Anomalous enhancement of thermoelectric power factor in multiple two-dimensional electron gas system

Toward drastic enhancement of thermoelectric power factor, quantum confinement effect proposed by Hicks and Dresselhaus has intrigued a lot of researchers. There has been much effort to increase power factor using step-like density-of-states in two-dimensional electron gas (2DEG) system. Here, we pay attention to another effect caused by confining electrons spatially along one-dimensional direction: multiplied 2DEG effect, where multiple discrete subbands contribute to electrical conduction, resulting in high Seebeck coefficient. The power factor of multiple 2DEG in GaAs reaches the ultrahigh value of ~100 μWcm-1 K-2 at 300 K. We evaluate the enhancement rate defined as power factor of 2DEG divided by that of three-dimensional bulk. The experimental enhancement rate relative to the theoretical one of conventional 2DEG reaches anomalously high (~4) in multiple 2DEG compared with those in various conventional 2DEG systems (~1). This proposed methodology for power factor enhancement opens the next era of thermoelectric research.

High-endurance micro-engineered LaB6 nanowire electron source for high-resolution electron microscopy

The size tunability and chemical versatility of nanostructures enable electron sources of high brightness and temporal coherence, both of which are important characteristics for high-resolution electron microscopy^1-3. Despite intensive research efforts in the field, so far, only conventional field emitters based on a bulk tungsten (W) needle have been able to yield atomic-resolution images. The absence of viable alternatives is in part caused by insufficient fabrication precision for nanostructured sources, which require an alignment precision of subdegree angular deviation of a nanometre-sized emission area with the macroscopic emitter axis⁴. To overcome this challenge, in this work we micro-engineered a LaB₆ nanowire-based electron source that emitted a highly collimated electron beam with good lateral and angular alignment. We integrated a passive collimator structure into the support needle tip for the LaB₆ nanowire emitter. The collimator formed an axially symmetric electric field around the emission tip of the nanowire. Furthermore, by means of micromanipulation, the support needle tip was bent to align the emitted electron beam with the emitter axis. After installation in an aberration-corrected transmission electron microscope, we characterized the performance of the electron source in a vacuum of 10^-8 Pa and achieved atomic resolution in both broad-beam and probe-forming modes at 60 kV beam energy. The natural, unmonochromated 0.20 eV electron energy loss spectroscopy resolution, 20% probe-forming efficiency and 0.4% probe current peak-to-peak noise ratio paired with modest vacuum requirements make the LaB₆ nanowire-based electron source an attractive alternative to the standard W-based sources for low-cost electron beam instruments.

Patchwork metasurface quantum well photodetectors with broadened photoresponse

Complex lightwave manipulation such as broadband absorption has been realized with metasurfaces based on laterally arranged metal-dielectric-metal cavities with different geometries. However, application of these metasurfaces for optoelectronic devices by incorporating functional dielectrics remains challenging. Here, we integrate a quantum well infrared photodetector (QWIP) with a metasurface made of a patchwork of square cavities with different dimensions arranged in a subwavelength unit cell. Our detector realizes wideband photoresponse approaching the entire responsivity spectrum of the QWIP-single-sized square cavities can utilize only 60% of the possible bandwidth-and external quantum efficiencies of up to 78% at 6.8 µm. Our highly flexible design scheme enables integration of photodetectors and metasurfaces with arbitrary arrangements of cavities selectively responding to incidence with a specific wavefront.

Synchronously wired infrared antennas for resonant single-quantum-well photodetection up to room temperature

Optical patch antennas sandwiching dielectrics between metal layers have been used as deep subwavelength building blocks of metasurfaces for perfect absorbers and thermal emitters. However, for applications of these metasurfaces for optoelectronic devices, wiring to each electrically isolated antenna is indispensable for biasing and current flow. Here we show that geometrically engineered metallic wires interconnecting the antennas can function to synchronize the optical phases for promoting coherent resonance, not only as electrical conductors. Antennas connected with optimally folded wires are applied to intersubband infrared photodetectors with a single 4-nm-thick quantum well, and a polarization-independent external quantum efficiency as high as 61% (responsivity 3.3 A W⁻¹, peak wavelength 6.7 μm) at 78 K, even extending to room temperature, is demonstrated. Applications of synchronously wired antennas are not limited to photodetectors, but are expected to serve as a fundamental architecture of arrayed subwavelength resonators for optoelectronic devices such as emitters and modulators.

Applications of metasurfaces for optoelectronic devices require wiring to each isolated antenna for biasing and current flow. Here, the authors report optimal wire interconnects design for controlling the optical properties and present antenna-enhanced mid-infrared photodetection incorporating a single quantum well.

Double-Sided Nonalloyed Ohmic Contacts to Si-doped GaAs for Plasmoelectronic Devices

There is increasing demand for the ability to form ohmic contacts without lossy intermediate layers on both the top and bottom sides of metal-semiconductor-metal plasmoelectronic devices such as quantum cascade lasers and metasurface photodetectors. Although highly Si-doped n-GaAs surfaces can allow an ohmic contact without alloying, conditions for realizing nonalloyed ohmic contacts to other n-GaAs surfaces, originally buried inside but exposed by removing the substrate, have yet to be studied. We discovered that nonalloyed ohmic contacts to initially buried surfaces with a practically low contact resistivity down to 77 K can be realized by fulfilling certain requirements, specifically keeping the Si-doping concentration within a narrow range of 7.5 × 1018 to 1.25 × 1019 cm-3 and setting the growth temperature of the succeeding upper layers to a low value of 530 °C.

Ultraviolet-nanoimprinted packaged metasurface thermal emitters for infrared CO2 sensing

Packaged dual-band metasurface thermal emitters integrated with a resistive membrane heater were manufactured by ultraviolet (UV) nanoimprint lithography followed by monolayer lift-off based on a soluble UV resist, which is mass-producible and cost-effective. The emitters were applied to infrared CO2 sensing. In this planar Au/Al2O3/Au metasurface emitter, orthogonal rectangular Au patches are arrayed alternately and exhibit nearly perfect blackbody emission at 4.26 and 3.95 μm necessary for CO2 monitoring at the electric power reduced by 31%. The results demonstrate that metasurface infrared thermal emitters are almost ready for commercialization.

Diffraction-unlimited optical imaging of unstained living cells in liquid by electron beam scanning of luminescent environmental cells

An environmental cell with a 50-nm-thick cathodoluminescent window was attached to a scanning electron microscope, and diffraction-unlimited near-field optical imaging of unstained living human lung epithelial cells in liquid was demonstrated. Electrons with energies as low as 0.8 - 1.2 kV are sufficiently blocked by the window without damaging the specimens, and form a sub-wavelength-sized illumination light source. A super-resolved optical image of the specimen adhered to the opposite window surface was acquired by a photomultiplier tube placed below. The cells after the observation were proved to stay alive. The image was formed by enhanced dipole radiation or energy transfer, and features as small as 62 nm were resolved.

Graphical template software for accurate micromanipulation in a scanning electron microscope

Micromanipulation techniques in a scanning electron microscope (SEM) have been utilized for assembling micrometer-sized structures. The precision of the assembled microstructures has been limited by the poor accuracy of the SEM image. We have developed a software to assist the operator in the accurate assembly of microstructures in a SEM, in which computer-generated outlines of the target structure [graphical templates (GTs)] are superimposed on the monitor. The displayed GTs are distorted on the basis of the image properties of the SEM evaluated in advance. As a consequence, the operator can construct microstructures with a high precision only by maneuvering the manipulator so that the outline of each object perfectly overlaps the GT without any alteration of the electron optics or circuits for improving the image accuracy.

Light propagation from a fluorescent particle embedded in a photonic cluster of micrometer-sized dielectric spheres

In self-assembled multilayer arrays of micrometer-sized spheres that include small amounts of fluorescent particles, unique six-dot-triangular and seven-dot-hexagonal patterns have been known to appear in the fluorescence microscopic images. Although it has been suggested that these two types of patterns correspond to local domain structures, i.e., face centered cubic (fcc) or hexagonal closed packed (hcp), no conclusive evidence has been provided to support this claim. In this study, we systematically investigated the relationship between the propagation patterns and the arrangement of the particles. Through a cross-check between an experiment using well-defined clusters fabricated by a micromanipulation technique and a rigorous calculation based on the expansion of vector spherical harmonics, we confirmed that the six-dot-triangular and seven-dot-hexagonal patterns correspond to the fcc and hcp domains, respectively. Further, we also found that the propagation patterns depend on the size of the clusters. As a result of a quantitative discussion on the light propagation in clusters with various sizes, it was clarified that a sufficient domain size is necessary for the appearance of clear triangular or hexagonal patterns.

[Subrenal capsule assay of nude mice for testing the effectiveness of UFT for rat prostatic carcinoma (R3327)]

Combination effect of UFT and CDDP against the rat prostatic carcinoma (R3327) was evaluated by the subrenal capsule assay in nude mice. The tumor growth of R3327 was inhibited in proportion to the UFT concentration. Anticancer effect of UFT was elevated by co-administration of CDDP. These data suggest a clinical usefulness of UFT administration with CDDP for hormonally refractory advanced prostatic carcinoma.

[Bladder tumors detected by transabdominal ultrasonography in the multiphasic health testing and services]

From August, 1989 to July, 1991, we carried out transabdominal ultrasonography for screening of abdominal and pelvic disease in regular health check-up at Mito Saiseikai Hospital. Of 5,706 screened examinees were ultrasonographically suspected of bladder tumor and subsequently received a cystoscopic examination. Cystoscopy confirmed bladder tumors in 9 of the 11 patients. Eight of the 9 cases showed no urological symptoms, while the remaining 1 case showed microscopic hematuria objectively. All of the 9 cases revealed a transitional cell carcinoma of low grade and low stage. The smallest tumor detected by ultrasonography was 5 mm in diameter. Transabdominal ultrasonography proved useful for diagnosis of the bladder tumor at its early stage in the mass screening. Therefore, it is recommended to perform lower abdominal ultrasonography as a routine examination of the multiphasic health testing and services.