Electron-Ion Relaxation, Phase Transitions, and Surface Nano-Structuring Produced by Ultrashort Laser Pulses in Metals

Fundamental physical phenomena in metals irradiated by ultrashort laser pulses with absorbed fluences higher than few tens of mJ/cm(2) are investigated. For those fluences, laser-produced electron distribution function relaxes to equilibrium Fermi distribution with electron temperature T-e within a short time of 10-100 fs. Because the electron subsystem has T-e highly exceeding much the ion subsystem temperature T-i the well-known twotemperature hydrodynamic model (2T-HD) is used to evaluate heat propagation associated with hot conductive electron diffusion and electron-ion energy exchange. The model coefficients of electron heat conductivity (?, T-e, T-i) and electron-ion coupling parameter (?, T-e) together with 2T equation of state E (?, T-e, T-i) and P (?, T-e, T-i) are calculated. Modeling with 2T-HD code shows transition of electron heat wave from supersonic to subsonic regime of prop-agation. At the moment of transition the heat wave emits a compression wave moving into the bulk of met al. Nonlinear evolution of the compression wave after its separation from the subsonic heat wave till spallation of rear-side layer of a film is traced in both 2T-HD modeling and molecular dynamics (MD) simulation. For fluences above some threshold the nucleation of voids in frontal surface layer is initiated by strong tensile wave following the compression wave. If the absorbed fluence is approximate to 30 % above the ablation threshold than void nucleation develops quickly to heavily foam the molten met al. Long-term evolution of the metal foam including foam breaking and freezing is simulated. It is shown that surface nano-structures observed in experiments are produced by very fast cooling of surface molten layer followed by recrystallization of supercooled liquid in disintegrating foam having complex geometry. Characteristic lengths of such surface nanostructures, including frozen pikes and bubbles, are of the order of thickness of molten layer formed right after laser irradiation. ((c) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)