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

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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