Materials Science
July 2017

Going meta

Sandia researcher taps high-performance computers to design metamaterials that mold electromagnetics.

An incoming, broad-spectrum light wave meets the broken-symmetry metasurface of cuboid resonators. (Top graph: broad spectrum.) After passing through the metasurface, the beam narrows due to the sharp resonances of the broken-symmetry metasurface (bottom graph:narrow spectrum). The swirling-arrows pattern represents the electric field distribution of light trapped in the resonators. Illustration courtesy of Sandia National Laboratories.

Metamaterials – substances engineered to have properties absent in nature – are made by assembling multiple parts made from metals and other composite substances. They’re designed on scales as small as nanometers – always smaller than the wavelengths of the phenomena they influence.

Metamaterials promise a bevy of potential applications in optical communication, radar, environmental sensing and nanophotonics – basically, lasing, imaging, spectroscopy and anything else involving the manipulation of light. They could even lead to invisibility cloaks – wave-bending shields that render aircraft, for instance, undetectable to radar.

“Metamaterials have properties that are based on repeating patterns of manmade atoms,” says Salvatore Campione, senior researcher at the Department of Energy’s Sandia National Laboratories in New Mexico. “Their exact shape, geometry, size, orientation and arrangement give them smart properties that are capable of manipulating electromagnetic waves. We can engineer the properties of these atoms to realize a desired functionality, which nature cannot do on her own.”

Suppose someone wanted to create subwavelength, lightweight optical lenses, replacing bulky, space-consuming lenses. “With metamaterials, they can be made in the micrometer or even millimeter range. You can imagine how small and lightweight they can be,” Campione says.

Campione and his colleagues model the electromagnetics of complex systems on two DOE supercomputers that can solve tens of millions of problems in hours: Trinity (housed at Los Alamos National Laboratory) and Sequoia (at Lawrence Livermore National Laboratory).

Electromagnetics studies the nature and interaction of static and dynamic electromagnetic fields occurring across a spectrum of wavelengths, from radio to infrared and optical frequencies. Every instance and application depends generating, guiding, radiating, receiving and detecting electromagnetic waves.

In two years, Campione has made impressive advances in metamaterials research that will boost the substances’ flexibility, efficiency, adaptability and other properties. He helped design active metamaterial devices that reconfigure themselves via external stimuli – an instrument, for example, might change operating frequency in response to a signal. In optical systems, this might involve a device that acts as a filter or modulator.

We can put nanostructures anywhere we want in three coordinate directions.

“Usually, the filters’ properties are static, and once fabricated they can’t be changed. As active metamaterial devices, the filter can be switched,” he says. “This versatility may allow the design of devices that also have multiple functionalities – for example, it’s not just about the filter, but also the modulator or the lens.”

Metamaterials, which started getting attention about 20 years ago, originally were made with metals such as gold as their base. Metals at shorter wavelengths (and corresponding high frequencies) became impractical, however, because of their lossy nature – that is, they tend to absorb energy and subsequently lose it.

“Ten to 12 years ago, gold was widely used in experiments, but it was so lossy that practical applications were hindered,” Campione says. “To counteract the drawback of metal-based metamaterials, we’ve been using all-dielectric metamaterials, which are less lossy. They seem to be promising building blocks for many devices.”

Dielectric substances are poor electrical conductors but nonetheless dependably support electrostatic fields. Common examples include oxides of various metals, such as titanium dioxide; semiconductors, such as gallium arsenide, mica, ceramics and silicon; and plastics and glass. In a recent Optics Express paper, Campione and coauthors described how they tailored metamaterials to include manmade atoms arranged in sub-wavelength sizes. In another recent work, in Applied Physics Letters, Campione and colleagues showed how they created three-dimensional metafilms made of artificial atoms with many degrees of freedom. “Three-D means that the atoms can be freely oriented in the three coordinate directions, so you can use the entire volume space. The third dimension is vertical, so we’re not limited to placing the atoms on a planar surface; we can place them on the walls, too. Three-D-engineered optical metafilms show promise to dramatically reduce the cost of infrared optical systems while also increasing functionality. We can put nanostructures anywhere we want in three coordinate directions.”

Campione has been in a career sprint ever since a summer 2012 internship with the lab’s Igal Brener (applied photonic microsystems) and Michael B. Sinclair (electronic, optical and nano). At age 31, Campione is author or co-author of more than 60 peer-reviewed journal articles and two book chapters. He’s received numerous honor for his achievements, including the IEEE 2016 HKN Outstanding Young Professional Award, an annual prize given to just one engineer worldwide.

The Italy native holds bachelor’s and master’s degrees in electronic engineering from Polytechnic of Turin and, in 2013, an electrical and computer engineering Ph.D. from the University of California, Irvine. Sandwiched between his Turin and Irvine degrees was a master’s in electrical and computer engineering from the University of Illinois, Chicago. He was a Sandia postdoctoral researcher at from January 2014 to January 2016. In February 2016 he converted to senior member of technical staff in electromagnetic theory, where he’s collaborated with Sandia’s Larry K. Warne and Lorena I. Basilio.

The computer science background has been particularly useful to Campione because he writes his own massively parallel codes. “Our in-house codes are written largely to function with the Department of Energy’s high-performance supercomputing platform,” he says. “We do use commercial codes to verify and validate our results, but we typically use our own codes to both simulate and analyze.”