Enhanced magnetic second-harmonic generation in an ultra-compact plasmonic nanocavity.

Observation of the second-harmonic generation (SHG) from subwavelength metallic structures is often hindered by the interrelations of higher-order multipolar contributions. In particular, the magnetic Lorentz contribution to SHG is often neglected due to the ineffective magnetic field enhancement in electrically resonant structures. Here, we demonstrate a strong Lorentz-driven SHG output at the plasmon-induced magnetic dipolar resonance in inversion-symmetry-broken plasmonic nanocavities. We observe experimentally tenfold enhancement in the SHG intensity when the magnetic dipole mode is excited, with polarization-resolved measurements confirming the significant role of the hydrodynamic Lorentz-driven second-order nonlinear response. The enhancement originates from a significant spatial overlap between the electric and magnetic fields within the nanometer-scale cavity gaps. Our findings outline the critical role played by the resonant Lorentz-driven optically induced magnetic nonlinearities in metallic nanocavities, and it paves the way towards developing highly efficient nanoscale nonlinear photonic devices.