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arXiv · 1002.2533

Diffraction by a small aperture in conical geometry: Application to metal coated tips used in near-field scanning optical microscopy

A. Drezet·J. C. Woehl·S. Huant
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Abstract

Light diffraction through a subwavelength aperture located at the apex of a metallic screen with conical geometry is investigated theoretically. A method based on a multipole field expansion is developed to solve Maxwell's equations analytically using boundary conditions adapted both for the conical geometry and for the finite conductivity of a real metal. The topological properties of the diffracted field are discussed in detail and compared to those of the field diffracted through a small aperture in a flat screen, i. e. the Bethe problem. The model is applied to coated, conically tapered optical fiber tips that are used in Near-Field Scanning Optical Microscopy. It is demonstrated that such tips behave over a large portion of space like a simple combination of two effective dipoles located in the apex plane (an electric dipole and a magnetic dipole parallel to the incident fields at the apex) whose exact expressions are determined. However, the large "backward" emission in the P plane - a salient experimental fact that remained unexplained so far - is recovered in our analysis which goes beyond the two-dipole approximation.

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A. Drezet, J. C. Woehl, S. Huant. 2010-02-12. Diffraction by a small aperture in conical geometry: Application to metal coated tips used in near-field scanning optical microscopy. https://doi.org/10.1103/physreve.65.046611

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