Polarization And Absorption Functional Devices Of Terahertz Metamaterials

Terahertz technology has superior properties such as low photon energy,high transmittance and broadband,with wide application prospects towards security detection,imaging,medical diagnosis and wireless communication.These applications are are closely associated with high-performance terahertz devices for its effective modulation of terahertz waves.However,with the weak electromagnetic response to terahertz waves by using converntial natural materials,high-performance terahertz devices are expensive to produce and few in number,causing the dilemma of not being able to meet the ever-increasing demand of terahertz systems.Metamaterials and metasurfaces provide an effective avenue to address the lack of terahertz functional components due to their flexible design,compact structure,and powerful optical field control capabilities.Metamaterials can effectively modulate multiple dimensions of terahertz waves,such as polarization,amplitude and phase and so on.In particular,polarization and amplitude manipulation is extremely important,because polarization and absorption devices are key devices in many terahertz application systems.At present stage,how to achieve multiband and broadband polarization and absorption modulation is a key problem that needs to be solved urgently in the research field of high-performance terahertz devices.This thesis studies a variety of terahertz functional devices with excellent performance focusing on this issue,the main contents are as follows:（1）Based on the design idea of combining multilayer wire grid into single-layered structure,a continuous single-layered"wave-shaped"metamaterial is proposed,which can realize broadband and wide angle quarter-wave plate.Employing the same design idea,we have proposed a single-layered"stepped"structure by embedding the phase change material vanadium dioxide（VO₂）into the design of the structure,which implements a temperature-controlled switchable single-layered wave plate device.At room temperature,it can be regarded as a broadband transmissive quarter-wave plate.At high temperature,it can realize the function of reflective half-wave plate.The single-layered structure of broadband waveplate devices is of great significance for the development of miniaturization and integration of terahertz systems.（2）We have proposed a novel design of double-layer chained chiral metamaterials based on the design principle of breaking the symmetry of the structure in the transmission direction.The terahertz component may realize dual-polarization,high-efficiency and broadband asymmetric transmission effect.The simulation,theoretical and experimental measurements are in good agreement.The high-performance asymmetric transmission effect maintains good stability up to 60°oblique incidence angle.The enhanced asymmetric transmission properties are quantificationally explained by a multiple reflection and transmission interference model and the transfer matrix method（TMM）.High-performance terahertz asymmetric transmission devices have important applications in polarization manipulation and communication.（3）We have proposed a bifunctional hybrid metamaterial based on the phase transition.The metamaterial is composed of bilayer twisted rectangular aluminum rods and a complementary VO₂ layer.The hybrid metamaterial is equivalent to an array of 45°-twisted bilayer rectangular rods when the VO₂ film is in the complete insulating state,and it can realize mutual dual-band AT of linearly polarized wave.When the VO₂ film is metallic state,the hybridmetamaterial becomes a typical sandwiched structure that contains rectangular rod and a continuous metallic plate separated by a cyclicolefin copolymer（COC）dielectric spacer,and it may act as a dual-band half-wave plate.Terahertz multifunctional polarization control devices have unique advantages in multi-channel polarization detection and integrated design of terahertz devices.（4）We have proposed a polarization-dependent hybrid metamaterial absorption device based on the local absorption properties of electromagnetic resonance.Dual-band absorbers are polarization insensitive under the insulating VO₂ state.While the absorption properties exhibits a polarization-dependent dual band to broadband switchable effect under the metallic VO₂ state.The physical mechanism of the switchable absorption effect is elucidated using the equivalent impedance theory and electric field density distribution.The work has potential applications in the fields of terahertz sensing,imaging,and polarization detection.