Theoretical Determination of the Laser Induced Damage Threshold for Ultrashort Pulse Laser Heating of Metal Films

Abstract

Non-equilibrium heating between electrons and the crystalline lattice has been observed in numerous ultrashort pulsed laser applications yielding unexpected temperature profiles. Typically, laser induced damage threshold (LIDT) values for thin metal films are predicted using the melting temperature and the standard heat diffusion equation. A method is presented which accounts for non-equilibrium heating by solving the Parabolic Two Step (PTS) heat conduction model and deriving an analytical expression for the laser power that causes the film temperature to exceed a critical value. This critical temperature is a design parameter dependent on the material properties of the film and often the repetition rate of the source. In non-equilibrium heating, the predicted peak lattice temperature is significantly lower and occurs at some time after the deposition of energy, which can be orders of magnitude greater than the pulse duration. This minimizes the effect of the temporal shape of the pulse and allows for an analytical solution. This solution and an equation which can be used to calculate the thermal damage threshold value for non-equilibrium laser heating are presented.