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M. Fujii,M. Fukui,M. Haraguchi,T. Okamoto 한국물리학회 2005 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.47 No.1
We have made a numerical analysis of whispering gallery modes (WGMs) excited by a gaussian beam by using the finite-difference time-domain method and have obtained the characteristics of the WGMs in spheroids for various incidence directions of the incident light. In the calculations, we have considered a microspheroid located in vacuum surrounded by a perfectly matched layer as absorbing boundaries. It was assumed that the refractive index of the spheroid was 1.590 and the aspect ratio, R, ranged from 1.00 to 1.20. The diameter for the short axis was 1.50 μm. For the polarized light launched along the long axis, the resonance characteristics of excited WGMs were similar to those of a microsphere. On the other hand, for the polarized light launched along the short axis, several weak resonances, which were not observed for a sphere, where found in addition to the main WGM resonances. From a plot of the resonance wavelength as a function of R, it is found that the wavelength for the prominent resonance agrees with that for the weak resonances at R = 1.00 and varies linearly with R. The wavelengths for the weak resonances shift toward the shorter side compared with the prominent one with increasing R. These results can be interpreted in that the degenerated states of WGMs in a sphere split off several states of WGMs in a spheroid because of spatial anisotropy.
T. Ogawa,M. Nakayama,M. Haraguchi,M. Kuwahara,M. Fukui,S. Matsuo,T. Okamoto 한국물리학회 2005 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.47 No.1
We have macroscopically and microscopically investigated the localized surface plasmons (LSPs)on Ag anoparticles embedded in porous TiO2 glass. We have prepared TiO2 glass containing Ag nanoparticles (Ag/TiO2) by the sol-gel process and changed the porosity of the TiO2 glass by drying under a high vacuum condition. Using a spectrometer, we have macroscopically measured the absorption spectra (ABS) on all Ag nanoparticles and using near-field scanning optical microscopy (NSOM), we have microscopically measured the scattering spectra on a single Ag nanoparticle in the Ag/TiO2 after each drying process. Fitting the spectra derived from the numerical calculation based on the Mie theory to that evaluated from the ABS and the NSOM measurements, we have evaluated the deviation of the full width at half maximum evaluated by the experimental measurements from that derived from the theoretical calculation (FWHM) and the bound (Hashin-Shtrikman bounds) of the porosity of the TiO2 glass by the use of the Hashin Shtrikman limit, similar to the literature [1]. It has been found that the FWHM of the scattering spectra on the single Ag nanoparticle can be much smaller than one of the ABS on all Ag nanoparticles in the Ag/TiO2. This result is due to that the ABS is influenced by the aggregation and the size distribution of all Ag nanoparticles. Furthermore, on extrapolating from the plot of FWHM vs porosity, the FWHM is reduced to zero when the porosity is about 15 %.