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

摘要

In this paper we investigate the role of two-temperature heating dynamics for formation of periodic structureson metal surfaces exposed to single ultrashort laser pulses.The results of two-temperature model (TTM) twodimensionalsimulations are presented on the irradiation of gold by a single 800-nm femtosecond laser pulsethe intensity of which is modulated in order to reproduce an initial electron temperature perturbation, whichcan arise from incoming and scattered surface wave interference. The growing (unstable) modes of the latticetemperature distribution along the surface may be significant in the laser induced periodic surface structuresformation. After the end of the laser pulse and before the complete coupling between lattice and electronsoccurs, the evolution of the amplitude of the subsequent modulation in the lattice temperature reveals differenttendencies depending on the spatial period of the initial modulation. This instabilitylike behavior is shown toarise due to the perturbation of the electronic temperature which relaxes slower for bigger spatial periods and thusimparts more significant modulations to the lattice temperature. Small spatial periods of the order of 100 nm andsmaller experience stabilization and fast decay from the more efficient lateral heat diffusion which facilitates therelaxation of the electronic temperature amplitude due to in-depth diffusion. An analytical instability analysis ofa simplified version of the TTM set of equations supports the lattice temperature modulation behavior obtained inthe simulations and reveals that in-depth diffusion length is a determining parameter in the dispersion relation ofunstable modes. Finally, it is discussed how the change in optical properties can intensify the modulation-relatedeffects.