Janus and Huygens' dipoles: near-field directionality beyond spin-momentum locking

Unidirectional scattering from circularly polarised dipoles has been demonstrated in near-field optics, where the quantum spin-Hall effect of light translates into spin-momentum locking. By considering the whole electromagnetic field, instead of its spin component alone, near-field directionality can be achieved beyond spin-momentum locking. This unveils the existence of the Janus dipole, with side-dependent topologically protected coupling to waveguides, and reveals the near-field directionality of Huygens' dipoles, generalising Kerker's condition. Circular dipoles, together with Huygens' and Janus sources, form the complete set of all possible directional dipolar sources in far- and near-field. This allows designing of directional emission, scattering and waveguiding, fundamental for quantum optical technology, integrated nanophotonics and new metasurface designs.