Other technologies will be needed to allow the suit to communicate with the outside world. Earlier this year, MIT’s Fink announced development of coated polymer threads that might be just the thing, enabling silent communication with remote allies or commanders through the use of visible or infrared light.Fink’s threads are able to selectively reflect or absorb different wavelengths of light, thanks to their coating, which incorporates numerous ultrathin layers of two transparent materials-one organic, the other inorganic. The two materials slow light at different rates. In the resulting riot of reflections within those layers, some wavelengths are strongly reflected back out of the fiber, and others are canceled out. Just which wavelengths get reflected depends on the thickness of the layers, which can range from 100 to 1,000 nanometers and can be precisely controlled.
While most photonics researchers are working on chips and other gadgets for optical telecommunications, Fink’s group is the first to build a photonic thread that could be made into a textile, says Eli Yablonovitch, an electrical engineer at the University of California, Los Angeles, and a pioneer in optical materials. One possible use for these threads: a portion of a combat uniform that strongly reflects a specific signature of ambient infrared light. During the confusion of a nighttime firefight, for example, such an “optical bar code” could identify a soldier as friend to fellow troops equipped with night vision goggles tuned to the right reflected light. And Fink’s team would also like to come up with a way of tuning these materials on the fly, so that the wavelength could be changed electrically (and remotely) in case an enemy got his hands on a uniform.
This presents a special challenge, Yablonovitch says. “There are many solutions. Just no good ones. They have their work cut out for them to make it practical for the army,” he says.
For now, Fink’s group is pushing ahead with several approaches to making the optical fibers tunable. One strategy involves creating a sort of stretching rack that could pull the fibers taut. The tension would thin the layers, changing the reflected wavelength (see “Fine Tuning,” below). A second approach takes advantage of the fact that one of the materials in the layers-arsenic triselenide-slows light at a different rate in the presence of an electric field; change the field and you change the reflection of the whole fiber. These approaches, Fink says, could produce a tunable fiber within two years.
|A cross section shows the outer layers of optical materials coating a polymer thread. The layers’ thickness determines how light is reflected.|
|Before: If the wavelength is different from the layers’ thickness, the light can pass through.|
|After: Stretching the thread could thin the layers so their width matches the red wavelength. At each boundary between layers, some red light would be reflected (broken line), and some would continue on.|