Calibrated plane–convex microcavity for room-temperature polaritons with geometric <i>g</i> -scaling

We present a plane–convex open microcavity that enables room-temperature polariton spectroscopy with a simple geometric handle on the coupling rate. The effective length L eff is absolutely calibrated from the free-spectral range, and piezo tuning is performed at near-normal incidence ( k ∥ ≈0) to avoid angle-induced degradation. Using spin-coated PEA 2 PbI 4 quasi-2D perovskites, we observe clear anti-crossings in reflection, with vacuum Rabi splittings up to 79 meV (reflection) and 84 meV (PL) near L eff ≈3.4 μm. A linewidth-corrected analysis converts the apparent splitting into the coherent exciton-photon coupling rate g , revealing a robust geometric scaling g ∝ L eff −1/2 across multiple longitudinal orders and spatial sites, consistent with the filled-mode thin-film limit where transverse area cancels in the mode volume. The platform establishes a compact, broadly compatible testbed for room-temperature polaritons and provides a practical design rule: shortening L eff is a reliable, geometric lever to strengthen collective coupling in plane–convex microcavities.