We demonstrate strong exciton-photon coupling of Frenkel molecular excitons at room temperature in an all-metal microcavity containing a thin film of crystalline tetracene, with micron scale domains. In contrast to previous reports of strong coupling in organic microcavities containing localized excitonic states, here we observe large Davydov splitting of the lowest excited state, indicating strong, intermolecular interactions. The angle-resolved reflectivity of the microcavity is measured and understood in terms of the anisotropic dielectric constant of tetracene single crystals. The anisotropy leads to the formation of additional normal modes, and a giant splitting of the lower polariton branch is observed due to birefringence along both Davydov components.

• Received 16 January 2008

DOI:https://doi.org/10.1103/PhysRevB.77.073205