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.

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.

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.

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.

The electronic structures in a single pair of InAs/ GaAs coupled quantum dots ͑CQDs͒ with various interdot spacings are investigated by performing photoluminescence ͑PL͒ and photoluminescence excitation ͑PLE͒ 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.

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.

We have proposed the basic device structures for an all-optical controlled two-qubit quantum logic gate using two excitons confined in InAs/GaAs coupled quantum dots ͑CQDs͒. A two-qubit quantum logic gate is constructed using a correlated two-exciton system. This simplest two-qubit system involves four states such as the crystal ground states, two distinguishable exciton states, and a correlated-exciton-molecule state consisting of two excitons. The formation of this correlated-exciton-molecule state, which consists of two different excitons in a CQD, is the most important and difficult technology. In this study, we demonstrate the formation of the four states involving this correlated-exciton-molecule state and confirm two-qubit exciton system in CQDs. Moreover, we show that a correlated exciton molecule is created by employing a cascade process.

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 mm emission with a low coupling coefficient of 22 cm À1 and a threshold current of 35 mA by using high-density and high-uniformity QDs.