Evolution and mechanism of the periodical structures formed on Ti plate under femtosecond laser irradiation

•Investigate the evolution and succession of three types of fs-laser induced periodic surface structures (FLIPSSs).•Achieve the laser processing window of each type of the FLIPSS.•Explain the formation and evolvement of the ripple structure in the respective of surface plasma wave (SPW).•Ascribe the interaction between the transmitted laser and the reflected laser at the two surfaces of the biconvex lens to the formation of ring structure.

This work investigates the femtosencond laser (fs-laser) induced periodical surface structures (FLIPSS) on titanium plate including the concentric rings, microgrooves and subwavelength ripples. The evolution of the three types of the structures at different laser fluence and shot number is investigated experimentally in detail. The competition mechanisms exist among the different FLIPSS. A processing window for each resulting FLIPSS is obtained. In order to give an overall understanding of the FLIPSS, the formation mechanisms of each type of FLIPSS are discussed. The formation of the ripples is well explained by the propagating of the surface plasma wave (SPW) on the air/Ti interface. The evolutions of the ripple distribution are well understood according to this model as well. It is concluded that the interaction of the scattered wave of the laser light with the surface wave is concluded to give rise to the microgroove structure. According to our observation, the shape of the concentric rings does not change with the variation of the laser fluence and pulse number. The structure could be originated from the optical interference between the transmitted and reflected laser beams by the two surfaces of the biconvex lens. This investigation could not only make a further understanding of the formations of FLIPSS but also provide the possibility to control the surface morphologies in laser processing.