Ryoichi Tohmon

Various types of nonbridging oxygen hole center in high‐purity silica glass

Optical absorption measurements of the 2.0‐eV band and photoluminescence measurements of the 1.9‐eV emission, excited by various excitation bands, were carried out on high‐purity silica glasses subjected to γ‐ray irradiation. Two, and possibly three, different forms of nonbridging oxygen hole centers were deconvoluted from the results of the isochronal annealing experiments. The difference in the peak wavelength of the 2.0‐eV absorption and 1.9‐eV luminescence bands among various forms of nonbridging oxygen hole centers is reported.

Defects and optical absorption bands induced by surplus oxygen in high-purity synthetic silica

The nature of excess oxygen in as‐manufactured and γ‐irradiated high‐purity synthetic silicas is investigated. Electron‐spin‐resonance measurements suggest that peroxy radicals ( 3/4 SiOO⋅) could be produced either by the cleavage of peroxy linkages ( 3/4 SiOOSi 3/4 ) or by the reaction of E’centers ( 3/4 Si⋅) with oxygen molecules. The excess oxygen is found to exist in the glass in two forms: as peroxy linkages and as interstitial molecular oxygen. The peroxy linkage is shown to be the cause of optical absorption at 3.8 eV. Heat treatment at 900–1000 °C results in the growth of the 3.8‐eV band, that is, the peroxy linkages, through the reaction of oxygen vacancies and interstitial dioxygen molecules. These results indicate that the 5.0‐ and 3.8‐eV bands (which are characteristic of ‘‘oxygen‐deficient’’ and ‘‘oxygen‐surplus’’ silica, respectively) can coexist in a glass, depending on the synthesis conditions.

Relation between the 1.9 eV luminescence and 4.8 eV absorption bands in high‐purity silica glass

Photoluminescence measurements of the 1.9 eV emission were carried out on high‐purity silica glasses subjected to γ‐ray irradiation. The time decay of the luminescence, when excited by the 4.8 eV band, indicates that the 4.8 eV absorption and the 1.9 eV luminescence are caused by two different defects, and that an energy transfer occurs between the two defects. Comparison with electron spin resonance observations shows that both the nonbridging oxygen hole center (responsible for the 1.9 eV luminescence) and another undetermined defect (responsible for the 4.8 eV absorption) must be present in the glass before the 1.9 eV luminescence band can be excited by 4.8 eV photons.

Effect of cladding material on 2-eV optical absorption in pure-silica core fibers and method to suppress the absorption

The growth of 2-eV optical absorption induced by γ-ray irradiation is larger in pure-silica core fibers with a dopedglass cladding than in the fibers with a silicone cladding, regardless of the manufacturing process. Precursors of nonbridging oxygen hole centers which abundantly exist at the core/cladding interface of the glass-cladding fibers are responsible for the difference in the absorption. A combined-treatment of hydrogen and γ-ray irradiation can remove these precursors and suppress the absorption.

Cause of the 5.0 ev absorption band in pure silica glass

The optical absorption band at 5.0 eV (the ‘B2 band’) is classified into two types (B2α: and B2β), each having different photoluminescence peak, half width, and decreasing characteristic by heat treatment. The 7.6 eV absorption band is found in samples with the B2α band. The 7.6 eV and the B2α bands are caused by the same oxygen vacancy in the form of Si-Si . The former band is due to a singlet-to-singlet transition, while the latter to a singlet-totriplet transition.

Hydrogen bond of OH-groups in silica glass and its relation to the 1.39 μm absorption

Temperature-induced change in optical absorption spectra around 1.39 μm, the first OH-stretching overtone, was examined in order to study how OH groups exist in pure silica glass. The spectra are found to consist of four components. A model for the bond state corresponding to each component is presented.

Spatial distribution of defects in high‐purity silica glasses

The spatial distribution of defects and impurities in a variety of high‐purity silica glass manufactured by different methods are studied. The defects investigated include those found in the as‐manufactured glass (oxygen vacancy and peroxy linkage), as well as those induced by ionizing radiation or ultraviolet light (E’ center and oxygen hole centers). A significant difference is observed in the distribution between silica manufactured by different methods. Furthermore, the defects induced by ionizing radiation or ultraviolet light have a spatial distribution relative to the geometry of the as‐manufactured boule, suggesting that these defects arise primarily from the activation of preexisting precursors.

New Insight Into the Structure of SiO2 Glass from a Point Defect Study

Formation of defect centers in pure silica glass (a-SiO2) depends greatly on the manufacturing process of the glass. Non-bridging oxygen hole centers (NBOHC: ≡Si-0•) are dominantly created by γ-ray irradiation in high OH-group content (≥ 700 ppm) silica made by the direct glass deposition process1. In low OH-group content silica made by the plasma or soot method, oxygen content in the glass strongly affects the defect formation2,3; drawing induced peroxy radicals, drawing induced NBOHC, ≥-ray induced peroxy radicals and the 1.52 μm band induced by hydrogen treatment are created in oxygen-rich silica, but the 5 eV (245 nm) band and drawing induced E′ center (≡Si•) are created in oxygen-deficient silica. Based on these experimental results, the authors proposed2 the defect in the form of ≡Si-Si≡ as a model of the oxygen deficiency. In the present paper, further experimental evidence which supports the authors’ model and its proof by numerical calculation are presented. The structure of glass is also discussed.

Red Luminescence In Pure Silica Glass

Photoluminescence measurements of the 1.9-eV (red) emission were carried out on high-purity silica glasses subjected to y-ray irradiation. The time decay of the 4.8-eV-excited-luminescence indicates that the 4.8-eV absorption and the 1.9-eV luminescence arise at two different defect sites, and that an energy transfer occurs between the two defects. Comparison with electron spin resonance observations suggests that the defect responsible for the 1.9-eV luminescence is the non-bridging oxygen hole center (NBOHC: ≡Si-O). The 4.8-eV absorption band increases when the sample is heated in an oxygen atmosphere prior to y-irradiation, suggesting that the defect responsible is related to some form of excess oxygen. The defect is tentatively identified as a negatively charged non-bridging oxygen (≡Si-0 ) which is formed when a peroxy linkage traps a i-induced electron, (≡Si-0-0-Si≡ + e → ≡Si-0- + •O-Si≡ ). Both the NBOHC and the defect responsible for the 4.8-eV absorption must be present in the glass for the 4.8-eV band excited 1.9-eV luminescence to occur.

Triplet State In High-Purity Silica Glasses

Electron spin resonance in the spectral region about g=4, believed to be due to a triplet state, is investigated in y-irradiated high-purity silica glasses. The signal in this spectral region was first reported by Griscom, who attributed it to a cavity-like triplet state formed by chlorines. The signal near g=4 in the present study is found only in oxygen deficient samples (samples with optical absorption at 5.0-eV and 7.6-eV prior to y-irradiation). One of these oxygen deficient samples contains no chlorine, but instead contains florine. The signal intensities of the triplet are in good agreement with the chlorine or florine contents. These results suggest that the signal is due to a biradical at an oxygen deficient site (≡Si• •Si≡). When the samples are heat-treated in hydrogen atmosphere prior to y-irradiation, the triplet signal disappears, but not when treated with hydrogen at room temperature. This is consistent with the behavior of the oxygen vacancy (judging from the 5.0-eV absorption intensity), which is terminated by hydrogen-treatment at elevated temperature but remains after hydrogen-treatment at room temperature.