Observation of terahertz magnon of Kaplan-Kittel exchange resonance in yttrium-iron garnet by Raman spectroscopy

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.