The influence of focus position on photonic nanojet formation for sub-micrometer scale pulsed laser ablation in water medium

The photonic nanojet (PNJ) has demonstrated the ability to break the optical diffraction limit, which restricts the use of conventional laser ablation processes in sub-micrometer machining. Therefore, by applying high laser energy to the dielectric microparticle, the generated PNJ can reach the ablation threshold of the targeted material while maintaining its small beam size. In a liquid medium, the PNJ propagates over a longer distance, with its peak intensity generated at a distance from the microparticle. By controlling the position of laser focusing on the microsphere, machining was performed on silicon underwater. Our results suggest that by controlling the laser focusing position, a very long depth of focus for PNJ machining was obtained. These results will aid better surface patterning using the PNJ mechanism.