER

X 0

= 106 p o t = 30 n» A = 0 061 cm*

a = NEAR NORMAL INCIDENCE , STATIONARY SUBSTRATES

b = -35° INCIDENCE, EMPLOYING PLANETARY ROTATION

ft ° *

Figure 16. Damage threshold versus number of periods in a discrete film system of ZnS and ThF^

at two differing angles of vapor incidence.

0.6 0.7 0 8 0.9 1.0 I.I 1.2 1.3 1.4 1.5 1.6 1.7 1.6 1.9 Zr> 2.1 2.2 2.3 2 4 23 WAVELENGTH (MICRONS) Figure 14. Spectral behavior of a four cycle

periodic inhomogeneous film system.

150
MIXTURES OF ZnS AND ThF4

IN APPROXIMATELY SAWTOOTH FUNCTION NEAR NORMAL INCIDENCE STATIONARY SUBSTRATE

MIXING RATIO 95-5 AT EXTREMITIES

}.„ - 1.06(1

fit = 30 ns FWHM

A = 0.061 cm2

A%OUT21

A, B, C , . SUBSEQUENT COATING RUNS

IN SAME CHAMBER 12 3 4 NUMBER OF FUNCTION CYCLES

Figure 18. Shows a moderate to heavy damage area with a general onset of damage plus some areas of complete removal.

Figure 17. Damage threshold versus number of cycles in a periodic inhomogeneous film system of

ZnS and ThF.

.

Figure 19. High magnification SEM picture of a removal area. The partially damaged film sur- rounding it shows a peculiar but uniform texture. Note the absence of fused materials around the removal site.

Figure 20. The edge of a damage area showing the same overall appearance as the previous sample but with some thermally induced cracks in the falloff

region.

percent MgF

2

) sample but with an increase in the Figure 22. Thermal crackiftg damage in the falloff

in the size of the granularity. region: Note the absence of granularity.

151
Figure 23. Damage in the same relative area as the previous sample. The appearance is generally

the same.

Figure 26. Edge of a moderate to heavy damage

area. No stress cracks at the edge of the area

are evident.

Figure 24. High magnification SEM picture from

the central area. A further increase in granule

size is evident

.

Figure 25. Stress cracking in the energy falloff

region.

Figure 27. High magnification SEM picture of the dark to light transition region of VPS 4 - 7

- 512X (N) . Complete fusion of the film is now

evident

.

152
Figure 28. Moderate damage area. Figure 29. High magnification picture from the center of the area again showing complete fusion

of the film.

Figure- 32. Damage site in a 100 percent ThF, file No pertinent details are evident.

Figure 33.1 Damaged area showing a removal site. No cracking of the films is evident.

153
Figure 34. Detail of a removal site. A nucleus

is evident but no evidence of fused material.

Figure 35'. Shows damage in the form of pitting

with lines showing the interaction of the thermal pulses for each pit

.

Figure 36. Detail of a damage pit. No nucleus is evident. It is not clear whether or not the crescent shaped cracks are in the film or in the substrate

.

Figure 37. SEM picture showing a pit in the same area

.

Figure 38. Damage occurring at two adjacent

nuclei. Coalesced thermal areas are evident.

154

Figure 39. Peculiar granular damage area possibly

the result of a mixing phenomenon between the two materials of the film.
Figure 40. Edge of a damaged area showing an area

of uniform damage.

Figure 41. Detail of the damage area showing a pattern similar to that observed in 100 percent

ThF, films in previous studies.

Figure 42. Granular damage in the same area. Spacial frequency of the small pits is higher than

in the VPM 5 samples.

%

L

i

r

Figure 43. Low magnification picture of a moderate to heavy damage site. The typical light damage annulus surrounds the main, heavy damage area

Figure 44. Detail of the edge of a similar area Figure 45. Detail of stress disruption. This

in the same sample. Stress cracking is evident in characteristic is typical of the 100 percent ZnS

the light damage area. films.

155
Figure 46. Detail in the same area showing some fusion but less evidence of cracking and disruption in the surrounding area.

Figure 47. Shows damage at nuclei and also some mechanical disruption typical of ZnS films.

r

Mr

1

Figure 48. Shows nucleus damage but also the nucleus plus radiating crack damage characteristic

of PIHF systems. This indicates that the film may

not be completely inhomogeneous and may contain

some stratification of the components.

Figure 49. Cracking and disruption similar to that observed in pure ZnS films. Some evidence of fused particles on the film surface is evident.

Figure 50. Damage occurring at nuclei with inter- Figure 51. A small damage pit with some evidence

acting thermal areas. of film cracking not necessarily extending through- out the film.

156
Figure 52. Detail of a fused particle on the film

surface surrounded by a more extensively fused

area more characteristic of 100 percent ThF^ films.

Figure 53. This photograph shows an overall view

of a damage site in a one group multilayer reflector. Two types of damage are evident, first small craters emanating seemingly from a tiny nucleus,

and secondly damage around a sleek in the sub- strate. It is not known which was the first to appear. The small crater damage will be seen to be the general damage pattern becoming increasingly complex as the complexity of the multilayer

increases in subsequent records. The fact that

the sites are clustered much more closely at the

fault clearly indicates a possibility the damage

nuclei in the film might be associated with sub- strate features. This may be true even if the nucleus is in the body of the film bearing in mind

that substrate features are faithfully reproduced

at every interface of a multilayer through as many

as 24 layers.

157
Figure 54. This shows damage sites from the same

area as the previous photograph at higher magnification. The thermal wave effects surrounding each nucleus are clearly evident.

Figure 55. This is a view of damage sites which

clearly indicates a crater type of delamination in the film.

Figure 56. The same as described above. In both pictures it is difficult to tell whether one of the multilayer components is failing in preference to another

.

Figure 57. Crater type damage at the edge of a damage area. The sites have basically the same appearance as in the previous pictures but are bigger and in many placed coalesced.

158
Figure 58. This is a higher magnification picture

of the same area as D2 - 64X. The nuclei of the damage sites are very clear in this picture.

Figure 59. Low magnification SEM picture clearly

showing film delamination

.

Figure 60. The picture shows a deep crater site Figure 61„ The crater damage pattern is still

extending right through the film system to the present in this 3 group (7 layer) multilayer

substrate; it is possible in this instance to see system. It is still more complex than in the more extensive damage in some of the films than in previous instances of systems with less layers,

others

.

Figure 62. High magnification picture from the same area as the previous picture.

Figure 63. Picture shows distinct evidence of at least one film having been completely fused causing cracks in the adjacent films. The molten film

or films are presumably the much weaker ZnS films.

The Thorium Fluoride films are cracked and split. Outer 3 films only are affected.

159
Figure 64. A crater type site showing evidence of preferential fusion of the ZnS films.

Figure 66. Similar appearance to D3 - 256X (N)

.

Figure 68. Higher magnification picture of the above site.

Figure 65. Nine layer system. Damage appearance

is similar to D3 - 64X (N) except for the addition

of fine cracks.

Figure 67. Clear evidence of the failure of internal ZnS films causing mechanical disruption

of the outer ThF^ film and removal of some internal ThF, films.

Figure 69. Heavy damage area in a 9 layer system.

Many crater sites have coalesced to form a con- fused pattern.

160
Figure 70. High magnification picture of the edge

of the site showing the presence of molten material thrown from within the area.

Figure 71. Low magnification SEM pictujre of the above area; the ejected molten material is clearly

shown.

Figure 72 . High magnification SEM picture of the extreme edge of the area. Molten ZnS films and cracked ThF. films can be clearly seen.

Figure 73. Appearance of damage sites of a 1 cycle PIHF optically equivalent to a one group discrete system. Instead of the crater sites

peculiar to the multilayer we observe nuclei with

a system of radiating cracks.

Figure 74. This high magnification picture

clearly shows the nature of one of the nucleus/

crack systems observed above. The nucleus is in fact a very small crater with smooth walls rather

than the 'stepped walls' observed in multilayer

structures. The remarkable feature of the crack

is that it does not penetrate the whole thickness

of the film and appears to extend only to the point where thermal failure occurred.

161
Figure 75. Nucleus crack systems in a 2 cycle

film. Some evidence of 'swelling' of the film

around the cracks is evident.

Figure 76. Gross damage in the 2 cycle film show

ing no evidence of the nucleus/crack systems but indicating some evidence of stratification.

Figure 77. Low magnification SEM picture of an area which appears to show ' delamination' even though the film is continuous and has interfaces

only at the substrate and air.

Figure 79. Shows a complex system of nuclei and

cracks. It is not clear in this picture whether

or not all of the cracks are at the same level in the film.

Figure 78. This shows clearly the peeling of an outer portion of the PIH film. It is evident that damage has occurred at a certain depth in the film

completely removing any film above it as indicated

by the bubbles under the peeling portion of the film which show how the removal is being affected.

/

Figure 80. Appearance similar to the previous

picture with the addition of concentration of damage sites along a probable fault.

162
Figure 81. Detail of the nucleus again showing

outer surface crack extending only to the level at which damage has occurred.

Figure 82. Detail of the fault time shown in PIHF

7 - 64X (N) . The nucleus crack structure is com- plex and cracking at two different levels in the film is evident. This indicates that damage has occurred simultaneously at 2 different levels in the film.

Figure 83. Complex damage area. Complete removal

of the film and complex crack patterns are evident

Figure 84. Light damage area in the same system.

Figure 85. High magnification SEM picture of a portion of PIHF 9 - 64X (N) . In addition to com- plete removal the multilevel damage and cracking

is clearly evident.

Figure 86. High magnification SEM picture of portion of PIHF 9-256 (N) showing details of frac- ture initiation sites.

163
Figure 87. Heavy damage in a 4 cycle PIHF [similar to PIHF 9 - 64X (N)]

.

Figure 88. Light damage area in the same system.

Figure 89. Perfect illustration of shear of the PIHF due to damage occurrence at a lower level

.

The curling of the film demonstrates its tensile

property clearly indicating that it is mostly

ThF/. .

Figure 90. Beautiful illustration of a section of

a 4 cycle PIHF. No evidence of stratification or periodicity can be seen. The columnar film growth'

characteristic is beautifully illustrated. This

columnar growth in ZnS crysolite multilayer film

systems has been observed by other workers. (6)

164
Fundamental Absorption Mechanisms in High-Power Laser Window Materials

Robert Hellwarth

Department of Electrical Engineering

University of Southern California

Los Angeles, California 90007

The thermal deformation and damage to windows and lenses caused by their

optical absorption poses serious limitations on the operation of high-power CW

and long-pulse lasers. For example, no known materials have both low enough

absorption coefficients and good enough mechanical and chemical properties to perform satisfactorily as windows in the large 10.6 ym lasers being developed

today. However, very many crystals and glasses would be good candidates for highpower IR windows, if their linear absorption coefficients could be made lower by

at least an order of magnitude. Here we review what is known of the physical

mechanisms that are responsible for, or may limit, the small residual absorption

in the "transparent" wavelength regions of the best candidate window materials.

From the primitive state of present knowledge, it seems likely that uncontrolled

impurity effects are presently limiting performance in the best window materials,

and that therefore their high-power performance can be improved further. We

identify theoretical gaps whose filling would greatly improve our understanding

of the mechanisms and limits of laser-window absorption.

Key Words: Absorption limit, extrinsic absorption, high-power laser window

materials, intrinsic absorption, optical absorption mechanisms, thermal

damage.

1. Introduction

When a light beam passes through a window (or other optical component) , a small fraction of the beam energy is unavoidably deposited as heat in the window material or its coatings. If the beam power,

and hence the heat absorbed, exceeds a certain critical value, the window will fail in its function.

This failure can take the form either of a reversible thermal deformation which causes an unacceptable

degree of phase distortion in the exiting beam, or of an irreversible deformation or even rupture in the window material or its coatings. The limitations on laser power and performance set by the thermal

deformations in its windows are a serious problem in the high-power CW and long-pulse CO2 lasers

presently under development. Given various physical characteristics of a window, one can predict these

limitations. Reviews of how these predictions are formulated and their application to some 10.6 urn window materials are given by Sparks in an accompanying paper and elsewhere [1].^ A more exhaustive

assessment of all interesting classes of window materials as potential high-power IR windows has recently been completed by an ad hoc committee of the National Materials Advisory Board [2]. It would

appear that no material available at present could be fabricated into a satisfactory high-power 10.6 ym

window of the sizes desired (> 10 cm diameter). It is also evident from this work that a critical

factor in the performance of a material as a high-power laser window is its absorption coefficient 3 at the laser wavelength. The main hope for fabricating satisfactory high power windows, both presently at 10.6 ym and probably at other wavelengths in the future, lies in producing materials with lower

absorption coefficient than are presently available. At 10.6 ym the best absorption coefficients in the mechanically satisfactory materials are around one percent per cm, and two-orders-of -magnitude lower

absorption is desired.

In this paper we review the relatively little that is known of the physical mechanisms that are responsible for the small but troublesome residual bulk absorption in the "transparent" wavelength

regions (3 < 10~2) between the lattice absorption and electronic excitation frequencies. We emphasize

the problems most pertinent to the fabrication of better laser windows in the infrared and especially

at the CO2 laser wavelength of 10.6 ym. In this examination, it is convenient to separate the mechanisms which give rise to this residual absorption into two classes, which we call "extrinsic" and

*

^ Refer to Section 6 at the end of this paper for Support and Acknowledgements.

Figures in brackets indicate the literature references at the end of this paper.

165
"intrinsic". Extrinsic mechanisms are those associated with unwanted impurity atoms and molecules,

deviations from stoichiometry , and also with those lattice imperfections (dislocations, twinning,

grain boundaries, etc.) which constitute deviations from a thermal equilibrium state of a glass or crystal. The intrinsic absorption mechanisms are those which give rise to the electronic and vibrational absorptions that would exist in an ideal crystal or glass of some specified composition and possessing the structure and vacancy system appropriate to its ambient temperature. Generally speaking,

extrinsic absorption mechanisms appear to be the more troublesome in the best contemporary materials.

For example, despite much effort to achieve intrinsic materials for 10.6 ym windows there is no clearcut case at this writing in which the absorption of a promising window material is in fact intrinsic at this wavelength. Little effort has been applied to understanding the absorption mechanisms at shorter

laser wavelengths.

In the next section (2) we review what is known of the dependence of the infrared absorption on extrinsic mechanisms. Then, in section 3 we review the as yet imperfect methods of estimating the intrinsic absorption well below the band gap and above the intrinsic vibrational resonance frequencies.

In section 4 we comment on what is known of the corresponding mechanisms underlying the possibly

troublesome absorption at surfaces, and also at thin anti-reflection or protective coatings. From these studies, we attempt to identify those areas where better physical understanding would aid in fabricating

materials having lower absorption coefficients. These conclusions are given in the final part (5).

We have found that all the important examples of extrinsic and intrinsic absorption, in bulk or on surface, are of three general kinds: a) absorption by vibrational excitations without electronic

change of state, b) free carrier absorption, and c) absorption by excitation of bound electrons. Usually, the relevant absorption by each mechanism is the "linear" absorption whose coefficient $ cm

- ^ is not dependent on the laser or ambient radiation intensity. However, some nonlinear free carrier

absorption mechanisms may be important. We now proceed to discuss some of the outstanding problems in each of these areas.

2. Extrinsic Absorption Mechanisms

In crystals and glasses of near-ideal structure for a given composition, the small residual

absorption due to impurities, dislocations, etc., can be considered as simply a superposition of the absorption from each such imperfection. That is, the absorption per cm is well-approximated at frequency to by

3(u) = Z

± N. o.(w) , (1)

where the sum is over the various types i of imperfections whose number densities are and absorption

cross sections are 0±. The processes which give rise to the 0-^(0)) may be divided into the aforementioned three types which we discuss below: 1) excitation of polar bond vibrations or local modes without

electronic excitation, 2) absorption by free carriers donated by impurities, and 3) excitation of boundelectron impurity states.

2.1 Excitation of Polar Vibrations

There has been a great deal of experimental and theoretical work on the infrared absorption

lines caused by the addition of various impurities to various crystals in various amounts. (See, for example, the review articles of Newman [3] and of Spitzer [4].) The impurities may enter the lattice

singly or multiply in a number of different types of configurations. Unfortunately, the impurity atoms

or molecules which appear to produce troublesome absorptions at 10.6 ym and other IR laser wavelengths

have not yet been studied adequately in the candidate materials for high-power windows. These

impurities are primarily oxygen and hydroxyl radicals covalently bonded in various configurations.

They typically produce absorption lines in the vicinity of 3, 9, and 13 ym [5,6]. The cross sections

a± at 10.6 ym for metal-oxygen bond absorptions generally lie in the region of 10~18 to 10~20 cm^. The absorption cross section of an 0-H bond around 3 ym is likely to be of the same order of magnitude.

In any given material and at any specific frequency a), the cross sections of relevant vibrations

of these and other impurity atoms, ions, molecules, vacancies, boundaries, precipitates, striations,

etc. , must be determined by experiment. An excellent review of current work on this is given in reference [7]. Theory can do little more than provide the phenomenological framework of eq (1) and suggest guidelines, such as that various covalent bond frequencies of a given atom or complex do not change much from one host to another.

There is another possible effect of impurities in the low absorption region of crystals, whose

importance has not yet been assessed. This is the change in the coupling constants of the electromagnetic field to the intrinsic phonons , and also of the intrinsic phonons to each other, due to the disruption of the lattice periodicity. Since, as we discuss in the next section, the intrinsic

166
absorption at IR laser wavelengths in the "tail" of a fundamental reststrahl band commonly involves the creation of 4 to 100 optical phonons with the annihilation of one photon, these coupling alterations

might well increase absorption. Most importantly, heavy-ion impurities could contribute to this effect

as much as the lighter impurities. When an accurate theory of the intrinsic tail absorption becomes

available, this type of impurity effect will be easier to assess.

2.2 Free Carrier Absorption

Free carriers donated by impurities contribute a term 3e to the optical absorption; this

absorption can be correlated roughly with the dc conductivity. Except in very pure nonpolar crystals,

the free carrier collision time T is of the order of an infrared period (i.e., got < 1) in which case

the infrared conductivity is not orders of magnitude different from the dc conductivity, and the absorption can be approximated by the Drude formula

where p is the resistivity in ohm-cm and n(w) is the real part of the refractive index. Therefore,

materials whose resistivity is less than, or of the order of, 10^ ohm-cm usually have not been considered for high-power windows. An important exception is germanium whose T is so large that the best

commercially available material (whose impurity concentration is around 10-^ cm

-

^ and dc resistivity is around 20 ohm-cm) has an absorption of only 0.017 cm

- -'- at 10.6 ym [8]. This absorption appears to be

partly from extrinsic free carriers, but mainly from extrinsic vibrational absorption (probably of oxygen). However, new fabrication techniques for germanium may reduce these absorption components to their intrinsic values, as we discuss in section 3.2.

2.3 Excitation of Bound Electronic Impurity States

Heavy impurities generally have their vibration frequencies so low that the wing of their vibrational absorptions contributes negligibly at 10.6 ym and shorter wavelengths. This would typically be

the case if av < 10~23 Cm2 for the vibrational cross section as the impurity density can be kept below

10l9 cm~3 and 3 ~ 10

-

^ cm

- -'- is often tolerable. However, heavy impurities might still absorb in the infrared via an electronic absorption tail. The electronic absorption cross section O e in the wing of

a Lorentzian line of half width at half maximum Aoo is given by

-2 -2 2 a (oj) = 47rfr c Aoj [ (oj-co ) - (oj+oj ) ]cm , (3)

e o o o

-13 where r is the classical electron radius 2.8 x 10 cm, c is the velocity of light, and f ~ 1 is the f-number of the transition. In the infrared, to « GO , and

o

0 + 2f r X o>/(to Q) (4) e o o o

where X 0 = 2ttc/w0 and Q = oj 0

/Aoj is the "Q" of the transition. It is well known that heavy-ion

electronic excitations can have a small Q (~ 10 to 100, as do the Cr^+ "pump" transitions in pink ruby

which have X Q

~ 4000 and 5500 A). Therefore, an electronic "tail" absorbtion cross section O e might

reasonably be of order 10- 18 ^2 in the infrared if the line is Lorentzian in its low frequency tail.

| If so, the tail of the electronic absorptions of heavy-ion impurities might well be an important

factor in the material absorption. This would depend largely on whether or not the absorption falls off much more rapidly than a Lorentzian line in the low frequency tail - a question which has not been

studied sufficiently in solids.

How the extrinsic absorption of a typical material might dominate the transparent wavelength

region of a crystal is shown schematically in the plot of 3 versus go in Figure 1. Whether or not there

are actually bumps in the extrinsic portion of 3 versus oj (as indicated in Figure 1) is not yet known;

the absorption below lO-

^ cm

- -'- is too low to have been scanned by conventional techniques. It would

obviously be of great value to understanding window material absorption to know the absorption over a wide range of frequencies in this low-absorption region.

3. Intrinsic Absorption Mechanisms

At present, very little is known of the fundamental lower limit on the absorption of crystals at

| wavelengths where they are the most transparent to radiation, i.e., between their electronic absorption

ledge and their lattice vibration absorption bands. Similarly as for extrinsic absorption, the

167
mechanisms that may be important in setting the intrinsic absorption limit may be divided into three categories: 1) absorption by lattice vibrations, which the electrons follow adiabatically and in which there is no electronic transition involved; 2) free-electron or free-carrier absorption; and 3) bound-electron absorption. We shall review what is known of each of these mechanisms separately below

for both ideal crystals and glasses.

3.1 Lattice Vibration Absorption

There have been a number of recent studies of how the infrared absorption drops off as the fre- quency becomes much greater than the fundamental lattice absorption frequencies, and also becomes much

greater than the prominent overtone or combination absorption frequencies [8-10]. These studies have

been mainly on the alkali halides and MF2 crystals. In the more highly purified samples, it appears

that the absorption coefficient 6(w) universally exhibits an exponential fall-off over the lowest two- to-four decades of 3 that are measurable. That is, for U) >> ^>jq>

-w/u) « e

0

, (5)

where 0Jo is of the order of the reststrahl frequency w^o an^ varies little with temperature at room temperature [11]. (For example, w0

~ 1/2 a>po for KBr.)

In some crystals (e.g., A£>203> SrF2» BaF2, LiF, and CaF2) the observed absorption at 10.6 Mm is clearly part of this tail, and therefore probably intrinsic. However, in these cases the absorption at 10.6 ym is so high that the crystals are not good window material candidates. It is probable, though

not certain, that the observed low absorption at 10.6 ym in many of the more interesting candidate

materials (KBr, T£(Br,I), KC£, ZnSe, CdTe, GaAs, and Ge crystals, and chalcogenide glasses) lies above

this intrinsic absorption tail, roughly as illustrated in Figure 1. This is certainly the case for the 2-5 Mm lasers.

whether this exponential absorption tail is characteristic of all other classes of materials,

including glasses and nonpolar crystals, is not yet known. Clearly, it would be desirable to establish

some sort of theoretical basis for (5) to understand when, and to how low an absorption this very sug- gestive behavior obtains.

3.2 Free-Carrier Absorption

The theory of 2.2 above and the approximate equation (2) may also be applied to intrinsic free- carrier absorption. The only interesting material for which this may be an important limiting factor

at 10.6 pm is "ultrapure" Ge. Inch-size ingots of germanium, whose room temperature properties may be intrinsic, have recently become available on^a limited basis [12]. Pure Ge has a hole concentration

at room temperature of around 1.5 x 10 cm" and hence a free-carrier absorption of order 10" at 10.6 \lm, about fifty percent lower than the best commercial germanium. Whether the total infrared

absorption is correspondingly lower is not yet known because there may still be extrinsic or intrinsic

lattice absorption of this order of magnitude in these "ultrapure" samples. That intrinsic free- carrier absorption may be reduced further in Ge by n-type compensation is pointed out in reference [11]. In fact the absorption of the best commercial Ge has been successfully reduced by such compensation [11].

It has been suggested that, even if free-carrier absorption is negligible, free-carriers may con- tribute significantly to absorption below the band gap by interband transitions in semiconductors [13]

.

The cross sections for such processes deserve further investigation. High intensities at laser and/or

visible and ultraviolet frequencies may increase the free-carrier concentration by photo-excitation.

The resulting intensity-dependent (nonlinear) absorption has not yet been analyzed.

3.3 Bound-Electron Excitation

We need consider only those materials which have a band gap (i.e., minimum nominal energy to excite bound electrons to a conduction band) that is much larger than the laser frequencies of interest.

It is, therefore, only the low frequency tail of the fundamental gap absorption that might be of importance in a low-absorption laser window. This tail 3t(w) to the fundamental electronic absorption

has been observed to be of the form

5fiWkJ

B (oi) <* e (6)

in a wide variety of ionic, covalent, and amorphous materials. (Here, C is a constant of the order

168
unity, kg is Boltzmann's constant, and T the absolute temperature.) This dependence was first noticed

by Urbach in alkali halides and has been found to be so widespread that it is called Urbach's rule

[13,14]. The most recent and promising of many attempts to construct a theoretical basis for this

general behavior is that of Dow and Redfield [15].

In any event, it would seem that we may estimate with some confidence the contribution 3 t to the intrinsic absorption from electronic transitions by extrapolating measurements made just below the band

gap [16]. The Urbach tail for a typical material is indicated in the 3 versus u) plot of Figure 1. The

Urbach tail is so steep that it could produce significant absorption only for those materials whose

band gap is not more than ~20 kgT above the infrared frequency one wishes to transmit.

4. Surface and Coating Absorption

Surface and coating absorption can affect high-power window performance in several ways. The

surface heating can cause thermal distortions of the optical phase front. If fragile surfaces, such as

of the alkali halides, are subjected to high-power beams, the surface cracks and imperfections may propagate or enlarge, or become hygroscopic. When anti-reflection or passivating coatings are used,

surface absorption may tend to dislodge or even evaporate the coatings. The theory of surface and coating absorption may be thought of in terms of the same six categories that we have used in discussing

bulk absorption; by vibrational, bound-electronic, or absorption free-electronic excitations of either

extrinsic or intrinsic types. We comment separately on problems in 1) surface and 2) thin-film

coatings.

4.1 Surface Absorption

There is some evidence that surface absorption by oxygen bonds has been observed near 2.7 urn in various crystals [8]. In this case, we would, as in 2.1, assign an absorption cross section 0± to the

ith impurity bond type and write the fraction of the beam Fg absorbed at the surface as

F

s

(03) = E. M. a.(w) , (7)

where is the number per unit surface area of this type of imperfection. For example, to estimate the surface absorption at 10.6 ym due to a monolayer of oxygen, we suppose that the cross section Oj_ is of the order of the bulk absorption cross section (~10~18 Cm2) and the number per unit area is the two- thirds power of the atomic density of the crystal (~10^) . In this case we estimate Fg to be of order

10~3. Although this is a significant fractional absorption in high power applications, it is very difficult to measure with existing techniques. Perhaps if the Oj_ could be determined for the important

impurities, the more easily measured surface density would give a useful estimate for the surface

absorption Fg of a given sample.

Both linear and nonlinear absorption by surface free-carriers and surface bound-electron states may well be important, but have received little attention to date.

4.2 Thin-Film Coatings

In addition to introducing extra surfaces, and their associated absorptions, thin-film coatings

have a "bulk" absorption that may often be allowed to be significant in order to obtain other characteristics required of a useful coating. Unfortunately, it has become evident that the absorption

coefficient 3 in films is generally much larger than the 3 for the same nominal material in bulk (see, for example, the article 14 of reference [7]). The real part of the refractive index measured in a film is also different than that measured in bulk. These discrepancies have been observed to be a function of film preparation and the mechanisms responsible are not yet understood [17], Nonlinear

absorption processes are more likely to be important in films than in bulk, but estimates of these have

not been made.

5. Conclusions

It is probable that the low, but deleterious, infrared absorption that has been measured in the best candidate materials for high-power laser windows is extrinsic in origin. Therefore, the possibility exists that more careful preparation and purification of these materials will lower their

absorption. Unfortunately, the materials with the best thermal and mechanical properties tend to need

the most improvement in absorption. From our review of the gaps in theoretical knowledge of mechanisms

of absorption, we conclude that a better theoretical grasp of the following facets of the absorption

problem would aid the choice and fabrication of a satisfactory high-power infrared laser window. The

list is arranged roughly as we see the relative priorities of the theoretical problems to window fab- rication.

169
5.1

A practical limit to the improvement in window absorption that is possible in many cases of interest exists by virtue of the nearness of the laser frequency to intrinsic lattice vibrational fre- quencies and their overtones. The strength of this lattice-absorption tail has been traced experimentally so far mostly in polar crystals, where it has been found to fall exponentially with frequency over

the lowest decades of absorption measured. This behavior should be given a theoretical basis from

which to predice a) whether the exponential trend persists to lower absorptions, b) how the absorption

tail behaves with changing temperature and composition, and c) if this tail is to be expected in other classes of materials, especially glasses and nonpolar crystals.

5.2

It is clear how various species of ionic impurities can vibrate to cause "local mode" or bond absorption resonances above the reststrahl bands. However, these impurities may cause a much less obvious raising of the background absorption by virtue of the anharmonicity , disruption of periodicity,

and general change in phonon couplings, which they cause. An estimate of the changed background

vibrational absorption from these effects should be made.

5.3

At the high levels of both the infrared beam intensity and the ambient visible and ultraviolet

intensities that high-power infrared-laser windows may have to withstand, photoconductive free-carriers may introduce troublesome added (nonlinear) absorption. From existing data on the wavelength dependence

of photoconductivity, theoretical estimates should be made of this effect in candidate materials. These

effects of photoexcitation seem to deserve special attention at surfaces and in surface coatings.

5.4

Free-carriers, whether extrinsic, intrinsic, or photo-excited, may absorb by interband transsitions. Such absorption might well peak near the laser frequency and exceed normal (intraband) free- carrier absorption at wavelengths of interest. Some straightforward band theory ought to provide a prediction as to when such an effect might be important.

5.5

The absorption by electrons localized on impurities usually peaks at much shorter than infrared

wavelengths. Such absorption commonly has been observed to be so strong at its peak that it may well

contribute a small but significant absorption far from resonance at the longer laser wavelengths of interest here. The theory of the shape of these electronic absorption tails on their low frequency

side should be developed in analogy with the existing theory of the Urbach tail.

5.6

Although many of the discrepancies observed between values of absorption and refractive index

in bulk and in thin film can be attributed to "dirt" effects, there may be some intrinsic change

occasioned by the forced lattice distortions and proximity of surface states in the films. It would be helpful to the development of window coatings if some rules-of-thumb could be developed to predict the nature of these absorption and index discrepancies.

1 To this list many other potentially troublesome sources of absorption might be added. Studying

such subjects as the formation of color centers, radiation damage, and light scattering from impurities ; might well expose some important contribution to the eventual amount of heat that will be deposited

from a laser in the window material. We can only hope to have uncovered some of the more obvious problems and assessed them cursorily in a study so brief as this.

6. Support and Acknowledgements

This research has been supported in part by the Joint Services Electronics Program through the Air Force Office of Scientific Research under Contract F44620-71-C-0067 and in part by the Hughes

Research Laboratories, Malibu, California. The results are based partly on work undertaken for the Ad Hoc Committee on High-Power Infrared-Laser Windows of the National Materials Advisory Board. A complete report of this committee's findings is to be published shortly as NMAB Publication 292 (see reference [2]).

170
7. References

[1] Sparks, M. , J. Appl. Phys. 42, 5029 (1971).

[2] "High-Power Infrared-Laser Window, "report of

the Ad Hoc Committee on High-Power InfraredLaser Windows of the National Materials Advisory Board. (Publication NMAB-292, National

Academy of Sciences-National Academy of Engineering, to appear August 1972).

[3] Newman, R.C. , Advances in Physics, 18, 545 (1969).

[4] Spitzer, W.G., Advances in Solid State Physics,

(Festkorperprobleme) XI, 1 (1971).

[5] Nielson and Savage, Infrared Physics, 5_, 195

(1965) : "Chalcogenide Glasses Transmitting

in the Infrared between 1 and 20 ^lm."

[6] Vasko, A.,Lezal, D. and Srb , I., J. Non- Crystalline Solids, 4, 311 (1970): "Oxygen

Impurities and Defects."

[7] Proceedings of the Conference on High Power Infrared Laser Window Materials, (Oct. 1971).

Ed. by C.S. Sahagian and C.A. Pitha (Air Force Cambridge Research Laboratories, Special Re- port No. 127, Dec. 1971).

[8] Horrlgan, F.A. and Deutsch, T.F., Final Technical Report, Contract No. DAAH01-70-C-1251

,

(Sept. 1971), "Research in Optical Materials

and Structures for High-Power Lasers,"

Raytheon Research Division.

[9] Genzel, L., Fest Korper Probleme, 6, 32 (1967),

"Optische Absorption von Fest Korpern durch

Gitterschwingungen.

[10] Joos, G. , reproduced in Introduction to Solid State Physics (4th edition) by C. A. Kittel,

(John Wiley, New York, New York, 1970),

p. 155, from FIAT Rev. Ger. Sci. (1939-1946) Part II: The Physics of Solids, "Optische

Eigenschaften der Festen Korper."

[11] Horrigan, F. A. and Rudko , R.I., Final Technical Report, Contract DAAH01-69-C-0038

,

Raytheon Research Division, (September 1969),

"Materials for High Power C0„ Lasers."

[12] Hall, R.N., General Electric Research and De- velopment Center, Schenectady, New York,

(Private Communication.)

[13] Briggs, H.B. and Fletcher, R.C., Phys. Rev.

91, 1342 (1953); Kahn, A.H., Phys. Rev. 97, 1647 (1955); Wiley, J.D. and DiComenico, Jr.,

M.D., Phys. Rev. B3, 375 (1971).

[14] Urbach, F. Phys. Rev. 92, 1324 (1953). For a review of Urbach's rule, see J.J. Hopfield

Comments on Solid State Physics, I_, 16 (1968)

.

[15] Dow, J. D. and Redfield, D. , Phys. Rev. Letters, 26_, 762 (1971), "Theory of Exponential Absorption Edges in Ionic and Covalent

Solids."

[16] Martienssen.w. , J. Phys. Chem. Solids, 2, 257

(1957).

[17] Young, P. A., Thin Solid Films, 6, 423 (1970);

Young, P. A. and Thege, W. G. , J. Phys. Soc,

London, D4, 64 (1971).

8. Figures

1 \

FREQUENCY, cm"' Figure 1. Schematic plot of the optical absorption coefficient 3 versus optical frequency for a typical material that would be a good candidate for a high-power-laser window by virtue of its low extrinsic absorption (3 < 10" ) . The extrinsic-absorption-region spectrum of a good window

material has never been scanned and is drawn here entirely from imagination. As is discussed in

the text, the exponential forms of the intrinsic vibrational absorption tail and the electronic

absorption tail appear to be universal.

171
Recent Developments in High-Power Infrared-Window Research

M. Sparks

Xonics, Incorporated, Van Nuys , California 91406

The problem of failure of high-power infrared laser-system windows involves low intensities (a few hundred to a few thousand W/cm ). Thermally induced distortion of

the optical beam by a heated window and thermal fracture have been considered previously. Pressure-induced optical distortion is discussed. For large-diameter windows

of weak materials, the minimum window thickness is determined by pressure-induced

optical distortion, rather than by pressure-induced fracture. Previous figures of merit for rating candidate window materials assumed a constant thickness of 1 cm for all materials. New figures of merit, based on the minimum thickness required to withstand a given pressure, indicate a different preference for materials than that of

the previous figures of merit.

It is not known if the measured values of the optical absorption coefficient 8 for candidate materials are extrinsic or intrinsic. The intrinsic value of B is needed in order to determine if material-improvement programs could produce materials

with sufficiently low values of 8. Information on extrinsic processes is needed in order to identify the sources of the extrinsic contributions to 8 so that they may be removed. Measurements of 8 over ranges of frequency and temperature should be in- valuable in obtaining this information. Feasibility studies indicate that emissivity measurements should yield the required values of 8, which cannot be obtained by current transmission or calorimetric measurements.

Key Words: Infrared absorption, infrared laser systems, thermal fracture,

window materials.

The availability of high-power infrared lasers during the last few years has caused a shift of in- terest in infrared window material. For low-power applications, previous interest was in the so-called

cut-off wavelength, at which a material becomes opaque, roughly speaking. For a 1 cm-thick sample,

absorption becomes measurable in transmission when the optical absorption coefficient 6 reaches a value

of 8 — 0.01 cm~^ (approximately one percent absorption). For low-power applications, the absorption of several percent of the incident power usually is tolerable.

For currently envisioned high-power applications, the absorption of even a small fraction of one percent of the incident power can cause the window to melt, fracture, distort the JLaser beam or otherwise

cause the system to function improperly. Thus, values of 8 at least as low as 10" cm~l are of current

interest. As more powerful lasers become available, even smaller values of 3 will be needed.

The chief current interest in high-power infrared window materials is in obtaining a sufficiently

low value of 8 for materials that are otherwise satisfactory. Alkali halides have the lowest values of

8, but they have poor physical, thermal, and chemical properties. Semiconductor materials have good

physical, thermal, and chemical properties, but only fair optical properties, while glasses have good

chemical and physical properties, but poor thermal and optical properties.

Before considering several interesting results from the past year, recall the effect of thermally

induced optical distortion.fi] As illustrated in figure 1, an incident laser beam having greater in- tensity at the center of the window causes a greater temperature rise at the center than at the rim of

This research was supported by the Advanced Research Projects Agency of the Department of Defense and was monitored by the Defense Supply Service - Washington under Contract No. DAHC15-72-C-0129.

*

Figures in brackets indicate the literature references at the end of this paper.

172
the window. The bulging of the window and the change in the index of refraction cause the window to become a lens with finite focal length and nonzero aberration. This effect is severe in long-focallength systems since small angular deviations (10 microradians in figure 1) can cause significant de- gradation in the intensity at the target.

A similar effect is caused by a pressure differential across the window. The bowing of the window

caused by the pressure gives rise to optical distortion. At the suggestion of H. V. Winsor [2], M. Sparks and M. Cottis [3] examined the pressure-induced optical distortion-problem. A simple solution of

the mechanical -deformation problem indicates that the window displacement

^ contains terms proportional

to p^, p^* , and , where p is the radial coordinate (see figure 2). The p and p terms were considered

because the optical distortion for these terms is known. [1] The effects of the two terms are of the same order of magnitude, but the p^ term causes slightly more distortion. The p^ term was formally ne- glected since the optical analysis for this term has not been made. A straightforward calculation was performed to determine the values of thickness t at which an originally diffraction-limited target spot

is reduced in intensity by a factor of two.

The results are illustrated in table 1. The first column of figures on the left shows the value

of window thickness required to keep the window from fracturing under pressure P = 7.3 psi with a safety

factor of 4. The values of K> in the three right-hand columns are those required to keep the pressureinduced optical distortion from halving the target intensity. The minimum value of K> is the greater of

K> r and KiQ. An underscored value of -t» o indicates that -t > Lr. For example, in order to prevent optical

distortion from halving the target intensity, a 100 cm-diameter window of GaAs must be 3.18 cm thick,

which is larger than the value of 1.65 cm required to keep the window from fracturing. The minimum value

of t is 3.18 cm. A 1 cm-diameter window of GaAs must be at least 0.165 cm thick, which is larger than the value of 0.126 cm required to keep the pressure-induced optical distortion from halving the target intensity. The thickness of large-diameter windows of strong materials is limited by pressure-induced optical

distortion, while the thickness of small-diameter windows of weak material is limited by pressure-induced

fracture. Great thicknesses are required for the alkali halides; a 100 cm-diameter window of KBr with

minimum thickness of 18.5 cm weighs 880 lbs.

A second effect of pressure is to modify the previous figures of merit of window materials. In the figures of merit for thermally induced optical distortion, it was assumed that all materials were 1 cm

thick. [1] C. M. Stickley [4] pointed out that weaker materials require greater thicknesses, thereby

causing greater optical distortion. New figures of merit have been obtained by replacing the value of t = 1 cm by If or t0 , whichever is larger. The results are given in table 2. Here the figure of merit

is defined simply as the average value of intensity the window can withstand without thermal fracture

(second column of figures) or undue thermally induced optical distortion (last three columns). The listed

values of the figures of merit apply only under the following conditions: diameter D = 10 cm, P = 7.3 psi,

wavelength X = 10.6 \Jm, and safety factor F

g = 4. The underscored value of 3.76 kw/cm for ZnSe indicates

that the intensity which can be transmitted without undue thermally induced optical distortion is less than the intensity at which the window fractures thermally. The underscored value of 3.02 kW/cm for KC1

indicates that a 10 cm-diameter window of KC1 fractures thermally before the thermally induced optical

distortion becomes severe.

The materials in table 2 are arranged in the order of decreasing magnitude of these underscored

values. This preference of materials is different from the previous one which was based on the values

of intensity for 1 cm-thick materials. For example, ZnSe is first in the current rating compared with

fourth in the previous rating. Potassium bromide is on the bottom of the present list because the demonstrated value of 8 has been used. If the estimated value of 5 X

10"-' cm" is used, KBr is at the top of

the current list. It should be emphasized that in general the rating is changed when the values of diameter, wavelength, pressure, and safety factor are changed. For example, increasing the diameter from

10 to 100 cm and the pressure from 7.3 to 14.7 psi and decreasing the safety factor from 4 to 1 increases

the ratio (^Af^^tf by a factor of 28>

There has been an increased interest in alkali-halide materials based on the belief that these materials can be strengthened and coated to protect them from the atmosphere. There are several methods

of increasing the strength of the alkali halides, including fabrication of polycrystalline materials with

small grains and internally formed grain boundaries, using solid solutions such as KCl-NaCl, or adding

impurity ions such as Sr to crystals such as KBr. Assuming that the strengthening and coating is suc- cessful, the crucial question remaining is whether these processes will increase the optical absorption

to unacceptable values.

Large (~15 X 15 cm) samples of chemical-vapor-deposited polycrystalline ZnSe with 0 = 0.004 cm

were grown by Raytheon under contract to the Air Force Materials Laboratory. [5] This value of 8 is an

order of magnitude lower than the previous experimental value but is still not sufficiently low for many high-power applications. There are three cardinal questions concerning ZnSe. Is the value of 8 = 0.004

cm" intrinsic (already at the basic physical limit) or extrinsic? If it is extrinsic, what imperfection

causes 8 to be greater than its intrinsic value? If extrinsic, what is the intrinsic value? If the current value of 8 is intrinsic, no material-improvement program to reduce the value of 8 can succeed.

If the value is extrinsic, an estimate of the intrinsic value is needed in order to determine if a mate- rial-improvement program could in principle yield the value of 8 required in a particular application.

173
Table 1. Minimum Window Thickness

Material Fracture

10~2£

f/D

Pressure-induced optical distortion 10~ z

£ /D

o' D = 1 cm D = 10 cm D = 100 cm

ZnSe 3.00 1.23 1. 96 3. 10 CdTe 8. 03 1.66 2.63 4. 17 GaAs 1. 65 1.26 2.00 3. 18 Ge 2. 01 1.24 1.96 3. 10 KC1 12. 9 1.40 2.21 3.51 KBr 18.5 1.49 2.36 3. 74 NaCl 12.5 1.25 1.99 3. 15 KRS-5 (TIBr-TlI) 3. 80 2.23 3.53 5.60 Ge

28 Sb12 Se60

glass 4.27 1. 84 2.92 4.63

Table 2. Figures of merit for pulse mode in 10 cm diameter windows

Thermally induced optical distortion

Material

Measured

values of 8 in cm "1

Thermal

fracture

< 1 >tf

I by pressure

fracture

<I>,

f 0

4 by pressure

distortion

(I),

i = 1 cm

(previous

f of m)

<I>,1cm

kW/cm2 kW/cm2 kW/cm2 kW/cm2

ZnSe 5 x 10" 3 20.4 3.76 5.76 1. 13 KC1 4.8 x 10"4 3.02 37. 8 220. 48.8 CdTe 1.5 x 10" 3 14.6 1.07 3.25 0.859

NaCl 1.3 x 10" 3 0.971 23.

1

145. 29.0

GaAs 5 x 10" 3 40.5 1. 15 0.949 0. 190 KRS-5 5 x 10" 3 3.62 0. 377 0.406 0. 143 Ge

28

Sb

12 Se60

glass 0.02 1.56 0. 171 0.251 0.073

Ge 0.025 4.9 0. 149 0. 154 0.030 KBr 4 x 10" 3

0. 144 3. 75 29.6 6.98

174
Knowledge of the source of the extrinsic value of 8 would change the material-improvement program from

a cut-and-try experiment to one of removing the known imperfection. These three key questions actually

apply to every material of current interest, not just to ZnSe.

It appears that we are now in a position to obtain answers to these questions by a joint theoretical

and experimental program. In figure 3, the heavy curve and the heavy dot at 10.6 |_lm are experimental

values obtained at 300 K, and the remaining portion of the curve is a schematic illustration of the re- mainder of the absorption curve. There is a region of very small intrinsic absorption bounded by the fundamental lattice peak and the electronic absorption edge. At sufficiently small values of intrinsic

absorption, B is expected to be dominated by extrinsic processes, as illustrated schematically on the figure.

To illustrate how the three questions above would be answered , assume that the schematic illustration

in figure 3 is the experimental result. The different character of the curve to the right and left of

uu/u)^ » 6 suggests that the curve to the left is intrinsic and to the right is extrinsic. This can be verified by measuring the temperature dependence, which is well understood for the intrinsic multiphonon

absorption. The temperature dependence illustrated in figure 3 suggests that identification of the tran- sition from intrinsic to extrinsic behavior was correct. The temperature and frequency dependence of the extrinsic processes is needed in order to identify the physical sources of the absorption coefficient in the extrinsic region.

Sufficient power to perform calorimetric measurements is not available at all frequencies of interest.

Transmission measurements cannot yield values of B below approximately 10"^cm~^ for 1 cm-thick samples.

Thus, the light curve to the right of uu/u)f s» 6 cannot be obtained by calorimetric or transmission measurements. However, preliminary calculations indicate that it should be possible tg obtain these values of 6 by emissivity measurements. These calculations indicate that values of 0 = 10 cm" and possibly even

lower can be measured by the emissivity technique.

The recent shift of interest to very small values of the absorption coefficient and the possibility

of measuring these small values by emissivity measurements suggest a greatly increased use of emissivity

measurements over the next few years.

References

Dayton, Ohio, private communication.

[1] Sparks, M. , J. Appl. Phys. 42, 5029 (1971). [4] Stickley, C. M. , Advanced Research Projects

Agency, Washington, D. C. , private communica-

[2] Winsor, H. V., AFWL , Kirtland Air Force Base, tion. New Mexico, private communication.

[5] Donlan, V., Air Force Materials Laboratory,

[3] Sparks, M. and Cottis,M. , "Pressure-Induced

Optical Distortion in Laser Windows," unpublished . Figures

Distorted Ray

Undistorted Ray

1<V rad

Heated Wind

Cool Windo

Figure 1. Thermally induced optical distortion

Distorted Ra

Figure 3. Study to determine intrinsic and ex- trinsic values of 6

Figure 2. Pressure-induced optical distortion

175
Low Emittance and Absorptance Measurements

of Windows and Mirrors

Geert Wijntjes

Norman J.E. Johnson

J. Morris Weinberg

Block

19 Bl

Cambridge

,

Engineerin

ackstone S Massachus

g, Inc. treet

etts 02139

A cryogenic interferometric spectrometer has been developed

which can achieve a noise equivalent spectral radiance (NESR) of

1.2 x 10~H watts/cm2 -ster-cm-1 at 10 ym in 10 seconds of integration, with spectral resolution of 1 cm

-1

. The application of this instrument to the measurement of low

level emittance or absorptance in optical elements is described.

In such measurements, the specimen is kept at 300°K and its sur- roundings at 77°K. The measurement is limited by the sensitivity

of the instrument, the accuracy of the specimen temperature measurement, and the dynamic range of the measurement. The first

limitation is expressed by the noise equivalent spectral emittance.

(NEAe ) = R^CT)

where (T) is the blackbody radiance at temperature T. With reasonable xntegration times the determination of emittance is feasible

to a precision approaching a part in 10^, which represents the fundamental limitation to such measurement. The other limitations are proportional to the average emittance over the spectral range con- sidered, and for average emittances below 0>01» permit absolute

emittance measurements to a few parts in 10°. The high spectral

resolution of this instrument permits examination of absorption

band structure in substrate and coating materials, and the identification of contaminant organic materials. The measurement of bulk

scattering in transmissive materials is also possible.

Key Words: Absorptance measurement, bulk scattering, coating measurement,

emittance measurement, infrared absorptance, laser damage.

1. Introduction

The advent of high power infrared lasers with the requirement of high reflectivity mirrors and high transmission windows places crucial emphasis on precise measures

176
of the weak absorptance of such optical elements. Sparks described how significant

optical distortion is produced by distributed heating resulting from bulk absorption.

Localized heating due to impurities can result in serious damage to optical elements

and antireflection coatings. These phenomena may be difficult to evaluate at infrared

wavelengths using contemporary techniques; e.g. an absorptive particle embedded in a visually opaque window may become the nucleus of a destructive hot spot, while its

effect on the overall transmission may be less than one part in ten thousand. This

paper describes an advanced technique for the determination of the spectral absorpance of such optical elements to a precision of one part per million in the 6-14

micron region, as well as means of evaluating the bulk scattering of transmissive

elements

.

A cryogenic interferometric spectrometer has been developed by Block Engineering,

Inc. for the NASA Manned Spacecraft Center, which operates at liquid nitrogen temperature (77°K) with a detector cooled to liquid helium temperature (5°K) . This instrument

has a spectral range of 5 - 15 microns with a constant highest spectral resolution of

0.5 cm"! over the spectral range. A single scan produces an interferogram in approximately 0.5 seconds, providing information to produce the complete spectrum. Commonly,

many scans are made and accumulated in computer storage or on tape to reduce noise in

the interferogram and the resulting spectrum. The Fourier transformation of an interf erogram to a spectrum is performed by a computer, which can also subtract the background radiation spectrum from that of the combined source and background radiation

spectrum. Typically, a transformation utilizing 2K of core memory takes about 15

seconds, and the spectrum can be computer plotted linearly with wavelength or wavenumber.

The instrument was calibrated in a cold chamber at Arnold Engineering Development

Center (AFSC) . On the basis of this calibration, an NESR of 1.2 x 10"11 watts/cm^-

ster-cm~l can be realized in practice at 10 microns in a ten second integration time,

with a spectral resolution of 1 cm

- -'- (0.01 microns at 10 microns). The development of

a cryogenic interferometer with this performance capability permits the evolution of

techniques, such as those described here, to new levels of refinement.

2 . Technique

[2] As shown by Stierwalt and Potter , the measurement of emission is an effective way

to determine the absorptance of optical materials. In their method, the spectral

radiance of a specimen at a known and controlled temperature is measured with a prism

spectrometer relative to the radiance of a blackbody source at known temperature. The

resulting difference between specimen and blackbody reference radiance is a direct

measure of the specimen's emittance, and thus its absorptance. No indication of the

precision of their measurements was given, but this is clearly a function of the precision with which the temperature and emittance of the blackbody reference and the

temperature of the specimen are known, and of the sensitivity of the detector.

The great superiority of interf erometric spectrometers to dispersive spectrometers

lies in the multiplex (Fellgett's) advantage. This gain can be realized by commercially available instruments at infrared wavelengths in the 10 micron region, together

with the built-in computing capability provide immediate spectrum integration and

transformation. The application of this type of instrument to emittance measurements

provides a great increase in sensitivity without sacrifice of spectral resolution.

Further gains in sensitivity are possible by cooling the interferometer to a temperature such that its own radiation for the particular wavelength region approaches zero. Cooling also increases detector sensitivity by as much as three orders of magnitude

[ and, if one cools the specimen chamber as well, the sole remaining source of radiation

is the specimen.

Figures in brackets indicate the literature references at the end of this paper.

177
The sensitivity of a spectrometer can be expressed in terms of its noise equivalent spectral radiance

NESR = — watts/cm ster cm" . 0

i

n

i yff 6v

Where the NEP is the effective noise equivalent power for the detector, 0j_ is the optical throughput (area x solid angle) and rii the optical efficiency of the interferometer, T is the total time for Ns scans of the spectrum, and 6v is the spectral reso- lution of the instrument in wavenumbers (cm- -'-)

. We may describe the NESR as the spectral radiance giving unity signal-to-noise ratio averaged over the time T of the measurement. (The use of wavenumber rather than wavelength here is common in interferometry and results in some simplification of expression.) The spectral radiance of the

specimen is defined in terms of a blackbody as

R

v

(T) = e v R° (T) watts/cm2 ster cm" 1

where R°(T) is the spectral radiance of a blackbody at temperature T and e v is the

spectral emittance of the specimen. The minimum detectable emittance difference is

then

(NEAe) = NESR

R°(T)

V

The noise equivalent emittance is shown in figure 1 as a function of wavenumber

for an interferometer equipped with a HgCdTe detector cooled to 77 °K viewing a 300 °K

specimen. The interferometer is uncooled, and it is assumed that the detector views a 300 °K background. (It would be necessary, of course, to provide a cooled background

behind transmissive and in front of reflective specimens in order to distinguish the

specimen emittance.

)

In figure 2, the noise equivalent emittance is given for an interferometer

equipped with a Ge:Hg detector cooled to 5°K. The interferometer and blackbody background are cooled to 77°K, while the specimen is maintained at 300°K. The dramatic

improvement here can be achieved in practice because the 77 °K background (shown by

the dashed line) can be subtracted from the measured spectral emittance as indicated,

permitting emittance differences smaller than a part in a million to be measured.

System sensitivity is fundamentally limited by detector noise, arising both from

background radiation (photon noise) and from noise mechanisms within the detector itself. Modern cooled infrared detectors have internal noise equivalent powers (NEP^)

of 10~13 to 10~14 watts, and are photon noise limited for backgrounds at elevated

temperatures. The multiplex advantage of the interferometer is not fully realized

until the detector is limited by internal noise and no further improvement is possible

by reducing background radiation. The point at which the transition occurs, for a photoconductor , may be characterized by the product

1.259 x 10 22 (NEP

i

) 2 qe

, e (v--v-) $ G. = cm . V 21 1 v Q.n- R°(T) Oil v Here qe is the detector quantum efficiency, V]_ and \>2 the limits of the spectral band,

v 0 the band center, and R§ (T) , assumed to vary slowly, is averaged over the band.

Typically the factor is about 0.1, and the compromise between average emissivity

and spectral band is obvious. Figure 3 shows the optimization of a spectral band for

emissivity measurement. As the spectral interval is increased beyond V]_ and V2/ both rv and (^y~vx

) are increasing, and G^ is easily exceeded.

The absolute accuracy of an emittance measurement is related primarily to the error in the evaluation of the temperature of the specimen. From the Planck radiation

law, one can obtain

ZfT _|" x A

e v Ll-e-x

J ^T where x = 1.438 v/T.

178
At 1000 cm

-1

(10 microns), with a 300°K specimen, the factor x/(l-e~x ) is 4.83, showing the sensitivity of this factor. For a specimen with good thermal conductivity or

very low overall emittance, however, it should be possible to reduce AT/T to better

than a part in ten thousand. The spectral emittances at different wavelengths are as precise as indicated in figure 2, relatively speaking.

For the curves, a spectral resolution of 1 cm"^ was assumed, corresponding to 0.01

microns at 10 microns. This is in excess of the line spacing of a CO2 laser, which is

about 1.6 cm

- -'- , and is sufficient to allow identification of oil and aerosol contaminants. An integration time of 40 seconds was arbitrarily assumed as a reasonable

period in which to obtain complete spectral information over the 6-14 micron region

considered.

The values of NESR used to obtain these curves are based on calibrations of

actual interferometers built at Block Engineering, Inc. The NESR and (NEAe) v can be

improved linearly by decreasing the resolution, and as the square root by increasing

the integration time. One can also heat the samples to a higher temperature, increasing their radiance and improving the emittance measurement at shorter wavelengths.

Better detectors have recently become available for low background applications,

offering improvement in sensitivity by more than a factor of five and providing a limiting value for (NEAe) v approaching 10~7.

For practical reasons, the total dynamic range of measurement is limited relative

to the amplitude of the average radiance in the spectral band, giving the emittance

uncertainty

Ae,

e R° (T) v v(v. -v

R° (T) V 6v 2 M VnTT ft

where M is the number of bits in the digitization and N is the number of resolution

elements 6v in the spectral interval. (Other limitations in practice exist, but this

probably gives a better feeling for the nature of the limitation.) The range of the

emissivity measurement can be reduced to give full (NEAe) v sensitivity at emissivities

of greatest interest. The spectral range can also be reduced, which can provide similar improvement to that discussed earlier in reference to figure 3. We may compare these factors in the measurement of a 300° ± 0.03°K specimen in 40 scans with a 14 bit digitization and 1 cm~l resolution:

x 2 -x

1

e v

£ v

(v 2-V

l' T A£ R

Case A 6-14ym 0.5 476.0 cm" 1 2. 4 x 10~4 1 5 x 10~ 4 Case B 10-llym 0.01 0.91 cm" 1 4. 8 x 10~ 6 9 2 x 10~ 7 Case C 10.55-10. 65ym 0.01 0.09 cm" 1 4. 8 x 10" 6 1 4 x 10~ 7 Case D 10.55-10. 65ym 0.002 0.02 cm"1

9. 6 x 10~ 72 8 x 10" 8

The reduction of the spectral band is usually accomplished by placing a cooled

narrow band filter between the detector and the interferometer. Clearly any spectral

band within the limits of the detectors spectral response could have been chosen simply

by changing the filter, however, the interferometer can be optimized for a given band.

3. Instrumentation

The generalized measurement technique is shown in figure 4, where a large vacuum

chamber contains a cooled inner chamber in which the interferometer is mounted. The

detector, mounted in its own dewar, views the specimen through the interferometer. An

aperture, which defines the interferometer field of view, permits examination of the

specimen while shielding the specimen support and heater from the interferometer.

Behind the specimen is a blackbody calibration source, which is maintained at the 77°K

background temperature during specimen measurement. This blackbody provides the transmitted radiation through the specimen, while the cold interferometer itself provides

the reflected radiation. The system is calibrated by measuring this blackbody emission

with the specimen removed. Generally, the calibration would be made with a cold grid

in front of the blackbody to provide a precise attenuation of its radiation (probably

300°K). If it is desirable to examine small areas of the specimen, several scanning

techniques are possible. A sliding cold mask with a small aperture could be used with

some performance degradation, or the more complicated optical system suggested in the

179
figure, which would preserve optical throughput and system sensitivity could be in- corporated.

The most critical aspect of these measurements is the support, heating, and tem- perature measurement of the specimen. As indicated earlier, the temperature of the specimen must be uniform over its clear aperture, stable during the measurement process, and measurable to a high degree of accuracy. The problem is simplified for mxrrors which are opaque throughout the spectral region of interest, since support and heating can be accomplished from the back. windows must be supported and heated

at their edges by structure providing a minimum of extraneous radiation. One possible design utilizes a thin polished silver or aluminum tube, within which runs a heater wire and one or more temperature sensing elements. This tube nearly encircles the specimen, maintaining good thermal contact with it, and is attached to the wall at its ends. Another technique utilizes a number of blunt tipped, polished metal support and heater fingers which press radially against the specimen. If the specimen has poor thermal conductivity, induction, laser, or electron beam heating may be necessary in addition. The use of a laser of the type for which the specimen is intended may pro- vide some useful information regarding the dynamic behavior of absorptance, with the laser turned on between scans of the interferometer.

Bulk scattering in windows may be evaluated by placing a highly emissive strip around the outer edge of the specimen, providing approximate blackbody radiation out- side of the interferometer field of view. If external reflections have been carefully eliminated, the only significant contribution from this radiation source will be through scattering within the specimen. It should be noted that the representation in the figure is not intended as a working design, e.g., provision would be made for the

removal of the specimen without re-evacuating and re-cooling the interferometer chamber.

I 4. Measurement Error

The primary source of error arises in radiation from the heater support which may

enter through multiple reflection, specimen surface scattering, and specimen bulk scattering. These effects may be minimized by keeping the heater area small relative to

the sample aperture, and by providing the lowest possible surface emissivity for the

structure. For an encircling ring heater, the ratio of areas effective is approximately 4 t/d, where t is the width of the heater element and d is the specimen diameter.

The emissivity of the heater support can be reduced to 0.05 or less fairly easily.

The reduction of leakage radiation around the edges of the specimen and off the

background blackbody (for a window) is essentially a problem in geometry. A reduction

of nn

A^r^fii is achieved by n reflections of effective area A^ and effective solid

angle for each surface of reflectance r^. No difficulty should arise in reducing

this leakage by a factor of a million, through low reflectance cold baffling.

Specimen surface and bulk scattering are not controllable, although minimizing

heater radiance provides some improvement. This contribution may be precisely evaluated by measurement of specimen emissivity with the reflective heater support and

again with a high emissivity heater support.

The radiation from the background at 77 °K is less than a millionth of 300 °K

blackbody radiation at 10 microns, but is increasing very rapidly toward longer wavelengths. Its effect may be reduced by more than an order of magnitude by subtracting

the spectrum of the background without the specimen from that with the specimen in

the normal measurement mode.

Finally, the interferometer chamber and specimen housing are surrounded by a 300 °K vacuum housing, and this radiation is large due to the large emitting area involved. No difficulty should arise if very effective baffling is used at the junction

between the specimen housing and the interferometer chamber.

A graphic illustration of measurement errors is given in figure 5. Surface

scattering is indicated as less than a part in a million, which assumes high quality

optical surfaces are present in the specimen.

180
5. Conclusion

The application of a cooled interferometer system to the measurement of low

emittance (absorptance) specimens provides remarkable improvement in sensitivity and

spectral resolution. In addition, bulk scattering in specimens may be evaluated and

scanning is possible to locate and evaluate inclusions and imperfections in substrates

and coatings. These techniques may be applied easily to windows and mirrors, and are suitable for curved surfaces, since they do not depend on the focusing properties of

the specimens. The only requirement is the capability of heating, supporting, and

measuring the temperature of the specimens without introducing spurious radiation.

Materials with low melting points may be evaluated without difficulty, since all specimens are maintained at room temperature.

The measurement of a mirror or window with an average emissivity of 0.01 measured

in an 0.1 micron band at 10.6 microns with a spectral resolution of 1 cm"-1- can be made

with an uncertainty in emissivity less than 5 x 10~6 i n absolute terms and 6 x 10~7 in

relative terms over the spectral interval. The absolute error due to temperature stability is proportional to the emittance, and is less than 1 x 10"^ for an emissivity

of 0.002. The spectral resolution considered is sufficient to permit analysis of or- ganic impurities in transmissive substrates or on surfaces, as well as inorganic spectral band structure. The rapid spectral scan capability of this type of instrument,

used in conjunction with a pulsed laser, allows controlled investigation of transient

phenomena such as surface reactions and hot spot formation.

6. References

[1] Sparks, M. , J. Appl. Phys. 42, 5029 (1971). [2] Stierwalt, D. L. and Potter, R. F. , Emittance

Studies, Chapt. 3, Semiconductors and Semimetals, Vol. 3, Academic Press, New York, New York (1967).

7. Figures

WAVELENGTH (MICRONS)

12 5 10 8 5

DLM.LTOP £ 77°K

_1_

6

S - 4 - 3

io- 400 600 BOO 1000 li.00 1400 IS00 IC00 2000 200 iMOO WAVENUMBERS tM" 1 ) Figure 1. Noise equivalent emittance (NEAe). 300° K Grey Body

300° K Background

T = 40 Seconds

= 1 cm" 1 HgCdTe Detector @ 77°

K

WAVELENGTH (MICRONS) _8 5

400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 WAVENUMBERSCM-I)

Figure 2. Noise equivalent emittance.

300° K Grey Body S ource

77° K Background

T = 40 Seconds

AV = 1 cm" 1 Ge:Hg Detector @ 5° K

181
182
Investigation of Pulsed CO2 Laser Damage of

Metal and Dielectric-Coated Mirrors*

V. Wang, A. I. Braunstein, M. Braunstein, and J. Y. Wada

Hughes Research Laboratories

Malibu, California 90265

Recent progress made in pulsed high power CO2 lasers has

increased the power handling capabilities of mirrors and mirror

coatings. Discussed in this paper are surface damage mechanisms

based upon simple physical models (i.e., surface heating,

thermally induced stress, absorption centers, etc.) and the

experimental results obtained using a pulsed CO2 laser. Various

substrates (Mo, Cu, Ni , quartz, graphite, etc.), metal films

(Au, Ag, and Cu) , and dielectric coatings (Ge, ThF4 , CdTe) were

exposed to 10.6 urn pulses of 1 to 10 ysec duration with energy

fluxes of up to 2 00 J/cm2 . The measured threshold values

(> 140 J/cm^ for 10 ysec pulses) of polished metal mirrors were generally in agreement with the predicted values based on

the simple model. Thin film coated mirrors exhibited slightly

lower threshold values (Z 75 to 140 J/cm2 for 10 ysec pulses)

than that of simple metal metals. The thresholds for those

limited numbers of dielectric mirrors tested were even lower

(~ 10 J/cm2 for pulses of 1 to 10 ysec lengths); this low

performance is attributed to inclusions which act as local

absorption centers within the dielectric.

Key Words: Damage mechanisms, inclusions in dielectrics, multilayer

dielectric mirrors, pulsed CC>2 laser damage, thin film coated metal,

uncoated metal mirrors.

1. Introduction

Recent rapid progress made in pulsed high energy CO2 lasers has substantially increased the power handling capabilities of mirrors and mirror coatings. Demands for

low-loss mirrors capable of withstanding 10.6 ym laser flux of tens to hundreds of

joules per square centimeter are becoming more critical.

In this paper, we have examined damage mechanisms of the mirror surface based upon

a simple physical model. The response characteristics of the surface of a mirror are discussed in Section 2 for a typical liquid cooled metal mirror subjected to an incident flux over a wide range of pulselengths extending from submicroseconds to

essentially cw conditions (a few seconds) . Particular attention is directed to the

range of 1 to 50 ysec in which high energy operation of atmospheric pressure electrical

CO2 lasers are most common. In this range of pulsewidth, flux loading is limited by

thermally induced stresses which produce cracking and bond separation of coatings, by

excessive temperature rise which may lead to increased surface absorption, or by beam

scattering (caused by oxidation and melting effects) . The figures of merit of pulse

mirrors applicable in this range of operation are reviewed.

Measurements of damage threshold values of various substrates (Mo, Cu, Ni, quartz,

etc.), metal films (Au, Ag, and Cu) , and dielectric coatings (ThF^/CdTe) were performed

using a pulsed CO2 laser which is described in Section 2. The mirror samples were

Part of this work was conducted under the sponsorship of the Air Force Weapons

Laboratory under Contract F29601-71-C-0101.

183
exposed to 10.6 ym pulses of 1 to 10 usee duration with energy fluxes of up to 200 J/

cm-4

. The measured threshold values of polished metal mirrors were generally in agree- ment with the predicted value based on the simple model. Thin film coated mirrors exhibited slightly lower threshold values than that of simple metal mirrors. A

limited number of low loss multilayer dielectric coated mirrors were examined, and the threshold values were even lower than those of the thin metal film coated mirrors.

In Section 4, the experimental data and interpretation of these results are presented.

2. Power Limitations of Pulsed Mirrors

A typical high power mirror designed to operate in a cw mode of operation is shown in fig. 1. This mirror consists of a thin faceplate which is supported off

a rigid backup plate by columns. The faceplate structure is liquid cooled in order

to prevent excessive heating and distortion of the mirror surface.

The calculated power and energy density limitations of this type of mirror over a wide range of pulselengths are shown in figs. 2(a) and 2(b), respectively. As shown

in these figures, the pulse response characteristics can be divided into the following

three regions:

-2

• Quasi-cw limited (x > 10 sec)

• Faceplate distortion limited

(10-4 sec < t < 10~ 2 sec)

• Surface thermal effect limited

(x < 10-4 S ec)

2.1 Quasi-Cw Limited Region

In this region, the pulsewidth of the incident flux is sufficiently long for the

heat transfer rate between the faceplate and the liquid coolant and, consequently, the

faceplate temperature, to reach a steady-state condition. The time constant which

characterizes the minimum pulselength required to reach a near steady state is established by the film heat transfer coefficient of the faceplate coolant interface. The

maximum incident flux in this quasi-cw region is limited by the thermal distortion of

the faceplate, support column, and backup plates resulting from absorbed flux. A

surface figure requirement of about 1/20 of wavelength (at 10.6 ym) will limit the

allowable absorbed energy flux of available mirrors to a few hundred watts per cm2 .

2.2. Faceplate Distortion Limited

In this intermediate pulse region, the pulsewidth is shorter than the time con- stant associated with the heat transfer between the faceplate and the coolant. The

liquid cooling becomes ineffective in keeping the faceplate from heating excessively.

The allowable incident flux in this region is limited by the out-of-plane distortion

of the faceplate caused by the absorbed flux. Since the heat transferred to the

coolant is small over the duration of the pulse, the absorbed energy will be stored,

in the thin faceplate. The faceplate distortion is simply proportional to the

absorbed energy density. This region, therefore, is characterized by an allowed

energy density which remains at a constant value while the allowed incident power

increases inversely proportional to the pulsewidth.

2.3. Surface Thermal Effect Limited

As the pulsewidth is made narrower, the depth of heat penetration over the duration of the applied pulse will be limited by the diffusibility of material to a depth

only a small fraction of the faceplate thickness (^1 mm) . The allowable flux in this

region is no longer bounded by the out-of-plane distortion criterion; the surface temperature rise becomes a dominant factor. Although catastrophic damage may not occur over one pulse, it is possible that the repeated pulsing might lead to a gradual

deterioration of the dielectric coatings and the surface finish. Accordingly, the

400°C surface temperature has been used as a guideline upper limit value for those

data plotted in fig. 2. Stress yield conditions in many metals occur at temperatures

below 400°C. It is particularly significant with copper, as shown in fig. 2(b).

Typically, the heat penetration (or thermal skin) depths of good mirror materials

(in a 50 ysec period) are approximately 0.1 mm or smaller. Because of this small heat

184
penetration depth, conventional cooling techniques are ineffective over the duration

of the pulse. For repetitive pulsing, however, liquid cooling must be included.

2.4. Figures of Merit for 1 to 50 ysec Pulsewidths

In the range of 1 to 50 ysec pulsewidths, thermal effects occur over a distance

which is large compared to the electromagnetic skin depth for penetration of the

incident radiation into a metallic reflector (^600 8). Thus, the heat applied to the

mirror can be reasonably approximated as being applied at the front surface of the

mirror and diffused into the faceplate.

The thermal behavior of a simple mirror substrate can be described by the diffusion equation

|? = K V

2 T (1)

T = temperature

t = time

k = thermal diffusivity

K = conductivity

p = density

C = specific heat

P

* For most cases where the diffusion of the heat into the solid occurs over an area wide

compared to the depth of penetration, the following one-dimensional equation will be

valid:

21 = K ^

(2) 3t ,2 U;

dx

The solution to the diffusion equation for a uniform step input of laser radiation of

pulselength t is given in table 1 . The corresponding values of the expansion-induced

stress are also summarized in the same table.

Table 1 . Figures of Merit

Surface Temperature (kpC )

P AT = — (~) (kpC T)" 1/2

/? A p

Surface Expansion (pC /a):

P

» - ^= <S» toy-1

Thermal Stress Limit {^) :

a - .? E, I ( £) fknC Tl" 1/2

eA Energy per unit area

T Pulse length

k Thermal conductivity

P Density

CP Specific heat

V Poisson's ratio

ay Yield stress

a Coefficient of thermal

expansion

E Young's modulus

The surface temperature rise of a substrate material is seen to be a function of

the product (kpCQ

) . Among metals, the thermal conductivity is most widely varying of

these three coefficients , so high conductivity materials generally give the lowest temperature rise. Also, for a given absorbed energy, the temperature rise is expected to

be proportional to the square root of the pulse length. Surface expansion, on the

other hand, is independent of both pulselength and thermal conductivity. Also presented in table 1 the thermally induced stress can be shown to be proportional to the

coefficient of thermal expansion, the modulus of elasticity, and the temperature rise.

185
The limitations imposed upon various mirror materials by the temperature and

stress yield limits are as shown in fig. 2 and discussed in the preceding section.

3. Test Apparatus

The test apparatus consists of a transverse discharge pulsed C02 laser operating

at 10.6 ym as shown in fig. 3. A stable resonator is used with a partially transmitting output mirror and a 6.4 cm germanium lens to focus the output beam upon the test

samples. Typical temporal profiles of the output beam are shown in figs. 4(a) and 4(b),

respectively, for pulses of 1 and 10 ysec; these pulselengths are measured at the halfpower points. Beam energy measurements were made using pyroelectric and calorimetric

methods

.

The test samples are placed at varying distances near or in front of the focal

point. The test samples are mounted upon a rotary table in a common plane along with

plexiglas targets which are used to determine the approximate beam diameter of the

laser beam at a test sample surface. Beam patterns burned into these plexiglas

samples are used to determine the area, and therefore the energy density of the

incident beam. The threshold of mirror damage was determined by visual observation

of the irradiated areas (for changes such as oxidation or enhanced diffused scattering)

and microscopic inspection of the mirror sample after testing.

4. Test Results and Discussion

4.1. Uncoated Metal Mirrors

Several metal substrate mirrors including copper, molybdenum and electroless

nickel were tested. Copper was observed to fail through localized melting and oxidation which resulted in noticeable discolorations and enhanced diffused scattering.

Figure 5 shows a sample of the affected area where a copper substrate was exposed to

a 2 ysec pulse of approximately 200 J/cm2 . Visible in this sample are two damaged

sites about 10 -2 cm in diameter. Spot size of the beam as defined earlier is about

0.8 mm in this test. In this case, power levels at the focus are high enough to consistently cause air breakdown when the sample pieces are positioned. It appears

that the surface heating and its effect triggered air breakdown near the focus.

On the basis of the physical model discussed earlier, copper can be expected to have the lowest temperature rise but to undergo thermal stress yielding at relatively

low temperatures (^130°C) . Since copper would yield by plastic deformation due to thermally induced compression at the surface, this stress effect appears not to cause

any noticeable damage to the mirror in a single pulse operation. Repetitive pulse

operation may, however, result in earlier failure due to fatigue. Brittle materials

such as BeO would be more susceptible to failure at their thermal stress limit by

fracture

.

Two electroless nickel mirror samples on copper substrates were also tested.

Cracking of electroless nickel plating occurs at a threshold of damage of roughly

30 J/cm2 for 10 ysec. Electroless nickel (Kanigen) is a relatively thick (8 x 10~3 cm)

cataly tically deposited amorphous nickel-phosphorous alloy. After heat treating, its

high degree of hardness and lack of crystal structure make a desirable substrate for

polishing. However, with a thermal conductivity approximately one-one hundredth that

of copper, temperature rise can be expected to be an order of magnitude higher, making

it a poor choice for pulsed mirrors

.

The results of the metal mirror tests are summarized in table 2. As expected,

high thermal conductivity substrates exhibit better resistance to damage.

4.2. Thin Film Coated Mirrors

Since the absorption coefficient of thin films of Cu or Ag is generally lower than

that of simple refractory substrates, improvements in damage resistance might be expected by employing low loss metallic films over a substrate, thus lowering the absorbed

energy. Figures 6, 7, and 8 illustrate some of the damage mechanisms seen in Au and Cu

thin films over various substrate combinations which were exposed near their threshold

of damage. Figure 6 is a molybdenum substrate coated with 5 x 10 - 3 cm of Au alloy and

overcoated with 7.5 x 10 -5 cm of pure gold as the reflecting surface. This thickness

of Au alloy is approximately equal to the thermal penetration depth of the energy

during a pulse. Damage occurs at 75 J/cm2 for 10 ysec pulses and the damage appears to be the result of melting the Au. The diameter of the damage area in this micrograph is

186
Table 2. Metal Mirrors

Substrate Coating Absorption

Tested Energy Density*

at Threshold & Pulse Length

Copper None *\» 2 %

)

> Xfiu j/ cm ( iu usee; Mo None 2% > 140 J/cm2

( 2 psec)

Electroless

Nickel

None 8.6% 3 10-30 J/cm ( 1 ysec)

* ± 30%

approximately 1.2 mm. The underlying Mo substrate with its high melting temperature

can be seen intact in the micrograph while the resultant gold droplets show indication

of surface tension effects during melting.

_2 In fig. 7, a composite mirror consisting of a Mo substrate with approximately 10 cm of Cu plating and a vacuum deposited low-loss Cu reflecting surface about 10~5 cm j_ n thickness has been exposed to an average flux of 140 J/cm^ . in this case, the Cu layer

is sufficiently thick to essentially constitute an infinitely thick substrate from the

standpoint of pulsed thermal effects. With this mirror, oxidation and loss of the vacuum deposited reflective layer after exposure to 140 J/cm^ can be seen in fig. 7.

In contrast to this, a similar mirror employing a sputtered copper thin film

showed no visual signs of damage at 140 J/cm^ . It may be speculated that this sputtering process results in better thermal and mechanical bonding of the reflective film to

the substrate, thus enhancing the heat transfer from the reflector into the substrate

material

.

A similar mirror using Ag and a thin ThF^ protective overcoat was also tested.

The TI1F4 overcoat is sufficiently thin (^1000 R) that the electric field within the

dielectric is low due to the standing wave effect of the metal reflector. Heating

would be expected to originate in the metal reflector rather than from absorption in

the dielectric. At 'WO J/cm^ initial signs of scattering were visible in the ThF4

overcoating, presumably due to substrate heating.

A clear example of softening of a ThF4 overcoat can be seen in a Quartz/Ag/ThF4

mirror shown in fig. 8. Here, energies of 10 J/cm2 and above cause extensive damage

including the wrinkled pattern visible away from the central region of oxidized Ag.

In the case of an electroless nickel substrate and an Ag reflector without an overcoat

shown in fig. 9, the damage mechanism at 40 J/cm^ is cracking and delamination of the

Ag film. Another type of thin film damage, similar in nature to the Au/Mo mirror

mentioned previously, can be seen in a vacuum deposited thin film of Cu on Mo (fig. 10)

.

At 140 J/cm^ , surface tension forces after melting caused by beading of the Cu upon

the Mo substrate, as seen in fig. 10. Table 3 summarizes the thresholds measured

for thin film metallic reflectors. Copper reflectors upon copper substrates exhibited

the best resistance to damage. Gold and molybdenum substrates exhibited similar

results. The presence of a dielectric overcoat, which allows easier handling and

cleaning of metal mirrors, reduces the threshold only moderately. Electroless nickel

and quartz fared the most poorly.

4.3. Multilayer Dielectric Coated Mirrors

The third type of mirrors tested has the potential for the lowest absorption and

thus the possibility for very high incident energy fluxes. These dielectric enhanced

multilayer mirrors consist of a metallic reflector coated with quarter-wave layers of

alternately high and low index dielectric materials. In addition to materials limitations stemming from melting and oxidation of the dielectric, and the possibility of

delamination from either poor bonding, residual stresses, or thermal expansion induced

stress, there also exists damage by localized pitting due to inhomogeneous thin films.

This last mechanism presently limits performance of this type of mirror to well below

the performance of simple metal mirrors.

187
Table 3 . Thin Film Metallic Mirrors

Substrate Coating Absorption

Tested Energy Density* at

Threshold & Pulse Length

Copper Sputtered Cu % 2% > 140 J/cm2 (10 ysec)

Copper Vacuum deposited Cu

^ 1% ^ 140 J/cm2 (10 ysec)

Copper Ag/ThF4 ^ 1% 40-70 J/cm2 (10 ysec)

Mo/braze 50 pin Au "o 1.5% ^ 75 J/cm2 (10 ysec)

Mo Vacuum deposited Cu

1% 75-140 J/cm2 (10 ysec)

Electroless

nickel

Ag 0 . 7%

2 20-40 J/cm ( 2 ysec)

Quartz Ag/ThF4 ^ 1%

2 ^ 10 J/cm ( 2 ysec)

* +30%

Some examples of the damage seen in a two-layer dielectric enhanced mirror consisting of CdTe, ThF4

, and Cu are shown in figs. 11 through 15. Because of the more

complex structure, the failure of this type of reflective coating is striking, as shown

in figs. 11 and 12. The effects of melting of the dielectric, and surface tension

forces at the threshold are shown in fig. 13. The phenomena of local pitting, which

occurs on both high and low conductivity substrates, independent of the number of

2 layers of dielectric and the pulselength over the range tested, occurs around 10 J/cm .

| Figure 14 is a photograph of a two-layer CdTe/ThF^/Cu mirror on an Mo substrate with

an absorption of 0.5%. After exposure to 10 J/cm2 the copper reflector can be seen to

be damaged in the bottom of the pit in the dielectric, but the damage does not extend t the Mo substrate. The size of the local damage sites are about 10~2 cm, far smaller

than the beam diameter of 3 mm. A six-layer CdTe/ThF 4 mirror upon an Ag reflector and

electroless nickel substrate with an absorption below 0.2% also begins to pit at about

10 J/cm2 . in this case, more structure is visible in the pitted region due to the

larger number of layers of dielectric as shown in fig. 15. The nucleus of the damage

appears to be scattered at various heights within the dielectric as a result of the

varying depths of the craters , as evidenced by the number of rings visible within each

site.

Results of these tests upon dielectric multilayer mirrors are summarized in table

4. In all cases, damage begins to occur at about 10 J/cn\2. if a simple thermal

model of heating is assumed for an electroless nickel substrate and a temperature

limitation of 400°C is assumed, then the relationship between incident energy, pulselength, and absorption is illustrated in fig. 16. The experimental points are indicated by the dot symbols, for electroless nickel, and the square symbols for

molybdenum substrates.

Table 4. Dielectric Mirrors

Substrate Coating (No. of

Dielectric Layers

Absorption Tested Energy Density* at

Threshold & Pulse Length

Electroless

nickel

Cu/ThF

4 /CdTe (2) 0.5%

2 10 J/cm (2 ysec)

Electroless

nickel

Ag/ThF./CdTe (6) 0.2% 2 10 J/cm (1 ysec)

Electroless

nickel

Ag/ThF

4 /CdTe (6) 0.2% 2 10 J/cm (8 ysec)

Molybdenum Cu/ThF

4 /CdTe (2) 0.5%

2 10 J/cm (10 ysec)

* +30%

188
A model which assumes localized absorption centers imbedded within or between the

dielectric films which then cause local heating and damage to the dielectric would be

consistent with these observed results. First, the measured average absorption of a dielectric mirror is decreased by the use of additional layers. While decreasing the

flux incident upon the metallic reflector, the flux incident upon the first few layers,

and thus incident upon absorption centers located within these first layers, remains

independent of the number of layers. Second, the substrate would not be expected to

have an appreciable effect upon small absorption centers located within the dielectric,

due to a relative lack of thermal conduction between the absorption center and the substrate. Finally, as long as the thermal conductivity and the size of an absorption

center are sufficiently small, thermal diffusion of absorbed energy does not significantly modify the local temperature rise, thus making the process independent of

pulselength

.

A similar problem of small inclusions within a dielectric has been encountered

(and solved) in the development of high power 1.0 6 ym laser glass as well as in the

development of high reflectance mirrors in the visible. The solution is basically one

of careful control of process and materials.

5. Summary

The status of the three types of mirrors tested thus far can be summarized in

table 5. At present, simple metal mirrors have exhibited the highest resistance to

damage with threshold values in excess of 140 J/cm2 for 8 to 10 ysec pulses. The

range of incident flux which was predicted to cause damage agrees well with the

measured threshold values. A simple extrapolation of these results indicates that

an incident flux in excess of 500 J/cm2 for pulses of 50 ysec can be handled with

simple metal mirrors.

Table 5. Summary of Measurements

Laser Beam

( 8-10 ysec)

Metal

Thin Film

Coated Metal Multilayer Dielectric

Mirror Material

Absorption

Measured Threshold (Av)

Cu

^2%

>14 0 J/cm2

Cu on Cu

^1%

75-140 J/cm2

CdTe-ThF

4 ~Cu on Mo

^0.5%

7-10 J/cm2

For thin metal mirrors, performance has been comparable to simple metal mirrors,

with measured thresholds of 75 to 140 J/cm2 for the best mirrors. As with the simple

reflecting substrate mirrors, damage resistance is a function of thermal conductivity.

In addition, however, the thermal and mechanical bond of the thin film to the substrate plays an important role in the performance of these mirrors.

At present, dielectric enhanced multilayer mirrors exhibit the lowest damage

resistance with threshold values of approximately 10 J/cm2 . This low performance is

attributed to local absorption centers within the dielectric which nucleate damage sites

in the didlectric. It is expected that removal of these centers will considerably

improve their performance. With the provision that the expected low absorption of

these mirrors can be maintained uniformly over the surface, the ultimate performance

of these mirrors may be far superior to metal reflectors , especially at short pulselengths ('v-10~9 sec) .

6. Acknowledgment

We wish to acknowledge the many helpful discussions enjoyed with several

colle'agues, especially Drs . 0. P. Judd, P. O. Clark, and F. J. McClung at the

Hughes Aircraft Company. Thanks are extended to J. F. Moyer, J. E. Fitzgerald,

J. L. Schmid and L. J. McNulty for their able assistance in preparation of mirror

samples and experiments

.

189
7. Figures

SUPPORTING POSTS MIRROR SURFACE PLATE

Figure 1. CW water-cooled metal mirror.

PULSE" LENGTH, sec.

TRANSVERSE FLOW PULSED 10.6/t LASER

Pulse Length 1-10 ysec Pulse Energy <200 J/cm2 Beam Size 0.8 mm - 3 mm Beam Mode Multimode

Figure 2. Calculated maximum mirror loading versus pulselength

focal POINT Figure 3. Schematic of test apparatus.

6.4 cm LENS

5O

1 1 1 1 1 1 1 1 1 t

1 1 1 1 1 1 1 1

V\ T=I.O/isec rrLU t = 10.0/Asec

o

CL —1—1

J i i i i —r-N- 1 1 i i i i i i 0.5 /^sec/cm—) \<- 2.0 /tsec/cm-M \*~

TIME — TIME —

Figure 4. Typical laser output pulse shape.

190
Figure 5. Two damage sites on a simple copper substrate 200 J/cm2 (375X)

.

191

Figure 13. CdTe/ThF^ two layer dielectric en- hanced mirror, threshold 10 J/cm2 (375X)

.

Figure 14. CdTe/ThF

4 two layer dielectric enhanced

mirror, threshold 10 J/cm2 (37X)

.

Figure 15. CdTe/ThF

4 six-layer dielectric en- hanced mirror, threshold 10 J/cm2 (37X)

.

1 1

0.2% ABSORPTION

1 / ( CALC. ON BASIS OF / / f SUBSTRATE HEATING MODEL / I ( KANIGEN SUBSTRATE)

\ N % ABSORPTION ^0.5 % ABSORPTION

,0.5% OBSERVED DIELECTRIC / • 0.5% 0.2%

1 1

MIRROR THRESHOLDS —

1 10 100

t, /isec.

Figure 16. Damage threshold pulse energy versus pulsewidth for dielectric mirrors.

193
Appendix

Participants

N. C. Anderholm

L. J. Aplet

R. R. Austin

E. D. Baird

M. Bass

H. & J. Bennett

R. A. Berg

E. A. Blankenship

E. S. Bliss

N. Boling

H. S. Boyne

P. Braunlich

C. J. Broomer W . F . Brunn-er

P. Bryce

Div 5214, Sandia Lab., Box 5800, Albuquerue NM 87115 Hughes Aircraft, Culver City CA 90230 Perkin-Elmer , Norwalk CT 06897

Lawrence Livermore Lab., P 0 Box 808, Livermore CA 94550 Raytheon, 28 Seyon St., Waltham MA 02154 Michelson Lab., Code 6018, Naval Weapons Center, China Lake CA 93555 Lawrence Livermore Lab., P 0 Box 808, Livermore CA 94550 USAF Weapons Lab.(SYX), Kirtland AFB NM 87117 AFCRL/USAF, Hanscomb Field, Bedford MA 01730 Owens-Illinois, 1700 N. Westwood, Toledo OH 43651 Div 271, National Bureau of Standards, Boulder CO 80302 Bendix Research Lab., Bendix Center, Southfield MI 48076 Broomer Research Corp. , 23 Sheer Plaza, Plainview NY 11803 Lawrence Livermore Lab., P 0 Box 808, Livermore CA 94550 Alberta University, Edmonton, Canada

A. Y. Cabezas

V. Costich

T. G. Crow

2342 Selby Ave., Los Angeles CA 90064 Coherent Radiation, 3210 Porter Dr., Palo Alto CA 94304

ILS Inc., 3404 N. Orange Blossom Tr., Orlando FL 32805

J. Davis son

G. DeBell

L. DeShazer

R. Doussain

G. Dube

US Naval Research Lab., Washington DC 20390 Spectra-Physics, 1250 W. Middlefield, Mt . View CA 94040

Dept. of Physics, USC, Los Angeles CA 90007 Laboratoire National d'Essars Paris, 1 Rue Gaston Boissier, Paris, France Owens-Illinois, 1700 N. Westwood, Toledo OH 43651

S. J. Ekstrom Lawrence Livermore Lab., P 0 Box 808, Livermore CA 94550

A . Feldman

J. A. Fleck

D. Fradin

D. L. Franzen

A259 Materials, National Bureau of Standards, Washington DC 20234 Lawrence Livermore Lab., P 0 Box 808, Livermore CA 94550 McKay Lab., Harvard University, 90 Oxford St., Cambridge MA 02138 Div 271, National Bureau of Standards, Boulder CO 80302

T. Gallager

C . Giuliano

A. J. Glass

A. M. Glass

A. Glassman

H. H. Grimes

A. H. Guenther

Valpey, 36 Pope Rd., Holliston MA 01746 Hughes Research Labs., Malibu CA 90265 Wayne State University, 5050 Anthony Wayne Dr., Detroit MI 48202 Bell Labs., Mountain Ave., Murray Hill NJ 07976 Lawrence Livermore Lab., P 0 Box 808, Livermore CA 94550 NASA-Lewis , 21000 Brookpark Rd . , Cleveland OH 44135 USAF Weapons Lab./DYD, Kirtland AFB NM 87117

194
B. Guscott KMSF, Research Park, Ann Arbor MI 48108

Y. H. Hahn

N . F . Harmon

T. Harris

N. L. Hecht

R. W. Hellwarth

L. M. Hobrock

S . J . Holmes

D. Horowitz

R. C. Hyer

Laser Energy, Inc., 325 Mt. Read Blvd., Rocester NY 14611 The MITRE Corp., P 0 Box 208, Bedford MA 01730

Spacerays, 4 Standish Circle, Andover MA 01810

Univ. Dayton Research Institute, University of Dayton, Dayton OH 45409

USC, University Park, Los Angeles CA 90007

Hughes Aircraft Co., Culver City CA 90230

Northrop, 3401 W. Broadway, Hawthorne CA 01970

A259 Materials, National Bureau of Standards, Washington DC 20234

Los Alamos Scientific Lab., Los Alamos NM 87544

T. Izumitani Hoya Glass Works, 58-685 Hodokubo Hino, Tokyo, Japan

D. A. Jennings

C. J. Johnson

E. Jones

Div 271, National Bureau of Standards, Boulder CO 80302

II-VI, Inc., 207 Garden Lane, Glenshaw PA 15116 Div 5214, Sandia Corp., Box 5800, Albuquerque NM 87115

A. Kahan

J. M. Khan

AFCRL, Hanscomb Field, Bedford MA 01730 Lawrence Livermore Laboratory, P 0 Box 808, Livermore CA 94550

w. J. Lange Westinghouse Research, Pittsburgh PA 15235

J. K. Larin Laser Optics, Inc., P 0 Box 3, Danbury CT 06810

R. Lawson Dept. of Elect. Engineering, University of Alberta, Edmonton, Albert,

C. Layne Lawrence Livermore Laboratory, P 0 Box 808, Livermore CA 94550

H. Lee Owens-Illinois, 1700 N. Westwood, Toledo OH 43651

H. R. Leider Lawrence Livermore Laboratory, P 0 Box 808, Livermore CA 94550

R. A. Lindberg NASA-Lewis Research Center, 21000 Brookpark Road, Cleveland OH 44135

G. M. Loiacono Isomet Corp., 103 Bauer Drive, Oakland NJ 07436

P. Mace Los Alamos Scientific Lab., Los Alamos NM 87544

R. W. MacPherson DREV, P 0 Box 880, Courcelette, P. Q., Canada

A. Maimoni Lawrence Livermore Laboratory, P 0 Box 808, Livermore CA 94550

J. Marburger Dept. of Physics & Elect. Engineering, USC, Los Angeles CA 90007

B. Martin Dept. P14, International Business Machines, Inc., Endicott NY 13760

J. McMahon National Radiation Laboratory, Washington DC 20390

P. J. Mendoza Union Carbide-Korad Dept., 2520 Colorado Ave., Santa Monica CA 90404

J. de Metz CEA Li meil, PO 27 Villenenve St. Georges, 91 France

D. Milam ARCRL/USAF, Hanscomb Field, Bedford MA 01730

P. Miles Raytheon Co. -Research Div., 28 Seyon St., Waltham MA 02154

S. A. Miller NELC, 271 Catalina Blvd., San Diego CA 92106

J. Moffat ILC, Inc., 164 Commercial St., Sunnyvale CA 94022

L. P. Mott Optical Coating Lab., 5789 Giffen Ave., Santa Rosa CA 95403

J. Murachver EG&G, 35 Congress St., Salem MA 01970

195 (
B. E. Newnam

P. E. Nielsen

3121 Veteran Ave., Los Angeles CA 90034 AFWL/SYT, Kirtland AFB NM 87117

R. O'Neil

D. Osborn

A. Owyoung

MIT/Lincoln L-113, Wood St., Lexington MA 02173

Sylvania, 100 Fergeson Dr., Mt. View CA 94040 Div 5214, Sandia Lab., P 0 Box 5800, Albuquerque NM 87115

J. H. Parks Physics Dept., USC, Los Angeles CA 90007

R. Pollack LASL, Box 1663, Los Alamos NM 87544

R. Puttbach Airtron Div. -Litton, 200 E. Hanover Ave., Morris Plains NY 10453

K. D. Pyatt, Jr. Systems Science and Software, Box 1620, La Jolla CA 92037

W. Reichelt

R. L. Rewski

L. W. Riley

0. Roth

P. Rushworth

LASL(L-l), Los Alamos NM 87544 USAF Avionics Lab., AFAL/TEO Wright-Patterson AFB OH 45433 Apollo Lasers, 6365 Arizona Circle, Los Angeles CA 90045

Dept. 28Z, IBM, Box 1900, Boulder CO 80302 Martin Marietta, Box 5837, Orlando FL 32805

T. Saito USAF Weapons Lab/SYX, Kirtland AFB NM 87117

J. R. Schneider Physics Dept., University of Dayton, Dayton OH 45409

D. Segawa Hoya Glass Works, P 0 Box 210, Menlo Park CA 94025

L. L. Shaw Lawrence Livermore Lab., p 0 Box 808, Livermore CA 94550

C. Y. She Dept. of Physics, Colorado State University, Fort Collins CO 80521

J. Sieman USAF-Hughes Aircraft, Mail Sta. C-160, Culver City CA 90230

J. L. Smith US Army Missile Command/AMSMI-RRD , Redstone Arsenal AL 35809

M. J. Soileau USAF Weapons Lab/LMO, Kirtland AFB NM 87117

L. H. Solomon Itek Corp., 10 Maguire Road, Lexington MA 02173

M. Sparks Xonics, Inc., 6837 Havenhurst Ave., Van Nuys CA 91406

R. Speck Lawrence Livermore Laboratory, P 0 Box 808, Livermore CA 94550

K. Steinbruegge Westinghouse Resarch Labs. , Pittsburgh PA 15235

D. F. Stevison AFML/LP, Wright-Patterson AFB OH 45433

C. M. Stickley ARPA, Materials Sciences, 1400 Wilson, Arlington VA 22209

D. L. Stierwalt NELC, 271 Catalina Blvd., San Diego CA 92106

D. Sullivan Hughes Aircraft, Meul Station B-156, Culver City CA 90230

B. R. Suydam LASL, Box 1663, Los Alamos NM 87544

E. Teppo Naval Weapons Center, Code 4051, China Lake CA 93555

J. Terrell LASL, Box 1663, Los Alamos NM 87544

P. D. Thomas Dept. 52-22, Lockheed Research, 3251 Hanover, Palo Alto CA 94304

H. M. Thompson Cornell Aero Lab., Inc., 4455 Genesee, Buffalo NY 14221

J. H. Tillotson KMS Fusion, Box 1567, Ann Arbor MI 48106

196
V. Wang Hughes Research Lab., 3011 Malibu Canyon Road, Malibu CA 90265

s. W. Warren ITT Components, Edinburgh Way, Harlow, Essex, England

B. Watt LASL, Box 1663, Los Alamos NM 87544

R. Webb Allied Chemical Corp., P 0 Box 1021-R, Morristown NJ 07960

E. D. West Div 271, National Bureau of Standards, Boulder CO 80302

R. V. Wicks USAF Weapons Lab/SYX, Kirtland AFB NM 87117

J. D. Wilcox Lawrence Livermore Laboratory, P 0 Box 808, Livermore CA 94550

E. Woodbury Hughes Aircraft, 18621 Tarzana Drive, Tarzana CA 91356

R. Woodcock American Optical Corp., Southbridge MA 01550

6. Wyntj es Block Eng., Inc., 19 Blackstone St., Cambridge MA 07138

197

FORM NBS-114A (l-7t)

II c nppT rip rnMM 1. PIIRI irATION OR RFPORT NO 2. Gov't Accession BIBLIOGRAPHIC DATA

NBS_Sp

_

372

N°'

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Laser Induced Damage in Optical Materials ; 1972

5, Publication Date October 1972

6. Performing Organization Code

7. author(s) Various; Edited by Alexander J. Glass, Wayne State

University and Arther H. Guenther, Kirtland AFB. New Mexico

8. Performing Organization

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS, Boulder Labs.

DEPARTMENT OF COMMERCE

Boulder, Colorado 80302

10ivs* Prniprr/Ta<;lf/WorW r IUJC- 1/ Ida rv/ »> Li 1 K Unit U 11 11 liU Nn • 2710585/2710900

11. Contract/Grant No.

12. Sponsoring Organization Name and Address American Society for Testing and Materials and by the

National Bureau of Standards (Div. 271)

Boulder Laboratories, Boulder, Colorado 80302

13. Type of Report & Period C overe d Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant bibliography or literature survey, mention it here.) The fourth ASTM Symposium on Laser Induced Damage in Optical Materials was held at

the National Bureau of Standards in Boulder, Colorado on June 14-15 of this year. This

symposium is held as part of the activity of Subcommittee II on Lasers and Laser

Materials, of the ASTM. Subcommitte II is charged with the responsibility of formulating standards for laser materials, components, and devices. The chairman of Subcommittee II is Haynes Lee, of Owen-Illinois, Inc. Co-chairmen for the damage symposia are

Dr. Arthur H. Guenther, of the Air Force Weapons Laboratory, and Professor Alexander J. Glass, Chairman of the Department of Electrical Engineering at Wayne State University.

Approximately 125 attendees at the symposium heard 21 papers on topics relating to laser- induced damage in glass, crystalline materials, nonlinear optical materials, thin

film dielectric coatings, and infrared components. Particular attention was given to

the process of plasma formation at dielectric surfaces, the role played by self -focusing in bulk damage in solids, damage morphology of thin film coatings, and the role of

absorption in IR component failure . The proceedings of these Symposia represent the major source of information in the

field of laser-induced damage in optical materials. The Symposia themselves, along

with the periodic meetings of Subcommittee II, provide a unique forum for the exchange

of information regarding laser materials specifications among the manufacturers and users of laser devices, components, and systems. The Symposium also serves as a mechanism of information gathering, to enable the Subcommittee to write informed and re- alistic specifications.

17. KEY WORDS (Alphabetical order, separated by semicolons)

IR windows and mirrors; laser damage-, laser materials; self-focusing; thin films.

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