Mie plasmons: modes volumes, quality factors and coupling strengths   (Purcell factor) to a dipolar emitter
  Using either quasi-static approximation or exact Mie expansion, we characterize the localized surface plasmons supported by a metallic spherical nanoparticle. We estimate the quality factor $Q_n$ and define the effective volume $V_n$ of the $n^{th}$ mode in a such a way that coupling strength with a neighbouring dipolar emitter is proportional to the ratio $Q_n/V_n$ (Purcell factor). The role of Joule losses, far-field scattering and mode confinement in the coupling mechanism are introduced and discussed with simple physical understanding, with particular attention paid to energy conservation. 
G. Colas des Francs, S. Derom, R. Vincent, A. Bouhelier, A. Dereux
physics.optics quant-ph