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Tunable, Stretchable Optical Materials

• Katherine Bourzac via Technology Review
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.

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