Engineers at the University of California San Diego have created a thin, flexible, light-absorbing material that absorbs more than 87 percent of near-infrared light. The technology could someday support the development of solar cells; transparent window coatings to keep cars and buildings cool; and lightweight shields that block thermal detection.

The "near-perfect broadband absorber" provides 98 percent near-infrared absorption at 1,550 nanometers, the wavelength for fiber optic communication. The material is capable of absorbing light from every angle, and can theoretically be customized to absorb certain wavelengths of light while letting others pass through.

The absorber relies on optical phenomena known as surface plasmon resonances, which are the collective movements of free electrons that occur on the metal nanoparticles' surface upon interaction with certain wavelengths of light. Metal nanoparticles can carry many free electrons, and therefore exhibit strong surface plasmon resonance — mainly in visible light, not in the infrared.

UC San Diego engineers reasoned that if they could change the number of free electron carriers, they could tune the material’s surface plasmon resonance to different wavelengths of light. The team designed and built an absorber from zinc oxide semiconductors — materials that could be modified, or doped, to carry a different amount of free electrons.

The materials were deposited one atomic layer at a time on a silicon substrate to create an array of standing nanotubes, each made of alternating concentric rings of zinc oxide and aluminum-doped zinc oxide. The result: a material that is thin, flexible, and transparent in the visible.

The particle-based design, according to researchers, can be scaled up to make large surface area devices, like broadband absorbers for large windows.

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