Morphology and microstructure in fused silica induced by high fluence ultraviolet 3ω (355 nm) laser pulses

The morphology and microstructure induced in high quality fused silica by UV (355 nm) laser pulses at high fluence (10–45 J/cm2) have been investigated using a suite of microscopic and spectroscopic tools. The laser beam has a near-Gaussian profile with a 1/e2 diameter of ∼0.98 mm at the sample plane and a pulse length FWHM (full width at half maximum) of 7.5 ns. The damage craters consist 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, which also compact the crater wall, ∼10 μm thick and ∼20% higher in density. The size of the damage crater varies with laser fluence, number of pulses, and laser irradiation history. In the compaction layer, there is no detectable change in the Si/O stoichiometry to within ±1.6% and no crystalline nano-particles of Si were observed. Micro- (1–10 μm) and nano- (20–200 nm) cracks are found, however. A lower valence Si3+ species on the top 2–3 nm of the compaction layer is evident from the Si 2p XPS. The results are used to construct a physical model of the damage crater and to gain critical insight into laser damage process.