Masayoshi Ojima

Properties of liquids and liquid crystals in nano-scale space

Three-dimensional periodic structures of synthetic opals and various inverse opals with interconnected regular array of nano-scale voids in which liquids and liquid crystals can be infiltrated have been prepared for the study of liquids and liquid crystals confined in nano-scale spaces. One-dimensional periodic structure containing a defect layer in which liquids and liquid crystals can be introduced have also been prepared by the multi-layered SiO2 and TiO2 stacks. These three-dimensional and one-dimensional periodic structures containing liquids and liquid crystals exhibited novel properties as tunable photonic crystals. Utilizing these phenomena a simple apparatus to evaluate the refractive index of liquids precisely have been proposed, with which sorts of dielectric liquids can be distinguished. Kerr effect and non-linear optical effects of liquids contained in the defect layer of the one-dimensional systems have also been discussed. When the opals and inverse opals contain liquid crystals, the transmission dip and reflection peak wavelengths can be tuned by applied field. The response characteristics are different for opals and inverse opals, which can be interpreted by taking the interaction of liquid crystal molecules with the inner surface of the void into consideration. Utilizing one-dimensional systems containing liquid crystals in the defect layer a ultra-fast switching of a laser light and also in systems containing liquid crystals and fluorescent dyes a voltage tunable lasing in the wide spectral range have been demonstrated

Electric Field Tuning of Plasmonic Absorption of Metallic Grating with Twisted Nematic Liquid Crystal

Tunable surface plasmon absorption was achieved by applying an electric field to a metallic grating with a twisted nematic-liquid crystal (TN-LC) layer. The metallic grating caused the surface plasmon resonance (SPR) and aligned the TN-LC layer. A TN-LC layer was achieved with the metallic grating and rubbed counter substrate. Transverse electric (TE)-mode incident light was irradiated from the counter substrate and converted to transverse magnetic (TM) mode at the metallic grating surface. An electric field was used to control polarization direction rotation. The SPR with the TE-mode incident light was simulated using a finite-difference time-domain method; the experimental and theoretical results agreed well.

Pinning Effect of Mixed Cellulose Ester Membrane on Appearance of Cholesteric Blue Phases

Temperature range of cholesteric blue phase I (BP I) was expanded by infiltrating liquid crystal (LC) that exhibits BP I and BP II into a mixed cellulose ester membrane (MCEM). Cooling rate dependence of the BP I temperature range was investigated, which indicated that the expansion of BP temperature range upon infiltrating LC into MCEM was induced by the pinning effect at network surfaces in the MCEM. In particular, the temperature range of BP I infiltrated in 0.2-µm MCEM is eight times wider than that of pure BP LC compound. Pore size dependence of expansions was also investigated.

Pore size dependence of field emission from nanoscale porous carbon

Carbon inverse opals with three-dimensional nanoporous structures are fabricated by a template method using synthetic opals formed by the sedimentation of SiO2 spheres. The pore size of the carbon inverse opals ranges widely from about 74 nm to 550 nm depending on the diameter of the SiO2 spheres. These nanoporous structures of carbon exhibit excellent characteristics as electron emitters. As the pore size of porous carbon decreases, the effective emission area of field emission increases. The main emitter sites of porous carbon are interpreted to be the edges formed in the boundary of the neighboring pores. The emission characteristics have drastically improved upon heat treatment at high temperatures (about 2760 °C).

Finite-Difference Time-Domain Analysis of Polarization-Dependent Transmission in Cholesteric Blue Phase II

The photonic band structure and circular-polarization dependence of the transmission properties of cholesteric blue phase II were analyzed using a finite-difference time-domain method based on a double-twist cylinder model. The polarization dependence of the calculated band structure was not recognized in the same manner as that in previous studies. However, it can be clearly observed that the calculated transmission spectra depend on the circular polarization; this result agrees well with experimental results. On the basis of the circular-polarization dependence of the transmission spectra in the case of a thick sample, it can be indicated that a total reflection band appears in the selective reflection band.

Fluorescence Enhancement of Conducting Polymer Coated on Biharmonic Metallic Grating

We investigated the fluorescence enhancement of a conducting polymer coated on a biharmonic metallic grating. The biharmonic metallic grating with two harmonic grating components was fabricated using an interference system, and the formation of a one-dimensional plasmonic band gap was confirmed. The directional emission of fluorescence was enhanced by the band edge effect of the conducting polymer coated on the metallic grating surface. The enhancement factor of the fluorescence emitted from the band edges was found to be dependent on the electric field distribution at the metal–dielectric interface.

Effect of Mixed Cellulose Ester Membrane Structure on Appearance of Cholesteric Blue Phases

Temperature range of cholesteric blue phase (BP) was expanded by infiltrating liquid crystal (LC) that exhibits blue phases into mixed cellulose ester membrane (MCEM). The expansion of the BP temperature range was induced by pinning effect on the network surfaces of membrane. In particular, the temperature range of BP I in the LC/MCEM composite is four times wider than that of pure BP LC compound. Solvent treatment of MCEM with aqueous solution of ethanol leads thinner network of MCEM and bigger expansion of BP I temperature range. Polarizing microscopic observations and reflection spectrum measurements of the expanded BP have been carried out. Cooling rate dependence of the BP temperature range was also investigated, which indicated that the expansion of the BP temperature range upon infiltrating LC into MCEM was induced by the pinning effect at network surfaces in the MCEM.

Fabrication and Field Emission Properties of C60 Nanorod Formed by Spin-Cast Treatment

Nanorod-shaped surfaces of C60 films with thicknesses ranging from 100 to 400 nm and lengths ranging from 2 to 3 µm have been successfully obtained by a spin-cast treatment with trans-1,2-dichloroethylene. Results of observations by scanning electron microscopy and field emission (FE) measurements show an obvious correlation between the shape of the C60 nanorods and FE characteristics. By optimizing the conditions of the fabrication processes, the turn-on electric field was lowered from 25–30 to 17–20 V/µm. The nanorod-shaped surfaces of C60 films were still maintained even after a heat treatment under 250°C.

SURFACE PLASMON ENHANCED PHOTOLUMINESCENCE ON BIHARMONIC GRATING STRUCTURE

We compare the fluorescence enhancement from a conducting polymer film placed on top of a sinusoidal and biharmonic metallic grating. On the biharmonic grating, which possesses a surface plasmon polariton band-gap, the enhancement factor was larger than that on a sinusoidal grating, even though the coupling efficiency of the surface plasmon polariton was lower for the biharmonic grating. We investigate the mechanism of fluorescence enhancement numerically by finite difference time domain analyses and attribute the larger fluorescence enhancement to the electric field of the short wavelength band-edge mode extending longer into the dielectric region where the fluorophore is placed.

Fabrication of ZnO Nanopillars and Their Application to Organic Photovoltaic Cells Based on Conducting Polymer and Fullerene

The fabrication of ZnO nanopillars by electrodeposition and their application to an organic photovoltaic cell have been studied. The growth of ZnO nanopillars on indium-tin-oxide coated quartz substrates strongly depends on the conditions of electrodeposition. The use of a ZnO nanopillar in organic photovoltaic cells with a C60/poly(3-hexylthiophene) interpenetrating interface resulted in improved performance.