Normally the light waves emitted by different points of a thermal source cannot interfere with each other because they are produced out of step with one another. In contrast, different points on an antenna emit waves that interfere constructively in particular directions, producing lobes of radiation restricted to small solid angles. Physicists have shown recently, however, that the radiation emitted by a thermal source made of a polar material is partially coherent at around 10 to 100 nanometres.

Greffet and co-workers increased this coherence length by etching a nanoscale grating structure on the surface of a piece of silicon carbide, which is a polar material. This grating couples the radiation propagating away from the silicon carbide with the electromagnetic waves that automatically form on the surface of the material. These surface waves are coherent because they result from a collective motion of the atoms within the sample.

According to the researchers, this technique could allow scientists to modify the radiative heat transfer properties of some materials. By etching a grating on the surface of the silicon carbide, they were able to transform it from a mirror into a perfect absorber. At infrared wavelengths silicon carbide has a reflectivity of 94%, but with the grating this drops to almost zero. The same process could be applied to glass – another polar material that strongly reflects infrared radiation – allowing it to lose more heat by “radiative cooling”.