The net differential phase shift Δt introduced between the orthogonal p and s linear polarizations after four successive total internal reflections inside an in-line chevron dual-Fresnel-rhomb retarder is a function of the first internal angle of incidence φ and prism refractive index n. Retardance of 3λ/4 (i.e., Δt=270°) is achieved with minimum angular sensitivity when φ=45° and n=1.900822. Several optical glasses with this refractive index are identified. For Schott glass SF66 the deviation of Δt from 270° is ≤4° over a wavelength range of 0.55≤λ≤1.1 μm in the visible and near-IR spectrum. For a SiC prism, whose totally reflecting surfaces are coated with an optically thick MgF2 film, Δt=270° at two wavelengths: λ1=0.707 μm and λ2=4.129 μm. This coated prism has a maximum retardance error of ≈5°over>three octaves (0.5 to 4.5μm) in the visible, near-, and mid-IR spectral range. Another mid-IR 3λ/4 retarder uses a Si prism, which is coated by an optically thick silicon oxynitride film of the proper composition, to achieve retardance that differs from 270° by <0.5° over the 3-5 μm spectral range.
R. M. A. Azzam and H. K. Khanfar, "In-line broadband 270° (3λ/4) chevron four-reflection wave retarders," Appl. Opt. 47, 4878-4883 (2008).