Enhancing Second-Harmonic Generation of Plasmonic Nanocavities by Invoking Magnetic Lorentz Force and Overlapping Electric and Magnetic Resonances

The boosting of nonlinear optical effects in nanostructured materials relies on the strong light–matter interaction, which is often associated with the optical near-field spatial confinement beyond the diffraction limit and resonant intensity enhancement. Localized surface plasmon resonances in metal nanostructures may enhance the nonlinear response of the metal and that of nearby nonlinear materials. However, the centrosymmetric crystal structure of many metals (such as gold and silver) forbids the bulk occurrence of certain nonlinear processes, such as second-harmonic generation (SHG), within the electric-dipole approximation. Beyond this approximation, SHG can be substantially enhanced in metal, dielectric, and metal–dielectric hybrid nanostructures with higher-order multipole moments (such as electric quadrupoles and magnetic dipoles), generating larger nonlinear conversion efficiencies that are crucial for potential applications in optical biosensing, nanoscale laser devices, and light sources.Although Mie-type magnetic resonances have recently been exploited to enhance the SHG of all-dielectric nanostructures comprising high-index semiconductors, there have been few studies on metallic nanostructures, probably due to the physical constraint of generating pronounced magnetic resonance in visible and near-infrared regions. This constraint can be alleviated by assembling individual metallic nanostructures into twoor three-dimensional resonant nanoclusters. For instance, we have recently achieved strong plasmon-induced in-plane magnetic dipolar resonance in an ultrasmall plasmonic nanocavity comprising a gold nanosphere dimer closely spaced from a gold thin film. Inspired by this preliminary observation, we propose to realize magnetic resonance-enhanced SHG in such plasmonic nanocavities and similar ones, and determine the physical mechanism of this effect. Theoretical analysis suggests that magnetic SHG results from the Lorentz force exerted on metal electrons, and that the nonlinear polarization is proportional to the cross-product of electric and magnetic near-fields, indicating that the nonlinearity is induced by magnetoelectric coupling. A microscopic hydrodynamic model has revealed that the overall SHG conversion efficiency is enhanced by collectively maximizing the Lorentz contribution, the nonlinear Coulomb term and the convective term. We propose to collaborate with Prof. Peter Nordlander at Rice University (USA) to perform stringent calculations to quantify the magnitude of the Lorentz force-induced SHG and other nonlinear sources as functions of cavity geometry and excitation conditions. We will then perform polarization-resolved, angledependent single-nanocavity-level SHG mapping and spectroscopy measurements to verify our theoretical predictions. This will delineate novel physical mechanisms for subwavelength control of nonlinear light scattering in metal nanostructures, thereby affording new physical guidelines for the design of efficient nonlinear nanoplasmonic metadevices.