Figure 3. Femtosecond-resolved phonon nucleation and launch at a crystal step-edge.

(a) Bright-field image highlighting two step-edges (indicated by the partial dashed white lines) in the WSe₂ flake. The coloured lines represent regions from which the mean intensity was quantified and used to generate the time traces in g. The images were acquired with a 50-kHz repetition rate and a 20-s integration time per frame (see also the captions for Supplementary Videos 3 and 4 for further experimental details). (b–f) Select processed micrographs revealing the nucleation and emergence of a localized phonon wavefront (red). The dotted white lines indicate the position of the step-edge from which the wavefront emerges. (g) Intensity measurements obtained at the coloured lines in a and plotted as a function of time delay. The slope of the line passing through the peaks of the first oscillation period reflects the wavefront velocity of 5.5 nm ps⁻¹. Scale bars, 200 nm.