Direct and real-time probing of both coherent and thermal lattice motions

We report direct measurement of laser-induced ultrafast structural dynamics on the atomic time and length scales in a 20-nm Al film by taking real-time snapshots of transmission electron diffraction patterns. The damped single-mode breathing motion of the Al film along the surface normal was recorded as coherent and in-phase oscillations of all the Bragg peak positions, which had a period of 6.4 ps. The concurrent lattice heating with a time constant of 630 fs was measured by tracking the associated Bragg peak intensity attenuation. These results suggest a direct approach to identify the mechanisms of laser-induced ultrafast solid-liquid phase transition, as well as thermal or non-thermal melting, via probing the temporal evolution of their diffraction patterns.