Nature of damage in fused silica induced by high-fluence 3-omega 355-nm laser pulses, a multiscale morphology microstructure, and defect chemistry study

The morphology and microstructure of damage sites in high quality fused silica induced by high power UV (355nm) laser light have been investigated using a suite of microscopic and spectroscopic tools. These include SEM, TEM, microprobe analysis, XPS, SIMS and x-ray micro-tomography utilizing intense synchrotron radiation. Systematic SEM examinations show that the damage sites consist primarily of a molten core region (thermal explosion), surrounded by a near concentric region of fractured material. The latter arises from propagation of lateral cracks induced by the laser-generated shock waves. The size of the overall crater is dependent of the laser fluence, number of pulses, damage history and environment. In particular, differences in morphology of the damage sites are identified: air vs. vacuum; exit (more severe) surface vs. entrance surface; and regular polish (more severe) vs. super polish surfaces. A compaction layer, ∼10 microns thick and ∼20% higher in density has been identified with x- ray tomography. This layer has further been substantiated by micro-Raman spectroscopy. High resolution microprobe analysis shows that there is no variation in the Si/O stoichiometry of silica in the compaction layer to within ±1.6%. High resolution TEM indicates the absence of crystalline nano-particles of Si in the compaction layer. Macro- (10-0.1μm) and micro-cracks (200-20nm) are found, however, in the bright field images. The Si 2p XPS spectra indicates that there is a lower Si ³⁺ species on at least the top 2-3 nm of the compaction layer. These findings are critical to the design of a knowledge-based mitigation process for laser damage growth.