image: Figure 1 | Topological structure of toron. The isospin fibration structure of a photonic toron emerges from a red monopole, spirals through space, and vanishes into a blue antimonopole (left), with a structure similar to a rainbow pinwheel.
Credit: H. Wu, N. Mata-Cevera et al.
Imagine blowing on a toy pinwheel and watching the blades spin faster, slower, or suddenly reverse. Now replace the toy with pure light, shrink it to the width of a hair, and give it two microscopic “north” and “south” poles that can be steered at will. That, in essence, is what recently reported by a research team led by Prof. Yijie Shen from NTU Singapore.
Reporting in Physical Review Letters, the scientists sculpted the photon’s spin—the tiny twirl that gives light its rotational energy—into a three-dimensional knot called a toron. The knot looks like a rainbow pinwheel frozen mid-twirl, anchored at each end by elusive point-defect “monopoles” with only one pole each. Such three-dimensional topological excitations that combine point-defect monopoles with swirling skyrmion tubes, had only previouly been seen in liquid crystals. Surprisingly, now which can be made by light.
The trick lies in a tabletop device no larger than a shoebox. A laser passes through a programmable hologram (think of it as the world’s tiniest movie projector) that paints complex spirals onto the beam. By adjusting only two sliders on a computer screen—no lenses to realign, no crystals to swap—the researchers can make the pinwheel spin up, spin down, or gracefully dissolve into other shapes such as hopfions and skyrmion tubes.
Because the entire sculpture is encoded in light’s own rotation, it is virtually immune to dust, vibrations and even the wobble of the laser itself. That robustness makes torons promising “Lego bricks” for future optical circuits that could carry far more data than today’s fiber links, or act as invisible tweezers to steer nanoparticles inside living cells.
Even the researchers can’t resist a little show-and-tell. “We have videos where you watch the pinwheel untie itself into two floating monopoles, then re-tie into the opposite handedness—like a magic trick in slow motion,” says senior author Yijie Shen.
He added “For the first time, we have not only created a photonic toron but also experimentally realized a family of topologies—skyrmions, hopfions, and monopoles—using the spin of light, and we can deterministically switch among them via topological phase transitions. Unlike previous realizations that were confined to materials, these structures now live in free space and can be transported at will as novel optical information carriers.”
"I believe the findings should be interesting for a broad physics community not only in optics, light-matter interactions, optical information and communication. For instance, having an optical platform to generate and study texture topologies can be useful for areas such as condensed-matter physics, for if the singularities are topologically protected, they can mimic quasiparticles." commented by the PRL Referee
Their next step is to see whether the same pinwheel dance can be coaxed from sound waves, plasmons on metal surfaces, or even ultracold atoms, and interacted with material based torons, e.g. liquid-crystal torons. If so, the humble pinwheel may become the universal mascot for a new era of topological technology.
Journal
Physical Review Letters