The Design Of Electromagnetic Metamaterials And Its Absorption Characteristics

Electromagnetic metamaterial (EM MM), a new theory for designing material, possesses exotic physical properties not available in naturally occurring materials and thus has become a hot issue in international scientific fields during the past ten years. With the rapid development of the MM, its absorption characteristics also have been received more and more attentions. Compared with the conventional absorber, the metamaterial absorber (MA) has several unparalleled advantages, such as high absorptivity, thin thickness, light weight, tunable frequency, desired EM parameters and many potential applications in undamaged detections, THz imaging and stealth technology. In this thesis, based on the absorption mechanism and research methods of the EM MM, we construct multi-band MAs in the microwave regime using Jerusalem crosses (JCs) and rotated square rings (RSRs), and then introduce the multi-reflection interference theory to interpret their absorption mechanism. Furthermore, we adopt the diagonal and nested arrangements to design broadband MAs in the terahertz region, and investigate numerically their absorption characteristics at wide angles of incidence for both transverse electric (TE) and transverse magnetic (TM) waves.In the first part of this thesis, we present the design, simulation and measurement of multi-band microwave MAs, and then introduce the multi-reflection interference theory to analyze the absorption mechanism. Firstly, we present the influence of the gap on the absorption performance of the conventional split ring resonator (SRR) absorber, which reveals that the geometry of the square SRR can be equivalent to a JC resonator and its corresponding MA is changed to a JC absorber. By simply assembling several JCs with slightly different geometric parameters next to each other into a unit cell, a perfect multi-band absorption can be effectively obtained. The multi-reflection interference theory is introduced to calculate the reflection and transmission coefficients at the JC interface, and the calculated absorption rates using the interference model agree well with the simulated and experimental results at the normal incidence.Secondly, we constrct a multi-band microwave MA based on a nested arrangement of three RSRs, it exhibits a polarizaiton-insensitive and wide-angle triple-band absorption. Therefore, this arrangement can reduce the effect of the polarization direction and incident angle of the external electromagnetic wave on the metamaterial’s absorption characteristics. And then the power-loss desity distributions at the resonance frequencies are illustrated to explain its absorption mechanism. Finally, the calculated absorption rates for the strongly coupled MA are obtained by finally optimized multi-reflection interference model and improved unit cell at the RSR interface. It is concluded that these calculated absorption rates concide well with the simulated results at wide angels of incidence for both TE and TM waves.In the second part of this thesis, the design and characterization of broadband terahertz MAs and their absorption characteristics at wide angels of incidence for both TE and TM waves are presented. Firstly, we design a single-band terahertz MA using metallic curcular patch and introduce surface current and z-component electric field distributions on the two metallic layers at the resonance frequency to investigate its absorption mechanism. By simply assembling several circular patches with slightly different geometric parameters in a diagonal arrangement into a unit cell, we propose a broadband and ultra-thin terahertz absorber. These simulated results indicate that as the metallic circular patch adds, the absorption bandwidth of the metamaterial also increases but its total absorption performance decreases.Secondly, we design a coplanar broadband terahertz MA consisiting of a periodic array of two nested square rings. And the surface current distributions on the two metallic layers at the resonance frequency are monitored to interpret its absorption mechanism. The simulated absorption rates at wide angels of incidence for both TE and TM waves are given. At last, we discuss the loss contributions of each part of the broadband terahertz MA.