Photobleaching of sol-gel-derived germanium oxide glass thin films
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Photobleaching of sol-gel-derived germanium oxide glass thin films Jae Hyeok Jang, Junmo Koo, and Byeong-Soo Bae Laboratory of Optical Materials and Coating (LOMC), Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Taejon 305-701, Korea (Received 23 April 1999; accepted 15 November 1999)
Photobleaching of the optical absorption band in the 5-eV region of sol-gel-derived germanium oxide glass thin films was examined with the irradiation of the 5-eV light. The photobleaching represented by the saturated absorption coefficient change (−⌬␣) and the ratio of the neutral oxygen monovacancies to neutral oxygen divacancies concentrations for the germanium oxide were 175 cm−1 and 0.113, respectively. These values are larger than those of the pure germanium oxide bulk glass as well as germanosilicate thin films. The changes in bonding configuration around Ge atom by ultraviolet illumination were analyzed using x-ray photoelectron spectroscopy.
A permanent increase in the refractive index of GeO2 doped silica waveguides by ultraviolet (UV) illumination can be used in the fabrication of Bragg gratings. In 1978, K.O. Hill et al.1 first observed the photosensitivity in germanium doped silica glass. It has been shown that this photosensitivity is closely associated with germaniumrelated oxygen defect centers.2 In 1958, Cohen and Smith3 first showed that the presence of a strong absorption band near 5 eV was attributed to neutral oxygen vacancies caused by the reduction of Ge4+ to Ge2+. Hosono et al.4 proposed that the 5-eV absorption bands were composed of two types of neutral oxygen vacancies. One has a peak at 5.06 eV and is due to neutral oxygen monovacancies (NOMV) coordinated by two Ge ions. Another component of the absorption spectra has a peak at 5.16 eV, which is assigned to neutral oxygen divacancies (NODV) coordinated by two oxygens and having two lone pair electrons. Only the absorption band at 5.06 eV of NOMV absorption band can be bleached and converted into Ge E⬘ centers by UV illumination.5 In the color center model,6 the refractive index change is thought to arise from bleaching of NOMV and the growth of other absorption bands above 5.5 eV. The increase in intensity of the latter is much larger than the decrease in that of the former. As a consequence, the net absorption change enhances positive refractive index changes through the Kramers– Kronig conjugate mechanism.7 The higher photosensitivity is made by the higher concentration of the photobleachable NOMV which is dependent on the composition and the fabrication processing. It has been known that the higher germanium oxide content4 or heat treatment under reduced atmosphere8 enhances the photosensitivity. Thus, it is expected that oxygen deficient germanium 282
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J. Mater. Res., Vol. 15, No. 2, Feb 2000 Downloaded: 03 Apr 2015
oxide glass produces the higher photosensitivity. It was found that melted germanium oxide bulk glas
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