Nanosecond UV laser damage and ablation from fluoride crystals polished bydifferent techniques

Ablation thresholds and damage behavior of cleaved and polished surfaces of CaF2, BaF2, LiF and MgF2 subjected to single-shot irradiation with 248 nm/ 14 ns laser pulses have been investigated using the photoacoustic mirage te chnique and scanning electron microscopy. For CaF2, standard polishing yiel ds an ablation threshold of typically 20 J/cm(2). When the surface is polis hed chemo-mechanically, the threshold can be raised to 43 J/cm(2), while po lishing by diamond turning leads to intermediate values around 30 J/cm(2) C leaved surfaces possess no well-defined damage threshold. When comparing di fferent fluoride surfaces prepared by diamond turning it is found that the damage resistivity roughly scales with the band gap. We find an ablation th reshold of 40 J/cm(2) for diamond turned LiF while the MgF2 surface can wit hstand a fluence of more than 60 J/cm(2) without damage. The damage topogra phy of conventionally polished surfaces shows flaky ablation across the las er-heated area with cracks along the cleavage planes. No ablation is observ ed in the case of chemo- mechanical polishing; only a few cracks appear. Di amond turned surfaces show small optical absorption but mostly cracks and a blation of flakes and, in some cases, severe damage in the form of craters larger than the irradiated area. The origin of such different damage behavi or is discussed.