Co-doping of LiYF4 crystal: a virtuous effect of cooling efficiency

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, two samples 5% Yb:YLF and 5% Yb-0.0080% Tm:YLF, grown with the same starting material in order to avoid the difference in chemical composition of impurity. Some contaminants, like iron, are very detrimental for cooling efficiency and they make it difficult to compare grown crystals with different raw powders. We demonstrate that the presence of a small concentration of Tm3+ ions permits it to decrease the background absorption and to increase the cooling efficiency.