Toroidal dipole-induced transparency in core-shell nanoparticles

The scattering of nanoparticles plays a profound role in the recently flourishing fields of plasmonics and metamaterials. However, current investigations into nanoparticle scattering are based on the electric and magnetic resonances only, where their toroidal counterparts are usually not considered. The inclusion of toroidal terms can render new explanations for some fundamental scattering properties and thus may stimulate further breakthroughs in both scattering-related basic researches and applications. Here we revisit the most fundamental problem of Mie scattering by individual spherical nanoparticles and show that compared to conventional interpretations in terms of electric and magnetic responses, the roles played by their toroidal counterparts are indispensable. Based on the demonstration of efficient toroidal dipole excitation in homogeneous dielectric particles, we reveal that the extensively studied scattering transparencies of core-shell nanoparticles can actually be classified into two categories: (i) the trivial transparency with no effective multipole excitations and (ii) the non-trivial transparency induced by the destructive interferences of excited electric and toroidal multipoles. The incorporation of toroidal multipoles offers new insights into the study of nanoparticle scattering in both near and far fields, which may shed new light on many applications, such as biosensing, imaging, nanoantennas, photovoltaic devices, and so on.