Role of the temperature dynamics in formation of nanopatterns upon single femtosecond laser pulses on gold

In this paper we investigate the role of two-temperature heating dynamics for formation of periodic structures on metal surfaces exposed to single ultrashort laser pulses. The results of two-temperature model (TTM) twodimensional simulations are presented on the irradiation of gold by a single 800-nm femtosecond laser pulse the intensity of which is modulated in order to reproduce an initial electron temperature perturbation, which can arise from incoming and scattered surface wave interference. The growing (unstable) modes of the lattice temperature distribution along the surface may be significant in the 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 instabilitylike behavior 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 behavior 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.