  Magnons in ferrimagnetic insulators such as yttrium iron garnet (YIG) have
recently emerged as promising candidates for coherent information processing in
microwave circuits. Here we demonstrate optical whispering gallery modes of a
YIG sphere interrogated by a silicon nitride photonic waveguide, with quality
factors approaching $10^6$ in the telecom c-band after surface treatments.
Moreover, in contrast to conventional Faraday setup, this implementation allows
input photon polarized colinearly to the magnetization to be scattered to a
sideband mode of orthogonal polarization. This Brillouin scattering process is
enhanced through triply resonant magnon, pump and signal photon modes - all of
whispering gallery nature - within an "optomagnonic cavity". Our results show
the potential use of magnons for mediating microwave-to-optical carrier
conversion.
