Defect study on several fluoride coatings

Abstract

Defect in thin film is one of the most important factors influencing laser-induced damage in infrared laser systems, and always a major concern. The infrared thin film is required for reflecting infrared and visible wavelengths, and the materials should be low-absorbing in the spectral wavelengths as well. In general, this multilayer coating consists of a number of high and low reflective index materials with alternate layers. Zinc Sulfide is used as high index material because it has good visible and IR optical properties. Low index material can be Thorium Fluoride, but ThF4 is radioactive and toxic, several fluoride coatings are studied in order to select an appropriate material instead of ThF4 in this paper. Single layer of YbF3, LaF3,YBC(a new combined fluoride) and PrF2 coating are produced by thermal evaporation, and classifications and derivation of defects in these thin films are introduced firstly. Then, the influence of depositing method on surface defect density of laser thin film is analyzed. Finally, result shows that YbF3 is an appropriate fluoride in place of ThF4, and the multilayer thin film deposited by YbF3 and ZnS material can be applied in infrared laser systems.