Research On Terahertz Polarization Control Device Based On Metasurface

Terahertz waves(0.1-10 THz)have many unique properties and are of great application value in many fields,such as next-generation wireless communication,biomedicine,and radar detection.Polarization,as the essential property of the electromagnetic wave,often carries critical information about the electromagnetic wave.It is of great significance for the development of terahertz technology to control the polarization state of terahertz waves freely.Due to structure size and material loss,the traditional device structure or design working in the low-frequency band is difficult to directly apply to developing terahertz polarization control devices.Metasurface has the advantages of small size,lightweight,easy manufacture,and low loss,bringing new opportunities for the research of terahertz technology.This dissertation focuses on the metasurface to control the polarization characteristics of terahertz waves and mainly studies the asymmetric transmission effect,polarization conversion effect,and polarization selective absorption effect in the terahertz frequency band,and tries to solve significant problems such as the narrow working bandwidth,single function and large size of traditional terahertz polarization control devices.The main content and innovation of this dissertation include the following aspects:(1)Single-band and dual-band Terahertz polarization selective devices based on chiral metasurface.Most existing terahertz polarization selective metasurfaces can only operate in a single band,and their circular dichroism(CD)effect is relatively weak.First,we designed and experimentally verified a single-band terahertz polarization selective metasurface based on a metal strip and a split rectangular ring structure to solve these problems.The working principle of the metasurface was analyzed with a reflective multiple interference model,and the influence of critical structural parameters on the polarization selection effect was discussed.On this basis,the structure of the single-band terahertz polarization selective metasurface is optimized and improved.Two kinds of polarization selective metasurface(named Type 1 dual-band polarization selective metasurface and Type 2 dual-band polarization selective metasurface)that can work in two frequency bands simultaneously are obtained using the resonant superposition principle.For the type 1 dual-band polarization selective metasurface,the measured CD spectra reach two peaks of 0.44 and 0.515 at 0.49 THz and 0.65 THz,respectively.For the type 2 dual-band polarization selective metasurface,the CD spectra can get the first peak of 0.52 at 0.59 THz and the second peak of-0.89 at 0.81 THz.The proposed dual-band polarization selective metasurfaces have promising applications for multi-band terahertz communication,biosensing,and circular polarization detection.(2)Research on metasurface-based dual-function terahertz polarization converter.Most existing terahertz polarization conversion devices can only realize one kind of polarization conversion effect.One device can only realize linear polarization conversion or linear-circular polarization conversion alone.We designed a terahertz polarization conversion metasurface to solve this problem based on the metallic grating and a Z-shaped structure.With the assistance of the metallic grating structure,an F-P-like cavity is formed between the two metal layers of the proposed polarization conversion metasurface.Then,the proposed polarization conversion metasurface can achieve broadband linear-tocircular and linear polarization conversion in two different bands.From 0.34-0.48 THz,the x-polarized incident wave is converted to a left-handed circularly polarized(LCP)wave with a measured 34.2%relative bandwidth,and the conversion efficiency is about 70%.From 0.53-0.85 THz,the x-polarized incident wave can be converted to a y-polarized transmitted wave with a measured polarization conversion ratio higher than 80%.This work integrates two polarization conversion functions into one device,which is beneficial for reducing system complexity and cost.(3)Research on metasurface-based terahertz dual-band and wideband asymmetric transmission devices.Aiming at the problem that most terahertz asymmetric transmission metasurfaces can only work in a single band and lack experimental verification,the dual-band and broadband asymmetric transmission metasurface operating in the terahertz band is fabricated.The reported dual-band asymmetric transmission metasurface usually realizes the asymmetric transmission effect on the incident wave with two orthogonal polarization states in two operating bands,respectively.In this work,we experimentally demonstrate a dual-band asymmetric transmission effect only for one-polarized linear wave in the terahertz band.When the incident wave is x-polarized,the measured crosspolarization transmission coefficient owns two peaks of 0.715 and 0.548 at the frequency of 0.74 THz and 1.22 THz,respectively.However,when the incident wave is y-polarized,the transmission coefficient is lower than 0.2 in the wide frequency range from 0.5 THz to 1.5 THz.Obviously,the metasurface shows a dual-band asymmetric transmission effect.Despite the dual-band asymmetric transmission metasurface,this dissertation also experimentally demonstrates an asymmetric transmission metasurface that can realize a broadband asymmetric transmission effect in the terahertz band.For the x-polarized incident wave,the measured cross-polarized transmission coefficient is higher than 0.6 from 0.55 to 0.82 THz and reaches a peak of 0.714 at 0.62 THz.However,the cross-polarized transmission coefficient is lower than 0.2 from 0.4 to 0.9 THz when the incident wave is y-polarized.As a result,the metasurface shows a broadband asymmetric transmission effect.(4)Research on metasurface-based tunable terahertz polarization control device.Firstly,aiming at the problem that most tunable terahertz asymmetric transmission metasurfaces lack dual-band modulation function,this dissertation proposes a design strategy to realize tunable dual-band asymmetric transmission effect.By changing the chemical potential of the graphene,the asymmetric transmission effect and the transmission circular dichroism effect can be dynamically tuned in the two operating bands.Secondly,aiming at the problem that most tunable polarization selective metasurface’s operating mode is not rich enough,this dissertation proposes a switchable multi-functional polarization selective metasurface based on vanadium dioxide(VO2).By switching between the metallic state and the dielectric state of VO2,the metasurface can operate in four modes.It can dynamically control the polarization selection absorption(or reflection)effect on circular and linear polarized waves.The design of the tunable asymmetrical transmission metasurface and the switchable multifunctional polarization selective metasurface can provide an essential strategy for designing tunable terahertz polarization control devices in the future.