| In recent years,optical imaging systems are developing rapidly towards the characteristics of integration,lightweight and miniaturization.The traditional refractive optical systems are based on geometric optics,and their performance mainly depends on the material dispersion,geometric shape,surface errors and other characteristics of the optical elements.Due to the reason above,the traditional refractive optical systems encounter many difficulties in adapting to the new requirements of integration,lightweight and miniaturization.Metalens,as a new kind of planar lightweight imaging element,has emerged many excellent research results thanks to its constituent units owning the ability to freely tailor electromagnetic response and the structural characteristics of adapting to micro-nano processing technology.However,the research of the metalens in long-wavelength infrared(LWIR)needs to be further enriched,especially for the question of improving the imaging quality when the aperture size of the metalens is enough large.Therefore,this thesis mainly focuses on improving the imaging performance of LWIR metalens.Through imaging measurement and theoretical analysis,it is concluded that the diffractive optical element characteristics of the metalens are the main factor that results in the decline of the imaging performance,especially for the metalens with a large diameter.Aiming at this problem,the following research are carried out.Some design strategies to improve the imaging performance of the LWIR metalens are studied in this dissertation,which are the all-Si achromatic metalens based on deep silicon etching,bandpass filter integrated metalens based on electromagnetically induced transparency(EIT)and the wide field of view(WFOV)Ge-based metalens.The main research work in this dissertation is as follows:1.The first part of the research content mainly focuses on the basic characteristics of the LWIR metalens.Firstly,the basic principle of the focusing effect and design method of the metalens are analyzed.Secondly,through systematic experimental exploration,the reactive ion etching(RIE)method of Ge-based microstructures is successfully mastered,the manufacturing process of the LWIR metalens with a large diameter is determined,and the automatic generation program of the mask pattern applicable to the metalens with a larger diameter is compiled.Finally,the imaging experiments of the LWIR metalens are carried out aiming at different targets in the different backgrounds.According to the measurement results,the reason why the imaging performance of the LWIR metalens with a large diameter declining is analyzed theoretically,and the simulation calculation is carried out to verify the conclusion.2.In order to solve the problem of the obvious diffractive chromatic aberration of the metalens,the design method of the LWIR achromatic metalens is studied in this dissertation.First,from the point of view of Fermat’s principle,we analyze the design method of the LWIR achromatic metalens.With a fixed aperture size,the value range of the equivalent optical thickness of the non-dispersive metasurfaces constructing the achromatic metalens determines the minimum f-number.The fabrication characteristic with high aspect ratio of deep silicon etching amplifies the difference value of optical thickness between different meta-atoms by increasing the propagation distance of the propagation mode,which ensures a small f-number to obtain a better imaging resolution.Secondly,A 280-μm-diameter silicon achromatic metalens with a f-number of 1 and the average focusing efficiency of 27.66% has been designed and simulated to validate the feasibility of this strategy.The height of the metasurfaces constructing the metalens is20 μm.The simulation results show that the maximum focal length deviation percentage from the target value between the wavelength of 8.6 and 11.4 μm is 1.6%.In contrast,the maximum focal length deviation percentage of the diffractive metalens with the same aperture,the same material,and the same f-number is 15.0%.This achromatic metalens design has good feasibility due to introduction of the process characteristics of deep silicon etching.3.A high Q-factor narrow-band bandpass filter integrated metalens based on EIT for LWIR imaging is designed to achieve a better imaging performance by eliminating stray light.Firstly,the design constraints of the LWIR EIT metasurface are established through analogy analysis using equivalent circuit method,and the proper LWIR EIT metasurface element is designed accordingly.Secondly,based on the same material system,we choose the all-dielectric low-profile Huygens metasurface as the subunits of the metalens.The introduction of the EIT metasurface into the design of the LWIR metalens achieves the design of the bandpass filter integrated metalens.Finally,with the purpose of validating the feasibility of this design method,we design a 300-μmdiameter integrated metalens whose f-number is 0.8 and the simulation is carried out.The introduction of EIT metasurface does not affect the focusing near the diffraction limit at the target wavelength,and greatly reduces the influence of stray light caused by non-target wavelength incident light.And the Q-factor corresponding to the center wavelength reaches 663.4.In order to solve the problem that the field of view of the metalens is too small,from the perspective of geometric optics,we regard the design of the phase equation of the metalens as the process of tailoring deflection angle of the ray passing through the different parts of the metalens,and the design method of the LWIR WFOV metalens is verified.Then,the advantages of selecting Ge as the base material of the metasurface are discussed.Finally,a 201 μm diameter LWIR WFOV metalens is designed to operate at the wavelength of 9.3 μm,and focal length and maximum angle of view are 50 μm and 30° respectively.The corresponding simulation is carried out.The focusing ability of the metalens is hardly influenced by the change of the incident angle,and it is relatively consistent in a wide field of view.The calculation results verify the feasibility of the design method. |
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