Correlative Ultrafast Imaging of a Photodriven Phase Transition Using 4D Scanning Transmission Electron Microscopy

Oxides exhibiting insulator-metal transitions are promising candidates for next-generation ultrafast electronic switching devices. However, critical gaps remain in understanding the onset of strain and its dynamics as these materials undergo structural transitions, particularly in nanostructured configurations. Here, we present ultrafast four-dimensional scanning transmission electron microscopy enabling virtual imaging and strain mapping at every point in space and time. Using this technique, we directly probe a laser-excited phase transition in the prototypical material vanadium dioxide (VO₂). This direct imaging capability reveals the dynamics of the structural phase transition and connects it to the resulting strain formation on picosecond time scales. We find that the transient in-plane strain reaches ∼1% within ∼20 ps, an order of magnitude larger than expected from thermal expansion of the monoclinic phase. This indicates that the dominant strain contribution originates from the evolving structural phase transformation. Our results reveal the coupling between electronic, structural, and mechanical responses in correlated oxides under nonequilibrium conditions.