Unidirectional launching of surface plasmons with subwavelength metallic gratings around the plasmonic critical angle

We find and verify that unidirectional surface plasmon polaritons (SPPs) are highly excited by subwavelength metallic gratings with back-side illumination around the plasmonic critical angle (PCA). The conditions and physical origins of the SPP excitation around the PCA are clarified. Moreover, the experimental results support our numerical calculations very well. Potential applications are also suggested.