Generation and bleaching of intrinsic color centers in photo-thermo-refractive glass matrix

Photo-thermo-refractive (PTR) glass is a photosensitive multicomponent silicate glass that imparts refractive index change after exposure to ionizing radiation and thermal treatment. The origin of this photosensitivity is explained by thermal precipitation of sodium fluoride crystals controlled by atomic silver nucleation centers. These centers are produced by photo-reduction of silver ions resulted from photoionization (photo-oxidizing) of cerium ions. This feature of PTR glass is successfully used for high efficiency phase hologram recording. However, the mechanism of electric charge exchange (transfer) between these ions is not known and the role of intrinsic electron and hole centers was not studied. To elucidate this problem, pure PTR glass matrix with no dopants was prepared and the subsequent absorption spectra of color centers were studied after excitation of the intrinsic absorption edge of glass matrix. Color centers were bleached by thermal treatment and by optical excitation of induced absorption bands. The analysis of the structure of absorption spectra at different stages of coloration and bleaching led to the deconvolution of complex spectra to Gaussian components. Based on this analysis and comparison with simple silicate glasses, induced absorption bands were assigned to different intrinsic electron and hole centers. Interconversion of different centers is also studied.