Two-dimensional (2D) ferroelectric materials like NbOI$_{2}$ have garnered significant interest, yet their temporal response and synergetic interaction with light remain underexplored. Previous studies on the polarization of oxide ferroelectrics have relied on time-resolved optical second harmonic generation or ultrafast X-ray scattering. Here, we probe the laser-induced polarization dynamics of 2D NbOI$_{2}$ nanocrystals using ultrafast transmission electron diffraction and deflectometry. The deflection of the electron pulses is directly sensitive to the changes in the polarization, while the diffraction signal captures the structural evolution. Excited with a UV laser pulse, the polarization of NbOI$_{2}$ is initially suppressed for two picoseconds, then it recovers and overshoots, leading to a transiently enhanced polarization persisting for over 200 ps. This recovery coincides with coherent acoustic phonon generation, triggering a piezoresponse in the NbOI$_{2}$ nanocrystals. Our results offer a new method for sensing the ferroelectric order parameter in femtosecond time scales.
Efficient high-harmonic generation in van der Waals ferroelectric NbOI$_2$ crystals
International Center for Quantum Materials, School of Physics, Peking University（北京大学物理学院量子材料国际中心）；School of Science, Nanjing University of Posts and Telecommunications（南京邮电大学理学院）；Institute of Ultrafast Optical Physics, Department of Applied Physics, Nanjing University of Science and Technology（南京理工大学应用物理系超快光学研究所）；Beijing Academy of Quantum Information Sciences（北京量子信息科学研究院）
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