Ultrafast Near-Field Dynamics in Silver Nanowires Driven by Few-Cycle Short-Wave Infrared Pulses.

Time-resolved imaging of plasmonic near fields is well-established at visible wavelengths but remains largely unexplored in the short-wavelength infrared (SWIR) range. Here, we use time-resolved photoemission electron microscopy (TR-PEEM) to directly visualize ultrafast plasmon dynamics in silver nanowires driven by a few-cycle SWIR pulses. We observe strong electric-field enhancement localized at the nanowire ends, oscillating synchronously with the SWIR optical cycle. Both the local intensity and the temporal response vary within and between individual nanowires. Time-dependent linear modeling reveals that subwavelength variations in nanowire length significantly affect the plasmon dynamics, while the precise end morphology influences both dynamics and field enhancement. Electromagnetic simulations predict additional substrate-induced plasmon modes beyond the nanowire's fundamental longitudinal resonances. Because the imaged photoemission signal depends highly nonlinearly on the local electric field at the wire-vacuum interface, it is dominated by emission from the nanowire ends, suppressing weaker substrate-induced modes. The photoemission yield shows a lower-than-expected power-law dependence with incident field intensity, suggesting the potential onset of strong-field effects beyond simple perturbative multiphoton emission. These results establish silver nanowires as efficient local SWIR field concentrators and demonstrate that their tunable ultrafast plasmonic responses can be imaged and filtered using photoelectrons, offering promising avenues for nanoscale photonic applications and ultrafast control of electron emission.