Multimodal\nNonlinear Optical Imaging of MoS₂ and MoS₂‑Based\nvan der Waals Heterostructures

Abstract

van der Waals layered structures,\nnotably the transitional metal\ndichalcogenides (TMDs) and TMD-based heterostructures, have recently\nattracted immense interest due to their unique physical properties\nand potential applications in electronics, optoelectronics, and energy\nharvesting. Despite the recent progress, it is still a challenge to\nperform comprehensive characterizations of critical properties of\nthese layered structures, including crystal structures, chemical dynamics,\nand interlayer coupling, using a single characterization platform.\nIn this study, we successfully developed a multimodal nonlinear optical\nimaging method to characterize these critical properties of molybdenum\ndisulfide (MoS₂) and MoS₂-based heterostructures.\nOur results demonstrate that MoS₂ layers exhibit strong\nfour-wave mixing (FWM), sum-frequency generation (SFG), and second-harmonic\ngeneration (SHG) nonlinear optical characteristics. We believe this\nis the first observation of FWM and SFG from TMD layers. All three\nkinds of optical nonlinearities are sensitive to layer numbers, crystal\norientation, and interlayer coupling. The combined and simultaneous\nSHG/SFG-FWM imaging not only is capable of rapid evaluation of crystal\nquality and precise determination of odd–even layers but also\nprovides <i>in situ</i> monitoring of the chemical dynamics\nof thermal oxidation in MoS₂ and interlayer coupling in\nMoS₂–graphene heterostructures. This method has\nthe advantages of versatility, high fidelity, easy operation, and\nfast imaging, enabling comprehensive characterization of van der Waals\nlayered structures for fundamental research and practical applications.