Absolute up- and down-conversion luminescence efficiency in hexagonal Na(Lu/Y/Gd)F4: Yb, Er/Tm/Ho with optimized chemical composition

Up- and down-conversion luminescence efficiency of hexagonal NaYF4, NaLuF4 and NaGdF4, prepared by a novel hydrothermal method using succinic acid and codoped with Yb and Er/Tm/Ho, was investigated over a broad spectral range 350 – 1640 nm. For this synthesis method NaLuF4 matrix provided the best up-conversion efficiency, while NaYF4 and NaGdF4 give about twice and eight-times reduced efficiency, respectively. We systematically varied dopant concentrations of Yb3+ and Er3+/Tm3+/Ho3+ in order to find the optimum composition. The best performing material was found to be NaLuF4: Yb3+ 15%, Er3+ 1.5%, for which we performed detailed study of absolute luminescence power efficiency under different excitation power using an integrating sphere approach. The integrated up-converted signal reaches about 17% in the excitation power range of 70-100 W/cm2, while the down-converted emission power efficiency is about 10%. An important fraction of absorbed power is (re)emitted close to the excitation wavelength. This close-to-resonance emission represents 2–3 (anti-Stokes) and 10–30% (Stokes shifted) of absorbed power depending weakly on the excitation power. In order to prove Yb3+ luminescence-related origin of this loss channel we exploited the time-resolved luminescence kinetics using modulated laser beam (both 405 and 978 nm), which allows also determination of the absorption cross section in the range 1 – 3 × 10-21 cm2 at 978 nm matching the published values for Yb3+ ion. The determined efficiency parameters are comparable to the best values reported so far and we discuss the way to further increase the efficiency limit.