An absorbing substrate can be coated with a transparent thin film of refractive index N1 (within a certain range) and thickness d such that the ratio of complex reflection coefficients for the p_and s polarizations of the film-covered substrate ρ = Rp/Rs is the inverse of that of the film-free substrate ρ¯ = R¯p/R¯s at an angle of incidence ø. A method to determine the relationship among ø, N1, and d that inverts ρ (i.e., makes ρ = 1/ρ¯) for a given substrate at a given wavelength is described and is applied to aluminum and silver substrates at 0.6328- and 10.6-μm wavelengths, respectively. Sensitivity of the inversion condition to incidence-angle and film-thickness errors is analyzed. ρ-inverting layers can be applied to one of the two metallic mirrors of a beam displacer or axicon to preserve the polarization state of incident monochromatic radiation.
R. M. A. Azzam, "Inverting the ratio of the complex parallel and perpendicular reflection coefficients of an absorbing substrate using a transparent thin-film coating," J. Opt. Soc. Am. A 1, 699-702 (1984) http://www.opticsinfobase.org/josaa/abstract.cfm?URI=josaa-1-7-699 .