Breakdown of spallation in multi-pulse ultrafast laser ablation

TL;DR

This study investigates how homogeneous spallation in ultrafast laser ablation of metals changes from single to multiple pulses, revealing it is primarily a single-pulse phenomenon with diminishing effects over successive pulses.

Contribution

The paper demonstrates that homogeneous spallation occurs mainly during the first pulse and diminishes with subsequent pulses, challenging the assumption it persists in multi-pulse regimes.

Findings

Homogeneous spallation dominates the first pulse.

Spallation effects reduce significantly after the second pulse.

By the fourth pulse, spallation signatures fully saturate into a phase-explosion-like state.

Abstract

Ultrashort-pulse laser ablation of metals near damage threshold is governed by homogeneous spallation, in which tensile unloading releases a nanometre-thin liquid film whose optical signatures are temporally evolving concentric Newton rings in pump--probe experiments. This well-established picture rests almost exclusively on single-pulse results obtained on ideally flat surfaces, yet application-oriented processing invariably operates in a multi-pulse regime in which each pulse irradiates a surface progressively modified by preceding pulses. Whether homogeneous spallation persists under these conditions has remained an open question. Here we resolve this question using time-resolved pump-probe interferometry applied pulse by pulse to austenitic stainless steel. We show that homogeneous spallation dominates the first pulse, while its contribution is strongly reduced for the second pulse. By the third pulse, Newton rings vanish and sustained surface bulging collapses, with the optical transients fully saturating into a phase-explosion-like signature by the fourth pulse. Fourier-domain coherence analysis rules out roughness-induced decoherence as an optical artefact. Four independent observables, spanning time-resolved and final-state measurements, converge on the same transition after three to four pulses. Spallation-layer formation, widely invoked to explain ultrashort-pulse ablation of metals, is thus a single-pulse phenomenon rather than a multi-pulse ablation mechanism.