Effective Upconversion in Alkaline Earth Fluorides Activated by Yb3+–Ho3+ Ions

Abstract: A study is performed of optical spectra (absorption, excitation, luminescence, energy yield) associated with the upconversion of infrared excitation of 980 nm into visible radiation in crystals of alkaline–earth fluorides CaF2, SrF2, and BaF2, co-doped with YbF3 (0.01–10 mol %) and HoF3 (0.01–0.3 mol %). In the 0.1–10 W cm−2 range of excitation power densities, the intensity of the upconversion bands at 542, 650, and 752 nm grows as the square of the power. A quadratic dependence of the intensity of upconversion on the concentration of Yb3+ (0.03–3 mol %) and a weak dependence on the Ho3+ concentration (0.01–0.3 mol %) with a maximum at 0.1–0.15 mol % are observed. It is assumed that the upconversion is due to consecutive energy transfer from two closely spaced excited ytterbium ions to the holmium ion. © 2019, Allerton Press, Inc.