Nonlinear absorption mechanisms during femtosecond laser surface ablation of silica glass

Spatial profiles of single-shot microcraters produced by tightly focused femtosecond laser pulses with variable pulse energies are measured by means of a laser confocal microscope. Dependences of crater depth on laser intensity at different pulse energies appear as overlapping saturating curves with the same threshold, indicating the presence of nonlinear absorption and absence of nonlocal ablation effects. A monotonic twofold increase in absorption nonlinearity is related to the transition from minor defect-band absorption to fundamental band-to-band absorption.