Atomistic simulation study of short pulse laser interactions with a metal target under conditions of spatial confinement by a transparent overlayer

Abstract

The distinct characteristics of short pulse laser interactions with a metal target under conditions of spatial confinement by a solid transparent overlayer are investigated in a series of atomistic simulations. The simulations are performed with a computational model combining classical molecular dynamics (MD) technique with a continuum description of the laser excitation, electronphonon equilibration, and electronic heat transfer based on two-temperature model (TTM). Two methods for incorporation of the description of a transparent overlayer into the TTM-MD model are designed and parameterized for Ag-silica system. The material response to the laser energy deposition is studied for a range of laser fluences that, in the absence of the transparent overlayer, covers the regimes of melting and resolidification, photomechanical spallation, and phase explosion of the overheated surface region. In contrast to the irradiation in vacuum, the spatial confinement by the overlayer facilitates generation of sustained high-temperature and highpressure conditions near the metal-overlayer interface, suppresses the generation of unloading tensile wave, decreases the maximum depth of melting, and prevents the spallation and explosive disintegration of the surface region of the metal target. At high laser fluences, when the laser excitation brings the surface region of the metal target to supercritical conditions, the confinement prevents the expansion and phase decomposition characteristic for the vacuum conditions leading to a gradual cooling of the hot compressed supercritical fluid down to the liquid phase and eventual solidification. The target modification in this case is limited to the generation of crystal defects and the detachment of the metal target from the overlayer. V

Generation of subsurface voids and a nanocrystalline surface layer in femtosecond laser irradiation of a single-crystal Ag target

“…In particular, the equilibrium melting temperature, T m , determined in liquid-crystal coexistence simulations, is 1139 ± 2 K [8], about 8% below the experimental values of 1235 K [36]. The threshold temperature for the onset of the explosive phase separation into liquid and vapor, T*, determined in simulations of slow heating of a metastable liquid, is found to be ~3450 K at zero pressure and ~4850 K at 0.5 GPa [37]. The onset of the phase explosion can be expected at 10% below the critical temperature [38][39][40] and the values of T* calculated for the EAM Ag material are not in conflict with the range of experimental values of the critical temperature of Ag spanning from 4300 K to 7500 K [41].…”

Electron-phonon thermalization in a scalable method for real-time quantum dynamics

“…This together with the introduction of Langevin thermostats [17] has proved successful in interpreting measured quantities [18,19]. This approach remains of active interest and, in recent years, * Corresponding author: vrizzi01@qub.ac.uk it has evolved into more elaborate methodologies where the nuclear motion is taken into account via classical molecular dynamics simulations, while electrons are treated at increasing levels of sophistication [20][21][22][23][24][25].…”

Atomistic Simulation Study of Short Pulse Laser-Induced Generation of Crystal Defects in Metal Targets

“…These studies have recently been complemented by atomistic molecular dynamics (MD) simulations of short pulse laser interactions with bulk metal substrate covered by a transparent overlayer [56]. The atomistic simulations have provided important insights into the effect of the spatial confinement on the kinetics and mechanisms of laser-induced phase transformations.…”

“…At higher laser fluences, when the surface region of the metal substrate is brought into the supercritical state, the confinement by the overlayer prevents the expansion and phase decomposition of the supercritical fluid, which undergoes rapid cooling to the liquid state, contraction, and detachment from the overlayer. The relatively small lateral size of the computational systems used in the first simulations [56], however, prevented detailed analysis of some of the important processes inferred from the simulation results, such as the evolution of voids leading to the detachment at the metal-overlayer interface or the nucleation and growth of multiple crystallites leading to the formation of nanocrystalline structure in the interfacial region.…”

“…In order to elucidate the fundamental physics of the surface nanostructuring, the laserinduced formation of nanostructures has been investigated in different environments, like vacuum [54], background gas [39], or liquid [55], as well as under conditions of spatial confinement by a transparent overlayer [56]. One of the numerous examples that can be addressed here is the work by Stratakis et al [55] where the formation of nanostructures by short pulse laser ablation of Aluminum (Al) in liquids, water, and ethanol was investigated.…”