Abstract
Optical third-order polarizations close to the bandgap of bulk GaAs are investigated by degenerate four-wave mixing using 100 fs pulses. Spectrally and temporally resolved experiments reveal excitonic and free-carrier components both occurring within the bandwidth of the femtosecond pulses. The excitonic component dominates at excitation densities below 1016 cm-3 and shows a time behaviour governed by many-body effects. At higher carrier concentrations, we observe an additional contribution due to non-thermalized free carriers that shows a spectrum resonant to the pulse and a photon-echo-like time behaviour. Theoretical calculations based on the semiconductor Bloch equations and a Monte Carlo simulation of the scattering dynamics are in good agreement with the experimental results.
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