Ichiro Tanahashi

Optical nonlinearities of Au/SiO2 composite thin films prepared by a sputtering method

Third‐order nonlinear optical properties of Au/SiO2 composite thin films have been investigated by means of a degenerate four‐wave mixing at room temperature. In the optical‐absorption spectra the absorption peak due to the surface plasmon resonance of Au particles is observed at the wavelength around 530 nm. With increasing the mean diameter of Au particles, the absorption at the peak is increased and the full width at half‐maximum of the absorption band is decreased from 130 to 80 nm. The third‐order nonlinear susceptibility χ(3) exhibits a peak at the wavelength of the absorption peak and the maximum value of χ(3) is obtained to be 2.0×10−7 esu. The size‐dependent enhancement of the χ(3) in Au particles with the mean diameter of 3.0–33.7 nm has been also investigated. The value of χ(3)/α (α: absorption coefficient) for the films is increased upon an increase of mean diameter of Au particles. This is explained by the size dependences of the local‐field factor and the imaginary part of dielectric constan...

Effect of concentration of surface acidic functional groups on electric double-layer properties of activated carbon fibers

Abstract The relationship between electric double-layer properties in an organic electrolytic solution and a concentration of surface acidic functional groups (surface acidity) of a phenolic resin-based activated carbon fiber (ACF) was characterized in a system of ACF/electrolytic solution/ACF The lower surface acidity of ACF resulted in the lower apparent leakage current of the electric double layer. The capacitance and the dc resistance of the layer were almost independent of the surface acidity. Further heat treatment of ACF in nitrogen atmosphere decreased the surface acidity of ACF. Types of C-O bonds in surface acidic functional groups were identified as carboxy and ester groups for ACF as-prepared and as phenolic hydroxy and quinone groups for ACF heat treated. The ACF obtained from the phenolic resin-based fibers showed an extremely low surface acidity in comparison with other kinds of ACF. The polarizable electrodes of the phenolic resin-based ACF resulted in electric double-layer capacitors with high capacitance and low leakage current.

Ultrafast Optical Switching in a Silver Nanoparticle System

An ultrafast response of a silver nanoparticle system, as fast as 360 fs, was observed in a femtosecond optical-Kerr-shutter experiment. The ultrafast response is attributed to a self-diffraction of a pump pulse due to transient grating. We estimated χ(3) of the silver nanoparticle system to be 1.5 × 10-7 esu in the femtosecond region.

Effects of heat treatment on Ag particle growth and optical properties in Ag/SiO 2 glass composite thin films

Small Ag particles were embedded in SiO 2 glass thin films by a multi-target sputtering method. The mean diameter of Ag particles in the as-deposited film with 28.0 at. % of Ag was estimated to be 4.4 nm and it was increased to 24.0 nm when the film was heat-treated at 700 °C for 3 h. The diameter was proportional to the cube root of the heat-treatment time, suggesting that the Ag particles grew in the supersaturated solid solution. In the optical absorption spectra of the heat-treated films, the absorption peak due to the surface plasmon resonance of Ag particles was observed about 410 nm. The peak intensity became large and the full width at half maximum of the absorption band was decreased with increasing the diameter of Ag particles.

Activated carbon fiber sheets as polarizable electrodes of electric double layer capacitors

Abstract Activated carbon fiber sheets (ACF sheets) have been developed as polarizable electrodes of electric double layer capacitors. The ACF sheets were prepared by a paper-making process from activated carbon fibers (ACF), carbon fibers (CF), and pulps. The specific surface area of ACF sheets depended proportionally on the content of ACF and their electrical resistance decreased drastically with addition of the pitch-based CF. Natural pulps were suitable material as a binder of ACF sheets. The electric double layer capacitance of the capacitors with ACF sheets was proportional to the content of ACF and the impedance was mainly influenced by the content and the type of CF. The capacitors showed good performance over a wide temperature range (−25 to 80°C) and good charge-discharge cycle characteristics (10,000 cycles).

Preparation and optical properties of silica gels in which small gold particles were grown by photoreduction

Abstract Small Au particles were grown in silica gels by a photoreduction technique. Photoreduction of AuCl − 4 ions in the silica gel was carried out by ultraviolet light irradiation at room temperature. X-ray photoelectron spectroscopy and X-ray diffraction measurement indicated the formation of Au particles in the silica gel. From transmission electron microscopy observation of the Au/silica gel with 1 wt% of Au, the diameter of Au particles was estimated to be ∼ 30 nm. Subsequent to ultraviolet light irradiation for 10 min, a stable absorption band at 550 nm gradually developed at room temperature, indicating that the number of Au particles were increasing. The peak is due to the surface plasmon resonance absorption of Au particles.

Optical Properties of CdS Microcrystallite-Doped SiO2 Glass Thin Films

The optical properties of SiO2 glass thin films containing CdS microcrystallites have been investigated. The thin films were deposited onto SiO2 and GaAs substrates using an rf-sputtering technique. X-ray photoelectron spectroscopy and X-ray diffraction measurement indicated that the Cd and S atoms were doped as CdS with a hexagonal system in SiO2 glass matrixes. In the optical absorption spectra, the absorption edge of the films with 18.5 at% of CdS clearly exhibited a blue shift by about 0.13 eV in comparison with that of the bulk CdS. In accordance with this result, the photoluminescence spectra of the films exhibited a blue band-edge emission at a higher energy than that of the bulk CdS. The shift to higher energy seems to be a low-dimensional quantum size effect due to confinement of electrons and holes in CdS microcrystallites.

Characterization of activated carbon fiber cloths for electric double-layer capacitors by adsorption method

Abstract Two types of phenolic-resin based activated carbon fiber cloths, ACFC (a) and (b), have been characterized for polarizable electrodes of electric double-layer capacitors by the gas adsorption method. Results have been analyzed by means of the Dubinin-Astakhov equation, the t-method, and the micropore analysis method. The BET surface areas (SBET) of ACFC (a) and (b) were 1798 and 1061 m2g−1, respectively. In the pore size distribution curve of ACFC, the most popular pore diameter was 0.84 and 0.7 nm for ACFC (a) and (b). Both samples of ACFC had micropores only. With increasing SBET, the pore diameter became large and the C-constant in the BET equation became small. The electric double-layer formed on the surface of ACFC (a) in an organic electrolyte was stable and that of ACFC (b) was unstable. In accordance with the results, the temperature dependence of capacitance in the capacitors with ACFC (a) was small, while that with ACFC (b) was large. As polarizable electrodes of the capacitors, the ACFC (a) was a more suitable material than the ACFC (b).

Mechanism of a Terahertz Optical Kerr Shutter with a Gold Nanoparticle System

We observed an ultrafast response as fast as 240 fs in a gold nanoparticle system, in a femtosecond optical Kerr shutter experiment. We successfully demonstrate 2.5 THz optical switching. The ultrafast response of the Kerr signal is explained by a simple model taking into account the self-diffraction of the pump pulse by the generated transient grating.

Preparation and Nonlinear Optical Properties of Ag/SiO2 Glass Composite Thin Films

A multitarget sputtering method was applied to prepare Ag particles embedded in SiO2 glass thin films. In the optical absorption spectra of the films, the absorption peak due to the surface plasmon resonance of Ag particles was clearly observed at the wavelength of 390–406 nm. The full width at half-maximum of the absorption band is decreased with increasing diameter of Ag particles. The third-order nonlinear susceptibility at 400 nm was estimated to be 1.6×10-8 esu for the film with 9.0 at% of Ag by means of a degenerated four-wave mixing.