Infrared to near-infrared and visible upconversion mechanisms in LiYF4: Yb3+, Ho3+

Upconversion materials have regained interest in recent years due to their potential to enhance the efficiency of solar cells. The research has focused on lanthanide based upconverters, especially Er3+- and Yb3+-doped materials. In this paper we report Ho3+ and Yb3+ upconversion after excitation into the 5I6 Ho3+ state in LiYF4. Infrared excitation into the 5I6 Ho3+ level at 1150 nm induces both visible 5F5→5I8 Ho3+ and near-infrared 2F5/2→2F7/2 Yb3+ upconversion luminescence, around 650 nm and 1000 nm, respectively. The concentration dependence was investigated and the most efficient Ho3+ upconversion was found for LiYF4: 2%Yb3+, 20%Ho3+. Time-resolved spectroscopy as well as temperature dependent measurements were used to unravel the mechanism underlying the upconversion processes. Experimental 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.