Research On High-resolution Imaging Based On The Terahertz Jet Effect

As one of the key technologies in the terahertz range,Terahertz(THz)imaging technology has shown great advantages and potential applications in biomedicine,safety inspection,aerospace and other fields.However,according to the Rayleigh criterion,the resolution of conventional far-field THz imaging technology is limited by the diffraction limit(0.61?/NA),which is generally on the order of millimeters.Thus it is difficult to meet the needs of practical applications.Furthermore,the existing THz near-field imaging technology usually cannot overcome the contradiction between the improvement of resolution and the loss of signal throughput and spectral bandwidth.In order to improve the spatial resolution with-out the loss energy and bandwidth,this thesis proposes a dielectric semi-sphere coupled THz high-resolution imaging method.Using"terajet"effect produced by a dielectric semi-sphere with a suitable refractive index the sub-wavelength THz light can break through the diffraction limit of spatial resolution,and obtain the multi-dimensional time domain and frequency domain information of the sample.This thesis mainly includes the following parts:Firstly,based on the finite element numerical simulation method,the transmission characteristics of the terajet beam generated by the dielectric semi-sphere are studied.The dielectric semi-spheres with different diameters,refractive indices and di-layer structures are simulated by CST software at different frequencies,the THz imaging resolution geometric structure of dielectric semi-sphere are explored.The simulation results show that the structure of the dielectric semi-sphere can not only be used for high-resolution imaging,but also the focal length is relatively large,about 1.4 mm from the sphere,and hence the operation space is relatively large during the experiment.the di-layer structure is used to increase the diameter of the focused beam.It is about ?/2,but its jet distance is shorter than the media hemisphere,about 0.8 mm.In order to verify the above simulation results,the performance of the dielectric semi-sphere coupled THz imaging system is experimentally measured.It is found that the signal flux of the system is obviously enhanced and the spectral bandwidth is almost no loss after the dielectric semi-sphere,which is proved to be a high-resolution,high-throughput,broad-bandwidth far-field THz imaging method.Then we use the knife-edge method to measure the beam diameter when there is a dielectric semi-sphere.It is found that the dielectric semi-sphere reduces the spot diameter of the system by 55.1%and the resolution of the system to a minimum of 100?m.Finally,an all-dielectric silicon grating and a new version of 100 yuan were imaged using a 3 mm polytetrafluoroethylene dielectric semi-sphere.The imaging results show that,with the dielectric semi-sphere,both the time domain-mode and the frequency-domain mode can effectively distinguish the raster stripe and the words "100" in the RMB security line.However,if the sample is imaged in free space,it cannot be effectively distinguish.This shows that 3 mm dielectric semi-sphere coupled THz imaging can effectively improve the imaging resolution of the THz system.The dielectric structure coupled terahertz imaging method proposed in this thesis increases the resolution of conventional far-field THz imaging by an order of magnitude.In addition,the method is simple in operation,low in cost,and has no restrictions on the test sample(both dielectric and metal),and the method is a high-throughput and broad-bandwidth imaging technique,thereby improving the THz imaging resolution for the future.This work is of great significance for the improvement of the THz far-field imaging resolution.