Metasurface-based Electromagnetic Wave Modulation With Extended Near-field Coverage

Microwave near-field wave modulation can flexibly manipulate the amplitude,phase,polarization of electromagnetic(EM)wave,thus promoting the development of microwave near-field imaging detection,microwave wireless energy transfer technologies to the large-range,high-precision,and multiplexing directions.Expanding the near-field modulation coverage of the microwave component and integrating EM multiplexing performance are the the key factors driving the development and innovation of microwave near-field wave modulation.The traditional component requires more stringent material selection and processing accuracy,and the modulation coverage is limited to the narrow axial range in the paraxial region on one side of the component.In addition,the lack of EM multiplexing performance restricts its further application in complex scenarios such as high-quality EM near-field wave front reconstruction and energy allocation transmission.Compared with the traditional near-field component,metasurfaces can realize flexible regulation of EM parameters such as phase,amplitude,and polarization,and has the characteristics of flatness,easy-integration,and low-loss,which provides a new technical way for the near-field wave modulation to realize high-quality and multi-freedom wave front modulation.However,the existing array profile retrieval algorithms are only applicable to specific numerical aperture(NA)conditions,which makes it difficult to balance the transverse field-of-view/axial depth-of-focus coverage and the precise wave modulation.Additionally,constrained by the physical structure symmetry in the EM transmission direction,it’s difficult for metasurfaces to realize the asymmetric response under bidirectional EM excitation via the subwavelength periodic structure,which restricts the near-field modulation coverage expanding from half-space to full-space.This dissertation summarizes the EM response mechanisms,physical structure realization forms,and array profile retrieval methods in the existing literature on metasurfaces,and analyzes the development claims of microwave near-field wave modulation on the physical size,near-field coverage,and multiplexing performance of the components.From the perspective of EM multi-freedom modulation via metasurfaces,this dissertation explores the new array profile retrieval algorithms and multi-freedom modulation structure operating mechanisms,and investigates how to expand the transverse field-of-view/axial depth-of-focus covered by EM near-field wave modulation and how to expand the near-field modulation coverage to the bidirectional full-space via the subwavelength periodic structure.Firstly,this dissertation designs a variety of numerical experiments to verify the NA limitations of the Rayleigh-Sommerfeld diffraction theory and the weighted Gerchberg-Saxton algorithm(GSW)in the design for wave front reconstructing.Additionally,this dissertation introduces signal-to-noise ratio and structural similarity parameters to measure the noise level and accuracy,and compares the applicability and relative merits of the two methods under different typical conditions by means of theoretical analysis and numerical calculation.On this basis,this dissertation constructs a complete designing and analyzing process of “Array profile solving→Diffraction field calculating→Wave front quality measuring”,which provides theoretical guidance and methodological support for the subsequent research for expanding the microwave near-field wave modulation coverage.Secondly,the wave front accuracy of the existing metasurface in the non-paraxial region is constrained by NA,and the transverse field-of-view and noise level need to be improved.This dissertation investigates a complex-amplitude iterative algorithm,which is applicable to the accurate near-field modulation in wide field-of-view.By setting the relationship between complex-amplitude iteration and amplitude/phase iteration,the amplitude and phase profiles can be solved simultaneously.Among them,the introduction of the weighting factor varying with the diffraction field distribution can realize the dynamic adjustment of the metasurface profiles,which improves the iterative mechanism and enhances the wave front accuracy in the non-paraxial region,thus expanding the transverse field-of-view while ensuring the accuracy of the wave front.To validate the algorithm,this dissertation designs a compact Sine-shaped complex-amplitude modulation meta-atom in the microwave band,and constructs a wide field-of-view and high-precision metasurface,whose mean signal-to-noise ratio and structural similarity parameter are improved by nearly 23% and 17% compared with the conventional method.On this basis,this dissertation designs the frequency-multiplexed complex-amplitude modulation meta-atom,and tests the processed complex-amplitude modulation metasurface at 10 GHz and 13 GHz.Thirdly,to improve the applicability of the existing methods for large NA focusing design and expand the axial coverage of metasurfaces,this dissertation combines the phase iterative solution and polarization-multiplexing modulation method,and allocates the different focusing field to the independent polarization channels.In each channel,the phase iteration method relaxes the constraint of NA,thus realizing the expansion of the axial depth-of-focus.On the one hand,this dissertation improves the GSW algorithm,which constructs the multiplanar iterative mechanism by discretizing the quasi-3D target into the multiplanar cross-sectional field,achieving enlarged depth-of-focus energy focusing range.On the other hand,this dissertation designs a birefringent structure,which can achieve independent modulation of the horizontal-and vertical-polarizated transmission phase with the phase coverage of over 300°.On this basis,this dissertation designs a large-depth-of-focus,high resolution metasurface,and analyzes the axial near-field energy focusing performance of the metasurface at 10 GHz(experimental depth-of-focus of 41λ).Finally,this dissertation investigates the bidirectional asymmetric complex-amplitude modulation method by non-interleaved structures,i.e.,breaking the bidirectional symmetry of the EM transmission,unlocking the independent amplitude-phase modulation via the subwavelength periodic structure,and extending the near-field modulation coverage from the half-space to the full-space.This dissertation establishes the hybrid relationship between the composite geometric phase response and the propagation phase response,derives the decoupling relationship between the bidirectional transmission amplitude and phase,and constructs a bidirectional asymmetric complex-amplitude modulation model to guide the actual meta-atom structure design.On this basis,this dissertation designs a series of multiplexing,high-quality,full-space metasurfaces,and analyzes the near-field modulation performance in the functional form of energy allocation focusing and asymmetric wave front reconstructing(mean signal-to-noise ratio better than 14.71 d B).Furthermore,this dissertation investigates the transmission-reflection duplex model for the coplanar modulation scenario,and processes an asymmetric metasurface for wave front reconstructing with a peak signal-to-noise ratio better than 16.61 dB.