Deciphering the driving mechanisms of incubation in ultrashort pulse laser ablation

Incubation, the systematic reduction of the ablation threshold with pulse number, critically influences ultrashort pulse laser micromachining, yet its microscopic origin remains insufficiently understood despite its widespread relevance in applications. Here, multi-pulse experiments (500 fs pulse duration, 1040 nm wavelength) with fluences ranging from 0.75 to e2 times the ablation threshold and repetition rate of 1 Hz on aluminum and stainless steel were combined with pulse-resolved absorptance from Finite-Difference-Time-Domain simulations to disentangle the roles of global absorption, crater-edge near-field enhancements, and microscopic material weakening. For aluminum, surface roughening leads to an absorption increase reciprocal to the threshold, providing a sufficient explanation of incubation. In stainless steel, however, the threshold decreases despite nearly constant absorption, demonstrating that increased absorption is not a necessary condition for incubation. Edge-localized near-field enhancements provide an early but limited contribution, saturating after a few pulses. A porosity-based description within classical nucleation theory demonstrates that material weakening can only be explained microscopically by defect-induced reductions of the effective penetration depth together with pulse-dependent nucleation rates. These findings establish a microscopic and quantitative framework for incubation, advancing the physical understanding of the transition from single- to multi-pulse ablation, providing the basis for predictive models of multi-pulse ablation with ultrashort-pulses.