Boosted second-harmonic generation in the LiNbO$\mathrm{_3}$ metasurface governed by high-Q guided resonances and bound states in the continuum

TL;DR

This paper demonstrates a novel LiNbO3 metasurface that leverages high-Q guided resonances and bound states in the continuum to achieve record-high second-harmonic generation efficiency at low pump intensities.

Contribution

It introduces a new approach using BICs and guided resonances in LiNbO3 metasurfaces to significantly enhance SHG efficiency, even in low-index nanostructures.

Findings

Achieved 5% SHG efficiency at 0.4 MW/cm² pump intensity.

Demonstrated strong field confinement and tunable Q-factors via BICs.

Revealed the effects of nonlinear resonances and crystal rotation on SHG.

Abstract

To date, second-harmonic generation (SHG) at nanoscale has been concentrated on employing high-refractive-index nanostructures, owing to the strong field confinement at deep subwavelength scales based on optically resonant effects. However, low-index nanostructures generally exhibit weaker resonant effects and lower field confinement. To address this issue, by harnessing the large nonlinearity of LiNbO$\mathrm{_3}$, we propose a novel approach to employ guided resonances and bound states in the continuum (BICs) with a LiNbO$\mathrm{_3}$ metasurface consisting of a LiNbO$\mathrm{_3}$ disk array sitting on a LiNbO3 thin film. Such a system can transform the guided modes supported by LiNbO$\mathrm{_3}$ thin film into high-quality guided resonances which can be excited directly under plane-wave illumination. Importantly, we further demonstrate strong field confinement inside LiNbO3 thin film with tailorable Q-factor by realising 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 $\mathrm{MW/cm^{2}}$. 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 new route to constructing enhanced SHG based on high-Q guided resonances and BICs, including low-index and high-index nonlinear materials.