Short and Ultrashort Pulsed Laser Processing of Zinc : Resolidification Morphology of Ablated Craters

Laser surface texturing of polycrystalline zinc in bulk form and as galvanised coatings is performed with single, as well as multiple nanosecond and picosecond laser pulses at a wavelength of 1030 nm. By varying the laser fluence over a wide range, as well as varying the number of pulses on the same location, thousands of ablated craters can be fabricated and their morphology studied. From the analysis thereof, it can be concluded that, within the boundaries of our experimental (laser) conditions, ablated craters always show molten features. Depending on the pulse fluence and duration, the final solidifed morphology can feature jets with meso- and micro scale spherical endings, micro rims and protrusions, as well as nano-roughness or a rim around the ablated crater. If the laser fluence is high, one observes the evidence of surface waves and finger formation, resulted from different types of hydrodynamic instablity. In contrast to nanosecond laser processing, it is found that craters formed using picosecond pulses deviate significantly from the Gaussian shape of the laser intensity distribution at high laser intensity, due to different temperature gradients in the electronic and lattice subsystems.