Among the I.R.-transmittive materials, germanium takes an important place owing to its excellent optical and mechanical properties which compensate for the fact that it is rather expensive. Germanium can be easily ground and polished into appropriate shapes. Its high refractive index, varying only slowly with wavelength, makes it extremely valuable for lenses if treated with an antireflective coating. Its absorption coefficient between 2 and 12 micrometer wavelength is very low. It is not hygroscopic and shows in general a high chemical resistance. For all these reasons the use of Ge as an I.R. optical material has grown considerably. Most of the germanium used in infra-red optics up to now is polycristalline. A number of measurements, however, proves that monocrystalline Ge is optically more uniform and also has higher mechanical resistance. The difference in homogeneity can be explained by the production techniques described in this paper. The characteristics of mono- and polycrystalline Ge are also compared in detail.
L. H. De Laet,
"Advantages Of Monocrystalline Germanium For Infrared Optics", Proc. SPIE 0109, Advances in Optical Production Technology I, (14 October 1977); doi: 10.1117/12.955499; https://doi.org/10.1117/12.955499