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

Layered halide perovskites host exceptionally strong excitons, whose optical signatures are usually read 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 thickness-dependent evolution, 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.