This PDF file contains the front matter associated with SPIE Proceedings Volume 1438, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

It is an essential aspect of science, technology and commerce to be able to communicate in a common language of units, procedure and phenomenology. Presently, there exist no such standards for the field of damage testing, leading to the present problems in scientific communication, comparison of test results and commercial activities. A necessary precursor to the standardization and proper cataloging of laser damage test data is a common nomenclature. There are many cases where parameters, phenomenology and test results are denoted by identical names. Moreover, there are cases where a single idea is known by multiple labels. This lack of standardization in the definitions and nomenclature leads to difficult, tedious and frequently unfruitful literature searches. This paper is intended to be the first step in the standardization of the field of laser induced damage by reviewing the literature and assigning in one review paper, a_set of exclusive definitions or labels to the various definitions of damage, threshold determination, spot size and pulse width. In making these assignments, a vehicle is provided for the necessary seminal .discussions leading to eventual adoption of accepted standards of procedure and definition for laser damage testing.

Refractive indices of transparent materials can be measured with high accuracy on small laboratory samples using null ellipsometry. Measurement precision in both n and k obtained ellipsometrically is ±0.0004 for semi- transparent samples. Systematic errors in ellipsometric characterization of optical constants for transparent materials can result from back-surface reflection as well as from front-surface scattering caused by surface roughness. An analysis of the contribution of these errors and the methods of eliminating them are discussed. We conclude that careful ellipsometric characterization can give indices of refraction to three decimal places in the infrared for materials with low k.

Efforts to optimize precision machining emphasize form and surface finish as the primary objectives. Minimizing subsurface damage is no less important for high power optics. This paper reports our usage of instrumented microindentation for subsurface damage assessments. Our instrument is an example of a new class of diagnostic tool. All apply controllable loadings to an indenter and continuously measure the load and resulting penetration. The utility of microindentation depends on appropriate interpretations of load-depth curves. The technique provides insight into the physical nature of the subsurface and is a means to obtain a variety of engineering properties from the subsurface region. We use it to profile flow stress and elastic modulus with depth, and to gauge creep and anelastic behaviors. Instrumented microindentation promises rapid and sensitive property assessments. We will use a range of examples to show this potential is realizable.

A set-up for automated measurement of damage thresholds on UV optical components is presented, being part of the EUREKA program "High Power Excimer Lasers". It includes on-line monitoring of probe beam parameters as well as sample condition, using digital image processing techniques for both laser beam profiling and high sensitivity damage detection. The latter is performed with a video microscopy system by pixel-to-pixel comparison of the video frames taken before and after the test laser pulse.

We are completing a thorough expansion of the laser-induced damage-test capabili- ties at LLNL which allow us to conduct tests under a variety of parameter conditions. We have nine different laser systems which cover the following parameter space:

In conjunction with our diversification of laser damage testing capabilities (see "Expanded Damage Test Facilities at LLNL" this conference), we have expanded upon a database of threshold measurements and parameter variations at 1064 nm. This includes all tests at low pulse-repetition frequencies (PRF) ranging from single shots to 120 Hz. These tests were conducted on the Reptile laser facility since 1987 and the Variable Pulse Laser (VPL) facility since 1988. Pulse durations ranged from 1 to 16 ns. The table below summa- rizes the test data scaled to 10-ns pulses.

Recently, we have expanded our efforts to develop state-of-the-art optical compo- nents for use in large-scale, high-peak-power, solid state lasers. Laser-induced damage to many of these components sets critical constraints on construction costs and limits the peak powers attainable in current and proposed devices. Increasing the damage threshold when- ever possible by improving materials fabrication technology is therefore the ultimate goal of our work. The optical components and technological developments we are investigating are:

We present results of a study of the laser induced damage thresholds of T13AsSe3 at 10 pm using nanosecond and picosecond pulses. The damage threshold was found to be ne 10 J/cm2 with 130 ns (FWHM) and no damage was observed using 60 ps pulses.

Single and multiple shot laser induced damage threshold (LIDT) values were determined for Schott's 0G-550 optical absorption glass using Q-switched 32 nm laser radiation. Bulk discoloration occurred for a single sot LIDT of 55 mJ/cm while surface pitting resulted at a single shot LIDT of 5.6 J/cm . Multiple shot LIDTs were determined at 10, 5, and 2 Hz. For all three repetition rates, the multiple shot LIDTs decreased exponentially from the single shot LIDT, and level off at 35% of that value.

Preliminary analysis has indicated that laser radiation may alter microwave propagation in transparent solids by both electron avalanche and thermally induced plasmas. These effects may occur at fluence levels below that at which visible damage occurs. Temperature rises well below the melting point may create sufficiently dense plasmas to affect propagation. The dependence of propagation effects upon plasma density and size has been modeled. Effects of varying the initial temperature have been studied. Irreversible changes in microwave absorption are also considered. Effects are analyzed for a CdTe microwave modulator for CO2 laser radiation. Plasmas with densities 1015-1016 cm -3 are generated at temperatures 500-700 °C in CdTe. At these densities, microwave propagation may be significantly affected. Microwave measurements of simulated plasmas have been carried out using metallic and resistive materials of different dimensions Comparison is made with theoretical models. An experimental setup using a high energy pulsed CO2 laser and a sensitive microwave reflectometer for measuring laser induced effects will be discussed.

Laser-induced damage in two types of silicon photosensor array has been studied. The samples were MOS CCD time delay integration (TDI) sensors with a 2048x96 element array of pixels and CID photodiode arrays of 512x1 pixels. The laser source was a Q-switched 1064 nm Nd:YAG laser (10 Hz rep rate, 10 ns pulses with a 250 µm spot radius). Tests for morphological and electrical damage to the CCD arrays have been reported previously. In new experiments, the micro-damage morphology is examined and correlated with both the observed electrical degradation and newly observed stress effects. We report the observation of surface deformation and lattice defects due to laser-induced stresses in the SiO2 and poly-silicon thin films on the silicon substrate. Measurements of damage for the CID arrays show them to be more resistant to laser damage than MOS structures such as CCD arrays. In addition, electrical degradation of these arrays was observed which affected the video output signal from the devices.

We present a sensitive single beam technique for measuring nonlinear refraction in a variety of materials that offers simplicity, sensitivity and speed. The transmittance of a sample is measured through a finite aperture in the far-field as the sample is moved along the propagation path (z) of a focused Gaussian beam. The sign and magnitude of the nonlinearity is easily deduced from such a transmittance curve (Z-scan). Employing this technique a sensitivity of better than A/300 wavefront distortion is achieved in n2 measurements of BaF2 using picosecond visible laser pulses.

We have characterized the nonlinearities observed in suspensions of carbon black particles in liquids (CBS). We have developed a preliminary explanation of the optical limiting characteristic of the CBS that qualitatively explains the low limiting thresholds. We have found that the limiting depends primarily on the input optical fluence (J/cm2) rather than irradiance (W/cm2). We have monitored transmission, side scattered light, and the photoacoustic response of the CBS simultaneously. The nonlinear scattered light appears to be the dominant nonlinearity. Additionally, we have observed that the nonlinearities disappear after repeated laser firings. Thus, in essense, we are performing a laser induced damage experiment, and we have prepared a material with a low damage threshold. These data have led us to the following model. The carbon first linearly absorbs the input light efficiently. The carbon is rapidly heated, vaporizes and ionizes to form a rapidly expanding microplasma. This plasma absorbs and scatters subsequent light, thus limiting the transmittance.

Heavy metal fluoride (HMF) ylasses are of considerable interest for use as multispectral optical components. HMF glass samples of different compositions were prepared and subjected to different doses of gamma radiation at room temperature. Optical absorption measurements showed that strong damage occurred in the UV and visible regions and no significant losses were detected in the infrared region.

A significant enhancement of the single shot Laser Induced Damage Threshold of CaF2 and fused silica and a moderate enhancement for GaAs and Al has been observed as the result of laser annealing using an excimer laser operating at 248 nm. This phenomenon is primarily attributed to a reduction of the residual surface roughness of the samples.

The detection of damage associated shock pulses via the deflection of a probe laser beam is exploited to study multiple-shot damage thresholds in CaF2, PMMA, copper, and aluminium. We find a reduction of threshold intensity as compared to single-shot con- ditions. The experimental results point towards deviations from the previously postu- lated fluence dependence FN = FINS-1 [see Ref.4]. They are, however, compatible with a simple model relating the threshold reduction to defect accumulation via non-linear energy absorption.

The application of the Ronchi ruling beam characterization method to axially symmetric optical beams is analyzed. Specific results are derived for the Airy and focused annulus diffraction patterns. Plots of the ratio of minimum to maximum transmitted optical power versus the first null radius of the beam functions show that for the Airy pattern and other focused annuli with obscuration ratios smaller than approximately 0.30, the method should be just as useful as with Gaussian beams.

In this method, the laser induced damage threshold for a material is obtained by determining the fluence within the damaging beam at a radial distance equal to the radius of the damage spot. The energy profile of the laser pulse is deter- mined by translating a slit across the beam and reading the energy passing through the slit with2position. The laser profile is then quantified into fluence squares of area 400um in a two dimensional fluence grid. The radius of the damage site is then applied to the fluence grid, indicating the fluence square that corresponds to the edge of damage and it is this value that is the predicted damage threshold.

The construction and performance of a system for the spatial characterization of a CO2 pulsed laser beam is described. Using thermally quenched fluorescence a direct imaging technique has been developed for single shot CO2 laser pulse beam profiling. A CCD video camera and framestore interfaced to a PC computer are used to image the spatial intensity distribution produced by a CO2 pulse incident upon a temperature dependent luminescent screen. The resolution is similar to much more expensive pyroelectric detector array techniques with all the advantages of two dimensional beam processing.

The characteristics of laser-induced electrical failure in biased silicon avalanche photodiodes have been observed. The samples were RCA reach-through avalanche photodiodes with antireflection coatings. They were biased at typical operating voltages during irradiation. The laser source was a Q-switched 1064 nm Nd:YAG pulsed laser operating at 10 Hz with a 10 ns pulse length and with a 300 gm spot radius. The current-voltage characteristics were monitored for permanent change as a function of laser fluence and the degradation thresholds were found. Two types of change were observed. The first type was a large increase in bulk leakage current. It may be modeled by the introduction of defects into the depletion region by deep melting transients. The second type was catastrophic failure in which the devices were electrically shorted after irradiation. It may be modeled by excessive current density in the photodiode junction. The type of failure was determined by the parameters of the biasing circuit.

The development of subsurface defects in optical components has been extensively investigated by the authors and others over the past few years. Investigation of optical components using nondestructive photon backscattering (PBSTM), delineation etch/ optical microscopy and charge decoration/delineation techniques has shown laterally variable spatial distributions of subsurface defects in the form of lineated remnant polishing traces, voids, and microscopic impurity clusters introduced during the surface finishing process. In this paper, we report on the correlation between nondestructive PBSTM mapping and delineation etch/optical microscopy techniques for detection of subsurface defects, as well as the effect of subsurface defects qn the initiation of microscopic or "incipient" laser damage (A= 248 nm) at levels below the point of macroscopic or catastrophic failure observed at the surface. The detection of laser induced microscopic cracking on nucleation sites within nondestructively identified areas provides additional insight into the progressive develop- ment of failure sites in distributed subsurface regions of low RMS, optically finished surfaces.

The damage to optics from high power laser radiation depends in part upon the surface films on the system optics. Approaches to hardening these surfaces to these types of radiation have focused on layered film compositions and upon the reduction of structural defects in the films and the substrates. The importance of heavy metal contamination at trace levels (less than one thousandths of a monolayer) in the top few monolayers of the surfaces, or at the film interfaces, has been minimized because of insufficient analytical technology to detect these impurities. These localized heavy metals may absorb radiation, ionize to high positive states, and become the source of defects leading to damage. This paper will describe a new analytical technique, Total Reflection X-Ray Fluorescence (TXRF), which is capable of quantitatively detecting heavy metals (Z<11) on the surface (top few nm's) of the substrates or films, with detection limits down to 1011 atoms/cm2 (in most cases several orders of magnitude better than ESCA or AES, and quantitative, in contrast to SIMS.)

An increasingly important tool in the characterization and measurement of optical materials and components in the Wyko TOPO Noncontact Surface Profiler. Optically, it consists of a low- to medium- power microscope in which the objective, depending upon the power, is a visible light interferometer of either Michelson, Mireau, or Linnik design. By integrating a detector array into the viewing system and attaching the reference arm of the interferometer to a piezoelectric transducer, the technique of surface profiling by phase shifting is accomplished by computer. The potential utility of this system to the investigator interested in the fundamental mechanisms of laser-induced damage is enormous, provided that the user is aware of its proper use and limitations. This paper is intended to act as a user's guide to the TOPO system in such cases and to document its precision and accuracy limitations given a variety of surface types and measurement configurations.

It has been very important to reduce the machining cost of optical materials in the laser fusion program. LHG-8 laser glass was ground with the ultra-precision surface grinder having a glass-ceramic spindle of zero-thermal expansion in order to remove or shorten the optical polishing process in manufacturing high power laser optics. The laser damage thresholds were measured at 1 ns and 30 ns in pulse width and 1.053 µm in wavelength on ground surfaces as well as optically-polished surfaces. 0.5 nm rms in surface roughness was measured on the ground surface with WYKO's TOPO-3D. The laser damage thresholds on the ground surface were very similar to that on the optically-polished one.

Reaction bonded silicon carbide (RB SiC) can be readily fabricated to near net shape and mirror blanks produced by this method can potentially be less costly than those fabricated by chemical vapor deposition (CVD). However, RB SiC is two phase, SiC and up to 30% silicon (Si), and can not normally be directly polished to low scatter and roughness levels due to the difference in hardness of the two phases. We have investigated the polishability of RB SiC as a function of Si content and microstructure. Our results show that with a favorable microstructure, RB SiC can be polished to less than 10A rms. For reduced roughness and lower scatter surfaces, we have developed low temperature deposition techniques to apply polishable, single phase coatings to the figured two phase surfaces. Scatter and roughness measurements show levels are comparable to CVD SiC.

While the theoretical possibility of achieving near-perfect diffuse reflectance in a purely scattering material is longstandineu, no attempt seems to have been made to develop scatterers with albedos of 0.999 and beyond. Such ultra-high albedo diffuse scatterers are now of interest because they offer the possibility for the development of shields against high levels of laser radiation(2).

We report observation results about damages on SiO2 mirror surfaces. The damages were made when the mirrors were used as a cavity reflector for the Ar and Kr excimer lasers. The surface profile, transmission and reflectance spectra, and X-ray photoelectron spectra show that bulk Si is isolated in the surface layer which was exposed to 9.8eV photons from the Ar excimer laser. The Kr excimer laser, whose photon energy is 8.5eV, does not induce such a phenomenon. The Ar excimer laser photons, surmounting the fundamental band gap of 5i02, 9eV, are considered to create high density excitons that induce the Si-0 bond breaking and Si isolation.

A major concern in the operation of the LLNL 120 kJ Nova laser system is damage to the nominal 1-meter diameter mirrors (i.e. HR coatings). The damage appears to originate from microscopic defects that vary in size up to approximately 30 pm in diameter. These defects produce damage sites up to 250 pm and, in some cases, covering 20% or more of the mirror surface. Defects due to particulates that are in or under the coating are highly unlikely due to the fact that coating thickness is only 5 pm thick and particles up to 30 pm would produce surface roughness that could be easily detected. Consequently, the defects must be plate-like (i.e. "2-D") in nature. A by-product of the damage is a haze left on the surface of the mirror. This haze is analyzed and shown to be blown-off particles of Zr02/Si02 coating material. The particle morphology suggests nucleation from a vapor phase which implies localized heating in the coating to temperatures on the order of 3000°K. Some possible damage mechanisms are also explored.

Thermal transport data on thin metal coatings are presented. The data illustrate the ability of the measurement technique to characterize the thermal transport properties of single samples. The method offers insight on coating survivability. key words: adhesion; coatings; damage; delamination; laser hardness; laser-induced dam- age; nondestructive evaluation; nuclear hardness; optical coatings; thermal conductivity; thermal diffusivity; thermal properties; thermal transport; thin films.

The application of total internal reflection microscopy (TIRM) to the inspection of subwavelength optical features on surfaces has been well documented.-3 This paper discusses recent experiments testing the feasibility of using TIRM to study dielectric coatings in the nucleation stage. Samples were coated with thin films of different thicknesses to allow optical scattering levels to be determined as a function of coating thickness. An increase was observed in scatter levels with increasing coating thickness, and is assumed to be related to thin film microstructure. Thus, TIRM provides a means of detecting film defects in the early stages of coating growth, as well as identifying possible precursors to nodules that are often observed in thin and thick films.

The "scattering characterization" that was developed in Marseilles enables us today, with the help of complementary tools (causal model, isotropy degree, ...), to point out the origin of micro-roughness in optical coatings without any ambiguity. Each material can be associated to a key parameter that characterizes its scattering. These parameters can be determined through measurements performed on one single layer, and then can be used to predict scattering in more complex systems. Experimental results (IAD, IP) are presented and show that in most cases, scattering measurement of substrate is sufficient to predict scattering from any coating with a high accuracy. However, at very low scattering levels (under 10-6), this prediction is limited by the presence of local defects in the coatings.

Linear phosphorus-nitrogen based polymers have been synthesized and deposited as thin films from solution on silica or silicon substrates using dip-coating or spin-casting methods. Most materials transmit well into the ultraviolet region of the spectrum and have refractive indices (1.5 to 1.7 at 500 nm) which are controlled by the functionality of chemical substituents introduced at the phosphorus atom. Films exposed to pulsed laser irradiation (1064 nm , 8ns pulsewidth) exhibit damage morphologies ranging from severe cratering to delamination. The chloro-substituted polymer shows a spheroidal surface morphology after exposure to a 25 J/cm2 pulse suggesting melting or condensation of ablated material. The relative stability of these materials to laser irradiation will be discussed in terms of chemical bond ionicity which is influenced by atom electronegativity and the nature of substituent groups. Preliminary measurements of the non-linear optical response observed in several of these materials (SHG) will also be discussed.

As optical coatings become more complex, partly due to improved deposition processes and more stringent design criteria, the number of layers in the coating increases, with a corresponding risk of interfacial instability. This can lead to partial film delamination, arising from residual stress effects, from interdiffusion of different chemical species across the film boundaries, or from the presence of impurity species at the interface itself. These effects are very relevant to the application of such coatings in laser systems, especially where the newer digital techniques are used to synthesise graded designs. One useful way of exploring the effect is to incorporate a large number of interfaces within a Fabry-Perot etalon cavity, and to temperature cycle the structure, whilst measuring its spectral response. The changes observed can be correlated with information derived from X-ray photoelectron spectroscopic examination of discrete interfaces specifically fabricated to simulate those present in the etalon cavity.

With the recent advances in surface analytical instrumentation, a methodology now exists which enables optical thin films to be studied in considerable detail. Scanning Auger spectroscopy with argon ion etching capabilities in conjunction with scanning electron microscopy, backscatter electron microscopy, optical microscopy, and light element energy dispersive x-ray spectroscopy have been successfully used at the University of Dayton to study coating defects and radiation induced damage in a variety of specimens. A brief description of the capabilities and limitations of some of these techniques will be given. Ball cratering, a relatively obscure technique by which underlying layers in an optical stack can be exposed, has proven to be a power- ful tool which has extended the range of the above mentioned analytical techniques. Some examples of adhesion failures, crack phenomena, substrate and film failures, and Auger depth profiling will be presented.

Results are presented that show the damage thresholds of e-beam deposited multi- layer HfO2/SiO2 thin films can be permanently increased by a factor of 2 to 3 by illumination with subthreshold fluences of laser light. This sub-threshold illumination procedure is referred to as "laser conditioning". The films used in this study were prepared by three different physical-vapor-deposition techniques: ion-beam sputtering, plasma plating and e- beam evaporation. Only the e-beam deposited films showed consistent and significant improvement with laser conditioning. Of the material pairs examined (Hf02/Si02, ZrO2/SiO2 and TiO2/SiO2), Hf02/Si02 gave the greatest and most consistent damage improvement with conditioning. The number of layers and the reflective or transmissive characteristics of the HfO2/SiO2 films were found to have little` impact on laser conditioning of the film. The results show that the damage thresholds of a wide range of e-beam deposited coatings (e.g. HR's, polarizers, etc.) can be improved by laser conditioning. Several possible conditioning mechanisms are examined.

The laser conditioning of dielectric thin film HR coatings has been studied as a practical method for the improvement of the damage thresholds of large area ( 1.1 m dia.) high power 1064 nm laser mirrors on the LLNL 120 kJ, 100 TW Nova laser system. Both Hf02/Si02 and ZrOJSiO2HR coatings were conditioned by rastering with a small (-0.2 mm) diameter beam from a pulsed (18 Hz, 8 ns) Nd-YAG laser (1064 nm), The samples were rastered at various fluences below the unconditioned damage threshold and subsequently damage tested. Large area conditioning studies were also performed using a large aperture beam of the Nova laser. The laser conditioning effect was found to be permanent. Improvements in damage threshold due to conditioning were as high as a factor of 2.7 and were dependent on the conditioning parameters. A model for the conditioning effect is proposed based on the emptying of electronic defect levels within the bandgap of the dielectric materials.

In this paper we report on an empirical study of the population distributions of conditioned laser damage thresholds on antireflection (AR) coated BK-7 glass while varying the laser spot size. Two different 1.06 gm AR coating designs were tested. All damage tests were conducted at the design wavelength of 1.06 gm. To obtain the population distribution, 20 determinations of the conditioned damage threshold were performed. This procedure was repeated for each spot size on each of the samples. For control purposes, the unconditioned damage threshold was also measured for each spot size and coating design. The data was analyzed statistically by describing the population distribution and testing for differences as a function of spot size and coating design.

The GLORIA VUV facility at Spectra Technology, Inc. has been used to measure the damage threshold and reflectivity of first surface Al+MgF2 mirrors and second surface MgF2+Al+MgF2 mirrors. The mirrors were illuminated by high power VUV coherent radiation at 130 nm. Mirror reflectivity was monitored for several thousand pulses for each optic studied. Two nanosecond long pulses and energy densities as high as 20 mJ cm-2 were used for the measurements. In addition, measurements of "color center" formation rates in transmissive optics have been obtained. The GLORIA VUV facility, experimental measurement system, and damage results will be discussed in detail.

Laser-induced damage thresholds of dielectric single layers and reflectors were measured at 193 nm. Layers of A1203 and SiO2 were prepared by electron beam evaporation and by ion beam sputtering; layers of NaF, AlF3, MgF2, GdF3, LaF3, NdF3 and YF3 were prepared by thermal evaporation. Spectrophotometric methods were used to evaluate optical constants and inhomogeneity coefficients in the spectral range between 150 nm and 250 nm. The dependence of refractive indices and absorption coefficients on process para- meters and deposition methods was analyzed in order to prepare low loss reflectors for 193 nm and 157 nm.

Newnam et al. [1] reported experiments showing that the angular dependence of 351-nm laser damage thresholds in HfO2/SiO2 multilayer dielectric reflectors was much weaker than even the 1/cosh expected from simple geometric fluence dilution. Several plausible explanations were suggested, but none were convincing. We propose a simple geometric model based on a cylindrical form for the coating defect responsible for damage initiation. We have measured 248-nm damage thresholds for bare fused silica, evaporated aluminium films, and HfO2/SiO2 and A1203/SiO2 dielectric reflectors at angles out to 85°. The measured data agree well with our simple model.

Pulse-width dependence of optical coating damages at 1053 nm laser wavelength is studied. Quarterwave-thick single layer films of high-index materials of A1203, ZrO2 and TiO2, and low-index materials of MgF2 and SiO2 were tested. Damage thresholds of the MgF2 and SiO2 films follow the square root dependence on the pulse-width T (VT scaling) over the pulse widths of 0.35-90 ns. Although the damage thresholds of the Al2O3, ZrO2 and TiO2 films followed the NA scaling from 0.35 to 1 ns, they become independent of the pulse-width for the pulse-widths longer than 6 ns. That is, the pulse-width dependence of the damage threshold is determined by the absorption property of the coating layer, which controls the damage mechanism.

Multilayer dielectric optical coatings, nominally consisting of 1000 or more optical-quarterwave layers, have been prepared by reacting SiC14 and a halogenated dopant (e.g. GeCl4) with 02 in a microwave-driven plasma. The dopant concentration is such that the index difference (An) between adjacent quarterwave layers is about 0.02 or less. The deposition is carried out at high substrate temperatures (850 to 1100°C) producing a fully dense, fused silica coating. Surface damage thresholds of high reflectivity (HR) coatings prepared by this plasma process are comparable to those for fused silica. For example, at 1.06 p.m and 16-ns we measure surface damage thresholds greater than 45 J/cm2 compared to about 45-60 J/cm2 for super-polished optical fused silica.

We have designed and built a high-temperature, plasma-assisted, chemical vapor deposition system to deposit multilayer optical coatings of SiO2 and doped-SiO2 on flat substrates. The coater concept and design is an outgrowth of our recent work with Schott Glaswerke demonstrating the use of plasma assisted CVD to prepare very high damage threshold optical coatings. (That work is reported in a companion paper at this Symposium).

The use of molecular beam technology (MBD) under ultra high vacuum conditions (UHV) has proved to be a highly controllable process capable of producing dense films which are free of porosity, low in impurities and having bulk like optical properties. These distinctions are requisite for coatings which require a high laser damage threshold, low scatter and stable spectral characteristics. The application also lends itself to the formation of novel distributed Bragg reflectors (DBR) and rugate structures which require a highly controllable deposition process to form the many, often very thin, layers involved.

The electric field distribution within a fabry-perot etalon is readily variable by changing the angle of incidence or the thickness of the etalon. The laser damage threshold (LDT) of Fabry-Perot etalons was measured as a function of electric field distribution, monitored through the transmittance. Two types of etalon were used - thin film Fabry-Perot devices with high finesse were investigated at 1.064µm, and a simple dielectric slab etalon (Germanium) was investigated at 10.6µm.

Our study of highly reflective (HR) dielectric coatings prepared by the sol-gel process has indicated that hydrated alumina and silica are the materials of choice for the high and low index components respectively.

In continuation to our search program on laser-damage resistant coatings and in synergy with the Lawrence Livermore National Laboratory, we have prepared porous single layer coatings of ZrO2 on fused silica and BK-7 substrates from suitable aqueous colloidal suspensions of crystalline zirconia at room temperature. By adding judiciously a soluble inorganic binder to the suspension prior to application, it has been possible to increase substantially both the coating refractive index and the abrasion-resistance. The optimum binder concentration was about 30 % and at this level the optical properties and laser damage threshold of the zirconia coating were satisfactory. Multilayer high reflectivity dielectric coatings were also elaborated by laying down quaterwave-thick, alternating coats of this binder-aided zirconia with silica, also prepared from colloidal suspension. To achieve 99 % reflectivity, 17-19 layers were required. Single shot (1- on-1), laser damage tests at 1064-nm wavelength with a pulse length of 3-ns were carried out on both the single layer and multilayer systems. The thresholds averaged 8.6 and 8.2 J/cm2 respectively. With a laser- annealing post-treatment, n-on-i threshold values were higher than the 1-on-1 figures by a factor of about 1.5.

Unusual behavior has been reported for D20/H20-dosed CaF2 optical thin films. This behavior includes resistance to laser-induced damage at 2.7-µm radiation, improved damage morphology, and reduced optical absorption in the water band for even 1-120-dosed CaF2 optical thin films. Analysis of the films strongly suggests that the modification of film properties is not the result of inclusion of the D20/1420 into the film matrix or chemical contamination. Rather, the act of dosing has modified the structural properties of the films, thus producing the effects observed.

It is the purpose of this paper to put recent novel experimental efforts to understand dielectric breakdown in wide-band-gap materials in various fields into a common framework and outline significant changes in the understanding of dielectric breakdown at optical frequencies and free electron heating under DC conditions. New experimental techniques used to measure multiphoton absorption and energy deposition in wide-band- gap alkali halides in the prebreakdown regime have led to hard evidence refuting the avalanche model of laser-induced damage at visible laser wavelength. The experiments show that virtually all lattice heating occurs via nonlinear absorption of laser photons by multiphoton excited free electrons. Direct measurements of free electron heating by DC fields in thin SiO2-films and direct measurements of electron-phonon scat- tering rates of energetic free electrons impose a new understanding of carrier heating. The scattering of free electrons with non-polar acoustic phonons is found to be the dominant interaction in preventing the free carriers from reaching energies high enough to cause impact ionization and initiate avalanche breakdown. These results unambiguously show that the role of avalanche breakdown under DC conditions has been overestimated in the past.

The values of the laser induced surface damage thresholds for samples of silicon, fused quartz and soda lime glass at both 0.248pm (UV) and 10.6pm (IR) are presented. Experiments are then described where laser damage is induced using crossed UV-IR beams, with varying time delays imposed between the arrivals of the two laser pulses at the sample surface. The results of these crossed beam experiments are then discussed. Order of magnitude reductions in the IR damage thresholds of silicon and glass are observed when the beams are crossed, no such reduction is observed for quartz. Further, only in the case of glass samples is a reduction in the UV damage threshold seen when the beams are crossed. The reduction in IR threshold on silicon has been seen with delays between the laser pulses of up to 100ns. The reduction in IR threshold for glass was seen for delays up to 1.2ms. These results lead to a discussion of the role of so-called seed electrons in the damage process and the mechanisms operative at the different wavelengths.

We discuss the first results obtained from a new testing fa- cility for measuring the induced absorption in multilayer dielec- tric coatings exposed to intense UV radiation. The absorption loss is measured in-situ during exposure, as a function of time for var- ious UV photon energies, intensities, and sample materials. The sample is irradiated by the direct beam from the TOK undulator at NSLS and measured in-vacuo. The undulator is typically run at K = 1 (so that the harmonic emission is low) to simplify the data anal- ysis. The test chamber is separated from the undulator beamline by a differentially pumped line which permits the introduction of car- bonaceous gases into the chamber at pressures up to 10-5 Torr. By varying the current and energy of the stored electrons, one can ad- just the intensity and photon energy of the undulator first harmonic in the range up to 1.2 w/cm2, and between about 5 eV and 35 eV.

Multilayer dielectric high-reflectance coatings were investigated for potential application in a high-average-power, 1-1.1m wavelength, free-electron laser oscillator. Of concern are the ultra- violet harmonics of the primary wavelength generated within the oscillator that tend to degrade the dielectric coatings. These coatings are required to be high reflecting and low absorbing with respect to the 1-µm wavelength and resist degradation/damage from the ultraviolet harmonics.

Radiation damage in single crystal CsI(T1) and polycrystal CsI was assessed by measuring the changes in radioluminescent intensity caused by successive neutron and gamma ray pulses from a TRIGA nuclear reactor. The radioluminescent intensity from the single crystal decreases within nine pulses by 30% in the near infrared range (0.7-4.5 µM) and by 25% in the visible range (0.2-1 pm) before staying constant. For the polycrystal CsI, the emission in the visible range decreases by 60% within 6 pulses and then remains constant, whereas no measurable emission is observed in the infrared. Finally, a heating and cooling cycle is shown to repair the damage for both crystals.

The inclusion-dominated model of laser-induced damage is reexamined in light of recent mea- surements of thin film thermal conductivities. In particular, the effects of varying the thermal conductivity and imaginary part of the index of refraction are explored. Two different values of the index of refraction are shown to be associated with a given damage threshold, and physical mecha- nisms are proposed for each value. Thermal conductivity is demonstrated to affect only weakly the value of the imaginary part of the index of refraction corresponding to a particular damage threshold.

Third order hyperpolarizabilities were calculated using the Huckel Hamiltonian and third order perturbation theory for a series of phosphonitrilic compounds, (X2P-N)n, as a function of bond length alternation, ligand substitution and backbone conformation. Phosphonitrilic compounds show hyperpolarizabilities comparable to those reported for organic species, and are modulated by ligand group electronegativity. In contrast to organic polyenes, the difference in it orbital energy between phosphorus and nitrogen is critical to determining the onset of saturation and the magnitude of the hyperpolarizability. Conformation effects are smaller than those seen in polyenes.

We present evidence from nonlinear refractive index measurements, two-photon absorption measurements, and four- wave mixing measurements on semiconductors showing that the bound electronic nonlinearity can be calculated from two-photon absorption dispersion via a simple Kramers-Kronig analysis. This analysis shows n2 changing from positive to negative as the photon energy approaches the band-gap energy, consistent with observations. Additionally, this simple calculation, which assumes two parabolic bands, gives good agreement with measured values of n2 in wide- gap dielectrics that are 2 to 3 orders of magnitude smaller than in semiconductors.

ZnS and ZnSe are important materials for laser windows and optical thin- film coatings. Understanding the laser-induced damage mechanism in optical materials provides the ability to make improved damage-resistant materials. Photoconductivity (PC) techniques have demonstrated the capability to provide information on carrier production that can lead to electron avalanche by single or multiple photon absorption processes produced by a high-intensity laser beam. A study of the linear PC using a monochromated Xe lamp and the nonlinear PC using various lasers as excitation sources will be presented for chemically vapor-deposited ZnS and ZnSe.

Complete model of a multiactuator flexible mirror,made as a thin plate with discrete piezo-actuators is described. Experimental results validated the efficiency of proposed model are presented.New concept of adaptive optical system with flexible mirror and linear wavefront sensor is developed.

Development and creation of powerful technological lasers re- quires complex solution of problems of optics, quantum elec- tronics and thermal physics. The achieving of stable in time energetic parameters of technological lasers with cooled ele- ments of power (EPO) and adaptive (EAO) optics can be achieved only by a correct choice of their cooling modes, eliminating or compensating negative effects of heating EPO by laser radia- tion because of principal difference of reflectance of the mir- ror surface from I. The convective regime of EPO or EA0 coo- ling is mainly used in laser technique and carried out by pum- ping the coolant through a penetrable disperse compact heat ex- changer, on the work surface of which a thermally thin layer, separating radiation and the coolant liquid in heat changer, is made /1,2/. The intensity of heat transter and therefore, the degree of thermostatting of the reflecting surface of EPO are determined by a large number of design and technological factors not always reproducible in the EPO manufacturing. Ne- vertheless, for EPO, made on the basis of powder porous struc- tures, the level of removing heat fluxes reach 2 . 10' wt/cm2 under the thermal deformation of the mirror surface less than 1pm, the effective heat transfer factor for a heating mirror surface equal to r%, 105 wt/m2 . °C in this case /2/.