Circular Huygens Dipoles: Unidirectional Spin-Angular Momentum from Achiral Nanoparticles

Abstract

Simultaneous control over the directionality and spin of light at the nanoscale is a central goal in nanophotonics with applications ranging from quantum information to advanced biosensing. We introduce the concept of the Circular Huygens Dipole and numerically demonstrate its realization in a single Si nanocuboid. We show that the polarization of an incident linear wave controls the interference between co-located circular electric and magnetic dipoles excited in phase quadrature. This enables deterministic switching of the forward-scattered radiation between purely right- and left-circularly polarized states. The system also functions as a directional spin-to-linear polarization converter. Our findings establish a robust, passive method for reconfigurable spin-directional control in a simple, monolithic silicon nanostructure, opening avenues for chip-scale spin-optics, chiral quantum interfaces, and novel sensing platforms.