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Your search keyword '"Kitada, Takahiro"' showing total 238 results
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238 results on '"Kitada, Takahiro"'

1. Terahertz wave emission with 1.5 μm pump from photoconductive antenna using stacked Er-doped-InAs quantum dot layers with ultrafast carrier relaxation.

Author

Minami, Yasuo, Abe, Hiromutsu, Lu, Xiangmeng, Kumagai, Naoto, and Kitada, Takahiro

Subjects
SUBMILLIMETER waves, ANTENNAS (Electronics), QUANTUM dots, TERAHERTZ time-domain spectroscopy, TERAHERTZ spectroscopy, QUANTUM cascade lasers, LIGHT absorption, OPTICAL communications
Abstract

We fabricated a photoconductive antenna (PCA) with Er-doped InAs quantum dot (QD) layers on a GaAs substrate and demonstrated THz wave generation with 1.5 μm pump. In terahertz time-domain spectroscopy (THz-TDS), PCAs require high mobility and short lifetimes of photocarriers since the carriers need to respond rapidly to optical pulses for broad THz generation and detection with a high time resolution. In this study, to achieve PCA operation at an optical communication wavelength band of 1.5 μm, a substrate for a PCA was fabricated using stacked Er-doped-InAs QD layers. Then, photocurrent measurements were performed to observe optical absorption in the 1.5 μm band. The results indicated the high capability of the PCA for THz wave generation and detection, in fact, which enabled us to generate the THz wave in the 1.5 μm band optical system. [ABSTRACT FROM AUTHOR]

Published
2023
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3. Electrical properties and energy band alignments of p-Si/n-Ga2O3 and p+-Si/n-Ga2O3 heterostructures fabricated by surface-activated bonding.

Author

Wang, Zhenwei, Kitada, Takahiro, Takatsuki, Daiki, Liang, Jianbo, Shigekawa, Naoteru, and Higashiwaki, Masataka

Subjects
*ELECTRICAL energy, *TWO-dimensional electron gas, *HETEROSTRUCTURES, *CAPACITANCE-voltage characteristics, *NUMERICAL calculations
Abstract

We fabricated p-Si/n-Ga2O3 and p+-Si/n-Ga2O3 heterostructures by surface-activated bonding (SAB) and investigated their electrical properties. Current density–voltage measurement was performed before and after thermal annealing at 450 °C. The current density substantially increased after annealing, which was attributed to thinning of an intermediate layer formed by the bonding process. Distinctive two-stage capacitance–voltage characteristics were observed for p-Si/n-Ga2O3 heterostructures, which were well reproduced by numerical calculation considering the effect of two-dimensional electron gas formed at the heterointerface. These results indicate that Ga2O3-based p–n heterostructures with good interface properties and large-area uniformity can be fabricated using SAB. [ABSTRACT FROM AUTHOR]

Published
2023
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7. Fabrication of n-Si/n-Ga2O3 heterojunctions by surface-activated bonding and their electrical properties.

Author

Wang, Zhenwei, Takatsuki, Daiki, Liang, Jianbo, Kitada, Takahiro, Shigekawa, Naoteru, and Higashiwaki, Masataka

Subjects
CONDUCTION bands, DISTRIBUTION (Probability theory), THERMAL equilibrium, ACTIVATION energy, HETEROJUNCTIONS, ELECTRIC capacity
Abstract

We studied electrical properties of n-Si/n-Ga2O3 heterojunctions fabricated by surface-activated bonding. The energy barrier heights (qϕb) at the Si/Ga2O3 interface for different reverse voltages (Vrev) were derived from temperature-dependent current density–voltage (J–V–T) characteristics. With shifting Vrev to the negative direction, qϕb gradually decreased and reached a constant value due to negatively charged interface states. The conduction band offset at the heterointerface was estimated to be 0.18 eV from the Vrev dependence of qϕb. The qϕb calculated from a capacitance of the heterojunction at thermal equilibrium was larger than those derived from the J–V–T characteristics, attributing to spatially inhomogeneous qϕb caused by the non-uniform distribution of the charged interface states. The density of shallow interface states was also extracted from the reverse J–V–T characteristics, which was estimated to be about 6 × 1012 cm−2 eV−1. [ABSTRACT FROM AUTHOR]

Published
2022
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10. Spin–orbit parameters derivation using single-frequency analysis of InGaAs multiple quantum wells in transient spin dynamics regime.

Author

Shida, Hiroki, Kawaguchi, Kohei, Saito, Yasuhito, Takazawa, Ichirota, Fukasawa, Toshiki, Iizasa, Daisuke, Saito, Takahito, Kitada, Takahiro, Ishitani, Yoshihiro, Kohda, Makoto, and Morita, Ken

Subjects
TRANSIENTS (Dynamics), QUANTUM wells, SCATTERING (Mathematics), MAGNETIC fields, TIME-frequency analysis
Abstract

The deriving method of spin–orbit (SO) parameters using a single-frequency analysis was examined in a transient regime of diffusive spin dynamics in InGaAs/InAlAs multiple quantum wells. Transient regime of diffusive spin dynamics is the time regime when the spin precession frequency induced by SO magnetic fields decreases and changes with time. Recently, we have established a method of deriving SO parameters by scanning time-resolved Kerr rotation microscopy in this transient regime [Kawaguchi et al., Appl. Phys. Lett. 115, 172406 (2019)] using the time-dependent spin precession frequency analysis. Although reliable SO parameters were derived, time-independent single-frequency analysis is still attractive because of its simplicity. In this paper, SO parameters' derivation was performed by the single-frequency analysis comparing the experiment and the Monte Carlo (MC) simulation. The best fit of the simulation to the measurement for the SO-induced frequency yields the derivation of SO parameters; however, the derived values were different from the reliable SO parameters derived by the time-dependent analysis. This discrepancy arises from a spin relaxation time difference between the experiment and MC simulation. After intentionally adjusting the spin relaxation time of the MC simulation to the experiment, the SO-induced frequency obtained by the MC simulation with reliable SO parameters reproduced the experiment well. We found that the spin relaxation time adjustment of the MC simulation to the experiment is necessary to obtain accurate SO parameters from the single-frequency analysis comparing the experiment with the MC simulation. [ABSTRACT FROM AUTHOR]

Published
2020
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20. Velocity enhancement of cryogenically cooled InP-based HEMTs on (411)A-oriented substrates

Author

Watanabe, Issei, Shinohara, Keisuke, Matsui, Toshiaki, Kitada, Takahiro, Shimomura, Satoshi, Mimura, Takashi, Yamashita, Yoshimi, and Endoh, Akira

Subjects
High-electron-mobility transistors -- Design and construction, Indium -- Electric properties, Phosphorus compounds -- Electric properties, Business, Electronics, Electronics and electrical industries
Abstract

In0.75Ga0.25As/In0.52Al0.48As high electron mobility transistors (HEMTs) on (411)A-oriented InP substrates is developed using molecular beam epitaxy (MBE) and their dc and RF characteristics at room and cryogenic temperatures are investigated. Delay-time analysis is conducted to investigate the causes of the high maximum transconductance, [g.sub.m max], of 2.25 S/mm and a cutoff frequency, [f.sub.T] of 310 GHz at 16 K for the 195-nm gate HEMT fabricated on the (411)A InP substrate.

Published
2006

22. Surface Emitting Devices Based on a Semiconductor Coupled Multilayer Cavity for Novel Terahertz Light Sources (Special Section on Fabrication Technologies Supporting the Photonic/Nanostructure Devices)

Author

Kitada, Takahiro, Ota, Hiroto, Lu, Xiangmeng, Kumagai, Naoto, and Isu, Toshiro

Subjects
two-color lasing, coupled multilayer cavity, terahertz source, Physics::Optics, frequency conversion
Abstract

Compact and room-temperature operable terahertz emitting devices have been proposed using a semiconductor coupled multilayer cavity that consists of two functional cavity layers and three distributed Bragg reflector (DBR) multilayers. Two cavity modes with an optical frequency difference in the terahertz region are realized since two cavities are coupled by the intermediate DBR multilayer. In the proposed device, one cavity is used as the active layer for two-color lasing in the near-infrared region by current injection and the other is used as the second-order nonlinear optical medium for difference-frequency generation of the two-color fundamental laser light. The control of the nonlinear polarization by face-to-face bonding of two epitaxial wafers with different orientations is quite effective to achieve bright terahertz emission from the coupled cavity. In this study, two-color emission by optical excitation was measured for the wafer-bonded GaAs/AlGaAs coupled multilayer cavity containing self-assembled InAs quantum dots (QDs). We found that optical loss at the bonding interface strongly affects the two-color emission characteristics when the bonding was performed in the middle of the intermediate DBR multilayer. The effect was almost eliminated when the bonding position was carefully chosen by considering electric field distributions of the two modes. We also fabricated the current-injection type devices using the wafer-bonded coupled multilayer cavities. An assemble of self-assembled QDs is considered to be desirable as the optical gain medium because of the discrete nature of the electronic states and the relatively wide gain spectrum due to the inhomogeneous size distribution. The gain was, however, insufficient for two-color lasing even when the nine QD layers were used. Substituting two types of InGaAs multiple quantum wells (MQWs) for the QDs, we were able to demonstrate two-color lasing of the device when the gain peaks of MQWs were tuned to the cavity modes by lowering the operating temperature.

Published
2017

23. Time-resolved measurements of two-color laser light emitted from GaAs/AlGaAs-coupled multilayer cavity

Author

Minami, Yasuo, Ogusu, Kotaro, Lu, Xiangmeng, Kumagai, Naoto, Morita, Ken, Kitada, Takahiro, Minami, Yasuo, Ogusu, Kotaro, Lu, Xiangmeng, Kumagai, Naoto, Morita, Ken, and Kitada, Takahiro

Abstract

We measured the two-color laser oscillation from a GaAs/AlGaAs-coupled multilayer cavity at 18–42 °C using current injection. We confirmed simultaneous lasing by detecting the sum frequency generation signal generated by the two-color laser light, and performed time-resolved measurement using a streak camera with a spectrometer. From the observed time transient of the spectra at various temperatures, it it is clarified that the temperature change of the device, induced by current injection, modulates the effective cavity length. Therefore, the temperature control of the device is a key factor in stable two-color lasing and THz wave generation.

Published
2019

24. Lateral photoconductivity of InAs/GaAs quantum dots for 1.5 μm-wavelength excitation photoconductive terahertz antenna devices

Author

Kaizu, Toshiyuki, Kojima, Osamu, Minami, Yasuo, Kitada, Takahiro, Harada, Yukihiro, Kita, Takashi, and Wada, Osamu

Abstract

We report lateral photoconductive properties of multilayer-stacked undoped InAs/GaAs quantum dots (QDs) for the application of photoconductive terahertz (THz) antenna devices that operate in a 1.5 μm-telecom-wavelength band. The excitation power-dependent photocurrent showed a high value without saturation under high excitation power for the excitation wavelength of 1460 nm. From the reflection pump-probe signal, a fast photocarrier lifetime was derived. These results, together with the low dark current characteristic, support the applicability of the multilayer-stacked undoped InAs/GaAs QDs to photoconductive THz antennas operating in a 1.5 μm-wavelength band.

Published
2024
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34. Room-temperature two-color lasing by current injection into a GaAs/AlGaAs coupled multilayer cavity fabricated by wafer bonding

Author

Kitada, Takahiro, Lu, Xiangmeng, Minami, Yasuo, Kumagai, Naoto, Morita, Ken, Kitada, Takahiro, Lu, Xiangmeng, Minami, Yasuo, Kumagai, Naoto, and Morita, Ken

Abstract

Room-temperature two-color lasing was demonstrated by current injection into a GaAs/AlGaAs coupled multilayer cavity for terahertz emitting devices utilizing its difference-frequency generation (DFG) inside the structure. We prepared two epitaxial wafers with (001) and (113)B orientations and they were directly bonded to form the coupled multilayer cavity. The (001) side cavity contains two types of InGaAs multiple quantum wells to realize optical gain for two-color lasing while a single GaAs layer of the (113)B side cavity is a nonlinear medium for efficient DFG. Lasing behavior was clearly observed for two modes under pulsed current conditions at room temperature. We found that intensity relation between two-color lasing was dependent on the pulse duration due to the different temporal profiles of emission intensity. Simultaneous two-color lasing in the device was also confirmed by measuring the sum-frequency generation signal using a beta barium borate crystal.

Published
2018

41. Current-injection two-color lasing in a wafer-bonded coupled multilayer cavity with InGaAs multiple quantum wells

Author

Minami, Yasuo, Ota, Hiroto, Lu, Xiangmeng, Kumagai, Naoto, Kitada, Takahiro, Isu, Toshiro, Minami, Yasuo, Ota, Hiroto, Lu, Xiangmeng, Kumagai, Naoto, Kitada, Takahiro, and Isu, Toshiro

Abstract

Current-injection two-color lasing has been demonstrated using a GaAs/AlGaAs coupled multilayer cavity that is a good candidate for novel terahertz-emitting devices based on difference-frequency generation (DFG) inside the structure. The coupled cavity structure was fabricated by the direct wafer bonding of (001)- and (113)B-oriented epitaxial wafers for the efficient DFG of two modes in the (113)B side cavity, and two types of InGaAs multiple quantum wells (MQWs) were introduced only in the (001) side cavity as optical gain materials. The threshold behavior was clearly observed in the current–light output curve even at room temperature. Two-color lasing was successfully observed when the gain peaks of MQWs were considerably tuned to the cavity modes by the operating temperature.

Published
2017

42. Sublattice reversal in GaAs/Ge/GaAs heterostructures grown on (113)B GaAs substrates

Author

Lu, Xiangmeng, Kumagai, Naoto, Minami, Yasuo, Kitada, Takahiro, Lu, Xiangmeng, Kumagai, Naoto, Minami, Yasuo, and Kitada, Takahiro

Abstract

GaAs/Ge/GaAs heterostructures were grown on high-index (113)B GaAs substrates by molecular beam epitaxy. Sublattice reversal in GaAs/Ge/GaAs was identified by comparing the anisotropic etching profile of the epitaxial sample with that for reference (113)A and (113)B GaAs substrates. The shape of the resulting mesa for the lower GaAs layer was similar to that for the reference (113)B GaAs substrate, whereas that for the upper GaAs layer was similar to that for the reference (113)A GaAs substrate. An atomic-resolution analysis was also conducted by mapping using energy-dispersive X-ray spectroscopy, whereby the sublattice reversal was directly observed through the atomic arrangements.

Published
2017

46. Two-color surface-emitting lasers using a semiconductor coupled multilayer cavity

Author

Kitada, Takahiro, Ota, Hiroto, Lu, Xiangmeng, Kumagai, Naoto, Isu, Toshiro, Kitada, Takahiro, Ota, Hiroto, Lu, Xiangmeng, Kumagai, Naoto, and Isu, Toshiro

Abstract

Two-color surface-emitting lasers were demonstrated, employing a GaAs/AlGaAs coupled multilayer cavity composed of two cavity layers and three distributed Bragg reflector (DBR) multilayers. InGaAs multiple quantum wells (MQWs) with two different well widths were introduced only in the upper cavity, sandwiched between p-type and n-type DBRs. This current-injection type device exhibited two-color lasing in the near-infrared region under room temperature pulsed conditions. This two-color lasing was obtained when the lower cavity had an optimal thickness relative to the upper cavity thickness and the MQW emission properties.

Published
2016

47. Photoconductivity of Er-doped InAs quantum dots embedded in strain-relaxed InGaAs layers with 1.5µm cw and pulse excitation

Author

Murakumo, Keisuke, Yamaoka, Yuya, Kumagai, Naoto, Kitada, Takahiro, Isu, Toshiro, Murakumo, Keisuke, Yamaoka, Yuya, Kumagai, Naoto, Kitada, Takahiro, and Isu, Toshiro

Abstract

We fabricated a photoconductive antenna structure utilizing Er-doped InAs quantum dot layers embedded in strain-relaxed In0.35Ga0.65As layers on a GaAs substrate. Mesa-shaped electrodes for the antenna structure were formed by photolithography and wet etching in order to suppress its dark current. We measured the photocurrent with the excitation of ~1.5 µm cw and femtosecond pulse lasers. Compared with the dark current, the photocurrent was clearly observed under both cw and pulse excitation conditions and almost linearly increased with increasing excitation power in a wide range of magnitudes from 10 W/cm2 to 10 MW/cm2 order.

Published
2016

48. Fabrication of two-color surface emitting device of a coupled vertical cavity structure with InAs quantum dots formed by wafer bonding

Author

Ota, Hiroto, Lu, Xiangmeng, Kumagai, Naoto, Kitada, Takahiro, Isu, Toshiro, Ota, Hiroto, Lu, Xiangmeng, Kumagai, Naoto, Kitada, Takahiro, and Isu, Toshiro

Abstract

We fabricated the two color surface emitting device of a coupled cavity structure which is applicable to terahertz light source. GaAs/AlGaAs vertical multilayer cavity structures were grown on a (001) and (113)B GaAs substrates and the coupled multilayer cavity structure was fabricated by wafer-bonding of them. The top cavity contains self-assembled InAs quantum dots (QDs) as optical gain materials for two-color emission of cavity-mode lights. The bonding position was optimized for the equivalent intensity of two-color emission. We formed a current injection structure, and two-color emission was observed by current injection, although lasing was not observed.

Published
2016

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