2.0-μm emission and energy transfer of Ho³⁺/Yb³⁺ co-doped LiYF₄ single crystal excited by 980 nm

Ho³⁺/Yb³⁺ co-doped LiYF₄ single crystals with various Yb³⁺ concentrations and ～ 0.98 mol% Ho³⁺ concentration are grown by the Bridgman method under the conditions of taking LiF and YF₃ as raw materials and a temperature gradient (40 ℃/cm–50 ℃/cm) for the solid–liquid interface. The luminescent performances of the crystals are investigated through emission spectra, infrared transmittance spectrum, emission cross section, and decay curves under excitation by 980 nm. Compared with the Ho³⁺ single-doped LiYF₄ crystal, the Ho³⁺/Yb³⁺ co-doped LiYF₄ single crystal has an obviously enhanced emission band from 1850 nm to 2150 nm observed when excited by a 980-nm diode laser. The energy transfer from Yb³⁺ to Ho³⁺ and the optimum fluorescence emission around 2.0 μm of Ho³⁺ ions are investigated. The maximum emission cross section of the above sample at 2.0 μm is calculated to be 1.08× 10^-20 cm² for the LiYF₄ single crystal of 1-mol% Ho³⁺ and 6-mol% Yb³⁺ according to the measured absorption spectrum. The high energy transfer efficiency of 88.9% from Yb³⁺ to Ho³⁺ ion in the sample co-doped by Ho³⁺ (1 mol%) and Yb³⁺ (8 mol%) demonstrates that the Yb³⁺ ions can efficiently sensitize the Ho³⁺ ions.