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Design and Characterization of Liquid Crystal–Based Tunable Metasurfaces for Microwave Holographic Antenna Applications

Pezhman, Niloofar | 2025

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 58680 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Ahmadi Boroujeni, Mehdi
  7. Abstract:
  8. Liquid crystals are an integral part of modern technologies. Although they have a long history of application, their use in microwave and millimeter-wave regimes is relatively recent and has attracted increasing research interest in recent decades. The effective utilization of liquid crystals in microwave engineering requires an accurate understanding of their electromagnetic response, particularly their anisotropic behavior under applied electric fields. In this thesis, a novel method is proposed for retrieving the electromagnetic parameters of nematic liquid crystals, with a primary focus on the dielectric permittivity tensor. Since nematic liquid crystals are anisotropic, non-magnetic dielectric materials, their relative permeability can be assumed to be approximately unity, allowing the characterization process to concentrate on permittivity estimation. The proposed approach is based on a narrowband resonant microstrip sensor featuring a dual-band, dual-mode configuration implemented using coupled-line resonators. These resonators support two orthogonal resonant modes, namely even and odd modes, whose distinct electric-field distributions enable independent sensitivity to different tensor components of the dielectric permittivity. Unlike conventional techniques that rely on external DC biasing to reorient liquid crystal molecules, the proposed method exploits polarization rotation of the electric field, enabling simultaneous extraction of the tensorial permittivity components without the need for an external bias voltage. To accurately analyze the resonant behavior, both active and passive scattering parameters are defined, and analytical relationships for their transformation and for the separation of even and odd modes are derived. By correlating the resonance frequency shifts of these modes with the dielectric properties of the material under test, a high-accuracy linear sensor model is developed. The results demonstrate that the proposed sensor enables reliable retrieval of the dielectric permittivity tensor of nematic liquid crystals with a reduced number of measurements and high precision. Furthermore, the presented methodology establishes a robust platform for anisotropic material characterization and provides a promising foundation for future applications in tunable metasurfaces and microwave holographic antennas
  9. Keywords:
  10. Nematic Liquid Crystal Droplet ; Microwave Resonant Sensor ; Anisotropic Dielectric Characterization ; Passive Scattering Parameters ; Active Scattering Parameters ; Dual-Mode Coupled-Line Resonator

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