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Tunable, Stretchable Optical Materials

Active metamaterials might be used in solar cells that change properties with the weather.

The field of metamaterials has yielded devices that seem to come from science fiction–invisibility cloaks, highly absorbent coatings for solar cells and ultra-high-resolution microscope lenses. Metamaterials are precisely tailored to manipulate electromagnetic waves–including visible light, microwaves, and other parts of the spectrum–in ways that no natural materials can.

Tunable materials: When this array of optical resonators, made of silver printed on a stretchy polymer, is strained, the filters respond to different frequencies in the infrared spectrum.

With few exceptions, however, these materials work in a very limited range of wavelengths of light, making them impractical–an invisibility cloak isn’t very useful if it only redirects light of one color but can be readily seen under others. Now researchers at Caltech have shown that by mechanically stretching an optical filter made from a metamaterial, they can dynamically change which wavelength of infrared light it responds to.

Metamaterials that could be tuned, rather than working solely in a fixed wavelength, might lead to thermal photovoltaics that change their properties with the weather to maintain high efficiency, goggles that respond to blinding glare to block it out, or devices for processing optical signals to speed telecommunications, for example.

Instead of building a metamaterial on rigid materials, the Caltech researchers made an array of silver resonators on a stretchy polymer film. These resonators “ring” when struck with a particular wavelength of light, and act as a strong filter for that wavelength. Each resonator is shaped like a “C” next to an “l”; the distance between the tip of the “C” and the “l,” about 50 nanometers in the test devices, determines the wavelength of light at which it will resonate.

Researchers led by Harry Atwater, a professor of applied physics and materials science, found that they could stretch the polymer sheets by as much as 50 percent, changing the distance between the two parts of the resonator, without warping the dimensions of the silver “C” and “l.” This let them dynamically change which wavelength of light the material would respond to over a broad swath of the infrared spectrum. The work is described online in the journal Nano Letters.

“What’s nice about this is, it’s relatively easy to tune over a very broad band with simple mechanical means,” says Willie Padilla, professor of physics at Boston College. Padilla is also working on tunable metamaterials. He says those that work in the infrared spectrum could have applications in thermal photovoltaic cells that adjust their properties as light and heat levels change.

Stretching is both simpler and more effective than other approaches developed for tuning metamaterials, researchers say. Other scientists have made active metamaterials that can be tuned by applying a voltage, or bombarding the material with laser light, for example. These approaches require a lot of power and only subtly change the metamaterial’s properties. “This is much easier to control,” says Steven Cummer, professor of electrical and computer engineering at Duke University.

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.

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