Nanoneedle formation via doughnut beam-induced Marangoni effects

Recently, nanosecond pulsed optical vortices enables the production of a unique chiral and sharp needle-like nanostructure (nano-needle). However, the formation process of these structures has been unsolved although mass transport by angular momentum would contribute to the chirality. Here, we reveal that another key factor in the formation of a sharp nano-needle is the Marangoni effect during the melting condition at high temperature. Remarkably, the thickness and height of the nano-needle can be precisely controlled within 200 nm, corresponding to 1/25 of beam radius (5 µm) beyond the diffraction limit by ring-shaped inhomogeneous temperature rise. Our finding will facilitate the development of advanced nano-processing with a variety of structured light beams.