1D to 2D Growth of NaF Crystals in Photothermo-Refractive Glasses

The precipitation of NaF crystals with a low refractive index only in the UV exposure region is essential to obtain high refractive index change in photothermo-refractive (PTR) glass for producing high-performance volume Bragg gratings (VBGs). However, the precision control of the growth of NaF crystals in the UV exposure region of PTR glass remains challenging. In this work, the effect of Al₂O₃ on the crystallization behavior of NaF crystals in PTR glass was investigated using the nonisothermal crystallization kinetic method. After photothermal-induced nucleation, the crystallization activation energy (E_a) of PTR glass decreased first and then increased with the rise in the Al₂O₃ content. The appropriate amount of Al₂O₃ (4 mol %) is helpful for reducing the E_a and promoting the formation of NaF crystals. The crystal growth index (n) and crystal growth dimension (m) suggest that the crystallization behavior of NaF crystals was photothermally induced nucleation and crystallization of 1D growth. The increasing trend of the m values indicates that the crystals tend to transit from 1D to 2D growth with the increase in Al₂O₃ content. When Al₂O₃ is 6 mol %, n equals m + 1, which implies the NaF crystals crystallize spontaneously rather than growing on the Ag nuclei. The crystals observed using scanning electron microscopy images showed that irregular precursors appeared first and then grew into needle-like crystals. When the NaF crystals grew sufficiently long, they transformed into lamellar crystals. This work elucidates the crystallization process of NaF crystals in PTR glass and provides guidance for the production of high-performance VBGs.