Scientists have created tiny “optical tornadoes” – swirling beams of light that twist like miniature whirlwinds using a surprisingly simple setup based on liquid crystals. Instead of relying on complex nanotechnology, the team used self-organising structures called torons to trap and manipulate light, causing it to spiral and rotate in intricate ways.
Even more impressively, they achieved this effect in light’s most stable, lowest-energy state, making it far easier to generate laser-like beams with these unusual properties.
Can light spin like a whirlwind? Researchers have now shown that it can. Scientists from the Faculty of Physics at the University of Warsaw, the Military University of Technology, and the Institut Pascal CNRS at Universite Clermont Auvergne have created swirling “optical tornadoes” inside an extremely small structure.
“Our solution combines several fields of physics, from quantum mechanics, through materials engineering, to optics and solid-state physics,” explains Prof. Jacek Szczytko from the Faculty of Physics at the University of Warsaw, the leader of the research group.
“The inspiration came from systems known from atomic physics, where electrons can occupy different energy states. In photonics, a similar role is played by optical traps, which confine light instead of electrons,” added Szczytko.
What is an optical vortex?
“You can think of it as an optical vortex,” says Dr. Marcin Muszynski from the Faculty of Physics at the University of Warsaw and Department of Physics City College of New York, the first author of the study.
“The light wave twists around its axis, and its phase changes in a spiral manner. Moreover, even the polarisation- the direction of oscillation of the electric field begins to rotate,” added Marcin.
These structured light states are attractive for applications such as quantum communication and controlling microscopic objects. However, producing them has typically required complicated nanostructures or large experimental systems.
Liquid crystals offer a simpler path
The team chose a different strategy. “Instead of building complex systems, we used a liquid crystal, a material with properties intermediate between a liquid and a solid.
Although it can flow like a liquid, its molecules arrange themselves in an ordered way, maintaining a fixed orientation and relative positions, much like in a crystal,” explains Joanna Medrzycka, a nanotechnology student at the Faculty of Physics, University of Warsaw, who, together with Dr. Eva Oton from the Military University of Technology, prepared the liquid crystal samples.
Within this material, special defects known as torons can form. “They can be imagined as tightly twisted spirals, similar to DNA, along which the liquid crystal molecules are arranged. If such a spiral is closed by joining its ends into a ring resembling a doughnut, we obtain a toron,” Medrzycka explains.
“These structures act as microscopic traps for light. A key step was creating an equivalent of a magnetic field for photons. Although light does not respond to magnetic field like electrons do, a similar behavior can be achieved for light by other means.”
A “synthetic magnetic field” for light
“Spatially variable birefringence, that is, the difference in the propagation of different polarisations of light, acts like a synthetic magnetic field,” explains Dr. Piotr Kapuscinski of the Faculty of Physics at the University of Warsaw.
“We call it ‘synthetic’ because its mathematical description resembles the behavior of a magnetic field, even though physically it isn’t there. As a result, light begins to ‘bend,’ much like electrons moving in cyclotron orbits.”
To strengthen the effect, the toron was placed inside an optical microcavity, a structure made of mirrors that repeatedly reflects light and keeps it confined for longer periods. “This makes the field much stronger,” says Dr. Muszynski. “Additionally, we can control the size of the trap, and thus the properties of the light, using an external electric voltage.”
Stable light vortices in the ground state
The most striking result came next. “In typical systems, light carrying orbital angular momentum appears in excited states,” explains Prof. Guillaume Malpuech from Universite Clermont Auvergne and CNRS. “For the first time, we managed to obtain this effect in the ground state, i.e., the lowest-energy state. This is significant because the ground state is the most stable and the easiest for energy to accumulate in.”
“This makes it much easier to achieve lasing,” emphasises Prof. Szczytko. “Light naturally ‘chooses’ this state because it is associated with the lowest losses.” To confirm this, the researchers introduced a laser dye into the system.
“We obtained light that not only rotates but also behaves like laser light: it is coherent and has a well-defined energy and emission direction,” says Dr. Marcin Muszynski. (ANI)
