Enhanced Anisotropic Second Harmonic Generation in Type-II van der Waals Heterostructures of g-C3N4/BiVO4

Due to the remarkably strong second-harmonic generation (SHG), two-dimensional materials have excellent potential applications in nonlinear optics (NLOs). However, their SHG efficiency remains limited due to the short light–matter interaction length. Through van der Waals heterostructure (vdWHS) engineering, it is possible to elongate light–matter interacting length and control structural symmetries and anisotropies. Here, vdWHSs were built by noncentrosymmetric monolayer g-C₃N₄ and centrosymmetric BiVO₄ slabs with varying layers. Their structural stabilities, electronic band structures, band alignments, and SHG susceptibilities have been systematically simulated by DFT calculations. Interestingly, BiVO₄ slabs with two or more layers form stable type-II heterostructures with g-C₃N₄, exhibiting the interlayer separation of photogenerated carriers. Besides, vdWHSs broke the centrosymmetry of BiVO₄, resulting in remarkable and complex SHG responses. The in-plane SHG susceptibility of g-C₃N₄/BiVO₄ vdWHSs exceeds 100 pm/V in the 700–800 nm range and gradually decreases as the thickness increases. Significant polarized out-of-plane SHG was introduced by vdWHS. The χ_zxx⁽²⁾, χ_zxy⁽²⁾, and χ_zyy⁽²⁾ components show multiple high peaks at 400–780 and 780–1240 nm, with intensities 3 times larger than that of LiNbO₃. These indicate that such vdWHS composites hold considerable potential for NLO in both visible and infrared light regions, which are important for advanced optical communication and photonic computing systems.