Can light rotate like a whirlwind? Researchers have shown that it is possible. Scientists from the Department of Physics at the University of Warsaw, the Military Institute of Technology, and the Pascal CNRS Institute at the University of Clermont-Auvergne have created a “light tornado” that swirls inside an extremely small structure. This advance represents a new way to build compact light sources with complex geometries, which could support simpler and more scalable photonic devices for optical communications and quantum technologies.
“Our solution combines several fields of physics, from quantum mechanics to materials engineering, optics and solid-state physics,” explains research group leader Professor Jacek Szczytko from the Department of Physics at the University of Warsaw. “Inspiration comes from systems known in atomic physics where electrons can occupy different energy states. In photonics, optical traps that trap light instead of electrons play a similar role.”
What is a vortex of light?
“You can think of it as a vortex of light,” says Dr. Marcin Muszynski, lead author of the study, from the Department of Physics at the University of Warsaw and the Department of Physics at the City University of New York. “The light wave twists around its axis and its phase changes in a spiral manner. Moreover, even the polarization, the direction of vibration of the electric field, begins to rotate.”
These structured optical states are attractive for applications such as quantum communications and control of tiny objects. However, producing them typically required complex nanostructures or large-scale experimental systems.
LCD offers a simpler path
The team chose a different strategy. “Instead of building a complex system, we used liquid crystals, which are materials with properties intermediate between liquids and solids. Liquid crystals can flow like liquids, but their molecules align in an ordered way, maintaining a constant orientation and relative position, just like crystals,” explains nanotechnology student Joanna Mendřicka from the Faculty of Physics at the University of Warsaw. He prepared the liquid crystal samples with Dr. Eva Oton of the Army Institute of Technology.
Special defects known as trons can form within this material. “You can imagine them as tightly twisted helices, similar to DNA, along which liquid crystal molecules are arranged. If you close the ends of such a helix by joining them in a donut-like ring, you get a tron,” Mendožicka explains. “These structures act as microscopic traps for light. The key step was to create the equivalent of a magnetic field for photons. Light does not respond to magnetic fields like electrons, but similar behavior can be achieved for light by other means.”
“Synthetic magnetic field” of light
“Spatially varying birefringence, or the difference in the propagation of different polarizations of light, acts like a composite magnetic field,” explains Dr. Piotr Kapuscinski from the Department of Physics at Warsaw University. “We call it ‘synthetic’ because its mathematical description resembles the behavior of a magnetic field, even though there is no physical presence of a magnetic field. As a result, the light begins to ‘bend’, much like an electron moving in a cyclotron orbit.”
To enhance this effect, the tron was placed inside an optical microcavity. This optical microcavity is a structure made of mirrors that repeatedly reflects light and traps it for long periods of time. “This makes the magnetic field even stronger,” says Dr. Muszynski. “Furthermore, we can use an external voltage to control the size of the trap, and thus the properties of the light.”
Stable light vortex in ground state
The most shocking result came next.
“In a typical system, light carrying orbital angular momentum appears in an excited state,” explains Professor Guillaume Malpouch of the University of Clermont-Auvergne and CNRS, who together with Professor Dmitry Solnyshkov and postdoctoral fellow Daniil Bobilev developed a theoretical model of this phenomenon. “For the first time, we have been able to obtain this effect in the ground state, the state with the lowest energy. This is important because the ground state is the most stable and the easiest for energy to accumulate.”
“This makes laser oscillation much easier,” emphasizes Professor Szczytko. “Light naturally “selects” this state because losses are minimal. ”
To confirm this, the researchers introduced a laser dye into the system. “We now have light that not only rotates, but also behaves like a laser beam. The light is coherent and has a well-defined energy and direction of emission,” says Dr. Marcin Muszyński.
Towards simpler photonic and quantum technologies
“What’s interesting is that our approach takes inspiration from very advanced theories involving so-called vector charges,” adds Professor Dmitry Solnyshkov. “So, in a sense, we’ve managed to make photons behave less like electrons and more like quarks, the charged particles that make up protons.
“This discovery opens a new path for creating miniature light sources with complex structures. It shows that instead of relying on complex nanotechnology, self-assembling materials can be used. In the future, this could enable simpler and more scalable photonic devices, for example in optical communications or quantum technologies,” concludes Professor Wiktor Piček from the Military Institute of Technology.
