## 1 Introduction
[18] processed YSZ with pore channels of several tens of nanometers in diameter by the freeze-casting process. It is possible to obtain sintered zirconia with nanostructures by using various sintering methods, but these methods can make the entire bulk porous. Laser processing is one of the effective methods for creating nanostructures [19]. Many techniques have been studied to fabricate nanostructures, such as nanoholes and nanogrooves on the scale of several hundred nanometers or less, including: processing high aspect ratio nanoholes in SiO\({}_{2}\) substrates using femtosecond pulsed lasers [20; 21; 22]; fabrication of nanoholes, periodic nanostructures, and nanodot structures in metals using femto/picosecond pulsed lasers [23; 24]; and drilling submicron holes in polymers using femtosecond pulsed lasers at extreme ultraviolet (XUV) wavelengths [25]. There are many reports on new nanostructure-processing methods and patterning that exceed the diffraction limit of laser light; however, to our knowledge, there have been no sufficient studies on polycrystalline zirconia for nanoholes, except for the formation of nanogrooves [26; 27; 28]. Therefore, in our previous study, we proposed a new method to fabricate nanoscale pores in YSZ. By irradiating the femtosecond pulsed laser, generation of nanopores in the crystal grains of the YSZ top layer succeeded [29]. A large number of pores can be generated on the surface with a single laser pulse irradiation without causing damage to the bulk by using this technique. These nanopores could be effectively used to load various materials on the YSZ surface and impart new functionalities. Therefore, in this study, a novel method was proposed to embed material into nanopores formed in YSZ using laser irradiation. Figure 1 shows the structure of the target YSZ sample. First, a femtosecond pulsed laser was irradiated onto the polycrystalline YSZ substrate to form nanopores within the surface crystal grains. Using the pose as an anchor, we attempted to create a functional material by locally filling and fixing any material within the nanopores. This nanopore structure is a new structure that we have realized, and the material has never been supported in the pores. If this method is realized, it can be expected to provide a new approach to surface functionalization for zirconia, which is difficult to microfabricate using conventional methods and suffers from damage during processing. Silver nanoparticles were selected as the embedding material. YSZ has excellent chemical stability and does not interact with silver, and silver has antibacterial properties [9], so YSZ has advantages in its use in biomaterials and daily necessities of people. For example, when implanted into the body as a biomaterial, YSZ's biocompatibility will allow it to bond well with bone and tissue, and the antibacterial and bactericidal effects of silver inhibit bacterial growth, increasing the stability of YSZ products. Since the base material is YSZ, excessive elution of metal ions and base material deterioration can be suppressed compared to metal base materials. In addition, the health risks associated with the absorption of silver ions are extremely low [30], and by introducing it into the pores, it can be expected to prevent desorption from the sample surface and provide a sustained effect. Furthermore, by generating pores at arbitrary locations on the surface, it becomes possible to selectively control the location where the material is supported. Material delivery systems have been actively studied in recent years [31], and if this technology, which takes advantage of the biocompatibility of YSZ itself, can be applied not only to silver but also to other materials, it may have further potential for application. Thus, in this study, the feasibility of metal nanoparticle loading into nanopores in zirconia surfaces by femtosecond pulsed laser irradiation, which has never been reported before, and the fundamental characteristics of the process were investigated.