When lasers illuminate material it usually warms up, so laser beams are used for cutting sheet steel, for welding or even as scalpels. But this effect can also be reversed. When the frequency of the laser beam makes the irradiated material just not absorbing its light and slightly more energy (of the photons, as physicists call the light particles) is needed for that, this photons “take” this missing energy from the oscillation energy of the material’s atoms.
Such oscillation energy (“phonons”) is equivalent to the vibration of atoms which is also called temperature and which is slightly reduced by this: the material is cooled down. A team of scientists from Technische Universität Dortmund and Ruhr-Universität Bochum has just carried out the first detailed experimental study regarding this process (known as “photoluminescence up-conversion”) in semiconductor nanostructures. Based on this, the development of a vibration-free cooling of semiconductors might be possible.
The scientists determined the optimal laser wave-length as a function of temperature. They found out that the cooling efficiency of any laser beam increases with the temperature, analog to conventional cooling systems.
The temperature in the material, which has to be slightly lower than the photon energy, is adjusted when the gallium-arsenide layers are created, which are embedded in aluminum-gallium-arsenide layers. The thickness of the gallium-arsenide layer, usually a few dozens atom layers, determines this energy.
This so-called “quantum wells”, which can be created with the precision of one atom layer can also be applied to the latest semiconductor-laser generation. This technology can therefore be used to produce the sending laser as well as the cooling material.
The study has been carried out at the Chair for Experimental Physics III, Technische Universität Dortmund, by Dr. Soheyla Eshlaghi, Wieland Worthoff and Prof. Dr. Dieter Suter as well as Prof. Dr. Andreas D. Wieck from the Chair for Applied Solid-State Physics, Ruhr-Universität Bochum.