New materials for optical cooling

Well-characterized solid-state laser materials are evaluated for performance in optical refrigeration as well as radiation-balanced laser systems. New figures-of-merit are developed and applied to ytterbium-doped materials. Superior performance is predicted for high-cross-section tungstate materials. Photothermal deflection experiments on samples of Yb3+-doped KGd(WO4)(2) confirm anti-Stokes fluorescence cooling. This is the first observation of optical cooling in a crystal.