The original damage ring pattern at the exit surface of fused silica induced by highly modulated nanosecond infrared laser pulses demonstrates the time dependence of damage morphology. Such a damage structure is used to study the dynamics of the plasma issued from open cracks. This pattern originates from electron avalanche in this plasma, which simultaneously leads to an ionization front displacement in air and a silica ablation process. Experiments have shown that the propagation speed of the detonation wave reaches about 20 km/s and scales as the cube root of the laser intensity, in good agreement with theoretical hydrodynamics modeling. During this presentation, we present the different phases and the associated mechanisms leading to this peculiar morphology: • During an incubation phase, a precursor defect provides energy deposit that drives the near surface material into a plasma state. • Next the silica plasma provides free electrons in the surrounding air, under laser irradiation an electron avalanche is initiated and generates a breakdown wave. • Then this breakdown wave leads to an expansion of the air plasma. This latter is able to heat strongly the silica surface as well as generate free electrons in its conduction band. Hence, the silica becomes activated along the breakdown wave. • When the silica has become absorbent, an ablation mechanism of silica occurs, simultaneously with the air plasma expansion, resulting in the formation of the ring patterns in the case of these modulated laser pulses. These mechanisms are supported by experiments realized in vacuum environment. A model describing the expansion of the heated area by thermal conduction due to plasma free electrons is then presented. Next, the paper deals with the two damage formation phases that are distinguished. The first phase corresponds to the incubation of the laser flux by a subsurface defect until the damage occurrence: an incubation fluence corresponds to this phase. The second is related to the damage expansion that only refers to the energy deposit feeding the activation mechanism up to the end of the pulse: an expansion fluence corresponds to this phase. A striking feature is that the damage diameters are proportional to the fluence of expansion at a given shot fluence. Indirectly, the fluences of incubation by the precursors are then determined.
In order to control laser-induced shock processes, two main points of interest must be fully understood: the laser–matter interaction generating a pressure loading from a given laser intensity profile and the propagation of induced shock waves within the target. This work aims to build a predictive model for laser shock-wave experiments with two grades of aluminum at low to middle intensities (50 to 500 GW/cm2) using the hydrodynamic Esther code. This one-dimensional Lagrangian code manages both laser–matter interaction and shocks propagation. The numerical results are compared to recent experiments conducted on the transportable laser shocks generator facility. The results of this work motivate a discussion on the shock behavior dependence to elastoplasticity and fracturation models. Numerical results of the rear surface velocity show a good agreement with the experimental results, and it appears that the response of the material to the propagating shock is well predicted. The Esther code associated to this developed model can therefore be considered as a reliable predictive code for laser ablation and shock-wave experiments with pure aluminum and 6061 aluminum in the mentioned range of parameters. The pressure–intensity relationship generated by the Esther code is compared to previously established relationships.
In the context of high power laser applications, we study the effect of a heat treatment on CO2 laser mitigation of laser damage sites on fused silica samples. The isothermal annealing in a furnace is investigated and then compared to the local annealing by CO2 laser irradiation that is applied to enhance laser damage resistance on mitigated sites. Before and after isothermal annealing, we study the sites morphology, the evolution of residual stress and the laser-induced damage threshold measured at 355nm, 3ns. The results show that the initial laser damage probabilities were significantly improved after annealing at 1050°C for 12 hours. These results are compared to simulations with a thermo-mechanical model based on finite-element method.
Seeded nanosecond Q-switched Nd:YAG lasers working with an unstable resonator and a variable-reflectivity-mirror are widely used for they represent useful sources for stable and repeatable light-matter-interaction experiments. Moreover, in most setups, the fundamental wavelength is converted to higher harmonics. When the injection seeder is turned off, random longitudinal mode beating occurs in the cavity, resulting in strong variations of the temporal profile of the pulses. The generated spikes can then be ten times higher than the maximum of equivalent seeded pulses. This strong temporal incoherence is shown to engender spatial incoherence in the focal plane of such unseeded pulses leading to an instantaneous angular displacement of tens of µrad. This effect is even more pronounced after frequency conversion.
The influence of vacuum on nanosecond laser-induced damage at the exit surface of fused silica components is investigated at 1064 nm. In the present study, as previously observed in air, ring patterns surrounding laserinduced damage sites are systematically observed on a plane surface when initiated by multiple longitudinal modes laser pulses. Compared to air, the printed pattern is clearly more concentrated. The obtained correlation between the damage morphology and the temporal structure of the pulses suggests a laser-driven ablation mechanism resulting in a thorough imprint of energy deposit. The ablation process is assumed to be subsequent to an activation of the surface by hot electrons related to the diffusive expansion of a plasma formed from silica. This interpretation is strongly reinforced with additional experiments performed on an optical grating in vacuum on which damage sites do not show any ring pattern. Qualitatively, in vacuum, the intensity-dependent ring appearance speed V ∝ I1/2 is shown to be different than in air where V ∝ I1/3 . This demonstrates that the mechanisms of formation of ring patterns are different in vacuum than in air. Moreover, the mechanism responsible of the propagation of the activation front in vacuum is shown to be outdone when experiments are performed in air.
Scratches at the surface of fused silica optics can be detrimental for the performance of optical systems because they initiate damage on the optic but also they perturb the amplitude or phase of the transmitted laser light. Removing scratches by conventional polishing techniques can be time consuming as it is an iterative and long process, especially when hours of polishing time are required to obtain very high surface accuracy. So we have investigated ways to remove them with local laser processing. The silica is then heated at temperature higher than the softening point to heal the cracks.
The morphology of laser-induced damage sites at the exit surface of fused silica is tightly correlated to the mode
composition of the nanosecond laser pulses at 1064 nm. In the single longitudinal mode (SLM) configuration, a molten
and fractured central zone is surrounded by a funnel-shaped surface modification. Ring patterns surround the damage
sites when these are initiated by multiple longitudinal modes (MLM) laser pulses. In this last mode configuration, the
pulses temporal profiles as well as the damage ring patterns differ from pulse to pulse. The appearance chronology of the
rings is found to be closely related to the temporal shape of the laser pulses. This supports that the damage morphology
originates from the coupling of a laser-supported detonation wave propagating in air with an ablation mechanism in
silica. In our experiments, the propagation speed of the detonation wave reaches about 20 km/s and scales as the cube
root of the laser intensity, in good agreement with theory.
Localized CO2 laser heating of silica glass has demonstrated its ability to mitigate surface damage on optics used for
high power laser applications. The parameters for this process such as the power, the beam size and the exposition time
are however critical and some fundamental studies on the silica behavior under CO2 laser irradiation are required to
develop the processes. It is necessary for instance to understand the silica transformation, the material ejection and the
thermo-mechanical stresses induced by the laser heating and subsequent cooling. A thermo-mechanical model based on
finite-element method has been used to calculate the temperature of silica heated by CO2 laser irradiation and the
residual stress after cooling of the samples. The model, as the different parameters used for calculations, are detailed in
this paper and the numerical results are compared to different dedicated experimental studies.
The laser induced damage densities measured on fused silica surface are found to be higher when produced with multiple longitudinal mode pulses than those produced by single longitudinal mode pulses at 1064 nm. The enhancement of the three-photon absorption due to the intensity spikes related to longitudinal mode beating might favor the damaging process at this wavelength. At 355 nm the picture is different. The absorption is supposed to be linear and an opposite behavior occurs. Evidences of a process leading to the possible annealing of a part of absorbent defects are discussed in this paper.
Various scratches on fused silica optics after polishing have been characterized with confocal microscopy and then tested
with nanosecond UV laser. Scratches are identified as a major contributor to laser damage even if they are only a few
micrometers wide. We propose a process in order to remove these scratches whose depth ranges from 2 to 16 μm. We
use a CO2 laser to heat the scratched areas at high temperature which will heal fractures due to viscous flow. The
characterizations were completed by laser damage tests that finally prove the effectiveness of the repair. We conclude
also that this repair process proves to be fast, localized to the scratch and clean because no debris are generated.
CO2 laser is used to prolong the lifetime of large optics for high power lasers such as the NIF and LMJ.
Indeed, on silica optical components, damaged sites, whose diameter is in the order of tens of microns, appear at
high UV laser fluence, and the size of such sites increases exponentially with each UV laser shot. An intense
heat by CO2 laser ejects the material from the surface of the optical component and removes all fractures around
the damaged site so that this site will not be damaged at fluences of operation of the UV laser. A crater is formed
at the site of initial damage. But the intense heat creates debris and residual stress around this crater. Due to these
debris and stress, the optical component is again weakened. We show here that a second heating process, done
with different settings of the CO2, named here laser annealing, eliminates the debris and reduce stress. The
results presented here establish that annealing significantly improves the resistance of laser optics.
We investigate the residual stress field created near mitigated sites and its influence on the efficiency on the CO2 laser
mitigation of damage growth process. A numerical model of CO2 laser interaction with fused silica is developed that
take into account laser energy absorption, heat transfer, thermally-induced stress and birefringence. Specific photoelastic
methods are developed to characterize the residual stress near mitigated sites in fused silica samples. The stress
distribution and quantitative values of stress levels are obtained for sites treated with the CO2 laser in various conditions
of energy deposition (beam size, pulse duration, incident power). The results obtained also show that the presence of
birefringence/residual stress around the mitigated sites has a critical effect on their laser damage resistance.
The lifetime of silica optics in high power laser facility as the Laser MégaJoule (LMJ) is typically limited by the
initiation of surface damages and their subsequent growth. To prevent this problem, a mitigation technique is used: it
consists in a local melting of silica by CO2 laser irradiation on the damage site. Because of the difficulty to produce
efficient mitigated sites with large depth, the characterization of damage site to mitigate is very important. In this
context, confocal microscopy appears to be an efficient solution to detect precisely cracks present under the damage site.
Significant improvement in polishing processes of fused silica optical components, has increased optics lifetime at the
wavelength of 351 nm. Nonetheless, for large laser operation facilities like the Laser MegaJoule (LMJ), zero defect
optics are not yet available. Therefore a damage mitigation technique has been developed to prevent the growth of
initiated damage sites: this technique consists in a local melting and evaporation of silica by CO2 laser irradiation on the
damage site. Because of the difficulty to produce efficient mitigated sites with large depth, the initial depth of damage to
mitigate is a critical issue. An aim of our work was to determine the real extension of the damage site (including
fractures) for different laser pulse durations between 3 ns and 16 ns and at different laser fluences. The fractures are nondetectable
in conventional microscopy. The depth of the damage can thus be underestimated. Hence confocal microscopy, was used to observe these sub-surface fractures and to measure precisely the depth of damage. Results show that the damage is 2 to 4 times wider than deeper and this ratio is independent of the pulse duration and of the fluence. With this new information, the mitigation process can now be optimized.
Pollution is known to play an important role in optical performances of high power laser components. In this study, we propose a method to characterize the particulate pollution and to evaluate its impact on the optical component lifetime. We applied it at the Ligne d'Intégration Laser (LIL), which is a prototype installation of the future Laser Mega Joule facility. By systematic examination of each particle, we obtain, in number, size, and chemical composition, an overview of pollution deposited on optical surfaces. Then we deduce the origin of the principal pollutions. After irradiation of the polluted samples, with conditions close to the maximum of the LIL operation, no laser-induced damage is observed. In contrast, irradiation induces an important cleaning.
Contamination by metallic particles has been known to reduce the laser damage threshold on high power laser
optics. To simulate the presence of metallic particle on the Ligne d'Integration Laser optics, silica substrates
were arti.cially polluted by square aluminum dots of 5 × 5 micron2 and 50 × 50 micron2, respectively. The metallic dot
sites were irradiated by a Nd:YAG laser at 1064 nm with different fluences. The sites were analyzed by Nomarski
microscopy, optic profilometry and photothermal microscopy. For both sizes of metallic dots, vaporization of
metal can be observed. We study in this paper the dot size influence on the surface cleaning process and the
effect of the pre-irradiation mode (1 shoot or several shots).
To evaluate the impact of particulate contamination in laser induced damage of optical material, an
experimental program is established. The first step consists in the Ligne d'Integration Laser (LIL) particle
contamination sampling. Carbonated cellophane tapes, antireflection coated and uncoated silica samples were
inserted in the LIL laser chain, in six different zones to collect particles. The second step is the pollution
characterization. Polluted cellophane tapes are analysed by Scanning Electron Microscopy and Energy
Dispersive Spectrometry. The density and the nature of particles collected in the Amplification Section are
found to be homogenous throughout this section. The pollution collected in the Frequency Conversion and
Focusing system is more complex. One of its features is a larger proportion of silica particles. The last step
consists in the silica samples irradiation. Antireflection coated and uncoated silica samples are examined by
optical microscopy, then irradiated at 1064 nm or 355 nm and examined again. No damage growing under
several irradiations is observed. We show a cleaning effect efficient for particles larger than 20 microns.
During the life of a high-power laser chain, optical components may be damaged due to local high fluence levels in the
inhomogeneous beam. The origin of the laser damage can be impurities, surface defects or flaws and cracks resulting
from polishing, or it may be produced by self-focusing in the component. The aim of this study is to better understand
the correlation between a surface crack on a silica optical component and laser damage. To accomplish this, calibrated
indentations were made on silica samples. Observations of the sites were made with an optical microscope, and three
different morphologies were recognized. Then the zones containing the indentations were irradiated (single shot mode)
with a Nd Yag laser at 355 nm for various fluences. Subsequent observations of the sites were made with an optical
microscope, with the aim of correlating site morphology and laser-induced damage. Some sites were believed to have
undergone laser conditioning. They were further irradiated (raster scan mode) at high fluence, and some evidence for a
laser conditioning effect was obtained.
Correctly determining the lifetime of optical components is a major issue in the operation of high power laser facilities such as the Laser Megajoule developed by the Commissariat a l'Energie Atomique (CEA). Laser damage that occurs at the surface is a main cause of optical aging, and may lead to dramatic degradation of the focal spot. To estimate the effect of such defects, we measured and calculated the distortion of the focal spot induced by "model defects." These "model defects" are circular silica dots randomly distributed on a silica substrate. The experiments were conducted in the ANTALIA facility at the Centre d'Etude Scientifique et Technique d'Aquitaine (CESTA). We performed numerical calculations of beam propagation with the Miro software, developed by the CEA. We obtain a remarkable correlation between measurements and simulations in the central part of the focal spot for large defects. However, experimental noise and measurement dynamics become serious problems when we confine our attention to smallerdefects (<500 micron) or to the diffuse light around the central part of the focal spot. We present some modifications of the ANTALIA experimental setup designed to overcome these problems.
A major issue in high power lasers for fusion is laser-induced damage on optical components. Since damage is often initiated by a surface crack resulting from polishing, it is important to understand the physics involved in this phenomenon. In this study, calibrated surface scratches have been realized on two silica samples using an indenter-scratcher apparatus. A variety of scratches have been tested by applying different speeds and forces on the scratcher needle. Optical microscope observations show that the scratches made at highest speed create irregular dashed lines. In addition, we have observed, at intermediate speed, an evolution in time of the scratches due to local stresses in silica.
One of the samples was irradiated by a Nd:YAG laser beam at 355 nm with the scratches on the exit surface. Microscope observations were made before and after irradiation.
Strong dependence on the scratch speed was observed on the local laser damage. Again, temporal evolution of the damage has been observed.
One of the major issues met in the operating of high power lasers concerns the cleanliness of laser components. In this context, in order to assess laser-induced damage in presence of metallic particulate contamination, we study the behaviour of aluminum on a silica substrate. Model samples containing calibrated aluminum square dots of 50 x 50 μ2 have been deposited by photolithography on a silica substrate. The sample was irradiated by a Nd:YAG laser at 1064 nm with different fluences and also different numbers of shots on each dot. Then the initial aluminum dot zone and the surrounding silica were analyzed using Nomarski microscopy, profilometry and photothermal microscopy. Laser fluence is revealed to be a very important parameter for the behaviour of aluminum dots. For example, it is possible to find a fluence of irradiation where aluminum dots are blown off the substrate and only small modifications occur to silica. In this case, increasing the number of shots doesn't significantly affect the silica surface.
To obtain better understanding of particulate contamination, chromium dots (50 x 50 μm2) were deposited on a silica substrate by photolithography. The aim in using this sample is to observe the mechanism of damage initiation that can be attributed to surface contamination of micro-metric size. A Nd:YAG laser irradiated the sample at 1064 nm for different fluences and also different numbers of shots. Several methods were used to characterise the laser effects on the chromium dots and the silica substrate: "Nomarski", "atomic force" and photothermal microscope observations.
The laser fluence is found to be the most important parameter for the behaviour of the chromium dots. At low fluence (<1 J/cm2), they become cracked (fractured). At medium fluence (around 1 J/cm2) chromium fusion is reached and chromium oxide appears. Finally at higher fluence (3 J/cm2), although chromium dots are blown off the substrate and small damage to silica occurs on the first shot, the subsequent shots do not lead to a dramatic increase in the damage.
A major issue in high power lasers for fusion is laser-induced damage on optics and its evolution in time after a large number of shots. Since damage is often characterized by an initial surface crack, its surface usually increases, following an exponential law.
Surface scratches have been made on silica samples in order to get calibrated fractures. Then, to test different experimental conditions, we made a variety of scratches in terms of length and depth. The samples are then irradiated by a Nd:YAG laser first at 1064 nm (1w) then at 355 nm (3w). They are successively tested with the scratches facing the laser beam or placed with the scratches on the back surface.
In order to understand the role played by nanodefects in optical breakdown of dielectrics, the interaction of an intense laser field with model dielectric samples containing metallic nanoparticles is studied both theoretically and experimentally. A theoretical study of the metal conduction electrons dynamics in the laser field predicts an efficient injection of carriers from the metallic inclusion to the conduction band of the dielectric, which leads to a strong local increase of the optical absorption in the initially transparent matrix. This prediction is tested experimentally by using time-resolved spectral interferometry to measure excitation densities as a function of the laser intensity in silica samples doped with gold nanoparticles, which are compared with similar measurements in pure silica.
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.
In the context of high power laser applications like the French "Laser Megajoule" or the "National Ignition Facility" in the United States, laser-induced-damage in fused silica appears to be a limitation. Although it has been established that nano-sized defects are suspected to be responsible of the damage initiation in optical
components, the induced-breakdown process is still unknown. The specific apparatus developed at the Institut Fresnel permits to study in real time the laser interaction on an isolated absorbing defect (around 100 nm): Indeed the coupling of a nanosecond focused laser (beam diameter about 10μm) and a high resolution photothermal
microscope (beam diameter 1μm) in a unique facility permits to highlight the evolution of the local absorption of the defect versus irradiation energy density. Furthermore, the morphology of this modification is followed thanks to a "refraction mapping" (photothermal deflection measured at frequency 0). An in-situ
dark field microscope and an ex-situ AFM complete the observations. In order to simulate the presence of the nano-defects, engineered sub-micronic gold particles (100-600 nm) were embedded in silica samples. The whole observations contribute in the understanding of the different stages of the damage initiation and more particularly, a determination of a "pre-damage" threshold can be performed.
A photothermal microscope has been combined with an experimental
set-up allowing damage threshold measurements at the same
wavelength. The microscope is based on photothermal deflection of
the transmitted probe beam: the CW pump beam (1.06 μm
wavelength) and the probe beam are collinear and focused through
the same objective. The diameter of the pump beam on the sample
surface is 1 μm. Laser damage thresholds are measured thanks
to a pulsed beam (1.06 μm wavelength and 6 nanosecond pulse)
and the spatial position of the pulsed beam is controlled by a CCD
camera. This experimental setup has been used to study the
behavior of metallic inclusions in dielectric materials in laser
damage processes. Results are presented with gold inclusions of
about 600 nm in diameter in silica.
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.
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.
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.
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.
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