Activated Self‐Trapped Excitons via Sb³⁺ Doping in Lead‐Free Perovskite Cs₃Cu₂I₅ for Enhanced Nonlinear Optics and Ultrafast Mode‐Locking Operation

Abstract Lead‐free zero‐dimensional (0D) metal halide perovskites have garnered considerable research interest owing to their pronounced self‐trapped exciton emission and exceptional stability. In this study, micron‐sized particulate Cs 3 Cu 2 I 5 and Sb‐Cs 3 Cu 2 I 5 are synthesized using the anti‐solvent method. For the first time, the broadband nonlinear optical properties of these materials in the near‐infrared region are systematically investigated. Compared to pristine Cs 3 Cu 2 I 5 , Sb‐Cs 3 Cu 2 I 5 exhibited superior nonlinear optical performance, demonstrating a modulation depth of 17.7% and a nonlinear absorption coefficient of −(4.28 ± 0.05) cm MW −1 at 1 µm, as well as a modulation depth of 13.5% and a nonlinear absorption coefficient of −(3.25 ± 0.06) cm MW −1 at 1.5 µm. Capitalizing on its exceptional nonlinear optical response, Sb‐Cs 3 Cu 2 I 5 is employed as a saturable absorber in both Yb‐doped and Er‐doped fiber lasers. In the Yb‐doped fiber laser, noise‐like pulse mode‐locking is achieved with a central wavelength of 1038.4 nm and a pulse width of 721 fs, while in the Er‐doped fiber laser, conventional soliton mode‐locking is realized with a central wavelength of 1557.7 nm and a pulse width of 853 fs. This work highlights the impact of Sb doping on the nonlinear optical features of Cs 3 Cu 2 I 5 , providing a novel design strategy for advanced nonlinear optical materials and applications in ultrafast photonic devices.