Phase%20transitions%20in%20femtosecond%20laser%20ablation

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

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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

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Stages of ultrashort ablation
1. Pulse ?L 100 fs
t 0
10 nm
2. Energy absorption by conduction band
electrons
t lt 1 ps
3. Heat conductivity electron-lattice
collisions
100 nm
V gt 10 km/s
t 5 ps
4. Thermal decomposition and SW and RW
generation
RW
SW
V 1 km/s
t gt 10 ps
RW
5. Mechanical fragmentation
V lt 1 km/s
t 100 ps
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Two-temperature multi-materialEulerian
hydrodynamics
Basic equations
Mixture model
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Two-temperature semi-empirical EOS
Stable EOS
Metastable EOS
bn
bn
bn
sp
unstable
kinetic models
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Thermal decomposition of metastable liquid
Metastable liquid separation into liquid-gas
mixture
unstable
dP/dt -(P-Peq)/?M dT/dt -(T-Teq)/?T
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Model of homogeneous nucleation
0.9TcltTltTc
unstable
V.P. Skripov, Metastable Liquids (New York
Wiley, 1974).
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Mechanical spallation (cavitation)
P
P
P
unstable
liquid voids
Time to fracture is governed by the confluence of
voids
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Spallation criteria
Minimal possible pressure
P lt -Y0
Energy minimization
D. Grady, J. Mech. Phys. Solids 36, 353 (1988).
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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).
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Results with stable and metastable EOS
F 5 J/cm2
(l)
unstable
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Ablation of Al target
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Ablation of Au target
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Ablation of Cu target
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Ablation of Ni target
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Ablation depth vs. fluence
Experiment M. Hashida et al. SPIE Proc. 4423,
178 (2001). J. Hermann et al. Laser Physics
18(4), 374 (2008).
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Mechanisms of ablation
unstable
Y. Hirayama, M. Obara Appl. Surf. Science
197-198 (2002)
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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