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 χzxx(2), χzxy(2), and χzyy(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.