Strategies for improving the regularity of high spatial frequency laser-induced periodic surface structures on titanium materials upon sub-ps-laser areal scan processing

High spatial frequency laser-induced periodic surface structures (HSFL) exhibiting sub‐100 nm spatial periods – far below the optical diffraction limit – can be easily manufactured on metals by ultrashort pulse laser‐based irradiation. Such single step processing of laser-induced nanostructures enables various surface functionalities for applications in medicine, tribology, and energy conversion. These applications may crucially depend on the regularity of the HSFL. One of the most relevant metals for such applications forming pronounced HSFL is titanium and its alloys. In this work we systematically study the impact of areal laser processing parameters and strategies for improving the intrinsic regularity of HSFL on different titanium materials (Ti, Ti6Al4V). This includes the investigation of the relevance of the materials grain structure through polished and over-coated bulk samples. Also, the influence of laser processing parameters, such as fluence, number of effective pulses, scan line orientation, polarization, pulse repetition rate, and number of over-scans is explored. For quantifying the regularity of HSFL morphologies / topographies, a detailed analysis of scanning electron and atomic force microscopic images is performed with the recently developed freely available ReguΛarity software tool. Based on Fourier analysis methods, it allows objective derivation of relevant parameters such as the spatial periods, its relative spreads and angular dispersion. Based on these strategies, the optimal parameters for the homogeneous areal processing of HSFL on titanium materials using ultrashort near infrared (NIR) laser pulses are determined in order to achieve the highest possible regularity.