Efficiently Unidirectional Coupling of Surface Plasmons Based on Asymmetric Cascaded Nanoslits

A compact unidirectional plasmonic coupler based on asymmetric cascaded nanoslits is proposed and theoretically analyzed. The incident photons illuminated on the structure are unidirectionally coupled into the metal–insulator–metal waveguide. The operation principle is clarified and a universal theoretical model based on coupled-mode equations is developed to facilitate structure design. Proof-of-principle demonstrations show that an ultrahigh extinction ratio ER $= 43.8$ dB at a specific wavelength and broadband of 160 nm covering a wide angular range from −20° to 15° with ER ≥ 10 dB are achieved.