Fano resonances and fluorescence enhancement of a dipole emitter near a plasmonic nanoshell

We analytically study the spontaneous emission of a single optical dipole emitter in the vicinity of a plasmonic nanoshell, based on the Lorenz-Mie theory. We show that the fluorescence enhancement due to the coupling between optical emitter and sphere can be tuned by the thickness ratio of the core-shell nanosphere and by the distance between the quantum emitter and its surface. In particular, we demonstrate that both the enhancement and quenching of the fluorescence intensity are associated with plasmonic Fano resonances induced by near- and far-field interactions. These Fano resonances have asymmetry parameters whose signs depend on the orientation of the dipole with respect to the spherical nanoshell. We also show that if the atomic dipole is oriented tangentially to the nanoshell, the interaction exhibits saddle points in the near- field energy flow. This results in a Lorentzian fluorescence enhancement response in the near field and a Fano line shape in the far field. The signatures of this interaction may have interesting applications for sensing the presence and the orientation of optical emitters in close proximity to plasmonic nanoshells.