This PDF file contains the front matter associated with SPIE Proceedings Volume 7842, including the Title Page, Copyright information, Table of Contents, International Program Committee, Symposium Welcome, Summary of Meeting, Abstracts, and Participant List listing.

Scandium oxide is an attractive candidate for the engineering of interference coatings, although not widely explored. This paper describes the ion beam sputtering of Sc₂O₃. It is shown that the structural properties of the material are affected by the deposition conditions. Laser damage in different regimes of pulsewidths is investigated. These results show that the 1-on-1 laser damage fluence, in both the thermal and deterministic regimes, varies with deposition conditions but this is not the case for S-on-1, indicating that laser-induced defects are important.

In this study, we report on our recent progress in research of single layer mixed zirconia-silica and niobia-silica composite coatings prepared by Ion Beam Sputtering technique. All coatings of the same optical thickness were characterized in terms of reflection/transmission spectrometry, X-ray diffraction, atomic force and optical microscopy, optical back-scattering and optical resistance (laser-induced damage threshold - LIDT) in subpicosecond mode. The optical resistance, TIS and LIDT results reveal clear dependence on high refractive index material content in composite coating and its crystalline structure. The results are interpreted and discussed by the means of different models available in literature.

One TiO₂/SiO₂ high reflector with absorption of 300 ppm and two HfO₂/SiO₂ high reflectors with absorption of 40 and 4.5 ppm were fabricated using electron beam evaporation method. The influence of average absorption on laser induced damage threshold (LIDT) at 1064 nm of these coatings was studied using r-on-1 test mode. The LIDT of high absorbing TiO₂/SiO₂ coating is only 6.2 J/cm² at 3 ns, whereas, two weak absorbing HfO₂/SiO₂ high reflectors got almost the same LIDT that was over 100 J/cm² at 3 ns. Then a raster scan test method was employed to find the typical damage morphologies of these coatings with different absorption level. For TiO₂/SiO₂ high reflector with high average absorption, the representative damage morphologies are shallow pits that were probably caused by absorbing centers. However, for HfO₂/SiO₂ high reflectors with low average absorption, the dominant damage morphologies are micrometer-sized nodules ejected pits and the delamination initiating from the pits. The absorption of HfO₂/SiO₂ coatings is low enough to have minor influence on the laser induced damage.

UV antireflection coatings are a challenging coating for high power laser applications as exemplified by the use of uncoated Brewster's windows in laser cavities. In order to understand the current laser resistance of UV AR coatings in the industrial and university sectors, a double blind laser damage competition was performed. The coatings have a maximum reflectance of 0.5% at 355 nm at normal incidence. Damage testing will be performed using the raster scan method with a 7.5 ns pulse length on a single testing facility to facilitate direct comparisons. In addition to the laser resistance results, details of deposition processes and coating materials will also be shared.

After several investigations in laser induced damage behavior of oxide mixtures of different compositions, also HfO₂ could be steplessly mixed with SiO₂. A study of SiO₂/HfO₂ IBS single layers and high reflectors is presented. Damage testing has been performed at 800nm and 355nm on an extensive set of single layers employing different mixture ratios of silica and hafnia. The analysis of the response of optical single layer coatings to femtosecond and nanosecond pulse exposure provides input for further coating designs, in particular for the optimization in respect to the damage threshold properties. A deeper understanding of the damage mechanisms is gained by comparing the ns and fs pulse results as a function of the mixing ratio.

Sandia's Large Optics Coating Operation has extensive results of laser induced damage threshold (LIDT) testing of its anti-reflection (AR) and high reflection coatings on substrates pitch polished using ceria and washed in a process that includes an alumina wash step. The purpose of the alumina wash step is to remove residual polishing compound to minimize its role in laser damage. These LIDT tests are for multi longitudinal mode, ns class pulses at 1064 nm and 532 nm (NIF-MEL protocol) and mode locked, sub-ps class pulses at 1054 nm (Sandia measurements), and show reasonably high and adequate laser damage resistance for coatings in the beam trains of Sandia's Z-Backlighter terawatt and petawatt lasers. An AR coating in addition to coatings of our previous reports confirms this with LIDTs of 33.0 J/cm² for 3.5 ns pulses and 1.8 J/cm² for 350 fs pulses. In this paper, we investigate both ceria and zirconia in doublesided polishing (common for large flat Z-Backlighter laser optics) as they affect LIDTs of an AR coating on fused silica substrates washed with or without the alumina wash step. For these AR coated, double-sided polished surfaces, ceria polishing in general affords better resistance to laser damage than zirconia polishing and laser damage is less likely with the alumina wash step than without it. This is supported by specific results of laser damage tests with 3.5 ns, multi longitudinal mode, single shot pulses at 1064 nm and 532 nm, with 7.0 ns, single and multi longitudinal mode, single and multi shot pulses at 532 nm, and with 350 fs, mode-locked, single shot pulses at 1054 nm.

A Petawatt facility called PETAL (PETawatt Aquitaine Laser) is under development near the LIL (Ligne d'Integration Laser) at CEA Cesta, France. PETAL facility uses chirped pulse amplification (CPA) technique. This system needs large pulse compression grating exhibiting damage threshold of more than 4 J/cm² normal beam at 1.053μm and for 500fs pulses. In this paper, we study an alternative design to the classic multilayer dielectric (MLD) grating called "mixed metal-multilayer dielectric grating" (MMLD). This design consists in a gold reflective layer coated with a few pairs of HfO₂/SiO₂. The top low index SiO₂ layer of the stack is then engraved to receive the grating. We evidenced in a previous work that leads to high efficient pulse compression gratings. We have shown in last Boulder Damage Symposium that mixed mirror is equivalent to a "classic" MLD mirror. We herein detail the damage performances obtained on the MMLD gratings and compare them with these of MLD gratings.

Nanosecond-pulse UV-laser-damage initiation in multilayer coatings comprised from metal oxide as a high-index component, and silica oxide as a low-index material, is strongly linked to metal oxide. The nature of the absorbing species and their physical properties remain unknown because of extremely small sizes. Previous experimental evidence provided by high-resolution mapping of damage morphology points to a few-nanometer scale of these absorbers. This work demonstrates submicrometer mapping of 355-nm absorption in HfO₂ monolayers using a recently developed photothermal heterodyne imaging technique. Comparison of absorption maps with spatial distribution of UV pulsed-laser- induced damage morphology allows one to better estimate the size and densities of nanoscale absorbing defects in hafnia thin films. Possible defect-formation mechanisms are discussed.

The laser induced damage thresholds (LIDT), N-on-1 test at 266 nm HfO₂/SiO₂ AR coatings, were measured for a 2 layer and 4 layer antireflection (AR) coating designs on fused silica. LIDT values for the 2 layer AR coating exhibited a constant threshold level over a wide range of increasing number of laser pulses. LIDT values for the 4 layer AR coating decreased relatively rapidly with increasing pulses in comparison. The projected lifetime of the 2 layer coating design was thus determined to be much longer than that of the 4 layer design. To explain the observed LIDT performance differences, this study effort employed the following metrological techniques: 1) Measurement of the surface roughness with a surface profile interferometer, 2) Analysis of material crystal structure with X-ray diffractometry, 3) Examination of surface damage morphology, 4) Spectrophotometric analysis of the reflectance of AR coatings, 5) Investigation of the electric filed distribution utilizing optical coating design software, and 6) Calculation of the maximum temperature rise.

HfO₂/SiO₂ dichroic mirrors were prepared using reactive electron beam evaporation process. The mirrors have high reflectance at 532 nm (S polarization) and high transmittance at 1064 nm (P polarization) for angle of incidence of 45°. Here we report the laser damage behaviors of the dichroic mirrors that were irradiated by 532 nm and 1064 nm nanosecond laser pulse separately. The influence of substrate polishing quality on the laser damage resistance of HfO₂/SiO₂ dichroic mirrors was also discussed. At 1064 nm, the nano-sized absorbers between coatings and substrate interface or subsurface are the precusors for creating damage. And the poor substrate polishing quality significantly decreases the laser induced damage threshold (LIDT). At 532 nm, two distinct damage behaviors initiating from visible nodules and nano-sized absorbers were observed. Nodules are ejected at low fluence but the ejected pits are very stable until quite high fluence around 20J/cm² (1 ns). The nano-sized absorbers trigger damage at medium fluence around 14J/cm² (1 ns), and this kind of damage grows very fast during subsequent irradiations. The substrate polishing quality has minor influence on the LIDT at 532 nm.

Two kinds of multilayer HfO₂/SiO₂ high reflectors, the "standard" and "modified" designs with different electric field distribution, were investigated in this paper. A femtosecond laser system was applied to test the laser-induced damage thresholds (LIDTs) of these two kinds of coatings. Optical microscopy and scanning electron microscopy (SEM) were employed to study the morphology and structural information of the damage site. Compared the LIDTs of "modified" design with that of "standard" design, it was found that reducing electric field at layer interface could improve the LIDTs of multilayer coatings. Furthermore, the improvement efficiency showed certain dependency on the pulse width. Diverse roles of several conceivable ionization mechanisms played in the damage process were discussed to explain the relation between the LIDTs and electric field distribution.

In order to evaluate the laser induced damage threshold (LIDT) of our HfO₂/SiO₂ high reflectance films prepared by reactive electron beam evaporation process at 1064nm and 532nm, the four popular test methods are performed according to the ISO 11254 and other relevant standards. The improvement of laser conditioning effect and influence of cumulative effect have been studied and estimated during the tests by comparing the deviations of the thresholds along the damage probability curves and relationship between the thresholds and the pulses number. Moreover, the details are carefully inspected during raster scan especially at 1064nm with all the >2μm nodules and damage sites marking and recording, then the ejection probability and growth rate of nodules are given. Note that attention should be paid to the submicron absorbing particulates at 532nm which are probable to trigger the damage.

Laser-induced damage thresholds of HR and AR coatings prepared by Japanese optics makers were measured at 10-ns pulse and 1064-nm wavelength. The data would be an important database to improve the damage threshold for makers, and to guide the designs of laser systems for optics users.

The paper presents results of investigations in evaporated LaF₃-MgF₂ and LaF₃-AlF₃ classical high-reflecting multilayers deposited on super-polished CaF₂ substrates. In addition to typical spectroscopic inspections up to the band edge of fluoride substances in the VUV spectral range, the work is dedicated to the determination of laser-induced damage threshold at moderate pulse numbers for the wavelength 193 nm. Further on, dose dependent irradiation tests are performed well below the fluence level of damage onset indicating changes for the spectral transfer functions. These experimental observations are discussed in order to find a correlation to the characteristic damage behavior of both material combinations. In contrast to the standard evaporation process, partial-reflecting fluoride coatings have been deposited under ion-assisted conditions with fluorine gas. Results of damage tests will show excellent performance to high fluence levels.

In this paper, we present test results and involved procedures of a comprehensive test campaign for S on 1 testing of laser optics with large test areas allowing the generation of a profound test database for further analysis. This database will serve as a starting point for an empirical study of the lifetime of laser optics, which will be discussed in companion paper somewhere in these proceedings. The optics are designed to operate as anti-reflective or high-reflective components at the respective test wavelengths for 0° angle-of-incidence. Both, coatings and substrates of 2.0 inch diameter are produced from the same batches to be as identical as possible. There were two different coating technologies used, e-beam and IAD e-beam, to explore a possible effect of the coating process on the long term laser irradiation behavior. The laser damage test bench is operated with a laser source delivering laser pulses in a single longitudinal mode at a repetition frequency of 100 Hz. The beam profile is of a Gaussian-shape and of high spatial quality at the fundamental Nd:YAG laser wavelength with a pulse duration of 3.5 ns at 1064 nm. Typical beam diameters on the samples were 400 μm, and usually more than 500 test sites are irradiated in one test to achieve statistical significance. The laser test procedure itself is adapted from the ISO standard 11254-2 for multiple pulse irradiations, and the LIDT evaluation is done accordingly.

Single shot LIDT of single layer coatings of different deposited materials (SiO₂, HfO₂, Ta₂O₅ and Nb₂O₅) have been studied. We report dependence of the damage threshold with different operational and material parameters (pulse duration, nature of the deposited material, deposition process or thickness of the layer). For interpretation a model dedicated to optical coatings and based on the conduction band electron rate equation is used. The simulations are compared to experiments. The theoretical approach is in good accordance to the experimental data.

The processes involved at the onset of damage initiation on the surface of fused silica have been a topic of extensive discussion and thought for more than four decades. Limited experimental results have helped develop models covering specific aspects of the process. In this work we present the results of an experimental study aiming at imaging the material response from the onset of the observation of material modification during exposure to the laser pulse through the time point at which material ejection begins. The system involves damage initiation using a 355 nm pulse, 7.8 ns FWHM in duration and imaging of the affected material volume with spatial resolution on the order of 1 μm using as strobe light a 150 ps laser pulse that is appropriately timed with respect to the pump pulse. The observations reveal that the onset of material modification is associated with regions of increased absorption, i.e., formation of an electronic excitation, leading to a reduction in the probe transmission to only a few percent within a time interval of about 1 ns. This area is subsequently rapidly expanding with a speed of about 1.2 μm/ns and is accompanied by the formation and propagation of radial cracks. These cracks appear to initiate about 2 ns after the start of the expansion of the modified region. The damage sites continue to grow for about 25 ns but the mechanism of expansion after the termination of the laser pulse is via formation and propagation of lateral cracks. During this time, the affected area of the surface appears to expand forming a bulge of about 40 μm in height. The first clear observation of material cluster ejection is noted at about 50 ns delay.

Previously we have shown that the size of laser induced damage sites in both KDP and SiO₂ is largely governed by the duration of the laser pulse which creates them. Here we present a model based on experiment and simulation that accounts for this behavior. Specifically, we show that solid-state laser-supported absorption fronts are generated during a damage event and that these fronts propagate at constant velocities for laser intensities up to 4 GW/cm². It is the constant absorption front velocity that leads to the dependence of laser damage site size on pulse duration. We show that these absorption fronts are driven principally by the temperatureactivated deep sub band-gap optical absorptivity, free electron transport, and thermal diffusion in defect-free silica for temperatures up to 15,000K and pressures < 15GPa. In addition to the practical application of selecting an optimal laser for pre-initiation of large aperture optics, this work serves as a platform for understanding general lasermatter interactions in dielectrics under a variety of conditions.

Knowing the ultimate surface morphology resulting from CO₂ laser mitigation of induced laser damage is important both for determining adequate treatment protocols, and for preventing deleterious intensification upon subsequent illumination of downstream optics. Physical effects such as evaporation, viscous flow and densification can strongly affect the final morphology of the treated site. Evaporation is a strong function of temperature and will play a leading role in determining pit shapes when the evaporation rate is large, both because of material loss and redeposition. Viscous motion of the hot molten material during heating and cooling can redistribute material due to surface tension gradients (Marangoni effect) and vapor recoil pressure effects. Less well known, perhaps, is that silica can densify as a result of structural relaxation, to a degree depending on the local thermal history. The specific volume shrinkage due to structural relaxation can be mistaken for material loss due to evaporation. Unlike evaporation, however, local density change can be reversed by post annealing. All of these effects must be taken into account to adequately describe the final morphology and optical properties of single and multiple-pass mitigation protocols. We have investigated, experimentally and theoretically, the significance of such densification on residual stress and under what circumstances it can compete with evaporation in determining the ultimate post treatment surface shape. In general, understanding final surface configurations requires taking all these factors including local structural relaxation densification, and therefore the thermal history, into account. We find that surface depressions due to densification can dominate surface morphology in the non-evaporative regime when peak temperatures are below 2100K.

We compare force fields (FF's) that have been used in molecular dynamic (MD) simulations of silica in order to assess their applicability for use in simulating IR-laser damage mitigation. Although pairwise FF's obtained by fitting quantum mechanical calculations such as the BKS and CHIK potentials have been shown to reproduce many of the properties of silica including the stability of silica polymorphs and the densification of the liquid, we show that melting temperatures and fictive temperatures are much too high. Softer empirical force fields give liquid and glass properties at experimental temperatures but may not predict all properties important to laser mitigation experiments.

Nano-sized absorbing defects take a part in the initiation of laser damage phenomena, especially in the nanosecond range of pulse duration. A special interest has been devoted to the modeling of photo-induced thermal effects in optical components. In this paper we present a 3D model based on the FEM method (Finite Elements Method) which allows the calculation of 3D temperature distribution in nanostructures with complex geometries irradiated by a laser. We present examples of application of the method in the field of laser damage. The purpose of these examples is to show the relevance of taking into account as precisely as possible the full opto-thermal and geometric characteristics of a system.

In preparation for experiments on the ORION laser facility, a number of investigations have been performed to characterise debris and shrapnel plumes arising from laser target interactions. These interactions may arise from the use of long [ns] pulse or short [ps] pulse laser beams with mainly solid targets. Plume characteristics depend on the target material and geometry. We describe interactions with metal or polymer targets and geometries including planar foils, cylinders, wires, or complex combinations. Characterisation techniques were based mainly on glass witness plates or aerogel catchers with subsequent analysis by optical or electron microscopy, spectro-photometry, and image analysis.

The aim of this preliminary study is to provide a simple model for estimating the laser-induced damage formation in potassium dihydrogen phosphate crystals (KH₂PO₄ or KDP) irradiated by nanosecond laser pulses operating at 351nm. In our modelling approach, a damaged site is assumed to be induced from a nanometric existing defect, i.e. a precursor defect. It makes it possible to absorb an important part of the incident laser energy which results in a damage formation by some processes which combine heating and hydrodynamic processes. In our model, the main expected features of the damage scenario are accounted for: the defect-assisted laser absorption and subsequent plasma formation and evolution, the plasma absorption, heat transfer and hydrodynamic processes via a simple Equation Of State (EOS). In these calculations, a crystal zone is assumed to damage since it undergoes high enough density variations. Calculations shows that a nanometric precursor defect can effectively lead to damaged site of several tens of micrometers in size as observed experimentally. Also, we demonstrate the reliability of the long-standing assumption regarding the precursor defect size. Furthermore, a particular morphology of the damaged site exhibiting various regions is obtained. These estimates have now to be confirmed especially by improving the EOS and by introducing an elasto-plastic behavior.

Nanosecond Laser-Induced Damage (LID) in potassium dihydrogen phosphate (KH₂PO₄ or KDP) remains an issue for light-frequency converters in large-aperture lasers such as NIF (National Ignition Facility, in USA) and LMJ (Laser MegaJoule, in France). In the final optic assembly, converters are simultaneously illuminated by multiple wavelengths during the frequency conversion. In this configuration, the damage resistance of the KDP crystals becomes a crucial problem and has to be improved. In this study, we propose a refined investigation about the LID mechanisms involved in the case of a multiple wavelengths combination. Experiments based on an original pump-pump set-up have been carried out in the nanosecond regime on a KDP crystal. In particular, the impact of a simultaneous mixing of 355 nm and 1064 nm pulses has been experimentally studied and compared to a model based on heat transfer, the Mie theory and a Drude model. This study sheds light on the physical processes implied in the KDP laser damage. In particular, a three-photon ionization mechanism is shown to be responsible for laser damage in KDP.

Numerous studies have investigated the role of photoionization in ultrafast laser-induced damage of bulk dielectrics. This study examines the role of spectral width and instantaneous laser frequency in laser-induced damage using a frequency dependent multiphoton ionization model and numerical simulation of an 800 nm laser pulse propagating through fused silica. When the individual photon wavelengths are greater than 827 nm, an additional photon is required for photoionization, reducing the probability of the event by many orders of magnitude. Simulation results suggest that this frequency dependence may significantly affect the processes of laser-induced damage and filamentation.

At ~10^-7 Torr, the multiple femtosecond pulse LIDT, F(∞), is about 10% of the single pulse damage fluence for hafnia and silica films compared to ~75% at 630 Torr. The 1-on-1 LIDT is pressure independent. The decrease of F(∞) is related to the water vapor and oxygen content of the ambient gas with the former having the largest effect. The decrease of F(∞) is associated with a change in damage morphology. In air, the damage "crater" starts at the center of the beam and grow in diameter as the fluence increases. In contrast, the damage starts at random "sites" within the exposed area under vacuum. Absorbing centers are created by the removal of oxygen from predisposed sites (for example grain boundaries) on the film surfaces, producing absorbing states.

We present a coupled study of laser-induced damage and ablation of fused silica in the femtosecond regime. Both thresholds are essentially different and investigations under a wide excursion of pulse duration (< 10 fs to 300 fs) and applied fluence (Fth < F < 10 Fth) provide quantitative knowledge on i) the strength of the so-called "deterministic" character of femtosecond laser damaging, linked to ionization mechanisms ; ii) the physical characteristics of surface ablation craters demonstrating that high selectivity and nanometric resolution is achievable.

A linear increase of the laser-induced damage thresholds in silica glasses with decreasing the temperature was reported in this conference at last year. Various nonlinear phenomena should be generated in silica glasses besides the damage in high intensity. Temperature dependences of the nonlinear refractive indices and the SBS (stimulated Brillouin scattering) thresholds in silica glasses at temperature 173 K to 473 K were measured with single-mode Q-switched Nd:YAG laser at fundamental wavelength. As the result, the nonlinear refractive indices increased with decreasing temperature. Because the change was not enough to explain the temperature dependence of laser-induced damage thresholds, the temperature dependence of nonlinear refractive indices would be negligible on laser-induced damage thresholds. On the other hand, the SBS thresholds also increased with decreasing temperature. This result means that acoustic phonons arise easily at high temperature. Probably, the SBS phenomenon is one of reasons for temperature dependence of laser-induced damage thresholds.

In this study, wet etch process was applied to expose the bulk damage sites in the Z-cut and X-cut KDP crystals to the surface, which gave us the direct access to the modified material for scanning electron microscopy (SEM) and optical microscopy. It's found that the morphology of damage site initiated with 1064nm laser was consisted of three distinct regions: a core, some oriented cracks spreading from the core, and a region of modified material surrounding the core. The 3D pattern of the crack was constructed from three distinct 2D projections, which were perpendicular to each other. The 355nm laser damage morphology was also investigated, and the wavelength dependence of laser damage morphology was presented. A simple model was proposed to explain the damage morphology, and the relation between the 3D crack patterns and crystal structure was discussed.

In the ultra short laser pulse regime, the damage process is driven by the interaction of the laser pulse with the electronic structure of the material. The way of excitations in dielectric materials is dominated by multi photon and avalanche ionization processes. Often, the complete theoretical description is limited by the lack of knowledge of the precise material properties. Usually, LIDT measurement data are only available for pure materials (e.g. TiO₂, Ta₂O₅ or SiO₂). The development of composite materials opens the way to vary material properties, continuously. Additionally, all material changes are based on the same chemical elements in different compositions. The paper compares measurement results of the University of New Mexico and Vilnius University performed on the same set of Ti_xSi_1-xO₂-mixtures to calculations based on Keldysh theory. When applying simple approximations for the physical properties of the mixture, the theoretical description agrees well with the measurement results.

We investigate the influence of THG-cut KDP crystal orientation on laser damage at 1064 nm under nanosecond pulses. This study makes a connection between precursor defects and the influence of their orientation on the laser damage. Previous investigations have already been carried out in various crystals and particularly for KDP, indicating propagation direction and polarization dependences. We performed experiments for two orthogonal positions of the crystal and results clearly indicate that KDP crystal laser damage depends on its orientation. We carried out further investigations on the effect of the polarization orientation, by rotating the crystal around the propagation axis. We then obtained the evolution of the damage probability as a function of the rotation angle. To account for these experimental results, we propose a model based on heat transfer, the Mie theory and a Drude model. The geometry of the precursor defects is assumed to be ellipsoid-shaped and we numerically introduce absorption efficiency calculations for this geometry. Modeling simulations are in good agreement with experimental results.

The multiple-pulse laser-induced breakdown behavior of dielectrics is modeled. The model is based on a critical conduction band (CB) electron density leading to dielectric breakdown. The evolution of the CB electron density during the pulse train is calculated using rate equations for the occupation and ionization of band and midgap states (native and laser induced). Using realistic estimations for the trap density and ionization cross-section, the model is able to reproduce the experimentally observed drop in the multiple-pulse damage threshold relative to the single-pulse value, as long as the CB electron density is controlled primarily by avalanche ionization seeded by multiphoton ionization of the traps and the valence band. The model shows that at long pulse duration, the breakdown threshold becomes more sensitive to presence of traps close (within one photon energy) to the conduction band. The effect of native and laser-induced defects can be distinguished by their saturation behavior. The model explains the principal behavior of the LIDT of a pair of pulses as a function of the temporal separation. Using the model, the observed transients can be related to rate constants of electrons leaving the CB and midgap states.

The study of laser induced damage threshold caused by series of identical laser pulses (LID-T-N) on gamma radiation resistant glasses and their analogs is performed applying know-how ultra stable laser radiation. The presented results and analysis of earlier received results show that nonlinear optical phenomena in extreme conditions of interaction are different from the traditional nonlinear optical processes, because they depend not only on intensity of electromagnetic field of laser radiation, but also on the pulse number in series of identical laser pulses. This range of laser intensities is not wide; it is different for each material and determines the range of Extreme Nonlinear Optics. The dependence of LID-T-N on pulse number N for different kinds of high quality transparent glasses was observed. The study of dynamics of these processes (i.e. the study of dependence on N) at different intensities in series of incident laser pulses provides new information about properties of the materials useful for studying laser damage fundamentals and their application. The expectation that gamma radiation resistant glasses could give useful information for technology of resistant optics for high power lasers has not proved. The received results well correspond with the earlier proposed model of laser damage.

Ultrashort-pulse laser irradiation may melt the target and, at higher intensities, lead to ablation. The state of the material shortly after irradiation is characterized by high temperatures and pressures (both tensile and compressive). In addition, the material may not yet be in thermal equilibrium. Molecular dynamics simulation is well suited to model the state of matter under these conditions. We give several examples of how molecular dynamics simulations have contributed to understanding the ablation phenomena after ultrashort-pulse laser irradiation.

Modeling of laser-induced ionization and heating of conduction-band electrons by laser radiation frequently serves as a basis for simulations supporting experimental studies of laser-induced ablation and damage of solid dielectrics. Together with band gap and electron-particle collision rate, effective electron mass is one of material parameters employed for the ionization modeling. Exact value of the effective mass is not known for many materials frequently utilized in experiments, e.g., fused silica and glasses. Because of that reason, value of the effective mass is arbitrary varied around "reasonable values" for the ionization modeling. In fact, it is utilized as a fitting parameter to fit experimental data on dependence of ablation or damage threshold on laser parameters. In this connection, we study how strong is the influence of variations of the effective mass on the value of conduction-band electron density. We consider influence of the effective mass on the photo-ionization rate and rate of impact ionization. In particular, it is shown that the photo-ionization rate can vary by 2-4 orders of magnitude with variation of effective mass by 50%. Impact ionization shows a much weaker dependence on effective mass, but it significantly enhances the variations of seed-electron density produced by the photo-ionization. Utilizing those results, we demonstrate that variation of effective mass by 50% produces variations of conduction-band electron density by 6 orders of magnitude. In this connection, we discuss the general issues of the current models of laser-induced ionization.

The mechanism of laser induced damage in optical materials under high power nanosecond laser irradiation is commonly attributed to the presence of precursor centers. Depending on material and laser source, the precursors could have different origins. Some of them are clearly extrinsic, such as impurities or structural defects linked to the fabrication conditions. In most cases the center size ranging from sub-micrometer to nanometer scale does not permit an easy detection by optical techniques before irradiation. Most often, only a post mortem observation of optics permits to proof the local origin of breakdown. Multi-scale analyzes by changing irradiation beam size have been performed to investigate the density, size and nature of laser damage precursors. Destructive methods such as raster scan, laser damage probability plot and morphology studies permit to deduce the precursor densities. Another experimental way to get information on nature of precursors is to use non destructive methods such as photoluminescence and absorption measurements. The destructive and non destructive multiscale studies are also motivated for practical reasons. Indeed LIDT studies of large optics as those used in LMJ or NIF projects are commonly performed on small samples and with table top lasers whose characteristics change from one to another. In these conditions, it is necessary to know exactly the influence of the different experimental parameters and overall the spot size effect on the final data. In this paper, we present recent developments in multiscale characterization and results obtained on optical coatings (surface case) and KDP crystal (bulk case).

Current methods for the manufacture of optical components inevitably leaves a variety of sub-surface imperfections including scratches of varying lengths and widths on even the finest finishes. It has recently been determined that these finishing imperfections are responsible for the majority of laser-induced damage for fluences typically used in ICF class lasers. We have developed methods of engineering subscale parts with a distribution of scratches mimicking those found on full scale fused silica parts. This much higher density of scratches provides a platform to measure low damage initiation probabilities sufficient to describe damage on large scale optics. In this work, damage probability per unit scratch length was characterized as a function of initial scratch width and post fabrication processing including acidbased etch mitigation processes. The susceptibility of damage initiation density along scratches was found to be strongly affected by the post etching material removal and initial scratch width. We have developed an automated processing procedure to document the damage initiations per width and per length of theses scratches. We show here how these tools can be employed to provide predictions of the performance of full size optics in laser systems operating at 351 nm. In addition we use these tools to measure the growth rate of a damage site initiated along a scratch and compare this to the growth measured on an isolated damage site.

The advantage of using mid-infrared (IR) 4.6 μm lasers, versus far-infrared 10.6 μm lasers, for mitigating damage growth on fused silica is investigated. In contrast to fused silica's high absorption at 10.6 μm, silica absorption at 4.6 μm is two orders of magnitude less. The much reduced absorption at 4.6 μm enables deep heat penetration into fused silica when it is heated using the mid-IR laser, which in turn leads to more effective mitigation of damage sites with deep cracks. The advantage of using mid-IR versus far-IR laser for damage growth mitigation under non-evaporative condition is quantified by defining a figure of merit (FOM) that relates the crack healing depth to laser power required. Based on our FOM, we show that for damage cracks up to at least 500 μm in depth, mitigation using a 4.6 μm mid-IR laser is more efficient than mitigation using a 10.6 μm far-IR laser.

In this paper, we report on the first steps in an empirical investigation into the nature of the laser survivability curve. The laser survivability curve is the onset threshold as a function of shot number. This empirical investigation is motivated by the desire to design a universal procedure for the measurement of the so-called S on 1 damage threshold. Analysis is carried on the test results for first results from a large set of planned measurements from identical samples produced for this investigation. The sample set and test conditions are discussed. A pair of measurements, one taken at atmospheric pressure and one at vacuum are introduced and analyzed as an example. Interim observations on the nature of the laser survivability curve, and its determination to be used in the remainder of this investigation based on this initial look, are presented at the conclusion of this paper.

Customized spatial light modulators have been designed and fabricated for use as precision beam shaping devices in fusion class laser systems. By inserting this device in a low-fluence relay plane upstream of the amplifier chain, "blocker" obscurations can be programmed into the beam profile to shadow small isolated flaws on downstream optical components that might otherwise limit the system operating energy. In this two stage system, 1920 × 1080 bitmap images are first imprinted on incoherent, 470 nm address beams via pixelated liquid crystal on silicon (LCoS) modulators. To realize defined masking functions with smooth apodized shapes and no pixelization artifacts, address beam images are projected onto custom fabricated optically-addressable light valves. Each valve consists of a large, single pixel liquid cell in series with a photoconductive Bismuth silicon Oxide (BSO) crystal. The BSO crystal enables bright and dark regions of the address image to locally control the voltage supplied to the liquid crystal layer which in turn modulates the amplitude of the coherent beams at 1053 nm. Valves as large as 24 mm × 36 mm have been fabricated with low wavefront distortion (<0.5 waves) and antireflection coatings for high transmission (>90%) and etalon suppression to avoid spectral and temporal ripple. This device in combination with a flaw inspection system and optic registration strategy represents a new approach for extending the operational lifetime of high fluence laser optics.

The Final Optics Damage Inspection (FODI) system automatically acquires and utilizes the Optics Inspection (OI) system to analyze images of the final optics at the National Ignition Facility (NIF). During each inspection cycle up to 1000 images acquired by FODI are examined by OI to identify and track damage sites on the optics. The process of tracking growing damage sites on the surface of an optic can be made more effective by identifying and removing signals associated with debris or reflections. The manual process to filter these false sites is daunting and time consuming. In this paper we discuss the use of machine learning tools and data mining techniques to help with this task. We describe the process to prepare a data set that can be used for training and identifying hardware reflections in the image data. In order to collect training data, the images are first automatically acquired and analyzed with existing software and then relevant features such as spatial, physical and luminosity measures are extracted for each site. A subset of these sites is "truthed" or manually assigned a class to create training data. A supervised classification algorithm is used to test if the features can predict the class membership of new sites. A suite of self-configuring machine learning tools called "Avatar Tools" is applied to classify all sites. To verify, we used 10-fold cross correlation and found the accuracy was above 99%. This substantially reduces the number of false alarms that would otherwise be sent for more extensive investigation.

Laser-induced contamination (LIC) is a phenomenon that can lead to the degradation of the properties of optical components in vacuum due to the formation of deposits in the area irradiated by a laser beam. The deposit growth is proposed to be the result of photochemical and photothermal mechanisms triggered by the interaction of UV laser radiation and outgassing species from polymeric materials on the surface of the optics. In the framework of ESA's ADM-Aeolus satellite mission, a successful test campaign has been performed, which has demonstrated the efficiency of several mitigation techniques against LIC for the ALADIN laser. These include the standard contamination control methods of identification of materials with particular propensity to cause LIC, reduction of the outgassing of organic materials by vacuum bakeout and shielding of optical surfaces from contamination sources as well as novel methods such as in-situ cleaning. These methods are now being applied at satellite level in order to guarantee the success of the mission. The subject of this paper is to summarise the various mitigation techniques from the large number of studies that have been performed and is applicable to any use of high power pulsed lasers in vacuum in the presence of organic contaminants.

This work is focused on the design of a 250W high-intensity continuous-wave fibre optic laser with a 15μm spot size beam and a beam parameter product (BPP) of 1.8 for its use on Laser-assisted Cold Spray process (LCS) in the micro-machining areas. The metal-powder deposition process LCS, is a novel method based on Cold Spray technique (CS) assisted by laser technology. The LCS accelerates metal powders by the use of a high-pressure gas in order to achieve flash welding of particles over substrate. In LCS, the critical velocity of impact is lower with respect with CS while the powder particle is heated before the deposition by a laser beam. Furthermore, LCS does not heat the powder to achieve high temperatures as it happens in plasma processes. This property puts aside cooling problems which normally happen in sintered processes with high oxygen/nitrogen concentration levels. LCS will be used not only in deposition of thin layers. After careful design, proof of concept, experimental data, and prototype development, it should be feasible to perform micro-machining precise work with the use of the highintensity fibre laser presented in this work, and selective deposition of particles, in a similar way to the well-known Direct Metal Laser Sintering process (DMLS). The fibre laser consists on a large-mode area, Yb³⁺-doped, semi-diffraction limited, 25-m fibre laser cavity, operating in continuous wave regime. The fibre shows an arguably high slope-efficiency with no signs of roll-over. The measured M² value is 1.8 and doping concentration of 15000ppm. It was made with a slight modification of the traditional MCVD technique. A full optical characterization will be presented.

Multilayer dielectric (MLD) diffraction gratings for Petawatt-class laser systems possess unique laser damage characteristics. Details of the shape of the grating lines and the concentration of absorbing impurities on the surface of the grating structures both have strong effects on laser damage threshold. It is known that electric field enhancement in the solid material comprising the grating lines varies directly with the linewidth and inversely with the line height for equivalent diffraction efficiency. Here, we present an overview of laser damage characteristics of MLD gratings, and describe a process for post-processing ion-beam etched grating lines using very dilute buffered hydrofluoric acid solutions. This process acts simultaneously to reduce grating linewidth and remove surface contaminants, thereby improving laser damage thresholds through two pathways.

Laser-induced damage is a key lifetime limiter for optics in large laser facilities. After tested on a large-aperture high-power laser facility, a damaged fused silica component is disassembled and conditioned to receive damage test on a small-aperture laser. The damage threshold and growth behavior show the corners on the component are less damage resistant. The acid etch on corner has not effectively increased the damage threshold but lowered the damage growth coefficient. A statistic-based model is presented to extrapolate the threshold data in small-aperture test to predict the damage threshold under functional conditions.

Two different concepts are introduced and compared to measure for the first time residual absorption in pure and doped fused silica fiber preform raw materials at 940 nm and 1550nm directly by means of the laser induced deflection (LID) technique in order to analyze the minimal achievable attenuation in high power fiber lasers based on the doped fused silica raw materials. It is found, that attaching a thin disk sample of the preform raw material to a practically non-absorbing substrate at the wavelength of investigation is the best measurement concept with respect to material consumption, absolute calibration and impact of the strong scattering in the doped raw materials on the measurement. For several Yb doped laser active raw materials the initial absorption value at 1550 nm (ranging from less than 50 dB/km to about 1800 dB/km) is compared to the total loss achieved for the manufactured fiber at 1200 nm, 1315 nm and 1600 nm, respectively. For some of the chosen materials the fiber loss is very comparable to residual raw material absorption indicating that the initial absorption is the dominant loss mechanism in the manufactured fiber. In contrast, for some fibers the total loss exceeds the values of the raw material absorption which allows the conclusion that additional loss mechanism like scattering, stress, geometrically fluctuation and micro or macro bending contributes to the fiber attenuation.

As high energy laser systems evolve towards higher energies, fundamental material properties such as the laserinduced damage threshold (LIDT) of the optics limit the overall system performance. The Z-Backlighter Laser Facility at Sandia National Laboratories uses a pair of such kiljoule-class Nd:Phosphate Glass lasers for x-ray radiography of high energy density physics events on the Z-Accelerator. These two systems, the Z-Beamlet system operating at 527nm/ 1ns and the Z-Petawatt system operating at 1054nm/ 0.5ps, can be combined for some experimental applications. In these scenarios, dichroic beam combining optics and subsequent dual wavelength high reflectors will see a high fluence from combined simultaneous laser exposure and may even see lingering effects when used for pump-probe configurations. Only recently have researchers begun to explore such concerns, looking at individual and simultaneous exposures of optics to 1064 and third harmonic 355nm light from Nd:YAG [1]. However, to our knowledge, measurements of simultaneous and delayed dual wavelength damage thresholds on such optics have not been performed for exposure to 1054nm and its second harmonic light, especially when the pulses are of disparate pulse duration. The Z-Backlighter Facility has an instrumented damage tester setup to examine the issues of laser-induced damage thresholds in a variety of such situations [2] . Using this damage tester, we have measured the LIDT of dual wavelength high reflectors at 1054nm/0.5ps and 532nm/7ns, separately and spatially combined, both co-temporal and delayed, with single and multiple exposures. We found that the LIDT of the sample at 1054nm/0.5ps can be significantly lowered, from 1.32J/cm² damage fluence with 1054/0.5ps only to 1.05 J/cm² with the simultaneous presence of 532nm/7ns laser light at a fluence of 8.1 J/cm². This reduction of LIDT of the sample at 1054nm/0.5ps continues as the fluence of 532nm/7ns laser light simultaneously present increases. The reduction of LIDT does not occur when the 2 pulses are temporally separated. This paper will also present dual wavelength LIDT results of commercial dichroic beam-combining optics simultaneously exposed with laser light at 1054nm/2.5ns and 532nm/7ns.

We report on femtosecond (fs) laser experiments yielding the time constants τ_rel for the non-radiative relaxation from optically excited high energy M_Na** states to the fluorescent M_Na* state in CaF₂ samples. The values obtained with the third and second harmonics of the fs laser amount to τ_rel (262 nm) = (3.0 ± 0.3) ps and to τ_rel (392 nm) = (1.0 ± 0.1) ps for the two selected M_Na** states at 4.7 eV (262 nm) and 3.2 eV (392 nm) excitation energy, respectively. These time constants were derived from depletion processes of the fluorescence at 740 nm (M_Na* state) using fs laser pulses of the NIR fundamental wavelength (785 nm) at variable delay relative to the UV fs laser pulses. In addition, photobleaching of the MNa centers upon UV fs laser irradiation is observed and simulated by assuming a constant fraction of MNa bleaching per pulse for a given laser fluence. This fraction ranges from 0.14% per pulse at 392 nm and 0.28mJ/cm² to about 1% per pulse at about 6 mJ/cm².

The measurement of absorption in dielectric materials is one of the most important methods for the qualification of the losses of optical components. For this reason, various procedures were developed for the measurement of the absorption losses. One of the most sophisticated and established technique is the laser calorimetric measurement according to ISO11551. The method allows to measure the absolute value of absorption losses. In the presented measurement campaign, two sets of samples are investigated by laser calorimetric measurements. The study displays the results of the linear and non-linear absorption measurements of Ti_xSi_1-xO₂ -single layers coated by ion beam sputtering. For the determination of the linear absorption behaviour a quasi CW-Laser at the wavelengths 532nm and 1064nm was applied Additionally, the non linear absorption is measured by a Ti:Sapphire CPA-Laser at 790nm.

Previous work on KDP has shown that thermal annealing could improve laser damage resistance of KDP optics at 3w. However, the improvement varies with the pulse length: whereas a strong improvement was observed at 16ns, no improvement at all was observed for a pulse length of 2.5ns. Whatever the pulse length, though, combinations of laser conditioning and thermal annealing led to better results than laser conditioning alone. The goal of this study is to verify if these results also hold for DKDP. A major difference is that, due to quadratic to monoclinic high temperature transition, the annealing temperature considered for KDP cannot be applied to KDP. This paper reports the temperature range considered for DKDP as well the modifications brought by thermal annealing on laser damage resistance at 12ns and 2.5ns.

The artificially grown calcium fluoride is one of key materials for microlithography and used for excimer laser optics etc. Such calcium fluoride is required high laser durability and laser induced bulk damage threshold (LIDT). However, the artificially grown calcium fluoride is not a complete crystal, and there are a lot of sub-grain boundaries inside the crystal that have the possibility of causing degradation of laser durability and LIDT. Moreover, mechanical properties of calcium fluoride are different according to the crystal axis, therefore there is a possibility that mechanical properties influences LIDT. In this study, we examined the relation between crystal structure and LIDT. First, we examined the relation between the crystal axis and LITD of single crystal calcium fruoride. The relation between the crystallographic axis and LIDT that the laser enters was examined. The ArF excimer laser and the fifth high harmonic of the Nd:YAG laser at 213nm were used for the irradiation source of light. We prepared samples that optical axes were <111>, <110> and <001> from the same crystal. From the result of this examination, when the laser irradiated in <111> axis, LIDT was the highest. Next, we observed the damage with polarizing microscope and optical microscope. The result of this observation suggested that the laser damage of calcium fluoride was related to the crystal orientation. Finally, we investigated the damage mechanism of calcium fluoride. It is thought that the laser irradiation induced stress is relaxed most easily when the optical axis is <111>. Therefore, LIDT of calcium fluoride is supposed to be highest when the optical axis is <111>.

Over the last eight years we have been developing advanced MRF tools and techniques to manufacture meter-scale optics for use in Megajoule class laser systems. These systems call for optics having unique characteristics that can complicate their fabrication using conventional polishing methods. First, exposure to the high-power nanosecond and sub-nanosecond pulsed laser environment in the infrared (>27 J/cm² at 1053 nm), visible (>18 J/cm² at 527 nm), and ultraviolet (>10 J/cm² at 351 nm) demands ultra-precise control of optical figure and finish to avoid intensity modulation and scatter that can result in damage to the optics chain or system hardware. Second, the optics must be super-polished and virtually free of surface and subsurface flaws that can limit optic lifetime through laser-induced damage initiation and growth at the flaw sites, particularly at 351 nm. Lastly, ultra-precise optics for beam conditioning are required to control laser beam quality. These optics contain customized surface topographical structures that cannot be made using traditional fabrication processes. In this review, we will present the development and implementation of large-aperture MRF tools and techniques specifically designed to meet the demanding optical performance challenges required in large aperture high-power laser systems. In particular, we will discuss the advances made by using MRF technology to expose and remove surface and subsurface flaws in optics during final polishing to yield optics with improve laser damage resistance, the novel application of MRF deterministic polishing to imprint complex topographical information and wavefront correction patterns onto optical surfaces, and our efforts to advance the technology to manufacture largeaperture damage resistant optics.

Substrate scratches can limit the laser resistance of multilayer mirror coatings on high-peak-power laser systems. To date, the mechanism by which substrate surface defects affect the performance of coating layers under high power laser irradiation is not well defined. In this study, we combine experimental approaches with theoretical simulations to delineate the correlation between laser damage resistance of coating layers and the physical properties of the substrate surface defects including scratches. A focused ion beam technique is used to reveal the morphological evolution of coating layers on surface scratches. Preliminary results show that coating layers initially follow the trench morphology on the substrate surface, and as the thickness increases, gradually overcoat voids and planarize the surface. Simulations of the electrical-field distribution of the defective layers using the finite-difference timedomain (FDTD) method show that field intensification exists mostly near the top surface region of the coating near convex focusing structures. The light intensification could be responsible for the reduced damage threshold. Damage testing under 1064 nm, 3 ns laser irradiation over coating layers on substrates with designed scratches show that damage probability and threshold of the multilayer depend on substrate scratch density and width. Our preliminary results show that damage occurs on the region of the coating where substrate scratches reside and etching of the substrate before coating does not seem to improve the laser damage resistance.