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

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