Spectroscopic study of thulium-activated double sodium yttrium fluoride Na0.4Y0.6F2.2:Tm3+ crystals: I. Intensity of spectra and luminescence kinetics

Na0.4Y0.6F2.2:Tm3+ crystals with a thulium content from 1 to 100 at % have been grown by the Stockbarger-Bridgman method. The optical spectra of Na0.4Y0.6F2.2:Tm3+ crystals were investigated in detail at room and low (10 K) temperatures, and the luminescence kinetics was analyzed using different excitation methods. The structure of the Stark splitting of thulium levels as “quasi-centers,” characterized by inhomogeneous broadening of the Stark components, is determined from analysis of the absorption spectrum at 10 K. The oscillator strengths of the transitions from the ground state to excited multiplets are determined from the absorption cross-section spectra at 300 K for ten transitions in the range 5000–38 500 cm−1 and seven transitions in the range 5000–28 500 cm−1. The transition intensity parameters Ω t , obtained by the Judd-Ofelt method from the spectra due to the transitions to ten and seven excited levels, were found to be, respectively, (i) Ω2 = 1.89 × 10−20, Ω4 = 2.16 × 10−20, and Ω6 = 1.40 × 10−20 cm2 and (ii) Ω2 = 2.04 × 10−20, Ω4 = 2.01 × 10−20, and Ω6 = 1.44 × 10−20 cm2. These values of the intensity parameters were used to calculate the radiative transition probabilities and branching ratios and to estimate the multiphonon nonradiative transition probabilities for NYF:Tm. The luminescence decay kinetics from thulium radiative levels upon their selective excitation by nanosecond laser pulses has been studied and the lifetimes of thulium radiative levels in NYF crystals have been found.