Ultrafast coherent dynamics of impulsively excited inter-valence band polarizations in bulk GaAs

Quantum coherence between heavy and light hole states of bulk GaAs is created via an impulsive excitation of the interband absorption continuum with 20 fs pulses. Free carrier quantum beats are observed on a 100 fs time scale by monitoring the interband transmission with weak spectrally dispersed probe pulses. Experiments on differently doped samples, with various excitation densities and excitation bandwidths give direct insight into the properties of coherent inter-valence band polarizations. Amplitude and phase of the oscillatory transmission changes show a strong dependence on the polarization of pump and probe pulses. Damping of the quantum beats involves both inhomogeneous broadening by the dispersion of heavy-light hole splittings in the optically coupled range and scattering processes. Theoretical calculations based on the semiconductor Bloch equations for a three band scheme account for the data.