Solid-state Laser Cooling of Ytterbium-doped Tungstate Crystals

Anti-Stokes fluorescence can be used to cool a material if its radiative quantum efficiency is nearly unity. Until recently, the only solid which had shown net cooling was an ytterbium-doped heavy metal fluoride glass. Reasons for the lack of success with other materials include energy transfer to unwanted impurities, trapping of the fluorescence by internal reflection and reabsorption, and nonradiative relaxation via low-frequency accepting modes. However, substantial progress has recently been achieved in cooling new classes of rare-earth-doped solids. Photothermal deflection spectroscopy of Yb3+ in the two laser crystals KGd(WO4)2 and KY(WO4)2 demonstrates optical cooling. These materials have high figures of merit for radiation-balanced lasing, in which the Stokes heating by stimulated emission is offset by the anti-Stokes cooling due to spontaneous emission. However, approximately 1.3% of the relaxations from the excited to the ground states are nonradiative via energy transfer to impurities. In addition, about 0.3% of the pump light is absorbed per centimeter of travel by species such as transition-metal ions which dump that energy directly into heat.