Engineering Strong Exciton-Photon Coupling via Molecular Orientation in Organic Microcavities*

Exciton-polaritons are formed by strong coupling between the exciton in a semiconductor and a confined cavity photon mode. Organic semiconductors are attractive for exciton-polariton formation due to their large exciton binding energy and oscillator strengths. Indeed, prior work has led to demonstrations of polariton condensation, lasing, and applications in light-emitting and detecting devices. To enhance potential applications, a better understanding of how to engineer coupling strength at a material level is needed. This study focuses on tuning the exciton-photon interaction through intentional control over molecular transition dipole moment (TDM) orientation in the glassy, organic thin film absorber. Specifically, ultrastrong coupling is demonstrated in a metal reflector microcavity containing a thin film of 4, 4'-bis[(N-carbazole)styryl]biphenyl(BSB-Cz) with a Rabi splitting greater than 1.0 eV. Building upon prior literature showing the tunability of molecular orientation with substrate temperature during film deposition, BSB-Cz optical microcavities have been used to show that tuning molecular orientation via processing conditions can lead to a ~20% variation in the Rabi splitting. These results will be discussed in the context of absorber design rules for tuning polariton behavior, and in informing potential applications.