  One-dimensional junctions between parallel and closely arranged multiple
silver nanowires (NWs) exhibit a large electromagnetic (EM) enhancement factor
(FR) owing to both localized and surface plasmon resonances. Such junctions are
referred to as one-dimensional (1D) hotspots (HSs). This study found that
two-photon excited emissions, such as hyper-Rayleigh, hyper-Raman, and
two-photon fluorescence of dye molecules, are generated at the edge of 1D HSs
of NW dimers with continuous-wave near-infrared (NIR) laser excitation and
propagated through the 1D HSs; however, they were not generated from the
centers of 1D HSs. Numerical EM calculations showed that FR of the NIR region
for the edges of 1D HSs was larger than that for the centers by approximately
104 times, resulting in the observation of two-photon excited emissions only
from the edge of 1D HSs. The analysis of the NW dimer gap distance dependence
of FR revealed that the lowest surface plasmon (SP) mode, compressed and
localized at the edges of the 1D HSs, was the origin of the large FR in the NIR
region. The propagation of two-photon-excited emissions was supported by
higher-order coupled SP mode.
