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

由于玻色-爱因斯坦凝聚（BEC）和激子超流动性的诱人可能性，强光场诱导的高密度电子-空穴（e-h）系统的性质受到了相当大的关注。实验技术的最新发展使光谱和输运性质的同时测量成为可能。特别是在Cu_2O和BiI_3中，高密度激子的快速相干传播被观察到。这些现象是由凝聚态激子系统的宏观量子相干性引起的，因为随着激子密度的增加，快速激子输运更加明显。然而，由于极化效应、激子密度的空间非均匀性、激子的有限寿命、声子效应等复杂的情况，目前对这些结果的理解仍然是尝试性的和不明确的。在本研究项目中，我们首次从理论上研究了具有大振子强度的二维激子的异常输运现象，考虑了激子-激子相互作用和极化子效应。在保持宏观量子相干性的前提下，通过解耦近似导出了激子极化算子期望值相对于宏观量子态的运动方程。数值解与简单的弹道（或扩散）极化子传播有明显的偏差。这一结果表明，激子-激子相互作用大大增强了激子极化子在高密度状态下的快速相干传播。这一分析为我们提供了高密度激子系统输运现象不可用实验的正确解释。

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.