The role of defects in laser surface damage thresholds of fluoride crystals

The energy of the acoustic pulse generated by laser-surface interactions and measured by probe beam deflection was used to investigate laser surface damage thresholds of fluoride crystals with optical quality. It was found that damage thresholds decrease with increasing density of surface states. The defect density also controls the energy absorption mechanism: for surfaces with few defects, like polished MgF₂ and CaF₂, avalanche breakdown occurs at above 1 GW/cm₂, whereas for materials with lower damage thresholds, such as LiF, BaF₂, and roughened or incubated surfaces of CaF₂, multiphoton absorption across the band gap is observed.