Resonant nonlinear nanostructured grating in an unstructured lithium niobate on insulator platform

Lithium niobate (LN) is a versatile material for applications in integrated nonlinear optics; however, due to its stable physical and chemical properties, efficient nonlinear conversion with nanostructured LN is not easy to implement. Here, we propose a resonant nanostructure composed of a SiO2 grating anda lithium niobate on insulator (LNOI) layer, which has been proven to support bound states in the continuum (BIC) modes through simulations. By experimentally measuring the angle-resolved transmittance spectra of the fabricated sample, we confirm that our proposed nanostructure exhibits quasi-BIC modes transitioning from BIC modes, resulting in strong localized field confinement. In the nonlinear experiment, our results show the significant potential of the nanostructure in enhancing second harmonic generation (SHG). By pumping at the resonant wavelength, we achieve a SHG enhancement factor of 6400, compared to a flat LNOI thin film under a fundamental wave (FW) peak intensity of 460 MW/cm2. The SHG conversion efficiency reaches up to 1.741 x 10-7, leading to a normalized conversion efficiency of 3.785 x 10-7 cm2/GW. Although the experimental results are two orders of magnitude lower than the theoretical maximum SHG conversion efficiency of approximately 10-5, we believe that our proposed etchless LNOI nanostructure highlights the tremendous potential for applications in nonlinear resonant nanophotonics.