Laurent Gallais

Statistical study of single and multiple pulse laser-induced damage in glasses

Single and multiple pulse laser damage studies are performed in Suprasil silica and BK-7 borosilicate glasses. Experiments are made in the bulk of materials at 1.064microm with nanosecond pulses, using an accurate and reliable measurement system. By means of a statistical study on laser damage probabilities, we demonstrate that the same nano-precursors could be involved in the multiple shot and single shot damage process. A damage mechanism with two stages is then proposed to explain the results. Firstly, a pre-damage process, corresponding to material changes at a microscopic level, leads the precursor to a state that can induce a one-pulse damage. And secondly a final damage occurs, with a mechanism identical to the single shot case. For each material, a law is found to predict the precursor life-time. We can then deduce the long term life of optical elements in high-power laser systems submitted to multipulse irradiation.

Localized pulsed laser interaction with sub-micronic 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.

LIDT improvement of multilayer coatings by accurate analysis of fabrication steps

We present Laser Induced Damage Threshold (LIDT) results on multilayer components with different optical functions for near infrared applications. In this paper, we investigate the different fabrication steps of such functions. In particular, we show experimental results on surface preparation (polishing, cleaning,) and on deposition techniques (Ion beam assisted process) related to the materials involved in coatings. Laser damage tests are performed at 1064-nm with a 5-ns pulse Nd:Yag laser and experiments are made at surfaces of optical components using a 12 μm diameter focused beam. Accurate damage probability curves are plotted thanks to a reliable statistical measurement of laser damage. Use of a statistical model permits to deduce the densities of laser damage precursors. A systematic analysis of all the steps involved in fabrication allows then to build multilayer components with high laser induced damage resistance.

Comparison of numerical simulations with experiment on generation of craters in silica by a laser

The validation of numerical simulations of laser induced damage of fused silica requires detailed knowledge of the different parameters involved in the interaction. To approach the problem, we have performed simulations of laser energy deposition in spherical metallic defects and the surrounding fused silica. Our code DELPOR takes into account various laser/defect induced absorption mechanisms of SiO2, such as radiative ionization, avalanche and multiphotonic ionization. We have studied crater formation produced by the absorber explosion with a 2-D Lagrange-Euler code taking into account crack formation and propagation in the brittle material. To validate our simulations, we have made and tested samples of ultra-pure silica thin film, containing gold nanoparticles of diameter 0.6 μm. The fused silica coating could have three different thickness. We compare experiment and simulations for two laser irradiations at wavelengths 0.351 and 1.053 μm.

Toward an absolute measurement of LIDT

The improvement of LIDT value of optical components had lead to develop a lot of experimental setups and procedures of test around the world. In this context it is often very difficult to make accurate comparisons of laser damage threshold values between the different apparatus. The differences are due to the procedure of test, the spatial and temporal beam variations, the laser damage criterion and others. A specific laser damage testing apparatus, with an accurate damage initiation detection and allowing a real time acquisition of the different shot parameters, lead to exhibit the influence of each parameter on the damage process. Laser beam profiling is performed in real time and give access to the fluence for one pixel (0.2μm2), therefore an effective fluence or a pixel fluence can be calculated in order to reach an “absolute” threshold value. The metrology developed will be detailed and some results obtained on silica and BK7 at 1064nm and 355nm with different procedures of test will be presented and discussed to illustrate the aim of this study.

Laser ablation of fused silica induced by gold nanoparticles: comparison of simulations and experiments at λ=351 nm

Simulations of laser-fused silica interactions at 0.351 μm are a key issue in predicting and quantifying laser damage in large laser systems such as LIL and LMJ. Validation of numerical simulations requires detailed knowledge of the different parameters involved in the interaction. To concentrate on a simple situation, we have made and tested a thin film system based on calibrated gold nanoparticles (0.2-0.8 μm diameter) inserted between two silica layers. The fused silica overcoat was either 2 or 10 microns thick. We have performed simulations of laser energy deposition in the engineered defect (i.e. nanoparticle) and the surrounding fused silica taking into account various laser/defect induced absorption mechanisms of SiO2 (radiative ionization, avalanche and multiphotonic ionization). We have studied crater formation produced by the absorber explosion with a 2D Lagrange-Euler code taking into account crack formation and propagation in the brittle material. We discuss the influence of the defect depth (with respect to the surface) on the damage morphology. The simulations are compared with our experimental results.

Quantitative study of laser damage probabilities in silica and calibrated liquids: comparison with theoretical prediction

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, SL and SH, 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.

Multi-Wavelength Optical Monitoring for Infrared Complex Functions: Application to Process Improvement

Use of an infrared direct optical monitoring is presented for realization of complex stacks requiring optical performances on different spectral ranges for infrared applications. Such system is used for process improvement in laser damage application.

Calculated and thermally measured laser damage in metallic thin films as a function of pulse duration

Laser damage in metallic thin films is investigated in materials such as Gold and Nickel with different pulse durations from 1s to 10ns. In these materials, damage phenomena can be clearly investigated through thermal effects. Then, the heat equation is solved for monolayers which offers the prediction of transient temperature in each case. These results of calculations are compared to LIDT measurements at 1.O6μm and associated to morphology studies made by Atomic Force Microscope. Different mechanisms are highlighted, depending on the layer thickness. The experimental and theoretical results are found to agree well in the range of milliseconds to nanoseconds. This study offers a significant understanding for the design of metal-dielectric multilayers, such as broadband light absorbers, filters or mirrors.

Photothermal microscopy and laser damage in optical components

The development of applications of high power lasers require new characterization techniques for studying behavior of optical materials under intensive illumination, laser damage phenomena. Destructive investigations in silica have led to the conclusion that absorbing defects, typically a few nanometers in size, were responsible for laser damage initiation. The measured precursor densities are very low. The understanding of the true nature of these defects and damage phenomena requires the development of non destructive evaluation techniques with both high spatial resolution and high detectivity. The capability of collinear photothermal deflection to reach sub-micrometric resolution by reduction of the pump beam diameter has been theoretically and experimentally explored. Its ability to detect single absorbing particles has been studied. Currently 100-nm-diameter gold inclusions can be imaged with a signal-to-noise ratio of 8 at the wavelength 1064nm. Such a photothermal microscope has been coupled with an experimental set-up allowing damage threshold measurement at the same wavelength. Thus behavior of 100-nm-gold inclusions in silica can be studied under irradiation. Further improvements by decreasing wavelength, increasing modulation frequency and by using piezoelectric translation stages, will allow to study 10-nm-inclusions. We present an overview of last developments in the field of photothermal microscopy in connection with laser damage.