Terahertz magnetoelectric resonance enhanced by mutual coupling of electromagnons

Both electric- and magnetic-dipole active spin excitations, i.e., electromagnons, which mediate the dynamical magnetoelectric effect, have been investigated for a multiferroic perovskite of manganite by optical spectroscopy at terahertz frequencies. Upon the magnetoelectric resonance at 1 meV in the multiferroic phase with the bc-plane spin cycloidal order, a gigantic dynamical magnetoelectric effect has been observed as a nonreciprocal directional dichroism or birefringence. The light k-vector-dependent difference (Δκ=κ(+)-κ(-)) of the extinction coefficient (κ(±)) is as large as Δκ~1 or 2Δκ/(κ(+)+κ(-))~0.7 at the lowest-lying electromagnon energy. We clarified the mutual coupling of the E(ω)[parallel]a-polarized electromagnons of the different origins, leading to the enhancement of the magnetoelectric resonance.