Quasi-Bound States in the Continuum Enhanced Second-Harmonic Generation in Thin-Film Lithium Niobate

This work reports the second-harmonic generation (SHG) in a z-cut thin-film lithium niobate (LN) under nearly normal-incident linearly-polarized pumping. To utilize the largest nonlinear term which is along the out-of-plane direction in the z-cut LN film, the film is patterned with a 2D periodic array to support symmetry-protected bound states in the continuum (BICs) bearing strong out-of-plane electric field component. Enabled by such optical resonances near the BIC condition at the pumping wavelength, significant enhancement in SHG compared to an unpatterned film is observed. This approach, combining transparent thin-film nonlinear materials with tailored BICs, offers prospects for engineering compact coherent light sources in a broad wavelength range.