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

摘要

Light diffraction through a subwavelength aperture located at the apex of ametallic screen with conical geometry is investigated theoretically. A methodbased on a multipole field expansion is developed to solve Maxwell's equationsanalytically using boundary conditions adapted both for the conical geometryand for the finite conductivity of a real metal. The topological properties ofthe diffracted field are discussed in detail and compared to those of the fielddiffracted 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 areused in Near-Field Scanning Optical Microscopy. It is demonstrated that suchtips behave over a large portion of space like a simple combination of twoeffective dipoles located in the apex plane (an electric dipole and a magneticdipole parallel to the incident fields at the apex) whose exact expressions aredetermined. However, the large "backward" emission in the P plane - a salientexperimental fact that remained unexplained so far - is recovered in ouranalysis which goes beyond the two-dipole approximation.