Thin glass is widely used in modern manufacturing. Glasses are brittle and heat sensitive, so they require specific care to be processed. Ultrafast laser technology, which has the unique capacity to produce either surface ablation, bulk modification or backside ablation, is already used to process glasses but combining quality and throughput is still a key issue. Compared to frontside ablation, backside ablation enables (i) to produce sharper rims featuring lower taper angles on deep ablation craters, (ii) to limit the effects of ablation plume shielding, and (iii) to minimize deposition of debris during and after the interaction. Conversely to front ablation where the energy deposition occurs onto the surface, backside ablation leads to in-volume energy deposition prior to the theoretical focus due to non-linear absorption. The drilling speed is defined by the ablation front velocity in top-down drilling whereas it is fixed by the vertical velocity of the focus in bottom-up drilling. Finally, backside ablation requires high numerical apertures to go through and low fluences to avoid ablation at the front surface which stops the drilling process.

We report here on time-resolved pump-probe shadowgraphy of bottom-up percussion drilling of alkali-free alumina-borosilicate 300 μm thin glasses using single and double femtosecond laser pulses. We used a probe under Brewster angle incidence to see the transient bulk modification during laser drilling. The influence of numerical aperture, focus position and fluence have been investigated. We compare single to double pulse laser irradiation. The results are discussed in terms of drilling speed and aspect ratio. We observed a significant improvement of drilling speed (x 1.7) and aspect ratios by introducing a vertical and temporal shift between the two subsequent sub-pulses. We note that the drilling could suffer from a dropout if the vertical velocity is too high, even before any front ablation crater appearance