laser-induced damage in optical materials: sixteenth astm symposium

Laser-induced damage in optical materials:sixteenth ASTM symposium

Harold E. Bennett, Arthur H. Guenther, David Milam, and Brian E. Newnam

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

sium) was held at the National Bureau of Standards in Boulder, CO, 15-17 Oct. 1984. The Symposium was

held under the auspices of ASTM Committee F-1, Subcommittee on Laser Standards, with the jointsponsorship of NBS, the Defense Advanced Research Project Agency, the Department of Energy, the Office of

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

Symposium, including representatives from England, France, The Netherlands, Scotland, and West Germa-ny. The Symposium was divided into sessions concerning Materials and Measurements, Mirrors andSurfaces, Thin Films, and Fundamental Mechanisms. As in previous years, the emphasis of the paperspresented at the Symposium was directed toward new frontiers and new developments. Particular emphasiswas given to materials for high-power apparatus. The wavelength range of prime interest was from 1 0.6 ,um to

the UV region. Highlights included surface characterization, thin-film-substrate boundaries, and advancesin fundamental laser-matter threshold interactions and mechanisms. Harold E. Bennett of the U.S. NavalWeapons Center, Arthur H. Guenther of the U.S. Air Force Weapons Laboratory, David Milam of the

Lawrence Livermore National Laboratory, and Brian E. Newnam of the Los Alamos National Laboratorywere cochairmen of the Symposium.

1. Introduction and Principal Conclusions

The Sixteenth Annual Symposium on Optical Mate-rials for High Power Lasers (Boulder Damage Sympo-sium) was held, as in previous years, at the NationalBureau of Standards in Boulder, CO, 15-17 Oct. 1984.The Symposium was held under the auspices of theASTM Committee F-1, Subcommittee on Laser Stan-dards, with the joint sponsorship of NBS, the DefenseAdvanced Research Projects Agency, the Departmentof Energy, the Office of Naval Research, and the AirForce Office of Scientific Research. Approximately180 scientists attended the Symposium, including rep-resentatives from England, France, The Netherlands,Scotland, and West Germany. The Symposium wasdivided into sessions concerning Materials and Mea-surements, Mirrors and Surfaces, Thin Films, and fi-nally, Fundamental Mechanisms. In all, approxi-mately 45 technical presentations were made.

Harold Bennett is with U.S. Naval Weapons Center, China Lake,California 93555; A. H. Guenther is with U.S. Air Force WeaponsLaboratory, Kirtland Air Force Base, New Mexico 87117; D. Milamis with University of California, Lawrence Livermore National Lab-oratory, Livermore California 94550; and B. E. Newnam is withUniversity of California, Los Alamos National Laboratory, Los Ala-mos, New Mexico 87545.

Received 2 September 1986.

The cataloging of properties and damage thresholdsof bulk materials continued to be of interest. Authorsat this conference discussed studies of liquid crystals,plastics, cubic zirconia, and a laser host crystal, galliumscandium gadolinium garnet. Of particular interestwas the successful fabrication of polarizers from liquidcrystals.

Laboratories for studying laser-induced damagehave continued to become more sophisticated. Fourauthors described automated experiments in whichcomputers control the operating parameters of thesource laser, the devices used to diagnose the laserpulses, the manipulation of the damage sample, andthe determination as to whether damage occurred at aparticular site. A multiple-bounce reflectometer us-able with intense 248-nm laser pulses and a variableattenuator for the 248-nm wavelength were also de-scribed.

Detection of the defects responsible for damage ini-tiation has been an elusive goal. Low-energy electrondiffraction (LEED) was used to study recrystallizationof sites on metal surfaces that had been irradiated atfluences below those required to cause apparent dam-age. A closely related paper described a study bytransmission electron microscopy of metal samplesthat were thinned after having been damaged. Local-ized damage was found to have occurred at grain-boundary sites that frequently contained impuritiessuch as Fe.

1 March 1987 / Vol. 26, No. 5 / APPLIED OPTICS 813

A shift in emphasis from single-pulse one-on-oneexperiments to the much more difficult multipulselaser damage was evident. The exploration of themechanism of multipulse damage utilizing electron orion emission from the damaged surface continued.Apparently, simple probability as evidenced by the1972 lucky-electron theory of Bass and Barrett will notexplain multipulse damage, and the cumulative effectof excitations must be considered. Damage levelswere related to pulse energy but not to pulse repetitionrate and may be related to evaporation at localized hotspots, at least in silicon. A generalized theoreticalframework for analyzing multipulse thermal eventswas provided, but the mechanism of initiation remainsunknown. In addition, surface-potential measure-ments were explored as an alternative to charged parti-cle emission. It was demonstrated that the influenceof multipulse damage extends far beyond the diameterof the initiating laser beam, at least for small spot sizes.Further exploration of multipulse damage using thesetools can be anticipated.

Other, more traditional questions in the field of laserdamage were also clarified. Careful work on the the-ory of two-photon absorption resulted in theoreticalvalues calculated from material parameters which arein good agreement in all cases with experimental val-ues. Eleven semiconductor materials of various typeswere studied. Several other longstanding problemswere attacked in the same spirit.

Self-focusing was examined utilizing a theory re-ported at this meeting last year. In cases where suffi-cient data were available, the experimental results ofvarious authors were recalculated, yielding improvedvalues. It was shown that previous analyses gave re-sults which may be in error by an order of magnitude.Ripples associated with single- and multiple-pulsedamage were also investigated, and it was shown that,whereas single-pulse-derived ripples are correctly pre-dicted by present theory, multipulse-generated ripplesare not. Finally, multipulse damage measurementsusing 30-ps pulse lengths at 1.06 m on ruled andholographic gratings were reported. These data are ofimmediate importance in optical system design. Atheoretical analysis of these results has yet to be re-ported, but is anticipated shortly.

It was gratifying to note that, along with improveddamage ratings, increased resistance to adverse envi-ronments was being addressed as well. This was par-ticularly true for corrosive chemicals and electronbeams as in excimer lasers as well as resistance toabrasion and humidity in outdoor applications. How-ever, true advances have come in improved under-standing of the effect of deposition parameters (partic-ularly in the novel techniques of molecular-beamepitaxy, laser evaporation, sputtering, and ion-beam-assisted processes) on film structure such as substratetemperature, photon or particle energy, background-gas composition, and pressure, as well as postdeposi-tion techniques like annealing. This work is leading todenser, more damage-resistant, robust films with low-er absorption.

We heard about experimental correlation of damagesusceptibility with residual stress in films and identifi-cation of initiating defect types and location by themorphology of damage. Also, the benefit of diagnostictechniques well suited to films, e.g., Raman scatteringto map structure and time resolve thermal processesduring laser irradiation, was appreciated. These com-plement other techniques to measure the thermophy-sical properties of thin films, like thermal conductiv-ity, the results of which are then compared with bulkvalues.

An outstanding development in optical coatings hasbeen the process for producing porous-silica antire-flection coatings. When properly made, they haveexhibited damage thresholds equal to that of barefused silica surfaces. This year their usefulness fromthe ultraviolet to the near infrared (0.2-1.2 Mm) wasdemonstrated.

Not yet understood, however, is the physics behindthe wavelength-dependent influence of overcoats. Asreported at the last conference and again this year, useof either overcoats of MgF2 or SiO2 resulted in a mod-erate reduction (2-20%) in damage threshold at 355nm. This contrasts with the 50-100% average im-provements previously demonstrated at 248 and 1064nm.

One controversial topic of discussion was the theo-retical vs actual performance of intracavity, grazing-incidence reflectors, now considered essential for high-average power operation of free-electron lasers (FEL).The optimized designs for multilayer dielectric reflec-tors for use at grazing-incidence angles, advocated atthis meeting, predict large (>100X) increases in dam-age threshold over that possible at normal incidence.However, careful damage tests at 351 nm, also reportedat this conference, revealed a very modest (only 2.5X)increase in threshold at 850 compared to normal inci-dence. (About double this enhancement was mea-sured in other unpublished tests at 248 nm.) Possibleexplanations were suggested, but these results are stillnot entirely understood. Until future tests with otherdielectric mirrors show considerable improvement,metal mirrors must be considered the conservativechoice for grazing-incidence applications, particularlyin the presence of the other high-energy radiation envi-ronments of FEL resonators.

11. Summary of Papers

The subject matters of the Sixteenth Annual Sym-posium covered four broad areas of interest to thehigh-power laser community: (1) Materials and Mea-surements, (2) Mirrors and Surfaces, (3) Thin Films,and (4) Fundamental Mechanisms. These conferenceproceedings are organized accordingly. In this sec-tion, a concise summary of each paper is provided, andclosely related papers are discussed together, whenev-er possible. The interested reader is referred to thecomplete manuscript of any paper for further details.Our intention here is to provide the reader with anoverview of the Symposium and to identify the topics

814 APPLIED OPTICS / Vol. 26, No. 5 / 1 March 1987

of current interest, the authors, and their organiza-tions. To highlight the presented papers, each topicalarea is discussed with a brief statement of the underly-ing problems and the status of understanding withinthe area of interest.

A. Materials and Measurements

In the category of optical materials and measure-ment techniques, four general topics received atten-tion this year. These included: (1) liquid crystalswhose properties are being exploited for use as polariz-ers, electrooptical shutters, and modulators, (2) bulkdamage resistance of new laser crystals and windows,(3) application of two types of electron microscopy foranalysis of metal surfaces to deduce the cause of laserdamage, and (4) automation of laser damage facilitiesincluding development of advanced diagnostic mea-suring devices.

Development of novel materials for laser applica-tions continues to be a major goal of those participat-ing in these annual symposia. Installation of polariz-ers made from liquid crystals in the high-power glassdevelopment laser (GDL) at the University of Roches-ter was discussed by S. Jacobs, K. Cerqua, T.Kessler, W. Seka, and R. Bahr of the Laboratory forLaser Energetics. The polarizers consisted of an 11-,m thick layer of a base nematic doped with either left-handed (LH) or right-handed (RH) chiral additive toproduce a helical twist structure in the crystal. Acircularly polarized beam with wavelength X0 = nP,where n is the average refractive index and P is thepitch dimension of the helix, is diffusely scattered bythe crystal if wave rotation is in the same sense as thehelix but is transmitted when the wave rotation andhelix are opposed. This property of liquid-crystal po-larizers allows them to be used in polarizer/analyzerroles with circularly polarized light. Extinction of-104 for the crossed polarization case was reported.They have been installed in GDL as the crossed polar-izers associated with Pockels cell shutters and as singleelements to serve as isolators for waves propagatingbackward through the chain (Fig. 1).

The reported advantages of liquid crystal polarizersover Brewster-angle thin-film polarizers were low costand easy replacement due to in-house fabrication, in-sensitivity to tip angles as large as 200 off normal,insensitivity to temperature change (0.35-1.4 nm/ 0 Cin the position of the central maxima), high transmit-tance (-98%), and the capability to propagate circular-ly polarized light over most of the path length in a laserarm. Laser damage thresholds (1054-nm, 1-ns) were 3J/cm2 for the RH units, but <1 J/cm2 for LH units, aproblem that the authors circumvented by fabricatingLH polarizers as RH polarizers on quartz halfwaveplates.

Additional data on liquid crystals were presented byS. Wu and L. Hess of Hughes Research Laboratories.They described a study at 10.6 ,um of the nonlinearityof the optical rotatory power of a 900 twisted nematicliquid crystal. The crystal (BDH-E7) exhibited theexpected dependence of rotatory power on crystal

0

1.0

0.8

0.6

I~~~~~~~~~ r I

1.8'- -, =1064 .m

1.4- Crossed Pair

1.0 l0.4 ~~~~~I I ,

Single Elements LH buk- RH

_.2 _

300 500 700 900 1100 1300 1500 1700 1900

WAVELENGTH (em)

Fig. 1. Spectral transmission through liquid crystal polarizers.Single LH and RH element scans indicate that, apart from regions ofincreased optical density due to selective reflection, the spectra arefeatureless. The wavelength at which the crossed polarizers exhibitblocking extinction is determined by tuning the liquid crystal pitch

with composition mixing.

thickness when subjected to low power (<0.3 kW/cm 2 )irradiation at 10.6 Am. When the intensity was in-creased, the rotatory power decreased due to align-ment of the liquid crystal molecules with the polariza-tion of the laser beam. The incident power required toobtain a given loss of rotatory power depended on theinverse square of the crystal length. The loss of rota-tory power was reversible for intensities up to 7.5 kW/cm2 , at which level a crystal with 52-Am thicknessunderwent a phase change into an isotropic material.Recovery of the anisotropic state required -2 min. Ata higher intensity, -8.5 kW/cm2 for the 52-Am crystal,permanent damage occurred, but crystals with thick-nesses of either 24 or 13.5 Am were not altered at thatpower level. The mechanism for the permanent dam-age was not known.

Other authors discussed their efforts to produce andcharacterize improved crystals for use as laser hosts orwindows. N. Mansour, M. Soileau, and E. VanStryland of North Texas State University reportedmeasurements with 45-ps, 1064-nm pulses of the bulkdamage thresholds for cubic zirconia crystals stabi-lized by the addition of 9.4-21% of Y203 . The beamdiameter at the waist formed by their focusing lens was7.2 Am (halfwidth at Io/e). Breakdown thresholdswere the same for either linearly or circularly polarizedbeams, indicating that self-focusing was not a factor,and thresholds were also independent of the Y203 con-centration. When the beam waist was increased insize to 10 Am, self-focusing dominated the experiment.Measurement of the critical power for self-focusingyielded an estimate of the value of n2 in the stabilizedzirconia of 8 X 1013 esu. This value is in reasonableagreement with a theoretical estimate made by theauthors of 10 X 1013 esu, which value is the same aspredicted using an empirical equation, derived previ-ously, that relates n2 to the Abbe number and linearrefractive index.

M. Acharekar and D. McCarthy of Litton Systemsand R. Blachman of Material Progress Corp. de-scribed damage tests of crystals of gadolinium scandi-um gallium garnet doped with both chromium andneodymium (Nd:Cr:GSGG), and the performance of


I

I

lasers using this crystal. Their Q-switched laserequippedwith a 5- X 65-mm rod provided 135 mJ/pulseat 1061 nm with 2.2-mrad divergence when pumpedwith 8-J pulses at 10 Hz. The typical pulse width(FWHM) was 17 ns. These 17-ns pulses were used tomeasure both the bulk and surface damage thresholdsfor crystals of Nd:Cr:GSGG that had been polishedwith 0.3-ym alumina powder. Beam size3 used in thetest were not specified. The bulk damage thresholdfor this laser crystal was reported to be greater thanthat of BK-7 glass but less than that of fused silica. Itwas also reported that a 10% reduction in Nd concen-tration led to a 13% increase in bulk damage threshold.The surface damage threshold was 14 J/cm 2 .

Methacrylate polymers were studied in a collabora-tive effort by R. O'Connell and R. Ellis of the Univer-sity of Missouri at Columbia, A. Romberger of Penn-sylvania State University (Berks Campus), T. Deaton,K. Siegenthaler, A. Shaffer, and B. Mullins of theU.S. Air Force Academy, and T. Saito of LawrenceLivermore National Laboratory. These authors pre-pared samples of polymethyl methacrylate (PMMA),polyethyl methacrylate (PEMA), and polycyclohexylmethacrylate (PCMA), some containing up to 10 wt. %of a plasticizer, dibutyl phthalate (DBP). The mono-mers were initially distilled and, in some cases, alsopassed through filters with pore sizes of 0.40 or 0.22ym. Both heating (400C) and ultraviolet light wereused to obtain polymerization. The samples weretested with 1064-nm, 8-ns pulses focused to a beam sizeof 33 m (radius at Io/e2), and tests were conducted toinsure that the experiments were not influenced byself-focusing. Thresholds for either single-shot ormultiple-shot tests were defined to be the fluence nec-essary to cause damage at 50% of the sites, a conventionthat leads to an overestimate of the large-area damagethresholds. Using this convention, the authors foundsingle-shot thresholds of 29, 67, and 86 J/cm2 for sam-ples of PMMA made from distilled monomer and fil-tered to contain, respectively, 7.1, 0.5, and 0.1 parti-cles/mm3, which correlated well with the thresholds.However, the lowest fluences for which damage oc-curred in the samples, 12, 28, and 21 J/cm2 , respective-ly, only partially correlated with dust content, andthese minimum fluences are the best indication of howthe materials would perform in actual usage.

These authors also reported that single-shot thresh-olds of UV-polymerized samples were systematicallygreater than those of thermally polymerized samples,and that multiple-shot (250) thresholds were indepen-dent of the technique used for polymerization andmuch less than single-shot thresholds. Multiple-shotthresholds were also found to be independent of theamount of plasticizer used, although the authors feltthis observation was possibly invalid due to unequaldust concentrations in the plasticized samples.

A portion of each preceding conference has beengiven to discussions of experimental techniques forcharacterizing materials and measuring damagethresholds. At this conference, A. Helms, C. Cho, andS. Bernasek of Frick Chemical Laboratory at Prince-

ton University described analysis by low energy elec-tron diffraction (LEED) of sites on molybdenum sur-faces that had been laser irradiated. They studied twosamples (A and B) that had carefully prepared 100-oriented surfaces and a third (C) with similar orienta-tion whose surface had been disordered by Ar+ ionbombardment. The surfaces were initially analyzedby both LEED and by Auger electron spectroscopy(AES), and then moved under vacuum into a samplechamber which was used to transport the sample undervacuum to a separate laboratory for laser irradiation.One half of each surface was irradiated by 140-ns sec-ond-harmonic pulses from a Nd:YAG laser operatingat 5 kHz. The laser spot size was -25 ,um. Areacoverage was obtained by a raster motion of the sam-ples. The two crystalline surfaces A and B were irradi-ated at, respectively, 15 and 60 MW/cm2 , intensitiesthat are below and above that necessary to producecratering. The ion-bombarded surface was irradiatedat 75 MW/cm2. Subsequent LEED evaluation of sur-faces A and B indicated that both contained randomlydistributed islands of raised 100-oriented materialwhose edges were oriented with crystalline planes, in-dicating both the occurrence of damage during irradia-tion at subthreshold intensities and the regrowth of100-oriented material from melts produced at higherintensities. Laser annealing of the disoriented surfaceon sample C produced a partial recovery of crystallin-ity.

H. Hurt of the Michelson Laboratory at the U.S.Naval Weapons Center described the use of transmis-sion electron microscopy (TEM) to investigate thedefects responsible for site-selective damage on coppermirrors. The surfaces studied were on small (3-mmdiam X 0.25-mm thick) samples of either OFHC cop-per or OFHC copper plated with a UBAC (ultrabrightacid copper) copper film. These surfaces were dia-mond turned at NWC using tool rake angles of either-5° or -15°, and then damaged with 100-ns, 10.6-,umCO2 laser pulses. The damaged areas were thinnedfrom the back and examined by TEM. Sites on aUBAC surface which appeared by Nomarski microsco-py to have a wavy slip were found to have been melted.One pit at a selective damage site was centered on agrain boundary and contained small particles of iron.In tests of additional 3.8-cm (1.5-in.) diam samplesapparent slip was observed as the limiting damagemechanism and occurred at 8 and 10 J/cm2 in OFHCcopper turned with rake angles of, respectively, -5°and -15°, and at 19 J/cm2 in UBAC copper.

Several papers discussed automated laser damageexperiments or improved apparatus for use in damagestudies. J. Franck, S. Seitel, V. Hodgkin, W. Faith,and J. Porteus of the U.S. Naval Weapons Centerhave automated an infrared damage facility equippedwith C02, HF, and DF lasers. A computer systemcontrols the fluence of the laser pulses and raster scan-ning of the sample to allow multiple-site irradiation(Fig. 2). Their system automatically detects occur-rence of damage by detecting a laser-induced alter-ation of the scattering of light from the beam of a He-


NEGATIVE - - S PIN HOLE \ SC

PROBEBEAM

COLLECTINGOPTIC

'RE-IMAGEDSCATTER SOURCE

Fig. 2. Schematic of the scatter-probe arrangement. The focused

specular beam is blocked by a negative pinhole, and the light scat-tered by the laser-induced pit is collected by a lens system, spatially

filtered, and sent to a photodiode detector.

Ne laser that irradiates the test site. For samples inwhich damage consisted of micropits, this techniquefor detecting damage was found to give results equiva-lent to those obtained by postirradiation inspection oftest sites by Nomarski microscopy. Scattering was aless suitable detector of spatially uniform damage, buta cursory examination of the sample was able to detectwhether failure by this mode had influenced the auto-mated damage detection (Fig. 3). The stated intent ofthe group was to speed testing so that 400-800 sitescould be tested on each sample, thereby providing anaccurate and statistically meaningful large-areathreshold.

P. Filbert, J. Guttman, C. Navoda, and B. Watsonof the Lockheed Palo Alto Research Center reporteduse of a computer-automated IR-sensitive camera torecord fluence distributions in the beam of a largeNd:glass laser being used in damage tests. A fractionof the beam was directed to a diffusely reflectingscreen, and a down-magnified image of this screen wasrecorded by a 100 X 200 pixel silicon-photodiode array.The computer-processed image of the beam profile,appropriately scaled in size, was superimposed on anenlarged photograph of the damage site, allowing thedamage to be correlated with fluence contours in thebeam. The authors indicated that the ability to mea-sure thresholds with large beams of varied profilewould allow determination of the importance of mate-rial stress gradients in damage.

A high-precision multiple-pass ultraviolet reflec-tometer was described by L. Jolin and S. Foltyn ofLos Alamos National Laboratory. The instrumentwas based on the White cell and allowed computationof the net loss in a dielectric reflector through measure-ment of the intensity of a 351-nm laser beam as afunction of the number of times (>100) it had interact-ed with the reflector. To obtain damage resistance,dielectric thin-film reflectors were used as the second-ary mirrors needed in a White cell arrangement. Re-peated measurements by several operators indicatedthat the precision in reflectance measurements was+0.0007. The authors reported 351-nm reflectancesranging from 0.8498 to 0.9976 that were measured onseventeen multiple-layer HR coatings.

In another instrument paper from Los Alamos Na-tional Laboratory, S. Foltyn, J. Griggs, L. Jolin, J.Roberts, and D. Keaton described a new attenuator

for controlling the fluence in the divergent and usuallyunpolarized beams emitted by excimer lasers. Theirdevice employs two reflectors operated at incidenceangles near 40°. The incident beam is deflected by thefirst reflector onto the second reflector, which is ori-ented to restore the initial direction of propagation.Because the reflectance of a multilayer dielectric re-flector varies with incidence angle, beam attenuationcould be accomplished by tuning the incidence anglesat the reflectors. The reflectors were mounted in anassembly that allowed angle tuning by stepper motorcontrol. Transmittance at 248 nm was measured as afunction of incidence angle for an attenuator equippedwith 49-layer A12 03 /SiO2 reflectors designed to be re-flective at 248 nm for normally incident beams.Transmittance varied from 99% to 3% as incidenceangle was tuned from 340 to 440, and beam steeringwas <0.5 mrad. A transmissive version of the devicewas also described.

Variable pulse width was an important feature of therecently completed CO2 laser damage test facility atBarr & Stroud, Ltd., as described by D. Gibson and A.Wilson. Through variations in the gas fill, their lasercan be made to provide pulses with durations of -100-ns, either with or without a 2-As tail. Introducing alocking signal from a small CO2 laser into the principalCO2 source laser provided 6-/is pulses. Pulses withtunable duration <90 ns were obtained with a plasmashutter in which the attenuating plasma was inducedby irradiation of a dielectric. The laser provided out-put energies ranging from 10 J at 10 Hz to 15 J at 1 Hz,which allowed deposition of damaging fluences (1-100J/cm2) over areas -1 mm in diameter. Their statedintent was to compile a data base suitable for use inlaser design applications, and initial test results werereported for a wide range of window materials andoptical films (Fig. 4).

B. Mirrors and Surfaces

Three themes were evident in the contributions con-cerning the effects of intense laser irradiation of sur-faces. The first was a continuation of the experimentsto extend our understanding of the physical processes

DAMAGEFREQUENCY

1.0 -

0.9

0.8 _

0.7 -

0.6

0.5 _

0.4

0.3 -

0.2 -

0.1 _

0.0 -0.0 0.2 0.4 0.6 0.8

AXIAL FLUENCE (ARBITRARY UNITS)1.0

Fig. 3. Laser-induced damage frequency vs axial fluence where thevisible damage determined by Nomarski microscopy has been re-solved into two different morphological types (erosion and increasedscatter). For pitting damage, the scatter probe (Fig. 2) providedmore sensitive detection. The sample was a single-layer A120 3

coating on plated and diamond-turned Cu tested at 2.7 um.


* EROSION (UNIFORM)

PITTING SELECTIVE)+ INCREASED SCATTER

* +

* +

l L t I + l

3 ,

LDT(jc,2)

Pj_ ,

* . ...ludAR3.FYC I.00

.Baser F I 1rlm 4.1s

X ZnS

(Absorptace,,%f1

Fig. 4. Laser-induced damage threshold at 10.6gum vs the inverse ofabsorptance of several antireflection coatings on Ge. The laser

pulse had a 100-ns initial spike followed by a 2-us tail.

MOTOR IZEDPROBE X___MO T ION

MANUAL/ IRON

L~l PREAMP CIRCUIT BOARD

Y COORDINATEX S" STEM

STA INLESSPROBE ARM TIP VIBRATION:

... ~rP IIT PA

25 HR CLOSEST APPROACHIRIE U , !A APL E TO SAMPLE SURFACE

~~~~ASER~ ~ ~ ~ ~ ~~~i MIOTORIZED

BEAME - 1 SAMPLE MOTIONAXIS/

STAINESPROBE TIP SAMPLE ANDIR. DIAMETER MOUNT GROUNDED

Fig. 5. Mechanical diagram of the Kelvin probe for measuringlaser-induced changes in the surface potential or work function.

changes in surface potential. Although the laser beamhad a diameter of only 0.39-mm FWHM, changes insurface potential extended over a 4-6-mm range andsmall changes could be observed several millimetersbeyond that, suggesting care in site spacing for multi-ple-site damage tests, at least on dielectrics (Fig. 6).Decay relaxation times for the modified surface poten-tials were over an hour for dielectrics and permanent,i.e., long duration, changes at damage sites were ob-served.

Y. Jhee, M. Becker, and R. Walser of the Universi-ty of Texas continued to explore the mechanism ofmultiple-pulse laser damage of silicon using pulsetrains of 60-ps, 1.06-,gm pulses produced by a cw mode-locked Nd:YAG laser. Initial damage as seen using anSEM was correlated to the onset of electron and ionemission. This emission was subsequently monitoredas a damage indicator. They encountered multiple-pulse damage thresholds below the one-on-one dam-age threshold for crystalline silicon. In addition, theprobability of damage for single events at various siteswas not the same as that for multiple-shot damage (i.e.,temporal and spatial probabilities were unequal).Thus, the 1972 lucky-electron theory of Bass and Bar-rett could not explain these results. No evidence ofslip accumulation as evidenced by an increase in sur-face roughness as a precursor of damage was foundeither. They conjectured that a cumulative effect ofexcitations, not simple probability, is required to ex-plain the results, even though damage does depend onpulse energy but not on pulse repetition frequency.They concluded that a multiphoton process is unlikelyand the damage mechanism may have been as simpleas thermal evaporation at localized hot spots in Si.

involved at high laser intensities. This included eval-uation of surface potential as a predamage indicatorand multiple-pulse accumulation effects in silicon.Two papers treated the topic of laser-induced ripplestructure, primarily for semiconductors. Adequateunderstanding of this phenomenon will undoubtedlylead to practical applications. The third and relatedtopic was picosecond-pulse damage to manufactureddiffraction gratings, both ruled and holographic.

Surface-potential modification as a precatastrophicdamage indicator was studied by M. Becker of theUniversity of Texas, J. Kardach, of the Air ForceInstitute of Technology, and A. Stewart and A.Guenther of the U.S. Air Force Weapons Laboratory.Two-dimensional surface-potential measurementswere made using a vibrating Kelvin probe (Fig. 5) onOFHC diamond-turned copper mirrors, single-crystalsilicon, and MgF2 , HfO2 , and ThF 4 halfwave films onfused SiO2, oriented MgF2, and crystalline and amor-phous quartz substrates. In damaged areas a positivesurface potential was observed for the diamond-turned Cu, while all others gave negative values. Ex-cept in the case of one copper sample, no predamagechanges in surface potential on irradiated surfaceswere detected, and not all damaged areas exhibited

Fig. 6. Surface potential change contours, measured by the Kelvinprobe (Fig. 5), for a MgF2 thin film irradiated by ten 1064-nm laser

pulses at 69 J/cm 2.


> s n

.....

2c

1.

The charged particle emission followed an Arrheniusequation, supporting the concept of laser-induced de-fect sites that then become evaporation sites. Themechanism for formation of these sites and their char-acter was not identified.

N. Mansour, G. Reali, P. Aiello, and M. Soileau,all of North Texas State University, examined in detailthe ripple structure formed near the onset of laserdamage in wide gap dielectrics and the intermediate-gap semiconductors ZnS and ZnSe. For the dielectricsthe ripple spacing is consistently (within 5%) equal tothe wavelength of incident light in the medium. Theripple direction is normal to the direction of polariza-tion of the incident light for smooth surfaces but can beperturbed by scratches or surface defects. The resultscan be explained by a simple electrostatic-dipole mod-el but not by the competing surface excitation model.By contrast, results for the intermediate-gap semicon-ductors are not easily explained by any model. Theydiffer at front and back surfaces, spacings vary by overa factor of 2 at the same surface for ZnSe, and the backsurface ripples are not oriented perpendicular to thepolarization and may be parallel to it. No explanationis offered for these anomalies.

The analysis of laser-induced surface ripples for Si,Ge, GaAs, and Cu in terms of the interference betweenthe incident light and surface waves in the sample waspresented by P. Fauchet of Princeton University andA. Siegman of Stanford University. Unfortunately,the authors did not adequately reference previous andalternate explanations of the phenomena. Single-pulse ripples fit the presented theory quite well formaterials studied. However, ripples formed by multi-pulse illumination were not explained by the simpletheory. Irradiation levels below which ripples did notform under multipulse illumination were found to beone-third of the single-pulse threshold for crystallinematerials and as low as one-tenth the single pulsethreshold for amorphous materials. The authorspointed out that two possible hypotheses reported byothers are not inconsistent with their data, but theyoffered no experimental data, such as an increase insurface roughness or small crystallite formation, whichwould support these hypotheses.

The results of extensive measurements of the laserdamage resistance of diffraction gratings were present-ed for the first time at these symposia. At the LosAlamos National Laboratory, D. Gill and B. Newnamtested holographic and ruled gratings using 30-ps,1.06-,um wavelength radiation. Holographic gratingshad thresholds 1.5-5 times higher than ruled gratingsand the replicas had higher damage thresholds still,sometimes by over a factor of 2, than did the masterholographic grating. S-polarized light had a thresholdfrom 1.5 to 6 times higher than P-polarized light, andfirst-order gratings had as much as 5 times the damagethreshold of more finely ruled gratings used at thesame angle in third order. At 1.06-Mm gold-coatedgratings had 1.4-9.7 times higher damage thresholdthan aluminum-coated gratings. No quantitative the-oretical analysis of these results was given.

C. Thin Films

Thin films continued to lead the interest at theseannual symposia with over 50% of the presented pa-pers addressing various aspects of this complex topic.A full third of these dealt with evaluation of the prop-erties of films produced by new processes and compar-ing them with those of standard deposition methods.Others examined the possible correlation of laser dam-age with coating absorption and variations of filmstress over small areas. Multiple papers also ad-dressed the requirements for intracavity reflectors forhigh-average power operation of free-electron lasers.Of special interest was a measurement study of thethermal properties of films, including thermal conduc-tivity and specific heat, and the comparison with bulkproperties.

We begin by considering the numerous papers in-volving development, demonstration, and evaluationof advanced coating deposition methods. K. Lewis, J.Savage, A. Cullis, N. Chew, L. Charlwood, and D.Craig of the Royal Signals & Radar Establishment inMalvern, England, have pioneered the growth of opti-cal thin films by molecular-beam techniques. Thefilms, primarily ZnSe, had negligible values of absorp-tion at 10.6 Am and exhibited very high damage thresh-olds. The authors attributed the latter, in part, to thehigh-density character of the films of polycrystalline,columnar morphology whose crystallite size is depen-dent primarily on the deposition temperature. Also,the films showed no evidence of pinholes and had avery low impurity level.

As an indication of how good these films are, thelaser-induced damage threshold of 1-Am thick ZnSefilms on polycrystalline ZnSe substrates was in the 60-73-J/cm2 range (l/e2 diameter -102 gm) which com-pared well with 50-60 J/cm 2 for the uncoated region ofthe sample. This observed difference was real andrepeatable. For these tests, the temporal pulse fromthe gain-switched CO2 laser included an initial spike,33-ns wide FWHM, followed by a long tail extending to1.7 is. For simple pulses of -340-ns duration, damageof the films occurred at -31-38 J/cm2 (180-Am diame-ter). Finally, although no voids could be detected inthe films, high densities of microtwins and stackingfaults were evident. Thus, further improvements indamage threshold may be feasible.

Variations of novel deposition techniques and well-regarded procedures continue to be investigated forpotential improvements on present-day methods. T.Raj, L. Weaver, S. Tuenge, J. Price, and K. Jun-gling of the Martin-Marietta Aerospace Co. reportedon coatings of ZrO2 produced in ultrahigh vacuum(UHV) of 5 X 1010 Torr using either electron-beam orlaser evaporation by a 100-W cw CO2 laser. Sub-strates were maintained at either 1500 C or 2500C whilebackground 02 pressures were varied between 0 and 5X 10-5 Torr. Coatings were evaluated for stoichio-metry and impurities by AES and for refractive indexby ellipsometry (Figs. 7 and 8). Some coatings weredamage tested at 351 nm. Results obtained led to theconclusion that the stoichiometry of ZrO2 films depos-


,2.02 -\

1.98

1.94

1.90

1 2 3 4 6

02 Pressure (106 Torr)

Fig. 7. Refractive index of ZrO2 coatings, deposited by a cw 10.6-gm laser, vs 02 backfill pressure. Substrate temperatures were

1500C and 2500C.

2.06

2.02

0

e

11

C

11

Z

1.98

1.94

1.90

1.86

As we continue progressing in our separation of thecontrolling variables in the coating process, we canfocus on optimization of the variables if they are suffi-ciently independent. In ion-beam deposition of re-fractory oxides, one such well-established variable isbeam composition. H. Demiryont and J. Sites ofColorado State University have identified a criticallevel of oxygen necessary in the beam to produce stoi-chiometric films of TiO2, Ta2O5, and SiO2 when usingboth elemental or oxide targets (Fig. 9). The thresh-old was easily seen in the film composition, refractiveindex, and optical band-gap character. There wasstrong evidence that an oxide forms on elemental tar-gets and this is subsequently sputtered onto the sub-strate.

The demand for higher quality thin films has stimu-lated experimentation with a number of novel coatingdeposition techniques that offer the possibility of finecontrol of the film properties. A. Stewart and A.Guenther of the U.S. Air Force Weapons Laboratoryconducted a limited survey of characteristics of single-layer films of ZrO2 and A1203 produced by a number ofthese processes. Besides the conventional electron-beam technique, the other processes evaluated includ-ed electron beam in ultrahigh vacuum, electron beamwith ion-beam assist, cw laser in ultrahigh vacuum,and an advanced epitaxial technique. Characteriza-tion of the film properties included total-integratedscatter, film transmittance near the UV absorptionedge, and x-ray diffractometry. Multiple-pulse laser

ID 0.1LO l.q?

0C

0

C

x

w

1 2 3 4 5

02 Pressure (10 6 Torr)

Fig. 8. Refractive index of ZrO2 coatings, deposited by an electronbeam, vs 02 backfill pressure.

ited by either technique is not affected by variations in02 backfill pressure to 5 X 10-5 Torr. However, stoi-chiometry was superior at 1500C than for higher sub-strate temperatures, and the refractive index was clos-er to bulk values at this lower temperature. Anotherobservation was that coatings produced in UHV condi-tions had at least an order-of-magnitude lower con-tamination level than coatings produced in normal HVchambers. In all cases, free metal and suboxides ofZrO2 were evident. Unfortunately, damage trendswere inconclusive. These investigations should becontinued with a larger sized sample set.

8.0

6.01x

> 4.0

ir- tL0

0.3 0.5 0.7 0.9Wavelength (m)

Fig. 9. Dispersion curves for tantalum oxide films deposited by ion-beam sputtering with varying amounts of oxygen in the primarybeam. Percentages refer to the fractional amount of oxygen back-

ground pressure.


4.u

100

N

-6 20

I 15

E 10ci

1 2 3 4 5 10Pulse duration, ns

Fig. 10. 1064-nm laser damage thresholds of solgel antireflectioncoatings on fused silica as a function of laser pulse duration r. Thedashed line describes the scaling of the median thresholds as 10.8

TO.56

.

damage thresholds varied by only a factor of 2 for eachmaterial. The results of this study will serve as abenchmark for future progress. As the parameters ofthe newer processes are optimized for maximum dam-age resistance over the next several years, any relativeadvantages should become manifest.

Substantial progress in the development of poroussilica antireflection coatings for large-scale optics usedin high-power laser systems was reported by I. Thom-as, J. Wilder, W. Lowdermilk, and M. Staggs ofLawrence Livermore National Laboratory. To maxi-mize laser damage resistance by minimizing thechances of any carbonaceous residue, a highly purifiedorganic silicate, tetraethyl orthosilicate, Si(OC2 H5 )4 ,was hydrolyzed by a basic catalyst to yield a colloidalsuspension of silica particles in anhydrous ethanol.When applied in quarterwave thicknesses to opticalcomponents made of fused silica and KDP by a dip orspin process at room temperature, no further treat-ment was necessary. Damage thresholds measured at248,346, and 1064 nm were approximately the same asfor the uncoated substrates.

In a second experimental study of porous-silica an-tireflection coatings, D. Milam, L. Thomas, C. Wein-zapfel, and J. Wilder of Lawrence Livermore Nation-al Laboratory determined the pulse-duration scalingof laser damage for a series of coatings using pulsedurations of 1, 5, and 9 ns. Ten porous-silica antire-flection coatings were deposited from both methanoland ethanol solutions containing silica particles withdiameters of 10-20 nm. The median thresholds mea-sured at the three pulse durations were 11,26.5, and 38J/cm2 , respectively. The observed threshold scalingto the 0.56 power of the pulse width is, within experi-

mental error, close to the 0.5 power previously mea-sured for both bare, polished glass surfaces and thosewith graded-index made by etching (Fig. 10).

Porous silica coatings can be deposited from alcohol-ic solutions containing either silica microspheres withdiameters of 10-20 nm or smaller, polymer silica mole-cules. To further develop the versatility of these asantireflection coatings, D. Milam and G. Peterson ofLawrence Livermore National Laboratory studiedhow a mixture of these two forms could be optimized tomaximize the spectral bandwidth of high transmissionover the 0.2-1.2 -Mm range. Calculations were madefor three single-layer density gradients: (1) homoge-neous refractive index (Fig. 11), (2) index profiles thatincrease monotonically with distance into the coating,and (3) index profiles that rise in discrete steps. Theydetermined that the maximum bandwidth occurs forthe linear-ramp coating whose index rises linearlyfrom that of air to that of the substrate. In currentpractice, however, this design cannot be realized be-cause a sufficiently porous film at the air-film inter-face cannot be attained. For currently attainable frac-tional silica contents, estimated to range from 0.4 to0.5, the optimum bandwidth results from a coatingwith a single discrete step in its index profile.

Besides the study of new coating procedures, newcoating materials and combinations are being investi-gated for specific applications as more novel tech-niques become commonplace. To this end, multilayerAR coatings of AlN3 /A12 03 , prepared by reactive sput-tering, were characterized by L. Koshigoe, L. John-son, T. Donovan, and C. Marrs of the MichelsonLaboratory of the Naval Weapons Center. Previousstudies had shown these materials to have adequatestability and scratch resistance. A variety of sputter-ing techniques were employed from the use of dc mag-netrons to ion-beam and rf diodes. An importantaspect of this study was a determination of the resis-

100

99

98

97

_

1 96

E5 95I-

94

93

92

0.2 0.4 0.6 0.8 1.0 1.2

Wavelength (microns)

Fig. 11. Calculated two-surface transmittance of a silica windowwith porous homogeneous coatings. Also shown is the transmit-tance of the bare substrate and the silica content of the coating as a

function of the physical thickness.


91

tances of these materials and coatings to laser radia-tion, fluorine, electron-beam irradiation, and humid-ity, while coating composition and stoichiometry wereevaluated by SAM, SEM, EDX, XPS, and Nomarskimicroscopy. Substrates included fused silica, silicon,and calcium fluoride. The first tests on these filmswere encouraging. However, considerable work needsto be done to reduce the number of damage-initiatingdefect sites. It was pointed out that in most cases, theoxide layer should be the outer layer because it is moreimpervious to environmental degradation.

Raman scattering continues to gain in utility as aroutine diagnostic technique for characterizing thinfilms. Its use to study annealing and phase transfor-mation, such as crystallization, was reported by R.Rujkorakarn, L. Hsu, and C. She of Colorado StateUniversity. Two types of low-loss low-scatter amor-phous submicron-thick titania film were thermally an-nealed in air. The transition to anatase/rutile crystal-line phases coincided with a decrease in opticaltransmission, small changes in refractive indices andenergy gap in the film, along with a 10OX increase ininelastic scattering.

Raman scattering has also found considerable use asan analytical measurement technique to probe thestructure, thickness, and homogeneity of optical thinfilms. G. Exarhos of Battelle Pacific Northwest Lab-oratory and P. Morse of the U.S. Air Force WeaponsLaboratory added a new twist by employing time reso-lution to investigate laser-induced damage in TiO2coatings. The technique involved two lasers whichallowed one to follow surface transformation/relax-ation phenomena, etc. One laser was used as a Ramanprobe while another pulsed laser stimulated the dis-turbance. The authors observed that anatase-phasecoatings have lower damage thresholds than those withthe rutile phase, and the former failed via an irrevers-ible phase transition to the rutile structure at tempera-tures below 9100C. On the other hand, rutile-phasecoatings apparently accommodate energy in an excitedelectronic state resulting in nonequilibrium phasetransformation to an anataselike state.

In support of high power laser development for oper-ation in the near ultraviolet, A. Stewart of the U.S. AirForce Weapons Laboratory and D. Gallant of Rocket-dyne Division of Rockwell International have assem-bled a laboratory of instrumentation to characterizelow-loss optical surfaces and thin-film coatings.These were used to measure scatter, absorption, andreflectance at 351 nm of maximum reflectors depositedby commercial vendors on superpolished silicon andfused silica substrates. Ion-beam sputter-depositedreflectors realized the lowest losses and exhibited thehighest reflectance of 0.999 on the average. However,laser damage tests on these reflectors, conducted by S.Foltyn's group at Los Alamos National Laboratory,determined that electron-beam-produced coatings arepresently more damage resistant by a factor of 2.Since the ion-beam deposition process is in the earlyphases of optimization for damage resistance, largeimprovements may well be attained by persistent ef-

fort.As reported at several previous Boulder Damage

Symposia, the damage resistance of multilayer-dielec-tric reflectors is substantially increased by the addi-tion of halfwave thick overcoats of low-index material.Experiments at 1064 and 248 nm have demonstratedthat overcoats result in average improvements of -50%and 100%, respectively. However, at the 1983 Sympo-sium, C. Carniglia and T. Hart of the Optical CoatingLaboratory, Inc., and M. Staggs of the Lawrence Liv-ermore National Laboratory reported that halfwaveovercoats of SiO2 on high reflectors containing ZrO2and Ta 2O5 caused about a 20% reduction in damagethresholds at the intermediate wavelength of 355 nm.This very surprising result prompted these authors toconduct another and even more careful series of coat-ing and damage experiments using a scandia/silica/magnesium fluoride reflector design which had experi-enced the 100% threshold enhancement with overcoatsof both MgF 2 and SiO2. Again, on the average theovercoated reflectors had slightly lower (2-7%) thresh-olds. The physics behind the wavelength-dependentinfluence of overcoats is still not understood and offersthe coating researcher a challenge.

In a paper of considerable interest to the thin-filmdamage community, D. Decker, L. Koshigoe, and E.Ashley of the Michelson Laboratory, U.S. NavalWeapons Center, reported on measurements of thethermal properties of optical material in thin-filmform and compared them with tabulated bulk materialproperties. Materials initially analyzed were the im-portant refractory oxides A1203 and iO2. Measuredvalues of SiO2 thin-film thermal conductivity werefound to be lower by a factor of 5-8 when compared tobulk fused silica while the observed decrease for A1203was of the order of 80. The large variations wereascribed to the greater disorder present in thin filmsthan in bulk materials. In point of fact, the A1203films were quite amorphous suggesting support for theworkers' contention. Specific-heat values for thefilms were quite comparable with bulk values since thisproperty should be largely independent of structuralcharacteristics and density. The conclusions drawn inthis work have far-reaching implications when model-ing the laser interaction and thermal response in thin-film damage experiments. Clearly, more work is need-ed to corroborate the present results, determine theextent of variation in other materials, such as fluorides,and to address discontinuities between layers and thesubstrate/film system boundary. We were mostpleased to see this work under way and reported.

For many years, scientists have been trying to relateabsorption to damage sensitivity. While this has beenrelatively easy in windows and for some surfaces, nostrong correlation has been found for thin films per-haps because all films have been of uniformly poorquality. However, a new attack on this problem wasreported this year by D. Ristau, X. Dang, and J.Ebert of the University of Hanover, Federal Republicof Germany. Employing an infrared, scanning-linetechnique, they measured the temporal and spatial


development of surface-temperature increases of coat-ed material irradiated with a cw Nd:YAG laser. In thisway, they were able to separate the interface and bulkabsorption of single and multiple layers and relatethem to laser damage thresholds for 15-ns, 1.064-Mmpulses. All oxide samples of Ti, Ta, Hf, Al, and Si wereprepared by electron-beam and ion-beam assisted de-position. Thermal conductivities much lower thanbulk values were recorded for the oxides of Si and Al,with the e-beam-deposited films exhibiting higher ab-sorption and lower thermal conductivity. Interesting-ly, absorption was much higher in the bulk of the filmsthan at interfaces between layers.

With the exception of HfO2 and the coevaporatedTiO2 layers, there was a strong correlation betweenabsorption and damage threshold. Damage morphol-ogy affords evidence of inclusions in the bulk while forstacks, interface damage is frequently manifest, prob-ably due to it being a region of high stress. Coevapora-tion may help alleviate this problem.

A. Heiney, J. Eastman, and C. Gabel of the Lab-oratory for Laser Energetics at the University of Roch-ester attempted a study of the effect of anisotropicstress in thin films on laser-induced damage sensitiv-ity. Use was made of a microscope-based modulatedellipsometer to determine the directional dependenceof the stress in the plane of the film via induced bire-fringence. Interestingly, areas with positive (tensile)anisotropic stress were found to have higher damageresistance than regions of negative (compressive) an-isotropic stress. The authors reported that damagethresholds changed from 1.3 to 6.9 J/cm 2 within 100 Mm.The width of measured bands in this work ranged from0.5 to 2.5 J/cm2 . Pulse lengths were 30ps at 1.06/Mm, andthe test samples were generally TiO2/SiO2 reflectors.

Intracavity grazing-incidence mirrors appear neces-sary to handle the high-average powers expected infuture free-electron laser (FEL) oscillators. Withslight curvature of the figure, these mirrors diverge thereflected beam to tolerable intensity levels at the cavi-ty end mirrors. However, these intracavity reflectorsnow become potentially very susceptible to laser-in-duced damage and/or thermal distortion. W. South-well of Rockwell International Science Center evalu-ated the advantages of multilayer-dielectric reflectorsfor this purpose and presented an optimized design forthe layer thicknesses. With the standard design usingquarterwave layer thicknesses, very high reflectancewas predicted for s-polarized radiation with drasticallyreduced internal standing-wave (SW) electric fields.For a reflector designed for 860 incidence and film-material pairs with refractive indices of 2.0 and 1.38,the calculated peak SW fields squared were reduced bya factor of 160 relative to normal-incidence reflectors,but only for s polarization. A design with improvedperformance for both s and p polarizations was alsodevised, but a smaller field-squared reduction factor of40 was predicted. Based on previous correlations oflaser damage and the internal SW electric fields, theauthor predicted that these designs should be muchless susceptible to laser damage and thermal distor-

tion.With FELs still in mind, the theoretical design of

intracavity grazing-incidence reflectors using multi-layer dielectric coatings was also treated by J. Shellanof W. J. Schafer Associates. His criterion was to maxi-mize the reflectance of the multilayer stack to mini-mize the power transmitted to an absorbing substrate.With coating absorption assumed negligible comparedto that of the substrate, he derived analytical expres-sions for the thickness of the outermost layer of other-wise all quarterwave-layer reflectors and bench-marked the predicted reflectance against thatgenerated using numerical methods of nonlinear opti-mization. For 100% s-polarized light, maximum re-flectance requires the outer layer to have quarterwaveoptical thickness, whereas it should have halfwavethickness for 100% p polarization. Shellan's expres-sions are most helpful for analyzing the most likelyconfiguration where the fraction of p-polarized light issmall, but nonzero. Even if only 0.1% of the incidentflux is p polarized, optimization of the top layer thick-ness will reduce the reflector transmittance, and there-by the substrate absorption, by at least an order ofmagnitude.

One would expect that the angular dependence ofdamage to thin-film coatings would be something onecould calculate in a straightforward, if somewhat in-volved, manner. But as B. Newnam, S. Foltyn, D.Gill, and L. Jolin of Los Alamos National Laboratoryshowed, first principle calculations and experimentalverifications do not always agree. The results of theirstudy were not only surprising but have wide-reachingimplications since it was expected that near-grazingangles of incidence could be employed to diverge thetightly focused beams such as are present in free elec-tron lasers and thereby realize a higher intensitythroughput. Their work centered on measuring thedamage resistance of HfO2/SiO2 multilayer reflectors

9 130X

I) I1dir

I- --, 9

Wj _w -LW 5L_2 S 5

3-J

IXI I I I I I I

10° 30° 50B 70°ANGLE OF INCIDENCE

90,

Fig. 12. Measured damage thresholds normalized to the averageresult for normal incidence are compared to the predictions of thethree models (1) /cosk dilution of the fluence at the reflectorsurface, (2) inverse of the normalized peak electric-field-squared inthe top HfO2 layer for S-polarized light, and (3) for P polarization.The measured damage thresholds fell far below the model predic-

tions.


S-POLARIZATION TESTS A ATHfOz/SiO2 REFLECTORS , 85 |351-nm, 0-ns PULSES -AT 35 pps

MODELS , |[gs(I)/g(o )]ph k /

----- [gP(e)1'pP(0_)] -2 /

pk

subjected to a 10-ns pulsed 351-nm XeF laser operat-ing at 35 pps. Several angles of incidence between 00and 850 were employed using an S-polarized beam.Calculations of the expected damage threshold in-crease with angle of incidence included both areal in-tensity decrease (1/cos6) and appropriate electric fieldvariations in the thin-film-substrate system. In onecase, a multilayer designed and tested at 850 affordedonly a 2.5X increase in damage threshold compared tonormal-incidence reflectors while one could conceiv-ably expect a factor of 1OX (Fig.12). Potential expla-nations for this discrepancy included: effects due toincreased irradiation area at large angles and physical-ly thicker layers, both resulting in the interception ofmore defects, plus scattered- and trapped-light effects.Obviously, more work needs to be accomplished toshed light on this problem.

It is readily accepted that defects are bad in opticalelements used in high-power/high-energy laser sys-tems, so that anything we can learn about them isuseful whether it enables us to determine their originor to model their influence. The powerful use of scan-ning-electron microscopy was employed by L. John-son, E. Ashley, T. Donovan, J. Franck, R. Wool-ever, and R. Dalbey of the Michelson Laboratory ofthe U.S. Naval Weapons Center to study damage-initiating defects in ZnSe/ThF4 and SiH/SiO2 multi-layers.

For ZnSe/ThF4 multilayer mirrors, oblong-shapeddamage sites, oriented perpendicular to the laser elec-tric field, were associated with particulates at or nearthe surface while circular-shaped damage sites wererelated to those defects below the top layer. A thirdtype of damage was identified as pinholes. Stress-related cracking was frequently observed as well. Fre-quently, damage related to nodule defects was seen inSiH/SiO 2 multilayers. Good correlation was seen be-tween nodule density and damage sensitivity. Excel-lent micrographs of the different types of damage mor-phology related to different types of defect werepresented.

Laser damage data for pulse widths longer than afew hundred nanoseconds are scarce, especially forvisible-wavelength or near-infrared lasers. Thus, theresults of a multilayer reflector damage survey using 8-Ms (FWHM) pulses from an atomic iodine laser at 1.315,m, presented (manuscript only) by T. Deaton of theU.S. Air Force Academy, were especially interesting.Large 2.8-mm spot diameters were used in the single-shot-per-site mode. A number of coating materialcombinations and designs from several vendors, eachdeposited on silicon and molybdenum substrates, weretested. Using the onset of pitting visible by Nomarskimicroscopy as the damage criterion, threshold fluencesranged widely from 5 J/cm 2 to over 140 J/cm 2 , withuncertainties typically -15%. Titania/silica coatingshad some of the highest thresholds, and reflectors in-corporating lead fluoride layers had the lowest damageresistance. As might be expected, very little depen-dence on the substrate material was evident. Hope-fully, this survey will be followed by optimization of

the best reflector designs.A contribution from H. Deng, M. Bass, and N.

Koumvakalis of the Center for Laser Studies at theUniversity of Southern California concerned the mea-surement of the 10.6-gm damage resistance of multi-layer-dielectric reflectors comprised of ZnSe and ThF4as a function of substrate material, reflectance, anddefect density. Testing was conducted with a CO2laser restricted to operate in the TEMoo mode with apulse duration of 230 ns. The single-shot-per-sitemode of testing with a small-diameter beam (120um atIo/e level) was used. For these conditions, the averagethreshold was 90 J/cm2 with no apparent dependenceon substrate material (Si, Ge, and ZnSe), reflectance(ranging from 65% to 100%), or defect density. Antire-flection coatings of these two materials on ZnSe had amuch lower threshold (40 J/cm 2 ), which was probablydue to laser absorption by embedded impurities at thesubstrate-coating interface. Additional tests to relatevisible-light scatter to damage susceptibility revealedno correlation.

The problem of economical recovery of damagedcoated optics for recoating has been awaiting a satis-factory solution. This is especially important whenlarge-diameter substrates, figured to the tolerancesnecessary to ultraviolet coatings, are involved. J. As-mus and J. Oldenettel of Maxwell Laboratories eval-uated the effectiveness of a pulsed xenon flashlamp forremoval of a number of dielectric coatings includingSiO2, LaF3, MgF2, and HfO2. They found that a trainof exposures with 30-J/cm 2 fluence and 6 0 0-Ms pulsewidth is sufficient to dislodge these coatings withoutdamaging fused silica or Zerodur substrates. The au-thors suggested that coating removal results from ab-sorption near the ultraviolet band edge with conse-quent differential heating and expansion. In someinstances, a thin coating residue remained after irra-diation indicating the need for further refinement ofthis technique.

D. Fundamental Mechanisms

With the practical problems of thin-film coatingsdrawing so much attention this year, only three papersaddressed the fundamental physics questions of im-portance. These included calculations of the thermalresponse of thin films under repetitive-pulse irradia-tion, theoretical analysis, and critical review of pastwork concerning self-focusing in transparent solids,and calculation of two-photon absorption coefficientsfor semiconductors.

Extending their thermal modeling work for dielec-tric thin films M. Lange and J. McIver of the Univer-sity of New Mexico and A. Guenther of the U.S. AirForce Weapons Laboratory solved the thermal diffu-sion equation for repetitive pulses. The possibility ofirreversible changes occurring with each pulse was in-cluded in the theory, an example of which is shown inFig. 13. In this case, the spatial dependence of theabsorption from pulse to pulse is a key parameter.The special case of spherical or cylindrical absorptioncenters was shown to lead to an elegant closed-form


IN= I

*N=10N=1001. 2

1.01

E(A2) 081

It

0

tp= 10- Os

'd =100I;--

0.2 0.4 0. 6 0. 8 1. 0 1. 2 1. 4

(Xk2 FILM AT X= 1 uM) RADIUS a(#X10-4cm)

Fig. 13. Computed damage threshold vs radius of an absorbinginclusion in a 0 .5 -jum thick thorium fluoride film for various numbersof i-Mm, 100-ps laser pulses at 108 Hz. Although minimal thermalbuildup with increasing number of pulses is evident at this repetition

rate, a large effect is predicted for 109-Hz irradiation.

Table I. n2 Measurements in Liquids and Solids

n2 n2

Wavelength (X 10-13 esu) (X 10-13 esu)Material (gm) This work Other workers

CS2 1.06 128 + 10 125 + 30a0.53 123 ± 10

NaCl 1.06 1.37 + 0.15 1.22 0.2 1 b0.53 1.38 ± 0.13

SiO2 1.06 0.62 ± 0.03 0.95 i 0.10b0.53 0.60 + 0.04

BK-7 1.06 1.45 + 0.150.53 1.01 ± 0.08

a Time-integrated interferometry at 1.32 gm; Ref. 1.b Time-resolved interferometry at 1.06 Mm; Ref. 2.

solution. Various models of absorption proportionalto number of pulses were explored and the basis waslaid for further work on multipulse damage analysis.

M. Soileau, W. Williams, and E. Van Stryland ofNorth Texas State University continued to addressnonlinear electrooptical processes. Theoretical anal-ysis, indicating that the critical parameter of self-fo-cusing is the second critical power P2 (presented in1983), was applied to various experimental studiesreported in the literature and to measurements madeon SiO2 , NaCl, BK-7 glass, and CS2 . In cases wheresufficient data were available the recalculations weremade, yielding improved values (Table I). It wasshown that some previous analyses yield results whichare both logically inconsistent and in error by as muchas an order of magnitude.

An analytical relationship for the two-photon ab-sorption coefficient based on material parameters suchas energy gap, linear index of refraction, wavelength,and a parameter f was derived by E. Van Stryland, H.Vanherzeele, M. Woodall, M. Soileau, A. Smirl, S.Guha, and T. Boggess of North Texas State Universi-ty. (The parameter f is a function of the assumed

band structure and hence those optical states whichare coupled.) Using this relation, the two-photon ab-sorption coefficients of ZnTe, CdTe, CdS/Se mixtures,GaAs, ZnS, ZnSe, CdS, and ZnO were calculated usingpublished parameters and compared against measuredpulses. Good agreement was found in all cases, al-though a dependence on crystal type and single orpolycrystalline structure was observed. Calculatedvalues for ZnS, ZnSe, CdS, and ZnO were reported for awavelength of 0.53,Mm, and values for the others for awavelength of 1.06,gm (Fig. 14). Data on InSb, report-ed previously, were shown to fit the theory, also. Thenonlinear real part of the index was calculated assum-ing that it arises from two-photon generated free carri-ers. An optical limiter made of GaAs, which employsboth two-photon absorption and self-defocusingtransmitting 33% of the incident light at low intensi-ties, was demonstrated. For energies above 10,MJ thetransmitted energy was constant at 10,MJ over an inci-dent energy range of at least 20. In comparison, GaAsmelted above an energy of 100 MJ (0.9 J/cm 2 ), but thedevice continued to function. Response time wasshown to be in the picosecond range.

Ill. Recommendations for the Future

We should retain a commitment to the extensive andlengthy development required to adapt promising newmaterials to practical applications. The successfuland novel use of liquid crystals as polarizers is a signifi-cant example of the advantage obtained through con-centrated study of a new material. In addition, in-creased use of surface diagnostic devices, particularlythose capable of studying damage precursors, is need-ed to find the causes of damage. We hasten to add thatautomation of damage test laboratories will provide aneeded increase in the national capability for signifi-cant damage testing. However, such labs should useprocedures that contain adequate redundancy to in-sure detection of error due to malfunctions of the appa-ratus.

Fig. 14. Two-photon absorption coefficient 2 (cm/GW) scaled byn2/(Ep)1/2F2vs energy gap for a variety of semiconductors. Parabol-ic band structure was assumed which affects the optical couplingfactor F2, n is the refractive index, and E, is nearly material indepen-dent (19-26 eV). The solid line is a least-squares fit of the data to aline of slope -3 (omitting ZnTe). The various x shown for GaAs,CdTe, CdSe, and ZnTe are data from Ref. 3. The laser wavelength

was either 1.06 or 0.53 gm as indicated.


The increasing use of diffraction gratings in high-power laser resonators motivates more laser damagetesting for a wide range of laser pulse widths. Basedon the results reported at this conference, an especiallyvaluable achievement would be development of a pro-cess for fabricating large-size, blazed holographic grat-ings directly on high-conductance substrates such assilicon, silicon carbide, or molybdenum. Eliminationof the photoresist or epoxy layer used in current fabri-cation would greatly increase the damage threshold forlong-pulse and cw laser applications.

While it was gratifying to note the many correlationsbetween deposition process variables and film struc-ture on one hand and optical properties on the other,insufficient attention was paid to correlating damageresistance with these process-dependent characteris-tics. We all agree that lower absorption, denser, less-stressed films are good goals. However, there are someconflicting, or at least not readily consistent, resultsregarding the magnitude of the stress dependence ofdamage. This should be pursued with attention givento stress compensation in multilayers. Continued de-velopment of new processes, materials, and new diag-nostics is warranted. UHV and molecular-beam epi-taxy look promising as does ion-beam-assisted deposi-tion; too little is known about laser evaporation as yet,but laser-induced desorption or substrate modifica-tion appear promising. The most advantageous of thediagnostics is Raman scattering, particularly whentime-resolved, and surface emission as an indicator ofsurface cleanliness or precatastrophic damage. Butmost importantly, we must resolve the issue of thin-film thermophysical properties. For example, ther-mal conductivities for some film materials are of simi-lar magnitude as those of bulk materials while they areorders-of-magnitude lower for other films. Precisemeasurements of these properties are important forthermal failure modeling, material selection, and filmdesign. They must certainly be structure dependent.

Finally, two unexplained results concerning thedamage resistance of multilayer dielectric reflectors,described at the beginning of this review, should bepursued vigorously with careful experimentation andanalysis. These are the wavelength-dependent influ-ence of overcoat layers and the angular dependence,especially at angles beyond 60°.

The editors would like to acknowledge the invalu-able assistance of Aaron A. Sanders and the otherinvolved staff members of the National Bureau ofStandards in Boulder, CO, for their interest, support,and untiring efforts in the professional operation of thesymposium. Particular thanks go to Susie Rivera forher part in the preparation and publication of theproceedings and to Pat Whited of the U.S. Air ForceWeapons Laboratory for conference coordination.

BibliographyA. J. Glass and A. H. Guenther, Eds., "Damage in Laser Glass,"

ASTM Special Technical Publication 469 (ASTM, Philadelphia,

1969).A. J. Glass and A. H. Guenther, Eds., "Damage in Laser Materials,"

Natl. Bur. Stand. U.S. Spec. Publ. 341 (1970).N. Bloembergen, "Fundamentals of Damage in Laser Glass," Na-

tional Materials Advisory Board Publ. NMAB-271 (NationalAcademy of Sciences, Washington, DC, 1970).

A. J. Glass and A. H. Guenther, Eds., "Damage in Laser Materials:1971," Natl. Bur. Stand. U.S. Spec. Publ. 356 (1971).

N. Bloembergen, "High Power Infrared Laser Windows," NationalMaterials Advisory Board Publ. NMAB-356 (National Academyof Sciences, Washington, DC, 1971).

A. J. Glass and A. H. Guenther, Eds., "Laser Induced Damage inOptical Materials: 1972," Natl. Bur. Stand. U.S. Spec. Publ. 372(1972).


A. J. Glass and A. H. Guenther, "Laser Induced Damage in OpticalMaterials, 1973: a Conference Report," Appl. Opt. 13, 74 (1974).


A. J. Glass and A. H. Guenther, "Laser Induced Damage in OpticalMaterials: Sixth ASTM Symposium," Appl. Opt. 14,698 (1975).


A. J. Glass and A. H. Guenther, "Laser Induced Damage in OpticalMaterials: Seventh ASTM Symposium," Appl. Opt. 15, 1510(1976).


A. J. Glass and A. H. Guenther, "Laser Induced Damage in OpticalMaterials: Eighth ASTM Symposium," Appl. Opt. 16, 1214(1977).


A. J. Glass and A. H. Guenther, "Laser Induced Damage in OpticalMaterials: Ninth ASTM Symposium," Appl. Opt. 17, 2386(1978).


A. J. Glass and A. H. Guenther, "Laser Induced Damage in OpticalMaterials: Tenth ASTM Symposium," Appl. Opt. 18, 2112(1979).

H. E. Bennett, A. J. Glass, A. H. Guenther, and B. E. Newnam, Eds.,"Laser Induced Damage in Optical Materials: 1979," Natl. Bur.Stand. U.S. Spec. Publ. 568 (1979).

H. E. Bennett, A. J. Glass, A. H. Guenther, and B. E. Newnam,"Laser Induced Damage in Optical Materials: Eleventh ASTMSymposium," Appl. Opt. 19, 2375 (1980).

H. E. Bennett, A. J. Glass, A. H. Guenther, and B. E. Newnam, Eds.,"Laser Induced Damage in Optical Materials: 1980," Natl. Bur.Stand. U.S. Spec. Publ. 620 (1981).

H. E. Bennett, A. J. Glass, A. H. Guenther, and B. E. Newnam,"Laser Induced Damage in Optical Materials: Twelfth ASTMSymposium," Appl. Opt. 20, 3003 (1981).

H. E. Bennett, A. H. Guenther, D. Milam, and B. E. Newnam, Eds.,"Laser Induced Damage in Optical Materials: 1981," Natl. Bur.Stand. U.S. Spec. Publ. 638 (1983).

H. E. Bennett, A. H. Guenther, D. Milam, and B. E. Newnam,"Laser-Induced Damage in Optical Materials: ThirteenthASTM Symposium," Appl. Opt. 22, 3276 (1983).



H. E. Bennett, A. H. Guenther, D. Milam, and B. E. Newnam,"Laser-Induced Damage in Optical Materials: FourteenthASTM Symposium," Appl. Opt. 23, 3782 (1984).


H. E. Bennett, A. H. Guenther, D. Milam, and B. E. Newnam,"Laser-Induced Damage in Optical Materials: Fifteenth ASTMSymposium," Appl. Opt. 25, 258 (1986).

References

1. K. J. Witte, M. Galanti, and R. Volk, "n2-Measurements at 1.32Am of Some Organic Compounds Usable as Solvents in a Satura-ble Absorber for an Atomic Iodine Laser," Opt. Commun. 34, 278(1980).

2. M. J. Weber, D. Milam, and W. L. Smith, "Nonlinear RefractiveIndex of Glasses and Crystals," Opt. Eng. 17, 463 (1978).

3. J. H. Bechtel and W. L. Smith, "Two-Photon Absorption inSemiconductors with Picosecond Laser Pulses," Phys. Rev. B 13,3515 (1976).

15-29 Disordered Solids: Structures & Processes course,Erice, Italy B. Di Bartolo, Dept. Physics, BostonColl., Chestnut Hill, MA 02167

12-15 Int. Symp. on Optical Particle Sizing: Theory and Prac-tice, Rouen G. Gouesbet, Dept. Chem. Eng., INSAde Rouen, BPO8, 76130 Mont-Saint-Aignan, France

13-15 7th Int. Workshop on Expert Systems & Their Applica-tions, Avignon Agence de l'Informatique, TourFiat,92084 Paris-La Defense, Cedex 16, France

17-22 40th Ann. SPSE Conf. & Symp. on Hybrid ImagingSystems, Rochester D. Wilkalis, P.O. Box 821, Fair-port, NY 14550

17-22 Optics, Electro-Optics, & Sensors, Tech. Symp. South-east, Orlando SPIE, P.O. Box 10, Bellingham, WA98827

18-20 SIAM Conf. on Optimization, Houston Soc. for Indus-trial & Applied Mathematics, 14th Fl., 117 S. 17thSt., Phila., PA 19103

18-21 Modern Experimental Spectroscopy course, Las CrucesLaser Inst. of Amer., 5151 Monroe St., Toledo, OH43623

22-26 9th Int. Exhibition for Environmental Pollution Con-trol, Measuring & Testing Equipment, Seoul SHKInt. Services Ltd., 22/F., Tian An Centre, 151Gloucester Road, Wanchai, Hong Kong

27-29 6th Int. IPAT Mtg., Brighton D. Cowan, Tech. MediaServices, 62 Kelvingrove St., Glasgow G3 7SA, UK

June

17-25 21st Session of the Int. Commission on Illumination,Venice W. Budde, NRC, Ottawa, Canada KIA OR6

21-25 5th National Synchrotron Radiation InstrumentationConf., Madison R. Ward, Synchrotron RadiationCtr., 3725 Schneider Dr., Stoughton, WI 53589

22-26 Laser Mtg., Munich Kallman Assoc., 5 Maple Ct.,Ridgewood, NJ 07450

22-26 Optical Image Processing & Pattern Recognition course,Madison F. Drake, Eng. Professional DevelopmentDept., U. WI, 432 N. Lake St., Madison, WI 53706

22-26 8th Int. Conf. on Laser Spectroscopy, Are S. Svanberg,Box 118, S-221 00 Lund, Sweden

22-26 Advanced Infrared Technology course, Ann Arbor Eng.Summer Confs., 200 Chrysler Ctr., N. Campus, U. ofMI, Ann Arbor, MI 48109

23-29 Int. Environmental Control, Measuring & TestingEquipment Exhibition, Beijing SHK Int. ServicesLtd., 22/F., Tian An Centre, 151 Gloucester Road,Wanchai, Hong Kong

28-7 July Optical Instabilities & Chaos course, Lucca M. Ingus-cio, Director, Dipartimento di Fisica, U. di Pisa, Pi-azza Tirricelli, 56100. Pisa, Italy

29-30 Color Appearance Top. Mtg., Annapolis OSA Mtgs.Dept., 1816 Jefferson Pl., N.W., Wash., DC 20036

1-5 5th European Fiber Optic Communications & LocalArea Networks Expo., Basel Info. Gatekeepers, Inc.,214 Harvard Ave., Boston, MA 02134

9-13 Deutsche Gesellschaft fur angewandte Optik Mtg.,Berchtesgaden/Bavaria R. Torge co C. Zeiss, Post-fach 1369/1380, D-7082 Oberkochen, FRG

15-18 The Effects of Modes of Formation on the Structure ofGlass Conf., Nashville D. Kinser, Box 1689-B, Van-derbilt U., Nashville, TN 37235

15-19 ISDN/Broadband Networks for the Future Mtg., Atlan-ta Info. Gatekeepers, Inc., 214 Harvard Ave., Bos-ton, MA 02134

15-19 Infrared Technology Fundamentals & System Applica-tions course, Ann Arbor Eng. Summer Confs., 200Chrysler Ctr., N. Campus, U. of MI, Ann Arbor, MI48109

July

0-0 The Life, Work, & Milieu of Robert Hooke, London M.Hunter, Dept. of History, Birbeck Coll., U. of London,Malet St., London WCIE 7HX, UK

1-3 Color Vision Deficiencies Int. Symp., Annapolis B.Drum, Johns Hopkins U., School of Medicine, Wil-mer Ophthalmological Inst., 601 N. Broadway B-27,Baltimore, MD 21205

1-4 Computer Assisted Radiology Mtg., Berlin IEEE, 345E. 47th St., New York, NY 10017

continued on page 884


Meetings continued from page 812

1987

May

laser-induced damage in optical materials: sixteenth astm symposium

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