  The collective spontaneous emission of two point-dipole emitters near a
plasmonic core-shell nanosphere is theoretically investigated. Based on the
expansion of mode functions in vector spherical harmonics, we derive closed
analytical expressions for both the cooperative decay rate and the
dipole-dipole interaction strength associated with two point dipoles close to a
sphere. Considering a plasmonic nanoshell containing a linearly amplifying
medium inside the core, the second-order correlation function for the two
emitters shows that it is possible to tune the photon emission, selecting
either photon bunching or antibunching as a function of the polarization and
position of the sphere. This result opens vistas to applications involving
tunable single-photon sources in engineered artificial media.

Optics