Spot size dependence of the ablation threshold of BK7 optical glass processed by single femtosecond pulses

Abstract A comparative study of the evolution diameter and depth ablated craters, while varying systematically both pulse energy spot size is reported. It reveals that surface ablation threshold fluences double-checked by diameter-regression method a multiphoton-absorption-based fit show clear dependence on radius processing laser beam BK7 optical glass surface. Tighter focusing results in rise from 6.5 J/cm 2 up to nearly 11 within 26.1–10.1 µm domain. The time-integrated reflected part every single with rising indicates three types response material, depending actual pulse. At where first second section join, which shifts lower energies decreasing size, initial reflectivity, assigned permanent reflectivity glass, suddenly increases. This plasma mirror formation target enhancement thresholds very similar. values remain equal for radii at above 13.5 µm, followed size. could be affirmed as dominant process behind our case. appearance double-crater structures consistent air ionization, resulting highest energies. assignment further corroborated proving defect model hardly accounted results. findings support consideration control parameter.