Thermomechanical wave propagation in gold films induced by ultrashort laser pulses

Abstract

The thermomechanical behavior of gold films caused by ultrashort pulse lasers is investigated using a combined continuum-atomistic approach with the inclusion of the hot electron blast force. During the very early laser-metal interaction, a nonequilibrium thermal state and a significant hot electron blast wave are generated in the films. The laser heating leads to an initial compressive stress with the peak occurring near the irradiated side of the films. For a 100-nm film, a conversion from compression to tension starts in the mid-portion, the resulting tensile stress converts back to compression, and then the above stress conversions repeat. For the thicker films of 500 nm and 1 μm, the tensile stress starts from the irradiated side due to the free surface stress reflection, and the resulting two-fold shock wave comprising tension and compression propagates towards the rear surface and then reflects back to the front-side. The effect that the thermal stress decreases with increase of the pulse duration is more pronounced in thinner films. Based on the simulated dynamic stress responses, nonthermal damage to the 100-nm film could occur in its mid-depth region while the material could be nonthermally removed from either the front or rear-side of the 500-nm and 1-μm films.