Masafumi Kondo

Reduction in Threshold Current Density of Quantum Well Lasers Grown by Molecular Beam Epitaxy on 0.5° Misoriented (111)B Substrates

GaAs/AlGaAs graded-index, separate confinement heterostructure quantum-well lasers with an extremely low threshold current density (Jth) were grown by molecular beam epitaxy on 0.5° misoriented (111)B substrates. The Jth of these lasers was reduced by 20 A/cm2 compared with that of (100)-oriented devices, which results from the quantization along the direction. The minimum Jth of 158 A/cm2 was achieved for a 490 µm long device.

Nuclear magnetic resonance imaging with improved image quality and operation efficiency

A nuclear magnetic resonance imaging with improved image quality and operation efficiency. It uses an active shield coil with an outer shielding coil having at least two coil turns which are provided on a plurality of separate layers and mutually connected electrically, in order to reduce the heat generation. The eddy current in the shielding coil is reduced by providing a slit portion thereon. The eddy current compensation is achieved by using an appropriate eddy current compensation pulse to minimize a residual eddy current magnetic field within a desired field of view according to a model of a general expression of the residual eddy current magnetic field after the eddy current compensation. The optimum setting of the system parameters for various imaging pulse sequences is achieved for the undetermined gradient coil configuration as well as for the predetermined gradient coil configuration, according to a mathematical model analysis of various imaging pulse sequences.

An evaluation of an in vivo local sound speed estimation technique by the crossed beam method

An in vivo local sound speed estimation technique, using the crossed beam method, has been proposed and its applicability was evaluated. At first, the potential of this technique was studied by a mapping simulation using the ray tracing technique followed by an experiment with a cylindrical agar phantom. The simulation result showed that an exact measurement of local sound speed values was difficult, but the sound speed information for the local region (its relative magnitude to the surrounding medium) was emphasized as a refraction mapping pattern. The experimental results agreed well with the calculation results. Furthermore, a clinical application was performed, using the clinical system (modified electronic linear scanner), on two liver tumor patients.

Enhancement of Heavy-Hole-Related Excitonic Optical Transitions in (111)-Oriented Quantum Wells

Photoluminescence excitation spectra have been measured at low temperatures on high quality GaAs/Al0.3Ga0.7As and GaAs/AlAs multiple quantum wells grown by molecular beam epitaxy along the [111] and [100] crystallographic axes. Comparisons of these spectra have shown clear evidence of the enhancement of the heavy-hole-related excitonic optical transitions relative to the light-hole-related transitions in (111)-oriented quantum wells in comparison with (100)-oriented quantum wells.

A Fundamental Evaluation of In Vivo Local Velocity Measurement by Crossed Beam Method

The possibility of in vivo local velocity measurements by crossed beam method is described. The local velocity in the homogeneous medium is calculated from the known beam geometry and the difference in propagation time measured by several pairs of transducers. Each propagation time is determined by the center of moment for the echo from the cross region. Experimental results show the estimated error to be about 4%, but corrected values agree well with true values and the effectiveness of this new method is confirmed.

High Reliability in AlGaAs Laser Diodes Prepared by Molecular Beam Epitaxy on 0.5°-Misoriented (111)B Substrates

Slip lines in the molecular beam epitaxial wafer bonded with In during the growth have been eliminated by using (111)-oriented GaAs substrates instead of the usual (100)-oriented ones. AlGaAs double-heterostructure and quantum well lasers were grown on (100)- and 0.5°-misoriented (111)B substrates. The yield of reliable lasers is much higher for (111)-oriented devices than that for (100)-oriented ones, possibly due to the elimination of local defects such as slip lines.

Enhancement of the Capture Rate of Carriers in (111)-Oriented GaAs/AlGaAs Quantum Well Structures

A detailed analysis on the dependence of the photoluminescence spectrum on the excitation density in (111)- and (100)-oriented single-quantum-well (SQW) structures is presented. The results show that the capture rate of photo-excited carriers in the barrier region by the SQW and the radiative recombination rate in the SQW are enhanced in (111)-oriented SQW structures compared to those in (100)-oriented ones.

A Fundamental Evaluation of in Vivo Sound Speed Mapping Technique by Crossed Beam Method

Recently, several kinds of in vivo sound speed measurement techniques, using a pulse echo method, have been developed for the purpose of ultrasound tissue characterization.1,2,3The crossed beam method was proposed as a simple method by Haumschild and Greenleaf4 and Nishimura et al.5. This method uses two single probes. One probe is used for transmitting the ultrasound pulsed wave and the other for receiving the wave scattered from the region where the beams from the two probes cross. From the propagation time of the pulsed wave, the sound speed value is calculated. This method can also be realized using a linear array probe.6

A Fundamental Evaluation of In Vivo Local Sound Speed Measurement by Crossed Beam Method (2nd report)

The possibility of in vivo sound speed mapping by the crossed beam method was studied by a calculation using the ray tracing technique and a fundamental experiment. The calculation result is that the sound speed image is shown as a very characteristic pattern, which gives useful information about in vivo sound speed distribution. It was confirmed that experimental values agreed with calculated values.