Transient Terahertz Oscillations During Photoinduced Polarization Topology Reconfiguration in Ferroelectric Superlattices

ABSTRACT Terahertz resonances embedded in crystalline heterostructures could close a spectral gap between conventional electronics and photonics while opening new windows on phenomena in non‐equilibrium lattice dynamics. We show that femtosecond optical screening of the depolarization field in epitaxial PbTiO ₃ /SrTiO ₃ superlattices launches a collective polar mode that oscillates near 1 THz and coherently spans the entire mini‐Brillouin zone. Wave‐vector‐resolved pump–probe X‐ray diffraction resolves a nearly dispersion‐less oscillation at 0.87 and 0.94 THz at the zone boundary and zone center, respectively, persisting for ∼2.5 ps, corresponding to a weakly damped resonance. Dynamical phase‐field simulations reveal the origin of the mode to mesoscopic rotation of closure‐domain textures during the photoexcited transition from an unscreened to a screened electrostatic state. Varying the PbTiO ₃ and SrTiO ₃ ratio tunes the mode frequency continuously from 0.9 to 1.4 THz, providing a quantitative design rule for frequency‐selectable THz oscillators in ferroelectric heterostructures. By coupling nanoscale polarization reconfiguration to long‐wavelength coherent dynamics, this work establishes depolarization‐field engineering to topology‐driven THz functionality and expanding the landscape of collective lattice dynamics.