  Backscattering Raman spectroscopic investigations were performed on an
yttrium-iron garnet single crystal using linearly and circularly polarized
light. A terahertz (THz) magnon of the Kaplan-Kittel (KK) exchange resonance
was discovered, which had been regarded as unobservable via optical methods.
The KK exchange resonance had a 7.8-THz frequency at 80 K, and the polarization
selection rule led to an antisymmetric Raman tensor of the $A_{2}$ mode.
Moreover, the assignment of all the Raman-active phonon modes, $3A_{1g}$,
$8E_{g}$, and $14T_{2g}$, was proposed. This study will stimulate further
investigation of the coupling of THz magnons and phonons and pave the way
toward THz optomagnonics.
