Fibre laser points to woven 3D displays

13 March 2012

A fibre that can emit light along its length in any direction may herald flexible 3D displays

A fibre that can emit light along its length in any direction may herald flexible 3D displays.

Most light emitters, from candles to light bulbs to computer screens, look the same from any angle. But in a paper published on the Nature Photonics website, MIT researchers report the development of a new light source – a fibre only a little thicker than a human hair – whose brightness can be controllably varied for different viewers.

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 paper 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 US Army and the National Science Foundation, through MIT’s Institute for Soldier Nanotechnologies and Center for Materials Science and Engineering.

The newly developed fibre has a hollow core and surrounding this core are alternating layers of materials with different optical properties, which together 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-degree 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. Yet despite the complexity of its structure, the fibre is only 400 micrometers across; A human hair is usually somewhere around 100 micrometers in diameter.

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 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 says. “The process is very scalable.”

As a display technology, the fibres have the obvious 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,” says 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.”

Loncar also thinks that the problem of pumping the fluid droplet back and forth to produce images is probably soluble. “There are entire lasers that depend on microfluidics,” he says. “The handling of fluids on a small scale nowadays is a pretty developed technology. So I don’t see this as a major obstacle.”

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