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.