# Femtosecond Pulse Laser Near-Field Ablation of Ag Nanorods
## 1 Introduction  
Because of their wide range of potential applications in biomedicine [1; 2], analytical chemistry [3; 4], and photovoltaics [5], plasmonic nanoparticle research is an attractive field [6; 7; 8]. Silver nanoparticles (Ag NPs) play an essential role in catalysts because of their high thermal conductivity [9]. Surface plasmon resonance (SPR) is a crucial property and originates from the collective oscillation of surface free electrons under the coupling of light and plasmonic NPs [10]. The SPR band of Ag NPs is dependent on their optical properties, which are sensitive to NP morphology and size [11]. Ultrafast laser ablation is crucial for controlling morphology and has therefore been extensively investigated [12].  
Exposure of Ag NPs to single- and multiple-pulse lasers results in ablation [13; 14; 15], and three mechanisms were proposed for interpreting this ablation process [16]. [PERSON] et al. attributed the fragmentation of NPs to laser-induced thermal evaporation and confirmed that fragmentation occurs only if the NPs have a temperature higher than their boiling point [17]. [PERSON] et al. conducted a pulsed laser ablation experiment using a laser at wavelength of 355 nm, and concluded that NPs were fragmented layer by layer under the photothermal effect [18]. In support of the thermal evaporation mechanism, [PERSON] et al. performed an experiment and combined the results with melting calculations, concluding that the mechanism is valid when using a laser of low intensity [19]. In contrast, [PERSON] et al. adopted the Coulomb explosion mechanism, which considers fragmentation to result from charge repulsion caused by the ejection of numerous free electrons; in pulsed laser ablation of Ag NPs, hydrated electrons were detected [20]. Subsequently, [PERSON] et al. conducted 355 nm nanosecond laser ablation of NPs and also observed hydrated electrons and charged ions. The results indicated that although the wavelength of 355 nm is not in the SPR band, fast absorption of interband electrons enhanced fragmentation [21]. Furthermore, [PERSON] et al. performed picosecond laser-induced fragmentation of NPs using two temperature modes and concluded that Coulomb explosion plays a larger role than does laser-induced thermal evaporation [22].  
In addition to the two aforementioned mechanisms, a near-field ablation mechanism based on plasmonic field enhancement has been proposed; this process would be nonthermal excitation because the ablation temperature is lower than the melting point [23; 24]. [PERSON] et al. reported that the photomechanical effect can result in ablation without a thermal phase transformation, and the thermoelastic stress can overcome the physical strength of NPs [25]. [PERSON] et al. conducted laser ablation of a single NP on a silicon substrate, and two lobe-shaped structures were observed that matched the enhanced field distribution [26]. [PERSON] et al. discussed the complicacy of the near-field distribution on the basis of ultrafast laser ablation of gold nanotriangles [27].  
The modification of various substrates through laser-induced near-field ablation has been widely investigated [28; 29]. However, in most research, the NPs have completely destroyed and removed from the substrates after a single high-intensity pulse. Little information presently exists on the morphology evolution of silver nanorods (Ag NRs) subject to near-field ablation. In this study, we demonstrated the dependence of the near-field distribution on laser wavelength and polarization by performing a femtosecond pulse laser ablation experiment, achieving Ag NRs morphology control by tuning the electronic field distribution. The evolution mechanism was analyzed through both experimentation and simulation. In addition, the effect of the laser polarization angle on junction plasmonic welding was also determined. Furthermore, this study proposes a useful tool for fabricating nanostructures, including bone-shaped NRs, T-shaped NRs, and nanodots.