First-Principle Calculations and Experimental Investigation of Structure, Defect, and Spectra Properties of Yb3+-doped NaGd(1–x)La(x)(MoO4)2 Composite Laser Crystals

Inhomogeneous broadening in disordered crystals has been widely recognized as a successful approach to enhance the spectral bandwidth of rare earth ions in optical spectra, and the Yb3+-doped mixed laser crystals are known as ultrafast pulse materials and have emerged as a hotspot recently. In this study, mixed sodium gadolinium/lanthanum double molybdate disordered laser crystals NaGd(1–x)La(x)(MoO4)2 with x = 0.3, 0.5, and 0.9 were grown by the Czochralski (Cz) method. The structure of as-grown crystals was investigated by XRD, SEM, FT-IR, and Raman experiments. The spectroscopic properties, including absorption, emission, and fluorescence decay characteristics, were investigated thoroughly. The NaGd0.5La0.5(MoO4)2 (NGLM) composite crystal exhibited an exceptionally broad absorption which reaches 70 nm (fwhm) at 980 nm, exceeding those of NaGd­(MoO4)2 (NGM) and NaLa­(MoO4)2 (NLM). Additionally, the electronic structures (band structure and density of states) of NGM, NGLM, and NLM were determined from the first-principles method combined with the PBE function. The calculations of defect formation energies under O-poor and O-rich conditions demonstrate that the +2 defects are easily generated in the undoped NGM crystal prepared under reducing atmospheres, and the calculation of absorption properties reveals that the black color of NGM without annealing is mainly attributed to intraband transitions between defect induction states and VBM or CBM. Moreover, the lifetimes of excited energy level 2F5/2 of Yb3+ were examined under different experimental environments to obtain the intrinsic radiative lifetime, and the emission cross section was calculated as 2.25 × 10–20 cm2 at 1016 nm. The features of the Yb3+:NaGd(1–x)La(x)(MoO4)2 crystal indicate the potential applications for ultrafast and broad tunable laser through slight regulation of the composite engineering strategy.