Optical nanofocusing on tapered metallic waveguides

Tapered metal wires show a remarkable ability to 'squeeze' the lateral extent of a propagating surface-plasmon-polariton mode as it travels toward the tip of the taper. The transformation can be continued well below the diffraction limit to terminate at a nanoscale apex where intense near-fields are created. We perform the first full numerical simulations to investigate and quantify this phenomenon. We find optimal angles for maximal tip-field enhancement on conical wires by considering absorption, scattering to radiation and reflection. The optimal parameters we obtain contradict the conditions for adiabatic tapering, thereby advocating the use of numerical simulations. Despite the influence of losses, nanofocusing is still highly efficient for a broad range of practical metals, visible wavelengths and taper geometries. Diverse nano-optic applications can benefit directly and significantly from the results.