Publication date: 8th July 2026
Exciton-polaritons in two-dimensional (2D) lead-halide perovskites have emerged as a promising platform for room-temperature polaritonics owing to their large exciton binding energies and high oscillator strength.[1,2] Despite rapid progress in the field, room-temperature polariton condensation in solution-processed pure-phase n=1 2D perovskites remains elusive, primarily because the microscopic mechanisms governing polariton relaxation and population build-up are still poorly understood.[3,4]
Here, we report the first demonstration of strong exciton-photon coupling in pure-phase n=1 butylammonium lead iodide (BA2PbI4) microcavities and employ femtosecond transient absorption spectroscopy to directly probe exciton-reservoir-to-polariton relaxation dynamics. By combining angle-resolved optical spectroscopy, transfer matrix method (TMM) modelling and femtosecond transient absorption measurements, we identify and asign three distinct dynamical regimes governing the nonequilibrium response of the system. An ultrafast component (<1 ps) is dominated by many-body excitonic effects, including phase-space filling, Coulomb screening, and biexciton formation, leading to transient reshaping of the cavity resonances. An intermediate timescale (5–15 ps) reveals delayed exciton-to-polariton scattering and exciton-exciton annihilation, manifested by a transient spectral response associated with the lower polariton branch. Finally, long-lived dynamics (>50 ps) are governed by dark-state and trap-mediated relaxation processes.
Our measurements reveal that efficient population accumulation at the bottom of the lower polariton branch is strongly inhibited by ultrashort polariton lifetimes, incomplete reservoir feeding, and non-radiative trapping channels. These findings provide direct ultrafast spectroscopic insight into the relaxation pathways and kinetic bottlenecks that limit polariton population build-up in solution-processed 2D perovskite microcavities, and offer important design guidelines for achieving room-temperature polariton condensation and nonlinear polaritonic phenomena in pure-phase n=1 perovskite systems
1. Horizon Europe research and innovation program under the Marie Skłodowska-Curie European Fellowships (MSCA-EF) Grant Agreement No. 101066752.
2. Research and Innovation Foundation (RIF) of Cyprus through the “Bilateral Collaborations” programme (Grant Agreement No. “BILATERAL/ISRAEL(MOST)/0224/0023”, SOLARITONICS).
3. Engineering and Physical Sciences Research Council (EPSRC) Programme Grants EP/T012455/1 (Molecular Photonic Breadboards), EP/M025330/1 (Hybrid Polaritonics), and EP/X039285/1 (Understanding the Electronic Structure Landscape in Wide Band Gap Metal Halide Perovskites).
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