DYNAMICS OF SCREENING AND FIELD FLUCTUATIONS ON ULTRASHORT TIME SCALES

The short-time dynamics of screening in semiconductors is considered a fter excitation with an ultrashort laser pulse of several 10 fs. While the kinetics of carriers is considered within standard approximations (semiconductor Bloch equations) the plasmon kinetics includes particu larly memory effects due to non-Markovian behaviour and retardation as well as interaction between plasmons and the coherent interband polar ization induced by the laser pulse. For pulses shorter than the relaxa tion time of carriers pronounced oscillations of the plasmon density ( intensity of plasma oscillations) are found which are strongly enhance d by the coherent interband polarization. It is concluded that the not ion of plasmons as quasiparticles fails on an ultrashort time scale, b ecause it is essentially an adiabatic concept and experiments in the f s-domain should be reinterpreted in this respect.