particle. We can then consider that the incident light “sees” the hole as a ( P , M ) combi- nation. The general form of Eq. 1 is certainly conserved for various quasi circular opening in planar geometry and must essentially be independent on the aperture shape of subwave- length holes. As a good staring point we can anticipate the generality of this result, and we can admit that in any problem of diffraction by subwavelength holes in non planar screens the emission zone can be described by dipoles terms similar to Eq. 1. Nevertheless, numerous differences certainly arise from boundary effects when we look at the field near the metallic surface. In particular, the dipole orientations may change with the screen geometry as we will show later on. Model - In the case of an NSOM fiber tip, the aperture is situated on the apex of a conical, metal-coated structure (see Fig. 1). For eigenvalue-problems of conical symmetry around some origin, it is convenient to express the fundamental solutions in spherical coordinates ( r, θ, φ ). The separation of the angular and radial variables in the Helmholtz equation fol- lows the “Hankel(r) · spherical-harmonics( θ, φ )” solution[ ? ] ψ ( r ) = h$_{ν}$ ( kr ) · Y$_{ν,m}$ ( θ, φ ) with the modification that in this geometry the parameter ν can have a non-integer value which is determined by the boundary conditions. In analogy to the method of multipole expan- sions with spherical harmonics used in free space, we can develop our propagating field as an expansion in electric and magnetic “quasi-multipoles”. The principal difference to the classical multipole expansion[9] consists in the substitution l → ν . We find that in general, a propagating electromagnetic field in conical geometry produces an angular distribution of radiated power of the form with L = 1 $_{i}$r × ∇ . Here, ν$_{E}$ and ν$_{M}$ refer to the electric (TM) and magnetic (TE) quasi- multipoles, respectively. The coefficients a$_{ν,m}$ depend on charge and current distribution on the metallic cone. It can be shown that for two roots ν and ν ′ (with ν ≤ ν $^{′}$), the ratio a$_{ν}$ ′ /a$_{ν}$ is comparable to ( a/λ ) ν $^{′}$− ν ≪ 1 which can be neglected. Therefore, we can omit all terms in the expansion except for the first two roots ν$_{E,}$$_{0}$ , and ν$_{M,}$$_{0}$ . Specific boundary conditions can be written for a good metal such as aluminum. They depend only on its electric permittivity ϵ ( ω ) ( ≃ − 34 . 5 + 8 . 5 i for λ = 488 nm). If we dP d Ω = c 8 πk 2 · ‖ ( − i ) ν$_{E}$ +1 ∑ ν$_{E}$,m a ( E ) ν$_{E}$$_{,m}$. L Y$_{ν}$$_{E}$$_{,m}$ ( θ, φ ) √ ν$_{E}$ ( ν$_{E}$ + 1) × e$_{r}$ + ( − i ) ν$_{M}$ +1 ∑ ν$_{M}$,m a ( M ) ν$_{M}$ ,m · L Y$_{ν}$$_{M}$$_{,m}$ ( θ, φ ) √ ν$_{M}$ ( ν$_{M}$ $_{+ 1)}$‖ $^{2}$, (2) 4