We investigate the optical magnetoelectric (ME) effects which lead to the nonreciprocity of photons arising from the enhanced dynamical ME coupling of the electromagnon resonances in the spin-cycloidal phases of multiferroic perovskite manganites ${\mathrm{Eu}}_{1-x}{\mathrm{Y}}_x{\mathrm{MnO}}_3$ ($x=0.3$, 0.4, 0.45, 0.5) by means of terahertz time-domain polarimetry. The correlation between the optical ME effects and the associated order parameters is examined by changing $x$. Nonreciprocal directional dichroism (NDD) is demonstrated in the $ab$- and $bc$-plane cycloidal phases. The resonantly enhanced NDD in the $ab$-plane cycloidal phase shows correlation with the relevant coupled order parameter composed of spontaneous polarization $\mathit{P}$ and magnetization $\mathit{M}$ as $\mathit{P}\times \mathit{M}$. The NDD is further enhanced in the $bc$-plane cycloidal phase due to the mode-mode coupling of electromagnons with distinct origins. The nonreciprocal optical rotation stemming from gyrotropic birefringence (GB) is also demonstrated in the $ab$-plane spin cycloidal phase. The magnitude of GB shows clear deviations from the relevant coupled order parameter of $\mathit{P}·\mathit{M}$. The GB is remarkably enhanced near the phase boundary ($x=0.3$) between the cycloidal phase and the canted antiferromagnetic phase. As a result, the dynamical diagonal ME coupling approaches 0.23 (770 ps/m) for $x=0.3$. These different $x$ dependences between NDD and the GB are discussed on the basis of the mechanisms of the dynamical ME coupling of collective spin excitations.

• Received 27 February 2023
• Accepted 29 June 2023

DOI:https://doi.org/10.1103/PhysRevB.108.024418