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 chan ge in silica glass which results from densification of the SiO2 matrix arou nd photo-excited neutral oxygen vacancies (oxygen-deficient centers ODC). S uch densification is supposed to be due to a 'local phase transition' induc ed by a transformation of one of the valence sp-electrons of the ODC from a localized to a partially delocalized state. Theoretically, the latter stat e arises in constructing the energy spectrum of a singly excited, relaxed O DC and may be interpreted as an intermediate-radius exciton-like state in t he framework of the Falicov model for mixed-valence systems. It is shown th at the partially delocalized state can become energetically favorable as co mpared to the localized one. This mechanism of the photorefractive effect e liminates the contradiction between a large refractive index change and a r elatively small ODC concentration in silicas, because it does not invoke th e assumption of a photo-induced change in polarizability of point defects. (C) 1998 Elsevier Science B.V. All rights reserved.