Asymmetric Electromagnetic Wave Propagation In Spiral Based Chiral Metamaterials

Metamaterial does not exist in nature, but is constituted by artificial structural units. The electromagnetic properties of metamaterial depends more on the cell structure, rather than the material itself. The chiral metamaterial is one category of metamaterials. For ordinary metamaterial, the electromagnetic properties are designed by changing the equivalent permittivity, equivalent permeability and the scale of the geometric dimension of the structure. Unlike ordinary metamaterial, since the mirror symmetry of chiral metamaterial is broken either in the propagation direction or in the perpendicular plane, it leads to the interaction of electromagnetic wave radiation with the structural chirality in the metamaterial. Compared to ordinary metamaterial, chiral metamaterial offers more ability to regulate the electromagnetic properties of the metamaterial. The chrial metamaterial has attracted more and more attention in recent years. Thank to the novel properties of chiral metamaterial, it can find extensive applications in many areas like microwave absorber, antenna design, and other microwave devices. Therefore it is important to undertake extensively study of the chiral metamaterial.The presented thesis investigates the EM wave propagation characteristics of chiral metamaterial, as well as some relatively theoretical and experimental analysis, especially focuses on the asymmetric EM wave transmission within the chiral metamaterial. Based on the results of previous studies, this thesis has an in-depth exploration of the intrinsic electromagnetic properties of chiral metamaterial based on spiral metallic ring structure and give theoretical analysis for the physical mechanisms of the asymmetric transmission characteristics.This thesis mainly concludes the following parts:1. Design the chiral metamaterial composed of unit cell of a spiral metallic ring and a dielectric layer. We have analyzed that such chiral metamaterial slab enables a dual-band asymmetric transmission effect for different linearly polarized waves. Strong one-way transmission of two orthogonal polarized waves at different frequency bands have been confirmed through test on fabricated prototype sample at the microwave band. The mechanism that supports such asymmetric transmission has been analyzed which is caused by the induced asymmetric current distributions that enable strong polarization conversion and cross-polarization transmission. We also designed another chiral metamaterial structure, spiral chiral metamaterial without metalizing via hole, which also has a good single-band asymmetric transmission effect for particular linearly polarized wave.2. Based on the spiral chiral metamaterial with or without metalizing via hole, we put forward an idea to design a structure which can achieve dynamic switch of the asymmetric transmission effect for particular polarized wave within certain frequency band. We explored the design in detail and achieved some progress which laid the foundation for the next research step.3. Design the double spiral chiral metamaterial and square spiral chiral metamaterial. These two chiral metamaterial structures both have a very good single-band asymmetric transmission effect for one particular linearly polarized EM wave, and may have good application potential in EM wave controlling areas.