During the academic semesters the plasma physics department hosts seminars on Tuesday at 2:30 pm.
If you have questions or want to suggest a speaker/topic, please contact Prof. Olga Rosmej or Dr. Paul Neumayer.

Plasmaphysik Seminar

Relaxation dynamics of nonequilibrium electrons in laser-excited solids

by Prof. Baerbel Rethfeld (Technische Universitaet Kaiserslautern)

Europe/Berlin
Seminarraum Theorie (GSI Darmstadt)

Seminarraum Theorie

GSI Darmstadt

Description
When an ultrashort laser pulse of visible light is absorbed by a solid, mainly the electrons in the material are excited. In metals, free electrons in the conduction band can directly absorb photons. In semiconductors and dielectrics, on the other hand, a band gap has to be overcome first, as almost no free electrons are present at room temperature in the unexcited material. Due to this excitation, the electronic system, or the so-called electron-hole plasma, is in a nonequilibrium state. A sequence of different relaxation processes transfers the material into a new equilibrium. Depending on the interaction associated with the particular relaxation process, it occurs on a characteristic timescale. On the basis of complete Boltzmann-type collision integrals, we calculate the transient distribution functions of electrons and phonons in different materials. We consider electron-electron interaction, different ionization processes, as well as electron-phonon coupling. By that we trace the relaxation cascade of nonequilibrium electrons after ultrafast heating. Distinct material properties enter through the density of states of the electrons in the conduction band. We study in particular noble metals, dielectrics and ferromagnets. In noble metals and ferromagnets, d-electrons play an important role, whereas in dielectrics two separated bands govern the dynamics and the ionization state may differ from equilibrium. We show, that the electron distributions deviate from Fermi distributions for timescales up to a few picoseconds. While the initial thermalization within one band has an intrinsic timescale of typically only a few tens of femtoseconds, nonequilibrium occupations of the different bands may persist. Moreover, continous electron-phonon coupling drive the conduction electrons out of equilibrium for much longer times. We present in detail the mutual influence of different interaction and relaxation processes.