Growth and downconversion luminescence of Ho3+/Yb3+ codoped α-NaYF4 single crystals by the Bridgman method using a KF flux

Downconversion (DC) luminescence with emission at about 1000 nm under excitation of 448‐nm light in Ho3+/Yb3+ codoped α‐NaYF4 single crystal is realized. The crystal was grown by the Bridgman method using KF as an assisting flux in a NaF‐YF3 system. The energy‐transfer process and quantum cutting (QC) mechanisms are presented through the analysis of the spectra. The energy‐transfer processes of first‐ and second‐order cooperative DC are responsible for the increase of the emission intensity at 1000 nm, and it is the first‐order cooperative DC that is dominant for the DC process. When the Ho3+ concentration is fixed at about 0.8 mol%, the optimal concentration for ∼1000 nm emission is 3.02 mol% Yb3+ in the current research. The energy‐transfer efficiency and the total quantum efficiency are analyzed through the luminescence decay curves. The maximum quantum cutting efficiency approaches to 184.4% in α‐NaYF4 single crystals of 0.799 mol% Ho3+ and 15.15 mol% Yb3+. However, the emission intensity at 1000 nm decreases while the energy‐transfer efficiency from Ho3+ to Yb3+ increases, which may result from the fluorescence quenching between Ho3+ and Yb3+ ions, Yb3+ and Yb3+ ions. The present study involved Ho3+/Yb3+ codoped α‐NaYF4 single‐crystal growth, structure analysis, luminescence properties, luminescence decay curves and calculating data of the energy‐transfer efficiency and the quantum cutting efficiency. The energy‐transfer processes of first‐ and second‐order cooperative DC are responsible for the increase of emission intensity at 1000 nm, and it is the first‐order cooperative DC that is dominant for the DC process.