  We investigated the spectral properties of electromagnetic (EM) enhancement
of one-dimensional hotspots (1D HSs) generated between silver nanowire (NW)
dimers. The EM enhancement spectra were directly derived by dividing the
spectra of ultrafast surface-enhanced fluorescence (UFSEF) from single NW
dimers with UFSEF obtained from large nanoparticle aggregates, which
aggregate-by-aggregate variations in the UFSEF spectra were averaged out. Some
NW dimers were found to exhibit EM enhancement spectra that deviated from the
plasmon resonance Rayleigh scattering spectra, indicating that their EM
enhancement was not generated by superradiant plasmons. These experimental
results were examined by numerical calculation based on the EM mechanism by
varying the morphology of the NW dimers. The calculations reproduced the
spectral deviations as the NW diameter dependence of EM enhancement. Phase
analysis of the enhanced EM near fields along the 1D HSs revealed that the
dipole-quadrupole coupled plasmon, which is a subradiant mode, mainly generates
EM enhancement for dimers with NW diameters larger than ~80 nm, which was
consistent with scanning electron microscopic measurements.
