icantly different. The most attractive feature of the wavefield is its high localization in the metal-dielectric interfaces, leading to fast decays when moving away from the surfaces and thus forming wedge-like shapes. This is in agreement with the patterns shown in Figs. 4(a) and (b) contributing to the integral (5) at x = 0. Although the highest peak is attained at y = 0, a large one also arises on the other side of the central metallic film, y = w . There, the Bloch modes h$_{K}$ ( y ) exp ( iKy ) are as strong as in focus and they interfere nearly in-phase (dephase < 0 . 29 π rad) giving a secondary focus. Ignoring this sidelobe, the FWHM of the figure is Δ$_{y}$ = 45 . 99 nm, well below λ$_{d}$ . Apart from surface resonances, radial asymmetry from focus is also attributed to a high concentration of light along certain directions in the xy plane. This directional enhancement of the radiated power may be explained by means of an effect coined as photon focusing [35], a term which first was used onto the strong anisotropy of heat flux in crystalline solids and that takes other names in optics like self-collimation [36], self-guiding [37], and subdiffractive propagation [38]. The estimation of the value(s) of the azimuthal coordinate θ where photon focusing is manifested is based on the stationary-phase principle. Firstly note that the phase factor exp ( ik$_{x}$x + iKy ) of Eq. (5) varies rapidly when moving away from the origin, r → ∞ , being ( x, y ) = r (cos θ, sin θ ). This phase term also introduces strong oscillations for running k$_{x}$ except in the vicinities of those spatial frequencies k$_{xs}$ satisfying x + K ′ $_{s}$y = 0, where K ′ = dK/dk$_{x}$ and the subindex s stands for the value given at k$_{xs}$ . Setting K ′ s = tan φ$_{s}$ we have the solutions φ$_{s}$ = θ ± π/ 2; consequently the tangent of the curve K ( k$_{x}$ ) at the stationary points k$_{xs}$ is normal to the vector position of the observation point ( x, y ). Moreover, the asymptotic behavior of Eq. (5) depends exclusively on those stationary points k$_{xs}$ ; substituting K by K$_{s}$ in every term of the integrand excepting the phase factor for which we use a quadratic expansion K ≈ K$_{s}$ + K ′ $_{s}$( k$_{x}$ − k$_{xs}$ ) + K ′′ $_{s}$( k$_{x}$ − k$_{xs}$ ) $^{2}$/ 2, finally yields where 12 h$_{x}$ ( x, y ) ≈ ∑ K,k$_{xs}$ a$_{K}$$_{s}$h$_{K}$$_{s}$ ( y ) exp ( ik$_{xs}$x + iK$_{s}$y ) I$_{K}$$_{s}$ ( y ) , (28) I$_{K}$$_{s}$ ( y ) = ∫ ∞ −∞ exp [ iK ′′ $_{s}$( k$_{x}$ − k$_{xs}$ ) $^{2}$y/ 2 ] dk$_{x}$ = √ 2 π | K ′′ $_{s}$y | exp [ i π 4 sign ( K ′′ $_{s}$y ) . (29)