Artificial nonreciprocal photonic materials at GHz-to-THz frequencies

Lorentz reciprocity governs the symmetry with which electromagnetic signals travel in space and time. A reciprocal channel supports signal transport in two directions with the same transmission properties. Nonreciprocal devices do not obey this general symmetry, and therefore enable isolation and circulation, offering fundamental functionalities in modern GHz-to-THz photonic systems. While most nonreciprocal devices to date are based on magneto-optical phenomena, significant interest has been raised by approaches that avoid the use of magnetic materials, instead relying on artificial materials and circuits that mimic magnetically biased ferrites, enabling compact, light, integrated, and significantly cheaper nonreciprocal devices. Here, we review recent progress in and opportunities offered by artificial nonmagnetic materials that break reciprocity, revealing their potential for compact nonreciprocal devices and systems.