Enhanced drilling of transparent materials with ultrashort pulses

Ablative processes for transparent materials processing still remains a hot topic, as current techniques still have their limitations in ablation rates and quality, particularly for features on the order of a millimeter or less. In this publication, we present results of recent work on laser micromachining of sapphire with ultrashort pulsed lasers in the sub-picosecond regime. We accomplish this by using bottom-up processing with laser ablation, which has demonstrated the ability to generate zero-taper features with relatively high aspect ratios (>1:1) in many transparent materials. In previous work with <1ps (1030nm) sources, we have demonstrated the ability to drill ≤250µm diameter holes in 430µm thick c-plane sapphire. Here, we extend the scope of this work to examine additional methods and laser parameters presented here. In particular we examine the benefits of frequency conversion to 515nm wavelength, allowing smaller spots and higher fluence levels is compared to trials at the fundamental wavelength. We find no compelling differences in minimum achievable taper or drilling time for processes with 1030nm vs. 515nm performed with similar fluences, but the smaller spot size and increased fluence afforded with 515nm light allows for faster bottom-up drilling. Additionally, we test the effects of water-assisted processing by placing the sapphire wafer in contact with a water bath for better extraction of particulate matter during processing. Processes with a water bath are found to allow holes with 2° taper to be drilled in ≤5 seconds, which is a decrease in cycle time of ≥50% for holes with ≤2° taper in air.