Unidirectional transmission enabled by magnetic vortex in a compactly packed nanoparticle array

Abstract Substantial miniaturization without compromising optical performance remains a central challenge in nanophotonics. While densely packed nanoparticle array offers a promising platform, their potential is hindered by complex near-field coupling and the lack of precise diagnostic tools. Here, we decode the plasmonic coupling in a compactly packed three-dimensional array of silver nanoparticles by introducing magnetic vortex visualization in the H-plane as a new diagnostic standard. This approach reveals two distinct evolution pathways: a cascade mode, where magnetic vortices coherently span multiple nanoparticles, and a discrete mode, with vortices confined to individual particles. We demonstrate that the cascade mode harnesses collective dark modes to drive circulating displacement currents, generating magnetic dipoles orthogonal to the electric dipoles. Together, they form a complementary dipole pair that satisfies the Huygens-source condition, enabling highly unidirectional transmission with an exceptional front-to-back ratio of up to 29.1 dB and strongly directional forward emission. This work establishes compactly packed spherical array not only as an ultra-dense integration platform but as a functional medium for advanced wavefront control, opening avenues for miniaturized optical devices in beam shaping, sensors, and on-chip photonic circuits.