Effect of Ag–AgBr Nuclei on Photoinduced Crystallization Mechanism in Photo-Thermo-Refractive Glass

Photo-thermo-refractive (PTR) glass used for hologram writing is a photosensitive sodium–zinc–aluminum–silicate glass doped with cerium, silver, fluorine, and bromine. Photoinduced crystallization of NaF results in refractive index variations in UV exposed areas. It is considered that bromine, which probably forms AgBr, plays a key role during the precipitation of nuclei and the growth of subsequent NaF crystals. However, previous reports merely indicated the presence of AgBr through a red-shift in the surface plasmon resonance (SPR) absorption band associated with Ag nanoparticles (NPs). They overlooked the fact that the size, shape, and distribution of Ag NPs can also tune the position of the SPR band. In this study, the optical, thermal, and microscopic properties of PTR glass were investigated by varying nucleation temperature/duration. Transmission electron microscopy and selected area electron diffraction patterns clearly identified AgBr nuclei in a well-controlled nucleated PTR glass sample. The results demonstrate that the formation of AgBr quenches the luminescence of Ag NPs or molecular clusters via nonradiative recombination, which introduced nuclei centers during photoinduced crystallization. This study elucidates the chemical composition and growth dynamics governing the nucleation and crystallization processes in PTR glass.