An improved two-temperature model for metal thin film heating by femtosecond laser pulses

In femtosecond laser processing of metals, the well-known two-temperature models have been widely used to calculate the photon-electron and electron-lattice interactions and energy transport. However, the estimations of some critical parameters, such as the electron heat capacity and reflectivity, in the models are limited to low fluences and cannot correctly predict the damage threshold without using fitting variables. This paper extends the existing two-temperature models by using quantum treatments for free electrons to calculate the time and space dependent optical and thermal properties, including the electron heat capacity, electron relaxation time, electron conductivity, reflectivity and absorption coefficient. The improved two-temperature model is employed to investigate the heating process of thin gold films until melting occurs, which is assumed to be the initiation of damage. The predicted damage threshold fluences for gold films using the proposed new model are in agreement with published experimental data. The effect of pulse duration on the damage threshold fluence is also studied.