Fig. 1.

Schematic of coherent atomic motions leading to transverse waves in a nanostructure. A transverse elastic wave packet is created at time zero and detected later in space by using ultrafast electron packets. The initial strain originated at a distant point travels in the acoustic waveguide with a group velocity υ_g and wave vector k_// and crosses the “observation point,” i.e., the nanoscale electron probe, which enables us to measure the spatiotemporal characteristic of the transient shear strain. For the first antisymmetric shear-horizontal eigenmode of the waveguide, the atoms at the top and bottom halves of the slab slide in opposite directions (here emphasized with the red ribbon) and characterized by the polarization vector . Along the surface normal, the standing wave condition results in a wave vector k_⊥. The transverse motion is color coded with increasing strain from blue to orange. The cross-sectional view is at the peak of the transient. The dimensions are not to scale.