Mapping relaxation pathways and bottlenecks in 2D perovskite microcavities

Exciton–polaritons in two-dimensional (2D) perovskites offer a promising platform for room-temperature polaritonics; however, their relaxation and scattering mechanisms remain only partially understood. Here, we report, for the first time, strong exciton–photon coupling in pure phase n = 1 butylammonium lead iodide (BA2PbI4) microcavities and employ femtosecond transient absorption (TA) spectroscopy to directly probe the underlying exciton-reservoir-to-polariton relaxation dynamics. Global analysis of the TA spectra reveals three distinct characteristic timescales: t1 (< 1 ps), governed by ultrafast excitonic many-body interactions, including phase-space filling, Coulomb-screening, biexciton formation, and cavity-resonance reshaping; t2 (5–15 ps), arising from exciton-reservoir-to-polariton scattering and exciton–exciton annihilation (EEA), together with bandgap renormalization (BGR); and t3 (> 50 ps), associated with long-lived dark-state or trap-state relaxation. The decay-associated spectra (DAS) further indicate that although transient polariton-associated spectral response emerges on picosecond timescales, efficient population accumulation within the lower polariton branch (LPB) remains strongly limited by ultrashort polariton lifetimes, incomplete reservoir-to-LPB feeding, and nonradiative trapping processes. Taken together, these results provide a detailed ultrafast spectroscopic insight into the nonequilibrium relaxation pathways and kinetic bottlenecks governing 2D perovskite microcavities and offer guidance for future optimization strategies aimed at enabling room-temperature polariton condensation and nonlinear effects in pure phase n = 1 2D perovskite systems.