Free-electron generation in laser-irradiated dielectrics

When transparent solids are irradiated with laser or particle beams an electron-hole plasma may be generated, which consists of free electrons in the conduction band of the solid and holes in the valence band. If the electron density reaches values close to the critical plasma density (at which the plasma frequency of the free electron gas equals the laser frequency), the dielectric becomes highly absorbing. The large energy transfer to the solid leads to dielectric breakdown and further to phase transitions and ablation. We compare different models describing the temporal evolution of the free-electron density under laser irradiation. The role of different ionization processes is investigated in dependence on laser pulse duration and intensity. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.