Optical defects produced in fused silica during laser-induced breakdown

Abstract

Fused silica irradiated with $\text{∼3-}\mathrm{ns}$ 1064-, 355-, and 266-nm laser pulses as well as with $\text{∼120-}\mathrm{fs}$ 825-nm pulses is studied by a combination of photoluminescence (PL) and Raman scattering spectroscopies. Results show that, for laser fluences above the laser-induced breakdown threshold, in all the cases studied, irradiation results in the formation of four defect-related PL bands centered on $\text{∼1.9}$ (655), 2.2 (565), 2.7 (460), and 4.3 eV (290 nm). Bands centered on 1.9, 2.7, and 4.3 eV are attributed to nonbridging oxygen hole centers (1.9 eV) and oxygen-deficiency defects (2.7 and 4.3 eV). However, defects giving rise to a broad band at $\text{∼2.2\hspace{0.17em}}\mathrm{eV}$ are unknown. For all the laser-modified samples studied, Raman spectroscopy reveals a dramatic increase in the intensity of $D_1$ and $D_2$ lines, associated with in-phase breathing motions of oxygen atoms in puckered four- and planar three-membered ring structures, respectively. This indicates laser-induced material densification. Based on these results, we discuss physical processes occurring during the catastrophic laser-induced material breakdown, leading to material densification and the formation of point defects.