Rydberg exciton--polaritons in a Cu2O microcavity

Giant Rydberg excitons with principal quantum numbers as high as n{\thinspace}={\thinspace}25 have been observed in cuprous oxide (Cu2O), a semiconductor in which the exciton diameter can become as large as ∼1{\thinspace}$\mu$m. The giant dimension of these excitons results in excitonic interaction enhancements of orders of magnitude. Rydberg exciton--polaritons, formed by the strong coupling of Rydberg excitons to cavity photons, are a promising route to exploit these interactions and achieve a scalable, strongly correlated solid-state platform. However, the strong coupling of these excitons to cavity photons has remained elusive. Here, by embedding a thin Cu2O crystal into a Fabry--P{\'e}rot microcavity, we achieve strong coupling of light to Cu2O Rydberg excitons up to n{\thinspace}={\thinspace}6 and demonstrate the formation of Cu2O Rydberg exciton--polaritons. These results pave the way towards realizing strongly interacting exciton--polaritons and exploring strongly correlated phases of matter using light on a chip.