Femtosecond Laser Manipulation of Multistage Phase Switching in Two-Dimensional In₂Se₃ Visualized via an In Situ Transmission Electron Microscope

Phase transitions critically determine material properties for applications, making them central to material science. The two-dimensional (2D) van der Waals material In₂Se₃ has been extensively studied as a model system for multiphase switching due to its intricate phase transition behaviors and outstanding ferroelectric properties for device applications. However, the lack of an efficient method for precise phase control and the poorly defined conditions for multiphase transitions have severely hindered its practical use. Here, we report that the femtosecond (fs) laser can serve as a potent tool for fast and precisely manipulating multiphase transitions in In₂Se₃ thin flakes. Using a transmission electron microscope capable of in situ fs laser irradiation, we realize controllable fast phase switching between four phases of 2D In₂Se₃ by controlling the laser fluence, including the transition from the ferroelectric α phase to the antiferroelectric β' or paraelectric β phase, reversible switching between antiferroelectric β' and paraelectric β phases at room temperature, as well as reversible transformation between the ferroelectric α' phase and antiferroelectric β' or paraelectric β phase at liquid nitrogen temperature. Notably, these multiphase transitions are accompanied by rapid formation and annihilation of domain structures and superlattices, resulting in fast changes in electric conductivity. Our first-principles calculations verify the multiphase transition pathways and reveal that the conductivity change stems from electronic band structure variation among the different phases. This work systematically investigates the phase transition behaviors in In₂Se₃ through spatially and temporally resolved characterization methods, providing foundational insights into memory device optimization.