Study on Bose-Einstein Condensation of Biexciton Systems by Phase-conjugation Spectroscopy

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An excitonic particle such as an exciton or a biexciton in a semiconductor crystal has a boson-like nature. It has been expected that the transition to Bose condensed phase of the excitonic particles occurs under a high density photo-excitation by laser light. The porpose of this research is to examined the expectation experimentally by means of a nonlinear optical aim of phase-conjugation spectroscopy.In the biexciton system in CuCl, following results were obtained :1. distributions of biexcitons were reproduced by Bose distribution functions in an incoherent excitation regime,2. under the condition a remarkable enhancement of the phase-conjugate (PC) signal intensity was observed. The enhancement was observed correlatively when the biexciton density was beyond the critical density for the Bose-Einstein condensation, and3.100% enhancement of the PC signal intensity was realized at the maximam under the injection of incoherent biexcitons.In the exciton system in Cu_2O,following results were obtained :1. distributions of excitons were well reproduced by Bose distribution functions,2. a new method to generate very cold and spatially uniformed ortho-excitons using a two-photon resonant excitation was developed,3. the distribution of ortho-excitons under the two-photon resonant excitation is reproduced by a Bose distribution function with a low temperature. The result suggested that the condition of the ortho-excitons was very close to the phase boundary for the Bose-Einstein condensation,4. it was confirmed that the phase-conjugation spectroscopy were applicable to the study of the Bose-Einstein condensation in ortho-exciton system from the measurement of the polarization characteristics of the two-photon transition.