Hiroshi Kakiuchida

Optical Properties of Vanadium Dioxide Film during Semiconductive–Metallic Phase Transition

The optical constants of vanadium dioxide (VO2) films were determined at visible and near-infrared wavelengths at various temperatures during a semiconductive–metallic phase transition by ellipsometric analysis with Lorentz-oscillator formulae. The reversible changes in optical constants against temperature due to thermochromism were observed at around 70 °C. The wavelength dispersions of the optical constants were well expressed by the sum of three oscillators and their oscillating energies were attributed to photon-excited transitions. The variation in band structure during the phase transition was monitored, and the relationship between the band structure and optical properties was discussed.

Precise determination of fictive temperature of silica glass by infrared absorption spectrum

In silica glass, the peak position of infrared absorption spectrum at around 2260 cm−1, which is related to the average of distribution of Si–O–Si bonding angle, is useful for determining frozen-in temperature of glass structure, the so-called fictive temperature. We precisely determined relation between the peak position and fictive temperature up to 1550 °C and found that at fictive temperatures higher than 1400 °C the present relation differs from the relation which has been extrapolated from the previous data at fictive temperatures below 1400 °C. The peak position was found to simply shift to lower wave numbers with raising fictive temperature. This suggests that at fictive temperatures up to about 1600 °C there is no maximum in density, which has been reported to exist at around 1500 °C.

Light-scattering study of the glass transition in silica, with practical implications

The glass transition in silica glass was investigated by light-scattering measurements, and relations between the glass transition temperature, fictive temperature, cooling rate, and viscosity in silica glass were clarified. Furthermore, the effect of OH ions on the structural relaxation was found to work in two ways: The OH ions reduce the activation energy of viscoelastic relaxation, and they also generate a new relaxation due to local structure change. Application of such a relaxational process due to impurities should be effective for controlling the performances of silica glass.

Refractive Index, Density and Polarizability of Silica Glass with Various Fictive Temperatures

The refractive index and density of silica glass were measured for various fictive temperatures. They both increase without any local maxima, as the fictive temperature increases up to 1550°C. From the relationship between the refractive index and the density, polarizability was estimated using the Lorentz-Lorenz formula. The polarizability decreases with increasing density, while the refractive index increases. The variation in refractive index with fictive temperature depends on changes in not only the density but also the polarizability. The polarizability was found to contribute to the variation in refractive index to the extent of about one half of that from the density. With increasing density, the refractive index at a longer wavelength increases at a higher rate, which originates from a lower rate of decrease in polarizability at a longer wavelength.

Limit of the Rayleigh scattering loss in silica fiber

The limit of the Rayleigh scattering and total losses in silica fiber was determined from the results of the Rayleigh scattering and structural relaxation measurements. Annealing condition during fiber drawing was optimized, and much potential to reduce the Rayleigh scattering loss has been verified. Total loss can be reduced below 0.15 dB/km at 1.55 μm, for example, even when annealing time is 0.1 s. Total loss below 0.14 dB/km can be realized by increasing the annealing time.

Investigation of the origin of the Rayleigh scattering in SiO2 glass

To investigate the factors determining the Rayleigh scattering in silica glass, the polarized and depolarized light scattering intensities were measured in liquid, supercooled liquid and glassy states of silica. On the heating and cooling processes between glass and liquid, the behavior of the polarized scattering intensity against temperature changed at the temperature corresponding to the melting point of crystobalite. The isothermal compressibility and the depolarization ratio are almost constant in the supercooled liquid state. These results strongly suggest the existence of microcrystalline regions in supercooled liquid and glassy states of silica. The existence of microcrystallites is an important factor in silica glass properties and determines the Rayleigh scattering intensity.

Analysis of Anisotropic Diffraction Gratings Using Holographic Polymer-Dispersed Liquid Crystal

Highly polarized gratings based on a holographic polymer-dispersed liquid crystal (HPDLC) are realized by interferometric exposure. The resulting volume gratings exhibit a diffraction efficiency of 80% and a distinctive ratio of diffraction efficiency of 300. Phase separation by photo polymerization for forming the gratings is studied by evaluating the anisotropic diffraction property and configuration of the separated phase of the gratings. The distinctive ratio of diffraction efficiency in polarization parallel to the grating vector to that in polarization perpendicular to the grating vector increases with grating formation temperature. The microscopic origin of the anisotropic property is investigated by optical polarization microscopy and scanning electron microscopy (SEM). Observations strongly suggest that by increasing the grating formation temperature, the layers of liquid-crystal (LC) and cured-polymer phases in the gratings are well formed and a coalesced LC droplet configuration with small droplets is obtained. LC molecules are considered to be more strongly oriented in the small droplets, and consequently, the LC orientation produces a highly polarized diffraction.

Dielectric relaxation in silica glass

Dielectric dispersion in silica glasses with various OH concentrations were investigated from 20 Hz to 1 MHz in the temperature range from 30 to 1000 °C. Dielectric relaxation, which could be attributed to the elementary process of structural secondary relaxation caused by OH motion, has the activation energy between 2.3 and 2.6 eV. ac and dc electrical conductivities and diffusion coefficient of OH have been deduced from the imaginary part of the dielectric constant. The ac electrical conductivity shows the characteristic feature as is usually observed in amorphous solids. The dc electrical conductivity and diffusion coefficient of OH derived therefrom obey the Arrhenius law with the activation energy of 1.0±0.2 eV in the temperature range from 350 to 1000 °C.

Refractive index and density in F- and Cl-doped silica glasses

The refractive index and density of fluorine- and chlorine-doped silica glasses were measured as functions of fictive temperature. The halogen concentrations were observed to have a refractive index or density that is independent of the fictive temperature were found. This implies that these properties are not affected by any heat-treatment conditions.

Effect of Chlorine on Rayleigh Scattering Reduction in Silica Glass

The Rayleigh-scattered light intensities of silica glasses containing chlorine with concentrations from 0 to 1.8 mol% were measured. The scattered intensity was found to change linearly to the fictive temperature but was independent of chlorine concentration within experimental error. The result indicates that chlorine dopant influences neither density nor concentration fluctuation. From the present result, how much the Rayleigh scattering can be reduced by heat treatment and doping of chlorine was estimated, and chlorine was found to be an effective dopant for achieving the reduction.