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

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 Cs₃Cu₂I₅ and Sb-Cs₃Cu₂I₅ 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 Cs₃Cu₂I₅, Sb-Cs₃Cu₂I₅ 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-Cs₃Cu₂I₅ 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 Cs₃Cu₂I₅, providing a novel design strategy for advanced nonlinear optical materials and applications in ultrafast photonic devices.