We have studied the electronic states of closely stacked InAs/GaAs quantum dots (QDs) with a 4.0-nm spacer layer using linearly polarized photoluminescence (PL) and time-resolved PL measurements. An increase in the stacking-layer number (SLN) leads to an increase in the linear polarization anisotropy in the (001) plane; the [−110]-polarization component becomes dominant. These SLN-dependent polarization characteristics result from the valence-band mixing induced by the vertically coupled electronic states. The PL spectrum of the stacked QDs shows clear blueshifts with an increase in the excitation power because of the band filling. In addition, the radiative recombination lifetime has been found to obey the T 1/2 dependence, which directly confirms the one-dimensional translational motion of excitons in the closely stacked QDs.

Anisotropic magnetic-field evolution of the valence-band states in ideal Cd 1−x Mn x Te quantum wire structures have been studied theoretically by using multiband effective-mass method. The heavy-and light-hole bands show significant mixing owing to both the one-dimensional quantum confinement and the p-d exchange interaction. Because of the anisotropy of the initial quantization condition determined by the one-dimensional confinement, the Zeeman diagram of the valence bands exhibits anisotropic characteristics depending on the direction of the external magnetic field. According to the magnetic-field evolution of the valence-band states, the optical transition probability shows a dramatic change in the polarization.

We studied polarization anisotropy observed in photoluminescence from closely stacked InAs/GaAs quantum dots (QDs). As the number of stacked layers was increased, the anisotropy in the (001) plane became drastically larger and the [001]-polarization component became larger than the [110] component when observed from the [ 110] direction. However, the polarization intensity of the [ 110] component remained stronger than that of the [001] component in the stacked QDs. Such varied polarization anisotropies depending on the observation direction have been found to result from the valence-band mixing in the vertically coupled electronic states. #

We studied the two-step photon absorption (TSPA) process in InAs/GaAs quantum-dot superlattice (QDSL) solar cells. TSPA of subband-gap photons efficiently occurs when electrons are pumped from the valence band to the states above the inhomogeneously distributed fundamental states of QDSLs. The photoluminescence (PL)-excitation spectrum demonstrates an absorption edge attributed to the higher excited states of the QDSLs in between the InAs wetting layer states and the fundamental states of QDSLs. When the absorption edge of the excited state was resonantly excited, the superlinear excitation power dependence of the PL intensity demonstrated that the electron and hole created by the interband transition separately relax into QDSLs. Furthermore, timeresolved PL measurements demonstrated that the electron lifetime is extended by thereby inhibiting recombination with holes, enhancing the second subband-gap absorption.

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