Ultrafast Laser Texturing of Metal Surfaces: Effects of Process Parameters on Surface Reflectance and Possibility of Hierarchical Structuring

Abstract

Ultrafast laser processing has been widely studied for surface texturing. The complex interaction between the laser energy, plasma, and surface chemistry produces complex morphologies including Laser-Induced Periodic Surface Structures and random higher aspect ratio geometries. Laser texturing allows engineering of metallic surface’s wettability as well as the reflectance on either broadband or narrowband basis. This paper experimentally maps the laser process parameters to the surface morphology and diffuse reflectance for stainless steel, aluminum, and copper substrates. All experiments are conducted with a 1030 nm wavelength, 230 fs pulse length laser in an ambient environment. The results show how the common morphological regimes shift with material and how the reflectance varies with morphology. To further decrease the reflectance, hierarchical structures are generated by first locally micromachining the surface to form a lattice of trenches using the focused laser beam, before texturing the surface with a rastered, defocused laser beam. The micromachined features interact with laser texturing and increase light trapping on the surface. This is a function of the depth and periodicity of the hierarchical structures as well as the surface topography. This approach provides the ability to lower the surface reflectance and add an extra level of control for directing deep micro-cavities along the surface.