Light funneling mechanism explained by magneto-electric interference

We investigate the mechanisms involved in the funneling of the optical energy into sub-wavelength grooves etched on a metallic surface. The key phenomenon is unveiled thanks to the decomposition of the electromagnetic field into its propagative and evanescent parts. We unambiguously show that the funneling is not due to plasmonic waves flowing toward the grooves, but rather to the magneto-electric interference of the incident wave with the evanescent field, this field being mainly due to the resonant wave escaping from the groove.