Ab Initio Optical Properties of Tribological/Engineering Surfaces

A two-scale approach to the diffuse light scattering from rough surfaces is proposed and applied for the first time. On the microscopic scale, the inter- and intra-layer contributions to the complex optical conductivity tensor are quantum mechanically calculated ab initio, based on the Luttinger formalism and by means of a contour integration within the spin-polarized relativistic screened Korringa-Kohn-Rostoker bandstructure method. These contributions are then properly gathered to determine the layer-resolved permittivities, which in turn uniquely characterize, from an optical point of view, the complete surfaced system of interest and hence yield, together with the known roughness of the surface, the set-up of the next, macroscopic part of the proposed approach. On the latter length scale, the previously developed 2 x 2 matrix technique, which properly accounts for all possible reflections and optical interferences within a layered system, is carefully generalized to also account for the roughness of the surface layer. Applied to a semi-infinite bcc Fe/Fe(100) tribological sample, this two-scale approach has shown that the pointwise normal refraction vector of the rough surface layer, e.g., closely follows the inverse magnitude of the surface normal, at least for linearly polarized light and normal incidence.