Ultra-short pulse laser ablation of metals: threshold fluence, incubation coefficient and ablation rates

In this paper we present femtosecond laser ablation studies of the metals copper, silver and tungsten. Measurements of the threshold fluence determined from the hole diameters versus fluence provide incubation coefficients of the three materials, which are found to be equal within one standard deviation. Furthermore, we have determined the single-shot threshold fluences to be 1.7 +/- 0.3 J/cm(2), 1.5 +/- 0.4 J/cm(2) and 0.44 +/- 0.02 J/cm(2) for copper, silver and tungsten, respectively. These are in good agreement with theoretical values calculated neglecting heat diffusion. The ablation rates in the regimes of high and low fluence have been examined. In the low-fluence regime, a logarithmic dependence has been found, while the high-fluence data are consistent with a linear dependence. A comparison of the high-fluence data with an approximate analytical solution provides the effective absorptance of the samples, i.e. the fraction of the laser energy that is contributing to the ablation process.