LASER DAMAGE OF ALKALINE-EARTH FLUORIDES AT 248 NM AND THE INFLUENCE OF POLISHING GRADES

Damage behaviour and thresholds for single 248 nm/14 ns excimer laser pulses have been investigated for single crystals of CaF2 and BaF2 wit h (111) surface orientation. The probe beam deflection technique was a pplied as a sensitive tool for detecting the onset of single-shot dama ge. Below the plasma threshold, we observed one- and two-photon absorp tion for CaF2 and BaF2, respectively, When testing the influence of di fferent polishing techniques, we found the lowest thresholds for conve ntional hard-polish. Advanced methods as ductile machining or chemical polishing lead to a distinct increase in damage threshold up to and e ven better than what is observed for cleaved surfaces, SEM investigati ons of irradiated areas show that damage preferably takes place at res idual steps or other structural defects.