Phys. Rev. Lett. 107, 045901 (2011)

An ideal absorber and emitter of thermal radiation — a black body — could be used to increase the efficiency of thermophotovoltaic devices. But in reality, thermal emission from a natural material is lower than in this perfect case. It now seems, however, that artificial materials can be almost ideal. And what's more, they can be designed to emit at a selected wavelength with a bandwidth much narrower than a black body at the same temperature.

Metamaterials are engineered structures made up of a repeated pattern, or unit cell, with dimensions smaller than the wavelength of interest. The optical properties of a metamaterial can be selected by altering the dimensions of this unit cell. Xianliang Liu and colleagues constructed a metamaterial of gold crosses separated from a metallic ground sheet by a dielectric spacer. They then measured the infrared radiation given off by their metamaterial at various temperatures up to 300 °C. The emitted light was a spectrally narrow peak centred on a wavelength of 5.8 μm. Impressively, the peak emissivity was 98% of the theoretical black-body limit.