Research On Terahertz Sensors Based On Electromagnetic Metamaterials

Terahertz wave attracts the extensive attention of scientists at home and abroad by virtue of the excellent electromagnetic characteristics,such as high penetration,large bandwidth,non-ionization and so on.The terahertz technology plays an important role in the fields of information and communication technology,space science,etc.Moreover,biochemical molecules exhibit a wealth of characteristic fingerprint spectra in such bands,which enables terahertz detection techniques to be used as efficient detection tools for spectroscopy,environmental monitoring,security,medical imaging,biosensing,etc.However,terahertz detection techniques are less sensitive than required when dealing with biochemical solutions at low concentrations and analytes at deep subwavelength scales.Metamaterials feature flexibly tunable resonant responses,and on resonance excitation,incident electromagnetic field shows localized enhancement at subwavelength scale within metamaterials,which can be utilized to amplify the terahertz detection signals.This makes terahertz metamaterial sensing an effective solution for new,highly sensitive,label-free,non-destructive testing.The use of various metamaterial designs to enhance the sensitivity of terahertz sensors has also become one of the research hotspots in the field of terahertz technology.On the basis of in-depth study of the existing theories and applications on terahertz metamaterial sensing,we propose vertical split ring metamaterials to function as novel terahertz sensors,design a sensing method for metamaterials to simultaneously measure dual attributes of the analytes and optimize the fabrication process of metamaterial absorbers,aiming to achieve highly sensitive and efficient terahertz metamaterial sensing.The main research findings and innovations are as follows:1)The sensing mechanisms of various terahertz metamaterial sensors are deeply analyzed,and the key factors restricting the improvement of sensing performance are explored.In order to obtain excellent sensing performance,a vertical split ring resonator metamaterial sensor is proposed.In such a metamaterial design,the resonantly enhanced sensing region is lifted in the vertical direction,and hence the concentrated resonant field region is far away from the substrate,which reduces the loss caused by the substrate contact and enhances the sensing performance.2)A vertical dual-gap ring metamaterial is proposed and a terahertz sensor based on the structure is designed.The resonance characteristics of the metamaterial and the sensing performances of the sensor based on the metamaterial are analyzed theoretically,and the underlying physical mechanisms are revealed.The results show that the double-gap design maintains the structural symmetry,eliminating the bianisotropic response in the resonators,and results in a resonance with a Q factor of about 20.Moreover,the top gap in the vertical double-gap structure is far away from the substrate,which reduces the dielectric loss caused by the substrate contact and greatly enhances the sensing performance of the sensor.The maximum sensitivity is up to 788 GHz/RIU.In addition,such 3D terahertz sensor is insensitive to fabrication errors and wide incidence angles.3)A method to excite a dark resonance mode by introducing structural asymmetry in the vertical dual-gap ring metamaterials is proposed.Mathematical model fitting and comparative analysis are used to explore the resonance excitation and hybridization processes and the physical meanings are revealed.The results show that the Fano lineshapes in the spectra are induced by the resonance hybridization between the flexibly tunable dark mode and other resonances,which narrows the resonance linewidths.A narrowband resonance with a Q factor of 327 and a linewidth of 5.9 GHz is obtained at 1.93 THz.Moreover,in addition to high Q resonance,the asymmetric vertical double-gap ring metamaterials also feature the high sensitivity inherent in such vertical ring metamaterials,resulting in an excellent comprehensive sensing performance.4)The spectral characteristics and electromagnetic field distribution features of the low-order and high-order magnetic resonance modes commonly existed in vertical split ring resonators are simulated and analyzed.The results show that under the specific incident polarization,the dual magnetic resonance modes can be excited in such vertical split ring metamaterials benefiting from the special geometrical configuration of the vertical split ring resonators.The sensing potential of the vertical single gap ring metamaterials is analyzed numerically,which exhibits a weaker sensing performance than that of the vertical double-gap design.5)An algorithm based on metamaterial absorber for simultaneous measurement of refractive index and conductivity of analytes is proposed.The nonlinear equations set between spectral response variables(resonannce frequency shifts and amplitude modulations)and refractive index and conductivity of the analyte is obtained by three rounds of mathematical fittings.By virtue of the linear expression between resonance frequency shift and refractive index,the expressions of the dual attributes of the analyte on the spectral response variables are obtained by inversely solving the equations set.A simple metamaterial absorber is selected for simulation analysis.The results show the maximum error percentage is only 0.83%,which fully demonstrates the accuracy and feasibility of the algorithm.6)The traditional fabrication process of metamaterial absorbers is optimized.The new fabrication process reduces the fabrication steps and the fabricated metamaterial absorbers exhibit better sensing performance while maintaining the absorption performance.Two common metamaterial absorber designs are chosen as examples.The traditional and new-style metamaterial absorbers are compared in terms of absorption and sensing performances,respectively.The results are explained by simulation analysis,which show that the sensing performance is enhanced as the new-style metamaterial absorbers can expose more resonant fields to analytes,which induces adequate coupling.In addition,this new fabrication method can be used to fabricate complex metamaterial absorbers and is insensitive to fabrication errors.