Fundamentals of energy cascade during ultrashort laser-material interactions

During an ultrashort laser pulse, numerous photons are emitted in a very short period of time leading to very high peak power. The photons can excite free electrons in the material to very high temperatures (heating) or strip bound electrons from the atoms (ionization). In ultrashort laser heating there is a time lag between the electron heating and the lattice heating. The two-temperature model has been proposed to calculate the electron temperature and the lattice temperature and the related damage threshold for metals. On the other hand, ablation models based on impact ionization and photoionization have been proposed to predict material removal rates for semiconductors and dielectrics. However, in existing heating or ablation models, some critical thermal and optical properties of the material are assumed to be time, space, and fluence independent or the estimations are limited to temperatures much lower than the Fermi temperature. In this paper, the quantum theories are employed to calculate the free electron heating, free electron relaxation time, and the temporal and spatial dependent thermal and optical material properties. The improved two-temperature model is used to predict damage fluences of gold thin films. The new ablation model based on the Fokker-Planck equation can predict ablation depth and crater shape of semiconductors and dielectrics. The predicted results are in good agreement with experimental data.