Compact Plasmonic Unidirectional Coupler With High Performance Based on an Asymmetric Slit-Groove Structure

A new scheme for unidirectional coupling of surface plasmon polaritons (SPPs) from freely propagating light into a metal-insulator-metal waveguide is proposed, which is composed of an asymmetric double-slit of different widths sandwiched by symmetric grooves. A theoretical model based on coupled-mode equations is developed to facilitate structure design and elucidate the underlying physical mechanism. With the model, the asymmetric double-slit with different widths is designed for unidirectional launching of SPPs with an ultrahigh extinction ratio of 44.8 dB at the wavelength 800 nm. A method of stepwise design is proposed for simplicity of design and convenience of analysis. Proof-of-principle demonstrations show that an ultrahigh extinction ratio of 37.1 dB and significant field enhancement for the proposed whole structure are obtained. In addition, an ultracompact plasmonic dichroic splitter based on the asymmetric double-slit with high extinction ratios of 24.2 and 21.8 dB at splitting wavelengths of 800 and 900 nm is achieved, and the lateral dimension of the plasmonic dichroic splitter is only 760 nm.