Optimum wavefront control using metasurfaces through surface electric and magnetic impedance adjustment

Abstract

The ability to manipulate and transmit electromagnetic (EM) waves with high efficiency is crucial for a wide range of applications, such as wireless communication, imaging, and sensing. Metasurfaces, two-dimensional arrangements of sub-wavelength elements, have emerged as a promising platform for achieving such control. When metasurfaces interact with a source of EM waves, they induce surface currents that act as secondary sources. These surface currents can be analyzed using multipole expansion, wherein they are expressed as combinations of multipole moments. Hence, fields emitted from metasurfaces can be calculated by superimposing contributions of these components, which are often dominated by lowest- order moments, i.e., electric and magnetic dipoles, especially in far zones. Accordingly, manipulating the properties of the constitutive moments offers a powerful solution to control EM waves. Therefore, in the context of this dissertation, a range of methods are presented to design and analyze metasurfaces from the perspective of moments. These methods aim to manipulate EM waves and enhance transmission while minimizing design complexity. The most basic moment-based metasurface introduced in this dissertation aims to enhance the radiation properties of an antenna. This is achieved by converting surface waves into radiating waves through the conversion of electric dipole moments. By arranging straight metallic posts perpendicular to the ground, TM surface waves are absorbed to excite electric dipoles on the posts. A thin strip parallel to the ground is connected to each post at one end to form an inverted-L shape, which converts the current flow to the horizontal direction. This interaction results in the generation of an electric dipole having the same radiation properties as the antenna. As a result, surface wave reflection and transmission are effectively reduced, leading to significant improvements in isolation and bore-sight gain, as confirmed by simulation and experimental results. While electric dipoles emit EM waves omnidirectionally, the integration of orthogonal electric and magnetic dipoles results in unidirectional radiation. This characteristic prevents backscattering and enhances transmission efficiency. Therefore, by utilizing the straightforward implementation of dipole moments, the second design approach utilizes orthogonal components of Huygens’ dipole, namely electric and magnetic dipoles, to alter the propagating direction of an EM plane wave with high refraction efficiency and low design complexity. This is achieved by designing the unit cell with printable wire and loop structures that exclusively induce electric and magnetic dipoles, respectively. The presence of a dielectric substrate separating the two structures allows independent control of moment components by adjusting only the wire length and loop radius. Although achieving multiband operation is challenging, this approach offers a broad bandwidth, thanks to its natural implementation of the Huygens’ source. Experimental results of the proposed wire-loop metasurface demonstrate an improvement in the refraction efficiency at wide angles of the refracted wave. To facilitate the control of propagation direction at multiband, a new implementation method is introduced. The third moment-based metasurface presented in this dissertation is designed to spatially separate superimposed beams of the first three odd-order harmonic waves generated from a nonlinear metasurface, allowing for their simultaneous use. To control multi-frequency band responses and simplify the design, the proposed metasurface uses a cascaded structure of three impedance sheets, which combine to provide equivalent electric and magnetic effects. Simple I-shaped patterns of three different sizes are imposed on each layer, playing a key role in adjusting the sheet impedance at each frequency with reduced overall design complexity. Despite the existence of unexpected inter-layer coupling, this approach shows great design flexibility in manipulating multiple frequency waves since the combined effects of electric and magnetic dipoles are treated as scalar circuit impedances. In the attempt to scale down the aforementioned metasurface for operation in sub-THz bands, it is noted that the impact of material losses becomes more pronounced and cannot be neglected at these frequency ranges. Therefore, in addition to the above approaches, the final section of this dissertation introduces a method for achieving electromagnetic transparency of lossy media using parity-time (PT) symmetry-based metamaterial. The proposed approach introduces a ‘gain’ layer that balances the material losses, enabling the system to satisfy the necessary conditions for achieving electromagnetic transparency through PT symmetry. As ‘gain’ media is not naturally occurring, equivalent structures are developed to reveal its properties. Despite not relying on multipole moments in its design, this approach holds substantial potential for enhancing signal propagation in complex environments, making it promising for future wireless communication applications.