Spin-orbit interactions of light
Spin-orbit interactions of light “are usually associated with tight focusing, interfaces, anisotropic media, or nanostructures,” says Forbes. “In these cases, the spin and orbital degrees of freedom become coupled because the beam interacts strongly with a lens, a surface, or a material.”
One surprising part for the team was seeing a measurable spin-orbit effect within the paraxial (weakly focused/collimated) regime. “In free space, for example, the laser beams roughly maintain the same size,” Forbes says. “And even focused with lenses, the light bends at small angles relative to the direction of propagation/travel—so the effect isn’t caused by a material after the beam is prepared.”
It’s induced by propagation. “The Pancharatnam topological charge determines how the two circular polarization components evolve,” says Forbes. “These components then spread differently as the beam propagates, which creates a radial separation of opposite circular polarizations—essentially a free-space optical Hall effect.”
The beam begins with no local spin, but after propagation one handedness dominates near the center while opposite handedness dominates further out. By simply changing the sign of the topological charge, you can reverse this handedness pattern.
Theory, numerical modeling, experiments
The team’s work combines theory, numerical modeling, and experiments. “Our simulations show how the polarization ellipses, Stokes parameters, spin density, and Poincaré-sphere coverage evolve as the beam propagates,” says Nape, a senior lecturer and researcher at Wits School of Physics. “A useful way to visualize the result is that the beam starts with polarization states lying around the equator of the Poincaré sphere, which corresponds to linear polarization. As it propagates, the states move away from the equator—meaning circular and elliptical polarization components have appeared. Eventually, the beam can populate a much larger region of the sphere, which shows that a wide range of local polarization states has emerged.”
One of the most striking parts of their discovery of a hidden property of light is that “the beam starts with zero local spin and zero local chirality, but develops both just by propagating,” says Nape. “Our ‘aha’ moment was when we realized the Pancharatnam topological charge wasn’t just a label describing the beam—it actively controls the spin-orbit interaction. In other words: An integer winding number inside the input beam determines where left- and right-handed light appear later in propagation.”
This connects topology, spin, chirality, and propagation in a direct way. “If we change the topology, the beam’s local handedness reorganizes,” Nape says. “Importantly, this all occurs generically in free space.”