Influence of Synthesis Conditions for Yb:CaWO₄ Single Crystals on the Down-Conversion Luminescence of Yb³⁺ Ions in These Crystals

Abstract—Yb:CaWO₄ and Yb, Nb:CaWO₄ single crystals have been grown by the Czochralski method in air and in protective atmospheres and subjected to additional annealing in air, in CO/CO₂ atmosphere, and in forevacuum. The optical absorption spectra in the range from 250 to 1500 nm and the luminescence spectra of these crystals in the visible and near-IR spectral regions upon UV excitation have been investigated. It is shown that additional introduction of Nb⁵⁺ ions into Yb:CaWO₄ crystal increases by an order of magnitude (almost to unity) the Yb³⁺ distribution coefficient between CaWO₄ crystal and melt. It is found that optical excitation of the crystals in the range of 260–355 nm induces down-conversion luminescence of Yb³⁺ ions from the ²F_5/2 level in the vicinity of 1 µm. An increase in the oxidative potential of synthesis atmosphere, as well as the introduction of niobium into the crystal composition, weakens this luminescence. A consistent pattern explaining the nature of the donor centers involved in down-conversion population of excited state ²F_5/2 of Yb³⁺ in the crystals is proposed. Within this pattern, Yb²⁺ ions play the role of these donor centers. Another (much less efficient) mechanism of population of the ²F_5/2 level is intracenter relaxation from the higher-lying charge transfer excited state within Yb³⁺ ions. At the same time, it is confirmed that color centers based on oxygen vacancies and partially reduced tungsten ions, as well as self-trapped excitons on tungstate complexes, are not involved in population of the excited state ²F_5/2 of Yb³⁺ ions, and the optical centers formed in Yb:CaWO₄ crystals as a result of vacuum annealing suppress ytterbium luminescence.