  We develop a theory of cooperative emission mediated by cooperative energy
transfer (CET) from an ensemble of quantum emitters (QE) to plasmonic antenna
at a rate equal to the sum of individual QE-plasmon energy transfer rates. If
the antenna radiation efficiency is sufficiently high, the transferred energy
is radiated away at approximately the same cooperative rate that scales with
the ensemble size. We derive explicit expressions, in terms of local fields,
for cooperative Purcell factor and enhancement factor for power spectrum valid
for plasmonic structures of any shape with characteristic size smaller than the
radiation wavelength. The radiated power spectrum retains the plasmon resonance
lineshape with overall amplitude scaling with the ensemble size. If QEs are
located in a region with nearly constant plasmon local density of states
(LDOS), e.g., inside a plasmonic nanocavity, we demonstrate that the CET rate
scales linearly with the number of excited QEs, consistent with the experiment,
and can be tuned in a wide range by varying the excitation power. For QEs
distributed in an extended region saturating the plasmon mode volume, we show
that the cooperative Purcell factor has universal form independent of the
system size. The CET mechanism incorporates the plasmon LDOS enhancement as
well, giving rise to possibilities of controlling the emission rate beyond
field enhancement limits.
