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

Optical third-order polarizations close to the bandgap of bulk GaAs ar e investigated by degenerate four-wave mixing using 100 fs pulses. Spe ctrally and temporally resolved experiments reveal excitonic and free- carrier components both occurring within the bandwidth of the femtosec ond pulses. The excitonic component dominates at excitation densities below 10(16) cm-3 and shows a time behaviour governed by many-body eff ects. At higher carrier concentrations, we observe an additional contr ibution due to non-thermalized free carriers that shows a spectrum res onant to the pulse and a photon-echo-like time behaviour. Theoretical calculations based on the semiconductor Bloch equations and a Monte Ca rlo simulation of the scattering dynamics are in good agreement with t he experimental results.