Diffraction by a small aperture in conical geometry: Application to metal-coated tips used in near-field scanning optical microscopy
- 2 April 2002
- journal article
- research article
- Published by American Physical Society (APS) in Physical Review E
- Vol. 65 (4), 046611
- https://doi.org/10.1103/physreve.65.046611
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 has remained unexplained so far—is recovered in our analysis, which goes beyond the two-dipole approximation.Keywords
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