Super-Elastic and Plastic Shock Waves Generated by Lasers

It is proved that in the femtosecond (fs) laser experiments there is an elastic shock wave (SW), which propagates before the strong plastic shock with plastic pressures of up to few Megabars. We have found that an initial speed of the melting front is larger than a sound speed because of the fast (supersonic) propagation of an electronic thermal wave into metal. It is shown that the steep part of the elastic compression wave separates from the supersonic melting front when the melting front transits from supersonic to subsonic speed of propagation. Later in time this steep part of the elastic compression wave overturns as result of intersections of elastic characteristics and the elastic SW appears. The elastic SW (ElSW) forms from the steep part which corresponds to the intersection of an isochoric line 0    with a melting curve ) (p Tm . Therefore the elastic pressure ) , ( 0 m ElSW T p p   behind the ElSW is independent from the value of absorbed laser energy abs F and the value of plastic pressure, while the latter is proportional to fluence abs F ; here ) , ( T p  is one-temperature equation of state. This remarkable property may be used for experimental definition of melting curve ) (p Tm . The thermal wave transfers heat into bulk from the skin-layer of metal, where laser energy was absorbed. The thermal wave propagates fast at an early stage when electrons are much hotter than ions: i e T T  . In the case of aluminum this is the first 2-3 ps after the fs heating laser pulse. By hydrodynamics and atomistic simulations and comparison of results of modeling with fs experiments, it is found that there is a continuation of an elastic shock Hugoniot to the pressures, which are much higher (10-50 times) than the usually accepted values for the Hugoniot elastic limit. We have shown that fs lasers are very efficient generators of shocks in condensed matter. We have calculated the elastic and plastic branches of the Hugoniot curves and the spall strength for shocks created by such generators. A new structure of elastic-plastic SW in metals has been discovered [Zhakhovsky et al., PRL, 107, 135502 (2011)]. It is a stationary single structure of elastic and plastic shocks, where radiation of strong elastic pressure pulses from the plastic shock front supports the elastic shock front on the fixed distance from the plastic SW. This means that plastic shock front pushes away the elastic wave, instead of overrun it. Prior to that, people thought that the generated together elastic and plastic shock waves can only propagate in non-stationary split wave regime, where the plastic wave lags behind the elastic wave with time. We have studied the fs laser ablation of aluminum, gold, and nickel films. Formation and reflection of pressure and tensile waves, melting, bubble nucleation and cavitation in the molten layer have been studied. It is shown that ablation of molten metal is caused by cavitation and foaming of melt both taking place within the frontal layer of the film irradiated by a laser pulse. We have analyzed the process of freezing of the cavitation bubbles within the surface layer of the metal films. Spall phenomena caused by reflection of a shock wave are investigated on the rear side of the film. Authors (NAI, SIA, VAK, and MBA) acknowledge support from RFBR grant No. 10-02-00434-а. 1. Super-elastic trace of supersonic melting and laser elastic-plastic shocks Shocks propagating through crystals may have an elastic component and therefore they differ qualitatively from shocks in liquids and gases because the last are purely plastic. It was shown [1-4] that femtosecond (fs) lasers produce an elastic precursor ahead a very strong plastic shock. This has been proved for aluminum and nickel and for plastic pressures up to a few Megabars. We show here that there are three reasons listed below for this surprising behavior (it is unexpected/surprising that an elastic wave survives at so high pressures). Namely, there is supersonic propagation of a twotemperature electron heat conduction wave, first [5-7]. This is shown in Fig. 1 (a).