"Heterogeneous" versus "homogeneous" nucleation and growth of microcones on titanium surface under UV femtosecond-laser irradiation

Two separate sets of microcones were observed on titanium surface under various ablative 248-nm femtosecond-laser exposures near rough spallation crater edges and in crater centers, respectively. “Heterogeneous” nucleation of microcones occurred on crater edge spikes above the corresponding cumulative spallation thresholds, with their gradual growth proceeding through redeposition of the ablated matter. In contrast, quasi-periodical central microcones emerged through a “homogeneous” nucleation process owing to ablative surface instability, with average cone volume increasing exponentially versus the shot number over the broad fluence range until its stabilization, and logarithmically versus laser fluence at the different shot numbers above the instability threshold fluence of about 30 mJ/cm2. The instability was assigned to a positive optical feedback, resulting from enhanced laser energy deposition and ablation within the inter-cone valleys, as previously suggested and demonstrated by our numerical modeling of near-surface electromagnetic intensity distribution.