Excitonic versus free-carrier contributions to ultrafast pump-probe signals at the band edge of bulk GaAs

The nonequilibrium dynamics of photoexcited semiconductors occurs on ultrafast time scales and is initially governed by coherent polarizations. Impulsive excitation of a coherent superposition of heavy- (HH) and light-hole (LH) states leads to pronounced beating phenomena, which were first found for excitons in quantum wells and also recently in the free-carrier continuum of bulk gallium arsenide (GaAs). At the absorption edge of GaAs, femtosecond pump-probe experiments are determined by both excitonic and free-carrier contributions. A direct experimental separation of the two components is still missing but mandatory for a complete interpretation of pump-probe data. In this paper, we study HH-LH quantum beats at the band edge of a slightly strained bulk GaAs sample in spectrally and time-resolved pump-probe experiments with 20-fs pulses from a Ti:sapphire laser.