## 3 Results  
In order to adjust the area and period of rippled structures induced by using a femtosecond laser, the effects of the processing steps on the area and period of rippled structures on a titanium surface are experimentally investigated in this study. For the experiments,the total number of pulses (N) is fixed at 100 and the incident fluence is about 0.20 J/cm\({}^{2}\). In order to study the effect of two-step processing on surface morphology, the following three processing sequences are designed: N = 100, N = 40 + 60, and N = 60 + 40. For example, N = 100 represents 100 pulses to induce the surface micro-nano structure in one step. N = 40 + 60 represents two series of femtosecond laser pulses to irradiate the titanium surface. The first series consists of 40 pulses, and the second series consists of 60 pulses. The time interval between the two series is a few seconds to tens of seconds. This process is called a two-step method. Figure 1 shows the SEM images of ablation morphology and subwavelength ripples on titanium obtained following the two-step fabrication method. The areas circled in orange represent regions with surface rippled structures, as shown in Figure 1a,c,e. The size of the area circled in orange is represented by its diameter, D. The diameters of the areas circled in orange are about 18.9 \(\pm\) 0.1, 20.0 \(\pm\) 0.08, and 21.9 \(\pm\) 0.05 \(\upmu\)m under the processing sequences N = 100, N = 40 + 60, and N = 60 + 40, respectively. As shown in Figure 1b,d,f, the periods of ripples with a perpendicular oriented laser polarization are in the range of 480 \(\pm\) 20 nm, 465 \(\pm\) 20 nm, and 430 \(\pm\) 30 nm for the processing sequences N = 100, N = 40 + 60, and N = 60 + 40, respectively. Similar rippled structures have also been reported in [22]. The experimental results show that the areas of rippled structures induced by using a femtosecond laser can be effectively increased following a two-step fabrication method. The periods of ripples formed on the titanium surface have also been effectively adjusted by the designed processing sequence. The experimental results indicate that it is possible to fabricate large area surface structures with small periods following a two-step method. For the processing sequences N = 40 + 60 and N = 60 + 40, the formation of rippled structures induced following a two-step method can be divided into two processes: rippled structures induced by the first series pulses (the first process) and rippled structures induced by second series pulses (the second process). For the first process (N = 40 + 60), the initial surface plasmon (SP)-laser interference plays an important role, based on our previous study [37]. At the interface between a metal with permittivity \(\varepsilon_{\rm M}\) and air with permittivity \(\varepsilon_{\rm A}\), if the value of the real component of the complex dielectric function is less than \(-1\), surface plasmons can be resonantly excited by the coupling between the surface electrons of the dielectric and the incident field, which is characterized by surface electromagnetic waves (also called the surface plasmon field). For metals, the excitation conditions of the surface plasma field are always satisfied after the incident femtosecond laser irradiation. Because of the interference between the absorbed laser field and the surface plasmon field, the coupling field intensity is enhanced in some areas and reduced in other areas. The absorbed laser beam profile is reshaped by the presence of surface plasmons, which displays periodic patterns. Then, energy with spatially periodic patterns leads to the periodic patterns of the free electron temperature distribution and lattice  
Figure 1: SEM images of ablation morphology and subwavelength ripples on titanium obtained by one- and two-step fabrication. (**a,b**) N = 100; (**c,d**) N = 40 + 60. (**e,f**) N = 60 + 40.  
temperature distribution. Due to the interactions of laser and materials, subwavelength ripples are formed in some areas.