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Controlling the Polarization of Terahertz Waves by the Meta-Surface for the Imaging Applications

Hajitabar Marznaki, Shiva | 2020

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 52599 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Ahmadi Boroujeni, Mehdi
  7. Abstract:
  8. The purpose of this thesis is the analysis, design and fabrication of polarization converters in the millimeter-wave and terahertz regions using metasurfaces in order to generate different polarizations in the transmitter of terahertz stand-off imaging systems. In this project, the design and fabrication of two linear cross-polarizers and a circular polarization converter have been done at the frequency of 100 GHz. These polarizers are easy to fabricate by printed circuit board (PCB) technology and are capable of converting the polarization of a linear incident wave to the cross-polarized transmission wave and circular polarization, respectively. Linear polarizers are double-layer and composed of Asymmetric Split Ring Resonator (ASRR) as the one layer and S-shaped resonators or wire-grid as the other layer. By optimization of the geometrical parameters and full-wave simulation using the CST software, a polarization conversion ratio (PCR) of over 85% and a cross-transmission coefficient of more than 0.8 have been achieved over a wide frequency range. The co-transmission coefficient is also found to be extremely low in this frequency region. In addition, the performance of the proposed linear polarizers remains acceptable in a wide range of the incidence angles. The designed circular polarizer also has a transmission coefficient of more than 0.9 and low reflection coefficient in its operational bandwidth. These characteristics make it possible to generate arbitrary slant linear polarizations by cascading two of them. Finally, the proposed structures were fabricated on a low-loss substrate using lithography technology and were measured using a terahertz photomixing-based continuous-wave (CW) spectroscopy system in the frequency range of 50 GHz to 200 GHz. A good agreement between simulated and measurement results was observed which validates the procedure of design and fabrication. In the stand-off imaging systems, separating the targets from the background and recognizing the shape of the target are the main challenges that can be partially overcome by analyzing the polarization of scattered waves. Therefore, the results of this thesis can find application in such polarimetric imaging systems
  9. Keywords:
  10. Metasurfaces ; Terahertz Technology ; Stand-Off Imaging ; Terahertz Imaging ; Polarization Converter

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