## 3 Results and Discussion
### Surface Morphology  
#### 3.2.1 Effect of Laser Fluence and Scanning Speed  
After the nano-silver dispersion was applied to the pore-processed surface, the processing characteristics of the second laser irradiation (Figure 2c) were evaluated. To investigate the effect of the laser conditions, line irradiation was conducted. The laser beam was scanned a single time at a pulse width of 256 fs with different laser fluences and scanning speeds. Figure 5 shows the SEM images of the irradiated surface at a scanning speed of 10 mm/s with various laser fluences. From low magnification images (Figure 5(a1-c1)), the dried dispersion film was completely removed at the laser-scanned area and the YSZ surface with pre-processed nanopores was exposed. At 0.4 J/cm\({}^{2}\) and 0.5 J/cm\({}^{2}\), only dried film was removed from the surface; however, by increasing laser fluence to 0.7 J/cm\({}^{2}\), the YSZ substrate was also machined and the surface with nanopores was partially peel off. In general, the ablation threshold for YSZ is larger than 0.7 J/cm\({}^{2}\), but it is thought that the roughening of the surface due to nanopore formation would cause processing even at smaller fluences. Therefore, the surface tended to be more removed in the pore-generated area than in the unprocessed area. From high-magnification images, it was clear that many nanopores kept their shape after laser irradiation and were filled with some material at 0.4 J/cm\({}^{2}\) (Figure 5(a2)). When the laser was irradiated at 0.5 J/cm\({}^{2}\), periodic structures were partially formed around the apertures of pores (Figure 5(b2)). At 0.7 J/cm\({}^{2}\), the periodic structures formed a larger area, and the grain surface also had periodic spallation marks (Figure 5(c2)). These structures were considered to be the laser-induced periodic surface structures (LIPSS) that form by irradiating ultrashort laser pulses near the ablation threshold [33]. In general, LIPSS are classified into two types based on their periodicity. LIPSS with a period on the same scale as the irradiated laser wavelength are low-spatial frequency LIPSS (LSFL), and those with  
Figure 4: (**a**) Relationship between water dilution ratio and pH; (**b**) change in zeta potential of each sample. (The two dashed lines indicate the pH range of the water in (**a**)). a period of less than half of the wavelength are high-spatial frequency LIPSS (HSFL). In this study, LIPSS were formed perpendicular to the laser polarization direction, and the periodicity was approximately 100-200 nm, which was much smaller than half of the laser wavelength (\(\lambda\)/2 = 514 nm). This indicates that the structures were HSFL. In general, LIPSS have advantages in improving and enhancing surface functionalities such as wettability, biocompatibility, and wear resistance, and have a wide range of applications [34, 35]. Our previous study also revealed that LIPSS formation enhances the hydrophilicity of the YSZ surface [26]. However, in this study, the irradiation condition where clear LIPSS formed (Figure 5c) resulted in the removal of silver and surface layers. Change in pore diameter irradiated with different fluences was measured, as shown in Table 2. As a result, there was no change in pore diameter due to changes in laser fluence. It was shown that surface processing such as LIPSS occurred around the pore as the fluence increased, but it did not affect the size or shape of the pore itself. According to these results, the material different from the substrate was successfully embedded only within the pre-made nanopore by laser process by adjusting irradiation conditions. It was also found that lower laser fluence can suppress surface damage such as LIPSS formation and removal of the surface layer of YSZ substrate to retain nanopore shape. The effect of laser scanning speed was investigated. Figure 6 shows the SEM images of the irradiated surface at a laser fluence of 0.5 J/cm\({}^{2}\) with various scanning speeds. At low scanning speed, dried film was completely removed at the irradiated area (Figure 6a).