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While knots are typically associated with flexible materials like ropes and laces, a recent breakthrough in physics has shown that even a beam of light can take on a knotted form.
This fascinating concept can be illustrated with the analogy of dropping multiple rocks into a pond simultaneously. The resulting concentric ripples converge into a complex pattern. If one could manipulate the shape and speed of these ripples, intricate three-dimensional designs could emerge.
In an innovative research effort over the past few years, scientists have been effectively creating such constructs with light. By overlapping multiple laser beams that possess specific properties and directing them through intricate lens systems, they have been able to produce stationary “optical knots,” visually akin to a web of smoke rings suspended in space.
Engineers at Duke University have recently unveiled a holographic device capable of dividing a single laser beam into five tailored beams to generate an optical knot. Their findings, published in respected journals such as Nature Communications and Photonics Research, suggest potential applications for these optical structures in transmitting encoded data and measuring air turbulence.
The researchers demonstrated that the information contained within these optical knots can endure the challenges posed by traveling through turbulent air, although the process is not as straightforward as initially thought.
“Initially, it was assumed that these mathematically stable shapes could seamlessly navigate complex environments,” stated Natalia Litchinitser, a professor of electrical and computer engineering at Duke. “We learned, however, that while they can be made more stable, they are not infallibly resilient.”
To explore this phenomenon, the team employed a device resembling a small convection oven. Although fiber optic setups can simulate the effects of turbulence, the researchers aimed to observe the behavior of light over longer distances. While a collaborating team in South Africa has set up experiments spanning two buildings, the Duke team had to conduct their tests on a table-sized platform.
“We designed a compact apparatus with a hot plate and fans to induce air turbulence,” explained Danilo Gomes Pires, a postdoctoral researcher in Litchinitser’s lab. “We then manipulated the light beam by bouncing it off mirrors, creating a simulation of it traveling nearly 1,000 feet.”
If the beams were unaffected during their journey, the resulting knot should appear as a continuous loop with three intertwined segments. Contrary to this expectation, the research indicated that increased turbulence often resulted in the knot transforming into two linked circles or a single ring, which led to the loss of the contained information.
However, the researchers were encouraged to find that this degradation was avoidable. By introducing additional twists and curves to the knot’s structure—similar to enhancing a simple waterslide into a complex design—they established more reference points within the knot, enabling its stability over longer distances.
Although the application of optical knots remains in its early developmental stages—having been identified only about 20 years ago—they hold significant promise. The shapes could be used for long-distance data transmission, and the extent of their disruption could serve as a measure of turbulence encountered along the way. Moreover, these sophisticated knots may have the potential to capture and manipulate minuscule particles in three-dimensional environments.
“Before we can harness optical knots for practical uses, further investigation into their behavior is necessary,” Litchinitser noted. “Our work marks the first observation of these knots propagating through authentic turbulence, paving the way for future explorations into their performance in open environments.”
This research received funding from the Office of Naval Research and the Army Research Office.
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