Infrared to near-infrared and visible upconversion mechanisms in LiYF _4: 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 Yb ³⁺-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 ²F_5/2→²F_7/2 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%Yb ³⁺, 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.