  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.
