  Controlling directionality of emission, scattering and waveguiding is an
important requirement in quantum optical technology, integrated photonics and
new metasurface designs, as well as radio and microwave engineering. Recently,
several approaches have been developed to achieve unidirectional scattering in
the far-field relying on Huygens' dipolar sources, and in waveguided optics
based on spin-Hall effects involving circularly polarised electric or magnetic
dipoles, all of which can be realised with plasmonic or dielectric
nanoparticles. Here we show that there exists a dipolar source complimentary to
Huygens' dipole, termed Janus dipole, which is not directional in the
far-field, but its coupling to waveguided modes is topologically protected so
that it is allowed on one side of the dipole but not on the opposite side. The
near field directionality of the Huygens' dipole is also revealed and a
generalised Kerker's condition for far- and near-field directionality is
introduced. Circular electric and magnetic dipoles, together with Huygens' and
Janus dipolar sources, form a complete set of directional dipolar sources in
far- and near-field, paving the way for promising applications.
