A new fiber developed by Yoel Fink’s group emits blue laser light only at a precisely controlled location. Image: Greg Hren |
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,
Massachusetts Institute of Technology (MIT) researchers report the development
of a new light source—a fiber only a little thicker than a human hair—whose
brightness can be controllably varied for different viewers.
The
fiber thus opens the possibility of 3D displays woven from flexible fibers that
project different information to viewers’ left and right eyes. The fiber 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 United
States Army and the National Science Foundation, through MIT’s Institute for
Soldier Nanotechnologies and Center for Materials Science and Engineering.
The
newly developed fiber has a hollow core; 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
fiber. 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 fiber is only 400 um across.
In
experiments, the researchers simultaneously activated liquid crystals on
opposite sides of the fiber 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 fiber 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 fibers have the obvious drawback that each of them
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.
Even
before the researchers answer that question, however, the fiber 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 kilometers of
these things. It’s remarkable.”
Loncar
adds, “And they envision this being used for surgeries and things like that,
where it would be really hard to use any other laser approach.”
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.”
