UV-induced coloration and decreasing of the fast component's intensity of UV-excited luminescence in unirradiated silica glasses and UV-bleaching and decreasing of the fast component's intensity of the luminescence in gamma- irradiated silica glasses have been studied. In the experiments, unirradiated and irradiated by the gamma-ray dose 10⁵ rad in 1987 high purity multicomponent silica glasses and N₂-laser ((lambda) equals337 nm, (tau) approximately equal to 9 ns, repetition frequency 40 Hz, P approximately 6 MW/cm²) were used. The glasses had 5-6 eV band gap and less than 10^-5 wt.% impurity concentration. During 20 min of laser irradiation the absorption coefficient of the unirradiated glasses at 337 nm is increased by 0,25 cm^-1 and luminescence intensity was decreased by 35-40%. In gamma-irradiated silica glasses UV-irradiation leads to bleaching of gamma-induced coloration and decreasing of the intensity of UV-irradiation excited luminescence. After laser irradiation of the gamma- irradiated glasses during 20 min, their absorption coefficient at 337 nm is decreased by 0,13 cm^-1 and the fast component's intensity is decreased by 30-40%. It should be pointed out, that both unirradiated and gamma- irradiated samples of glasses have the luminescence intensities of an equal value, but the absorption coefficients are different. These effects and luminescence time resolved spectra depend on the UV-irradiation intensity and are explained by the two photon absorption of laser irradiation in glasses.

Single-wall carbon nanotubes are formed by Nd:YAG laser vaporization of a graphite/(1 at. % Ni, 1 at. % Co) target into flowing argon (500 Torr) within a quartz tube furnace (1000 degree(s)C). Here, this process is investigated for the first time with time-resolved laser-induced luminescence imaging and spectroscopy of Co atoms, C₂ and C₃ molecules, and clusters. These measurements under actual synthesis conditions show that the plume of vaporized material is segregated and confined within a vortex ring which maintains an approximate 1 cm³ volume for several seconds. Using time-resolved spectroscopy and spectroscopic imaging, the time for conversion of atomic and molecular species to clusters was measured for both carbon (200 microsecond(s) ) and cobalt (2 ms). This rapid conversion of carbon to nanoparticles, combined with transmission electron microscopy analysis of the collected deposits, indicate that nanotube growth occurs during several seconds of time from a feedstock of mixed nanoparticles in the gas-suspended plume. By adjusting the time spent by the plume within the high- temperature zone using these in situ diagnostics, single- walled nanotubes of controlled length were grown at an estimated rate of 0.2 micrometers /s. Ex situ annealing of short, 100-200 nm-long SWNT seeds collected after limited growth inside the hot oven resulted in continued growth of longer SWNT bundles, supporting the condensed phase conversion mechanism for SWNT growth.

Color centers contributing to the optical absorption spectrum of synthetic silica glass in the near infrared to vacuum UV range are reviewed. Intrinsic defects related to oxygen deficiency are characterized: E'-centers, oxygen vacancies, divalent silicon and defects related to oxygen excess: oxygen dangling bonds, peroxy radicals, interstitial oxygen and ozone molecules. The optical properties of common impurities/dopants in synthetic silicas used in laser optics are discussed.

Materials that have periodic microstructure on a given length scale display a strong modification of electromagnetic density of states for radiation wavelength at the corresponding scale. These structures are known as photonic crystals. Under certain circumstances the density of states vanishes completely for a range of wavelengths, and the material is said to have a photonic band gap, whereby optical propagation is completely suppressed. This affords the possibility of optical control useful for a range of applications including novel filters, waveguides, and efficient laser structures. In this paper, a range of fabrication methods of these crystals is described, together with basic theory and some properties and applications. Particular attention is given to two-dimensionally periodic materials in the form of optical fibre (the photonic crystal fibre and photonic band gap fibre), which have the potential for high-power optical guidance. Three-dimensionally periodic materials designed to control microwave radiation are also described.

Intrinsic, extrinsic (transition metals and hydroxyl), and induced (color centers) absorption of multicomponent silicate glasses in UV, visible and near IR spectral regions are described. Parasitic effects influencing absorption measurements are discussed. Excitation to the intrinsic absorption band results in intrinsic luminescence and ionization followed by color center generation and phosphorescence. The thresholds of electron and hole mobility in glass network are found in the far UV region. The hole-centers generation as a criterion of substance ionization is proposed. A number of nonlinear mechanisms of glass ionization are discussed. Two-photon ionization was detected in alkaline-silicate glasses exposed to high-power laser radiation in nano- and picosecond regimes. Three- photon ionization was detected in lead-silicate glasses. No reliable data on multiphoton ionization (with number of photons more than 3) of glasses are found. Two- and three- photon cooperative self-multiplication of color centers was found in CuCl-doped glasses. Glass matrix ionization by spectral broadening of femtosecond IR pulses is described. Thermal and surface ionization of glass under intense irradiation by pulsed lasers is described.

At high optical power densities, materials that would normally be classed transparent, break down. The mechanisms by which high purity fused silica optical fibers fail are discussed in this paper. Multimode fibers with a core diameter of 400 micrometers have been tested with a Nd:YAG laser with a view to transmitting the maximum amount of energy. The importance of surface finish has been verified by implementing polishing schedules of varying degrees. The front face of many of the fibers would be improved during laser testing, due to plasma formation which acts to anneal the surface. It has been found that the energy level at which this effect first occurs gives a good indication of the initial surface roughness. Atomic force microscopy has been used to confirm surface roughness measurements as low as 3 nm and excellent agreement between high power transmittance and surface quality has been found.

Interaction of high intensity laser radiation with fused silica mainly introduces strong temperature increase in the region where this radiation is absorbed. The local temperature rises up to 2500 K or higher. At these temperatures high concentrations of thermal intrinsic point defects are generated. Estimated concentrations of equilibrium oxygen excess defects at 2500 K reaches 7(DOT)10¹⁸cm^-3 and the main part of these defects are non-bridging oxygen atoms. Before equilibrium large part of oxygen excess defects are oxygen molecules. Presence of oxygen molecules in concentration 3(DOT)10¹⁸cm^-3 creates the local internal pressure 1,0 atm. at 2500 K. As the result the boiling and evaporation of the material from the region where high intensity laser radiation is absorbed occurs. Remaining material contains increased concentrations of oxygen deficit point defects, such as three- and two-fold coordinated silicon atoms, as well as oxygen excess point defects which are tightly bonded to the glass network, such as non- bridging oxygen atoms. Presence of these defects causes the strong increase of UV-wavelength laser radiation absorption. This stimulates additional laser interaction with fused silica. New features in defect generation appears when very short (femtosecond) laser pulses are used. In this case the laser pulse length is comparable with the period of main atomic vibrations in fused silica. Such conditions open new sub-threshold intrinsic defect generation mechanisms.

In the fullerene-doped polymer dispersed liquid crystals the holographic recording and optical limiting effect have been established to be intermediate between those for fullerene- doped liquid crystals and for fullerene-doped polymers. It has been shown that fullerene-doped polymer-dispersed liquid crystals could be applied for optical limiting at the incident laser power of 0-3-0.4 J cm^-2. They can be used as effective diffractive optical elements.

The fullerene-doping effect on spectral and nonlinear optical properties of (pi) -conjugated organic systems based on 2-cyclooctylamino-5-nitropyridine, 2-(n-prolinol)-5- nitropyridine, N-(4-nitrophenyl)-(L)-prolinol, and polyimide 6B has been investigated. Under nanosecond laser irradiation at the wave length of 532 nm, reverse saturable absorption has been examined. To clarify peculiarities of optical laser power limiting, the reverse saturable absorption, the Foerster mechanism, two-photon and free-carrier absorption, as well as the complex formation must be taken into consideration. The compounds investigated have been found to be efficient absorbers and they could be applied to limit the laser irradiation at the energy density of more than 4-5 J/cm^-2.

The morphology and microstructure of damage sites in high quality fused silica induced by high power UV (355 nm) laser light have been investigated using a suite of microscopic and spectroscopic tools. These include SEM, TEM, microprobe analysis, XPS, SIMS and x-ray micro-tomography utilizing intense synchrotron radiation. Systematic SEM examinations show that the damage sites consist primarily of a molten core region (thermal explosion), surrounded by a near concentric region of fractured material. The latter arises from propagation of lateral cracks induced by the laser- generated shock waves. The size of the overall crater is dependent of the laser fluence, number of pulses, damage history and environment. In particular, differences in morphology of the damage sites are identified: air vs. vacuum; exit (more severe) surface vs. entrance surface; and regular polish (more severe) vs. super polish surfaces. A compaction layer, approximately 10 microns thick and approximately 20% higher in density has been identified with x-ray tomography. This layer has further been substantiated by micro-Raman spectroscopy. High resolution microprobe analysis shows that there is no variation in the Si/O stoichiometry of silica in the compaction layer to within +/- 1.6%. High resolution TEM indicates the absence of crystalline nano-particles of Si in the compaction layer. Macro- (10-0.1 micrometers ) and micro-cracks (200-20 nm) are found, however, in the bright field images. The Si 2p XPS spectra indicates that there is a lower Si³⁺ species on at least the top 2-3 nm of the compaction layer. These findings are critical to the design of a knowledge-based mitigation process for laser damage growth.

Theoretical study of ultrafast laser induced damage by short pulses ((tau) <1 ps) is carried out on large-band-gap dielectric in an effort to understand the complex physical processes involved. The numerical method of solving a general time-dependent Fokker-Planck type equation for free electron production is discussed in detail. The calculation shows that the collisional avalanche ionization competes with the multiphoton ionization even for pulse length shorter than 25 fs. Sensitivity tests of all the rates in the equation are performed and the most critical ones are identified. From these tests we obtain valuable information in developing new materials that have the desired damage fluence for specific applications. To describe the relaxation of electron plasma, a three body recombination rate is included. Thus, the temporal behavior of the electron density due to a single pulse is treated, as well as the case of exposure to two laser pulses with a time delay between them. The model is only partially successful in reproducing the recent experimental data. Effect of the presence of a linear decay term and optical defects on the damage threshold is considered in the context of the rate equation input.

There are considered two effects of nonlinear light propagation that can play important role in laser-induced damage of transparent materials: self-induced variations of light polarization of focused laser beam and developing of field instability in non-absorbing microinclusions. The effect of self-induced variations of light polarization is considered qualitatively for focused laser beam with arbitrary focal spot. Detailed study of the effect is fulfilled for Gaussian beams of low order. It is shown that initial light polarization can turn into elliptic one with inhomogeneous distribution of polarization-ellips parameters. Developing of field instability in transparent microinclusions is the other considered nonlinear effect. It is shown that transparent microinclusion can initiate large local field increase accompanied by positive feedback resulting in further field increasing near the inclusion. If the electric field strength exceeds damage threshold during nonlinear evolution in the inclusion then developing of field instability results in damage of microinclusion before electric field reaches certain upper level determined by ionization processes.

Ablation from amorphous SiO₂ (silica glass) was induced using ArF excimer laser. Threshold fluences of 2.5 J/cm² was apparently necessary to commence ablation by a single pulse. Ablation was also observed after the number of pulses at below the threshold fluence. Microscopic structural change was examined with x-ray photoelectron spectroscopy, Raman spectroscopy, optical absorption spectroscopy in vacuum ultraviolet (vuv) region. A regular puckered four membered ring as well as Si³⁺ structure were introduced with irradiation. Repeat of flash heating and quenching by the pulse laser irradiation might generate regular puckered four membered rings and Si³⁺ species in silica glass. Increase of Si³⁺ concentration would reduce threshold fluence and number of pulse for ablation.

A densely packed bed of alkaline earth fluoride particles percolated by a fluid medium has been investigated as potential index-matched optical limiter in the spirit of a Christiansen-Shelyubskii filter. Marked optical limiting was observed through this transparent medium under conditions where the focused second-harmonic output of a Q-Switched Nd:YAG laser was on the order of about 1 J/cm². An open- aperture Z-scan technique was used to quantify the limiting behavior. In this case, the mechanism of optical limiting is thought to be a nonlinear shift in the fluid index of refraction, resulting in an index mismatch between the disparate phases at high laser fluence. This induced mismatch appears to be promoted by localized electric field enhancement present near the sharp edges at the crystallite/fluid boundaries. Index mismatch between the two phases leads to multiple reflections, loss of coherence, and a significant transmission decrease due to Mie scattering. The presence of many boundaries significantly amplifies the effect. The role of thermally induced changes in refractive index for this system appears to be relatively small in pulsed-laser experiments. However, cw-laser blocking was achieved by a thermal mechanism when an absorber (iodine) was dissolved in the liquid phase. Fundamental studies of such systems are used to verify theoretical predictions of the limiting effect, and aid in the design and development of improved limiters based upon this optical deflection approach.

We investigate damage of purified silica (transmission band down to 160 nm) by sub-ps light pulses having a wavelength of 795 nm. Illumination by 350 fs duration pulses focused by a high numerical aperture NA equals 1.35 microscope objective results in one of the lowest reported values for the single-shot bulk light-induced damage threshold (LIDT) of 5 J/cm², well below the critical self-focusing power in silica. We have also investigated peculiarities of damage by two coincident laser pulses (duration 440 fs) having power of about 0.5 x LIDT, and linearly cross-polarized to avoid interference effects. The reduction of LIDT in silica is demonstrated for an elevated lattice temperature T equals 400 K, at which the thermal linear/volume expansion coefficient has its maximum. Comparison between the LIDT values obtained from the numeric simulation and experiments demonstrates that the critical density of optically generated free carriers corresponding to LIDT n_cr approximately equal to 10²¹cm^-3 is reached during the first half time of the laser pulse illumination (0.2ps).

The formation of thin film single layers and multilayers from sol-gel materials at room temperature inherently yields a coating with residual organic impurities and trapped solvent. Both of these features have the potential to act as initiators or secondary activators of laser damage. The laser induced damage threshold (LIDT) of single layer anti- reflective silica and single layer high index zirconia coatings have been assessed as a function of curing temperature. A coating of 200-nm thickness deposited by dip coating was exposed to elevated temperatures in an air atmosphere from 20 degree(s)C to 350 degree(s)C and cooled to room temperature and pressure (RTP) before testing. The effect of this rise in temperature on the LIDT is discussed. Ultra Violet-Visible transmission spectra of the thin films are also presented. A clear densification process was found to occur leading to both a change in thickness and refractive index as a function of temperature. The absence of this trend in the low index silica layer is believed to be a direct consequence of the different chemistry during formation as a similar solvent is used in both the high and low index layers. The implications of this with respect to the effect on multilayers are also discussed.

Anti-reflective thin films used on optics within the target chambers of high power lasers used for plasma physics experiments degrade significantly in use. The cause of this may not be attributed to any single influence. Coating from target debris, absorption of vacuum chamber vapors, and the exposure to vacuum conditions and X rays will all contribute. To simulate some of these conditions, sol-gel derived optical film ageing studies have been performed on a range of anti-reflective coatings over a 4-month period. Similarly the use of sol-gel High Reflectivity coatings for mirrors need to be durable and stable with time in their likely working environments. The coatings have been assessed with regard to their laser induced damage threshold (LIDT), transmission properties, refractive index and thickness at three wavelengths (355 nm, 532 nm and 1064 nm). Although exact replication of the conditions is impossible some of the parameters have been controlled including pressure and humidity as a function of time. We report on the changes experienced by these coatings under these conditions and discuss possible reasons for the observed trends and the effect this could have on coating selection.

The CEA/DAM megajoule-class pulsed Nd:glass laser devoted to Inertial Confinement Fusion (ICF) research is requiring 240 cavity-end mirrors. The mirror design is based on 44-cm square highly-reflective (HR)-coated deformable substrates. Such large dielectric mirrors are using interference quarterwave stacks of SiO₂ and ZrO₂-PVP (PolyVinylPyrrolidone) thin films starting from sol-gel colloidal suspensions (sols). The colloidal/polymeric ratio of the ZrO₂-PVP composite system has been optimized regarding refractive index value, laser damage threshold and chemical interactions have been studied using FT-IR spectroscopy. Therefore a promising deposition technique so- called Laminar Flow Coating (LFC) has been associated to sol-gel chemistry for HR sol-gel coating development. The as-designed LFC prototype machine has been used for coating solution wave deposition by transportation of a tubular dispense unit under the substrate flat surface. Thin film so created by the solvent evaporation was then dried at room temperature or using short wavelength UV-curing built-in station. Optimization of parameters such as optical layer number, coating uniformity, coating edge effect, 1053-nm reflectance and laser damage threshold is discussed. Demonstration has been made that this novel coating method is a competitive way for large-area optical deposition compared to dipping or spinning techniques. Association of sol-gel colloidal suspensions to LFC process appear to be a promising cheap way of producing high power laser optical coatings.

Non-erasable fine pitched grating structures were successfully encoded in amorphous SiO₂ glasses and amorphous SiO₂ thin films on silicon wafers by colliding a pair of focused pulses split from a single femtosecond pulse from a 10 Hz mode-locked Ti:sapphire laser with a regenerative amplifier (wavelength: 800 nm; pulse duration: 100 fs; pulse energy: approximately 3 mJ/pulse). This pattern was not observed for cases in which the relative time delay of the two pulses was over 0.2 ps. The encoded periodic spacing was changed by varying the angle between the two crossed pulses. A minimum periodic spacing of approximately 430 nm was achieved for a laser wavelength of 800 nm. Structural alternations of silica network induced by intense fs-laser irradiations were observed. Laser ablation processes and volume compaction in amorphous SiO₂ are origin of the formation of grating structures.

The experimental setup developed in Marseille for the laser damage testing allows a localized study. Indeed the use of a 25 micrometers for the waist of the focused beam, permits to de-correlate the extrinsic damage due to the micronic defects (visible under microscope) for the intrinsic ones (non-detectable before damage with conventional imaging systems). The probability of damage versus incident fluence is an S curve given in the range of two thresholds, S_L and S_H, the low and high damage thresholds. Most often the shape of probability damage curves are different between the intrinsic and the extrinsic cases. In our arrangement the beam size and the extrinsic defect size are in the same range, so by pointing at these visible defects it is possible to determine their specific threshold, and the density of defect is directly obtained from the optical image. Therefore a specific study of the intrinsic zones by pointing the beam at a zone free of extrinsic point, allows us to focus our attention only on these invisible defects. These particles are supposed to be nano-sized. The highlight and the identification of these nono-precursors is the aim of this paper.

To understand laser damage mechanisms using nano-second pulsed laser, different tools have been developed. Indeed, one challenge today is to exhibit nano-absorbing centers, supposed to be the main precursors of damage. In general, for practical reasons these tools involve a shopped CW laser, associated with a lock-in amplifier in order to exhibit local absorption in materials. Most often, no evident correlation appears between the zone revealed by the CW pump and the damage site created with pulsed laser shoot. The aim of this paper is to investigate this point, by similar experiments using both CW and pulsed laser. We will show results obtained on materials with different absorption level, using standard CW/pulsed photothermal techniques and atomic force microscopy morphology studies. A direct consequence of this study is also to evaluate the contribution of thermal effect in the laser damage process, to highlight if the case arises with other kind of mechanisms.

Changes in optical absorption of lithographic-grade SiO₂ glass containing hydrogen were examined when irradiated intermittently by ArF excimer laser (6.4 eV) under a simulated operating mode of lithography laser. Absorption intensity at 6.4 eV was increased during the irradiation and was decreased gradually after the termination of the irradiation. However, when re-irradiated, the absorption intensity was recovered instantly to the extent just before the termination of the irradiation. These observed phenomena could be explained by the formation and restoration of E' centers. E' centers were generated through two kinds of processes, dissociation of strained Si-O-Si bonds and dissociation of photo-induced ODCs, while E' centers were converted into SiHs by chemical reaction with hydrogen in SiO₂ glass. The phenomenon of fast re-darkening was due to photolysis of SiHs into E' centers.

Absorption and scattering present local defects which can be imaged simultaneously in exactly the same conditions, allowing a precise study of correlation between absorption and scattering spatial variations in thin film materials and surfaces. Absorption A is linearly related to the extinction coefficient whereas scattering S is mainly due to surface profile and refractive index variations. Furthermore spatial frequencies involved in these variations take a great part in scattering. However imaginary and real parts of the complex index are related by KK relations, and correlation A/S depending on the wavelength can be expected. Simulations in fused silica are presented. Studies of correlation give information about nature of defects.

The purpose of this paper is to present the effect of a spherical inclusion in SiO₂ under pulsed laser irradiation. The 3D electromagnetic field distribution (E, B) in the inclusion and the SiO₂ bulk is calculated using the Mie theory extended to radially inhomogeneous media. The effects of electric field enhancement can be investigated for any size and any type (metallic or dielectric) of inclusion. This E-field enhancement may lead to direct breakdown of SiO₂. The laser energy coupled with the inclusion and the host material is computed with the numerical code DELPOR along the laser duration. DELPOR is a 1D hydrodynamic code used for spherical geometry taking into account the laser solid- to-plasma interaction, thermal diffusion and phase transitions. During the laser pulse, the 1D hydrodynamic calculation is coupled with a 3D Lagrangian-Eulerian code to investigate the mechanical effects involved in the blow-up of the inclusion near the SiO₂ slab surface. For a set of experimental conditions, we attempt to determine the role of mechanical effects and E-field enhancement. Our ultimate goal is to predict laser damage morphology as a function of the physical and geometrical parameters (inclusion type, size and depth) of the inclusion and to compare it with experimental data.

Results are reported from recently performed bulk-damage, pulse-scaling experiments on DKDP tripler samples taken from NIF-size, rapid-growth boule BD7. The tests were performed on LLNL's Optical Sciences Laser. A matrix of samples was exposed to single shots at 351 nm (3(omega) ) with average fluences from 4 to 8 J/cm² for pulse durations of 1, 3 and 10 ns. The damage sites were scatter-mapped after testing to determine the damage evolution as a function of local beam fluence. The average bulk damage microcavity (pinpoint) density varied nearly linearly with fluence with peak values of approximately 16,000 pp/mm³ at 1 ns, 10,000 pp/mm³ at 3 ns and 400 pp/mm³ at 10 ns for fluences in the 8-10 J/cm² range. The average size of a pinpoint was 10(+14,-9) micrometers at 1 ns, 37+/- 20 micrometers at 3 ns and approximately 110 micrometers at 10 ns, although all pulse durations produced pinpoints with a wide distribution of sizes. Analysis of the pinpoint density data yielded pulse-scaling behavior of t^0.35. Significant planar cracking around the pinpoint as was observed for the 10 ns case but not for the 1 and 3 ns pulses. Crack formation around pinpoints has also been observed frequently for Zeus ADT tests at approximately 8 ns. The high pinpoint densities also lead to significant eruption of near-surface bulk damage. Measurements of the damage site area for surface and bulk gave ratios (A_surf/A_bulk) of 2:1 at 1 ns, 7:1 at 3 ns and 110:1 at 10 ns.

Over the course of testing a substantial number of KDP and DKDP crystals from rapid and conventional growth processes, we have discovered that there is a consistent difference in the value of the damage resistance between z-cut and tripler, x-cut and y-cut crystals for a given test fluence. This increase in damage probability for tripler, x and y-cut crystals is consistent for both conventional and rapid growth KDP and well as DKDP. It also holds for unconditioned (S/1) and conditioned (R/1) tests and has values of 2.1+/- 0.6 and 1.5+/- 0.3 respectively. Testing has also revealed that there is no sensitivity to incident laser polarization. This is in direct contradiction to models based on simple, non-spherical absorbers. This result plus new information on the size and evolution of bulk damage density (see Runkel et al., this proceedings) has led to a reinterpretation of the growth parameter data for rapid growth NIF boules. It now appears that variations in impurity concentration throughout the boule do not affect the damage probability curve as dramatically as previously thought, although this is still a topic of intensive investigation.

This paper discusses the results of thermal annealing and in-situ second harmonic generation (SHG) damage tests performed on six rapid growth KDP type 1 doubler crystals at 1064 nm (1(omega) ) on the Zeus automated damage test facility. Unconditioned (S/1) and conditioned (R/1) damage probability tests were performed before and after thermal annealing, then with and without SHG on six doubler crystals from the NIF-size, rapid growth KDP boule F6. The tests revealed that unannealed, last-grown material from the boule in either prismatic or pyramidal sectors exhibited the highest damage curves. After thermal annealing at 160 degree(s)C for seven days, the prismatic sector samples increased in performance ranging from 1.6 to 2.4X, while material from the pyramidal sector increased only modestly, ranging from 1.0 to 1.3X. Second harmonic generation decreased the damage fluence by an average of 20 percent for the S/1 tests and 40 percent for R/1 tests. Conversion efficiencies under test conditions were measured to be 20 to 30 percent and compared quite well to predicted behavior, as modeled by LLNL frequency conversion computer codes.

An international round robin study was conducted on the absorption measurement of laser-quality coatings. Sets of optically coated samples were made by a reactive DC magnetron sputtering and an ion beam sputtering deposition process. The sample set included a high reflector at 514 nm and a high reflector for the near infrared (1030 to 1318 nm), single layers of silicon dioxide, tantalum pentoxide, and hafnium dioxide. For calibration purposes, a sample metalized with hafnium and an uncoated, superpolished fused silica substrate were also included. The set was sent to laboratory groups for absorptance measurement of these coatings. Whenever possible, each group was to measure a common, central area and another area specifically assigned to the respective group. Specific test protocols were also suggested in regards to the laser exposure time, power density, and surface preparation.

Linear and nonlinear absorptance in Al2O3 films of different optical thicknesses are investigated using an ArF laser calorimeter. While the linear absorptance at 193 nm shows the linear increase expected for homogenous layers coated with identical process parameters, nonlinear absorptance increases nonlinearly with increasing film thickness. Thus, it cannot be described by a constant nonlinear absorption coefficient. The experimental findings are explained by a simple phenomenological approach using excited states with a finite interaction length longer than the actual film thickness. Due to the observed quadratical increase a new material constant is introduced which describes the nonlinear absorptance behavior correctly.

In this paper, we present our new Coblentz Hemisphere based Angular Resolved and Integral Scattering Measurement Apparatus (CHARISMA). It is based on the combination of the well known Coblentz-Hemisphere used for integral scattering measurements and highly sensitive camera system developed at the Laser-Laboratorium Goettingen. Using CHARISMA, single- shot determination of the bidirectional reflectance distribution (BRDF) of optical samples is possible. Due to the spherical aberrations in the high-precision machined Coblentz-Hemisphere, the light scattered from a sample is not imaged onto a single spot but rather onto a stratified area. From this light distribution monitored by the camera system the angular-resolved scattering distribution can be evaluated unambiguously using a sophisticated complicated mathematical transformation.

Complicated dielectric coatings consist of a large number of layers and thus have many interfaces, that may contribute to the total absorption of the coatings. We examined this contribution of the interfaces using two different approaches. For the determination of the absorption of the first interface between the substrate and the coatings we varied the thickness of dielectric single layers. For the examination of the influence of the interfaces within a dielectric stack, coatings consisting of (lambda) /2-layers were produced and their absorption was measured.

Laser induced breakdown of a high-quality mirror consisting of alternating (lambda) /4 layers of Ta₂O₅ and SiO₂ and a single 500-nm thin film of Ta₂O₅ were studied with amplified and unamplified femtosecond pulses. The experimental data can be fitted with a model taking into account multiphoton absorption, impact ionization, and local intensity enhancements due to interference effects in the films. The results indicate that state of the art, high- quality thin films show a damage behavior that is similar to bulk materials. Defects and impurities play a negligible role. Incubation effects are found to reduce the damage threshold when the coatings are damaged with multiple pulses from a femtosecond oscillator. Time-resolved pump-probe reflection and transmission experiments indicate a decay of the excited electron plasma with characteristic time constants of 4 ps, 60 ps, and 700 ps.

Fused silica samples from six suppliers were irradiated with a range of fluences (0.004 mJ/cm² to 0.2 mJ/cm²) using an ArF 193-nm excimer laser. The test was performed in an effort to determine fluence level dependency of induced wavefront distortion and birefringence. Each sample was irradiated with four beams of different fluence levels for 22 billion pulses over a period of 133 days. Wavefront distortion in the irradiated areas was observed for all samples. The sign and magnitude of the distortion were dependent upon the fluence level and the particular sample under irradiation. Birefringence measurements were also made. The birefringence characteristic varied among the samples, possibly as a function of fluence level and material. FTIR spectrum measurements were made and were correlated with wavefront distortion measurements. A description of the test and measurements is presented along with data covering a pulse count of 22 billion pulses.

An example of a multilayer component for 248-nm excimer laser applications is explained in this contribution. By means of a polarizing beam splitter the potential of plasma- assisted deposition is shown. Such a coating shows a high reflecting and a high transmitting function for different polarization at a certain angle of incidence. High reflection requires a considerable number of layer pairs with a large total optical film thickness. Despite large film thickness, the optical absorption can be minimized and high transmission can be obtained. The laser induced damage threshold of such a coating is in the order of 2.5 J/cm² for single shot measurements at 20 ns pulse length. The life time is greater than 5X10⁹ shots at a fluence of 10mJ/cm². Very stable deposition conditions must be maintained to produce these coatings within small tolerance of the optical function. It was found that plasma-assisted deposition is a more reliable technology for producing these coatings compared to classical evaporation.

The use of high power dye laser beams during long periods is difficult because the performances of the optical components are decreasing with time. To understand this phenomenon, a set-up for research of experimental conditions that can minimize this dramatic trend has been built. After being irradiated by a laser beam of some kW/cm², the optical components are locally covered by droplets. The size of the droplets depends on experimental conditions. They can be eliminated by cleaning. The high absorption of the deposits (200 ppm to 1000 ppm) leads us to search experimental conditions to limit the layer deposition speed. XPS measurements prove that the contamination is merely made up of organic compounds and hydrogenated carbon. The experiments show that a clean room environment under a controlled airflow is not sufficient to assure a very slow deposition. Vacuum from 10^-2 to 10^-6 mbar is even worse than room conditions and only the presence of oxygen can limit or eliminate these deposits.

Simulations of laser-silica interaction at 1.053 and 0.351 micrometers is a key issue in predicting and quantifying laser damage in large laser systems such as LIL, LMJ or NIF. Laser induced damage will occur in many locations. Pure intrinsic (defect free materials) laser damage of real world fused silica does not exist in the nanosecond pulse length range. That is why many attempts to model laser damage using intrinsic properties failed. It does not mean that intrinsic phenomena (avalanche ionization...) do not play a role in laser damage. We have introduced extrinsic features of fused silica in our calculations. Surface defects are modeled in terms of electronic density gradients. We use Monte Carlo simulation to extract avalanche ionization coefficients and collision frequencies. We use fluid equations to determine electric conductivity and compute the electric field distribution with Helmholtz equation in our 1D hydrodynamic DELPOR code where Joule heating, thermal conduction and electron diffusion are taken into account.

A model thin-film system based on SiO₂ coating with artificially introduced gold nanoparticles was investigated for the mechanism of 351-nm, pulsed-laser-radiation interaction with well-characterized nanoabsorbers. Damage morphology, represented by craters, provides strong evidence of the important role of the melting and vaporization processes. Measured crater volumes and numerical estimates based on them suggest that crater formation cannot proceed through laser-energy absorption confined within the particle. It instead starts in the particle and then, due to energy transfer, spreads out to the surrounding matrix during the laser pulse.

Volume holographic gratings and ion-etched dielectric diffraction gratings have been designed with the goal of improving the efficiency and damage threshold when used in CPA laser compression scheme. Two damage threshold measurement techniques have been implemented, including one method based on the statistical distribution of damage fluences. We first tested samples with submillimeter laser spots in the subpicosecond regime. We could demonstrate large sample gratings in a complete CPA configuration with over 96% efficiency per pass and a damage threshold twice the one observed on gold coated gratings.

Waveguide writing using a train of femtosecond laser pulses at 850 nm, in the transparent spectral region of bulk As₄₀S₆₀ glasses is reported. Wave guides were written by translating the glass sample along the optical axis of a strongly focused laser beam. Refractive index variation, linear absorption spectra, and Raman spectra of the exposed region were measured. The chemical mechanism responsible for the index variation has been correlated to a breakage of the glass' As-S bonds and formation of As-As and S-S bonds and increase of the disorder of the glass network. The nonlinear optical origin leading to this phenomenon has been shown, and possible mechanisms are discussed.

The 157 nm F₂ laser is becoming the workhorse for lithography tools for the 70 nm technology node. In this paper we review our recent advances in technology and reliability of 157 nm lasers. We discuss the improved lifetimes of main laser components and their impact on Cost of Ownership (CoO) of the F₂ laser. The typical lifetime of Lambda Physik Novaline laser discharge tube, coated CaF₂ optics, and energy monitors exceeds 3 billion, 2 billion, and 2.5 billion respectively. The CoO of the F₂ lasers reaches that of ArF lasers. We also report the results of our very thorough studies on the various line-narrowing arrangements, and feasibility of amplification at 157 nm, in the context of our recent studies of the fundamental spectral properties of F₂ lasers.

While the 157 nm optical lithography has become recognized as the most promising solution for the 70-nm node of Semiconductor Industry Association Roadmap, the reliable metrology for this spectral range still remains one of the biggest challenges. We report the results of our long-term exposure measurements, which led to improved performance of the optical components of F₂ laser. We discuss in detail the development of the reliable energy monitors degrading less than 3% per billion pulses. We also report the development of novel VUV laser beam profiling tool, measuring the divergence and the width of the beam with accuracy of 0.05 mrad, and 0.10 mm respectively. The compact size makes this tool useful for applications outside the optical laboratory. We also discuss present status and our recent contributions to the high resolution VUV spectroscopy and demanding requirements imposed by 157 nm lithography.

In this paper we describe the interaction of an intense laser beam with metallic nanoparticles embedded in glass. Energy deposition in the metal is calculated on the basis of Mie's scattering theory, using an accurate model for the dielectric function which involves interband transitions. As is shown by two-temperature modeling of the laser-heated metal, nonequilibrium thermodynamics must be used to describe such systems, even in the nanosecond laser pulse range. Taking into account the particle cooling process by heat diffusion in the glass matrix, the model provides a useful tool for the understanding of laser damage initiation by metallic nanoinclusions.

Storage Ring Free Electron Laser (FEL) are attractive, full of promise, tuneable and powerful laser sources for the UV range. High reflectivity dielectric mirrors should be produced in order to allow lasing at very short wavelength, with a long stability in a strongly harsh environment and to optimize the extracted FEL power required for most of the newest applications. The front mirror of the laser cavity receives all the synchrotron radiation (SR) emitted by the wiggler, which is responsible for the mirror degradation, combined with the contamination by the vacuum residuals. We are tackling the problem of tests and manufactures of reliable robust mirrors and explore themes such as resistance analysis of UV mirrors to FEL multiscale power, broadband (X-UV) mirror robustness. Under drastic SR conditions, multiscale wavelength damages could be observed. Specific measurement techniques, able to investigate localized spatial modification induced by the non-uniform synchrotron radiation are presented. A local crystalline structure modification of the high index material appears together with a severe increase of the roughness.

We developed a novel coating method using chemical reactions of gaseous reactants on a surface. A self-limiting nature of surface chemical reactions allows precisely controlled growth of films with high uniformity and controllability of thickness over large area. The nonuniformity of thickness distribution was under 1% over 240 mm in diameter. The film thickness had proportional relationship with a number of chemical reactions. TiO₂ films at growth temperature of 25 degree(s)C had a laser-induced damage threshold of 5 J/cm² for 1-ns, 1.06-micrometers laser pulses. The laser damage resistance of TiO₂ films decreased at higher growth temperature. TiO₂ films grown at the high temperature had higher crystallinity. We clarified that the laser damages resulted from the local sites that absorbed the laser energy.

The aim of this work is to investigate the influence of the standing-wave electric field profile on the laser damage resistance of HfO₂ thin films. To this end, HfO₂ thin films of different optical thickness and deposited by the electron beam evaporation technique at the same deposition conditions have been analyzed. Laser damage thresholds of the samples have been measured at 308 nm (XeCl laser) by the photoacoustic beam deflection technique and microscopic inspections. The dependence of the laser damage threshold on the standing-wave electric field pattern has been analyzed.

Laser-induced breakdown is believed to be a primary retinal damage mechanism for sub-50 fs laser pulses. Recent studies of ultrashort pulse ocular effects indicate that with frequency chirp compensation, damage thresholds for the retina can be reduced. However, the reductions in threshold do not follow trends predicted by strictly input pulse duration-dependent models. We present a study of the effects of dispersion and the effects of spherical and chromatic aberrations in the propagation of ultrashort laser pulses. We consider optical models of the eye and also common laboratory optical configurations that mimic the eye. Intensity profiles in the focal volume of the optical system are computed for various materials, models, and amounts of aberration. A comparison of relative peak intensities is used to estimate trends in laser-induced breakdown (LIB) thresholds, based upon computation models previously published. These trends in LIB thresholds are compared to experimental data collected in our laboratory.

We built a bi-directional scatter diagnostic to measure and quantify losses due to scattering and absorption in harmonic generation crystals (DKDP) for the National Ignition Facility. The diagnostic performs angle-resolved photometry at 351 nm, and is capable of both near-specular transmission and large angle scatter measurements. In the near-specular configuration, the transmission can be measured with variable acceptance angle ranging from +/- 65 (mu) rad up to +/- 60 mrad. A silicon photo detector and a scientific- grade CCD camera quantify total energy and energy distribution. A linear swing arm detection system enables large angle scatter measurements of 360 degree(s), in principal, with step sizes as small as 0.01 degree(s) and variable collection angle ranging from 1 to 20 mrad. The design of this instrument and its application to the measurement of optical scatter from laser damage and final finishing process of DKDP are discussed.

Laser conditioning has been shown to improve the laser damage threshold of some optical coatings by greater than 2x. Debate continues within the damage community regarding laser-conditioning mechanisms, but it is clear that nodular ejection is one of the byproducts of the laser conditioning process. To better understand why laser conditioning is so effective, photothermal microscopy was used to measure absorption of coating defects before and after laser exposure. Although a modest absorption reduction was expected due to the lower electric field peaks within a pit and the absence of potentially absorbing nodular seeds, surprisingly, absorption reductions up to 150x were observed. Photothermal microscopy has also been successfully used to correlate laser-induced damage threshold and absorption of defects in hafnia/silica multilayer optical coatings. Defects with high absorption, as indicated by high photothermal signal, have low damage thresholds. Previously a linear correlation of damage threshold and defect photothermal signal was established with films designed and damage tested at 1(omega) (1053 nm) and Brewster's angle (56.4 degree(s)), but characterized by photothermal microscopy at 514.5 nm and near-normal angle of incidence (10 degree(s)). In this study coatings designed, characterized by photothermal microscopy, and damage tested at the same wavelength, incident angle, and polarization did not have a correlation between defect photothermal signal and absorption.

Laser conditioning by raster scanning DKP and DKDP crystals using Nd:YAG and XeCl excimer laser systems was demonstrated. The laser systems were evaluated to determine their respective feasibility of improving the damage thresholds of the harmonic materials for use on the National Ignition Facility (NIF). Crystals were first evaluated using an Nd:YAG laser (355 nm, 7.6 ns) by scanning 2 x 2 cm² areas with sub-damage threshold fluences and then performing unconditioned (S/1) damage tests at 355-nm in the respectively scanned regions. Subsequently, five KDP and DKDP samples of various damage quality were raster scanned in a similar fashion at MicroLas GmbH (Goettingen, Germany) using a commercial Lambda Physik Excimer system (XeCl, (lambda) equals 308 nm, 20 ns). The samples treated in Germany were then tested at Livermore National Laboratory (LLNL) at 355 nm to demonstrate the excimer's potential as an alternative conditioning source.

Damage growth in optical materials used in large aperture laser systems is an issue of great importance when determining component lifetime and therefore cost of operation. Understanding the mechanisms and photophysical processes associated with damage growth are important in order to devise mitigation techniques. In this work we examined plasma-modified material and cracks for their correlation to damage growth on fused silica and DKDP samples. We employ an in-situ damage testing optical microscope that allows the acquisition of light scattering and fluorescence images of the area of interest prior to, and following exposure to a high fluence, 355-nm, 3-ns laser pulse. In addition, high-resolution images of the damage event are recorded using the associated plasma emission. Experimental results indicate that both aforementioned features can initiate plasma formation at fluences as low as 2 J/cm². The intensity of the recorded plasma emission remains low for fluences up to approximately 5 J/cm² but rapidly increases thereafter. Based on the experimental results, we propose as possible mechanisms leading to damage growth the initiation of avalanche ionization by defects at the damage modified material and presence of field intensification due to cracks.

Recent LLNL experiments reported elsewhere at this conference explored the pulse length dependence of 351 nm bulk damage incidence in DKDP. The results found are consistent, in part, with a model in which a distribution of small bulk initiators is assumed to exist in the crystal, and the damage threshold is determined by reaching a critical temperature. The observed pulse length dependence can be explained as being set by the most probable defect capable of causing damage at a given pulse length. Analysis of obscuration in side illuminated images of the damaged region yields estimates of the damage site distributions that are in reasonable agreement with the distributions experimentally directly estimated.

General physical relations connect the expected size and depth of laser damage induced craters to absorbed laser energy and to the strength of the material. In general, for small absorbers and instantaneous energy release, one expects three regions of interest. First is an inner region in which material is subjected to high pressure and temperature, pulverized and ejected. The resultant crater morphology will appear melted. A second region, outside the first, exhibits material removal due to spallation, which occurs when a shock wave is reflected at the free surface. The crater surface in this region will appear fractured. Finally, there is an outermost region where stresses are strong enough to crack material, but not to eject it. These regions are described theoretically and compared to representative observed craters in fused silica.

We investigated chemical etching as a possible means to mitigate the growth of UV laser-induced surface damage on fused silica. The intent of this work is to examine the growth behavior of existing damage sites that have been processed to remove the UV absorbing, thermo-chemically modified material within the affected area. The study involved chemical etching of laser-induced surface damage sites on fused silica substrates, characterizing the etched sites using scanning electron microscopy (SEM) and laser fluorescence, and testing the growth behavior of the etched sites upon illumination with multiple pulses of 351- nm laser light. The results show that damage sites that have been etched to depths greater than about 9 micrometers have about a 40% chance for zero growth with 1000 shots at fluences of 6.8-9.4 J/cm². For the etched sites that grow, the growth rates are consistent with those for non-etched sites. There is a weak dependence of the total fluorescence emission with the etch depth of a site, but the total fluorescence intensity from an etched site is not well correlated with the propensity of the site to grow. Deep wet etching shows some promise for mitigating damage growth in fused silica, but fluorescence does not seem to be a good indicator of successful mitigation.

The effective lifetime of optics in the UV is limited both by laser induced damage and the subsequent growth of laser initiated damage sites. We have measured the growth rate of laser induced damage in fused silica in both air and vacuum. The data shows exponential growth in the lateral size of the damage site with shot number above threshold fluence. The concurrent growth in depth follows a linear dependence with shot number. The size of the initial damage influences the threshold for growth; the morphology of the initial site depends strongly on the initiating fluence. We have found only a weak dependence on pulse length for growth rate. Low fluence conditioning in air may delay the onset of growth. Most of the work has been on bare substrates but the presence of a sol-gel AR coating has no significant effect.

Multi-shot investigations of Ti:sapphire laser (wavelength (lambda) approximately equals 800 nm) induced damage were performed in three different laboratories (BAM, Berlin; LZH, Hannover; UNM, Albuquerque). The ablation behavior of a high reflecting mirror consisting of alternating (lambda) /4- layers of Ta₂O₅ and SiO₂ was studied. Fused silica served as substrate. The influence of the pulse duration ((tau) equals 13 - 130 fs), the pulse number (30 - (infinity) ) and the repetition rate (10 Hz - 100 MHz) on the damage threshold will be discussed.

Previous investigations indicate that oxide coatings exhibit non-linear absorption phenomena below 200 nm. Hereby, absorption data of Al₂O₃ thin film coatings has been determined absolutely by laser calorimetry (LCA) at 193 nm in the low fluence regime. As an alternative, on the basis of the pulsed surface thermal lens technique (STL), photothermal measurements allow to determine the absorption relatively at fluence levels both in the subdamage fluence range far from the damage onset and close to the LIDT. By combining the two measurement techniques, the absolute determination of linear as well as multiphoton absorption can be achieved also in the vicinity of the laser damage fluences. This is of crucial interest because the initiation of damage onset can be observed immediately. Absolute absorption data of Al₂O₃ coatings at different laser fluences stating of some mJoule/cm² will be presented for the wavelength 193 nm. Thus, the correlation between the increase of absorption and the onset of breakdown can be illustrated impressively. The evaluation and discussion of the experimental results are focused on the degree of non-linearity of the investigated absorption behavior of oxide single layers initiating the optical breakdown of UV oxide coatings.