We have investigated the temperature dependence of photoluminescence (PL) dynamics in CdS quantum dots (QDs) prepared by a colloidal method. A size-selective photoetching process and a surface modification technique with a Cd(OH) 2 layer enabled us to prepare size-controlled CdS QDs with high PL efficiency. The PL decay profiles became slower with an increase in temperature, contrary to an ordinary behavior. We have revealed that such anomalous temperature dependence of the PL-decay profile is explained by a three-state model consisting of a ground-state and two excited states: a lower-lying bound-exciton state and a higherlying free-exciton state (the "dark-exciton state") having an optically inactive triplet nature.
We have investigated the active-layer-thickness dependence of exciton-photon interactions in planar CuCl microcavities with HfO 2 /SiO 2 distributed Bragg reflectors. The active layer thickness was changed from λ/32 to λ/4, while the cavity length was fixed at λ/2. We performed angle-resolved reflectance measurements and clearly detected three cavity-polariton modes, originating from the lower, middle, and upper polariton branches, in a strong-coupling regime of the Z 3 and Z 1,2 excitons and cavity photon. The incidence-angle dependence of the cavity-polariton modes was analyzed using a phenomenological Hamiltonian for the strong coupling. It was found that the interaction energies of the cavity-polariton modes, the so-called vacuum Rabi splitting energies, are systematically controlled from 22(37) to 71(124) meV for the Z 3 (Z 1,2 ) exciton by changing the active layer thickness from λ/32 to λ/4. The active-layer-thickness dependence of the Rabi splitting energy is quantitatively explained by a simple theory for quantum-well microcavities.
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