Ultrafast laser direct drilling of high aspect ratio microchannels in fused silica: Insights on channel self-termination mechanisms

Ultrafast lasers have attracted extensive interests for their capability in drilling microchannels in transparent materials. However, direct drilling by ultrafast laser has been believed insufficient to achieve satisfactory aspect ratio. Moreover, the underlying mechanism of self-termination in drilling is still unclear. We present a comprehensive study on direct drilling of fused silica by ultrafast laser (190 fs) with different sample moving speeds (20-500 mu m/s) and laser fluences (244.8-1958.5 J/cm2). We demonstrate the feasibility of drilling uniform and taper-free channels longer than 2000 mu m, with an aspect ratio of-35:1 and decent sidewall roughness of-0.66 mu m. The dependences of the channel aspect ratio and quality on sample moving speeds and laser fluences are demonstrated, and optimal speeds are found to increase with laser fluences. We propose and testify new mechanisms of early termination in drilling at nonoptimal sample moving speeds. Insufficient laser energy density causes drilling to end prematurely at high sample/beam moving speeds. When the moving speed is too low, excessive laser energy deposition leads to a nontransparent region above the channel, which blocks the laser beam and halts the drilling process. This work highlights the capability of ultrafast lasers to directly drill channels with satisfactory aspect ratios in transparent materials and unveils new drilling self-termination mechanisms.