Recently, femtosecond (fs) laser operating in GHz burst pulses has emerged as a highly efficient tool for high-performance materials processing in terms of processing quality and speed over the conventional irradiation scheme of fs laser pulses (single-pulse mode). Our group has demonstrated that linearly polarized GHz burst mode fs pulses can fabricate unique lattice-like, two-dimensional (2D) periodic structures on Si and Ti surfaces. This is significantly different from the stripe-patterned (1D) periodic structures created by the single-pulse mode. Meanwhile, it is known that the single-pulse mode with circular polarization can create 2D dot-patterned structures on various solid material surfaces, which could offer different surface functionality compared to the structures created by the linearly polarized beams. In this study, we aimed at surface nanostructuring by circularly polarized fs GHz burst pulses to explore the possibility of forming novel surface structures for surface functionalization.


In the experiment, circularly polarized fs laser pulses (wavelength: 1030 nm, pulse duration:  220 fs) in the single-pulse and GHz burst modes (intra-pulse repetition rate: 4.88 GHz) were focused onto Ti surfaces. The single-pulse mode created the 2D array of dot-pattered structures with a diameter and a period of approximately 500 nm and 700 nm, respectively, as already reported by some groups. Interestingly, we found that novel rock-surface and tetrapod-like 2D nanostructures were fabricated by the GHz bursts with two and three intra-pulses, respectively. Moreover, we have found that the surface morphology formed with the GHz bursts strongly depends on the number of intra-pulses.