Effect of ultraviolet exposure and thermal treatment on NaF micro-crystallization mechanism in photo-thermo-refractive glasses

Photo-thermo-refractive (PTR) glass is a promising recording medium for volume-holographic devices. The recording processes involve a series of steps including ultraviolet (UV) exposure, nucleation, and crystallization thermal treatment. Sodium fluoride (NaF) microcrystals precipitate after crystallization, and modulate the refractive index locally of the glass matrix. In this study, to simulate the volume Bragg grating recording processes and study the effect of photothermal-induced processes on the crystallization kinetics in PTR glass, Si-AlNa-Zn-K PTR glasses with and without Br ? ions were prepared using the melt-quenching method. The UV exposure duration, thermal treatment temperature and crystallization time were regulated during subsequent treatment. During these processes, Br-free samples were used for comparison to demonstrate the potential roles of Br-ion in the nucleation and crystallization. The Ag nanoparticle (NP) nuclei surrounded by AgBr (AgF) microcrystals were characterized using UV-visible absorption spectroscopy and transmission electron microscopy, and the changes in the precipitated NaF were characterized using X-ray diffraction, Raman spectroscopy as well as XPS. The mechanism of NaF micro-crystallization is disclosed based on above experimental results. It is found the formation of Ag nanoparticle surrounded by AgBr (AgF) microcrystals inhibits the crystallization kinetics of NaF microcrystals in PTR glass.