Kinetics of laser-induced melting of thin gold film: How slow can it get?

Abstract

Melting is a common and well-studied phenomenon that still reveals new facets when triggered by laser excitation and probed with ultrafast electron diffraction. Recent experimental evidence of anomalously slow nanosecond-scale melting of thin gold films irradiated by femtosecond laser pulses motivates computational efforts aimed at revealing the underlying mechanisms of melting. Atomistic simulations reveal that a combined effect of lattice superheating and relaxation of laser-induced stresses ensures the dominance of the homogeneous melting mechanism at all energies down to the melting threshold and keeps the time scale of melting within ~100 picoseconds. The much longer melting times and the prominent contribution of heterogeneous melting inferred from the experiments cannot be reconciled with the atomistic simulations by any reasonable variation of the electron-phonon coupling strength, thus suggesting the need for further coordinated experimental and theoretical efforts aimed at addressing the mechanisms and kinetics of laser-induced melting in the vicinity of melting threshold.

Melting Is Well-Known, but Is It Also Well-Understood?

“…For such rates, heterogeneity in the temperature distribution results as well as wave propagation within the core, which causes oscillation in pressure and temperature due to thermoelastic coupling, although the effect on the melting temperatures and the maximum attainable temperature of Al at the interface attained before the fracture of the oxide shell appears to be relatively small for the heating rates used. Experiments on Au films with a relatively weak electron− phonon coupling were done by Arefev et al 624 This leads to an increased time scale of lattice heating and a separation of nonthermal effects defined by the electronic excitation from thermally driven atomic dynamics and phase transformations. The authors also discussed the effect of the amount of superheating, as this affects the melting time significantly.…”

“…The threshold energy density for complete melting ε mel can be evaluated by integration of the temperature-dependent heat capacity c P (T) from 300 K to T mel and addition of the enthalpy of melting Δ m H. Consequently, the superheating energy density ε is often expressed in terms of ε mel . The following discussion is largely taken from the corresponding one by Arefev et al 624 Measurements performed for 20 nm Au films irradiated by 200 fs laser pulses ε = 1.5ε mel and ε = 1.7ε mel revealed a melting process that starts at about 7 ps and takes approximately 3 ps to complete. 625 These results are consistent with a melting time of about 7 ps reported for 35 nm Au film irradiated by a 90 fs laser pulse at ε = 1.8ε mel .…”

“…The apparent disagreement of the above results with the results of the two-temperature MD simulations, where the melting time remains below 100 ps as the deposited energy density decreases down to ε mel , has been attributed to the inaccuracies of interatomic potentials and overestimation of the strength of the electron−phonon coupling. 612,631 The aim of Arefev et al 624 was to check the hypothesis that the discrepancy between the time of complete melting observed in the experiments and that predicted in earlier simulations can be eliminated by using an improved interatomic potential and assuming a lower strength of the electron−phonon coupling. However, their calculations indicate that the long melting times in the vicinity of the melting threshold and the contribution of the heterogeneous melting inferred from the experiments cannot be reconciled with the atomistic simulations by any reasonable variation of the electron−phonon coupling parameter.…”

“…Consequently, the superheating energy density ε is often expressed in terms of ε mel . The following discussion is largely taken from the corresponding one by Arefev et al…”

“…Experiments on Au films with a relatively weak electron–phonon coupling were done by Arefev et al This leads to an increased time scale of lattice heating and a separation of nonthermal effects defined by the electronic excitation from thermally driven atomic dynamics and phase transformations. The authors also discussed the effect of the amount of superheating, as this affects the melting time significantly.…”