# The Effect of Laser Pulse Widths on Laser--Ag Nanoparticle Interaction: Femto- to Nanosecond Lasers
## 4 Results and Discussion
### Nanoparticle Size Distribution
The mean diameters of the nanoparticles produced using laser pulse widths of 4, 32, 64, and 100 ns were 75-85, 100-110, 95-105, and 120-130 nm, respectively as shown in Figure 7. The smallest value were seen for the femtosecond pulse, and the values tended to increase with the pulse width up to nanoseconds. The rate of diameter size change increased markedly when the pulse width changed from picosecond to nanosecond. The analysis of these results revealed that the region of particle size change according to the pulse width could be divided into the femtosecond and picosecond region, and the nanosecond region. This supports the fact that the mechanism of near-field enhancement andCoulomb explosion as described above and the mechanism of photothermal ablation are different in nanoparticle generation. In addition, the rate of increase of the particle size was larger in the nanosecond region, which is a characteristic of the laser thermal ablation related to the surface area. Conversely, it could be inferred that the surface area and fragmentation of the particles had a weak relationship in the femtosecond and picosecond regions. Figure 5: Transmission electron microscopy (TEM) images of Ag nanoparticles after being dried on a glass surface. (**a**) 164 fs, (**b**) 5 ps, (**c**) 4 ns, (**d**) 32 ns, (**e**) 64 ns, and (**f**) 100 ns pulse widths. The yield rate of nanoparticles according to the laser pulse width can be confirmed by two methods. The first is the yield rate according to the total mass of nanoparticles per unit area and unit time. The second is the yield rate according to the total number of nanoparticles per unit area and unit time. The nanoparticle weight production rate was higher using lasers with nanosecond pulse widths (0.71 \(\times\) 10\({}^{-15}\)-2.79 \(\times\) 10\({}^{-15}\) g/min) compared to that obtained using lasers with femtosecond and picosecond pulse widths (4.78 \(\times\) 10\({}^{-17}\)-8.06 \(\times\) 10\({}^{-17}\) g/min). However, this result is directly related to laser ablation of the target in water. As previously described, nanoparticles generated from nanosecond laser pulses have larger mean diameters. Therefore, even if the same number of nanoparticles is generated, the total mass of the nanoparticles generated from the nanosecond laser pulses is larger than that of the nanoparticles generated from the femtosecond or picosecond laser pulses. In our experiment, the number of nanoparticles generated for each parameter was measured to determine the particle generation rate according to the laser pulse width (Figure 8). This showed that the number of nanoparticles increased as the pulse width was shortened. Therefore, laser fragmentation is more efficient in the femtosecond or picosecond region. Considering these results, the laser pulse width that can be applied depends on the size of the nanoparticles required. Moreover, the change in nanoparticle size in relation to the pulse width is more sensitive in the nanosecond region. Figure 6: Size distribution of Ag nanoparticles. (**a**) 164 fs, (**b**) 5 ps, (**c**) 4 ns, (**d**) 32 ns, (**e**) 64 ns, and (**f**) 100 ns pulse widths.