Infrared Filters of Antireflected Si, Ge, InAs, and InSb*

Vacuum-deposited single-, double-, and triple-layer infrared antireflection coatings for Si, Ge, InAs, and InSb have been developed for the 1-μ to 15-μ region. MgF2, didymium fluoride, SiO, ZnS, CeO2, and Si were used for the preparation of the coatings. All of the coatings are hard and durable and can be cleaned and boiled in water for several hours with little or no damage. With these coatings, the transmittance of Si and Ge plates can be increased to a maximum value of nearly 100%, and stays above 90% over a wavelength interval the limits of which are in the ratio of 1.5:1, 2.7:1, and 3:1 for single-, double,- and triple-layer coatings, respectively. InAs and InSb are slightly absorbing, but more than 90% transmittance can be obtained with sufficiently thin antireflected plates. The temperature dependence of absorption in all four of these materials has been measured in the range from 25°C to 250°C.