Observation of Two‐Photon Resonant Raman Scattering via the Biexciton Level in Two‐Dimensional Halide Perovskite (C ₆ H ₅ C ₂ H ₄ NH ₃ ) ₂ PbI ₄

ABSTRACT 2D halide perovskites provide an excellent system for investigating fundamental properties of excitons due to strong quantum and dielectric confinements, yet biexciton‐related phenomena remain largely unexplored. Here, we report two‐photon resonant Raman scattering (TRRS) in (PEA) 2 PbI 4 (PEA = C 6 H 5 C 2 H 4 NH 3 ), enabled by the biexciton level acting as an intermediate state. TRRS shows a distinct threshold behavior, causing an emergent sharp Raman peak superimposed on the biexciton photoluminescence spectrum. The TRRS threshold is 88.5 MW/cm 2 at 10 K, which gradually increases with temperature up to 379.7 MW/cm 2 at 90 K. Unlike coexisting amplified spontaneous emission, this Raman peak shifts by 2Δω 1 when the excitation frequency (ω 1 ) is tuned by Δω 1 near 2329.9 meV, which is the resonant two‐photon energy for the direct generation of biexcitons. Consistent with two‐photon selection rules, TRRS is inactive under co‐circularly polarized excitation. Our findings highlight strong nonlinear optical properties of the perovskite and underscore its potential for biexciton‐based applications.