Anomalous behavior of the high-density excitons and their nonlinear optical response in a quasi-two-dimensional space in a layered crystal BiI3

Spatial and temporal behavior of excitons heavily excited in a quasi-two-di mensional space of a stacking fault interface in BiI3 has been studied by s pace- and time-resolved luminescence measurements. The luminescence spectra show the peak-energy shift due to high-density effects. Under the heavier excitation, an anomalous propagation of excitons is observed on the tempora l profiles of the luminescence intensity and the peak-shift values at each detecting position. From a comparison with a model calculation, it is seen that transport mechanisms of high-density excitons are basically explained by both diffusion and polariton transport suffering the scattering, while t here exist high-density exciton-polaritons which propagate with the polarit on group velocity. These results are discussed in terms of a collective mot ion and coherent transport of excitons. (C) 2000 Elsevier Science B.V. All rights reserved.