# Femtosecond Pulse Laser Near-Field Ablation of Ag Nanorods
## 3 Results and Discussion
### Controlling the Morphology of Ag NRs by Tuning the Laser Polarization and Wavelength  
Figure 3a shows a TEM image of an as-prepared Ag NR before laser ablation. The mean aspect ratio of the Ag NRs was 3.9 and could be controlled by modifying the concentration of the trisodium citrate solution. The surface plasmons of the NRs were divided into two distinct modes--transverse and longitudinal--because of light polarization. To analyze the relationship between the extinction  
Figure 2: Morphology of Ag NPs after femtosecond pulse laser illumination. **(a)** Melting occurred at the ends of the Ag NRs, and a grain boundary was evident. **(b)** TEM image of the transformation from as-prepared NRs into small NRs. **(c)** TEM image of the transformation from nanospheres into bone-shaped NRs. **(d)** Melting occurred at the junction between two adjacent rods. The scale bars are 20 and 50 nm. peak wavelength and NR aspect ratio, Mie-Gans theory was considered, and the extinction coefficient \(\gamma\) for Ag NRs was calculated using the dipole approximation as follows [35]:  
\[\gamma=\frac{2\pi NV\varepsilon_{m}^{3/2}}{3\lambda}\frac{\sum\limits_{j}{\frac{(1 /p_{j}^{2})\varepsilon_{2}(\omega)}{\left(\varepsilon_{1}(\omega)+(1-p_{j}) \varepsilon_{m}/p_{j}\right)^{2}+\varepsilon_{2}(\omega)^{2}}}}, \tag{1}\]  
\[p_{A}=\frac{1-e^{2}}{e^{2}}(\frac{1}{2e}\ln(\frac{1+e}{1-e})-1), \tag{2}\]  
\[p_{B}=p_{C}=\frac{1-p_{A}}{2}, \tag{3}\]  
\[e=\sqrt{1-\left(\frac{B}{A}\right)^{2}}, \tag{4}\]  
where \(N\) and \(V\) are the number of nanoparticles per unit volume and the volume of single particle, respectively; \(\varepsilon_{m}\), \(\varepsilon_{1}\), and \(\varepsilon_{2}\) represent the dielectric constant of medium, the real, and complex part of the material dielectric function, respectively; \(p_{j}\) are the depolarization factors of the nanorod axes (\(A\), \(B\), \(C\)). The aspect ratio \(R\) is \(A/B\), and \(\lambda\) is the laser wavelength. The relationship between the extinction coefficient and light wavelength at three aspect ratios is plotted in Figure 3b. The extinction peak wavelength of the longitudinal mode increased with increasing Ag NR aspect ratio, and the resonance band was much more pronounced at the highest ratio, with 120 nm width. This calculated result was in agreement with an analysis of plasmonic NPs [36]. Conversely, the extinction peak wavelength of the transverse mode was blueshifted with increased aspect ratio, and the extinction coefficient was almost constant. In addition, the rising shoulder at lower wavelengths (less than 360 nm) corresponds to interband absorption between the 4d and 5 sp bands [37]. When the aspect ratio was 3.9, the transverse plasmon absorption peak wavelength was 380 nm, and the longitudinal plasmon resonance peak was 790 nm. When using parallel polarization and 800 nm wavelength, which is near the longitudinal plasmon resonance band, a dipolar lobe morphology of the Ag NRs after illumination was observed, as shown in Figure 4a. This indicated that ablation occurred only at the ends, but not on the two sides, of the  
Figure 3: (**a**) TEM image of as-prepared Ag NR; the mean aspect ratio of all NRs was 3.9. The scale bar is 20 nm. (**b**) Calculated extinction spectra of Ag NRs with varying aspect ratios \(R\). For \(R\) = 3.9, the transverse plasmon absorption band was at 380 nm, whereas the longitudinal plasmon resonance band was at 790 nm. NRs, because small clusters formed at the ends under the enhanced electronic field. This agrees with the simulation results presented in Figure 4b, and the enhancement factor was 13. As illustrated in Figure 4c, a notch in the middle of the NRs could be observed when using parallel polarization and 400 nm laser wavelength, which is far from the longitudinal plasmon resonance band. Thus, greater ablation occurred in the middle of the NRs than at any other part, and the electronic field had the distribution displayed in Figure 4d.