Dynamical Processes of Electron-Spin Systems Photo-Excited in Semiconductor Quantum Dots

本项目的目的是(1)了解非磁性和磁性半导体中量子受限激子的单态-三态自旋转换和受限激子磁极化子的驰豫动力学；(2)探讨受限激子-自旋复杂系统所固有的新的现象。材料为CuCl型半导体量子点和CdMnTe半磁性半导体的混晶。在后一种系统中，激子被限制在合金势垒中，即自然生长的量子点中。CuCl量子点中的受限激子：(1)在4K的磁致发光光谱中，发现在Faraday和Voigt几何构型下，局域激子发光的强度增强和光谱调制都发生了。这些结果清楚地表明，由于激子的限制，自旋三重态(暗激子态)的部分光学跃迁被允许，并且在纵向和…之间发生混合更多的横向激子。(2)通过双光子共振激发成双激子态，得到了与双激子自旋三重态相对应的稳定J=2态。此外，J=0双激子显示出非常有效的激光，这来自于双激子的超辐射性质。(3)测量磁致发光和吸收的时间行为和温度依赖性可以阐明自旋三重态激子在其平移运动中的作用和动力学。CdMnTe中的局域受限激子：(1)在合金势垒中局域受限激子的强烈、选择性激发下，发现了一个新的非线性发光带，它表现出巨大的磁光效应和超快衰减。这种发光及其自旋记忆可能与局域激子磁极化子的某种多体效应有关。(2)有必要通过测量相关电子-自旋系统的瞬时吸收及其与磁场的关系来阐明它们的超快弛豫过程。较少

The purpose of this project is (1) to know the singlet-triplet spin conversion of quantum-confined excitons and the relaxation dynamics of confined exciton magnetic polarons in non-magnetic and magnetic semiconductors and (2) also to approach to new kinds of phenomena inherent to confined exciton-spin complex systems. The materials are CuCl semiconductor quantum dots and mixed crystals of CdMnTe semi-magnetic semiconductor. In the latter system, excions are confined in alloy potential wells, that is, naturally grown quantum dots.1. Confined excitons inCuCl quantum dots : (1) In the magneto-luminescence spectra at 4K, it has been found that the intensity enhancement and the spectral modification of the localized exciton luminescence occur both under Faraday and Voigt geometries. These results clearly show that, due to the exciton confinement, the optical transition to/from the spin-triplet state (dark exciton state) is partially allowed and the mixing occurs between the longitudinal and … More transverse excitons. (2) By means of two-photon resonance excitation into biexciton states, there has been found the stable J=2 state which corresponds to the spin-triplet state of biexciton. Furthermore, the J=0 biexciton shows very efficient lasing which comes from the superradiant nature of the biexciton. (3) The measurement of temporal behavior and temperature dependence of the magneto-luminescence and absorption may clarify the role and the dynamics of spin-triplet excitons confined in their translational motion.2. Locally confined excitons in CdMnTe : (1) Under the intense, selective excitation of locally confined excitons in alloy potential wells, there has been found a new nonlinear luminescence band which shows giant magneto-optical effects and ultra-fast decay. The luminescence and their spin memory may be related with a certain many-body effect of localized exciton magnetic polarons. (2) It is necessary to clarify the ultra-fast relaxation processes of the correlated electron-spin systems through the measurement of transient absorption and their magnetic field dependence. Less