  Terahertz time-domain spectroscopy was performed to directly probe the
low-energy (1-5 meV) electrodynamics of triangular lattice antiferromagnets
CuFe1-xGaxO2 (x = 0.00, 0.01, and 0.035). We discovered an electromagnon
(electric-field-active magnon) excitation at 2.3 meV in the paraelectric
up-up-down-down collinear magnetic phase, while this electromagnon vanishes in
the ferroelectric helimagnetic phase. Anti-correlation with noncollinear
magnetism excludes the exchange-striction mechanism as the origin of dynamical
magnetoelectric coupling, and hence evidences the observation of spin-orbit
coupling mediated electromagnon in the present compound.
