PERIODIC AND QUASI-PERIODIC NONQUARTERWAVE MULTILAYER COATINGS FOR 90-DEG REFLECTION PHASE RETARDATION AT 45-DEG ANGLE OF INCIDENCE

All possible solutions for a periodic stack of 10 ZnS-ThF4 bilayers on a Ag substrate that produce 90-deg differential reflection phase shif t at 45-deg angle of incidence and 3.39-mu m wavelength are determined , The angular and wavelength sensitivity of several of these designs i s considered. To increase the reflectance, reduce the diattenuation, a nd improve the sensitivity of the 20-layer reflection quarterwave reta rder (QWR), we also consider quasiperiodic stacks. These designs are o btained by iteration starting from an initial high-reflectance, 10-bil ayer, non-QWR, periodic structure and adjusting the thicknesses of the two films of the topmost bilayer to realize the desired 90-deg retard ance, Again multiple solutions are obtained for each initial design an d their angular and wavelength sensitivity are analyzed, It is found t hat the best sensitivity corresponds to the lowest and nearly equal op tical thicknesses of the two films of the topmost bilayer, Performance comparable to that reported in the literature, obtained for more comp licated stacks in which the thicknesses of all 20 films are different, is reported.