Osamu Wada

Impurity doping in self-assembled InAs/GaAs quantum dots by selection of growth steps

We carry out Si doping in InAs/GaAs quantum dots (QDs) by selecting appropriate steps of the self-assembling growth process. The self-assembling growth process of QDs consists of nucleation, assembling, self-limiting, and dissolving steps. The electrical conductivity of the QDs doped at the various growth steps has been investigated by conductive atomic force microscopy. The two-dimensional current images demonstrate that the spatial carrier distribution remarkably depends on the growth steps. When Si impurities are introduced into QDs during the assembling step, carriers are preferentially incorporated in the QDs. Furthermore, the doped QDs lead to enhancement of the photoluminescence intensity and to suppression of the temperature quenching of the intensity.

Temperature-dependent carrier tunneling for self-assembled InAs/GaAs quantum dots with a GaAsN quantum well injector

We have investigated the carrier tunneling process in a quantum-dot (QD) tunnel injection structure, which employs a GaAs1−xNx quantum well (QW) as a carrier injector. The influence of the barrier thickness between the GaAs1−xNx well and InAs dot layer has been studied by temperature-dependent photoluminescence. Although the 2.5 nm barrier sample exhibits the best tunneling efficiency, a 3.0 nm thickness for the barrier is optimum to retain good optical properties. The carrier capture time from the GaAs1−xNx QW to QD ground states has been evaluated by time-resolved photoluminescence. The result indicates that efficient carrier tunneling occurs at temperatures above 150 K due to the temperature dependent nature of phonon-assisted processes.

Detailed Design and Characterization of All-Optical Switches Based on InAs/GaAs Quantum Dots in a Vertical Cavity

We propose an all-optical switch based on selfassembled InAs/GaAs quantum dots (QDs) within a vertical cavity. Two essential aspects of this novel device have been investigated, which includes the QD/cavity nonlinearity with appropriately designed mirrors and the intersubband carrier dynamics inside QDs. Vertical-reflection-type switches have been fabricated with an asymmetric cavity that consists of 12 periods of GaAs/Al0.8Ga0.2As for the front mirror and 25 periods for the back mirror. All-optical switching via the QD excited states has been achieved with a time constant down to 23 ps, wavelength tunability over 30 nm, and ultralow power consumption less than 1 fJ/μm2 . These results demonstrate that QDs within a vertical cavity have great advantages to realize low-powerconsumption polarization-insensitive micrometer-sized switching devices for the future optical communication and signal processing systems.

Temperature dependence of photoluminescence characteristics of excitons in stacked quantum dots and quantum dot chains

We have investigated the effects of temperature on the photoluminescence (PL) characteristics of excitons in ordinary stacked quantum dots (QDs) and QD chains in which QDs are interconnected along the growth direction. While the temperature dependence of the PL intensity of both samples is similar, that of the PL decay time is different. In addition, the PL decay times of both samples monitored at 150 K clearly depend on the detection energy. This result is attributed to lateral QD coupling. From these results, in ordinary stacked QDs, the exciton transfer owing to the lateral coupling is the only cause of the increase in the PL decay time. On the other hand, in QD chains, the interconnection along the chain direction as well as the lateral coupling is considered to cause the change in the PL characteristics and induce the extremely long exciton lifetime.

Magneto‐Photoluminescence Spectroscopy of Exciton Fine Structure in Nitrogen δ‐Doped GaAs

We have studied the magnetic‐field evolution of the fine structure splitting of the exciton bound to nitrogen (N) pairs in GaAs in the Faraday configuration. With applying the magnetic field, the photoluminescence (PL) spectrum splits into several signals and changes their intensities. The observed magneto‐PL spectra indicate the mixing between the split exciton states having the orthogonal linear polarization components in the zero‐magnetic field, which demonstrates the characteristic Zeeman effects of the anisotropic exciton structure bound to the N pairs.

All-optical switch using InAs quantum dots in a vertical cavity

We have investigated at the first time an all-optical switch using self-assembled InAs/GaAs quantum dots (QDs) within a vertical cavity structure. The optical nonlinearity of the QD switch has been optimized by an asymmetric cavity to achieve the maximum differential reflectivity. Optical switching via QD excited states exhibits a fast decay with a time constant down to 23 ps and a wavelength tunability over 30 nm. By compared to the theoretical design, the absorption strength of QD layers within the cavity has been determined.