| Metasurface,the 2D case of metamaterial,is composed of several meta-particles arranged periodically or quasi-periodically to manipulate electromagnetic(EM)waves artificially and flexibly.Compared with the bulky metamaterials,metasurfaces enjoy a series of advantages such as low-profile,low-quality,low-cost,easily conformal and integrated.Because traditional metasurfaces adopt the continual manipulations on EM properties,they can be regarded as “analog metasurfaces”.In 2014,Prof.CUI Tiejun originally proposed the concept of “digital coding metamaterials/metasurfaces”,which could represent EM characteristics via discrete digital states and control EM waves by coding sequences or coding patterns.This concept simplifies the procedure and reduces the difficulty in designing metasurface,providing more novel manipulating approaches.More importantly,coding and digital metasurfaces build the bridge between physics and information science,implying that the theories in digital signal processing can be implemented in metasurface designs.Furthermore,combined with the active devices and Field Programmable Gate Array(FPGA),digital coding metasurfaces can even realize real-time controls on EM information,indicating more potential applications in communicating,imaging and radar systems.The topic of this thesis focuses on novel digital coding metasurfaces,including theoretical analysis,meta-particle designs and functionality innovations.The main contents and contributions of this thesis are summarized as follows:1)Making some achievements in transmission-type metasurface especially digital coding metasurface.Firstly,an original dual-band transmissive meta-particle is proposed,working at 12 GHz and 18 GHz independently and simultaneously without any mutual influence,which successfully leads to a high-gain dual-band transmitarray.Then,inspired by a novel multi-layer metasurface structure,a novel broadband 1-bit transmission-type digital coding metasurface is proposed,which is available in the whole X-band from 8.1-12.5GHz.The proposed design can not only form a single highdirectivity beam with low side lobes in the wide band,but also achieve frequencydependent beam-scanning effect with a range of deflecting angle.2)Through manipulating reflection and transmission simultaneously,a novel concept of reflection-transmission amplitude-phase coding scheme is introduced,in which the amplitude codes “0” and “1” represent the full reflection and transmission.The proposal of amplitude codes and the combination with traditional phase codes can prominently improve the information capacity and control full-space EM information.Two approaches,polarization control and active-device control,are proposed to construct the amplitude-phase code,realizing the simultaneous or dynamically realtime manipulations reflecting and transmitting EM properties.3)To inprove EM manipulation capacities,amplitude coefficients are introduced in digital coding metasurface design.Firstly,an isotropic amplitude controllable digital coding metasurface is originally realized to manipulate the transmitting amplitude and phase simultaneously and independently without changing the working polarization.Two prototypes with different properties are accomplished accordingly,including a design with fully controllable amplitude and 1-bit phase codes,as well as 3-bit particles with very high transmittance.Then,it is improved to anisotropic design,which can manipulate amplitude and phase responses of both x-and y-polarized EM incidence simultaneously and independently.Furthermore,the working frequency can be tuned by varying the range of geometrical sizes.Illustrated by the full-wave simulations and experiments,this advanced design is capable to achieve digitally multi-beam radiation and multi-focal meta-lens.4)Inspired by the fundamental EM theory,the concept of complex codes is proposed,in which the whole complex phase part is taken into consideration to construct the phase codes.A novel addition theorem on complex codes is also established,which is especially important for programmable metasurface to construct complicated radiating patterns via only phase codes.On the unit-cell level,it means information addition of two complex codes;on the metasurface-system level,it means superposition of two coding patterns,implying that two far-field scattering patterns will appear simultaneously with bifunctions.The excellent performances in the theoretical analysis,full-wave simulations and measurements of several samples can verify the feasibility of addition theorem. |
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