Upconversion cooling of Er-doped low-phonon fluorescent solids
  We report on a novel mechanism for laser cooling of fluorescent solids based on infrared-to-visible upconversion often found in rare-earth-doped low-phonon materials. This type of optical cooling presents some advantages with regards to conventional anti-Stokes cooling. Among them, it allows to obtain cooling in a broader range of frequencies around the barycenter of the infrared emitting band. 
Angel J. Garcia-Adeva, Rolindes Balda, and Joaquin Fernandez