Surface plasmons and strong light-matter coupling in metallic nanoshells

A theory of the interaction between a radiating dipole and the plasmonic excitations of a spherical metallic nanoshell in the quasistatic approximation is formulated. After a derivation of surface plasmon frequencies and a comparison with the corresponding modes of metal spheres and cavities, we introduce an expression for the effective volume for any position of the dipole inside or outside the nanoshell, describing the local electromagnetic field enhancement in analogy to other cavity-QED systems. The modification of the dipole decay rate is calculated as a function of frequency for various geometrical parameters, and it reflects the spectrum of spherelike and cavity-like surface plasmon excitations. We then give a formulation of emission spectra, suitable for describing light-matter interaction beyond perturbation theory, and study the conditions for the strong coupling regime to occur. By suitably tuning the geometrical parameters of the nanoshell and by choosing the order of surface plasmon modes to minimize the effective volume, a vacuum Rabi splitting can occur in emission spectra for dipole oscillator strengths as small as a few units, which can be easily achieved with organic molecules or quantum dots. The most favorable situation for strong coupling is when the dipole is located inside the nanoshell. Surprisingly, this dipole couples with spherelike modes more strongly than with cavity-like ones, if the shell is thin enough. As a conclusion, metallic nanoshells turn out to be a suitable platform in order to investigate the strong-coupling regime of light-matter interaction by exploiting surface plasmon resonances.