Compact slit-based couplers for metal-dielectric-metal plasmonic waveguides.

We introduce compact wavelength-scale slit-based structures for coupling free space light into metal-dielectric-metal (MDM) subwave-length plasmonic waveguides. We first show that for a single slit structure the coupling efficiency is limited by a trade-off between the light power coupled into the slit, and the transmission of the slit-MDM waveguide junction. We next consider a two-section slit structure, and show that for such a structure the upper slit section enhances the coupling of the incident light into the lower slit section. The optimized two-section slit structure results in ∼ 2.3 times enhancement of the coupling into the MDM plasmonic waveguide compared to the optimized single-slit structure. We finally consider a symmetric double-slit structure, and show that for such a structure the surface plasmons excited at the metal-air interfaces are partially coupled into the slits. Thus, the coupling of the incident light into the slits is greatly enhanced, and the optimized double-slit structure results in ∼ 3.3 times coupling enhancement compared to the optimized single-slit structure. In all cases the coupler response is broadband.