Boosting second-harmonic generation in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi mathvariant="normal">LiNbO</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:math> metasurface using high-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>Q</mml:mi></mml:math> guided resonances and bound states in the continuum
Ze Zheng, Lei Xu, Lujun Huang, Daria A. Smirnova, Peilong Hong, Cuifeng Ying, Mohsen Rahmani
Abstract
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 ${\mathrm{LiNbO}}_{3}$, we propose an approach to employ guided resonances and bound states in the continuum (BICs) with a ${\mathrm{LiNbO}}_{3}$ metasurface consisting of a ${\mathrm{LiNbO}}_{3}$ disk array sitting on a ${\mathrm{LiNbO}}_{3}$ thin film. Such a system can transform the guided modes supported by a ${\mathrm{LiNbO}}_{3}$ 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 ${\mathrm{LiNbO}}_{3}$ 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 $\mathrm{MW}/{\mathrm{cm}}^{2}$. 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.