Spectrally and Time-Resolved Ultrafast Exciton Dynamics in GaAs Thin Film Using Non-Degenerate Four-Wave Mixing

We studied the coherent and incoherent exciton dynamics in a 470-nm thick GaAs film using non-degenerate three-beam Four Wave Mixing (FWM) at 10 Kelvin with 150 fs pulses. Heterodyne FWM measurements in "photon echo" configuration show a mainly homogeneously broadened exciton transition enabling the determination of the exciton dephasing time. In the "lifetime configuration", the measurements provide an exciton lifetime of 42 ps. The heterodyne FWM amplitude reveals a distinct beat structure which originates from strain-induced splitting of the heavy- (hh) and light-hole (lh) excitons as demonstrated by polarization-dependent spectrally resolved FWM experiments. These experiments further enable the measurement of individual hh- and lh-exciton dephasing times, as well as hh-lh interexciton coherence times. Excitation density-dependent measurements give an insight into the influence of excitation-induced effects and enable estimating the exciton-exciton and exciton-carrier scattering parameters. The experimental studies are accompanied by numerical calculations of the optical Bloch equation for an eight-level model which agree well with the experimental findings. This work contributes to a deeper understanding of the exciton dynamics in bulk semiconductors.