Periodic surface structures on Ni-Fe film induced by a single femtosecond laser pulse with diffraction rings

This Letter reports the formation of periodic surface structures on Ni-Fe film irradiated by a single femtosecond laser pulse. A concave lens with a focus length of -150 mm is placed in front of an objective (100x, NA = 0.9), which transforms the Gaussian laser field into a ring distribution by the Fresnel diffraction. Periodic ripples form on the ablation area after the irradiation of a single femtosecond laser pulse, which depends on the laser polari-zation and laser fluence. We propose that the ring structure of the laser field leads to a similar transient distribution of the permittivity on the sample surface, which further launches the surface plasmon polaritons. The interaction of the incident laser with surface plasmon polaritons dominates the formation of periodic surface structures.