# The Effect of Laser Pulse Widths on Laser--Ag Nanoparticle Interaction: Femto- to Nanosecond Lasers
## 2 Background  
### Effect of Laser Pulse Width  
Irradiation of a solid target in a liquid with a laser beam results in the generation of metal flakes by laser fragmentation. The particles are refined by irradiation with the laser beam a second time. The laser beam reduces the size of the generated nanoparticles or produces particles with a specific size distribution. The interaction of the laser beam with the liquid also affects particle formation. Bubbles and cavitation result from laser-induced plasma generation and affect nanoparticle fragmentation [36; 37]. In addition, the aqueous solution influences the formation of clusters of nanoparticles [38]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Material** & **Wavelength** & **Pulse Width** & **Fluence or Pulse Energy** & **Particle Size** & **Reference** \\ \hline Au & 400 and 532 nm & 150 fs & 7.3 and 3.6 mJ/cm\({}^{2}\) & –60 nm & [33] \\ Au & 1025 nm & 450 fs & 10–130 mJ & 7–50 nm & [27] \\ Ag & 355 and 532 nm & 6 ns and 18 ps & 10 and 2–3 mJ & 5–60 nm & [28] \\ Au & 355 nm & 30 ps & 6.3 and 17 mJ/cm\({}^{2}\) & NA & [32] \\ Au/Ag & 532 and 1064 nm & –5 ns & 35 mJ & –10 nm or less & [20] \\ Ag & 355 and 1064 nm & 7 and 5 ns & NA & –26 nm & [34] \\ Au-Ag alloy & 355 and 532 nm & 6 ns & 150 and 130 mJ & –10 nm & [35] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characteristics of the nanoparticles produced using various laser parameters. Nanoparticle size and dispersion can be controlled by varying the laser parameters. The wavelength of the laser pulse also affects nanoparticle generation [20]. When a laser of a different wavelength with the same pulse width is used, the fragmentation effect increases with decreasing pulse wavelengths. This is because the shorter the pulse wavelength, the greater the absorption cross section [28]. In addition, liquids other than deionized water may be used, and they can contribute to nanoparticle formation and stability [16; 20]. Another study attempted to control the size of nickel (Ni) nanoparticles by using water containing other nanoparticles such as silica particles [39]. Laser pulse width is a predominant determinant of nanoparticle size and distribution. Laser pulse widths in the femtosecond, picosecond, and nanosecond regions can be used to induce laser fragmentation. Several studies have evaluated the interactions of picosecond or femtosecond laser pulses with materials [27; 32]. Such ultrafast laser pulses cause nonlinear multiphoton absorption due to their high peak power. Because of this, energy cannot be transferred from the electrons to the ion grids in femtoseconds or picoseconds, which enables the processing of materials that are little affected by heat [40]. Irradiation with a nanosecond pulsed laser beam results in thermal diffusion at the particle surface. It is important to compare diffusion time and pulse width. In general, thermal diffusion in metal nanoparticles requires 50 to 100 ps, and the diffusion time can be used to classify laser fragmentation mechanisms into short and long pulse-width categories [41]. At a pulse width of less than a few picoseconds, the electron and phonon temperatures in the particle are different. In this case, to determine the particle temperature distribution, a model with different electron and phonon temperatures should be applied [42; 43]. Also, because the laser beam absorption time is shorter than that of heat diffusion, it is appropriate to assume that the boundary condition of particles is adiabatic. In this model, the energy absorption by laser irradiation is a function of the fluence and absorption cross-sectional area of the laser [44]. This correlation affects the electron temperature and phonon temperature of the nanoparticles; that is, as the pulse width decreases, the peak laser power and the electron temperature increase. This results in a release of electrons from the particle; moreover, a Coulomb explosion may occur [28; 33]. Coulomb explosions are a major mechanism of femtosecond laser fragmentation.