Strong emission of particulates towards the incident beam direction in pulsed-laser ablation experiments

Theoretical predictions suggested that particulates (large clusters and droplets) in pulsed-laser ablation deposition (PLD) move towards the surface normal and constitute a small fraction of the total plume mass. Contrary to expectations, here we report that, independently of the laser beam direction, large clusters are ejected towards the laser direction of incidence, which generally differs from the surface normal. Moreover, fragments and droplets constitute the major fraction of the ablated mass. Cross-sectional SEM investigations performed on the Si targets show that the direction of growth of the columns follows the laser beam direction. These observations have been explained by the change of the microscopic ablation mechanism from monomer evaporation at low local laser fluences to phase explosion at higher local fluences.