Space-resolved luminescence and degenerate four-wave-mixing spectra of heavily photo-created exciton states in a layered crystal GaSe

Abstract

High-density effects on the exciton states in a layered GaSe single crystal have been studied by observing photoluminescence and degenerate four-wave-mixing (DFWM) signals. The luminescence peak of the lowest exciton states shifts from lower energy to the higher lying absorption peak of the exciton owing to the site-filling effects of the weakly bound exciton states under heavy excitation. For heavier excitation, the recoil (red-shifted) luminescence due to exciton–exciton and exciton–carrier scattering becomes dominant. The free exciton shows remarkable spatial propagation under the heavy excitation on the space- and time-resolved luminescence spectra, giving a group velocity ∼3×10 8 cm/s. The time correlated DFWM signals having a single exponential decay for lower excitation densities come to have multiple components due to many-body effects under higher excitation densities. In the negative time domain, the DFWM signals reveal the response from the excitonic molecule states. From the DFWM spectra in the negative time domain, the binding energy of the excitonic molecule is estimated to be ∼8 meV.