We report the first direct visualization of gap-SPPs propagating on a AgNW dimer. A self-assembled AgNW dimer loaded with a monolayer of molecules is locally excited to launch the SPPs, and the wide-field microscopy maps of surface-enhanced Raman scattering (SERS) of the molecules are acquired. The SERS images, representing the gap-field intensity distributions, reveal that the gap-plasmons of AgNW dimers with a few nm of gap can propagate up to ~8 um. The images also show oscillating components with periods of 400 ~ 800 nm, arising from the mode-beating of the two gap-SPP modes. Through a close comparison with electrodynamics simulations of NWs, we identify that the two modes are the monopole-monopole gap-modes with a propagation length of 0.5 ~ 2 um), and the dipole-dipole gap-modes with a propagation length of 5 ~ 8 um.
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