Investigation of the role of pulse duration and film thickness on the damage threshold of metal thin films irradiated with ultrashort laser pulses

The precise evaluation of the damage threshold constitutes a key issue for efficient material processing with ultrashort laser pulses. One particular area of increasing interest which is largely, still, unexplored is the laser-based processing of thin metal materials and, more particularly, films of thickness comparable to the optical penetration depth. In this work, we present a detailed investigation of the combined effects of the pulse width and the thickness of the irradiated target on the damage threshold of thin films. Through an analysis of the optical response of thin Nickel films and the employment of a Two Temperature Model (TTM), the interplay of the two competing mechanisms that account for the energy dissipation (i.e. the electron diffusion and electron–phonon coupling) as a function of the pulse duration and the thickness is discussed. The ultrafast dynamics and thermal response of Nickel films (of thicknesses ranging from 10 nm to some hundreds nanometres) upon irradiation with ultrashort laser pulses (of pulse width ranging from sub-100 fs to some tens picoseconds) were explored. The simulation results reveal a non-linear connection between the damage threshold and the two aforementioned parameters while the investigation of the role of those parameters in the electron diffusion length and relaxation time yields remarkable dependencies. Experimental results attained for a range of values of the pulse duration manifest that the damage threshold exhibits a minimum for a pulse width comparable to the electron–phonon relaxation time regardless of the film thickness.