Research On Dynamic Control Characteristics Of Directional Scattering Field Based On Nano-antenna Multipole Coupling

Due to the optical nano-antenna can excite abundant resonance modes,it has become a hotspot in the field of micro-nano photonics.The optical nano-antenna has a wide range of applications in laser,solar cell,photoelectric detection,biomedicine and near-field optics,which makes it become an important part of modern nano photonic devices.In the study of the optical properties of nano-antennas,the light scattering excited by nanoparticles is an important branch.The complex and interesting optical resonance modes supported by nano-antennas provide more possibilities for the design of nanostructures and the control of scattering field.Studying the interaction between electromagnetic multipoles is the basic method to analyze the light field,and it is particularly important to effectively control the scattering field directivity based on the excitation of electromagnetic resonance.Therefore,the research on the regulation of scattering field characteristics of optical nano-antenna based on multipole coupling plays a very important role in the field of micro-nano photonics.Based on the multipole expansion theory,this paper analyzes the electromagnetic resonance response of the optical nano-antenna and the coupling between the multipoles,and studies the manipulation of the scattering field characteristics.Firstly,we designed a kind of nano-antenna with a sector-shaped hole in the silicon disk structure,which can generate the magnetic dipole resonance along the wave vector direction of the incident field under the excitation of plane wave.The wavelength,resonance intensity and phase of the magnetic dipole resonance can be adjusted by adjusting the size parameters.Secondly,the nano-antenna is designed by the phase change materials whose refractive index can be dynamically adjusted by changing temperature,and then the coupling effect between the electromagnetic resonance can be affected,furthermore the forward/backward unidirectional scattering of the incident light field will be effectively controlled.The main contents of this paper are as follows:(1)The multipole expansion method of nanoparticles with different structures in free space and in the presence of dielectric substrate is verified.By calculating the multipole decomposition(to the third order)of the induced polarization current in the scatterer of nanoparticles with arbitrary shape,the corresponding multipole expression of the scattering field can be obtained,and finally the contribution of the electromagnetic multipole in the scattering spectrum is obtained.As the interaction between multipoles is the essential reason that affects the characteristics of the scattering field,the multipole expansion theory is an important tool to analyze the scattering field.This paper shows the part of the calculation results,which is in good agreement with the results of the relevant references,and verifies the correctness of the programmed codes of multipole expansion theory.(2)A nano-antenna with a sector-shaped hole in the silicon disk structure is designed to generate a magnetic dipole moment along the wave vector direction of the incident field when the plane wave is normal incidence.The research analyzed the electromagnetic resonance response characteristics of the nano-antenna,and summarized the influence of the structure size parameters on the electric/magnetic dipole resonance.By adjusting the amplitude and phase of the magnetic dipole and electric dipole resonance,it is expected to realize the directional scattering of the far field.(3)Two kinds of nano-antennas are designed by using phase change materials,one is a simple spherical structure composed of Ge Te,the other is a core-shell structure composed of gold and Ge Te.By adjusting the structure parameters and the crystallinity of the material,the relationship between the amplitude and phase of electromagnetic resonance and the coupling between them are analyzed.Finally,by adjusting the crystallinity,the direction of the scattering field can be adjusted at the same wavelength.Therefore,the scattering field directivity of the designed nano-antenna can be dynamically adjusted by changing the temperature of the phase change material.