Refractive index change dependence on Ge(1) defects in γ-irradiated Ge-doped silica

We present an experimental study regarding the effects of the γ radiation on silica glass doped with Ge up to 10 000 ppm molar produced by the sol-gel technique. We have determined the irradiation-induced changes in the refractive index (Δn) as a function of the oxygen deficiency of the samples, evaluated from the ratio between the germanium lone pair centers (GLPC) and the Ge content. Δn at 1500 nm have been estimated using optical-absorption spectra in the range 1.5-6 eV. We have found that Δn is independent of Ge differences for GLPC/Ge values <10-4, while it depends on Ge for larger oxygen deficiencies. In details, the oxygen deficiency can reduce the induced Δn of the investigated materials and our studies evidence that the photosensitivity of the GeO2-SiO2 glass is reduced until the GLPC concentration reaches values of 2x1017-5x1017 defects/cm3. We have also investigated the induced concentration of paramagnetic point defects [Ge(1), Ge(2), and E'Ge] using the electron-paramagnetic-resonance (EPR) technique. From the comparison of the optical and EPR data we have further found a relation between the induced optical-absorption coefficient at 5.8 eV and Ge(1) defects, a linear correlation between Ge(1) and Δn and the absence of a correlation between the other paramagnetic defects and Δn. These findings suggest that the Δn phenomenology is closely related to the Ge(1) generation mechanisms and this latter is affected by the oxygen deficiency.