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)

Abstract

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]approximate to 0.05-0.6x10(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 Ho3+ 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) Ho3+ configuration for NaBiW and NaBiMo crystals. Very satisfactory correlations were obtained between experimental and calculated crystal-field levels, with rms deviations sigma=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.