Microwave-to-optical transduction using magnon-exciton coupling in a layered antiferromagnet

Abstract

Coherent interfaces between microwave-frequency quantum systems and low-loss optical links are essential for quantum networks. However, existing microwave-optical transducers often trade conversion efficiency against added noise, bandwidth, and device integrability. Here, we demonstrate coherent microwave-to-optical transduction based on magnon-exciton coupling in the layered antiferromagnet CrSBr. Driving the antiferromagnetic resonance with microwave signals imprints coherent modulation on a reflected optical probe, generating optical sidebands that are resonantly enhanced near excitonic transitions. While prior magnon-based approaches to microwave-to-optical transduction have typically relied on intrinsically weak off-resonant magneto-optical effects (e.g., Faraday rotation), our scheme exploits strong light-matter interactions at exciton resonances. Even in a bulk crystal without cavity enhancement, we observe coherent conversion over an intrinsically broadband window of ~ 300 MHz. We further show that multiple exciton-polariton resonances inherit the magnon-coupled response, suggesting a route to broaden the usable optical detuning range and to mitigate optical dissipation. Our results establish magnon-coupled excitons in layered magnets as a scalable platform for broadband microwave-optical interfaces, with pathways to higher cooperativity via reduced magnetic volume and cavity integration.

Figures (4)

• Figure 1: Transduction platform and magneto-optical characterization.a, Schematic of microwave-to-optical conversion mediated by coupling between magnon and exciton. b, Device schematic. A bulk CrSBr crystal is placed on a coplanar waveguide (CPW) inside a cryostat at 2 K. The crystal $b$-axis aligns with the transmission line, with the $c$-axis perpendicular to the CPW plane. Resonant microwave excitation drives magnons, while an optical pump excites excitons. An external, static field $B_\text{ext}$ is applied perpendicular to the CPW. Magnon–exciton coupling produces an optical modulation at magnon frequency. c, Crystal and magnetic structure for $B_\text{ext} < B_\text{sat}$, where $B_\text{sat}$ is the saturation field. Spins in adjacent layers are antiferromagnetically aligned and canted toward the field, setting a canting angle $\theta$. The microwave drive excites a uniform precession of the spins (magnons). Exciton wavefunctions respond to spin alignment, enabling coupling to magnons. d, Frequency-domain representation of microwave-to-optical transduction. Optical photons are generated as a sideband at frequency $\omega_o$, offset by the microwave frequency $\omega_{\mu}$ from the optical pump $\omega_p$. e, Microwave transmission change $\Delta S_{21}$ as a function of the magnetic field showing magnon dispersion. Lower and upper branches correspond to optical and acoustic magnon modes, respectively. Above the saturation field ($B_\text{sat} \approx 1.9$ T), a single ferromagnetic resonance branch is observed. f, Normalized optical reflectance ($R/R_\text{BG}$) versus the static magnetic field showing exciton dispersion, where $R_\text{BG}$ is the reflectance of the CPW metal plane. Both high- and low-energy excitons show energy redshifts with increasing field magnitude, evidencing coupling between exciton and the magnetic order.
• Figure 2: Magnon-induced reflectance change.a, Normalized optical reflectance ($R_\text{OFF}/R_{BG}$) versus probe photon energy at $B_\text{ext}=0$ T and $0.5$ T, with the microwave drive off. b, Microwave-induced reflectance change, $(\Delta R/R_\text{OFF})= (R_\text{ON}-R_\text{OFF})/R_\text{OFF}$, for both fields. Here, $R_\text{ON}$ ($R_\text{OFF}$) is the reflectance with microwave drive on (off). The microwave drive frequency is 23.2 GHz and 22.5 GHz at $B_\text{ext}=0$ T and $0.5$ T, respectively. The response is strongest at the exciton resonances. c, d, Maps of $(\Delta R/R_\text{OFF})$ versus probe energy and microwave drive frequency at $B_\text{ext}=0$ T (c) and $B_\text{ext}=0.5$ T (d). e, f, Line cuts of $\Delta S_{21}$ from Fig. 1e at $B_\text{ext}=0$ T (e) and $B_\text{ext}=0.5$ T (f). The highlighted frequency window corresponds to the modulation band in (c, d). g,(right, positive B) Field dependence of the drive frequency yielding the maximal reflectance change (peak of $\Delta R/R_\text{OFF}$). The shaded band indicates the frequency range over which an appreciable reflectance change is observed ($\Delta R/R_\text{OFF}>0.0015$). The resulting dispersion agrees with the magnon mode dispersion extracted from microwave spectroscopy (Fig. 1e, left, negative B).
• Figure 3: Homodyne detection of coherent microwave-to-optical conversion.a, Microwave power of the homodyne signal measured on a spectrum analyzer as a function of probe-frequency detuning from the driven magnon frequency. A peak at zero detuning indicates coherent optical modulation at the magnon frequency. b, Two-dimensional map of microwave power versus microwave drive frequency and detuning. The modulation persists over a frequency range of $\sim$300 MHz. c, Magnetic-field dependence of the drive frequency for which the maximal homodyne signal is observed, consistent with the magnon dispersion from microwave spectroscopy (Fig. 1e). d, Converted optical sideband photon flux versus incident microwave photon flux. The solid line is a linear fit. Measurements in Fig.\ref{['fig:fig3']}a, b, and d were performed with $B_\text{ext}=0.5$ T.
• Figure 4: Coupling between magnons and exciton-polaritons.a, Schematic of microwave-to-optical conversion mediated by coupling between magnon and exciton-polariton. b, Normalized reflectance $R/R_\mathrm{BG}$ versus probe photon energy, taken at $B_\text{ext}=-1.3$ T, showing multiple polariton resonances. c, Microwave-induced differential reflectance $\Delta R/R_\mathrm{OFF}$ at $B_\text{ext}=-1.3$ T as a function of microwave drive frequency and probe photon energy, revealing multiple polariton-assisted transduction features.