FIG. 2: An idealized metal-coated fiber tip with its characteristic funnel shape. The top angle β is inferior to 15 ◦ , and the aperture diameter 2 a is of the same order of magnitude as the aluminum coating thickness, i. e., 100 nm. FIG. 3: The electric (a) and magnetic (b) fields in the near-field zone of a two-dimensional cone, calculated rigourously with adapted complex potentials. The assumption is made that the topology of the 3D problem can be obtained by deforming the 2D case. In the 3D case, (a) and (b) planes are perpendicular to each other. In addition, the magnetic field in (a) and the electric field in (b) are perpendicular to both planes for symmetry reasons. 18