Infrared absorption and laser spectroscopy of Ho 3 + doped K 2 YF 5 microparticles

High-resolution absorption and laser spectroscopy are used to determine electronic energy levels for Ho 3 + ions in K 2 YF 5 microparticles. A total of 72 crystal-field energy levels, distributed among 8 multiplets, are assigned. This optical data is used for crystal-field modelling of the electronic structure of Ho 3 + in K 2 YF 5 . Partially-resolved hyperfine splittings are accurately reproduced by the model. The temperature dependence of the fluorescent lifetime of the 5 F 5 multiplet is measured and the temperature dependence of the non-radiative relaxation is modelled by a five-phonon process. Preliminary measurements of infra-red to visible upconversion in microparticles co-doped with Ho 3 + and Yb 3 + is reported. • Hydrothermal synthesis of high brightness K 2 YF 5 :Ho 3+ microparticles. • Detailed spectroscopic study of the optical centre in K 2 YF 5 :Ho 3+ . • Crystal-field analysis of experimental energy levels and ground-state hyperfine splitting. • Modelling of temperature-dependent non-radiative relaxation. • Infra-red to visible upconversion in co-doped Ho 3+ /Yb 3+ microparticles.