A Visible-Frequency Excitonic Reststrahlen Band in (PEA)₂PbI₄ Slabs

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Layered halide perovskites host exceptionally strong excitons, whose optical signatures are usually interpreted as absorptive resonances on a smooth dielectric background. Strong excitons, however, can also reshape the dielectric response itself and drive the real permittivity negative, opening a reflective band: the visible, excitonic analogue of an infrared Reststrahlen band. Whether bare (PEA)₂PbI₄ slabs reach this regime has remained unclear. Here we show that low-temperature transmission of (PEA)₂PbI₄ slabs, driven by the intralayer-exciton manifold, evolves with increasing thickness from an excitonic dip into a broad near-zero-transmission interval with compressed Fabry-Pérot-like fringes. Transfer-matrix analysis with an effective Lorentz-oscillator dielectric response reproduces this crossover, reconstructs a finite negative-Re(ε) window, and implies near-ultrastrong exciton-photon coupling. Calculated field maps show suppressed in-plane field penetration within this interval and a driven longitudinal response near the high-energy (ε=0) edge. These results identify (PEA)₂PbI₄ slabs as a cavity-free visible-frequency excitonic Reststrahlen material.