Efficiently spatial field localization enabled second-harmonic and sum-frequency generation in an etchless LiNbO₃ layer by guided resonant quasi-bound states in the continuum

High-Q resonances on metasurfaces have yielded a feasible approach for nonlinear frequency conversion in recent years. In this work, we propose a highly efficient nonlinear metasurface based on an etchless lithium niobate (LiNbO₃) layer by combining periodic silicon nanodisks. These nanodisks introduce two kinds of capabilities for achieving ultra-sharp spectral lineshapes owing to the high-Q resonances of bound states in the continuum and simultaneously attain a significantly enhanced spatial field localization within the LiNbO₃ cavity owing to guided resonant behaviors. This distinctive mode engineering yielded a total efficiency of conversion of 2.8% in the second-harmonic generation and 1% for sum-frequency generation when the peak intensity of the fundamental pump was as low as 1 MW/cm². Our method eliminates challenges to the manufacture of etched LiNbO₃ thin films and allows for the efficient conversion of frequency. It is suitable for various applications, such as integrated, nonlinear nanophotonic chips, and high-speed communication.