Excited oxygen-deficient center in silicon dioxide as a structurally non-rigid, mixed-valence complex

A possible mechanism is proposed for UV light induced refractive index change in silica glass which results from densification of the SiO2 matrix around photo-excited neutral oxygen vacancies (oxygen-deficient centers ODC). Such densification is supposed to be due to a 'local phase transition' induced by a transformation of one of the valence sp-electrons of the ODC from a localized to a partially delocalized state. Theoretically, the latter state arises in constructing the energy spectrum of a singly excited, relaxed ODC and may be interpreted as an intermediate-radius exciton-like state in the framework of the Falicov model for mixed-valence systems. It is shown that the partially delocalized state can become energetically favorable as compared to the localized one. This mechanism of the photorefractive effect eliminates the contradiction between a large refractive index change and a relatively small ODC concentration in silicas, because it does not invoke the assumption of a photo-induced change in polarizability of point defects. (C) 1998 Elsevier Science B.V. All rights reserved.