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. Bi 2 O 2 Se exhibits exceptional electronic properties as a promising semiconductor, yet its nonlinear optical response under low excitation intensities hinders its practical applications. Therefore, precise Sb 3 ⁺ doping in Bi 2 O 2 Se (Bi 1.9 Sb 0.1 O 2 Se) 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 Sb 3 ⁺ 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 1.9 Sb 0.1 O 2 Se 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 Bi 2 O 2 Se, 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.