## 3 Results
For the second process (N = 60 + 40), the initial rippled structures induced by the first series of pulses are equivalent to the preset grating structures in our previous study [30]. The existing ripples can significantly enhance the energy absorption and the SP-laser coupling, and the SP-laser coupling intensities are much more intense in the grooves, which makes the fabrication of subwavelength ripples more efficient [30]. In order to investigate the detailed effects of the processing steps on the area and period of rippled structures, a series of experiments are designed as shown in Figure 2a-c. Polarization of an incident laser is a key factor associated with the formation of subwavelength ripples. Hence, the effects of laser polarization on the area and period of rippled structures on a titanium surface are experimentally investigated in this study. Figure 2 shows the illustrated process of the experiments. Subwavelength ripples are induced by 2N femtosecond laser pulses on the surface of the sample, as shown in Figure 2a, which is denoted by 2N(0\({}^{\circ}\)), where N is the number of incident laser pulses and 0\({}^{\circ}\) means the laser polarization rotated 0\({}^{\circ}\) with respect to the original polarization. Then, we apply two series of femtosecond laser pulses to irradiate the same place on the sample surface, as shown in Figure 2b. First, the titanium sample is irradiated by the first series of femtosecond laser pulses, and then by a second series of femtosecond laser pulses on the same place. For example, the laser processing shown in Figure 2b is denoted by N(0\({}^{\circ}\)) + N(0\({}^{\circ}\)). In order to investigate the change of rippled structures, the second series of pulses with the laser polarization rotated 90\({}^{\circ}\) with respect to the original polarization are used to irradiate the same place on the sample surface, as shown in Figure 2c, which is denoted by N(0\({}^{\circ}\)) + N(90\({}^{\circ}\)). Figure 2d-f are the corresponding experimental results of Figure 2a,b. The experimental results shown in Figure 2d,e indicate the effects of pulse number on the area and period of rippled structures on the titanium surface, which is attributed to the incubation effects during the multi-shot regime [6]. As shown in Figure 2e,f, the polarization of a femtosecond laser significantly affects the surface topography, especially the direction of the rippled structure. Figure 2e,f show the processing of recording, erasing, and rewriting of subwavelength ripples induced by the femtosecond laser. The detailed SEM images of the ablation morphology and subwavelength ripples on titanium obtained by following a two-step method for femtosecond laser fabrication are shown in Figure 3. The diameters of rippled regions are about 20.8 \(\pm\) 0.06 and  
Figure 2: Schematic of experimental processing of ripples on a titanium surface. (**a**) Subwavelength ripples induced using a 2N femtosecond laser pulses; (**b**) Subwavelength ripples induced using two series of femtosecond laser pulses, denoted by N(0\({}^{\circ}\)) + N(0\({}^{\circ}\)); (**c**) Subwavelength ripples induced using two series of femtosecond laser pulses, denoted by N(0\({}^{\circ}\)) + N(90\({}^{\circ}\)); (**d–f**) are the corresponding experimental results of (**a–c**). 19.6 \(\pm\) 0.09 \(\upmu\)m under the conditions 50(0\({}^{\circ}\)) + 50(0\({}^{\circ}\)) and 50(0\({}^{\circ}\)) + 50(90\({}^{\circ}\)), respectively. The periods of subwavelength ripples are in the range of 450 \(\pm\) 25 nm to 560 \(\pm\) 15 nm under the conditions 50(0\({}^{\circ}\)) + 50(0\({}^{\circ}\)) and 50(0\({}^{\circ}\)) + 50(90\({}^{\circ}\)), respectively. By comparing with the experimental results shown in Figure 1a, it can easily be observed that the diameters of rippled regions under the conditions N = 50(0\({}^{\circ}\)) + 50(0\({}^{\circ}\)) and N = 50(0\({}^{\circ}\)) + 50(90\({}^{\circ}\)) are much larger than the diameter under the condition of N = 100(0\({}^{\circ}\)).