## 4 Experimental Results and Discussion  
### Electric Field Concentration Simulation Analysis between Nanoparticles  
Simulation software (COMSOL Multiphysics; COMSOL Inc., Burlington, MA, USA) was used to analyze electromagnetic waves; the results are shown in Figure 3. On laser irradiation of metal nanoparticles, the conductive electrons oscillate because of SPR, thus  
Figure 2: Schematic of process; (**a**) Pulsed laser beams irradiate the target and the laser interacted with the target surface to cause ablation. (**b**) The plasma and bubble with nanoparticles generate from the target. (**c**) Bubble condensation and heterogeneous nanoparticles occur. (**d**) The detached nanoparticles were either further fragmented by laser pulses or reacted with elements in the plasma to form novel fused nanoparticles.  
concentrating the electric field near the nanoparticles. We analyzed the intensity changes of the electric field distributions around nanoparticles according to the distance between, and relative sizes of, the particles. We simulated cases 1-3 in Table 1. Circular conductive particles with mesh sizes of 3.7 and 133 nm were modeled. The meshes were free triangles.  
Case 1 explored the effects of nanoparticle diameter (40-200 nm); as the diameter increased, the maximum electric field distributions around the particles were calculated as shown in Figure 3a. The characteristic particle length in this region was shorter than the wavelength, and the effect of hot spots with intense electromagnetic field enhancement according to absolute particle size was relatively constant [9; 45; 46; 47]. The effect of the distance between nanoparticles on electric field concentration was analyzed in Case 2 (Figure 3b). The nanoparticle diameter used in the calculation was 40 nm. As the particle spacing increased from 10 to 160 nm, the maximum electric field intensity decreased dramatically. Therefore, the closer same-sized particles are to each other, the greater the electric field. Nanoparticles of different sizes were analyzed in Case 3 (Figure 3c). The size ratio of central and surrounding particles was 1:2.5. When the central particles were larger than the surrounding particles, the maximum electric field intensity was about 6-30% greater than in the opposite case. Thus, the simulations revealed the behaviors of differently sized nanoparticles as the electric field distribution varied. When particles were relatively close and small particles surrounded large particles, the hot spots with electromagnetic field enhancement were stronger than when large particles surrounded small particles. Thus, when nanoparticles of different sizes absorb laser energy, the electric field concentration is more efficient than when the particles are of the same size. This promotes nanoparticle generation and growth. If the pulse is relatively long, the electric field distribution acceler  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters** & **Values** & **Classification** \\ \hline Particle Diameter (nm) & 40–200 & Case 1 \\ Distance between particles (nm) & 4–160 & Case 2 \\ The ratio of diameters between particles & 2d\(\rightarrow\)4d, 1/2d–1/4d & Case 3 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Simulation parameters.  
Figure 3: Simulated electric fields distribution and electric field vs. particle diameter. (**a**) Case 1, (**b**) Case 2, and (**c**) Case 3.  
ates the interactions between particles by instantaneously increasing the temperature of the particle surfaces, because the electric field becomes concentrated on the surfaces.