Research On Key Technologies Of Multi-band Terahertz Security Screening Systems

Terahertz radiation comprises electromagnetic waves with wavelengths between the millimeter band and infrared,with good penetration and resolution.The photon energy of terahertz radiation is much lower than the ionizing energy of biological tissues,giving it high safety and a wide application in areas such as security screening,biological detection,wireless communication and the medical field.Terahertz security screening systems receive terahertz radiation that is emitted or reflected from an imaged target for imaging.According to the presence or absence of terahertz sources,they can be divided into active and passive categories.In both active or passive security screening systems,the power spectral density of the terahertz radiation in the area where the contraband is located is different from the surrounding area,thus showing the shape and location of the contraband on the body image.Compared with singleband imaging,multi-band imaging can provide richer information,so it is becoming an important direction for current research on terahertz security screening systems.This thesis presents an in-depth study of the key technologies of multi-band terahertz security system from three aspects:operating band,device model,and detector.Two high contrast multi-band design schemes and a distributed quasi-static model of field-effect transistor(FET)are proposed,and two quad-band terahertz detectors are designed and implemented.The main research contents of this thesis are as follows:(1)Study on the operating band of high-contrast passive multi-band terahertz security screening systemsFirstly,the concept of absolute and relative contrast is proposed for the detection of concealed metal contraband,and the expressions of the relationship between absolute and relative contrast and system operating band,human skin reflectivity,clothing transmittance,human body temperature,environmental temperature and atmospheric transmittance are given by system modeling.Then,computer simulation results for the corresponding absolute and relative contrast frequency characteristics are presented using the reflectivity of human skin and the transmittances of four kinds of popular clothing,and the relationship between absolute and relative contrast and operating bands is explored.Finally,based on the results of modeling and simulation analysis of passive terahertz security screening system,10 detection windows with high absolute contrast are given,and a reconfigurable multi-band single-system operating band design scheme is proposed to achieve high relative contrast for multi-type clothing scenarios,and a fixed multi-band dual-system operating band design scheme to achieve a compromise between relative contrast,imaging time,and design difficulty.(2)Study on the distributed quasi-static model of FETFirstly,based on the classical quasi-static model of FET,a distributed quasi-static model of FET is proposed for the first time,and the differential equations and analytical expressions of terahertz radiation power sensors based on gate-source and drain-source coupled FET are derived.Then,the DC voltage response,leakage length and input impedance are calculated and compared with the lumped model and the hydrodynamic model.The simulation and experimental results show that the proposed model is significantly better than the lumped model and very close to the hydrodynamic model.This model extends the classical quasi-static model of FET to the terahertz band,which can continue to prolong the vitality of the classical quasistatic model,thus reducing the development cost of the circuit simulation tools of FET in the terahertz band.(3)CMOS quad-band terahertz microbolometer developmentFirstly,the temperature equation of the microbolometer in a passive terahertz security screening system is derived by system modeling.Then,the main performance of the proposed quad-band terahertz on-chip antenna is given by modeling and simulation.Then,the physical structure of the CMOS terahertz microbolometer is proposed,the noise equivalent power(NEP)expression is derived,and the computer simulation results are presented.Finally,the proposed detector fabricated based on 180 nm CMOS process is tested.The experimental results show that the NEP of the microbolometer can reach 1 pW/(?)level under liquid nitrogen cooling and no DC bias,which can meet the needs of both passive terahertz radiation detection and large-scale array designs.(4)Impedance-matched common-gate differential quad-band NMOS terahertz detector developmentFirstly,based on the analysis of the working principle of the conventional common-gate differential NMOS sensor,an impedance-matched common-gate differential NMOS sensor is proposed and analyzed.Then,a terahertz radiation power detector consisting of a quad-band terahertz on-chip antenna and an impedance-matched common-gate differential NMOS sensor is designed and implemented.The experimental results show that the return loss of the quadband terahertz on-chip antenna is significantly reduced by the impedance matching design.At room temperature,the NEP of the detector is less than 100 pW/(?)in the three operating bands and close to 100 pW/(?)in the other operating band,which is significantly better than the performance of the reported multi-band terahertz detectors at room temperature.