Dissertation Defence: Microwave Resonant Sensors for Characterizing Electrical Properties at Microwave and Optical Frequencies

Ali Maleki Gargari, supervised by Dr. Loic Markley, will defend their dissertation titled “Microwave Resonant Sensors for Characterizing Electrical Properties at Microwave and Optical Frequencies” in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Electrical Engineering.

An abstract for Ali Maleki Gargari dissertation is included below.

Examinations are open to all members of the campus community as well as the general public. Registration is not required for in person defences.

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ABSTRACT

Materials exhibit diverse electromagnetic responses across a broad spectrum, characterized by parameters such as permittivity, permeability, and refractive index. This thesis investigates microwave resonant structures as high-sensitivity sensors for material property detection. Traditional cabled connections constrain accessibility and introduce noise to microwave structures, driving the development of wireless systems. Furthermore, integrating these structures with light-sensitive materials extends their sensitivity to optical properties.To address cabled limitations, two periodic resonant structures were designed for antenna-based passive material detection. The first design features a passive multi-layered structure based on a mushroom high-impedance surface, enhancing sensitivity and enabling a low-profile antenna-based material detection system. This system wirelessly tracks resonance frequency shifts caused by changes in the effective permittivity of the middle sensing layer. Notably, it achieved a high sensitivity of 150 MHz per unit relative permittivity at a monitoring distance of 430 mm. Subsequently, a new sensor design was proposed, increasing the sensitivity up to 1.46 GHz per unit relative permittivity while maintaining a minimal MUT profile (0.033λ0).Moreover, this thesis introduces integrating low-cost light-dependent resistors (LDR) into microwave resonant structures, enabling terahertz property tracking within the microwave frequency range. A red-light-responsive double-gap split ring resonator (DGSRR) exhibited sensitivity to visible light at microwave frequencies. Illuminating the structure with red light (intensity of 27.46 mW/m2) induced an 85 MHz frequency downshift and a 13.9 dB amplitude reduction. In an experiment involving 3600 uL water and 20 uL commercial green food coloring, the LDR-integrated structure demonstrated the potential to distinguish liquid colors at microwave frequencies. Additionally, a passive photosensitive microwave structure was developed to simultaneously monitor light intensity variation in three wavelength regions (green, red, and blue) for visible light sensing. Finally, an LDR-integrated periodic resonant structure was designed for non-contact material detection beyond the resonator’s near-field. Placing a thin layer of dust 5 cm above the resonant surface, the resonant profile was remotely monitored by a horn antenna situated 25 cm above the dust layer. Differences in the measured resonant frequency and amplitude of the structure in the presence and absence of visible light showcased the potential of LDR-integrated microwave resonators for non-contact material detection within wireless microwave sensing systems.