M. Hosoda

Degeneracy breaking of optical resonance modes in rolled-up spiral microtubes

The authors investigate the optical resonance modes (ORMs) generated in semiconductor microtubes of spiral symmetry by using finite-difference time-domain methods. Even though the tube-wall thickness is much smaller than the light wavelength, modes having an electric field parallel to the tube wall can loop around the tube periphery as ORMs. In addition, the degenerated ORMs present in conventional cylindrical cavities such as whispering gallery modes are broken and separated into two modes due to the spiral asymmetry of the rolled-up microtubes.

Quantum-well microtube constructed from a freestanding thin quantum-well layer

We fabricated and experimentally investigated a nanostructure known as a quantum-well (QW) microtube, which is a fine tube with a micron- or nanometer-order diameter fabricated by rolling a semiconductor GaAs QW. Although the wall thickness is only 40 nm, the system retains the quantum properties of a QW, and photoluminescence from the QW subband can be clearly observed. Even though the QW width is sufficiently small to make the QW subband type-II band-aligned, a type-II to type-I transition caused by uniaxial strain in the microtube allows for optical emission.

Photoluminescence of GaAs/AlGaAs micro-tubes containing uniaxially strained quantum wells

Abstract We fabricated micro-tubes containing two GaAs/AlGaAs quantum wells (QWs) in a section of the tube layer, and studied the optical properties of the embedded QWs. A multilayer structure composed of GaAs and AlGaAs layers and a lattice-mismatched InGaAs layer was epitaxially grown on a GaAs substrate by MBE. This multilayer structure rolled up by the built-in strain when it was freed from the substrate. By measuring the photoluminescence peak shift of the QWs caused by the uniaxial strain, we were able to determine the radial profile of the strain within the micro-tube wall.

Current self‐oscillations in photoexcited type‐II GaAs‐AlAs superlattices

Self‐oscillations of the photocurrent have been observed in type‐II GaAs‐AlAs superlattices. In addition to the fundamental frequency, several higher harmonics are present. The frequency of the oscillations can be tuned for a fixed carrier density from 15 to 120 MHz by simply changing the applied bias. The frequency distribution within a certain voltage range can be varied by changing the density of photoexcited carriers. For larger carrier densities, higher frequencies are observed in a different voltage range. This system could therefore be used as a high‐frequency oscillator, which can be controlled by two external parameters, the applied voltage and the light intensity.

Influence of Γ-X resonances on Γ1 ground state electron occupation in type-I GaAs/AlAs superlattice

The influence of Γ-X resonances on Γ1 ground state electron occupation in type-I direct-gap GaAs/AlAs superlattices was studied under an applied electric field. Photoluminescence and photocurrent-voltage characteristics showed anomalous behaviors at corresponding Γ-X resonance voltages. The experimental results demonstrate that Γ1-Xn transfer degrades the sweep-out of carriers, while X1-Γ2 transfer promotes it.

Formation of electric-field domains in an asymmetric double-quantum-well GaAs/AlAs superlattice

We report the observation of electric-field domain formation in an asymmetric double-quantum-well GaAs/AlAs superlattice. Photoluminescence (PL) spectra exhibit several branches with increasing bias voltage. These steeply red-shifted PL branches can be attributed to Stark-ladder transitions between the heavy hole ground state in the narrow quantum well and the electron ground state in the wide quantum well in the high-field domain. According to our calculations, electrons and holes in the high-field domain are separated into different quantum wells, resulting in a large reduction of the radiative recombination energy with increasing electric field. Each PL branch most likely corresponds to the moving of the domain boundary.

Avalanche breakdown mechanism originating from Γ–X–Γ transfer in GaAs/AlAs superlattices

An avalanche breakdown mechanism in GaAs/AlAs type-I superlattices is demonstrated. This mechanism shows its power at a bias voltage where both of the following two conditions are met. One is electron transfer from the Γ ground state to the X ground state (Γ1-X1), and the other is the escape of electrons from the X1 state to the second Γ state (X1-Γ2). Under both conditions, because the AlAs barriers become transparent for electron transport due to the Γ1-X1-Γ2 path, the drift speed (i.e., the acceleration of electrons) grows, and then the superlattice shows the phenomenon of avalanche breakdown. From our experimental results for various GaAs/AlAs superlattices, it is thought that such avalanche breakdown frequently occurs when type-I GaAs/AlAs superlattices have thick barrier widths.

Delayed photocurrent affected by Γ‐X resonance in GaAs/AlAs type‐I short‐period superlattices

Delayed photocurrents were observed in GaAs/AlAs type‐I short‐period superlattices by measuring time‐resolved photoresponses under ultrashort optical pulse excitation. According to the envelope function calculations, the X1 state in AlAs barriers resonates with the Γ2 state in the adjacent GaAs wells at a bias voltage where the delayed photocurrents were conspicuous. These results strongly suggest that the dynamic carrier transport process is significantly influenced by X1‐Γ2 resonance effects in the superlattices.

Efficient short-wavelength light emission from asymmetric double quantum wells by using electron and hole collection into the same narrow quantum well

We propose an effective method for carrier injection into the high-energy Γ ground state in a narrow quantum well (QW) in asymmetric double QW systems using Γ–X–Γ electron transfer and simultaneous hole tunneling. Although the high-energy state is type-II band-aligned for the electrons, our method enables an efficient injection of both electrons and holes into the same narrow QW, and it thus demonstrates relatively strong light emission from the higher energy state.

Photoluminescence property of uniaxial strained GaAs/AlGaAs quantum wells contained in a micro-tube

Abstract Photoluminescence (PL) properties of uniaxially strained GaAs/AlGaAs quantum wells contained in a micro-tube have been investigated. The PL peak is clearly redshifted due to the strain effect.