Yoichi Kurokawa

Controlled plasmon resonance in closed metal/insulator/metal nanocavities

The controlled plasmon resonance in nanometer-sized optical cavities with a closed end has been demonstrated. A nanosheet plasmon cavity is a metal/insulator/metal waveguide with a finite length. Its lowest-order transverse-magnetic guided mode is reflected at the ends and exhibits the Fabry-Perot resonance. In this study, one of the ends was closed by an obliquely evaporated Au film, and the so-called organ pipe resonances were observed as reflection dips. Since such closed configurations offer a higher field enhancement and higher detection efficiency of the scattered light, they are promising as fundamental structures for Raman enhancement.

Photonic material for designing arbitrarily shaped waveguides in two dimensions

We investigate numerically optical properties of novel two-dimensional photonic materials where parallel dielectric rods are randomly placed with the restriction that the distance between rods is larger than a certain value. A large complete photonic gap (PG) is found when rods have sufficient density and dielectric contrast. Our result shows that neither long-range nor short-range order is an essential prerequisite to the formation of PGs in the novel photonic material. A universal principle is proposed for designing arbitrarily shaped waveguides, where waveguides are fenced with side walls of periodic rods and surrounded by the novel photonic materials. We observe highly efficient transmission of light for various waveguides. Due to structural uniformity, the novel photonic materials are well suited for filling up the outer region of waveguides of arbitrary shape and dimension comparable with the wavelength.

Light diffraction from a bilayer lattice of microspheres enhanced by specular resonance

Anomalously strong diffraction from a bilayer lattice of dielectric microspheres, previously reported by the authors, has been attributed to the enhancement of diffraction by specular resonance in constituent bispheres. On the basis of rigorous calculations and experiments, specular resonance from bispheres is found to be dominant even in the scattering from a cluster of several microspheres. As a consequence, a diffraction model, in which a bilayer lattice of microspheres is viewed as a two-dimensional array of bispheres, is constructed. By incorporating the specular resonance into a diffraction theory of two-dimensional lattices as a structure factor, observed diffraction behavior is explained. In the diffraction from a bilayer lattice of microspheres, the specular resonance functions as a blazing mechanism. Possible configurations and suitable parameters for efficient gratings are discussed, and a self-assembled bilayer lattice of dielectric microspheres with a diameter 1.6–3.2 times as large as the wa...

How Can a Resonant Nanogap Enhance Optical Fields by Many Orders of Magnitude

In a resonant cavity or a gap, with a nanometer-sized width and depth on an Au surface, filled up with SiO2, the electric field intensity is enhanced by many orders of magnitude by the illumination of a plane wave. Graphical representations of power flow are employed to elucidate how the cavity harvests energy from the incident wave inside to give rise to the enormous field enhancement. The power income from the incident fields into the cavity and the power expense as scattered fields and absorption are discussed on the basis of three Poynting vectors, which correspond to the extinction, scattering, and absorption cross sections. The streamlines of the Poynting vectors distinctly visualize that the cavity collects light from an area much wider than its own geometrical width. In addition, the presence of an alternating power flow accumulated in the cavity for a certain time duration is unveiled on the basis of the real and the imaginary parts of the complex Poynting vector. Both the spatial squeezing and the temporal accumulation contribute to the high power density in the cavity. A clear insight into the underlying physics acquired on the basis of a pictorial understanding is expected to play a critical role in designing a plasmon nanocavity with a higher field enhancement in a rational way.

Observation of SERS of picolinic acid and nicotinic acid using cellulose acetate films doped with Ag fine particles

Abstract Surface enhanced Raman (SER) spectra of picolinic acid and nicotinic acid were observed using cellulose acetate (CA) films doped with Ag fine particles. The spectra obtained match those reported for silver colloids though some differences in SER band intensity were observed. The ease of preparation and handling of the CA film method renders it more useful than the colloid method for the observation of SER spectra.

Surface-enhanced Raman scattering (SERS) using polymer (cellulose acetate and Nafion) membranes impregnated with fine silver particles

Abstract Cellulose acetate and Nafion membranes impregnated with fine silver particles were prepared. The mean size of the fine silver particles in the cellulose membrane was in the range of 10 nm. Surface enhanced Raman scattering (SERS) spectra of pyridine, Cl − and SCN − ions were observed using the impregnated cellulose acetate membrane. A SERS spectrum of pyridine was also obtained using the impregnated Nafion membrane but due to amorphous carbon broad bands were observed in the range of 1200-1600 cm −1 . The SERS method using the impregnated members was found to be applicable to the detection of the adsorbates.

Optical nonlinearities of Al2O3 glasses with CdS microcrystallines

Fabrication of Al2O3 glasses with CdS microcrystallites by means of the sol-gel method, as well as their linear and nonlinear optical characteristics, is reported for the first time. The shift in the wavelength of the absorption edge through a quantum size effect was clearly observed for the CdS-doped Al2O3 prepared by changing the annealing temperature. A large nonlinear absorption and refractive index were obtained because a high CdS microcrystallite concentration was achieved through the sol-gel method.

Nanoscopic particle-doped polymer membrane prepared by the counter diffusion method and its properties

Abstract A nanometer-size particle-doped membrane has been prepared by a counter diffusion method. It was obtained by inserting a finely porous gel membrane between solutions of different reactants. The resultant membrane looks like a solid solution. The particle size is primarily determined by the extent of the gel network of the supporting gel membrane. The particle content in the membrane is in the range 3–20 wt% and the particle size is in the range 3–20 nm. Some membrane properties have been examined.

New preparation technique for a highly dispersed nickel-alumina catalyst and its application to liquid phase hydrogenation

Abstract A highly dispersed nickel-alumina catalyst has been prepared by the sol-gel process using hydrous alumina sol derived from an aluminum salt solution. The size of the nickel particles is controlled by the dimensions of the network of the homogeneous precursor alumina gel. This new catalyst shows higher selectivity in the liquid phase hydrogenation of 1,3 and 1,5-cyclooctadiene and methyl linolenate than either Raney nickel or impregnated nickel-alumina catalysts. However, it shows lower activity than Raney nickel. This may be due to the tendency of the nickel particles to be encapsulated within the alumina particles.

Preparation of a transparent alumina film doped with CdS and its nonlinear optical properties

We have examined the preparation of transparent alumina film and CdS-doped alumina film prepared by the sol-gel process using aqueous sol starting from AlCl3. The resulting alumina films obtained by sol-gel transformation are transparent in the visible and near infrared up to 1000 °C. Homogeneous solution of alumina sol and Cd(CH3COO)2 was converted into gel film by dehydration. Then, on exposure to H2S stream at 250–600 °C, a CdS-doped film was obtained. It shows relatively sharp blue-shifted visible absorption that is attributed to size quantization effect. Large nonlinear absorption and refractive index have been obtained, because high concentration of CdS particles can be achieved by using this method.