CC BY 4.0Pashaei-Adl, HamidHamidPashaei-AdlBennenhei, ChristophChristophBennenheiStruve, MartiMartiStruvePeksa, PaulinaPaulinaPeksaDyksik, MateuszMateuszDyksikBaranowski, MichałMichałBaranowskiSong, Kok WeeKok WeeSongGittinger, MoritzMoritzGittingerLienau, Christoph T.Christoph T.LienauKönig, Jonas K.Jonas K.KönigFitzgerald, Jamie M.Jamie M.FitzgeraldJasti, Naga PrathibhaNaga PrathibhaJastiEilenberger, FalkFalkEilenbergerPłochocka, PaulinaPaulinaPłochockaMalić, ErminErminMalićKyriienko, OleksandrOleksandrKyriienkoEsmann, MartinMartinEsmannSchneider, ChristianChristianSchneider2025-10-082025-10-082025https://publica.fraunhofer.de/handle/publica/497136https://doi.org/10.24406/publica-565810.1002/adom.20250139210.24406/publica-56582-s2.0-105016335605Layered perovskites are an emergent class of materials, which feature extraordinarily large light–matter coupling, driven by excitons with binding energies significantly beyond the thermal energy at room-temperature. In this work, widely tunable room-temperature cavity exciton polaritons are demonstrated at the cross-over from the strong coupling to the very strong coupling regime in mechanically exfoliated crystals of quasi-2D Ruddlesden–Popper iodide perovskite (BA)2(MA)2Pb3I10 embedded in an open microcavity. The coupled exciton-cavity system features a Rabi-splitting up to ΩR ≃155 meV, exceeding the experimentally determined exciton binding energy of Eb = 100 ± 10 meV and thus operates at the onset of the very strong coupling regime, in which the light–matter coupling alters the interaction of electron and hole. This combined experimental-theoretical effort provides a consistent microscopic picture successfully describing the observed peculiar scaling of the Rabi-splitting with an increasing effective cavity length. These findings provide a foundation for future on-chip applications involving tunable polaritonic and nonlinear optical devices based on strongly coupled perovskite systems.entrueexciton-polaritonopen optical microcavityquasi-2D perovskitesvery strong coupling regimejournal article