Magneto-optical coupling in whispering gallery mode resonators

We demonstrate that yttrium iron garnet microspheres support optical whispering gallery modes similar to those in non-magnetic dielectric materials. The direction of the ferromagnetic moment tunes the resonant optical frequency via the Voigt effect, dependent on the angle of the magnetization with respect to the plane of the whispering gallery mode. This parametric coupling of the magnetization to the optical mode may enable analogous experiments to those performed in cavity optomechanics. In addition, the Faraday effect couples the two ordinarily linear polarized modes, split by the geometrical birefringence due to the boundary conditions at the surface. This results in a polarization rotation of the light emitted from the cavity. Our results extend recent work on the strong coupling of microwave photons to magnetization dynamics into the optical domain. An understanding of the magneto-optical coupling in whispering gallery modes, where the propagation direction rotates with respect to the magnetization, is fundamental to the emerging field of cavity optomagnonics. [arXiv:1510.06661].