Ultra-short laser induced electron excitation/relaxation kinetics

International audienceLaser-induced electronic excitation, absorption and relaxation are the key issues in ultra-short laser interactions with dielectric materials. To numerically analyze these processes, several approaches are typically used. First, several detailed non-equilibrium models are based on a system of the detailed kinetic Boltzmann equations. Then, Fokker-Planck equations are also used. Finally, much more simplified rate equations are typically used in engineering modelling [1,2]. These models require additional sub-models to account for photo-ionization, electron-impact ionization, defect formation, recombination and other relaxation processes. In these sub-models, many parameters are rather unknown and are calculated based on additional considerations. One of such parameters is electron collision frequency [1-3], which was found to be crucial in determination of laser absorption and hence of laser damage.Boltzmann-based calculations are performed including all possible collisional processes. As a result, electron energy distributions are obtained allowing a better analysis of ultra-short laser interactions. The results reveal an effect of the laser-field on collision frequencies resulting in smaller free-carriers absorption than the one predicted by commonly used rate-equation models. Both electron-electron and electron-phonon relaxation are then examined, and the mean energy density of the electron sub-system is investigated as a function of laser fluence and pulse duration. Because efficient bond breaking requires energy, these calculations provide the required thresholds [4]. The dependency of the calculated damage threshold on laser pulse duration is compared with the available experimental data. The developed model is useful for many laser applications including high precision in laser treatment, laser-assisted atomic probe analysis, and for the development of new powerful laser systems.References[1] B. Chimier, O. Utéza, N. Sanner, M. Sentis, T. Itina, P. Lassonde, F. Légaré, F. Vidal, and J. C. Kieffer. "Damage and ablation thresholds of fused-silica in femtosecond regime." Physical Review B 84 (9), 094104, (2011).[2]T.E. Itina, N.S. Shcheblanov, N. Electronic excitation in femtosecond laser interactions with wide-band-gap materials. Applied Physics A, 98(4), 769-775 (2010).[3] C. Xie, V. Jukna, C. Milián, R. Giust, I. Ouadghiri-Idrissi, T. Itina, J. M. Dudley, A. Couairon, F. Courvoisier. "Tubular filamentation for laser material processing. Scientific reports, 5 (2015).[4] N. S. Shcheblanov, T. E. Itina, Appl. Phys. Femtosecond laser interactions with dielectric materials: insights of a detailed modeling of electronic excitation and relaxation processes, Appl. Phys. A, 110(3), 579-583 (2013)