Title: Fano resonances and fluorescence enhancement of a dipole emitter near a plasmonic nanoshell,
Abstract: 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 aspect 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.