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

摘要

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.