Gyrotropic birefringence via electromagnon resonance in multiferroic manganite

The magnetoelectric (ME) coupling has been exemplified in the certain classes of crystalline solids including multiferroics and topological insulators. Since the ME coupling modifies the Maxwell’s equations, such ME materials give rise to the exotic electromagnetic phenomena. In particular, the light-matter interaction can induce the unconventional gyrotropy, which is called gyrotropic birefringence (GB), whereas its observation remains highly limited. We focus on the electrically active spin excitation i.e. electromagnon, inherent to the multiferroics, which potentially exhibits the enhanced optical ME effect. Here we report the first observation of GB on multiferroic manganite with spin-induced ferroelectricity by use of the time-domain terahertz polarimetry. The enhanced optical rotation arising from the GB was found on the electromagnon resonance. It is demonstrated that the GB is scaled by the bilinear coupling of ferroic order parameters P･M (P: Electric polarization, M: magnetization) in the magnetic field dependence. In addition, the quantitative spectral analysis reveals that the electromagnon and antiferromagnetic resonance cooperatively produce the enhanced GB, indicating the important role of inter-mode coupling for the enhanced dynamical ME effect.