Structurally-tolerant vertical directional coupling between metal-insulator-metal plasmonic waveguide and silicon dielectric waveguide

Vertical directional coupling between a metal-insulator-metal (MIM) plasmonic waveguide and a conventional dielectric waveguide is investigated. The coupling length, extinction ratio, insertion loss and coupling efficiency of the hybrid coupler are analyzed. As an example, when the separation between the two waveguides is 250 nm, a maximum coupling efficiency of 73%, an insertion loss of -1.4 dB and an extinction ratio of 16 dB can be achieved at a coupling length of 4.5 microm at 1.55 microm wavelength. A particular feature of this hybrid coupler is that it is highly tolerant to the structural parameters of the plasmonic waveguide and the misalignment between the two waveguides. The performance of this hybrid coupler as a TM polarizer is also analyzed and a maximum extinction ratio of 44 dB and an insertion loss of -0.18 dB can be obtained. The application of this hybrid coupler includes the signal routing between plasmonic waveguides and dielectric waveguides in photonic integrated circuits and the polarization control between TE and TM modes. In addition, it provides an approach for efficiently exciting MIM plasmonic modes with conventional dielectric modes.