# Femtosecond Pulse Laser Near-Field Ablation of Ag Nanorods
## 3 Results and Discussion
### Near-Field Ablation Evolution of Ag NRs  
Regarding the femtosecond pulse laser near-field ablation process, a possible thermo-force coupling mechanism was proposed based on the melting and curving of the Ag NRs. First, laser energy is absorbed by the electrons, and accumulated heat is then transferred from electrons to the lattice through electron-phonon coupling within 1-4 ps [39]. Strong excitation with large intensity can increase the time required for energy transfer to the lattice [28]. When the lattice temperature is higher than the boiling temperature of Ag NRs, fragmentation starts to occur. The near-field force caused by localized electronic field enhancement promotes fragmentation. As shown in Figure 5a,b, multipolar oscillation resulted in the formation of a notch, and the simulated electronic field in the middle of the Ag NR was enhanced when the next pulse arrived. Subsequently, the notched NR broke, yielding two small NRs, as illustrated in Figure 5c. Figure 5d shows the electronic field distribution of these two small NRs under the laser pulse effect, revealing that the field in the gap and at the middle was enhanced. This resulted in further fragmentation and nanodot formation, as indicated in Figure 5e. In addition, the fusion of small clusters was observed. Thus, small clusters fragmented from NRs were melted after laser illumination and subsequently agglomerated. The effect of the electric field caused by the small clusters on the ablation process was not taken into account in this study, because it was not the main factor behind the fragmentation of NRs. As indicated in Figure 5f,g, under the near-field force effect, the NR with a notch was curved because of the electronic field gradient, and the force was sufficiently strong to remove the NPs [40]. Figure 5h,i show the further fragmentation of the dimer and subsequent formation of nanodots.  
Nanojoints between two metal structures have been widely studied, such as end to end junction and side to side junction [41; 42]. However, side-to-end T-shaped junctions have rarely been reported. The success of plasmonic welding is dependent on the gap and angle between the two metal structures, and perpendicular structures have superior bonding [43]. Through femtosecond laser near-field ablation of two noncontact Ag NRs, T-shaped nanojoints were obtained. As displayed in Figure 6a,  
Figure 5: Schematic of the morphology evolution from NRs to nanodots. Corresponding TEM images and calculated electric field distributions of Ag NRs after irradiation with light of wavelength 400 nm. (**a,b**) The middle section of the material was ablated, and the electric field was enhanced in this area. (**c,d**) This resulted in separation into two small NRs and ablation enhancement in each particle. (**e**) Finally, nanodots were obtained. Another possible evolution process is also proposed. (**f,g**) Electric field gradient induced bending of Ag NRs, with the intensity enhanced in the middle. (**h,i**) This resulted in fragmentation and separation. The scale bar is 50 nm.  
the angle between the two NRs was 90\({}^{\circ}\), and the laser polarization angle was 45\({}^{\circ}\) along the NR. Figure 6c illustrates the calculated electronic field under these conditions, and the field was found to differ from the field of a single NR because of field coupling between the NRs. The field in the gap was enhanced and symmetric. Because of the intense field in the gap, the material near the gap melted and then flowed into the gap and fused. Some small clusters were observed around the nanojoint because of material explosion. Figure 6b,d presents the asymmetric field distribution in the gap, which may have resulted in inhomogeneous melting and could have weakened the bonding. As displayed in Figure 6e, the field on the other side of the NR was enhanced and could destroy the structure of the NR. This indicates that plasmonic welding of T-shaped NRs is dependent on the polarization direction related to the spatial structure of the NRs.