Research And Design On Flat Wide-angle Metalens For Passive Millimeter-wave Imaging

At present,passive millimeter-wave focal plane array(PMMW-FPA)imaging technology has been rapidly evolving and innovating towards large field-of-view(FOV),high resolution,real-time imaging,miniaturization,and low cost,and applied in many important scenarios,such as security screening,non-destructive evaluation,non-invasive medical diagnosis,and remote sensing detection,etc.Wide angular FOV,diffraction-limited focusing,compact wideband focusing block is the key factor for PMMW-FPA imaging system to realize high-resolution real-time imaging over a large FOV while maintaining its miniaturization and low cost.The traditional dielectric lens has a special surface topological structure,large volume,heavy weight,complex processing and assembly,and is affected by Seidel aberration including Spherical aberration,Coma,Astigmatism,Petzval field curvature,and Distortion.Compared with the traditional dielectric lens,metalens can enable to flexibly manipulate the amplitude,phase,and polarization state of the electromagnetic(EM)wave at the subwavelength scale,and in turn provides a new development opportunity for the PMMW imaging technology and its large-scale application.However,angle-dependent Seidel aberrations severely limits the angular FOV and the EM focusing performance of metalens.Spherical metalens or metalens doublet can effectively suppress Seidel aberrations,but increases its profile size and difficulties in fabrication,assembly,and system integration.Ultra-thin flat metalens is easy to process and integrate,but has the relatively narrow angular FOV,reduced focusing performance,and low focus ing efficiency.After extensive analysis of the existing wide-angle metalens in terms of concrete realization form,focusing performance,design methodology,and analysis method,etc.,take the PMMW-FPA imaging technology as the application background and the flat wide-angle metalens as the research object,by means of theoretical analysis,numerical calculation and experimental measurement,this dissertation focuses on how to effectively suppress Seidel aberration,improve the focusing performance,and expand the angular FOV of the metalens while maintaining its flat structure configuration of subwavelength thickness,and further how to create the required metalens accurately and efficiently.The main contents of this dissertation are as follows:Firstly,in order to verify that geometric optics(GO)and Helmholtz-Kirchhoff diffraction integral have some deviations in the design and analysis the near-field metalens operating at the quasi-optical low frequency,a flat metalens irradiated by a Ka-band pyramidal horn antenna is designed by the GO,processed,and measured.The measured results are further compared with the numerical calculation ones based on Helmholtz-Kirchhoff diffraction integral and the three-dimensional full-wave simulation ones based on time-domain finite integral technique.In order to further verify the focusing performance of the designed metalens and its applicability in PMMW imaging,a single channel PMMW imaging camera is constructed,and then its spatial resolution test and imaging exp eriment are carried out.Secondly,in order to improve the accuracy and efficiency of the metalens design and analysis,this dissertation explores the scattering mechanism of the subwavelength unit cell on the metasurface and each point on the target field,then mathematically models the metasurface and the target field based on the equivalent dipole radiation wavelet model,and further formulates dipole-based wavelet superposition method while theoretically deriving its effective computational domain.In order to verify the validity and accuracy of the proposed method in the design and analysis of metalens,in this dissertation,the full-wave simulation and the numerical calculation methods(Huygens’ s principle,Huygens-Fresnel principle,and the proposed method)are adopted to compare and analyze several typical numerical experiments on the metalens design and analysis.Thirdly,in order to suppress Seidel aberration,improve the focu sing performance,and expand the angular FOV of the metalens,this dissert ation starts from the propagation of the EM wave to derive and analyze the intrinsic mechanism of generating the focal spot array with consistent focusing performance over a wide angular FOV by the metalens,and then based on the planning and management of the EM waves through the metalens this dissertation proposes a high-efficiency flat wide-angle metalens optimization design method,i.e.the incident-exit angle pairing method(IEAPM),and further carries out the theoretical analysis and mathematical modeling of its optimization design priciple.Then,this dissertation constructs a subwavelength regular hexagon slots with compact structure and superior EM response,designs a dual-linear polarization conical corrugated horn antenna,optimally synthesizes a high-efficiency,diffraction-limited focusing,wide angular FOV,and wideband flat metalens by the IEAPM,and further compares and analyzes the 35 GHz and wideband(33 GHz-37 GHz)focusing performance of the designed metalens by using numerical calculation and experimental measurement.Finally,in order to further expand the angular FOV of the flat metalens and improve its focal spot array consistency,on the basis of in-depth exploration on the anaberration metalens doublet and the Fraunhofer diffraction pattern of diffraction aperture,this dissertation proposes a highly-consistent focusing flat wide-angle metalens optimization design method,i.e.the virtual diffraction aperture method(VDA),and further carries out the theoretical analysis and mathematical modeling of its optimization design priciple.On this basis,a highly-consistent focusing flat wide-angle metalens for PMMW-FPA imaging is optimally designed by adjusting the position,size,and shape of the virtual diffraction aperture and the field p rofiles on it,and further compares and analyzes the 35 GHz and wideband(33 GHz-37 GHz)focusing performance of the designed metalens by using numerical calculation and experimental measurement.Aiming at suppressing Seidel aberration of the metalens,imp roving its focusing performance,and in turn creating a flat wide-angle metalens,in this dissertation,an accurate and efficient numerical calculation method for the metalens design and analysis is formulated,two optimization design methods for creating the flat wide-angle metalens are proposed,and these methods as well as the designed metalenses are numerically analyzed and experimentally verified.The research in this dissertation provides theoretical support and new design ideas for the optimization design and rapid prototyping of the flat wide-angle metalens,and further contributes to promoting the development and innovation process of the PMMW-FPA imaging system towards large FOV,high-resolution,high quality,real-time imaging,miniaturization,and low cost.