A simple method to dramatically enhance the optical magnetoelectric (ME) effect, i.e., nonreciprocal directional birefringence, is proposed and demonstrated for a polar ferrimagnet ${\mathrm{GaFeO}}_3$ as a typical example. We patterned a simple grating with a period of $4\mu \mathrm{m}$ on a surface of ${\mathrm{GaFeO}}_3$ crystal and used the diffracted light as a probe. The optical ME modulation signal for the Bragg spot of the order $n=1$ becomes gigantic in the photon energy 1–4 eV and reaches 1–2% of the bare diffracted light intensity in a magnetic field of 500 Oe. This is amplified by more than 3 orders of magnitude compared to that for the reflection of bulk ${\mathrm{GaFeO}}_3$. Fabricating a photonic crystal will make it possible to lead the way for the practical use of the optical ME effect.

• Received 21 October 2005

DOI:https://doi.org/10.1103/PhysRevLett.96.167202