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.