Ultrabroad resonance of localized plasmon on a nanoparticle coupled with surface plasmon on a nanowire enabling two-photon excited emission via continuous-wave laser.

This study found that plasmonic hotspots (HSs) between silver nanoparticles (NPs) and silver nanowires (NWs) generated two-photon excited emissions, including hyper-Rayleigh, hyper-Raman, and two-photon fluorescence of dye molecules with continuous-wave (CW) near-infrared (NIR) laser excitation. A comparison between experimental results and electromagnetic (EM) calculations revealed that a large EM enhancement factor (FR) at the HS appears in the visible to NIR regions owing to EM coupling between localized plasmons of the NP and surface plasmons of the NW. This ultrabroad resonance of the coupled plasmons enables the observation of two-photon-excited emissions with CW laser excitation. A comparison of the dependence of the NP diameter, NW diameter, and gap distance on two-photon-excited emissions further demonstrated that the large HS-by-HS variations in the emission intensities were mainly induced by the sensitivity of FR to the gap distance at the HSs. We also experimentally and theoretically investigated the propagation of two-photon-excited emission light to a neighboring NP on the NW via surface plasmons.