Femtosecond laser induced surface swelling in poly-methyl methacrylate.

We show that surface swelling is the first step in the interaction of a single femtosecond laser pulse with PMMA. This is followed by perforation of the swollen structure and material ejection. The size of the swelling and the perforated hole increases with pulse energy. After certain energy the swelling disappears and the interaction is dominated by the ablated hole. This behaviour is independent of laser polarization. The threshold energy at which the hole size coincides with size of swelling is 1.5 times that of the threshold for surface swelling. 2D molecular dynamics simulations show surface swelling at low pulse energies along with void formation below the surface within the interaction region. Simulations show that at higher energies, the voids coalesce and grow, and the interaction is dominated by material ejection.