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)₂Pb₃I₁₀ 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.