Generalized Langevin dynamics for anisotropic spallation in ultrafast laser-material interactions

This work presents a computational framework for ultrafast laser ablation in metals that incorporates generalized Langevin dynamics (GLD) into the established two-temperature model-molecular dynamics (TTM-MD) approach. Unlike conventional Langevin dynamics (CLD) with uniform phonon damping, GLD introduces wavevector-dependent electron-phonon coupling, enabling anisotropic modeling of pressure-wave dynamics crucial for spallation-driven ablation. In nickel, simulations reveal up to 20% lower ablation yield along the ⟨110⟩ direction compared to ⟨100⟩ at threshold fluence, due to stronger shock attenuation and shorter phonon lifetimes. Moreover, the GLD-enhanced model, when integrated with the Helmholtz formulation for optical absorption, predicts an ablation threshold of 300 mJ/cm², consistent with experimental values (270-340 mJ/cm²). This framework reduces dependence on ad hoc parameter tuning and captures orientation-sensitive laser-matter interactions, making it well-suited for microstructure-sensitive predictions in ultrafast laser processing of polycrystalline materials.