Interface second harmonic generation enhancement in bulk WS2/MoS2 hetero-bilayer van der Waals nanoantennas.

Layered van der Waals (vdW) materials have emerged as a promising platform for nanophotonics due to large refractive indexes and giant optical anisotropy. Unlike conventional dielectrics and semiconductors, the absence of covalent bonds between layers allows for novel degrees of freedom in designing optically resonant nanophotonic structures down to the atomic scale: from the precise stacking of vertical heterostructures to controlling the twist angle between crystallographic axes. Specifically, although monolayers of transition metal dichalcogenides exhibit giant second-order nonlinear responses, their bulk counterparts with 2H stacking possess zero second-order nonlinearity. In this work, we investigate second harmonic generation (SHG) arising from the interface of WS₂/MoS₂ hetero-bilayer thin films with an additional SHG enhancement in nanostructured optical antennas, mediated by both the excitonic resonances and the anapole-driven field enhancement. When both conditions are met, we observe up to 10² SHG signal enhancement, compared to unstructured bilayers, with SHG conversion efficiency reaching ≈ 10^-7. Our results highlights vdW materials as a platform for designing unique multilayer optical nanostructures and metamaterial, paving the way for advanced applications in nanophotonics and nonlinear optics.