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TITLE: MAGNETIC FIELD ENHANCEMENT IN METAMATERIALS: FROM THEORY TO APPLICATION.

ABSTRACT: Metamaterials, defined as artificially constructed materials composed of subwavelength meta-atoms, have emerged as a promising tool to manipulate electromagnetic (EM) waves due to their extraordinary responses to incident EM waves at deep subwavelength scales. Efforts in developing metamaterials have progressed from initial demonstrations of breaking the generalized limitations of refraction and reflection of natural materials, to their current use in facilitating a range of practical applications. One flourishing research direction is the application of metamaterial in electromagnetic field enhancement. The enhanced magnetic fields are attractive for a variety of applications such as wireless power transfer, near field communication, magnetic induction tomography, and magnetic resonance imaging (MRI), among others. While promising, the reported applications of metamaterials remain impractical and fail to realize the full potential of these unique materials. The common thread across our work is incorporating conventional and novel methods to create functional magnetic metamaterials for field enhancement, while considering the practical application scenarios. In the case of radiofrequency identification (RFID), a magnetic metamaterial composed of an array of unit cells featuring metallic wires or helices, distributed in a hexagonal configuration, is proposed to enhance magnetic field strength. The insertion of a metamaterial between the transmitter and receiver antennas serves to amplify the magnetic flux density and, thus, increase the coupling coefficient, ultimately improving the wireless power transfer efficiency. In the second part of our work, we integrate the magnetic metamaterial with auxetic structures and design a tunable metamaterial, which enables a marked boost in RF field strength, ultimately yielding a dramatic increase the signal to noise ratio (SNR) in its application to MRI. The tunability resulting from these geometric structures, without the introduction of additional control units or circuits, makes magnetic metamaterials more feasible in clinical medical imaging scenarios. Furthermore, benefiting from the design flexibility of auxetics, arbitrarily shaped 3D tunable metamaterials are designed based on auxetic structures that may conform against the curved surfaces of human body parts such as the brain, breast, or musculoskeletal system (knee, ankle, etc.). In the future, in order to further increase the communication distance in wireless systems or increase penetration depth in MRI, I plan to further explore gradient metamaterials, which are characterized by a continuous spatial variation of their properties. Gradient metamaterials provide a promising approach to manipulate the phase/amplitude of the incident wave and further improve device functionality.

COMMITTEE: ADVISOR/CHAIR Professor Xin Zhang, ME/ECE/BME/MSE; Professor Chuanhua Duan, ME/MSE; Professor Richard Averitt Physics, University of California San Diego; Professor Lei Tian, ECE/BME; Dr. Stephan Anderson, BUSM/ME.