Properties and generation by irradiation of germanium point defects in ge-doped silica

Ge doped amorphous silicon dioxide (Ge doped silica) has attracted the attention of researchers for more than 50 years. This material is used in many different technological fields from electronics, to telecommunication, to optics. In particular, it is widely used for the production of optical fibers and linear and nonlinear optical devices. The optical fibers, which allow to transmit optical signals with high speed avoiding interferences, are constituted by two regions with different refractive index values: core (inner part) and cladding (external part). To increase the refractive index of the core with respect to that of cladding, Ge doping of silica is commonly used. Moreover, in the Ge doped fiber two main radiation effects are observed: the photosensitivity and the second harmonic generation (SHG). The photosensitivity permits the induction of a spatial modulation of the core refractive index (Fiber Bragg Grating (FBG)) and together with the SHG, is useful to produce a great number of devices. The Ge related point defects are considered relevant for the photosensitivity and the SHG, but at the same time they are also causes of the degradation of the fiber transmission properties. For these reasons and for new application fields as the silicabased systems for nuclear environments, the defect structures, their properties, their associated optical activities, their generation and conversion processes have been widely studied and represent crucial arguments for ongoing research. These arguments have double valence, as they can be considered from a physical point of view and from the technological one. In this chapter, we will consider many aspects of the generation processes of the defects by irradiation, with particular attention to paramagnetic defects Ge(1), Ge(2) and E'Ge, and to oxygen deficient optically active defects as the Germanium Lone Pair Center (GLPC). Some connections between material properties and defects generation will be investigated too. We will start providing a background on the literature dealing with the Ge doped glasses, their applications, the effects of the irradiation and the principal types of defects. After that, the principal properties of the investigated materials will be briefly described. The main part of the chapter will be dedicated to the presentation and the discussion of experimental data. These data concern the γ and the β radiation effects on various samples and the studies on some properties of the induced defects. Finally, we will present the main conclusions that can be derived by the data.