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We have systematically investigated the photoluminescence (PL) dynamics of free excitons in GaAs/Al0.3Ga0.7As single quantum wells, focusing on the energy relaxation process due to exciton-acoustic-phonon scattering under non-resonant and weak excitation conditions as a function of GaAs-layer thickness from 3.6 to 12.0 nm and temperature from 30 to 50 K. The free exciton characteristics were confirmed by observation that the PL decay time has a linear dependence with temperature. We found that the free exciton PL rise rate, which is the reciprocal of the rise time, is inversely linear with the GaAs-layer thickness and linear with temperature. This is consistent with a reported theoretical study of the exciton-acoustic-phonon scattering rate in the energy relaxation process in quantum wells. Consequently, it is conclusively verified that the PL rise rate is dominated by the exciton-acoustic-phonon scattering rate. In addition, from quantitative analysis of the GaAs-layer thickness and temperature dependences, we suggest that the PL rise rate reflects the number of exciton-acoustic-phonon scattering events. [ABSTRACT FROM AUTHOR]

We present a numerically stable computational method to calculate multiple subband states in quantum wells based on the variational minimization of the total Hamiltonian involving the nonlinear interaction terms such as the Hartree and exchange-correlation interactions. Application of a nonlinear multidimensional minimization procedure to the total Hamiltonian enables us to realize multiple state computations of improved convergence. As an example of the multiple state calculation, subband states of interacting electrons and holes in single quantum wells are computed. We demonstrate that the dependence of the overlap integral of electron and hole wave functions on the electron–hole density is significantly modified by the exchange-correlation interaction. Furthermore, we show the effects of the nonlinear interactions on the carrier population and chemical potential in multiple subbands at various temperatures and carrier concentrations. This method is expected to be a convenient tool for investigating the high-density behavior of carriers in semiconductor low-dimensional structures. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report on the observation of the coherent optical phonon in a hexagonal YMnO3 epitaxial film using a reflection-type pump-probe technique at various temperatures, excitation powers, and energies. We detected an oscillatory structure with a frequency of ~5.1 THz, which is assigned to the coherent optical phonon with A1 symmetry, in a time-domain signal. It was found that the coherent optical phonon can be observed at temperatures from 10K to room temperature, while the oscillation amplitude markedly decreases with an increase in temperature around ~70K corresponding to the Néel temperature. The temperature dependence of the oscillation amplitude indicates that the coherent optical phonon is sensitive to the spin-lattice coupling connected with the antiferromagnetic transition. [ABSTRACT FROM AUTHOR]

The authors investigated the scattering effect of photogenerated carriers on the decay of the coherent longitudinal optical (LO) phonons in an undoped GaAs/n-type GaAs epitaxial structure using a terahertz time-domain spectroscopic technique. The terahertz wave from the coherent LO phonon was observed together with those emitted both by the ultrafast photocurrent and by the coherent LO-phonon-plasmon coupled (LOPC) mode. The simultaneous observation of the coherent LO phonon and coherent LOPC mode originates from the fact that the photogenerated carrier density laterally distributes around the surface of the undoped GaAs layer owing the Gaussian pro- file of the pump beam. The authors found that the terahertz wave from the coherent LO phonon lives up to 5.0 ps, whereas those from the coherent LOPC mode and ultrafast photocurrent disappear within 1.0 ps. The decay time of the coherent LO phonon monotonically decreases with an increase in the pump power. This finding indicates that the dephasing of the terahertz wave from the coherent LO phonon is dominated by the photogenerated carrier scattering in the time range shorter than 1.0 ps in which photogenerated carriers remain. [ABSTRACT FROM AUTHOR]

We have investigated the temperature dependence of photoluminescence-excitation (PLE) spectra of Eu3+ emission due to the intra-4f shell transitions in a Eu-doped GaN epitaxial film from the viewpoint of the energy transfer process by carriers and excitons from the host GaN to Eu3+ ions. It was found that the excitonic band of the PLE spectrum disappears in a low temperature region below ~140K in spite of the fact that the optical transitions of the A and B excitons are clearly observed in a reflectance spectrum. The excitonic PLE band becomes remarkable with an increase in temperature. This fact indicates that carriers originating from the thermal dissociation of photogenerated excitons contribute to the Eu3+ emission. In other words, excitons play no role in the energy transfer process. Furthermore, the PLE spectrum at room temperature exhibits an oscillatory structure resulting from longitudinal-optical phonon emission in a hot carrier relaxation process. [ABSTRACT FROM AUTHOR]

We have investigated the photoluminescence (PL) properties of a CuBr microcavity at 10 K, including the temporal profiles, from the viewpoint of cavity-polariton condensation. The excitation energy density dependence of the PL intensity (band width) of the lower polariton branch at an in-plane wave vector of k// = 0 exhibits a threshold-like increase (decrease). A large blueshift in the PL energy of ~1 0 meV caused by the cavity-polariton renormalization is correlated with the excitation energy density dependence of the PL intensity. The estimated density of photogenerated electron-hole pairs at the threshold is two orders lower than the Mott transition density. These results consistently demonstrate the occurrence of cavity-polariton condensation. In addition, we found that the PL rise and decay times are shortened dramatically by the cavity-polariton condensation, which reflects the bosonic final state stimulation in the relaxation process and the intrinsic cavity-polariton lifetime in the decay process. [ABSTRACT FROM AUTHOR]