Stacking\nEngineering toward Giant Second Harmonic\nGeneration in Twisted Graphene Superstructures

The nonlinear optical response in\ngraphene is finding increasing\napplications in nanophotonic devices. The activation and enhancement\nof second harmonic generation (SHG) in graphene, which is generally\nforbidden in monolayer and AB-stacked bilayer graphene due to their\ncentrosymmetry, is of urgent need for nanophotonic applications. Here,\nwe present a comprehensive study of SHG performance of twisted multilayer\ngraphene structures based on stacking engineering. It is found that\nthe modulation of in-plane and out-of-plane SHG susceptibility components\nby stacking few-layer graphene is essential in producing giant SHG\nresponse in twisted multilayer graphene. Giant SHG intensity in twisted\nmultilayer graphene is observed, reaching nearly 10 times that of\nmonolayer MoS<sub>2</sub> under 1064 nm excitation, which significantly\noutperformed graphene structures reported to date. Our findings present\na facile and effective approach to enhance SHG in graphene structures,\nshowing promise for future application of graphene in second harmonic\nnanophotonic devices as well as prospects for the study of SHG among\ntwo-dimensional (2D) structures in general.