In-gap polaritons in uniformly filled microcavities

We examine the nature and the dispersion of new polariton modes, which appear in the polariton gap (i.e. within the Rabi splitting), considering the spectrum of polaritons in a planar microcavity uniformly filled with a resonant material and exhibiting the strong exciton-photon coupling regime. These additional modes are the consequence of the quantization of the excitonic states caused by the confinement. We consider these in-gap modes in materials typically used in microcavities and link the obtained results to experiments, where such in-gap states have indeed been observed. We show that the splitting between the in-gap modes and their bandwidth depend on the bandwidth of the exciton out of the microcavity and on the oscillator strength of the excitonic transition.