Mapping and Optically Writing Nanogap Inhomogeneities\nin 1‑D Extended Plasmonic Nanowire-on-Mirror Cavities

Tightly confined\nplasmons in metal nanogaps are highly sensitive\nto surface inhomogeneities and defects due to the nanoscale optical\nconfinement, but tracking and monitoring their location is hard. Here,\nwe probe a 1-D extended nanocavity using a plasmonic silver nanowire\n(AgNW) on mirror geometry. Morphological changes inside the nanocavity\nare induced locally using optical excitation and probed locally through\nsimultaneous measurements of surface enhanced Raman scattering (SERS)\nand dark-field spectroscopy. The increasing molecular SERS intensity\nand corresponding redshift of cavity plasmon modes by up to 60 nm\nindicate atomic-scale changes inside the nanocavity. We correlate\nthis to diffusion of silver atoms into the nanogap, which reduces\nthe nanogap size and enhances the optical near-field, enhancing the\nSERS. These induced changes can be locally excited at specific locations\nalong the length of the nanowire and remain stable and nonreversible.\nPolymer surface coating on the AgNW affects the power threshold for\ninducing atom migration and shows that strong polyvinylpyrrolidone\n(PVP)– Ag binding gives rise to higher power thresholds. Such\nextended nanogap cavities are an ideal system to provide robust SERS\nwhile withstanding high laser powers. These results provide insights\ninto the inhomogeneities of NW nanocavities and pave the way toward\nspatially controlled NW lithography in ambient conditions.