EXCITONIC AND FREE-CARRIER POLARIZATIONS OF BULK GAAS STUDIED BY FEMTOSECOND COHERENT SPECTROSCOPY

The transient third-order polarization at the band gap of undoped and p-doped GaAs is investigated by spectrally and temporally resolved fou r-wave mixing. Excitonic and free-carrier contributions simultaneously excited within the bandwidth of the 100-fs pulses are clearly disting uished by their different spectral envelopes. The excitonic part domin ates at carrier densities below 10(16) cm-3 and shows a time evolution governed by exciton-free-carrier scattering and by many-body effects. At higher density, the free-carrier polarization has a strength simil ar to the exciton contribution and exhibits a spectrum resonant to the femtosecond pulses with a photon-echo-like temporal behavior. The dat a are analyzed by a numerical solution of the semiconductor Bloch equa tions including an ensemble Monte Carlo simulation of the scattering d ynamics of the carriers. The theoretical model is in good agreement wi th the experimental results.