Clarifying the origin of third-harmonic generation from film-coupled nanostripes

The resonance associated with plasmonic nanostructures strongly enhances local optical fields, and can thus dramatically enhance the nonlinear response of the composite structure. However, the origin of the nonlinear signal generated from hybrid nanostructures consisting of both metallic and dielectric components can be ambiguous when all constituents possess nonlinearities. In this paper, we introduce a method for specifically identifying the third harmonic generation (THG) originating from different nonlinear sources in a film-coupled nanostripe. The nanostripe consists of a metallic patch separated from a metallic film by a dielectric spacer. By considering the THG from each nonlinear source separately, we show that the near- and far-field behaviors of the THG generated within the various constituents of the nanostripe are distinguishable due to fundamental differences in the THG radiation properties. The THG signal from the metal is shown to be suppressed by the structure itself, while the THG signal from the spacer is enhanced by the gap plasmon modes supported by the structure. The total THG signal is found to be the sum of all nonlinear sources, with the far-field radiation pattern determined by the ratio between the third-order susceptibilities of the dielectric and the metal.