Raman measurement and optical properties of Nd:GdCa ₄O(BO ₃) ₃ crystals

X-ray diffraction measurements on a 0.65 at. % Nd-doped GdCa ₄O(BO ₃) ₃ (Nd:GdCOB) crystal revealed a noncentrosymmetric monoclinic space group Cm with lattice parameters a=8.091±0.001, b=16.023±0.002, c=3.559±0.006Å, and β=101.30±0.01°. The fitting primitive cell contained two Nd:GdCOB units. Raman data show that the characteristic spectra of Nd:GdCOB are mainly associated with BO ₃ triangles and partly by Ca(1)O ₆ octahedra, which implies that the structural rigidity of Nd:GdCOB is mainly associated with the B-O bond stretching and bending modes and partly by the Ca(1)-O bonds. The large distortions of the Gd/NdO ₆ and Ca(2)O ₆ octahedra intensify the polar forces and anisotropic lattice forces, which indicates excellent nonlinear properties for Nd:GdCOB. Simultaneously, the large distortions of the Gd/NdO ₆ octahedra also provide a suitable crystal field for the activated Nd ³⁺ ions, with the site symmetry m lacking inversion symmetry. This condition increases the probability of the parity-forbidden f→f transitions. Also due to the greater number of anion groups in the primitive cell, this crystal would be expected to be excellent as a self-frequency doubled material. The absorption parameters calculated by Judd-Ofelt theory show that the transitions ⁴I _9/2→ ⁴F _5/2 and ⁴I _9/2→ ²H _9/2 have larger absorption cross sections and enable the laser-diode pumping band around 808 nm to be more than 30 nm in bandwidth. In the calculated luminescence parameters, the radiation ⁴F _3/2→ ⁴I _11/2 transition at wavelength of 1060 nm has the largest fluorescence branch-ratio and differential stimulated-emission cross section.