Multimodal Nonlinear Optical Imaging of MoS₂ and MoS₂-Based van der Waals Heterostructures

Abstract

van der Waals layered structures, notably the transitional metal dichalcogenides (TMDs) and TMD-based heterostructures, have recently attracted immense interest due to their unique physical properties and potential applications in electronics, optoelectronics, and energy harvesting. Despite the recent progress, it is still a challenge to perform comprehensive characterizations of critical properties of these layered structures, including crystal structures, chemical dynamics, and interlayer coupling, using a single characterization platform. In this study, we successfully developed a multimodal nonlinear optical imaging method to characterize these critical properties of molybdenum disulfide (MoS2) and MoS2-based heterostructures. Our results demonstrate that MoS2 layers exhibit strong four-wave mixing (FWM), sum-frequency generation (SFG), and second-harmonic generation (SHG) nonlinear optical characteristics. We believe this is the first observation of FWM and SFG from TMD layers. All three kinds of optical nonlinearities are sensitive to layer numbers, crystal orientation, and interlayer coupling. The combined and simultaneous SHG/SFG-FWM imaging not only is capable of rapid evaluation of crystal quality and precise determination of odd-even layers but also provides in situ monitoring of the chemical dynamics of thermal oxidation in MoS2 and interlayer coupling in MoS2-graphene heterostructures. This method has the advantages of versatility, high fidelity, easy operation, and fast imaging, enabling comprehensive characterization of van der Waals layered structures for fundamental research and practical applications.

Stacking-controllable interlayer coupling and symmetric configuration of multilayered MoS2

“…The FWM signals increased almost linearly with an increasing layer thickness, as previously reported in the literature, while the correlated SHG signal varied according to the stacking configuration. 6 The 2H-like even-numbered layers with AB(A...)-stacking showed little SHG intensity due to the existence of inversion symmetry and followed the trend indicated by the blue dashed line. The 3R or AA(A...)-type stacked multilayer crystals presented intense SHG intensities and followed the trend of the red dashed line.…”

“…[10][11][12][13]18 Furthermore, studying the nonlinear optical properties, such as SHG or four-wave mixing (FWM), as a function of the layer number and stacking orientation of the multi-stacked crystals grown by CVD have scarcely been sought. [4][5][6] The minute variations of the band structures according to the geometry and order of stacking are sufficient to modulate the nonlinear optical susceptibility, and hence, versatile MoS 2 crystals synthesized by CVD play a crucial role in understanding fundamental optoelectronic properties.…”

Non-linear excitation of quantum emitters in hexagonal boron nitride multiplayers

“…[1][2][3][4][5][6][7][8] For instance, studies of non-linear absorption or refraction in transition metal dichalcogenides have been promising for applications in mode locking, ultrafast photonics, and optical switching. [9][10][11][12] Hexagonal boron nitride (hBN) is another layered material 13,14 that has recently been subject to an increased research due its ability to host room-temperature quantum emitters. [15][16][17] While the origin of these emitters is still under investigation, they exhibit remarkable properties such as ultra-high brightness, full polarization, and tunable emission 18 making them very interesting for quantum sensing and optical communications.…”

Production Methods of Van der Waals Heterostructures Based on Transition Metal Dichalcogenides

“…Britnell et al [62] first successfully obtained WS 2 -graphene heterojunction by combining the mechanical exfoliation and dry transfer method. MoS 2 -graphene heterostructures are also obtained by using the similar procedure by transferring MoS 2 onto the graphene surface (Figure 3d,e) [63]. By using the same strategy, Moriya et al [64] To probe interlayer interactions in transition metal dichalcogenide heterostructures [52], Rigosi's group [53] successfully prepared the MoS2-WS2 and MoSe2-WSe2 heterostructures, respectively, by simply exfoliating the relevant crystals and then constructing the heterostructures by using mechanical transfer technique.…”

“…Britnell et al [62] first successfully obtained WS2-graphene heterojunction by combining the mechanical exfoliation and dry transfer method. MoS2-graphene heterostructures are also obtained by using the similar procedure by transferring MoS2 onto the graphene surface (Figure 3d,e) [63]. By using the same strategy, Moriya et al [64] fabricated the graphene-MoS2-metal vertical field effect transistor, which showed superior electrical performance when compared with those of other graphene based vertical transistors.…”