Two-temperature hydrodynamics of laser-generated ultrashort shock waves in elasto-plastic solids

Abstract

View the article online for updates and enhancements. Abstract. Shock-wave generation by ultrashort laser pulses opens new doors for study of hidden processes in materials happened at an atomic-scale spatiotemporal scales. The poorly explored mechanism of shock generation is started from a short-living two-temperature (2T) state of solid in a thin surface layer where laser energy is deposited. Such 2T state represents a highly non-equilibrium warm dense matter having cold ions and hot electrons with temperatures of 1-2 orders of magnitude higher than the melting point. Here for the first time we present results obtained by our new hybrid hydrodynamics code combining detailed description of 2T states with a model of elasticity together with a wide-range equation of state of solid. New hydro-code has higher accuracy in the 2T stage than molecular dynamics method, because it includes electron related phenomena including thermal conduction, electron-ion collisions and energy transfer, and electron pressure. From the other hand the new code significantly improves our previous version of 2T hydrodynamics model, because now it is capable of reproducing the elastic compression waves, which may have an imprint of supersonic melting like as in MD simulations. With help of the new code we have solved a difficult problem of thermal and dynamic coupling of a molten layer with an uniaxially compressed elastic solid. This approach allows us to describe the recent femtosecond laser experiments. IntroductionShock compression of solids was being studied intensively last several decades [1]. Hugoniot Elastic Limit (HEL) is a key concept in this branch of science. Above the HEL uniaxially deformed state becomes impossible -instead, isotropization of stresses and deformations takes place. For such metals as aluminum, nickel, and so on, the usual values of HEL are small -this means that an elastic shock wave (SW) driven by piston at a stress near HEL has velocity D el only 1 − 2% higher than elastic sound speed c el . Until recently, before works [2-6], the elastic branch of Hugoniot was widely accepted as a linear function for stresses below the common HEL. This is why a strong elastic SW firstly observed in the excellent pump-probe experiments with femtosecond lasers done by Evans et al. [7] and Gahagan et al. [8] was not recognized as an elastic wave. Such an elastic SW moves with velocity notably higher the longitudinal sound speed, and notably faster than the plastic SW with the same amplitude of pressure. Estimating those papers we can say today that they were well ahead of their time. Recent calculations [9] show that in the experiments [7] degree of nonlinearity of elastic SW was significant D el /c el −1 ≈ 0.1−0.2. In

Atomistic simulation of laser-pulse surface modification: Predictions of models with various length and time scales

“…One of the ways to simulate this situation is a continuum approximation with the two-temperature equation of state. [15][16][17] Such approach is a powerful tool for study of the surface modification by laser pulse. Nevertheless, this method does not take into account the phenomena existing at the atomistic level (such as decay of metastable phase or nucleation) that are essential for correct description of the surface modifications at threshold fluences.…”

Picosecond-Nanosecond Laser Flash, Formation of powerful Elastic Waves in Crystals, and Shock Peening

“…The complexity here is in the end-to-end consideration of the elastic-plastic behavior together with the phenomena of phase transitions of the first kind: melting/crystallization and condensation. Previously, this problem was solved for the case of ultrashort pulse: see work with molecular dynamics [7][8][9]11], as well as hydrodynamic studies in which multiphase and phase transitions are embedded together with elasto-plasticity [52,59,60].…”