Crystal field analysis and emission cross sections ofHo3+in the locally disordered single-crystal laser hostsM+Bi(XO4)2(M+=Li,Na;X=W,Mo)

Abstract

The spectroscopic properties of ${\mathrm{Ho}}^{3+}$ laser channels in locally disordered tetragonal $\mathrm{Na}\mathrm{Bi}{(\mathrm{W}{\mathrm{O}}_{4})}_{2}$ (NaBiW), $\mathrm{Na}\mathrm{Bi}{(\mathrm{Mo}{\mathrm{O}}_{4})}_{2}$ (NaBiMo), and $\mathrm{Li}\mathrm{Bi}{(\mathrm{Mo}{\mathrm{O}}_{4})}_{2}$ (LiBiMo) single crystals grown by the Czochralski method have been studied in the $5--300\text{\ensuremath{-}}\mathrm{K}$ temperature range using several holmium concentrations $[\mathrm{Ho}]\ensuremath{\approx}0.05--0.6\ifmmode\times\else\texttimes\fi{}{10}^{20}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. Here $5\text{\ensuremath{-}}\mathrm{K}$ polarized optical absorption and photoluminescence measurements have been used to determine the energy position of 85, 56, and 39 ${\mathrm{Ho}}^{3+}$ 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 $4{f}^{10}$ ${\mathrm{Ho}}^{3+}$ configuration for NaBiW and NaBiMo crystals. Very satisfactory correlations were obtained between experimental and calculated crystal-field levels, with rms deviations $\ensuremath{\sigma}=8.8$ and $7.3\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ for NaBiW and NaBiMo, respectively. The radiative properties and emission cross sections of ${\mathrm{Ho}}^{3+}$ laser channels in these hosts were calculated by the Judd-Ofelt theory and compared with experimental results. The emission cross sections of ${\mathrm{Ho}}^{3+}$ 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 ${\mathrm{Ho}}^{3+}$-doped double tungstates and double molybdates feasible materials for tunable and short-pulse laser operation.