Observation of single ultrashort laser pulse generated periodic surface structures on linelike defects

The formation of self-assembled laser induced periodic surface structures (LIPSSs) after ultrashort pulsed laser ablation is still a matter of controversy in the literature. There is agreement that at least two different physical driving forces lead to ripples with distinguishable spatial periodicity. High spatial frequency LIPSSs with periodicity well below the incident wavelength are discriminated from low spatial frequency LIPSSs (LSFLs) revealing longer periodic structures. In general, both types of LIPSS appear after multipulse irradiation with the linear polarization direction on all material classes from metals to dielectrics. However, single-pulse induced LSFLs at 540 ± 35 nm periodicity with subpicosecond pulse are observed at linelike surface defects, e.g., scratches and grain boundaries. Depending on the difference in orientation between the electric field vector and the scratch direction, LIPSSs evolve upon ablation with 515 nm and 1 ps pulses near the threshold. This corroborates the theory proposed by Sipe et al. [Phys. Rev. B 27, 1141–1154 (1983)], where the impinging electromagnetic wave interacts with a collectively excited surface electron wave of the respective material at a surface defect. The observations on oxygenfree pure copper, zirconia, and a stainless steel substrate are discussed. Moreover, LSFLs generated with circular polarization at defects after single pulse ablation of wide bandgap zirconia ceramic are presented. In application, this phenomena affects the attainable surface quality, where LSFLs appear at defects such as scratches, grain boundaries, and, generally, material inhomogeneity. The absorptivity and ablation characteristic change leading to an altered material-laser interaction at the surface. This could be the root cause of conelike protrusion structures observed on stainless steel.