We present our theoretical and experimental study of strong coupling between excitons and thick cavities, for which several lungitudinal resonant modes may exist and interact with the excitonic transition. We show that cavities having identical coupling strengths but different sizes can exhibit very different behavior, contrary to common belief. Our studies show that in multimode cavities, a distinct transition between two types of strong coupling occurs at a critical length, which is determined by the balance between the velocity of light within the cavity and the decoherence rate in the excitonic medium. Below the transition, the cavity modes can intermix via their mutual coupling with the medium, while above the transition this mixing disappears, such that the resulting in fomration of fully decoupled polaritonic branches. Moreover, we find that such systems cannot be modeled over both sides of this transition simultaneously by either one of the currently used Hamiltonians, which implies that a new Hamiltonian, which takes into account propagation effects within the cavity, should be sought after.

New type of cavity-QED transition in multimode cavities under strong coupling

ACS Spring 2022

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