A design procedure is described to determine the thicknesses of single-layer coatings of a given dielectric on a given metallic substrate so that a specified net phase retardance (and/or a net relative amplitude attenuation) between the p and s polarizations is achieved after three reflections from a symmetrical arrangement of three mirrors that maintain collinearity of the input and output beams. Examples are presented of halfwave and quarterwave retarders (HWR and QWR) that use a ZnS-Ag film-substrate system at the CO2-laser wavelength λ = 10.6 µm. The equal net reflectances for the p and s polarizations are computed and found to be high (above 90%) for most designs. Sensitivity of the designs (deviation of the magnitude and phase of the ratio of net complex p and s reflection coefficients from design specifications) to small film thickness and angle-of-incidence errors is examined, and useful operation over a small wavelength range (10–11 µm) is demonstrated.
T. F. Thonn and R. M. A. Azzam, "Three-reflection halfwave and quarterwave retarders using dielectric-coated metallic mirrors," Appl. Opt. 23, 2752-2759 (1984)