Kirchhoff's law of radiation: violation of a law of nature measured directly for the first time
What absorbs radiation well also radiates well - that's what Kirchhoff's radiation law says. A material now breaks this rule. Among other things, this could help to improve solar cells.
For a long time, it was assumed that the absorption and emission of radiation were strictly linked. A body that absorbs radiation well also emits radiation well. In 1860, the German physicist Gustav Kirchhoff summarised this relationship in the radiation law named after him today: In thermal equilibrium, radiation absorption and emission are equal for a given wavelength and angle to the surface. This means that a black object that absorbs all light is also the most effective emitter - which is why an ideal radiation spectrum is also known as blackbody radiation. For a long time, this relationship was central to the design of devices that absorb and emit radiation. However, theoretical considerations and simulations have long suggested that the relationship is not strictly valid.
Now a research group led by Harry A. Atwater from the California Institute of Technology in Pasadena has reported that it has indeed directly detected a violation of Kirchhoff's radiation law. They used a so-called magneto-optical material whose optical properties change when a magnetic field is applied. The magnetic field increases the ability to emit radiation at a certain angle, while reducing absorption at the same angle, the team reports in its publication in the journal "Nature Photonics". "This is the first experimental evidence that the law can be broken," says first author Atwater, according to a press release from his institute.
The result confirms earlier studies by this and other research groups, according to which magneto-optical materials should be able to disprove Kirchhoff's law. In recent years, a number of experimental set-ups have been proposed, calculated and simulated for this purpose. However, it has never been possible to measure the difference between emission and absorption directly. The material used by the working group consists of two layers: a silicon layer with fine grooves etched into it at 500 nanometre intervals and an underlying layer made of the semiconductor indium arsenide, which is doped with impurity atoms and has a dielectric constant close to zero at a certain wavelength.
"The device combines a material that responds strongly to a magnetic field with a patterned structure that enhances absorption and emission in infrared wavelengths," explains Atwater's employee Komron Shayegan, according to the press release. Without a magnetic field, the structure obeys Kirchhoff's radiation law. However, as soon as a moderate magnetic field is applied, the emission and absorption efficiency are no longer the same at certain angles. Such materials could improve the efficiency of energy-harvesting systems such as solar cells, says Shayegan. "If a solar cell were not to emit radiation back to the source, but instead to another energy collector, a larger proportion of the energy could be converted," explains the researcher.
Spectrum of Science
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