Formation of controllable 1D and 2D periodic surface structures on cobalt by femtosecond double pulse laser irradiation

Controlling the surface morphology at the subwavelength scale is one of the cornerstones of modern nanophotonics. Femtosecond laser-induced surface structuring is a viable technique for the large-scale formation of nano- and microscale structures. A typical example is the formation of one-dimensional laser-induced periodic surface structures (LIPSSs), which can lead to strong modifications of optical and wetting properties of the material surface. Creating two-dimensional (2D) patterned structures, however, is a more challenging and rewarding task. Here, we demonstrate a single step method for fabricating various subwavelength structures on the cobalt (Co) surface using different laser fluences (0.12-0.24J/cm(2)) and time delay (0-30 ps) between double pulses. More importantly, we can control the geometry and organization of the formed structures demonstrating spherical, triangular, rhombic, and high spatial frequency LIPSSs using two temporally delayed orthogonally polarized femtosecond laser beams. We show that the laser fluence and delay time between the two beams are the controlling parameters for realizing the different surface morphologies. We provide a numerically supported, phenomenological model to explain the formed 2D structures. Our model employs elements from both the scattered surface-wave interference and the self-organization theories that are commonly used to explain uniform surface structures.