Takao Koda

Morphology and photonic band structure modification of polystyrene particle layers by reactive ion etching

Morphology of self-assembled polystyrene particle layers has been modified by reactive ion etching. The etched layers have two-dimensional periodic structures in submicron scale, the period of which is determined by the initial size of the particles, and the shape of the etched particles has been gradually changed to a thinner ellipsoid depending on the etching time. Resonant phenomenon between incident light and electromagnetic eigenmodes of the photonic band of the etched layers has been observed in transmission measurement. The resonant frequencies have gradually shifted according to the etching time, i.e., the photonic band structure of the layers has been successfully modified. Various kinds of applications can be expected due to the electromagnetic resonant phenomenon and the characteristic surface structure of the layers.

Strong localization of Bloch photons in dual-periodic dielectric multilayer structures

We predict that singular photonic Bloch modes with extremely narrow bandwidths occur in dual-periodic dielectric multilayer structures. The electromagnetic field of these modes is shown to be strongly localized around the troughs of undulated periodic dielectric structures, which leads to an extraordinary high-Q value effect in the dual-periodic one-dimensional photonic crystals. We also propose several real systems suitable for experimental confirmation of the features predicted.

Scanning near-field optical images of ordered polystyrene particle layers in transmission and luminescence excitation modes

Scanning near-field optical images of hexagonally close-packed layers of polystyrene spherical particles with a diameter of 1.0 microm have been investigated. The layers were composed of particles that were doped either totally or partially with an organic fluorescent dye. Observations were made in the transmission and luminescence excitation modes with a scanning near-field optical microscope (SNOM) with a spatial resolution shorter than the wavelength of light. The patterns observed in the SNOM images are significantly dependent on the microstructures of layers, that is, the layers are either single or double layered, and the particles are either totally or partially doped. These results are discussed in terms of specific modes of electromagnetic waves transmitting across and along the layers after the local excitation at the tip end of the scanning microprobe.

Near-field optical images of ordered polystyrene particle layers and their photonic band effect

To study the origin of the SNOM image contrast patterns observed for samples composed of transparent materials only, we have investigated the transmission SNOM images of an ordered polystyrene particles layer for various wavelengths of incident light. We have found a drastic change in the SNOM image patterns as the incident wavelength is scanned across the resonant wavelength of the photonic bands of the layer. Observed wavelength dependence of the near-field intensity of the layer was found to be in a good qualitative agreement with the theoretical calculation in terms of the photonic band effect.

Observation of local light propagation in ordered latex layers by scanning near-field optical microscope

Abstract Under the local excitation by a scanning near-field optical microscope, we have observed the transmission and luminescence excitation images of the hexagonally close-packed single layers of ordered, dye-doped polystyrene particles (LATEX) of 1.0 and 0.37 μm in diameter. The transmission images with the spatial resolution shorter than the optical wavelength dramatically changes, depending not only on the diameter of the particles but also on the aperture size of the probe tip end of the microscope. The luminescence excitation images for the single layers composed of particles, all or a part of which is dye-doped, give the information how the excitation light propagates through the layers. From these results we propose that the localized and extended modes of electromagnetic waves with large wave vectors propagating across and along the layers, respectively, are generated under the local excitation by the near-field effect

Optical Modes in Two-dimensionally Ordered Dielectric Spheres

We have experimentally observed the optical modes of periodic two-dimensionally ordered polystyrene spheres with a diameter of 5.15 µm using the attenuated total reflection method. The dispersion relation of such optical modes clearly shows that there are two types of propagating modes in the ordered spheres. We point out that one of the two optical modes is related to the guided modes in optical fibers and the periodicity of the spheres plays an important role in its excitation mechanism. The other mode propagates along spherical surfaces.

Self-Assembled Polystyrene Microparticle Layers as Two-Dimensional Photonic Crystals

Abstract Optical properties of ordered polystyrene microsphere layers have been investigated and the results are discussed in terms of the two-dimensinal photonic band effect.

Nanosecond and picosecond dynamics of photo-induced phase-transition in low dimensional organic crystals

Abstract We report the dynamics of a unique photo-induced phenomenon in quasi-one-dimensional organic crystals; i.e. phase transitions triggered by photo-excitation in charge-transfer crystal tetrathiafulvalene-p-chloranil (TTF-CA). Domain wall movement in the single crystal has been investigated by means of nanosecond and picosecond time-resolved reflection spectroscopy.

Photonic band effect in ordered polystyrene particle layers

Abstract Quasi two-dimensional (2D) photonic band structures have been investigated on ordered polystyrene particle layers by means of polarized transmission spectra for oblique incidence of light. The results are discussed in terms of quasi-2D photonic band effect.

Real-time observation of the dielectric constant variation of evaporated polydiacetylene films during photopolymerization and photochromic transitions

Abstract The change of the dielectric constants and thicknesses has been measured in-situ on alkyl-urethane-substituted polydiacetylene (PDA) films grown on a silver (Ag) film during a photopolymerization and photochromic change by using the attenuated total reflection geometry. Photopolymerization and photochromic change in the vacuum-deposited PDA films from the monomer to the blue, the red, and the bleached forms, have been readily achieved by ultraviolet light irradiation with a D 2 lamp. Surprisingly, the values of the dielectric constant of the thin PDA films deposited on the Ag film were found to be comparable with those of oriented films. The dielectric constants of PDA films grown on a BK7 glass have been measured by using a normal-reflection method in the wavelength region from 400 to 1400 nm at room temperature.