(a) A transient laser grating triggers a structural phase transition, resulting in different lattice structures in separate domains of the same lamella. (b) Ultrafast diffraction-contrast imaging using various virtual masks (right panel). Different virtual masks highlight distinct specimen properties.

Ultrafast Imaging of a Propagating Photo-Driven Phase Transition Using 4D STEM

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. In recent study, we present ultrafast four-dimensional scanning transmission electron microscopy enabling virtual imaging and strain mapping in space and time. Using this technique, we directly probe a laser-excited phase transition in the prototypical material vanadium dioxide (VO₂), recording its spatiotemporal propagation. This direct imaging capability reveals the dynamics of the structural phase transition and connects it to the resulting strain formation on picosecond timescales. This correlation reveals how atomic-scale symmetry breaking inherently generates lattice distortions, which then propagate to govern macroscopic property changes. Our findings provide new insights into the coupling between electronic, structural, and mechanical responses in correlated oxides under non-equilibrium conditions.

The work was carried out by Arthur Niedermayr, Jianyu Wu and Jonas Weissenrieder at UEM lab located at KTH.