Microwave-optical coupling via Rydberg excitons in cuprous oxide

We report exciton-mediated coupling between microwave and optical fields in cuprous oxide (${\mathrm{Cu}}_{2}\mathrm{O}$) at low temperatures. Rydberg excitonic states with principal quantum number up to $n=12$ were observed 4 $\mathrm{K}$ using both one-photon (absorption) two-photon (second harmonic generation) spectroscopy. Near resonance an state, the addition of a field significantly changed absorption line shape, added sidebands frequency coherent second harmonic. Both effects showed complex dependence on $n$ angular momentum $l$. All these features are semiquantitative agreement model based intraband electric dipole transitions exciton states. With simple antenna we already reach regime where (Rabi frequency) is comparable nonradiatively broadened linewidth excitons. The results provide additional way manipulate states, open possibility cryogenic transducer