Subpicoliter Magnetoptical Cavities

Abstract

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 ($100\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}{\mathrm{m}}^{3}$) optical mode volume, approximately $50$ times smaller than previous devices. From this, we estimate that the magnon single-photon coupling rate is $G\ensuremath{\approx}50$ 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.