Laser cooling (also known as optical cooling) is a well-established technique…but mostly for gases. The basic idea is to disperse the thermal energy of the atoms through shining light on them: the frequency of the laser is set to be slightly lower than the energy of transition between two configurations in the atoms, so that atoms’ motion provides that extra bit to lead to absorption. That converts energy of motion (kinetic energy) into light, slowing the atoms down. However, the trick doesn’t work for solids, because the kinetic energy takes the form of phonons, quasiparticles of sound. However, researchers figured out a way to annihilate the phonons using a laser, in a particular type of semiconductor. Using that trick, they cooled the material down by 40 degrees, opening the way to rapid refrigeration of some solids.

The authors of the new study used cadmium sulphide (CdS), a material known as a group-II-VI semiconductor. Commonly used in digital electronics, semiconductors are insulators under normal conditions, but can be induced to conduct electricity when impurity atoms are added. Group-II-VI semiconductors host both strong phonons, and an additional type of particle-like excitation known as an exciton. Excitons are created through interactions between electrons and “holes” that the electrons left behind. [Read more...]