Undirectional launcher of surface plasmon polaritons based on subwavelength slits with side-illumination and backside-illumination

We propose a metal-insulator-metal (MIM) model of undirectional launcher of surface plasmon polaritons (SPPs) based on subwavelength slits with side-illumination and backside-illumination in theory. The electromagnetic field intensities along one direction can be enhanced or suppressed by surface plasmon interference. The periodical change of the splitting ratio is analyzed. The high-efficiency undirectional propagation of SPPs is achieved by utilizing the adjusted double two slits structure with side-illumination. The best splitting ratio is about 30. The finite element method (FEM) illustrates our theoretical design.