Study of many exciton states and homogeneous width of confined exciton in CuCl quantum dots

Biexciton and triexciton states in CuCl quantum dots embedded in NaCl crystals were studied by means of time-resolved size-selective pump-and-probe technique. A clear induced absorption band was observed on the high-energy side of the excitation photon energy. The new induced absorption was assigned to the transition from the exciton ground state to one of the weakly correlated exciton pair states which were theoretically predicted to exist and to play an important role in nonlinear optical processes. Its pump energy dependence and temporal evolution strongly support this assignment. Under highdensity or two-color excitation condition,a triexciton state in quantum dots was observed for the first time.Homogeneous linewidth of CuCl quantum dots was studied by using accumulated photon echo method. Two kinds of samples were used to clarify the influence of the matrix.namely CuCl quantum dots in NaCl crystals and CuCl quantum dots in borosilicate glasses. It was found that the homogeneous linewidth at low temperature was very sharp compared with that previously measured by other experimental techniques. Their temperature dependence were similar at relatively high temperature region (T > 10K) for NaCl sample and glass sample, but they were very different at lower temperature region.This indicates that at low temperatures matrix strongly affects the dephasing of the excitons confined in quantum dots. The temperature dependent homogenenous linewidth was explained by the interaction of the confined acoustic phonon and that of two-level systems (TLS).Thesis (Ph. D. in Science)--University of Tsukuba, (B), no. 1542, 1999.7.23Includes bibliographical references (p. 70-73)"List of publications": p. 78-79Study of many exciton states and homogeneous width of confined exciton in CuCl quantum dots ~ 池沢，道

EXTREMELY-NARROW LINEWIDTH OF THE CONFINED EXCITONS IN CuCl QUANTUM DOTS

Homogeneous linewidth of the excitons confined in CuCl quantum dots embedded in a NaClcrystal was studied by heterodyne-detected accumulated photon echo technique. At 2K, photonecho signal shows an exponential decay with a decay time of 160ps except for a slow rise. Thehomogeneous linewidth calculated from the dephasing time was found to be much smaller thanthat reported previously by the other methods. At 1.5K, we observed the linewidth of 1.6meV. Asfar as we know, this is the narrowest exciton linewidth in quantum dots.24th International Conference on the Physics of Semiconductors : Jerusalem, Israel August 2-7, 199

Longitudinal optical phonons in the excited state of CuBr quantum dots

The size dependence of the longitudinal optical (LO) phonons in the excited state of CuBr quantum dots (QD’s) in glass and NaBr crystals in the intermediate confinement regime was studied by means of persistent spectral hole burning spectroscopy. The phonon-exciton coupled states were clearly observed at a photon energy of about 2.993 eV when the LO phonon energy is close to the energy difference between the ground 1S and excited 1P states of CuBr QD’s in glass. The energies of the LO phonons observed in smaller CuBr QD’s in glass and NaBr crystals were determined to be about 18.6 and 17.6 meV, respectively, which are smaller than that of LO phonons in the bulk CuBr material. The energy softening of the LO phonons was explained in terms of the phonon renormalization

Biexciton and Triexciton States in Quantum Dots in the Weak Confinement Regime

Biexciton and triexciton states in CuCl quantum dots were studied by means of time-resolved size-selective pump-and-probe technique. A clear induced absorption band is observed on the high-energy side of the excitation photon energy. The new induced absorption is assigned to the transition from the exciton ground state to one of the weakly correlated exciton pair states which were theoretically predicted to exist and to play an important role in nonlinear optical processes. Its pump energy dependence and temporal evolution strongly support this assignment. Under high-density or two-color excitation condition, a triexciton state in quantum dots is observed for the first time

Complementary detection of confined acoustic phonons in quantum dots by coherent phonon measurement and Raman scattering

Coherent acoustic phonon oscillation is observed in PbSe quantum dots embedded in phosphate glass by femtosecond pump-and-probe. The size dependence of the oscillation is investigated. Distinct low-frequency peaks are observed in Raman spectrum for the same samples. The size-dependence of the frequencies is well explained by elastic sphere model, but the observed modes are different to each other for coherent phonon and Raman scattering. Coherent phonon measurement and Raman scattering are found to give complementary information on confined acoustic phonons in this system

Tunneling Induced Dephasing And Pauli Blocking In InP Quantum Dots

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

Quantum interference of two photons emitted from a luminescence center in GaAs:N

The indistinguishability of photons emitted from a nitrogen luminescence center in GaAs is investigated by two-photon interference under nonresonant optical excitation. A clear dip is observed in a parallel polarization configuration for consecutively emitted two photons with a 2-ns time interval. The indistinguishability is approximately 0.24, and is found to be independent of the time interval between 2 ns and 4 ns. These results suggest the existence of a very fast dephasing mechanism within 2 ns

Submillisecond electron spin relaxation in InP quantum dots

Light-induced orientation of electron spins in the negatively charged InP quantum dots is found to persist longer than 100 µs. We have proved experimentally that the long-lived orientation is due to slow relaxation of the electron spins rather than to the dynamic nuclear polarization effects

Single-photon generation from a nitrogen impurity center in GaAs

We have demonstrated single-photon emission from a nitrogen luminescence center in GaAs. An inhomogeneously broadened luminescence band formed by localized centers was observed in the spectral range from 1480 meV to 1510 meV at 5 K in nitrogen delta-doped GaAs. Optical properties of the individual centers were investigated by steady-state and time-resolved micro photoluminescence. We have found that a bright luminescence center emits single photons with a radiative lifetime of 650 ps, which is much shorter than the lifetime of NN pairs in previous reports