May 2 2018
Scientists at Duke University have developed the first metal-free, dynamically tunable metamaterial for manipulating electromagnetic waves. The method could form the foundation for technologies spanning from optimized security scanners to new types of visual displays.
Artistic representation of the new metasurface technology. Rays of light (red) bombard the silicon cylinders, changing their electromagnetic properties to precisely tune how they interact with electromagnetic waves. (Credit: Duke University)
The results have been published in the April 9 issue of the Advanced Materials journal.
A metamaterial is an artificial material that controls waves like sound and light through properties of its structure instead of its chemistry. Scientists can manipulate these materials to have unnatural or rare properties, like the ability to bend light backward or to absorb particular ranges of the electromagnetic spectrum.
“These materials are made up of a grid of separate units that can be individually tuned. As a wave passes through the surface, the metamaterial can control the amplitude and phase at each location in the grid, which allows us to manipulate the wave in a lot of different ways.
Willie Padilla, Professor, Electrical and Computer Engineering, Duke
In the latest technology, each grid location has a miniature silicon cylinder just 50 µm in height and 120 µm in width, with the cylinders spaced 170 µm apart from each other. While silicon is not usually a conductive material, the scientists bombard the cylinders with a precise frequency of light in a process known as photodoping. This imbues the usually insulating material with metallic properties by exciting electrons present on the cylinders’ surfaces.
These recently freed electrons make the cylinders to interact with electromagnetic waves traveling through them. The size of the cylinders commands what frequencies of light they can interact with, while the angle of the photodoping impacts how they control the electromagnetic waves. By decisively producing these details, the metamaterial can manipulate electromagnetic waves in a number of different ways.
For this research, the cylinders were sized to interact with terahertz waves—a band of the electromagnetic spectrum that fits between infrared light and microwaves. Regulating this wavelength of light could enhance broadband communications between satellites or result in security technology that can scan through clothing without difficulty. The method could also be modified to other bands of the electromagnetic spectrum—like visible light or infrared—just by scaling the size of the cylinders.
We’re demonstrating a new field where we can dynamically control each point of the metasurface by adjusting how they are being photodoped. We can create any type of pattern we want to, allowing us to create lenses or beam-steering devices, for example. And because they’re controlled by light beams, they can change very fast with very little power.
Willie Padilla, Professor, Electrical and Computer Engineering, Duke
While current metamaterials manipulate electromagnetic waves using their electric properties, the new technology can also control them using their magnetic properties.
“This allows each cylinder to not only influence the incoming wave, but the interaction between neighboring cylinders,” said Kebin Fan, a research scientist in Padilla’s laboratory and the paper’s first author. “This gives the metamaterial much more versatility, such as the ability to control waves traveling across the surface of the metamaterial rather than through it.”
We’re more interested in the basic demonstration of the physics behind this technology, but it does have a few salient features that make it attractive for devices. Because it is not made of metal, it won’t melt, which can be a problem for some applications. It has subwavelength control, which gives you more freedom and versatility. It is also possible to reconfigure how the metamaterial affects incoming waves extremely quickly, which has our group planning to explore using it for dynamic holography.
Willie Padilla, Professor, Electrical and Computer Engineering, Duke
The Department of Energy (DE-SC0014372) and the Army Research Office (ARO W911NF-16-1-0361) supported the study.
Citation: “Photo-Tunable Dielectric Huygens’ Metasurfaces,” Kebin Fan, Jingdi Zhang, Xinyu Liu, Gufeng Zhang, Richard D. Averitt, and Willie J. Padilla. Advanced Materials, April 9, 2018. DOI: 10.1002/adma.201800278