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Broken spatial and time reversal symmetries in materials often give rise to new emergent phenomena in the interaction between light and matter. The combination of chirality and broken time reversal symmetry in a magnetic field leads to magneto–chiral phenomena, such as the nonreciprocity of transmission. Here, we construct a terahertz hybrid metamaterial that combines the natural optical activity of a chiral metallic gammadion bilayer and the magneto-optical activity of semiconductor indium antimonide in a magnetic field. We report a resonant magneto–chiral effect that leads to polarization-independent nonreciprocal optical transmittance. Furthermore, we discover a magneto-optical Faraday effect that is resonantly controlled by the natural optical activity of the chiral gammadion bilayer. Unlike optical activity due to chirality, the novel Faraday effect is odd under time reversal. Both phenomena are activated by a modest magnetic field, which may open doors for their potential applications in polarization-independent optical isolation and highly efficient polarization control at terahertz frequencies. 

We report the observation of magneto-optical nonreciprocity in Faraday geometry in a hybrid metamaterial consisting of an Archimedean spiral metasurface and semiconductor InSb that serves as the magneto-optical medium. None of the metamaterial constituents possesses chirality, which is usually a necessary ingredient for optical nonreciprocity in natural materials when the light travels along the magnetic field direction. We also find that our metamaterial can serve as an optical element for polarization control via magnetic field. Another significant property of our hybrid metamaterial is the emergence of the four different transmittance states, which are observed for the four combinations of the positive and negative magnetic field and the direction of the wavevector of light.