A special type of double-layer antireflection coating for infrared optical materials is described. This coating has two unusual features: the total optical thickness of the double layer is less than one-quarter wavelength; and the film with the higher index of refraction is on the outside. The reduction in reflectance in the low reflecting region is similar to that of a single-layer coating. Measured values of transmittance are given for coatings on Si and Ge. An interesting point in one of the examples is the use of a Ge film in the antireflection coating for Ge. The coatings are very durable.
J. T. Cox, "Special Type of Double-Layer Antireflection Coating for Infrared Optical Materials with High Refractive Indices," J. Opt. Soc. Am. 51, 1406-1408 (1961)