Quantum Kinetics and Femtosecond Spectroscopy — The Discovery of Slowness

Abstract

For time intervals which are shorter than characteristic periods of the medium, collisions can no longer be regarded as instantaneous. They have to be described instead by non-Markovian scattering integrals. In this early time regime the quantum mechanical coherence of the photo-excited carriers is not yet destroyed. In this regime quantum kinetics with its memory effects yields an excellent description of femtosecond four wave mixing (FWM) and differential transmission spectroscopy (DTS) experiments. The essential features of carrier-phonon scattering and the carrier-carrier scattering via a time-dependently screened Coulomb potential are discussed. Particularly the time-dependent build-up of screening by the optically excited carriers is itself a highly interesting quantum kinetic problem. The determination of the density-dependent dephasing time and the observation of the quantum kinetically predicted phonon-plasmon beats with coherent control techniques are discussed.