Researchers discover a new way to control light in empty space

Light does not usually surprise people. It travels, it reflects, it bends. In labs, researchers can twist it into more exotic forms, but that has often required special surfaces, unusual materials, or intense focusing with powerful optics.

This time, the surprise came from free space itself.

Scientists at the University of East Anglia, working with colleagues in South Africa, report that light can develop a kind of handedness as it moves through empty space, without mirrors, materials, or special lenses shaping it along the way. The work, published in Light: Science & Applications, points to a new way of controlling light by using its internal geometry.

That matters because handedness, also called chirality, sits at the center of chemistry and biology. Many molecules, including some used in medicines, come in left- and right-handed forms that can look nearly identical while behaving very differently in the body.

“Our work shows that light can naturally develop this handed behaviour all on its own,” said Dr. Kayn Forbes from UEA’s School of Chemistry, Pharmacy and Pharmacology. “You just have to prepare it in the right way.”

The study focuses on structured light, a type of light whose shape, brightness, and direction are arranged in precise ways.

One version of structured light can twist like a corkscrew as it travels. Researchers call that an optical vortex. Light can also spin depending on how it is polarized, and that spin can have a left-handed or right-handed form. Those two features, the twist and the spin, belong to what physicists describe as the angular momentum of light.

For years, interactions between the spin and twist of light were thought to be very weak unless researchers used tightly focused beams or carefully designed materials. In the new work, the team argues that this picture missed something important.

They found that when a beam is prepared in a carefully balanced state, spin can emerge naturally as the light propagates through free space in the paraxial regime, the gentler regime where light is not tightly focused.

“It starts off with no spin at all,” explained MSc student Light Mkhumbuza, who carried out key experiments. “But as the beam travels forward, spinning regions appear and separate out, almost as if the spin was hiding and then revealed itself.”

That change did not require any extra surface or medium to trigger it. The light simply moved forward.

The explanation, the team says, lies in topology, the branch of mathematics that deals with properties that stay the same even when shapes stretch or deform.

Dr. Isaac Nape of the University of the Witwatersrand in Johannesburg offered a familiar image: “To explain it, imagine a mug and a doughnut. You can morph one into the other without tearing it, because they both have one hole. That hole is a topological feature.”

In the light beam studied here, the hidden feature is a topological fingerprint linked to how the beam’s polarization and phase wind together. The paper identifies a quantity called the Pancharatnam topological index, written as ℓp, as the key control parameter. According to the researchers, that index stays embedded in the beam and quietly steers how the light evolves during propagation.

At the starting plane, the beam has no local spin and no optical chirality density. But as it travels, the two circular components of the beam evolve differently. They pick up different Gouy-phase and divergence behavior, and that pushes them into different radial regions. The result is a measurable separation between left- and right-circular polarization components and the appearance of local spin.

“This gives us a completely new tuning knob for light. By adjusting its topology, we can decide how and where chirality appears,” said Dr. Nape.

The team tested beams with different values of the Pancharatnam topological charge, including 1, -1, 2, and -2. They found that the sign of the charge switched which handedness dominated near the center of the beam, while the magnitude changed the radial profile of the spin density.

The experimental setup used horizontally polarized Laguerre-Gaussian modes and a q-plate to prepare the desired beam. The researchers stress that the q-plate was a convenient preparation tool, not the source of the effect itself. They say other beam-shaping methods, including spatial light modulators and interferometric schemes, could also create the same kind of initial state.

What mattered was the topology built into the beam at the start.

When ℓp equaled 0, the far-field measurements showed no spin separation. Once ℓp was not zero, right- and left-circular components separated in the beam, producing what the paper describes as a topology-driven optical Hall effect. The researchers also reconstructed how the polarization states filled out the Poincaré sphere as the beam propagated. At the start, only linear polarization states appeared. Farther along, the beam gradually occupied the full sphere, showing that all possible spin states had emerged.

The paper argues that this is not the same as earlier demonstrations where spin splitting was already built in at the source plane. Here, both circular components began with identical radial amplitudes and zero local spin. The spin and chirality appeared during propagation because of the Pancharatnam topological index alone.

That claim is one of the main reasons the work challenges long-standing assumptions in optics.

“For something so familiar, light is proving to be far richer, stranger and more powerful than anyone imagined,” said Dr. Forbes. “And astonishingly, this new behaviour has been there all along, just waiting to be seen.”

The practical appeal is easy to see. Chirality-sensitive light plays a role in identifying molecules, probing biological systems, and manipulating tiny particles. The usual methods often depend on fragile materials or demanding optical setups. A material-free route could simplify some of that work.

Dr. Forbes said the research “could lead to simpler and more sensitive medical tests, especially in drug development.”

The team also points to optical sensing, data transmission, optical manipulation, and quantum communication. Because the method lets researchers tune spin and chirality with a single topological parameter, it could offer a compact way to encode information in light without relying on precision-engineered surfaces.

The study also notes possible uses in polarization-tailored trapping and manipulation, high-dimensional information encoding, and chiral sensing.

This work suggests that some future optical tools may become simpler to build and easier to tune. Instead of depending on exotic materials or strong focusing, researchers may be able to prepare a beam once and let free-space propagation do the rest. That could help in chiral sensing, where left- and right-handed molecules need to be distinguished, and in communication systems that pack more information into structured beams.

The findings also offer a new design rule for photonics. If the topology of a beam can predict when spin and chirality will appear, engineers could use that to build new light sources, sensors, and information systems around geometry itself.

The paper also includes important limits. The authors did not directly measure the orbital angular momentum content of each polarization component. They also note that some polarization ellipses seen at the source plane likely came from experimental errors induced by the waveplates, which they say could be improved through calibration.

And although the beams later show transverse profiles similar to optical skyrmions, the study does not aim to engineer optical skyrmions or reproduce earlier paraxial skyrmion constructions.

Research findings are available online in the journal Light: Science & Applications.

The original story "Researchers discover a new way to control light in empty space" is published in The Brighter Side of News.

Like these kind of feel good stories? Get The Brighter Side of News' newsletter.