Ultrafast-electron-diffraction studies of matter in extreme conditions: from ultrafast melting to dynamic compression*

Measuring atomic-resolution images of materials using time-resolved diffraction techniques has transformed our understanding on material behaviors and phase transition dynamics under extreme conditions. In particular, the advances in X-ray Free Electron Lasers and field-accelerated ultrafast electrons have allowed unprecedented explorations in this area of research, enabling femtosecond visualization of transient dynamics at atomic length scales. In this talk, I will present our recent work of using time-resolved diffraction technique based on relativistic electrons, also known as ultrafast-electron diffraction (UED), to study matter in extreme conditions created by ultrafast-laser excitation of solids.

I will first present results of ultrafast melting of solids induced by femtosecond laser excitation. By measuring melt time with UED, we resolved melting mechanism transition between heterogeneous and homogeneous melting regimes in warm dense Au [1]. These results provided direct comparison with molecular-dynamics (MD) simulations and revealed missing physical phenomena that will need to be included in simulations. Using the same technique, we also investigated the impact of radiation-driven defects on ultrafast melting [2]. More recently, we have performed UED experiments to visualize lattice response of single-crystal Al to ultrafast laser-induced compression. We observed lattice transitioning from a purely elastic to a plastically relaxed state. From the transverse plastic strain evolution, we found two distinct regimes of lattice relaxation: dislocation nucleation and transport. Our MD simulations showed excellent agreement with the experimental results. These results provided important insights into the dislocation dynamics during incipient plastic deformation.
[1] M. Mo, et al. Science 360, 1451 (2018).
[2] M. Mo, et al. Science Adv. 5, eaaw0392 (2019).