Ultrafast Dynamics Reconstruction of Invar Alloy Based on Molecular Dynamics Coupled with the Two-Temperature Model and Multilayer Film Optics

The exploration of the interaction process between femtosecond lasers and metals through experimental and theoretical methods has become a major research focus. However, the comprehensive analysis of the transient thermal and optical information inside the ablated metal induced by a femtosecond laser is a challenging task. Ultrafast dynamics processes for an Invar alloy under femtosecond laser irradiation are reported in this work. Ablation mechanisms including ultrafast melting, expansion, spallation, and transition mode between spallation and phase explosion for Invar alloy are analyzed through experiments and simulations. Adopting the theoretical framework of molecular dynamics coupled with a two-temperature model (MD-TTM) and multilayer film optics, the relative reflectivity images reconstructed are in good agreement with pump–probe experiments. 3D transient complex refractive index tomography of Invar alloy after femtosecond laser excitation has been obtained. By coordinating the experiments with theories, the electro-thermodynamic pathways constructed on a four-state diagram (reflectivity, temperature, pressure, and density) for transient ablation of Invar alloy offer a means of quantifying the photothermal information. The methods employed precisely bridge experiments with theories, which may contribute to investigating ultrafast ablation in different material systems, meeting photothermal data requirements for high-temperature states, and precisely controlling material eruption and redeposition by adjustable subpulse intervals and repetition rates.