Directional sources realised by toroidal dipoles

TL;DR

This paper explores the use of toroidal dipoles as a novel mechanism for achieving directional optical sources, demonstrating their potential to replace electric dipoles and enable tunable, asymmetric light coupling in nanophotonics.

Contribution

It introduces pseudo directional dipoles based on toroidal dipoles, expanding the design space for directional optical sources beyond conventional electric and magnetic dipoles.

Findings

Toroidal dipoles can replace electric dipoles in directional sources.

PDDs enable tunable asymmetric coupling with waveguides.

New degrees of freedom for optical directionality control.

Abstract

Directional optical sources can give rise to the directional excitation and propagation of light. The directionality of the conventional directional dipole (CDD) sources are attributed to the interference of the electric and/or magnetic dipoles, while the effect of the toroidal dipole on optical directionality remains unexplored.} Here, we numerically and analytically investigate the directional properties of the toroidal dipole. We show that the toroidal dipole can replace the electric dipole in the CDD sources to form the pseudo directional dipoles (PDDs), which can be applied to achieve analogous near-field directional coupling with a silicon waveguide. Moreover, the directionality of the PDDs can be flexibly controlled by changing the geometric parameters of the toroidal dipole, leading to tunable asymmetric coupling between the sources and the waveguide. These new types of directional sources provide more degrees of freedom for tailoring the optical directionality compared to the conventional sources. The results open new possibilities for directional light manipulation and can find applications in on-chip optical routing, waveguiding, and nanophotonic communications.