Phase transformations initiated by optical breakdown in bulk of fused silica

Abstract

Optical breakdown inside a transparent dielectric triggered by a tightly focused laser beam has a character of microexplosion. As a result, extreme thermodynamic conditions are generated in the material. Both high pressure and high temperature combine in causing major structural transformations (phase transitions). This work presents mapping of such structural changes initiated by microexplosion in bulk of fused silica. The high-resolution cross-sectional imaging with high-resolution electron microscopy (HRTEM) revealed transition area between the amorphous and crystalline phases of the material affected by high pressure and temperature. It was found that the crystalline phase is created in the form of nano-crystals (crystallites) embedded in an amorphous matrix and is located mainly in the vicinity of the geometrical focus. The range of this finding was extended by the spectral analysis indicating redistribution, spatial segregation, and glass network reorganization paths leading to material densification. Closer inspection of the densified area with help of micro-photoluminescence (PL) confirmed appearance of the crystalline structure. Physical mechanisms included in such a transformation process and the main features of the created glass ceramic are also discussed.