Direct fabricating large-area nanotriangle structure arrays on tungsten surface by nonlinear lithography of two femtosecond laser beams

Abstract

Two-dimensional arrays of periodic nanostructures are fabricated on bulk tungsten surface within a single step using collinear propagation of two time-delayed femtosecond laser beams with orthogonal polarizations. It is surprisingly found that the geometric profile of the structure unit exhibits a triangle shape in hundred nanometer scales, and its spatial dimension can be modulated by the ambient air pressure ranging from 1 atm to 10^-3 Pa. As the ambient air pressure decreases, the obtained surface structures display a large depth covered with nanowires. Physically, the formation of such triangle structures is originated from the transient physical correlations between the two laser-matter interaction processes, and also affected by the heat transfer effects of the surrounding air. In addition, the experimental measurements reveal that the minimum reflectivity of the nanotriangle surface structures is unprecedentedly reduced to as low as ~2.9% especially within the visible-infrared range.