Microwave-optical coupling via Rydberg excitons in cuprous oxide

TL;DR

This paper demonstrates microwave-optical coupling in cuprous oxide via Rydberg excitons, showing controllable interactions and potential for cryogenic transducers, with experimental results aligning with a dipole transition model.

Contribution

It introduces a new method for manipulating excitonic states using microwave fields in cuprous oxide, advancing towards microwave-to-optical transduction at cryogenic temperatures.

Findings

Observation of Rydberg excitons up to n=12 at 4 K

Microwave fields significantly alter absorption and generate sidebands

Coupling strength comparable to exciton linewidths

Abstract

We report exciton-mediated coupling between microwave and optical fields in cuprous oxide (Cu$_2$O) at low temperatures. Rydberg excitonic states with principal quantum number up to $n=12$ were observed at 4~K using both one-photon (absorption) and two-photon (second harmonic generation) spectroscopy. Near resonance with an excitonic state, the addition of a microwave field significantly changed the absorption lineshape, and added sidebands at the microwave frequency to the coherent second harmonic. Both effects showed a complex dependence on $n$ and angular momentum, $l$. All of these features are in semi-quantitative agreement with a model based on intraband electric dipole transitions between Rydberg exciton states. With a simple microwave antenna we already reach a regime where the microwave coupling (Rabi frequency) is comparable to the nonradiatively broadened linewidth of the Rydberg excitons. The results provide a new way to manipulate excitonic states, and open up the possibility of a cryogenic microwave to optical transducer based on Rydberg excitons.