Many plants and animals have tiny surface structures that absorb certain wavelengths of light. These nanostructures provide the colors in butterfly wings, camouflage for cicadas and enable moths to capture as much light as possible when flying at night.

Now, scientists from six institutions in Taiwan have created nanostructure surfaces that absorb almost all incident light.1 These man‑made ‘anti-reflection’ structures work by controlling the optical refractive index of the composite materials—that is, the amount by which the speed of light is reduced as it passes into a surface.

“Reflection occurs when light from one medium falls on another,” explains project member Surojit Chattopadhyay at National Chung Hsing University in Taichung. “Ideally, if two substances have the same refractive index, there is no reflection. The fictional Invisible Man has a refractive index close to unity—the refractive index of air—so there is no reflection from his body and we cannot see him.”

Fig. 1: Huang, Y. F. et al. Improved broadband and quasi-omnidirectional anti-reflection properties with biomimetic silicon nanostructures. Nature Nanotechnology 2, 770-774 (2008).

The Taiwanese team built surfaces consisting of millions of tiny silicon spikes, called nanotips (Fig. 1). Because the spikes are very narrow at the top, the refractive index of the surface is practically the same as air, and light passes straight through. The nanotips get gradually thicker as light passes deeper into the surface, so the refractive index increases, but the increase is so slow that very little light is reflected. This makes silicon—usually a polished grey color—appear pitch black.

The silicon nanotips absorb far more light than natural structures. “The eyes of a moth work only for visible [light], whereas [our] structures can work very well up to far infrared and beyond,” says Chattopadhyay.

The nanotip surfaces are also capable of absorbing light from wide angles and could, for example, increase the amount of light converted into electricity in solar panels.

Anti‑reflection materials could even be used for military technology of the future. The low reflectance at infrared wavelengths means that nanotip clothing could hide soldiers from thermal imaging‑making them effectively invisible at night. In addition, the team has developed aluminum nanotip surfaces that might have applications for military vehicles.