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 with (111) surface orientation. The probe beam deflection technique was applied as a sensitive tool for detecting the onset of single-shot damage. Below the plasma threshold, we observed one- and two-photon absorption for CaF2 and BaF2, respectively. When testing the influence of different polishing techniques, we found the lowest thresholds for conventional hard-polish. Advanced methods as ductile machining or chemical polishing lead to a distinct increase in damage threshold up to and even better than what is observed for cleaved surfaces. SEM investigations of irradiated areas show that damage preferably takes place at residual steps or other structural defects.