Ultrafast laser stress generation landscape in fused silica

Although stress is a critical factor in many ultrafast laser-based fabrication techniques, its relationship to different laser operating parameters remains poorly understood. Here, we investigate the stress landscape within fused silica generated by ultrafast laser pulses, with repetition rates of 100 - 900 kHz and pulse energies of 200 - 4000 nJ, by measuring all three components of the stress-induced curvature change of fused silica plates. We find that for all repetition rates, there is an inflection in the stress when the average laser power is about 60 mW, and this inflection is not correlated with the morphological transition from nanogratings to melting, as observed from cross-section imaging. The equibiaxial and antibiaxial components of stress exhibit a characteristic average ratio of about -1.65 up until the visually observed onset of melting within the modifications, which occurs when the average laser power is about 423 mW. We conclude that nanogratings produce a characteristic stress state, with a maximum magnitude that is reached at lower pulse energy than nanograting erasure. Beyond nanograting erasure, the stress state is more variable and distinct from the nanograting-induced stress state.