Structural Design And Performance Control Of Tunable Terahertz Metasurface

Terahertz waves have a unique position in the electromagnetic spectrum,between microwaves and infrared waves.Therefore,terahertz waves have the advantages of both long-wave and short-wave.The spectral characteristics of terahertz wave make it has strong potential applications in medical,radar and communications.In the past thirty years,high-power terahertz sources and high-sensitivity terahertz detectors have made great progress with the unremitting efforts of scientific researchers.However,due to the lack of functional materials that can respond to terahertz waves in nature,the development of terahertz functional modulation devices and equipment is very slow.The developmental imbalance of modulation devices has greatly hindered the integration and further functional research and development of terahertz systems.As an artificially designed array of structures,the metasurface defines the electromagnetic properties through the interaction of the microstructures and electromagnetic waves,and realizes specific electromagnetic response phenomena.Among them,the terahertz metasurface has become an important research direction of terahertz functional devices because it can perfectly solve the problem of the lack of functional materials in the terahertz band.This dissertation mainly studies the electromagnetic response characteristics of metasurfaces in the terahertz band,explores the combination effects of different natural materials and metasurfaces,and then realizes the development of metasurface-based terahertz modulation devices in multiple application scenarios.The specific research contents are as follows:First,based on the plasmon-induced transparency（PIT）effect,a liquid crystal terahertz metasurface was designed and its modulation mechanism was studied.Through the theoretical analysis of the surface lattice resonance（SLR）mode,the metal strip metasurface is used to make the film substrate into a lattice,and a novel non-local SLR is induced.The influence of the structure size and refractive index on the frequency and intensity of the SLR mode are studied respectively.The thin film metasurface is converted into the form of a liquid crystal cell,and the PIT effect at 1.75 THz is induced by the bright-dark coupling of the dipole resonance and SLR mode,which breaks the limitation that the PIT effect requires multiple resonators units to excite.Based on the dependence of SLR intensity on the difference between the refractive index of liquid crystal and the substrate,the large depth modulation with PIT effect over 90%is realized by electronically changing the refractive index of liquid crystal,which proves a new modulation method that the refractive index directly controls the resonant intensity rather than the resonant frequency position.Secondly,based on the deformation characteristics of polydimethylsiloxane（PDMS）under applied stress,the stretchable metasurfaces with variable period and morphology in terahertz band was designed.The periodical characteristics of the cross metal metasurface under tension are analyzed.The frequency tuning of 0.14 THz and the intensity modulation effect of 90%are achieved by using the electric dipole resonance,and the dynamic tuning from 2.3 THz to 1.9 THz is achieved by using the period-sensitive m SLR mode.The frequency shift capability of up to 0.4 THz is better than most current terahertz metasurface.The deformation characteristics of single-layer and double-layer stretchable metasurfaces were verified through the combination of experiment and theory,and the deformation model under streching was established.The single-layer and double-layer metasurface respectively excite the PIT effect through the bright-bright mode coupling between the metal strip and the U-shaped ring,and the bright-dark mode coupling between the upper metal strip and the lower metal wire pair.Further,by controlling the coupling strength between the resonant units,the modulation of the PIT transparency peak induced by the coupling is achieved.The deformable metasurface provides a novel PIT effect modulation method and a terahertz wave control method.In addition,an optically controlled polarization-independent terahertz metasurface is designed based on the photoconductive effect of silicon wafers.The PIT effect induced by the metasurface composed of a ring and a split ring resonator is analyzed.The reverse current between the resonators will interfere with the loop current on the split ring and cause the magnetic attenuation of the magnetic dipole,which greatly reduces the strength of the PIT effect.By connecting the ring and the split ring,the generation of reverse current is eliminated from the root,and a more significant PIT transparent window has been successfully stimulated.This method of using structural connections to suppress non-demand coupling is expected to be used in other metasurface designs.Furthermore,the polarization-independent PIT metasurface is formed by the quadruple rotation of the connected half-ring and split-ring resonators.Experiments and simulations are used to demonstrate the polarization-independent ability and slow light effects of metasurfaces.The polarization-independent metasurface prepared on the silicon substrate produces a significant transparent peak at 0.6 THz.The transmission peak decreases as the power of the pump light increases,achieving a modulation depth of 36%.Finally,a terahertz double-layer chiral metasurface with asymmetric transmission capability and polarization conversion effect is proposed.Combining the reciprocity theorem and structural symmetry,the design scheme of the polarization conversion metasurface with asymmetric transmission capability is deduced.Based on the scheme,the anisotropic double split ring and the isotropic ring are combined to construct a double-layer chiral metasurface with the upper and lower layers twisted by 90°.Through simulation,it is found that the polarization conversion ability of the metasurface comes from the coupling effect of the electric resonance and magnetic resonance.The chiral metasurface has significantly different responses to the terahertz wave propagating in the opposite direction.The metasurface can effectively convert the y-polarized wave propagating in the forward direction into the x-polarized wave.At 1.1 THz,the cross polarization transmittance exceeds 97%,and the polarization conversion rate is as high as 99%.However,the backward propagating y-polarized wave is blocked,and the asymmetric transmission parameter can reach 0.97.Furthermore,the introduction of VO₂material with metal-insulating phase transition demonstrates the temperature modulation ability of metasurface polarization conversion and asymmetric transmission effect.This kind of polarization conversion metasurface scheme that uses an anisotropic structure to introduce chirality and uses an isotropic structure to construct a reverse current greatly improves the flexibility of design.