Akira J. Ikushima

Second-harmonic generation in germanosilicate glass poled with ArF laser irradiation

We report on a remarkably large second-harmonic generation (SHG) in a germanosilicate glass (15.7 GeO2–84.3 SiO2 in mol %), which was induced by poling while the sample was irradiated with the ArF laser at the wavelength of 193 nm. The SHG coefficient, d, thus induced was measured at various poling electric-field strengths using the Maker-fringe technique. The value of d increases with increasing poling field and then saturates around 1.5×105 V/cm. The largest d coefficient value obtained in this work, which is 3.4±0.3 pm/V, seems to exceed d22 of LiNbO3. Furthermore, polarization dependence of the induced SHG shows an anisotropy of the refractive index.

Silica glass: A material for photonics

Recent studies on two aspects of silica glass as a photonic material will be described. Part A of this review will be focused on structural disorder and structural relaxations in silica glass. With regard to the structural disorder, investigations have been made to improve transparency and to shift the optical absorption edge in the ultraviolet towards shorter wavelengths. Remarkable advances have been achieved in the understanding of both light scattering, which is a dominant factor in the optical losses in silica fibers, and the absorption edge. Freezing of the structural disorder was observed, and structural relaxations are found to be important for improving the transparency, whereas for the absorption edge thermal vibration effects seem to be more predominant than the structural disorder. From the results, the present authors have tried to control the structural relaxation for developing silica glass with an ultimate optical transparency, finding that a very tiny amount of the proper impurity species...

Effects of fluorine on structure, structural relaxation, and absorption edge in silica glass

The effects of fluorine (F) doping on silica glass structure, structural relaxations, and vacuum ultraviolet (VUV) absorption edge were investigated by the infrared and vacuum ultraviolet absorption measurements. Linewidth of 2260 cm−1 absorption band, which is one of the good indicators of structural disorder, decreases proportionally to the fictive temperature (Tf). On the other hand, it is irrespective of F concentration. Structural relaxations are strongly stimulated by F doping. The transparency near the absorption edge depends on both F concentration and Tf. When Tf is constant, absorption edge shifts to a shorter wavelength proportionally to the F concentration. These results strongly suggest that there are two main factors that control the VUV absorption edge of F-doped silica glass; band gap widening by increasing Si–F bond and reduction of structural disorder. The latter factor is not directly caused by structural change by F doping. F enhances the structural relaxation, and it is easy to obtain...

Valence and conduction band edges of metal-phthalocyanines and carrier behavior

Abstract The energy levels of the valence and conduction band edges of various metal-phthalocyanines (MPcs) have been determined by using photoelectron emission and optical absorption measurements. Carrier behaviors at the interface of MPcs and a metallic material such as Au or Al have been investigated by measuring current-voltage characteristics of a metal/MPc/metal system with changing MPc. The current-voltage characteristics do not seem to be explained simply by the usual Schottky barrier considerations.

DEFECT FORMATION IN GEO2-SIO2 GLASS BY POLING WITH ARF LASER EXCITATION

We have investigated formation of Ge-related defects in a GeO2–SiO2 glass fiber preformed by poling with ArF laser excitation. Electric field dependence of the induced-defect concentrations was measured by means of the optical absorption. Several color centers such as the germanium electron-trapped centers and the Ge E′ center were induced. Concentrations of the induced germanium electron-trapped centers and Ge E′ center increase with increasing the electric field. Conversion efficiency from the germanium electron-trapped centers to the Ge E′ center was found to be independent of the electric field. The present result strongly suggests that the poling with ArF laser excitation is effective in the present GeO2–SiO2 glass for the formation of germanium electron-trapped centers.

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.

Raman spectra of fluorine-doped silica glasses with various fictive temperatures

We have demonstrated the change of the Raman spectra in F-doped silica glasses with various fictive temperatures, and have also presented a useful peak-deconvolution technique for evaluating the relative concentration of D1 and D2 bands that could be attributed to four- and three-membered ring structures. The intensities of D1 and D2 bands are mainly dominated by the fictive temperature, and they are not almost affected by the F content at the same fictive temperature. It has been made clear that the intensity of D1 band as well as D2 band has a linear relationship with the IR peak position around 2260 cm−1 that is correlated with the average Si–O–Si bond angle, irrespective of the F content. The “activation energies” of D1 and D2, estimated from the Arrhenius plot, are 0.26 and 0.41 eV, respectively.

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.

Generation of second harmonics in Ge-doped SiO2 thin films by ultraviolet irradiation under poling electric field

Thin films of Ge-doped silica were prepared by radio-frequency sputtering in a controlled atmosphere of Ar and O2 and then subjected to ultraviolet irradiation under poling electric field (UV poling). Effects of oxygen mass flow content during the sputtering on the second-order nonlinearity of the films were investigated. Second harmonic generation (SHG) was observed and its intensity was found to depend greatly on the oxygen mass flow content with an optimum at 1 cm3/min. The annealing in a vacuum enhanced SHG intensity and values as high as 12.5±0.6 pm/V have been achieved with films containing 50 mole % of GeO2. This is on one hand, and we also found that loading thin films with hydrogen prior to UV poling largely improved the decay of SHG. A decay time as long as 7 years has been obtained. The change in second-order optical nonlinear properties of the films is discussed based on structural defects.