Infrared-to-visible upconversion inLaCl3:1%Er3+: Energy-level and line-strength calculations

Single crystals of ${\mathrm{LaCl}}_3{:1\%\mathrm{}\mathrm{Er}}^{3+}$ were grown by the Bridgman technique. ${\mathrm{Er}}^{3+}$ substitutes for ${\mathrm{La}}^{3+}$ and has a site symmetry of $C_{3h}$ in ${\mathrm{LaCl}}_3.$ Seventy-three energy levels of ${\mathrm{Er}}^{3+}$ were determined up to $45000{\mathrm{cm}}^{\mathrm{-}1}$ from absorption, excitation, and luminescence spectra measured over the temperature range 4.2–293 K. A correlated crystal-field analysis was done by fitting 16 atomic, four crystal-field, and one correlated crystal-field parameters to the experimental Stark levels. The fit is excellent with a standard deviation of $9.0{\mathrm{cm}}^{\mathrm{-}1}.$ On the basis of the wave functions thus obtained the oscillator strengths for the transitions from the ${}^4{I}_{15/2}\mathrm{}\left(1\right)\mathrm{}$ ground state to all the levels up to $45000{\mathrm{cm}}^{\mathrm{-}1}$ were calculated by fitting seven complex $A_{t,p}^{\lambda }$ intensity parameters to 47 observed oscillator strengths. The intensity distribution among the crystal-field components was then calculated for the inter-excited-state transition ${}^4{I}_{11/2}{\to }^4{F}_{7/2}$ which plays an important role in an excited-state-absorption upconversion process.