Numerical Analysis of Ultrashort Pulse Laser-Induced Thermomechanical Response of Germanium Thin Films

Ultrashort pulse laser heating of a germanium nanofilm was simulated using a combined continuumatomistic method that couples the molecular dynamics and a self-consistent energy model for ultrafast lasersemiconductor interaction. Both a single pulse and a pulse burst were considered. To accurately describe laser energy deposition, the transient optical properties were computed based on the Drude formula. It was found that for a single pulse at low fluence (e.g., 0.02 J/cm2), the pulse duration had little impact on the lattice temperature response. In contrast, a higher lattice temperature could be obtained for a longer pulse (e.g., 5 ps) at higher fluences (e.g., 0.06 J/cm2) due to lower surface reflectivity. A strong thermal stress wave could be induced by the laser heating, with its maximum compression and tension occurring in the front and rear film regions, respectively. The investigations of laser burst heating revealed that a laser burst not only can retain the advantages of ultrashort pulse lasers but can enhance the photon efficiency for material melting as well.