  Directional excitation of guidance modes is central to many applications
ranging from light harvesting, optical information processing to quantum
optical technology. Of paramount interest, especially, the active control of
near-field directionality provides a new paradigm for the real-time on-chip
manipulation of light. Here we find that for a given dipolar source, its
near-field directionality can be toggled efficiently via tailoring the
polarization of surface waves that are excited, for example, via tuning the
chemical potential of graphene in a graphene-metasurface waveguide. This
finding enables a feasible scheme for the active near-field directionality.
Counterintuitively, we reveal that this scheme can transform a circular
electric/magnetic dipole into a Huygens dipole in the near-field coupling.
Moreover, for Janus dipoles, this scheme enables us to actively flip their
near-field coupling and non-coupling faces.
