One-way edge mode in a gyromagnetic photonic crystal slab.

We demonstrate that one-way electromagnetic modes could be sustained by the edge of a gyromagnetic photonic crystal slab of triangular lattice under an external dc magnetic field. The applied magnetic field breaks the time-reversal symmetry of the three-dimensional system, and thus the original degeneracy point in k space, at which two dispersion surfaces intersect, is lifted, resulting in a photonic band gap below the light cone. At this band gap, the one-way mode is localized horizontally to the slab edge, while confined by the index contrast in the vertical direction.