Ultrafast deflection of the electron beam and light-induced acoustic waves in ferroelectric NbOI₂ investigated with ultrafast electron diffraction

NbOI₂ is a low-symmetry 2D van der Waals material that exhibits ferroelectricity along the niobium-oxygen atom chains. It has a large in-plane anisotropy of electrical, optical, and mechanical properties and a strong optical nonlinearity leading to enhanced second-harmonic generation. In this study, the NbOI₂ samples were excited with a femtosecond UV pulse, and then the transient non-thermal states of the samples were probed by a femtosecond electron pulse in an ultrafast transmission electron diffraction geometry. Here, we report a transient deflection of the electron probe, followed by large modulations to the diffraction intensity. After the laser excitation, the electron diffraction pattern is transiently deflected along the polar direction, returning to the initial state within a few picoseconds, indicating the fast response of the polarization to the optical pump. Subsequently, we observe large oscillations in the diffraction intensity, which we attribute to the generation of coherent acoustic phonons due to thermal stress. These findings provide a temporally resolved characterization of the thermal response of ferroelectrics and offer a new paradigm for sensing the change of polarization in ferroelectrics on a picosecond time scale.