Space-Time-Coding Digital Metasurfaces And Applications

Electromagnetic(EM)metasurfaces are artificial composite materials formed by periodic or aperiodic arrangement of sub-wavelength unit cells on a two-dimensional plane or curved surface.They have the advantages of ultra-thin thickness,extremely low loss,and high integration.Therefore,in recent years,metasurfaces have been extensively studied and used to control EM waves,and many novel physical phenomena and applications have been produced.As the digital version of metasurfaces,digital coding and programmable metasurfaces make it possible to study metasurfaces from the perspective of information science,in which binary codes are innovatively used to characterize the metasurfaces.Coding metasurfaces could greatly simplify the design and optimization procedures,and manipulate EM wave in real time by changing the coding sequences.The digital representation of coding metasurfaces also builds a bridge between the physical world and the digital world,which is convenient to be combined with related theories in the information science.This thesis systematically studies the general theory and practical applications of space-coding,time-coding and space-time-coding digital metasurfaces.The characterization parameters of the coding metasurfaces can be encoded in the space and time domains,which can manipulate the radiation or scattering characteristics of EM wave in the both space and frequency domains.Hence,coding metasurfaces can finally achieve multi-dimensional control of EM waves in the space,time and frequency domain.The main research contents and contributions of this thesis are summarized as follows:1)A Fabry-Perot resonant cavity antenna based on space-coding metasurface is proposed.The special structural design of coding metasurface makes the antenna obtain high gain and keep low radar cross section simultaneously.This new antenna has a compact structure and a low profile,and can achieve a low-scattering stealth effect in a wide frequency band.2)The principle of geometric phase in optics is used to design the space-coding metasurface,that is,the unit cells with the same structure but different rotation angles are used to construct multi-bit phase coding.Combined with the convolution operation of the scattering patterns,the space-coding metasurface based on geometric phase can realize flexible controls of circularly polarized waves,forming spin-controlled multibeams in free space,including vortex beams carrying orbital angular momentum and pencil beams with high-directivity.3)A transmission-reflection-integrated multifunctional space-coding metasurface is proposed.Thanks to the special design of multilayered anisotropic unit cells,this fixed space-coding metasurface can realize multiple different functionalities by changing polarization and propagation direction of the incident waves,including anomalous reflection,diffuse scattering,and vortex beam generation.This integrated design of space-coding metasurface can work in both reflective mode and transmissive mode,and hence simultaneously control the reflected and transmitted wavefronts on the same aperture.4)The concept and general theory of space-time-coding(STC)digital metasurfaces are proposed.The characterization parameters of the coding metasurfaces are encoded in both space and time domains,which can manipulate the radiation and scattering characteristics of EM waves in both space and frequency domains,i.e.to control propagation direction and harmonic spectrum distribution simultaneously.Hence,STC digital metasurfaces can finally achieve multi-dimensional control of EM waves in the space,time and frequency domains.Based on the proposed concept and theory,three illustrative examples of harmonic beam steering,beam shaping and scattering reduction are presented to demonstrate the characteristics and advantages of STC digital metasurfaces.5)Based on the theory of STC digital metasurfaces,Lorentz reciprocity and time reversal symmetry is broken by designing a suitable STC matrix.Combined with the dynamic programmability of digital metasurfaces,such reciprocity effects can be controlled dynamically and harmonic frequency conversion is also realized.6)Based on the theory of STC digital metasurfaces,a general method for realizing arbitrary multi-bit equivalent programmable phase based on a physical layer 2-bit programmable metasurface is proposed.Besides,an analysis method of vector synthesis is also presented to design the required phase value at specific harmonic frequencies.Specifically,the 4-bit and arbitrarily higher-bit coding phases are synthesized by designing 2-bit time-coding sequences.Beam deflection is realized at the fundamental frequency and positive first harmonic frequency by using the 4-bit equivalent phase,which shows the advantage of high-bit digital coding.7)Based on the theory of STC digital metasurfaces,a new information encoding scheme is proposed to construct a two-channel wireless communication system with space-and frequency-division multiplexing characteristics,in which the original information can be directly encoded at the metasurface level.Two different pictures are directly transmitted to two designated users located at different positions simultaneously and independently,without the need of digital-analog conversion and mixing processes in traditional technologies.The STC digital metasurface can realize the functions of information modulation and energy radiation at the same time,and can manipulate the spatial and frequency spectrum characteristics of electromagnetic waves,which greatly expands the application range of the metasurfaces.