Polarization dependent nanostructuring of silicon with femtosecond vortex pulse

We fabricated conical nanostructures on silicon with a tip dimension of similar to 70 nm using a single twisted femtosecond light pulse carrying orbital angular momentum (l = +/- 1). The height of the nano-cone, encircled by a smooth rim, increased from similar to 350 nm to similar to 1 mu m with the pulse energy and number of pulses, whereas the apex angle remained constant. The nano-cone height was independent of the helicity of the twisted light; however, it is reduced for linear polarization compared to circular at higher pulse energies. Fluid dynamics simulations show nano-cones formation when compressive forces arising from the radial inward motion of the molten material push it perpendicular to the surface and undergo re-solidification. Simultaneously, the radial outward motion of the molten material re-solidifies after reaching the cold boundary to form a rim. Overlapping of two irradiated spots conforms to the fluid dynamics model. (C) 2017 Author(s).