Analytical Modeling of Waveguide-fed Metasurfaces for Microwave Imaging and Beamforming

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2018

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Abstract

A waveguide-fed metasurface consists of an array of metamaterial elements excited by a guided mode. When the metamaterial elements are excited, they in turn leak out a portion of the energy traveling through the waveguide to free space. As such, a waveguide-fed metasurface acts as an antenna. These antennas possess a planar form factor that offers tremendous dexterity in forming prescribed radiation patterns; a capability that has led to revolutionary advances in antenna engineering, microwave imaging, flat optics, among others.

Yet, the common approach to model and design such metasurfaces relies on effective surface properties, a methodology that is inspired by initial metamaterial designs. This methodology is only applicable to periodic arrangements of elements, and the assumption that the neighboring elements are identical. In the scenarios where the metasurface consists of an aperiodic array, or the neighboring elements are significantly different, or the coupling to the waveguide structure changes; the aforementioned approaches cannot predict the electromagnetic response of the waveguide-fed metasurface. In this thesis, I have implemented a robust technique to model waveguide-fed metasurfaces without any assumption on the metamaterial elements' geometry or arrangement. The only assumption is that the metamaterial elements can be modeled as effective dipoles, which is usually the case given the subwavelength size of metamaterial elements.

Throughout this document, the simulation tool will be referred to Dipole Model. In this framework, the total response of each dipole, representing a metamaterial element, depends on the mutual interaction between elements, as well as the perturbation of the guided mode. Both effects are taken into account and, by using full-wave simulations, I have confirmed the validity of the model and the ability to predict radiation patterns that can be used for beamforming as well as for microwave imaging.

Once the capabilities of the dipole model are compared with full wave simulations of both traditional antenna designs as well as more elaborated waveguide-fed metasurfaces, I develop an analysis on the use of these metasurfaces for microwave imaging systems. These systems are used to form images of buried objects, which is crucial in security screening and synthetic aperture radar (SAR). Traditionally, the hardware needed for many imaging techniques is cumbersome, including large arrays of antennas or bulky, moving parts. However, one attractive alternative to overcome these problems is to use dynamic metasurface antennas. By quickly varying the radiation patterns generated by these antennas, enough diverse measurements can be made in order to produce high quality images in a fraction of the time.

The compact size and speed come with a trade-off: a computationally intensive optical inverse problem has to be solved, which has so far prohibited these antennas from enjoying widespread use. I address this problem by reformulating the problem to make it similar to a SAR scenario, for which fast image reconstruction algorithms already exist. By adapting an algorithm known as the Range Migration to be compatible with these metasurfaces, I can cut down on real-time computation significantly. The computer simulations performed are highly promising for the field of microwave imaging, since it is demonstrated that diffraction-limited images can be acquired in a fraction of the time, in comparison with other imaging techniques.

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Pulido Mancera, Laura Maria (2018). Analytical Modeling of Waveguide-fed Metasurfaces for Microwave Imaging and Beamforming. Dissertation, Duke University. Retrieved from https://hdl.handle.net/10161/16941.

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