OBSERVATION OF SINGLE ULTRA-SHORT LASER PULSE GENERATED PERIODIC SURFACE STRUCTURES ON LINE-LIKE DEFECTS: Paper # Micro 302

摘要

The formation of self-assembled laser induced periodic surface structures (LIPSS) after ultra-short pulsed laser ablation is still a matter of controversy in literature. It is agreed that there are at least two different physical driving forces depending on the spatial periodicity. High spatial frequency laser induced periodic surface structures (HSFL) with a periodicity well below the incident wavelength are discriminated from low spatial frequency structures (LSFL) revealing longer periodic structures. In general, both kind of LIPSS appear after multi-pulse irradiation with linear polarization direction on all material classes from metals to dielectrics. However, single-pulse induced LSFL at 540nm+/-35nm periodicity with sub picosecond pulse are observed at line-like surface defects, e.g. scratches. Depending on the difference in orientation between the electric field vector and the scratch direction, LIPSS evolve upon ablation with 515nm and 1ps pulses near the threshold. This corroborates the theory proposed by Sipe, where the impinging electromagnetic wave interacts with a collectively excited surface electron wave of the respective material at a surface defect. The observations on oxygen-free pure copper, zirconia and a stainless steel substrate are discussed. In application this phenomena has an impact on the attainable surface quality. The LSFL appear at defects like scratches, grain boundaries, and, generally, material inhomogeneity. Following, the absorptivity and ablation characteristic changes, which leads to an altered material modification at the surface. This could be the cause of cone-like protrusion structures on stainless steel. Additionally, LSFL are generated at defects after single pulse ablation of a zirconia ceramic even with circular polarization.