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

## TL;DR

This paper analytically investigates how a dipole emitter's fluorescence near a plasmonic nanoshell can be enhanced or quenched through Fano resonances, with tunable effects based on geometry and orientation, relevant for sensing applications.

## Contribution

It provides an analytical framework for understanding and controlling fluorescence enhancement and quenching near plasmonic nanoshells via Fano resonances, considering emitter orientation and geometry.

## Key findings

- Fluorescence enhancement and quenching are linked to plasmonic Fano resonances.
- The sign of the Fano asymmetry parameter depends on dipole orientation.
- Saddle points in near-field energy flow occur for tangential dipole orientations.

## 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.