The Rabi frequency Ω accounts for the coupling between the electric field and the two-level system via Ω = µE 0 /ℏ where µ describes the transition dipole moment. The non-linearity that distinguishes eq 3 from its linear counterpart eq 2 originates from the Rabi frequency in the denominator. This textbook formula is a more correct approach to describe the response of a two-level system to an electric field. Why is it usually not applied in excitonic material models? For weak interactions between light and matter, Ω can be neglected in eq 3 and the classical Lorentz solution is recovered. For larger interaction strengths, Ω causes the so-called power broadening. As Ω ∝ E 0 , it is usually argued that a non-negligible Ω requires many photons to reach sufficiently high field strengths. However, it is the electric field that scales the Rabi frequency and not the number of photons. Plasmonic nanoparticles provide such small mode volumes that even for very low photon numbers, the electric fields can be very strong. In fact, cavity QED determines that even the vacuum state |0⟩ in an optical cavity is of finite energy. This leads to a finite coupling energy between light and matter that is expressed by the vacuum Rabi frequency Ω 0 as 23