Experimental Study On Electromagnetic Metamaterial Cloaks

Electromagnetic invisibility cloaks, or the capability without being discovered by human's eyes or detectors, have been seeking for a long history. In the past ten years, invisibility cloaking design strategies (e.g. transformation optics, conformal mapping method and scattering cancellation method) and metamaterials developed rapidly, which boost the development of electromagnetic invisibility cloaks. However, achieving invisibility cloaks with a single design strategy meets bottlenecks (e.g. dispersive material, loss and superluminal propagation) in practice. Based on advantages of previous electromagnetic invisibility cloak design methods, this dissertation presents a novel inverse design methodology of invisibility cloaks, and shows the robustness of the inverse design methodology with theoretical analysis and experimental verification. The dissertation extends transformation optics to the surface wave and presents a broadband solution of surface wave propagating smoothly across obstacles without any loss experimentally. The dissertation explores the pertinent applications of invisibility cloaks and other novel electromagnetic devices based on metamaterials. The contents can be cataloged as follows:1. The concept of inverse invisibility cloaking design methodology is proposed with combining the advantages of transformation optics, conformal mapping method and scattering cancellation method. In the inverse design methodology which is a target-orienting method based on Mie scattering theory, electromagnetic parameters can be obtained with the help of genetic algorithm and the cloaking materials are constructed by broadband non-resonant metamaterials. The experimental demonstration of a subwavelength cylindrical invisibility cloak and a one-wavelength-large cylindrical invisibility cloak proved the validity of this inverse design method. The inverse design methodology can be extended to multilayered spherical invisibility cloaks with anisotropy.2. A 2D cylindrical invisible antenna is experimentally demonstrated with inverse design method. Combining the invisibility cloak and microwave antenna, the invisible antenna can decrease the antenna's scattering greatly in all the azimuthal angles under transverse electric (TE) polarization incidence; meanwhile the antenna can still receive outer information without being discovered.3. An electromagnetic carpet cloak design is proposed based on strict transformation optics. Since the metamaterials, which construct the carpet cloak, have the stable property of magnetic anisotropy over a broad frequency range, the carpet cloak can achieve broadband invisibility under TE polarization incidence.4. A broadband solution of surface wave propagating smoothly across obstacles without any loss is presented based on transformation optics. With the adopt of spoof SPP structure and metamaterials, corner cloaks and a carpet cloak for surface waves are demonstrated experimentally. This technology can improve the working efficiency of photonic and plasmonic devices and therefore has potential application value in practice.5. A deep subwavelength imaging strategy is proposed based on singular media. By backward thinking of metamaterial cloaking strategy, we squeez the whole space to thin rod, the k-surface of transformed space changes to two paralleled lines. Both numerical simulation and microwave measurement show the solid effectiveness of deep subwavelength imaging.6. Kinds of THz metamaterials are introduced, which might be used for THz cloaking.