Terahertz Plasmon Surface Enhanced Sensing Mechanism And Devices

As a transition band between microwave and infrared,terahertz（THz）wave has the characteristics of broadband,low energy and strong spectroscopic analysis,which shows a wide application prospect in the qualitative and quantitative sensing of matter.However,the absorption cross-sections of molecules are miniscule compared with THz wavelengths,resulting in rather weak interactions between molecules and THz radiation.For this reason,the sensitivity of THz spectroscopy in sensing applications is greatly impeded.In recent years,due to its unique surface plasmon resonance（SPR）response ability,the micro-nano electromagnetic structure surface provides an effective means to enhance the interaction between THz wave and matter,and has become a hotspot of high sensitivity THz sensing research.Based on terahertz plasmon surface（TPS）,including corrugated metal microstructure spoof plasmon surface（SPS）and graphene plasmon surface,highly sensitive and tunable THz sensing mechanism and THz sensors were develop by exciting a highly confined SPR mode to generate significant electricfield enhancement effects.The main research contents include:In allusion to the limitation of discrete electricfield enhancement distribution in conventional metal SPS,a highly confined spoof surface plasmon polartions（SSPPs）mode supported on periodic grooves array decorated metallic SPS was reported to achieve highly sensitive THz sensing.Through the combination of lithography,deep eactive ion etching,and magnetron sputtering coating,the precision machining of metallic grooves SPS with deep trench has been accomplished,which solved the problem of the precision machining of the metal surface structure with high aspect ratio in femtosecond laser micro-processing technology.The results show that the metallic grooves SPS has a flexible resonance frequency design freedom,and the THz electromagnetic field can be gathered and enhanced along the groove,resulting in strong THz-matter interaction.In view of the challenge of polar liquids detection in traditional THz-SPR sensors,based on the metallic grooves SPS,a hypersensitized THz sensor for micro polar liquids detection via combining high-resistivity silicon prism was proposed.The experimental results show that using a high refractive index prism as an external surface plasmon coupler,the large momentum matching and the complete coupling between incident THz wave and SSPPs can be achieved.Besides,the strong absorption of electromagnetic field energy by polar liquids can be alleviate effectively by prism coupling,which solved the problem of polar liquids sensing detection in traditional THz-SPR sensors.In addition to being a highly sensitive liquid sensing platform,the grooved SPS also can be extended to the tunable THz metasurface perfect absorbers（MPAs）by combining low refractive index PE prism.The modulation of absorption frequency and absorption width of MPAs can be achieved simultaneously by changing the dielectric environment of the groove with polar liquids filling.In order to better characterize the change of high loss media such as polar liquids in THz sensing,a high accuracy and high figure of merit（FOM）sensing characterization method based on THz phase-jump responses under SPR condition was presented.The experimental results show that on the basic of the build sensor with metallic grooves SPS,by introducing THz phase-jump transformation as an output response,a clearer picture of SPR state can be observed,presenting a remarkable improvement in the identification of sensing resonance frequency compared with that of amplitude detection.In particular,when the liquids sensing is carried out under the optimal coupling gaps,the ratio of phase-based Q-factor to its corresponding amplitude-based Q-factor generally falls within the range of 4～26,and the sensitivity S_n and FOM based on THz phase readout up to 0.370 THz/RIU and 24.7RIU^-1,respectively.Besides,the interaction between THz wave and matter can be further enhanced by increasing the spatial coincidence degree between the highly confined SSPPs electromagnetic field and sample.As a result,an improved sensitivity for THz sensing can be obtained.Compared with the SSPPs controlled by metallic microstructure,the surface plasmon polartions（SPPs）supported by graphene possess a unique dynamic tunable characteristics,and become an important developing direction to the adjustable THz-SPR sensors.Thus,based on a large continuous doped graphene,two kinds of TPS,including suspended graphene and graphene/insulator stack were further proposed.On this basis,three kinds of ultrasensitive and tunable THz refractive index sensors in a total internal reflection geometry were designed.The results show that the S_n and FOM of the proposed Otto type suspended graphene based adjustable THz sensor up to 6.65THz/RIU and 1,187 RIU^-1,respectively,which are at least one order of magnitude higher than conventional metal SPS based sensors.What’s more,the resonance frequency increases from 2.43 THz for E_F=0.4 eV to 7.33 THz for E_F=1.2 eV,which means that this THz sensor can be dynamically tunable in a large frequency range.Besides,it was found that the optimization of the graphene/insulator stack can maintain high graphene carrier mobility,support more significant electricfield enhancement,and improve THz sensing performance.At the same time,the THz phase also presents jump transformation response in the sensing method of angle modulation.A FOM as high as171 was achieved based on THz phase detection,which is about 16 times of the FOM obtained via traditional THz amplitude based measurement method.The main innovation of this thesis is that we proposed a high-resistivity silicon prism coupled highly confined SSPPs mode on metallic grooves SPS to achieve high sensitivity THz sensing of polar liquids.This provide an adequate solution for the traditional THz-SPR sensors which detection range are limited to low loss media.Meanwhile,we had delve into the THz phase sources that are often neglected in SPR state,and presented a THz phase-jump response based sensing characterization method to free from the spectral offset interference in the traditional method with THz amplitude measurements.In addition,the utilization of large continuous graphene can effectively avoid the substantial degradation of carrier mobility caused by patterned graphene processing in tunable THz sensors.The results not only provide a fire-new solution for the design and processing of hypersensitized and adjustable THz sensors,but also give an original idea for the accurate sensing characterization of high loss media,which is of great significance in promoting the development of THz sensing devices.