Optical EngineeringPeriodic and quasiperiodic nonquarterwave multilayer coatings for 90-deg reflection phase retardance at 45-deg angle of incidence
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All possible solutions for a periodic stack of 10 ZnS-ThF4 bilayers on a Ag substrate that produce 90-deg differential reflection phase shift at 45-deg angle of incidence and 3.39-μm wavelength are determined. The angular and wavelength sensitivity of several of these designs is considered. To increase the reflectance, reduce the diattenuation, and improve the sensitivity of the 20-layer reflection quarterwave retarder (QWR), we also consider quasiperiodic stacks. These designs are obtained by iteration starting from an initial high-reflectance, 10-bilayer, non-QWR, periodic structure and adjusting the thicknesses of the two films of the topmost bilayer to realize the desired 90-deg retardance. Again multiple solutions are obtained for each initial design and their angular and wavelength sensitivity are analyzed. It is found that the best sensitivity corresponds to the lowest and nearly equal optical thicknesses of the two films of the topmost bilayer. Performance comparable to that reported in the literature, obtained for more complicated stacks in which the thicknesses of all 20 films are different, is reported.