Cooling to 208K by optical refrigeration

We report cooling to record low temperatures by optical refrigeration with ytterbium-doped zirconium–barium–lanthanum–aluminum–sodium-fluoride glass. The glass cooling element was mounted in vacuum in a low-thermal-emissivity chamber and pumped with the light from a diode-pumped solid state Yb:YAG (ytterbium-doped yttrium-aluminum-garnet) laser. Starting from room temperature, the glass cooling element reached a minimum temperature of ∼208K when pumped with ∼10W of 1026-nm light. The heat load at minimum temperature was ∼29mW.