Ultrafast dynamics and ablation mechanism in femtosecond laser irradiated Au/Ti bilayer systems

The significance of ultrafast laser-induced energy and mass transfer at interfaces has been growing in the field of nanoscience and technology. Nevertheless, the complexity arising from non-linear and non-equilibrium optical-thermal-mechanical interactions results in intricate transitional behaviors. This complexity presents challenges when attempting to analyze these phenomena exclusively through modeling or experimentation. In this study, we conduct time-resolved reflective pump-probe imaging and molecular-dynamics coupled two-temperature model (MD-TTM) simulations to investigate the ultrafast dynamics and ablation mechanism of Au/Ti bilayer systems. The calculated energy absorption curves indicate that Au film reduces the energy deposition in the underlying Ti layer, resulting in reduced melting and evaporation rate of Ti. The phase transition process induces different mechanical responses. The potential energy patterns indicate that the expansion of vapor Ti extrudes the surface Au layer outward. In simulated stress distribution images, the Au layer can hamper the expansion of the vapor-phase Ti and brings dynamic compressive stress to the residual Ti layer. When the compressive stress transforms into tensile stress, the material is removed through mechanical damage. Therefore, both Au and Ti in the 20 nm Au-covered Ti are completely removed. Our approach elucidates the ablation mechanism within the Au/Ti bilayer system and offers fresh insights into managing thermo-mechanical responses within analogous systems.