Maskless formation of uniform subwavelength periodic surface structures by double temporally-delayed femtosecond laser beams

Surface structures with nanoscale size and periodicity are important in controlling the flow of light as well as light-matter interaction. Fabrication of these periodic nanostructure requires sophisticated and expensive nanofabrication methods that limits large scale production of such structures. Laser induced periodic surface structures (LIPSS) is a scalable and cheap method to create periodic structures. However, they suffer from lacking a long-range order. Here, we present a maskless fabrication technique of creating highly uniform sub-wavelength structures on nickel surface, using two collinear femtosecond laser beams at various temporal delays. Our femtosecond laser induced periodic surface structures (FLIPSSs) show a high spatial uniformity with a grating period ranging between 320-350 nm. The high spatial uniformity is a consequence of using two pulsed beams with short inter-beam delay time due to reduction in the propagation length of excited surface plasmon polaritons (SPPs). The sub-wavelength period is due to grating splitting mechanism where the two beams sequentially interact with existing structures to create a superimposed grating. The demonstrated fabrication method enables large scale fabrication of regular sub-wavelength structures.