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Two-color lasing in a GaAs/AlAs coupled multilayer cavity has been studied for a novel type of terahertz emitting devices. In the structure, two cavity layers based on GaAs were connected by the intermediate GaAs/AlAs distributed Bragg reflector multilayer and three layers of self-assembled InAs quantum dots (QDs) were inserted only in the top-side cavity layer as optical gain materials. The broadened QD emissions around 1.25 µm are very useful to realize the optical gain because they could cover the wide range of the mode frequency difference. Two mode emissions with an optical frequency difference of 3.2 THz were successfully demonstrated under room temperature cw operation by optical pumping. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

We have observed the electrical conduction properties of silicon carbide (SiC) that was locally modified by femtosecond laser. The current-voltage (I-V) characteristics of laser-modified regions were measured. Intriguingly, when the polarization of the laser beam was parallel to the scanning direction, the resistance of the modified region decreased with increasing the irradiated fluence. The resistance of the region irradiated at a fluence of 86 J /cm² decreased by more than six orders of magnitude compared with the non-irradiated one. In contrast, when the polarization of the laser beam was perpendicular to the scanning direction, the resistance of the modified region did not show the significant reduction. From the scanning electron microscope observations and Raman spectroscopy, we suppose that the difference of the resistance for each polarization direction is due to the difference of the chemical composition generated in laser modified region. [ABSTRACT FROM AUTHOR]

An extremely high maximum transconductance gm max of 2.25 S/mm and a cutoff frequency ƒT of 310 GHz was achieved at a cryogenic temperature (16 K) in a 195-nm-gate In0.75Ga0.25As/In0.52Al0.48As high electron mobility transistor (HEMT) fabricated on a (411)A-oriented InP substrate by molecular beam epitaxy, compared with room temperature values (fro max = 1.78 S/mm and ƒT = 245 GHz). These significantly enhanced gm max and ƒT values are attributed to a high electron velocity of up to 4.9 × 107 cm/s due to suppressing phonon scattering in the In0.75Ga0.25As/In0.52Al0.48As HEMT with (411)A super-flat InGaAs/InAlAs interfaces (effectively atomically flat heterointerfaces over a wafer-size area). [ABSTRACT FROM AUTHOR]

We investigated flatness of growth-interrupted InAlAs and InGaAs surfaces using In0.53Ga0.47As/In0.52Al0.48As quantum wells (QWs) with well widths (Lw) of 0.6–12 nm grown on a (4 1 1)A-oriented InP substrate by molecular beam epitaxy. In the (4 1 1)A QWs with growth interruption (GI) at the InGaAs/InAlAs interface (InAlAs surface), full width at half maximum (FWHM) of a photoluminescence peak of the 1.2-nm-thick InGaAs well was as narrow as 15.6 meV, which was almost the same as that of the corresponding well without GI (14.4 meV). On the other hand, the FWHM of the (1 0 0) QWs with GI at the InAlAs surface was 24.1 meV (Lw=1.2 nm), which was 55% larger than that of the (4 1 1)A QWs with GI at the InAlAs surface. This result indicates that the effectively atomically flat interfaces [(4 1 1)A super-flat interfaces] are successfully kept in spite of introducing GI, while GI makes the (1 0 0) interface much rough. [Copyright &y& Elsevier]

Single-particle relaxation times in In0.7Ga0.3As/In0.52Al0.48As quantum well high electron mobility transistor (QW-HEMT) structures for two subbands (i=0 and 1) are studied by fast Fourier transform analysis of Shubnikoov-de Haas (SdH) oscillations measured at 4 K in order to investigate remote impurity (RI) scattering in the QW-HEMT structure with effectively atomically flat interfaces over a wafer-size area ((4 1 1)A super-flat interfaces) grown on a (4 1 1)A InP substrate by molecular beam epitaxy. The (4 1 1)A QW-HEMT structure with thin spacer thickness of 3 nm shows two times longer single-particle relaxation times (τs(0)=0.71×10−13 s and τs(1)=1.08×10−13 s) compared to those (τs(0)=0.37×10−13 s and τs(1)=0.49×10−13 s) for the corresponding QW-HEMT structure with conventional (1 0 0) interfaces, indicating that RI scattering is much reduced in the (4 1 1)A sample. Calculations of τs(0) limited by RI scattering assuming random distribution of sheet-doped impurities show that the enhancement of τs(0) for the (4 1 1)A QW-HEMT structure cannot be explained by laterally uniform spacer thickness resulting from the (4 1 1)A super-flat interfaces. Much reduced RI scattering in the (4 1 1)A QW-HEMT structure is considered to be mainly caused by extremely uniform distribution of sheet-doped impurities. [Copyright &y& Elsevier]

Effectively atomically flat interfaces over a macroscopic area (“(411)A super-flat interfaces”) were successfully achieved in In0.53Ga0.47As/In0.52Al0.48As quantum wells (QWs) grown on (411)A InP substrates by molecular beam epitaxy (MBE) at a substrate temperature of 570°C and V/III=6. Surface morphology of the In0.53Ga0.47As/In0.52Al0.48As QWs was smooth and featureless, while a rough surface of those simultaneously grown on a (100) InP substrate was observed. Photoluminescence (PL) linewidths at 4.2 K from the (411)A QWs with well width of 0.6–12 nm were 20–30 % narrower than those grown on a (100) InP substrate and also they are almost as narrow as each of split PL peaks for those of growth-interrupted QWs on a (100) InP substrate. In the case of the (411)A QWs, only one PL peak with very narrow linewidth was observed from each QW over a large distance (7 mm) on a wafer. [ABSTRACT FROM AUTHOR]

In order to characterize interface roughness scattering, two-dimensional electron gas (2DEG) mobility in (4 1 1)A and (1 0 0) selectively-doped In0.53Ga0.47As/In0.52Al0.48As quantum well (QW) HEMT structures, which were grown on InP substrates by molecular beam epitaxy, with a gate contact was measured at 20 K as a function of sheet electron concentration (Ns) by changing gate bias. Thickness of the QW was designed to be rather small (6 nm) for enhancing the interface roughness scattering for both samples. 2DEG mobilities of the (4 1 1)A sample were 21,000–51,400 cm2/V s in the range of Ns=0.7–1.7×1012 cm−2, which are about more than 2 times higher than the mobilities (8,700–25,400 cm2/V s) of the conventional (1 0 0) sample. 2DEG mobility was calculated by taking into account interface roughness scattering and ionized remote impurity scattering. By fitting the calculated results to the observed ones, lateral size (Λ) and height (Δ) of the interface roughness of the (4 1 1)A sample were determined to be 3.5 and 0.23 nm, respectively, which are 30% and 50% smaller than the corresponding values (Λ=5.0 nm and Δ=0.43 nm) of the (1 0 0) sample. [Copyright &y& Elsevier]

Spatial and temporal diffusive spin dynamics in the transient regime for 10-nm-wide InGaAs/InAlAs quantum wells were investigated using time-resolved optical Kerr rotation (TRKR) microscopy. The transient regime of diffusive spin dynamics is a specific regime which appears after local excitation but before the formation of a spin mode. In this regime, the spin precession frequency induced by the spin–orbit (SO) magnetic field decreases with time since the diffusive velocity decreases as the distribution of spins expands. In this study, by decreasing the spot size of the excitation pulse, we examined the spin dynamics in the transient regime. We carefully analyzed the TRKR signals using time-dependent frequency equations established by Kohda et al. [Appl. Phys. Lett. 107, 172402 (2015)]. All the TRKR signals could be well reproduced by the time-dependent frequency equations. Our analysis reveals that the spin precession frequencies arising from the SO interaction gradually decrease with time, and they allowed the extraction of the Rashba and Dresselhaus SO parameters. We revealed that SO parameters can be extracted accurately via scanning measurements in the diffusive motion of local spin excitation even in the time periods before the appearance of the spin mode. [ABSTRACT FROM AUTHOR]

Two-color surface-emitting lasers were fabricated using a GaAs-based coupled multilayer cavity structure grown by molecular beam epitaxy. InGaAs/GaAs multiple quantum wells were introduced only in the upper cavity for two-mode emission in the near-infrared region. Two-color lasing of the device was successfully demonstrated under pulsed current operations at room temperature. We also observed good temporal coherence of the two-color laser light using a Michelson interferometer. A coherent terahertz source is expected when a wafer-bonded coupled cavity consisting of (0 0 1) and non-(0 0 1) epitaxial films is used for the two-color laser device, in which the difference-frequency generation can be enabled by the second-order nonlinear response in the lower cavity. [ABSTRACT FROM AUTHOR]

We have investigated the effects of Sb-soak on InAs quantum dots (QDs) grown on (001) and (113)B GaAs substrates by molecular beam epitaxy. Surface morphologies of the QDs were characterized by atomic force microscopy. The optical properties of buried QDs were investigated by photoluminescence (PL). We showed that effects of Sb-soak on density and PL of (001) and (113)B QDs were quite different. The increased density and blue-shift of (001) QDs can be explained by the surfactant effect of Sb atoms which increase the areal density of the kinks for nucleation. On the other hand, for (113)B QDs, the incorporation effect should be responsible for the red-shift because the Sb atoms may be diffused into QDs. [ABSTRACT FROM AUTHOR]

Abstract: A strong and ultrafast optical Kerr signal at has been demonstrated in a GaAs/AlAs multilayer cavity containing self-assembled InAs quantum dots (QDs) embedded in strain-relaxed barriers. Time-resolved optical measurements using 100 fs pulses with 100kHz repetition rate were carried out in the various excitation powers at room temperature. Although only 2 layers of the InAs QDs were inserted in the half-wavelength cavity layer, the strongly enhanced optical Kerr signal was observed compared to that of a GaAs cavity which had no QDs, in the whole range of excitation power (1–10mW). The signal enhancement becomes more significant with decreasing excitation power because two-photon absorption is suppressed in the cavity consisting of the 2 QD layers and strain-relaxed barrier. In the low-excitation power regime of 1–2mW, the optical Kerr signal was about 2 orders of magnitude larger than that of the GaAs cavity due to the large nonlinearity of the resonant InAs QDs. [ABSTRACT FROM AUTHOR]

The surface migration length of As4 molecules on (4 1 1)A surfaces during molecular beam epitaxy has been determined from lateral profiles of As content (x) in GaAsxP1−x layers grown on (4 1 1)A GaAs substrates with a (1 0 0) side-slope using As4 and P2 molecular beams. The migration length of As4 molecules on (4 1 1)A surfaces at a substrate temperature of 535°C was found to steeply increase from 4 to 10 μm and saturate to 10 μm with increasing V/III flux ratio. This V/III ratio dependence of migration length of As4 is understood by a proposed model where an As4 molecule is dissociated and incorporated into the crystal phase when an As4 molecule encounters a Ga adatom on the (4 1 1)A surface. Quantitative analysis is made using rate equations based on the model and the experimental results are reproduced with reasonable fitting parameter. [Copyright &y& Elsevier]

We determined the lateral mobility of photocarriers and activation energy of the photocurrent (PC) in stacked InAs quantum dot (QD) layers with ultrafast carrier relaxation times and 1.5-μm band absorption. Lateral mobility is one of the important parameters in the application of QD layers to photoconductive antennas operating with 1.5-μm gating light for terahertz (THz) wave detection. The activation energy obtained from temperature dependence of PC spectroscopy is important for elucidating the mechanism, and it is an indicator of the temperature range for antenna operation. The dependence of the mobility on the photocarrier density was evaluated from the dependence on the pump power. The mobility sharply increased from 42 cm2/V･s in the dark to 416 cm2/V･s upon laser illumination and then decreased with the increase in the number of generated photocarriers. The broad PC peak from InAs QD layers at around 1400 nm was continuously monitored in the temperature range of 4–292 K. The activation energy of the PC from the InAs QD layers was evaluated in the temperature range of 159–240 K where the PC significantly increased. In the broad PC peak in the wavelength range of 1200–1600 nm, the activation energy increased from 48 to 62 meV with the wavelength. The PC observed at lower temperatures was not derived considering the thermal escape of electrons, but it was determined through the Auger process. The InAs QD layer with ultrafast carrier relaxation has considerable potential in photoconductive antenna applications for THz wave detection. • Lateral mobility increased: 42 cm2/V･s in darkness to 416 cm2/V･s with illumination. • Thermally stable and broad peak of InAs QD layers occurred during PC spectroscopy. • From 159 to 240 K, PC increased and activation energy of the PC depended on QD sizes. • At cryogenic temperatures, PC was derived using the Auger process. [ABSTRACT FROM AUTHOR]

The production of aroma compounds by the microbial fermentation of whey was studied. Seven strains of the yeast Wickerhamomyces pijperi were used for the fermentation of glucose-added whey (whey-g). Twelve aroma compounds (isobutanol, isoamyl alcohol, 2-phenylethanol, acetaldehyde, ethyl acetate, propyl acetate, isobutyl acetate, isoamyl acetate, ethyl butyrate, ethyl propionate, ethyl hexanoate and ethyl benzoate) were identified in the fermented broth using headspace gas chromatography mass spectrometry analysis. The major components were ethyl acetate (several tens to hundreds ppm), acetaldehyde (several tens ppm) and isoamyl alcohol (about 10 ppm). The strong fruity odor of ethyl benzoate (about 1 ppm) was detected in the broth of W. pijperi YIT 8095 and YIT 12779. The balance of aroma compounds produced was varied depending on the media used, and ethyl benzoate was only produced when using whey-g. The variation in the production of the aroma compounds over time using W. pijperi YIT 12779 at various culture temperatures (from 15-30°C) was also studied. From the results we propose that W. pijperi could be used as a novel microorganism for production of aroma compounds from whey.