Optimization of second-harmonic generation from touching plasmonic wires

We employ transformation optics to optimize the generic nonlinear wave interaction of second-harmonic generation (SHG) from a pair of touching metallic wires. We demonstrate a six orders of magnitude increase in efficiency with respect to SHG from a single wire, and a ten orders of magnitude increase in the second-harmonic scattered power by increasing the background permittivity. These results have clear implications for the design of nanostructured metallic frequency-conversion devices. Finally, we exploit our analytic solution of a nontrivial nanophotonic geometry as a platform for performing a critical comparison of the strengths, weaknesses, and validity of other prevailing theoretical approaches previously employed for nonlinear wave interactions at the nanoscale.