Etsuo Maeda

ZnO dense nanowire array on a film structure in a single crystal domain texture for optical and photoelectrochemical applications

A single crystal domain texture quality (a unique in-plane and out-of-plane crystalline orientation over a large area) ZnO nanostructure of a dense nanowire array on a thick film has been homogeneously synthesized on a-plane sapphire substrates over large areas through a one-step chemical vapor deposition (CVD) process. The growth mechanism is clarified: a single crystal [02(-)1] oriented ZnAl(2)O(4) buffer layer was formed at the ZnO film and the a-plane sapphire substrate interface via a diffusion reaction process during the CVD process, providing improved epitaxial conditions that enable the synthesis of the high crystalline quality ZnO nanowire array on a film structure. The high optoelectronic quality of the ZnO nanowire array on a film sample is evidenced by the free exitonic emissions in the low-temperature photoluminescence spectroscopy. A carrier density of ~10(17) cm(-3) with an n-type conductivity of the ZnO nanowire array on a film sample is obtained by electrochemical impedance analysis. Finally, the ZnO nanowire array on a film sample is demonstrated to be an ideal template for a further synthesis of a single crystal quality ZnO-ZnGa(2)O(4) core-shell nanowire array on a film structure. The fabricated ZnO-ZnGa(2)O(4) sample revealed an enhanced anticorrosive ability and photoelectrochemical performance when used as a photoanode in a photoelectrochemical water splitting application.

Optical hydrogen detection with periodic subwavelength palladium hole arrays

The extraordinary transmission of infrared light through subwavelength rectangular hole arrays of palladium is used to detect hydrogen. The main resonance peak of rectangular hole arrays is found to shift upon exposure to hydrogen. Experimental evidence of the change in the Pd phase, producing a shift toward longer wavelengths of the main resonance peak, is presented and supported by simulations that agree with experimental observation. The all-optical and selective detection scheme of hydrogen produces large peak shifts that enable the detection of hydrogen concentration near the lower flammability threshold in air.

Coupling of localized surface plasmons to U-shaped cavities for high-sensitivity and miniaturized detectors

We report numerical analysis of the coupling of localized surface plasmons to the modes of U-shaped cavities. The coupling results in intense resonance for which the electric field is strongly enhanced on the cavity surfaces. As a result, an optical vortex in the power flow is formed in the cavities and a sharp and strong resonance dip is observed in the reflectance spectrum. High sensitivity of the dip wavelength to change in the refractive index of the surrounding medium is reported. The high sensitivity is realized with a small number of cavities, thus enabling miniaturization of detectors based on U-shaped cavities.

Analysis of hydrogen exposure effects on the transmittance of periodic sub-wavelength palladium hole arrays

In a previous study, we have reported transmittance variations of sub-wavelength palladium hole arrays upon exposure to hydrogen. The main resonance peak of the microfabricated palladium hole arrays exhibiting the extraordinary transmission effect decreases when exposed to the lower flammability concentration of hydrogen in air. This variation in transmittance was attributed to a combination of two effects, namely, a change in the dimension of the array holes due to the expansion of palladium upon hydrogen absorption, and/or a change in the permittivity of the palladium layer upon formation of the palladium hydride phase. In this study, we examine the relative strength of the two effects by finite-difference- time-domain simulation. Our simulation results show that the transmittance variation upon exposure to hydrogen is predominantly caused by the lateral expansion of the palladium layer, which induced a decrease in the hole width of the array.

Hydrogen absorption effects on the transmittance of sub-wavelength palladium hole arrays with different thicknesses

The far-field extraordinary optical transmission (EOT) of palladium (Pd) sub-wavelength hole arrays in the infrared region is used to detect hydrogen near the lower flammability threshold in air. Upon exposure to hydrogen, the Pd layer of the hole array expands, causing changes in the hole structure, and the Pd permittivity decreases. These two effects shift the main EOT transmittance peak of the Pd hole array to longer wavelengths. In this report, the effect of the Pd layer thickness on the redshift is analyzed by the rigorous coupled wave analysis technique and experimental observation. Our computational and experimental results show that the hole structural effect on the peak shift is dominant in the opaque region of the Pd layer transmission, whereas the Pd permittivity effect is dominant in the semi-transparent region. The optimum Pd layer thickness for hydrogen sensing is found to be at the boundary between the semi-transparent and the opaque regions of the Pd layer.

Sub-picometer multi-wavelength detector based on highly sensitive nanomechanical resonator

The wavelength division multiplexing (WDM) method for near infrared (NIR) optical fiber (1530–1565 nm) is the system that is wildly used for intercontinental communication. WDM achieves high-speed and large-capacity communication, but costs a lot because the high-resolution (∼10 pm) wavelength locker for wavelength stabilization only corresponds to a single wavelength. In this report, we propose a highly sensitive sub-picometer multi-wavelength detector that substitutes a typical single-wavelength detector for WDM. Our wavelength detector consists of a narrow band (FWHM   20 000) nanomechanical resonator. The photonic absorber confines and transforms the illuminated NIR light wave into thermal stress, and then, the thermal stress in the nanomechanical resonator will appear as the eigenfrequency shift of the nanomechanical resonator. Through experimental works with an NIR laser and optical Doppler vibration meter, the sens...

Sub-wavelength palladium antenna arrays for hydrogen optical detection in the infrared region

Sub-wavelength scaled metallic structures have been studied as sensing elements in new optical devices because these structures enable strong enhancement of the electric field. Among these structures, nano-antenna arrays play a special role for antennas are known to realize both functions of source and detection for radiation. In this paper, rectangular shaped palladium (Pd) sub-wavelength nano-antenna arrays were applied to the detection of permittivity change of the antennas made of Pd that forms Pd hydride when exposed to hydrogen (H2). The dip of the extinction spectrum was shifted toward longer wavelengths. The shape, periodicity, and permittivity dependence of the extinction spectrum of the nano-antenna arrays were investigated through computational and experimental studies. The peak position and sharpness of the extinction spectrum were tailored by varying the period of the arrayed structure. Extinction dip was shifted by 164 nm when exposed to 2% H2.

Hole shape effect induced optical response to permittivity change in palladium sub- wavelength hole arrays upon hydrogen exposure

Sensing with sub-wavelength hole arrays is being actively researched as a means to improve detection sensitivity and reduce the size of the developed sensor. One of the approaches to sensing with hole arrays is to use a shift of the main transmittance peak generated by analyte exposure. In this report, the effect of the shape of the holes on the peak shift is investigated with a view to improve further the main transmittance peak shift. Rectangular holes are studied by simulation and experiments with a palladium metallic matrix. Palladium permittivity is varied by exposure to hydrogen and generates main transmittance peak shifts toward longer wavelengths. The simulation results of the propagation constant and electric field distribution revealed that the peak shift is controlled by the short side length of the rectangular holes. The short side of the rectangular holes normalized by the peak wavelength should be below 1/10 for the rectangular holes to achieve their maximum effect.

On-Chip Sorting of Long Semiconducting Carbon Nanotubes for Multiple Transistors along an Identical Array

Ballistic transport and sub-10 nm channel lengths have been achieved in transistors containing one single-walled carbon nanotube (SWNT). To fill the gap between single-tube transistors and high-performance logic circuits for the replacement of silicon, large-area, high-density, and purely semiconducting (s-) SWNT arrays are highly desired. Here we demonstrate the fabrication of multiple transistors along a purely semiconducting SWNT array via an on-chip purification method. Water- and polymer-assisted burning from site-controlled nanogaps is developed for the reliable full-length removal of metallic SWNTs with the damage to s-SWNTs minimized even in high-density arrays. All the transistors with various channel lengths show large on-state current and excellent switching behavior in the off-state. Since our method potentially provides pure s-SWNT arrays over a large area with negligible damage, numerous transistors with arbitrary dimensions could be fabricated using a conventional semiconductor process, leading to SWNT-based logic, high-speed communication, and other next-generation electronic devices.

A 3D metallic structure array for refractive index sensing with optical vortex

A simple and large-scale fabrication technique for three dimensional structure arrays using a photolithography process was applied to realize an array of high-aspect-ratio metallic fins. The fin array enables light confinement between the high-aspect-ratio fins, thus generating optical vortices. The light confinement between the fins produces sharp dips in the reflection spectrum of the array. We show that the position of the dip wavelength is sensitive to change in the refractive index of the surrounding medium. Sensitivity to change in the refractive index was quantified by optical simulation and experimental measurements.