Tunable Room‐Temperature Polaritons in the Very Strong Coupling Regime in Quasi‐2D Ruddlesden–Popper Perovskites

Layered 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) 2 Pb 3 I 10 embedded in an open microcavity. The coupled exciton‐cavity system features a Rabi‐splitting up to Ω R ≃155 meV, exceeding the experimentally determined exciton binding energy of E b = 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.