Picosecond versus femtosecond-laser ablation of silicon in atmosphere

Laser ablation (LA) using nanosecond (ns) or femtosecond (fs) pulse widths is well-established for the volatilization of a liquid or solid for applications ranging from micromachining to sampling for compositional analysis. Far less work has examined laser ablation in the intermediate picosecond regime (ps-LA), which corresponds to the approximate timescale for the transfer of energy from laser-excited electrons to the lattice. 213 and 355 nm ps-LA of silicon (Si) with Gaussian beam profiles is compared here to 800 nm fs-LA with both Gaussian and flat-top beam profiles, all performed at or above the ablation threshold with 20 000-67 000 laser pulses. The morphology and composition of the ablation spots are examined using scanning electron microscopy and energy dispersive x-ray spectroscopy (EDS), respectively. 213 nm ps-LA yields more visible nanostructures compared to those ablated by 355 nm ps-LA, but both form central craters with surrounding nanostructures due to resolidified material. The flat-top fs beam creates protruding nanostructures isolated near the rim of the crater and an inside-out umbrella-like structure at the center. The Gaussian fs-LA region displays a relatively smooth conical crater, albeit with some nanostructure at the rim of the crater. EDS finds that these nanostructures are at least partly composed of silicon oxide or suboxides. The invisibility of these nanostructures to optical profilometry is consistent with black-silicon. The ablation crater results from optical profilometry for 213 nm ps-LA are close to those for 800 nm flat-top fs-LA, and both are consistent with cylindrical craters.