Dimensional Crossover of Strong Exciton–Photon Coupling in a Tunable Microcavity with Layered Quasi​-2D Perovskites

We report room-temperature strong exciton--photon coupling in a reconfigurable, open-access plane--concave microcavity incorporating layered quasi-2D perovskites with quantum-well numbers $n=1$ and $n=2$. Piezo-controlled cavity-length tuning sweeps the photonic mode through the exciton resonance, yielding clear anticrossings in white-light reflection. Coupled-oscillator fits give minimum vacuum Rabi splittings of $\sim$100\,meV ($n=2$). Despite broader exciton absorption and a slightly lower cavity $Q$ for $n=1$, the extracted coupling rate $g$ remains smaller than for $n=2$, indicating an intrinsically reduced effective oscillator strength and/or optical overlap in the thinner-well film. Transfer-matrix field profiles and Hopfield analysis quantify hybridization versus detuning and effective cavity length. This materials-focused comparison establishes BA-based layered perovskites as viable strong-coupling media and provides design rules for tuning light--matter interaction via quantum-well thickness; the same platform is compatible with molecular and gas-phase systems for cavity-enhanced spectroscopy.