| How to effectively convert between the propagation field and the radiation field has always been a key issue in wireless communication systems.An optical antenna which is a medium for electromagnetic waves,can effectively convert the propagation field energy and the local field energy to each other in the sub-wavelength range.Since the core-shell optical antenna contains both dielectric and metal materials,it can support both electrical resonance and magnetic resonance modes.In addition,it also can support more complex electromagnetic resonance mode by changing the size and material of the core-shell nanostructure such that it can achieve directional radiation,near-field enhancement and enhance the radiation efficiency of quantum source and so on.Therefore,the research of the shell-core optical antenna can have great academic significance and practical value in quantum light sources,near-field microscope,biosensing,and so on.This thesis aims to explore the near-filed and far-field properties of hybrid material shell-core structure nanosphere with a notch optical antenna,asymmetric core-shell structure coupled optical antennas,and symmetrical multi-layer nanostructure coupled optical antennas.This thesis carries out the following studies:(1)A metal-dielectric-metal multilayer core-shell structured nanosphere optical antenna is designed and optimized to achieve the excitation of electromagnetic multipole moments and obtain directional radiation with ultra-narrow beam widths.This type of antenna is made by making a notch on the surface of the shell-core nanosphere,and placing the dipole source in the notch for near-field excitation.The influence of antenna geometric parameters and dipole source position on the radiation performance of the antenna is analyzed based on Mie scattering theory,multilayer scattering theory,and numerical simulation software system.Under the optimized parameters,the maximum directivity of the antenna is more than 13 with about 60%radiation efficiency,while the beam width is narrow under 35~0.Comparing with array optical antennas and other single-structure optical antennas,this antenna has compactness and flexibility design and also has high directivity.(2)An asymmetric core-shell nanosphere coupled optical antenna is proposed,and the mechanism and scattering performance of the optical antenna are systematically analyzed.In addition,the front-to-back ratio of the antenna can be greatly enhanced,and the maximum front-to-back ratio of the antenna can be achieved with the maximum scattering intensity by adjusting the width of the gap in the coupled antenna,while the antenna has low energy loss.Besides,comparing with single core-shell nanosphere antennas and all-metal coupled antennas,it can be seen that the electric dipole and magnetic multipole moment in the asymmetric hybrid material coupled antenna effectively couple in the gap.(3)The properties of symmetric multilayer metal-dielectric-metal nanosphere coupled optical antennas are researched.This antenna has double coupling effects.One coupling effect is the surface plasmon resonance coupling effect in the single multilayer nanosphere and the other coupling effect is between the nanospheres.This type of optical antenna can have the lager near-field enhancement at different frequencies under the excitation of the plane wave and the radiation characteristics of total reflection under the excitation of the dipole source with adjusting the structural parameters of the optical antenna and the position parameters of the dipole source. |
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