| Metasurfaces are 2D planar artificial electromagnetic(EM)structures,which are comprised of sub-wavelength unit cells.They have attracted wide attentions in recent decades for their powerful abilities of manipulating EM waves and achieved a series of applications in the area of microwave and optics,including anomalous deflection,RCS reduction,holography,and ultra-thin lens.Furthermore,with the introduction of time-domain digital coding metasurfaces,metasurfaces obtain the capability of controlling EM harmonic distributions for the first time,and are gradually applied in the realm of harmonic beamforming,broadband spectra camouflage,Doppler cloak,and new-architecture wireless communications.In addition,some researches start to explore the possibility of controlling spatial EM waves' group delay to realize direct analog signal processing at the interface of EM waves and materials.Under this backdrop,this thesis mainly concentrates on the research of information modulating and signal processing based on metasurfaces.The main contents and contributions of this thesis are listed below:1.A mechanism to realize broadband and accurate control over harmonic amplitudes and phases is proposed.Specifically,the precise harmonic manipulation is achieved via loading digital coding sequences with different combinations of duty ratio and time delay to the metasurfaces.Meanwhile,dispersive effect of the reflection coefficients has no impact on relative harmonic distributions under the proposed coding strategy,hence enabling broadband harmonic manipulation.We design and fabricate a 1-bit digital coding metasurface operating at 22-33 GHz to conduct the experiment.The reflection phase of the metasurface is determined by the on/off state of the PIN diodes embedded in the unit cells.Finally,by loading voltage waveforms to the metasurface with the control platform,we realize extremely accurate control of harmonic amplitudes and phases as well as broadband harmonic beamforming in the experiment,thus verifying the proposed method.2.A high-order wireless communication system operating at millimeter-wave(mm Wave)band is built based on the above theory of harmonic manipulation.Concretely speaking,we can synthesize arbitrary constellation diagrams at the +1st-order harmonic by loading coding sequences with different combinations of duty ratio and time delay to the metasurfaces.In the experiment,we realize 256 quadrature amplitude modulation(QAM)along with other schemes in the broad frequency band of 27-29 GHz,vastly boosting the ability to modulate information of metasurface-based wireless transmitters.3.A method of designing spatial phasers is proposed based on metasurfaces.A phaser,also known as dispersive delay structure,is an integral part of analog signal processor.A phaser is a circuit,whose group delay can be engineered to obtain the desired response as a function of frequency.Traditionally,phasers have been implemented in circuits or waveguide structures,but they cannot deal with EM waves in free space due to limitation of natural materials.To resolve this drawback,a novel metasurface-based structure,spatial phaser,is proposed for analog signal processing in the spatial domain.To demonstrate the theory,we devise three spatial phasers with different group-delay responses and fabricate one of them to conduct the experiment.The experimental and simulated results agree well with each other.The work will facilitate the advancement of real-time spatial analog signal processing. |
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