Ultrafine Fiber Tip Etched in Hydrophobic Polymer Coated Tube for Near-Field Scanning Plasmonic Probe

Abstract

We propose a novel one-step etching technique for the preparation of an optical fiber tip using a hydrophobic polymer coated tube (HP) containing 48% hydrofluoric acid (HF) to etch photosensitive single-mode fiber (PSMF). Dipping of PSMF in an HP tube suppresses the convex meniscus (CM) of the HF solution in the tube due to formation of a concave surface tension meniscus (SM) around the PSMF. The continuous etching reduces the diameter of PSMF and shortens the SM along PSMF resulting in elevation of CM around PSMF. The elevation of the CM and faster etching rate of the PSMF core digs a capillary ring along the fiber core-cladding boundary. Capillary action fills the ring with HF, which finally recedes and creates a tip at the apex of the fiber. We have fabricated a typical tip with typical features of tip height ~ 4 μm, base diameter ~ 3 μm , cone angle ~ 25°, and aperture ~ 30 nm. For the theoretical analysis, a tip similar to the fabricated one is modeled to estimate surface plasmon (SP) enhancement, ~ 200, at the end of the tip with a silver coating.