We have just posted a paper on arXiv describing a potential new criterion for strong light-matter coupling using spectroscopic ellipsometry. Strongly coupled systems are usually characterised using intensity data (transmittance, reflectivity, etc). Spectroscopic ellipsometry allowed us to study the combined amplitude and phase response instead. We found we could observe the transition from weak to strong coupling through a series of quite beautiful plots of the ellipsometric parameter “rho” (see photo). We are now working to study other systems in the same way, to see what insights this approach to visualising ellipsometric data might provide.