Cavity model. (a) Sketch representing the cavity model including the reflection coefficients of the gap-plasmon on the edges of the patch. (b) Map of the magnetic field (modulus) for a single 75 nm wide square patch, separated from the metallic film by a 10 nm thick dielectric layer with an optical index of 1.54 illuminated from above in normal incidence at resonance. The field profile in the region which is enclosed in the white rectangle is accurately predicted by the model (center). The inset shows the map of the magnetic field (modulus) as predicted by the cavity model when both excitation coefficients A
are taken equal, at resonance.