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Compact and efficient 2D and 3D designs for photonic-to-plasmonic coupler

Two ultra-compact designs of wideband photonic to metal-dielectric-metal (MDM) couplers are introduced. The two proposed couplers show a high efficiency of 92% and 94% at unexpected very small coupling lengths, using MDM as a coupling waveguide. Emerging from the half-wavelength transformer concept, when the characteristic impedances of the coupled waveguides are equal in magnitude but have a phase difference, we report a slight shift in the coupler length of the first maximum. Relying on this concept, the lengths of the proposed couplers are 10 nm and 15 nm, respectively, to match the phase shift. The proposed couplers also enjoy a very wide bandwidth of 1785 nm for the 10 nm design and 1865 nm for the 15 nm design because of the short coupling length. Also, we introduce a 3D design for the coupler based on a quasi-MDM waveguide of a 20 nm silver layer to facilitate the fabrication process. The simulation results are produced by 2D finite element methods and verified by 3D finite difference time domain, but with incomparable computational cost.