Near-Complete Radiation of Plasmonic Mode From Nano-Slit to Free Space

We propose here an optical antenna structure that can efficiently out-couple metal-insulator-metal plasmonic mode to free space. For high emission efficiency, we focus on suppressing backward reflection and leakage to nonradiating surface mode. To block nonradiating surface mode, we design a trench structure at the exit plane of the nanoslit. By properly balancing the reflectance at the nanoslit exit and additional reflectance due to the returning surface mode, near-zero reflection characteristics are obtained by destructive interference. The design rules for parameter optimization are derived by using the scattering coefficients of the nanoslit and the trench. The proposed scheme can be used to determine the conditions for near-complete transmission through subwavelength slits.