Localized Probe of Molecular Interaction under Strong Light-Matter Coupling

Strong light-matter coupling gives rise to hybrid polariton states, offering new opportunities to control dynamics and reactivity. While spectra show dramatic polaritonic signatures, it remains unclear whether molecular-scale interactions are altered under strong coupling. Here, we probe the localized environment of strongly coupled systems using electronic energy transfer (EET), which provides insight beyond conventional far-field spectroscopy. By introducing a probe molecule and employing femtosecond transient absorption, we measure the EET rate between the probe and strongly coupled molecules inside a Fabry-Pérot cavity. Remarkably, the EET rate remains unchanged, despite substantial spectral modifications under strong coupling. This result indicates that near-field interactions governing molecular energy transfer are largely unaffected, helping explain why photophysics in such systems often reflect uncoupled molecules. Our approach is simple and general, offering a valuable tool for probing local interactions in polaritonic materials and related applications.