Kaya Nagasawa

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.

Visible photoluminescence from Si clusters in γ-irradiated amorphous SiO2

Visible photoluminescence (PL) bands around 2 eV were studied in 60Co γ‐irradiated (dose<1 MGy) oxygen‐deficient‐type amorphous SiO2 (a‐SiO2) excited by 2–4 eV photons. In addition to the well‐known 1.9 eV PL band due to nonbridging oxygen hole centers, another PL band was observed at 2.2 eV when excited by 3.8 eV photons. The intensity of the 2.2 eV band increases with decreasing oxygen partial pressure during the sample preparation. Electron‐spin‐resonance measurements show that the intensity of the 2.2 eV band is correlated with the concentration of the Eδ′ center, a paramagnetic state of a cluster of silicons. After much higher γ irradiation with a dose up to 10 MGy, a new PL band was induced at 1.75 eV under excitation by 2.5 eV photons, as well as the 1.9 and 2.2 eV PL bands. By comparing its spectral shape and excitation energy with known PL band in Si‐implanted a‐SiO2, it is suggested that the 1.75 eV band is associated with Si nanocrystals formed from Si clusters in a‐SiO2 by the high‐dose γ irra...

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.

Radiation effects on pure silica core optical fibers by γ-rays: relation between 2 eV band and non-bridging oxygen hole centers

The defect center causing the 2 eV absorption band induced in pure silica core fibers by 60Co γ-rays is discussed. The 2 eV band and non-bridging oxygen hole center (NBOHC, SI–O) show almost the same thermal annealing characteristics. The peak wavelength of the 2 eV band in low-OH (6 ppm) fibers is 630 nm, but shifts to shorter wavelengths according to the amount of OH group the sample contains, and appears at 600 nm in high-OH (700 ppm) fibers. The peak shape is broader in high-OH fibers than in low-OH fibers. This shift and broadening is attributable to a hydrogen bond between NBOHC and a hydrogen atom. Based on the above results, the defect center responsible for the 2 eV band is considered to be NBOHC.

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.

Green photoluminescence band in γ-irradiated oxygen-surplus silica glass

Previous studies have reported the existence of a green photoluminescence (PL) band in oxygen-deficient silica glass when exposed to ultraviolet (UV) light. Oxygen deficient defects (Eδ′) were thought to be the origin of this PL band. In this article, we describe the characteristics of a green PL band in oxygen-surplus silica glass (excited by visible and UV light). At room temperature, we observed the full width at half maximum and lifetime of this PL band to be about 0.2 eV and 300 ns, which differed from the previously reported values of 0.4 eV and 20–30 ns, respectively. We propose that the peroxy radical (O3≡Si–O–O↑,↑: unpaired electron) or other oxygen surplus defects are the cause of this green PL band rather than oxygen deficient defects.

Point defects in high purity silica induced by high-dose gamma irradiation

The defects induced by high‐dose (10 MGy) gamma irradiation (60Co) are studied in various types of high‐purity silica glasses [including synthetic crystal (α‐quartz)]. While the defects induced by gamma irradiation of up to 1 MGy have been reported to be generated through the bond breaking of manufacturing‐method‐dependent point defect sites (precursors), such precursor dependency disappears or at least weakens in the defects induced by 10 MGy gamma irradiation. Electron spin resonance, optical absorption, and luminescence investigations suggest that at high‐dose irradiation the defects are created mainly by radiolysis or bond breaking, and associated oxygen diffusion occurred at silicon–oxygen bonds other than at point defect sites. Crystalline α‐quartz shows much higher radiation resistivity than amorphous silica glasses, suggesting that strained silicon–oxygen bonds are the breaking sites.

Gamma-Ray-Induced Absorption Bands in Pure-Silica-Core Fibers

Radiation-induced optical absorption bands in pure synthetic silica-core fibers were studied. The effect of Cl contamination on the absorption change of the irradiated fibers in relation to the radiation resistance was examined, and it was deduced that low-Cl fibers have good radiation resistance even when they have a low OH content. The effect of the OH content on the radiation response in low-Cl fibers was thus investigated. The radiation-induced 630 nm band is enhanced in higher-OH-content silica-core fibers. The 660 nm and 760 nm bands were also observed in certain irradiated fibers. The former increases with dose up to 0.1~2 kGy but decreases at higher doses. The mechanisms of the radiation-induced bands are discussed.

Characteristic red photoluminescence band in oxygen-deficient silica glass

We studied a red photoluminescence (PL) band at about 1.8 eV with full width at half maximum of 0.2–0.4 eV in a series of oxygen deficient-type silicas before and after γ irradiation. The decay lifetime of the PL was estimated to be about 200 ns. The PL excitation peak was found to be located at 2.1 eV. The intensity of the 1.8 eV band was enhanced by about 100 times after γ irradiation up to a dose of 10 MGy. These results suggest that the 1.8 eV PL is associated with oxygen deficient states in silica glass, which were introduced during manufacture and were enhanced further by the γ irradiation. Comparison of the PL properties was made with other luminescent Si-based materials in terms of the peak energy, lifetime, and temperature dependence.

Correlation between the green photoluminescence band and the peroxy radical in γ-irradiated silica glass

We studied the origin of a green photoluminescence (PL) in silica glass at about 2.25 eV using electron spin resonance. The 2.25 eV PL band (lifetime: ∼300 ns, full width at half maximum: ∼0.2 eV) appears strong in γ-irradiated high-oxygen content silica glass suggesting that this PL is associated with a large surplus of oxygen states in silica glass. We investigated the correlation between the 2.25 eV PL band and the peroxy radical by the isochronal annealing method. Peroxy radicals are formed by the O3≡Si–O–O• bond (•: unpaired electron) and the small “peroxy radical” (SPR) bond in silica glass. Based on the annealing characteristics of the 2.25 eV PL band and the peroxy radical, we suggest that the PL band is associated with a surplus of oxygen defects formed by the SPR rather than the O3≡Si–O–O• bond.