Calibrated Plane--Convex Microcavity for Room-Temperature Polaritons with Geometric g-Scaling

We present a plane--convex open microcavity that supports room-temperature polariton spectroscopy and offers 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 $k_parallel!approx!0$ to avoid angle-induced degradation. Using spin-coated PEA2PbI4 quasi-2D perovskites, we observe clear anti-crossings in reflection, with vacuum Rabi splittings up to 71 meV (reflection) and 87 meV (PL) near L{eff}!approx!3 μm. A linewidth-corrected analysis converts the apparent splitting into the coherent exciton--photon coupling rate g, revealing a robust geometric scaling g propto L{eff}^-1/2 across multiple longitudinal orders and spatial sites, consistent with the filled-mode thin-film limit where the 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.