High performance step-down AR coatings for high refractive-index IR materials

A numerical thin film synthesis program has been used to design high performance antireflection coatings of the step-down type for a number of IR window materials. In the calculations, the dispersion of the optical constants of all the materials is accounted for. Various trade offs between the width of the AR region and the maximum reflection within that width are possible. For example, an AR coating for germanium has been found in which the reflectance is <0.1% over a 1.4:1 range of wavelengths. Similar designs have been developed for silicon, cadmium telluride, zinc selenide, and zinc sulfide substrates. The experimentally measured performance of several coatings will be given.