A phenomenon observed in a miniature light-scattering system composed of an ultrathin layer of silicon nitride on a chip could lead to improved optical communications and sensors. Researchers at the National Institute of Standards and Technology (NIST) studied such a system, which was additionally etched with a series of closely spaced, periodic grooves. The grooves created a grating 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-cm length of the grating. Traditionally, when shining a flashlight into a murky pond, the beam will only penetrate so far; absorption and scattering rapidly diminish the intensity of the light beam, which loses a fixed percentage of energy per unit of distance traveled. That decline, known as exponential decay, is true for light traveling through any fluid or solid that readily absorbs and scatters electromagnetic energy. A team at NIST 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. The work supports optical communications and sensing applications. Courtesy of S. Kelley/NIST. The grating used in the NIST team’s work scattered light — mostly upward, perpendicular to the device — much like pond water does. In most of their experiments, the NIST scientists observed 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 team adjusted the width of the grooves so that they were nearly equal to the spacing between them, the researchers found that when they used 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 upward from the grating. Whenever the intensity of light along the grating shifted from exponential to linear decline, the light scattered upward formed a wide beam that had the same intensity throughout. A broad light beam of intensity is a highly desirable tool for many experiments involving clouds of atoms. When the behavior was first observed by electrical and computer engineer Sangsik Kim and NIST scientist Vladimir Aksyuk in 2017, they thought they had made a mistake. 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 team several years to develop a theory that could explain the phenomenon. The team found that it has 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 upward. At some critical junctional, known as the exceptional point, all of these factors intersect to dramatically alter the loss of light energy, changing it from exponential to linear decay. The researchers were surprised to realize that the phenomenon they observed with infrared light is a universal property of any type of wave traveling through a lossy periodic structure, whether the waves are acoustic, infrared, or radio. The finding could enable researchers to transmit beams of light from one chip-based device to another without losing as much energy, which could be a boon for optical communications. The broad, uniform beam created by the exceptional point is also useful for studying a cloud of atoms. The light induces the atoms to jump from one energy level to another; its width and uniform intensity enable the beam to interrogate the rapidly moving atoms for a longer period of time. Precisely measuring the frequency of light emitted as the atoms make the transitions is a key step in building highly accurate atomic clocks and creating precise navigation systems based on trapped atomic vapors. More generally, Aksyuk said, the uniform beam of light makes it possible to integrate portable, chip-based photonic devices with large-scale optical experiments, reducing their size and complexity. Once the uniform beam of light probes an atomic vapor, for instance, the information can be sent back to the photonic chip and processed there. Yet another potential application is environmental monitoring. Because the transformation from exponential to linear absorption is sudden and exquisitely sensitive to the wavelength of light selected, it could form the basis of a high-precision detector of trace amounts of pollutants. If a pollutant at the surface changes the wavelength of light in the grating, the exceptional point will abruptly vanish and the light intensity will swiftly transition from linear to exponential decay, Aksyuk said. The research was published in Nature Nanotechnology (www.doi.org/10.1038/s41565-022-01114-3).
