| Recently,more and more attention has been paid to the researches and the applications of millimeter wave(MMW)/terahertz(THz)technologies.MMW/THz band has the characteristics of large absolute bandwidth and small wavelength.It can be used to realize high-speed/ultra high-speed wireless communication and high resolution imaging radar.In addition,THz waves also have broad application prospects in imaging,safety inspection,material identification,and medical imaging.In a variety of MMW/THz systems,devices for transmitting and collecting electromagnetic energy,directly determined the performance of the systems,e.g.,communication capacity,transmission distance,and imaging quality.In these systems,conventional parabolic antennas,dielectric lens antennas,parabolic mirrors and other structures with empty feed structures are commonly used.However,these devices are limited by their large volumes,heavy weights,non-planar structures,and requirements of high fabrication accuracy.They are thus hard to be used in the compact MMW and THz systems.As alternatives,transmitting and reflecting metasurfaces based on periodic structures,such as reflectarrays,transmitarrays,flat lenses and mirrors are good candidates for the MMW/THz applications,because they not only have the advantages of no feeding network,ease to obtain high gain and subwavelength focus,but also have the characteristics of small size,easy fabrication,flexible design,low cost and flat structure.Therefore,it is very important to study the MMW/THz metasurfaces based on periodic structures and to fully explore their potential technical advantages and applications.Based on these purposes,this thesis mainly studies the devices based on periodic structure such as high-gain MMW/THz transmitarray antennas,frequency-controlled THz focus scanning mirrors,and THz beam splitters,as well as their potential applications.The main work and innovations of this thesis are as follows:1.In MMW band,especially the THz band,the high-gain antennas are always need high fabrication precision and high cost.To overcome these issues,newly transmitarray antennas are proposed based on the 3D printed and printed circuit board(PCB)technologies.Finally,low-cost high-gain MMW/THz transmitarray antennas as well as frequency-controlled beam-steering transmitarrays are realized.Firstly,a MMW transmitarray antenna using 3D printing technology is proposed.A periodic matching layer is designed at the interface between the dielectric element and the air to improve the transmiting efficiency and antenna gain.A gain of 23.5dBi is achieved by the designed transmitarray antenna at 60 GHz.Besides,by using the multifrequency matching method,a 3D printed transmitarray with the ability of frequencycontrolled beam scanning is realized in 60 GHz frequency band with a 15° scanning angle.Then,the design and fabricated methods are applied to the 275 GHz frequency band.The fixed beam and beam-scanning 3D printed transmitarrays are designed to achieve a gain of 27 dBi and a scanning range of 12°,respectively.Afterwards,two kinds of flat elements are proposed for designing THz transmitarrays which have simple structures and low precision requirement.The first one is with two metal layers and the second one is with three metal layers.Coventional PCB technology is employed to fabricate them,and the results show that gains of 28.8dBi and 28.7dBi are achieved by the two kinds of transmitarray at 250 GHz,respectively.2.For the purposes of low-cost and rapid THz imaging,three kinds of frequencycontrolled focus scanning mirrors are proposed which provide good solutions for the lowcost and rapid THz imaging.First,a one-dimensional(1D)focus-scanning mirror is designed,by analyzing the relationship between the desired phase-frequency curve and focus scanning coverage range,the principle of the focus scanning mirror is revealed.Then,a dual-resonance element is used for phase compensation in the frequency band of 225-300 GHz,The results of the designed mirror show that the focus scanning coverage of 8mm is achieved in the frequency band.Furthermore,the design principle can be extended to implement longitudinal scanning or a wideband REM without focus dispersion.Secondly,to expand the application field of the focus scanning mirror,a twodimensional(2D)focus scanning mirror is proposed.The desired phase range of the proposed mirror is much larger than the 1D one,so here a series of three-resonance element are proposed for the phase compensation and the results show that 2D focus scanning is achieved in the frequency band of 225-300 GHz.Finally,in order to enlarge the coverage area of focus scanning,a dual-band focus scanning method is proposed,with the low frequency band of 220-260 GHz,and the high frequency band of 370-430 GHz.In the design,the focus positions in each frequency band can be controlled independently,thus 1D focus scanning can be achieved in each frequency band.By combining them,a 2D region can be covered.3.Two kinds of beam splitter with wavefront-controlled abilities are proposed in this thsis to improve the system of terahertz beam splitter and make the design of terahertz systems more flexible.The two split beams of the first kind of beam splitter are operating in reflection and transmission states,respectively.Two kinds of elements are proposed for this purpose,this first one can achieve simultaneous control of the transmitted and reflected wavefronts.A four-layered element is proposed based on the first kind of element,the beam splitter based on the proposed four-layered element can control the transmitted and reflected wavefront independently with reflected waves deflected in the 20° direction and transmitted waves focused in the near field at 250 GHz.Then,the second beam splitter is proposed with two transmitted beams deflected to different directions and another four-layered element is proposed for this purpose.The transmission phases of x-and y-polarized waves can be tuned by different parameters of the elements.Furthermore,a beam splitter is designed at 1THz based on the proposed element.It is fabricated by the standard micro-fabrication method and measured.The results show that the x-polarized waves are deflected to 25° in the xz plane and the ypolarized waves are deflected to 30° in the yz plane.In addition,the proposed four-layered element can be used for polarization conversion.The polarization convertors based on the proposed element are designed operating at 1THz with the abilities of linear polarization to circular polarization or linear polarization to its cross polarization,and the proposed design is verified by experiment. |
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