Electronic excitation and relaxation processes in femtosecond laser interactions with insulators: the role of laser wavelength, intensity and different absorption mechanisms
Résumé
Electronic excitation-relaxation processes induced by ultra-short laser pulses are studied numerically for a crystalline targets (α quartz and, MgO). A detailed kinetic approach is applied to the calculations accounting for electron-photon-phonon, electron-phonon, electron-electron, electronand electron ion scatterings. In addition, both laser field photoionization ranging from multi-photon to tunneling one, and electron impact ionization (avalanche process) processes are included in the model. The importancerole of the electron-ion collisions oin the calculation of laser energy absorption and ionization is demonstrated. Under a powerful femtosecond laser irradiation, the electron energy distribution is far from equilibrium state. Theis intialinitial excitation stage and following electron relaxation stage depend on the laser parameters, such as: wavelength, intensity, pulse duration and target properties. Based on the performed calculations we study the thermalization time as a function of laser parameters. Futhermore, Based on the performed calculations we presented comparing the thermal and optical breakdown new laser damage criteria derived from our calculations and based on electron energy. Comparison with the available experimental data confirms the validity of the calculations.