Strong Light–Matter Interaction of Molecular Aggregates with Two Excitonic Transitions

We demonstrate simultaneous strong light–matter coupling of J- and H-like transitions of molecular aggregates of a merocyanine dye placed in a planar microcavity. The molecules form a uniaxial crystal with the transition dipole moments of the two transitions oriented along and normal to the cavity mirrors. Both transitions are spectrally narrow, allowing one to reach the strong-coupling regime between the excitons and cavity photons of the 3λ/2- and 1λ modes of the cavity. Our work explores the strong light–matter coupling of H-transitions, which has so far not been properly studied. We show how the number of coherently coupled molecules in the molecular aggregates influences the strong-coupling allowing us to relate the degree of aggregation to the observed Rabi energy. Our polaritonic system forms a platform to study the photophysics of complex strong-coupling phenomena.