UEA sees new route to chiral sensing and optical manipulation

A collaboration between the University of East Anglia (UEA) and the University of the Witwatersrand (Wits) has developed a new route to generating and controlling light's chirality and spin angular momentum.

Described in Light Science & Applications, the findings could lead to novel opportunities for tunable optical manipulation and chiral sensing.

"Light's spin and orbital angular momentum (SAM and OAM) are both are inherently chiral," wrote the project in its paper. "Together they form the total angular momentum of light, a central concept in modern photonics."

Chirality is also a significant factor in the imaging of biological specimens and tissues, and in the study of pharmaceutical molecules, so for researchers the generation of chiral light and control of its properties in a structured light beam is a key concern. This control is often achieved using novel light-matter interactions, because the spin-orbit effects behind these properties are too weak to observe under normal conditions.

The new project aimed to investigate the control of a light beam's topology without the need for special materials, metasurfaces or very tight focusing. It focused on two key properties of light: spin, linked to the handedness of light’s polarisation; and 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," commented Kayn Forbes from the University of East Anglia. "Light can twist by many integer amounts, creating a much larger alphabet for information encoding."

A simpler and more flexible way to control light

In experiments at Wits, researcher Light Mkhumbuza and colleagues showed that these effects can appear naturally as a specially prepared beam travels through free space. This approach 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," said Mkhumbuza.

Isaac Nape of the Structured Light Laboratory at Wits commented that the real breakthrough lies in topology, a built-in property of the beam that remains preserved as the beam changes during propagation. A parameter termed the Pancharatnam topological index is crucial, indicating how the polarization states in light are structured and acting as a tunable parameter for control of that structure.

"You can smoothly remould a mug into a donut without tearing or gluing, so the number of holes stays the same," said Nape, drawing a common analogy. "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, commented the international project team, and points to practical ways light can be controlled for future sensing and information technologies.