Excitation and Relaxation Dynamics in Laser-Excited Semiconductors and Dielectrics

Beschreibung

The interaction of an ultrashort laser pulse with solid matter initiates a complex cascade of interconnected processes: the electronic system is driven out of its thermodynamic equilibrium leading to a state where no temperature can be defined. At the same time, in semiconductors and dielectrics, the free-carrier density in the conduction band increases tremendously. This in turn strongly affects the optical properties of the material influencing photoabsorption and hence the entire excitation process. Collisions among electrons strive to counteract the laser-induced nonequilibrium. In semiconductors and dielectrics, both collisions within a single band, so-called intraband collisions, and collisions among electrons originating from different bands, i.e., interband collisions, come into account. The latter may even lead to further particle exchange between valence band and conduction band. Thus, interband collisions again influence the free-carrier density. Furthermore, electrons may interact with lattice vibrations, so-called phonons, transferring energy to the crystal lattice. Finally, heat and particles are transported away from the excited region. Yet, none of these processes is independent of the others but all of them, though happening on different timescales, are closely interconnected. Only the interplay of excitation and relaxation processes finally decides whether the material is damaged. This thesis deals with the theoretical treatment of the excitation and relaxation dynamics in semiconductors and dielectrics based on numerical simulations. Different numerical models that are focused on different stages of the dynamics following the impact of an ultrashort laser pulse are presented. In the framework of this thesis, these models are extended significantly and validated against experimental data.

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Anika Rämer

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Laser-Excitation Semiconductors Dielectrics

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