Study On Near Field Enhancement And Far Field Characteristics Of Nanoantennas

Nanoscale manipulation of the directionality of light scattering by nanoantennas in the field of micro-nano optics is the cornerstone in a variety of applications such as surface-enhanced Raman scattering,chemical biological sensor,plasmonics solar cells and optical microscope.Additionally,the scattering of nanoantennas and radiation of the electromagnetic fields can interfere in the far-field,thereby modifying the radiation pattern of the emission.Therefore,the mechanism of directional scattering and the field enhancement effect are not well understood.To fully explore the localized field enhancement and far-field characteristics,including scattering efficiency of the light wave,the directionality of the scattering,and the polarization characteristics,the near and far-field properties of metal-dielectric core-shell nanodimers,all dielectric nanoantennas and metal-dielectric patch nanoantennas are investigated systematically by the simulation method based on the principle of LSPR.The relationship between multi-mode interaction of nanoantenna and far-field directional scattering provides theoretical guidance for the design and application of new nanoantennas.A kind of Au-ITO-Ag symmetry-breaking multilayered nanoshell dimers is proposed.The results show that the polarization direction and wavelength of incident light,the size of structure have a significant effect on the LSPR resonance mode and the far-field distribution.The dipole symmetry mode redshift as the radius of the gold core increases,and the resonant wavelength changes in the visible to near-infrared wavelength range.The analyzed electric enhancement contour indicates that the gap part of Ag outer nanocup has the most prominent electric field intensity.Symmetry-breaking multilayered nanoshell can not only regulate the extinction efficiency of light field but also affect the directivity of far-field scattering.All-dielectric nanocross dimer,hollow nanodisk and metal-dielectric core-shell nanodisk antenna are designed to explore the underlying mechanism of high magnetic field concentrations.The results of the far-field distribution show that the silicon nanocross dimer antenna and the hollow nanodisk can support multiple resonance modes such as electric dipole moment,magnetic dipole moment,electric quadrupole moment and magnetic quadrupole moment,realizing far-field unidirectional scattering.Besides,the response characteristics of the different resonant modes of the nanoantennas are modulated under the radiation of electric and magnetic dipole sources.The Purcell factor can be greatly enhanced,and it is shown that the radiation attenuation of MD emission can be significantly enhanced.The metal-dielectric core-shell nanodisk antenna greatly reduces metal loss and maintains LSPR properties.At specific wavelengths of 804 nm,924 nm and 952 nm,the multiple Kerker's type condition is satistied for achieving multi-wavelength far-field unidirectional scattering,which arises from the interference of different multipolar moments.The nanoantennas not only produces unidirectional scattering by far-field excitation,but also enhanced directionality for close-by dipole excitation.A PT symmetric system was constructed by introducing gain medium and absorption medium into the nanoantenna.The near and far-field characteristics of PT symmetric metal-dielectric core-shell nanocube are studied.The results show that by adjusting the imaginary part of the dielectric material of PT symmetric system,the far-field unidirectional characteristics of the nanoantenna can be effectively changed.When imaginary part k=0.38 and k=0.76,forward-scattering and back-scattering are realized respectively,which are characterized by dual-wavelength far field directional radiation.The properties of cavity modes in the PT-symmetric metal-dielectric-metal patch nanodisks and nanocube are explored with the excitation of electric dipole.It is found that the cavity modes could be selectively excited using a dipole source.The excitation of cavity modes is shown to be highly sensitive to the dipole position,which utilizes the symmetry principle and the degree of field overlap between the dipole source radiation field and the cavity mode pattern.Furthermore,the coupling between electric and magnetic resonance of PT-symmetric metal-dielectric-metal patch nanocube leads to unidirectional scattering and the realization of Kerker's type condition.Finally,it is possible to control the unidirectional emissions in opposite directions at different wavelengths by adjusting the imaginary part of the PT symmetric dielectric material.The research methods and results in this paper provide theoretical guidance for designing and manipulating optical properties of nanoantennas,and have some reference value for exploring application field of LSPR and improving the performance of biochemical sensors.