The Theory And Design Of Metamaterial Absorbers

Electromagnetic absorbing materials have attracted continual attentions all over the world due to their potential applications in the fields of military and civil aspects. However, with development of material synthesis and microstructure characterization technologies, further improvement of absorption properties for traditional absorbing materials is limited. The emergence and fast development of metamaterials, that is, artificial engineered materials with exotic properties not found among natural materials, opens a door for developing new types of electromagnetic absorbers. In this dissertation, we focus on the analytical modeling, design principle and bandwidth broadening of metamaterials in the application of electromagnetic absorbing materials. Many derived conclusions have been verified by numerical and experimental methods. The main contributions of this dissertation are described as follow.1. Using the transmission line equivalent circuit model, we have discussed the influence of the surface resistance on absorption properties of high impedance surface absorbers. It is found that an upper value of the surface resistance exists for a given reflection threshold. This can serve as a guideline for determining the bound of optimized parameters in numerical investigation. In addition, an upper bound for the bandwidth of high impedance surface absorbers has been also derived. It is shown that the maximum bandwidth is linearly determined by the thickness and permeability of grounded substrates, and the frequency dispersive of substrates is the key factor that determines if the maximum achievable bandwidth can be close to the limit. Finally, inspired by circuit analog absorbers with broadband absorption, we have presented a broadband high impedance surface absorber based on a metamaterial substrate, which is designed using metallic split ring resonators vertically embedded into an ultrathin dielectric substrate. Both simulated and experimental results display an expanded absorption bandwidth of less than-10 d B compared to the conventional absorbers.2. On the basis of the cavity-like resonance, an equivalent circuit model has been developed firstly for wire-based metamaterial absorbers to reveal the relationship between constitutive parameters and absorption properties. The analytical and numerical results show that the absorption frequency is determined by the wire length and the spacer electromagnetic parameters, whereas the absorption level by the spacer thickness and losses which include Ohmic loss of the metal as well as dielectric loss of the spacer. It is also well explained why small losses have hardly any effect on the absorption frequency, but can result in the near-unity absorption. Based on the strategy for designing broadband absorbers derived from the circuit model, the three dimensional metamaterial structure, that is, corrugated metamaterials, has been presented to achieve the broadband absorber. Results show that incident wave with a certain frequency can be strongly absorbed by the corrugation region where the height is about λ/4, with λ being the corresponding wavelength in the dielectric. Assembling various heights of corrugations together to form a graded corrugation structure can excite many distinct absorption modes and their overlapping with each other results in a broadband absorption. It should be emphasized that our proposed broadband metamaterial structures are scalable and can be applied at millimeter wave, terahertz frequencies and infrared band as well.3. Based on the feasibility that carbon fibers could be employed to construct metamaterials for electromagnetic wave absorption instead of metal, we have fabricated several absorbers which are respectively consisting of unidirectional, cross-shaped, multi-layered and three-dimensional metamaterial structures. Both numerical and experimental results show that the absorbers allow a choice between the strong absorption of all polarizations or only one linear polarization, of single band or multi-band, and of narrow or broad band by tuning the topology and geometry of carbon fibers in the metamaterials. Our study opens a possibility of the design-fabrication-testing application of carbon fibers in absorbing materials, avoiding the traditional rule-of-thumb approach.