Funnel-Shaped Arrays of Metal Nano-Cylinders for Nano-Focusing

We analyse funnel-shaped arrays of metal nanocylinders that can be potentially used as waveguides for nano-focusing of light. The proposed structures consist of Ag nanocylinders with gradually changed radii and discrete spacing arranged like side-view funnels with different angles. Finite-difference and time-domain simulations demonstrate that the proposed structures with different spacings and funnel angles have versatile light propagation characteristics. These structures can focus the incident Gaussian light beam (200 nm at its full width at half maximum) into beam sizes of 10 nm, 15 nm, and 20 nm, respectively, which corresponds to the transmission efficiencies of 38%, 40%, and 80%.