Thermal emission goes coherent
Mar 8, 2002
Unlike lasers, thermal sources such as light-bulb filaments usually emit incoherent radiation in many directions over a wide range of wavelengths. But now Jean-Jacques Greffet of the Laboratoire EM2C in France and colleagues have observed coherent emission from a thermal source. They found that a specially prepared sample of silicon carbide emits coherent infrared light in well-defined directions like an antenna (J Greffet et al 2002 Nature 416 61).
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”.
About the author
Edwin Cartlidge is News Editor of Physics World