Metamaterials are engineered materials which consist of a periodic arrangement of subwavelength scatterers or unit cells, whose size is much smaller than wavelength λ of the incident electromagnetic (EM) wave. The effective EM properties of metamaterials depends on the design and arrangement of unit cells in contrast to material composition in atomic scale like that of conventional materials. Hence, they can be tailored to have specialized EM characteristics which are difficult or impossible to achieve with lenses made of conventional dielectric materials. A lens made of negative refractive index metamaterials can overcome the diffraction limited resolution of conventional lenses at far-field working distances. This dissertation focuses on the implementation of such metamaterial lenses for microwave nondestructive evaluation (NDE) applications. In the first part of the dissertation, negative index metamaterial (NIM) lens design consisting of split-ring resonators (SRR) and thin wires unit cells is studied and implemented at two frequencies of operation in the microwave S and C bands. A novel NIM lens imaging sensor system using homodyne detection measurements is proposed in this work. Coherent homodyne detection scheme provides a simple, low-cost, and highly sensitive NIM lens imaging system that can be used in the field under practical conditions. Using the proposed sensor system, subwavelength focusing by negative refraction is verified experimentally at both frequencies of operation. Subwavelength focal spot of sizes 0.82λ and 0.65λ are obtained with the 3.5 GHz (S band) and 6.3 GHz (C band) designs respectively. Imaging resolution enhancement by a factor of 2.24 is obtained at a distance of 1.67λ with the C band lens design. The sensor system was further used to perform microwave NDE experiments of subwavelength defects inside Teflon and glass fiber reinforced (GFRP) composite samples. Defects comprising subwavelength hole of diameter 0.25λ and a groove of dimensions 0.17λ x 0.06λ placed at the focal plane of the lens was imaged both in the transmission and reflection mode using the proposed sensor system. In the second part of the dissertation, an alternative approach using gradient index (GRIN) metasurface lens consisting of electric-LC (ELC) unit cells is studied. Metasurfaces are 2D counterparts of 3D metamaterials and provide an attractive alternative to metamaterials as they take less physical space and exhibit lower losses. Although GRIN lens operation is distinct from that of NIM lenses and does not rely on negative refraction, they offer various advantages including planar design, wideband operation, and no restrictions on source to focal plane distances. The design, simulation, and experimental validation of a GRIN metasurface lens operating at 8 GHz is reported in this dissertation. The proposed lens has an aperture of size 119 mm (3.2λ) x 119 mm (3.2λ) and thickness of only 0.6 mm (.016λ). The metasurface lens is designed and analyzed using full-wave finite element (FEM) solver. A prototype of the proposed GRIN metasurface lens was fabricated for experimental verification. A focal spot size of 1.1λ is achieved with the proposed GRIN lens with a resolution enhancement factor of 1.5 at a distance of 8.37λ. Microwave NDE imaging results of a defect of dimensions 0.4λ kept at the focal plane of the GRIN lens is also reported. The work is concluded by presenting a comparative discussion of the two approaches (NIM lens and GRIN lens) for high resolution microwave imaging along with remarks on the future direction of the work.
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