We report on femtosecond time-resolved transmission and reflectivity measurements on bulk GaAs. Spectral-hole burning is observed, to our knowledge, for the first time in GaAs at room temperature. Carrier thermalization occurs within 200 fs and shows no significant dependence on excitation density or excess energy in the range from 2 x 10 cm to 2 x 10 cm and 35 meV to 90 meV, respectively. Calculations of the carrier dynamics are performed and include full dynamic screening of the carrier-carrier and the carrier LO-phonon interaction. The calculated thermalization times agree well with the experimental results. Negative transmission changes above the spectral hole are mainly caused by a complementary increase of reflectivity, but not by an increase of absorption.

We report on new experimental results on the internal thermalization of the electron-hole plasma (EHP) in bulk GaAs. Time-resolved differential transmission spectra have been systematically studied for different pump photon energies (1 .45 eV