Exceptional Photonic Second‐Harmonic Generation From the Breathing Kagome Lattice Nb 3 TeI 7

ABSTRACT 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 Nb 3 TeI 7 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 Nb 3 TeI 7 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 [Nb 3 ] 9+ Kagome lattice. Furthermore, we demonstrate that the SHG intensity of Nb 3 TeI 7 is highly tunable through high pressure and halogen‐substitution. In particular, the SHG intensity of bulk Nb 3 TeI 7 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.