## 3 Results and Discussion
The periods of ripple structures reduce with the increasing number of pulses. For 10 and 30 shots, the periods of ripples are approximately 620 and 570 nm, respectively, which is within the range of 0.7 \(\lambda\)in \(<\lambda<\lambda\)in. The ripples with periods in the range of 0.6 \(\lambda\)in \(<\lambda<\) 0.7 \(\lambda\)in are formed when the pulse number increases from 40 shots to 100 shots. The aforementioned experimental results are consistent with those of previous studies and literature [38]. The femtosecond laser-induced ripple evolution is observed from shot to shot in a multishot regime. When the incident laser fluence is slightly higher than the ablation threshold, several main stages of ripple dynamic evolution, including primary ripples or random nanoroughness (N = 10-30) and secondary ripples (N \(>\) 30), are observed. This phenomenon is termed the blue shift of ripple periods, which is attributed to the shift of the dispersion curve of surface plasmon-polariton (SPP) dispersion generated by the scattering of SPPs [39]. To study incubation effects during a multishot regime, the effects of the femtosecond laser pulse irradiation modes on ablation morphology and those of subwavelength ripples on titanium are experimentally studied. The formation of rippled structures by two consecutive series of pulses is observed. First, the sample is exposed to a series of pulses comprising several pulses. The second series of pulses comprising remaining pulses is used to irradiate the same place of the sample surface. All experiments are performed under the same number of total pulses. The aforementioned process is termed laser pulse irradiation modes. For example, in our experiments, the total number of laser pulses, which is denoted by N, is fixed at 10. Figure 4a,a1 shows that the femtosecond laser pulse irradiation mode N = 10 + 0 refers to the samples irradiated using 10 consecutive shots. The femtosecond laser pulse irradiation mode N = 5 + 5 refers to the samples irradiated using the first five shots and remaining five shots at the same place. Figures 4 and 5 indicate that laser pulse irradiation modes considerably affect ablation morphology and subwavelength ripples on the metal titanium. The respective ablation diameter (D) and the periodicity (\(\lambda\)) of the ripples obtained at those modes:  
N = 10 + 0 (D \(\approx\) 11.7 \(\mu\)m, \(\lambda\approx\) 620 nm)  
N = 5 + 5 (D \(\approx\) 14.2 \(\mu\)m, \(\lambda\approx\) 600 nm)  
N = 4 + 6 (D \(\approx\) 14.8 \(\mu\)m, \(\lambda\approx\) 550 nm)  
N = 6 + 4 (D \(\approx\) 14.4 \(\mu\)m, \(\lambda\approx\) 500 nm)  
N = 2 + 8 (D \(\approx\) 17.0 \(\mu\)m, \(\lambda\approx\) 580 nm)  
N = 8 + 2 (D \(\approx\) 16.5 \(\mu\)m, \(\lambda\approx\) 540 nm)  
Figure 3: Dimensions and periods of multiple-shot ablated regions with subwavelength ripples as a function of pulse number. The laser pulse energy is 0.20 J/cm\({}^{2}\) in all cases. Comparing the femtosecond laser pulse irradiation modes N = 10 + 0 and other modes, superior surface morphologies characterized by larger ablation areas and finer rippled structures, can be obtained at the modes of N = 5 + 5, N = 4 + 6, N = 6 + 4, N = 2 + 8, and N = 8 + 2. Hence, the experimental results indicate that samples first irradiated using several shots and then the remainder shots have superior surface morphologies, including larger ablation areas and finer rippled structures, because of different incubation coefficients. An incubation coefficient is used to describe the dependence of multi-shot ablation threshold \(\Phi_{th,N}\) of metals on the number \(N\) of impinging pulses and the threshold fluence with a single pulse \(\Phi_{th,1}\)[40,41]. Incubation is absent for S = 1 and the ablation threshold stays constant irrespective of the number of pulses. As far as the damage accumulation mechanism reduces the ablation threshold, the incubation coefficient gets lower [37,42,43].