Effect of cooling on the optical properties and crystallization of UV-exposed photo-thermo-refractive glass

Photo-thermo-refractive (PTR) glass is a multi-component silicate glass that undergoes a refractive index change after UV-exposure and thermal treatment. This photo-thermo-refractivity is due to the precipitation of sodium fluoride nano-crystals; thus the glass remains highly transparent in the visible and near-IR regions. Up to now, most studies focused on the influence of temperature and duration of thermal treatment on the PTR glass properties, but no attention was given to the cooling step after thermal treatment. In this paper, the influence of cooling on crystallization and resulting optical properties of UV-exposed PTR glass is studied. We show that cooling between the nucleation and growth treatments is a mandatory step to achieve the full benefits of the first heat-treatment, i.e., a large number of small crystals. We also show that the main part of the refractive index change occurs on the cooling path after pre-nucleation. Non-isothermal DSC study associated with in situ pre-nucleation treatment shows that pre-nucleation enhances crystallization only if the temperature is decreased below T_g before the second (development) treatment. Using high temperature photometric measurements of the absorption spectra of UV-exposed PTR glasses, we tentatively associate that effect with the presence of liquid drops of a silver containing phase during regular pre-nucleation treatment. This fact explains the necessity to cool such drops below their melting point to obtain nucleation centers for efficient precipitation of NaF nano-crystals.