First demonstration of optical refrigeration efficiency greater than 4% at room temperature

In this work, we present the cooling efficiency of a LiYF₄ (lithium yttrium fluoride,YLF) sample co-doped with 10at.% ytterbium (Yb³⁺) and 0.0040at.% thulium (Tm³⁺). For the first time at room temperature, the cooling efficiency of a sample overcomes the 4%-barrier, and it must be compared with the best values reported in literature, about 3%, obtained with a YLF:10at.%Yb sample. We also investigated the frequency behaviour of energy transfer mechanisms between Yb and Tm ions in order to have a better understanding of the contribution of phonons to the cooling cycle. These mechanisms can explain the cooling efficiency enhancement in the co-doped system.