Reflectance and preparation of front-surface mirrors for use at various angles of incidence from the ultraviolet to the far infrared

Abstract

The increasing use of reflecting optics in ultraviolet (UV), visible, and infrared (IR) devices has stimulated a great amount of research on coatings for front-surface mirrors. Methods for measuring the reflectance of front-surface mirrors at various wavelengths and angles of incidence are discussed, and techniques for preparing reflecting films with maximum reflectance and durability are described. Data on the UV, visible, and IR reflectance of the most frequently used mirror coatings, Al, Ag, Au, and Rh, are presented. The application of single-layer and multilayer dielectric overcoatings for increasing their durability and normal-incidence reflectance are described, and the effect of these coatings on reflectance at higher angles of incidence is treated. It is shown that Al and Ag overcoated with rather thin layers of silicon oxides or Al2O3 (t = 1000–2000 A) have, in the IR from 8 to 12 μm, practically the same high reflectance as the unprotected metal at close to normal incidence but greatly decreased reflectance at angles larger than 40°. Since only the parallel component is responsible for the IR-reflectance decrease, such film combinations are suitable for producing highly efficient reflection polarizers for the IR, such as for CO2 10.6-μm laser radiation. The effect of water absorption in dielectric overcoatings on the mirror reflectance at 3 μm is discussed.