Cummer was part of the group that demonstrated the first invisibility cloak, which works in the microwave region of the spectrum and pulls off its feat by bending the waves around an object. He’s currently working on tunable metamaterials for the microwave and radio regions, which he hopes will lead to antennas that dynamically block out interfering frequencies. Metamaterials that work with this band of the spectrum are easier to tune because they’re built like circuit boards and can be switched with small bursts of power. It’s more challenging to make active materials that work with higher frequencies like visible light and infrared light, where Atwater’s materials work.
Cummer and Padilla note that the use of flexible materials also sets Atwater’s work apart and makes it possible to imagine future applications such as infrared camouflage that integrates into a soldier’s clothing, rendering him invisible to night-vision goggles. “A lot of this work is aiming to add to the tool belt,” Cummer says. “As we get closer to applications, someone will want a flexible material.”
The mechanical tuning concept is likely to work with many metamaterials designs, not just the particular resonator Atwater used. “This is quite a powerful approach,” says Vladimir Shalaev, professor of electrical and computer engineering at Purdue University. “You could use different designs, depending on the properties you want, and build them on a stretchable material,” he says.
To demonstrate one potential application of the tunable materials, Atwater’s group made a simple chemical sensor. They designed the resonators in the flexible array to detect whether a particular type of carbon-hydrogen bond associated with a particular wavelength of light was present in a sample. The researchers tested the sensitivity of the array under different levels of strain and found that they could improve its sensitivity by stretching it. It’s a proof of principle now, but it could be a first step toward a sensor that could detect multiple chemicals, which wouldn’t even need to be known when the sensor was fabricated.