Crystal field analysis and emission cross sections of Ho~(3+) in the locally disordered single-crystal laser hosts M~+Bi(XO_4)_2 (M~+=Li,Na; X=W,Mo)

摘要

The spectroscopic properties of Ho~(3+) laser channels in locally disordered tetragonal NaBi(WO_4)_2 (NaBiW), NaBi(MoO_4)_2 (NaBiMo), and LiBi(MoO_4)_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 X 10~(20) cm~(-3). Here 5-K polarized optical absorption and photoluminescence measurements have been used to determine the energy position of 85, 56, and 39 Ho~(3+) Stark levels in NaBiW, NaBiMo, and LiBiMo crystals, respectively. These energy levels were labeled with irreducible representations corresponding to the S_4 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 4f~(10) Ho~(3+) configuration for NaBiW and NaBiMo crystals. Very satisfactory correlations were obtained between experimental and calculated crystal-field levels, with rms deviations σ-=8.8 and 7.3 cm~(-1) for NaBiW and NaBiMo, respectively. The radiative properties and emission cross sections of Ho~(3+) laser channels in these hosts were calculated by the Judd-Ofelt theory and compared with experimental results. The emission cross sections of Ho~(3+) 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 Ho~(3+)-doped double tungstates and double molybdates feasible materials for tunable and short-pulse laser operation.