Exceptional Photonic Second-Harmonic Generation From the Breathing Kagome Lattice Nb₃TeI₇

The combination of broken inversion symmetry and spatially localized electron density of states from a flat energy band in a Kagome lattice could provide an intriguing platform for giant nonlinear optical responses. In this work, we report giant second-harmonic generation (SHG) in thin flakes and a monolayer of Nb₃TeI₇ with non-centrosymmetric structure and breathing Kagome lattice. For both the 2H- and 1T-phases, the effective SHG coefficient χ⁽²⁾ increases monotonically upon reducing thickness from six layers to monolayer samples. Remarkably, the monolayer Nb₃TeI₇ exhibits a χ⁽²⁾ as high as 1572 pm V⁻¹ under excited 1064 nm, which is one of the highest values among reported nonlinear optical crystals. Density functional theory calculations suggest a correlation between this exceptional χ⁽²⁾ and electronic resonances involving flat bands in the strongly polarized [Nb₃]⁹⁺ Kagome lattice. Furthermore, we demonstrate that the SHG intensity of Nb₃TeI₇ is highly tunable through high pressure and halogen-substitution. In particular, the SHG intensity of bulk Nb₃TeI₇ can be enhanced fivefold at an external pressure of ∼ 4 GPa. All these results demonstrate the potential in nonlinear optical generation in non-centrosymmetric two-dimensional Kagome crystals, in which the flat bands could be engineered through twisting or stacking in favor for stronger SHG and applications.