Ultrafast carrier dynamics and third-order nonlinear optical response in two-dimensional Ruddlesden-Popper perovskites (PEA)2(MA)n-1PbnI3n+1

Two-dimensional (2D) Ruddlesden-Popper (RP) perovskites offer a compelling platform for tailorable optoelectronic properties through quantum and dielectric confinement. However, a systematic understanding of how layer thickness governs their ultrafast carrier dynamics and third-order nonlinear optical response remains lacking, limiting their rational design for integrated photonics. Here, we synthensize high-quality (PEA) 2 (MA) n-1 Pb n I 3n+1 ( n = 1, 2, 3) crystals via an antisolvent-assisted crystallization approach. Increasing the inorganic layer number drives a transition from exciton-dominated to free-carrier-dominated relaxation, accompanied by a pronounced enhancement of saturable absorption revealed by femtosecond transient absorption spectroscopy(fs-TAS). The modulation depth increases from 7.4% (n=1) to 10.5% (n=3), while the saturation intensity decreases from 107.7 to 35.8 kW·cm -2 . Leveraging these layer-tunable nonlinear properties, we integrate these perovskites as saturable absorbers into erbium-doped fibre lasers, achieving stable mode-locking in the 1.5 μm band. The n = 3 variant further enables both the conventional soliton and dissipative soliton resonance operations, underscoring its versatility. Our work establishes layer-number engineering as a powerful strategy for tailoring the carrier dynamics and nonlinear optical responses in solution-processable nanomaterials, paving the way for their use in on-chip ultrafast photonic devices.