  Exotic spin-multipolar ordering in spin transition metal insulators has so
far eluded unambiguous experimental observation. A less studied, but perhaps
more feasible fingerprint of multipole character emerges in the excitation
spectrum in the form of quadrupolar transitions. Such multipolar excitations
are desirable as they can be manipulated with the use of light or electric
field and can be captured by means of conventional experimental techniques.
Here we study single crystals of multiferroic Sr$_2$CoGe$_2$O$_7$, and show
that due to its nearly isotropic nature a purely quadrupolar bimagnon mode
appears in the electron spin resonance (ESR) spectra. This non-magnetic
spin-excitation couples to the electric field of the light and becomes
observable for a specific experimental configuration, in full agreement with a
theoretical analysis of the selection rules.
