Non-equilibrium phononic phenomena on nm × ps scale revealed by ultrafast transmission electron microscopy*

Transmission electron microscopy (TEM) has been playing an important role in materials science owing to the high spacial resolution (< nm) and sensitivity to crystal structures. On the other hand, typical temporal scales of phononic phenomena on nanometer-scale are in the order of picoseconds, which is far beyond the temporal resolution of conventional TEM being limited to milliseconds by the shutter speed of the camera. Recent advances in ultrafast TEM techniques open a pathway to revealing non-equilibrium phononic phenomena on nm × ps spatiotemporal scale.

In this talk, we demonstrate how we can visualize and analyze the optically-excited nanometric acoustic waves propagating through the thin plate, based on the ultrafast bright-field movie and diffraction measurements [1]. In addition, we will present the finding of the unusual acoustic modes appearing in a transition metal dichalcogenide VTe₂, arising from the ultrafast breaking of the peculiar V-V-V trimer bonding [2]. Further quantitative estimation of nanometric acoustic wave amplitude and polarization based on the recently developed five-dimensional scanning TEM (5D-STEM) [3] will also be demonstrated.