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RESEARCH PRODUCT

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

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We present an experimental study regarding the effects of the $\ensuremath{\gamma}$ radiation on silica glass doped with Ge up to $10\text{ }000$ ppm molar produced by the sol-gel technique. We have determined the irradiation-induced changes in the refractive index $(\ensuremath{\Delta}n)$ 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. $\ensuremath{\Delta}n$ at 1500 nm have been estimated using optical-absorption spectra in the range 1.5--6 eV. We have found that $\ensuremath{\Delta}n$ is independent of Ge differences for GLPC/Ge values $l{10}^{\ensuremath{-}4}$, while it depends on Ge for larger oxygen deficiencies. In details, the oxygen deficiency can reduce the induced $\ensuremath{\Delta}n$ of the investigated materials and our studies evidence that the photosensitivity of the ${\text{GeO}}_{2}{\text{-SiO}}_{2}$ glass is reduced until the GLPC concentration reaches values of $2\ifmmode\times\else\texttimes\fi{}{10}^{17}--5\ifmmode\times\else\texttimes\fi{}{10}^{17}\text{ }\text{defects}/{\text{cm}}^{3}$. 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 $\ensuremath{\Delta}n$ and the absence of a correlation between the other paramagnetic defects and $\ensuremath{\Delta}n$. These findings suggest that the $\ensuremath{\Delta}n$ phenomenology is closely related to the Ge(1) generation mechanisms and this latter is affected by the oxygen deficiency.