Phase transitions in femtosecond laser ablation

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

3. Setup parameters  = 0.8 mkm, L = 100 fs, ( FWHM ) F = 0.110 J/cm2 Single pulse, Gaussian profile targets: Al, Au, Cu, Ni laser • 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. Stages of ultrashort ablation 1. Pulse L ~ 100 fs t = 0 ~10 nm 2. Energy absorption by conduction band electrons t < 1 ps 3. Heat conductivity + electron-lattice collisions V > 10 km/s ~100 nm t ~ 5 ps 4. Thermal decomposition and SW and RW generation RW SW V ~ 1 km/s t > 10 ps RW 5. Mechanical fragmentation V < 1 km/s t ~ 100 ps

5. Two-temperature multi-materialEulerian hydrodynamics Basic equations Mixture model

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

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

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

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

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

11. Dynamics of ablation of Al target F = 5 J/cm2   P P  M T  P P M. E. Povarnitsyn et al. Phys. Rev. B 75, 235414 (2007).

12. P ~ 0 SW P ~ Pmin<0 P ~ 0 SW Results with “stable” and “metastable” EOS F = 5 J/cm2 (l) unstable

13. Ablation ofAl target

14. Ablation ofAu target

15. Ablation ofCu target

16. Ablation ofNi target

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

19. 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 • Usage of “metastable” and “stable” EOS allows to take into account kinetics of metastable liquid decomposition • Observed mechanisms of ablation: • thermal decomposition in the vicinity of critical point • cavitation in liquid phase at high strain rate and negative pressure • Ablation depth correlates with the melted depth • Kinetics of melting is in sight