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.
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. The intensity declined linearly with the distance traveled rather than exponentially.
The scientists were just as intrigued by a property of the infrared light scattered upwards from the grating. Whenever the intensity of light along the grating shifted from exponential to linear decline, the light scattered upwards formed a wide beam that had the same intensity throughout. A broad light beam of uniform intensity is a highly desirable tool for many experiments involving clouds of atoms.
Electrical and computer engineer Sangsik Kim had never seen anything like it. When he first observed the strange behavior in simulations he performed at NIST in the spring of 2017, he and veteran NIST scientist Vladimir Aksyuk worried that he had made a mistake. But two weeks later, Kim saw the same effect in laboratory experiments using actual diffraction gratings.
If the wavelength shifted even slightly or the spacing between the grooves changed by only a tiny amount, the system reverted back to exponential decay.
It took the NIST team several years to develop a theory that could explain the strange phenomenon. The researchers found that it has its roots in the complex interplay between the structure of the grating, the light traveling forward, the light scattered backward by the grooves in the grating, and the light scattered upwards. At some critical juncture, known as the exceptional point, all of these factors conspire to dramatically alter the loss in light energy, changing it from exponential to linear decay (see animation).
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