Optical Coatings 2-6 Microns

This report presents and discusses results of an investigation of optical coating materials for the mid infrared with emphasis on the 3.8 micrometers DF and 5.3 micrometers CO wavelength regions. A data base of experimental information generated under the contract, comprising crystal structure and preferred orientation, deposition parameters, relative growth rates, refractive indices, and absorption coefficients of ten candidate optical thin film materials is presented. This listing of properties is utilized to identify five coating materials, ThF4, PbF2, SiO, ZnSe, and ZnS for intensive study and application in fabricating laser optics. Intensive studies include optimization of deposition conditions, absorptance and film thickness, effects of polycrystalline substrates upon coating structure and absorptance, and determination of survivability in an HF environment. Results of these investigations and of measurement of absorptance, structure, and preferred orientation in multilayer coatings employing these materials are presented and discussed. In addition, the relative merits of CaF2 and SrF2 substrates, critical chemical characteristics of substrate surfaces and cleaning procedures, and effects of substrate crystallographic orientation upon single layer film properties are discussed in detail.