THE ROLE OF ULTRAFAST INTERCARRIER PROCESSES IN SUBPICOSECOND LASER STUDIES OF SEMICONDUCTORS

We present an attempted consistent description of the interplay of coherent and incoherent electronic processes during the transient non-linear optical response of a laser-pulse-excited electron-hole plasma in a semiconductor. By applying the density-matrix formalism of Stahl's 'coherent band-edge dynamics' we follow the evolution of the carrier system, going all the way back to the instant of the photogeneration of the individual electron-hole pairs. The basic objects of the theory are expectation values of the various carrier-carrier pair correlation functions in r space. Their time evolution is given by Bloch-type rate equations with phenomenological time-dependent damping terms, which in tum are obtained from a parallel k-space ensemble Monte Carlo simulation of the detailed scattering dynamics of the non-equilibrium carrier-phonon system. We thereby treat both the coherent and the incoherent part of the carrier dynamics on the same footing. This improves on the conventional use of time-independent damping constants in this type of analysis, a doubtful procedure in view of the transiently changing scattering dynamics typical for the semiconductor response to an ultrashort and high-intensity laser pulse. We apply this novel algorithm to experimental scenarios recently realized in the sub-picosecond laser-pulse spectroscopy of hot carriers in GaAs-type polar semiconductors.