Spectroscopy of Er₂O₃ crystals for efficient microwave-to-optical quantum transduction

Fully concentrated (or stoichiometric) rare-earth oxide crystals present a promising platform for microwave-to-optical quantum transduction owing to their strong magneto-optic nonlinearity. At cryogenic temperatures, magnetic ordering in these crystals suppresses noise, enabling narrow optical inhomogeneous linewidths and collective magnetic excitations in the microwave band. Here, we perform spectroscopy on both bulk polycrystalline and epitaxially grown single-crystal Er₂O₃ for transduction applications. Optical spectroscopy in the telecom C-band reveals optical inhomogeneous linewidths as narrow as 3.5 GHz, along with magnon sidebands for both C₂ and C_3i symmetry sites. We find homogenous linewidths on the order of 20 kHz for the C_3i symmetry site and investigate spectral diffusion. Furthermore, we perform antiferromagnetic resonance measurements by coupling the crystals to microwave resonators to determine the magnon coupling strengths and linewidths. Finally, we propose a device design and operating protocol for implementing microwave-to-optical transduction using Er₂O₃.