Excellent Anapole by Decoupling Electric Multipoles of Ag/Si Core-Shell Nanoparticles

The interplay of Mie's multipole resonances provides a huge opportunity for discovering new phenomena in nanophotonics and developing nanophotonic devices of new functionalities. One of the keys to this task is to control the spectral overlap or separation between resonances, for which Ag/Si core-shell nanostructures, that is, nanospheres and nanodisks, are proposed here. The Ag core and the dielectric shell have different roles in dominating spherical electric dipole (ED) and electric quadrupole (EQ) resonances with different variational trends to the change of the structural parameter such as the core radius. Thus, the EQ and ED may be decoupled in a great spectral separation, such that an excellent anapole can be achieved and lies in a broad valley of the scattering spectrum. The anapole mode displays a perfect far-field radiation pattern of the Cartesian ED and toroidal dipole with destructive interference and radiates more than 70 times less than that of the solid nanodisk of the same size. Only one tightly focused radially polarized beam is needed for illumination in order to avoid excitation of magnetic-type resonances. This work of the spectral characteristics of Ag/Si core-shell nanoparticles provides detailed physical insight into experimental demonstration of nonradiating scatterers and suggests a scheme to study the particular electromagnetic modes in a hybrid nanostructure.