Mode Number Dependent Photophysical Dynamics under Electronic Strong Coupling

Abstract Light-matter hybrid states known as polaritonic states emerge when molecules are strongly coupled to optical cavity modes. While strong coupling has been shown to modify various photophysical processes, the mechanistic origin and the role of individual cavity parameters are poorly understood. Here, we demonstrate that the cavity mode number critically governs polariton dynamics, independent of the Rabi splitting magnitude. Fabry–Perot cavities with mode numbers of 1, 2, 4, and 5 were fabricated using rhodamine B in a polymer matrix at a constant Rabi splitting. Ultrafast transient absorption spectroscopy reveals that the slow decay component increases with cavity mode number, accompanied by a long-lived excited state observed exclusively in higher-mode cavities. These observations can be rationalized by entropic stabilization of dark states, which enables population trapping and back energy transfer to the lower polariton. These findings establish cavity mode number as a new experimental handle for designing strongly coupled systems for photochemical applications.