Total polarization conversion using subwavelength laser-induced periodic surface structures on stainless steel

Stainless steel is a basic raw material in industry. It can be customized by generating laser-induced periodic surface structures (LIPSS) as subwavelength gratings. Here, we present the capabilities of LIPSS on stainless steel to modify the polarization state of the reflected radiation at the infrared . These structures have been modeled using the finite element method and fabricated by femtosecond laser processing. The Stokes parameters have been obtained experimentally and a model for the shape has been used to fit the simulated Stokes values to the experimental data. The birefringence of the LIPSS is analyzed to explain how they modify the polarization state of the incoming light. We find the geometry of the subwavelength grating that makes it work as an optical retarder that transforms a linearly polarized light into a circularly polarized wave. Also, the geometrical parameters of the LIPSS are tuned to absorb selectively one of the components of the incoming light, becoming a linear axial polarizer. An appropriate selection of the geometrical parameters and orientation of the fabricated LIPSS makes possible to obtain an arbitrary pure polarization state when illuminated by any other pure polarization state. The overall reflectance of these transformations reaches values up to 60% with respect to the incident intensity, that is the same reflectivity obtained for non nanostructured stainless steels flat surfaces.