Efficient TE-polarized mode coupling between a plasmonic tunnel junction and a photonic waveguide

Nanoscale plasmonic gaps are useful structures both electrically, for creating quantum tunnel junctions, and optically, for confining light. Inelastic tunneling of electrons in a tunnel junction is an attractive source of light due to the ultrafast response rate granted by the tunneling time of electrons in the system as well as the compact dimensions. A main hurdle for these light emitting tunnel junctions, however, is their low external efficiency given by both low electron-to-plasmon conversion as well as low plasmon-to-photon conversion. Inversely, coupling light into a nanogap for high confinement and field enhancement can be difficult due to the size mismatches involved. We show a 3 nm gap metal-insulator-metal plasmonic tunnel junction evanescently coupled to the fundamental TE mode of a standard silicon waveguide in a tapered directional coupler configuration with a transmission efficiency of 54.8% at λ =1.55 μ m and a 3-dB coupling bandwidth of 705 nm. In the inverse configuration, we show an electric field enhancement of | E |/| E 0 | ≈120 within a plasmonic tunnel junction in the technologically important optical telecommunications band.