The role of surface roughness in the formation of LIPSS on metals

Abstract

The present work addresses the controlled formation of Laser Induced Periodic Surface Structures (LIPSS) using femtosecond laser pulses in metals: copper and steel. In the former, we have identified a processing regime which produces two different periods formed at different local fluences1. Given that the simple plasmonic model2 overestimates the periods experimentally observed at oblique angles of incidence, we have introduced a novel predictive model. This model takes into account the influence of the surface¿s roughness on the surface plasmon polariton (SPP) formation. The model results fit adequately the ripples periods so obtained1. We have also investigated the formation of LIPSS in steel and discovered a new effect when processing the surface at oblique angle while scanning the sample, featuring a dependence of the ripple period on the scan direction. This phenomenon, coined quill writing or non-reciprocal writing3, allows us to select with high precision the ripples period just by inverting the scan direction4. The underlying mechanism is related to the asymmetry the roughness indiced upon exposure at oblique incidence. As a consequence, different roughness at different positions generate different wavevectors of the surface plasmon polariton when scanning in different directions. In addition, we have adapted the roughness-plasmonic model to absorbent materials such as steel, using an effective dielectric constant. Notes and References 1 Y. Fuentes-Edfuf, et al., ACS Omega 4 6939 (2019) 2 A. M. Bonch-Bruevich, Opt. Eng. 31 7181 (1992) 3 P. G. Kazansky, et al., APL. 15 151120 (2007) 4 Y. Fuentes-Edfuf, et al., ASS 493 948 (2019)