Nonlinear Optical Response Modulated by Lattice Reconstruction in Small-Angle Twisted Bilayer MoS₂

Subtle perturbations in interlayer twist angles offer a novel investigational avenue for revealing intriguing optical characteristics within the domain of 2D materials. In this study, a homogeneous bilayer structure featuring precisely defined interlayer twist angles is meticulously engineered by stacking monolayer single-crystal MoS2 in a controlled, layer-by-layer manner. Scanning tunneling microscopy (STM) reveals an unexpected lattice reconstruction, particularly at twist angles of <= 2(degrees), which induces the emergence of expansive rhombohedral-stacked large triangular domains. Furthermore, a second-harmonic generation (SHG) is utilized as a sensitive modality to demonstrate profound transformations in the local electron band structures. By combining the linear spectra with SHG, the change in the C-exciton optical response in the band-nesting region of the small-angle twisted bilayer MoS2 before and after crossing a critical twist angle of 2(degrees) is discussed. The study shows that SHG can be used as a nondestructive method to characterize the lattice structure of small-angle twisted bilayer samples. Furthermore, determining the potential relationship between the lattice reconstruction and the linear and nonlinear spectra further demonstrates the regulatory effect of the interlayer twist angle on the nonlinear optical effects of 2D materials.