2017-10-31
171
The field of metamaterials has yielded devices that seem to come from science fiction – highly absorbent coatings for solar cells, ultra-high-resolution microscope lenses etc. 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. Researchers at Caltech have shown that by mechanically stretching an optical filter made from a metamaterial, they can dynamically change the 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," (50 nm) 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 cover a broad swath of the infrared spectrum.
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.
http://www.technologyreview.com/energy/26482/
