Since Ω 0 grows stronger for decreasing mode volumes, this criterion can easily be fulfilled in plasmonic cavities. We therefore propose that eq 3 should be applied when modelling the spectra of nanoparticles with a plasmonic core and an excitonic shell. A similar relation is often expressed with the term "saturation photon number" n 0 ≈ γ 2 /Ω 2 0 in cavity QED. The switch to this slightly more complicated permittivity model of eq 3 is further motivated by the fact that the third peak (in the extinction spectra of Figure and) results to a considerable extent from the particle's absorption and not its scattering. The sole absorption of the excitonic shell can be straightforwardly quantified within a numerical Maxwell solver (see supplementary information and Ref. 15). In the classical model, this absorption can be even stronger than that of the metallic core although the number of involved charge carriers is significantly lower. It is plausible that a correction of the classical model includes a mechanism that suppresses the emergence of this presumably unphysical peak via some form of damping which is more or less the effect of Ω in eq 3 (see also the discussion section). Now, the remaining question is which value needs to be inserted for Ω. First, we want to restrict ourselves to the low-excitation limit. If not utilizing well-designed ultrashort laser pulses, it is unrealistic to excite the plasmonic systems twice within the typical plasmon lifetime of about 100 fs. We therefore consider the case that on average, only individual photons probe the otherwise unexcited system, and therefore propose that it is the vacuum Rabi frequency Ω 0 that must be inserted in eq 3. Moreover, the shell does not consist of only one but several two-level systems. Yet, it is argued that these form a collective system (sometimes refered to as "giant oscillator") in the plasmonic cavity. This collective nature and the corresponding coherence means that each individual subsystem is directly affected by the electric field induced by the respective others. Consequently, Rabi oscillations and the vacuum Rabi splitting of individual two-level systems as well as for example chemical reactivity 8 are directly modified by the number of involved entities. We therefore suggest that the value for Ω that has to be inserted in the permittivity is retrieved from the collective coupling strength of the complete core-shell system. Until now, we regard this approach (i.e. using eq (3) and inserting the collective vacuum Rabi frequency for the light-matter coupling) as a heuristic quantum model (HQM) that is derived from the semi-classical approach. We assume that quantum mechanical treatments that also capture the effect discussed here can be derived more rigorously. We suspect that the reason, why this effect is still not discussed in literature might be the vast difference in effective rates i.e. coupling strength and exciton/plasmon/photon decay rates between traditional quantum electrodynamics (QED) systems and plasmon-exciton coupling. We believe that this effect is not necessarily exclusive though much more pronounced in plasmon-exciton systems. In the following, we want to present the consequences of applying this new HQM to the model of the core-shell sphere with a core formed by a Drude metal which is illustrated in Figure .