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

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 s hifts 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) luminesc ence due to exciton-exciton and exciton-carrier scattering becomes dominant . The free exciton shows remarkable spatial propagation under the heavy exc itation on the space- and time-resolved luminescence spectra, giving a grou p velocity similar to 3 x 10(8) cm/s. The time correlated DFWM signals havi ng a single exponential decay for lower excitation densities come to have m ultiple components due to many-body effects under higher excitation densiti es. In the negative time domain, the DFWM signals reveal the response from the excitonic molecule states. From the DFWM spectra in the negative time d omain, the binding energy of the excitonic molecule is estimated to be simi lar to 8 meV. (C) 2000 Published by Elsevier Science B.V. All rights reserv ed.