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Fast electron transfer from PbSe quantum dots (QDs) to the porous anatase TiO2 film was observed in transient absorption, when the lowest unoccupied molecular orbital level of PbSe QDs is higher than that of TiO2. In PbSe QDs 2.7nm in diameter linked to the TiO2 film the bleaching recovery decay shortened to 1ps from 650ps observed in the non-linked PbSe QDs. The electron transfer from the quantum state in small PbSe QDs to TiO2 takes place fast and efficiently.

We report on the measurement of the exciton homogeneous linewidth in nitrogen impurity centers in GaAs:N. Fourier spectroscopy on a single center revealed a long coherence time over 300 ps at low temperature. The narrowest linewidth obtained at liquid helium temperature is 3.5 μeV, which is comparable with that of semiconductor quantum dots. The linewidth increases with increasing temperature, showing a thermally activated behavior with activation energies of 2∼5 meV.

The authors studied luminescence polarization of individual NN centers in GaP:N and discussed the atomic configurations. In addition to the widely known green luminescence centers, NN1∼NN10, new luminescence centers were observed in the infrared spectral region, which may be due to the true nearest‐neighbor NN pair.

We have studied the optical orientation and polarization relaxation dynamics of electron spins in Ga‐doped and undoped ZnO films by time‐resolved Kerr rotation (TRKR). In the Ga‐doped film, we have observed a long‐lived spin signal more than 10 ns when the pump‐probe energy is resonant to the neutral donor‐bound excitons (D° X). It is not observed in the undoped film. Meanwhile, when the energy is resonant to free excitons, faster spin relaxation of about 2 ns is observed for itinerant electrons in both the films. The optical spin orientation is always parallel to the c‐axis in the ZnO films, and independent of the pump incidence angle. This contrasts with the observation in crystals with the zincblende structure.

Coherent precession of electron spins and hole spins in one‐electron‐doped InP quantum dots is triggered by the circularly polarized pump pulse and succeeding precession is studied by means of time‐resolved Kerr rotation. Formation of trion states induces prompt coherent precession of trion (hole) spins and spin flip of the hole in the trion followed by annihilation of trion induces delayed coherent precession of electron spins. The circularly polarized pump pulse triggers prompt coherent precession of electron spins via spin flip Raman process, too. We found two spin initialization processes of doped electrons in InP quantum dots.

Spin relaxation of electrons doped in InP quantum dots was studied by means of luminescence pump-probe and Hanle measurements. Optical pumping makes spins of doped electrons to be oriented in parallel to the helicity of the circularly polarized excitation. The luminescence pump-probe showed the spin orientation of the doped electrons decay on a millisecond time-scale. Hanle measurement clarified the spin dephasing relaxation time of doped electrons is 1.7 ns which is explained by the frozen fluctuation of nuclear spins., Comment: 3 pages, 2 figures, presented in ICPS-2006

application/pdf, We observed the tunneling induced dephasing and Pauli blocking in InP quantum dots (QDs). A highly-sensitive heterodyne-detected photon echo method enabled us to observe the signal from one layer of self-assembled InP QDs in charged or neutral condition controlled by the electric field. The photon echo signal decreased much with the increase of electron doping. The photon echo of neutral InP QDs under the electric field showed tunneling-induced dephasing, which decays non-exponentially reflecting the non-Markovian nature of the tunneling process.

application/pdf, The persistent spectral hole burning (PSHB) phenomena were discovered in many kinds of nanocrystalline semiconductors, such as CdSe, CdS, CuCl, CuBr and CuI, embedded in crystals, glass or polymers. Hole burning in the luminescence spectrum was also observed. The observed PSHB phenomena are explained by the ionization of nanocrystals. The photoionization of nanocrystals takes place by tunneling through potential barriers with broadly distributed barrier height and thickness. The energy of photoionized nanocrystals is changed from the original energy and their new energies depend on the spatial arrangement of trapped carriers. Quantum confinement of carriers and resulting strong Coulomb interaction between confined carriers and trapped carriers cause the energy change.

application/pdf, We have measured photoluminescence (PL) spectra and time-resolved PL in CdTe quantum dots (QDs) under the longitudinal magnetic field up to 10T. Circular polarization of PL increases with increasing magnetic field, while its linear polarization is absent under linearly polarized excitation. Time-resolved PL measurements clarified that this behavior is caused by the suppression of spin relaxation induced by the longitudinal magnetic field. We believe that this behavior is related to the hyperfine interaction of electron spin with magnetic momenta of lattice nuclei.

application/pdf, We observed a light-hole-to-electron transition of strain-induced quantum dots (SIQDs) in a strongly strained GaAs quantum well (QW). Circularly-polarized luminescence excitation spectroscopy showed a pronounced counter-circularly polarized component below the heavy-hole exciton transition in the QW. This special feature can be explained by the existence of the light-hole-to-electron transition at this energy level. On the other hand, a very high co-circular polarization (=0.8) was obtained near the lowest transition of SIQD.

application/pdf, Periodic oscillation is observed in transient four-wave mixing signal in strain-induced GaAs quantum dots. The oscillation is assigned to biexcitonic beat based on its polarization dependence and its initial phase, and the biexciton binding energy is deduced. Since strain-induced dots are produced by adding lateral confinement to a quantum well, direct comparison of the biexciton binding energy in 2D and in 0D system is possible. The beat period in quantum dots is found to be shorter than that in quantum well, and the enhancement factor by the reduced dimensionality is 1.5. Magnetic field dependence of the biexciton binding energy is also studied. It is independent of the magnetic field up to 8 T, and exciton g-factor in the strain-induced dots is obtained.

application/pdf, Excitonic lasing from single ZnO nanobelts was observed at room temperature, which was due to the exciton-exciton scattering processes appearing under intense light excitation. Morphologies of the nanobelts are rectangular shapes, so that crystalline facets of the nanobelts acted as the lasing cavity mirrors. We demonstrated that mode spacings correspond to cavity lengths of the respective nanobelts, and directly observed the lasing from single ZnO nanobelts by mapping of the luminescence intensity.