Research On The Tunable THz Polarization Converter Based On Dirac Semimetals Metasurface

Terahertz waves generally refer to electromagnetic waves with a frequency in the range of 0.1-10 THz,between microwave and infrared.The special position of the THz spectrum determines its broad application prospects in many applications such as communication technology,medical imaging,security monitoring,spectroscopy,and biosensing.To develop terahertz technology,it is necessary to develop high-quality terahertz sources,detectors and terahertz functional devices.Terahertz polarization devices have important applications in THz switching,wave plates,polarization conversion,optical rotators,asymmetric transmission,wavefront control and other fields.Three-dimensional(3D)Dirac semimetals,also known as"3D graphene",have ultra-high carrier mobility and strong photoelectric properties,and have become an important preparation material for terahertz functional devices.This thesis focuses on the terahertz polarization conversion device based on Dirac scmimetals.The main research contents of this thesis are as follows:1.A dynamically tunable broadband linear-to-circular cross polarization converter based on Dirac semimetals was proposed.The proposed converter unit cell consists of a center-cut cross-shaped metallic patterned structure with a sandwiched Dirac semimetal ribbon on a polyimide substrate bottomed with gold.The proposed system converts linear waves to right hand circular polarized waves in the frequency ranges of 1.5-2.8 THz or to left-hand circular polarized waves in two narrow frequency ranges of 1.20-1.25 and 3.04-3.07 THz.The polarization conversion is dynamically tunable by varying the Fermi energy without re-optimizing the microstructures,which may further tunable polarizers and polarization switchers developments.2.A single-layer metasurface based on Dirac semimetals was investigated,which features tunable,broadband terahertz cross-polarization conversion in transmission mode.This effect can be actively tuned by changing the Fermi energy of the Dirac semimetals with no change to the geometry of the resonators.Tunability of the broadband polarization conversion is observed in the 3.82-7.88 THz range,over which the polarization conversion rate mostly remains above 80%.The proposed design offers a potentially effective approach to developing terahertz polarization control devices for terahertz imaging,sensing,and communications.3.A dynamic metasurface based on Dirac semimetals was presented and capable of realizing broadband and tunable asymmetric transmission in the terahertz region.The Dirac semimetal resonators have a chiral structure patterned with double-T resonators that results in partial polarization conversion of waves incident upon the material,leading to asymmetric transmission across a wide frequency range 1.2-1.6THz.We show how the gradual shift of the Fermi energy of Dirac semimetal permits a method of control over the asymmetric transmission.4.A three-layer complementary strip Dirac semimetal polarizer is proposed to achieve polarization rotation,perfect polarization conversion,and asymmetric transmission.This device can realize broadband polarization operation of linear polarized waves in the terahertz frequency domain.By changing the Fermi level of the Dirac semimetal,the polarization angle of linearly polarized light in the frequency range of 1.3 to 1.63 THz can be rotated from 0° to 90°,and the device has good robustness for linearly polarized light with different incident angles.When used as an asymmetric transmission device,its polarization conversion efficiency reaches more than 98%and the asymmetric transmission parameter reaches more than 50%in the range of 1.3-1.63 THz.