Heavy-Light Hole Quantum Beats in the Band-to-Band Continuum of GaAs Observed in 20 Femtosecond Pump-Probe Experiments

Coherent polarizations in the band-to-band continuum of bulk GaAs are studied in pump-probe experiments with 20 fs pulses. For the first time, we observe quantum beats on a 100 fs time scale that are due to an impulsively excited quantum coherence between heavy and light hole states. The beat frequency is determined by the heavy-light hole energy splitting changing continuously with the energy separation between the laser and the band gap. Theoretical calculations of the coherent response based on the semiconductor Bloch equations for a three-band scheme account for the data.