Shape effects in tapered metal rods during adiabatic nanofocusing of plasmons

We studied adiabatic nanofocusing of strongly localized surface plasmons in tapered metal rods with parabolic perturbations of the conical shape of the taper. Typical plasmon propagation parameters and local field enhancements are determined along the tapered rod, depending on the shape and plasmon frequency (wavelength). In particular, it is demonstrated that metal rods with tapered convex shape are more efficient in achieving larger local field enhancements at the tip, as compared to rods with the concave shape of the taper. This is explained by weaker dissipative losses in tapered rods with convex shape. In addition, plasmon nanofocusing in tapered rods with convex shape is less sensitive to further variations of the shape. As a result, it is demonstrated that tapered rods with the convex shape could be advantageous for practical applications where strong local field enhancement near the tip is required, and/or where metal rods with relatively large dissipation are used. The obtained results will be important for optimization and practical fabrication of efficient nanofocusing structures using tapered metal rods.