Colossal infrared nonlinear optical anisotropy in a 2D charge-transfer Mott insulator

Mott insulators are a unique class of materials whose insulating state originates from strong electron-electron correlations: the interactions localize charge carriers, and the resulting on-site Coulomb repulsion opens a charge gap, fundamentally different from conventional insulators, making these systems an exceptional platform for exploring exotic physical phenomena. Significantly, the interplay between strong correlations and charge transfer not only stabilizes the antiferromagnetic ground state but also endows the material with enriched properties, particularly in optics. Herein, we demonstrate a 2D antiferromagnetic charge-transfer Mott insulator, Vanadium Oxychloride (VOCl), which shows giant third-harmonic generation (THG) anisotropy (ρ_THG = I_x/I_y, where I_x and I_y represent the THG intensities corresponding to the excitation polarization parallel to crystal's x- and y-axes), with ρ_THG reaching up to 187 at 1280 nm excitation wavelength. Notably, it is the highest THG anisotropic ratio within the van der Waals materials family. The nonlinear anisotropy is further modulated across a broadband infrared (IR) excitation wavelength range from 2028 to 1280 nm, during which ρ_THG rises from 2.6 to 187, corresponding to a 72-fold enhancement relative to its value at 2028 nm. Additionally, VOCl demonstrates layer-independent third-order susceptibilities (χ⁽³⁾ ~ 10^-19 m²/V²) and band structures attributed to its extremely weak interlayer electronic coupling. Moreover, the colossal THG anisotropic ratio in 2D VOCl can be ascribed to the synergistic effect of the correlated charge-transfer Mott insulator behavior and intrinsic C₃ symmetry breaking, as supported by theoretical calculations. The colossal nonlinear optical anisotropy in 2D VOCl positions it as an excellent candidate for nanophotonic and optoelectronic applications, enabling next-generation nanodevices based on 2D correlated Mott insulators.