High-Reflectivity Mirrors for Use at 10.6 μm

The advent of high-efficiency, high-power lasers, operating at various vibrational-rotational transitions of the carbon dioxide molecule in the vicinity of 10 μm, has created a need for mirrors which have high reflectivity in this region. Reflectivities between 98% and 99% can be realized with metallic mirrors of copper, silver, or gold; however, in the case of silver and copper (which have the highest reflectivities) dielectric protective coatings are required to prevent atmospheric degradation of reflectivity and to permit cleaning of the mirror. Reflectivities greater than 99% can be achieved with “enhanced metallic reflectors,” which consist of metals coated with pairs of alternating, quarter-wavelength thick, low and high index of refraction dielectrics or semiconductors. In principal this type of mirror can have a reflectivity approaching 99.9%, as has been achieved for multilayer dielectric mirrors for use at the 0.633-μm He-Ne laser line in the visible. Performance is limited, however, by the difficulty of finding materials which have suitable physical and optical properties when used in quarter-wavelength thicknesses. The present paper includes a discussion of experimental equipment for making accurate and reproducible measurements of absorption in high-reflectivity mirrors at 10.6 μm, of comparative absorption data on a variety of metals produced in different ways, and of the problems involved in producing enhanced dielectric mirrors along with some preliminary results.