## 3 Results and Discussion
### Mechanism of Silver Implantation  
Based on the aforementioned results, a nanoparticle-embedded process in the nanopore during laser irradiation was summarized. The mechanism of change in surface morphology is schematically shown in Figure 16.  
After pore fabrication, the YSZ surface is slightly positively charged. In addition, the surface becomes hydrophilic as reported in our previous study [29]. Therefore, when a nano-silver dispersion liquid is dropped, the liquid spreads widely on the surface, and the negatively charged silver nanoparticles are adsorbed by Coulomb force (Figure 16a). Since the samples are placed in a vacuum desicator, it is assumed that the degassing action will allow the dispersion to penetrate the pores. After the water is gone and dried, the dispersant PVP remains with a large amount of silver nanoparticles (Figure 3). In this study, when the laser irradiation was performed at a fluence extremely smaller than the ablation threshold of the YSZ, only the surface PVP film was removed without machining the YSZ surface. PVP is one of the most commonly used materials for dispersion of metal nanoparticles. Removal by combustion has been reported in many cases. For example, [PERSON] et al. [40] proposed a method to remove PVP from Pt nanocolloids by multiple washing with solvent and combustion. In the case of this study, the removal of PVP was realized by the role of laser irradiation as one of the methods for combustion. Since metal nanoparticles enhance the combustion of PVP, the glass transition temperature of PVP is about 180 \({}^{\circ}\)C, and the decomposition temperature is about 380 \({}^{\circ}\)C [41]. The  
Figure 16: Difference in material behavior on the surface: (**a**) dropping nano-silver dispersion on YSZ surface after nanopore generation, (**b1**) irradiation by femtosecond pulsed laser, (**b2**) irradiation by nanosecond pulsed laser, (**b3**) heating in an electric furnace. melting point of bulk silver is about 960 \({}^{\circ}\)C [42], but the melting point of silver nanoparticles may have changed due to nanoparticulation. [PERSON] et al. [43] reported that the melting point of single nanoparticles or nanoparticle clusters significantly decreased compared to the bulk material due to the size effect. Therefore, it is assumed that the melting point of silver nanoparticles used in this study is also smaller than that of bulk silver. In laser processing of polymer materials, ablation by laser pulses breaks down the polymer into gases and removes it from the surface [44]. When metal nanoparticles are included, the interfacial interaction between the particles and the polymer matrix in contact with them promotes the heating and melting of the polymer, which changes the ablation properties [45; 46]. Therefore, it is considered that the polymer decomposition proceeds through a complex combination of photochemical and thermal reactions acting simultaneously. From the above, the phenomena on the sample surface during laser irradiation are considered as follows. When irradiated by laser under appropriate conditions, silver nanoparticles aggregate in PVP, which reaches the glass transition point, and melt to form large clumps. Then, PVP decomposes and disperses into the atmosphere, leaving mainly silver on the YSZ. Here, at any pulse width and laser fluence, the surface morphology did not change when the scanning speed was greater than a certain value. This indicates that PVP cannot be decomposed by single pulse irradiation but is gradually decomposed by repeated irradiation at the same location. When an ultrashort pulsed laser is used (Figure 16(b1)), the silver clumps remaining on the YSZ surface are gradually ablated and removed due to multiple pulse irradiation. However, small clumps deep within the elongated nanopores surrounded by zirconia, which has a very low thermal conductivity compared to silver, are less affected by the laser beam and remain within the pore as clumps.