Topological Control of Chirality and Spin with Structured Light

TL;DR

This paper demonstrates that structured light with engineered topological properties can control spin angular momentum and chirality purely through intrinsic topology, revealing a free-space optical Hall effect driven by topological mechanisms.

Contribution

It introduces a method to control spin-orbit interaction and chirality in free space using higher-order Poincaré modes without material interfaces or tight focusing.

Findings

Modulation of topological charge drives spatial separation of circular polarization states.

Identifies differential Gouy phase shifts and radial divergence as mechanisms for the effect.

Reveals spin-orbit effects in free space without non-paraxial conditions.

Abstract

Structured light beams with engineered topological properties offer a powerful means to control spin angular momentum (SAM) and optical chirality, key quantities shaped by spin-orbit interaction (SOI) in light. Such effects are typically regarded as emerging only through light-matter interactions. Here, we show that higher-order Poincar\'e modes, carrying a tunable Pancharatnam topological charge $\ell_p$, enable precise control of SOI purely from the intrinsic topology of the light field, without requiring any material interface. In doing so, we reveal a free-space paraxial optical Hall effect, where modulation of $\ell_p$ drives spatial separation of circular polarization states - a direct signature of SOI in a regime previously thought immune to such behaviour. Our analysis identifies two propagation-induced topological mechanisms underlying this effect: differential Gouy phase shifts between orthogonal components, and radial divergence of the beam envelope. These results overturn the common view that spin-orbit effects in free space require non-paraxial conditions, and establish a broadly tunable route to generating and controlling chirality and SAM without tight focusing. This approach provides new opportunities for optical manipulation, chiral sensing, and high-dimensional photonic information processing.