In order to calculate a$_{λ}$ , we need to know the local field B (0) , E (0) at the center of the aperture. We would like to mention that a rigorous representation of the near-field can be drawn for a two-dimensional tip as shown in Fig. 3. Such a tip is described by magnetostatic and electrostatic potentials which can be found using the well-known complex potential method valid for the Laplace equation in two dimensions [29]. Numerical calculations by Novotny et al . [30] shows the same field behavior. The field topology in the 3D case can be anticipated from this 2D result. The choice of this field topology is based on the assumption that the mode entering the conical part of the tip is the one in the preceeding hollow, circular cylinder. This is principally the polarized TE11 mode used in near-field microscopy. Turning back to the 3D case, we remark that, despite the lack of a rigorous analytical model, the near-field can be computed numerically using different methods. These calculations confirm the intuitive field topology obtained in our 2D result. Using our 2D model and numerical computations obtained by other authors, we postulate that the unknown near- field components are linked to each other by the “plane wave” relation : where z is the cone symmetry axis. This condition may be intuitively understood by as- suming that the incident light propagating in the fiber is linearly polarized. This means that the symmetry of the field is conserved at the aperture as shown in Fig. 3. The elec- tric and magnetic amplitudes at the center of the aperture domain (i. e. at the top of the cone) can be determined from the Poynting theorem[18] which imposes the equality ∫ V ‖ B ‖ 2 = ∫ V ‖ E ‖ 2 − i c ω ∮ ( E × B ∗ ) r rd Ω in the spherical domain of the aperture zone. If the surface integral is equal to zero in this formula, i. e. if most of the energy is either transmitted or reflected but not stocked or dissipated in the near-field zone, we obtain E (0) ≃ B (0) as a good approximation. Hence, with this symmetry condition, the field is entirely determined and the calculation shows that the only non-vanishing terms have m = 1. The normalized angular distribution of radiated power is then in which only the ν values ν$_{E}$ and ν$_{M}$ have been considered . It is worth noting here that our derivation of Eq. 17 can be obtained in a similar way by means 1 P dP d Ω ≃ 2 5 ∥ L Y$_{ν}$$_{E}$$_{,}$$_{1}$ ( θ, φ ) √ ν$_{E}$ ( ν$_{E}$ + 1) × ˆ r + 2Re L Y$_{ν}$$_{M}$$_{,}$$_{1}$ ( θ, φ ) √ ν$_{M}$ ( ν$_{M}$ $_{+ 1)}$∥ ∥ , (17) 〈 E 〉 = 〈 B 〉 × ˆ z , (16) 9