Thermomechanical Interaction in Metal Films during Ultrashort Laser Heating

Abstract

An ultrafast thermoelasticity based on a hyperbolic two-step heat conduction model with electron-phonon interaction is applied to investigate the thermomechanical response of five immense homogeneous, isotropic gold films (20 nm, 100 nm, 200 nm, 500 nm, and 2 μm) impacted under femtosecond laser pulses by using finite element method (FEM). Finite element governing equations are established and solved in the time domain directly. The results, including electron temperature, lattice temperature, displacements, stresses, and strains, are presented graphically. The effect of thickness of thin films is studied; in addition, characteristics of stress evolution and displacement development in thin films are obtained. Finally, the influence of hot-electron blast force and thermal-mechanical coupling term are studied.

Keywords:

• ultrafast thermoelasticity
• femtosecond laser
• hot-electron blast force
• thermal-mechanical coupling
• metal films

Nomenclature

Ae,Bl	=	constants of electron scattering rate
Cl	=	lattice specific heat
Ce	=	electronic specific heat
G	=	electron phonon coupling constant
G	=	electron and lattice coupling factor
Kl	=	lattice thermal conductivity
Ke	=	electronic thermal conductivity
qe	=	electronic heat flow vector
ql	=	lattice heat flow vector
Te	=	electron absolute temperature
Tl	=	lattice absolute temperature
T0	=	reference temperature
λ, μ	=	lame coefficients
Λ	=	electron-phonon coupling factor
τe	=	electron thermal relaxation time
τl	=	lattice thermal relaxation time
Q	=	heat source
ρ	=	density
αt	=	linear expansion coefficient
σij	=	stress tensor
eij	=	strain tensor
ui	=	displacement component
γ	=	(3λ + 2μ)αt

Subscripts and Superscripts

(1, 2)	=	material layer
i(j, k)	=	Cartesian coordinates subscript
0	=	value at reference temperature  T0