QUANTUM COHERENCE OF CONTINUUM STATES IN THE VALENCE-BAND OF GAAS QUANTUM-WELLS

Quantum beats of heavy and light holes in GaAs quantum wells an invest igated in femtosecond time-resolved four-wave mixing and transmission experiments as a function of optical excitation energy. Under nonexcit onic excitation conditions, the four-wave mixing signal disappears due to the immediate loss of the interband coherence of continuum states. In the transmission experiment, the quantum beats are observed up to excess energies of 75 meV above the exciton resonances. The experiment al data clearly demonstrate the coherence of continuum states in the v alence band. Changes of the beat frequency with the excitation energy are due to the dispersion of the valence bands.