High-aspect-ratio crack-free microstructures fabrication on sapphire by femtosecond laser ablation

We report a novel laser processing strategy to realize high-aspect-ratio crack-free microstructures fabrication on sapphire substrates by combining femtosecond laser-induced plasma-assisted ablation and subsequent direct laser ablation. The laser-induced plasma assisted ablation allows us to create a precursor for microstructure generation on smooth sapphire at a low laser fluence. Then we demonstrate a precise control of the direct laser ablation operating at a critical fluence to deepen the previous microstructures. The precise control and optimization of the processing parameters are able to effectively reduce laser heating and avoid crack formation inside the substrate. Crack-free and taper-free micro-grooves are achieved on sapphire with a maximum high aspect-ratio up to 10:1. The capability of this method to fabricate a variety of functional microstructures is also experimentally demonstrated.