Seminars

Ultrafast electron kinetics in semiconductors and metals irradiated with VUV-XUV femtosecond laser pulse

by Nikita A. Medvedev, Technical University of Kaiserslautern and OPTIMAS Research Center

Europe/Berlin
AER 19, Room 3.11

AER 19, Room 3.11

Description
When ultrashort laser pulses irradiate a solid, photoabsorption by electrons in conduction band produces non-equilibrium highly energetic free electrons gas. We study the ionization and excitation of the electronic subsystem in a semiconductor and in a metal (on examples of solid silicon and aluminum, respectively). The irradiating femtosecond laser pulse has a photon energy in a range of tens of eV (VUV-XUV regime). We apply the classical Monte Carlo method and extend it to take into account the electronic density of states and band structure, as well as Pauli's principle for electrons excited into the conduction band. In the case of semiconductors this applies to the holes as well. Conduction band electrons and valence band holes induce secondary excitation and ionization processes which we simulate event by event. We discuss the transient electron dynamics with respect to the differences between semiconductors and metals. For metals the electronic distribution demonstrates two branches on femtosecond timescales: a low energy distribution as a slightly distorted Fermi-function and a long non-equilibrium high energy tail. For the case of semiconductors it is split into two parts by the band gap. To thermalize, these excited electronic subsystems need longer times than the characteristic pulse duration. Thus, we conclude that an analysis of experimental data with femtosecond lasers must be based on non-equilibrium concepts.