Phosphorus-doped silica is a material of paramount importance in microelectronics, opto-electronics and in fiber optics technology. P-codoping is employed in the production of fiber amplifiers for it prevents the clustering of typical rare-earth dopants such as erbium, ytterbium and cerium. Moreover P-doped optical fibers are among the best candidate for the development of radiation detectors and dosimeters . Such a development will benefit from an improved understanding of the radiation induced defects as well as of their generation mechanisms.
In this talk we present an investigation of the optical properties of several diamagnetic P centers in P-doped silica  by means of first-principles calculations, including many-body perturbation theory (GW and Bethe-Salpeter Equation) techniques . The calculated absorption spectra indicate that the 6.9 eV band is originated from the large number of [(O-)3P(=O)]0 tetrahedra which constitute the usual form of P inclusion in silica. As far as concerns the [(O-)2P(=O)2]− tetrahedral unit structures the present calculations suggest that
they could contribute to the broadening of the 6.9 eV band. Moreover we provide evidence in favor of another precursor of the so-called r-POHC: a [(O-)3P=O]0 unit sharing a bridging oxygen with a penta-coordinated silicon atom. The calculated absorption spectra also indicate that substitutional P atoms as well as three-fold P defects do not substantially contribute to the optical absorption band at 6.9 eV. Finally we will also discuss of some issues  related to oxygen vacancies in ytterbium-doped silica and present a few preliminary results (atomic structures and formation energies) in the frame of the Talents3 project.
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