Understanding the effect of vibronic interactions in organic molecules for polaritons

A molecular exciton polariton is a hybrid light and matter state which forms from the coupling of a photon and an exciton, the photoexcited state of an organic semiconductor or dye. Typically, these states are studied by confining a layer of dye between closely spaced mirrors in a Fabry-Perot microcavity. In the case of a simple polariton system, the interaction between one photon state and one exciton state in resonance can produce an upper and lower polariton band. This resonance is known to be collective across all dipoles available inside the cavity and the formed coherent states delocalize over the field volume. The newly formed hybrid states possess angular dispersion from the photonic mode of the cavity, which forces an anticrossing at the exciton-photon resonance. When more excitons are coupled to the photonic mode of the cavity, these interactions can give rise to a ladder of polariton states with varying degrees of photonic versus exciton character. In this study, we consider three different organic molecules with prominent vibronic structure, and thus multiple strong absorbing exciton transitions, in their steady-state absorption. The molecules DPP, PDI, and PTCDA were deposited via thermal evaporation within silver mirrors. Strong coupling was attained in all structures, and we used angle-dependent reflectivity measurements to determine the dispersions of the polaritonic states for comparison to optical modeling. Studying the relation between the vibronic structure and the dispersion of the polariton branches is useful to aid in the development of devices based on exciton polaritons, like low-threshold lasers, and lays the groundwork for future studies with more sophisticated cavity architectures.