Toroidal-dipole-assisted realization of directional sources by coupled chiral particles

Conventional directional dipoles (CDDs), which consist of electric and/or magnetic dipoles that satisfy specific amplitude and phase conditions, can give rise to directional coupling and radiation in both the near and far fields. Theoretically, a toroidal dipole can replace an electric dipole of the CDDs and form pseudo-directional dipoles (PDDs), providing more degrees of freedom for directional manipulation of light. However, the realization of PDDs in practical structures has yet to be reported due to the difficulty of achieving desirable toroidal dipoles. Here, we employ coupled helices to achieve PDDs and demonstrate flexible manipulation of light propagation in a silicon waveguide. By tuning the incidence and helix geometry, we can control the relative amplitude and phase between the toroidal dipole, electric dipole, and magnetic dipole. This enables the realization of pseudo-circular dipole, pseudo-Huygens dipole, and pseudo-Janus dipole, providing three types of directional sources with distinct near-field properties. The near-field directionality of these sources can be tuned by changing the separation distance between the helices and the waveguide, leading to tunable asymmetric excitation of guided light. The proposed realization may facilitate the design of directional sources and switches in photonic integrated circuits, with broad applications in on-chip information processing and optical communication.