Unidirectional sub-diffraction waveguiding based on optical spin-orbit coupling in subwavelength plasmonic waveguides

TL;DR

This paper demonstrates a method to achieve unidirectional sub-diffraction waveguiding in plasmonic structures using circular polarization, enabling polarization-controlled light routing without additional nano-antennas.

Contribution

It introduces a novel approach to break symmetry in plasmonic waveguides via spin-orbit coupling, achieving high unidirectionality controlled by light polarization.

Findings

Up to 94% of light directed into a single waveguide channel.

Unidirectional guiding is switchable by polarization.

No need for additional nano-antennas for direction control.

Abstract

Subwavelength plasmonic waveguides show the unique ability of strongly localizing (down to the nanoscale) and guiding light. These structures are intrinsically two-way optical communication channels, providing two opposite light propagation directions. As a consequence, when light is coupled to these planar integrated devices directly from the top (or bottom) surface using strongly focused beams, it is equally shared into the two opposite propagation directions. Here, we show that symmetry can be broken by using incident circularly polarized light, on the basis of a spin-orbital angular momentum transfer directly within waveguide bends. We predict that up to 94 \% of the incoupled light is directed into a single propagation channel of a gap plasmon waveguide. Unidirectional propagation of strongly localized optical energy, far beyond the diffraction limit, becomes switchable by polarization, with no need of intermediate nano-antennas/scatterers as light directors. This study may open new perspectives in a large panel of scientific domains, such as nanophotonic circuitry, routing and sorting, optical nanosensing, nano-optical trapping and manipulation.