In the range of intensity 1013-1014 W/cm2, material processing with ultrashort laser pulses is characterized by the absence of a plasma shielding effect and a small heat affected zone. The understanding of sub-picosecond laser pulse effects involves detailed study of laser-solid interaction processes. The dependence on fluence and pulse duration of the ablation rate can be studied experimentally. To analyse the laser-metal ablation mechanisms, we have inserted an electron-ion decoupling into a hydrodynamic code. The hydrodynamic simulations reveal many details of the processes yielding ablation.
In order to understand the mechanisms of laser damage initiation, we study “model” samples constituted of pure silica seeded with 3 nm gold particles. Numerical simulations are performed with a 1-D hydrodynamic code to determine the laser light absorption by a spherical nanoparticle. This code also simulates the thermal conduction, radiative transfer and ionization by UV light emitted by the heated metallic particles. The setup used for experimental studies is a high resolution, high sensitivity photothermal microscope. This setup allows correlation between optical absorption and laser irradiation. We observe the silica transformation in terms of absorption modification as a function of the irradiation fluence. The morphology of irradiated samples surface is observed thanks to “Nomarski” and “atomic force” microscopy and compared to photothermal microscopy results. A correlation is observed between flaked silica and strongly absorbing areas.
In the context of high power laser applications, laser-induced-damage in fused silica is still a limitation which need more study. To obtain better understanding of induced-breakdown processes, pure silica "model" samples, seeded with 100-250 nm diameter gold nanoparticles have been prepared. The aim in using these samples is to observe the mechanism of damage initiation that can be attributed to inclusions of nano-metric size. These samples were studied in a series of experiments using a photothermal microscope coupled with an experimental set-up allowing damage threshold measurement at wavelength 1064 nm. This installation is of great interest because it enables us to combine the laser irradiation of the sample with the optical absorption measurement. An evaluation of the silica transformation as a function of the fluence of irradiation can thus be obtained from the experimental results. These experimental data are completed with "Nomarski" and "atomic force" microscope observations, and then interpreted. Finally, we compare our results to numerical simulations performed with a 1-D hydrodynamic code. These simulations indicate that the threshold for melting the gold inclusion as a function of the incident laser fluence exceeds the threshold at which the absorption of the inclusion decreases.
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.
With the aim of observing and simulating laser initiated damage, ultra-pure silica "model" samples, seeded with gold nanoparticles (diameters 2 - 5 nm), were prepared and exposed to 3 ns laser pulses at wavelength 355 nm. These samples enable us to study the mechanism of damage initiation caused by inclusions of nanometric size, which is the typical size of defects occurring in optical quality glass. The samples were studied in a series of experiments using a time-of-flight mass spectrometer at Argonne National Laboratory. This installation is of great interest because it enables us to combine the laser irradiation of the sample with the chemical identification of material ejected from the exposed surface. An evaluation of the quantity of gold atoms emitted during irradiation can thus be obtained from the experimental results. These experimental data are completed with “Nomarski” and “atomic force” microscope observations, and then interpreted. In particular, a comparison is made to numerical simulations obtained with our DELPOR code. An encouraging result is the existence of a pre-damage phase at very low fluences that is not detectable by classical optical devices.
In order to exhibit the role of laser damage precursor centers in silica, gold particles ranging from 0.2mm to 0.8mm have been included between two coatings of SiO2 deposited on silica substrates. UV and IR nano-second pulsed irradiations on samples with different SiO2 over-layer thicknesses (2, 5, 10 mm) has been performed. The damage morphologies observed with Nomarski and atomic force microscopes have shown to be dependent on fluence, wavelength and SiO2 over-layer thickness. In addition a localized irradiation study using 6mm spot size allows to aim accurately on an isolated particle. The measurement is compared to the experiments presented for a few hundred micrometers spot size. Indeed in this case we have to consider that several particles are simultaneously irradiated. A comparison between the respective morphologies could inform us on potential collective effects of the particles. The choice of experimental test conditions, and the whole associated results will be presented and discussed.
We have performed simulations of laser energy deposit in sub-micrometric spherical defects and the surrounding fused silica. We have studied crater generation produced by the absorber explosion with a 2D/3D Lagrange-Euler code taking into account crack formation and propagation in the brittle material. The comparison of the 2D simulations with experiment shows quite good agreement for shallow defects (depth < 2 μm). We have observed experimentally that the explosion of deeper absorbers results in a more complex crater morphology. Therefore we have begun performing 3D simulations in order to reproduce these features.
Depending on the defect depth, the cracks may not reach the surface and a crater doesn't appear. Nevertheless, those cracks or pre-existing cracks can contribute to efficient electric field enhancement and breakdown on the surface. Different types of cracks (size, inclination, filled with a material or not) were investigated and the 2D or 3D electromagnetic field distributions were computed using a finite element code.
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.
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.
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.
Simulations of laser matter interaction at intermediate energies is a key issue in predicting and quantifying laser damage in the LMJ or the NIF facilities. We have done simulations of laser interaction with several metals from the solid state to the plasma by solving the Helmholtz equation in our hydrodynamic code DELPOR. We are comparing our result to time dependent reflectivity measurements on Aluminum and Iron and discuss the influence of the transitions of phase of the materials in laser mater interaction.
KEYWORDS: Spatial filters, National Ignition Facility, Tantalum, Near field, Interferometers, Modulation, Streak cameras, Diagnostics, Near field optics, Solids
Beamlet measurements show that cone pinholes outperform other types tested with regard to both closure and back reflections. A +/- 150 (mu) rad stainless steel cone remained open for a 15.5 kJ, 10:1 contrast shaped pulse with +/- 7.5 (mu) rad of SSD divergence, which more than meets the requirements for a NIF ignition pulse. Measurements also showed the maximum tolerable pressures in the NIF spatial filters to be a few milliTorr, leading to recommendations of 0.1 nd 0.6 mTorr for the NIF transport and cavity spatial filters, respectively.
Absolute measurement of spherical surface by use of point diffraction interferometer (PDI) has been studied both theoretically and experimentally. By the estimation of optical error, 10-3 (lambda) rms can be expected as absolute accuracy. Experimental results with high accuracy have been obtained.
A major concern in Megajoules and NIF laser studies is the spatial filter pinhole closure. The answer to this problem is a new pinhole architecture and suitable materials. The conical pinhole design at LLNL is the best candidate tested on the NIF prototype: Beamlet. We have investigated the material dependence in terms of laser-solid and laser-plasma interaction. We have demonstrated that a proper choice of material delays the plasma build-up to the end of the ICF drive pulse. To model the pre-plasma step, it is necessary to take into account optical and thermal parameters of the material, and processes of transition, fusion and vaporization. Preliminary calculations predicted the observed material dependence.
On the basis of a microscopic theory for the interaction of intense laser irradiation with free electrons over the range from visible to near infrared, we calculate space-time energy deposition in SiO2 for several wavelengths and laser pulsewidths. Most of the parameters used in the simulations are extracted from a Monte Carlo integration of the Boltzmann transport equation. We investigate the relative roles of multiphoton ionization, avalanche multiplication and free carrier absorption as intrinsic processes of carrier generation and pulse absorption for different laser irradiation conditions. The intensity profile distortion in the time domain of the transmitted pulse due to the rapid build-up of the plasma density is discussed. In the field of ultrashort laser pulses, damages exhibit a morphology dramatically different from that observed with long pulses: thin layers of material are removed by ablation. Therefore, we interfaced the space-time energy deposition with a 1D Lagrangian hydrodynamic code and estimated thermomechanical effects: stress evolution, spalling.
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