Efficient and durable AR coatings for Ge in the 8–115-μm band using synthesized refractive indices by evaporation of homogeneous mixtures

The development of an efficient and durable medium band antireflective coating on germanium for the 8–11.5-μm wavelength region is described. The minimal average reflectance of a double-layer stack is calculated as a function of a varying index of one of the layers, while the second chosen index is kept constant. In this way a comprehensive display of the outcome of various material combinations can be obtained. A homogeneous CdTe/CeF3 double layer is compared to improved coatings, in which the outer layer’s index is synthesized by single-source evaporation of mixtures of CeF3 + SrF2 and CeF3 + SrF2 + CaF2. The addition of a Y2O3 binding layer and the inhomogeneous mixing of the two layer materials at the phase interface by coevaporation significantly improved the durability of the coating.