1

Phase transitions in femtosecond laser ablation

M. Povarnitsyn, K. Khishchenko, P. Levashov

Joint Institute for High Temperatures RAS, Moscow, Russiapovar@ihed.ras.ru

E-MRS 2008 Spring MeetingStrasbourg, France

27 May, 2008

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

Setup parameters

laser

targets: Al, Au, Cu, Ni = 0.8 mkm,L = 100 fs, ( FWHM )F = 0.110 J/cm2

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

Stages of ultrashort ablation

t = 01. 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

SWRW

RW

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Two-temperature multi-materialEulerian hydrodynamics

Basic equations Mixture model

6

Two-temperature semi-empirical EOS

“Stable” EOS

1

10

1

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Density, g/cm3

Al

l+g

Te

mp

era

ture

, kK

s

lg

s+g

s+l

CP

kinetic models

1

10

1

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Density, g/cm3

l+g

(s)

(g)

(s+l)

(l)

Te

mp

era

ture

, kK

Al

s

lg

s+g

s+l

CP

sp

bnbn

unstable

bn

“Metastable” EOS

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1

10

1

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Density, g/cm3

l+g

(s)

(g)

(s+l)

(l)

Te

mp

era

ture

, kK

Al

s

lg

s+g

s+l

CP

Thermal decomposition of metastable liquid

Metastable liquid separation into liquid-gas mixture

dP/dt = -(P-Peq)/M

dT/dt = -(T-Teq)/T

unstable

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Model of homogeneous nucleation

V.P. Skripov, Metastable Liquids (New York: Wiley, 1974).

1

10

1

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Density, g/cm3

l+g

(s)

(g)

(s+l)

(l)

Te

mp

era

ture

, kK

Al

s

lg

s+g

s+l

CP

0.9Tc<T<Tc

unstable

9

1

10

1

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Density, g/cm3

P = 0 GPa P = -2 GPa P = -5 GPa

l+g

(s)

(g)

(s+l)

(l)

Te

mp

era

ture

, kK

s

lg

s+g

s+l

CP

Mechanical spallation (cavitation)

P

P

P

Time to fracture is governed by the confluence of voids

liquid + voidsunstable

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Spallation criteria

Minimal possible pressure

D. Grady, J. Mech. Phys. Solids 36, 353 (1988).

P < -Y0

Energy minimization

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Dynamics of ablation of Al target

0

10

20

30

40

50

0

10

20

30

40-200 0 200 400 600

0

1

2

3

4D

ensi

ty (

g/cm

3 )

10 ps

-200 0 200 400 600

20 ps

-200 0 200 400 600

0

1

2

3

4

x (nm)

Den

sity

(g/

cm3 )

30 ps

-200 0 200 400 600

x (nm)

80 ps

Pre

ssur

e (G

Pa)

Pre

ssur

e (G

Pa)

TM

P

P

P

P

F = 5 J/cm2

M. E. Povarnitsyn et al. Phys. Rev. B 75, 235414 (2007).

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Results with “stable” and “metastable” EOS

1

10

1

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Density, g/cm3

Al

l+g

Te

mp

era

ture

, kK

s

lg

s+g

s+l

CP

1

10

1

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Density, g/cm3

l+g

(s)

(g)

(s+l)

(l)

Te

mp

era

ture

, kK

Al

s

lg

s+g

s+l

CP

SW

P ~ 0

P ~ Pmin<0

SW

P ~ 0

(l)

unstable

F = 5 J/cm2

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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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1

10

1

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Density, g/cm3

Al

l+g

Te

mp

era

ture

, kK

s

lg

s+g

s+l

CP

1

10

1

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Density, g/cm3

l+g

(s)

(g)

(s+l)

(l)

Te

mp

era

ture

, kK

Al

s

lg

s+g

s+l

CP

Mechanisms of ablation

Y. Hirayama, M. Obara Appl. Surf. Science 197-198 (2002)

unstable

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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