Nonlinear absorption transition governed by exciton and defect states in 2D (PEA)2PbI4 from visible to near-infrared

The active control of nonlinear absorption (NLA), particularly the dynamic transition between saturable absorption (SA) and reverse saturable absorption (RSA), is essential for advanced photonics such as all-optical logic gates. However, the mechanisms governing these transitions remain poorly understood, especially concerning the roles of excitonic and defect states. This work systematically investigates the broadband NLA properties of two-dimensional (2D) perovskite (PEA)2PbI4 films from 470 to 1440 nm. Abnormal RSA to SA transition is observed at 470 nm (near the continuous state absorption shoulder) under a low pump intensity of ∼20.5 GW/cm2, attributed to the competition between many-body effect induced above band edge absorption enhancement and bleaching caused by non-thermalized carriers. At 520 nm (near the exciton resonance), SA dominates due to exciton bleaching. In the 560–650 nm range, RSA dominates due to the two-photon absorption, and the coefficient dispersion can be well-described by a two-band model. Notably, a clear transition from SA to RSA appears in the near-infrared region (1240–1440 nm), where defect-state filling induces SA, followed by RSA driven by three-photon absorption. The threshold for this transition is as low as ∼57.5 GW/cm2 at 1340 nm. These findings provide a mechanistic understanding of wavelength-dependent NLA transitions in 2D perovskites, highlighting the potential of (PEA)2PbI4 for multi-band nonlinear photonic applications.