Damage to cooper mirrors by high-power CO2 laser radiation

A focussed, cw CO2 laser is used to irradiate polished Cu mirrors at power densities near 40 kW/qcm. Uncoated mirrors and mirrors having a reflection-enhancing Au coating with a protective Y2O3 layer are tasted. During irradiation insitu measurements of the temperature distribution in the surface are made and the morphology is monitored by interferometry. Chemical changes are determined by ex-situ x-ray photoemission and scanning Auger measurements. Damage to uncoated mirrors is connected with irreversible deformation, an increase in thermal gradients under irradiation (attributable to increased absorption), and oxidation of the surface layer, with formation of Cu2O and CuO. While the whole surface becomes oxidized, the effect is strongest in the laser focus, where an enhanced carbon content is also found. Similar thermal and morphological changes occur in coated mirrors under analogous irradiation conditions, but the relatively inert Y2O3 protective layer is not chemically changed. St ructural changes are present in the surface and may contribute to the changed absorption.