Coherent dynamics of excitons in an island-inserted GaAs/AlAs quantum well structure: Suppression of phase relaxation and a deep quantum beat

The degenerate four-wave mixing technique in the self-diffraction geometry is employed to investigate a novel GaAs/AlAs quantum well microstructure. Time-integrated transient measurements are carried out to study the coherent dynamics of excitons in a GaAs quantum well with AIAs islands inserted at the center of the well (IIQW) in excitation densities from 6 × 109 to 1.5 × 10⁹ cm^-2. The decay of the time-integrated self-diffracted signal shows slower dephasing for the IIQW compared to that of a normal quantum well (NQW) without island insertion. The dephasing rate is found to increase slightly in the IIQW, while it is found to increase sharply in the NQW with the excitation density. A deep beat with a period of 1.45 ps, which disappears for co-circular polarization configuration, is observed in the decay of the self-diffracted signal in the IIQW. The detcction-energy-resolved measurement reveals that the modulation of the beat increases up to full depth at low detection energy with respect to the peak energy of the self-diffracted signal. The dependence on detection energy and on polarization configuration shows that the beat is a quantum beat due to biexcitonic effect, which is enhanced by localization due to island insertion.