Planar Metasurfaces for Leaky-Wave Antennas and Anomalous Scattering

This PhD thesis explores the application of planar metasurfaces (MTSs) in the design of advanced electromagnetic devices, focusing on leaky-wave antennas (LWAs), Bessel beam (BB) launchers, and anomalous refractors. The research is conducted under the supervision of Prof. Enrica Martini, with co-supervision from Prof. Matteo Albani and Prof. Stefano Maci at the University of Siena, and includes a seven-month visiting period at KTH Royal Institute of Technology under Prof. Oscar Quevedo-Teruel. The thesis is structured into five main parts. The first part introduces the concept of MTSs, both modulated and non-modulated, and their application in controlling electromagnetic waves. The Floquet theorem is introduced for the analysis of periodic structures, and homogenized impedance models are developed to describe MTSs at both microscale and macroscale. A significant contribution is the discovery that sinusoidally modulated anisotropic MTSs can completely suppress the open-stopband (OSB) effect in circularly polarized LWAs, enabling continuous beam scanning from backward to forward directions without frequency blindness. The second part investigates the use of OSB suppression in near-field applications, particularly in the generation of ultra-long nondiffractive range Bessel beams. By mitigating the OSB, the nondiffractive range of a BB generated by a leaky-wave launcher is significantly extended, achieving a range of approximately 25 meters at 30 GHz, which is 50 times the aperture diameter and 2500 vacuum wavelengths. This advancement is particularly relevant for applications in near-field communications and wireless power transfer. The third part focuses on the design of anomalous refractors using modulated MTSs. Three different design approaches—Impedance-Equalized Huygens' (IEH), Omega-Type Bianisotropic Metasurfaces (O-BMS), and Floquet-Mode Optimization (FM-OPT)—are compared. The FM-OPT approach, based on the optimization of surface field harmonics, is shown to outperform the others in terms of performance and accuracy, especially for larger refraction angles and thicker substrates. The fourth part focuses on the Multimodal Transfer-Matrix Method (MMTMM) for the Bloch analysis of periodic structures. Here it is proposed to linearize the nonlinear eigenvalue problem associated with 2-D and 3-D periodic structures, allowing for efficient computation of complex modes, stopband rejection, and leakage. The method is validated through the analysis of various structures, including 1-D mushroom-type stopband structures, 2-D LWAs and meandered microstrip lines, and 3-D double-wire-mesh metamaterials. In conclusion, this thesis demonstrates the potential of planar metasurfaces in advancing the design of electromagnetic devices, offering new solutions for beam steering, near-field focusing, and wavefront control. The developed theoretical models and numerical methods provide a foundation for future research and practical applications in the field of metamaterials and metasurfaces.