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

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

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

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

Molecular phylogeny of the higher and lower taxonomy of the Fusarium genus and differences in the evolutionary histories of multiple genes

<p>Abstract</p> <p>Background</p> <p>Species of the <it>Fusarium </it>genus are important fungi which is associated with health hazards in human and animals. The taxonomy of this genus has been a subject of controversy for many years. Although many researchers have applied molecular phylogenetic analysis to examine the taxonomy of <it>Fusarium </it>species, their phylogenetic relationships remain unclear only few comprehensive phylogenetic analyses of the <it>Fusarium </it>genus and a lack of suitable nucleotides and amino acid substitution rates. A previous stugy with whole genome comparison among <it>Fusairum </it>species revealed the possibility that each gene in <it>Fusarium </it>genomes has a unique evolutionary history, and such gene may bring difficulty to the reconstruction of phylogenetic tree of <it>Fusarium</it>. There is a need not only to check substitution rates of genes but also to perform the exact evaluation of each gene-evolution.</p> <p>Results</p> <p>We performed phylogenetic analyses based on the nucleotide sequences of the rDNA cluster region (rDNA cluster), and the β-tubulin gene (<it>β-tub</it>), the elongation factor 1α gene (<it>EF-1α</it>), and the aminoadipate reductase gene (<it>lys2</it>). Although incongruence of the tree topologies between <it>lys2 </it>and the other genes was detected, all genes supported the classification of <it>Fusarium </it>species into 7 major clades, I to VII. To obtain a reliable phylogeny for <it>Fusarium </it>species, we excluded the <it>lys2 </it>sequences from our dataset, and re-constructed a maximum likelihood (ML) tree based on the combined data of the rDNA cluster, <it>β-tub</it>, and <it>EF-1α</it>. Our ML tree indicated some interesting relationships in the higher and lower taxa of <it>Fusarium </it>species and related genera. Moreover, we observed a novel evolutionary history of <it>lys2</it>. We suggest that the unique tree topologies of <it>lys2 </it>are not due to an analytical artefact, but due to differences in the evolutionary history of genomes caused by positive selection of particular lineages.</p> <p>Conclusion</p> <p>This study showed the reliable species tree of the higher and lower taxonomy in the lineage of the <it>Fusarium </it>genus. Our ML tree clearly indicated 7 major clades within the <it>Fusarium </it>genus. Furthermore, this study reported differences in the evolutionary histories among multiple genes within this genus for the first time.</p

GaAs/AlAs triple-coupled cavity with InAs quantum dots for ultrafast wavelength conversion devices

We have investigated a GaAs/AlAs triple-coupled multilayer cavity structure with InAs quantum dots for an ultrafast wavelength conversion device. Three cavity modes with the resonance frequencies ω1, ω2, and ω3 were used for efficient wavelength conversion via a four-wave mixing (FWM) process. Identical frequency separation between two adjacent modes (ω1 − ω2 = ω2 − ω3) was successfully realized using a controlled lateral thickness variation across the wafer. Time-resolved FWM signals from the triple-coupled multilayer cavity were measured using 100 fs laser pulses. The incident laser pulses were divided into two pulses and each of them was spectrally shaped individually so that the input and control pulses only covered the ω1 and ω2 modes, respectively. The wavelength-converted FWM signal with a frequency of ω3 (= 2ω2 − ω1) was clearly observed when the sample was simultaneously irradiated with the input and control laser pulses

Fabrication of two - color surface emitting device of a coupled vertical cavity structure with InAs QDs formed by wafer - bonding

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

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

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

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

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

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

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

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)

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

En bloc excision of giant polycystic liver with hepatic cava and its auto-transplant caval reconstruction as a safe surgical procedure for liver transplantation

Safely excising a giant liver while leaving the hepatic inferior vena cava intact is difficult. Hata et al. present images and videos describing their novel technique consisting of total hepatectomy including the hepatic cava; extracorporeal retrieval; and auto‐transplant inferior vena cava reconstruction, for an extremely enlarged polycystic liver weighing 24 kg.[Image: see text