  The demonstration of enhanced spontaneous emission of nanoscaled optical
emitters near metallic nanoparticles and the recent realization of a nanolaser
based on surface plasmon amplification by stimulated emission of radiation
(spaser) encourage the search for strong coupling regime at the nanoscale. Here
we propose the concept of nanopolaritons. We demonstrate with accurate
scattering calculations that the strong coupling regime of a single quantum
emitter (a semiconductor quantum dot) placed in the gap between two metallic
nanoparticles can be achieved. The largest dimension of the investigated system
is only 36 nm. Nanopolaritons will advance our fundamental understanding of
surface plasmon enhanced optical interactions and could be used as
ultra-compact elements in quantum-information technology.
