Modulation of vibronic transitions in chlorophyll a through strong light-matter coupling

Abstract

Strong coupling between molecular electronic transitions and the electromagnetic field of an optical resonator gives rise to hybrid light-matter states known as exciton-polaritons. Polariton states exhibit distinct energies and photophysical properties from their unhybridized molecular counterparts, making exciton-polaritons a powerful platform for engineering photophysical processes in molecular materials. In this work, we demonstrate strong light-matter coupling between a confined photon mode in a metallic Fabry-Pérot microcavity and electronic transitions of chlorophyll a (Chl a), the primary light-harvesting chromophore in natural photosynthesis. In addition to its strong absorption, Chl a features prominent vibronic structure in its absorption spectrum, making it a compelling model system for exploring how vibronic transitions are influenced by cavity coupling. Angle-dependent reflection spectra clearly resolve upper- (UP), middle- (MP), and lower-polariton (LP) branches, indicating that multiple vibronic sub-levels of the Q_y state of Chl a contribute to the light-matter coupling in the system, which had not been resolved in previous studies. Furthermore, the narrowing of the LP peak width suggests that strong light–matter interaction can suppress the energetic disorder present in the embedded Chl molecules. These findings present promising opportunities for tailoring vibronic excited-state manifolds and associated relaxation pathways in light-harvesting systems via cavity-mediated interactions.