  Vanadium dioxide (VO2) exhibits an insulator-to-metal transition accompanied
by a structural transition near room temperature. This transition can be
triggered by an ultrafast laser pulse. Exotic transient states, such as a
metallic state without structural transition, were also proposed. These unique
characteristics let VO2 have great potential in thermal switchable devices and
photonic applications. Although great efforts have been made, the atomic
pathway during the photoinduced phase transition is still not clear. Here, we
synthesized freestanding quasi-single-crystal VO2 films and examined their
photoinduced structural phase transition with mega-electron-volt ultrafast
electron diffraction. Leveraging the high signal-to-noise ratio and high
temporal resolution, we observe that the disappearance of vanadium dimers and
zigzag chains does not coincide with the transformation of crystal symmetry.
After photoexcitation, the initial structure is strongly modified within 200
femtoseconds, resulting in a transient monoclinic structure without vanadium
dimers and zigzag chains. Then, it continues to evolve to the final tetragonal
structure in approximately 5 picoseconds. In addition, only one laser fluence
threshold instead of two thresholds suggested in polycrystalline samples was
observed in our quasi-single-crystal samples. Our findings provide new
essential information for a comprehensive understanding of the photoinduced
ultrafast phase transition in VO2.
