Visualizing Nanoscopic Acoustic Mode Competition in van der Waals Ferroelectric
Understanding how low-dimensional ferroelectrics respond to ultrafast excitation at nanoscales is essential for controlling energy flow and mechanical functionality in next-generation polar devices, yet the nanoscopic structural response to ultrafast depolarization remains unresolved, obscuring the microscopic pathways of acoustic decoherence and energy dissipation. Here, we spatiotemporally resolve lattice motion in the van der Waals ferroelectric NbOI2 using combined ultrafast electron microscopy and diffraction, revealing three acoustic phonons: two transverse shear modes and one longitudinal breathing mode. The transverse mode that shears the layers perpendicular to the in-plane polar axis dominates over that along the polar axis, reflecting anisotropic polarization-strain coupling. Real-space mapping uncovers spatially correlated heterogeneity in mode amplitudes and lifetimes. Regions dominated by a single shear mode exhibit significantly longer acoustic lifetimes than multimode regions, suggesting acoustic phonon-phonon scattering as a major source of decoherence. Our results provide a microscopic understanding of ultrafast depolarization-driven acoustic dynamics and spatially heterogeneous energy dissipation in van der Waals ferroelectrics.