Electromagnetic Field Shaping through Static and Active Metasurfaces at Radiofrequency: Theoretical Approaches and Applications

Low-frequency electromagnetic fields (EMF) manipulation through metasurfaces has emerged as a pivotal research area with significant implications for sensing technologies, wireless power transfer (WPT), and magnetic resonance imaging (MRI). This dissertation presents an extensive investigation about the design, theoretical principles, and experimental validation of magnetic metasurfaces, bidimensional structures specifically conceived to interact with and modulate the magnetic component of EMFs in the near-field domain. By tailoring the interaction between metasurfaces and electromagnetic waves, these advanced materials provide unparalleled control over field distributions, enabling innovative solutions across various technological domains. The study is structured around two principal types of metasurfaces: static and actively reconfigurable. More in detail, Chapter 1 provides a historical and technical overview, detailing the evolution of metasurfaces from three-dimensional metamaterials to compact two dimensional configurations. It highlights the emerging role of metasurfaces in various key applications, including sensing, energy transfer systems, and biomedical imaging. Chapter 2 examines static metasurfaces, characterized by fixed electromagnetic properties. The analysis of these structures employs two theoretical frameworks: the Biot-Savart law and elementary magnetic dipole theory. The implemented case studies explore their use in advanced sensing and ultra-focused WPT applications, respectively, demonstrating their ability to reshape near-field magnetic distributions and improve the overall system efficiency. Experimental validations confirm the robustness of these models. Chapter 3 shifts the attention to reconfigurable metasurfaces, integrating electronically adjustable components such as varactor diodes and digitally controlled capacitors to enable dynamic adaptability. These metasurfaces allow real-time manipulation of magnetic field distributions, making them a promising tool for advanced WPT and MRI applications where operational conditions vary. Part of the research detailed in this chapter was conducted during a 7-month research period as a visiting research scholar at the Fraunhofer MEVIS research center in Bremen, Germany (1 June – 22 December 2023). This part of the work specifically focused on developing reconfigurable metasurfaces tailored to enhance MRI systems, particularly in optimizing field uniformity and improving the signal-to-noise ratio (SNR) in dynamic imaging scenarios. Throughout the dissertation, a consistent emphasis is placed on bridging theoretical approaches and practical implementation. Numerical simulations guide the metasurfaces design, while experimental validations, conducted on prototypes fabricated through PCB technology, confirm their effectiveness in real-world scenarios. Throughout this thesis, the main objective consists in outlining a path that, starting from general notions about metasurfaces, focuses specifically on magnetic metasurfaces. Particular attention is given to the distinctive characteristics of static metasurfaces, which are well-suited to applications with stable operating conditions, and to reconfigurable metasurfaces, which are fundamental for dynamic and adaptable environments. While research on metasurfaces is well-advanced, several application areas still require further exploration. This thesis aims to serve as a foundational reference for the integration of magnetic metasurfaces into real-scenario devices, spanning from industrial to medical scenarios.