According to the team at Dukes this approach could form the basis for technologies ranging from improved security scanners to new types of visual displays.
A metamaterial is an artificial material that manipulates waves like light and sound through properties of its structure rather than its chemistry. Designs can be created with these materials to have rare or unnatural properties, like the ability to absorb specific ranges of the electromagnetic spectrum or to bend light backward.
"These materials are made up of a grid of separate units that can be individually tuned," explained Willie Padilla, professor of electrical and computer engineering at Duke. "As a wave passes through the surface, the metamaterial can control the amplitude and phase at each location in the grid, allowing us to manipulate the wave in a lot of different ways."
Each grid location contains a tiny silicon cylinder just 50 microns tall and 120 microns wide, with the cylinders spaced 170 microns apart. While silicon is not normally a conductive material, the researchers bombard the cylinders with a specific frequency of light in a process called photodoping. This imbues the material with metallic properties by exciting electrons on the cylinders' surfaces.
These electrons then cause the cylinders to interact with electromagnetic waves passing through them. The size of the cylinders dictate what frequencies of light they can interact with, while the angle of the photodoping affects how they manipulate the electromagnetic waves.
By purposefully engineering these details, the metamaterial can control electromagnetic waves in many different ways.
In this particular study conducted by researchers at Duke, the cylinders were sized to interact with terahertz waves -- a band of the electromagnetic spectrum that sits between microwaves and infrared light.
According to Padilla, controlling this wavelength of light could improve broadband communications between satellites or lead to security technology that can easily scan through clothing. The approach could also be adapted to other bands of the electromagnetic spectrum, simply 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," Padilla said. "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."
