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.