The technique could lead to inexpensive devices that can find radio wave devices like cellular phones or Wi-Fi emitters, or cameras that capture images using radio waves. Their results appear online in Optica.
UK team sounds off with metamaterial bricks
"In this paper we achieved spectral images of microwave noise sources themselves, which means we can locate radio and microwave sources, like antennas, while simultaneously characterising what frequencies they are emitting over," said Aaron Diebold, an electrical and computer engineering research assistant at Duke University, North Carolina, who led the research. "At optical frequencies, that would be like getting a colour image of a hot object like a stove burner. While that is pretty simple optically, it takes different techniques in the radio and microwave regime."
Locating sources of these types of waves is already possible, but the techniques and equipment required are complex. Such devices traditionally use an array of antennae that cause them to become bulky and expensive. According to Duke, radio waves are much larger than light waves, so the methods used in optical frequencies are prohibitively complex and would result in extremely large detectors and other machinery.
In the new paper, the researchers turned to a metamaterial composed of a collection of squares containing inlaid wires in specific shapes that can be dynamically tuned to interact with radio waves passing through them.
By having some squares allow radio waves to pass through and others that block them, the researchers can create a coded aperture.
"We use the different patterns to encode data into a single measurement, which boosts the signal strength relative to what you would get with just a single, small antenna," said Mohammadreza Imani, a research scientist at Duke. "We also use the metamaterials to 'stamp' the different frequencies of the data, which allows us to tease them apart."
The metamaterials also modulate various frequencies differently as they pass through the coded aperture, which allows the researchers to deduce the frequencies of the waves being detected.
In the paper, the researchers showed that they can "see" and identify the shape of radio waves emitted by a smiley-face-shaped antenna. They then showed that their system can work in the real world by locating radio wave sources in three dimensions relative to one another.
The researchers plan to continue to refine their methods in the hope of eventually being able to take "pictures" of objects and scenes with nothing more than the radio waves bouncing off them.
"Passive imaging occurs in situations where you don't control the source, like taking a photo using light from the sun or light bulbs," said David R. Smith, the James B. Duke Distinguished Professor of Electrical and Computer Engineering at Duke. "At microwave frequencies, there are lots of signals bouncing around constantly. These ambient RF waves could provide enough illumination for a metasurface imager to reconstruct images using the techniques described in this research."
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