Material swelling as the first step in the ablation of metals by ultrashort laser pulses

The ablation from a single-crystal Al(111) surface with single ultrashort near-infrared laser pulses has been investigated under ultrahigh vacuum conditions. Scanning-electron and atomic-force microscopy of the irradiated surface reveal a surprising development of the material response at increasing fluence: at low fluence, swelling exceeding one hundred nanometers dominates. At higher fluences, a hole is gradually formed in the swollen material, which eventually reaches below the original surface level. The observations indicate the significance of mechanical effects during ablation.