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 at an angle of incidence ϕ. A method to determine the relationship among ϕ, N1, and d that inverts ρ (i.e., makes ρ = 1/ ) 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.
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