Nonreciprocal Huygens’ Metasurfaces Based on Bound States in the Continuum

Magneto-optical effects are typically very weak in the optical spectrum, and there- fore, nonreciprocal devices based on them are very bulky. We demonstrate that optical metasurfaces supporting quasi-bound states in the continuum provide a powerful way for achieving strong nonreciprocal responses in a sub-wavelength geometry. Using the multi-mode temporal coupled-mode theory, we synthesize and analyze a nonreciprocal metasurface with Huygens’ meta-atoms based on conventional ferrite materials exhibiting nearly-unity contrast in transmission for circularly polarized light.