Self‐Trapped Excitons and Nonlinear Dielectric Modulation in Van Der Waals Ferroelectric CuInP ₂ S ₆

ABSTRACT Van der Waals (vdW) ferroelectrics offer a unique platform for exploring light–matter interactions and developing advanced optoelectronic devices. However, the excited‐state properties of these materials, particularly those involving self‐trapped excitons (STEs), remain largely unexplored. Here, we report the observation of STEs emission in the vdW ferroelectric CuInP 2 S 6 (CIPS), which is activated by in‐plane lattice distortions induced by Cu deficiencies. The STEs exhibit a broad photoluminescence (PL) range from 500–700 nm, a large Stokes shift of ≈ 0.7–0.8 eV, and a nanosecond‐scale lifetime. Temperature‐dependent PL measurements reveal strong electron–phonon coupling, characterized by a high Huang–Rhys factor of 8.54 and an exciton binding energy of 318 meV. Using excited‐state‐sensitive Goos–Hänchen (GH) shift spectroscopy, we identify polarization‐dependent optical responses of STEs, with a distinct displacement peak emerging only under s ‐polarized illumination. Furthermore, the optical conductivity extracted from GH spectra exhibits nonlinear dispersion near the energy levels of the self‐trapped states, indicating modified dielectric behavior in the excited state. The coexistence of ferroelectricity and visible‐light activity endows Cu‐deficient CIPS with strong potential for broadband light emission, photodetection, and neuromorphic optoelectronic applications.