Effect of laser field enhancement on collision frequencies and absorption during ultra-short laser interactions with dielectric materials

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, a detailed non-equilibrium model is developed [1] based on the kinetic Boltzmann equations without any appeal to the classical Drude model. The 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 a remarkable effect of the laser-field on collision frequencies resulting in smaller free-carriers absorption than the one predicted by commonly used rate-equation models. In addition, our calculations clearly demonstrate laser intensity limits for the applicability of Keldysh’s equation for the photoionization process [2]. 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. The dependency of the calculated fluence threshold on laser pulse duration is compared with the available experimental data. The obtained results also explain several recent pump-probe experiments. 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.Furthermore, a finite-difference time-domain (FDTD) solution of Maxwell’s equations is numerically coupled with time-dependent electron carrier density equation for dielectric material (fused silica) with a small inclusion. The process of plasma generation near the inclusion is studied and local field distribution is obtained. The generated plasma is studied and the roles of the electron collisional frequency and the irradiated wavelength are demonstrated.[1] N. S. Shcheblanov and T. E. Itina, Appl. Phys. A DOI: 10.1007/s00339-012-7130-0 (2012)[2] N. S. Shcheblanov and T. E. Itina, CECAM 201