Wits and UEA researchers reveal a new free-space route to chiral light for sensing and photonics

Chirality is the property that makes something left- or right-handed. It is important in chemistry, biology and medicine because two molecules can have the same composition but behave very differently in the body. In pharmaceuticals, one version of a molecule may work as intended, while its mirror image may be less effective or even harmful.

 

Because of this, scientists use chiral light to study chiral matter. In the new study, published in Light: Science & Applications, the researchers show that the interaction between light’s spin and twist can be controlled by the beam’s own internal topology.

 

“This is why chiral light is widely used to probe chiral matter in techniques such as optical activity and circular dichroism,” says Dr Kayn Forbes, one of the co-principal investigators from the University of East Anglia.

 

The team focuses on two key properties of light. The first is spin, linked to the handedness of light’s polarisation. The second is twist, created when light is shaped into an optical vortex with a corkscrew-like structure.

 

“Unlike polarisation, these twists are not limited to a simple left/right pair — light can twist by many integer amounts, creating a much larger alphabet for information encoding,” says Forbes.

 

For years, scientists believed these spin-orbit effects were too weak to observe under normal conditions and needed special materials, metasurfaces or very tight focusing to become visible.

 

“To make them visible, researchers have generally relied on anisotropic materials, metasurfaces, or extremely tight focusing. These enhanced regimes enable striking effects, such as particles spinning or orbiting differently depending on where they sit in the beam,” says Dr Isaac Nape of the Structured Light Laboratory at Wits.

 

The new work challenges that view.

In experiments at Wits, MSc student Light Mkhumbuza and colleagues show that these effects can appear naturally as a specially prepared beam travels through free space. The team starts with a structured beam whose polarisation changes across the beam, even though it has no circularly polarised component at the start.

 

“As the beam propagates, its components naturally evolve so that spin accumulates locally and separates into different regions, effectively revealing spin where none was initially present,” says Mkhumbuza.

 

Nape says the real breakthrough lies in topology, a built-in property of the beam that remains preserved as the beam changes during propagation.

 

“You can smoothly remould a mug into a donut without tearing or gluing, so the number of holes stays the same,” he says, explaining the idea through a well-known topology example.

 

He says the same principle helps explain what is happening in the beam. “The Pancharatnam topological index characterises the beam’s global polarization-phase topology, acting like a conserved topological fingerprint that reveals itself as the beam propagates.”

 

The researchers say this gives scientists a simpler and more flexible way to control light for chiral sensing and optical manipulation. It could also support new ways of encoding information in both the polarisation and spatial structure of light, with possible use in classical and quantum communication.

 

The work highlights a growing research partnership between South Africa and the United Kingdom in advanced photonics and quantum science, and points to practical ways light can be controlled for future sensing and information technologies.

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