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

A series of 10 at% Yb3+, 5 at% Er3+ co-doped NaGd(MoxW1-xO4)2 (x = 0.5, 0.7 and 0.9) single crystals were grown by Czochralski method. X-ray diffraction (XRD), Rietveld refinement and FT-IR analyses were employed to comprehensively analyze the crystal structure. 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 deep into uncovering how the crystal's composition impacts its spectroscopic properties. As the crystal composition changes, there is a notable inhomogeneous broadening of the spectral. A maximum full width at half-maximum (FWHM) at 1535 nm was obtained to be 83 nm in (MoO4)1.0(WO4)1.0 composition crystal. However, the maximum absorption and emission cross-sections were achieved in (MoO4)1.4(WO4)0.6 component crystal with values of 1.15 × 10-20 cm2 and 1.05 × 10-20 cm2, respectively. Besides, for (MoO4)1.4(WO4)0.6 composition crystal, a positive gain cross-section is obtained at around 1535 nm for a population inversion ratio β above 0.4. The results show that double molybdate/tungstate mixed crystals co-doping with Yb3+ and Er3+ are promising for 1.5 μm near-infrared solid-state lasers.