In this work, we demonstrate the implementation of a nonreciprocal perfect absorber (NPA) made of composite magnetic metamaterials (MMs) consisting of an array of dielectric core loaded (DCL) ferrite rods with either hollow or dielectric cores. The NPA can be functionalized as a PA for the incident beam at a specified direction, while at the symmetric direction the absorption is very weak so that a strong reflection is observed due to the excitation of nonreciprocal magnetic surface plasmon. Interestingly, it is shown that the material loss might be beneficial to the absorption, but it will result in the degradation of nonreciprocal performance. For the delicately designed MMs, only a very small material loss is necessary and simultaneously ensures the high nonreciprocal performance of NPA. To interpret the high quality of NPA, we developed a generalized effective-medium theory for the composite MMs, which shows the direct consequence of the DCL ferrite rods with optimized core size and core permittivity. The partial wave analysis indicates that the nonreciprocal dipole resonance in DCL ferrite rod plays a crucial role in improving the nonreciprocity. The narrow band feature and the angular sensitivity make the NPA promising for the diode-like functionalities. In addition, by controlling the magnitude and orientation of bias magnetic field both the operating frequency and the nonreciprocity can be flexibly controlled, adding an additional degree of freedom. The concept proposed in this research is promising for microwave photonics and integrated photonics.

We demonstrate reconfigurable unidirectional propagation of electromagnetic waves in waveguide channels sandwiched by two dielectric photonic crystal (PC) slabs or magnetic PC (MPC) slabs, where the extrinsic and intrinsic responses emerge in two kinds of systems. Concretely, the unidirectionality of the MPC system originates from the time-reversal symmetry breaking nature of magnetic material, while in the dielectric PC system, the unidirectionality is achieved by active control of two separated line sources. The results indicate that in the MPC based system, not only the amplitude but also the directionality of the guiding electromagnetic waves is flexibly regulated by controlling the separation and phase difference of two active line sources. However, the directionality in the MPC based system is determined by the magnetization due to the intrinsic unidirectionality of the magnetic system, but for the dielectric PC based system, directionality is controlled by active sources. The introduction of a second line source in the MPC based system can result in two remarkably different consequences. On one hand, the directionality can be switched from forward to backward propagation by reversing magnetization. On the other hand, the same operation results in the transition from unidirectional propagation to localization of the electromagnetic waves by dynamically controlling two active sources. The functionality and flexibility in the present systems might find potential applications in microwave photonics and integrated photonics.

We present a zero-index–based heterostructured magnetic metamaterial (HSMM) composed of two arrays of ferrite rods with different radii and lattice separations, which exhibits unidirectional propagation of electromagnetic (EM) waves, and the unidirectionality is reconfigurable dependent on the bias magnetic field (BMF). By calculating the photonic band diagrams and the effective constitutive parameters, it is shown that, for the MMs with two groups of lattice separations and ferrite rod radii, the effective refractive index is switched either from effective zero index (EZI) to effective positive index (EPI) by decreasing the BMF for one MM or from EZI to effective negative index (ENI) for the other MM by increasing the BMF. As a result, two kinds of HSMMs can be constructed with the combination of either EZI and ENI or EZI and EPI, both of which can be used to implement the unidirectional transport of EM waves and exhibit reconfigurable unidirectionality by either decreasing or increasing the BMF, thus providing us with more degrees of freedom. The concept put forward in the present work can be possibly extended to the heterostructured metamaterials made of phase-change materials and realize reconfigurable EM properties in optical frequency by tuning the temperature.

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In this work, a cylindrical symmetric gradient-index two-dimensional electromagnetic systems is constructed by using of the magnetic metamaterials consisting of an array of ferrite rods. With the change of the bias magnetic field the different gradient-index systems can be obtained, based on which a flexible beam steering is demonstrated. Based on the effective-medium theory, the effective electric permittivity and the effective magnetic permeability can be retrieved and thus the effective refractive index is obtained straightforwardly. It is shown that with the variation of the ferrite rod radius the effective index profile with particular gradient can be realized, which exhibits the electromagnetic "black hole" like effect. Especially, the gradient index profile is also designed by introducing the gradient bias magnetic field, which, in principle, results in the refractive index profile with many different gradients. Finally, the propagation of a Gaussian beam in the gradient-index systems is simulated with the use of multiple scattering theory. A few different phenomena are observed such as the "black hole" effect, the interior beam deflection, the exterior beam deflection, and the beam splitting. Furthermore, the functionalities can be switched between each other by controlling the bias magnetic field, adding an additional degree of freedom for beam steering.