Nonlinear Strong Coupling by Second-Harmonic Generation Enhancement in Plasmonic Nanopatch Antennas*

Electromagnetic enhancements generated in plasmonic nanocavities have enabled many interesting photonic phenomena which are widely applicable to emerging technologies in both the linear and nonlinear optical regimes. In the present study, strong second-harmonic generation (SHG) is demonstrated in plasmonic nanopatch antennas (NPAs) formed with silver nanocubes separated from a smooth gold film by a nm-scale zinc oxide gap layer. When compared to the SHG generated by the zinc oxide layer on gold film, the NPAs display a 10⁴-fold increase in the intensity of the SHG signal at resonant plasmon frequency. More importantly, by integrating quantum emitters with an absorption energy matched to the fundamental frequency of the NPAs, a second-order exciton-polariton strong coupling response with a Rabi splitting energy of 19 meV is reported. As such, the nonlinear frequency-converting NPAs fabricated in this study could be utilized for the control of the light−matter interactions in both weak and strong coupling regimes, providing a powerful tool for use in emerging optical engineering and information processing technologies.