| Due to their novel electromagnetic properties that have not been observed in nature,metasurface,composed of the artifical electromagnetic microstructure,have become one of the hot research topics in optical and materials science.These properties are mainly attributed to the geometric structure of the metasurface,and adjusting properly the electromagnetic metasurface' geometrical features,one can achieve properties such as negative refractive index,perfect absorption,and enhanced optical nonlinearities.Driven by these properties,many novel applications have been proposed,including the anomalous refraction and reflection,beam shaping,transformation between propagating waves and surface waves and holograms.Therefore,understanding the optical properties of the basic elements(such as near-field enhancement properties,phase generation mechanism,etc.)is a prerequisite of designing functional metamaterials and metasurfaces.In addition,considering the high-dimensional structure parameter space of the basic element and the technical limitations of industry,the introduction of optimization algorithm provides the possibility to obtain the suitable structure parameters quickly and efficiently.Starting from the optical field control of the artifical electromagnetic microstructure,this thesis aims to design and optimize different functional devices by using the metasurface.This thesis carries out following studies:The properties of metasurface are determined by the physical properties of its basic elements,which can be regarded as nanoantennas.Based on that,the scattering ability of a metal-dielectric multilayer patch nanoantenna is discussed.Combining the multipole expansion method and the mode field properties,it is shown that the structure supports electric dipole(ED)or electric multipole and magnetic dipole(MD)components.By adjusting the geometric parameters and material material,low loss directional scattering can be achieved by the interaction between the multiple modes.Furthermore,a scheme based on geometric parameter optimization to realize low loss superior directional scattering at one or multiple wavelengths has been proposed.The results suggest that combining a well-established electromagnetic solver and a custom optimization algorithm,the geometric parameters of the structure that meet the Kerker condition or anti-Kerker condition at one wavelength or directional scattering in opposite directions at different wavelengths can be achieved.Based on those results,the influence of a substrate on the directional scattering properties of the metal-dielectric multilayer patch nanoantenna is studied.It is shown that although the substrate will affect the directional scattering of the nanoantenna,it does not fundamentally alter the optical properties.Lastly,by using the optimization algorithm,the directional scattering of the nanoantenna situated on a substrate can also be optimized.In the microwave band,the mode properties of cylindrical metal structures corrugated by very long,curved grooves are studied.And a Huygens metasurface is designed according to the electromagnetic response.The unit of the metasurface consists of two corrugated metal disks with the same radius placed up and down.By altering the geometry and permittivity of the filled material,the MD mode of one disk have nearly the same resonant frequency with that of the ED mode of the other.The results show that when the vertical distance between the two disks is small,one of them will induce the other to resonate,which will lead to the mode hybridization between the same modes.Then the EDs and MDs are separated from each other,resulting in no significant directional scattering and the transmission of the metasurface composed of this basic elements is very low.While if the vertical distance between the two disks is large enough,the near-field mode hybridization disappears.a nearly zero backscattering happens due to the coherent effect of ED mode and MD mode in the far field and the transmission of the metasurface composed of this basic elements is approximate to 1.By using frequency selective surface(FSS),a full optimization procedure for designing mantle cloaks enclosing arbitrary objects is proposed.The accuracy and efficiency of the design method are evaluated by comparing several methods.The results show that the optimized result of the mantle cloak for the infinite cylinder is almost the same as the analytical solution.And for the random two-dimensional object that can not resolved by theory,the optimized result is consistent with that of the parameter scanning method,while the calculation times are far less than that of the parameter scanning method.Then,by showing the cases of using different FSS to design mantle cloak for several complex two-dimensional(three-dimensional)objects,the universality of the design scheme is proved.A full-color holographic imaging technology using geometric metasurface is proposed.Starting from the concept of Jones matrix,the basic principle of geometric metasurface is discussed.Particularly,the transmission of the unit cell is optimized.Using Fraunhofer diffraction theorem to calculate the hologram of monochromatic image,the importance of both phase and amplitude modulation is emphasized by comparing with only phase modulation.Furthermore,the holograms of two-dimensional and three-dimensional full-color images based on the holographic multiplexing are studied.And polarization,position,and angle can be regarded as the channel keys.The research shows that the full-color holographic imaging design based on holographic multiplexing can perfectly reconstruct the original object without increasing the complexity of design and production.In summary,starting with the Bayesian optimization of the resonance characteristics for the basic unit of the metasurface elements and considering the left and right effects of the array comprehensively,this thesis proposes an optimization method of metasurface for specific functions based on different optimization algorithms.And it has proved that this kind of method has a good application in the improvement of transmission and reflection of metasurface,the research of holographic metasurface and the design of FSS for mantle cloak.This kind of optimization algorithm can be further extended to the fast and efficient design of more functional metasurface. |
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