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K: Kurzzeitphysik

K 3: Laser beam - matter interaction

K 3.4: Talk

Thursday, March 20, 2003, 12:15–12:30, F04

Evolution of free electron density in dielectrics under ultrashort laser pulse interaction — •Bärbel Rethfeld — Institute for Laser- and Plasmaphysics, University of Essen, Germany

Dielectrics may show strong absorption when being irradiated with intensities above a certain threshold. The absorption is due to an increased free electron density caused by electrons transferred from the valence band to the conduction band by microscopic ionization processes. Photoionization and electron–electron impact ionization are two competing processes. With help of a kinetic approach we have shown that the photoionization is the dominating process for pulse durations below about 100 fs and that the electron–electron impact ionization can not be described by a simple rate equation [1]. Here we propose an extended rate equation much easier to apply than the full kinetic approach, taking the energy of each electron and thus the distribution function in a simplified way into account. Our model is valid for any pulse duration and intensity. In certain limiting cases analytical solutions directly enlight the fundamental characteristics of the electron avalanche and the applicability of the common rate equation.

[1] A. Kaiser, B. Rethfeld, M. Vicanek, and G. Simon, “Microscopical processes in dielectrics absorbing a subpicosecond laser pulse,” Phys. Rev. B 61(17), pp. 11437–11450, 2000.