Second-harmonic generations in a plasmonic two-wire transmission-line

A plasmonic two-wire transmission-line is experimentally exploited for the demonstration of second-harmonic generation (SHG) despite of material and geometric symmetries. Interestingly, regardless of the two potential modes excited at the fundamental frequency, the resulting SHGs are only coupled into the asymmetric mode. This characteristic is explained through surface nonlinear polarization calculations.