Search

Hyperfine interaction in a quantum dot produces a spin-coupled system between an injected electron and a few tens of thousands of nuclei in the electron wavefunction. We report here that the double bistable phenomenon of nuclear spin polarization (NSP) can be observed by sweeping three control parameters. All the observations indicate that the NSP has three stable branches under a longitudinal magnetic field, whereas the number of the stable branches of NSP has been believed to be two at most. The incorporation of the electron spin relaxation due to the nuclear spin fluctuation enables reproduction of the observed features of the NSP and the electron spin polarization.

Semiconductor quantum dots provide a spin-coupled system of an electron and nuclei via enhanced hyperfine interaction. We showed that the nuclear spin polarization in single quantum dots can have three stable branches under a longitudinal magnetic field. The states were accompanied by hysteresis loops around the boundaries of each branch with a change in the excitation condition. To explain these findings, we incorporated the electron spin relaxation caused by the nuclear spin fluctuation into the previously studied dynamic nuclear spin polarization mechanism. The model reproduces the features of nuclear spin polarization and the associated strong reduction in the observed electron spin polarization, and can refer to the tristability of nuclear spin polarization.

We study stable and efficient coupling of single photons generated from a quantum dot (QD) into a single-mode fiber (SMF) prepared in a fiber couple module (FCM). We propose a method to focus the objective lens to a sample surface without imaging with the help of laser reflection. By assembling all the constituents, i.e., a pair of lenses, the SMF, and the optical alignment to the QD source, we demonstrate stable single-photon count rate and a high collection efficiency of 43.5% of the photons emitted in air from the QD into the collection lens in the FCM.

GaSb based quantum wells (QWs) show promising optical properties in near-infrared spectral range. In this paper, we present photoluminescence (PL) spectroscopies of InxGa1−xSb/AlyGa1−ySb QWs and discuss the possible thermal quenching and non-radiative carrier recombination mechanisms of the QW structures. The In and Al concentrations as well as the QW thicknesses were precisely determined with the X-ray diffraction measurements. Temperature dependent time-integrated and time-resolved PL spectroscopies resulted in the thermal activation energies of ∼45 meV, and the overall self-consistent calculation of the band parameters based on the measured physical values confirmed that the activation energies are due to the hole escape from the QW to the barriers. The relation of the present single carrier escape mechanism with the other escape mechanisms reported with other material systems was discussed based on the estimated band offset. The relation of the present thermal hole escape to the Auger recombination was also discussed.

We have realized metal-coated semiconductor nanostructures for a stable and efficient single-photon source (SPS) and demonstrated improved single-photon extraction efficiency by the selection of metals and nanostructures. We demonstrate with finite-difference time-domain (FDTD) simulations that inclination of a pillar sidewall, which changes the structure to a nanocone, is effective in improving the photon extraction efficiency. We demonstrate how such nanocone structures with inclined sidewalls are fabricated with reactive ion etching. With the optimized design, a photon extraction efficiency to outer airside as high as similar to 97% generated from a quantum dot in a nanocone structure is simulated, which is the important step in realizing SPS on-demand operations. We have also examined the direct contact of such a metal-embedded nanocone structure with a single-mode fiber facet as a simple and practical method for preparing fiber-coupled SPS and demonstrated practical coupling efficiencies of similar to 16% with FDTD simulation.

We have investigated the optical properties of a single InAsP quantum dot embedded in a standing InP nanowire. Elongation of the transverse exciton-spin relaxation time of the exciton state with decreasing excitation power was observed by first-order photon correlation measurements. This behavior is well explained by the motional narrowing mechanism induced by Gaussian fluctuations of environmental charges in the nanowire. The longitudinal exciton-spin relaxation time is evaluated by the degree of the random polarization of emission originating from exciton states confined in a single-nanowire quantum dot by using Mueller calculus based on Stokes parameters representation. The reduction in the random polarization component with decreasing excitation power is caused by suppression of the exchange interaction of electron and hole due to an optically induced internal electric field by the dipoles at the wurtzite and zinc-blende heterointerfaces in the InP nanowire.

We investigate the photoluminescence temperature dependence of individual InAs/InGaAlAs quantum dots emitting in the optical telecommunication bands. The high-density dots are grown on InP substrates and the selection of a smaller dot number is done by the processing of suitable nanometer-sized mesas. Using ensembles of only a few dots inside such mesas, their temperature stability, inter-dot charge transfer, as well as carrier capture and escape mechanisms out of the dots are investigated systematically. This includes the discussion of the dot ensemble and individual dots. Among the single-dot properties, we investigate the transition of emission lines from zero-phonon line to acoustic phonon sideband-dominated line shape with temperature. Moreover, the presence of single recombination lines up to temperatures of about 150K is demonstrated.

The spin interaction between an electron and nuclei was investigated optically in a single self-assembled InAlAs quantum dot (QD). In spin dynamics at the initial stage, the fluctuation of nuclear field and the resulting electron spin relaxation time play a crucial role. We examined a positively charged exciton in a QD to evaluate the key physical quantities directly via the temporal evolution measurements of the Overhauser shift and the degree of circular polarization. In addition, the validity of our used spin dynamics model was discussed in the context of the experimentally obtained key parameters.

Superconductivity and optoelectronics have developed almost independently and had very rare interactions with each other in science and technologies. However recent interdisciplinary research opens up the potential for developing new optoelectronic devices. This review paper presents our recent theoretical and experimental demonstrations that superconductivity significantly modifies and accelerates photon generation processes. We have prepared superconducting light emitting diodes (LEDs) emitting at ∼1.6-µm optical-fiber communication band for the experimental demonstrations. This new-type LED operation is based on a unique physics related to Cooper-pair interband transition in a semiconductor, and further research leads to the solid-state simultaneously generated photon-pair sources for the potential application in quantum information and communication.

We experimentally demonstrate Cooper pairs’ drastic enhancement of the band-to-band radiative recombination rate in a semiconductor. Electron Cooper pairs injected from a superconducting electrode into an active layer by the proximity effect recombine with holes injected from a p-type electrode. The recombination of a Cooper pair with p-type carriers dramatically increases the photon generation probability of a light-emitting diode in the optical-fiber communication band. The measured radiative decay time rapidly decreases with decreasing temperature below the superconducting transition temperature of the niobium electrodes. Our results indicate the possibility to open up new interdisciplinary fields between superconductivity and optoelectronics.

High efficiency in the extraction, transmission and detection of single and entangled photons is one of the most significant factors to provide general usage and to suppress the bit-error rate in optical communication networks. We propose and realize metal embedded nanostructures with quantum dots (QDs) as photon sources to meet these challenges on the emitter side. Advantages of the process are the ability of fast nanometer-scale fabrication and the high reproducibility and yield. Mesas with typical taper angles of 20–30° and diameters between 100 nm and 1 µm were produced; these nanostructures are referred to as “cones” in this work. The dependence of the taper angle on the composition of the etched cone material is discussed, focusing on the contribution of indium. The nano-cones used as photon sources were embedded in highly reflective metal and turned upside down to provide efficient and partly directed photon extraction. We present the selection of 1.35 µm photons emitted from a single QD as a result of embedding only a small number of dots in a nanometer sized cone.

Photon emission with nonclassical photon statistics is discussed with a single InAlAs quantum dot. The deterministic single-photon generation in which the emitted photon wavelength matches well to the highly sensitive wavelength region of highly efficient, low-noise Si-single-photon detectors and also to an atmospheric transmission window is demonstrated. Competing transition processes between neutral and charged exciton species originating from an exclusive formation in the same single quantum dot are clarified. It was found that suppressing the charged exciton formation is possible by a quasi-resonant excitation for a deterministic monochromatic single-photon generation. Polarization-dependent photoluminescence spectroscopy clearly indicates the preservation of photon polarizations between photons emitted by biexciton/exciton recombinations. Furthermore, the deterministic polarization-correlated photon pair generation with biexciton-exciton cascaded transition occurring in a single quantum dot is directly confirmed by the polarized second-order photon correlation measurements. This indicates a longer polarization flip time than the exciton lifetime, which is an essential requirement for the deterministic Einstein-Podolsky-Rosen photon pair generation under the present biexciton-exciton cascaded transition scheme.

Triggered single-photon generation from InAlAs quantum dot (QD) was demonstrated for the first time. Emitted photon energy coincides with high detection efficiency range of Si single-photon detectors, which is highly suitable for free-space communication. Single-QD spectroscopy and crossed photon correlation measurements unambiguously revealed that several emitting lines observed in a single mesa structure originated from the identical QD, and two temporary competing decay processes associated with neutral states and charged states were identified. Presence of the competing process is also inferred from an analysis of steady-state photoluminescence intensities. Formation process of charged exciton in QD is also discussed.

Single-photon emitters and detectors are the key devices to realize secure communications with single-photon-based quantum cryptography and single-photon-based quantum computing. InAlAs quantum dots (QDs) cover the wavelength range with high quantum efficiencies of Si-based single-photon detectors. Clear photon antibunching was observed from an InAlAs single QD under weak excitations. To realize single-photon emitters on demand, complete population of the QD energy states before the photon emission events is necessary, but the measured antibunching properties were dependent substantially on the photo-excitation powers. The physical origin of this problem is discussed and the criterion to distinguish the real deviation from the photon antibunching condition and the artifact of the measurements is clarified. The capability of single-photon emissions on demand will be demonstrated with photon antibunching under pulsed operations.

Single-photon emitters and detectors are key devices to realize secure communications with single-photon-based quantum cryptography and single-photon-based quantum computing. InAlAs quantum dots (QDs) cover the wavelength range with high quantum efficiencies of Si-based single-photon detectors. Clear photon antibunching was observed from an InAlAs single QD under weak excitations. To realize single-photon emitters on demand, complete population of the QD energy states before the photon emission events is necessary, but the measured antibunching properties were dependent substantially on the photo-excitation powers. The physical origin of this problem is discussed. The criterion to distinguish the real deviation from the photon antibunching condition and the artifact of the measurements is clarified. The capability of single-photon emissions on demand will be demonstrated with photon antibunching under pulsed operations.