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.
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.