PhD Oral Exam - Amirashkan Darvish, Electrical and Computer Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Nothing comes without costs. Electromagnetic interference (EMI) has been one of the first costs humans started identifying after initial achievements on how electromagnetic waves interact with different substances. Shielding against EMI emissions became one of the prominent challenges for the Electromagnetic Compatibility (EMC) community. The extent of the benefits of EMI shielding ranges from healthcare applications to commercial/industrial ones. While the term, EMI shield, conventionally refers to a metallic enclosure that completely/partially covers susceptible products, there are many different materials and structures that are being used for similar purposes. Polymers, graphene, composites, silicon, artificial porous materials, frequency-selective surfaces, and active components could be incorporated into creating electromagnetic shields. Since the two latter cases provide spectrum selectivity, designers are more intrigued by their potential.

This study aims to solve two major shielding problems in the context of passive and active methods. The first problem is the nearfield shielding characterization of FSSs, where the near-field performance of FSS structures is analyzed and differentiated from far-field performance. After introducing sufficient numerical and analytical models and techniques to characterize shields' near-field responses, we will study to reveal the fundamental failure of conventional FSS-based shields in providing NF shielding. Then, we will introduce a new flexible bandstop FSS that provides stable near-field (NF) characteristics in the X-band. Since NF shielding performance is also a function of incident angle, edge diffraction, bending effects, and wave polarization, another analysis is carried out to find ways to minimize these effects. Finally, the measurement results justify the reliable NF shielding performance of the fabricated prototype.

This dissertation further investigates the active methods and techniques that enable the user to harness the incident signal while providing the required shielding. This part of the study involves an investigation on reconfigurable intelligent surfaces for shielding or amplification, where space-time-modulated (STM) medium is studied as a potential solution for green shielding purposes. Since the presence of losses helps shields absorb waves, we will develop the methods to analyze the dispersion, attenuation, amplification, and the area of solutions of electromagnetic waves in a lossy progressively disturbed medium. Then, the green shielding possibilities are investigated through four different mechanisms. It turns out that STM media can be influential for green shielding purposes, encouraging future attempts to realize the shields. The research further explores other types of shielding by finding useful links to cloaking techniques and reducing radar cross-section (RCS) using STM media.