UEA Team Shows Structured Light Acquires Spin Angular Momentum in Free Space via Geometric Phase

The experiments used vector vortex beams prepared with balanced orbital and polarization degrees of freedom. As the beams propagated, the intrinsic geometric phase accumulated differently across the transverse plane, converting initially zero net spin into distinct left- and right-handed domains. This occurs because the beam's transverse momentum distribution couples the spin and orbital angular momenta through the Berry curvature of the light field itself. The result overturns the textbook assumption that significant spin-orbit interactions require tight focusing or anisotropic media.

Prior work on spin-orbit effects in light, such as the 2016 Nature Photonics paper by Bliokh et al. on transverse spin, relied on evanescent waves or high-numerical-aperture optics. The UEA result shows the same physics can be accessed in paraxial free-space propagation when the initial state is prepared on the Poincaré sphere equator. This opens routes to chiral light sources that avoid lossy metamaterials, directly relevant to next-generation polarimetry for pharmaceutical enantiomer detection.

The finding aligns with emerging geometric-phase metasurface research but bypasses fabrication entirely. If scalable, it could simplify generation of high-purity twisted light for quantum key distribution protocols that encode information in both orbital and spin angular momentum. A key open question is whether the effect survives atmospheric turbulence at the levels encountered in satellite links.

Follow-on experiments at UEA and collaborating South African labs are already testing whether the induced spin pattern can be used for label-free detection of chiral molecules at concentrations below current commercial circular-dichroism limits.