GE AND SN DOPING IN SILICA: STRUCTURAL CHANGES, OPTICALLY ACTIVE DEFECTS, PARAMAGNETIC SITES

Abstract

Doped silica is a key material in several technological fields. Its physical properties can be suitably tuned for obtaining specific features by introducing dopants atoms substituting for silicon in the network. In fiberoptics, doping with germanium atoms permits the creation of the radial profile of refractive index which is essential for an optical guide. Ge and Sn substitutional doping can also induce interesting photochromic features which may be used to write longitudinally modulated refraction index patterns for obtaining filters or for compensating dispersion effects in silica-based optical fibers (Hill gratings [1,2]). It is not yet clear the main role of dopants in these processes. In fact, beyond the introduction of additional electronic levels peculiar of the dopant species, dopants can influence the medium range order of the SiO2 network, also favouring the formation of intrinsic defects (as oxygen vacancies) by changing the thermochemical conditions during the material synthesis [2]. Indeed, Ge and Sn doping cause perturbations in the structural and defect-related properties, and these are important for the occurrence of photoconversion of optically active defects and photoinduced structural densification [2-5]. Nevertheless, the effect of dopant atoms on native structural stresses and coordination defects may also depends on the preparation method. In fact the amorphous silica network cannot be related to a well defined structure, and a continuum of possible structures may be compatible with the same stoichiometry but differentiated as medium range coordination features. Defect occurrence and defect properties may in turn be affected by structural modifications. As a result, different situations may be experimentally found in nominally identical materials owing to variations in the topological properties of the matrix. For this reason, in the physics of SiO2 defects careful attention is to be paid in the synthesis parameters. Doping itself, no matter which atomic species are involved, should be sought as a perturbation of the structure, able to induce a change of average structural features in the silica network. For these reason, before