Role of an Extended Filamentation Focus during the Ultrashort-Pulse Laser Ablation of the Silicon Surface in an Aqueous Medium

The laser ablation of a silicon target in distilled water is carried out by single femto- and picosecond pulses upon focusing with an objective lens with a numerical aperture NA = 0.25 as a function of the focus position relative to the target surface. A plasma channel in water is visualized for the given numerical aperture used. The ablation surface relief is characterized using scanning and probe electron microscopy. The maximum crater depths and volumes are detected and analyzed as functions of the position of the linear and extended nonlinear foci.