NONLINEAR OPTICAL PHENOMENA GENERATED BY SPATIALLY SEPARATED LIGHT BEAMS IN EXCITON SUPERFLUID STATE

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The nature of high-density electron-hole (e-h) systems induced by an intense optical field has received considerable attention because of the attractive possibility of the Bose-Einstein condensation (BEC) and superfluidity of excitons. Recent developments in the experimental techniques make it possible the simultaneous measurements of the spectroscopic and transport properties. In particular, the fast and coherent propagat ions of high-density excitons were observed in Cu_2O and BiI_3. These phenomena arise from the macroscopic quantum coherence of condensed exciton systems, because the fast exciton transport is more pronounced with increasing the exciton density. However, current understanding of these results still remain tentative and unclear because of the complicated situation, such as the polariton effect, the spatial inhomogeneity of exciton density, the finite life-time of excitons, the phonon effect and so forth. In this research project, we have *oretically investigated for the first time the anomalous transport phenomenon of two-dimensional excitons with large oscillator strength, taking into account the exciton-exciton interaction and the polariton effect. The equation-of-motion for expectation value of the exciton polariton operator with respect to the macroscopic quantum state has been derived by the decoupling approximation assuming that the macroscopic quantum coherence is maintained. The numerical solution shows the significant deviation from the simple ballistic (or diffusive) polariton propagation. This result indicates that the exciton-exciton interaction considerably enhances the fast coherent propagation of exciton polaritons in the high-density state. This analysis provides us with the proper interpretation of not iceable experiments on the transport phenomena in high-density exciton systems.