Study On Electromagnetic Coupling Characteristics Of Nanoantenna Based On Anapole Mode

Metal nanoantennas can localize light waves in nano-scale space,and show strong light absorption characteristics and local field enhancement characteristics in visible light band due to local surface plasmon resonance(LSPRs)effect.Therefore,it plays an important role in surface enhanced Raman scattering(SERS),fluorescence enhancement and other fields.However,the high intrinsic loss of metal nanoantenna limits its application.The corresponding dielectric nanoantenna is considered as a good substitute for metal nanoantenna because of its low loss.Particularly,dielectric nanoantennas with specific sizes support anapole mode under certain conditions,and the dielectric nanoantennas in this mode have attracted wide attention of researchers because of their unique strong near-field enhancement characteristics and extremely small far-field scattering characteristics.In addition,the excitation of anapole mode can highly localize the electric field in dielectric nanoantennas,which has stronger localized electric field enhancement than other dielectric nanoantennas supporting traditional Mie-type resonance modes.Therefore,the electromagnetic coupling characteristics of nanoantenna based on anapole mode are studied,which provides a way to further enhance the electric field intensity and realize high Q nano-optical devices.The research contents of this paper are as follows:(1)A dielectric-metal composite nanoantenna composed of slotted silicon disk and Au nano-dimer is designed.It is proved that it can significantly improve the electric field intensity of the nanoantenna system.By optimizing the reasonable structural parameters,the composite nanoantenna shows an enhanced resonant electric field intensity(> 3700 times)at the gap center of the dimer nanoantenna,which is 30 times higher than that of the single dimer nanoantenna.Using numerical simulation and multipole decomposition analysis,the far-field scattering characteristics and near-field electromagnetic field distribution are systematically studied,and the electric field enhancement mechanism is clarified.The results show that the high electric field intensity in the composite nanoantenna system is mainly attributed to the mode coupling between the plasma resonance of the gold nanoantenna and the radiation-free anapole in the slotted silicon nano-disk.The radiation attenuation rate of fluorescent molecules and the Raman enhancement factor of the composite nanoantenna are also calculated,which shows that the proposed composite nanoantenna can be used as an effective platform to amplify the radiation decay rate and Raman scattering intensity of fluorescent molecules.(2)A slotted silicon nanodisk array structure based on anapole mode is designed,considering the influence of structural elements on the electromagnetic coupling characteristics of the system.The results show that both EIT-like effect and Fano line pattern can be observed in the reflectance spectrum when the slotted nanodisks supporting anapole mode are placed in the array.Through near-field studies and theoretical calculations,two distinct spectral characteristics at the resonant position of the system are due to the hybrid coupling of cavity mode,anapole mode and diffraction mode in the system.Starting with the parameters on which EIT-like effect depends,the coupled mode theory(CMT)is used to prove that the formation and evolution of EIT-like linetype is essentially the coupling of modes at two resonant frequencies.In addition,by adjusting the groove length L,the reflectivity at the EIT-like position is changed.At the same time,the FWHM and quality factor at EIT-like position are calculated.We also explore the influence of disk radius and array period at fixed height.Finally,the sensitivity of the corresponding resonant position with the ambient refractive index is calculated,and the results show that the designed nanoantenna is expected to be applied to optical refractive index sensors.