Gyrotropic Metamaterials: Advancing Nonreciprocal Photonics and Electronics

Ferrite materials biased by an external magnetic field are the most common gyrotropic media used in nonreciprocal electromagnetic devices such as isolators, circulators, and nonreciprocal phase shifters. This traditional approach involves using bulky magnets, which can pose a problem for compact devices and those with large apertures. Alternatively, permanently magnetized materials such as hard ferrites and ferromagnets with high coercivity can be utilized. These materials can produce a nonreciprocal response, like Faraday rotation, even in the absence of an external bias field. However, magnetized magnetic materials generate a demagnetizing field that can be nonuniform, depending on their shape. This nonuniformity can significantly impact the performance of the nonreciprocal device. Another issue with magnetized materials is the presence of stray magnetic fields around the device. Some critical applications, such as inertial navigation and quantum sensing, cannot tolerate even tiny magnetic fields; yet, they still require a nonreciprocal component to function properly. To overcome these problems, we propose a fundamentally different approach to achieving Faraday rotation: using gyrotropic metamaterials with tailored magnetization, including configurations with zero net magnetization, which do not require a bias field. Without bias and demagnetizing fields, the aperture of this Faraday rotator can be virtually unlimited. Utilizing this method, we demonstrate uniform 45-degree Faraday rotation and effective isolation across a microwave band.