Research On The Terahertz Functional Devices Based On Tunable Materials

Due to the emergence of metamaterials,the development of science and technology related to the field of terahertz has become very fast,and the use of metamaterials to complete the design of devices in related fields has become the focus of scientific research in recent years,such as the use of metamaterials to design the absorber,polarization converter,modulator,etc.In this paper,different tunable materials combined with metamaterials are used as a starting point to form composite devices,such as broadband single tunable,broadband multi-tunable,and multi-band multi-functional metamaterial terahertz functional devices are investigated respectively.The physical reasons behind the designed devices are explained in more detail using the corresponding theories.The main research work of this paper is as follows:（1）Graphene-based broadband perfect absorbers and dual tunable broadband absorbers based on graphene and vanadium dioxide are designed respectively.The designed metamaterial devices have a classical"sandwich"structure,and the voltage-modulated Fermi energy method,which is a common method for changing the conductivity of graphene,is also applied in this paper.Since the material properties of graphene can be determined by its dielectric constant,then the purpose of tuning the corresponding properties can be achieved.In addition,the angular stability of the designed devices is also required due to the practical applications.Through simulation,it is found that the proposed two types of broadband absorber design methods are robustness to the incident angle and have polarization insensitivity,which makes them highly compatible for practical use.（2）A single-frequency to dual-frequency switchable absorber design is demonstrated and analyzed.Instead of the conventional three-layer structure,a five-layer resonant unit is composed of two materials,Vanadium dioxide（VO₂）and Bulk Dirac Semi-metal（BDS）,combined with metamaterials.Due to the reversible phase change of vanadium dioxide,the device can be"switched"during the operation of the device,i.e.,the metamaterial device changes to a different function when VO₂undergoes a phase change.Interestingly,changing the Fermi energy of the Dirac semimetal does not affect the absorption rate when the conductivity of vanadium dioxide is 200,000 S/m.In contrast,the peak absorption point is influenced by the Fermi energy when the conductivity of VO₂ is 10 S/m.Therefore,the device can simultaneously achieve dual functionality and active tunability of resonant frequency and absorption amplitude.（3）Two different broadband polarization converters are designed.One is to use VO₂ as a tunable material.The polarization converter consists of two semicircular gold rings and two T-shaped VO₂ nanostrips.Simulation analysis shows that the Polarization Conversion Ratio（PCR）in the frequency range of 1.474～3.532 THz can be maintained above 90%.By analyzing the currents at the three resonant frequency points,it is found that the reasons for the efficient polarization conversion are electrical resonance and magnetic resonance,respectively.In addition,by changing the conductivity of VO₂,the bandwidth and amplitude of polarization conversion efficiency can be adjusted.The second is to use BDS as a tunable material.The resonant structure of the device is three BDS split rings distributed on the main diagonal,and a gold film is used on the bottom surface.After CST simulation,it is found that the bandwidth of PCR>90%is 1.086 THz（1.965～3.051 THz）,while the frequency band of PCR>95%is 0.63 THz（2.07～2.7 THz）.The reason for the excellent performance conversion was also verified by field analysis at two different frequencies selected.（4）A trifunctional terahertz metasurface is discussed,which utilizes Dirac semi-metal and VO₂ as tunable materials,in which VO₂ and BDS are both"switching"function,using the phase transition mechanism of VO₂ and the electrically tunable mechanism of BDS,the function of the metasurface can switch between dual-band absorption,half-wave plate（HWP）and quarter-wave（Quarter Wave Plate.QWP）.It can be known from the simulation that the VO₂ conductivity corresponding to the dual-frequency absorption function is 2×10⁵ S/m,and two perfect absorption peaks appear at 4.094 THz and 4.894 THz.Interestingly,the positions and magnitudes of the two absorption peaks are hardly affected by the Fermi energy of the intermediate Dirac semimetal open ring.The VO₂ conductivity corresponding to the polarization converter is 10 S/m,and the change of Fermi energy can further switch the metasurface between QWP and HWP.When the Fermi energy of BDS is 85 me V,the line-to-circular polarization function of the metasurface can be realized in the frequency band of1.71～2.73 THz.And when the Fermi energy of BDS is increased to 180 me V,it has a cross-polarization function in the range of 1.82～2.88 THz.