太赫兹介质超表面实现双域多信道复用

Terahertz Dielectric Metasurface for Dual-Dimensional Multi-Channel Multiplexing

提出一种工作于0.25~0.35 THz的介质超表面,基于该超表面、结合轨道角动量(OAM)和频率域实现双域多信道复用,理论上可实现4×N(N为任意正整数)路携带不同信息的正交同轴波束的同时传输。超表面由介质柱阵列和介质基板构成,可基于PancharatnamBerry相位原理进行设计。选取0.25、0.3、0.35 THz三个频点进行仿真验证,结果表明:当3路具有不同频率和入射角的圆极化平面波沿4个不同方向斜入射至超表面上时,在垂直于超表面的方向上,12路交叉极化透射波被转换为彼此拓扑正交或频率正交的同轴波束,即结合OAM和频率域实现了12路信道复用。所提出的介质超表面在高速率大容量太赫兹通信领域具有潜在的应用价值。

Objective In recent years,the terahertz(THz)band has attracted extensive attention from researchers due to its potential of realizing highspeed and highcapacity wireless communication systems.The multiplexing technology has great research prospects in improving the communication rate and system capacity.The electromagnetic wave(EMW)carrying the orbital angular momentum(OAM)is called the OAM wave.OAM can be used as a new information carrier to provide an additional dimension for spatial multiplexing.The metasurface can effectively control the amplitude,phase,and polarization of EMW,and according to the main types of materials used,it can be divided into the metal and dielectric metasurfaces.Compared with the metal metasurface,the dielectric metasurface has the advantages of smaller ohmic loss,lower costs,easier processing and manufacturing,and higher transmission efficiency.Most previous research focused on generating an OAM beam or realizing OAM beam multiplexing by the metal metasurface,and hence,generating OAM beams and further realizing OAM beam multiplexing based on the dielectric metasurface have become the research hotspots.The methods of OAM beam multiplexing based on the dielectric metasurface have the disadvantages of OAM waves carrying the same information,the limited number of multiplexing channels,and the complexity and high cost of the communication system.Although the above problems can be effectively solved on the basis of the anglemultiplexed dielectric metasurface by converting multiple incident waves with different angles into orthogonal OAM coaxial beams,the current anglemultiplexed dielectric metasurface only works at a single frequency.Once the incident wave frequency changes,the generated OAM waves will deviate from the axis.In addition,the existing research focuses on the optical frequency band,and few studies combine two or more physical dimensions to achieve multiplexing.We need to study the realization of dualdimensional or multidimensional multichannel multiplexing in the terahertz band based on the dielectric metasurface and the expansion of the working bandwidth of the dielectric metasurface.Therefore,this paper proposes a dielectric metasurface,on the basis of which the dualdimensional multichannel multiplexing can be realized by the combination of OAM and frequency dimensions.Theoretically,the simultaneous transmission of 4×Nchannel(N is any positive integer)orthogonal coaxial beams can be realized.The proposed dielectric metasurface has potential application value in the field of highspeed and highcapacity terahertz communication.Methods First,the designed dielectric unit cells of the metasurface are composed of silicon pillars and substrates,and unit cells with different rotation angles are simulated on CST Microwave Studio.Periodic boundaries are set in the xaxis and yaxis directions;two Floquet ports are set in the zaxis direction,and the excitation is set as the left circularly polarized(LCP)wave in the negative direction of the zaxis.Then,for topological charges l1=0,l2=−2,l3=+2,and l4=−4,according to the theoretical formula,the required phase distribution of the proposed metasurface is calculated.After that,the metasurface is designed on the basis of the PancharatnamBerry(PB)phase principle and the dielectric unit cell.Then,to verify the designed metasurface,we take frequencies f1=0.35 THz,f2=0.3 THz,and f3=0.25 THz to calculate the corresponding incident angle simultaneously.Finally,farfield amplitude and phase patterns of right circularly polarized(RCP)transmission waves are simulated on CST Microwave Studio when threechannel circularly polarized(CP)plane waves with different frequencies and incident angles are obliquely incident on the metasurface in four directions.Results and Discussions When the rotation angleφrot of the unit cell is 0°,30°,70°,120°,and 180°,the variations of the simulated phase and normalized amplitude of the crosspolarized transmission coefficient TRL with frequency under LCP incident waves show that the 3 dB bandwidth of TRL of the dielectric unit cell is 0.1 THz,and the working frequency band is about 0.25-0.35 THz(with a relative bandwidth of 33.3%).In addition,as the rotation angle of the unit cell changes from 0°to 180°,the unit cell can cover the phase change of 0°-360°(Fig.2).Therefore,the metasurface can be realized on the basis of the PB phase principle and the designed dielectric unit cell.When LCP plane waves with the frequency f1 and angleθi(f1)are incident on the metasurface in±x and±y directions separately,simulated farfield amplitude and phase patterns of RCP transmission waves show that fourchannel beams are generated in the direction perpendicular to the metasurface.At the same time,according to the characteristics of OAM beam energy and phase distribution,the amplitude of one of the transmitted waves is in a solid distribution,with the phase unchanged,and thus,the topological charge l1 equals 0.For the other threechannel beams,the amplitude and phases are distributed in circular and spiral shapes,respectively,and when observed along the−zaxis,the phase changes by+4π,−4π,and+8πin a clockwise direction.In other words,the threechannel OAM beams with topological charges of l2=−2,l3=+2,and l4=−4 are generated.It can be seen that fourchannel topologically orthogonal coaxial beams with frequency f1 are generated in the direction perpendicular to the metasurface(Fig.5).Similarly,for fourchannel LCP plane waves with f2 andθi(f1),or f3 andθi(f3)incident on the metasurface along the±x and±y axes,four topologically orthogonal coaxial beams with frequency f2 or f3 in the direction perpendicular to the metasurface are generated(Figs.6 and 7).Conclusions In this paper,a dielectric metasurface working at 0.25-0.35 THz is proposed.When Nchannel CP plane waves with different frequencies and incident angles are obliquely incident on the metasurface in four directions,in the direction perpendicular to the metasurface,4×Nchannel crosspolarized transmission waves are converted into coaxial beams that are orthogonal to each other in topology or frequency,namely that the 4×N channel multiplexing is realized.For simulation verification,we assume f1=0.35 THz,f2=0.3 THz,and f3=0.25 THz.The simulations show that when fourchannel LCP plane waves with frequency f1,f2,or f3 are obliquely incident on the metasurface along the±x and±y axes,four topologically orthogonal coaxial beams with frequency f1,f2,or f3 in the direction perpendicular to the metasurface are generated.At the same time,the generated three groups of beams are orthogonal to each other in frequency.It can be seen that on the basis of the designed dielectric metasurface,12-channel incident waves are successfully converted into coaxial beams with topology or frequency orthogonality.In other words,12-channel multiplexing is realized by the combination of OAM and frequency dimensions.The designed dielectric metasurface has potential application value in the field of highspeed highcapacity terahertz communication.