Dynamic evolution of high spatial frequency femtosecond laser-induced periodic surface nanostructures on germanium thin films

We present a comprehensive theoretical study of the mechanisms lying behind the formation of high spatial frequency laser-induced periodic nanostructures (HSFL-LIPSS) on germanium thin films irradiated by 300 fs pulses (1030 nm wavelength). The study is based on a new model specifically designed (referred to as TTM++), consisting of a double extended two-temperature model coupled with a carrier density rate equation and a generalized plasmonic model. Our model allows for tracking the dynamic mechanisms (e.g., thermal and optical-plasmonic processes) during femtosecond laser irradiation, enhancing the understanding and the control of the very first phases of HSFL-LIPSS formation. We deduce that HSFL-LIPSS result from ultrafast processes like non-thermal melting, where the material lattice remains cold despite the change of state. Additionally, we propose to irradiate germanium films by burst mode at high repetition rate (500 GHz), inducing an ultrafast accumulation effect, and heating the material heating up to 92 %. As a result, the risk of thermal damage is minimized, and high-quality HSFL-LIPSS can be obtained at a very low laser fluence, which is reduced by up to 83 % with respect to conventional methods.