## 3 Results
This phenomenon is caused by the incubation effect due to the accumulation of laser-induced structural changes of the material during the femtosecond laser ablation. The existing rippled structure can improve absorption efficiency [6]. The period of subwavelength ripples under the condition N = 50(0\({}^{\circ}\)) + 50(0\({}^{\circ}\)) is much smaller than the periods under the condition of N = 100(0\({}^{\circ}\)). According to our previous study [30], after the first series of femtosecond laser pulse irradiation, the initial rippled structures are formed on the surface of the titanium. The existing ripples can strongly enhance the energy absorption of the second series of femtosecond laser pulses and the enhanced laser intensity is mainly localized in the grooves. When the polarization is perpendicular to the already existing ripples, the second series of laser pulses can further deepen the grooves, thus, accelerating the ripple forming process [30]. For the condition N = 50(0\({}^{\circ}\)) + 50(90\({}^{\circ}\)), the periods of the subwavelength ripples are larger than the periods under the condition N = 100(0\({}^{\circ}\)) and the orientation of ripples is perpendicular to the polarization direction of the second series of femtosecond pulses. As shown in Figure 3e,f, the period of the rippled region under the condition N = 50(0\({}^{\circ}\)) + 50(90\({}^{\circ}\)) is similar to that induced by N = 50(0\({}^{\circ}\)) (period in the range of 550 \(\pm\) 15 nm) femtosecond laser pulses, which shows the processing of recording, erasing, and rewriting of ripples during femtosecond laser induced subwavelength ripples. Similar phenomena have also been reported by [41]. During the rewriting process, when the number of pulses in the second sequence reach a certain number, the original ripples disappear. At the same time, the new ripples are already formed with an orientation of 90\({}^{\circ}\) to the original one, which means that two processes, including material removal and the formation of new ripples, simultaneously occur. Hence, as shown in Figure 3, we can easily observe the following; (1) the area and period of subwavelength rippled structures can be simultaneously adjusted by changing the processing sequence; (2) the area of rippled structure can be increased and its period can be decreased by two series of femtosecond laser pulses with the same polarization; (3) the recording, erasing, and rewriting of subwavelength ripples can be achieved using two series of femtosecond laser pulses with perpendicular polarization directions; (4) the period of subwavelength ripples under the condition 50(0\({}^{\circ}\)) +50(90\({}^{\circ}\)) is similar to that induced by N = 50(0\({}^{\circ}\)) femtosecond laser pulses. The subwavelength rippled structures are fabricated by following the two-step method using relatively fewer femtosecond laser pulses. The SEM images of ablation morphology and subwavelength ripples on titanium obtained at different femtosecond laser pulse irradiations are shown in Figure 4. The incident of the femtosecond laser is designed as follows: N = 20 (0\({}^{\circ}\)), N = 10(0\({}^{\circ}\)) + 10(0\({}^{\circ}\)), N = 10(0\({}^{\circ}\)) + 10(90\({}^{\circ}\)), and N = 10(0\({}^{\circ}\)). The diameters of subwavelength rippled structures induced using the four femtosecond laser  
Figure 3: SEM images of ablation morphology and subwavelength ripples on titanium obtained by following one- and two-step fabrication processes. (**a**, **b**) N = 50 (0\({}^{\circ}\)) + 50(0\({}^{\circ}\)); (**c**, **d**) N = 50(0\({}^{\circ}\)) + 50(90\({}^{\circ}\)); (**e**, **f**) N = 50 (0\({}^{\circ}\)). designs are about 14.8 \(\pm\) 0.16, 15.0 \(\pm\) 0.15, 13.6 \(\pm\) 0.22, and 14.0 \(\pm\) 0.2 \(\upmu\)m, respectively. The corresponding periods of the subwavelength ripples induced using the four femtosecond laser designs are 590 \(\pm\) 15, 570 \(\pm\) 15, 620 \(\pm\) 10, and 620 \(\pm\) 10 nm, respectively.