Laser radiation can induce cooling not only in dilute gases of atoms but also in certain transparent solids. The left panel of the figure shows the basic scheme: A laser photon excites a transition from an upper level of one state to a lower level of another, and a higher-energy photon is emitted, with phonons making up the difference. Now Mansoor Sheik-Bahae (University of New Mexico), Mauro Tonelli (University of Pisa), and colleagues have cooled a solid to 155 K, a new temperature record, using a laser-based system with no moving parts. The previous record, 208 K, was set in 2005 in the ytterbium-doped glass ZBLAN (composed of zirconium, barium, lanthanum, aluminum, and sodium fluorides). The Yb3+ ions’ lowest two energy levels are split by the surrounding atoms into seven sublevels, as shown in the right panel. ZBLAN’s appeal was that it had been synthesized to high purity for use in optical fibers. But its potential for cooling is limited: Its amorphous structure broadens the Yb3+ energy levels, so the peak absorption is weak. The new record was set using Yb-doped yttrium lithium fluoride (Yb:YLF), whose regular crystal structure makes the Yb3+ sublevels much sharper and the resonant absorption much stronger. But synthesis of high-purity Yb:YLF is relatively undeveloped, and existing high-power lasers fall just short of the Yb3+ E4--E5 transition’s 1020-nm peak. The researchers speculate that improvements in those areas should allow cooling to 77 K—the boiling point of liquid nitrogen. (D. V. Seletskiy et al., Nat. Photonics 4, 161, 2010.) —Johanna Miller
