The NIST team directed light into an ultrathin layer of silicon nitride etched with grooves to create a diffraction grating. If the separation between the grooves and the wavelength of light is carefully chosen, the intensity of light declines much more slowly, linearly rather than exponentially.
Credit: S. Kelley/NIST
Shine a flashlight into a murky pond water and the beam won’t penetrate very far. Absorption and scattering rapidly diminishes the intensity of the light beam, which loses a fixed percentage of energy per unit distance traveled. That decline—known as exponential decay--holds true for light traveling through any fluid or solid that readily absorbs and scatters electromagnetic energy.
But that’s not what researchers at the National Institute of Standards and Technology (NIST) found when they studied a miniature light-scattering system—an ultrathin layer of silicon nitride fabricated atop a chip and etched with a series of closely spaced, periodic grooves. The grooves create a grating—a device that scatters different colors of light at different angles—while the silicon nitride acts to confine and guide incoming light as far as possible along the 0.2-centimeter length of the grating.
The grating scatters light—most of it upwards, perpendicular to the device--much like pond water does. And in most of their experiments, the NIST scientists observed just that. The intensity of the light dimmed exponentially and was able to illuminate only the first few of the grating’s grooves.
However, when the NIST team adjusted the width of the grooves so that they were nearly equal to the spacing between them, the scientists found something surprising. If they carefully chose a specific wavelength of infrared light, the intensity of that light decreased much more slowly as it traveled along the grating ..
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