Chirality inversion for tailoring light generation in nonlinear van der Waals metasurfaces

3R-phase molybdenum disulfide (3R-MoS2) multilayers are emerging as a compelling platform for nonlinear nanophotonics, owing to their high refractive index, intrinsically broken inversion symmetry, low optical losses, and large second-order susceptibility. Here, we demonstrate highly efficient and chirality-sensitive second-harmonic generation (SHG) in 3R-MoS2 metasurfaces by coupling them to bound states in the continuum (BICs) characterized by elliptical polarization vortex singularities. We achieve SHG efficiencies approaching 1%—over six orders of magnitude higher than those in unpatterned 3R-MoS2 multilayers—by leveraging the quasi-BIC with Q-factor exceeding 104 under oblique incidence. Remarkably, the quasi-BIC exhibiting near-unity circular dichroism enables a three-orders-of-magnitude difference in SHG efficiency between left- and right-circularly polarized (LCP/RCP) excitation, manifesting a clear signature of photonic spin–dependent nonlinear generation. Intriguingly, the periodic helicity inversion of elliptical polarization states around the momentum-space BIC gives rise to azimuthal-angle-dependent chirality inversion, enabling efficient switching of SHG output between LCP and RCP excitation. Our results not only establish a versatile strategy for tailoring nonlinear optical responses via chirality reversal enabled by topological BICs in van der Waals metasurfaces but also hold promising potential for applications in chiral light sources, nonlinear holography, and optical switching.