Antoine Bourgeade

Modeling the Early Ionization of Dielectrics by Ultrashort Laser Pulses

In this paper, we present a model for propagation of intense and ultrashort laser pulses ionizing dielectrics. We consider early ionization so that this process is sufficiently weak to avoid requiring a complete description of the ionization process (e.g. the use of kinetic equations which are very expensive from a computational point of view). As the intensity of the field is small, one photon ionization is neglected. Ionization may only occur through multi-photonic and collisional ionization. The conduction band is discretized and multiple-rate equations are written for electron densities. The wave-field evolves through Maxwell equations.

Toward a New Laser Induced Hydrodynamic Forward Transfer Process with Femtosecond Lasers

Theoretical approaches to the photoionization of few-electron atoms are discussed. These include nonequilibrium Greens functions and wave function based approaches. In particular, the Multiconfiguration Time-Dependent Hartree-Fock method is discussed and applied to model a one-dimensional atom with four electrons. We compute ground state energies and the time-dependent photoionization by the field a strong laser pulse with two different frequencies in the ultraviolet (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Numerical methods for the bidimensional Maxwell-Bloch equations in nonlinear crystals

Two numerical schemes are developed for solutions of the bidimensional Maxwell-Bloch equations in nonlinear optical crystals. The Maxwell-Bloch model was recently extended [C. Besse, B. Bidegaray, A. Bourgeade, P. Degond, O. Saut, A Maxwell-Bloch model with discrete symmetries for wave propagation in nonlinear crystals: an application to KDP, M2AN Math. Model. Numer. Anal. 38 (2) (2004) 321-344] to treat anisotropic materials like nonlinear crystals. This semi-classical model seems to be adequate to describe the wave-matter interaction of ultrashort pulses in nonlinear crystals [A. Bourgeade, O. Saut, Comparison between the Maxwell-Bloch and two nonlinear maxwell models for ultrashort pulses propagation in nonlinear crystals, submitted (2004)] as it is closer to the physics than most macroscopic models. A bidimensional finite-difference-time-domain scheme, adapted from Yee [IEEE Trans. Antennas Propag. AP-14 (1966) 302-307], was already developed in [O. Saut, Bidimensional study of the Maxwell-Bloch model in a nonlinear crystal, submitted (2004)]. This scheme yields very expensive computations. In this paper, we present two numerical schemes much more efficient with their relative advantages and drawbacks.

Laser-Matter Interaction in Transparent Materials: Confined Micro-explosion and Jet Formation

High intensity laser beam was tightly focussed inside bulk of dielectrics at adjustable distance from the outer boundary (1-15 µm). Laser- matter interaction region is thus confined inside a cold and dense material, with and without boundary effects. In what follows we first describe self-consistently the relevant laser-matter interaction physics. At high intensity of the laser beam in a focal region (> 6 × 10 12 W/cm 2 ) the material is converted into a hot and dense plasma. The shock and rarefaction waves propagation, formation of a void inside the target are all described. Then, a model was developed to predict size of the voids in the bulk of materials, i.e. without boundary effects. Results were compared to experimental observations. The size of a void formed by 800 nm 150 fs laser pulses is ~0.2 µm 3 . Finally we present new results in confined geometries and we show that jets can develop sizes and expansion velocities depending both on energy laser and distance from the rear surface. This jet formation regime, apparently new, can be related to some LIFT process, with submicrometer diameter jets.

Process for repairing large scratches on fused silica optics

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.

Impact of FM-AM conversion on smoothing by spectral dispersion

In order to set the shape of the focal spot, high power lasers for inertial confinement fusion have a phase plate at the end of the chain. This produces hot spots that can be avoided by the use of optical smoothing. Smoothing consists either in reducing the number of high-energy hot spots by splitting the focal spot energy on two orthogonal states of polarization or in moving the speckle pattern sufficiently fast so that the focal spot seems more homogeneous over time. In the latter case, the spectrum is broadened by a temporal phase modulation and dispersed with a grating. However, because of propagation impairments (filtering functions, chromatic dispersion, frequency conversion,...), part of the frequency modulation is converted into detrimental amplitude modulation. This is called FM-AM conversion. Its impact on smoothing performance is considered here. Three main parameters may be affected: power fluctuations of the focal spot, size of the speckle hot spots (autocorrelation function) and dynamic of the evolution of the spatial contrast of the focal spot versus time. We show that depending on the features of the FM-AM conversion (frequency content of AM, type of filtering function) either one or more of these parameters may be affected. As a matter of fact, a low frequency AM induces power fluctuations while higher frequency AM induces variation of the autocorrelation function. Moreover, as opposed to an amplitude-filtering function, chromatic dispersion will not change the power spectral density of the pulse and thus the dynamic of the contrast.

CO2 laser microprocessing for laser damage growth mitigation of fused silica optics

Abstract. We report on the development of a mitigation process to prevent the growth of UV nanosecond laser-initiated damage sites under successive irradiations of fused silica components. The developed process is based on fast microablation of silica as it has been proposed by Bass et al. [Bass et al., Proc. SPIE 7842, 784220 (2010)]. This is accomplished by the displacement of the CO2 laser spot with a fast galvanometer beam scanner to form a crater with a typical conical shape to mitigate large (millimetric) and deep (few hundred microns) damage sites. We present the developed experimental system and process for this application. Particularly, we detail and evaluate a method based on quantitative phase imaging to obtain fast and accurate three-dimensional topographies of the craters. The morphologies obtained through different processes are then studied. Mitigation of submillimetric nanosecond damage sites is demonstrated through different examples. Experimental and numerical studies of the downstream intensifications, resulting in cone formation on the surface, are presented to evaluate and minimize the downstream intensifications. Eventually, the laser damage test resistance of the mitigated sites is evaluated at 355, 2.5 ns, and we discuss on the efficiency of the process for our application.

Laser produced nanocavities in silica and sapphire: a parametric study

We present a model, that describes a sub-micron cavity formation in a transparent dielectric under a tight focusing of a ultra-short laser pulse. The model solves the full set of Maxwells equations in the three-dimensional geometry along with non-linear propagation phenomenons. This allows us to initialize hydrodynamic simulations of the sub-micron cavity formation. Cavity characteristics, which depend on 3D energy release and non linear effects, have been investigated and compared with experimental results. For this work, we want to deeply acknowledge the numerical support provided by the CEA Centre de Calcul Recherche et Technologie, whose help guaranteed the achievement of this study.