Underwater persistent bubble-assisted femtosecond laser ablation for hierarchical micro/nanostructuring

In this study, we demonstrate a technique termed underwater persistent bubble assisted femtosecond laser ablation in liquids (UPB-fs-LAL) that can greatly expand the boundaries of surface micro/nanostructuring through laser ablation because of its capability to create concentric circular macrostructures with millimeter-scale tails on silicon substrates. Long-tailed macrostructures are composed of layered fan-shaped (central angles of 45 degrees -141 degrees) hierarchical micro/nanostructures, which are produced by fan-shaped beams refracted at the mobile bubble interface (>= 50 degrees light tilt, referred to as the vertical incident direction) during UPB-fs-LAL line-by-line scanning. Marangoni flow generated during UPB-fs-LAL induces bubble movements. Fast scanning (e.g. 1 mm s(-1)) allows a long bubble movement (as long as 2 mm), while slow scanning (e.g. 0.1 mm s(-1)) prevents bubble movements. When persistent bubbles grow considerably (e.g. hundreds of microns in diameter) due to incubation effects, they become sticky and can cause both gas-phase and liquid-phase laser ablation in the central and peripheral regions of the persistent bubbles. This generates low/high/ultrahigh spatial frequency laser-induced periodic surface structures (LSFLs/HSFLs/UHSFLs) with periods of 550-900, 100-200, 40-100 nm, which produce complex hierarchical surface structures. A period of 40 nm, less than 1/25th of the laser wavelength (1030 nm), is the finest laser-induced periodic surface structures (LIPSS) ever created on silicon. The NIR-MIR reflectance/transmittance of fan-shaped hierarchical structures obtained by UPB-fs-LAL at a small line interval (5 mu m versus 10 mu m) is extremely low, due to both their extremely high light trapping capacity and absorbance characteristics, which are results of the structures' additional layers and much finer HSFLs. In the absence of persistent bubbles, only grooves covered with HSFLs with periods larger than 100 nm are produced, illustrating the unique attenuation abilities of laser properties (e.g. repetition rate, energy, incident angle, etc) by persistent bubbles with different curvatures. This research represents a straightforward and cost-effective approach to diversifying the achievable hierarchical micro/nanostructures for a multitude of applications.