Directive antenna nanocoupler to plasmonic gap waveguides.

Optical nanocouplers matching a fiber microwaveguide to a plasmonic nanowaveguide are essential components for practical applications of nanophotonic systems. In this Letter we design an efficient nanocoupler using a directive nanoantenna in the visible range λ0=0.65 μm. The antenna has been optimized both in the configuration and in the matching element in order to achieve the maximum coupling efficiency (CE). In spite of the reduced size of the nanoantenna in comparison with the impinging fiber spot size (due to the localized plasmonic resonance at such high frequency), we have shown that a good efficiency can be achieved, improving the CE by a factor of 10 with respect to the nondirective antenna nanocouplers.