Liquid vortexes and flows induced by femtosecond laser ablation in liquid governing formation of circular and crisscross LIPSS

Orientations of laser induced periodic surface structures (LIPSS) are usually considered to be governed by the laser polarization state. In this work, we unveil that fluid dynamics induced by femtosecond (fs) laser ablation in liquid (fs-LAL) can easily break this polarization restriction to produce irregular circular-LIPSS (CLIPPS) and crisscross-LIPSS (CCLIPSS). Fs laser ablation of silicon in water shows formation of diverse LIPSS depending on ablation conditions. At a high power of 700 mW (repetition rate of 100 kHz, pulse duration of 457 fs and wavelength of 1045 nm), single/twin CLIPSS are produced at the bottom of macropores of several microns in diameter due to the formation of strong liquid vortexes and occurrence of the vortex shedding effect. Theoretical simulations validate our speculation about the formation of liquid vortex with an ultrahigh static pressure, which can induce the microstructure trenches and cracks at the sidewalls for fs-LAL of Si and tungsten (W) in water, respectively. At a low power of 50 mW, weak liquid vortexes are produced, which only give birth to curved LIPSS in the valleys of grooves. Consequently, it is deduced that liquid vortex plays a crucial role in the formation of macropores. Mountain-like microstructures induce complex fluid dynamics which can cause the formation of CCLIPSS on them. It is believed that liquid vortexes and fluid dynamics presented in this work open up new possibilities to diversify the morphologies of LIPSS formed by fs-LAL.