Kiyofumi Muro

High-power InGaAs/AlGaAs laser diodes with decoupled confinement heterostructure

High power InGaAs/AlGaAs laser diodes with decoupled confinement heterostructure (DCH) have been developed. Almost Al-free waveguide and cladding layers were realized in 980 nm DCH laser diodes without degrading temperature characteristics. The extremely low electrical and thermal resistances allowed high power and efficient operation. The maximum CW output power as high as 9.5 W was obtained with 100-micrometer-aperture broad area. DCH laser diode. The maximum efficiency was 55% at 2.5 W. The series resistance of 1.8-mm long cavity was 0.04(Omega) and internal loss was 1.5 cm-1. The characteristic temperature (T0) was 155 K. The substantially Al-free DCH structure enables easy fabrication of various index guided laser diodes. We have developed two types of real index guided laser diodes, buried- ridge and self-aligned structure. Buried-ridge laser diode presented 1.3 W maximum CW output power and 500 mW single mode operation. Self-aligned structure laser diodes showed 1.4 W CW output power and 700 mW single mode operation with better reproducibility.

Characteristics and reliability of high-power InGaAs/AlGaAs laser diodes with decoupled confinement heterostructure

In order to overcome catastrophic optical damage, decoupled confinement heterostructure (DCH) featuring a broadened waveguide and thin carrier block layers have been developed. Due to decoupling of carrier and optical confinement, a DCH laser can be designed more flexibly than a conventional separated confinement heterostructure laser, i.e., laser diodes can be designed with a variety of gain coupling factor (Gamma) (perpendicular), quantum-well number NW, keeping the beam divergence angle constant.

Spin polarization in quantum Hall state obtained by Kerr rotation spectra

We developed a novel system for a high sensitive measurement of Kerr rotation spectra, and Kerr rotation spectra of quantum Hall system in AlGaAs/GaAs 17 nm quantum well were measured. Spin polarization of quantum Hall states was obtained by integrating the spectra, since Kerr rotation spectra reflect spin population of electrons. The spin polarization decreased rapidly on both sides of ν = 1, which is ascribed to Skyrmion effect. However, the spin polarization present a flat region around ν = 1 which means a quantum Hall ferromagnet is not fully spin polarized.

High-sensitive optical measurement of spin polarization in a quantum Hall system

Spin polarization measurement is important for the study of a variety of spin states in quantum Hall system. Kerr rotation is proportional to spin polarization, so we developed a high sensitive measurement of Kerr rotation by using homodyne detection and a variety of modulation techniques. Furthermore, we developed Kerr rotation spectra measurement system base on the multi-channel homodyne detection, which enables the assignment of the optical transitions and semi-quantitative estimation of spin polarization by integrating the spectra over the specific optical transition. The spin polarization presents a rapid spin depolarization on both sides of ? = 1 due to Skyrmionic excitation. However the top of spin polarization presents a narrow flat region. Furthermore the spin polarization around ? = 3 also shows a rapid spin depolarization which suggest the existence of Skyrmion at higher odd filling factor.

High power and widely tunable external cavity diode laser with a single-angled-facet laser chip

High power external cavity diode lasers were developed in the wavelength region of 1µm by using single-angled facet laser diodes. 330mW output and 130nm tuning were achieved in the conventional Littman-Metcalf configuration.

Optical Imaging of Quantum Hall Current in GaAs/AlGaAs Two‐dimensional Electron System

We have measured the current distribution in GaAs/AlGaAs two‐dimensional electron system at quantum Hall effect regime, where the critical current shows sub‐linear dependence on the channel width. An optical‐fiber‐based potential imaging system by Pockels effect is improved by using a twin‐optical bridge detector which reduces retardation noise in the optical fiber. We have also mapped the resistance change by the local photo‐excitation in the QHE regime. Both the observed images show the evidence of current concentration at center region of the channel, and the extent of current concentration increases with increasing the current.

Time‐Resolved Kerr Rotation Spectroscopy of Spin Dynamics in a Quantum Hall System

A Time‐resolved Kerr rotation spectroscopy under selective excitation by narrow spectrum pump beam is applied to high‐mobility two dimensional (2D) electrons. The large non‐oscillating Kerr signal is ascribed to the formation of Skyrmions by photoexcited carrier, and is confirmed by the selective excitation in TRKR measurement. The collapse of the spin coherence (dip of T2*) near ν = 1 may be related by the formation of Skyrmions and anti‐Skyrmiosn pair under photoexcitation.

ELECTRON SPIN IMAGING IN QUANTUM HALL DEVICES BY KERR ROTATION MEASUREMENT

The electron spin polarization images in a single layer GaAs/AlGaAs quantum well in the quantum Hall effect (QHE) regime have been observed by using an optical fiber-based high sensitive Kerr rotation microscope. The observed Kerr signal image at the Landau level filling factor ν = 1 reveals the small electron density fluctuation less than 1% through the degree of electron spin polarization. We have also observed the Kerr rotation images in the current flowing QHE devices. At ν = 1 QHE plateau, the winding strip of just ν = 1 incompressible state, which contributes the dissipationless electron transport, has been clearly observed in the bulk region of the devices. The highly current dependent Kerr images have valuable information for studying current density distribution and the distribution of spin polarization in the QHE regime. The experimental results and techniques of the Kerr imaging in the QHE regime have been reported in detail.

OPTICALLY INDUCED DYNAMIC NUCLEAR SPIN POLARIZATION IN QUANTUM HALL REGIME OBSERVED BY A TIME-RESOLVED KERR ROTATION

The dynamics between electron and nuclear spins in quantum Hall regime is investigated by a time-resolved Kerr rotation (TRKR) spectroscopy carried down in tilted-field geometry. The spin-flip energy varies almost linearly with the applied magnetic field under the nuclear spin depolarizing radio frequency field and the helicity modulation of circularly polarized pump, while the spin-flip energy obtained under a circularly polarized pump deviates from linearly behavior. This deviation is ascribed to the Overhauser shift due to dynamic nuclear spin polarization (DNP). The DNP depends markedly on the filling factor and present peaks at even filling factors. These results can be well explained by a simple model based on the detailed balance between electron and nuclear spins.

Study of Spin Dynamics in Quantum Hall Regime by Time Resolved Kerr Rotation Spectroscopy

Time‐resolved Kerr rotation (TRKR) Spectroscopy has been applied to a two‐dimensional (2D) electron gas in GaAs/AlGaAs hetero‐junction to probe the electron and nuclear spin dynamics in quantum Hall regime. A large and long‐lasting Kerr rotation signal was ascribed to the high‐mobility 2D‐electron gas, since the Kerr signal from etched high purity GaAs substrate was much smaller and shorter. In a configuration where an out‐of‐plane magnetic field quantizes 2D‐electrons into Landau levels, the spin coherence time, T2* exhibits a drastic increase. Furthermore, both T2* and effective g factor exhibit an oscillation behavior as a function of the magnetic field. Under a circularly polarized pump, we observed an Overhauser shift up to 40 μeV, which corresponds to 30 % dynamic nuclear polarization (DNP).