Getting your polarisations in a twist (Image: Greenshoots Communications/Alamy)

Twist a two-dimensional strip of paper then tape its ends together and it transforms into a one-sided loop. It’s not magic; it’s a Möbius strip. These mathematical structures show up everywhere from M.C. Escher drawings to electrical circuits, but almost never in nature. Now, a team of physicists have shown for the first time that light can be coaxed into a Möbius shape.

“Light can kind of turn one-sided and single-edged under certain conditions,” says Peter Banzer of the Max Planck Institute for the Science of Light in Erlangen, Germany.

Banzer and his colleagues were following up on predictions made by Isaac Freund at Bar-Ilan University in Israel, who first suggested in 2005 that light’s polarisation, a property that describes how its electric field moves, could become twisted. If proved experimentally, the phenomenon could pave the way for fundamental studies of how light and matter interact, such as using light to trap tiny particles for biomedical purposes.


Let’s twist again

In 2010, Freund proposed a way to test this: prepare two polarised beams of light and allow them to interfere with each other in a particular way. The interference will cause the polarisation to twist, forming a Möbius strip.

Banzer’s team scattered two polarised green laser beams off a gold bead that was smaller than the wavelength of the light. The resulting inference introduced a polarisation pattern with either three or five twists, giving it a Möbius-like structure.

“These results are the first (experimental) proof that polarisation Möbius strips really exist, which has been a decade-long question in the community,” Banzer says. “These findings emphasise the richness of light and its properties.”

“The study is a brilliant tour de force at the cutting edge of optical technology,” says Freund. “The real significance of this study goes far beyond verifying a particular prediction, because it demonstrates that it is possible to measure the full three-dimensional polarisation structure of light.”

Journal reference: Science, DOI: 10.1126/science.1260635