• codoping ytterbium-thulium
• cooling efficiency
• Czochralski technique
• optical refrigeration
• Rare Earths

Anti-Stokes shift, provided by embedded rare earth (RE) ions in fluoride crystal host, is used to obtain a net cooling of a solid system. Yb3+ is currently the RE ion that presents the best cooling performance when inserted into a suitable host, like yttrium lithium fluoride (YLF). Recently, a new approach to laser cooling has been proposed, in which an enhancement of the cooling efficiency is reached by co-doping with Yb3+ and Tm3+. In this work, we compare, in terms of cooling efficiency, several samples doped with Yb and co-doped Yb-Tm. They were grown by employing the Czochralski technique. This specific technique allows to fabricating defect-free and large samples for such application. Samples possessing the same chemical composition of impurity and defect-free were compared denoting the goodness of the Tm-enrichment for the optical refrigeration. We demonstrate that the presence of a small concentration of Tm ions permits to decrease the background absorption and to increase the cooling efficiency. So far, a sample reached the important results of 4.2% in cooling efficiency overcoming the 3% barrier.