Simulation of phase transitions and material decomposition in ultrashort laser–metal interaction M. Povarnitsyn*, K. Khishchenko, P. Levashov Joint Institute for High Temperatures of RAS, Moscow, Russia *povar@ihed.ras.ru 15th APS Topical Conference on Shock Compression of Condensed Matter Kohala Coast, Hawaii 29 June, 2007

Motivation Laser machining, micro- and nanostructuring, laser-induced plasma spectroscopy (LIPS), nanoparticle synthesis in vacuum or in a liquid solution, medical imaging, laser surgery, etc. Outline Problem setup main parameters Mechanisms of ultrashort laser ablation Numerical model Basic equations Equations of state (EOSs) Thermal decomposition model (homogeneous nucleation) Mechanical decomposition model (spallation) Results Dynamics of ablation Analysis of phase trajectories Ablation in the case of different EOSs Conclusions and future plans

Setup parameters targets: Al, Au, Cu, etc.  = 0.8 mkm, L = 100 fs, ( FWHM ) F = 0.15 J/cm2 Single pulse, Gaussian profile targets: Al, Au, Cu, etc. laser Actual questions: Heat wave propagation ? Melted zone depth ? Cavitation and fragmentation ? Parameters of the plume ? Generation of nanoparticles, clusters and chunks ? Ablation depth vs laser flux ?

Stages of ultrashort ablation 1. Pulse L ~ 100 fs ~10 nm t < 1 ps 2. Energy absorption by conduction band electrons ~100 nm t ~ 5 ps 3. Heat conductivity + electron-lattice collisions V > 10 km/s t > 10 ps 4. Thermal decomposition and SW and RW generation V ~ 1 km/s t ~ 100 ps 5. Mechanical fragmentation V < 1 km/s

Basic equations Two-temperature single-fluid multi-material Eulerian hydrodynamics with sources of absorption and energy exchange

Interface reconstruction algorithm 2D 3D j+1 j j-1 i-1 i i+1 U*t U* (a) (b) (c) (d) D. Youngs (1987) D. Littlefield (1999) Symmetric difference approximation or some norm minimization is used to determine unit normal vector (e) Specific corner and specific orientation choice makes only five possible intersections of the cell

Two-temperature semi-empirical EOS Stable EOS Metastable EOS Sp Bn “instant relaxation”   0 “frozen relaxation”    kinetic models

Thermal decomposition of metastable liquid liquid + gas Metastable liquid separation into liquid-gas mixture Terms used: homogeneous nucleation; phase explosion; explosive boiling; critical point phase separation

Model of homogeneous nucleation 0.9Tc<T<Tc V. P. Skripov, Metastable Liquids (New York: Wiley, 1974). S. I. Tkachenko, V. S. Vorob'ev, and S. P. Malyshenko, J. Phys. D: Appl. Phys. 37, 495 (2004).

Mechanical spallation (cavitation) liquid + voids Time to fracture is governed by the confluence of voids

Spallation criteria P < -Y0 Minimal possible pressure Energy minimization D. Grady, J. Mech. Phys. Solids 36, 353 (1988).

Dynamics of ablation of Al target F = 5 J/cm2   P P  M T  P P

Ablation dynamics of Al target = 0.8 mkm  = 100 fs F = 5 J/cm2 Al

Results with stable and metastable EOSs P ~ 0 SW P ~ Pmin<0 P ~ 0 SW (l)

Ablation depth in Al target 1. Povarnitsyn et al, PRB 75, 235414 (2007); 2. Amoruso et al, Appl. Phys. 98, 044907 (2005); 3. Colombier et al, PRB 71, 165406 (2005); 4. Komashko et al, Appl. Phys. A 69, S95 (1999); 5. Vidal et al, PRL 86, 2573 (2001)

Conclusions and Outlook Simulation results are sensitive to the models used: absorption, thermal conductivity, electron-lattice collisions, kinetics of nucleation, fragmentation criteria, EOS, etc… Time-dependent criteria of phase explosion and cavitation in metastable liquid state were introduced into hydrodynamic model Observed decomposition of ablated substance is due to: thermal phase separation in the vicinity of critical point mechanical fragmentation of liquid phase at high strain rates and negative pressures 4. Usage of metastable and stable equations of state allows to take into account kinetics of metastable phase separation in metastable liquid Ablation depth correlates with the melted depth Treatment of individual droplets and bubbles will be introduced since their size may be comparable with the size of grid cells

Conclusions and Outlook Simulation results are sensitive to the models used: absorption, thermal conductivity, electron-lattice collisions, kinetics of nucleation, fragmentation criteria, EOS, etc… Time-dependent criteria of phase explosion and cavitation in metastable liquid state were introduced into hydrodynamic model Observed decomposition of ablated substance is due to: thermal phase separation in the vicinity of critical point mechanical fragmentation of liquid phase at high strain rates and negative pressures 4. Usage of metastable and stable equations of state allows to take into account kinetics of metastable phase separation in metastable liquid Ablation depth correlates with the melted depth Treatment of individual droplets and bubbles will be introduced since their size may be comparable with the size of grid cells