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

We present an experimental study regarding the effects of the gamma 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 (Delta n) 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. Delta n at 1500 nm have been estimated using optical-absorption spectra in the range 1.5-6 eV. We have found that Delta n 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 Delta n 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 2x10(17)-5x10(17) defects/cm(3). 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 Delta n and the absence of a correlation between the other paramagnetic defects and Delta n. These findings suggest that the Delta n phenomenology is closely related to the Ge(1) generation mechanisms and this latter is affected by the oxygen deficiency.