In-Situ Monitoring of Bottom-Up Percussion Drilling of Alkali-Free Alumina-Borosilicate Thin Glasses Using Single and Double Femtosecond Laser Pulses

We report on top-down and bottom-up percussion drilling of alkali-free alumina-borosilicate glass using single and double femtosecond laser pulses. The drilling process dynamics are studied by pump-probe shadowgraphy under Brewster angle which allows for visualizing transient effects in the 300-m thick glass. We show that the top-down drilling is very limited and does not allow for drilling through holes in contrast to the bottom-up approach. We study the influence of the laser parameters such as the fluence and focusing conditions which reveal to constitute key parameters in order to find an adequate process window for successful bottom-up drilling. We observe that the vertical velocity for bottom-up processing should be ideally in agreement with the ablation moving interface velocity. Interestingly, the shadowgraphs reveal that the drilling process may be interrupted even before any frontside ablation appears if the vertical velocity shift is too high. Moreover, we investigate the effects of introducing an additional spatial and temporal shift during the drilling process which lead to a significant increase in drilling rate by almost a factor of two.