Researchers turn gold black

Optical materials

May 8, 2008

Nanostructured surface designed to absorb light completely from any incident direction.(© 2008 Nature Publishing Group.)

Researchers from Spain, France, and the UK have created nanostructured metal surfaces that exhibit omnidirectional light absorption [Teperik et al., Nat. Photonics (2008) 2, 299].

Light-absorbing materials are sought after for many applications, but particularly for preventing crosstalk in optical interconnects and as thermal light emitters. However, current light absorbers are far from having perfect black-body performance.

“It is relatively common to observe total absorption associated with optical resonances, but this effect has so far only been realized for specific directions of incidence and outside the visible spectral region,” explains F. Javier García de Abajo of the Instituto de Óptica – CSIC in Spain.

To achieve total omnidirectional absorption of light, the researchers fabricated nanoporous surfaces by electrodepositing Au over a monolayer of latex spheres on a Au substrate. As the metal grows around the spheres, it forms a smooth cone with an aperture at the top. When the latex is removed, buried spherical voids are left behind.

“[For] total omnidirectional absorption of light, we rely on specifically engineered localized plasmon resonances in the nanostructured Au surfaces,” he says.

In effect, the Au voids trap light until it is absorbed – just like black-body absorption where light enters a cavity through a small hole and is reflected off the cavity walls multiple times until it is absorbed because of the very small probability of escaping out through the hole.

“The familiar metallic look of Au becomes as black as it gets, whichever way we observe it from,” says García de Abajo.

The researchers suggest that the nanostructured metal surfaces could be useful for efficient photovoltaic cells. “Our results also have important fundamental implications related to the possibility of obtaining black-body-like behavior in nanostructured environments,” he suggests.

Cordelia Sealy