  We propose a new mechanism for irreversibility of light assisted by surface
plasmon polaritons (SPPs). By achieving the different mutual conversion
efficiency between the radiation field and the SPPs on the opposite surfaces of
metal membrane, the unidirectional transmission is observed in a quite simple
asymmetric metallic sub-wavelength structures Ag(15nm)/AAO, in which the
difference of the two-way transmittance is large than 50%.
