Crystal field analysis and emission cross sections of Ho³⁺ in the locally disordered single-crystal laser hosts M⁺Bi(XO₄)₂ (M⁺=Li,Na; X=W,Mo)

The spectroscopic properties of Ho³⁺ laser channels in locally disordered tetragonal NaBi(WO₄)₂ (NaBiW), NaBi(MoO₄)₂ (NaBiMo), and LiBi(MoO₄)₂ (LiBiMo) single crystals grown by the Czochralski method have been studied in the 5–300-K temperature range using several holmium concentrations [Ho]0.05–0.6×10²⁰  cm⁻³. Here 5-K polarized optical absorption and photoluminescence measurements have been used to determine the energy position of 85, 56, and 39 Ho³⁺ Stark levels in NaBiW, NaBiMo, and LiBiMo crystals, respectively. These energy levels were labeled with irreducible representations corresponding to the S₄ local symmetry of an average optical center. Single-electron Hamiltonians combining together free-ion and crystal-field interactions have been used in the fit of experimental energy levels and in the simulation of the corresponding 4f¹⁰ Ho³⁺ configuration for NaBiW and NaBiMo crystals. Very satisfactory correlations were obtained between experimental and calculated crystal-field levels, with rms deviations =8.8 and 7.3  cm⁻¹ for NaBiW and NaBiMo, respectively. The radiative properties and emission cross sections of Ho³⁺ laser channels in these hosts were calculated by the Judd-Ofelt theory and compared with experimental results. The emission cross sections of Ho³⁺ in NaBiW are similar to those observed in other crystal laser hosts, and positive gain cross sections can be achieved in extended spectral ranges. These properties make the Ho³⁺-doped double tungstates and double molybdates feasible materials for tunable and short-pulse laser operation.

©2007 The American Physical Society