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Layered antiferromagnetism is the spatial arrangement of ferromagnetic layers with antiferromagnetic interlayer coupling. Recently, the van der Waals magnet, chromium triiodide (CrI 3 ), emerged as the first layered antiferromagnetic insulator in its few-layer form 1 , opening up ample opportunities for novel device functionalities 2-7 . Here, we discovered an emergent nonreciprocal second order nonlinear optical effect in bilayer CrI 3 . The observed second harmonic generation (SHG) is giant: several orders of magnitude larger than known magnetization induced SHG 8-11 and comparable to SHG in the best 2D nonlinear optical materials studied so far 12-15 (e.g. MoS 2 ). We showed that while the parent lattice of bilayer CrI 3 is centrosymmetric and thus does not contribute to the SHG signal, the observed nonreciprocal SHG originates purely from the layered antiferromagnetic order, which breaks both spatial inversion and time reversal symmetries. Furthermore, polarization-resolved measurements revealed the underlying C 2h symmetry, and thus monoclinic stacking order in CrI 3 bilayers, providing crucial structural information for the microscopic origin of layered antiferromagnetism 16-20 . Our results highlight SHG as a highly sensitive probe that can reveal subtle magnetic order and open novel nonlinear and nonreciprocal optical device possibilities based on 2D magnets.

Materials with massless Dirac fermions can possess exceptionally strong and widely tunable optical nonlinearities. Experiments on graphene monolayer have indeed found very large third-order nonlinear responses, but the reported variation of the nonlinear optical coefficient by orders of magnitude is not yet understood. A large part of the difficulty is the lack of information on how doping or chemical potential affects the different nonlinear optical processes. Here we report the first experimental study, in corroboration with theory, on third harmonic generation (THG) and four-wave mixing (FWM) in graphene that has its chemical potential tuned by ion-gel gating. THG was seen to have enhanced by ~30 times when pristine graphene was heavily doped, while difference-frequency FWM appeared just the opposite. The latter was found to have a strong divergence toward degenerate FWM in undoped graphene, leading to a giant third-order nonlinearity. These truly amazing characteristics of graphene come from the possibility to gate-control the chemical potential, which selectively switches on and off one-and multi-photon resonant transitions that coherently contribute to the optical nonlinearity, and therefore can be utilized to develop graphene-based nonlinear optoelectronic devices.

Great efforts have been made to develop efficient visible light-activated photocatalysts in recent years. In this work, a new nanocomposite consisting of anatase TiO 2 , Ag, and graphene was prepared for use as a visible light-activated photocatalyst, which exhibited significantly increased visible light absorption and improved photocatalytic activity, compared with Ag/TiO 2 and TiO 2 /graphene nanocomposites. The increased absorption in visible light region is originated from the strong interaction between TiO 2 nanoparticles and graphene, as well as the surface plasmon resonance effect of Ag nanoparticles that are mainly adsorbed on the surface of TiO 2 nanoparticles. The highly efficient photocatalytic activity is associated with the strong adsorption ability of graphene for aromatic dye molecules, fast photogenerated charge separation due to the formation of Schottky junction between TiO 2 and Ag nanoparticles and the high electron mobility of graphene sheets, as well as the broad absorption in the visible light region. This work suggests that the combination of the excellent electrical properties of graphene and the surface plasmon resonance effect of noble metallic nanoparticles provides a versatile strategy for the synthesis of novel and efficient visible light-activated photocatalysts.

Grain boundaries in as-grown polycrystalline MoS2 monolayers are revealed by second-harmonic-generation microscopy. Through the anisotropic polarization pattern and phase interference at the grain boundary, grain edge termination and boundary types are identified. Statistical analysis on hundreds of grains shows that grain-boundary formation is driven by kinetics and can be nicely described by the edge attachment growth model.