BILAYER COATINGS THAT PRODUCE A 90-DEGREES DIFFERENTIAL REFLECTION PHASE-SHIFT AT OBLIQUE-INCIDENCE - ALL POSSIBLE SOLUTIONS

For a system that consists of a transparent bilayer on an absorbing su bstrate of known optical properties, we determine all possible combina tions of the thicknesses of the two layers such that the differential reflection phase shift between the p and s polarizations Delta = +/-90 degrees for monochromatic light incident al a given angle phi. A ZnS- ThF4 bilayer on a Ag substrate at 3.39 mu m wavelength is considered a s an example of a high-reflectance system. Constant principal-angle co ntours are presented in the plane of the normalized thicknesses of the two films. Four distinct bilayers on Ag make \Delta\ = 90 degrees and equalize the reflectances for the p and s polarizations at phi = 65 d egrees, hence realize an ideal quarter-wave retarder (QWR). The QWR wi th the thinnest bilayer has the lowest sensitivity to incidence angle and wavelength shifts. A second example is that of a Si3N4-SiO2 bilaye r on Si at 633 nm wavelength. The results in this case are of interest in relation to the four-detector photopolarimeter, and also for princ ipal-angle ellipsometry on this technologically important material sys tem.