Energy transfer enhanced laser cooling in Ho³⁺ and Tm ³⁺-codoped lithium yttrium fluoride

We report a theoretical scheme for laser cooling of solids based on energy transfer usually found in rare-earth codoped materials. The cooling scheme enables a large enhancement in the cooling efficiency with regard to the standard anti-Stokes fluorescence cooling. A Ho³⁺ and Tm ³⁺-codoped low-phonon crystal (LiYF₄) sample is investigated to find that the cooling efficiency increases, and then decreases with the increasing of the resonant absorption. The optimal cooling efficiency is predicted to exceed 5%. The maximum cooling power density could be promoted greatly by applying the codoped cooling scheme. The cooling scheme is also valid for other rare-earth (for example, Tm³⁺ and Er³⁺, or Er³⁺ and Yb³⁺) codoped materials.