Dynamic of electronic subsystem of semiconductors excited with an ultrashort VUV laser pulse

Abstract

We investigate theoretically the interaction of a semiconductor with an ultrashort high-intensity VUV laser pulse produced by new light source FLASH at DESY in Hamburg. Applying numerical simulations of excitations and ionization of electronic subsystem within a solid silicon target, irradiated with femtosecond laser pulse (25 fs, photon energy of 38 eV), the transient distribution of electrons within conduction band is obtained. The Monte Carlo method (ATMC) was extended in order to take into account the electronic band structure and Pauli's principle for electrons excited into the conduction band. Secondary excitation and ionization processes were included and simulated event by event as well. In the presented work the temporal distribution of the density of excited and ionized electrons, the energy of these electrons and their energy distribution function were calculated. It is demonstrated that due to the fact that part of the energy is spent to overcome ionization potentials, the final kinetic energy of free electrons is much less than the total energy provided by the laser pulse. We introduce the concept of an 'effective energy gap' for collective electronic excitation, which can be applied to estimate the free electron density after high-intensity VUV laser pulse. The effective energy gap depends on properties of the material as well as on the laser pulse.