Laser-induced damage growth of fused silica optics near growth threshold at 351 nm.

Laser-induced damage growth on the exit surface of fused silica optics triggered by nanosecond pulses at 351 nm is widely described with exponential dynamics. In this Letter, a particular experimental setup allowed us to study damage growth with a large beam and fluences near damage growth threshold for a high number of shots. This allowed us to observe and characterize a regime with a slow and linear growth dynamic not documented in the literature and yet fundamental for the operation of high-power laser installations.

Pulse duration dependent optical nonlinearities of Bi2Se3 thin films.

Topological insulators, such as the Bi2Se3 material, exhibit significant optical nonlinearities. This work investigates the impact of the pulse duration on the nonlinear optical responses of Bi2Se3 layers. Scanning electron microscopy studies have been performed to reveal the crystalline structure of the samples. The nonlinear optical performance has been investigated for a wide range of pulse durations, from 400 fs to 10 ps, using 1030 nm laser excitation. The nonlinear absorption coefficients recorded in this study range from -1.45 x10-7 m/W to -4.86 x10-7 m/W. The influence of two different mechanisms on optical nonlinearities was observed and discussed. Identical experimental conditions have been employed throughout the studies allowing a direct comparison of the results.

Parametric study of laser-induced damage growth in fused silica optics with large beams at 351 nm. Part 1: stochastic approach.

Both the rate and probability of the growth of laser-induced damage sites in fused silica depend on several parameters. In this two-part paper, we investigate the impact of the laser parameters on damage growth. In Part I, we present statistical measurements of damage growth at different energy densities, pulse durations, and initial damage sizes. In Part II, we use fractal analysis to quantify the evolution of the damage morphology as a function of the laser energy density and pulse duration. Damage initiation is performed using phase masks. These phase masks allow for the initiation of evenly spaced damage sites that can then be exposed to the same laser beam, and, therefore, the same pulse duration. This configuration allowed the study of damage growth in a large population of more than 5000 damage sites. The results clearly indicate that both the probability and the rate at which a damage site will grow strongly depend on the laser pulse duration. These differences can be explained by hypotheses that we have developed from an observation of the bulk damage morphology. Such observations will be presented in detail in the second part of this article.

Parametric study of laser-induced damage growth in fused silica optics with large beams at 351 nm. Part 2: fractal analysis.

The impact of laser fluence and pulse duration on both the rate and probability of growth of laser-induced damage sites has been reported and analyzed statistically in a companion paper. In this paper, we report and analyze the volume morphology of damage sites during the growth process in fused silica optical components, at 351 nm, under various laser fluence and pulse durations. Fractal analysis has been used to quantify the bulk damage morphology. A clear link between the damage morphology and laser pulse duration has been observed. The results from fractal analysis allows for a better understanding of the results from the stochastic approach developed in our companion paper. More specifically, fractal analysis shows how the laser parameters such as fluence and pulse duration impact the phenomenology and the dynamics of the growth process.

Giant ultrafast optical nonlinearities of annealed Sb2Te3 layers.

The optimization of thin Sb2Te3 films in order to obtain giant ultrafast optical nonlinearities is reported. The ultrafast nonlinearities of the thin film layers are studied by the Z-scan technique. Giant saturable absorption is obtained, which is the highest ever reported, by means of the Z-scan technique.

Quantification of laser-induced damage growth using fractal analysis.

Lateral and longitudinal laser damage growth under subsequent irradiations at 351 nm in the nanosecond range from micrometric to millimetric scales is presented herein. Atypical behavior has been observed, showing the growth in the longitudinal direction, whereas the lateral growth does not evolve. We propose the use of fractal analysis to describe the evolution of the bulk damage morphology. The results indicate first a dependence between the damage fractal dimension and the laser parameters, such as the fluence and the pulse duration. Next, it seems from observations that the damage morphology modifications drive the growth rate changes.

Investigations on laser damage growth in fused silica with simultaneous wavelength irradiation.

The laser-induced damage growth phenomenon is experimentally studied for damage sites on the exit surface of fused silica. The sites are irradiated by nanosecond laser pulses at 1064 and 355 nm separately and also simultaneously. The results in the single wavelength configurations are expressed in terms of the probability of growth and growth coefficient. For growing sites, a fluence correction expression is proposed in order to take into account the millimetric Gaussian profile of the beams. The use of this expression is necessary to obtain results that are consistent with the ones obtained in the existing literature with large homogeneous beams. In the multiple wavelengths configuration, the results are expressed as a function of the laser fluences at each wavelength and are found to be closely related to the parameters determined in the single wavelength experiments. A coupling between the two wavelengths is quantified, and could originate from the formation and the expansion of a plasma produced both in the center and at the periphery of the damage sites.

Nanosecond UV laser-induced fatigue effects in the bulk of synthetic fused silica: a multi-parameter study.

Multiple-pulse S-on-1 laser damage experiments were carried out in the bulk of synthetic fused silica at 355 nm and 266 nm. Two beam sizes were used for each wavelength and the pulse duration was 8 ns. The results showed a fatigue effect that is due to cumulative material modifications. The modifications have a long lifetime and the fatigue dynamics are independent of the used beam sizes but differ for the two wavelengths. Based on the fact that, in the context of material-modification induced damage, the damage thresholds for smaller beams are higher than for larger beams, we discuss possible mechanisms of damage initiation.

Photoluminescence characterization of KH2PO4 crystal: application to three-dimensional growth-sector identification.

In this work, rapidly grown KH2PO4 (KDP) crystals extracted from the prismatic and the pyramidal growth sectors of crystal boules were analyzed using photoluminescence measurements. From the spectra, we deduced a robust criterion to discriminate between both growth sectors in an unknown KDP plate. Moreover, spatially resolved photoluminescence was shown to enable a local probing of different planes in the bulk of the material leading to accurate and nondestructive three-dimensional mapping of the sector boundary, which is often the weakest point in terms of laser-damage resistance in rapidly grown KDP crystals.

Contrasted material responses to nanosecond multiple-pulse laser damage: from statistical behavior to material modification.

This work is dedicated to the study of so-called fatigue effects upon nanosecond laser-induced damage of several crystalline materials and synthetic fused silica irradiated by multiple pulses. The obtained damage probability versus fluence and pulse number data are exploited to determine if the observed fatigue is due to statistics (the more often the material is irradiated, the higher the probability for it to be damaged) or to material modification under irradiation. Whereas 1064 nm irradiation seems to be responsible for statistic behavior, 355 nm irradiation generates material modifications in the case of synthetic fused silica.

Nanosecond-laser-induced damage in potassium titanyl phosphate: pure 532 nm pumping and frequency conversion situations.

Nanosecond-laser-induced damage measurements in the bulk of KTiOPO4 (KTP) crystals are reported using incident 532 nm light or using incident 1064 nm light, which pumps more or less efficient second harmonic generation. No damage threshold fatigue effect is observed with pure 532 nm irradiation. The damage threshold of Z-polarized light is higher than the one for X- or Y-polarized light. During frequency doubling, the damage threshold was found to be lower than for pure 1064 or 532 nm irradiation. More data to quantify the cooperative damage mechanism were generated by performing fluence ramp experiments with varying conditions and monitoring the conversion efficiency. All damage thresholds plotted against the conversion efficiency align close to a characteristic curve.

Multiple pulse nanosecond laser induced damage study in LiB3O5 crystals.

Multiple pulse nanosecond laser induced damage in the bulk of LiB3O5 (LBO) crystals was investigated at 1064 nm, 532 nm and 355 nm. Scanning electron microscopy of cleaved damage sites confirmed the presence of different zones that have already been reported in the case of KH2PO4 (KDP). Multi pulse measurements reveal a strong decrease of the damage threshold with increasing pulse number at 1064 nm (fatigue effect). A weaker fatigue effect was observed at 532 nm and no fatigue effect was found at 355 nm. This observation is best explained by an inherently statistical light matter interaction generating laser induced damage. Finally, a polarization dependent damage threshold anisotropy was evidenced at all three wavelengths, being strongest at 1064 nm. The results indicate the importance of Li+ vacancy stabilized color centers for the damage mechanism.

Laser-induced damage of KDP crystals by 1omega nanosecond pulses: influence of crystal orientation.

We investigate the influence of THG-cut KDP crystal orientation on laser damage at 1064 nm under nanosecond pulses. Since laser damage is now assumed to initiate on precursor defects, this study makes a connection between these nanodefects (throughout a mesoscopic description) and the influence of their orientation on laser damage. Some 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 res ts, we propose a laser damage model based on ellipsoid-shaped defects. This modeling is a refined implementation of the DMT model (Drude Mie Thermal) [Dyan et al., J. Opt. Soc. Am. B 25, 1087-1095 (2008)], by introducing absorption efficiency calculations for an ellipsoidal geometry. Modeling simulations are in good agreement with experimental results.

Nanosecond laser induced damage in RbTiOPO4: the missing influence of crystal quality.

Nanosecond laser induced damage in RbTiOPO(4) (RTP) an isomorphic material to the more widely known KTiOPO(4) (KTP) is studied in crystals with varying properties. The ionic conductivity along the z-axes of the tested crystals ranged from 1.5 10(-9) S/cm to 1.1 10(-12) S/cm. Further, different growth sectors with different absorption in the range of hundreds of ppm/cm and differing zones in inhomogeneous crystals have been investigated. Despite these important differences in crystal quality, no significant difference could be observed in the laser damage resistance at 1064 nm. Thus growth induced defects only play a minor role in nanosecond laser induced damage in RTP. Transient, laser induced defects are discussed in analogy with KTP as possible laser damage precursors.

Multiscale analysis of the laser-induced damage threshold in optical coatings.

We have investigated the influence of laser beam size on laser-induced damage threshold (LIDT) in the case of single- and multiple-shot irradiation. The study was performed on hafnia thin films deposited with various technologies (evaporation, sputtering, with or without ion assistance). LIDT measurements were carried out at 1064 nm and 12 ns with a spot size ranging from a few tens to a few hundreds of micrometers, in 1-on-1 and R-on-1 modes. These measurements were compared with simulations obtained with the statistical theory of laser-induced damage caused by initiating inclusions. We show how to obtain information on the initiating defect properties and the related physical damage mechanisms with a multiscale study. Under certain conditions, it is possible with this method to discriminate different defects, estimate their densities, and follow the evolution of the defects under multiple irradiation. The different metrology implications of our approach, particularly for obtaining a functional LIDT of optical components are discussed.

Laser damage resistance of hafnia thin films deposited by electron beam deposition, reactive low voltage ion plating, and dual ion beam sputtering.

A comparative study is made of the laser damage resistance of hafnia coatings deposited on fused silica substrates with different technologies: electron beam deposition (from Hf or HfO(2) starting material), reactive low voltage ion plating, and dual ion beam sputtering. The laser damage thresholds of these coatings are determined at 1064 and 355 nm using a nanosecond pulsed YAG laser and a one-on-one test procedure. The results are associated with a complete characterization of the samples: refractive index n measured by spectrophotometry, extinction coefficient k measured by photothermal deflection, and roughness measured by atomic force microscopy.

Effect of laser irradiation on silica substrate contaminated by aluminum particles.

A major issue in the use of high-power lasers, such as the Laser Megajoule (LMJ), is laser-induced damage of optical components. One potential damage initiator is particulate contamination, but its effect is hard to distinguish from that of other damage precursors. To do so, we introduced artificial contaminants typical of metallic pollution likely to be present on the optical components of the LMJ chains. More precisely, aluminum particles of two different sizes were placed on a silica sample. These dots were characterized by optical microscopy and profilometry. Then they were exposed to a laser beam with a pulse length of 6.5 ns at 1064 nm and fluences in the range from 1 to 40 J/cm(2). Each dot was characterized again with the same techniques and also by photothermal microscopy. To complete the experimental results, we performed numerical simulations with a one-dimensional Lagrangian hydrodynamics code. We show that the particle removal by laser irradiation produces a modification of the silica surface that does not evolve into catastrophic damage under subsequent irradiation. However, the effect does depend on the size of the dots. We demonstrate that a procedure exists that removes the dot and leaves the site capable of resisting high fluence.

High-resolution photothermal microscope: a sensitive tool for the detection of isolated absorbing defects in optical coatings.

The photothermal deflection technique allows us to highlight the presence of inhomogeneities of absorption in optical components. This nondestructive tool is of great interest to the study of the role of contaminants, inclusions, and impurities in the laser-induced damage process. We show that the detection of nanometer-sized isolated absorbing defects requires the development of an adapted photothermal setup with high detectivity and high spatial resolution. Thus it is essential to improve the resolving power up to its theoretical limit.

Optimized metrology for laser-damage measurement: application to multiparameter study.

An automatic test apparatus for refined testing of laser damage is presented that permits an in situ analysis of the tested area before, during, and after pulsed irradiation. Spatial and temporal beam profiling are performed in real time and give access to the localized fluence for each shot. Furthermore, an optimization of the initiation of damage detection is undertaken by use of image processing and yields a resolution better than 1 microm. Through several examples, these conditions are demonstrated to be useful for reaching an understanding of the laser-damage process. A complete study is undertaken of different kinds of glass that permits the main influence of test parameters (shot frequency, shot number, beam profile variation, temporal and spatial meshing, ...) on the damage procees to be shown. The study was made for different test procedures (1:1, S:1, R:1) and completed by atomic-force microscope analysis. Evidence indicates that the upgrading of metrology associated with an automatic process offers new opportunities for understanding laser-induced damage mechanisms and for emphasizing specific effects such as damage initiation, damage growth, and conditioning for repetitive shots.

Localized pulsed laser interaction with submicronic gold particles embedded in silica: a method for investigating laser damage initiation.

Laser damage phenomena in fused silica are currently under study because of numerous related high power laser applications. Nanosized defects are believed to be responsible for some laser damage initiation. In order to predict and to quantify this initiation process, engineered submicronic gold defects were embedded in silica. The study of these samples by localized pulsed irradiation of isolated gold particles coupled with Nomarski, atomic force and photothermal microscope observations permits us to discriminate between two distinct stages of material modification: one detectable at the surface and the second in the neighbourhood of the embedded particle. Comparison between the observations and simulations results in good agreement if we assume that inclusion melting initiates the damage.