The New Black: A nanoscale coating reflects almost no light

The velvet background on a painting of Elvis looks black because it reflects so little light. But getting a surface to reflect no light at all is surprisingly difficult. Now, researchers have created a virtually reflectionfree surface by coating it with filaments only a few billionths of a meter thick.

REFLECTIONFREE. Aluminum nitride coated with antireflection filaments appears nearly black (top left), while samples of aluminum nitride, silicon, and aluminum (from second left to right) reflect much more light. An electron micrograph of the coating (bottom) shows the layers of filaments that make the coating nonreflective. Schubert/Rensselaer Polytechnic Inst.

Improved antireflective surfaces might have many uses. For example, they could eliminate light-wasting reflections in fiber-optic telecommunications, or the surfaces could brighten low-power light-emitting diode (LED) lamps.

Applied to a clear surface, the coating would make a lens absorb more light, increasing its transparency. On an opaque surface, the filaments would make a silicon solar cell, for example, almost perfectly absorbing.

The coating creates “really a new class of materials,” says E. Fred Schubert, a member of the research team at Rensselaer Polytechnic Institute in Troy, N.Y.

Schubert and his colleagues set out to minimize light’s reflections. Light rebounds when it strikes the boundary between two materials that have different “refractive indices”—measures of how fast light travels through the substances. For example, sunlight bounces off the surface of a pond because light travels more slowly in water than in air. The greater the difference between the refractive indices of any two materials, the more light is reflected.

To prevent reflections, the team put a transparent piece of aluminum nitride in a vacuum and coated the surface with five layers of nanoscale filaments made either of silicon dioxide or titanium dioxide. Each layer resembles a rug with the yarns leaning at 45°. Together, the five layers are only about 700 nanometers thick—the wavelength of red light. The individual filaments are 20 to 30 nm wide, the team reports in the March Nature Photonics.

By altering the spacing between the filaments, the scientists gave each layer a slightly different refractive index. The top layer has so much space between filaments that its refractive index is nearly the same as that of air. The filaments in the other four layers are progressively denser, so the layers have increasing refractive indices. The bottom layer’s index of 2.15 is the same as that of the underlying surface.

This staggered transition replaces an abrupt boundary with gradual ones, greatly reducing the reflection of light. At its surface, the coating has a refractive index of 1.05, which is close to air’s index of 1.0. Even transparent solids such as glass have indices of at least 1.4.

The low-reflection coating works for visible light and all other wavelengths between near ultraviolet and near infrared.

“It certainly is an improvement over the existing state of the art,” comments Sri Sridhar, who studies nanophotonics at Northeastern University in Boston.

The current coating has a faint blue tinge, however. Schubert says that this results from diffraction, not reflection. He explains that the thickness of the layers happens to equal the wavelength of the blue light. His team is currently making the layers thinner to avoid this problem.

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