Spatiotemporal Imaging of Coherent Acoustic Phonons in Layered Ferroelectric NbOI₂

Understanding how lattices respond to nonequilibrium excitation in low-dimensional ferroelectrics is essential for controlling energy flow and mechanical functionality in next-generation ferroelectric devices. Here, we report spatiotemporally resolved lattice dynamics in the van der Waals ferroelectric NbOI₂ using ultrafast electron diffraction and microscopy. Above-band-gap optical excitation rapidly screens the in-plane polarization within a few picoseconds, launching three coherent acoustic phonons—two transverse shear 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 coupling between polarization and elastic strain. Furthermore, the dephasing times of the two shear modes exhibit stronger spatial variation than those of the longitudinal mode, indicating a higher susceptibility of shear modes to local mechanical inhomogeneity in the freestanding sample. Our results provide a microscopic understanding of polarization–strain coupling under ultrafast excitation in van der Waals Ferroelectrics.