Dynamical chemical etching for fabrication of optical fibre nanotips

Optical fibre nanotips are the key components in many applications such as nanoscale imaging and sensing. In this paper we describe and study a dynamic chemical etching method for the fabrication of optical nanoprobes. Our method allows controlling the shape and roughness of a fibre optical nanoprobe, by a combination of mechanical movements coupled to chemical etching. The stripped distal end of an optical fibre-end is dipped into a vial containing an aqueous hydrofluoric acid solution covered with a protection layer. The vial and the optical fibre are connected to different motors allowing them to rotate independently around the fibre and the vial axis respectively. The basic idea of our method is to use rotation movements to generate different kind of flows inside the vial, which will lead to different kind of shapes and surface characteristics of the obtained nanoprobes. Different regimes can be generated by changing the ratio between the angular velocities and the ratio between the radii of optical fibre and vial, ranging from laminar flow to the onset of chaotic flow (Taylor-Couette flow theory). Computational fluid dynamic analysis show that different flow regimes correspond to different shear forces acting on the forming nanotip, in agreement with experimental results.