Optimal mode matching in cavity optomagnonics

Inelastic scattering of photons is a promising technique to manipulate magnons but it suffers from weak intrinsic coupling. We theoretically discuss an idea to increase optomagnonic coupling in optical whispering gallery mode cavities by generalizing a previous analysis to include the exchange interaction. We predict that the optomagnonic coupling constant to surface magnons in yttrium iron garnet (YIG) spheres with radius 300μm can be up to 40 times larger than that to the macrospin Kittel mode. Whereas this enhancement falls short of the requirements for magnon manipulation in pure YIG, nanostructuring and/or materials with larger magneto-optical constants can bridge this gap.