Researchers have knots in light all tied up

Researchers from the Universities of Southampton and Glasgow have succeeded in tying knots in light beams.

Using a computer-designed hologram, they created threads of darkness embedded in a laser beam. The hologram bends the direction of optical energy flow, so these dark threads form loops. The loops can then be linked together, or tied into knots.

Dr Mark Dennis, a University of Southampton mathematician, worked in collaboration with Professor Miles Padgett, Dr Johannes Courtial, and Jonathan Leach in the Optics group in the University of Glasgow’s Department of Physics and Astronomy. Their findings are set out in a paper ‘Knotted threads of darkness’ which is published in Nature this week (11 November 2004).

Dr Dennis made the detailed calculations required to find the ideal mathematical form for the knotted laser beams while he was a Leverhulme Fellow based at the University of Bristol. He recently took up the post of Royal Society Research Fellow in the School of Mathematics at Southampton.

The role of the Glasgow group was designing the hologram that produced the exact combination of beams required to form the loops and knots. A laser was used to illuminate the hologram and the detailed structures recorded by a sensitive camera.

This work is experimental confirmation of earlier theoretical predictions made by Dr Dennis, with Professor Sir Michael Berry at the University of Bristol. Scientifically the work dates back to Lord Kelvin, who tried to formulate a theory for atoms made of loops and knots, embedded not within light, but in the fictitious ether.

Dr Dennis explains: ‘In the present day, the ability to synthesize such knots demonstrates the precise three-dimensional control it is possible to exert over light. These dark loops, links and knots are exciting structures in themselves. They could also be used as traps for quantum mechanical matter such as Bose-Einstein condensates which are matter waves on a macroscopic scale.’