Enhanced optical magnetoelectric effect in a patterned polar ferrimagnet

A simple method to dramatically enhance the optical magnetoelectric (ME) effect, i.e., nonreciprocal directional birefringence, is proposed and demonstrated for a polar ferrimagnet GaFeO$_3$ as a typical example. We patterned a simple grating with a pitch of 4 $\mu$m on a surface of GaFeO$_3 $ crystal and used the diffracted light as a probe. Optical ME modulation signal for 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 three orders of magnitude compared to that for the reflection of bulk GaFeO$_3$. Fabricating a photonic crystal will make it possible to lead a new route for the practical use of the optical ME effect.