A doughnut-shaped laser was used to create a 45-metre-long optical fibre out of air, and a pulse of light was then sent through it. The technology could eventually be used to detect radioactive materials from kilometres away.
Tight beams of light, like lasers, can transmit information and energy, but they tend to widen as they travel, which makes them lose both. This is why they are often confined to glass tubes, or optical fibres, when used for communication.
However, fibre optics have to be installed and very intense light pulses can melt them. So at the University of Maryland and his colleagues created an optical fibre out of air.
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To mould air into a fibre-like structure called an air waveguide, the team used a powerful laser pulse that only lasted for 300 quadrillionths of a second. It heated and re-arranged air particles so that the air bounced any subsequent light beam in the same way as walls of traditional optical fibres would. The waveguide stuck around after the first pulse and the researchers transmitted a second pulse that lasted about a thousand times longer.
Milchberg and his colleagues used a different laser to make an air waveguide but it only spanned about 70 centimetres. He says that they wouldn鈥檛 have been able to increase its length 65 times had they not mastered one crucial tweak 鈥� using a doughnut-shaped laser beam. Previously they used a laser that effectively built the 鈥渨alls鈥� of the waveguide from four beams arranged in a square.
The team knew the doughnut beam鈥檚 circular shape would create a more filled-out waveguide 鈥渨all鈥�, but predicting the details of how the air would actually react in the moment was a very complex problem, Milchberg says. To create the waveguide, the team had to develop methods for simulating the process on a computer and confirm the beam鈥檚 shape by letting it hit a sample of helium, which created something like an imprint. 鈥淚t would melt most mirrors and cameras if we tried to image it that way. Everything about it was a new physics problem,鈥� he says.
The researchers鈥� method could be used to create kilometre-long air waveguides. However, that would require more energetic lasers and perfectly even and symmetric doughnut beams, which could be expensive and technically challenging, says at the Polytechnic Institute of Paris in France. In in January 2023, he and his colleagues used a laser to create an air waveguide that caught and guided lighting, therefore creating a lightning rod from air.
Milchberg says that long air waveguides could be used for remote detection of radioactive materials. When hit with a guided light, the radiation emitted from such materials could become energetic enough for remote detectors to discern. 鈥淵ou could be on an aeroplane above a city, shooting lasers into air to determine whether there is plutonium or uranium there without having to walk the streets,鈥� he says.
Physical Review X
