There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Diarylethenes for Memories and Switches

The historical beginnings of photosensitivity and fiber Bragg grating (FBG) technology are recounted. The basic techniques for fiber grating fabrication, their characteristics, and the fundamental properties of fiber gratings are described. The many applications of fiber grating technology are tabulated, and some selected applications are briefly described.

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Fiber Bragg grating technology fundamentals and overview

Lanthanide-based luminescent hybrid materials.

With the goal of being able to create optical devices for the telecommunications industry, we investigated the effects of 810-nm, femtosecond laser radiation on various glasses. By focusing the laser beam through a microscope objective, we successfully wrote transparent, but visible, round-elliptical damage lines inside high-silica, borate, soda lime silicate, and fluorozirconate (ZBLAN) bulk glasses. Microellipsometer measurements of the damaged region in the pure and Ge-doped silica glasses showed a 0.01–0.035 refractive-index increase, depending on the radiation dose. The formation of several defects, including Si E′ or Ge E′ centers, nonbridging oxygen hole centers, and peroxy radicals, was also detected. These results suggest that multiphoton interactions occur in the glasses and that it may be possible to write three-dimensional optical circuits in bulk glasses with such a focused laser beam technique.

Writing waveguides in glass with a femtosecond laser

The sources of an absorption band at \ensuremath{\sim}5 eV observed in ${\mathrm{SiO}}_{2}$:${\mathrm{GeO}}_{2}$ and ${\mathrm{GeO}}_{2}$ glasses have not been unambiguously identified. Results reported here are consistent with the source of two types of neutral oxygen vacancies. Samples of ${95\mathrm{S}\mathrm{i}\mathrm{O}}_{2}$:${5\mathrm{G}\mathrm{e}\mathrm{O}}_{2}$ and ${90\mathrm{S}\mathrm{i}\mathrm{O}}_{2}$:${10\mathrm{G}\mathrm{e}\mathrm{O}}_{2}$ were prepared by a chemical vapor deposition soot-remelting method. Optical-absorption and electron paramagnetic resonance spectra were measured. An absorption band centered at 5 eV in as-prepared ${\mathrm{SiO}}_{2}$:${\mathrm{GeO}}_{2}$ glasses is composed of two components. One has a peak at 5.06 eV and a FWHM (full width at half maximum) of 0.38 eV. Illumination with uv light bleached this band, and generated Ge E' centers. A linear relation was found between the decrement in the intensity of the 5.06-eV component and the concentrations of uv-induced Ge E' centers. This relation is a basis for attributing the defect responsible for this component to the precursors of uv-induced Ge E' centers. We propose that the 5.06-eV band is due to neutral oxygen monovacancies (NOV's) coordinated by two Ge ions. The oscillator strength of this band was evaluated to be approximately 0.4\ifmmode\pm\else\textpm\fi{}0.1 assuming that the NOV's are converted into Ge E' centers by absorption of uv quanta. The activation energy for this conversion process was of the order of ${10}^{\mathrm{\ensuremath{-}}2}$ eV.The second component of the absorption spectra has a peak at 5.16 eV and a FWHM of 0.48 eV. This band is not bleached but emits luminescence bands at 3.2 eV (intense) and 4.3 eV (weak) when irradiated with 5-eV light. Based on other research, we assign this band to ${\mathrm{Ge}}^{2+}$ ions coordinated by two oxygens and having two lone pair electrons (neutral oxygen divacancies). The concentrations of ${\mathrm{Ge}}^{2+}$ ions were much larger than those of the NOV's and the ratio of the NOV's to ${\mathrm{Ge}}^{2+}$ ions increases with increasing ${\mathrm{GeO}}_{2}$ content. A similarity was found in the characteristics of these two types of oxygen-deficient defects to those in ${\mathrm{SiO}}_{2}$ glasses.

Nature and origin of the 5-eV band in SiO2:GeO2 glasses

The effects of ArF excimer laser irradiation on dehydrated high-purity silica glass were investigated on both the optical-absorption bands due to oxygen-deficient centers (ODC) and the formation of ${E}^{\ensuremath{'}}$ centers. With an intense uv flux from an excimer laser, an ${E}^{\ensuremath{'}}$-center density of the order of ${10}^{15}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ was created. The 7.6-eV absorption band remains at the original level, while the 5.0-eV absorption band having the 4.3-eV emission band decreased. Both bands were reduced by heat treatment in an ${\mathrm{O}}_{2}$ atmosphere. These results suggest that there exist two types of ODC: ODC(I), which is responsible for the 7.6-eV band, and ODC(II), for the 5.0-eV band. The concentrations of ODC(I) and ODC(II) were evaluated to be 1 \ifmmode\times\else\texttimes\fi{} ${10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ through an analysis of the gas treatment data and of the order of ${10}^{14}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ through the growth curve of the ${E}^{\ensuremath{'}}$ centers, respectively. The structural origin of ODC(I) has been attributed to the Si-Si bond, judging from the fact that the peak energy and cross section of the absorption were in close agreement with those of the ${\mathrm{Si}}_{2}$${\mathrm{H}}_{6}$ molecule. Theoretical calculations on defect energy levels by O'Relly and Robertson supported the structural model for IDC(I) and suggested an unrelaxed oxygen vacancy for ODC(II). These assignments were also consistent with the results of a quenching experiment in which the fictive temperature of samples was changed; the equilibrium between concentrations of ODC(I) and ODC(II) shifted in a reasonable manner.

Two types of oxygen-deficient centers in synthetic silica glass

Superoxide radical ion O/sub 2//sup -/ is an active species of oxygen and is known to play important roles in many chemical reactions involving O/sub 2/. The formation of O/sub 2//sup -/ on or in solid oxides is usually achieved with the aid of energetic photons such as uv, X-ray, or ..gamma..-ray radiation. It has been found that O/sub 2//sup -/ occurs in the 12CaO 7Al/sub 2/O/sub 3/ crystal prepared by the solid-state reaction between CaCO/sub 3/ and Al/sub 2/O/sub 3/ or Al(OH)/sub 3/ powder without any accompanying irradiation; the concentration of O/sub 2//sup -/ goes up to ca. 4 x 10/sup 18/ units/g.From the change in the line shape with temperature, the O/sub 2//sup -/ is concluded to be included in the structural cavity inherent in the crystal lattice. A drastic reduction in the concentration of O/sub 2//sup -/ on anion substitution and on heating under an oxygen-free atmosphere strongly suggests that the occurrence of O/sub 2//sup -/ is closely related with free oxygen, which is a unique characteristic of the 12CaO 7Al/sub 2/O/sub 3/ crystal. Such characteristics found for 12CaO 7Al/sub 2/O/sub 3/ are analogous to those for ultramarine, Na/sub 8/(Si/sub 6/Al/sub 6/O/sup 24/)S/sub 2/. 25 references, 4more » figures, 2 tables.« less

Occurrence of superoxide radical ion in crystalline calcium aluminate 12CaO.7Al2O3 prepared via solid-state reactions

Eu 3+ ion fluorescence spectra were compared in glasses of the Al 2 O 3 SiO 2 system prepared by a sol—gel process. The gels synthesized by the hydrolysis of Si(OC 2 H 5 ) 4 , Al(OC 4 H 9 sec ) 3 , and EuCl 3 ·6H 2 O were heated in air from 200 to 1000°C to form x Al 2 O 3 ·(100 - x )SiO 2 ( x = 0, 5, 10 mol%) glasses containing 1, 2 and 5 wt% of Eu 2 O 3 . Eu 3+ -doped Al 2 O 3 SiO 2 glasses had an intense fluorescence, two times larger than that of glass without Al 2 O 3 . The phonon sideband (PSB) peaks associated with the 7 F 0 → 5 D 2 transition of Eu 3+ ion were observed at 910 and 600 cm −1 , which were assigned to the SiO − and AlO − bonds, respectively. As the content ratio of Al 3+ to Eu 3+ ion decreases, the PSB intensities decrease and the band at 910 cm −1 disappears in samples of 1 and 2 wt% of Eu 2 O 3 . It is concluded that Eu 3+ ion is preferentially coordinated with AlO − bonds, where Al 3+ ions constitute the AlO 6 octahedra.

Properties of sol—gel-derived Al2O3SiO2 glasses using Eu3+ ion fluorescence spectra☆

Defect formation in dehydrated silica glasses was investigated using various excimer lasers with different energies. The ArF laser (6.4 eV) generates the E’ center more effectively than the KrF laser (5.0 eV), while the XeCl laser (4.0 eV) generated no centers. Defect generation was found to be proportional to the square of the laser photon density, indicating that it occurs dominantly due to a two‐photon process which makes band‐to‐band excitation possible. The E’ center probably originated from oxygen‐deficient centers. Contributions to the E’ center formation from a process involving direct absorption at the sites of intrinsic defects in SiO2 glass were discussed on the basis of the excitation energy dependence and a comparison with the effect of a low‐pressure mercury lamp.

Yoshihiro Abe

Papers

Nature and origin of the 5-eV band in SiO2:GeO2 glasses

Two types of oxygen-deficient centers in synthetic silica glass

Occurrence of superoxide radical ion in crystalline calcium aluminate 12CaO.7Al2O3 prepared via solid-state reactions

Properties of sol—gel-derived Al2O3SiO2 glasses using Eu3+ ion fluorescence spectra☆

Two‐photon processes in defect formation by excimer lasers in synthetic silica glass