Dynamics of femtosecond laser-induced melting of silver

Abstract

We use optical third-harmonic generation to measure the melting dynamics of silver following femtosecond laser excitation. The dynamics reveals an unusual two-step process that is associated with the extreme electronic temperatures and very short time and length scales. In the first, which lasts a few picoseconds, the electron and phonon systems begin to equilibrate, and a thin surface layer undergoes melting. Heat conduction during this period is strongly suppressed by electron scattering from $d$-band excitations. In the second stage, the surface region remains above the melting temperature for a surprisingly long time, 20–30 ps, with the melt front propagating into the bulk at a velocity of $\approx 350\text{m}{\text{s}}^{-1}$. In this stage, the electron and phonon systems again fall out of equilibrium and conduction of heat away from the surface region is now limited by the weak electron-phonon $\left(e\text{−}p\right)$ coupling. From our model calculation, we propose that the melt depths in noble metals irradiated by femtosecond lasers are limited to thicknesses on the order of two to three times of the optical-absorption depth of the light.