We present and discuss results outlining the use of fullerenes for optoelectronics and photonics. These applications are particularly compelling with the observation of such promising properties as photoluminescence, electroluminescence, large nonresonant optical nonlinearity, and superconductivity. We focus on nonlinear optical properties and their application to high- speed integrated all-optical switching. We present measurements on the dispersion and dynamics of the nonlinear optical coefficients in the near infrared and the figure of merit for photonic switching, indicating very favorable results. The first demonstration of photonic switching using fullerene thin films as the nonlinear medium is presented. Our results show many advantages of fullerenes and fullerene devices, including the simplicity of processing into guided wave structures for nonlinear integrated optics, large nonlinear coefficients, effectively nonlatency,and potentially terabit/sec operation in the near infrared. Comparisons are made with the fiber optic switching approach. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-48.

The techniques utilized to study the surface and bulk properties of KTiOPO₄ (KTP) were Rutherford backscattering (RBS), particle induced x-ray emission (PIXE), secondary ion mass spectrometry (SIMS), optical absorption and emission spectroscopy, and controlled laser damage. RBS and SIMS results provide strong evidence for potassium ion and titanium ion migration from the bulk to the electrode surface under an applied DC voltage. Optical measurements suggest the presence of Ti³⁺ ions in pristine, EC and PC damages KTP. Catastrophic damage was induced models will be presented to rationalize the RBS, PIXE and SIMS data for the impurities, and a damage mechanism consistent with the findings of the laser damage and optical absorption and emission experiments will be discussed.

We report damage threshold results for potassium titanyl phosphate (KTP) and potassium titanyl arsenate (KTA) which are cut for use as nonlinear crystals in noncritically phase matched optical parametric oscillators. Controlled experiments are described that shed light on the damage process in these nonlinear crystals in terms of the pump, signal, and idler wavelengths. Both surface and bulk material damage morphology is described for each crystal type and for each wavelength. Both materials were evaluated with our optical parametric oscillator damage test facility, which operates at a wavelength of 1574 nm and our Q-switched Nd:YAG test facility that operates at a wavelength of 1064 nm. A comparison of the relative merits of these crystal types as OPO materials is also given.

The effect of particle desorption from Al mirror surfaces by the influence of pulsed UV laser radiation has been studied. The investigations are closely related to the demands of astronomers, who are looking for a more effective way of cleaning the Al coatings of future very large telescope mirrors. A systematic parameter study has been performed in order to determine the irradiation conditions which yield the highest dust removal efficiency (i.e. reflectivity increase) on contaminated samples, taking particularly into account laser-induced damage and degradation effects of coating and substrate. The particle removal rate increases with increasing laser fluence, being limited however by the damage threshold of the coating. Therefore, parameters influencing the damage threshold of metal coatings like wavelength, pulse width, and number of pulses have been studied in detail. Data indicate that on Al coated BK7 and Zerodur samples KrF laser radiation yields the optimum result, with cleaning efficiencies comparable to polymer film stripping. The initial reflectivity of the clean coating can nearly be reinstalled, in particular when an additional solvent film on the sample surface is applied. Hence, laser desorption seems to be a viable method of cleaning large Al mirrors for telescopes.

The Laser Damage and Conditioning Group at LLNL is evaluating diagnostics which will help make damage testing more efficient and reduce the risk of damage during laser conditioning. The work to date has focused on photoacoustic and scattered light measurements on 1064-nm wavelength HfO₂/SiO₂ multilayer mirror and polarizer coatings. Both the acoustic and scatter diagnostics have resolved 10 micrometers diameter damage points in these coatings. Using a scanning stage, the scatter diagnostic can map both intrinsic and laser-induced scatter. Damage threshold measurements obtained using scatter diagnostics compare within experimental error with those measured using 100x Nomarski microscopy. Scatter signals measured during laser conditioning can be used to detect damage related to nodular defects.

The scanning probe microscope is an exciting new analytical instrument and a recent addition to the material scietntist's toolbox. SPM's record digital topographic data with horizontal and vertical resolutions from 0.1 angstrom to 12 micrometers , and 1 angstrom to 150 micrometers , respectively. These dimensions are ideal for thin film structure and defect analysis, as well as substrate characterization. The films we examine are vacuum deposited multiple layer assemblies of inorganic dielectric, metallic or organic materials, ranging in total film thickness from 10 nm to 100 micrometers . Understanding thin film and substrate microstructure is necessary for deposition process control, in research and manufacturing, as well as for characterizing the final thin film device properties. Additionally, the digital nature of the SPM image allows for detailed numerical analysis, which aids qualitative and quantitative data interpretation.

The reported uncertainty is a key parameter in comparing laser damage test results for statistically significant differences. Yet, this important statistic is frequently determined in a nonrigorous manner. This paper presents a Monte Carlo based calculation of the expected distribution of results from a single laser damage threshold (LDT) measurement. The uncertainties in the measured damage frequency and test fluence are used to estimate the probability distributions used in the Monte Carlo model. The model is run many times and the resulting distribution is analzed to yield and estimate the uncertainty in the LDT measurement. It is demonstrated that the distribution of results is not symmetric thus invalidating the use of more conventional closed form formulations for threshold uncertainty.

In this paper are the results of investigations of the mechanisms of photo-induced changes of alkali-silicate (crown) and lead-silicate (flint) glasses optical parameters upon the exposure to intense laser radiation, and the basic regularities of these processes are reported. These investigations were performed in Research Center 'S.I. Vavilov State Optical Institute' during the last 15 years. The kinetics of stable and unstable CC formation and decay, the effect of widely spread impurity ions on these processes, the characteristics of fundamental and impure luminescence, the kinetics of refractive index change under conditions of multi-photon glass matrix excitation, and other properties are considered. On the basis of analysis of received regularities it was shown that the nonlinear coloration of alkali-silicate glasses (the fundamental absorption edge is nearly 6 eV) takes place only as a result of two-photon absorption. Important efforts were aimed at the detection of three or more photon matrix ionization of these glasses, but they failed. However it was established that in the lead silicate glasses, the long-wave carriers mobility boundary is placed considerably higher than the fundamental absorption edge of material matrix. This results in that the linear color centers formation in the lead silicate glasses is not observed. The coloration of these glasses arises only from the two- or three-photon matrix ionization, and the excitation occurs through virtual states that are placed in the fundamental absorption region. In the report the available mechanisms of photo-induced changes of glasses optical parameters, and some applied aspects of this problem are discussed.

Ion figuring is an optical fabrication method that provides deterministic surface figure error correction of previously polished surfaces by using a directed, inert, and neutralized ion beam to physically sputter material from the optic surface. Considerable process development has been completed and numerous large optical elements have been successfully final-figured using this process. The process has been demonstrated to be highly deterministic, capable of completing complex-shaped optical element configurations in only a few process iterations, and capable of achieving high- quality surface figure accuracies. A review of the neutral ion beam figuring process will be provided, along with discussion of processing results for several large optics. Most notably, processing of Keck 10 meter telescope primary mirror segments and correction of one other large optic where a convergence ratio greater than 50 was demostrated during the past year will be discussed. Also, the process has been demonstrated on various optical materials, including fused silica, ULE, zerodur, silicon, and chemically vapor deposited (CVD) silicon carbide. Where available, results of surface finish changes caused by the ion bombardment process will be discussed. Most data have shown only limited degradation of the optic surface finish, and that it is generally a function of the quality of mechanical polishing prior to ion figuring. Removals of from 5 to 10 micrometers on some materials are acceptable without adversely altering the surface finish specularity.

Coated ZnSe optical components are irradiated with high-power, pulsed CO₂ laser radiation at fluences up to 250 J/cm². The components are characterized at various stages of irradiation by optical microscopy, interferometric microscopy, profilometry, surface chemical analysis (x-ray photoemission and Auger electron spectroscopy), and surface structural analysis (micron-Raman spectroscopy). Two types of coating damage occur within the irradiated area of the component: a breaking apart of the ZnSe overlayer of the coating system over relatively large areas resulting in a network structure, and the formation of isolated craters of diameter approximately 30-50 micrometers extending in depth of approximately 5 micrometers through the coating system down to the ZnSe substrate. Chemically, the irradiated area is characterized by an oxidation of both Zn and Se and an increase in the stoichiometric ratio of Zn and Se. These effects are especially pronounced at the crater defects, and are attributed to localized optical absorption, leading to thermal stress and chemical reactions of Zn and Se with atmospheric or absorbed water and/or oxygen. Structually, the coatings exhibit a polycrystalline structure with no orientation of the individual grains. During irradiation the grain size diminishes giving, in addition, indication for built-in stress and partial melting at high laser fluences.

The model of generation of periodic structures of defects in solids under action of laser radiation is performed. It is shown the possibility of defect organizing into periodic structures under action of space-periodic temperature and light fields at nonequilibrium conditions, in particular, at presence of gradients of temperature or charge-carrier density. The period and orientation of this structures are determined the properties of target and characteristics of light interference fields.

Phase plates are required to remove aberrations from laser beams caused by inhomogeneities in the optical components of the laser. The first type of plate that we prepared consisted of a bilevel optical component that caused spatial smoothing of the beam by breaking it up into a fine scale spatial structure. This was made by etching a pattern directly into the substrate using HF/NH₄F. Components up to 80 cm in diameter were prepared but these are only 85% efficient because of beam losses in secondary maxima. Multilevel designs are more efficient and we have prepared 5 inch diameter samples with 16 levels. These require four separate etch steps but have efficiencies greater than 90%.

The development of processes for depositing diamond in large sizes offers the promise of optics with a combination of intrinsic superior properties. These properties include optical transmission over a large wavelength range due to a wide electronic bandgap and forbidden first order phonon induced absorption, the ability to withstand high laser powers due to high optical transmissivity and high thermal conductivity, and great mechanical strength due to large elastic moduli. Polycrystalline diamond of high optical quality can now be produced by various enhanced chemical vapor deposition (CVD) techniques in sizes many cm in diameter and in thickness of the order of one mm. Key to the use of diamond as an optical material is our ability to evaluate the defects that influence the properties. A variety of techniques are used to investigate the defect content. These include optical absorption spectroscopy, Raman spectroscopy, transmission electron microscopy, electronic carrier lifetime, and thermal conductivity. Use of CVD diamond in an aircraft environment of rain and sand erosion is currently limited by the strength of the material which is in the range 200 to 400 MPa. Despite its lower than expected strength, diamond has excellent resistance to thermal shock. Even if all of the technical limitations for producing high quality CVD diamond for optical applications are overcome, the cost of production is still very high; large scale use will require a significant drop in price. A particular impediment is the high cost of polishing diamond.

For most infrared—transmitting materials the primary cause of failure in a ri5si1e window/dome application can be attributed to brittle fracture induced by tensile stresses originating from instantaneous terperature gradients generated by aerodynamic heating. To describe the thermal shock I rely on the well known expression for the maximum stress experienced by a clamped plate (or a complete sphere), if there is a linear temperature variation across the thickness and both surfaces are free to expand:

The growth of large area artificial diamond by the chemical vapor deposition technique has led to the possibility of the deployment of diamond components, especially as the output windows of dual-band laser radar and viewing systems used in adverse atmospheric conditions. The characterization of this material, and in particular the measurement of its power and energy handling capability, is therefore of interest to a wide spectrum of workers in the high power laser field. This paper summarizes the published laser threshold data and attempts to correlate these with the laser wavelength and pulse length and material absorption in order to gain a consistent picture. This analysis shows that although much of the early material was limited by both particulate and lattice absorption there is now material available that approximates to Type IIa natural diamond and reaches the theoretical power handling capability.

We have measured n₂ and two-photon absorption coefficients in type IIa diamond at (lambda) equals 1064, 532, 355, and 266 nm using Z-scan technique and picosecond pulses from a modelocked Nd:YAG laser. The observed despersion is compared to the two-band theory of semiconductors.

The thermal and electrical conductivities of natural diamond and of diamond films were measured between room temperature and 1000 c. Natural type IIa diamond, which is the purest diamond, was found to have a room temperature thermal conductivity of 24-25 W/cm-K which drops to 4-5 w/cm-K at 1000 C. Two diamond films (2 mm thick) grown by microwave plasma CVD were found to have thermal conductivities that fell exactly on the curve for the natural type IIa diamond. The electrical resistivity of both insulating natural diamond and good-quality diamond films is around 10¹⁶(Omega) -cm, which has been shown to be the apparatus limited value. The resistivity decreases with increasing temperature to a value of 10⁵ to 10⁷(Omega) -cm at 1000 C. the best diamond films have electrical resistivities two orders of magnitude greater than that for natural diamond over the whole temperature range. These conductivity results, which are determined by the impurities and defects in the sample, thus indicate that the best diamond films currently being synthesized are as pure or purer than the best natural diamonds.

Thin nanocrystalline diamond films promising for IR optical applications were grown on Si substrates from methane-hydrogen gas mixture in a DC arc plasma CVD reactor. Three stages for the synthesis of the highly smooth noncrystalline diamond films are important: (i) substrate pretreatment with ultrafine diamond powder, (ii) excimer laser irradiation of seeded substrates, and (iii) two-step deposition process. A correlation between optical properties of the films and growth conditions has been established by means of Raman spectroscopy, spectroscopic ellipsometry and optical transmission spectroscopy techniques. Surface roughness, which was R_a equals 8 - 40 nm for the 1 micrometers thick films, significantly decreased the transmission in the visible because of light scattering, but it had a negligible effect in the IR range. The films are transparent in the IR and have optical constants n equals 2.34-2.36 and k equals 0.005- 0.03. The hydrogen incorporation in the films in amounts up to 1.5% have been deduced from intensity of C-H absorption band around 2900 cm(superscript -1.

The use of pure diamond as a high power optical material will depend on its absorption, the effects of nonlinear processes (high peak power beams), and on the ablility to remove absorbed energy from the diamond. This paper speculates on how acoustic matching in diamond could be used at low temperature to permit escape of ballistic phonons generated by megawatt beams.

Synchroton facilities worldwide provide scientists with useful radiation in the ultraviolet to the x-ray regime. Third-generation synchrotron sources will deliver photon fluxes in the 10¹⁵ photons/s/0.1%BW range, with brilliance on the order of 10¹⁸ photons/s/0.1%BW/mrad²mm². Along with the increase in flux and brilliance is an increase in the power and power densities of the x-ray beam. Depending on the particular insertion device, the x-ray beam can have total power in excess of 10 kW and peak power density of more than 400 W/mm². Such high heat loads are a major challenge in the design and fabrication of x-ray beamline components. The superior thermal and mechanical properties of diamond make it a good candidate as material in these components. Single crystal diamonds can be used as x-ray monochromators, while polycrystalline or CVD diamonds can be used in a variety of ways on the front-end beamline components. This paper will discuss the issues regarding the feasibility of using diamond in third-generation synchrotron beamline components.

Advances in optical coating and materials technology have made possible the development of instruments with substantially improved efficiency in the extreme ultraviolet/far ultraviolet (EUV/FUV) spectral region. For example, the development of chemical vapor deposited (CVD) SiC mirrors provides an opportunity to extend the range of normal incidence instruments down to 60 nm. The EUV performance and some applications of optical coatings including MgG₂ protected aluminum, CVD- SiC, SiC films, boron carbide films, and multilayer coatings will be discussed. Contamination sensitivity and cleaning will be addressed.

High performance AL₂O₃/SiO₂ mirror coatings for 248 nm have been investigated with respect to their excimer laser damage resistivity. Global damage thresholds (in the range of 10-20 J/cm²) averaged over large areas were determined with the pulsed photoacoustic mirage detection method. With a raster scanning technique utilizing the same detection scheme, the local damage behavior was studied with 100 micrometers spatial resolution. It was found that the local damage threshold at specific sites was lower than the global damage threshold and it was assumed that this phenomenon was associated with micrometer-scale defects in the multilayer coating. To test this hypothesis photothermal displacement microscopy with micrometers lateral resolution was performed on the investigation regions prior to excimer laser light irradiation. Photothermal images revealed an extremely small background absorption and a small number of absorbing defect sites. For a number of such sites a clear correlation between the local absorption and the onset of laser damage at that specific location was found. We conclude that the crucial factor determining the damage resistivity of the high quality coating systems are defects and contaminants and that it will be possible to predict their damage thresholds by a complete microscopic photothermal inspection.

The CEA Limeil-Valenton Inertial Confinement Fusion (ICF) program is currently addressing, in close collaboration with LLNL, the critical physics and technology issues for demonstrating and exploiting high-gain ICF. We believe that a compelling strategy for our national ICF program is to demonstrate ignition and moderate gain at the beginning of the next century with an upgrade to the existing Phebus laser to provide 1.8 MJ of 0.351- micron light in a nanosecond regime. The proposed MJ-range laser system will consist of 288 optically independent and individually targetable beams each having a final optical aperture of approximately 35 centimeters. While Phebus used a multistage, single-pass amplifier design, the proposed MJ laser will use a single-stage multipass amplifier design so-called L-Turn. In the L-Turn architecture, a laser pulse enters the cavity and makes 4 passes in the resonator using different pathways before being directed out. The pulse reflects off a pick-off 6 x 6 cm square mirror that may withstand very high fluences. (Abstract truncated.)

A polarizing beamsplitter used for the OMEGA Upgrade, was produced using the hafnia/silica combination. These coatings have stringent optical requirements, are placed in the stages of the laser with the highest fluence at 1054 nm, and are required to have a low net stress to produce low wavefront distortion. Hafnia/silica coatings are also more stable than other film combinations such as tantala/silica. Hafnia/silica films were investigated for other applications. A triple wavelength antireflection coating was developed for calorimeter absorption glass. The metal-converted hafnia is also used on selected transport mirrors used at 351 nm and angles of incidence up to 45 degrees. Damage test results for 1054 nm, 1 ns, and 351 nm, 0.7 ns will be presented.

Recent work is reported on the growth and characterization of boron nitride thin films on 1 cm² Si (100) substrates by a newly developed reactive laser ablation technique. The exact nature of the resulting films is highly process dependent and is analyzed by ion channeling and Fourier transform infrared spectroscopy (FTIR). The thermal properties of these films are studied by thermal wave analysis, and they are found to be highly dependent on the crystallographic structure. This value is believed to be the best thermal conductivity measured for boron nitride films to date.

PECVD is an attractive technique for depositing coatings of refractory optical materials because of the ability to control composition (to assure low absorption), the low substrate temperature required, and the high deposition rate attainable. Deposition processes have been established that achieve thin films with low absorption, low scatter, good thickness uniformity, and environmental durability. Multilayer dielectric- enhanced IR reflectance mirrors using a variety of silicon-based refractory materials have been fabricated and evaluated. Results of optical and physical characterizations and environmental testing will be presented.

Nodular defects in multilayer dielectric coatings have been computer modeled to characterize the electromechanical responses to laser pulses with wavelengths of 1.06 micrometers and pulse lengths between 1 and 20 ns. The simulation begins with an axisymmetric electric field model using AMOS, a full-wave Maxwell solver with lossy (dispersive) electric and magnetic material models. Electric fields calculated by this code determine the spatial distribution of absorbed laser energy in the vicinity of the nodule. This data is linked to a thermal/stress model and mechanical calculations are executed using the general purpose finite element code COSMOS/M. The simulation estimates the transient temperature response of the nodule and the surrounding medium and predicts the dynamic stresses caused by the thermal impulse. This integrated computer process has been exercised to characterize failure of nodules as a function of defect characteristics, including seed size and depth.

High resolution 400ns TEA carbon dioxide laser pulses with a 100ns rise time were used to observe the 10.6 micrometers radiation transmitted by polycrystalline vanadium oxide coatings deposited on germanium substrates. The variation in the transmission and reflection of the vanadium oxide coatings was simultaneously observed throughout the duration of the incident pulse over a range of incident fluences. The observation of fluence related changes in the behavior of both the transmitted and reflected pulses showed that the coatings exhibited a semiconductor-to-metallic phase transition that was power related and not energy dependent.

One of the main problems related to optical thin film materials used in high power laser environments is the catastrophic damage caused to them due to laser irradiation. While the influence of ion bombardment on the optical properties of oxide thin films is now a well understood subject, the morphology and crystalline behavior of these films under ion incidence is not so well studied. Hence, it is of great importance to investigate the effects of ion bombardment during growth on the microstructure and crystalline behavior of oxide materials. In this paper, the authors present the results of a detailed investigation of alumina, ceria, titania, and zirconia thin films prepared by ion-assisted deposition. It has been found that the crystalline behavior of the films is strongly material dependent. So that, while ceria crystallizes in situ independent of deposition parameters, all the other materials require thermal annealing for crystallization to occur. Both in zirconia as well as alumina, otherwise high temperature modifications; cubic zirconia and (alpha) -Alumina, have been synthesized at low temperatures. Some reasons for this behavior is discussed. The stress and grain sizes seem to be interrelated and also momentum dependent. The stress in particular was found to be dependent on the ion energy, current density, ion species and substrate temperature. Some theoretical models have been developed to explain the observed behavior.

High LIDT (laser induced damage threshold) optical coating deposition techniques are of large practical interest for those who develop compact and high power pulsed lasers. Currently several technologies are used to deposit such coatings. But parameters and conditions of such technologies usually are not discussed in detail because of commercial reasons. Authors of this paper have developed efficient methods to find optimum conditions for high LIDT optical coatings deposition. This method is based on utilizing high precision and reproducibility LIDT measurement system and ion milling optical coating deposition system placed at the same laboratory for real time feedback.

Single-shot laser induced breakdown, in wide band gap materials such as SiO₂ and MgF₂, has been studied over almost 5 orders of magnitude in duration from 150 fs to 7 ns. A Ti:sapphire chirped pulse amplification system was used in this experiement, so the pulse duration could be continuously adjusted without changing any other parameters. The damage threshold was detected by looking at the plasma formation and the change of material transmission coefficient. The avalanche mechanism was found to dominate over the entire pulse-width range even for 150 fs pulses where we would expect multi-photon processes to take over. A strong departure from the conventional fluence threshold scaling law is observed for pulses shorter than 10 ps, where beyond this point the fluence threshold increases. Also, it is observed for the first time that for short pulses the damage threshold becomes very accurate and less statistical than that for longer pulses.

The microstructure of dielectric films provides a significant influence on the electric field distribution in these materials. In this paper, we focus on the relationship between the electric field distribution and organization of film constituents. Using our self- consistent determination of the local electric field in inhomogeneous media, we have shown that enhanced fields can result from columnar microstructures such as typically generated by CVD-type fabrication processes, and low dielectric components in optical coatings. In addition to the microstructural enhancement, a surface specific enhancement due to presence of low dielectric components is observed.

A correct approach to description of nonlinear electrodynamic phenomena for tight focused light beams is considered in the present paper. A new self-influence effect connected with changing of polarization state of tight focused light beam is discussed and illustrated.

Five different fused silica types were evaluated for their resistance to UV-induced compaction and color center formation at 193-nm. Real-time monitoring of color- center-induced absorption showed three distinct dependencies of transmission on pulse count. The initial rates of color center formation varied by well over a factor of ten between the materials tested while compaction-induced birefringence rates varied by at most a factor of four. Of the likely candidates for lithographic applications, Corning Excimer Grade 7940 fused silica was the least prone to color center formation while Suprasil 311 showed the lowest compaction rates. The rates of compaction-induced birefringence and color-center-induced absorption from 213-nm radiation were found to increase dramatically under elevated sample temperature conditions. Since a two- photon absorption mechanism is believed to be the catalyst for UV damage to fused silica, two-photon absorption coefficients were characterized at elevated temperatures. The two-photon coefficients at 213-nm for all materials measured including crystalline quartz and under all applied conditions were statistically equivalent, leading to the conclusion that the energy dissipation mechanism, in addition to two-photon absorption, is important to UV damage to fused silica.

Our extensive measurements of damage thresholds for fused silica and several fluorides (LiF, CaF, MgF, and BaF) at 1053 and 526 nm for pulse durations, (tau) , ranging from 275 fs to 1 ns are reported elsewhere at this meeting. A theoretical model based on electron production via multiphoton ionization, Joule heating, and collisional (avalanche) ionization is in good agreement with experimental results.

The dependence of laser-induced damage (LID) threshold of K8 (Russia) and BK7 (USA) glasses on laser pulse duration in the wide region from 4 X 10^-11 to 3 X 10^-8 s has measured. Strong attention was paid to methods of measurements. It was indicated to strong influence of laser radiation statistics on the results of the threshold measurements. It was shown that intrinsic LID threshold of glasses does not depend on pulse duration when there is not self-focusing. However, the behavior of LID threshold time dependence is determined by different types of self-focusing under focusing in spots more than laser radiation wavelength. The self-focusing threshold power is changed with the increasing of spot diameter.

A CO₂ master oscillator power amplifier (MOPA) laser chain has been developed capable of producing approximately 2J per pulse at 2kHz. This leads to a unique combination of high peak and average power in a single laser system. As the laser is to be used in a process where it must be capable of prolonged periods of operation, a method had to be devised to allow constant monitoring of the beam profile and behavior of the optical components to ensure that the laser induced damage thresholds (LIDT) of the optical components are not exceeded. A photoacoustic detector (PAD), that allows determination of damage to optical components, and a M², or beam quality detector for accurate calculation of beam profiles in the CO₂ laser chain have been developed. The results obtained with these detectors will be discussed.

we report the development of an optical parametric oscillator (OPO) laser exposure testing facility operating at a wavelength of 1.57 microns. This facility consists of a singly resonant optical parametric oscillator employing KTP in a noncritically phase-matched condition. The test facility is pumped by a Q- switched Nd:YAG laser operating at 1064 nm. The overall layout and design of the facility is described. Standard operating parameters are described as well as a summary of damage threshold data of typical OPO laser system components. The systems was primarily designed to test optical components in OPO laser systems for the rapidly developing field of eye-safe laser systems.

This paper presents an argument leading to estimate the minimal acceptable test sample size. In most cases the size of the test sample determines the number of possible test sites, since the other factors are usually fixed by test procedure. If the sample is too small, not enough data points are available and the resulting execution of an ISO standard or equivalent test, returns inaccurate and inprecise results. By use of a computer-based model, a 'single' optic is repeatedly tested and the results plotted versus sample size. This data is used to determine the curve of test accuracy as a function of sample size. The paper concludes with some 'rules of thumb' for determining the minimum acceptable test sample size for a given level of desired accuracy.

An easy to use, nondestructive method for evaluating subsurface damage in polished substrates has been established at LLNL. Subsurface damage has been related to laser damage in coated optical components used in high power, high repetition rate laser systems. Total Internal Reflection Microscopy (TIRM) has been shown to be a viable nondestructive technique in analyzing subsurface damage in optical components. A successful TIRM system has been established for evaluating subsurface damage on fused silica components. Laser light scattering from subsurface damage sites is collected through a Nomarski microscope. These images are then captured by a CCD camera for analysis on a computer. A variety of optics, including components with intentional subsurface damage due to grinding and polishing, have been analyzed and their TIRM images compared to an existing destructive etching method. Methods for quantitative measurement of subsurface damage are also discussed.

For a number of years we have been investigating laser-induced damage mechanisms that can occur during the transmission of Q-switched, Nd/YAG laser pulses through fused silica fibers. We have found that fiber end-face characteristics, laser characteristics, and aspects of the laser-to-fiber injection typically determine dominant damage mechanisms. However, an additional damage process has been observed occasionally at internal sites where fibers were experiencing significant local stresses due to fixturing or to bends in the fiber path. A transmission reduction prior to damage was typically not measureable at these sites. Damage would not always occur during initial testing, but sometimes occurred later in time at laser levels that previously had been transmitted without damage. In these cases the time at stress appeared to be more important than the number of transmitted shots prior to damage. A possible relation between internal damage thresholds at stressed sites and the total time under stress is suggested by the fact that silica fibers experience static fatigue processes. These processes involve the slow growth of local defects under tensile stress at rates that depend upon environmental conditions. Defects reaching sufficient size and having appropriate location could be sites for reduced laser-induced damage thresholds. This possibility could have important implications for high-power fiber transmission systems that must satisfy extended lifetime requirements. The needs of the telecommunications industry have motivated extensive studies into initial fiber defect characteristics and their likely growth mechanisms. The present work used the understanding developed in these studies to guide a preliminary experimental investigation into the possibility that static fatigue processes can affect damage thresholds. The experiments used a laser injection and fiber routing configuration that produced significantly elevated fluences within fiber core regions under tensile stress. In one set of experiments, internal damage thresholds were determined in available fiber samples that had been assembled in stress-imposing fixtures for periods up to 24 months. A decline in mean thresholds with time was observed, although measured values showed significant scatter. In order to establish initial strength and fatigue properties for these fibers, a number of additional samples were used to generate time-to-failure data at various stress levels. Based on these results, other fiber samples were subjected to conditions that greatly accelerated fatigue processes. Internal damage thresholds were then measured in these fibers and compared to thresholds measured in fresh fibers. Conclusive comparisons were frustrated by sample-to-sample and lot-to-lot variations in fiber defects.

The Geoscience Laser Altimeter System (GLAS) is being developed by NASA/GSFC to measure the dynamics of the ice sheet mass balance, land, cloud, and atmospheric properties. An instrument altimetric resolution of 10 cm per shot is required. The laser transmitter will be a diode-pumped, Q-switched, Nd:YAG laser producing 1064 nm, 100 mJ, 4 ns pulses at 40 Hz repetition rate in a TEM(infinity) mode. A minimum lifetime goal of 2 billion shots is required per laser transmitter. The performance of the GLAS laser can be limited by physical damage to the optical components caused by the interaction of intense laser energy with the optical coatings and substrates. Very little data exists describing the effects of long duration laser exposure, of 4 ns pulses, on an optical component. An Accelerated GLAS Exposure Station (AGES) is being developed which will autonomously operate and monitor the GLAS laser at an accelerated rate of 500 Hz. The effects of a large number of laser shots will be recorded. Parameters to be monitored include: laser power, pulsewidth, beam size, laser diode drive current and power, Q-switch drive voltage, temperature, and humidity. For comparison, one set of AGES sister optical components will be used in the nonaccelerated GLAS laser and another will be evaluated by a commercial optical damage test facility.

Studies at Litton Laser Systems Division (LLSD) and other laser vendors have found that in a hermetically sealed laser, trace levels of gas phase contaminants can lead to photo-induced optical damage. This damage has been shown to occur at power densities as low as 20 MW/cm² on uncoated quartz substrates. Since contamination-induced optical damage can be a significant factor in reducing laser reliability and lifetime, it is important to understand the mechanisms by which it occurs. In this paper we will describe investigations performed at LLSD on the mechanisms of contamination-induced optical damage. The starting point for these studies is a simple test developed at LLSD to evaluate the potential of materials in sealed optical compartments to cause contamination-induced damage. A systematic study of the morphology, composition, chemical species dependence, and initiation time of contamination-induced optical damage allow a number of qualitative conclusions concerning the damage mechanism to be deduced.

Investigated is the influence of isothermal annealing on the real structure and laser damage threshold of KDP single crystals. It is shown that the annealing of the samples at temperatures close to that of phase transition results in a pronounced decrease of the number of defects in the crystals, their optical homogeneity being improved. The chosen optical annealing conditions allow to raise the values of mechanical strength and laser damage threshold by 2 or 3 times and thereby to reduce the spread of these characteristics over the bulk of the crystals.

This paper is devoted to studying the influence of Ca, Si, Pb, and Cr impurities (possessing no absorption bands at the wavelength of acting laser irradiation) on the value of bulk laser damage threshold and UV absorption in KDP single crystals. It is shown that for the investigated concentration range (1 divided by 10^-5 1 divided by 10^{-2 mass%} laser damage threshold essentially decreases with raising the concentration of impurity ions in the crystal lattice. The maximal value of the said characteristic (approximately 40 J/cm²) is found to be achieved in the case when the concentration of impurities is not less than 1 divided by 10^-5 mass%.

Breakdown in SiO₂ is studied versus fluence using an intensified CCD spectrometer. Broad-band photoluminescence spectra were measured versus number of laser pulses. Before the breakdown of fused silica, the intensity of this photoluminescence increases. After breakdown, a plasma is formed and ablated Si emission lines are measured. The plasma is characterized by its emission spectra and excitation temperature temporal profiles. The temperature profiles of the plasma are calculated by the Bolzmann method. These data are studied to provide fundamental information of breakdown mechanisms in optical materials.

This paper deals with the study of characteristics of damage and damage onset of mirrors and substrates at 10.6 micrometers wavelength by means of photoacoustics using laser pulse irradiation with up to 10 pulses per on site. One group of the mirrors which have been investigated are commercially available polished uncoated Mo- and Cu- mirrors. A second set of mirrors consists of copper mirrors coated with NiCu, or Au_max, layer systems for enhanced reflectivity. NaCl and ZnSe substrates were selected as IR transparent materials. For measuring the photoacoustic waves generated by laser pulse irradiation, a piezoceramic detector is used. The amplified signal of the detector is sampled by a digital oscilloscope.

The photothermal deflection technique (PDT) is applied to the optical and thermal characterization of optical components for high-power CO₂ lasers. Besides bare copper and silicon substrates with different metallic and dielectric coatings, ZnSe optics were investigated at 10.6 micrometers using the laser-induced sample surface doformation to deflect a HeNe probe laser beam. The measured deflection signal is compared to laser-calorimetrically measured absorption whereas the surface deformation profile is used to investigate the thermal behavior of the different coating and substrate types. A finite-element-analysis (FEA) has also been performed, which allows the calculation of the surface temperature and the deformation profile for bare substrates and two additional coating layers with respect to the thermo-optical properties. These results are used as additional information for the sample characterization. The accuracy of the measurements of the thermal conductivity using the PDT technique is discussed in detail.

Photothermal techniques are widely used for measuring optical absorption of thin film coatings. In these applications the calibration of photothermal signal is typically based on the assumption that the thermal properties of the thin film make very little contribution. In this paper we take mirage technique as an example and present a detailed analysis of the influence of thin film thermal properties on absorption measurement. The results show that the traditional calibration method is not valid on surprisingly many situations. Our theoretical calculations are verified by experimental results, and the thin film coatings investigated in this work include both single and multiple layer samples. Based on the detailed studies, new calibration methods are proposed for absorption measurements by using thermal wave analysis.

High reflecting mirrors for the wavelength of 514 nm were produced with ion-beam sputtering using different process parameters. The optical characteristics were determined by cavity decay time measurement, laser calorimetry, and total integrated scatter measurements. The results for single layer of SiO₂ and Ta₂O₅ were evaluated with respect to an optimization of the deposition process. Furthermore, coatings were investigated by IR spectroscopy and RBS. Laser induced damage thresholds of coatings were measured with a pulsed Nd:YAG laser. Mirrors produced with optimized parameters show absorption values less than 5 ppm, and the scatter losses are in the range of 6 ppm at 514 nm. Adding the transmission loss of the mirrors, this is in good agreement with the results of the cavity decay time measurement. The damage threshold values and the optical losses are discussed and compared to the results of conventional coatings.

The results of measurements of the optical density, induced at the moment of influence of the e-beam on crystal MgF₂, CaF₂, BaF₂, Al₂O₃, and also on different silica glasses, performed using laser radiation at 193, 248, and 353 nm, are presented and discussed.

This paper reviews the experimental works dedicated to the development of mirrors and antireflection coatings for the UV spectrum range carried out in the Quantum Radiophysics Division of P.N. Lebedev Physical Institute, Russian Academy of Sciences. The methods of obtaining the single-layer and multilayer optical coatings are given. The effect of the optical coating construction as well as various technological factors of coating process on damage threshold and chemical stability of mirrors is investigated.

Photoexcitation and recombination of nonequilibrium charge carriers in both natural gemstone diamonds and CVD (chemical vapor deposition) polycrystalline diamond films in UV spectrum regions have been investigated. Transient picosecond photoconductivity technique applied permitted to conduct measurements with the time resolution better than 200 picoseconds and to register a charge carrier concentration value as low as 10²⁰ - 10¹³ cm^-3. The dependencies of photocurrent amplitude as a function of incident laser radiation intensity in the range from 10³ to 10¹⁰ W/cm² have been obtained. Charge carrier lifetimes had been measured and charge carrier drift mobility were estimated. It is shown that the electronic properties of high quality thick CVD diamond films are comparable to those of the most perfect natural type IIa crystals. Investigation of Raman and luminescence spectra of diamonds have been performed along with scanning electron microscopy studies to characterize bulk and surface structure of tested specimens.

The continual need for microelectronic devices that operate under severe electronic and environmental conditions (high temperature, high frequency, high power, and radiation tolerance) has sustained research in wide bandgap semiconductor materials. The properties suggest these wide bandgap semiconductor materials have tremendous potential for military and commercial applications. High frequency bipolar transistors and field effect transistors, diodes, and short wavelength optical devices have been proposed using these materials. Although research efforts involving the study of transport properties in GaN and diamond have made significant advances, much work is still needed to improve the material quality so that the electrophysical behavior of device structures can be further understood and exploited. Electron beam induced current (EBIC) measurements can provide a method of understanding the transport properties in GaN and diamond. This technique basically consists of measuring the current or voltage transient response to the drift and diffusion of carriers created by a short-duration pulse of radiation. This method differs from other experiemental techniques because it is based on a fast transient electron beam created from a high- speed, laser-pulsed photoemission system.

First, the results of available laser-induced damage threshold (LIDT) measurements performed on type IIa single crystals of natural or synthetic origin are reveiwed in the light of an incident irradiance versus laser wavelength plot. It is shown that the multiplicity of the across-the-gap photon absorption essentially determines the nature of the damage process, but true bulk dielectric breakdowns appear to be difficult to achieve because diamond targets are 'optically thin' in most test configurations. The same observation holds for chemically vapor-deposited (CVD) diamond, especially since massive deposits of good optical quality have only recently become available. Upon using the Bettis-House-Guenther (BHG) scaling law for obtaining normalized LIDT values, it is seen that damage thresholds recorded for CVD diamond over a wide range of spot sizes and pulse durations correlate remarkably well with single-crystal surface breakdown data. For picosecond pulses impinging on single-crystal natural or polycrystalline CVD diamonds it is shown that the fieldstrength at the damage threshold (E_DB) exhibits a spot size (2(omega) ) dependence best described as follows: E_DB equals A(root)2(omega) with A approximately equals 24 MV(root)micrometers /cm in the visible and A approximately equals 48 MV(root)micrometers /cm in the infrared. This demostrates that the optical strength of diamond is comparable to the strength of well established wide-bandgap laser-windows material candidates.

Aluminum nitride (AlN) thin films are grown by a newly developed plasma source molecular beam epitaxy (PSMBE) system. The films were grown on Al₂O₃ (1102), Al₂O₃ (0001), Si (111), and Si (100) substrates. Structural characterization of the films were performed by x-ray diffraction (XRD), atomic force microscopy (AFM), and high resolution electron microscopy (HREM). The XRD pattern indicates highly textured films. Cross-sectional HREM reveals epitaxy on AlN on most substrates. The Si (111) and Al₂O₃ (0001) plane is lattice matched to the c-plane growth of AlN and the Al₂O₃ (1102) plane is lattice matched to the a-plane growth of AlN. The optical and thermal properties of these films and studied by ellipsometry and thermal wave analysis. The quality of the films is evidenced by the low optical absorption, bulk-like optical index, and bulk-like thermal conductivity.

The studies of laser damage resistance of silica glass and fluoride crystals performed at ArF, KrF, and XeF excimer laser wavelengths with a pulse duration of approximately 80 ns have given the following data: the dependence of the bulk damage threshold on irradiated spot size, ranging in value from 6 to 1200 micrometers ; temperature dependence of laser damage resistance; the influence of metal admixtures on damage threshold; the influence of ionizing radiation on damage resistance; the coefficients of nonlinear absorption; frequency dependence of the bulk damage threshold for silica glass in the UV region.

Optical property changes of newly developed silica by both KrF and ArF excimer laser irradiation will be reported. X-2-01 has been provided for high power laser application for several years and has proved its high performance against KrF laser irradiation. The new material X-1-04 shows superior optical stability in transmittance and refractive index change by KrF and ArF laser irradiation. The application of these materials will be discussed considering their properties of geometry, homogeneity, and laser resistance.

The inclusion of particulates within multiilayer thin films structures has been highlighted as one of the key factors influencing the perfomance of optical coatings at high laser fluences. The problem is especially severe for a number of refractory materials, where the particulates arise from a number of sources in the coating process, but their density is significantly enhanced by the strong electrostatic fields present within most conventional equipment. While advanced techniques such as molecular beam evaporation have demonstrated significant gain in the realization of coatings with high laser damage thresholds, the Knudsen sources generally employed in such equipment do not allow the evaporation of refractory materials, especially oxides. This paper addresses this shortcoming by examining the feasibility of using vapor phase precursors in various activated processes. Currently available techniques are reviewed and some of the obstacles that have to be overcome highlighted. While such techniques can also be susceptible to particulate formation for different reasons to those met in electron beam evaporation, it has been found that under controlled conditions, particularly when critical supersaturation conditions are avoided, it is possible to deposit films which are ostensibly free from inclusions. Moreover, it has been found that it is possible to deposit films at lower temperatures, so avoiding problems associated with thermal stress in conventional processes. The properties of titania films deposited by such a process are described.

The effect of ultraviolet laser radiation (248 nm) on dielectric thin films can be separated into intrinsic absorption of the film material, absorption at structure faults of the layer like grain boundaries or F-centers, and the absorption at inclusions of different materials. Besides this, the film substrate interface as well as the substrate material by itself can act as absorbing regions. Our investigations to the damage morphology of laser induced changes at oxide thin films show typical damage structures for the different sites, where the main part of the laser radiation was absorbed. In some cases we found surprising figures, e.g. lens formed film detachments between damage pits originating from absorbing inclusions. Together with calculations of the temperature field, which is generated by the laser pulse, the dominating damage mechanisms are estimated. At the oxide films under test, the damage is released by inclusion absorption with additional film ablation by overlapping electron avalanches.

Post-mortem atomic force mocroscopy (AFM) analysis of 1 micrometers thick, monolayer, delectric coatings of HfO₂, Y₂O₃, and Ta₂O₅ , all prepared by conventional e-beam deposition, was carried out on irradiated sites of moderate-level damage after 1054-nm irradiation by identical laser pulses. Analysis of all maps shows that growth nodules, long believed to be the prime laser-damage drivers, are few in all of our films and are irrelevant to damage here. Damage morphology was represented by micrometer-scale craters and domes. Crater sizes and depths, linked to the sizes and positions of absorbing defects in the film media, indicate that average absorber size increases from HfO₂ to Y₂O₃ to Ta₂O₅. HFO₂ also has the shallowest craters, while Ta₂O₅ samples consistently damage near the film- substrate interface, with yttria falling in between these extremes. Crater cross sections reveal a predominance of conical wall formations, pointing towards a thermal- explosion mechanism of crater formation. Domes that were found to be entirely absent in Ta₂O₅, occur with approximately 2% probablility in Y₂O₃, i.e., 2% of all mapped defects were domes with more than 25% probablility in HfO₂ films. Obtained results suggest that domes are crater precursors, i.e., arrested damage events because of lack of sufficient energy transfer from the absorbing inclusion.

The European Union (Common Market) has devised a number of initiatives to encourage collaboration between Member States. The Human Capital and Mobility Programme has been established to encourage the exchange of young scientists, information, and results through the formation of 'Networks of Laboratories'. The current network comprises laboratories from France (2), Germany (2), Italy, Spain (2), and the United Kingdom. The program involves the deposition of coatings followed by the exchange of components and characterization and testing using a wide range of techniques.

Transparent YAG ceramics with nearly the same optical characteristics as those of a single crystal were fabricated by a solid-state reaction method using high-purity powders. The average grain size and relative density of the 1.1 at % Nd:YAG ceramics obtained were about 50 micrometers and 99.98% respectively. An oscillation experiment was performed on a cw laser by the diode laser excitation system using the fabricated ceramics. The experimental results indicated an oscillation threshold and a slope efficiency of 309 mW and 28% respectively. These values were equivalent or superior to those of 0.9 at % Nd:YAG single crystal fabricated by the Czochralski (CZ) method.

There were several references at the 1993 damage conference back to the early days of laser damage studies and in particular to the theoretical paper by E. X. Bliss at the first damage symposium in 1969 on the pulse duration dependence of laser damage mechanisms. However, because of the variations, variability and differences inherent in the measurement of laser- induced damage thresholds this theory has not been used to its full potential. Although the measurement techniques have now been standardized and there is a wider understanding of the mechanisms of LID, the data banks exist mainly in narrow pulse width and wavelength domains. This paper has been written in order to show how the laser test pulse duration affects the measured laser damage threshold and to give, at least, reasonable guidelines for the LIDT's which may be expected for typical materials over a range of pulse lengths.

The temperature dependence of reflectance and transmittance of commonly used CO₂ laseroptics is investigated and presented. The knowledge of this dependence is not only important for a better understanding of damage mechanisms in the regime of long pulse and cw-operation, it may also be useful for engineering aspects of high power lasers. The measurements are performed for highly reflective and transmissive optics with an accuracy of +/- 2qq10^-4 and a resolution better than 50ppm. The investigations are done in air to take into account the effects of the environment. Due to convection cooling, the maximum temperature is restricted to 200 degree(s) C. One observation is that oxidation of copper mirrors does not necessarily reduce the relfectance at 10.6 micrometers . Aluminum and silicon mirrors, ZnSe windows, and reflectors and germanium windows with different multilayer coatings are also investigated. The results for the uncoated metal mirrors are compared to the Drude theory.

The damage threshold of five different types of quartz glass obtained from NSG Precision Cells, Inc. was determined using nanosecond pulses at 532 nm. It was found that the damage threshold of one of the glasses is more than twice that of others.

LaF₃/MgF₂-dieletric thin film combinations can be applied in optics for wavelengths down to 150 nm. Several such HR systems for a wavelength of 248 nm were investigated. In these coatings, the influence of laser conditioning on damage threshold and absorptivity was found to be remarkable. XPS- and TEM-investigations showed that the conditioning effect is related to structural and stoichiometric changes in the multilayers, especially in the near-surface-sublayers.

For application in UV thin film optics the thermal contribution to the laser-induced optical breakdown was investigated utilizing time-resolved photothermal probe beam deflection (MIRAGE) technique. The potentiality of this method for the determination of both the subdamage range and the onset of single-shot-damage of Al₂O₃/SiO₂ and LaF₃/MgF₂ high-reflective coatings by using the thermal branch of the MIRAGE technique could be demonstrated. Examining the dielectric mirrors by 248 nm KrF laser irradiation, distinct damage precursor features were found. Thus, the physical origin of the UV-pulsed radiation breakdown in HR coatings can be elucidated.

The samples were irradiated using single CO₂ TEA laser pulses of increasing energy. After each irradiation the laterally resolved aborption was measured. All investigations were carried out both at normal pressure and in vacuum. We observed that a marked decrease in absorption occurs within an area which was larger than the irradiated area. Outside that area the absorption increases. These tendencies were observed on NaCl samples stored and irradiated both in vacuum and in dry air. Using humid air as ambience, however, the absorption increases also inside the irradiated area. The laser damage thresholds and the laterally resolved absorption of NaCl are discussed considering the parameters of investigation.

The environmental effect on the aging behavior of NaCl and KCl windows was studied. Laser windows were aged at different relative humidities in a controlled climate-chamber. Degradation is monitored with a microscope inspection system equipped with a computer controlled image processing board. The temporal development of surface defect density under different atmospheric conditions was investigated with respect to optical absorption and damage thresholds of the windows at 10.6 micrometers . Laser windows coated with single layers of NaF deposited by an adapted IAD-technique were analyzed. The performance of the coated and uncoated laser windows is discussed under consideration of typical applications. In comparison to the bare samples, the coated windows show an improved resistivity against environmental influences. Accelerated testing theory is employed to model the aging behavior of the samples. An approach to deduce a qualified acceleration factor is made in order to extrapolate the lifetime of alkali halide laser window under normal conditions.

The mechanisms of photo-induced changes of refractive index of the (Tau) (Phi) lead- silicate glasses (analogous with the SF glasses from Schott catalog) under the effect of high power laser radiation with quantum energy less bandgap have been studied. It is shown that the laser-induced color centers results in increase of refractive index into the exposed bulk during the laser pulse action. This leads to considerable redistribution of irradiance and decrease of laser radiation brightness even in the case of optical elements less 1 mm thickness. The observed effect may be connected both with radiation-induced dilation of matter and heating of interaction region owing to absorption of radiation by color centers. Comparing the kinetics of refractive index change of the glass after exposure by laser pulse at 0.53 micrometers and the kinetics of color centers decay allows us to draw a conclusion about heat character of observed changes.

Accumulation effect in silicate glasses under single frequency laser radiation (0.53 and 1.06 micrometers ) has been investigated. It is shown that this effect in lead silicate glasses at 0.53 micrometers is the result of the multiphoton matrix ionization, the accumulation of color centers and radiation absorption by them, and the decrease of thermal self- focusing threshold. However, under radiation at 1.06 micrometers in the same materials the multiphoton ionization and color centers formation are not observed. It is established that in this case accumulation effect is connected with densification of glasses in the interaction bulk. This densification is the result of the action of large electrostrictional pressure. It leads to the modification of physical properties of glasses without electronic excitations and the decrease of the self-focusing threshold. In this connection, the accumulation effect takes place only if electrostrictional pressure exceeds threshold value of pressure required to densification of glasses.

This paper considers the possibility of locally increasing electromagnetic field amplitude in transparent dielectric. It is shown that there exists threshold amplitude of light field such that exceeding it can lead to electrodynamic instability.

A phenomenological model for accumulation of irreversible changes initiated by absorbing inclusion in transparent dielectric material under laser radiation is proposed. Kinetics of the accumulation process is investigated for both highly absorbing and low absorbing inclusions. Main regularities of the accumulation effect are derived, particularly the dependence of multishot damage threshold upon a number of pulses. Results of the theoretical analysis are compared with experimental data.

The effect of absorbing inhomogeneities on the laser damage threshold at 1.06 micrometers is well known. We investigated the influence of laser irradiation below the damage threshold on the film absorption. Two regions of different defect denity were tested, a 'defect area' and a 'defect free area'. For those regions,the damage thresholds, and moreover, the thermal effects of irradiation below the damage threshold are different. First, the local distribution of absorption of the unirradiated 'defect free area' was measured. After the irradiation of this region, using a fluence short above the damage threshold, absorption was measured once more. A considerable decrease was observed. However, further pulses did not lead to a measureable change. The absorption increases distinctly after the 8th pulse for this sample, and the damage was registered after the next pulse. In contrast, irradiation of the 'defect area' with fluence below the damage threshold did not cause any changes of absorption. Nevertheless, after the 6th pulse the coating was damaged.

We report extensive laser-induced damage threshold measurements on pure and multilayer dielectrics at 1053 and 526 nm for pulse durations, (tau) , ranging from 140 fs to 1 ns. Qualitative differences in the morphology of damage and a departure from the diffusion-dominated (tau) ^1/2 scaling indicate that damage results from plasma formation and ablation for (tau) <EQ10 ps and from conventional melting and boiling for (tau) >50 ps. A theoretical model based on electron production via multiphoton ionization, Joule heating, and collisional (avalanche) ionization is in good agreement with both the pulsewidth and wavelength scaling experimental results.

A study is reported of the influence of temporal fluctuations of laser radiation on the development of thermal explosion of absorbing inclusions and on the statistical properties of the laser induced damage in tranparent dielectrics. A fluctuation time scale in which the fluctuations effect the thermal explosion of inclusions is established. An analysis is made of the conditions ensuring control of temporal fluctuations of laser radiation so as to eliminate their influence on the experimental statistical relationships of absorbing inclusions in the bulk and on the surface of a sample.

Experimental studies of the laser element's radiation strength show that optical damage of transparent dielectrics may be accompanied by the formation of surface periodic structures (SPS). SPS with ripples oriented normally to the strength vector of the incident electric field were found on the output surface of a dielectric plate (alkali- halide crystal) under focusing near or middle IR laser radiations of a microsecond duration. This relief was assumed to arise from heating caused by interference between the incident light and the wave scattered from the surface defects on the assumption that scattered wave represents a rapidly decreasing field of the Coulomb type. More correct SPS model developed by V. S. Makin proposes participation of surface electromagnetic waves (SEW). As known, optical damage is accompanied by the development of plasma flash. When emission of electronics from solid surface is strong, the plasma dielectric constant runs out to be negative with its modulus exceeding the dielectric constant of the transparent medium. This causes the generation conditions for SEW to be fulfield on the dielectric-plasma boundary, which results in interference between the incident light and SEW, thus leading to formation of SPS. The model explains reasonably well, why these SPS can be observed only on the output surface when developing plasma produces no screening effect on the surface. For CO₂ laser irradiation, the necessary electronic concentration is high but reasonable value and amounts about 10¹⁹ cm^-. However, SPS formed by the short-wavelengths radiation cannot find correct explanation in the framework of this model, since electronic concentration at the wavelengths (lambda) equals micrometers should be no less than 10²¹ cm^-3 in this case.