The creation of highly non-equilibrium conditions in solids by irradiation with ultrashort laser pulses has been a topic of extensive research in recent years. Advances in experimental characterization techniques, such as ultrafast electron diffraction (UED), has enabled structural data to be obtained with femtosecond resolution. The measured intensities of the Bragg peaks reflect, not only the lattice temperature effects, but also the structural changes due to phase transitions such as melting. De-convoluting these effects from the integrated structural data is challenging; therefore, a detailed description of the structural evolution is difficult to achieve.
We recently carried out a combined experimental/ modelling study of femto-second laser irradiated of thin gold films and obtained a detailed description of the atomistic dynamics of the photo-induced solid to liquid phase transition. We used UED to measure the time evolution of the Bragg peak intensities and a coupled two temperature - molecular dynamics (2T-MD) model to simulate the atomistic dynamics following laser irradiation. Crucially, the time and length scales of the experiment and modelling were the same, therefore we can compare the Bragg peak evolution obtained from the two methods directly.
We obtained excellent agreement between the calculated and experimental Bragg peak intensities over the full experimental time scale for all fluences, which suggests that the modeled atomistic dynamics are a true representation of the structural dynamics of the laser irradiated gold films.
We identified three distinct types of melting dynamics induced by the different experimental fluences. In the low fluence regime we found heterogeneous melting, where melting initiated at the free surfaces and the melt front propagated to the interior of the film. At the intermediate fluence we observed homogeneous melting in which molten seeds grew and coalesced in the interior of the film. For high fluence irradiation we identified non-thermally accelerated melting.
In this case the reduced screening due to the redistribution of the conduction electrons had a significant effect on the interatomic interactions, which resulted in rapid expansion of the film. We will present the details of the experimental and modelling methods, as well as the calculated atomistic dynamics of the gold films in the three fluence regimes.
University College London, University College London
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