Effect of Pressure Relaxation on the Mechanisms of Short-Pulse Laser Melting

The kinetics and microscopic mechanisms of laser melting of a thin metal film are investigated in a computational study that combines molecular dynamics simulations with a continuum description of the laser excitation and subsequent relaxation of the conduction band electrons. Two competing melting mechanisms, homogeneous nucleation of liquid regions inside the crystalline material and propagation of melting fronts from external surfaces, are found to be strongly affected by the dynamics of the relaxation of the laser-induced pressure.

Figure 1

Contour plots of lattice temperature (a) and pressure (b) for simulation of laser melting of a 50 nm Ni film irradiated with a 200 fs laser pulse at an absorbed fluence of $430\mathrm{J}/{\mathrm{m}}^2$. Solid and dashed lines show the beginning and end of the melting process. Rectangles A and B in (a) show the areas of the film and times for which snapshots are shown in Fig. 3. Laser pulse is directed along the Y axes, from the bottom of the contour plots.

Figure 2

Conditions of equilibrium and nonequilibrium melting observed in simulations performed for EAM Ni material. Dark diamonds correspond to the conditions of equilibrium melting obtained in liquid-crystal coexistence simulations. The slope of the solid line is calculated from the Clapeyron equation. Blue dashed lines are isentropes plotted for four different values of entropy. Dark triangles connected by the black dashed line correspond to the maximum overheating of crystal observed in simulations performed with 3D periodic boundary conditions. Color circles show the conditions leading to the onset of homogeneous melting in the simulation of laser melting. The conditions for the onset of homogeneous melting are realized along the dashed line in Fig. 1 for the depth up to 35 nm. The circles are connected by lines and colored from red to blue in the order of increasing depth under the surface.

Figure 3

Snapshots from the simulation of laser melting for times and locations marked by rectangles A (a) and B (b) in Fig. 1a. Atoms are colored according to the local order parameter—red atoms have local crystalline surroundings, blue atoms belong to the liquid phase. Animated sequences of snapshots from the simulation can be found at 14.