1. 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
15th APS Topical Conference on Shock Compression of Condensed Matter
Kohala Coast, Hawaii
29 June, 2007

2. 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

3. 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 ?

4. 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

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

6. Interface reconstruction algorithm2D 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

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

8. 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

9. 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).

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

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

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

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

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

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

16. 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

17. 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