Design And Investigation Of Graphene Based Terahertz Tunable Metamaterials

Terahertz(THz)wave,defined as the electromagnetic wave with frequency from 0.1 to 10 THz,has found numerous potential applications in many fields,therefore,the THz science and technology has become increasingly important over the past decade.However,many natural materials have weak responses to THz wave,so that the techniques to efficiently manipulate THz waves are still lagging behind,which are demanded in many practical applications.On the other hand,artificial metamaterials can enable more flexible manipulation of the EM waves and are easily scaled to work at THz frequency.As such,considerable attention at THz frequencies has been focused on metamaterials,especially the tunable THz metamaterials.Among the different tuning schemes for THz devices,graphene,the newly discovered two-dimensional material,has revealed its potential application in high-performance tunable THz and infrared devices,since it produces a largely-tunable surface conductivity with respect to the external electrostatic biasing.However,the investigation on graphene based terahertz tunable metamaterials has just begun,thus their fascinating physical properties and potential applications leave much to be further explored.This thesis presents design and analysis of a series of terahertz tunable metamaterials with high performance,which are based on traditional metal/dielectric metamaterials combined with graphene.The main contributions of this dissertation are listed below:1.We combine the metamaterial having unit cell of cross-shaped metallic resonator with the double layer graphene wires to realize polarization independent absorber with spectral tuning at terahertz frequency.The absorption performance with a peak frequency tuning range of 15%and almost perfect peak absorption has been demonstrated by controlling the Fermi Energy of the graphene that can be conveniently achieved by adjusting the bias voltage on the graphene double layers.The mechanism of the proposed absorber has been explored by a transmission line model and the tuning is explained by the changing of the effective inductance of the graphene wires under gate voltage biasing.Further more,we also propose a polarization modulation scheme of terahertz wave by applying similar polarization dependent absorbers.Through the proposed polarization modulator,it is able to electrically control the reflected wave with a linear polarization of continuously tunable azimuth angle of the major axis from 0° to 90° at the working frequency.These design approaches enable us to electrically control the absorption spectrum and the polarization state of terahertz waves more flexibly.2.We propose a scheme to design switchable quarter-wave plate for terahertz Wave that is composed of graphene based grating and metallic grating structures.The proposed active device can dynamically switch the transmission wave among left-handed,right-handed circular polarization and linear polarization states by electrically controlling the Fermi energy of the graphene grating.The device is analyzed with grating circular polarizer theory and its performance is investigated through full wave simulations on practically realizable geometry.The proposed quarter-wave plate having a subwavelength thickness demonstrates a wide angle of incidence tolerance,and a broad bandwidth operation.We also propose a tunable broadband terahertz polarization rotator,which is composed of four graphene gratings and isolating layers.This device can continuously vary the polarization rotation angle of transmission wave by controlling the conductivity of the graphene that can be conveniently achieved by adjusting the bias voltage on the graphene gratings.We employ numerical simulation to demonstrate that the proposed device could achieve continuous tunability of the polarization rotation angle from 20° to 70°,broad bandwidth,and high efficiency.These concepts offer a further step in developing tunable polarizers and polarization switchers,which may be applied in practical terahertz image and communication systems.3.We present a scheme to achieve broadband backward scattering reduction through diffuse terahertz wave reflection by a flexible metasurface.The diffuse scattering of terahertz wave is caused by the randomized reflection phase distribution on the metasurface,which consists of meta-particles of differently sized metallic patches arranged on top of a grounded polyimide substrate simply through a certain computer generated pseudorandom sequence.Both numerical simulations and experimental results demonstrate the ultralow specular reflection over a broad frequency band and wide angle of incidence due to the re-distribution of the incident energy into various directions.The diffuse scattering property is also polarization insensitive and can be well preserved when the flexible metasurface is conformably wrapped on a curved reflective object.Afterwards,we combine graphene layers with such metasurface to achieve tunable backward scattering by electrically controlling the Fermi energy of graphene layers.Numerical simulations prove that the metasurface shows continuously tunable property of radar cross section over a large region.The proposed design opens up a new route for specular reflection suppression,and may be applicable in stealth and other technology in the terahertz spectrum.4.We propose a metasurface consisting of a meta-particle array of metal rectangular-ambulatory-plane on a grounded polymer substrate embedded with graphene layers to control reflected beam at terahertz regime.By changing the dimension of the inside rectangular,the meta-particle may achieve reflection phase range from 0 to 2? and introduce abrupt phase shifts along the metasurface,resulting in anomalous reflection.On this basis,by electrically controlling the Fermi energy of the graphene grating,the reflection beam will be steered.Numerical simulations show that both single-beam switching between 0° or 26°and dual-beam switching either 0° or ±36°is achieved at THz frequency with fast operation speed.This approach may be applied to manipulation of radiation in the THz and infrared region.