We offer a unified approach to several phenomena related to theelectromagnetic vacuum of a complex medium made of point electric dipoles. Tothis aim, we apply the linear response theory to the computation of thepolarization field propagator and study the spectrum of vacuum fluctuations.The physical distinction among the local density of states which enter thespectra of light propagation, total dipole emission, coherent emission, totalvacuum energy and Schwinger-bulk energy is made clear. Analytical expressionsfor the spectrum of dipole emission and for the vacuum energy are derived.Their respective relations with the spectrum of external light and with theSchwinger-bulk energy are found. The light spectrum and the Schwinger-bulkenergy are determined by the Dyson propagator. The emission spectrum and thetotal vacuum energy are determined by the polarization propagator. An exactrelationship of proportionality between both propagators is found in terms oflocal field factors. A study of the nature of stimulated emission from a singledipole is carried out. Regarding coherent emission, it contains two components.A direct one which is transferred radiatively and directly from the emitterinto the medium and whose spectrum is that of external light. And an indirectone which is radiated by induced dipoles. The induction is mediated by one (andonly one) local field factor. Regarding the vacuum energy, we find that inaddition to the Schwinger-bulk energy the vacuum energy of an effective mediumcontains local field contributions proportional to the resonant frequency andto the spectral line-width.
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