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Notes: A new and effective method for developing ultralow-loss glasses for long-distance optical fiber systems is presented. The idea is based on the observed strong effect of a very small amount of alkaline or alkaline-earth impurities on structural relaxation of "frozen-in" density fluctuations. Efficiency of this method is demonstrated in the case of silica glass with Na2O of less than 50 wt. ppm, where the light scattering loss was reduced by 13% by the addition of only 10 wt. ppm ((approximate)1×10−3 mol %) Na2O. Various problems in developing ultralow-loss fiber materials in multicomponent glasses are avoidable in this method, since concentrations of dopants are very small. © 1997 American Institute of Physics.

Notes: Degradation of top electrodes is one of the most important factors to determine the lifetimes of organic electroluminescence (EL) devices. An organic EL device [indium thin oxide (ITO)/N,N'-diphenyl-N,N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4, 4'-diamine (TPD)/tris(8-hydroxy- quinoline)aluminum (Alq3)/Al] was prepared and a morphological change of the Al top electrode was observed during and/or after applying voltage by atomic force microscopy and scanning electron microscopy (SEM). The change in the electrode surface, i.e., the increase in surface roughness was observed during the current flow. The degradation process started from faint dark core parts and propagated into disks with different rates depending on the magnitude of applied voltage. Degraded sites of the Al electrode, which were analyzed as aluminum oxide by Auger electron spectroscopy, protruded into the air on the organic layers. In SEM images of a life-end electrode, discontinuities due to crevasse formation in the organic layers sandwiched by the ITO base and the metal top electrodes were observed in many places. These results confirm that one of the most crucial factors of the degradation process was deformation of metal and organic layers due to heat, gas evolution, and oxidation caused by applied voltage.

Notes: © 2000 American Institute of Physics.

Notes: 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. © 1999 American Institute of Physics.

Notes: 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. © 1998 American Institute of Physics.

Notes: 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 gives rise to structural subrelaxations, which are effective in reducing the Rayleigh scattering. The scattering was reduced by 13% by addition of only 10 wt ppm Na2O, for example. In part B of this review the second-order optical nonlinearity induced in Ge-doped silica glass will be described based on recent experiments carried out by the group of present authors. A large second-order optical nonlinearity has been successfully induced in the glass by simultaneous applications of a high dc electric field and ultraviolet (UV) irradiation, so-called UV poling. The nonlinearity induced by UV poling in bulk and film samples has achieved a magnitude of χ(2), comparable to or even larger than those of LiNbO3 and other crystals. Surprisingly enough, the nonlinearity induced by this method then decays after the UV poling as an exact single-exponential function of time, very much unlike the usual decay processes observed in glasses. Evidence is presented associating the nonlinearity with GeE′ defect centers created from oxygen deficient vacancies through photochemical reactions. The decay or degradation can be made much slower with the addition of proper impurities which work as electron scavengers. In addition, we have found that crystallites are generated in the glass by the UV poling, which leads to an increase in the third-order nonlinearity, χ(3), approximately 15 times larger than before the treatment. As a whole, the evidence strongly suggests that a major origin of the second-order nonlinearity induced in the glass is a combined effect of a large third-order nonlinearity associated with the crystallites and an internal space-charge field, where the charges to build up the field are produced during the formation of GeE′ centers. © 2000 American Institute of Physics.

Notes: 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. © 2000 American Institute of Physics.

Notes: Effects of preirradiation by ArF-excimer laser and thermal annealing on photoinduced defects creation in synthetic silica glass were investigated by optical absorption measurements. It has been confirmed that the durability for ArF-excimer laser irradiation can be improved by the preirradiation followed by annealing. The absorption spectra strongly suggest that a dominant role of the preirradiation should be to assist relaxations of weak parts in the glass such as strained Si–O–Si bonds. Furthermore, the photoinduced defects creation can be divided into two processes, and the preirradiation only suppresses the photoinduced defects creation in the first process. © 1999 American Institute of Physics.

Notes: 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 samples with lower Tf, resulting in reducing structural disorder. Furthermore, a way to determine Tf of F-doped silica glass is offered. © 2002 American Institute of Physics.

Notes: The effect of Cl ions on the structural relaxation in silica glass was investigated by observing the fictive temperature, which was determined from the position of infrared absorption peak around 2260 cm−1 related to the average Si–O–Si bond angle. It has been found that Cl ions reduce the relaxation time of the long-range cooperative relaxation due to viscous flow, the so-called α relaxation. Moreover, Cl ions reduce the relaxation time of a subrelaxational process, caused by local structural relaxation. Although Cl has been used in industry only for dehydrating silica glass for optical fibers, these structural relaxation enhanced by Cl may be used advantageously to develop ultralow-loss fiber glass. © 1998 American Institute of Physics.