Such a vacuum effect would also allow for intriguing applications. Optical nonlinearities in the few photon limit could be harvested as each single photon would further change the permittivity. Similarly, the strong coupling regime allows for two-photon nonlinearites with the same origin: The Rabi frequency changes for the number of excitations in the system. Yet, for the nonlinearities that occur in the HQM, the strong coupling regime is no prerequisite. The strong coupling regime requires a combination of sufficiently low linewidths of both emitters (γ) and cavities (κ). Typically, this criterion is quantified to 2Ω > (κ + γ)/2 . On the other hand, the comparison between the vacuum field and the threshold intensity for power broadening I pb previously showed, that the requirement of power broadening does not regard the cavity linewidth at all, since only Ω 2 > γ 2 /2 is necessary. Power broadening is not about pushing energy back and forth between the two subsystems but about the energy that is fed into the emitters. In principle, it should be possible to also construct weakly coupled systems in which power broadening is relevant and low photon number nonlinearities can be exploited. This is an exclusive property of lossy (bad) cavities and thus of plasmonic systems. In high-finesse optical cavities, the onset of a cavity induced power broadening comes along with the onset of strong coupling, which already can be exploited for two photon nonlinearites. In a plasmonic system, Ω 2 > γ 2 /2 can be fulfilled far before strong coupling occurs, i.e. plasmonic systems indeed may lead to qualitatively different features with respect to cavity QED than conventional high quality cavities.