Michio Ikezawa

Single Photon Emission from Individual Nitrogen Pairs in GaP

Luminescence from single isoelectronic traps in nitrogen δ-doped gallium phosphide was studied by micro-photoluminescence spectroscopy. These single traps were found to emit photons that were nearly identical in both emission energy and brightness. Furthermore, single photon emission from a single nitrogen pair is confirmed by a strong antibunching under continuous optical excitation at 30 K. These results imply that single nitrogen pairs in GaP might be useful for realizing unique sources of energetically-defined single photons, and even indistinguishable photons.

Type-II behavior in wurtzite InP/InAs/InP core-multishell nanowires

We study optical transitions from a periodic array of InP/InAs/InP core-multishell nanowires (CMNs) having a wurtzite crystal structure by using photoluminescence (PL) and PL excitation (PLE) spectroscopy. Observing a large Stokes shift between PL and PLE spectra, a blueshift of the PL peak with a cube-root dependence on the excitation power and a slow and nonexponential decay of PL with an effective decay time of 16 ns suggest a type-II band alignment. Band-offset calculation based on the “model-solid theory” of Van de Walle [Phys. Rev. B 39, 1871 (1989)] supports type-II band lineup if the InAs layer in the wurtzite CMNs is assumed to sustain compressive strain in all directions.

Size-dependent picosecond energy relaxation in PbSe quantum dots

We measured the size dependence of the energy relaxation time of PbSe quantum dots at room temperature. The radius of the dots was evaluated by small-angle x-ray scattering. We found a monotonic decrease of the relaxation time from 25 to 1 ps with a decrease of the radius r from 2.9 to 1.4 nm. The 1/r 3 dependence of the relaxation rate was indicated.

Ultrafast carrier dynamics in CuInS2 quantum dots

The ultrafast carrier dynamics in CuInS2 (CIS) quantum dots (QDs) was studied by means of femtosecond transient absorption (TA) spectroscopy. The size-dependent 1S transition energy determined from bleaching spectra is in agreement with that calculated on the finite-depth-well model in the effective mass approximation. The TA bleaching comes from filling of electron quantized levels, allowing us to know the dynamics of the 1S electron in CIS QDs. The sub-100-ps electron trapping at surface defects in bare QDs accelerates with decreasing QD size, while is effectively suppressed in well-passivated CIS/ZnS core/shell QDs.

Efficient energy transfer from hole transporting materials to CdSe-core CdS/ZnCdS/ZnS-multishell quantum dots in type II aligned blend films

We studied the energy transfer between CdSe core/shell quantum dots (QDs) and hole transporting materials (HTMs) in type II aligned inorganic/organic blend films by using steady-state and time-resolved photoluminescence (PL) spectroscopy. The lengthening and shortening in PL lifetimes of the QDs in HTMs under resonant excitation condition were explained by energy transfer and charge separation processes. Surprisingly, the maximum energy transfer efficiency from 4,4′,4″-Tris (carbazol-9-yl)-triphenylamine (TcTa) to CdSe/CdS/ZnCdS/ZnS core/multishell QDs was determined to be 86% by calculating the excited state lifetime of the TcTa molecules participating in the energy transfer process.

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.

Observation of excited biexciton states in CuCl quantum dots: Control of the quantum dot energy by a photon

Experimental observations of the excited biexciton states in CuCl quantum dots embedded in a NaCl crystal are presented. An increased absorption due to the transition from the exciton state to the excited biexciton states was clearly observed on the high-energy side of the exciton resonance in subpicosecond pump-and-probe spectra at 77 K. The energy difference between it and the exciton resonance largely increased with decreasing dot size. Temporal evolution of the induced absorption band was studied and the results were consistent with the identification of the excited biexciton states.

Millisecond-Range Electron Spin Memory in Singly-Charged InP Quantum Dots

We report millisecond-range spin memory of resident electrons in an ensemble of InP quantum dots (QDs) under a small magnetic field of 0.1 T applied along the optical excitation axis at temperatures up to about 5 K. A pump-probe photoluminescence (PL) technique is used for optical-orientation of electron spins by the pump pulses and for study of spin relaxation over the long time scale by measuring the degree of circular polarization of the probe PL as a function of pump-probe delay. Dependence of spin decay rate on temperature and magnetic field suggests two-phonon processes as the dominant spin relaxation mechanism in these QDs at low temperatures.

Ultrafast Carrier Dynamics and Hot Electron Extraction in Tetrapod-Shaped CdSe Nanocrystals.

The ultrafast carrier dynamics and hot electron extraction in tetrapod-shaped CdSe nanocrystals was studied by femtosecond transient absorption (TA) spectroscopy. The carriers relaxation process from the higher electronic states (CB2, CB3(2), and CB4) to the lowest electronic state (CB1) was demonstrated to have a time constant of 1.04 ps, resulting from the spatial electron transfer from arms to a core. The lowest electronic state in the central core exhibited a long decay time of 5.07 ns in agreement with the reported theoretical calculation. The state filling mechanism and Coulomb blockade effect in the CdSe tetrapod were clearly observed in the pump-fluence-dependent transient absorption spectra. Hot electrons were transferred from arm states into the electron acceptor molecules before relaxation into core states.

The enhanced binding energy for biexcitons in InAs quantum dots

We observed that the biexciton binding energy in InAs quantum rhombic disks (QRDs) is enhanced by twice compared with that for InAs quantum dots (QDs) so far reported around 1.24 μm nearby the telecommunication wavelength. The heterodyne-detected four-wave-mixing detected the exciton-biexciton quantum beat superposed on photon echo decay, giving the biexciton binding energy of 3.4 meV to 3 monolayer (ML) InAs QRDs and 4.1 meV to 4 ML InAs QRDs, respectively. The largest biexciton binding energy of 4.1 meV in InAs QDs is ascribed to increased electron-hole overlap in confined geometry with a minimized strain distribution.