Nature 493, 504–508 (2013)

Optical refrigeration, also known as laser cooling, is being explored as a solid-state technology for reducing the temperature of small devices and samples. This approach relies on using optical emission at a shorter wavelength than an incident pump laser to extract thermal energy in the form of phonons from a suitable material system. It has been demonstrated in rare-earth-doped glasses, but net cooling in semiconductors has remained elusive. Now, Jun Zhang and co-workers from Nanyang Technological University in Singapore report cooling by up to 40 K from room temperature and by 15 K from 100 K by pumping cadmium sulphide (CdS) nanobelts with 514 nm or 532 nm light with a power in the milliwatt range. The cooling effect, which has an estimated efficiency of the order of 1–2%, is attributed to strong coupling between excitons and longitudinal optical phonons in CdS, which allows resonant annihilation of several longitudinal optical phonons in a luminescence up-conversion process. According to the researchers, the demonstration indicates that group II–VI semiconductors have a promising future for cooling applications.