Beamforming Transmission Metasurface Design Using High-Efficiency Huygens’ Unit Cells
- Metamaterials are substances with subwavelength structures that are engineered to have specific electromagnetic properties. The metamaterials show significant control on the properties of constituent materials and thus introduce numerous physical phenomena. However, metamaterials deem as less attractive due to their excessive losses, bulk size, and fabrication difficulty. These limitations have led to flourishing two-dimensional metamaterials, which are known as metasurfaces. Metasurfaces can bring medium discontinuities to the wavefronts of electromagnetic waves. As a result, it enables the tailoring of electromagnetic fields across a subwavelength scale.
The building blocks of metasurfaces are polarizable particles, which support strong current near resonance and control polarization and wavefront. This surface can be made as either reflective or transmissive surfaces. In this thesis, concentration is given on high-efficiency transmission metasurface unit cells based on surface electric and magnetic impedances derived from Huygens’ Principle. However, unit cells for low transmission loss (< 1 dB) over a wide transmission phase range require at least three metallic layers, which complicates the unit cell design process. This work introduces high-efficiency Huygens’ metasurface unit cell topologies in double-layer FR4 printed circuit board (PCB) by implementing surface electric and magnetic current using the top and bottom metallic patterns and via drills. Eleven unit cells were optimized for wide phase coverage (−150° to 150°) with a low average transmission loss of −0.82 dB at 10 GHz. Then, the high-efficiency of the designed unit cells has been demonstrated by designing and fabricating two focusing lenses with dimensions of near 150 × 150 mm (5λ × 5λ) to focus a spherical beam radiated from short focal distances (f = 100 and 60 mm). The fabricated focusing lens showed 12.87 and 13.58 dB focusing gain for f = 100 and 60 mm at 10 GHz, respectively, with a 1 dB fractional gain bandwidth of near 13%.
Next, as an extension of the above work, reconfigurable/tunable unit cells based on the highly efficient unit structure have been constructed. In this extended work, recent development in the area of tunable metasurface designs has been highlighted. The surface reconfigurability in real-time has been realized by many tunable components (MEMS, Vanadium Dioxide, Graphene, Liquid Crystal, GaAs switch, and Schottky diode). However, the addition or realization of these components makes the structure bulky and thus hinders the construction of a compact device. As an alternative, we used PIN (positive-intrinsic-negative) and varactor diodes combined with unit cells structure to make the unit cell less bulky, fabrication friendly, and highly transmissive.
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