Research On Dynamic Manipulation Of Metasurface

Metasurfaces,planer artificial materials composed of subwavelength unit cells,have shown superior abilities to manipulate the wavefronts of electro-magnetic waves.In the last few years,metasurfaces have been a burgeoning field of research,with a large variety of functional devices,including planar lenses,beam deflectors,polarization converters,and metaholograms,being demonstrated.Up to date,the majority of metasurfaces cannot be tuned postfabrication.Yet,the dynamic control of optical properties of metasurfaces is highly desirable for a plethora of applications including free space optical communications,holographic displays,and depth sensing.Recently,much effort has been made to exploit active materials,whose optical properties can be controlled under external stimuli,for the dynamic control of metasurfaces.The tunability enabled by active materials can be attributed to various mechanisms,including but not limited to thermo-optic effects,free-carrier effects,and phase transitions.This potentially allows us to achieve the fast control of metasurfaces without reconfiguring their structures at optical frequencies.Therefore,people concentrate on the tunability enabled by active materials like semiconductors,transparent conducting oxides(TCOs),graphene,a variety of phase-change materials(PCMs).Moreover,utilizing special technique when fabricating metasurfaces,such as introducing stretchable substrate material and reversible electrochemisry deposition,can also achieve the tunability of metasurface..In this dissertation,the research on dynamically manipulating the polarization and amplitude of electromagnetic wave of tunable material based metasurfaces are performed by using numerical simulation method.We proposed some metasurfaces that are based on tunable materials such as graphene,Dirac semimetal,and vanadium dioxide,and investigated the dynamic tunabilities in polarization conversion effect,plasmon induced transparency effect and the switchable multi-functions of the metasurface devices.Our findings are beneficial in the areas of biological sensing,light manipulation,and light slowing.In this dissertation,the following researches are included.For the polarization manipulation of electromagnetic wave,the metal-graphene-metal metasurface is investigated because of its controllable linear asymmetric transmission and perfect polarization conversion in THz regime.An on-to-off control of asymmetric transmission and perfect polarization conversion is achieved by changing the Fermi energy of graphene.By gradually shifting the Fermi energy of graphene,two functions are realized,i.e.,controllable linear asymmetric transmission and controllable total transmission with near perfect polarization conversion.Besides,a switchable THz metamaterial that can be switched between two functions of half-wave plate and quarter-wave plate is investigated.The two switchable functions can be simply achieved by inserting a VO₂ film in the meta-material design.The proposed metamaterial can convert x-polarized incident wave to y-polarized reflected wave when VO₂ is at metal phase,and convert x-polarized wave to circularly polarized wave when VO₂ is at insulator phase.The metamaterial performs well in the two functions.For the amplitude manipulation of electromagnetic wave,this dissertation mainly focus on the manipulation of the response spectrum of metasurface to the incident wave.First,a bulk Dirac semimetal(BDS)based metamaterial that can realize the tunable multiple plasmon-induced transparency(multi-PIT)effect is investigated.By breaking the periodic property of the unit cell,three-band PIT effect is achieved.By changing the Fermi energy of the BDS,the tunable multi-PIT spectrum and tunable multi-band group delay can be achieved.The multi-PIT peaks are also sensitive to the background refractive index and can lead high FOM.Then,we proposed a graphene and strontium titanate(STO)integrated terahertz metasurface that can achieve the flexible manipulation of plasmon-induced transparency(PIT)effect.By separately biasing the graphene antennas,the bright-bright mode coupled PIT effect is achieved.By flexibly configuring the Fermi energy distributions on the graphene antennas,the shape of PIT spectrum is also flexibly manipulated.Moreover,the working frequency band can be shifted by changing the environmental temperature due to the temperature dependent permittivity of the STO film.For the simultaneous manipulation of amplitude and polarization of electromagnetic wave,we first proposed a planar THz hybrid bulk Dirac semimetal(BDS)-metal metasurface in which a spectral window that possesses high reflectance with no polarization conversion effect arises in the polarization conversion pass band.Interestingly,though the proposed structure possesses no symmetry property,the effects are nearly the same for x-and y-polarization incidences.The breaking window can be independently and dynamically manipulated by changing the Fermi energy of the BDS rods,without breaking the background polarization conversion pass band.Moreover,a vanadium dioxide(VO₂)-integrated multi-functional metamaterial structure that consists of three metallic grating layers and two VO₂films separated by Si O₂ dielectric spacers.The proposed structure can be flexibly switched among three states by adjusting temperature,incident direction,and polarization.In state 1,the incident wave is strongly transmitted and perfectly converted to its orthogonal polarization state.In state 2,the incident wave is perfectly absorbed.In state 3,incident wave is totally reflected back.The working frequency of the multi-functional metamaterial can be arbitrarily tuned within a broad pass band.