Study On Tunable Absorption And Multifunctional Focusing Performance Of Terahertz Metamaterials

Terahertz waves are between microwaves and infrared light.The special electromagnetic spectrum position makes it widely used in important fields such as biomolecular detection,medical imaging,and high-speed space communication.However,it is usually difficult for materials in nature to produce effective electromagnetic response to terahertz waves,which limits the development of terahertz device.In recent years,the emerging metamaterial technology can use special structural design to achieve high-efficiency response in any electromagnetic waveband,which provides a new way for the development of various terahertz functional devices.As the key element of the terahertz detection system,the terahertz focusing lens and tunable absorption device have always received special attention from researchers.This thesis studies the absorption and focusing devices such as terahertz perfect absorption of multi-frequency bands,tunable absorption of large frequency domain,multi-functional focusing and polarization detection,and shaping vector focal curve with arbitrary spatial trajectory,which provides new ideas for the development of terahertz focusing lens and tunable absorption device.The research contents of this thesis are as follows:(1)A liquid crystal tuning triple-band terahertz metal metamaterial absorber is designed,and the multi-band near-linear absorption characteristics and perfect absorption characteristics of the absorber are numerically analyzed.The absorber is composed of a conductive metal plane layer,a liquid crystal electrical tuning layer and a gold disk resonator array layer.The calculation of the terahertz absorption spectrum shows that in the gap plasmon resonance coupling mechanism,the three absorption bands have theoretical absorption efficiency of more than 99%.The three resonance peak frequencies of the absorber show continuous linear tunability as the refractive index of the liquid crystal changes.In the liquid crystal tuning operation,the peak absorption rates of the three absorption bands theoretically always remains at a level of more than 99%.This scheme can simultaneously perform multi-band spectrum tuning,wide-angle incidence,no polarization dependence and theoretical absorption efficiency of more than 99%.The designed liquid crystal tuning triple-band terahertz metal metamaterial absorber has great application potential in the field of biomolecular spectral selective terahertz imaging and sensing.(2)A liquid crystal assisted tuning dual-band terahertz graphene metamaterial absorber is designed,and the hybrid tuning absorption characteristics of the absorber are numerically analyzed.The absorber is composed of a conductive metal plane layer,a liquid crystal electrical tuning layer and a graphene disk resonator array.Using the changes in the refractive index of the liquid crystal and the Fermi level of the graphene,the absorber can be double-tuned to increase the spectral scanning limit of the terahertz absorber.The simulation results show the characteristics of near-linear wide tuning,wide incident operating angle,incident polarization independence and theoretical absorption efficiency of more than 99% in the liquid crystal tuning region.Tuning the Fermi level of graphene,the dynamic range of resonant frequency scanning can theoretically be as high as ?f / f = 50%,while ensuring that the absorption efficiency exceeds 90%.The graphene/liquid crystal hybrid tuning scheme can expand the working band of the terahertz metamaterial absorber,and has potential application prospects in the fields of integrated tunable terahertz detection,imaging and sensors.(3)A reflective terahertz metalens with four focal points is designed,and its performance in detecting the ellipticity and polarization major axis of the terahertz wave is numerically analyzed.The metalens is composed of a metal reflective layer,a dielectric spacer and a zigzag metal resonator array.The resonators have different orientations in space to produce the required Pancharatnam-Berry phase.The polarization states at the four focal points include left-handed circular polarization,right-handed circular polarization,incident polarization,and polarization states with the major axis rotated by ?/4 compared to the incident polarization.The handedness,ellipticity and polarization major axis direction of the incident polarization state can be determined based on the light intensity at the four focal points.The uniqueness of the designed terahertz metalens makes it very attractive for applications in compact terahertz polarization detection,communication and information processing.(4)The design method of metalens for generating the terahertz vectorial focal curve is proposed,and the generation of terahertz focal curves with arbitrary polarization profiles and shapes is numerically analyzed.Based on the design of the phase profile of the multi-foci metalens,enough continuous focal points are generated along the predesign focal curve trajectory,resulting in two-dimensionally distributed terahertz focal curves.In simulation,a focal curve image with an effect similar to holography is obtained,which can realize the polarization control of the metalens at the pixel level.This kind of metalens with arbitrary shape and polarization profile focal curve can greatly expand the application of integrated optics in polarization topology and light alignment.(5)The vectorial focal curve metalens samples are designed and prepared,and the experiment verifies the design scheme of the metalens to generate the vector focal curve.The metalenses are used to generate various shapes of vector focal curves in the focal area,including simple ring focal curves,an Archimedean spiral focal curve,a complex trefoil knot focal curve.The detection and analysis of the distributions of various polarization profiles on the focal curves are completed,including the linear,nonlinear and discontinuous evolution relationship between the polarization rotation angle and the position along the focal curve.The experimental results are in good agreement with the simulation design results,which shows that the vector focal curve metalens design method is robust.In summary,terahertz tunable absorbers studied in this paper achieve multi-band perfect tuning of absorption efficiency and expand the tuning bandwidth,and multifunctional focusing devices achieve the polarization control of the multi-foci and the generation of the vector focal curves,which have potential application value in the fields of terahertz detection,sensing and imaging.