Ultrafast solid-liquid-vapor phase change of a thin gold film irradiated by femtosecond laser pulses and pulse trains

Abstract

Effects of different parameters on the melting, vaporization and resolidification processes of thin gold film irradiated by femtosecond pulses and pulse train were systematically studied. The classical two-temperature model was adopted to depict the non-equilibrium heat transfer in electrons and lattice. The melting and resolidification processes, which was characterized by the solid-liquid interfacial velocity, as well as elevated melting temperature and depressed solidification temperature, was obtained by considering the interfacial energy balance and nucleation dynamics. Vaporization process which leads to ablation was described by tracking the location of liquid-vapor interface with an iterative procedure based on energy balance and gas kinetics law. The parameters in discussion included film thickness, laser fluence, pulse duration, pulse number, repetition rate, pulse train number, etc. Their effects on the maximum lattice temperature, melting depth and ablation depth were discussed based on the simulation results.

Keywords

melting / evaporation / nucleation dynamics / nanoscale heat transfer

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Acknowledgments

This work was supported by the U.S. National Science Foundation (NSF) (Grant No. CBET-0730143) and the National Natural Science Foundation of China (Grant No. 51129602).

Notations

Be	Coefficient for electron heat capacity/(J·m-3·K-2)
C	Heat capacity/(J·m-3·K-1)
cp	specific heat/(J·kg-1·K-1)
frep	Repetition rate/Hz
G	electron-lattice coupling factor/(W·m-3·K-1)
h	Latent heat of phase change/(J·kg-1)
Ji	Single pulse fluence/(J·cm-2)
Jt	Total energy of a pulse train/(J·cm-2)
k	Thermal conductivity/(W·m-1·K-1)
L	Thickness of the metal film/m
M	Molar mass/(kg·kmol-1)
q''	Heat flux/(W·m-2)
R	Reflectivity
Rg	Specific gas constant/(J·kg-1·K-1)
Ru	Universal gas constant/(J·kmol-1·K-1)
s	Interfacial location/m
S	Intensity of the internal heat source/(W·m-3)
t	Time/s
tp	Pulse width/s
tsep	Separation time/s
T	Temperature/K
TF	Fermi temperature/K
Tm	Melting point/K
u	Interfacial velocity/(m·s-1)
V0	Interfacial velocity factor/(m·s-1)
x	Coordinate/m
Greek Symbols
δ	Optical penetration depth/m
δb	Ballistic range/m
ϵ	Total emissivity
ρ	Density/(kg·m-3)
σ	Stefan-Boltzmann constant/(W·m-2·K-4)
Superscripts
0	Last time step
Subscripts
0	Initial condition
e	Electron
eq	Thermal equilibrium state
i	Pulse sequence
l	Lattice
ℓ	Liquid
R	Thermal radiation
s	Solid
sur	Surface
∞	Ambient environment

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