Ultrafast laser ablation of silicon with ∼GHz bursts

The authors report on processing silicon with bursts of hundreds of subpicosecond pulses with an intraburst pulse repetition frequency of 0.86 GHz at 515 nm. They find that the burst-to-burst overlap is a key parameter in optimizing the ablation efficiency for the line scribing and milling processes, contrary to traditional nonburst ultrafast processes. A nanosecond laser reference experiment and the corresponding multiphase simulations demonstrate that this behavior is directly related to the hydrodynamic effects of the molten material generated during the laser processing. Exploring the hole and scribe morphology with scanning transmission electron microscopy and selective area electron diffraction yields a surprise: holes show no sign of a polycrystalline or amorphous heat affected zone, while scribed lines clearly do. The multiphase modeling provides a likely explanation-it is not "ablation cooling."