Research On Highly Sensitive Terahertz Sensors Based On Graphene Metamaterial

Terahertz wave possesses unique physical properties such as low energy,high penetration,and fingerprint spectra,and many biomolecules exhibit rich characteristic absorption spectra in this band,but when detecting analytes at subwavelength scales or low concentrations,terahertz wave cannot fully interact with the sample,resulting in many limitations in the practical application of conventional terahertz detection technology.As an artificially designed non-natural material,metamaterials can locally enhance the incident terahertz wave at the subwavelength scale,and they are extremely sensitive to changes in the dielectric constant of the surrounding environment,which leads to an effective increase in sensing sensitivity.Therefore,metamaterial-based terahertz sensors arise at the historic moment,which can not only break the resolution limit but also realize label-free and rapid trace detection.However,metal-based metamaterials have some drawbacks,such as high loss,poor localization,and lack of tunable characteristics.Based on this,in this thesis,transmissive and reflective terahertz sensors are designed and investigated using graphene metamaterials instead of conventional metallic metamaterials to improve their sensing sensitivity and realize multi-band substances detection.The main research results of this thesis are as follows:Firstly,transmissive terahertz metamaterial sensor based on graphene plasmon-induced transparency.1.A transmissive terahertz graphene metamaterial sensor based on polarizationinsensitive single-window plasmon-induced transparency(PIT)is proposed.The patterned graphene of the sensor is composed of a central cross and four surrounding strips,and the whole structure maintains a high degree of centrosymmetry,so that a consistent singlewindow PIT phenomenon can be observed when the electric field is along the x and y polarization directions,respectively,exhibiting good polarization-insensitive properties.In addition,by tuning the Fermi level of graphene,the amplitude and resonant frequency of the transparent window can be adjusted simultaneously.The results show that the maximum normalized sensitivity,Q(Quality)factor,and FOM(Figure Of Merit)of the sensor can reach 0.44/RIU(Refractive Index Unit),34.67,and 17.28/RIU,respectively.2.A transmissive terahertz graphene metamaterial sensor based on polarization-sensitive dual-window PIT is proposed,and its patterned graphene consists of a simple continuous“king” shape.When the electric field is along the x-polarization direction,the three brightmode resonators generate a destructive interference phenomenon through the bright-bright coupling,and two transparent windows can be observed within 0.75～2 THz,but the spectral amplitude changes significantly after the polarization angle exceeds 50°.Likewise,the sensor shows good tunability and amplitude modulation characteristics when the Fermi level is changed.Furthermore,the intermediate dielectric column added in the middle of this sensor increases its contact range with the analyte,and its maximum normalized sensitivity is increased by 39.77% compared with the first single-window PIT sensor.Secondly,reflective terahertz metamaterial sensor based on graphene narrowband absorber.1.A reflective dual-band terahertz graphene metamaterial sensor based on a continuous dielectric layer is proposed with the following unit structure from top to bottom: patterned graphene-continuous dielectric layer-metal substrate.The sensor is capable of producing two ultra-narrow perfect absorption peaks within 3～9 THz,and the structure of the sensor is well isotropic,maintaining excellent polarization angle and wide incidence angle insensitivity in both Transverse Magnetic(TM)and Transverse Electric(TE)modes.In addition,by tuning the Fermi level of graphene,the resonant frequency of the sensor can be varied in a wide range,thus enabling the detection of substances in different frequency bands.The results show that the maximum normalized sensitivity,Q factor,and FOM of the sensor are0.287/RIU,216.29,and 76.89/RIU,respectively.2.A reflective five-band terahertz graphene metamaterial sensor based on a continuous dielectric groove is proposed with the following unit structure from top to bottom: patterned graphene-continuous dielectric groove-continuous dielectric layer-metal substrate.Five ultra-narrow absorption peaks of the sensor can be found within 2～10 THz,meanwhile,thanks to its centrosymmetric structure,the sensor can maintain the polarization angle and wide incident angle insensitivity in both TM and TE modes.Moreover,the resonant frequency of the sensor absorption peaks can be effectively tuned by adjusting the Fermi level of graphene.Due to the addition of a continuous dielectric groove,the contact range between the sensor and analyte is greatly increased,and the results show that the maximum normalized sensitivity,Q factor,and FOM that the sensor can achieve are 0.612/RIU,355.94,and 215.25/RIU,respectively.Compared with the first dual-band sensor,its maximum normalized sensitivity,Q factor,and FOM are improved by 113.24%,64.57%,and 179.95%,respectively.