Composition-engineered Yb3+ and Er3+ co-doped bimolybdate and bitungstate mixed crystals for promising solid-state infrared laser application†

A series of 10 at% Yb³⁺ and 5 at% Er³⁺ co-doped NaGd(Mo_xW_1−xO₄)₂ (x = 0.5, 0.7 and 0.9) single crystals was grown via the Czochralski method. X-ray diffraction (XRD), Rietveld refinement and FT-IR analyses were employed to comprehensively analyze the crystal structures. The results firmly established that the crystals belong to the tetragonal system, with the space group identified as I4₁/a. In addition, the 1.5 μm spectral properties of the crystals were investigated, including emission, absorption and fluorescence decay characteristics. Importantly, the study delved deeply into uncovering how the crystal's composition impacts its spectroscopic properties. As the crystal composition changed, there was a notable inhomogeneous broadening of the spectral line. A maximum full width at half-maximum (FWHM) of 83 nm at 1535 nm was obtained for the Yb,Er:NaGd(Mo_0.5W_0.5O₄)₂ crystal. However, the maximum absorption and emission cross-sections were achieved in the Yb,Er:NaGd(Mo_0.7W_0.3O₄)₂ crystal, with values of 1.15 × 10⁻²⁰ cm² and 1.05 × 10⁻²⁰ cm², respectively. Besides, for the Yb,Er:NaGd(Mo_0.7W_0.3O₄)₂ crystal, a positive gain cross-section was obtained at around 1535 nm for a population inversion ratio (β) of >0.4. These results show that double molybdate/tungstate mixed crystals co-doped with Yb³⁺ and Er³⁺ are promising for 1.5 μm near-infrared solid-state laser application.