Dynamics of Optically Established Quantum Condensate Phases in Semiconductors

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In layered crystals BiI_3, and GaSe, we have studied spatial and temporal behavior of high-density exciton masses and their nonlinear optical response in connection with coherent motion of the exciton masses in a photo-induced quantum condensed phase such as the Base-Einstein condensation (BEC), by observing spatial expansion of the resonance luminescence and the degenerate four-wave-mixing (DFWM) light induced by spatially isolated two laser-light spots on the sample surface in the space-resolved regime.In BiI_3, spatial expansion of the excitons confined in a quasi tow-dimensional stacking fault interface (SFE) was examined in the space- and time-resolved photoluminescence under the spot excitation with a picosecond laser light tuned to the SFE state. The space and time evolution of the luminescence intensity was analyzed by applying a nonlinear Schrodinger equation (Gross-Pitaevski equation) by considering weakly repulsive exciton-exciton interaction. It shows that the expansion of … More the exciton masses is reproduced in accordance with the observed results. The DFWM light was emitted owing to the coherent transport of the exciton masses from one to the other spot sites showing evidently the SFE masses being in a condensed phase. On the other hand, in GaSe, the exciton luminescence under heavy excitation is characterized by the recoil process due to scattering with the other excitons or carriers. In the luminescence process at high density, the inelastic scattering between excitons with relatively large energies governs dominantly the dynamical process of exciton masses revealing the recoil luminescence, and showing also the remarkable response on the DFWM excitation spectra. It turns out that this strongly repulsive interaction between excitons prevents the establishment of their coherent quantum condensed phase, in contrast to the case of the BiI_3 SFE system.In the quantum wire structures of GaAs/AlAs system, dynamical behavior of the heavily excited excitons and their relaxation process have been examined. It is found that a repulsive interaction between excitons brings about remarkable spectral blue-shifts and site-filling effects. The curious behavior is understood to be a characteristic in low dimensional exciton systems.In other materials, such as Alq_3, light-harvesting dendrimers, π-conjugated conducting polymer : polythiophene, highly luminescent hexanucler 6-methylpyridinetholato copper(1), we have investigated exciton transitions and luminescence spectra under high density excitation. In these materials some interesting optical responses and relaxation processes are made clear with respect to the exciton-exciton interaction and many-body effects. The details are future problems. Less