Continuously Tuning Strong Exciton–Photon Coupling via Molecular Orientation in Organic Microcavities

Exciton-polaritons in organic microcavities are applied in devices including lasers, light-emitting devices, and photodetectors, as well as in structures capable of tuning exciton kinetics and energy transfer. To enable a broader tailoring of polariton properties, it is important to develop means to better control molecular orientation and tune the intensity of the exciton–photon interaction. Vapor-processed, glassy organic thin films are previously shown to have tunable molecular orientation as evidenced by phenomena including birefringence and transition dipole moment (TDM) alignment. Here, this tunability in TDM orientation with thin film processing conditions is exploited to continuously vary the interaction between the exciton and confined cavity photon mode. By embedding a thin film of 4,4′-bis[(N-carbazole)styryl]biphenyl (BSB-Cz) in a metal-reflector microcavity, ultrastrong coupling and hybridization of multiple electronic transitions of BSB-Cz are demonstrated with a common cavity mode. Increasing the temperature during BSB-Cz deposition tunes the TDM orientation from predominantly in-plane to random to slightly vertical. This leads to a corresponding ≈30% variation in the associated Rabi splitting, consistent with theoretical predictions. This work demonstrates a means to continuously tune coupling strength from a materials perspective while also providing a handle to tune orientation disorder in thin film.