Prediction of damage threshold fluences for metal films by an ultrashort laser pulse

At laser intensities near the threshold fluence, the electron temperatures in metals heated by an ultrashort pulse can be comparable to the Fermi temperature. As the existing approximations for material properties used in the two-temperature models are limited to electron temperatures that are much lower than the Fermi temperature, the models are suitable only for low fluences. This paper extends the existing estimations for optical and thermal properties to high electron temperatures by the following improvements: 1) using the Fermi-Dirac distribution, the heat capacity of metal free electrons is calculated; 2) the free electron relaxation time and electron conductivity are determined by using a quantum model derived from the Boltzmann transportation equation for dense plasma; and 3) the free electron heating and interband transition are both taken into account using a modified Drude model with quantum adjustments to calculate the reflectivity and the absorption coefficient. The proposed two-temperature model is employed to calculate the heating process of thin metal films until melting occurs, which is assumed to be the initiation of damage. The predicted damage threshold fluences for 200 nm gold film using the proposed model are in good agreement with published experimental data. The damage threshold fluence as a function of pulse duration is also studied.