Snapshots from a simulation of the generation of a nanobump on a 20 nm Ni film located on a transparent substrate and subjected to a localized excitation by a 200 fs laser pulse. The size of the region of the optical excitation is 10 nm. The snapshots show the MD part of the combined atomistic-continuum model, with pressure-transmitting boundary conditions applied circumferentially around the MD circular slab and continuum equations for the lattice and electron temperatures solved in a wide region extending more than 300 nm from the center of the computational domain. Atoms are colored according to their potential energy, from blue color corresponding to a low potential energy of −4.2 eV to red color corresponding to a high potential energy of −3.6 eV (the cohesive energy of the EAM Ni fcc crystal is 4.45 eV). A sector of the film is blanked to show the distribution of the potential energy inside the film.
Contour plots showing the spatial and time evolution of the electron (a) and lattice (b) temperatures, pressure (c), and local order parameter (d) in a simulation illustrated in Fig. 1. The solid red and dashed white lines in (b) show the equilibrium melting temperature and glass transition temperature isotherms, respectively. The dashed-dotted black line in (b) shows the position of the liquid/crystal interface separating the liquid region from the original crystalline film, as identified from calculation of the local order parameter shown in (d). The apparent steps in the position of the solidification front at , , and in (d) are reflecting the procedure used in the data analysis and visualization rather than a real physical effect. The black dashed line in (b) corresponds to the onset of homogeneous crystallization in the undercooled liquid region, defined through the centrosymmetry parameter as the time when the number of atoms that belong to the configurations with local fcc symmetry exceeds 10% (see Fig. 5).
PDF calculated for EAM Ni at different temperatures (a) and the ratio of the magnitudes of the first minimum to the first maximum of the PDFs, , used for identification of the glass transition temperature (b).
Snapshots of the central region (nanobump) of the irradiated film illustrating the crystallization process during the final 50 ps of the simulation. All atoms that do not belong to the configurations with local fcc symmetry are blanked. All fcc atoms are shown in the left frames, whereas, only 7 nm slices cut through the center of the MD domain are shown in the right frames. The configurations are quenched using the velocity dampening technique for 2 ps prior to the structural analysis to reduce thermal noise in atomic positions.
The fraction of atoms with local fcc structure in the central region (nanobump) of the irradiated film. The arrows schematically show the contributions of the epitaxial regrowth of the original crystalline film and nucleation of new crystallites inside the undercooled liquid.
Snapshot of the central region (nanobump) of the irradiated film taken at 250 ps after the laser pulse. The atoms are colored according to their CSP. Atoms that belong to the configurations with local fcc symmetry are colored blue, atoms that belong to the undercooled liquid and crystal defects (grain boundaries, dislocations, and point defects) are colored green. Surface atoms are blanked in the image. The configurations are quenched using the velocity dampening technique for 2 ps prior to the structural analysis to reduce thermal noise in atomic positions.
Article metrics loading...
Full text loading...