Mechanism of the optical unidirectional transmission in metal subwavelength grating with different surfaces

The mechanism of the unidirectional transmission in metal subwavelength gratings with different surfaces is investigated theoretically. This kind of unidirectional transmission belongs to the extraordinary optical transmission assisted by the surface plasmon polariton (SPP). The SPP wave comes through the multi-reflection process and finally transforms into the transmission wave. The difference of amplitude transmissivities and reflectivities at different surfaces in opposite incident directions results in the unidirectional transmission. A division method is proposed to calculate the parameters of subwavelength gratings with asymmetric surfaces.