Arbitrary Field Transformations Using Practical Cylindrical Metasurfaces

Abstract

For the past 160 years, since James Clerk Maxwell first formulated his famous equations that brought electromagnetism to humankind, electromagnetism has played a central role in the development of new technologies. Establishing full control over electromagnetic waves has become the holy grail of electromagnetic and optical engineering research. Metasurfaces, which are engineered surfaces composed of intricately designed unit cells, possess the potential to achieve this goal. Previous works have demonstrated that by simply cascading several metasurfaces, even the most general form of constitutive relations, bianisotropy, can be realized. In other words, cascaded metasurfaces do not merely allow artificial media with stipulated material parameters but also provide a prescription for tailoring electromagnetic waves in unprecedented ways. Despite the large body of literature focused on cascaded planar metasurfaces, their cylindrical counterparts have not been as thoroughly researched. As cascaded cylindrical metasurfaces become needed in scenarios for aerodynamic, hydrodynamic or aesthetic reasons, a suitable theory as well as design principles for these structures come to be indispensable.

The main goal of this dissertation is to achieve arbitrary field transformations through cascaded cylindrical metasurfaces. In other words, the intent is to allow the design of devices based on cascaded cylindrical metasurfaces that can transform a known excitation field to a desired output field. The design of practically realizable, cascaded cylindrical metasurfaces has been hampered by several difficulties. In this dissertation, these issues are addressed one by one. First, coupling between metasurface layers and between unit cells was not accurately captured by earlier modeling methods. Here, a comprehensive multimodal wave matrix theory is derived in order to account for these effects. Secondly, idealized (fictitious) line current sources were often assumed as excitations for cylindrical metasurfaces, which neglects interactions between sources and metasurfaces. On the contrary, a realistic coaxial feed, which is located at the center of the cascaded cylindrical metasurfaces, has been considered in this dissertation. Its scattering properties have been characterized and it has been integrated into the design of cylindrical metasurfaces. Next, earlier research efforts were restricted to single-input, single-output devices. To design a more useful multiple-input, multiple-output device, a displaced feed has been theoretically characterized, and the work extended to include multiple feeds. Finally, the spatial dispersion of the patterned metallic claddings, typically used to realize metasurfaces, has not been properly accounted for in design. In this dissertation, spatial dispersion effects of the patterned claddings are studied and incorporated into the theoretical design. With all these advances, rigorous analysis and efficient synthesis of cylindrical-metasurface-based devices are made possible.

In order to demonstrate the ability to perform arbitrary field transformations, a realizable device based on the aforementioned advances is proposed. Concentrically-cascaded cylindrical metasurfaces are placed within a radial waveguide and fed by a central coaxial feed. The cascaded cylindrical metasurfaces consist of several patterned metallic claddings separated by dielectric spacers. As an example, a practically realizable azimuthal mode converter is presented and validated through full-wave simulation. The proposed device converts an azimuthally-symmetric coaxial excitation (0th azimuthal order) to a radiated field of 1st azimuthal order. This azimuthal mode converter can be used to generate orbital angular momentum to increase channel capacity in near field communications. The outlined design principles are widely applicable and can also be employed to engineer various cylindrical-metasurface-based devices such as beam-shaping shells, illusion devices and even multifunctional metasurfaces.