The Research On Nonreciprocal Optical Proper-Ties Of Magnetic Metamaterials

It is well known that the electric permittivity and magnetic permeability of natural materials are restricted in an extremely narrow range of the electromagnetic (EM) parame-ter space, resulting in a serious bound in molding the propagation of EM wave. Metamate-rial, a kind of artificial composite material consisting of subwavelength building blocks, can possess effective constitutive EM parameters nearly in the whole parameter space by state-of-art design of the architecture with appropriate nature materials. It offers us an un-precedented manipulation of EM waves, giving rise to various exotic phenomena. In chap-ter one, we demonstrate the idea of metamaterial design and its significance. Meanwhile, we present a brief introduction on negative permittivity, negative permeability, and nega-tive index materials. Magnetic metamaterials (MMs), made of ferrite building blocks, can achieve particular EM properties such as tunability by external magnetic field and nonrec-iprocity due to time reversal symmetry (TRS) broken. The research presented in this dis-sertation is concentrated on the nonreciprocal EM properties of MMs. The framework of the dissertation is as follows:In chapter two, we provide the magnetic permeability of ferrite materials. It is a gyro-tropic second rank tensor under saturation magnetization, which is responsible for nonre-ciprocal EM properties. By employing Mie scattering theory, the Mie scattering coeffi-cients of a single ferrite rod can be derived, based on which the multiple scattering theory and effective-medium theory are also obtained. With these theoretical approach, we can calculate the effective permittivity, effective magnetic permeability, scattering coefficient, partial wave scattering amplitude, and band structure of a designed MM. Furthermore, we can simulate the transmission spectra and electric field pattern when an EM wave incidents on a finite MM structure.In chapter three, we demonstrate a design of a perfect nonreciprocal absorber which exhibits different absorptance for a light beam incident with two geometrically symmetric angles. At one direction, the absorptance nearly reach1, while at the other direction, a strong reflection occurs. This nonreciprocity can be used to design a perfect absorber with switching effect. Importantly, it is found that the working frequency of the perfect absorber is near the magnetic surface plasmon (MSP) resonance where the imaginary part of the ef-fective magnetic permeability is larger than2, leading to the high absorptance. In addition, this nonreciprocity originates from the TRS broken of the MMs, due to which an incident beam can be absorbed completely at a particular direction, while at the symmetrically op-posite direction an obvious reflected is observed. The working frequency can be controlled as well by tuning the external magnetic field, which will increase with the increasing mag-netic field. Finally, this absorber is immune to position and size disorders of the ferrite rods, suggesting the robustness of nonrecipropcity.In chapter four, we emphasize on the design of single-layer MMs and investigate the nonreciprocity of a particular single-layer MM structure. It is demonstrated that when a Gaussian beam is incident to the single-layer MM structure at a particular angle, the trans-mitted beam is refracted negatively, on the same side of the surface normal. While at oppo-site or symmetrically opposite direction, a nearly total reflected beam is observed. The oc-currence of nonreciprocal negative transmission and total reflection is due to the0-th and±2-th Mie resonances, which can be corroborated by calculating the Mie scattering coeffi-cients of a single rod and the scattering field patterns. Besides, the nonreciprocal property can be flexibly manipulated by reversing magnetization or tuning the magnitude of the ex-ternal magnetic field.Finally, we summarize the dissertation and present an outlook of the future research. We expect the experimental demonstration of the properties shown in present work and design optical waveguide devices with even better performance.