On the origin of third harmonic light from hybrid metal-dielectric nanoantennas

Near field amplification of electric fields magnifies the nonlinear optical signals generated in the metal volume of plasmonic nanoantennas, as well as in the surrounding media. We investigate the third harmonic light emitted by a hybrid nanoantenna composed of two metallic rods separated by a small gap filled with a dielectric. Despite the large cubic nonlinear susceptibility of metals, the presence of a hot spot in the antenna's gap may easily transform weaker dielectric nonlinearities into the dominant source of third harmonic light. This has led to diverse and sometimes opposite interpretations of the nature of the nonlinear response, which are further complicated by the limited knowledge of the intrinsic nonlinear susceptibilities of the constituent materials. Here, a quantitative description of third harmonic generation is provided as a function of the ratio between the dielectric and the metal nonlinear susceptibilities in a hybrid metal-dielectric nanoantenna. We find a spectral discriminator that allows us to determine the origin of third harmonic light unambiguously.