A multidirectional fiber laser could herald flexible 3-D displays and medical tools that activate therapeutic compounds with bursts of light. Most light emitters, from candles to lightbulbs to computer screens, look the same from any angle. But the brightness of a new light source developed at MIT can be controllably varied for viewers, enabling medical devices that can be threaded into narrow openings to irradiate diseased tissue, and selectively activating therapeutic compounds while leaving healthy tissue untouched. The fiber, developed by materials science and electrical engineering professor Yoel Fink and his group, has a hollow core surrounded by alternating layers of materials with different optical properties that, together, act as a mirror. A droplet of fluid within the core can be moved up and down the fiber, and when “pumped” — in experiments, the researchers used another laser to pump the droplet — emits light. The light bounces back and forth between the mirrors, emerging from the core as a 360° laser beam. The core is surrounded by four channels filled with liquid crystals, which vary the brightness of the emitted light. Each channel is controlled by two electrode channels running parallel to it. Although complex in structure, the fiber measures only 400 µm across. During experiments, Fink's group simultaneously activated liquid crystals on opposite sides of the fiber to determine whether a transparent woven display would present the same image to viewers on both sides, rather than the mirror images which a display that emitted light uniformly would present. A fiber developed by Yoel Fink's group emits blue laser light at a precisely controlled location only. Courtesy of Greg Hren. They discovered that, in principle, there was no reason that a fiber could not have many liquid-crystal channels that vary the light intensity in several directions. “You can build as many of these liquid-crystal channels as you want around the laser,” said Alexander Stolyarov, a Harvard University graduate student who is doing his postdoctoral research with Fink's group. “The process is very scalable.” Although the new fibers hold promise for future applications, they have an obvious drawback as a display technology: Each fiber provides only one image pixel. To make the fibers 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 colored point. Before they even answer that question, however, the fiber could prove useful in photodynamic therapy, a method that uses light to activate 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. “The technology that they used to do it — basically, they can make kilometers of these things. It's remarkable.” The findings appeared online in Nature Photonics (doi: 10.1038/nphoton.2012.24).