1. 1 Phase transitions in femtosecond laser ablation M. Povarnitsyn, K. Khishchenko, P. Levashov Joint Institute for High Temperatures RAS, Moscow, Russia povar@ihed.ras.ru E-MRS 2008 Spring Meeting Strasbourg, France 27 May, 2008

2. 2 1.Introduction 2.Setup parameters 3.Mechanisms of ultrashort laser ablation 4.Numerical model Basic equations Equation of state (EOS) Thermal decomposition model (homogeneous nucleation) Mechanical decomposition model (cavitation) 5.Results Dynamics of ablation Analysis of phase states Sensitivity to EOS 6.Conclusions and future plans Outline

3. 3 Setup parameters laser targets: Al, Au, Cu, Ni = 0.8 mkm,  L = 100 fs, ( FWHM ) F = 0.1  10 J/cm 2 Single pulse, Gaussian profile Actual questions: Heat affected zone (melted zone) Shock wave formation Parameters of the plume Cavitation and fragmentation Generation of nanoclusters Ablation depth vs. laser fluence

4. 4 Stages of ultrashort ablation t = 0 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 t > 10 ps 4. Thermal decomposition and SW and RW generation t ~ 100 ps 5. Mechanical fragmentation V > 10 km/s V ~ 1 km/s V < 1 km/s SW RW

5. 5 Two-temperature multi-material Eulerian hydrodynamics Basic equationsMixture model

6. 6 Two-temperature semi-empirical EOS “Stable” EOS kinetic models sp bn unstable bn “Metastable” EOS

7. 7 Thermal decomposition of metastable liquid Metastable liquid separation into liquid-gas mixture dP/dt = -(P-P eq )/  M dT/dt = -(T-T eq )/  T unstable

8. 8 Model of homogeneous nucleation V.P. Skripov, Metastable Liquids (New York: Wiley, 1974). 0.9Tc<T<Tc unstable

9. 9 Mechanical spallation (cavitation) P P P Time to fracture is governed by the confluence of voids liquid + voids unstable

10. 10 Spallation criteria Minimal possible pressure D. Grady, J. Mech. Phys. Solids 36, 353 (1988). P < -Y 0 Energy minimization

11. 11 Dynamics of ablation of Al target T M    P P P P  F = 5 J/cm 2 M. E. Povarnitsyn et al. Phys. Rev. B 75, 235414 (2007).

12. 12 Results with “stable” and “metastable” EOS SW P ~ 0 P ~ P min <0 SW P ~ 0 (l) unstable F = 5 J/cm 2

13. 13 Ablation of Al target

14. 14 Ablation of Au target

15. 15 Ablation of Cu target

16. 16 Ablation of Ni target

17. 17 Ablation depth vs. fluence Experiment: M. Hashida et al. SPIE Proc. 4423, 178 (2001). J. Hermann et al. Laser Physics 18(4), 374 (2008).

18. 18 Mechanisms of ablation Y. Hirayama, M. Obara Appl. Surf. Science 197-198 (2002) unstable

19. 19 Conclusions and outlook 1.Simulation results are sensitive to the models used: absorption, thermal conductivity, electron-lattice collisions, kinetics of nucleation, fragmentation criteria, EOS, etc… 2.Time-dependent criteria of phase explosion and cavitation in metastable liquid state were introduced into hydrodynamic model 3.Usage of “metastable” and “stable” EOS allows to take into account kinetics of metastable liquid decomposition 4.Observed mechanisms of ablation: thermal decomposition in the vicinity of critical point cavitation in liquid phase at high strain rate and negative pressure 5.Ablation depth correlates with the melted depth 6.Kinetics of melting is in sight