Microwave-optical coupling via Rydberg excitons in cuprous oxide

Abstract

We report exciton-mediated coupling between microwave and optical fields in cuprous oxide (Cu2O) 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.

High resolution nanosecond spectroscopy of even-parity Rydberg excitons in Cu$_{2}$O

“…Lastly we note that the highly coherent, narrowband nature of the SHG produced in our experiment makes it very useful for applications, even within the currently observed range of principal quantum numbers. An example is the recent observation of the coherent modulation of the second harmonic by an applied microwave electric field [26].…”

“…Unlike one-photon absorption, where interactions with optical phonons [25] gives rise to strong background absorption, SHG is a background-free process that disappears above the bandgap of the material. These properties were essential to a recent demonstration of the microwave modulation of an optical carrier in cuprous oxide [26]. It is therefore an important question whether the even-parity spectrum may be resolved up to the same high principal quantum numbers (n > 25) observed in one-photon excitation of the nP series [1,18].…”

“…For SHG at the even parity states, the amplitude S n,L ∝ Q nl→VB M VB→nl [26]. Here Q nl→VB is the quadrupole matrix element between the Rydberg state and the valence band, which is predicted to scale as Q nl→VB 2 ∝ n −3 [52,57].…”