Fibre laser research paves way for flexible 3D displays

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Researchers have developed a fibre that can emit light along its length in any direction, paving the way for flexible 3D displays and medical tools that activate therapeutic compounds with bursts of light.

According to a team from MIT's Research Laboratory of Electronics, the new light source - a fibre only slightly thicker than a human hair – has a brightness that can be controllably varied for different viewers. Most light emitters look the same from any angle. The team believes that the fibre opens the possibility of 3D displays woven from flexible fibres that project different information to viewers' left and right eyes. The fibre could also enable medical devices that can be threaded into narrow openings to irradiate diseased tissue, selectively activating therapeutic compounds while leaving healthy tissue untouched. The new fibre has a hollow core, surrounded by alternating layers of materials with different optical properties that act as a mirror. In the core is a droplet of fluid that can be moved up and down the fibre. When the droplet receives energy, or is 'pumped' – in experiments, the researchers used another laser to pump the droplet – it emits light. The light bounces back and forth between the mirrors, emerging from the core as a 360° laser beam. Surrounding the core are four channels filled with liquid crystals, which vary the brightness of the emitted light; each liquid crystal channel is controlled by two electrode channels running parallel to it. According to MIT, despite the complexity of its structure, the fibre is just 400µm across. In experiments, the researchers simultaneously activated liquid crystals on opposite sides of the fibre to investigate a hypothetical application in which a transparent, woven display would present the same image to viewers on both sides — not mirror images, as a display that emitted light uniformly would. But in principle, Stolyarov says, there's no reason why a fibre couldn't have many liquid crystal channels that vary the light intensity in several different directions. "You can build as many of these liquid crystal channels as you want around the laser," Stolyarov said. "The process is very scalable." As a display technology, the fibres have the drawback that each of them provides only one image pixel. To make the fibres more useful, the researchers are investigating the possibility that the single pixel — the droplet of water — could oscillate back and forth fast enough to fool the viewer into perceiving a line rather than a coloured point. Even before the researchers answer that question, however, the fibre could prove useful in the burgeoning field of photodynamic therapy, in which light activates injected therapeutic compounds only at targeted locations. "The coolest thing about this work, really, is the way it's made," said Marko Loncar, an associate professor of electrical engineering at Harvard University. "The technology that they used to do it, basically, they can make kilometres of these things. It's remarkable. And they envision this being used for surgeries and things like that, where it would be really hard to use any other laser approach. There are entire lasers that depend on microfluidics. The handling of fluids on a small scale nowadays is a pretty developed technology. So I don't see this as a major obstacle." The research paper, published in Nature Photonics is the work of seven researchers affiliated with MIT's Research Laboratory of Electronics (RLE), including Yoel Fink, a professor of materials science and electrical engineering and the RLE's director; John Joannopoulos, the Francis Wright Davis Professor of Physics; lead author Alexander Stolyarov, a graduate student at Harvard University who is doing is PhD research with Fink's group; and Lei Wei, a postdoc at RLE. The work was funded by the U.S. Army and the National Science Foundation, through MIT's Institute for Soldier Nanotechnologies and Center for Materials Science and Engineering.