  Recent works dealt with the optical transmission on arrays of subwavelength
holes perforated in a thick metallic film. We have performed simulations which
quantitatively agree with experimental results and which unambiguously evidence
that the extraordinary transmission is due to the excitation of a
surface-plasmon-polariton (SPP) mode on the metallic film interfaces. We
identify this SPP mode and show that its near-field possesses a hybrid
character, gathering collective and localised effects which are both essential
for the transmission.
