Light can be coaxed to tie itself into knots, but new research suggests that the knot-tying can happen spontaneously – under the right conditions. Australian National University physicists have produced a model from physics and mathematical concepts that generates optical vortices with dark cores in a bright laser beam, which tangle into knots. The light knots have potential applications in quantum computing, laser beams and advanced modern optics. “Apart from their curiosity value, what’s really interesting and useful about these knots of darkness is that they show you what the power flow is doing,” said study co-author Dr. Anton Desyatnikov at ANU’s Nonlinear Physics Centre. “It is part of the incredible progress science is making in the field of optics; we’re beginning to do things with light that would have once seemed impossible.” An initially deformed vortex-free soliton develops vortex lines during propagation in a self-focusing saturable medium. (Green arrow is the optical Z-axis). Here we distinguish isolated vortex rings (unknots) in pink, linked vortex rings (Hopf links) in green and orange, and vortex (trefoil) knots in blue. Unlike previous research that produced knots in light artificially, the new research focuses on models in which knots form spontaneously, similar to the tangles in power cords. Unlike electrical cables, light does not form knots easily, Desyatnikov said. “Scientists have found that inducing knots to form in laser beams by introducing perturbations in the form of laser speckle only very rarely induces knots,” he said. “Our models suggest that you have to get the key parameters of the light in a certain range before you can easily tie the light in knots, but once you do, the knots are virtually guaranteed.” It is not possible, to predict where exactly the knots will form, he said. “Just that under these specific circumstances, the optical vortices will spontaneously nucleate and tie themselves into little knots.” The study appeared in Scientific Reports (doi: 10.1038/srep00771). For more on “Light Tied in Knots,” visit www.photonics.com/a40798.