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 300 mu m 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.