The role of the femtosecond laser induced nano/micro structures on the optical features of the steel surface; experimental polarimetry study

We are witnessing that ultrashort laser material processing (ULMP) is getting an established technique to modify many characteristics of a surface. Being applicable for various materials with any geometry or physical characteristics, besides being rapid and having simple experimental set-ups makes ULMP appropriate for small and large scale controlled modifications. Using this method to adjust the optical features of a surface has been also suggested in last years. So that mimicking the functional surfaces existing in nature which have distinctive surface morphologies and thus have special optical features, efforts are going on to create similar samples using ULMP. Here in this study, the potential of ULMP technique in changing the optical properties of metallic substrates is explored. Selecting 316L stainless steel as a substrate, five samples which are irradiated in different circumstances are fabricated. The role of the laser induced nano/micro structures on the optical features of these samples are studied systematically. Visible light backscattering in the range of 0°-65° was studied at first. Extracting the Mueller matrix elements for all samples at two different incident angles of 45° and 60°, the polarimetry parameters including retardancy, depolarization and diattenuation are calculated at the next step. Due to the multi-scale nature of the morphologies, more than the illuminating light angle, dependence of the samples optical response to its orientation is also discussed in detail Results clearly reflect that the optical response of laser induced periodic surface structure (LIPSS) coated layers have an anisotropic behavior regarding the incident angle and the polarization of the illuminating light. Furthermore, the surface orientation itself can play very serious role on the final response of the sample. It comes out that the characteristics of a surface such as its morphology, roughness or the correlation of the LIPSS structures has important effect in these regards.