  Unidirectional backward and forward scattering of electromagnetic waves by
nanoparticles are usually interpreted as the interference of conventional
multipole moments (i.e., electric and magnetic dipole, electric quadrupole,
etc.). The role of toroidal dipole moments in unidirectional scattering is
generally overlooked. In this work, we investigate the unidirectional
scattering for the system of three plasmonic nanospheres. It is found that the
unidirectional backward scattering is caused by the interference between the
toroidal dipole moment and other conventional multipole moments. Tunable
primary backward and forward scattering can be achieved under some specific
configurations. Our results can find applications in the design of
nanoantennas.
