Amplification and regulation of periodic nanostructures in multipulse ultrashort laser-induced surface evolution by electromagnetic-hydrodynamic simulations

The formation of periodic structures in ultrafast laser-irradiated surfaces implies dynamic coupling between the incoming light and the light-driven material. To capture the mutual influence and feedback between light and evolving surface topographies, we investigate numerically the evolution of metal surfaces irradiated by multiple femtosecond laser pulses of sub-, near-, and slightly above-threshold ablation fluence. The multiphysical model combines Maxwell equations and thermohydrodynamic approach based on electron-ion heat transfer and compressible Navier-Stokes equations and allows us to account for interpulse feedback on the resulting surface topographies. First pulses of the subthreshold energy lead to material swelling, nanocavitation few tens of nanometers below the surface, and, as a result, nanoroughness formation on the initially flat surface. Further pulses contribute to the development of periodic surface structures, enhanced absorption, and increased removal rate. Cavitation in the tips of ripples is found to play a crucial role in modification and regulation of surface topography for sub- and near-threshold ablation fluences. At higher laser pulse energy, thermal ablation is mostly involved in surface modification, and the ablation rate per power reaches its maximum at three to five times the ablation threshold fluence, resulting from the optimal heat penetration depth for laser ablation. The numerical results offer a better understanding of the surface topography modifications upon multipulse femtosecond laser irradiation.