Digitalized Coding Metasurface And Its Applications

Metasurfaces are plannar artificial materials and are composed of an array of subwavelength scatterers periodically distributed on a two-dimensional plane.Various manipulations to electromagnetic(EM)wave can be easily realized through the equivalent electric/magnetic surface impedances and abrupt phase discon-tinuities,such as anomalous reflection/refraction,absorption,cloaking,polarization conversion,etc.Like metamaterials,the unit cell of conventional metasurface is still characterized by effective medium parameters with continuous value(homogeneous or inhomogeneous).Hence,they can be considered as an analogy to the analogue circuit from the circuit perspective.To impelement digital-type metasurface,Prof.Tie Jun Cui from Southeast University proposed the concept of digitalized coding metasurface in 2014,which could manipulate the EM wave through pre-designed coding sequences.The emergence of coding metasurface have signifi-cantly simplified the design procedure and increased the degree of freedom in the manipulation of EM wave in more aspects.Most importantly,it has become a bridge linking metasurfaces with information science,making it possible to combine most exsiting algorithms in digital signal processing with coding metasurfaces.This thesis focuses on the theoretical analysis,numerical simulations,sample fabrication,experimental ver-ification and practical applications of coding metasurface,showing its powerful manipulation to EM wave with a series of fascinating new materials and devices.The main contents and contributions of this thesis are summarized as follows:1)Based on conventional metasurfaces,we design a quad-band terahertz absorber by distributing four metallic square loops on two polyimide layers,and a broadband terahertz absorber by arranging multiple metallic bars on three polyimide layers.Both designs are experimentally demonstrated at terahertz frequency,showing high absorption and wide incident angle tolerance.2)Based on the scattering cancellation technique,we design and experimentally demonstrate an ultrathin mantle cloak featuring tunable cloaking frequency at microwave frequency by employing varactor diode.By tuning the applied DC volatege,the cloak can adaptively change its cloaking frequency,in which the bi-static Radar Cross Section(RCS)of an electrically small conducting cylinder can be effectively suppressed in a certain observation plane.Making a careful investigation of the surface impedance required to minimize each Mie scattering term,we elaborately design an ultrathin mantle cloak using interdigital structure and experimentally evaluate its performance in reducing the RCS of an electrically large conducting cylinder at microwave frequency.3)To avoid the blockage of radiation caused by the feeding antenna in reflection-type coding metasur-face,we seek to design a transmission coding metasurface at terahertz frequency using triple-layered split ring resonator structures.The ultrathin and flexible sample fabricated at terahertz frequency features excel-lent antiwear property and corrosion resistance.Owning to the free-standing design,the measured results shows much higher efficiency in refracting terahertz wave compared with previously reported designs.4)We design a frequency-dependent dual-functional I-bit coding metasurface based on the orthogo-nality of different frequencies,and experimentally verify its independent manipulation of terahertz waves at two distinct frequencies.Using numerical simulations,we discuss the influence of metal conductivity and dielectric loss on the reflection amplitude of coding particle,and analyze the mechanism of super unit cell in helping obtain accurate phase responses.The appearance of back scattering in the radiation pattern is also explained from the perspective of Fourier transform.5)We propose the theory of anisotropic coding metasurface,and experimentally demonstrate a 2-bit anisotropic coding metasurface at terahertz frequency by designing a dumbbell-shaped structure that has in-dependent digital state for the x and y polarizations.The anisotropic coding metasurface can have independent manipulations to EM wave under two orthogonal polarizations.Later,we continue to study the conversion from spatial wave to surface wave using anisotropic coding metasurface,and propose a compensation coding sequence which allows the anisotropic coding metasurface to have exactly the same radiation pattern under oblique incidence.6)We propose the theory of tensor coding metasurface which could take full control of the EM wave including polarization and wave vector.A rectangular-shaped split ring resonator structure is designed as the coding particle to constitute a tensor coding metasurface,which can manipulate the polarization of EM wave and convert spatial wave to transverse electric(TE)or transverse magnetic(TM)mode surface waves.We present the first experimental demonstration for the conversion from spatial wave to surface wave under the normal illumination of terahertz wave.A rigorous derivation of the dispersion relation of the EM wave in an infinitely large dielectric board is provided,which serves as a cross validation of the TM and TE mode surface wave observed in both simulations and experiments.7)We initially introduce convolution operation from digital signal processing into the design of coding metasurface,and propose a new coding scheme which can rotate a radiation pattern to a desired direction with negligible distortion.The radiation can continuously scan the entire upper-half space with only a limited number of coding digital states,which overcomes the restrictions of descrete radiation angles supported by the previous coding scheme.Based on this coding scheme,we present the design of a coding metasurface antenna that can generate cone-shaped radiation pattern with arbitrary shape,as well as a controllable random surface.These new devices and materials possessing exotic physical phenomena will have potential applications in many fields including wireless communication,radar detection,diffusion materials in both EM and acoustic fields.8)We propose a field programmable metasurface based digital wireless communication system with an entirely new architecture.A channel estimation algorithm and channel optimization algorithm are proposed to estimate and optimize the available digital states of the communication system in real-time.More detailed analyses are provided for the modulation order under different sampling numbers and background noises.In addition,the advantage of secrete communication of the system is discussed.9)We propose to measure the amount of information carried by a coding metasurface using the in-formation entropy.We reveal the proportional relationship between the geometrical entropy and physical entropy,which are defined to evaluate the amount of information for the coding pattern and far-field pattern,respectively.These new findings provide a theoretical foundation for the further study of the programmable metasurface based wireless communication system.