  Strong light-matter interactions in localized nano-emitters when placed near
metallic mirrors have been widely reported via spectroscopic studies in the
optical far-field. Here, we report a near-field nano-spectroscopic study of the
localized nanoscale emitters on a flat Au substrate. We observe strong-coupling
of the excitonic dipoles in quasi 2-dimensional CdSe/CdxZnS1-xS nanoplatelets
with gap mode plasmons formed between the Au tip and substrate. We also observe
directional propagation on the Au substrate of surface plasmon polaritons
launched from the excitons of the nanoplatelets as wave-like fringe patterns in
the near-field photoluminescence maps. These fringe patterns were confirmed via
extensive electromagnetic wave simulations to be standing-waves formed between
the tip and the emitter on the substrate plane. We further report that both
light confinement and the in-plane emission can be engineered by tuning the
surrounding dielectric environment of the nanoplatelets. Our results lead to
renewed understanding of in-plane, near-field electromagnetic signal
transduction from the localized nano-emitters with profound implications in
nano and quantum photonics as well as resonant optoelectronics.
