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

半导体晶体中的激子或双激子等激子粒子具有类似玻色子的性质。在激光的高密度光激发下，激子粒子向玻色凝聚相跃迁。本研究的目的是利用相位共轭光谱的非线性光学目标，对这一期望进行实验检验。在CuCl双激子系统中，得到了以下结果：用玻色分布函数再现了双激子在非相干激励下的分布。在此条件下，相位共轭（PC）信号强度显著增强。当双激子密度超过玻色-爱因斯坦凝聚的临界密度时，可以观察到相应的增强，在注入非相干双激子的情况下，PC信号强度的最大增强达到3.100%。在Cu_2O的激子体系中，得到了以下结果：玻色分布函数很好地再现了激子的分布。提出了一种利用双光子共振激发产生极冷和空间均匀正交激子的新方法。在低温条件下，用玻色分布函数再现了双光子共振激发下的正激子分布。结果表明，邻激子的条件非常接近玻色-爱因斯坦凝聚的相边界。通过对双光子跃迁偏振特性的测量，证实了相共轭光谱法适用于邻激子体系中玻色-爱因斯坦凝聚的研究。

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.