Nonlinear optical phenomena induced by ultra-fast lasers in two-dimensional (2D) materials have recently attracted significant research interest. Herein, we investigate monolayer bismuthene using first-principles simulation based on the real-time Time-Dependent Density Functional Theory (rt-TDDFT). The results indicate that it exhibits a pronounced optical Kerr effect, with a negative nonlinear refractive index (NNRI) $n_2=-1.194\times {10}^{-9}{\mathrm{cm}}^2/\mathrm{W}$, whose absolute value is comparable to that of typical two-dimensional materials. By analyzing the electron transition probabilities among the conduction band minimum (CBM), valence band maximum (VBM), and $\mathrm{VBM}+1$, combined with perturbation theory for multi-level problems, we attribute the NNRI to the competition between two-level and three-level processes. This study clarifies the microscopic origin of NNRI in monolayer bismuthene, laying a theoretical foundation for unraveling the nonlinear optical regulation law of elemental two-dimensional bismuth-based nanomaterials.

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• Density functional theory
• Kerr effect
• Nonlinear optical susceptibility
• Photocurrent
• Third order nonlinear optical processes