Abstract: Upconversion materials have regained interest in recent years due to their potential to enhance the effciency of solar cells. The research has focused on lanthanide based upconverters, especially Er3 þ- and Yb3 þ-doped materials. In this paper we report Ho3 þ and Yb3 þ upconversion after excitation into the 5I6 Ho3 þ state in LiYF4. Infrared excitation into the 5I6 Ho3 þ level at 1150 nm induces both visible 5F5-5I8 Ho3 þ and near-infrared 2F5/2-2F7/2 Yb3 þ upconversion luminescence, around 650 nm and 1000 nm, respectively. The concentration dependence was investigated and the most effcient Ho3 þ upconversion was found for LiYF4: 2%Yb3 þ, 20%Ho3 þ. Time-resolved spectroscopy as well as temperature dependent measurements were used to unravel the mechanism underlying the upconversion processes. Experi- mental results and rate equation modeling provides evidence for infrared to near-infrared/visible upconversion mechanisms that are different from those previously reported in the literature.

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