It has been found that vacuum-deposited films of CsI, AgCl, TlBr, and TlCl are useful as antireflection coatings for silicon over a broad spectral range in the infrared. Measurements performed on a plane-parallel silicon plate coated with various thicknesses of the above materials yield transmittance values ranging from 99% at λ = 2.9 µm to 88% at λ = 27 µm. Experimental details of the coating process and properties of the coatings are discussed.
G. H. Sherman and P. D. Coleman, "Antireflection Coatings for Silicon in the 2.5–50-µm Region," Appl. Opt. 10, 2675-2678 (1971)