Optical fibre nanotips fabricated by a dynamic chemical etching for sensing applications

Abstract

Nanoprobe tips are key components in many applications such as scanning probe microscopes, nanoscale imaging, nanofabrication and sensing. This paper describes a dynamic chemical etching method for the fabrication of optical nanoprobes. The tips are produced by mechanically rotating and dipping a silica optical fibre in a chemical etching solution (aqueous hydrofluoric acid) covered with a protection layer. Using different dynamic regimes of the mechanical movements during the chemical etching process, it is possible to vary the cone angle, the shape, and the roughness of the nanoprobes. It is found that the tip profiles are determined by the nonlinear dynamic evolution of the meniscus of the etchant near the fibre. Computational fluid dynamic simulations have been performed, showing that different flow regimes correspond to different shear forces acting on the forming nanotip, in agreement with experimental results. With this method, a high yield of reproducible nanotips can be obtained, thus overcoming the drawbacks of conventional etching techniques. Typical tip features are short taper length (∼200 μm), large cone angle (up to 40°), and small probe tip dimension (less than 30 nm).

On the CFD Analysis of a Stratified Taylor-Couette System Dedicated to the Fabrication of Nanosensors

“…As previously stated, it is a 3D closed domain with rotating boundaries that reproduces the experimental setup of [13]. Half of the vial is filled with an aqueous solution of hydrofluoric acid (HF) which has a density of 970 kg/m 3 and a dynamic viscosity of about 0.00092 Pa·s.…”

“…Rotational velocity influences also surface roughness: lower ripples are present in the fast rotation case, with a decrease from 10 nm to 7 nm in the averaged root-mean-square roughness with respect to the slow case. The present work proposes an extension of the research described in [13], hereafter referred to as previous work, where the innovative methodology for fibers nanotips production was introduced and a preliminary fluid dynamic analysis of the system performed. The aim is to use Computational Fluid Dynamics (CFD) to improve the knowledge about the fluid dynamic behavior of the mechanical-chemical etching apparatus based on Taylor-Couette systems characterized by unconventional radius ratios with respect to what is found in literature (for the present case, radius ratio and aspect ratio before the etching process are respectively η = 0.0417 and Γ = 1.27).…”

“…Results have shown potential in terms of characteristics control. Figure 1, which derives from a previous work by the same research group [13], shows the comparison between different regimes of fiber and vial rotation, generically referred to as slow and fast rotation (these generically fast and slow rotation regimes are not to be confused with the TCN1 and TCN2 cases that will be further introduced). Figure 1a,b show a linear and a curved profile obtained respectively by slow and fast rotation.…”

“…Moreover, some of these specific applications has motivated and driven the research towards more realistic analysis. An example of this interaction is represented by the work of Ali et al [12], where the linear stability analysis accounts for the effects of a dilute suspension of spherical particles, inspired by rotating filtration devices, and by the work of Barucci et al [13], where the setup of a realistic fibers nanotips production device is numerically simulated.…”

“…The innovative method called mechanical-chemical etching (or dynamic etching) [11] is based on a combination of mechanical movements, which, coupled with chemical etching, allows better control of the shape and roughness of a fiber optical nanoprobe (for more details see [13]). The basic idea is to use rotation movements to generate different kinds of flows inside a vial, which, modifying the shear stresses acting on the fiber due to fluid viscosity and the diffusivity of the system, should allow to control shape and surface characteristics of the obtained nanotip, formed on the fiber inserted into the vial.…”