Femtosecond time-resolved pump-probe measurements on percolating gold in the ablation regime

We report on femtosecond laser ablation experiments on percolating gold layers deposited on a glass substrate. In our experiments, we measure changes in optical transmission and reflection induced by single, high-intensity infrared laser pulses as a function of the time delay between the pump and the probe. For the highest pump intensities we find that on a time scale of about 150 ps after excitation, the transmission and reflection approach values close to the substrate values. We attribute this rapid ablation to vaporization of the entire layer when the injected energy exceeds the cohesive energy of the material. This vaporization results in the rapid transformation of the gold layer into a sufficiently dilute mist of atoms and nano-particles which renders the material almost optically transparent to the probe pulse. SEM images of the surfaces show how the morphology of the films changes at relatively low excitation intensities and show the complete removal of the gold at high intensities. We find that the ablation threshold for percolating Au on glass is 2.3 × 10¹¹ W/cm², which is two orders of magnitude lower than the damage threshold for continuous gold layers as reported in the literature.