Keishiro Goshima

Observation of interdot correlation in single pair of electromagnetically coupled quantum dots

The interdot correlation in a single pair of InAs∕GaAs barrier-coupled quantum dots (QDs) is investigated by microphotoluminescence spectroscopy, in which each QD is individually excited at unique energy levels. Surprisingly, we observe an anomalous increase in the luminescence intensity when the two QDs are excited simultaneously. This remarkable finding can be interpreted in terms of the electromagnetic coupling between QDs with thick barrier layers.

Optical characteristics of InAs/GaAs double quantum dots grown by MBE with the indium-flush method

We fabricated InAs/GaAs double quantum dot (QD) structures by molecular beam epitaxy (MBE) with the Indium-Flush method, where the energy separation between the electron levels of two QDs was less than the longitudinal optical (LO) phonon energy with a different barrier thickness. We confirm the peak energy shift between the double QDs in the photoluminescence (PL) spectra and assign this shift to the wave function coupling effect between the double dots. We also measured the time resolved PL spectra and observed the carrier transfer from smaller QDs to larger ones in the time domain. By estimating the tunneling time between double QDs, we obtain a tunneling time that is longer than the exciton decay time in single QD. Additionally, we mention the fade-out of the electron LO phonon interaction with the electron wave function coupling between double QDs based on the result of photoluminescence excitation measurements. These results suggest that our structures are attractive for quantum information processing.

Electronic structures in single pair of InAs∕GaAs coupled quantum dots with various interdot spacings

The electronic structures in a single pair of InAs∕GaAs coupled quantum dots (CQDs) with various interdot spacings are investigated by performing photoluminescence (PL) and photoluminescence excitation (PLE) measurements. Luminescence from the bonding (X+) and antibonding (X−) states caused by electron-wave-function coupling was observed in the micro-PL spectra of the CQDs. We indicate the contribution of the hole excited states to the PL spectra in QDs based on the results for the spectral dependence on circularly polarized light and the structures of PLE spectra. PLE spectra reveal the electronic structures of the CQD system at higher energy states where both the common excited levels due to the level sharing between the electron excited states and the individual excited levels related to the hole excited states coexist. In addition, we mention that the energy-level mixing due to the strong-wave-function coupling between two QDs influences the decoherence of the carrier relaxation processes.

Observation of exciton molecule consisting of two different excitons in coupled quantum dots

We propose and experimentally investigate an exciton molecule consisting of two different excitons in coupled quantum dots (QDs). Quantum mechanical coupling between double QDs leads to the creation of bonding and antibonding states and should yield an exciton molecule consisting of two excitons that originate from these two states. We prepared a quantum mechanically coupled QD system and succeeded in observing a single exciton molecule in a single pair of coupled QDs by means of a two-color excitation photoluminescence measurement.

Pulse area control of exciton rabi oscillation in InAs/GaAs single quantum dot

We investigated the optical properties of an exciton and a charged exciton in an InAs/GaAs single quantum dot (QD) with truncated pyramidal shape by microspectroscopy, and clarified the difference of sub-band structure between the exciton and the charged exciton in the same single QD. We observed the exciton population of the excited states by monitoring the luminescence of the ground state exciton and succeeded in the experimental demonstration of Rabi oscillation of the exciton and the charged exciton. The transition dipole moments estimated from experimental results in a pure InAs QD are 32 and 40 D for the charged exciton and exciton, respectively, which were comparable to those in InGaAs QD.

Electronic structures and carrier correlation in single pair of coupled quantum dots

The electronic structures and carrier correlation in a single pair of InAs/GaAs coupled quantum dots (QDs) are investigated by performing photoluminescence (PL), one-color photoluminescence excitation (PLE) and two-color PLE measurements. Luminescence from the bonding (X+) and anti-bonding (X-) states due to the wave function coupling was observed in the micro-PL (µ-PL) spectra of the coupled QDs. One-color PLE spectra reveal the electronic structures of the coupled QD system in which there is the coexistence of both common excited level series between the X+ and X- states and individual excited level series for each state. In two-color PLE measurement, the suppression of PLE peak intensity at the energy separation of the longitudinal optical (LO) phonon suggests a carrier correlation through the screening effect of the carrier–LO-phonon interaction in the coupled QD system. Additionally, we demonstrate the control of the energy state in the coupled QDs using two-color excitations.

Coherent Control of Exciton in a Single Quantum Dot Using High-Resolution Michelson Interferometer

We developed a high-resolution Michelson interferometer with a He–Ne two-frequency laser positioning system, and measured the coherent carrier dynamics of a single InAs self-assembled quantum dot (SAQD) using a micro-spectroscopy system. The phase-locked double pulses were stabilized, with the maximum deviation being below 10 nm during the long measurement time of 1 h. Using this system, coherent control of an exciton in an InAs SAQD with very fine phase stabilization was demonstrated. The dephasing time of the single quantum dots was 9.5 ps which is close to that estimated from the homogeneous linewidth in the photoluminescence excitation (PLE) spectrum.

Exciton Rabi Oscillation in Single Pair of InAs/GaAs Coupled Quantum Dots

We measured the optical properties of exciton states and the exciton Rabi oscillation in a single pair of coupled quantum dots (CQDs). We compared the transition dipole moments of exciton in a single quantum dot (SQD) and CQDs. The transition dipole moment estimated from the experimental result in a single pair of CQDs was 72 D, which is larger than that for SQD. We concluded that the large transition dipole moment is attributed to the large coherence volume in CQDs.

1.3 µm Distributed Feedback Laser with Half-Etching Mesa and High-Density Quantum Dots

We propose a quantum dot laser with a new distributed feedback (DFB) structure, namely, a half-etching mesa (HEM) DFB laser that employs a vertical grating structure and requires no regrowth process. The features of the HEM structure are a low coupling coefficient and a low threshold current achieved by suppressing the large scattering loss in the quantum dots (QDs) active layer. This structure is realized by etching vertical gratings only as far as the center of the active layer. We demonstrated fundamental mode control for a 1.3 µm emission with a low coupling coefficient of 22 cm-1 and a threshold current of 35 mA by using high-density and high-uniformity QDs.

Optical Control of 2-Qubit Exciton States in a Coupled Quantum Dot

We investigated the formation dynamics of an artificial exciton molecule state, which consists of two different excitons confined in each quantum dot of a pair of quantum dots, namely, a coupled quantum dot (CQD). By the selective optical excitation of the inherent energy states of two different excitons using two-color pulsed excitation, we created an artificial exciton molecule state and investigated the transient dynamics, and monitored the photoluminescence, namely, the population of the artificial exciton molecule states. By employing the power dependence of the two colors, we tried the two-qubit logic gate operation and succeeded in controlling the population of an artificial exciton molecule (pulse area ?