  We present a model for exciton-plasmon coupling based on an energy exchange
mechanism between quantum emitters (QE) and localized surface plasmons in
metal-dielectric structures. Plasmonic correlations between QEs give rise to a
collective state exchanging its energy cooperatively with a resonant plasmon
mode. By defining carefully the plasmon mode volume for a QE ensemble, we
obtain a relation between QE-plasmon coupling and a cooperative energy transfer
rate that is expressed in terms of local fields. For a single QE near a sharp
metal tip, we find analytically the enhancement factor for QE-plasmon coupling
relative to QE coupling to a cavity mode. For QEs distributed in an extended
region enclosing a plasmonic structure, we find that the ensemble QE-plasmon
coupling saturates to a universal value independent of system size and shape,
consistent with the experiment.
