Second Harmonic Generation in van der Waals Heterostructure of Centrosymmetric ReS₂ and Graphene

Van der Waals (vdW) heterostructures of two-dimensional (2D) materials promise exotic properties beyond the reach of existing material systems and open unprecedented opportunities for device applications. Here, it is demonstrated that vdW stacking of centrosymmetric ReS₂ and graphene into heterostructure could induce strong second harmonic generation (SHG), though there is not any SHG from the individual ReS₂ or graphene. It could be attributed to the interfacial charge transfer in the heterostructure. With the distorted 1T crystal structure of ReS₂ and electrostatic screening effect, the charge transfer gives rise to non-uniform charge distribution across the ReS₂'s atomic layers, resulting in the broken centrosymmetry for the second-order hyperpolarizability. At the specific pump wavelength of 1550 nm, the strength of the induced SHG in a trilayer ReS₂/graphene vdW heterostructure is approximately twice of that from a monolayer MoS₂. This work reveals vdW stacking is a simple and efficient method for inducing SHG in 2D materials with centrosymmetry, which could be considered as another unique attribute of 2D materials. It also indicates that SHG spectroscopy is a valid technique for probing charge transfer and distribution in heterostructures of 2D materials.