Electron kinetics in semiconductors and metals irradiated with VUV-XUV femtosecond laser pulses

In solids under irradiation with femtosecond laser pulses, photoabsorption produces a strongly nonequilibrium highly energetic electrons gas. We study theoretically the ionization of the electronic subsystem of either a semiconductor (silicon) or a metal (aluminum) target, exposed to an ultra-short laser pulse (pulse duration similar to 10 fs) of VUV-XUV photons. We developed a numerical simulation technique, based on the classical Monte-Carlo method, to obtain transient distributions of electrons within conduction band. We extend the Monte-Carlo method in order to take into account quantum effects such as the electronic band structure, Pauli's exclusion principle for electrons in the conduction band and for holes within the valence band (for semiconductors), and free-free electron scattering (for metals). In the presented work, the temporal distribution of the energy density of excited and ionized electrons were calculated. The transient dynamics of electrons is discussed regarding the differences between semiconductors and metals. It is demonstrated that for the case of semiconductors, since a part of the energy is spent to overcome ionization potentials, the final kinetic energy of free electrons at the end of the laser pulse is much less than the total energy provided by the laser pulse. In contrast, for metals all the energy is present as kinetic energy in the electronic subsystem, unless the photon energy is greater that an ionization potential of a deep atomic shell. In the latter case, a part of the energy is shortly kept by deep-shell holes, and is released back to the electrons by Auger-processes on femtosecond timescales.