Fundamental defect centers in glass: Electron spin resonance and optical absorption studies of irradiated phosphorus-doped silica glass and optical fibers

Defect centers induced by ionizing radiation (50–100-keV x rays, 60Co γ rays) in high purity P-doped silica glass have been observed and elucidated by ESR spectroscopy. Four generic species are well characterized on the basis of the observed 31P hyperfine splittings and g values as defects analogous to PO2−3 (phosphoryl), PO4−4 (phosphoranyl), PO2−2 (phosphinyl), and PO2−4 radicals. The latter species, also termed the phosphorus-oxygen-hole center (POHC), is shown to occur in two variants comprising holes trapped on one or two nonbridging oxygens. Radiation-induced Si E′ centers with and without P next-nearest-neighbors were also identified, and a singlet resonance S due to E′ type defects such as (OSi2)Si⋅ and/or (O2Si)Si⋅ was observed to grow in with annealing above ∼800 K, regardless of whether or not the sample was irradiated. The structures, formation mechanisms, and precursors of these defects have been determined or inferred for all centers. Radiation-induced optical absorption spectra over the range 0.5–6.2 eV have been obtained for bulk glass and fiber samples after irradiation and following anneals to various temperatures up to 1250 K. Gaussian resolutions of the spectra into component bands have been performed and isochronal anneal data have been used to identify optical absorptions of the PO2−3, PO4−4, PO2−2, POHC, and S centers. The PO2−3 defect was found to absorb at ∼0.8 eV in the region of interest for fiber optic communication. Oscillator strengths are calculated for all bands.