Optical nanofocusing by tapering coupled photonic-plasmonic waveguides.

We have numerically designed a compact structure to concentrate optical waves from a silicon-on-insulator waveguide to a deep sub-wavelength and high intensity surface plasmon focal spot at the tip of a metallic strip taper. A systematic design approach is developed to obtain an optimal photon-surface plasmon side coupling efficiency of up to 50% at 1.55 μm wavelength, and an over 50 times increase in electric field intensity in a focal region of around 20nm × 20nm × 7nm in size. The length of the whole device is 2.2 μm. We expect fabrication of the proposed device to be simpler than devices in previous work.