Elusive Pure Anapole Excitation in Homogenous Spherical Nanoparticles with Radial Anisotropy

For homogenous isotropic dielectric nanospheres with incident plane waves, Cartesian electric and toroidal dipoles can be tunned to cancel each other in terms of far-field scattering, leading to the effective anopole excitation. At the same time however, other multipoles such as magnetic dipoles with comparable scattered power are simultanesouly excited, mixing with the anopole and leading to a nonnegligible total scattering cross-section. Here, we show that, for homogenous dielectric nanospheres, radial anisotropy can be employed to significantly suppress the other multipole excitation, which at the same time does not compromise the property of complete scattering cancallation between Cartesian electric and toroidal dipoles. This enables an elusive pure anopole excitation within radially anisotropic dielectric nanospheres, which may shed new light on many scattering related fundamental researches and applications.