Nanosecond UV laser damage and ablation from fluoride crystals polished by different 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 technique and scanning electron microscopy. For CaF2, standard polishing yields an ablation threshold of typically 20 J/cm(2). When the surface is polished chemo-mechanically, the threshold can be raised to 43 J/cm(2), while polishing by diamond turning leads to intermediate values around 30 J/cm(2) Cleaved surfaces possess no well-defined damage threshold. When comparing different fluoride surfaces prepared by diamond turning it is found that the damage resistivity roughly scales with the band gap. We find an ablation threshold of 40 J/cm(2) for diamond turned LiF while the MgF2 surface can withstand a fluence of more than 60 J/cm(2) without damage. The damage topography of conventionally polished surfaces shows flaky ablation across the laser-heated area with cracks along the cleavage planes. No ablation is observed in the case of chemo- mechanical polishing; only a few cracks appear. Diamond turned surfaces show small optical absorption but mostly cracks and ablation of flakes and, in some cases, severe damage in the form of craters larger than the irradiated area. The origin of such different damage behavior is discussed.