The unique role of bromine in NaF crystallization of photo-thermo-refractive glass

Photo-thermo-refractive (PTR) glass generates a refractive-index modulation through photothermally induced NaF nanocrystallization, making it essential for volume Bragg gratings. The halide co-dopant determines how silver nanoparticle templates (Ag NPs@AgX, X = Cl, Br, I) steer NaF nucleation, however, a systematic comparison of Cl - , Br - , and I - under identical photo-thermal protocols has been missing. Here, we compared KCl-, KBr-, and KI-doped PTR glasses using DSC, XRD, UV–Vis spectroscopy, SEM/EDS, and ellipsometry under one- and two-step heat treatments. Iodine’s low solubility and high volatility accelerate NaF coarsening and cause opacity. Chlorine, despite favorable lattice matching with NaF, suffers volatilization during prolonged heat treatment. Bromine balances these extremes: its moderate polarizability and superior melt retention sustain a steady supply of Ag NPs@AgBr templates, yielding controlled NaF precipitation without transparency loss. Bromine-based PTR glass thus offers a balanced crystallization pathway and transparency, providing a mechanistic basis for fabricating high-efficiency volume Bragg gratings.