Toroidal dipole resonances enhanced second-harmonic generation with shallow etching of lithium niobate metasurface

Lithium niobate (LiNbO₃) has shown great potential for applications in nonlinear metasurfaces, thanks to its large second-order nonlinear coefficients and high integration capabilities. Optical resonances play a crucial role in further enhancing the nonlinear optical responses of LiNbO₃ metasurfaces (LNMS). In this study, both numerically and experimentally, we designed and fabricated a metasurface structure that supports toroidal dipole (TD) resonance to enhance second-harmonic generation (SHG). This structure, which consists of an array of shallow-etched square columns on a continuous thin film, intensifies the SHG signal at 400 nm within the LiNbO₃ film by means of strong local field confinement. Experimental results indicate that this signal is ten times stronger compared to that of lithium niobate on insulator (LNOI). These findings emphasize the potential of TD resonance in enhancing the performance of LiNbO₃ in integrated nonlinear nanophotonic applications.