Loss processes on crystal cooling efficiency

In the perspective of building a solid state cryocooler based on optical refrigeration, it is strategic to develop high purity crystals and to have a full understanding of the mechanisms that can reduce cooling efficiency. In this paper we highlight two loss mechanisms that can hinder the optical refrigeration performance of a Yb:YLF monocrystal: the presence of impurities and fluorescence reabsorption. We show how the presence of impurities can be inferred by measuring the mean lifetime of the Yb optical transition. We measure how a few ppms of other rare earth ions can be detrimental for optical refrigeration by exciting a cooling Yb:YLF sample with two laser sources. We also report the first practical demonstration of the negative effect of fluorescence reabsorption on the cooling performance.