Enhancing second-harmonic generation using dipolar-parity modes in non-planar plasmonic nanocavities

There is an active demand to develop efficient nanoscale nonlinear sources for applications in photonic circuitry, quantum optics, and biosensing. To this end, plasmonic systems have been utilized to boost the nonlinear signal generation; however, high efficiencies of frequency conversion for realistic applications remain a challenge. Metal-insulator-metal (MIM) nanocavities are good candidates for strongly concentrating the fields at the nanoscale to enhance the optical nonlinearities; however, they typically suffer from the requirement to have a quadrupolar resonance at the emission wavelength. Here we introduce non-planar MIM nanocavities with a nonlinear spacer that can strongly enhance the second-harmonic generation (SHG), despite having fundamental and emission modes of the same parity. Our experimental and numerical results indicate that the enhancement is due to the non-planar design of the cavities and the bulk nonlinearity of the spacer layer.