Visualization of ultrafast melting with femtosecond electron diffraction*

Understanding the structural dynamics of ultrafast laser-induced melting is important for applications ranging from laser micromachining to warm dense matter experiments.Direct experimental observations of ultrafast melting have previously not been possible because of the small time and length scales involved. However, recent advances in ultrafast-electron-diffraction (UED) techniques [1] have opened up an exciting opportunity to study the transient atomic dynamics with femtosecond temporal resolution.

Here we report on the melting dynamics studies of femtosecond-laser irradiated metallic nanofilms with UED at relativistic energies. In our experiments, we employed 400nm, 130fs (FWHM) laser pulses as the heater and 3.2MeV, 350fs (FWHM) electrons as the probe. With this pump-probe technique, we have recently resolved for the first time the transition between heterogeneous and homogeneous melting regimes in warm dense gold [2]. These results provided a direct testing to predictions from molecular-dynamics (MD) simulations and revealed the melting sensitivity to nucleation seeds. We have also applied this technique to study the melting dynamics of tungsten with externally-driven defects from radiation damage [3]. We found that radiation-damaged tungsten liquefied at a lower temperature than pristine tungsten. Combining the experimental data with MD simulations allowed us to quantify, for the first time, how the ultrafast melting process is affected by radiation damage.

[1] S. P. Weathersby, et al. RSI 86, 073702 (2015).
[2] M. Z. Mo, et al. Science 360, 1451 (2018).
[3] M. Z. Mo, et al. Science Adv. 5, eaaw0392 (2019).