Design and experimental evaluation of cascaded metasurfaces

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Gohel, Jayesh
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Tailoring electromagnetic waves has various applications in wireless communications, medical imaging, and remote sensing. However, to be suitable for a given application, the spatial properties of electromagnetic waves need to be manipulated to fit that application. This is generally done by designing antennas based on the required performance criteria. Designing antennas for complicated performance requirements can be quite time-consuming as it may require extensive full-wave optimization. Alternatively, a metasurface can be used as it can systematically transform electromagnetic waves to what the designer desires. Electromagnetic metasurfaces are sheet-like structures consisting of artificial “atoms” called meta-atoms or unit cells, that are small compared to the operational wavelength. When electromagnetic waves interact with these unit cells, they are transformed based on the properties of the unit cells. The design of lossless and passive metasurfaces requires enforcing total power conservation (TPC) and local power conservation (LPC) to make sure that the power is conserved in the whole metasurface and at each individual unit cell. If a single metasurface cannot satisfy the required constraints, a metasurface pair, also known as cascaded metasurfaces, can be considered. In this thesis, a fully automated design approach is developed for cascaded metasurface system designs with external sources based on an existing design approach. The overall design is divided into macroscopic and microscopic steps. In the macroscopic step, an automated conjugate gradient (CG) optimization algorithm is developed with analytical expressions to arrive at the required surface properties of the cascaded metasurface system. In the microscopic step, the unit cells are physically realized by copper traces determined using a lookup table which was further optimized to achieve the desired forward scattering parameters for each unit cell. The electromagnetic wave transformation is then verified by modeling the cascaded metasurface system in Ansys HFSS software and performing the full-wave simulation. Finally, a cascaded metasurface system is fabricated and measured. This serves as one of the few fabricated cascaded metasurfaces reported in the literature. Planar near-field antenna range (PNFR) measurements are conducted to experimentally evaluate the performance of the cascaded metasurface system. The successful simulation and experimental verification confirm the working of the fully automated cascaded metasurface design algorithm.
Metasurfaces, Metamaterials, LPC, Antennas, Electromagnetics