Polarization-Dictated Exciton Resonance Engineering for Phase-Programmable Nonlinear Photonics in 2D Ferroelectrics

Monolayer two-dimensional ferroelectric CuInP₂S₆ (CIPS) exhibits promising optoelectronic applications owing to its room-temperature ferroelectricity, although the dynamic modulation mechanism of its optical properties remains elusive. The mechanism governing the optical response of monolayer CIPS is systematically investigated through first-principles calculations. Reversible structural phase transitions from a paraelectric (PE) to an out-of-plane polarized ferroelectric (FE) state result from lateral Cu displacement induced by a moderate external electric field. Subsequent removal of the field triggers spontaneous reversion to the thermodynamically stable, macroscopically nonpolar antiferroelectric (AFE) ground state. These transitions alter exciton localization and optical absorption properties through coordinated charge transfer and band structure rearrangement. Exciton binding energies (0.48-0.91 eV) vary with polarization states, indicating strong excitonic effects in the visible spectrum. Crystal symmetry reduction during structural phase transitions activates distinct second-harmonic generation (SHG) responses: the FE phase exhibits out-of-plane SHG mediated by vertical polarization, whereas the AFE phase maintains comparable in-plane SHG through nonlinearity enabled by sublattice polarization. This work establishes electric-field-controlled Cu positioning as a versatile strategy for dynamically tuning both linear and nonlinear optical properties in low-dimensional ferroelectrics, providing a platform for reprogrammable optoelectronic memories.