Extension of Bethe's diffraction model to conical Geometry: application to near field optics

TL;DR

This paper extends Bethe's diffraction model to conical geometries, providing an analytical approach that accurately predicts near-field optical behavior of coated fiber tips used in microscopy.

Contribution

It introduces a simple multipole expansion model for small apertures in conical geometries, enhancing the understanding of light diffraction in near-field optics.

Findings

Model reproduces experimental results

Insensitive to cone angle and aperture radius

Includes empirical two-dipole model as a first approximation

Abstract

The generality of the Bethe's two dipole model for light diffraction through a subwavelength aperture in a conducting plane is studied in the radiation zone for coated conical fiber tips as those used in near field scanning optical microscopy. In order to describe the angular radiated power of the tip theoretically, we present a simple, analytical model for small apertures (radius < 40 nm) based on a multipole expansion. Our model is able to reproduce the available experimental results. It proves relatively insensitive to cone angle and aperture radius and contains, as a first approximation, the empirical two-dipole model proposed earlier.