Symmetry-Forbidden Second-Harmonic Generation in a Fully Centro-Symmetric Plasmonic Nanocircuit

All nonlinear optics textbooks state that second-harmonic generation requires a non-centro-symmetric crystal structure in bulk. In the last decade, asymmetric nanoparticles of centro-symmetric material have been used to generate second-harmonic. We introduce a third way of breaking the symmetry, by a structure that offers suitable optical modes which are not plane waves.