To realize fast switching of a few μs and low-power consumption, we apply a differential control method to a thermo-optic Mach-Zehnder interferometer optical switch loaded with a direct-current injection-type phase shifter using multimode interference. We achieve ultrafast switching of several tens of ns, which is considerably higher than the conventional switching speed. With differential control, we demonstrate extremely fast optical switching with τ rise = 36 ns and τ fall = 24 ns. This method can be widely applied to similar devices and can contribute significantly to the speedup of other thermo-optic switches.

We fabricated a Si-SiO2-Si horizontal slot waveguide using u-Transfer Printing method. Supercontinuum generation ranging from 1.35 to 2.05 μm was demonstrated with a coupled energy of as low as 10 pj.

High repetition FMCW distance measurements were performed using direct modulation of a hybrid wavelength tunable laser at a repetition rate of 25 kHz. A minimum standard deviation of 0.3 cm at a bandwidth of 1.6 GHz was obtained for the same distance measurement in free space. A measurement error of less than 1 cm was achieved for the 3D imaging, demonstrated fast and accurate FMCW distance measurement.

Thermal crosstalk of a compact, low-power, high-speed thermo-optical MZI switch was evaluated using a ring resonator placed nearby. Thermal crosstalk is small enough to maintain sufficient classification accuracy even in a 64 × 64 size optical neural networks, which significantly reduces the integrated circuits footprint compared to conventional micro-heater switches.

We fabricated large scale silicon passive Optical Phased Array consisting of 128 channel Arrayed Waveguide Grating. The beam divergence was less than 0.3° and scan range was over 40°. This passive optical phased array is effective for realization of small size, low power consumption and high-resolution beam scanner.

We report a hybrid wavelength tunable laser connected to a high Q fiber Fabry-Perot resonator. The coupling efficiency is controlled by an integrated reflectivity variable mirror. It was optimized by setting the reflectivity to 0.718, and narrow linewidth below 10 kHz was confirmed due to the self-injection locking effect.

A curved directional coupler with small wavelength dependence of the coupling efficiency was designed, and the characteristics of a hybrid wavelength tunable laser diode using it were evaluated. An ultra-wide wavelength tunable range of 112.6 nm and a side mode suppression ratio of over 40 dB were demonstrated.

Optical phased arrays (OPAs) have received considerable attention as solid-state beam scanners. However, conventional OPAs that actively control the phase difference between arrays are characterized by excessive power consumption for high-precision beam emission. In this study, we fabricated an OPA comprising Bragg grating and arrayed waveguide grating (AWG). Multi-mode waveguide is used in AWG to reduce the effect of manufacturing error. This device realizes wide and high-resolution two-dimensional beam steering only by sweeping wavelength. FWHM of the emitted beam is 0.534° × 2.27°, and the steering range is 43.9° × 13.5° with 1/64 of the power consumption of conventional OPA.

We propose a carrier-depletion type Si optical modulator for transverse electric (TE) polarization loaded with an integrated polarizer. The integrated TE-pass polarizer with a bent waveguide has a compact and simple structure and demonstrate a polarization extinction ratio of 41.3 dB and insertion loss of 0.67 dB. This Si optical modulator loaded with the integrated polarizer achieved a maximum extinction ratio of 54.5 dB. It is expected to have a wide range of applications in digital coherent optical communication and optical sensing fields.

A silicon photonics hybrid wavelength tunable laser diode with a phase shifter capable of high-speed frequency modulation was fabricated. High-precision, high-repetition FMCW measurement was demonstrated using the hybrid tunable laser diode.

We fabricated a 100 nm broadband hybrid wavelength tunable laser diode using a silicon photonics wavelength filter and a quantum-dot semiconductor optical amplifier (QD-SOA). Wavelength filter consists of double ring resonators using curved directional couplers.

A differential control method was applied to a thermo-optic MZI optical switch loaded with MMI phase shifters capable of high-speed, low-power-consumption switching operation. The obtained switching time was 28 ns for τrise and 20 ns for τfall, extremely fast optical switching operation has been demonstrated.

In this paper, we fabricated a 6 × 1.5 mm2 1-chip beam steering device by integrating a laser diode with an OPA that doesn't have any phase shifters. Power consumption for beam steering is 65 mW. Beam steering range is 42.2 ° × 9.54 °.

The compact, high-speed, low-power consumption Mach-Zehnder interferometer (MZI)-type optical switch using thermo-optical effect is a key device in future optical integrated circuits. In this study, we achieved ultra-high-speed switching operation of 0.4 μs and low electrical power operation of 23 mW with a small device footprint by loading an asymmetric doped MZI structure and an integrated heat sink. This optical switch can be fabricated in the standard manufacturing process of silicon photonic devices, has high practicality and will be a useful tool in various optical integrated circuits.

We demonstrated a Si-photonics-based heterogeneous tunable laser in the 1.55-µm-band with quantum dot reflective semiconductor optical amplifier. Relatively low threshold current could be achieved owing to high gain characteristic.

A C-band tunable dual-wavelength laser for a RoF light source is demonstrated. The difference frequency was tunable in the range of 22.6 GHz to 192.0 GHz. The generated dual-wavelength laser amplified by EDFA could be converted to a corresponding radio frequency through a high-speed photodetector.

A hybrid laser with an optical negative feedback circuit integrated on a silicon photonics chip was investigated. Our numerical analysis verified that the laser can function as a wavelength-tunable laser with a spectral linewidth of less than 10 kHz. The result was supported by experiments.

We propose a thermo-optic phase shifter utilizing multimode interference in a silicon waveguide. The electrodes were attached to the section where the multimode interference had the minimum optical electric field around the sidewalls. We designed the multimode waveguide width to reduce optical propagation loss in the C-L wavelength band (1.53-1.625 μm). We succeeded in developing a low optical loss thermo-optic phase shifter with less than 0.1 dB in the wavelength range of 1.5 to 1.6 μm. Moreover, wavelength dependence was reduced by adjusting the number of electrodes. An optical switch with the designed thermo-optic phase shifter was fabricated and used to estimate the optical switching time. Switching time of several microseconds was obtained by normal pulse operation, and ultrafast optical switching of about 500 ns was realized by overdrive control.

We investigate a structure to reduce the wavelength-dependence of a ring resonator's transmission characteristics by replacing the conventional directional coupler of the racetrack resonator with a curved directional coupler. We show its potential as a filter for broadband wavelength tunable single mode lasers

We propose a thermo-optic phase shifter which uses multimode interference. Electrodes are attached to the section where the multimode interference has minimum optical power in the sidewalls. We use the Silicon large thermo-optic constant to create an integrated optical device to control the state of light in several microseconds, for both heating and cooling times.

Thermo-optic coefficients of organic-inorganic hybrid polymers with rutile-TiO2 nanoparticles were carefully estimated using SOI ring resonators. Athermal operation of the SOI ring resonators with TiO2 hybrid-polymer claddings was achieved. The hybrid-polymer cladding can be applied to diverse silicon waveguide structures to control the temperature dependence of silicon photonic devices.

We experimentally present a Fabry-Perot cavity that uses two-row photonic crystals in a multimode waveguide as the reflecting elements in an add-drop configuration to achieve fine FSR tuning and maximum footprint efficiency.

In this work, 1 GHz video data was collected by a CMOS camera and successfully transmitted by the electro-optic (EO) modulator driven by an external modulation module integrated onto the same chip. For this application, the EO modulator component included a polymer waveguide modulator, which performed a 20 GHz bandwidth, clear eye diagram opening with a Q factor of 10.3 at 32 Gbit/s and a drive voltage of 1.5 V pp . By utilizing a thermally stable EO polymer, the wide-band polymer modular can yield a photonic integrated camera sensor system which is a reliable processing platform for real-time data processing.

In this paper we present a compact direct current injection thermo-optic switch based on a Mach-Zehnder Interferometer configuration that is suitable for autonomous vehicle applications as it has a low heating resistance value of 97 Ω, a rapid 2.16 μs switching time constant, and a Pπ of 28 mW. The device relies on multimode interference to achieve low optical insertion losses of less than 1.1 dB per device, while allowing direct current injection to heat the waveguide and achieve fast operation speeds. Furthermore, the total resistive value can be tailored as the heating elements are placed in parallel.

In this paper we experimentally demonstrate the possibility of detecting an object using a rectangular resonator with a surface area of 200 μm2. The optical biosensor is fabricated on the standard silicon-on-insulator platform, and the object is sensed with the evanescent field that extends from the top of the device. It is possible to detect the presence of the object by measuring the shift of the resonant wavelengths in the transmission spectrum. We compare the performance of the device when it is operated in TE and TM polarization, obtaining superior performance with TM polarization, whereby it achieves a shift of 1.5 nm when a large element is placed on its surface.

In this paper, we experimentally characterize the sensitivity of different types of multimode Bragg gratings in Silicon-On-Insulator and demonstrate a high sensitivity of 120 nm per refractive index unit using Bragg gratings for TM polarized light.

We propose a tunable two-wavelength heterogeneous quantum dot laser diode. The tunable two-wavelength laser consists of a quantum dot semiconductor optical amplifier and an external cavity fabricated from silicon photonics technology as the two-wavelength tunable filter. We successfully demonstrated two-wavelength lasing oscillation by tuning the difference frequency from approximately 34 GHz to 400 GHz.

We propose a tunable dual-wavelength heterogeneous quantum dot laser diode. The tunable dual-wavelength laser consists of a quantum dot semiconductor optical amplifier as the optical gain medium and an external cavity fabricated from silicon photonics technology as the wavelength tunable filter. We successfully demonstrated dual-wavelength lasing oscillation by tuning the difference frequency from approximately 34 to 400 GHz.

We demonstrated wavelength tunable laser diodes with Si-wire waveguide ring resonators as an external optical cavity. The footprint of the optical cavity including the semiconductor optical amplifier is as small as 0.8 mm x 0.6 mm which is less 1/10 of those made of silicon oxinitride material. Wavelength tuning range of approximately 55 nm was demonstrated, which covers entire L-band of the optical communication wavelength system. Furthermore, a spectral linewidth narrower than 100 kHz was obtained by optimizing the design of the external optical cavity. The optical nonlinearity of silicon is described and the influence on laser oscillation is also discussed.

A heterogeneous wavelength-tunable laser diode combining quantum dot and silicon photonics technologies is proposed. A compact wavelength-tunable filter with two ring resonators was carefully designed and fabricated using silicon photonics technology. The tunable laser combining the wavelength-tunable filter and an optical amplifier, which includes InAs quantum dots, achieved a 44.0 nm wavelength-tuning range at around 1250 nm. The broadband optical gain of the quantum dot optical amplifier was effectively used by the optimized wavelength-tunable filter. This heterogeneous wavelength-tunable laser diode could become a breakthrough technology for high-capacity data transmission systems. (C) 2016 The Japan Society of Applied Physics

The silicon photonic wavelength tunable laser diode is a good candidate for the integrated light source of the digital coherent optical communication system. However, optical nonlinear effects easily occur due to the strong light confinement of Si photonic wire waveguides. The impact on laser operation caused by nonlinearity was discussed.

A new wavelength-tunable laser diode combines quantum-dot (QD) technology and silicon photonics with large optical gains around the 1310 nm telecom window and is amenable to integration of other passive and active components towards a truly integrated photonic platform.

Compact, wavelength-tunable light sources are desired for the enhancement of information communication technology and bio-imaging applications. We propose a compact, wavelength-tunable laser diode with a wide wavelength-tunable range around 1230 nm consisting of a quantum-dot optical amplifier and a silicon photonic tunable filter. High-quality InAs quantum dots grown with the sandwiched sub-nano separator technique were used as the optical gain medium. The wavelength-tunable filter was constructed with ring resonators fabricated using silicon photonics. The single-mode laser oscillation was demonstrated with a 28.5-nm wavelength-tunable range. (C) 2015 The Japan Society of Applied Physics

We investigated the waveguide-core size distribution of ring resonators fabricated on a 300mm silicon-on-insulator (SOI) wafer using a CMOS-compatible process featuring ArF immersion lithography. These ring resonators were constructed in a Si-wire waveguide with a standard core size of 400nm width and 220nm height. The group refractive indices of the waveguide were derived from the transmission spectra of the ring resonators. From the deviation of these group refractive indices, the waveguide-core width distribution was estimated to be 5 nm, and the waveguide-core height distribution was estimated to be 1 nm. Moreover, the device property distribution of various Si-wire waveguide depended on the estimated fabrication error was calculated. The waveguide core with the smallest device property distribution had a 540nm width and a 160nm height, and this waveguide has a device property distribution of 2/3 value compared with the standard core size. (C) 2015 The Japan Society of Applied Physics

We propose a narrow-spectral-linewidth silicon photonic wavelength-tunable laser with a novel external wavelength-tunable filter, which consists of two silicon ring resonators with different circumferences and a highly asymmetric Mach-Zehnder interferometer (MZI), the two optical paths of which have significantly different lengths. Calculations and experimental results indicated that the gain difference between longitudinal modes was increased by the highly asymmetric MZI. Consequently, a narrow spectral linewidth of 12 kHz and a stable single-mode oscillation were obtained. (C) 2015 Optical Society of America

Si-photonic hybrid ring external cavity wavelength tunable lasers by passive alignment techniques with more than 100-mW fiber-coupled power and linewidth narrower than 15 kHz along the whole C-band are demonstrated. These attractive performances are achieved due to very low loss Si-wire waveguides, of which loss is lower than 0.5 dB/cm. Obtained results show excellent features of Si-photonics toward commercial products.

We present a wavelength-tunable laser diode with a 99-nm-wide wavelength tuning range. It has a compact wavelength-tunable filter with high wavelength selectivity fabricated using silicon photonics technology. The silicon photonic wavelength-tunable filter with wide wavelength tuning range was realized using two ring resonators and an asymmetric Mach-Zehnder interferometer. The wavelength-tunable laser diode fabricated by butt-joining a silicon photonic filter and semiconductor optical amplifier shows stable single-mode operation over a wide wavelength range. (C) 2015 AIP Publishing LLC.

We fabricated wavelength-tunable laser diodes using a Si photonic wavelength filer that consists of ring resonators and an asymmetric Mach-Zehnder interferometer. The footprint of the optical cavity including the semiconductor optical amplifier is small, 2.6 mm x 0.5 mm, which is about 1/9 of those for tunable laser diodes made of silicon oxynitride. The wavelength could be tuned over approximately 62 nm, which covers the entire L-band of the optical communication wavelength range. The maximum output power reaches 42.2mW. Furthermore, a spectral line width narrower than 100 kHz was obtained. Such tunable laser diodes with narrow spectral line widths are suitable as light sources integrated into other digital coherent devices.

We fabricated wavelength-tunable laser diodes with external cavity consist of Si photonic wire waveguide ring resonators. About 51.5 nm wavelength tuning operation, which covers the entire L-band of the optical communication wavelength range, was obtained. The cavity length dependence of the spectral line width was verified to obtain narrower spectral linewidth. The observed spectral linewidth of 3.78 mm long cavity and 8.19 mm long cavity are 131.0 and 64.8 kHz, respectively. The advantage of long external cavity to design the narrow spectral linewidth Si photonic wavelength tunable laser diode was confirmed. (C) 2014 The Japan Society of Applied Physics

We investigated the wide-band wavelength tuning operation to increase the possibility of silicon photonic wavelength tunable laser diodes which is composed semiconductor optical amplifier and silicon photonic waveguide filter. The wide-band wavelength tuning operation of 94.1 nm was demonstrated.

Si-photonic Hybrid Ring-filter External Cavity (SHREC) wavelength tunable lasers by passive alignment techniques with over 100-mW fiber-coupled power and linewidth narrower than 15 kHz along the whole C-band are demonstrated. Obtained results show excellent features of Si-photonics towards commercial products.

Titanium dioxide (TiO2) has several attractive properties such as high linear and nonlinear refractive indices and the wide bandgap for applying the nonlinear optical devices. We have been studied channel waveguides with TiO2 as the submicron size core to verify their feasibility as the nonlinear optical devices. In this study, we analyzed optical propagation in the TiO2 photonic crystal waveguide and demonstrated the enhanced optical nonlinearity due to slow light effect.

We observed photonic band-gap disappearance on Bragg-grating wavelength filters with Si photonic-wire waveguides, and studied the physical mechanism with theoretical and numerical analyses. This is a unique phenomenon observed in channel waveguides with very high index-contrast between the waveguide core and cladding materials. The photonic band-gap disappearance was observed in structures where two different optical field distributions of standing wave degenerate.

We fabricated wavelength-tunable laser diodes with Si wire waveguide ring resonators as an external optical cavity. The footprint of the optical cavity including the semiconductor optical amplifier is as small as 2.73 x 0.89 mm(2), which is about 1/5 of those made of silicon oxinitride material. About 44-nm-wavelength tuning operation, which covers the entire L-band of the optical communication wavelength range, was obtained. Furthermore, spectral linewidth narrower than 100 kHz was obtained by optimizing the design of the external optical cavity. The tunable laser diodes with narrow spectral linewidth are suitable as light sources for digital coherent optical transmission systems. (C) 2012 The Japan Society of Applied Physics

We describe a phase-matching scheme for generating the second harmonic in ZnO channel waveguides using a modal phase-matching (MPM) method. We found that MPM can be achieved between the TM00 mode of the fundamental wave and the TM02 mode of the second-harmonic wave by using structural dispersion of the waveguide mode. Furthermore, we calculated the normalized conversion efficiency by taking the overlap integral of each waveguide mode and the effective area of the fundamental wave into account, and obtained a sufficiently high conversion efficiency. These results show that ZnO channel waveguides are very attractive for generating second-harmonic devices. (C) 2012 The Japan Society of Applied Physics

In this paper, we analyze the nonlinear optical parameter gamma estimated from experimental results of spectral broadening in ZnO channel waveguides. The effective area was calculated for waveguides of various dimensions and using the resulting data, the nonlinear optical parameter gamma was obtained analytically. We compare these values with those estimated from measurements, and found good agreement between them. We extended the analysis to calculate the optimal size for maximum nonlinearity and found that the fabricated waveguides were very close to the optimal size. (C) 2011 The Japan Society of Applied Physics

We studied the feasibility of an integrated optical circuit consisting of Si-wire waveguides and silica-based waveguides. In order to realize such a hybrid waveguide optical circuit, we investigated a waveguide junction structure based on adiabatic mode conversion with tapered Si and a waveguide crossing structure of heterogeneous waveguides by numerical analysis and measurements. We estimated the optical coupling loss between the waveguides to be 0.39 dB from the measurements. We also estimated the insertion loss of the waveguide crossing to be less than 0.7 dB, and the crosstalk between the waveguides to be less than -35 dB by numerical analysis. (C) 2011 The Japan Society of Applied Physics

We studied a characteristics trimming technique in Si photonic-wire waveguide devices. In order to trimming device properties, we utilized refractive index change of amorphous silicon when it crystallizes by annealing. We fabricated MZI devices with amorphous silicon waveguides, and demonstrated the trimming of the transmission spectra by thermal annealing and also laser annealing after finished the device fabrication process. We observed 5.2 % of the refractive index change owing to crystallization by annealing in a nitrogen atmosphere and 5.8% of change by laser crystallization.

We fabricated wavelength tunable laser diodes consisting of ring resonator wavelength filters with Si-wire waveguides. More than 45 nm wavelength tuning operation was obtained by about 100 mW heating power. We also measured spectral linewidth of the tunable laser, and observed about 200 kHz of spectral linewidth.

We investigated a possibility of making rutile TiO(2) channel waveguides for nonlinear optical applications. Single mode conditions, the group velocity dispersion and the nonlinear-optical parameters for the rutile TiO(2) channel waveguides were calculated using the finite-element method. We also fabricate channel waveguides with rutile TiO(2) using reactive ion etching. The propagation loss measured by the cut-back method was around 6 dB/mm. According to our simulation, around 70 nm of spectral broadening owing to the self-phase modulation will be expected by a pumping optical pulse with 360 W peak power.

We present a fabrication procedure for ZnO channel waveguides intended for nonlinear optical applications. Ar ion milling was used to etch the single crystal thin film samples, and the effects of bias power, chamber pressure and Ar flow rate were investigated, finding optimal parameters for waveguide fabrication. The effect of sidewall roughness was estimated by comparing the results of cut-back measurements and an analytical model. We show an easy and effective method for the fabrication of ZnO channel waveguides.

We report the observation of nonlinear optical effects in the visible spectrum using ZnO channel waveguides. We demonstrate sixfold spectral broadening and a 2 pi phase shift due to self-phase modulation in ZnO channel waveguides using femtosecond optical pulses with a center wavelength of 840 nm. We measured a value for the nonlinear parameter gamma of 13.9 +/- 3.0 W-1 m(-1), which is more than 1300 times that of a highly nonlinear fiber. The calculated intensity-dependent refractive index is found to be consistent with previously reported values of bulk single-crystal ZnO. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3480422]

We have systematically studied Bragg Grating Filters in Si photonic-wire waveguides. Highly uniform samples have been fabricated in a specialized foundry service for Si photonics. The reflected wavelength has been controlled over an 180 nm range. By changing the design parameters we controlled the photonic band-gap to be between 6 to 16 nm. Thermal tuning of the reflected wavelength was also measured, resulting in a wavelength shift of more than 3 nm.

ZnO channel waveguides were fabricated for the first time, for the purpose of being used in nonlinear optical applications. Wide-bandgap ZnO was chosen for the core material to avoid two-photon absorption at wavelengths above 800nm which are used for biomedical applications. We calculated the single-mode conditions and group-velocity dispersion of the propagating modes for the waveguides. We also fabricated channel waveguides using single-crystal ZnO thin film grown on sapphire substrate. Measured propagation losses with a cut-back method were approximately 3 to 6 dB/mm for single-mode waveguides. (C) 2010 The Japan Society of Applied Physics

For the study of spin-dependent transport in narrow-gap semiconductor nanostructures, we fabricated a few-electron quantum dot based on an In0.56Ga0.44As resonant tunneling diode structure with an Al2O3 gate insulator formed by atomic layer deposition and an air-bridge drain electrode. This gated quantum dot device allows us to control the number of electrons from zero to a few dozen without leakage problem. We describe the processing techniques and the characteristics of the low-temperature transport. (C) 2007 Elsevier B.V. All rights reserved.

The electronic transport properties of a gated vertical quantum dot fabricated of an asymmetrical InGaAs/AlGaAs double-barrier resonant tunneling heterostructure are studied experimentally. At a temperature of 15 mK, a series of small current peaks are observed far below the voltage of a main resonance peak. The voltage position of these peaks appeared to be strongly dependent on the presence of magnetic field oriented perpendicular to the plane of the barriers. The occurrence of the peaks is attributed to tunneling mechanisms involving inter- Landau- level resonant tunneling, longitudinal-optical (LO)-phonon-assisted tunneling and to electrostatic effects such as Coulomb blockade (C) 2007 Elsevier B.V. All rights reserved.

Using an Al2O3 gate insulator by atomic layer deposition and air-bridge drain electrode, we fabricated a quantum dot with few electrons based on an In0.56Ga0.44As resonant tunneling diode structure. Artificial atomic properties manifested themselves in magnetotransport, enabling the determination of effective electron g factors. Results show that the insulating gate structure used here is effective for realizing quantum dots made of narrow-gap semiconductors for studying spin-related phenomena. (c) 2007 American Institute of Physics.

Discrete phase libraries of thermoelectric compounds, Mg(x)Si(y)Ge(1-y), were fabricated by a combinatorial pulsed laser deposition followed by annealing as a thin film form on an integrated ceramic substrate. In the substrate are embedded four probe electrical contacts to each sample, lead wires and pads to be accessed by needle probes. Resistivity and Seebeck coefficient were evaluated electrically, while temperature difference was locally given to each sample by a local heater also embedded in the substrate. The sample temperature (300-673 K) was controlled by a heating stage and temperature difference at the two contact points for each sample was evaluated by an infrared camera. The dependences of polarity and absolute values of Seebeck coefficient on the composition agree well with the data in literature. (C) 2007 Elsevier B.V. All rights reserved.

We observed Shubnikov-de Haas oscillation and the quantum Hall effect in a high-mobility two-dimensional electron gas in polar ZnO/Mg(x)Zn(1-x)O heterostructures grown by laser molecular beam epitaxy. The electron density could be controlled in a range of 0.7 x 10(12) to 3.7 x 10(12) per square centimeter by tuning the magnesium content in the barriers and the growth polarity. From the temperature dependence of the oscillation amplitude, the effective mass of the two-dimensional electrons was derived as 0.32 +/- 0.03 times the free electron mass. Demonstration of the quantum Hall effect in an oxide heterostructure presents the possibility of combining quantum Hall physics with the versatile functionality of metal oxides in complex heterostructures.

The authors fabricated a gated-vertical (In,Ga)As quantum dot with an Al2O3 gate insulator deposited using atomic layer deposition and investigated its electrical transport properties at low temperatures. The gate voltage dependence of the dI/dV-V characteristics shows clear Coulomb diamonds at 1.1 K. The metal-insulator gate structure allowed the authors to control the number of electrons in the quantum dot from 0 to a large number estimated to be about 130. (c) 2007 American Institute of Physics.

We investigated injection of spin polarized electrons in a (Ga,Mn)As/n(+)-GaAs Esaki diode (ED) by using a three-terminal device integrating a (Ga,Mn)As ED and a light emitting diode (LED). Electroluminescence polarization (P-EL) from the LED was measured under the Faraday configuration as a function of bias voltages applied independently to the Esaki diode and to the LED. The maximum P-EL of 32.4% was observed when the valence electrons near the Fermi energy of (Ga,Mn)As are ballistically injected into the LED. (c) 2006 American Institute of Physics.

We have investigated the pulse-width and magnetic-field dependences of current-induced magnetization switching in a (Ga,Mn)As/GaAs/(Ga,Mn)As magnetic tunnel junction. Critical current to induce magnetization switching shows a linear dependence on pulse width from 10 to 1000 mu s. The magnetic-field dependence appears to indicate that the observed current-induced magnetization switching proceeds through metastable magnetization structures. (C) 2006 American Institute of Physics.

We have fabricated a three terminal device integrating a (Ga,Mn)As Esaki diode and a light emitting diode (LED) to investigate the bias-voltage dependence of the injection of spin polarized electrons. The electroluminescence polarization (P-EL) from the LED was measured under Faraday configuration as a function of bias voltages applied independently to the Esaki diode and to the LED. The polarization shows strong dependence on the bias applied to the Esaki diode when the LED bias is fixed. The maximum P-EL of 32.4% was observed when the valence electrons near the Fermi energy of (Ga,Mn)As are injected into the LED. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Current-driven magnetization reversal in a ferromagnetic semiconductor based (Ga,Mn)As/GaAs/(Ga,Mn)As magnetic tunnel junction is demonstrated at 30 K. Magnetoresistance measurements combined with current pulse application on a rectangular 1.5x0.3 mum(2) device revealed that magnetization switching occurs at low critical current densities of 1.1-2.2x10(5) A/cm(2) despite the presence of spin-orbit interaction in the p-type semiconductor system. Possible mechanisms responsible for the effect are discussed.

We study nonmagnetic impurity effects in MgB2 using the quasiclassical equations of superconductivity for a weak-coupling two-band model. Parameters in the model are fixed so as to reproduce experiments on MgB2 as closely as possible. The quasiparticle density of states and the specific heat are calculated for various values of the interband impurity scattering. The density of states changes gradually from a two-gap structure into the conventional single-gap structure as the interband scattering increases. It is found that the excitation threshold is not a monotonic function of the interband scattering. Calculated results for the specific heat are in good agreements with experiments on samples after irradiation.

Two types of quantum point contacts (QPCs), side-gate quantum wire and split-gate QPCs, were fabricated at high In-content InGaAs/InAlAs single heterojunction which confines two-dimensional electron gas revealing strong Rashba spin-orbit interaction. In the side-gate quantum wire QPC, the conductance quantization in unit of 0.5(2e(2)/h) was mostly observed in the various samples with different dimensions. This is in contrast to the result in the split-gate QPC, where we confirmed usual (2e(2)/h) conductance quantization. Since those results were observed under zero magnetic field, the 0.5(2e(2)/h) quantization could be attributed not to the spin splitting but to the possible spin polarized transport due to the mode coupling between the one-dimensional subbands expected in sufficiently narrow adiabatic constrictions. (C) 2003 Published by Elsevier B.V.

In 1996, it was suggested by K.J. Thomas et al. (Phys. Rev. Lett. 77, 135 (1996)) that 0.7(2e(2)/h) conductance structure in Quasi-1DEG (Q1DEG) at GaAs/AlGaAs heterostructure might be related to spin polarization at zero magnetic field. We have recently studied spontaneous spin-splitting in the 2DEGs formed at normal metamorphic In0.75Ga0.25As/In0.75Al0.25As heterojunctions grown on GaAs substrates and obtained the value of less than or similar to10 meV as the zero-field splitting at Fermi level (Y. Sato et al., J. Appl. Phys. 89, 8017 (2001)). In this work, we attempted to observe spin-related phenomena in this heterojunction Q1DEGs at zero magnetic field. We observed e(2)/h conductance steps in low electron concentration side-gate point contact.

Conductance fluctuations in quasi-ballistic quantum wires formed at spontaneously spin-splitted heterojunctions have been measured and discussed. The fluctuation amplitude was found to likely decrease when an applied magnetic field increased and when a gate voltage decreased. Both the applications of magnetic field and negative gate voltage are known to decrease the spin-orbit coupling constant in our heterojunction and they are expected to increase the fluctuation amplitude in anti-localization regime. The fluctuation results could, however, be explained, if we pay attentions to the fact that despite the large zero field spin-splitting,, the quantum wires am classified into those with tranport regimes of 'localization" and "weak' spin-orbit scattering due to the high mobilities.

Spin-injection experiments in NiFe/two-dimensional electron gas (2DEG)/NiFe four-terminal devices are described. The 2DEG was confined at a modulation-doped In0.75Ga0.25As/In0.75Al0.25As heterojunction interface. NiFe source-drain electrodes were located similar to1 mum apart and two voltage probes were fabricated between them. The separation between the NiFe electrodes was thus almost less than both the mean free path (similar to2 mum) and the spin-diffusion length (> 1 mum) of the 2DEG. In nonlocal four-terminal measurements, where a constant current (i) was sent through one NiFe to one ohmic electrode, the two-terminal magnetoresistance (R-2t = exhibited a spin-valve-like effect, while R-4t = V-4t/i showed a resistance hysteresis. The amplitude of the latter amounted to almost 12% of the R-4t (B = 0) at 1.5 K, the extent of which was one order of magnitude larger than those so far reported. These results suggest the importance of a quasi-ballistic as well as a spin-coherent coupling between the two NiFe electrodes, which may crucial in the future operation of spin field-effect transistors (FETs).

We have studied critical layer thickness (CLT) in an In0.75Ga0.25As resonant tunneling diode structure grown on a GaAs substrate, ia an InAlAs step-graded buffer (SGB) with two types of inverse step(IS)-SGB. We have observed red or blue shift in photoluminescence spectra and CLT change depending on SGB condition. This change reflects from the difference of residual strain InAlAs SGBs adopted here.,(C) 2001 Elsevier Science B.V. All rights reserved.

We have studied electronic and structural characterizations of high indium content metamorphic InGaAs/InAlAs modulation-doped heterostructures. An improved low-temperature electron mobility of mu (e) = 5.45 x 10(5) cm(2) Vs in [(1) over bar 1 0] was confirmed for a directed Hall-bar sample when Si-doped InAlAs layer was slightly etched. In addition, inplane mobility anisotropy of 40% between [(1) over bar 1 0] and [1 1 0] directions. We confirmed. This anisotropy seems to br originated fi om the different undulation period between[(1) over bar 1 0] and [1 1 0] directions. We theoretically calculated electron mobility taking both alloy disorder scattering and background impurity scattering into account. It is found that the calculated and experimental values are in good agreement. (C) 2001 Elsevier Science B.V. All rights reserved.

Amounts of spontaneous spin splittings were estimated from low-temperature magnetoresistances in two-dimensional electron gases created at In0.75Ga0.25As/In0.75Al0.25As heterojunctions under a gate bias. Typical sheet electron densities and mobilities in the raw wafers were similar to1.0 x 10(12)/cm(2) and 2-5 x 10(5) cm(2)/V s at 1.5 K, respectively. A maximum spin-orbit coupling constant alpha (zero) of similar to 30(x10(-12) eV m) was obtained for the van der Pauw sample. In gated Hall-bar samples, a decrease in the alpha (zero) value with decreasing gate voltage (V-g) was first confirmed in a normal heterojunction. The main origin for such a large alpha (zero), which is a few times larger than any previously reported, was found to be a structure-dependent so-called interface contribution in the Rashba term. (C) 2001 American Institute of Physics.

We have estimated spin-splitting energy of two-dimensional electron gases formed at metamorphic In0.75Ga0.25As/In0.75Al0.25As heterojunctions by analyzing magneto-resistance traces up to 10 T. This heterojunction often reveals a large spontaneous spin-splitting at low fields, while at high fields, Zeeman splitting becomes dominant. We have investigated and compared the two sanples; one reveals beating oscillation and hence has a spontaneous or zero-field spin-splitting and another does not. In the former sample, if we assume the equal sign for both the spin-splittings, the dependency of the absolute splitting energy on the magnetic field is found to have a minimum (Delta = 2 meV) at about 3.5 T, in which we also confirmed a zero-field spin-splitting of Delta (0) = 11.04meV and very much enhanced g* = - 42.0 at 1.5 T. In contrast, effective g-factor (g*) obtained at high fields in the latter sample is g* = - 7.7 at 1.5 T, which is fairly smaller than that in the former. Origins of those features are discussed and the possibility of linear spin-splitting dependency on magnetic field is pointed out. (C) 2001 Published by Elsevier Science B.V.

In this work, we are studying the effects of interface strain on the amount of spin-orbit coupling constant of the two-dimensional electron gas formed at the heterointerface, high In-content InGaAs/InAlAs. Tuning of the interface strain is carried out by changing the step-graded buffer (SGB) layer with additional inverse steps (ISs). Estimations of photoluminescence, far-infrared absorption and magnetoresistance measurement suggest that, although a slight extent of residual strain still remains in a case of normal SGB, the residual strain is found to decrease, if an appropriate IS layer is adopted, Associating with this, the decreases of the band gap and electron effective mass were observed together with the increase of the spin-orbit coupling constant.

We study the electrical spin-injection and detection of spin-polarized electrons in ferromagnetic electrode - two-dimensional electron gas (2DEG) system. The 2DEG was formed at In0.75Ga0.25As/In0.75Al0.25As heterointerface and NiFe electrodes were deposited on the side surface of the mesa containing channel layer. The NiFe electrodes (width=F1:0.5, and F2:3mum) were located 1 mum apart, which was less than the mean free-path (similar to2mum) of the 2DEG, and two voltage probes were fabricated between them. We confirmed by SdH oscillations that electrons indeed transport from F1 to F2 through the 2DEG. In non-local geometry below 1.5K, we observed a spin-valve like effect of similar to0.2% in source-drain resistance of despite including the single ferromagnet/2DEG interface in the current pass. We also observed the hysterisis behavior of similar to12% in four-terminal resistance below 1.5K, Both signals were found to disappear at 200K. This temperature dependency strongly suggests that these signals could not be explained only by the local Hall effect, since the sheet electron density, N-S, in our heterostructure is almost kept constant from 0.3 to 200K.