Cryogenic optical refrigeration

We review the field of laser cooling of solids, focusing our attention on the recent advances in cryogenic cooling of an ytterbium-doped fluoride crystal (Yb3+ : YLiF4). Recently, bulk cooling in this material to 155 K has been observed upon excitation near the lowest-energy (E4-E5) crystal-field resonance of Yb3+. Furthermore, local cooling in the same material to a minimum achievable temperature of 110 K has been measured, in agreement with the predictions of the laser cooling model. This value is limited only by the current material purity. Advanced material synthesis approaches reviewed here would allow reaching temperatures approaching 80 K. Current results and projected improvements position optical refrigeration as the only viable all-solid-state cooling approach for cryogenic temperatures. (C) 2012 Optical Society of America