  An enhancement in Brillouin light scattering of optical photons with magnons
is demonstrated in magneto-optical whispering gallery mode resonators tuned to
a triple resonance point. This occurs when both the input and output optical
modes are resonant with those of the whispering gallery resonator, with a
separation given by the ferromagnetic resonance (FMR) frequency. The
identification and excitation of specific optical modes allows us to gain a
clear understanding of the mode-matching conditions. A selection rule due to
wavevector matching leads to an intrinsic single-sideband excitation. Strong
suppression of one sideband is essential for one-to-one frequency mapping in
coherent optical-to-microwave conversion.
