“Optical rotatum” is the term researchers coined to describe a newly discovered structure of light.
Beams of light that can be shaped into corkscrew-like forms, known as optical vortices, are used today in a variety of applications. Pushing the boundaries of structured light, applied physicists at Harvard’s John A. Paulson School of Engineering and Applied Sciences (SEAS) have reported a new type of optical vortex beam. This beam not only twists as it travels but also changes at different rates in different regions, creating unique patterns. Its behavior resembles the spiral shapes often seen in nature.
Drawing from classical mechanics, the researchers nicknamed their never-before-demonstrated light vortex an “optical rotatum” to describe how the torque on the light’s corkscrew shape gradually changes. In Newtonian physics, “rotatum” refers to the rate of change in torque on an object over time.
The optical rotatum was created in the lab of Federico Capasso, the Robert L. Wallace Professor of Applied Physics and the Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS. “This is a new behavior of light consisting of an optical vortex that propagates through space and changes in unusual ways,” Capasso said. “It is potentially useful for manipulating small matter.” The research is published in Science Advances.
Light That Echoes Nature’s Spirals
In a peculiar twist, the researchers found that their orbital angular momentum-carrying beam of light grows in a mathematically recognizable pattern found all over the natural world. Mirroring the Fibonacci number sequence (made famous in The Da Vinci Code), their optical rotatum propagates in a logarithmic spiral that is seen in the shell of a nautilus, the seeds of a sunflower, and the branches of trees.
“That was one of the unexpected highlights of this research,” said first author Ahmed Dorrah, a former research associate in Capasso’s lab, now an assistant professor at Eindhoven University of Technology. “Hopefully we can inspire others who are specialists in applied mathematics to further study these light patterns and gain unique insights into their universal signature.”
More Control Through Metasurfaces
The research builds on previous work in which the team used a metasurface, a thin lens etched with light-bending nanostructures, to create a light beam with controlled polarization and orbital angular momentum along its propagation path, converting any input of light into other structures that change as they move. Now, they’ve introduced another degree of freedom to their light, in which they can also change its spatial torque as it propagates.
“We show even more versatility of control, and we can do it continuously,” said Alfonso Palmieri, a graduate student in Capasso’s lab and co-author of this research.
Potential use cases for such an exotic beam of light include the control of very small particles, such as colloids in suspension, by introducing a new type of force in accordance with the light’s unusual torque. It could also enable a precise optical tweezer for micro-manipulation of small things.
While others have demonstrated torque-changing light using high-intensity lasers and bulky setups, the Harvard team made theirs with a single liquid crystal display and a low-intensity beam. By showing they can create a rotatum in an industry-compatible, integrated device, the barrier to entry for their technology to become reality is much lower than previous demonstrations.
Reference: “Rotatum of light” by Ahmed H. Dorrah, Alfonso Palmieri, Lisa Li and Federico Capasso, 11 April 2025, Science Advances.
DOI: 10.1126/sciadv.adr9092
