A New Signature for Strong Light–Matter Coupling Using Spectroscopic Ellipsometry

[Image: see text] Light–matter interactions can occur when an ensemble of molecular resonators is placed in a confined electromagnetic field. In the strong coupling regime the rapid exchange of energy between the molecules and the electromagnetic field results in the emergence of hybrid light–matter states called polaritons. Multiple criteria exist to define the strong coupling regime, usually by comparing the splitting of the polariton bands with the line widths of the uncoupled modes. Here, we highlight the limitations of these criteria and study strong coupling using spectroscopic ellipsometry, a commonly used optical characterization technique. We identify a new signature of strong coupling in ellipsometric phase spectra. The combination of ellipsometric amplitude and phase spectra yields a distinct topological feature that we suggest could serve as a new criterion for strong coupling. Our results introduce the idea of ellipsometric topology and could provide further insight into the transition from the weak to strong coupling regime.