Microwave-to-optical conversion via ferromagnetic magnons has so far been limited by the optical coupling rates achieved in millimeter-scale whispering-gallery-mode resonators. Toward overcoming this limitation, we propose and demonstrate an open magneto-optical cavity containing a thin film of yttrium iron garnet (YIG). We achieve a 0.1 pL ($100\mu {\mathrm{m}}^3$) optical mode volume, approximately $50$ times smaller than previous devices. From this, we estimate that the magnon single-photon coupling rate is $G\approx 50$ Hz. This open-cavity design offers the prospect of wavelength-scale mode volumes, small polarization splittings, and good magneto-optical mode overlap. With achievable further improvements and optimization, efficient microwave-optical conversion and magnon cooling devices become a realistic possibility.

• Received 8 July 2020
• Revised 26 August 2020
• Accepted 8 September 2020

DOI:https://doi.org/10.1103/PhysRevApplied.14.044005