  In this paper we investigate whether the periodic structures on metal
surfaces exposed to single ultrashort laser pulses can appear due to an
instability induced by two-temperature heating dynamics. The results of
two-temperature model (TTM) 2D simulations are presented on the irradiation of
gold by a single 800 nm femtosecond laser pulse whose intensity is modulated in
order to reproduce a small initial temperature perturbation, which can arise
from incoming and scattered surface wave interference. The growing (unstable)
modes of the temperature distribution along the surface may be responsible for
the LIPSS (Laser Induced Periodic Surface Structures) formation. After the end
of the laser pulse and before the complete coupling between lattice and
electrons occurs, the evolution of the amplitude of the subsequent modulation
in the lattice temperature reveals different tendencies depending on the
spatial period of the initial modulation. This instability-like behaviour is
shown to arise due to the perturbation of the electronic temperature which
relaxes slower for bigger spatial periods and thus imparts more significant
modulations to the lattice temperature. Small spatial periods of the order of
100 nm and smaller experience stabilization and fast decay from the more
efficient lateral heat diffusion which facilitates the relaxation of the
electronic temperature amplitude due to in-depth diffusion. An analytical
instability analysis of a simplified version of the TTM set of equations
supports the lattice temperature modulation behaviour obtained in the
simulations and reveals that in-depth diffusion length is a determining
parameter in the dispersion relation of unstable modes. Finally it is discussed
how the change in optical properties can intensify the modulation-related
effects.
