Extension of Bethe’s diffraction model to conical Geometry : application to near field optics A. Drezet, J. C. Woehl, and S. Huant Laboratoire de Spectromtrie Physique, CNRS UMR5588, Universit´e Joseph Fourier Grenoble, BP 87, 38402 Saint Martin d’Hres cedex, France (Dated: October 31, 2018) 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. Abstract arXiv:1002.0951v1 [physics.optics] 4 Feb 2010 1