Resolving Excited-State Dynamics of Two-Dimensional Layered (PEA) ₂ SnI ₄ Polycrystalline Thin Films in Fabry–Pérot Cavities

Exciton–photon hybrid states in cavities based on 2D hybrid organic–inorganic perovskites (2D-HOIPs) offer a promising route toward low-threshold lasing, yet the excited-state dynamics of these hybrid states remain largely unexplored, particularly in disordered polycrystalline films where dark states’ (DS) negligible photon fraction may become spectroscopically active. Here, using sub-20 fs fs-TA spectroscopy, we reveal the transient signature and detuning-dependent decay pathways of hybrid states in Fabry–Pérot cavities employing (PEA) 2 SnI 4 polycrystalline films. A short-lived upper polariton (UP) population is identified with decay rate scales with photon fraction. Crucially, a long-lived negative band was observed in negatively detuned cavities and identified as DS-like states by disentangling from optical artifacts. Our work provides direct evidence that DS-like states contribute to the long-lived TA response of cavities and clarifies the fs-TA signatures of hybrid states in (PEA) 2 SnI 4 polycrystalline films, offering a new photophysical insight for engineering low-threshold lasers based on scalable 2D-HOIP films.