In-situ observation of the hole formation during deep drilling with ultrashort laser pulses

摘要

We report on the in-situ observation of the laser drilling process using ultrashort laser pulses. Our technique is based on transmission imaging of a silicon sample at 1060 nm. For drilling, we used a laser system that provides pulses with a duration of 8 ps at 1030 nm. This wavelength is below the band edge and silicon shows linear absorption. The beam is focused on the sample surface perpendicular to the transillumination. Therefore, the temporal evolution of the longitudinal silhouette of the hole can be visualized during the drilling progress. Our observations show a change of the drilling dynamics in the depth of the material. Effects like the decrease in ablation rate, the formation of bulges, deviations in drilling direction and finally a branching of the hole end occur due to the influence of the previously excavated capillary. That causes a perturbation of the beam by internal reflections and the interaction with ablation products as well as their additional abrasive effect. The dependence of hole depth and shape on the process parameters, especially fluence and pulse energy, is studied. The maximum achievable hole depth in deep drilling is chiefly determined by the pulse energy but largely independent of fluence.