Boosting second-harmonic generation in the LiNbO3 metasurface using high- Q guided resonances and bound states in the continuum

To date, second-harmonic generation (SHG) at nanoscale has concentrated on employing high-refractive-index nanostructures, owing to the strong field confinement at deep subwavelength scales based on optically resonant effects. However, nanostructures with lower index contrast between the structures and the surroundings generally exhibit weaker resonant effects and lower field confinement. To address this issue, by harnessing the large nonlinearity of LiNbO3, we propose an approach to employ guided resonances and bound states in the continuum (BICs) with a LiNbO3 metasurface consisting of a LiNbO3 disk array sitting on a LiNbO3 thin film. Such a system can transform the guided modes supported by a LiNbO3 thin film into high-quality guided resonances which can be excited directly under plane-wave illumination. Importantly, we further demonstrate strong field confinement inside a LiNbO3 thin film with a tailorable quality factor (Q factor) by realizing a Friedrich-Wintgen BIC. Such a unique mode engineering enables a record-high SHG efficiency of 5% under a pump intensity as low as 0.4 MW/cm2. Moreover, we reveal the influence of nonlinear resonances and cross coupling on the SHG by showing the anomalous SHG and efficiency tuning with the rotation of the crystal axis. Our work offers a route to construct enhanced SHG based on high-Q guided resonances and BICs, including low-index and high-index nonlinear materials.