Numerical simulations of germanium nanofilm under femtosecond pulse laser heating

Femtosecond pulse laser heating of a germanium nanofilm is simulated by a method coupling the molecular dynamics and an energy transfer model for ultrafast laser interaction with semiconductors. Simulations demonstrate that the carrier temperature and density drastically evolve at the early heating time, while the lattice temperature gradually rises until the carrier-lattice thermal equilibrium is reached. The surface reflectivity dynamically changes as the carrier density evolves. The femtosecond laser heating can cause a strong thermal stress wave in the film. Initially, a compressive wave is yielded with the peak compression near the irradiated surface. Then, the compression wave transforms into a two-fold wave including compression and tension. At the rear film side, a strong tensile wave occurs with the maximum tension near the back surface. It is also found that shorter laser wavelength brings not only higher carrier temperature and density but also higher lattice temperature and larger thermal stresses.