Hiromasa Shimizu

Fabrication and characterization of an InGaAsP/InP active waveguide optical isolator with 14.7 dB/mm TE mode nonreciprocal attenuation

The authors have fabricated transverse electric (TE) mode InGaAsP/InP active waveguide optical isolators based on the nonreciprocal loss shift and demonstrated improved TE mode isolation ratio of 14.7 dB/mm with reduced insertion loss at a wavelength of 1550 nm for monolithically integrable optical isolators. The wavelength dependence of the isolation ratio and the propagation loss were also measured. An isolation ratio greater than 10 dB/mm was realized over the entire wavelength range of 1530-1560 nm. These results lead to the monolithic integration of semiconductor waveguide optical isolators with edge-emitting semiconductor lasers and highly functional photonic integrated circuits with many cascaded optical devices.

Magnetic and transport properties of III–V based magnetic semiconductor (GaMn)As: Growth condition dependence

We have studied growth condition dependence of magnetic and transport properties of magnetic semiconductor (GaMn)As grown by low-temperature molecular-beam epitaxy (LT-MBE). With increasing substrate temperature and decreasing As overpressure during the growth of (Ga1−xMnx)As with x=0.043, the hole concentration increased, the conduction behavior changed from semiconducting to metallic, and the ferromagnetic transition temperature became higher. This is explained by a decrease in the compensation of Mn acceptors by the reduction of excess As related defects in the LT-MBE grown (GaMn)As. Our experimental results indicate that the selection of the MBE growth parameters is very important for better controlling the electronic and magnetic properties of (GaMn)As.

Structural and electronic transport properties of polycrystalline p-type CoSb3

The structural and electronic transport properties of polycrystalline p-type CoSb3 with different grain sizes (about 3 and 3×102 μm) were investigated. The magnetic susceptibility was also measured. Samples were characterized by x-ray diffractometry, electron-probe microanalysis, and optical microscope observation. Samples were found to be stoichiometric and homogeneous. The Hall carrier concentration of the samples is of the order of 1018 cm−3 and weakly dependent on the temperature. The temperature dependence of the Hall mobility suggests that the predominant scattering mechanism drastically changes depending on grain size: for large grain size a combination of the neutral impurity scattering and the acoustic phonon scattering, and for small grain size the ionized impurity scattering. The magnetic susceptibility was found to be essentially diamagnetic independently of grain size, and to vary slightly with temperature. The weak temperature dependence of the susceptibility can be explained by taking into ...

Magneto-optical properties of a GaAs:MnAs hybrid structure sandwiched by GaAs/AlAs distributed Bragg reflectors: Enhanced magneto-optical effect and theoretical analysis

We have fabricated a multilayer structure consisting of a semiconductor-magnetic hybrid material GaAs:MnAs and GaAs/AlAs distributed Bragg reflectors, that is a one-dimensional semiconductor-based magnetophotonic crystal. Significant enhancement of magneto-optical effect was demonstrated at a controlled wavelength at room temperature. Magneto-optical spectra of this multilayer system were well explained by theoretical calculations, and the mechanism for the enhancement of magneto-optical effect was shown. Also, the required conditions for application to optical isolators are discussed and some potential solutions are indicated. Since our structure is composed of all semiconductor-based materials, the present results potentially lead to semiconductor based magneto-optical devices integrated with III–V-based optoelectronics.

Tunneling spectroscopy and tunneling magnetoresistance in (GaMn)As ultrathin heterostructures

Abstract We have studied tunneling spectroscopy of two types of ultrathin magnetic semiconductor heterostructures; Au/Al 2 O 3 /(GaMn)As, and (GaMn)As/AlAs/(GaMn)As. The latter is all MBE grown heterostructures with abrupt and smooth interfaces and atomically controlled barrier thickness. It was found that (GaMn)As has a small energy gap, which is similar to the soft Coulomb gap, near the Fermi energy, like heavily doped semiconductors. Temperature dependence of the tunneling conductance, which is proportional to the transverse density-of-state of the electrode, follows the power law T 0.5 . Furthermore, large tunneling magnetoresistance (TMR) was observed in a (GaMn)As/AlAs/(GaMn)As tunnel junction.

Anisotropic tunneling magnetoresistance in GaMnAs/AlAs/GaMnAs ferromagnetic semiconductor tunnel junctions

We have observed very large tunneling magnetoresistance (TMR) in Ga1−xMnxAs/AlAs/Ga1−xMnxAs ferromagnetic semiconductor tunnel junctions. A TMR ratio as high as 75% was obtained in a junction with a thin (1.5 nm) AlAs tunnel barrier when the magnetic field was applied along the [100] axis in the film plane. The TMR ratio decreased when the applied magnetic field direction was along the [110] and [110]. This anisotropic TMR was found to be explained by the single-domain theory assuming cubic magnetic anisotropy with the easy axis of 〈100〉, which is induced by the zincblende-type Ga1−xMnxAs crystal structure.

Monolithic Integration of a Waveguide Optical Isolator With a Distributed Feedback Laser Diode in the 1.5-

We present first monolithic integration of a waveguide optical isolator with a distributed feedback laser diode (DFB LD) in the 1.5-mum wavelength range. The integrated devices are composed of 0.25-mm-long index-coupled DFB LDs and 0.75-mm-long semiconductor active waveguide optical isolators. The semiconductor active waveguide optical isolators are based on the nonreciprocal loss in the semiconductor optical amplifier (SOA) waveguides with ferromagnetic metals (Fe). The fabrication process consists of two steps of metal-organic vapor phase epitaxy to grow the DFB LD/SOA layer structures, one dry-etching process for the waveguide stripe formation, and three steps of electron-beam evaporation for the electrodes and ferromagnetic metals deposition. They showed single-mode emission at 1543.8nm and 4-dB optical isolation under a magnetic field of 0.1 T.

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We developed a 1.5-microm band TM-mode waveguide optical isolator that makes use of the nonreciprocal-loss phenomenon. The device was designed to operate in a single mode and consists of an InGaAlAs/InP ridge-waveguide optical amplifier covered with a ferromagnetic MnAs layer. The combination of the optical waveguide and the magnetized ferromagnetic metal layer produces a magneto-optic effect called the nonreciprocal-loss phenomenon--a phenomenon in which the propagation loss of light is larger in backward propagation than it is in forward propagation. We propose the guiding design principle for the structure of the device and determine the optimized structure with the aid of electromagnetic simulation using the finite-difference method. On the basis of the results, we fabricated a prototype device and evaluated its operation. The device showed an isolation ratio of 7.2 dB/mm at a wavelength from 1.53 to 1.55 microm. Our waveguide isolator can be monolithically integrated with other waveguide-based optical devices on an InP substrate.

m Wavelength Range

A 1.5-µm nonreciprocal-loss waveguide optical isolator having improved transverse-magnetic-mode (TM-mode) isolation ratio was developed. The device consisted of an InGaAlAs/InP semiconductor optical amplifier waveguide covered with a ferromagnetic epitaxial MnSb layer. Because of the high Curie temperature (Tc=314 °C) and strong magneto-optical effect of MnSb, the nonreciprocal propagation of 11–12 dB/mm has been obtained at least up to 70 °C.

1.54-μm TM-mode waveguide optical isolator based on the nonreciprocal-loss phenomenon: device design to reduce insertion loss

We have studied the growth and properties of quaternary alloy magnetic semiconductor (InGaMn)As grown on both GaAs substrates and InP substrates by low-temperature molecular-beam epitaxy (LT-MBE). (InGaMn)As thin films were ferromagnetic below ~ 30 K, exhibiting a strong magneto-optical effect. The lattice constant of [(InyGa1-y)1-xMnx]As, whose Mn concentration x is below 4%, is slightly smaller than that of InyGa1-yAs with the same In/Ga content ratio. We have also obtained very smooth surface morphology of nearly lattice-matched (InGaMn)As thin films grown on InP substrates, which is important for application to thin-film-type magneto-optical devices integrated with III–V opto-electronics.