Tsunenobu Onodera

Light propagation within colloidal crystal wire fabricated by a dewetting process.

We present a colloidal crystal wire composed of thousands of connected microspheres that is fabricated by a simple dewetting process utilizing a drain phenomenon, and we directly observe the light propagation within the wire by near-field scanning optical microscopy. The optical properties of propagation light suggest that the propagation mechanism was attributed mainly to nanojet-induced mode coupling for the straight propagation component and partly to whispering-gallery mode coupling within the colloidal crystal wire.

Crystal Size Dependence of Fluorescence Spectra from Perylene Nanocrystals Evaluated by Scanning Near-Field Optical Microspectroscopy

The fluorescence spectra of perylene nanocrystals, the crystal size of which is in the range of about 300 nm to 1 µm, were measured with the scanning near-field optical microspectroscopy. It has become apparent for the first time that the emission peaks from the self-trapped exciton state were evidently blue-shifted with decreasing crystal size. On the other hand, the emission spectra from the free exciton state were scarcely dependent on crystal size. These spectral changes were discussed semi-quantitatively on the basis of the strong coupling model between excitons and lattice for a molecular crystal. The influence of the substrate was also suggested.

Observation of light propagation across a 90° corner in chains of microspheres on a patterned substrate

To demonstrate light-path manipulation in arbitrary shapes we fabricated coupled-resonator optical waveguides (CROWs) having a 90°-corner structure on a lithographically patterned substrate. The spectra of propagation light within the CROWs were directly measured by guide-collection-mode near-field scanning optical microscopy. The spectra revealed that the propagation light through the CROWs has a larger transverse-magnetic polarization mode than a transverse-electric (TE) one. The most plausible cause of the lower intensity in the TE mode is that light leaks out to the Si substrate.

Micro‐demultiplexer of Coupled Resonator Optical Waveguide Fabricated by Microspheres

How does light propagate within a chain of transparent spheres? In the case of microspheres with diameters of a few micrometers, the feature of an optical resonator should be included in the discussion because the diameters are nearly the same order of magnitude as the wavelength of light. In microspheres, light goes around the circumference of those with whispering gallery modes (WGMs), and indicates a spectrum that has sharp and discrete peaks. [ 1–4 ] Moreover, waveguides that utilize a weak coupling between resonators, such as microrings, microdisks, or microspheres, are called coupled-resonator optical waveguides (CROWs). [ 3 , 5 , 6 ] By controlling the coupling effi cient, this concept can be applied to on-chip optical buffer memory. [ 7–9 ] Another particularly appealing feature of CROWs is the possibility of making loss-less and refl ection-less bends with a wavelength-scale curvature because the microresonators can couple at an arbitrary point on the circumference of light going around. When we use microspheres as the microresonators, CROWs allow us to utilize a self-assembly phenomenon in colloidal suspension to align and connect microspheres with non-elaborate processing. [ 10–15 ] Moreover, if we use a lithographically patterned substrate which is formed lines of dimples in order to trap the microspheres, we can fabricate arbitraryshape waveguides for on-chip optical circuits. Since the prevalent waveguide concepts [ 16–19 ] require very elaborate technology to fabricate such sharp bends, or require a delicate assembling technique, the CROW concept might partly replace the prevalent concepts. In this article, we utilize a self-assembly phenomenon

Temperature- and size-effects on optical properties of perylene microcrystals

Abstract Perylene microcrystals with different sizes were prepared by the reprecipitation and subsequent microwave irradiation method. Their excitonic absorption peaks were blue-shifted with decreasing crystal size in the size range of sub-micrometer. Perylene microcrystal films were also fabricated by means of the electrostatic adsorption method to investigate the dependence of the excitonic absorption spectra on temperature. As a result, the temperature-dependence of half-width of the excitonic absorption spectra was well explained by Toyozawa’s theoretical prediction, which was based upon an interaction between exciton and lattice vibration. In addition, it was revealed that the peak position of emission spectra from the self-trapped exciton (STE) state was relatively blue-shifted in comparison with the bulk crystal, and that the lifetime of STE emission became shorter in perylene microcrystal in any temperature.

Influence of micro-joints formed between spheres in coupled-resonator optical waveguide.

Light propagation is simulated through coupled-resonator optical waveguides (CROWs) composed of seven transparent polystyrene microspheres, including micro-joints formed between the spheres. In nanojet-induced mode (NIM) light propagation, the micro-joints increased the optical coupling between microspheres drastically, and the light confinement by individual microspheres weakened as the micro-joint diameter increases. These results suggest that we can control NIM light propagation by changing the micro-joint diameter; this amounts to a nanojet throttle valve.

Positioning dependent anion recognition by thiourea-based chromoionophores via hydrogen bonding in aqueous vesicle solutions

A cationic vesicle interface exhibited a filter function for less hydrophobic anions, and highly selective anion recognition via hydrogen bonding was achieved by thiourea-based chromoionophores (Cn-TU) located deep inside the vesicle.

Reversible and efficient anisotropic orientation of dispersed aromatic hydrocarbon nanocrystals in a magnetic field

We have prepared an aqueous dispersion of perylene nanocrystals by use of the reprecipitation method and have measured their orientation in a magnetic field spectroscopically. We observed reversible and efficient contrast changes upon applying the magnetic field.

Ordered Array of Polymer Microspheres on Patterned Silicon Substrate Fabricated Using Step-by-Step Deposition Method

Two-dimensional ordered arrays of polystyrene microspheres (PSMSs) with a large-scale area were successfully fabricated on a lithographically patterned substrate using a so-called tapered cell. The repetition of the deposition procedure provided the subsequent second array on the first array as a template. Consequently, we succeeded in the fabrication of a Kagome structure on the hexagonally patterned substrate, and evidently observed through the corresponding fluorescence microscopy images that two types of fluorescent PSMSs were, respectively, separately arranged in the dimple and on the terrace. Namely, a potential application of the present method using a patterned substrate would be the step-by-step deposition of microspheres with different refractive indexes, and optoelectronic and magnetic properties.

Silver-Deposited Polydiacetylene Nanocrystals Produced by Visible-Light-Driven Photocatalytic Reduction

We have successfully demonstrated for the first time the fabrication of Ag-nanoparticle-deposited polydiacetylene (PDA) nanocrystals by a visible-light-driven photocatalytic reduction. Namely, a large number of Ag nanoparticles of 5–15 nm in size could specifically be deposited only on the surface of the PDA nanocrystal core, prepared by the conventional reprecipitation method, through the photocatalytic action of PDA itself. The present fabrication technique is expected to be applied to various combinations of π-conjugated organic materials and novel metals for novel optoelectronic device applications.