  Nanoparticles supporting a distinct series of Mie resonances have enabled a
new class of nanoantennas and provide efficient ways to manipulate light at the
nanoscale. The ability to flexibly tune the optical resonances and scattering
directionality are particularly essential for various applications ranging from
biosensing to nanolasers. In this paper, we investigate the core-shell
nanoparticles that support both electric and magnetic Mie resonances and for
the first time systematically reveal the mode evolution from a pure high-index
dielectric nanosphere to a pure plasmonic one, where it has a sudden transition
when core-shell ratio increases from 0.4 to 0.5. Furthermore, by engineering
the electric and magnetic resonances, we demonstrate the unidirectional forward
and backward scattering in such a system and reveal its tunability via
geometric tuning.
