Research And Application Of Time-domain Metasurface

Coding metasurfaces are functional surfaces that are designed and characterized from the perspective of digital information.The emergence of coding metasurfaces greatly simplify the design procedures of the microwave devices by only optimizing the coding sequence of the meta-atoms.In addition,some traditional concepts in signal processing are introduced in the design of metasurfaces,bringing new functionalities and phenomena as a result.Since2014,much attentions have been paid to the spatial coding metasurfaces,resulting in a number of exciting applications such as beamforming,holography,ultra-thin lens,diffusion surface,etc.In this dissertation,a new kind of coding metasurface is developed in the time domain,namely,the time-domain digital coding metasurface.Its operation mechanism is intensively investigated through theoretical derivation,numerical simulation,and experimental verification.The potentials for wireless communication are also discussed in detail.The main contents and contributions of this dissertation are summarized as follows:· The theory and the design method is presented for the time-domain digital coding metasurface.The mechanism of electromagnetic manipulation by the coding metasurfaces with time-varying characteristics is discussed in detail.Some unique functionalities are revealed,such as nonlinear harmonic generation and frequency conversion.In the experiment,a reflective metasurface is designed with good phase tenability that depends on the biasing voltages on the loaded varactor diodes within the meta-atoms.A customized control platform is utilized to output the biasing voltage waveforms.The experimental results show that the metasurface can regulate the harmonic amplitude of the reflected wave as expected,which verifies the correctness of the theoretical model.· Based on the time-shift characteristic of Fourier Transform theory,an independent harmonic amplitude and phase regulation method is proposed by adding a proper delay to the control signal of the time-domain digital coding metasurface.Combined with the traditional spatial coding strategies,it is possible to control the scattering patterns and harmonic intensities simultaneously.A prototype reflective metasurface is designed for demonstration.By applying different control signals to the metasurface,the feasibility of independent harmonic amplitude/phase control is clearly demonstrated in the experiment.· An efficient frequency synthesis method based on time-domain metasurface is proposed,which takes the linear time-varying reflection phase to efficiently generate the harmonic of the specific order while greatly suppress the remaining harmonics.The energy conversion efficiency is used as an important measure to evaluate the frequency synthesis performance of the metasurface,which is proved to be very high from the measurement results.· A simplified architecture of wireless communication transmitter is proposed based on time-domain digital coding metasurface,in which the baseband signals are loaded on the carrier wave by controlling the time varying reflection coefficients of the metasurface.Two kinds of transmitters with different modulation schemes are designed respectively to verify the modulation ability and evaluate the communication performance.The real-time video transmission is demonstrated via the proposed system in a real indoor scenario.· A method of realizing multi-modulation schemes for wireless communication is proposed by utilizing the time-domain digital coding metasurface.The baseband signals are modulated on the carrier wave with high energy conversion efficiency by controlling the linear time-varying reflection phase and the additional time delay of the control signal at the same time.High-order modulation schemes can also be implemented similarly with this method.During the experiment,three modulation schemes are implemented respectively,and the communication performance is also measured and analyzed.