Quasi-BIC Resonant Enhancement of Second-Harmonic Generation in WS2 Monolayers.

Atomically thin monolayers of transition metal dichalcogenides (TMDs) emerged as a promising class of novel materials for optoelectronics and nonlinear optics. However, the intrinsic nonlinearity of TMD monolayers is weak, limiting their functionalities for nonlinear optical processes such as frequency conversion. Here, we boost the effective nonlinear susceptibility of a TMD monolayer by integrating it with a resonant dielectric metasurface, which supports pronounced optical resonances with high quality factors: bound states in the continuum (BICs). We demonstrate that a WS$_2$ monolayer combined with a silicon metasurface hosting BICs can enhance the second-harmonic intensity by more than three orders of magnitude compared to a WS$_2$ monolayer on top of a flat silicon film of the same thickness. Our work suggests a pathway to employ high-index dielectric metasurfaces as hybrid structures for enhancement of TMD nonlinearities with applications in nonlinear microscopy, optoelectronics, and signal processing.