Defect‐Free Sb‐Doping in Bi ₂ O ₂ Se Achieves Two‐Order‐of‐Magnitude Reduction in Saturation Intensity While Preserving High Carrier Mobility

Doping generally introduces performance trade-offs in materials, yet overcoming this fundamental limitation remains crucial for advancing materials research. Bi₂O₂Se exhibits exceptional electronic properties as a promising semiconductor, yet its nonlinear optical response under low excitation intensities hinders its practical applications. Therefore, precise Sb³⁺ doping in Bi₂O₂Se (Bi_1.9Sb_0.1O₂Se) is achieved for the first time via solid-state reaction and systematically studies its impact on the electronic structure and optical properties through first-principles calculations and experimental. The results reveal that Sb³⁺ substitution slightly reduces the bandgap without introducing defect states, and transient absorption spectroscopy further confirms prolonged carrier relaxation. At 1.5 µm, the modulation depth from 8.8% to 10.1% while dramatically reducing the saturation intensity from 47.2 to 0.53 kW cm^- ². This improvement is attributed to the stable linear absorption characteristics after doping, the synergistic effect between prolonged relaxation time and free-carrier-induced optical loss. In a mode-locking system, Bi_1.9Sb_0.1O₂Se achieves a broader 3-dB and shorter pulse duration at substantially reduced pump intensities. This work achieves defect-free energy level optimization in Sb-doped Bi₂O₂Se, where the material's high carrier mobility is not only preserved but further enhanced, while the saturation intensity is declined by about two orders of magnitude, enabling a low-power, high-performance nonlinear photonic devices.