Infrared Absorption of Thin Metal Films

Abstract

The maximum infrared absorption that can be obtained in an unsupported metal film in air is 50 percent. It is shown that the absorption may be increased by supporting the metal on a dielectric of high refractive index, provided that the radiation passes through the dielectric to the metal. Under certain conditions there is then no reflection from the layer. If the combination is reversed so that the radiation is incident on the metal first, a low absorption results. If the supporting dielectric is of low refractive index neither direction will lead to high absorption; in this case the use of another system, with metal on each side of the support is preferable. Using whichever of the two systems is appropriate to the supporting dielectric, an absorption of 70 percent is always obtainable.