Ablation-Induced Stresses in Fused Silica by 157-nm F₂-Laser irradiation

Abstract

The F2laser is a promising source for direct etching of microstructures and the precise shaping of optical-grade surfaces on wide bandgap materials such as fused silica. We report here on residual tensile stresses induced in fused silica (Coming 7940, UV grade) by 157-nm laser ablation. Plastic strain of 160-mm thick rectangular strips, monitored with an optical interferometric microscope, revealed the presence of residual tensile stresses in the near-ablated surface. HF chemical thinning of the sample showed the thickness of ablation-affected layer provoking strain was ∼275 nm, a value independent of laser fluence (1.9-4.7 J/cm2) and scanning speed (94 - 220 µm/s). A near-surface mean residual tensile stress of ∼80 MPa was inferred from a thin film-substrate approximation.

Effect of defocusing on laser ablation plume observed by laser-induced fluorescence imaging spectroscopy

“…A decrease in ablation mass can be taken as evidence that the ablation depth has a saturation property with respect to the pulse-energy density. This property would be caused by the exponential dependence of the optical penetration depth in a solid [30][31][32][33][34] or the plasma shielding effect, which is very complicated. [35][36][37] Considering the irradiation area, the peak ablation mass should occur under the defocused condition.…”

F2-laser ablation of Fabry–Perot cavities in optical fibres: chemical sensors

“…The LIF band as shown in figure 5 has a range, 360-500 nm, with peak intensity around 410 nm. The effective absorption coefficient (α) of fused silica at 157 nm, obtained from the ablation rates was reported to be 1.7 × 10 7 m −1 [11]. Therefore the penetration depth of the radiation is within the sub-micrometre regime.…”

“…Microstructures devoid of micro-cracks were produced with a depth resolution of the order of a few nanometres. Konovolov and Herman [11] observed convex craters after ablating fused silica (Corning 7940) using a 157 nm laser. These features were attributed to the compaction of the material as a consequence of laser ablation and the development of tensile stresses in the thin layers immediately beneath the ablated surface.…”