Damage to coated ZnSe optical components by high-power CO2 laser radiation

Coated ZnSe optical components are irradiated with high-power, pulsed CO2 laser radiation (Lambda = 10.6 mym, pulse length about 100 ns) at fluences up to 210 J/qcm. The components are characterized at various stages of irradiation by thermography, optical microscopy, stylus profilometry, and surface chemical analysis (x-ray photoemission and Auger electron spectroscopy). During irradiation no temperature rise in the component surface is observed. Two types of coating damage occur within the irradiated area of the component: a braking apart of the ZnSe overlayer of the coating system over relatively large areas, and the formation of isolated craters of diameter about 30-50 mym extending in depth about 3 mym through the coating system down to the ZnSe substrate. Chemically, the irradiated area is characterized by an oxidation of both Zn and Se and an increase in the stoichiometric ratio of Zn to Se. These effects are especially pronounced at the crater defects, and are attributed to loc alized optical absorption, leading to thermal stress and chemical reactions of Zn and Se with atmospheric or adsorbed water and/or oxygen.